Dithiolene Chemistry: Synthesis, Properties, and Applications

DITHIOLENE CHEMISTRY PROGRESS IN INORGANIC CHEMISTRY VOLUME 52

Advisory Board JACQUELINE K. BARTON CALIFORNIA INSTITUTE OF TECHNOLOGY, PASADENA, CALIFORNIA THEODORE J. BROWN UNIVERSITY OF ILLINOIS, URBANA, ILLINOIS JAMES P. COLLMAN STANFORD UNIVERSITY, STANFORD, CALIFORNIA F. ALBERT COTTON TEXAS A & M UNIVERSITY, COLLEGE STATION, TEXAS ALAN H. COWLEY UNIVERSITY OF TEXAS, AUSTIN, TEXAS RICHARD H. HOLM HARVARD UNIVERSITY, CAMBRIDGE, MASSACHUSETTS EIICHI KIMURA HIROSHIMA UNIVERSITY, HIROSHIMA, JAPAN NATHAN S. LEWIS CALIFORNIA INSITITUTE OF TECHNOLOGY, PASADENA, CALIFORNIA STEPHEN J. LIPPARD MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MASSACHUSETTS TOBIN J. MARKS NORTHWESTERN UNIVERSITY, EVANSTON, ILLINOIS EDWARD I. STIEFEL PRINCETON UNIVERSITY, PRINCETON, NEW JERSEY KARL WIEGHARDT ¨ LHEIM, GERMANY MAX-PLANCK-INSTITUT, MU

DITHIOLENE CHEMISTRY Synthesis, Properties, and Applications Special volume edited by

EDWARD I. STIEFEL Department of Chemistry, Princeton University Princeton, New Jersey

PROGRESS IN INORGANIC CHEMISTRY Series edited by

KENNETH D. KARLIN Department of Chemistry, Johns Hopkins University Baltimore, Maryland

VOLUME 52

AN INTERSCIENCE PUBLICATION JOHN WILEY & SONS, INC.

Copyright # 2004 by John Wiley & Sons, Inc. All rights reserved. Published by John Wiley & Sons, Inc., Hoboken, New Jersey. Published simultaneously in Canada. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning, or otherwise, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, 978-750-8400, fax 978-646-8600, or on the web at www.copyright.com. Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, (201) 748-6011, fax (201) 748-6008. Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best efforts in preparing this book, they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives or written sales materials. The advice and strategies contained herein may not be suitable for your situation. You should consult with a professional where appropriate. Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. For general information on our other products and services please contact our Customer Care Department within the U.S. at 877-762-2974, outside the U.S. at 317-572-3993 or fax 317-572-4002. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print, however, may not be available in electronic format. Library of Congress Catalog Card Number 59-13035 ISBN 0-471-37829-1 Printed in the United States of America 10 9 8

7 6 5 4

3 2 1

Dieter Sellmann (1941–2003) With the agreement of all of the authors, this volume is dedicated to our colleague Dieter Sellmann, who died unexpectedly shortly after completing his contribution to this book. Dieter was a marvelous synthetic chemist whose beautiful molecules are amply displayed in Chapter 11 (written with Jo¨ rg Sutter) of this collection. But, Dieter was more than someone who created novel and beautiful molecules. He brought insight and understanding to these structures and, especially, to their reactivity with important small molecules. He was inspired by biology, by the reactions of nitrogenase, and by nitrogen oxide interconversions. Indeed, some of his synthetic creations laid bare the possibilities of binding of reactive intermediates in glorious detail, about which others could only speculate. His molecules will continue to give us important clues about the reactivity of enzymes and other catalytic systems. We will miss Dieter’s creativity, insights, and good humor, but we also remember and honor a life of great scientific accomplishment, which his article in this volume beautifully represents. Dieter was an outstanding scientist, a wonderfully warm and vibrant human being, and a good friend. He will be sorely missed. v

Preface This volume of Progress in Inorganic Chemistry documents the intense current interest and bright future prospects for research on the chemistry and uses of dithiolene complexes. Over the last forty years, complexes of these remarkable ligands have gone from an important and interesting subclass of inorganic coordination chemistry to a field that, while generating continued interest in structure, bonding, and reactivity, now has impact on a far larger stage. The findings that dithiolene complexes have useful reactivity and sensing properties, that they are at the core of a large number of biologically essential enzymes, and that they display remarkable (super)conductivity, optical, and magnetic properties in the solid state, together have given great impetus to work in this field. These new and, in many cases, quite unexpected findings are documented in this volume side by side with continued discussions of the basic synthetic, structural, spectroscopic, bonding, and reactivity properties of the complexes. The size and scope of this volume and the quality of the individual contributions reveal a vital field that is just entering its prime. It is our hope that, by collecting comprehensive reviews on the various subfields of dithiolene chemistry in a single place, we will contribute toward the stimulation of this now burgeoning field of interdisciplinary research. Although there had been some very early work (pre-1960) on the use of certain dithiolene ligands in quantitative analysis of metal ions, these initial studies were largely empirical and never explored the highly colored complexes at the structural level. The modern era of dithiolene research started in the early 1960s with contributions from three research groups: those of Schrauzer and co-workers (at Munich and the University of California at San Diego); Gray and co-workers (at Columbia); and Davison and Holm and co-workers (at Harvard). The combined work of these research groups first established the square-planar nature, redox activity, and broad scope of the highly colored bis(dithiolene) complexes of the late transition metals. From the outset, there was contention about the electronic structures of these complexes, as the redox capacity of the ligands (i.e., their noninnocence) made the assignment of oxidation state difficult, or, in some cases, ambiguous, at best. Interest in the area was further heightened by the synthesis and structural characterization of the tris(dithiolene) complexes of early transition metals. In addition to sharing the unusual properties of the bis complexes, members of the tris(dithiolene) family were shown to be the first molecular examples of trigonal-prismatic coordination by Eisenberg and Ibers (at Columbia and Brookhaven). vii

viii

PREFACE

Work on dithiolene chemistry continued through the 1960s and 1970s fueled by continued interest in the remarkable coordination chemistry of dithiolene complexes. However, in the last 20 years tremendous added impetus to research in the area arose from discoveries in materials science, enzymology, analytical science, and reactivity that broadened the impact and import of dithiolene chemistry. This volume seeks to capture the interplay of basic work on dithiolene complexes with the growing biological, sensor, reactivity, and materials science implications and applications that have made dithiolene chemistry a vibrant and growing field. Chapter 1 deals with synthesis, where we learn that there are many ways to make dithiolene complexes, either from preformed ligands or through the chemical reactivity of bound sulfur species. Synthesis is at the core of most of the coordination chemistry that has been done on dithiolene complexes. Chapter 2 deals with structures and structural trends of the most common simple dithiolene complexes. Indeed, it was the square-planar nature of most late transition metal bis(dithiolene) complexes and the unprecedented trigonalprismatic six-coordination of some of the tris(dithiolene) complexes that was one of three major drivers for early work in the field. In addition to structure, early work was also driven by two other prominent features: electronic structural uniqueness and one-electron redox activity. The second major driver, the electronic structural uniqueness of dithiolene complexes of the transition metals, was manifested in their highly colored nature (i.e., large extinction coefficients). This feature lead to extensive spectroscopic, magnetic, and theoretical studies, which continue through the present with great intensity. Electronic structural studies, reviewed in Chapter 3, reveal the many intricate and interesting features of dithiolene complexes, including the oxidation state ambiguity that can arise from the noninnocence of the ligands. Chapter 4 deals explicitly with the vibrational spectroscopy (IR and Raman), where the spectra are extremely valuable in probing the binding in the ligands and complexes. The material in Chapters 3 and 4 also show the great utility of the spectroscopic and computational tools in probing the molybdenum and tungsten dithiolene cofactors found in biological systems. The third feature of dithiolene chemistry that attracted early attention was the chemical and electrochemical one-electron redox reactivity of the complexes, which allowed a given complex stoichiometry (M/L ratio) to be isolated with several different charges (i.e., different states of oxidation, albeit not necessarily different oxidation states of the metal). Chapter 5 deals with the electrochemical and chemical reactivity of dithiolene complexes, wherein it is seen that the chemical reactivity goes beyond simple redox reactivity and includes reactions that are often ligand, rather than metal, based. Chapters 6 and 7 show how the unique electronic structural features of dithiolene complexes manifest themselves in luminescent and photochemical behavior. Chapter 6 reveals that the excited states of dithiolene complexes and

PREFACE

ix

their photochemistry can be understood in those cases where the luminescent activity can be dissected in detail. Chapter 7 describes particular examples wherein the luminescent behavior has been exploited in the development of effective sensors for molecular oxygen and, with great promise, for other molecules as well. Chapter 8 reviews the considerable work that has been done on solid-state systems. These systems combine the structural features of planar dithiolene complexes, wherein specifically discovered and/or designed ligands form complexes that coalesce into extended lattices, with unusual conductive, magnetic, and/or optical properties. The extensive interest in this field is nurtured by the truly unusual nature of the extended structures, which, in turn, clearly exploit some of the unique structural and electronic structural features of the simpler dithiolene complexes. Chapters 9 and 10 deal with our now extensive knowledge of dithiolene centers in molybdenum and tungsten enzymes and in their chemical model systems, respectively. Chapter 9 introduces the families of molybdenum and tungsten enzymes that contain the pyranopterin dithiolene ligand, reveals the array of reactions catalyzed by these enzymes, and describes the active site protein structures that have come to light in recent years through X-ray crystallography. The enzyme work provides great impetus and added importance to studies of model systems outlined in Chapter 10. Work on these simpler systems reveals structural trends, electronic structural details, and reactivity modes that are essential to the full understanding of the structures, spectra, and reactivity of the enzymes, many of which are important in medical, agricultural, and environmental systems. Last, but not least, Chapter 11 reveals how the dithiolene unit has been used as a building block to construct more complex organic ligands. These ligands form a remarkable variety of novel complexes (see dedication) that display new forms of reactivity, which may yet reveal ways in which important small molecules are activated and converted by enzyme systems in the transition metal dithiolene family. The 11 chapters in this volume reveal a vigorous field that may just be entering its prime. The new results from synthesis, structure elucidation, spectroscopy, biology, bioinorganic chemistry, analytical science, solid-state materials chemistry, and reactivity define a rich field that has far to go before reaching maturity. It is also clear that, as we learn more about dithiolene complexes, we will see new applications arising that exploit our fundamental understanding of chemical, material, and biological systems. It is our hope that this monograph, by bringing together the myriad aspects of dithiolene chemistry in a single volume, will serve as a comprehensive archival reservoir, stimulate further advancement of the field, and impel its growing interface with diverse areas of science and technology. Princeton, New Jersey

EDWARD I. STIEFEL

Contents Chapter 1

Synthesis of Transition Metal Dithiolenes T. B. Rauchfuss

Chapter 2

Structures and Structural Trends in Homoleptic Dithiolene Complexes C. L. Beswick, J. M. Schulman, and E. I. Stiefel

1

55

Chapter 3

The Electronic Structure and Spectroscopy of Metallo-Dithiolene Complexes 111 M. L. Kirk, R. L. McNaughton, and M. E. Helton

Chapter 4

Vibrational Spectra of Dithiolene Complexes M. K. Johnson

Chapter 5

Electrochemical and Chemical Reactivity of Dithiolene Complexes K. Wang

267

Luminescence and Photochemistry of Metal Dithiolene Complexes S. D. Cummings and R. Eisenberg

315

Metal Dithiolene Complexes in Detection: Past, Present, and Future K. A. Van Houten and R. S. Pilato

369

Solid-State Properties (Electronic, Magnetic, Optical) of Dithiolene Complex-Based Compunds C. Faulmann and P. Cassoux

399

Chapter 6

Chapter 7

Chapter 8

213

Chapter 9

Dithiolenes in Biology S. J. N. Burgmayer

Chapter 10

Chemical Analogues of the Catalytic Centers of Molybdenum and Tungsten Ditholene-Containing Enzymes 539 J. McMaster, J. M. Tunney, and C. D. Garner xi

491

xii

Chapter 11

CONTENTS

Dithiolenes in More Complex Ligands D. Sellmann and J. Sutter

585

Subject Index

683

Cumulative Index, Volumes 1–52

723

CHAPTER 1

Synthesis of Transition Metal Dithiolenes THOMAS B. RAUCHFUSS School of Chemical Sciences University of Illinois at Urbana-Champaign Urbana, IL CONTENTS I. INTRODUCTION

2

II. SYNTHESIS FROM PREFORMED ALKENEDITHIOLATES, 1,2-DITHIONES, OR THEIR EQUIVALENT A.

B.

C.

D.

From Benzenedithiol and Related Derivatives / 4 1. Arene Derivatives / 4 2. Linked Bis(benzenedithiolate) Complexes / 8 3. Heterocyclic and Heteroatomic Dithiolates / 10 From 1,2-Alkenedithiolates / 10 1. Via Reductive Dealkylation / 10 2. By Base Hydrolysis of Dithiocarbonates (Dithiole-2-ones) and Related Derivatives / 11 From Selected 1,2-Alkenedithiolate Dianions / 15 1. 4,5-Dimercapto-1,3-dithiole-2-thione (dmit2
) / 15 2. Inorganic Dithiolates Related to dmit2
/ 17 3. Tetrathiafulvalene (TTF)-Derived Dithiolenes / 19 4. From the Thiacarbons [CnSn]2
and Related Derivatives / 20 5. 1,2-Maleonitrile 1,2-dithiolate (mnt2
) / 21 Via Thiophosphate Esters (from a-hydroxyketones and a-diketones) / 21

Dithiolene Chemistry: Synthesis, Properties, and Applications, Progress in Inorganic Chemistry, Vol. 52 Special volume edited by Edward I. Stiefel, Series editor Kenneth D. Karlin ISBN 0-471-37829-1 Copyright # 2004 John Wiley & Sons, Inc. 1

4

2

THOMAS B. RAUCHFUSS E. F. G.

From 1,2-Dithietes / 22 From 1,2-Dithiones, Including Dithiaoxamides and Esters of Tetrathiaoxalate / 23 Via Intermetallic Dithiolene Transfer / 25 1. Non-Redox Routes / 25 2. Redox Routes / 26

III. TRANSITION METAL PROMOTED ROUTES TO DITHIOLENES A. B. C. D. E. F.

29

Addition of Electrophilic Alkynes to Metal Sulfides / 29 Addition of Unactivated Alkynes to Metal Sulfides / 32 From Metal Sulfides and a-Haloketones and Related Precursors / 37 By Dehydrogenation of Alkanedithiolates / 38 From Dithiocarbonates / 39 Specialized Routes to Dithiolenes / 40 1. S-Dealkylation / 40 2. Insertion into Metal–Alkyne Bonds / 41 3. C

C Coupling Pathways / 42 4. From Alkynes and Thiocarbonyl Derivatives / 43 5. Dithiolene Coupling / 43 6. From Alkynyl Anions / 44

IV. SUMMARY AND OUTLOOK

44

ACKNOWLEDGMENTS

44

ABBREVIATIONS

44

REFERENCES

45

I.

INTRODUCTION

Research on metal dithiolenes has remained continuously active since its inception in the early 1960s. Initially, the area was driven by the distinctive redox and structural characteristics of these coordination compounds. After this discovery phase, dithiolene chemistry was fueled by its connections to materials science with respect to photonics and electronic conductors. These developments paralleled growth in the area of organic metals; in fact, the preparative chemistry of dithiolene complexes has greatly benefited from advances in tetrathiafulvalene chemistry (1). This direction remains active (2, 3). In the 1990s, research on dithiolenes was energized by recognition that virtually all Mo- and W-containing enzymes feature dithiolene ligands, which are in turn incorporated into the heterocyclic pyranopterins (4–7). Parallel with the aforementioned developments—discovery, materials-driven studies, and biologically driven studies—dithiolene complexes continue to appear in many contexts, often unexpected ones, due to the great stability of the MS2C2R2 ring.

SYNTHESIS OF TRANSITION METAL DITHIOLENES

3

This chapter discusses the synthesis of transition metal dithiolene complexes and is current to late 2002. Dithiolene chemistry has been reviewed several times previously, but this is the first review dedicated to synthetic aspects. Emphasis is placed on more contemporary methods, and the reader should consult the older reviews, especially those by Mueller-Westerhoff et al. (8) and McCleverty (9) for discussions of earlier literature. An effort was made to be comprehensive with respect to methods and the range of complexes examined, but the chemistry of metal dithiolenes is so vast that it is not practical to be exhaustive. In this chapter, the term dithiolene refers to a ligand of the formula R2C2S2, which depending on one’s formalism could be described as an alkene-1,2dithiolate dianion, a 1,2-dithione, or some oxidation state between these two extremes (Fig. 1). Benzenedithiolates, their derivatives, and analogues are also included. This chapter is divided into two main parts. The first part focuses on reactions where the dithiolene ligand is generated independently of the metal center. For the most part, these preparations give alkenedithiolate dianions, which ordinarily are treated with metal electrophiles to form dithiolene complexes. In the second part, transition metals actively participate in the assembly of the dithiolenes, usually via the reaction of a metal sulfido species with an alkyne or hydrocarbon in an equivalent oxidation state. When considering the synthesis of a dithiolene complex, it is essential to bear in mind that dithiolenes vary widely in their electronic properties. If one simply seeks an unsaturated chelating dithiolate, the most convenient options are benzenedithiolate and the inorganic dithiolenes 1,3-dithiole-2-thione-4,5-dithiolate (dmit2
) and 1,2-maleonitrile-1,2-dithiolate (1,2-dicyanoethene-1,2-dithiolate) (mnt2
). Large-scale syntheses of these ligands are available. Dithiolenes

Figure 1. Relationships and nomenclature for common dithiolene precursors.

4

THOMAS B. RAUCHFUSS

such as mnt2
and dmit2
that have electronegative substituents behave like bidentate pseudohalides, and their complexes are usually synthesized via simple salt metathesis reactions. Akyl-substituted dithiolenes (e.g., [Me2C2S2]2
), are powerful p donors, useful for stabilizing metals in high formal oxidation states. Syntheses of complexes of such strongly donating dithiolenes often require redox steps after the initial formation of a metal dithiolene complex.

II.

SYNTHESIS FROM PREFORMED ALKENEDITHIOLATES, 1,2-DITHIONES, AND THEIR EQUIVALENT A.

From Benzenedithiol and Related Derivatives 1.

Arene Derivatives

Arene-1,2-dithiols are completely stable and are valuable precursors to dithiolene complexes. Benzenedithiol is the most common member of this class of ligands, but related derivatives include toluene-3,4-dithiol (10), 3,4,5,6tetrachlorobenzenedithiol (11), 3,4,5,6-tetramethylbenzenedithiol (12), 2,3naphthalenedithiol (13), and quinoxalinedithiol (14) (Fig. 2). Benzenedithiols are traditionally prepared by reductive dealkylation of 1,2C6R4(SR0 )2, which in turn are obtained by treatment of dibromobenzenes with alkali metal or cuprous thiolates. The methodology continues to be used, for example, for crown ether-appended derivatives (15). A newer and more powerful synthesis of 1,2-benzenedithiol and its derivatives has been developed (16). This method (17) involves reaction of the benzenethiol with 2 equiv of BuLi to give 2-LiC6H4(SLi), which reacts with elemental sulfur to give the dithiolate (Eq. 1). SH

1) BuLi 2) S8 3) H+

SH

ð1Þ SH

This method has been extended to the synthesis of the bulky benzenedithiol 3(Ph3Si)C6H3-1,2-(SH)2 (18) as well as a series of mixed chalcogenides such as [1,2-C6H4(S)(Te)]2
(19). Typically, transition metal benzenedithiolates (and related derivatives) are prepared by the following methods: salt elimination reactions using a metal halide and the dithiolate dianion, thiol exchange, and condensation of the free thiol with oxo, alkoxo, and amido precursors. In one example, the dithiol was

SYNTHESIS OF TRANSITION METAL DITHIOLENES

5

Figure 2. Structures of important arenedithiolate and related ligands.

treated with a metal methyl compound concomitant with the elimination of methane (Eq. 2) (20). WMe6 þ 3 C6 H4 ðSHÞ2 ! WðS2 C6 H4 Þ3 þ 6 CH4

ð2Þ

Homoleptic dithiolene complexes, for example, [Ni(S2C6R4)2]z and [M(S2C6R4)3]z (M ¼ Mo, W), are generally prepared by reaction of the metal halide and the dithiolate, often followed by oxidation of the initially formed complexes. An illustrative study is the synthesis of [Ni(S2C6H2(t-Bu)2)2]2
, which can be oxidized to the monoanion and neutral derivatives using air and iodine, respectively (21). Reactions of MoCl5 and WCl6 with the benzenedithiolate salts give M(S2C6R4)3 (15, 22) or reduced derivatives. Similarly, treatment of Ti(NMe2)4 with C6H4(SH)2 gives (NMe2H2)2[Ti(S2C6H4)3], wherein the amido ligand serves not only as a proton acceptor but also generates the countercation (23). The tris(dithiolenes) are so robust and so easily formed that they plague syntheses of the oxo-dithiolenes [MO(S2C6R4)2]z (24). In the case of Mo derivatives, the metathetical reactions can be conducted

6

THOMAS B. RAUCHFUSS

[WO(SPh)4]-

C6H4(SH)2

[WO(S2C6H4)2]BH4-

[WO2(S2C6H4)2]2Figure 3.

Me3NO

[WO(S2C6H4)2]2-

Illustrative syntheses of oxo-tungsten dithiolenes (25).

in the presence of donor ligands, which inhibit the formation of the tris(dithiolene) complexes. For example, treatment of Na2(S2C6R4) with MoCl4(MeCN)2 in the presence of donor ligands L affords Mo(S2C6R4)2L2 (L ¼ PPh2Me or MeNC; R ¼ H, Me) (12). These derivatives are closely related to the corresponding dicarbonyls (L ¼ CO; see Section II.G.2). Compounds of the type [MO(S2C6H4)2]n
(M ¼ Mo, W) have received much attention. A direct approach to [WO(S2C6H4)2]
proceeds via the reaction of WOCl3(thf)2 (where thf ¼ tetrahydrofuran) benzenedithiol, and Et3N (24). Alternatively, thiol exchange routes can be advantageous as a means to minimize redox processes and formation of [M(S2C6H4)3]n
. For example, [WO(SPh)4]
and benzenedithiol give [WVO(S2C6H4)2]
, which can be subsequently reduced with NaBH4 to give [WIVO(S2C6H4)2]2
(Fig. 3) (25). Related thiol exchange reactions involve conversion of [MoO(SC6H4R)4]n
(R ¼ Cl, n ¼ 2; R ¼ H, n ¼ 1) into [MoO(S2C6H3R)2]n
(R ¼ H, Me, Ph3Si) (18, 26). The oxo-bis(dithiolenes) are amenable to further reactions. Treatment of [MO(S2C6H4)2]2
Me3NO (13) affords [MVIO2(S2C6H4)2]2
(M ¼ Mo, W) (25, 27). Silylation of the oxo-bis(dithiolene) complexes gives [MVI(OSiR3)(S2C6H4)2]
(M ¼ Mo, W), which are versatile precursors to diverse coordination sets on the bis(dithiolene) framework (12, 24). For example, such [MVI(OSiR3)(S2C6H4)2]
derivatives can be oxidized using Me3NO to give [MVI(O)(OSiR3)(S2C6H4)2]
(M ¼ Mo, W) and, in the W case, sulfided using dibenzyltrisulfide to give [WVI(S)(OSiR3)(S2C6H4)2]
. The mildly electrophilic complex MeReO3 condenses with 2 equiv of benzenedithiol to give MeRe(O)(S2C6H4)2 (28), a rare alkyl metal dithiolene. Thiol exchange has been employed to probe the strength of metal ligand bonds as illustrated by the reactions of dicysteinyl peptide-bound derivatives of [Fe2S2(SR)4]2
with toluene-3,4-dithiol. For strongly chelating dipeptides, [Fe(SR)2(S2C6H3Me)]
derivatives (R ¼ protein) form with H2S elimination, whereas less strongly chelating dipeptides are displaced by this dithiol to give [Fe2S2(S2C6H3Me)2]2
(29). Since several molybdoenzymes feature a single dithiolene ligand active site (30), the synthesis of monodithiolene complexes has been of interest. Two general approaches can be envisioned. Stepwise installation of dithiolene

SYNTHESIS OF TRANSITION METAL DITHIOLENES O S

7 _

S

Mo

S

S PhSeCl _

O S

Mo

S

Cl

Et3N/Et4NOH

S S

O Mo

Mo O

RS

_

O

2-

O

O

Cl

S

S

Mo

S

S

SR SR

RS = 2,4,6-(i-Pr)3C6H2S adamantyl-2-S

Figure 4. Routes to monodithiolene derivatives of Mo (30).

ligands or removal of dithiolenes from bis- and tris(dithiolene) precursors. One dithiolene can be removed from [MoO(S2C6H4)2]
using PhSeCl to give monodithiolene [MoOCl2(S2C6H4)]
together with (PhSeS)2C6H4 (30). The chloro ligands in [MoOCl2(S2C6H4)]
undergo ready substitution to give diverse mixed-ligand derivatives as shown in Fig. 4. Thiolate–siloxide exchange was employed to prepare [MoO2(OSiPh3)(S2C6H4)]
via the reaction of MoO2(OSiPh3)2 with Li2S2C6H4. Benzenedithiolates can also be prepared by H2 elimination when using lowvalent precursor complexes (Eq. 3) (31). Cp2 V þ C6 H4 ðSHÞ2 ! Cp2 VS2 C6 H4 þ H2

ð3Þ

This H2-elimination route from 1,2-benzenedithiol was also employed in the synthesis of Fe2(S2C6H4)(CO)6 (32) and the coordinatively unsaturated [Mn(CO)3(S2C6H4)]
(33). A series of related coordinatively unsaturated species [Cr(CO)3(S2C6R4)]2
(R ¼ H, Cl, Me) were prepared by displacement of the

8

THOMAS B. RAUCHFUSS

(OC)2(NC)Fe

-

H N S

2Fe2(CO)9

C6H4(SH)2

C6H4(SH)2

S

S Fe(CO)2(CN)

(CO)2(CN)Fe Et 4NCN S

S

S

S Fe(CO)3

(OC)3Fe

Et4NCN 2(OC)(NC)2Fe

S

2-

-CO

S

(OC)2(NC)2Fe

S S

+CO

Figure 5. Synthetic routes to Fe(S2C6H4)–CO complexes (38, 39).

solvento ligands from Cr(CO)3(MeCN)3. At higher metal stoichiometry, one obtains the binuclear [Cr2(CO)6(m-CO)(m-Z2:Z2-S2C6R4)]2
(34). Other coordinatively unsaturated dithiolenes have been prepared by salt-forming methods, for example, [triphos]Fe(S2C6H4) (35), [Z4-C4Me2(t-Bu)2]Pd(S2C6H4) (36), and (C5Me5)Ir(S2C6H4) (37). The species Fe2(S2C6H4)(CO)6 undergoes ready degradation upon treatment with Et4NCN, giving rise to both mono- and diiron derivatives (Fig. 5) (38). 2.

Linked Bis(benzenedithiolate) Complexes

Relatively elaborate benzenedithiol ligands have been prepared via ortho lithiation of 1,2-benzenedithiol with 3 equiv BuLi, which affords 3-LiC6H3(SLi)2. This trilithiated compound undergoes carbonation to give 2,3-dimercaptobenzoic acid. This carboxy-functionalized dithiolene can be linked via amide formation to give the bis(benzenedithiol), isolated as its Cp2TiIV derivative (see also Section II.G.1) (40, 41). Such ligands can be converted to chelating bis(dithiolene) complexes (Fig. 6). The use of 2,3-dimercaptobenzoic acid derivatives is inspired by the naturally occurring chelators derived from 2,3dihydroxybenzoate (42). An improved and very promising methodology to such linked dithiolenes begins with the ortho lithiation of 1,2-C6H4(S-i-Pr)2, generated on a large scale

SYNTHESIS OF TRANSITION METAL DITHIOLENES

9

CH2OH SH SH

S-i- Pr

1) PBr3 2) Mg 3) Na/C10H8

SH

S-i- Pr

SH

(CH2O)n Li

Cl NaS-i- Pr

S-i- Pr

Cl

S-i-Pr

S-i- Pr

BuLi

S-i- Pr CO2

O HS

HS

NH

NH

CO2H

O SH

SH

1) SOCl2

S-i- Pr

2) C2H4(NH2)2 3) Na/C10H8

S-i- Pr

Figure 6. Hahn’s methodology to bis(benzenedithiolates).

from 1,2-C6H4Cl2, to give the versatile nucleophile 3-LiC6H3-1,2-(S-i-Pr)2 (43). This revised metalation procedure was employed in the synthesis of the ethylene-linked dithiolene 1,2-C2H4[3-C6H3-1,2-(SH)2]2, which forms bimetallic complexes with a staircase-like structure (Fig. 7).

   4
Figure 7. Structure of Ni2 ðS2 C6 H3 Þ2 C2 H4 2 (40).

10

THOMAS B. RAUCHFUSS

3.

Heterocyclic and Heteroatomic Dithiolates

Heterocyclic analogues of benzenedithiolates are also available. 3,4-Thiophene-dithiolate is generated from the corresponding dibromothiophene (44). The isoelectronic but inorganic 1,2,5-thiadiazole-3,4-dithiolate, [SN2C2S2]2
(tdas, see Fig. 2) can be prepared by sulfidation of SN2(CCl)2; the dianion forms bis(chelate) derivatives of Ni(II) and Fe(III) (45). Complexes of 1,10 -ferrocenedithiolate ([FcS2]2
) exhibit properties like arenedithiolates, one difference being the potential for dative Fe ! M bonding (46). Ferrocenedithiol and its salts are well known and have been widely employed as ligands. Some illustrative complexes are Ni(S2Fc)(PMe2Ph) (47) and TpRe(O)[S2Fc] (48). The olefin polymerization precatalysts FcS2M(NMe2)2 (M ¼ Ti, Zr) were prepared by treatment of M(NMe2)4 with Fc(SH)2, concomitant with elimination of HNMe2 (49). 1,2,10 ,20 -Ferrocenetetrathiol (50) could in principle be employed for the synthesis of multimetallic derivatives. 1,2-Dicarboranedithiolate can be generated by deprotonation of 1,2-dicarborane followed by sulfidation (51–53). The resulting Li2S2C2B10H10 reacts with metal dihalides to give the corresponding dithiolates.

B. 1.

From 1,2-Alkenedithiolates Via Reductive Dealkylation

In contrast to arenedithiols, 1,2-alkenedithiols are usually unstable. The corresponding alkenedithiolate dianions are, however, valuable precursors to dithiolene complexes, although they vary widely in their ease of manipulation. Salts of the required cis-[C2R2S2]2
can be generated by the reductive cleavage of cis-1,2-bis(benzylthio)alkenes using Na/NH3 (few alkali metal salts of dithiolenes have in fact been characterized in any detail). The reductive dealkylation was discussed above as a route to benzene- and thiophenedithiolates (15, 43, 44). Generally speaking, alkenedithiolates derived by this dealkylation route are strongly reducing and should be handled with complete exclusion of oxidants and electrophiles (e.g., chlorinated solvents, water). After their isolation as solids, the disodium dithiolates are generally treated with the metal halide to give the corresponding dithiolene complexes. In some cases, intermediate anionic dithiolene complexes are allowed to undergo oxidation by air or solvent prior to isolation of the final complex. Illustrative is the traditional route to the M(S2C2H2)n complexes (M ¼ Ni, n ¼ 2; M ¼ Mo, n ¼ 3) (54, 55), the synthesis of which begins with the Na/NH3 cleavage of cis-1,2-bis(benzylthio)ethene,

SYNTHESIS OF TRANSITION METAL DITHIOLENES

11

which can be made on a large scale from cis-dichloroethene and benzylthiolate salts (56). Solutions of cis-C2R2S2Na2 are treated with divalent metal salts (e.g., Ni, Co, Fe, Cu) to give intermediate anionic species that are subsequently oxidized to give neutral or monoanionic complexes. Third-row metal centers resist reduction, thus treatment of (C5Me5)TaCl4 with cis-C2H2S2Na2 gives the expected (C5Me5)Ta(S2C2H2)2 (57). The compound cis-1,2-C2H2[SC(O)Me]2, prepared by treatment of cis-1,2-C2H2(SNa)2 with acetyl chloride (54, 58), is a promising if untested precursor to the dithiolene dianion (55). The reductive cleavage of the corresponding trans-1,2-bis(benzylthio)ethene gives trans-C2H2(SNa)2, which does not normally form molecular complexes (55) (a complex derived from a trans-alkenedithiolate is described in Section III.F.3). The cleavage of benzylthioethers has more recently been used to generate nonplanar dithiolene ligands shown in Eq. 4 (59). OMe PhCH2S

MeO

PhCH2S

S

1) Na/NH3 2) MCl2L2 or NiCl2, air oxidation or MoCl5, air oxidation

MeO

M S

n

MeO

M = Ni(PR3)2, Pd(b py); n = 1 M = Ni; n = 2 M = Mo, n = 3 (2 isomers)

ð4Þ Because of the nonplanarity of these dithiolenes, the Mo(dithiolene)3 derivative exists as two isomers, one with C3 symmetry and the other with Cs symmetry.

2.

By Base Hydrolysis of Dithiocarbonates (Dithiole-2-ones) and Related Derivatives

A powerful route to dithiolene complexes employs alkenedithiolate dianions generated by the hydrolysis of cyclic unsaturated dithiocarbonates, which are formally called 1,3-dithiole-2-ones. Representative of the many examples (60), the base hydrolysis route has been used to prepare the ferrocene-substituted dithiolene Ni[S2C2H(C5H4)FeCp]2 (61), the sulfur-rich dithiolene [Ni(S2C2S2C2H4)2]
(62), the cyano(dithiolenes) trans-{Ni[S2C2H(CN)]2}n (n ¼ 1, 2) (63), 2,3-thiophenedithiolates [Au(S2C4H2S)2]
(64), and the tris(styryldithiolate)

12

THOMAS B. RAUCHFUSS Br

R2

S R1

O

S

O

S

S

O

Me2CHOCS2R2

R2

OCHMe2

S

R2

R1

AIBN

R2

S

1) OR-

S MLn

O S R1

O

R1

S

2) Ln MX2

R1

S

AIBN = azoisobutyrylnitile

Figure 8. Synthetic routes to alkene dithiocarbonates (dithioles) and their conversion to dithiolene complexes.

Mo[S2C2H(Ph)]3 (65). As is typical throughout dithiolene chemistry, initially produced anionic dithiolene complexes are often allowed to undergo airoxidation to give more conveniently isolated derivatives, for example, of the type [Ni(S2C2R2)2]
and Mo(S2C2R2)3. The required 1,3-dithiol-2-ones can be prepared in several ways. Commonly used precursors are a-haloketones, which are often commercially available or can be prepared by halogenation of the ketones (66, 67). The overall procedure involves a series of efficient steps with well-defined intermediates (Fig. 8). A key step is the acid-catalyzed cyclization of a a-ketoxanthate ester [RC(O)CH2SC(S)OR] in neat H2SO4 to give the dithiocarbonate (68). The corresponding reaction using a-keto dithiocarbamate ester [RC(O)CH2SC(S)NR2] generates the iminium analogues of the cyclic dithiocarbonates (69), although the xanthate approach still appears preferable. 1,3-Dithiole-2-ones have also been efficiently prepared from alkynes via the addition of the equivalent of ‘‘CS2O’’, which in turn is derived from diisopropyl xanthogen disulfide (70) (Fig. 8). The reaction, which is effected using the free radical initiator AIBN, has been used to prepare 2-thienyl substituted dithiolenes, which can undergo subsequent electropolymerization (71). Xanthate derivatives of hydroxymethylalkynes (e.g., HOCH2C2R) also convert to 1,3dithioles (72). The use of 1,3-dithiole-2-ones is compatible with functionalized backbones, for example, the attachment of heterocyclic side groups to the dithiolene backbone (70, 72).

SYNTHESIS OF TRANSITION METAL DITHIOLENES

13

The 1,3-dithiole-2-ones can be prepared via displacement of ethylene from ethylenetrithiocarbonate with electrophilic alkynes (73). The modified trithiocarbonate is then converted to the corresponding dithiocarbonate, base hydrolysis of which provides [(MeO2C)2C2S2]2
. This route was employed in the synthesis of {Ni[S2C2(CO2Me)2]2}
(73), although such complexes are more routinely generated by the addition of C2(CO2Me)2 to metal sulfido complexes (see Section III.A). Displacement of ethylene from ethylenetrithiocarbonate using the electrophile di(2-thienoyl)acetylene (74) gives 2,20 -dithienyldithiolenes (Eq. 5), which are susceptible to electropolymerization (75, 76).

Z S S

C2Z2

Z

S

- C2H4

Z

S S

S

S

Z

ð5Þ

S

S [Z = CO2Me, C(O)-2-C4H3S]

The dithiole-2-thione (NC)2C2S2CS, which is derived from mnt2
(see II.C.5), has been converted to a variety of dithiolene precursors such as the amide [H2NC(O)]2C2S2CS, the diacid (HO2C)2C2S2CS, and the unsubstituted derivative H2C2S2CS (77). Butadiene-1,2,3,4-tetrathiolate [S4C4H2]4
has been prepared from the bis(dithiocarbonate). This tetraanion is a precursor to the coordination polymer [Ni(S4C4H2)]n (78). The conversion of the dithiocarbonates into alkenedithiolates involves base hydrolysis, which is usually effected with sodium alkoxides in alcohol. With the dianion in hand, the synthesis of complexes follows the usual course, as described above. Obviously, oxophilic metal centers, for example, Ti(IV) and Nb(V) (62), are incompatible with the usual alcohol solutions of in situ generated alkenedithiolates. In such cases, the anhydrous salts Na2S2C2R2 are employed in nonhydroxylic solvents, although after complex formation protic solvents are typically employed for cation exchange. The dithiocarbonate methodology has been used to prepare a number of molybdenum–dithiolene complexes. In these syntheses, particular attention must be paid to the molybdenum precursor in order to avoid formation of the highly stable (and biologically irrelevant) tris(dithiolene) species. For example, treatment of Ph(H)C2S2Na2 with MoO2(pentane-2,4-dionate)2 gave M[S2C2H(Ph)]3n
(M ¼ V, Mo, W) with displacement of the oxo group (79). The use of [MoO2(CN)4]4
inhibits the formation of the tris complex (80), allowing one to

14

THOMAS B. RAUCHFUSS

obtain the mixed-ligand complex {MoO[S2C2H(Ar)]2}2
, which exists as both cis and trans isomers (Eq. 6).

H

S

S

S

H

S

R

Mo

X R

2-

O

H

1) OH2) [MoO2(CN)4]4-

S (X = NMe2+, O)

R

S

(and trans isomer)

ð6Þ

O N

N MeN

R = Ph; 2-, 3-, 4-C5H4N, N

H2N

N

N

The use of cyanide ligands to suppress persubstitution by dithiolenes has also been applied to the synthesis of [Ni(CN)2(dithiolene)]2
(81). Whereas complexes of ethylenedithiolate [H2C2S2]2
are typically prepared by the reductive S-dealkylation of cis-H2C2(SCH2Ph)2 (Section II.B), a viable alternative route involves base hydrolysis of 1,3-dithiol-2-one, H2C2S2(CO).The parent H2C2S2CO can in turn be prepared on a multigram scale from chloroacetaldehyde (82). This 1,3-dithiol-2-one can be functionalized via deprotonation followed by C-alkylation (72), thus opening the way to a variety of functional dithiolenes (Eq. 7).

H

H

S

S

ð7Þ

1) LiNR2 MLn

O H

S

2) E+ 3) OR4) Ln MX2

E

S

A versatile route to RS-substituted dithiolenes entails S-alkylation of the trithiocarbonate dmit2
(see Section II.C.1), which provides an efficient means to introduction of diverse functionality to the dithiolene backbone. Subsequent to
CS2C2(SR)2 is converted to the dithiocarbonate S-alkylation, the resulting S


O CS2C2(SR)2 with Hg(OAc)2 in acetic acid (63, 83, 84). Such dithiocarbonates are more easily hydrolyzed than the trithiocarbonates (72, 85). This approach has been used for the synthesis of Ni[S2C2(S(CH2)nMe)2]2 (n ¼ 2–11) (86) and related complexes with pendant alkene substituents (Eq. 8) (87).

SYNTHESIS OF TRANSITION METAL DITHIOLENES

S

S

S

S

1) RBr

S

S

SR

S

SR

1) MeO-

S

SR

S

SR

LnM

O 2) Hg2+

15

2) LnM2+

(dmit2-)

ð8Þ The direct reaction of [Zn(dmit)2]2
with 1,2,-dibromoethylether affords the ethoxy-substituted trithiocarbonate, which eliminates ethanol to give a sulfurrich dithiolene with extended unsaturation (Eq. 9) (88). EtO

Br

Br

S 1) [Zn(dmit)2]2H+

2) (- EtOH) 3) KOH 4) Ni2+ 5) air

S

_ Ni

S

S

ð9Þ

2

Recently, it was found that treatment of [Zn(dmit)2]2
with certain alkylating
CS2C2Hagents gives unsymmetrically functionalized derivatives such as S

S scission (SR) (R ¼ 3-CH2C5H4N, C2H4CN). Although mechanism of the C
remains obscure, these trithiocarbonates are promising precursors to unsymmetrical dithiolenes (89). C.

From Selected 1,2-Alkenedithiolate Dianions

1.

4,5-Dimercapto-1,3-dithiole-2-thione (dmit2
)

The heterocycle dmit2
, occasionally referred to as [a-C3S5]2
, is one of the most important dithiolene ligands. The literature on dmit2
is vast, but an overview of the ligand chemistry including many useful experimental procedures is available (1) as are reviews on specific aspects of the coordination chemistry (2, 3, 90–92). Most studies on dmit2
are directed toward applications in materials chemistry, for example, the photonic or electronic properties (90, 93). The synthesis of dmit2
involves treatment of a dimethylformamide (DMF) solution of CS2 with Na to give a mixture of dmit2
and CS32
. Recent work has shown that in the presence of CS2, dmit2
actually exists as its deep red thioxanthate, [dmit CS2]2
(94). The breakthrough discovery that enabled the proliferation of this ligand was the finding by Hoyer and co-workers (95) that dmit2
can be conveniently isolated in multigram scale as quaternary ammonium salts of [Zn(dmit)2]2
(Fig. 9).

16

THOMAS B. RAUCHFUSS

Figure 9. Synthetic interrelationships involving dmit2
and other CS2-derived species.

In principle, oxidized derivatives of dmit2
(C3S5)n and [(C3S5)2]2
could be employed for the synthesis of dmit complexes (83). Salts of [Zn(dmit)2]2
are air stable in contrast to alkali metal salts of dmit2
itself (94). Diverse organic cations, for example, [CpFe(C5H4CH2NMe3)]þ, have been used in the isolation of [Zn(dmit)2]2
(96). The basic Hoyer– Steimecke synthesis of [Zn(dmit)2]2
has been subjected to numerous optimizations, mainly aimed at large-scale syntheses (>50 g) (1, 97–99), although the original procedure (95) is excellent. The method has been revised so that it consumes CS2 more efficiently, facilitating its preparation from 13CS2 (94). In one interesting modification, CS2 and Na are first combined in the reaction flask and the electron-transfer process is controlled by the addition of DMF (98, 99). Although the reductive coupling of CS2 has long been assumed to cogenerate equimolar amounts of dmit2
and [CS3]2
, recent reports show that under appropriate conditions, formation of [CS3]2
can be suppressed (97). Complexes of dmit2
are commonly generated either via ligand transfer from [Zn(dmit)2]2
(see Section III.G) or by salt metathesis using Na2dmit. Alkali metal salts of dmit2
are prepared by hydrolysis of the thioester dmit[C(O)Ph)]2. This dithioester is prepared by treatment of [Zn(dmit)2]2
with PhC(O)Cl and

SYNTHESIS OF TRANSITION METAL DITHIOLENES

17

isolated as yellow crystals with favorable stability and solubility (1). Ionic complexes of dmit2
are usually synthesized in a three-step, one-pot procedure: (a) dmit[C(O)Ph)]2 is hydrolyzed with NaOMe in MeOH; (b) an alcoholic or aqueous solution of the metal cation, for example, NiCl2(H2O)6, is then added to give the alkali metal salt of the metal dithiolene complex, for example, Na2[Ni(dmit)2]; and (c) an aqueous or methanolic solution of a quaternary salt, for example, R4NCl or Ph4PCl ¼ quatþCl
, is added to precipitate (quat)m[M(dmit)n] salts which are amenable to recrystallization from MeCN solution. This route is employed to access derivatives of [Ni(dmit)2]2
, the subject of many hundreds of publications (2, 100), which are discussed by Cassoux in chapter 8 of this volume (100). Starting from dmit[C(O)Ph)]2, one can also isolate the anhydrous and air-sensitive Csþ and NMe4þ salts of dmit2
(1, 101, 102). Numerous complexes of dmit2
have been prepared using in situ generated Na2dmit (103–115), for example, [Mn(dmit)2]24
(113), [Re2(dmit)5]2
(114), and [Rh(dmit)2]
(103). With more inert precursors such as Cp2MoCl2 (116), alkali metal salts of dmit2
are used, whereas for more reactive metal electrophiles the transfer (see Section II.D) of dmit2
from [Zn(dmit)2]2
is convenient, for example, for the synthesis of Cp2Ti(dmit) from Cp2TiCl2 (117). Note that the M

Cp linkage is not immune to substitution as shown by the conversion of Cp2TiCl2 into [CpTi(dmit)2]
using Na2dmit (118). A related strategy for the preparation of dmit2
complexes involves the reaction of preformed [Ni(dmit)2]2
and [Ni(MeCN)6]2þ in a 3:1 ratio to give [Ni2(dmit)3]2
, whose structure is shown in Fig. 10 (94).

Figure 10. Structure of [Ni2(dmit)3]2
(94).

18

THOMAS B. RAUCHFUSS

Whereas mixed-ligand dmit complexes are generally prepared by reaction of dmit2
sources with substituted metal halides, the displacement of dmit2
from homoleptic complexes represents an alternative route. For example, treatment of [Ni(dmit)2]2
with triphos gives [Ni(dmit)(triphos)] (119).

2.

Inorganic Dithiolates Related to dmit2


The coordination chemistry of the oxa derivative of dmit2
, [OCS2C2S2]2
(dmid2
), has been well developed although the electronic properties of the resulting complexes have elicited only modest attention. This ligand is generated by the base-degradation of the bis(dithiocarbonate) tetrathiapentalenedione (TPD) (120). Otherwise, TPD has played an important role in the development of tetrathiafulvalenes and dithiolenes (121, 122); the reader is referred to Sections II.C and III.E for related methodology. The corresponding iminecontaining dithiolates [RNCS2C2S2]2
would be interesting ligands. Isomeric with dmit2
is dmt2
, (dmt2
¼ 4,5-dimercapto-1,2-dithiole-3-thione) wherein the three carbon atoms are contiguous (see Fig. 10) (2). The synthesis of dmt2
starts with the dmit2
preparation followed by heating at 120–140 C to effect the Steimecke rearrangement (123, 124). The complex (NEt4)2[Zn(dmt)2] is isolated as an oil before conversion to the thioester dmt[C(O)Ph)]2, which is usually purified prior to conversion to its metal complexes. One unusual feature of dmt2
is the reactivity that is latent in the ligand backbone, illustrated by its reaction with dimethylacetylene dicarboxylate (DMAD) (123). Tetrathiooxalate, [C2S4]2
(see Fig. 10), is not a true enedithiolate and strictly speaking falls outside the scope of this chapter. Nonetheless, the exploration of this ligand is closely tied to dithiolene chemistry. Early researchers mistook dmit2
for [C2S4]2
(125), not realizing that C2 S2
4 reacts with CS2 to give dmit2
(126). Hydrolysis of the bis(dithiocarbonate) C2(S2CO)2 yields [OCS2C2S2]2
(see above) (127), not [C2S4]4
as has been claimed (128, 129) (see Section II.C). The alkene C2(SMe)4 has been generated by treatment of basic solutions of C2(S2CO)2 with methylating agents, but this reaction proceeds via the intermediacy of OCS2C2(SMe)2. An easy gram-scale route to (Et4N)2C2S4 has been developed (130), based on a simplification of Jeroschewski’s electrosynthesis (125). Numerous bi- and polynuclear complexes with M2(C2S4) cores (monometallic derivatives of [C2S4]n
are unknown) are assigned as ethylenetetrathiolate --derivatives as judged by structural criteria, specifically the C

C distance. Such complexes are often prepared by reductive coupling of CS2 using low-valent metal complexes such as those of Ni(I), Fe(I), and Ti(II) (131). Tetrathiooxalate complexes, generated by salt metathesis from (Et4N)2C2S4 (130, 132), undergo reduction to give ethylenetetrathiolato complexes (Eq. 10) (133).

SYNTHESIS OF TRANSITION METAL DITHIOLENES +C2S42–

Cl Cp*Rh Cl

S

Cl RhCp*

Cl

S

Cl

+2e–

S

RhCp*

Cp*Rh +Cl2

Cl

S

S

19

S RhCp*

Cp*Rh S

+Cl2

S

ð10Þ Partial oxidation of C2 S2
4 proceeds with loss of sulfur and coupling to give the vinylidene dithiolate derivative of dmit2
. This planar dianion C4S62
is isolated as its blue-purple Et4Nþ salt (130). Treatment of this salt with metal halides affords di- and polymeric complexes, for example, [C4S6][RuCl(arene)]2 and the semiconducting [NiC4S6]n (134) (Eq. 11). S

S 2-

S

S

I2

S

S

S-

S

S

S-

Ni2+

S

S

S

S

S

S

Ni

n

ð11Þ

3.

Tetrathiafulvalene (TTF)-Derived Dithiolenes

Because of the close structural and preparative connections between the TTFs and dithiolene chemistry, it is only natural that extended dithiolenes have been developed with a TTF-like core. These complexes are generally prepared via the corresponding TTF-based di- or tetrathiolates. Tetrathiafulvalenetetrathiolate, with the formula [C6S8]4
or [S2C2S2C

CS2C2S2]4
, has long been known, but only recently have well-defined molecular complexes been described. Solutions of [C6S8]4
can be generated by hydrolysis of the corresponding bis(dithiole) OCS2C2S2C

CS2C2S2CO (see Section II.C) or by lithiation–sulfidation of TTF itself. The former method has been applied to the synthesis of polymeric complexes (129). The latter method was employed in the synthesis of C6S8[TiCp2]2 (135), which has been fully characterized. In contrast to the binucleating character of tetrathiafulvalenetetrathiolate, a variety of chelating tetrathiafulvalenedithiolates are also known, and these species give rise to complexes with especially interesting electrical properties. The synthetic routes to this family of ligands typically begin with dmit2
as illustrated in Fig. 11. The synthesis of this hybrid TTF–dithiolene illustrates the use of the cyanoethyl group to protect the sulfur atoms of the dithiolene (81, 136, 137). The trimethylene-capped tetrathiafulvalenetetrathiolate forms a molecular

20

THOMAS B. RAUCHFUSS

S

S

2C3H6Br2

Zn

S S

S

S

S

S

S

S

1) BrC2H4CN 2) Hg(O2CMe)2/HO2CMe

S

S

CN

S

S

CN

S

S

S

S

S

S

S

S

P(OEt)3

O

CN NC nS

S

S

S

S

S

1) NMe4OH 2) Ni2+

Ni S

S

2

Figure 11. Preparation of trimethylenetetrathiafulvalenedithiolate complexes (136).



CS2C2S2(CH2)3]2}2
. Electro-oxidization species of the type {Ni[S2C2S2C

(138) is commonly employed to secure single crystals of dithiolene-based 2
organic metals. When applied to {Ni[S2C2S2C

CS2C2S2(CH2)3]2} , one obtains crystals of the charge-neutral species [Ni(dithiolene)2], an unusual single component metalloorganic electrical conductor (139). 4.

From the Thiacarbons [CnSn]2
and Related Derivatives

The coordinating properties of the thiacarbons [CnSn]2
have been of intermittent interest (140). Beck et al. investigated the coordination chemistry of tetrathiasquarate, [C4S4]2
(141, 142). This dianion forms an extensive series of bimetallic complexes [C4S4][MLn]22
, where MLn ¼ Rh(PPh3)2þ, Pt(PPh3)2, PdCl2, Pt(PEt3)2þ (141), and Au(PMePh2)þ (142). Metal carbonyls form similar complexes as well as derivatives where the squarate is unidentate (143). Complexes with terminal [C4S4]2
ligands, for example, L2MC4S4 are apparently unknown. Benzenehexathiolato complexes include multimetallic complexes such as C6S6[Au(PPh3)]6 (144), C6S6[Pt(PR3)2]3 (145), and C6S6[TiCp2]3 (146). Such species are prepared by salt-metathesis reactions.

SYNTHESIS OF TRANSITION METAL DITHIOLENES

5.

21

1,2-Maleonitrile 1,2-dithiolate (mnt2
)

An easily prepared, versatile, and time-honored dithiolene ligand is maleonitriledithiolate, [(NC)2C2S2]2
, or mnt2
. The subject of numerous studies, mnt complexes are well described in earlier reviews (9, 147). The sodium salt of mnt2
species arises via the reaction of alkali metal cyanide with CS2 followed by the spontaneous coupling of the intermediate [S2CCN]
concomitant with loss of sulfur (148). Quaternary ammonium salts of mnt2
do not appear to have been synthesized. Most complexes of mnt2
are prepared by straightforward salt metathesis reactions, as expected for this pseudohalide-like dithiolene. Recent studies on oxo Mo/W derivatives use less obvious methods. Sarkar and co-workers (149) synthesized [MoO2(mnt)2]2
via the reaction of aqueous [MoO4]2
with Na2mnt buffered with citrate and phosphate. The phosphate buffer plays a significant role in this synthesis. The corresponding reaction with HSO3
in place of the buffer afforded [MoO(mnt)2]2
, isolated as its quaternary ammonium salt (Eq. 12). [MoO4]2

mnt2-

[MoO2(mnt)2]2-

PPh3

[MoO(mnt)2]2-

ð12Þ

Direct access to [MoIVO(mnt)2]2
involves the use of [MoOCl5]2
(150), analogous to the routes to [TcO(mnt)2]
, [OsN(mnt)2]
, and related mixed -dithiolene complexes (151–153). Treatment of [MoOCl(MeCN)4]þ with a mixture of [H2C2S2]2
and mnt2
gives [MoO(mnt)(S2C2H2)]2
(12), which, like [MoO(S2C6H4)2]2
(154), can be oxidized with Me3NO. The complex [WIVO(mnt)2]2
has attracted attention because, like the tungsten-containing enzyme acetylene hydratase, it catalyzes the hydration of alkynes. This complex is synthesized using aqueous [WO4]2
, mnt2
, and dithionite (155). The corresponding reaction of [WO4]2
, mnt2
, and HSO3
gave [WVIO2(mnt)2]2
. The use of HSO3
is curious as it is normally considered a reductant, but the less oxidizing W(VI) center apparently resists reduction. At low pH, both the [WOn(mnt)2]2
derivatives convert to the [W(mnt)3]2
, especially in the presence of excess mnt2
. A rare example of a unidentate dithiolene is Ru(k1-mnt)(CO)2(terpy) (terpy is 0 0 00 2,2 ,6 ,2 -terpyridine), prepared from the corresponding [RuCl(CO)2(terpy)]þ (156). The stability of this complex reflects the relatively low nucleophilicity of mnt2
. D. Via Thiophosphate Esters (from a-Hydroxyketones and a-Diketones) A historically significant route to dithiolenes starts from a-hydroxyketones (also called acyloins) (157). This methodology is well suited for the large-scale

22

THOMAS B. RAUCHFUSS

synthesis of homoleptic dithiolene complexes, especially those with aryl and simple alkyl substituents. Perhaps the most important complexes of this type are Ni(S2C2R2)2, where R ¼ Me and Ph, which have gained recent attention as dithiolene-transfer agents for the synthesis of bis(dithiolene) derivatives of Mo and W (see Section II.G.2) (158). In the thiophosphate strategy, an 1,2-enedithiol is recognized as a tautomer of an a-mercaptothione, which in turn is related via S-for-O exchanges to the corresponding a-hydroxyketone (Eq. 13). R

R

OH

S

PS2 1) H2O

+ P4S10 - H2S, "P4S8O2" R

O

R

S

Ni(S2C2R2)2

2) Ni2+

n

ð13Þ Thus, treatment of a-hydroxyketones with P4S10 gives intermediate species described as thiophosphate esters (159), although such species have not been rigorously characterized. Hydrolysis of these thiophosphates followed by treatment with metal sources, for example, [WO4]2
(160), NiCl2  (H2O)6 (160), or Cp2NbCl2 (161) gives dithiolene complexes. Schrauzer and Mayweg (160) describe a reliable, large-scale (45 g) procedure to Ni(S2C2R2)2, where R ¼ Me and Ph. 1,2-Diketones (e.g., derivatives of benzil) can be used in place of ahydroxyketones, a modification that broadens the utility of this method (71, 162), despite the fact that the dithione is the incorrect oxidation state to combine with metal salts. Large numbers of nickel diaryldithiolenes have been prepared via this sulfiding method (163–165). Representative of the dithiolene complexes prepared by the P4S10 /diketone route are Ni[S2C2(Ph)(C6H4NMe2)]2, W[S2C2(C6H4NMe2)2]3, and Ni[S2C2(C6H4OC11H23)2]2, which have interesting acid–base (162, 166–168) and liquid-crystal properties (65, 169). E.

From 1,2-Dithietes

1,2-Dithietes (170–172) are four-membered R2C2S2 rings with adjacent sulfur atoms. Such heterocycles are isomeric with 1,2-dithiones and formally result from the two-electron oxidation of 1,2-alkenedithiolates (Fig. 1). Among the few known 1,2-dithietes, bis(trifluoromethyl)dithiete, (CF3)2C2S2, played a key role in the early stages of dithiolene chemistry. Preparation of this volatile (and poisonous) liquid dithiete involves the reaction of hexafluoro-2-butyne with molten sulfur (9). Oxidation of the dithiolene Cp2TiS2C2(CO2Me)2 (Section III.A) gives the dithiete (MeO2C)2C2S2, which has been characterized crystallographically (173).

SYNTHESIS OF TRANSITION METAL DITHIOLENES

23

Figure 12. Representative complexation reactions involving 1,2-bis(trifluoromethyl)dithiete.

Because of its solubility in nonpolar solvents and its oxidizing character, (CF3)2C2S2 is well suited for synthesis of dithiolenes starting with nonpolar, low-valent organometallic precursors, for example, metal carbonyls (8). The synthesis of dithiolenes from dithietes is illustrated by the reaction of [CpMo(CO)3]2 and (CF3)2C2S2 to afford [CpMo{S2C2(CF3)2}]2 via a dicarbonyl intermediate (Fig. 12) (174). An unusual method of exploiting the oxidative character of (CF3)2C2S2 involves its reaction with [MS4]2
, which gives {M[S2C2(CF3)2]3}2
(M ¼ Mo, W) (175). Starting from mixed-oxo-metal sulfides one obtains oxo-dithiolenes such as {MoO[S2C2C2(CF3)2]2}2
(Fig. 12). The same oxo-molybdenum species can be obtained by reduction of (CF3)2C2S2 followed by salt metathesis (12). F.

From 1,2-Dithiones, Including Dithiaoxamides and Esters of Tetrathiaoxalate

Few dithiolenes are prepared via reactions involving 1,2-dithioketones, a rare class of compounds prone to oligomerization. The first stable 1,2-dithione, 1,2-bis(4-dimethylaminophenyl)ethane-1,2-dithione, was generated by photolysis

24

THOMAS B. RAUCHFUSS

of the corresponding dithiocarbonate. The resulting dithione exists in equilibrium with the dithiete (176, 177). The corresponding diphenyl derivative exists exclusively in the dithiete form, indicating that p-donor substituents stabilize the dithione form. Cyclohexanedithione (178) (or its dithiete tautomer), has been trapped in situ with Mo(0) to give the poorly soluble tris(dithiolene) (Eq. 14). S

SMe

hν

"C6H8S2"

- MeSCN

N

S

Mo(CO)6

Mo

- 6 CO S

S

ð14Þ

3

A subset of the 1,2-dithiones are dialkyl tetrathiooxalate esters, C2S2(SR)2. The parent C2S2(SMe)2 exists in dynamic equilibrium with its dimer (Eq. 15) (179). MeS

S S SMe

MeS

S

S

SMe

S

SMe

M(CO)x

2 - x CO

S

SMe

S

SMe

M

SMe S

n

M = Ni, n = 2 M = Mo, n = 3

ð15Þ The compound C2S2(SMe)2 exhibits oxidizing character; for example, it reacts photochemically with Ni(CO)4 and Mo(CO)6 to give dithiolene complexes Ni[S2C2(SMe)2]2 and Mo[S2C2(SMe)2]3 (an alternative synthesis of such complexes is described in Section III.E) (180, 181). The thermal reaction of Ni(cyclooctadiene)2 and C2S2(SMe)2 in the presence of bidentate ligands affords the mixed-ligand complexes Ni[S2C2(SMe)2]L2 (L2 ¼ tmeda, bpy or 2,2-bipyridine). The required S2C2(SMe)2 is derived from dmit2
via methylation, conversion to the dithiocarbonate OCS2C2(SMe)2 followed by photodecarbonylation (179). In principle, this methodology could give a range of SR-substituted dithiolenes. Related to the dithioesters of dithiooxalic acid are the diamides R2NC(S)C(S)NR2, which are relatively more stable than ordinary dithiones (182, 183). These 1,2-dithiones are mildly oxidizing as illustrated by their reactions with Mo(CO)2(PR3)2(MeCN)2 with displacement of MeCN (184). Crystallographic studies (185) show that Mo(CO)2(PBu3)2[S2C2(NC5H10)2] (NC5H10 ¼ pipe˚ piperidinyl) has some enedithiolate character (r C


C ¼ 1.37 A). In contrast, the
corresponding tetracarbonyls, Mo(CO)4[S2C2(NR2)2], arising from Mo(CO)4(OPPh3)2, are described as Mo (0) derivatives (184).

SYNTHESIS OF TRANSITION METAL DITHIOLENES

25

Nickel complexes of unstable cyclic dithioamides are generated by sulfurizing the corresponding diamide in the presence of Ni powder using (MeOC6H4)2P2S4, a sulfiding agent akin to P4S10 (Eq. 16). R

S R

N

N

R

S

[ArPS2]2, Ni

O

ð16Þ

S

Ni (R = Et, i-Pr)

O

N

S

N R

2

Yields are diminished if the thiation is conducted prior to the addition of the metal, indicative of the thermal instability of these dithioamides. Furthermore, the yields are lower when Ni(II) salts are used in place of Ni powder, consistent with the oxidative character of the dithioamide. Nickel dithiolenes of the dithiooxamides exhibit extraordinary extinction coefficients ( 80,000 dm3 mol
1cm
1 at 1000 nm) (186, 187). G.

Via Intermetallic Dithiolene Transfer

Two basic types of dithiolene exchange reaction are practiced, (1) non-redox reactions, which usually involve use of ZnII and Cp2TiIV based reagents, and (2) redox reactions, which commonly involve neutral bis(dithiolene) complexes of Ni. 1.

Non-Redox Routes

Dithiolenes of Ti(IV) and Zn(II) (see Section III.A) transfer their alkenedithiolate to softer metals. Chelate-transfer reactivity under mild conditions was first demonstrated with Cp2TiS2C2R2 (Z ¼ CO2Me, CF3), which reacts with a variety of metal dichloro complexes, for example, [RhCl2(CO)2]
, NiCl2(PR3)2, to give the corresponding late metal dithiolene and titanocene dichloride, which can be removed by filtration through silica gel with which it reacts (188). Related zinc complexes, for example, Zn(S2C2R2)(tmeda), where tmeda ¼ tetramethylethylenediamine, also display this chelate-transfer reactivity and are perhaps still more versatile (see Section III.A) (189). Both the zinc and titanocene dithiolenes react with main group halides such as CXCl2 reagents (X ¼ O, S), to give the corresponding XCS2C2R2. The hexametallic dithiolene [PdS2C2(CO2Me)2]6 (Fig. 13) was prepared by ligand transfer involving PdCl2(cyclooctadiene) and Zn[S2C2(CO2Me)2](tmeda), concomitant with dissociation of the diene from Pd (190). Organic salts of [Zn(dmit)2]2
readily undergo dithiolene-transfer reactions with metal chlorides. For example, this dianion reacts with Cp2TiCl2, NbCl5,

26

THOMAS B. RAUCHFUSS

Figure 13. Structure of [Pd6S2C2(CO2Me)2]6.

VCl3, and AuCl(PPh3) to afford Cp2Ti(dmit) (92, 117), [V(dmit)3]2
(191), [Nb2S4(dmit)4]2
(115), and dmit[Au(PPh3)]2 (192), respectively. Although other dmit complexes are rarely employed for dithiolene transfer CpNi(dmit) was prepared by treatment of [Ni(dmit)2]
with Cp2Niþ (193). 2.

Redox Routes

Schrauzer et al. (194) showed that Ni(S2C2R2)2 and metal carbonyls react upon photolysis resulting in transfer of dithiolene ligands. For example, Ni(S2C2R2)2 and Fe(CO)5 react to give the binuclear dithiolene complexes Fe2(S2C2R2)(CO)6 (R ¼ H, Me, Ph); these species can also be prepared by the photoaddition of Fe2S2(CO)6 to alkynes (195). More recent studies on this kind of reaction have revealed examples of incomplete transfer of dithiolene ligands, resulting in the formation of heterometallic complexes, Eq. 17 (Cp0 ¼ C5Me5, C5H4SiMe3) (196). 0:5 ½FeðS2 C2 Ph2 Þ2 2 þ 0:5 Cp02 Ru2 ðCOÞ4 ! Cp0 RuFeðS2 C2 Ph2 Þ3 þ    ð17Þ Photolysis of M(CO)6 (M ¼ Mo, W) and Ni(S2C2R2)2 gives modest yields of M(S2C2R2)2(CO)2 (R ¼ Me, Ph) (197). As expected for M(IV) derivatives, the carbonyl ligands are labile and can be displaced with a variety of donor ligands. For example, the chelating 1,2-bis(diphenylphosphino)ethane (dppe) reacts with W(CO)2(S2C2R2)2 to give M(S2C2R2)2(dppe) (Fig. 14). A monodithiolene derivative, W(S2C2Me2)(CO)4, was also described, although the behavior of

SYNTHESIS OF TRANSITION METAL DITHIOLENES

CO M(S2C2R2)3 dppe

M(S2C2R2)2(dppe)

X dppe

M(S2C2R2)2(CO)2

27

1) S2C2R'22Mo(S2C2Ph2)2(S2C2R2) 2) HCl/air (R = H, CN)

PR'3

M(S2C2R2)2(CO)(PR'3) (R' = Bu, t-Bu)

SH-, H+

Mo2(S2C2R2)4E2 (R = Ph, C6H4-4-Me)

Figure 14. Selected reactions of M(S2C2R2)2(CO)2 and related derivatives (M ¼ Mo, W) (198–200).

this complex has not been examined [see related work on dithiaoxamide complexes of Mo (Section II.G)]. After a long hiatus, the dithiolene-transfer reaction involving Ni(S2C2R2)2 was rejuvenated by Holm and co-workers (198) who sought new routes to bis (dithiolene) complexes of molybdenum and tungsten as models for metalloenzymes. As discussed above, a synthetic challenge in the chemistry of molybdenum and tungsten dithiolenes is often preventing formation of the tris(dithiolene) complexes, which are substitutionally inert. Dithiolene transfer from Ni(S2C2R2)2 to Mo/W (0) proceeds more efficiently when conducted thermally using preformed M(CO)3(MeCN)3. At the stoichiometry of Ni(S2C2R2)2/M(CO)3(MeCN)3 ¼ 2, the yields of M(CO)2(S2C2R2)2 are 30% (Mo) (198) and 70% (W) (165, 199) (Eq. 18). 2 NiðS2 C2 R2 Þ2 þ MðCOÞ3 ðMeCNÞ3 ! 2 ½NiðS2 C2 R2 Þ n þ MðCOÞ2 ðS2 C2 R2 Þ2 ð18Þ The required Ni(S2C2R2)2 complexes can be prepared on a large scale using the dithiophosphate route (Section II.E). The use of the labile M(CO)3(RCN)3 reagents, which are easily generated thermally (201), facilitates this chelate-transfer reaction. The resulting M(S2C2R2)2(CO)2 complexes have trigonal-prismatic structures, as anticipated by the characterization of the related Mo(CO)2(Se2C6H4)2 (202). The series [M(S2C2R2)2(CO)2]n
(n ¼ 0, 1, 2) has been characterized, the dianionic species having been generated by reduction of the neutral complex with potassium anthracenide, and the monoanion was obtained by comproportionation (203). The carbonyl ligands in M(S2C2R2)2(CO)2 are readily substituted by sources of O2
, S2
, and Se2
(Fig. 15) (198, 199). Furthermore, phenoxides, arylthiolates, and arylselenoates (198, 204, 205) also displace one or both of the carbonyl ligands, the determining factor apparently being the steric crowding around the M(IV) center.

28

THOMAS B. RAUCHFUSS

Figure 15. Representative reactions of M(S2C2R2)2(CO)2 (M ¼ Mo, W) with anions (198, 199, 203–205).

The resulting anionic complexes closely resemble the proposed active site structures of the molybdopterin-based O-atom transfer enzymes such as dimethyl sulfoxide reductase (DMSOR) (4–6), which characteristically converts dimethyl sulfoxide (DMSO) to Me2S. The complexes [M(S2C2Me2)2(OR)]
, for example, deoxygenate Me3NO, DMSO, and Ph2SeO, to give the reduced substrates and the oxo-metallates [MO(S2C2Me2)2(OR)]
(M ¼ Mo, W) (158, 200, 205). One dithiolene can be removed from [MoO(S2C2Me2)2]
using PhSeCl to give mono(dithiolenes) [MoOCl2(S2C2Me2)]
(30), analogous to the more extensively developed [MoOCl2(S2C6H4)]
(Fig. 4). These monodithiolenes undergo substitution of the chlorides to give diverse alkoxy and thiolato derivatives, which are structural analogues of the active sites of sulfite

SYNTHESIS OF TRANSITION METAL DITHIOLENES

C(9)

C(4) C(5)

C(3)

C(10)

C(8)

C(6)

C(2)

29

C(11)

C(7)

C(12)

S(2) C(1)

S(3) S(4)

Mo(1) S(1)

C(17)

C(20)

Co(1) C(24) C(13)

C(16)

C(23)

Co(2)

C(21)

O(2) C(19)

C(15)

C(14)

O(1)

C(22) C(18)

Figure 16. Structure of Mo(CO)2[CpCo(S2C6H4)]2 (206).

oxidase and assimilatory nitrate reductase (30). As usual, the main challenge in the preparation of mono(dithiolene) Mo complexes is preventing formation of the very stable tris(dithiolene) derivatives. Some insights into the details of dithiolene-transfer reactions are provided by a study of the reaction of CpCo(S2C6H4) with Mo(CO)3 sources [e.g., Mo(CO)3(py)3/BF3; py ¼ pyridine]. The product is the trimetallic species Mo(CO)2[CpCo(S2C6H4)]2 (Fig. 16) (206). Structurally elated complexes have been prepared from [Ni(S2C6H4)2]
and sources of Cp*Ruþ (207).

III.

A.

TRANSITION METAL PROMOTED ROUTES TO DITHIOLENES Addition of Electrophilic Alkynes to Metal Sulfides

Metal per- and polysulfido complexes react with electrophilic alkynes to give dithiolenes. The readily available diester DMAD is most commonly employed

30

THOMAS B. RAUCHFUSS

in this reaction (12, 188, 208–222). Other electrophilic alkynes that have been used in this context are C2(CF3)2 (175, 223, 224), HC2CO2Me (189), C2[C(O)Ph]2 (225), and C2[C(O)NH2]2 (12, 226). Still more elaborate alkynes have been employed in the synthesis of pterin-related dithiolenes (217, 227). Terminal sulfido complexes also are known to add electrophilic alkynes as illustrated by additions to both Tp*WS2(EPh), where E ¼ O, Se; Tp* ¼ hydrido tris(3,5-dimethylpyrazolyl)borate (228) and WS2(OSiPh3)2(Me4phen) (229), where Me4phen is tetramethylphenanthroline. The prototype reaction of DMAD with sulfur-rich metal complexes involves treatment of Cp2TiS5 with DMAD to give Cp2TiS2C2(CO2Me)2 (Eq. 19) (188). S S Ti

S S

S

S C2Z2

S Ti

Z Z

ð19Þ

The reaction in Eq. 19 is first order in alkyne and polysulfido complex and is probably initiated by nucleophilic attack of a coordinated sulfur atom at the alkyne carbon followed by attack of the incipient carbanion on another part of the polysulfido chain. In some reactions, vinylpersulfido or sulfur-rich derivatives of dithiolenes are obtained, for example, the ‘‘b isomer’’of Cp2TiS2C2(CO2Me)2 (214) and Cp2MoS3C2R2 (217). These convert to dithiolenes upon heating or treatment with PR3, respectively. The dithiolenes Cp2TiS2C2R2 are distinctively green in color and are readily purified. These and related titanocene complexes are of synthetic value because the dithiolene ligand can be removed as the free dianion or transferred to a ‘‘softer’’ metal center (Section II.H.1). DMAD is a highly reactive electrophile, so caution should be exercised in using this reagent. Illustrative of the complications that one can encounter, the reaction of Mo(S2)(S2CNEt2)3 with DMAD gives ‘‘melded’’ dithiolene com
S bond plexes wherein the Et2NC fragment has inserted into the dithiolene M
(230). Subsequent to the development of the titanocene dithiolenes, related results were obtained starting with ZnS6(tmeda), where tmeda ¼ tetramethylethylenediamine and ZnS4(pmdta) (189), where pmdta ¼ pentamethyldiethylenetriamine (ligand). These species react with DMAD and HC2CO2Me to give dithiolene complexes, and the dithiolene can also be readily removed from the zinc center. The zinc complexes are more potent dithiolene-transfer agents than the titanocene complexes. The pentacoordinate complex ZnS4(pmdta) reacts more rapidly with alkynes than the tmeda derivative (Fig. 17). The reaction of phenylethynylquinoxaline and [Mo(S)(S4)2]2
gives {Mo[S2C2(Ph)(C8H5N2)]3}2
(227), which probably exists as a mixture of rapidly

SYNTHESIS OF TRANSITION METAL DITHIOLENES S

31

S S

S

Me2 N

S

S

Zn Me2N

pmdta Me2N S S

RC2CO2Me (R = H, CO2Me)

S NMe

Zn S

MeO2C

Me2N

Me2 N

S

R S

RC2CO2Me (R = H, CO2Me)

Zn N Me2

pmdta

MeO2C

S Zn

R

XCl2 (X = Cp2Ti, CO)

Me2N NMe MeO2C

S

S X

Me2N R

S

Figure 17. Preparative relationships involving amine-supported zinc polysulfides and zinc dithiolenes (189).

interconverting fac and mer isomers (231). These species are unremarkable except that they undergo oxidation (I2) to produce thiophenic derivatives that are structurally related to the products resulting from the oxidative degradation of molybdopterin (5). Although the mechanism of this conversion remains unclear, the cyclization step is fairly general as illustrated by related oxidations of families W(IV) dithiolenes (Fig. 18) (228). The conversion of dithiolenes into thiophenes has been known for many years (232). A related thiophene-forming reaction has also been observed using intact dithiolenes (Eq. 20) (233).

ð20Þ

32

THOMAS B. RAUCHFUSS

Me

Me Ph

S

Ph

N

N 3 BH

W

N Tp*WS2(EPh) N

N

EPh

S N I2

O

SR S

N HN Ph H2N

N H

N urothione

N Ph

S N

S

2

Figure 18. Synthesis of a molybdenum dithiolene (via a [3 þ 2] pathway) and its conversion to a thiophene derivative. Structure of urothione, the oxidative degradation product of a molybdopterin (228).

The ethanedithiolate complex ½Mo2 S4 ðS2 C2 H4 Þ2 2
reacts with DMAD to give the corresponding dithiolene Mo2 S2 ½S2 C2 ðCO2 MeÞ2 2
2 (215), via a process that involves loss of ethylene. Precedent for this reaction is the reaction of ethylenetrithiocarbonate with DMAD as discussed in Section II.B.2 (see Eq. 5).

B.

Addition of Unactivated Alkynes to Metal Sulfides

Alkynes, even those lacking electron-withdrawing substituents, add to Cp2Mo2S4 and derivatives to give dithiolenes. This discovery marked one of the seminal developments in the chemistry of metal sulfides because it foreshadowed the extensive reactivity of sulfido ligands toward diverse small molecule substrates. Alkynes displace alkenes from the bis(alkanedithiolates) Cp2Mo2(S2C2H3R)2 to give the bis(dithiolene) derivatives Cp2Mo2(S2C2R0 2)2 (234). For example, treatment of Cp2Mo2(S2C2H4)2 with acetylene gives ethylene and Cp2Mo2(S2C2H2)2 (Eq. 21). These reactions proceed via the initial loss of the alkene followed by the binding of the alkyne to the sulfido ligands. In contrast to the Cp2Mo2S4 system, most metal sulfides react only with electrophilic alkynes (see Section III.A)

SYNTHESIS OF TRANSITION METAL DITHIOLENES

33

S S (C5R5)Mo

Mo(C5R5)

S

S R2C2 R S

R S

S + R2C2

S

+ R2C2 R

- C2H4

H S

S Mo(C5R5)

(C5R5)Mo S

S

R - H2

H

ð21Þ

Mo(C5R5)

(C5R5)Mo

S

S Mo(C5R5)

(C5R5)Mo S

S

The dithiolene complex can be hydrogenated (2 atm, 60 C) to re-form the starting ethanedithiolate complex, thereby defining an alkyne–alkene hydrogenation cycle. An alternative entry into these dithiolenes involves reaction of alkyne with (MeC5H4)2Mo2(m-S)2(m-SH)2 with displacement of H2 (235). Furthermore, it was found that anti-(MeC5H4)2Mo2(m-S)2(S)2 and species described as (MeC5H4)2Mo2Sx react with acetylene to give (MeC5H4)2Mo2(S2C2H2)2 (236). Treatment of Cp2Mo2S2(SH)2 with 1 equiv of PhC2H in the presence of oxygen gives Cp2Mo2(O)(m-S)2[S2C2H(Ph)], which is also reactive toward H2 (237). Similar oxo-dithiolenes can be obtained in low yields when Cp2Mo2(S)(O)(m-S)2 is treated with acetylene (238). Of the many modifications of the Cp2Mo2S4 system, perhaps the most widely studied is the methanedithiolate Cp2Mo2S2(S2CH2), wherein the reactivity is focused on the pair of m-S ligands. This species forms dithiolenes upon treatment with the alkynes C2H2, C2Ph2, and C2Et2, but following a general trend, it binds alkenes more weakly (239). Functionalized Cp ligands have been introduced, for example, C5H4R, where R ¼ CH2CH2NMe2, CH2CO2Me, CH2CO2
(240); these allow dithiolene

34

THOMAS B. RAUCHFUSS

formation to occur in water. Similarly, the complexes can be water-solubilized by functionalization of the methylenedithiolate, e.g., Cp2Mo2S2(S2CHCH2CO2
). Alkynes also add to organotungsten and rhenium sulfides, although, in contrast to the Cp2Mo2S4 chemistry, these reactions are complicated by sulfur atom transfer processes. For example, Cp2W2S2(m-S)2 reacts with acetylene over the course of days (room temperature) to give low yields of Cp2W2(S2C2H2)2. Longer reaction times favor the formation of more complex products such as Cp2W2S3(S2C2H2) and Cp2W2S2(S2C2H2)2; in the latter two examples, the dithiolene ligands are nonbridging (241). The dithiolene ligands in Cp2W2S2(S2C2H2)2, but not Cp2W2(S2C2H2)2, are hydrogenated under mild homogeneous conditions; the reaction forms ethylene and Cp2W2(S)(m-S)2(S2C2H2), that is, with net change in the S/W ratio. As with the analogous Mo systems, alkynes displace ethylene from Cp2W2(S)(m-S)2(S2C2H4) and Cp2W2S2(S2C2H4). Acetylene also binds to [(C5Me4Et)2Re2(m-Z2:Z2-S2)2]2þ (242), but does not displace alkenes from Cp2V2(S2C2H3R)2 (241). In contrast to Mo analogues, Cp2V2S4 adds only electrophilic alkynes (223). The inorganic clusters [Mo3(m3-S)(m2-E)(m2-S)2(H2O)9]4þ (E ¼ O, S) add acetylene at room temperature in aqueous 1 M HCl solution to give the dithiolene (243). The dithiolene ligand adopts an unusual bonding mode, wherein each thiolate sulfur atom bridges two metals (Fig. 19). The cluster

O21 O22

O23 Mo2

C2

S3 O33

C1

H2 S1

Mo3

S2 H1 Mo1

O31 O32

O12

O11

O O13

Figure 19. Structure of the C2H2 adduct of [Mo3(m3-S)(m-O)(m-S)2(H2O)9]4þ (243).

SYNTHESIS OF TRANSITION METAL DITHIOLENES

35

[Mo3(m3-S)(m2-E)(m2-S2C2H2)(H2O)9]4þ is a rare example of a M-dithioleneH2O complex. A related alkyne addition involves the addition of PhC2H to [Mo(Et2dtc)]2[Rh(PPh3)2]S4Cl, where dtc ¼ dithiocarbamate which gives a dithiolene-bis(vinylthiolato) derivative, wherein one sulfur of the dithiolene is triply bridging and the second sulfur is doubly bridging. The reaction is thought to proceed via initial dissociation of PPh3 (244). The anion [ReS4]
, which is isoelectronic with the well-known d0-oxotransfer agents [MnO4]
and OsO4, binds a variety of unactivated alkynes (Fig. 20). The alkyne þ [ReS4]
reaction depends on the reactant ratio; additionally, the presence of elemental sulfur has a strong influence. A monomeric dithiolene [ReS2(S2C2R2)]
is initially produced, but it subsequently dimerizes (the corresponding monomeric alkanedithiolate [ReS2(S2C7H9)]
is stable) (245, 246). With a deficiency of alkyne, one obtains tetrametallic [Re4S12(S2C2R2)2]4
with an alkyne/Re ratio of 0.5. These derivatives arise via the addition of [ReS4]
to [ReS2(S2C2R2)]
followed by dimerization. Treatment of [ReS4]
with 2 equiv of alkyne in the presence of elemental sulfur gives [ReS(S2C2R2)2]
; such

Figure 20. Synthetic relationships of dithiolenes derived from [ReS4]
.

36

THOMAS B. RAUCHFUSS

species adopt a square-pyramidal geometry common to d2 complexes (247). When the alkyne addition is conducted in the presence of RSH (R ¼ H, alkyl, aryl) one obtains related derivatives [ReS(S2C2R2)(SH)(SR)]
(R2 ¼ PhC2,H; Ph, Ph). Furthermore, by using ethanedithiol it is possible to prepare mixed dithiolene–dithiolate complexes, for example, {ReS[S2C2(tms)2](S2C2H4)}
(248), where tms ¼ trimethylsilyl. Related to these results is the finding that [Cp*WS3]
adds Ph2C2 to give the corresponding tungsten(IV) dithiolene (249). In contrast to [ReS4]
, the dianion [WS4]2
does not bind Ph2C2. The reactivity of metal sulfido complexes toward alkynes thus correlates with the charge on the metal sulfide: neutral complexes (e.g., Cp2M2S4) being more reactive than monoanions, [Cp*WS3]
and [ReS4]
, which in turn are more reactive than [WS4]2
. The reaction of metal carbonyl complexes with elemental sulfur in the presence of alkynes has long been known to afford dithiolenes, for example, of Fe, Mo, and Ni, although usually in low yields (250). This route to dithiolenes is mechanistically interesting because binary metal sulfides are unreactive toward alkynes under mild conditions, thus dithiolene formation indicates the occurrence of reactive MSx intermediates. Insights into this reaction have mostly come from studies on Cp metal carbonyls. Sulfidation of Cp2 Fe2(CO)4 in the presence of alkynes gives dithiolenes, for example, Cp2 Fe2(CO)S2C2(CO2Me)2 and the cubanes Cp2 [Ph(R)C2S2]2Fe4S4 (R ¼ Ph, Et, Me) and Cp3 (Ph2C2S2)Fe4S5 (251–253). These species are representative of other abiological Fe

S ensembles that feature dithiolenes in place of thiolate terminal ligands (254, 255). Insight into the trapping of reactive metal sulfides with alkynes is provided by studies on the desulfurization of Cp2 Ru2S4 in the presence of alkynes (256). This reaction gives Cp2 Ru2(m-Z2:Z4-S2C2R2) via the intermediate Cp2 Ru2S2. Otherwise, Ph2C2 and Cp2 Ru2S4 do not react, and when the desulfurization S Cp*Ru

S RuCp*

S

R R

-S

S

Cp*Ru Cl Cp*Ru

S S

H S S H

S

R2C2 RuCp*

Cp*Ru

S

RuCp*

-HCl RuCp* Cl

Figure 21. Generation of Cp2 Ru2S2 and its trapping with acetylene to give an Z2:Z4-dithiolene complex (256, 257).

SYNTHESIS OF TRANSITION METAL DITHIOLENES

37

agent (PBu3) is added prior to the addition of the alkyne, only the unreactive Cp2 Ru4S6 cluster, not dithiolenes, results. The intermediacy of Cp2 Ru2S2 in this reaction has been independently confirmed because dehydrohalogenation of Cp2 Ru2Cl2(m-SH)2 in the presence of alkynes gives the same dithiolenes (Fig. 21) (257). The CpCo/Rh promoted reaction with sulfur and alkynes is the basis of a catalytic synthesis of thiophenes (258).

From Metal Sulfides and a-Haloketones and Related Precursors

C.

This method is somewhat related to the thiophosphate method (Section II.E). Metal complexes of the type LnM(SH)2 react with a-halogenated ketones to give the corresponding dithiolenes. The bis(thiol) complexes include Cp2Mo(SH)2 and M(SH)2(dppe) (M ¼ Ni, Pd, Pt) (259). The pathway for this dithiolene synthesis probably begins with the alkylation of one SH ligand, taking advantage of the nucleophilicity characteristic of such ligands (260, 261). The halide leaving group can be replaced by phosphate esters (262) (Eq. 22). The latter may have implications for the biosynthesis of molybdopterin cofactors, the precursors to which are a-phosphorylated ketones. Note that molybdopterin cofactor does not contain molybdenum, it is the organosulfur component that binds Mo and W. R1

X

SH +

Ln M SH

S

R1

S

R2

Ln M O

R2

- H2O, HX

LnM = X = Br, M(Ph2PC2H4PPh2) tosyl, OPO(OEt)2 Cp2Mo

ð22Þ

R1 = H, Me, Ph N

N

N

N

R2 = N

This method has found particular use in the preparation of pyridine-substituted dithiolene complexes, which exhibit pH sensitive luminescence properties (263). The a-halocarbonyl starting materials could include related precursors used in the synthesis of unsaturated dithiocarbonates described in Section II.C.

38

THOMAS B. RAUCHFUSS

D. By Dehydrogenation of Alkanedithiolates Given the considerable stability of dithiolenes, it is not surprising that they can be generated by dehydrogenation of alkanedithiolato complexes. Indeed, Pt(S2C2H2Ph2)(bpyR2), where bpyR2 is a substituted 2,2-bipyridine, dehydrogenates upon photolysis of its oxygenated solutions. Photooxidation of the corresponding ethanedithiolate gives S-oxygenated products instead (264). Conventional dehydrogenation agents, for example, dichloro-dicyanoquinone, do not appear to have been applied to the dehydrogenation of alkanedithiolates. Dehydrogenation has been employed in the synthesis of thiophenedithioles (Section II.B.2) from the corresponding tetrahydrothiophene derivatives (64). Electrophilic alkenes react with ZnS4(pmdeta) to give dithiolene complexes Zn(S2C2R2)(pmdta). The following alkenes were employed in this reaction: cis- and trans-C2H2(CO2Me)2, C2H3CO2Me, C2H3CN, 1,2-C2H2Me(CN), C2H3CHO, and 1,2-C2H2(CN)(Ph). The reaction proceeds via the reversible formation of a dipolar intermediate, as indicated by the ability of the polysulfido complexes to catalyze the cis–trans isomerization of C2H2(CO2Me)2 (Eq. 23) (265). Such dipolar intermediates are proposed to undergo ring closure to give alkanedithiolato intermediates. Independently prepared alkanedithiolate Zn[S2C2H2(CO2Me)2](pmdta) reacts with elemental sulfur to give the dithiolene Zn[S2C2(CO2Me)2](pmdta). The dithiolene ligands can be removed from the Zn center, for example, with phosgene trimer (COCl2)3 and Cp2TiCl2 to give OCS2C2H(CN) and Cp2TiS2C2H(CN), respectively (Section II.G.1). NMe

NMe

Me2N

Me2N Zn

S

Zn

+ S8, -H2S

Me2N

R

S

S

ð23Þ

Me2N R

S Z

Z

An esoteric example of a dehydrogenative route to a dithiolene involves the thermolysis of Cp2TiS5. In the product, two H atoms on one Cp ring have migrated to allow the formation of a cyclopentene-1,2-dithiolate derivative (Eq. 24) (266).

S

S

S

S S

S Ti

∆

S

S Ti

S

S

ð24Þ

SYNTHESIS OF TRANSITION METAL DITHIOLENES

39

The Kajitani–Sugimori group, which has conducted numerous studies on the ligand-centered reactions of dithiolenes (267), synthesized various CpCo(S2C2RR0 ) derivatives via the reaction of CpCo(CO)2, electrophilic alkenes, and elemental sulfur (268). The reaction of [TcCl6]2
with ethanedithiol is claimed to generate small amounts of [Tc2(S2C2H4)2(S2C2H2)2]2
wherein the dithiolate, not the dithiolene is bridging (269), although the structure assignment has been disputed (270). The ethanedithiolate Cp*Re(S2C2H4)Cl2 upon thermolysis or treatment with O2 gives the dithiolene Cp*Re(S2C2H2)Cl2 (271). Similarly, heating Cp*Re(S2C2H3Et)Cl2 and Cp*Re(S2C2H2Me2)Cl2 gives the corresponding alkyl-substituted dithiolenes without C

S bond scission. The dehydrogenation follows first-order kinetics. Such reactions are relevant to the ability of metal sulfides to catalyze hydrogen-transfer reactions. E.

From Dithiocarbonates

As discussed in Section II.C, the hydrolysis of certain unsaturated dithiocarbonates, that is, dithiole-2-ones, gives alkenedithiolate dianions that react further with metal cations to give dithiolenes. In selected cases, dithiolenes can be prepared directly from reactions of metal complexes with dithiole-2-ones. This process has been demonstrated on two occasions using the electrophilic tetrathiapentalenedione. Treatment of TPD with [MoS4]2
affords [Mo(C3S4O)3]2
and COS. Oxidation of this tris(dithiolene) complex to the charge-neutral Mo(VI) derivative followed by hydrolysis in the presence of BuBr gives Mo[S2C2(SBu)2]3 (Eq. 25) (272). S

S O

O S

S

[MoS4]2-

2-

S

Mo _

COS, S

O S

S

S

3

(TPD) 1) [Cp2Fe]+ 2) NaOMe/RX

S

SR

S

SR 3

Mo

ð25Þ

In a similar reaction, [MoS4]2
reacts with C4S5O, the monothiocarbonyl derivative of TPD, to give the same products as above, showing the preferential reactivity of the thiometalate for the dithio- versus the trithiocarbonate. It was also found that the tetrasulfido complex ZnS4(pmdta) (Section III.A) reacts with TPD to give Zn(S2C2S2CO)(pmdta) (189).

40

THOMAS B. RAUCHFUSS

Relevant to the quest for molybdopterin-related ligands, heterocyclesubstituted trithiocarbonates were shown to give modest yields of the CpCo (dithiolene) when treated with CpCo(cod) (cod ¼ 1,5-cyclooctadiene, Eq. 26) (85). S S H N

S H N

S CpCo(alkene)2

N

O

Me

S

N

H

CoCp

O

Me

H

BnO2C

BnO2C

ð26Þ The CpCo platform has been employed to display and interrogate the dithiolene unit with respect to the ligand-based reactivity. The dithiolene ligand cannot, however, be readily removed from the CpCo site. The reaction of dmit-derived trithiocarbonates (see Eq. 8) with Cp2Mo2[C2(CO2Me)2](CO)4 followed by treatment with elemental sulfur gives Cp2Mo2(S)(m-S)2(S2C2R2) [R ¼ CO2Me, SMe, SC(O)Ph] (273). The latter is a unique example of a complex of the tto-derived ligand {S2C2[SC(O)Ph]2}2
. F.

Specialized Routes to Dithiolenes

Dithiolenes have been prepared by many unplanned or unusual methods, often involving organometallic intermediates (274). Whereas these transformations are presently classified as specialized, future work may show that these methods enjoy more significance than presently appreciated. The fact that dithiolenes appear in the products of so many reactions is testament to their considerable stability. 1.

S-Dealkylation

Complexes of alkyl-linked o-benzenedithiolates eliminate ethylene to give benzenedithiolato complexes (Eq. 27) (275–277). This reaction is the reverse of the known addition of alkenes to bis(dithiolene) complexes (278–281). Photolysis of the S-benzylated dithiolenes M(PhCH2S2C2Ph2)2 (M ¼ Ni, Pd, Pt) results in S-dealkylation with elimination of benzyl radicals (282). In general, however, the properties of S-alkylated dithiolenes have not been

SYNTHESIS OF TRANSITION METAL DITHIOLENES

41

thoroughly investigated (199, 283, 284) beyond the work on CpCo derivatives (267).

S S

PEt3 PEt3

-C2H4

Os PEt3

S

S

S

S

S

S

ð27Þ

Os PEt3

1,4-Dithiin derivatives (dithiabenzenes) have long been known to react with metal carbonyls to give dithiolenes (285). A recent illustration is provided by the finding that treatment CpCo(CO)2 with 2-nitro-3,5-diphenyl-1,4-dithiin affords CpCoS2C2(Ph)H (286). 2.

Insertion into Metal–Alkyne Bonds

Alkyne complexes of the early metals that are nucleophilic at carbon (i.e., have metallacyclopropene character) insert electrophiles such as elemental sulfur. Thus, zirconocene benzyne complexes, which are generated upon thermolysis of diarylzirconocene complexes, can be trapped in the presence of sulfur to give benzenedithiolato complexes (287, 288). Some insights into the mechanism of this process is provided by the finding that the vinylperthiolate form of Cp2TiS2C2(CO2Me)2 rearranges intramolecularly to give the dithiolene (214, see also 215) (Eq. 28). S

S

S Z

Ti Z

S Ti

Z Z

ð28Þ

Addition of sulfur to the alkyne-bridged complex Cp2Mo2(CO)4(C2R2) efficiently affords dithiolenes Cp2Mo2(S)(m-S)2S2C2R2 [R2 ¼ H2; H, Me; H, Ph; Et2; (CO2Me)2] in good yields (273). Such complexes had previously been prepared by alkyne addition to Cp2Mo2Sx derivatives (238). Related dithiolene complexes have been made by the successive treatment of the same alkyne complexes with diorganotrithiocarbonates and elemental sulfur (273).

42

THOMAS B. RAUCHFUSS

3.

C

C Coupling Pathways

A remarkable method for the assembly of dithiolenes proceeds via coupling of the two RCS halves of the ligand mediated by a metal center. Reaction of [Mn(CO)5]
with ArC(S)Cl (Ar ¼ C6H5, 4-ClC6H4, 4-ClC6H4) gives the unusual example of a trans (or E-) dithiolene complex. This transformation has been rationalized by invoking a thioacyl Mn[C(S)Ph](CO)5 intermediate, which due to the carbenoid character of the thioacyl ligand, undergoes C

C coupling even at very low temperatures (Eq. 29). Cl

Mn(CO)5

S NaMn(CO)5

Ar

Ar Ar

S 25 °C - 4 CO

S

S (CO)3Mn

S

Mn(CO)3

Ar

(CO)5Mn

ð29Þ Decarbonylation of (trans-S2C2Ph2)[Mn(CO)5]2 occurs at
30 C to give (Z2:Z5-S2C2Ph2)[Mn2(CO)6 (289). Other examples of Z2:Z5-dithiolenes have the formula Cp2 Ru2S2C2R2 (256, 290). Reduction of Fe[Z2-SCSMe)(CO)2L2]þ gives the dithiolenes Fe2[S2C2(SMe)2](CO)4L2 [L ¼ PPh3, P(OMe)3] (291). These species degrade in air to the deeply colored, 16 e
monometalic derivatives Fe[S2C2(SMe)2](CO)2(PPh3), which are formally derived from dimethyltetrathiooxalate (Section II.F). Conceivably, these C

C coupling reactions are related to the reductive coupling of CS2 by low-valent metal complexes, which leads to tetrathiooxalate (or ethylenetetrathiolate) complexes, for example, from Cp2TiII and (triphos)RhI sources (131, 292, 293) (see also Section II.C.2). The bis(alkylidyne) clusters Cp3Co3(CR)2 react with elemental sulfur to give CpCoS2C2R2 (R ¼ Bu, Ph, CO2Me, C2SiMe3). Labeling studies show that the two alkylidyne units from the same cluster are coupled in this reaction, which probably begins with attack of sulfur on a Co

Co bond (294). Werner reported the formation of a CpCo(dithiolene) complexes via a multistep process that begins with C-alkylation of CpCo(PMe2Ph)(CNMe) followed by reaction with CS2 and further sulfidation with elemental sulfur. The product is structurally related to CpCo(dmit) (295). 4.

From Alkynes and Thiocarbonyl Derivatives

Dithiocarbamate complexes are known to react with alkynes to give low yields of dithiolenes and related ligands via pathways that can be difficult to

SYNTHESIS OF TRANSITION METAL DITHIOLENES

43

rationalize (296). For example, the bis(alkyne) complex W(dtc)2(PhC2H)2 reacts photochemically with phenylacetylene to give WðC5 Ph2 H2 NMe2 ÞðS2 C2 HðPhÞÞðdtcÞ via a proposed dimethyamino carbyne intermediate (297). The electron-rich complex Fe(CO)2[(P(OMe)3]2(Z2-CS2) adds electrophilic alkynes via an apparent 1,3-dipolar addition process to give carbene derivatives that in turn undergo efficient air oxidation to afford the 16 e
dithiolene complexes Fe(CO)[(P(OMe)3]2(S2C2RR0 ) (R, R0 ¼ CO2Me, Ph, CHO, etc.) (298). 5.

Dithiolene Coupling

Unsubstituted dithiolenes can be coupled via dehydrogenation. Examples of this behavior come from studies on derivatives of CpCo, a standard platform for exploring the reactivity of dithiolenes (267). Treatment of CpCoS2C2H(Ph) with AlCl3 gives small amounts of the 1,2,3,4-tetrathiobutadiene derivative [CpCo]2(S4C4Ph2) (299). The related benzenehexathiolate complex arises via dehydrogenative coupling of 3 equiv of CpCoS2C2H2 (Eq. 30) (300). S

S

CpCo S

H

CpCo

S

S

CoCp

CpCo

CoCp

S

S

+ S

S

S

H H

H

S

S Co Cp

ð30Þ Although inefficient and mechanistically mysterious, these transformations highlight the reactivity inherent in the C

H bond of unsubstituted dithiolenes, a fertile area for further research (see Eq. 20). Benzenehexathiolate complexes can also of course be prepared by reactions of metal salts with C6(SH)6 or its salts (see Section II.A) (144, 146). Hoffman has shown that mnt2
complexes form phthalocyanines that are decorated on their exteriors with metal dithiolenes (301, 302). The reaction is analogous to the conversion of phthalonitriles to phthalocyanines. 6.

From Alkynyl Anions

The complex Fe2(S2)(CO)6 reacts with alkynyllithium reagents to give dithiolenes after treatment with electrophiles (303). The synthesis proceeds

44

THOMAS B. RAUCHFUSS

via m-alkynylthiolato complexes, which add electrophiles (Hþ, Me3SiCl, PhCHO) at the carbon adjacent to sulfur.

IV.

SUMMARY AND OUTLOOK

Dithiolene complexes can be prepared by incredibly diverse routes, the range of which is testament to the stability of the MS2C2R2 ring. These routes are of interest not only for their synthetic utility but also for the insights that they provide on the electronic structure of the host–metal complex. Whereas dithiolenes occur widely as biological cofactors, the chemistry of synthetic catalysts based on dithiolenes remains minimal—the development of catalytic chemistry of metal dithiolenes will likely spawn many new synthetic methods.

ACKNOWLEDGMENTS Our research in this area has been supported by NIH and NSF. I wish to thank the following for helpful comments on the review: M. Fourmigue´ (Nantes), J. A. Joule (Manchester), E. I. Stiefel (Princeton), and C. J. Young (Melbourne). This review is dedicated to the memory of Dieter Sellmann.

ABBREVIATIONS AIBN bpy cod Cp Cp DMAD DMF dmid2
dmit2
DMSO DMSOR dmt2
dtc dppe Me4phen mnt2


Azoisobutyronitrile 2,20 -Bipyridine 1,5-Cyclooctadiene Cyclopentadienyl Pentamethylcyclopentadienyl Dimethylacetylene dicarboxylate Dimethylformamide 4,5-Dimercapto-1,3-dithiole-2-one 4,5-Dimercapto-1,2-dithiole-3-thione-4,5-dithiolate Dimethyl sulfoxide Dimethyl sulfoxide reductase 4,5-Dimercapto-1,2-dithiole-3-thione Dithiocarbamate Bis(diphenylphosphino)ethane Tetramethylphenanthroline Maleonitrile-1,2-dithiolate (1,2-dicyanoethane-1,2-dithiolate)

SYNTHESIS OF TRANSITION METAL DITHIOLENES

OAc pmdta py quat tdas terpy thf tmeda tms Tp* TPD triphos TTF tto2


45

Acetate Pentamethyldiethylenetriamine Pyridine Tetraalkylammonium (tetraarylphosphonium) 1,2,5-Thiadiazote-3,4-dithiolate 2,20 ,60 ,200 -Terpyridine Tetrahydrofuran Tetramethylethylenediamine Trimethylsilyl Hydridotris(3,5-dimethylpyrazoly)borate Tetrathiapentalenedione 1,1,1-Tris(diphenylphosphinomethyl) ethane Tetrathiafulvalene Tetrathiooxalate

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CHAPTER 2

Structures and Structural Trends in Homoleptic Dithiolene Complexes COLIN L. BESWICK Engelhard Corporation Iselin, NJ JOSHUA M. SCHULMAN and EDWARD I. STIEFEL Department of Chemistry Princeton University Princeton, NJ CONTENTS I. INTRODUCTION

56

II. DITHIOLENE LIGANDS AND COMPLEXES

57

A. Bonding / 57 B. Nomenclature / 58 C. Distribution / 59 III. HOMOLEPTIC BIS(DITHIOLENE) COMPLEXES A. Transition Metal Homoleptic Bis(dithiolene) Complexes / 59 1. Geometrical Aspects of Bis(dithiolene) Complexes / 60 2. Typical Bond Lengths and Angles / 63 3. Ligand Types / 64 4. Multimeric Molecular Structures / 73

Dithiolene Chemistry: Synthesis, Properties, and Applications, Progress in Inorganic Chemistry, Vol. 52 Special volume edited by Edward I. Stiefel, Series editor Kenneth D. Karlin ISBN 0-471-37829-1 Copyright # 2004 John Wiley & Sons, Inc. 55

59

56

COLIN L. BESWICK ET AL. 5. Ni, Pd, Pt / 76 6. Cu, Au / 77 7. Ag, Zn, Cd, Hg / 78 8. Cr, Mn, Fe, Co / 78 B. Main Group Homoleptic Bis(dithiolene) Complexes / 79

IV. HOMOLEPTIC TRIS(DITHIOLENE) COMPLEXES

80

A. Transition Metal Homoleptic Tris(dithiolene) Complexes / 80 1. Geometrical Aspects of Tris(dithiolene) Complexes / 84 2. Typical Bond Lengths and Angles / 87 3. Ligand Bending in Tris(dithiolene) Structures / 88 B. Main Group Homoleptic Tris(dithiolene) Complexes / 92 V. HOMOLEPTIC MONO(DITHIOLENE) COMPLEXES

92

VI. SUMMARY

95

ACKNOWLEDGMENTS

95

ABBREVIATIONS

96

REFERENCES

96

I.

INTRODUCTION

Dithiolene complexes display unusual structural, electronic, photophysical, photochemical, and reactivity features. The base ligand units, 1,2-ethenedithiolate, 1,2-dithione, and 1,2-benzenedithiolate, are depicted in Scheme 1 as A, B, and C. To underpin the syntheses, properties, and applications of dithiolene molecules, this chapter presents a comprehensive discussion of structurally characterized homoleptic dithiolene complexes. That is, the structural unit must contain dithiolene ligands as the only ligand type, and there must be only one type of dithiolene ligand. Emphasis is placed on structural aspects such as coordination geometries, bond distances and angles, and on identifying trends related to the specific dithiolene ligand and the identity and formal oxidation S− S− A

−2 e−

S

S−

+2 e−

S

S−

B

C

Scheme 1. Ligand oxidation states.

STRUCTURES AND TRENDS IN HOMOLEPTIC DITHIOLENE COMPLEXES

57

state of the central atom. The goal is to provide a framework of structural characteristics and trends for the full range of dithiolene complexes. A number of earlier reviews address structurally characterized dithiolene complexes (1–4). Eisenberg (1) elegantly reviews roughly 20 crystallographically characterized complexes prior to 1970 and Mu¨ ller-Westerhoff and Vance (4) discuss additional examples. The crystallographic reports used in this chapter were retrieved and tabulated predominantly through the Cambridge Structural Data Center (CSDC) (5) as of early 2003. Crystallographic figures were generated using the Ortep3 software package (6).

II.

DITHIOLENE LIGANDS AND COMPLEXES A.

Bonding

Many characteristics of dithiolene compounds can be rationalized in terms of the structure and bonding of the bidentate S chelate of the dithiolene ligand. Unlike saturated 1,2-dithiolate ligands, dithiolene ligands in complexes form relatively rigid and roughly planar five-membered rings with considerable electronic flexibility. This electronic flexibility allows the redox state of the complex to be varied without significant alteration of the basic geometry of the structure. Scheme 2 depicts various bonding descriptions of a representative complex in which the formal oxidation states of the metal and ligand vary. Such electronic versatility may make it difficult to establish, by inspection, a first-order bonding description of a dithiolene complex. However, bond distances, such as the S

C lengths, have been used as an indicator of the electronic ˚ are configuration of a dithiolene complex (7): long S

C distances of 1.77 A characteristic of ligand bonding in the dithiolate form (A and D); and short S

C ˚ , are more characteristic of dithione bonding (B and distances, as low as 1.64 A F). Even when these indicators are considered, the oxidation state of the fivemembered ring is often better described as a combination of dithiolate and S

IV

S

S

S

S D

II

S

S

M

M S

II

S

M S

S

E

S E

S

S M

S

S G

Scheme 2. Complex oxidation states.

S 0 S M S S F

58

COLIN L. BESWICK ET AL.

dithione forms (D, E, and F) as in G, where electron density is distributed over the five-ring atoms. Despite the ambiguity in dithiolene ligand and metal oxidation state, a metal oxidation state formalism will occasionally be used in this chapter, as it often is in the literature. This formalism considers the ligands as ‘‘dithiolates’’ (A or C) that form complexes as in D. A discussion of this formalism is presented by Alvarez et al. (8). It is important to understand that such a formal description may not reflect the true electronic configuration and bonding of the metal or ligands within a particular complex. To distinguish this formal reckoning, we place the metal oxidation designation in single quotes, for example, ‘Cu(III)’ or ‘Ni(IV)’. B.

Nomenclature

There is considerable variability in the naming of dithiolenes within the literature. Nomenclature such as 1,2-ethenedithiolate or 1,2-benzenedithiolate as base terminology is useful as a consistent naming practice for the free ligands. However, the term dithiolate does not indicate that dithiolenes are different from saturated 1,2-dithiolate ligands and does not group structures that have related electronic configurations and bonding tendencies. The term dithiolene, initially introduced by McCleverty and co-workers (2, 9) and Balch et al. (10) has gained acceptance as it groups together a number of structural −

S

−S

H

−S

S

−S

−S

H

−S

S

bdt 1,2-Benzenedithiolate

O

edt 1,2-Ethenedithiolate

dmio 1,3-Dithiole-2-one-4,5-dithiolate

−S

S

−

S

S

−S

S

−S

S

S

dddt 5,6-Dihydro-1,4-dithiine-2,3-dithiolate

dmit 1,3-Dithiole-2-thione-4,5-dithiolate

−S

CN

−S

−S

CN

−S

mnt 1,2-Maleonitrile-1,2-dithiolate cis-2,3-Dimercapto-2-butenedinitrile 1,2-Dicyanoethene-1,2-dithiolate

CH3

tdt Toluenedithiolate Toluene-3,4-dithiolate

−S −S

O O

Dithiosquarate 3,4-Dioxocyclobutene-1,2-dithiolate

Figure 1. Dithiolene ligand structures, abbreviations, and nomenclature for common examples.

STRUCTURES AND TRENDS IN HOMOLEPTIC DITHIOLENE COMPLEXES

59

types that behave in a similar manner. It is nevertheless important to recognize that many different names may be present in the literature for the same ligand. Abbreviations for some common dithiolene ligands are given in Fig. 1. C.

Distribution

There are roughly 500 crystallographic reports in the CSDC that contain homoleptic metal dithiolene units. There are 421 transition metal and 14 main group reports of bis(dithiolene) complexes, which contain two dithiolene ligands about a central atom. There are 49 transition metal and 9 main group based reports of tris(dithiolene) complexes. There are also three types of homoleptic mono(dithiolene) structures reported based on Ag, Pd, and Tl. More than one structural unit may appear within a given crystal structure and more than one crystal structure may appear in a single publication. We are unaware of structurally characterized homoleptic tetrakis(dithiolene) complexes, although heteroleptic complexes containing dithiolene ligands do occur where the central element has a high coordination number.

III.

HOMOLEPTIC BIS(DITHIOLENE) COMPLEXES

The earliest complete structural reports of homoleptic bis(dithiolene) complexes are [Ni(mnt)2]2
by Eisenberg et al. (11), and Ni[S2C2(Ph)2]2 by Schrauzer and Mayweg (12) and Sarain and Truter (13a). Interest in these and other related species was heightened by their ability to undergo multiple reversible one-electron-transfer reactions (1). Later, Ni(dmit)2 was discovered as a structural unit in the first molecular superconducting material (see Chapter 8 in this volume) (13b). Many structural reports have followed these early discoveries. Aspects of geometry, metrical parameters, the relative constancy of the five-membered ring, and extended packing arrangements have each contributed to important discussions of dithiolene complex structures. A.

Transition Metal Homoleptic Bis(dithiolene) Complexes

There are roughly 421 reports of homoleptic bis(dithiolene) units based on transition metal elements. The approximate tally of the structures as a function of central metal atom is outlined in Fig. 2. The examples predominantly contain late transition metals. The majority of complexes are Ni based, partially because of interest in these complexes for materials applications. Other common central elements are Cu, Pd, Pt, Au, and Zn. There are also a few Fe and Co complexes and a small number of structures based on Cr, Mn, Ag, Cd, and Hg.

60

COLIN L. BESWICK ET AL.

1-3 Examples 4-6 Examples

C

>10 Examples

Si

Cr Mn Fe* Co* Ni * Cu Zn Pd* Ag Cd

Ge Sb Te

Pt * Au Hg Tl Pb Bi

* Some additional bis(dithiolene) complexes based on these elements exhibit strong intermolecular interactions (see Section III.A.4). Figure 2. Distribution and frequency of homoleptic bis(dithiolene) structures.

1.

Geometrical Aspects of Bis(dithiolene) Complexes

The coordination geometry about the central transition metal atom in the majority of homoleptic bis(dithiolene) structures is square planar or near tetrahedral. There are a few cases where the coordination geometry may be better described as square pyramidal as a result of significant intermolecular interactions (see Section III.A.4). An excellent measure of the geometry is the dihedral angle (l) between the two SMS planes, each defined by two S atoms within a ligand and the central atom. Thus l ¼ 0 and 90 represent true squareplanar and bis(chelated)-tetrahedral geometries, respectively. Although most complexes are either square planar ðl ¼ 0 – 10 Þ or near tetrahedral ðl ¼ 80 – 90 Þ, there are examples that demonstrate intermediate values ðl ¼ 10 – 80 Þ. Distortions, other than twisting of the SMS planes, can be defined in part by
M

c2 angle, where c1 and c2 are midpoints between the two S atoms of the c1
each dithiolene ligand (Scheme 3). For the vast majority of complexes, where
M

c2 angles are nearly strong intermolecular interactions are not present, c1
180 and are rarely <174.0 . In other words, l is a reasonable descriptor of
M

c2 distortion in the coordination geometry. Significantly nonlinear c1
angles indicate specific intermolecular interactions such as dimer formation (see Section III.A.4), lone-pair effects (in the case of main group complexes, see Section III.B.), or distortions due to crystal packing.

S c1 S

S M

c2 S

Scheme 3. The c1

M

c2 angle.

STRUCTURES AND TRENDS IN HOMOLEPTIC DITHIOLENE COMPLEXES

S η

S

C

61

C

M Scheme 4.

Ligand bend.

Another characteristic measure of geometry is the bend between the SMS and SCCS planes of a dithiolene ligand chelate (Z) as shown in Scheme 4. Over 90% of bis(dithiolene) structures demonstrate average values of Z <6 , indicative of the highly planar nature of the dithiolene-ligand chelate. A representative example of a structure with square-planar coordination geometry is the anionic portion of the Ni based mnt structure, [N(C2H5)4][Ni(mnt)2] (14), as shown in Fig. 3. Figure 3(a) shows the example anion to be
Ni

c2 ¼ 179:9 ; Z ¼ 0:7 Þ. The square-planar virtually planar ðl ¼ 1:8 ; c1
arrangement of the dithiolene ligands about the Ni atom is clear from Fig. 3(b).
Ni

S angles The sum of the S

Ni

S angles is 360.1 with the intraligand S
only slightly wider than their interligand counterparts. Selected ring metrics, also given in Fig. 3, confirm the five-membered rings to be highly symmetric. Distances for Ni

S bonds are typical of other ‘Ni(III)’ bis(mnt) structures. In contrast, the anionic portion of [Na([2.2.2]-cryptand)]3[Ag(mnt)2] (15) is shown in Fig. 4. The coordination geometry about the Ag atom is distorted
Ag

c2 ¼ 178:5 Þ and the anion has crystallogratetrahedral ðl  85:2 ; c1
phically imposed C2 symmetry. Despite the twisting of the SMS planes, each

Distances (Å) Ni—S1 Ni—S2 Ni—S3 Ni—S4 S1—C1 S2—C2 S3—C5 S4—C6 C1—C2 C5—C6

2.148 2.151 2.149 2.148 1.726 1.721 1.706 1.725 1.367 1.370

Angles (°) S1—Ni—S2 S3—Ni—S4 Ni—S1—C1 Ni—S2—C2 Ni—S3—C5 Ni—S4—C6 S1—C1—C2 S2—C2—C1 S3—C5—C6 S4—C6—C5 S1—Ni—S3 S2—Ni—S4

92.5 92.5 103.6 103.3 103.4 103.3 119.9 120.7 120.9 119.8 86.5 88.6

Figure 3. Anionic portion of [NEt4 ][Ni(mnt)2 ]. (a) View showing the planar orientation of atoms. (b) View showing atom labels and structure. Selected bond distances and angles are given. See (14) for estimated standard deviation (esd) values.

62

COLIN L. BESWICK ET AL. Distances (Å) Ag—S1 Ag—S2 S1—C1 S2—C2 C1—C2

2.549 2.577 1.727 1.732 1.386

Angles (°) S1—Ag—S2 86.5 Ag—S1—C1 98.8 Ag—S2—C2 98.7 S1—C1—C2 128.9 S2—C2—C1 127.0

Figure 4. Atom positions and bonds for the anionic portion of {Na([2.2.2]-cryptand)}3 [Ag(mnt)2 ] (15). Selected distances and angles are reported. The twist angle (l) between the SMS ligand planes is 85.2 .

five-membered ring is roughly planar and the two Ag

S bond lengths are ˚ ), as are the two S
˚ ). similar (2.549 and 2.577 A
C distances (1.727 and 1.732 A The geometry is representative of other near-tetrahedral bis(dithiolene) examples, although the M

S bond lengths in other examples are shorter. Table I compiles typical geometries according to the ‘‘formal’’ d-orbital electronic configuration of the central metal. Distorted-tetrahedral arrangements, only occur for all d 10 systems, whereas for d 4, d 8 , and d 9 systems square-planar arrangements occur exclusively. For d 5, d 6 , and d 7 systems, square-planar arrangements are most common with some isolated distorted-tetrahedral examples. Under specific conditions, the flat square-planar units can sometimes form strongly joined dimers or trimers (Section III.A.4). In these cases, the coordination geometry about the central atom is best described as square pyramidal. Table I Geometrical Distribution of Metal Complexes with Respect to the Formal Electronic Configuration of the Metala Electronic Configuration

Distorted Tetrahedral

d10

Zn(II), Cd(II), Hg(II), Ag(I)

d9 d8 d7 d6 d5 d4 a

Co(II) Fe(II) Mn(II)

Square Planar

Square Pyramidal

Cu(II) Ni(II), Pd(II), Pt(II), Cu(III), Au(III) Co(II), Ni(III), Pd(III), Pt(III), Au(IV) Pd(III)b , Pt(III)b Fe(II), Co(III), Ni(IV), Pd(IV), Pt(IV) Co(III), Pd(IV), Pt(IV) Fe(III) Mn(II), Fe(III), Co(IV) Cr(II), Mn(III) Fe(IV)b

The formal oxidation state is assigned considering the net charge on the structural unit with the ligands considered as dithiolates. See text for further details and a discussion of formal, rather than literal, use of oxidation states and electronic configurations. In the text formal oxidation states are placed in single quotes [e.g., ‘Ni(III)’] to indicate the formality. b Occur in mixed oxidation state examples (see Section III.A.4).

STRUCTURES AND TRENDS IN HOMOLEPTIC DITHIOLENE COMPLEXES

2.

63

Typical Bond Lengths and Angles

Element Identity

The central metal exerts an influence on metric parameters of bis(dithiolene) complexes, especially M

S bond lengths. Figure 5 outlines, for each element, the range of M

S bond lengths found among the existing structural examples. Bond distances for each unit are the average of the four M

S bonds. Overall, ˚ . Complexes of average M

S bond distances range between 2.101 and 2.563 A Ni demonstrate the shortest average M

S bond distances and the small number of complexes based on Mn, Hg, Cd, and Ag, demonstrate the longest M

S bond distances. Within complexes of one element, the M

S bond lengths roughly correlate in an inverse fashion with the charge on the complex. Less oxidized (i.e., more negatively) charged complexes typically have longer M

S bond lengths. For example, all ‘Cu(II)’ complexes exhibit longer Cu

S bond lengths than ‘Cu(III)’ complexes. The single Ag complex (Fig. 4) is the only bis(dithiolene) unit where the metal has a formal oxidation state of þ1. The ‘Ag(I)’ structure has a distorted-tetrahedral geometry and contains the longest average M

S bond ˚ ) of all mononuclear bis(dithiolene) complexes. length (2.563 A Other bond lengths for bis(dithiolene) transition metal units are similar over a large number of examples. This constancy is an important characteristic of dithiolene complexes. Ranges in bond and angle parameters are outlined in ˚ range (1.642–1.777 A ˚ ), Fig. 6. Average S

C distances fit within a 0.135 A 1
˚ excluding two questionable reports of larger values; [Au(dmit)2] at 1.849 A 1
˚ (16), and [Cu(tdt)2] at 1.832 A (17). Average C

C distances fall within a ˚ range (1.284–1.520 A ˚ ), excluding some outliers. 0.236 A

Zn Pt Pd Ni Fe Cu Co Au

2.311 2.368 2.240 2.319 2.232 2.336 2.101 2.223 2.194 2.227 2.389 2.167 2.308 2.162 2.198 2.297 2.263 2.344

Mn Hg Cr Cd Ag 2.00

2.10

2.20

2.30

2.40

2.50

2.60

Bond Distance (Å) Figure 5.

Ranges of average M

S bond lengths for bis(dithiolene) transition metal structures.

64

COLIN L. BESWICK ET AL.

92.1 107.3 117.7 86.5 129.5 96.4

2 2. .101 56 3

S

S 1.642 1.77 7 1.284 1.520

M

S

S 2.964 3.559

˚ ) and angles ( ) for bis(dithiolene) transition metal Figure 6. Ranges of average bond distances (A structures.

Angle parameters for the dithiolene five-membered rings are shown in Fig. 6. Average SMS angles range between 86.5 and 96.4 . The SMS angles tend to be more acute for longer M

S distances and within complexes with distortedtetrahedral coordination rather than square-planar geometry. Ranges for MSC and SCC angles are between 92.1 and 107.3 and 117.7 and 129.5 , respectively. As shown in Fig. 7, larger MSC angles are offset by smaller SCC angles. 3.

Ligand Types

An extensive listing of homoleptic bis(dithiolene) structural units appears in Table IIA–C. The variability in ligand structure is significant. Table IIA–C shows a wide range of substituents that extend the basic

S

C

S


C
framework including simple electron-withdrawing and -accepting substituents such as CN, CF3, Ph, and Me. Other substituents are also found, including

Average SCC Angle (°)

131

126

121

116 90

95

100

105

110

Average MSC Angle (°) Figure 7. Plot showing the inverse correlation between average MSC and SCC angles within homoleptic bis(dithiolene) transition metal complexes.

STRUCTURES AND TRENDS IN HOMOLEPTIC DITHIOLENE COMPLEXES

65

heteroatoms and extended p systems. The charge on bis(dithiolene) units ranges from
3 to þ2, although the charge on the vast majority of units ranges from
2 to neutral. Information regarding the unit charge is not reported in Table IIA–C but can be obtained from the original references. Despite the diversity of ligand types, more than one-half of all structurally characterized homoleptic bis(dithiolene) complexes contain the mnt or dmit ligands (Table II A and B, Entries 1 and 30; Fig. 1). Extended packing arrangements of some mnt complexes are addressed by Lewis and Dance (18). Investigations of the dmit ligand and associated derivatives with a wide range of different cations account for the numerous examples. Examples of bis(mnt) structures have been shown in Figs. 3 and 4. Figure 8 shows the structural solution for the anionic portion of [N(n-Bu)4][Au(dmit)2] (242). Figure 8(a) illustrates that the structure is planar. Geometrical parameters
Au

c2 ¼ 180:0 . Coordination geofor the AuS4 core are l ¼ 0:0 and c1
metry about the Au atom, viewed in Fig. 8(b), is square planar with the sum of the S

Au

S angles equal to 360.0 . Dithiolene ligand substituents affect the bonding parameters of dithiolene complexes. Substituents that extend the p conjugation of the dithiolene ligand tend to lengthen M

S bond distances, presumably because more electron density from the dithiolene S atoms is tied up in ligand p conjugation rather than ligand–metal bonding. Further lengthening of M

S bonds is observed if electron-withdrawing substituents are present in the p-conjugated system. Electron-donating ligands tend to have less of an effect. The shortest M

S distances are typically realized with alkyl or aryl substituents such as Entries 2–4, 9, 10–12, 14, and 16 in Table IIA. The longest average M

S bond lengths for structures based on a particular element occur with the dithiosquarate ligand, S2C4O2
2 (Table IIC, Entry 52; Distances (Å) Au—S1 Au—S5 S5—C3 S1—C1 C1—C3

2.324 2.322 1.742 1.751 1.314

Angles (°) S1—Au—S5 91.5 Au—S1—C1 99.0 Au—S5—C3 99.2 S1—C1—C3 124.9 S5—C3—C1 125.4 S1—Au—S5' 88.5

Figure 8. Atom positions and bonds for the anionic portion of [N(n-Bu)4 ][Au(dmit)2 ] (242). There are two distinct, yet similar, [Au(dmit)2 ]1
anions within the unit cell, each with crystallographically imposed inversion symmetry. Only one [Au(dmit)2 ]1
unit is shown. Selected distances and angles are reported. The dihedral angle (l) between the SMS ligand planes is 0.0 . (a) Side view that shows the planar nature of the anion, (b) Structural view.

66

OMe

O(CH2)3Me

R¼

1dd

1cc

16

15

1aa 2bb 1y

R1 ¼ R4 ¼ Ph R1 ¼ R4 ¼ CN
Ph, R2

R3


(CH2)3Me R1
R4
S
O R1
R3
S OMe

1y

12 13 14

CH2 1z

R4

7p 3s 1t 1u 1v 1w

56d

R

CN
R


Ph R

CF3
Me R
R

COOMe
SMe R
R

S(CH2)3Me
S(CH2)4Me R
R1

R3
Me

(CH2)7Me

S

R3


R4


R1



M

S

11

S

S

R1
R4

Ph, R2 ¼ R3 ,

R2

R1

240

Ni

Total No.c

b Dithiolene Complex (MLn
2
)

10

1 2 3 4 5 6 7 8 9

Entry

12e 6m

43

Pd

27f 2n 1q

44

Pt

1x

9g 3o

30

Cu

1r

9h

35

Au

3i

12

Zn

Table IIA Listing of Structurally Characterized Bis(dithiolene) Complexesa

1j

4

Fe

4k

6

Co

Agl

7

Miscellaneous

67

R5

S

S

M

S

S

R5


H RAll


Me Me

HO N N CN Cu O N N

3ff 1kk

1ee


R4
R5

t-Bu 3rr R1



COOMe R4 R5



R4

Me R1


R3 R1


R4

Me


R1 ¼ R4 ¼ CN, R2 ¼ R3 ¼

Me

Pd

Pt

1gg

Cu

4hh 1ll 1pp

Au

Zn

1qq

1ii

Fe

1mm

Co

Crjj Mnnn , Cdoo

Miscellaneous

b

Nondefined substituents are

H. The charges associated with structures range betwen
3 and þ2, more typically between
2 and 0, but are not individually listed for a variety of reasons (including uncertainty, in a significant number of structures). c Total for Table IIA–C. See the following footnotes (d–rr) for references: d l u dd mm (11, 14, 18–63). (15). (117). (126). (138). e m v ee nn (57, 64–71). (12, 49, 101–104). (118, 119). (18). (139, 140). f n w ff oo (21, 27, 28, 30, 32, 56, 63, (105). (118). (127–129). (141). o x gg pp (102, 106, 107). (17, 120). (17). (142). 69, 72–81). g p y hh qq (18, 32, 82–88). (28, 108–112). (121). (130–133). (143). h q z ii rr (30, 65, 68, 89–94). (113). (122). (134). (144). i r aa jj (18, 95, 96). (114). (123). (135). j s bb kk (97). (115). (124). (136). k t cc ll (98–100). (116). (125). (137).

a

18 19 R1 20 21 R2 22

17

Me

Ni

68

38 39 40 41

23 24 25 26 27 28 29 30 31 32 33 34 35 36 37

S

S

=

S

S

S

S

C

M


¼
CH2
¼
(CH2)2

¼
(CH2)3

¼
(CH2)4

S ¼
CH2CF2CH2


¼
C(H)
C(H)
¼
C(O)

S ¼
C(S)

¼
C(Se)

¼
CH2C(CH2)CH2

¼
CH2OCH2

¼
CH2SCH2
¼ (trans)

CH(Me)CH2

¼ (trans)

CH(Ph)CH2
¼
CH(Ph)CH(Ph)

—————————————— R ¼ S(CH2)7Me R R ¼ SCH Me S 2 RR ¼
S(CH2)3S

RR ¼
(CH2)3
S R

b Entry Dithiolene Complex (MLn
2
)

1 4ll 1nn

jj

1c 7d 6i 1k 2l 1m 3n 89p 9x 2y 1aa 1cc 2ff 1gg 1hh

Ni

1ii

17

q

3e

Pd

1dd

5

r

3f

Pt

1z

1z 1bb 1ee

1mm

9

2ee

t

2h

Au

s

2

2g 3j 1k

Cu

2o 6u

Zn

TABLE IIB Listing of Structurally Characterized Bis(dithiolene) Complexes (Continued)a Fe

1v

Co

Hgkk

Cdw

Miscellaneous

69

N

N

S

S

N

N

S

S

M

M

S

S

S

M

S

S

S

S

S

M

S

S

N

N

S

S

S

S

N

N

NBu

N(H)Bu

RR ¼

1uu

2rr

1oo

Pd

1qq

Pt

3ss

Cu

1tt

1pp

Au

Zn

Fe

Co

Miscellaneous

b

Nondefined substituents are

H. The charges associated with structures range between
3 and þ2, more typically between
2 and 0, but are not individually listed. See the following for references: c l u dd mm (145). (169). (237, 246, 247). (156). (267). d m v ee nn (146–152). (170). (248). (258). (268). e n w ff oo (153–155). (171–173). (249). (259, 260). (269). f o x gg pp (156–158). (174). (224, 250–252). (261). (270). g p y hh qq (159). (156, 175–224). (253, 254). (262). (271). h q z ii rr (151, 160). (156, 175, 213, 225–232). (253). (263). (272, 273). i r aa jj ss (161–165). (177, 233–236). (255). (264). (274–276). j s bb kk tt (164, 166, 167). (237, 238). (256). (265). (274). k t cc ll uu (168). (16, 183, 239–245). (257). (266, 267). (277).

a

BuN

S

Bu(H)N

46 S

45

44

43

42

Ni

70

52

51

S

50

S

O

O

S

S

47 48 49

R2

N

N

MeO

R1

S

S

S

M

M

S

S

S

S

S

S

N R1

R2 N

S

S

S

O

O

MeO OMe

M

S

S

S

OMe

M

S

S

S

S

S

b Entry Dithiolene Complex (MLn
2
)



R2

Et R1

R1

R2
i-Pr R1

Me, R2
i-Pr

2h

1g

1f

4d 1e

Ni

4i

2c

Pd

2j

Pt

1k

Cu

3l

Au

1m

Zn

TABLE IIC Listing of Structurally Characterized Bis(dithiolene) Complexes (continued)a

1n

Fe

Co

Miscellaneous

71

HO HO

NC

NC

S

S

Cu

N

N

O

O O

O

N H

S

S

M

S

S

S

S

S

S

S

S

M

M

S

S

S

S

M

O

O

S

S

N

N

O

O

S

Cu

S OH OH

CN

CN

N

H N

2r

1r

1p

1o

Pd

Pt

1q

Cu

1s

1r

Au

Zn

Fe

Co

Miscellaneous

b

Nondefined substituents are
H. The charges associated with structures range between
3 and þ2, more typically between
2 and 0, but are not individually listed. See the following footnotes (c–s) for references: c h m r (278). (285, 286). (286). (296). d i n s (279–281). (287, 288). (292). (297). e j o (282). (288, 289). (293). f k p (283). (290). (294). g l q (284). (288, 291). (295).

a

56

55

54

53

N

Ni

72

COLIN L. BESWICK ET AL.

O

S

12

O

S

23

4

S

1.462

O 1.509

94.19˚

Ni 2.

S 1.399

2.2

1.467

O

Figure 9. Sketch of the almost planar anionic portion of K2 [Ni(S2 C4 O2 )2 ] (285). The Ni atom falls ˚ ) and the S
on an inversion center. Selected bond distances (in A
Ni

S angle have been included.

Fig. 1). Bond strain of the four-membered ring results in wider SCC and narrower MSC angles than typically found in bis(dithiolene) chelates. Ligand bite angles range between 91.4 and 96.4 for dithiosquarate ligands, which is near the upper limit of those typically found (86.5–96.4 ; Fig. 6). The larger bite angle also appears to favor a distorted-tetrahedral geometry as two of the three dn homoleptic bis(dithiolene) complexes, where n < 8, that display distortedtetrahedral geometry are based on the dithiosquarate ligand; Co(II) and Fe(II) in Table I. Electronic explanations for longer M

S bonds, other than those based on the strain of the four-membered ring, seem less important as similar long bonds are not observed in larger ring systems such as entries 47–49, 54, and 55 in Table IIC. Note that entry 53 also has a four-membered ring and exhibits ˚ ) and wide SMS angles (93.0 ). unusually long M

S bonds (2.325 A Figures 9 and 10 show S2C4O2
2 examples with some details. The anionic portion of K2[Ni(S2C4O2)2] (285) is sketched in Fig. 9. The NiS4 core has a
Ni

c2 ¼ 180 Þ and all atoms in the square-planar arrangement ðl ¼ 0:0 , c1
dianion fall nearly in the same plane. Selected bond lengths and the S

Ni

S angle (94.19 ) are included in the figure. It is also of note that the Kþ ions interact with the ketone oxygen atom substituents. The anionic portion of [PPh4]2[Fe(S2C4O2)2] (292) is shown in Fig. 10. In contrast to the Ni-based structure in Fig. 9, the coordination geometry about the
Fe

c2 ¼ 172:8 Þ. It is also Fe center is distorted tetrahedral ðl ¼ 85:8 , c1
apparent in Fig. 10 that there are bends in the ligands ðZ ¼ 16:0 and 4.0 ; see Scheme 4Þ. Average bond and angle parameters are included in Fig. 10. Most notable and characteristic of dithiosquarate complexes are the long average ˚ ) and wide S
Fe

S distance (2.389 A
Fe

S angle (95.7 ).

Distance (Å) Fe—S 2.389 S—C 1.689 —C 1.397 C—

Angles (°) S—Fe—S 95.7 Fe—S—C 92.1 —C 129.5 S—C —

Figure 10. The anionic portion of [PPh4 ]2 [Fe(S2 C4 O2 )2 ] (292). The S

Fe

S planes are twisted by 85.8 . Selected average bond lengths and angles are given.

STRUCTURES AND TRENDS IN HOMOLEPTIC DITHIOLENE COMPLEXES

4.

73

Multimeric Molecular Structures

Some bis(dithiolene) structures are part of a more expansive molecular stacking arrangement. The nature of crystal packing is important as it can be a strong indicator of whether a dithiolene material may exhibit beneficial conductive properties [see Chapter 8 in this volume (13b)]. In some cases, intermolecular interactions between bis(dithiolene) units are sufficiently strong to form distinct, well-defined molecular dimers and trimers (10). These aggregates are held together by strong intermolecular M

S or M

M bonds in orientations illustrated in Fig. 11. Table III lists strongly bound dimeric and trimeric structures. A number of structures reported in the literature as ‘‘dimers’’ have not been included in Table III as distances between the monomeric units would constitute an uncharacteristically long bond. This distinction does not suggest that an important electronic interaction is absent within these excluded structures, but instead reflects ambiguity in distinguishing between a weakly joined dimer and a distorted asymmetric stacking arrangement. For almost all dimeric examples, the central metals have formally d5 through d7 electronic configurations. Within the dimer, each monomeric bis(dithiolene) unit retains similar values for bond distances and angles when compared to related, nondimeric species. ˚ However, metal atoms are drawn out of the monomeric unit plane by 0.1–0.45 A and the coordination geometry about the central atom is best described as square
M

c2 pyramidal. A consequence of metal atom displacement is that all c1
angles (Scheme 3) in dimeric structures are smaller than those for monomeric

R R

S S

M

S S

R R

R R

S S

M

S S

R R

S S

R R

M−M Dimer

R R

S S

M

S S

M

S S

R R

R R

M−S Dimer R R

S S

M

S S

R R

S S

M

S S

R R

S S

M

S S

R R

R R

M−S Trimer Figure 11. Structural types of strongly bound, multimeric bis(dithiolene) units.

R R

74

COLIN L. BESWICK ET AL. TABLE III Crystallographically Characterized Dimeric and Trimeric Bis(dithiolene) Complexes

Entrya

Multimeric Dithiolene Complex

D(M

S) ˚) (A

D(M    M) ˚) (A

References

2.756 3.079 3.065 3.043 3.096 3.148 3.252 3.168 3.166 3.120 2.781 3.152 3.222 3.139 3.220 3.149 3.124 3.148 3.199 3.104 3.166 3.715

298 299 300 301 87 302 120 303 304 305 306 63 63 303 18 87 87 300 307 308 296 309

3.227 3.330

310 311

M

S Dimers 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78

1


[{Fe[S2C2(CF3)2]2}2] {[Fe(mnt)2]2 }2
(4)b

{[Fe(bdt)2]2}2
{[Fe(tdt)2]2}2
{[Fe(dmit)2]2}2
(2)b [{Fe{S2C2(COOMe)2]2}2]2
{Co[S2C2(CF3)2]2}2 {[Co(mnt)2]2}2
(7)b

{[Co(dddt)2]2}2
{[Co(S2C6Cl4)2]2}2
[{Co[S2(CO)2C6H4]2}2 2
c {[Mn(dmit)2]2}4


2.315 2.474 2.494 2.477 2.487 2.477 2.513 2.478 2.489 2.466 2.383 2.368 2410 2.394 2.392 2.392 2.352 2.430 2.327 2.405 2.412 2.533 M

S Trimers

79 80

3


{[Co(mnt)2]3} {[Ni(dddt)2]3}2þ

2.312/2.550 2.657/3.053 ˚) D(M

M) (A M

M Dimers

81 82 83 84 85 a

[Pd(edt)2]2 {[Pd(dmit)2]2}1
[Pt(edt)2]2 {[Pt(dmit)2]2}0=1
[Pt(dmit)2]2

2.790 3.177 2.749 3.015/3.122 2.935

312 313 312 190, 314 179

The numbering is continued from Table II. The values in parentheses are the number of different reports of an anion structure. c {Bis[cobalt bis(naphthoquinonedithiolate)]}2
. See monomeric structure in Table IIC, Entry 55. b

STRUCTURES AND TRENDS IN HOMOLEPTIC DITHIOLENE COMPLEXES

75

bis(dithiolene) complexes. For M

M dimers, c1

M

c2 angles range between
S dimers (146– 169 and 175 , and excluding the Mn based dimer values for M

M

c2 angle for {[Mn(dmit)2]2 g4
(136 ) is 163 ) are even smaller. The c1
smaller yet and corresponds to the formal ‘Mn(II)’ center, the least oxidized of all the listed dimer structures. Examples in Table III show that formation of well-defined dimeric and trimeric structures is promoted by dithiolene ligands that contain an electrondeficient p system, and lack steric bulk outside of the monomeric ligand plane. Examples are mnt, 1,2,3,4-tetrachloro-5,6-benzenedithiolate (S2C6Cl2
4 ), and . In some cases, dimeric species will only form under specific S2C2(CF3)2
2 conditions. Strong ligands such as NO, phosphines, or even strongly coordinating solvents, can act to break apart a dimer to form heteroleptic dithiolene complexes (315–317). Table III shows that dimers joined through strong M

S interactions are based on ‘Fe(II)’, ‘Co(III)’, and ‘Mn(II)’. Apical M

S bond distances joining the ˚ in [{Fe[S2C2(CF3)2]2}2 ]1
(Table III, monomeric units range from 2.315 A ˚ Entry 57) to 2.533 A in f[Mn(dmit)2]2 g4
(Table III, Entry 78). Most of the M   M distances within the M

S dimers are outside what would be considered a primary bond. The M  M distance in f[Mn(dmit)2]2 g4
(Table III, Entry 78; ˚ ) is very long as the ‘Mn(II)’ center and the moderate p acidity of the 3.715 A dmit ligand do not strongly promote the intermolecular interaction. However, M   M distances reported in Table III for {{Fe[S2C2(CF3)2]2}2}1
(Table III, ˚ ) are ˚ ) and {Co[S2C2(CF3)2]2}2 (Table III, Entry 67, 2.781 A Entry 57, 2.756 A indicative of significant interactions in these highly electron-deficient structures. In the [S2C2(CF3)2]2
structures, additional metal–metal interactions stabilize the high formal electronic state of ‘Co(IV)’ and the mixed state of ‘Fe(III)/ Fe(IV)’. Figure 12 shows the structure of {[Co(S2C6Cl4)2]2}2
(308) as an example of an M

S dimer (Table III, Entry 76). The structure contains inversion symmetry relating each monomeric {Co(S2C6Cl4)2}1
unit, as is the case with almost all

Distances (Å) Co—S S—C —C C— Co—S3′ Co…Co′

2.189 1.759 1.406 2.405 3.104

Angles (°) S1—Co—S2 S3—Co—S4 Co—S—C —C S—C — Co—S3—Co′

90.4 90.7 104.8 118.9 84.9

Figure 12. Selected example of the M

S dimeric structure, [N(n-Bu)4 ]2 [Co(S2 C6 Cl4 )2 ]2 (308). The dianion is on an inversion center. Selected bond distances and angles are included. The
c2 ¼ 160:3 , and monomeric units demonstrate some distortions from planarity (l ¼ 22:1 , c1
Co
 average Z ¼ 11:5 ).

76

COLIN L. BESWICK ET AL.

Distances (Å) Pt—S 2.296 S—C 1.684 — C 1.396 C— Pt—Pt′ 2.749

Angles (°) S1—Pt—S4 88.1 S2—Pt—S3 87.9 Pt—S—C 103.7 S—C — 122.2 —C

Figure 13. Selected example of the M

M dimeric structure, [Pt(edt)2 ]2 (312). The molecule is located on a crystallographic inversion center. Selected bond distances and angles are given.

dimeric structures in Table III. Immediately striking is that each bis(dithiolene) unit is distorted from planarity in comparison to monomeric square-planar bis(dithiolene) units (see Figs. 3, 8, 9). The dihedral angle between ligand SMS
S dimer structures as a whole planes (l) is 22.1 , which is representative of M

Co

c2 angle is 160.3 and there are significant ðl ¼ 19:1–41:7 Þ. The c1

Co

S and ligand bends [Z (see Scheme 4) ¼ 5.3 and 17.6 ] between the S
SCCS planes of the dithiolene ligands. The average ligand bend for all M

S dimers is 8.9 , which is significantly larger than that for monomeric bis(dithiolene) structures. Selected specific and average metrics for {[Co(S2C6Cl4)2]2}2
(308) are ˚ ) is longer than included in Fig. 12. The bridging Co

S30 bond (2.405 A ˚ the average Co

S bond length (2.189 A) within the monomeric unit. The Co

S3

Co0 angle (84.9 ) is close to perpendicular and demonstrates the ability of dithiolene ligands to bridge to another metal center without breaking the ˚ ) is represenfive-membered chelate. The average S

C bond distance (1.759 A tative of M

S dimers and is as long or longer than those found in monomeric bis(dithiolene) structures (Fig. 6). Table III shows that dimers joined through M

M interactions are based on ‘Pd(IV)’ and ‘Pt(IV)’ (in some cases ‘Ni(IV)’ structures could also be argued to fall in this category) and Fig. 13 gives the structure of [Pt(edt)2]2 (312) as an example. The structure contains inversion symmetry and a Pt

Pt0 bond length of ˚ 2.749 A. The five-membered rings are not as distorted as in M

S dimers. The Pt centers are still displaced from the plane of the four S atoms resulting in a
Pt

c2 angle (169.2 ). There is also a slight twisting ðl ¼ 10:9 Þ nonlinear c1
within the Pt(edt)2 monomeric units but no significant ligand bend is observed ðZ ¼ 0:6 Þ. Selected bond distances and angles reported in Fig. 13 are similar to monomeric bis(dithiolene) structures. 5.

Ni, Pd, Pt

The group 8 (VIII) metals comprise almost 80% of the structurally characterized homoleptic bis(dithiolene) units. Nickel complexes alone account for

STRUCTURES AND TRENDS IN HOMOLEPTIC DITHIOLENE COMPLEXES

77

over one-half, numbering 240 (see Table IIA–C). The Ni–dithiolene units ˚ , Fig. 5) demonstrate the shortest average M

S bond distances (2.101–2.223 A of all metal dithiolene units. The geometry is square planar (l near 0 ) and, with few exceptions, does not deviate significantly. Intraligand SMS angles range between 89.6 and 94.5 . Fifty-six of the Ni entries are ½Ni(mnt)2 n
, 28 when n ¼ 1 and 28 when n ¼ 2. Consistent with a lower formal oxidation state, Ni

S distances for ˚] ½Ni(mnt)2 n
structures tend to be longer when n ¼ 2 [‘Ni(II)’, 2.159–2.181 A ˚ ]. A [Ni(mnt)2]1
example is shown compared to n ¼ 1 [‘Ni(III)’, 2.138–2.155 A in Fig. 3. The shortest Ni

S bonds, and therefore the shortest M

S bonds overall typically occur with ½Ni(S2C2R2)2 n
ðn ¼ 0Þ such as in Entries 2–4, 10– 12, 14, and 16 in Table IIA. Despite the large number of ½Ni(mnt)2 n
units, the most investigated Ni dithiolene unit is ½Ni(dmit)2 n
with 89 examples. The discovery of the Ni–dmit based molecular superconducting material spurred the investigation of metal complexes of dmit and dmit derivatives such as those listed in Table IIA–C [see Chapter 8 in this volume (318)]. ˚ ) and Pt
˚ ) are Bond distances for Pd

S (2.232–2.336 A
S (2.240–2.319 A longer than those of Ni

S (Fig. 5). Square-planar coordination geometries for formal ‘Pd(II)’ and ‘Pt(II)’ units are highly preferred as expected for formal
M dimers in more oxidized Pd and Pt d 6=d7 d8 systems. Formation of M
systems result in square-pyramidal arrangements (Section III.A.4, Tables I and III, and Fig. 13).

6.

Cu, Au

Copper dithiolene structures are generally square planar with SMS angles ranging from 90.5 to 95.8 . Deviations from square-planar geometry ðl > 10 Þ are sometimes observed, especially in the case of some formal ‘Cu(II)’ ðd9 Þ examples. The Cu

S bond lengths correlate strongly with the formal oxidation ˚ ) are all shorter state of the metal: ‘Cu(III)’ complexes (2.167–2.185 A ˚ than ‘Cu(II)’ examples (2.250–2.308 A). The longest Cu

S distances are encountered within structures containing the dithiosquarate ligand (Table IIC, Entry 52). There are 35 Au dithiolene units with formal metal oxidation states mostly as ‘Au(III)’ and some as ‘Au(IV)’. The complexes are often considered alongside isoelectronic Pt bis(dithiolene) examples. Coordination geometries for all the Au structures are square planar as is the case for the [Au(dmit)2 ]1
example shown ˚ (Fig. 5) and in Fig. 8. Average Au

S bond distances range from 2.263–2.344 A the longest Au

S bonds are observed with the dithiosquarate ligand (Table IIC, Entry 52).

78

COLIN L. BESWICK ET AL.

7.

Ag, Zn, Cd, Hg

Metals with a formal d10 electronic configuration, as simple bonding theory predicts, all tend toward distorted-tetrahedral rather than square-planar coordination geometry (see Table I). The combination of near-tetrahedral geometry, which limits M

S p interaction, and the low formal oxidation state of þII make ˚ ). The small range of Zn
the Zn

S bonds rather long (2.311–2.368 A
S bond lengths is likely due to the single ‘Zn(II)’ formal oxidation state. The small number of ‘Cd(II)’ and ‘Hg(II)’ dithiolene units demonstrate the longest M

S bond lengths (see Fig. 5) of all bis(dithiolene) structures except for the single ‘Ag(I)’ example. Intraligand SMS angles are larger for Zn units (93.7– 94.9 ) compared with smaller values for Ag (86.5 ), Cd (86.7–89.9 ), and Hg (88.4 ). The single homoleptic bis(dithiolene) example based on Ag and shown in Fig. 4 is quite remarkable. The structure demonstrates the longest average M

S ˚ ) of all the homoleptic bis(dithiolene) complexes. The bond length (2.563 A structural unit has a
3 charge relating to a formal oxidation state of ‘Ag(I)’. The geometry is distorted tetrahedral with l ¼ 85:2 . 8.

Cr, Mn, Fe, Co

The first-row transition metal elements, Cr, Mn, Fe, and Co, comprise a small number of monomeric structures. The only structural report based on Cr is [Cr(bdt)2 ]2
(Table IIA, Entry 18). The geometry is distorted tetrahedral ˚ is rather long, the ninth longest
S bond length of 2.364 A ðl ¼ 83:9 Þ. The Cr
of all metal bis(dithiolene) structures. The report by Henkel and co-workers (140) on [Mn(tdt)2 ]n
complexes (n ¼ 1 and 2; Table IIA, Entry 19) demonstrates the subtle energetic difference between distorted-tetrahedral and square-planar geometry. The dithiolene planes of the ‘Mn(II)’ complex, [Mn(tdt)2 ]2
, are nearly perpendicular ðl ¼ 89:0 Þ and ˚ . Oxidation to the monoanion, the average M

S bond length is 2.417 A 1
[Mn(tdt)2 ] , results in a transition to square-planar coordination geometry ˚ ) has decreased by 0.14 A ˚. where the average M

S distance (2.281 A Most bis(dithiolene) Fe and Co complexes are found as strongly joined M

S dimers (see Section III.A.4, Table III, and Fig. 12) with square-pyramidal geometry about the central metal atom. In the M

S dimers, the five-membered dithiolene rings are more distorted than with unbridged monomeric units as
M

c2 , and ligand bend (Z) angles. determined from dihedral (l), c1
There are a small number of homoleptic Fe and Co bis(dithiolene) examples that are found as monomeric structures. The average Co

S bond lengths of five ˚ . The average Co
square-planar complexes range from 2.162 to 2.198 A
S bond ˚ . Three length of the single distorted-tetrahedral example is longer at 2.297 A

STRUCTURES AND TRENDS IN HOMOLEPTIC DITHIOLENE COMPLEXES

79

˚ ) that are square-planar Fe structures have Fe

S bond lengths (2.194–2.227 A ˚ ). The S
shorter than the one distorted-tetrahedral example (2.389 A
Fe

S angles for the square-planar complexes are near 90 , whereas for the distortedtetrahedral complex the angle is larger (95.8 ). B.

Main Group Homoleptic Bis(dithiolene) Complexes

In the CSDC there are 14 structurally characterized homoleptic bis(dithiolene) complexes centered on main group elements. The distribution of the reports according to central element is shown in Fig. 2. Structural units based on C(319, 320), Si(321), Ge(322), Sb(323–325), Tl(326), Pb(327), Bi(328, 329), and Te(330, 331) are depicted in Fig. 14. Coordination geometries for main group homoleptic bis(dithiolene) units are predominantly distorted tetrahedral. Dihedral angles, l, between the dithiolene ligands for C, Si, Ge, and Tl units are nearly 90 . Ligands in the Pb, Sb, and Bi
M

c2 angles examples are twisted (l ¼ 76–88 ), but in these cases the c1
(Scheme 3) are not linear as a result of ‘‘nonbonding electron-pair density’’. The Te structures, [Te(mnt)2 ]2
ðl ¼ 13:4 Þ and [Te(dithiosquarate)2 ]2
ðl ¼ 0:8 Þ in Fig. 14, are different than the other main group complexes as they are better described as square-planar complexes. [Te(mnt)2 ]2
also demonstrates a non
Te

c2 angle (162.0 ). linear c1


2−

NC

S

NC

S

S

CN

S

S

CN

S

M

n

S M

M = Te, Pb

S

M = "C", Si, Ge; n = 0 M = Sb, Tl; n = -1 S

S

S

S

S

1−

S

S

S

S

M

S

M = Sb(3), Bi(2) 2−

F3C

S

F3C

S

S

CF3

S

CF3

O

S

C

S

O

Te O

S

S

O

Figure 14. Homoleptic bis(dithiolene) units centered on main group elements (see text for references).

80

COLIN L. BESWICK ET AL.

IV.

HOMOLEPTIC TRIS(DITHIOLENE) COMPLEXES

The majority of all six-coordinate complexes exhibit an octahedral coordination geometry minimizing ligand interactions. The initial structural report of Re(S2 C2 Ph2 )3 by Eisenberg and Ibers (332, 333), and shortly thereafter the reports of Mo(edt)3 by Smith et al. (334) and V(S2 C2 Ph2 )3 by Eisenberg and Gray (335) were the first six-coordinate complexes to exhibit near trigonalprismatic geometries (336, 337). Along with observed reversible electrontransfer reactions in tris(dithiolene) complexes, these geometric discoveries led to great interest in synthetic and structural aspects of tris(dithiolene) complexes. Explanations for stable trigonal-prismatic arrangements that are also consistent with experimental observations have been described. Specifically, overlap of metal dz2 with ligand ph orbitals, and metal dxy and dx2
y2 with ligand pv orbitals (7, 338). These p interactions can be weakened to favor octahedral arrangements in the presence of bulky dithiolene substituents or high metal d-orbital energies. A detailed discussion of bonding descriptions is found in Chapter 3 of this volume (339). A.

Transition Metal Homoleptic Tris(dithiolene) Complexes

There are roughly 50 homoleptic tris(dithiolene) complexes reported in the CSDC (5). The elemental distribution of these structures is outlined in Fig. 15. As opposed to bis(dithiolene) complexes, tris(dithiolene) complexes are based predominantly on early transition metal elements. Many of the tris(dithiolene) complexes are centered on V, Mo, and W. There are also complexes of Ti, Zr, Nb, Ta, Cr, Tc, Re, Ru, and Os. In addition, there are tris(dithiolene) complexes of Fe and Co, elements that also form homoleptic complexes with two dithiolene ligands. A detailed listing of the structural units along with references and geometrical parameters (to be discussed) is given in Table IV.

1-3 Examples 4-6 Examples >10 Examples Ti

V Cr

Fe Co

Zr Nb Mo Tc Ru

In Sn Sb

Ta W Re Os Figure 15. Distribution and frequency of homoleptic tris(dithiolene) complexes.

81 171.3 169.9 169.4 172.2 163.2

82.4 82.0 86.3 86.7

88.2 87.8 86.5 87.1 84.5

[V(mnt)3]2
[Mo(mnt)3]2
[W(mnt)3]2
[Cr(mnt)3]2
[Cr(mnt)3]3
[Fe(mnt)3]2
ð3Þf

[Ru(mnt)3]2
[Ru(mnt)3]3
[Tc(mnt)3]2


158:6e 155.4 155.1 168.1 173.1

81.7 85.9 80.2 81.4

V(S2C2(Ph)2]3 W(S2C2(Ph)2]3 {W[S2C2(Ph)2]3}1
Re[S2C2(Ph)2]3

137.5 151.6 136.8 137.8

137.1 136.2 136.6 137.1 136.5

81.3 80.4 79.2 80.4 79.5

Mo(S2C2Me2)3 [Mo(S2C2Me2)3]1
[Mo(S2C2Me2)3]2
[W(S2C2Me2] [W(S2C2Me2)3]2


137.0 136e

136 180

SMStrans ( )b;c

82.8 82.5e

SMSintra ( )b

V(edt)3 Mo(edt)3

Trigonal Prism Octahedron

Dithiolene Complex

a

178.2 177.8 176.5 177.1 174.5

173e 172.4 172.0 176.3 176.7

171.7 175.9 170.2 171.4

171.3 170.4 169.2 170.4 169.5

172.8

ecorr ( )



84 81 83 88 71

61 53 53 80 91

4 39 2 5

3 1 2 3 1

3 1

0 100

TP!OCT (%)

0.3/2.2 1.0/2.4 0.8/2.7 0.0/0.0 1.0/2.3

2.51/1.4 1.8/1.4 1.7/2.2 1.2/1.1

6.3/0.3 8.3/1.5 4.4/0.1 3.8/0.7

2.9/0.2 1.9/0.4 1.7/0.2 1.8/0.6 1.9/0.2

2.2/0.3


SMStrans( )/ d ( )

TABLE IV Transition Metal Homoleptic Tris(dithiolene) Complexes

62 63 62 60 69

73 73 64 59

85 77 89 86

88 90 87 89 88

89

90 55

f ( Þb

28 28 45 52 60e 49 48 47 50 35

8 25 2 6

3 1 5 2 3

2 0e

0 60

y ( )b;d

6.9 5.6 5.5 0.0 3.0

2.2 2.0 4.9 9.1

2.3 1.6 12.0 2.2

15.9 4.0 0.9 7.1 1.3

0.7 18e

344 345 345 18 18 346 18 18 347 347 348

335 343 343 333

341 341 342 342 342

340 334

Z ( )b;d Reference

82 153.1 146.6 139.7 139.3 161.9

82.9

83.3 85.1 81.9 82.5 82.0

82.4

81.9 81.9 81.3 82.7

[V(tdt)3]2


[V(dmit)3]1
[V(dmit)3]2
[Mo(dmit)3]2
W(dmit)3 [W(dmit)3]2


[W(dmio)3]2


V(dddt)3 [V(dddt)3]1
ð2Þf

Ti(dddt)3]2


[Zr(bdt)3]2
[Ta(bdt)3]1
[Tc(bdt)3]1
[Nb(bdt)3]1
[Os(bdt)3]2


[W(bdt)32
ð2Þf

83:1e 82.1 81.9 81.7 80.9 81.4 79.8 80.8 81.8 80.3 85.2

[V(bdt)3]n
Mo(bdt)3 [W(bdt)3]1
ð2Þf

139.3 161.8 147.2 136.7 146.5

162.7

161:7e 136.4 158.2 156.1 136.6 151.6 162.5 154.7 141.1 136.2 166.4

142.6

80.5

{[Mo[S2C2(CO2Me)2]3}2


SMStrans ( )b;c

SMSintra ( )b

Dithiolene Complexa

171.9 171.9 171.3 172.7

172.4

173.3 175.1 171.9 172.5 172.0

172.9

173.1 172.1 171.9 171.7 170.9 171.4 169.8 170.8 171.8 170.3 175.2

170.5

ecorr ( )

29 10 9 71

47

9 66 31 2 29

72

26 1 62 56 2 44 78 54 14 1 77

19

TP!OCT (%)

Table IV (Continued)

4.0/0.9 0.9/0.1 0.7/0.8 0.8/0.8

0.2/0.2

4.4/0.4 4.6/7.2 0.4/0.4 0.3/0.3 0.8/0.8

3.2/4.4

4:4e 0.3/0.3 3.0/2.2 5.7/2.7 2.4/0.2 2.7/1.1 7.9/4.1 11.5/11.8 4.8/0.6 1.0/0.4 4.7/2.9

3.1/1.5


SMStrans ( )/ d ( )

81 86 88 69

75

88 63 81 89 81

68

90 72 74 87 76 68 63 83 88 63

86

f ( Þb

16 7 3 34

25

3 45 16 1 15

36

37e 0 30 28 4 23 35 44 12 3 45

7

y ( )b;d

0.4 23.4 22.4 7.0

1.2

24.8 12.0 3.7 10.6 4.2

8.5

23.0 2.1 1.0 2.0 2.7 3.1 17.3 2.3 24.9 10.7

4.3

363 363 364 365

362

359 360 361 361 361

340

340 350 351 352 353 354 7 355 356 357 358

349

Z ( )b;d Reference

83

g

f

e

d

c

b

a

S

S

S

S

S

S

S

S

S

O

O

Ni

O

O

N

N

3

O

3–

3

NH NH NH NH

3

1–

3

2–

3+

86.5

91.1

81.6

81.2

173.9

177.9

138.7

140.4

176.5

181.1g

171.6

171.2

94

95

8

13

6.5/4.1

0.4/1.9

1.0/0.2

2.6/0.7

56

56

88

87

56

60

3

5

9.4

10.0

6.0

22.7

Values for only one structural unit have been reported in cases where multiple units are contained within the unit cell. ‘TP!OCT’ is a value. Average values. The three largest interligand SMS angles are used to calculate SMStrans. In some cases, this may overestimate SMStrans values by 1–2 . Values may vary slightly from cited literature depending on the calculation method. Data from original reference. The values in parentheses are the number of different reports of an anion structure. See text for details regarding the value >180 .

Cr

Co

Mo

V

S

369

368

367

366

84

COLIN L. BESWICK ET AL.

1.

Geometrical Aspects of Tris(dithiolene) Complexes

Stereochemical arrangements of homoleptic tris(dithiolene) complexes range between trigonal-prismatic (TP) and octahedral (OCT) geometries. A number of methods can be used to describe the coordination geometry in tris(dithiolene) complexes (345). For example, dihedral angles between ligand SMS planes are indicative of structural tendencies between TP (120 ) and OCT (90 ) geometries, and a compression ratio has been used to describe the approach of SSS trigonal faces on twisting from TP to OCT (336). Other measures such as twist angles and SMS angles are discussed in Sections IV.A.1.a and IV.A.1.b. In Section IV.A.1.c, specific structural examples are shown to display some of the different measures and aspects of these geometries. a. Determination of Geometry by Trigonal Twist Angle. Stiefel and Brown suggested that the trigonal twist angle (y) between S atoms in the twodimensional (2D) projection along the threefold axis shown in Fig. 16 would describe the coordination geometry of a structure between TP ðy ¼ 0 Þ or OCT ðy ¼ 60 Þ extremes (336). Subtleties of y calculations and their sensitivity to structural distortions have been described (370). Values of y can be constrained from reaching the full OCT limit (60 ) by chelating ligands with small bite angles. Calculations for corrected OCT limits have been suggested (336, 371) and are discussed below. In order to generate a listing of y values for the tris(dithiolene) structures, the average dihedral angle (f) between the ligand SMS and trigonal SSS planes outlined in Fig. 17 has been calculated for each tris(dithiolene) structure. For each structure, average values of f are listed in Table IV and can be used as a direct measure of a structure’s tendency toward TP (90 ) or OCT ( 55 ) geometry (345). Calculated values for y based on f are also shown in Table IV. Note that there may be small variations in the listed y values with those reported in the literature, as different methods of calculation differ in their sensitivity to particular distortions in geometry (370).

θ

Figure 16. Twist angle projected perpendicular to the molecular threefold axis.

STRUCTURES AND TRENDS IN HOMOLEPTIC DITHIOLENE COMPLEXES

85

SMS Plane φ SSS Plane S

S

S M

S

S

S

Figure 17. Depiction of one of the dihedral angles (f) between ligand SMS and trigonal SSS planes. As a structure distorts from TP to OCT geometry, the ligands twist and f becomes <90 .

An informative measure of structural distortion is the deviation from 0 of the dihedral angle between the two SSS planes (d); each plane defined by three S atoms comprising a trigonal face (Fig. 18). Values for d are listed in Table IV and are effective in identifying structures with significant distortions. See Section IV.A.1.c for an example. b. Determination of Geometry by trans-SMS Angle. Even though significant information regarding coordination geometry for tris(dithiolene) structures can be obtained from y, f, and d, calculation of these useful values is rather complex within a large database and further complicated as values for ‘true’ OCT limits in complexes that contain ligands with restrictive bite angles are difficult to obtain. A simpler and more straightforward method reported in the literature involves trans-SMS angles (SMStrans ), where the two S atoms are from different dithiolene ligands and are nearly opposite each other in the complex (7, 344). Regular TP and OCT geometries have SMStrans values of 136 and 180 , respectively. As in the case for twist angles, SMStrans may be constrained from Top SSS Plane S

S

S M

S

S

S Bottom SSS Plane

Figure 18. Depiction of the top and bottom trigonal SSS planes. The dihedral angle between the planes (d) is a measure of distortion in the complex.

86

COLIN L. BESWICK ET AL.

reaching the OCT limit by small chelate bite angles. The complement of the chelate angle (i.e., SMSintra ) must be approximately equal to the supplement of SMStrans (7). So ½ecorr ffi 180
ð90
SMSintra Þ , where ecorr is the SMStrans value expected for an OCT geometry constrained by SMSintra . Note that ecorr only considers the geometrical constraints of the ligand in determining the OCT limit. Table IV lists average SMStrans values for each transition metal homoleptic tris(dithiolene) complex along with ecorr values. To simplify the data, a straightforward measure of coordination geometry (‘TP ! OCT’) has also been included in Table IV where ‘TP ! OCT’ ¼ (½ðSMStrans
136 Þ=ðecorr
136 Þ  100%). Values of ‘TP ! OCT’ range between 0 and 100%, which are representative of TP and OCT ligand arrangements, respectively. Structural distortion can also mislead interpretation of ‘TP ! OCT’ values. A simple method of estimating structural distortions (besides calculating values for d) is to examine the range of SMStrans values within a particular complex. If
SMStrans is large, significant structural distortion is present and average values describing coordination geometry must be used with caution. Values for
SMStrans are listed in Table IV and correlate, in a gross sense, with values for d. c. Selected Examples of Octahedral and Trigonal-Prismatic Structures. Figure 19 shows the anionic portion of [PPh4 ]3 {Co[S2 C2 (CO)3 ]3 } [(368); second to last entry in Table IV]. The dithiolene ligands are arranged in an octahedral fashion about the central Co atom. The structural view is down the D3 axis making the twist angle ðy ¼ 60 Þ readily apparent. Listed values for TP ! OCT (95%) and fð56 Þ also reflect the OCT coordination geometry

Selected Average Parameters Distances (Å) Co—S 2.265 S—C 1.675 C— —C 1.402

Angles (°) S—Co—S 91.1 Co—S—C 100.7 S—C — —C 123.1

Figure 19. Atom positions and bonds for the anionic portion of [PPh4 ]3 {Co[S2 C2 (CO)3 ]3 }. Selected distances and angles are reported. See (368) for esd values. The geometry around the Co atom is nearly OCT (‘TP ! OCT’ ¼ 95%; y ¼ 60 ).

STRUCTURES AND TRENDS IN HOMOLEPTIC DITHIOLENE COMPLEXES

87

Selected Average Parameters Distances (Å) Re—S 2.324 S—C 1.685 —C 1.338 C—

Angles (°) S—Re—S 81.4 Re—S—C 109.0 —C 120.0 S—C —

Figure 20. Atom positions and bonds for Re(S2 C2 Ph2 )3 (333). Selected distances and angles are reported. The geometry around the Re atom is nearly TP (‘TP ! OCT’ ¼ 5%; y ¼ 6 ).

about the Co atom. The calculated value of SMStrans (‘‘181.1 ’’) is clearly larger
Co

S angle than the limit of 180 and is a result of the larger than normal S
ðSMSintra ¼ 91:1 Þ, which in turn results from the constraints of the fivemembered ring. Measures of geometric distortion in the structure ð
SMStrans ¼ 0:4 ; d ¼ 1:9 Þ are small. Figure 20 shows Re(S2 C2 Ph2 )3 (333), which was the first crystallographically solved six-coordinate TP complex. The view down the approximate C3 axis in this near D3h complex, shows the average twist angle to be near, though not exactly, zero (Table IV, y ¼ 6 ). Other geometrical parameters [TP ! OCT (5%) and fð86 Þ] also suggest the near TP geometry. One of the three ligands is twisted slightly, which results in an elevated distortion value ð
SMStrans ¼ 3:8 Þ. It is not always the case that the dithiolene ligands are arranged in such a symmetrical fashion about the central metal atom. For example, the anionic portion of [AsPh4 ][Ta(bdt)3 ] (355) is shown in Fig. 21. The structure demonstrates two dithiolene ligands, defined by atoms S2 and S3, and S4 and S5, are closer to the TP extreme (y ¼ 16 and 16 , respectively). The third ligand, defined by atoms S1 and S6, is twisted toward the OCT extreme ðy ¼ 54 Þ. This asymmetric distortion leads to large distortion parameters as outlined in Table IV ð
SMStrans ¼ 11:5 ; d ¼ 11:8 Þ. 2.

Typical Bond Lengths and Angles

Average bond length and angle parameters for the homoleptic tris(dithiolene) complexes are less sensitive to the identity of the transition metal or dithiolene ligand than those of bis(dithiolene) structures. Ranges of values are summarized ˚ , with in Fig. 22. Average M

S bond lengths cluster between 2.263 and 2.543 A 2
˚ values for two [Fe(mnt)3 ] units (2.263 and 2.269 A) (18), fCo[S2 C2 (CO)3 ]3 g3


88

COLIN L. BESWICK ET AL.

Selected Average Parameters Distances (Å) Ta—S 2.430 S—C 1.748 C— — C 1.382

Angles (°) S—Ta—S 80.8 Ta—S—C 106.6 S—C — —C 120.4

Figure 21. Atom positions and bonds for the anionic portion of [AsPh4 ][Ta(bdt)3 ] (355). The structure is distorted (
SMStrans ¼ 11:5 ; d ¼ 11:8 ) with two ligands (defined by S4 and S5, and S2 and S3) nearer TP geometry than the third.

˚ ; second to last entry in Table IV, Fig. 19) (368) and [Zr(bdt)3 ]2
(2.265 A ˚ ) (7) falling at the outer extremes. Metal centers with higher formal (2.543 A charges and complexes with electron-withdrawing dithiolene substituents tend
to result in shorter M

S bond lengths. Ranges for S

C and C


C distances are ˚ , respectively. 1.665–1.785 and 1.304–1.524 A Average intraligand SMS angles (SMSintra ) range between 79.2 and 91.1 as shown in Fig. 22 and a detailed listing is found in Table IV. The three largest average SMSintra angles correspond to the Fe and Co complexes identified above, which have the shortest M

S bond lengths. Average MSC and SCC angle ranges are 100.7–111.1 and 118.5–123.9 , respectively. Average interligand trans-SMS angles are also listed in Table IV. 3.

Ligand Bending in Tris(dithiolene) Structures

An additional structural characteristic found in some tris(dithiolene) structures is the bend (Z) between the SMS and SCCS planes in the five-membered S

M

3 26 2. 543 2.

S

1.66 1.78 5 5

1.304 1.524

118.5 123.9

100.7 111.1 79.2 91.1

M

S

S

˚ ) and angles ( ) for tris(dithiolene) transition metal Figure 22. Ranges of average bond distances (A structures. 

STRUCTURES AND TRENDS IN HOMOLEPTIC DITHIOLENE COMPLEXES

(a) D3h No ligand bend

89

(b) C3h Ligand bend

Figure 23. View looking down the threefold axis of a TP tris(dithiolene) complex. (a) The HOMO orbital is comprised of the ligand pv orbitals centered about the dz2 metal orbital. (b) Energy minimization of the HOMO occurs if ligand bending takes place improving the bonding orbital overlap. [Figure adapted from (372).]

ring (see Section III.A.1, Scheme 4 for graphical description). A detailed listing of average Z values for tris(dithiolene) structures is shown in Table IV. Bending may not be uniform for all ligands within a structural unit. For example, individual ligand values for [Ta(bdt)3 ]1
(355) shown in Fig. 21 are roughly 1.6, 18.7, and 31.8 . Rationalization for why ligand bending takes place in some tris(dithiolene) complexes has been addressed recently by Campbell and Harris (372) and previously by others (7, 357). In TP structures, the bend is thought to result from a second-order Jahn–Teller distortion where the highest occupied molecular orbital (HOMO) is lowered in energy as a result of improved overlap between the ligand pv and the metal dz2 orbitals as depicted in Fig. 23 (372). Ligand bending is observed most prominently in formal d0 systems, where the stabilized molecular orbital is doubly occupied, although a reduced effect is still observed when the orbital is singly occupied. The ligand bend is expected to be significantly reduced in examples where the analogous destabilized lowest unoccupied molecular orbital (LUMO) becomes singly occupied, and the bend is eliminated when the LUMO is doubly occupied. For example, the recent structures, [Mo(S2 C2 Me2 )3 ]n , where n ¼ 0,
1, and
2, all show near TP stereochemistry. Ligand bending is most severe when n ¼ 0 (d 0 , Z ¼ 15:9 ), still significant when n ¼
1 (d1 , Z ¼ 4:0 ), and is effectively absent when n ¼
2 (d2 , Z ¼ 0:9 ) (341, 342). Large dithiolene substituents, such as Ph, can introduce steric interactions that limit the degree of ligand bending, and crystal packing forces may cause or mitigate ligand bending in some cases. Two further examples are shown in Figs. 24 and 25. The formally d0 complex Mo(bdt)3 (350) is shown in Fig. 24. The view down the near C3 axis reveals the TP configuration of the MoS6 core (TP ! OCT ¼ 1%; y ¼ 0 ) and the bends in each of the Mo

bdt rings (Z ¼ 16:1, 22.6, and 30.3 , values differ slightly from

90

COLIN L. BESWICK ET AL.

Selected Average Parameters Distances (Å) Mo—S 2.367 S—C 1.727 —C 1.411 C—

Angles (°) S—Mo—S 82.1 Mo—S—C 106.2 —C 119.5 S—C —

Figure 24. Structure of the formally d0 complex Mo(bdt)3 (350) exhibiting significant ligand bends. The molecular symmetry is nearly C3h. The view is down the threefold axis with the reflection plane passing through the Mo atom and perpendicular to the axis.

those reported as only the five-membered ring atoms were used in this calculation). The neutral V(edt)3 complex is shown in Fig. 25. The view down the threefold axis in this near D3h example shows the TP configuration of the complex (TP ! OCT ¼ 3%; y ¼ 2 ), but unlike Mo(bdt)3 , there is no significant bending apparent in the five-membered rings ðZ ¼ 0:7 Þ. The V(edt)3 is also a good example of the difficulty in defining the metal oxidation state in dithiolene complexes as ‘V(VI)’ is clearly not a reasonable proposition. Thus, there must be at least some dithione character (B, see Scheme I) to the edt ligands in this complex, which is strongly supported by the presence of the ˚ ) of all the tris(dithiolene) shortest average S

C bond distance (1.665 A

Selected Average Parameters Distances (Å) V—S 2.347 S—C 1.665 C— —C 1.356

Angles (°) S—V—S 82.8 V—S—C 107.0 S—C — —C 121.6

Figure 25. Structure of V(edt)3 (340). The nearly D3h symmetry is shown with the view down the threefold axis.

91

83.0 81.9 84.0 82.1

83.2

[Sn(dmit)3]2
(3)e

[Sn(dmio)3]2


e

d

c

b

a

172.8

165.2 171.3 170.7 172.5

170.3

173.2

173.0 171.9 174.0 172.1

174.0

175.3

173.5 171.5

ecorr ( )



99

79 98 91 101

90

89

86 94

0 100

TP!OCT (%)

3.5/0.8

2.3/0.0 4.7/4.3 6.9/0.2 4.3/3.6

5.7/2.2

7.2/2.8

2.8/2.5 3.1/2.5


SMStrans ( )/ d ( )

57

66 60 62 56

57

62

63 59

90 55

f ( )b

51

39 45 44 52

52

45

45 48

0 60

y ( ) b;d

39.4

26.1 44.6 28.3 47.3

17.8

17.4

24.4 30.3

Z ( ) b;d

Values for only one structural unit have been reported in cases where multiple units are contained within the unit cell. Average values. The three largest interligand SMS angles are used to calculate SMStrans. In some cases, this may overestimate SMStrans values by 1–2 . Values may vary slightly from the cited literature depending on the calcuation method (370). The number of reports based on the anion structure.

[Sb(dmit)3]2


84.0

[Sb(tdt)3]1


85.3

[Sb(bdt)3]1


171.0

168.2 169.3

83.5 81.5

[Sn(mnt)3]2
[ln(mnt)3]3


SMStrans ( )b;c

136 180

SMSintra ( )b

Trigonal Prism Octahedron

Dithiolene Complex

a

TABLE V Main Group Homoleptic Tris(dithiolene) Complexes

376

376 376 376 377

375

323

373 374

References

92

COLIN L. BESWICK ET AL.

complexes. Arguably, V(edt)3 should also exhibit a measurable ligand bend, but it has been suggested that stabilization is not realized due to a large difference between the metal and ligand orbital energies. Whatever the reason, the absence of a ligand bend is a reminder that some aspects of dithiolene bonding are subtle and difficult to predict. B.

Main Group Homoleptic Tris(dithiolene) Complexes

There are nine homoleptic tris(dithiolene) structures reported in the CSDC based on main group elements, specifically Sn, Sb, and In. The elemental distribution is shown in Fig. 15 and a detailed listing of the structures is given in Table V. Table V for main group examples is analogous to Table IV for transition metal units. Listed values of TP ! OCT (79–101%), f (56–66 ), and y (39– 52 ) all indicate that the tris(dithiolene) main group structures tend toward OCT stereochemistry. Values of Z (17.4–47.3 ) indicate significant ligand bending in all structures. Values for M

S distances follow the charge on the unit: 2.440 and ˚ for n ¼
1; 2.531–2.558 A ˚ for n ¼
2; and 2.604 A ˚ for n ¼
3. Other 2.478 A ˚ ˚. ring bond distance ranges are S

C, 1.702–1.796 A; and C

C, 1.343–1.411 A  Although SMS angles (81.5–85.3 ) are similar to transition metal examples, most MSC and SCC angles are shorter (92.8–104.2 ) and longer (120.4–126.4 ), respectively.

V.

HOMOLEPTIC MONO(DITHIOLENE) COMPLEXES

There are only three types of crystallographically solved homoleptic mono(dithiolene) structures: {[Tl(bdt)]2 }2
(326); two examples of {[Ag(mnt)]4 }4
with different cations (15); and {[Pd(S2 C2 (COOMe)2 )]6 }n , where n ¼ 0 and
2 (378). The structures are all comprised of distinct mono(dithiolene) units that satisfy the coordination sphere of the central atoms through S atoms that form additional bridging bonds. The anionic portion of the dimeric structure, (NEt4 )2 [Tl(bdt)]2 (326), is shown in Fig. 26. The dianion is comprised of two [Tl(bdt)]1
units related by an inversion center. Metric parameters for the five-membered rings are also shown in Fig. 26. The rings exhibit considerable ligand bends ðZ ¼ 37:2 and opposite, 50.5 Þ. There are additional bonds bridging each ligand S atom to the ˚ , although this later distance is large to opposing Tl atom (3.074 and 3.347 A consider as a significant interaction). Intraligand Tl

S bonds are shorter (2.876 ˚ ), but are still longer than those found in bis(dithiolene) complexes. and 2.858 A A significant deviation from bonding parameters of bis or tris(dithiolene) structures is the acute S

Tl

S angle of 71.8 , in part due to the rather long

STRUCTURES AND TRENDS IN HOMOLEPTIC DITHIOLENE COMPLEXES

Distances (Å) Tl—S1 2.876 Tl—S2 2.858 Tla—S1 3.074 Tla—S2 3.347 S1—C1 1.776 S2—C2 1.769 C1—C2 1.408 Tl…Tla 3.512

93

Angles (°) S1—Tl—S2 71.8 Tl—S1—C1 102.4 Tl—S2—C2 102.3 S1—C1—C2 123.0 S2—C2—C1 123.9 Tl—S1—Tla 72.3 Tl—S2—Tla 68.4

Figure 26. Atom positions and bonds for the dimeric mono(dithiolene) structure {[T1(bdt)]2 }2
(326). Selected distances and angles are reported. Significant ligand bends are observed (Z ¼ 37:2 and opposite, 50.5 ).

˚ ) is not indicative of significant Tl

S bonds. The Tl   Tla distance (3.512 A bonding interaction. The anionic portion of [N(n-Bu)4 ]4 [Ag(mnt)]4 is shown in Fig. 27. The four ˚) [Ag(mnt)]1
units are joined via Ag

S bridge bonds (average ¼ 2.450 A between one S atom of each dithiolene unit and a Ag atom of another unit. ˚ ) within the five-membered rings The average distance for Ag

S bonds (2.516 A is slightly longer. The four Ag atoms form a square plane with a single S atom bridging each pair of adjacent Ag atoms. The average distance between adjacent ˚ . The average Ag
Ag atoms is 3.002 A
S

Ag bridging angle is 73.9 , which is similar to that observed in f[Tl(bdt)]2 g2
. Two of the ligands exhibit significant bends (Z ¼ 0:6, 4.8, 20.4, and 22.2 ).

Selected Average Parameters Distances (Å) Ag—S 2.516 Ag—Sbr 2.450 S—C 1.738 —C C— 1.371 Ag—Ag 3.002

Angles (°) S—Ag—S 87.6 Ag—S—C 97.6 S—C— 127.4 —C Ag—S—Ag′ 73.9

Figure 27. Bonds and atom positions for the tetrameric mono(dithiolene) structure f[Ag(mnt)]4 g4
(15). There are two distinct, yet similar, tetramers. Only one is shown here. Selected average distances and angles are reported. See original reference for esd values.

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COLIN L. BESWICK ET AL.

Selected Average Parameters Distances (Å) Pd—S 2.299 Pd—Sbr 2.370 S—C 1.771 C— —C 1.337

Angles (°) S—Pd—S 87.5 Pd—S—C 103.3 S—C— —C 121.4 Pd—S—Pd′ 90.8

Figure 28. Structure of the hexameric mono(dithiolene) complex, [PdS2 C2 (COOMe)2 ]6 (378). The structure contains a crystallographically imposed inversion center. In order to highlight the monomeric units and clarify the structure: (a) each of the symmetry related ligands have been made identical to aid in identifying each monomeric PdS2 C2 (COOMe)2 unit; (b) Pd atoms are large, S
OMe) groups have atoms are medium, and C atoms are small; (c) ester oxygen(

O) and methoxy (
been omitted for clarity; and (d) bonds within monomeric units are thickened to distinguish from bridging bonds. Selected average distances and angles are reported. Average adjacent and opposite ˚ , respectively. The ligand bends (Z) are 15.7, 17.5, and 18.3 . Pd

Pd distances are 3.325 and 4.703 A

The structure of the neutral hexameric complex [Pd(S2 C2 (COOMe)2 )]6 is shown in Fig. 28. The molecule contains an inversion center. Similar to
S bridge {[Tl(bdt)]2 }2
and {[Ag(mnt)]4 }4
, the Pd6 S12 core results from Pd
bonds that form between Pd[S2 C2 (COOMe)2 ] units. The six Pd atoms form an octahedral array and the 12 S atoms lie on the edges of an imaginary cube in which the Pd atoms are centered in the six faces. Each of the two S atoms of a monomeric unit makes one bridge to a Pd atom of another monomeric unit. For example, in the labeled ligand in Fig. 28, one S atom bridges to a shaded Pd atom and the other S atom bridges to a nonshaded Pd atom. The bridge bonds are nearly perpendicular to the plane of the five-membered ring (90.8 ). A great deal can be learned about the bonding characteristics of dithiolene ligands from the three types of multimeric mono(dithiolene) complexes. A single dithiolene ligand is apparently not sufficient to satisfy the coordination sphere of a central atom. Yet, aggregations of monomeric units held together by auxiliary bonds can lead to well-defined molecular units. The presence of the M

S bridge bonds does not grossly alter the standard parameters of the five-membered ring. The ring remains symmetrical and roughly planar, although some dramatic ligand bending can be observed such

S, S

C, and C
as in the case of {[Tl(bdt)]2 }2
. Moreover, M


C distances and related angles are similar to what would be anticipated from bis(dithiolene)

STRUCTURES AND TRENDS IN HOMOLEPTIC DITHIOLENE COMPLEXES

95

structures and the identity and formal oxidation state of the central element. It therefore seems clear that bridge bonding primarily relies on sulfur orbitals that are nonbonding or weakly bonding with respect to the monomeric metal– dithiolene unit. There is a clear relationship between bridge bonding in multimeric mono(dithiolene) structures and the formation of M

S stacked dimers and trimers of bis(dithiolene) units (see Section III.A.4). In both cases, S atoms in the metal– dithiolene chelates can bridge to a second metal center. Despite the bridging, the five-membered ring parameters remain largely unchanged although some distortions are apparent. Further, stacked M

S bis(dithiolene) dimers can be broken apart in the presence of strongly binding ligands such as CO, NO, and phosphines. Initial indications are that mono(dithiolene) aggregates can also be disaggregated (378).

VI.

SUMMARY

Examination of structurally characterized dithiolene complexes identifies a number of important features of dithiolene ligand systems. The ligands largely maintain their shape, bond distances, and angles over a wide range of metalbased complexes. Even additional bridging bonds found in bis(dithiolene) dimers and multimeric mono(dithiolene) structures do not greatly alter the metal–dithiolene chelate. Bis(dithiolene) structures demonstrate square-planar (the majority), distortedtetrahedral, and square-pyramidal coordination geometries about the central atom, and are the most common homoleptic dithiolene structure type. Tris (dithiolene) structures range between trigonal-prismatic and octahedral geometries and have been described by a number of geometrical parameters such as twist angles, ligand bends, and SMStrans angles. There are also well-defined molecular aggregates of monomeric dithiolene units that have been crystallographically characterized such as dimeric bis(dithiolene), and multimeric mono(dithiolene) complexes. The structural information collected here will hopefully provide helpful knowledge and potential insight into the syntheses, characterization, and properties of dithiolene structures as a whole, including heteroleptic coordination environments where dithiolene ligands are found.

ACKNOWLEDGMENTS We thank Dr. Henry H. Murray, Dr. Douglas M. Ho, and members of the CSDC for contributions that made this chapter possible.

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COLIN L. BESWICK ET AL.

ABBREVIATIONS bdt Bu Me CF3 CN CSDC Dithiosquarate 2D dddt dmio dmit edt esd Et HOMO i LUMO mnt n OCT Ph PPhþ 4 S2 C6 Cl4 S2 (CO)2 C6 H4 t (or tert) tdt TP TP ! OCT

1,2-Benzenedithiolate Butane Methyl Trifluoromethyl Cyano Cambridge Structural Data Center 3,4-Dioxocyclobutene-1,2-dithiolate Two dimensional 5,6-Dihydro-1,4-dithiine-2,3-dithiolate 1,3-Dithiole-2-one-4,5-dithiolate 1,3-Dithiole-2-thione-4,5-dithiolate 1,2-Ethenedithiolate Estimated standard deviation Ethyl Highest occupied molecular orbital Iso Lowest unoccupied molecular orbital 1,2-Maleonitrile-1,2-dithiolate (1,2-Dicyanoethene-1,2-dithiolate) Normal Octahedral Phenyl Tetraphenyl phosphonium 1,2,3,4-Tetrachloro-5,6-benzenedithiolate Naphthoquinonedithiolate Tertiary Toluene-3,4-dithiolate Trigonal prismatic A measure of the degree of closeness to trigonal prismatic or octahedral structure (quantitative expression in text)

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CHAPTER 3

The Electronic Structure and Spectroscopy of Metallo-Dithiolene Complexes MARTIN L. KIRK, REBECCA L. McNAUGHTON, and MATTHEW E. HELTON The Department of Chemistry MSC03 2060 The University of New Mexico Albuquerque, NM CONTENTS I. II.

INTRODUCTION

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METALLO-MONO(DITHIOLENES)

116

A. Introduction / 116 B. Metallo-mono(dithiolene) Complexes that Possess a Strong-Field Axial Ligand / 117 1. Bonding Description and Origin of the Sdithiolene ! Mo Charge Transfer / 117 2. The Electronic Buffer Effect in Oxo-metal-mono(dithiolenes) / 128 C. Ligand-to-Ligand Charge Transfer in Metallo-mono(dithiolenes) / 138 III.

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Introduction / 142 Square-Planar Metallo-bis(dithiolenes) / 143 1. A Simple Molecular Orbital Description of Square-Planar Metallo-bis(dithiolene) Bonding / 143 2. Ground-State Spectroscopic Probes / 146

Dithiolene Chemistry: Synthesis, Properties, and Applications, Progress in Inorganic Chemistry, Vol. 52 Special volume edited by Edward I. Stiefel, Series editor Kenneth D. Karlin ISBN 0-471-37829-1 Copyright # 2004 John Wiley & Sons, Inc. 111

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C.

IV.

METALLO-TRIS(DITHIOLENES) A. B. C. D.

V.

3. Excited-State Spectroscopic Probes / 149 Metallo-bis(dithiolenes) Possessing Axial Oxo Ligands / 166 1. A Comparison between Square-Planar and Square-Pyramidal Metallo-bis(dithiolene) Bonding / 166 2. Excited-State Spectroscopic Probes / 167 173

Introduction / 173 Molecular Orbital Description of Metallo-tris(dithiolene) Bonding / 174 Distortions from the Ideal D3h Trigonal-Primatic Geometry / 179 Spectroscopic Studies of Metallo-Tris(dithiolenes) / 188 1. EPR Spectroscopy / 188 2. Electronic Absorption Spectroscopy / 192

CONCLUSIONS

196

ACKNOWLEDGMENTS

197

ABBREVIATIONS

197

REFERENCES

198

I.

INTRODUCTION

Determining the electronic structure of metallo-dithiolenes is pivotal in developing a detailed understanding of their role in bioinorganic chemistry (1–110) as well as in materials science (111–248). Numerous metallo-dithiolenes have been synthesized and, depending on the choice of the transition metal and the nature of the dithiolene, possess lumophoric (249–265) magnetic (113, 114, 120, 143, 145, 158, 159, 163, 191, 192, 199, 203, 211, 213, 215, 216, 218, 266, 267), and conducting properties (111, 113, 121, 123, 124, 130–133, 136– 138, 144, 155, 158, 159, 173, 175, 183, 185, 189–191, 193–195, 197, 198, 201– 203, 205, 206, 217–219, 240, 241, 268–279). Perhaps the most fascinating function of transition metal dithiolenes is their ability to catalyze a variety of formal oxygen atom transfer (OAT) reactions in specific model systems and at the active sites of pyranopterin molybdenum and tungsten enzymes. The numerous properties of metallo-dithiolene complexes are a direct consequence of the fantastic redox interplay between the redox-active metal and the dithiolene ligand. A considerable amount of experimental evidence over the last 40 years has pointed to the fact that dithiolenes are highly noninnocent ligands (20, 86, 110, 113, 280–282). That is, they may be viewed in a valence bond description as existing somewhere between the extremes of neutral (dithione/dithiete) and dianionic (dithiolate) forms (Fig. 1). This variable formal

METALLO-DITHIOLENE COMPLEXES

ene-1,2-dithiolate

R

S

1,2-dithione

−

113

1,2-dithiete

R

S

R

S

R

S

R

S

2e− S−

R 6 πe−

4 πe−

4 πe−

Figure 1. Valence bond description of the various dithiolene ligand forms. Two-electron oxidation of the dianionic ene-1,2-dithiolate results in the formation of the 1,2-dithione and 1,2-dithiete resonance forms.

oxidation state behavior is somewhat different than that observed for their oxygen counterparts, the quinone–semiquinone–catecholate series, where discrete oxidation states are assigned to the ligand. Note that the neutral form of the ligand has two limiting resonance structures, one of which (dithiete) points toward the possibility of disulfide bond formation within the ligand. The fundamental difference between dianionic and neutral dithiolene ligands is the number of p electrons in the C2S2 unit, which amounts to four in the case of the neutral dithiolene and six for the dianionic form. Irrespective of the dithiolene p-electron count, there are also six s electrons for both the neutral and dianionic forms of the ligand. Hoffmann and co-workers (283) provided a very nice description of the valence orbitals of p symmetry for the dithiolene ligand, and a qualitative molecular orbital (MO) description of the dithiolene p valence orbitals depicted in Fig. 2. The ligand orbitals are labeled w1– 4, with w1 and w3 being symmetric

Figure 2. Qualitative MO description of the four dithiolene p valence orbitals. Electron occupancies are given for the 1,2-dithione/1,2-dithiete resonance forms (A) and the dianionic ene1,2-dithiolate (B). [Adapted from (283)].

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MARTIN L. KIRK ET AL.

with respect to the syz mirror plane, and w2 and w4 being asymmetric with respect to the same plane. As expected, the energy ordering is commensurate with the nodal character and w1 lies at the deepest binding energy with w4 being the most destabilized. If we consider a dianionic dithiolene possessing six p electrons, the ligand orbitals w1–3 are each doubly occupied and w4 is empty, while in the neutral form of the ligand w3 is vacant. Typically, the dithiolene ligand is thought of as dianionic in exercises involving formal electron designation, and there is support for this approach in simple metallo-dithiolene complexes. For example, a fragment MO analysis of [Ni(S2C2H2]2 yields the population of the w1– 4 ligand orbitals in the complex, and these are w1 ¼ 1:98, w2 ¼ 2:00, w3 ¼ 1:72, and w4 ¼ 0:02 electrons, respectively (283), which is in complete agreement with the 6 p-electron description of a dianionic dithiolene. However, with more complex dithiolene ligands, particularly those that possess highly electron-withdrawing substituents, this interpretation should be regarded with extreme caution. Furthermore, caution should also be exercised when considering the dithiolene electron count in all complexes possessing redox active metals, as well as metal complexes possessing more than one coordinated dithiolene donor, see below. A variant on the butadienoid orbitals depicted in Fig. 2 was initially suggested by Hoffmann and co-workers (283) and utilized by Solomon and co-workers (19) in their seminal analysis of oxo-molybdenum mono(dithiolene) electronic spectra. This model (Fig. 3) utilizes two isolated C C p molecular

Figure 3. An alternative MO description of the dithiolene valence p orbitals. These are essentially the four SALCs that form the basis for the MOs depicted in Fig. 2.

METALLO-DITHIOLENE COMPLEXES

115

orbitals and two effective ‘‘lone-pair’’ orbitals localized on each of the sulfur atoms. These are essentially the four symmetry-adapted linear combinations (SALCs), which form the basis for the molecular orbitals in Fig. 2. In this model, the w0 1 dithiolene orbital is composed entirely of the symmetric combination of C C p p molecular orbitals and is greatly stabilized in energy due to the strength of the ethylene C C bonding interaction. The antibonding * counterpart of this orbital is the C C p p orbital, w0 4, which is highly destabilized and always unoccupied. The stabilization of w0 1 and the destabilization of w0 4 leave the symmetric and antisymmetric sulfur-based MOs, w0 2 and w0 3, available for bonding with the metal. This two-orbital description of the principal ligand valence orbitals being comprised primarily of sulfur p orbital character is supported by photoelectron spectroscopic studies that suggest the valence MOs of dithiolenes are predominantly sulfur in character, with little contribution from the ethylene carbons (16, 284–286). Finally, a third dithiolene ligand model has been utilized with success in order to understand the electronic structure and spectroscopy of a number of oxo-molybdenum mono- and bis(dithiolenes) (23). This dithiolene ligand bonding description utilizes the symmetric and antisymmetric out-of-plane Sop p p orbitals, in addition to the corresponding in-plane symmetric and antisymmetric Sip p orbitals. Ab initio and density functional theory (DFT) calculations have been performed on the simple dithiolene dianion, [S2C2H2]2 (23), in order to illustrate the details of this four orbital model and electron density contours of the four MOs are presented in Fig. 4. These calculations result in an isolated set of four filled dithiolene orbitals, and these are the ligand

Figure 4. The four highest energy occupied MOs of the ethene-1,2-dithiolate (edt2) ligand as determined by ab initio and DFT calculations. Note that both in-plane and out-of-plane orbitals are present.

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MARTIN L. KIRK ET AL.

wave functions that can energetically mix and form SALCs with the d orbitals of appropriate symmetry localized on the metal. These calculations indicate that the electron density is largely localized on the sulfur atoms, in agreement with experimental results from photoelectron spectroscopy (16, 284–286). Furthermore, these four orbitals may be divided into two sets with respect to their orientation to the plane containing the constituent atoms of the ene-1,20 00 dithiolate. These are the in-plane components, jaip and jaip , and the out-of-plane a0 a00 0 00 components jop and jop . Here, the superscripts a and a refer to the symmetry of the wave function with respect to a mirror plane oriented perpendicular to the ene-1,2-dithiolate plane and bisecting the C C bond. Obviously, determining which of these dithiolene bonding descriptions is most useful in developing a detailed understanding of complicated metallodithiolene spectra will depend on the precise nature of the ligand and metal, as well as the number of dithiolene chelates bound to the metal in the complex. Interestingly, the evolution of metallo-dithiolene electronic structure has proceeded in somewhat of an ‘‘inverted’’ manner. Metallo-bis- and metallotris(dithiolenes) were the first metallo-dithiolene complexes to be subjected to extensive spectroscopic and theoretical study. Although the comparably simpler metallo-mono(dithiolenes) possess only a single dithiolene ligand bound to the metal, this class of molecules has been the last of the metallo-dithiolene complexes that have been subjected to detailed spectroscopic and electronic structure studies. This situation has arisen, in part, because of the inherently low symmetry of metallo-mono(dithiolenes) and the fact that the structures of molybdenum-mono(dithiolene) sites in biology have only recently been elucidated (11, 13, 17, 18, 45, 48, 59, 64, 69, 75–79, 89, 91–96, 287–294).

II.

METALLO-MONO(DITHIOLENES) A.

Introduction

The most detailed spectroscopic and electronic structure studies of metallomono(dithiolenes) have focused on the nature of ligand-to-ligand charge transfer (LLCT) excitations in [M(diimine)(dithiolene)] complexes (112, 250–257, 262, 264, 295–301) and in monooxo molybdenum dithiolenes (19, 20, 22, 23) as models for pyranopterin molybdenum enzymes such as sulfite oxidase (SO). Since metallo-mono(dithiolenes) generally possess little or no symmetry, detailed spectrosopic and electronic structure studies of this class of metallodithiolenes have only recently begun to appear. The analysis of the spectroscopic data has been aided by the fact that the dithiolene-to-metal charge

METALLO-DITHIOLENE COMPLEXES

117

transfer (CT) transitions involve only a single dithiolene ligand and are generally the lowest energy ligand-to-metal charge transfer (LMCT) transitions observed in these molecules or sites. Futhermore, the presence of a single terminal oxo donor in oxo-metal-mono(dithiolenes) has allowed detailed band assignments to be made in the absence of single-crystal polarization data from electronic absorption and magnetic circular dichroism (MCD) spectroscopies. The assignments have been facilitated by the interpretation of resonance Raman (RR) excitation profiles as resonance enhancement of the nM   O mode is indicative of O a CT excitation to the metal dxz;yz orbital set, which is strongly M antibonding in nature (23, 106, 302–305). Next, we discuss the electronic structure and spectroscopy of metallo-mono(dithiolene) complexes that possess a strong-field axial oxo or nitrosyl ligand, and the electronic origin of LLCT excitations in oxo-metal-mono(dithiolene) complexes.

B.

1.

Metallo-mono(dithiolene) Complexes that Possess a Strong-Field Axial Ligand

Bonding Description and Origin of the Sdithiolene ! Mo Charge Transfer

S) [(L-N 3) ¼ hydrotris(3,5-dimethyl-1-pyrazolyl)borate] The (L-N 3)MoO(S compounds possess nearly perfect Cs symmetry with respect to the donor atoms that comprise the first coordination sphere. However, an argument can be made that effective C4v symmetry may be utilized as a starting point for spectroscopic interpetation due to the presence of the strong-field terminal oxo ligand (23). This finding is readily apparent in the nearly axial metal hyperfine tensors determined from single-crystal electron paramagnetic resonance (EPR) studies on (L-N 3)MoOCl2 (306), and EPR simulations of (L-N 3)MoO(bdt) (307, 308) and (L-N 3)MoO(ead) (309) frozen solution spectra, where bdt ¼ benzene1,2-dithiolate. From a ligand-field perspective, this implies a nearly degenerate dxz;yz orbital set in the ground state. Therefore, a C4v ! C2v ! Cs descent in symmetry has been used in the analysis of MCD and electronic absorption spectra for (L-N 3)MoO (dithiolene) compounds (19, 23), keeping in mind the strong axial nature of the ligand field and the anticipated small anisotropy within the equatorial plane. At this point, it is of interest to discuss the relationship between MO theory and the intensity of electronic transitions. The oscillator strength of an electronic absorption band is proportional to the square of the transition dipole moment integral, jhcG jrjcE ij2 , where cG and cE are the ground- and excited-state wave functions, and r is the dipole moment operator. In a one-electron approximation, jhcG jrjcE ij2 ¼ jhca jrjcb ij2 , where ca and cb are the two MOs involved in the one-electron promotion ca ! cb . Metal–ligand covalency results in MO wave

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functions that can be expanded in terms of both metal- and ligand-centered functions:

where;

ca ¼ C1 jM þ C2 jL þ cb ¼ C10 jM þ C20 jL þ X Cn wn jL ¼ n

Here, the Ci are the coefficients of the metal (jM ) and ligand (jL ) orbitals that constitute the ca and cb ground-state MO wave functions. With respect to the oxo-molybdenum mono(dithiolenes), jM are the Mo d 0 orbitals (e.g., jaxy etc.); jL are the four orthogonal ene-1,2-dithiolate orbitals 0 0 00 00 (jaip , jaop , jaip , and jaop ) of Figs. 3 and 4, to which the dominant contributors are the wn for the atomic sulfur p orbital functions. Experimental studies have shown that the dominant contributor to the oscillator strengths of all transitions are integrals of the form hwn jrjw0n i (310 –312), which, in the limit of no overlap between the two sulfur p orbitals on different atoms reduces to (311). X Cn Cn0 rL hwn j w0n i n

Here, Cn and Cn0 are the sulfur atomic orbital coefficients in ca and cb , rL is the position vector, and hwn j w0n i is an overlap integral. The upshot of this is that the same type of dithiolene MO (e.g., hjip j jip i or hjop j jop i) must be present in both the ground-state and excited-state wave functions for enhanced CT transition intensity, and this intensity is a direct consequence of the dmetal  Sdithiolene covalency. The ligand-field splitting of the d-orbital manifold in (L-N 3)MoO(dithiolene), as well as other high-valent metal oxo compounds (see below), is ˚ M  dominated by the presence of a short 1:7  1:9 A  O bond (19, 23). Experimentally, the Mo t2g orbital set is found to be split by 1.5–2.0 eV. This enormous splitting is 10Dq for most first-row transition metals. This splitting results from the fact that the terminal oxo ligand is an extremely strong s and p donor, and in the presence of a moderate-to-weak equatorial ligand field, the d 0 00 0 orbital splitting diagram in Fig. 5 results. Here, the caz2 and caxz , cayz orbitals are strongly destabilized by s- and p-antibonding interactions with the terminal oxo S) complexes, all of the d orbitals may act as ligand. In d 1 (L-N 3)MoO(S acceptor orbitals in low-energy LMCT interactions involving the coordinated dithiolene. Therefore, the energy of these CT transitions will be strongly affected by the oxo-mediated destabilization of the acceptor orbitals. Figure 5 also depicts the anticipated relative energy of the lowest ligand-field and CT excitations. As a result of the Mo d orbital splitting pattern, the predicted

METALLO-DITHIOLENE COMPLEXES

119

Figure 5. A simple spectroscopically effective MO diagram for the (L-N 3)MoO(S S) complexes. The MO energies are not to scale. The cxz;yz orbitals are shown as degenerate due to the dominance of the strongly bound oxo ligand on the z axis. [Adapted from (23).]

0

lowest energy CT transitions will terminate in the Mo caxy orbital followed by 00 0 transitions to the caxz , cayz orbital set. The LMCT transitions originate from the four dithiolate based jL molecular orbitals of Fig. 5, two of which are oriented 0 00 0 within the dithiolene plane (jaip and jaip ) and two that are orthogonal to it (jaop 00 and jaop ). It can be seen from this diagram that the stabilization of these ligandbased MOs will result from specific metal–ligand and sulfur–sulfur bonding interactions. The room temperature solution electronic absorption spectrum of (L-N 3) MoO(bdt) is presented in Fig. 6. This spectrum is representative of virtually all (L-N 3)MoO(dithiolene) complexes (19, 23) with the possible exception of (L-N 3)MoO(qdt) (20, 22), where qdt ¼ quinoxaline-2,3-dithiolate, see below. However, the transitions observed for (L-N 3)MoO(tdt) (19), where tdt ¼ toluene-1,2-dithiolate, are generally shifted to slightly lower energies relative

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MARTIN L. KIRK ET AL.

6400 6

5600

ε(M1cm−1)

4800 4000 5

3200 2400 4

1600 800 0 5,000

1 10,000

2

3 15,000

20,000

25,000

30,000

35,000

Energy (wavenumbers) Figure 6. Room temperature solution electronic absorption spectrum of (L-N3)MoO(bdt). [Adapted from (23).]

to the corresponding bands in (L-N 3)MoO(bdt). The low-energy region of the spectrum consists of three distinct spectral features (bands 1, 2, and 4) below

20,000 cm1, and we have found these bands to be characteristic of (L-N 3)MoO(dithiolene) compounds. Band 3 is very weak, and only discernible in the low-temperature MCD spectra (see below) (20, 22). The transition energies and molar extinction coefficients for some of these (L-N 3)MoO(dithiolene) complexes have been tabulated and are presented in Table I. Band 4 is of particular interest, as it is the lowest energy absorption feature possessing appreciable intensity characteristic of a LMCT transition. The 5 K mull MCD–absorption overlay of (L-N 3)MoO(bdt) is shown in Fig. 7. The electronic absorption spectrum exhibits five distinct features in the

TABLE I S) Complexes in Dichloroethane Summary of Electronic Absorption Data for (L-N 3)MoO(S Emax ðcm1 Þ ½eðM 1 cm1 Þ Band 1 2 4 a

Reference 23. Reference 19. c Reference 20. b

(L-N 3)MoO(bdt)a 9,100 (360) 13,100 (270) 19,400 (sh,1220)

(L-N 3)MoO(tdt)a; b 9,100 (490) 13,000 (270) 19,600 (sh,1320)

(L-N 3)MoO(qdt)c (L-N 3)MoO(ead)a; b 11,300 (170) 11,800 (160) 13,700 (130) 15,500 (220) 19,100 (sh,1050) 20,000 (sh,570)

METALLO-DITHIOLENE COMPLEXES

121

Figure 7. The 5 K mull MCD–absorption overlay of (L-N3)MoO(bdt) in the solid state. Thin line: MCD spectrum; thick line: electronic absorption spectrum. [Adapted from (23).]

solid state with weak to significant intensity. Although the 21,500 cm1 band is observed as a shoulder in the mull absorption of (L-N 3)MoO(bdt), it is absent in the corresponding absorption spectrum of (L-N 3)MoO(tdt) (23). The general similarity of the solution and mull absorption spectra for (L-N 3)MoO(bdt) indicate that only minor structural changes accompany solvation, and this is true for all of the (L-N 3)MoO(dithiolene) compounds that we have studied to date. The MCD spectrum of (L-N 3)MoO(bdt) is comprised of temperature-dependent C- and pseudo-A terms (313a). The relationship between observed MCD and electronic absorption bands in (L-N 3)MoO(bdt) is given in Table II. The MCD mull spectra of (L-N 3)MoO(bdt) and (L-N 3)MoO(tdt) are compared in

TABLE II Calculated Oscillator Strengths of (L-N 3)MoO(bdt) and the Relationship between MCD and Electronic Absorption Bandsa Band No.

E(soln)max (cm1)

Oscillator Strength

E(mull)max (cm1)

1 2 3 4 5 6

9,100 13,100 15,800 19,400 22,100 25,100

5:6  103 3:3  103

8,500 12,700

1:6  102 1:7  102 9:2  102

19,200 21,500 24,600

a

Reference 23.

E(MCD)max (cm1)

MCD Term

12,400 15,700 19,300 21,000 24,300

C þC þC þpseudo-A þpseudo-A

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MARTIN L. KIRK ET AL.

Normalized MCD Intensity (mdeg)

1000

500

0

−500

−1000 10,000

15,000

20,000 25,000 30,000 Energy (wavenumbers)

35,000

40,000

Figure 8. The 5 K solid-state MCD spectra of (L-N3)MoO(bdt) (solid line) and (L-N3)MoO(tdt) (dashed line). [Adapted from (23).]

Fig. 8. These MCD spectra are quite similar, and the spectral band pattern is a  characteristic feature of (L-N 3)MoO(dithiolene) complexes, where the Mo  O bond is oriented cis to a single dithiolene ligand (19, 23). Vibrational spectroscopy, as discussed in detail in Chapter 4 of this volume (313b), has played a fundamental role in elucidating the excited-state electronic structure of (L-N 3)MoO(dithiolene) complexes. The infrared (IR) data for (L-N 3)MoO(bdt) and (L-N 3)MoO(tdt) display intense peaks at 932 and  926 cm1, respectively. This vibration has been assigned as the Mo  O stretching vibration (23). The 514.5-nm solution Raman spectrum of (L-N 3)MoO(bdt) in benzene is given in Fig. 9, revealing only three observable vibrational modes between 250 and 1000 cm1. Vibrational bands in the 300–400-cm1 region for transition metal ene-1,2-dithiolates have been collectively assigned as Mo S stretching vibrations (23, 31, 50, 88, 106, 209, 287, 302, 303, 314–324). A qualitative depolarization study has been performed on (L-N 3)MoO(bdt) and yielded depolarization ratios of 0.40 (362 cm1 ), 0.22 (393 cm1 ), and 0.01 (932 cm1 ) (23). Because the symmetry of (L-N 3)MoO(dithiolene) complexes is very close to Cs , these depolarization ratios are characteristic of the totally symmetric a0 modes. Raman spectra for (L-N 3)MoO(bdt) and (L-N 3)MoO(tdt) have also been collected at low temperature in the solid state. Again, three vibrational bands were observed at 362, 393, and 931 cm1 for (L-N 3)MoO(bdt) and at 342, 376, and 926 cm1 for (L-N 3)MoO(tdt). No significant ground-state vibrational frequency shifts occur between the solid and solution spectra, providing strong evidence that the structural integrity of these complexes is maintained in a solution environment.

Raman Intensity

METALLO-DITHIOLENE COMPLEXES

123

ν6 ν3

ν1

300

400

500 600 700 800 900 Raman-shift (wavenumbers)

1000 1100

Figure 9. Resonance Raman spectrum of (L-N3)MoO(bdt) in benzene using 514.5-nm excitation. The unmarked bands are those of the solvent. [Adapted from (23).]

Resonance Raman spectroscopy has been used to directly probe the origin of CT transitions through the construction of RR excitation profiles, which have been collected for (L-N 3)MoO(bdt) and (L-N 3)MoO(tdt) in the solid state at 140 K using laser excitation at wavelengths between 457.9 and 528.7 nm (23). This wavelength range encompasses the absorption envelopes of bands 4 and 5. As can be observed in Fig. 10, the three vibrational bands for (L-N 3)MoO(bdt) 0.7

Absorbance

0.5 1.5 0.4 6 5

0.3

1

4

0.2 0.5 0.1 0 16,000

18,000

20,000

22,000

Mean Relative Raman Intensity

2

0.6

0 24,000

Energy (wavenumbers)

Figure 10. Solid-state RR excitation profiles of (L-N3)MoO(bdt) superimposed on the Gaussianresolved mull absorption spectrum. The circles, diamonds, and squares correspond to the 362, 393, and 932 cm1 bands, respectively. [Adapted from (23).]

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MARTIN L. KIRK ET AL.

Vibrational Symmetry Coordinates O

O

O

Mo

S

Mo

S

S ν2

ν3 O

O

O S

Mo

S

Mo

ν5

S S

S

S ν4

S S

S

ν1

Mo

Mo

ν6

Figure 11. The six vibrational symmetry coordinates of a simple Cs four atom model applicable to the vibrational spectra of (L-N3)MoO(dithiolene) complexes. The actual normal modes will be a linear combination of symmetry coordinates possessing the same symmetry. [Adapted from (23).]

are resonantly enhanced when pumping into the absorption envelopes of bands 4 and 5. It is important to note that extremely selective RR enhancement patterns are evident for excitation into band 4 (362- and 393-cm1 modes) and band 5 (931-cm1 mode), respectively. The spectroscopic innocence of the hydrotris(3,5-dimethyl-1-pyrazolyl)borate ligand results in all LMCT transitions below 28,000 cm1 being S ! Mo in origin, and (L-N 3)MoO(dithiolene) complexes may be treated as an effective four-atom Cs MoOS2 chromophore. This four-atom vibrational model yields the 3N  6 ¼ 6 symmetry coordinates, which are listed in Fig. 11. These symmetry coordinates form an effective basis for the six normal modes of vibration. In this simplified model, S ! Mo CT  transitions result in excited-state distortions along the Mo S bonds.  O and Mo It is these excited-state distortions that form the basis for A-term RR enhancement of totally symmetric normal modes (325). Since the depolarization studies indicate that all three vibrational modes are totally symmetric a0 modes, Fig. 11 can be used to assign the nature of these vibrations. The high-frequency n3 mode  has been assigned as the totally symmetric Mo  O stretch on the basis of its depolarization ratio (rk =r? ¼ 0:01), high-frequency, strong IR absorption, and analogy to similar oxo-molybdenum compounds (20, 22, 23, 106, 107, 110, 302, 326). As there exists only one totally symmetric a0 nS Mo S stretch, the stronger 393-cm1 band has been assigned as this vibrational mode (n2), and the Mo S bend (n6). It was 362-cm1 mode was assigned as the symmetric S argued that the n6 bending mode was expected to occur at a considerably higher Mo Cl bend in frequency in (L-N 3)MoO(bdt) than the corresponding Cl (L-N 3)MoOCl2 (23). The rationale for this argument was that free S S

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125

movement is impeded by the presence of the dithiolene chelate ring, increasing the frequency of this totally symmetric mode. As mentioned previously, a marked difference in the (L-N 3)MoO(bdt) RR enhancement patterns for in-plane (n6 and n1) and out-of-plane (n3) modes was found to exist. The two in-plane modes, n6 and n1, are resonantly enhanced within the envelope of band 4, while n3 is resonantly enhanced within the envelope of band 5. When these profiles are superimposed on the Gaussian resolved mull absorption spectrum in Fig. 10, it becomes clear that the electronic origins of bands 4 and 5 are radically different from one another. These remarkably orthogonal RR enhancement patterns are indicative of LMCT transitions strongly localized in the dithiolate-Mo plane (band 4) and perpendi O bond (band 5). Thus, the resonance enhancement of cular to it along the Mo  n6 and n1 provide direct evidence for band 4 being an in-plane LMCT transition, 0 and this was the first direct evidence for the involvement of the jaip dithiolate orbital in the low-energy CT spectra of biomimetic oxo-molybdenum and oxo-tungsten dithiolene compounds (23). A combination of descent in symmetry (C4v ! C2v ! Cs ) and orbital overlap considerations have been utilized in order to assign the six lowest energy bands in the (L-N 3)MoO(dithiolene) series. The inherently low oscillator strengths of the Mo ligand-field bands in oxo-molybdenum complexes (e < 100 M 1 cm1), coupled with the presence of low-energy LMCT excitations, result in ligand-field transitions that are difficult to observe in (L-N 3)MoO(dithiolene) compounds. As a result, LMCT bands are anticipated to dominate the absorption spectra of these complexes. The unique electronic structure of these (L-N 3)MoO(dithiolene) complexes arises from two basic factors. The first is the strong axial s- and p-donor properties of the terminal oxo ligand, which dominates the ligand field and predetermines the energy of the Mo-based dxz ; dyz ; and dz2 acceptor orbitals. The second is the equatorial dithiolene sulfur donors, from which the low-energy LMCT transitions arise. Dithiolene covalency contributions to the electroactive 0 caxy , or redox, orbital can be directly probed via the relative oscillator strengths 0 0 0 0 of the caop ! caxy and caip ! caxy transitions (see above). These three wave functions may be expanded in terms of Mo- and dithiolene sulfur-based functions: 0

0

0

0

0

0

0

0

0

0

0

caxy ¼ c1 jaxy þ c2 jaip þ c3 jaop caip ¼ c01 jaxy þ c02 jaip þ c03 jaop 0

caop ¼ c001 jaxy þ c002 jaip þ c003 jaop 0

0

The oscillator strength ( f) of the (L-N 3)MoO(dithiolene) caip ! caxy LMCT transition was determined to be 2.6–6.0 times more intense than the lower

126

MARTIN L. KIRK ET AL. TABLE III Absorption Band Maxima and Assignments for (L-N 3)MoO(bdt)a

Band

a

Emax (cm1)

f

Assignment 3

0

00

Comments 0

1

9,100

5:6  10

cop ða =a Þ

cxy ða Þ

2

13,100

3:3  103

cop ða0 =a00 Þ

cxy ða0 Þ

3 4

15,800 19,400

1:6  102

cxy ða0 Þ cip ða0 Þ

cxz ða00 Þ, cyz ða0 Þ cxy ða0 Þ

5

22,100

1:7  102

cip ða00 Þ

cxz ða00 Þ, cyz ða0 Þ

6

25,100

9:2  102

cop ða00 Þ

cxz ða00 Þ, cyz ða0 Þ

Out-of-plane to in-plane transition results in low intensity Out-of-plane to in-plane transition results in low intensity Ligand field transition First intense LMCT transition, in-plane to in-plane transition, cxy ða0 Þ ! cx2 y2 ða00 Þ ligand field transition expected in this region also MCD pseudo-A, RR enhancement of nMo¼O , Poor in-plane– out-of-plane overlap gives weak intensity MCD pseudo-A, good out-of-plane– out-of-plane overlap gives large intensity

Reference 23.

0

0

0

0

energy caop ! caxy transitions (19, 23). The square root of the oscillator strength 0

0

0

0

for the caop ! caxy transition is proportional to c2 c002 hjaip j jaip i þ c3 c003 hjaop j jaop i, 0

0

since hjaip j jaop i ¼ 0 due to the orthogonality of the S pz and px;y atomic 0 0 orbitals. Similarly, f 1/2 for the caip ! caxy transition is proportional to 0 0 0 0 c2 c02 hjaip j jaip i þ c3 c03 hjaop j jaop i. Clearly, the oscillator strength ratio indicates 0 that c2 > c3 in the ground state, and covalency contributions to caxy are strongly 0 dominated by the jaip orbital. An approximation has been made (23, 310–312), 0 0 0 0 where the oscillator strengths of the caip ! caxy and caop ! caxy transitions are a0 determined solely by the ligand coefficients in cxy . This results in a c22 =c23 ratio of 2.6–6.0, providing an estimate of anisotropic (in-plane vs. out-of-plane) 0 covalency contributions to Mo–dithiolene bonding in the ground-state caxy highest occupied molecular orbital (HOMO) wave function. The results of these recent detailed spectroscopic studies on simple (L-N 3)MoO(dithiolene) complexes (19, 23) have allowed the assignment of

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low-energy dithiolene ! Mo CT transitions (Table III), which are the dominant spectral features in these oxo-molybdenum-mono(dithiolene) compounds. In particular, Kirk and co-workers (23) showed that RR spectroscopy is an extremely useful tool in making band assignments in oxo-molybdenum-mono(00 00 0 0 0 dithiolenes). Specifically, the important caop ! caxz ; cayz and caip ! caxy LMCT transitions were definitively assigned by virtue of their orthogonal RR excitation S modes profiles (23). A significant distortion along both the n1 and n6 Mo 0 0 accompanies the caip ! caxy transition, and this is indicative of a substantial change in in-plane Mo–dithiolene bonding accompanying this one-electron 0 00 00 0 promotion to the caxy orbital. Similarly, the caop ! caxz ; cayz transition results in O an excited-state distortion along n3 and a concomitant weakening of the Mo bond. Resonance Raman enhancement of n4 was not observed, presumably because the distortion along this out-of-plane Mo S symmetric bending mode is very small. 0 0 The definitive assignment of band 4 as the caip ! caxy bonding-to-antibonding transition has been extremely important. The nature of the bonding and 0 0 antibonding combinations between the Mo jaxy and in-plane S jaip orbitals is 0 detailed in Fig. 12. Here it is readily observed that the jaip orbital of the 0 dithiolene ligand is oriented such that good overlap with the Mo jaxy orbital occurs, resulting in a special three-center pseudo-s type bonding interaction. 0 0 The energy of the caip ! caxy LMCT transition reflects the strength of this bonding interaction, and the intensity of the transition directly probes the pseudo-s mediated Mo–dithiolene covalency.

Figure 12. A MO diagram showing the psuedo-s bonding and antibonding combinations of the Mo 0 0 jaxy and dithiolate jaip orbitals. [Adapted from (23).]

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The energy of the caxy redox orbital is primarily affected by the nature of the equatorial ligand field such that strong pseudo-s Mo–dithiolate bonding will 0 ) raise the energy of this orbital. Additionally, the effective nuclear charge (Zeff of the metal is also a property of the donor atoms, and appreciable changes in 0 for Mo result from the highly polarizable dithiolene sulfur donor ligands. Zeff 0 00 0 00 00 0 The intensity of the caop ! caxz ; cayz and caop ! caxz ; cayz LMCT transitions in (L-N 3)MoO(dithiolene) complexes reflects the p-donor character of the dithio0 due to efficient p-mediated charge lenes and the concomitant reduction in Zeff donation to the metal. Thus, the relative p-donor properties of different dithiolene ligands (see below) may be anticipated to modulate the reduction potential of the metal center by affecting the valence ionization energy of the 0 0 (20, 22). caxy redox orbital through changes in the metal Zeff 0 0 It has recently been observed that the oscillator strength of the caip ! caxy transition for (L-N 3)MoO(dithiolene) compounds also depends on the nature of the ligand backbone. The intensity of this transition for (L-N 3)MoO(ead), which possesses a dithiolate ligand that lacks the ene C C bond, is a factor of 2 less than that observed for comparable (L-N 3)MoO(dithiolene) compounds that a0 a0 possess the dithiolene C C bond (19). The large cip  cxy in-plane covalency in oxo-metal-mono(dithiolenes) suggests that this interaction, in concert with ligand p-delocalization, plays a fundamental role in the unusual ability of these ligands to support multiple redox states (86, 87, 110, 281, 327–331) and to electronically buffer (16) metal centers to the large changes in charge that accompany these redox changes. Another role hypothesized for this in-plane covalency is to facilitate electron transfer in pyranopterin molybdenum and tungsten enzymes (20–24, 106, 110). This bonding interaction represents a remarkable three-center pseudo-s type bonding interaction that is hypothesized to couple 0 the jaxy redox orbital directly into the in-plane orbitals of the pyranopterin ene1,2-dithiolene chelate in members of these enzyme families (20–24, 106, 110). 2.

The Electronic Buffer Effect in Oxo-metal-mono(dithiolenes)

Photoelectron spectroscopy (PES) has been shown to provide a convenient probe of metal ion effective nuclear charge and the nature of the metal–ligand bond via the energy of valence-electron photoionizations (16, 20, 22, 284, 285, 312, 332–334). Recently, PES spectroscopy has been employed in the study of oxo-molybdenum compounds of the type (L-N 3)MoE(X,Y) [E ¼ O, S, NO; X, Y ¼ halide, alkoxide, or thiolate] in order to evaluate the synergy between the axial (E) and equatorial (X,Y) donors in affecting the ionization energy of the HOMO localized on the Mo center (16, 284, 334). These studies have conclusively shown that equatorial dithiolene coordination electronically buffers the Mo center in (L-N3)MoE(tdt) (Fig. 13) from the severe electronic perturbations associated with the enormous variation in the p-donor/acceptor properties

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Figure 13. General structure of (L-N3)MoE(dithiolene) complexes, where E ¼ O, S, or NO.

of the axial ligand, E, and it has been proposed that this is a primary role for the pyranopterin ene-1,2-dithiolate chelate during the course of enzymatic catalysis. Gas-phase ultraviolet (UV) PES is a particularly effective probe of the bonding in transition metal complexes, including metallo-dithiolenes. Electronic absorption, MCD, and EPR spectroscopy have each indicated that the bonding scheme in (L-N3)MoO(dithiolene) complexes is very covalent (19, 20, 22, 23). Variable energy PES provides a convenient probe of metal–ligand covalency, since the photoionization cross-section is a function of the energy of the incident photons, and the azimuthal quantum number, ‘, of the orbital from which the photoejected electron originates (312). Therefore, the relative ionization intensities as a function of incident photon energy are expected to differ significantly for photoelectrons originating from Mo 4d and S 3p orbitals. Interestingly, initial PES studies comparing members of the alkoxide series (LN3)Mo(E)(OR)2 (E ¼ O, S, NO) indicate that the formally Mo(II) nitrosyl complex is more difficult to ionize than the corresponding Mo(V) oxo complex by 0.83 eV (334). This counterintuitive behavior can be explained by the strong p-donor properties of the nitrosyl ligand, which strongly stabilizes the dxz;yz orbitals in this low-spin d 4 complex. Thus, the valence ionization energy of the dxy redox orbital for the Mo(V)–oxo complex is considerably less than that for (L-N3)Mo(NO)(OR)2 (Fig. 14). The first ionization energy of (L-N3)Mo(NO) (OR)2 is 6.57 eV, 0.38 eV lower than that observed for (L-N3)MoO(tdt), indicating that the dithiolate ligand significantly stabilizes the redox orbital in the latter complex. Consequently, it is interesting to note that the first ionization energy of the corresponding (L-N3)MoE(dithiolene) complexes are independent of the nature of the axial ligand (Fig. 15), in stark contrast to that observed for the corresponding (L-N3)Mo(E)(OR)2 complexes (16). This result points to a critical role for dithiolene ligands in modulating the effective nuclear charge of

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Figure 14. Frontier molecular orbital diagrams of (a) metal-oxo and (b) metal-nitrosyl p-bonding interactions. [Adapted from (16).]

Figure 15. The He I photoelectron spectra of (a) (L-N3)MoO(tdt), (b) (L-N3)MoS(tdt), and (c) (L-N3)MoNO(tdt). [Adapted from (16).]

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metallo-dithiolene complexes, overcoming both the large formal oxidation state changes at the metal and the effect of p-acceptor mediated stabilization of the dxz;yz orbital set in (L-N3)Mo(NO)(tdt). The variable-energy PES spectra for (L-N3)MoO(tdt), (L-N3)MoS(tdt), and (L-N3)Mo(NO)(tdt) are presented in Figs. 16–18, and the relative areas of the first two ionizations for this series are presented in Table IV. Electronic spectroscopies, the results of bonding calculations, and the magnitude of Mo hyperfine parameters in the EPR spectra of (L-N3)MoO(dithiolene) complexes indicate that the first ionization should originate from a MO of predominantly Mo character, while the second ionization should occur from a ligand-based

Figure 16. The Ne I, He I, and He II photoelectron spectra of (L-N3)MoO(tdt). [Adapted from (16).]

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Figure 17. The Ne I and He I photoelectron spectra of (L-N3)MoS(tdt). [Adapted from (16).]

dithiolene Sop orbital. Therefore, variable-energy PES should provide considerable information concerning the atomic orbital character of these MOs through an analysis of their relative intensity patterns. Theoretical studies have indicated that the photoelectron cross-sections for ionizations from Mo 4d orbitals should increase twofold over that for S 3p ionizations when comparing He II and He I sources (16). This situation is reversed when comparing spectra obtained using Ne I and He I sources. Here, the cross-section for ionizations from the Mo 4d orbitals is expected to decrease by a factor of 4 when compared with crosssections for S 3p ionizations (16). These ratios will be reduced by Mo S covalency effects in proportion to the Mo and S atomic orbital coefficients in the HOMO and HOMO-1. Inspection of Figs. 16–18 reveal no such relative intensity changes as a function of the incident photon energy or the axial ligand. Thus, both the HOMO and HOMO-1 possess nearly equivalent Mo and S atomic orbital character, and are representative of a highly covalent bonding scheme. Since the HOMO is predominantly an in-plane MO, and the HOMO-1 an out-of-plane orbital, there appears to be no anisotropic covalency contributions to the bonding. As this is in contrast to the electronic absorption and MCD

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Figure 18. The Ne I, He I, and He II photoelectron spectra of (L-N3)MoNO(tdt). [Adapted from (16).]

studies, it likely reflects the lower resolution of the PES technique for quantifying anisotropic covalency contributions to the bonding scheme of metallo-dithiolenes. Considerable effort has been expended in determining how the inherent electronic structure of a dithiolene donor can affect and control important properties of transition metal complexes (20, 22, 332, 335–341). Recently,

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TABLE IV First Ionization Potentials and Relative Areas of Ionizations 1 and 2 for Dithiolene Complexesa

Complex

First Ionization Potential (eV)

Ionization

Relative Area (%) ————————————— Ne I He I He II

(L-N3)MoO(tdt)

6.95

1 2 þ 20

1.00 1.44

1.00 1.53

(L-N3)MoS(tdt)

6.88

1 2 þ 20

1.00 1.12

1.00 1.39

(L-N3)Mo(NO)(tdt)

6.90

1 þ 10 2 þ 20

1.00 0.88

1.00 0.88

a

1.00 1.24

1.00 1.25

Reference 16.

research has focused on how simple variations in the donor ability of a coordinated dithiolene affect the reduction (7, 8, 20, 22) and ionization potentials of oxo-molybdenum dithiolenes. Photoelectron spectroscopy has revealed that the Mo dxy redox orbital of (L-N3)MoO(qdt) is stabilized by

0.8 eV compared to that of (L-N3)MoO(tdt) (20, 22). This work has shown that different dithiolene ligands can have a marked effect on the reduction and ionization potentials of the metal center by varying the donor ability of the coordinated dithiolene. Therefore, an electron-withdrawing effect, induced by an oxidized pyrazine ring fused to the dithiolene donor, results in highly anisotropic covalency contributions to M S bonding, dramatically affecting the valence ionization energy of the oxo-molybdenum HOMO (20, 22). The 6–11 eV He I PES of (L-N3)MoO(tdt) and (L-N3)MoO(qdt) are displayed in Fig. 19. The general shape of the spectral profile for (L-N3)MoO(qdt) between 8.5 and 10.5 eV is very similar to that of alkoxide complexes such as (L-N3)MoO(OEt)2 (284, 334). The ionizations in this spectral region are primarily associated with the orbitals of the (L-N3) ligand and the quinoxaline ring of the qdt2 ligand, and the spectral differences between (L-N3)MoO(tdt) and (L-N3)MoO(qdt) are due to the presence of the tolyl p ionizations at 9.5 eV in the spectrum of (L-N3)MoO(tdt). The most striking differences between (L-N3)MoO(tdt) and (L-N3)MoO(qdt) occur in the region <8.5 eV. For (L-N3)MoO(tdt), there are two ionization bands between 6 and 8 eV that are energetically isolated from the other ligand-based ionizations. Since the Mo ion in these complexes is in the V oxidation state (d1), band 1 of (L-N3)MoO(tdt) in Fig. 19a has been attributed to the ionization of this Mo valence electron (16, 334). The second band, which has been fit with two Gaussians (2 and 3 in Fig. 19), is associated with ionizations from the symmetric and antisymmetric combinations of out-of-plane sulfur p orbitals (Sop) on the tdt2 ligand. However, the dxy and Sop orbitals of (L-N3)MoO(qdt) are

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Figure 19. The He I photoelectron spectra of (L-N3)MoO(tdt) (a) and (L-N3)MoO(qdt) (b), where band 1 is the ionization of the Mo dxy electron and bands 2 and 3 are ionizations of the two electrons localized in the out-of-plane Sop orbitals of the dithiolene. [Adapted from (20).]

considerably stabilized with respect to (L-N3)MoO(tdt) such that they are partially obscured by the (L-N3) ligand and quinoxaline ring ionizations (Fig. 19b). These ionizations are observed as a shoulder on the low-energy side of the ligand ring ionizations, and are fit with two Gaussian bands (1 and 2). The position of band 1 is stabilized by 0.77 eV relative to the first ionization for (L-N3)MoO(tdt), which reveals the dramatic influence of different dithiolene donors on the valence ionization energy of the electroactive dxy redox orbital. The reduction potentials for a series of (L-N3)MoO(dithiolene) complexes as a function of the Mulliken charge on the dithiolene sulfur are presented in Fig. 20. A linear relationship exists between the solution reduction potentials and the calculated Mulliken charge per S atom for the four dithiolate dianions, ead2, tdt2, bdt2, and qdt2 (20). Interestingly, the more negative the charge localized on sulfur for a given dithiolate donor, the more difficult the corresponding (L-N3)MoO(S-S) complex is to reduce. This linear correlation

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Mulliken Charge (per S)

−0.4 −0.5 −0.6

LMoO(qdt)

−0.7

LMoO(bdt) LMoO(tdt)

−0.8 LMoO(ead) −0.9

−1 −1.2 −1.1 −1.0 −0.9 −0.8 −0.7 −0.6 Reduction Potential (V)

−0.5

−0.4

Figure 20. Plot of the reduction potential of (L-N3)MoO(dithiolene) complexes as a function of calculated Mulliken charge per S atom of the dithiolene. [Adapted from (20).]

indicates that the Mulliken charge calculated for a dithiolene S donor in the dianionic ligand is an excellent predictor of the solution reduction potential of the complex. Since a linear relationship exists between the reduction and ionization potentials in the (L-N3)MoE(OR)2 series (284), it can be inferred that the calculated charge localized on the sulfur donors is an excellent predictor of metallo-dithiolene ionization potentials as well. Electronic spectroscopies have also been used to probe orbital contributions to the observed differences in ionization and reduction potentials of (L-N3)MoO(dithiolene) complexes at higher resolution. The 298 K solution absorption and 5 K, 7 T mull MCD spectra of (L-N3)MoO(tdt) and (LN3)MoO(qdt) are presented in Fig. 21. Spectral assignments for (L-N3)MoO(qdt) were based on the earlier studies of (L-N3)MoO(tdt) and (L-N3)MoO(bdt) (19, 23). The similar intensities of the Sip ! Mo dxy CT transitions for (L-N3)MoO(tdt) and (L-N3)MoO(qdt) reflect a nearly equivalent pseudo-s mediated charge donation (covalency) between the in-plane orbitals of these two dithiolene donors and the oxo-molybdenum dxy orbital. Since the in-plane bonding interaction is essentially equivalent in (L-N3)MoO(tdt) and (LN3)MoO(qdt), this cannot be the origin of the marked differences in the reduction and ionization potentials between these two complexes. However, the electronic absorption spectra reveal that the Sop ! Mo dxz;yz transitions for (L-N3)MoO(tdt) and (L-N3)MoO(bdt) are much more intense than those observed for (L-N3)MoO(qdt). These differences have been suggested to result from a substantial change in the nature of Mo–dithiolene bonding between these complexes, which was attributed to the greater Sop donor ability of the tdt2

METALLO-DITHIOLENE COMPLEXES

6000

(a)

137

100

5000

50

4000

0

ε(M−1*cm−1)

2000

−50

1000

−100 150

(b) 5000

100

MCD Intensity (mdeg)

3000

4000 50

3000 2000

0

1000

−50

10,000

15,000 20,000 25,000 30,000 Energy (wavenumbers)

Figure 21. Electronic absorption (dashed line) and MCD (solid line) spectra of (L-N3)MoO(qdt) (a) and (L-N3)MoO(tdt) (b). [Adapted from (20).]

ligand relative to qdt2 (20). This study revealed that the large differences observed in the reduction and ionization potentials of these complexes result from a highly anisotropic bonding scheme. Effectively, the Sop donor ability of the qdt2 ligand is severely compromised relative to tdt2, which results in an increase in the effective nuclear charge on the Mo ion in (L-N3)MoO(qdt). Detailed spectroscopic, electrochemical, and computational studies support the conclusion that oxo-metal-mono(dithiolene) reduction potentials can be dramatically affected by the nature of the dithiolene ligand bound to the metal. Furthermore, the origin of this behavior lies in dithiolene-dependent differences in Mo S covalency. Although the ‘‘electronic buffer’’ effect (16) has been suggested to be a general property of dithiolene donors, more recent studies (20) indicate that the qdt2 ligand possesses considerable contributions from the dithione resonance structure (Fig. 22) limiting the ability of the qdt2 sulfur atoms to donate charge to the metal. As a result, the electronic buffer effect may be considerably reduced in charge-deficient dithiolenes. Therefore, [(L-N3)MoO]2þ fragments coordinated to charge-deficient dithiolenes such as

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H

(a)

N

SH

N

S

N

SH

N

S

H H2qdt(one)

H2qdt(ol) (b) N

S

N

S

N

S

N

S

1

2 2

Figure 22. Structures of the dianionic qdt tautomeric forms of the H2qdt ligand (a).

ligand (b) and the dithiol (1) and dithione (2)

qdt2 and mnt2 (mnt ¼ 1,2-maleonitrile-1,2-dithiolate) should be harder to oxidize and possess higher ionization potentials. This trend results from an increase in the effective nuclear charge on the metal center brought about by the poor donating ability of these charge deficient dithiolenes. Clearly, electronic structure studies on oxo-metal-mono (dithiolenes) have provided deeper insight into the nature of metal–dithiolene bonding interactions (19, 20, 22, 23), and are of considerable use in interpreting the spectra of more complicated metallobis(dithiolenes) and metallo-tris(dithiolenes) (see below). C.

Ligand-to-Ligand Charge Transfer in Metallo-mono(dithiolenes)

There has been considerable interest and activity in understanding the excited-state electronic structure of metallo-mono(dithiolene) complexes that possess LLCT transitions (112, 258, 259, 262, 263, 295, 297, 342–346a). The LLCTs represent the rarest type of CT transition found in metal–ligand complexes. The intense focus on LLCT complexes stems primarily from the ability to generate CT excited states with considerable charge separation, a critical component of artificial photosynthetic devices capable of harnessing light energy and converting it into a technologically useful form of chemical energy [see Chapter 6 in this volume (346b)]. Many of these complexes are luminescent and for square-planar M(diimine)(dithiolene) complexes (Fig. 23) the nature of the LLCT transition has been recast in terms of a mixed-metal– ligand-to-ligand charge-transfer (MMLL0 CT) transition. This refinement results from the fact that there appears to be at least some metal character in the HOMO and lowest unoccupied molecular orbital (LUMO) wave functions involved in

METALLO-DITHIOLENE COMPLEXES

N

S

R

S

R

139

M N

Figure 23. General structure of M(diimine)(dithiolene) complexes.

the MMLL0 CT transition. The assignment of these solvatochromatic MMLL0 CT transitions has been based upon a combination of semiempirical MO calculations and spectral shifts as a function of the ligand (252). The luminescent properties of metallo(diimine)(dithiolene) complexes are extensively discussed by Eisenberg and co-workers in Chapter 6 of this volume (346b), and will not be discussed further here. Our primary emphasis will focus on the unique electronic structure of metallo(diimine)(dithiolene) complexes, which lead to the observation of LLCT (MMLL0 CT) transitions. Square-planar metallo(diimine)(dithiolene) complexes generally display intense, solvatochromatic absorptions in the visible region of the spectrum that are not found in the corresponding metallo-bis(dithiolene) or metallo-bis (diimine) complexes. Futhermore, the LLCT transition energy does not vary appreciably as a function of the metal ion. Extended Hu¨ ckel calculations on Ni, Pt, and Zn metallo(diimine)(dithiolene) complexes indicate that the HOMO is comprised almost entirely of dithiolene w3 orbital character (Figure 2), while the LUMO was found to possess essentially all diimine p orbital character (112, 252, 268). In stark contrast to the spectra of square-planar Ni and Pt metallo (diimine)(dithiolene) complexes, the psuedo-tetrahedral complexes of Zn possess extremely weak LLCT transitions. Now, it is of interest to discuss the differences in LLCT intensity as a function of geometry from a MO point of view. This discussion should help to explain important orientation-dependent differences in photoinduced electron delocalization and charge separation. Figure 24 displays the high energy (E > 25,000 cm1) region of the room temperature electronic absorption spectrum for Zn(bpy)(tdt), where bpy ¼ 2,20 -bipyridine. The LLCT transition occurs at 22,470 cm1 (445 nm) with very weak absorption intensity (e ¼ 72 M 1cm1). The origin of the weak LLCT is a function of the symmetry of this psuedo-tetrahedral complex. A MO diagram for Zn(bpy)(tdt), derived from extended Hu¨ ckel calculations, is presented in Fig. 25. Irrespective of whether the metallo(diimine)(dithiolene) complex is square-planar or psuedo-tetrahedral, the point symmetry is C2v , and all intermediate geometries possess C 2 symmetry. When the dithiolene and diimine planes are orthogonal (psuedo-tetrahedral geometry) the HOMO ! LUMO transition represents a b2 ! b1 one-electron promotion and is electric dipole forbidden. However, the HOMO ! LUMO transition in a square-planar

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Figure 24. Absorption spectrum of Zn(bpy)(tdt) in CH2Cl2. [Adapted from (268).]

geometry is a dipole allowed, z-polarized, b1 ! b1 transition. Thus, the oscillator strength of the LLCT transition is a function of the dithiolene-diimine torsion angle (268). Within the approximations discussed at the beginning of this section, the intensity of a CT transition is proportional to the square of the overlap integral involving the donor and acceptor orbitals, which in this case are the HOMO and LUMO. ILLCT / hfHOMO j fLUMO i2 Furthermore, this overlap should vary as a function of the projection of the dithiolene p orbitals onto the diimine p orbitals, and this possesses a simple cos y dependence. hfHOMO j fLUMO i / cos y Therefore, the intensity of the transition as a function of the dithiolene-diimine torsion angle should approximate a cos2 y function, and ILLCT / cos2 y Since the I LLCT for square-planar (y ¼ 0 ) Pt(diimine)(dithiolene) complexes ranges from 5,000–10,000 M 1cm1, the average torsion angle for Zn(bpy)(tdt) can be directly calculated from the molar extinction coefficient of this complex (e ¼ 72 M 1cm1). This simple calculation reveals a torsion angle y between 83 and 85 , which is in excellent agreement with that predicted for psuedo-tetrahedral Zn(diimine)(dithiolene) complexes.

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Figure 25. Molecular orbital energy diagram for Zn(diimine)(edt). [Adapted from (268).]

Extended Hu¨ ckel calculations have been used to probe the nature of the HOMO and LUMO wave functions for Pt(diimine)(dithiolene) complexes (252). These calculations reveal a HOMO orbital composition for Pt(bpy)(mnt), which is 27% Pt, and 72% dithiolene. The LUMO for this complex contains dominant contributions from the bpy ligand with 2% Pt and 98% bpy character. The appreciable degree to which Pt orbitals contribute to the HOMO is the origin of the MMLL0 CT description for these complexes. These results compare

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nicely with extended Hu¨ ckel calculations on planar Ni(dim)(edt) (dim ¼ 1,2ethanediimine), which has been shown to possess 13% Ni, 81% dithiolene, and 6% diimine character in the HOMO and 4% Ni, 3% dithiolene, and 93% diimine character in the LUMO (268). Similar calculations on psuedo-tetrahedal Zn(dim)(edt) yield 0.6% Zn, 98.3% dithiolene, and 1.1% diimine in the LUMO and 0.3% Zn, 0.9% dithiolene, and 98.8% diimine character in the HOMO, while calculations on the hypothetical planar Zn(dim)(edt) show a slight increase in electron delocalization (268). Summarizing, semiempirical MO calculations on metallo(diimine)(dithiolene) complexes indicate that the HOMO is predominantly dithiolene in character, while the LUMO is a diimine p-type orbital. Therefore, the low-energy visible transition observed for these complexes displays considerable LLCT character with a large change in the dipole moment accompanying the electronic excitation. The one-electron description of these LLCT excitations indicates that excited-state distortions should occur along both diimine and dithiolene normal modes. Resonance Raman spectroscopy has been used to quantitate these molecular distortions in Ni[biacetylbis(aniline)](mnt) by using a time-dependent formalism (297). The largest distortions on the mnt2 ligand are those along the C S bonds, while the largest distortions that occur within the C and C biacetylbis(aniline) ligand are along the C N bonds. Interestingly, considerable metal–ligand bond distortions were found to accompany the LLCT in Ni[biacetylbis(aniline)](mnt). This distortion was attributed to a combination of metal–ligand bond covalency and electrostatic changes (297). The fact that the RR studies indicate distortions along both the Ni N and Ni S bonds may suggest that the diimine and dithiolene orbitals are more highly mixed in the HOMO and LUMO than predicted by extended Hu¨ ckel calculations.

III.

METALLO-BIS(DITHIOLENES) A.

Introduction

The electronic structure and spectroscopy of metallo-bis(dithiolenes) are considerably more complicated than that of the metallo-mono(dithiolenes) discussed in Section II.C because there are now two dithiolene donors, which result in twice as many sulfur-based MOs that contribute to the overall metal– ligand bonding scheme. The result is an increase in the density of states in the valence region, with a concomitant increase in the number of Sdithiolene ! M CT excitations. Nevertheless, numerous spectroscopic studies and bonding calculations have been undertaken in order to explain the unique electronic properties of these molecules. The fact that two dithiolene ligands are now coordinated to

METALLO-DITHIOLENE COMPLEXES

143

the metal allows for the possibility of interligand S S bonding interactions that may stabilize particular structures and/or affect specific molecular distortions. Herein, we will discuss in detail the bonding descriptions of two common types of metallo-bis(dithiolenes); four-coordinate square-planar complexes with Ni, Pt, or Pd as the central metal ion, and five-coordinate square-pyramidal Mo and W complexes that possess a strong p-donor axial oxo ligand. These are arguably the most intensely studied and best understood of the metallobis(dithiolenes). A detailed understanding of their electronic structure and spectroscopy will certainly provide much needed insight into the bonding descriptions of new and more complex metallo-bis(dithiolene) complexes. Nickel-bis(dithiolenes) are very strong chromophores that possess an extremely intense visible or near-infrared (NIR) transition that has been observed at energies as low as 7000 cm1 with molar extinction coefficients as high as 80,000 M 1cm1 (115, 120, 140, 145, 147, 148, 202, 209, 211, 275–277, 328, 345, 347–360). Extensive electronic structure and spectroscopic studies of metallo-bis(dithiolenes) have been undertaken as a result of their potential involvement in electrically conducting (115, 116, 142, 145, 146, 201, 202, 270, 359, 361–363) and optoelectronic device technologies (113, 119, 160, 248, 364– 374), as well as their presence in the active sites of certain pyranopterin Mo and pyranopterin W enzymes (11, 13, 17, 19, 27, 28, 36, 45–49, 59, 60, 64, 69, 75– 79, 86, 87, 89, 91–96, 287–292, 320, 375–378a). Most metallo-bis(dithiolene) sites in enzymes are involved in formal OAT reactions with a wide variety of substrates, and are coupled to two one-electron, electron-transfer (ET) processes. The optoelectronic device applications of metallo-bis(dithiolenes) include their potential use in supramolecular arrays, optical storage systems, optical switches, second-order nonlinear optical devices, and NIR and Qswitching laser dyes. Many metallo-bis(dithiolenes) based on dmit2 and related extended dithiolene ligands behave as superconductors [see Chapter 8 in this volume (378b)] (dmit ¼ dimethylimidazolidine-2,4,5-trithione). B. 1.

Square-Planar Metallo-bis(dithiolenes)

A Simple Molecular Orbital Description of Square-Planar Metallo-bis(dithiolene) Bonding

There have been a considerable number of MO calculations performed on four-coordinate square-planar metallo-bis(dithiolene) complexes at various levels of theory (283, 327, 328, 379–384). The most important differences in the results of these various calculations rests in the energy level ordering of the valence MOs as well as the degree of metal–sulfur covalency. This point is important as the nature of the MO scheme has greatly affected how the results of both ground and excited-state spectroscopic studies on very similar

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metallo-bis(dithiolenes) have been interpreted. It is therefore paramount that the results of these bonding calculations be evaluated in terms of the experimental data, and not vice versa. The results of calculations, when calibrated to the available experimental data, will ensure that a proper electronic structure description is obtained for the molecule in question. Along these lines, Hoffmann and co-workers (283) provided an elegant, yet simple description of the bonding in neutral [Ni(edt)2]. This result will be used as an initial basis for the discussion of square-planar metallo-bis(dithiolene) electronic structure. The simple MO diagram for [Ni(edt)2], derived from Hoffmann’s extended Hu¨ ckel calculations, is given in Fig. 26. Note that there are now twice as many dithiolene-based orbitals compared to the corresponding metallo-mono(dithiolenes), which is simply due to the fact that we have now doubled the number of dithiolene donors. Although this electronic structure description is quite complex, the [Ni(edt)2] MO diagram presents a very satisfying picture for most chemists. The dxy and dxz orbitals are both empty in this scheme, and six of the eight out-of-plane p-type (only the six highest energy orbitals are shown) and all four of the in-plane s-type dithiolene ligand orbitals are occupied. This electronic structure description is exactly what one would expect for two dianionic dithiolene ligands bound to Ni(IV). Furthermore, the filled ligand orbitals lie at energies below the five d orbitals of Ni, indicating that they are primarily acting as s- and p-donor ligands. Note that the (þ) and () combinations of the completely bonding p-type orbitals are not shown, as these two orbitals lie at considerably deeper binding energies. Finally, the energy ordering of the five Ni d orbitals is that anticipated for a square-planar molecule. The dxy (b1g ) orbital is raised considerably in energy due to a high degree of mixing with a s-type dithiolene orbital of the same symmetry. The severe destabilization of the dxy orbital precludes electron occupation of this orbital, and neither Ni(I) (d9) or Ni(0) (d10) electron configurations are anticipated to be encountered for these complexes. There are numerous valence bond structures that can be considered for [Ni(edt)2] (283, 328), and it is therefore of interest to consider the various possible formal oxidation states that the metal ion and the dithiolene ligands may assume. Furthermore, the simple MO scheme depicted in Fig. 26 is consistent with a low-to-moderate degree of metal–sulfur covalency, as Ni is formally in the IV oxidation state. Since sulfur ligands form very covalent metal–ligand bonds (23, 106, 110, 310, 332, 385, 386), we should examine how an increase in metal–ligand covalency affects the bonding description of these metallo-bis(dithiolenes). The Ni oxidation state can, in principle, range from Ni(0) to Ni(IV) in square-planar metallo-bis(dithiolenes) depending on the oxidation state of the two dithiolene ligands. Provided the two dithiolene donors are dianionic in [Ni(edt)2], the metal ion is formally in the IV oxidation state and possesses a d 6 electron configuration (see Fig. 26). However, depending on the

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Figure 26. Molecular axes appropriate for [Ni(dithiolene)2]x complexes and a MO diagram for Ni(S2C2H2)2. [Adapted from (283).]

nature of the coordinating dithiolenes, one or both of these ligands could be thought of as existing in the radical anion or neutral dithione (dithiete) form. Finally, when the valence ionization energy of the dithiolene ligand is similar to that of the metal, a highly covalent metal–ligand bonding picture emerges and a description of formal oxidation states for Ni and the dithiolene becomes somewhat ambiguous. We now show how this issue has been addressed and

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used to define appropriate bonding descriptions for metallo-bis(dithiolenes) through the use of ground- and excited-state spectroscopic probes of their electronic structures in addition to detailed bonding calculations implementing various levels of theory. 2.

Ground-State Spectroscopic Probes

The nickel-bis(dithiolene) complexes [Ni(mnt)2]0,1,2 and [Ni(tfd)2]0,1,2 display a rich redox behavior while maintaining a square-planar geometry over all three oxidation states (tfd ¼ bis(trifluoromethyl)-1,2-dithiete). Therefore, paramagnetic monoanionic [Ni(mnt)2]1 and [Ni(tfd)2]1 provide excellent systems for probing the nature of the MO that contains the unpaired spin (SOMO) by techniques such as EPR, electron–nuclear double resonance (ENDOR), and electron spin echo envelope modulation (ESEEM) spectroscopies. A principal driving force for electronic structure studies on squareplanar metallo-bis(dithiolenes) has been to understand how changes in their oxidation state occur without displaying appreciable changes on the geometric structure, and whether the observed redox behavior is best described as metal or ligand based. Some of the earliest EPR work in this area was conducted by Maki, Holm, and co-workers (387, 388) over 30 years ago, and these studies involved the measurement of 61Ni hyperfine and 33S ligand hyperfine parameters in single crystals of [Ni(mnt)2]1. These researchers observed a rhombic g tensor (gx ¼ 2:14, gy ¼ 2:04, gx ¼ 1:99) with the principal components of the g tensor coincident with the molecular axes given in Fig. 26. The observed 61Ni hyperfine parameters were interpreted in the context of a d7 Ni3þ ion with 25–50% of the spin density localized in a b2g orbital possessing metal dyz orbital character. These early natural abundance 33S studies revealed that 50– 80% of the spin density resided on the four sulfurs of the dithiolene ligand. However, the precise percentage depends on whether the parallel and perpendicular hyperfine coupling constants possess the same or opposite sign. Regardless, the results point to a very covalent metal–ligand bonding scheme with dominant ligand-based redox chemistry for [Ni(mnt)2]0,1,2 and [Ni(tfd)2]0,1,2. Very similar results regarding the nature of the ground-state wave function were determined by Heuer et al. (389) by measuring the Ak and A? 77 Se hyperfine coupling constants for bis((trifluoromethyl)ethylenediselenalato)nickelate(III), [Ni(tds)2]1. These authors assumed that Ak and A? possessed opposite sign, and deduced that 70% of the unpaired spin density was localized on the four Se atoms of the two diselenolate ligands. Recall that the simple MO interpretation depicted in Fig. 26 indicated that this b2g redox orbital was predominantly metal dyz in character. Obviously, the EPR results strongly suggest a very covalent Ni S bonding scheme, and perhaps even an inverted one with occupied dithiolene-based MOs at lower binding energies than certain Ni d orbitals.

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TABLE V Spin Densitiesa Nuclei 61

Ni S [NiS4] 13 C1 13 C2 19 F 14,15 N 33

F(exp)

F(Xa)c

F(BLYP)c

F(EHMO)c

0.25b 0.13b 0.85 0.025 0.003 0 0.01

0.179 0.161 0.82 0.029 0.002

0.175 0.144 0.75 0.047 0

0.16 0.13 0.68 0.06 0.01

0.013

0.014

0.02

a

Reference 381. Spin density reported as a range by Maki and co-workers (387 and 388) determined from the 61Ni hyperfine and spin density for S depends on the sign the 33S hyperfine coupling constant (0.13 vs. 0.20) as discussed in (381). c Xa Spin densities are from Sano et al. (355). The EHMO and BLYP calculations are described in (381). b

Recently, the spin density (r) localized on the cyano nitrogens, ethylene carbons, and cyano carbons in [Ni(mnt)2]1 were determined by Hoffman and co-workers (381) using a combination of ENDOR and ESEEM spectroscopies. These studies were important, in that they were able to quantify the extent of spin delocalization onto the olefinic carbon atoms. The [Ni(mnt)2]1 study was complimented by 19F ENDOR studies on [Ni(tfd)2]1 which, when interpreted in the context of the [Ni(tds)2]1 data (389), allowed a detailed account of the p spin delocalization partitioned on each atomic center of [Ni(mnt)2]1 and [Ni(tds)2]1 (381). The experimental and calculated spin densities for the constituent atoms of [Ni(mnt)2]1 and [Ni(tds)2]1 are presented in Table V. The ENDOR and ESEEM results show that 10% of the unpaired spin density for [Ni(mnt)2]1 is localized on the olefinic C atoms, which is in reasonable agreement with that predicted from Xa calculations (355). The remaining spin density delocalized onto the ligand periphery is dominantly on the cyano N (4%) with the cyano carbon possessing 1% of the unpaired spin density. This yields a value for r(NiS4) of 0.85, indicating that the bulk of the unpaired spin density is localized on the NiS4 core. The assumption of opposite signs for the parallel and perpendicular dipolar Ni hyperfine coupling constants by Maki and co-workers (387, 388) resulted in 25–50% of the spin density being localized on the Ni center. The results of Hoffman et al. (381) provide an estimate of r(Ni) 0.32, which is on the low side of that suggested by Maki and co-workers (387, 388). Since the total spin density localized on the NiS4 core is 0.85, this results in 50–60% of the spin density localized on the four dithiolene sulfur atoms. It is comforting to note that the results of extended Hu¨ ckel, Xa, and DFT calculations support the experimental results, in that the bulk of the spin density is localized on the NiS4 core, with the majority located on the four S atoms of the two mnt2 ligands (355, 381). The large sulfur contribution to the b2g

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N S

S

C

C

S

S

C

C C

S

SS

C C C

N

N

CC N N

CC

N

N C

S

S

C

S

C

Ni C N

S

N

Figure 27. Molecular orbital contour of the HOMO for [Ni(mnt)2]2. [Adapted from (389).]

SOMO is clearly evident in the electron density contour plots presented in Fig. 27. It is also interesting to compare the spin density ratio of specific atoms on the mnt2 ligand in order to assess the degree of p delocalization over the entire ligand in [Ni(mnt)2]1 and [Ni(tds)2]1. Specifically, the r(Cethylene)/r(S) ratio is 0.16, which indicates the remarkable ability of the S 3pz orbitals to efficiently couple with the 2p orbitals of the ethylenic carbon pz orbitals to promote extensive p-type delocalization (381). To summarize, the key result of these ENDOR and ESEEM studies, with respect to the redox chemistry of [Ni(mnt)2]1 and [Ni(tds)2]1, is that the electroactive b2g redox orbital is primarily ligand based. The ability of the dithiolene ligand to promote in-plane spin delocalization in metallo-bis(dithiolenes) has been determined using [Cu(mnt)2]2, which possesses an unpaired electron in an in-plane b1g (dxy ) orbital (390). This electronic structure allows a detailed comparison with the out-of-plane p-type spin delocalization present in square-planar nickel bis(dithiolenes). Interestingly, the measured spin delocalization onto the dithiolene ligands in [Cu(mnt)2]2 is remarkably different from that determined for [Ni(mnt)2]1 and [Ni(tds)2]1. The ESEEM studies on the [Cu(mnt)2]2 ion doped in a diamagnetic [Ni(mnt)2]2 host have determined that 50% of the unpaired spin

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149

density is localized in the Cu b1g (dxy ) orbital (390). The ESEEM data clearly indicate that an extremely small amount of the Cu(II) spin is delocalized onto S covalency must the C and N atoms of the mnt2 ligands. Therefore, the Cu be quite large, and 50% of the spin density has been estimated to be delocalized onto the four sulfur donor atoms (390). This makes sense, as the S bonds in this complex, and Cu dxy orbital is directed along the four Cu overlaps with the S atoms in a s fashion. The collective spin density studies suggest that spin delocalization onto the olefinic carbons and beyond in metallobis(dithiolenes) is facilitated only by the p-type orbitals, with s spin density localized primarily within the MS4 first coordination sphere. 3.

Excited-State Spectroscopic Probes

As was the case for the ground-state studies, the most extensive electronic spectral studies of square-planar metallo-bis(dithiolene) complexes are those of nickel. Although analogous Pt and Pd complexes have also been studied, a thorough interpretation of their electronic spectral data has been limited, which is primarily due to complications arising from the use of more elaborate dithiolene ligands, in addition to the occurrence of metal–metal bonding interactions and p stacking in these complexes. A pictorial representation of the extreme noninnocence of the dithiolene ligand in the [M(dithiolene)2]0,1,2 electron-transfer series is presented in Fig. 28, where formally high-valent Ni R

S

S

S

R 2−

R

S

S

R

S

S

L2− MII L2−

R

S S

R

R

1−

S

S

R

M

e−

S

R

R

e−

R

S

L2− MIV L2−

L2− MIII L2−

R

M R

S

S

R

M S

R

M

1−

R

S

S

R

L2− MII L0

R

M R

S

S

R

R

S

L 2− MII L−

S

R

M S

R L−

S

R

MII L−

Figure 28. Noninnocence of the dithiolene ligand in the [M(dithiolene)2]0,1,2 electron-transfer series. [Adapted from (328).]

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(Ni2þ, Ni3þ, and Ni4þ) is tacitly assumed due to the large destabilization of the metal dxy orbital (328). Although crystallographic and electronic structure evidence points toward a Ni(II) oxidation state for the dianionic [M(dithiolene)2]2 complexes, considerable metal–ligand redox interplay is anticipated in the more oxidized members of this series (328). Such interplay allows various oxidation state assignments to be postulated for both the metal and the associated dithiolene ligands. Although considerable information regarding ground-state electronic structure and spin delocalization has been obtained by ground-state magnetic resonance techniques such as EPR, ENDOR, and ESEEM spectroscopies, detailed information regarding the proper description of the overall bonding in metallo-bis(dithiolenes) is best obtained from excited-state spectral studies. Furthermore, detailed spectroscopic studies can be used effectively to evaluate and calibrate the results of electronic structure calculations (see below). Very recently, a combined DFT and X-ray crystallographic study of the [Ni(S2C2Me2)2]0,1,2 electron-transfer series was undertaken by Holm and co-workers (328). This study has proved to be extremely important, in that it details the intimate relationship between the geometric and electronic structures of all members of this electron-transfer series at parity of the dithiolene ligand. The (S2C2Me2)2 ligand is one of the simplest of the ene-1,2-dithiolates, with p delocalization limited solely within the S C S fragment. The relative C simplicity of the dithiolene ligand in these compounds allows for a comprehensive description of the electronic structure and bonding in the [Ni(S2C2 Me2)2]0,1,2 series. Therefore, results obtained for the [Ni(S2C2Me2)2]0,1,2 electron-transfer series may effectively be used as a basis for the interpretation of other [M(dithiolene)2]0,1,2 complexes as a function of the metal ion and increasing complexity of the dithiolene ligand. The solution electronic absorption spectra for all three [Ni(S2C2Me2)2]0,1,2 complexes in acetonitrile are given in Fig. 29. The most notable spectroscopic feature in this figure is the presence of a single intense low-energy transition in the NIR region for [Ni(S2C2Me2)2]0 and [Ni(S2C2Me2)2]1, and the complete absence of a similar low-energy absorption feature in the reduced dianionic [Ni(S2C2Me2)2]2 complex. The occurrence of an intense low-energy absorption can be understood by considering the simple MO diagram that was presented in Fig. 26. Assuming a dianionic description for both of the dithiolene ligands, the oxidation state of Ni varies as Ni4þ ! Ni3þ ! Ni2þ in [Ni(S2C2Me2)2]0,1,2, and this corresponds to d electron configurations of d6, d7, and d 8, respectively. This affects the occupancy of the b2g redox orbital [formally metal dxz (283)], which is vacant, half-occupied, and fully occupied in [Ni(S2C2Me2)2]0, [Ni(S2C2Me2)2]1, and [Ni(S2C2Me2)2]2, respectively. Lowenergy, formally S ! Ni charge-transfer transitions involving one-electron promotions to the the b2g acceptor orbital are possible only when it is not fully

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Figure 29. Room temperature electronic absorption spectra of the [Ni(S2C2Me2)2]0,1,2 complexes in acetonitrile. [Adapted from (328).] Solid line: [Ni(S2 C2 Me2 )2 ]0 ; thin line: Ni(S2 C2 Me2 )2 ]1 ]; dashed line: Ni(S2 C2 Me2 )2 ]2 ].

occupied, which is the case for [Ni(S2C2Me2)2]0 and [Ni(S2C2Me2)2]1. Dipole selection rules in D2h symmetry are governed by the symmetry properties of the electric dipole moment operator, which transforms as b3u ðxÞ, b2u ðyÞ, and b1u ðzÞ. Therefore, an initial description of the low-energy absorption feature for [Ni(S2C2Me2)2]0 and [Ni(S2C2Me2)2]1 is consistent with either a b1u ! b2g (x polarized), au ! b2g (y polarized), or b3u ! b2g (z polarized) dipole-allowed one-electron promotion, and the lack of such a transition in [Ni(S2C2Me2)2]2 results because the b2g orbital is fully occupied. According to Fig. 26 (283), the lowest energy dipole allowed LMCT transition is the x-polarized b1u ! b2g (dithiolene ! dxz ) transition. Even though this is a satisfying description, there is a significant problem with the assignment of this band as a LMCT transition. Typically, LMCT transitions occur at progressively lower energies as the metal becomes increasingly more oxidized. This trend results from an increase in the valence ionization energy of the metal-based acceptor orbitals that accompany metal-centered oxidation processes. However, inspection of Fig. 29 reveals that as [Ni(S2C2Me2)2]1 is oxidized to [Ni(S2C2Me2)2]0, the transition energy of the intense low-energy absorption band increases by 2000 cm1. Further insight into the nature of these low-energy transitions may be gleaned from the atomic orbital compositions of the b2g acceptor orbital and the highest occupied ungerade donor orbital of b1u symmetry (Table VI). The DFT calculations indicate that the b2g redox orbital in [Ni(S2C2Me2)2]0 and [Ni(S2C2Me2)2]1 is a dominantly ligand-based MO, possessing 60% S 3pz

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MARTIN L. KIRK ET AL. TABLE VI Composition of Selected Orbitals of [Ni(S2C2Me2)2]z

MO

z

5b2g

0 1 2

6b1u

0 1 2

4b3g

0 1 2

Ni(3dxz )b

Ni(3dyz )b

Ni(4pz )b

13.31 19.52 38.91 5.61 5.17 4.31 62.87 67.92 77.32

a

S(3pz )b

C(2pz )b

H(1s)b

59.20 59.56 49.48

24.97 20.02 11.64

2.14 1.64 1.03

43.37 50.32 58.09

41.82 38.25 33.01

5.00 4.40 4.17

24.96 20.75 12.12

8.68 8.31 7.83

1.42 1.34 1.45

a

Reference 328. Only contributions of >1.0% are taken into account; slight contributions of S(3d) (<2%) are not listed. b

atomic orbital character (328). This calculation is in complete agreement with the ENDOR and ESEEM results of Hoffman and co-workers (381), where they determined that the b2g orbital of [Ni(mnt)2]1 possessed 50–60% S 3pz character. These latter DFT calculations indicate that the b2g orbital is energetically isolated from the lower energy occupied orbitals, and the b2g electron density contour is shown in Fig. 30. Again, note the similarity of this orbital to that determined from the calculations of Hoffman. Together, they reveal a relatively small contribution from the Ni 3dxz atomic orbital to this MO, and Table VI shows that the 3dxz orbital accounts for only 13% of the total b2g orbital composition in [Ni(S2C2Me2)2]0 and 20% for [Ni(S2C2Me2)2]1. The

˚ above the NiS4 Figure 30. Molecular orbital contour for the b2g orbital of [Ni(S2C2Me2)2] at 0.8 A plane. [Adapted from (328).]

METALLO-DITHIOLENE COMPLEXES

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calculated dxz character in the b2g SOMO is in good agreement with the ENDOR derived spin density on Ni in [Ni(mnt)2]1. The b1u donor orbital is predominantly dithiolene based in [Ni(S2C2Me2)2]0 and [Ni(S2C2Me2)2]1, with no metal character. This orbital is comprised of nearly equal contributions from dithiolene S 3pz and C 2pz orbitals. Thus, although the nature of the low-energy NIR transition is complex, it can reasonably be assigned as a b1u ! b2g ligand ! ligand (intraligand) one-electron promotion that possesses a small degree of LMCT character. The Holm and co-workers (328) study provided a very nice correlation between the electronic and geometric structures of the entire [Ni(S2C2Me2)2]0,1,2 electron-transfer series (Fig. 31), providing key electronic structure contributions to the redox behavior of these molecules. The Ni S bond

S1′

S2

C4

C3′ C1′

C2 91.35(6) 2.128(1)

C1

1.714(1)

C3

1.365(9)

Ni1 C2 1.515(6)

S2

S1

C4

[Ni(S2C2Me2)] S1

S2

C4 C2

C1′ 1.342(6)

90.64(4) 2.143(1)

C1 C3

1.737(4)

Ni1

S1

C3′

C2 S2

1.512(4)

C4

1−

[Ni(S2C2Me2)] S1

C3

S2′

C1

Ni1

C2′ 90.1(2)

C2 C4

2.179(4) 1.761(4)

S2

C4′

1.337(7) C1′

S1

1.508(9)

C3′

[Ni(S2C2Me2)]2− Figure 31. X-ray crystal structures of [Ni(S2C2Me2)2]0,1,2. [Adapted from (328).]

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TABLE VII ˚ ) and Angles (deg) from X-Ray Data and Optimized Geometrical Selected Mean Bond Lengths (A Parameters for the Complexes [Ni(S2C2Me2)2]z with z ¼ 0, 1 , or 2 a X-Ray Data Optimized Parameters ————————————————— ——————————————————— z ¼ 2 b z¼0 z ¼ 1 z ¼ 2 z¼0 z ¼ 1 Ni S C S C C C C(CH3) S Ni Sc

2.128(1) 1.714(1) 1.365(9) 1.515(6) 91.35(6)

2.143(1) 1.737(4) 1.342(6) 1.512(4) 90.64(4)

2.179(4)d 1.761(4)e 1.337(7)e 1.508(9) 90.1(2)

2.137 1.704 1.377 1.492 90.76

2.165 1.732 1.356 1.497 90.52

2.209 1.757 1.347 1.499 89.81

a

Reference 328. Mean values. c Bite angle of the chelate rings. d Range of 10, 2.1744(7)–2.1884(8). e Range of 10, 1.754(3)–1.769(2). f Range of 5, 1.329(4)–1.345(4). b

lengths are found to increase upon reduction, indicating that the redox orbital possesses some Ni S antibonding character. Selected mean bond lengths for the [Ni(S2C2Me2)2]0,1,2 series are given in Table VII, and the data clearly show that the C S bonds increase upon reduction, while the ene C C bond distances decrease. The electron density contour in Fig. 30 shows that the dithiolene character in this MO derives from an antisymmetric combination of w3 (Figure  C bonding and C 2) dithiolene MOs. The w3 ligand orbital is C S antibonding,  and when this orbital is fully occupied, the ligand is in the reduced, dianionic dithiolene state. Therefore, the two-electron reduction of [Ni(S2C2Me2)2]0 is a ligand-based reduction yielding a [Ni(S2C2Me2)2]2 complex possessing two dianionic dithiolene ligands, which necessitates that Ni is in the II formal oxidation state for the most reduced member of this series. Furthermore, inspection of Table VI reveals that the doubly occupied b2g redox orbital of [Ni(S2C2Me2)2]2 possesses 39% Ni 3dxz character, and a significant reduction in the contribution of dithiolene C and S pz atomic orbital character to this MO. Collectively, it appears as if the [Ni(S2C2Me2)2]0 ! [Ni(S2C2Me2)2]1 reduction process is essentially ligand based, and nearly saturates the ability of the dithiolene to absorb additional electron density in the [Ni(S2C2Me2)2]1 ! [Ni(S2C2Me2)2]2 reduction (328). This points to a substantial change in the atomic orbital character of the b2g acceptor orbital as a function of oxidation state, with the redox orbital becoming more metal based as the complex is successively reduced (Fig. 32). Additional insight into the bonding scheme of metallo-bis(dithiolenes) derives from intermediate neglect of differential overlap (INDO) linear

METALLO-DITHIOLENE COMPLEXES

155

4A1u

2404

5b3g 19a1g 5b3g

100 15026 14329 13b1g

13b1g

298 3707

8808

13b1g

8776 5b2g

7439 5b2g

5b2g

9222 5116

8490 1956

6b1u 4b3g 18a1g

1799

16b3u 19a1g

676 269

[Ni(S2C2Me2)]0

18a1g 4b3g

1113

4b3g 6b1u 18a1g

4481 6b1u

[Ni(S2C2Me2)]1−

[Ni(S2C2Me2)]2−

Figure 32. Molecular orbital energy level diagrams for [Ni(S2C2Me2)2]0,1,2 from DFT calculations. [Adapted from (328).]

combination of atomic orbitals–molecular orbital (LCAO–MO) configuration interaction (CI) calculations on neutral [Ni(S2C2H2)2] (384), which possesses the simplest of the dithiolene ligands. The results of the CI study indicate that

4% of an 1Ag excited-state (originating from a ðb1u Þ2 ! ðb2g Þ2 double excitation) mixes with the 1Ag ground state. The authors suggest that this CI picture reflects an appropriate valence bond description of [Ni(S2C2H2)2] that consists of the resonance hybrid structure given in Fig. 33. This resonance hybrid is formally a 14 p-electron system, with 10 p electrons originating from a

S

S M

S

S

S

II

MII S

S

S

Figure 33. Resonance hybrid description for M(dithiolene)2 complexes.

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formally localized mixed-valent ligand system and 4 dp electrons on the divalent nickel ion. A number of other excited-state configurations were found to account for the remaining 9% excited-state character mixed into the selfconsistent field (SCF) ground-state wave function. The b1u orbital of [Ni(S2C2H2)2] is predominantly ligand based, being comprised of 46% S pz and 52% C pz atomic orbital character. This finding is in remarkable agreement with the DFT calculations of Holm and co-workers (328) on [Ni(S2C2Me2)2]. The INDO LCAO–MO CI calculations yield a b2g redox orbital that is comprised of 55% S pz , 29% C pz , and 16% Ni dxz orbital character. A Mulliken population analysis of the occupied orbitals of [Ni(S2C2H2)2] shows that the charge on Ni and S are þ1.86 and 0.59, respectively. It has been suggested that the similarity of the calculated charge on Ni to a formal charge of þ2 was indicative of a near cancellation of forward and backward charge donation between the dithiolene ligands and formally divalent nickel. This cancellation is exemplified by the fact that the atomic orbital populations of the [Ni(S2C2H2)2] dxy and dxz orbitals differ substantially from 2 (dz2 ¼ 1:90, dx2 y2 ¼ 1:99, dxy ¼ 1:01, dxz ¼ 1:55, and dyz ¼ 2:00). This was specifically suggested to arise from a combination of significant s-type L ! M forward donation and p-type M ! L back donation. The occurrence of p-type M ! L back-donation implies that the ligand is formally oxidized and the dithiolate w3 orbital is not fully occupied. This result is consistent with both the p-acceptor nature of oxidized dithiolene ligands and the dominant ligand-based redox behavior in nickel-bis(dithiolene) complexes. A fundamental conclusion that stems from the INDO LCAO–MO CI derived atomic orbital electron population analysis is that [Ni(S2C2H2)2] behaves as a 14 p-electron system, which is in full agreement with the recent DFT study on the [Ni(S2C2H2)2]0,1,2 ET series (328). However, the proper description of how to partition the 14 p-electrons between the metal and the two dithiolene ligands remains somewhat ambiguous in the more oxidized members of the series. This finding is exemplified by the increased number of valence bond structures that can be drawn for [Ni(S2C2H2)2] as electrons are successively removed from the complex. The absorption spectrum of [Ni(S2C2H2)2] has been analyzed in detail, and band assignments have been made based on the results of these INDO LCAO– MO CI calculations (384). The spectrum of [Ni(S2C2H2)2] in hexane is presented in Fig. 34, where it is readily observed to be extremely similar to the solution spectrum of [Ni(S2C2Me2)2] (328). Namely, an intense band is observed at 13,900 cm1, with two weaker bands at 18,180 and 27,000 cm1. A shoulder is evident at 22,500 cm1, on the high-energy side of the 18,180-cm1 band. Two additional high-energy bands are also present, and these are observed at 34,480 and 47,620 cm1, with a clear shoulder to the blue side of the

METALLO-DITHIOLENE COMPLEXES

157

Figure 34. Absorption spectrum of [Ni(S2C2H2)2] in hexane. [Adapted from (328).]

low-energy band. This shoulder is clearly resolved in the electronic absorption spectrum of [Ni(S2C2Me2)2] (328). As was the case for the oxo-molybdenum-mono(dithiolenes) (19, 20, 22, 23, 110), the occurrence of intense CT and/or intraligand transitions in [Ni(S2C2H2)2]0, [Ni(S2C2Me2)2]0, and [Ni(S2C2Me2)2]1 make the observation and assignment of ligand-field transitions extremely difficult due to their inherently low oscillator strengths. All of the recent bonding calculations indicate that the lowest energy electronic transition in these square-planar nickel bis(dithiolenes) likely involves a one-electron promotion to the lowest energy vacant orbital (Fig. 30), which possesses b2g symmetry. Thus, the intense 13,900-cm1 band observed for [Ni(S2C2H2)2], as well as similar transitions in other square-planar metallo-bis(dithiolenes) (115, 116, 202, 316, 328, 345, 351, 353), may be assigned as an intraligand b1u ! b2g one-electron promotion. Transitions of this type are expected to be quite intense, as both of these MOs possess large atomic orbital coefficients on the C and S atoms of the ene-1,2dithiolate ligand. Therefore, the energy and intensity of this band should not be particularly sensitive to the nature of the metal ion, but may be anticipated to vary considerably with increasing complexity and delocalization in the dithiolene ligand (see below). Interestingly, the oscillator strength for this band, predicted from a singles-only calculation, is nearly five times larger than the experimental value (384). However, the inclusion of double excitations in these INDO LCAO–MO CI calculations reveals the origin of the observed differences between the calculated and experimental oscillator strengths (384). Recall that the electronic ground-state possesses 4% of a 1Ag excited state that originates

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from a ðb1u Þ2 ! ðb2g Þ2 double excitation. As such, this is expected to attenuate the intensity of all transitions that involve one-electron promotions to the b2g acceptor orbital. Zerner and co-workers (384) assigned the higher energy absorption bands at 18,180, 22,500, and 27,000 cm1 as low intensity 1 Ag ! 1 B2u transitions. At energies greater than 30,000 cm1, the density of states increases dramatically. The total oscillator strength was calculated in this near continuum region and the transitions were determined to be dominantly x polarized. Specifically, the oscillator strength was found to be partitioned as 1.34 along x, 0.13 along y, and 0.02 along z, which is in accord with the two-fold (D2h ) symmetry of these metallo-bis(dithiolene) molecules. These researchers also determined that the dominant contributions to the intensity of these higher energy transitions principally derive from dipole allowed 1 Ag ! 1 B3u transitions (384). However, the precise determination of the one- and two-electron origin of the higher energy excitations was not attempted. The spectroscopic studies and bonding calculations of square-planar nickelbis(dithiolenes) suggest that the intense low-energy 1 Ag ! 1 B3u transition results from a b1u ! b2g one-electron promotion between predominantly ligand-based MOs. However, the metal does make a contribution to the b2g LUMO, and this is found to vary considerably as a function of the metal oxidation state. As such, it is of interest to specifically investigate the effects of metal-ion substitution in the metallo-bis(dithiolene) framework, as well as perturbations of the dithiolene ligand. The nickel-bis(dithiolene) results point toward dithiolene ligand modifications affecting the energy and intensity of the low-lying 1 Ag ! 1 B3u transition to the greatest extent, however, metal-ion substitution may provide an effective avenue for fine tuning the electronic structure of these molecules. Although spectroscopic and electronic structure studies of square-planar nickel-bis(dithiolenes) dominate the metallo-bis(dithiolene) literature, studies of Pt- and Pd-bis(dithiolene) complexes primarily focused on complexes possessing more extensively delocalized dithiolene ligands, or dimeric species possessing interdimer M M and M S interactions. Lauterbach and Fabian (382) used DFT and full singles ab initio calculations to explore the electronic properties of metallo-bis(dithiolenes) that possess simple dithiolene ligands such as edt2 and mnt2 as a function of the central metal ion. Interestingly, their population analysis (Table VIII) differs considerably from that obtained by Zerner and co-workers (384) using INDO LCAO– MO CI calculations on neutral [Ni(S2C2H2)2] (384). Zerner’s calculations yielded a charge of þ0.86 on Ni, while the natural orbital population analysis of Lauterbach and Fabian results in a charge of þ0.463 on the metal center (Table IX). As the central metal is changed from Pd (þ0.334) to Pt (þ0.351), the nuclear charge is noticeably reduced, reflecting the increased S ! M charge donation with the softer metals (382). A similar result was obtained for the metallo-bis(diselenolenes), where the atomic charges on Ni, Pd, and Pt are

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159

TABLE VIII Selected Natural Atomic Orbital Occupancies of p-Type Orbitals Calculated by Natural Orbital Population Analysisa Speciesc

R

pp (X)

pp (M)

dyz (M)

dxz (M)

ptotal (M)

pp (C)

ptotal b

1c 1c 1c 1d 2c 2d 3c 3c 3d 3d 4

H SH CN H H H H H H H

1.500 1.571 1.464 1.498 1.497 1.493 1.496 1.505 1.492 1.503 1.531

0.027 0.022 0.026 0.024 0.023 0.020 0.021 0.029 0.018 0.025 0.024

1.981 1.982 1.975 1.987 1.981 1.985 1.983 1.972 1.986 1.979 1.984

1.810 1.859 1.801 1.798 1.816 1.814 1.835 1.742 1.834 1.743 1.773

3.818 3.863 3.802 3.809 3.820 3.819 3.839 3.743 3.838 3.747 3.781

1.045 1.119 1.081 1.049 1.048 1.052 1.044 1.058 1.049 1.060 1.041

14 14.6 14 14 14 14 14 14 14 14 14.1

a

Reference 382. This is the total p-type orbital population on the present complex. c Metallo-bis(dithiolenes) 1-3 have the general formula Ni(X2 C2 R2 )2 . 1: M ¼ Ni; 2: M ¼ Pd; 3: M ¼ Pt; a: X ¼ NH; b: X ¼ O; c: X ¼ S; d: X ¼ Se. Compound 4 is Ni(bdt)2 . b

determined to be þ0.351, þ0.239, and þ0.257, respectively (382). Thus, these DFT calculations suggest a far greater amount of S ! M charge donation than that predicted by the INDO LCAO–MO CI calculations, and this is reflected in positive charges on the S and Se atoms. Therefore, the DFT calculations reflect a situation where the dithiolene sulfurs are either better donors and/or poorer acceptors than in the INDO LCAO–MO CI description. However, the DFT TABLE IX Total Atomic Charges Calculated by Natural Orbital Population Analysisa Speciesb

R

X

M

1c 1c 1c 1d 2c 2d 3c 3c 3d 3d 4

H SH CN H H H H H H H

0.046 0.030 0.149 0.133 0.079 0.163 0.074 0.096 0.156 0.181 0.045

0.463 0.489 0.465 0.351 0.334 0.239 0.351 0.278 0.257 0.172 0.492

a

C 0.384 0.414 0.284 0.443 0.384 0.444 0.382 0.389 0.442 0.447 0.211

R 0.221 0.261 0.019 0.222 0.221 0.221 0.221 0.224 0.221 0.223 0.085

Reference 382. Metallo-bis(dithiolenes) 1-3 have the general formula Ni(X2 C2 R2 )2 . 1: M ¼ Ni; 2: M ¼ Pd; 3: M ¼ Pt; a: X ¼ NH; b: X ¼ O; c: X ¼ S; d: X ¼ Se. Compound 4 is Ni(bdt)2 . b

160

MARTIN L. KIRK ET AL. TABLE X Energy Differences of the Kohn–Sham Frontier Orbitals and CIS Excitation Energiesa

Complexe R 1a 1b 1c 1c 1c 1c 1d 2a 2b 2c 2d 3a 3ad 3b 3bd 3c 3cd 3d 3dd 4 5

H H H SH SMe CN H H H H H H H H H H H H H H

eHOMO (au)b

eLUMO (au)b


e (au)b


E (eV)

E (nm)


ECIS (eV)c

E (nm)

20.15037 20.20313 20.22805 20.20737 20.18519 20.28825 20.22971 20.15019 20.20223 20.22391 20.22477 20.14576 20.1544 20.19993 20.20899 20.22022 20.22653 20.22104 20.22664 20.22888 20.21786

20.08315 20.15266 20.15912 20.15672 20.13753 20.22461 20.15967 20.08835 20.15734 20.16302 20.1631 20.08928 20.08106 20.15966 20.15573 20.16608 20.15795 20.16602 20.15812 20.16522 20.1109

0.06722 0.05047 0.06893 0.05065 0.04766 0.06364 0.07004 0.06184 0.04489 0.06089 0.06167 0.05648 0.07334 0.04027 0.05326 0.05414 0.06858 0.05502 0.06852 0.06366 0.10696

1.829 1.373 1.876 1.378 1.297 1.732 1.906 1.683 1.222 1.657 1.678 1.537 1.996 1.096 1.449 1.473 1.866 1.497 1.865 1.732 2.91

678 903 661 900 956 716 651 737 1015 748 739 807 621 1131 856 842 664 828 665 716 426d

2.182 2.141 1.991 1.619 1.546 1.829 1.924 2.217 1.872 1.623 1.561 1.854 2.243 1.743 1.943 1.414 1.858 1.358 1.797 1.734 d

568 579 623 766 802 678 645 559 662 764 794 669 553 711 638 877 667 913 689 715

a

Reference 382. HOMO and LUMO energies obtained by DFT. c CIS excitation energies based on DFT optimum geometries of Pt instead of the ECP60MHF. d More than one intense absorption between 200 and 300 nm (3.5 and 6 eV). e Metallo-bis(dithiolenes) 1-3 have the general formula Ni(X2 C2 R2 )2 . 1: M ¼ Ni; 2: M ¼ Pd; 3: M ¼ Pt; a: X ¼ NH; b: X ¼ O; c: X ¼ S; d: X ¼ Se. Compound 4 is Ni(bdt)2 . Compound 5 is bis(3-thioxopropene-1-thiolato)nickel. b

calculations are in reasonable agreement with those of Zerner and co-workers (384) in that both strongly suggest that these neutral metallo-bis(dithiolenes) are 14 p-electron systems (Table VIII). Full configuration interaction—singles-only (CIS) calculations were performed at the ab initio level of theory using the DFT optimized geometries for many of the complexes listed in Table X in order to probe the effects of metal-ion substitution on the energy of the low-energy transition in these complexes (382). The results of these calculations are in complete agreement with the other metallo-bis(dithiolene) studies, as only a single intense Ag ! B3u low-energy electronic transition is predicted in the visible or NIR region of the spectrum (Table X). The CIS calculation indicates that this transition is essentially a pure HOMO ! LUMO (b1u ! b2g ) one-electron transition, but the oscillator strengths are, again, calculated to be too large. As Lauterbach and

METALLO-DITHIOLENE COMPLEXES

161

Figure 35. Calculated absorption wavelengths for the intense low-energy transition (Ag ! B3u ; corresponding to a b1u ! b2g one-electron promotion) of various [M(X2C2H2)2] complexes. [Adapted from (382).]

Fabian performed a singles-only calculation, the overestimation of the oscillator strengths may originate from the neglect of double excitations such as ðb1u Þ2 ! ðb2g Þ2 (384). The energies of the Kohn–Sham orbitals do not have the same physical meaning as those derived from a Hartree–Fock calculation, therefore Koopman’s theorem does not hold for DFT calculations. However, the DFT HOMO–LUMO gap more accurately reproduces the experimental Ag ! B3u ðb1u ! b2g one-electron promotion) transition energy than the results of the CIS calculation (Table X). The transition energies of the Ag ! B3u lowenergy electronic transition derived from CIS calculations are presented in Fig. 35. Finally, it is extremely interesting to note that even though there is essentially no metal character in the HOMO, a small amount in the LUMO, and very little predicted one-electron LMCT character mixed into the ground state, the effect of the metal ion is predicted to shift the energy of the Ag ! B3u transition by as much as 4600 cm1 (382). Perhaps the most extensively studied metallo-bis(dithiolenes) that possess more elaborated dithiolene donors are those that utilize the dmit ligand, and DFT calculations have recently been used to investigate the nature of the ground-state one-electron levels of [Ni(dmit)2)], [Pt(dmit)2)], and [Pt(dmit)2)] (380). Despite the increased complexity of the dithiolene ligands in these complexes (Fig. 36), their electronic structure is remarkably similar to that of the simple or ‘‘parent’’ metallo-bis(dithiolenes) that have been discussed previously. This finding is most evident in the electron density contour plots of the HOMOs and LUMOs for [Ni(S2C2Me2)2], [Ni(dmit)2], and [Ni(H2timdt)2)], and these are presented in Fig. 37. An atomic orbital population analysis reveals that the HOMO possesses b1u symmetry, and is almost entirely comprised of out-of-plane ligand pp orbitals with only a few percent metal

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MARTIN L. KIRK ET AL.

R −

S

−

N

S

N

S −S

S

N

−S

N R

dmit2−

R2timdt2−

Figure 36. Structures of the dmit2 and H2timdt2 (where R ¼ H) dithiolene ligands.

Ni(S2C2Me2)2

Ni(dmit)2

Ni(H2timdt)2 Figure 37. The HOMOs and LUMOs for Ni(S2C2Me2)2, Ni(dmit)2, and Ni(H2timdt)2 resulting from hybrid-DFT calculations. [Adapted from (116).]

METALLO-DITHIOLENE COMPLEXES

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TABLE XI Percentage Contribution of Individual Atoms to Selected Orbitalsa of M(dmit)2 M ¼ Ni, Pd, Ptb; c e (eV) 8b1g

5b2g

4b1u

3b3g

7b2u

7b1g

11a1g

4b2g

2a1u

3b1u

10a1g

3b2g

2b3g

1b3g

Ni(dmit)2 Pd(dmit)2 Pt(dmit)2 Ni(dmit)2 Pd(dmit)2 Pt(dmit)2 Ni(dmit)2 Pd(dmit)2 Pt(dmit)2 Ni(dmit)2 Pd(dmit)2 Pt(dmit)2 Ni(dmit)2 Pd(dmit)2 Pt(dmit)2 Ni(dmit)2 Pd(dmit)2 Pt(dmit)2 Ni(dmit)2 Pd(dmit)2 Pt(dmit)2 Ni(dmit)2 Pd(dmit)2 Pt(dmit)2 Ni(dmit)2 Pd(dmit)2 Pt(dmit)2 Ni(dmit)2 Pd(dmit)2 Pt(dmit)2 Ni(dmit)2 Pd(dmit)2 Pt(dmit)2 Ni(dmit)2 Pd(dmit)2 Pt(dmit)2 Ni(dmit)2 Pd(dmit)2 Pt(dmit)2 Ni(dmit)2 Pd(dmit)2 Pt(dmit)2

4.68 4.34 4.21 5.34 5.37 5.41 6.1 5.98 6.03 6.47 6.78 6.69 6.64 6.41 6.59 6.68 6.47 6.66 6.69 7.23 7.28 6.8 6.77 6.86 7.27 7.38 7.41 7.6 7.38 7.46 7.86 8.49 8.44 7.92 8.55 8.54 8.12 8.53 8.42 9.19 9.49 9.46

M 35.0 (3dxy ) 33.0 (4dxy ) 33.0 (5dxy ) 13.0 (3dxz ) 9.0 (4dxz ) 10.0 (5dxz ) 2.0 (4pz ) 3.0 (5pz ) 2.0 (6pz ) 58.0 (3dyz ) 34.0 (4dyz ) 36.0 (5dyz ) 0.0 0.0 0.0 2.0 (3dxy ) 1.0 (4dxy ) 2.0 (5dxy ) 94.0 (3dz2 , 4s) 81.0 (4dz2 , 5s) 85.0 (5dz2 , 6s) 29.0 (3dxz ) 13.0 (4dxz ) 15.0 (5dxz )

2.0 (4pz ) 2.0 (5pz ) 2.0 (6pz ) 80.0 (3dx2 y2 , dz2 ) 67.0 (4dx2 y2 , dz2 ) 65.0 (5dx2 y2 , dz2 ) 52.0 (3dxz ) 63.0 (4dxz ) 57.0 (5dxz ) 28.0 (3dyz ) 32.0 (4dyz ) 30.0 (5dyz ) 11.0 (3dyz ) 30.0 (4dyz ) 30.0 (5dyz )

S1

S2

S3

C1

C2

53.0 55.0 52.0 52.0 54.0 53.0 24.0 27.0 30.0 27.0 41.0 40.0 4.0 2.0 4.0 2.0 2.0 1.0 6.0 19.0 12.0 7.0 13.0 12.0 60.0 59.0 55.0 40.0 41.0 40.0 15.0 22.0 25.0 15.0 5.0 10.0 18.0 4.0 4.0 41.0 40.0 42.0

0.0 3.0 4.0 5.0 6.0 6.0 20.0 18.0 19.0 11.0 20.0 19.0 8.0 6.0 10.0 10.0 8.0 9.0 0.0 0.0 3.0 25.0 25.0 29.0 37.0 38.0 43.0 17.0 15.0 19.0 0.0 0.0 0.0 7.0 2.0 3.0 50.0 56.0 57.0 29.0 16.0 15.0

7.0 4.0 5.0 10.0 11.0 11.0 28.0 32.0 27.0 0.0 0.0 0.0 84.0 (4py ) 86.0 (4py ) 84.0 (4py ) 83.0 (4py ) 85.0 (4py ) 83.0 (4py ) 0.0 0.0 0.0 34.0 45.0 40.0 0.0 0.0 0.0 29.0 31.0 32.0 0.0 0.0 0.0 14.0 8.0 11.0 0.0 0.0 0.0 0.0 0.0 0.0

5.0 3.0 4.0 18.0 17.0 17.0 23.0 21.0 23.0 4.0 5.0 5.0 0.0 3.0 0.0 0.0 2.0 2.0 0.0 0.0 1.1 4.0 2.0 3.0 3.0 2.0 2.0 6.0 5.0 3.0 5.0 6.0 10.0 7.0 14.0 13.0 4.0 7.0 9.0 19.0 12.0 13.0

0.0 2.0 2.0 2.0 3.0 3.0 3.0 0.0 1.0 0.0 0.0 0.0 4.0 3.0 2.0 3.0 2.0 3.0 0.0 0.0 2.8 1.0 2.0 2.0 0.0 0.0 0.0 6.0 6.0 4.0 0.0 0.0 0.0 5.0 7.0 6.0 0.0 0.0 0.0 0.0 0.0 0.0

a The core electrons and the Ni 3s and 3p valence electrons are not included in the numbering of the MOs. b The contributing atomic orbitals (AOs) on S and C are py in the b2u type orbitals, pz in the b2g , b3g , a1u , and b1u , and mostly px;y with some s orbital characters in the a1g and b1g orbitals. c Reference 380.

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MARTIN L. KIRK ET AL.

character. Therefore, it is not surprising that the HOMO energies are nearly independent of the metal. The b2g LUMO possesses from 9 to 13% metal dxz character, and the largest difference in the HOMO–LUMO gap between members of the series is only 1200 cm1. This value is considerably smaller than the 4600-cm1 gap determined by Lauterbach and Fabian (382) for the parent dithiolene series [M(edt)2)]. The filled MOs of b2g symmetry that possess appreciable metal dxz orbital character are between 1.5 and 3.0 eV below the LUMO. Thus, the low-lying LUMO possesses some p-acceptor character, which is reflected in 0.28, 0.22, and 0.26 electrons donated to the vacant b2g ligand orbital for the Ni, Pd, and Pt complexes, respectively. Similarly, the s-donor ability of the dmit2 ligand is reflected in dxy orbital populations of 1.26, 1.24, and 1.18 electrons. The computational results for the Mulliken population analyses for the [M(dmit)2)] series are summarized in Table XI. A highly elaborated dithiolene ligand, R,R0 timdt (R,R0 timdt ¼ monoanion of a disubstituted imidazolidine-2,4,5-trithione) (Fig. 36), has been used to synthesize neutral [M(R,R0 timdt)2] complexes of Ni, Pd, and Pt for comparison with simple metallo-bis(dithiolenes) as well as the [M(dmit)2] series (116). The electronic spectra of [M(R,R0 timdt)2] (R ¼ R0 ¼ Et, M ¼ Ni, Pd, Pt) complexes are presented in Fig. 38. For these complexes, the characteristic Ag ! B3u lowenergy electronic transition occurs at 1000 nm, compared with 720, 785, and

80,000

ε (M1 cm−1)

60,000

40,000

20,000

0 8,000

16,000

24,000 Energy (cm−1)

32,000

40,000

Figure 38. Electronic absorption spectra of [M(R,R0 timdt)2] (R ¼ R0 ¼ Et, M ¼ Ni, Pd, Pt) complexes in CHCl3. Thin line: [Ni(R,R0 timdt)2], Heavy line: [Pt(R,R0 timdt)2], Dashed line: [Pd(R,R0 timdt)2]. [Adapted from (116).]

METALLO-DITHIOLENE COMPLEXES

165

Figure 39. Observed linear relationship between the calculated versus experimental energies for the low-energy Ag ! B3u transition in various metallo-bis(dithiolenes). (1) Ni(S2C2Me2)2, (2) Pt(S2C2Me2)2, (3) Pd(S2C2Me2)2, (11) Ni(dmit)2, (12) Pt(dmit)2, (13) Pd(dmit)2, (14) Ni(H2timdt)2, (15) Pt(H2timdt)2, (16) Pd(H2timdt)2. Note that the absorption maxima have not yet been measured for 11–13. [Adapted from (116).]

680 nm for the Ni, Pd, and Pt parent dithiolate complexes, respectively. Amazingly, the [M(R,R0 timdt)2] complexes possess molar extinction coefficients that approach 80,000 M 1cm1. Furthermore, these transitions display very little shift in the energy of the Ag ! B3u transition as a function of the metal (300 cm1) compared with that of the parent dithiolate series [M(edt)2)] ( 2000 cm1). Hybrid DFT electronic structure calculations have been performed for the [M(R,R0 timdt)2] series (R ¼ R0 ¼ H) and, as anticipated, the HOMO possesses b1u symmetry and is almost entirely ligand based (116). The b2g LUMO is dominantly dithiolene sulfur in nature, with some metal dxz orbital character mixed into the wave function. The calculated HOMO–LUMO gap is lowest for the [M(R,R0 timdt)2] complexes, when compared with [M(edt)2)] and [M(dmit)2)], and this agrees with the spectroscopic data. The observation that the energies of the Kohn–Sham orbitals reflect meaningful quantities is exemplified in Fig. 39, where the calculated HOMO–LUMO gap is plotted as a function of the experimental Ag ! B3u transition energy. Amazingly, a linear correlation results allowing the Ag ! B3u transition energies for the [M(dmit)2)] complexes to be estimated at 11,100, 10,500, and 11,250 cm1 for the Ni, Pd, and Pt complexes, respectively (116). Finally, the calculated orbital energies for [M(H2timdt)2], [M(edt)2)], and [M(dmit)2)] (Fig. 40) allow for a simple

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MARTIN L. KIRK ET AL.

Figure 40. Calculated orbital energies for selected orbitals of 1–3, 11–16 (1: Ni(S2C2Me2)2, 2: Pt(S2C2Me2)2, 3: Pd(S2C2Me2)2, 11: Ni(dmit)2, 12: Pt(dmit)2, 13: Pd(dmit)2, 14: Ni(H2timdt)2, 15: Pt(H2timdt)2, 16: Pd(H2timdt)2). The HOMO (b1u ) and LUMO (b2g ) orbitals are in italics. [Adapted from (116).]

understanding of their redox properties. The LUMO is most stabilized for the [M(dmit)2)] series, and these complexes are observed to be the most easily reduced. Electrochemical oxidations have only been possible for the [M(R,R0 timdt)2] complexes, and this is reflected in their considerably destabilized HOMOs. Summarizing, the redox properties of square-planar metallobis(dithiolenes) appear to be solely a function of the HOMO and LUMO energies, both of which are highly dependent on the nature of the dithiolene ligand but only a weak function of the metal. C. 1.

Metallo-bis(dithiolenes) Possessing Axial Oxo Ligands

A Comparison between Square-Planar and Square-Pyramidal Metallo-bis(dithiolene) Bonding

Although considerably less effort has been expended on the spectroscopy of square-pryamidal metallo-bis(dithiolenes), notable differences in the electronic structure and bonding result from the presence of a strong s/p donor oxo ligand in the axial position. The most obvious differences between these two metallobis(dithiolene) classes is the fact that the p-type dxz;yz orbitals are strongly destabilized due to the strong interaction between the metal and the terminal oxo

METALLO-DITHIOLENE COMPLEXES

167

dz 2

E

dx 2 −y 2 dxz,yz

dxy Sop (nb) Sop (b) Sip (nb) Sip (b) Figure 41. Qualitative MO energy diagram for [MoO(bdt)2]1. Here, (b) represents a bonding MO and (nb) represents a non-bonding MO. [Adapted from (106).]

ligand, resulting in very large splittings within the t2g orbital set for the latter. Therefore, the dxz;yz orbitals in oxo-metallo-bis(dithiolenes) are unoccupied, and higher oxidation states are favored by the metal. Furthermore, the low-energy CT transitions are found to be L ! M instead of intraligand in nature and the  M  O bonding scheme presented in Fig. 41 forms a good basis for understanding the spectroscopic properties of oxometallo-bis(dithiolene) complexes. Finally, the presence of the terminal oxo donor results in the metal residing ˚ out of the bis(dithiolene) S4 plane, providing considerably less

0.5 A dithiolene S pp  M dp orbital overlap. This reduction in orbital overlap results in LMCT transitions that are an order of magnitude less intense than the intraligand CTs that are characteristic of square-planar metallo-bis(dithiolenes). 2.

Excited-State Spectroscopic Probes

The best understood oxo-metallo-bis(dithiolenes) possess the general formula [MO(dithiolene)2]n. Although dioxo [MO2(dithiolene)2]2 (33, 34, 70, 314, 391–393) and desoxo [MX(dithiolene)2]n (X ¼ OR, SR, or SeR) (58, 327, 394) complexes have also been studied, considerably less is known of their spectroscopic properties and electronic structures. Therefore, they will not be

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Figure 42. Electronic absorption and MCD spectra of [MoO(bdt)2]1. [Adapted from (106).]

discussed further. Monooxo-molybdenum(V)–bis(dithiolene) compounds possess a distinctive broad low-energy absorption feature with maxima ranging from 729 to 842 nm and a higher energy feature at 446–575 nm (31, 33, 58, 70, 106, 393, 395). The absorption spectrum of [MoO(bdt)2]1 is given in Fig. 42, where the two lowest energy absorption bands are observed to occur at 730 nm (band 1) and 500 nm (band 2) (106), and a combination of electronic absorption, MCD, and RR spectroscopies have been used to understand the electronic origin of these low-energy transitions. The MCD spectrum of [MoO(bdt)2]1 is overlayed on the absorption spectrum in Fig. 42. The spectrum displays temperature-dependent MCD C-term features at 735 and 515 nm that are consistent with electronic transitions involving one-electron promotion from a dithiolene MO to a nondegenerate (dxy ) acceptor orbital localized on molybdenum. Although three Mo S vibrational modes (344, 358, 377 cm1) were found to be resonantly enhanced in the RR spectrum of [MoO(bdt)2]1, there was virtually no enhancement of the Mo O stretch (106). This result is consistent with the assignment of bands 1 and 2 as LMCT transitions to the inplane Mo dxy acceptor orbital, which possesses no contributions from the axial oxo. The greater relative RR enhancement of the Mo S modes upon excitation in band 2 is indicative of the greater Mo S bond distortion anticipated to result from a bonding ! antibonding electronic transition. These spectroscopic data have collectively been used to evaluate the results of DFT calculations on the related electronic structure model MoO(edt)2 in both the þ5 and þ6 oxidation states (106). It is important to note that the calculations

METALLO-DITHIOLENE COMPLEXES

169

accurately reproduce the spectroscopically observed splitting of the Mo d orbital manifold resulting from terminal oxo ligation and support the energy level diagram depicted in Fig. 41. The eight highest energy occupied ligand-based MOs are primarily S in character, representing in-plane (Sip) and out-of-plane (Sop) linear combinations of S 3p orbitals. All of the Sip molecular orbitals are more energetically stabilized than the Sop molecular orbitals. Therefore, the calculations and spectroscopic results have allowed for the assignment of the low-energy LMCT transitions in [MoO(bdt)2]1 and DMSORox as Sip(nb) ! Mo dxy (band 1) and Sip(b) ! Mo dxy (band 2). The assignment of the two lowest energy absorption features in [MoO(bdt)2]1 as Sip ! Mo dxy LMCT transitions is important as the intensity of the bands provides a direct probe of in-plane dithiolate S character covalently mixed into the Mo dxy redox orbital. This electronic structure has been postulated to play a vital role in modulating the Mo reduction potential in DMSORox and facilitating electron-transfer regeneration of the enzyme active site following OAT from the DMSO substrate (106). Specifically, the high oscillator strengths of the Sip ! Mo dxy transitions reflect a considerable amount of Sip  Mo dxy orbital mixing. Interestingly, inspection of the bonding Sipða1 Þ–Mo dxy molecular orbital in Fig. 43 reveals that the Sip p orbitals are rotated off the Mo S bond axes and toward one another. This rotation has the effect of localizing a large amount of S electron density between the two S atoms of each dithiolene, and results in a pseudo-s bonding interaction that may effectively couple the Mo dxy redox orbital into the Sip orbitals of the coordinated dithiolene in enzymes (23, 106). This unique bonding interaction was previously postulated for oxo-Mo mono(dithiolenes) (23), including the ‘‘very rapid’’ intermediate in xanthine oxidase (XO) (21), where it has been proposed to couple the Mo dxy redox orbital into effective ET pathways involving the s system of the pyranopterin. These ideas result from the fact  that the Sip–Mo dxy interaction is maximized when the Mo  O bond is orthogonal to an ene-1,2-dithiolate plane. This idea has previously been referred to as the oxo-gate hypothesis (23, 106).

Figure 43. Molecular orbital contour of the Sip(a1)–Mo dxy (a1 ) bonding orbital resulting from a DFT calculation on [MoO(edt)2]. [Adapted from (106).]

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The electronic structure studies on the [MoO(bdt)2]1 ion provide a basis for the spectral interpretation of other oxo-molybdenum-bis(dithiolenes), particularly those possessing more elaborated, or highly redox-active dithiolene ligands. The latter dithiolene ligand systems are of particular interest, since the synergistic interactions that can occur between transition metal ions and their redox-active ligands form the basis for a myriad of complex phenomena including facile redox behavior (16, 82, 281, 396), enzymatic catalysis (397, 398), novel optical properties (248, 399), and valence tautomerism (400–404). The judicious choice of metal and ligand allows for the occurrence of accessible low-lying electronic states possessing considerable CT character. This metal– ligand redox interplay can, in principle, be exploited to construct novel molecular- and molecule-based multiproperty materials, with the ultimate goal of being able to control these various properties and ‘‘switch’’ between them via an external perturbation. One of the most fascinating properties displayed by transition metal complexes possessing redox active ligands is valence tautomerism (redox isomerism). Until recently, all of the complexes that display valence tautomerism have utilized ene-1,2-diolate (dioxolene) donors, or their Schiff-base variants, to facilitate a thermally induced intramolecular redox reaction utilizing the catecholato–semiquinonato couple. It has been suggested that the redox interconversion is dependent on a low degree of covalency in the M L bonding scheme (402). However, as detailed above, metallo-dithiolenes possess a highly delocalized bonding description. As a result, valence tautomerism has not been anticipated for metallo-dithiolene complexes. Interestingly, variable-temperature absorption data for the oxo-molybdenum(dithiolene) complex [MoO(quinoxaline-2,3-dithiolate)2] reveals the highly thermochromic nature of this compound. Although the observed thermochromism may be explained within the context of valence tautomerism, a more general description has recently been put forth, namely, thermally driven intramolecular CT, as this more accurately reflects the highly covalent nature of the Mo S bonds and extreme noninnocence of the quinoxaline-2,3-dithiolate ligand (110, 250, 329–331, 405). Substoichiometric addition of ferrocenium hexafluorophosphate to a dry 70:30 toluene/dimethylformamide solution of the dark blue Mo(IV) complex (PPh4)2[MoO(qdt)2] has been found to generate one-electron oxidized [MoO(qdt)2] (110). The bottle green [MoO(qdt)2] is highly thermochromic, and turns dark orange almost instantaneously upon lowering the temperature. The observed thermochromism is readily evident in the variable temperature electronic absorption spectra presented in Fig. 44. The low temperature (89 K) absorption spectrum displays two characteristic low-energy CT features at 14,750 and 18,000 cm1, and the observation of C-term MCD intensity (Fig. 45) at low temperatures confirms that the oxidized species is paramagnetic,

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171

Figure 44. Variable temperature electronic absorption spectra of [MoO(qdt)2]1 detailing the observed thermochromism. [Adapted from (110).]

and in the Mo(V) oxidation state. As such, the anion has been described as [MoVO(qdt)2]1 (110). The tight isosbestic points present in Fig. 44 indicate that [MoVO(qdt)2]1 interconverts with a single species as the temperature is increased. Interestingly, the low-energy S ! Mo dxy CT transitions essentially disappear and the very intense bands at 26,800 and 25,700 cm1 are attenuated

Figure 45. The 5 K MCD spectrum of [MoO(qdt)2]1. [Adapted from (110).]

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MARTIN L. KIRK ET AL.

N

N

O

S N

Mo

S

N

S

V

S

N

1−

O

S

S

N

1−

Mo

N

S

IV

S

N

Figure 46. Valence tautomeric forms of the [MoO(qdt)2]1 ion. [Adapted from (110).]

by roughly one-half upon increasing the temperature. These latter transitions were assigned as qdt p ! p transitions due to their similarity with bands assigned in (L-N 3)MoVO(qdt) (20, 22, 110). The lack of low-energy S ! Mo dxy CT transitions in the high-temperature absorption spectrum is very similar to the low-energy spectra of various [MoIVO(dithiolene)2]2 complexes, including [MoIVO(qdt)2]2 (8). Oxo-molybdenum(IV) complexes are low spin and diamagnetic, possessing ðdxy Þ2 electronic configurations. No low-energy S ! Mo dxy CT transitions occur in Mo(IV) oxo-molybdenum-bis(dithiolene) complexes since the dxy acceptor orbital is completely filled. Therefore, it has been proposed that a thermally induced intramolecular electron/charge transfer (valence tautomeric) process is operable in [MoVO(qdt)2]1 and responsible for the observed thermochromism. Such a S ! Mo dxy CT process formally results in the conversion of [MoVO(qdt)2]1 to the Mo(IV) species [MoIVO(qdt)(qdt)]1 (Fig. 46), which is consistent with the attenuation of both the S ! Mo dxy CT and the dianionic qdt p ! p bands. The normalized intensity of the 18,000-cm1 band has been plotted as a function of temperature in Fig. 47. Clearly, the [MoVO(qdt)2]1 ! [MoIVO(qdt)(qdt)]1 interconversion is very abrupt (
T 25 K) with a T1=2 near room temperature (270 K). Since low-energy S ! Mo dxy CT transitions are only observed for [MoVO(qdt)2], the intensity of this band is directly proportional to the mole fraction ( fMoV ) of [MoVO(qdt)2]1 present at a given temperature. The thermodynamic parameters for this thermally induced CT process have been obtained by fitting the data in Fig. 47 to Absorption intensity / fMoV ¼

1 expð
H=RT 
S=RÞ þ 1

Interestingly, this interconversion appears to be entropically driven, and the best fit parameters for the [MoVO(qdt)2]1 ! [MoIVO(qdt)(qdt)]1 interconversion were determined to be
H ¼ 74.5 kJ mol1 and
S ¼ 264 J K1 mol1

METALLO-DITHIOLENE COMPLEXES

173

Normalized Absorption Intensity

1.2 1 0.8 0.6 0.4 0.2 0 50

100

150

200

250

300

350

Temperature (K) Figure 47. Normalized intensity of the 18,000-cm1 band as a function of temperature for the [MoO(qdt)2] ion. [Adapted from (110).]

(110). This entropic term is quite large and its origin undoubtedly arises from distortions along Mo S and intraligand qdt normal modes that occur as a result of the thermally induced charge redistribution.

IV.

METALLO-TRIS(DITHIOLENES) A.

Introduction

The results of numerous structural, spectroscopic, and theoretical studies strongly suggest that a complex combination of dithiolene ligand constraint, ligand-field stabilization energy, inter- and intraligand S S interactions, energy matching between metal- and dithiolene-based orbitals, dithiolene p bonding, and overall complex charge all play key roles in stabilizing the trigonal prismatic (TP) coordination of metallo-tris(dithiolenes). The seminal studies of Gray and co-workers (280, 406), and Schrauzer and co-workers (407) represent the earliest attempts to employ MO theory toward understanding the electronic absorption spectra of six-coordinate TP metallo-tris(dithiolenes). However, only the former researchers made concrete attempts to understand the key electronic structure factors that stabilize the TP geometry over the more common octahedral arrangement. Furthermore, the MO description of Gray and co-workers (280, 406) has met with more general acceptance due to its ability to explain the electronic origin of the EPR derived spin-Hamiltonian parameters (gi and Ai ) for numerous TP metallo-tris(dithiolenes) (387, 388, 408, 409), as

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MARTIN L. KIRK ET AL.

Figure 48. The TP structure typical of M(dithiolene)3 complexes.

well as their effective magnetic moments and the salient features of their electronic absorption spectra (280, 406). B.

Molecular Orbital Description of Metallo-Tris(dithiolene) Bonding

The complex Re(S2C2Ph2)3 is, in many respects, the prototypical metallotris(dithiolene) (280), and the X-ray structure of this complex shows that it possesses almost ideal trigonal prismatic MS6 coordination geometry (D3h symmetry) (Fig. 48). The early bonding calculations for Re(S2C2Ph2)3 utilized 5d, 6s, and 6p valence orbitals on Re, and sp2 hybrid orbitals on sulfur for the inC S plane dithiolene Ss and Sh MOs while the p functions for the S C v fragment (the ene-1,2-dithiolate pv orbitals, 6 of which are S ) were derived from Hu¨ ckel calculations (Fig. 49) (280). This results in a total of 33 valence orbitals. The phenyl rings were neglected in the calculation since their C S orthogonal orientation results in poor conjugation with the S C skeleton. More recent Fenske–Hall (410) and density functional calculations on the [Mo(dithiolene)3]n series support the original bonding description proposed by Gray and co-workers (280, 406). However, only 17 of the original 33 valence orbitals (specifically the 5 metal 4d, 6 Sh, and 6 Sv orbitals) are found to be necessary to describe the key factors influencing the geometry and lowenergy spectral features of most metallo-tris(dithiolenes), and the resultant 17-orbital MO scheme is depicted in Fig. 50. The Fenske–Hall and DFT calculations indicate that there are minimal contributions of the six Ss orbitals (due to their primary involvement in S C s bonding), the six Cph orbitals, and the metal 5s and 5p functions to the lowest energy donor and acceptor MOs of [Mo(dithiolene)3]n. Thus, these 17 MOs represent an adequate basis for describing the spectroscopic properties of metallo-tris(dithiolenes). The metal-based orbitals are split by the trigonal-prismatic D3h ligand field to yield, in order of increasing energy, the dz2 ða01 Þ, dx2 y2 ;xy ðe0 Þ, and dxz;yz ðe00 Þ

METALLO-DITHIOLENE COMPLEXES dxz,yz

4e"

dx2−y2,xy

5e'

3πv dz2 3πv

3a 1 ' 2a 2 '

175

6s

dx2−y2,xy 5d

3πv

4e'

3πv

πh πh πh

3e" 3e' 2a2"

πh

Figure 49. Molecular orbital diagram of metallo-tris(dithiolene) complexes as proposed by Gray and co-workers. [Adapted from (280).]

orbitals (411). Many metallo-tris(dithiolenes) display deviations from ideal D3h symmetry, and it is informative at this stage to show the relationship between the d orbital functions in D3h , D3 , and Oh symmetries. This distortion sequence describes the synchronous rotation of the thiolate ligands about the primary axial three-fold symmetry axis. The relevant expressions that relate these metal d orbital wave functions are given below (408, 411): Mðeb Þ1 ¼ bd2  adþ1 Mðeb Þ2 ¼ bdþ2  ad1 Mðea Þ1 ¼ ad2 þ bdþ1 Mðea Þ2 ¼ adþ2 þ bd1 Mða1 Þ ¼ d0

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MARTIN L. KIRK ET AL.

Figure 50. Molecular orbital energy diagrams resulting from a Fenske–Hall calculation on MoVI(S2C2H2)3 [adapted from (410)] and DFT calculations [unpublished work of the authors] on MoVI(S2C2H2)3 and MoIV(S2C2H2)3. Occupied MOs are in bold.

where the limiting values ðOh ! D3 ! D3h Þ are a2 ¼ 23 and b2 ¼ 13 for Oh symmetry, and a2 ¼ 1 and b2 ¼ 0 for D3h symmetry. Therefore, in Oh symmetry ea and a1 become components of the t2g orbital set and eb becomes eg , while in D3h a1 becomes a01 , while ea and eb transform as e0 and e00 , respectively. The construction of MOs involving these d orbital functions results in the following LCAO functions on the metal (M) and ligands (L): ðeb Þ1 ¼ aM Mðeb Þ1 þ aL Lðeb Þ1 ðeb Þ2 ¼ aM Mðeb Þ2 þ aL Lðeb Þ2 ðea Þ1 ¼ bM Mðea Þ1 þ bL Lðea Þ1 ðea Þ2 ¼ bM Mðea Þ2 þ bL Lðea Þ2 ða1 Þ ¼ gM Mða1 Þ þ gL Lða1 Þ

METALLO-DITHIOLENE COMPLEXES

R

S

S M

R

S

C R

177

M

α

Figure 51. The sulfur-fold distortion of the five-membered MS2C2 chelate ring. Distortions along this mode modulate dithiolene S S orbital mixing.

The Lða1 Þ ligand function is comprised of a totally symmetric combination of dithiolene Sv orbitals, which interact with the metal dz2 orbital in a pseudo-s fashion. The dominant contributions to the individual Lðea Þ and L(eb ) ligand functions are the dithiolene Sv and Sh orbitals, respectively. Therefore, the ea orbitals may be considered as p* with respect to M L bonding while the eb orbitals possess predominantly s* character in the D3h limit. It becomes readily apparent that, upon lowering the symmetry to D3 , it is increasingly difficult to completely separate s- and p-bonding contributions since the four highest energy metal-based orbitals now possess e symmetry, and are therefore allowed to mix. Additional complications to the bonding description may occur in the presence of a ‘‘sulfur-fold’’ distortion (Fig. 51), which results in the [C2S2] ligand fragment becoming nonplanar with a concomitant increase in Sv Sh dithiolene orbital mixing. Thus, the character of the metallo-(tris)dithiolene ea and eb ligand orbitals, which derive from interactions between the metal and the three independent dithiolene ligand orbitals, must be carefully considered in any description of TP bonding. Obviously, such bonding considerations play a fundamental role in determining which key factors stabilize a particular geometry, determine the accessibility of multiple redox states and facile redox behavior, and the oscillator strengths (intensites) and energies of the observed LMCT bands (see below). The recent Fenske–Hall (410) and DFT calculations (412) performed on D3h metallo-tris(dithiolenes) provide additional insight into the LCAO description of metallo-tris(dithiolene) bonding as well as the degree of metal–sulfur covalency, particularly with respect to how these factors relate to stabilization of the TP structure. The metal a01 orbital has previously been suggested to be comprised of primarily metal dz2 character, and this is supported by a rather small dz2 –S overlap population of 0.017 calculated for this MO in a D3h ligand-field (Table XII) (410). The M S s overlap involving the dxz;yz ðe00 Þ orbitals is observed to be greater than the M S p overlap utilizing the dx2 y2 ;xy ðe0 Þ orbitals. Although this result is obviously anticipated, the calculations show that the p type overlap is extremely efficient, being 85% of the s-type overlap and supporting

178

MARTIN L. KIRK ET AL. TABLE XII Metal–Sulfur Orbital Overlap Populations in Mo(S2C2H2)3a Metal Orbital ——————————————————————— ———————— x 2  y2 xy xz yz z2

D3h geometry C3h geometry Net change a

0.017 0.088 0.071

0.217 0.215 0.003

0.217 0.215 0.003

0.256 0.256 0.000

0.256 0.256 0.000

Reference 410.

arguments favoring p bonding as a dominant stabilizing component of the TP geometry. Inspection of the metal e0 and e00 wave functions basically confirm the LCAO picture of dominant p* and s* bonding interactions for these orbitals. However, the DFT calculations do reveal a noticeable amount of Sv and Sh ligand orbital mixing in the e0 orbitals, while the metal e00 level is almost entirely Sh orbital mixing will be dxz;yz -Sh s* (412). Experimental verification of Sv reflected in the oscillator strengths of specific LMCT bands. However, this conclusion must await additional detailed analysis of the electronic absorption spectra for trigonal prismatic metallo-tris(dithiolenes). Ideally, studies utilizing polarized single-crystal electronic absorption spectroscopy would shed considerable light on the bonding picture for metallo-tris(dithiolenes). However, this would represent an extremely daunting endeavor as difficulties are anticipated to arise due the high extinction coefficients of the CT bands, necessitating the doping of these chromophores into suitable hosts with accessible spectroscopic windows. The 12 predominantly dithiolene ligand orbitals of D3h metallo-tris(dithiolenes) may be grouped into three general catagories: M L bonding, M L nonbonding, and M L antibonding (Fig. 52). In addition to the direct M L overlap, a considerable influence to the overall bonding scheme involves inter- and intraligand S S bonding interactions. It is the nature of these bonding interactions that contribute to electronic structure factors that stabilize the TP geometry and promote facile redox behavior. Calculations also indicate that the a01 orbital lies very close in energy to the nonbonding dithiolene a02 orbital while the remaining 11 ligand-based orbitals are found at considerably deeper binding energy (410, 412). These lower energy dithiolene orbitals are stabilized by a combination of strong M S s and p bonding in addition to considerable interligand S S bonding interactions, both of which have been postulated to stabilize the TP structure. The stabilizing interligand S S bonding interactions are strongly apparent in the lowest energy dithiolene orbital, which possesses essentially pure Sh orbital character. Since this a01 orbital is only weakly bonding

METALLO-DITHIOLENE COMPLEXES

179

Figure 52. The 17 valence (frontier) MOs that describe metallo-(tris)dithiolene bonding. These orbitals result from a DFT calculation on MoVI(S2C2H2)3. [Unpublished work of the authors.]

with the metal in D3h , the dominant contribution to the stability of this orbital Sh must be the strong intraligand, and to a lesser extent interligand, Sh interactions. These interactions are clearly observable in Fig. 53. C.

Distortions from the Ideal D3h Trigonal-Primatic Geometry

As has been previously mentioned, the two most important electronic structure factors that have been suggested to stabilize the TP geometry in

Sip−dxz,yz(e'')

Sop−dxy,x2−y2(e')

Sip−dz2(a1')

Sop−dz2(a1')

Sip−Sip

Figure 53. Schematic view of key orbital interactions in M(dithiolene)3 complexes.

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MARTIN L. KIRK ET AL.

metallo-tris(dithiolenes) are strong metal–sulfur p bonding as well as very effective interligand (S S) bonding interactions. The latter effect may be potentially offset somewhat by the fact that ligand–ligand repulsions are maximized in the TP geometry. Inspection of the ligand-field correlation diagram presented in Fig. 54 points to the conclusion that there are no geometries spanning

α

D

D

3h

O

3

e 6

e

h

g

b

4

e' '

Dq

2

0

e' e

-2

a

a ' 1

-4 0

10

20

a

t

1

30

40

50

2g

60

T wist An gle Figure 54. Correlation diagram depicting the dithiolene ligand twist effects on the energy of the metal orbitals of M(dithiolene)3 complexes. [Adapted from (413).]

METALLO-DITHIOLENE COMPLEXES

C3h

30° S fold

D3h

30° twist

D3

181

30°° twist

Oh

Figure 55. Geometric distortions of M(dithiolene)3 complexes that result in a change of symmetry from trigonal-prismatic D3h .

the range of distortions, Oh ! D3 ! D3h ! C3h (Fig. 55), where the TP geometry is favored based solely on ligand-field stabilization energies (LFSE) (Fig. 54) (413). The lack of stabilization for the TP geometry is clearly evident for d n>2 electron configurations, where population of the e0 orbital would be expected to drive a distortion toward an Oh geometry. The distortion toward an Oh geometry is described by a ‘‘trigonal twist’’ angle, a, which ranges from 0 (D3h ) to 60 (Oh ) and describes the synchronous rotation of the dithiolene chelates relative to a C2 axis perpendicular to the primary C 3 axis (Fig. 55). However, within this LFSE framework, population of the nonbonding a01 orbital by one or two electrons should yield no electronic preference for a particular coordination geometry. Therefore, for high-valent metal centers possessing d0, d1, or d2 electronic configurations an alternative description is necessary to explain the occurrence of any low-symmetry distortions. The two most common distortions from the idealized TP D3h geometry are brought about by (1) the aforementioned trigonal twist about the C 3 axis and (2) an intraligand S S ‘‘envelope fold’’ (Fig. 56). While the former distortion eventually drives the geometry to an ‘‘octahedral’’ limit via a D3 intermediate structure, the latter results, in the absense of a trigonal twist, in complexes that possess C3h symmetry. Metal–ligand p overlap is greatest in the TP geometry, and the overlap is reduced as the D3h complex distorts by rotating about the C3 axis. Thus, the degree of this distortion is related to the availability of the metal d orbitals for p bonding.

Figure 56. Change in dz2 –Sop orbital overlap resulting from twisting of the dithiolene ligand (trigonal twist) and folding of the MS2C2 chelate (sulfur-fold).

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MARTIN L. KIRK ET AL.

This distortion is best exemplified using the example of the metallotris(catacholates), [Fe(bct)3]3, [Al(bct)3]3, and [Ga(bct)3]3 [bct is the macrocyclic tris(catecholate)ligand bicapped (TRENCAM)] (413). All five of the Fe d orbitals are available for bonding in high-spin [Fe(bct)3]3, and the complex possesses a TP geometry. Detailed spectroscopic studies have quantified the degree of p bonding in these complexes, and it was determined that it is the strong Fe O p bonding that is the driving force for the TP geometry (414). There are no d orbitals available for bonding in [Ga(bct)3]3, and the energy of the d orbitals in [Al(bct)3]3 render them inaccessible for bonding interactions with the catacholate ligands. The geometries of the latter two complexes support this argument, as they are distorted toward octahedral, with a ¼ 34:2 and 40.3 , respectively (413). However, many metallo-tris(dithiolenes) possess vacant e0 orbitals that are energetically accessible for p bonding. Although this would be expected to stabilize the TP structure for these complexes, distortions toward the octahedral limit are still readily observed. Therefore, stabilization of the TP geometry in metallo-tris(dithiolenes) may reflect either the intrinsic differences between diolate and dithiolene p bonding, the high oxidation state of metal ions coordinated by the latter, or additional factors such as interligand S S bonding. Studies on the [M(benzenedithiolate)3] series (M Nb, Ta, Sb) provide evidence that M L p bonding does play an important role in stabilizing the TP geometry in metallo-tris(dithiolenes) (415, 416). As anticipated, distortions from TP to octahedral geometry are in the order Sb > Ta > Nb (415). This trend has been suggested to result from a good energy match between metal and dithiolene orbitals of p symmetry, facilitating stronger M S p bonding in [Nb(bdt)3] (415). Additional evidence for p bonding effects derives from the preference for unsaturated 1,2-dithiolates to assume TP geometries when compared to their saturated analogues. The trigonal twist angle observed for [Nb(bdt)3] is found to be 0.7 while that for [Nb(edt)3] is considerably larger (33.3 ), nearly halfway between TP and octahedral geometries (415). Recall from our earlier discussion concerning the electronic structure of ene-1,2dithiolate ligands that edt2 should be about as good a s- and psuedo-s donor (4e s-type donor) as bdt2. However, Martin and Takats (415) suggest that the p-bonding characteristics of these two ligands differ considerably since the p electrons in bdt2 are delocalized over the p system of the benzene ring, resulting in the destabilization of the 3pv bdt2 HOMO relative to that of edt2. The authors point out that this has two important implications with respect to the relative p-donor properties of these ligands, namely, that p delocalization results in (1) a reduction in the effective nuclear charge on S and (2) an increase in p-donor ability due to the destabilization of the dithiolene 3pv antibonding orbital. Ab initio calculations performed on various dithiolene dianions clearly support point 1 above, as the Mulliken charge per dithiolate sulfur donor is

METALLO-DITHIOLENE COMPLEXES

183

found to decrease as a function of increased p conjugation (20). Martin and Takats (415) hypothesize that destabilization of the dithiolate p orbital is the dominant contributor to the effective p donor ability of these ligands. As a result, the saturated dithiolate ligands were predicted to be better s donors than unsaturated dithiolates, but poorer p-donor ligands. As we discussed previously, detailed spectroscopic studies on the oxo-molybdenum-mono(dithiolenes) (L-N3)(MoO(bdt), (L-N3)(MoO(tdt), and (L-N3)(MoO(qdt) provide evidence that the strength of s bonding may not always be able to be predicted a priori (20, 22, 23). The extinction coefficients of the Sop ! Mo dxz;yz LMCT excitations for the unsaturated bdt2 and tdt2 ligands are considerably more intense than those of (L-N3)(MoO(qdt), and this results from the poorer p-donor character of the qdt2 sulfurs (20, 22). The reduced covalency in the Mo S p bonds of (L-N3)(MoO(qdt) clearly show that the p-donor ability of the dithiolene does not always parallel p delocalization induced destabilization of the ligand 3pv orbital (20, 22). The effective nuclear charge reduction on the S donors of the qdt2 ligand appears to stabilize the dithiolene 3pv orbital to a greater extent than the destabilizing influence of intraligand antibonding interactions (20). Therefore, the effective nuclear charge reduction accompanying p delocalization may be, in certain cases, the primary arbiter of dithiolene p-donor ability. An additional important factor must be considered when comparing the effects of saturated and unsaturated dithiolate ligands on stabilization of TP metallo-tris(dithiolene) structures, namely, C S and S C C S fragments since severe the planarity of the S C chelate ring puckering in the latter may modulate the M S p overlap and affect distortions from the idealized D3h symmetry. Thus, saturated dithiolates may be better p donors than their unsaturated analogues, but only under conditions where geometric parity is not realized. Finally, the degree to which a coordinated dithiolene ligand tends toward the oxidized dithione or dithiete resonance forms could affect the M S p-bonding description, since the antibonding 3pv ligand orbital is now unoccupied and can play a p-acceptor role. This p-acceptor character may be particularly important in stabilizing the TP geometry in complexes of the later transition metals. Understanding both the p donor and p-acceptor properties of the coordinated dithiolene provides a basis for understanding how electron delocalization can stabilize the TP geometry. Considering a noncovalent (ionic) bonding model involving high-valent metal centers and reduced unsaturated dithiolates in the dianionic limit, ligand–ligand repulsion terms dominate, and the distortions of metallo-tris(dithiolenes) will tend toward an octahedral geometry in order to minimize interligand S S interactions. Metallo-tris(dithiolenes) that possess the best dithiolene donors will have tris(dithiolene) SALCs composed of the ligand 3pv orbitals that are closer to the metal d orbitals. If one now allows for metal– ligand covalency, the ligand electron density is delocalized over the half-filled or

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empty metal d orbitals, resulting in a reduction of interligand S S interactions and increased stabilization of the TP geometry. Although considerable focus has been given to M S p-bonding, the s-bonding scheme cannot be ignored and it has even been suggested that the TP geometry may be stabilized by considering only M S s-bonding interactions in d 0–d 2 metals (415). Finally, consider the fact that the interligand S---S distances in [Mo(mnt)3]2 and ˚ , respectively, while the [Mo(qdt)3]2 are nearly identical at 3.19 and 3.14 A structure of the former is distorted TP and that of the latter is nearly perfect TP (330, 417, 418). This geometric incongruity seems to argue that the presence of interligand S---S interactions alone do not stabilize the TP geometry, and M S p bonding may be a dominant factor in determining the overall stability of TP structures. Recall that Gray’s early bonding scheme possessed very stabilized metal orbitals of comparable energy to the highest energy ligand-based functions comprised of dithiolene 3pv orbitals. This energy ordering suggests a highly covalent interaction between the metal dz2 orbital and ligand 3pv orbitals of a1 symmetry. The recent DFT (412) and Fenske–Hall (410) calculations indicate only the nonbonding dithiolene a02 orbital is of similar energy to the metal dz2 (a01 ) orbital, and the other ligandbased orbitals are found at considerably deeper binding energy. This was suggested by Martin and Takats (415), who also proposed that the metal dz2 orbital is essentially nonbonding, and the TP geometry is a function of the degree of interaction between metal dxy and dx2 y2 orbitals and filled dithiolene p orbitals of e0 symmetry. The very common ‘‘sulfur-fold’’ distortion from the idealized TP geometry results from a folding of the dithiolene ligand about the intraligand S S axis. The degree of this distortion is often found to vary among each of the three dithiolene ligands in metallo-tris(dithiolenes) (Fig. 56). Consider the complexes [Mo(S2C2H2)3] and [Mo(S2C6H4)3], which both display bending along the sulfur-fold. The bending angle, a, is 18 for [Mo(S2C2H2)3], while the bending of [Mo(S2C6H4)3] is irregular with a angles of 13.1 , 21.1 , and 30.0 . The same distortion along the sulfur-fold is also present in [Nb(S2C6H4)3] and [Mo(Se2C2(CF3)2)3], but is not observed in [W(Se2(COOMe)2)3]2, [Re(S2C2Ph2)3], and [V(S2C2Ph2)3]. Undoubtedly, crystal packing forces play a role here, but the electronic contributions certainly cannot be ignored. It has been suggested that the principal electronic driving force for distortions about the sulfur-fold in metallo-tris(dithiolenes) is an increase in the p bonding between dithiolene molecular orbitals of e0 symmetry, constructed from the three ligand 3pv orbitals, and the metal dxy and dx2 y2 orbitals, which also possess e0 symmetry (415). Other arguments for the observation of a sulfur-fold have centered on changes in the hybridization of the dithiolene sulfur atoms and p-type interac tions between the ene-dithiolate C  C p orbitals and metal d orbitals of appropriate symmetry.

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A very powerful argument detailing the primary driving force for the presence of a sulfur-fold has been put forth recently by Campbell and Harris using Fenske–Hall MO calculations (410). Recall that in D3h symmetry the metal dz2 (a01 ) and ligand Sv (a02 ) are nearly degenerate in energy. Furthermore, in D3h symmetry these orbitals do not mix, as they do not possess the same symmetry. Distortions about the sulfur-fold lower the symmetry of the molecule and, in the limit of a symmetric distortion (a1 ¼ a2 ¼ a3), the symmetry is reduced to C3h (Fig. 55). Now the metal dz2 and the highest occupied ligand Sv orbital possess the same symmetry ða0 Þ and strongly mix due to their near degeneracy. These geometric distortions, which occur as a result of orbital mixing and energy stabilization in the distorted structure, are known as secondorder Jahn–Teller distortions. The increased dz2 –Sv orbital overlap that accompanies this distortion is shown in Fig. 56, where the synchronous rotation of the S pv orbitals upon ligand bending results in a net bonding interaction for dn metals whenever n < 2. The orbital mixing results in the formation of bonding and antibonding combinations of these orbtials with a concommitant splitting in their energies, stabilizing the bonding combination and increasing the Mo S bond strength when the antibonding combination is not fully occupied (Fig. 57).

−5

a′

Energy (eV)

a1′ a2′

a′ −10

e′ e′ a1′′ a2′′ e′′ e′′ e′ a1′

−15 −5

0

a′′ e′′ a′′ e′ e′′ a′ 5 10 15 20 25 30 35 Mo–S–C Bend Angle

Figure 57. Fenske–Hall calculated correlation diagram relating the sulfur-fold and MO energy of MoVI(S2C2H2)3. [Adapted from (410).]

186

MARTIN L. KIRK ET AL. TABLE XIII Mo S Overlap Populations as a Function of the Sulfur-Fold Anglea

Mo S C bend angle (deg) Overlap populationb a b

0 1.491

6 1.555

12 1.564

18 1.576

24 1.587

30 1.588

Reference 410. The sum of the overlap populations between all sulfur orbitals and all metal orbitals.

Interestingly, the ligand-based MOs that are located at considerably deeper binding energies are found to be relatively unaffected by the sulfur-fold. The fact that only two orbitals appreciably change their energies upon bending of the dithiolene along the S S vector suggests that the primary driving force for the distortion is the energy stabilization of the Mo S bonding combination. The Fenske–Hall studies do not support the argument that the principal reason for the C3h distortion derives from an increase in overlap between the ligand 3pv orbitals possessing e0 symmetry and the e0 metal dxy and dx2 y2 orbitals (410). The calculated Mo S overlap populations for these orbitals remain virtually unchanged in going from a D3h to C3h geometry (Table XII), as do the S–dxz ; dyz overlap populations. The largest overlap population difference is observed in the S–dz2 interaction, which again is consistent with the driving force for the distortion resulting from a second-order Jahn–Teller effect. That these effects are principally a function of the S 3pv–Mo d overlaps is detailed in Tables XII and XIII, where it is immediately evident from the S–dxz ; dyz and S bonding occurs S–dxy ; dx2 y2 overlap populations that an increase in M concommitantly with the ligand bend. It has been observed that maximal sulfur-fold distortions occur in molecules where only two electrons are available for occupation of the nearly isoenergetic metal dz2 ða01 Þ and ligand Sv ða02 Þ orbtitals, as this provides the greatest energy stabilization. This stabilization is fully consistent with the description of distortions about the sulfur-fold occuring via a second-order Jahn–Teller effect. As was discussed previously, neither [V(S2C2Ph2)3] or [Re(S2C2Ph2)3] show evidence for a C3h distortion along the sulfur-fold (410). Since the [V(S2C2Ph2)3] complex possesses a ða01 ; a02 Þ1 electronic configuration, the driving force for the distortion should be markedly less than that of an ða01 ; a02 Þ2 configuration. Furthermore, there should be a weaker electronic driving force for a D3h ! C3h distortion in [Re(S2C2Ph2)3], which possesses an ða01 ; a02 Þ3 electron configuration, since the third electron must now occupy an energetically destabilized antibonding orbital. Note that we have been careful here to not make any a priori assumptions regarding the energy ordering of the a01 and a02 orbitals, as this does not affect the nature of the second-order Jahn–Teller distortion. Obviously, for the ða01 ; a02 Þ1 and ða01 ; a02 Þ3 electron configurations

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another effect, such as the presence of bulky phenyl substituents, must counteract the propensity to distort along the sulfur-fold. Recall that some researchers have argued that crystal packing forces are a primary reason for the observation of distortions along the sulfur-fold coordinate. Campbell and Harris (410) examined the crystal structure data for [Mo(S2C6H4)3], [Ph3As][Nb(S2C6H4)3], [V(S2C2Ph2)3], and [Re(S2C2Ph2)3] and observed that there is a relationship between the crystal packing and the steric bulk of the ligand. They argued that the presence of phenyl groups on the ligand periphery could impede ligand bending. This idea is consistent with the data for [Mo(S2C6H4)3] and [Ph3As][Nb(S2C6H4)3], which possess less bulky dithiolenes, and show deviations from planarity. Therefore, both the crystal packing force hypothesis and the second-order Jann–Teller description are consistent with the data. Although the authors clearly favor the second-order Jahn–Teller description, they nevertheless conceded that a more systematic study is necessary to properly differentiate between these two ideas (410). More recently, Stiefel and co-workers (281) performed a key experiment that addresses this issue. These researchers examined the structural differences between the Mo(IV) complex [Mo(tfd)3]2 and the Mo(VI) complex [Mo(tfd)3]. The former has an ða01 ; a002 Þ4 electronic configuration while [Mo(tfd)3] possesses a ða01 ; a02 Þ2 configuration. In agreement with the second-order Jahn– Teller hypothesis of Campbell and Harris (410), the X-ray structure of [Et4N]2[Mo(tfd)3] does not reveal a distortion along the sulfur-fold, but [Mo(tfd)3] does with a bend angle of 18 . Interestingly, although the structure of [Mo(tfd)3] is best described as C3h , inspection of the molecular structure of [Mo(tfd)3] indicates that this molecule no longer possesses a sh plane of symmetry and is distorted from D3h toward an Oh geometry (twisted TP). The actual symmetry is D3 , and the trigonal twist angle is 18 . The structure of    [Ta(C  CSi(t-Bu3))6] also indicates an 18 twist angle, and theoretical studies     on a [Ta(C CH)6] model reveal that trigonal twists of up to 20 are energetically feasible provided that the ligand is sterically bulky (419). Therefore, trigonal twist distortions up to 20 may occur due to steric constraints without approaching an appreciable electronic barrier. Recent DFT calculations (412) also confirm the fundamental results of the Fenske–Hall studies (410) and support the conclusion that the energies of the metal dz2 ða01 Þ and ligand Sv ða02 Þ orbitals are nearly degenerate in D3h . However, the DFT results, which were undertaken using both Mo(VI) and Mo(IV) as the central metal, place the ligand S pv orbital slightly above dz2 (412). These orbital energies are opposite the Fenske–Hall results of Campbell and Harris (410). Provided that this behavior is a function of the metal, its oxidation state, or the nature of the ligand, this orbital switching may conveniently explain many of the controversies that have arisen in the EPR and electronic spectroscopies of these systems (see below). Furthermore, since one of the orbitals in question is

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predominantly metal in character and the other predominantly ligand, an external perturbation that effectively changes the energy ordering or elicits enhanced mixing as a function of the sulfur-fold may result in drastic changes in the physical and optical properties of these molecules. A dynamic change in the energy ordering shifts electrons from the metal to the ligand (or vice versa) and is akin to a perturbation-induced ET (induced internal redox) process. These induced internal redox events are widely known in the literature on cobalt diolates as valence tautomerism. External perturbations that dynamically induce a distortion about the sulfur-fold modulate the degree of M S mixing, which is reminiscent of a CT process. This latter behavior has actually been observed in the metallo-bis(dithiolene) complex [MoO(qdt)] (110) (see above) and evidence for this behavior has recently been found in at least one metallotris(dithiolene) as well (see below). D. Spectroscopic Studies of Metallo-Tris(dithiolenes) 1.

EPR Spectroscopy

Multiple oxidation states are readily accessible for metallo-tris(dithiolenes) via chemical and electrochemical redox processes. Thus, for the high-valent d0, d1, and d2 electronic configurations the nature of the a01 and a002 orbitals are of particular importance, as these are the putative redox orbitals involved in these transformations. As for the metallo-mono(dithiolenes) and metallo-bis(dithiolenes), the EPR spectroscopy of metallo-tris(dithiolenes) has been particularly informative, if not controversial, in the determination of the singly occupied HOMO of these complexes. Among the earliest EPR studies in this area are those of Maki and co-workers (387, 388, 409, 420). These researchers immediately realized that the highly covalent nature of the bonding in metallotris(dithiolenes) resulted in questioning the applicability of formal oxidation state assignments for the metal and dithiolene ligands. A number of Cr, Mo, W, and V metallo-tris(dithiolenes) complexes were studied and the g and A values may be expressed in terms of the familiar spin-Hamiltonian, H ¼ gk be Hz Sz þ g? be ½Hx Sx þ Hy Sy  þ Ak Iz Sz þ A? ½Ix Sx þ Iy Sy  and presented in Table XIV. The initial assumption of these workers was that the unpaired electron occupied a metal-based a01 dz2 orbital. Assuming a degree of metal–ligand covalency is present, the a01 molecular orbital can be described as a linear combination of metal and ligand functions of appropriate symmetry. h pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffii cða01 Þ ¼ CM adða01 Þ  1  a2 ðsÞ  CL sða01 Þ  CL0 pða01 Þ

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TABLE XIV EPR Parameters for Various Metallo-Tris(dithiolenes)a Complex [CrS6C6(CF3)6] [MoS6C6(CF3)6] [WS6C6(CF3)6] [VS6C6(CF3)6] a

hgi

hAi

gk

g?

Ak

A?

1.994 2.0097 1.9910 1.980

16.3 12.2

1.995 2.011 1.987 1.974

1.995 2.009 1.993

17.4

9.6

100

45

63.3

References 387, 388, 409, and 420.

Here, C M is the total metal character in the a01 HOMO, a is the metal dz2 orbital coefficient that differs from C M by virtue of the fact some metal s character may be mixed into the MO. Both CL and CL0 are the coefficients of ligand orbitals that form s or p bonds with the metal dz2 orbital. Inspection of the values in Table XIV immediately makes clear that these complexes possess extremely isotropic g tenors. This observation led to the speculation that the HOMO may not be primarily a ðdz2 Þ1 configuration. However, assuming a ðdz2 Þ1 configuration in a D3h (or D3) geometry, Griffith and McGarvey have shown, in the limit of
1  j j, that gk ¼ 2:00

and 2Rg j j 4Rg j j  g? ¼ 2:00 
1
2 where  is the one-electron spin–orbit coupling constant for the metal, Rg is an orbital reduction factor, and
1 and
2 are the energies of the dz2 ! dxy ; dx2 y2 and dz2 ! dxz ; dyz ligand-field transitions, respectively. Here, covalent contributions to the bonding result in a deviation of Rg from unity (0:5  Rg  1:0). Maki and co-workers (387, 388, 409, 420) showed that the measured g values did not agree well with the theory, as unrealistically small, or negative, values of Rg were required to explain the data. Further analysis of these complexes focused on the hyperfine coupling. McGarvey (421) also developed expressions for the anisotropic hyperfine interaction and the reduced form of these equations were used by Maki and co-workers (420) in their analysis of the hyperfine coupling in metallotris(dithiolenes). These equations are given below: ak ¼ 4Ph1=r3 iave hRa =7i 3

a? ¼ 2Ph1=r iave hRa =7i

and

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MARTIN L. KIRK ET AL.

Here Ra is another orbital-reduction factor (0:5  Ra  1:0), P ¼ 2gN be bN . The anisotropic components of the hyperfine coupling tensor (ak and a? ) are defined as ak ¼ Ak  hAi a? ¼ A?  hAi Again, the calculated values of Ra were unrealistically small, casting additional doubt on a dz2 ground-state description for these complexes. Furthermore, the measured hyperfine values in the frozen glass spectra yield Ak > A? , which is opposite the order predicted by McGarvey (421) for a ðdz2 Þ1 configuration. Finally, the observed small values for the isotropic component, hAi, of the hyperfine coupling constant were also determined to be inconsistent with a ðdz2 Þ1 configuration. Therefore, the EPR results for [Cr(S2C2(CF3)2)3], [Mo(S2C2(CF3)2)3], [W(S2C2(CF3)2)3], and [V(dithiolene)3]2 were interpreted in terms of an ða01 Þ2 ða002 Þ1 configuration with the dz2 ða01 Þ orbital being doubly occupied (420). Recall that the a002 MO is nonbonding with respect to the metal in both D3h and D3 symmetries, and does not mix with the a01 dz2 orbital. Thus, the unpaired electron was proposed to be in an orbital that is fully localized on the ligand. Interestingly, the complex [V(S2C2Ph3)] possesses a nearly isotropic g tensor and hAi values very similar to the [V(dithiolene)3]2 complexes. This result is very surprising since [V(S2C2Ph2)3] possesses two fewer electrons than [V(dithiolene)3]2. Therefore, Maki and co-workers (409) suggested that the electronic configuration of [V(S2C2Ph2)3] is ða002 Þ1 ða01 Þ0 . This electronic configuration requires that the a002 and a01 orbitals switch their energy order as a function of the electron count in the complex. Recall that the only significant difference between the results of the Fenske–Hall (410) and DFT (412) calculations is a reversal in the energy ordering of the dithiolene based a002 and metal-based a01 orbitals. This bonding description suggests that, at least for these molecules, the dithiolene ligands are incapable of stabilizing the M5þ and M6þ valence states. Therefore, a valence bond treatment of these complexes requires that at least one of the dithiolenes are formally oxidized. Stiefel and co-workers (408) reinvestigated the nature of the [V(mnt)3]2 HOMO by performing a detailed single-crystal EPR study of the [V(mnt)3]2 ion doped into an isomorphous (Ph4As)2[Mo(mnt)3] host. The single-crystal study enabled these researchers to determine the magnitude and sign of the anisotropic hyperfine parameters and relate the single-ion g and A tensors to MO covalency parameters. Provided that the [V(mnt)3]2 ion adopts a similar molecular structure to its [Mo(mnt)3]2 host, the ion adopts a D3 structure with a trigonal twist angle of 28 . Thus the geometric structure of the ion is approximately halfway between TP and octahedral. The EPR data confirm axial symmetry for the complex with g and A tensors nearly colinear with the C 3 symmetry axis.

METALLO-DITHIOLENE COMPLEXES

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TABLE XV Single-Crystal EPR Parameters of [V(mnt)3]2

a

gk

g?

hgi

gsolution

Ak

A?

hAi

hAisolution

2.000

1.974

1.982

1.980

10

100

63.3

63.5

a

Reference 408.

The results of Maki and co-workers (409) (Table XIV) yielded absolute values of Ak and A? of 100 and 45 G, respectively (Aiso ¼ 63:3 G) for [V(mnt)3]2. However, the results of the single-crystal study allowed for the unambiguous determination of the sign for the principle components of the hyperfine tensor, yielding Ak ¼ 10 G and A? ¼ 100 G (Table XV) (408). Note that hAi is now 63.3 G, in agreement with the results of Maki except for the sign of hAi, which was not determined   in their frozen glass study. Significantly, gk is now > g?, and jA? j is > Ak , in agreement with the theory of McGarvey. Slightly different versions of the McGarvey equations were used by Stiefel and co-workers (408) in the analysis of the hyperfine coupling and g values, and these are given below: 4 1 Ak ¼ K þ b2 P  ðg?  2:0023ÞP 7 7 2 2 13 A? ¼ K  b P  ðg?  2:0023ÞP 7 14 gk ¼ ge  2 2 2  a b a b2 g2 b2 g? ¼ ge  6l þ
Eb
Ea where the constants have their usual meaning, l ¼ 250 cm1 for V(IV), and a, b, g, a, and b are MO coefficients that were defined previously. Since the geometry of the ion is D3, values for a2 and b2 were estimated as 0.82 and 0.18, respectively. Both
Ea and
Eb are the energies of the dz2 ! dxy ; dx2 y2 , and dz2 ! dxz ; dyz ligand-field transitions, which were assigned at 10,850 and 23,400 cm1, respectively. The analysis of the anisotropic hyperfine coupling parameters yielded g2M ¼ 0:65, or 65% V dz2 character in the HOMO. Therefore, this indicates that the HOMO, although highly covelent, is still predominantly metal-based, with 35% ligand character. The determination of the remaining covalency perameters resulted from an analysis of the g tensor using the equations listed above. The calculated amount of metal character in the s-antibonding e00 ðdxz;yz Þ orbital was determined directly from g? , yielding a2M ¼ 0:53. Since the authors made the assumption that b ¼ g, a unique determination of the p-antibonding MO (e0 , dxy;x2 y2 ) was not attempted (408).

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MARTIN L. KIRK ET AL.

The results of this work were deemed applicable to all metallo-tris(dithiolenes) in the near TP limit, including [V(S2C2Ph2)3], [Cr(S2C2Ph2)3], [Mo(S2C2Ph2)3], [Mo(S2C2(CF3)2)3], [W(S2C2Ph2)3], [Re(S2C2Ph2)3], and [Re(S2C7H6)3], for which solution and frozen glass EPR studies have been reported (408). All of these complexes display a very small anisotropy in the g tensor and, provided we are in the trigonal prismatic D3h limit, reduces to g k ¼ ge g ? ¼ ge 

6la2 b2
Eb

since b2 ¼ 0 for a trigonal prism. Thus, it is readily apparent that the energy of the lowest lying ligand-field transition, dz2 ! dxy ; dx2 y2 , does not contribute and the orbital angular momentum associated with this excited-state configuration does not mix into the 2A01 ground state. Since the dz2 ! dxz ; dyz ligand-field transition is estimated at energies
Eb > 3 eV and the product a2b2 is considerably less than unity due to the large degree of metal–ligand covalency, the deviation in g? is expected to be small and very close to the free-electron g value of 2.0023. In agreement with the earlier work of Maki and co-workers (409), this still requires an orbital reversal on reduction of [V(S2C2Ph2)3] to [V(S2C2Ph2)3]2, however, the orbital change is now from an ða002 Þ1 ða01 Þ0 ground-state configuration to ða002 Þ2 ða01 Þ1 . 2.

Electronic Absorption Spectroscopy

Schrauzer and co-workers (407, 422) conducted initial electronic structure studies of metallo-tris(dithiolenes) at about the same time that parallel studies were being under taken by Gray and co-workers (280, 406, 423). Detailed assignments of the electronic absorption spectra were conducted by both groups based upon the results of their bonding calculations. As the initial MO picture presented by Gray and co-workers (280), and later modified by other researchers, is generally accepted as the more accurate of the two, we will discuss band assignments and the electronic structure of a variety of metallo-tris(dithiolenes) using an abbreviated version of this general scheme (Fig. 50). Electronic absorption data for the neutral complexes reveal that two intense electronic transitions dominate the absorption spectra. The solution spectra of [Re(S2C2Ph2)3], [W(S2C2Ph2)3], and [Mo(tdt)3] are presented in Fig. 58. The first band is the more intense of the two and is generally found at 15,000 cm1 (e ¼ 20; 000–30,000 M 1cm1) while the second is observed at 24,000 cm1 (e ¼ 15; 000 M 1cm1). The general similarity of the electronic absorption

METALLO-DITHIOLENE COMPLEXES

193

Figure 58. Solution electronic absorption spectra of Re(bdt)3 (thin line), W(bdt)3 (thick line), and Mo(bdt)3 (dashed line). [Adapted from (280).]

spectra for these neutral complexes leads to the assumption that they are all close to the TP limit (as opposed to Oh ) and that they all possess a similar electronic structure description. Furthermore, the solid-state absorption spectra reveal very similar features to the solution spectra, indicating the TP structure is maintained in solution. Band assignments have been proposed for [Re(S2C2Ph2)3], [W(S2C2Ph2)3], and [Mo(tdt)3] (280). The weak band observed at 8230 cm1 (e ¼ 1090 M 1cm1) in [Re(S2C2Ph)3] has been assigned as a 2A01 ! 2 E0 transition originating from an a01 ! e0 one-electron promotion (Figs. 49 and 50). Supporting this assignment is the lack of such a low-energy transition in the Mo and W complexes, which are both d0 M(VI) ions. This excitation is formally a dz2 ! dxy ; dx2 y2 ligand-field transition, but the intensity is larger than one might anticipate for a d–d band. Although [Re(S2C2Ph2)3] belongs to a noncentrosymmetric point group, allowing for metal d–p mixing, the intensity likely derives from configurational mixing with the intense 14,050 cm1 (e ¼ 24; 000 M 1cm1) CT band. This band has also been assigned as an 2 0 A1 ! 2 E0 transition that arises from a a02 ! e0 one-electron promotion, and is present in [W(S2C2Ph2)3] and [Mo(tdt)3] as well. The last characteristic band that was assigned in [Re(S2C2Ph2)3] is a combination of 2A01 ! 2 E0 and 2A01 ! 2 A002 transitions that occur at 23,400 cm1 (e ¼ 12; 300 M 1cm1) resulting from e0 ! a01 and a002 ! a01 one-electron promotions. Provided that these band assignments are correct, the high intensity of the transitions indicate a very large degree of S character in both the donor

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MARTIN L. KIRK ET AL.

and acceptor orbitals involved in the individual one-electron promotions. Finally, the LMCT nature of the transitions is evident from the fact that the [W(dithiolene)3] transitions always occur at higher energy than their corresponding [Mo(dithiolene)3] counterparts. Thus, the predominantly metal-based MOs of Mo are stabilized relative to those of W and are located at deeper binding energies. Finally, a small dependence on band energies is observed as a function of the dithiolene donor, revealing that the valence ionization energies of the donor ligands also function to modulate the band positions and affect reduction potentials. An alternative assignment for the bands observed in neutral [M(dithiolene)3] complexes has been proposed, and is based upon resonance Raman excitation profiles (317). However, we should first briefly discuss specific aspects of the observed vibrational Raman spectra for metallo-tris(dithiolenes). Group theory dictates that the vibrational modes of [M(dithiolene)3]0,1,2 complexes in D3h and D3 are D3h vib

¼ 6a01 þ 2a02 þ 3a001 þ 6a002 þ 9e0 þ 8e00

¼ 9e0 þ 6a002 0 0 00 Raman ¼ 6a1 þ 9e þ 8e ir

for the isolated MS6 unit; 0 00 0 00 stretch ¼ a1 þ a2 þ e þ e and the a01 þ e0 þ e00 modes are Raman active D3 ¼ 9a1 þ 8a2 þ 17e ¼ 8a2 þ 17e

vib ir

Raman

¼ 9a1 þ 17e

for the isolated MS6 unit; stretch ¼ a1 þ a2 þ 2e and a1 þ 2e modes are Raman active Typically, three to four resonantly enhanced modes are observed in the 250– 400-cm1 region, which presumably derive from M S stretching modes, of which one is polarized and more intense than the rest. The expected depolarization value (r) for a totally symmetric mode in resonance with a doubly degenerate excited state is 0.125, and the polarized totally symmetric mode for these complexes typically fall in the 0.14–0.17 range. Thus, since only one totally symmetric M S stretching mode is anticipated for these complexes,

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it is surprising that two totally symmetric stretches are observed in [V(S2C2Ph2)3] at 343.4 and 379.5 cm1 in the solid state (317). A similar situation is observed in [(L-N3)MoO(dithiolene)] complexes (see above), where the additional totally symmetric mode has been assigned as a symmetric S M S bending mode that, together with the symmetric stretch, define the two symmetry coordinates that comprise the two M(dithiolene) ring breathing modes. Solution RR excitation profiles for the 343 cm1(a01 ), 375 cm1(a01 ), and 406 cm1 (e0 or e00 ) modes of [V(S2C2Ph2)3] show resonance enhancement of V S modes. Clearly, this indicates that the electronic excitations involve considerable distortions along these modes, as would be expected for oneelectron promotions to vacant or half-filled metal-based orbitals that are highly M S antibonding in character. These orbitals could, in principle, be any one of the five metal d orbitals, which possess a01 , e0 , or e00 symmetry in D3h symmetry. Clark and Turtle (317) assigned the lowest energy 9850-cm1 transition in [V(S2C2Ph2)3] as originating from an a01 ! a02 one-electron promotion, in contrast to the corresponding transition assigned by Gray and co-workers (280) in [Re(S2C2Ph2)3]. Furthermore, the 13,310-cm1 band was assigned as an 2A01 ! 2 E0 transtion arising from an e0 ! a01 one-electron promotion, again in contrast with the Gray assignment. Both groups agreed that the second intense transition is assignable as 2A01 ! 2 E0 ðe0 ! a01 Þ. Clearly, there is some disagreement in the precise origin of the absorption features observed in neutral [M(dithiolene)3] complexes. However, the fundamental conclusions drawn from both studies are basically the same, namely, that the bonding scheme is highly covalent. The complicated nature of the electronic origins of observed optical transitions in metallo-tris(dithiolenes) is exemplified by the complex [NBu4]2[Mo(mnt)3]. Clark and Turtle (317) used RR spectroscopy in conjuction with solid-state and solution electronic absorbance spectrosocopy to assign the absorption bands in a number of reduced [M(dithiolene)3]2 complexes, including [NBu4]2[Mo(mnt)3]. Interestingly, the reduced metallo-tris(dithiolene) complexes possess considerably lower extinction coefficients than their fully oxidized [M(dithiolene)3] counterparts, and the reason for this has yet to be elucidated. Nevertheless, three Mo S modes, including the totally symmetric stretch at 354 cm1, are shown to be resonantly enhanced with excitation into the envelope of the lowest energy transition at 14,980 cm1 (e ¼ 5800 M 1cm1). The authors have assigned this transition as 1A01 ! 1 E00 resulting from an a01 ! e00 one-electron promotion. This assignment is intriguing in light of the intensity of this band, because the a01 ! e00 excitation is formally a dz2 ! dxz ; dyz ligand-field transition and is dipole forbidden in the parent D3h symmetry. However, the X-ray structure of the [Ph4As]2[Mo(mnt)3] salt indicates the structure to be distorted TP with a ¼ 18 . To the extent that this is also the structure in solution, a reduction in symmetry toward D3 renders the transition

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as 1A1 ! 1 E, and provides a mechanism by which the transition can gain some electric dipole intensity. Provided that this is the correct assignment, both the a01 Mo dz2 orbital and the metal dxz ; dyz orbitals must be highly covalent and, more specifically, each possess a high degree of dithiolene ligand Sv character in their respective MOs. The electronic origin of the 20,100 cm1 transition is even more complicated. Excitation into this band shows resonance enhancement of 1 N stretches, which are both the 1497 cm1 C C and the 2197 cm C 2 S modes localized on the mnt ligands. Very little enhancement of the Mo was observed under these conditions, and this transition was left unassigned (317). However, in the W complex, [NBu4]2[W(mnt)3], an analogous band at S, 20,410 cm1 (e ¼ 6700 M 1cm1) shows resonance enhancement of W  N modes. The dominant contribution to the intensity of this band C  C, and C was tentatively assigned as the 1A1 ! 1 E component of the 4e0 ! 5e0 excitedstate configuration (317). As was discussed above, the bonding picture for metallo-tris(dithiolenes) indicates that they possess eight dithiolene-based MOs (a02 , 2e0 , a001 , a002 , 2e00 , and a01 ) and three metal orbitals (a01 , e0 , and e00 ). Let us consider the case of a complex formally consisting of a d0 metal bound to three dianionic dithiolenes in D3h symmetry. Thus, a total of 8  3 ¼ 24 one-electron promotions are possible, leading to a possible 44 LMCT excited states in the absence of lowsymmetry distortions or spin–orbit splitting of E0 and E00 states! This number increases to 46 if the two ligand-field states (for d1 and d 2 configurations) are included. Although not all of these transitions are electric dipole allowed in D3h symmetry, one can readily see the problem here. Summarizing, the combination of a very covalent M S bonding scheme, a corresponding high density of states, low-symmetry distortions and spin–orbit coupling have conspired to provide a high degree of difficulty in making firm band assignments.

V.

CONCLUSIONS

The electronic structure and spectroscopy of metallo-dithiolenes continues to be an active area of research, having made pivotal contributions to our understanding of their role in bioinorganic and materials chemistry. Metallodithiolenes have been synthesized that possess, depending on the choice of the transition metal and the nature of the dithiolene, lumophoric, magnetic, conducting, and catalytic properties. That metallo-dithiolenes possess such important and multifaceted properties is certainly a result of the complex, yet fantastic redox interplay between the metal and the dithiolene ligand. This research area remains highly active, and the determination of more elaborate dithiolene bonding descriptions continues with the eventual goal of obtaining more detailed and accurate descriptions of their underlying electronic structures.

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ACKNOWLEDGMENTS The author would like to acknowledge the Petroleum Research Fund and the National Institutes of Health (GM-057378) for continued support of much of the work detailed in this contribution. The author would also like to thank Mr. Antonio Williams for assistance with many of the figures in this Chapter.

ABBREVIATIONS b Bct bdt bpy BLYP CI CIS CT DFT dim dmit DMF DMSORox ead edt edta EHMO ENDOR EPR ESEEM ET HOMO INDO IR LCAO LFSE LLCT LMCT L-N3 LUMO MCD mnt

Bonding The macrocyclic tris(catecholate) ligand bicapped (TRENCAM) Benzene-1,2-dithiolate 2,20 -Bipyridine Becke–Lee–Yang–Parr functional Configuration interaction Configuration interaction—singles only Charge transfer Density functional theory 1,2-Ethanediimine Dimethylimidazolidine-2,4,5-trithione Dimethylformamide Oxidized dimethyl sulfoxide reductase Ethane-1,2-dithiolate Ethene-1,2-dithiolate Ethylenediaminetetraacetic acid Extended Hu¨ ckel molecular orbital Electron–nuclear double resonance Electron paramagnetic resonance Electron spin echo envelope modulation Electron transfer Highest occupied molecular orbital Intermediate neglect of differential overlap Infrared Linear combinatin of atomic orbitals Ligand-field stabilization energy Ligand-to-ligand charge transfer Metal-to-ligand charge transfer Hydrotris (3,5-dimethyl-1-pyraazolyl) Lowest unoccupied molecular orbital Magnetic circular dichroism 1,2-Maleonitrile-1,2-dithiolate

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MMLL0 CT MO nb NIR OAT PES qdt RR R,R0 timdt S-S Sip Sop SALC SCF SO SOMO T tds tdt tfd TP UV XO jM jL 0 jaip 00

jaip 0 jaop 00

jaop cn wn

Mixed-metal–ligand-to-ligand charge-transfer Molecular orbital Non-bonding Near-infrared Oxygen atom transfer Photoelectron spectroscopy Quinoxalene-2,3-dithiolate Resonance raman Monoanion of a disubstituted imidazolidine-2,4,5-trithione Dithiolene or dithiolate chelate In-plane dithiolene S p orbital Out-of-plane dithiolene S p orbital Symmetry adapted linear combination Self-consistent field Sulfite oxidase Singly occupied molecular orbital Tesla (Trifluoromethyl)ethylenediselenato Toluene-2,3-dithiolate Bis(trifluoromethyl)-1,2-dithiete Trigonal prismatic Ultraviolet Xanthine oxidase Metal-based function Ligand-based function Symmetric in-plane dithiolene molecular orbital Antisymmetric in-plane dithiolene molecular orbital Symmetric out-of-plane dithiolene molecular orbital Antisymmetric out-of-plane dithiolene molecular orbital Molecular orbtital functions comprised of wn Atomic orbital functions that form the cn REFERENCES

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NOTE ADDED IN PROOF Recently, Solomon and coworkers (R. K. Szilagyi, B. S. Lim, T. Glaser, R. H. Holm, B. Hedman, K. O. Hodgson, and E. I. Solomon, J. Am. Chem. Soc., 125, 9158 (2003)) have used S K-edge XAS to probe the ground state wave function of Ni dithiolenes ([NiL2]z, L ¼ 1,2-Me2C2S2; z ¼ 2-, 1-, 0). These researchers have developed a transition dipole integral for the dithiolene sulfur and determined that the ground state wave functions of the [NiL2]z series possess greater than 50% S character. Therefore, the bonding description of [NiL2]2-,1-,0 is said to be ‘‘inverted’’, with dithiolene valence orbitals lying at higher energy than the Ni d-orbital manifold. This further exemplifies the noninnocent behavior of dithiolene ligands in metallo-dithiolene complexes.

CHAPTER 4

Vibrational Spectra of Dithiolene Complexes MICHAEL K. JOHNSON Department of Chemistry and Center for Metalloenzyme Studies University of Georgia Athens, GA CONTENTS I. INTRODUCTION

214

II. FOUR-COORDINATE BIS(DITHIOLENE) METAL COMPLEXES

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III. SIX-COORDINATE TRIS(DITHIOLENE) METAL COMPLEXES

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IV. OXO-METALLO-MONO(DITHIOLENE) COMPLEXES

239

V. OXO-METALLO-BIS(DITHIOLENE) COMPLEXES VI. PYRANOPTERIN DITHIOLENES IN MONONUCLEAR Mo AND W ENZYMES A. B. C. D.

241 246

Xanthine Oxidase Family / 247 Sulfite Oxidase Family / 248 Dimethyl Sulfoxide Reductase Family / 250 Aldehyde Oxidoreductase Family / 258

VII. CONCLUDING REMARKS

261

ACKNOWLEDGMENTS

262

ABBREVIATIONS

262

REFERENCES

263

Dithiolene Chemistry: Synthesis, Properties, and Applications, Progress in Inorganic Chemistry, Vol. 52 Special volume edited by Edward I. Stiefel, Series editor Kenneth D. Karlin ISBN 0-471-37829-1 Copyright # 2004 John Wiley & Sons, Inc. 213

214

MICHAEL K. JOHNSON

I.

INTRODUCTION

Dithiolenes have attracted the interest of inorganic chemists for more than four decades since they provide redox-active ligands with the ability to form highly electron-delocalized complexes. Formal descriptions of the redox states of the dithiolene ligand are shown in Fig. 1, with the dianionic ene-1,2-dithiolate form being oxidized by two electrons to yield the neutral 1,2-dithione or 1,2dithiete. The interplay of ligand- and metal-based redox chemistry in transition metal dithiolene complexes leads to rich and versatile redox chemistry, as well as highly delocalized electronic structures with low energy electronic transitions. These attributes have lead to the preparation of discrete complexes and polymeric materials with interesting luminescence, magnetic, and conducting properties. In addition, dithiolene ligation is utilized in biological systems to facilitate the formal oxygen atom transfer (OAT) reactions that are catalyzed by mononuclear Mo and W enzymes. Hence, detailed understanding of the properties and chemistry of dithiolene complexes is essential in both the materials and biological research areas. The objective of this chapter is to summarize the results of vibrational studies of metal dithiolene complexes obtained using infrared (IR), Raman, and resonance Raman (RR) spectroscopies and thereby to demonstrate the ability of vibrational spectroscopy to assess the redox state of the coordinated dithiolene and the strength of the metal–S bonds. The utility of this approach is then extended to the study of mononuclear Mo/W enzymes and the use of RR spectroscopy for addressing the role of the pyranopterin dithiolene (ppd) ligand(s) in the catalytic cycle. Overall, the focus of this chapter is on understanding the ground-state vibrations of dithiolene ligands. Resonance Raman also facilitates assignment of electronic excited states via excitation profiles, and this application of Raman spectroscopy is discussed in more detail in Chapter 3 in this volume (1). II.

FOUR-COORDINATE BIS(DITHIOLENE) METAL COMPLEXES

By far, the most complete and rigorous vibrational assignments that are available for metallodithiolenes are for square-planar bis(dithiolene) metal R

S−

R

S−

Ene-1,2-dithiolate

−2e− +2e−

R

S

R

S

R

S

R

S

1,2-Dithione

1,2-Dithiete

Figure 1. Valence bond structures for the dithiolene ligand.

VIBRATIONAL SPECTRA OF DITHIOLENE COMPLEXES

215

2−

N C

N

C C S S C Ni C S S C C C

x

N z

y

N

Figure 2. Structure of the {Ni[S2C2(CN)2]2}2
anion.

complexes and the results serve as a paradigm for interpreting the vibrational modes of dithiolene ligands in general. The vibrational spectra of {Ni[S2C2(CN)2]2}2
will initially be discussed in detail in order to illustrate the rationale behind and the limitations of the available assignments. The availability of IR (2, 3), Raman, and RR (4) spectra, coupled with assignments based on 58Ni/62Ni isotope shifts (2) and normal coordinate analyses (2, 5), make {Ni[S2C2 (CN)2]2}2
one of the best characterized metal dithiolene complexes in terms of vibrational properties. These assignments are readily generalized to other members of the square-planar [M(S2C2R2)2]z family of bis(dithiolene) complexes and thereby facilitate observation and rationalization of trends in key vibrations as a function of M (Ni, Pd, Pt, Co, Cu, or Zn), z (0, 1
, or 2
), and R (H, Me, CF3, Ph, or CN). The planar {Ni[S2C2(CN)2]2}2
anion has D2h symmetry (see Fig. 2) and group theoretical analysis indicates that the 45 normal vibrational modes can be classified as 8Ag þ 7B1g þ 3B2g þ 3B3g þ 4Au þ 4B1u þ 8B2u þ 8B3u with the g modes Raman active and the u modes IR active, except for Au modes that are inactive in both Raman and IR. The in-plane Raman-active modes are Ag and B1g and the in-plane IR-active modes are B2u and B3u . In-plane and out-of-plane vibrations cannot mix due to symmetry considerations. Moreover, out-of-plane modes are expected to be weak, as a result of small dipole and polarizability changes, and to lie at low frequencies since the modes involve bond deformations involving relatively heavy atoms. Hence, they can largely be ignored in making assignments. A comparison of the IR and Raman spectra of [N(n-Bu)4]2{Ni[S2C2(CN)2]2} is shown in Fig. 3, along with key vibrational assignments. Both IR and Raman data are essential for determining the frequencies of the Ni

S stretching modes.
S) modes both involve movement The IR-active asymmetric B2u and B3u n(Ni
of the Ni, and hence occur to higher frequencies than Raman-active Ag and B1g modes, which do not involve movement of the central Ni atom (see Fig. 4 for the
S) modes, vibrational modes of a planar D2h NiS4 unit). In contrast to the n(Ni
the dominant higher frequency vibrational modes are localized on the dithiolene chelate rings and show accidental degeneracy with analogous frequencies for the Ag and B2u modes and for the B1g and B3u modes. The in-plane stretching modes

216

MICHAEL K. JOHNSON

IR

B2u

B3u

IR

B3u

B2u B3u ν(Ni-S)

ν(C-S)

ν(C-C) + ν(C-S)

ν(C=C)

ν(C≡N)

B2u

ν(C-S) + ν(C-C)

B3u B2u

Ag

Raman

B1g Ag

B1g

Raman 2400

2000

Ag

1600 1200 Wavenumber (cm−1)

B1g

B1g

Ag

800

500 400 300 200 Wavenumber (cm−1)

Figure 3. Infrared and Raman spectra of [N(n-Bu)4]2{Ni[S2C2(CN)2]2} showing key vibrational assignments for the {Ni[S2C2(CN)2]2}2
anion. Raman spectrum obtained using 647-nm excitation for a sample prepared as a KBr disk attached to a cold finger at 17 K. The Fourier transform IR (FTIR) spectra obtained at room temperature as a KBr disk in the 700–2400-cm
1 region and as a CsI disk in the 150–500-cm
1 region.

of a dithiolene ligand under C2v symmetry are depicted in Fig. 5. The IR and Raman modes corresponding to these modes in {Ni[S2C2(CN)2]2}2
differ only in the phasing of the local motions in the two chelate rings. Hence, their frequencies differ only to the extent that there is kinematic or electronic

S

S

S

M S

S

S

M S

S

S

S

M S

S

S M

S

S

S

Ag ν(M-S)

B1g ν(M-S)

B3u ν(M-S)

B2u ν(M-S)

S

S

S

+S

S M

S

S M

S

Ag δ(SMS)

S

S M

S

B3u δ(SMS')

S

x

S+

+ S

MS

B2u δ(SMS)

+S

SM

S+

B1u π

y

z

-S

S+

Au π

Figure 4. Schematic depiction of the normal vibrational modes of a D2h MS4 unit.

VIBRATIONAL SPECTRA OF DITHIOLENE COMPLEXES

C

C

S

C

C C S A1 ν(C=C) C C

C C

C

S

C C S A1 ν(C-S)

C

S

C C S A1 ν(C-C)

S

C

S

C

B1 ν(C-C) + ν(C-S)

C

217

C C

S S

B1 ν(C-S) + ν(C-C)

Figure 5. Schematic depiction of the in-plane stretching modes of a cis-C2C2S2 dithiolene unit under C2v symmetry.

coupling between the two rings and the near-coincident frequencies indicate that such coupling is negligible provided there is not significant mixing with ring deformation modes. The above discussion serves to demonstrate that either IR or Raman can be used to determine the vibrational frequencies localized on the dithiolene ligands, but that both IR and Raman data are required for complete analysis of Ni

S vibrational frequencies. However, relatively few of the published vibrational studies of dithiolene complexes report both IR and Raman data and consequently the assignments of M

S vibrational modes are incomplete and of limited utility for assessing trends in dithiolene ligation. Assignments for the in-plane vibrational modes of {Ni[S2C2(CN)2]2}2
are tabulated in Table I. The IR active modes are assigned based on normal coordinate analysis (2). Raman modes are assigned based on depolarization ratios (Ag modes are polarized and B1g modes are depolarized) and the dithiolene-base modes are readily assigned by analogy with the IR assignments (4). Earlier IR studies had suggested that several bands in the 250–400-cm
1 region contain contributions from Ni

S stretching (3) and normal mode calculations of a 1:1 metal–ligand model of the ion assigned a shoulder at
S stretching modes (5). Subse498 cm
1 and a band at 453 cm
1 to the Ni
quently, the 58Ni/62Ni isotope shifts determined by Schla¨ pfer and Nakamoto established that the band at 357 cm
1 and the shoulder at 365 cm
1 (58Ni minus 62 Ni shifts of 4.0 and 6 cm
1, respectively) have the largest contributions from
S) modes (see Fig. 4). However, normal mode calculathe B3u and B2u n(Ni
tions indicated that these modes are extensively mixed with the chelate ring and CCN deformation modes (see Table I). The Raman-active Ag and B1g modes (see Fig. 4) are assigned at 335 and 283 cm
1, respectively, based on polarization data, but normal mode calculations are not available to assess the extent of mixing with in-plane deformations. Overall, the relative frequencies of the n(Ni

S) modes in {Ni[S2C2(CN)2]2}2
ðB3u =B2u > Ag > B1g Þ are in

218

MICHAEL K. JOHNSON TABLE I IR and Raman Frequencies (cm
1) and Assignments for [NEt4]2{Ni[S2C2(CN)2]2}a

Assignment

Infraredb ————————————————— Symmetry Observed Calculated

n(C

N) n(C

C) n(C

S) þn(C

C) Ring deformation þ n(C

S) n(Ni

S) þ d(CCN) n(Ni

S) þ d(CCN) n(Ni

S) d(SNiS) d(SNiS) þ ring deformation d(CCN) þ ring deforrmation

B2u B2u B2u B2u B2u B2u

B2u B2u

219 (2.2) 107 s

216 (2.3) 107


n(C


N) n(C

C) þ n(C

S) n(C

S) Ring deformation þ d(CCN) Ring deformation þ n(Ni

S) n(Ni

S) þ ring deformation n(Ni

S) d(CCN) d(SNiS0 )

B3u B3u B3u B3u B3u B3u

2205 sh 1170 s 865 s 505

2202 1207 898 486 (0.2) 434 (1.8) 343 (4.5)

B3u B3u

177 (1.2) m

2195 s 1485 s 1055 m 535 m 457 (3.0) w 365 (6) sh

357 (4.0) s

2198 1486 1013 531 (0.1) 444 (1.0) 364 (5.7)

Ramanc —————————— Symmetry Observed Ag Ag Ag Ag

2193 p 1480 p 1063 p 498 p

Ag Ag

335 p 177 p

B1g B1g B1g B1g B1g

2211 sh 1147 dp 866 507 dp 416 dp

B1g

283 dp

181 (0.6) 42 (0.3)

a

Cation bands, out-of-plane vibrational bands and combination–overtone bands are not listed. Numbers in parentheses indicate 58Ni minus 62Ni isotope shifts, s ¼ strong, m ¼ medium, w ¼ weak, and sh ¼ shoulder. Data taken from (2). c Here p and dp refer to polarized and depolarized, respectively. Data taken from (4). Assignments are not based on normal coordinate analysis and are made on the basis of polarizations and by analogy with the IR data. b

excellent agreement with those established by isotope shifts and normal model calculations in square-planar MX4 complexes with M/X mass ratios similar to Ni/S (6) and in related square-planar Ni chelate complexes. The latter is best illustrated by the Ni(II)–dithiooxalato complex, [Ni(S2C2O2)2]2
, which has B2u n(Ni

S) at 362 cm
1 (7 cm
1 58Ni/62Ni isotope shift), B3u n(Ni

S) at 353 cm
1
1 58 62
1 (5-cm Ni/ Ni isotope shift), Ag n(Ni

S) at 317 cm , and B1g n(Ni

S) at 217 cm
1, and normal mode calculations indicate that differential mixing with deformation modes of the chelate rings is partly responsible for the splitting of the n(Ni

S) modes (7). The in-plane Ag and B2u d(SNiS) modes in {Ni[S2C2(CN)2]2}2
(see Fig. 4) are tentatively assigned to a polarized Raman band at 177 cm
1 and an IR band at 219 cm
1 with a 2.2-cm
1 58Ni/62Ni isotope shift (see Table I). Both are likely to be extensively mixed with ring and/or CCN

VIBRATIONAL SPECTRA OF DITHIOLENE COMPLEXES

219

deformation modes. The remaining in-plane bending mode, B3u d(SNiS0 ) involves symmetric bending of the dithiolene chelate rings (see Fig. 4) and is predicted to occur at 42 cm
1 by normal mode calculations. This mode has not been observed experimentally. Resonance Raman excitation profiles have been determined for [NEt4]2{Ni[S2C2(CN)2]2} in the 450–625-nm region (4). A comparison of the nonresonantly enhanced spectrum of [N(n-Bu)4]2{Ni[S2C2(CN)2]2} obtained with 647-nm excitation and the resonantly enhanced spectrum of [N(n-Bu)4]2{Ni[S2C2(CN)2]2} obtained with 488-nm excitation is shown in Fig. 6. Analogous Raman spectra have been reported for [NEt4]2{Ni[S2C2(CN)2]2} using 635- and 458-nm excitation (4). While both B1g and Ag modes are observed with similar intensities in the nonresonantly enhanced spectrum, Ag
1 modes, particularly those involving n(Ni

S) (335 cm
1), n(C

C) (1480 cm ),
1 and n(C

N) (2193 cm ), undergo preferential enhancement in resonance with the 476-nm (21,000-cm
1) visible absorption band (4). This indicates an

Absorbance

0.2

0.1

0 400 500 Wavelength (nm)

600

647-nm excitation

Ag

Ag

488-nm excitation

2000

1500

Ag

1000

Ag

Ag

Ag

500

Raman shift (cm−1) Figure 6. Low temperature (17 K) Raman spectra of [N(n-Bu)4]2{Ni[S2C2(CN)2]2} recorded in KBr disk with 647- and 488-nm excitation. The Ag modes are marked in the 488-nm spectrum to indicate the selective enhancement of these modes in resonance with the 476-nm absorption band. The inset shows the absorption spectrum in acetonitrile.

220

MICHAEL K. JOHNSON

allowed electronic transition with A-term resonance enhancement of Ag modes that mimic the distortion in the electronic excited state (8). Hence, the RR data were initially interpreted in terms of the 1Ag ! 1 B2u metal-to-ligand chargetransfer (MLCT) assignment for the 476-nm absorption band (9). Moreover, the
strong enhancement of the Ag n(C


N) mode demonstrates extensive delocalization of the dithiolene molecular orbitals extending over the CN group. In contrast to the Ag modes, which have excitation profiles that approximately follow the 476-nm absorption band, the B1g modes generally exhibit excitation profiles that increase monotonically throughout the 450–625-nm region (4). These nontotally symmetric modes are generally enhanced via a B-term mechanism involving vibronic mixing of two electronic excited states (8). Since a B1g mode is effective in vibronic mixing of B2u and B3u excited states, the excitation profiles are consistent with assignment of the higher energy electronic transition at 26,400 cm
1 to a 1 Ag ! 1 B3u ; B2u transition with ligand-to-metal charge transfer (LMCT) band (9). As discussed in detail in Chapter 3 in this volume (1), these electronic assignments deduced from Raman excitation have largely been confirmed by subsequent calculations, although the transitions have been shown to involve predominantly ligand-based molecular orbitals with variable mixing of metal orbitals into the excited-state molecular orbitals. The IR studies of [Ni(S2C2R2)2]z complexes with R ¼ H, Ph, CF3, or CN and z ¼ 0, 1
, or 2
conducted by Schla¨ pfer and Nakamoto (2) constitute the most comprehensive vibrational study of any series of metal-dithiolene complexes and supercede the earlier partial IR assignments made by Schrauzer and Mayweg (10, 11) and Adams and Cornell (3). Moreover, this IR study provides the basis for assessing and interpreting changes in the vibrational spectra as a function of the R group and the charge on the complex. For each complex, detailed assignments of the in-plane B3u and B2u modes in the mid- and far-IR regions were made based on 58Ni/62Ni isotope shifts and normal coordinate analyses using the GF matrix method and Urey–Bradley force fields. In the case of [Ni(S2C2H2)2], the IR spectrum calculated by density functional theory (DFT) was found to be in good agreement with the observed spectrum (12) and assignments. These calculations, based on animation of the calculated vibrations, are in excellent agreement with those made by Schla¨ pfer and Nakamoto (2). In order to illustrate trends in key vibrational modes, Table II lists the frequencies of n(C

S), and n(Ni

S) modes based on IR studies
C), n(C
of [Ni(S2C2H2)2]0,1
(2), {Ni[S2C2(CF3)2]2}0,1
,2
(2), [Ni(S2C2(Ph2)2]0,1
,2
(2), {Ni[S2C2(CN)2]2}1
,2
(2), and [Ni(S2C2Me2)2]0,1
,2
(13), and Raman studies of [Ni(S2C2S2CS)2]0,1
,2
in which (S2C2S2CS)2
is 1,3-dithiol-2thione-4,5-dithiolato (dmit) (14). In each complex, n(C

C) shows a systematic increase and both n(C

S), and n(Ni

S) show systematic decreases, as the negative charge on the complex anion increases. This trend unequivocally indicates more ene-dithiolate character upon reduction of all six complexes

VIBRATIONAL SPECTRA OF DITHIOLENE COMPLEXES

221

TABLE II Dithiolene Ligand Vibrational Frequencies (cm
1) as a Function of Charge in Square-Planar (Bis)dithiolene Ni Complexes Vibrational mode
C) n(C


n(C

S)a

n(Ni

S)b

z¼0

z ¼ 1


z ¼ 2


[Ni(S2C2H2)2] [Ni(S2C2Me2)2]z {Ni[S2C2(CF3)2]2}z [Ni(S2C2Ph2)2]z {Ni[S2C2(CN)2]2}z [Ni(S2C2S2CS)2]z

1350 1377, 1332 1425 1365

1435 1508 1485 1475 1435 1403

1594 1535 1533 1485 1435

2 13 2 2 2 14

[Ni(S2C2H2)2]z [Ni(S2C2Me2)2]z {Ni[S2C2(CF3)2]2}z {Ni[S2C2Ph2]2}z {Ni[S2C2(CN)2]2}z [Ni(S2C2S2CS)2]z

878, 798 799, 565 860 880

840, 790 768, 553 847 870 885, 535 933

744, 543 836 870 865, 535 907

2 13 2 2 2 14

[Ni(S2C2H2)2]z [Ni(S2C2Me2)2]z {Ni[S2C2(CF3)2]2}z [Ni(S2C2Ph2)2]z {Ni[S2C2(CN)2]2}z [Ni(S2C2S2CS)2]z

398, 428 465, 435 425, 465 454, 408, 475

385, 412 418, ndc 415, 449 428, 406, 465 365, 396, 468 358

nd, nd 394, 422 418, 401, 450 357, 365, 457 351

2 13 2 2 2 14

Complex z

1329

950

364

Reference

a

Vibrational modes with major contributions from n(C

S). Frequencies correspond to the IR-active B3u and B2u modes, respectively, for [Ni(S2C2H2)2]z, [Ni(S2C2Me2)2]z, and {Ni[S2C2(CN)2]2}z, the IR-active B3u mode for {Ni[S2C2(CF3)2]2}z and [Ni(S2C2Ph2)2]z, and the Raman-active Ag mode for [Ni(S2C2S2CS)2]z. b Vibrational modes with major contributions from n(Ni

S). Frequencies correspond to the IR-active B3u and one or two B2u modes, respectively, except for [Ni(S2C2S2CS)2]z, where the frequencies correspond to the Raman active Ag mode. c Not determined ¼ nd.

and suggests significant ligand-based redox chemistry. However, it is difficult to provide a more quantitative assessment of the extent of ligand-based redox chemistry based on frequencies alone because of vibrational coupling. Normal mode calculations show that n(C

C) couples little with other vibrational modes, and hence the frequency of this mode provides a useful and convenient assessment of the redox state of the dithiolene ligand (2). However, the n(C

S) and n(Ni

S) modes often couple extensively with in-plane deformation modes, rendering the absolute frequencies of dubious value, particularly for comparisons between different complexes (2). In order to overcome this problem, Schla¨ pfer and Nakamoto (2) discussed 0,1

, the trends in Ni

S, C

S, and C


C bond orders in [Ni(S2C2H2)2] 0,1
,2
0,1
,2
1
,2
, [Ni(S2C2Ph2)2] , and {Ni[S2C2(CN)2]2} {Ni[S2C2(CF3)2]2}

222

MICHAEL K. JOHNSON TABLE III ˚
1) for Selected Urey–Bradley Bond Stretching Force Constants (mdyn A a Square-Planar Bis(dithiolene) Ni Complexes

Vibrational Mode

z ¼ 1


z ¼ 2


[Ni(S2C2H2)2] {Ni[S2C2(CF3)2]2}z [Ni(S2C2Ph2)2]z {Ni[S2C2(CN)2]2}z

4.46 5.02 4.44

5.36 5.68 5.30 4.99

6.24 6.12 5.49

[Ni(S2C2H2)2]z {Ni[S2C2(CF3)2]2}z [Ni(S2C2Ph2)2]z {[Ni[S2C2(CN)2]2}z

3.34 2.95 3.24

3.05 2.84 2.93 3.20

2.69 2.88 2.98

[Ni(S2C2H2)2]z {Ni[S2C2(CF3)2]2}z [Ni(S2C2Ph2)2]z {Ni[S2C2(CN)2]2}z

1.46 1.52 1.64

1.34 1.40 1.39 1.53

1.22 1.33 1.27

z

K(C

C)

K(C

S)

K(Ni

S)

a

z¼0

Complex

Data taken from (2).

in terms of the Urey–Bradley force constants for stretching of the Ni

S, C

S, and C

C bonds (see Table III). In accord with the trends in frequencies, K(C

S) and K(C

S) both progres
C) progressively increases, whereas K(Ni
sively decrease, as the charge on the complex increases. The bond orders can be gauged by comparison of Urey–Bradley force constants for pure C

C
C and C
˚
1, respectively), C

S single bonds (2.4–2.6 mdyn bonds (7.4 and 2.5 mdyn A ˚
1) in [Ni(CS3)2]2
that are known to ˚
1), and the Ni

S bonds (1.41 mdyn A A have some double-bond character. Hence, the dinegative complexes are shown to approach an ene-dithiolate formulation and the two-electron reduction of the neutral complexes can formally be represented in terms of the valence bond representations shown in Fig. 7. The vibrational analysis of Schla¨ pfer and Nakamoto (2) has been confirmed and extended based on structural and vibrational studies and DFT calculations of [Ni(S2C2Me2)2]0,1
,2
by Holm and co-workers (13). This series is the only one for which high-resolution structural data is available in all three charge states. For the neutral, monoanionic, and dianionic complexes, the mean C

C

R

S C R C S Ni C S S C R R

2e−

R

S C R C S Ni C S S C R R

2−

Figure 7. Valence bond representation of two-electron reduction of neutral square-planar Ni– bis(dithiolene) complexes.

VIBRATIONAL SPECTRA OF DITHIOLENE COMPLEXES

223

˚ , 1.342(6) A ˚ , and 1.337(7) A ˚, distances progressively decrease [1.365(9) A ˚ , 2.143(1) A ˚ , and respectively], while the mean Ni

S distances [2.128(1) A ˚ , respectively] and mean C
˚ , 1.737(4) A ˚ , and 2.179(4) A
S distances [1.714(1) A ˚ , respectively] progressively increase. Similar trends have been 1.761(4) A reported for the neutral and monoanionic forms of [Ni(S2C2Ph2)2]0,1
(15–17) and the monoanionic and dianionic forms of {Ni[S2C2(CN)2]2}1
,2
(18). The bond lengths in [Ni(S2C2Me2)2]2
are in close agreement with mean 2 ˚ ] and S
˚]
C(sp2) distances [1.751(17) A (sp2)C

C(sp ) distances [1.331(9) A in the crystallographic database [Chapter 2 in this volume (19)]. Hence, the enedithiolate formulation appears to be a particularly good approximation for [Ni(S2C2Me2)2]2
. This conclusion is borne out by the vibrational data, which
1
S) indicate the highest n(C

C) frequency (1594 cm ) and lowest n(C

1 frequency (744 cm ) of any of the dianionic complexes investigated thus far (see Table II). The DFT calculations indicate that the electroactive orbital (5b2g ) is highly delocalized and has mainly Sð3pz Þ character for all members of the [Ni(S2C2Me2)2]0,1
,2
series of complexes, making the redox reactions largely ligand-based events. However, the metal d-orbital contribution to this orbital is not constant and increases from 13 to 20 to 39% on going from the neutral to the monoanionic to the dianionic forms of [Ni(S2C2Me2)2]0,1
,2
. Hence, the first reduction appears to almost saturate the ligand capacity and a significant change in orbital composition must occur to accommodate the second electron. This conclusion is borne out by the four times larger decrease in the C

C distance and two times larger increase in the n(C

C) frequency (see Table II) for the redox transition between [Ni(S2C2Me2)2]0,1
compared to the redox transition between [Ni(S2C2Me2)2]1
,2
. Predominantly ligand-based reduction is also apparent for other members of the family of [Ni(S2C2R2)2]0,1
,2
complexes on the basis of the force constant data in Table III. For example, the C

C stretching force constants increase by 13 and 10% on going from the neutral to monoanionic and monoanionic to dianionic forms of {Ni[S2C2(CF3)2]2}0,1
,2
, respectively, and by 19 and 15% on going from the neutral to monoanionic and monoanionic to dianionic forms of [Ni(S2C2Ph2)2]0,1
,2
, respectively. However, with R group substituents such CF3, Ph, and CN, which are net electron withdrawing via inductive or resonance effects, the vibrational results suggest that reduction to the dianion does not saturate the ligand reduction capacity, since the stretching force constant and frequencies for the C

C bond are significantly lower than that of a pure double bond. Only with R ¼ Me, which is weakly electron donating, is the ligand redox capacity exceeded on reduction to the dianion form leading to C

C stretching frequencies and bond lengths characteristic of a pure double bond, enedithiolate formulation. Further evidence for predominantly ligand-based reduction in {M[S2C2(CN)2]2}1
,2
,3
complexes has been provided by monitoring the

224

MICHAEL K. JOHNSON

2220 Pt Ni 2210 Wavenumber(cm−1)

Fe

2200 Pd

Co Mn

2190

Zn

Au Cu 2180

−1

−2 −3 Charge per [M(mnt)2]

1
,2
,3

Figure 8. Frequencies of the principal IR component of n(C


N) in {M[S2C2(CN)2]2} complexes (M ¼ Mn, Fe, Co, Ni, Cu, Zn, Pd, Pt, Au) as a function of charge on the complex. [Reproduceed by permission of the Royal Society of Chemistry (20).]



n(C


N) frequency in IR spectroelectrochemical studies (20). The C


N stretch
is only weakly coupled to other vibrations. Consequently, the n(C


N) frequency provides a direct monitor of p electron density on the dithiolene chelate ring and thereby is an indirect assessment of the metal–ligand p interactions. Reduction results in increased p-electron density on the dithiolene chelate ring,
and therefore is expected to result in a decrease in the n(C


N) frequency, as CN * is a p acceptor ligand by virtue of its p orbitals. This expectation is borne out by
the experimental studies with an 15 cm
1 decrease in n(C


N) on reduction of the monoanion to the dianion (M ¼ Fe, Co, Ni, Cu, Pd, Pt, Au) and an additional 12 cm
1 decrease on reduction of the dianion to the trianion (M ¼ Cu, Au) (see Fig. 8). The superconducting ability of [M(dmit)2] complexes, see Fig. 9, has prompted experimental and theoretical vibrational studies of [Ni(dmit)2]z and [Pd(dmit)2]z complexes (z is in the range 0–2
), in order to understand the mechanism of superconductivity in terms of electron-intramolecular and electron-intermolecular vibrational couplings (14, 21). These studies have

VIBRATIONAL SPECTRA OF DITHIOLENE COMPLEXES

225

Figure 9. Calculated frequencies and schematic depiction of the Ag vibrational modes of [Ni(dmit)2]. [Adapted from (21)]. The vibrational assignments for [Ni(dmit)]z and [Pd(dmit)2]z for z ¼ 0, 0.5
, 1
, 2
that are indicated to the right of the figure are taken from (14).

226

MICHAEL K. JOHNSON

focused on characterization of the eight totally symmetric Ag fundamental modes, since antisymmetric combinations of these modes couple with low-lying intermolecular electronic charge-transfer transitions in the solid state. Totally symmetric modes were identified in the Raman spectrum based on resonance enhancement and depolarization ratios (14) and were assigned based on previous vibrational studies of square-planar bis(dithiolene) metal complexes and ab initio calculations of the frequencies of the normal modes (21). The calculated frequencies are in good agreement with the observed frequencies after allowing for the 10% overestimation of the vibrational frequencies by the computational methods used in this work (21). Depictions of the Ag modes together with calculated frequencies for [Ni(dmit)2] and assignments for [Ni(dmit)2]z (z ¼ 0, 0.5
, 1
, 2
), and [Pd(dmit)2]z (z ¼ 0.5
, 1
, 2
) are shown in Fig. 9. Remarkably, the frequencies of the Ag modes for both [Ni(dmit)2]z and [Pd(dmit)2]z correlate linearly with charge and exhibit the same trends as for the simpler Ni bis(dithiolene) complexes discussed above for the n(C

S), n(M

S) modes (see Table II). Moreover, the n(C

C), n(C

C) and n(C

S) frequencies are the lowest and highest, respectively, compared to the equivalent [Ni(S2C2R2)2]z complexes with same overall charge on the complex (see Table II) in accord with dmit exhibiting the most extensive p delocalization. Resonance Raman excitation profiles of the most intense Raman bands at 1440, 520, and 320 cm
1 in [Ni(dmit)2]2
(acetonitrile solution) and at 1420, 530, and 345 cm
1 in [Zn(dmit)2]2
(chloroform solution) have been measured in the 450–700-nm region (21). For both complexes, these modes were assigned
S) modes, respectively, and each was found to to Ag n(C

C), n(ring), and n(M
exhibit an enhancement maximum close to the maximum of the intense lowenergy absorption bands centered at 595 and 515 nm in [Ni(dmit)2]2
and [Zn(dmit)2]2
, respectively, indicating an A-term enhancement mechanism. On
S) modes, the basis of the assignment of the 345- and 320-cm
1 bands to n(M
both transitions were therefore considered to have MLCT character, albeit with smaller metal contribution for the Zn complex. Indeed, analysis via timedependent theory was used for quantitative assessment of the increases in the ˚ for Zn
M

S bond lengths in the electronic excited state, 0.016 and 0.027 A
S and Ni

S, respectively (23). This result was particularly surprising for [Zn(dmit)2]2
, since the visible electronic transition centered at 515 nm had previously been assigned to a ligand-based p ! p* transition, based on the observation of an analogous transition in the sodium salt of the ligand (24). The more recent vibrational assignments for [Ni(dmit)2]2
discussed above and shown in Fig. 9, facilitate resolution of this confusing situation since the 320-cm
1 band in [Ni(dmit)2]2
and, by analogy, the 345-cm
1 band in [Zn (dmit)2]2
are both assigned to in-plane Ag ligand deformation modes. Hence the 515-nm absorption band in [Zn(dmit)2]2
is assigned to a pure ligand-based p ! p* transition. In contrast, the 595-nm band in [Ni(dmit)2]2
appears to have some MLCT

VIBRATIONAL SPECTRA OF DITHIOLENE COMPLEXES

227

character, since the Ag n(Ni

S) mode at 351 cm
1 is also enhanced in resonance with this transition (14, 25), although the excitation profile for this transition has yet to be reported. Vibrational studies of [M(Et2timdt)2] (M ¼ Ni and Pd; Et2timdt ¼ monoanion of 1,3-diethylimidazolidine-2,4,5-trithione), which are related to the [M(dmit)2] class of dithiolenes (see Fig. 9), have also been reported in the M

S stretching region (26). While the published assignments are confusing due to incorrect symmetry labels for some of the in-plane bending modes, vibrational assignments can be made by analogy with the assignments for [Ni(dmit)2] discussed above. These complexes have intense absorptions in the near-IR region centered at 1000 nm, and RR studies using excitation into this absorption envelop reveal bands at 327 and 435 cm
1 for [Ni(Et2timdt)2] and at 341 and 431 cm
1 for [Pd(Et2timdt)2]. These bands are assigned to the Ag n(Ni

S) and ligand deformation modes, respectively, which occur at 364 and 343 cm
1 in [Ni(dmit)2] (see Fig. 9). The higher frequency for the ligand deformation mode, which involves out-of-phase deformations of the peripheral and metal-containing five-membered rings, is presumably a consequence of the mass effect of replacing S with N in the peripheral ring. The corresponding out-of-phase IR-active B2u components of this in-plane ligand deformation mode are observed at 435 and 428 cm
1 in the IR spectra of [Ni(Et2timdt)2] and [Pd(Et2timdt)2], respectively. The asymmetric B2u and B3u n(M

S) modes are tentatively assigned to broad bands at 378 and 392 cm
1 for [Ni(Et2timdt)2] and [Pd(Et2timdt)2], respectively. Relatively few studies have addressed the effect of changing the metal on the vibrational frequencies of square-planar bis(dithiolene) metal complexes. The most comprehensive study reported IR frequencies for {M[S2C2(CN)2]2}1
with M ¼ Ni, Pd, Pt, Co, and Cu, and {M[S2C2(CN)2]2}2
with M ¼ Ni, Pd, Pt, Co, Cu, and Zn (3). Although the spectra were not assigned in detail, the ligandbased modes appear relatively insensitive to the nature of the metal. For example, in the dianionic series with M ¼ Ni, Pd, Pt, Co, Cu, and Zn, the
n(C

S) frequencies were observed in the range 2190–
C), and n(C


N), n(C
2206, 1472–1486, and 859–866 cm
1. A more detailed IR spectroelectrochem1
,2
,3

complexes ical study of the n(C


N) frequencies in {M[S2C2(CN)2]2} (M ¼ Mn, Fe, Co, Ni, Cu, Zn, Pd, Pt, Au) was reported by Best et al. (20) (see
Fig. 8). On progressing from Ni to Pd and from Pd to Pt, the n(C


N)
1 frequencies increase by 0 and 3.8 cm , respectively, for the monoanions and by 2.9 and 1.9 cm
1, respectively, for the dianionic forms. These small increases are indicative of increased metal–ligand p-back-donation concomitant with the increasing size of the metal d-orbitals. However, the opposite trend is observed for Cu and Au complexes, with slightly higher frequencies for Au complexes in both the monoanionic and dianionic forms. This anomalous result was rationalized in terms of a change in the highest occupied molecular orbital

228

MICHAEL K. JOHNSON

(HOMO) for the Cu and Au complexes, compared to the Ni, Pt, and Pd, complexes (20). The most pronounced effects are expected for the M

S stretching modes due to changes in the metal d–ligand p orbital overlap. However, the IR active M

S modes have only been rigorously assigned based on metal isotope shifts for the Ni complexes (2). Moreover, evaluating trends in IR-active M

S stretching modes in terms of changes in the M

S bond strength must take into account the mass of the metal, since the metal moves in the asymmetric stretching modes
S stretching modes (see Fig. 4). Hence, identification of the Ag and B1g M
provides the best method for investigating changes in M

S bonding. In accord with the IR results, Clark and Turtle (4) observed only small changes in the ligand-based vibrational frequencies in the Raman spectra of {M[S2C2(CN)2]2}2
with M ¼ Ni, Pd, and Pt. In contrast, the frequencies of the Raman-active n(M

S) vibrations were found to increase markedly on progres
S), Ni (335 cm
1) < Pd (349 cm
1) < Pt sing from M ¼ Ni to Pd to Pt: Ag n(M

1
S), Ni (283 cm
1) < Pd (292 cm
1) < Pt (310 cm
1) (4), (378 cm ); B1g n(M
reflecting increasing metal d–ligand p orbital overlap. A similar result is evident
S) modes increases from 327 to for [M(Et2timdt)2], for which the Ag n(M
341 cm
1 on going from M ¼ Ni to Pd (26). The much smaller increases of 4–5 cm
1 that are observed for [M(dmit)2]1
,2
on progressing from M ¼ Ni to Pd (see Fig. 9) indicate decreased metal d–ligand p interaction. This result is likely to be a consequence of the more extensive ligand p delocalization that characterize this interesting ligand.

III.

SIX-COORDINATE TRIS(DITHIOLENE) METAL COMPLEXES

Vibrational spectroscopy has not been extensively used in the characterization of tris(dithiolene) metal complexes. Moreover, complete assignments based on both IR and Raman spectra, isotope shifts, and normal mode calculations are not available for any individual complex. However, the available data suggest that the trends in M

S and dithiolene ligand vibrational modes as a function of the metal, the charge on the complex, and dithiolene substituents, closely parallel those discussed above for square-plane bis(dithiolene) metal complexes. Accordingly, the vibrational data are consistent with highly delocalized complexes with predominantly ligand-based redox chemistry. Tris(dithiolene) metal complexes have trigonal-prismatic (D3h ) coordination geometry with varying degrees of trigonal twist distortion to yield structures with coordination geometry intermediate between trigonal prismatic and octahedral corresponding to D3 symmetry [see Chapter 2 in this volume (19)]. Under idealized D3h symmetry, the 51 normal modes of a trigonal-prismatic

VIBRATIONAL SPECTRA OF DITHIOLENE COMPLEXES

S S

S

S M

S

S

S

S M

S

S

A 1′ Raman

S S

S

A 2′′ IR

S

S M

S

S S

S

S

229

S M

S

E′ Raman/IR

S S

E′′ Raman

Figure 10. Schematic depiction of the stretching modes of a D3h MS6 unit and their IR–Raman activity.

MðC2 S2 R2 Þ3 complex can be classified as 6A01 þ 2A02 þ 3A001 þ 6A002 þ 9E0 þ 8E00 , with one A01 , A002 , E0 , and E00 mode corresponding to the stretching vibrations of the MS6 unit (see Fig. 10). The A01 , E0 , and E00 modes are Raman active and the A002 and E0 modes are IR active. As for the square-planar bis(dithiolene) complexes, the vibrational modes located primarily on the dithiolene ligands are expected to exhibit accidental degeneracy with analogous frequencies for IR and Raman active components, provided there is no significant kinematic or electronic coupling between the dithiolene rings. However, both IR and Raman spectra and metal isotope shift data are certainly required for complete analysis of the M

S stretching modes irrespective of the extent of distortion from idealized D3h symmetry. As illustrated by the schematic D3h /D3 /Oh correlation diagram shown in Fig. 11, a trigonal twist distortion to yield effective D3 symmetry results in IR and Raman activity for both E modes, while preserving only IR activity for the A2 mode and only Raman activity of the A1 mode. The relative frequencies of the M

S stretching modes in tris(dithiolene) complexes can be assessed based on IR and Raman studies and normal coordinate analysis of the NbS6 unit in the structurally characterized tris(ethanedithiolate) Nb(V) complex, [Nb(S2C2H4)3]1
(27). The complex has D3

IR T 1u R R

Eg A 1g

A2 E E A1

IR IR,R R

A2

A 2" IR

E E A1

E' IR,R E'' R A 1' R

M

M

M

Oh

D3

D3h

Figure 11. Correlation diagram and IR–Raman activity for stretching modes of a MS6 unit under Oh , D3 , and D3h symmetry.

230

MICHAEL K. JOHNSON

Figure 12. Far-IR ðaÞ and Raman ðbÞ spectra of [NEt4][Nb(S2C2H4)3]. [Reprinted with permission from K. Tatsumi, I. Matsubara, Y. Sekiguchi, A. Nakamura, and C. Mealli, Inorg. Chem., 28, 773 (1989). Copyright # 1989 American Chemical Society.]

symmetry, with a trigonal twist (2y) of 30 and a bidentate bite (b) of 1.31 as defined by Kepert (28). Far-IR and Raman spectra of [NEt4] [Nb(S2C2H4)3] are shown in Fig. 12 and the intense Raman band at 348 cm
1 is readily assigned
S) mode, while the two closely spaced IR bands at 354 and to the A1 n(Nb
338 cm
1 are assigned to the A2 and E modes (derived from the degenerate T1u mode under idealized Oh symmetry). Vibrational analysis based on the GF matrix method was used to assess how the M

S vibrational frequencies vary with the trigonal twist of the MS6 skeleton, using force constants derived from fitting the Raman A1 and the IR A2 frequencies for the structurally derived geometric parameters (see Fig. 13). A trigonal-prismatic MS6 polyhedron

VIBRATIONAL SPECTRA OF DITHIOLENE COMPLEXES

231

Figure 13. Calculated n(M

S) frequencies for trigonal-prismatic MS6 unit as a function of trigonal-twist distortion. Calculation based on a Urey–Bradley force field for M ¼ Nb with the ˚
1 and F ¼ 0.22 mdyn A ˚
1. [Reprinted with permission bidentate bite b ¼ 1.31, K0 ¼ 1.57 mdyn A from K. Tatsumi, I. Matsubara, Y. Sekiguchi, A. Nakamura, and C. Mealli, Inorg. Chem., 28, 773 (1989). Copyright # 1989 American Chemical Society.]

corresponds to y ¼ 0 , but an ideal octahedron is not obtained at y ¼ 30 , since the bidentate bite angle 6¼ 90 (i.e., b 6¼ 2) [see Chapter 2 in this volume (19)]. This analysis predicts the E modes to occur at 342 and 288 cm
1 for y ¼ 15 , which compare well with the observed IR band at 338 cm
1 and the Raman band at 285 cm
1. Hence, for tris(dithiolene) complexes of second- or third-row transitional metals with trigonal-prismatic (D3h ) or trigonally twisted trigonalprismatic (D3) coordination geometry, the frequencies of the MS6 stretching are expected to be A002 ðA2 Þ > A01 ðA1 Þ > E0 ðEÞ > E00 ðEÞ with the higher energy E mode being more intense in the IR and the lower energy E mode being more intense in the Raman. The above analysis suggests that the separation (A2
E) of the dominant M

S stretching modes in the IR spectrum has the potential to provide a

232

MICHAEL K. JOHNSON

convenient measure of the magnitude of the trigonal twist distortion in tris(dithiolene) metal complexes. Although this may be an oversimplification, because vibrational coupling between dithiolene modes is not included in this analysis, it is noteworthy that the n(M

S) (A2
E) IR separation is much larger in [Nb(S2C2H2)3]1
(49 cm
1) and [Ta(S2C2H2)3]1
(47 cm
1) (27) than in {V[S2C2(CN)2]3}2
(20 cm
1). The former two complexes are expected to have trigonal-prismatic coordination geometry by analogy with the structurally characterized, isoelectronic complex [Mo(S2C2H2)3] (y ¼ 0 ) (29), while the latter complex has pronounced trigonal-twist distortion (y ¼ 17 ) (30). More extensive far-IR studies of crystallographically defined tris(dithiolene) metal complexes with M

S stretching mode assignments based on metal isotope shifts are clearly needed to test the validity of this correlation. As with the square-planar bis(dithiolene) complexes, the best characterized tris(dithiolene) metal complexes in terms of vibrational properties involve the 1,2-dicyanoethylene-1,2-dithiolate ligand. Resonance Raman studies of {M[S2C2(CN)2]3}2
(M ¼ V, Mo, W) have provided vibrational assignments and insight into electronic structures via excitation profiles (31) and spectroelectrochemical IR studies of {M[S2C2(CN)2]3}z (M ¼ V, Cr, Mn, Fe, Mo, W,
Re; z ¼ 1
, 2
, 3
, and 4
) have investigated trends in n(C

S)

N) and n(M
frequencies as a function of the metal and the charge on the complex (32, 33). In contrast to the RR spectra of square-planar bis(dithiolene) complexes, which are dominated by dithiolene ligand modes (see Fig. 6), the RR spectra of {M[S2C2(CN)2]3}2
(M ¼ V, Mo, W) using visible excitation are dominated by M

S vibrations in the 250–400-cm
1 region [see Fig.14(a)]. This result indicates more S-to-M CT character for the visible electronic transitions, which in turn suggests higher formal oxidation states for the metals. Three or four medium-to-strong Raman bands are generally enhanced in the M

S stretching region of {M[S2C2(CN)2]3}2
complexes and the strongest band is invariably polarized (depolarization ratios, r ¼ 0.14–0.17), and therefore assigned to the A1 n(M

S) vibration. On this basis, the bands at 314 cm
1 in {V[S2C2(CN)2]3}2
, 353 cm
1 in {Mo[S2C2(CN)2]3}2
, and 367 cm
1 in {W[S2C2(CN)2]3}2
have
S) and the increasing frequencies for first-, second-, been assigned to A1 n(M
and third-row transition metals are attributed to increasing metal–ligand p interaction (31). Excitation profiles for these modes in the 450–700-nm region parallel the absorption spectra for the lowest energy bands, indicating A-term enhancement via low energy LMCT bands. A more detailed discussion of the excitation profiles and their implications for electronic excited-state assignments can be found in Chapter 3 in this volume (1). More complete assignment of the M

S stretching modes in {M[S2C2(CN)2]3}2
(M ¼ V, Mo, W) requires parallel far-IR studies and/or metal isotope shift data, which are not available for the Mo and W complexes. Far-IR spectroelectrochemical studies of {V[S2C2(CN)2]3}2
revealed redox

VIBRATIONAL SPECTRA OF DITHIOLENE COMPLEXES

233

Figure 14. Solid-state RR spectra of (a) {V[S2C2(CN)2]3}2
(647-nm excitation), (b) [V[S2C2Ph2]3]1
(568-nm excitation), and ðcÞ [V[S2C2Ph2]3] (568-nm excitation). [Reproduced by permission of The Royal Society of Chemistry (31).]

sensitive bands at 354, 334, and 265 cm
1, which are logically assigned to n(M

S) modes (33) (see Fig. 15). Since depolarized Raman bands are observed at 350 and 271 cm
1 (31), it seems reasonable to assign the corresponding IR
S), the IR band at 334 and Raman bands near 350 and 270 cm
1 to E n(M

S), and the Raman band at 314 cm
1 to A1 n(M

S). The cm
1 to A2 n(M
frequency for the A1 mode relative to E and A2 modes, compared to the secondand third-row transition metal complexes discussed above, is readily rationalized in terms of the mass effect of the metal (see Fig. 10).

234

MICHAEL K. JOHNSON

Figure 15. Reduction- and/or oxidation-induced changes in the far-IR spectra of {M[S2C2(CN)2]3}2
(M ¼ V, Mn, Re) as determined by spectroelectrochemistry in dichloromethane. [Reprinted with permission from S. P. Best, R. J. Clark, R. C. S. McQueen, and J. R. Walton, Inorg. Chem., 27, 884 (1988). Copyright # 1989 American Chemical Society.]

Extensive vibrational data are available for assessing the trends in the IR z
n(C


N) frequencies in {M[S2C2(CN)2]3} (M ¼ V, Mn, Re, Cr, Fe, Mo, W; z ¼ 1
, 2
, 3
, 4
) complexes as a function of charge on the complexes (32,
33) (see Table IV). For first-row transition metals, the n(C


N) frequency

TABLE IV IR Frequencies (cm
1) for n(CN) Modes in {M[S2C2(CN)2]3}z Complexesa Mode

Metal

n(CN)

V Cr Mn Fe Mo W Re

a

z ¼ 1
2216, 2224(sh) 2214

2216, 2224(sh) 2217, 2225(sh) 2216, 2226(sh)

Data taken from (32 and 33).

z ¼ 2
2203, 2215(sh) 2203 2201, 2211(sh) 2200, 2211(sh) 2203, 2213(sh) 2202, 2213(sh) 2203, 2218(sh)

z ¼ 3
2188 2191 2192, 2205(sh) 2185 2178 2170, 2193(sh) 2179, 2200(sh)

z ¼ 4
2164

2134 2133

VIBRATIONAL SPECTRA OF DITHIOLENE COMPLEXES

235

decreases by 11–13 cm
1 on reduction from the monoanion to the dianion, by an additional 9–15 cm
1 on reduction from the dianion to the trianion, and by an additional 24 cm
1 on reduction from the trianion to the tetraanion in the V complex. Larger progressive decreases are observed for second- and third-row transition metals: 13–15 cm
1 on reduction from the monoanion to the dianion; 25–32 cm
1 on reduction from the dianion to the trianion; 44–46 cm
1 on reduction from the trianion to the tetraanion. As for the square-planar bis (dithiolene) complexes, the progressive decrease in the n(C

N) frequency on reduction is best interpreted in terms of predominantly ligand-based reduction resulting from CN acting as a p-acceptor ligand by virtue of its p* orbitals. The larger decreases in n(C

N) frequencies on reduction of complexes with secondand third-row transition metals are accordingly rationalized in terms of increased metal–ligand p interaction. Compared to square-planar bis(dithiolene) complexes (Table II), there have been fewer systematic studies of the trends in n(C

S), and n(M

S)
C), n(C
vibrational modes in [M(S2C2R2)3]z complexes as a function of M, z, and R. Moreover, the assignments of the bands attributed to predominantly C

S and M

S stretching vibrations must be considered tentative, since these modes are likely to be extensively mixed with in-plane ring deformation modes and the available assignments are not based on normal mode calculations or metal isotope shifts. With this caveat in mind, the available data for [M(S2C2R2)3]z complexes are summarized in Table V. The majority of data is based on solidstate (KBr) IR studies and is taken from the seminal paper by Schrauzer and Mayweg (34), with the assignment of the predominantly C

S stretching mode modified to be consistent with the subsequent more detailed studies of squareplanar bis(dithiolene) nickel complexes conducted by Schla¨ pfer and Nakamoto (2). The data for the IR-active n(M

S) frequencies in {M[S2C2(CN)2]3}z (M ¼ V, Mn, Re; z ¼ 1
, 2
, 3
) are taken from the far-IR spectroelectrochemical studies shown in Fig. 15 (33). Unfortunately, it was not possible to monitor redox trends in the n(C

S) modes in {M[S2C2(CN)2]3}z
C) and n(C
complexes via spectroelectrochemical IR studies due to the presence of strong absorption from the solvent or supporting electrolyte. The Raman data are taken from the RR studies of Clark and Turtle (31) (see Fig. 14). Given the limitations of the available data (Table V), the trends in the n(C

S), and n(M

S)
C), n(C
frequencies in tris(dithiolene) complexes as a function of the charge on the complex, the metal, and the nature of terminal dithiolene substituent, are generally in good agreement with those discussed above for square-planar bis(dithiolene) complexes. In particular, the increase in n(C

C) with concomitant decreases in n(C

S) and n(M

S) as the charge on the complex increases, is indicative of ligand-based reduction resulting in decreased electron delocalization over the chelate ring and increased ene-dithiolate character of the dithiolene ligands (see Fig. 7).

236

MICHAEL K. JOHNSON TABLE V Dithiolene Vibrational Frequencies (cm
1) in [M(S2C2R2)3]z Complexesa

Mode
n(C


C)

a

z¼0

z ¼ 1


[V(S2C2H2)3]z [Mo(S2C2H2)3]z [W(S2C2H2)3]z [Re(S2C2H2)3]z [V(S2C2Ph2)3]z [Cr(S2C2Ph2)3]z [Mo(S2C2Ph2)3]z [W(S2C2Ph2)3]z [Re(S2C2Ph2)3]z {V[S2C2(CN)2]3}z {Mo[S2C2(CN)2]3}z {W[S2C2(CN)2]3}z {Mo[S2C2(CO2Me)2]3}z {Mo[S2C2(CF3)2]3}z

1347 1402 1408 1418 1372, 1399 1398 1400 1422 1430

1416

z

n(C

S)

n(M

S)

Complex

[V(S2C2H2)3] [Mo(S2C2H2)3]z [W(S2C2H2)3]z [Re(S2C2H2)3]z [V(S2C2Ph2)3]z [Cr(S2C2Ph2)3]z [Mo(S2C2Ph2)3]z [W(S2C2Ph2)3]z [Re(S2C2Ph2)3]z {Mo[S2C2(CO2Me)2]3}z b

z

[V(S2C2H2)3] [Mo(S2C2H2)3]z [W(S2C2H2)3]z [Re(S2C2H2)3]z [V(S2C2Ph2)3]z [Cr(S2C2Ph2)3]z [Mo(S2C2Ph2)3]z [W(S2C2Ph2)3]z [Re(S2C2Ph2)3]z {V[S2C2(CN)2]3}z {Mn[S2C2(CN)2]3}z {Mo[S2C2(CN)2]3}z {W[S2C2(CN)2]3}z {Re[S2C2(CN)2]3}z {Mo[S2C2(CO2Me)2]3}z

z ¼ 3


1494

1481 1469 1492 1497 1475, 1488, 1525 1538 849

799

34 34 34 34 34 34 34 34 34 35

824 869

702 385, 361 380, 354 369, 329 338, 333 406, 346, 343 421, 356 403, 356 403, 359 373, 359

392, 363

367, 350

361, 333 398, 349, 351

361, 350 375, 346

340, 325

354, 334, 314 349 354 367 321, 308 365

References 34 34 34 34 31, 34 34 34 34 34 31 31 31 35 36

1450 1428, 1459

1455 894 866 854 856 892 891 878 872 879

z ¼ 2


328

294

34 34 34 34 31, 34 34 34 34 34 31, 34 33 31 31 33 35

Infrared frequencies are in regular type and Raman frequencies are in italic. Assignments for the n(C

S) and n(M

S) modes are tentative, as described in the text. b Infrared modes correspond to A002 and E0 modes and Raman modes correspond to A01 modes under D3h symmetry.

VIBRATIONAL SPECTRA OF DITHIOLENE COMPLEXES

237

Resonance Raman studies of tris(dithiolene) metal complexes have focused exclusively on {M[S2C2(CN)2]3}z complexes with the exception of one study on
S) [NEt4]2{Mo[S2C2(CO2Me)2]3} (35). Comparison of the frequencies n(Mo
2
and n(C

C) modes observed in the RR spectra of {Mo[S2C2(CO2Me)2]3} 2
and {Mo[S2C2(CN)2]3} , using excitation into the intense absorption bands centered near 650 nm, is particularly interesting because {Mo[S2C2(CO2Me)2]3}2
has a coordination geometry close to that of an idealized trigonal prism (37), while {Mo[S2C2(CN)2]3}2
has a trigonal twist of 30 (38). The difference in the geometries of these complexes provides a rationalization of the anomalously
S) at 365 cm
1) and the high frequency for the MoS6 breathing mode [A01 n(Mo

1 observation of multiple n(C

C) modes (1475, 1488, and 1525 cm ) in the RR 2
spectrum of {Mo[S2C2(CO2Me)2]3} (see Table V). The 11-cm
1 upshift of the MoS6 breathing mode in {Mo[S2C2(CO2Me)2]3}2
compared to
S) at 354 cm
1) is at first surprising, since {Mo[S2C2(CN)2]3}2
[A1 n(Mo
˚ the Mo

S bonds are 0.02 A longer in {Mo[S2C2(CO2Me)2]3}2
than in {Mo[S2C2(CN)2]3}2
. However, this finding can be rationalized in terms of a larger S--S interaction within each dithiolene ring in {Mo[S2C2(CO2Me)2]3}2
, ˚ in since the trigonal twist increases the intraligand S--S distance from 3.09 A 2
2
˚ to 3.14 A in {Mo[S2C2(CN)2]3} . In the RR {Mo[S2C2(CO2Me)2]3} spectrum of {Mo[S2C2(CN)2]3}2
, only one band attributable to C

C stretching was observed at 1492 cm
1, in resonance with the weak absorption band centered at 500 nm (31). The assignment of the bands at 1525 and 1488 or 1475 cm
1 in the 647-nm excitation RR spectrum of {Mo[S2C2(CO2Me)2]3}2
to the A01 and E0 n(C

C) modes, respectively, implies significant electronic coupling between the dithiolene rings (35). Kinematic coupling is expected to be negligible since the n(C

C) vibration is not extensively mixed with other modes and the C

C bonds are separated by four bonds. Electronic coupling requires metal–ligand p overlap and is expected to be optimal when the metal dp orbitals are empty as in Mo(IV) complexes. Hence, the absence of significant electronic coupling in square-planar bis (dithiolene) complexes (see above) is rationalized in terms of filled metal dp orbitals and the apparent enhancement in electronic coupling in {Mo[S2C2(CO2Me)2]3}2
compared to {Mo[S2C2(CN)2]3}2
is explained by decreasing metal–ligand p overlap with increasing trigonal twist. The case for highly delocalized frontier orbitals and predominantly ligandbased redox chemistry in tris(dithiolene) metal complexes has been most convincingly made by the recent structural, vibrational, and DFT results for members of the related [M(CO)2(S2C2Me2)2]0,1
,2
and [M(S2C2Me2)3]0,1
,2
(M ¼ Mo, W) series of complexes (39). The latter is the only set of tris(dithiolene) complexes that has been isolated and structurally characterized in three oxidation states. While the vibrational studies focused on IR spectra of the [M(CO)2(S2C2Me2)2]0,1
,2
series of complexes, the close parallel in

238

MICHAEL K. JOHNSON TABLE VI Selected IR Frequencies (cm
1) for [M(CO)2(S2C2Me2)2]0,1
,2
and Selected Mean Bond ˚ ) in [M(CO)2(S2C2Me2)2]0,1
,2
and [M(S2C2Me2)3]0,1
,2
(M ¼ Mo, W)a Lengths (A [Mo(CO)2(S2C2Me2)2]z [Mo(S2C2Me2)3]z ——————————————————— z¼0 z ¼ 1
z ¼ 2


[W(CO)2(S2C2Me2)2]z [W(S2C2Me2)3]z —————————————————— ——— z¼0 z ¼ 1
z ¼ 2


d(M

S) d(M

S)

2.380(1) 2.365(2)

2.408(1) 2.375(2)

2.457(1) 2.397(2)

2.376(1)

2.406(2) 2.376(2)

2.452(1) 2.389(1)



C) d(C


d(C


C) n(C

C)

1.367(4) 1.357(9) 1453

1.345(5) 1.354(11) 1520

1.335(6) 1.334(8) 1586

1.366(7)

1.344(8) 1.357(15) 1546

1.330(9) 1.326(10) 1590

d(C

S) d(C

S) n(C

S)

1.726(3) 1.714(5) 931

1.745(3) 1.725(8) 925

1.758(5) 1.755(5) 901

1.727(6) 930

1.747(6) 1.729(9) 926

1.764(6) 1.758(6) 901


d(C


O) n(C

O)

1.143(4) 2026, 1962

1.159(5) 1952, 1885

1.179(7) 1875, 1757

1.138(10) 2023, 1951

1.155(8) 1942, 1865

1.175(8) 1864, 1742

d(M

C)

2.025(3)

1.979(5)

1.918(7)

2.030(7)

1.979(7)

1.931(6)

a

1472

z

Data taken from (39). Bond lengths in italics are for [Mo(S2C2Me2)3] and [W(S2C2Me2)3]z.

the structural parameters of the dithiolene ligands in corresponding redox states (see Table VI) suggests that vibrational results are likely to be directly applicable to the [M(S2C2Me2)3]0,1
,2
series. On progressing through the [M(CO)2(S2C2Me2)2]0,1
,2
series in the reducing direction, a systematic

C), d(M
trend in vibrational frequencies and bond distances emerges: n(C

S),
d(S

C), and d(C

S), d(M

C) and
O) increase, whereas n(C

O), n(C
d(C

C), d(C

C) decrease (see Table VI). The trends in the d(M

O), and n(C

O) are consistent with the classical concept of CO as a p-acceptor ligand. The same trend in d(M

S), d(S

C), and d(C

C) is observed in the [M(S2C2Me2)3]0,1
,2
series (see Table V). In the most reduced members of the series, [M(CO)2(S2C2Me2)2]2
and [M(S2C2Me2)3]2
, the bond distances and vibrational frequencies are indicative of ligation by the ene-dithiolate form of ligand. For both the [M(CO)2(S2C2Me2)2]0,1
,2
and [M(S2C2Me2)3]0,1
,2
series, DFT calculations identify an electroactive orbital with 80% ligand character, which becomes increasingly occupied on reduction and has nearly constant composition. Overall, the picture that emerges for the [M(CO)2(S2C2Me2)2]0,1
,2
series is very similar to that discussed above for the [Ni(S2C2Me2)2]0,1
,2
series, except that the p-acceptor CO ligands prevent saturation of the ligand reduction capacity on going from the monoanionic to the dianionic forms. Taken together, the vibrational data in Tables V and VI indicate that the
C) frequency correlates with the electron-donating ability of the dithiolene n(C


VIBRATIONAL SPECTRA OF DITHIOLENE COMPLEXES

239

R group substituents (Me > H  Ph > CN). This correlation is further supported by IR studies of tris(dithiolene) tungsten complexes with asymmetrically substituted dithiolenes, [W(S2C2R0 R00 )3] (R0 ¼ H or Ph and R00 ¼ p-Me2NC6H4, p-MeOC6H4, p-MeC6H4, Ph, p-ClC6H4, or p-BrC6H4) (40). In both the R0 ¼ H and Ph series the n(C

C) frequencies increased with the electron-donating ability of the R00 substituents (p-Me2NC6H4 > p-MeOC6H4 > p-MeC6H4 > Ph > p-ClC6H4 > p-BrC6H4). For example, with R0 ¼ H, the n(C

C) frequencies were 1389 ( p-BrC6H4), 1391 ( p-ClC6H4), 1418 (Ph), 1430 (> p-MeC6H4)

C) frequencies were and 1452 cm
1 (p-MeOC6H4), and with R0 ¼ Ph, the n(C

1425 ( p-MeC6H4), 1427 ( p-MeOC6H4), and 1441 cm
1 ( p-Me2NC6H4).

IV.

OXO-METALLO-MONO(DITHIOLENE) COMPLEXES

Although oxo-molybdenum centers coordinated by a single dithiolene ligand constitute the redox-independent core of the active sites of both the xanthine

ppd

S S

S

ppd S

O

Mo

O

S

O

Mo

OH2

S(Cys)

XO family [Se(Cys)] [S(Cys)] O(Ser)

O M

S ppd

SO family

S

ppd

ppd

DMSOR family (M = Mo or W)

O HN H2N

N

W

S

S S

OH2 ?

O

H N N H

S

S S

ppd

AOR family

S− S− OPO32−

O H

ppd

Figure 16. Consensus oxidized active-site structures of the xanthine oxidase (XO), sulfite oxidase (SO), and DMSO reductase (DMSOR), and aldehyde oxidoreductase (AOR) families of mononuclear molybdenum and tungsten enzymes and the structure of the common ppd cofactor (41, 42). The question mark in the AOR structure indicates uncertainty in the presence of a coordinated water molecule.

240

MICHAEL K. JOHNSON

oxidase (XO) and sulfite oxidase (SO) classes of mononuclear molybdenum enzymes (see Fig. 16), the vibrational properties of the dithiolene moieties in appropriate analogue complexes are relatively unexplored. Holm and coworkers recently prepared and characterized a series of excellent structural analogues of the dioxo-Mo(VI) and monooxo-Mo(V) square-pyramidal forms of the sulfite oxidase active site using benzene-1,2-dithiolate (bdt) or 1,2-dimethylethylenedithiolate as the dithiolene ligand and adamantyl-2-thiolate or
SC6H22,4,6-i-Pr3 as the thiolate ligand (43). Vibrational characterization has thus far

O stretching frequencies. The IR been limited to IR identification of the Mo

data clearly demonstrate stronger Mo

O bonds in square-pyramidal mono (dithiolene) complexes than in octahedral bis(dithiolene) complexes. For example, the symmetric ns(Mo

O) and asymmetric nas(Mo

O) stretching frequencies of the cis MoO2 unit in [MoVIO2(SC6H2-2,4,6-i-Pr3)(bdt)]1
are 920 and 885 cm
1, respectively (43), compared to ns(Mo

O) and nas(Mo

O) frequencies of 858 and 831 cm
1, respectively, for [MoVIO2(bdt)2]2
(44). Raman spectra have been reported for oxo-bridged [{WO2(bdt)}2(m-O)]2
and [{WO2(ndt)}2(m-O)]2
anions (bdt ¼ benzene-1,2-dithiolate; ndt ¼ naphthalene-2,3-dithiolate) (45), which contain the square-pyramidal core of the dioxo-W(VI) form of the W-substituted form of SO (46) (see Fig. 16). However, analysis has thus far been limited to identification of the ns(W

O) mode at 953 cm
1 in [{WO2(bdt)}2(m-O)]2
and at 946 cm
1 in [{WO2(ndt)}2(m-O)]2
and tentative assignment of the bands at 367 cm
1 in [{WO2(bdt)}2(m-O)]2
and at 365 and 334 cm
1 in [{WO2(ndt)}2(m-O)]2
to W

S stretching modes. Once again the data attest to stronger W

O bonds than

O) was observed at in the octahedral bis(dithiolene) counterparts, since ns(W

885 and 883 cm
1 in the Raman spectra of [WO2(bdt)2]2
and [WO2(ndt)2]2
, respectively (47, 48). Moreover, a comparison of Mo

O and W

O stretching frequencies for the equivalent mono(dithiolene) or bis(dithiolene) W and Mo complexes indicates significantly stronger W

O than Mo

O bonds. The effects of changes in the metal (M) and the number of dithiolene ligands on the strength of the M

O bonds are readily rationalized in terms of stronger M

O bonding as a result of either improved dp –pp orbital overlap for W compared to Mo and the replacement of a bidentate dithiolene with a monodentate S or O ligand resulting in one M

O bond that is not weakened by the trans influence of a dithiolene S ligand (see below). The only serious attempt to assign Mo

S vibrational modes in oxomolybdenum mono(dithiolene) complexes has come from RR studies of LMoO(bdt) and LMoO(tdt) (L ¼ hydrotris-(3,5-dimethyl-1-pyrazolyl)borate; tdt ¼ toluene-1,2-dithiolate), which provide six-coordinate analogues of the square-pyramidal Mo(IV) form of SO with a spectroscopically innocent facial tridentate ligand ensuring cis arrangement of the oxo and dithiolene ligands (49). Intense bands at 932 and 926 cm
1 in the IR spectra of LMoO(bdt) and
O stretching modes. Three LMoO(tdt), respectively, were assigned to Mo


VIBRATIONAL SPECTRA OF DITHIOLENE COMPLEXES

241

modes were enhanced with 514-nm excitation in the RR spectra: 362, 392, and 931 cm
1 for LMoO(bdt); 342, 376, 926 cm
1 for LMoO(tdt). On the basis of qualitative depolarization ratios for LMoO(bdt), r ¼ 0.40, 0.22, and 0.01 for the bands at 342, 376, 926 cm
1, respectively, all three modes were attributed to totally symmetric A0 modes, under the effective Cs symmetry. The near-coincident excitation profiles for the 362- and 392-cm
1 bands, with maxima close to the shoulder in the absorption spectrum at 19,000 cm
1, are also consistent with assignment to totally symmetric modes that are enhanced by an A-term mechanism. Hence, the two low-frequency bands at 362 and 392 cm
1 were assigned to the symmetric bending and stretching modes of the MoS2 unit, respectively. The frequency of the symmetric bending mode of the MoS2 unit is high compared to the well-characterized square-planar bis(dithiolene) complexes involving simple dithiolene ligands (see Table I). However, by analogy with the vibrational studies of the [M(dmit)2]z (M ¼ Ni or Pd; z ¼ 0, 1
, 2
) complexes (Fig. 9), the high frequency for the symmetric bending mode may be a consequence of extensive p delocalization over both the dithiolene and benzene rings, and hence may be unique to bdt and related dithiolene ligands involving fused aromatic ring systems. Although the Mo

S vibrational assignments in LMoO(bdt) and LMoO(tdt) require confirmation via 34S isotope shifts, far-IR spectra, and normal mode calculations, they have provided the basis for assigning the electronic spectrum on the basis of the Raman excitation profiles. Hence, the 19,000-cm
1 absorption band has been assigned to the dithiolate S pin-plane ! Mo dxy bonding– antibonding transition. This unique three-center pseudo-s bonding interaction between in-plane dithiolate S p orbitals and the Mo dxy redox orbital is proposed to be a dominant contributor to the electronic structure of the enzyme active sites by modulating the valence ionization energy of the metal acceptor orbital (49). The excitation profile of the Mo

O stretching mode has a maximum near 22,000 cm
1 and enhancement is attributed to an A-term mechanism involving dithiolate S pout-of-plane ! Mo dxz ; dyz transitions, since these transitions increase electron density in Mo-based orbitals that are strongly antibonding with respect to the Mo

O bond (49). A more detailed discussion of the electronic assignments for this complex can be found in Chapter 3 in this volume (1).

V.

OXO-METALLO-BIS(DITHIOLENE) COMPLEXES

Numerous oxo-molybdo-bis(dithiolene) and oxo-tungsto-bis(dithiolene) complexes have been synthesized and characterized as potential structural analogues of the active sites of the dimethyl sulfoxide reductase (DMSOR) and aldehyde oxidoreductase (AOR) families of mononuclear Mo and W enzymes [see Fig. 16 and Chapter 10 in this volume (50)]. The available IR and Raman data for the Mo and W complexes are summarized in Tables VII and VIII,

242

MICHAEL K. JOHNSON TABLE VII Vibrational Frequencies (cm
1) for Oxo-molybdo-bis(dithiolene) Complexesa

Mo(VI) {MoO2[S2C2(CN)2]2}2
{MoO2[S2C2(CO2Me)2]2}2
{MoO2[S2C2(CO2NH2)2]2}2
[MoO2(S2C6H4)2]2
[MoO2(S2C6H3Me)2]2
[MoO2(S2C6H3SiPh3)2]2
[MoO(OPh)(S2C2Ph2)2]1
Mo(V) [MoO(S2C2Ph2)2]1
[MoO(S2C2Me2)2]1
[MoO(S2C6H4)2]1
{MoO[S2C6H3SiPh3]2}1
Mo(IV) {MoO[S2C2(CN)2]2}2
{MoO[S2C2(CO2Me)2]2}2
{MoO[S2C2(CO2NH2)2]2}2
[MoO(S2C6H4)2]2
[MoO(S2C2Me2)2]2
a b

354, 330, 313, 320 378, 356, 381, 358, 363,

336 322 329, 318 328


O)b n(Mo




C) n(C



n(Mo

S)

Complex

1472, 1471 1503 1468

377, 356, 344 365 344, 335 393 367 356

1491, 1482 1535, 1530 1540, 1530

885, 851, 890, 870, 838 907, 867 858, 829, 858, 863, 835, 864, 858, 827, 856, 917(43)

References

855

831 838 827

47, 51 46 52 44, 47 44, 47 44 43, 53

926 910 940(44), 934 937, 930

54 55 56–58 56

948, 928 910 948, 937 904 889

47, 51 35 52 47 55

Infrared frequencies are in regular type and Raman frequencies are in italic type. The 18O downshifts are given in parentheses. TABLE VIII Vibrational Frequencies (cm
1) for Oxo-tungsto-bis(dithiolene) Complexesa

Complex W(VI) {WO2[S2C2(CN)2]2}2
{WO(S2)[S2C2(CN)2]2}2
[WO2(S2C2Me2)2]2
[WO2(S2C6H4)2]2
[WO2(S2C10H6)2]2
[WO(OPh)(S2C2Ph2)2]1
[WO(Oi-Pr)(S2C2Ph2)2]1
W(V) [WO(S2C2Me2)2]1
[WO(S2C2Ph2)2]1
[WO(S2C6H4)2]1
[WO(S2C6H3SiPh3)2]1
W(IV) {WO[S2C2(CN)2]2}2
[WO(S2C2Me2)2]2
[WO(S2C2Ph2)2]2
[WO(S2C6H4)2]2
[WO(S2C10H6)2]2
a b

n(W

S)

n(C

C)

b n(W

O)

310

1476 1470

906, 860 920 876, 833 885, 843, 888, 847 883, 832, 884, 835 895(47) 885

59 59 60 47, 48 48 61 61

896 940 954, 953 944, 943

55 62 56 56

935 897 886 905, 906 920

59 62 62 47, 48 47

370, 352, 326 368, 357, 339

369 371 325

370 369

1483

Infrared frequencies are in regular type and Raman frequencies are in italic. The 18O downshifts are given in parentheses.

References

VIBRATIONAL SPECTRA OF DITHIOLENE COMPLEXES

243

Figure 17. Solid-state RR spectra for (PPh4)2{MoO2[S2C2(CN)2]2} with 633-nm excitation (a) and (PPh4)2{MoO[S2C2(CN)2]2} with 515-nm excitation (b). [Adapted from (47).]

respectively, and representative RR spectra for {MoO2[S2C2(CN)2]2}2þ and {MoO[S2C2(CN)2]2}2þ and for [MoO2(S2C6H4)2]2þ and [MoO(S2C6H4)2]2þ,

C), n(Mo


O), and n(Mo
illustrating the assignments of the n(C

S) modes,

are shown in Figs. 17 and 18, respectively. The data are limited to tentative assignment of multiple Mo

S or W

S stretching modes in the 300–400-cm
1 region primarily on the basis of RR data and excitation profiles, and assignment of RR and IR bands in the 1460–1540-cm
1 region to C

C stretching modes and in the 820–960-cm
1 region to Mo

O and W

O stretching modes. Except 1
for the Mo

O or W

O stretching modes in [MoO(OPh)(S2C2Me2)2] (54), 1
1
[WO(OPh)(S2C2Me2)2] (61), and [MoO(S2C6H4)2] (58), the assignments are not based on isotope shifts. Moreover, normal mode calculations are not
C, available for any of these complexes. However, the assignments of the C


244

MICHAEL K. JOHNSON

νs(Mo=O) 858

(Et4N)2[MoVIO2(S2C6H4)2] ν(Mo-S) 322 356

νas(Mo=O) 829 ν(Mo=O) 904 (Et4N)2[MoIVO(S2C6H4)2]

ν(Mo-S) 356

200

400

600 Raman Shift (cm−1)

800

1000

Figure 18. Solid-state RR spectra for (NEt4)2[MoO2(S2C6H4)2] and (NEt4)2[MoO(S2C6H4)2] using 530-nm excitation. Samples prepared as KBr disks attached to a cold finger at 17 K.

Mo

O, and W

O stretching modes are generally reliable, since these modes give rise to strong IR bands and usually exhibit strong resonance enhancement with visible excitation in the Raman spectrum. Furthermore the C

C, Mo

O, and W

O stretching modes do not undergo significant kinematic mixing with other modes, and hence are useful in assessing trends in bond distances. In contrast, the assignments of the Mo

S and W

S stretching modes must be viewed as tentative in the absence of 34S isotope shifts, depolarization ratios, and normal mode calculations to assess the extent of vibrational mixing. Hence, they are of limited use for assessing trends in Mo

S and W

S bond lengths. The majority of the oxo-molybdo-bis(dithiolene) and oxo-tungsto-bis(dithiolene) complexes that have been investigated by vibrational spectroscopies have octahedral [MO2(dithiolene)2]2
structures in the M ¼ Mo(VI) or W(VI) states and square-pyramidal [MO(dithiolene)2]2
,1
structures with an apical oxo group in the M ¼ Mo(IV,V) or W(IV,V) states. In both the W and Mo compounds, the M

O stretching frequencies progressively increase on going from [MVIO2(dithiolene)2]2
to [MIVO(dithiolene)2]2
to [MVO(dithiolene)2]1
,
O bonds. This finding is best illustrated by indicating strengthening of the M


VIBRATIONAL SPECTRA OF DITHIOLENE COMPLEXES

245



O the series with benzene-1,2-dithiolate as the dithiolene ligand, since M

stretching frequencies are available in all three oxidation states with M ¼ Mo and W (see Tables VII and VIII). For example, Raman data indicate that the ns(Mo

O) and nas(Mo

O) stretching frequencies of the cis MoO2 unit in [MoVIO2(bdt)2]2
are 858 and 829 cm
1, respectively (45), whereas the IV 2
and [MoVO(bdt)2]1
n(Mo

O) stretching frequencies in [Mo O(bdt)2]
1 complexes are 904 and 940 cm , respectively (45, 53) (see Table VII). An analogous trend is apparent for the equivalent W complexes (see Table VII)

O albeit with higher frequencies due to the stronger dp –pp interactions in W

bonds compared to Mo

O bonds. The increase in Mo

O stretching frequencies with decreasing anionic charge for the isostructural [MoO(bdt)2]2
,1
complexes is consistent with predominantly molybdenum-based redox chemistry since greater s and dp –pp bonding interactions are expected for MoV

O than for MoIV

O. The dramatic decrease in average Mo

O stretching for the octahedral [MoVIO2(bdt)2]2
is attributed to the strong mutual trans influence of the dithiolene S and terminal oxo groups that compete for the same Mo orbitals for both s and dp –pp interaction. A strong trans influence is evident in the structural studies of a wide range of octahedral [MO2(dithiolene)2]2
(M ¼ Mo ˚ longer for the dithiolene S atoms that or W) that show M

S distances 0.17 A are trans rather than cis to the oxo groups (42, 45, 46). The trans influence of dithiolene ligands in destabilizing terminal oxo groups is clearly important in facilitating OAT chemistry in enzymes such as SO. The paucity of n(C

C) stretching frequencies for oxo-molybdo-bis(dithiolene) and oxo-tungsto-bis(dithiolene) complexes with discrete C

C bonds (see Tables VII and VIII), severely inhibits the utility of vibrational data for assessing trends in the extent of dithiolene redox chemistry as a function of the dithiolene substituents. However, increases in the n(C

C) stretching frequencies do accompany reduction of [MoVIO2(S2C2R2)2]2
to [MoIVO(S2C2R2)2]2
(R ¼ CN, Me, NH2) (see Table VII) and {WVIO2[S2C2(CN)2]2}2
to {WIVO[S2C2(CN)2]2}2
(see Table VIII), suggesting more ene-dithiolate character. Hence, although the reduction is primarily metal based, it does appear to involve some ligand-based reduction. Moreover, the influence of dithiolene substituents in determining electronic delocalization in oxo-molybdo-bis(dithiolene) and oxo-tungsto-bis(dithiolene) complexes is clearly demonstrated by the inverse correlation that has observed between the M

O and C

C stretching frequencies as a function of aryl substituent (R) in asymmetric dithiolenes [MIVO(dithiolene)2]2
[M ¼ Mo and W; dithiolene ¼
SC(H)C(R)S
] (63, 64) (see Table IX). As the net electron-withdrawing ability of the R group increases as a result of inductive and resonance effects (phenyl  pyridin-3-yl < pyridin2-yl  pyridin-4-yl < quinoxalin-2-yl), n(C

C) decreases and n(M

O) increases in accord with greater p delocalization in the extended dithiolene ligand
O bond. and increased pp –dp interaction in the M


246

MICHAEL K. JOHNSON TABLE IX
1 2
The M

C Stretching Frequencies (cm ) for Asymmetic [MO(dithiolene)2]
O and C


a Complexes with M ¼ Mo or W and Dithiolene ¼ SC(H)C(R)S [MoO(dithiolene)2]2
———————— ——————— n(C
n(Mo

C)
O)

R group Phenyl Pyridin-2-yl Pyridin-3-yl Pyridin-4-yl Quinoxalin-2-yl a

879 902 882 900 905

1516 1503 1515 1503 1499

[WO(dithiolene)2]2
——————————— n(C
C) n(W

O) 884 905 888 905 903

1519 1507 1518 1508 1498

See (63, 64).

The recent high-resolution crystallographic studies of Rhodobacter sphaeroides DMSOR (65), coupled with the preceding X-ray absorption and RR studies (66, 67), have provided convincing evidence that the functional form of the active site redox cycles between a serine-ligated monooxo-Mo(VI)-bis (dithiolene) form with distorted trigonal-prismatic coordination geometry (see Fig. 16) and a serine-ligated desoxo-Mo(IV)-bis(dithiolene) species. Hence, [MVIO2(dithiolene)2]2
and [MIVO(dithiolene)2]2
complexes (M ¼ Mo or W) do not provide structural analogues of the DMSOR family of mononuclear Mo and W enzymes. However, Holm and co-workers (53–55, 60, 61) recently reported synthesis and characterization of an extensive series bis(dithiolene) Mo(IV,VI) and W(IV,VI) complexes that mimic both the structural and functional properties of the oxidized and reduced forms of DMSOR active site. Thus far vibrational studies of this series of compounds have been limited to IR studies to determine the W

O and Mo

O stretching frequencies in distorted octahedral complexes of the type [MVIO(OR)(S2C2Me2)2]1
(M ¼ Mo or W; R ¼ Ph or i-Pr) (see Tables VII and VIII) and the use of 18O-labeled substrates in order to demonstrate an OAT mechanism. More detailed vibrational studies are required in order to facilitate comparison with the RR studies of DMSOR discussed below and thereby assess differences and similarities in the synthetic and biological catalytic centers and the role of the dithiolene ligands in the catalytic mechanism. The results of detailed vibrational studies of these synthetic complexes are awaited with great interest.

VI.

PYRANOPTERIN DITHIOLENES IN MONONUCLEAR Mo AND W ENZYMES

The utility of RR for investigating the ppd ligated oxo-Mo or oxo-W active sites of each of the four classes of mononuclear Mo or W enzymes (see Fig. 16),

VIBRATIONAL SPECTRA OF DITHIOLENE COMPLEXES

247

is often limited by the presence of additional prosthetic groups, such as flavins, Fe

S clusters or hemes, that exhibit intense fluorescence and/or RR spectra. Nevertheless, significant progress has made over the past 10 years, and RR has made important contributions to understanding active-site structures and mechanisms, as well as the role and ligation of the ppd, particularly in the SO and DMSOR families of enzymes (see Fig. 16). These results are summarized below for each of the four families of mononuclear Mo and W enzymes with particular emphasis on the vibrations of the ppd ligands. At the outset, it is important to emphasize that the available crystallographic evidence has shown that the Moor W-ligated ppd units are distinctly nonplanar, in both the central pyrazine and the pyran rings, and correspond to the fully reduced tetrahydropterin oxidation state (68). In particular, the pyran ring adopts a half-chair conformation, and the best plane defined by the pyran ring is tilted by up to 40 from the plane of the conjugated part of the pterin ring (68). Hence, the dithiolene C

C is not conjugated to the pterin ring and, from a vibrational and electronic viewpoint, the ppd shown in Fig. 16 is best modeled as a 1,2-dimethylethylenedithiolate.

A.

Xanthine Oxidase Family

In addition to the Mo center shown in Fig. 16, XO contains one flavin adenine dinucleotide (FAD) and two [2Fe–2S]2þ,þ clusters (69) and the visible absorption spectrum is dominated by the flavin and Fe

S chromophores. Hence, RR spectra are dominated by Fe

S stretching modes and flavin modes (70). However, in a very recent RR study (71), the MoVI

S stretch was identified at 474 cm
1 via 12-cm
1 downshift when the terminal sulfido was specifically
1 labeled with 34S, and the MoVI

O stretch was identified at 899-cm based on
1 loss of this band and the appearance of a band at 892 cm when the terminal sulfido was replaced by a terminal oxo in the inactive desulfo form of the enzyme. Moreover, turnover experiments in H2 18 O showed that the oxo ligand is not labile during catalytic cycling, and therefore corresponds to a spectator oxo group. This lends further support to the proposal that this family of Mo enzymes functions via a hydroxlase-type mechanism in which the catalytically labile oxygen atom that is incorporated in the product is a metal-bound water or hydroxide rather than terminal oxo group (41). With the exception of a weak band at 1513 cm
1 that was only observed with 515-nm excitation and was tentatively attributed to the C

C stretch of the pyranopterin dithiolene, modes that could be attributed to the ppd ligand were not observed using exciting lines in the range 400–600 nm. Hence, although the primary role is likely to be in mediating electron transfer from Mo, little is currently known concerning the involvement of the ppd in the catalytic cycle.

248

MICHAEL K. JOHNSON

B.

Sulfite Oxidase Family

1532

In addition to the Mo center shown in Fig. 16, SO contains an N-terminal domain with a b5-type cytochrome (72) that dominates the visible absorption and RR spectra of the holoenzyme. Hence, RR characterization of the Mo center has been confined to studies of the Mo-domain of recombinant human SO. Resonance Raman spectra of the Mo-domain obtained with 488-nm excitation for samples prepared by tryptic cleavage of the overexpressed and purified K108R variant of the holoenzyme (73) and by overexpression and purification of the His-tagged Mo-domain (74), are compared in Fig. 19. Of particular importance is that the bands at 1006, 1161, and 1532 cm
1 in the Mo-domain samples prepared by tryptic cleavage [Fig. 19(a)] are no longer observed in the

1161

(a)

1006

289 362 *

903

(b)

362 416

*

400

762

864 881

* 289

Raman Intensity

864 881 903

*

800

P P

1200

P

1600

Raman shift (cm−1) Figure 19. Resonance Raman spectra of the oxidized Mo domain of recombinant human SO. (a) The Mo domain prepared by tryptic cleavage of the K108R variant of the holoenzyme. (b) Histagged Mo-domain. Spectra recorded using 488-nm excitation for samples (0.5–1.0 mM in Mo) in 50-mM tricine buffer, pH 8.0, frozen at 17–25 K. Bands marked an asterisk correspond to lattice modes of ice and bands marked with P correspond to nonresonantly enhanced protein modes. A linear ramp has been subtracted to correct for a sloping fluorescence background.

VIBRATIONAL SPECTRA OF DITHIOLENE COMPLEXES

249

His-tagged Mo-domain samples [Fig. 19(b)]. These bands were enhanced with excitation wavelengths 514 nm and were attributed to coupled C

S and C

C stretching modes (1006 and 1161 cm
1) and the C

C stretching modes (1532 cm
1) of the coordinated ppd (70). However, this assignment now seems unlikely, since these bands are not observed in the purer and more active His-tagged Mo-domain preparations (75). Hence, they are attributed to an impurity or a degradation product involving the dissociated ppd ligand. The observation of analogous bands in the initial samples of native Rh. sphaeroides DMSOR (67), which were not observed in subsequent redox-cycled native and recombinant preparations (see below), lends support to the latter interpretation. The above discussion leads to the conclusion that dithiolene vibrational modes have yet to be definitively identified in either of the two families of Moenzymes containing a single ppd ligand, that is, the XO or SO families. Moreover, by analogy with the cysteine-coordinated mononuclear Cu center in type-1 (blue) copper proteins (8, 76, 77) and mononuclear Fe center in superoxide reductases (78), the weak bands at 289, 362, and 762 cm
1 in the spectra of the Mo-domain of human SO that are enhanced in resonance with the broad absorption band centered at 480 nm, can rationally be interpreted in terms
Cb

Ca bending, Mo

S stretching and Sg

Cb stretching modes, of the Sg
respectively, of the coordinated cysteine residue (see Fig. 16). In accord with this interpretation, the 480-nm absorption band is lost and the bands at 289, 362, and 762 cm
1 are not enhanced with 488-nm excitation when the coordinated cysteine residue is mutated to serine in the C207S variant of the Mo-domain (73). Hence, in the oxidized Mo-domain of SO, the visible absorption band centered at 480 nm is attributed primarily to (Cys)S
-to-Mo(VI) CT. Resonance Raman studies of the Mo-domain of human sulfite have been particularly effective in probing the catalytic mechanism via the Mo

O stretching modes that are enhanced using excitation into the 480-nm CT band (73). The bands at 903 and 881 cm
1 have been identified as the ns(Mo

O) and nas(Mo

O) stretching modes, respectively, of the cis-MoO2 unit on the basis of 18 O isotope shifts for samples redox cycled in H2 18 O. These bands shift by 13 and 33 cm
1, respectively, and when taken together, the combined 18O shift of the 46 cm
1 is approximately one-half of that expected based on a mass effect if both oxo groups were exchanged. This result indicates that only one oxo group is exchangeable with water during redox cycling. The asymmetrical distribution of the 18O isotope shift over the symmetric and asymmetric Mo

O stretching modes on labeling only one of the two oxo groups has been observed in model complexes and rationalized based on normal mode calculations (79). Hence, the RR results support an OAT mechanism involving the water-exchangeable terminal oxo ligand, with the nonexchangeable (spectator) oxo group playing a crucial electronic role in stabilizing the transition state by increased bonding in the Mo(IV) state (formal increase in bond order from 2 to 3) (80).

250

MICHAEL K. JOHNSON

C.

Dimethyl Sulfoxide Reductase Family

Resonance Raman studies of DMSOR (67) and biotin sulfoxide reductase (BSOR) (81) have played a critical role in assessing active site heterogeneity, addressing the role of each of the two ppd ligands and establishing the catalytic mechanism in the DMSOR family of mononuclear Mo/W enzymes shown in Fig. 20. Before discussing the RR results and their significance in terms of the proposed catalytic mechanism, some historical perspective is required. Crystallographic studies of the highly homologous DMSORs from Rh. sphaeroides and Rhodobacter capsulatus resulted in three distinct oxidized active structures that differed in terms of the number of terminal oxo groups and the coordination of the two ppd ligands (82–84). This confusing situation was further compounded by RR studies of Rh. sphaeroides DMSOR (67), which indicated that the functional form of the active site in solution cycles between monooxo and desoxo forms with both ppd ligands remaining ligated throughout the catalytic cycle; oxidized active-site structures that were not well represented by any of the three crystallographically defined structures. Resolution of this controversy and validation of the RR results was subsequently forthcoming via a high˚ ) crystal structure of Rh. sphaeroides DMSOR (65), which resolution (1.3 A revealed a discretely disordered structure involving two distinct Mo(VI) coordination environments. One is analogous to the functional form deduced from solution RR studies and involves a hexacoordinate serinate-ligated monooxoMo(VI) species with both ppd coordinated (see Fig. 16). The other corresponds to the structure deduced by Huber and co-workers (83) and involves a

S OVI S Mo S S O(Ser)

H+,e−

S

OH V

S H

+,e−

DMS/biotin

S

VI

S

S O(Ser)

S

H 2O IV

S

BSO/DMSO

S

Mo S

O

H+,e−

H2O

S

S

S O(Ser)

H+,e−

S

Mo

Mo S

DMS/biotin

S O(Ser)

O IV

S

Mo S BSO/DMSO

S O(Ser)

Figure 20. Proposed catalytic cycle for DMSOR and BSOR.

VIBRATIONAL SPECTRA OF DITHIOLENE COMPLEXES

251

pentacoordinate dioxo-Mo(VI) form analogous to that of the SO family with serinate in place of cysteinate (see Fig. 16). One of the ppd ligands dissociates and is partially oxidized to a dithiete form. The original structures published by Rees and co-workers (82) and Bailey and co-workers (84) can both be rationalized in terms of a superposition of these two active-site structures and the pentacoordinate form appears to be induced by the presence of glycerol, 2methyl-2,4-pentanediol or N-(2-hydroxyethyl)piperazine-N 0 -ethanesulfonic acid (Hepes) buffer during crystallization (65). The Rh. sphaeroides DMSOR and BSOR are particularly attractive enzymes for RR studies designed to address the catalytic mechanism and the role of the ppd ligands in this family of mononuclear Mo enzymes, since they have no other prosthetic groups other than the Mo active sites. Moreover, homogeneous forms of active enzymes are readily obtained for both native and recombinant enzymes following redox cycling. Indeed, the ability of RR spectroscopy to monitor the number, fate, and exchangeability of terminal oxo groups has provided the most definitive evidence to date for an OAT mechanism involving monoxo-Mo(VI) and desoxo-Mo(IV) species for this family of mononuclear Mo enzymes (see Fig. 20). Vibrations involving Mo

O stretching have been identified by isotopic labeling of the terminal oxo group, either by reduction and reoxidation with 18Olabeled substrate or by redox cycling after exchange into H2 18 O buffer (67, 84). An example of the former type of experiment for DMSOR is shown in Fig. 21. Both procedures yield the same result, that is, a 43-cm
1 18O downshift for the band at 862 cm
1, which identifies this band as the n(Mo

O) mode of a monooxo-Mo(VI) species and demonstrates that the terminal oxo group is exchangeable with both H2O and substrate during catalytic cycling (see Fig. 20). A desoxo-Mo(IV) formulation for the dithionite-reduced enzymes is indicated by the complete absence of a band that could be attributed to a n(Mo

O) mode in the 800–950-cm
1 region using excitation wavelengths in the range 406– 676 nm (see Fig. 22) (67, 81). In the case of DMSOR, addition of the product dimethyl sulfide (DMS) to the oxidized enzyme under anaerobic conditions results in reduction and the appearance of an intense CT band centered near 550 nm (67, 85). Resonance Raman studies using excitation within this absorption envelop leads to enhancement of bands at 862 and 497 cm
1 that are readily assigned to the Mo

O and S

O stretching modes of a DMSObound Mo(IV) intermediate (see Fig. 20) based on 18O downshifts of 29 and 18 cm
1, respectively (67). In contrast, addition of products such as biotin or DMS to oxidized BSOR results in perturbation of the n(Mo

O) frequencies of the monoxo-Mo(VI) species rather than reduction, suggesting stabilization of a product-associated monoxo-Mo(VI) intermediate (see Fig. 20) (81). A recent Raman study of Rh. capsulatus DMSOR using only 752-nm excitation (86) has provided additional evidence in support of catalytic cycling between monooxo-Mo(VI) and desoxo-Mo(IV) for this class of mononuclear

(b )

200

400

600

P

P

1724 2ν(Mo=O)

P P

1527 ν(C=C) 1578 ν(C=C)

1212 νs(Mo-S)+ν(Mo=O)

ν(C-C)+ν(C-S) 1047 1079 1126

#

858 ν(C-S)

536

416

862 ν(Mo=O)

#

(a)

1005 1023

* 250 *

Raman Intensity

700 2νs(Mo-S) 764 ν(C-S)

MICHAEL K. JOHNSON 350 νs(Mo-S)

252

800

1000

1200

1400

1600

1800

−1)

Raman Shift (cm

Figure 21. Resonance Raman spectra of Rh. sphaeroides DMSOR after reduction and reoxidation with DMS16O (a) and DMS18O (b). The 18O downshifts of 43, 43, and 86 cm
1 are observed for the bands at 862, 1212, and 1724 cm
1, respectively. The spectra were recorded using 568-nm excitation for samples (3 mM) in 50-mM tricine buffer, pH 7.5, frozen at 25 K. Bands marked by an asterisk correspond to lattice modes of ice; bands marked with # correspond to residual DMSO; bands marked with P correspond to nonresonantly enhanced protein modes. A linear ramp has been subtracted to correct for a sloping fluorescence background and the data has been smoothed with 5-point running average.

Mo enzymes. However, in contrast to the multi-wavelength RR studies of Rh. sphaeroides DMSOR (67), a band at 818 cm
1 was assigned to the S

O stretching mode of bound DMSO in the DMS-reduced enzyme and a band at

O stretch of an unprecedented 1210 cm
1 was inexplicably attributed to the S

metal sulfoxide formed on one of the ppd sulfur atoms. In our opinion, both assignments are erroneous. The assignment of the 818-cm
1 band is based on unconvincing 18O-isotope shift data and is very unlikely since excitation profiles have shown that the vibrational modes of bound DMSO in the DMS-reduced derivative of DMSOR are only significantly enhanced in resonance with the CT band centered at 550 nm (64). As shown in Fig. 20, the 1212-cm
1 band in Rh.
O)
S) þ n(Mo
sphaeroides DMSOR had previously been assigned as a ns(Mo

1 18 O-isotope combination band based on the accurately determined 43-cm shift. There is no compelling reason to change this assignment (67).

1005 1017 1047 1075 1118

768 ν(C-S)

(a)

P

P P

Raman Intensity

*

ν(C-C)+ν(C-S)

367 ν(Mo-S) 396 385 346

459 416 402 513

*

253

1572ν(C=C)

VIBRATIONAL SPECTRA OF DITHIOLENE COMPLEXES

(b)

862

497

*

*

(c)

*

200

*

400

600

800

1000

1200

1400

1600

Raman Shift (cm−1) Figure 22. Resonance Raman spectra of reduced samples of Rh. sphaeroides DMSOR: (a) Dithionite-reduced; (b) DMS-reduced after oxidation with DMS16O; (c) DMS-reduced after oxidation with DMS18O. The 18O downshifts of 29 and 18 cm
1 are observed for the bands at 862 and 497 cm
1, respectively. The spectra were recorded using 530-nm excitation and the data collection and handling procedures are as described in Fig. 21. Bands marked by an asterisk correspond to lattice modes of ice and bands marked with P correspond to nonresonantly enhanced protein modes.

254

MICHAEL K. JOHNSON TABLE X Comparison of Dithiolene Ring Stretching Frequencies (cm
1) in Rh. sphaeroides DMSOR and BSORa

C) n(C



Oxidized Mo(VI) DMSOR BSOR Dithionite-reduced Mo(IV) DMSOR BSOR

n(C

S)

n(C

C), n(C

C) þ n(C

S), n(C

S) þ n(C

C)

1527, 1578 1529, 1573

764, 858b 760,860b

1005, 1023, 1047, 1079, 1126 1004, 1017, 1041, 1090, 1128

1572 1582

768c 760c

1005, 1017 1047, 1075, 1118 1002, 1041, 1085, 1124

a

Assignments based on data taken from (67 and 81).

16O stretching mode and only clearly observed in Mo

18O samples. Obscured by Mo

c Obscured by overtones of Mo

S stretching modes and most clearly observed with 488-nm excitation when the Mo

S stretching modes are less strongly enhanced. b

Unlike Mo enzymes with a single ppd ligand, the vibrational modes associated with the two ppd ligands in the DMSOR family exhibit significant resonance enhancement in both the oxidized and reduced states using visible excitation (67, 81, 87, 88). Representative spectra of oxidized and reduced forms of Rh. sphaeroides DMSOR in the 200–1800-cm
1 region are shown in Figs. 21 and 22, respectively. Assignments of dithiolene ring stretching frequencies for both the DMSOR and BSOR from Rh. sphaeroides are given in Table X. The assignments are revised from those previously published (67), since bands at 1005, 1160, and 1527 cm
1 that were observed to be selectively and strongly enhanced in both the oxidized and reduced samples using excitation wavelengths 514 nm (73) are no longer attributed to modes of a coordinated ppd. Although oxidized samples of Rh. sphaeroides DMSOR invariable exhibit Raman bands at 1005 and 1527 cm
1 with excitation profiles that maximize near 650 nm, subsequent RR studies of purer and more active samples of redoxcycled native and recombinant redox-cycled samples of oxidized and reduced Rh. sphaeroides DMSOR have been unable to confirm the presence of bands at 1005, 1160, and 1527 cm
1 that are most strongly enhanced with 488-nm excitation. Since similar modes were also observed in early samples of the SO molybdenum domain (see above) these bands are attributed to a degradation product involving a dissociated ppd ligand. The five distinct types of ppd ring modes that might be expected to exhibit significant resonant enhancement are shown in Fig. 5. Although detailed assignments of the n(C

C), n(C

S) þ n(C

C) and n(C

C) þ n(C

S) modes to individual ppd ligands are not possible, the assignments of the n(C

S) and n(C

C) modes indicate that the two ppd ligands are distinct in the oxidized Mo(VI) form, but that only one type of ppd is observed in the dithionite- or DMS-reduced Mo(IV) forms (see Table X and Figs. 21 and 22). On the basis of

VIBRATIONAL SPECTRA OF DITHIOLENE COMPLEXES

255

the bis(dithiolene) and tris(dithiolene) complexes discussed above, the n(C

S) and n(C

C) modes in oxidized DMSOR and BSOR can be rationally assigned to ppd ligands with different degrees of delocalization. Hence, the bands at
S) and n(C
760 and 1580 cm
1 are assigned to the n(C

C) modes of a ppd that closely approximates an ene-dithiolate formulation, whereas the bands at
S) and n(C
860 and 1530 cm
1 are assigned to the n(C

C) modes, of a more p-delocalized ppd (see Fig. 20). The observation of only the enedithiolate-type ppd in the reduced samples, that is, n(C

S) at 760 cm
1 and
1

C) at 1580 cm , suggests that one of the dithiolene partially or fully n(C

dissociates on reduction or that the p-delocalized ppd is redox active and is converted to an ene-dithiolate-type ppd on reduction. On the basis of the close similarity in the pattern of Mo

S stretching bands in the oxidized and reduced forms (see below), the latter explanation is considered more likely. Hence, the Mo(VI)/(IV) redox chemistry is predominantly, but not exclusively, Mo based. The electron-donating ability of the p-delocalized ppd ligand is likely to play an important role in lowering the activation barrier for the formation of the terminal oxo group in the substrate-bound Mo(IV) to monooxo-Mo(VI) step in the catalytic cycle (see Fig. 20). In many respects, the proposed ability of the pdelocalized ppd ligand to lower the energy of the transition state by stabilizing the monooxo-Mo(VI) state has parallels to the role of the spectator oxo group in the SO family. This ability is believed to stabilize the transition state in the dioxo-Mo(VI) to monooxo-Mo(IV) step by increased bonding in the Mo(IV) state (80). The discovery that only one of ppd ligands undergoes redox changes during catalytic cycling suggests distinct roles for the two ppd ligands in the DMSOR family. Hence, the redox-active, p-delocalized ppd ligand is proposed to play an important catalytic role in facilitating reduction of the bound substrate, while the redox-inert ene-dithiolate ppd ligand is proposed to function in mediating electron transfer to Mo. The Mo

S(dithiolene) stretching modes and dithiolene ring deformations are strongly enhanced with visible excitation in DMSOR (67, 87, 88) and in BSOR (81). While these modes are likely to be extensively mixed, they can be distinguished to a first approximation on the basis of sensitivity to metal oxidation state, excitation dependence, and 34S-isotope shifts (87–89). Four modes have been assigned to predominantly Mo

S(dithiolene) stretching in the oxidized Mo(VI), glycerol-inhibited Mo(V), and dithionite-reduced Mo(IV) forms of DMSOR (67) (see Fig. 23), as well as the oxidized and dithionitereduced forms of BSOR (81) (see Table XI). On the basis of electron paramagnetic resonance (EPR) studies, the stable glycerol-inhibited Mo(V) species provides a close analogue of the Mo(V) state that can be observed during catalytic turnover, with a bidentate glycerol in place of the hydroxide and serinate ligands (90). In each case, the most intense band is assigned to the

256

MICHAEL K. JOHNSON

Figure 23. Resonance Raman spectra of Mo(VI), Mo(V), and Mo(IV) forms of Rh. sphaeroides DMSOR in the Mo

S stretching region: (a) as prepared Mo(VI) DMSOR with 676-nm excitation; (b) glycerol-inhibited Mo(V) DMSOR with 676-nm excitation; (c) dithionite-reduced Mo(IV) DMSOR with 568-nm excitation. Data collection and handling procedures are as described in Fig. 21. Bands marked with an asterisk correspond to lattice modes of ice.

totally symmetric stretching mode and the pattern of bands is as expected for a square-pyramidal MoS4 unit under idealized C 4v symmetry [i.e.,
S) > n(B1)(Mo

S) (6)], with the E mode split on n(E)(Mo

S) > n(A1)(Mo
lowering the symmetry to C2v as a result of the chelating dithiolene ligands (see Table XI).

257

A2 A1 B1,B2

B1 A1 E

358 2.44

336 (3) 350 (7) 370 (4), 377 (3) 362 2.40

349 355 364, 379

b

Vibrational assignments taken from (67). Vibrational assignments taken from (81). c 34 S isotope shifts from (88) are given in parentheses; nd ¼ not determined. d As determined by Mo–EXAFS measurements (66, 91).

a

Average n(Mo

S) Average Mo

S distanced

C2v

Mode

C4v

375 2.33

346 (7) 367 (6) 385 (4), 402 (nd)

DMSORa —————————————————————————————— Glycerol Inhibited Dithionite Reducedc Oxidizedc Mo(VI) Mo(V) Mo(IV)

358 2.41

331 355 368, 378

375 2.33

345 363 386, 405

BSORb —————————————————— Oxidized Dithionite Reduced Mo(VI) Mo(IV)

TABLE XI ˚ ) in DMSOR
S Distances (A Comparison of Mo

S Stretching Frequencies (cm
1) and Average Mo
and BSOR from Rh. sphaeroides as a Function of Redox State

258

MICHAEL K. JOHNSON

The close similarity in terms of the number and pattern of Mo

S stretching modes in the RR spectra of the oxidized, glycerol-inhibited, and dithionite reduced forms (see Fig. 23), is clearly consistent with both ppd ligands remaining coordinated during catalytic cycling between the Mo(VI)/(V)/(IV) states (see Fig. 20). However, the trend in the Mo

S stretching frequencies as a function of Mo oxidation state suggests that changes in coordination accompany reduction. The average Mo

S(dithiolene) stretching frequency progressively increases on reduction, Mo(VI) < Mo(V) < Mo(IV), indicating shortening of the Mo

S bonds with decreasing Mo oxidation state. Moreover, this trend is in qualitative agreement with the average Mo

S bond distances as determined by Mo-extended X-ray absorption fine structure (EXAFS) analysis of Rh. sphaeroides DMSOR (66) and BSOR (91) (see Table XI). A significant decrease in the Mo

S bond length between the Mo(VI) and Mo(V) forms is expected due to the loss of the short Mo

O bond. However, a further shortening on going from Mo(V) and Mo(IV) is more difficult to explain without invoking a decrease in coordination number. In light of the near-equivalence in the C

C stretching frequencies of the two ppd ligands in the Mo(IV) form of both DMSOR and BSOR, it is therefore proposed that the desoxo-Mo(IV) form adopts a fivecoordinate square-pyramidal coordination geometry with an apical serinate ligand (see Fig. 20). D. Aldehyde Oxidoreductase Family Crystal structures have been reported for two members of the W-containing AOR family, both purified from the hyperthermophilic archaeon Pyrococcus furiosus: A general AOR with broad substrate specificity (92) and a formaldehyde ferredoxin oxidoreductase (FOR) that has optimal activity for low molecular weight aldehydes (93). While the structures clearly show W ligation by two ppd ligands, additional ligands are not well resolved due to the combination of active-site heterogeneity as revealed by spectroscopic studies (94, 95) and truncation problems associated with the high electron density of W (96). However, W-EXAFS studies have provided evidence for one terminal oxo group in active preparations of P. furiosus AOR (42). In addition to the active site shown in Fig. 16, this family of W enzymes also contains a redox active [4Fe

4S]2þ,þ cluster in close proximity to one of the two ppd ligands (42, 92, 93). Thus far, no RR studies have been reported for members of the W-containing AOR family (or any W-containing metalloenzyme) and the results presented below are offered in the spirit of a progress report in order to demonstrate the potential of RR for addressing the active-site structure and catalytic mechanism. Our initial attempts to obtain high-quality RR spectra for W-containing AORs were impeded by high fluorescence backgrounds and active-site heterogeneity. These problems have recently been circumvented by working with highly

VIBRATIONAL SPECTRA OF DITHIOLENE COMPLEXES

259

purified samples of P. furiosus FOR, which appears to be the most homogeneous of all the W-containing AORs investigated thus far, as judged by EPR redox titrations. The RR spectrum of thionine-oxidized P. furiosus FOR in the highfrequency region is shown in Fig. 24, together with that of DSMO-reoxidized

1578

p p

p

p

(b) Oxidized P. furiosus FOR (C = C ) 1576 1595

874 (W=O)

530 nm

(C=C)

705 722 742 758

600

800

1023 1045 1089 1125

(C-C+C-S) (2x W-S+ C-S)

1527

1005 1023 1047 1079 1126

s

(C-C+C-S)

764 (C-S)

s

(a) DMSO-reoxidized Rh. sphaeroides DMSOR

1212 (Mo-S+Mo=O)

700 (2x Mo-S)

862 (Mo=O)

568 nm

1000

p p p

1200

p

1400

1600

1800

Raman Shift (cm−1)

Figure 24. (a) Comparison of the high-frequency RR spectra of DMSO-reoxidized Rh. sphaeroides DMSOR obtained with 568-nm excitation and (b) of thionine-oxidized P. furiosus FOR obtained with 530-nm excitation. The spectra were recorded using samples (3 mM) frozen at 25 K. The buffering medium was a 50 mM Tris buffer, pH 7.8 for FOR and 50 mM tricine, pH 7.5 for DMSOR. Bands marked s correspond to residual DMSO and bands marked with p correspond to nonresonantly enhanced protein modes. A linear ramp has been subtracted to correct for a sloping fluorescence background.

366

MICHAEL K. JOHNSON

(a) Reduced P. furiosus FOR

488 nm 373

260

397

429

355

*

373

270

*

488 nm

431

366

(b) Reduced P. furiosus AOR

200

250

284

250

389

*

*

300 350 Raman Shift (cm−1)

400

450

Figure 25. Resonance Raman spectra of dithionite-reduced P. furiosus (a) FOR and (b) AOR in the W

S stretching region. The spectra were recorded using 488-nm excitation for samples (3 mM) in 50 mM Tris buffer, pH 7.8, frozen at 25 K. Bands marked with an asterisk correspond to lattice modes of ice. A linear ramp has been subtracted to correct for a sloping fluorescence background.

Rh. sphaeroides DMSOR to facilitate comparison. The C

C, C

S, and C

C stretching modes of two ene-dithiolate-type ppd ligands [n(C

C) ¼ 1576 and 1595 cm
1) are readily identified and the intense band at 874 cm
1 provides evidence for a monooxo-W(VI) center with similar coordination to the monooxo-Mo(VI) center in DMSOR. The W

O stretching frequencies are usually slightly higher than Mo

O stretching frequencies in equivalent complexes due to stronger p bonding (6). Experiments involving H2 18 O buffers that are designed to address the exchangeability of the terminal oxo group on reduction and during catalytic turnover under physiological conditions (85 C) are

VIBRATIONAL SPECTRA OF DITHIOLENE COMPLEXES

261

currently in progress. The ability to enhance the activity fivefold by treating the enzyme with sulfide under reducing conditions (97) has raised the possibility that the functional form of the enzyme contains a terminal sulfido rather than a terminal oxo group. This possibility is also being addressed via RR studies of samples activated with 32S2
and 34S2
. Resonance Raman studies of dithionite-reduced samples of P. furiosus AOR and FOR in the W

S stretching region both show four or five bands in the 350–450-cm
1 region (see Fig. 25), that are excellent candidates for W

S(dithiolene) stretching and dithiolene ring-deformation modes. As in the case of DMSOR, detailed assignments will require 34S-isotope shifts, and attempts to grow cells in 34S-enriched media are in progress. Reduced
S stretching [4Fe

4S]þ clusters have negligible resonance enhancement of Fe
modes with visible excitation. However, the characteristic bands of [4Fe

4S]2þ clusters are observed, superimposed on the W

S bands shown in Fig. 25, in thionine-oxidized samples (not shown). The absence of significant frequency shifts in the W

S(dithiolene) stretching modes on oxidation does, however, suggest that the majority of the W is in the W(VI) oxidation state in dithionitereduced samples. Attempts to reduce the W center using lower potential reductants and under more physiologically relevant conditions are in progress.

VII.

CONCLUDING REMARKS

This chapter has demonstrated the important, albeit underutilized, role of vibrational spectroscopy in characterizing the extent of p delocalization and metal- versus ligand-based redox chemistry in both inorganic and bioinorganic dithiolene complexes. For this potential to be fully realized, there is clearly a pressing need for parallel IR and Raman studies of structurally defined representatives of each type of complex, with assignments based on metal, sulfur, and oxygen (where appropriate) isotope shifts and rationalized in terms of force fields developed using DFT and/or normal mode calculations. This approach is required for meaningful assignments and interpretation of modes involving metal–S and C

S stretching in particular, because these modes are extensively mixed with stretching and deformation modes of the dithiolene ring and substituents. In the absence of such detailed vibrational studies, the C

C stretching frequency serves as the most useful and convenient monitor of the redox state of the dithiolene ligand, since this mode is not extensively mixed with other vibrational modes and is generally easy to identify in both the IR and Raman spectra. The observed range of frequencies for n(C

C) modes is 1329– 1594 cm
1, with the low end corresponding to the most oxidized and extensively delocalized form and the high end corresponding to the most reduced and almost completely localized ene-dithiolate formulation. On this

262

MICHAEL K. JOHNSON

basis, the vibrational studies of bis(dithiolene) and tris(dithiolene) metal complexes are consistent with highly delocalized structures with predominantly ligand-based redox chemistry, provided the reductive capacity of the ligand is not exceeded. In contrast, the available RR data indicate that the ppd ligands in mononuclear Mo and W enzymes are best considered as ene-dithiolates and the redox chemistry during catalytic cycling is predominantly metal based. Thus far the utility of RR for understanding the role of the ppd ligands in mononuclear Mo and W enzymes has only been fully exploited in the DMSOR family of enzymes. In particular, the observation of redox-inert and redox-active ppd ligands has led to the proposal that the two dithiolene ligands play distinct roles with one mediating electron transfer to Mo and the other acting as an electron donor–electron acceptor to facilitate reduction–oxidation of a bound substrate. Detailed vibrational studies of the recently characterized structural and functional analogues for the active sites of the DMSOR (53–55, 60, 61) and SO (43) families with 1,2-dimethylethylenedithiolate ligands should be very informative in further characterization of the catalytic mechanisms and assessing the roles the dithiolene ligands in both families of enzyme. Likewise, RR characterization of the vibrational modes of the ppd ligands in oxidized and reduced forms of enzymes in the XO, SO, and AOR families will clearly be important in assessing the involvement of the dithiolene ligand(s) in the catalytic cycle. Such studies will be particularly important for addressing the possibility that a redox-inert ene-dithiolate-type ppd mediates electron transfer to or from the Mo/W center in all mononuclear Mo/W enzymes.

ACKNOWLEDGMENTS Vibrational studies of dithiolene complexes and mononuclear Mo/W enzymes in the author’s laboratory have been supported by a grant from the National Science Foundation (MCB98008857). I am indebted to my collaborators, K. V. Rajagopalan, M. W. W. Adams, M. J. Barber, and R. H. Holm, for supplying inorganic complexes and enzymes for spectroscopic investigations, and to the graduate and postdoctoral students in my laboratory who have contributed to the work presented herein: S. G. Garton, I. K. Dhawan, B. P. Koehler, R. C. Conover, W. Fu, H. Oku, and B. R. Crouse. Special thanks are due to R. C. Conover for help in preparing the figures.

ABBREVIATIONS AOR bdt BSOR DFT

Aldehyde ferredoxin oxidoreductase Benzene-1,2-dithiolate Biotin sulfoxide reductase Density functional theory

VIBRATIONAL SPECTRA OF DITHIOLENE COMPLEXES

DMS DMSO dmit EPR Et2timdt EXAFS FAD FOR FTIR Hepes HOMO LMCT MLCT ndt OAT ppd RR SO tdt XO

263

Dimethyl sulfide Dimethyl sulfoxide 1,3-Dithiole-2-thione-4,5-dithiolato Electron paramagnetic resonance Monoanion of 1,3-diethylimidazolidine-2,4,5-trithione Extended X-ray absorption fine structure Flavin adenine dinucleotide Formaldehyde ferredoxin oxidoreductase Fourier transform infrared N-(2-hydroxyethyl)piperazine-N-ethane sulfonic acid Highest occupied molecular orbital Ligand-to-metal charge transfer Metal-to-ligand charge transfer Naphthalene-2,3-dithiolate Oxygen atom transfer Pyranopterin dithiolene Resonance Raman Sulfite oxidase Toluene-1,2-dithiolate Xanthine oxidase REFERENCES

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CHAPTER 5

Electrochemical and Chemical Reactivity of Dithiolene Complexes KUN WANG Corporate Strategic Research ExxonMobil Research and Engineering Co. Annandale, NJ CONTENTS I. INTRODUCTION

268

II. BIS(DITHIOLENE) COMPLEXES

270

A. Redox Properties / 270 B. Chemical Reactivity / 277 1. Ligand-Exchange Reactions / 278 2. Ligand Addition and Substitution Reactions / 278 3. Alkylation–Protonation / 281 4. Cycloaddition with Unsaturated Hydrocarbons / 284 5. Addition Reactions with Other Unsaturated Compounds / 287 6. Miscellaneous Reactions–Applications / 288 III. TRIS(DITHIOLENE) COMPLEXES

290

A. Redox Properties / 290 B. Chemical Reactivity / 296

Dithiolene Chemistry: Synthesis, Properties, and Applications, Progress in Inorganic Chemistry, Vol. 52 Special volume edited by Edward I. Stiefel, Series editor Kenneth D. Karlin ISBN 0-471-37829-1 Copyright # 2004 John Wiley & Sons, Inc. 267

268

KUN WANG

IV. HETEROLEPTIC (MIXED-LIGAND) DITHIOLENE COMPLEXES A. B. C. D. E.

299

Carbonyl Complexes / 299 Nitrosyl Complexes / 301 Other Mixed-Ligand Dithiolene–Donor Complexes / 302 Dithiolene Complexes with Metal–Ligand Multiple Bonds / 303 Organometallic Complexes / 305 1. Cyclopentadienyl Complexes / 305 2. Other Organometallic Complexes / 308

V. CONCLUSIONS AND FUTURE OUTLOOK

308

ACKNOWLEDGMENTS

309

ABBREVIATIONS

309

REFERENCES

310

I.

INTRODUCTION

The past four decades has witnessed a tremendous increase of interest in the chemistry of dithiolene complexes. Although the use of dithiols and dithiolates (e.g., benzenedithiolate) for analytical purposes was known as early as the 1930s, the field did not become active and expand rapidly until the 1960s when three groups (Davison/Holm at Harvard, Gray at Columbia, and Schrauzer at Munich) independently came to realize the unique nature of dithiolene ligands. Early interest focused primarily on the remarkable reversible redox behavior and unique electronic structures of this class of complexes (1–3). Recent interest has spanned a wide range of areas such as optical (4–8) and electromagnetic materials (9–13), chemical sensors (14–18), Q-switch laser dyes (19, 20), and bioinorganic chemistry (21–26). Each of these topics is reviewed separately in this special volume. A qualitative description of the bonding characteristics in dithiolene complexes, as exemplified by a neutral square-planar complex in Scheme 1, involves structures (1–3) in which the metal assumes a formal oxidation state of 0, II, or IV, while the ligands assume either a neutral ‘‘dithiodiketone’’ or a dianionic ‘‘dithiolate’’ form. The structure of a bis(dithiolene) complex is perhaps best considered to be a resonance hybrid of the limiting structures 1–3. The ligand p orbitals interact with the metal dp orbitals to give frontier orbitals of mixed-ligand and -metal character. In both the bis- and tris(dithiolene) complexes, electrons are not localized at the ligands, but are delocalized throughout the metal–dithiolene five-membered rings and exhibit a certain degree of aromaticity. The name

ELECTROCHEMICAL AND CHEMICAL REACTIVITY

S

S M

S

S

II

S

S

S

S

M0 S

M S

269

S

S

S

IV

S

M II S 1

S 2

3

Scheme 1

‘‘dithiolene’’ is chosen to describe these compounds without giving bias toward any of the limiting structures. The focus of this chapter is on the electrochemical and chemical reactivity of dithiolene complexes. The scope is limited mostly to mononuclear complexes with relatively simple 1,2-dithiolene ligands. The work summarized in this chapter is largely concerned with the redox and chemical reactions of complexes
S)3] (S

S ¼ a dithiolene ligand) coordination having the [M(S

S)2] or [M(S
sphere (i.e., homoleptic complexes). Mixed-ligand (i.e., heteroleptic) complexes containing at least one dithiolene ligand are only briefly surveyed. Emphasis is placed on important recent results. Readers interested in earlier work are referred to existing reviews (2, 3, 27). The general approach is to discuss the redox chemistry first, followed by discussions of chemical reactivity and potential applications. Selected redox potential data are listed to illustrate the effect of ligands and possible interpretations are given when sufficient data are available. A wide range of electrochemical techniques, working electrodes, solvents, and reference electrodes has been employed in investigating the electrochemistry of dithiolene complexes. In compiling the electrochemical data, redox potentials are listed as reported in the reference (versus the reference electrodes– couples used). To help compare studies employing different reference electrodes– couples, the redox potentials are also converted versus a common reference electrode–saturated calomel electrode (SCE). The conversion factors applied are listed in Table I (28). However, caution should be exercised in strictly comparing the converted numbers since the original values are measured in nonaqueous solvents; and some involve irreversible electrochemical processes. The redox processes are also particularly sensitive to a host of factors such as electrode, solvent, electrolyte, and cell configuration, which in turn affect capacitance,

270

KUN WANG TABLE I Conversion Factors for Different Reference Electrodes or Redox Couplesa SCEb (V)

SCEb Ag/AgClc Fc/Fcþd NHEe Ag/Agþf

Ag/AgClc (V)

Fc/Fcþd (V)

NHEe (V)

þ0.045


0.307
0.352

þ0.241 þ0.197 þ0.549


0.045 þ0.307
0.241 þ0.559

þ0.352
0.197 þ0.604


0.549 þ0.252

Ag/Agþf (V)
0.559
0.604
0.252
0.800

þ0.800

a

Adapted from (28). To convert from one reference electrode (RE1) to another (RE2), find RE1 in the left column and read across the row to find the number corresponds to RE2 and add the number. b Hg/Hg2Cl2 in saturated KCl solution. c In saturated KCl solution. d Ferrocene/ferrocenium (Fc/Fcþ) couple in 0.2 M LiClO4/MeCN (29, p. 701). e Normal hydrogen electrode (NHE). f Reference (29), page 699.

adsorption, and cell resistance (iR drop) (29). Indeed, converted values for a given redox couple can vary substantially (more than can be accounted for by experimental errors) in different systems. It should also be pointed out, however, that the conversion factors listed in Table I are for the purpose of discussion only, since different conversion factors are used by different groups (30). The reader is referred to a number of references that discuss the complications involved in rigorously converting reference systems (31).

II.

BIS(DITHIOLENE) COMPLEXES

Most of the late transition metals (such as Fe, Co, Rh, Ir, Ni, Pd, Pt, Cu, Au, and Zn) have been found to form bis(dithiolene) complexes. A significant amount of work has been reported on the electronic structures and spectroscopy (32), redox properties (2), as well as the conductivity (33) of bis(dithiolene) complexes. Far less has been reported on their chemical reactivity. A.

Redox Properties

The extensive electron delocalization in bis(dithiolene) complexes makes it possible for them to exist in a range of charge levels. It also makes oxidation state assignment of the metal and ligands potentially ambiguous. Mononuclear bis(dithiolene) complexes can undergo one, two, or even three reversible

ELECTROCHEMICAL AND CHEMICAL REACTIVITY

271

one-electron redox steps according to Eq. 1, where the dithiolene ligand is represented by S

S. [M(S (S

+ e−

S)2]+

− e−

[M(S

S)2]0

+ e− −

e−

[M(S

S)2]−

+ e− − e−

[M(S

S)2]2−

ð1Þ

S = dithiolene)

Redox potentials for a group of selected bis(dithiolene) complexes are listed in Table II. Neutral iron and cobalt bis(dithiolene) complexes exist in the dimeric form. The dimers stay intact when partially reduced and dissociate into monomers when fully reduced (34, 35). The potentials listed in Table II for Fe and Co complexes are therefore for the redox couples (0/
1,
1/
2, etc.) of the dimer (Eq. 2). [M(S

S)2]20 (S

+ e− −

e−

[M(S

S)2]2−

S = dithiolene, M = Fe, Co)

+ e− −

e−

[M(S

S)2]22−

+ 2e− − 2e−

2 [M(S

S)2]2−

ð2Þ

The redox potentials can serve as a guide to the synthesis of a given charge level of dithiolene complexes. Based on polarographic observations, several generalizations about the synthesis and chemical behavior of the planar and dimeric dithiolene complexes had been made [(36); all potentials below are referenced to SCE]: 1. For couples with E1=2 < 0:00 V, the reduced species is susceptible to air oxidation in solution; in couples with E1=2 > 0:00 V, the reduced species is air-stable. 2. When E1=2 > 0:20 V, the oxidized species can be reduced by weakly basic solvents (e.g., ketone or alcohol); but when
0:12 < E1=2 < þ0:20 V, the oxidized form can be reduced by stronger bases (e.g., aromatic amines). 3. When E1=2 <
0:12 V, the oxidized species is readily reduced by strong reducing reagents such as hydrazine, sodium amalgam, and NaBH4. 4. The reduced form can be oxidized by iodine when E1=2 < þ0:40 V; otherwise, stronger oxidants, such as Ni(tfd)2 [tfd ¼ 1,2-bis(trifluoromethyl)ethylenedithiolate] are required. It must be emphasized, however, that the values quoted above are approximate. In the course of synthesis involving oxidation or reduction reactions, proper selection of oxidizing or reducing agents, in addition to judicious choice of reaction conditions, is crucial. Side reactions, such as ligand exchange, may occur; and the reducing agents, such as amines, may function as coordinating ligands. Note that reduction of the oxidized form in couples with E1=2 <
0:95 V,

272

S2C2(CF3)2 S2C6H4 S2C6H3Me S2C2(CN)2

Cod

Rh Ni

S2C6H3Me S2C2(CN)2

Mn Fed

S2C2H2 S2C2HPh S2C2Me2

S2C2(CF3)2

S2C2(CF3)2 S2C6H4 S2C6H3Me S2C6H2Me2 S2C6Cl4 S2C2Ph2 S2C2(CN)2 S2C2(CN)2

S

S

M

1/0

1.22(1.17) 1.02 1.05(1.0) 0.92 1.03(0.98) 0.12(0.075) 0.115(0.07)
0.107(
0.15)

1.19

0.48 0.92(0.87) (ir)

1.24

0/-1


1/
2
0.69
0.12(
0.16) 1.06 0.67
0.99(
1.23) 0.16 0.08(0.03)
0.02 1.03 0.52
1.38(
0.82)
1.41(
0.85)
1.46(
0.90)
0.85(
0.29)
1.57(
1.01)
0.7 0.25(0.21) 0.23
0.26(
0.30)
0.12
0.088(
0.13)
0.92(
0.966)
0.879(
0.92)
1.114(
1.16)

Redox Potential (V)b


2.44


1.68


1.37


1.83


0.96(
1.0) (ir)


2/
3

TABLE II Redox Potential for Bis(dithiolene) Complexes M(S

S)2 a

SCE Ag/AgCl SCE SCE NHE SCE Ag/AgCl SCE SCE SCE Ag/Agþ Ag/Agþ Ag/Agþ Ag/Agþ Ag/Agþ SCE Ag/AgCl SCE Ag/AgCl SCE Ag/AgCl Ag/AgCl Ag/AgCl Ag/AgCl

Reference Electrode

CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 DMSO CH2Cl2 CH2Cl2 THF CH2Cl2 CH2Cl2 DMF DMF DMF DMF DMF THF CH2Cl2 MeCN DMF MeCN DMF DMF DMF DMF

Solventc

138 40 35 35 67 35 40e 39 35 35 139 139 139 139 139 38 40 140 97 140 97 97 97 97

Reference

273

Pt

Pd

S2C2(p-C6H4OMe)2 S2C2(CN)2

0.996(0.95) 0.165(0.12)
0.06(
0.105) 0.02(0.327) 0.182(0.137) 0.086(0.041) 1.15(1.105)

S2C2(CF3)2 S2C2H2 S2C2Me2 S2C2Ph2

1.17(1.48) (ir)

0.083(0.038) 0.035(
0.01) 0.218(0.173) 1.13(1.09)

0.134(0.09) 0.02(0.33)

0.45


0.119(
0.164)
0.121(
0.166)
0.151(
0.196)
0.60(
0.29)
0.60(
0.29)
0.61(
0.30)

S2C2(p-C6H4Me)2 S2C2(p-C6H4OMe)2 S2C2(p-C6H4Cl)2 S2C2(CN)2

1.15(1.46) (ir)

S2C2Et2 S2C2n-Pr2 S2C2i-Pr2 S2C2Ph n-Bu S2C2PhR, R ¼ Cyclopentylmethyl S2C2PhR, R ¼ 4-Pentylcyclohexyl S2C6H4 S2C6H3Me S2C6H2Me2 S2C6H2(OMe)2 S2C6Cl4 S2C2Ph2

0.243(0.198)


2.33


1.90

Ag/AgCl Ag/AgCl Ag/AgCl Ag/AgCl SCE Ag/AgCl Ag/AgCl Ag/AgCl Ag/AgCl Fc/Fcþ Ag/AgCl Ag/AgCl Ag/AgCl SCE Ag/AgCl


0.96(
1.01)
0.945(
0.99)
0.757(
0.80) 0.33(0.28) 0.473(0.43) 0.114(0.07)
0.718(–0.76)
0.87(
0.915)
0.61(
0.303)
0.636(
0.68)
0.72(
0.765) 0.23(0.185)

Ag/AgCl Ag/AgCl Ag/AgCl Fc/Fcþ Fc/Fcþ Fc/Fcþ Ag/Agþ Ag/Agþ Ag/Agþ SCE Ag/Agþ Ag/Agþ Ag/AgCl Fc/Fcþ


1.138(
1.18)
1.154(
1.2)
1.204(
1.25)
1.42(
1.11)
1.43(
1.12)
1.44(
1.13)
1.05(
0.49)
1.07(
0.51)
1.14(
0.58)
0.45
0.532(0.03)
1.24(
0.68)
0.881(
0.93)
0.80(
0.49) DMF DMF DMF CH2Cl2 MeCN DMF DMF DMF DMF CH2Cl2 DMF DMF CH2Cl2 MeCN DMF

DMF DMF DMF CH2Cl2 CH2Cl2 CH2Cl2 DMF DMF DMF CH2Cl2 DMF DMF DMF CH2Cl2

97 97 97 40 141 97 97 97 97 41 97 97 40 141 97

97 97 97 7 7 7 139 139 139 105 139 139 97 41

274

S2C2(p-C6H4Me)2 S2C2(p-C6H4OMe)2 S2C2(CN)2 S2C6H4 S2C6H3Me S2C6H2Me2 S2C6Cl4 S2C2Ph2 S2C2(CN)2 S2C2(CF3)2 S2C6H4 S2C6H3Me S2C6H3t-Bu S2C6H2Me2 S2C6Me4 S2C6Cl4 S2C10H6 (naphthalenyl)

S2C2(CF3)2 S2C2Me2 S2C2Ph2

S

S

1.20(1.51) (ir)

0.51(0.817)

1.51(1.465) 1.32 0.53 (qr) 0.46 (qr) 0.59 (qr) 0.22(0.527) 0.13(0.437)

0.852(0.807) 0.133(0.088)
0.06(0.247) 0.09(0.045) 0.043(
0.002)
0.004(
0.049) 1.28(1.235)


1.67(
1.11)


0.234(
0.28)
1.069(–1.11)
0.80(
0.493)
0.844(
0.89)
0.90(
0.945)
0.919(
0.96) 0.31(0.265)
1.14(
0.58)
1.15(
0.59)
1.21(
0.65)
0.752(
0.19)
1.41(
0.85)
0.54(
0.585)
0.97
1.57
1.61
1.59

Redox Potential (V)b


1.33 (ir) (
1.38)


0.76 (
0.81)

b

Numbers in parentheses are vs. SCE, converted using conversion factors listed in Table I. irreversible ¼ ir; quasireversible ¼ qr. c Dimethyl sulfoxide ¼ DMSO, tetrahydrofuran ¼ THF, dimethylformamide ¼ DMF. d Potentials are assigned for the redox couples of the dimeric species [M(S

S)2]2 (see Eq. 2) unless otherwise indicated. e For the redox couples of the monomeric species [M(S

S)2].

a

Au

Cu

M

TABLE II (Continued )

Ag/AgCl Ag/AgCl Fc/Fcþ Ag/AgCl Ag/AgCl Ag/AgCl Ag/AgCl Ag/Agþ Ag/Agþ Ag/Agþ Ag/Agþ Ag/Agþ Ag/AgCl SCE SCE SCE SCE Fc/Fcþ Fc/Fcþ Ag/Agþ Fc/Fcþ

Reference Electrode DMF DMF CH2Cl2 DMF DMF DMF CH2Cl2 DMF DMF DMF DMF DMF CH2Cl2 CH2Cl2 MeCN MeCN MeCN DMF DMF DMF DMF

Solventc

97 97 41 97 97 97 40 139 139 139 139 139 40 35 47 47 47 43 43 139 43

Reference

ELECTROCHEMICAL AND CHEMICAL REACTIVITY

275

or oxidation of the reduced species in couples with E1=2 > þ0:95 V, is generally difficult by normal chemical means. A powerful oxidant, capable of effecting the electron-transfer reaction, may also destroy the complex. In these cases, electrochemical methods have been proved useful (37). The redox properties of [Co(mnt)2]
and [Rh(mnt)2]
[mnt ¼ S2C2(CN)2, 1,2-maleonitrile-1,2-dithiolate] had been examined both chemically and electrochemically by Vlcek and Vlcek (38, 39). Reduction of the dianion to the trianion was achieved in THF at a dropping mercury electrode fE1=2 ¼
1:37 V for [Rh(mnt)2]2
/3
and
1.83 V for [Co(mnt)2]2
/3
(SCE)}. The trianionic species can be generated chemically by reduction of the dianion with LiAlH4 under anaerobic conditions. The trianion [Rh(mnt)2]3
is air-sensitive, which regenerates [Rh(mnt)2]2
upon exposure to O2. Oxidation of the dianion to the monoanion can be achieved chemically by using I2 or TCNE (tetracyanoethylene). The monoanion [Rh(mnt)2]
is in the singlet spin state, while [Co(mnt)2]
is a spin triplet. The monoanions [M(mnt)2]
readily dimerize to [M(mnt)2]22
(M ¼ Rh, Co) in solution (Scheme 2). Trianionic species [M(mnt)2]3
of Cu and Au have also been observed electrochemically (40). Although the [Cu(mnt)2]2
/3
couple is reversible, the corresponding couple for the Au complex is irreversible. The mono-cationic species [M(sdt)2]þ (sdt ¼ stilbeneditholate; M ¼ Ni, Pd, Pt) has been observed by cyclic voltammetry (CV) at E1=2  1.2 V (vs. Fc/Fcþ, ferrocene/ferrocenium) (41). The [M(sdt)2]0/þ couple is irreversible since the cationic species is highly reactive, which decomposes to give other radical species on the time scale of controlled potential electrolysis even at
40  C (41). Based on chemical, electrochemical, structural, and spectroscopic studies (2, 3), it has been concluded that the electron density in the metal orbitals does not change significantly as the charge level on the bis(dithiolene) complex is changed. Therefore, the accessibility of a range of charge levels of dithiolene complexes is perhaps more related to the accessibility of a number of formal oxidation states of the dithiolene ligands. Electrochemical and electron spin resonance (ESR) studies by Bowmaker et al. (41) support this view. One-electron

[M(mnt)2]−

S = 0 (Rh) 1 (Co)

+ e− − e−

[M(mnt)2]2−

S = 1/2

[M(mnt)2]22−

+ e− − e−

[M(mnt)2]3−

S=0

(M = Rh, Co) Scheme 2

276

KUN WANG

reduction of M(sdt)2 gives a product in which the unpaired electron is delocalized over both ligands and, to some extent, the metal. The species [M(sdt)2]
may be considered having one dithioketyl radical anion ligand and one dithiolate ligand (Eq. 3). − Ph

S

S

Ph

M II Ph

S

S

Ph

+ e− − e−

S

Ph

Ph

S

Ph

ð3Þ

M II

. Ph

S

S

Similarly, electron-transfer oxidation in toluene-3,4-dithiolate (tdt) complexes of Cu(II), Ni(II), Co(II), Fe(II), and Mn(II) has also been assigned ligand centered (42). Best et al. (40) reported infrared (IR) spectroelectrochemical studies of a group of dithiolene complexes [M(mnt)2]z
(M ¼ Ni, Pd, Pt, Cu, Au; z ¼ 1, 2). It was found that the CN stretching frequency nCN is relatively insensitive to the nature of the metal among the same type of complexes: nCN 2212 cm
1 for the monoanions ðz ¼ 1Þ and 2198 cm
1 for the dianions ðz ¼ 2Þ regardless of the number of metal d electrons. This finding suggests that the participation of metal d electrons in ligand p-acceptor orbitals is relatively insignificant. However, the fact that nCN decreases significantly as the charge of the complex goes from
1 to
2 (2212 vs. 2198 cm
1) suggests that the redox primarily center on the ligands, even though there is a certain degree of electron delocalization. A large number of bis(dithiolene) complexes has been prepared for group 10(VIII) (Ni, Pd, Pt) and group 11 (IB) (Cu, Au) metals. For a given metal, the ease of oxidation of [M(S2C2R2)2]z
decreases in the order R ¼ H, alkyl > aryl > CF3 > CN. This series parallels the electron-donating and -withdrawing ability of the substituent group R. For the dianions ðz ¼ 2Þ, the oxidative stability increases across the first-row transition metals: Fe < Co < Ni < Cu, indicating participation of metal d orbitals in the frontier orbitals of the dianionic species. Similarly, for [M(bdt)2]z
(bdt ¼ benzene-1,2-dithiolate) and [M(sdt)2]z
, the redox potential is dependent on the electronic properties of the substituents on the aromatic ring (Table II): The potential for the 0/
1 couple increases as the substituent group becomes more electron withdrawing. Square-planar bis(dithiolene) complexes have also been the subject of theoretical investigations (43–46). For example, density functional theory (DFT) calculations indicated that the highest occupied molecular orbital (HOMO) for Ni(S2C2H2)2 is primarily a ligand-based orbital comprising of four 3pz orbitals of sulfur, perpendicular to the molecular xy plane, and four 2pz orbitals of carbon with opposite phases. The lowest unoccupied molecular orbital (LUMO) is a mix of ligand–metal orbitals, but still mostly of the ligand

ELECTROCHEMICAL AND CHEMICAL REACTIVITY

277

Figure 1. Sketches of frontier orbitals for Ni(S2C2H2)2. [Adapted from (45)].

character (Fig. 1) (45). The results from molecular orbital calculations are consistent with experimental findings. For example, change of the metal does not affect the redox potential significantly for the [M(mnt)2]0/
1 couple (e.g., Ni, 1.22 V; Pd, 1.13 V; Pt, 1.15 V; Cu, 1.28 V vs. Ag/AgCl). This observation is not surprising since one-electron reduction of the neutral species adds the electron to the LUMO that has insignificant contribution from the metal d orbitals. Similarly, self-consistent field–Hartree–Fock (SCF–HF) calculations of the anion [Au(bdt)2]
predict that the HOMO is primarily a ligand-based p orbital, while the LUMO is a mixed-ligand–metal ( 50% of Au dxy ) orbital (43). The HOMO should therefore be destabilized by electron-donating substituents, which make the mono-anionic species easier to oxidize with more electrondonating substituents (43, 47). In contrast, the LUMO may be less sensitive to substituent electronic effects, which is consistent with the trend that the [Au(bdt)2]
1/
2 couple is not sensitive to the substituents on the dithiolene ligand (43, 47). In general, electrochemical data again support the molecular orbital description (Table II). In summary, all bis(dithiolene) complexes are redox active; most of them undergo two or three reversible, one-electron redox reactions. The dithiolene ligand itself is also redox active, which contributes significantly to the redox properties of the metal complex. Molecular orbital pictures derived from quantum mechanical calculations are consistent with the observed redox potential data. B.

Chemical Reactivity

Compared to the large body of electrochemical data, there have been fewer studies on the chemical reactivity of bis(dithiolene) complexes. In light of the rich redox chemistry of bis(dithiolene) complexes and the redox-active nature of the dithiolene ligands, it is not surprising that much of the reactivity observed is related to the redox properties and is often centered on the dithiolene ligands.

278

KUN WANG

In the absence of other reagents, most dithiolene complexes decompose at temperatures slightly above their melting points [e.g., 292 C for Ni(S2C2Ph2)2] to give low yields of organic sulfur-containing materials (48). 1.

Ligand-Exchange Reactions

Although most dithiolene complexes are stable compounds and many do not react with strong acids or bases, they are nevertheless reactive enough to undergo ligand exchange–displacement reactions as well as other reactions at the ligands. Similar to other square-planar metal complexes, bis(dithiolene) complexes undergo ligand-exchange reactions with other bis-chelating ligands in poorly coordinating solvents (Eq. 4) (49–53). M1 ðS

SÞ2 þ M2 ðL

LÞ2 Ð M1 ðS

SÞðL

LÞ þ M2 ðL

LÞðS

SÞ

(4Þ

(M1 , M2 ¼ Fe, Co, Ni, Pd, Pt, Cu, Au; S

S ¼ dithiolene; L

L ¼ bidentate ligands such as dithiolene, dithiocarbamate, a, b-diimine, diphosphine, etc.)

Reaction (4) is an equilibrium reaction and is generally slow at room temperature. The proposed reaction mechanism involves the formation of a stacking dinuclear intermediate (4) that isomerizes through the intermediate (5) to a mixed-ligand dinuclear species (6). Dissociation of 6 forming the mixedligand complexes is the rate-limiting step (Scheme 3). Consistent with the proposed mechanism, addition of a base such as a phosphine or an arsine, which is likely to suppress the dissociation of 6, inhibits the reaction. The equilibrium constant is not affected by the presence of an excess base (53). Ligand exchange–displacement reactions employing bis(dithiolene) complexes have been used to synthesize mixed-ligand complexes (Eq. 5) (54, 55). The reaction proceeds smoothly in dichloromethane with good yields (60–70%) (55). 2 Ni(S2C2R2)2 + W(CO) 3(MeCN)3 (R = Me, Ph)

W(CO)2(S2C2R2)2 + 2/n [Ni(S2C2R2)]n + 3 MeCN + CO

ð5Þ

2.

Ligand Addition and Substitution Reactions

Bis(dithiolene) complexes of group 8 (VIII) and 9 (VIII) metals generally exist in the dimeric form [M(S2C2R2)2]2z (M ¼ Fe, Co, Rh; z ¼ 0,
1,
2). Treating [M(S2C2R2)2]2z (M ¼ Fe, Co) with Lewis bases such as pyridine, phosphine,

ELECTROCHEMICAL AND CHEMICAL REACTIVITY

S S

S M1

S

L

+

S

L M2

L

279

S M1

S

L

L

S M2

L

L L

4 S

S M1

S

S L

M2

L

S

S M1 L

L

S

L

L

S

L

6 L

M1

M2

L

5

S

S

L

S

+

S S = a dithiolene

S

L M2

L

L L = a bidentate ligand Scheme 3

stilbine, CN
, or N3
results in dissociation of the dimer and formation of fivecoordinate adducts, [M(L)(S2C2R2)2]z/2 or [M(L0 )(S2C2R2)2](z/2
1) where L is a neutral and L0 is a monoanionic Lewis base (56–58). The adducts are, in most cases, also redox active (see below). McCleverty and Ratcliff (59) reported that passing NO through a suspension of [Fe(S2C2Ar2)2]2 [Ar ¼ Ph, 4-MeC6H4, 2-, 3-, or 4-MeOC6H4, 3,4-CH2O2C6H3, or 2,5-(MeO)2C6H3] breaks up the dimer giving the soluble NO adducts [Fe(NO)(S2C2Ar2)2]. The adducts adopt a square-pyramidal structure in which NO occupies the apical position. Neutral nickel bis(dithiolene) complexes undergo ligand substitution reactions with a variety of amines to form mixed-ligand dithiolene complexes (52, 60). Dance and Miller (60) pointed out that reduction of electron-poor bis(dithiolene) complexes (E1=2 > 0 V vs. SCE) by halides or pseudo-halides, unhindered amines, or common weakly Lewis basic solvents (such as acetonitrile)

280

KUN WANG

proceeds by initial nucleophilic displacement of a dithiolene ligand, which induces rapid subsequent reactions leading to overall disproportionation. The following reaction scheme was proposed (Scheme 4): rds

Step 1

Ni(tfd)2 + 2 MeCN

Step 2

Ni(tfd)2 + [(MeCN)2Ni(tfd)]

Step 3

Ni(tfd)2 + [(MeCN)2Ni(tfd)]+

Step 4

[(MeCN)2Ni(tfd)]2+ + 4 MeCN

Overall:

3 Ni(tfd) 2 +

[(MeCN)2Ni(tfd)] + tfd0 fast

[Ni(tfd)2]− + [(MeCN)2Ni(tfd)]+

fast

[Ni(tfd)2]− + [(MeCN)2Ni(tfd)]2+

fast

[Ni(MeCN)6]2+ + tfd0

2 [Ni(tfd)2]− + [Ni(MeCN)6]2+ + 2 tfd0

6 MeCN

tfd = 1,2-bis(trifluoromethyl)ethylenedithiolate tfd0 = 1,2-bis(trifluoromethyl)-1,2-dithiete rds = rate-determining step

Scheme 4

The seemingly simple ligand-exchange reactions in Steps 1 and 4 obviously involve electron transfer, since the charges on the free (tfd0) and on the coordinated tfd are different. Mixed-ligand complexes such as nickel dithiolene diimine can be prepared via ligand substitution reactions (Eq. 6) (52). Ni(S (S

S)2 + N

N

S = dithiolene, N

Ni(S

S)(N

N) + S

S

ð6Þ

N = diimine)

These complexes are redox active. The two one-electron reductions resemble more closely the reduction of the corresponding bis(diimine) complex than those of the corresponding bis(dithiolene) complex. The redox potentials are more sensitive to diimine ligand variation than to dithiolene variation. Oneelectron oxidation is relatively insensitive to diimine ligand variation. However, the dependence of one-electron oxidation on dithiolene variation has not been assessed directly due to the limited amount of data available on the oxidation of the corresponding dithiolene complexes. It has been proposed that the LUMO of the neutral mixed-dithiolene diimine complexes possesses more diimine than dithiolene character and that the HOMO is mainly metal d orbital in nature (52).

ELECTROCHEMICAL AND CHEMICAL REACTIVITY

3.

281

Alkylation–Protonation

Reduction of dithiolene complexes to their dianions increases the nucleophilicity of the sulfur atoms, facilitating electrophilic attack by alkyl halides. Schrauzer et al. (61, 62) described the formation of neutral complexes when [Ni(sdt)2]2
reacts with methyl iodide or other alkyl halides. The neutral product was first formulated as Ni(S2C2Ph2)(R2S2C2Ph2), with one dithiolate and one dithioether ligand, which was later determined to be a minor product (63). This

(Ph)CSMe and minor product is unstable and decomposes into MeSC(Ph)

[Ni(sdt)]n (n > 1). The major product (7) has the methyl groups bound at the sulfur atoms in two different ligands. The Ni

S4 moiety remains essentially planar with the bond lengths consistent with a localized electronic structure. The methyl groups are in trans positions, one above and one below the Ni

S4 plane, as revealed by the X-ray crystal structure (63). Further reaction with MeI results in full methylation of all four sulfur atoms, yielding Ni(Me2S2C2Ph2)2I2 (8) (63), the structure of which has also been determined by X-ray diffraction. The six-coordinate Ni(II) is equatorially coordinated by two molecules of cis˚ , only slightly bis(mercapto)stilbene. The average Ni

I bond length is 2.799 A shorter than the sum of the ionic radii, suggesting outer-sphere iodide coordination to nickel. Complex 8 is relatively labile, decomposing upon contact with protic solvents to give NiI2 and MeSC(Ph)

(Ph)CSMe. Me

Me Ph Ph

S S

Ni

7

Ph

S S Ph Me

Ph Ph

S S Me

I

Me

Ni

S S

I

Me

Ph Ph

8

Compared to reactions with methyl halides, reactions of [M(sdt)2]2
(M ¼ Ni, Pd, Pt) with benzyl halides are more complex (64). Three interconvertible isomers (9–11) have been observed in solution by low-temperature nuclear magnetic resonance (NMR) spectroscopy (Scheme 5). The trans–anti isomers (9) for all three metals have been isolated and are isomorphous (64). Dynamic behavior in solution is revealed by variable temperature NMR measurements and is primarily due to inversion at sulfur. The trans–anti and trans–syn isomers are the major species in the solution of the nickel complex. In solutions of Pd and Pt complexes, the cis–anti isomers are also detectable. The cis–anti isomer of the Pt complex has been isolated and is more stable than that of the Pd analogue (64). Hypothetically, a fourth isomer, the cis–syn isomer, may also exist. However, this species may not be stable even at temperatures as low as
50  C. Inspection of the crystal structure of the trans–anti isomer

282

KUN WANG

R

R Ph Ph

S S

M

trans-anti-

S S

Ph

Ph

S

Ph S

Ph

R

M

R S S

Ph Ph

trans-syn10

9 R Ph Ph

S

S M

S

R S

Ph Ph

cis-anti11

(M = Ni, Pd, Pt)

Scheme 5

suggests that the cis–syn isomer may be sterically less favorable. All products from the reaction of [M(sdt)2]2
(M ¼ Ni, Pd, Pt) with benzyl halides are light sensitive in solution. When M ¼ Ni, photolysis of 9 results in homolytic cleavage of the C

S bonds, generating Ni(sdt)2 and organic products derived from benzyl radicals (64). In contrast, the complex [Ni(mnt)2]2
was initially reported to be unreactive toward MeI (61); and it was speculated that the sulfur atoms are not sufficiently nucleophilic due to presence of the strongly electron-withdrawing CN group. Later investigations by Vlcek (65) reveal that the alkylation reaction does occur upon treatment with MeI, but the adduct decomposes rapidly in solution. The net reaction is described by Eq. 7. [M(mnt)2]2− + 4 MeI

2 Me2mnt + MI42−

ð7Þ

(M = Co, Ni, Cu, Zn)

Reactivity toward alkyl halides decreases significantly when going from dianion [M(mnt)2]2
to the corresponding monoanion [M(mnt)2]
, indicating a strong dependence of the reactivity on the electron density of the mnt ligands. However, the rate constant for the alkylation reaction of [M(mnt)2]2
is rather insensitive to the nature of the metal, again highlighting ligand-dominated reactivity in these complexes. Protonation of bis(dithiolene) complexes often results in the protons being added to the metal of the dithiolene complexes. Based on the similar electrochemical behavior to other metal hydrides, Vlcek and Vlcek (66) concluded that protonation of [M(mnt)2]3
(M ¼ Rh, Co) leads to addition of a proton at the metal, giving hydride complexes [M(H)(mnt)2]2
.

ELECTROCHEMICAL AND CHEMICAL REACTIVITY S− S

S

=

S

2

S−

S

2−

S

FeII

283

S

+ 2H+

H S

2

S

−

S

FeII S

fast

H

2−

S

Fe

S

S

II

S S S

S

FeII S H

H

H

S

S

FeII

S

H

0 +2

H+

S

S

H

H 0

2−

Fe

S

S

S

III

− 2H+

S S S

S

S S

S

FeIII S

S

FeIII

S

S

FeIII S H

Scheme 6

S

Fe II S H

H2

S

S

S

S

H

Fe

S

FeII

0

S

II

S

S

S

H

S

H

284

KUN WANG

In search of model systems for iron hydrogenases, Sellmann et al. (67) investigated the interaction of [Fe(bdt)2]2
with Hþ, H2, and H
. Formation of H2 was observed in the reaction with Hþ. The reaction mechanism was proposed to follow a step-wise protonation, forming a thiol–hydride complex; and H2 is proposed to form via heterolytic elimination from the metal hydride species (Scheme 6). Theoretical calculations suggest that concerted H2 elimination from a dithiol species is thermally forbidden (67).

4.

Cycloaddition with Unsaturated Hydrocarbons

Cycloaddition with unsaturated hydrocarbons such as alkenes and alkynes have been explored (68–74). Schrauzer and Mayweg (74) first reported that M(sdt)2 (M ¼ Ni, Pd, Pt) reacts with alkynes and alkenes via cycloaddition to the dithiolene ligand. With alkynes, dithiane is formed via decomposition of the adduct (Scheme 7). For dienes such as norbornadiene and butadiene, a 1:1 adduct with the Ni(S2C2R2)2 (R ¼ Ph, CF3) is formed. The adduct was originally proposed to have the structure of (12) (62). Subsequent study showed that only 13 is formed (2,3-dimethylbutadiene is used to illustrate the structures) (69, 71). Crystal structure of the adduct between 1,3-cyclohexadiene and Pd(sdt)2 was unambiguously established in later work by Clark et al. (73) (14). The olefin binds to the sulfur atoms across the ligands. Me

R R

Me S Ni S

S S

R = Ph, CF3

12

Me

Me R

R

R

R

S Ni S

S

R

S

R

Ph

S

S

Ph

S

Ph

Pd Ph

S

R = Ph, CF3

14 13

Wing et al. further investigated the reaction of olefins with dithiolene complexes and found that Ni(tfd)2 not only reacts with dienes, but also reacts with highly strained mono-olefins such as norbornene, forming a 1:1 adduct (69). The reaction with conjugated dienes is fast and thermally reversible. Reaction with nonconjugated olefins is slow and follows a second-order rate law. There is evidence that the adducts with nonconjugated olefins are light sensitive, dissociating into a photostationary equilibrium between the olefin and the adduct (62, 72, 74, 75).

ELECTROCHEMICAL AND CHEMICAL REACTIVITY

285

R R Ph

S

S

Ph

S

S

Ph

Ph

S

S

Ph

R

S

Ph

S

R

S

Ph

S

+

M Ph

Ph

M

Ph

(M = Ni, Pd, Pt)

S

S

+ RC CR

M Ph

Ph

n

Scheme 7

The norbornadiene adduct with Ni(tfd)2 was reported to possess two reversible one-electron redox processes (69): an oxidation at E1=2 ¼ 0:83 V and a reduction at E1=2 ¼
0:14 V (vs. SCE). The redox potentials are similar to those for [Ni(tfd)2]
, indicating an increase of electron density on the inorganic moiety upon olefin coordination. A reinvestigation of the norbornadiene adduct by Geiger (76) shown that the electrochemical behavior attributed to the adduct actually arises from a minor amount of the olefin free anion [Ni(tfd)2]
, produced during the reaction of Ni(tfd)2 with norbornadiene. The correct reduction potential of the adduct is 0.7 V more negative than the previously reported value. Furthermore, reduction of the olefin adduct is irreversible, leading to a rapid loss of norbornadiene and formation of the dianion [Ni(tfd)2]2
through an ECE (electron-transfer–chemical reaction–electron transfer) process. Interestingly, no reaction of bis(dithiolene) complexes with simple, monoolefins such as aliphatic olefins (ethylene, propylene, 1-hexene, etc.) had been reported until recently (77–79). Simple olefins such as ethylene, propylene, and 1-hexene have been found to react with Ni(tfd)2 under ambient conditions. The reaction is clean, selective, and reversible. The dithiolene complex does not react with H2O, CO, C2H2, H2, or low concentrations of H2S under the same conditions. The reaction could therefore be useful in cleaning up petrochemical olefin feeds in which these molecules are present as contaminants. If the olefin binding occurs at the metal center through p complexation, C2H2 and CO would compete strongly with olefins for the p interaction. It is therefore likely that the olefin binds at the ligand sulfur atoms rather than at the metal (Eq. 8), which is consistent with the observed reactivity pattern. The reaction is second order and the rate is significantly enhanced in polar solvents. Presence of electron-withdrawing groups in the olefin decreases the reaction rate. The reaction thus can be viewed as electrophilic addition of the complex to the

286

KUN WANG

H2C CH2 F3C

S

S

CF3

F3C +

Ni F3C

S

S

S

H2C CH2

S

CF3

S

CF3

Ni

CF3

F 3C

S

ð8Þ olefin. The DFT calculations suggest that the reaction is a two-step process, in which the trans product (15) is formed first from the direct addition of the olefin, while the thermodynamically more stable cis product (16) involves the isomerization of (15) (Scheme 8) (80).

R

S

S

R

S

R

H2C CH2

Ni R

S

H2C CH2 S R R S Ni R S S R 16

H2C CH2

R R

S S

H2 R CS Ni S R C H2 15

Scheme 8

Mechanistic studies suggest that the olefin binding and release rates may be significantly affected by changes of the electronic environment of the dithiolene complex. Electrochemical oxidation or reduction provides a way to tune the electronic properties of the dithiolene complex, and thereby may affect its affinity for olefins. Indeed, the binding and release of olefins can be controlled electrochemically (77) (Scheme 9). The starting dithiolene complex, Bu4N [Ni(mnt)2], is electrochemically oxidized to generate the neutral species, Ni(mnt)2, which reacts with olefin (ethylene, propylene, or 1-hexene) rapidly forming the olefin adduct. The olefin is rapidly released when the olefin adduct is reduced electrochemically. This controllable electrochemical reactivity with olefins may provide a basis for possible electrochemical processes to separate and purify olefins. The complex Ni(tfd)2 was also found to react with arenes such as perylene and pyrene forming 1:1 adducts (68). The resulting donor–acceptor complexes consist of stacks of alternating arene and Ni(tfd)2 molecules and are electrically

ELECTROCHEMICAL AND CHEMICAL REACTIVITY

S

NC

S

CN

S

CN

H2C CH2 S CN S NC Ni NC S S CN

H2C CH2

Ni NC

S

Binding

e−

Release

Recycle e−

NC

S

NC

S

287

S

CN −

S

CN

H2C CH2

Ni

Scheme 9

conducting in the solid state. Further discussions regarding these materials can be found in Chapter 8 in this volume (33). 5.

Addition Reactions with Other Unsaturated Compounds

Addition of other unsaturated compounds to bis(dithiolene) complexes has also been investigated. Reversible association with SO2 has been reported for [Au(tdt)2]
(81). The 1H and 13C NMR, as well as Raman spectroscopy, reveal weak association with SO2. It is suggested that SO2 bind either at the ligand sulfur atoms or at the toluene ring, possibly as a weak Lewis acid–Lewis base complex, rather than directly at gold. Whereas the reaction of molecular oxygen with [Ni(sdt)2]2
in weakly alkaline or neutral solutions leads to quantitative recovery of the neutral Ni(sdt)2, irreversible oxidation occurs with [Ni(sdt)2]2
in strongly alkaline solutions yielding the anion [Ni(O2S2C2Ph2)2]2
(17) (82). The anion has been isolated as a solvated sodium salt and its crystal structure has been determined. The ligands are in a cis configuration and the Ni is in a nearly planar environment. Clearly, oxygen reacts at the ligand sulfur atoms forming monosulfinate ligands (Scheme 10).

Na2

Ph

S

Ph

S

S

Ph

S

Ph

Ni

MeOH/ MeONa

2−

+

Na2

2 O2

2 PhCCPh + 2 SO2 + Na2NiS2

∆

2 PhCCPh + 2 SO2 + 2 H2S

Scheme 10

2− O OO O Ph S S Ni S S Ph Ph 17

Ph

2 H+

− Ni2+

Ph

SO2H

Ph

SH

288

KUN WANG

Compound 17 is the first example of a complex of cis-2-mercaptostilbene-1sulfinic acid, although the acid itself cannot be isolated (apparently it is not stable in the free state) (82). 6.

Miscellaneous Reactions–Applications

Nickel bis(dithiolene) complexes have been proposed as models for the active centers of hydrodesulfurization catalysts. Ab initio molecular orbital calculations on Ni(S2C2H2)2 indicate that the formally Ni(IV) (d6 ) state in the complex is stabilized upon H2S adsorption. The calculated adduct structure has H2S coordinating at nickel via the sulfur atom. The original planar structure of the dithiolene complex is distorted upon H2S binding, with the nickel atom ˚ . The S
being shifted from the basal plane by 0.35 A
C bond in the dithiolene ligand is weakened, while the C

C bond is strengthened upon H2S adduct formation, indicating that the ligand possesses more dithiolate character in the H2S adduct (83). Reynolds and co-worker (84, 85) reported a series of polymers containing nickel bis(dithiolene) linkages randomly inserted along the main chain of poly(phenylene) (18). S

S

R

z-

Ni S

S

n

R = O, S, CH 2, (CH2)10, (CH2)22, or (OCH2CH2)3O

18

The polymer with short flexible linkages is highly soluble in both aqueous and polar organic solvents in the reduced (dianionic) form and is slightly soluble in the oxidized or neutral form. Increasing the length of the organic flexible linkage in the polymer main chain increases the solubility of the polymers in the oxidized form. The polymers are electroactive and charge levels of the nickel complexes equivalent to [(NiL2)2
]n, [(NiL2)
]n, and [(NiL2)0]n can be observed electrochemically. A variety of bis(dithiolene) complexes has been reported as catalysts for the isomerization between quadricyclane and norbornadiene (Eq. 9) (86). ð9Þ Unlike conventional homogeneous catalysis involving relatively low oxidation state transition metals, the proposed mechanism involves successive

ELECTROCHEMICAL AND CHEMICAL REACTIVITY

289

pseudo-½4 þ 2 cycloaddition of quadricylane to the metal dithiolene in the dithioketone form. It has been proposed that the only function of the metal is to stabilize the dithioketone form of the R2C2S2 ligand in order to generate the necessary reactivity toward pseudo-½4 þ 2 cycloaddition (86). Oxidation of [Pd(mnt)2]2
by H2O2 produces a sulfonyl-containing anion {Pd(mnt)[O2S2C2(CN)2]}2
(19) (87), which retains a planar structure except for the sulfonyl oxygen atoms. Addition of AgClO4 to 19 forms {AgPd(mnt) [O2S2C2(CN)2]}22
(20), which has a double-decker structure consisting of two palladium dithiolene anions bridged by two Ag(I) ions through Ag

S bonds (87) (Scheme 11).

2−

NC

S

S

CN

excess H2O2

NC

S

Pd NC

S

S

CN

S

CN

Pd S

CN

NC

S

NC

2−

O

O

19

Ag+

NC

S S

O

O S

2−

CN

Pd Ag S CN

Ag S CN Pd NC S S CN O O NC

S

20

Scheme 11

Certain bis(dithiolene) complexes have been shown to possess antioxidant activities. For example, Ni(sdt)2 was found to be an effective antioxidant at temperatures up to 290 C for carboxy-terminated polybutadiene and polypropylene (88). This activity may be attributed to the fact that Ni(sdt)2 efficiently catalyzes the decomposition of peroxides. For example, Ni(sdt)2 has been shown to catalyze hydroperoxide (e.g., cumene hydroperoxide) decomposition (89–91). Reaction of Ni(sdt)2 with cumene hydroperoxide has been proposed to occur via a two-stage process. The first step is catalytic and involves formation of a dithiolene-hydroperoxide adduct where the hydroperoxide binds to the metal occupying the axial position (89, 90). In this stage, homolytic decomposition of hydroperoxide dominates and free radical intermediates are formed. This stage is followed by a stoichiometric, heterolytic decomposition (91). The cumylperoxy radical generated in the first step oxidizes the sulfur atoms in the dithiolene ligands leading to destruction of the dithiolene complex. A variety of fragments such as 1,2-diphenylacetylene, benzoic acid, and SO2, as well as possibly SO3 and H2SO4, can be formed. Dithiolene complex destruction was proposed to go through step-wise oxidation of the sulfur ligands (Scheme 12). Anionic iron bis(dithiolene) complexes have been reported to catalyze the autoxidation of phosphine, arsine, and cumene (92). No evidence for formation of molecular oxygen complexes has been found. The catalytic activity has been proposed to be a consequence of the redox activity of the complex. On this

290

KUN WANG

O Ph

S

Ph

S

S

Ph

RO2

RO

Ph

S

S

Ph

S

Ph

Ph

O Ph

S

Ph

S

S

Ph

S

Ph

RO2

RO

Ni

Ni

2RO2

2RO

S

O Ph

O

Ni S

nRO2

Ph

S

S

nRO

Ni Ph

Ph

O

S

S O

O

Ph

PhCCPh + PhCOOH + (SO2, SO3, H2SO4, NiSO4) Scheme 12

ground, other transition metal dithiolene complexes might also be active redox catalysts. Indeed, Dance (93) had shown that [Co(mnt)2]22
is an excellent catalyst for the autoxidation of thiols. Also related to their redox activity, a series of copper and manganese bis(dithiolene) complexes has been reported to catalyze the oxidative polymerization of 2,6-dimethylphenol to poly[oxy(2,6-dimethyl-1,4-phenylene)] (Eq. 10) (94, 95). OH O

n

ð10Þ n

In summary, bis(dithiolene) complexes are clearly distinct from traditional inorganic or organometallic complexes in which the chemical reactivity is dominated by the metal center. The unique properties of dithiolene ligands such as redox activity, aromaticity, and unsaturation of the metal–ligand chelate rings, in combination with the metal-centered reactivity paths, have generated many unusual reactivity patterns for this class of complexes. III.

TRIS(DITHIOLENE) COMPLEXES A.

Redox Properties

Tris(dithiolene) complexes are formed by group 4–6(IVB–VIB) metals, and by Mn, Re, Fe, Ru, Os, and Co. Most of the neutral tris(dithiolene) complexes

ELECTROCHEMICAL AND CHEMICAL REACTIVITY

291

adopt a trigonal-prismatic structure, although structures for the anions vary [see Chapter 2 (96) in this volume for structural trends]. Like the bis(dithiolene) complexes, the tris(dithiolene) complexes also undergo multiple one-electron redox reactions. In many tris(dithiolene) complexes, it is possible to observe four- or five-membered electron-transfer series corresponding to species with a charge of
3;
2;
1; 0, and þ1. The relationship between redox potential and chemical stability of tris(dithiolene) complexes is similar to that of the bis(dithiolene) complexes. The redox potentials (Table III) in general reflect the stabilization of the more highly charged (
2 or
3) species by electron-withdrawing substituent groups such as CF3 or CN on the dithiolene ligands. Early polarographic studies by Schrauzer and co-workers (97) revealed that the redox potentials for the 0/
1 and the
1/
2 couples of V, Mo, and W complexes are strongly dependent on the ligand substitution. A linear correlation between E1=2 values and Taft’s s (inductive substituent constant) was demonstrated (97). Similar trends have been observed for both symmetrically (R1 ¼ R2) and asymmetrically (R1 6¼ R2) substituted tris(dithiolene) complexes M(S2C2R1R2)3 (M ¼ Mo, W) (98). Redox potentials are highly dependent on the central metal ion, and for the same metal, on the nature of the ligand. The E1=2 values for both the 0/
1 and the
1/
2 couples for the Mo complexes are more positive than those of analogous W complexes by > 0:1 V, indicating that the Mo complexes are more easily reduced than their W analogues. For asymmetrically substituted W complexes of the type W[S2C2H (p-XPh)]3, the nature of X has a profound effect on the redox potentials (Table III). The ease of reduction for both processes (0/
1 and
1/
2) is the following: Cl > Br > H > Me > OMe Analogous behavior has been observed for the corresponding asymmetrically substituted Mo complexes Mo[S2C2H(p-XPh)]3. As indicated in Table III, a few tris(dithiolene) complexes exhibit oxidation waves corresponding to the formation of the monocations: Re(sdt)3þ, Re(tdt)3þ, and M(sdt)3þ (M ¼ Mo, W). However, none of these species has yet been isolated, although the oxidation potentials for generating these monocationic complexes appear to be accessible via chemical reagents. Similar to the case in the bis(dithiolene) complexes, the metal tfd complexes have been found to be one of the most powerful one-electron oxidizing agents isolated. For example, Mo(tfd)3 is an effective oxidizing agent for generating [V(mnt)3]
from the corresponding dianion [V(mnt)3]2
(99). Possible oxidation states of the metal in neutral tris(dithiolene) complexes have been considered. Upon complexation, the ‘‘ethylenic’’ bond in the dithiolene ligand has been observed to increase in length, and the ‘‘C–S’’

292

S2C6H4 S2C6Cl4 S2C2(CN)2 (estd) S2C6H3Me S2C2(CN)2

Ti

S2C2(CF3)2 S2C6Cl4 S2C2Ph2 (estd) S2C2(CN)2

Nb Ta Cr

Mo

S2C2(CF3)2

S2C2(CF3)2 S2C6Cl4 S2C2Ph2 S2C2HPh S2C2H2 S2C2(p-C6H4Me)2 [S2C2(p-C6H4OMe)2]3 S2C6H4 S2C6H4 S2C2(CN)2

Zr V

S

Sb

M

þ1/0

0.71(1.27) 1.13(1.09) 0.58 0.28(0.59) 0.98(0.93)

0.65 0.85 0.00

0.33(0.285) 0.26 0.25(0.205) 0.323(0.278) 0.269(0.215)

1.20

0/
1

0.48 0.36(0.92) 0.72(0.675)
0.01 0.17
0.68(
0.725)
0.665
0.722(
0.77)
0.745(–0.79)
0.783(
0.83)
0.38(0.179c )
0.71(
0.151c ) 0.76 0.25(0.81) 0.66(0.615)
0.09 0.11
0.7 0.49 0.30(0.86) 0.69(0.64) 0.05
0.28(0.03) 0.39(0.34)

0.56 0.96


1/
2


0.36
1.20
1.15
1.37(
0.81)
1.02(
1.06)

0.05
0.89(
0.33) 0.17(0.125)


2/
3
1.71(
1.15)
1.05
0.6
1.71(
1.15)
0.61
0.26(0.3)
0.32(
0.365)
1.06
0.97

Redox Potential


2.06(
1.5)
1.8(1.84)


1.57(
1.62)

TABLE III Redox Potential for Tris(dithiolene) Complexes M(S

S)3 a


3/
4 Ag/AgClO4 SCE SCE Ag/AgClO4 SCE Ag/AgClO4 Ag/AgCl SCE SCE Ag/AgCl SCE Ag/AgCl Ag/AgCl Ag/AgCl Ag/AgI Ag/AgI SCE Ag/AgClO4 Ag/AgCl SCE SCE SCE SCE Ag/AgClO4 Ag/AgCl SCE Fc/Fcþ Ag/AgCl

Reference Electrode MeCN CH2Cl2 CH2Cl2 MeCN CH2Cl2 MeCN CH2Cl2 CH2Cl2 CH2Cl2 DMF CH2Cl2 DMF DMF DMF CH2Cl2 CH2Cl2 CH2Cl2 MeCN CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 MeCN CH2Cl2 CH2Cl2 MeCN DMF

Solvent

102 142 142 102 142 143 104 142 142 97 107 97 97 97 102 102 142 143 103 142 142 142 142 143 104 142 144 97

Reference

293

W

S2C2(p-C6H4Me)2 S2C2(p-C6H4OMe)2 S2C2Me2

S2C2(CF3)2 S2C6Cl4 S2C6H3Me S2C6H2(OMe)2 S2C2H2 S2C2Ph2

S2C2Ph(p-MeOPh) S2C2(CN)2

S2C6H3Me S2C6H2(OMe)2 S2C2H(p-MeOPh) S2C2H(p-MePh) S2C2HPh

S2C2H2 S2C2Me2 S2C6Cl4 (estd) S2C2Ph2

1.10

1.10

0.91(0.86) 0.22(0.53) 0.54 0.02 0.04
0.133(
0.18)
0.34
0.54(0.02)
0.041(
0.086) 0.145(0.1)
0.091(
0.136)
0.138(
0.183)
0.333(
0.38)


0.30
0.49(0.07) 0.009(
0.035)
0.005(
0.05) 0.29 0.03 0.07(0.025) 0.122(0.077) 0.144(0.1) 0.010 0.135(0.09)


0.09(
0.135)
0.307(
0.352)


0.745(
0.79)
0.936(
0.98) 0.10
0.87
1.10(
0.54)
0.617(
0.66)
0.485(
0.53)
0.37
0.44
0.29(
0.335)
0.247(
0.29)
0.239(
0.28)
0.365
0.315(
0.36) 0.43 0.23(0.79) 0.35(0.30)
0.11(0.2)
0.05
0.54
0.56
0.845(
0.89)
0.87
1.14(
0.58)
0.684(
0.73)
0.32(
0.365)
0.681(
0.64)
0.751(
0.80)
0.994(
1.04) Ag/AgCl Ag/AgCl SCE SCE
2.92(
2.36) Ag/AgClO4 Ag/AgCl Ag/AgCl SCE SCE Ag/AgCl Ag/AgCl Ag/AgCl SCE Ag/AgCl
1.52 SCE
1.64(
1.08)
2.19(
1.63) Ag/AgClO4 Ag/AgCl Fc/Fcþ SCE SCE SCE Ag/AgCl SCE <
2.9(
2.34) Ag/AgClO4 Ag/AgCl Ag/AgCl Ag/AgCl Ag/AgCl Ag/AgCl

DMF DMF CH2Cl2 CH2Cl2 MeCN DMF CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 MeCN DMF MeCN CH2Cl2 CH2Cl2 CH2Cl2 DMF CH2Cl2 MeCN DMF CH2Cl2 DMF DMF DMF

97 97 142 142 143 97 98 105 105 98 98 98 107 98 142 143 97 144 142 142 105 97 142 143 97 98 97 97 97

294

1.34 0.43

0.85

S2C6Cl4 S2C2Ph2

S2C6H3Me S2C2(CN)2

S2C6Cl4 S2C2(CN)2 S2C2(CN)2

Fe

Ru Co

0.95(1.51) 1.48(1.43) 0.71
0.41
0.34(0.22) 0.39

0.61 0.18 1.00


0.068(
0.113) 0.06(0.015) 0.13(0.085)
0.035 0.195(0.15) 0.16(0.115) 0.097(0.05)
0.008(
0.05)

0.40
0.91
0.30

b


0.90


1.12

MeCN CH2Cl2 CH2Cl2 CH2Cl2 MeCN DMF CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 MeCN


0.72


0.35(0.21)
1.08(
0.52)
2.09(
1.53) Ag/AgClO4 0.13(0.085)
0.55(
0.59)
1.64(
1.69) Ag/AgCl
0.68
1.35 SCE
1.40 SCE
1.18(
0.62)
2.59(
2.03) Ag/AgClO4
0.91 SCE 0.53
0.38
1.18 SCE
0.07(
0.115) Ag/AgCl 0.27
0.77
1.51 SCE
0.70(
0.39)
1.71(
1.38) Fc/Fcþ 0.12 SCE


0.35
0.05(
0.095)
0.92

Solvent CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2 CH2Cl2

Reference Electrode Ag/AgCl Ag/AgCl Ag/AgCl SCE Ag/AgCl Ag/AgCl Ag/AgCl Ag/AgCl SCE Ag/AgCl SCE SCE SCE


0.483(
0.53)
0.41(
0.455)
0.32(
0.365)
0.42
0.18(
0.225)
0.26(
0.305)
0.367(
0.41)
0.455(
0.5)

Redox Potential (V)

(Continued )

Numbers in parentheses are vs. SCE, converted using conversion factors listed in Table I. Estimated ¼ estd. c Conversion factor for Ag/AgCl is used for the Ag/AgI reference electrode.

a

Tc Re

0.59 1.80

S2C2H(p-ClPh) S2C2H(p-BrPh) S2C2Ph(p-MeOPh) S2C2Ph(p-Me2NPh) S2C2(CN)2

S2C2H(p-MeOPh) S2C2H(p-MePh) S2C2HPh

S

Sb

S2C6Cl4 S2C6H3Me S2C2(CN)2 (estd)

Mn

M

TABLE III

143 104 142 142 143 108 142 103 142 145 146

98 98 98 107 98 98 98 98 142 103 142 108 142

Reference

ELECTROCHEMICAL AND CHEMICAL REACTIVITY

295

bond length decreases, thus strongly suggesting that the ligands have considerable ‘‘dithioketonic’’ character, especially in the neutral complexes. This observation implies that a formal oxidation state of VI for the metal is incorrect. The overall bonding nature can probably be represented by some combination of the valence bond forms shown below (Eq. 11). These representations appear to be consistent with molecular orbital calculations on these complexes (100, 101). [See Chapter 3 in this volume for details of the electronic structures of the tris(dithiolene) complexes (32).] S S S M S S S

S

S

S

S M S S S

S

S M S S S

S

S

S M S S S ð11Þ

The electrochemistry of group 4(IVB) and 5(VB) metal tris(dithiolene) complexes (Ti, Zr, Nb, and Ta) has been investigated by Martin and Takats (102). The reduction potential shifts to a more negative range with increasing metal d-orbital energies, supporting the molecular orbital description that the LUMO is of substantial metal character as suggested by Gray and co-workers (101). A series of mnt complexes [M(mnt)3]z
has been studied by Best et al. (103, 104) using spectroelectrochemical techniques. For complexes with z ¼ 1 or 2, the insensitivity of nCN to the identity of the metal indicates that there is little interaction between the metal d orbitals and the ligand pv orbitals. Therefore, the stability of the complex is largely a consequence of the strong metal–sulfur s or ph bonding. For z ¼ 3, however, significant metal–ligand p back-bonding occurs. The sensitivity of nCN to the nature of the metal for [M(mnt)3]4
(M ¼ V, Mo, Re) is even greater than that found for the corresponding trianions, suggesting increased metal contribution to the frontier orbitals in the tetraanions. Molybdenum and tungsten complexes with three crown ether benzenedithiolene ligands (21) have been reported (105); and the effect of alkali ion binding has been probed by CV (106). Upon binding with Liþ, Naþ, or Kþ, positive shifts in the redox potential have been observed for all complexes. This observation suggests that the tris(crown ether benzodithiolene) complexes of Mo and W may potentially be useful as sensors for alkali metal cations (106). O S

O

S

O

O

M O O M = Mo, W

21

3

296

KUN WANG

B.

Chemical Reactivity

Compared to bis(dithiolene) complexes, there are fewer reports on the chemical reactivity of tris(dithiolene) complexes. The 1H NMR studies of tris(dithiolene) complexes with asymmetrically substituted dithiolenes such as styrene-a,b-dithiolene reveal that the proton on the metalladithiolene (MS2C2) ring is subject to  3 ppm downfield shift and the ortho protons in the phenyl ring to a smaller downfield shift (107). This pattern is consistent with the presence of an induced diamagnetic ring current in the metalladithiolene ring, which is attributed to ‘‘aromaticity’’ of the metalladithiolene ring (107). Neutral tris(dithiolene) complexes M(tdt)3 (M ¼ Re, Mo, Tc) undergo drastic color changes when dissolved in polar solvents. Similar color changes also occur in the reactions with organic bases such as triphenylphosphine or o-phenanthroline (108). The reactions can be interpreted as one-electron reduction of the dithiolene complexes by the solvent or the base (Eq. 12). M(tdt)3

+

M(tdt)3−

B

ð12Þ

B+

+

Alkylation reaction is a common theme for both bis- and tris(dithiolene) complexes. For example, reaction of the dianion [W(sdt)3]2
with MeI forms [W(sdt)2(Me2S2C2Ph2)] (Eq. 13); and reaction of [Re(sdt)3]
with MeI gives a mono-methylated product [Re(sdt)2(MeS2C2Ph2)] (Eq. 14) (109). 2−

Ph

Ph

S S Ph Ph

2 MeI

S W S S S

Ph

Ph −

Ph

S S

Ph

Ph

Ph Ph

Ph

Ph

S Re S S S

Ph

S W S S S

ð13Þ Ph

Ph Ph

Ph

Me S S

MeI

Ph

Ph

Me S S Me

Ph Ph

S Re S S S

ð14Þ Ph Ph

Sellmann et al. (110) reported that the neutral tris(dithiolene) complex [W(bdt)3] undergoes nucleophilic alkylation by carbanions via intramolecular electron transfer (Scheme 13). Reaction of [W(bdt)3] with LiMe, depending on the concentration of the latter, yields either [W(bdt)2(MeSC6H4S)]
by alkylation (at high concentrations of LiMe) or [W(bdt)3]
by reduction (at low

ELECTROCHEMICAL AND CHEMICAL REACTIVITY 1/

2

H2 + C2H4

−

S W

S

3

.) nc co

3

3

(h

ig

h

S

S 22

Li M e

W

Me W

LiMe (low conc.)

−

S

−

S

24

H

297

−

S Et W

S

3

S

LiEt

W

S

−

3

S 2 LinBu

S

W

S

Me S

S

S 23

25

3

Me3 O

S

BF4

2−

S W

2M

e3 O

BF

4

S

S

W

S

Me S

S

S

Me 26 Scheme 13

concentrations of LiMe). It has been proposed that, at high concentrations, the methyl anion CH3
primarily attacks the tungsten center giving a sevencoordinate species (22), followed by methyl shift from metal to sulfur (23) accompanied by a formal reduction of W(VI) to W(IV). At low concentrations

298

KUN WANG

the CH3
functions as a one-electron reducing agent, giving rise to the formal W(V) complex 24 and possibly a methyl radical; the latter can abstract a hydrogen atom from the solvent forming methane (Scheme 13). With LiEt, reduction to 24 and formation of H2 and C2H4 are observed. The reaction was proposed to go through a tungsten–ethyl intermediate that eliminates a bhydrogen, forming a hydrido tungsten ethylene complex. Dissociation of C2H4 and cleavage of the W

H bond gives 24 (110). With other alkyllithium reagents LiR (R ¼ CH2t-Bu or Ph), only reduction reactions take place. The reduced species [W(bdt)2 (MeSC6H4S)]
(23) or [W(bdt)3]2
(25) reacts with carbocations such as Me3OBF4 forming a doubly alkylated product (26) (Scheme 13). This novel reaction type demonstrates the versatile reactivity of dithiolene ligands coordinated at transition metal centers. This reaction pattern may be important in understanding the trans-methylation reactions catalyzed by oxidoreductases with sulfur-coordinated transition metal centers such as CO dehydrogenase. The complex [Ru(mnt)3]3
is readily prepared by mixing Na2mnt with RuCl3 in water and is easily oxidized to [Ru(mnt)3]2
using I2 (111). Reactions of [RuIII(mnt)3]3
with triphenylphosphine or pyridine is sluggish while reactions of [RuIV(mnt)3]2
with these nucleophiles are complex, involving reduction by the displaced mnt2
ligand, to give trans-[RuIII(mnt)2(PPh3)2]
(27) and trans-[RuIII(mnt)2(py)2]
(28), respectively. A formally Ru(IV) complex, trans[RuIV (mnt)2Br2]2
(29), is formed upon treatment of [RuIV(mnt)3]2
with Br2 (111).

L NC NC

S S Ru S S L

−

CN

NC

CN

NC

Br S Ru S S S Br

2−

CN CN

27 (L = PPh3) 28 (L = Py)

29

The monoanionic tungsten complex W[S2C2Ph(p-MeOPh)]3
has been reported to catalyze formation of hydrogen from water, using free radicals derived from methyl viologen as the source of electrons (112). The catalytic cycle has been proposed to involve sequential electron and proton transfer. The complex Mo(tfd)3 was reported by King and co-workers (86) to catalyze the conversion of quadricyclane to norbornadiene (Eq. 9), polymerization of quadricyclane or norbornadiene, as well as the addition of water to quadricyclane to give nortricyclanol (Eq. 15). The reactions appear to involve pseudo½4 þ 2 cycloaddition to the sulfur atoms in the dithioketone form of the metal

ELECTROCHEMICAL AND CHEMICAL REACTIVITY

299

dithiolene. The same group also reported that Mo(tfd)3 catalyzes the oligomerization of acetone to Me2C(OH)CH2C(O)Me (Eq. 16). Electron transfer is likely involved in these reactions, although possible reaction mechanisms have not been discussed.

+

H2O

Mo(tfd)3

ð15Þ OH

O 2 Me

IV.

OH O

Mo(tfd)3

Me

Me Me

Me

ð16Þ

HETEROLEPTIC (MIXED-LIGAND) DITHIOLENE COMPLEXES

There is a large number of heteroleptic dithiolene complexes—complexes that contain at least one dithiolene and at least one other ligand. This chapter is focused on mononuclear and relatively simple dinuclear complexes. The electrochemistry of heteroleptic dithiolene complexes is only briefly discussed in view of the limited amount of data available. Oxodithiolene complexes are often studied as structural models for Mo and W enzymes and are reviewed in Chapter 10 in this volume (113). Focused here is the chemical reactivity of selected types of heteroleptic dithiolene complexes.

A.

Carbonyl Complexes

Ultraviolet (UV) or visible light irradiation of a mixture of group 6(VI) metal carbonyls M(CO)6 (M ¼ Cr, Mo, W) and Ni(S2C2Z2)2 (Z ¼ alkyl or aryl) affords the carbonyl complexes [M(CO)4(S2C2Z2)] and [M(CO)2(S2C2Z2)2] (54). The yield is improved significantly and no light is required when W(CO)3(MeCN)3 is used as the starting material (Eq. 5) (55). Polarographic investigations of W(CO)2(S2C2Ph2)2 reveal no evidence of redox behavior. However, the carbonyl groups in the bis(carbonyl)–bis(dithiolene) complexes are sufficiently labile and can be displaced by a variety of other ligands such as chalcogenide (55, 114), phosphine, dithiolene, or halogen ligands (115) (Scheme 14).

300

KUN WANG R

R

2−

S S S W S S S

Ph

R3P

Ph

Ph

Q

=

Ph Q− O, S, Se

Ph

PR3

S W S S S

Ph

3

Ph

PR

S W S S S

Ph [S2C2R2]2−

Ph

Ph

−

PhQ CO

Ph

Ph Ph

OC CO Ph

S W S S S

Ph

Ph Ph

2−

Q

X

e

=

2

S S, O,

Q

Q Ph

Ph

2−

S W S S S

Ph

Ph

Ph Ph

S X S S S W W S Ph S S X S Ph (X = Cl, Br, I)

MeI

Q

−

W S Ph Ph S S S Ph Me Ph

Scheme 14

The cis-dicarbonyl dithiolene complex [Rh(CO)2L]
(L ¼ mnt, tdt) can be prepared by treating Rh2(CO)4Cl2 with the appropriate dithiolene dianions. The complex [Rh(CO)2(mnt)]
undergoes substitution reactions with phosphines (Eq. 17) or bis(trifluoromethyl)dithiete, S2C2(CF3)2 (Eq. 18) (116). [Rh(CO)L(mnt)]− + CO

[Rh(CO)2(mnt)]− + L

(L = PPh3, P(OPh)3, PEt3)

ð17Þ [Rh(CO)2(mnt)]−

F3C

S

F3C

S

+

[Rh(mnt)(tfd)]nn−

+

CO

(n ≥1)

ð18Þ

ELECTROCHEMICAL AND CHEMICAL REACTIVITY

301

The products from Eq. 17 are redox active and undergo one-electron oxidation giving the neutral species (Eq. 19) (116). [Rh(CO)L(mnt)]−

− e−

B.

Rh(CO)L(mnt)

(L = CO, PPh 3)

ð19Þ

Nitrosyl Complexes

McCleverty and co-workers (117, 118) found that the dimeric dianion [Fe(S

S)2]22
is a useful intermediate in the synthesis of iron mono-nitrosyl complexes. Treating the dimer with NO in nonpolar or weakly coordinating solvents gives [Fe(NO)(S

S)2]
. These species are stable toward aerial oxidation but are often attacked by two-electron donor ligands such as pyridine(py), DMF, or PPh3, resulting in expulsion of NO and formation of the corresponding adduct [Fe(L)(S

S)2]
(L ¼ py, DMF, PPh3). Complexes [Fe(NO)(S2C2R2)2]z (z ¼
1; R ¼ CN, CF3, or Ph) contain two redox active ligands (NO and dithiolene) and a redox active metal, they can undergo extensive one-electron redox reactions, comprising a five-membered electron transfer series with z ¼ þ1, 0,
1,
2, or
3 (118). The redox potential for the couple [Fe(NO)(S2C2R2)2]z=z
1 is dependent on the nature of the substituent group on the dithiolene ligand. The relative oxidative stability of the reduced species is S2C2(CN)2 > S2C2(CF3)2 > S2C6Cl4 > S2C6H3Me > S2C2Ph2. The redox potentials have been used as a guide in choosing proper chemical reagents for the preparation of the species with z ¼ 0,
1, or
2 (118). The aryl dithiolene complexes have also been prepared and the electrochemistry studied. Complexes [Fe(NO)(S2C2Ar2)2] [Ar ¼ 4-MePh; 2-, 3-, or 4-MeOPh; 3,4-CH2O2Ph; or 2,5-(MeO)2Ph] (59) are part of a four- and possibly fivemembered redox series: [Fe(NO)(S2C2Ar2)2]z, z ¼
2;
1; 0; þ1, or þ2. Redox potentials for the couple [Fe(NO)(S2C2Ar2)2]z/z
1 are dependent on the nature of the aryl group, as shown in Table IV. The E1=2 decreases with electrondonating substituents at the phenyl ring, indicating decreased oxidation stability of complexes with electron-rich dithiolene ligands. The NO stretching frequency nNO in infrared (IR) is dependent on the overall charge z of the complexes. The nNO for the dianion is 140 cm
1 lower than that of the monoanion, which is 30 cm
1 lower than that of the neutral species. The trend is anticipated simply on the grounds that the degree of back-bonding to NO is decreased as the negative charge is removed from the complex. The nature of the substituent group on the dithiolene ligands also has a strong effect on nNO in both the mono- and dianionic species. As the relative electronaccepting ability of the dithiolene ligand decreases, the extent of the backdonation to the NO group increases and nNO decreases. The order of decreasing

302

KUN WANG TABLE IV Redox Potential for Iron and Cobalt Nitrosyl Dithiolene Complexes, [M(NO)(S

S)2]z

M

S

S

Fe

S2C2(CN)2 S2C2(CF3)2 S2C6Cl4 S2C6H3Me S2C2Ph2 S2C2(4-MePh)2 S2C2(4-MeOPh)2 S2C2(3-MeOPh)2 S2C2(2-MeOPh)2 S2C2(3,4-CH2O2Ph)2 S2C2[2,5-(MeO)2Ph]2 S2C2(CN)2

Redox Potential þ2/þ1

Co a

0.95

þ1/0

0/
1

0.71 0.54 0.46 0.62 0.45 0.88 0.31

0.84 0.74 0.27
0.02
0.15
0.17
0.10
0.22
0.23
0.16

a

Reference
1/
2 0.03
0.07
0.24
0.64
0.83
0.85
0.9


2/
3
1.34
0.36

118

59


0.42
0.48 0.16


1.32

118

Reference electrode ¼ SCE; solvent ¼ CH2Cl2.

nNO is S2C2(CN)2 > S2C2(CF3)2 > S2C6Cl4 > S2C6H3Me > S2C2Ph2, consistent with the trend observed for redox potentials (118). Interestingly, for the neutral diaryl dithiolene complexes such as Fe(NO)(S2C2Ar2)2, nNO has little dependence on the nature of the substituent group on the aryl ring (59), suggesting that there is significant difference in the electronic structures of the anions and the neutral species. Cobalt mono-nitrosyl complexes, [Co(NO)(S

S)2]
, can be obtained simi2
larly by treating [Co(S

S)2]2 with NO. However, these species are significantly less stable than their iron analogues (118). The electrochemical transformation of a molybdenum nitrosyl complex [Mo(NO)(dttd)] [dttd ¼ 1,2-bis(2-mercaptophenylthio)ethane] (30) is rather interesting (119). Ethylene is released from the backbone of the sulfur ligand upon electrochemical reduction. The resulting nitrosyl bis(dithiolene) complex reacts with O2 to give free nitrite and a Mo–oxo complex. Multielectron reduction of 30 in the presence of protons releases ethylene and the NO bond is cleaved, forming ammonia and a Mo–oxo complex (Scheme 15). The proposed reaction mechanism involves successive proton-coupled electrontransfer steps reminiscent of schemes proposed for Mo enzymes (120). C.

Other Mixed-Ligand Dithiolene–Donor Complexes

Other donors that can form mixed-ligand complexes with dithiolene ligands include phosphorous-, nitrogen-, oxygen-, and other sulfur-based ligands. The resultant complexes are abundant and most of them are redox active. However,

ELECTROCHEMICAL AND CHEMICAL REACTIVITY 2−

NH3 6 e−, 3 H+

S S NO Mo S Cl S

303

Cl− +

S S Mo O S S

NO2−

C2H4 O2

30

3− 4 e−

Cl− +

S S Mo NO S S

Scheme 15

the dithiolene is, to a certain degree, a spectator ligand in many of these reactions, which are mostly based on the other donor ligands or on the metal. Therefore the electrochemical and chemical reactivity of these complexes are not reviewed here. Interested readers are referred to an earlier review (2). D.

Dithiolene Complexes with Metal–Ligand Multiple Bonds

Many molybdenum and tungsten oxo and sulfido complexes are structurally related to molybdenum and tungsten enzymes. The chemistry of this class of complexes can be found in Chapter 10 in this volume (113). Other metal-oxo complexes such as V, Cr, Tc, and Re oxo complexes have been isolated and characterized. The five-coordinate vanadyl complex, [VO(mnt)2]2
, reacts with excess Na2mnt forming [V(mnt)3]2
(121). Bis(dithiolene)-oxo complexes of Tc and Re, [MO(mnt)2]
(M ¼ Tc, Re), have been proposed as possible medical imaging agents (122). Redox potentials for the Re complexes are lower than those for the Tc analogues, indicating that the former are harder to reduce than the Tc analogues. Metal-nitrido complexes containing dithiolene ligands have been reported recently. Reaction of electrophiles such as R3OBF4 (R ¼ Me, Et) and Ph3CPF6 with [M(N)(bdt)2]
(M ¼ Ru, Os) has been studied by Sellmann et al. (123) While the Ru-nitrido complex gives an intractable mixture of products, the Osnitrido complex yields clean products (Eqs. 20–21). Alkylation at sulfur is

304

KUN WANG

observed when Meþ or Etþ is used, while alkylation at the nitrido ligand is observed with much bulkier electrophiles, such as the trityl cation. Even though the different electronic properties between Ph3Cþ and Meþ or Etþ may contribute to the difference in reactivity, it is more likely that the steric factors are responsible. The bulkier trityl cation preferentially attacks the more exposed nitrogen atom. N Os S S S

S

−

+ R3O+

N S Os S S S

CH2Cl2 r.t.

+ R2O

R

(R = Me, Et; r.t. = room temperature)

ð20Þ

CPh3 N S Os S S S

−

+ Ph3C+

N S Os S S S

CH2Cl2 r.t.

ð21Þ

(r.t. = room temperature)

Reaction of strong Lewis acids such as B(C6F5)3 with [Os(N)(bdt)2]
yields a product in which the Lewis acid is added to the nitrogen atom (31). The strength of [Os

N]

B interaction was probed using a series of Lewis bases such as NEt3, PMe3, and THF. In the presence of a strong Lewis base such as NEt3 or PMe3, the borane can be removed and the parent complex regenerated quantitatively. Further reaction of 31 with MeOTf (OTf
¼ OSO2CF3
, triflate anion) results in alkylation at the sulfur atom (Eq. 22) (124). B(C6F5)3 N S Os S S

S

−

N S Os S S S

B(C6F5)3 CH2Cl2

31

−

ð22Þ

B(C6F5)3 MeOTf CH2Cl2

Me S S

N Os

S S

Complex [Mo(N)(bdt)2Cl]2
can be synthesized by the reaction of [Mo(N)Cl4]
with two equivalents of Li2bdt (125). The complex reacts readily with O2

ELECTROCHEMICAL AND CHEMICAL REACTIVITY

305

or H2O, even in the solid state, to produce [Mo(O)(bdt)2]2
(Eq. 23). The extreme air-sensitivity of [Mo(N)(bdt)2Cl]2
shows that it has higher reactivity
N bond is toward O2 compared to [Mo(N)Cl4]
, suggesting that the Mo
weakened by dithiolene coordination. N − Mo Cl Cl

Cl Cl

2 Li2bdt

S S

N Mo

2−

S S

Cl

ð23Þ O O2

S S

solid state

2−

S Mo S

E.

Organometallic Complexes

1.

Cyclopentadienyl Complexes

A large number of cyclopentadienyl (Cp) complexes, both neutral and charged, has been prepared. Most of the complexes reported are for early to mid-transition metals. Rich redox and chemical reactivity has been reported (126). Extensive work on the Cp dithiolene complexes has been carried out by Sugimori et al. (127) and has recently been reviewed. A few representative examples are discussed below. The coexistence of aromaticity and unsaturation in the metalladithiolene ring presents unique reactivity for this class of complexes. The cobaltadithiolene ring in [CpCo(S2C2HR)] undergoes electrophilic (Eq. 24), radical (Eq. 25), and ionic (Eq. 26) substitution reactions that are typical for aromatic rings. Co

S

H

S

R

MeCOCl (AlCl3)

Co

S

COMe

S

R

ð24Þ

(R = H, Ph, COOMe)

Me Co

S S

H R

AIBN 80oC, benzene

Co

S S

C

Me

Me

CN + Me C R CN

Co

S

H

S

R

(R = H, Ph, COOMe; AIBN = 2,2'-Azobisisobutyronitrile)

ð25Þ

306

KUN WANG

O

O

N X

Co

S S

H

O

Co

o

30 C, CHCl3

Ph

ð26Þ

S

N

S

Ph

O

(X = Br, I)

In addition to aromaticity, the metalladithiolene ring also possesses the characteristics of unsaturation. The metalladithiolene ring in the [CpCo(S2C2XY)] type of complexes has been found to react with a variety of reactive molecules (Scheme 16) (127). R2

R1 C

X

S Co S

Y

32 X

Co S P(OR)3

∆ or hν

Y

X N3R (-N2) or

S

S

[RN=I-Ph]

X

ν/

RN

O

2

P(

O

R)

3

N2CR1R2 (-N2)

S

h

Co

Co S

S

Y

Z

Z

S

X

S

X

Co

Co S

hν

ZC

C

Z

hν

Y

S

Y

Y

(for pentamethylcyclopentadienyl compounds) Scheme 16

Most of these reactions are reversible under either thermal or photochemical conditions. Interestingly, the bridging alkylidene complex (32) is redox active.

ELECTROCHEMICAL AND CHEMICAL REACTIVITY

307

Electrochemical reduction or oxidation causes the elimination of the bridging alkylidene moiety (128, 129). Cyclopentadienyl dithiolene complexes also undergo ligand-transfer reactions (Eq. 27) (130). This type of reaction provides a useful synthetic tool for late transition metal dithiolene complexes. The driving force for this reaction is the formation of the more stable Cp2TiCl2 as compared to the less stable Cp2Ti(bdt) (presumably due to the ‘‘hard–soft mismatch’’ between metal and ligands in the latter). The reaction is believed to proceed via an associative pathway involving a heterobimetallic intermediate with bridging dithiolene ligands (130). Cp2Ti(bdt)

+

Cp2TiCl2 +

Pt(cod)Cl2

Pt(cod)(bdt)

ð27Þ

(cod = 1,5-cyclooctadiene)

Similar to homoleptic dithiolene complexes, alkylation of Cp2M(S2C2Z2) results in the alkyl group being added to the ligand sulfur atom (Eq. 28) (131).

Me CO2Me

S M S

CO2Me

+ MeI

M

S S

+ CO2Me

I−

CO2Me

(M = Mo, W)

ð28Þ Oxidation of complex Cp2Ti(S2C2Z2) (Z ¼ CO2Me) with sulfuryl chloride affords 1,2-dithiete and its oligomers (132). The reaction, if proved general, could be employed as a synthetic route to dithietes (Eq. 29). Z S Ti S

Z

SO2Cl2

Z

S

Z

benzene

Z

S

(Z = CO2Me)

Z + Z

S S S S

Z

Z + Z

Z

S S Z

Z S S

S

S S S

Z

ð29Þ Z

Z

Cyclopentadienyl dithiolene complexes such as CpCo(S2C2R2) and Cp2Ti(S2C2R2) have been shown to catalyze the isomerization of quadricyclane to norbornadiene (Eq. 9) (133). The catalytic activity is closely related to the reduction potential of the complexes: Complexes with higher E1=2 are more active. This result suggests that the reaction involves a certain degree of charge transfer between quadricyclane and the dithiolene complex. The complex Cp2Ti(mnt) has recently been reported to show anti-tumor activity (134).

308

KUN WANG

2.

Other Organometallic Complexes

Eisenberg and co-workers (135–137) investigated dithiolene-containing organometallic complexes of Rh and Ir. Addition of an alkyl halide to [Rh(CO)(PPh3)(mnt)]
forms a Rh(III) halo acyl complex. The halide can be stripped off by Agþ salts (135) (Eqs. 30 and 31). [Rh(CO)(PPh3)(mnt)]− + RX [RhX(COR)(PPh3)(mnt)]−

[RhX(COR)(PPh3)(mnt)]−

Ag+

ð30Þ

[Rh(COR)(PPh3)(mnt)(sol)]

X−

ð31Þ

(X = Cl, Br, I; sol = coordinating solvent)

Migration of the alkyl group between the acyl carbon atom and a sulfur donor atom has been observed (136). The reaction was proposed to involve successive 1,2-migrations with a Rh(III) alkyl dithiolene intermediate (Scheme 17). O L'

C

R

S Rh L S

CN

− L'

CN

O

R C S Rh S L

CN

OC

CN

L

R Rh

S

CN

S

CN

OC L

Rh

R S

CN

S

CN

(L, L' = PPh3, CO)

Scheme 17

While both the alkylation–dealkylation at a sulfur donor ligand and the insertion–deinsertion of a carbonyl group at a metal center are well-known processes, the rhodium carbonyl dithiolene complex is unique in having both processes accessible within the same system. Migration of the R group within the coordination sphere of these complexes involves internal redox reactions in which the Rh center is reduced while the dithiolene ligand is formally oxidized. The alkyl group migration reaction of these Rh dithiolene complexes thus may have interesting implications in the area of small molecules (e.g., alkanes, CO) activation. V.

CONCLUSIONS AND FUTURE OUTLOOK

Although it has already been studied extensively, the redox and redox-related chemistry remains a dominant theme in dithiolene chemistry. Much of the chemical reactivity reported so far is associated with the dithiolene ligand and is, in many cases, related to the redox properties. Clearly, the range of accessible charge levels of dithiolene complexes may be exploited for new reaction

ELECTROCHEMICAL AND CHEMICAL REACTIVITY

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chemistry. However, most homoleptic dithiolene complexes isolated so far are anionic; only a small number of neutral complexes have been isolated while no cationic species have been isolated. In applications where electrophilic interactions are necessary, such as the activation of alkanes and the binding of olefins, species in the neutral or the cationic form are obviously of greater interest– importance due to their higher reactivity. Therefore, generation and isolation of neutral or cationic dithiolene complexes may lead to interesting reactivity and possible new applications. Electrochemical techniques have shown great advantages in this regard. Complexes with charge levels that are not easily accessed chemically can be accessed electrochemically; and novel reactivity pattern can sometimes be observed with these electrochemically generated species (37, 77–79). Electrochemical modulation of the chemical reactivity of dithiolene complexes is likely to remain a fruitful area. ACKNOWLEDGMENTS I would like to acknowledge many of my current and former ExxonMobil colleagues for their advice and support during the preparation of this chapter. I especially would like to thank Dr. Ed Stiefel for his advice and mentoring, Dr. John Robbins for critical reading of the manuscript. Dr. Jose Santiesteban and Dr. Mike Matturro are acknowledged for their understanding and support. I would also like to thank Dr. Raquel Terroba and Dr. Colin Beswick for sharing their data prior to publication. Dr. Hal Murray is acknowledged for his help with the literature search.

ABBREVIATIONS AIBN bdt cod Cp CV DFT DMF DMSO dttd ECE ESR Fc Fcþ HF HOMO

2,20 -Azobisisobutyronitrile Benzene-1,2-dithiolate 1,5-Cyclooctadiene Z5-Cyclopentadienyl anion Cyclic voltammetry Density functional theory Dimethylformamide Dimethyl sulfoxide 1,2-Bis(2-mercaptophenylthio)ethane Electron-transfer–chemical reaction–electron-transfer process Electron spin resonance Ferrocene Ferrocenium ion Hatree–Fock Highest occupied molecular orbital

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KUN WANG

IR LUMO L

L mnt NHE NMR N

N OTf
pdt Py rds r.t. SCE SCF sdt S

S TCNE tdt tfd tfd0 THF UV

Infrared Lowest unoccupied molecular orbital Bidentate ligand 1,2-Maleonitrile-1,2-dithiolate Normal hydrogen electrode Nuclear magnetic resonance Diimine ligand Trifluoromethanesulfonate anion 1,2-Diphenyl-ethylene-1,2-dithiolate Pyridine Rate-determining step Room temperature Saturated calomel electrode Self-consistent field Styrene-a,b-dithiolate Dithiolene ligand Tetracyanoethylene Toluene-3,4-dithiolate 1,2-Bis(trifluoromethyl)ethylene-1,2-dithiolate 1,2-Bis(trifluoromethyl)-1,2-dithiete Tetrahydrofuran Ultraviolet REFERENCES

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CHAPTER 6

Luminescence and Photochemistry of Metal Dithiolene Complexes SCOTT D. CUMMINGS Department of Chemistry Kenyon College Gambier, OH RICHARD EISENBERG Department of Chemistry University of Rochester Rochester, NY CONTENTS I. INTRODUCTION AND BACKGROUND 8

II. SQUARE-PLANAR d COMPLEXES A.

B. C.

316 320

Square-Planar Bis(dithiolene) Complexes / 320 1. Excited States and Luminescence / 320 2. Ion-Pair Charge-Transfer Photochemistry / 324 3. Photoproduction of Hydrogen / 328 4. Photooxidation Chemistry / 330 5. Photochemical Radical Formation / 334 Square-Planar Mixed-Ligand Dithiolene–Donor Complexes / 335 Square-Planar Mixed-Ligand Dithiolene–Diimine and Related Complexes / 339

Dithiolene Chemistry: Synthesis, Properties, and Applications, Progress in Inorganic Chemistry, Vol. 52 Special volume edited by Edward I. Stiefel, Series editor Kenneth D. Karlin ISBN 0-471-37829-1 Copyright # 2004 John Wiley & Sons, Inc. 315

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Excited States and Luminescence / 339 Photoinduced Electron-Transfer Reactions / 344 Self-Quenching / 346 Photooxidation Chemistry / 348 Structural Variation / 351

III. TETRAHEDRAL AND DISTORTED FOUR-COORDINATE d10 COMPLEXES A. B.

Tetrahedral Bis(dithiolene) Complexes / 353 Tetrahedral Mixed-Ligand Dithiolene Complexes / 354

IV. OCTAHEDRAL d n COMPLEXES WITH n
6 A. B.

353

356

Homoleptic Complexes / 356 Mixed-Ligand Complexes / 357

V. CONCLUSIONS

360

ACKNOWLEDGMENTS

361

ABBREVIATIONS

361

REFERENCES

362

I.

INTRODUCTION AND BACKGROUND

Since their initial popularity in the 1960s (1–4), metal dithiolene complexes have been studied extensively, stimulated in part by their real or potential applications in diverse areas such as light energy conversion schemes (5–9), nonlinear optics (10–14), Q-switch laser dyes and light-driven information devices (15, 16), and their biological relevance as models for molybdenum pterin cofactors (17–24). All of these applications derive from the interesting electronic structures that metal dithiolenes possess that are often characterized by facile redox behavior of the complexes. As is true for all of the chapters in this volume, we focus here on 1,2-dithiolene complexes containing the unsaturated five-membered MS2C2 chelate ring 1, as distinct from other unsaturated or saturated dithiolate systems. In this chapter, the luminescence properties and photochemical behavior of 1,2-dithiolene complexes are reviewed. During the early days of bonding analysis of metal dithiolene complexes, the systems often appeared to defy conventional oxidation state formalisms with overall complex reduction leading in some cases to formal metal oxidation (1, 25–36). In this context, 1,2-dithiolene ligands were described as ‘‘noninnocent,’’ with highest occupied and lowest unoccupied ligand orbitals interacting with metal d functions to give frontier orbitals of mixed character to varying degrees. This aspect may be contrasted with more conventional coordination complexes in which the extent of metal–ligand delocalization is lower or more

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restricted. Part of this difference arises from the relative energies of the sulfur donor valence orbitals compared with those of the metal bonding functions and part results from the nature of the p structure of the five-membered metal– dithiolene chelate ring illustrated in part by the enedithiolate and dithione resonance structures of 1. S

S

M (n-2)+

M n+

S

S 1

A semiempirical molecular orbital (MO) treatment of the p structure of the 1,2-dithiolene ligand mnt2 (mnt ¼ 1,2-maleonitrile-1,2-dithiolate) and its 1,1dithiolate isomer denoted as i-mnt2 (i-mnt ¼ 2,2-dicyanoethylene-1,1-dithiolate) is instructive in considering the key metal–dithiolate interactions that determine the excited states of their respective complexes. The most important points are that: (1) the highest filled p orbital for mnt2 lies significantly higher in energy than the corresponding p orbital for i-mnt2, and (2) the lowest unoccupied p* orbital for mnt2 lies lower in energy than the analogous function for i-mnt2. These orbitals are shown in Fig. 1. The consequence of the former is that mnt2 is in principle a better p donor than i-mnt2, when interacting with a metal dp orbital of like symmetry while the effect of the latter is to make the mnt2 ligand a better p acceptor than i-mnt2. Both of these features lead to greater electron delocalization in 1,2-dithiolene complexes relative to 1,1-dithiolate analogues and lower energy electronic transitions involving charge-transfer behavior for the former. The individual sulfur s donor orbitals for mnt2 and i-mnt2 are similar in energy but because of differences in the relative proximity of the S donors, symmetric and antisymmetric combinations are more affected in the latter relative to the former. These differences mean that the two linear combinations of S s donor atomic orbitals in i-mnt2 (the symmetric and antisymmetric combinations) bracket the corresponding functions for mnt2 and make the 1,1dithiolate ligand a stronger s donor. The 1,1-dithiolate ligand may thus be viewed as a somewhat stronger field ligand than the corresponding 1,2dithiolene system. This notion is supported by spectroscopic evidence obtained for various metal dithiolene and analogous 1,1-dithiolate complexes. For example, the lowest energy d–d transition for [Ni(mnt)2]2 (2) first reported by Gray and co-workers nearly 40 years ago (37) is seen at 11,700 cm1 while that for the isomeric complex [Ni(i-mnt)2]2 (3), occurs at 15,700 cm1, corresponding to a difference in the splitting parameter
1 of 4000 cm1 (38, 39). Each of these complexes also exhibits charge-transfer (CT) transitions

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SCOTT D. CUMMINGS AND RICHARD EISENBERG

Figure 1. Highest occupied (HOMO) and lowest unoccupied (LUMO) p molecular orbitals for mnt2 (left) and i-mnt2 (right). The vertical positions correspond approximately to relative energies. For i-mnt2, the highest occupied p orbital is the (HOMO-1) orbital with the HOMO being the antisymmetric sulfur s donor combination.

with the lowest energy band for the transition from the metal-based HOMO to a dithiolate p* orbital being  1100 cm1 lower in energy for 2 relative to 3. This result provides experimental support for the notion mentioned above that the lowest p* orbital of the dithiolene ligand is more stable than the corresponding orbital of the 1,1-dithiolate ligand system. From semiempirical PM3tm MO calculations, it is seen that the HOMO and LUMO orbitals for 2 and 3 have similar compositions and that the latter has a slightly larger HOMO–LUMO gap consistent with the experimentally observed stronger field splitting imposed by i-mnt relative to mnt (37–39). One aspect that emerges from inspection of the frontier orbitals of 2 is that the HOMO possesses both ligand and metal character (37), which is expected to vary in relative

LUMINESCENCE AND PHOTOCHEMISTRY OF METAL DITHIOLENE COMPLEXES

N C

S

2–

N

S

C

S

C

319

Ni N

C

S

N

2 N C

S

S

N

2–

C

Ni C N

S

S

C N

3

amounts for different complexes, thereby influencing the nature of the CT involving this orbital. In this chapter, we focus on the luminescence properties and photochemistry of metal dithiolene complexes. The excited states in these systems are in large measure determined by the coordination and bonding of the dithiolene ligands. In metal complexes, long-lived excited states are generally of CT or intraligand (IL) character. The former may be metal–ligand, ligand–metal, or for complexes containing dissimilar ligands, ligand–ligand. Excited states of the intraligand type are usually indicated by a corresponding emission from free ligand or closely related model compounds of the coordinated ligand, whereas CT excited states exhibit trends in emission energies with metal or ligand substitution and often appear at significantly lower energies than corresponding IL transitions. Both types of emission may exhibit evidence of vibronic structure, especially at low temperatures, wherein multiple maxima are observed, separated in energy by amounts corresponding to one or more major vibrational modes of the complex. Well-established examples of vibronic structure for both types of emission are known in the literature, particularly for complexes containing 2,20 bipyridine (bpy) and 1,10-phenanthroline (phen) ligands (40–45). Since the dithiolene ligands and closely related derivatives are not significantly emissive (aside from one class of systems from Pilato’s group discussed below and in chapter 7 in this volume) (46, 47a,b), the observed luminescence from metal dithiolene complexes is of a CT origin. As an aside, Pilato’s systems possess an intraligand CT rather than one between the metal center and a ligand or between two different ligands. For late metal dithiolene systems, the HOMO is either metal based or mixed in character between metal and dithiolene. In the case of homoleptic dithiolene complexes, the ligand provides a relatively low

320

SCOTT D. CUMMINGS AND RICHARD EISENBERG

lying p* orbital that can serve as an acceptor during CT excitation In these systems, luminescence and/or photochemistry thus originate from either a metal–ligand charge transfer (MLCT) excited state or one with a mixedmetal–dithiolate HOMO and a p*(dithiolate) LUMO. On the other hand, for mixed-ligand complexes the nature of the emissive state is determined by which ligand p* orbital lies lower in energy. As we will see, when a diimine such as bpy or phen is present in the complex, its lowest unoccupied p* orbital is positioned below the corresponding dithiolate p* orbital leading to a charge transfer to diimine excited state. These orbital energies mean that for the mixedligand–Pt diimine dithiolate complexes, the excited state involves a HOMO of Pt d, S p, and dithiolate character and a p* diimine LUMO (48–50). In accord with the notion of relative HOMO energies for 1,2-dithiolene and 1,1-dithiolate metal complexes, the excited states for the Pt diimine dithiolene complexes lie at lower energies than those for the analogous 1,1-dithiolate systems. For purposes of organization, we classify the reported luminescence properties and photochemical reactivity of dithiolene complexes according to dn configuration first, with subsequent groupings based on whether the complexes are homoleptic or have mixed ligands. For mixed-ligand systems, the presence or absence of delocalization in the other ligand(s) and their energetics are crucial in determining the nature of the excited state. By far, for emissive or photochemically active dithiolene complexes, the largest group contains d8 metal ions and square-planar coordination. Our analysis will begin with these systems, followed by related d10 complexes and then octahedral dn complexes for n
6.

II.

SQUARE-PLANAR d8 COMPLEXES

Square-planar d8 complexes containing dithiolene ligands are subdivided into three classes based on coordination environment and distinct excited-state properties: (1) homoleptic bis(dithiolene) complexes; (2) mixed-ligand complexes having a chelating dithiolene ligand and the remaining two coordination sites occupied by donor ligands such as phosphine, CO, CN, or 1,5-cyclooctadiene; and (3) mixed-ligand complexes having a chelating dithiolene ligand and a chelating diimine or arylpyridine p-acceptor ligand. A.

Square-Planar Bis(dithiolene) Complexes 1.

Excited States and Luminescence

A great amount of research has focused on the electronic structure, spectroscopy, redox properties, and conductivity of homoleptic bis(1,2-dithiolene) complexes of d8 transition metal ions (51). The compounds are often highly

LUMINESCENCE AND PHOTOCHEMISTRY OF METAL DITHIOLENE COMPLEXES

321

colored and possess delocalized p-electron systems to varying extents in the ground state. Comparative studies using ultraviolet–visible (UV–vis) absorption spectroscopy have been useful in determining the orbital nature of the lowenergy excited states (30, 37, 52–54) while several computational studies have yielded calculations of the molecular orbitals (MOs) of these systems (55). The photoluminescence and photochemistry observed for bis(dithiolene) complexes depend on the nature of the excited states of these systems and the ability of these complexes to have several different stable oxidation states. Prototypical of this class of complexes are [Ni(mnt)2]2 (2) and its Pt(II) analogue. The deep red color of solutions of [Pt(mnt)2]2 results from absorption bands in the 475–550-nm region originally assigned as a d(Pt)–p*(mnt) MLCT transition (37). Very weak d–d transitions occur at longer wavelengths (639 and 694 nm) and mnt-localized p–p* transitions and ligand–metal charge transfer (LMCT) occur at shorter wavelengths (336–228 nm). For related compounds, the relative orderings of orbital energies depend on the specific dithiolene, the metal ion, and its oxidation state. Because of the extent of delocalization in the bonding in these systems, unambiguous assignment of the low-energy bands as arising from ‘‘pure’’ d–d, MLCT, LMCT, or p–p* excited states is difficult. The low-energy electronic transitions for the [M(mnt)2]2 complexes with M ¼ Ni, Pd, Pt reported by Langford and co-worker (56) are summarized in Table I. Despite a great deal of information from absorption spectroscopy and theory on the low-energy excited states of bis(1,2-dithiolene) complexes, understanding the nature of the excited state involved in luminescence and photochemistry has been challenging. Much of the difficulty arises from the very short lifetimes of the excited states of this class of dithiolene complexes. In fact, only a few reports have addressed the excited-state lifetimes. Through the use of transient absorption spectroscopy with an excitation wavelength of 355 nm, Langford and co-workers (57) reported that the 3MLCT excited state of [Pt(mnt)2]2 decays within  10 ns in acetonitrile, and even faster in water. The excited state of the Ni analogue decays at much faster rates and the Pd analogue displays no transients between 20 ps and 10 ns. Based on indirect methods of photoelectrochemistry, the researchers later estimated the lifetimes of the Ni and Pt bis(mnt) complexes to be 4 and 43 ns, respectively (56). Photoluminescence from ambient temperature solutions of metal bis(1,2dithiolenes) is rarely observed, and there have been only a few reports on luminescence of any type from these compounds. In addition to the weak emission (f ¼ 105 ) seen for [Pt(mnt)2]2 ðlmax ¼ 775 nmÞ, a similarly weak emission is observed for [Pt(qdt)2]2 (qdt ¼ quinoxaline-2,3-dithiolate, 4) but at significantly higher energy (lmax ¼ 606 nm) (58). Both absorption and emission spectra for the latter complex are highly dependent on solution pH, with protonation of one of the qdt nitrogen atoms leading to the shifts shown in

322

SCOTT D. CUMMINGS AND RICHARD EISENBERG TABLE I Assignments of the Electronic Absorption Spectra of [M(mnt)2]2 (M ¼ Ni, Pd, Pt) Complexes in Acetonitrilea Wavelengthb (e)c

Complex

Assignment

2

855 571 519 476 378 319 270

(30) (570) (1,250) (3,800) (6,600) (30,000) (50,000)

d–d d–d MLCT MLCT LMCT p–p* LMCT

[Pd(mnt)2]2

637 440 387 325 295

(64) (5,700) (2,840) (20,200) (47,000)

d–d LMCT MLCT p–p* LMCT

[Pt(mnt)2]2

694 639 540 473 336 309 228

(49) (56) (1,220) (3,470) (15,600) (13,400) (43,500)

d–d d–d d–d/MLCT MLCT p–p* p–p* LMCT

[Ni(mnt)2]

a

Ref. 56. In nanometers (nm). c In M1 cm1. b

Fig. 2. The isosbestic points at 446 and 556 nm in the absorption spectra are matched by an isoemissive point at 685 nm indicating only two species present in solution, both of which are emissive. The shift in emission maximum from 606 nm in neutral solutions to 728 nm upon addition of acid may have interesting sensor applications. The results for 4 stand in contrast with results from dppz-containing Ru(II) tris diimine complexes, where dppz ¼ dipyridoipyridophenazine, in which reversible protonation of quinoxaline N atoms leads to quenching of emission. Luminescence in frozen solvent glasses for 4 at 77 K is much stronger (f ¼ 0.044 for the qdt complex), but still broad and without resolved structure.

N

S

2-

S

N

S

N

Pt N

S 4

LUMINESCENCE AND PHOTOCHEMISTRY OF METAL DITHIOLENE COMPLEXES

323

Figure 2. Changes in (a) absorption and (b) emission spectra of (Bu4 N)2[Pt(qdt)2] upon addition of 10-mL aliquots of 4.25 102 M MeCOOH in methanol.

Luminescence of [Bu4N]2[Pt(mnt)2] in the solid state at low temperature was first reported by Johnson et al. (59), and has been the topic of several studies by Gliemann and co-workers (60–62). Polarized emission spectra of single crystals of [Bu4N]2[Pt(mnt)2] were measured at 2
T
300 K (62). Spectra at low T display highly resolved vibronic structure in the 700–850-nm range. Both magnetic field effects and MO calculations support an assignment of this emission as originating from a manifold of three triplet states of MLCT character. The emission lifetimes were reported as 8.3 ms at T ¼ 2 K and 1.2 ms at 200 K. The nature of the cation also has an effect on the vibronic structure of the emission. A polarized emission spectrum of crystalline [Et4N]2[Pd(mnt)2] at 3 K was also reported and the vibronic structure was assigned. The emission in the 800–900-nm region was assigned as a d–d phosphorescence (3 B3g ! 1 Ag ). R H

N+

R S

S

N

S

+ N H

H

Pt

-

X H

N R

S

5

X-

R

A class of related complexes are the Pt(II) bis(dithiooxamide) cations. Rosace et al. (63) reported the luminescence properties of tight contact ion

324

SCOTT D. CUMMINGS AND RICHARD EISENBERG

pairs shown as 5 having the formula {Pt(H2R2dtox)2X 2 }, where dtox ¼ 1,2dithiooxamide and R corresponds to an alkyl group and X ¼ Cl, Br, or I. In fluid solutions at 298 K, the compounds have an unstructured luminescence band centered in the 700–730-nm range with emission lifetimes varying from 18 to 60 ns. In frozen solvent glasses at 77 K, the emission blue shifts to 640–650 nm and the lifetimes increase to 345–510 ns. The emission is assigned to a phosphorescence from a 3MLCT excited state. Some of the complexes also exhibit ligand-centered emission in the 610–620-nm range, and the counteranion appears to play a role in the equilibrium between the two emitting states. Luminescence has been observed in fluid solution for the norbornadiene adduct of Pt(II) bis(1,2-diphenyl-1,2-dithiolene) (64). The broad, featureless band at 520 nm (f ¼ 4.5 103) in deaerated dichloromethane has been assigned to phosphorescence from a 3IL/MLCT excited state. 2.

Ion-Pair Charge-Transfer Photochemistry

Much of the photochemistry of metal dithiolene complexes involves electrontransfer (ET) reactions that result in oxidation of the complex. Bis(1,2-dithiolene) complexes, especially those that are dianionic, are good electron donors in the excited state. Ground-state oxidation is also facile, with stable 1 and 0 oxidation states for some of the complexes. Early work in the area of intermolecular CT interactions included studies involving (R4N)2[M(SS)2] donors [R ¼ Et or Bu, M ¼ Co, Ni, Pd, Pt, Cu, Zn, and SS ¼ mnt or scf (scf ¼ 1,2-diperfluoromethylenedithiolate)] and organic acceptors (A) such as 2,3-dichloro-5,6-dicyanobenzoquinone, 7,7,8,8-tetracyanoquinodimethane, and tetracyanoethylene (65). In dichloromethane solutions of donor and acceptor, new bands arise in the 500–700-nm region that are relatively independent of the nature of the metal ion, dithiolene, alkylammonium cation, or charge of the complex. The observed transition was therefore assigned to absorption by the anion of the organic acceptor A, the result of thermal ET from the metal dithiolene complex. A related system is that of [Ni(tim)][M(mnt)2] (M ¼ Ni, Pd, Pt; tim ¼ 2,3,9,10-tetramethyl-1,4,8,11-tetra-azacyclotetradeca-1,3,8,10-tetraene), for which broad and modestly intense (e  104 M1 cm1) absorption bands in the 820–840-nm region corresponding to optical CT are observed in the reflectance spectra (54). The cation Ni(tim)2þ is the unsaturated tetraaza macrocycle 6. Analogous outer-sphere charge-transfer bands are not seen for related systems having as the cation Ni(cyclam)2þ (cyclam ¼ 1,4,8,11-tetraazacyclotetradecane [14] and N4 ), which is a saturated tetraaza analogue and does not serve as an electron acceptor. In more recent years, a great number of studies have been reported by Kisch and co-workers (66–71) that involve ion-pair charge-transfer (IPCT) photochemistry of complex salts of the type A[M(SS)2], where A ¼ organic acceptors

LUMINESCENCE AND PHOTOCHEMISTRY OF METAL DITHIOLENE COMPLEXES

325

2+

Me

N

N

Me

N

Me

Ni Me

N

6

such as dialkylated bipyridinium cations (viologens), where M ¼ Ni, Pd, and Pt (as well as tetrahedral complexes with Cu and Zn), and SS corresponds to a number of different dithiolenes, including mnt, dmit (1,3-dithiole-2-thione-4,5dithiolate; see 7), dmid (2-oxo-1,3-dithiol-4,5-dithiolate; see 8), and dmt (1,2dithiole-3-thione-4,5-dithiolate. Weak interionic interactions occur between the square-planar metal dithiolene complexes and the aromatic organic acceptors in solutions of polar solvents such as dimethyl sulfoxide (DMSO) or dimethylformamide (DMF), as evidenced by small blue shifts in the absorption bands of the dithiolene complex as compared to those seen for the corresponding Bu4Nþ salts. No such interaction is proposed for the related tetrahedral Zn or Cu complexes. In concentrated DMSO solutions, or in solutions with a large excess of A2þ, IPCT bands appear for the Ni, Pt (and Zn) complexes, which correspond to photoinduced CT from donor to acceptor as indicated by Eq. 1. hn

½MðSSÞ2 2 þ A2þ ! ½MðSSÞ2  þ Aþ

ð1Þ

The IPCT band energy and intensity depends on the metal ion and acceptor as well as the solvent (addition of THF leads to a bathochromic shift and increase in intensity). Absorption maxima are highly dependent on the nature of the dithiolene, ranging from 450 to 730 nm for the A[M(mnt)2] complexes, and from 620 to 950 for the A[M(dmit)2] complexes [[M(dmit)2]2 is shown as 7]. A general trend is observed for the complexes of the type A[Ni(SS)2], where A ¼ methylviologen (MV2þ) for which the IPCT band energies decrease with increasing electron-donating ability (or HOMO energy) of the dithiolene along the Ni(SS)2 2 series, where SS ¼ dmid (8) < dmit (7) < mnt (2) < dto (dto ¼ 1,2-dithiooxalate; 9) (72). Molar absorptivities are small, typically < 50 M 1 cm1 in DMSO. These transitions are also observed in the solid-state diffuse reflectance spectra. Interestingly, the IPCT bands do not appear for the corresponding monoanionic metal dithiolene complexes, although it appears

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SCOTT D. CUMMINGS AND RICHARD EISENBERG

that an IPCT has been observed for the monoanionic complex Ir(CO)2(mnt) with methylviologen (see below). 2-

S

S

S

S

S

S

S

S

S

S

S

M

S S

S 7

S

S

2-

O

M

O S

S 8

2-

O

S

S

O

S

O

Pt O

S 9

Application of Hush theory to the observed IPCT bands yielded information about the relationship between optical and thermal ET in these systems. The redox potentials of both the metal dithiolene donors and the viologen acceptors can be systematically varied, which, in turn, tunes the thermodynamic driving force for electron transfer. The researchers found that the IPCT band energy increases linearly with more positive free energy
G for ET, and that the reorganization energy (w) remains constant with variation in the metal or cation redox potentials (66, 67). Laser flash photolysis of solutions of Pt(mnt)2 2 and a 30-fold excess of N,Ndimethyl-4,40 -bipyridinium (MV2þ) at 347 nm leads to a transient species that absorbs at l > 800 nm and has been characterized as the CT pair {MVþ, [Pt(mnt)2]} (68). This assignment is supported by the fact that the absorption spectra of these species (generated electrochemically) match that of the transient obtained by direct excitation and that the transient species is not observed when MV2þ is replaced with NBuþ 4 . The intensity of the transient is increased with increasing concentration of acceptor or decreasing ionic strength. In addition, the transient is not observed with the related Ni(mnt)2 2 complex, lending some support to the proposal that the photochemistry originates from the triplet state of the metal dithiolene complex, which is much more efficiently populated for

LUMINESCENCE AND PHOTOCHEMISTRY OF METAL DITHIOLENE COMPLEXES

*(MV2+[Pt(mnt)2 ]2- ) hν

327

-

(MV+ [Pt(mnt)2 ] )

kET

k'd

l/τ

k'-d

k –ET MV+ + [Pt(mnt)2 ]

(MV2+[Pt(mnt) 2] 2-)

-

Scheme 1

the Pt complex than the Ni complex. The lack of IPCT in solutions without an excess of MV2þ suggests that the forward ET occurs from a contact ion-pair complex. An observation that the decay of the transient follows second-order kinetics (k ¼ 1 1010 M1 s1) supports a mechanism with back ET occurring from a solvent separated ion pair that must first recombine by diffusion. This mechanism is summarized in Scheme 1. Similar IPCT photochemistry is observed for salts involving other organic acceptors, with results indicating that the cage escape efficiency of the CT pair is sensitive to the charge of the acceptor, being 1.0 for Aþ but only 0.1 for A2þ. The related tetrahedral complex MV[Zn(mnt)2] displays similar CT photochemistry, although the decay of the transient species occurs with a secondorder rate constant of k ¼ 3 109 M 1 s1 (69). It is possible to ‘‘intercept’’ the reactive CT pair of the IPCT chemistry, despite the short lifetimes. Continuous photolysis of aerated solutions of A[Pt(mnt)2] leads to clean oxidation of the metal dithiolene complex to form [Pt(mnt)2] (70). Similar photoreactivity is not observed in nitrogen saturated solutions, and net oxidation only occurs in cases when the reduction potentials of the viologens are more negative than 0.6 V. These results all support a mechanism (Scheme 2) in which excitation of the metal dithiolene complex leads to IPCT to the viologen acceptor, followed by reduction of molecular oxygen by the viologen radical cation, provided that the redox potential of A2þ is sufficient to reduce oxygen. Superoxide has been trapped using 5,5-dimethyl1-pyrroline-N-oxide. The overall photoreaction only occurs for the platinum

{A2+[Pt(mnt)2]2-} A + + [Pt(mnt)2]A + + O2

hν k -1 k2 Scheme 2

A + + [Pt(mnt) 2]A 2+ + [Pt(mnt) 2]2A 2+ + O 2-

328

SCOTT D. CUMMINGS AND RICHARD EISENBERG

dithiolene complex. It has been postulated that rapid deactivation of the metal dithiolene excited state by metal-centered d–d states occurs in the Ni and Pd complexes (71). The IPCT work on A[M(mnt)2] represents an interesting example of the ability to photochemically generate reactive superoxide with proper design of the metal dithiolene–acceptor salt. The use of cycloalkylated biimidazolium dications with [M(mnt)2]2 complexes (M ¼ Ni, Pd, Pt, and Zn) lead to similar IPCT absorption bands, although the packing of ions in the solid-state structure is highly dependent on the geometry of the cation (73). While irradiation of the biimidazolium acceptors in the presence of ethylenediaminetetraacetic acid (EDTA) can produce strongly reducing radical cations, and these in turn can reduce protons to hydrogen in the presence of colloidal platinum, no sensitization of the photoreaction to generate H2 was observed using the metal dithiolene complexes. 3.

Photoproduction of Hydrogen

One of the most interesting, yet elusive, aspects of photochemistry research involving metal bis(dithiolene) complexes is the photochemical production of hydrogen. A short but intriguing communication by Kisch and co-workers in 1980 (74) reported the production of hydrogen from water using metal bis(dithiolene) photocatalysts. Three octahedral tris(dithiolene) complexes were also included in the paper. Irradiation of 11.5:1 THF/H2O solutions of the neutral complex Ni(S2C2Ph2)2 (10) using l 290-nm light led to the formation of 225 equiv of H2 after 483 h. Interestingly, irradiation in anhydrous THF also led to hydrogen production, whereas irradiation in the presence of D2O yielded HD and D2 (but not H2) formation. Irradiation with higher energy UV light (l 254 nm) increased the rate of hydrogen production. Photolysis also led to the formation of the dianionic [Ni(S2C2Ph2)2]2 species, but the nature of the actual photocatalyst and the mechanism were not discussed. A footnote in the paper also describes the generation of > 2 L of hydrogen gas after 24 h of irradiation of the related tetrahedral [Zn(mnt)2]2 complex. However, this photoproduction of hydrogen using [Zn(mnt)2]2 was later attributed to the formation of the semiconductor n-ZnS upon UV irradiation of the dithiolene complex, itself an interesting photochemical transformation (75, 76).

S

S Ni

S

S 10

LUMINESCENCE AND PHOTOCHEMISTRY OF METAL DITHIOLENE COMPLEXES

329

Further work on the photoproduction of hydrogen from H2O/THF solutions using metal dithiolene complexes was reported in 1983 (5). Numerous squareplanar bis(1,2-dithiolene) complexes of Ni, Pd, and Pt were investigated, along with many other types of metal dithiolenes, for photocatalytic generation of hydrogen. Almost all of the complexes underwent photobleaching at early reaction times, indicating that the complexes were serving as catalyst precursors. The nature of the actual catalyst was not identified, although a discussion of evidence for homogeneous and or heterogeneous catalysts was presented. Although the experimental evidence suggests that the observed photogeneration of hydrogen may not involve homogeneous metal dithiolene photocatalysts, a theoretical study by Alvarez and Hoffmann addressed possible mechanisms for hydrogen elimination from d8 square-planar bis(dithiolene) complexes (77). Concerted elimination of H2 from protonated sulfur atoms in the complex was proposed to be a thermally forbidden but photochemically allowed pathway, and protonation of a metal hydrido complex was also considered, as shown in Scheme 3. H S S

H +

hν

S M

S

H

H

S

S M

S

S

H

H

S S

S M

hν

S

H

S S

H2 S

M S

Scheme 3

A related complex, bis(2-chlorodithiobenzil)nickel(II) (11), was described by Katakis and co-workers (78, 79) as a photocatalyst for hydrogen production from water. A series of photolysis experiments were run using acetone–THF solutions of the neutral dithiolene complex, along with EDTA as a sacrificial electron donor and methyl viologen (MV2þ) as an ET relay. Irradiation of the complex with EDTA present led to one-electron reduction of the complex. With MV2þ also present, hydrogen was produced using wavelengths > 350 nm. When the excitation energy was limited to l ¼ 593 nm, no hydrogen was detected, even though the complex absorbs strongly at 604 and 840 nm and the monoanion absorbs strongly at 504 and 920 nm. Quantum yields depended on concentrations of components in solution and ranged from 0.004 to as high as

330

SCOTT D. CUMMINGS AND RICHARD EISENBERG

0.360. The mechanism was proposed to involve reduction of the neutral nickel bis(dithiolene) complex by methyl viologen radical cation, followed by dimerization to form the active catalyst {[Ni(SS)2]2}2, as shown in Eqs. 2–6: Cl S

S Ni

S

S

11

Cl

MVþ þ NiðSSÞ2 ! MV2þ þ NiðSSÞ 2 f½NiðSSÞ2 2 g

2

2NiðSSÞ 2 þ þ

!

ð3Þ 2

þ H þ MV ! ½NiðSSÞ2 2 H

½NiðSSÞ2 2 H

2

½NiðSSÞ2  2

þ

þH ! þ

þ MV !

ð2Þ

½NiðSSÞ2 2 2 þ MV

½NiðSSÞ2  2 þ H2 2þ ½NiðSSÞ2 2 2 þ MV

2þ

ð4Þ ð5Þ ð6Þ

Although the production of hydrogen from water using a simple homogeneous system of photocatalyst and electron relays is very attractive and would represent an important step in the field of solar energy conversion schemes, the square-planar metal bis(dithiolene) complexes have not received attention beyond these few reports. The research raises interesting questions regarding the nature of the photoreactive excited states of the complexes and their photochemical transformations to active catalysts. After these results have been replicated and shown not to be the result of artifacts in the systems, further mechanistic, photochemical, and photophysical studies need to be done to clarify the chemistry behind the photoproduction of hydrogen and whether bis(dithiolene) complexes are playing a photochemical role in this chemistry or are serving simply as precursors for the generation of colloidal metal sulfide semiconductor particles. 4.

Photooxidation Chemistry

Photochemical oxidation of square-planar bis(1,2-dithiolene) complexes of Ni, Pd, and Pt is by no means limited to IPCT excitation. Photooxidation also occurs in halocarbon solvents. In 1982, two separate reports addressed the photochemistry of metal bis(1,2-dithiolene) complexes. Vogler and Kunkely (80) investigated complexes of the type M(S2C2R2)z2 , where M ¼ Ni, Pd, Pt,

LUMINESCENCE AND PHOTOCHEMISTRY OF METAL DITHIOLENE COMPLEXES

331

R ¼ CN, z ¼ 2 or M ¼ Ni, Pt, R ¼ Ph, z ¼ 0 or M ¼ Ni, R ¼ Ph, z ¼ 1. Although photostable in most solvents, the complexes that can be oxidized at potentials between 0.1 and 0.5 V vs. SCE undergo clean photooxidation in chloroform using irradiation wavelengths of 300–350 nm. Quantum efficiencies are as high as 0.25 at lex ¼ 313 nm for [Ni(mnt)2]2, but drop off sharply as the excitation wavelength increases. The photoreactivity depends strongly on the nature of the halocarbon solvent, with the region of light sensitivity shifting to longer wavelengths in the stronger oxidant carbon tetrachloride, and shifting to shorter wavelengths in the weaker oxidant dichloromethane. Excitation at even higher energies also leads to photoreactions, and difference absorption spectra in acetonitrile and chloroform solvents reveal low-intensity features in the 300– 350-nm regions. The excited state responsible for the observed photoreactions, therefore, was assigned as a charge-transfer–solvent (CTTS) state. Excitation into the lowest energy MLCT absorption bands, in the 440–550-nm region, did not lead to photooxidation. The photochemical oxidation of square-planar bis(1,2-dithiolene) complexes in halocarbon solvents thus represents an apparent exception to Kasha’s rule, which states that the lowest energy excited state is the origin of photoreactivity. Dooley and Patterson (81) reported further investigations of the photooxidation of [M(mnt)2]2 (M ¼ Ni, Pd, Pt, Co, Cu) in a 24:1 chloroform/acetonitrile solvent mixture. Again, clean photochemistry was observed, with several isosbestic points in the overlaid UV–vis spectra shown in Fig. 3. For

Figure 3. Electronic absorption spectral changes during 254-nm photolysis of [Pt(mnt)2]2 in CHCl3/MeCN (24:1 v/v) at 23  C. Isosbestic points appear at 540, 435, 393, and 315 nm. Reproduced by permission from reference 81.

332

SCOTT D. CUMMINGS AND RICHARD EISENBERG

[Ni(mnt)2]2, quantum yields increase from 7.3 104 at 405 nm to 0.71 at 254 nm. Quantum yields also vary with the nature of the halocarbon in a manner that matches the oxidizing power of the solvent. Photooxidation quantum yields do not, however, correlate with the oxidation potential of the metal complexes. Subsequent studies by Dietzsch et al. (82) found that the quantum yields for the photooxidation of the corresponding [M(dmit)2]2 (M ¼ Ni, Pt) complexes, were lower than their mnt analogues even though the former are more easily oxidized electrochemically. Another unusual result of their study was that the quantum yields, in general, are significantly lower in a 24:1 chloroform/ acetonitrile solvent mixture than those reported by Vogler and Kunkely (80) in pure chloroform, even though the reduction potential of the solvent would not be expected to be significantly different. These results may indicate that coordination of acetonitrile may affect the efficiency of photooxidation, or that other types of excited states may be involved in the observed redox chemistry. Langford and co-workers (83) reinvestigated the effect of solvent on [M(mnt)2]2 (M ¼ Ni, Pt), using mixtures of chloroform and acetonitrile with mole fractions of the components ranging from of 0 to 1.0. For both complexes, the quantum yield of photooxidation increases with increasing mole fraction of chloroform up to a limiting value, after which quantum yields are independent of increasing CHCl3 mole fraction. Again, the researchers found that quantum yields increase with decreasing excitation wavelength. Normalization of the quantum yield data to the limiting value for each excitation wavelength allowed for direct comparison of solvent and wavelength effects. The researchers found that the solvent dependence is independent of the excitation wavelength for both complexes. Picosecond spectroscopy was used to better understand the role of chloroform as a reactant. Transient absorption spectra were used to follow the decay of the [M(mnt)2]2 excited state, which were identical in chloroform or acetonitrile. With primary quantum yields found to be very close to the overall steady-state photolysis quantum yields, the results indicate that the wavelength dependence is not a result of a classical competition between radical recombination and cage escape. With no evidence for primary radical formation, the origin of the solvent-dependent quantum yields may lie in differences in solvent reorganization or dissipation of thermal energy through vibrational modes. The photoinduced ET reactions of metal bis(mnt) complexes have also been studied by Persaud and Langford (56) using photoelectrochemistry. Their research addressed a fundamental aspect of the CT photochemistry of the metal bis(dithiolene) complexes: whether or not photooxidation can occur from the lower energy MLCT excited states. Acetonitrile solutions of [M(mnt)2]2 (M ¼ Ni, Pd, Pt, and Cu) were investigated using optically transparent SnO2 electrodes. For the Ni and Pt complexes, weak (nA) cathodic currents were

LUMINESCENCE AND PHOTOCHEMISTRY OF METAL DITHIOLENE COMPLEXES

333

detected when the visible MLCT bands were irradiated. No photocurrent was detected when the low-energy LMCT band of [Pd(mnt)2]2 was irradiated, however. Excitation of the MLCT band of [Cu(mnt)2]3 generated a cathodic current, but the same did not occur for [Cu(mnt)2]2. The results indicate that the lower energy MLCT excited states, like the higher energy CTTS excited states, can be involved in photooxidation of the complexes. The very small currents are an indication of the short excited-state lifetimes of the complexes, decay from which competes efficiently with diffusion to the electrode in these experiments. The Ni and Pt complexes can also be incorporated into polymer films of quaternized poly(vinylpyridine) (PVP) and deposited onto the transparent electrode (84). Photocurrents are enhanced to microamps (mA), an increase that may be attributed to either the effect of immobilization of the complexes near the electrode surface or an increase of the excited-state lifetimes in the polymer matrix. However, the effective concentrations of the complexes in this study were much greater than for the acetonitrile solutions in their earlier work. The polymer films are not stable to continuous photolysis, and voltammograms of the films are quite sensitive to anions used in the supporting electrolyte. The system can be stabilized by using a polymer blend of PVP and a copolymer containing quaternary ammonium ion and including [Fe(CN)6]4 in the electrolyte solution (85). Upon irradiation of the visible MLCT bands of [M(mnt)2]2 (M ¼ Ni, Pt), photocurrents are produced. The mechanism (Scheme 4) is believed to involve photooxidation of the metal bis(dithiolene) triplet state by the SnO2 electrode, followed by [Fe(CN)6]4 reduction of the monoanion, with completion of the ET cycle as ferricyanide, [Fe(CN)6]3, diffuses to the other electrode and is reduced.

*Ni(mnt)2 2-

hν

eSnO2

Ni(mnt)2

Ni(mnt)

2-

1-

Fe(CN)6

Fe(CN)6

Scheme 4

3-

4-

Fe(CN)6

Fe(CN)6

3-

4-

Pt

334

SCOTT D. CUMMINGS AND RICHARD EISENBERG

5.

Photochemical Radical Formation

Although the charge-transfer–solvent photoreactions of metal dithiolene complexes involve net one electron oxidation, radical chain mechanisms have not been proposed and reactive organic intermediates have not been detected. In contrast, the formation of long-lived metal complex–organic radicals is characteristic of the photochemistry of S,S0 -dialkylated derivatives of bis(1,2diphenyl-1,2-ethanedithiolato) complexes of Ni, Pd, and Pt such as the benzyl system 12. Photoreactivity of these complexes was reported as early as 1968 by Schrauzer (1) and more recently by Sugimori (86). Irradiation of benzene solutions of the nickel complex leads to formation of the dealkylated nickel bis(dithiolene) complex and dibenzyl. The neutral metal dithiolene complex is characterized by its deep blue color and absorption maximum at 850 nm. To elucidate the reaction mechanism, Yamauchi and co-workers (87) investigated the kinetics of radical formation. Benzyl radicals can be trapped from solution while a strong, long-lived (thousands of seconds) electron spin resonance (ESR) signal attributable to the metal complex radical is obtained. By studying the effect of 2,2,6,6-tetramethyl-1-1-piperidinyloxy (TEMPO) radical traps on reaction rates and polarization of the ESR spectrum, the mechanism shown in Scheme 5 was proposed.

Ph CH2 Ph

S

Ph

S

hν

Ni Ph

S

S

1(12)*

3(12)*

Ph

CH2 Ph Ph

12

CH2

Ph

S

Ph

S

S

Ph

Ph

S

S

Ph

Ph

S

Ni

S

Ph

S

Ph

Ni CH2

10

Ph (τ ~ 1000 s) Scheme 5

LUMINESCENCE AND PHOTOCHEMISTRY OF METAL DITHIOLENE COMPLEXES

335

The triplet excited state of the Ni S-benzylated complex 12 dissociates into a radical pair composed of a singly alkylated metal dithiolene complex and a benzyl radical. The metal complex radical decomposes further to the dealkylated nickel bis(dithiolene) complex 10 or it undergoes geminate recombination with the benzyl radical to re-form the starting complex. The metal ion has a large effect on the efficiency of the dissociation chemistry, with platinum being approximately 10 times more efficient than the nickel complexes and palladium being the least efficient. Other alkyl derivatives are also photoreactive but with efficiencies that depend on the stability of the alkyl radical formed. This work, with its unusually long-lived metal complex radicals, again demonstrates that efficient photochemistry can arise from the short-lived excited states of metal bis(dithiolene) complexes. Although only rarely luminescent in ambient fluid solutions, square-planar transition metal bis(dithiolene) complexes do display significant and varied photochemical reactivity. Much of the photoreactivity described above for dianionic bis(dithiolene) complexes involves excited-state oxidation and often leads to radical formation. In addition, the excited states of these complexes are receiving attention for their potential as materials for optical (15), nonlinear optical (10–13), and electrooptical (16) devices. The relevance of this work to those applications is addressed in other parts of chapter 8 in this volume (87b). B.

Square-Planar Mixed-Ligand Dithiolene–Donor Complexes

While luminescence from the MLCT excited states of square-planar bis (dithiolene) complexes is relatively uncommon, complexes in which one of the dithiolenes has been replaced by a ‘‘nonchromophoric’’ donor ligand often exhibit strong photoluminescence. Among the first examples of photoluminescence from square-planar complexes were iridium(I) complexes of the general formula [Ir(L)(L0 )(mnt)] (13), where L and L0 are electron-pair donors, and related Rh(I) analogues (59). All of the iridium compounds luminesce detectably in the solid state at 298 K, but none of the compounds are luminescent in fluid solution at room temperature. At 77 K in the solid state or in frozen solvent glasses, the emission and excitation spectra are highly structured, displaying vibrational progressions that correlate to the C C vibration of the mnt ligand. Lifetimes at 77 K are in the 10–400-ms range. On the basis of changes in the emission and absorption band energies with variation in the electron-donor ability of the L,L0 ligands, the emitting state was assigned as d–p*(mnt) 3MLCT. The iridium mnt complexes (13) also undergo outer-sphere ET reactions with methyl viologen (88), related in some ways to that found by Kisch and co-workers (77) for A[M(SS)2] systems via IPCT (see above). Depending on the nature of the L and L0 ligands in 13, the reducing ability of the metal dithiolene complex can be varied in such a manner that the ET reactions can

336

SCOTT D. CUMMINGS AND RICHARD EISENBERG

L'

S

–

CN

M = Rh, Ir L = L' = CO, P(OPh) 3 ,

M L

S

1 — dppe 2

[1,2-

bis(diphenylphosphino)ethane] L = CO; L' = PAr 3 , CN-

CN

13

occur thermally or photochemically. For 13 with L ¼ CO, L0 ¼ CN, the ET is thermal with clear spectroscopic evidence for MVþ in solution, whereas for 13 with L ¼ CO, L0 ¼ PPh3, the ET is photochemically driven leading to the binuclear Ir(II) product 14. For the least reducing complex (13) studied (L ¼ L0 ¼ CO), only an optical CT in the form of a broad, weak absorption at 470 nm was seen. NC

CN

S Ph 3P OC

CO

S Ir

Ir S

NC

PPh 3 S

CN

14

A different aspect of the photochemical reactivity of the Ir(I) precursor Ir(CO)(PPh3)(mnt) is illustrated by the synthesis of the Ir(III) complex IrBr(CO)(PPh3)2(mnt) (89). Specifically, the Ir(III) bromide complex forms in the reaction of Br2 with Ir(CO)(PPh3)(mnt) followed by the addition of an equivalent of PPh3. The most notable feature about this reaction is the fact that the solution turns black before clearing to orange 10 min after the start of the reaction. The intense color is indicative of formation of a CT complex with an associated intermolecular CT absorption. The nature of the species that gives rise to this absorption is thought to be [Br2    Ir(CO)(PPh3)(mnt)] with the optical transition corresponding to electron transfer from Ir(I) to Br2. Mixed-ligand square-planar platinum(II) complexes of the type [Pt(L) (L0 )(mnt)] (15), where L ¼ L0 ¼ P(OPh)3, P(OEt)3, PPh3; L þ L0 ¼ 1,5cyclooctadiene (cod), 1,2-bis(diphenylphosphino)methane (dppm) have also been investigated for their photoluminescent properties (59, 90, 91). These complexes luminesce in the solid state at 298 K and at 77 K in frozen solvent

LUMINESCENCE AND PHOTOCHEMISTRY OF METAL DITHIOLENE COMPLEXES

337

glasses or the solid state. The highly structured emission in rigid matrices was again used to assign the emission as phosphorescence from a d–p*(mnt) 3MLCT excited state. The mnt complexes were also compared with related 1,1-dithiolate complexes. In contrast to the mnt complexes, emission bands for the 1,1dithiolate complexes are broad and featureless and maxima are shifted to higher energy. The emission spectrum of the complex Pt(cod)(qdt) in frozen glasses at 77 K also displays a highly structured luminescence with a vibrational progression of 1370 cm1 corresponding to the C C vibrational mode of the dithiolene. The spectrum is nearly identical to the related mnt complex, but blue-shifted by 2500 cm1 even though the absorption band is red-shifted by 3500 cm1. These energy differences are also observed when comparing the complexes [Pt(mnt)2]2 and [Pt(qdt)2]2, indicating the dithiolene ligand affects the relative energies of the singlet and triplet MLCT excited states. Protonation of the quinoxolinedithiolate ligand has large effects on the emission for the complexes [Pt(qdt)2]2, but smaller effects for Pt(cod)(qdt) (92). S

L

CN

Pt L

L = P(OR) 3, PAr3 ,1/2 dppe, 1/2 cod S

CN

15

Pilato and co-workers (46, 93) investigated the luminescence of squareplanar platinum(II) complexes of dppe with dithiolenes that contain a pyridinium or quinoxaline ring. Their work represents the first examples of solution emission at room temperature from platinum(II) phosphine dithiolene complexes. The lowest energy excited state has been characterized as a 1,2enedithiolate p ! heterocycle p* ILCT, based on a solvatochromic absorption band that is independent of substituting Pt with Pd or Ni but sensitive to changes on the heterocycle ring. Both fluorescence and phosphorescence can be detected in deaerated solutions of (dppe)Pt{S2C2(2-quinoxaline)(H)} (16), although the phosphorescence can be completely quenched by oxygen. In changing the solvent polarity from DMSO to toluene, the emission energy increases from 660 to 540 nm while the quantum yield for emission decreases. The ratio of fluorescence to phosphorescence quantum efficiencies also shifts with solvent, with 3f/1f ¼ 1 in toluene and 3f/1f ¼ 3 in DMSO. The excited-state decay was fit to fast (0.08–0.18 ns) and slower (350–4700 ns) lifetimes, which are also solvent dependent and are attributed to relaxation from the 1ILCT* and 3ILCT* states, respectively. Because visible excitation leads to increased charge density on the heterocycle, the excited state of 16 is more basic than the ground state (46). The

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SCOTT D. CUMMINGS AND RICHARD EISENBERG

Ph

Ph P

S Pt S

P Ph

N

Ph

16

N

long-lived 3ILCT* state is capable of bimolecular quenching by organic electron donors and acceptors, as well as by weak organic acids and H-atom donors. Quenching rate constants, as calculated from a Stern–Volmer analysis of the data, decrease as the acidity of the quencher decreases (i.e., as the pK a of the quencher increases). Because the 3ILCT* state is both a better electron and proton acceptor than the ground state, it is expected to serve as an excited state H-atom acceptor as well. Quenching of the phosphorescence by p-methoxyphenol and p-dihydroquinone, for example, is attributed to H-atom transfer. Protonation of the ground-state complex leads to a loss of the emission, though this may be due to the fact that it shifts to long wavelengths beyond the detectors limits. Luminescence from related complexes (dppe)Pt{S2C2(2-pyridine)(H)} and (dppe)Pt{S2C2(4-pyridine)(H)} display remarkable sensitivity to protonation (94). The neutral complexes are nonemissive, as their excited states are rapidly deactivated by low-lying metal centered d–d states. Upon protonation of the pyridine, the 1ILCT* and 3ILCT* excited states drop substantially in energy (6700 cm1 for the 4-pyridine complex), allowing for efficient fluorescence and phosphorescence. The protonated complexes are also capable of excited-state ET reactions with suitable organic electron donors or acceptors. The use of alkyl spacers between the 1,2-enedithiolate donor unit and the heterocycle acceptor unit leads to substantial changes in the luminescence intensity, suggesting that the two must be coplanar for emission in room temperature solutions (95). Transient absorption studies have also been performed on complexes of this type (96). The triplet–triplet absorption spectra change with variation in the pendant heterocyle, but not with variation in the phosphine ligand or with protonation. Decay of the transient absorption band was used to determine the lifetime of the 3 ILCT state, which ranged from 0.7 to 15.9 ms. The luminescence and photochemistry associated with this new family of luminescent square-planar metal dithiolene complexes may make them suitable as molecular probes and sensors. Their use in ratiometric oxygen sensing, which takes advantage of the disparate lifetimes of the singlet and triplet emission bands, has been reported (97).

LUMINESCENCE AND PHOTOCHEMISTRY OF METAL DITHIOLENE COMPLEXES

C.

339

Square-Planar Mixed-Ligand Dithiolene–Diimine and Related Complexes 1.

Excited States and Luminescence

The combination of dithiolene and diimine chelating ligands in square-planar d8 complexes gives rise to a unique CT excited state, and complexes of this class have been the subject of a rich and growing amount of research in recent years. The Pt(diimine)(dithiolene) complexes were among the earliest examples of emission from square-planar metal complexes in fluid solution. Luminescence from room temperature solutions of Pt(diimine)(mnt) complexes with diimine ¼ bpy, phen, or an alkyl- or aryl-substituted derivative was reported in 1990 by Zuleta et al. (98, 99), following an initial report on similar complexes with a 1,1-dithiolate. Complexes of Pt(II) containing a heteroaromatic diimine and either a 1,2dithiolene or a 1,1-dithiolate ligand possess a common but distinct type of excited state that is involved in the luminescence and bimolecular excited-state reactivity of these systems. The hallmark of the Pt(diimine)(dithiolate) chromophore is a moderately intense solvatochromic absorption band (molar extinction coefficients e of 5000–10,000 M 1 cm1) in the 450–700-nm region of the spectrum that shifts to higher energy with increasing solvent polarity. Based on spectroscopic changes as a result of simple ligand modification and semiempirical MO calculations on model systems, the solvatochromic transition was assigned as a CT from an orbital of mixed metal and dithiolate composition to a lowest unoccupied orbital localized on the diimine ligand. This assignment, which has been designated as both MMLL0 CT (for ‘‘mixed-metal/ligand-toligand’’ charge transfer) and more simply, ‘‘charge transfer-to-diimine’’, may be compared with a ligand–ligand charge-transfer (LLCT) assignment made for related nickel diimine dithiolate systems first described by Dance and for Pt(bpy)(tdt) reported by Vogler (100, 101). LLCT absorption bands have also been reported for several other examples of mixed-ligand diimine dithiolate complexes of Ni(II), Pd(II), and Pt(II) (102) and their partially oxidized products (103–105). Both spectroscopic and computational results, however, indicate that there is significant metal d orbital character in the HOMO and metal involvement in the ground-state dipole moments and solvatochromism for Ni(diimine) (dithiolate) and related platinum(II) complexes (106). A review of many other types of metal complexes possessing LLCT and related excited states has appeared (107). Systematic variation in the nature of both the diimine and dithiolate ligands can be used to ‘‘tune’’ the photoluminescent and excited-state ET properties. In order to understand the molecular design factors that influence the energy,

340

SCOTT D. CUMMINGS AND RICHARD EISENBERG

lifetime, emission quantum yield, and redox potentials of the emissive excited state with the purpose of developing the Pt(diimine)(dithiolate) chromophore for use in light-driven reactions, a comprehensive study of the system was conducted (108). Specifically, two series of Pt(diimine)(dithiolate) complexes were synthesized, characterized, and investigated. The first consists of Pt(diimine)(tdt) complexes (17–24), where tdt ¼ toluene-3,4-dithiolate and the diimines are substituted alkyl, aryl, and carboalkoxy bipyridines and phenanthrolines,

Me Me

t-Bu S

N

Pt

N Me

S

Me

S

N

t-Bu

17

Me

S

N

Pt

Me

18

Me Pt

Pt N Me

S

N

Me

S

S

Cl

S

N

Pt N

Me

20

19

N

Pt S

N

Me

S

Me

22

21 Cl

EtOOC S

N N

S

N

Pt

Cl

S

N

S

N

Pt S

23

Me

N EtOOC

S

24

Me

LUMINESCENCE AND PHOTOCHEMISTRY OF METAL DITHIOLENE COMPLEXES

t-Bu

t-Bu N

S

N

CN

Pt S

t-Bu

S

N

Me

S

N

Me

Pt

N

N

CN

t-Bu

25

26

t-Bu

t-Bu N

N

S

N

S

CN

S

COO- t-Bu

Pt

Pt

t-Bu

341

N

S

t-Bu

27

28

t-Bu N

S

CN

S

PO(OEt)2

Pt N

t-Bu

29

while the second series corresponds to Pt(dbbpy)(dithiolate) complexes (18, 25– 29), where dbbpy ¼ 4,40 -di-tert-butyl-2,20 -bipyridine and the dithiolates are mnt, tdt, 6,7-dimethyl-quinoxaline-2,3-dithiolate (dmqdt), ethane-1,2-dithiolate (edt), as well as two 1,1-dithiolates. All of the compounds display solvatochromic absorption bands and broad, unstructured solution luminescence. For the Pt(diimine)(tdt) series, lowering the pdiimine LUMO with substituents on the diimine leads to a lowering of the absorption and emission energies as seen in Fig. 4. A plot of the CT absorption energy versus the Hammett substituent constant of the 4,40 -X2-2,20 -bpy ligand yields an excellent correlation. For the Pt(dbbpy)(dithiolate) series, the change in energy spans 5000 cm1 with the energies increasing according to the series tdt < mnt < dmqdt  edt < 1,1-dithiolates, consistent with the weaker field splitting of the 1,2-dithiolenes relative to the 1,1-dithiolates. The results of Table II show that the energy of the excited state can be tuned by nearly 1 eV through systematic ligand modification. From emission lifetimes and quantum yields of emission, the radiative and nonradiative decay rate constants were

342

SCOTT D. CUMMINGS AND RICHARD EISENBERG TABLE II Photoluminescent Properties of Pt(diimine)(dithiolate) Complexes 17–29 in Dichloromethane at 298 K

Compound ———————————— Formula Number Eem (eV)a Pt(tmphen)(tdt) Pt(dbbpy)(tdt) Pt(dmbpy)(tdt) Pt(bpy)(tdt) Pt(phen)(tdt) Pt(Cl-phen)(tdt) Pt(Cl2-bpy)(tdt) Pt(EC-bpy)(tdt) Pt(dbbpy)(tbcda) Pt(dbbpy)(cpdt) Pt(dbbpy)(edt) Pt(dbbpy)(dmqdt) Pt(dbbpy)(mnt)

17 18 19 20 21 22 23 24 28 29 27 26 25

1.94 1.93 1.87 1.86 1.84 1.81 1.68 1.58 2.51 2.50 1.97 2.30 2.04

bem

t(ns)c

t0(ms)

57.0 10.8 7.4 3.1 6.7 2.6 0.43 0.04 12 2.2 2.8 64 1.0

1020 504 381 291 517 315 157 68 2.0 1.5 10 80 3

178 465 517 933 773 1207 3639 18970 1.6 0.6 32 12.4 29

kr(ms1)d 0.0056 0.0022 0.0019 0.0011 0.0013 0.00083 0.00027 0.000053 0.62 0.15 0.028 0.081 0.035

knr(ms1)d 0.98 2.0 2.6 3.4 1.9 3.2 6.4 15 499 690 111 12.4 333

a

From emission (em) maxima in butyronitrile at 77 K. 104. c 10%. d Nonradiative ¼ nr and radiative ¼ r. b

Figure 4. (a) Charge-transfer-to-diimine absorption bands for Pt(tmphen)(tdt) (17), Pt(dbbpy)(tdt) (18), Pt(Cl-phen)(tdt) (22), Pt(Cl2 bpy)(tdt) (23) and Pt(EC-bpy)(tdt) (24, 4,40 -di(ethoxycarbonyl)2,20 -bipyridine) in dichloromethane (absorbance maxima normalized). (b) Charge-transfer–diimine emission bands for Pt(tmphen)(tdt) (17), Pt(dbbpy)(tdt) (18), Pt(Cl-phen)(tdt) (22), Pt(Cl2bpy)(tdt) (23), and Pt(EC-bpy)(tdt) (24) in frozen butyronitrile at 77 K (emission intensities normalized).

LUMINESCENCE AND PHOTOCHEMISTRY OF METAL DITHIOLENE COMPLEXES

343

TABLE III Ground- and Excited-State Redox Properties of Pt(diimine)(dithiolate) Complexes 17–29a Compound ————————————— Formula Number Pt(tmphen)(tdt) Pt(dbbpy)(tdt) Pt(dmbpy)(tdt) Pt(bpy)(tdt) Pt(phen)(tdt) Pt(Cl-phen)(tdt) Pt(Cl2-bpy)(tdt) Pt(EC-bpy)(tdt) Pt(dbbpy)(tbcda) Pt(dbbpy)(cpdt) Pt(dbbpy)(edt) Pt(dbbpy)(dmqdt) Pt(dbbpy)(mnt)

17 18 19 20 21 22 23 24 28 29 27 26 25

Eem (eV) 1.94 1.93 1.87 1.86 1.84 1.81 1.68 1.58 2.51 2.50 1.97 2.30 2.04

E(Pt0= )b 1.495 1.398 1.371 1.339 1.319 1.257 1.043 0.962 1.302 1.274 1.484 1.334 1.266

E(Ptþ=0 )b

E(Pt= )d

0.347 0.389 0.390 0.376e 0.376e 0.359e 0.380 0.412 0.963 0.957 0.431 0.814 0.944

0.45 0.54 0.50 0.52 0.52 0.56 0.64 0.62 1.21 1.23 0.49 0.97 0.77

E(Ptþ= )d 1.60 1.55 1.48 1.49 1.46 1.46 1.30 1.17 1.55 1.54 1.54 1.49 1.10

a

All potentials are in V vs. NHE unless otherwise noted. E1/2 from reversible couple. c Ep anodic peak potential from irreversible couple. d Estimated according to Fig. 5. e Quasireversible couple. b

calculated, leading to intrinsic radiative lifetimes for these complexes in the microsecond range (see Table II), consistent with a formally spin-forbidden decay. Additional support for the orbital nature of the emissive state in the Pt(diimine)(dithiolate) complexes is obtained from the electrochemical data given in Table III. The formation of the emissive state formally involves oxidation of the HOMO having dithiolate and metal character and reduction of the LUMO, which is diimine localized. There should therefore exist a correlation between the energy of the excited state and the difference between the oxidation and reduction potentials for each complex. Indeed, such a linear correlation was found for the emission energies of the complexes; a similar correlation was obtained when absorption energies for the solvatochromic transition were plotted (108). In addition to their effects on emission energy, the diimines and dithiolates also influence the emission lifetime and quantum yield of the Pt(diimine) (dithiolate) chromophore. The complexes display lifetimes ranging from 1 ns to > 1 ms and em ranging from < 105 up to 6.4 103, indicating such an influence on the kinetics of excited-state decay (see Table II). The tdt complexes have lifetimes that are significantly longer than those measured previously for

344

SCOTT D. CUMMINGS AND RICHARD EISENBERG

other Pt(diimine)(dithiolate) complexes. An analysis of the decay rates given in Table II shows that two trends are evident for the Pt(diimine)(tdt) series. Nonradiative decay rate constants knr increase while radiative decay rate constants kr decrease in going from the complex of highest excited-state energy to lowest. The excited-state energies for the Pt(diimine)(tdt) complexes have been approximated using low-temperature emission (em) maxima, and a plot of ln (knr) versus Eem for these complexes exhibits good qualitative agreement with the energy gap law, which predicts that the rates of nonradiative decay increase as the energy gap separating the ground and excited state decreases. This correlation suggests that the charge-transfer-to-diimine excited states of the tdt complexes all have similar vibrational and electronic components, although with very minor differences noted between the sets of tdt complexes where the diimine is a derivative of bpy versus a derivative of phen. For the series of Pt(dbbpy)(dithiolate) complexes 18, 25–29, the structural variation between the dithiolates is more extensive. Consequently, there appears to be no correlation between ln (knr) and Eem, indicating that factors in addition to the energy gap are important to the nonradiative decay pathways for this series of complexes. In addition to major differences in the important normal modes of 18 and 25–29 as a result of different dithiolate structures, the MMLL0 CT excited states of some of the complexes are higher in energy so that deactivation by metal centered d–d states becomes a possible decay pathway. It was also found that the radiative decay rate constant kr increases with Eem throughout the series of Pt(dbbpy)(dithiolate) complexes. While the importance of kr on emission lifetimes increases at higher energies, the excitesstate lifetimes of Pt(diimine)(dithiolate) complexes having the highest Eem are still the most strongly influenced by knr rather than kr. 2.

Photoinduced Electron-Transfer Reactions

In the initial studies of the luminescence properties and photochemical behavior of Pt(diimine)(dithiolate) complexes, it was found that these complexes undergo both oxidative and reductive ET quenching (48, 98, 99). The complexes appeared to be stable under reductive quenching conditions (i.e., irradiation in the presence of an electron donor such as N,N,N0 ,N0 -tetramethyl-pphenylenediamine) consistent with the reversible reductions the complexes were found to undergo electrochemically, but they exhibited slow decomposition under oxidative quenching conditions in accord with the irreversible oxidations that the complexes undergo. A detailed study of ET quenching of the related 1,1-dithiolate complex Pt(dpphen)(ecda) (30, dpphen ¼ 4,7-diphenylphenanthroline, ecda ¼ 1-(ethoxycarbonyl)-1-cyanoethylene-2,2-dithiolate) thought to have the same emitting state as the 1,2-dithiolene derivatives was conducted for the purpose of determining the excited-state reduction potential and comparing

LUMINESCENCE AND PHOTOCHEMISTRY OF METAL DITHIOLENE COMPLEXES

345

Figure 5. Simplified thermochemical analysis to estimate excited-state redox potentials.

it with a value obtained from a simple thermochemical cycle (109). In this way, it was found that the excited-state reduction potential E(Pt= ) for 30 determined experimentally [0.93 V vs. saturated colomel electrode (SCE) or 1.17 V vs. normal hydrogen electrode (NHE)] agreed well with the value estimated from the thermochemical cycle of embodied in Fig. 5 ( 0.98 V vs. SCE or 1.22 V vs. NHE). Ph N

S

CN

Pt N Ph

S

COOEt

30

Based on the above analysis validating Fig. 4 for estimating excited-state redox potentials, values of E(Pt= ) and E(Ptþ= ) were estimated for both series of Pt(diimine)(dithiolate) complexes studied by Cummings and Eisenberg (109) using electrochemical data and emission energy maxima at 77 K to estimate E00. The results are summarized in Table III. It was found that ligand variation

346

SCOTT D. CUMMINGS AND RICHARD EISENBERG

could be used to tune the excited-state reduction potential E(Pt= ) from 1.21 to 0.45 V, while the excited-state oxidation potential E(Ptþ= ) could be varied from 1.60 to 1.17 V. Consistent with the assignment of the excited state in Pt(diimine)(dithiolate) complexes as 3[Pt(d)/S(p)/dithiolate–p*diimine], variation of the diimine was found to moderate the excited-state oxidation potential, whereas variation of the dithiolate influenced E(Pt= ) most markedly. Parenthetically, Base and Grinstaff (110) reported that the related complex Pt(dpphen)(1,2-dithiolato-1,2-dicarba-closo-dodecaborane) is a strong excitedstate oxidant, on the basis of a 1.09-V excited-state reduction potential estimated as in Fig. 4 from spectroscopic and electrochemical data. Through a series of quenching experiments and Stern–Volmer analyses of the quenching data, rate constants for photoinduced ET between Pt(diimine) (dithiolate) complexes and various electron donors and acceptors were determined. These quenching rate constants were found to range from 4 106 to > 1010 M1 s1, increasing as the thermodynamic driving force increased, in agreement with the Rehm–Weller equations (Eqs. 7a and b) where
Gel is the driving force, Q is the redox quencher, and Wp is a Coulombic work term (the work term for neutral molecules of similar size and shape does not vary significantly).
Gel ¼ nF½EðQþ=0 Þ  EðPt= Þ  Wp
Gel ¼ nF½EðPt

þ=

0=

Þ  EðQ

Þ  Wp

ð7aÞ ð7bÞ

The study by Cummings and Eisenberg thus shows that the excited-state properties of Pt(diimine)(dithiolate) complexes can be altered in a predictable manner through systematic ligand modification (109). These properties include excited-state energies that have been varied by > 1 eV, excited-state redox potentials with diimines modulating E(Ptþ= ) and dithiolates affecting E(Pt= ), and excited-state dynamics. The ET quenching studies are totally consistent with these results and the driving force dependence of ET. 3.

Self-Quenching

An important aspect of the photophysics of the Pt(diimine)(dithiolate) photochemistry that has received increasing attention is the ability of the excited-state complexes to undergo self-quenching. Initial work by Connick and Gray (111) showed that the lifetime of the complex Pt(bpy)(bdt) (bdt ¼ benzene-1,2-dithiolate, 31) decreased with increasing solution concentration. The bimolecular self-quenching rate constant, calculated from a Stern–Volmer quenching analysis, was found to be 9.5 109 M1 s1 in acetonitrile and 4 109 M1 s1 in chloroform. However, no evidence of excimer formation

LUMINESCENCE AND PHOTOCHEMISTRY OF METAL DITHIOLENE COMPLEXES

347

was found in emission spectra of 31, possibly because excimer emission would occur beyond spectrometer detector limits.

S

N Pt N

S

31 The notion of self-quenching in luminescent Pt diimine complexes was generalized in a study by Connick et al. (112) that included work on several Pt(diimine)(tdt) systems. The complexes Pt(phen)(tdt), Pt(dmbpy)(tdt) (dmbpy ¼ 4,40 -dimethyl-2,20 -bipyridine), Pt(tmphen)(tdt), and Pt(dbbpy)(tdt) all display concentration-dependent lifetimes, and self-quenching rate constants (kq) were calculated from Stern–Volmer analyses of the data. Quenching rate constants are in the range of 1–4.7 109 M1 s1 in dichloromethane, indicating that the association of the excited-state complex with a ground-state complex is an efficient process, near solvent diffusional limits. Increasing steric bulk on the diimine has a minor but discernible effect on lowering the selfquenching rate. Self-quenching has also been observed for other Pt(II) diimine complexes, including some that also display excimer emission in concentrated solutions. In independent work by Che et al. (113) and Vogler and co-worker (114), Pt(diimine)(CN)2 complexes were found to exhibit a broad, red-shifted emission with a discernible rise-time from more concentrated solutions (>104 M) relative to that seen from very dilute solutions. More recent studies on luminescent Pt(diimine)(arylacetylide)2 complexes have also shown decreasing lifetimes with increasing concentrations, and in a few cases, evidence has been obtained for excimer emission (9, 112, 115). For the Pt(diimine)(dithiolate) complexes examined in these studies, however, there was no evidence for excimer emission using steady-state luminescence spectroscopy. The notion of self-quenching was probed more extensively using ‘‘cross-quenching’’ experiments with mixtures of metal complexes and quenching experiments with naphthalene and phenanthroline (112). The results are consistent with the notion of excimer formation via a Pt   Pt interaction, a factor that should be considered in the photochemistry of Pt square-planar complexes including dithiolene systems. However, it is expected that charged systems will show either little or no tendency to self-quench because of Coulombic repulsions. The study by Connick et al. (112) not only points to the importance of investigating self-quenching of luminescence from square-planar platinum(II)

348

SCOTT D. CUMMINGS AND RICHARD EISENBERG

excited-state complexes, but also helps to clarify the nature of the emission observed for complexes in the Pt(diimine)(dithiolate) family. Based on measurements of temperature dependence of the emission of concentrated frozen solutions of related Pt(diimine)(1,1-dithiolate) complexes, Crosby and Kendrick (116) concluded that the emissive excited state is 3ds*ps, resulted from stacking of the square-planar complexes in solution, rather than 3MMLL0 CT. However, while there is evidence for self-quenching in concentrated solutions of certain Pt(diimine)(dithiolate) systems, the steady-state emission spectra obtained from optically dilute solutions with concentrations of 106–105 M give no evidence of excimer emission. Recent work by Hupp and co-workers (117) using Stark effect emission spectroscopy provides additional evidence for a CT excited state and further supports the assignment of the emissive excited state as a 3CT state of the monomer.

4.

Photooxidation Chemistry

The long lifetimes of CT excited states of the Pt(diimine)(dithiolate) complexes allow for bimolecular photochemistry, often involving oxidation of the complex. The earliest report of photoreactivity of these complexes dealt with the photooxidation of Pt(bpy)(tdt) (20) following excitation at 577 nm in chloroform (118). The reaction proceeds with a quantum yield of f ¼ 0.03 and was attributed to ET to the halocarbon solvent (Eq. 8) similar to the CTTS photooxidation chemistry of the platinum bis(dithiolate) dianions described above. PtðbpyÞðtdtÞ þ CHCl3 ! PtðbpyÞðtdtÞþ þ Cl þ CHCl2

ð8Þ

ESR spectroscopy provided evidence for the radical ion of the oxidized tdt ligand, but the metal complex cation was not isolated nor were the products of halocarbon reduction identified. Interestingly, the related complex Ni(phen) (S2C2Ph2) was reported to undergo similar photooxidation when irradiated at higher energy, but not when irradiated in the low-energy CT band (118). Srivastava and co-workers (119–121) investigated several Pt(diimine) (dithiolate) complexes as part of the studies on the generation of 1O2 using metal complexes as photosensitizers. They found that continuous photolysis of the complexes in oxygenated solutions led to their decomposition. Evidence of singlet oxygen (1O2 ) generation came from experiments using the singlet oxygen trap 2,2,6,6-tetramethyl-4-piperidinol and quenching by NaN3. The complexes Pt(bpy)(tdt) (20) and Pt(phen)(tdt) (21) produced 1O2 upon photolysis of the low-energy CT absorption band (520–620 nm). The platinum

LUMINESCENCE AND PHOTOCHEMISTRY OF METAL DITHIOLENE COMPLEXES S

Ph

S

S

base

(NN)PtII

(NN)Pt

-H+

R

electron transfer

R = Ph R= H

Ph

+ S

R = Ph

R + O2–

(NN)PtII S R R = PH R= H

(NN)PtII S

+ HO2–

S

path a O2

S

Ph

II

Ph

+

+

O S (NN)Pt

R path b O2 energy transfer

NN = dbbpy

O

II

+

R

R

= 1O 2

S

R

R = Ph R= H

R= H O

O

O

II

S

R

S (NN)Pt

349

O S

S (NN)PtII

(NN)PtII

S

S

Scheme 6

reaction products were not identified in these studies, but the chemistry was hypothesized to involve oxidation of the dithiolene ligand by singlet oxygen. Further investigations into the photooxidation chemistry of Pt(diimine) (dithiolate) complexes were presented in a 1996 report by Schanze and coworkers (122). The luminescent complexes Pt(dbbpy)(dpdt) (dpdt ¼ meso-1,2diphenyl-1,2-ethenedithiolate, 32) and Pt(dbbpy)(edt) (27) are both photostable in deoxygenated solutions but react in oxygenated solutions when photolyzed in the visible CT absorption band. Interestingly, the two complexes follow different reaction pathways. The complex Pt(dbbpy)(dpdt) undergoes dehydrogenation to yield the dithiolene species Pt(dbbpy)(1,2-diphenyl-1,2-ethenedithiolate), while the complex Pt(dbbpy)(edt) is converted to the sulfinate product. The mechanism, shown in Scheme 6, is believed to involve ET from the CT excited state of the Pt(diimine)(dithiolate) complex to oxygen to form singlet oxygen. The 1O2 then reacts with the ground-state complex to yield a sulfoperoxide intermediate, which can decay by one of two pathways to the two products. A mechanism of ET from the excited-state metal complex to form superoxide seems less consistent with experimental results involving singlet oxygen quenchers, the effect of deuterated solvents and the use of methylene blue as a singlet oxygen photosensitizer.

350

SCOTT D. CUMMINGS AND RICHARD EISENBERG

t-Bu

t-Bu N

N

S

S Pt

Pt N

t-Bu

N

S

S

t-Bu

32

27

While 27 and 32 are not ‘‘dithiolene’’ complexes but rather saturated dithiolate analogues, similar photooxidation chemistry has been reported for the complex Pt(bpy)(bdt) (31) by Connick and Gray (111). Like many of the Pt(diimine)(dithiolate) complexes previously studied, this complex is luminescent in fluid solution, with 3CT emission in the 700–750-nm range and with a lifetime of 460 ns in acetonitrile. Spectroscopy and cyclic voltammetry were used to estimate the excited-state redox potentials as E(Pt= ) ¼ 0.6 V and E(Ptþ= ) ¼ 1.7 V, making the complex an especially strong photoreductant (111). Irradiation of oxygenated acetonitrile solutions with lex > 450 nm led to oxidation of the dithiolene ligand and conversion of the complex to the monosulfinated and then disulfinated complexes 33 and 34, as shown in Scheme 7.

O

N

S Pt

N

S

hν O2

N

O

S Pt

N

S

hν O2

S Pt

N

S O

31

33

O

O

N

O

34

Scheme 7

In contrast to these findings, Cocker and Bachman (123) reported that the nickel complex [Ni(bpy)(bdt)] is photooxidized to yield an octahedral disulfonate complex. Cocrystals of a monosulfinate (bpy)2Ni(bdtO2)Ni(bdt) and disulfinate (bpy)2Ni(bdtO4)Ni(bdt) bimetallic complexes were isolated from solution, indicating ligand disproportionation chemistry. A follow-up study investigated the role of the central metal ion in complexes of this type, by exploring the photoreactivity of [Pd(bpy)(bdt)] (124). Photooxidation in DMF afforded the monosulfinate complex [Pd(bpy)(bdtO2)], with minor amounts of the disulfinate complex [Pd(bpy)(bdtO4)], whereas chemical oxidation yielded only the latter.

LUMINESCENCE AND PHOTOCHEMISTRY OF METAL DITHIOLENE COMPLEXES

351

The authors suggest that the ‘‘anomalous’’ reactivity of the nickel complex can be attributed to greater flexibility in the coordination geometry. 5.

Structural Variation

With increasing interest in the long-lived and redox active excited states of the Pt(diimine)(dithiolate) family of compounds, attention has also turned to other types of related complexes. One variation that has been explored is replacement of the diimine with a cylometalated arylpyridine chelating ligand. Balashev and co-workers (125) recently prepared and characterized the complexes (Bu4N) [Pt(ppy)(mnt)] (35) and (Bu4N)[Pt(tpy)(mnt)] (36), where ppy ¼ C-deprotonated phenylpyridine and tpy ¼ C-deprotonated thienylpyridene. Complex 36 is luminescent with lmax ¼ 668 nm in DMF solutions at 298 K and an unusually long lifetime of 1.4 ms. In contrast, the phenylpyridine complex is emissive only in the solid state at 298 K, with lmax ¼ 663 nm. It is interesting to note that both emission wavelengths are significantly longer than other platinum(II) arylpyridine complexes (126). The excited state has been characterized as a similar CT exited state as the Pt(diimine)(mnt) complexes described above, but with a p* orbital of the arylpyridine ligand acting as the LUMO. Based on spectroscopic and electrochemical data, the excited-state redox potentials have been estimated as E(Pt= ) ¼ 0.5 V and E(Ptþ= ) ¼ >1.5 V, indicating that the complexes are strong photoreductants, but unusually weak photooxidants, especially when compared to other platinum(II) arylpyridine complexes (126). – N

S

–

CN

N

Pt

S

CN

S

CN

Pt S

35

CN

S

36

In early work on M(diimine)(dithiolate) systems, Miller and Dance (100) examined Ni(II) and Pd(II) complexes in addition to the Pt(II) systems already discussed. While the Ni and Pd complexes exhibit the low-energy solvatochromic absorption attributable to a CT- to diimine discussed above, the complexes show no solution emission, which may indicate the importance of the third-row metal ion for efficient intersystem crossing to the triplet CT state and diminished nonradiative decay. In order to probe the effect of metal ion on the excited-state properties of square-planar diimine dithiolate complexes, two Au(III) complexes containing tdt and a diimine or phenylpyridine have been prepared recently and

352

SCOTT D. CUMMINGS AND RICHARD EISENBERG

their photophysical properties investigated (127). The complexes [Au(dbbpy) (tdt)]PF6 (37) and Au(Z2-C,N-ppy)(tdt) (38) were prepared from their dichloride precursors, [Au(dbbpy)Cl2]PF6 and Au(Z2-C,N–ppy)Cl2. The neutral C,N–ppy complex exists in two isomeric forms denoted by the position of the Au-bound phenyl C atom relative to the tdt methyl substituent (cisoid or transoid). Whereas the precursor dichloride complexes do not absorb in the visible region of the spectrum, 37 and 38 possess mildly solvatochromic absorption bands in the visible region with molar extinction coefficients of 2300 and 3200 M1 cm1 in CH2Cl2. The absorption bands at 444 and 408 nm, respectively, were tentatively assigned as CT–diimine transitions in both complexes. Unlike their Pt(II) analogues, however, neither of the Au(III) complexes, luminesces in solution or in rigid media at low temperature when lex 300 nm. t-Bu

+ S

N Au N

t-Bu

S

N Au

S

37

Me

S

Me

38

The above observations suggest that Au(III) orbitals are substantially more stabilized than the Pt(II) orbitals, leading to a reduced intensity in the solvatochromic absorption and the absence of emission in 37 and 38. It is posssible that the increased charge on the d8 metal ion serves to draw the energy of the ds* orbital low enough to make nonradiative d–d states comparable in energy to the CT–diimine states so that observable emission is eliminated. Thus, while the Au(III) complexes are isostructural with their Pt(II) analogues in a molecular sense, their electronic structures exhibit significant differences that can be tied to the relative energies of the metal valence orbitals and the results underscore the influence of the metal center on the lowest excited states of these complexes. In contrast, the precursor Au(III) dichloride complexes do exhibit luminescence in low-temperature glass matrices, but in both cases, the emission is at high energy with significant vibrational structure showing spacings between 1300 and 1500 cm1, characteristic of C C and C N vibrational modes of the diimine or phenylpyridine ligand. These high-energy emissions are assigned to intraligand p–p* transitions in both complexes (127). As part of an effort to build covalently linked multicomponent assemblies for light–chemical energy conversion schemes, the Pt(diimine)(tdt) chromophore has been incorporated into bimetallic complexes with platinum(II) and ruthenium(II) diimine chromophores using dipyridocatecholate (dpcat) or

LUMINESCENCE AND PHOTOCHEMISTRY OF METAL DITHIOLENE COMPLEXES

353

tetrapyridophenazine (tppz) as bridging ligands (128). Weak luminescence from (tdt)Pt(dpcat)Pt(dbbpy) (39) is observed in frozen solvent glass at 77 K, but not in fluid solution unlike the solution emissive analogue Pt(phen)(tdt) (21). Excitation of the Ru tris(diimine) absorption band at 430 nm in [(dbbpy)2 Ru(tppz)Pt(tdt)]2þ (40) leads to energy transfer to and emission from the lower energy Pt(diimine)(tdt) moiety at low temperature. t-Bu S

N

O

Pt S

N Pt

N

O

N

t-Bu

39 2+ t-Bu

N

N

N

N

t-Bu

N

N

N

S Pt

Ru N

S

2 40

With rich luminescent properties, long-lived CT excited states and a variety of bimolecular photochemical reaction pathways, the mixed-ligand squareplanar diimine dithiolene complexes of d8 metal ions show great promise for solar-energy conversion, as luminescent probes or in photocatalytic applications. Complexes of this type have also received attention for the nonlinear optical properties, such as second harmonic generation, related to the MMLL0 CT excited state (14, 129). III.

TETRAHEDRAL AND DISTORTED FOUR-COORDINATE d10 COMPLEXES A.

Tetrahedral Bis(dithiolene) Complexes

Homoleptic bis(dithiolene) complexes of d10 metal ions such as Cu(I), Zn(II), Cd(II), and Hg(II) are known. The tetrahedral zinc bis(dithiolene) complexes are among the best studied, and display much of the same CT photochemistry as the square-planar bis(dithiolene) complexes of the d8 metal ions (65, 66). The use of

354

SCOTT D. CUMMINGS AND RICHARD EISENBERG

[Zn(mnt)2]2 as a photocatalyst for hydrogen production from water was described briefly in Section II.A. The actual photocatalyst is believed to be zinc sulfide formed from the metal dithiolene in a photoinitiation process (75, 76). Photoluminescence from tetrahedral bis(dithiolene) complexes is apparently rare. The complexes [M(mnt)2]2 (M ¼ Zn, Cd, Hg), [Zn(dmit)2]2, [Zn (qdt)2]2, and [Zn(tdt)2]2 were investigated by Fernandez and Kisch (130). All of the complexes display both fluorescence and phosphorescence in frozen glasses of ethanol or 2-methyltetrahydrofuran at 77 K from excited states characterized as dithiolene localized IL in nature. The qdt and tdt complexes are also emissive in fluid solution at room temperature. The fluorescence is attributed to a photochemically produced monodithiolene complex. The compounds luminesce over a wide range of wavelengths (350–730 nm) with quantum efficiencies in the range of 102–104, depending on the dithiolene. For the complexes [M(mnt)2]2, the emission shifts to lower energy while quantum efficiency changes substantially along the series M ¼ Zn, Cd, Hg, indicating that the central metal ion influences in some way the intraligand excited-state emission. B.

Tetrahedral Mixed-Ligand Dithiolene Complexes

Mixed-ligand diimine dithiolate complexes of Zn(II) were among the first compounds classified as having a LLCT excited state (131). The complexes Zn(phen)(tdt) (41), Zn(bpy)(tdt) (42), and Zn(biq)(tdt) (43), where biq ¼ 2,20 biquinoline, were reported to have absorption bands at wavelengths of 475 nm (80 M1 cm1), 465 nm (65 M1 cm1), and 590 nm (40 M1 cm1), respectively. The LLCT transition is attributed to a HOMO that is localized on the dithiolene and a LUMO that is localized on the diimine, with a clear relationship to the MMLL0 CT transition described for the square-planar M(diimine)(dithiolate) complexes (M ¼ Ni, Pd, Pt) described in Section II.C. However, the relative orientations of the two planar ligands in the d8 square planar and the d10 pseudotetrahedral complexes are quite different, and have a profound influence on the absorption and emission properties.

S

N N

Me

N

S

N

Zn S

41

S

N

Zn

Zn S

42

Me

N

S

Me

43

LUMINESCENCE AND PHOTOCHEMISTRY OF METAL DITHIOLENE COMPLEXES

355

Benedix et al. (132), presented a theoretical study of the effect of the torsion angle between the dithiolene and diimine ligands on the orbital energies and electronic coupling between the unsaturated chelate rings. Their findings indicate that for a true tetrahedral orientation, with a 90 torsion angle, the LLCT transition is symmetry forbidden, because the p orbitals are orthogonal. However, even a minor deviation from tetrahedral symmetry, which requires little energy, makes the transition partially allowed, leading to the small measured molar absorptivities for this solvatochromic transition. Photoluminescence has been reported for Zn(bpy)(tdt) in the solid state and in CH2Cl2/ethanol solvent glass at 77 K (Benedix, CPL 1990). Weak blue (lmax ¼ 432–462 nm) and red (lmax ¼ 620 nm) emissions were observed but both were attributed to impurities, specifically, Zn(bpy)2þ and Zn(tdt) species. The compounds Zn(phen)(tdt) (41) and Zn(bpy)(tdt) (42) were reinvestigated more recently with room temperature solid-state emission being reported (133). Broad, featureless emission bands were observed at lower energy (595 and 568 nm, respectively) for the two complexes, as well as a highly structured feature in the 380–450-nm region of the spectrum. Lifetimes of  25 ms were reported. The LLCT excited states of the complexes Zn(phen)(tdt), Zn(bpy)(tdt), and Zn(biq)(tdt) were reported to be ‘‘nonluminescent’’ by Noy et al. (134), but capable of reducing both methyl viologen (to MVþ) and oxygen (to O 2 ). The photoluminescent behavior of a complex of the type Zn(diimine)(dtsq), where diimine ¼ 2,20 -biquinoline, phen (44), or 4,7-diphenyl-2,9-dimethyl-phen (batho) and dtsq ¼ dithiosquarate, have been reported by Gronlund et al (135). The phen and batho complexes display broad, featureless luminescence spectra in the solid state at room temperature. Upon cooling to 77 K, the emission spectrum of Zn(batho)(dtsq) resolves into three sharp peaks overlapping the broad emission feature; these sharp peaks are assigned to a diimine localized p–p* emission. The Zn(diimine)(dithiolate) solids degrade upon UV laser excitation, which has inhibited accurate lifetime measurements.

S

N

O

Zn N

S

O

44

A great number of studies have addressed the photophysical properties of related Zn(diimine)(SR)2 complexes. Crosby and co-workers (136, 137) investigated many mixed-ligand complexes of Zn(II) and Cd(II) containing diimine ligands and monodentate aromatic thiolate ligands. They note that the rotational

356

SCOTT D. CUMMINGS AND RICHARD EISENBERG

motion of the thiolate ligands has an important effect on the photophysical properties. With only a few examples of chelating 1,2-dithiolene ligands used, and differences in the reports on their photoluminescent properties, more work appears necessary in order to fully characterize the emission from them.

IV.

OCTAHEDRAL dn COMPLEXES WITH n
6 A.

Homoleptic Complexes

To date, none of the tris(1,2-dithiolene) complexes has been found to be luminescent, so excited states have been assigned based solely on absorption spectra. An intriguing series of reports on the photochemistry of tris(dithiolene) complexes comes from Katakis and co-workers (7, 78, 138, 139a) on the use of trigonal-prismatic tungsten systems to promote the photochemical splitting of water. The results, if confirmed, have significant implications for light–chemical energy storage. In these experiments, various unsymmetrical W(S2C2RR0 )3 complexes in 1:1 water/acetone (or a higher boiling ketone) solutions are irradiated with 350–500-nm light in the presence of methyl viologen or another electron acceptor. The products of the photolyses are reported to be H2 and O2, the former detected by gas chromatography and the latter determined indirectly using Cr2þ and subsequent titration. Of > 30 dithiolene complexes tried, three (shown as 45), were reported to be effective in the generation of O2 with sufficient stability to be catalytic. Each of the effective complexes possesses an aryl group with a strongly electron-donating substituent in the para position. The W(S2C2RR0 )3 complexes were nonemissive so no information about the photochemically active excited state in these systems was described. The investigators estimated excited-state reduction potentials for the W(S2C2RR0 )3 using a thermochemical analysis similar to that used for the platinum diimine dithiolate complexes as shown in Fig. 5, but with the excited-state energy E00 estimated from the absorption band since the complexes were nonemissive. The resultant values are flawed because of the overestimation of E00. S

R

W S X

45 R = H, Ph; X = NMe2, OMe

3

LUMINESCENCE AND PHOTOCHEMISTRY OF METAL DITHIOLENE COMPLEXES

357

A recent transient absorption study of the active W(S2C2RR0 )3 systems has led to the observation of different transients, the assignments of which are without basis. Proposals by Katakis and co-workers (139b) are based on supramolecular assemblies in analogy with natural photosynthesis, but the models appear unrealistic. Independent attempts to repeat the water splitting results have been unsuccessful to date (140), so the use of W(S2C2RR0 )3 complexes as photocatalysts in this important reaction, while intriguing, remains unconfirmed. As with the Ni(II) and Zn(II) bis(dithiolene) complexes, it is possible that photolysis of 45 leads to the generation of tungsten sulfide, which in turn generates water splitting products on photolysis. B.

Mixed-Ligand Complexes

Reaction of the luminescent Ir(I) complexes [Ir(CO)L(mnt)] with alkyl halides in the presence of 1 equiv of L leads to the formation of Ir(III) organometallic complexes of the general formula IrR(CO)L2(mnt) (46, R ¼ Me, Et, CH2CN; L ¼ PAr3) (141). The reaction appears to proceed by alkylation followed by coordination of L to yield the neutral Ir(III) product complex. The species for R ¼ Me and L ¼ PPh3 has been characterized crystallographically and is shown in Fig. 6. The Ir(III) complexes IrR(CO)L2(mnt) luminesce in fluid solution at ambient temperature, making these systems a rare set of luminescent alkyl complexes. The 298 K emission gives evidence of vibronic structure that becomes more pronounced in rigid media as shown in Fig. 7. The major progression has a spacing of  1100 cm1. There are two possible assignments of the emitting state in the IrR (CO)L2(mnt) systems. Both involve CT to mnt but differ in the nature of the HOMO. One assignment assumes that the highest occupied orbital is of Ir d character based on shifts in lem with phosphine donor, yielding a d–pmnt MLCT excited state, while the other draws from the work of Wrighton and Watts in which the HOMO of an alkyl or aryl complex is assumed to be the organic moiety’s sb orbital, thus giving a sb–pmnt sigma bond-to-ligand charge transfer (SBLCT) excited state (142–144). The IrR(CO)L2(mnt) complexes also exhibit photochemical reactivity that was examined in detail because as alkyl carbonyl complexes, they have potentially competing photochemical pathways in the forms of CO photodissociation and M R bond homolysis (89). Prior work on alkyl carbonyl complexes such as CpFeR(CO)2 and CpW(Bn)(CO)3 had shown that despite net chemistry arising from metal–alkyl cleavage, CO photodissociation was the preferred photochemical process (145–149). The photolysis of IrMe(CO)(PPh3)2(mnt) in the presence of radical traps such as PrSH was found to proceed as in Eq. 9 indicating the generation of Me radical (89, 141). Execution of Eq. 9 in the presence of 13CO showed no exchange of CO during the early formation of CH4, *

358

SCOTT D. CUMMINGS AND RICHARD EISENBERG

Figure 6. Molecular structure of IrMe(CO)(PPh3)2(mnt) (46-Me). Reproduced by permission of ‘‘The Royal Society of Chemistry’’.

Figure 7. Emission and absorption spectra of IrMe(CO)(PPh3)2(mnt) (46-Me). Spectrum (a) is the electronic absorption spectrum in chloroform; spectrum (b) (___) is the emission spectrum with lexc at 440 nm from the solid state at 77 K; spectrum (c) (– – –) is the emission spectrum with lexc at 440 nm from EPA glass at 77 K; and spectrum (d) (---) is the emission spectrum with lexc at 440 nm from CHCl3 solution at room temperature. Reproduced by permission of ‘‘The Royal Society of Chemistry’’.

LUMINESCENCE AND PHOTOCHEMISTRY OF METAL DITHIOLENE COMPLEXES

359

demonstrating that the preferred photochemical pathway for IrMe(CO) (PPh3)2(mnt) is metal–alkyl homolysis in contrast to that found for the CpMR(CO)x systems. Photolysis of the corresponding ethyl complex IrEt(CO) (PPh3)2(mnt) leads to rapid b-elimination, Eq. 10, which proceeds faster than exchange with 13CO, again supporting metal–alkyl homolysis as the principal photochemical reaction path. The generation of IrH(CO)(PPh3)2(mnt) from the radical pair {Ir(CO)(PPh3)2(mnt) , Et }formed on photolysis is thought to be thermodynamically favorable because the bond dissociation energy of the methyl C H bond of the ethyl radical is relatively weak. *

PPh 3 OC

Ir

Me

S

C

S

C

N

h ν,

C6 D6

*

PPh 3 S Ir S H PPh 3

OC

N

PPh 3

C C

N

+ CH4 + N

S S

ð9Þ

46-Me PPh 3 OC Et

Ir

S

C N

S

C

PPh 3

N

h ν, C6 D6 5 min

OC H

PPh 3 S Ir S PPh 3

C N C N

+

C2 H4

ð10Þ

46-Et

Photodissociation of CO appears to play a role in other reactions of metal dithiolene carbonyl complexes and in the formation of dithiolenes from metal carbonyls and other dithiolene systems. Early work by Schrauzer et al. (150) revealed the generation of M(CO)n(S2C2R2)3n/2 (n ¼ 0, 2, 4; M ¼ Mo, W) from M(CO)6 and Ni(S2C2R2)2 (R ¼ Me, Ph) in a process undoubtedly involving CO photodissociation. A follow-up study by Miller and Marsh (151) on the formation of W(S2C2Me2)3 from the photolysis of W(CO)2(S2C2Me2)2 suggested CO photodissociation to generate W(CO)(S2C2Me2)2 and formation of a m-S bridged dinuclear intermediate prior to dithiolene ligand transfer. An observed inverse square-root dependence of the quantum yield for the formation of W(S2C2Me2)3 on light intensity supported either the direct coupling of two photogenerated species or the photochemical promotion of dithiolene transfer between an intermediate formed from W(CO)(S2C2Me2)2 and the starting dicarbonyl complex. The other products in addition to W(S2C2Me2)3, however, were not observed or characterized. A set of photochemically promoted elimination reactions has been described for the d6 dithiolene complex CpCo(PBu3)(mnt) (47) and several closely related complexes 48–50 (152). Complexes 48 and 49 form by the reaction of the

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corresponding CpM(dithiolene) complex with quadricyclane to give the norbornylene adducts via cationic rearrangement and S-alkylation, while 50 is generated by the addition of dimethyl acetylenedicarboxylate (DMAD) to the corresponding Cp*M(dithiolene) derivative. While 48–50 are not formally dithiolene complexes since one of the sulfur donors is alkylated, they are closely related to their dithiolene precursors. Irradiation of 47 under UV irradiation (254 nm) leads to the generation of 1O2 and elimination of PBu3 as the phosphine oxide, but the nature of the triplet excited state that sensitizes 1 O2 formation was unspecified. Analogous photoelimination reactions of 48–50 to yield the 16 e complexes CpCo(bdt), CpRh(S2C2Ph2), and Cp*Co(S2C2Z2), respectively, are observed but as with 47, the nature of the excited state leading to the photoreaction in each case remains undetermined. For the norbornylenecontaining complexes 48 and 49, the quantum yield for reaction falls off as lirrad increases from 254 to 365 nm. N S

C

S

C

Co PBu3

S Co

S

S Rh

Ph Ph

S

Z

S

Z

Co

S Z

N

Z

47

48

49

50 Z = COOMe

V.

CONCLUSIONS

It is evident from the research described throughout this chapter that metal dithiolenes and their closely related derivatives exhibit a varied and moderately rich photochemistry. The photochemically active excited states of these complexes are invariably CT in origin. For many d8 bis(dithiolene) complexes and monodithiolene systems having two other ligands such as phosphines, CO, and isocyanides, the nature of observed emissions generally appears to be a MLCT or a metal/ligand-to-ligand charge transfer in which the acceptor orbital is p*(dithiolate) in character. For the series of MLL0 (mnt) complexes, the variation of lem with the donor ability of L and L0 supports the notion that the HOMO in these camplexes corresponds to a metal d orbital, or a delocalized orbital with substantial metal d character. While the bis(dithiolene) complexes are generally not luminescent in fluid solution, emission has been detected in rigid media at low temperatures. The most notable photochemistry for d8 bis(dithiolene) dianions corresponds to photoinduced oxidation in halocarbon solvents via a CTTS excited state with the observation of clean isosbestic points

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361

during the reaction. The dianionic complexes, as well as other monoanionic systems, also provide examples of IPCT excited states in the presence of cationic electron acceptors such as methyl viologen MV2þ.

ACKNOWLEDGMENTS Over the years, research from the Eisenberg laboratory at the University of Rochester on the photochemistry and photophysics of metal dithiolene complexes has been supported by the Department of Energy, Division of Chemical Sciences, for which we are most grateful. Numerous collaborators have helped in our endeavors and their names are mentioned in the citations from the Eisenberg laboratory. We also thank Ms. Arlene Bristol for help with manuscript preparation.

ABBREVIATIONS batho bdt biq bpy cod CT CTTS cyclam dbbpy diimine DMAD dmbpy DMF dmid dmit dmqdt DMSO dmt dpcat dpdt dppe dpphen dppm dppz dto dtox

4,7-Diphenyl-2,9-dimethyl-1,10-phen Benzene-1,2-dithiolate 2,20 Biquinoline 2,20 -Bipyridine 1,5-Cyclooctadiene (ligand) Charge transfer Charge-transfer solvent 1,4,8,11-Tetraazacyclotetradecane[14]ane N4 4,4-Di-tert-butyl-2,20 -bipyridine 2,20 Biquinoline phen Dimethyl acetylenedicarboxylate 4,40 -dimethyl-2,20 -bipyridine Dimethylformamide 2-Oxo-1,3-dithiol-4,5-dithiolate 1,3-Dithiole-2-thione-4,5-dithiolate 6,7-Dimethyl-quinoxaline-2,3-dithiolate Dimethyl sulfate 1,2-Dithiole-3-thione-4,5-dithiolate Dipyridocatecholate meso-1,2-diphenyl-1,2-ethenedithiolate 1,2-Bis(diphenylphosphine)ethane 4,7-Diphenylphenanthroline 1,2-Bis(diphenylphosphino)methane Dihyridophenazine 1,2-Dithiooxalate 1,2-Dithiooxamide

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dtsq ecda edt EDTA em ESR ET HOMO IL I-mnt IPCT LMCT LUMO MLCT MMLL0 CT mnt MO NHE nr phen ppy PVP qdt SBLCT SCE scf TBA tdt TEMO tim tppz tpy UV vis

Dithiosquarate 1-(Ethoxycarbonyl)-1-cyanoethylene-2,2-dithiolate Ethane-1,2-dithiolate Ethylenediaminetetraacetic acid Emission Electron spin resonance Electron transfer Highest unoccupied molecular orbital Intraligand 2,20 -Dicyanooethylene-1,1-dithiolate Ion-pair charge transfer Ligand-to-metal charge transfer Lowest unoccupied molecular orbital Metal-to-ligand charge transfer Mixed metal/ligand-to-ligand charge transfer 1,2-Maleonitrile-1,2-dithiolate Molecular orbital Normal hydrogen electrode Nonradioactive 1,10-Phenanthroline C-deprotonated phenylpyridine Poly(vinylpyridine) Quinoxaline-2,3-dithiolate Sigma bond-to-ligand charge transfer Saturated colomel electrode 1,2-Diperfluoromethylethenedithiolate Tetrabutyl ammonium Toluene-3,4-dithiolate 2,2,6,6-Tetramethyl-1-piperidinyloxy 2,3,9,10-Tetramethyl-1,4,8,11-tetraazacyclotetradeca1,3,8,10-tetraene Tetrapyridophenazine C-deprotonated thienylpyridine Ultraviolet Visible

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134. A. Z. Noy, A. M. Galin, Y. V. Razskazovskii, and M. Y. MelNikov, Proc. Indian Acad. Sci.Chem. Sci., 107, 789 (1995). 135. P. J. Gronlund, W. F. Wacholtz, and J. T. Mague, Acta Crystallogr. Sect. C-Cryst. Struct. Commun., 51, 1540 (1995). 136. G. A. Crosby, R. G. Highland, and K. A. Truesdell, Coord. Chem. Rev., 64, 41 (1985). 137. K. J. Jordan, W. F. Wacholtz, and G. A. Crosby, Inorg. Chem., 30, 4588 (1991). 138. D. Katakis, Pure Appl. Chem., 60, 1285 (1988). 139. (a) E. Lyris, D. Argyropoulos, C. A. Mitsopoulou, D. Katakis, and E. Vrachnou, J. Photochem. Photobiol. A-Chem., 108, 51 (1997). 139. (b) R. Humphry-Baker, C. A. Mitsopolou, D. Katakis, and E. Vrachnou, J. Photochem. Photobiol. A-Chem., 114, 137 (1998). 140. E. Fujita, personal communication. 141. P. Bradley, C. E. Johnson, and R. Eisenberg, J. Chem. Soc., Chem. Commun., 255 (1988). 142. J. C. Luong, R. A. Faltynek, and M. S. Wrighton, J. Am. Chem. Soc., 101, 1597 (1979). 143. J. C. Luong, R. A. Faltynek, and M. S. Wrighton, J. Am. Chem. Soc., 102, 7893 (1980). 144. P. I. Djurovich and R. J. Watts, Inorg. Chem., 32, 4681 (1993). 145. D. B. Pourreau and G. L. Geoffroy, Adv. Organomet. Chem., 24, 249 (1985). 146. J. P. Blaha and M. S. Wrighton, J. Am. Chem. Soc., 107, 2694 (1985). 147. R. J. Kazlauskas and M. S. Wrighton, J. Am. Chem. Soc., 102, 1727 (1980). 148. R. J. Kazlauskas and M. S. Wrighton, J. Am. Chem. Soc., 104, 6005 (1982). 149. D. R. Tyler, Inorg. Chem., 20, 2257 (1981). 150. G. N. Schrauzer, V. P. Mayweg, and W. Heinrich, J. Am. Chem. Soc., 88, 5174 (1966). 151. J. S. Miller and D. G. Marsh, Inorg. Chem., 21, 2891 (1982). 152. M. Kajitani, T. Fujita, N. Hisamatsu, H. Hatano, T. Akiyama, and A. Sugimori, Coord. Chem. Rev., 132, 175 (1994).

CHAPTER 7

Metal Dithiolene Complexes in Detection: Past, Present, and Future ROBERT S. PILATO Lumet LLC Bethesda, MD KELLY A. VAN HOUTEN Sensors for Medicine and Science Inc. Germantown MD CONTENTS I. WHY METALLO-1,2-ENEDITHIOLATES AND DETECTION II. THE GENERATION OF METALLO-1,2-ENEDITHIOLATES, AN EARLY METHOD FOR METAL ANALYSIS

370 371

III. DUAL-EMITTING HETEROCYCLIC-SUBSTITUTED METALLO-1,2-ENEDITHIOLATES

374

IV. THE DEVELOPMENT OF METALLO-1,2-ENEDITHIOLATES AS OXYGEN PROBES

376

A. B. C. D.

Metallo-1,2-enedithiolates Metallo-1,2-enedithiolates Metallo-1,2-enedithiolates Metallo-1,2-enedithiolates

as Dual-Emitting Oxygen Probes / 379 as Phase-Based Oxygen Probes / 381 as Double-Frequency Modulation -Based Probes / 383 versus Other Oxygen Probes / 384

Dithiolene Chemistry: Synthesis, Properties, and Applications, Progress in Inorganic Chemistry, Vol. 52 Special volume edited by Edward I. Stiefel, Series editor Kenneth D. Karlin ISBN 0-471-37829-1 Copyright # 2004 John Wiley & Sons, Inc. 369

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ROBERT S. PILATO AND KELLY A. VAN HOUTEN

V. METALLO-1,2-ENEDITHIOLATES AND A NEW METHOD FOR THE DETECTION OF ACTIVATED PHOSPHATES VI. PROTONATION STATE DEPENDENT EMISSION, A BASIS FOR pH MONITORING

387

391

A. Enabling Emission from Metallo-1,2-enedithiolates by Protonation / 391 B. Quenching of the 3ILCT Emission by Protonation / 393 VII. CONCLUSION

393

ACKNOWLEDGMENTS

394

ABBREVIATIONS

394

REFERENCES

395

I.

WHY METALLO-1,2-ENEDITHIOLATES AND DETECTION

The chemical, electrochemical, and photophysical properties of metallo1,2-enedithiolates make them of potential use in analyte detection. This finding was first realized in the 1930s when colorimetric metal–ion identification using 1,2-enedithiolates was first developed (1–2). As visible spectrophotometers became commonplace in the 1960s, this colorimetric method for metal qualification became a spectrophotometric method for metal quantification (3–16). Both methods relied upon the inherent reactivity of 1,2-enedithiolates (aryl-1,2-dithiolates) with metal ions and the strong visible absorptions of the resulting metal complexes. Metallo-1,2-enedithiolates did not again come to the attention of those developing new analytical methods until the late 1990s. It was the chemical and photophysical properties of luminescent heterocyclic-substituted platinum1,2-enedithiolates that drew the attention. Variations in emission intensity, lifetime, and a unique fluorescent/phosphorescent ratio have all been used for signal transduction with emissive members of this class. Much of this recent work was augmented by the development of new electronic devices for specific use with this class of metallo-1,2-enedithiolates. Further development of these complexes and the accompanying devices will undoubtedly lead to their use in many new detection strategies. At present, these systems are under development as oxygen, temperature, and pH probes as well as dual-emitting luminescent tags (17–24). This chapter serves to highlight several of the chemical and photophysical properties that make 1,2-enedithiolates of utility in detection. It also outlines detection methods that have been successfully developed based upon the unique properties of these ligands and metal complexes.

METAL DITHIOLENE COMPLEXES IN DETECTION: PAST, PRESENT, AND FUTURE 371

II.

THE GENERATION OF METALLO-1,2-ENEDITHIOLATES, AN EARLY METHOD FOR METAL ANALYSIS

1,2-Enedithiolates (aryl-1,2-dithiolates) have been used since the 1930s for the qualification of metal ions (1–2). Toluene-3,4-dithiol (tdt) was the first commercially available dithiol used as an organic spot-test reagent to qualify metals (14). Prior to ultraviolet–visible (UV–vis) spectrophotometers being commonplace, color differentiation of the precipitated complexes was used for metal identification (Scheme 1). For example, the following metals precipitate from 4 M HCl using tdt: palladium, red; molybdenum yellow-green; tungsten, blue-green; lead, yellow; cobalt, black; platinum, violet; ruthenium, brown (25). If the sample was a mixture, separation of the metals using standard methods was required (14). A typical scheme to distinguish first-row metals is shown in Scheme 1. In this scheme both tdt and [Zn(tdt)2]2
are used as a source of the tdt ligand. In the late 1950s and 1960s, as UV–vis spectrophotometers became available, tdt was superseded by 2,3-quinoxalinedithiol (qdt) for the quantification of metal ions (Table I) (3, 5–8, 11). Using qdt, metal ion concentration in the 0.1–10 ppm range can be determined with errors of < 5%. However, metal mixtures require separation prior to analysis except in those cases where simultaneous Beer’s law equations can be solved (5, 8–9, 12).

Scheme 1

372

ROBERT S. PILATO AND KELLY A. VAN HOUTEN TABLE I Electronic Spectral Data for qdt Metal Complexes

Complex

Color

[Ni(qdt)2]2
[Ni(qdt)2]2
[Co(qdt)3]3
[Cu(qdt)2]3
[Pd(qdt)2]2
[Pt(qdt)2]2
Os(qdt)a

Blue Pink Red Blue Orange-red Blue Green

a

Solvent

lmax(nm)

e(Lmol
1cm
1)

aq. EtOH, HCl Ammonia aq. EtOH, HCl aq. EtOH, pH2 aq. DMF, HCl aq. DMF, HCl aq. DMF, HCl

656 520 510 625 548 624 560

17,500 20,650 36,400 22,500 30,600 14,100 17,300

Reference 8 26 8 9 5 11 12

The exact stochiometry and charge of the osmium complex was not established (12).

2,3-Quinoxalinedithiolate was first prepared in 1956 by Morrison and Furst (4) who observed that qdt formed colored complexes with metals in aqueous ammonium hydroxide. Nickel was first quantitated using qdt in 1958 by Skoog et al. (26) in liquid ammonia. Silver, copper, cobalt, and manganese were found to interfere with nickel detection. In particular, under the conditions of Skoog et al., the absorbance of [Co(qdt)3]3
(lmax ¼ 475 nm) significantly overlapped with the absorbance of [Ni(qdt)2]2
at 520 nm. Burke and Yoe (8) described the simultaneous spectrophotometric determination of cobalt and nickel in acidic ethanol. An analogous procedure in acidic dimethylformamide (DMF) was described by Ayers and Annand (3). By simultaneously solving Beer’s law equations, the concentration of each metal can be determined (Eqs. 1 and 2). In general, the results of these two methods were similar. However, Burke and Yoe (8) found that iron and copper interfered with the measurement while Ayers and Annand (3) found interference from manganese. A505 ¼ eðCoÞ505 ½Co þ eðNiÞ505 ½Ni

ð1Þ

A650 ¼ eðCoÞ650 ½Co þ eðNiÞ650 ½Ni

ð2Þ

where A ¼ absorbance and e ¼ extinction coefficient Furthermore, qdt can be used for the simultaneous determination of cobalt, nickel, and copper in acidic ethanol solutions by monitoring the absorbance maxima (lmax) at 510, 656, and 625 nm, and solving the simultaneous equations at each wavelength (Scheme 2) (9). In this method, thiourea is added to generate Cu(I), the reactive form of copper. At pH 6, in the absence of thiourea, nickel and cobalt can be determined without interference from copper. Palladium and platinum can also be determined simultaneously in acidic DMF with qdt (5, 6, 11). In these methods, [Pd(qdt)2]2
is measured at 548 nm while [Pt(qdt)2]2
is measured at 624 nm. A variety of metals interfere with the

METAL DITHIOLENE COMPLEXES IN DETECTION: PAST, PRESENT, AND FUTURE 373

Scheme 2

quantification of Pd and Pt and require prior separation. A protocol for osmium detection has also been described (12), where the resulting osmium complexes absorb at 560 nm in acidic DMF. Early in the development of these analytical methods, it was realized that the dithiolate groups of both tdt and qdt were sensitive to oxidation and that qdt was light sensitive (14, 25). Oxidative and photoinduced decomposition of the ligand increases background interference in metal analysis. Where it is possible to use [Zn(tdt)2]2
, the background absorbance is substantially reduced relative to an analysis using tdt (see Scheme 1) (13, 14, 16, 25). An alternative to qdt, (S)-2-(3-mercaptoquinoxalinyl)thiourinium, is stable and soluble in aqueous ethanol solutions unlike qdt (27). At pH 10 in ammonia– ammonium chloride buffer, this reagent hydrolyzes to qdt. (S)-2-(3-Mercaptoquinoxalinyl)thiourinium has been used for the simultaneous detection of nickel and cobalt and the determination of palladium (27, 28). A related reagent, 6-nitro-(S)-2-(3-mercaptoquinoxalinyl)thiourinium has also been used in metal analysis (7). This reagent is hydrolyzed in ammonia buffer to generate 6-nitro2,3-quinoxalinedithiol (nqdt). Following adjustment to pH 2.0, the mixture is extracted with methyl isobutyl ketone and spectrophotometrically analyzed. 6-Nitro-(S)-2-(3-mercaptoquinoxalinyl)thiourinium has been used for the simultaneous spectrophotometric determination of nickel and cobalt by the quantification of [Ni(nqdt)2]2
(710 nm, E ¼ 20,700 L mol
1cm
1) and [Co(nqdt)2]2
(530 nm, E ¼ 40,000 L mol
1cm
1), respectively.

ð3Þ

Today, the use of tdt, qdt, and their derivatives to quantify metals is considered a ‘‘classical’’ method. Detection limits well below the ppm level achieved with qdt are now readily available using other tequniques. However, metal quantification using qdt is still useful and cost-effective. It is also

374

ROBERT S. PILATO AND KELLY A. VAN HOUTEN

important for those working in this field to recognize the historical progression of efforts using metallo-1,2-enedithiolates.

III.

DUAL-EMITTING HETEROCYCLIC-SUBSTITUTED METALLO-1,2-ENEDITHIOLATES

While metallo-1,2-enedithiolates have long been used in metal analysis, it was not realized until the 1990s that they had unique photophysical properties (10, 17–24, 29–33). It was not until 2000 that low-cost devices were created to take full advantage of the excited-state properties of the complexes (20, 21, 24, 29). An entire class of heterocyclic-substituted platinum 1,2-enedithiolates (Fig. 1) was found to be unique among inorganic lumiphores in that they were room temperature dual emitters (17–19, 22–24, 29–32). The dual emission is from a long-lived state with considerable triplet character (phosphorescence) 3 ILCT* and a short-lived state with considerable singlet character (fluorescence), 1ILCT*. Both states are thought to arise from an intraligand charge-transfer (ILCT) transition with considerable 1,2-enedithiolate p to heterocycle p* character (17–19, 22, 31). This transition is generally in the 400–500 nm (25,000– 20,000 cm
1) range and is the lowest lying band in the emissive heterocyclicsubstituted platinum 1,2-enedithiolates. The ILCT assignment was based upon the following observations. First, the energy of the absorption band is solvent sensitive, supporting the CT assignment. Second, the band’s energy is nearly identical for the corresponding Ni, Pd, and Pt complexes and is unaffected by varying the phosphine ligand. This consistency supports an intraligand CT assignment, and rules out assignment to

Figure 1. Representative members of the heterocyclic (het)-substituted platinum 1,2-enedithiolate dual emitters, where L2 ¼ dppm, 1,1-bis[diphenyldiphosphino)methane]; dppe, 1,2-bis[diphenyl diphosphino)ethane]; dppp, 1,3-bis[(diphenyldiphosphino)propane].

METAL DITHIOLENE COMPLEXES IN DETECTION: PAST, PRESENT, AND FUTURE 375

Figure 2. A plot of aromatic reduction potential versus the energy of the ILCT transition for the (dppe)Pt{S2C2(2-Het)(H)} complexes (&). The approximate energy of the d to d transition for the (dppe)Pt{S2C2(2-Het)(H)} complexes (&).

either a metal–ligand change transfer (MLCT) or ligand–metal charge transfer (LMCT) (31, 33). Third, the band is red shifted by both methyl substitution of the 1,2-enedithiolate and protonation of the heterocycle. Furthermore, a plot of ILCT energy versus the reduction potential of the appended heterocycle (or aromatic) is linear, suggesting the accepting orbital resides upon the heterocycle (Fig. 2) (17). Excitation of room temperature deaerated solutions of selected heterocyclicsubstituted platinum-1,2-enedithiolates leads to a dual emission, which is characteristic of this class of molecules (Fig. 3) (17–19, 22–24, 29–31). As can be seen in Fig. 3, the triplet emission is diffusionally quenched by oxygen while the singlet emission is insensitive to oxygen. As can be seen in Table II, the singlet emissions are generally in the 610–713 nm (16,500–14,000 cm
1) range while those of the triplet are in the 660–750 nm (15,000–13,000 cm
1) range (17–19, 22–24, 29–31). The room temperature singlet lifetimes range from 0.02–3.1 ns while those of the triplet range from 720–15900 ns. Given the lifetime of the triplet, it is diffusionally quenched at

376

ROBERT S. PILATO AND KELLY A. VAN HOUTEN

Figure 3. Representative emission of a dual-emitting heterocyclic-substituted platinum 1,2enedithiolate. The emission spectra of [(dppe)Pt{S2C2(CH2CH2-N-2-pyridinium)}][BPh4], in dimethyl sulfoxide (DMSO) at 298 K (no instrument correction applied); Solid line, emission spectrum under N2 (emission from both the 1ILCT* and 3ILCT*); Dashed line, emission spectrum under air (emissions from the 1ILCT*).

analyte concentrations of chemical and biological interest (10
3–10
6 M). Given its short lifetime, the singlet is not diffusionally quenched at concentrations of quencher <10 M. As such, the singlet emission serves as an internal standard during triplet quenching events (18, 30). Complexes listed in Table II have triplet emissions that are quenched by proton, hydrogen atom, electron, and energy transfers that do not effect either the singlet or the ground state (17–19, 22–24, 29–31). The range of quenching modes and variations among members of this class makes their incorporation into complex detection strategies increasingly likely, while the dual nature of the emission makes new modes of signal transduction possible. To date, heterocyclic-substituted platinum 1,2-enedithiolates have been used in analyte detections that rely on changes in triplet lifetime (18–20), the 3 ILCT*/1ILCT* intensity ratio (21), the ratio of modulation amplitudes (20), and polarized excitation (34). While most of this work has been directed at new methods for monitoring oxygen, these complexes could be important for many luminescent detection methods given the range of phosphorescence quenching modes (18, 19, 30).

IV. THE DEVELOPMENT OF METALLO1,2-ENEDITHIOLATES AS OXYGEN PROBES Given the relatively low concentration of oxygen in aqueous media and its role as the terminal electron acceptor in aerobic pathways, oxygen concentrations reflect upon the health of an environment, cell culture, or organism. The

377

0.001 0.002 0.003 0.003 0.003 0.003 0.003 0.002 0.004 0.002 0.007 0.006

f

1

0.003 0.009 0.009 0.03 0.004 0.011 0.016 0.010 0.020 0.010 0.020 0.002

f

3

550 677 553 589 585 572 578 586 610 677 620 660

lmax (nm)

1

680 732 731 742 764 740 758 775 730 732 700 748

lmax (nm)

3

0.05 4.3 0.3 3.1 0.1 0.2 0.2 0.2 0.2 0.2 0.1 0.2

1 t (ns)

710a 2,010c 3,520c 7,500c 2,800c 15,900c 12,500c 11,100c 8,500c 8,660c 4,500c 4,700c

3 t (ns)

b

Lifetimes in nanoseconds (ns) was determined by frequency modulation. Quenching agent ¼ A ¼ oxygen, B ¼ electron donor; C ¼ proton donor; D ¼ hydrogen atom donor; E ¼ emission enabled by protonation. c Lifetimes determined by transient absorption spectroscopy.

a

[(dppm)Pt{S2C2(2-Pyridinium)(H)}]þ [(dppe)Pt{S2C2(2-Pyridinium)(H)}]þ [(dppm)Pt{S2C2 (4-Pyridinium)(H)]þ [(dppe)Pt{S2C2 (4-Pyridinium)(H)]þ [(dppp)Pt{S2C2 (4-Pyridinium)(H)]þ [(dppm)Pt{S2C2 (N-Methyl-4-pyridinium)(H)]þ [(dppe)Pt{S2C2 (N-Methyl-4-pyridinium)(H)]þ [(dppp)Pt{S2C2 (N-Methyl-4-pyridinium)(H)]þ [(dppm)Pt{S2C2(CH2CH2-N-2-Pyridinium)}]þ [(dppe)Pt{S2C2(CH2CH2-N-2-Pyridinium)}]þ [(dppp)Pt{S2C2(CH2CH2-N-2-Pyridinium)}]þ (dppe)Pt{S2C2(2-Quinoxaline)(H)}

Complex

TABLE II Luminescent Properties and Quenching Modes of Heterocyclic-Substituted Platinum 1,2-Enedithiolates

A,B,E A,B,E A,B,E A,B,E A,B,E A,B A,B A,B A,B A,B A,B A,B,C,D

Quench Agent

378

ROBERT S. PILATO AND KELLY A. VAN HOUTEN

accurate measurement of dissolved oxygen is important to environmental, industrial, and biomedical monitoring, and such oxygen detection is a potential application of a long-lived emitter such as the heterocyclic-substituted platinum 1,2-enedithiolates. While the use of the modified Clark electrode is a long standing polarographic method for oxygen detection in solution, it is adversely affected by signal drift, electrical interference, agitation of the media, and the inherent problem of oxygen consumption (35–44). Given these limitations, optical oxygen detection methods are now finding use in biomedical, bioprocess, environmental, and military applications (37, 39– 41, 45– 47). In bioprocess applications, electrochemical methods generally require that an insert probe breach a sterile bioprocess vessel. New methods in luminescent detection have removed this limitation and are a major advance in fields where a sterile environment is required (45, 48). Once polymer encapsulated, several emissive metal complexes can be used to measure oxygen in the gas phase as well as in aqueous or biomedia (29, 43, 44, 49–64). Oxygen concentrations can be determined using emission intensity by adapting the Stern–Volmer IO =I ¼ kq tO ½O2  þ 1 IO =I ¼ KO2 ½O2  þ 1 ½O2  ¼ fIO =I
1g=KO2

ð4Þ ð5Þ ð6Þ

equation (Eq. 4), where I o is the emission intensity in the absence of a quenching agent (oxygen), I is the emission intensity at some oxygen concentration [O2], tO is the lifetime of the emitter in the absence of oxygen, and kq is the oxygen quenching rate constant. It is not a prerequisite that either the lifetime or the quenching rate constant be known, only that the Stern–Volmer constant KO2 be generated by standardization of the sensor (Eqs. 5 and 6). The system is standardized by determining I o and plotting I o/I
1 versus [O2], where the slope is KO2 (Eq. 5). Once the system is standardized, the only oxygen-dependent variable is I. While the simplicity of measuring absolute intensity is appealing, it is well established that simply relying on an oxygen-induced change in emission intensity is not adequate to accurately determine an oxygen concentration in most settings (45, 49, 65). Intensity-based measurements are adversely affected by changes in optical clarity, fluctuations in the source and detector, and photobleaching of the emitter (54, 65– 68). These nonanalyte induced variations in absolute intensity make continual restandardization a must. To circumvent these problems, several luminescent detection methods have been developed. Of these methods, two have been developed specifically for use with dual emitting metallo-1,2-enedithiolates (20, 21).

METAL DITHIOLENE COMPLEXES IN DETECTION: PAST, PRESENT, AND FUTURE 379

A.

Metallo-1,2-enedithiolates as Dual-Emitting Oxygen Probes

Since the heterocyclic-substituted platinum 1,2-enedithiolates are dual emitters with only one emission that is oxygen quenched, ratiometric oxygen analysis is possible (see Fig. 3) (21, 29). While fluorescence and phosphorescence intensities vary with changes in optical clarity, fluctuations in the source and detector, and photobleaching of the emitter, the intensity ratio of a dual emitter (phosphorescence–fluorescence) does not. As such, the 3I/1I intensity ratio can be used in place of I, in Eq. 3 to generate Eq. 7 and 8 (29–31, 69). ð3 IO =1 IO Þ=ð3 I=1 IÞ ¼ kq tO ½O2  þ 1

ð7Þ

ð3 IO =1 IO Þ=ð3 I=1 IÞ ¼ KO2 ½O2  þ 1

ð8Þ

3

I O/1I O and 3I/1I are the triplet–singlet emission intensity ratios in the absence of oxygen and at some oxygen concentration [O2], respectively. In an analytical setting, the intensity ratio in the absence of quencher, 3I O/1I O, is a constant defined as RO. The parameter kqtO is again consolidated into the term KO2 , which is obtained by standardization of the sensor. The system is standardized by determining RO, and by plotting RO/(3I/1I)
1 versus [O2], where the slope is KO2 . Once the system is standardized, the only oxygen-dependent variable is the triplet–singlet ratio 3I/1I (Eq. 9). ½O2  ¼ fRO =ð3 I=1 IÞ
1g=KO2

ð9Þ

While the approach described in Eqs. 6–8 can be used with a standard fluorometer and cuvette, a low-cost ratiometric oxygen analyzer was created to advance the technique (21). The device was designed to use the dual-emitting metallo-1,2-enedithiolate, [(dppe)Pt{S2C2(CH2CH2-N-2-pyridinium)}] [BPh4] (22), and a diode-based two-wavelength detector (21). However, the device can be readily adapted to other dual emitters. The detector components, in particular the through-pass filters and light-emitting diode (LED), were chosen to excite and measure the relative fluorescence and phosphorescence of the metallo-1,2-enedithiolate [(dppe)Pt{S2C2(CH2CH2-N-2-pyridinium)}] [BPh4] (Fig. 4). An output profile from this device is seen in Figure 5. While similar detection methods (without the accompanying device) have been developed using both a fluorescent and phosphorescent molecule as well as with two phosphorescent molecules, these are inferior to a method using a dual-emitting metallo-1,2-enedithiolate (70–72). In the two-dye method, photobleaching of either dye leads to gross changes in the intensity ratio, and hence these systems are no better than measuring absolute intensity under conditions where photobleaching can occur. In studies where the dual emitter was purposely

380

ROBERT S. PILATO AND KELLY A. VAN HOUTEN

Figure 4. Instrument schematic for a ratiometric analyzer designed to monitor oxygen using polymer immobilized [(dppe)Pt{S2C2(CH2CH2-N-2-pyridinium)}] [BPh4]. The LED, 470-nm light emitting diode; F(1), 460 30-nm bandpass; SP, sensing patch; F(2), 570 40 nm; F(3), 680

22 nm; PD1, PD2, photodiodes; TA1, TA2, transimpedance amplifiers; SW1, SW2, switches; BPF, bandpass filter; Abs, Absolute value detector; X(
1) multiplier; Int, integrator; TH, trigger with hysteresis; Rcf, RC filter. [Adapted from (21).]

Figure 5. Output from a ratiometric intensity analyzer designed to monitor oxygen using [(dppe)Pt{S2C2(CH2CH2-N-2-pyridinium)}] [BPh4], immobilized in cellulose acetate plasticized with 75%/wt. triethyl citrate. [Adopted from (21).] The instrument response voltage correlates directly to the triplet/singlet ratio of the dual emitter at various oxygen concentration.

METAL DITHIOLENE COMPLEXES IN DETECTION: PAST, PRESENT, AND FUTURE 381

photobleached, it was demonstrated that the ratio at a given oxygen level was unaffected by emitter losses as large as 35% (21, 29). B.

Metallo-1,2-enedithiolates as Phase-Based Oxygen Probes

Since few room temperature dual emitters were known prior to the heterocyclic-substituted platinum-1,2-enedithiolates, methods, other than those described in Eqs. 7–9, have been developed to circumvent problems with absolute luminescence intensity measurements. These methods generally require a measurement that is correlated to emitter lifetime. The ratio of the lifetime in the absence of the quencher,tO, and at some quencher concentration, t is defined in Eq. 10, where kq is the oxygen-quenching rate constant. Plotting tO/t
1 versus [O2] allows kqtO (KO2 as defined in Eq. 4) to be determined from the slope (Eq. 11). ð10Þ tO =t ¼ kq tO ½O2  þ 1 ½O2  ¼ ðtO =t
1Þ=KO2

ð11Þ

One of the most convenient methods for determining lifetime, as well as one of the best suited to low-cost applications, involves using frequency-modulated excitation (49). Upon excitation by a frequency modulated source, the finite lifetime of the emitter causes a phase-shift and demodulation of the emission relative to the excitation waveform as shown schematically in Fig. 6.

Figure 6. A schematic representation of a modulated excitation wave form (solid line) and the time delayed modulated emission waveform (dashed line). The parameters
j and
A represent the change in phase and the change in modulation amplitude of the two wave forms.

382

ROBERT S. PILATO AND KELLY A. VAN HOUTEN

Figure 7. The modulation and phase profiles for [(dppe)Pt{S2C2(CH2CH2-N-2-pyridinium)}] [BPh4], immobilized in cellulose acetate plasticized with 75%/wt. triethyl citrate. ( ) Phase in air, (*) modulation in air. (~) Phase in N2, (
) modulation in N2. [Adapted from (21).]

The lifetime is related to phase shift, j, at the modulation frequency o by Eqs. 12 and 13. Applying this relationship yields Eq. 14. The sensor is standardized by plotting Tan j versus [O2] at a set frequency, o. This method of oxygen detection works well with a number of long-lived emitters with lifetimes >1 ms (49, 65). Figure 7 shows the tan j ¼
ot

ð12Þ


tan j=o ¼ t

ð13Þ

½O2  ¼
1= tan j=okq þ 1=tan jO =okq

ð14Þ

phase and demodulation of [(dppe)Pt{S2C2(CH2CH2-N-2-pyridinium)}][BPh4] (22) versus frequency in both air and nitrogen. As can be seen the phase shift change from air to nitrogen for [(dppe)Pt{S2C2(CH2CH2-N-2-pyridinium)}][BPh4], immobilized in cellulose acetate plasticized with 75%/wt. triethyl citrate is >20 over much of the frequency range from 10–50 kHz.

METAL DITHIOLENE COMPLEXES IN DETECTION: PAST, PRESENT, AND FUTURE 383

From this study, the average 3ILCT* lifetime of [(dppe)Pt{S2C2(CH2CH2-N-2pyridinium)}] [BPh4], immobilized in cellulose acetate plasticized with 75%/wt. triethyl citrate is 14 and 4.6 ms when the emitter is under nitrogen and air, respectively. Commercially available patches for use in bioprocess applications that include [(dppe)Pt{S2C2(CH2CH2-N-2-pyridinium)}] [BPh4], have phaseshift differences from air to nitrogen of 40 at 20 kHz (79). This phase-shift difference is among the largest recorded for the 0–20% oxygen range.

C.

Metallo-1,2-enedithiolates as Double-Frequency Modulation Based Probes

When using frequency-modulated excitation for detection, it has been the general practice to rely on the phase shift rather than demodulation of the emission, relative to the excitation waveform (see Fig. 6) (45, 49, 59, 66–68, 73, 74). However, a recent method developed using [(dppe)Pt{S2C2(CH2CH2-N-2pyridinium)}] [BPh4], and monitoring the modulation amplitude of two different frequency-modulated emissions has been successfully applied to oxygen analysis (20). As can be seen in Eqs. 12–24, the triplet–singlet emission intensity ratio 3I/1I, can be defined in terms of the ac modulation amplitudes, A. This proof starts with Eq. 15, where AAC ¼ ac amplitude, 3I and 1I are the steadystate intensities, AAC ¼ f3 I=½1 þ o2 ð3 tÞ2 1=2 g þ f1 I½1 þ o2 ð1 tÞ2 1=2 g

ð15Þ

The parameters 3t, 1t are the triplet and singlet lifetimes, respectively, and o is the modulation frequency. This equation defines the relationship between the modulation amplitude and the singlet and triplet lifetimes and intensities. Rather than using a single excitation frequency, two excitation frequencies oa and ob are used to generate amplitudes, Aoa and Aob as shown in Eqs. 16–20 and 21–25, respectively. The lifetimes used in the proof where 3t > 1000 1t are representative of the singlet and triplet lifetimes of a heterocyclic-substituted platinum-1,2-enedithiolate. The frequencies, oa ¼ 0:1=3 t and ob ¼ 100=3 t, were selected since they reflect the lifetime differences between the singlet and triplet states. As can be seen in Eqs. 16–20, when the excitation is modulated at low frequency (oa ¼ 0:1=3 t) the amplitude, Aoa , is the sum of the singlet and triplet intensities. However, when the excitation is modulated at high frequency (ob ¼ 100=3 t), Eqs. 21–25, the resulting amplitude is due to only the singlet intensity. Combining Eqs. 20 and 25 allows the 3I/1I to be determined (Eqs. 26 and 27). As described in Eqs. 9–12, the triplet/singlet intensity ratio can be used in place of intensity to generate a Stern–Volmer equation.

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ROBERT S. PILATO AND KELLY A. VAN HOUTEN

Aoa Aoa ¼ f3 I=½1 þ o2 ð3 tÞ2 1=2 g þ f1 I=½1 þ o2 ð1 tÞ2 1=2 g oa ¼ 0:1=3 t

ð16Þ ð17Þ

Aoa ¼ ½3 I=ð1:01Þ1=2  þ ½1 I=ð1:00000001Þ1=2  3

1

ð18Þ

Aoa ¼ ð0:995Þ I þ I

ð19Þ

A oa ¼ 3 I þ 1 I

ð20Þ

Aob ¼ f3 I=½1 þ o2 ð3 tÞ2 1=2 g þ f1 I=½1 þ o2 ð1 tÞ2 1=2 g

ð21Þ

Aob ob ¼ 100=3 t

ð22Þ

Aob ¼ ½3 I=ð10; 001Þ1=2  þ ½1 I=ð1:01Þ1=2  3

1

Aob ¼ ð0:01Þ I þ ð0:995Þ I A ob ¼

1

ð23Þ ð24Þ ð25Þ

I

Therefore Aoa =Aob ¼ ð3 I þ 1 IÞ= 1 I 3

1

I= I ¼ Aoa =Aob
1

ð26Þ ð27Þ

A computer driven double-modulation system with the accompanying software was designed to use [(dppe)Pt{S2C2(CH2CH2-N-2-pyridinium)}] [BPh4], as an oxygen sensor (20). Figure 8 shows the hardware schematic, and the software routine used in this evaluation. The frequencies used in the device where oa ¼ 400 Hz and ob ¼ 100 kHz. The oxygen standardization curve for [(dppe)Pt{S2C2(CH2CH2-N-2-pyridinium)}] [BPh4], immobilized in cellulose acetate plasticized with 75%/wt. triethyl citrate using the double-modulation amplitude method is shown in Fig. 9. As a ratiometric method, doublemodulation has advantages over double-intensity based measurements. These advantages include a single excitation source, single filter, and single detector. As can be seen in Fig. 4, double-intensity methods require two filters and two detectors and as such are more costly to build.

D. Metallo-1,2-enedithiolates versus Other Oxygen Probes As presented in Sections IV. A–C, instruments are under development to take full advantage of the heterocyclic-substituted platinum-1,2-enedithiolates as

METAL DITHIOLENE COMPLEXES IN DETECTION: PAST, PRESENT, AND FUTURE 385

Figure 8. Instrument schematic for a ratiometric modulation and the corresponding software to monitor oxygen using polymer immobilized [(dppe)Pt{S2C2(CH2CH2-N-2-pyridinium)}] [BPh4]. [Adapted from (20).] LED-D, LED-frequency driver; LED, 470 light emitting diode; F(1), 470

40 nm bandpass filter; SP, sensing patch; F(2), 550 nm longpass; PD photodiodes; TIA transimpedance amplifiers. The frequencies used were oa ¼ 400 Hz and ob ¼ 100 kHz.

Figure 9. A graph of IO =I
1 vs. % oxygen at 298 K for [(dppe)Pt{S2C2(CH2CH2-N-2pyridinium)}] [BPh4], immobilized in cellulose acetate plasticized with 75%wt. triethyl citrate used to standardize the double modulation device. [Adapted from (20).]

386

ROBERT S. PILATO AND KELLY A. VAN HOUTEN TABLE III Oxygen Sensitivity of Emitters Encapsulated in GE RTV118, GE RTV108, and Cellulose Acetate Butyrate

Emitter [Ru(dpp)3(ClO4)2]a [(dppe)Pt{S2C2(CH2CH2N-2-pyridinium)}(BPh4)] [Ru(bpy)3(ClO4)2]a [(dppe)Pt{S2C2(CH2CH2N-2-pyridinium)}(BPh4)] Pt-ocatethylporphyrin [(dppe)Pt{S2C2(CH2CH2N-2-pyridinium)}(BPh4)] [Ru(dpp)3(BPh4)2] a

Encapsulating Medium

PðO2 Þ1=2 (Torr)

Reference

GE RTV118 GE RTV118

29.8 51

60 29

GE RTV118 GE RTV108 Cellulose acetate butyrate Cellulose acetate butyrate Cellulose acetate butyrate

377 63 12.9 70 102

60 29 50 29 52

0

2,2 -Bipyridine ¼ bpy, 4,7-diphenyl-1,10-phenanthroline ¼ dpp.

dual emitters in oxygen detection. As such, the platinum 1,2-enedithiolates have to be compared to other molecules of utility in luminescent oxygen detection. It is generally accepted that both the immobilizing polymer and the lifetime of the emitter determine the useful range of detection (41, 43, 44, 50–52, 57, 58, 63, 75–77). This range is generally defined by the point at which one-half of the emission is quenched, PO2 ð1=2Þ . The choice of polymer also effects the linearity of the Stern–Volmer plot (47, 59–62, 78) This effect is thought to reflect the solubility of the emitter in the polymer and the formation of emitter crystallites, which lead to inhomogeneity in the emitter lifetime (47, 59–62, Pilato, unpublished results). It is also generally accepted that for a given polymer, the sensing film thickness controls the response and recovery times of the sensor (42, 44, 50–52,76). Table III includes data for different emitters in various polymer–plasticizer combinations used for oxygen detection and allows comparison of the emitter molecules. As can be seen, [(dppe)Pt{S2C2(CH2CH2-N-2-pyridinium)}] [BPh4], has PO2 ð1=2Þ values in RTV118 and cellulose acetate butyrate that are similar to [Ru(dpp)3]2þ, a common emitter used in oxygen detection. This PO2 ð1=2Þ is ideal for environmental, biomedical, and bioprocess oxygen monitoring in the range of 0–100% O2. The heterocyclic-substituted platinum 1,2-enedithiolates have PO2 ð1=2Þ values that are substantially higher than the platinum group metalloporphyrins making them less useful for measuring oxygen at very low levels (<10 Torr) (50, 55, 79, 80). In several of the commercial silicones (including RTV 108 and 118), the platinum 1,2-enedithiolates are more prone to photobleaching than the ruthenium-based emitters. However, these complexes are quite robust in a large range of other polymers (64).

METAL DITHIOLENE COMPLEXES IN DETECTION: PAST, PRESENT, AND FUTURE 387

V.

METALLO-1,2-ENEDITHIOLATES AND A NEW METHOD FOR THE DETECTION OF ACTIVATED PHOSPHATES

Unlike [(dppe)Pt{S2C2(2-pyridinium)(H)}]þ, complexes of the general structural type [(P2)Pt{S2C2(2-pyridinium)(CH2CH2OH)}]þ, where P2 ¼ dppm, dppe, dppp are nonemissive in room temperature solution (3f < 10
5). However, the complexes (P2)Pt{S2C2(2-py)(CH2CH2OH)} (py ¼ pyridine), react rapidly with a range of fluoro-, chloro-, and cyanoorganophosphates (Scheme 3) as well as with organosulfonates (23, 24). The luminescent products from these reactions, [(P2)Pt{S2C2(CH2CH2-N-2-pyridinium)}]þ, are generated in >95% spectroscopic yield (22). It is the organophosphate induced conversion of the nonemissive (P2)Pt{S2C2(2-py)(CH2CH2OH)}, to the emissive [(P2)Pt{S2C2(CH2CH2-N-2-pyridinium)}]þ that serves as the basis for a new sensor for activated phosphates (23, 24).

Scheme 3

The difference in the quantum yields for [(dppe)Pt{S2C2(CH2CH2N-2-pyridinium)}]þ and [(dppe)Pt{S2C2(2-pyridinium)(H)}]þ from that of [(dppe)Pt{S2C2(2-pyridinium)(CH2CH2OH)}]þ supports an ILCT* excited state where the CT formally oxidizes a 1,2-enedithiolate sulfur and reduces the pyridinium nitrogen. This results in a double bond between the 1,2-enedithiolate and the heterocycle. For such a resonance form/excited state to be stabilized, the 1,2-enedithiolate and heterocycle must approach coplanarity (Scheme 4). As confirmed crystallographically (22), this criterion is forced upon [(dppe)Pt{S2C2 (CH2CH2-N-2-pyridinium)}]þ, even in the ground state. However, in the protonated complexes, the ability of the 1,2-enedithiolate and heterocycle to be coplanar is controlled by the bulk of the R0 group. It appears that when R0 ¼ H the heterocycle and 1,2-enedithiolate can approach coplanarity and the

388

ROBERT S. PILATO AND KELLY A. VAN HOUTEN

Scheme 4

excited state is stabilized leading to a long-lived emissive state. However, the bulk of the CH2CH2OH group impedes coplanarity and destabilizes the excited state (17, 22). When immobilized in a polymer–plasticizer matrix, (dppe)Pt{S2C2(2-py) (CH2CH2OH)}, can be used for the rapid selective detection of volatile fluoro-, chloro-, and cyanophosphates (Table III, Fig. 10). As with many film immobilized lumiphores, the reactivity is controlled by the immobilizing matrix (39–44, 46, 50–52, 57, 58, 63, 75–77) Such an effect is seen as the sensitivity to phosphate esters increases as plasticizer (triethyl citrate, TEC) concentration increases in the cellulose acetate CA/TEC film (Table IV). While reaction of (dppe)Pt{S2C2(2-py)(CH2CH2OH)} with (O)P(OEt)2X, where X ¼ F, Cl, CN, and [(OPh)2P(O)Cl] at room temperature are facile, the reaction of (dppe)Pt{S2C2(2-py)(CH2CH2OH)} with (OPh)(OC6H4pNO2)2P(O), (OPh)2(OC6H4p-NO2)P(O), and (OEt)2(SPh)P(S) are extremely slow (Table V). The reaction specificity is key to differentiating organophosphates used as pesticides and those used as chemical weapons. The specificity of (dppe)Pt{S2C2(2-py)(CH2CH2OH)} reflects the poorer leaving groups found in the latter phosphates, which do not allow for phosphorylation of CH2CH2OH. The leaving group sensitivity of this reaction leads to the selectivity in phosphate detection. The phosphate (t-BuO)P(O)F was also investigated. While this phosphate rapidly phosphorylated the CH2CH2OH group, the resultant phosphoester could not be displaced by the 2-pyridyl group appended to the 1,2-enedithiolate at room temperature. This finding is explainable by the steric interference of the nucleophilic attack at the carbon a to the phosphate. The use of (dppe)Pt{S2C2(2-py)(CH2CH2OH)} in phosphate detection is further complicated by the nature of the emissive states. The triplet emission of

METAL DITHIOLENE COMPLEXES IN DETECTION: PAST, PRESENT, AND FUTURE 389

Figure 10. The luminescence spectra of (dppe)Pt{S2C2(2-py)(CH2CH2OH)} (0.3%/wt) immobilized in a cellulose acetate–150% triethylcitrate film (0.5 mm thick): - - - Control film. ——— Film exposed to 99 Torr OP(OEt)2F in N2 for 15 s —   — Film exposed to HCl.

[(dppe)Pt{S2C2(CH2CH2-N-2-pyridinium)}]þ is sensitive to oxygen. Unlike oxygen detection, the presence of the long-lived emissive state is not desirable and detracts from the organophosphate detection method. Since the largest quantum yield is due to the triplet (see Fig. 8), either oxygen must be eliminated or an immobilizing polymer, which allows phosphate diffusion and not oxygen diffusion must be found. Since it is undesirable to eliminate oxygen, (dppe)Pt{S2C2(2-py)(CH2CH2OH)} will likely not see use as sensor for the detection of chemical warfare agents. However, the phosphate induced fluorescent detection method developed has led to the design and study of several organic molecules, which contain similar reactive groups to the metal complexes where fluorescence can be turned on or off in the presence of activated phosphates (34). These organic 1,2-enedithiolate analogues are more likely candidates for this new and exciting selective phosphate detection method (34).

390

ROBERT S. PILATO AND KELLY A. VAN HOUTEN TABLE IV The Conversion of (dppe)Pt{S2C2(2-py)(CH2CH2OH)} to [(dppe)Pt{S2C2(CH2CH2-N-2-pyridinium)}]þ in Various Polymer–Plasticizer Combinations Minimum Exposure Time (s)b

Polymer–Plasticizera

Phosphate Ester

CA CA/ 25% TEC CA/ 50% TEC CA/ 100% TEC CA/ 150% TEC GE-RTV108 GE-RTV118 CA/ 150% TEC CA/ 150% TEC

(O)P(OEt)2Cld (O)P(OEt)2Cld (O)P(OEt)2Cld (O)P(OEt)2Cld (O)P(OEt)2Cld (O)P(OEt)2Cld (O)P(OEt)2Cld (O)P(OEt)2Fe (O)P(OEt)2CNf

Not observed >600 15 <15 <15 <15 <30 <15 <15

Complete Exposure Time (s)c Not observed Not observed 600 40 30 15 180

a Percentages listed are for TEC weight percent of CA. The complex (dppe)Pt{S2C2(2py)(CH2CH2OH)} loading, 0.3%/weight. Silicone films impregnated in CH2Cl2 solution containing 0.1% NEt3. b Minimum exposure required for luminescent detection of a deaerated sample, 470 nm excitation, 570 and 675 emission. c Minimum exposure required for maximum emission. d (O)P(OEt)2Cl at 70 Torr in an N2 flow of 50 mL s
1. e (O)P(OEt)2F at 99 Torr in an N2 flow 50 mL s
1. f (O)P(OEt)2CN at 41 Torr in an N2 flow of 50 mL s
1.

TABLE V Rates of Conversion in Solution of (dppe)Pt{S2C2(2-py)(CH2CH2OH)} to [(dppe)Pt{S2C2(CH2CH2-N-2-pyridinium)}]þ Relative to the Rate of (OEt)2P(O)Cl Phosphate Ester (OEt)2P(O)X X¼F X ¼ Cl X ¼ CN (OEt)2P(S)Cl (Me)2P(O)Cl (OPh)2P(O)Cl (OPh)(OC6H4p-NO2)2P(O) (OPh)2(OC6H4p-NO2) P(O) (OEt)2(SPh)P(S)

Rates Relative (OEt)2P(O)Cla

1.1b 1.0 0.71 0.24 1.1 1.0 0.069 <0.0003 <0.0003

a Rates are relative to those required for the conversion of (dppe)Pt{S2C2(2-py)(CH2CH2OH)} (10
4 M) to [(dppe)Pt{S2C2(CH2CH2-N-2-pyridinium)}]þ in the presence of triazole (10
2 M) and 10
3 M (OEt)2P(O)Cl at 20  C in CH2Cl2. All phosphate ester concentrations are 10
3 M. The maximum conversions are 70–90%. Under the pseudo-first-order conditions listed, [(dppe)Pt{S2C2(CH2CH2-N-2-pyridinium)}]þ is generated with (OEt)2P(O)Cl at a rate of 1.2  10
5 Ms
1. b Generated from (OEt)2P(O)Cl and benzoyl fluoride and used without purification (81).

METAL DITHIOLENE COMPLEXES IN DETECTION: PAST, PRESENT, AND FUTURE 391

VI.

PROTONATION STATE-DEPENDENT EMISSION, A BASIS FOR pH MONITORING

For many of the reasons listed in the oxygen-detection section of this chapter, it would be desirable to monitor pH using optical methods. As with oxygen detection, this could be particularly useful in settings where sterile containment is desirable. As with oxygen, current electrode-based pH probes require that a sterile environment be breached by the probe and its leads. A relatively large number of organic lumiphores and luminescent polypyridyl ruthenium complexes have been developed as luminescent pH probe molecules (82–84). The heterocyclic-substituted platinum-1,2-enedithiolates have also been developed in this regard (18, 19). There are heterocyclic-substituted platinum 1,2-enedithiolates that have ILCT emissions that are either enabled by protonation of the ground state or quenched by protonation of the excited states. Which process is operative is dependent on the heterocycle appended to the 1,2-enedithiolate, and hence the electron affinity of the accepting orbital in the ILCT transition (see Fig. 2). A.

Enabling Emission from Metallo-1,2-enedithiolates by Protonation

Both (dppe)Pt{S2C2(2-py)(H)} and (dppe)Pt{S2C2(4-py)(H)} are emissive only upon protonation of the appended pyridine. As noted in Section III (Fig. 2), the energy of the ILCT absorption is dependent on the reduction potential (electron affinity) of the appended heterocycle. Protonation of the pyridine in either (dppe)Pt{S2C2(2-py)(H)} or (dppe)Pt{S2C2(4-py)(H)} shifts the reduction potential to a more positive value and increases the electron affinity of the accepting orbital in the ILCT transition. This protonation is also accompanied by a decrease in ILCT transition energy of 6100 and 6700 cm
1 for the 2-pyridin(ium) and 4-pyridin(ium) substituted complexes, respectively. In general, a red shift of this magnitude should lead to a loss of emission intensity due to an increase in the nonradiative decay consistent with the energy gap law (85). However, in this class of complexes room temperature protonation of the pyridine shifts the energy of the ILCT transition below that of a nonemitting state, and hence the complexes are emissive. The nonemitting state is thought to be a d to d state (22, 31). The pKa of pyridine itself is too low to be of use in measuring physiological pH. However, it has been demonstrated that binding either 2- or 4-pyridinium to the 1,2-enedithiolate leads to a 2–3 pKa shift (17, 86) of the appended pyridinium relative to free pyridinium. This moves the pKa of [(dppe)Pt{S2C2(2pyridinium)(H)}]þ and [(dppe)Pt{S2C2(4-pyridinium)(H)}]þ to a range where their emission is enabled at or near physiological pH making these molecules extremely interesting as pH sensing molecules. This pKa shift has been

392

ROBERT S. PILATO AND KELLY A. VAN HOUTEN

Scheme 5

attributed to resonance stabilization of the protonated heterocycle by the 1,2enedithiolate as shown in Scheme 5. Noted that it is essential that the polymers used to immobilize the emitter be amenable to the application (44, 47, 50–52). An additional factor that plagues both luminescent polypyridyl ruthenium complexes and the 2- or 4-pyridine substituted platinum 1,2-enedithiolate in this application is the oxygen-induced triplet quenching. A protonation state dependent emission similar to that described above has been observed by Eisenberg and co-worker (10, 87) for [Pt(qdt)2]2
as described in Chapter 6 in this volume (88). B.

Quenching of the 3ILCT Emission by Protonation

Unlike the pyridyl complexes described in Section VI.A, the quinoxaline substituted complex, [(dppe)Pt{S2C2(2-quinoxaline)(H)}], is emissive when neutral. As described in Section III, the difference of the quinoxaline-substituted complex from the pyridyl-substituted complexes arises from the ease of reduction of quinoxaline relative to pyridine. However, protonation of the quinoxaline complex leads to a loss of triplet emission, which is due to a substantial decrease in the emission energy of [(dppe)Pt{S2C2(2-quinoxaline) (H)}]. This observation is consistent with the energy gap law (85). Like other CT excited states, the ILCT excited state of [(dppe)Pt{S2C2(2-quinoxaline)(H)}] localizes electron density, and hence negative charge upon the accepting orbital in the electronic transition (18, 89). The accepting orbital is thought to have considerable p* character, and hence has considerable contribution from the nitrogen-based atomic orbitals of the quinoxaline. As such, the quinoxaline is expected to be more basic in the excited state than in the ground state. Given that the lifetime of the 3ILCT of [(dppe)Pt{S2C2(2-quinoxaline)(H)] is well into the microsecond region (18), protonation of the 3ILCT* excited state is possible on the diffusional time scale. The 1ILCT* excited state is short lived and unaffected by proton concentrations that do not effect the ground-state molecule. Since this protonation results in the selective loss of the emission of the 3ILCT*, the 3I/1I ratio serves as a measure of proton-induced quenching much in the same way that the same ratio serves to measure oxygen quenching. The extent of

METAL DITHIOLENE COMPLEXES IN DETECTION: PAST, PRESENT, AND FUTURE 393

Figure 11. Dependence of the quenching rate constant, kq (log scale) versus pK a of the quenching agent associated with the quenching of the 3ILCT* of [(dppe)Pt{S2C2(2-quinoxaline)(H)] in MeCN. Proton donors are indicated by filled triangles, and hydrogen atom donors are indicated by filled squares. [Adapted from (18).]

quenching observed is dependent on the pKa of the protonating acid as well as the concentration of the acid (Fig. 11) (18). To date, the proton quenching of the 3 ILCT has only been studied in organic solvents and these studies would have to be extended to immobilizing polymers to determine the utility of this molecule in monitoring pH in practical settings.

VII.

CONCLUSION

As detection needs continually move to the specificity offered only by biological and chemical recognition, reactive chemical entities will play an increasingly important role. The discovery of new metallo-1,2-enedithiolates and the further study of the reactivity of these complexes is important to their potential use in detection. Much of the work described in this chapter is <5 years old, which underlies the importance of continued study. The reactivity of complexes with important analytes such as olefins (90) [see Chapter 6 in this volume (88)], oxygen, and organophosphates make metallo-1,2-enedithiolates of potential use as components in analyte detection strategies. Of particular importance is the study of photophysical properties and the coupling of emission behavior to chemical reactivity. In addition, the further study of the heterocyclic-substituted platinum-1,2-enedithiolates as room temperature dual emitters is warranted. The dual emitters not only allow ‘‘conventional’’ detection methods but new ratiometric, double-modulation and polarization-based methods to be incorporated into detection strategies (34, 91, 92). At least two of these methods were development solely for the heterocyclic substituted 1,2-enedithiolates. While these detection methods evolved to monitor molecular oxygen, it is likely, given their unique

394

ROBERT S. PILATO AND KELLY A. VAN HOUTEN

photophysical properties, that these complexes will be further developed for use in other luminescent detection strategies. With such a bright future in detection, it was worthy in this chapter to also remind researchers in the field of the early application of these complexes. The use of 1,2-enedithiolates to generate metal complexes for metal analysis predates the advent of modern spectroscopy. These colorimetric methods evolved to spectrometric methods as UV–vis spectrophotometers became commonplace in the 1960s. While the sensitivity of these classical methods is not that of modern analytical techniques, these methods are still important in metal analysis and are important to our understanding of this class of molecules.

ACKNOWLEDGMENTS We acknowledge Dr. Edward I. Stiefel for his dedication to this research and researchers in this field.

ABBREVIATIONS bpy CA DMF DMSO dppe dppm dpp dppp Het ILCT LED LMCT MLCT nqdt py qdt tdt TEC UV–vis

2,20 -Bipyridine Cellulose acetate Dimethylformamide Dimethyl sulfoxide 1,2-Bis(diphenylphosphino)ethane 1,2-Bis(diphenylphosphino)methane 4,7-Diphenyl-1,10-phenanthroline 1,2-Bis(diphenylphosphino)propane Heterocycle Intraligand charge-transfer Light-emitting diode Ligand–metal charge transfer Metal–ligand change transfer 6-Nitro-2,3-quinoxalinedithiolate Pyridine 2,3-Quinoxalinedithiolate Toluene-3,4-dithiolate Triethylcitrate Ultraviolet–visible

METAL DITHIOLENE COMPLEXES IN DETECTION: PAST, PRESENT, AND FUTURE 395

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CHAPTER 8

Solid-State Properties (Electronic, Magnetic, Optical) of Dithiolene Complex-Based Compounds CHRISTOPHE FAULMANN and PATRICK CASSOUX ‘‘Mole´cules et Mate´riaux’’, Laboratoire de Chimie de Coordination CNRS, Toulouse Cedex 04, France CONTENTS I. INTRODUCTION

400

II. ELECTRICAL PROPERTIES

406

A. Background / 406 B. Compounds Based on 1,2-Dithiolene Complexes / 408 1. Compounds Exhibiting Metal-Like Behavior / 408 2. Superconductors Based on 1,2-Dithiolene Complexes / 422 C. Related Compounds Based on 1,1-Dithiolene Complexes / 427 D. Potential Applications and Patents / 428 1. Dithiolene Complex-Containing Films / 428 2. Survey of Patents on Electrical Properties of Dithiolene-Based Systems / 431 III. MAGNETIC PROPERTIES

431

A. Spin-Peierls Systems / 432 B. Spin-Ladder Systems / 433 1. The First System: ( p-EPYNN)[Ni(dmit)2] / 433 2. The Second System: (DT-TTF)2[Au(mnt)2] / 436 3. [Cp2M(dithiolene)](TCNQF4) / 437 C. Ferromagnetic Systems / 441

Dithiolene Chemistry: Synthesis, Properties, and Applications, Progress in Inorganic Chemistry, Vol. 52 Special volume edited by Edward I. Stiefel, Series editor Kenneth D. Karlin ISBN 0-471-37829-1 Copyright # 2004 John Wiley & Sons, Inc. 399

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Systems Exhibiting Ferromagnetic Interactions / 441 Bulk Ferromagnets / 448

IV. OPTICAL PROPERTIES

453

A. Strong Near-IR Absorption / 453 B. Nonlinear Optical Properties / 458 1. Second-Order NLO / 458 2. Third-Order NLO / 461 C. Optical Information / 463 V. TOWARD MULTIFUNCTIONAL DITHIOLENE COMPLEX-BASED COMPOUNDS

464

A. Conductivity and Magnetism / 465 B. Conductivity and Optical Properties / 465 VI. CONCLUSION

467

ACKNOWLEDGMENTS

467

ABBREVIATIONS

468

REFERENCES

470

I.

INTRODUCTION

Only in recent decades has it become clear that inorganic molecule-based compounds can possess remarkable physical properties. These properties can be electrical, magnetic, or optical. One of the first typical examples is KCP (kalium tetracyanoplatinat, in German), K2[Pt(CN)4X0.3]
nH2O; X ¼ Cl, Br), which was prepared as early as 1842 (1, 2). This complex was shown to be a metal-like conductor more than a century later (3, 4). Likewise, but within a shorter period of time, the use of metal complexes of 1,2-dithiolene ligands (Scheme 1), first -

S Z

S

Z

S

S Z

-

S

Z

-

Z

-

-

S Z

1,1-Dithiolene 1,2-Dithiolene 1,3-Dithiolene

Scheme 1

prepared and studied on and after the early 1960s (5–8), without consideration of its relation to solid-state physics, eventually led to the characterization of conductors (9 and references cited therein) and superconductors (10). As far as electrical properties are concerned, the interest in using dithiolene complexes was actually inspired by the development of ‘‘organic metal’’

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

401

systems that originate from the discovery of (TTF)
(TCNQ) (TTF ¼ C6H4S4, tetrathiafulvalene; TCNQ ¼ C12H4N4, tetracyanoquinodimethane) (11, 12) and the derived TTF-like based conducting and superconducting systems (13 and references cited therein). In the course of this research, a number of structural and electronic criteria required for the formation of conducting, molecular onedimensional (1D) systems was suggested (14). These criteria include, (a) the stacking of planar molecules along one direction; (b) an electronic content, orbital symmetry, and close packing allowing good overlap between stacked molecules; and (c) the partial filling of the conduction band through either partial oxidation or partial electron transfer. Indeed, by virtue of their overall planar molecular structure and tendency to 1D stacking (15), and their extended p-molecular orbital (MO) system (16), dithiolene complexes share with TTFs and TCNQs the capacity for electronically significant, intermolecular p-orbital interaction in the solid state. Moreover, they also possess the capacity for reversible electron-transfer reactions that yield stable ion–radical species, exhibit, like TTF, sulfur atoms on the periphery, and may be actually be considered as isolobal to TTF (17). Section II will deal with the electrical properties of dithiolene complexes. Electron transfer, as mentioned above, is one of the prerequisites for observing conducting properties in a molecule-based solid, and may also result in systems with interesting magnetic properties, including spin-Peierls (SP) transitions (18–20), ferro- and ferrimagnetism (20, 21), and spin-ladder behavior (22–24). Remember, in low-dimensional, poorly conducting compounds, interstack spin-phonon coupling drives a magnetoelastic distorsion of the lattice, that is, the SP transition, which induces a magnetic ordering at Tc (SP) > 0 K. Below Tc (SP), the spin system is ‘‘dimerized’’, forming a singlet ground state with a magnetic gap (18, 19). The existence of the SP transition, (i.e., this progressive, temperature-dependent ‘‘‘dimerization’’ of the spins) was theoretically predicted in 1962 by McConnell and co-workers (18, 19) and observed for the first time in the (TTF)[MS4C4(CF3)4] dithiolene complexes with M ¼ Cu, Au (25, 26). Mechanisms for ferromagnetic exchange were proposed in the 1960s by McConnell (27, 28) and Mataga et al. (29). However, the first realization of ferromagnetic ordering in a molecule-based solid, (FeCp2 )(TCNQ), (Cp ¼ C10H15, pentamethylcyclopentadienyl) was reported in 1979 (30). The first bulk ferromagnet, (FeCp2 ) (TCNE), (TCNE ¼ C6H4, tetracyanoethylene), was characterized in 1985 (31–34). In dithiolene complex-based compounds, the spin may originate from the metal d orbitals or from the delocalized p system of the associated ion–radical. Short-range ferromagnetic interactions have been observed in several dithiolene complex-based compounds, but only two bulk ferromagnets have been described (Section III.C.2). On the other hand, molecule-based compounds consisting of assemblies of S ¼ 12 chains, one next to the other, have been designated as spin-ladder systems. These lately

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fashionable systems may exhibit strikingly different properties such as short- to long-range ordering with or without spin gap (22–24). Dithiolene complexes being prone to 1D stacking interactions (i.e., the leg of the ladder) and to interstack interactions (i.e., the rungs of the ladder), have also been shown to exhibit spin-ladder behavior. Section III will concern the magnetic properties of dithiolene complexes. Finally, in Section IV dithiolene complexes with interesting optical properties, such as strong near-IR (NIR) absorption, nonlinear optical behavior, and the use of some dithiolene complexes for optical data storage will be reviewed. In Section V, attempts toward coupling conducting, and magnetic or optical properties will be discussed briefly. The number of papers on dithiolene complexes has increased so sharply within the past 5 years (>180/year references in and after 1995 compared to 120/year between 1990 and 1994, and 40/year between 1966 and 1989) that we had to organize each section of this chapter according to the concerned property (electrical, magnetic, optical), to the type (1,1-, 1,2-, or 1,3-) of the dithiolene ligand(s) and to the number of coordinated ligands. As a number of reviews on these aspects of the subject are available, and as long as they were not out-of-date, we will sum up their more significant parts in order to concentrate on the most recent relevant findings. The dithiolene ligands considered in this chapter are depicted in Scheme 1. 1,1-Dithiolene ligands form complexes with four-membered rings, 1,2-dithiolene ligands form complexes with five-membered rings, and 1,3-dithiolene ligands form complexes with six-membered rings (Scheme 2).

Z

S

S Z

S

Z

S

Z

M

S

S

S

Z

M

S

Z

Bis(1,1-dithiolene) complex

Bis(1,2-dithiolene) complex

Z Z Z

S

S

S

S

Z

M

Z

S

S

Z S Z

Z Bis(1,3-dithiolene) complex

S

Z

S

Z

M S

Z Z Tris(1,2-dithiolene) complex (n = 3)

Scheme 2

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

403

Two main different kinds of dithiolene complexes may be identified: 1. Homoleptic dithiolene complexes such as MLn, where L ¼ dithiolene ligand (1,1-, 1,2- or 1,3-), and n ¼ 2 [bis(dithiolene)] or 3 [tris(dithiolene)] (see Scheme 2). Within this class of compounds, a further distinction should be made between symmetrical compounds in which all dithiolene ligands are identical [see, e.g., M(mnt)2 in Scheme 3;

NC

S

S

NC

S

S

CN

M

S

S S

S

CN

S

CN

M S

CN

M(mnt)2

S

M(dmit)(mnt)

S

S

S

S

N

S

N

M S

S

S

M(dmit)(tdas)

Scheme 3

mnt2 ¼ [(NC)2C2S2]2, 1,2-maleonitrile-1,2-dithiolato), and unsymmetrical compounds in which at least one ligand is different from the other one(s) {see, e.g., [M(dmit)(mnt)] or [M(dmit)(tdas)] in Scheme 3} (dmit2 ¼ [C3S5]2, 1,3-dithiole-2-thione-4,5-dithiolate; tdas2 ¼ [C2N2 S3]2, 1,2,5-thiadiazole-3,4-dithiolato). 2. Heteroleptic dithiolene complexes such as XmMLn, where m ¼ 1, 2 or 3, n ¼ 1, 2, and X is a non-dithiolene ligand (see examples in Scheme 4).

S

N

S

M

S S

S M

S

N

S

CpM(dmit)

M(bpy)(bdt) where bpy = 2,2'-bipyridine and bdt = benzene-1,2-dithiolato

Scheme 4

Most of the dithiolene ligands considered in this chapter are depicted in Schemes 5–6 with their abbreviated designation.

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CHRISTOPHE FAULMANN AND PATRICK CASSOUX

-

S

-

S

-

NC

S

NC

S

edt2-

-

mnt

S

S

S

-

S

S

S 2-

-

S

S

-

S

-

NC

S

NC

S

S

S

-

-

S

-

S

S

S

S

S

-

-

S dmt2-

mdt2-

S pddt2-

dmise2-

dmid2-

-

bdt2-

S

Se

-

-

S

-

dddt

O

S

-

S

S

F3C

S

tfd2-

S

S

S

2-

S

dmit2-

-

F3C

S

-

where bdt2- = benzene-1,2-dithiolato dddt2- = 5,6-dihydro-1,4-dithiin-2,3-dithiolato dmid2- = 2-oxo-1,3-dithiole-4,5-dithiolato dmise2- = 2-selenoxo-1,3-dithiole-4,5-dithiolato dmit2-= 2-thioxo-1,3-dithiole-4,5-dithiolato dmt2- = 3-thioxo-1,2-dithiole-4,5-dithiolato edt2- = ethylene-1,2-dithiolate mdt2- = 2H-1,3-dithiole-4,5-dithiolato mnt2- = maleonitrile dithiolate pddt2- = 6,7-dihydro-5H-1,4-dithiepinin-2,3-dithiolato tfd2- = trifluoro-ethylene-1,2-dithiolate Scheme 5

S R2N C S

S RO C S

S RS C S

R2dtc or R2-dithiocarbamate

RXant or R-xanthates

RSxant or R-thioxanthate

SRN C

S-

Dithiocarbimate

SCN C

SS C

S-

1,1-Ethenedithiolate

S-

S R CS

Trithiocarbonate Dithiocarboxylate

Scheme 6

The electronic structure of dithiolene complexes is of crucial importance in determining the electron transfer in the derived compounds exhibiting interesting physical properties. Bis(1,2-dithiolene) metal(II) complexes, for example, may be isolated as dianions, monoanions, and neutral species

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

S

S

NiII S S

2-

S

S

NiIII S S

-

405

S

S NiIV S S

Scheme 7

(Scheme 7), which can be converted into one another by either chemical or electrochemical reactions (35, 36). When considering the structures depicted in Scheme 7 for [Ni(S2C2H2)2]n, the oxidation state of the nickel should be Ni(II), Ni(III), or Ni(IV), for n ¼ 2, 1, and 0, respectively. However, it is rather unlikely that highly polarizable sulfur donor atoms could stabilize the Ni(IV) high oxidation state in the neutral species. In one seminal work on dithiolene compounds, Schrauzer and Mayweg (37) considered three possible structures for neutral dithiolene nickel complexes in which the oxidation state of nickel may be Ni(0), Ni(II), or Ni(IV) [Scheme 8; S

S

S

Ni0 S

S

S

NiII S

S

S NiIV

S

S

S

Scheme 8

the Ni(I), and Ni(III) intermediate oxidation states may be also contemplated (38)]. The Ni(IV)-containing structure was dismissed because of the reason discussed above, and the expected tetrahedral Ni(0)-containing structure was unlikely because of the planarity of most of the bis(1,2-dithiolene) nickel complexes (15). The Ni(II)-containing structure where nickel is coordinated to two delocalized spin-paired monoanions seems to be most likely. Indeed, as will be seen later, most of the bis(dithiolene) complex-based compounds discussed in this chapter are M(II) species. Nevertheless, the situation may be different in heteroleptic complexes because of geometrical distortion upon oxidation or reduction (38). Finally, in addition to Sections II (electrical properties), III (magnetic properties), and IV (optical properties), Section V will be devoted to dithiolene complexes exhibiting coexisting or coupled properties (electrical and magnetic, or electrical and optical). Information on the synthesis of the dithiolene ligands and their complexes may be found in Chapter 1 of this volume (39). Likewise, information on the preparation of the dithiolene complex-based compounds described in this chapter may be found in previous articles and reviews (9, 10, 40–46) and will not be discussed here.

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II.

ELECTRICAL PROPERTIES A.

Background

As mentioned in the Introduction, molecule-based systems must meet several, apparently necessary, but certainly not sufficient criteria for exhibiting a semiconducting or metal-like behavior:  One-dimensional stacking of the molecules may provide a framework for possible band structure formation. In this case, repeat units should preferably have an overall planar geometry. This concept can be extended to two-dimensional (2D) structural arrangements where interactions between the molecules are found along two directions as in cation–radical salts derived from TTF-like molecules such as BEDT-TTF (BEDT-TTF ¼ C10H8S8, bis(ethylenedithio)-tetrathiafulvalene) (13), or even to threedimensional (3D) systems such as the doped AxC60 fullerides (47).  The stacked molecules should have a high-energy MO (preferably unfilled) with a large extension perpendicular to the plane of the molecule allowing good overlap between sites. A close stacking is desired for increasing this overlap, that is, increasing the bandwidths. Even with unfilled orbitals, meeting both of the first criteria may result in semiconductors, provided that the energy gap between the valence band and the conduction band is minimized.  In order to ensure a metallic state, partial filling of the conduction band is required. This may be obtained either through partial oxidation, as in nonintegral oxidation state (NIOS) salts such as KCP (14, 48–52), or through partial charge transfer, as in donor–acceptor (D–A) adducts such as (TTF)
(TCNQ) involving donor (such as TTF) and acceptor (such as TCNQ) molecules (11–13). These guidelines have been applied for selecting transition metal containing molecule-based materials as a source of conducting compounds, including linear-chain iridium complexes (53), macrocyclic metal complexes (essentially phthalocyanine metal complexes) (54), and dithiolene complexes (55). Indeed, dithiolene complexes meet the first two criteria mentioned above because of their overall planar molecular structure and their extended p-MO system. Moreover, the availability of stable ion–radical species at various redox potentials, depending on the nature of the metal or the substituents, allows control of the oxidizing strength of the neutral species, and thus the electron transfer in derived NIOS salts or D–A compounds. It was also expected that, in addition to their extended p-electron system, dithiolene complexes might take advantage of the d orbitals of the transition metal (as in KCP) for enhancing intermolecular interaction, but this expectation has not been realized (10).

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

407

Although the first dithiolene ligands were synthesized as early as 1957 (mnt2 (56), tfd2 in 1960 {tfd2 ¼ [(CF3)2C2S2]2} (57), and edt2 in 1964 (edt2 ¼ [C2H2S2]2 (58), it was only in 1969 that electrical properties of some bis- and tris(dithiolene) complexes were reported (59). All these complexes exhibit low conductivity values at room temperature (RT) ranging from 103 to 1015 S cm1 . Since then, a great deal of research has focused on the quest for electrical conductors (and superconductors) using dithiolene complexes, mainly bis(dithiolene) square-planar d8 metal complexes. However, the first dithiolene complex exhibiting metal-like conductivity, (Per)2[Pt(mnt)2] (Per ¼ C20H12, perylene) (60), was prepared and characterized in 1980. Ironically, in this compound it is not the dithiolene complex component that is responsible for the conductivity, but actually the perylene organic part. The first reported dithiolenebased conductor in which metal-like behavior is really due to intermolecular interaction between metal complex anions is (H3O)0.33Li0.82[Pt(mnt)2]
1.67H2O (61). In addition to the mnt2 ligand, a number of dithiolene ligands were developed in the 1970s. In particular, the dmit2 ligand reported in 1975 by Steimecke et al. (62) was intensively used in the preparation of metal complexes, which subsequently proved to be precursors to conducting and/or magnetic molecular systems. The intense effort on these systems is reflected in the large number of papers produced (320 for mnt and 380 for dmit complexes), which were devoted to these materials between 1980 and today. Another good candidate ligand for the preparation of conducting complexes is dddt2 ([C4H4S4]2) (63). According to the concept of isolobality developed by Hoffmann and co-workers (17) (a metal d 8 M2þ ion is isolobal to an ethylene C4þ 2 fragment), dithiolene complexes should be structurally and electronically closely related to TTF-like molecules. In particular, bis(dddt) complexes greatly resemble the BEDT-TTF donor molecule (Scheme 9), that has been extensively used in the preparation of molecular organic superconductors (13).

S

S

S

S

S

S

S

S

S

S

S

S

S

S

M S

S

M(dddt)2

BEDT-TTF

Scheme 9

As with most molecule-based solids, most of the dithiolene complex-based solids are insulators, or, at best, semiconductors, which can hardly be considered as a remarkable property. Therefore, in the following sections we focus on systems exhibiting metal-like or superconducting behavior.

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B.

Compounds Based on 1,2-Dithiolene Complexes 1.

Compounds Exhibiting Metal-Like Behavior

In spite of the large number of dithiolene complexes synthesized, 50 of their derived compounds exhibit a metal-like behavior at a temperature close to RT and within a significant temperature range. These compounds, listed in Tables I–IV, together with the relevant references and some details about their electrical behavior are either NIOS salts or D–A adducts. For the NIOS salts of dmit and mnt complexes, the associated counterions are mostly closed-shell cations such as alkyl-ammonium or sulfonium, or alkaline metal cations. For the D–A adducts derived from dmit and mnt acceptor complexes, the associated donors are TTF-like molecules and Per, respectively. The dddt complexes deserve special mention as the derived metal-like conductors involve [M(dddt)2]xþ cation radicals (instead of anion–radicals as in dmit or mnt systems). The (cation)x[M(dithiolene)2] NIOS salts are prepared either by chemical or electrochemical oxidation of the appropriate (cation)[M(dithiolene)2] salt. The D[M(dithiolene)2]y D–A compounds are prepared either by electrochemical oxidation of the donor molecule D in the presence of the appropriate (cation)[M(dithiolene)2] salt or by metathesis reaction between the appropriate D(anion) and (cation)[M(dithiolene)2] salts (10). a. Compounds Based on dmit. More than one-half of the known dithiolenebased metal-like conductors are derived from dmit complexes (Table I). A review on dmit complex-based conductors dating from 1991 is available (9). Though being a semiconductor (nevertheless having an interesting RT conductivity of 10 S cm1 ), the first NIOS salt in this series, (n-Bu4N)0.29[Ni(dmit)2], was reported in 1983 (102). Following this promising result, systematic variation of the nature of the alkyl ammonium cation in NIOS dmit-based compounds was carried out. It was soon observed that a slight, even minute, modification of the cation may result in important, or even drastic, changes in the conducting behavior. A good example of this is given by the comparison of the (Me4N)0.5[Ni(dmit)2] and (HMe3N)0.5[Ni(dmit)2] NIOS salts for which the sole chemical variation consists of substituting just one hydrogen atom for one methyl group. Although (HMe3N)0.5[Ni(dmit)2] has a higher RT conductivity {140 S cm1 cf. 50 S cm1 for (Me4N)0.5[Ni(dmit)2]}, it undergoes a MI transition at 220 K (64), whereas (Me4N)0.5[Ni(dmit)2] remains metallic down to 100 K (103), and even undergoes a superconducting transition under pressure (see below) (104, 105). These differences can be rationalized in terms of subtle differences in the crystal, electronic band structures, and Fermi surfaces of the two compounds (64). Although the overall structural arrangement of the

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

409

TABLE I Compounds of dmit Exhibiting Metal-Like Behavior

Compounds

Electrical Propertiesb ————————————————— sRT ðS cm1 Þa

(HMe3N)0.5[Ni(dmit)2] a-(Me2Et2N)[Ni(dmit)2]2 g-(Me2Et2N)[Ni(dmit)2]2 a-(dmp)[Ni(dmit)2]2 (AcrH)[Ni(dmit)2]3 Na[Ni(dmit)2]2 K0.4[Ni(dmit)2] (n-Bu4N)0.5[Pd(dmit)2] (n-Bu4N)0.33[Pd(dmit)2] Cs[Pd(dmit)2]2 (MeEt2S)[Pd(dmit)2]2 a-(Et4N)0.5[Au(dmit)2] b-(n-Bu4N)x[Au(dmit)2] (n-BuEt3N)x[Au(dmit)2] (n-Bu2Et2N)x[Au(dmit)2] b-(Me3S)x[Au(dmit)2] (Me3SC3H5)x[Au(dmit)2] (n-Bu3S)x[Au(dmit)2] (Me4N)[Pt(dmit)2]2 (HMe3N)[Pt(dmit)2]3 d-TTF[Pd(dmit)2]2 g-(EDT-TTF)[Ni(dmit)2] a0 -(EDT-TTF)2[Pd(dmit)2]2 g-(EDT-TTF)[Pd(dmit)2] (IEDT-TTF)2[Pd(dmit)2]2 (TFBDM-TTF)[Ni(dmit)2] TTF[Ni(dmit)(mnt)] a-(EDT-TTF)[Ni(dmit)(mnt)] TTF[Ni(dmit)(tdas)] EDT-TTF[Ni(dmit)(tdas)] a b

140 45 45 NR 45 1–100 10–230 12 150 200 100 500 30 50 200 300 300 40 10 140 100 100 55 NR NR 900 900 900 900 900

MI at 220 K Metallic down to 0.5 K Metallic down to 2 K Metallic down to 0.5 K Metallic down to 0.4 K Metallic down to 0.025 K MI at 20 K MI at 120 K MI at 240 K (irreversible) MI at 70 K MI at 130 K (irreversible) Metallic down to 1.5 K Metallic down to 1.5 K MI at 220 K MI at 120 K MI at 90 K MI at 100 K MI at 4.2 K MI at 220 K MI at 180 K MI at 120 K MI at 100 K Metallic down to 0.5 K MI at 100 K MI at 4.2 K MI at 220 K MI at 30 K MI at 30 K Metallic down to 4.2 K MI at 50 K

References 64–66 67–76 76 72, 75–77 78, 79 80 81, 82 83 83 82, 84–87 88, 89 90 90 90 90 90 90 90 64 91 92 93 94–98 93 99, 100 89 101 101 101 101

Not reported ¼ NR. Metal–insulator ¼ MI transitions.

Ni(dmit)2 units is almost identical in both (HMe3N)0.5[Ni(dmit)2] and (Me4N)0.5[Ni(dmit)2], these compounds are not isostructural (space groups are P1 and C2/c, respectively): There is only one stacking direction, [1 0 0], in (HMe3N)0.5[Ni(dmit)2] compared to two, [1 1 0] and [1 1 0], in (Me4N)0.5[Ni(dmit)2] (Fig. 1). Consequently, the Fermi surfaces of the two compounds are nearly identical, but the Fermi surface of (Me4N)0.5[Ni(dmit)2] actually is a superposition of two identical Fermi surfaces corresponding to the two symmetry

410

CHRISTOPHE FAULMANN AND PATRICK CASSOUX

Figure 1. Crystal structures of (a) (HMe3N)0.5[Ni(dmit)2] [drawn from the data available in (64)] and (b) (Me4N)0.5[Ni(dmit)2] [Adapted from the data available in (110)] (showing the single stacking direction in (HMe3N)0.5[Ni(dmit)2] and the two stacking directions in (Me4N)0.5[Ni(dmit)2]). (Dotted lines indicate S


S contacts shorter than the sum of the van der Waals radii.)

related layers of Ni(dmit)2 stacks (Fig. 2). Since these Fermi surfaces are 1D and perpendicular to the stacking directions, structural distortions associated with charge density wave (CDW) condensations (106–109) could quite possibly drive the MI transitions that are observed experimentally in both (HMe3N)0.5[Ni(dmit)2] and (Me4N)0.5[Ni(dmit)2]. Remember that a CDW instability results from an electron–phonon interaction that induces a periodical distortion of the crystal lattice, and hence a spatial modulation of the charge density (106). In low-dimensional solids, a CDW instability may be due to the formation of electron–hole pairs resulting in the periodical lattice distortion, thus driving a

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

411

Figure 2. Fermi surface of a Ni(dmit)2 layer of (a) (HMe3N)0.5[Ni(dmit)2] and (b) (Me4N)0.5[Ni(dmit)2]. (c) Superposition of the two Ni(dmit)2 layers of (Me4N)0.5[Ni(dmit)2]. [Adapted from (64).]

MI transition designated as the Peierls transition (107–109). However, such a CDW condensation could occur in (Me4N)0.5[Ni(dmit)2] if and only if the nesting vectors of the two Fermi surfaces were close enough so that the same distortion would destroy both Fermi surfaces simultaneously, which does not seems very likely. Note that predictions in improvement of the conducting properties connected with the morphology of the countercation (e.g., by decreasing its size for hopefully closer packing of the structure and better overlap) have often not been confirmed. Another difficulty encountered throughout this research on dithiolene-based conductors is the frequent occurence of several different structural modifications for the same compound. A good example of such polymorphism is given by (Me2Et2N)[Ni(dmit)2]2, which crystallizes in three different phases (a, b, and g) (67, 76, 111). Two of these phases exhibit metal-like behavior (a and g), while the third (b) is a semiconductor (67). Although not isostructural, the a and g phases adopt a similar stacking arrangement, called ‘‘spanning overlap’’, in which one Ni(dmit)2 unit simultaneously overlaps with two neighboring Ni(dmit)2 units (Fig. 3). This results in a 2D electronic structure promoting metal-like behavior. Note, in passing, that one of the main area of current interest for molecular conductors is directed toward fermiology, that is, the exploitation of magnetooscillatory effects in the magnetic field dependent resistivity (Shubnikov– de Haas) or magnetization (de Haas–van Alphen) to study the Fermi surface topology (112). Along this line, it is interesting to note that a-(Me2Et2N)[Ni(dmit)2]2 is the first dithiolene-based conductor in which Shubnikov–de Haas oscillations have been observed (69, 70). In addition to ammonium cations, several cyclic, saturated, or unsaturated cations (77, 113–116) such as N, N-dimethylpiperidinium [dmpþ ¼ [C13H10N]þ (72, 75–77) and acridinium [(AcrH)þ ¼ [C13H10N]þ] (78, 79) yielded metal-like

412

Figure 3.

CHRISTOPHE FAULMANN AND PATRICK CASSOUX

‘‘Spanning Overlap’’ in a-(Me2Et2N)0.5[Ni(dmit)2]. [Adapted from the data in (110).]

conductors (Scheme 10). The complex a-(dmp)[Ni(dmit)2]2 is isomorphous to a-(Me2Et2N)[Ni(dmit)2]2. It was expected that the use of the dmpþ cation would allow us to avoid the disorder observed in the (Me2Et2N)þ salt. The a-(dmp)[Ni(dmit)2]2 complex adopts a spanning overlap mode (see above) and remains metallic down to 0.5 K, but no superconducting state is observed. A structural change from C2/c to Cc progressively takes place <225 K. Although the 1:3 stoichiometry of (AcrH)[Ni(dmit)2]3 is different from that usually found in dithiolene-based conductors, such a stoichiometry was previously reported for (PPh4)[Ni(dmit)2]3 (117) and (Cp2Co)[Ni(dmit)2]3 (Cp ¼ C5H5, cyclopentadienyl) (118), which are insulators. The apparently favorable spanning overlap mode observed in the structure of (AcrH)[Ni(dmit)2]3 is consistent with a decently high RT conductivity of 45 S cm1 and a metal-like behavior down to the very low temperature of 0.5 K. However, conductivity measurements under pressures up to 13 kbar do not indicate superconducting behavior (79). In spite of the large number of dithiolene palladium complex-based compounds with tetraalkylammonium cations that have been prepared, only a few

N N H

dmp+

Scheme 10

AcrH+

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

413

members of this series exhibit metal-like behavior. Among these, the (nBu4N)x[Pd(dmit)2] (x ¼ 0.33 or 0.5) NIOS salts were among the first to be reported (83). A rather high RT conductivity, 200 S cm1 , and a metal-like behavior down to 70 K has been observed for Cs[Pd(dmit)2]2 (Fig. 4) (82, 87). Its structure consists of slipped ‘‘dimers’’ (or dyads face-to-face dimerized anions) stacking in parallel columns, with an intra- and interdimer separation of ˚ , respectively (84). Although structural features and transport 3.299 and 3.762A properties of Cs[Pd(dmit)2]2 are reminiscent of those of the a-(Me4N)[Ni(dmit)2]2 superconductor, no superconducting transition has been observed in this compound, even under pressures up to 9 kbar (84). The first M(dmit)2-based salts with sulfonium cations were prepared by Yagubskii et al. (119) and Kato et al. (67). The only member of this series exhibiting metal-like behavior is (MeEt2S)[Pd(dmit)2]2 (88). Its electrical behavior is metallic down to 130 K. The corresponding metal–semiconductor transition does not seem to be reversible, and when the compound is reheated, it remains semiconducting over almost the entire temperature range, recovering its metallic state at a higher temperature than that of the metal–insulator transition observed during the first cooling cycle. Nevertheless, when cooled again, the sample retains its metallic behavior. Conductivity measurements under pressures up to 9 kbar do not show any indication of a superconducting transition (120). Several compounds based on Au(dmit)2 show a metal-like behavior (see Table I) (90). Among them, three promising phases involving the (Et4N)þ, (n-Bu4N)þ, and (n-BuEt3N)þ cations remain metallic down to low temperatures (4.2 or 1.5 K). Unfortunately, further characterization of these compounds has not been reported, but the use of gold in the complex anion may be an interesting strategy (121). Very few dithiolene platinum complex-based compounds have been reported and most of them exhibit poor electrical conductivity (9, 122, 123). The complex (Me4N)[Pt(dmit)2]2 does show metal-like behavior (RT conductivity ¼ 10 S cm1 ) but only >220 K (123). Another exception is (NHMe3)[Pt(dmit)2]3. This compound, which has the unusual 1:3 stoichiometry, behaves as a metal down to 180 K, with a room temperature conductivity of 140 S cm1 (91). The Pt(dmit)2 units form pairs with an eclipsed overlap mode. Unlike the analogue Ni derivatives (see above) substituting one hydrogen atom for one methyl group in the (Me4N)þ cation leads to an improvement in the electrical properties. The M(dmit)2-based D–A compounds with Dþx open-shell cation–radicals may also exhibit metal-like (and superconducting, see below) behavior. Polymorphism was also observed in these compounds. For example, the association of TTF with Pd(dmit)2 affords no less than five different phases, designated a, a0 , b, g, and d, with the same 1:2 stoichiometry (92). Three of them, a, a0 , and d, exhibit a metal-like behavior and two of them undergo a superconducting

414

CHRISTOPHE FAULMANN AND PATRICK CASSOUX

Figure 4. The complex Cs[Pd(dmit)2]2. (a) temperature-dependent conductivity. [Adapted from (82).] (b) projection of the crystal structure onto the ac plane. [Drawn from the data available in (84).] (c) Projection of the crystal structure along the c axis. [Drawn from the data available in (84).]

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

415

transition under pressure (a, a0 ) (see below). The third d-(TTF)[Pd(dmit)2]2 complex has a RT conductivity of 100 S cm1 , but undergoes a MI transition at 120 K (92). Another example of polymorphism is given by the three phases obtained when associating the asymmetric donor EDT-TTF (C8H6S6, ethylenedithiotetrathiafulvalene) (Scheme 11) and Ni(dmit)2, namely, the a-, b-, and F S

S

S

S

S

S

I

S

S

S

S

S

S

Me

S

S

Me

S

S

F F F

EDT-TTF

IEDT-TTF

TFBDM-TTF

Scheme 11

g-(EDT-TTF)[Ni(dmit)2] phases. The g-phase is metallic down to 100 K, at which temperature it undergoes a MI transition due to a CDW distortion (93). The b-phase behaves as a semiconductor (124). The a-phase exhibits a metallike behavior down to 1.3 K (124) (and is an ambient pressure superconductor; see Section II.B.2). An anomalous resistivity maximum is observed at 14 K for this phase (125), and at 40 K for the corresponding a0 -(EDT-TTF)2[Pd(dmit)2]2 congener (97). Pressure-dependent and non-ohmic transport behavior (Fig. 5), observation of supper-lattice spots in diffuse X-ray scattering pattern, reduction of the magnetic spin susceptibility, and tight-binding band structure calculations strongly indicate that CDW instability affects the EDTTTF stacks of (EDT-TTF)2[Pd(dmit)2]2 (94, 126). Likewise, the resistivity anomaly observed at 14 K for a-(EDT-TTF)[Ni(dmit)2] is ascribed to a CDW instability. It is interesting to note that, whereas (EDT-TTF)2[Pd(dmit)2]2 remains metallic down to 500 mK, but does not become superconducting even under pressures up to 10 kbar (97), a-(EDT-TTF)[Ni(dmit)2] undergoes a superconducting transition <1.3 K at ambient pressure (see below) (127). With the aim of increasing the dimensionality through I


S interaction, Imakubo et al. (99) used the IEDT-TTF donor molecule (IEDT-TTF ¼ C8H5S6I, iodoethylenedithiotetrathiafulvalene) (see Scheme 11) derived from EDT-TTF by substituting an iodine for one hydrogen atom on the TTF moiety. Indeed, the ˚, crystal structure of (IEDT-TTF)[Pd(dmit)2] reveals I


S distances of 3.308(4) A ˚ shorter than the sum of the van der Waals radii (4.0 A), indicating strong I


S interactions. Due to these I


S interactions, the donor and the Pd(dmit)2 units are almost parallel, which is not the case in the parent (EDT-TTF)2[Pd(dmit)2]2 compound (97, 98). The complex (IEDT-TTF)[Pd(dmit)2] behaves like a metal down to 4.2 K, whereas (EDT-TTF)2[Pd(dmit)2]2 remains metallic at lower temperatures (500 mK) (97).

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CHRISTOPHE FAULMANN AND PATRICK CASSOUX

Figure 5. The complex a0 -(EDT-TTF)[Pd(dmit)2]. Pressure-dependence of the resistivity a, ambient pressure; b, 2; c, 5; d, 10 kbar) and (insert) non-ohmic behavior at ambient pressure for an injected current of 150 mA (


) and 1.5 mA (___). [Adapted From (97).]

The association of the asymmetrical TFBDM-TTF donor molecule (TFBDM-TTF ¼ C12H6S4F4, tetrafluorobenzo-dimethyl-tetrathiafulvalene) (see Scheme 11) with Ni(dmit)2 affords a conducting species with a record high RT conductivity (sRT ¼ 900 S cm1 ) that remains metallic down to 220 K (89). To date, however, no additional characterization has been reported for this compound. Some homoleptic unsymmetrical (dmit/mnt, dmit/tdas) dithiolene nickel complex-based D–A compounds with D ¼ TTF and EDT-TTF also exhibit metal-like conductivity (see Table I) (101). Their molecular structure is shown in Scheme 3. The unsymmetrical tetraalkylammonium salts [ML1L2] (M ¼ Ni, Pd, Pt) have been prepared by ligand exchange reaction between tetraalkylammonium salts of [ML1] and [ML2] (128, 129) and the D–A compounds have been synthesized by electrooxidation. Among these complexes, only the Ni derivatives exhibit metallic-like properties, namely, TTF[Ni(dmit)(mnt)] (metallic down to 30 K), a-EDT-TTF[Ni(dmit)(mnt)] (metallic down to 30 K), TTF[Ni(dmit)(tdas)] (metallic down to 4.2 K), and EDT-TTF[Ni(dmit)(tdas)] (metallic down to 50 K) (see Table I). The complex a-EDT-TTF[Ni(dmit)(mnt)] is isostructural (130) to a-EDT-TTF[Ni(dmit)2)] [ambient pressure superconductor, Section II.B.2 (124)]. Under pressure, conductivity measurements up to 18 kbar show a monotonous decrease of the resistivity but do not reveal any superconducting transition (101).

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

417

TABLE II Compounds of mnt Exhibiting Metal-Like Behavior

Compounds Per2[Au(mnt)2] Per2[Co(mnt)2] a-Per2[Cu(mnt)2] Per2[Fe(mnt)2] a-Per2[Ni(mnt)2] Per2[Pd(mnt)2] Per2[Pt(mnt)2] Per[Co(mnt)2]
0.5 MeCN Li0.8(H3O)0.33[Pt(mnt)2]
1.67H2O Cs0.83[Pd(mnt)2]
0.5H2O

a

Electrical Propertiesa ——————————————— sRT ðS cm1 Þ 700 200 700 200 700 300 700 60 30–200

MI at 12.2 K MI at 73 K MI at 32 K MI at 58 K MI at 25 K MI at 28 K MI at 8.2 K MI at 272–277 K MI at 200 K (Peierls transition) 5 (at ambient Metallic down pressure) to 1.2 K under pressure (18 kbar)

References 132–134 135 136, 137 135 136, 137 132, 138, 139 60, 132, 138 140 61, 86, 141–143 144, 145

Metal–insulator ¼ MI transitions.

Metal-like behavior has been briefly mentioned by Underhill and co-workers for Ni(dmit)2-based NIOS salts with Naþ (80) and Kþ cations (81, 82). Poor quality samples have prevented these authors from fully characterizing these phases. b. Compounds Based on mnt. Most of the mnt-based metal-like conductors are D–A compounds in which the donor molecule D is Per (Table II). As mentioned above, in these (Per)2[M(mnt)2] compounds only the Per stacks are responsible for metal-like behavior. A recent review on mnt-based compounds exhibiting interesting conducting and magnetic properties is available (131). The crystal structure of all a-(Per)2[M(mnt)2] consists of segregated stacks of ˚ between the planes of the Per Per and M(mnt)2 units, with a spacing of 3.32 A molecules. The temperature dependencies of the electrical conductivity indicate metal-like behaviors down to temperatures ranging from 73 to 8.2 K. At those temperatures, a sharp transition to an insulating state occurs. This transition is due to a gap opening at the Fermi level due to a Peierls distortion. This metal– insulator transition is observed whatever the electronic structure (d7 or d8 ) of the metal involved in the M(mnt)2 unit, that is, whatever the magnetic character (paramagnetic or diamagnetic) of the M(mnt)2 units is. In the (Per)2[M(mnt)2] compounds (M ¼ Ni, Pd, Pt), for example, a large paramagnetic susceptibility is due to the large paramagnetic contribution from the M(mnt)2 component (in addition to the Pauli susceptibility from p conduction electrons in the Per chains). At low temperatures, the Peierls transition affecting the conduction

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CHRISTOPHE FAULMANN AND PATRICK CASSOUX

chains (Per) may be accompanied, and this is especially conspicuous for the Pt compound, by a spin-Peierls transition affecting the M(mnt)2) chains. However, it is not clear whether these two simultaneous 1D electronic and magnetic instabilities are actually coupled, and the existence of a real interplay is not yet established [Section III.A and (146)]. The sole mnt-based compound in which metal-like behavior is really due to intermolecular interaction between metal complex anions (also the first reported dithiolene complex-based compound of this kind) is (H3O)0.33Li0.82[Pt(mnt)2]
1.67H2O (61). This compound was first synthesized in 1981 (141) by the air oxidation of H2[Pt(mnt)2] in the presence of LiCl. It behaves like a metal down to 215 K, at which temperature it undergoes a MI transition (143). The degree of hydration in the crystal deeply affects the electrical properties and alters the temperature dependence of the conductivity. Increasing hydration results in a decrease of the conductivity. The Pt Pt repeat distance along the ˚ (143), which suggests a small overlap between the Pt stacking axis is 3.60 A atoms, opposite to what is observed in KCP in which the Pt Pt distance is ˚ (4, 147). Thus, the conduction pathway probably is through the ligand2.89 A centered p system, which is composed of predominantly S 3pz orbitals. The overlap integrals along face-to-face stacking directions are much larger than the interstack overlap integrals and the resulting large anisotropy in conductivity indicates that (H3O)0.33Li0.82[Pt(mnt)2]
1.67H2O actually is a 1D metal, with the p electrons system of the mnt ligand mainly responsible for the conductivity. A related compound, Cs0.83[Pd(mnt)2]
0.5H2O is also metallic down to 1.2 K, but only when applying pressures up to 18 kbar (144, 145). At ambient pressure, or even below 5 kbar, this compound behaves like a semiconductor. c. Compounds Based on dddt. As stressed in the Introduction, more than any other dithiolene complexes, the dddt complexes are closely related to TTFlike molecules, and especially BEDT-TTF (see Scheme 9). Moreover, squareplanar metal complexes based on the dddt ligand share with BEDT-TTF the capacity for existing as cation–radicals [and not only as neutral species and anion–radicals like most other dithiolene complexes, except mdt complexes (mdt2 ¼ [H2CS2C2S2]2) (148, 149)]. Vance et al. (63) reported the synthesis of the dddt2 in 1985. The first anion– radical salts of dddt complexes were prepared as alkylammonium salts (R4N)n[M(dddt)2] (M ¼ Ni, Pd, Pt, Au, Cu, Co; n ¼ 2 or 1) (63, 150–157). Chemical oxidation of the (R4N)[M(dddt)2] salts gives the neutral species [M(dddt)2] (151, 153, 158). Electrochemical oxidation of the neutral species affords NIOS cation-radical salts, viz., [M(dddt)2](anion)x [M ¼ Ni, Pt, Au; anion ¼ (ClO4), (BF4), (IBr2), (AuBr2), . . . ; x ¼ 0:5; 0:66] (Table III) (153, 159, 160).

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

419

TABLE III Compounds of dddt Exhibiting Metal-Like Behavior

Compounds [Ni(dddt)2]3(HSO4)2 [Ni(dddt)2]3(AuBr2)2 [Ni(dddt)2]AgxBry xy3 [Pd(dddt)2]Ag1.54Br3.50 [Pd(dddt)2]2PF6 [Pd(dddt)2]2SbF6 [Pd(dddt)2]2SnClx a

Electrical Propertiesa —————————————————— sRT ðS cm1 Þ

References

60–300 35–100 ð//acÞ 100 ð//aÞ 20–40

Metallic 300–25 K Metallic down to 1.3 K

161–167 167, 168

Metallic down to 4.2 K

167

15 1 1–2 1

Metallic down to 1.3 K MI at 220 K MI at 200 K MI at 140 K

167, 169 167 167 167

Metal–insulator ¼ MI transitions.

Detailed electrochemical studies have shown that the mechanism of formation of such [M(dddt)2](anion)x salts involves one-electron transfer leading to the [M(dddt)2]þ cation–radical, followed by a chemical reaction of [M(dddt)2]þ with the [M(dddt)2] neutral species (170, 171). Not surprisingly, the crystal structures of most of the [M(dddt)2](anion)x NIOS salts are similar to the corresponding salts derived from the BEDT-TTF molecule, except the Ni(dddt)2 complexes with (ClO4) and (BF4). The first dddt-based compound of this series exhibiting metal-like properties, [Ni(dddt)2]3(HSO4)2, was synthesized in 1992 by Yagubskii et al. (166). Its RT conductivity is 60–300 S cm1 (166), that is, even higher than that of the corresponding isostructural (BEDT-TTF)3(HSO4)2 salt (10–40 S cm1 ) (172– 174). It behaves like a metal down to 25 K [cf. with 130 K for (BEDTTTF)3(HSO4)2 (173, 174)]. It was suggested that these differences could be due to the contribution of the nickel atom (163). Within the Ni(dddt)2 stacks, a possible overlap between the Ni d orbitals and the S p orbitals could induce a stabilization of the structure, resulting in a lattice distortion occurring at a lower temperature than in (BEDT-TTF)3(HSO4)2 (161, 163). The [Ni(dddt)2]3(AuBr2)2 complex has the same 3:2 stoichiometry as the previous compound, which is observed for all the Ni(dddt)2 complexes, except [Ni(dddt)2]AgxBry (see below) but the structure is different: The packing in [Ni(dddt)2]3(HSO4)2 involves [Ni(dddt)2]3 ‘‘trimers’’ (or triads) with nonequivalent stacking distances, whereas in [Ni(dddt)2]3(AuBr2)2 the stacking of the Ni(dddt)2 units is uniform. Therefore, in [Ni(dddt)2]3(AuBr2)2, only the orbitals of the S atoms of the inner and planar rings overlap with the orbitals of the metal atom of the adjacent unit, opposite to [Ni(dddt)2]3(HSO4)2 in which the S atoms of the outer and nonplanar rings are involved in the same

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CHRISTOPHE FAULMANN AND PATRICK CASSOUX

interaction. This nonplanarity leads to an unfavorable orientation of the p orbitals of the S atoms relative to the d orbitals of the Ni atoms. The regular arrangement in [Ni(dddt)2]3(AuBr2)2 may explain the stability of the metallic state down to a lower temperature (1.3 K) than in [Ni(dddt)2]3(HSO4)2 (25 K) (168). The poorly characterized [Ni(dddt)2]AgxBry product, which probably is a polymeric silver bromide containing species, behaves like a metal down to 4.3 K (167). The Pd analogue, [Pd(dddt)2]Ag1.54Br3.50, is metallic down to 1.3 K (169). Its crystal structure consists of layers of [Pd(dddt)2] moieties alternating with layers of silver bromide complex anions. A noticeable feature lies in the existence of a uniform stacking of Pd(dddt)2 in the conduction layers, instead of the stacking of dyads usually encountered in Pd complexes (9, 160, 175–177). Several other dddt Pd compounds exhibiting metal-like properties have been reported but poorly characterized (see Table III). Interestingly, none of the many Pt(dddt)2-based NIOS compounds that have been prepared exhibits metal-like behavior (160), which is probably related to the ability of the Pt(dddt)2 moieties to form Pt


S


Pt bonds, inducing a strong dimerization in the conducting stacks (160, 175, 176). Doublet et al. (176) showed that the partially filled band of the M(dddt)2 salts is derived either from the highest occupied molecular orbital (HOMO) or from the lowest unoccupied molecular orbital (LUMO) of the monomer unit depending on the extent of dimerization, high conductivity being expected for weakly dimerized M(dddt)2 systems. d. Other 1,2-Dithiolene-Based Compounds. Several additional 1,2-dithiolenebased compounds exhibiting metal-like behavior have been reported (Table IV). TABLE IV Other Dithiolene Compounds Exhibiting Metal-Like Behavior

Compounds Cs[Pd(dmise)2]2 (dmp)[Ni(dmise)2]2 (NH2Me2)[Ni(dmise)2]2 (NHMe3)[Ni(dmise)2]2 (C8H6S8)2Ni (BMDT-TTF)x[Ni(dcit)2]a (n-Bu4N)0.29[Ni(dmbit)2]b

a

Electrical Properties ——————————————————— sRT ðS cm1 Þ NR 70 100 30 0.1 20–40 1

Metallic down to 4 K Metallic down to 2 K Weak metal 300 K Semimetal down to 100 K Metallic 300–275 K Metallic 300–120 K Metallic 300–220 K SC 220–45 K Metallic 45–20 K 2

3,4-Dimercapto-5-cyanoisothiazole ¼ dcit . 2-Thione-1,3-dithiole-benzo[d]-4,5-dithiolate ¼ dmbit2 . c Semiconductor ¼ SC. b

References 178 179 178 178 180 181

182

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

421

The dmise2 ligand (dmise2 ¼ [C3S4Se]2), may be considered as being derived from the dmit2 ligand by substituting a selenone group for the terminal thione group (see Scheme 5) (183). It was expected that such a substitution would result in an increase of the dimensionality of the derived systems through enhanced intermolecular Se


Se or Se


S interaction. The complex (Me4N)[Ni(dmise)2]2, the selenone analogue of the superconducting (Me4N)[Ni(dmit)2]2 (see below), had been first regarded as a metal (184) between 300 and 140 K, but was later shown to be a semiconductor (185). The complex Cs[Pd(dmise)2]2, which behaves as a metal down to 4 K (178), is not isostructural (84) to the dmit-analogue Cs[Pd(dmit)2] (see Table I), which undergoes a MI transition at 70 K. In this case, substituting Se for S might really have enhanced the dimensionality of the system. On the other hand, (dmp)0.5 [Ni(dmise)2] (179), which is isostructural to the dmit-analogue (dmp)0.5[Ni(dmit)2], remains metallic down to 2 K {0.5 K for (dmp)0.5[Ni(dmit)2]; see Table I}: The Ni(dmise)2 units are arranged in the so-called ‘‘spanning overlap mode’’ (as in the dmit analogue; see Fig. 3). The neutral Ni complex of an extended dithiolene ligand, (C8H6S8)2 (Scheme 12) has been reported (180, 186). The Ni(C8H6S8)2 complex may be NC MeS

S

S

S

S

S

S

SMe

S S

Ni MeS

S

S

S

S

S

S

N

SMe

Ni(C8H6S8)2 S

S

S

S

S

S

S S

N

S

S S CN

M(dcit)2 S

S S

S Ni

S

S

S

S

Ni S

S

S

Ni(dmbit)2

BMDT-TTF Scheme 12

prepared by stirring an acetonitrile suspension of (Me4N)[Ni(C8H6S8)2] in air. The Ni(C8H6S8)2 complex exhibits a rather high RT conductivity (101 S cm1 ), and a metallic behavior between 300 and 275 K. This behavior was not expected for a neutral complex and may be due to an extended network of intermolecular overlap favored by the large extension of the ligand p system and the large number of peripheral S atoms. Substituting ethyl and butyl groups for terminal methyl groups in Ni(C8H6S8)2 only yields semiconductors. The (BMDT-TTF)x[Ni(dcit)2] compound (see Scheme 12) involves the (BMDT-TTF) donor [C8H4S8, bis(methylenedithio)tetrathiafulvalene] (187) and the Ni(dcit)2 heterocyclic dithiolene complex (dcit2 ¼ [C4N2S3]2). Room temperature conductivity was reported to be 20–40 S cm1 and the

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CHRISTOPHE FAULMANN AND PATRICK CASSOUX

behavior is metallic down to 120 K. Unfortunately, as this compound has been obtained as plate-shaped crystals of insufficient quality, X-ray structural data is not available for discussion of the conducting properties (181). The (n-Bu4N)0.29[Ni(dmbit)2] NIOS salt (dmbit2 ¼ [C7H2S5]2) (see Scheme 12) was prepared by several authors. The different preparation procedures include chemical oxidation with iodine of (n-Bu4N)2[Ni(dmbit)2] (188), chemical oxidation with bromine or (n-Bu4N)Br3 of (n-Bu4N)[Ni(dmbit)2], and galvanostatic oxidation of (n-Bu4N)[Ni(dmbit)2] (182). It was reported that this compound behaves as a metal down to 220 K, as a semiconductor down to 45 K, and as a metal again down to 22 K. However, as no single crystals of sufficient quality have been obtained, a better understanding of this unusual behavior was not possible. 2.

Superconductors Based on 1,2-Dithiolene Complexes

The ultimate (admitted or not) goal of the research reviewed in Section II.B.1 was, and still is, the preparation of compounds not only exhibiting metal-like properties, but also possibly undergoing a superconducting transition thus showing zero resistivity below a critical temperature Tc . Remember that a superconducting material placed in a magnetic field at temperatures below Tc, is perfectly diamagnetic. However, above a critical field Hc , the metallic state is restored. Injection of electrical current densities higher than a critical value, Jc , also destroys superconductivity. The most commonly accepted theory, BCS [developed by Bardeen, Cooper, and Schrieffer (189, 190)], involves the pairing of two electrons with opposite spins and wave vectors (the Cooper pair). This pairing is believed to be due to electron–phonon interactions. In order to synthesize such a superconducting material, a large number of dithiolene-based complexes have been synthesized, which explains the variety of 1,2-dithiolene ligands used (and 1,1- and 1,3-dithiolene ligands as well; see below). Disappointingly, there is only one dithiolene ligand that has led to superconductors (viz., dmit2). To date, only nine superconductors have been characterized: five (cation)x[M(dmit)2] NIOS salts and four D[M(dmit)2] D–A compounds (Table V). Historically, the first reported superconductor in this series was [TTF][Ni(dmit)2]2, whose synthesis was first described in 1981 (230). Its crystal structure and conducting behavior at ambient pressure were reported in 1986 (122). Both TTF and Ni(dmit)2 moieties are arranged in uniform segregated stacks along the [010] direction (Fig. 6). Since the M(dmit)2 component possesses 10 peripheral sulfur atoms, it is not surprising that an extended network of intermolecular S


S interactions (evidenced by inter˚ sum of the van der Waals radii) is atomic distances shorter than the 3.6 A observed. These S


S interactions involve S atoms on both types of molecules and occur between units in adjacent stacks, leading to an apparent 3D network

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

423

TABLE V The Nine Superconductors Derived from the dmit Ligand Compound

Tc (K) P (kbar)

Type

Electrical Properties

b -(Me4Sb)0.5[Pd(dmit)2]

NIOS

sRT ¼ not reported 2–3.5 SC at ambient pressure 6.1 to 10

191, 192

b0 -(Et2Me2P)0.5[Pd(dmit)2]

NIOS

191, 193–198

a-[EDT-TTF][Ni(dmit)2]

D–A p

(Et2Me2N)0.5[Pd(dmit)2]

NIOS

sRT ¼ 10 S cm1 1.8 or 4 SC at ambient pressure 6.9 or 10.4 sRT ¼ 100 S cm1 1.3 MI at 20 K then SC Ambient down to 20 K and pressure metallic down to 4.2 K sRT ¼ 10–80 S cm1 4 SC at ambient pressure 2.4

b-(Me4N)0.5[Pd(dmit)2]

NIOS

sRT ¼ 30 S cm1 SC at ambient pressure

6.2 6.5

203–208

a-[TTF][Pd(dmit)2]2

D–A p

sRT ¼ 750 S cm1 MI at 100 K (hysteresis)

1.7 22

92, 209–213

a0 -[TTF][Pd(dmit)2]2

D–A p

sRT ¼ 750 S cm1 MI at 100 K (reversible)

5.93 24

92, 209–214

(Me4N)0.5[Ni(dmit)2]

NIOS

sRT ¼ 50 S cm1 Metallic down to 100 K Below 100 K, the conductivity is direction dependent

5 7

103–105, 211, 215–218

[TTF][Ni(dmit)2]2

D–A p

sRT ¼ 300 S cm1 Metallic down to 4 K

1.62 7

87, 209–211, 213, 219–233

0

References

93, 100, 124, 127, 199–202

75, 203–205

Superconducting critical temperature ¼ Tc .

of closely spaced molecules. However, subsequent band structure calculations indicated that this compound (and all other dmit-based superconductors) are best considered as quasi-1D systems, as all types of S


S interactions are not electronically efficient (211). At ambient pressure, [TTF][Ni(dmit)2]2 behaves as a metal down to low temperatures (conductivity 300 at 290 K and 1.5 105 S cm1 at 4 K). Under pressure, it undergoes a complete transition to a superconducting state (226, 227). This compound has been the subject of a great number of physical studies (pressure-dependent transport measurements, diffuse X-ray scattering, 1H and 13C NMR (nuclear magnetic resonance), ESR (electron spin resonance), magnetic susceptibility measurements, band structure calculations, etc.) (91, 122, 209, 211, 213, 219–231, 233–242). The reader is directed toward a detailed review that has appeared on this subject (10). Shortly after the first reports on the [TTF][Ni(dmit)2]2 D–A compound, Kobayashi and co-workers (126) reported the synthesis and X-ray structure of the (Me4N)0.5[Ni(dmit)2] NIOS salt (103). At ambient pressure, this salt behaves

424

CHRISTOPHE FAULMANN AND PATRICK CASSOUX

Figure 6. Structure of TTF[Ni(dmit)2]2. (a) Projection onto the (010) plane. (b) Parallel view along the [010] direction. [Adapted from the data available in (122).]

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

425

as a metal down to 100 K, the temperature at which a metal–semimetal transition occurs. Further studies showed that this salt undergoes a superconducting transition under pressure: at 3.2 kbar, Tc ¼ 3:0 K; at 7 kbar, Tc ¼ 5:0 K (104, 105). As with the previously discussed [TTF][Ni(dmit)2]2 D–A superconductor, (Me4N)0.5[Ni(dmit)2], the first reported dmit-based NIOS superconductor has been extensively studied (211, 215, 216, 218) (along with the other superconductors listed in Table V). The reader is again directed toward the relevant references and a detailed review on these compounds (10). At ambient pressure, the b0 -(Me2Et2P)0.5[Pd(dmit)2] phase behaves as a semiconductor with a RT conductivity of 10 S cm1 (191). Application of pressures >5 kbar induces a metallic behavior and the system undergoes a superconducting transition within the 6.9–10.4-kbar pressure range with Tc ¼ 4:0–1.8 K (196). At higher pressures (from 11 up to 16.2 kbar), the compound looses its metallic character and the larger the pressure, the larger the resistivity (196). Thus, the superconducting region is embedded between two nonmetallic states. The crystal and electronic structures as well as the ESR and NMR properties (197) have been studied at low temperature (8 K) (195). Very recently, the discovery of the ninth superconductor of this series, b0 -(Me4Sb)0.5[Pd(dmit)2], has been claimed (M. Tamura, personal communication). Preliminary results indicate Tc values ranging from 2 to 3.5 K at pressures from 6 to 10 kbar. Considering the small number of its members, the series of dmit-based superconductors is very important in terms of original properties and new concepts:  It is the only series of dithiolene-based compounds (and the only series of molecular metal complex-based compounds) exhibiting superconducting behavior.  The superconducting critical temperature Tc remains low (the highest Tc being 6 K, to be compared with the record Tc of 12.8 K, for a moleculebased superconductor, detained by k-(BEDT-TTF)2{Cu[N(CN)2]Cl} (243, 244).  In most cases, superconductivity in dmit-based systems requires application of pressure, which, however, does not mean that all of these systems are under a generalized curse. Indeed, a-(EDT-TTF)[Ni(dmit)2] undergoes a superconducting transition at ambient pressure.  The M(dmit)2 component alone is responsible for superconductivity, what is conspicuous in NIOS salts, but was also proved in D–A compounds with TTF-like D components, in which the donor does not contribute to the superconductivity.  Contrary to most ‘‘organic’’ TTF-like molecule-based superconductors, for some dmit-based superconductors, namely, [TTF][Ni(dmit)2]2 and

426

CHRISTOPHE FAULMANN AND PATRICK CASSOUX

Figure 7. Temperature–pressure phase diagram of [TTF][Ni(dmit)2]2 showing the increase of Tc with pressure. [Adapted from (223).]

(Me4N)0.5[Ni(dmit)2], the superconducting critical temperature Tc increases with increasing pressure, as shown in Fig. 7. The temperature–pressure phase diagram in Fig. 7 allows the visualization of the domains of temperatures and pressures in which a system may be in its various possible states (insulating, metallic, or superconducting). In such a phase diagram, the boundary line between two domains refers to the critical temperature at a given pressure, or the critical pressure at a given temperature at which the transition from one to the other state occurs.  The CDW states are often observed (in contrast to the situation in most of the ‘‘organic’’ superconductors where SDW states [SDW ¼ spin density wave) are most commonly observed (13)]. Remember that the SDW instability is related to magnetic exchange interactions. Below a given temperature, Ts , the material shows an itinerant antiferromagnetic state connected with the coupling in pairs of two electrons with opposite spins. This coupling induces a spatial modulation of the spin density, which results in a gap opening at the Fermi level and a MI transition (245, 246). These CDW states are either in competition (as in a0 -[TTF][Pd(dmit)2]2)

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

427

or coexistence (as in [TTF][Ni(dmit)2]2) with the superconducting state. In the second case, this coexistence explains why a CDW instability may not inevitably induce a MI transition. In [TTF][Ni(dmit)2]2 for example, a CDW instability, experimentally observed by X-ray diffuse scattering (87, 213), occurs at 40 K, and yet, this compound remains metallic down to the much lower temperature of 1.5 K (223).  Many of these properties may be rationalized in terms of a unique multisheet Fermi surface based on both the HOMO and the LUMO bands (211, 221, 247) and not solely on the HOMO band as in organic superconductors (13). Tight-binding band calculations have shown that the nature of the conduction band depends on the degree of ‘‘dimerization’’ within the stacks. This band originates either from the LUMO for weakly ‘‘dimerized’’ systems, or from the HOMO for strongly ‘‘dimerized’’ systems (221).  The superconducting properties of these dmit-based salts have prompted extensive studies of related dithiolene-based systems obtained by changing the nature of the metal, the cation in NIOS salts, or the donor in D–A compounds, and by using other ligands resembling dmit2. To date, no superconductor has been characterized in any of these related series. C.

Related Compounds Based on 1,1-Dithiolene Complexes

To our knowledge, there are only three compounds based on 1,1-dithiolene ligands that exhibit metal-like behavior: (Per)2[Au(i-mnt)2] (248, 249), (BEDTTTF)x[Ni(i-mnt)2] {i-mnt2 ¼ [(NC)2C2S2]2} (250), and (Per)2[Au(cdc)2] (cdc2 ¼ [C2N2S2]2) (251) (Scheme 13). For both i-mnt2 and cdc2 ligands, NC

S

NC

S

S

CN

S

CN

M

S

NC N

M(i-mnt)2

S M

S

N S

CN

M(cdc)2

where i-mnt2- = -, iso-maleonitrile dithiolate and cdc2- = cyanodithiocarbimate

Scheme 13

only the gold complexes have been synthesized, other metal complexes (Pt, Pd) being unstable. The RT conductivity of (Per)2[Au(i-mnt)2] is notably high (1000 S cm1 ) and metal-like behavior is retained down to 9 K (248, 249). Since the physical properties of (Per)2[Au(i-mnt)2] are very similar to those of (Per)2[Au(mnt)2],

428

CHRISTOPHE FAULMANN AND PATRICK CASSOUX

their crystal structures might be similar (248, 252, 253). The (BEDTTTF)x[Ni(i-mnt)2] complex has an RT conductivity of 22 S cm1 and behaves like a metal down to 185 K (250). No further characterization of this compound has been reported. Although the crystal structure of (Per)2[Au(cdc)2] is not known, its cell parameters suggest a different packing than that of (Per)2[M(mnt)2] compounds. The RT conductivity is 150 S cm1 and a poorly defined MI transition is observed at 50 K (251). As in the (Per)2[M(mnt)2] series of compounds (see above), the metallic properties of both i-mnt- and cdc-based compounds are due to either the Per or the BEDT-TTF component. Additional information on these compounds and other 1,1-dithiolene-based compounds may be found in (159). D. Potential Applications and Patents 1.

Dithiolene Complex-Containing Films

The production of electrically conducting Langmuir–Blodgett (LB) films for possible electronic device application has become of great interest over the last decade. One of the main expected advantages of the LB films compared to the materials based on discrete molecules is the possible formation of highly organized ultrathin layers of materials, with a controllable thickness. The first LB films that were shown to exhibit electrical properties were built on pure organic molecules, such as derivatives of TTF (254, 255) and TCNQ (256, 257), adapted for the deposition as LB films through the attachment of long alkyl chains. However, as these systems are 1D, they are subjected to Peierls instabilities. In order to increase the dimensionality, the same strategy used for the organic systems as well as for the dithiolene-based systems, that is, the use of molecules with a large number of appropriate peripheral atoms allowing contacts in several directions, also has been applied to LB films processing. Dithiolene complex-containing films may be obtained by electrodeposition. a. Conducting LB Films. To our knowledge, there are only two LB films containing a dithiolene ligand that exhibit metallic behavior. In both cases, the concerned metal complex is Au(dmit)2 (surprisingly, the films obtained with other metals do not show any metallic behavior). The first dithiolene-based LB film exhibiting metal-like properties was reported in 1989 by Nakamura et al. (258, 259). This LB film was obtained by using [(C10H21)3NMe][Au(dmit)2] {designated here as 3C10-Au(dmit)2; 3C10 ¼ [(C10H21)3NMe]þ, tridecylmethylammonium}. The preparation of this film proceeds as follows: The (3C10)[Au(dmit)2] salt (260) is mixed with icosanoic acid [Me(CH2)18COOH] in an acetonitrile–benzene (1:1) solution.

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

429

This mixture forms a monolayer at the air–water interface. This monolayer is then transferred on a substrate (glass, quartz, CaF2) by a horizontal lifting method. The neutral films so obtained are oxidized, that is, ‘‘doped’’, in situ with gaseous bromine or by galvanostatic electrochemical oxidation at constant current (261). The RT conductivity of the films is 30 S cm1 , and they behave as a metal down to 200 K, below which temperature the behavior remains quasimetallic. However, after aging in vacuum, the film behaves like a semiconductor from RT down to 10 K. Such films have been extensively studied in order to understand the origin of the metal-like behavior. The results of ESR studies (259) indicate a low amount of disorder or a high degree of dimensionality, compared to what is observed for other LB films. Although a number of materials based on Au(dmit)2 with different attached alkyl chains have been synthesized and studied, the influence of the length of the alkyl chain is still not clear (262). Thermopower measurements have shown that the material may consist of highly conducting metallic regions separated by thin weakly conducting areas (263–265). Attempts to characterize the molecular arrangement by AFM (266, 267) have shown a structural change of the film before and after oxidation. The conductivity has also been measured under pressure up to 17 kbar: The conductivity increases with pressure and the transition temperature is lowered (268, 269). Conducting LB films of ditetradecyldimethylammonium-Au(dmit)2, {[(C14H29)2N(CH3)2][Au(dmit)2]}, have been fabricated without the use of matrix molecules (270). The in-plane direct current (dc) RT conductivity is as high as 40 S cm1 after electrochemical oxidation. A metal-like behavior was observed in the temperature range of 250–300 K. At lower temperatures, the material becomes a semiconductor. b. Dithiolene-Based Films Used as Electrical Sensors. As Chapter 7 (271) of this volume deals with the use of dithiolene complexes as sensors, we shall only briefly mention some examples. Films, appropriate for this application, may be obtained either with the LB technique or by electrodeposition. Depending on their composition, they can be used to detect gases, such as hydrazine (N2H4), ammonia (NH3), and nitric oxide (NO). It is the change in the electrical conductivity of the films that allows the detection of a given gas in their vicinity. Mono- or multilayer LB films of BDN-stearyl alcohol {BDN ¼ [Et2N(C6H4)(Ph)C2S2]2Ni ¼ bis[(4-diethylamino)dithiobenzyl]nickel, Scheme 14] have been used for the detection of N2H4 (272–274). The conductivity of the films is sensitive to 0.5 ppm of gas at room temperature. The response and recovery time of the sensor are < 30 s. Mixed films containing BDN and stearyl alcohol are electrically conducting and derived ‘‘chemiresistor’’ sensors were fabricated by depositing layers of this material onto interdigital electrodes. When exposed to NH3 or ppb levels of hydrazine,

430

CHRISTOPHE FAULMANN AND PATRICK CASSOUX

Et2N

NEt2 S

S Ni

S

S

BDN Scheme 14

the current through these sensors increases by several orders of magnitude. These sensors are promising for use in alarm devices for hydrazine. Electrodeposited films of (n-Bu4N)x[Ni(dmit)2] can be used as sensors for detecting SO2 or NO (275, 276). The resistance of the films increases on exposure to SO2; these changes are reversible in argon. The films are even more sensitive to NO, although these changes are largely irreversible. Elemental analysis indicates the presence of small amounts of NIOS species in the film. Exposure to NO results in the oxidation of these species into the less conducting neutral [Ni(dmit)2]0. Sulfur dioxide in turn is presumed to reduce the conducting NIOS species into the less conducting (n-Bu4N)[Ni(dmit)2] monovalent salt. c. Dithiolene-Based Films Used for Memory Switching. Uniform amorphous thin films of the semiconducting (NCTA)2[Ni(dmit)2] salt [NCTA ¼ Me(CH2)15NþMe3 ¼ cetyltrimethylammonium] have been electrodeposited between an indium tin oxide (ITO) and an Al electrode (277, 278). The resulting device may switch between a high-impedance (OFF) state and a low-impedance (ON) state simply by changing the polarity of an applied electric field across the device. A plausible microscopic switching mechanism is proposed based on the generation and elimination of semiconducting NIOS species through an electric field-induced solid-state redox reaction. The same behavior had been previously observed for electrodeposited films of (n-Bu4N)2[Zn(dmit)2] salts on ITO (279). Electrical switching and memory phenomena have also been reported for a device consisting of a layer of the (n-Bu4N)x[Ni(dmid)2] NIOS salt (dmid2 ¼ [C3OS4]2, 2-oxo-1,3-dithiole-4,5-dithiolato), ‘‘sandwiched’’ either between Al and Cu electrodes (280), or between Al and Pt electrodes (281). For the sandwich structure Al/(n-Bu4N)x[Ni(dmid)2]/Cu, a rapid switching is observed from the OFF state (high impedance) to the ON state (low impedance) only when the applied field across the sample surpasses a threshold value, opposite to Al/(n-Bu4N)x[Ni(dmid)2]/Pt, in which the switching goes from the ON to the OFF state with an applied field. Restoration of the initial state (OFF for Al/(n-Bu4N)x[Ni(dmid)2]/Cu) or ON for Al/(n-Bu4N)x[Ni(dmid)2]/Pt is obtained by removing the applied field. The switching

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

431

mechanism for these two systems are different: Although it is not known in Al/(n-Bu4N)x[Ni(dmid)2]/Cu (280), the character of the switching is similar to what is observed in M-TCNQ (M ¼ Cu, Ag) (282). In Al/(n-Bu4N)x[Ni(dmid)2]/ Pt, it is believed to be a field-induced, solid-state, reversible electrochemical redox reaction, wherein NIOS complexes are present in the ON state but integer oxidation state complexes are present in the OFF state (281). 2.

Survey of Patents on Electrical Properties of Dithiolene-Based Systems

In spite of the promising breakthroughs mentioned above, only a small number of patents have issued that exploit the conducting properties of dithiolene-based systems, compared to the number of patents connected with the competitor ‘‘organic’’ conductors, which have also been deposited as conducting films on various substrates. Some examples of issued patents involve: coating insulating surfaces such as plastics [poly(ethylene terephthalate)] with a mixture of Na2mnt/NiCl2
6H2O/CuSO4
5H2O that makes these surfaces conductive and more adhesive (283); powders for developing fingerprints based on Ni(tfd)2 complexes (284); and conducting polymers containing bis(dithiolene) complexes obtained by mixing the appropriate complex with sebacoyl chloride for 3 days in DMF (285).

III.

MAGNETIC PROPERTIES

Antiferromagnetism is the most commonly observed magnetic behavior for dithiolene complexes. Because all of the results along this line that are reported in the literature would by far exceed the length limitation of this chapter, this research will not be discussed in this section. In spite of the large number of dithiolene complexes, very few exhibit unusual and remarkable behaviors, such as undergoing a spin-Peierls transition, behaving like spin-ladder, exhibiting ferromagnetic interactions, or behaving like a bulk ferromagnet. Once again, note that almost all of the interesting dithiolene complex-based magnetic compounds that are known are bis(1,2-dithiolene) complexes, except complexes of molybdenum that are tris(1,2-dithiolene) complexes and heteroleptic complexes of the type Cp2M(dithiolene). To our knowledge, no 1,1- or 1,3-dithiolene complexes have been reported as exhibiting any one of the unusual remarkable magnetic properties listed above. This section is devoted to four main topics: spin-Peierls systems, spin-ladder systems, systems exhibiting ferromagnetic interaction, and bulk ferromagnets. To our knowledge, no dithiolene-based films (including LB films) have been studied for their ferromagnetic properties and no patent has claimed the use of these materials in this area.

432

CHRISTOPHE FAULMANN AND PATRICK CASSOUX

A.

Spin-Peierls Systems

The spin-Peierls transition (18, 19) is a peculiar magnetoelastic transition that may take place in a limited series of quasi 1D insulating systems (20, 286). It is interesting to note that, for once, this behavior had been theoretically predicted for organic free radicals (18, 19) long before its experimental observation. It happened that the first experimental observation of a spin-Peierls system was reported for dithiolene systems, namely, (TTF)[M(tfd)2], where M is Cu or Au (25, 26). However, since bis(1,2-dithiolene) complex-based compounds have a tendency to form 1D stacks (see above), this priority is not truely amazing. In the isostructural (TTF)[M(tfd)2] compounds with M ¼ Cu, Au, discrete (TTF)þ and [M(tfd)2] ions alternate along the three nearly orthogonal directions of a NaCl-like lattice (287). The (TTF)þ cations bear one unpaired spin. Although the [M(tfd)2] anions with M ¼ Cu, Au are diamagnetic, thus theoretically precluding any direct magnetic interactions along the stacks, a subtle structural change occurring below room temperature (288, 289) allows magnetic interactions between the (TTF)þ cations to develop along a diagonal direction. This behavior (Fig. 8) corresponds to an S ¼ 12, 1D uniform Heisenberg antiferromagnetic chain. Below 12 K for M ¼ Cu, and 2 K for M ¼ Au, the magnetic susceptibility decreases sharply (whatever the orientation of the crystal in the magnetic field). This decrease is related to a ‘‘dimerization’’ of the spins, that is, a spin-Peierls transition that progressively takes place forming a singlet ground state with a magnetic gap (25, 26).

Figure 8. Temperature dependence of the magnetic susceptibility of (TTF){M[S2C2(CF3)2]2} compounds with M ¼ Cu, Au. [Adapted from the data available in (20).]

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

433

A large number of papers have appeared on this research reporting additional theoretical and experimental studies on the (TTF)[M(tdf)2] (M ¼ Cu, Au) compounds, and detailed reviews are available (20, 286). Note, however, that these dithiolene complex-based compounds were the first members of the quite limited series of spin-Peierls systems. Among these, another related, selenium analogue dithiolene complex-based compound, (TTF){Cu[Se2C2(CF3)2]2}, also undergoes a spin-Peierls transition at 6 K (290). A spin-Peierls transition also has been reported for (Per)2[Pt(mnt)2] (131, 291) (Section II.B.1.b). B.

Spin-Ladder Systems

Spin-ladder systems consist of assemblies of increasing width of S ¼ 12 chains one next to the other. These may be considered as intermediate between the 1D chain systems with quasi-long-range antiferromagnetic (AF) order and the 2D lattices with true long-range AF order. The transition from the 1D systems (chains) to 2D systems (plane) is not smooth. Depending on the width of the ladder (i.e., the number of coupled chains), the strength of the coupling within the chain (i.e., along the leg), and between the chains (i.e., along the rungs), the magnetic properties of these systems may be quite different: from short- to long-range spin order, with or without spin gap. Ladders made from an even number of legs show purely short-range spin correlation. Ladders with an odd number of legs display properties similar to those of single chains. These systems have recently become very fashionable because of their potential applications in the area of quantum magnets (292–294) and because it has been predicted that holes doped into even-leg ladders may pair and possibly superconduct (295–300). A large number of works on this subject have been reported in the last few years [for a review, see (22–24)]. Remember here (Section II) that, depending on the counterion, some [M(dithiolene)2] systems have a tendency to form stacks with side-by-side S


S interactions between stacks. Therefore, such systems with a spin S ¼ 12 seemed to be good candidates for the construction of ladder-like structures. 1.

The First System: (p-EPYNN)[Ni(dmit)2]

Imai and co-workers (301–303) were the first to use the [Ni(dmit)2] system in combination with a cation radical of a-nitronyl nitroxide, namely, p-N-ethylpyridinium a-nitronyl nitroxide (p-EPYNN) (Scheme 15). The (p-EPYNN)[Ni(dmit)2] compound is obtained by metathesis reaction between p-EPYNN.I and (n-Bu4N)[Ni(dmit)2] in acetonitrile. It crystallizes in the triclinic system, ˚, space group P1 with a ¼ 11:647ð4Þ, b ¼ 11:986ð3Þ, c ¼ 12:047ð6Þ A

434

CHRISTOPHE FAULMANN AND PATRICK CASSOUX

ON+

+ NH Et

S

S

S

S

S

S

N O. p-EPYNN

DT-TTF Scheme 15

˚ 3. The struca ¼ 103:25ð3Þ, b ¼ 106:01ð3Þ, g ¼ 109:91ð2Þ , V ¼ 1419:4ð9Þ A tural arrangement (Fig. 9) consists of chains of head-to-tail arranged p-EPYNN, ˚ separated by two slightly dimerized with N


O contacts of 2.95 and 3.38 A  chains of [Ni(dmit)2] . Within these chains, the [Ni(dmit)2] units are connected through short S


S contacts along the c axis, forming the leg of the ladder. In addition, there are plane-to-plane p overlaps between [Ni(dmit)2] units of the adjacent chain. This interaction only involves two chains, and consequently corresponds to the rungs of the ladder. This structure can be described as a two-leg ladder, in which the [Ni(dmit)2] ladder is sandwiched between 1D p-EPYNN chains. The temperature dependence of the magnetic susceptibility shows two distinct regimes (Fig. 10): (a) from 300 K to  150 K, wT decreases; (b) below 40 K, wT increases. Between 150 and 40 K a plateau is observed. The high-temperature regime has been assigned to the spin-ladder chains of [Ni(dmit)2] anions, which become nonmagnetic at lower temperature (150 K),

Figure 9. Structure of (p-EPYNN)[Ni(dmit)2], showing 1D chain of p-EPYNN and 1D ladder chain ˚ ). [Drawn from the data of Ni(dmit)2 (dashed lines shows S


S contacts between 3.2 and 3.7 A available in (304).]

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

435

Figure 10. Temperature dependence of the magnetic susceptibility of (p-EPYNN)[Ni(dmit)2]. Black and gray shaded areas represent the contribution of the (p-EPYNN)þ and [Ni(dmit)2] ion radicals, respectively. [Adapted from (304).]

as expected for antiferromagnetic spin-ladder that produce a spin-gap (=kB ¼ 940 K). The low-temperature regime is attributed to the p-EPYNN chains, which exhibit ferromagnetic coupling <40 K, as confirmed by EPR measurements between 300 and 4 K (304). Only one single line is observed over the whole temperature range. The g value and the width of this signal are temperature dependent. Below 150 K, they are typical of an a-nitronyl nitroxide radical (g ¼ 2:010, narrow). Above 150 K, g and the width of the signal begin to increase, as observed for [Ni(dmit)2] radicals. The wT values between 40 and 150 K (plateau) are close to the Curie constant, indicating that only one-half of the component radicals contribute in this temperature region. The related (p-EPYNN)[Au(dmit)2] complex has also been synthesized. However, its crystal structure does not correspond to a true spin-ladder system but rather to 1D chains of [Au(dmit)2] alternating with 1D chains of ( pEPYNN)þ still presenting ferromagnetic interactions at low temperature (304). Some attempts have been made to dope (p-EPYNN)[Ni(dmit)2] by the partial replacement of paramagnetic [Ni(dmit)2] with nonmagnetic [Au(dmit)2] (305), which led to the loss of ferromagnetic coupling within the ( p-EPYNN)þ chains.

436

CHRISTOPHE FAULMANN AND PATRICK CASSOUX

2.

The Second System: (DT-TTF)2[Au(mnt)2]

Like the dmit complexes, the mnt complexes are also promising candidates for the synthesis of spin-ladder systems. As previously discussed (Section II.B.1.b), metal-like D–A conductors have been obtained when associating M(mnt)2 and Per (159). Spin-ladder D–A systems have been obtained when associating M(mnt)2 and the dithiopheno-tetrathiafulvalene (DT-TTF ¼ C10H4S6) donor (Scheme 15). All (DT-TTF)2[M(mnt)2] (M ¼ Ni, Pt, Au) D–A compounds are isostructural (306) and crystallize in the monoclinic system, space group P21 =n. The structure can be described as uniform segregated stacks of [M(mnt)2] and DTTTF molecules along the b direction. The donor stacks are paired owing to strong interaction through short S


S contacts, and these pairs alternate with single stacks of [M(mnt)2] along the ac direction (Fig. 11). This structural arrangement forms a two-leg ladder. As expected for a (DT-TTF)þ1/2 cation-radical, thermopower measurements indicate a hole transport for (DT-TTF)2[Au(mnt)2], which behaves like a semiconductor with a slight change in the conductivity regime 220 K. Diffuse X-ray scattering studies show that the donors ‘‘dimerize’’ along the b stacking direction, two DT-TTF molecules sharing one electron. Since [Au(mnt)2] is

Figure 11. Projection of the structure of (DT-TTF)2[M(mnt)2] onto the bc plane. [Adapted from the data available in (307).]

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

437

diamagnetic, only the (DT-TTF)þ 2 units are responsible for the magnetic behavior of this complex. The magnetic susceptibility increases from 300 to 70 K and decreases down to 20 K, at which temperature it becomes dominated by a Curie tail. This behavior is typical of a system with localized S ¼ 12 spins antiferromagnetically coupled together with a gap in the low-temperature region. The susceptibility can be fitted by a two-leg spin-ladder model (308, 309), which gives the values of 83 K for Jk =kB, 142 K for J? =kB, and 78 K for =kB. Moreover, mSR measurements confirmed that (DT-TTF)2[Au (mnt)2] is a molecular material with a two-leg spin-ladder configuration (310). The results corroborate theoretical expectations for the absence of magnetic order in even-leg spin ladders in which a quantum spin-liquid state is realized. Although isostructural to the Au complex, the Ni and Pt salts do not form spin-ladder systems. This may be due to slight differences in the transfer integrals within the DT-TTF stacks and also to the paramagnetism of [M(mnt)2] (M ¼ Ni, Pt) that may interact with the DT-TTF system. Related Co and Fe compounds have also been reported (311), but they are just simple ionic salts as shown by their 1:1 stoichiometry (instead of the 2:1 stoichiometry for the Au, Ni and Pt compounds). Very recently, Ribera et al. (307) published an excellent and detailed report on this family of compounds.

3.

[Cp2M(dithiolene)](TCNQF4)

Complexes with the general formulas [Cp2M(dithiolene)]0,þ1, [CpM(dithiolene)2]1,0, [CpM(dithiolene)], and [CpM(dithiolene)2]2 have been reported recently by Fourmigue et al. (38, 312). These complexes are heteroleptic complexes with one or two Cp rings and two or one dithiolene or diselenolene ligand, respectively. The D–A compounds with the general formula [Cp2M(dithiolene)](TCNQF4) (TCNQF4 ¼ C12F4N4, tetrafluorotetracyanoquinodimethane; dithiolene ¼ dmit, dmid) have been recently reported as well (313). These complexes are synthesized by the chemical oxidation of the neutral [Cp2MIV(dithiolene)] by TCNQF4 (when using Mo and dmit2, the majority phase is a 2:1 adduct and the minority phase is a 1:1 adduct containing solvent molecules). In the solid state, these complexes adopt a folded structure of the MS2C2 plane, as shown in Fig. 12. The folding angle varies from 10.2(1) to 27.56(1) , depending on the metal (Mo < W) and the dithiolene ligand (dmit < dmid). The [Cp2M(dithiolene)]þ cations form head-to-tail dimers held together by ligand overlap interactions (short S


S contacts of 3.805 ˚ ). The (TCNQF4) anions are also strongly dimerized into and 3.823 A diamagnetic [(TCNQF4)2]2 moieties. The complexes [Cp2Mo(dmid)](TCNQF4) and [Cp2W(dmid)](TCNQF4) are isostructural. In these compounds,

438

CHRISTOPHE FAULMANN AND PATRICK CASSOUX

Figure 12. Lateral view of [Cp2M(dithiolene)]þ showing the folding angle of the MS2C2 plane and the head-to-tail dimers. [Adapted from the data available in (313).]

each [Cp2M(dmid)]þ cation interacts with another [Cp2M(dmid)]þ cation ˚ ) (Fig. 13). through one short lateral S


S contact (3.585 A Thus, the molecular arrangement of the [Cp2M(dmid)]þ gives structural ladders, the rungs of these ladders consist of the interactions within the [Cp2M(dmid)]þ dimers and the legs are formed by the lateral S


S contacts between the [Cp2M(dmid)]þ cations. The complex [Cp2Mo(dmit)](TCNQF4)(CH2Cl2)0.33 also adopts the same structural arrangement. The magnetic susceptibility temperature dependence of these dmid complexes reveals antiferromagnetic interactions with a gap in the low-temperature region. Two-leg spin-ladder systems characterized within this series show the following magnetic parameters: =kB ¼ 74 K, J? =kB ¼ 106:8 K and Jk =kB ¼ 40:7 K for [Cp2Mo(dmid)](TCNQF4); and =kB ¼ 13 K, J? =kB ¼ 23:4 K, and Jk =kB ¼ 15:6 K for [Cp2W(dmid)](TCNQF4). The variations observed for these parameters when going from Mo to W have been related to differences observed in the stacking of the cations, that is, folding angle, interplanar distance, and so on (313). As in the dmid-based systems discussed above, the [Cp2W(dmit)]þ cations in the related [Cp2W(dmit)](TCNQF4) compound are also arranged as head-to-tail dimers. However, these dimers are not face-to-face (dmit over dmit) but laterally ˚ (Fig. 14)]. These displaced, with short S


S contacts [3.433(4) and 3.749(5) A interactions within the dimers form the rungs of the ladder. The leg of the ladder is built on the interaction between the Cp rings and the dmit ligand (Fig. 15). The spin gap value is 40 K. This class of heteroleptic compounds is of interest since several factors (M, ligands, net charge) can be modulated in order to obtain complexes with

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

439

Figure 13. Ladder structure of [Cp2M(dmid)]þ in [Cp2M(dmid)](TCNQF4) (M ¼ Mo, W). [Adapted from the data available in (313).]

Figure 14. Top view of the [Cp2W(dmit)]2 dimer. [Adapted from the data available in (313).]

440

CHRISTOPHE FAULMANN AND PATRICK CASSOUX

Figure 15. Ladder-like structure of [Cp2W(dmit)]þ in [Cp2W(dmit)](TCNQF4). [Adapted from the data available in (313).]

different electronic localization and possibly spin-ladder systems with various parameters. As an example, the ‘‘doping’’ of [Cp2W(dmid)](TCNQF4) by [Cp2Mo(dmid)] may lead to a series of [Cp2W(dmid)]1x [Cp2Mo(dmid)]x(TCNQF4) compounds with  and J values controlled by the value of x. C.

Ferromagnetic Systems

Molecular magnetism, and especially intermolecular magnetic interactions in molecule-based solids, have attracted considerable interest as it involves an ever increasing number of organic and/or inorganic materials. Consequently, an overwhelming number of papers, reviews, and books have been devoted to this major subject, and, if needed, the reader is directed toward the most recent

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

441

and comprehensive ones (21, 314–317). Even though antiferromagnets are not uninteresting, the major effort in this field recently has been focused on compounds exhibiting ferromagnetic interactions, and especially ferromagnets. This preference partly results from prospects of application in various advanced technologies such as magnetic imaging or magnetooptics. The barrier, which is overcome when going from compounds just exhibiting ferromagnetic interactions to ferromagnets, is related to the length along which two spins are ferromagnetically correlated. A short-range order is observed in simply ferromagnetically coupled compounds. In turn, a long-range order is observed in ferromagnets, and below a critical temperature, Tc , the correlation length becomes infinite and the material acquires a spontaneous magnetization. 1.

Systems Exhibiting Ferromagnetic Interactions

Short-range ferromagnetic interactions have been observed in several (seven, to our knowledge) dithiolene complex-based series of compounds with thedithiolene ¼ mnt2, dmit2, dmid2, tfd2, edt2, and bdt2 {bdt2 ¼ [(C6H4)C2S2]2, benzene-1,2-dithiolato}. Historically, mnt2 and tfd2 were the first ligands involved in metal complexes aimed at promoting cooperative ferromagnetic interaction (318). a. Systems Based on mnt. Following on the discovery of bulk ferromagnetism in the planar cyano–radical anions of decamethylferrocenium, (Cp2 Fe)(TCNE) (31, 319), work has been carried out with the idea of substituting planar metal–organic complexes for the organic TCNE acceptor molecule and varying the nature of the metallocene. The (Cp2 Fe)[M(mnt)2] (M ¼ Ni, Pt) compounds were synthesized in 1989 by reaction of (Cp2 Fe)(BF4) with (n-Bu4N)[M(mnt)2] (318). The Pt salts exist as a- and b-forms. None of these salts are isostructural. Among these compounds, only the b-(Cp2 Fe)[Pt(mnt)2] phase possesses parallel


DþADþA


chains (Fig. 16) similar to those observed in the (Cp2 Fe)(TCNE) ferromagnet (34). Indeed, owing to the presence of four independent entities in the asymmetric unit, orthogonal chains of


DþAADþAA


are observed in the structural arrangement. Field-dependent susceptibility characteristic of ferromagnetic coupling is observed for this complex, but no bulk ferromagnetic behavior. Stabilization of the ferromagnetic ground state is thought to result from configuration mixing of the ground state with the lowest charge-transfer excited state (28). According to the model for ferromagnetic alignment proposed by McConnell, which is based on configuration interaction along a


DþADþA


chain (28), and confirmed by studies on (Cp2 Fe)(TCNE) and b-(Cp2 Fe)[Pt(mnt)2] mentioned above, an alternation between the acceptor anion–radical

442

CHRISTOPHE FAULMANN AND PATRICK CASSOUX

Figure 16. Crystal structure of b-(Cp2 Fe)[Pt(mnt)2], showing the


DþADþA


arrangement. [Adapted from the data available in (318).]

and the donor cation–radical seems to be necessary for the development of cooperative ferromagnetic interactions. The [Fe(Z5-C5Me4St-Bu)2]þ cation radical (Scheme 16) is derived from the decamethylferrocenium cation by S

t-Bu S

N

N

S

S

S

S

S t-Bu OMeFc

BDNT Scheme 16

substituting one thiobutyl substituent for one methyl group on each Cp* ring, thus lowering the original symmetry (320). The [Fe(Z5-C5Me4St-Bu)2][M(mnt)2] (M ¼ Ni, Pt) compounds are isomorphous (321). Their structure (Fig. 17) consists of alternating layers of [M(mnt)2] and ferrocenium units, affording the desired


DþADþA


structural motif.

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

443

Figure 17. Projection of the structure of [Fe(Z5-C5Me4St-Bu)2][M(mnt)2] (M ¼ Ni, Pt) onto the bc plane. [Adapted from the data available in (320).]

The magnetic measurements reveal an increase of wT at low temperature, indicative of weak ferromagnetic coupling (320). Several related compounds have been studied by Zu¨ rcher et al. (320) in which the cation may contain a methyl group instead of the t-Bu group and/or Co may be used instead of Ni or Pt. Nevertheless, the [Fe(Z5-C5Me4St-Bu)2][M(mnt)2] (M ¼ Ni, Pt) compounds are the only ones exhibiting ferromagnetic coupling and the


DþADþA


molecular arrangement. Several compounds involving the BDNT donor molecule {BDNT ¼ 4,9-bis(1,3-benzodithiol-2-ylidene)-4,9-dihydronaphtho[2,3-c][1,2,5]thiadiazole} (322) (see Scheme 16) and the M(mnt)2 (M ¼ Ni, Pd, Pt, Au) acceptor complexes have been obtained by diffusion methods or electrocrystallization (323). Within this series, the (BDNT)2[Ni(mnt)2] compound distinguishes itself by its 2:1 stoichiometry, whereas the Pd, Pt, and Au congeners are 1:1 or 1:2 compounds. The (BDNT)2[Ni(mnt)2] complex exhibits an antiferromagnetic behavior between 300 and 125 K, at which temperature ferromagnetic interactions occur in the material. It was concluded from EPR experiments that [Ni(mnt)2] is responsible for ferromagnetic interaction. In spite of its relatively high RT conductivity (s ¼ 0:5 S cm1 ), this compound is a semiconductor, and,

444

CHRISTOPHE FAULMANN AND PATRICK CASSOUX

as in the (Per)[M(mnt)2] systems (Section II.B.1.b), only the BNDT-based cation-radical component is responsible for the transport properties. In view of the magnetic and transport properties, it seems that the Ni(mnt)2 and BDNT components do not interact. For example, there is no anomaly in the conductivity plot versus temperature 125 K. The structure of this compound is not known. Owing to the nonplanarity of BDNT, a mixed


DþADþA


structure is not likely, and a segregated 1D structure is expected. Then, ferromagnetic coupling would not require the presence of the alternating


DþADþA


structural motif in this peculiar compound. b. Systems based on tfd. The (Cp2 Fe)[Ni(tfd)2] complex is prepared by reaction of the neutral (Cp2 Fe) species with [Ni(tfd)2] in dichloromethane (318). The structure (Fig. 18), which is similar to that of b-(Cp2 Fe)[Pt(mnt)2] (Section III.C.1.a), is built on alternating units of cations and anions. The effective moment rises when cooling down the sample, suggesting ferromagnetic interaction. Magnetization data at 2.2 K do not show any hysteresis or remnant magnetization. In order to minimize unfavorable A/A interchain interaction, which has been considered as responsible for the crossover from ferromagnetic to

Figure 18. Crystal structure of (Cp2 Fe)[Ni(tfd)2] ) onto the ac plane. [Adapted from the data available in (318).]

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

445

antiferromagnetic behavior in (Cp2 Fe)[Ni(dmit)2] (Section III.C.1.c), the strategy followed by Heuer et al. (324) consisted in building a structure in which each S ¼ 12 cation would be surrounded by six S ¼ 12 anions. To do so, they associated the [Mo(tfd)3] acceptor anion with (Cp2 M)þ (M ¼ Cr, Fe) and with (Cp2Fe)þ. The resulting compounds obey the Curie–Weiss law with positive y values for (Cp2 M)[Mo(tfd)3] (M ¼ Cr, Fe) and (Cp2Fe)n[Mo(tfd)3] ðn ¼ 2; 1Þ. The crystal structure of (Cp2 Fe)[Mo(tfd)3] consists of discrete linear stacks of alternating S ¼ 12 (Cp2 Fe)þ cations and S ¼ 12 [Mo(tfd)3] anions, with the anions adopting a nearly perfect trigonal-prismatic geometry. As mentioned above, these positive values of y indicate ferromagnetic interactions, arising from the coupling between spins localized on the donor and acceptor within the


DþADþA


chains (324). Chronologically following the discovery of the second bulk ferromagnet, (Cp2 Mn)(TCNQ) by Broderick et al. (325), it was claimed that the use of higher spin cations may lead to higher ordering temperatures. Consequently, various (Cp2 Mn)[M(tfd)2] (M ¼ Ni, Pd, Pt) compounds were prepared from (Cp2 Mn)(PF6) and the appropriate (n-Bu4N)[M(tfd)2] (326). Powder X-ray data indicate that these three salts are isostructural, but only the structure of the Ni derivative has been elucidated. The complex (Cp2 Mn)[Ni(tfd)2] is also isostructural to (Cp2 Fe)[Ni(tfd)2] (see Fig. 18). The structure consists of an array of parallel stacks of alternating (Cp2 Mn)þ and [Ni(tfd)2] moieties. All (Cp2 Mn)[M(tfd)2] compounds obey the Curie–Weiss law with a positive y value. Their magnetic moment at room temperature is larger than the spin-only value calculated for isolated two-spin systems, probably due to an orbital contribution of (Cp2 Mn)þ to the g factor. Below 20 K, wM T increases rapidly, indicating ferromagnetic interactions. For each compound, the wM susceptibility decreases to 2.5 K, indicating an antiferromagnetic phase transition. This field-induced transition is evidenced by field-cooled magnetization. Thus, the (Cp2 Mn)[M(tfd)2] compounds should be regarded as metamagnets, for which the application of a magnetic field causes a transition to a ferromagnetic state. The question arises as to why (Cp2 Mn)[M(tfd)2] compounds are metamagnets, whereas (Cp2 Fe)[Ni(tfd)2] is not. Broderick et al. (326) suggest that a strong intrachain ferromagnetic coupling between the donor and the acceptor associated with a weak antiferromagnetic interchain interaction could lead to a metamagnetic behavior with a Ne´ el temperature depending on the strengths of these interactions (327, 328). Thus, using a (Cp2 Mn)þ system with an S ¼ 1 value larger than the S ¼ 12 value for the (Cp2 Fe)þ may lead to stronger intrachain interactions, resulting in the development of a metamagnetic state. c. Systems Based on dmit and Isolog Ligands. We reviewed the spin-ladder behavior of the (p-EPYNN)[Ni(dmit)2] compound (Section III.B.1) and already

446

CHRISTOPHE FAULMANN AND PATRICK CASSOUX

Figure 19. Crystal structure of (Cp2 Fe)[Ni(dmit)2] showing the face-to-face anions and the sideby-side cations. [Adapted from the data available in (329).]

mentioned that this compound exhibits ferromagnetic interaction <40 K associated with the p-EPYNN chains (304). Other dmit-based compounds exhibit ferromagnetic coupling. For example, the (Cp2 Fe)[Ni(dmit)2] compound was synthesized by combining (Cp2 Fe) (BF4) and (n-Bu4N)[Ni(dmit)2] (329). The structure (Fig. 19) of this complex consists of


DþDþAADþDþAA


stacks with side-by-side cations alternating with face-to-face dimerized anions (dyads). Although this arrangement is not strictly identical to the


DþADþA


arrangement recommended by McConnell (28), this compound does exhibit ferromagnetic interactions: The wM T product is almost constant down to 25 K, below which temperature it increases very rapidly. The maximum of wM T is reached at 2.1 K. Below this temperature, it decreases down to 1.5 K. This antiferromagnetic transition might be due to an antiferromagnetic coupling between A


A, favored by short S


S contacts within the dyads. The magnetic moment of the palladium analogue compound, (Cp2 Fe)[Pd(dmit)2], remains almost constant down to 20 K, below which temperature it increases rapidly, indicating ferromagnetic interactions. The structure has not yet been reported, and no magnetic ordering has been observed down to 2 K. The dmid2 ligand may be considered as deriving from the dmit2 ligand by substituting oxygen for the terminal thione sulfur atom. Although this variation seems quite minor, the structure of (Cp2 Fe)[Ni(dmid)2]
MeCN (330) is totally

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

447

different from that of (Cp2 Fe)[Ni(dmit)2] (329). The structure consists of 2D layers built on


DþDþADþDþA


stacks. These stacks are separated from each other by sheets of perpendicular [Ni(dmid)2] (A) that neutralize the charge. The magnetic behavior of {Cp2 Fe)[Ni(dmid)2]
MeCN obeys the Curie–Weiss law with y ¼ 1:97 K and presents typical ferromagnetic interaction superimposed on weak antiferromagnetic interactions. However, Fettouhi et al. (330) suggested that the ferromagnetic interaction occurs through the overlap between the negative spin density of the Cp units and the positive spin density of the S atoms of the dmid ligand (spin polarization effect). This mechanism seems to be opposite to that based on coupling between DþA and D2þA2 proposed by Broderick et al. (329) (the so-called second McConnell mechanism) (318). d. Systems Based on Other Ligands. Several metallocenium-containing compounds associated with other dithiolene-based complexes recently have been synthesized by Gama et al. (331). Their general formula is (Cp2 M0 )(NiL2) (M0 ¼ Fe, Mn, Cr, L ¼ edt, bdt) (see Scheme 5). These complexes are obtained from the appropriate metallocenium cations (Cp2 M0 )þ and (n-Bu4N)(NiL2). In this series of compounds, antiferromagnetic interactions are observed at high temperatures, but in several cases ferromagnetic interactions may become dominant at low temperatures. The structure of (Cp2 Fe)[Ni(edt)2] was claimed to be characterized by a


DþADþA


stacking arrangement similar to that observed in the structure of the above mentioned (Cp2 M)[Ni(tfd)2] compounds [M ¼ Fe (318), Mn (326)] (see Section III.C.1.b). The magnetic behavior of (Cp2 Fe)[Ni(edt)2] obeys the Curie–Weiss law, with y ¼ 9:8 K indicating that antiferromagnetism dominates at high temperatures. Moreover, the minimum in the w1 versus T curve observed at 4 K is typical of an antiferromagnetic transition. At lower temperatures, the magnetization measurements indicate a field-induced transition to a metamagnetic state with a Ne´ el temperature of TN ¼ 3:2 K and a critical field of Hc 4 G at 2 K. The crystal structures of (Cp2 Fe)[Ni(bdt)2] and (Cp2 Cr)[Ni(bdt)2] show a molecular arrangement different from that of (Cp2 Fe)[Ni(edt)2], but reminiscent of that of (Cp2 Fe)[Ni(dmid)2]
MeCN (see above) (330). These structures consist of D–A mixed layers, composed of


DþDþADþDþADþDþ


stacks separated by layers of [Ni(bdt)2] cations neutralizing the charge. All (Cp2 M)[Ni(bdt)2] (M ¼ Fe, Mn, Cr) compounds exhibit ferromagnetic interaction at low temperatures, but for M ¼ Fe and Cr, no spontaneous magnetization, saturation, or hysteresis is observed. However, (Cp2 Mn) [Ni(bdt)2] was shown to behave like a metamagnet, with TN ¼ 2:3 K and Hc 200 G at 2 K. The (n-Bu4N)2[Cu(dcmdtcroc)2] (dcmdtcroc)2 ¼ dithiodicyanomethanecroconato or 4- (dicyanomethylene)-1,2-dimercaptocyclopent-1-ene-3,5-dionato-S,S0

448

CHRISTOPHE FAULMANN AND PATRICK CASSOUX

O

N C

C C C N

O

O N C S S C C C Cu C C C S S C C C N O Cu(dcmdtcroc)2 Scheme 17

compound (Scheme 17) was synthesized in 1989 by Venkatalakshmi and coworkers (332). In the structure of this compound, the nonplanar anions ˚ (Fig. 20). [Cu(dcmdtcroc)2]2 form dimers with a Cu


Cu distance of 4.883(1) A Magnetic measurement data have been analyzed in terms of an antiferromagnetic interdimer exchange value of J ¼ 0:041 cm1 and a ferromagnetic intradimer exchange value of J ¼ 6 cm1. 2.

Bulk Ferromagnets

As described above, the first concepts for the design of molecule-based magnets were proposed by McConnell in 1963 (333). In 1967, the second set of concepts, involving a


DþADþA


stacking arrangement of donor and acceptor ion radicals (28), were proposed. An alternative mechanism was also proposed by Mataga et al. (29) involving triplet diphenylcarbene moieties, but this mechanism could hardly be used for designing dithiolene complex-based magnets. The first experimental confirmation of the existence of a moleculebased bulk ferromagnet was reported in 1985 with the characterization of (Cp2 Fe)(TCNE) (31). Several additional series of molecule-based magnets have since been introduced, including organic nitronyle nitroxides (334), (TDAE)(C60) [TDAE ¼ tetrakis(dimethylamino)ethylene] (335), and others (21). Room temperature ferromagnetic ordering has recently been achieved in the V(TCNE)x compounds (336–338). Nevertheless, to our knowledge only two dithiolene complex-based bulk ferromagnets have been described, (NH4)[Ni(mnt)2]
H2O and (Cp2 Mn)[Ni(dmit)2]. a. (NH4)[Ni(mnt)2]
H2O. This compound, extensively studied at one time (339–344), was first reported in 1977 by Perez-Albuerne et al. (345) in its hydrated as well as in its non-hydrated form. In 1980, Isett et al. (346), on the sole basis of the unit cell determination, proposed a structure involving uniform stacks. Clemenson et al. (347) finally elucidated the structure in 1988. The structure contains uniform stacks (Fig. 21), which is unusual for (cation)[M(mnt)2] salts (348, 349). The [Ni(mnt)2] anions stack along the b axis and are tilted by 26 with respect to the stacking axis. The overall structural

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

449

Figure 20. (a) Projection of the structure of (n-Bu4N)2[Cu(dcmdtcroc)2] onto the ab plane. (b) Dimers formed by the nonplanar [Cu(dtcroc)2]2 anions. [Adapted from the data available in (332).]

arrangement consists of sheets of [Ni(mnt)2] anions parallel to the ab plane, separated from each other along the c direction by sheets of hydrogen bonded cations and water molecules. These water molecules are also hydrogen bonded to the N atoms of the CN groups of the mnt2 ligand. Although the structure at 98 K has not been fully solved, it appears that the c parameter doubles,

450

CHRISTOPHE FAULMANN AND PATRICK CASSOUX

Figure 21. (a) Projection of the structure of (NH4)[Ni(mnt)2]
H2O onto the ac plane. [Adapted from the data available in (347).]

indicating a dimerization of the anions in an eclipsed mode. The compound has an antiferromagnetic behavior between 300 and 100 K, which can be fitted with the Curie–Weiss law. At 100 K, a broad transition occurs and a fit to the data between 70 and 20 K gives a positive temperature intercept ðy ¼ 7 KÞ, indicative of ferromagnetic coupling. Long-range magnetic ordering appears <5 K. Polycrystalline samples as well as single crystals of this salt were studied down to 2 K in a superconducting quantum interference device (SQUID) magnetometer. On polycrystals, the remnant and coercive fields reach values of 1500 G and 100 Oe at 2 K, respectively. On a single crystal oriented perpendicular to the field, a coercive field of 100 Oe and a magnetization of 1800 emu then G mol1 at 2000 Oe have been determined from magnetization data. Magnetic measurements have also been performed under pressure: The Curie temperature rises with increasing pressures up to 6.8 kbar. Above 7.4 kbar, no ferromagnetic transition is observed, indicating a structural phase transition above this pressure. This pressure dependence is believed to arise from a competition between ferromagnetic coupling (resulting from nickel–sulfur intermolecular spin interactions), and antiferromagnetic coupling (resulting from nickel–nickel interactions). The origin of the ferromagnetism in this compound has been explained according to the McConnell mechanism (342). b. (Cp2 Mn)[Ni(dmit)2]. As previously mentioned (Section III.C.1.b), it is believed that the use of a high-spin cation, such as (Cp2 Mn) in (Cp2 Mn)(TCNQ) (325), could lead to ferromagnetic compounds with a high ordering temperature. The (Cp2 Mn)[Ni(dmit)2] compound, first reported in 1999 (350), together with (Cp2 Mn)[Fe(dmit)2] and (Cp2 Ni)[Ni(dmit)2], can be prepared

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

451

Figure 22. Crystal structure of (Cp2 Mn)[Ni(dmit)2] showing the side-by-side cations alternating with face-to-face anions (dyads).

by a metathesis reaction between (Cp2 Mn)(PF6) and (n-Bu4N)[Ni(dmit)2] or Na[Ni(dmit)2]. The complex (Cp2 Mn)[Ni(dmit)2] is isostructural (351) to the (Cp2 Fe)[Ni(dmit)2] compound that was shown to exhibit ferromagnetic coupling (329) (Section III.C.1.c). The overall structural arrangement (Fig. 22) consists of


DþDþAADþDþAA


stacks comprising side-by-side cations alternating with dyads. The magnetic behavior has been studied on microcrystalline samples. The parameter wM T increases when the temperature decreases and reaches a maximum at 3 K. Below this temperature, the magnetic susceptibility becomes field dependent, indicating long-range magnetic ordering. This compound exhibits spontaneous magnetization, as shown by the magnetization curves ZFC, FC, and REM (zero-field cooled, field cooled, and Remnant, respectively), for which the ordering temperature is 2.5 K [Fig. 23(a)]. The magnetization versus field at 2 K increases very rapidly at low fields, as expected for a magnet, and increases smoothly at higher fields to reach the value of 12,900 cm3 G mol1. This saturation value is, however, lower than the expected value of 18,000 cm3 G mol1, but much larger than the value expected for a ferrimagnet (5800 cm3 G mol1) [Fig. 23(b)]. Thus, unlike (Cp2 Fe)[Ni(dmit)2], which only exhibits ferromagnetic coupling (Section III.C.1.c), (Cp2 Mn)[Ni(dmit)2] is a bulk ferromagnet, as confirmed by the hysteresis observed at 2 K (Fig. 24) (though not very impressive as far as coercive field is concerned). The determination of the origin of this bulk ferromagnetism has been recently explained (351). Note, however, that the use of the larger S ¼ 1 spin

452

CHRISTOPHE FAULMANN AND PATRICK CASSOUX

Figure 23. (a) Zero-field cooled, FC, and REM magnetization curves for (Cp2 Mn)[Ni(dmit)2]. (b) Magnetization curve versus field at 2 K for (Cp2 Mn)[Ni(dmit)2].

system, (Cp2 Mn)þ, instead of the S ¼ 12 spin system, (Cp2 Fe)þ, actually led to an improvement in the magnetic properties. Indeed, whereas (Cp2 Mn)[Ni(dmit)2] is a bulk ferromagnet, the (Cp2 Mn)[M(tfd)2] (M ¼ Ni, Pd, Pt) compounds are metamagnets (326), and (Cp2 Fe)[Ni(dmit)2] (329) or (Cp2 Fe)[Ni(tfd)2] (318) only exhibit short-range ferromagnetic coupling (Section III.C.1).

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

453

Figure 24. Hysteresis for (Cp2 Mn)[Ni(dmit)2].

IV.

OPTICAL PROPERTIES

Since Chapter 6 (352) in this volume is devoted to the luminescence and photochemistry of dithiolene complexes, these aspects will not be developed here. In this section, we restrict ourselves to the review of dithiolene complexbased compounds exhibiting optical properties, such as strong infrared (IR) absorption and second- and third-order nonlinear optical properties. In the last part of this section, we will review the use of some dithiolene complexes for optical data storage (printers, developer, CD-ROM, etc.). Contrary to what was observed throughout the two previous sections, no particular dithiolene complex really dominates the domain of the molecular materials exhibiting interesting optical properties. A.

Strong Near-IR Absorption

The very strong absorption of the dithiolene complexes makes them excellent candidate systems for producing Q-switch laser dyes [for recent reviews, see (353, 354) and references cited therein], and for their use in laser devices. The use of these devices allows the generation of short and very intense pulses. In short, dithiolene complexes are especially appropriate for such applications because of (a) their delocalized structure (Section I), their very intense electronic absorption >700 nm (p–p*) (16); (b) their various oxidation states (Section I); and (c) their high thermal and photochemical stability. Moreover, due to the subtle changes that can be introduced on the ligands, the absorption can be smoothly tuned over the desired wavelength range. Most of the

454

CHRISTOPHE FAULMANN AND PATRICK CASSOUX

complexes used in these systems are derived from Ni because of the higher delocalization within these complexes compared to the Pd and Pt analogues (355–357). A detailed review by Mu¨ ller-Westerhoff et al. (354) is available on this topic. A number of Ni-based dithiolene complexes with absorption at wavelengths >700 nm have been described (43, 353, 354, 358–380) and several patents have been issued for the use of dithiolene complexes for Q-switch laser dyes (381– 392). Due to the large number of these complexes, we will focus on those showing high performance, that is, absorption wavelength >1000 nm and absorbance at least equal to 20,000 dm3 mol1 cm1. Some of the ligands involved in dithiolene complexes exhibiting such strong IR absorption are shown in Schemes 18–23. Since the discovery of the first example of such compounds, bis(p-dimethylaminostilbenedithiolato)nickel ðlmax ¼ 1060 nm; e ¼ 28; 000 dm3 mol1 cm1 Þ (1 in Scheme 18) (379), many research works have been devoted to the study of the influence of the ligand on the properties of the materials (375, 378, 393–395). These works were aimed at increasing the intensity of the absorption maximum at lower energy. This research led to the conclusion that dithiolene complex should contain (a) extended p systems, with electron-donating substituents as coplanar as possible with the Ni(dithiolene) unit; and (b) sterically bulky substituents in order to increase the solubility. With these ideas in mind, several symmetrical and unsymmetrical Ni complexes were synthesized by Mueller-Westerhoff and co-workers (Scheme 18) (43, 353, 354, 375). All of the complexes are neutral. The presence of an amino group on each phenyl group in ligand 2 (see Scheme 18) results in the shift of the absorption in the corresponding complex to longer wavelengths (1120 nm) compared to that of the unsymmetrical ligand 1 with an amino group on only two of the four phenyl groups (1060 nm). The effect of coplanarity is observed when comparing the complexes of ligands 3 and 4. In both 3 and 4, the amino substituent is not allowed to rotate and is constrained to coplanarity by incorporation in a saturated ring. As the number of coplanar substituents increases when going from 3 to 4, l and e increase. In the complex of ligand 5, the lack of an electron-donating group is compensated by a totally delocalized and planar system. Comparison of complexes of ligands 2 and 6 allowed observation of the positive effect due to the forced planarity of the aromatic substituents in 6, which are linked to each other. The coplanarity of the amino groups in 7 shifts the maximum of the absorption of the corresponding complex to lower energy compared to 6, and also results in narrowing of the absorption band (353). The longest absorption wavelengths are reached with complexes of ligands 8 (375) and 9 (353). In the complex of ligand 9, in which the substituents and the dithiolene unit are coplanar, an increase in l is observed when compared with the complex of ligand 8 in which they are free to rotate. Although all the

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

455

N

N S-

S-

S-

S-

N SS-

N 1 1060 nm (28,000)

3 or JUL1 1180 nm (35,000)

2 1120 nm (28,000) N

N SS-

S-

S-

S-

SN

5 1140 nm (30,000)

N

6 1340 nm 4 or JUL2 1270 nm (45,000) N N

N N

N S-

S-

S

SN

N N

7 1380 nm

S-

-

8 or DETHQ 1370 nm (42,000)

SN N

9 1440 nm

Scheme 18

complexes derived from the ligands shown in Scheme 18 exhibit large l values, some of them (6 to 9) are air sensitive and some are poorly soluble, which prevents further purification. The same authors have also used another approach consisting of totally changing the nature of the substituents on the dithiolene complex. This approach led to the synthesis of the neutral tetraferrocenyl dithiolene complex 10 (Scheme 19), which absorbs at 1310 nm (e not reported) (353).

456

CHRISTOPHE FAULMANN AND PATRICK CASSOUX

Fe

Fe

S− S−

Fe

S

S−

S

S−

Fe

10 1310 nm

11 1155 nm (11,480) CH2Cl2 Scheme 19

Very recently, Lee and Noh (367) reported the synthesis of (n-Bu4N)[Ni(dfcvdt)2] (dfcvdt2 ¼ {[Fe(C10H9)2]2C2H2C2S4}2, 2,3-diferrocenyl-1,4dithiin-5,6-dithiolato) (see 11 in Scheme 19) involving the same ferrocenyl substituent as in 10, but separated from the dithiolene moiety by an additional sulfur containing ring. This compound shows an absorption at higher energy than 10 ðl ¼ 1155 nmÞ. Zuo et al. (368) reported neutral and monoanionic Ni complexes based on ligands such as phdt2 (12) (phdt2 ¼ 5,6-dihydro-5-phenyl-1,4-dithiin-2,3dithiolato) and medt2 (13) (medt2 ¼ [C5H6S4]2, 5,6-dihydro-6-methyl-1,4dithiin-2,3-dithiolato) (396) (Scheme 20), which are analogous to the dddt2 Ph

S

S-

S

S-

Me

S

S-

S

S-

S

S-

S

S-

12 phdt2- 1028 nm

13 medt2- 1029 nm

14 bddt2- 1035 nm

(43,000) C6H6

(36,000) C6H6

(63,000) C6H6

Scheme 20

ligand (Section II.B.1.c) with one phenyl or methyl group substituted for one hydrogen. In benzene, the neutral compounds exhibit absorption >1000 nm with absorbance of 43,000 and 36,000 dm3 mol1 cm1, for 12 and 13, respectively. The monoanionic complexes also show a strong absorption >1000 nm, but with a much lower absorbance (15,000 dm3 mol1 cm1). Note that the monoanionic complexes of [Ni(dmit)2] and [Ni(dddt)2] exhibit energy transitions at 1137 and 1175 nm, with extinction coefficients of 45,000 and 15,600 dm3 mol1 cm1, respectively (368). Addition of an outer ring on the dddt2 ligand leads to the bddt2 ligand (14) (Scheme 20) (bddt2 ¼ [C8H10S4]2, 4a,5,6,7,8,8ahexahydro-1,4-benzodithiin-2,3-dithiolato), which can form the neutral

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

457

Ni(bddt)2 compound (366). This compound shows a remarkable absorption at 1035 nm with e ¼ 63; 000 dm3 mol1 cm1 and is therefore an excellent candidate as a near-IR dye for Q-switching neodymium lasers. Attaching thioalkyl groups on the ethylene-dithiolene unit has been performed by Charlton and co-workers (372, 397). This led to the synthesis of eight neutral compounds with a formula [TTCn-Ni(edt)2] ðn ¼ 411Þ (15) (Scheme 21). All of them exhibit strong absorption around 1000 nm with an absorbance of  40,000 dm3 mol1 cm1. As suggested by Mu¨ ller-Westerhoff (354), the presence of electron-donating substituents should shift the absorption maximum to lower energies, compared to the unsubstituted dithiolene complex. This has been accomplished by several groups (371, 398–401) who prepared neutral [M(R2timdt)2] compounds (M ¼ Ni, Pd, Pt) (see compound 16 in Scheme 21; R2timdt2¼ 1,3-dialkylimindazoline-2,4,5-trithione), which are reminiscent of the [M(dmit)2] systems, but in which donor NR groups (R ¼ Et, i-Pr, Bu, etc.) have been substituted for

R

R

S

S-

S-

N

S-

S

S-

S

R N

R 16 or R2timdt R = Et, i-Pr, Bu 1000 nm (80,000) CHCl3

15 or TTCn-edt R = H2n+1Cn n = 4-11 1000 nm (40,000) CH2Cl2 NC

S

S

NC

S

S

-

S

S

CN

S

S

CN

Ni

17 1135 nm (21,880) CHCl3

X X

Y S

S

X S

Ni S

X X

X

S Ni

S

S Y

S n

X X

18 X = Cl, F; Y = H, Cl, F; n = 1, 4, 5, 9 1190-1900 nm (59,300-200,000) DMF Scheme 21

458

CHRISTOPHE FAULMANN AND PATRICK CASSOUX

the endocyclic S atoms, without altering the planarity of the ligand. The properties of these compounds are quite remarkable, since they absorb 1000 nm with a molar extinction coefficient of 80,000 dm3 mol1 cm1, one of the largest values reported in the literature. Substitution of the terminal S atom of the dmit ligand has been performed by Gompper et al. (370). They synthesized the monoanionic dicyanomethylene complex (n-Bu4N)[Ni(C6S4N2)2] (17) (see Scheme 21), which shows a strong absorption band around 1135 nm ðe ¼ 21; 880 dm3 mol1 cm1 Þ. An interesting approach has also been made by Campbell et al. (402), by using anionic polynuclear Ni compounds 18, shown in Scheme 21. These compounds exhibit a low-energy electronic transition (between 1190 and 1900 nm) with e ranging from 59,300 to 2,000,000 dm3 mol1 cm1. B.

Nonlinear Optical Properties

General considerations about nonlinear optical properties (NLO) can be found in (403). It is interesting to note that, while a large number of papers have appeared on organic molecule-based compounds exhibiting NLO properties, especially for establishing structure–property relationships for second harmonic generation (SHG) (404–411), this is not the case for inorganic molecule-based materials (412–414). 1.

Second-Order NLO

As far as SHG is concerned, a so-called ‘‘two-state model’’ proposed by Oudar (415) predicts that compounds with an intense and low-frequency chargetransfer absorption band may possess a large molecular hyperpolarizability b. Therefore, dithiolene complexes are of interest for NLO properties owing to their delocalized electronic configuration and the possibility of the transfer of electron density between the metal and the ligand, which induces an intense near-IR absorption transition (416). Recently, reviews on metal-containing materials exhibiting NLO properties have appeared (417–419). Since SHG requires materials lacking a center of symmetry, symmetrical homoleptic dithiolene complexes involved in SHG were often used solely as spectator counterions for balancing the charge of the push–pull NLO active molecule such as the hemicyanine dye (HCD) (19) shown in Scheme 22. However, as will be R2N

C C H H

NH C18H37

19 or HCD R = Me, Et, Bu, etc. Scheme 22

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

459

discussed in Section V.B, attempts have been made to use symmetrical homoleptic dithiolene complexes as the conductive component in multifunctional NLO materials. Though it could be expected that such dithiolene complexes themselves would not exhibit NLO properties, several authors have nevertheless reported the fabrication of LB films of HCDs combined with dithiolene complexes exhibiting improved SHG properties (420–422). Indeed, HCDs are well known for their large second-order molecular hyperpolarizabilities (423). Replacement of the iodide anion in (HCD)I by a dmit-based complex dianion has been claimed to yield compounds of the general formula [(HCD)2[M(dmit)2] (M ¼ Cd, Ni (420), Zn (421, 422)]. The LB films of these compounds may exhibit improved SHG when compared to those of the corresponding (HCD)I. For example, the nonlinear susceptibility w2 of (HCD)2[Zn(dmit)2] was reported to be about three times larger than w2 of the corresponding (HCD)2I (421, 422), whereas w2 for the Cd derivative lies in the same range while w2 for the Ni derivative is lower (420). Although the structure of these films is not known, the Zn complex may act not only as a counterion but also as a spacer avoiding aggregation and/or dispersion of the active chromophore (HCD) and allowing an ordered segregation of HCD in the films. It is clear, however, that unsymmetrical homoleptic (20–23) (Scheme 23), or heteroleptic dithiolene complexes (24–27) (Scheme 24), which have a nonzero dipole moment, seem to be better candidate systems for SHG application than

R

R N

S

N

CN

S

S

Ni N

S

S

CF3

S

CF3

Ni S

S

N

CN

R

R 20

21

OMe N

MeO S

S

CF3

S

CF3

Ni S

MeO

S

MeO

S

OMe 23

Scheme 23

CF3

S

CF3

Ni

MeO 22

S

460

CHRISTOPHE FAULMANN AND PATRICK CASSOUX

N

N

R

S S

N

S

R

S

R'

M

Pt N

24

25

O O N

N

CF3

S

N

S Pt

Ni S

N

CF3

S

O O 26

27

N

S

R

S

R

M N

28

Scheme 24

homoleptic symmetrical dithiolene complexes. Note, however, that even if the space group of a compound built on unsymmetrical molecules is noncentrosymmetrical, and if these molecules are packed in a pseudo-centrosymmetrical arrangement, will the second order susceptibility wð2Þ be close to zero, with no SHG effects will be observable. The most promising, and the most extensively studied compounds are heteroleptic Ni(diimine)(dithiolate) complexes, whose general formula (28) is shown in Scheme 24 (412, 414, 417, 424–432). Cummings et al. (412) in 1997 studied a series of such M(diimine)(dithiolate) complexes (M ¼ Ni, Pd, Pt, Zn), where the dithiolate ligand is derived from either a 1,2-dithiolene or 1,1dithiolene framework. These authors conclude that, in order to increase the molecular hyperpolarizability of the complexes, the best candidate system should be Pt based, with electron-donating substituents on the dithiolate ligands (to increase the donor strength), whereas electron-withdrawing substituents should be placed on the diimine as far as possible from the Pt atom. Depending on the substituents and the metal atom, the studied compounds exhibit b0 values within the 0 to 16  1030 esu range. The same strategy was followed by Chen

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

461

et al. (424–427, 430), who attached numerous electron-acceptor substituents on the diimine in complexes such as Ni(diimine)(tfd) (430) [(26), Scheme 24]. The corresponding compounds display b0 values of about 30  1030 esu. For the two previous series of compounds, only values of the molecular hyperpolarizability, b0 , have been determined. To our knowledge, no bulk nonlinear susceptibility, wð2Þ , values have been reported for these compounds. Actually, the X-ray structures reported for some of these compounds show that they crystallize in centrosymmetrical space groups, which results in canceling the molecular nonlinearities.

2.

Third-Order NLO

Third-order NLO effects are of interest in numerous fields such as opticalphase conjugation (to restore distorted images) and all-optical-switching and computing (403). A number of metal-containing materials with third-order NLO properties have been reported (418, 419). Although third-order NLO effects do not require any symmetry restriction, the structure–property relationships that govern third-order NLO polarization are not well defined. Moreover, the different methods used to determine some characteristic parameters do not always give the same values [(433–435) versus (436)], although recent work seems to clarify the situation (431). In spite of this fuzziness, a large amount of research devoted to the use of metal dithiolene complexes in third-order NLO has been reported in the last 10 years (368, 397, 431, 433, 434, 436–456). As seen in Section IV.B.1, and since no useful guidelines have been proposed to select appropriate candidate dithiolene complex-based systems (should they be symmetrical or unsymmetrical, homoleptic or heteroleptic, should they contain aryl- or alkyl- or halo-group, etc.?), many different systems seem to have been studied. One of the main difficulties seems to arise from the necessary balance between a value of the second hyperpolarizability wð3Þ as large as possible (for device application) and a linear absorption coefficient a as low as possible (457). Although several authors have reported on symmetrical and unsymmetrical homoleptic dithiolene complexes for third-order NLO applications (368, 436– 440), the most complete and significant studies have been carried out by Underhill and co-workers (397, 431, 433, 434, 441–448). Two of these systems are shown in Scheme 25. One of the conclusions (434) is that an optimized material should absorb 800 nm (433) in order to have the highest ratio wð3Þ /a. For obtaining the material in a useful form, it is also proposed to embed the dithiolene complexes as guests in polymers such as poly(methylmethacrylate) (PMMA) (434, 449, 450). Along this line, the most efficient material in this series is the bis[1-butyl-2-phenylethene-1,2-dithiolato(2)-S,S0 ]nickel(II) compound (29) shown in Scheme 25 (451).

462

CHRISTOPHE FAULMANN AND PATRICK CASSOUX

R S S

S

S

S

Ni

Ni S

S

S

n

R 30 Oligomers R = H, Bu

29

Scheme 25

Unfortunately, although the NLO properties of the dithiolene complex– polymer composite for loadings up to 20% in PMMA are almost identical to those observed for the corresponding dithiolene complex in solution, they are severely degraded for higher concentrations, probably due to intermolecular interaction (451). The use of sol–gel materials instead of PMMA-based composites allows an increase in the content of dithiolene complexes (452), but to the detriment of the linear absorption coefficient a. Another material processing that has been explored consists in synthesizing oligomers of dithiolene complexes such as 30 in Scheme 25, and embedding these oligomers in PMMA (453, 454). It was hoped that such a ‘‘copolymerization’’ would allow a higher concentration of the dithiolene complex without a degradation of the properties. Indeed, this technique increases the dithiolene complex content within the matrix, but the ratio wð3Þ /a still needs to be improved. Very recently, Zuo et al. (368) reported third-order NLO properties of a compound previously studied for its strong near-IR absorption, namely, (n-Bu4N)[Ni(phdt)2] (Section IV.A and 12 in Scheme 20), as well as compounds previously studied for their electrical conductivity, (n-Bu4N)[Ni(dddt)2] and (n-Bu4N)[Ni(pddt)2] (pddt2 ¼ [C5H6S4]2, 6,7-dihydro-5H-1,4-dithiepinin2,3-dithiolato) (Section II and Scheme 5). Recently, other attempts have been reported by Dai et al. (439) using the complexes 31 (Scheme 26). These authors admit that the efficiency of their materials needs to be improved by optimizing the balance between absorption and nonlinearity. Third-order NLO properties of M(dmit)2 and M(mnt)2 complexes with sandwiched organometallic cations [CpFe(Z-C6H6)]þ have also been reported very recently (440). Some heteroleptic dithiolene complexes studied for their third-order NLO properties and involving the dmit or the mnt ligands are shown in Scheme 27 (455, 456). Only the dmit-containing complexes exhibit a large third-order optical nonlinearity, due to their larger planar conjugated system compared to the mnt analogues.

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

R1

463

R2 S

S Ni

S

S R2

R1 31 R1 = H, Cl; R2 = H, Cl, Me, Et, CHMe2

Scheme 26

R L S

CN

S

CN

L

M

S

L

S

M S

S

S

CN

S

CN

M

S L

M = Ni, Pt, Pd; 2L = bpy, phen where phen = 1,10-phenanthroline

R M = Ni, Pt, Pd; R = H, Me

Scheme 27

Although none of the above mentioned systems has ever been used for the fabrication of a device with optical purpose, these studies have helped us to obtain a better insight and some guidelines for the synthesis of materials, which hopefully will play an important role in the optical communication systems. C.

Optical Information

Dithiolene complexes, and more specifically the nickel derivatives, are involved in materials used for optical data storage, such as compact disc or laser disc read-only memory (CD- or LD-ROM), and also in copiers or photography related devices. In the latter case, it is the IR-absorbing property that is exploited. Some of these compounds can be found in reviews by MuellerWesterhoff et al. (353, 354). Recent patents are given as examples in references (458–481). In the field of optical storage, short reviews were published in 1988 (482) and 1990 (362, 483). Here, the dithiolene complexes act as inhibitors of the laser-induced fading of the colored thin layers of the optical discs. They also act as an antioxidant and increase the photostability of the cyanine dyes that constitute the recording layer. Contrary to what was observed for the two

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previous topics reviewed in this chapter (conductivity and magnetism), the number of basic research-oriented publications related to the optical properties of dithiolene complex-based materials is not very high, but the number of patents devoted to their application is comparatively quite large, reflecting their interest for the fabrication of recording layers. Just for the year 1985, Nakazumi counted >50 patents (482). More recent patents are given in references (484– 524). Due to the large number of dithiolene complexes involved in these numerous patents, and since no really practical guidelines seem to have emerged from these works, it can be only noted that the complex may be either symmetrical (with the same ligand) or unsymmetrical (two different ligands). The metal can be any transition metal (but Ni derivatives are the most commonly used). The substituents on the dithiolene ligands can be either alkyl, or aryl (and substituted aryl), or dialkylamino groups, or halides. Miscelleanous related works on the optical properties of dithiolene complexes have been published, and will be cited here just for recollection: photoconductivity (525, 526); photoelectrical conversion and photoswitching (420, 527–529); and photosensitive materials (530).

V.

TOWARD MULTIFUNCTIONAL DITHIOLENE COMPLEX-BASED COMPOUNDS

Except for the M(dmit)2 systems discussed in Section II, most of the >100 molecule-based superconductors described during the past 20 years are salts derived from organic donor molecules such as TMTSF (C10H12Se4, tetramethyltetraselenafulvalene), BEDT-TTF, and others, including in particular, BEDT-TSF, usually designated as BETS (C10H8S6Se4, bis(ethylenedithio)tetraselenafulvalene). During the same period, a parallel research effort was devoted to the design and preparation of molecule-based magnets. As described above, after the discovery in 1985 of the first molecule-based ferromagnet, [FeCp2 ](TCNE) (31), several new systems having ordering temperatures above room temperature, such as V(TCNE)x
y(solvent) were introduced (336–338). Therefore, one may now consider that molecule-based superconductors and magnets are no longer extremely rare. Thus, the development of multifunctional molecular systems, such as magnetic conductors and superconductors, or conductors exhibiting optical properties, or magnetic NLO materials, has evolved as a new challenge. In these systems, the coexistence of both properties, and better still an effective interplay of these properties, is sought after from a basic research perspective as well as because these multifunctional compounds might be useful for the development of optoelectronic or magnetooptic devices. Naturally, the use of dithiolene complex-based systems for meeting these new challenges recently also has been examined.

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

A.

465

Conductivity and Magnetism

Some of the compounds discussed in the previous sections were originally synthesized with the aim of obtaining materials with coexisting or coupled properties, such as conductivity and magnetism. One of the first examples is the (NH4)[Ni(mnt)2]
H2O compound, reported in 1977 and selected for studies of its possible conductive properties (345), which were disappointingly those of a semiconductor. In addition, the magnetic properties were also extensively studied because of the ferromagnetic interactions observed in this compound (342 and references cited therein) (Section III.B.2.a). Coupled 1D electronic and magnetic properties have been reported in (Per)[M(mnt)2] complexes (131). Some of these systems undergo simultaneous Peierls and spin-Peierls transitions, but the existence of a real interplay is not yet established. This work was reviewed in Sections II and III. In Section III, we also discussed the spin-ladder behavior of dithiolene complex-based compounds, namely, (p-EPYNN)[Ni(dmit)2], (DT-TTF)2[Au(mnt)2], and [Cp2M(dmid)](TCNQF4). All of these compounds display semiconducting behavior and no interplay of electrical and magnetic properties was reported. Likewise, the first dithiolene complex-based magnet, (Cp2 Mn)[Ni(dmit)2] (350), is a semiconductor, as could be expected from its stoichiometry, and the magnetic properties of further oxidized conducting species (Cp2 Mn)x [Ni(dmit)2] remain to be determined (531). Recently, Fettouhi et al. (532) reported the synthesis of (BEDT-TTF)2[Fe(mnt)2]2, in which BEDT-TTF is expected to give rise to conductive properties, whereas [Fe(mnt)2] should afford magnetic properties. This compound behaves like a semiconductor, but the magnetic properties have not been reported yet. In conclusion, at this point one must admit that no dithiolene complex-based compound exhibits a real interplay of conductivity and magnetism. This finding should not result in some kind of inferiority complex, as in only one single Ga, or Cl Br molecule-based system, l-(BETS)2FeCl4 and derived mixed Fe species, has it been unambiguously proven that conduction electrons may interact with localized spins (533). B.

Conductivity and Optical Properties

The design and synthesis of novel materials exhibiting NLO properties is an active current research area because of possible applications in photonic technologies (404, 405). In Section IV, we reviewed the results obtained along this line when using dithiolene complexes.

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However, as far as interplay of conductivity and second-order NLO in hybrid molecular materials is concerned, very few studies are available (534, 535). As previously mentioned, both conducting and second-order NLO properties are formally connected to the same concept of charge transfer, though intermolecular in conductors but intramolecular in compounds exhibiting second-order optical nonlinearity. Attempts to associate the [Ni(dmit)2] anion with cationic cyanine dyes known to exhibit second-order optical non-linearity such as DAMSþ (4-dimethylamino-1-methylstibazolium), DAMPþ (4-dimethylamino1-methylpyridinium) and NOMSþ (40 -nitro-1-methylstibazolium) (Scheme 28)

N+

O

N+

N O

N

NOMS+

DAMS + N+

N

DAMP + N+ N

+

N N HPMS+

H3CO

MPMS+

HO Scheme 28

(536, 537) led to the preparation of semiconducting 1:1 salts where NLO properties were lost at the bulk level because of a centro-symmetrical molecular arrangement (538, 539). As a matter of fact, this could have been predicted beforehand as most of the M(dmit)2-based compounds are centrosymmetrical (9, 10). In order to overcome the difficulty related to this ubiquitous centrosymmetrical molecular arrangement, and as chirality is known to provide a means of ensuring crystallization in noncentrosymmetrical space groups, chiral chromophores such as HPMSþ and MPMSþ {HPMSþ ¼ 40 -[2-(hydroxymethyl)pyrrolidinyl]-1-methylstibazolium; MPMSþ ¼ 40 -[2-(methoxymethyl)pyrrolidinyl]-1-methylstibazolium} (see Scheme 28) have been associated with the the [Ni(dmit)2] anion (540). Disappointingly, both resulting semiconducting 1:1 salts show zero NLO efficiency, probably due to an antiparallel molecular arrangement resulting in the cancellation of the non-zero b hyperpolarisability. Likewise, the use of chiral ferrocenyl-based cations led to the

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

467

preparation of Ni(dmit)2 salts crystallizing indeed in a noncentrosymmetrical space group but having a pseudo-centrosymmetrical molecular arrangement, which induces an almost zero second-order susceptibility (540). As previously mentioned in Section IV, some dithiolene complexes might exhibit under light irradiation induced properties such as photocurrent generation or photoelectric conversion (420, 527–529). Such behaviors are discussed in details in Chapter 6 of this volume (352).

VI.

CONCLUSION

The chemistry of dithiolene complexes started in the early 1960s. These complexes may be considered today as forming one of the most important family of inorganic molecular compounds because of their unique structural, spectral, and redox properties, together with the great versatility of their chemistry for the design of molecular assemblies with functional capabilities. In particular, as shown in this chapter, they have remarkable physical properties, including metallic conductivity and superconductivity, unusual magnetic behaviors (spin ladder and magnets), and interesting optical performance such as nonlinearity. Thus, one may expect, without being too presumptuous, that such features are promising for the use of these highly stable and relatively easily prepared compounds in a variety of important applications. Indeed, a number of patents may be found in the reference list of this chapter. Naturally, some difficulties remain to be overcome. The first difficulty is related to the rather low critical temperatures at which some unusual but useful behaviors are observed, that is, superconductivity or bulk ferromagnetism. On the other hand, to date attempts to obtain dithiolene complex-based systems exhibiting the interplay of two properties (conductivity and magnetism or optics) have not led to applicable results. Nevertheless, as shown by the publication rate in the concerned areas, this research is still attracting great interest and enjoying a continuous expansion. This is certainly due to the expectations of potential practical applications, which is also related to the still necessary deepening of the understanding of the unique electronic content of dithiolene complexes. A good illustration is given by the following, unanswered question: Why, among all the numerous dithiolene complex-based compounds, do only those complexes derived from the dmit ligand display superconductivity?

ACKNOWLEDGMENTS This chapter is dedicated to the memory of Olivier Kahn, and never has such a dedication been more justified. The editor of this volume never would have asked PC to

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contribute this chapter if Olivier Kahn had not existed. Indeed, when PC went through a difficult stage in his career, it was Olivier Kahn who comforted, supported, and interested him in the field of compounds exhibiting unusual solid-state properties. We also wish to pay tribute to all co-workers, postdoc fellows, and students of the PC group, and especially Lydie Valade, who contributed to our work in the field of this review, thus increasing our expertise, to the point of making it possible for us to write this chapter.

ABBREVIATIONS 1D 2D 3C10 3D AcrHþ AF BCS bddt2 BDN BDNT bdt2 BEDT-TTF BEDT-TSF BETS BMDT-TTF bpy cdc2 CD-Rom CDW Cp Cp* D–A DAMPþ DAMSþ dc dcit2 dcmdtcroc2 dddt2

One dimensional Two dimensional Tridecylmethylammonium, [(C10H21)3NCH3]þ Three dimensional Acridinium, [C13H10N]þ Antiferromagnetic Bardeen, Cooper, and Schrieffer 4a,5,6,7,8,8a-Hexahydro-1,4-benzodithiin-2,3-dithiolato, [C8H10S4]2 Bis[(4-diethylamino)dithiobenzyl]nickel, Et2N(C6H4)(Ph)C2S2 4,9-Bis(1,3-benzodithiol-2-ylidene)-4,9dihydronaphtho[2,3-c] [1,2,5]thiadiazole) Benzene-1,2-dithiolato, [(C6H4)C2S2]2 Bis(ethylenedithio)tetrathiafulvalene, C10H8S8 Bis(ethylenedithio)tetraselenafulvalene, C10H8S4Se4 See BEDT-TSF Bis(methylenedithio)tetrathiafulvalene, C8H4S8 2,20 -Bipyridine Cyanodithiocarbimate, [C2N2S2]2 Compact disc-read-only memory Charge density wave Cyclopentadienyl, C5H5 Pentamethylcyclopentadienyl, C10H15 Donor–acceptor 4-Dimethylamino-1-methylpyridinium 4-Dimethylamino-1-methylstibazolium Direct current 3,4-Dimercapto-5-cyanoisothiazole, [C4N2S3]2 4-(Dicyanomethylene)-1,2-dimercaptocyclopent-1-ene3,5-dionato, [C8N2O2S2]2 5,6-Dihydro-1,4-dithiin-2,3-dithiolato, [C4H4S4]2

SOLID-STATE PROPERTIES OF DITHIOLENE COMPLEX-BASED COMPOUNDS

DETHQ dfcvdt2 dmbit2 dmid2 DMF dmise2 dmit2 dmpþ dmt2 dtc DT-TTF edt2 EDT-TTF FC Hc HCD HOMO HPMSþ IEDT-TTF i-mnt2 IR ITO JUL1 JUL2 KCP LB LD-Rom LUMO mdt2 medt2 MI mnt2 MO MPMSþ NCTA NIOS NIR NLO

469

N,N 0 -Diethyl-6,60 -tetrahydroquinoxalyl 2,3-Diferrocenyl-1,4-dithiin-5,6-dithiolato, {[Fe(C10H9)2]2C2H2C2S4}2 2-Thione-1,3-dithiole-benzo[d]-4,5-dithiolato, [C7H2S5]2 2-Oxo-1,3-dithiole-4,5-dithiolato, [C3OS4]2 Dimethylformamide 2-Selenoxo-1,3-dithiole-4,5-dithiolato, [C3S4Se]2 2-Thioxo-1,3-dithiole-4,5-dithiolato, [C3S5]2 N,N-Dimethylpiperidinium, [C13H10N]þ 1,3-Dithiole-2-thione-4,5-dithiolate, [C3S5]2 Dithiocarbamate Dithiopheno-tetrathiafulvalene, C10H4S6 Ethylene-1,2-dithiolate, [C2H2S2]2 Ethylenedithiotetrathiafulvalene, C8H6S6 Field cooled Critical field Hemicyanine dye Highest occupied molecular orbital 40 -[2-(Hydroxymethyl)pyrrolidinyl]-1-methylstibazolium Iodoethylenedithiotetrathiafulvalene, C8H5S6I 1,2-iso-maleonitrile-1,2-dithiolate, [(NC)2C2S2]2, 2,2-dicyanoethylene-1,1-dithiolate Infrared Indium tin oxide Bi-julodinyl nickel dithiolene Tetra-julodinyl nickel dithiolene Kalium tetracyanoplatinat (German) Langmuir–Blodgett Laser disc-read-only memory Lowest unoccupied molecular orbital 2H-1,3-Dithiole-4,5-dithiolato, [H2CS2C2S2]2 5,6-Dihydro-6-methyl-1,4-dithiin-2,3-dithiolato, [C5H6S4]2 Metal–insulator transition 1,2-maleonitrile-1,2-dithiolate, [(NC)2C2S2]2 (1,2-dicyanoethane-1,2-dithiolate) Molecular orbital 40 -[2-(Methoxymethyl)pyrrolidinyl]-1-methylstibazolium Cetyltrimethylammonium, C19H42Nþ Nonintegral oxidation state Near infrared Nonlinear optical

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NOMSþ pddt2 p-EPYNN Per phen phdt2 PMMA R2dtc R2timdt2 REM RSxant RT RXant SC SP SDW SHG SQUID Tc TCNE TCNQ TCNQF4 TDAE tdas2 TFBDM-TTF tfd2 THG TMTSF TTF ZFC

40 -nitro-1-methylstibazolium 6,7-Dihydro-5H-1,4-dithiepinin-2,3-dithiolato, [C5H6S4]2 p-N-Ethylpyridinium a-nitronyl nitroxide Perylene, C20H12 1, 10-Phenantroline 5,6-Dihydro-5-phenyl-1,4-dithiin-2,3-dithiolato Polymethylmethacrylate Dithiocarbamate 1,3-Dialkylimindazoline-2,4,5-trithione Remnant R-Thioxanthate Room temperature R-Xanthates Semiconductor Spin-Peierls Spin density wave Second harmonic generation Superconducting quantum device Superconducting critical temperature Tetracyanoethylene, C6H4 Tetracyanoquinodimethane, C12H4N4 Tetrafluorotetracyanoquinodimethane, C12F4N4 Tetrakis(dimethylamino)ethylene 1,2,5-Thiadiazole-3,4-dithiolate, [C2N2S3]2 Tetrafluorobenzo-dimethyl-tetrathiafulvalene, C12H6S4F4 1,2-Bis(trifluoromethyl)ethylenedithiolate, [(CF3)2C2S2]2 Third harmonic generation Tetramethyltetraselenafulvalene, C10H12Se4 Tetrathiafulvalene, C6H4S4 Zero-field cooled

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523. S. Maeda, Y. Kurose, and T. Ozawa, Mitsubishi Chemical Industries Co. Ltd., Japan, JP Patent 62 246 590 (1987); Chem. Abstr., 109, 83574. 524. W. Schrott, P. Neumann, and B. Albert, BASFA.-G., Germany, Ger. Offen., DE Patent 3 505 750 (1986); Chem. Abstr., 106, 25863. 525. H. Kisch, Comments Inorg. Chem., 16, 113 (1994). 526. H. Meier, W. Albrecht, H. Kisch, I. Nunn, and F. Nuesslein, Synth. Met., 48, 111 (1992). 527. Y. Umezawa, T. Yamamura, and A. Kobayashi, J. Electrochem. Soc., 129, 2378 (1982). 528. J. Zhai, L.-B. Gan, and C.-H. Huang, Appl. Surf. Sci., 140, 223 (1999). 529. W.-S. Xia, C.-H. Huang, and D.-J. Zhou, Langmuir, 13, 80 (1997). 530. Z. Peng, Z. Wang, X. Ni, Y. Li, Q. Pan, and Y. Wei, Gongneng Cailiao, 29, 183 (1998). 531. E. Rivie`re, personal communication. 532. M. Fettouhi, A. Waheed, S. Golhen, N. Helou, L. Ouahab, and P. Molinie, Synth. Met., 102, 1764 (1999). 533. H. Kobayashi, A. Kobayashi, and P. Cassoux, Chem. Soc. Rev., 29, 325 (2000). 534. P. G. Lacroix and K. Nakatani, Adv. Mater., 9, 1105 (1997). 535. K. Sutter, J. Hulliger, and P. Gu¨ nter, Solid State Commun., 74, 867 (1990). 536. S. R. Marder, J. W. Perry, and C. P. Yakymyshyn, Chem. Mater., 6, 1137 (1994). 537. S. R. Marder, J. W. Perry, and W. P. Schaefer, Science, 245, 626 (1989). 538. I. Malfant, N. Cordente, P. G. Lacroix, and C. Lepetit, Chem. Mater., 10, 4079 (1998). 539. I. Malfant, R. Andreu, P. G. Lacroix, C. Faulmann, and P. Cassoux, Inorg. Chem., 37, 3361 (1998). 540. R. Andreu, I. Malfant, P. G. Lacroix, P. Cassoux, K. Roque, E. Manoury, J.-C. Daran, and G. G. A. Balavoine, C. R. Acad. Sci., Ser. IIc: Chim., 2, 329 (1999).

NOTES ADDED IN PROOFS Since this chapter was written and during the editing procedures of this volume devoted to the dithiolene metal complexes, a review on metal 1,2-bisdithiolene complexes has been published by N. Robertson and L. Cronin, in Coord. Chem. Rev., 227, 93 (2002). Another point which deserves to be noted is the publication of the first neutral metal 1,2-bisdithiolene compound, namely Ni(tmdt)2, (tmdt2 ¼ trimethylenetetrathifulvalenedithiolate), which exhibits a metallic character down to 0.6 K (H. Tanaka, Y. Okano, H. Kobayashi, W. Suzuki and A. Kobayashi, Science, 291, 285 (2001). S

S

S

S

S

S

S

S

S

Ni S

S

S

Ni(tmdt)2

CHAPTER 9

Dithiolenes in Biology SHARON J. NIETER BURGMAYER Department of Chemistry Bryn Mawr College Bryn Mawr, PA CONTENTS I. INTRODUCTION

492

II. METAL DITHIOLENES IN NATURE A.

493

Structural Classification of Dithiolene-Containing Enzymes / 496 1. Molybdenum Enzyme Families / 496 2. Tungsten Enzyme Families / 498 3. Nomenclature Difficulties / 498 4. The Dithiolene Ligand / 499 5. Redox Reactions of Pterins / 501

III. PROPERTIES OF THE DITHIOLENE IN BIOLOGY A. B.

C.

The Dithiolene Unit as Revealed by Degradation Studies of Molybdopterin / 504 X-Ray Crystallography of the Enzyme at the Dithiolene-Molybdenum Site / 507 1. Crystal Structures of the XDH/XO Family / 508 2. Crystal Structures of the SO Family / 510 3. Crystal Structures of the DMSOR Family / 511 4. Crystal Structures of the AOR Family / 514 Spectroscopic Probes of the Dithiolene-Molybdenum Unit / 515 1. Resonance Raman Spectroscopy / 515 2. X-Ray Absorption Spectroscopy / 516

Dithiolene Chemistry: Synthesis, Properties, and Applications, Progress in Inorganic Chemistry, Vol. 52 Special volume edited by Edward I. Stiefel, Series editor Kenneth D. Karlin ISBN 0-471-37829-1 Copyright # 2004 John Wiley & Sons, Inc. 491

504

492

SHARON J. NIETER BURGMAYER 3. 4. 5.

Electronic Spectroscopy / 517 Paramagnetic Spectroscopies / 518 Magnetic Circular Dichroism / 518

IV. POSSIBLE ROLES OF THE DITHIOLENE LIGAND IN BIOLOGY A. B.

519

Overview of Enzyme Mechanisms / 519 The Function of Molybdopterin / 522

V. BIOSYNTHESIS OF THE DITHIOLENE COFACTOR

527

VI. CONCLUSION

531

ACKNOWLEDGMENTS

000

ABBREVIATIONS

531

REFERENCES

532

I.

INTRODUCTION

In contrast to their long history of study within inorganic chemistry, dithiolenes have only recently become of interest to those scientists studying biological systems. The first chemical evidence that a dithiolene chelate is involved in certain metalloenzymes was published in 1987 with definitive structural characterization not forthcoming until 1995. Within approximately 15 years, the dithiolene was demonstrated to be a structural motif retained through evolution from archaebacteria to humans. While the discovery of dithiolenes in biology is a recent scientific accomplishment, they may represent some of the oldest ligands designed by Nature to bind transition metals for biochemical transformations. Many of the general characteristics of dithiolene ligands and their metal complexes described in other chapters of this volume are also useful to Nature. A recurring theme of dithiolene coordination chemistry is their ability to stabilize multiple redox states for a wide variety of metals. The access to multiple redox states combined with a propensity for substantial electronic delocalization likely influenced Nature’s ‘‘choice’’ of dithiolene as a chelating ligand over other potential bidentate sulfur donors. In this chapter, the occurrence, behavior, and roles of dithiolene complexes in biology are explored. This chapter opens with introductions of those biological systems that use metal dithiolenes for catalysis. Next the physical properties of the dithiolene-containing component of these systems are described. Following the behavior and characteristics of these bioinorganic dithiolene species,

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493

speculations on the role of the dithiolene in bioinorganic catalysis is discussed. Finally, the information available regarding the biosynthesis of the metal dithiolene unit is summarized.

II.

METAL DITHIOLENES IN NATURE

Biomolecules containing dithiolene ligands can be found in organisms throughout the entire kingdom of life (1). These biomolecules are enzymes involved in the catalysis of chemical transformations essential to the host organism. Dozens of enzymes have been identified that share a number of common traits (2, 3). The metal chelated by the dithiolene ligand is either molybdenum or its heavy congener, tungsten. The core dithiolene ligand is conserved, with minor variations remote from the metal-chelate site. The enzymes are, with one exception, involved in redox reactions. Members within this large group of enzymes have been referred to as molybdenum or tungsten enzymes, names that focus on the presence of the transition metals Mo or W. However, it is certainly the partnership of the metal and the unique dithiolene that is, along with the protein, responsible for the biological function in these enzymes. Molybdenum and tungsten enzymes have great importance to the biosphere. Through catalysis of simple chemical reactions, they participate in the global balance (cycling) of carbon (4–6), nitrogen (6, 7), and sulfur (8, 9) compounds. A perusal of the sampling of molybdenum and tungsten enzymes in Table I illustrates the diversity of organisms and substrates involved. For the organism dependent on one or more molybdenum enzymes, a few examples show why these enzymes are essential for the organism’s health. All plants depend on nitrate reductase to accomplish the seemingly trivial reaction of nitrate reduction to nitrite, often the first step of nitrogen assimilation into compounds required for growth (5, 22). Many bacteria use molybdenum or tungsten enzymes in anaerobic respiration where the terminal electron acceptor is a reducible molecule other than oxygen, such as nitrate (2, 50), polysulfide (51), trimethylamine oxide (33, 52) or dimethyl sulfoxide (DMSO) (2, 29, 30). Mammals have several molybdenum enzymes. In humans, genetic diseases involving these Mo enzymes lead to severe, albeit rare and usually fatal neurological problems (26). Patients do not generally survive childhood, if infancy. In contrast to the impact of molybdenum enzymes to our contemporary, largely aerobic, biogeochemistry, the biochemistry and host organisms of the tungsten enzymes likely point back to an ancient evolutionary era. The biogeochemistry during that era was anaerobic and life persisted through extracting energy from carbon and sulfur sources (53, 54). Many of the known

494

(Sulfite oxidase)

SO [Nitrate reductase (assimilatory)]

XO/XDH (Xanthine oxidase, xanthine dehydrogenase) (Aldehyde dehydrogenase) (Aldehyde oxidoreductase) (Carbon monoxide dehydrogenase)

Enzyme Family (Enzyme)

Plants, fungi, bacteria Mammals, fowl, plants

Mammals, fowl, bacteria Mammals Bacteria Bacteria

Source

Mo(O)2(MPT)(S-Cys)

Mo(O)2(MPT)(S-Cys)

þ  2 SO2 3 þ H2O ! SO4 þ 2 H þ 2 e

Mo(S)(O)(MPT) Mo(S)(O)(MCD) Mo(O)(OH)(MCD)(S-Cu)

RC(O)H þ H2O ! RC(O)OH þ 2 Hþ þ 2 e RC(O)H þ H2O ! RC(O)OH þ 2 Hþ þ 2 e CO þ H2O ! CO2 þ 2 Hþ þ 2 e

þ   NO 3 þ 2 H þ 2 e ! NO2 þ H2O

Mo(S)(O)(MPT)(OH)

Metal Environmenta

Xanthine þ H2O ! uric acid þ 2 Hþ þ 2 e

Reaction Catalyzed

TABLE I Representative Members of Dithiolene-Containing Molybdenum and Tungsten Enzymes

Fe2S2, cyt b

FAD, cyt b

2 Fe2S2, FAD 2 Fe2S2 Fe2S2, FAD

2 Fe2S2, FAD

Other Cofactorsb

26–28

22–25

13–15 16–18 19–21

10–12

References

495

Bacteria Bacteria, algae Bacteria

Mo(?)(MGD) Mo(?)(MPT) Mo(MPT)(?)

þ  2 SeO2 4 þ 2 H þ 2 e ! SeO3 þ H2O

W(O)(MPT)2

(Sx )2 þ x Hþ þ x e ! xSH þ   ClO 3 þ 2 H þ 2 e ! ClO2 þ H2 O

RC(O)H þ H2O ! RC(O)OH þ 2 Hþ þ 2 e

Mo(O)(MGD)2(OH)

þ  3 AsO 2 þ 2 H2O ! AsO4 þ 4 H þ 2 e

Bacteria

Bacteria

Mo(O)(MGD)2(O-Ser) Mo(O)(MGD)2(O-Ser) Mo(O?)(MGD)2(Se-Cys) Mo(MGD)2(S-Cys)

(Me)SO(Me) þ 2 Hþ þ 2 e ! S(Me)2 þ H2O N(Me)3 þ H2O ! ON(Me)3 þ 2 Hþ þ 2 e þ  HCO 2 ! CO2 þ 2 H þ 2 e þ    NO3 þ 2 H þ 2 e ! NO2 þ H2O

Bacteria Bacteria Bacteria Bacteria

Fe2 S2

Unknown 2 FAD, 2 cyt b

Fe4 S4

Fe3S4, Rieske Fe2 S2

None None Fe4S4 Fe4S4

49

46 47, 48

44, 45

42, 43

40, 41

29–32 33, 34 35–37 38, 39

Abbreviations used in this table: MPT designates molybdopterin; MGD designates MPT guanine dinucleotide; MCD designates MPT cytosine dinucleotide; NHis, O-Ser, S-Cys, Se-Cys designate attachments from protein residues histidine, serine, cysteine and selenocysteine. b Flavin adenine dinucleotide ¼ FAD. c Dimethyl sulfoxide ¼ DMSO.

a

(Selenate reductase)

Unclassified Enzymes (Polysulfide reductase) (Chlorate reductase)

AOR (Aldehyde ferredoxin oxidoreductase, formaldehyde ferredoxin oxidoreductase)

DMSOR (DMSOc reductase) (Trimethylamine oxidase) (Formate dehydrogenase) [Nitrate reductase (dissimilatory)] (Arsenite oxidase)

496

SHARON J. NIETER BURGMAYER

tungsten enzymes are isolated from hyperthermophilic archae and bacteria— organisms that may be our evolutionary ancestors—which thrive at elevated temperatures possibly typical of the ancient environment (3, 5). The dithiolene ligand for tungsten in these ancient metalloenzymes has changed little, if at all, through evolution as it became incorporated into the catalytic site of molybdenum enzymes in higher organisms. This conservation of the basic structure is perhaps the most convincing evidence for the value of the metal-dithiolene group in Nature, where it carries out a variety of chemical jobs. A.

Structural Classification of Dithiolene-Containing Enzymes

Dithiolene-containing molybdenum and tungsten enzymes are classified into four families according to the active site structure (2, 3, 55). These families are now described and will be used throughout the remainder of this chapter. This classification system is a very useful mechanism for making clear distinctions among the >60 molybdenum and tungsten enzymes reported thus far, as well as those that will be undoubtedly discovered in the future. This classification scheme has been made possible within the last decade through structural information from X-ray diffraction that has emerged on numerous proteins. The system is based on the common structural characteristics among the molybdoenzymes, but it incorporates some functional similarities as well. 1.

Molybdenum Enzyme Families

All of the dithiolene-containing molybdenum enzymes have a single metal at the catalytic site. For this reason, they are referred to as mononuclear molybdenum enzymes to distinguish them from nitrogenase that has a polynuclear Mo/Fe/S cluster but does not contain a dithiolene group (7, 56, 57). The mononuclear enzymes are sorted into three main families based on the number of oxo ligands and dithiolene ligands (Fig. 1) (2). The common attributes retained among the three families are the dithiolene ligand (with minor variations remote from Mo), the cycling of molybdenum oxidation states  O) in at least between IV, V, and VI, and the presence of an oxo ligand (Mo  one of the oxidation states of the enzyme in nearly every member. The latter characteristic is the reason that the mononuclear molybdenum enzymes are often referred to as oxo-molybdenum enzymes. The three main families are differentiated according to oxo versus sulfido ligation at molybdenum, the number of associated dithiolene ligands, and the type of additional prosthetic groups involved in electron transfer. The xanthine dehydrogenase/xanthine oxidase family (XDH/XO) is characterized by the presence of oxo and sulfido ligands in the Mo(VI) state, a single dithiolene ligand, an oxygen-donor ligand such as water or hydroxide, and the

DITHIOLENES IN BIOLOGY

497

S S

xanthine dehydrogenase / xanthine oxidase (XDH/XO) family

O Mo

S

OH / H2O O

S

sulfite oxidase (SO) family

Mo S

S-Cys O

DMSO reductase (DMSOR) family

O

S

X

Mo S

S

S

X = O-ser, S-cys or Se-cys

aldehyde ferredoxin oxidoreductase (AOR) family

O S S

H N

H2N

where

S

S S

O

N S

W

NH

HN

S O

S

molybdopterin (MPT)

Px Px = phosphate or dinucleotide

Figure 1. The four families of dithiolene-containing molybdenum and tungsten enzymes based on the structures of the catalytic reaction centers.

presence of iron–sulfur (Fe2 S2 ) clusters as well as FAD as electron-transfer prosthetic groups (10–20). The sulfite oxidase (SO) family displays a dioxo coordination at Mo(VI), a single dithiolene ligand, and a monodentate thiolate ligand from the amino acid cysteine that forms a covalent attachment to the protein (23–28). This family occasionally makes use of heme or FAD to assist electron transfer. The members of the third family, dimethyl sulfoxide reductase (DMSOR), are distinct from the remaining mononuclear Mo enzymes in their requirement for two dithiolene ligands, a single oxo ligand in the Mo(VI) state, and attachment to the protein via a serine, cysteine, or selenocysteine protein residue (31–42). In this family, there are some members, such as DMSOR, whose sole redox active component is the molybdenum center (31). Other

498

SHARON J. NIETER BURGMAYER

enzymes in the DMSOR family vary in their requirement for additional redox centers, which may be met by Fe4 S4 clusters or hemes (33, 35, 40). In addition to the structural differences between the three mononuclear molybdenum enzymes, these families are differentiated by their substrate reactions. The enzymes of the XO/XDH family are true hydroxylases adding an  OH group to a carbon center of the substrate. In contrast, enzymes of the SO and DMSO reductase families add to or remove from the substrate a single oxygen atom. This formal chemical manipulation of a sole oxygen atom is often referred to as oxygen atom transfer (OAT) although mechanistically the reaction may or may not be that simple. In the SO family, this OAT type reactivity operates on inorganic substrates, whereas the DMSOR family of enzymes utilizes both inorganic and organic substrates. 2.

Tungsten Enzyme Families

Tungsten enzymes are subdivided into two classes whose distinction is based on function, rather than structure (3). All but one of the tungsten enzymes identified to date have similar catalytic sites consisting of two dithiolene ligands chelating in a distorted, pseudo-trigonal-prismatic geometry (42–45). These tungsten enzymes share the common function of catalyzing the oxidation of carbon compounds but they are subdivided into those enzymes (the majority) that oxidize aldehydes to carboxylic acids and those that activate CO2 . Although the substrate reactions—aldehyde oxidation—are obviously related to those of the molybdenum enzymes in the XO/XDH family, the distinct protein sequences and different catalytic site compositions are the reasons that the tungsten enzymes are considered a separate family. Distinct from the tungsten enzymes catalyzing organic oxidation reactions is the tungsten enzyme acetylene hydratase whose function is to add a molecule of water across the triple carbon bond of acetylene (58). Included in Table I are molybdenum enzymes that are as yet unclassified due to their partial characterization (46–49, 58). These enzymes includes polysulfide reductase that accomplish sulfur reduction to sulfide (46), underlining its role in the global sulfur cycling. Chlorate and selenate reductase are examples of relatively rare enzymes using simple oxyanions of third-row elements as substrates (47–49, 58). 3.

Nomenclature Difficulties

The nomenclature used by scientists working in the mononuclear molybdenum enzyme field has been a cause of confusion and contention (1, 59). For many years, the molybdenum atom and its ligands at the catalytic site has been referred to as ‘‘the molybdenum cofactor’’ and ‘‘Moco’’ or ‘‘Mo-co’’ (1, 60, 61).

DITHIOLENES IN BIOLOGY

499

The idea that the same cofactor species operated in all Mo enzymes originated from a reconstitution assay. In this assay method, the isolated Moco from one enzyme, such as XO, is inserted into a cofactor-free mutant (Nit-1) of nitrate reductase from Neuraspora crassa, where it can reactivate or ‘‘reconstitute’’ normal nitrate reductase catalytic activity. It is now recognized that the Mo at the active site has many different coordination environments, as has been illustrated for the three Mo families in Fig. 1. In this context, the mutant nitrate reductase assay experiment is interpreted as involving some reprocessing of the inserted molybdenum cofactor from foreign enzymes to obtain the correct form of the cofactor for nitrate reductase catalysis. The Moco designation, if it is to be used, must refer to the family of sites present in Moco enzymes. A second point of confusion concerns the name given to the dithiolene ligand on all Mo and W enzymes. Rajagopalan and Johnson (62) suggested the name molybdopterin, often abbreviated MPT, which does not itself contain molybdenum but rather is named to designate the special ligand for molybdenum and, as it later turned out, also tungsten. Molybdopterin, however, is not a unique molecule, since it is found in several forms that differ in the phosphate terminus of the side chain, as specified in Table I [and see Fig. 2(c)]. In addition, the numbering of the atoms in molybdopterin ligand varies from structure to structure and paper to paper, causing confusion for readers (63). As the process of resolving these inconsistencies continues, persons delving into the field are cautioned to read with care. 4.

The Dithiolene Ligand

The dithiolene structural unit that is common and required by all Mo and W enzymes was only recently identified definitively. After several decades of active research on molybdenum enzymes, conclusive structural evidence for the dithiolene piece was available as recently as 1995 (42). Prior to the initial suggestion in 1984, based on chemical evidence, that a dithiolene was a necessary component of the so-called molybdenum cofactor, the molybdenum coordination sphere was presumed to be characterized by several thiolate or thioether ligands and one or more oxido and sulfido ligands. This coordination environment had been deduced from matching spectroscopic characteristics obtained from the enzymes with those from synthetic compounds prepared by inorganic chemists. Electron spin resonance (EPR) and extended X-ray absorption fine structure (EXAFS) were the methods most informative in this process of teasing out the metal coordination sphere, but they were not capable of revealing the dithiolene (1, 60, 64). As detailed later, the dithiolene remained hidden from analytical view until methodology for producing and characterizing the many pterin degradation products was developed (65). Throughout the evolution of our knowledge of the molybdenum site structure,

500

N

O

N H

H N

O

S

S

N

O

N H

H N

O

S S O

OPO3

P 2O5 O

HO

HO O

N

N

H 2N

N

NH

(b)

O O

N

N

N

H H H C C C OH OH OH

O P2O5

(e)

HO

O

HO

Flavin adenine dinucleotide (FAD)

HN

O

(c)

O

N

N N

N

Figure 2. The ligand common to all molybdenum and tungsten enzymes, MPT, is shown here in several formats: (a) in common stick notation; (b) as a ball and stick; (c) an orientation rotated 90 from view (b) to emphasize the spacial relationship between the pterin plane and the dithiolene–pyran ring portion; (d ) MGD in common stick notation and for comparison, (e) FAD, a common electron-transfer prosthetic group. Coordinates ˚ for the views in (b) and (c) are taken from the data deposited in the Protein Data Bank (PDB) for the 1.3-A resolution structure of DMSO reductase from Rhodobacter sphaeroides.

(d)

Molybdopterin guanine dinucleotide (MGD)

H 2N

HN

(a)

Molybdopterin (MPT)

H 2N

HN

NH2

DITHIOLENES IN BIOLOGY

501

the considerable body of Mo/S coordination chemistry from inorganic studies provided a wealth of examples for the special capability of Mo plus S pair and this was understood to be a crucial contribution to molybdenum-dependent biochemistry. The discovery of the dithiolene has augmented this view that additional specialized features of the dithiolene ligand, as compared to dithiolate ligands, are critical for the catalytic function of these Mo and W enzymes. The name molybdopterin was assigned to this unique dithiolene ligand during the early discovery of its existence, a name chosen to designate the special ligand for molybdenum that contains a pterin (62). Subsequently, it was discovered that there is not just one form of molybdopterin (1, 66). These additional derivatives are differentiated at the phosphate portion of molybdopterin where the simple monophosphate terminus of the minimal molybdopterin [Fig. 2(a)] is appended to a dinucleotide, such as is depicted for a guanosine form in view d in Fig. 2. In addition to guanine, other nucleic acids appended at the dinucleotide include adenine, cytosine, and hypoxanthine. Conventional abbreviations used are MPT for the core molybdopterin ligand, MGD, MCD, molybdopterin adenosine dinucleotide (MAD), and molybdopterin inosine dinucleotide (MID) for the guanine, cytosine, adenine, and hypoxanthine, respectively. These modified versions of molybdopterin are found solely in enzymes from bacterial sources. Note that an alternative suggestion is to redefine MPT as the metal binding pyranopterin dithiolene (1). The particular dithiolene chosen by Nature for catalytic systems may, indeed, appear very strange. The dithiolene within molybdopterin bears substituents that are more complicated than found on other dithiolenes. Figure 2 provides several views of molybdopterin to aid the reader to understand the pieces and shape of this odd ligand. The question of why Nature has evolved this particular dithiolene for molybdenum and tungsten chelation in enzymes will be addressed in Section IV following a discussion of its chemistry, structure, and properties in Section III. It will suffice here to mention several important points about its composition. First, one side of the dithiolene connects to a pterin substituent that is a redox active fragment. Second, the substituents on either side of the dithiolene are joined through a pyran ring formed from an a-hydroxyl on the phosphate side chain connected to the pyrazine ring of the pterin. Third, the sole point of variability among molybdopterin observed in all X-ray structures of either Mo or W enzymes is the composition of the structure at the phosphate terminus remote from the pterin. The simple phosphate terminus observed from higher organisms is replaced by a dinucleotide of several different nucleic acids. 5.

Redox Reactions of Pterins

As an introduction to the chemistry of the pterin piece of molybdopterin, to be elaborated in Section III, pterin redox reactivity is now addressed. Pterins

502

SHARON J. NIETER BURGMAYER O

H N

N

N H

HN

Reduced

H2N

quinonoid 6,7-dihydropterin tautomers: A, B, C - 2 e -/ 2 H +

O

5,6,7,8-Tetrahydropterin + 2 e -/ 2 H +

HN HN N

H2N

N H

O

O N

HN

Semireduced

A N

H2N

O

N

N

H N

N

HN N

N H

H2N

B

N

N

C rearrange

N H

O

rearrange

5,8-Dihydropterin

N

HN H 2N

+ 2 e -/ 2 H +

N

N H

7,8-Dihydropterin

O

Oxidized

HN 4

5

N 6

3

H2N 2 N 1

N

7

8

Pterin Figure 3. A general scheme illustrating oxidation and reductions reactions of the pterin ring system, including tautomeric forms of the semireduced states.

appear in biological systems in one of two functions, either as a pigment or as a redox component (67–69). Their participation in the second role results from their nitrogen heterocyclic structure, which supports multiple reduction levels and tautomeric forms. Figure 3 incorporates all of these possibilities into a general scheme for a simple unsubstituted pterin. On the left side of Fig. 3 are the oxidized, semireduced and most reduced forms of a simple pterin as generated in electrochemical experiments. On the right side of the diagram are the various tautomers of the semireduced, dihydro- form of pterin and their interconversion. Two major points can be extracted from the complexity of pterin redox reactions, which will be important throughout this chapter. First, there are three reduction levels for pterin—the tetrahydro-, dihydro-, and oxidized pterin—each accessed by sequential two electron/two proton processes. Second,

DITHIOLENES IN BIOLOGY

503

the intermediate semireduced dihydropterin may occur in many tautomeric structures of which the 7,8-dihydropterin is the thermodynamically most stable. Additional substitution on the pterin, such as the dithiolene in molybdopterin, may increase the number of tautomers possible and/or stabilize a particular tautomer. There is little data to suggest how these redox reactions of the pterin system in molybdopterin will change as a result of appending a dithiolene and a fused pyran ring to the pterin system. Degradation and derivative products have been isolated from some enzymes subjected to oxidizing or other denaturing conditions (see Section III) and suggest some reactivity characteristics of molybdopterin (62, 65, 66). Still, specific details of how the redox-rich pterin system of molybdopterin interacts with oxidation state changes at the molybdenum or tungsten atom, as well as by the redox active dithiolene unit, are entirely unknown. From reactivity studies on a related hydroxylated pyranopterin, it is known that the pyranopterin system behaves as a dihydropterin species, which is reduced two electrons above the oxidized core (70). Figure 4 includes both the pyran ring and the dithiolene along with possible molybdopterin interconversions based on know pterin chemistry. The intent of Fig. 4 is to suggest a variety of reactions but these certainly do not exhaust the possibilities. For molybdenum and tungsten enzymes, the unknown redox reactivity of metal-coordinated molybdopterin is one big mystery remaining to be solved.

M O HN N

H2N

H N N H

S

O

H N

HN R

O

H2N

N

N

H2N

N

N

O S HN

HO

Oxidized pterin- form

HN R

HO

N

H 2N

H N N H

S S

R

H 2N

N

H N N H

5,8-Dihydro- form

oxidation ? - 2 e- , - 2 H+

rearrangement?

M

S

reduction ? + 2 e- , + 2 H+

S

HO

Tetrahydro- form

R

HO

M

M S N

tautomerization?

S

reduction ? + 2 e- , + 2 H+

- 2 e- , - 2 H+

HN

O

S

5,6-Dihydro- form

Molybdopterin pyrano- form

O

M

M

reversible ring opening ?

S

R H2N

S

O

S

N

HN N

N H

HO

7,8-Dihydro- form

Figure 4. Some hypothetical reactions of metal-bound molybdopterin.

R

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SHARON J. NIETER BURGMAYER

III.

PROPERTIES OF THE DITHIOLENE IN BIOLOGY

It is tempting to regard the dithiolene of molybdopterin within its host proteins and ask the same questions that are posed about the dithiolene unit in small molecules. Unfortunately, the answers to these questions are not as easily obtained due to the difficulties of probing the dithiolene while it is enveloped in protein. Physical properties such as the electronic and vibrational spectroscopy, electrochemical behavior, and structural variation as a function of metal oxidation state may be elusive data and must be garnered from carefully designed experiments. Moreover, other components held within the protein may interfere with certain measurements. Since electron transfer and redox reactions are the major functions of dithiolene-containing enzymes, it is common to find other prosthetic groups included in the protein for electron transfer. Such species include Fe2 S2 and Fe4 S4 clusters, FAD, and hemes and each of these possesses its own characteristic electronic absorption spectrum. These intense spectral absorptions effectively mask the weaker features expected from the Mo- and W-bound dithiolene fragment. Removal of the metal dithiolene complex from the protein would seem to be an alternative method for collecting physical data in the absence of protein and prosthetic group interference. Unfortunately, the isolated, protein-free moiety is highly unstable and prone to decomposition. This extreme instability has limited studies on the free dithiolene metal complex. Nonetheless, considerable information on molybdopterin has been accumulated. Much of the early information on molybdopterin was gained through studies of its decomposition products formed under controlled conditions. Subsequently, enzymes have been identified that do not possess any other prosthetic groups except molybdopterin and Mo at the catalytic site and these have been investigated by several spectroscopic methods with success. For those enzymes having additional prosthetic groups, clever experimental design has produced useful spectroscopic results, for example, when the enzyme redox state is poised to dampen the interference of other strongly absorbing groups. By far, most of the structural information on molybdopterin and its binding at Mo or W has been provided by X-ray crystallography on proteins. The remainder of this section will describe how molybdopterin was identified through protein degradation studies, the picture of molybdopterin within the protein as viewed by X-ray crystallography, and finally, selected examples of how spectroscopic investigations have complemented—or contradicted—the conclusions from X-ray crystallography. A.

The Dithiolene Unit as Revealed by Degradation Studies of Molybdopterin

The first information suggestive of a dithiolene chelate for Mo in molybdoenzymes emerged from identification of the decomposition products of the

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Moco (66). The pieces of this degradation puzzle, illustrated in Fig. 5, eventually fit together to implicate a reduced tetrahydropterin substituted at the 6-position by a four-carbon chain including a dithiolene on the a and b carbons, a hydroxyl at the g carbon and a terminal phosphate group (62, 65). This novel pterin was named molybdopterin. All the species in Fig. 5 contributed in some way to deducing the nature of molybdopterin, although only a few will receive special comment below. The dithiolene group of molybdopterin was initially inferred from the formation of a degradation product named form B and the human metabolite urothione (Fig. 5) (62). Both molecules contain a thiophene ring fused to pterin at positions 6 and 7. Thiophene formation was envisioned to result from dithiolene dissociation from Mo, a cis–trans isomerization of the dithiolene  C  bond, followed by attack of the b-thiolate sulfur atom at the pterin C7 C position to accomplish an intramolecular cyclization producing the thiophene ring. When induced by heat, this cyclization reaction leads to form B, while normal biochemical metabolism in humans produces urothione. Under abnormal conditions in patients with combined oxidase deficiency who lack one or more molybdoenzymes, urothione is absent. The speculation that a thiophene product from oxidation reactions masked a latent dithiolene group was strongly supported by model chemistry where several examples of Mo-dithiolene oxidation also yielded thiophene products (Fig. 6) (71). Note that these thiophene oxidation products are only observed in model compounds when the dithiolene bears a N-heterocycle substituent, either quinoxaline or pterin (72). Stronger chemical evidence for the presence of a dithiolene in molybdopterin was obtained when the mild alkylation reagent iodoacetamide effectively trapped the dithiolene (65). This reaction yielded a derivative whose characterization by FAB mass spectrometry and nuclear magnetic resonance (NMR) was consistent with the structure shown in Fig. 5. The method appeared to leave the side chain intact and preserved the pterin oxidation state. From this experiment the view persisted that molybdopterin is a disubstituted dithiolene bearing a reduced pterin and a short chain terminated with a phosphate. The reduction state of the pterin was a point of uncertainty throughout these studies of molybopterin derivatives. The absence of fluorescence in anaerobic molybdopterin samples suggested a reduced pterin. Redox titration of XO and SO both indicated that the pterin could undergo a two-electron oxidation reaction (73, 74). Sulfite oxidase, for example, produced the fluorescence characteristic of an oxidized pterin after addition of 2 equiv of ferricyanide. However, titrating XO was problematic due to interfering redox processes of the iron–sulfur clusters. Ultimately, the proposed molybdopterin structure in Fig. 5 was verified by protein crystallography that finally revealed the pyran ring whose signature was lost during the degradation studies (42). Conclusive evidence for a dithiolene chelate at the active site came first from the protein crystal structure of a

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molybdenum enzyme

N

H2N

COOH

N

HN

oxidized pterin

N

MnO4[O]

O

N

H2N

C

N

HN

CHOHCH2OPO32-

Form A

N

heat, KI/ I2 O N

HN

heat,

human metabolism

CHOHCH2OPO32N

H2N

S

N

Form B SMe

O N

HN H2N

CHOHCH2OH N

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N

H2NCOH2C

O2 / I-CH2CONH2

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N

HN

CH2OPO32N

H2N

CH2CONH2

S

O

urothione

HO

N

trapped dithiolene

Mo O HN H 2N

N

H N N H

Mo

S S

S O

CDP

molybdopterin structure from X-ray diffraction:

O

H N

N

N H

HN H2N

S CH2OPO32HO

proposed molybdopterin structure

Figure 5. Key degradation products of molybdopterin that led to its initial proposed structure.

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2N

N C S

S

C

C S

Mo

[O]

N

S

N

S

N

C

N

S S

S

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C C

N

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(piv)HN

2NH

N N

O N C

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Mo S

O N

S S

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N N

NH(piv) NH O

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NH(piv) NH

N

[O]

C

C

N

N

N

S

C C

HN (piv)HN

N

N

piv = -C(O)C(CH3)3

Figure 6. Oxidative degradation of model molybdenum dithiolene complexes that produce thiophene compounds related to form B and urothione.

tungsten enzyme, aldehyde ferredoxin oxidoreductase, and was soon afterward confirmed for the Mo enzyme aldehyde oxidoreductase (17, 42). The mysteries of molybdopterin were to continue beyond its identity from crystallography, however, and the story of the variety of molybdopterin-containing protein structures is told in Section III.B. B.

X-Ray Crystallography of the Enzyme at the Dithiolene-Molybdenum Site

The indirect chemical evidence described above was masterfully interpreted to suggest the dithiolene chelate and substituents of molybdopterin. Nevertheless, it was protein crystallography that provided definitive proof of the intact dithiolene chelate in the molybdenum and tungsten enzymes. Improvements both in protein crystal growth, diffraction data collection, and in computation

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and solution methods have greatly accelerated the rate of appearance of new crystal structures. For example, the first crystal structure determination for a tungsten enzyme appeared in 1995; two molybdenum enzymes were reported in 1995 and in 1996; during the year 2000, 11 structure files were deposited in the PDB for six different enzymes crystallized in one or more forms. As of mid2001, 26 crystal structures have been reported, at least one for a member of each of the three classes of dithiolene-containing molybdenum enzymes and at the outset of 2003, >60 structures of molybdenum enzymes or proteins related to molybdoenzymes biosynthesis have been deposited in the PDB. There are two examples of tungsten protein structures. These are described below organized according the enzyme family categories defined in Section II. The picture of the active site in oxo-molybdenum enzymes as developed through the technique of protein X-ray diffraction studies has gradually come into sharper focus. Unlike diffraction studies of small inorganic molecules, where interatomic distances and spacial relationships are (typically) determined without ambiguity, the precise and unequivocal definition of the inner coordination sphere of Mo in enzymes has only rarely been achieved. Reasons for the uncertainty are many: heterogeneity of the protein sample, sometimes caused by the protein isolation or crystal growth conditions, less than optimal crystal diffraction and alteration in the X-ray beam that affects the resolution of the data, among others (75, 76). In particular, the appearance of conflicting structures, particularly within the DMSOR class, came as a surprise, with consensus structure(s) slowly emerging. 1.

Crystal Structures of the XDH/XO Family

There are now three different proteins of the XDH/XO family whose structures have been determined by X-ray protein crystallography. The structure of aldehyde oxidoreductase from the bacterium Desulfovibrio gigas was the first X-ray structure determined for an oxo-molybdenum enzyme (17) and has been followed by structures of XO/XDH (10) and carbon monoxide dehydrogenase (CODH) (19, 21). Although the three enzymes are placed in the same family, there are structural differences among them at the active site and at the phosphate remote from the Mo center. Aldehyde oxidoreductase and XO/dehydrogenase share the same approximate square-pyramidal geometry at the Mo atom created by an equatorial dithiolene chelate, an apical sulfido ligand, an oxo ligand trans to one dithiolene sulfur in the equatorial plane and a hydroxo or water group in the remaining equatorial site. The structure of aldehyde oxidoreductase was determined to higher resolu˚ ) and in several different forms (oxidized, sulfo, reduced, desulfo, tion (1.8 A i-PrOH bound) thereby providing a detailed description of changes at the Mo

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site (18). From this study, it was observed that the sulfido ligand was easily lost so that in the oxidized Mo(VI) form, there was only partial occupancy of the sulfido in the Mo( O)( S) core. This information is used to explain the lack of sulfido ligand observed in the XDH/XO crystal structures, despite prior evidence for such a group from EXAFS (77–79). The crystal structure with bound isopropyl alcohol indicates the likely position of substrate binding in the active site (18). The propanol hydroxyl group forms a hydrogen bond to the equatorial hydroxo/water ligand on Mo. It is assumed that when the aldehyde substrate binds in this position, a similar hydrogen bond is formed between the aldehyde oxygen thereby orienting the carbonyl oxygen for nucleophilic attack and aldehyde oxidation. Several intriguing differences were observed between the oxidized and reduced/sulfurated forms of aldehyde oxidase (18). First, there is a significantly ˚ in the oxidized different distance between the dithiolene sulfur atoms, 3.0 A ˚ form as compared to 3.5 A in the reduced form. The short S S separation has been interpreted as a partial bond, whose precedent exists in Mo bis(thiolato) complexes (80, 81). The importance of a partial disulfide resides in the possibility of dithiolene ligand-based redox that accompanies turnover or otherwise occurs in these enzymes. The longer bonds in the reduced form seem to effect a puckering of the MoS2 C2 ring of the dithiolene chelate. The pyranopterin conformation in the aldehyde oxidoreductase structure is common to all molybdopterin enzymes. A roughly 40 angle between the plane of the pterin rings (the coplanar pyrimidine and pyrazine rings) and the best plane through the pyran ring unambiguously requires saturated bridgehead carbons at the pyran–pyrazine junction (Fig. 2). The crystal structures of XDH/XO were accomplished at lower resolution ˚ ) and provide little additional detail about the Mo environment (10). The (2.1 A significance of this pair of structures resides in the topological change that accompanies the cysteine oxidation or proteolysis, which changes XDH to an oxidase. This change in XDH reactivity results from a shift in a portion of the protein chain that blocks access to the FAD by its normal external redox partner, nicotinamide adenine dinucleotide (NADþ ), thus causing the oxidized form to use oxygen as external electron acceptor. As observed in D. gigas aldehyde oxidase, the XDH/XO structures possess the sequential arrangement of the redox cofactors: Mo center; two Fe2 S2 clusters; flavin (in XDH/XO). This configuration provides important clues for establishing the purpose of the pterin substituent in molybdopterin [addressed in detail in Section IV (Fig. 12)]. The history of CODH crystallography is one illustration of how the limitations of protein X-ray crystallography can lead to erroneous interpretation. The initial structural report of CODH (19) described the active site of CODH as having several important differences from the other members of its family. It did not possess the apical sulfido ligand but instead a second oxo ligand. Second, a

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˚ from Mo, in the S-selanylcysteine residue was identified in the active site, 3.7 A space occupied by the isopropyl alcohol in aldehyde oxidase. However, a ˚ ) (21) using multiple wavelength subsequent high-resolution structure (1.1 A anomalous dispersion identified a Cu atom at the position formerly assigned to the Se atom and reassigned one of the terminal oxo ligands as a sulfide bridging the Mo and Cu atoms. The resultant Mo coordination sphere consists of a dithiolene chelate from MCD, an apical oxo ligand, an equatorial hydroxo ligand, and an equatorial  SCu group. This structure represents the first bimetallic cluster within the oxo-molybdoenzymes. Both the oxo and the  SCu group are suggested to be required for reaction of the substrate CO. These two differences at the active site notwithstanding, CODH is considered a member of the XDH/XO family primarily because of its homologous protein sequence. 2.

Crystal Structures of the SO Family

Sulfite oxidase, the prototype enzyme of this class, is the only protein in this family that has been structurally characterized by X-ray diffraction (27). The structural study was performed on chicken liver SO and this avian protein has high homology to mammalian forms (28). There is exact conservation of the amino acid sequences involved in hydrogen bonding to the active site of sulfite oxidase among all organisms and, with one exception, with all assimilatory nitrate reductases. Consistent with predictions from spectroscopy and EXAFS (78, 82), the active site structure is closely related to that observed in the XDH/ XO family proteins. A single dithiolene chelates to Mo. The molybdopterin ligand terminates with a phosphate rather than a dinucleotide. The dithiolene occupies an equatorial position in an approximate square-pyramidal geometry where the apical site is taken by the single oxo ligand. The remaining two equatorial sites are filled by the thiolate of Cys-185 and by a hydroxo or water ligand. This cysteine is conserved in all SO and nitrate reductase (NR) enzymes and its mutation to a serine, which donates a hydroxylate ligand in place of a thiolate to Mo, results in inactivation of the enzyme (83, 84). The appearance of one, not two, oxo ligands in the initial crystal structure was surprising since the diffraction crystals were grown from as-isolated protein in the oxidized form. Prior EXAFS results had detected two oxo substituents in oxidized SO protein and only one in reduced protein samples (82). Either reduction of the protein by trace sulfite during crystallization or photoreduction in the X-ray beam are the hypotheses offered for the observation of a reduced Mo in the active site. Evidence for the former involves the presence of a mixture of sulfite and the reaction product sulfate within hydrogen-bonding distance of the equatorial hydroxo/water ligand. From these interactions, it is presumed that the hydroxo/water ligand marks the site of the catalytically active oxo (unobserved crystallographically) in the oxidized Mo(VI) state that is involved

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in substrate oxidation. This interpretation places the catalytically active oxo in the equatorial plane. 3.

Crystal Structures of the DMSOR Family

Of the data sets for Mo and W enzymes containing dithiolene ligands deposited in the PDB in 2001, more than one-half described proteins within the DMSOR family, which share the characteristic of a molybdenum-bis(molybdopterin dinucleotide) unit in enzyme active sites. The DMSOR was one of the first structurally characterized molybdenum proteins (85) and this one protein has continued to be the focus of a disproportionate number of crystal structure studies (31, 86–88). Indeed, structures of DMSOR alone represent over onethird of the reported PDB structures in 2001 for dithiolene-containing Mo and W enzymes. The reason for this plethora of structural studies is the disagreement among the various reports concerning the details of the active site structure in DMSOR. Figure 7 illustrates some of the active site structures reported. The controversy generated by these conflicting Mo environments spawned many subsequent studies of DMSOR protein structures isolated in various forms and analyzed at different resolution. Rather than attempt a detailed overview or history of all the discrepancies, we focus on the converging picture of the Mo environment. Afterward, the remaining important discrepancies will be addressed. Five years after the first DMSOR structure was reported, an X-ray determi˚ ) provided key nation of DMSOR from Rh. sphaeroides at high resolution (1.3 A

Mo

O(ser)

S

O(ser)

Mo

S

S S

O

O

O S

S

S

oxidized form

O

S

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O(ser) Mo

S S

oxidized form

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oxidized form

O Mo

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dithionite reduced form

O(ser)

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O(ser)

S

Mo

S S

OH

S

dithionite reduced form

DMSOR (1.82 Å) DMSOR (2.2 Å) Rees and co-workers 1996 (85) Bailey and co-workers 1997 (87) Figure 7.

DMSOR (1.88 Å) Schneider et al. 1996 (86)

Some of the DMSO active sites observed by X-ray crystallography that are in conflict.

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SHARON J. NIETER BURGMAYER

information needed to address how many of the Mo environments shown in Fig. 7 actually exist for this enzyme (31). The high resolution of this study permitted the identification of two different Mo environments at the active site. One environment is presumed to be the normal (i.e., catalytically active) Mo environment consisting of symmetrically coordinated dithiolene chelates of two molybdopterin guanine dinucleotide ligands at a monooxo-Mo(VI) unit in the oxidized enzyme. The sixth coordination site is occupied by the hydroxylate group of a protein serine residue. A second Mo environment was refined where one of the two dithiolene chelates has dissociated, accompanied by the addition of a second oxo ligand. These two environments, shown in Fig. 8, were refined as contributors to a disordered structure with 40% occupancy of the hexacoordinate environment and 60% occupancy for the dithiolene-dissociated, pentacoordinate structure. The crystals used in this study were grown in the presence of hydroxyethylpiperazineethanesulfonic acid (HEPES) buffer and the acquired structure showed the presence of a HEPES molecule in the active site with a 60% partial occupation similar to the occupancy of the pentacoordinate structure. Accordingly, it was assumed that HEPES only bound to the pentacoordinate

˚ X-ray structure of DMSOR. (a) The Figure 8. The two molybdenum sites observed in the 1.3-A six-coordinate catalytic site; (b) the five-coordinate, inactive, oxidized site. (The views were prepared using coordinates deposited in the PDB.)

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structure. The occupation of HEPES near the active site is suggested to cause the dissociation of one dithiolene and the (subsequent) migration of the Mo atom toward the remaining molybdopterin chelate. The molybdenum atom is shifted ˚ toward the single coordinated ligand (P-pterin) in the altered form. 1.6 A With these observations, it was possible to reinterpret the prior conflicting structures and the main conclusions follow. The protein crystals may be, indeed quite easily, isolated as heterogeneous mixtures at the Mo center (89). Hetero˚ structure and is inferred geneity was induced by the HEPES buffer in the 1.3-A to result from other diol coprecipitants employed in the earlier structures. This heterogeneity is the cause of spurious positions of additional oxo ligands and variable coordination numbers observed in early structures. Therefore, the disorder at Mo remains hidden unless very high resolution data is obtained. The prior Rhodobacter capsulatus structure of Bailey and co-workers (87, 88) can be superimposed onto the hexacoordinated structure so that the second oxo group is coincident with the second Mo site, consistent with the suggestion that this oxygen atom was an artifact. The other Rh. capsulatus structure of Huber and co-workers (86) can be almost perfectly superimposed by the pentacoordinated structure with the exception that the shortened S S distance in the ˚ in the Rh. capulatus structure. This short S dissociated (Q) pterin is 2.5 A S distance strongly suggests that dithiolene dissociation was accompanied by its oxidation forming a (partial) disulfide bond. Other members of this family that have been structurally determined by X-ray diffraction include formate dehydrogenase (FDH), trimethylamine oxidase (TMAO), dissimilatory nitrate reductase(NAP), and most recently, arsenite oxidase (AsO). Only the distinctive points of their structures will be briefly described here. The active site of the oxidized form of TMAO is nearly identical to that of DMSOR with two molybdopterin ligands and a coordinated serine  OH group (33). Two oxo ligands to Mo were included in the refinement but, given the low ˚ ) and what has been learned about resolution data used in this structure (2.5 A variable O-donor ligands in DMSOR, this is unlikely to be correct. Another peculiarity of this structure is the large deviation from planarity within the Mo-dithiolene five-membered chelate ring but this observation also remains in ˚ resolution. doubt at 2.5-A ˚ ) offering a more Arsenite oxidase was solved at higher resolution (1.64 A reliable view of the active site (40). Two dithiolene chelates are symmetrically ˚ ) and a single oxo ligand is observed at 1.6 A ˚. bound at normal distances (2.4 A The absence of any other covalent link from the protein leaves the Mo as five coordinate (alanine replaces the aminoacid position normally occupied by the coordination of serine, cysteine or selenocysteine residues). This Mo environment was interpreted as indicating a reduced Mo site, possibly from photoreduction in the X-ray beam.

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FDH has been structurally analyzed in the oxidized, reduced, and nitrite˚ resolution, respectively (35). All three inhibited forms at 2.8-, 2.3-, and 2.9-A structures have two fully chelated molybdopterin ligands and the active site is highly similar to that of DMSOR with the exception that the serine  OH ligand is now occupied by a Se atom of selenocysteine. Another striking result from this study is the large change in the orientation of the molybdopterin ligands on oxidation from Mo(IV) to Mo(VI) as the pterin portion of one molybdopterin rotates nearly 30 away from the equatorial plane. Unlike oxidized DMSOR and ˚ from Mo rather TMAO, oxidized FDH was refined with a  OH ligand at 2.2 A than an oxo group at a shorter distance, producing a coordination sphere comprised of two dithiolenes, one SeCys and one hydroxyl in a trigonalprismatic geometry. By using the nitrite-bound structure to model formate ˚) binding to the Mo(VI) form, the formate proton is observed to sit close (1.5 A to the Se atom. It is hypothesized that Se protonation precedes transfer of the formate proton to the nearby imidazole base of histidine. The dissimilatory NAP active site mirrors that of FDH with the replacement of Cys for SeCys in the NAP structure (90). 4.

Crystal Structures of the AOR Family

The crystal structure of aldehyde ferredoxin oxidoreductase (AOR) from the hyperthermophile Pyrococcus furiosus was the first of any molybdenum or tungsten enzyme (excepting nitrogenase) (42). The AOR was adopted as the parent name for the family of tungsten enzymes. The structure of formaldehyde ferredoxin oxidoreductase (FOR) has recently been solved (44). The structure of AOR was important for several reasons. It showed the first detailed structural information for molybdopterin chelated through its dithiolene group to a metal and it presented two surprises to researchers in the field. The structure revealed the presence of a pyran ring bearing the dithiolene fused to the expected reduced pterin system and the presence of two, not one, molybdopterin ligands chelated to the W atom, which were related by an approximate C2 axis. The two tungsto enzyme structures, AOR and FOR, share a number of similarities. Neither AOR nor FOR have any covalent link to the protein through coordination of an amino acid residue in contrast to the subsequently discovered bis(molybdopterin) active sites observed in the DMSOR family. Both enzyme active sites use a magnesium cation to bridge the phosphate termini of the two molybdopterin ligands. Unique to FOR among all the Mo and W sites is the coordination of a calcium cation to the pterin carbonyl oxygen atom. Since ˚ ) and not aligned with the putative the calcium is remote from the W atom (7.8 A electron-transfer pathway (see below) it is believed to have a structural role in the enzyme. The FOR structure was solved at higher resolution than that of AOR but still lacks precision at the W site. The imprecise tungsten environment is

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partly due to the high electron density of the tungsten atom and partly to the heterogeneous nature of the crystal, which is believed to have a random population of tungsten in oxidation states IV, V, and VI. Hence, the precise position of the oxo ligand to tungsten remains unclear. The W O group was observed by ˚ bond distance (91). No oxo ligand was detected in the AOR EXAFS at a 1.85-A ˚ was included. structure while in the FOR refinement, electron density at 2.1 A It might be expected that some structural similarity would exist between the molybdenum and tungsten aldehyde oxidoreductases. The FOR structural study includes a glutarate-bound form of the enzyme where the glutarate is presumed to occupy the active site. When this structure is superimposed onto the structure of the propanol-bound form of the functionally related molybdoenzyme aldehyde oxidoreductase, a remarkable coincidence of atoms suggested a similarity in the mechanism of aldehyde oxidation between the monomolybdopterin Mo enzyme and the bis(molybdopterin) tungsten enzyme, a surprising result given the lack of sequence homology. C.

Spectroscopic Probes of the Dithiolene-Molybdenum Unit

Valuable spectroscopic studies on the dithiolene chelated to Mo in various enzymes have been enhanced by the knowledge of the structure from X-ray diffraction. Plagued by interference of prosthetic groups—heme, flavin, iron– sulfur clusters—the majority of information has been gleaned from the DMSO reductase system. The spectroscopic tools of X-ray absorption spectroscopy (XAS), electronic ultraviolet/visible (UV/vis) spectroscopy, resonance Raman (RR), MCD, and various electron paramagnetic resonance techniques [EPR, electron spin echo envelope modulation (ESEEM), and electron nuclear double resonance (ENDOR)] have been particularly effective probes of the metal site. Of these, only MCD and RR have detected features attributable to the dithiolene unit. Selected results from a variety of studies are presented below, chosen because their focus is the Mo-dithiolene unit and organized according to method rather than to enzyme or type of active site. 1.

Resonance Raman Spectroscopy

Resonance Raman was the first spectroscopic tool wielded to expose the dithiolene moiety in a holoenzyme (92, 93). Dimethyl sulfoxide reductase, and more recently, biotin sulfoxide reductase (BSOR) are the enzymes of choice for this method because they are free of interfering spectral absorptions from other prosthetic groups (94, 95), but SO has also been successfully investigated using RR (96). The initial RR investigations of DMSOR produced assignments based  on established spectral signatures of the Mo S, Mo  O, and dithiolene ring   C S and C C bond vibrations observed in small molecule model compounds.

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In the midst of the controversy surrounding the conflicting DMSOR active site structures from crystallography, RR produced results supporting the Mo(VI) (oxo)bis(dithiolene)(serine) and desoxo Mo(IV)bis(dithiolene)(serine) cores and offered no evidence in support of a dioxo-Mo core, the core observed in the X-ray structure of Rh. capsulatus DMSOR (86). Both the vibrational mode patterns and frequencies expected for a square-pyramidal MoS4 unit were observed in accordance with all dithiolene sulfur atoms remaining fully bound to Mo during the substrate reaction. In light of the definitive high-resolution structure of DMSOR, one can conclude that these RR results were completely accurate in describing the Mo site. Subsequently, RR was used to successfully detect structural changes between the oxidized and reduced forms of both DMSOR and BSOR that are consistent with the proposed oxygen atom transfer mechanism of the catalytic reaction (95, 97). These experiments make use of the readily measurable isotopic shifts in vibration frequency between 16 O Mo and 18 O Mo to follow the fate of the oxygen atom removed from DMSO (or BSO) by the Mo. In this way, the clean transfer of 18 O from DMS18 O to Mo(IV) to yield the oxidized form of the active site as Mo(VI) 18 O was directly observed as well as the substrate-bound intermediate, (DMS18 O)Mo(IV). Further discussion of the technique of RR applied to metal dithiolenes and dithiolene-containing enzymes is included in Chapter 4 in this volume (98). 2.

X-Ray Absorption Spectroscopy

X-ray absorption spectroscopy, primarily EXAFS and Mo K-edge methods, has a long history of revealing atoms and coordination numbers for metals in biological systems and has been especially useful in molybdenum enzymes where the Mo atom is attached to a variety of heavy (i.e., S) and light (i.e., O) atoms at varying distances (1, 77–79). It enjoyed the preeminent position of being the only technique able to supply metrical information until techniques of X-ray protein crystallography had advanced to their current productive stage. The EXAFS experiments first identified the distinctive metal coordination environments that now are used to distinguish the XDH/XO and SO enzyme families and earlier reviews of Mo and W enzymes are replete with references to this work. Given the covert problems of active site heterogeneity due to instability under crystallization conditions and photoreactions, it is clear that the XAS technique still has a unique and valuable capability for producing precise metal–ligand distances and coordination environments, albeit as an average, that can complement the protein structural work (76). Two examples will be presented in the following paragraphs. The XAS molybdenum K-edge spectroscopy gives unequivocal evidence for the six-coordinate structures of the active site in DMSOR (99) and in the closely

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related BSOR (100). The spectra obtained for the oxidized and dithionitereduced forms are consistent with a description of the Mo(VI) and Mo(IV) states as oxo-Mo(VI)-bis(dithiolene)-serinate and aqua/hydroxo-Mo(IV)-bis(dithiolene)-serinate, respectively. The XAS results were important because they predicted the correct active site structures for DMSOR prior to the highresolution X-ray crystal structures that later would resolve the earlier discrepancies between DMSOR structural analyses (31). A striking example of the power of XAS to reveal structural features missed by X-ray diffraction studies was recently described for FDH (101). Use of Mo ˚ ) but significant and Se K-edge EXAFS permitted detection of a long (2.12 A Se S interaction between the Mo-coordinated selenocysteine and an adjacent dithiolene sulfur uniquely in the oxidized form of the enzyme. A Mo O group is also present in the oxidized form. Both of these moieties, the Mo O and the Se S bonds, disappear on reduction. The interpretation is that a reversible Se S bond formation, a ligand-based redox reaction, may have mechanistic import for this enzyme. This study was the first report of ligand-based redox observed in an intact Mo or W enzyme. Consideration of reversible Se S bond formation together with the desoxo structure observed in oxidized Escherechia coli FDH leads to the hypothesis that ligand-based redox may replace Mo-based redox in FDH enzymes and that the oxidized form of the enzyme contains a desoxo Mo(IV), not a desoxo Mo(VI), as suggested in the original crystallographic interpretation (35). 3.

Electronic Spectroscopy

Use of the chemist’s routine tool of electronic spectroscopy to study Mo and W enzymes had been a fruitless endeavor because of the antagonistic problems of intense absorptions from hemes, Fe2 S2 clusters, and flavin. One way to circumvent these problems is to remove the interfering prosthetic group. This strategy was applied to sulfite oxidase, where tryptic cleavage of the heme domain allowed detection of the S(cys)- Mo(VI) charge-transfer absorption at 480 nm (83). This absorption was observed to disappear in a catalytically dead mutant where serine replaces the ligated cysteine, demonstrating the requirement of a third thiolate donor for proper electronic tuning of the Mo site. A second way around the interference of strongly absorbing groups is to study an enzyme that does not require them. Electronic spectroscopy recently provided important results in a study of different forms of DMSOR. Visible spectroscopy tracked changes due to active site inactivation induced by the certain buffers and by oxygen (89). Sulfonate buffers, especially HEPES, can occupy the active site causing the loss of low-energy absorption at 720 nm attributed to dithiolene dissociation by analogy to mono(dithiolene) enzymes. Dithiolene loss interferes with DMS oxidation (backward reaction) but not

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DMSO reduction (forward reaction). Oxygen is required for dithiolene loss and inactivation in the presence of HEPES. The investigation proved that dithiolene dissociation, a structural feature observed in some early DMSOR X-ray structures, is an artifact and not indicative of structural changes involved in the catalytic cycle. The study concludes with the warning that, because the O2 damaged species lack characteristic long wavelength absorptions, these species can easily escape detection and may have been present in earlier spectroscopic work, thereby explaining some of the conflicting results in the literature. 4.

Paramagnetic Spectroscopies

The paramagnetic nature of the intermediate Mo(V) redox state has been probed by EPR, ESEEM, ENDOR, and MCD. Historically, EPR was the first technique to give any information regarding the molybdenum center in the enzymes (12). For example, the presence of thiolate ligands was indicated by the high g values and coupling to nearby protons was proved using isotopically labeled samples (102, 103). Recently, an EPR study has detected a new signal attributed to a trihydropterin radical in aldehyde dehydrogenases (103). Pulsed and double resonance EPR techniques (ESEEM, ENDOR) have the capability of detecting nearby nuclei (105–108). The ESEEM technique has detected coordinated phosphate at Mo in SO and quantified proton nuclei in SO (109) while ENDOR shows promise for revealing protons of coordinated cysteine (110). 5.

Magnetic Circular Dichroism

Of the paramagnetic techniques listed above, magnetic circular dichroism (MCD) is one of the few techniques that can identify electronic structure specific to the dithiolene chelate bound to Mo/W (111, 112). Since at this point only a handful of studies have been accomplished, the salient points of each will be summarized. The first enzyme investigated by MCD was DMSOR where the Mo center is the sole chromophore (113). The Mo(V) state of DMSOR is formed only in substoichiometric amounts, making it impossible to study its features by electronic spectroscopy or MCD. An inactive, glycerol-inhibited form of the enzyme trapped in the Mo(V) state, which was robust under the required experimental conditions, was studied instead. The resultant MCD spectrum consisted of six transitions whose behavior (temperature dependence, magnetization) mapped precisely onto the expected six transitions of a dithiolene chelated to Mo. These results were duplicated for the tungsten enzyme AOR in P. furiosus (114). However, note that the interpretation of these results was based on the (then) prevailing hypothesis that DMSOR was a monomolybdopterin

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enzyme. In light of the eventual crystallographic evidence for two dithiolene chelates on Mo, the interpretation may be more complicated and merit revision. Still, it demonstrates the capability of the MCD technique to ‘‘see’’ electronic structure wherein is buried orbital information from beyond the first coordination sphere, that is, the technique can distinguish a dithiolene from two thiolates. Quite recently MCD has been successfully employed with two enzymes, XO (115) and SO (116), which possess additional prosthetic groups. In the SO study, a two-electron reduction of the enzyme using the natural substrate sulfite under anaerobic conditions trapped the Mo(V)–Fe(II) state, where both the Mo and heme iron are reduced by one electron (116). The resultant absorption spectrum showed negligible interference from the heme. A surprising lack of low energy S charge-transfer transitions absorptions (<17,000 cm1 ) were detected where Mo (CT) are expected for both the molybdenum-dithiolene unit and the thiolate of coordinated cysteine. This result was interpreted with respect to the orientation and geometry of the dithiolene and cysteinate ligands. The extent of CT between cysteinate sulfur and Mo depends on favorable overlap, which in turn depends on the oxo-Mo-S(cys)-Ca dihedral angle. The spectroscopic results confirm the orthogonality of this angle as observed in the crystal structure. Similarly, dithiolene-based CT absorption intensities depend on orbital mixing of dithiolene sulfur and Mo based on spectroscopic benchmarks from model compounds. On this basis, the oxo-Mo-S(dithiolene) angles of 109 observed in the crystal structure are expected to produce appropriate orbital mixing and significant CT absorptions. The reasons why this expectation is not met are not yet understood. Since XO possesses both a FAD and a Fe2 S2 cluster that threaten severe spectral interference, useful MCD spectra were obtained by trapping the Mo(V) intermediate when bound to a nonnatural purine substrate (115). The resulting spectrum showed a remarkable similarity to that of a model compound with a cis-oxo-Mo-dithiolene unit, thereby strongly suggestive of a cis-oxo-Modithiolene arrangement in this intermediate redox state of the enzyme. Because such an arrangement is in disagreement with the crystallography of aldehyde oxidoreductase (see above), which shows a cis-sulfido-mo-dithiolene active site, the proposal has been made that a conformation change is required during catalytic turnover that effectively rotates the sulfido atom out and the oxo ligand into a cis position adjacent to the dithiolene chelate plane. IV.

POSSIBLE ROLES OF THE DITHIOLENE LIGAND IN BIOLOGY A.

Overview of Enzyme Mechanisms

The mechanisms proposed for molybdenum and tungsten enzymes have focused primarily on the Mo site with little, if any, consideration of the reasons

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why the molybdopterin ligand is essential for catalytic activity. Therefore a brief overview of mechanisms proposed for various enzymes will be discussed prior to suggesting how the molybdopterin ligand might be involved. A quick perusal of the many substrate reactions is listed in Table I showing that a common feature of these biochemical transformations is a substrate redox reaction involving a net gain or loss of an oxygen atom that originates from water. The substrate reaction is coupled to molybdenum redox that, for many Mo enzymes, is well established as a cycle between Mo(IV) and Mo(VI) states through an intermediate Mo(V) in part of the catalytic cycle. Coupling substrate and oxygen-based redox with a catalytic Mo (or W) center is most frequently described by two half reactions as mechanistic steps. The first of these is OAT (Eq. 1) and the second is coupled electron proton transfer (CEPT) (Eq.2). IV  O MVI   O þ substrate ! M þ substrate IV VI  þ O þ 2H þ 2e M þ H2 O ! M  

ð1Þ ð2Þ

Figure 9(a) illustrates how a combination of OAT (117) and CEPT (118) describes the substrate reaction catalyzed by SO and accounts for observed intermediates. Note that the regeneration of the oxidized Mo(VI) state by a CEPT process on the left side of Fig. 9(a) occurs in two discrete one-electron steps via a Mo(V) state. Evidence to support participation of a Mo(V) intermediate in SO, as well as in other enzymes, comes from EPR spectroscopy (102, 103). In the case of SO, the electrons lost in the CEPT process are sequentially transferred to a Fe(III) center of heme b located within the protein and subsequently to cytochrome c (61). A cycle for substrate reduction can be devised by beginning at the reduced Mo(IV) state and reversing the direction of OAT and CEPT steps, as illustrated for the nitrate reductase reaction in Fig. 9(b). Electrons are provided to reduce Mo(VI) to Mo(IV) via cytochrome b and FAD. The above schemes work reasonably well for certain enzyme reactions, especially for substrates where oxygen addition/loss occurs at a main group element (e.g., N, S, Se, Cl, see Table I). In addition to SO and nitrate reductase, key examples are DMSOR, trimethylamine oxide reductase, chlorate reductase, and selenate reductase. In the case of enzymes catalyzing C-based redox reactions of organic molecules, notably XDH and aldehyde oxidase, a direct OAT step is unlikely and is replaced by mechanistic steps typical of hydroxylation (2). The essential features of the mechanism are shown in Fig. 10 for xanthine dehydrogenase/oxidase. The mechanisms presented in Figs. 9 and 10 are attractive for their simplicity and symmetry [e.g., in Fig. 9(a) and (b)] but they should not be considered definitive nor limiting models. Recent X-ray structures have provided more

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oxygen

coupled electron proton transfer

O S

atom

+ SO32-

transfer

MoVI O

S

O S

O

- H+, - e -

O S S

O S

Mo

S

O O

MoV OH O

- H+, - e -

S

- SO42-

MoIV

S O

O S S

O O S O O

S

MoIV O H

MoIV

S

H

(a)

+ H2O

coupled

(b)

electron proton transfer

- NO2-

O S S

MoVI O O S

+ H+, + e -

S

O

MoVI O

N O

O S S

MoV OH + H+, + e -

O S S - H2O

O S S

MoIV O H H

O N

MoIV O

O

O S

MoIV

S

transfer

+ NO3-

atom oxygen

Figure 9. Catalytic cycles that combine OAT and CEPT to accomplish (a) sulfite oxidation and (b) nitrate reduction. To traverse each cycle, begin at the outlined arrow.

detailed views of catalytic site environments that demand new ideas about mechanistic steps in these enzymes (see Section III). For example, the crystal structure of SO includes two molecules of sulfate near the catalytic site [Fig. 11(a)]. If one of these sulfate ions indicates the position of substrate ˚ ) to accomplish binding, the sulfite is too remote from Mo (Mo S > 5 A the inner-sphere OAT steps of the mechanism as presented in Fig. 9(a). The crystal structure reveals a network of hydrogen-bound water from the sulfite to

522

SHARON J. NIETER BURGMAYER O N

HN O S

+ O

OH

H N H

N H

xanthine

MoVI

S

S O

- H+, - e -

O S S

O

OH

S

Mo

N S

S

NH

H N H

OH

N H

V

O

Mo

S

Fe2S2

FAD

- H +, - e H N

O O S

OH2

+ H 2O

S

NH

O

S

MoIV

O

N H

Mo

S

SH

N H

O

SH O HN O

H N O

N H

N H

uric acid

Figure 10. One possible mechanism for xanthine hydroxylation by XDH.

the Mo atom suggesting that a net transfer of an oxygen atom could occur through a hydrogen-bond mediated system (27). Figure 11(b) is one depiction of how an oxygen atom from water is transferred to sulfite in parallel with electron transfer to Mo and proton transfers to a hydrogen-bonded Mo O and to the water network. Other enzymes whose X-ray structures led to a consideration of other mechanistic steps are CODH (21) and FDH (35), both briefly addressed in Section III. These two enzymes share the feature of additional atoms—the— SCu group in CODH and the Se in FDH—in the active site in locations suggestive of their involvement in the substrate reactions. B.

The Function of Molybdopterin

The vast majority of studies, either on molybdenum and tungsten enzymes or on small molecule models for them, have focused on the metal and probed its

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(a) Mo SO4 2MPT

w

w

w

w

w

w SO4

2-

(b)

O S S

Mo

O VI

S

O

S

O H

S O H

O H

H

O O

Mo

IV

O

H

O O S O H O

net 2 e- transfer

H O H

OH H

H O H

O S

IV

Mo

S

O

H O

H+ transfer, re-establish H-bonds

H O H

O S O O

H OH H

Figure 11. A CEPT-based mechanism for sulfite oxidation. (a) This view, prepared from PDB coordinates, illustrates the two sulfate ions and associated hydrogen-bonded water molecules. (b) Use of second coordination sphere water as a source of an oxygen atom for forming sulfate.

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SHARON J. NIETER BURGMAYER

spectroscopic signatures, its reactivity, and its coordination environment. It should be clear, however, that the wondrously unique dithiolene ligand in these metalloenzymes must play a significant role to explain why it has been conserved through evolution as an essential cofactor for so many organisms. Experiments that directly probe this role have not yet been devised, so we can only deduce what this role might be based on clues from protein crystallography and from related pterin molecules in biology. Elsewhere in biology, enzyme cofactors having a pterin or pteridine system function either in molecular oxygen activation (e.g., tetrahydrobiopterin and flavin) or in methyl group transfer (e.g., folates) (119–121). Both processes involve oxidation and reduction of the pterin or pteridine ring system. With this background of pterin chemistry established, it is difficult not to consider some similar redox role for the pterin within molybdopterin in the molybdenum and tungsten enzymes. Evidence in support of this redox role for molybdopterin has come from protein crystallography. In every protein structure that includes iron–sulfur clusters as additional prosthetic groups, either a direct or short hydrogen-bonded interaction exists to connect a nitrogen atom of the pterin to a sulfur ligand of the cluster. Several examples are illustrated in Fig. 12. The two structures at the top of Fig. 12, aldehyde oxidoreductase (AO) and CODH, exhibit the same hydrogen-bonding between the pterin amino group and the nearby iron–sulfur cluster. A second iron–sulfur cluster occupies the same relative position in the protein for both AO and CODH . This second iron–sulfur cluster is positioned to transfer electrons to an adjacent FAD molecule in CODH, whereas the FAD binding domain is absent in AO. The structure of the catalytic site of AOR at the bottom of Fig. 12 illustrates a different configuration of molybdopterin- iron– sulfur cluster interaction. Here, a hydrogen-bond is formed between a nitrogen atom in the middle, pyrazine ring of pterin to a cysteinyl sulfur atom bound to an iron of an cubane iron–sulfur cluster. The implication of these structures, impossible to ignore, is that the pterin serves as the conduit to pass electrons gained at the metal through substrate oxidation to waiting electron accepting iron–sulfur clusters that in turn transfer the electrons to a flavin, if present, or the external electron acceptor. While the evidence is undeniable for electron transfer via the pterin system for enzymes in the XO/XDH and AOR families, comparable structural features are not observed in SO. The additional electron-transfer group, the heme, is ˚ ) prohibiting an quite distant from the pterin ring system (Mo Fe  32 A efficient electron transfer between these cofactors in the solid state. Because a flexible polypeptide chain connects the two domains housing the heme and the Moco, one postulation under investigation is that in solution the heme domain moves to position the heme closer to the pterin system to receive electrons during catalysis.

DITHIOLENES IN BIOLOGY

Mo

525

Cys

3.5 Å Fe2S2 cluster MCD

Cys

Aldehyde oxidoreductase

8.7 Å

CO Dehydrogenase 12.4 Å

Mo

5.4 Å

FAD

Fe2S2 clusters

MCD

Fe4S4 cluster

MPT

3.1 Å W

Formaldehyde ferredoxin oxidoreductase

Cys MPT

Figure 12. Electron-transfer pathways between Mo or W and other electron-transfer prosthetic groups in the enzymes. The views were prepared using coordinates deposited in the PDB.

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SHARON J. NIETER BURGMAYER

If crystallography has all but proved that molybdopterin is an effective ‘‘wire’’ for directing electrons away from the metal and the site of catalysis, the mechanism of how that electron transfer occurs is yet in question. One critical piece of data to be accounted for in any hypothesis is that a coulometric titration of XO shows reduction of the enzyme by six electrons and these are accounted for the molybdenum (two electrons), two iron–sulfur cluster (one electron each), and the flavin (two electrons) (122). No net reduction of the pterin is required or exhibited by this enzyme. The plethora of redox paths available to molybdopterin, some of which were shown in Fig. 4, involve primarily an opening of the pyran ring and electronic redistribution through the pi system of the pterin. Given the rigid constraints imposed on the pterin conformation by the many hydrogen bonds from protein backbone or residues, it seems highly unlikely that oxidation involving any bridgehead carbons between the pyran ring and the adjacent pyrazine ring would be favorable. The change in geometry associated with the subsequent rehybridization of these carbons would require a substantial movement of the pyran ring, including the dithiolene. One might imagine that this is triggered only after substrate binding or turnover. Recently, a proposal has been made for net electron transfer through the pterin via the sigma bonds using a ‘‘superexchange’’ mechanism (123, 124). The basis for this approach to funneling electrons through pterin originates at the dithiolene through consideration of what types of metal and dithiolene orbitals are involved in receiving electron(s) after substrate oxidation. The lowest unoccupied molecular orbital (LUMO) for electrons is a metal d orbital that feeds into sulfur p orbitals in the plane of the dithiolene chelate, that is, orbitals that participate in the sigma or in-plane pi bonding framework, not the out-ofplane pi bonding network. Electron tunneling through the sigma bonds also makes sense since they eventually are transmitted to the next electron acceptor through sigma-type hydrogen bonds. Given the elaborate structure of molybdopterin, it can be expected to serve in several roles for the metal and the enzyme. Previous paragraphs have described issues related to the putative electron transfer through the ‘‘wire’’ of molybdopterin. The ligand is also able to function as a capacitor of sorts, ‘‘storing’’ charge that may build up on the Mo during the course of catalytic turnover. This capability has been demonstrated in small molecules where a dithiolene chelate is able to absorb or release electron density in response to changing electronic character of ancillary ligands on the metal. This ability to dampen the change in electron density at the metal was dubbed ‘‘the electronic buffer effect’’ (125). It is the highly covalent bonding in a metal dithiolene that accomplishes the ‘‘buffering’’, where the redox-flexible dithiolene can effect partial reduction or oxidation the metal as the dithiolene shifts electronically between a enedithiolate and a dithioketone.

DITHIOLENES IN BIOLOGY

S

R

Mo

S

R

S

R'

Mo S

Mo+n(ene

527

R'

-1,2,-dithiolate) (oxidized metal; reduced ligand)

Mo+(n-2)(1,2-dithioketone) (reduced metal; oxidized ligand)

The facile electronic shift between the above resonance structures allows the dithiolene to participate in adjusting the redox potential of the metal center, effectively serving as a rheostat for the metal. Molybdopterin has another function besides participating in electron transfer between the site of catalysis and other electron-acceptor groups. It serves as an anchor for the active site where a multitude of hydrogen bonds between the pterin (and, if present, the dinucleotide) and the protein provide a secure tether for the reactive metal site (17). Evidence for the immobility conferred by the pterin(s) embedded in the protein is found in a comparsion of the DMSOR structures from both Rhodobacter sources. Regardless of the Mo coordination environment, the MGD ligands are nearly superimposable (75). This similarity ˚ structure, where the Mo of pterin structure is most clearly observed in the 1.3-A atom dissociated and shifted away from one pterin ligand, which otherwise was unaffected. The nucleotide tails on MGD, MCD, and other derivatives of molybdopterin also contribute to locking the molybdenum catalyst in position. Lastly, recent evidence suggests that the dithiolene may particpate in the substrate reaction in special cases. The short Se S contact in FDH (35) observed by XAS is interpreted as an interaction that possibly replaced Mo-based redox with selenide–sulfide redox.

V.

BIOSYNTHESIS OF THE DITHIOLENE COFACTOR

The final topic addressed in this chapter is the biosynthesis of the dithiolene cofactor ligand and its coordination to molybdenum and tungsten in the enzymes. Nature has clearly devised a synthetic process to overcome the twin difficulties of building a reactive dithiolene unit bearing a complicated and equally reactive pterin substituent. Molecular biology has been the tool to elucidate the steps in this complex process. Although the dithiolene formation step remains mainly a subject of conjecture, definitive information about the reagent molecule that will eventually be converted to a dithiolene is known. Several decades ago, the earliest genetic work in molybdenum enzymes identified mutants of two fungi, Aspergillus nidulans (125) and Neurospora crassa (126) that lacked all molybdenum enzyme activities, specifically, nitrate reductase, XDH, and aldehyde oxidase. The mutant N. crassa produces an

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SHARON J. NIETER BURGMAYER

sulfite oxidase

dissociation of Mo-cofactor

nitrate reductase

apo SO

apo NR

xanthine dehydrogenase

apo XDH

aldehyde oxidase

apo AO

reconstitution of Nit-1 apoprotein

dissociated Moco

Nit-1

Nit-1

Nit-1

Nit-1

active nitrate reductase Figure 13. The reconstitution of apo nitrate reductase from mutant nit-1 by dissociated molybdenum cofactor isolated from different enzymes.

inactive nitrate reductase protein, named nit-1 (126). It was determined that the inactive nit-1 consists of the intact apoprotein, but lacks the Moco and is the source of its inability to reduce nitrate. An important outcome of this finding was the development of an assay specific to the Moco using the nit-1 mutant. The assay, diagrammed in Fig. 13, consists of removing the molybdenum cofactor from a variety of molybdoenzymes, transferring the cofactor to nit-1, and quantifying the level of nitrate reduction. Moreover, this assay indicated a common Moco among the different Mo enzymes. Now it is understood that there is not a single molybdenum cofactor structure conserved among all

DITHIOLENES IN BIOLOGY

529

molybdenum enzymes but that the essential components of Mo and molybdopterin ligand(s) can be reprocessed by nit-1 to give a functional NR enzyme (1). Further genetic studies in plants, predominantly Nicotiana (tobacco), maize, and Arabidopsis thaliana, identified the participation of six genes (127). One of these genes produced protein that can reconstitute nit-1 if Mo is included in the assay, demonstrating intact molybdopterin without molybdenum bound. The inability of the other five gene products to reconstitute nit-1 is therefore due to a defect in the molybdopterin ligand biosynthesis. Parallel investigations of bacterial genetics in E. coli have elucidated the overall steps in the biosynthesis of molybdopterin (Fig. 14) (128, 129). This biosynthetic pathway consists of three main stages. The molecular predecessor of molybdopterin is a nucleic acid, a guanosine phosphate derivative. In the first stage guanosine-X-phosphate (GXP) is converted to precursor Z, where all of the carbon atoms of GXP are incorporated in the product by an as yet undetermined mechanism. The second stage produces the dithiolene by transfer of sulfur to the keto-phosphoester groups at the a,b carbons. The enzyme catalyzing this stage, MPT synthase, was proposed to use a thiocarboxylate for the sulfuration of precursor Z and this has been verified by the recently X-ray structure of MPT synthase. Two sulfur atoms must be incorporated to build a dithiolene and the hypothesis is that the thiocarboxylate of MPT synthase is resulfurated by another enzyme, MPT synthase sulfurylase before the second sulfur of MPT is added. The molybdopterin ligand is complete after this stage and the ligand remains associated with the MPT synthase protein. Since the exact nature of the dithiolene sulfurs remains unknown, several possibilities are included in Fig. 14. The third stage is the molybdenum insertion into MPT synthase for Mo chelation by molybdopterin. A similar pathway has been established for plants, albeit directed by different genes (127, 130). Once the Mo MPT synthesis is accomplished, the Moco must be transported to the apoprotein. Molybdenum cofactor so produced might use the protein responsible for the Mo insertion for transport or, as other evidence suggests, a special carrier protein may distribute Mo MPT to its attendant proteins. In humans, gene analogues to the six genes coding for Moco biosynthesis identified in bacteria and plants are known, suggesting a similar biosynthetic pathway to Mo MPT and its enzymes (131, 132). When known mammalian proteins were tested for homology with the E. coli MPT synthase protein, a surprising match was found to the human protein gephyrin (132). The established function of gephyrin is to anchor neurotransmittor receptors to the cytoskeleton, a function with little apparent correlation to Moco biosynthesis. The disparate functions notwithstanding, gephyrin has been subsequently shown to bind MPT strongly and to restore Moco biosynthesis in deficient mutants. Since the molybdopterin structure has been conserved a billion years since its first appearance in the Archaebacteria, it is possible that gephyrin was a product

530

SHARON J. NIETER BURGMAYER

O N

HN

a

guanosine-X-phosphate

N H2N N RO f O b e d c OHOH Stage 1

O HN H 2N

N

H HO OH O N O a P c d Oe b f O N O H

precursor Z

+S

MPT Synthase Sulfurylase

MPT Synthase

Stage 2

+S

O HN H 2N

N

H N

SH

N H

O

SH OPO32- molybdopterin

+ Mo source

Stage 3

O Mo X O HN H 2N

N

H N

S

N H

O

S OPO32-

Moco (excluding ancillary ligands)

+ apoproteins

molybdenum enzymes Figure 14. The three major stages of the biosynthesis of the Moco determined for bacteria, plants, and mammals.

DITHIOLENES IN BIOLOGY

531

of evolution where its original function was in Moco biosynthesis. Its better known role as neurotransmittor receptor binding in the central nervous system may be a more recent evolutionary assignment. VI.

CONCLUSION

The versatile and useful dithiolene ligand is the topic of this volume and other chapters have amply documented its impact on inorganic and materials chemistry. From the biological perspective, the value of the dithiolene and its appended pterin system must be immense for Nature to retain it unchanged through millennia, as archaebacteria evolved to homo sapiens. In fact, it seems that chemists have once again adapted for their own purposes a unit that Nature developed long ago. In light of the many discoveries of new dithiolenecontaining molybdenum and tungsten enzymes, particularly from bacteria, we can indeed wonder how many more dithiolene enzymes will be found and what biochemical roles they will play in the health of the host organism. Given the general affinity of metals for the dithiolene ligand, we can also wonder whether dithiolene-containing enzymes might exist that use metals other than Mo and W. A recent discovery of a vanadium-containing nitrate reductase (133) may foreshadow the next era of discovering new examples of metal-dithiolenes that are important to biology. ACKNOWLEDGMENTS The inspiration and support of Ed Stiefel over many years has been vital to the writing of this chapter. I also thank Paul for all his patience during this project.

ABBREVIATIONS AO AOR AsO BSOR CEPT CODH CT DMSO DMSOR ENDOR EPR ESEEM

Aldehyde oxidoreductase Aldehyde ferredoxin oxidoreductase Asenite oxidase Biotin sulfoxide reductase Coupled electron proton transfer Carbon monoxide dehydrogenase Charge transfer Dimethyl sulfoxide Dimethyl sulfoxide reductase Electron nuclear double resonance Electron paramagnetic resonance Electron spin echo envelope modulation

532

SHARON J. NIETER BURGMAYER

EXAFS FAB–MS FAD FDH FOR GXP HEPES i-PrOH LUMO MAD MCD MGD MID Moco MPT NAD Nit-1 NMR NR OAT PDB piv RR SO TMAO UV–vis XAS XDH XO

Extended X-ray absorption fine structure Fast atom bombardment mass spectrometry Flavin adenine dinucleotide Formate dehydrogenase Ferredoxin oxidoreductase Guanosine-X-phosphate Hydroxyethylpiperazineethanesulfonic acid Isopropyl alcohol Lowest unoccupied molecular orbital molybdopterin adenosine dinucleotide molybdopterin cytosine dinucleotide or magnetic circular dichroism Molybdopterin guanosine dinucleotide molybdopterin inosine dinucleotide Molybdenum cofactor Molybdopterin (or metal binding pyranopterin dithiolene) Nicotinamide adenine dinucleotide Cofactor-free mutant Nuclear magnetic resonance Nitrate reductase Oxygen atom transfer Protein Data Bank Pivaloyl Resonance Raman Sulfite oxidase Trimethylamine oxidase Ultraviolet–visible X-ray absorption spectroscopy Xanthine dehydrogenase Xanthine oxidase

REFERENCES 1. R. S. Pilato and E. I. Stiefel, in Bioinorganic Catalysis, J. Reedijk and E. Bouwman, Eds., Marcel Dekker, New York, 1999, p. 81. 2. R. Hille, Chem. Rev., 96, 2757 (1996).

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CHAPTER 10

Chemical Analogues of the Catalytic Centers of Molybdenum and Tungsten DithioleneContaining Enzymes JONATHAN McMASTER, JOSEPHINE M. TUNNEY and C. DAVID GARNER The School of Chemistry The University of Nottingham Nottingham NG7 2RD, U. K. CONTENTS I. II. III.

INTRODUCTION

540

MOLYBDENUM AND TUNGSTEN OXO-CHEMISTRY

544

DITHIOLENE COMPLEXES AND THE DEVELOPMENT OF CHEMICAL ANALOGUES OF THE CATALYTIC CENTERS OF THE MPT ENZYMES

547

A. B. IV. V.

Synthesis and Characterization / 547 Oxygen Atom Transfer / 560

DITHIOLENES THAT INCLUDE ASPECTS OF MPT

569

CONCLUSIONS

575

ACKNOWLEDGMENTS

577

ABBREVIATIONS

577

REFERENCES

578

Dithiolene Chemistry: Synthesis, Properties, and Applications, Progress in Inorganic Chemistry, Vol. 52 Special volume edited by Edward I. Stiefel, Series editor Kenneth D. Karlin ISBN 0-471-37829-1 Copyright # 2004 John Wiley & Sons, Inc. 539

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I.

INTRODUCTION

Our understanding of the structure and function of enzymes that involve molybdenum or tungsten as their catalytic center is undergoing rapid development, stimulated by recent crystal structure determinations of representative enzymes (1–24a). Molybdenum and tungsten are unique in being the only 4d (Mo) and 5d (W) transition metals that are required for the normal metabolism of biological systems; indeed molybdenum appears to be an essential trace element for all living systems. The high concentration of [MoO4]2
in sea water (1  10
2 mg L
1) means that Mo is readily available to biological systems, despite its relatively low terrestrial abundance; the concentration of [WO4]2
in sea water is much lower at 1  10
4 mg L
1. There are >40 distinct molybdenum enzymes that occur in all classes of living systems and are especially important in the biochemical cycles of carbon, nitrogen, and sulfur (24b). The majority of the molybdenum enzymes, with notable exceptions including the nitrogenases (25–28) and a 2-hydroxyglutarylCoA dehydratase (10), catalyze a conversion of the type [Eq. 1], that is, the net effect of the catalysis corresponds to the transfer of an oxygen atom to or from the substrate. QO þ 2 Hþ þ 2 e
$ Q þ H2 O

ð1Þ

Well-characterized examples of these enzymes include the following:  Dimethyl sulfoxide reductases (DMSOR) of bacteria and fungi that catalyze the reduction of DMSO to dimethyl sulfide (DMS). These enzymes play a significant role in the global sulfur cycle, not least because DMS is volatile and is the precursor of the methylsulfonate aerosols that nucleate cloud formation (29). Furthermore, the distinctive smell of DMS acts as a guide to certain seabirds who use it to locate productive regions of the ocean (30).  Various bacteria grow anaerobically using trimethylamine-N-oxide (TMAO) as an alternative electron acceptor of a respiratory transport chain (31, 32). The energy-yielding reaction involves the conversion of TMAO to tetramethylamine (TMA) catalyzed by a TMAO reductase (TMAOR).  Nitrate reductases (14) are found in a wide range of eukaryotes and prokaryotes and have a crucial role in nitrogen assimilation (33, 34) and dissimilation (35). These reductases catalyze the reduction of NO
3 to NO
2 . For the assimilatory nitrate reductases this reaction is followed by

ANALOGUES OF Mo AND W DITHIOLENE-CONTAINING ENZYMES











541

nitrite reduction to ammonia; dissimilatory nitrate reductases catalyze this reduction for respiration, to generate a transmembrane potential gradient. Sulfite oxidases (21) catalyze the physiologically vital oxidation of sulfite to sulfate; the terminal reaction in the oxidative degradation of the sulfurcontaining amino acids cysteine and methionine. In humans, a genetic deficiency of sulfite oxidase can occur for two reasons (36). The first is a defect in the synthesis of metal-binding pyranopterin dithiolene (MPT). This deficiency also affects the production of xanthine and aldehyde oxidase. The second defect affects sulfite oxidase alone and is caused by mutations in the gene-encoding for this enzyme. Individuals suffering from either genetic defect exhibit the same symptoms, including severe neurological abnormalities, mental retardation, and in several cases, attenuated growth of the brain. Xanthine oxidases occur in the liver and kidneys of humans and animals, and cow’s milk is a good source of the enzyme (16, 17, 37). These enzymes catalyze the final step in purine metabolism in primates, the production of uric acid. An excess of uric acid crystallizes in joints producing inflammation (i.e., gout); this condition can be relieved by treatment with allopurinol, an inhibitor of xanthine oxidase. Aldehyde oxidases (19) occur in the liver of mammals and catalyze the oxidation of RCHO!RCO2H; for R ¼ Me, this reaction represents the second step in the conversion of ethanol to acetic acid; the first step is catalyzed by the Zn enzyme, alcohol dehydrogenase. The CO dehydrogenase of the carboxidotrophic bacterium Oligotropha carboxidovorans is a molybdenum-containing iron–sulfur flavoprotein that catalyzes the oxidation of CO to CO2, generating a proton gradient across the cytoplasmic membrane (19, 20). Formate dehydrogenases occur in anaerobic bacteria (20) and catalyze the oxidation of formate to carbon dioxide.

Although less prominent than their molybdenum counterparts, several tungsten-containing enzymes have been isolated and characterized (2, 10, 20, 23, 24a). A notable aspect of the biochemical role of these enzymes is that many occur in hyperthermophilic archea that live at temperatures of 100  C. The known tungsten enzymes can be classified into three functional and phylogenetically distinct families, the representative members being: aldehyde ferredoxin oxidoreductase; formaldehyde ferredoxin oxidoreductase; and gyceraldehyde-3phosphate ferredoxin oxidoreductase (23). Thanks to several protein crystallographic studies and a wealth of spectroscopic information, we now have a good basis for understanding the structure– function relationships of the molybdenum and tungsten enzymes. In each

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O HN H2N

N

H N

SH

N O H MPT

SH OPO32-

Figure 1. Structure of MPT, the special ligand that binds molybdenum or tungsten at the catalytic center of enzymes.

enzyme, the metal center is mononuclear and is ligated by one or two molecules of a special cofactor, MPT (Fig. 1). Note: This cofactor was originally named ‘‘molybopterin’’ and given the abbreviation MPT by Rajagopalan et al. (38, 39) prior to its structural characterization. However, now that MPT is known to bind tungsten and its structure has been established (4, 13), it is better described as the metal-binding pyranopterin dithiolene with preservation of the abbreviation MPT. This moiety consists of a pterin core with a dithiolene group that is located on a pyran ring fused to a reduced pyrazine ring. The MPT may exist in either the mononucleotide form (where R ¼ H), as found in many eukaryotic Mo

MPT enzymes, or as the dinucleotide of adenine, cytosine, guanine, or hypoxanthine, as in prokaryotic Mo

MPT enzymes. The form of MPT found in W

MPT enzymes varies in a less predictable manner and appears to depend more on the enzyme’s functionality than its source. The nature of MPT is remarkably conserved from protein to protein; both the pyrazine and pyran rings are distinctly nonplanar and each of the three chiral carbon atoms of the pyran ring is in the (R) configuration. The pyran ring adopts a half-chair conformation that deviates significantly from the plane of the pterin system. In the enzymes, the best plane defined by the pyran ring is tilted 40 from the plane of the pterin ring. However, the relationship between the rings is not precisely determined since there is some conformational flexibility in the way the pyran ring is tilted out of the plane of the conjugated part of the pterin. Additional conformational flexibility is present in respect of the phosphorylated hydroxymethyl side chain, leading to a wide distribution of positions for the phosphate group with respect to the pterin system. In addition to providing the dithiolene group that ligates the Mo or W and modulating the properties of the metal center, especially its redox potential, the possible roles for MPT include the following:  The provision of basic sites to handle the protons that are an integral component of the oxidation (or reduction) process.  Providing a route for electron transfer to or from the Mo or W during enzyme turnover (40, 41).  Lowering the activation energy for oxygen-atom transfer by strong S!M s and p donation (42).

ANALOGUES OF Mo AND W DITHIOLENE-CONTAINING ENZYMES O

S

S

S Mo

O

Sulfite oxidase family O

S

=

HN H2N

N

H N

S

N H

O

S

R = H (MPT); R = nucleoside

S

S S Cys

OH

Xanthine oxidase family

L O Mo

O

S

S

S

S Mo

543

OPO(OH)(OR)

S

DMSOR family L = OSer for DMSOR, TMAO reductase L = SCys for dissimilatory nitrate reductase L = SeCys for formate dehydrogenase

Figure 2. A structural classification of the families of the mononuclear Mo

MPT enzymes (1).

The term ‘‘molybdenum cofactor’’ (or Moco) refers to the metal center and its inner coordination sphere. Moco is not a single, unique, moiety, rather it is a diverse collection of protein-bound sites that have certain common features. Thus, one or two MPTs are coordinated to the metal via the dithiolene group and the remainder of the metal’s coordination sphere is taken up by non-protein ligands (e.g., oxo, hydroxo, water, or sulfido groups) and, in some cases, an amino acid side chain is coordinated. Hille (1) has shown that the mononuclear Mo

MPT enzymes of molybdenum can be classified into three families, on the basis of the nature of the inner coordination sphere of the oxidized form of the enzyme (Fig. 2).  Members of the DMSOR family have the Mo ligated by two MPTs, plus a
O), sulfido (Mo

S), or selenido (Mo

Se) group and terminal oxo (Mo
the donor atom from the side chain of an amino acid residue, S of cysteine, O of serine, or Se of selenocysteine.  Members of the sulfite oxidase family involve one MPT bound to a cis-MoO2 center with an additional coordination site occupied by a cysteinyl residue.  Members of the xanthine oxidase family have MPT coordinated to a fac-MoOY(H2O) (Y ¼ O or S) center with no amino acid residue bound. All of the W

MPT enzymes so far identified involve the metal bound to two MPTs, that is, they are members of the DMSOR family although there is no homology (23). In each of the Mo

MPT and W

MPT enzymes, catalysis is effected at the metal center and the catalytic cycle involves an interconversion between the

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JONATHAN McMASTER ET AL.

M(VI) and M(IV) oxidation states. The M(V) state is generated by a oneelectron reduction of the M(VI) state, or the one-electron oxidation of the M(IV) state, and occurs during the catalytic cycle—en route to the regeneration of the catalytically active state. Spectroscopic studies of the Mo

MPT enzymes, notably electron spin resonance (EPR) investigations of the Mo(V) state, have clearly demonstrated that the substrate interacts directly with the metal center (37). The first structural characterization of a substrate-bound complex was achieved for the DMSOR from Rhodobacter capsulatus; DMS was added to the as-isolated enzyme to generate a complex with DMSO that was O-bound to the molybdenum (43).

II.

MOLYBENUM AND TUNGSTEN OXO-CHEMISTRY

An awareness of the presence of molybdenum and tungsten at the catalytic centers of MPT enzymes has stimulated development of the coordination chemistry of these metals for > 30 years (44, 45). A whole host of new compounds have been synthesized and characterized; many of these systems are chemically significant in their own right. In addition, the availability of this wide range of compounds has provided valuable calibrations of the information content and accuracy of the spectroscopic and structural techniques used to investigate the nature of the metal centers in the biological systems. As the nature of the metal centers in the MPT enzymes has become better defined, chemists have directed their efforts to synthesize compounds that are effective structural and, in some cases, functional analogues of the catalytic centers of the natural systems. Investigations of these chemical systems have provided valuable insights into the factors that determine the reactivity of the catalytic centers in the enzymes. Some aspects of the ‘‘textbook’’ chemistry of the higher oxidation states of molybdenum and tungsten (M) are clearly relevant to the roles of these metals at the catalytic centers of the MPT enzymes. Thus, one or two (M

O) or oxogroups are ubiquitous in complexes of these metals in their higher ( IV)

O; this should oxidation states. [Note: Oxo groups are generally depicted as M

not be read literally, that is, as indicative of the presence of a double bond, since there is some flexibility in the nature of these bonds since they involve one s component and up to two p components (46).] Furthermore, reduction from M(VI) to M(IV) [or M(V)] invariably involves the loss of one oxo group and vice versa. Generally, this involves the conversion of cis-dioxo{MVIO2}2þ to a monooxo {MVO}3þ (or a {MIVO}2þ) center, and vice versa. However, the conversion of a monooxo {MVIO}4þ to a desoxo M(V) [or M(IV)] center, and vice versa, has also been established. Facile oxygen atom transfer (OAT), for example, from NO
3 to a Mo(IV) center (see Fig. 3) requires a good overlap

ANALOGUES OF Mo AND W DITHIOLENE-CONTAINING ENZYMES

545

Figure 3. Overlap of the lowest occupied molecular orbital (LUMO) of the oxidant—the p* orbital of NO
3 —with the highest occupied molecular orbital (HOMO) of the reductant—the dxy orbital of an {MIVO}2þ center (47).

between the LUMO of the oxidant (here, the p* orbital of NO
3 ) and the HOMO of the reductant [here, the dxy orbital of an {MoIVO}2þ center] (47). Similar concomitant changes in the oxidation state of the Mo or W and the number of oxo groups are supported by a variety of ligands, notably S-donors, see [Eq. 2] (48–50). 1 2

O2 þ ½MoOðS2 CNEt2 Þ2  ! ½MoO2 ðS2 CNEt2 Þ2 

½MoO2 ðS2 CNEt2 Þ2  þ Ph3 P ! ½MoOðS2 CNEt2 Þ2  þ Ph3 PO

ð2Þ

An important feature of the chemistry of molybdenum and tungsten in their higher oxidation states is the linking of metal centers through m-oxo groups to form dimeric or oligomeric assemblies that are generally chemically inert. Such interactions are not possible for the molybdenum and tungsten enzymes, as the metals are embedded in the protein matrix. Moreover, the development of the chemistry of these metals has been advanced by the use of chelates that prevent the metal centers becoming linked by one (or more) m-oxo-group(s). Furthermore, the reactivity of a Mo or a W center can be controlled by the presence of a chelate, especially as this can restrict and/or direct the manner in which a potential ligand interacts with the metal center. Holm and co-workers (51, 52) developed sterically hindered NS2 - and NSdonor ligands designed to suppress the comproportionation reaction [Eq. 3] that leads to a m-oxo-molybdenum(V) dimer and showed that these systems were capable of catalyzing OAT reactions, including those involving some substrates of the Mo

MPT enzymes. fMoVI O2 g2þ þ fMIV Og2þ ! fOMoV OMoV Og4þ

ð3Þ

546

JONATHAN McMASTER ET AL.

Studies of hydrotris(3,5-dimethylpyrazol-1-yl)borate, (L-N3 ), and related tripodal N-donor ligands (53) have had a significant impact on the synthesis of small molecule analogues of the active sites of the Mo

MPT and W

MPT enzymes (54–56). These ligands provide a fac-coordination to a metal center with a pseudo-octahedral geometry and access to the three remaining sites is influenced by the nature of the substituents on the pyrazole rings. Substitution at the 3position of the (L-N3 ) pro-ligands has facilitated the isolation of a variety of monomeric M(VI), M(V), and M(IV) (M ¼ Mo or W) complexes—including systems based on {MVIO2}2þ, {MVIOS}2þ, {MVO2}þ, {MVO}3þ, and {MoIVO}2þ centers. The structural, spectroscopic, redox, and chemical properties of these systems are tuned by the nature of (a) the metal center, (b) the substituents on (L-N3 ), and (c) the coligands. Representatives of this series of compounds are capable of catalyzing OAT reactions and a catalytic cycle has been developed (Fig. 4) for the oxidation of PPh3 to Ph3PO by H2O and an external oxidant, in this case O2, as catalyzed by [(L-N3 )MoO2(SPh)] (57). This catalysis involves the transfer of an oxygen atom from an {MoO2}2þ center to a PPh3 molecule, to produce an {MoO}2þ center and Ph3PO. The first step of the catalytic cycle involves the HOMO of the phosphine ‘‘substrate’’ introducing electron density into the LUMO of the {MoO2}2þ center, a p* orbital, thereby weakening an Mo

O bond and facilitating OAT (42). The second, ‘‘spectator’’, oxo group helps drive the reaction by forming stronger s and p bonds to the resultant Mo(IV) center and the Ph3PO produced is replaced in the coordination sphere of the metal by an H2O molecule. The [(L-N3 )MoO2(SPh)] complex can be regenerated by two coupled electron–proton transfer (CEPT) processes, each involving the one-electron oxidation of the molybdenum center accompanied by a deprotonation of the water molecule, converting the {MoIVO(OH2)}2þ center,

Figure 4. (a) (57).

A catalytic cycle (b) for the oxidation of PPh3 to Ph3PO catalyzed by [(L-N3)MoO2(SPh)]

ANALOGUES OF Mo AND W DITHIOLENE-CONTAINING ENZYMES

547

via {MoVO(OH)}2þ, to a {MoVIO2}2þ center. The {MoVO(OH)}2þ center has a d1 configuration and can be detected by EPR spectroscopy. This reaction sequence was the first chemical system to manifest the full cycle proposed for OAT to a substrate, as accomplished by the Mo

MPT and W

MPT enzymes.

III. DITHIOLENE COMPLEXES AND THE DEVELOPMENT OF CHEMICAL ANALOGUES OF THE CATALYTIC CENTERS OF THE MPT ENZYMES A.

Synthesis and Characterization

Dithiolenes were introduced in the 1930s as analytical reagents for metals (58). An extensive series of investigations of d-transition metal complexes of dithiolenes commenced in the 1960s, stimulated by their facile redox chemistry, the intriguing ‘‘noninnocence’’ of these ligands, and the novel, trigonalprismatic geometry of tris(dithiolene) complexes (59). Prior to a consideration of oxo-molybdenum and oxo-tungsten bis(dithiolene) complexes, and as a link to the pyrazolylborato complexes discussed in the latter part of Section II, mention will be made of the dithiolene complexes, [(LN3 )MoO(dithiolene)] [dithiolene ¼ toluene-3,4-dithiolate (tdt) (60), benzene-1,2dithiolate (bdt) (61, 62), and quinoxaline-2,3-dithiolate (qdt) (63)]. The [(LN3 )MoO(bdt)] complex has been structurally characterized and the metal shown to have a distorted octahedral stereochemistry with fac-coordination of (L-N3 ) (59). Detailed spectroscopic studies of these [(L-N3 )MoO(dithiolene)] complexes have been carried out, in order to improve our understanding of their electronic structure and to relate this to the electron transfer and OAT reactions of the molybdenum and tungsten oxo-transferase enzymes (63). A highly covalent interaction was identified between the redox-active (or frontier) orbital 0 (dxy ) on the molybdenum and the sulfur in-plane orbital ðfa Þ of the dithiolene group. This bonding interaction would be expected to occur between the metal and the dithiolene ligand in the Mo

MPT and W

MPT enzymes. The threecenter, pseudo s-type, bonding would couple the metal’s redox-active dxy orbital with the in-plane sulfur p orbitals of MPT and play an important role in modulating the redox potential of the center by raising the energy of the metal’s dxy orbital. The Mo

O group controls the orientation of dxy and the fða0 Þ-dxy overlap is maximized when the dithiolate chelate is oriented cis to the Mo

O bond. This electronic structure analysis has led to the development of an ‘‘oxogate hypothesis’’, whereby the metal’s reduction potential and the coupling of the dxy orbital to electron-transfer (ET) pathways that involve the s system of
O bond(s) relative to the MPT are dictated by the orientation of the Mo


548

JONATHAN McMASTER ET AL.

dithiolene group (63). This oxo-gate hypothesis has important implications for the function of the catalytic centers of the Mo

MPT and W

MPT enzymes. Thus, for sulfite oxidase, the catalytically labile oxo group is located in the ‘‘equatorial’’ plane and the presence of an axial oxo group appears to be essential to facilitate the ET necessary for regeneration of the active site of the enzyme. In addition to tris(dithiolene) complexes of Mo and W (64), early investigations led to the synthesis of [MoO(mnt)2]2
(65) and a range of related monooxo bis(dithiolene) complexes of Mo(IV) and W(IV) have been prepared. The first oxo-bis(dithiolene) complexes to be structurally characterized, [PPh4][MoO(bdt)2] and [NEt4]2[MoO(bdt)2] (66), provided the first structural comparison between the two components of an MoVOS4/MoIVOS4 redox couple. [PPh4][MoO(bdt)2] was prepared by the reaction of [PPh4][MoO(SPh)4] with H2bdt (1:2) in CH2Cl2. The complex [MoO(bdt)2]2
was prepared by the reaction of K4[MoO2(CN)4  6H2O] with H2bdt (1:2) in H2O/EtOH (1:1) and isolated as its [NEt4]þ salt. The [MoO(bdt)2]
and [MoO(bdt)2]2
anions have virtually identical structures; both possess a square-pyramidal geometry (Fig. 5) with the oxygen atom at the apex and the molybdenum atom raised slightly about the basal (S4 ) plane. The dimensions of the two anions are very similar ˚ ] and Mo
˚] and the Mo

S [2.377(1)–2.388(2) A
O [1.668(3)–1.699(6) A distances closely correspond to the values obtained from molybdenum K-edge extended X-ray absorption fine structure (EXAFS) studies on reduced forms of the Mo

MPT enzymes (67). The addition of one electron to [MoO(bdt)2]
results in a slight increase in all of the metal–ligand bond lengths, while the dimensions of the bdt ligands are not changed to any significant extent. These results, when taken together with the electronic structure of {MoVO}3þ centers (46), are consistent with the electron being added to the molybdenum 4dxy

Figure 5. (66).

A comparison of the structures of the [MoO(bdt)2]
(a) and the [MoO(bdt)2]2
(b) anions

ANALOGUES OF Mo AND W DITHIOLENE-CONTAINING ENZYMES

549



O bond upon reduction is reflected orbital. The increase in the length of the Mo


1 in the decrease in the n(Mo

O) stretching frequency from 944 to 905 cm and presumably arises from a reduction in the O(2pp )!Mo(4dp ) donation. Other [MoO(dithiolene)2]2
complexes have been synthesized, including dithiolene ¼ S(R)C

C(R)S; R ¼ C(O)Ph (68) and CO2Me (69), and dithiolene ¼ 3-(triphenylsilyl)-1,2-benzenedithiolate and 5-(triphenylsilyl)-3,4toluenedithiolate (70). Analogous tungsten complexes have been synthesized, including [WO(bdt)2]n
(n ¼ 1 or 2) (71). Beyond the demonstration of the M(V)/M(IV) redox couple (see above) for several of these systems, an important development, for comparison of these systems with the nature and function of the catalytic centers of the Mo

MPT and W

MPT enzymes, has been the synthesis of [MO2(dithiolene)2]2
complexes for both metals. One convenient route to these M(VI) centers is the oxidation of the corresponding M(IV) complex with an amine oxide (e.g., Et3NO). Known examples of these M(VI) complexes include the following:  M ¼ Mo; dithiolene ¼ bdt (72, 73) mnt ¼ 1,2-maleonitrile-1,2-dithiolate (69, 74), 3-(triphenylsilyl)-1,2-benzenedithiolate or 4-methyl-1,2-benzenedithiolate (73).  M ¼ W; dithiolene ¼ bdt (71) or 2,3-napthalenedithiolate (75). The structures of several of these [MO(dithiolene)2]2
complexes have been determined, including [MoO(mnt)2]2
(73, 74), [MoO(bdt)2]2
(73), and [WO2(bdt)2]2
(71). Each of these anions has a distorted octahedral geometry,

O groups significantly longer than those cis with the M

S bonds trans to the M

2

S distances trans to the Mo
to the M

O
O groups; in [MoO2(bdt)2] , the Mo
˚ groups are 2.588(3) and 2.608(3) A and those cis to the Mo

O groups are ˚ (73). 2.417(3) and 2.434(3) A The identification of the monooxo {MVIO(OSer)(dithiolate)2} and the desoxo {MIV(OSer)(dithiolate)2} (M ¼ Mo, W) centers as key intermediates in the catalytic cycle of the DMSOR (1, 33, 76, 77) has prompted Holm and coworkers (78–80) to investigate the synthesis and properties of chemical analogues of these centers. The complexes resulting from these endeavors have provided important structural, spectroscopic, and functional benchmarks that have significantly improved our understanding of the nature and function of the catalytic centers of the DMSOR family of enzymes. Des-oxo-bis(dithiolene)M(IV) (dithiolene ¼ L ¼ bdt or edt; M ¼ Mo or W) complexes, [M(OSiPh2t-Bu)(L)2]
have been prepared by silylation of the corresponding [MO(L)2]2
complex (see Fig. 6) and characterized by Xray crystallography. Each anion possesses a square-pyramidal stereochemistry in which the metal atom is coordinated by an axial silyloxide and two dithiolene ˚ longer and the average Mo
ligands (79). The Mo

O bond is 0.14 A
S bond

550

JONATHAN McMASTER ET AL.

Figure 6. Silylation of [MO(bdt)2] to produce [M(OSiPh2t-Bu)(bdt)2] (a) and of [MO2(bdt)2] to produce [MO(OSiPh2t-Bu)(bdt)2] (b) (79, 80).

˚ shorter than their [MoO(bdt)2]2 and [MoO(edt)2]2 lengths are
0.05 A counterparts, consistent with a lowering of the order of the Mo O bond upon silylation and an associated enhancement of the Mo S dithiolene ligation (79). Treatment of [WO(bdt)2]2 with RR0 2SiCl (R ¼ R0 ¼ Me or R ¼ t-Bu, R0 ¼ Me, Ph) yields the corresponding [W(OSiRR0 2)(bdt)2] complex. Like their Mo counterparts, these anions possess a square-pyramidal geometry with a bent O Si angle, depending on the (R ¼ R0 ¼ Me) or linear (R ¼ t-Bu, R0 ¼ Ph) W steric demands of the silyloxide ligand. A stable {WVIO(OSiR3)} moiety appears to require bulky substituents; for example, treatment of [WO2(bdt)2]2 with Me3SiCl yields [WO(bdt)2Cl] and (Me3Si)2O, since [WO(OSiMe3)(bdt)2] reacts with a second equivalent of Me3SiCl (80). Silylation of [MO2(bdt)2]2 (M ¼ Mo or W) with RR0 2SiCl yields the corresponding [MO(OSiRR0 2)(bdt)2] complex (see Fig. 6) (79, 80); The metal center of each of these anions possesses a distorted octahedral geometry with the oxo- and silyloxo-groups being mutually cis. Both the [Mo(OSiPh2t-Bu)(bdt)2] and [MoO(OSiPh2t-Bu)(bdt)2] complexes are good structural analogues of the {MoIV(OSer)(dithiolene)2} and {MoVIO(OSer)(dithiolene)2} centers of the DMSOR from Rh. capsulatus and Rhodobacter sphaeroides (76). Similarly, the immediate tungsten environment of W in [WO(OSiPh2t-Bu)(bdt)2] corresponds to that (Fig. 7) of the oxidized form of the W DMSOR of Rh. capsulatus (77). Furthermore, the ultraviolet–visible (UV–vis) absorption spectra of [M(OSiPh2t-Bu)(bdt)2] (M ¼ Mo or W) (Fig. 8) possess similar profiles

ANALOGUES OF Mo AND W DITHIOLENE-CONTAINING ENZYMES

Figure 7. (77).

551

Structure of the tungsten center of the as-isolated form of W

DMSOR of Rh. capsulatus

with the absorptions being blue shifted for W versus Mo; similar comments apply to the UV–vis spectra of [MO(OSiPh2t-Bu)(bdt)2]
(M ¼ Mo or W) (79). Also, it is important to note that the UV–vis spectra of [MoO(OSiPh2t-Bu)(bdt)2]
and [Mo(OSiPh2t-Bu)(bdt)2]
manifest absorptions at wavelengths that are similar to those observed, respectively, for the oxidized and dithionitereduced Mo

DMSOR of Rh. sphaeroides (81). Similarly, the UV–vis spectrum
DMSOR of [WO(OSiPh2t-Bu)(bdt)2]
closely resembles that of (oxidized) W
of Rh. capsulatus (77).

552

JONATHAN McMASTER ET AL.

Figure 8. The UV–vis spectra of [M(OSiPh2t-Bu)(bdt)2] (a) and [MO(OSiPh2t-Bu)(bdt)2] (b) (M ¼ Mo or W). [Reproduced with permission from J. P. Donahue, C. R. Goldsmith, U. Nadiminti, and R. H. Holm, J. Am. Chem. Soc., 120, 12869 (1998). Copyright # 1998 American Chemical Society.]

Several important advances in the chemistry of Mo and W dithiolene complexes have been accomplished by Holm and co-workers (81–86), including the development of new synthetic routes to mononuclear and binuclear des-oxobis(dithiolene)M(IV) and monooxo-bis(dithiolene)M(V) complexes. The compounds [M(CO)2(S2C2R2)2] (M ¼ Mo or W; R ¼ Me or Ph) have been shown to be valuable starting materials for these syntheses and these complexes can be

ANALOGUES OF Mo AND W DITHIOLENE-CONTAINING ENZYMES

553

prepared by reacting [M(CO)3(MeCN)3] with [Ni(S2C2R2)2] (81, 87). Reactions of [Mo(CO)2(S2C2R2)2] (R ¼ Me or Ph) with a range of nucleophiles (81, 84) are summarized in Fig. 9. Thus, [Mo(CO)2(S2C2Me2)2] reacts with [NEt4]OH to produce the square-pyramidal complex, [MoO(S2C2R2)2]2
that is readily oxidized to [MoO(S2C2Me2)2]
by treatment with I2. Also, [Mo(CO)2(S2C2R2)2] (R ¼ Me or Ph) reacts with an arene oxide (ArO
) to form the corresponding des-oxo [Mo(OAr)(S2C2R2)2]
complex, the UV–vis spectra of which exhibit some similarities with that of the dithionite-reduced state of DMSOR from Rh. sphaeroides (88). Structural characterizations of several [Mo(OAr)(S2C2R2)2] complexes have established that the Mo has a squarepyramidal coordination geometry with the arene oxide in the axial position and ˚ above the S4 plane. The length of Mo

S bonds is between the metal 0.77 A ˚ ˚ ) and C
2.31 and 2.33 A; the lengths of the dithiolene C

C ( 1.33 A
S ˚ ( 1.77 A) bonds are consistent with these ligands being present at the ene-1,2dithiolate oxidation level. The axial Mo

OAr bond length varies according to ˚] the basicity of this ligand and, for R ¼ Me, the longest distance [1.933(3) A
˚ was observed for the least basic (C6F5O ) and the shortest distance [1.843(2) A] for the most basic (i-PrO
) (81, 84). Electrochemical investigations have shown that [Mo(OAr)(S2C2R2)2]
complexes exhibit two reversible redox processes, at (versus SCE) E1=2 ¼
1:95 and 0.10 V (R ¼ Me; Ar ¼ C6H3-2,6-i-Pr2) and
1.74 and 0.30 V (R ¼ Ph; Ar ¼ C6H3-2,6-i-Pr2), that are attributed to the Mo(IV/III) and Mo(V/IV) couples, respectively. The very negative potential that is required to produce the Mo(III) state of these systems clearly suggests that this oxidation state is unlikely to be accessible to corresponding molybdenum centers of the Mo

MPT enzymes. The [Mo(CO)2(S2C2R2)2] complexes react with an arylthiolate (ArS
) to yield one of two products (81) (Fig. 9). With R ¼ Me and Ar ¼ Ph, the trigonalprismatic, monocarbonyl [Mo(CO)(SPh)(S2C2Me2)2] complex is formed but, for both R ¼ Me and Ph, with Ar ¼ 2,4,6-i-Pr3C6H2S
, the monocarbonyl complex formed initially loses CO to produce the corresponding [Mo(SC6H2-2,4,6-iPr3)(S2C2Me2)2]
complex, presumably due to the steric constraints exerted by this Ar group. The [Mo(SC6H2-2,4,6-i-Pr3)(S2C2R2)2]
(R ¼ Me or Ph) complexes possess a square-pyramidal coordination geometry, with Mo

SAr ¼ ˚ , Mo
2.338(1) and 2.320(1) A
S

C(Ar) ¼ 103.7(1) and 102.6(1) , and ˚ , respectively. The coordination Mo

S(dithiolene) ¼ 2.31(7) and 2.313(6) A sphere of the Mo in these complexes is reminiscent of that proposed for the active sites of the reduced forms of the Ser147Cys mutant of the DMSOR from Rh. sphaeroides (76e) and the dissimilatory nitrate reductase from D. sulfuricans (89). The UV–vis spectra of these protein centers manifest some similarities with those of [Mo(SC6H2-2,4,6-i-Pr3)(S2C2R2)2]
(R ¼ Me or Ph) complexes.

554

Me Me

Me Me

S

S

S Mo

S

O

S

S

I2

S

S Mo

O

2-

R R

R

S

S R

R

O

S

S S

S

O

Me Me

Me Me

-

S

S Mo

O F

F

S Mo

R R

-

F

F

Ph Ph

-

Me Me

-

CH2Cl2

F

S

S

S

S

R’O-

S Mo

S

R = Me, Ph

S

S

Me Me

O S Mo

S

O

Ph Ph

Me Me

Ni

S Mo

S

S

S

S

S

S

S

Me Me

-

R = Me, Ph

S

S

S

S

CO

Ar S-

S

S

CO

S Mo

S

S Mo

S

PhSSPh LiBHEt 3

R R

Me Me

R R

S Mo

OC

R R

-

Me Me

-

R R

R R

Me Me

ArSe-

Se

S

S

CO

S

S

R = Me, Ph

S

S Mo

S

S Mo

Se CO

PhSeSePh LiBHEt 3

R = Me, Ph

R R

-

Me Me

-

S

S

Mo

S

Mo

S

Ph Ph

S

S

Ph Ph

Me Me

S

S

Me Me

PhSe-

Ph

Se

Se

Ph

Y

Y

S

S

S

S

S

S

Ph Ph

S

Mo

Ph Ph

Me Me

S

Mo

Me Me

Na 2S Li 2Se

Figure 9. Scheme showing synthetic routes to mono- and binuclear bis(dithiolene)molybdenum(IV) and (V) complexes, starting from [Mo(CO)2(S2C2R2)2] (R ¼ Me or Ph) (81, 84).

Me Me

-

Me Me

[Et 4 N]OH

[Mo(CO)3(MeCN)3] +

R

2-

Y = S, Se

2-

ANALOGUES OF Mo AND W DITHIOLENE-CONTAINING ENZYMES

555

-

Me Me

OC CO S Mo S S

S

Me Me

2-AdO-

Y

2-AdS-

S Mo S S S

LiBHEt3

Me Me

Y = O, S, Se Me Me

(2-AdSe)2 Figure 10. Synthetic route to [Mo(YAd)(S2C2Me2)2]
(Ad ¼ 2-adamantyl; Y ¼ O, S, or Se) complexes from [Mo(CO)2(S2C2Me2)2] (84).

The [Mo(CO)2(S2C2Me2)2] (R ¼ Me or Ph) complexes react with ArSe
(Ar ¼ Ph or C6H2-2,4,6-i-Pr3) to form the corresponding trigonal-prismatic [Mo(SeAr)(CO)(S2C2R2)2]
complex; that is, the CO group is retained in ˚ vs. each case, presumably because the longer Mo

SeAr bond (by 0.15 A
Mo

SAr) reduces the steric interactions that, for ArS , lead to decarbonylation. Attempts to use a [Mo(CO)2(S2C2R2)2] complex to synthesize the corresponding [MoX(S2C2R2)2]2
(X ¼ S, Se) complex, by reaction with Na2S or Li2Se, were unsuccessful and resulted in the formation of dimers containing a MoV 2 (m-X)2 core (Fig. 9) (81). In a further development of this research (84), the reaction of [Mo(CO)2(S2C2R2)2] with the nucleophiles R0 Y
(R0 ¼ Ph, i-Pr, or C6F5O, Y ¼ O; R0 ¼ 2-Ad; Y ¼ O, S or Se) (see Fig. 10) was shown to form the corresponding des-oxo [MoIV(YR0 )(S2C2Me2)2]
complex. These complexes have considerable potential as structural analogues for the reduced forms of the catalytic centers of the DMSOR and the TMAOR (Y ¼ O), the dissimilatory nitrate reductase (Y ¼ S), and the formate dehydrogenase (Y ¼ Se) (13). Thus, the spectroscopic and electrochemical properties and reactivity of these complexes will provide useful calibrations of the corresponding behavior of the catalytic centers of the MPT enzymes. The new synthetic procedure to form [W(CO)2(S2C2R2)2] (R ¼ Me or Ph), by reacting [W(CO)3(MeCN)3] with [Ni(S2C2R2)2] (87), has facilitated an exploration of the reactions of these complexes with a range of nucleophiles (see Fig. 11) (83, 85–87). Thus, [W(CO)2(S2C2R2)2] (R ¼ Me, Ph) reacts with R0O
, R0 S
, and R0 Se
to yield the corresponding, square-pyramidal, des-oxo complex, [WIV(YR0 )(S2C2R2)2]
. The crystal structures of [W(OR0 )(S2C2R2)2]
(R ¼ Ph, R0 ¼ Ph; R ¼ Me, R0 ¼ Ph, 2-Ad, i-Pr, or p-C6H4NH2) have been determined. Like their Mo counterparts, each of these tungsten centers possesses a square-pyramidal coordination geometry with the OR0 group in the axial ˚ above the S4 plane; the W

OR0 bond distances [1.826(4)– position 0.75 A ˚ 1.868(2) A] show only a slight dependency on the nature of R0 . Also, as for their

556

Me Me

R R

S

S

Me Me

X

W

O

S

S

S

S

S

S

W

O

W

O

-

-

Me Me

R R

-

R

S

S

Ph Ph

Me Me

Ni

Me Me

R'O-

S

S

S

S

O W

S

S

S

S

R

R

S S

O

W

O

W

O

S

S

S

S

Me Me

Ph Ph

-

Me Me

-

-

CH2Cl2

Me Me

S

S

R R

Me Me

S

S W S

S

S

S

CO

Y

W

S S

R R

Me Me

R R

-

-

Me Me

-

R = Me, Ph

R = Me, Ph

W

Y

Y S

S

R'Y Y = S, Se

R R

OC CO S W S S S

Me Me

-H2O

+2OH-

S

S

S W

Me Me

I2

S

S

-H2S

S

2-

S

Me Me

W

+2SH-

S

S

O

S

S

W

Me Me

S

S

Me Me

Me Me

2-

S

S

S Me Me

S

W

S S

MeMe

S22-

Figure 11. Scheme showing synthetic routes to mono- and binuclear bis(dithiolene)tungsten(IV) and (V) complexes from [W(CO)2(S2C2R2)2] (R ¼ Me or Ph) (83, 85–87).

Me Me

S

S

S

S

[W(CO)3(MeCN)3] +

R

Y = S, Se

2-

ANALOGUES OF Mo AND W DITHIOLENE-CONTAINING ENZYMES

557

Mo counterparts, the dimensions of the dithiolene ligands suggest that each is present at the ene-1,2-dithiolate oxidation level. In a similar manner (83, 86), [W(CO)2(S2C2Me2)2] reacts with R0Y
(R0 ¼ 2Ad, Ph, or C6H2-2,4,6-i-Pr3; Y ¼ S, Se) to yield the corresponding [W(YR0 )(CO)(S2C2Me2)2]
complex. In contrast to the corresponding chemistry observed for molybdenum, attempts to prepare the carbonyl-free derivatives (e.g., by using more forceful conditions) were unsuccessful. These [W(YR0 )(CO)(S2C2Me2)2]
(Y ¼ S, Se) complexes possess a distorted trigonalprismatic coordination geometry with C

C and C

S bond distances that are consistent with the dithiolenes being present as ene-1,2-dithiolates and, therefore, these are (formally) W(IV) complexes. Each complex undergoes a reversible reduction in the potential range –1.45 to –1.57 V vs. SCE that has been assigned to the W(IV/III) couple (83). The complex [W(CO)2(S2C2Me2)2] reacts with YH
(Y ¼ O or S) (83, 86) to form the corresponding square-pyramidal [WY(S2C2Me2)2]2
complex. These, formally W(IV), complexes exhibit two redox couples at potentials that vary little with the nature of Y and are attributed to the W(V/VI) and W(VI/V) couples. Both [WY(S2C2Me2)2]2
complexes react with I2 to generate a W(V) complex; for Y ¼ O, [WO(S2C2Me2)2]
is formed, but for Y ¼ S a dinuclear complex containing the W2(m2-S)2 unit is obtained (Fig. 11) (86). The [W(OPh)(S2C2Me2)2]
complex reacts with PhYH (Y ¼ S or Se) (see Fig. 12) to form [W(YPh)2(S2C2Me2)2]
. These compounds possess a distorted trigonal-prismatic geometry with the YPh ligands occupying mutually cis

OR'

-

IV

Me Me

S

W

S

S S

Me Me

Me3NO MeCN

YH

(PhCH2S)2S -

-

Y Me Me

S

W

S

O VI

Me Me

Y S S

Y = S, Se

Me Me

S W OR' S S S Me

S VI

Me Me

S W OR' S S S Me

Me

Me

R' = Ph R' = Ph,i- Pr, 2-Ad, p-C6H4-X (X = CN, Br, Me, OMe, NH2)

Figure 12. Synthetic routes to bis(dithiolene)W(V) and (VI) complexes (86).

558

JONATHAN McMASTER ET AL.

positions and represent a new type of tungsten dithiolene complex (86). The monooxo [WO(OR0 )(S2C2Me2)2]
[R0 ¼ Ph, i-Pr, 2-Ad, or p-C6H4-X (X ¼ CN, Br, Me, OMe, NH2)] complexes have been prepared by an OAT reaction between the corresponding [W(OR0 )(S2C2Me2)2]
complex and TMAO (85). The direct nature of this OAT was demonstrated using infrared (IR) spectroscopy, by monitoring the reaction of Ph2Se18O with [W(OPh)(S2C2Me2)2]
to show that [W(18O)(OPh)(S2C2Me2)2]
was formed. The [WO(OPh)(S2C2Me2)2]
complex has been characterized by X-ray crystallography (Fig. 13) and shown

(a)

S3

S1

O2

W

S4

O1 S2

Rc-W-DMSOR active site (oxidized) (b) [WO(OPh)(S2C2Me2)2]−

Figure 13. Structure of (a) [WO(OPh)(S2C2Me2)2]
[Reproduced with permission from K.-M. Sung and R. H. Holm, J. Am. Chem. Soc., 123, 1931 (2001). Copyright # 2001 American Chemical Society.] and (b) a comparison of the coordination sphere of this complex with that of the tungsten center in the as-isolated state of the W

DMSOR from Rh. capsulatus (77).

ANALOGUES OF Mo AND W DITHIOLENE-CONTAINING ENZYMES

559

to possess a distorted octahedral coordination geometry with a cis-arrangement of the oxo- and phenolato groups. The trans influence of the oxo group leads to ˚ ] being 0.07 A ˚ longer than the average length of the W

S1 bond [2.492(1) A the three other W

S bonds; other distortions from regular octahedral geometry include a ‘‘trans’’ S2

W

S3 angle of 153.49(5) and the dihedral angle between the two dithiolene rings of 99.8 . This structure is analogous to that of the complexes [MO(OSiPh2t-Bu)(bdt)2]
(M ¼ Mo, W) (79, 80) and the coordination sphere of the tungsten center in the oxidized state of the W

DMSOR from Rh. capsulatus (Fig. 7) (77), as illustrated in Fig. 13. ˚ and W
Furthermore, the bond distances, W

O ¼ 1.76 and 1.89 A
S ¼ 2.44 ˚ A obtained from W LIII-edge EXAFS studies of the enzyme site (77) correspond closely to those of [WVIO(OPh)(S2C2Me2)2]
[W ¼ O ¼ 1.728(3), ˚ ]. W

O(Ph) ¼ 1.994(4), W

Sav ¼ 2.434(1) A The active sites of the sulfite oxidase and xanthine oxidase enzyme families contain only one MPT bound to the Mo and the synthesis of mono(dithiolene) molybdenum complexes as prospective chemical analogues of these centers has represented a significant synthetic challenge. The reaction of [MoO(S2C2Me2)2]
or [MoO(bdt)2]
with PhSeCl results in removal of one dithiolate ligand and formation of [MoOCl2(S2C2Me2)]
or [MoOCl2(bdt)]
. The key step involves the conversion of a bis(dithiolene) to a monodithiolene [Eq. 4]; two electrophilic PhSeCl molecules bind to one bdt to form a bis(selenosulfide) and the two chlorides occupy the vacated coordination sites. ½MoOðbdtÞ2 
þ 2 PhSeCl ! ½MoOCl2 ðbdtÞ
þ C6 H4 ðSSePhÞ2

ð4Þ

The chlorides of the product can be substituted by other ligands and mono(dithiolene) complexes [MoO(AdS)2(S2C2Me2)]
, [MoO(SR)2(bdt)]
(R ¼ Ad or 2,4,6-i-Pr3C6H2) and [MoOCl(SC6H2-2,4,6-i-Pr3)(bdt)]
have been synthesized and characterized (90). These complexes represent the first examples of five-coordinate monodithiolene MoVO complexes; each has a square-pyramidal structure with an apical oxogroup. Representatives of these MoVO centers exhibit rhombic EPR spectra that show some similarities to signals exhibited by the low- and high-pH forms of chicken liver sulfite oxidase (91). The Mo(VI) mono(dithiolene) complexes have also been synthesized (90). The complex [MoO2(OSiPh3)2] reacts with Li2(bdt) in tetrahydrofuran (THF) to form [MoO2(OSiPh3)(bdt)]
and the reaction of this product with 2,4,6-iPr3C6H2SH in MeCN forms [MoO2(SC6H2-2,4,6-i-Pr3)(bdt)]
. These complexes possess a square-pyramidal coordination geometry with apical and basal oxo ligands and the MoVIO2S3 moiety of the latter closely resembles the active site of chicken liver sulfite oxidase (92) and, by analogy, assimilatory nitrate reductases from Neurospora crassa, Chlorella vulgaris, and spinach (1).

560

JONATHAN McMASTER ET AL.

These studies represent the first structural analogues of the catalytic centers of members of the sulfite oxidase family in their oxidized state. The oxidized form of the catalytic center of each member of the xanthine oxidase family of enzymes involves a cis-MoVIOS center bound to one MPT (1, 93). The corresponding desulfo-form of this center, involving a cis-MoVIO2 center, is catalytically inactive. Beyond the intrinsic interest of producing mixed oxo– sulfido complexes, the development of chemical systems that improve our understanding of the nature and reactivity of cis-MoVIOS centers has represented a significant challenge (94). However, the development and investigation of such systems are in prospect, especially since a convenient synthesis of dithiolene complexes has been developed by reacting a cis-MS2 (M ¼ Mo or W) center with an activated alkyne (95). This type of synthesis was reported earlier by Pilato et al. (96, 97). B.

Oxygen Atom Transfer

As indicated in Section II, a concomitant change in the oxidation state of, and the number of oxo groups bound to, a Mo or W center [e.g., Eq. (1)] is supported by S-donor ligands. These ligands include dithiolenes and the interconversion of [MO(dithiolene)2]2
and [MO2(dithiolene)2]2
complexes is well developed. Thus, [MoO(mnt)2]2
is oxidized, for example, by TMAO, to [MoO2(mnt)2]2
(69, 74) and corresponding oxidations of other [MoO(dithiolene)2]2
[e.g., dithiolene ¼ bdt (70, 74) and S2C2(CO2Me)2 (69)] complexes have been investigated. The resultant [MoO2(dithiolene)2]2
complexes are effective 2
and oxidants. Thus, [MoO2(mnt)2]2
oxidizes HSO
3 , forming [MoO(mnt)2]
HSO4 (74, 98) and the kinetics observed for this reaction have been interpreted in terms of the initial formation of a seven-coordinate complex, [MoO2(HSO3)(mnt)2]3
, followed by OAT. Further investigations of this reaction, in MeCN and various MeCN/H2O ratios, showed that the rate was increased by the presence of H2O. This result implied that a more reactive intermediate was formed, perhaps one in which a H2O molecule is hydrogen bonded to an oxo group of the {MoO2}2þ center. However, in contrast, [WO2(mnt)2]2
does not oxidize HSO
3 in MeCN/H2O, but the reaction does proceed in anhydrous MeCN (96, 99). The rate of an OAT reaction from [MoO2(mnt)2]2
to PPh3
xEtx (x ¼ 0–3) has been shown to depend on the basicity of the phosphine with the steric influence of the substituents on the phosphorous being of minor importance (98). The OAT reactions between [MO2(mnt)2]2
(M ¼ Mo or W) and (RO)3
nR0 nP (n ¼ 0, R ¼ Me; n ¼ 1, R ¼ Me, R0 ¼ Ph; n ¼ 1, R ¼ Et, R0 ¼ Me) have been examined. The activation parameters obtained,
H z

10 kcal mol
1 and
Sz
33 eu, are consistent with an associative
M bond. transition state that involves the formation of an (RO)3
nR0 nP   O


ANALOGUES OF Mo AND W DITHIOLENE-CONTAINING ENZYMES

561

The relative rates of reaction, kMo > kW , at 298 K, kMo =kW 102 –103 are attributed to the relative ease of (a) reducing the Mo(VI) center versus the VI W(VI) center and (b) cleaving an MoVI

O bond versus a W

O bond (100). The [MO2(dithiolene)2]2
[M ¼ Mo (73) or W (101); dithiolene ¼ bdt or tdt] complexes will oxidize benzoin [Eq. (5)] and the rates observed for the initial OAT were found to vary with the dithiolene. ½MoO2 ðdithioleneÞ2 2
þ PhCHðOHÞCðOÞPh ! ½MoOðdithioleneÞ2 2
þ PhCðOÞCðOÞPh þ H2 O

ð5Þ

Also, oxidation of [MoO(dithiolene)2]2
[dithiolene ¼ mnt, S2C2(CO2Me)2, or S2C2(CONH2)2], has been monitored alone and in the presence of 1,3-bis(isobutyrylamino)benzene. The presence of the latter accelerates the rate of oxidation and it has been suggested that this effect arises due to an enhancement 2
of the strength of the Mo

O bonds of the product, [MoO2(dithiolene)2] , by the formation of NH   S hydrogen bonds (96). Corresponding NH   S interactions could facilitate the catalysis of OAT by the Mo and W centers of the MPT enzymes (102). Holm and co-workers (79, 80, 84, 85, 103) carried out an extensive series of investigations of OAT reactions of mononuclear bis(dithiolene) complexes of Mo and W. The results of these studies are, not only important in their own right, but also provide information that is relevant to the function of the catalytic centers of the Mo

MPT and W

MPT enzymes (Table I), especially the DMSORs and TMAORs. The [M(OSiRR0 2)(bdt)2]
(M ¼ Mo or W; RR0 2 ¼ Me3, t-BuMe2, or t-BuPh2) complexes are capable of undergoing oxygen atom addition (79, 80). However, [Mo(OSiPh2t-Bu)(bdt)2]
has a surprisingly limited reactivity toward sulfoxides and, even under forcing conditions, oxidation of this complex by TMAO is sluggish and incomplete (79). In contrast, [W(OSiPh2t-Bu)(bdt)2]
is oxidized rapidly (<10 min at 20 C) by, for example, TMAO, Nmethylmorpholine N-oxide, or Ph2SeO, to form [WO(OSiPh2t-Bu)(bdt)2]
. The direct nature of these OAT reactions has been demonstrated by the use of Ph2Se18O and PhI18O; the formation of a W

18 O bond being confirmed by an appropriate shift in the W

O stretching frequency (from that of W

16 O) and by mass spectrometry. In contrast to [Mo(OSiRR0 2)(bdt)2]
complexes, [Mo(OR0 )(S2C2R2)2]
0 (R ¼ Ph, Ad, or i-Pr; R ¼ Me or Ph) complexes react readily with N-, S-, and Se-oxides (Fig. 14) and a detailed kinetic investigation of these OAT reactions has been accomplished (84, 85). Equation (6) summarizes the overall reaction sequence and the first step has been shown to proceed by direct OAT,

562

JONATHAN McMASTER ET AL. TABLE I Relative Reactivities of Molybdenum and Tungsten Isoenzymesa

Isoenzyme FMDH

Organism Methanobacterium thermoautotrophicum

(N-formylmethanofuran dehydrogenase) Methanobacterium wolfei

Reaction O

O

NHCHO +

W/Mo Reactivity Ratio 0.2

H 2O

NH3+ + CO2 +

H+ + 2 e-

QO Me3NO

+ O

O

N

2.2 4.1

Me NO

TMAOR Escherichia coli (trimethylamine N-oxide reductase)

QO þ 2 Hþ þ 2 e
! Q þ H2 O

4.2

Me NO

2.0

Me2SO No reaction for Mo kcat =KM ¼ 2:7  104 M
1 s
1 for W Ph2SO No reaction for Mo kcat =KM ¼ 2:7  104 M
1 s
1 for W

SO No reaction for Mo kcat =KM ¼ 2:0  104 M
1 s
1 for W DMSOR (dimethyl sulfoxide reductase) a

Rh. capsulatus

Me2 SO þ 2 Hþ þ 2 e
! Me2 S þ H2 O

17 (forward reaction) 0.06 (backward reaction)

See Ref. 85.

since oxidation of [Mo(OPh)(S2C2Me2)2]
by Ph2Se18O or C6H4(CH2)2S18O produces [Mo18O(S2C2Me2)2]
. ½MoðOR0 ÞðS2 C2 R2 Þ2 
þ QO ! ½MoOðOR0 ÞðS2 C2 R2 Þ2 
! ½MoOðS2 C2 R2 Þ2 
ð6Þ These OAT reactions involve second-order kinetics, an associative transition state, and an appreciable enthalpy of activation (8–15 kcal mol
1). The reaction rates are highly substrate dependent, with k2 values that span a range of 108

ANALOGUES OF Mo AND W DITHIOLENE-CONTAINING ENZYMES

Q

OR'

QO

-

IV

R R

S Mo S S S

R R

‡ O

S Mo OR' S S S R

R R

k2

563

R

Q

O

-

V

Me Me

S Mo S S S

Me Me

k1

R'O

R R

O VI S Mo OR' S S S R R

H-donor R'OH Figure 14. Summary of the oxo-transfer reaction sequence, commencing with [Mo(OR0 ) (S2C2R2)2]
(R0 ¼ Ph, Ad, or i-Pr; R ¼ Me or Ph; QO ¼ N or S-oxide), see Eq. (5) (84).

(Table II) and, for those reactions for which activation parameters were derived, Tj
Sz j contributes 40–50% of
Gz . Significantly faster rates are observed for OAT from N-oxides than S-oxides and this observation can be related to the relative magnitudes of the gas-phase dissociation energies and the basicities of the substrates. For TMAO, D(N

O) is estimated as 61–71 kcal mol
1 (104, 105) and, for DMSO and TMSO, D(S

O) is estimated as 86–87 kcal mol
1 (106). The gas-phase proton affinity of TMAO (235 kcal mol
1) is substantially larger than that of DMSO (211 kcal mol
1) (107) and (hence) TMAO is a stronger base (pKBH ¼ 4.6) (108) than DMSO (pKBH ¼
1.8) (109) and tetramethylene S-oxide (TMSO) (pKBH ¼
1.3) (110). Thus, the rate of OAT is faster when the Q

O bond is weaker and the basicity of QO is greater. Furthermore, the fastest rate of OAT from TMSO occurs for [Mo(OC6F5)(S2C2Me2)2]
, the complex (of those investigated) that contains the most electron-withdrawing axial ligand. These data are consistent with a concerted transition state that involves Q

O bond weakening and Mo

O bond making (84).

564

JONATHAN McMASTER ET AL.

TABLE II Rate Constants and Activation Parameters for OAT from QO to [Mo(OR0 )(S2C2R2)2]
Complexesa R0

R

Ph

Me

QO

k2298 ðM
1 s
1 Þ


H z ðkcal mol
1 Þ

Me3NO

2:0ð1Þ  102

8.1(6)


21(2)

1:8ð1Þ  102

f

f

16(6)

9.5(1)


21(1)

1:5ð2Þ  10
4

10.1(4)


39(1)

14.8(5)


36(1)

+ O O N Me (PhCH2)3NO


Sz ðeuÞ

Ph

Me

Ph

Me

Ph

Me

Ph

Me

Ph

Ph

SO

3:4ð2Þ  10
4

f

f

C6F5

Me

SOd

5:2ð2Þ  10
3

11.8(3)


30(5)

i-Pr

Me

SOe

5ð1Þ  10
6

f

f

a b c d e f

SOb Me2SOb

c

1:3  10
6

See Ref. (84). See Ref. (82). Calculated value from Eyring equation. Measured in THF. Measured in neat substrate. Not measured.

With respect to the second step of Eq. (6), reaction of the [Mo(OR0 )(S2C2Me2)2]
complexes with TMAO produces [MoO(S2C2R2)2]
in 90–94% yield, together with R0OH (i.e., PhOH, 2-adamantol, or 2-propanol). Thus, it appears that the product of the OAT reaction, [MoO(OR0 )(S2C2R2)2]
, decomposes by an internal redox process, to form [MoO(S2C2Me2)2]
and R0O ; the phenoxyl or alkoxyl radical then captures a hydrogen atom from the solvent, or a trace of water, to form R0OH (84). The [W(OR0 )(S2C2R2)2]
(R0 ¼ Ph, 2-Ad, i-Pr, or p-C6H4X, where X ¼ CN, Br, Me, OMe, or NH2; R ¼ Me or Ph) complexes react with an N-, P-, As-, S-, or Se- oxide (QO) to form the corresponding [WO(OR0 )(S2C2R2)2]
complex (85). As for the reactions described above, the direct nature of the OAT reaction has been demonstrated by the transfer of 18O to the W center from Ph2Se18O. These reactions are second order and involve a large negative entropy of activation (Table III), indicative of an associative transition state. The relative rates of the reactions vary with the substrate as: *

ANALOGUES OF Mo AND W DITHIOLENE-CONTAINING ENZYMES

565

TABLE III Kinetic Data for OAT from QO to [W(OR)(S2C2R2)2]
Complexesa k2298 ðM
1 s
1 Þb


H z ðkcal mol
1 Þ


Sz ðeuÞ

3:9ð4Þ  10
5

14.4(2)


30(1)

9:0ð3Þ  10
4

11.6(4)


33(1)

3.2(1)

9.4(5)


24(2)

0.34(1)

9.9(9)


27(4)

SO

3:5ð1Þ  10
2

10.8(1)


29(1)

Me

SO

4:2ð1Þ  10
3

p-C6H4Me

Me

SO

6:2ð1Þ  10
4

p-C6H4OMe

Me

SO

5:8ð1Þ  10
4

p-C6H4NH2

Me

SO

3:8ð7Þ  10
4

15(1)


24(4)


4  10 0.93(5)

12(5)


19(3)

0.71(9)

11(6)


23(5)

R0

R

Ph

Me

Ph

Me

Ph

Me

Ph

Me

p-C6H4CN

Me

p-C6H4Br

QO Me2SO

SO Ph3AsO

NO

6c

i-Pr i-Pr

Me Me

Me3NO

i-Pr

Me

O

+ ON Me

a

Ref. (85). A MeCN solution, 298 K. c Measured in neat substrate at 298 K. b

TMAO > Ph3AsO > pyO > R2SO Ph3PO(py ¼ pyridine). The reaction rate is increased by a weaker Q

O bond and a greater QO basicity. A change in the dithiolene substituents from R ¼ Me to R ¼ Ph leads to a small rate enhancement [k2(Ph)/k2(Me) ¼ 3.3]. However, a more basic axial ligand, R0 ¼ i-Pr vs. Ph, significantly retards the rate [k2(i-PrO
)/k2(PhO
) ¼ 0.0045]. The rate of
p-X)(S2C2Me2)2]
comthe OAT reaction between TMSO and a [W(OC6H4
plex correlates with the Hammett constant of X. Each of these variations is consistent with a considerable amount of W   OQ bond making being involved in the transition state.

566

JONATHAN McMASTER ET AL. -

X' O

IV

X

S W S S S

X X

X [Br4, CN]

[Br4, Br]

[Br4, H]

[Br4, Me]

[Br4, NH 2 ]

[F4, CN]

[F4, Br]

[F4, H]

[F4, Me]

[F4, NH2]

[X4, X'] = [H4, CN]

[H4, Br]

[H4, H]

[H4, Me]

[H4, NH2]

[Me4, CN]

[Me4, Br]

[Me4, H]

[Me4, Me]

[Me4, NH2]

[(OMe)4, CN]

[(OMe)4, Br]

[(OMe)4, H]

[(OMe)4, Me]

[(OMe)4, NH2]

Figure 15. The range of {W(OC6H4

p-X0 )[S2C2(C6H4

p-X)2]2}
complexes synthesized by Sung and Holm (103).

Similar conclusions were obtained by Sung and Holm (103), in respect of the results obtained from the investigation of the OAT reactions of TMSO with an
p-X0 )[S2C2(C6H4

p-X)2]2}
complexes [see extensive series of {W(OC6H4
Fig. 15 and Eq. (7)].
p-X0 Þ½S2 C2 ðC6 H4

p-XÞ2 2 g
þ TMSO ! fWðOC6 H4

p-X0 Þ½S2 C2 ðC6 H4

p-XÞ2 2 g
þ TMS fWOðOC6 H4


ð7Þ

The
Sz values for each of these reactions is large and negative, consistent with an associative transition state. The rate of the OAT reaction varies with the nature of the substituents X and X0 and Fig. 16 shows that the relative rates correlate with the Hammett constants of X and X0 in the sense that electronwithdrawing groups accelerate the rate of OAT and vice versa. This result is interpreted on the basis of the electron density distribution at the W(IV) center; that is, the electronic effects of the substituents (X and X0 ) are transmitted through the phenyl rings to the metal (103). The results obtained have been interpreted on the basis of an oxo-transfer pathway for the OAT process (Fig. 17), from the substrate QO to the W(IV) center that involves two transition states and one intermediate. The first transition state ðT1 Þ primarily involves
OQ WIV   OQ bond making. The intermediate (I), possessing a discrete WIV
bond and a distorted octahedral stereochemistry, is then formed, followed by the
Q bond is transition state T2 in which: (a) electron transfer occurs, (b) the O


ANALOGUES OF Mo AND W DITHIOLENE-CONTAINING ENZYMES

567

4σp −1

−0.5

0

0.5

1

1.5 2.5

2.5

2

2

CN 1.5

1

1

Br

0.5

0.5 Br

OMe Me

0

F

H

0

Me

−0.5

log (k [X4,X ′]/k [H4,X′])

log (k [X4,X ′]/k [X4,H])

1.5

−0.5

NH2

−1

−1

−1.5

−1.5 −1

−0.5

0

0.5

1

1.5

σp 0

Figure 16. Hammett plots of the relative rates of reaction (6) as log(k½X4 ;X  =k½X4 ;H ) vs. sP 0 0 (., variable X0 at specified X) and logðk½X4 ;X  =k½H4 ;X  Þ vs. 4sP (o,variable X at specified X0 ). [Reproduced with permission from K.-M. Sung and R. H. Holm, J. Am. Chem. Soc., 124, 4312 (2002). Copyright # 2002 American Chemical Society.]

weakened; and (c) a multiple bonded WVI

O interaction develops. The reaction is completed by the separation of the reduced substrate Q from the complex, which adopts the stereochemistry of [WO(OPh)(S2C2Me2)2]
. With reference to Fig. 17, electron-withdrawing substituents on the phenoxide and dithiolene ligands increase the rate of reaction and promote substrate binding by rendering the W(IV) center more electrophilic. The proposal of two transition states, with T1 involving the rate-determining step, follows from a lack of a correlation between the observed rate constants and the ‘‘oxidizability’’ of the W(IV) center. The latter was estimated by measuring the potential of the [W(CO)2L2]0/1
and [W(CO)2L2]1
/2
(and the [NiL2]0/1
and [NiL2]1
/2) [L ¼ S2C2
p-X)2; X ¼ H, Me, OMe, F, or Br] couples. Electron-withdrawing (C6H4
groups stabilize the lower oxidation state of the metal and therefore, with

568

JONATHAN McMASTER ET AL.

Q R R

1−

‡

O

IV

S W O S R′ S S R R

T1

EDG ‡ Q

1−

O S W O S R′ S R S R R T R

G

EWG

∆G ‡1

2

EWG

R R

R′ O IV S W S S S

1−

R

EWG

∆G ‡2

∆G+

R EDG

+ QO

Q

R R

1−

O IV S W O S R′ S S R R

I

EDG O R R

1−

VI

S W O S R′ +Q S R R

S

reaction coordinate Figure 17. Proposed qualitative reaction coordinate for reaction (7) with a schematic representation of the nature and relative free energies of the starting complex, the transition states (T1,T2), the intermediate (I), and the product. The bold line refers to the reaction sequence for [W(OPh)(S2C2Ph2)2]
and the other lines to complexes [W(OR0 )(S2C2R2)2]
containing electrondonating groups (EDG) or electron-withdrawing groups (EWG) X/X0 in the substituents R0 ¼ p-C6H4X0 and R ¼ p-C6H4X. [Reproduced with permission from K.-M. Sung and R. H. Holm, J. Am. Chem. Soc., 124, 4312 (2002). Copyright # 2002 American Chemical Society.]

reference to Eq. (7), are expected to stabilize W(IV) relative to W(VI). If the rate-determining step of this OAT involved ET, the rate should be slower when X and X0 are electron withdrawing, whereas the reverse is the case. The studies of Holm and co-workers (84, 85, 103) described above also provide an insight into the relative reactivity of Mo versus W for OAT

ANALOGUES OF Mo AND W DITHIOLENE-CONTAINING ENZYMES

569

reactions. Thus, the reaction described by Eq. (8) in MeCN has kW =kMo ¼ 6 for R ¼ Me and 8.8 for R ¼ Ph (81, 84, 85). ½MðOPhÞðS2 C2 R2 Þ2 
ðM ¼ Mo or WÞ þ TMSO ! ½MOðOPhÞðS2 C2 R2 Þ2 
þ TMS

ð8Þ

As a comparison, for single turnover, reaction (9) of Rh. capsulatus DMSOR isoenzymes has kW =kMo ¼ 17 (77, 111).
DMSORðM ¼ Mo or WÞ þ DMSO ! MVI

DMSOR þ DMS MIV


ð9Þ

Although Holm and Sung (103) have not carried out a comparable study of reaction (7) for the corresponding Mo analogues. These authors argue that the qualitative reaction pathway of Fig. 17 applies to the OAT reactions of [Mo(OPh) (S2C2R2)2]
complexes, but kMo < kW , which is explained by proposing that the binding of a substrate to Mo(IV) would be weaker than to W(IV).

IV.

DITHIOLENES THAT INCLUDE ASPECTS OF MPT

The synthesis of dithiolene complexes of molybdenum and tungsten, the inner-coordination sphere of which closely resembles that of the catalytic centers of particular enzymes, and the demonstration that such complexes can effect facile OAT reactions, are significant achievements. Also, collectively, the results of these investigations represent considerable progress toward the goal of synthesizing chemical analogues of the catalytic centers of the Mo

MPT and W

MPT enzymes. However, it is important to note that the chemical studies reported above have so far not included dithiolenes that possess the functional groups of MPT (Fig. 1). Thus, MPT is: (a) an asymmetric dithiolene (i.e., the two sulfur-bearing carbons are inequivalent), (b) involves the dithiolene group as a substituent of a pyran ring, and (c) possesses a pterin nucleus. Pterins are redox active in their own right and can adopt one of several oxidation levels, for example, fully oxidized, dihydro, or tetrahydro (Fig. 18). The stereochemical nature of MPT observed in each protein crystallographic study (13) is equivalent to that of the fully reduced, or tetrahydropterin, state. This argues against the existence of dihydropterin states such as 5,6-dihydropterin or 7,8-dihydropterin, and one of the various quininoid forms of the dihydropterin, in the crystalline forms of these enzymes that have been characterized by X-ray crystallography. However, it is important to note that, for the tricyclic structure of MPT, the tetrahydropterin state is equivalent to that of a dihydropterin, as manifest in the pyran ring-opened form of the bicyclic

570

JONATHAN McMASTER ET AL.

Dihydropterin

Tetrahydropterin O HN H2N

N

Fully oxidised pterin

O

H N

HN

N H

N

N H

O

H N

S

N

H2N

N

H2N

N

N

S

N O H MPT

O HN

N

HN

H2N

HN

H2N

O N

OPO(OH)(OR)

S N

S

N HO H

OPO(OH)(OR)

Ring-opened MPT Figure 18. Oxidation levels of pterins and the pyran ring-closed and ring-opened forms of molybdopterin (MPT).

pterin (Fig. 18). This ring opening could lead to a p interaction between the pyrazine and the metal–dithiolene rings, effectively coupling the redox behavior of the metal–dithiolene center to that of the pyrazine. As an initial step toward the synthesis of MPT, a general strategy for the synthesis of asymmetrically substituted dithiolenes has been developed (Fig. 19) (111, 112). This step has been used to generate a series of [MO(dithiolene)2]2
(M ¼ Mo or W) complexes (Fig. 20) which, like their analogues involving symmetrical dithiolenes (see Fig. 5), have a square-pyramidal structure with the oxygen atom at the apex (Fig. 21, showing the oxo-group hydrogen bonded to an ethanol molecule of crystallization). The physical properties of each of these complexes are consistent with retention of the MOS4 core in solution. The positions of the dithiolene 1H and 13C NMR (nuclear magnetic resonance), the

n(M


O) and n(C


C) stretching frequencies, and the E1=2 values for the M(V)/ M(IV) redox couple are affected by the nature of the substituent on the

ANALOGUES OF Mo AND W DITHIOLENE-CONTAINING ENZYMES KS O

O O

S

c. H2SO4

Br

Ar

571

S

Ar

S

O

O S

Ar

S OH-

SS-

Ar

NMe2

NaS O Ar

O

S

c. H2SO4

Br

S

Ar

S

NMe2 Ar

S OH-

+ NMe2

S S-

Ar

S-

Figure 19. A general strategy for the synthesis of asymmetrically substituted dithiolenes (111).

2-

2-

O

O S S

M = Mo, W

M

S S

+

R

S S

M

S S R

R

R

N R=

N

sdt

N

N

2-pedt

3-pedt

4-pedt

H2N

N

qedt

O

O MeN

N

N

MeN

N

N

NH2-ptedt

Me2N

N

N

N

NC(H)(Me)2-ptedt

Figure 20. Monooxo-bis(dithiolene)M(IV) (M ¼ Mo or W) complexes involving asymmetrically substituted dithiolene ligands; the dithiolenes can adopt a cis or a trans disposition at the M
O center (111, 112).

572

JONATHAN McMASTER ET AL.

Figure 21. Structure of [MoO(sdt)2]2
[sdt ¼ S2C2(H)Ph] with an ethanol molecule of crystallization hydrogen bonded to the oxo-oxygen (111).

dithiolene. These results clearly indicate that MPT may play an important role in the catalysis effected by the Mo

MPT and W

MPT enzymes, beyond providing the dithiolene group that coordinates to the metal. The chemical shifts of the dithiolene 1H resonances in these [MO (dithiolene)2]2
(M ¼ Mo or W) complexes vary with the nature of the substituents as dithiolene ¼ qedt > NC(H)N(Me)2-pedt 2-pedt NH2-ptedt 4pedt > 3-pedt > sdt (see Fig. 20) and the values observed correlate with the Hammett constants of the substituents. These variations in chemical shift are considered to arise due to both inductive effects and the relative ability of the R group to stabilize a resonance form in which positive charge is localized at the C atom carrying the dithiolene proton (111, 112). Similar comments apply to the variations observed in the 13C NMR chemical shifts of the dithiolene carbon that carries the dithiolene proton and the trends observed in n(C

C) and n(M

O) stretching frequencies. Thus, the n(C

C) stretching frequency varies from 1499 in [MoO(qedt)2]2
to 1516 cm
1 in [MoO(sdt)2]2
and 1498 in [WO(qedt)2]2
to 1519 cm
1 in [WO(sdt)2]2
. The reverse order applies for n(M

O) with values ranging from 905 cm
1 in [MoO(qedt)2]2
to 879 cm
1 in [MoO(sdt)2]2
and 905 cm
1 in [WO(qedt)2]2
to 884 cm
1 in [WO(sdt)2]2
. Therefore, it appears that, as the electronic structure (Fig. 22) draws charge onto the nitrogen heterocycle, this leads to a greater (S)C

C(S) contribution (i.e., a

ANALOGUES OF Mo AND W DITHIOLENE-CONTAINING ENZYMES O

O 2-pedt N

S

S

+

M

M

N

S

S

S

S

N

S

S

573 + O S S M S S

Form A

O 4-pedt

S

+

S

M S N

+ O S S M S S

O S

S M

S

S -

S -N

N Form B

Figure 22. Resonance forms of the asymmetrically substituted dithiolene ligands 2-pedt and 4-pedt (see Fig. 20) (111, 112).

lower n(C

C) stretching frequency); the Spp ! Modp (dxy ,dyz) bonding will be reduced and the Opp ! Modp bonding increased (i.e., a higher n(M

O) stretching frequency). It is of interest to note that the n(C

C) stretching frequencies of these complexes are 50–70 cm
1 lower than the band attributed
1 to n(C

C) stretching at 1568 cm observed in the resonance Raman (RR) spectrum of the reduced [Mo(IV)] form of DMSOR from Rh. sphaeroides (113). This latter observation has been interpreted as providing evidence for the presence of an ene-1,2-dithiolate group. The variation in the E1=2 values for the M(V)/M(IV) couples of the [MO(dithiolene)2]2
(M ¼ Mo or W) complexes in Fig. 20 can be understood from the arguments presented above. Thus, the complexes of sdt and qedt, the least and most electron-withdrawing ligands, define the limiting values, the corresponding E1=2 values for the Mo(V)/Mo(IV) and W(V)/W(IV) couples being
480 and
280 mV (vs. SCE) and
705 and
520 mV (vs. SCE), respectively. The redox properties of tris(quinoxaline-2,3-dithiolato)molybdate(IV), [Mo(qdt)3]2
, in the presence of protons provides a clear demonstration of the chemical versatility that is possible for a redox-active metal dithiolene center that involves a pyrazine ring linked to the dithiolene group. In an aprotic solvent, two reversible, Nernstian, waves are observed that (formally) correspond to the Mo(V)/Mo(IV) and Mo(IV)/Mo(III) couples. However, on addition of trifluoroacetic acid (Htfa), the Mo(V)/Mo(IV) couple slightly shifts to a higher potential and becomes non-Nernstian and a new three-electron, quasireversible, couple occurs some 900 mV less negative than the original Mo(IV)/ Mo(III) couple. The latter is attributed to the addition of one electron and one

574

JONATHAN McMASTER ET AL.

proton to each qdt, to form a ‘‘Mo(IV)’’ complex that contains three quinoxalinium radicals, that is, [Mo(Hqdt)3]2
(114). The first molybdenum–pterin complex was synthesized by Burgmayer and Stiefel (115). This involved a syn-{Mo2O5}2þ core bound to two doubly deprotonated xanthopterin (H2L) ligands; each L2
coordinates one Mo through the pyrimidine oxygen and a nitrogen of the pyrazine ring. Burgmayer and co-workers (116–119) have extended the range of Mo–pterin complexes. For example, [MoOCl2(detc)2] (detc ¼ diethyldithiocarbonate) reacts with H4dmp (H4dmp ¼ 6,7-dimethyl-5,6,7,8-tetrahydropterin) to yield [MoOCl(detc)(H3dmp)]Cl, in which the pterin is coordinated through the carbonyl oxygen and pyrazine ring nitrogens. The results of X-ray photoelectron studies are consistent with a significant delocalization of electron density over the molybdenum–pterin framework (119). These results suggest that the pterin nucleus of MPT may play an important role in the electron transfer to, or from, the catalytic centers of Mo

MPT and W

MPT enzymes. The reaction of [Cp2MoS4] with alkynes has been used to good effect by Pilato et al. (96), to synthesize the interesting dithiolene complex, Cp2Mo{S2C2(2-quinoxaline)[C(O)Me]}, and the unprecedented ‘‘ene-1,2-trithiolate’’ complex, Cp2Mo{S3C2(2-quinoxaline)[C(O)Me]}. The existence of the latter ‘‘trithiolene’’–Mo(IV) complex raises the interesting possibility of an internal redox isomerism (Fig. 23) that could be relevant to the function of the xanthine oxidase family of enzymes (120). This chemistry may be relevant to the nature and function of the molybdenum center of E. coli formate dehydrogenase. The Mo K-edge EXAFS of both the oxidized and reduced form of this enzyme were found to be very similar, each inolving a des-oxo-molybdenum site with four Mo

S bonds ˚ , (probably) one Mo
˚ , and one Mo
at 2.35 A
O bond at 2.1 A
Se interaction ˚ . The Se K-edge EXAFS showed clear evidence for a S
at 2.62 A
Se contact ˚ , presumably indicative of a novel seleno-sulfide ligand to the of 2.19 A molybdenum (121).

Cp

S MoIV

Cp

S

Cp

COMe

MoVI

COMe Cp

S

S S

S

2-quinoxaline

2-quinoxaline Figure 23. The proposed redox isomerism of the ‘‘ene-1,2-trithiolate’’ complex, Cp2Mo{S3C2(2quinoxaline)[C(O)Me]} (96).

ANALOGUES OF Mo AND W DITHIOLENE-CONTAINING ENZYMES

V.

575

CONCLUSIONS

The information presented above clearly indicates that chemists have made considerable progress in the synthesis and characterization of dithiolene complexes that provide benchmarks for the structural, spectroscopic, and reactivity of the catalytic centers of the Mo

MPT and W

MPT enzymes. However, several significant challenges remain before a comprehensive understanding of the structure–function relationships of these enzymes can be claimed. Now that a protected form of MPT is available (122, 123), it will be of interest to see whether Mo and W complexes of MPT can be obtained and, if so, how closely their structures and properties resemble those of the catalytic centers of the Mo

MPT and W

MPT enzymes. Furthermore, such studies should provide valuable information concerning the roles of the various functional groups of MPT in optimizing the properties of the catalytic centers of the natural systems. One important property of these catalytic centers is the similar potentials of the M(VI)/M(V) and M(V)/M(IV) (M ¼ Mo or W) couples, that is, the centers are poised for a two-electron change. This redox behavior is probably a consequence of one (or both) of these redox couples involving e
/Hþ transfer, which could arise from the coupling of the electron addition to (loss from) the metal center with Hþ gain by (loss from) MPT and/or an appropriate functional group of the protein. A further challenge is to understand why a natural system selects Mo over W (or vice versa) as the catalytic center of an MPT enzyme. Such selectivity would not appear to operate on the basis of size, given that equivalent Mo(N) and W(N) centers (N ¼ IV, V, or VI) possess very similar radii and corresponding dithiolene complexes involve Mo

S and W

S (and Mo

O and W

O) bonds of essentially the same length. One possibility is that the ‘‘natural selection’’ is based on a difference in the redox potentials of corresponding Mo(N)/ MoðN
1Þ and W(N)/WðN
1Þ (N ¼ V or VI) couples. For example, in the case of the Mo

and W

DMSOR of Rh. capsulatus, Mo(VI)/Mo(V) and/ DMSOR Mo(V)/Mo(IV) are 325 mV more positive than their W counterparts (77, 124). Similar differences in redox potential (Mo vs. W) are observed for [MO(dithiolene)2]1
/2
couples (dithiolene ¼ bdt) (71) and a series of asymmetrically substituted derivatives (111, 112)) and for the couples [MO(S2C2Me2)2]1
/2
and [MO(S2C2Me2)2]0/1
(82, 87). These differences are significantly less than those observed between equivalent Mo(V)/Mo(IV) and W(V)/W(IV) couples with N-, and halogen-donor ligands and, as discussed by Holm and co-workers (100), is presumably due to the covalency of the M

Sdithiolene interactions that delocalize the change in the electron density over the complex. Holm and co-workers (84, 85, 103) have shown that, for corresponding OAT
O center, the reactions in which an M(IV) center is converted into an MVI


576

JONATHAN McMASTER ET AL.

relative rates are kW > kMo . This result has been interpreted as a consequence of the M

Osubstrate bond in the transition state being stronger for M ¼ W than M ¼ Mo. However, it is important to note that this sense of relative rates for OAT does not necessarily translate into a faster reduction of the substrate at a tungsten center of an oxo-transferase enzyme, as compared to its molybdenum counterpart (see Table I). In vivo, Rh. capsulatus W

DMSOR reduces both DMSO and TMAO at a significantly slower rate than the corresponding Mo

DMSOR the relative rates are 9 and 22%, respectively (125). In each

O case, the rate-determining step is considered to be the conversion of the MVI

center to the M(IV) center. The reduction of a MVI

O center and the regeneration of a M(IV) center is expected to proceed at a slower rate for M ¼ W than M ¼ Mo since: (a) a lower reduction potential is required to reduce a W(VI) center than its Mo(VI) counterpart (see below); (b) more energy is required to deform a W

Ooxo counterpart (100). The
Ooxo bond than its Mo
relative reactivity of an Mo

MPT enzyme versus its W counterpart may have influenced the natural selection of one metal over the other for a particular biological role. The elegance of the structure–function relationships operating at the catalytic centers of Mo

MPT and W

MPT enzymes is illustrated by the results of theoretical studies accomplished by Hall and co-workers (126, 127). These authors used density functional calculations to investigate the course of OAT reactions, including that from DMSO to [Mo(OMe)(S2C2Me2)2]
, that serves as a chemical analogue of the reduced form of the catalytic center of Mo

DMSOR. In agreement with the chemical investigations of this reaction (84), the theoretical studies suggest that OAT should occur by an associative mechanism (127). An important aspect of the theoretical study was the indication that formation of the intermediate complex is assisted by a bonding interaction between the sulfur of the DMSO and the oxygen of the methoxy group. This observation is significant, especially as a corresponding interaction, between the serinate oxygen and the sulfur of the coordinated DMSO, has been observed in the crystal structure of the ‘‘substrate-bound’’ form of Rh. capsulatus Mo

DMSOR (43). This ‘‘substrate-bound’’ form of the enzyme was generated by the addition of an excess of DMS to the as-isolated enzyme. The SDMSO    OMe ˚ ) and distances calculated for the ‘‘substrate-bound’’ complex (2.447 A ˚ the transition state (2.444 A) compare favorably with the crystallographic value ˚ ) (43). The ‘‘substrate-bound’’ form proceeds through a transition state (2.65 A (þ8.8 kcal mol
1) that transfers the DMSO oxygen to the Mo, concomitantly breaking the S

O bond of DMSO to form an [MoO(OMe)S2C2Me2]
center and DMS. The overall exoergicity of the reaction is calculated as þ19.9 kcal mol
1, on the basis of separated reactants and products (127). In addition to the SDMSO    OR interaction, another important aspect of these theoretical calculations is the arrangement of the dithiolene ligands obtained in

ANALOGUES OF Mo AND W DITHIOLENE-CONTAINING ENZYMES

577

the transition state. The values calculated for the S

Mo

S transoid angles (involving sulfur atoms in different chelate rings) of 146.7 and 150.7 compare favorably with values obtained by protein crystallographic investigations of the DMSORs—in particular, those for the ‘‘substrate-bound’’ form of the Rh. capsulatus enzyme are 151.7 and 146.7 (43). This result suggests that an entatic state principle (128) is operative for this OAT. Thus, it is postulated that, by using the pterin ligands as ‘‘anchors’’, the geometry of the catalytic center of the enzyme is poised in a geometry that closely resembles the transition state for OAT from DMSO (123). This conclusion points to an elegance in the structure– function relationships that apply to the catalytic centers of the Mo

MPT and W

MPT enzymes, such that the nature of each system is optimized for the execution of its biological role.

ACKNOWLEDGMENTS Each of us is grateful for the friendship, guidance, and assistance received from many colleagues during the researches that have contributed to the scientific perspective presented in this chapter. Also, we acknowledge the support provided by the EPSRC, BBSRC, other funding agencies, and the Universities of Nottingham, Manchester, and Arizona.

ABBREVIATIONS Ad Ar bdt t-Bu CEPT Cp detc DMF DMS DMSO DMSOR EDG edt EPR ET EWG EXAFS

2-Adamantyl Arene Benzene-1,2-dithiolate Tertiary butyl Coupled electron–proton transfer Z5-Cyclopentadienyl Diethyldithiocarbamate N,N-Dimethylformamide Dimethyl sulfide Dimethyl sulfoxide Dimethyl sulfoxide reductase Electron-donating group Ethene-1,2-dithiolate Electron paramagnetic resonance Electron transfer Electron-withdrawing group Extended X-ray absorption fine structure

578

JONATHAN McMASTER ET AL.

FMDH H4dmp Htfa IR HOMO (L-N3) LUMO Me mnt Moco MPT OAT Ph i-Pr py pedt qdt qedt RR Ser sdt tdt THF TMS TMSO TMA TMAO TMAOR UV–vis

N-Formylmethanofuran dehydrogenase 6,7-Dimethyl-5,6,7,8-tetrahydropterin Trifluoroacetic acid Infrared Highest occupied molecular orbital Hydridotris(3,5-dimethyl-1-pyrazolyl)borate Lowest unoccupied molecular orbital Methyl 1,2-Maleonitrile-1,2-dithiolate (1,2-dicyanoethylene-1,2-dithiolate) Molybdenum cofactor Metal-binding pyranopterin dithiolate Oxygen atom transfer Phenyl Isopropyl Pyridine 1-(Pyridin-2-yl)-ethene-1,2-dithiolate Quinoxaline-2,3-dithiolate 1-(Quinoxalin-2-yl)-ethene-1,2-dithiolate Resonance Raman Serine Styrene-1,2-dithiolate Toluene-3,4-dithiolate Tetrahydrofuran Tetramethylene sulfide Tetramethylene S-oxide Trimethylamine Trimethylamine-N-oxide Trimethylamine-N-oxide reductase Ultraviolet–visible

REFERENCES 1. R. Hille, Chem. Rev., 96, 2757 (1996). 2. M. K. Johnson, D. C. Rees, and M. W. W. Adams, Chem. Rev., 96, 2817 (1996). 3. D. Collison, C. D. Garner, and J. A. Joule, Chem. Soc. Rev., 25, 25 (1996). 4. D. C. Rees, Y. Hu, C. Kisker, and H. Schindelin, J. Chem. Soc., Dalton Trans., 3909 (1997). 5. H. Schindelin, C. Kisker, and D. C. Rees, JBIC, 2, 773 (1997). 6. K. V. Rajagopalan, JBIC, 2, 786 (1997). 7. C. Kisker, H. Schindelin, D. Baas, J. Retey, R. U. Meckenstock, and P. M. H. Kroneck, FEMS Microbiol. Rev., 22, 503 (1998).

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CHAPTER 11

Dithiolenes in More Complex Ligands ¨ RG SUTTER DIETER SELLMANN and JO Institut fu¨r Anorganische Chemie Friedrich-Alexander-Universita¨t Erlangen-Nu¨rnberg Erlangen, Germany CONTENTS I. INTRODUCTION

587

A. Metal–Sulfur Interactions / 587 B. Scope and Coverage / 587 II. DITHIOLENE DERIVED LIGANDS A. B. C. D. E. F.

589

Introduction / 589 Tridentate Ligands / 590 Tetradentate Ligands / 592 Pentadentate Ligands / 596 Hexadentate Ligands / 598 Miscellaneous Ligands / 599

III. SYNTHESIS OF COMPLEXES WITH DITHIOLENE DERIVED MULTIDENTATE LIGANDS

601

A. Introduction / 601 B. Syntheses / 602 IV. STRUCTURE, BONDING, AND GENERAL PROPERTIES OF [M(L)(Sn )] COMPLEXES

Dithiolene Chemistry: Synthesis, Properties, and Applications, Progress in Inorganic Chemistry, Vol. 52, Special volume edited by Edward I. Stiefel, Series editor Kenneth D. Karlin ISBN 0-471-37829-1 Copyright # 2004 John Wiley & Sons, Inc. 585

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586 A. B. C. D. E.

Introduction / 604 Mononuclear versus Polynuclear Structures / 606 Oxidation States and the 18 Valence Electron Rule / 612 Chirality / 617 Electronic Effects / 620 1. High-Spin versus Low-Spin [M(Sn )] Complexes / 620 2. Benzene Ring Substituent Effects / 622 F. Protonation, Alkylation, and Redox Chemistry of Complexes with [M(Sn )] Fragments / 623 G. C

S Bond Cleavage / 626 H. Characterization / 629 V. CONVERSION OF SMALL MOLECULES IN THE COORDINATION SPHERE OF [M(Sn )] COMPLEXES

630

A. Introduction / 630 B. Nitrosyl Complexes / 630 1. 18 and 19 VE Nitrosyl Complexes / 630 2. 16, 17, and 18 VE Nitrosyl Complexes / 632 3. Conversions of NO into NPR3 , H2 NO, and HNO Ligands / 632 4. A Cycle for the NO ! NH2 OH Reduction / 636 C. Conversion of CO, CO2 , and SO2 / 637 D. H2 S and S2 Complexes / 639 E. Diazene Complexes / 640 1. Introduction / 640 2. Synthesis and Structures of [M(Sn )]–N2 H2 Complexes / 641 3. Hydrogen-Bridge Diastereoisomerism and PR3 Exchange Reactions of [m-N2 H2 {M(PR3 )(S4 )}2 ] Complexes / 647 VI. [M(Sn )] COMPLEXES MODELING REACTIONS OF [MS] ENZYMES

652

A. B. C. D.

Introduction / 652 Hydrogenase Models / 653 Carbon Monoxide Dehydrogenases / 658 Nitrogenase Relevant Complexes and Reactions Leading to a Model for the FeMoco Function / 661 1. Introduction / 661 2. A Hypothetical Cycle for N2 Reduction with [Fe(Sn )] Complexes and Reversible Redox Reactions of Diazene Complexes / 662 3. Modeling the Nitrogenase Catalyzed N2 Dependent HD Formation with Diazene Complexes / 664 4. The Open-Side Model of FeMoco Functioning / 668 E. Dinitrogen Complexes with [M(Sn )] Cores / 670

VII. CONCLUDING REMARKS

672

ACKNOWLEDGMENTS

673

ABBREVIATIONS

673

REFERENCES

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DITHIOLENES IN MORE COMPLEX LIGANDS

I. A.

587

INTRODUCTION

Metal–Sulfur Interactions

Sulfur ligands have long been known to form strong bonds with many transition metals. Metal sulfide ores are an integral part of the lithosphere, and every beginner in chemistry learns how to separate metals by precipitating them with H2 S. More recent is the knowledge that metal–sulfur species play an important role in the biosphere. Molecular metal–sulfur ligand complexes form the active centers of numerous metal–sulfur enzymes that catalyze reactions fundamental for life. These enzymes range from electron transferases to oxidoreductases, for example, from ferredoxines to nitrogenase (1). Sulfur ligands in enzymes usually are sulfide (S2
), thiolates (cysteinate), and thioethers (methionine). Dithiolene ligands were counted among the typically abiological ligands until the ethylene dithiolate unit was discovered in the molybdopterin cofactor (2). Thus there are no strict borders between biological and artificial sulfur ligands. Common to sulfide, thiolate, thioether, or dithiolene ligands and distinguishing them from ligands with oxygen, nitrogen, or phosphorus donor atoms, are the size and electronegativity of sulfur. Sulfur is a relatively big atom [covalent radii (pm): S 104, O 66, N 70] and has an electronegativity (EN) comparable to that of carbon or hydrogen (ENs on the Allred-Rochow scale: S 2.44, C 2.50, H 2.20, N 3.07, O 3.50; metals 1.11–1.75). These data indicate that sulfur donors are highly polarizable (soft) and M

S bonds substantially covalent. M

S bonds certainly are much less polar than M

O or M

N bonds. In addition, lone pairs at the sulfur donor atoms and stability toward hydrolysis distinguishes metal thioether and metal thiolate bonds from metal phosphine, amine, phosphide, or amide bonds. These features may be more decisive for the bonding in complexes with metal–sulfur cores than the question whether the sulfur donors are linked by short or long bridges, saturated or unsaturated C2 units. They can be made accountable for many intriguing properties of metal–sulfur ligand complexes and the interest in this class of compounds, originating from such diverse fields as large-scale heterogeneous dehydrodesulfurization of petroleum, material sciences, organic synthesis, medicine, and, last but not least, bioinorganic chemistry (3). B.

Scope and Coverage

Much of the chemistry described in this article resulted from the search for complexes modeling structural and functional features of metal–sulfur oxidoreductases such as hydrogenases, CO dehydrogenases, and, in particular, nitrogenases. Complexes were sought that exhibited sulfur dominated coordination

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spheres and reacted with enzyme substrates or important potential intermediates. It quickly turned out that such complexes were difficult to obtain. Complexes with mono- or bidentate thiolate ligands frequently either are labile or tend to oligomerize via thiolate bridges such that vacant sites are blocked. In some cases, sterically bulky thiolate ligands enabled to solve this problem (4). Replacing mono- or bidentate thiolates by multidentate ligands containing thioether and thiolate donors proved to be an alternative method. Complexes with robust metal–sulfur [M(Sn )] cores formed, which, although also inclined to oligomerize via M

S

M bridges, exhibited vacant or readily accessible vacant sites for the coordination of enzyme relevant small molecules. As in the case of bulky thiolate ligands, the problem to be overcome was the synthesis of the respective ligands. Another serious problem resulted from the extremely poor solubility of numerous complexes in all common solvents, preventing any investigations in the homogeneous phase. The iron complex [Fe(CO)2 ({CH2 SC2 H4 S}2 )] (1) illustrates this finding. It forms in the template alkylation of [Fe(CO)2 (S2 C2 H4 )2 ]2
by 1,2-C2 H4 Br2, is entirely insoluble in all common solvents, and could be characterized by X-ray crystallography only because it crystallized in single crystals directly from the reaction solution (5). S S S

Fe

CO CO

S 1 [Fe(CO)2({CH2SC 2H4S} 2)]

Replacing alkanedithiolates by 1,2-benzenedithiolate as starting thiolate yielded better soluble complexes and offered a lot of other advantages. For example, by introducing substitutents in the benzene rings, either more lipophilic or more hydrophilic complexes could be synthesized. In addition, the planar [SCCS] chelates of 1,2-benzenedithiolate units in multidentate sulfur ligands gave rise to stabler five-membered metal chelate rings, fewer configurational isomers, and, not the least, better crystallizing complexes than the configurationally flexible alkane dithiolates. This article focuses on the synthesis, reactivity, and potential biological relevance of complexes with multidentate sulfur ligands. The sulfur ligands mainly derive from 1,2-benzenedithiol. As yet, 1,2-benzenethiolate has never been found as a ligand in biological systems, neither is it a typical dithiolene ligand because the unsaturated C

C bond between the two S donors is part of the aromatic benzene ring. However, taking into account the dominant influence of the first coordination sphere donors upon the properties of metal centers and complexes, complexes with 1,2-benzenedithiolate derived ‘‘more complex’’

DITHIOLENES IN MORE COMPLEX LIGANDS

589

ligands were expected to yield insights into how the active metal–sulfur centers in enzymes react, even if these active centers have entirely different structures. In order to strive for completeness with respect to the title and simultaneously to keep this contribution within bounds, the ligands and complexes considered will have to meet the following criteria: (1) The ligand denticity is > 2. (2) At least once the complexes should exhibit the most characteristic structural motif of dithiolene complexes; two sulfur donor atoms binding in the 1,2 position to an unsaturated C2 unit and coordinating to a metal. (3) Dithiolene complexes usually exhibit two-coordinate sulfur donor atoms. Accordingly, the complexes considered here had to contain at least one C

S

M unit with a two-coordinate sulfur atom. The following formula gives the structural motif resulting from these constraints, the two-coordinate sulfur atom is indicated by the square. C

... C

S

S M 2

This formula served as a literature search motif. It makes no restrictions on the formal charges of either the SCCS unit or the metal center. Neither does it confine the number of atoms binding to the other sulfur atom. The constraints 1–3 exclude unsaturated bidentate 1,2-dithiolates or the ‘‘real’’ dithiolenes, which are reviewed elsewhere in this volume. Ligands having only thioether donors are also excluded, even if the thioether donors are connected through unsaturated C2 units. However, important ligands that are closely related chemically will be mentioned in due course, even if they do not meet constraints 1–3. These ligands are preferably derived from ortho-substituted thiophenols o-C6 H4 (X)SH with X ¼ PH2 , NH2 , or OH. Reviews on thiolate complexes are numerous (6) but usually have not described the ligands and complexes treated here. Some aspects of this article have previously been discussed in different contexts (7).

II.

DITHIOLENE DERIVED LIGANDS A.

Introduction

A great deal of coordination chemistry has emerged because we are curious to find out what happens when metals are coordinated by ligands. New chemistry generated by new ligands is a strong incentive for designing and synthesizing new ligands. The goal to model the active sites of metal enzymes

¨ RG SUTTER DIETER SELLMANN AND JO

590

with respect to their electronic properties, structure, and reactivity represents another motive for ligand synthesis. Irrespective of the motive, however, design and synthesis of new ligands can be frustrating, because the ligands do not come up to expectations. When working with thiolate ligands, one of the most frequent experiences is that insoluble and intractable complexes result. It explains the considerable amount of organic synthesis spent on derivatizing the ligands described here. B.

Tridentate Ligands

The number of tridentate ligands meeting the identification criteria is small. These ligands exhibit S3 , S2 N, or S2 C donor atom sets and are summarized in Scheme 1.

S S

R SH

NEt2

R HS

SH

R=H : S 3−H2 R = SiMe 3 : Me3SiS3−H2 R = CO2H : HO2CS3−H2

Et2NS2−H

S

CH2

S S R RS3−H

HS

S

R = Me, Et, Cy Scheme 1.

M S2C

S

C M

S S2C

Tridentate ligands.

The notations, S3

H2 , R S3

H, and so on are introduced in order to indicate donor atom sets and to serve as ‘‘reading help’’. They are not to be confused with, for example, trisulfane HSSSH.
H2 was obtained in a four-step one-pot synthesis according to The ligand S3
Scheme 2 (8). Twofold ortho lithiation of diphenylsulfide (3) with n-BuLi in hexane/TMEDA gave 4. Subsequent reaction with elemental sulfur, LiAlH4 reduction of the
H2 . resulting polysulfide 5, and final acidification with HCl yielded S3

H2 and subsequent reaction with CO2 /HCl or Renewed lithiation of S3

H2 and HO2 C S3

H2 (9). Me3 SiCl gave the derivatives Me3 Si S3
R
H derivatives, in which one thiol function of S3

H2 has become The S3
selectively alkylated, could be obtained for R ¼ Me, Et, Cy, but were difficult

DITHIOLENES IN MORE COMPLEX LIGANDS

S

S

Li(TMEDA)

S

Li(TMEDA)

3

591

HCl S

(S S)n

4

SH

HS

S3−H2

5

TMEDA = N,N,N',N'-tetramethylethylenediamine

Li

Me3SiCl

S

Me3 Si

SH

SLi S3−H

n-BuLi

HS

SiMe3

Me 3SiS3-H2

S SLi Li

CO/HCl

S

HOOC

COOH

SH

HS

HO2CS3-H2

Scheme 2

to separate from byproducts (8). Alkylation of [Ni(S2 C6 H4 )2 ]2
with
H (8). NEt2 (C2 H4 Br) and subsequent acidolysis yielded the tridentate Et2 NS2
The ligands with S2 C donor atom sets have been found only in complexes and cannot be isolated in the free state. Sodium amalgam reduction of mononuclear [Ni(S4 C3 Me2 )] leads to removal of benzenedithiolate and formation of trinuclear [Ni(m-S2 C3 Me2 )]3 (Eq. 1) (10). The trinuclear complex shows cleavage and insertion reactions of interest as CO dehydrogenase model reactions (see below). Tetrahydrofurane ¼ THF.

S S S Ni S S

[Ni(S 4C3Me2)]

+ Na/Hg / THF −

2− C6H4S2

Ni S Ni S S

S Ni

S

ð1Þ

[Ni(µ-S2C3Me2)]3

Cyclopentadienyl cobalt and rhodium complexes with 1,2-benzenedithiolate or S2 C2 R2 type ligands readily add unsaturated carbon molecules to form tridentate ligands exhibiting S2 C donor sets. When nitrenes are added, dithiolene derived ligands with S2 N donor sets result. Scheme 3 shows examples of

¨ RG SUTTER DIETER SELLMANN AND JO

592

R1

R2 C S M S

NR S M S

R R

R R

1 2

N2CR R (− N2)

N3R(−N2)

S Co

S

R

S

R

M

S RO2C

CO2R

CO2R RO2C

S

H

R

H

S

R

S

R

M

M S

R

Scheme 3

these reactions and illustrates the structure of the quadricyclane adduct obtained with [CpCo(S2 C6 H4 )]. These reactions have been intensively studied from synthetic and mechanistic points of view and were reviewed recently (11, 12). C.

Tetradentate Ligands

Most of the tetradentate ligands have been synthesized starting from bidentate sulfur ligand precursors. Here, the principal problem was to find a practicable method to alkylate selectively one of two identical sulfur functions. For example, it is impossible to connect intermolecularly two 1,2-benzenedithiolate units by alkylation in order to obtain a tetradentate ligand. The intramolecular alkylation always prevails according to Eq. 2. −

2

−

S

−

S

S

+

1,2-C2H4Br2

S +

S

−

ð2Þ

S

In this regard, it was an important discovery that [M(S2 C2 R2 )2 ] complexes with M ¼ Ni, Pd, Pt reversibly reacted with olefins or diolefins to give adducts

DITHIOLENES IN MORE COMPLEX LIGANDS

593

(Eq. 3) (13). X-ray structure determinations showed that two dithiolene ligands had been bridged by C2 or C4 units (14). R R

S S M S S

R R

R R

+ C C

M = Ni, Pd, Pt; R = Ph, CF3;

=

C C S S M S S

R R

ð3Þ

,

These reactions demonstrated the selective alkylation of sulfur donors in dithiolene ligands. However, they appeared to be confined to nickel triad dithiolene complexes, and the resulting multidentate sulfur ligand could not be decoordinated. Alkylating 1,2-benzenedithiolate ligands bound to the [FeII (CO)2 ] templates proved to be a more broadly applicable method. It also enabled the dissociation of the resulting ligands (5, 15, 16).
H2 . Scheme 4 exemplarily illustrates the procedure for the parent ligand S4
Coordination of 1,2-benzenedithiolate to Fe2þ ions (from FeCl2  4 H2 O) in 2− −

Fe

2+

S MeOH + 2

−

S

S S S

Fe

CO CO

S

20 ˚C

S S Fe S S

2−

+ CO 20 ˚C S

S S

Fe

CO CO

+ 1,2-C2H4Br2 20 ˚C

S

+ exc. HCl reflux

S

S

FeCl2 + 2 CO + SH HS S4−H2

[Fe(CO)2(S4)]

Scheme 4

MeOH gives the dark yellow planar [Fe(S2 C6 H4 )2 ]2
ion. Passing a stream of CO through the solution yields, under rearrangement of the C6 H4 S2 2
ligands, cis-[Fe(CO)2 (S2 C6 H4 )2 ]2
. In cis-[Fe(CO)2 (S2 C6 H4 )2 ]2
, the thiolate donors have become nonequivalent by standing trans to either a CO or another thiolate. Subsequent alkylation with 1,2-C2 H4 Br2 gives neutral [Fe(CO)2 (S4 )], which precipitates and is readily separated in nearly quantitative yield. Treating [Fe(CO)2 (S4 )] with concentrated hydrochloric acid in boiling THF yields the
H2 , which is extracted with ether. free S4


¨ RG SUTTER DIETER SELLMANN AND JO

594

Basically by this method, a large number of ligands with an S4 donor sets could be obtained. In some cases, the planar [Ni(S2 C6 H4 )2 ]2
or the C2 symmetrical cis-[Mo(NO)2 (S2 C6 H4 )2 ]2
ion were used as templates as well. The ligands are summarized in Scheme 5. X R'

S

S

R'

SH HS R

HS

SH R

R = R' = H R = R' = tert-butyl R = CO2X, R' = H X = H, Me

S

S

XS4−H2

S4−H2 BuS4−H2 CO2XS4−H2

X = (CH2)n, n = 1−8, 12 X = CH2C(O)CH2, CH2CH(OH)CH 2, CH2CHR, R = Me, C9H18OH, CH2Ph

Scheme 5. Tetradentate ligands with S4 donor sets.

tert-Butyl (16) or CO2X (17) substituents in the benzene rings were intended to improve the solubility of corresponding complexes in organic solvents or water, respectively. The benzene dithiolate units can be bridged by short (C1 ) or long (C12 ) carbon chains (18), by carbon chains that exhibit ketone or alcohol functions (19), or by carbon chains containing stereogenic C atoms (20, 21). There is evidence that the selective alkylation of only one thiolate donor per benzenedithiolate ligand according to Scheme 4 is also controlled by the overall charge of the intermediate complexes. It influences the nucleophilicity of the thiolate donors, which is strongly decreased in the neutral complexes. Scheme 6 summarizes S4 donor set ligands that required other synthetic methods.

O

H N

X

S

SH HS

SH HS

SH HS

tpS4−H2

XS4−H4

S

H N

O

X = p-C6H4 X = (CH2)5 Scheme 6


H2 ligand exclusively contains o-phenylene bridges and was syntheThe tpS4
sized in order to probe the influence of rigid aromatic C2 versus flexible alkyl C2 bridges (22). Scheme 7 illustrates the route of synthesis.

DITHIOLENES IN MORE COMPLEX LIGANDS

595

−

S Na+

F 2

+

−

S Na+

NO2 a

S

S

S

b

NO2 O2N

S

c

NH2 H2N

S

S

SH HS tpS4−H2

Scheme 7. Synthesis of tpS4

H2 .

Nucleophilic aromatic substitution of the fluorine substituents by benzenedithiolate sulfur atoms (step a), reduction of the nitro compound (step b), diazotization, reaction with KS2 COEt, alkaline hydrolysis, and acidification
H2 (step c). It could be purified via the [Ni(tpS4 )]2 complex (Fig. 1), gave tpS4

H2 . which is readily hydrolyzed with dilute hydrochloric acid to give pure tpS4
Final purification by use of metal complexes was also applied in the
H4 . These ligands exclusively contain thiolate syntheses of the ligands XS4
donors and were prepared by Hahn et al. (23) using 2,3-dimercaptobenzoic acid as starting material (Scheme 8). Isopropyl or benzyl protection of the thiol functions, conversion into the acyl chlorides, reaction with a,o-diamines, and deprotection of the sulfur atoms enabled the connection of two 1,2-benzenedithiol units via carboxylic acid amide bonds. The crude ligands were purified via titanium complexes such as (NMe4 )[CpTiL] or [m-L{Cp2 Ti}2 ] (Cp ¼ cyclopentadienyl) (Scheme 9). Ligand-transfer reactions between the latter complex and (NEt4 )2 [CoCl4 ] enabled the synthesis

S7

S3 S4 S8 S2

Ni2

Ni1 S1

Figure 1.

S6

S5

Molecular structure of [Ni(tpS4 )]2 .

¨ RG SUTTER DIETER SELLMANN AND JO

596

O

OH

O

OH

SH

(1) + SO2Cl2 SR (2) + H2N−X−NH2

SH

SR

O

H N

H N

X

O

H N

O

X

SR RS

SH HS

SR

SH HS

RS

H N

O

X = p-C6H4 X = (CH2)5

Scheme 8

−

O H N NMe4

H N O

S

+

S

Ti

O

S S

S

O

X

N H

N H

S Ti

(NMe4)[CpTiL]

S Ti S

[µ-L{Cp2Ti} 2] Scheme 9

of mononuclear (NEt4 )[CoL] (24a). First results indicate that metal complexes of these ligands tend to exhibit similar coordination geometries as with 1,2benzenedithiolate. Steric strain caused by the bridges, however, influences electronic spectra, redox potentials, as well as reactivity of the complexes (24b). D.

Pentadentate Ligands

Scheme 10 summarizes pentadentate ligands. In most cases, these ligands were synthesized by using the template alkylation of 1,2-benzenedithiolates bound to [Fe(CO)2 ] groups. An example is illustrated in Scheme 11 (28). Scheme 11 shows that the alkylation proceeds stepwise and leads to loss of one CO group, which is probably labilized by the decreasing overall charge of the template. An isomerization of the resulting five-coordinate intermediate from trigonal bipyramidal to square pyramidal yields the proper coordination geometry of the Fe center to enable the coordination of the D donor and the second alkylation step. The final [Fe(CO)(DS4 )] species can adopt two different geometries, the achiral meso form with cis-thiolate donors or the chiral rac form with transthiolate donors. Which one of both forms results depends on the D donor. For example D ¼ S gives the meso form while D ¼ NH gives the rac form. Steric
CH2

S

CH2 chains effects such as the favored gauche conformation of CH2
have been invoked in order to explain these stereochemical differences of

DITHIOLENES IN MORE COMPLEX LIGANDS

597

D R

S

S

SH

HS

R

R

R

D = S(C 2H4)2

R=H R = t-Bu R = CO2X

S5−H2 Bu S5−H2 CO2XS5−H2

(25) (26) (27)

D = NH(C2H4)2

R=H R = t-Bu

NHS4−H2 Bu NHS4−H2

(28) (29)

D = NR'(C2H4)2

R' = Me; R = H R' = CH2CH2CO2R'', R = H − R' = CH2CH2CO2 , R = H

NMeS4−H2 NCH2CH2CO2R''S4−H2 NCH2CH2CO2− S4−H2

(30) (30) (30)

D = O(C2H4)2

R=H

OS4−H2

(28)

D=

R=H R = t-Bu R = CO2H

pyS4−H2 pyBuS4−H2 pyCO2HS4−H2

(31) (32) (27)

H2C

N

CH2

Scheme 10

2−

S S

Fe

1−

Br

S CO CO

+ D(C2H4Br)2 − Br

S

−

D

S S S

D

D

S S S

Fe

CO

S trigonal bipyramidal

− CO

CO CO

D

S

1−

Br

Fe

1−

Br

Br

S S

Fe

CO

S

1−

1−

D Br S S Fe S S C O

S S Fe S S C O square pyramidal

S

− Br

−

− Br

−

D S S Fe S S C O

D S S Fe S S C O

meso form D=S

rac form D = NH

Scheme 11

¨ RG SUTTER DIETER SELLMANN AND JO

598

[Fe(CO)(S5 )] and [Fe(CO)(NH S4 )] (28). Without doubt, however, electronical effects are also important. They will be discussed in detail for [Fe(L)(NH S4 )] complexes, which can assume the meso form when L represents s-ligands such as MeOH, N2 H4 , or NH3 . For a number of reasons, complexes with [MNS4 ] cores were needed that exhibit trans-thiolate donors and do not isomerize. This aim was reached with
H2 ligands. Their rigid 2,6-bis(methylenepyridine) bridges always the pyR S4
enforce a trans-coordination of the thiolate donors in [M(L)(pyR S4 )] complexes (py ¼ pyridine). R N S S M S S R R L 6 [M(L)( pyRS4)] (R = H, t-Bu)

R

A considerable number of the complexes with pentadentate ligands exhibit unusually high stabilities. For example, [Fe(CO)(S5 )] is thermally stable up to  150 C, and can be dissolved in concentrated sulfuric acid without decomposition. E.

Hexadentate Ligands

Only a few hexadentate ligands have been reported (Scheme 12). They belong either to the thioether-thiol type or the Hahn type, in which 1,2benzenedithiol units are connected through carboxylic acid amide bonds. S

S

S

SH

S HS

S6−H2

S

S

S

S

S

S

[Bzo2-18S6] 2+

1. Ru

−

2. + OH O

HN NH

HS HS

HN

SH

O SH

S

S

S

S

S

S

O

SH

SH S6−H6

Scheme 12

DITHIOLENES IN MORE COMPLEX LIGANDS

599

The template alkylation of [Fe(CO)2 (S2 C6 H4 )2 ]2
with (BrC2 H4 SC2 H4 SC2
H2 (15). The H4 Br)2 yielded the open-chain tetrathioether dithiol ligand S6

H6 ligand was obtained by the Hahn route described in Section hexathiol S6
III.C, using 1,3,5-tris(aminomethyl)benzene for connecting three 2,3-dimercaptobenzoic acid units. It could be purified via the trinuclear Ti complex
H6 with [Mo(O)2 (acac)2 ] (acac ¼ [{Cp2 Ti}3 L] (L ¼ S6
6 ). The reaction of S6
acetylacetonato) yielded a mononuclear [Mo(S6 )] complex that corresponds to the parent complex [Mo(S2 C6 H4 )3 ] (23a).
18-S-6] [Hexathio-dibenzo [18] crown-6] crown thioether has The [Bzo2

18-S-6)]Br2 exhibits a been included because its Ru(II) complex [Ru(Bzo2
base-induced S

C cleavage reaction. This reaction gives rise to a hexadentate vinylthioether-thiolate ligand (Scheme 13), which was identified only in
18-S-6 complexed form binding to Ru(II) (33a). The thia crown ether Bzo2
had been obtained by template alkylation of [Fe(CO)2 (S2 C6 H4 )2 ]2
with 2 equiv of S(C2 H4 Br)2 at elevated temperatures (33b). F.

Miscellaneous Ligands

Scheme 13 depicts a couple of ligands that do not meet the identification criteria defined above, but are chemically related to dithiolene derived ligands. The tetradentate ligands of the N2 H2 S2

H2 type having N2 S2 donor sets are
H2 ligands of Section II.C and formally result from isoelectronic with the R S4
replacing two thioether donors by two NR amine donors. The notation
H2 or N2 Me2 S2

H2 is used in order to indicate that not only the N2 H2 S2
SH, but frequently also the NH functions can be deprotonated to give amide
H2 ligands. Another one is that donors. This difference is important to the R S4
the amine donors, when coordinated, cause higher electron densities at the metal
H2 has been obtained centers than thioether donors. The parent ligand N2 H2 S2
by Corbin and Work (34a) according to the route indicated by Eq. 4. SH 2

O

O

H

H

+ NH2

H N S H C C H S N H

−

(1) + H

(2) + HCl

H N

H N

SH HS

ð4Þ Condensation of o-aminothiophenol with glyoxal, reduction of the resulting benzthiazolidine with NaBH4 or LiAlH4 , and final acidification gives
H2 . The derivatives with benzene ring substituents formed in principally N2 H2 S2

H2 , deprotonathe same way (35). Benzylation of the SH functions in N2 H2 S2
tion and subsequent methylation of the NH functions, and the final deprotection
H2 ligand (36). of the S donors gave the N2 Me2 S2


¨ RG SUTTER DIETER SELLMANN AND JO

600

X R

X N

N

H N

R

SH

SH HS R

N2H2S2−H2 (34) BuN2H2S2−H2 (35) N2Me2S2−H2 (36)

H N

X = NH X=S

Ph

N

P

SH HS

SH HS

Ni P

Ni

S

S

HS N3H3S2−H2 (38) N2H2S3−H2 (39)

II

S S

Fe

SH

P SH

P(o-C6H4SH) 3 (41)

2−

N C S

H N

PhP(o-C6H4SH) 2 (41)

P II II

X

HS

Me

NMe(o-C6H4SH) 2 (40)

S

HS

pyN2H2S2−H2 (31, 37)

SH

S

H N

R

R = H, X = H R = t-Bu, X = H R = H, X = Me

S

N

−

O C

S

Ph

P

Ph

S S

M

S

Ph

P

M = Ni(II), Fe(II) [Ni{P(o-C6H4S)3}]2 (42)

2−

[Fe(CN){P(o-C6H4S)3}]

(43)

−

[M(CO){P(o-C6H4S)3}] (44)

Scheme 13

The pyN2 H2 S2

H2 ligand was first synthesized by Vahrenkamp and coworkers (37) from 2,6-pyridinedialdehyde and o-aminothiophenol. Another synthesis is outlined in Scheme 14 (31). The synthesis illustrates the sometimes laborious procedures necessary for the synthesis of amine thiol ligands with terminal thiol functions. This method is indispensable for the syntheses of the
H2 (38) and N2 H2 S3

H2 (39), in which NH(C2 H4 Br)2 alkyl analogues N3 H3 S2
and S(C2 H4 Br)2 are used for alkylating 2(3H)-benzothiazolone. Another group of chemically related ligands comprises the compounds NMe(o-C6 H4 SH)2 , PhP(o-C6 H4 SH)2 , and P(o-C6 H4 SH)3 . They are prepared
H2 (Section II.B). Lithiation of by methods similar to the preparation of S3
NMePh2 , treatment with elemental sulfur, and acidification yielded NMe

DITHIOLENES IN MORE COMPLEX LIGANDS

N

H N 2

N

601

N

H N

N

H N

C OO C

C O S

S

SH

S

HS

pyN2H2S2−H2 Scheme 14. Synthesis of pyN2 H2 S2

H2 via alkylation of 2(3H)-benzothiazolone.

(o-C6 H4 SH)2 (40). Reactions of PhPCl2 or PCl3 with 2 or 3 equiv of lithium [2lithio-benzenethiolate] afforded PhP(o-C6 H4 SH)2 (41) and P(o-C6 H4 SH)3 (41). These ligands yielded nickel and iron complexes (40–44). With regard to enzyme model complexes and the coordination of small molecules, the redox active complexes [Ni{P(o-C6 H4 S)3 }]2 , [Fe(CN){P(o-C6 H4 S)3 }]2
, and [M(CO){P(o-C6 H4 S)3 }]
(M ¼ Ni, Fe) are of particular interest (42–44). For the sake of completeness, nickel complexes with bis(thiosquaramide) ligands such as [Ni(sq

S4 )]2 (sq ¼ squaric acid) (45) are to be noted. They exhibit thermochromic behavior in pyridine solution. Their dinuclear structure is concluded from elemental analyses and spectroscopic data. Complexes such as
H)2 ] serve as stabilizers in color photography (46a). They are assumed [Ni(S2 O
to exhibit six-coordinate Ni(II) centers. Quite recently, the coupling of sulfide and dithiocarbamate ligands by 0

acetylenes, for example, MeO2 CC


CCO2 Me (RC


CR ) facilitated to generate 0 0 a new type of sulfur ligand L ¼ SC(R )C(R )SC(NR2 )S in tungsten complexes such as [W(S2 )(S2 NR2 )(L)] (46b). 2− (n-Bu)2N

N

N

N(n-Bu)2

S

S

S

S

S

Ni

Ni

O

S

S

S

(n-Bu)2N

N

N

N(n-Bu)2

S S

[Ni(sq−S4)]2

III.

O Ni

S

R'

R'

S

[Ni(S 2O−H)2]

S

+

2H

R'

S S

S

R N R

S W S S

R'

N R

R

[W(S2)(S2NR2)(L)]

SYNTHESIS OF COMPLEXES WITH DITHIOLENE DERIVED MULTIDENTATE LIGANDS A.

Introduction

Major objectives for the synthesis of multidentate ligands with Sn donor sets have been complexes with robust [M(Sn )] cores and one or more sites for the

602

¨ RG SUTTER DIETER SELLMANN AND JO

additional coordination of small molecules. The ligand Sn donor sets could be supplemented by N, O or other donor atoms, either in order to study electronic and structural effects caused by varying donor sets, or to determine specifically the number of sites for coordinating small molecules. In the end, complexes were sought that were stable and highly reactive at the same time, which comes close to a contradiction in terms. In the search of such complexes, two major problems kept showing up. Vacant metal coordination sites were blocked by formation of M

S

M bridges, because all Sn ligands exhibit at least one thiolate donor. Auxiliary ligands L, intended to be labile in order to enable substitution reactions, proved inert. Solutions to these problems expectedly depended on many factors such as metal centers, steric requirements of the ligands, solubility of complexes, or the nature of the auxiliary ligands L. For example, switching from iron to homologous ruthenium complexes could be tantamount to switching from extremely labile to absolutely inert species. Nevertheless, a few general points concerning the synthesis of complexes with [M(Sn )] cores were found and are summarized here. B.

Syntheses

Starting compounds usually are metal halide precursors and alkali salts of the Sn ligands. Precipitation of alkali halides favors the formation of the [M(Sn )] complex fragments. The following cases can be distinguished. 1. Labile metal precursor complexes and Sn ligands give [M(L)(Sn )] complexes with a labile L ligand (Eq. 5) (47): 2−

FeCl2 . 4 H2O + NHS4

MeOH [Fe(MeOH)(NHS4)]

ð5Þ

The MeOH ligand exchanges for a large number of other ligands, ranging from CO to NH3 . 2. The metal precursor complex and the Sn ligand yield [M(L)(Sn )] complexes with stable M

L bonds. The M

L bonds can be selectively activated and replaced by M

L0 bonds (Eq. 6) (48). 2−

Mo(CO) 4Cl2 + S4

[Mo(CO)3(S4)]

+ PMe3 ∆ T or hν

[Mo(PMe3)(CO)2(S4)]

ð6Þ

In some cases, the L ! L0 substitution is achieved under reducing conditions (Eq. 7) (49). IV

[Mo (Cl)2(S4)] + 2 NO

+ Zn − ZnCl2

II

[Mo (NO)2(S4)]

ð7Þ

DITHIOLENES IN MORE COMPLEX LIGANDS

603

This reaction is a reductive nitrosylation of a [MoIV (S4 )] complex. (Throughout this contribution, NO ligands are considered as neutral 3e
donor ligands.) 3. Metal precursor complexes and Sn ligands give coordinatively saturated
S

M bridges. The M

S

M polynuclear [M(Sn )]x complexes with M
bridges either are labile and spontaneously dissociate in solution to yield coordinatively unsaturated [M(Sn )] species, or they are cleaved by nucleophiles to give [M(L)(Sn )] complexes (Eq. 8) (50). 2−

NiCl 2 . 6 H2O + S3

[Ni(S 3)]3

+3L

3 [Ni(L)(S3)]

ð8Þ

4. Oxidative additions of Sn

H2 ligands to low-oxidation state metal precursor complexes proved an efficient method to obtain hydride complexes (Eq. 9) (51). [Rh(Cl)(CO) 2]2 + 2 BuS4−H2

2 [Rh(H)(CO)(BuS4)] + 2 HCl + 2 CO

ð9Þ

5. The formation of coordinatively unsaturated [M(Sn )] species, which can be isolated in the solid state, is the truly exceptional case. It could be observed only for the reaction according to Eq. 10 (52): 2−

FeCl2 . 4 H2O + NHS4

THF

[Fe(NHS4)] . THF

ð10Þ

X-ray structure analysis established the solvate character of THF and the five coordination of the Fe center in [Fe(NH S4 )]. The only other fivecoordinate Fe center was observed in the related [Fe(pyN2 H2 S2 )] complex (Fig. 2a and b) (31). By far, the largest number of reactions between metal precursor complexes and Sn ligands yielded inert and frequently also more or less insoluble products. The products could be mononuclear [M(L)(Sn )] as well as polynuclear [M(Sn )]x or [M(L)(Sn )]x complexes. The M

L bonds in the [M(L)(Sn )] complexes could prove inert, even when they had been labile in the precursor complexes. One example is [Ru(PPh3 )(pyS4 )]. It forms according to Eq. 11 and contains a PPh3 ligand that could not be exchanged for any other ligand, even under drastic conditions, for example, 150 bar of CO at 120 C (31, 53).

2−

[Ru(PPh3)(Cl)2] + pyS4

N S S Ru S S PPh3 [Ru(PPh3)(pyS4)]

ð11Þ

¨ RG SUTTER DIETER SELLMANN AND JO

604

N1 S2

S3 Fe1 S1

S4

(a)

N3

N1

Fe1

N2

S1 S2 (b) Figure 2.

IV.

Molecular structures of (a) [Fe(NH S4 )] and (b) [Fe(pyN2 H2 S2 )].

STRUCTURE, BONDING, AND GENERAL PROPERTIES OF [M(L)(Sn )] COMPLEXES A.

Introduction

In spite of recurrent problems such as M

S

M bridging, sparing solubility, inert M

L bonds, and so on, a number of [M(Sn )] complex fragments could be found that bind and activate or stabilize small molecules or ions. Scheme 15 gives a survey on the resulting [M(L)(Sn )] complexes, metals, and molecules or ions acting as coligands L or L0 in one or the other of these [M(L)(Sn )] complexes. The metals demonstrate that 1,2-benzenedithiolate derived ligands with thioether donors are practically as versatile as 1,2-benzenedithiolate itself with regard to coordinating transition metals. The metals can be enzyme relevant as well as nonbiological metals, although emphasis has been put on

DITHIOLENES IN MORE COMPLEX LIGANDS

R

R R R

S S M L S

S

R = H, CO2H, CO2R, SiMe 3

R R

N S S S M S L

S

S N M N R' S

R

R

S M

R

R

R

S

R'

L' L

R = H, t-Bu, CO2H, CO2R

605

L' L

D S S M S S R L

R R

R

R R = H, t-Bu

R = H, t-Bu, CO2R D = NH, NMe; S, O

R R

R = H, t-Bu, CO2X M

= Fe, Ru, Os; Cr, Mo, W; Ni, Pd, Pt; V; Co, Rh, Ir

L or L' = N2, N2H2, N2H3, N2H4, NH, NH2, NH3; NO+, NO, HNO, NH2O, NHPR3, NPR3; H2, H−; CH3−, CR2, CO, CH3COSR, PR3; Cl−, Br−, I−, N3−, O2−; H2S, S2−, S2n−, RS− Scheme 15

biologically relevant metals and their congeners. The vanadium complexes of
H2 ligand have been investigated by others (54). the S4
The coligands range from s–p ligands such as CO or phosphines to hard ligands, for example, chloride or oxide. The coligands comprise species that are relevant to the natural nitrogen, hydrogen, carbon, and sulfur cycles. They also reflect a considerable electronic flexibility of the [M(Sn )] fragments. Scheme 16 suggests the intrinsic properties of sulfur donors to be the underlying cause for this flexibility. Sulfide, thiolate, and thioether ligands can act as s donor, s donor–p acceptor and, due to their lone pairs also as s donor–p donor ligands (7f, 55). X-ray structural evidence shows that p donation leads to M

S bond distance shortening. Calculations indicate that the p-acceptor capacity of thiolates and, in particular, thioethers may be due to low-lying empty sulfur d orbitals or s* S

C orbitals (56, 57). M' M

SR2

M

SR2

M

SR2

M

SR M''

σ-Donor π-acceptor

σ-Donor

σ-Donor π-donor

Bridging

Scheme 16. Bonding modes of sulfur ligands exemplified for thiother and thiolate ligands.

¨ RG SUTTER DIETER SELLMANN AND JO

606

Irrespective of the question of whether the sulfur donors are part of saturated 1,2-alkanedithiolates or unsaturated 1,2-dithiolenes, the versatility of sulfur ligands contrasts with the bonding modes of amines or phosphines. Amines act as s-donor ligand, phosphines as s-donor–p-acceptor ligands. The actual bonding mode of sulfur donors will depend, of course, on the occupation of the metal orbitals. Formation of S ! M p-donor bonds can be expected with electron deficient or high-oxidation state metal centers. The M ! S p-acceptor bonds should be favored with electronically saturated or electron-rich metal centers. The differences between thiolates and amines may be leveled when the amines are deprotonated to give amides. Amides and thiolates are isoelectronic. In the coordinated state, they both have lone pairs and can act as s-donor–pdonor ligands, and the bonding similarities of 1,2-benzenedithiolate and o-aminothiophenolate complexes have been noted earlier (58). Scheme 16 also illustrates the recurrent motif in metal sulfide and thiolate coordination chemistry (and highly unwelcome with regard to the binding of small coligands): The bridging of metal centers and the blocking of vacant coordination sites through M

S

M0 bonds. In summary, the electronic flexibility of [M(Sn )] fragments gives rise to very diverse complexes, structures, and reactivities. B.

Mononuclear versus Polynuclear Structures

Thiolate donors in [M(Sn )] fragments with coordinatively unsaturated metal centers may stabilize these metal centers by either p donation or formation of M

S

M bridges. The following examples illustrate that it is as yet difficult to predict which alternative is chosen. The actual structure that results can depend on the denticity, additional donor atoms, and sterical constraints of the Sn ligand, oxidation, and spin states of the metal centers, and, conceivably, the nature of the metal itself. The [Fe(S4 )] fragment has two vacant sites and tetramerizes in the solid state (59). The complicated structure of the resultant [Fe(S4 )]4 (Fig. 3) is rationalized by the ‘explosion’ drawing of Figure 4. Figure 4 shows that [Fe(S4 )]4 contains four homochiral fragments. In THF solution, stepwise dissociation of [Fe(S4 )]4 leads to two coordinatively unsaturated (and again homochiral) [Fe(S4 )]2 dimers and finally to four [Fe(S4 )] monomers, which can be detected and identified by coordination of CO yielding [Fe(CO)(S4 )]2 (60) and [Fe(CO)2 (S4 )] (60). The [Fe(S4 )]4 complex is a remarkable example of the tetramerization of homochiral complex fragments. It also represents the final product when [Fe(CO)2 (S4 )] is completely decarbonylated. Even more complicated structures can result when [Fe(Sn )] fragments completely dissociate giving rise to ‘‘naked’’ Fe centers. Such ‘naked’ Fe

DITHIOLENES IN MORE COMPLEX LIGANDS

607

S 11 S1 S2

S10 Fe3

S9 Fe1

S13 S4

S3

Fe2

S5

S14

S8

Fe4

S7

Figure 3.

S12

S16

S15

Molecular structure of [Fe(S4 )]4 .

S11

S11 S1

S1

S3

S5

S10 Fe3

S9

S2

S13

Fe1 S4

S12 S14

S3

S10 S9

S2 Fe1

Fe3

S12

S4 [Fe(S4)]4

[Fe(S4)]4

S13 S14

S15

Fe2

S16

Fe4

S5

S8

Fe2

S8

S7

S15

S16

S7

S6

S6 + CO

+ CO

S S

Fe C O

S

S

S S S

Fe C O

[Fe(CO)(S4)]2

S S

S S

Fe

CO CO

S [Fe(CO)2(S4)]

Figure 4. Explosion drawing of the [Fe(S4 )]4 structure demonstrating the stepwise dissociation into two homochiral [Fe(S4 )] dimers and four homochiral [Fe(S4 )] monomers.

¨ RG SUTTER DIETER SELLMANN AND JO

608

Fe3 Fe4

Fe2

Fe5

Fe1

Fe6

(a)

S7

S3 S2

S4

Fe1 S1

Fe2

S

S6 S5

S9

S S

Fe

S S S

Fe

S

(b)

S S

S Fe S

S

(c)

Figure 5. (a) Molecular structure of [Fe(S3 )]12 , (b) tetrahedral coordination of a Fe(II) atom bridging two [Fe(S3 )2 ] units, (c) alternate six- and four-coordinate Fe(II) centers in [Fe6 {Fe^ (S3 )2 }6 ] ¼ [Fe(S3 )]12 .

DITHIOLENES IN MORE COMPLEX LIGANDS

609

centers probably occur when the ‘iron wheel’ of Figure 5 forms. Figure 5 depicts the molecular structure of [Fe(S3 )]12 that is actually [Fe6 {Fe(S3 )2 }6 ] and forms as a minor byproduct when [Fe(PCy3 )(CO)2 (S3 )] (7) (61) was recrystallized from toluene.

S S S

Fe

CO PCy3

C O

7

[Fe(PCy3)(CO)2(S3)]

Formation of [Fe(S3 )]12 can be envisaged to occur in the following way: The labile PCy3 and CO ligands of [Fe(PCy3 )(CO)2 (S3 )] dissociate, the resulting [Fe(S3 )] fragments disproportionate into [Fe(S3 )2 ]2
and Fe(II) ions, and these ions aggregate in such a way that six octahedral [Fe(S3 )2 ] units are bridged by six four-coordinate Fe(II) centers. The bridging Fe(II) centers are tetrahedrally coordinated by four thiolate donors of two [Fe(S3 )2 ] units (Fig. 5). The Fe12 iron-wheel thus alternately exhibits octahedral FeS6 and tetrahedral FeS4 centers. The series of nickel(II) complexes (Scheme 17) demonstrates the variety of structures found when the Sn denticity, the bridge length between the benzenedithiolate units or the donor atoms D in DSn ligands (D ¼ additional donor atom) are varied. The [Ni(S3 )] fragments form [Ni(S3 )]2 (50) or [Ni(S3 )]3 (62) in the solid state, depending on the crystallization conditions. Mononuclear [Ni(L)(S3 )] complexes only result upon subsequent reaction with nucleophiles L ranging


from PR3 , CN
, RS
, N
3 , NHPR3 , NCO , NSO to Cl (Eq. 12). S [Ni(S 3)]2 or 3 + L

S

Ni L

ð12Þ

S 2
The monoalkylderivatives R S
3 of S3 do not act as tridentate, but only as bidentate ligands. Formation of 1:2 complexes is favored, illustrated by the
S4 )] comstructures of [Ni(MeS3 )2 ] or [Ni(Et2 NHS2 )2 ]Br2 (50). The [Ni(C3
plex is mononuclear (63), [Ni(Bu S4 )]3 , however, which contains the analogous C2 bridged Bu S4 ligand is trinuclear (64). The structures found with DS4 ligands comprise mononuclear [NiS5 ] (63), which is achiral and diamagnetic; dinuclear [Ni(NH S4 )]2 (65), which is

¨ RG SUTTER DIETER SELLMANN AND JO

610

S S Ni S S

S S

Ni

S

S

Ni S

S S Ni Ni S S S

S

[Ni(S 3)]2

[Ni(S 3)]3

Et H Et N Ni S

S

S

S

S MeS

2+

SMe

S Ni S S Me3P

Ni S

S

S

S N Et H Et [Ni(MeS 3)2]

S S

S Ni S

2+

[Ni(PMe3)(C3−S4)]

[Ni(Et2NHS2)2]

S

S Ni S S

[Ni(C3−S4)]

S S Ni S S

S

S

=

Ni S

S

[Ni( BuS4)]3 H N

S S S

Ni

S S

[Ni(S 5)]

S S

Ni

S S

S S

Ni

S S

S S

N H [Ni(NHS4)]2

Ni

S S O

O S S

Ni

S S

[Ni(OS 4)]2

Scheme 17

paramagnetic and contains chiral [Ni(NH S4 )] fragments; and the completely different [Ni(OS4 )]2 (63), which is diamagnetic and consists of two cofacial [NiS4 ] units with four-coordinate Ni centers connected by O(C2 H4 )2 bridges. The ready change of nickel coordination numbers and structures is also

DITHIOLENES IN MORE COMPLEX LIGANDS

611

H N

H N

H N

S Fe S S S

S S Fe S S O H Me

S S Fe S S C O

[Fe(NHS4)]

.

THF

[Fe(MeOH)(NHS4)]

[Fe(CO)(NHS4)] Me N S S Fe S S C O [Fe(CO)(NMeS4)]

N S S

S Fe S C O

[Fe(CO)(pyS4)]

N S S H2N

S Fe S NH2

[Fe(N2H4)(pyS4)]

CO2R S Fe S CO2R RO2C S S Fe S S RO2C N N

S S

[Fe(pyRO2CS4)]2

Scheme 18

evidenced by [Ni(PMe3 )(C3

S4 )] (66) resulting from [Ni(C3

S4 )] and PMe3 . The complex [Ni(PMe3 )(C3

S4 )] indicates the facile but labile addition of fifth ligands to square-planar Ni(II) complexes with [NiS4 ] cores. The NiS distance to the apical S donor being 261.31(14) pm is  40 pm longer than the basal NiS distances. Scheme 18 is comprised of coordinatively unsaturated, coordinatively saturated, labile, and inert Fe(II) complexes. The complex [Fe(NH S4 )]  THF is one of the extremely rare five-coordinate Fe(II) complexes with Sn ligands (52) (cf. Section III.B). The [Fe(MeOH)(NH S4 )] complex (47) crystallizes from MeOH, has a six-coordinate Fe center, is also paramagnetic, exhibits cis-thiolate donors, and possesses a very labile MeOH ligand. The complex [Fe(CO)(NH S4 )] (28) is diamagnetic, has trans-thiolate donors, and is nearly inert. Replacing the NH by a NMe group leads to [Fe(CO)(NMeS4 )] (28, 30), which is also diamagnetic and inert, but exhibits cis-thiolate donors. Structures, reactivity, and magnetism of [Fe(L)(NS4 )] complexes evidently depend on many factors.

¨ RG SUTTER DIETER SELLMANN AND JO

612

The ambivalence of [FeNS4 ] cores with respect to either cis- or trans-thiolate coordination is eliminated in the [Fe(pyR S4 )] complexes, which contain the rigid py(CH2 )2 bridge and thiolate donors that always adopt trans positions. This finding is demonstrated by [Fe(CO)(pyS4 )] (31), [Fe(N2 H4 )(pyS4 )] (67), and the dinuclear [Fe(pyRO2 C S4 )]2 (27). The analogous [Fe(pyS4 )]nþ 2 complexes could be obtained in the three oxidation states n ¼ 0, þ1, and þ2 (67). Note here that [Fe(CO)(NH S4 )] and [Fe(CO)(pyS4 )] have identical [FeCNS4 ] core coordination geometries. A pH dependent reversible dimerization was observed for complexes containing the [Ru(N2 H2 S2 )] fragment. The [Ru(N2 H2 S2 )] fragments are isoelectronic with [Ru(S4 )] fragments, but have NH donors that can be deprotonated. As in the case shown in Eq. 13, they can also exhibit a different coordination geometry with cis- instead of trans-thiolate donors. + H N 2

S S Ru N NO H PPh3

+

− 2 H , − 2 PPh3 +

+ 2 H , + 2 PPh3

N

O N

H

S S Ru Ru N S N S H N N O

ð13Þ

Deprotonation of one NH donor in [Ru(NO)(PPh3 )(N2 H2 S2 )]þ and loss of PPh3 reversibly yields the dinuclear [Ru(NO)(N2 HS2 )]2 . Equation 13 indicates the molecular structures of the two complexes as determined by X-ray crystallography (68, 69). C.

Oxidation States and the 18 Valence Electron Rule

The [M(Sn )] complexes with metal centers in low-to-medium oxidation states usually obey the 18 valence electron (VE) rule. This can be regarded as a consequence of the soft sulfur donors and covalent M

S bonds. It also rationalizes the tendency to form coordinatively saturated species. In line with this, Mo(II) complexes, and likewise the analogous Cr(II) and W(II) species, frequently have seven-coordinate metal centers, illustrated by the structures of [Mo(CO)3 (R S4 )] (70, 71) and [Mo(CO)2 (S5 )] (72) (Scheme 19). Replacing three 2e
donor CO ligands by two 3e
donor NO ligands gives six-coordinate species, for example, [Mo(NO)2 (S4 )] (49). The RS4 molybdenum complexes containing Mo(VI) to Mo(0) centers illustrate that Sn ligands can bind to highoxidation state as well as to medium- and low-oxidation state metal centers. From a different point of view, the [Mo(R S4 )] fragments demonstrate their ability to bind hard as well as soft coligands (73, 74).

DITHIOLENES IN MORE COMPLEX LIGANDS

613

R

R

S

S S Mo S

S CO CO CO

S S Mo S S C C O O

R

R

[Mo(CO)3(RS4)]

S V S S Mo V O O S Mo O S S S

[Mo(O)2(BuS4)]

S

[Mo(NO) 2(S4)]

S

VI O S Mo O S

S IV S Mo

NO NO

S

[Mo(CO)2(S5)]

S S

S

S S Mo

[µ-O{Mo(O)(BuS4)}2]

Cl Cl

S

S III S Mo

NO Cl

S

S II S Mo

CO CO CO

S

S

S

[Mo(Cl) 2(S4)]

[Mo(Cl)(NO)(S 4)]

[Mo(CO)3(S4)]

2− S OC 0 S Mo CO OC C O

O C 0 OC CO Mo CO S S

−

− 2 e , −2 CO −

+ 2 e , +2 CO

2−

[µ-S4{Mo(CO)4}2]

S

S I

S

I

OC Mo Mo CO S CO C O C C O O [µ-S4{Mo(CO)3}2]

Scheme 19

The high-oxidation state Mo complexes do not follow the 18 VE rule. However, they contain p-donor ligands such as O2
or Cl
. Metal donor distances obtained from X-ray structure determinations indicate that in these complexes p-donor bonds reduce the electron deficiency of the metal centers. Another example is [Mo(NMe2 )(NO)(S4 )] obtained according to Eq. 14.

614

¨ RG SUTTER DIETER SELLMANN AND JO

[Mo(Cl)(NO)(S 4)]

+ 2 NHMe2 − NH2Me2Cl

S

S S Mo

NO NMe2

ð14Þ

S

The [Mo(NMe2 )(NO)(S4 )] complex formally has a 16 VE configuration, but exhibits a Mo

NMe2 distance of 192.4(8) pm, which is short in comparison with Mo

N single bonds that range from 210 to 220 pm. Even more conclusively, proton nuclear magnetic resonance (1 H NMR) spectroscopy showed that the NMe2 ligand is rotationally stable up to temperatures of 140 C, which indicates a rotation barrier of Erot 84 kJ mol
1 and is most plausibly rationalized with a Mo

NMe2 double bond. The NMe2 ligand acts as a 4 e
donor and enables the Mo center to obtain an 18 VE configuration (75). The redox induced opening and closing of metal coordination sites, that is, the generation of vacant sites and their subsequent occupation by substrates, are elementary steps suggested for the active sites of many oxidoreductases. The couple [m-S4 {Mo(CO)4 }2 ]2
/[m-S4 {Mo(CO)3 }2 ] (Scheme 19) illustrates how these processes may occur in [MS] enzymes. The oxidation of [m-S4 {Mo(CO)4 }2 ]2
leads to loss of two CO ligands. The resulting vacant sites are occupied by bridging thiolate donors. The Mo(I) centers in
Mo bond, [m-S4 {Mo(CO)3 }2 ] are held together by thiolate bridges, a Mo
and, in addition, the ‘‘clamp’’ of the S4 ligand C2 H4 bridge. This clamp prevents the complete separation of the Mo centers upon reduction. Reduction leads to reopening of vacant sites, one at each Mo center, that are occupied by CO (76). In this respect, the C2 H4 bridge can be envisaged to take over the role of an enzyme protein holding together metal centers in the course of reactions.
Mo A Mo

Mo single bond in [m-S4 {Mo(CO)3 }2 ] is inferred from the Mo

Mo double distance [291.9(1) pm] and diamagnetism of the complex. A Mo
bond exists in the diastereomeric [Mo(CO)(S4 )]2 complexes 8–10 showing an MoMo distances of 267.3(1) pm (8) (77a), 265.7(1) pm (9) (77b), and 265.2(2) pm (10) (77c) (Scheme 20). Note that complexes 8 and 9 are diastereomeric because one [Mo(CO)(S4 )] entity in 9 contains cis-thiolate donors. The nitrosyl complex 11 demonstrates how replacement of two CO by two NO ligands,
Mo double bond of which contribute six instead of four electrons, turns the Mo
10 into a Mo

Mo single bond in 11 [291.4(2)pm] (77d). All complexes are diamagnetic, and the MoMo distances are easily rationalized with 18 VE configurations that the Mo centers achieve by formation of either Mo

Mo single or double bonds. Stabilization of high iron and ruthenium oxidation states has been achieved
H2 ligand. The reaction according to Eq. 15 yields diamagwith the N2 H2 S2
netic [Ru(PCy3 )(N2 S2 )] which formally contains a Ru(IV) center (78).

DITHIOLENES IN MORE COMPLEX LIGANDS

RuCl3 · 3 H2O + 5 PCy3 + Li2N2H2S2

S S

PCy3

MeOH

S S Mo S C O

S Mo S C O

8

O C

ð15Þ

S RuIV S N N

64 ˚C

S

S S Mo S C O

615

S S S

Mo

S S

C O 9

O N

S

S S S Mo Mo S S S S C O

S S S S Mo Mo S S S S N O

10

11 Scheme 20

Oxidation of the 18 VE complexes [FeII(PR3 )2 (N2 H2 S2 )] (79) yields [FeIV(PR3 )(N2 S2 )], (R ¼ Me, Et, Pr, Cy) (80a). The complex [FeIV(PR3 ) (N2 S2 )] could be further oxidized with I2 to give [FeV(I)(N2 S2 )](80b) according to Eq. 16. S N Fe N H S H

PR3 PR3 PR3

+ O2 THF/MeOH

S Fe S N N

I + I2 − PR3

S Fe S N N

ð16Þ In the course of these reactions, the N2 H2 S2
2 ligand is further deprotonated to give the amide-thiolate ligand N2 S4
2 . The complexes [Fe(PR3 )(N2 S2 )] and [Fe(I)(N2 S2 )] are both paramagnetic exhibiting S ¼ 1 and S ¼ 12 spin states. They formally contain Fe(IV) and Fe(V) centers and represent examples of discrete high-oxidation state molecular iron complexes. Short Fe

S (218 pm) and Fe

N (184 pm) distances indicate that in both complexes p donation from thiolate and amide donors is effective in reducing the electron deficiency at the Fe centers and stabilizing their high-oxidation states.

¨ RG SUTTER DIETER SELLMANN AND JO

616

S S M S L

S S (R)

(RR)

(Z)(RR)

S

S S M S L β

S M S L

(S)

(RS)

(E)(RR)

(Z)(RS)

S

13

S S M

14

(E)(RS)

S

S S M

L

S S

S

15

16

S S M

S S M

L

S

S

S S M

S

(Z)(RS)

S S

S

S S M

S

S

S

S

19

L S M

S

S S

S

L

S S

S

L S M

S

e

αα-(Z)(SS)

ββ-(Z)(RR)

e

ββ-(Z)(SS)

αβ-(Z)(RS)

e

βα-(Z)(RS)

S S M

d

S

S

L

S S

S

S S M

d

L S M

S

S S M

S

S S

L 18

S

S S M

L

S S

L 21

αβ-(E)(RR) = βα-(E)(RR) d

S

L

20

αα-(Z)(RR)

S

L

S

S S M

L

19

21

S

17

S

20

14

L

S S M

S S M

S

16

S S M

S

S

S S M

L

L 15

L S M

18

S

(E)(RS)

αααβ- = βαββ(E)(SS) (E)(SS) (Z)(SS) (Z)(SS)

13

S S M

L

17

L

L

12

α

(SS)

ααββαβ- = βααββαααββ(Z)(RR) (Z)(RR) (E)(RR) (E)(RR) (Z)(RS) (Z)(RS) (E)(RS) (E)(RS) 12

S

e

αβ-(E)(SS) = βα-(E)(SS)

αα-(E)(RS) (meso) ββ-(E)(RS) (meso)

Scheme 21. The theoretically possible 10 stereoisomers of [M(L)(S4 )]2 complexes (\ ¼ ophenylene rings) with their schematic representations and enantiomeric (e) or diastereomeric (d) relationships.

DITHIOLENES IN MORE COMPLEX LIGANDS

617

D. Chirality The [Mo(CO)(S4 )]2 complexes demonstrate that combining [M(L)(S4 )] entities can give rise to distinctly different dinuclear structures. One important reason is the chirality of the C1 symmetrical [M(L)(S4 )] fragments and the resulting stereochemical nonequivalence of the lone pairs at the thiolate donors. A stereochemical analysis (Scheme 21) shows that racemic [M(L)(S4 )] fragments theoretically are able to form 10 dinuclear [M(L)(S4 )]2 stereoisomers. The RR, RS, and SS combinations of [M(L)(S4 )] fragments each can give rise to four stereoisomers. Because two of these isomers are identical, a total number of 10 isomers results (60). These isomers differ in following features. (a) The L ligands point either into the same [(Z) isomer] or into the opposite [(E) isomer] direction. (b) The bridging of the M centers takes place via different thiolato-S atoms, which differ in one point: the thioether-S atom in the ortho position of the respective benzene ring stands either trans to the ligand L (a bridging) or trans to the second thiolate bridge (b-bridging). The different thiolate S atoms are indicated by a and b in the insert of Scheme 21. The analysis also shows that in the case of (RR) and (SS) isomers the (Z) form is possible only when aa or bb bridging occurs, and the (E) form requires ab or ba bridging. Vice versa, in the case of (RS) isomers, the (Z) form can only exist with ab or ba bridges and the (E) form only with aa or bb bridges. The centrosymmetric aa-(E)(RS) and bb(E)(RS) isomers (17 and 18 in Scheme 21) are meso isomers of [M(L)(S4 )]2 . Isomers 12, 13, 20, and 21 have C2 symmetry; the remaining isomers possess C1 symmetry such that all dimers except 17 and 18 are chiral. The stereochemical relationships between the resulting 10 isomers is summarized at the bottom of Scheme 21. Molecular models furthermore indicate that steric hindrance is most likely for (Z) isomers (12, 15, 16, and 20 in Scheme 21). In particular, this analysis made it surprising that [M(L)(S4 )]2 species frequently exhibit the (Z) form, which leads to a remarkably unequal spatial filling of the M coordination spheres. Figure 6 depicts a space-filling model of [Fe(CO)(S4 )]2 that exists as aa-(Z)(SS)/aa-(Z)(RR) pair of enantiomers. The upper moiety of the molecule is sterically overcrowded by the bulky S4 ligands, the lower moiety contains only the small CO ligands, and the whole molecule reminds of a two-legged UFO (which, in this case, could be identified). Note that not only [M(L)(S4 )] but also the C2 symmetrical [M(S4 )] fragments as well as numerous other [M(Sn )] fragments are chiral. When di- or polynuclear complexes form from these fragments, stereoisomers have to be taken into account in the interpretation of spectra, in particular, 1 H NMR spectra. Unambiguous and conclusive assignment of signals usually requires the availability of stereoisomerically pure complexes. The chirality of [M(S4 )] fragments also gives rise to the question as to whether certain ‘‘chiral elements’’ such as ‘‘centers, planes, or axes of chirality’’

¨ RG SUTTER DIETER SELLMANN AND JO

618

S6

S2 S5

S7

C1

C2 S8

S4 Fe1

Fe2 S1 O2

S3

O1

Figure 6. Space-filling model of [Fe(CO)(m-S4 )]2 (60).

could be assigned to individual atoms or sections of these fragments (20, 21). This question concerns the widely-held belief that the transfer of chiral information in metal-centered assymmetric reactions is particularly efficient, if the metal center itself is ‘‘chiral’’ or the complexes are ‘‘chiral at the metal’’ (81). These terms implicate such an influence of the metal center and are still commonly used, although they were shown by Mislow and Sigel (82) to be misleading, if not to say illogical. Mislow and Sigel suggested that, for example, the term ‘‘chiral center’’ should be replaced by the terms ‘‘stereogenic and/or chirotopic center’’. In fact, such a differentiation is indispensable for metal complexes with coordination numbers higher than 4. Scheme 22 demonstrates S S M L S L S

+R −H

S H S M L L R S S

+ L' −L

S H S M L L' R S S

22

23

24

Chirotopic M

Chirotopic and prostereogenic M

Chirotopic and stereogenic M

Scheme 22. Stereochemical properties of the metal center M in octahedral [M(S4 )] complexes.

that the metal centers in [M(S4 )] complexes can precisely be differentiated with regard to their chirotopicity and stereogenicity. All three complexes are chiral, but only 24 contains a stereogenic metal center. In the C2 symmetric complex 22, all atoms including the metal center are chirotopic (chiral), however, the

DITHIOLENES IN MORE COMPLEX LIGANDS

619

metal center is not stereogenic, because permutation of the identical ligands L does not give a new stereoisomer. Introduction of a substituent R into the C2 H4 bridge results in the C1 symmetrical [M(L)2 (RS4 )] complex 23. The R carrying C atom has become stereogenic (an asymmetric C atom), however, the M center is still not stereogenic, because permutation of the ligands L does not yield new stereoisomers. Nevertheless, the M center in 23 differs stereochemically from that in 22, which is seen when one L is replaced by L0 to give complex 24. Permutation of L and L0 now gives two different stereoisomers of complex 24. Accordingly, the metal center M in 24 is not only chirotopic but also stereogenic. In order to express the relationship between 23 and 24, the M center in 23 is termed prostereogenic: Any further desymmetrization of 23 causes the M center to become stereogenic. Experimentally probing the consequences of these considerations yielded several results. 1. Alkylation of chiral [Mo(NO)2 (S2 C6 H4 )2 ]2
with racemic 1,2-dibromopropane or 1,2-dibromobutane gives diastereospecifically the pair of [Mo(NO)2 (RS4 )] enantiomers with equatorial R substituents (Eq. 17) (20). (This pair is one out of four theoretically possible pairs of diastereomeric enantiomers.) 2− H CH2Br R

Br

S +

S Mo NO S NO S

−

− 2 Br

S H S O Mo N NO R * S S

ð17Þ

ligands (containing RC2 H3 2. Likewise, coordination of racemic RS2
4 bridges) to metals diastereospecifically affords only one pair of enantiomers (Eq. 18) (21), which is a special case of ligand stereospecificity (83).

2−

[Ru(Cl)2(PPh3)3] + RS4

R = Me, PhCH2, HO(CH2)2

S H S Ru R * S S

PPh3 PPh3

ð18Þ

3. There are no differences in distances and angles of [Ru(PPh3 )2 (RS4 )] complexes and the [Ru(PPh3 )2 (S4 )] parent complex that can be traced back to the R substituents or stereogenic C atoms in [Ru(PPh3 )2 (RS4 )]. Nevertheless, the ruthenium prostereogenicity in [Ru(PPh3 )2 (RS4 )] is clearly seen in the reactions with CO or PMe3 . These reactions yield high

¨ RG SUTTER DIETER SELLMANN AND JO

620

diastereomeric excesses of one (out of two possible) substitution products. For example, the reaction according to Eq. 19 gave the diastereomeric CO complexes in a ratio of 10:1 (21). S H S Ru PPh3 S PPh3 Me S

S H S O Ru C S PPh3 Me S

+ CO − PPh3

S H S PPh3 Ru + S CO Me S

10

E. 1.

:

ð19Þ

1

Electronic Effects

High-Spin versus Low-Spin [M(Sn )] Complexes

When [M(Sn )] complexes contain 4d or 5d metal centers, they are always in the low-spin state. In this regard, [M(Sn )] complexes behave like other 4d and 5d metal complexes. The [M(Sn )] complexes with the 3d metal nickel also exhibit normal magnetic behavior. Square-planar complexes with [NiS4 ] cores are diamagnetic, six-coordinate pseudo-octahedral complexes are always paramagnetic with S ¼ 1 ground states as expected from simple crystal field or molecular orbital theory. Five-coordinate Ni complexes, which could be either paramagnetic or diamagnetic, prefer the diamagnetic state, demonstrated by the [Ni(S5 )] or [Ni(PMe3 )(S4 )] complexes. In these complexes, however, one nickel–sulfur bond is considerably longer than the other nickel–sulfur bonds. The [FeII(L)(NS4 )] complexes are different. They formally obey the 18 VE rule but can be either paramagnetic or diamagnetic. The series of [Fe(L)(NH S4 )] and [Fe(L)(pyS4 )] complexes reveals how steric requirements of the NS4 ligand and the nature of the coligands L can influence the structures, magnetism, and reactivity of [Fe(Sn )] complexes. The five-coordinate [Fe(NH S4 )] is paramagnetic (S ¼ 2) and has a distorted trigonal-bipyramidal structure and an average Fe

S(thiolate) distance of 230.6 pm, which is short in comparison to the average Fe

S(thioether) distance (254.0 pm). The [Fe(NH S4 )] complex reversibly adds s-donor ligands such as MeOH, N2 H4 , NH3 , or N
3 to give paramagnetic (S ¼ 2) six-coordinate complexes (cf. Section IV.B). All of these
N (225 pm) and derivatives are Cs symmetrical, labile, and exhibit long Fe
long Fe

S (240–260 pm) distances. In contrast, the addition of s–p ligands such as CO, NOþ , and phosphines is irreversible and yields diamagnetic derivatives. These complexes have C1 symmetry, are inert, and exhibit short Fe

N (207 pm) and short Fe

S (220–230 pm) distances. Thus, the change from s to s–p ligands results in a structural rearrangement, a spin-state change, and a concomitant shortening of the [Fe(NH S4 )] core distances (47, 52).

DITHIOLENES IN MORE COMPLEX LIGANDS

621

H N

σL

H N S S

Fe

S S

σL (N2H4, NH3, N3 ) CS , S = 2 , long distances labile

S Fe S S S [Fe(NHS4)] eσ*

eσ* t2

aM eπ

σ−π L

H N

S S Fe S S σ−π L + (CO, NO , PR3) C1 , S = 0 , short distances inert

Figure 7

The electronic reason for these effects is the splitting and occupation of the frontier molecular orbitals in both types of complexes (Fig. 7). The energy gap between the nonbonding t2 and the es orbitals, which are antibonding with respect to the metal donor s bonds, is evidently small. If the coligand L is a s-donor ligand, four unpaired electrons occupy the t2 and es orbitals. If the coligand L is a s-donor p-acceptor, the t2 orbitals are stabilized, the energy gap widens, and six paired electrons result that either are involved in p back-bonding (ep ) or are nonbonding (aM ). In this regard, the NH S4 ligand represents a borderline case between weak- and strong-field ligands, and the nature of the coligand L determines which spin state is favored for the [Fe(L)(NH S4 )] complex (25). In contrast, the complexes with [Fe(pyS4 )] fragments always are low spin, not only with s–p, but also with s coligands.

N S S

Fe S L

S

25

[Fe(pyS4)L]

These complexes all exhibit a configuration in which the thiolate and thioether donors pairwise adopt trans positions, and have short Fe–donor bond distances indicating the absence of antibonding electrons, which is shown exemplarily by [Fe(N2 H4 )(pyS4 )] (67), [Fe(CO)(pyS4 )] (31), and [Fe(CO) (NH S4 )] (28) in Fig. 8. The pyridine N donor exerts a slightly stronger ligand field than the secondary amine NH donor, however, the major reason for favoring the low-spin

¨ RG SUTTER DIETER SELLMANN AND JO

622

2 1

S S

H2N

1N

S Fe S 3 NH2

4

2 1

[Fe(N2H4)(pyS4)] Fe−S1 Fe−S2 Fe−S3 Fe−S4 Fe−N1 Fe−L

229.9(1) 221.2(1) 222.7(1) 230.8(1) 195.5(3) 204.2

Figure 8.

S S

1N

S Fe S 3 C O

H N

4

[Fe(CO)(pyS4)] 231.1(2) 223.2(1) 222.5(1) 228.9(2) 201.4(2) 175.7(2)

2

1

S S Fe S S 3 1 C O

4

[Fe(CO)(NHS4)] 230.5(3) 222.5(3) 225.1(3) 229.8(3) 207.2(8) 175.3(12)

Comparison of bond distances in [Fe(L)(pyS4 )] and [Fe(CO)(NH S4 )] complexes.

state in [Fe(L)(pyS4 )] complexes might be the rigidity of the py(CH2 )2 bridge. This bridge sterically forces the thiolate and thioether donors into trans prositions and the [Fe(pyS4 )] fragment to retain short Fe-donor bonds, such that the Fe(II) center is prevented from assuming a high-spin state that would require long Fe-donor bonds due to antibonding electrons. 2.

Benzene Ring Substituent Effects

With regard to electronic effects, another important question concerns the electronic influence of benzene ring substituents in Sn ligands upon the electron density at coordinated metal centers. In other words, can electronic changes in the benzene rings be transmitted beyond the sulfur donors to the metal? The complexes of Table I demonstrate that such effects are negligible. The n(CO) or n(NO) frequencies that sensitively reflect electronic changes at metal centers are practically identical in corresponding complexes although the benzene rings exhibit either electron-donating or -withdrawing substituents. For example, four tertiary butyl groups in [Fe(CO)(Bu S5 )], eight methyl groups in [Fe(CO)(Me8 S5 )], four CO2 Me groups in [Fe(CO)(CO2 Me S5 )], and eight chloro groups in [Fe(CO)(Cl8 S5 )] do not cause any significant n(CO) shift when compaired with the parent complex [Fe(CO)(S5 )]. If there are n(CO) shifts occurring, these shifts are extremely small in comparison to the large n(CO) shifts caused by direct protonation or alkylation of thiolate donors (cf. Section IV.F).

DITHIOLENES IN MORE COMPLEX LIGANDS

623

TABLE I The n(CO) and n(NO) Frequencies of [M(L)x (Sn )] Complexes with Electron-Donating or -Withdrawing Benzene Ring Substituents Stretching Frequenciesa

Complex

Reference

[Fe(CO)2 (S4 )] [Fe(CO)2 (Bu S4 )] [Fe(CO)2 (S5 )] [Fe(CO)2 (Bu S5 )] [Fe(CO)2 (Me8 S5 )] [Fe(CO)2 (CO2 Me S5 )] [Fe(CO)2 (Cl8 S5 )]

2040/2000 2040/1995 1960 1960 1960 1963 1965

5 16 25 26 25 27 15

[Ru(CO)2 (S4 )] [Ru(CO)2 (Bu S4 )]

2055/2005 2048/1990

84 85

[Mo(CO)3 (S4 )] [Mo(CO)3 (Bu S4 )] [Ru(NO)(pyS4 )]þ [Ru(NO)(pyBu S4 )]þ

2020/1940 2020/1946 1892 1892

48 86 53 32

a

In KBr(cm
1 ).

F.

Protonation, Alkylation, and Redox Chemistry of Complexes with [M(Sn )] Fragments

An essential feature of reactions catalyzed by metal–sulfur oxidoreductases is the coupling of proton- and electron-transfer processes. In this context, an important question is how primary protonation of metal–sulfur sites influences the metal–sulfur cores, small molecules bound to them, and the subsequent transfer of electrons. In order to shed light upon this question, protonations, isoelectronic alkylations, and redox reactions of [M(L)n (Sn )] complexes were 2
; NH S2
investigated (M ¼ Fe, Ru, Mo; L ¼ CO, NO; Sn ¼ S2
4 , S5 4 ). The CO and NO ligands served as infrared (IR) probe for the electron density at the metal centers. Resulting complexes were characterized as far as possible by Xray crystallography. Scheme 23 shows examples of such complexes. + S S

S Mo S

S NO L

S S

Rh

H N

S PR3 CO

S S

S

Fe

CO CO

S

[Mo(L)(NO)(S4)] (79) [Rh(CO)(PR3)(S4)]+ (87)

[Fe(CO)2(S4)] (88)

Scheme 23

S S Fe S S C O [Fe(CO)(NHS4)] (89)

¨ RG SUTTER DIETER SELLMANN AND JO

624

Strikingly similar results were obtained for all complexes and revealed a structure-function relationship of [M(L)(Sn )] complexes that can be summarized as follows: Metal thioether thiolate complexes exhibit Brønsted acid–base behavior and can be reversibly protonated. Thiolate protonation drastically influences electron densities at the metal centers and redox potentials of complexes. However, in all these reactions and electronically changing situations, the metal–sulfur core of a specific [M(Sn )] complex remains invariant. This invariance of the core is illustrated by the iron complexes [Fe(CO)2 (S4 )] and [Fe(CO)2 (NH S4 )], which have been investigated particularly thoroughly. In CH2 Cl2 solution, the thiolate donors of both [Fe(CO)2 (S4 )] and [Fe(CO)2 (NH S4 )] can be protonated reversibly and in single steps by HBF4, as shown in Eq. 20.

S S S

Fe S

CO CO

S

+ 1 HBF4 CH2Cl2

S S

Fe S

+

H CO CO

S

+ 1 HBF4 CH2Cl2

S S

Fe S

2+

H CO CO

ð20Þ

H

The protonation leads to a strong increase of the n(CO) frequencies. The exactly analogous n(CO) frequency increase was observed when the thiolate donors were stepwise alkylated with Me3 OBF4 or Et3 OBF4. While the resulting thiol derivatives proved too labile to be isolated, the isoelectronic alkyl derivatives could be crystallized and characterized by X-ray structure analysis. Series of isoelectronic complexes were obtained, which comprise up to four species as in
Et)]þ , and the case of [Fe(CO)2 (S2 C6 H4 )2 ]2
, [Fe(CO)2 (S4 )], [Fe(CO)2 (S4
2þ
Et2 )] . [Fe(CO)2 (S4
Table II summarizes IR spectroscopy and X-ray structural results for [Fe(CO)2 (S4 )] and [Fe(CO)(NH S4 )] complexes. In all cases, each step of protonation or alkylation increases the n(CO) frequencies of the complexes by 35–40 cm
1 . The overall increase of the n(CO) frequencies that can reach 150 cm
1 , is remarkably large and reflects the weakening of Fe

CO p back-bonding resulting from a considerable decrease of electron density at the Fe centers. This observation suggested that there should be a correlation between n(CO) frequencies and Fe

S distances. However, Table II shows that such a correlation does not exist. The Fe

S distances remain virtually invariant. Note that in the case of [Fe(CO)2 (S4 )] and its ethyl derivatives, even the Fe

S(thiolate) and the Fe

S(thioether) distances are identical within standard deviations. The invariance of Fe

S distances that strongly contrasts with the difference in electron density at the Fe centers as indicated by the large n(CO) shifts can be rationalized by the following bonding scheme (Scheme 24).

DITHIOLENES IN MORE COMPLEX LIGANDS

625

TABLE II n(CO) Frequencies (cm
1 ) versus FeS Distances (pm) in [Fe(CO)2 (S4 )], [Fe(CO)(NH S4 )], and Their Alkylated Derivatives

ν (CO)av. d (Fe–S)

S S S

Fe

CO CO

S

2−

1939

232.7

2010

228.4(2) (88)

2049

228.9(2) (88)

2091

228.1(2) (88)

H N

2+

+

[Fe(CO)2(S4−Et)]

2+

[Fe(CO)2(S4−Et2)]

4

S S 3

H N Fe

2

4

S S 1

C O

2

S S Fe S S 3 Et 1 C Me O

ν(CO)

1954

2026

Fe(1)–C(1) Fe(1)–N(1) Fe(1)–S(1) Fe(1)–S(2) Fe(1)–S(3) Fe(1)–S(4)

175.3(12) 207.2(8) 229.8(3) 222.5(3) 225.5(3) 230.5(3)

177.2(12) 203.4(9) 227.1(4) 224.5(4) 224.4(4) 229.1(4)

SR

Fe +

+

(90)

[Fe(CO)2(S2C6H4)2] [Fe(CO)2(S4)]

R+

Fe

S

R R+

Fe

S

R R+

+

R = H , CH3 , C2H5

Scheme 24

Protonation, and likewise alkylation, leads to weakening of the respective S ! Fe s bonds and an inductive withdrawal of electron density from the Fe centers. The newly formed thiol or thioether donors have p-acceptor capability and partial Fe

S p back-bonding leads to a further decrease of electron density at Fe. However, the weakening of the Fe

S s bonds and formation of the Fe ! S p back-bonds compensate each other so that the Fe

S distances remain invariant. The decrease of electron density at the iron centers caused by protonating or alkylating thiolate donors strongly shifts the redox potentials of corresponding complexes, which were obtained from cyclic voltammograms. Table III summarizes the results (and assignments) for [Fe(CO)(NH S4 )] and its alkylated derivatives (89).

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626

TABLE III Redox Potentials of [Fe(CO)(NH S4 )] and Its Protonated or Alkylated Derivativesa;b

S S

Fe

+

H N

H N S S

C O

S S

Fe

R

C O

2+

H N

S S

S S

Fe

R

C O

S S R

Redox Couple Fe(III)/ Fe(II)

+ 0.35 q

Fe(II) / Fe(I) Fe(I) / Fe(0)

n.o. n.o.

a b

0.98 q R=H R = Et 1.02 q R = Et −1.34 i n.o.

R=H R = Et R = Et

1.46 i n.o. −0.54 q −1.44 i

−1

In V versus NHE, in CH 2Cl2, v = 100 mV s . q = quasireversible; i = irreversible; n.o. = no observed;

∧

=

.

The data of Table III reveal two important points. (1) Protonation and alkylation have identical consequences and anodically shift the redox potentials of corresponding redox couples by a remarkable 500–700 mV per protonation or alkylation step. (2) This redox potential shift makes the protonated (or alkylated) derivatives more difficult to oxidize, but easier to reduce than the parent complex. Irreducible complexes can become reducible upon protonation or alkylation. For example, the 18 VE complex [Fe(CO)(NH S4 )] shows no redox wave in the cathodic region down to
1.8 V, however, the doubly alkylated
Et2 )]2þ exhibits two cathodic redox waves at
0.54 derivative [Fe(CO)(NH S4
and
1.44 V. The wave at
0.54 V can be assigned to the reversible formation
Et2 )]þ . The data of Tables II and III of the 19 VE monocation [Fe(CO)(NH S4
allow the following conclusions: The invariance of Fe

S distances combined with the Broensted acid–base behavior and strong coupling of proton and electron flux is a genuine structure–function relationship of [MS] species. It facilitates redox reactions for kinetic and thermodynamic reasons, and this may also be true for the active centers of [MS] enzymes, even if these centers have completely different structures. The most important consequence for oxidoreductases such as nitrogenases could be that the reduction of N2 under mild conditions requires a primary protonation of the FeMo cofactor. G.

C

S Bond Cleavage

The cleavage of C

S bonds in [M(Sn )] complexes usually concerns S

C(alkyl) bonds and is an unwelcome reaction in most cases. This unwanted

DITHIOLENES IN MORE COMPLEX LIGANDS

627

reaction was one reason to synthesize the tpS4

H2 ligand, which exhibits only S

C(aromatic) bonds that are relatively inert toward S

C cleavage. Occasionally, S

C cleavage reactions in Sn ligands can lead to unusual species. Cleavage of S

C bonds in Sn ligands has been observed in the presence of strong reductants, in reactions with electron-rich metal complexes, under ultraviolet (UV)-irridiation, thermally, in the presence of bases, and in reactions involving complexes with unpaired electrons. The following examples demonstrate this. The [Mo(CO)4 (Cl)2 ] complex reacts with S2
4 to give the Mo(II) complex [Mo(CO)3 (S4 )] in a straightforward reaction. The analogous reaction of [Mo(PMe3 )4 (Cl)2 ] with S2
4 takes a different route. Here C2 H4 is released and the Mo(IV) complex, [Mo(PMe3 )2 (S2 C6 H4 )2 ], forms (91). The reaction is rationalized in Eq. 21.

2−

[Mo(PMe3)4Cl2]

+ S4

− PMe3, − Cl

−

S

S II S Mo

PMe3 PMe3

C2H4 +

S

S IV S Mo

S

PMe3 PMe3

S

ð21Þ Complex formation primarily yields [Mo(PMe3 )2 (S4 )]. The relatively poor pacceptors PMe3 cause an electron density at the Mo center much higher than in [Mo(CO)3 (S4 )]. This electron density is released by reducing the C2 H4 bridge of the S4 ligand to give ethylene. The reaction reverses the formation of S2
4 at the Fe(CO)2 templates. It is also related to the reactions of the Ni, Pd, and Pt dithiolene complexes described in Section II.C. However, the reaction according to Eq. 21 is irreversible, and [Mo(PMe3 )2 (S2 C6 H4 )2 ] could not be alkylated, either with ethylene or with 1,2-C2 H4 Br2 (92). The reaction mechanism suggested in Eq. 21 also explains the decomposition of [Os(PEt3 )2 (S4 )] yielding [Os(PEt3 )2 (S2 C6 H4 )2 ] and C2 H4 (93), as well as the conversion of [Mo(O)2 (Bu S4 )] into [Mo(PMe3 )2 (3,5-t-Bu-C6 H2 S2 )2 ] by treatment with an excess of PMe3 (73a). Though stable at room temperature, [Mo(CO)3 (S4 )] and [Mo(CO)3 (Bu S4 )] contain labilized C2 H4 bridges, which is seen when the complexes are heated to 80 C or UV irradiated at standard temperature (Eq. 22) (70).

[Mo(CO)3(BuS4)]

80 ˚C or UV

C2H4 +

S

S S Mo S

CO + CO CO

ð22Þ

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628

The compound C2 H4 is released and, starting from a Mo(II) complex, again a Mo(IV) species results, which, inter alia, is an interesting species showing the CO coordination to a high-oxidation state metal center. The C2 H4 release is accompanied by a decrease of the average MoS distances of 18 pm, indicating that in the high-oxidation state [MoIV (CO)2 (Bu S2 )2 ] complex S ! Mo p donation becomes effective (70). Similar C2 H4 bridge-dissociation reactions occured when [Fe(CO)2 (S4 )] was treated with LiBEt3 H or metallic sodium, or in reactions between [Fe(CO)5 ] or 2
[Fe(CO)3 (butadiene)] with S2
4 . These reactions yielded [Fe(S2 C6 H4 )2 ] , 2
2
and probably involve the [Fe(CO)(S2 C6 H4 )2 ] , or [Fe(CO)2 (S2 C6 H4 )2 ] [Fe0 (S4 )] intermediate species (94). Simple addition of bases resulted in a S

C cleavage in the thia crown ether complex [Ru(Bzo2 -18-S-6)]2þ (33a). A vinylthioether thiolate ligand formed and the reaction is rationalized by a base induced Hþ abstraction as indicated in Eq. 23. H H H S H S S Ru S S S [Ru(Bzo2-18S6)]

B 2+

+

S CH CH2

−

+ OH

S S Ru S S S

+

−H

ð23Þ

2+

A similar reaction takes place when [Ru(NO)(S5 )]Br is treated with LiNH2 (Eq. 24) (95). + Br−

S S S Ru S S N O

+ NH2

S CH CH2

−

S S Ru S S N O

− NH3 − − Br

ð24Þ

[Ru(NO)(S5)]Br

Unpaired electrons that occupy antibonding orbitals in the 19 VE intermediate
C cleavage according to Eq. 25 (96). {[Ru(NO)(PPh3 )(S4 ) ]}, might cause the S


[Ru(PPh3)2(S4)]

+ NO − PPh3

{[Ru(NO)(PPh3)(S4)·]}

−H

.

S S Ru H2C CH S S

PPh3 NO

ð25Þ

DITHIOLENES IN MORE COMPLEX LIGANDS

629

The reaction according to Eq. 26 is considerably more complicated. The product complex of Eq. 26 contains a C2 H4 SH group and formally results from two [Ru(NO)(Bu S5 )]Br, one of which loses CH2 CHBr, the other one NOBr (97). SH S [Ru(NO)(BuS5)]Br

+ LiBEt3H

S S Ru S NO S S Ru S S S

ð26Þ

The S

C bond-breaking and bond-making reactions take place when [Ru(CO)(S5 )] is treated with S(C2 H4 Br)2 to give the thia crown ether complex [Ru(CO)(Br)2 (Bzo-9-S-3)] (Bzo-9-S-3 ¼ Trithio-benzo[9]crown-6) according to Eq. 27 (98), where DMF ¼ dimethylformamide.

[Ru(CO)(S5)] + S(C2H4Br)2

DMF 140 ˚C

S S Br + other products Ru S Br C O

ð27Þ

[Ru(CO)(Br)2(Bzo-9S3)]

This reaction is rather different from the reaction between [Fe(CO)(S5 )] and S(C2 H4 Br)2 , which yielded the target thia crown ether complex [Fe(Bzo2 18-S-6)]Br2 (33b).

H.

Characterization

The inaccessibility of sulfur by common spectroscopic methods requires in most cases X-ray structure determinations in order to establish the structure of [M(Sn )] complexes unambiguously. Practically all complexes that are mentioned in this chapter have been characterized by X-ray crystallography. The IR spectra usually are suited only to serve as fingerprints. The same holds true for the complex splitting patterns observed in 1 H NMR spectra. However, IR spectroscopy can serve as a very convenient and rapid method for monitoring reactions of complexes that have characteristic n(CO) or n(NO) bands. In addition, n(CO) or n(NO) frequency shifts are very sensitive probes for detecting electronic changes at the metal centers. The 13 C NMR spectra have been valuable for determining basic symmetries. Simple counting of 13 C signal numbers, for example, enables one to distinguish between C1 and C2 or Cs

630

¨ RG SUTTER DIETER SELLMANN AND JO

symmetrical complexes. Mass spectra were mainly obtained with FD (field desorption) ionization. Characteristic peaks are the [M(Sn )] fragment ions. The UV–vis (visible) spectra are dominated by strong M

S charge-transfer bands and the p–p transition bands of the benzene rings. In the case of [m-S2 {M(Sn )}2 ] and [m-N2 H2 {M(Sn )}2 ] complexes, strong bands in the visible region were indicative of delocalized multicenter p-bond systems. Most of the complexes proved electrochemically active and enabled the aquisition of cyclic voltammograms.

V. CONVERSION OF SMALL MOLECULES IN THE COORDINATION SPHERE OF [M(Sn )] COMPLEXES A.

Introduction

Activation of stable molecules, stabilization of unstable species, and the transfer of charge comprising electrons, protons, and other ions are elementary steps of biological and industrial processes catalyzed by redox-active metal complexes. Detailed understanding of the elementary steps is a requirement for a mechanistic understanding of the overall processes as well as for the development of either ‘‘competitive’’ or more efficient catalysts. This section describes the properties of [M(L)(Sn )] complexes in which L represents various small molecules. The NO complexes, for example, are considered important intermediates in many reactions, ranging from waste water purifications to the bacterial nitrate–ammonia or nitrate–dinitrogen conversion or the mammalian bioregulation, in which NO acts as neurotransmitter (99–104). B. 1.

Nitrosyl Complexes

18 and 19 VE Nitrosyl Complexes

The majority of metal nitrosyl complexes have 18 VE configurations. For the elucidation of redox mechanisms, a point of considerable interest will be the consequence caused by one-electron reduction of 18 VE nitrosyl complexes to give 19 VE species. Complexes with 19 VE are usually highly labile and too unstable for isolation and complete characterization, but nitrosyl complexes with [M(Sn )] cores have yielded a few exceptions to this rule. One-electron reductions of [Fe(NO)(Bu S5 )]þ and [Fe(NO)(NH S4 )]þ give the labile but isolable 19 VE complexes [Fe(NO)(Bu S5 )] (105) and [Fe(NO)(NH S4 )] (106). Their lability is evidenced by the NO ! CO exchange, yielding the stable CO

DITHIOLENES IN MORE COMPLEX LIGANDS

631

complexes (Eq. 28). Nitrous oxide for CO exchange is a rare reaction, because usually only the reverse, CO for NO exchange, is possible. [Fe(NO)(XS 4)] + CO

[Fe(CO)(XS4)] + NO

X = S, N H

ð28Þ

The neutral 19 VE complex [Fe(NO)(NH S4 )] could be structurally characterized. In comparison with the 18 VE carbonyl complex [Fe(CO)(NH S4 )], it exhibits elongated Fe-donor bonds in the [Fe(NH S4 )] fragment. The FeNO group is bent (142 ). The frontier-orbital occupation according to Scheme 25 rationalizes these findings, demonstrating that the 18 and 19 VE complexes are both e*σ

e*σ −

+1e

18 VE

aM

aM

eπ

eπ 19 VE

Scheme 25. Frontier orbital occupation in 18 and 19 VE six-coordinate nitrosyl metal complexes.

low spin, and the odd electron must occupy a molecular orbital that is antibonding with respect to the Fe–donor s bonds (106). The [Fe(NO)(pyS4 )]þ=0 complexes enabled the first direct structural comparison of strictly homologous 18 and 19 VE nitrosyl complexes exhibiting identical atom connectivities (Fig. 9) (107). Addition of the 19th VE again leads to elongation of all Fe–donor bonds indicating the occupation of an antibonding s* type molecular orbital. The bending of the FeNO group from 179.5 to 150.4 and simultaneous n(NO) frequency decrease from 1901 to 1670 cm
1 indicate that this orbital must have a considerable contribution from a p*-type NO orbital. The density functional theory (DFT) calculations corroborate these conclusions and show that the occupied [Fe(NO)(pyS4 )]þ orbitals close to the highest occupied molecular orbital (HOMO) are sulfur dominated, forming a valence band of energetically closely spaced orbitals that provides flexibility for different binding modes of the FeNO moiety as well as the [Fe(pyS4 )] core. Due to the antibonding electron, [Fe(NO)(pyS4 )], too, shows a rapid NO ! CO exchange to yield the 18 VE complex [Fe(CO)(pyS4 )]. Furthermore, the complex is readily reoxidized to give the [Fe(NO)(pyS4 )]þ cation.

¨ RG SUTTER DIETER SELLMANN AND JO

632

+ 1N S S4 Fe S S 3 1 2N O 2

Fe−S1 Fe−S2 Fe−S3 Fe−S4 Fe−N1 Fe−N2 Fe−N2−O

−

+e −e

−

2

S S 1

1N

S Fe S 3 2N

4

O

18 VE

19 VE

231.3(2) 225.6(2) 225.4(2) 231.2(2) 200.5(3) 163.4(3) 179.5(3)

229.7(1) 230.1(1) 230.0(1) 229.7(1) 216.7(2) 171.2(3) 150.4(5)

Figure 9. Reversible 1 e
redox reactions and bond distance changes of the 18 and 19 VE [Fe(NO)(pyS4 )]þ=0 complexes.

2.

16, 17, and 18 VE Nitrosyl Complexes

The [Mo(NO)(S4 )] fragment yielded a series of three complexes with formal 16, 17, and 18 VE counts (Fig. 10). Although the chloro complex [Mo(Cl)(NO)(S4 )] potentially can be considered an 18 VE species due to p donation from the chloride ion (see above), the different electronic situations in the three complexes are clearly reflected by the n(NO) frequencies. However, there are no antibonding electrons, and the Mo

S (and other Mo donor) bond distances stay in the usual range. In addition, while Mo

S(thiolate) and Mo

S(thioether) distances in different [Mo(S4 )] complexes can vary considerably, the respective Mo

S distances in [Mo(Cl)(NO)(S4 )], [Mo(PMe3 )(NO)(S4 )], and [Mo(NO)2 (S4 )] are very similar. Figure 10 summarizes these results and correlations (108, 109).

3.

Conversions of NO into NPR3 , H2 NO, and HNO Ligands

Electron transfer coupled with atom or proton transfer converts NO into lower oxidation state N ligands. The first example achieved with [M(Sn )] nitrosyl complexes is given by Eq. 29 (49, 71).

DITHIOLENES IN MORE COMPLEX LIGANDS

S

S III S Mo

NO Cl

S

S II S Mo

S [Mo(Cl)(NO)(S 4)] VE count

NO PMe3

633

S

S II S Mo

NO NO

S

S

[Mo(PMe3)(NO)(S4)]

[Mo(NO)2(S4)]

16

17

18

ν(NO) cm

1670

1603

1760, 1665

d Mo-S(thiolate) pm

239.7(2)

242.5(2)

245.1(3)

d Mo-S(thioether) pm

253.8(2)

254.7(3)

253.6(3)

−1

Occupancy of frontier orbitals Figure 10. Correlations between formal VE counts, n(NO) frequencies, Mo

S distances and occupancy of frontier orbitals in [Mo(L)(NO)(S4 )] complexes.

[Mo(NO) 2(S4)] + 2 PR3

S

S S Mo

NO + OPR3 N PR3

ð29Þ

S

The [Mo(NO)2 (S4 )] complex has two relatively low-frequency n(NO) bands (1760 and 1665 cm
1 ). Such low-frequency NO ligands usually are not susceptible to nucleophilic additions (110). The [Mo(NO)2 (S4 )] complex, however, readily reacted with alkyl or arylphosphines PR3 to give phosphine iminato complexes. The second equivalent of PR3 serves to bind the oxygen atom of the NO ligand. If we consider the phosphine iminato group NPR
3 to be a mononegative ligand containing phosphorus P(V) and nitrogen N(
III), a formal electron count results in a Mo(II) ! Mo(III) oxidation and two times a P(III) ! P(V) oxidation. The resulting five electrons are used to reduce the N(þII) in the NO down to N(
III) in the NPMe3 ligand. The reaction according to Eq. 29, which at first sight looks like a nucleophilic PR3 addition to NO ligands, is probably better rationalized by the reaction pathway of Scheme 26. Attack of the Mo center by one PR3 leads to bending of one NO ligand. Such reactions are familiar to NO complexes that generally do not exceed 18 VE

¨ RG SUTTER DIETER SELLMANN AND JO

634

S

S S Mo

NO NO

S O + PR3 S Mo N PR3 S N S O

+ PR3

S

S O S Mo N PR3 S N S O PR3

S O S Mo N PR3 S N S O PR3

S

S S Mo S

NO N PR3 + O PR3

Scheme 26

configurations (110). Subsequent attack of the labilized N

O bond by a second PR3 , formation of OPR3 and a final Mo ! N shift of the molybdenum bound PR3 yield the final product. A similar mechanism is suggested for the NO reduction that yields a Z2 H2 NO (hydroxylaminyl) complex according to Eq. 30 (111, 112).

[Mo(NO) 2(S4)]

+ NaBH4 S

MeOH

S S Mo S

NO NH2 O

ð30Þ

Primary hydride attack of the Mo center, intramolecular two-electron transfer from H
via the Mo atom to the NO ligand, and protonation of the N atom with protons resulting from the (oxidized) hydride and solvent MeOH give the NH2 O complex (Eq. 31). −

2−

S −

[Mo(NO) 2(S4)] + H

S

S Mo S

S NO H N O

S

S Mo S

+ H+ NO N H+ O

S

S S Mo S

NO NH2 O

ð31Þ The reactions according to Eqs. 29 and 30 are both certainly favored by the readiness of Mo(II) to form seven-coordinate complexes. The formation of the N2 H3 complex [Mo(N2 H3 )(NO)(S4 )] does not involve a NO conversion, but is closely related from mechanistic and structural points of

DITHIOLENES IN MORE COMPLEX LIGANDS

635

view. The [Mo(N2 H3 )(NO)(S4 )] complex forms from [Mo(Cl)(NO)(S4 )] and 2 equiv of N2 H4 , probably also via a seven-coordinate intermediate (Eq. 32).

[Mo(Cl)(NO)(S 4)] + N2H4

S

S S Mo S

+ N2H4 NO NH2NH2 − N H Cl 2 5 Cl

S

S S Mo S

NO NH NH2

ð32Þ The methyl derivative [Mo(NHNMe2 )(NO)(S4 )] could be characterized by X-ray crystallography (Fig. 11) (113). The formation of [Mo(NH2 O)(NO)(S4 )] represents a [2 Hþ /2 e
] reduction of a [Mo(NO)2 (S4 )] complex. A [1 Hþ /2 e
] reduction of an NO complex takes place according to Eq. 33. + −

N S S Ru S S N O

N S S Ru S S N O H

+ H (NaBH4) MeOH +

ð33Þ

[Ru(HNO)(pyS4)]

[Ru(NO)(pyS4)]

This [1 Hþ /2 e
] reduction is equivalent to a nucleophilic hydride addition to a NO ligand, a reaction that is analogous to the well-known formation of metal

S3 Mo1

S4 O1 N1 H2 N2

S2 S1

N3

C1

C2 Figure 11. Molecular structure of [Mo(NHNMe2 )(NO)(S4 )].

¨ RG SUTTER DIETER SELLMANN AND JO

636

formyl species from M

CO complexes. Frequently postulated, but never observed with isolable species, the hydride addition to the NO complex yields a complex stabilizing HNO that is extremely unstable in the free state. The complex [Ru(HNO)(pyS4 )] is only the second HNO complex characterized by X-ray structure analysis (114). The first example was [Os(HNO)(Cl)2 (CO) (PPh3 )2 ], which formed by HCl addition to [Os(NO)(Cl)(CO)(PPh3 )2 ] (115). A Cycle for the NO ! NH2 OH Reduction

4.

The reactions described in the previous sections illustrate the robustness of the [M(Sn )] complex fragments and their simultaneous flexibility with regard to coordination numbers, oxidation states, and structures. Such properties are also required for homogeneous catalysts, and the [Mo(NO)(S4 )] fragment closes a cycle leading from NO to NH2 OH. The [Mo(NH2 O)(NO)(S4 )] complex yields [Mo(Cl)(NO)(S4 )] and NH2 OH upon treatment with HCl. The resulting [Mo(Cl)(NO)(S4 )] can be reductively nitrosylated in the presence of zinc to give the original starting complex [Mo(NO)2 (S4 )] (Eqs. 34 and 35) (116, 117). [Mo(NH2O)(NO)(S 4)] + HCl

[Mo(Cl)(NO)(S 4)] + NH2OH

[Mo(Cl)(NO)(S 4)] + NO

+ Zn − ZnCl2

ð34Þ ð35Þ

[Mo(NO)2(S4)]

The cycle of Scheme 27 demonstrates that the [3 Hþ /3 e
] reduction of NO to give NH2 OH can be separated into two distinct steps: A [2 Hþ /2 e
] reduction that involves no Mo electrons, and a [1 Hþ /1 e
] reduction in which one electron comes from the Mo center. Note that the [2 Hþ /2 e
] reduction step takes place first thereby skipping the N0 oxidation state with formation of thermodynamically favored dinitrogen.

− Cl

II

(S4)Mo

NO − + NO 2 H , 2 e

−

+ NO, e

III NO (S4)Mo Cl

NH2OH

S II

(S4)Mo

NO O NH2

S S Mo

NO

S [Mo(NO)(S 4)]

− + + H , Cl

Scheme 27. Cycle for the NO ! NH2 OH reduction in a [Mo(NO)(S4 )] coordination sphere.

DITHIOLENES IN MORE COMPLEX LIGANDS

C.

637

Conversion of CO, CO2 , and SO2

The nucleophilic addition of organolithium compounds to CO ligands is a characteristic reaction of metal carbonyl complexes. This reaction can also be observed when CO binds to [M(Sn )] fragments. The reaction according to Eq. 36 has been selected because the resulting benzoylato complex was characterized by X-ray crystallography (118). S THF

[Fe(CO)2(MeSC 6H4S) 2] + LiPh

Me S Me S

Fe

CO C O Li(thf)3

ð36Þ

S

More unusual are the reactions between CO, CO2 , and SO2 and [Ni(L)(S3 )]
complexes with L ¼ N
3 and N(SiMe3 )2 . The azide complex (NBu4 )[Ni(N3 ) (S3 )] is inert toward UV photolysis and thermolysis, however, it readily reacts with CO under standard conditions according to Eq. 37 (119). −

[Ni(N3)(S3)]

1 bar + CO

[Ni(NCO)(S 3)]

20 ˚C

−

ð37Þ

+ N2

Infrared spectroscopy and 13 CO experiments show that in this reaction a fivecoordinate nickel carbonyl complex with a n(CO) band at 2127 cm
1 forms as an intermediate. In this intermediate, the azide ligand, which had been inert in the [Ni(N3 )(S3 )]
starting complex, evidently is labilized so much that it spontaneously dissociates N2 to form the NCO ligand of the final [Ni(NCO)(S3 )]
complex. Scheme 28 illustrates the suggested reaction mechanism. O C S

S Ni

S

S S

Ni

O C

− S

N3

SO

Ni

S

−

C N

S S

− S N N2

S

Ni

SO

− − N2

C N N N

−

S Ni

S

NCO

Scheme 28. Proposed mechanism for the reaction between (NBu4 )[Ni(N3 )(S3 )] and CO.

Extrapolation of this result to [Ni(L)(S3 )]
complexes with L ¼ N(SiMe3 )
2, in which the nitrogen oxidation state is
3, enabled desoxygenations, entirely or

638

S

S

S

S

S

S

O

Ni

S

Ni

O C O

Ni

O C

O

S

S

S

S

S

S

− S N(SiMe3)2

S

Ni

O − S N(SiMe3)2

O

O O C − S Ni N(SiMe3)2

Ni

O C

S

S

S

S

S

S

Ni

Ni

Ni

−

−

−

S

S

− O(SiMe 3)2

− O(SiMe 3)2

S N(SiMe3)2 O

SO

C N(SiMe3)2 O

SO

C N(SiMe3)2

SO

S

S

S

S

Ni

Ni

−

−

+ O(SiMe 3)2

− S N

SO

C N

SO

CN

Ni

S

Scheme 29. Proposed mechanisms for the reactions between (NBu4 )[Ni{N(SiMe3 )2 }(S3 )] and CO, CO2 , and SO2 .

N(SiMe3)2

S

−

N(SiMe3)2

S

−

N(SiMe3)2

S

−

S

S

S

S

Ni

Ni

S

−

−

NCO

NSO

S

DITHIOLENES IN MORE COMPLEX LIGANDS

639

partially, of CO, CO2 , and SO2 . For example, the reaction with CO yields the cyano complex [Ni(CN)(S3 )]
and CO2 , while SO2 gives NCO and NSO complexes (Eqs. 38a–c) (119). + CO −

[Ni{N(SiMe 3)2}(S3)]

+ CO2 + SO2

−

[Ni(CN)(S 3)]

−

[Ni(NCO)(S 3)]

−

[Ni(NSO)(S 3)]

+ O(SiMe 3)2

ð38aÞ

+ O(SiMe 3)2

ð38bÞ

+ O(SiMe 3)2

ð38cÞ

The SiMe3 groups act as oxygen atom scavengers, and formation of O(SiMe3 )2 probably is a major driving force of the reactions. The mechanisms for the reactions with CO, CO2 , and SO2 must be different. Common to all three of them, however, might be the primary adduct formation and the change of the nickel coordination number from 4 to 5 as suggested in Scheme 29. D. H2 S and S2 Complexes Quite a number of the complexes mentioned before demonstrate that [M(Sn )] complex fragments are suited to stabilize unstable molecules. Hydrogen sulfide is not an unstable molecule, rather it is activated by coordination and H2 S complexes are usually very unstable. The H2 S complex [Ru(H2 S)(PPh3 )(S4 )], however, is stable enough for isolation, and the THF solvate has been the first example of H2 S complexes ever to be characterized by X-ray structure analysis. Figure 12(a) schematically depicts the molecular structure of [Ru(H2 S)(PPh3 )
H   S bridges forming between two molecules in the solid state (S4 )] and the S
(120). The H2 S complex is readily oxidized by aerial oxygen to give the S2 complex [m-S2 {Ru(PPh3 )(S4 )}2 ]. Monitoring the oxidation by 1 H NMR spectroscopy yielded evidence for the intermediate formation of [m-H2 S2 {Ru(PPh3 )(S4 )}2 ] (Eq. 39) (120b). 2 [Ru(H2S)(PPh3)(S4)]

[µ−H2S2{Ru(PPh3)(S4)}2]

[µ−S2{Ru(PPh3)(S4)}2]

ð39Þ While [Ru(H2 S)(PPh3 )(S4 )] is yellow, [m-S2 {Ru(PPh3 )(S4 )}2 ] is deep turquoise blue. The m-S2 bridge in [m-S2 {Ru(PPh3 )(S4 )}2 ] gives rise to ....S— ....S— ....M] 4center–6e
electron p bonding, which causes intense long[M— wave absorptions up to 1050 nm (e  14,000) in the UV–vis–near IR spectra. The [m-S2 {Ru(PPh3 )(S4 )}2 ] complex exhibits considerable electrochemical versatility, showing six redox waves in the cyclic voltammogram (120b, 121).

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640

S S Ru PPh3 S H S H S S H S Ru S H S Ph3P S (a)

[Ru(H2S)(PPh3)(S4)]

S S S (c)

(b)

[µ−S2{Ru(PPh3)(S4)}2]

E π 4

S

S Fe. S .Fe .. . ... S S

S S S Ru PPh3 .... S S.. S .. Ru S S S Ph3P S

M S S M

π4

π3

M

S S M

π3

π2

M

S S

M

π2

(d) π1

M

S S M

π1

S S

[µ-S2{Fe(S4)}2]

Figure 12. Molecular structures of (a) [Ru(H2 S)(PPh3 )(S4 )], (b) [m-S2 {Ru(PPh3 )(S4 )}2 ], and (c)
S
[m-S2 {Fe(S4 )}2 ] and (d) the localized 4c–6e
p system of the M

S–M cores.

The [m-S2 {Fe (S4 )}2 ] complex forms from FeCl2 , S2
4 , and elemental sulfur in boiling THF (122). It shows a different structure, but exhibits similar electronic spectra and cyclic voltammograms as [m-S2 {Ru(PPh3 )(S4 )}2 ]. E.

Diazene Complexes 1.

Introduction


NH, is the potential first reduction intermediate in N2 fixation, Diazene, HN
when molecular N2 is reduced in the [2 Hþ /2 e
] reduction steps. In the free state, N2 H2 is extremely unstable (
Hf ¼ þ212 kJ mol
1 ). Its formation according to Eq. 40 would require a very negative (and abiological) reduction potential, +

N2 + 2 H

−

+ 2e

HN=NH

E˚ = −1450 mV (pH 7)

ð40Þ

which is a consequence of the stability of N2 and its triple bond. Therefore breaking the ‘‘first’’ bond is the most difficult step in dissociating the N2 triple bond. It requires 523 kJ mol
1 , which is more than one-half of the total bonddissociation energy of N2 (944 kJ mol
1 ). For these reasons, N2 H2 is unlikely to occur in the free state in the course of N2 fixation. However, the situation may be

DITHIOLENES IN MORE COMPLEX LIGANDS

641

entirely different when N2 H2 forms an an enzyme-bound species coordinated to the metal–sulfur cofactors of nitrogenases. The N2 H2 compound can be stabilized by coordination to transition metals (123). The question of how N2 H2 is stabilized in the coordination spheres of the FeMo, FeV, or FeFe cofactors of nitrogenases confers significance on N2 H2 complexes with [M(Sn )] cores. 2.

Synthesis and Structures of [M(Sn )]–N2 H2 Complexes

All [M(Sn )]

N2 H2 complexes reported were synthesized by using hydrazine or hydrazine derivatives as a source of the N2 H2 ligand. (Note that the synthesis of N2 H2 complexes from N2 and mild reductants would solve one of the key problems of N2 fixation!) The first example of a [M(Sn )]

N2 H2 complex was [m-N2 H2 {Ru(PPh3 )(S4 )}2 ] (124). In minimum yields and milligram amounts, it formed through the
N2 H2 {Ru(PPh3 )air oxidation of [Ru(N2 H4 )(PPh3 )(S4 )]. The complex [m

N2 H2 (S4 )}2 ] exhibited features that later proved to be common to all [M(Sn )]
complexes and essential for the stabilization of N2 H2 . These are, in short, steric shielding of the diazene ligand by bulky [M(Sn )] fragments, 4c–6e
p systems ....NH— ....NH— ....M] entity, and strong bifurcated N
in the [M—
H   S(thiolate) bridges between diazene protons and thiolate donors. Scheme 30 schematically

S

S S

Ru

S

S

N

Ph3P H

S H PPh3

N S

S

S Ru S S

[µ-N2H2{Ru(PPh3)(S4)}2]

S S S

S H N N PPh3 H S

S H N N PPr H S

PPh3

Ru

Ru

S S

S

[µ-N2H2{Ru(PPh3)(BuS4)}2]

PPr3

Fe

3

S S

Fe S

[µ-N2H2{Fe(PPr3)(S4)}2]

S S S

S H N N PEt3 H S

PEt3

Fe

Fe

S S

S

[µ-N2H2{Fe(PEt3)(BuS4)}2]

Scheme 30. Schematical drawing of the structures of [m-N2 H2 {M(PR3 )(S4 )}2 ] complexes.

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642

C27

N1

C17

C28 C16 C10 C11

S1 C23 C24 C22 C21

C25

S4

Fe1

C20 S3

S2a

H2

.. .. S2 C15 C14 . N2 H2a .. .. N2a S3a .. . S1a

Fe1a

C12 C13

S4a

N1a Figure 13. Molecular structure of [m-N2 H2 {Fe(NH S4 )}2 ].

depicts the structures of [m-N2 H2 {M(PR3 )(S4 )}2 ] complexes as determined by X-ray structure analysis. The [m-N2 H2 {Ru(PPh3 )(S4 )}2 ] complex is extremely sparingly soluble (124). Aerial oxidation of [Ru(N2 H4 )(PPh3 )(Bu S4 )] gave the more soluble tert-butyl derivative [m-N2 H2 {Ru(PPh3 )(Bu S4 )}2 ], although also in very low yield (125). The [m-N2 H2 {Ru(PPh3 )(R S4 )}2 ] complexes contain abiological PR3 ligands and ruthenium centers. In this regard, it was significant to find that air oxidation of [Fe(N2 H4 )(NH S4 )] yielded [m-N2 H2 {Fe(NH S4 )}2 ], which exclusively contains biologically compatible metal centers and donor atoms (Fig. 13) (47, 126). This complex exhibits the same features stabilizing the N2 H2 ligand as the ruthenium complexes. It too was accessible only in minimum yields and is practically insoluble in all common solvents. However, it established that N2 H2 complexes do not need phosphine coligands to stabilize the N2 H2 ligand. For this reason, various phosphines could be tried to improve the yields and
N2 H2 complexes. In the course of these investigations solubilities of [M(Sn )]
the synthesis according to Eq. 41 was found (127).

S

2 Fe

2+

2−

+ 2 S4

+ O2 + 2 PPr3 + exc. N2H4

THF room temp.

S S

S H N N PPr3 H S

PPr3

Fe

Fe

S S

S

[µ-N2H2{Fe(PPr3)(S4)}2]

ð41Þ

DITHIOLENES IN MORE COMPLEX LIGANDS

643

Combining the starting materials gave brown solutions of [Fe(N2 H4 )(PR3 )(S4 )] complexes. Passing a stream of air through these solutions caused an instantaneous color change to deep blue. Subsequent workup provided the N2 H2 complexes in practically quantitative yield and amounts of up to 10 g per run. The complexes proved well soluble in organic solvents and exhibit molecular structures analogous to those of [m-N2 H2 {Ru(PPh3 )(R S4 )}2 ]. Yields and solubilities of the [m-N2 H2 {Fe(PR3 )(S4 )}2 ] complexes for the first time enabled the
N2 H2 complexes. systematic study of the chemistry of [M(Sn )]
Interesting and as yet not easy to explain is the finding that the syntheses according to Eq. 41 work well only with PnPr3 and PnBu3 . No success was granted with either PMe3 or PPh3 . Possibly, the same holds true for N2 H2 complexes as for N2 complexes, where subtle changes in the coordination sphere are able to cause instability of the M

N2 bond. A long-known example is the [Ir(Cl)(PR3 )2 ] fragment, where even such minor changes as the replacement of PPh3 by tri-p-tolylphosphine render the Ir site incapable of binding N2 (128). The high yields of [m-N2 H2 {Fe(PnPr3 )(S4 )}2 ] made the [Fe(PnPr3 )(S4 )] fragment a good candidate for trapping diazene that was generated by acidolysis of K2 N2 (CO2 )2 according to Eq. 42. +

+2H K2[O2C−N=N−CO2]

−2K

+

N2H2 + 2 CO2

ð42Þ

All such experiments with other complexes or organic compounds had remained unsuccessful in the past. However, ½m-N2 H2 {Fe(PnPr3 )(S4 )}2 ] formed in high yields, irrespective of whether the N2 H2 was generated in the same solution that contained the [Fe(PnPr3 )(S4 )] fragment or whether it was generated in another Schlenk tube and transferred into the THF solution of [Fe(PnPr3 )(S4 )] by a stream of nitrogen (129). The [Ru(PPh3 )(tpS4 )] fragment enabled the characterization of the corresponding dinuclear diazene and hydrazine complexes [m-N2 H2 {Ru(PPh3 )(tpS4 )}2 ] and [m-N2 H4 {Ru(PPh3 )(tpS4 )}2 ]. This couple is of interest because the molecular structures (Fig. 14) demonstrate that going from N2 H2 to N2 H4 complexes increases the number of intramolecular N

H   S bridges (130). The significance of such bridges with respect to N2 fixation mechanisms is discussed below. The N2 H2 complexes described so far are all centrosymmetric and contain enantiomeric [M(L)(S4 )] or [M(NH S4 )] fragments. The complex [mN2 H2 {Ru(PCy3 )(S4 )}2 ] is different. It is C2 symmetrical, contains two homochiral [Ru(PCy3 )(S4 )] fragments, and these fragments are connected by a transdiazene in such a way that the PCy3 ligands point in the same direction. Figure 15 depicting an Oak Ridge thermal ellipsoid plot (ORTEP) plot of the molecular structure shows that a molecule with considerable steric strain results (131).

¨ RG SUTTER DIETER SELLMANN AND JO

644

P1A

S3

S1A Ru1A

S4

Ru1

S2

N1

S2A

N1A S4A

S1 P1

S3A

(a)

S3

S2

S4

Ru1

N1 S1

P1

P1A S1A N1A S4A

Ru1A

S2A

S3A (b)

Figure 14. Molecular structures of [m-N2 H4 {Ru(PPh3 )(tpS4 )}2 ] (a) and [m-N2 H2 {Ru(PPh3 )(tpS4 )}2 ] (b).

This steric strain is caused mainly by the bulky PCy3 ligands. It explains why [m-N2 H2 {Ru(PCy3 )(S4 )}2 ] readily exchanges its PCy3 for smaller PR3 ligands and, in the absence of such ligands, catalyzes the heterolysis of molecular hydrogen (see below).

DITHIOLENES IN MORE COMPLEX LIGANDS

S23

S13

S21

S11 S12 Ru1 P1

645

N1N2 S14 H1 H2

Ru2 S24

S22

P2

Figure 15. The molecular structure of [m-N2 H2 {Ru(PCy3 )(S4 )}2 ].

In spite of the considerable steric strain, [m-N2 H2 {Ru(PCy3 )(S4 )}2 ] has a remarkably high tendency of formation and four different methods of preparation have been found. They involve aerial oxidation of [Ru(N2 H4 )(PCy3 )(S4 )], treatment of [RuIII (I)(PCy3 )(S4 )] with N2 H4 , and the reactions of either [Ru(dmso)(PCy3 )(S4 )] (dmso ¼ dimethyl sulfoxide) or [RuIII (I)(PCy3 )(S4 )] with excess N2 H2 generated in situ by acetolysis of K2 N2 (CO)2 (131). Several features are common to all [m-N2 H2 {M(Sn )}2 ] complexes. Bulky

NH

NH
[M(L)(Sn )] fragments sterically shield the [M

M] bridges and
presumably stabilize the diazene ligand kinetically. The diazene ligands always exhibit the trans configuration. The diazene N

N distances range between 130.0(7) ([m-N2 H2 {Fe(NH S4 )}2 ]) and 125.9(10) pm ([m-N2 H2 {Ru(PCy3 )(S4 )}2 ]). For trans-HN

NH in the gas phase, a distance of 126.6 pm has been calculated (132). Thus, the N

N distance stays practically unaltered when diazene becomes bound to two metal centers. The M

N distances in the [M

NH

M] entities are always shorter than M

N single bonds. In [m
NH

N(diazene) distances are 187.7(4) pm, N2 H2 {Fe(NH S4 )}2 ], for example, the Fe

N the Fe

N(NH S4 ) distances, however, which can be regarded to represent Fe
single bonds are 207.3(4) pm. The [M

NH

M] entities represent chro
NH
mophores and dinuclear N2 H2 complexes are intensely dark blue to dark green, exhibiting strong absorptions (e 10,000) in the visible region. These proper
NH

M] entities ties are rationalized by a 4c – 6e
p system in the [M

NH
(Fig. 16), which is analogous to the 4c – 6e
p system in [m-S2 {M(Sn )}2 ] complexes. In addition to the s bonds, four localized p molecular orbitals result from the combination of two occupied metal d orbitals and the diazene p orbitals. Three

646

¨ RG SUTTER DIETER SELLMANN AND JO

E

π4 π3 π2 π1 H M NN M H

....NH— ....NH— ....M] chromophore in Figure 16. Localized 4c–6e
p molecular orbitals of the [M— [m-N2 H2 {M(Sn )}2 ] complexes.

of these orbitals are occupied, and the strong absorptions in the visible region can be assigned to p3 ! p4 transitions. With respect to the NN bond, there are one antibonding and two bonding p orbitals such that one bonding p interaction results. As a consequence, the NN bond of the coordinated N2 H2 exhibits double-bond character like the NN bond of free diazene. The same considerations hold true for the M

N(diazene) p interactions and rationalize the considerable double-bond character of the M

N(diazene) bonds. In summary, N2 H2 is a s-donor p-acceptor ligand and gives rise to conjugated ....NH— ....NH— ....M] systems when bridging two metals. This certainly is the [M— major factor contributing to the stabilization of N2 H2 . The N

H   S hydrogen bridges between the diazene NH protons and the thiolate donors are the third feature common to [m-N2 H2 {M(S4 )}2 ] complexes. Figure 17 illustrates the planar or nearly planar arrangement of trans-thiolate donors, metal centers, and N2 H2 atoms found in all complexes and gives as example N

H    S distances retrieved from the molecular structure of [m-N2 H2 {Fe(NH S4 )}2 ]. Similar distances are found in the other N2 H2 complexes. Short and long N

H    S bridges can be distinguished. The short bridges range from 220 to 235 pm, the long bridges from 278 to 288 pm. In both cases, the distances are shorter than the sum of H and S van der Waals radii (305 pm)

S M S

278

H N 220

H

220

N 278

S M S

Figure 17. Intramolecular hydrogen bridges in the [m-N2 H2 {MS2 (thiolate)}2 ] cores of
H    S distances in pm were retrieved from [m-N2 H2 {Fe(NH S4 )}2 ]). [m-N2 H2 {M(Sn )}2 ] complexes (N


DITHIOLENES IN MORE COMPLEX LIGANDS

647

(133). The bond strength of these N

H   S bonds is not directly accessible by experiment. However, it has been estimated that they contribute 70 kJ mol
1 to the total stabilization of coordinated N2 H2 (47, 126). This estimate is corroborated by recent DFT calculations (134), which support the assumption that N

H   S bridges are a significant factor for stabilizing diazene in
N2 H2 complexes. [M(Sn )]
3.

Hydrogen-Bridge Diastereoisomerism and PR3 Exchange Reactions of [m-N2 H2 {M(PR3 )(S4 )}2 ] Complexes

The N

H   S bridges in [m-N2 H2 {M(Sn )}2 ] complexes give rise to a type of diastereoisomerism that was first observed with [m-N2 H2 {Fe(PnPr3 )S4 }2 ] (127). It proved to be a general property of m-N2 H2 complexes, in which chiral [M(Sn )] fragments are bridged by diazene. Since it is essentially caused by the hydrogen bridges in diazene complexes, it has been termed hydrogen-bridge diastereoisomerism. In Section IV.D, it was shown that chiral [M(L)S4 )] fragments potentially can yield 10 stereoisomeric dinuclear [M(L)(S4 )]2 complexes. One reason is that the
S

M bridging are stereochemically [M(L)(S4 )] thiolate donors used for M
nonequivalent. The same holds true for the thiolate donors in centrosymmetric [m-N2 H2 {M(L)(S4 )}2 ] complexes and is illustrated in Scheme 31. Scheme 31 shows two diastereomers of [m-N2 H2 {Fe(PR3 )(S4 )}2 ] (26 and 27). Both diastereomers are centrosymmetric. Their stereochemical difference

S S H Fe S N N H S Pr3P

S

S

PPr3

S

S Fe S S

S

Fe Pr3P

S

N H

H PPr3

N S

Fe

S S

S

26

27

S S S

Fe Pr3P

S S

S

..

S

N H

Fe

S

N ..

H

Pr3P

28

29

Scheme 31. Schematic representation of two diastereomeric centrosymmetric [m-N2 H2 {Fe(PR3 )(S4 )}2 ] complexes.

648

¨ RG SUTTER DIETER SELLMANN AND JO

becomes more apparent when they are symmetrically cleaved at the inversion center. Figures 28 and 29 depict the resulting ‘‘left’’ halves of the diastereomers 26 and 27. The NH groups remain in the plane defined by the N, Fe, and the two thiolate atoms. Due to the C1 symmetry of the [Fe(NH)(PR3 )(S4 )] fragments, the NH protons in 28 and 29 are nonequivalent and should give rise, for example, to different chemical shifts in the 1 H NMR spectrum. Likewise, the two centrosymmetric complexes 26 and 27 are expected each to yield one 1 H NMR diazene signal. The bridging thiolate donors in [M(L)(S4 )]2 complexes can be distinguished with respect to the thioether donors of the S4 ligand. In an exactly analogous way, the HN atoms of the fragments 28 and 29 can be differentiated as well. In fragment 28, the N atom carrying the proton is trans to that thioether S donor whose ortho-benzene-thiolate forms the S   HN bridge. In fragment 29, however, the respective thioether S donor is cis to the NH atom. Formulas 26 and 27, and likewise 28 and 29, thus are neither different projections of identical molecules nor enantiomers, but represent diastereomeric complexes. These considerations explain the 1 H, 31 P, and 13 C NMR spectra of [m-N2 H2 {Fe(PnPr3 )(S4 )}2 ] and analogous complexes. Figure 18 shows the 1 H NMR spectrum of a sample of [m-N2 H2 {Fe(PnPr3 )(S4 )}2 ]. This sample had been crystallized at
30 C, was redissolved in CD2 Cl2 at temperatures below
20 C, and its X-ray structure analysis had yielded the structure schematically given by diastereomer 26. One NH singlet at 15.4 ppm was observed, in correspondence with expectations for a centrosymmetric N2 H2 complex. If we warm this CD2 Cl2 solution up to room temperature, a second singlet appeared at 15.1 ppm, while the 15.4 ppm signal intensity decreased. The second singlet could be assigned to another centrosymmetric N2 H2 complex that had to be formed from the first N2 H2 complex. Lowering the temperature back to
30 C did not cause disappearance of the second NH signal. However, when the mixture was recrystallized the resulting crystals again showed only one 1 H NMR NH singlet at 15.4 ppm at
30 C. These findings are plausibly explained by formation of a second N2 H2 complex, which is also centrosymmetric but diastereomeric to the N2 H2 complex that had been structurally characterized. On inspection of structurally characterized analogous [m-N2 H2 {M(PR3 )S4 )}2 ] complexes (cf. Scheme 30), it was found that they can exhibit two diastereomeric structures. The [m-N2 H2 {Ru(PPh3 )(Bu S4 )}2 ] complex has a structure analogous to 26 (Scheme 31), [m-N2 H2 {Ru(PPh3 )(S4 )}2 ], however, has a structure analogous to 27. Which one of these diastereomeric forms is isolated, presumably depends on crystallization conditions and solubilities. The isomerization leading to a second diastereomeric N2 H2 complex was originally and tentatively explained by 1,2 shifts of the diazene protons (127). Experimental results obtained later suggest a different and more plausible

DITHIOLENES IN MORE COMPLEX LIGANDS

649

(b)

(a)

(a)

Figure 18. The 1 H NMR (left) and

(b)

31

P NMR (right) spectra of [m-N2 H2 {Fe(PnPr3 )(S4 )}2 ] (123).

mechanism. The PnPr3 ligands of [m-N2 H2 {Fe(PnPr3 )(S4 )}2 ] can readily be exchanged for PnBu3 under retention of the [m-N2 H2 {Fe(S4 )}2 ] core according to Eq. 43 (135). [µ-N2H2{Fe(PnPr3)(S4)}2]

+ exc. PnBu3 − PnPr3

[µ-N2H2{Fe(PnBu3)(S4)}2]

ð43Þ

The NMR spectroscopic monitoring gave evidence for the intermediate
N2 H2

Fe(PnBu3 )(S4 )], formation of the ‘‘mixed’’ species [(S4 )(PnPr3 )Fe
which exhibits different phosphine ligands at the Fe centers. Analogous observations were subsequently made with the [m-N2 H2 {Ru(PR3 )(S4 )}2 ] complexes (131).

¨ RG SUTTER DIETER SELLMANN AND JO

650

(S)

(R)

(S) S

S

S S

H PR S Fe N 3 N Fe S R3P H S (S)

− 2 PR3 S S

S

H N

S

+ 2 PR3

S N Fe H

30

S S

S

(R)

S H R 3P N Fe Fe PR 3N H S S S

S

S

S Fe

(R)

S S

+ 2 PR3

31

Scheme 32. Isomerization of [m-N2 H2 {Fe(PR3 )(S4 )}2 ] via an intermediate with five-coordinate Fe centers.

These findings suggest that the isomerization of [m-N2 H2 {M(PR3 )(S4 )}2 ] complexes is caused by dissociation of the PR3 ligands (Scheme 32). After dissociation of the PR3 ligands, irrespective of whether it takes place stepwise or simultaneously, a m-diazene complex with five-coordinate Fe centers results. The recoordination of phosphine ligands to these Fe centers can take place from two different directions, indicated by the arrows in the formula with fivecoordinate Fe centers. If the phosphines are added from the same direction into which they had left when dissociating, a stereoisomer results that has the same configuration as the starting complex. However, when they are added from opposite directions (from the front on the left and from the back on the right Fe center as indicated in Scheme 32), a different complex 31 results. It is worth
H   S noting that neither the [M(S4 )] configurations change nor is any N
bridge cleaved in the course of this isomerization. Since there is no symmetry operation that is able to transfer complex 31 into complex 30, complexes 30 and 31 must be diastereomeric. Note here that the diastereoisomerism of 30 and 31 can also be demonstrated through an analysis of the stereogenicity of the Fe centers in 30 and 31. For this purpose, 30 and 31 are split again into halves. Without hydrogen bridges, the Fe centers in the resulting S halves 32 and 33 are prostereogenic (cf. Section IV.D). Therefore, any further desymmetrization of 32 and 33 must cause the Fe centers to become stereogenic (20, 21). (S)

(S)

S

S S Fe

P N

S

S S Fe

S

S

32

33

N P

DITHIOLENES IN MORE COMPLEX LIGANDS

651

Formulas 34 and 35 show that this is the case when the N

H    S bridge, or, in other words, a fourth chelate ring, forms. (S)

(S) H

S S S

S

Fe N P S

S S

H P N

S

34

35

S3 S2

Fe

P1A S4 S1A N1

Fe1 S1

N1A S4A P1

Fe1A

S2A

S3A

(a)

S3 S2

S4 N1

Fe1

P1A N1A

S1

S4A P1

S1A Fe1A S2A S3A

(b)

Figure 19. Molecular structures of the hydrogen-bridge diastereomers (a) (R,S)-(a;a)[m-N2 H2 {Fe(PPr3 )(tpS4 )}2 ]  2 THF and (b) (R,S)-(b;b)-[m-N2 H2 {Fe(PPr3 )(tpS4 )}2 ]  4 CH2 Cl2 [The notation (a;a) and (b;b) corresponds with the notation introduced in Section IV.D for the thiolate atoms involved in bridges.]

¨ RG SUTTER DIETER SELLMANN AND JO

652

Now, permutation of N and P donors yields stereoisomers, while the (S) configuration of the [Fe(S4 )] fragment remains unaltered as before. As above, the difference between 34 and 35 can be expressed with respect to thioether and thiolate atoms standing cis or trans to the NH group. The underlying cause for the diastereoisomerism of 34 and 35 (or 30 and 31), however, is the stereogenicity of the Fe centers. Recently, the diazene complex [m-N2 H2 {Fe(PPr3 )(tpS4 )}2 ] allowed characterization of both diastereomers by X-ray structure determination. Figure 19 shows their molecular structures (136).

VI.

[M(Sn )] COMPLEXES MODELING REACTIONS OF [MS] ENZYMES A.

Introduction

It has been emphasized several times that the major part of chemistry concerning dithiolenes in more complex ligands served as a means to the end of finding robust [M(Sn )] complexes with high reactivity for modeling reactions catalyzed by [MS] oxidoreductases, in particular, nitrogenases, hydrogenases, and CO dehydrogenases (7). If such complexes would catalyze enzyme reactions with enzyme-like activity, they could be termed competitive catalysts. A couple of findings indicate that such competitive catalysts must differ structurally from the enzyme centers. Instead of being structural copies, competitive catalysts rather have to realize the principles that govern the reactivity of [MS] enzyme centers (7g). The relationships between structure and function of metal– sulfur complexes may reveal these principles. An important structural feature, common to both metal–sulfur enzyme centers and (hypothetical) low molecular weight competitive catalysts, certainly is the sulfur coordination of the metals. The type of donor atom represents the most basic structural feature of any metal complex. Functional features indispensable for competitive catalysts are determined by Eq. 44. It expresses the hydrogenase, CO dehydrogenase, and nitrogenase reactions in their most general form. +

A + xH

+ ye

−

B

ð44Þ

A competitive catalyst must enable the conversion of a substrate A into a product B by transfer of protons and electrons. In order to catalyze reactions according to Eq. 44, competitive catalysts have to (1) be robust and stable in the

DITHIOLENES IN MORE COMPLEX LIGANDS

653

absence of proteins, (2) exhibit vacant sites for the coordination and activation of substrates, (3) possess Brønsted acid–base properties for the transfer of protons, (4) be redox active for the transfer of electrons, and (5) enable the electron transfer at mild redox potentials. Conditions (1–4) are self-explanatory. Condition (5) concerns the redox potentials of biological reactions that lie in the range of about þ500 to
500 mV. This constraint explains why an in vitro reaction that needs metallic lithium or sodium is not compatible with biological reactions, even if it is catalytic. For nitrogenases, presumably a sixth condition has to be added. It concerns the evidence that nitrogenase substrates are reduced by multiples of [2 Hþ /2 e
] transfer steps.

B.

Hydrogenase Models

Hydrogenases catalyze the Hþ /H2 redox equilibrium and the D2 /Hþ exchange with protons from water according to Eqs. 45a and b (137). +

2H

−

ð45aÞ ð45bÞ

H2

+ 2e +

+

D2 + H

HD + D

X-ray structure analysis revealed the structures of [NiFe] and [FeFe] hydrogenases and established the metal thiolate coordination in their active centers. Scheme 33 depicts the active centers (138, 139). N C

NC

CO

Fe Cys S

X S Cys Ni

Cys S

Cys [Fe4S4]

S

S

OC NC

Fe

S C O

OH2 Fe

CO CN

S Cys

Scheme 33. Structures of the active centers in [NiFe] and [FeFe] hydrogenases.

The structure determinations did not solve, of course, the molecular mechanisms of the hydrogenase reactions (140). However, spectroscopic investigations and redox titrations of [NiFe] hydrogenases strongly indicated that in these enzymes the nickel–sulfur entity functions as a site of substrate activation and conversion (141). Thus nickel–sulfur complexes that could catalyze reactions 45a and b gained considerable interest. Reaction 45b, which does not require the intermolecular transfer of electrons and thus is ‘‘simpler’’ than reaction 45a, served as a test reaction for hydrogenase activity.

¨ RG SUTTER DIETER SELLMANN AND JO

654

The D2 /Hþ exchange reaction requires a heterolytic cleavage of the D2 molecule into Dþ and D
species and, vice versa, of H2 into Hþ and H
. When such a H2 heterolysis takes place at [MS] sites, plausible intermediates are metal hydride thiol species forming according to Eq. 46. M

S + H2

M S

M H

H H

S

ð46Þ

H

Complexes exhibiting this structural motif can be anticipated to prove or disprove hypotheses and to yield deeper insights into the reaction mechanism of hydrogenases. Nickel–sulfur hydride complexes were a primary research target. However, the few which were found did not catalyze the H2 heterolysis according to Eqs. 45b or 46 (142). In the quest for other metal–sulfur complexes exhibiting the [M(H)(SH)] motif, the Rh and Ru hydride complexes [Rh(H)(CO)(S4 )] (51, 143) and [Ru(H)(PCy3 )(S4 )]
(144) were found. They proved to catalyze a D2 / Hþ exchange according to Eq. 47. In order to do so, the Rh complex required the addition of catalytic amounts of Brønsted acids such as aqueous HCl or HBF4. − S S

III CO S Rh H S

[Rh(H)(CO)(S4)]

EtOH + D2

S S

II S Ru

PCy3 H

S −

[Ru(H)(PCy3)(S4)]

EtOD + HD

ð47Þ

This observation, and supporting IR and 1 H NMR studies of the protonation of [Rh(H)(CO)(S4 )] and its PCy3 derivative [Rh(H)(PCy3 )(Bu S4 )] at temperatures between
50 C and þ20 C have corroborated the mechanism outlined in Scheme 34 (143). The protonation of a thiolate donor, formation of a nonclassical Z2 -H2 complex, release of H2 and addition of D2 , and the heterolytic cleavage of this D2 by the concerted attack of the Lewis acidic metal center and the Brønsted basic thiolate donor are essential steps. The acidic thiol deuteron can exchange with EtOH protons. The resulting free protons and the deuteride complex yield HD and the coordinatively unsaturated species that is the actual catalyst. The detailed mechanism comprises a considerably larger number of steps (and equilibria) (143). For example, the occurrence of Z2 -D2 and [M(D)(SD)] intermediates that exchange with Hþ should give rise to [M(D)(SH)]

DITHIOLENES IN MORE COMPLEX LIGANDS

S S

S M

+

S

L H

S M

S

S

S

H +

S

S M

+ D2

+

S S

S M S

D + EtOH

H H

− H2

L

S +H

+ L

S M S

S − HD

S

+

S +H

L H

655

+

S L S

D

L D

S M SD

+

−H

+

S

+

S

S M

EtOD

S

L D D

þ

Scheme 34. Mechanism of the D2 /H exchange according to Eq. 47 catalyzed by [M(H)(PR3 )(R S4 )] complexes with M ¼ Rh(III), Ru(II).

intermediates. If these reactions are reversible, scrambling of hydride ligands and thiol protons according to Eq. 48 has to occur.

M D

M

S

S

H

D H

M S

H D

M H S

ð48Þ

D

This scrambling could be established by NMR spectroscopy. Measurement of the T1 relaxation time further served to identify the Z2 -HD complex [Ru(Z2 HD)(PCy3 )(S4 )] as an intermediate [1 J(HD) ¼ 32 Hz, T1 (min) ¼ 4 ms] (Fig. 20) (144). In the case of rhodium, the corresponding complex [Rh(Z2 -HD)(PCy3 )(S4 )]þ is cationic and presumably represents a transition state that could not be detected by NMR spectroscopy. The complexes [Rh(H)(CO)(S4 )] and [Ru(H)(PCy3 )(S4 )]
differ from the nickel site in [NiFe] hydrogenases with respect to structure and metal centers, however, they evidently meet several of the principles that must hold true for enzyme centers and competitive catalysts alike. In this case, the complexes

656

¨ RG SUTTER DIETER SELLMANN AND JO

–6.5 ppm 1

Figure 20. The HD signal in the H NMR spectrum of [Ru(Z2 -HD)(PCy3 )(S4 )].

[Rh(H)(CO)(S4 )] and [Ru(H)(PCy3 )(S4 )]
demonstrate that a vacant site at a Lewis acidic metal center and a Brønsted basic thiolate donor suffice to enable the heterolysis of H2 and to catalyze the D2 /Hþ exchange. Exactly the same holds true for the phosphorane–imine complex [Ni(NHPPr3 )(S3 )] (145, 146). It, too, catalyzes the D2 /Hþ exchange. Figure 21 depicts the structure of [Ni(NHPPr3 )(S3 )] and illustrates the typical geometry of [Ni(L)(S3 )] complexes. Due to the topology of the S3 ligand, all [Ni(L)(S3 )] complexes are neither square planar nor tetrahedral but strongly distorted and exhibit structures somewhere in between the two ideal geometries. However, all [Ni(L)(S3 )] complexes characterized so far are diamagnetic.

S2

S3

S1

Ni1 H1 N1 P1

Figure 21. Molecular structure of [Ni(NHPPr3 )(S3 )].

DITHIOLENES IN MORE COMPLEX LIGANDS

657

The NH group of the NHPPr3 ligand proved to be an excellent detector for probing the reactions of [Ni(NHPPr3 )(S3 )] with hydrogen and protons according to Eqs. 49 and 50. [Ni(NHPPr3)(S3)] + D2O

[Ni(NDPPr3)(S3)] + HDO

ð49Þ

[Ni(NHPPr3)(S3)] + D2

[Ni(NDPPr3)(S3)] + HD

ð50Þ

Reaction 49 established the acidic character of the NH proton, while reaction 50 proved that [Ni(NHPPr3 )(S3 )] also reacts with molecular H2 or D2 , respectively. All species shown in Eqs. 49 and 50 could be identified, and reactions 49 and 50 could be carried out successively. The results prove that [Ni(NHPPr3 )(S3 )] catalyzes the reaction according to Eq. 51, which is possible only when D2 is heterolytically cleaved into Dþ and D
. [Ni(NHPPr3)(S3)] D2 + H2O

ð51Þ

HD + HDO

The mechanism suggested for this reaction is based on the identification of the species shown in Eqs. 49 and 50, the results obtained with the Rh and Ru complexes [Rh(H)(CO)(S4 )] and [Ru(H)(PCy3 )(S4 )]
, the finding that the related [Ni(N3 )(S3 )]
complex can add CO (cf. Section V.C), and the wellknown fact that many four-coordinate nickel complexes add a labile fifth ligand. Scheme 35 illustrates the mechanism.

HDO

S

Ni S

S H NPPr3

D2 (a)

(e) H2O

S S

D D

S D Ni NPPr3

S

Ni S

HD (b) (d) D S S

H

D

S D Ni NPPr3

S

Ni S

(c)

Scheme 35

D S H NPPr3

S H NPPr3

¨ RG SUTTER DIETER SELLMANN AND JO

658

D2 adds to [Ni(NHPPr3 )(S3 )] to give a nonclassical Z2 -D2 complex (step a). The concerted attack of the Lewis acidic nickel center and one of the Brønsted basic thiolate donors cleaves D2 into D
and Dþ (step b). The acidic thiol deuteron complex releases HD (step d) and the exchange of the ND deuteron with H2 O reestablishes the starting catalyst complex (step e). Thus, this model shows that D2 /Hþ exchange can be catalyzed by surprisingly ‘‘simple’’ nickel complexes. No unusual nickel oxidation states or other peculiarities need to be invoked. As in the case of the Rh and Ru complexes [Rh(H)(CO)(S4 )] and [Ru(H)(PCy3 )(S4 )]
, all that is necessary are a Lewis acidic metal center and a Brønsted basic thiolate donor. The Lewis acidity of the nickel center or, in other terms, its capability to coordinate a fifth ligand are important. In this context, it is worth noting that the nickel coordination geometries of [Ni(NHPPr3 )(S3 )] and the [Ni(Cys)4 ] site in [NiFe] hydrogenase are distorted in a similar and characteristic way. Such distortions influence the relative energies of the nickel acceptor and donor orbitals that have to interact with the s and s* orbitals of H2 in order to enable primary coordination of H2 (147). Thus, in the end, there has to exist not only a vacant site but the metal center has to have orbitals suited for coordinating the relevant substrates. Recent attempts to model the iron site of [NiFe] hydrogenase centers led to (NEt4 )2 [Fe(CO)(CN)2 (S3 )] (61a). The neutral PMe3 derivative of this anion could be combined with a [Ni(S2 C6 H4 )] fragment to yield [(C6 H4 S2 )Ni(mS3 )Fe(CO)(PMe3 )2 ], which is the as yet closest approach to the dinuclear [S2 Ni(m-S2 )Fe(CO)(L2 )] core of [NiFe] hydrogenase centers (61b). N C (NEt 4)2

S S

Fe

PMe3 CN CO

S S

Ni

S S

S

(NEt 4)2[Fe(CO)(CN)2(S3)]

C.

Fe

PMe3 CO

S

[(C 6H4S2)Ni(µ-S3)Fe(CO)(PMe 3)2]

Carbon Monoxide Dehydrogenases

Nickel sulfur centers have been recognized as constituents of CO dehydrogenases (CODH). The exact number of sulfur donors and the nickel coordination geometries in these enzymes possibly vary depending on the enzyme source. However, it is taken for granted that the nickel–sulfur centers are

DITHIOLENES IN MORE COMPLEX LIGANDS

659

involved in the two basic reactions catalyzed by CODH, the reversible CO oxidation and the synthesis of acetyl-CoA from CO, a Me group, and coenzyme A (CoA) (Eqs. 52 and 53) (148). +

CO + H2O

CO2 + 2 H

CO + [Me] + CoA

−

ð52Þ

+ 2e

ð53Þ

MeCOCoA

Well-defined nickel–sulfur complexes that enable a stepwise combination of CO, alkyl, and thiol groups to give thioesters can be anticipated to yield deeper insight into the molecular mechanism of the acetyl-CoA synthesis (142, 149).
S4 )] afforded an example for such a thioester The complex [Ni(C3 Me2
synthesis. In principle, it is even catalytic and Scheme 36 summarizes the individual steps (10). −

+

−2e ,−2H − 4 CO S SH

S

S Ni

S

−

+ 2 e (Na / Hg)

S

S 36 [Ni(S 4-C3Me2)]

HS 41

−

S

−

S Ni(CO) 4

S

S

L +

Ni S Ni S Ni S S

S 40

S

O + 4 CO

S

O

Ni S

37

L

39 (L = PMe3)

S Ni + CO

+L (L = py, thf, PMe3)

S

L 38 (L = py)

Scheme 36. The [NiS] mediated thioester formation from alkyl, CO, and thiols. [The alkyl and acyl complexes 38 and 39 were characterized by X-ray structure analysis for L ¼ py (38) and L ¼ PMe3 (39).]


C3 Me2 )] (36) exhibits a distorted square-planar The starting complex [Ni(S4
[NiS4 ] core, is redox active, and can be electrochemically reduced reversibly to
C3 Me2 )] (36) by give a Ni(I) species. In contrast, chemical reduction of [Ni(S4
sodium amalgam yields the trinuclear Ni(II) alkyl complex 37, which can be cleaved by two-electron ligands (L) such as pyridine, THF, or PMe3 to give the mononuclear derivatives 38. Upon addition of 1 equiv of CO, the derivatives 38

¨ RG SUTTER DIETER SELLMANN AND JO

660

yield the acyl complexes 39. In the presence of excess CO, 39 releases the cyclic thioester 40 and gives [Ni(CO)4 ]. Significantly, formation of 40 and [Ni(CO)4 ] is likewise observed when complexes 37 or 38 are treated with excess CO. Because [Ni(CO)4 ] reacts with the neutral thioether thiol 41 to give the starting complex [Ni(S4

C3 Me2 )] (36), the cycle can be closed. Scheme 36 represents the first example for a [NiS] mediated formation of thioesters from alkyl, CO, and thiol groups in a cyclic way. All intermediates shown in Scheme 37 could be intercepted and characterized by spectroscopic methods and X-ray structure analysis. They permit a detailed insight into the individual steps of thioester formation that may take place in an analogous way at the active site of CODH when acetyl-CoA is synthesized. The net reaction of Scheme 37 can be expressed by Eq. 54.

S

S

SH

S +

CO

+

SH HS

SH

S

ð54Þ

O 41

40

A comparison of Eq. 54 and Scheme 36 reveals that the nickel center performs two functions. First, the nickel center acts as mediator of the two electron-transfer reactions during which S

C bonds are cleaved and formed (36 ! 37 and 39 ! 40). Second, it facilitates the formation of an acyl group from an alkyl group and CO (38 ! 39). This reaction is expected to be favored when the nickel center has a coordination number lower than 5. A low coordination number of nickel also facilitates the final release of the thioester in the consecutive reaction of 39 with excess CO, because intermediates such as complex 42 can readily form.

S S

Ni

O CO L

42

In conclusion, the [NiS] mediated formation of thioesters from alkyl, CO, and thiol groups lends support to an acetyl-CoA formation pathway that comprises CO insertion into a Ni

Me and an intramolecular S

C bond formation between nickel-bound acyl groups and thiolate ligands. These reactions are favored at square-planar nickel complexes that enable two-electron redox reactions and readily add fifth ligands.

DITHIOLENES IN MORE COMPLEX LIGANDS

661

D. Nitrogenase Relevant Complexes and Reactions Leading to a Model for the FeMoco Function 1.

Introduction

Biological N2 fixation is catalyzed by Fe/Mo, Fe/V, or FeFe (the Fe-only) nitrogenases (150, 151). The extremely different reaction conditions of the biological and the Haber–Bosch processes of N2 reduction, that is, standard temperature and pressure and biological redox potentials on the one hand, redhot temperatures and high pressures on the other hand, make the quest for low molecular weight competitive catalysts particularly challenging. Despite numerous chemical systems that coordinate and/or activate molecular dinitrogen, this challenge has not yet been met by chemistry. Nonenzymatic chemical systems reducing N2 usually need strong abiological reductants (e.g. alkaline metals) and none of these systems work truly catalytically. Even the elucidation of the molecular structure of the FeMo cofactor of nitrogenases has not afforded a solution to the problem. Thermodynamics, however, clearly states that the reaction according to Eq. 55 only needs a mild reduction potential that is less negative than that required for the reduction of protons to dihydrogen at pH 7 (
410 mV!) (150). +

N2 + 6 H

−

+ 6e

2 NH3

ð55Þ

Since 1964, when the first N2 complex was discovered, it has become more and more evident that the most difficult problem in competing with nitrogenases is not standard pressure–temperature, but the mild reduction potential at which N2 has to be reduced. Nitrogenases are very versatile enzymes. They reduce, in addition to N2 , a lot of other substrates, for example, protons, acetylene, azide, nitriles, and isonitriles. All of these substrates are reduced by multiples of [2 Hþ /2 e
] reductions. Both CO and NO inhibit nitrogenase activity. The apparent [2 Hþ /2 e
] multiplicity of substrate reductions and a couple of other findings strongly suggest diazene and hydrazine to be intermediates of the N2 ! NH3 reduction. The reduction of N2 is always accompanied by the reduction of protons to give H2 . This H2 evolution cannot be suppressed even by high pressures of N2 and is termed obligatory H2 evolution. It is another feature of nitrogenases that is difficult to explain. This chapter will describe nitrogenase relevant complexes and reactions achieved with complexes containing 1,2-benzenedithiolate derived multidentate ligands.

¨ RG SUTTER DIETER SELLMANN AND JO

662

2.

A Hypothetical Cycle for N2 Reduction with [Fe(Sn )] Complexes and Reversible Redox Reactions of Diazene Complexes

Scheme 37 shows a hypothetical cycle for the N2 reduction in the coordination sphere of [Fe(Sn )] complexes (7e–7g) (47), emphasizing the dominance of iron and sulfur in all nitrogenase cofactors. H N S S

− NH3

S S

N

−

+

+2H , +2e

N

+ N2

S S

Fe

S S

N H

H N S S

Fe

S S

S S

Fe

H N

H

S S

Fe N

N H

S S

N HHH

Fe N H

H N

− NH3 +

−

+2H , +2e

S S

Fe

S S

S S H

+

−

+2H , +2e

N H N H H

Scheme 37. Hypothetical cycle for N2 fixation catalyzed by [Fe(NH S4 )] fragments.

Activation of N2 through binding to [Fe(NH S4 )] fragments and the successive transfer of [2 Hþ /2 e
] reduction equivalents gives NH3 . The complexes exhibit no unusual Fe oxidation states, but formally only Fe(II). The N2 H2 , N2 H4 , and NH3 complexes in Scheme 37 could be isolated and completely characterized (47). The reader is reminded, however, that these complexes were not obtained from the N2 complex (see above). The diazene in [mN2 H2 {Fe(NH S4 )}2 ] is stabilized by the factors described in Section V.E. These are steric shielding by [Fe(NH S4 )] fragments, a strong 4c–6e
p bonding system between the Fe d orbitals and N2 H2 p system, and strong bifurcated
H   S2 (thiolate) bridges, N

H   S2 (thiolate) bridges. In particular, the N
which do not exist in the (hypothetical) N2 precursor, possibly help to stabilize the unstable N2 H2 to such an extent that even the first and most difficult step from N2 to N2 H2 becomes exergonic. This finding is indicated in Fig. 22.

DITHIOLENES IN MORE COMPLEX LIGANDS

663

∆Hf kJ mol

−1

200

N2H2 N2H4

100 0 −100

N2, H2 M+N2+H2

M-N2

M-N2H2

M-N2H4

2 NH3

Figure 22. Potential reaction diagram for N2 reduction in the absence and in the presence of metal– sulfur complexes.

The major problem of the cycle in Scheme 37 is the starting N2 complex. So far it has proved inaccessible. In fact, although there are numerous metal complex fragments binding N2 , and a very few of them also exhibit auxiliary sulfur ligands. Until recently metal–sulfur ligand complex fragments that were capable of coordinating molecular nitrogen under mild conditions had remained unknown. Other problems of the cycle in Scheme 37 are the low yield and insufficient solubility of the diazene complex, which prevent the detailed investigation of the chemical reactivity and, in particular, the redox properties of [mN2 H2 {Fe(NH S4 )}2 ]. A resolution to this problem was provided by the more accessible and soluble N2 H2 complexes of the type [m-N2 H2 {Fe(PR3 )(S4 )}2 ]. They exhibit the identical features of stabilizing the N2 H2 ligand and enabled the recording of cyclic voltammograms (CV). The CV of [m-N2 H2 {Fe(PPr3 )(S4 )}2 ] is shown in Fig. 23. Very similar CVs were obtained for [m-N2 H2 {Ru(PPh3 )(S4 )}2 ] and the other N2 H2 complexes of Section V.E. The CV of [m-N2 H2 {Fe(PPr3 )(S4 )}2 ] exhibits three reversible redox waves I, II, and III in the anodic region. The intensity of these waves depends on the temperature and the presence of protic solvents (MeOH) or bases (NaOMe), indicating that rapid and reversible protonation–deprotonation reactions of the N2 H2 ligand are associated with the redox processes. The redox waves can be assigned to the formation of the corresponding monocation (I), dication (II) and trication (III). The important point is that [m-N2 H2 {Fe(PPr3 )(S4 )}2 ] can be oxidized reversibly in two consecutive steps yielding the dication with two Fe(III) centers as indicated in Fig. 23. This finding affords the working hypothesis for attempts to achieve the reduction of N2 under mild and catalytic conditions.

¨ RG SUTTER DIETER SELLMANN AND JO

664

II

II

[ Fe-NH=NH-Fe ] I

a

[ Fe-NH=NH-Fe ]

I

b II

II +

III

25 µ A

II III 2+

III

c III

[ Fe-NH=NH-Fe ]

1.5

0.0

[V] vs. NHE

Figure 23. Cyclic voltammogram of [m-N2 H2 {Fe(PPr3 )(S4 )}2 ] (in CH2 Cl2 ,
20 C, v ¼ 100 mV s
1 ) and assignment of redox waves and Fe oxidation states. Normal hydrogen electrode ¼ NHE.

A different rendering of the relevant core atoms of the dication [Fe, N2 H2 , S(thiolate) donors] shows that the dicationic diazene complex B and the N2 complex C are potential redox isomers or valence tautomers (Scheme 38). S II

Fe

H N N

S

S

S

H

II

Fe

−2 e

−

+2 e S

43

−

III

Fe

H

S

N N

S

H 44

III

Fe

2+

S II

Fe

H

S

N N

II

Fe

2+

S

−2H

+

+

II

Fe

S N

N

0

II

Fe

+2H S

S

H

S

S

45

S 46

Scheme 38. Redox isomerism of the [m-N2 H2 {Fe(PPr3 )(S4 )}2 ]2þ ion.

It can be conceived that intramolecular electron transfer from N2 H2 to the Fe(III) centers, cleavage of NH bonds, and formation of SH bonds converts 44 into 45, which is a doubly protonated N2 complex. Deprotonation of 45 would yield the neutral N2 complex 46. The reversal of this sequence would convert 46 into 43 and realize the first 2 Hþ /2 e
reduction step of N2 fixation. Twofold protonation of 46 gives 45 in which all atoms necessary to form the neutral diazene complex 43 have already taken their positions. The above mentioned anodic redox potential shift upon protonation (cf. Section IV.F) could enable us to reduce species C at relatively mild redox potentials, in contrast with the neutral species 46, which might be irreducible when it is an 18 VE complex. 3.

Modeling the Nitrogenase Catalyzed N2 Dependent HD Formation with Diazene Complexes

In Section VI.D.2, the significance of [M(Sn )] diazene complexes was emphasized with regard to the as yet unattained reduction of N2 under mild

DITHIOLENES IN MORE COMPLEX LIGANDS

665

conditions. The [M(Sn )] diazene complexes are also of considerable importance for understanding another key reaction of nitrogenases, the N2 dependent HD formation. The reduction of N2 catalyzed by nitrogenases is always accompanied by proton reduction leading to H2 . This obligatory H2 evolution is different for the three types of nitrogenases. Equations 56–58 give the NH3 /H2 ratios observed with FeMo, FeV, and FeFe nitrogenases, respectively (150–152). +

+

+

+ 12 e

+

+ 21 e

N2 +

8H

N2 +

12 H

N2 +

21 H

8e

−

FeMo

−

FeV

−

FeFe

2 NH3 +

H2

ð56Þ

2 NH3 +

3 H2

ð57Þ

2 NH3 + 7.5 H2

ð58Þ

In 1960, Burris and co-workers (153) observed that the obligatory H2 evolved by FeMo nitrogenase contained HD when D2 had been added to the gas phase (Eq. 59). +

N2 + 8 H

−

+ D2

+ 8e

2 NH3 + 1 (H2, HD)

ð59Þ

Severe constraints were found to be imposed upon this HD formation (154). (1) Electron balance studies showed that one electron is needed per HD formed such that Eq. 60 holds. +

2H

−

+ 2e

2 HD

+ D2

ð60Þ

(2) The HD formation occurs only in the presence of N2 . (3) No D2 forms when HD is added instead of D2 . (4) Only a minimum amount of Tþ (2.4%) is released into the aqueous phase when T2 is added. These constraints compellingly require that N2 reduction and HD formation are integral parts of one and the same reaction and have to occur via a predominantly intramolecular reaction in or at the FeMoco. They also made it very difficult to suggest a plausible and consistent mechanism. Because no complex was found able to model either of the reaction(s) or any of the constraints, all mechanistic proposals had to be speculative and exhibited one or more grave shortcomings. This holds true even for the least speculative mechanism, the diazene mechanism. The diazene mechanism takes into account the electron balance and suggested a [2 Hþ /2 e
] reduction of enzyme bound N2 to N2 H2 . In order to explain the HD formation, it proposed a reoxidation of N2 H2 by D2 according to Eq. 61. Equation 61 sums up to a D2 catalyzed N2 H2 decomposition. +

Enz · N2

−

+2H ,+2e

Enz · N2H2

+ D2

Enz · N2 + 2 HD

ð61Þ

666

¨ RG SUTTER DIETER SELLMANN AND JO

This diazene mechanism raises the question of why N2 should first get reduced in order to be subsequently reoxidized by D2 . The D2 is rather an unusual oxidant and the intimate molecular mechanism of the N2 H2 oxidation by D2 had to remain open (154). The reactions of [m-N2 H2 {Ru(PCy3 )(S4 )}2 ] (cf. Section VI.B) with D2 and D2 O gave surprising insights and led to several important conclusions with regard to the N2 dependent HD formation. The complex [m-N2 H2 {Ru(PCy3 )(S4 )}2 ] reacts reversibly with D2 or D2 O to give [m-N2 D2 {Ru(PCy3 )(S4 )}2 ] and HD or HDO, respectively, according to Eqs. 62a and b (155). It is significant that the reaction with D2 O is about seven times slower than that with D2 . [µ-N2H2{Ru(PCy3)(S4)}2] + 2 D2

[µ-N2D2{Ru(PCy3)(S4)}2] + 2 HD

ð62aÞ

[µ-N2H2{Ru(PCy3)(S4)}2] + 2 D2O

[µ-N2D2{Ru(PCy3)(S4)}2] + 2 HDO

ð62bÞ

Combining Eqs. 62a and b demonstrates that the formation of HD from D2 and water protons (which are the hydrogen source for HD) can be mediated by a diazene species. Diazene, on the other hand, is the most plausible intermediate of a [2 Hþ /2e
] reduction of N2 , and it is a reduction intermediate that can form only from N2 and not from any other nitrogenase substrate. Scheme 39 outlines the suggested reaction mechanism. It takes into account two previous results (see above). (1) The [Ru(PCy3 )(S4 )] fragment catalyzes the heterolysis of H2 or D2 (144). (2) The complex [m-N2 H2 {Ru(PCy3 )(S4 )}2 ] exchanges its PCy3 for PPr3 ligands according to a likely dissociative mechanism (131), which suggests the occurrence of intermediate vacant Ru sites in the diazene complex. The individual steps of the mechanism in Scheme 39 are analogous to those discussed above for the hydrogenase model complexes. After the heterolysis step yielding Dþ and D
, an intramolecular and probably very rapid exchange between thiol deuterons and diazene protons has to take place in order to explain the experimental findings. These results show that a N2 dependent HD formation is possible on the N2 H2 level and, in that respect support a diazene mechanism of N2 reduction. They also demonstrate that D2 does not oxidize N2 H2 , which leads to the conclusion that in case of a diazene mechanism, two pathways of HD formation must exist in nitrogenase. The experimentally established electron balance for the nitrogenase reaction is given by Eq. 60. The model reaction of Eq. 62a, rewritten as Eq. 63, does not require any electrons, and it uses only one-half of the D2 for HD formation. The other one-half remains bound in the N2 D2 complex. These differences between the nitrogenase catalyzed reaction 60 and the model reaction 63 resolve when it is taken into account that the HD formation in

DITHIOLENES IN MORE COMPLEX LIGANDS

667

S S S S

N Ru

S

S

Ru

N S

H S

S S

H

PCy3

+ 2 D2 / − 2 HD + 2 H2 / − 2 HD

PCy3

S S

D N

Ru

S

S

D S

PCy3

− 2 PCy3

PCy3

+ 2 PCy3

S S

S

H

S N

Ru

D

Ru

N

N

Ru

S

D S

+ 2 D2

Ru

N

S

H S

− 2 HD S

S S

H N

S

Ru D

Ru

N

Ru

D

S

H

S H

D N N D

D heterolytic D2 cleavage

D

S

D

Ru S D H

S

Ru

S D

+

N H

S

D

S

H N

S

Ru

N

+

H /D exchange

Ru

Ru

S D D

S D

H N N H

S

D

Ru S D D

Scheme 39. Mechanism of the D2 /NH exchange of [m-N2 H2 {Ru(PCy3 )(S4 )}2 ].

nitrogenase is catalytic, that is, the electron flux does not stop when the N2 H2 /N2 D2 level has been reached, and the diazene bound deuterium, too, is utilized for HD formation. This is possible when the N2 D2 species is reconverted into the N2 H2 species by a [4 Hþ /4 e
] or 2  [2 Hþ /2 e
] reduction according to Eq. 64. Adding Eqs. 63 and 64 gives the experimentally established electron balance, and strongly indicates a nonreductive (Eq. 63) and a reductive (Eq. 64) HD formation pathway in nitrogenase. N2H2[M]2 + 2 D2

2 D2

−

+

+ 4e

+

+ 4e

N2D2[M]2 + 4 H + 4H

-

N2H2[M]2

N2D2[M]2 + 2 HD

ð63Þ

N2H2[M]2 + 2 HD

ð64Þ

4 HD

ð65Þ

How the D2 and D2 O reactions of [m-N2 H2 {Ru(PCy3 )(S4 )}2 ] also allow rationalization of the other constraints of the N2 dependent HD formation (156)

¨ RG SUTTER DIETER SELLMANN AND JO

668

has been described in detail elsewhere and shall not be repeated here. From our point of view, a particularly significant point of these reactions is the strong support they lend to the open-side model for the functioning of the FeMoco (FeMoco ¼ iron molybdenum cofactor) in nitrogenase. 4.

The Open-Side Model of FeMoco Functioning

The open-side FeMoco model (7f, 156, 157) takes into acount a large number of biological, biochemical, and chemical results. The results considered most important are that: iron is the dominant metal not only in Fe-only, but in all three nitrogenase types, the Fe centers in the FeMoco, and probably also in the FeV and FeFe cofactors, are undercoordinated, sulfur donors prevail; transition metal thiolate or sulfide bonds frequently are labile; so is isolated FeMoco with its t1=2 2 h in aqueous media; isolated FeMoco does not catalyze N2 reduction, indicating the essential role of the enzyme protein; all nitrogenase substrates are reduced in multiples of [2 Hþ /2 e
] steps; diazene is efficiently stabilized by transition metal sulfur complexes; diazene complexes can be electrochemically oxidized in several reversible steps; through primary protonation nonreducible (18 VE) transition metal sulfur ligand complexes become reducible in biologically compatible redox ranges of about
500 mV; the FeMo protein of FeMo nitrogenase contains two amino acids, Gln a191 and His a195, which are ˚ ), have N and O donors suited for located in close proximity to the FeMoco (5 A metal coordination, and are essential for nitrogenase activity; water molecules, which can also act as ligands, surround the FeMoco in native nitrogenase in large numbers (158); crystallographic studies have shown that the structures of enzymes in the resting state and active turnover state (modeled by inhibitors) can be considerably different, and last but not least, kinetic studies demonstrate that nitrogenase as isolated (in the dithionite reduced state) must take up a minimum of two electrons before it can bind (but not yet reduce) N2 . Putting together these findings, the open-side FeMoco model essentially suggests that the FeMoco structures are different in the resting and turn-over state. The primary uptake of a minimum of two electrons, necessary for the
S

Fe bridge. Oxygen and nitrogen binding of N2 , leads to cleavage of one Fe
donor atoms of the essential amino acids Gln a191 and His a195, and vicinal water molecules are added. One of these water molecules may result from protonation of the small interstitial atom, which has been found at the center of the FeMoco in the most recent X-ray structure determination of FeMo nitrogenase. This interstitial atom is possibly an O atom (158a), and may become an external H2 O ligand exactly like the reductive protonation of [m3 O{Fe(CH3 CO2 )2 }3 ]þ in water gives [Fe(H2 O)6 ]2þ ions (158b). Two unique fivecoordinate Fe centers result, which are in low-spin states through steric constraints of the surrounding protein. The unique five-coordinate Fe centers

DITHIOLENES IN MORE COMPLEX LIGANDS

669

Gln α191

O

Fe S S

H2O

S

S Mo

N H His α195

S S

Fe

N

H

Fe

Fe

H2 O

S

S

FeII

S

H

N

S

H2 O

S

Fe

+[2 H+ / 2 e–]

S Fe

S Fe

S

Fe

S

N

H2 O

S

N

S

Fe

H2 O

Fe N

Fe

Fe

S Fe

S

+2e– +N2

S

S H2O

S

H2 O

Fe

Fe

FeII

H2 O

Fe

S

Fe

Fe

S

H2O

O

(O) (Gln)

NH 2

Fe

S

S

S

Mo

Mo N

N (His)

Turnover state

Resting state

Figure 24. Opening the FeMoco and binding N2 to two five-coordinate low-spin Fe(II) centers.

have the proper distance in order to bind N2 as well as its reduction products formed subsequently (Fig. 24). The molecular structures of the diazene complexes, with their strong N

H    S bridges, for example, [m-N2 H2 {Fe(NH S4 )}2 ], enabled us to illustrate N2 H2 as the first [2 Hþ /2 e
] reduction intermediate of N2 . In the open-side FeMoco model, these hydrogen bridges involve one N

H   O bridge to a H2 O or OH
ligand. The diazene stage accomplishes the N2 dependent HD formation. Hard H2 O ligands binding to low-spin Fe(II) centers, which are relatively soft due to the sulfur coordination, can be anticipated to be labile. Exchange of these H2 O ligands for D2 , one at each Fe center, gives Z2 -D2 intermediates. The D2 ligands subsequently heterolyze and exchange with the NH protons according to exactly the same mechanism as outlined above for [m-N2 H2 {Ru(PCy3 ) (S4 )}2 ] (Fig. 25). O

O Fe S

Fe

H2 O

S Fe

S

H2 O

Fe H

N H

Fe D

S S

Fe

N

D H2 O

Fe

S

Fe

S Mo

N

S

Fe

+ 2 D2 – 2 H2O

S Fe

S H

N H D

S H2 O

S

Fe

S Fe

N

D2 heterolysis

S Fe

D S

S

Fe

S

S

Mo N

Figure 25. The D2 /H2 O exchange at the two unique Fe(II) centers of the open-side FeMoco

N2 H2 stage as the primary step of the subsequent D2 heterolysis and HD formation.

¨ RG SUTTER DIETER SELLMANN AND JO

670

In all these steps the rest of the FeMoco serves as a flexible spacer and electron relays for the two unique Fe centers. These Fe centers can shuttle between Fe(II) and Fe(III) oxidation states in the course of N2 reduction (and likewise of the N2 dependent HD formation) like the Fe centers in the reversible electrochemical oxidation of diazene complexes such as [m-N2 H2 {Fe(PPr3 ) (S4 )}2 ]. E.

Dinitrogen Complexes with [M(Sn )] Cores

It is obvious that the cope-stone for corroborating the open-side model of the FeMoco function and, more generally, the proposals of how N2 can be reduced under mild conditions, is still missing. This cope-stone represents N2 complexes with Brønsted basic sulfur coligands that form under truly mild conditions and can be protonated and reduced at mild redox potentials. Such complexes have been invoked in all other models for nitrogenase mechanisms as well, however, they remained unknown in spite of numerous intensive and long-lasting efforts. There are only 10 N2 complexes with ancillary sulfur ligands (159, 160). None of them form under mild conditions. Only two could be prepared directly from N2 and metal sulfur species. These are [Mo(N2 )2 (Me8 -16[ane]S4 )] (160a) and [Re(N2 )(SAr)3 (PPh3 )] (160f). Their preparation, however, required strong reductants or precursors prepared by use of strong reductants. Thus these complexes do not meet the severe constraints imposed on mild conditions that comprise standard temperature, standard pressure, and, most importantly, mild and biologically compatible redox potentials. The N2 complexes [Ru(N2 )(PiPr3 )(N2 Me2 S2 )] and [m-N2 {Ru(PiPr3 )(N2 Me2 S2 )}2 ] meet the requirements to exhibit Brønsted basic sulfur donors and to form under mild conditions, excluding strong reductants at any stage of the synthesis. Treatment of the precursor complex [Ru(MeCN)(PiPr3 )(N2 Me2 S2 )] with molecular N2 reversibly gives the corresponding N2 complex [Ru(N2 )(PiPr3 )(N2 Me2 S2 )] (161). Controlled removal of the N2 ligand yielded the dinuclear [m-N2 {Ru(PiPr3 )(N2 Me2 S2 )}2 ] (Eq. 66). The dinuclear complex formed as a racemate of (S,S) and (R,R) enantiomers that spontaneously separated upon crystallization (162). The N2 complexes could be completely characterized, and Fig. 26 shows their molecular structures.

S Me + N2 N Ru PiPr3 N N C Me Me 1 bar S 20 ˚C

[Ru(MeCN)(PiPr3)(N2Me2S2)]

S Me N Ru PiPr3 + Ar/− N2 N N N toluene Me S ~ 40 ˚C

S

Me

N S PiPr3 N Ru N N Ru NMe N S Me S P iPr3 Me

[Ru(N2)(PiPr3)(N2Me2S2)] (S,S)-[µ-N2{Ru(PiPr3)(N2Me2S2)}2]

ð66Þ

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N2 S2

N1

P1 Ru1 S1

N3 N4

(a)

S2 P1 N2

Ru1

N1A S1A

N3 N3A S2A

N1

N2A Ru1A

S1 P1A

(b) Figure 26. Molecular structures of (a) (R)-[Ru(N2 )(PiPr3 )(N2 Me2 S2 )] and (b) (S,S)-[m-N2 {Ru(PiPr3 )(N2 Me2 S2 )}2 ].

Mononuclear [Ru(N2 )(PiPr3 )(N2 Me2 S2 )] exhibits no peculiarities with respect to distances, angles, mode of binding the N2 ligand, and so on, when compared with other regular N2 complexes. For example, its n(N2 ) frequency at 2113 cm
1 (KBr) reflects a normal N2 activation with respect to the n(N2 ) frequency of gaseous N2 (2331 cm
1 ).

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The dinuclear counterpart [m-N2 {Ru(PiPr3 )(N2 Me2 S2 )}2 ], although at first glance looks like a classical dinuclear complex with an end-on bridging N2 ligand, exhibits unique structural and spectroscopical features. For example, the Ru

N bond trans to the N2 ligand is 6 pm shorter than the corresponding cisRu

N bond. The [Ru

N

N

Ru] angle [172.2(2) ] significantly deviates from linearity, possibly indicating a beginning sp2 hybridization of the N2 unit. The n(N2 ) frequency of 2042 cm
1 (Raman) is some 70 cm
1 lower than in mononuclear [Ru(N2 )(PiPr3 )(N2 Me2 S2 )]. This n(N2 ) frequency decrease is considerably larger than in any other couple of homologous mononuclear– dinuclear N2 complexes, and contradicts molecular orbital theory for such complexes (163). Ongoing research focuses on attempts to reduce the N2 ligands of these complexes. Irrespective of the results of these attempts, [Ru(N2 )(PiPr3 )(N2 Me2 S2 )] and [m-N2 {Ru(PiPr3 )(N2 Me2 S2 )}2 ] are unique and unprecedented with respect to the mild conditions under which N2 binds to metal thiolate complex fragments. This finding raises the question of why the attempts to synthesize such complexes have remained unsuccessful for such a long time. Two points can be made. (1) It is established that the coordination of N2 can depend on triflingly minor looking subtleties, demonstrated, for example, by the stability of [Ir(N2 )(Cl)(PPh3 )2 ] and nonexistence of [Ir(N2 )(Cl)(P{p-tolyl}3 )2 ] (128). Metal and N2 orbitals have to match precisely in order to warrant binding of N2 . (2) Thiolate (and also sulfide) donors are principally adverse to the coordination of N2 . Binding N2 to a metal center requires a vacant site of coordination. Metal thiolate (or sulfide) complexes, however, are notorious for saturating vacant sites by formation of M

S

M bridges and oligo- to polynuclear complexes. In nitrogenase, such N2 binding sites at the FeMoco may be kept vacant through steric strain and shielding provided by the enzyme protein. Low-molecular weight thiolate complexes, however, need to achieve simultaneously the electronic fine-tuning of the vacant site and the delicate balance between the competing reactions of blocking this site either by N2 or by a sulfur donor from another metal thiolate complex fragment. As yet, there is only the experimental way to elucidate the conditions necessary for meeting these constraints.

VII.

CONCLUDING REMARKS

Multidentate sulfur ligands mainly derived from 1,2-benzenedithiol have furnished a large number of transition metal complexes with multifarious structures, electronic properties, and reactivity features. This chapter emphasizes the reactivity of complexes that, in the end, were found in the quest of competitive catalysts for nitrogenases and other metal–sulfur enzymes. The

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ultimate goal has not yet been reached, however, in chemistry as in mountaineering the way can be the destination.

ACKNOWLEDGMENTS D. S. acknowledges gratefully the enthusiasm, competence, and hard work of many co-workers, diploma and doctoral students, and postdoctoral fellows. Their willingness to share his chemical obsessions and to walk with him on stony paths and frequently pathless territory is highly appreciated. D. S. also thanks the continuous financial research support from the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie.

ABBREVIATIONS acac Bzo-9-S-3 Bzo2 -18-S-6 CODH CoA Cp CV Cy DFT DMF dmso EN FD FeMoco 1 H NMR HOMO IR L Me NHE Ph py sq THF TMEDA UFO UV

Acetylacetonato Trithio-benzo[9]crown-3 Hexathio-dibenzo[18]crown-6 Carbon monoxide dehydrogenase Coenzyme A Cyclopentadienyl Cyclic voltammetry Cyclohexyl Density functional theory Dimethylformamide Dimethyl sulfoxide (ligand) Electronegativity Field desorption Iron molybdenum cofactor Proton nuclear magnetic resonance Highest occupied molecular orbital Infrared Ligand Methyl Normal hydrogen electrode Phenyl Pyridine Squaric acid Tetrahydrofurane N,N,N 0 ,N 0 -Tetramethylethylenediamine Unidentified flying object Ultraviolet

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Subject Index Ab initio calculations: bis(dithiolenes), reaction-application analysis, 288–290 metallo-bis(dithiolenes), 158–166 four-coordinate structures, 226–228 metallo-dithiolenes, 115–116 metallo-tris(dithiolenes), ideal D3h trigonalprismatic distortion, 182–188 Absorption spectra: metallo-bis(dithiolenes), 156–166 axial oxo ligands, 172–173 metallo-mono(dithiolenes), bonding parameters, 125–128 metallo-tris(dithiolenes), electronic absorption (EA) spectroscopy, 192–196 square-planar d8 bis(dithiolenes), excited states and luminescence, 321–324 Acetate butyrate, metallo-1,2-enedithiolates, 386 Acetylene: metal sulfides, unactivated alkyne additions, 34–37 metal–sulfur interactions, miscellaneous ligands, 601 Activated phosphates, metallo-1,2enedithiolates, 387–390 Addition reactions, bis(dithiolenes), unsaturated compounds, 287–288 Aldehyde oxidase (AO), structure and properties, 541 Aldehyde oxidoreductase (AOR): biological metallodithiolenes: crystalline structure, 514–515 XDH/XO crystalline structure, 508–510 magnetic circular dichroism (MCD) analysis, 518–519 molybdopterin ligand structure, 524–527

oxo-metallo-bis(dithiolenes), vibrational spectra, 241–246 oxo-metallo-mono(dithiolenes), vibrational spectra, 239–241 pyranopterin dithiolenes, mononuclear molybdenum/tungsten enzymes, 258–261 Alkanedithiolates, dehydrogenation, 38–39 Alkenedithiolates, transition metal dithiolene synthesis: benzenedithiol and related derivatives, 4–10 arene derivatives, 4–10 dicarborane- and ferrocenedithiolates, 10 dianion ligands: 4,5-dimercapto-1,3-dithiole-2-thione, 15–18 inorganic dmit2– dithiolate derivatives, 18–19 1,2-maleonitrile 1,2-dithiolate (mnt2
) tetrathiafulvalene (TTF)-derived dithiolenes, 19–20 thiacarbons and derivatives, 20 1,2-dithietes, 22–23 dithiocarbonate (dithiole-2-ones) and related derivatives, base hydrolysis, 11–15 1,2-dithiones, 23–25 intermetallic dithiolene transfer: non-redox routes, 25–26 redox routes, 26–29 reductive dealkylation, 10–11 thiophosphate esters, a-hydroxyketone and a-diketone derivatives, 21–22 Alkenes, bis(dithiolenes), hydrocarbon cycloaddition, 284–287 Alkylation reaction: bis(dithiolenes), 281–284 heteroleptic dithiolenes, organometallics, 307–308

Dithiolene Chemistry: Synthesis, Properties, and Applications, Progress in Inorganic Chemistry, Vol. 52 Special volume edited by Edward I. Stiefel, Series editor Kenneth D. Karlin ISBN 0-471-37829-1 Copyright # 2004 John Wiley & Sons, Inc. 683

684

SUBJECT INDEX

Alkylation reaction (Continued) hexadentate ligands, 599 [M(L)(Sn)] fragments: chirality, 619–620 protonation and electron transfer, 623–626 square-planar bis(dithiolenes), radical formation kinetics, 335 tetradentate ligands, 592–596 tris(dithiolenes), chemical reactivity, 296–299 Alkynes: bis(dithiolenes), hydrocarbon cycloaddition, 284–287 1,2-dithiolene assembly routes: metal–alkyne bonds, 41–42 thiocarbonyl derivatives, 43 in metal sulfides, 29–32 unactivated alkyne addition, 32–37 Alkynyl anions, dithiolene assembly, 44 Aluminum electrodes, dithiolene-based films, memory switching, 430–431 Amine donors, metal–sulfur interactions, ligand structures, 599–600 Ammonia, dithiolene-based films, electrical sensors, 429–430 Ammonium cations, dithiolene electrical properties: dmit compounds, 409–417 Langmuir–Blodgett (LB) conducting films, 428–429 superconductivity, 423–427 Analyte detection, metallo-1,2-enedithiolates, 370 Angular parameters: bis(dithiolene) complexes: bond lengths and angles, 63–64 coordination geometry, 60–62 tris(dithiolene) complexes, 88 Anionic structure: bis(dithiolene) complexes, 61–62 ligands, 72 mono(dithiolene) complexes, 92–95 Anisotropic covalency: metallo-mono(dithiolenes), electronic buffer effect, 134–138 metallo-tris(dithiolenes), electron paramagnetic resonance (EPR), 189–192 Antiferromagnetism, 1,2-dithiolenes, 431 spin-ladder systems, 433–440 Antimony, main group bis(dithiolenes), 79

Antioxidants, bis(dithiolenes), miscellaneous reactions/applications, 289–290 Arenedithiolate ligands, transition metal dithiolene synthesis, benzenedithiol compounds, 4–10 Arene oxides, metal-binding pyranopterin dithiolene (MPT), catalytic center chemical analogues, 553–560 Arsenite oxidase (AsO), X-ray diffraction studies, 513–514 A-term resonance enhancement: metallo-bis(dithiolenes), four-coordinate structures, 220–228 metallo-tris(dithiolenes), six-coordinate vibrational spectra, 232–239 oxo-metallo-mono(dithiolenes), 241 Atomic orbital calculations, metallobis(dithiolenes), square-planar bonding, 158–166 Axial oxo ligands, metallo-bis(dithiolenes), 166–173 excited-state spectroscopic probes, 167–173 square-planar–square-pyramidal bonding comparison, 166–167 Backbone structure, metallo-mono(dithiolenes), 128 Bacteria, trimethylamine-N-oxide (TMAO) reactions, 540 Band structures, dithiolene electrical properties, superconductivity, 423–427 Bardeen, Cooper, and Schrieffer (BCS) theory, 1,2-dithiolene complexes, superconductivity, 422–427 Base hydrolysis, transition metal dithiolene synthesis, dithiocarbonates and derivatives, 11–15 Beer’s law, metallo-1,2-enedithiolates, 371–374 1,2-Benzenedithiolates: metal–sulfur interactions, 588–589 [M(L)(Sn)] complexes, 604–606 pentadentate ligands, 596–598 tetradentate ligands, 592–596 tridentate ligands, 590–592 Benzenedithiols and derivatives, transition metal dithiolene synthesis, 4–10 arene derivatives, 4–10 dicarborane- and ferrocenedithiolates, 10

SUBJECT INDEX Benzene ring, [M(L)(Sn)] complexes, electronic substituent effects, 622–623 Benzyl halides, bis(dithiolenes), alkylation– protonation, 281–284 Bidentate bite angle, metallo-tris(dithiolenes), six-coordinate vibrational spectra, 231–239 Bimetallic complexes: square-planar mixed-ligand dithiolene– diimine complexes, structural variation, 352–353 transition metal dithiolene synthesis, arenedithiolate ligands, 8–10 Binuclear complexes, metal-binding pyranopterin dithiolene (MPT), catalytic center chemical analogues, 552–560 Biological dithiolene ligands: cofactor biosynthesis, 527–531 dithiolene-molybdenum spectroscopic probes, 515–519 electronic spectroscopy, 517–518 magnetic circular dichroism, 518–519 paramagnetic spectroscopy, 518 resonance Raman spectroscopy, 515–516 X-ray absorption spectroscopy, 516–517 enzyme mechanisms, 519–522 molybdopterin degradation studies, 504–507 molybdopterin function, 522–527 [MS] enzyme modeling, FeMoco function, nitrogenase complexes, 661–664 natural dithiolenes, 493–503 enzyme structural classification, 496–504 ligand structure, 499–501 molybdenum enzyme families, 496–498 nomenclature issues, 498–499 pterin redox reactions, 501–503 tungsten enzyme families, 498 research background, 492–493 X-ray crystallography, dithiolenemolybdenum enzyme site, 507–515 aldehyde ferrodoxin oxidoreductase (AOR) crystal structure, 514–515 DMSOR crystal structure, 511–514 sulfite oxidase crystal structure, 510–511 XDH/XO crystal structure, 508–510 Biosynthesis, dithiolene cofactor, 527–531 Biotin sulfoxide reductase (BSOR): pyranopterin dithiolenes, mononuclear molybdenum–tungsten enzymes, 250–258

685

resonance Raman (RR) spectroscopic probe, 515–516 X-ray absorption analysis, 516–517 Bis(1,3-benzodithiol-2-ylidene)-4,9dihydronaphtho[2,3-c][1,2,5]thiadiazole (BDNT), dithiolene magnetic properties, ferromagnetic mnt systems, 443–444 Bis(dddt) complexes, dithiolene electrical properties, 407 Bis[(4-diethylamino)dithiobenzyl]nickel (BDN), dithiolene-based films, electrical sensors, 429–430 Bis(dithiolenes), 277–290 distribution, 59 electrochemical–chemical reactivity, 268–270 alkylation–protonation, 281–284 ligand addition and substitution, 278–280 ligand-exchange reactions, 278 miscellaneous reactions/applications, 288–290 redox properties, 270–277 unsaturated carbon cycloadditions, 284–287 unsaturated compound addition reactions, 287–288 metal-binding pyranopterin dithiolene (MPT), catalytic center chemical analogues, 548–560 metallo-bis(dithiolenes): electronic–spectroscopic structural studies, 142–143 four-coordinate structures, vibrational spectra, 214–228 solvatochromatic absorption, 139 square-planar structures, 143–173 axial oxo ligands, 166–173 oxo-metallo-bis(dithiolenes), vibrational spectra, 241–246 ring structures, 402 square-planar structures: axial oxo ligands: excited-state spectroscopic probes, 167–173 square-pyrimidal bonding comparison, 166–167 d8 complexes, 320–335 excited states and luminescence, 320–324 hydrogen photoproduction, 328–330 ion-pair charge-transfer, 324–328

686

SUBJECT INDEX

Bis(dithiolenes) (Continued) photooxidation, 330–333 radical formation, 334–335 molecular orbital (MO) bonding calculations, 143–166 excited-state spectroscopic probes, 149–166 ground-state spectroscopic probes, 146–149 tetrahedral/distorted four-coordinate d10 complexes, 353–354 transition metal homoleptic dithiolenes, 59–79 bond lengths and angles, 63–64 chromium, manganese, iron and cobalt, 78–79 copper and gold, 77 geometrical properties, 60–62 ligand types, 64–72 multimeric molecular structures, 73–76 nickel, palladium, and platinum, 76–77 silver, zinc, cadmium, and mercury, 78 Bis(dithiooxamide) cations, square-planar d8 bis(dithiolenes), excited states and luminescence, 323–324 Bis(ethylenedithio)-tetrathiafulvalene (BEDTTTF): dithiolene electrical properties, 406–407 dddt compounds, 418–420 superconductivity, 425–427 multifunctional electromagnetic dithiolenes, 465 Bis(methylenedithiol)tetrathiafulvalene (BMDT-TTF), dithiolene electrical properties, metal-like behavior, 421–422 Bismuth, main group bis(dithiolenes), 79 Bonding parameters: dithiolene ligand bonding, 57–58 metallo-bis(dithiolenes), 154–166 metallo-mono(dithiolenes), strong-field axial ligand, 117–128 metallo-tris(dithiolenes), electronic absorption (EA) spectroscopy, 196 transition metals, 63–64 tris(dithiolene) complexes, 87–88 Bulk ferromagnets, dithiolene magnetic properties, 448–453 (Cp*2Mn)[Ni(dmit)2], 450–453 (NH4)[Ni(mnt)2]H2O, 448–450

Butadiene, bis(dithiolenes), hydrocarbon cycloaddition, 284–287 Cadmium complexes: bis(dithiolene), homoleptic structures, 78 mixed-ligand diimine dithiolates, 355–356 tetrahedral bis(dithiolenes), 353–354 C

C bond distances: metal-binding pyranopterin dithiolene (MPT), 571–574 metallo-bis(dithiolenes): four-coordinate structures, 218–228 square-planar bonding, 154–166 metallo-tris(dithiolenes): ideal D3h trigonal-prismatic distortion, 184–188 six-coordinate vibrational spectra, 235–239 oxo-metallo-bis(dithiolenes), vibrational spectra, 243–246 pyranopterin dithiolenes: dimethyl sulfoxide reductase (DMSOR)/ biotin sulfoxide reductase (BSOR), 254–258 xanthine oxidase (XO) family, 247 square-planar mixed-ligand dithiolene-donor complexes, 335–338 vibrational spectra, 261–262 Carbon–carbon coupling pathways, dithiolene assembly, 42 13 C NMR: bis(dithiolenes), unsaturated compound addition reactions, 287–288 hydrogen-bridge diastereoisomerism/PR3 exchange reactions, [m-N2H2{M(PR3)(S4)}2] complexes, 648–652 metal-binding pyranopterin dithiolene (MPT), chemical shift variations, 572–574 [M(L)(Sn)] fragments, 629–630

N stretching: C

metallo-bis(dithiolenes), four-coordinate structures, 224–228 metallo-tris(dithiolenes), six-coordinate vibrational spectra, 235–239 Carbon dioxide, [M(L)(Sn)] complexes, CO, CO2, and SO2 conversion, 637–639 Carbon monoxide (CO). See also NO
! CO exchange metal-binding pyranopterin dithiolene (MPT), catalytic center chemical analogues, 555–560

SUBJECT INDEX metal–sulfur interactions, pentadentate ligands, 596–598 [M(L)(Sn)] complexes: benzene ring substituent effects, 622–623 chirality, 617–620 CO, CO2, and SO2 conversion, 637–639 protonation, alkylation, and redox reactions, 624–626 octahedral dn complexes, mixed-ligand structures, 357–360 Carbon monoxide dehydrogenase (CODH): biological metallo-dithiolenes: enzyme mechanisms, 522–523 XDH/XO crystalline structure, 508–510 molybdopterin ligand structure, 524–527 [MS] enzyme modeling, 658–660 structure and properties, 541 Carbonyl complexes: heteroleptic dithiolenes, 299–301 octahedral dn complexes, mixed-ligand structures, 357–360 transition metal dithiolene synthesis, metal sulfides, unactivated alkyne additions, 36–37 Catalytic centers: metal-binding pyranopterin dithiolene (MPT) enzymes: chemical analogues, synthesis, and characterization, 547–560 dithiolene complexes, 569–574 future research issues, 575–577 oxygen atom transfer (OAT), 560–569 molybdenum/tungsten oxo-chemistry, 545–547 [MS] enzyme modeling, carbon monoxide dehydrogenase (CODH), 658–660 CCN deformation, metallo-bis(dithiolenes), four-coordinate structures, 217–228 Charge-deficient dithiolenes, electronic buffer effect, 137–138 Charge density wave (CDW) condensations, dithiolene electrical properties: dmit compounds, 410–417 superconductivity, 426–427 Charge transfer (CT): biological metallo-dithiolenes, magnetic circular dichroism (MCD) analysis, 519 dithiolene luminescence and photochemistry, 317–320

687

metallo-bis(dithiolenes), axial oxo ligands, 167 metallo-mono(dithiolenes), molecular orbital (MO) calculations, 118–128 square-planar bis(dithiolenes), ion-pair charge-transfer photochemistry, 324–328 square-planar mixed-ligands: dithiolene–diimine complexes: excited states and luminescence, 339–344 photooxidation, 348–351 self-quenching mechanism, 348 structural variation, 351–352 dithiolene–donor complexes, 336–338 tetrahedral bis(dithiolenes), 353–354 Charge-transfer-solvent (CTTS) state: metallo-dithiolenes, photochemical applications, 360–361 square-planar d8 bis(dithiolenes), photooxidation, 331–333 square-planar mixed-ligand dithiolene– diimine complexes, photooxidation, 348–351 C2H4 bridge-dissociation reactions, [M(L)(Sn)] fragments, C–S bond cleavage, 628–629 Chelate ring modes: metallo-bis(dithiolenes): four-coordinate structures, 216–228 luminescence and photochemistry, 317–320 pyranopterin dithiolenes, dimethyl sulfoxide reductase (DMSOR), 256–258 Chemical reactivity: bis(dithiolenes), 277–290 alkylation-protonation, 281–284 ligand addition and substitution, 278–280 ligand-exchange reactions, 278 miscellaneous reactions–applications, 288–290 redox properties, 270–277 unsaturated carbon cycloadditions, 284–287 unsaturated compound addition reactions, 287–288 metallo-1,2-enedithiolates, emission behavior and, 393–394 tris(dithiolenes), 296–299 ‘‘Chemiresistor’’ sensors, dithiolene-based films, 429–430 Chirality, [M(L)(Sn)] fragments, 617–620

688

SUBJECT INDEX

Chromium complexes: bis(dithiolenes), homoleptic structures, 78–79 dithiolene magnetic properties, metallocenium compounds, 447–448 heteroleptic dithiolenes: carbonyl complexes, 299–301 metal–ligand multiple bonds, 303–305 [M(L)(Sn)] fragments, 18 valence electron (VE) rule, 612–616 Cis–anti isomers, bis(dithiolenes), alkylation– protonation, 281–284 Cis–syn isomers, bis(dithiolenes), alkylation– protonation, 281–284 Cobalt complexes: bis(dithiolenes): homoleptic structures, 78–79 redox properties, 271–277 heteroleptic dithiolenes: cyclopentadienyls, 305–307 nitrosyl complexes, 302 metallo-1,2-enedithiolates, 372–374 tridentate ligands, 591–592 Coligands: [M(L)(Sn)] complexes, 605–606 18 valence electron (VE) rule, 612–616 Comproportionation reaaction, molybdenum– tungsten oxo-chemistry, 545–547 Configuration interaction—singles-only (CIS) calculations, metallo-bis(dithiolenes), 160–166 Cooper pairing, 1,2-dithiolene complexes, superconductivity, 422–427 Coordination geometry: main group bis(dithiolene) complexes, 79 metallo-tris(dithiolenes), six-coordinate vibrational spectra, 237–239 transition metal homoleptic dithiolenes: bis(dithiolene) complexes, 60–62 tris(dithiolene) complexes, 84–87 octahedral and trigonal-prismatic structures, 86–87 trans-SMS angle, 85–86 trigonal twist angle, 84–85 tris(dithiolenes), 84–87 trans-SMS angle, 85–86 trigonal twist angle, 84–85 Coordination spheres, [M(L)(Sn)] complexes, small molecule conversion, 630–652 carbon monoxide, carbon dioxide, and sulfur dioxide, 637–639

diazenes, 640–652 hydrogen-bridge diastereoisomerism and PR3 exchange, 647–652 [M(Sn)]–N2H2 complexes, 641–647 H2S and S2 complexes, 639–640 nitrosyl complexes, 630–636 nitric oxide conversions, 632–636 nitric oxide
! NH2OH reduction, 636 16,17, and 18 valence electron complexes, 632 18 and 19 valence electron complexes, 630–632 Cope-stone structure, dinitrogen complexes, [M(Sn)] cores, 670–672 Copper complexes: bis(dithiolenes): homoleptic structures, 77 miscellaneous reactions–applications, 290 redox properties, 275–277 dithiolene-based films, memory switching electrodes, 430–431 dithiolene magnetic properties, spin-Peierls (SP) systems, 432–433 metallo-bis(dithiolenes), square-planar bonding, 149 metallo-1,2-enedithiolates, 372–374 tetrahedral bis(dithiolenes), 353–354 Coupled electron proton transfer (CEPT): biological metallo-dithiolenes, enzyme mechanisms, 520–522 molybdenum–tungsten oxo-chemistry, 546–547 Covalency: metallo-mono(dithiolenes), electronic buffer effect, 129–138 metallo-tris(dithiolenes), electron paramagnetic resonance (EPR), 188–192 [Cp2M(dithiolene)](TCNQF4), dithiolene magnetic properties, spin-ladder systems, 437–440 Cp2Mo2S4: metal–alkyne bonds, dithiolene routes, 41–42 metal-binding pyranopterin dithiolene (MPT), 574 metal sulfides, unactivated alkyne addition, 32–37 Cp2TiS5: alkanedithiolate dehydrogenation, 38–39 DMAD, electrophilic alkyne reactions, 30–32

SUBJECT INDEX Critical temperature, 1,2-dithiolene complexes, superconductivity, 422–427 Crown ether derivatives: hexadentate ligands, 599 transition metal dithiolene synthesis, arenedithiolate ligands, 5–10 tris(dithiolenes), redox properties, 295 Crystal packing: bis(dithiolene) complexes, multimeric molecular structures, 73–76 metallo-tris(dithiolenes), ideal D3h trigonalprismatic distortion, 187–188 CS2, 4,5-dimercapto-1,3-dithiole-2-thione (dmit2
) synthesis, 15–18 C–S stretching: metallo-bis(dithiolenes): four-coordinate structures, 218–228 square-planar bonding, 154–166 metallo-tris(dithiolenes), six-coordinate vibrational spectra, 235–239 [M(L)(Sn)] fragments, 626–629 pyranopterin dithiolenes, dimethyl sulfoxide reductase (DMSOR)/biotin sulfoxide reductase (BSOR), 254–258 vibrational spectra, 261–262 Curie tails, dithiolene magnetic properties, spinladder systems, 437 Curie–Weiss law, dithiolene magnetic properties, bulk ferromagnets, 450 Cyanide ligands, transition metal dithiolene synthesis, dithiocarbonates, 14–15 Cyclic, saturated–unsaturated cations, dithiolene electrical properties, dmit compounds, 409–417 Cycloaddition, bis(dithiolenes), 284–287 Cyclometalated arylpyridine chelating ligand, square-planar mixed-ligand dithiolene– diimine complexes, 351–352 (Cp2 Mn)[Ni(dmit)2], dithiolene magnetic properties, 450–453 Cyclopentadienyl complexes, heteroleptic dithiolenes, 305–307 dddt2– ligand, dithiolene electrical properties, 407 metal-like behavior, 418–420 Degradation studies, molybdopterin-dithiolenes, 504–507 Dehydrogenation: alkanedithiolates, 38–39

689

1,2-dithiolene coupling, 44 Density functional theory (DFT): bis(dithiolenes): hydrocarbon cycloaddition, 286–287 redox properties, 276–277 metallo-bis(dithiolenes): axial-oxo ligands, excited-state spectroscopic probes, 168–173 four-coordinate structures, vibrational spectra, 222–228 square-planar bonding, 147–149 excited-state spectroscopic probes, 150–166 metallo-dithiolenes, 115–116 metallo-tris(dithiolenes): electron paramagnetic resonance (EPR), 190–192 ideal D3h trigonal-prismatic distortion, 184–188 molecular orbital bonding analysis, 174–179 [M(L)(Sn)] fragments, nitrosyl small molecule conversion, 18 and 19 valence electrons, 631–632 Desoxo-bis(dithiolene)M(IV) complexes, metalbinding pyranopterin dithiolene (MPT), catalytic center chemical analogues, 549–560 D2/Hþ exchange reaction, [MS] enzyme modeling, hydrogenase enzymes, 654–658 S-Dealkylation, dithiolene routes, 40–41 Diamides, dithione synthesis, 24–25 Dianionic alkenedithiolate synthesis: 4,5-dimercapto-1,3-dithiole-2-thione (dmit2– ), 15–18 inorganic dmit2– dithiolate derivatives, 18–19 1,2-maleonitrile 1,2-dithiolate (mnt2
), 21 tetrathiafulvalene (TTF)-derived dithiolenes, 19–20 thiacarbons and derivatives, 20 Diastereoisomerism, [M(L)(Sn)] fragments, hydrogen bridges, 657–652 Diazene complexes: [MS] enzyme modeling, FeMoco function, nitrogenase complexes: [Fe(Sn)], reversible redox reactions, 662–664

690

SUBJECT INDEX

Diazene complexes (Continued) nitrogen-dependent HD formation, 664–668 open-sided models, 668–670 [M(Sn)] fragments, small molecule conversion, 640–652 hydrogen-bridge diastereoisomerism and PR3 exchange, 647–652 [M(Sn)]–N2H2 complexes, 641–647 Dicarborane complexes, transition metal dithiolene synthesis, benzenedithiol and derivatives, 10 1,2-Dicyanoethylene-1,2-dithiolate ligand, metallo-tris(dithiolenes), six-coordinate vibrational spectra, 232–239 Dihydropterins, metal-binding pyranopterin dithiolene (MPT), 569–574 Diimines: bis(dithiolenes), ligand addition and substitution, 280 metallo-mono(dithiolenes), ligand-to-ligand charge transfer (LLCT), 138–142 mixed-ligand diimine dithiolates, 354–356 square-planar mixed-ligand dithiolene– diimine complexes, 339–353 excited states, 339–344 photoinduced electron transfer, 344–346 photooxidation, 348–351 self-quenching, 346–348 structural variation, 351–353 a-Diketones, thiophosphate esters, 21–22 4,5-Dimercapto-1,3-dithiole-2-thione (dmit2
): bis(dithiolene) complexes, transition metal ligand structures, 64–72 dithiolene electrical properties, 407 Langmuir–Blodgett (LB) conducting films, 428–429 metal-like behavior, 408–417 superconductivity, 422–427 dithiolene magnetic properties: p-EPYNN[Ni(dmit)2] spin-ladder systems, 433–435 ferromagnetic isolog ligands, 445–447 spin-ladder systems, 438–440 dithiolene nonlinear optical properties, thirdorder nonlinear optics, 462–463 dithiolene optical properties, strong infrared (IR) absorption, 458 inorganic dithiolates, 18–19

metallo-bis(dithiolenes), four-coordinate structures, 224–228 transition metal dithiolene synthesis, 15–18 Dimeric structures: bis(dithiolene) complexes, 73–76 dithiolene electrical properties, dmit compounds, 413–417 mono(dithiolene) complexes, 92–95 Dimethyl acetylene dicarboxylate (DMAD): electrophilic alkyne reactions, 29–32 inorganic dithiolates and dmit2
, 18–19 octahedral dn complexes, mixed-ligand structures, 360 Dimethylformamide (DMF): 4,5-dimercapto-1,3-dithiole-2-thione (dmit2
) synthesis, 15–18 metallo-1,2-enedithiolates, 372–374 square-planar bis(dithiolenes), ion-pair charge-transfer photochemistry, 325–328 Dimethyl sulfide (DMS): biological metallo-dithiolenes, electronic spectroscopy, 517–518 pyranopterin dithiolenes, dimethyl sulfoxide reductase (DMSOR)/biotin sulfoxide reductase (BSOR), 251–258 structure and properties, 540 Dimethyl sulfoxide (DMSO): metal-binding pyranopterin dithiolene (MPT), future research issues, 576–577 metallo-bis(dithiolenes), excited-state spectroscopic probes, 169–173 square-planar bis(dithiolenes), ion-pair charge-transfer photochemistry, 325–328 square-planar mixed-ligand dithiolene-donor complexes, 337–338 structure and properties, 540 Dimethyl sulfoxide reductase (DMSOR): biological metallo-dithiolenes: crystalline structure, 511–514 electronic spectroscopy, 517–518 molybdenum enzymes, 497–498 X-ray absorption analysis, 516–517 magnetic circular dichroism (MCD) analysis, 518–519 metal-binding pyranopterin dithiolene (MPT): catalytic center chemical analogues, 549–560

SUBJECT INDEX oxygen atom transfer (OAT), 563–569 transoid angles, 577 molybdenum-MPT enzymes, 543 oxo-metallo-bis(dithiolenes), vibrational spectra, 241–246 oxo-metallo-mono(dithiolenes), vibrational spectra, 239–241 pyranopterin dithiolenes, mononuclear molybdenum/tungsten enzymes, 250–258 aldehyde oxidoreductase (AOR) and, 260–261 sulfite oxidase (SO) family, 249 resonance Raman (RR) spectroscopic probe, 515–516 structure and properties, 540 Dinegative complexes, metallo-bis(dithiolenes), four-coordinate structures, 222–228 Dinitrogen complexes, [M(Sn)] cores, 670–672 Dipyridocatecholate (dpcat), square-planar mixed-ligand dithiolene–diimine complexes, 352–353 Dithiaoxamides, dithione synthesis, 23–25 1,2-Dithietes: alkenedithiolate synthesis, 22–23 metallo-dithiolenes, 112–113 1,4-Dithiin derivatives, metal carbonyl reactions, 41 Dithiocarbonates, transition metal dithiolenes: base hydrolysis, 11–15 promoted routes, 39–40 Dithioketonic ligands, tris(dithiolenes), redox properties, 295 Dithiolates: 1,2-benzenedithiolate, metal–sulfur interactions, 588–589 luminescence and photochemistry, 317–320 nomenclature, 58–59 square-planar mixed-ligand dithiolene– diimine complexes, photooxidation, 350–351 Dithiolene cofactor, biosynthesis, 527–531 1,2-Dithiolenes. See also Homoleptic dithiolenes; Transition metals dithiolenes; Tris(dithiolenes); specific Dithiolene-based compounds, e.g., Ferromagnetic systems; specific types, e.g., Bis(dithiolenes) biological ligands, 492–493 cofactor biosynthesis, 527–531

691

dithiolene-molybdenum spectroscopic probes, 515–519 electronic spectroscopy, 517–518 magnetic circular dichroism, 518–519 paramagnetic spectroscopy, 518 resonance Raman spectroscopy, 515–516 X-ray absorption spectroscopy, 516–517 enzyme mechanisms, 519–522 molybdopterin: degradation studies, 501–507 function, 522–527 natural dithiolenes, 493–503 enzyme structural classification, 496–504 ligand structure, 499–501 molybdenum enzyme families, 496–498 nomenclature issues, 498–499 pterin redox reactions, 501–503 tungsten enzyme families, 498 X-ray crystallography, dithiolenemolybdenum enzyme site, 507–515 aldehyde ferrodoxin oxidoreductase (AOR) crystal structure, 514–515 DMSOR crystal structure, 511–514 sulfite oxidase crystal structure, 510–511 XDH/XO crystal structure, 508–510 distribution, 59 electrical properties: applications and patents, 428–431 electrical sensors, 429–430 Langmuir–Blodgett (LB) films, 428–429 memory switching, 430–431 band structure, 406–407 metal-like behavior, 408–422 dddt-based compounds, 418–420 dmit-based compounds, 408–417 mnt-based compounds, 417–418 nickel-based compounds, 421–422 related compounds, 427–428 research background, 406–407 superconductors, 422–427 electrochemical and chemical reactivity, 268–270 ligand bonding, 57–58 ligand structures, 404–405 magnetic properties, 431–453 ferromagnetic systems, 440–453 bulk ferromagnets, 448–453 (Cp2 Mn)[Ni(dmit)2], 450–453

692

SUBJECT INDEX

1,2-Dithiolenes (Continued) (NH4)[Ni(mnt)2]  H2O, 448–450 dmit/isolog ligands, 445–447 metallocenium compounds, 447–448 mnt systems, 441–444 tfd systems, 444–445 spin-ladder systems, 433–440 [Cp2M(dithiolene)](TCNQF4), 437–440 DT-TTF2[Au(mnt)2], 436–437 p-EPYNN[Ni(dmit)2], 433–435 spin-Peierls systems, 432–433 metal-binding pyranopterin dithiolene (MPT) enzymes: catalytic center chemical analogues: oxygen atom transfer, 560–569 synthesis and characterization, 547–560 catalytic center properties, 569–574 multifunctional compounds, 464–467 conductivity and magnetism, 465 conductivity and optical properties, 465–466 nomenclature, 58–59 optical properties, 453–464 data storage, 463–464 nonlinear optical (NLO) properties, 458–463 second-order NLO, 458–461 third-order NLO, 461–463 strong near-infrared (IR) absorption, 453–458 specialized routes, 40–44 alkyne/thiocarbonyl derivatives, 43 alkynyl anions, 44 C–C coupling pathways, 42 coupling pathways, 43 S-dealkylation, 40–41 metal-alkyne bond insertion, 41–42 vibrational spectra, 214 Dithiole-2-ones. See Dithiocarbonates 1,2-Dithiones: metallo-dithiolenes, 112–113 transition metal dithiolene synthesis, alkenedithiolates, 23–25 Dithiopheno-tetrathiafulvalene (DT-TTF), dithiolene magnetic properties, spin-ladder systems, 436–437 Dmbit2– ligand, dithiolene electrical properties, metal-like behavior, 422 d8 metal ions, square-planar complexes, 320–352

bis(dithiolenes), 320–335 excited states, 320 hydrogen photoproduction, 328–330 ion-pair charge-transfer, 324–328 photooxidation, 330–333 radical formation, 334–335 mixed-ligands: dithiolene–diimine complexes, 339–353 excited states, 339–344 photoinduced electron transfer, 344–346 photooxidation, 348–351 self-quenching, 346–348 structural variation, 351–353 dithiolene–donor complexes, 335–338 d10 metal ions, tetrahedral/distorted fourcoordinate complexes, luminescence, 353–356 bis(dithiolenes), 353–354 mixed-ligand dithiolenes, 354–356 Dmid ligands, dithiolene magnetic properties: ferromagnetic systems, 446–447 spin-ladder systems, 437–440 Dmise2– ligand, dithiolene electrical properties, metal-like behavior, 420–421 Dmit2
. See 4,5-Dimercapto-1,3-dithiole-2thione (dmit2
) dn metal ions, octahedral complexes: homoleptic dithiolenes, 356–357 mixed-ligand complexes, 357–360 D2O, [MS] enzyme modeling, FeMoco function, nitrogenase complexes, nitrogendependent HD formation, 666–668 Donor–acceptor adducts: dithiolene electrical properties, 406–407 dmit compounds, 413–417 metal-like behavior, 408–422 mnt compounds, 417–418 superconductivity, 422–427 dithiolene magnetic properties: ferromagnetic mnt systems, 441–444 spin-ladder systems, 436–437 [Cp2M(dithiolene)](TCNQF4), 437–440 Donor atom sets: dithiolene-[M(Sn)] synthesis, 602–604 metal–sulfur interactions, miscellaneous ligands, 599–601 tetradentate ligands, 593–596 tridentate ligands, 590–592 d-orbital electronic configuration:

SUBJECT INDEX bis(dithiolene) complexes, geometrical distribution, 62 tris(dithiolene) complexes, ligand bending, 88–92 Double-frequency modulation, metallo-1,2enedithiolates, 383–384 Dual-emitting oxygen probes, metallo-1,2enedithiolates, 379–381 future research issues, 393–394 Effective nuclear charge (Z 0 eff), metallomono(dithiolenes), 128 electronic buffer effect, 129–138 18 valence electron (VE) rule, [M(L)(Sn)] fragments, 612–616 nitrosyl small molecule conversion, 630–632 protonation, alkylation, and redox reactions, 626 Electrical properties, 1,2-dithiolenes: applications and patents, 428–431 electrical sensors, 429–430 Langmuir–Blodgett (LB) films, 428–429 memory switching, 430–431 metal-like compounds, 408–422 dddt-based compounds, 418–420 dmit-based compounds, 408–417 mnt-based compounds, 417–418 nickel-based compounds, 421–422 multifunctional compounds: magnetism and conductivity, 465 optical properties and conductivity, 465–466 related compounds, 427–428 research background, 406–407 superconductors, 422–427 Electrical sensors, dithiolene-based films, 429–430 Electron delocalization: bis(dithiolenes), redox properties, 270–277 tris(dithiolenes), ideal D3h trigonal-prismatic distortion, 183–188 Electronegativity, metal–sulfur interactions, 587 Electronic absorption (EA) spectroscopy: metallo-bis(dithiolenes), square-planar bonding, excited-state spectroscopic probes, 150–166 metallo-dithiolenes, mono(dithiolenes), 117 metallo-mono(dithiolenes): bonding parameters, 117–128 electronic buffer effect, 132–138

693

metallo-tris(dithiolenes), 192–196 Electronic buffer: metallo-mono(dithiolenes), 128–138 molybdopterin ligand structure, 526–527 Electronic coupling, metallo-tris(dithiolenes), six-coordinate vibrational spectra, 237–239 Electronic effects, [M(L)(Sn)] fragments: benzene ring substituents, 622–623 high-spin vs. low-spin, 620–622 Electronic spectroscopy, biological metallodithiolenes, 517–518 Electron-nuclear double resonance (ENDOR) spectroscopy: biological metallo-dithiolenes, 518 metallo-bis(dithiolenes), square-planar bonding, 146–149, 152–166 Electron paramagnetic resonance (EPR) spectroscopy: biological metallo-dithiolenes, 518 metallo-bis(dithiolenes), square-planar bonding, 146–149, 152–166 metallo-mono(dithiolenes): bonding parameters, 117–128 electronic buffer effect, 131–138 metallo-tris(dithiolenes), 173–174 structural studies, 188–192 pyranopterin dithiolenes, dimethyl sulfoxide reductase (DMSOR), 255–258 Electron spin echo envelope modulation (ESEEM): biological metallo-dithiolenes, 518 metallo-bis(dithiolenes), square-planar bonding, 146–149, 152–166 Electron spin resonance (ESR): biological metallo-dithiolenes, ligand structure, 499–501 bis(dithiolenes), redox properties, 275–277 dithiolene electrical properties, Langmuir– Blodgett (LB) conducting films, 429 square-planar bis(dithiolenes), radical formation kinetics, 334–335 square-planar mixed-ligand dithiolene– diimine complexes, photooxidation, 348–351 Electron transfer (ET): dithiolene conducting properties, 401 metal-binding pyranopterin dithiolene (MPT): catalytic center chemical analogues, 547–560

694

SUBJECT INDEX

Electron transfer (ET) (Continued) oxygen atom transfer (OAT), 568–569 square-planar bis(dithiolenes): ion-pair charge-transfer, 324–328 photooxidation, 332–333 square-planar mixed-ligands: dithiolene–diimine complexes: excited states and luminescence, 339–344 photoinduced reactions, 344–346 dithiolene–donor complexes, 335–338 Electron-withdrawing groups: metal-binding pyranopterin dithiolene (MPT), oxygen atom transfer (OAT), 560–569 tris(dithiolenes), redox reactions, 291–295 Electrophilic alkenes, alkanedithiolate dehydrogenation, 38–39 Electrophilic alkynes, in metal sulfides, 29–32 Emission spectra: metallo-1,2-enedithiolates: chemical reactivity, 393–394 dual-emitting oxygen probes, 379–381 modulation-based probes, 383–384 oxygen probes, 378 phase-based oxygen probes, 381–383 protonation state-dependent emission, pH monitoring: ILCT emission quenching, 392–393 metallo-1,2-enedithiolates, 391–392 square-planar d8 bis(dithiolenes), excited states and luminescence, 321–324 Ene-1,2-dithiolates. See also Metallo-1,2enedithiolates metallo-bis(dithiolenes): four-coordinate structures, 222–228 square-planar bonding, 148–149 metallo-dithiolene complexes, 112–113 oxo-metallo-bis(dithiolenes), vibrational spectra, 245–246 ‘‘Envelope’’-fold, tris(dithiolene) complexes, ideal D3h trigonal-prismatic distortion, 181–188 Enzyme chemistry: biological metallo-dithiolenes, 519–522 molybdenum enzymes, structure and classification, 540–544 [M(Sn)] complexes, [MS] enzyme modeling reactions, 652–672 carbon monoxide dehydrogenase (CODH), 658–660 dinitrogen complexes, 670–672

hydrogenase, 653–658 nitrogenase, FeMoco function, 661–670 tungsten enzymes, structure and classification, 541–544 p-EPYNN[Ni(dmit)2] spin-ladder systems, 1,2dithiolene magnetic properties, 433–435 Ethylenediaminetetraacetic acid (EDTA), square-planar d8 bis(dithiolenes): hydrogen photoproduction, 329–330 ion-pair charge-transfer photochemistry, 328 Ethylene-dithiotetrathiafulvalene (EDT-TTF), dithioelene electrical properties: dmit compounds, 415–417 superconductivity, 425–427 Eukaryotes, nitrate reductases in, 540–541 Excited states: metallo-mono(dithiolenes): bonding parameters, 124–128 ligand-to-ligand charge transfer (LLCT), 142 square-planar d8 bis(dithiolenes), 320–324 square-planar mixed-ligand dithiolene– diimine complexes, 339–344 Excited-state spectroscopic probes, metallobis(dithiolenes): axial oxo ligands, 167–173 square-planar bonding, 149–166 Extended X-ray absorption fine structure (EXAFS): biological metallo-dithiolenes: ligand structure, 499–501 sulfite oxidase (SO) crystalline structure, 510–511 dithiolene-molybdenum unit analysis, 516–517 metal-binding pyranopterin dithiolene (MPT), catalytic center chemical analogues, 548–560 pyranopterin dithiolenes: aldehyde oxidoreductase (AOR), 258–261 dimethyl sulfoxide reductase (DMSOR), 258 fac coordination, molybdenum–tungsten oxo-chemistry, 546–547 fac isomers, electrophilic alkyne reactions, 31–32 Far-infrared spectra, metallo-tris(dithiolenes), six-coordinate vibrational spectra, 230–239

SUBJECT INDEX Fast atom bombardment (FAB) mass spectrometry, molybdopterin-dithiolene degradation, 505–507 FDH enzyme: biological metallo-dithiolenes, mechanisms, 522 X-ray absorption analysis, 517 Fe complexes. See Iron complexes Fenske–Hall calculations, metallotris(dithiolenes): electron paramagnetic resonance (EPR), 190–192 ideal D3h trigonal-prismatic distortion, 184–188 molecular orbital bonding analysis, 174–179 Fermi surfaces, dithiolene electrical properties: dmit compounds, 408–417 mnt compounds, 417–418 superconductivity, 426–427 Ferredoxin oxidoreductase (FOR), pyranopterin dithiolenes, 258–261 Ferrocenedithiolates, transition metal dithiolene synthesis, benzenedithiol, and derivatives, 10 Ferromagnetic systems, dithiolene magnetic properties, 401, 440–453 bulk ferromagnets, 448–453 (Cp2 Mn)[Ni(dmit)2], 450–453 (NH4)[Ni(mnt)2]H2O, 448–450 dmit/isolog ligands, 445–447 metallocenium compounds, 447–448 mnt systems, 441–444 tfd systems, 444–445 Fe–S stretching mode, pyranopterin dithiolenes, xanthine oxidase (XO) family, 247 Field desorption (FD) spectroscopy, [M(L)(Sn)] fragments, 630 Films, dithiolene complexes, 428–431 conducting Langmuir–Blodgett films, 428–429 electrical sensors, 429–430 memory switching, 430–431 Flavin adenine dinucleotide (FAD): electronic spectroscopy, 517–518 pyranopterin dithiolenes, mononuclear molybdenum/tungsten enzymes, 247 Fluorescence: metallo-1,2-enedithiolates: activated phosphate detection, 389–390 dual-emitting oxygen probes, 379–381

695

tetrahedral bis(dithiolenes), 354 Fluorine substituents, tetradentate ligands, 595–596 Force constants, metallo-bis(dithiolenes), fourcoordinate structures, 223–228 Formaldehyde ferrodoxin oxidoreductase (FOR), biological metallodithiolenes, crystalline structure, 514–515 Formate dehydrogenase (FDH): biological metallo-dithiolene X-ray diffraction, 513–514 structure and properties, 541 Four-coordinate structures: bis(dithiolenes), vibrational spectra, 214–228 tetrahedral/distorted four-coordinate d10 complexes, 353–356 bis(dithiolenes), 353–354 mixed-ligand dithiolenes, 354–356 Fourier transform infrared (FTIR) spectroscopy, metallo-bis(dithiolenes), four-coordinate structures, 215–228 Frequency-modulated excitation, metallo-1,2enedithiolates: double-frequency modulation-based probes, 383–384 phase-based oxygen probes, 381–383 Geometric properties. See Coordination geometry; Stereochemistry GF matrix, metallo-bis(dithiolenes), fourcoordinate structures, 220–228 Gold complexes: bis(dithiolenes): homoleptic structures, 77 redox properties, 275–277 dithiolene electrical properties: dddt compounds, 419–420 dmit compounds, 413–417 Langmuir–Blodgett (LB) conducting films, 428–429 room temperature conductivity, 427–428 dithiolene magnetic properties: spin-ladder systems: DT-TTF molecules, 436–437 p-EPYNN[Ni(dmit)2], 435 spin-Peierls (SP) systems, 432–433 mono(dithiolene) structures, 92–95 square-planar mixed-ligand dithiolene– diimine complexes, structural variation, 351–352

696

SUBJECT INDEX

Ground-state spectroscopic probes, metallobis(dithiolenes), square-planar bonding, 146–149 Guanosine-X-phosphate (GXP), dithiolene cofactor biosynthesis, 529–531 Haber–Bosch processes, [MS] enzyme modeling, FeMoco function, nitrogenase complexes, 661–664 Hahn route: metal–sulfur interactions, hexadentate ligands, 598–599 transition metal dithiolene synthesis, arenedithiolate ligands, 7–10 a-Haloketones, transition metal dithiolenes, metal sulfides, 37 Hammett constant, metal-binding pyranopterin dithiolene (MPT): chemical shift variations, 572–574 oxygen atom transfer (OAT), 565–569 Hartree–Fock calculation, metallobis(dithiolenes), 161–166 Hemocyanine dye, dithiolene nonlinear optical (NLO) properties, second-harmonic generation, 458–461 Heterocyclic-substituted complexes, metallo-1,2-enedithiolates, 374–376 modulation-based probes, 383–384 Heteroleptic dithiolenes. See Mixed-ligand dithiolenes ligand structures, 403 magnetic properties, spin-ladder systems, 438–440 nonlinear optical (NLO) properties, secondharmonic generation, 459–461 Hexadentate ligands, metal–sulfur interactions, 598–599 Highest occupied molecular orbital (HOMO): bis(dithiolenes): ligand addition and substitution, 280 redox properties, 276–277 dithiolene electrical properties: dddt compounds, 420 superconductivity, 427 metallo-bis(dithiolenes), 160–166 metallo-dithiolenes: luminescence and photochemistry, 318–320 photochemical applications, 360–361 metallo-mono(dithiolenes): electronic buffer effect, 128–138

ligand-to-ligand charge transfer (LLCT), 138–142 metallo-tris(dithiolenes): electron paramagnetic resonance (EPR), 188–192 ideal D3h trigonal-prismatic distortion, 182–188 molybdenum–tungsten oxo-chemistry, 544–547 octahedral dn complexes, mixed-ligand structures, 357–360 square-planar d8 bis(dithiolenes), ion-pair charge-transfer photochemistry, 325–328 square-planar mixed-ligand dithiolene– diimine complexes, excited states, and luminescence, 339–344 tetrahedral mixed-ligand dithiolenes, 354–356 High-spin complexes, [M(L)(Sn)] fragments, 620–622 HNO complexes, [M(L)(Sn)] fragments, nitric oxide conversion, 635–636 Homoleptic dithiolenes: bis(dithiolene) complexes: main group complexes, 79 transition metals, 59–79 bond lengths and angles, 63–64 chromium, manganese, iron, and cobalt, 78–79 copper and gold, 77 geometrical properties, 60–62 ligand types, 64–72 multimeric molecular structures, 73–76 nickel, palladium, and platinum, 76–77 silver, zinc, cadmium, and mercury, 78 electrical properties, dmit compounds, 416–417 electrochemical–chemical reactivity, 269–270 ligand structures, 403 mono(dithiolene) complexes, 92–95 nonlinear optical (NLO) properties: second-harmonic generation, 459–461 third-order nonlinear optics, 461–463 octahedral dn complexes, 356–357 structural properties, 56–57 transition metal dithiolene synthesis, arenedithiolate ligands, 5–10 tris(dithiolene) complexes, 80–92 main group complexes, 92–95

SUBJECT INDEX transition metals, 80–92 bond lengths and angles, 87–88 geometric properties, 84–87 octahedral and trigonal-prismatic structures, 86–87 trans-SMS angle, 85–86 trigonal twist angle, 84–85 ligand bending, 88–92 H2S complexes, [M(L)(Sn)] fragments, 639–640 Hu¨ ckel calculations: metallo-bis(dithiolenes), square-planar bonding, 144–166 metallo-tris(dithiolenes), molecular orbital bonding analysis, 174–179 transition metal dithiolenes, 139 Hush theory, square-planar d8 bis(dithiolenes), ion-pair charge-transfer photochemistry, 326–328 Hydride complexes, bis(dithiolenes), alkylation– protonation, 282–284 Hydrocarbon, bis(dithiolenes), cycloaddition, 284–287 Hydrodesulfurization, bis(dithiolenes), 288–290 Hydrogen: [M(L)(Sn)] fragments, diazenes: hydrogen-bridge diastereoisomerism, 657–652 N2H2 synthesis and structure, 646–647 [MS] enzyme modeling, FeMoco function, nitrogenase complexes, 661–664 nitrogen-dependent HD formation, 664–668 square-planar d8 bis(dithiolenes), photoproduction, 328–330 Hydrogenase enzymes, [MS] enzyme modeling, 653–658 Hydrotris-(3,5-dimethyl-1-pyrazolyl)borate, oxo-metallo-mono(dithiolenes), 240–241 Hydroxyethylpiperazineethanesulfonic acid (HEPES): biological metallo-dithiolenes, electronic spectroscopy, 517–518 dimethyl sulfoxide reductase (DMSOR) crystallography, 512–514 a-Hydroxyketones, thiophosphate esters, 21–22 Hydroxylaminyl complex (H2NO), [M(L)(Sn)] fragments, nitric oxide conversion, 634–636 Hyer–Steimecke synthesis, zinc(dmit2)2– synthesis, 16–18

697

Hyperfine coupling, metallo-tris(dithiolenes), electron paramagnetic resonance (EPR), 189–192 Ideal D3h trigonal-prismatic distortion, tris(dithiolene) complexes: ‘‘envelope’’-fold, 181–188 six-coordinate vibrational spectra, 228–239 Indium tin oxide, dithiolene-based films, memory switching, 430–431 Infrared (IR) spectroscopy: bis(dithiolenes), redox properties, 276–277 density functional theory and, 261–262 heteroleptic dithiolenes, nitrosyl complexes, 301–302 metallo-bis(dithiolenes), four-coordinate structure, 215–228 metallo-mono(dithiolenes), bonding parameters, 122–128 metallo-tris(dithiolenes), six-coordinate vibrational spectra, 229–239 [M(L)(Sn)] fragments, 629–630 oxo-metallo-bis(dithiolenes), 241–246 oxo-metallo-mono(dithiolenes), 240–241 Inorganic dithiolates, 4,5-dimercapto-1,3dithiole-2-thione (dmit2
) synthesis, 18–19 In-plane parameters: metallo-bis(dithiolenes): four-coordinate structures, 217–228 square-planar bonding, 148–149 metallo-mono(dithiolenes), 125–128 Interligand bonding, tris(dithiolene) complexes, ideal D3h trigonal-prismatic distortion, 180–188 Intermediate neglect of differential overlap (INDO) linear combination of atomic orbitals–molecular orbital (LCAO–MO) configuration interaction (CI), metallobis(dithiolene) complexes, 154–166 Intermetallic dithiolene transfer, alkenedithiolate synthesis: non-redox routes, 25–26 redox routes, 26–29 Intermolecular charge transfer, bis(dithiolene) complexes, 60–62 Intraligand charge transfer (ILCT): metallo-dithiolenes, luminescence and photochemistry, 319–320 metallo-1,2-enedithiolates:

698

SUBJECT INDEX

Intraligand charge transfer (ILCT) (Continued) activated phosphate detection, 387–390 dual-emitting heterocyclic-substituted complexes, 374–376 phase-based oxygen probes, 383 protonation state-dependent emission, pH monitoring: metallo-1,2-enedithiolates, 391–392 quenching, 392–393 square-planar d8 bis(dithiolenes), excited states and luminescence, 324 square-planar mixed-ligand dithiolene–donor complexes, 337–338 Iodine complexes, metal-binding pyranopterin dithiolene (MPT), catalytic center chemical analogues, 553–560 Ionization energy, metallo-mono(dithiolenes), electronic buffer effect, 129–138 Ion-pair charge-transfer (IPCT): square-planar bis(dithiolenes), 324–328 square-planar mixed-ligand dithiolene–donor complexes, 335–338 Iridium complexes: dinitrogen complexes, [M(Sn)] cores, 672 heteroleptic dithiolenes, 307–308 mixed-ligand octahedral dn complexes, 357–360 square-planar mixed-ligand dithiolene–donor complexes, 335–338 Iron complexes: bis(dithiolenes): alkylation–protonation, 284 homoleptic structures, 78–79 miscellaneous reactions–applications, 289–290 redox properties, 271–277 metal–sulfur interactions, 588 hexadentate ligands, 599 miscellaneous ligands, 601 pentadentate ligands, 596–598 [M(L)(Sn)] fragments: diazenes, N2H2 synthesis and structure, 643–647 electronic effects, high-spin vs. low-spin complexes, 620–622 H2S and S2 complexes, 639–640 mononuclear vs. polynuclear structures, 606–612 nitrosyl small molecule conversion, 18 and 19 valence electrons, 630–632

protonation, alkylation and redox reactions, 624–626 18 valence electron rule, 614–616 [MS] enzyme modeling: FeMoco function, nitrogenase complexes, 661–670 diazenes: [Fe(Sn)], reversible redox reactions 662-664 nitrogen-dependent HD formation, 664–668 open-sided model, 668–670 hydrogenase enzymes, 658 Isoenzymes, metal-binding pyranopterin dithiolene (MPT), oxygen atom transfer (OAT), 562–569 Isolability, dithiolene electrical properties, 407 Isolog ligands, dithiolene magnetic properties, dmit ferromagnetic systems, 445–447 Kalium tetracyanoplatinat (KCP), electrical properties, 400 nonintegral oxidation (NIOS), 406–407 a-Ketoxanthate ester, transition metal dithiolene synthesis, dithiocarbonates, and derivatives, 12–15 Kohn–Sham frontier orbitalas, metallobis(dithiolenes), square-planar bonding, 160–166 Labile complexes, dithiolene-[M(Sn)] synthesis, 602–604 Langmuir–Blodgett (LB) films: dithiolene metallic behavior, 428–429 dithiolene nonlinear optical (NLO) properties, second-harmonic generation, 459–461 Laser flash photolysis, square-planar d8 bis(dithiolenes), ion-pair charge-transfer photochemistry, 326–328 Lead complexes, main group bis(dithiolene), 79 Lewis acids–bases: bis(dithiolene) chemical reaactivity, ligand addition and substitution, 278–280 heteroleptic dithiolenes, metal–ligand multiple bonds, 304–305 [MS] enzyme modeling, hydrogenase enzymes, 655–658 Ligand addition and substitution: bis(dithiolene) chemical reaactivity, 278–280 heteroleptic dithiolenes, cyclopentadienyls, 306–307

SUBJECT INDEX Ligand-based reduction: metallo-bis(dithiolenes), four-coordinate structures, 223–228 metallo-tris(dithiolenes), six-coordinate vibrational spectra, 237–239 Ligand-exchange reactions, bis(dithiolene) chemical reaactivity, 278 Ligand-field stabilization energies (LFSE), tris(dithiolene) complexes, ideal D3h trigonal-prismatic distortion, 181–188 Ligand structures: biological metallo-dithiolenes, 499–501 bis(dithiolene) complexes, transition metals, 64–72 metal–sulfur interactions, 587 acetylenes, 601 hexadentate ligands, 598–599 [M(L)(Sn)] complexes, 604–630 characterization, 629–630 chirality, 617–620 C–S bond cleavage, 626–629 electronic effects, 620–623 benzene ring substituents, 622–623 high-spin vs. low-spin, 620–622 mononuclear vs. polynuclear structures, 606–612 [MS] enzyme modeling, 652–672 carbon monoxide dehydrogenase (CODH), 658–660 dinitrogen complexes, 670–672 hydrogenase models, 653–658 nitrogenase complexes, FeMoco function, 661–670 oxidation, 18 valence elctron rule, 612–616 protonation, alkylation, and redox chemistry, 623–626 model complexes, 587–590 multidentate ligand synthesis, 601–604 nickel complexes, 601 N2S2donor sets, 599–600 pentadentate ligands, 596–598 small molecule conversion, [M(Sn)] complexes, 630–652 carbon monoxide, carbon dioxide, and sulfur dioxide, 637–639 diazenes, 640–652 H2S and S2 complexes, 639–640 nitrosyl complexes, 630–636 tetradentate ligands, 592–596 tridentate ligands, 590–592

699

tris(dithiolene) complexes, 88–92 Ligand-to-ligand charge transfer (LLCT): metallo-dithiolenes, mono(dithiolenes), 116–117 metallo-mono(dithiolenes), 138–142 square-planar mixed-ligand dithiolene– diimine complexes, excited states and luminescence, 339–344 tetrahedral mixed-ligand dithiolenes, 354–356 Ligand-to-metal charge transfer (LMCT): metallo-dithiolenes: bis(dithiolenes): axial oxo ligands, 167 excited-state spectroscopic probes, 169–173 four-coordinate structures, 220–228 square-planar bonding, 151–166 mono(dithiolenes), 117 molecular orbital (MO) calculations, 118–128 tris(dithiolenes): electronic absorption (EA) spectroscopy, 194–196 ideal D3h trigonal-prismatic distortion, 183–188 molecular orbital bonding analysis, 178–179 six-coordinate vibrational spectra, 232–239 metallo-1,2-enedithiolates, dual-emitting heterocyclic-substituted complexes, 375–376 square-planar d8 bis(dithiolenes), excited states and luminescence, 321–324 Linear combination of atomic orbitals (LCAO). See also Intermediate neglect of differential overlap (INDO) linear combination of atomic orbitals– molecular orbital (LCAO-MO) configuration interaction (CI) metallo-tris(dithiolenes), molecular orbital bonding analysis, 176–179 LMoO(S–S) complex, metallomono(dithiolenes), molecular orbital (MO) calculations, 119–128 (L-N3)MoO complexes, metallomono(dithiolenes): electronic buffer effect, 128–138 molecular orbital (MO) calculations, 119–128

700

SUBJECT INDEX

Lowest unoccupied molecular orbital (LUMO): bis(dithiolenes): ligand addition and substitution, 280 redox properties, 276–277 dithiolene electrical properties: dddt compounds, 420 superconductivity, 427 metallo-bis(dithiolenes), square-planar bonding, 158–166 metallo-dithiolenes, luminescence and photochemistry, 318–320 metallo-mono(dithiolenes), ligand-to-ligand charge transfer (LLCT), 138–142 molybdenum/tungsten oxo-chemistry, 544–547 molybdopterin ligand structure, 526–527 square-planar mixed-ligand dithiolenediimine complexes, excited states and luminescence, 341–344 tetrahedral mixed-ligand dithiolenes, 354–356 tris(dithiolene) complexes, ligand bending, 89–92 Low-spin complexes, [M(L)(Sn)] fragments, 620–622 Luminescence, metallo-dithiolenes, 315–320 octahedral dn complexes: homoleptic dithiolenes, 356–357 mixed-ligand complexes, 357–360 square-planar d8 complexes, 320–352 bis(dithiolenes), 320–335 excited states, 320 hydrogen photoproduction, 328–330 ion-pair charge-transfer, 324–328 photooxidation, 330–333 radical formation, 334–335 mixed-ligand dithiolene–diimine complexes, 339–353 excited states, 339–344 photoinduced electron transfer, 344–346 photooxidation, 348–351 self-quenching, 346–348 structural variation, 351–353 mixed-ligand dithiolene–donor complexes, 335–338 tetrahedral–distorted four-coordinate d10 complexes, 353–356 bis(dithiolenes), 353–354 mixed-ligand dithiolenes, 354–356

Magnetic circular dichroism (MCD): biological metallo-dithiolenes, 518–519 metallo-bis(dithiolenes), excited-state spectroscopic probes, 168–173 metallo-mono(dithiolenes): bonding parameters, 117–128 electronic buffer effect, 132–138 Magnetic properties, 1,2-dithiolenes, 401, 431–453 conductivity and, 465 ferromagnetic systems, 440–453 bulk ferromagnets, 448–453 (Cp2 Mn)[Ni(dmit)2], 450–453 (NH4)[Ni(mnt)2]H2O, 448–450 dmit/isolog ligands, 445–447 metallocenium compounds, 447–448 mnt systems, 441–444 tfd systems, 444–445 spin-ladder systems, 433–440 [Cp2M(dithiolene)](TCNQF4), 437–440 DT-TTF2[Au(mnt)2], 436–437 p-EPYNN[Ni(dmit)2], 433–435 spin-Peierls systems, 432–433 Magnetization curves, dithiolene magnetic properties, bulk ferromagnets, 451–453 1,2-Maleonitrile 1,2-dithiolate (mnt2– ) ligands: alkenedithiolate synthesis, 21 bis(dithiolene) complexes, transition metal ligand structures, 64–72 dithiolene electrical properties, 407 metal-like behavior, 417–418 dithiolene magnetic properties: ferromagnetic systems, 441–444 spin-ladder systems, 436–437 dithiolene nonlinear optical properties, thirdorder nonlinear optics, 462–463 tris(dithiolenes), redox properties, 295 Mammalian enzymes, biological metallodithiolenes, 493–497 Manganese complexes: bis(dithiolenes): homoleptic structures, 78–79 miscellaneous reactions–applications, 290 metallo-1,2-enedithiolates, 372–374 McGarvey equations, metallo-tris(dithiolenes), electron paramagnetic resonance (EPR), 191–192 (cation)[M(dithiolene)2] salt, electrical properties, metal-like behavior, 408–422

SUBJECT INDEX Memory switching, dithiolene-based films, 430–431 (S)-2-(3-Mercapto-quinoxalinyl)thiourinium, metallo-1,2-enedithiolates, 373–374 Mercury complexes: bis(dithiolenes), homoleptic structures, 78 tetrahedral bis(dithiolenes), 353–354 mer isomers, electrophilic alkyne reactions, 31–32 Metal–alkyne bonds, dithiolene routes, 41–42 Metal analysis, metallo-1,2-enedithiolates, 371–374 Metal-binding pyranopterin dithiolene (MPT): dithiolene catalytic centers, 569–574 enzymes: chemical analogues, catalytic centers: oxygen atom transfer (OAT), 560–569 synthesis and characterization, 547–560 structure and properties, 542–544 sulfite oxidase deficiency, 541 future research issues, 575–577 molybdenum–tungsten oxo-chemistry, 544–547 Metal–ligand multiple bonds, heteroleptic dithiolenes, 303–305 Metal–ligand p overlap, tris(dithiolene) complexes, ideal D3h trigonal-prismatic distortion, 181–188 Metal/ligand-to-ligand charge transfer (MLL– CT), metallo-dithiolenes, photochemical applications, 360–361 Metal-like behavior, 1,2-dithiolenes: electrical properties, 408–422 dddt-based compounds, 418–420 dmit-based compounds, 408–417 mnt-based compounds, 417–418 nickel-based compounds, 421–422 superconductivity, 425–427 Metallo-bis(diimines), solvatochromatic absorption, 139 Metallocenium compounds, dithiolene magnetic properties, 447–448 Metallo-dithiolenes. See also Transition metal dithiolenes biomolecular distribution, 493–503 enzyme structural classification, 496–504 ligand structure, 499–501 molybdenum enzyme families, 496–498 nomenclature issues, 498–499 pterin redox reactions, 501–503

701

tungsten enzyme families, 498 bis(dithiolenes): electronic–spectroscopic structural studies, 142–143 four-coordinate structures, vibrational spectra, 214–228 square-planar structures, 143–173 axial oxo ligands, 166–173 excited-state spectroscopic probes, 167–173 square-pyrimidal bonding comparison, 166–167 molecular orbital (MO) bonding calculations, 143–166 excited-state spectroscopic probes, 149–166 ground-state spectroscopic probes, 146–149 electronic structure, 112–116, 404–405 luminescence and photochemistry, 315–320 octahedral dn complexes: homoleptic dithiolenes, 356–357 mixed-ligand complexes, 357–360 square-planar d8 complexes, 320–352 bis(dithiolenes), 320–335 excited states, 320 hydrogen photoproduction, 328–330 ion-pair charge-transfer, 324–328 photooxidation, 330–333 radical formation, 334–335 mixed-ligand dithiolene-diimine complexes, 339–353 excited states, 339–344 photoinduced electron transfer, 344–346 photooxidation, 348–351 self-quenching, 346–348 structural variation, 351–353 mixed-ligand dithiolene-donor complexes, 335–338 tetrahedral–distorted four-coordinate d10 complexes, 353–356 bis(dithiolenes), 353–354 mixed-ligand dithiolenes, 354–356 mono(dithiolenes): electronic–spectroscopic structural studies, 116–117 strong-field axial ligand, 117–142 bonding parameters, 117–128

702

SUBJECT INDEX

Metallo-dithiolenes (Continued) ligand-to-ligand charge transfer (LLCT), 138–142 oxo-metal complexes, electronic buffer effect, 128–138 tris(dithiolenes): electron absorption (EA) spectroscopy, 192–196 electronic–spectroscopic structural studies, 173–196 electron paramagnetic resonance (EPR) spectroscopy, 188–192 ideal D3h trigonal-prismatic geometry, distortions, 179–188 molecular orbital (MO) calculations, 174–179 six-coordinate structures, vibrational spectra, 228–239 Metallo-1,2-enedithiolates: activated phosphate detection, 387–390 analyte detection, 370 dual-emitting heterocyclic-substituted compounds, 374–376 future research issues, 393–394 metal analysis, 371–374 oxygen probes, 376–378 double-frequency modulation-based probes, 383–384 dual-emitting probes, 379–381 phase-based probes, 381–383 vs. other probes, 384–386 protonation state-dependent emission, pH monitoring, 391–393 ILCT emission quenching, 392–393 Metallo-tris(catecholates), tris(dithiolene) complexes, ideal D3h trigonal-prismatic distortion, 182–188 Metal sulfides, transition metal dithiolenes: dithiocarbonate synthesis, 13–15 electrophilic alkyne addition, 29–32 a-haloketones and precursors, 37 unactivated alkyne addition, 32–37 Metal–sulfur interactions, dithiolene-derived ligands: acetylenes, 601 hexadentate ligands, 598–599 [M(L)(Sn)] complexes, 604–630 characterization, 629–630 chirality, 617–620 C–S bond cleavage, 626–629

electronic effects, 620–623 benzene ring substituents, 622–623 high-spin vs. low-spin, 620–622 mononuclear vs. polynuclear structures, 606–612 [MS] enzyme modeling, 652–672 carbon monoxide dehydrogenase (CODH), 658–660 dinitrogen complexes, 670–672 hydrogenase models, 653–658 nitrogenase complexes, FeMoco function, 661–670 oxidation, 18 valence elctron rule, 612–616 protonation, alkylation, and redox chemistry, 623–626 small molecule conversion, 630–652 carbon monoxide, carbon dioxide, and sulfur dioxide, 637–639 diazenes, 640–652 H2S and S2 complexes, 639–640 nitrosyl complexes, 630–636 multidentate ligand synthesis, 601–604 nickel complexes, 601 N2S2donor sets, 599–600 pentadentate ligands, 596–598 tetradentate ligands, 592–596 tridentate ligands, 590–592 Metal-to-ligand charge transfer (MLCT): metallo-bis(dithiolenes), four-coordinate structures, 220–228 metallo-dithiolenes: luminescence and photochemistry, 320 photochemical applications, 360–361 metallo-1,2-enedithiolates, dual-emitting heterocyclic-substituted complexes, 375–376 octahedral dn complexes, mixed-ligand structures, 357–360 square-planar d8 bis(dithiolenes): excited states and luminescence, 321–324 photooxidation, 331–333 square-planar mixed-ligand dithiolene–donor complexes, 335–338 Mixed-ligand complexes: bis(dithiolenes), ligand addition and substitution, 280 carbonyl complexes, 299–301 diimine dithiolates, ligand-to-ligand charge transfer, 354

SUBJECT INDEX hydrogen-bridge diastereoisomerism/PR3 exchange reactions, [m-N2H2{M(PR3)(S4)}2] complexes, 648–652 mixed-ligand–donor complexes, 302–303 mixed-ligand multiple bonds, 303–305 nitrosyl complexes, 301–302 octahedral dn complexes, 357–360 organometallic complexes, 305–308 cyclopentadienyl complexes, 305–307 square-planar structures: dithiolene–diimine complexes, 339–353 excited states, 339–344 photoinduced electron transfer, 344–346 photooxidation, 348–351 self-quenching, 346–348 structural variation, 351–353 dithiolene-donor complexes, 335–338 tetrahedral mixed-ligand dithiolenes, 354–356 Mixed-metal-ligand-to-ligand charge-transfer (MMLL’CT) transition: metallo-mono(dithiolenes), 138–142 square-planar mixed-ligand dithiolene– diimine complexes: excited states and luminescence, 339–344 self-quenching mechanism, 348 tetrahedral mixed-ligand dithiolenes, 354–356 [M(L)(Sn)] complexes: small molecule conversion, coordination spheres, 630–652 carbon monoxide, carbon dioxide, and sulfur dioxide, 637–639 diazenes, 640–652 hydrogen-bridge diastereoisomerism and PR3 exchange, 647–652 [M(Sn)]–N2H2 complexes, 641–647 H2S and S2 complexes, 639–640 nitrosyl complexes, 630–636 16, 17, and 18 valence electron complexes, 632 nitric oxide conversions, 632–636 nitric oxide
! NH2OH reduction, 636 18 and 19 valence electron complexes, 630–632 structure, bonding, and general properties, 604–630 characterization, 629–630 chirality, 617–620

703

C–S bond cleavage, 626–629 electronic effects, 620–623 benzene ring substituents, 622–623 high-spin vs. low-spin, 620–622 mononuclear vs. polynuclear structures, 606–612 [MS] enzyme modeling, 652–672 carbon monoxide dehydrogenase (CODH), 658–660 dinitrogen complexes, 670–672 hydrogenase models, 653–658 nitrogenase complexes, FeMoco function, 661–670 oxidation, 18 valence elctron rule, 612–616 protonation, alkylation, and redox chemistry, 623–626 small molecule conversion, 630–652 carbon monoxide, carbon dioxide, and sulfur dioxide, 637–639 diazenes, 640–652 H2S and S2 complexes, 639–640 nitrosyl complexes, 630–636 Modified Clarke electrode, metallo-1,2enedithiolates, oxygen probes, 378 Molecular orbital (MO) theory: metallo-dithiolenes, 113–116 bis(dithiolenes), 142–143 excited-state spectroscopic probes, 168–173 luminescence and photochemistry, 317–320 square-planar bonding, 143–166 mono(dithiolenes): bonding parameters, 117–128 electronic buffer effect, 131–138 ligand-to-ligand charge transfer (LLCT), 139–142 tris(dithiolenes), 173–174 bonding analysis, 174–179 electronic absorption (EA) spectroscopy, 192–196 electron paramagnetic resonance (EPR), 189–192 six-coordinate vibrational spectra, 237–239 square-planar d8 bis(dithiolenes), excited states and luminescence, 320–324 square-planar mixed-ligand dithiolene– diimine complexes, excited states and luminescence, 339–344

704

SUBJECT INDEX

Molecular orbital (MO) theory (Continued) stacked molecules, electrical properties, 406–407 [(L-N3)MoO(dithiolene)], metal-binding pyranopterin dithiolene (MPT) enzymes, synthesis and characterization, 547–560 Molybdenum cofactor (Moco): biological metallo-dithiolenes, nomenclature protocols, 498–499 biosynthesis, 527–531 [MS] enzyme modeling, FeMoco function, nitrogenase complexes, 661–670 diazenes: [Fe(Sn)], reversible redox reactions 662–664 nitrogen-dependent HD formation, 664–668 open-sided model, 668–670 structure and properties, 543–544 Molybdenum complexes: biological metallo-dithiolenes, 493–503 enzyme mechanisms, 519–522 enzyme site X-ray crystallography, 507–515 aldehyde ferrodoxin oxidoreductase (AOR) crystal structure, 514–515 DMSOR crystal structure, 511–514 sulfite oxidase crystal structure, 510–511 XDH/XO crystal structure, 508–510 enzyme structural classification, 496–504 ligand structure, 499–501 molybdenum enzyme families, 496–498 nomenclature issues, 498–499 pterin redox reactions, 501–503 tungsten enzyme families, 498 dithiocarbonates, 39–40 1,2-dithiolene ligands: intermetallic transfer, redox routes, 26–29 research background, 2–4 heteroleptic dithiolenes: carbonyl complexes, 299–301 metal–ligand multiple bonds, 303–305 nitrosyl complexes, 302 hexadentate ligands, 599 metal-binding pyranopterin dithiolene (MPT): catalytic center chemical analogues, 547–560 future research issues, 575–577 structure and properties, 542–544 metallo-dithiolenes:

bis(dithiolenes), excited-state spectroscopic probes, 169–173 mono(dithiolenes), 116–117 electronic buffer effect, 129–138 tris(dithiolenes): electron paramagnetic resonance (EPR), 189–192 ideal D3h trigonal-prismatic distortion, 183–188 six-coordinate vibrational spectra, 232–239 [M(L)(Sn)] fragments: chirality, 617–620 C–S bond cleavage, 626–629 nitric oxide conversion: 16, 17, and 18 valence electron nitrosyl complexes, 632 NO
! NH2OH reduction, 636 NPR3, H2NO, and HNO ligands, 632–636 18 valence electron (VE) rule, 612–616 oxo-chemistry, 544–547 oxo-molybdenum bis(dithiolenes), vibrational spectra, 241–246 oxo-molybdenum mono(dithiolenes): electronic buffer effect, 128–138 molecular orbital (MO) calculations, 118–128 pyranopterin dithiolenes, mononuclear enzymes, 246–262 aldehyde oxidoreductase (AOR) family, 258–261 dimethyl sulfoxide reductase (DMSOR), 250–258 sulfite oxidase family, 248–249 xanthine oxidase family, 247 transition metal dithiolene synthesis: arenedithiolate ligands, 5–10 dithiocarbonates, 13–15 metal sulfides: electrophilic alkyne reactions, 31–32 a-haloketones, 37 unactivated alkyne additions, 32–37 tris(dithiolenes): chemical reactivity, 298–299 ligand bending, 89–92 redox reactions, 291–295 Molybdenum K-edge spectroscopy: dithiolene-molybdenum unit analysis, 516–517

SUBJECT INDEX metal-binding pyranopterin dithiolene (MPT), catalytic center chemical analogues, 548–560 Molybdopterin enzymes. See also Metal-binding pyranopterin dithiolene (MPT) biological metallo-dithiolenes: degradation studies, 504–507 nomenclature protocols, 499 redox reactions, 501–503 structural properties, 501, 504 functional properties, 522, 524–527 structure and classification, 540–544 synthase, dithiolene cofactor biosynthesis, 529–531 MoMonooxo-molybdenum(V) dis(dithiolene) compounds, excited-state spectroscopic probes, 167–173 Monoanionic complexes, dithiolene optical properties, strong infrared (IR) absorption, 456–458 Mono(dithiolene) complexes: homoleptic structures, 92–95 metallo-mono(dithiolenes): electronic–spectroscopic structural studies, 116–117 strong-field axial ligand, 117–142 bonding parameters, 117–128 ligand-to-ligand charge transfer (LLCT), 138–142 oxo-metal complexes, electronic buffer effect, 128–138 oxo-metallo-mono(dithiolenes), vibrational spectra, 239–241 Mononuclear structures: metal-binding pyranopterin dithiolene (MPT) enzymes: catalytic center chemical analogues, 552–560 oxygen atom transfer (OAT), 561–569 [M(L)(Sn)] complexes, 606–612 molybdenum–tungsten enzymes, pyranopterin dithiolenes, 246–262 aldehyde oxidoreductase (AOR) family, 258–261 dimethyl sulfoxide reductase (DMSOR), 250–258 sulfite oxidase family, 248–249 xanthine oxidase family, 247 tridentate ligands, 591–592

705

Monooxo-bis(dithiolene), metal-binding pyranopterin dithiolene (MPT), 570–574 Mo

O stretching frequencies: oxo-metallo-bis(dithiolenes), 244–246 oxo-metallo-mono(dithiolenes), 240–241 pyranopterin dithiolenes: aldehyde oxidoreductase (AOR) and, 260–261 dimethyl sulfoxide reductase (DMSOR)/ biotin sulfoxide reductase (BSOR), 251–258 sulfite oxidase (SO) family, 249 xanthine oxidase (XO) family, 247 Mo–S stretch: oxo-metallo-mono(dithiolenes), 240–241 pyranopterin dithiolenes: dimethyl sulfoxide reductase (DMSOR), 255–258 xanthine oxidase (XO) family, 247 M–S bond lengths: bis(dithiolene) complexes: multimeric molecular structures, 75–76 transition metals, 63–64 ligand structures, 64–72 metallo-bis(dithiolenes), four-coordinate structures, 226–228 metallo-tris(dithiolenes), six-coordinate vibrational spectra, 229–239 mono(dithiolene) structures, 94–95 oxo-metallo-bis(dithiolenes), vibrational spectra, 243–246 tris(dithiolene) complexes, 87–88 electronic absorption (EA) spectroscopy, 194–196 metallo-tris(dithiolenes), 175–179 [M(Sn)] complexes: dinitrogen complexes, cores in, 670–672 [MS] enzyme modeling reactions, 652–672 carbon monoxide dehydrogenase (CODH), 658–660 dinitrogen complexes, 670–672 hydrogenase models, 653–658 nitrogenase complexes, FeMoco function, 661–670 Mulliken charge: metallo-mono(dithiolenes), electronic buffer effect, 135–138 metallo-tris(dithiolenes), ideal D3h trigonalprismatic distortion, 182–188

706

SUBJECT INDEX

Multidentate ligands, dithiolene-[M(Sn)] synthesis, 601–604 Multifunctional dithiolenes, 464–467 conductivity and magnetism, 465 conductivity and optical properties, 465–466 Multimeric molecular structures, bis(dithiolene) complexes, transition metals, 73–76 M/X mass ratios, metallo-bis(dithiolenes), four-coordinate structures, 217–228 Near-infrared (NIR) spectroscopy, metallobis(dithiolenes), square-planar bonding, excited-state spectroscopic probes, 150–166 N2H2 complexes, [M(Sn)] fragments, synthesis and structures, 641–647 NH2OH, nitric oxide reduction, 6365 Nickel complexes: bis(dithiolenes): alkylation–protonation, 281–284 four-coordinate structures, 214–228 homoleptic structures, 76–77 hydrocarbon cycloaddition, 286–287 ligand addition and substitution, 279–280 metallo-bis(dithiolenes), 143 unsaturated compound addition reactions, 287–288 dithioamides synthesis, 25 dithiolene electrical properties: dddt compounds, 419–420 dmit compounds, 408–417 extended ligand structures, 421–422 dithiolene magnetic properties: p-EPYNN[Ni(dmit)2] spin-ladder systems, 434–435 ferromagnetic systems, 444–445 metallocenium compounds, 447–448 spin-ladder systems, 437 dithiolene nonlinear optical (NLO) properties, second-harmonic generation, 460–461 dithiolene optical properties, strong infrared (IR) absorption, 454–458 heteroleptic dithiolenes, carbonyl complexes, 299–301 intermetallic dithiolene transfer, redox routes, 26–29 metallo-bis(dithiolenes), square-planar bonding, 144–166 excited-state spectroscopic probes, 149–166

ground-state spectroscopic probes, 146–149 metallo-dithiolenes, electronic structure, 405 metallo-1,2-enedithiolates, 372–374 dual-emitting heterocyclic-substituted complexes, 374–376 metallo-mono(dithiolenes), Hu¨ ckel calculations, 139–142 metal–sulfur ligand structures, 601 tetradentate ligands, 592–596 [M(L)(Sn)] fragments: CO, CO2, and SO2 conversion, 637–639 mononuclear vs. polynuclear structures, 609–612 [MS] enzyme modeling: carbon monoxide dehydrogenase (CODH), 658–660 hydrogenase enzymes, 653–658 square-planar structures: d8 bis(dithiolenes): excited states and luminescence, 321–324 hydrogen photoproduction, 328–330 ion-pair charge-transfer photochemistry, 325–328 photooxidation, 330–333 radical formation, 334–335 mixed-ligand dithiolene–diimine complexes: photooxidation, 350–351 structural variation, 351–352 Ni–S bonding, metallo-bis(dithiolenes), fourcoordinate structures, 215–228 {Ni[S2C2 (CN)2]2}2– , four-coordinate structure, 215–228 19 valence electron (VE) rule, [M(L)(Sn)] fragments, nitrosyl small molecule conversion, 630–632 [Nb(S2C2H4)3]1– , metallo-tris(dithiolenes), sixcoordinate vibrational spectra, 229–239 nit-1 protein, dithiolene cofactor biosynthesis, 529–531 Nitrate reductase (NR): biological metallo-dithiolenes: sulfite oxidase (SO) crystalline structure, 510–511 X-ray diffraction studies, 513–514 structure and properties, 540–541 Nitrenes, tridentate ligands, 591–592 Nitric oxide (NO):

SUBJECT INDEX bis(dithiolene) chemical reactivity, ligand addition, and substitution, 279–280 dithiolene-based films, electrical sensors, 429–430 heteroleptic dithiolenes, nitrosyl complexes, 301–302 [M(L)(Sn)] fragments: benzene ring substituent effects, 622–623 small molecule conversion: 16, 17, and 18 nitrosyl complexes, 632 NO
! NH2OH reduction, 636 NPR3, H2NO, and HNO ligands, 632–636 18 valence electron (VE) rule, 612–616 Nitrogenase compexes, [MS] enzyme modeling, FeMoco function, 661–670 diazenes: [Fe(Sn)] and reversible redox reactions, 662–664 nitrogen-dependent HD formation, 664–668 open-sided model, 668–670 Nitrogen complexes: diazenes: HD formation, 664–668 reduction cycle, [Fe(Sn)] complexes, reversible redox reactions, 662–664 dinitrogen complexes, [M(Sn)] cores, 670–672 heteroleptic dithiolenes, 302–303 metal–ligand multiple bonds, 303–305 hydrogen-bridge diastereoisomerism/PR3 exchange reactions, [m-N2H2{M(PR3)(S4)}2] complexes, 647–652 [M(Sn)] fragments, diazene complexes, 640–652 N2H2 synthesis and structure, 641–647 [m-N2H2{M(PR3)(S4)}2] complexes, hydrogenbridge diastereoisomerism/PR3 exchange reactions, 647–652 (NH4)[Ni(mnt)2]  H2O: dithiolene magnetic properties, 448–450 multifunctional electromagnetic dithiolenes, 465 Nitrosyl complexes: heteroleptic dithiolenes, 301–302 [M(L)(Sn)] complexes, coordination spheres, small molecule conversion, 630–636

707

16,17, and 18 valence electron complexes, 632 nitric oxide conversions, 632–636 nitric oxide
! NH2OH reduction, 636 18 and 19 valence electron complexes, 630–632 NO
! CO exchange, [M(L)(Sn)] fragments, nitrosyl small molecule conversion, 18 and 19 valence electrons, 630–632 Noninnocent ligands: metal-binding pyranopterin dithiolene (MPT) enzymes, 547 metallo-bis(dithiolenes): luminescence and photochemistry, 316–317 square-planar bonding, excited-state spectroscopic probes, 149–166 Nonintegral oxidation (NIOS): dithiolene-based films: electrical sensors, 430 memory switching, 430–431 dithiolene electrical properties, 406–407 dddt compounds, 419–420 dmbit2– ligand, 422 metal-like behavior, 408–422 dmit compounds, 408–417 superconductivity, 422–427 Nonlinear optical (NLO) properties, dithiolenes, 458–463 second-order NLO, 458–461 third-order NLO, 461–463 Non-redox routes, intermetallic dithiolene transfer, 25–26 Norbornadiene, bis(dithiolenes): hydrocarbon cycloaddition, 284–287 miscellaneous reactions–applications, 288–290 Normal hydrogen electrode (NHE), squareplanar mixed-ligand dithiolene–diimine complexes, photoinduced electrontransfer, 345–346 N2S2 donor sets, metal–sulfur ligand structures, 599–600 Nuclear magnetic resonance (NMR) spectroscopy: bis(dithiolenes), alkylation–protonation, 281–284 molybdopterin-dithiolene degradation, 505–507

708

SUBJECT INDEX

Nuclear magnetic resonance (NMR) spectroscopy (Continued) [MS] enzyme modeling, hydrogenase enzymes, 654–658 Nucleophilic reactions: metal-binding pyranopterin dithiolene (MPT), catalytic center chemical analogues, 553–560 [M(L)(Sn)] fragments, CO, CO2, and SO2 conversion, 637–639 tetradentate ligands, 595–596 Oak Ridge thermal ellipsoid plot (ORTEP), [M(L)(Sn)] fragments, diazenes, N2H2 synthesis and structure, 643–647 Octahedral dn complexes, luminescence and photochemistry: homoleptic dithiolenes, 356–357 mixed-ligand complexes, 357–360 Octahedral (OCT) geometry: oxo-metallo-bis(dithiolenes), vibrational spectra, 244–246 tris(dithiolene) complexes, 84 examples, 86–87 ideal D3h trigonal-prismatic distortion, 181–188 trans-SMS angle, 85–86 trigonal twist angle, 84–85 Olefins, bis(dithiolenes), hydrocarbon cycloaddition, 284–287 One-dimensional molecular systems: basic criteria, 401–402 dithiolene electrical properties, 406–407 dmit compounds, 409–417 mnt compounds, 418 superconductivity, 423–427 dithiolene magnetic properties, spin-ladder systems, 433 p-EPYNN[Ni(dmit)2] spin-ladder systems, 433–435 One-electron reductions, [M(L)(Sn)] fragments, nitrosyl small molecule conversion, 18/19 valence electrons, 630–632 One-pot synthesis, tridentate ligands, 590–592 Open-sided models, [MS] enzyme modeling, FeMoco function, nitrogenase complexes, 668–670 Optical data storage, dithiolene optical properties, 463–464

Optical properties. See also Nonlinear optical (NLO) properties dithiolenes, 453–464 conductivity and, 465–466 data storage, 463–464 nonlinear optical (NLO) properties, 458–463 second-order NLO, 458–461 third-order NLO, 461–463 strong near-infrared (IR) absorption, 453–458 Organometallics: dithiolene electrical properties, 400–401 heteroleptic dithiolenes, 305–308 Ortho lithiation: metal–sulfur ligand structures, 600–601 [M(L)(Sn)] fragments, CO, CO2, and SO2 conversion, 637–639 transition metal dithiolene synthesis, arenedithiolate ligands, 8–10 tridentate ligands, diphenyl sulfide, 590–592 Oscillator strength: metallo-bis(dithiolenes), 157–166 metallo-mono(dithiolenes), bonding parameters, 125–128 Osmium complexes, [M(L)(Sn)] fragments, C–S bond cleavage, 626–629 Out-of-plane parameters: metallo-bis(dithiolenes), square-planar bonding, 148–149 metallo-mono(dithiolenes), 125–128 Oxidation states: dithiolenes: electrical properties, 406–407 ligand bonding, 57–58 [M(L)(Sn)] fragments, 18 valence electron (VE) rule, 612–616 Oxidative additions, dithiolene-[M(Sn)] synthesis, 603–604 Oxo-chemistry, molybdenum–tungsten complexes, 544–547 Oxo-dithiolenes: heteroleptic dithiolenes, metal–ligand multiple bonds, 303–305 metallo-dithiolenes, oxo-molybdenum dithiolenes, 114–116 transition metal dithiolene synthesis, arenedithiolate ligands, 5–10

SUBJECT INDEX ‘‘Oxo-gate’’ hypothesis, metal-binding pyranopterin dithiolene (MPT), catalytic center chemical analogues, 547–560 Oxo-gate hypothesis, metallo-bis(dithiolenes), 169–173 Oxo ligands. See Axial oxo ligands Oxo-metallo-bis(dithiolenes), vibrational spectra, 241–246 Oxo-metallo-mono(dithiolenes): electronic buffer effect, 128–138 oxo-molybdenum mono(dithiolenes): electronic buffer effect, 128–138 molecular orbital (MO) calculations, 118–128 vibrational spectra, 239–241 Oxygen atom transfer (OAT) reactions: biological metallo-dithiolenes: enzyme mechanisms, 520–522 molybdenum enzymes, 498 dithiolene vibrational spectra, 214 metal-binding pyranopterin dithiolene (MPT): catalytic center chemical analogues, 558–569 future research issues, 575–577 metallo-bis(dithiolenes), 143 excited-state spectroscopic probes, 169–173 metallo-dithiolene complexes, 112 molybdenum–tungsten oxo-chemistry, 544–547 pyranopterin dithiolenes: dimethyl sulfoxide reductase (DMSOR)/ biotin sulfoxide reductase (BSOR), 251–258 sulfite oxidase (SO) family, 249 18 Oxygen isotope shift, pyranopterin dithiolenes: dimethyl sulfoxide reductase (DMSOR)/ biotin sulfoxide reductase (BSOR), 251–258 sulfite oxidase (SO) family, 249 Oxygen ligands, heteroleptic dithiolenes, 302–303 O–Q bonding, metal-binding pyranopterin dithiolene (MPT), oxygen atom transfer (OAT), 566–569 Oxygen probes, metallo-1,2-enedithiolates, 376–378 activated phosphate detection, 389–390 double-frequency modulation-based probes, 383–384

709

dual-emitting probes, 379–381 vs. other probes, 384–386 phase-based probes, 381–383 Palladium complexes: bis(dithiolenes): alkylation–protonation, 281–284 homoleptic structures, 76–77 miscellaneous reactions–applications, 289–290 dithiolene electrical properties, dmit compounds, 412–417 dithiolene magnetic properties, dmit ferromagnetic systems, 446–447 metallo-bis(dithiolenes), square-planar bonding, 158–166 metallo-1,2-enedithiolates, 372–374 dual-emitting heterocyclic-substituted complexes, 374–376 metal–sulfur interactions, tetradentate ligands, 592–596 mono(dithiolene) structures, 94–95 square-planar structures: d8 bis(dithiolenes): excited states and luminescence, 321–324 hydrogen photoproduction, 329–330 photooxidation, 330–333 radical formation, 334–335 mixed-ligand dithiolene–diimine complexes: photooxidation, 350–351 structural variation, 351–352 Patent surveys, dithiolene electrical properties, 431 Pauli susceptibility, dithiolene electrical properties, mnt compounds, 417–418 PCy3 ligands, [M(L)(Sn)] fragments, diazenes, N2H2 synthesis and structure, 644–647 Peierls transition, dithiolene electrical properties: dmit compounds, 411–417 mnt compounds, 417–418 Pentadentate ligands, metal–sulfur interactions, 596–598 Pentamethyl diethylenetriamine (pmdta): alkanedithiolate dehydrogenation, 38–39 electrophilic alkyne reactions, 30–32 Perylene complexes: dithiolene electrical properties:

710

SUBJECT INDEX

Perylene complexes (Continued) mnt compounds, 417–418 room temperature conductivity, 427–428 dithiolene magnetic properties, spin-Peierls (SP) systems, 433 electrical properties, 407 metal-like behavior, 408–422 multifunctional electromagnetic dithiolenes, 465 Phase-based oxygen probes, metallo-1,2enedithiolates, 381–383 pH monitoring, protonation state-dependent emission: ILCT emission quenching, 392–393 metallo-1,2-enedithiolates, 391–392 Phosphates: metallo-1,2-enedithiolates, 387–390 metal–sulfur ligand structures, 600–601 Phosphines: metal-binding pyranopterin dithiolene (MPT), oxygen atom transfer (OAT), 560–569 [M(L)(Sn)] fragments: chirality, 619–620 nitric oxide conversion, NPR3, H2NO, and HNO ligands, 632–636 molybdenum/tungsten oxo-chemistry, 546–547 [M(Sn)] fragments, diazenes, N2H2 synthesis and structure, 641–647 Phosphorane–imine complexes, [MS] enzyme modeling, hydrogenase enzymes, 656–658 Phosphorescence: metallo-1,2-enedithiolates, dual-emitting oxygen probes, 379–381 tetrahedral bis(dithiolenes), 354 31 P NMR, hydrogen-bridge diastereoisomerism– PR3 exchange reactions, [m-N2H2{M(PR3)(S4)}2] complexes, 648–652 Phosphorus ligands, heteroleptic dithiolenes, 302–303 Photochemistry, metallo-dithiolenes, 315–320 octahedral dn complexes: homoleptic dithiolenes, 356–357 mixed-ligand complexes, 357–360 square-planar d8 complexes, 320–352 bis(dithiolenes), 320–335 excited states, 320 hydrogen photoproduction, 328–330

ion-pair charge-transfer, 324–328 photooxidation, 330–333 radical formation, 334–335 mixed-ligand dithiolene–diimine complexes, 339–353 excited states, 339–344 photoinduced electron transfer, 344–346 photooxidation, 348–351 self-quenching, 346–348 structural variation, 351–353 mixed-ligand dithiolene–donor complexes, 335–338 tetrahedral/distorted four-coordinate d10 complexes, 353–356 bis(dithiolenes), 353–354 mixed-ligand dithiolenes, 354–356 Photoelectrochemistry, square-planar bis(dithiolenes), photooxidation, 332–333 Photoelectron spectroscopy (PES), metallomono(dithiolenes), electronic buffer effect, 128–138 Photoinduced electron-transfer, square-planar mixed-ligand dithiolene–diimine complexes, 344–346 Photooxidation: square-planar d8 bis(dithiolenes), 330–333 square-planar mixed-ligand dithiolenediimine complexes, 348–351 Picosecond spectroscopy, square-planar d8 bis(dithiolenes), photooxidation, 332–333 p-conjugated systems, bis(dithiolene) complexes, ligand structures, 64–72 p-molecular orbitals: bis(dithiolenes), luminescence and photochemistry, 317–320 1,2-dithiolenes, electrochemical–chemical reactivity, 268–270 [M(L)(Sn)] fragments, 605–606 diazenes, N2H2 synthesis and structure, 645–647 high-spin vs. low-spin complexes, 620–622 protonation, alkylation and redox reactions, 625–626 18 valence electron (VE) rule, 613–616 Platinum complexes: bis(dithiolenes): alkylation–protonation, 281–284

SUBJECT INDEX homoleptic structures, 76–77 dithiolene-based films, memory switching electrodes, 430–431 dithiolene electrical properties: dddt compounds, 420 dmit compounds, 413–417 mnt compounds, 418 dithiolene magnetic properties: ferromagnetic systems, 444–445 spin-ladder systems, 437 dithiolene nonlinear optical (NLO) properties, second-harmonic generation, 460–461 [(dppe)Pt{S2C2(CH2CH2-N-2-pyridinium)}] [BPh4], metallo-1,2-enedithiolates: dual-emitting oxygen probes, 379–381 modulation-based probes, 383–384 phase-based oxygen probes, 382–383 metallo-bis(dithiolenes), square-planar bonding, 158–166 metallo-1,2-enedithiolates, 372–374 activated phosphate detection, 388–390 dual-emitting heterocyclic-substituted complexes, 374–376 dual-emitting oxygen probes, 379–381 modulation-based probes, 383–384 phase-based oxygen probes, 382–383 metallo-mono(dithiolenes), Hu¨ ckel calculations, 139–142 metal-sulfur interactions, tetradentate ligands, 592–596 square-planar mixed-ligand dithiolenediimine complexes: excited states and luminescence, 339–344 photoinduced electron-transfer, 344–346 photooxidation, 348–351 self-quenching mechanism, 347–348 structural variation, 351–353 square-planar structures: d8 bis(dithiolenes): excited states and luminescence, 321–324 hydrogen photoproduction, 329–330 ion-pair charge-transfer photochemistry, 325–328 photooxidation, 330–333 radical formation, 334–335 mixed-ligand dithiolene–donor complexes, 336–338

711

Polymer-plasticizer matrix, metallo-1,2enedithiolates, activated phosphate detection, 388–390 Poly(methylmethacrylate) (PMMA), nonlinear optical (NLO) properties, third-order nonlinear optics, 461–463 Polymorphism, dithioelene electrical properties, dmit compounds, 409–417 Polynuclear structures: dithiolene-[M(Sn)] synthesis, 603–604 [M(L)(Sn)] complexes, 606–612 Poly(vinylpyridine) (PVP), square-planar bis(dithiolenes), photooxidation, 333 Ppd ligands: pyranopterin dithiolenes, dimethyl sulfoxide reductase (DMSOR)/biotin sulfoxide reductase (BSOR), 251–258 Resonance Raman (RR) spectroscopy, 262 PR3 exchange reaction, hydrogen-bridge diastereoisomerism, 647–652 Prokaryotes, nitrate reductases in, 540–541 Promoted routes, transition metal dithiolenes: alkanedithiolate dehydrogenation, 38–39 dithiocarbonates, 39–40 metal sulfides: electrophilic alkyne additions, 29–32 a-haloketones and related precursors, 37 unactivated alkyne additions, 32–37 specialized routes: alkyne–thiocarbonyl derivatives, 43 alkynyl anions, 44 C–C coupling pathways, 42 S-dealkylation, 40–41 dithiolene coupling, 43 metal–alkyne bond insertion, 41–42 Protonation: bis(dithiolenes), 281–284 [M(L)(Sn)] fragmenets, 623–626 square-planar mixed-ligand dithiolene–donor complexes, 337–338 state-dependent emission, pH monitoring: ILCT emission quenching, 392–393 metallo-1,2-enedithiolates, 391–392 Proton NMR spectroscopy: bis(dithiolenes), unsaturated compound addition reactions, 287–288 hydrogen-bridge diastereoisomerism/PR3 exchange reactions, [m-N2H2{M(PR3)(S4)}2] complexes, 648–652

712

SUBJECT INDEX

Proton NMR spectroscopy (Continued) metal-binding pyranopterin dithiolene (MPT), 571–574 [M(L)(Sn)] fragments, 629–630 tris(dithiolenes), chemical reactivity, 296–299 Pseudo-s bonding interaction, metallomono(dithiolenes): bonding parameters, 127–128 electronic buffer effect, 136–138 Pterin structures: biological metallo-dithiolenes, redox reactions, 501–503 biological metallodithiolenes, XDH/XO crystalline structure, 509–510 metal-binding pyranopterin dithiolene (MPT), 569–574 molybdopterin-dithiolene degradation, 505–507 Pyranopterin dithiolenes. See also Metal-binding pyranopterin dithiolene (MPT) metallo-mono(dithiolenes), molybdenum enzyme, 128 mononuclear molybdenum/tungsten enzymes, 246–262 aldehyde oxidoreductase (AOR) family, 258–261 dimethyl sulfoxide reductase (DMSOR), 250–258 sulfite oxidase family, 248–249 xanthine oxidase family, 247 Pyran ring structure, metal-binding pyranopterin dithiolene (MPT), 542–544 pterin structures, 569–574 Pyrazine rings, metal-binding pyranopterin dithiolene (MPT), 542–544 Pyridyl complexes, protonation state-dependent emission, 391–392

QO bonding, metal-binding pyranopterin dithiolene (MPT), oxygen atom transfer (OAT), 565–569 Q-switch laser dyes, dithiolene optical properties, strong infrared (IR) absorption, 453–458 Quadricyclane, bis(dithiolenes), 288–290 Quantum yields, square-planar d8 bis(dithiolenes), photooxidation, 332–333

Quenching data, square-planar mixed-ligand dithiolene-diimine complexes, photoinduced electron-transfer, 344–346 Quinoxaline substituted complex, protonation state-dependent emission, 392–393 2,3-Quinoxalinedithiol (qdt), metallo-1,2enedithiolates, 371–374

Racemic compounds, [M(L)(Sn)] fragments, chirality, 619–620 Radical formation kinetics, square-planar bis(dithiolenes), photochemistry, 334–335 Raman spectroscopy: bis(dithiolenes), unsaturated compound addition reactions, 287–288 density functional theory and, 261–262 metallo-bis(dithiolenes), four-coordinate structure, 215–228 metallo-mono(dithiolenes), bonding parameters, 122–128 metallo-tris(dithiolenes), six-coordinate vibrational spectra, 229–239 oxo-metallo-bis(dithiolenes), 241–246 oxo-metallo-mono(dithiolenes), 240–241 Ratiometric oxygen analysis, dual-emitting metallo-1,2-enedithiolates, 379–381 Redox properties: biological metallo-dithiolenes, pterins, 501–503 bis(dithiolenes), 270–277 1,2-dithiolenes: electrochemical–chemical reactivity, 268–270 future research, 308–309 ligand bonding, 57–58 heteroleptic dithiolenes, nitrosyl complexes, 301–302 intermetallic dithiolene transfer, 26–29 metal-binding pyranopterin dithiolene (MPT), catalytic center chemical analogues, 553–560 metallo-dithiolene complexes, 112–113 bis(ditholenes), square-planar bonding, 151–166 metallo-bis(dithiolenes), 169–173 tris(dithiolenes): electron paramagnetic resonance (EPR), 188–192

SUBJECT INDEX six-coordinate vibrational spectra, 232–239 [M(L)(Sn)] fragments: protonation and electron transfer, 623–626 18 valence electron rule, 614–616 molybdopterin-dithiolene degradation, 505–507 molybdopterin ligand, 524–527 [MS] enzyme modeling, FeMoco function, nitrogenase complexes, [Fe(Sn)] reversible redox reactions, 662–664 oxo-metallo-bis(dithiolenes), vibrational spectra, 245–246 pyranopterin dithiolenes: dimethyl sulfoxide reductase (DMSOR), 255–258 sulfite oxidase (SO) family, 249 tris(dithiolenes), 290–295 Reduction potentials, metallomono(dithiolenes), electronic buffer effect, 136–138 Reductive dealkylation, transition metal dithiolene synthesis, 1,2-alkene dithiolates, 10–11 Rehm–Weller equations, square-planar mixedligand dithiolene–diimine complexes, photoinduced electron-transfer, 345–346 Resonance Raman (RR) spectroscopy: dithiolene-molybdenum unit, 515–516 metal-binding pyranopterin dithiolene (MPT), 573–574 metallo-bis(dithiolenes): excited-state spectroscopic probes, 168–173 four-coordinate structure, 215–228 metallo-mono(dithiolenes), 117 bonding parameters, 123–128 ligand-to-ligand charge transfer (LLCT), 142 metallo-tris(dithiolenes): electronic absorption (EA) spectroscopy, 194–196 six-coordinate vibrational spectra, 232–239 oxo-metallo-bis(dithiolenes), 243–246 ppd ligands, 262 pyranopterin dithiolenes, mononuclear molybdenum/tungsten enzymes, 246–262 aldehyde oxidoreductase (AOR), 258–261

713

dimethyl sulfoxide reductase (DMSOR), 250–258 sulfite oxidase (SO), 248–249 xanthine oxidase (XO) family, 247 Rh. sphaeroides, pyranopterin dithiolenes, dimethyl sulfoxide reductase (DMSOR)/ biotin sulfoxide reductase (BSOR), 250–258 Rhenium complexes: bis(dithiolenes), redox properties, 275–277 heteroleptic dithiolenes, metal–ligand multiple bonds, 303–305 metallo-tris(dithiolenes): electronic absorption (EA) spectroscopy, 192–196 ideal D3h trigonal-prismatic distortion, 186–188 molecular orbital bonding analysis, 174–179 metal sulfides, unactivated alkyne additions, 34–37 [MS] enzyme modeling, hydrogenase enzymes, 654–658 tris(dithiolenes), redox properties, 291–295 Rhodium complexes: heteroleptic dithiolenes: carbonyl complexes, 300–301 organometallic reactions, 307–308 [MS] enzyme modeling, hydrogenase enzymes, 655–658 tridentate ligands, 591–592 Rhodobacter capsulatus, metal-binding pyranopterin dithiolene (MPT), catalytic center chemical analogues, 550–560 Rhodobacter sphaeroides, metal-binding pyranopterin dithiolene (MPT), catalytic center chemical analogues, 550–560 Room temperature (RT) conductivity, dithiolene electrical properties, 407 dddt compounds, 419–420 Langmuir–Blodgett (LB) conducting films, 428–429 metal-like behavior, 408–422 dmit compounds, 408–417 related compounds, 427–428 R,R0 timdt ligand, metallo-bis(dithiolenes), 164–166 Ruthenium complexes: dinitrogen complexes, [M(Sn)] cores, 670–672

714

SUBJECT INDEX

Ruthenium complexes (Continued) dithiolene-[M(Sn)] synthesis, 603 heteroleptic dithiolenes, metal–ligand multiple bonds, 303–305 hexadentate ligands, 599 metallo-1,2-enedithiolates, 386 [M(L)(Sn)] fragments, 612 chirality, 619–620 C–S bond cleavage, 628–629 H2S and S2 complexes, 639–640 nitric oxide conversion, 635–636 18 valence electron rule, 614–616 [MS] enzyme modeling: FeMoco function, nitrogenase complexes, nitrogen-dependent HD formation, 666–668 hydrogenase enzymes, 654–658 [M(Sn)] fragments, diazenes, N2H2 synthesis and structure, 641–647 square-planar d8 bis(dithiolenes), excited states and luminescence, 322–324 square-planar mixed-ligand dithiolene– diimine complexes, 352–353 transition metal dithiolene synthesis, metal sulfides, unactivated alkyne additions, 36–37 tris(dithiolenes), chemical reactivity, 298–299 Saturated calomel electrode (SCE): dithiolene electrochemical–chemical reactivity, 269–270 square-planar mixed-ligand dithiolene– diimine complexes, photoinduced electron-transfer, 345–346 SCCS units, metal–sulfur interactions, 588–589 Second-harmonic generation, dithiolene nonlinear optical (NLO) properties, 458 multifunctional conductivity-optical compounds, 466–467 two-state model, 458–461 Second-order Jahn–Teller distortions, metallotris(dithiolene) complexes, ideal D3h trigonal-prismatic distortion, 185–188 Selenium, dithiolene magnetic properties, spinPeierls (SP) systems, 433 Self-consistent field (SCF) ground-state wave function, metallo-bis(dithiolene) complexes, 156–166

Self-consistent field (SCF)-Hartree–Fock (SCF– HF) calculations, bis(dithiolenes), redox properties, 277 Self-quenching mechanism, square-planar mixed–ligand dithiolene–diimine complexes, 346–348 Semiconductor compounds, dithiolene electrical properties, metal-like behavior, dmit compounds, 408–417 17 valence electrons, [M(L)(Sn)] fragments, small molecule conversion, nitrosyl complexes, 632 Short-range ferromagnetic interactions, dithiolene magnetic properties, 441–448 Shubnikov-de Haas oscillations, dithiolene electrical properties, dmit compounds, 411–417 Sigma bond-to-ligand charge transfer (SBLCT), octahedral dn complexes, mixed-ligand structures, 357–360 s-donor orbitals: bis(dithiolenes), luminescence and photochemistry, 317–320 [M(L)(Sn)] complexes, 605–606 diazenes, N2H2 synthesis and structure, 645–647 high-spin vs. low-spin complexes, 620–622 protonation, alkylation and redox reactions, 625–626 Silicones: metallo-1,2-enedithiolates, 386 [M(L)(Sn)] fragments, CO, CO2, and SO2 conversion, 637–639 Silver complexes: bis(dithiolenes): homoleptic structures, 78 miscellaneous reactions–applications, 289–290 dithiolene electrical properties, dddt compounds, 419–420 metallo-1,2-enedithiolates, 372–374 Silylation reactions, metal-binding pyranopterin dithiolene (MPT), catalytic center chemical analogues, 550–560 Singly occupied molecular orbital (SOMO) calculations, metallo-bis(dithiolenes), square-planar bonding, 146–149 16 valence electrons, [M(L)(Sn)] fragments, small molecule conversion, nitrosyl complexes, 632

SUBJECT INDEX Small molecule conversion, [M(Sn)] complexes, coordination sphere, 630–652 carbon monoxide, carbon dioxide, and sulfur dioxide, 637–639 diazenes, 640–652 hydrogen-bridge diastereoisomerism and PR3 exchange, 647–652 [M(Sn)]–N2H2 complexes, 641–647 H2S and S2 complexes, 639–640 nitrosyl complexes, 630–636 16, 17, and 18 valence electron complexes, 632 nitric oxide conversions, 632–636 nitric oxide
! NH2OH reduction, 636 18/19 valence electron complexes, 630–632 Solvatochromatic absorption: metallo-mono(dithiolenes), 139 square-planar mixed-ligand dithiolene– diimine complexes, excited states and luminescence, 341–344 Solvent effects, square-planar d8 bis(dithiolenes), photooxidation, 332–333 ‘‘Spanning overlap,’’ dithiolene electrical properties: dmit compounds, 411–417 metal-like behavior, 421–422 Spectroscopic probes, biological metallodithiolenes, dithiolenemolybdenum unit, 515–519 electronic spectroscopy, 517–518 magnetic circular dichroism, 518–519 paramagnetic spectroscopy, 518 resonance Raman spectroscopy, 515–516 X-ray absorption spectroscopy, 516–517 Spin density wave (SDW), dithiolene electrical properties, superconductivity, 426–427 Spin-ladder systems: 1,2-dithiolene magnetic properties, 433–440 [Cp2M(dithiolene)](TCNQF4), 437–440 DT-TTF2[Au(mnt)2], 436–437 p-EPYNN[Ni(dmit)2], 433–435 dithiolene magnetic properties, 401–402 multifunctional electromagnetic dithiolenes, 465 Spin-Peierls (SP) transition, dithiolene magnetic properties, 401 system structures, 432–433 Square-planar complexes:

715

bis(dithiolenes), 61–62 redox properties, 276–277 vibrational spectra, 214–228 d8 complexes, 320–352 bis(dithiolenes), 320–335 excited states, 320 hydrogen photoproduction, 328–330 ion-pair charge-transfer, 324–328 photooxidation, 330–333 radical formation, 334–335 mixed-ligand dithiolene–diimine complexes, 339–353 excited states, 339–344 photoinduced electron transfer, 344–346 photooxidation, 348–351 self-quenching, 346–348 structural variation, 351–353 mixed-ligand dithiolene–donor complexes, 335–338 electrochemical–chemical reactivity, 268–270 metallo-bis(dithiolenes), 143–173 axial oxo ligands, 166–173 excited-state spectroscopic probes, 167–173 square-pyrimidal bonding comparison, 166–167 molecular orbital (MO) bonding calculations, 143–166 excited-state spectroscopic probes, 149–166 ground-state spectroscopic probes, 146–149 metallo-mono(dithiolenes), ligand-to-ligand charge transfer (LLCT), 139–142 [MS] enzyme modeling, carbon monoxide dehydrogenase (CODH), 658–660 Square-pyramidal bonding: metal-binding pyranopterin dithiolene (MPT), catalytic center chemical analogues, 553–560 metallo-bis(dithiolenes), 166–167 S  S interactions, dithiolene electrical properties, superconductivity, 422–427 Stacked molecules: dithiolene electrical properties: dmit compounds, 409–417 mnt compounds, 417–418 electrical properties, 406–407 Stearyl alcohol, dithiolene-based films, electrical sensors, 429–430

716

SUBJECT INDEX

Stereochemistry: metal-binding pyranopterin dithiolene (MPT), 569–574 [M(L)(Sn)] fragments, chirality, 617–620 tris(dithiolene) complexes, 84–87 Steric hindrance, molybdenum/tungsten oxochemistry, 545–547 Stern–Volmer analyses: metallo-1,2-enedithiolates: modulation-based probes, 383–384 oxygen probes, 378 polymer properties, 386 square-planar mixed-ligand dithiolene– diimine complexes: photoinduced electron-transfer, 345–346 self-quenching mechanism, 346–348 Strong-field axial ligand, metallomono(dithiolenes), 117–142 bonding parameters, 117–128 ligand-to-ligand charge transfer (LLCT), 138–142 oxo-metal complexes, electronic buffer effect, 128–138 Strong infrared (IR) absorption, dithiolene optical properties, 453–458 Sulfite oxidase (SO): biological metallo-dithiolenes: crystalline structure, 510–511 magnetic circular dichroism (MCD) analysis, 519 molybdenum enzymes, 497–498 molybdopterin ligand structure, 524–527 X-ray absorption analysis, 516–517 metal-binding pyranopterin dithiolene (MPT), catalytic center chemical analogues, 559–560 metal-binding pyranopterin dithiolene (MPT)cis-MoO2 center, 543 oxo-metallo-mono(dithiolenes), 239–241 pyranopterin dithiolenes, mononuclear molybdenum–tungsten enzymes, 248–249 structure and properties, 541 Sulfonium cations, dithiolene electrical properties, dmit compounds, 413–417 Sulfur dioxide, [M(L)(Sn)] complexes, CO, CO2, and SO2 conversion, 637–639 ‘‘Sulfur-fold’’ distortion, metallo-tris(dithiolene) complexes:

ideal D3h trigonal-prismatic distortion, 184–188 molecular orbital bonding analysis, 177–179 Sulfur ligands: heteroleptic dithiolenes, 302–303 metal–sulfur interactions, 587 [M(L)(Sn)] complexes, 605–606 S2 complexes, 639–640 [MS] enzyme modeling, hydrogenase enzymes, 653–658 S–C bond, bis(dithiolenes), reaction–application analysis, 288–290 S2 –C donor atom set, tridentate ligands, 591–592 S-donor ligands, metal-binding pyranopterin dithiolene (MPT), oxygen atom transfer (OAT), 560–569 34 S isotope shifts: aldehyde oxidoreductase (AOR) and, 261 dimethylsulfoxide reductase (DMSOR), 255–258 S

O stretching frequencies, pyranopterin dithiolenes, dimethyl sulfoxide reductase (DMSOR)/biotin sulfoxide reductase (BSOR), 251–258 Superconducting quantum interference device (SQUID), dithiolene magnetic properties, bulk ferromagnets, 450 Superconductivity: 1,2-dithiolene complexes, 422–427 multifunctional compounds, 464–467 metallo-bis(dithiolenes), four-coordinate structures, 224–228 Symmetry-adapted linear combinations (SALCs): metallo-dithiolenes, molecular orbital (MO) calculations, 114–116 tris(dithiolenes), ideal D3h trigonal-prismatic distortion, 183–188 Symmetry coordinates: metallo-mono(dithiolenes), bonding parameters, 124–128 metallo-tris(dithiolenes), molecular orbital bonding analysis, 174–179 Technetium complexes, heteroleptic dithiolenes, metal–ligand multiple bonds, 303–305 Tetracyanoethylene (TCNE): bis(dithiolenes), redox properties, 275–277 dithiolene magnetic properties, 401

SUBJECT INDEX bulk ferromagnets, 448–453 ferromagnetic mnt systems, 441–444 Tetradentate ligands: metal–sulfur interactions, 592–596 N2S2 donor sets, 599–600 Tetraferrocenyl dithiolenes, dithiolene optical properties, strong infrared (IR) absorption, 455–458 Tetrafluorobenzo-dimethyl-tetrathiafulvalene (TFBDM-TTF), dithioelene electrical properties, dmit compounds, 416–417 Tetrafluorotetracyanoquinodimethane (TCNQF4), dithiolene magnetic properties, spin-ladder systems, 437–440 Tetrahedral/distorted four-coordinate d10 complexes, luminescence, 353–356 bis(dithiolenes), 353–354 mixed-ligand dithiolenes, 354–356 Tetrahydrofuran (THF), square-planar d8 bis(dithiolenes), hydrogen photoproduction, 328–330 Tetrahydropterins, metal-binding pyranopterin dithiolene (MPT), 569–574 Tetrakis(dimethylamino)ethylene (TDAE), dithiolene magnetic properties, bulk ferromagnets, 448–453 Tetramethylamine (TMA), TMAO reductase (TMAOR) catalyst, 540 Tetramethyl enediamine (tmeda), electrophilic alkyne reactions, 30–32 2,2,6,6-Tetramethyl-1-1-piperindinyloxy (TEMPO), square-planar bis(dithiolenes), radical formation kinetics, 334–335 Tetrapyridophenazine (Tppz), square-planar mixed-ligand dithiolene–diimine complexes, 353 Tetrathiafulvalene (TTF) compounds: dithiolates, alkenedithiolate synthesis, 19–20 dithiolene electrical properties: dddt compounds, 418–420 dmit compounds, 415–417 metal-like behavior, 408–422 superconductivity, 422–427 dithiolene magnetic properties, spin-Peierls (SP) systems, 432–433 Tetrathiafulvalene (TTF)  Tetracyanoquinodimethane (TCNQ), electrical properties, 401 donor–acceptor adducts, 406–407

717

Tetrathiaoxalate esters, dithione synthesis, 23–25 Tetrathiapentalenedione (TPD): dithiocarbonates, 39–40 inorganic dithiolates, dmit2
, 18–19 Tetrathiooxalate, inorganic dithiolates and dmit2
, 18–19 Tfd2 compounds, dithiolene magnetic properties: ferromagnetic systems, 444–445 spin-Peierls (SP) systems, 432–433 Thallium complexes, mono(dithiolene) structures, 92–95 Thermochromism, metallo-bis(dithiolenes), 170–173 Thioalkyl groups, dithiolene optical properties, strong infrared (IR) absorption, 457–458 Thiocarbon derivatives: alkenedithiolate synthesis, 20–21 dithiolene assembly routes, alkynes, 43 Thiophenes: molybdopterin-dithiolene degradation, 505–507 transition metal dithiolene synthesis, electrophilic alkyne reactions, 31–32 Thiophosphate esters, alkenedithiolate synthesis, 21–22 Third-order nonlinear optics, dithiolene nonlinear optical (NLO) properties, 461–463 Three-dimensional molecular structures, dithiolene electrical properties, superconductivity, 422–427 Three-electron quasirreversible coupling, metalbinding pyranopterin dithiolene (MPT), 573–574 Titanium complexes: hexadentate ligands, 599 tetradentate ligands, 595–596 Toluene-3,4-dithiolate (tdt), metallo-1,2enedithiolates, 371–374 Transition metals: bis(dithiolenes): four-coordinate structures, vibrational spectrosopy, 227–228 redox properties, 276–277 1,2-dithiolenes (See also Metallo-dithiolene complexes) distribution, 59 promoted routes:

718

SUBJECT INDEX

Transition metals (Continued) alkanedithiolate dehydrogenation, 38–39 dithiocarbonates, 39–40 metal sulfides: electrophilic alkyne additions, 29–32 a-haloketones and related precursors, 37 unactivated alkyne additions, 32–37 specialized routes: alkyne–thiocarbonyl derivatives, 43 alkynyl anions, 44 C–C coupling pathways, 42 S-dealkylation, 40–41 dithiolene coupling, 43 metal–alkyne bond insertion, 41–42 research background, 2–4 synthesis: 1,2-alkene dithiolates: 4,5-dimercapto-1,3-dithiole-2-thione, 15–18 dithiocarbonate (dithiole-2-ones) and related derivatives, base hydrolysis, 11–15 inorganic dmit2– dithiolate derivatives, 18–19 1,2-maleonitrile 1,2-dithiolate (mnt2
) reductive dealkylation, 10–11 tetrathiafulvalene (TTF)-derived dithiolenes, 19–20 thiacarbons and derivatives, 20 benzenedithiol and related derivatives, 4–10 arene derivatives, 4–10 dicarborane- and ferrocenedithiolates, 10 1,2-dithietes, 22–23 1,2-dithiones, 23–25 intermetallic dithiolene transfer: non-redox routes, 25–26 redox routes, 26–29 thiophosphate esters, a-hydroxyketone and a-diketone derivatives, 21–22 homoleptic structures: bis(dithiolene) complexes, 59–79 bond lengths and angles, 63–64 chromium, manganese, iron and cobalt, 78–79 copper and gold, 77 geometrical properties, 60–62 ligand types, 64–72

multimeric molecular structures, 73–76 nickel, palladium, and platinum, 76–77 silver, zinc, cadmium, and mercury, 78 tris(dithiolene) complexes, 80–92 bond lengths and angles, 87–88 geometric properties, 84–87 octahedral and trigonal-prismatic structures, 86–87 trans-SMS angle, 85–86 trigonal twist angle, 84–85 ligand bending, 88–92 mono(dithiolenes), electronic buffer effect, 133–138 tris(dithiolenes), six-coordinate vibrational spectra, 235–239 Transoid angles, metal-binding pyranopterin dithiolene (MPT), 577 Trans-SMS angle, tris(dithiolene) complexes, 85–86 Tridentate ligands, metal–sulfur interactions, 590–592 Trigonal-prismatic (TP) geometry: octahedral dn complexes, homoleptic complexes, 356–357 oxo-metallo-bis(dithiolenes), vibrational spectra, 246 tris(dithiolene) complexes, 84 examples, 86–87 metallo-tris(dithiolenes), 173–174 ideal D3h distortions, 179–188 molecular orbital bonding analysis, 174–179 six-coordinate vibrational spectra, 228–239 trans-SMS angle, 85–86 trigonal twist angle, 84–85 Trigonal twist angle, tris(dithiolene) complexes, 84–85 ideal D3h trigonal-prismatic distortion, 181–188 six-coordinate vibrational spectra, 228–239 Trimeric structures: bis(dithiolene) complexes, 73–76 dithiolene electrical properties, dddt compounds, 419–420 Trimethylamine-N-oxide (TMAO): metal-binding pyranopterin dithiolene (MPT): catalytic center chemical analogues, 555–560 oxygen atom transfer (OAT), 563–569 structure and properties, 540

SUBJECT INDEX Trimethylamine oxidase (TMAO), biological metallo-dithiolene X-ray diffraction, 513–514 TMAO reductase (TMAOR), structure and properties, 540 Trimethylsulfoxide (TMSO), metal-binding pyranopterin dithiolene (MPT), oxygen atom transfer (OAT), 563–569 Trinuclear structures: hexadentate ligands, 599 tridentate ligands, 591–592 Triplet-singlet emission intensity ratio, metallo1,2-enedithiolates, modulation-based probes, 383–384 Tris(diimines), square-planar d8 bis(dithiolenes), excited states and luminescence, 322–324 Tris(dithiolenes): chemical reactivity, 296–299 distribution, 59 homoleptic structures, 80–92 main group complexes, 92–95 transition metals, 80–92 bond lengths and angles, 87–88 geometric properties, 84–87 octahedral and trigonal-prismatic structures, 86–87 trans-SMS angle, 85–86 trigonal twist angle, 84–85 ligand bending, 88–92 metal-binding pyranopterin dithiolene (MPT), catalytic center chemical analogues, 547–560 metallo-tris(dithiolenes): electron absorption (EA) spectroscopy, 192–196 electronic–spectroscopic structural studies, 173–196 electron paramagnetic resonance (EPR) spectroscopy, 188–192 ideal D3h trigonal-prismatic geometry, distortions, 179–188 molecular orbital (MO) calculations, 174–179 six-coordinate complexes, vibrational spectra, 228–239 octahedral dn complexes, homoleptic complexes, 356–357 redox properties, 290–295 ring structures, 402

719

square-planar d8 bis(dithiolenes), hydrogen photoproduction, 328–330 Tungsten complexes (see also W): biological metallo-dithiolenes, 493–503 enzyme mechanisms, 519–522 enzyme site X-ray crystallography, 507–515 aldehyde ferrodoxin oxidoreductase (AOR) crystal structure, 514–515 DMSOR crystal structure, 511–514 sulfite oxidase crystal structure, 510–511 XDH/XO crystal structure, 508–510 enzyme structural classification, 498 ligand structure, 499–501 molybdenum enzyme families, 496–498 nomenclature issues, 498–499 pterin redox reactions, 501–503 tungsten enzyme families, 498 dithiolene ligands, research background, 2–4 enzyme chemistry, structure and classification, 540–544 heteroleptic dithiolenes: carbonyl complexes, 299–301 metal-ligand multiple bonds, 303–305 intermetallic dithiolene transfer, redox routes, 26–29 metal-binding pyranopterin dithiolene (MPT): catalytic center chemical analogues, 547–560 future research issues, 575–577 oxygen atom transfer (OAT), 560–569 structure and properties, 542–544 metallo-tris(dithiolenes): electronic absorption (EA) spectroscopy, 192–196 electron paramagnetic resonance (EPR), 190–192 six-coordinate vibrational spectra, 232–239 metal–sulfur interactions, 601 [M(L)(Sn)] fragments, 18 valence electron (VE) rule, 612–616 octahedral dn complexes: homoleptic complexes, 356–357 mixed-ligand structures, 359–360 oxo-chemistry, 544–547 oxo-metallo-bis(dithiolenes), vibrational spectra, 241–246 oxo-metallo-mono(dithiolenes), 240–241 pyranopterin dithiolenes, mononuclear enzymes, 246–262

720

SUBJECT INDEX

Tungsten complexes (Continued) aldehyde oxidoreductase (AOR) family, 258–261 dimethyl sulfoxide reductase (DMSOR), 250–258 sulfite oxidase family, 248–249 xanthine oxidase family, 247 transition metal dithiolene synthesis: arenedithiolate ligands, 6–10 metal sulfides: electrophilic alkyne reactions, 31–32 a-haloketones, 37 unactivated alkyne additions, 34–37 tris(dithiolenes): chemical reactivity, 296–299 redox reactions, 291–295 Two-dimensional molecular systems: dithiolene electrical properties, 406–407 dmit compounds, 411–417 dithiolene magnetic properties, spin-ladder systems, 433 Ultraviolet (UV) irradiation, heteroleptic dithiolenes, carbonyl complexes, 299–301 Ultraviolet (UV) photoelectron spectroscopy (PES), metallo-mono(dithiolenes), 129–138 Ultraviolet–visible (UV–vis) spectroscopy: metal-binding pyranopterin dithiolene (MPT), catalytic center chemical analogues, 550–560 metallo-1,2-enedithiolates, 371–374 square-planar d8 bis(dithiolenes), photooxidation, 331–333 Unsaturated compounds, bis(dithiolenes): addition reactions, 287–288 hydrocarbon cycloaddition, 284–287 Urey–Bradley force constants, metallobis(dithiolenes), four-coordinate structures, 220–228 Urothione, molybdopterin-dithiolene degradation, 505–507 Valence electron (VE) rules, [M(L)(Sn)] fragments: protonation, alkylation and redox reactions, 626 small molecule conversion: 16, 17, and 18 VE nitrosyl complexes, 632

18/19 VE nitrosyl complexes, 630–632 16 valence electron rule, 614–616 18 valence electron (VE) rule, 612–616 Valence orbitals, metallo-dithiolenes, molecular orbital (MO) calculations, 113–116 Valence tautomerism, metallo-bis(dithiolenes), 170–173 Vanadium complexes: heteroleptic dithiolenes, metal-ligand multiple bonds, 303–305 metallo-mono(dithiolenes), electronic buffer effect, 134–138 metallo-tris(dithiolenes): electron paramagnetic resonance (EPR), 190–192 six-coordinate vibrational spectra, 232–239 [M(L)(Sn)] complexes, 605–606 square-planar mixed-ligand dithiolene– diimine complexes, photoinduced electron-transfer, 345–346 tris(dithiolenes): ligand bending, 90–92 redox reactions, 291–295 Vibrational spectroscopy: 1,2-dithiolenes, 214 metallo-bis(dithiolenes), four-coordinate structures, 214–228 metallo-mono(dithiolenes), bonding parameters, 122–128 metallo-tris(dithiolenes), six-coordinate complexes, 228–239 oxo-metallo-bis(dithiolenes), 241–246 oxo-metallo-mono(dithiolenes), 239–241 Viologens, square-planar d8 bis(dithiolenes): hydrogen photoproduction, 330 ion-pair charge-transfer photochemistry, 325–328 W–H bond, tris(dithiolenes), chemical reactivity, 298–299 W=O stretching modes: oxo-metallo-bis(dithiolenes), 244–246 pyranopterin dithiolenes, aldehyde oxidoreductase (AOR) and, 260–261 W–S stretching modes: aldehyde oxidoreductase (AOR) and, 261 oxo-metallo-bis(dithiolenes), vibrational spectra, 243–246 Xanthate derivates, transition metal dithiolene synthesis, dithiocarbonates, 12–15

SUBJECT INDEX Xanthine dehydrogenase/xanthine oxidase (XDH/XO): biological metallodithiolenes: crystalline structure, 508–510 magnetic circular dichroism (MCD) analysis, 519 molybdenum enzymes, 496–498 molybdopterin ligand structure, 524–527 X-ray absorption analysis, 516–517 metal-binding pyranopterin dithiolene (MPT), catalytic center chemical analogues, 559–560 metal-binding pyranopterin dithiolene (MPT)fac-MoOY(H2O) center, 543 oxo-metallo-mono(dithiolenes), vibrational spectra, 239–241 pyranopterin dithiolenes, mononuclear molybdenum–tungsten enzymes, 247 structure and properties, 541 sulfite oxidase deficiency, 541 X-ray crystallography: dithiolene-molybdenum enzyme site, 507–515 aldehyde ferrodoxin oxidoreductase (AOR) crystal structure, 514–515 DMSOR crystal structure, 511–514 sulfite oxidase crystal structure, 510–511 XDH/XO crystal structure, 508–510 metallo-bis(dithiolenes), square-planar bonding, excited-state spectroscopic probes, 150–166

721

[M(L)(Sn)] fragments, 629–630 [MS] enzyme modeling, hydrogenase enzymes, 653–658 oxo-metallo-bis(dithiolenes), vibrational spectra, 245–246 pyranopterin dithiolenes, mononuclear molybdenum–tungsten enzymes, dimethyl sulfoxide reductase (DMSOR), 250–258 YPh ligands, metal-binding pyranopterin dithiolene (MPT), catalytic center chemical analogues, 557–560 Zinc complexes: alkanedithiolate dehydrogenation, 38–39 bis(dithiolene), homoleptic structures, 78 4,5-dimercapto-1,3-dithiole-2-thione (dmit2
) synthesis, 16–18 metallo-bis(dithiolenes), four-coordinate structures, 226–228 metallo-mono(dithiolenes), Hu¨ ckel calculations, 139–142 square-planar bis(dithiolenes): hydrogen photoproduction, 328–330 ion-pair charge-transfer photochemistry, 325–328 tetrahedral bis(dithiolenes), 353–354 tetrahedral mixed-ligand dithiolenes, 354–356

Dithiolene Chemistry: Synthesis, Properties, and Applications, Progress in Inorganic Chemistry, Vol. 52 Special volume edited by Edward I. Stiefel, Series editor Kenneth D. Karlin ISBN 0-471-37829-1 Copyright # 2004 John Wiley & Sons, Inc.

Cumulative Index, Volumes 1–52 VOL. Abel, Edward W., Orrell, Keith G., and Bhargava, Suresh K., The Stereodynamics of Metal Complexes of Sulfur-, Selenium and Tellurium-Containing Ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ´ s, Richard D, and Horva´th, Istva´ns, T., Novel Reactions of Metal Carbonyl Adam Cluster Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Adamson, A. W., see Fleischauer, P. D. Addison, C. C. and Sutton, D., Complexes Containing the Nitrate Ion . . . . . . Albin, Michael, see Horrocks, William DeW., Jr. Allen, G. C. and Hush, N. S., Intervalence-Transfer Absorption, Part I Qualitative Evidence for Intervalence Transfer Absorption in Inorganic Systems in Solution and in the Solid State . . . . . . . . . . . . . . . . . . . . . . . . Allison, John, The Gas-Phase Chemistry of Transition-Metal Ions with Organic Molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ardizzoia, G. Attillo, see La Monica, Girolamo Arnold, John, The Chemistry of Metal Complexes with Selenolate and Tellurolate Ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Asprey, L. B. and Cunningham, B. B., Unusual Oxidation States of Some Actinide and Lanthanide Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Baird, Michael C., Metal–Metal Bonds in Transition Metal Compounds . . . . Bakac, Andreja, Mechanistic and Kinetic Aspects of Transition Metal Oxygen Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Balch, Alan L., Construction of Small Polynuclear Complexes with Trifunctional Phosphin-Based Ligands as Backbones . . . . . . . . . . . . . . . . . . . . . . . . . . Balhausen, C. J., Intensities of Spectral Bands in Transition Metal Complexes Balkus, Kenneth J., Jr., Synthesis of Large Pore Zeolites and Molecular Sieves Barton, Jacqueline K., see Pyle, Anna Marie Barwinski, Almut, see Pecoraro, Vincent L. Barrett, Anthony G. M., see Michel, Sarah L. J. Basolo, Fred and Pearson, Ralph G., The Trans Effect in Metal Complexes . . Bastos, Cecilia M., see Mayr, Andreas Baum, Sven M., see Michel, Sarah L. J. Beattie, I. R., Dinitrogen Trioxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Beattie, J. K. and Haight, G. P., Jr., Chromium (IV) Oxidation of Inorganic Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Becke-Goehring, Von Margot, Uber Schwefel Stickstoff Verbindungen . . . . . . Becker, K. A., Plieth, K., and Stranski, I. N., The Polymorphic Modifications of Arsenic Trioxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Beer, Paul D. and Smith, David K., Anion Binding and Recognition by Inorganic Based Receptors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bennett, L. F., Metalloprotein Redox Reactions . . . . . . . . . . . . . . . . . . . . . . Beno, Mark A., see Williams, Jack M. 723

PAGE

32

1

33

127

8

195

8

357

34

627

43

353

2

267

9

1

43

267

41 2 50

239 251 217

4

381

5

1

17 1

93 207

4

1

46 18

1 1

724

CUMULATIVE INDEX, VOLUMES 1–52

Berg, Jeremy M., Metal-Binding Domains in Nucleic Acid-Binding and GeneRegulatory Proteins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bertrand, J. A. and Eller, P. G., Polynuclear Complexes with Aminoalcohols and Iminoalcohols as Ligands: Oxygen-Bridged and Hydrogen-Bonded Species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Beswick, Colin L., Structures and Structural Trends in Homoleptic Dithiolene Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bharadwaj, Parimal K., Laterally Nonsymmetric Aza-Cryptands . . . . . . . . . . Bhargava, Suresh K., see Abel, Edward W. Bickley, D. G., see Serpone, N. Bignozzi, C. A., Schoonover, J. R., and Scandola, F., A Supramolecular Approach to Light Harvesting and Sensitization of Wide-Bandgap Semiconductors: Antenna Effects and Charge Separation . . . . . . . . . . . . . . . . . Bodwin, Jeffery J., see Pecoraro, Vincent L. Bowler, Bruce E., Raphael, Adrienne L., and Gray, Harry B., Long-Range Electron Transfer in Donor (Spacer) Acceptor Molecules and Proteins . . . Bowman, Stephanie, see Watton, Stephen P. Bradley, D. C., Metal Alkoxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Bridgeman, Adam J. and Gerloch, Malcolm. The Interpretation of Ligand Field Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Brookhart, Maurice, Green, Malcom L. H., and Wong, Luet-Lok, CarbonHydrogen-Transition Metal Bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Brothers, Penelope, J., Heterolytic Activation of Hydrogen by Transition Metal Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Brown, Dennis G., The Chemistry of Vitamin B12 and Related Inorganic Model Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Brown, Frederick J., Stoichiometric Reactions of Transition Metal Carbene Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Brown, S. B., Jones, Peter, and Suggett, A., Recent Developments in the Redox Chemistry of Peroxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Brudvig, Gary W. and Crabtree, Robert H., Bioinorganic Chemistry of Manganese Related to Photosynthesis Oxygen Evolution . . . . . . . . . . . . . Bruhn, Suzanne L., Toney, Jeffrey H., and Lippard, Stephen J., Biological Processing of DNA Modified by Platinum Compounds . . . . . . . . . . . . . . . Brusten, Bruce E. and Green, Michael, R., Ligand Additivity in the Vibrational Spectroscopy, Electrochemistry, and Photoelectron Spectroscopy of Metal Carbonyl Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Burgmayer, Sharon J. Nieter, Dithiolenes in Biology . . . . . . . . . . . . . . . . . . Busch, Daryle H., see Meade, Thomas J. Canary, James W. and Gibb, Bruce C., Selective Recognition of Organic Molecules by Metallohosts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Caneschi, A., Gatteschi, D., and Rey, P., The Chemistry and Magnetic Properties of Metal Nitronyl Nitroxide Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . Cannon, Roderick D., White, Ross P., Chemical and Physical Properties of Triangular Bridged Metal Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . Carlson, K. Douglas, see Williams, Jack M. Carty, A., see Tuck, D. G. Carty, Arthur J., see Sappa, Enrico

VOL.

PAGE

37

143

21

29

52 51

55 251

44

1

38

259

2

303

45

179

36

1

28

1

18

177

27

1

13

159

37

99

38

477

36 52

393 491

45

1

39

331

36

195

CUMULATIVE INDEX, VOLUMES 1–52

Cassoux, Patrick, see Faulmann, Christophe Castellano, Felix N. and Meyer, Gerald J., Light-Induced Processes in Molecular Gel Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Catlow, C. R. A., see Thomas, J. M. Cattalini, L., The Intimate Mechanism of Replacement in d5 Square-Planar Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chaffee, Eleanor and Edwards, John O., Replacement as a Prerequisite to Redox Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chakravorty, A., see Holm, R. H. Chang, Hsuan-Chen, see Lagow, Richard J. Chapelle, Stella, see Verche`re, Jean-Franc¸ois Chaudhuri, Phalguni and Wieghardt, Karl, The Chemistry of 1,4,7Triazacyclononane and Related Tridentate Macrocyclic Compounds . . . . . Chaudhuri, Phalguni, and Wieghardt, Karl, Phenoxyl Radical Complexes . . . Chisholm, M. H. and Godleski, S., Applications of Carbon-13 NMR in Inorganic Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chisholm, Malcolm H. and Rothwell, Ian P., Chemical Reactions of Metal– Metal Bonded Compounds of Transition Elements . . . . . . . . . . . . . . . . . . Chock, P. B. and Titus, E. O., Alkali Metal Ions Transport and Biochemical Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chow, S. T. and McAuliffe, C. A., Transition Metal Complexes Containing Tridentate Amino Acids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Churchill, Melvyn R., Transition Metal Complexes of Azulene and Related Ligands Ciurli, A., see Holm, Richard M. Claudio, Elizabeth S., Godwin, Hilary Arnold, and Magyar, John S., Fundamental Coordination Chemistry, Environmental Chemistry and Biochemistry of Lead (II) Clearfield, Abraham, Metal-Phosphonate Chemistry . . . . . . . . . . . . . . . . . . . Codd, Rachel, see Levina, Aviva Constable, Edwin C., Higher Oligopyridines as a Structural Motif in Metal-Iosupramolecular Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Corbett, John D., Homopolyatomic Ions of the Post-Transition ElementsSynthesis, Structure, and Bonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cotton, F. A., Metal Carbonyls: Some New Observations in an Old Field . . . Cotton, F. A., see Wilkinson, G. Cotton F. A. and Hong, Bo, Polydentate Phosphines: Their Syntheses, Structural Aspects, and Selected Applicators . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cotton, F. A. and Lukehart, C. M., Transition Metall Complexes Containing Carbonoid Ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Coucouvanis, Dimitri, see Malinak, Steven M. Coucouvanis, Dimitri, The Chemistry of the Dithioacid and 1,1-Dithiolate Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Coucouvanis, Dimitri, The Chemistry of the Dithioacid and 1,1-Dithiolate Complexes, 1968–1977 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cowley, Alan H., UV Photoelectron Spectroscopy in Transition Metal Chemistry Cowley, Alan H. and Norman, Nicholas C., The Synthesis, Properties, and Reactivities of Stable Compounds Featuring Double Bonding Between Heavier Group 14 and 15 Elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

725 VOL.

PAGE

44

167

13

263

13

205

35 50

329 151

20

299

29

1

18

287

19 11

51 53

51

1

47

371

42

67

21 21

129 1

40

179

16

487

11

233

26 26

301 45

34

1

726

CUMULATIVE INDEX, VOLUMES 1–52 VOL.

Crabtree, Robert H., see Brudvig, Gary W. Cramer, Stephen P. and Hodgson, Keith O., X-Ray Absorption Spectroscopy: A New Structural Method and Its Applications to Bioinorganic Chemistry . . Crans, Debbie C., see Verche`re, Jean-Franc¸ois Creutz, Carol, Mixed Valence Complexes of d5–d6 Metal Centers . . . . . . . . . Cummings, Scott D., Luminescence and Photochemistry of Metal Dithiolene Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cummins, Christopher C., Three-Coordinate Complexes of ‘‘Hard’’ Ligands: Advances in Synthesis, Structure and Reactivity . . . . . . . . . . . . . . . . . . . . Cunningham, B. B., see Asprey, L. B.

PAGE

25

1

30

1

52

315

47

685

Dance, Ian and Fisher, Keith, Metal Chalcogenide Cluster Chemistry . . . . . . Darensbourg, Marcetta York, Ion Pairing Effects on Metal Carbonyl Anions . Daub, G. William, Oxidatively Induced Cleavage of Transition Metal-Carbon Bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dean, P. A. W., The Coordination Chemistry of the Mercuric Halides . . . . . . DeArmond, M. Keith and Fried, Glenn, Langmuir-Blodgett Films of Transition Metal Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dechter, James J., NMR of Metal Nuclides, Part I: The Main Group Metals . Dechter, James J., NMR of Metal Nuclides, Part II: The Transition Metals . . De Los Rios, Issac, see Peruzzini, Maurizio Deutsch, Edward, Libson, Karen, Jurisson, Silvia, and Lindoy, Leonard F., Technetium Chemistry and Technetium Radiopharmaceuticals . . . . . . . . . . Diamond, R. M. and Tuck, D. G., Extraction of Inorganic Compounds into Organic Solvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . DiBenedetto, John, see Ford, Peter C. Dillon, Carolyn T., see Levina, Aviva Doedens, Robert J., Structure and Metal-Metal Interactions in Copper (II) Carboxylate Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Donaldson, J. D., The Chemistry of Bivalent Tin . . . . . . . . . . . . . . . . . . . . . Donini, J. C., Hollebone, B. R., and Lever, A. B. P., The Derivation and Application of Normalized Spherical Harmonic Hamiltonians . . . . . . . . . . Dori, Zvi, The Coordination Chemistry of Tungsten . . . . . . . . . . . . . . . . . . . Doyle, Michael P. and Ren, Tong, The Influence of Ligands on Dirhodium (II) on Reactivity and Selectivity in Metal Carbene Reactions . . . . . . . . . . . . . Drago, R. S. and Purcell, D. F., The Coordination Model for Non-Aqueous Solvent Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Drew, Michael G. B., Seven-Coordination Chemistry . . . . . . . . . . . . . . . . . . Dunbar, Kim R. and Heintz, Robert A., Chemistry of Transition Metal Cyanide Compounds: Modern Perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Dutta, Prabir K. and Ledney, Michael, Charge-Transfer Processes in Zeolites: Toward Better Artificial Photosynthetic Models . . . . . . . . . . . . . . . . . . . . Dye, James L., Electrides, Negatively Charged Metal Ions, and Related Phenomena . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

41 33

637 221

22 24

375 109

44 29 33

97 285 393

30

75

2

109

21 8

209 287

22 28

225 239

49

113

6 23

271 67

45

283

44

209

32

327

Earley, Joseph E., Nonbridging Ligands in Electron-Transfer Reactions . . . . Edwards, John O. and Plumb, Robert C., The Chemistry of Peroxonitrites . . . Edwards, John O., see Chaffee, Eleanor Eichorn, Bryan W., Ternary Transition Metal Sulfides . . . . . . . . . . . . . . . . . . Eisenberg, Richard, see Cummings, Scott D.

13 41

243 599

42

139

CUMULATIVE INDEX, VOLUMES 1–52

727 VOL.

PAGE

Eisenberg, Richard, Structural Systematics of 1,1- and 1,2-Dithiolate Chelates Eller, P. G., see Bertand, J. A. Emge, Thomas J., see Williams, Jack M. Endicott, John F., Kumar, Krishan, Ramasami, T., and Rotzinger, Franc¸ ois P., Structural and Photochemical Probes of Electron Transfer Reactivity . . . . Epstein, Arthur J., see Miller, Joel S. Espenson, James H., Homolytic and Free Radical Pathways in the Reactions of Organochromium Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Evans, David A., see Rovis, Tomislav Everett, G. W., see Holm. R. H.

12

295

30

141

30

189

Fackler, John P., Jr., Metal B-Ketoenolate Complexes . . . . . . . . . . . . . . . . . . Fackler, John P., Jr., Multinuclear d5–d10 Metal Ion Complexes with SulfurContaining Ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Faulmann, Christophe, Solid-State Properties (Electronic, Magnetic, Optical) of Dithiolene Complex-Based Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . Favas, M. C. and Kepert, D. L., Aspects of the Stereochemistry of FourCoordination and Five-Coordination . . . . . . . . . . . . . . . . . . . . . . . . . . . . Favas, M. C. and Kepert, D. L., Aspects of the Stereochemistry of NineCoordination, Ten-Coordination, and Twelve-Coordination . . . . . . . . . . . . Feldman, Jerald and Schrock, Richard R., Recent Advances in the Chemistry of ‘‘d0’’ Alkylidene and Metallacyclobutane Complexes . . . . . . . . . . . . . . . . Felthouse, Timothy R., The Chemistry, Structure, and Metal-Metal Bonding in Compounds of Rhodium (II) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fenske, Richard F., Molecular Orbital Theory, Chemical Bonding, and Photoelectron Spectroscopy for Transition Metal Complexes . . . . . . . . . . . Ferguson, J., Spectroscopy of 3d Complexes . . . . . . . . . . . . . . . . . . . . . . . . . Ferguson, James, see Krausz, Elmars Figgis, B. N. and Lewis, J., The Magnetic Properties of Transition Metal Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Finn, Robert C., Haushalter, Robert C., and Zubieta, Jon, Crystal Chemistry of Organically Templated Vanadium Phosphates and Organophosphonates . . Fisher, Keith, see Dance, Ian Fisher, Keith J., Gas-Phase Coordination Chemistry of Transition Metal Ions Fleischauer, P. D., Adamson, A. W., and Sartori G., Excited States of Metal Complexes and Their Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Floriani, Carlo, see Piarulli, Umberto Ford, Peter C., Wink, David, and DiBenedetto, John. Mechanistic Aspects of the Photosubstitution and Photoisomerization Reactions of d6 Metal Complexes Fowles, G. W. A., Reaction by Metal Hallides with Ammonia and Aliphatic Amines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fratiello, A., Nuclear Magnetic Resonance Cation Solvation Studies . . . . . . . Frenking, Gernot, see Lupinetti, Anthony J. Fried, Glenn, see DeArmond, M. Keith Friedman, H. L., see Hunt, J. P. Fu, Lei, see Mody, Tarak D.

7

361

21

55

52

399

27

325

28

309

39

1

29

73

21 12

179 159

6

37

51

421

50

343

17

1

30

213

6 17

1 57

33

275

Garner, C. David, see McMaster, Jonathan Gatteschi, D., see Caneschi, A. Geiger, William E., Structural Changes Accompanying Metal Complex Electrode Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

728

CUMULATIVE INDEX, VOLUMES 1–52

Geiser, Urs, see Williams, Jack M. Geoffroy, George, L., Photochemistry of Transition Metal Hydride Complexes George, J. W., Halides and Oxyhalides of the Elements of Groups Vb and VIb George, Philip and McClure, Donald S., The Effect of Inner Orbital Splitting on the Thermodynamic Properties of Transition Metal Compounds, and Coordination Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gerfin, T., Gra¨ tzel, M., and Walder, L., Molecular and Supramolecular Surface Modification of Nanocrystalline TiO2 Films: Charge-Separating and Charge-Injecting Devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gerloch, M., A Local View in Magnetochemistry . . . . . . . . . . . . . . . . . . . . . Gerloch, M. and Miller, J. R., Covalence and the Orbital Reduction . . . . . . . Gerloch, Malcolm, see Bridgeman, Adam J. Gerloch, Malcolm and Woolley, R. Guy, The Functional Group in Ligand Field Studies: The Empirical and Theoretical Status of the Angular Overlap Model Gibb, Bruce C., see Canary, James W. Gibb, Thomas, R. P., Jr., Primary Solid Hydrides . . . . . . . . . . . . . . . . . . . . . Gilbertson, Scott R., Combinatorial-Parallel Approaches to Catalyst Discovery and Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gibney, Brian, R., see Pecoraro, Vincent L. Gillard, R. C., The Cotton Effect in Coordination Compounds . . . . . . . . . . . Gillespie, Ronald J., see Sawyer, Jeffery F. Glasel, Jay A., Lanthanide Ions as Nuclear Magnetic Resonance Chemical Shift Probes in Biological Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Glick, Milton D. and Lintvedt, Richard L., Structural and Magnetic Studies of Polynuclear Transition Metal b-Polyketonates . . . . . . . . . . . . . . . . . . . . . Godleski, S., see Chisholm, M. H. Godwin, Hilary Arnold, see Claudio, Elizabeth S. Gordon, Gilbert, The Chemistry of Chlorine Dioxide . . . . . . . . . . . . . . . . . . Gratzel, M., see Gerfin, T. Gray, Harry B., see Bowler, Bruce E. Green, Malcom L. H., see Brookhart, Maurice Green, Michael R., see Burstein, Bruce E. Grove, David M., see Janssen, Maurits D. Grubbs, Robert H., The Olefin Metathesis Reaction . . . . . . . . . . . . . . . . . . . Gruen, D. M., Electronic Spectroscopy of High Temperature Open-Shell Polyatomic Molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gultneh, Yilma, see Karlin, Kenneth D. Hahn, James E., Transition Metal Complexes Containing Bridging Alkylidene Ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Haight, G. P., Jr., see Beattie, J. K. Haim, Albert. Mechanisms of Electron Transfer Reactions: The Bridged Activated Complex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hall, Kevin P. and Mingos, D. Michael P., Homo- and Heteronuclear Cluster Compounds of Gold . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hall, Tracy H., High Pressure Inorganic Chemistry Hancock, Robert D., Molecular Mechanics Calculations as a Tool in Coordination Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Haushalter, Robert C., see Finn, Robert C.

VOL.

PAGE

27 2

123 33

1

381

44 26 10

345 1 1

31

371

3

315

50

433

7

215

18

383

21

233

15

201

24

1

14

119

31

205

30

273

32

237

37

187

CUMULATIVE INDEX, VOLUMES 1–52

Hayaishi, Osamu, Takikawa, Osamu, and Yoshida, Ryotaro, Indoleamine 2,3-Dioxygenase, Properties and Functions of a Superoxide Utilizing Enzyme Heintz, Robert A., see Dunbar, Kim R. Helton, Matthew E., see Kirk, Martin L. Hendry, Philip, and Sargeson, Alan M., Metal Ion Promoted Reactions of Phosphate Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hennig, Gerhart R., Interstitial Compounds of Graphite . . . . . . . . . . . . . . . . Henrick, Kim, Tasker, Peter A., and Lindoy, Leonard F., The Specification of Bonding Cavities in Macrocyclic Ligands . . . . . . . . . . . . . . . . . . . . . . . . . Herbert, Rolle H., Chemical Applications of Mo¨ ssbauer Spectroscopy . . . . . . Heumann, Andreas, Jens, Klaus-Joachim, and Re´ glier, Marius, Palladium Complex Catalyzed Oxidation Reactions . . . . . . . . . . . . . . . . . . . . . . . . . Hobbs, R. J. M., see Hush, N. S. Hodgson, D. J., The Structural and Magnetic Properties of First-Row Transition Metal Dimers Containing Hydroxo, Substituted Hydroxo, and Halogen Bridges . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hodgson, Derek J., The Stereochemistry of Metal Complexes of Nucleic Acid Constituents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hodgson, Keith O., see Cramer, Stephen P. Hoff, Carl, D., Thermodynamics of Ligand Binding and Exchange in Organometallic Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hoffman, Brian E., see Michel, Sarah L. J. Hollebone, B. R., see Domini, J. C. Holloway, John H., Reactions of the Noble Gases . . . . . . . . . . . . . . . . . . . . Holm, R. H., Everett, G. W., and Chakravorty, A., Metal Complexes of Schiff Bases and B-Ketoamines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Holm, R. H. and O’Connor, M. J., The Stereochemistry of Bis-Chelate Metal (II) Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Holm, Richard M., Ciurli, Stefano, and Weigel, John A., Subsite-Specific Structures and Reactions in Native and Synthetic (4Fe-4-S) Cubane-Type Clusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Holmes, Robert R., Five-Coordinated Structures . . . . . . . . . . . . . . . . . . . . . Hong, Bo, see Cotton, F. A. Hope, Hakon, X-Ray Crystallography: A Fast, First-Resort Analytical Tool . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Horrocks, William DeW., Jr. and Albin, Michael, Lanthanide Ion Luminescence in Coordination Chemistry and Biochemistry . . . . . . . . . . . . . . . . . . . . . . Horva´ th, Istva´ n T., see Adams, Richard D. Humphries, A. P. and Kaesz, H. D., The Hydrido-Transition Metal Cluster Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hunt, J. P. and Friedman, H. L., Aquo Complexes of Metal Ions . . . . . . . . . . Hush, N. S., Intervalence Transfer Absorption Part 2. Theoretical Considerations and Spectroscopic Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hush, N. S., see Allen, G. C. Hush, N. S. and Hobbs, R. J. M., Absorption-Spectra of Crystals Containing Transition Metal Ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Isied, Stephan S., Long-Range Electron Transfer in Peptides and Proteins . . . Isied, Stephan S., see Kuehn, Christa

729 VOL.

PAGE

38

75

38 1

201 125

33 8

1 1

42

483

19

173

23

211

40

503

6

241

7

83

14

241

38 32

1 119

41

1

31

1

25 30

145 359

8

391

10

259

32

443

730

CUMULATIVE INDEX, VOLUMES 1–52

Jagirdar, Balaji R., Organometallic Fluorides of the Main Group Metals Containing the C-M-F Fragment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . James, B. D. and Wallbridge, M. G. H., Metal Tetrahydroborates . . . . . . . . . James, David W., Spectroscopic Studies of Ion-Ion Solvent Interaction in Solutions Containing Oxyanions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . James, David W. and Nolan, M. J., Vibrational Spectra of Transition Metal Complexes and the Nature of the Metal-Ligand Bond . . . . . . . . . . . . . . . . Janssen, Maurits D., Grove, David M., and Koten, Gerard van, Copper(I) Lithium and Magnesium Thiolate Complexes: An Overview with Due Mention of Selenolate and Tellurolate Analogues and Related Silver(I) and Gold(I) Species . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Jardine, F. H., The Chemical and Catalytic Reactions of Dichlorotris(triphenylphosphine(II) and Its Major Derivatives . . . . . . . . . . Jardine, F. H., Chlrotris(triphenylphosphine)rhodium(I): Its Chemical and Catalytic Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Jeffrey, G. A. and McMullan, R. K., The Clathrate Hydrates . . . . . . . . . . . . Jens, Klaus-Joachim, see Heumann, Andreas Johnson, B. F. G. and McCleverty, J. A., Nitric Oxide Compounds of Transition Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Johnson, Michael K., Vibrational Spectra of Dithiolene Complexes . . . . . . . . Jolly, William L., Metal-Ammonia Solution . . . . . . . . . . . . . . . . . . . . . . . . . Jones, Peter, see Brown, S. B. Jorgensen, Chr., Klixbull, Electron Transfer Spectra . . . . . . . . . . . . . . . . . . . Jorgensen, Chr., Klixbull, The Nephelauxetic Series . . . . . . . . . . . . . . . . . . . Jurisson, Silvia, see Deutsch, Edward Kadish, Karl M., The Electrochemistry of Metalloporphyrins in Nonaqueous Media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kaesz, H. D., see Humphries, A. P. Kahn, M. Ishaque and Zubieta, Jon, Oxovanadium and Oxomolybdenum Clusters and Solids Incorporating Oxygen-Donor Ligands . . . . . . . . . . . . Kamat, Prashant V., Native and Surface Modified Semiconductor Nanoclusters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kampf, Jeff W., see Pecoraro, Vincent L. Kanatzidis, Mercouri G. and Sutorik, Anthony C., The Application of Polychalcogenide Salts to the Exploratory Synthesis of Solid-State Multinary Chalogenides at Intermediate Temperatures . . . . . . . . . . . . . . . Karlin, Kenneth D. and Gultneh, Yilma, Binding and Activation of Molecular Oxygen by Copper Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kennedy, John D., The Polyhedral Metallaboranes, Part I: Metallaborane Clusters with Seven Vertices and Fewer . . . . . . . . . . . . . . . . . . . . . . . . . . Kennedy, John D., The Polyhedral Metallaboranes, Part II: Metallaborane Clusters with Eight Vertices and More . . . . . . . . . . . . . . . . . . . . . . . . . . . Kepert, D. L., Aspects of the Stereochemistry of Eight-Coordination . . . . . . . Kepert, D. L., Aspects of the Stereochemistry of Seven-Coordination . . . . . . . Kepert, D. L., Aspects of the Stereochemistry of Six-Coordination . . . . . . . . . Kepert, D. L., Isopolytungstates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kepert, D. L., see Favas, M. C. Kesselman, Janet M., see Tan, Ming X.

VOL.

PAGE

48 11

351 99

33

353

9

195

46

97

31

265

28 8

63 43

7 52 1

277 213 235

12 4

101 73

34

435

43

1

44

273

43

151

35

219

32

519

34 24 25 23 4

211 179 41 1 199

CUMULATIVE INDEX, VOLUMES 1–52

Kice, J. L., Nucleophilic Substitution at Different Oxidation-States of Sulfur Kimura, Eiichi, Macrocylic Polyamine Zinc(II) Complexes as Advanced Models for Zinc(II) Enzymes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . King, R. B., Transition Metal Cluster Compounds . . . . . . . . . . . . . . . . . . . . Kingsborough, Richard P., Transition Metals in Polymeric p-Conjugated Organic Frameworks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kirk, Martin L., The Electronic Structure and Spectroscopy of MetalloDithiolene Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kitagawa, Teizo and Ogura, Takashi, Oxygen Activation Mechanism at the Binuclear Site of Heme-Copper Oxidase Superfamily as Revealed by Time-Resolved Resonance Raman Spectroscopy . . . . . . . . . . . . . . . . . . . . Klingler, R. J. and Rathke, J. W., Homogeneous Catalytic Hydrogenation of Carbon Monoxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kloster, Grant M., see Watton, Stephen P. Kolodziej, Andrew F., The Chemistry of Nickel-Containing Enzymes . . . . . . . Konig, Edgar. Structural Changes Accompanying Continuous and Discontinuous Spin-State Transitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . Koten, Gerard van, see Janssen, Maurits D. Kramarz, K. W. and Norton, J. R., Slow Proton-Transfer Reactions in Organometallic and Bioinorganic Chemistry . . . . . . . . . . . . . . . . . . . . . . Krausz, Elmars and Ferguson, James. The Spectroscopy of the [Ru(bpy)3]2þ System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kubas, Gregory J., see Vergamini, Philip J. Kuehn, Christa and Isied, Stephan S., Some Aspects of the Reactivity of Metal Ion-Sulfur Bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kumar, Krishan, see Endicott, John F. Kustin, Kenneth and Swinehart, James, Fast Metal Complex Reactions . . . . . Laane, Jaan and Ohlsen, James R., Characterization of Nitrogen Oxides by Vibrational Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lagow, Richard J. and Margrave, John L., Direct Fluorination: A ‘‘New’’ Approach to Fluorine Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lagow, Richard J., and Chang, Hsuan-Chen, High-Performance Pure Calcium Phosphate Bioceramics: The First Weight Bearing Completely Resorbable Synthetic Bone Replacement Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . Laibinis, Paul E., see Tan, Ming, X. La Monica, Girolamo, The Role of the Pyrazolate Ligand in Building Polynuclear Transition Metal Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . Lange, Christopher W., see Pierpont, Cortlandt G. Laudise, R. A., Hydrothermal Synthesis of Single Crystals . . . . . . . . . . . . . . Laure, B. L. and Schmulbach, C. D., Inorganic Electrosynthesis in Nonaqueous Solvents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lay, Peter A., see Levina, Aviva Ledney, Michael, see Dutta, Prabir K. Le Floch, Pascal, see Mezaillies, Nicolas Lentz, Dieter, see Seppelt, Konrad Leung, Peter C. W., see Williams, Jack M. Lever, A. B. P., see Donini, J. C. Levina, Aviva, Codd, Rachel, Dillon, Carolyn T., and Lay, Peter A., Chromium in Biology: Toxicology and Nutritional Aspects . . . . . . . . . . . . . . . . . . . . . .

731 VOL.

PAGE

17

147

41 15

443 287

48

123

52

111

45

431

39

113

41

493

35

527

42

1

37

293

27

153

13

107

27

465

26

161

50

317

46

151

3

1

14

65

51

145

732

CUMULATIVE INDEX, VOLUMES 1–52 VOL.

Lewis, J., see Figgis, B. N. Lewis, Nathan S., see Tan, Ming, X. Libson, Karen, see Deutsch, Edward Lieber, Charles M., see Wu, Xian Liang Liehr, Andrew D., The Coupling of Vibrational and Electronic Motions in Degenerate Electronic States of Inorganic Complexes. Part I. States of Double Degeneracy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Liehr, Andrew D., The Coupling of Vibrational and Electronic Motions in Degenerate Electronic States of Inorganic Complexes. Part II. States of Triple Degeneracy and Systems of Lower Symmetry . . . . . . . . . . . . . . . . . Liehr, Andrew D., The Coupling of Vibrational and Electronic Motions in Degenerate and Nondegenerate Electronic States of Inorganic and Organic Molecules. Part III. Nondegenerate Electronic States . . . . . . . . . . . . . . . . Lindoy, Leonard F., see Deutsch, Edward Lindoy, Leonard F., see Henrick, Kim Lintvedt, Richard L., see Glick, Milton D. Lippard, Stephen J., see Bruhn, Suzanne L. Lippard, Stephen J., Eight-Coordination Chemistry . . . . . . . . . . . . . . . . . . . Lippard, Stephen J., Seven and Eight Coordinate Molybdenum Complexes and Related Molybdenum (IV) Oxo Complexes, with Cyanide and Isocyanide Ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lippen, Bernhard, Platinum Nucleobase Chemistry . . . . . . . . . . . . . . . . . . . Lobana, Tarlok, S., Structure and Bonding of Metal Complexes of Tertiaryphosphine-Arsine Chalcogenides Including Analytical, Catalytic, and Other Applications of the Complexes . . . . . . . . . . . . . . . . . . . . . . . . . Lockyer, Trevor N. and Manin, Raymond L., Dithiolium Salts and Dithio-bdiketone Complexes of the Transition Metals . . . . . . . . . . . . . . . . . . . . . . Long, K. H., Recent Studies of Diborane . . . . . . . . . . . . . . . . . . . . . . . . . . . Lorand, J. P., The Cage Effect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Lukehart, C. M., see Cotton, F. A. Lupinetti, Anthony J., Strauss, Steven H., and Frenking, Gernot, Nonclassical Metal Carbonyl . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . McAuliffe, C. A., see Chow, S. T. McCleverty, J. A., Metal 1,2-Dithiolene and Related Complexes . . . . . . . . . . McCleverty, J. A., see Johnson, B. F. G. McClure, Donald S., see George, Philip MacDonnell, Frederick M., see Wright, Jeffrey G. McMaster, Jonathan, Chemical Analogues of the Catalytic Centers of Molybdenum and Tungsten Dithiolene-Containing Enzymes . . . . . . . . . . . . McMullan, R. K., see Jeffrey, G. A. McNaughton, Rebecca L., see Kirk, Martin L. Magyar, John S., see Claudia, Elizabeth S. Maier, L., Preparation and Properties of Primary, Secondary and Tertiary Phosphines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Malatesta, Lamberto, Isocyanide Complexes of Metals . . . . . . . . . . . . . . . . . Malinak, Steven M. and Coucouvanis, Dimitri, The Chemistry of Synthetic Fe-Mo-S Clusters and Their Relevance to the Structure and Function of the Fe-Mo-S Center Nitrogenase . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

PAGE

3

281

4

455

5

385

8

109

21 37

91 1

37

495

27 15 17

223 1 207

49

1

10

49

52

539

5 1

27 283

49

599

CUMULATIVE INDEX, VOLUMES 1–52

Manoharan, P. T., see Venkatesh, B. Margrave, John L., see Lagow, Richard J. Marks, Tobin J., Chemistry and Spectroscopy of f-Element Organometallics Part I: The Lanthanides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Marks, Tobin J., Chemistry and Spectroscopy of f-Element Organometallics Part II: The Actinides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Martin, Raymond L., see Lockyer, Trevor N. Marzilli, Lulgi G., Metal-ion Interactions with Nucleic Acids and Nucleic Acid Derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Marzilli, Luigi G., see Toscano, Paul J. Mathey, Francosis, see Mezaillies, Nicolas Mayr, Andreas and Bastos, Cecilia M., Coupling Reactions of Terminal TwoFaced p Ligands and Related Cleavage Reaction . . . . . . . . . . . . . . . . . . . McKee, Vickie, see Nelson, Jane Meade, Thomas J. and Busch, Daryle H., Inclusion Complexes of Molecular Transition Metal Hosts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mehrotra, Ram C. and Singh, Anirudh, Recent Trends in Metal Alkoxide Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Meyer, Gerald J., see Castellano, Felix N. Meyer, Thomas J., Excited-State Electron Transfer . . . . . . . . . . . . . . . . . . . . Meyer, T. J., Oxidation-Reduction and Related Reactions of Metal-Metal Bonds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Me´ zaillies, Nicolas, Mathey, Francois, and Le Floch, Pascal, The Coordination Chemistry of Phosphinines: Their Polydentate and Macrocyclic Derivatives Michel, Sarah L. J., Hoffman, Brian M., Baum, Sven M., and Barrett, Anthony G. M., Peripherally Functionalized Porphyrazines: Novel Metallomacrocycles with Broad Untapped Potential . . . . . . . . . . . . . . . . . . . . . . . Miller, J. R., see Gerloch, M. Miller, Joel S. and Epstein, Anhur, J., One-Dimensional Inorganic Complexes Mingos, D. Michael P., see Hall, Kevin P. Mirkin, Chad A., see Slone, Caroline S. Mitra, S., Chemical Applications of Magnetic Anisotropy Studies on Transition Metal Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mitzi, David B., Synthesis, Structure and Properties of Organic-Inorganic Perovskites and Related Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mody, Tarak D., Fu, Lei, and Sessler, Jonathan L., Texaphyrins: Synthesis and Development of a Novel Class of Therapeutic Agents . . . . . . . . . . . . . . . . Morgan, Grace, see Nelson, Jane Muetterties, E. L., see Tachikawa, Mamoru Murphy, Eamonn F., see Jugirdar, Balayi R. Natan, Michael J., see Wright, Jeffrey G. Natan, Michael J. and Wrighton, Mark S., Chemically Modified Microelectrode Arrays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nelson, Jane, McKee, V. and Morgan, G. Coordination Chemistry of Azacryptands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Neumann, Ronny, Polyoxometallate Complexes in Organic Oxidation Chemistry Nguyen, Sonbinh T., see Tan, Ming X. Nolan, M. J., see James, David W.

733 VOL.

PAGE

24

51

25

223

23

225

40

1

33

59

46

239

30

389

19

1

49

455

50

473

20

1

22

309

48

1

49

551

37

391

47 47

167 317

734

CUMULATIVE INDEX, VOLUMES 1–52 VOL.

PAGE

36

299

29

203

10

223

14 19

173 105

40 15 47

445 101 1

48 2

457 193

45

83

20

229

49

169

17

327

45

393

41

331

52

369

39

181

39

75

Norman, Nicholas, C., see Cowley, Alan H. Norton, J. R., see Kramarz, K. W. Oakley, Richard T., Cyclic and Heterocyclic Thiazines . . . . . . . . . . . . . . . . . O’Connor, Charles J., Magnetochemistry—Advances in Theory and Experimentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . O’Connor, M. J., see Holm, R. H. Ogura, Takashi, see Kitagawa, Teizo O’Halloran, Thomas V., see Wright, Jeffrey G. Ohlsen, James R., see Laane, Jaan Oldham, C., Complexes of Simple Carboxylic Acids . . . . . . . . . . . . . . . . . . . Orrell, Keith, G., see Abel, Edward W. Ozin, G. A., Single Crystal and Cas Phase Raman Spectroscopy in Inorganic Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Ozin, G. A. and Vande`r Voet, A., Cryogenic Inorganic Chemistry . . . . . . . . . Pandey, Krishna K., Coordination Chemistry of Thionitrosyl (NS), Thiazate (NSO), Disulfidothionitrate (S3N), Sulfur Monoxide (SO), and Disulfur Monoxide (S2O) Ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Parish, R. V., The Interpretation of 119 Sn-Mo¨ ssbauer Spectra . . . . . . . . . . . Parkin, General, Terminal Chalcogenido Complexes of the Transition Metals Paul, Purtha P., Coordination Complex Impregnated Molecular Sieves-Synthesis, Characterization, Reactivity and Catalysis . . . . . . . . . . . . . . . . . . . . . . . . Peacock, R. D., Some Fluorine Compounds of the Transition Metals . . . . . . . Pearson, Ralph G., see Basolo, Fred Pecoraro, Vincent L., Stemmler, Ann J., Gibney, Brian R., Bodwin, Jeffrey J., Wang, Hsin, Kampf, Jeff W., and Barwinski, Almut, Metallacrowns: A New Class of Molecular Recognition Agents . . . . . . . . . . . . . . . . . . . . . . . . . . Perlmutter-Hayman, Berta. The Temperature-Dependence of the Apparent Energy of Activation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Peruzzini, Maurizio, De Los Rios, Issac, and Romerosa, Antonio, Coordination Chemistry of Transition Metals and Hydrogen Chalogenide and Hydrochalcogenido Ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pethybridge, A. D. and Prue, J. E., Kinetic Salt Effects and the Specific Influence of Ions on Rate Constants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Piarulli, Umberto and Floriani, Carlo, Assembling Sugars and Metals: Novel Architectures and Reactivities in Transition Metal Chemistry . . . . . . . . . . Pierpont, Conlandt G. and Lange, Christopher W., The Chemistry of Transition Metal Complexes Containing Catechol and Semiquinone Ligands . . . . . . . Pilato, Robert S., Metal Dithiolene Complexes in Detection: Past, Present, and Future. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Plieth, K., see Becker, K. A. Plumb, Robert C., see Edwards, John O. Pope, Michael T., Molybdenum Oxygen Chemistry: Oxides, Oxo Complexes, and Polyoxoanions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Power, Philip P., The Structures of Organocuprates and Heteroorganocuprates and Related Species in Solution in the Solid State . . . . . . . . . . . . . . . . . . Prue, J. E., see Pethybridge, A. D. Purcell, D. F., see Drago, R. S.

CUMULATIVE INDEX, VOLUMES 1–52

735 VOL.

PAGE

Pyle, Anna Marie and Banon, Jacqueline K. Banon, Probing Nuclei Acids with Transition Metal Complexes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

38

413

Que, Lawrence, Jr., and True, Anne E., Dinuclear Iron- and Manganese-Oxo Sites in Biology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

38

97

39 52

259 1

12

1

43

533

50

1

35

437

34

65

4

275

3 52 29

49 585 167

17

391

3

129

Ralston, Diana M., see Wright, Jeffrey G. Ramasami, T., see Endicott, John F. Raphael, Adrienne L., see Bowler, Bruce E. Rathke, J. W., see Klingler, R. J. Rauchfuss, Thomas B., The Coordination Chemistry of Thiophenes . . . . . . . Rauchfuss, Thomas B., Synthesis of Transition Metal Dithiolenes . . . . . . . . . Re´ glier, Marius, see Heumann, Andreas Ren, Tong, see Doyle, Michael P. Rey, P. see Caneschi, A. Reynolds, Warren L., Dimethyl Sulfoxide in Inorganic Chemistry . . . . . . . . . Rifkind, J. M., see Venkatesh, B. Roesky, Herbert W., see Jagirdar, Balaji R. Roesky, Herbert W., see Witt, Michael Romerosa, Antonio, see Peruzzini, Maurizio Rothwell, Ian P. see Chisholm, Malcolm H. Rotzinger, Francois P., see Endicott, John F. Roundhill, D. Max. Metal Complexes of Calixarenes . . . . . . . . . . . . . . . . . . Rovis, Tomislav, and Evans, David A., Structural and Mechanistic Investigations in Asymmetric Copper(I) and Copper(II) Catalyzed Reactions . . . . . . . . . . Sappa, Enrico, Tiripicchio, Antonio, Carty, Anhur J., and Toogood, Gerald E., Butterfly Cluster Complexes of the Group VIII Transition Metals . . . . . . . . Sargeson, Alan M., see Hendry, Philip Sanon, G., see Fleischauer, P. D. Sawyer, Donald T., see Sobkowiak, Andrzej Sawyer, Jeffery F., and Gillespie, Ronald J., The Stereochemistry of SB(III) Halides and Some Related Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . Scandola, F., see Bignozzi, C. A. Schatz, P. N., see Wong, K. Y. Schmulbach, C. D., Phosphonitrile Polymers Schmulbach, C. D., see Laure, B. L. Schoonover, J. R., see Bignozzi, C. A. Schrock, Richard R., see Feldman, Jerald Schulman, Joshua M., see Beswick, Colin L. Schultz, Arthur J., see Williams, Jack M. Searcy, Alan W., High-Temperature Inorganic Chemistry . . . . . . . . . . . . . . . Sellmann, Dieter, Dithiolenes in More Complex Ligends . . . . . . . . . . . . . . . . Seppelt, Konrad and Lentz, Dieter, Novel Developments in Noble Gas Chemistry Serpone, N. and Bickley, D. G., Kinetics and Mechanisms of Isomerization and Racemization Processes of Six-Coordinate Chelate Complexes . . . . . . . . . Sessler, Jonhathan L., see Mody, Tarak D. Seyferth, Dietmar, Vinyl Compounds of Metals . . . . . . . . . . . . . . . . . . . . . . Singh, Anirudh, see Mehrotra, Ram C.

736

CUMULATIVE INDEX, VOLUMES 1–52

Slone, Caroline S., The Transition Metal Coordination Chemistry of Hemilabile Ligands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Smith, David K., see Beer, Paul D. Smith III, Milton R., Advances in Metal Boryl and Metal-Mediated B-X Activation Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sobkowiak, Andrzej, Tung, Hui-Chan, and Sawyer, Donald T., Iron- and CobaltInduced Activation of Hydrogen Peroxide and Dioxygen for the Selective Oxidation-Dehydrogenation and Oxygenation of Organic Molecules . . . . . Spencer, James, T., Chemical Vapor Deposition of Metal-Containing Thin-Film Materials from Organometallic Compounds . . . . . . . . . . . . . . . . . . . . . . . Spiro, Thomas G., Vibrational Spectra and Metal-Metal Bonds . . . . . . . . . . . Stanbury, David M., Oxidation of Hydrazine in Aqueous Solution . . . . . . . . . Stanton, Colby E., see Tan, Ming X. Stemmler, Ann J., see Pecoraro, Vincent L. Stiefel, Edward I., The Coordination and Bioinorganic Chemistry of Molybdenum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stiefel, Edward I., see Beswick, Colin L. Stranski, I. N., see Becker, K. A. Strauss, Steven H., see Lupinetti, Anthony J. Strouse, Charles E., Structural Studies Related to Photosynthesis: A Model for Chlorophyll Aggregates in Photosynthetic Organisms . . . . . . . . . . . . . . . . Stucky, Galen D., The Interface of Nanoscale Inclusion Chemistry . . . . . . . . Suggett, A., see Brown, S. B. Sutin, Norman, Theory of Electron Transfer Reactions: Insights and Hindsights Sutorik, Anthony C., see Kanatzidis, Mercouri G. Sutter, Jo¨ rg, see Sellmann, Dieter Sutton, D., see Addison, C. C. Swager, Timothy M., see Kingsborough, Richard P. Swinehart, James, see Kustin, Kenneth Sykes, A. G. and Weil, J. A., The Formation, Structure, and Reactions of Binuclear Complexes of Cobalt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tachikawa, Mamoru and Muetterties, E. L., Metal Carbide Clusters . . . . . . . Takikawa, Osamu, see Hayaishi, Osamu Tan, Ming X., Laibinis, Paul E., Nguyen, Sonbinh T., Kesselman, Janet M., Stanton, Colby E., and Lewis, Nathan S., Principles and Applications of Semiconductor Photochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tasker, Peter A., see Henrick, Kim Taube, Henry, Interaction of Dioxygen Species and Metal Ions—Equilibrium Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Taylor, Colleen M., see Watton, Stephen P. Templeton, Joseph L., Metal-Metal Bonds of Order Four . . . . . . . . . . . . . . . Tenne, R., Inorganic Nanoclusters with Fullerene-Like Structure and Nanotubes Thomas, J. M. and Callow, C. R. A., New Light on the Structures of Aluminosilicate Catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thorn, Robert J., see Williams, Jack M. Tiripicchio, Antonio, see Sappa, Enrico Titus, E. O., see Chock, P. B. Tofield, B. C., The Study of Electron Distributions in Inorganic Solids: A Survey of Techniques and Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

VOL.

PAGE

48

233

48

505

40

291

41 11 47

145 1 511

22

1

21 40

159 99

30

441

13

1

28

203

41

21

34

607

26 50

211 269

35

1

20

153

CUMULATIVE INDEX, VOLUMES 1–52

Tolman, William B., see Kitajima, Nobumasa Toney, Jeffrey, H., see Bruhn, Suzanne L. Toogood, Gerald E., see Sappa, Enrico Toscano, Paul J. and Marzilli, Luigi G., B12 and Related Organocobalt Chemistry: Formation and Cleavage of Cobalt Carbon Bonds . . . . . . . . . . Trofimenko, S., The Coordination Chemistry of Pyrazole-Derived Ligands . . True, Anne E., see Que, Lawrence Jr. Tuck, D. G., Structures and Properties of Hx2 and HXY Anions . . . . . . . . . . Tuck, D. G., see Diamond, R. M. Tuck, D. G. and Carty, A., Coordination Chemistry of Indium . . . . . . . . . . . Tung, Hui-Chan, see Sobkowiak, Andrzej Tunney, Josephine M., see McMaster, Jonathan Tyler, David R., Mechanic Aspects of Organometallic Radical Reactions . . . Vander Voet, A., see Ozin, G. A. Van Houten, Kelly A., see Pilato, Robert S. van Koten, see Janssen, Maurits D. van Leeuwen, P. W. N. M., see Vrieze, K. Vannerberg, Nils-Gosta, Peroxides, Superoxides, and Ozonides of the Metals of Groups Ia, IIa, and IIb . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Venkatesh, B., Rifkind, J. M., and Manoharan, P. T. Metal Iron Reconstituted Hybrid Hemoglobins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Verche`re, Jean-Francois, Chapelle, S., Xin, F., and Crans, D. C., MetalCarboxyhydrate Complexes in Solution . . . . . . . . . . . . . . . . . . . . . . . . . . Vergamini, Phillip J. and Kubas, Gregory J., Synthesis, Structure, and Properties of Some Organometallic Sulfur Cluster Compounds . . . . . . . . . . . . . . . . . Vermeulen, Lori A., Layered Metal Phosphonates as Potential Materials for the Design and Construction of Molecular Photosynthesis Systems . . . . . . . . . Vlek, Antonin A., Polarographic Behavior of Coordination Compounds . . . . Vrieze, K. and van Leeuwen, P. W. N. M., Studies of Dynamic Organometallic Compounds of the Transition Metals by Means of Nuclear Magnetic Resonance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Walder, L., see Gerfin, T. Wallbridge, M. G. H., see James, B. D. Walton, R., Halides and Oxyhalides of the Early Transition Series and Their Stability and Reactivity in Nonaqueous Media . . . . . . . . . . . . . . . . . . . . . Walton, R. A., Ligand-Induced Redox Reactions of Low Oxidation State Rhenium Halides and Related Systems in Nonaqueous Solvents . . . . . . . . . Wang, Hsin, see Pecoraro, Vincent L. Wang, Hua H., see Williams, Jack M. Wang, Kun, Electrochemical and Chemistry Reactivity of Dithiolene Complexes Ward, Roland, The Structure and Properties of Mixed Metal Oxides . . . . . . . Watton, Stephen P., Taylor, Colleen M., Kloster, Grant M., and Bowman, Stephanie C., Coordination Complexes in Sol–Gel Silica Materials . . . . . . Weigel, A., see Holm, Richard M. Weil, J. A., see Sykes, A. G. Weinberger, Dana A., see Slone, Caroline S. Whangbo, Myung-Hwan, see Williams, Jack M.

737 VOL.

PAGE

31 34

105 115

9

161

19

243

36

125

4

125

47

563

47

837

21

261

44 5

143 211

14

1

16

1

21

105

52 1

267 465

51

333

738

CUMULATIVE INDEX, VOLUMES 1–52

White, Ross R. see Cannon, Roderick D. Wieghardt, Karl, see Chaudhuri, Phalguni Wieghardt, Karl, see Chaudhuri, Phalguni Wigley, David E., Organoimido Complexes of the Transition Metals . . . . . . . Wilkinson, G. and Cotton, F. A., Cyclopentadienyl and Arene Metal Compounds Williams, Jack M., Organic Superconductors . . . . . . . . . . . . . . . . . . . . . . . . Williams, Jack M., Wang, Hau H., Emge, Thomas J., Geiser, Urs, Beno, Mark A., Leung, Peter C. W., Carlson, K. Douglas, Thorn, Robert J., Schultz, Arthur J., and Whangbo, Myung-Hwan, Rational Design of Synthetic Metal Superconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Williamson, Stanley M., Recent Progress in Sulfur-Fluorine Chemistry . . . . . Winchester, John W., Radioactivation Analysis in Inorganic Geochemistry . . Wink, David, see Ford, Peter C. Witt, Michael and Roseky, Herbert W., Sterically Demanding Fluorinated Substituents and Metal Fluorides with Bulky Ligands . . . . . . . . . . . . . . . . Wong, Luet-Lok, see Brookhart, Maurice Wong, K. Y. and Schatz, P. N., A Dynamic Model for Mixed-Valence Compounds Wood, John S., Stereochemical Electronic Structural Aspects of FiveCoordination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Woolley, R. Guy, see Gerloch, Malcolm Wright, Jeffrey G., Natan, Michael J., MacDonnell, Frederick M., Ralston, Diana, M., and O’Halloran, Thomas V. Mercury(II)-Thiolate Chemistry and the Mechanism of the Heavy Metal Biosensor MerR . . . . . . . . . . . . . . . . . Wrighton, Mark S., see Natan, Michael J. Wu, Xian Liang and Lieber, Charles M., Applications of Scanning Tunneling Microscopy to Inorganic Chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

VOL.

PAGE

42 1 33

239 1 183

35 7 2

51 39 1

40

353

28

369

16

227

38

323

39

431

24

251

Xin, Feibo, see Verche`re, Jean-Francois Yoshida, Ryotaro, see Hayaishi, Osamu Zubieta, J. A. and Zuckerman, J. J., Structural Tin Chemistry t-Coordination Zubieta, Jon, see Kahn, M. Ishaque Zubieta, Jon, see Finn, Robert C. Zuckerman, J. J., see Zubieta, J. A.