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 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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81. S. D. Garton, C. A. Temple, I. K. Dhawan, M. J. Barber, K. V. Rajagopalan, and M. K. Johnson, J. Biol. Chem., 275, 6798 (2000). 82. H. Schindelin, C. Kisker, J. C. Hilton, K. V. Rajagopalan, and D. C. Rees, Science, 272, 1616 (1996). 83. F. Schneider, J. Lo¨ we, R. Huber, H. Schindelin, C. Kisker, and J. Knablein, J. Mol. Biol., 263, 53 (1996). 84. A. S. McAlpine, A. G. McEwan, A. L. Shaw, and S. Bailey, J. Biol. Inorg. Chem., 2, 690 (1997). 85. B. Adams, A. T. Smith, S. Bailey, A. G. McEwan, and R. C. Bray, Biochemistry, 38, 8501 (1999). 86. A. F. Bell, X. He, J. P. Ridge, G. R. Hanson, A. G. McEwan, and P. J. Tonge, Biochemistry, 40, 440 (2001). 87. S. Gruber, L. K. Kilpatrick, N. R. Bastian, K. V. Rajagopalan, and T. G. Spiro, J. Am. Chem. Soc., 112, 8179 (1990). 88. L. K. Kilpatrick, K. V. Rajagopalan, J. C. Hilton, N. R. Bastian, E. I. Stiefel, R. S. Pilato, and T. G. Spiro, Biochemistry, 34, 3032 (1995). 89. R. S. Pilato, K. A. Eriksen, M. A. Greaney, E. I. Stiefel, S. Goswami, L. K. Kilpatrick, T. G. Spiro, E. C. Taylor, and A. L. Rheingold, J. Am. Chem. Soc., 113, 9372 (1991). 90. M. G. Finnegan, J. C. Hilton, K. V. Rajagopalan, and M. K. Johnson, Inorg. Chem., 32, 2616 (1993). 91. C. A. Temple, G. N. George, J. C. Hilton, M. J. George, R. C. Prince, M. J. Barber, and K. V. Rajagopalan, Biochemistry, 39, 4046 (2000). 92. M. K. Chan, S. Mukund, A. Kletzin, M. W. W. Adams, and D. C. Rees, Science, 267, 1463 (1995). 93. Y. Hu, S. Faham, R. Roy, M. W. W. Adams, and D. C. Rees, J. Mol. Biol., 286, 899 (1999). 94. B. P. Koehler, S. Mukund, R. C. Conover, I. K. Dhawan, R. Roy, M. W. W. Adams, and M. K. Johnson, J. Am. Chem. Soc., 118, 12391 (1996). 95. I. K. Dhawan, R. Roy, B. P. Koehler, S. Mukund, M. W. W. Adams, and M. K. Johnson, J. Biol. Inorg. Chem., 5, 313 (2000). 96. H. Schindelin, C. Kisker, and D. C. Rees, J. Biol. Inorg. Chem., 2, 773 (1997). 97. R. Roy, S. Mukund, G. J. Schut, D. M. Dunn, R. Weiss, and M. W. W. Adams, J. Bacteriol., 181, 1171 (1999).

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.

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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

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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

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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 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

309

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

310

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

316

SCOTT D. CUMMINGS AND RICHARD EISENBERG 1. 2. 3. 4. 5.

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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317

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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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

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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

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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

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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

338

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

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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)

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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

360

SCOTT D. CUMMINGS AND RICHARD EISENBERG

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

LUMINESCENCE AND PHOTOCHEMISTRY OF METAL DITHIOLENE COMPLEXES

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

362

SCOTT D. CUMMINGS AND RICHARD EISENBERG

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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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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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.

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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 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.

384

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 (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 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.

466

CHRISTOPHE FAULMANN AND PATRICK CASSOUX

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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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

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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).

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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

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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

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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,

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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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SHARON J. NIETER BURGMAYER O

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

S

N

H2NCOH2C

O2 / I-CH2CONH2

S

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.

DITHIOLENES IN BIOLOGY

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2N

N C S

S

C

C S

Mo

[O]

N

S

N

S

N

C

N

S S

S

S

N

C C

N

N

S

N

(piv)HN

2NH

N N

O N C

S S

S

S

Mo S

O N

S S

S

N

C

O

N N

NH(piv) NH O

N

O S

N

HN (piv)HN

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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SHARON J. NIETER BURGMAYER

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

DITHIOLENES IN BIOLOGY

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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

S

O(ser) Mo

S S

oxidized form

O S

oxidized form

O Mo

S S

S

dithionite reduced form

O(ser)

S

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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SHARON J. NIETER BURGMAYER

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 ( 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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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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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

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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

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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

542

JONATHAN McMASTER ET AL.

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

544

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 (