Dithiolene chemistry synthesis, properties, and applications (progress in inorganic chemistry vol 52

Although there had been some very early work pre-1960 on the use ofcertain dithiolene ligands in quantitative analysis of metal ions, these initialstudies were largely empirical and neve

PROGRESS IN

INORGANIC CHEMISTRY

VOLUME 52

DITHIOLENE CHEMISTRY Synthesis, Properties, and Applications

Special volume edited by

EDWARD I STIEFEL

Department of Chemistry, Princeton University

Princeton, New Jersey

Copyright # 2004 by John Wiley & Sons, Inc All rights reserved.

Published by John Wiley & Sons, Inc., Hoboken, New Jersey.

Published simultaneously in Canada.

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Library of Congress Catalog Card Number 59-13035

ISBN 0-471-37829-1

Printed in the United States of America

Dieter Sellmann(1941–2003)

With the agreement of all of the authors, this volume is dedicated to ourcolleague Dieter Sellmann, who died unexpectedly shortly after completing hiscontribution to this book Dieter was a marvelous synthetic chemist whosebeautiful molecules are amply displayed in Chapter 11 (written with Jo¨rg Sutter)

of this collection But, Dieter was more than someone who created novel andbeautiful molecules He brought insight and understanding to these structuresand, especially, to their reactivity with important small molecules He wasinspired by biology, by the reactions of nitrogenase, and by nitrogen oxideinterconversions Indeed, some of his synthetic creations laid bare the possibi-lities of binding of reactive intermediates in glorious detail, about which otherscould only speculate His molecules will continue to give us important cluesabout the reactivity of enzymes and other catalytic systems We will missDieter’s creativity, insights, and good humor, but we also remember and honor alife of great scientific accomplishment, which his article in this volumebeautifully represents

Dieter was an outstanding scientist, a wonderfully warm and vibrant humanbeing, and a good friend He will be sorely missed

v

This volume of Progress in Inorganic Chemistry documents the intensecurrent interest and bright future prospects for research on the chemistry anduses of dithiolene complexes Over the last forty years, complexes of theseremarkable ligands have gone from an important and interesting subclass ofinorganic coordination chemistry to a field that, while generating continuedinterest in structure, bonding, and reactivity, now has impact on a far largerstage The findings that dithiolene complexes have useful reactivity and sensingproperties, that they are at the core of a large number of biologically essentialenzymes, and that they display remarkable (super)conductivity, optical, andmagnetic properties in the solid state, together have given great impetus to work

in this field These new and, in many cases, quite unexpected findings aredocumented in this volume side by side with continued discussions of the basicsynthetic, structural, spectroscopic, bonding, and reactivity properties of thecomplexes The size and scope of this volume and the quality of the individualcontributions reveal a vital field that is just entering its prime It is our hope that,

by collecting comprehensive reviews on the various subfields of dithiolenechemistry in a single place, we will contribute toward the stimulation of thisnow burgeoning field of interdisciplinary research

Although there had been some very early work (pre-1960) on the use ofcertain dithiolene ligands in quantitative analysis of metal ions, these initialstudies were largely empirical and never explored the highly colored complexes

at the structural level The modern era of dithiolene research started in the early1960s with contributions from three research groups: those of Schrauzer andco-workers (at Munich and the University of California at San Diego); Gray andco-workers (at Columbia); and Davison and Holm and co-workers (at Harvard).The combined work of these research groups first established the square-planarnature, redox activity, and broad scope of the highly colored bis(dithiolene)complexes of the late transition metals From the outset, there was contentionabout the electronic structures of these complexes, as the redox capacity of theligands (i.e., their noninnocence) made the assignment of oxidation statedifficult, or, in some cases, ambiguous, at best Interest in the area was furtherheightened by the synthesis and structural characterization of the tris(dithiolene)complexes of early transition metals In addition to sharing the unusual proper-ties of the bis complexes, members of the tris(dithiolene) family were shown to

be the first molecular examples of trigonal-prismatic coordination by Eisenbergand Ibers (at Columbia and Brookhaven)

vii

Work on dithiolene chemistry continued through the 1960s and 1970s fueled

by continued interest in the remarkable coordination chemistry of dithiolenecomplexes However, in the last 20 years tremendous added impetus to research

in the area arose from discoveries in materials science, enzymology, analyticalscience, and reactivity that broadened the impact and import of dithiolenechemistry This volume seeks to capture the interplay of basic work ondithiolene complexes with the growing biological, sensor, reactivity, andmaterials science implications and applications that have made dithiolenechemistry a vibrant and growing field

Chapter 1 deals with synthesis, where we learn that there are many ways tomake dithiolene complexes, either from preformed ligands or through thechemical 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 simpledithiolene complexes Indeed, it was the square-planar nature of most latetransition metal bis(dithiolene) complexes and the unprecedented trigonal-prismatic six-coordination of some of the tris(dithiolene) complexes that wasone of three major drivers for early work in the field

In addition to structure, early work was also driven by two other prominentfeatures: electronic structural uniqueness and one-electron redox activity Thesecond major driver, the electronic structural uniqueness of dithiolene com-plexes 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 greatintensity Electronic structural studies, reviewed in Chapter 3, reveal the manyintricate and interesting features of dithiolene complexes, including the oxida-tion 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 ligandsand complexes The material in Chapters 3 and 4 also show the great utility ofthe spectroscopic and computational tools in probing the molybdenum andtungsten dithiolene cofactors found in biological systems

The third feature of dithiolene chemistry that attracted early attention was thechemical and electrochemical one-electron redox reactivity of the complexes,which allowed a given complex stoichiometry (M/L ratio) to be isolated withseveral different charges (i.e., different states of oxidation, albeit not necessarilydifferent oxidation states of the metal) Chapter 5 deals with the electrochemicaland chemical reactivity of dithiolene complexes, wherein it is seen that thechemical reactivity goes beyond simple redox reactivity and includes reactionsthat are often ligand, rather than metal, based

Chapters 6 and 7 show how the unique electronic structural features ofdithiolene complexes manifest themselves in luminescent and photochemicalbehavior Chapter 6 reveals that the excited states of dithiolene complexes and

their photochemistry can be understood in those cases where the luminescentactivity can be dissected in detail Chapter 7 describes particular exampleswherein the luminescent behavior has been exploited in the development ofeffective sensors for molecular oxygen and, with great promise, for othermolecules as well.

Chapter 8 reviews the considerable work that has been done on solid-statesystems These systems combine the structural features of planar dithiolenecomplexes, wherein specifically discovered and/or designed ligands form com-plexes that coalesce into extended lattices, with unusual conductive, magnetic,and/or optical properties The extensive interest in this field is nurtured by thetruly unusual nature of the extended structures, which, in turn, clearly exploitsome of the unique structural and electronic structural features of the simplerdithiolene 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 tungstenenzymes that contain the pyranopterin dithiolene ligand, reveals the array ofreactions catalyzed by these enzymes, and describes the active site protein struc-tures that have come to light in recent years through X-ray crystallography Theenzyme work provides great impetus and added importance to studies of modelsystems outlined in Chapter 10 Work on these simpler systems reveals structuraltrends, electronic structural details, and reactivity modes that are essential to thefull understanding of the structures, spectra, and reactivity of the enzymes, many ofwhich 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 aremarkable variety of novel complexes (see dedication) that display new forms ofreactivity, which may yet reveal ways in which important small molecules areactivated and converted by enzyme systems in the transition metal dithiolenefamily

The 11 chapters in this volume reveal a vigorous field that may just beentering its prime The new results from synthesis, structure elucidation,spectroscopy, biology, bioinorganic chemistry, analytical science, solid-statematerials chemistry, and reactivity define a rich field that has far to go beforereaching maturity It is also clear that, as we learn more about dithiolenecomplexes, we will see new applications arising that exploit our fundamentalunderstanding of chemical, material, and biological systems It is our hope thatthis monograph, by bringing together the myriad aspects of dithiolene chemistry

in a single volume, will serve as a comprehensive archival reservoir, stimulatefurther advancement of the field, and impel its growing interface with diverseareas of science and technology

C L Beswick, J M Schulman, and E I Stiefel

M L Kirk, R L McNaughton, and M E Helton

S D Cummings and R Eisenberg

K A Van Houten and R S Pilato

C Faulmann and P Cassoux

S J N Burgmayer

J McMaster, J M Tunney, and C D Garner

xi

Chapter 11 Dithiolenes in More Complex Ligands 585

D Sellmann and J Sutter

CHAPTER 1

Synthesis of Transition Metal Dithiolenes

THOMAS B RAUCHFUSS

School of Chemical Sciences

University of Illinois at Urbana-Champaign

Urbana, IL

CONTENTS

II SYNTHESIS FROM PREFORMED ALKENEDITHIOLATES,

A From Benzenedithiol and Related Derivatives / 4

1 Arene Derivatives / 4

2 Linked Bis(benzenedithiolate) Complexes / 8

3 Heterocyclic and Heteroatomic Dithiolates / 10

B From 1,2-Alkenedithiolates / 10

1 Via Reductive Dealkylation / 10

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

Related Derivatives / 11

C 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 [C n S n ]2
and Related Derivatives / 20

5 1,2-Maleonitrile 1,2-dithiolate (mnt2
) / 21

D 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

E From 1,2-Dithietes / 22

F From 1,2-Dithiones, Including Dithiaoxamides and Esters of Tetrathiaoxalate / 23

G Via Intermetallic Dithiolene Transfer / 25

1 Non-Redox Routes / 25

2 Redox Routes / 26

A Addition of Electrophilic Alkynes to Metal Sulfides / 29

B Addition of Unactivated Alkynes to Metal Sulfides / 32

C From Metal Sulfides and a-Haloketones and Related Precursors / 37

6 From Alkynyl Anions / 44

This chapter discusses the synthesis of transition metal dithiolene complexesand is current to late 2002 Dithiolene chemistry has been reviewed severaltimes previously, but this is the first review dedicated to synthetic aspects.Emphasis is placed on more contemporary methods, and the reader shouldconsult the older reviews, especially those by Mueller-Westerhoff et al (8) andMcCleverty (9) for discussions of earlier literature An effort was made to becomprehensive 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 beexhaustive.

which depending on one’s formalism could be described as an dithiolate dianion, a 1,2-dithione, or some oxidation state between these twoextremes (Fig 1) Benzenedithiolates, their derivatives, and analogues are alsoincluded

alkene-1,2-This chapter is divided into two main parts The first part focuses on reactionswhere the dithiolene ligand is generated independently of the metal center Forthe most part, these preparations give alkenedithiolate dianions, which ordin-arily are treated with metal electrophiles to form dithiolene complexes Inthe second part, transition metals actively participate in the assembly of thedithiolenes, usually via the reaction of a metal sulfido species with an alkyne orhydrocarbon 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 simplyseeks an unsaturated chelating dithiolate, the most convenient options arebenzenedithiolate and the inorganic dithiolenes 1,3-dithiole-2-thione-4,5-dithio-

Figure 1 Relationships and nomenclature for common dithiolene precursors.

such as mnt2
and dmit2
that have electronegative substituents behave likebidentate pseudohalides, and their complexes are usually synthesized via simple

are powerful p donors, useful for stabilizing metals in high formal oxidationstates Syntheses of complexes of such strongly donating dithiolenes oftenrequire redox steps after the initial formation of a metal dithiolene complex

1,2-DITHIONES, AND THEIR EQUIVALENT

3,4,5,6-Benzenedithiols are traditionally prepared by reductive dealkylation of

alkali metal or cuprous thiolates The methodology continues to be used, forexample, for crown ether-appended derivatives (15) A newer and more power-ful 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

This method has been extended to the synthesis of the bulky benzenedithiol

Typically, transition metal benzenedithiolates (and related derivatives) areprepared by the following methods: salt elimination reactions using a metalhalide and the dithiolate dianion, thiol exchange, and condensation of the freethiol with oxo, alkoxo, and amido precursors In one example, the dithiol was

treated with a metal methyl compound concomitant with the elimination ofmethane (Eq 2) (20).

halide and the dithiolate, often followed by oxidation of the initially formed

which can be oxidized to the monoanion and neutral derivatives using air and

wherein the amido ligand serves not only as a proton acceptor but also generatesthe countercation (23) The tris(dithiolenes) are so robust and so easily

(24) In the case of Mo derivatives, the metathetical reactions can be conducted

Figure 2 Structures of important arenedithiolate and related ligands.

in the presence of donor ligands, which inhibit the formation of the

Alter-natively, thiol exchange routes can be advantageous as a means to minimize

The oxo-bis(dithiolenes) are amenable to further reactions Treatment of

coordina-tion sets on the bis(dithiolene) framework (12, 24) For example, such

Thiol exchange has been employed to probe the strength of metal ligandbonds as illustrated by the reactions of dicysteinyl peptide-bound derivatives of

whereas less strongly chelating dipeptides are displaced by this dithiol to give

Since several molybdoenzymes feature a single dithiolene ligand active site(30), the synthesis of monodithiolene complexes has been of interest Twogeneral approaches can be envisioned Stepwise installation of dithiolene

ligands or removal of dithiolenes from bis- and tris(dithiolene) precursors.

diverse mixed-ligand derivatives as shown in Fig 4 Thiolate–siloxide exchange

low-valent precursor complexes (Eq 3) (31)

S

S

S O

S

S

SR O

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

solvento ligands from Cr(CO)3(MeCN)3 At higher metal stoichiometry, one

coor-dinatively unsaturated dithiolenes have been prepared by salt-forming methods,

derivatives (Fig 5) (38)

Relatively elaborate benzenedithiol ligands have been prepared via ortho

2,3-dimercap-tobenzoic acid This carboxy-functionalized dithiolene can be linked via amide

also Section II.G.1) (40, 41) Such ligands can be converted to chelatingbis(dithiolene) complexes (Fig 6) The use of 2,3-dimercaptobenzoic acidderivatives is inspired by the naturally occurring chelators derived from 2,3-dihydroxybenzoate (42)

An improved and very promising methodology to such linked dithiolenes

2

S S

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

bimetal-lic complexes with a staircase-like structure (Fig 7)

NaS-i- Pr

(CH2O)n

1) SOCl22) C2H4(NH2)23) Na/C10H8

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

Figure 7 Structure of Ni 2 ðS 2 C 6 H 3 Þ C 2 H 4

(40).

3 Heterocyclic and Heteroatomic Dithiolates

Heterocyclic analogues of benzenedithiolates are also available phene-dithiolate is generated from the corresponding dibromothiophene (44)

bis(chelate) derivatives of Ni(II) and Fe(III) (45)

(46) Ferrocenedithiol and its salts are well known and have been widely

(50) could in principle be employed for the synthesis of multimetallicderivatives

1,2-Dicarboranedithiolate can be generated by deprotonation of

metal dihalides to give the corresponding dithiolates

In contrast to arenedithiols, 1,2-alkenedithiols are usually unstable Thecorresponding alkenedithiolate dianions are, however, valuable precursors todithiolene complexes, although they vary widely in their ease of manipulation

dithiolenes have in fact been characterized in any detail) The reductive kylation was discussed above as a route to benzene- and thiophenedithiolates(15, 43, 44)

deal-Generally speaking, alkenedithiolates derived by this dealkylation route arestrongly reducing and should be handled with complete exclusion of oxidantsand electrophiles (e.g., chlorinated solvents, water) After their isolation assolids, the disodium dithiolates are generally treated with the metal halide togive the corresponding dithiolene complexes In some cases, intermediateanionic dithiolene complexes are allowed to undergo oxidation by air or solventprior to isolation of the final complex Illustrative is the traditional route to the

which can be made on a large scale from cis-dichloroethene and benzylthiolate

Ni, Co, Fe, Cu) to give intermediate anionic species that are subsequentlyoxidized to give neutral or monoanionic complexes Third-row metal centers

promising if untested precursor to the dithiolene dianion (55) The reductive

derived from a trans-alkenedithiolate is described in Section III.F.3) Thecleavage of benzylthioethers has more recently been used to generate nonplanardithiolene ligands shown in Eq 4 (59)

OMe MeO

ð4Þ

and Related Derivatives

A powerful route to dithiolene complexes employs alkenedithiolate dianionsgenerated by the hydrolysis of cyclic unsaturated dithiocarbonates, which areformally called 1,3-dithiole-2-ones Representative of the many examples (60),the base hydrolysis route has been used to prepare the ferrocene-substituted

Mo[S2C2H(Ph)]3 (65) As is typical throughout dithiolene chemistry, initiallyproduced anionic dithiolene complexes are often allowed to undergo air-oxidation to give more conveniently isolated derivatives, for example, of the

The required 1,3-dithiol-2-ones can be prepared in several ways Commonlyused precursors are a-haloketones, which are often commercially available orcan be prepared by halogenation of the ketones (66, 67) The overall procedureinvolves a series of efficient steps with well-defined intermediates (Fig 8) A

the iminium analogues of the cyclic dithiocarbonates (69), although the xanthateapproach still appears preferable

1,3-Dithiole-2-ones have also been efficiently prepared from alkynes via the

xanthogen disulfide (70) (Fig 8) The reaction, which is effected using the freeradical initiator AIBN, has been used to prepare 2-thienyl substituted dithio-lenes, which can undergo subsequent electropolymerization (71) Xanthate

1,3-dithioles (72) The use of 1,3-dithiole-2-ones is compatible with functionalizedbackbones, for example, the attachment of heterocyclic side groups to thedithiolene backbone (70, 72)

R1 R2

S

O S S

The 1,3-dithiole-2-ones can be prepared via displacement of ethylene from

trithiocarbonate is then converted to the corresponding dithiocarbonate, base

(see Section III.A) Displacement of ethylene from ethylenetrithiocarbonate

(Eq 5), which are susceptible to electropolymerization (75, 76)

Z

Z - C2H4 S

S S Z

Z

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

ð5Þ

II.C.5), has been converted to a variety of dithiolene precursors such as the

prepared from the bis(dithiocarbonate) This tetraanion is a precursor to the

The conversion of the dithiocarbonates into alkenedithiolates involves basehydrolysis, which is usually effected with sodium alkoxides in alcohol With thedianion in hand, the synthesis of complexes follows the usual course, asdescribed above Obviously, oxophilic metal centers, for example, Ti(IV) andNb(V) (62), are incompatible with the usual alcohol solutions of in situ

employed in nonhydroxylic solvents, although after complex formation proticsolvents are typically employed for cation exchange

The dithiocarbonate methodology has been used to prepare a number ofmolybdenum–dithiolene complexes In these syntheses, particular attentionmust be paid to the molybdenum precursor in order to avoid formation of thehighly stable (and biologically irrelevant) tris(dithiolene) species For example,

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

Mo S

S R

H O

The use of cyanide ligands to suppress persubstitution by dithiolenes has also

chloroacetal-dehyde (82) This 1,3-dithiol-2-one can be functionalized via deprotonationfollowed by C-alkylation (72), thus opening the way to a variety of functionaldithiolenes (Eq 7)

S

S O H

H

1) LiNR22) E+3) OR - 4) Ln MX 2

MLn S

S E

A versatile route to RS-substituted dithiolenes entails S-alkylation of the

to introduction of diverse functionality to the dithiolene backbone Subsequent to

dithiocarbo-nates are more easily hydrolyzed than the trithiocarbodithiocarbo-nates (72, 85) This

(86) and related complexes with pendant alkene substituents (Eq 8) (87)

1) MeO2) LnM2+

SR

ð8Þ

ethoxy-substituted trithiocarbonate, which eliminates ethanol to give a rich dithiolene with extended unsaturation (Eq 9) (88)

sulfur-Br

Br EtO

S

S

S Ni S

remains obscure, these trithiocarbonates are promising precursors to trical dithiolenes (89)

overview of the ligand chemistry including many useful experimental dures is available (1) as are reviews on specific aspects of the coordination

in materials chemistry, for example, the photonic or electronic properties (90, 93)

proliferation of this ligand was the finding by Hoyer and co-workers (95) that

In principle, oxidized derivatives of dmit2
(C3S5)nand [(C3S5)2]2
could beemployed for the synthesis of dmit complexes (83).

optimiza-tions, mainly aimed at large-scale syntheses (>50 g) (1, 97–99), although theoriginal procedure (95) is excellent The method has been revised so that it

and the electron-transfer process is controlled by the addition of DMF (98, 99)

Figure 9 Synthetic interrelationships involving dmit2
and other CS 2 -derived species.

isolated as yellow crystals with favorable stability and solubility (1) Ionic

give the alkali metal salt of the metal dithiolene complex, for example,

of many hundreds of publications (2, 100), which are discussed by Cassoux in

102)

whose structure is shown in Fig 10 (94)

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

Whereas mixed-ligand dmit complexes are generally prepared by reaction of

homoleptic complexes represents an alternative route For example, treatment of

resulting complexes have elicited only modest attention This ligand is ated by the base-degradation of the bis(dithiocarbonate) tetrathiapentalenedione(TPD) (120) Otherwise, TPD has played an important role in the development

gener-of tetrathiafulvalenes and dithiolenes (121, 122); the reader is referred toSections II.C and III.E for related methodology The corresponding imine-

wherein the three carbon atoms are contiguous (see Fig 10) (2) The synthesis

illustrated by its reaction with dimethylacetylene dicarboxylate (DMAD) (123)

strictly speaking falls outside the scope of this chapter Nonetheless, theexploration of this ligand is closely tied to dithiolene chemistry Early research-

electrosynthesis (125)

metal complexes such as those of Ni(I), Fe(I), and Ti(II) (131) Tetrathiooxalate

reduction to give ethylenetetrathiolato complexes (Eq 10) (133)

-S S

S

S S

S S

S

Ni 2-

n

ð11Þ

Because of the close structural and preparative connections between theTTFs and dithiolene chemistry, it is only natural that extended dithiolenes havebeen developed with a TTF-like core These complexes are generally preparedvia the corresponding TTF-based di- or tetrathiolates

Section II.C) or by lithiation–sulfidation of TTF itself The former method hasbeen applied to the synthesis of polymeric complexes (129) The latter method

characterized

In contrast to the binucleating character of tetrathiafulvalenetetrathiolate, avariety of chelating tetrathiafulvalenedithiolates are also known, and thesespecies give rise to complexes with especially interesting electrical properties

illustrated in Fig 11

The synthesis of this hybrid TTF–dithiolene illustrates the use of thecyanoethyl group to protect the sulfur atoms of the dithiolene (81, 136, 137).The trimethylene-capped tetrathiafulvalenetetrathiolate forms a molecular

species of the type {Ni[S2C2S2C


CS2C2S2(CH2)3]2}2
Electro-oxidization(138) is commonly employed to secure single crystals of dithiolene-based

component metalloorganic electrical conductor (139)

intermittent interest (140) Beck et al investigated the coordination chemistry

complexes as well as derivatives where the squarate is unidentate (143)

appar-ently unknown Benzenehexathiolato complexes include multimetallic

(146) Such species are prepared by salt-metathesis reactions

S

S S

S

S

S Ni

S S

S S

1) NMe4OH 2) Ni2+

2-1) BrC2H4CN

2) Hg(O2CMe)2/HO2CMe

2 n-

Figure 11 Preparation of trimethylenetetrathiafulvalenedithiolate complexes (136).

5 1,2-Maleonitrile 1,2-dithiolate (mnt2
)

An easily prepared, versatile, and time-honored dithiolene ligand is

complexes are well described in earlier reviews (9, 147) The sodium salt of

been synthesized

reactions, as expected for this pseudohalide-like dithiolene Recent studies onoxo Mo/W derivatives use less obvious methods Sarkar and co-workers (149)

ammo-nium salt (Eq 12)

PPh3[MoO4]2 [MoOmnt 2(mnt)2]2- [MoO(mnt)2]2-

2-ð12Þ

tungsten-containing enzyme acetylene hydratase, it catalyzes the hydration of

considered a reductant, but the less oxidizing W(VI) center apparently resists

A historically significant route to dithiolenes starts from a-hydroxyketones(also called acyloins) (157) This methodology is well suited for the large-scale

synthesis of homoleptic dithiolene complexes, especially those with aryl andsimple alkyl substituents Perhaps the most important complexes of this type are

dithiolene-transfer agents for the synthesis of bis(dithiolene) derivatives of Moand 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 thecorresponding a-hydroxyketone (Eq 13)

+ P4S10

- H2S, "P4S8O2"

ð13Þ

described as thiophosphate esters (159), although such species have not beenrigorously characterized Hydrolysis of these thiophosphates followed by treat-

and Ph

1,2-Diketones (e.g., derivatives of benzil) can be used in place of hydroxyketones, a modification that broadens the utility of this method (71,162), despite the fact that the dithione is the incorrect oxidation state to combinewith metal salts Large numbers of nickel diaryldithiolenes have been preparedvia this sulfiding method (163–165) Representative of the dithiolene complexes

acid–base (162, 166–168) and liquid-crystal properties (65, 169)

sulfur atoms Such heterocycles are isomeric with 1,2-dithiones and formallyresult from the two-electron oxidation of 1,2-alkenedithiolates (Fig 1) Among

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

crystallographically (173)

Because of its solubility in nonpolar solvents and its oxidizing character,

low-valent organometallic precursors, for example, metal carbonyls (8) Thesynthesis of dithiolenes from dithietes is illustrated by the reaction of

dicarbo-nyl intermediate (Fig 12) (174) An unusual method of exploiting the oxidative

(Fig 12) The same oxo-molybdenum species can be obtained by reduction of

and Esters of Tetrathiaoxalate

Few dithiolenes are prepared via reactions involving 1,2-dithioketones, arare 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

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

of the corresponding dithiocarbonate The resulting dithione exists inequilibrium with the dithiete (176, 177) The corresponding diphenyl derivativeexists exclusively in the dithiete form, indicating that p-donor substituentsstabilize 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).

3

h ν

S

SMe

M S

ð15Þ

com-plexes is described in Section III.E) (180, 181) The thermal reaction of

photode-carbonylation (179) In principle, this methodology could give a range ofSR-substituted dithiolenes

These 1,2-dithiones are mildly oxidizing as illustrated by their reactions with

Nickel complexes of unstable cyclic dithioamides are generated by ing the corresponding diamide in the presence of Ni powder using (MeO-

O O

ð16Þ

Yields are diminished if the thiation is conducted prior to the addition of themetal, indicative of the thermal instability of these dithioamides Furthermore,the yields are lower when Ni(II) salts are used in place of Ni powder, consistentwith the oxidative character of the dithioamide Nickel dithiolenes of the dithio-

Two basic types of dithiolene exchange reaction are practiced, (1) non-redox

redox reactions, which commonly involve neutral bis(dithiolene) complexes of Ni

Dithiolenes of Ti(IV) and Zn(II) (see Section III.A) transfer their dithiolate to softer metals Chelate-transfer reactivity under mild conditions was

to give the corresponding late metal dithiolene and titanocene dichloride, whichcan be removed by filtration through silica gel with which it reacts (188)

tetramethylethylenediamine, also display this chelate-transfer reactivity andare perhaps still more versatile (see Section III.A) (189) Both the zinc and

of the diene from Pd (190)

VCl3, and AuCl(PPh3) to afford Cp2Ti(dmit) (92, 117), [V(dmit)3]2
(191),

other dmit complexes are rarely employed for dithiolene transfer CpNi(dmit)

upon photolysis resulting in transfer of dithiolene ligands For example,

of reaction have revealed examples of incomplete transfer of dithiolene ligands,

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

Figure 13 Structure of [Pd 6 S 2 C 2 (CO 2 Me) 2 ] 6

this complex has not been examined [see related work on dithiaoxamidecomplexes of Mo (Section II.G)].

was rejuvenated by Holm and co-workers (198) who sought new routes to bis(dithiolene) complexes of molybdenum and tungsten as models for metalloen-zymes As discussed above, a synthetic challenge in the chemistry of molybde-num and tungsten dithiolenes is often preventing formation of the tris(dithiolene)complexes, which are substitutionally inert

ð18Þ

reagents, which are easily generated thermally (201), facilitates this chelate-transfer

characterized, the dianionic species having been generated by reduction of theneutral complex with potassium anthracenide, and the monoanion was obtained

by comproportionation (203)

arylthio-lates, and arylselenoates (198, 204, 205) also displace one or both of thecarbonyl ligands, the determining factor apparently being the steric crowdingaround the M(IV) center

M(S2C2R2)2(CO)2

CO X M(S2C2R2)3

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

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

Mo(S2C2Ph2)2(S2C2R2) (R = H, CN)

1) S2C2R'22) HCl/air dppe

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

The resulting anionic complexes closely resemble the proposed active sitestructures of the molybdopterin-based O-atom transfer enzymes such asdimethyl sulfoxide reductase (DMSOR) (4–6), which characteristically converts

mono-dithiolenes undergo substitution of the chlorides to give diverse alkoxy andthiolato derivatives, which are structural analogues of the active sites of sulfite

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

oxidase and assimilatory nitrate reductase (30) As usual, the main challenge inthe preparation of mono(dithiolene) Mo complexes is preventing formation ofthe very stable tris(dithiolene) derivatives.

Some insights into the details of dithiolene-transfer reactions are provided

TO DITHIOLENES

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

C(4)

C(6) S(2)

S(3)

S(4) Mo(1)

Co(2)

C(11)

C(10) C(9)

C(24)

C(21) C(20)

C(14) C(15)

C(16)

Co(1) S(1)

C(2)

Figure 16 Structure of Mo(CO) 2 [CpCo(S 2 C 6 H 4 )] 2 (206).

in this reaction (12, 188, 208–222) Other electrophilic alkynes that have been

have been employed in the synthesis of pterin-related dithiolenes (217, 227).Terminal sulfido complexes also are known to add electrophilic alkynes

The prototype reaction of DMAD with sulfur-rich metal complexes involves

S S

S S

S Ti

C2Z2

S S

distinctively green in color and are readily purified These and related titanocenecomplexes 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 inusing this reagent Illustrative of the complications that one can encounter, the

(230)

Subsequent to the development of the titanocene dithiolenes, related results

dithiolene complexes, and the dithiolene can also be readily removed from thezinc center The zinc complexes are more potent dithiolene-transfer agents than

more rapidly with alkynes than the tmeda derivative (Fig 17)