Progress in Inorganic Chemistry, Volume 58 ebook

CONTENTS

Cover

Advisory Board

Title Page

Copyright

Chapter 1: Tris(dithiolene) Chemistry: A Golden Jubilee

I. Introduction

II. Ligands

III. Complexes

IV. Structures

V. Theory

VI. Electrochemistry

VII. Magnetometry

VIII. Spectroscopy

IX. Summary

X. Conclusions

Acknowledgments

Abbreviations

References

Chapter 2: How to Find an HNO Needle in a (Bio)-Chemical Haystack

I. Introduction

II. Chemical and Biological Relevance of HNO

III. Azanone Detection Methods

IV. Conclusions and Future Perspectives

Acknowledgments

Abbreviations

References

Chapter 3: Photoactive Metal Nitrosyl and Carbonyl Complexes Derived from Designed Auxiliary Ligands: An Emerging Class of Photochemotherapeutics

I. Introduction

II. Metal Nitrosyl and Carbonyl Complexes as Nitric Oxide and Carbon Monoxide Donors

III. Photoactive Metal Nitrosyl Complexes

IV. Photoactive Metal Carbonyl Complexes

V. Conclusion

Acknowledgments

Abbreviations

References

Chapter 4: Metal—Metal Bond-Containing Complexes as Catalysts for C—H Functionalization

I. Introduction

II. Dirhodium and Diruthenium C—H Functionalization Chemistry

III. Dipalladium C—H Functionalization Chemistry

IV. Parallels Between Dirhodium and Dipalladium Systems

V. Summary

Acknowledgments

Abbreviations

References

Chapter 5: Activation of Small Molecules by Molecular Uranium Complexes

I. Introduction

II. Scope and Organization

III. Carbon Monoxide

IV. Nitrogen Monoxide

V. Dinitrogen

VI. Dioxygen

VII. Carbon Dioxide

VIII. Nitrous Oxide

IX. Water

X. Dihydrogen

XI. Saturated Hydrocarbons

XII. Alkenes and Alkynes

XIII. Arenes

XIV. Concluding Remarks

Acknowledgments

Abbreviations

References

Chapter 6: Reactive Transition Metal Nitride Complexes

I. Introduction

II. Scope

III. Previous Reviews

IV. Properties of the Nitride Ligand

V. Synthesis of Transition Metal Nitrides

VI. Reactivity

VII. Nitrides as Catalyst Precursors and Intermediates

VIII. Strategies for Increasing Nitride Reactivity

IX. Conclusions

Acknowledgments

Abbreviations

References

Subject Index

Cumulative Index

End User License Agreement

List of Tables

Table I

Table II

Table III

Table IV

Table V

Table VI

Table VII

Table VIII

Table IX

Table X

Table XI

Table XII

Table XIII

Table XIV

Table XV

Table XVI

Chart 1

Table I

List of Illustrations

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Guide

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Advisory Board

JACQUELINE K. BARTON

CALIFORNIA INSTITUTE OF TECHNOLOGY, PASADENA, CALIFORNIA

SHUNICHI FUKUZUMI

OSAKA UNIVERSITY, OSAKA, JAPAN

CLARK R. LANDIS

UNIVERSITY OF WISCONSIN, MADISON, WISCONSIN

NATHAN S. LEWIS

CALIFORNIA INSTITUTE OF TECHNOLOGY, PASADENA, CALIFORNIA

STEPHEN J. LIPPARD

MASSACHUSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MASSACHUSETTS

JEFFREY R. LONG

UNIVERSITY OF CALIFORNIA, BERKELEY, CALIFORNIA

THOMAS E. MALLOUK

PENNSYLVANIA STATE UNIVERSITY, UNIVERSITY PARK, PENNSYLVANIA

TOBIN J. MARKS

NORTHWESTERN UNIVERSITY, EVANSTON, ILLINOIS

JAMES M. MAYER

UNIVERSITY OF WASHINGTON, SEATTLE, WASHINGTON

DAVID MILSTEIN

WEIZMANN INSTITUTE OF SCIENCE, REHOVOT, ISRAEL

WONWOO NAM

EWHA WOMANS UNIVERSITY, SEOUL, KOREA

VIVIAN W. W. YAM

UNIVERSITY OF HONG KONG, HONG KONG

Progress in Inorganic Chemistry

Volume 58

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Library of Congress Catalog Number: 59-13035

ISBN 978-1-118-79282-7

Tris(dithiolene) Chemistry: A Golden Jubilee

Stephen Sproules

West CHEM, School of Chemistry, University of Glasgow, Glasgow, G12 8QQ United Kingdom

Contents

Introduction

Ligands

Arene Dithiolates

Alkene Dithiolates

Sulfur

Carbon Disulfide

Phosphorus Pentasulfide

Other Sulfur Sources

Dithiones

Complexes

Metathesis

Redox

Transmetalation

Structures

Beginnings

Neutral Complexes

Reduced Complexes

Isoelectronic Series

Redux

Trigonal Twist

Dithiolene Fold

Oxidized Ligands

Theory

Hückel

Fenske–Hall

Electrochemistry

Magnetometry

Spectroscopy

Vibrational

Electronic

Nuclear Magnetic Resonance

Electron Paramagnetic Resonance

Spin Doublet

Spin Quartet

X-Ray Absorption Spectroscopy

Metal Edges

Sulfur K-Edge

Mössbauer

Summary

Group 4 (IV B)

Group 5 (V B)

Group 6 (VI B)

Group 7 (VII B)

Group 8 (VIII B)

Group 9 (VIII B) and Beyond

Conclusions

Acknowledgments

Abbreviations

References

I. Introduction

The search for organometallic compounds with sulfur-donor ligands gave inorganic chemistry its first tris(dithiolene) coordination compound in 1963 (1). Anticipating a combination of CO and sulfur-donor ligands, King (1) apathetically described the product of the reaction of bis(trifluoromethyl)dithiete with molybdenum hexacarbonyl as a hexavalent metal coordinated to three as yet unidentified dithiols. The first bis(dithiolene) homologues with late transition metals appeared in the literature the previous year (2,3). Seduced by the remarkable properties exhibited by these compounds, three research groups led the investigation in the early 1960s: Gray and his cohort at Columbia and then at Caltech; Schrauzer and co-workers in Munich, the Shell Development Company, and University of California at San Diego; and finally the Harvard quartet of Davison, Edelstein, Holm, and Maki. The competitive environment that ensued significantly advanced this emerging field into what we now know as transition metal dithiolene chemistry. Bis(dithiolene) compounds elicited greater interest than their tris(dithiolene) analogues despite both being strongly chromophoric, exhibiting multiple reversible electron-transfer processes, and possessing unprecedented molecular geometries. Bis(dithiolenes) were found to be persistently square planar (3–5), an outcome that could only arise from ligand participation in the frontier orbitals. Therefore, this sulfur–donor ligand with an unsaturated carbon backbone is regarded as the first noninnocent chelating ligand as it can exist in one of three forms: a dianionic dithiolate, a monoanionic dithienyl radical, and a neutral dithione (Scheme 1). Gray and co-workers (6,7) worked on the premise that these were metal stabilized radical-ligand systems. Schrauzer and co-workers (8,9) could never escape calling these dithioketones, whereas Holm, Maki, and co-workers (4,10) avoided applying such definitive terms. An innovative compromise was brokered by McCleverty (11,12) when he introduced the term dithiolene, obviating the need to specifiy discrete oxidation levels (13,14). Only the metal was assigned, and for the archetypal bis(dithiolene) complexes of group 10 (VIII B) metals, it was unanimously agreed that a low-spin d8 ion lay at the center of the neutral, monoanionic, and dianionic species, with each differing in the occupation of ligand-based valence orbitals.

Scheme 1. The three oxidation levels for a dithiolene ligand (L = dithiolene).

The prosperity enjoyed by transition metal dithiolene complexes abruptly faded at the end of the 1960s. Interest in bis(dithiolene) compounds continued given their structural resemblance to tetrathiafulvalene (TTF), which pushed this field into the new areas of photonics and conductitvity (15). In contrast, tris(dithiolenes) chemistry languished until an unexpected revival in the mid-1990s. The spark came from the discovery of dithiolene ligands in biological systems (16,17). The protein structure of an aldehyde:ferridoxin oxidoreductase consisting of a tungsten ion coordinated by two dithiolate ligands did for dithiolene chemistry what the Cambrian Explosion did for life on Earth (18). Oxo-molybdenum and -tungsten bis(dithiolene) synthetic dead ends were now in vogue as small molecule analogues for the active sites of oxotransferase enzymes (19–23). The fortunes of tris(dithiolene) compounds were similarly transformed. The last decade has been the most insightful in the 50-year history of tris(dithiolene) chemistry as these old compounds became the subject of scrutiny by new techniques, modern instrumentation, and advanced computational methodology (24–26). The focus was on their molecular and electronic structures that had never been completely resolved. The following account tracks the history and evolution of tris(dithiolene) chemistry in this its golden jubilee year.

II. Ligands

Dithiolene ligands can be categorized into three groups based on the nature of the C—C bond in the elementary {S2C2} unit: (1) arene dithiolates where the double bond is part of an aromatic system (Fig. 1a); (2) alkene dithiolates with an olefinic double bond (Fig. 1b); and (3) neutral dithiones with a C—C single bond and unsaturated S—C bonds (Fig. 1c). The synthetic route to dithiolene ligands depends largely on the metal ion it binds. Arene dithiolates are traditionally isolated as dithiols. The analogous alkene-1,2-dithiols are unstable (27–29), and preferably handled in situ as pro-ligands or alkali salts before combining with an appropriate metal reagent. The recurrent step in all ligand synthesis is protection of the 1,2-dithiolate unit, which takes on a multitude of forms from simple protonation, to alkyl, ketyl, thione, and silyl entities that prevent exposed sulfur atoms from partaking in counterproductive side reactions. The protecting groups also allow the carbon backbone to be functionalized so that the physical properties of the final dithiolene ligand and complex are tailored to suit the desired application.

Figure 1. General classes of dithiolene ligand.

A. Arene Dithiolates

The archetypal member of this group of ligands is the ubiquitous benzene-1,2-dithiolate, (bdt)2−. It first appeared in the literature in 1966 in the synthesis of square-planar bis(dithiolene) complexes with Co, Ni, Cu (30), and was generated by treating o-dibromobenzene with cuprous butyl mercaptan (31,32), followed by cleavage of the thioether with sodium in liquid ammonia (Scheme 2a) (33). A high-yielding synthesis involving the addition of sodium 2-propanethiolate to 1,2-dichlorobenzene in dimethylacetamide at 100 °C produces 1,2-bis(isopropylthio)benzene, which is readily deprotected to form benzene-1,2-dithiol, bdtH2 (Scheme 2b) (34,35). The procedure was updated some 30 years later with the ortholithiation of thiophenol, and subsequent reaction with elemental sulfur followed by acidification, which gave large quantities of dithiol (Scheme 2c) (36,37). In both procedures, specific functional groups can be introduced in the first stage, for example, the preparation of veratrole-4,5-dithiol (vdtH2) from 1,2-dimethoxybenzene following Scheme 2a (38), and 3,4,5,6-tetrafluorobenzene-1,2-dithiol (bdtF4H2) from 1,2,3,4-tetrafluorobenzene following Scheme 2b (39).

Scheme 2. Preparation of benzene-1,2-dithiol.

All known arene dithiolate ligands utilized in the formation of tris(dithiolene) complexes are presented in Fig. 2. Dithiol versions of toluene-3,4-dithiolate, (tdt)2−, xylene-4,5-dithiolate, (xdt)2−, and the crown ether homologues can be prepared from their corresponding o-dibromo precursors following Scheme 2a (30,38,40). Alkyl protection of the thiolate groups followed by ortholithiation leads to 3,6-bis(trimethylsilyl)ethene-1,2-dithiolate, (tms)2− (41). This methodology has been exploited by Kreickman and Hahn (42) to generate an inventory of (bdt)2− moieties linked to each other or catecholate, (cat)2−, via an amide bridge. The ligands pertinent to this topic are displayed in Fig. 3.

Figure 2. Arene-1,2-dithiolate ligands and their abbreviations. (See list of abbreviations for ligand identification.)

Figure 3. Benzene-1,2-dithiolate-based polydentate ligands. (See list of abbreviations for ligand identification.)

Tetrachlorobenzene-1,2-dithiol (bdtCl4H2) is prepared by boiling hexachlorobenzene with sodium sulfide and iron powder in N,N′-dimethylformamide (DMF) (30,43). Addition of base precipitates the iron compound, [Fe(bdtCl4)2]n, and treatment with ZnO in boiling MeOH liberates bdtCl4H2. The four-step synthesis of 3,5-di-tert-butylbenzene-1,2-dithiol, tbbdtH2, starts with commercially available 3,5-di-tert-butyl-2-aminobenzoic acid as outlined in Scheme 3 (44). Sulfur-rich 2,5-dithioxobenzo[1,2-d:3,4-d′]bis[1,3]dithiolene-7,8-dithiolate, (dbddto)2−, begins with thiolation of hexachlorobenzene with benzylmercaptan to form hexakis(benzylthio)benzene (45). Treatment with sodium in liquid ammonia followed by protonation affords benzenehexathiol. Only the hexathiol reacts with carbon disulfide in pyridine to give nearly quantitative yields of the pyridinium salt of 7-mercapto-2,5-dithioxobenzo[1,2-d:3,4-d′]bis[1,3]dithiole-8-thiolate, the precursor to (dbddto)2− (46). Quinoxaline-2,3-dithiol, qdtH2, is conveniently formed in the reaction of 2,3-dichloroquinoxaline with excess thiourea in refluxing ethanol (47).

Scheme 3. Preparation of tbbdtH2.

B. Alkene Dithiolates

The tremendous variety of dithiolene ligands with an alkene backbone presented in Fig. 4 highlight the desire to adapt the basic motif in order to equip the complex with specific properties. They have been grouped here based on the sulfur source used in preparing the ligand, principally elemental sulfur, phosphus pentasulfide, and carbon disulfide.

Figure 4. Alkene-1,2-dithiolate ligands and their abbreviations. (See list of abbreviations for ligand identification.)

1. Sulfur

With the notable proficiency by which sulfur atoms readily bond to each other, it is rather surprising to find only one genuine example of a dithiolene ligand that is introduced to a metal as a dithiete (Fig. 5a) (1,10). The synthesis of bis(trifluoromethyl)dithiete is conducted under conditions that would violate modern Health and Safety protocols: hexafluoro-2-butyne is bubbled through molten sulfur, after which the malodorous and poisonous dithiete is obtained as a liquid via fractional distillation of the reaction mixture (48). Its structure as that of a dithiete rather than a dithione was confirmed by vapor-phase X-ray diffraction revealing an S—S distance of 2.05 Å (49). Bis(trifluoromethyl)dithiete is an oxidizing agent and it is readily reduced by sodium to form the dithiolate, Na2tfd (50).

Figure 5. Representative structures of (a) bis(trifluoromethyl)dithiete, (b) dimethyl-1,2-dithiete dicarboxylate, and (c) the equilibrium between diphenyl-1,2-dithiete and dithiobenzil.

The paucity of dithiete entities stems from the propensity of sulfur atoms to target neighboring molecules to form oligomeric and polymeric mixtures (51). The reported preparation of benzene-1,2-dithiete (52) was revised as a mixture of sulfur-bridged species, the smallest being bis(o-phenylene)tetrasulfide (53). Sulfuryl chloride oxidation of [Cp2Ti(dmm)] (54), where (dmm)2− is dimethylmaleate-2,3-dithiolate and Cp is cyclopentadienyl, releases dimethy-1,2-dithiete dicarboxylate (Fig. 5b) (55). This dithiete is sufficiently stable for X-ray diffraction studies (Fig. 6), and has been characterized with an S—S bond length of 2.07 Å.

Figure 6. Molecular structure of dimethyldithiete dicarboxylate.

Both dithiobenzil and 4,4′-bis(dimethylamino)dithiobenzil have been generated by irradiation of the corresponding dithiocarbonate releasing CO (56). It is speculated that the former exists exclusively as diphenyl-1,2-dithiete, whereas the latter is a dithione (Fig. 5c). In the presence of [Mo(CO)6], dark green, neutral tris(dithiolene) compounds are isolated from the reaction (56). Similarly, photolysis of 3-(methylthio)-5,6-tetramethylene-1,4,2-dithiazine yields tetramethylenedithiete/cyclobutanedithione, which is scavenged by [Mo(CO)6] to form [Mo(cydt)3] (cydt2− = cyclohexene-1,2-dithiolate) (57).

Schrauzer and Mayweg (3,58) stumbled into dithiolene chemistry via the esoteric reaction of nickel sulfide and “tolan”, more commonly known as diphenylacetylene, which produced a black crystalline solid formulated NiS4C4Ph4. The one-pot reaction produced the first neutral bis(dithiolene)nickel complex that the authors described as square planar and diamagnetic (3). Although Schrauzer's laboratory would divert to an alternative method of complex synthesis (8,9), their approach was used by others investigating the reaction of unsaturated organic molecules with sulfur in the presence of metal ions. Dimethyl- and diethyl-acetylene dicarboxylate are considered activated because the highly electron-withdrawing ester substituents weaken the triple bond. Therefore, they are primed to react with metal–sulfur units (e.g., {MS2} and {M(S4)}) to form a five-membered ring: A metallodithiolene. This procedure has successfully produced V, Mo, and W complexes with three (dmm)2−, diethylmaleate-2,3-dithiolate, (dem)2−, dibenzoylethene-1,2-dithiolate (dbzdt)2−, bis(trifluoromethyl)ethene-1,2-dithiolate (tfd)2−, and 1-quinoxalyl-2-phenylethene-1,2-dithiolate (qpdt)2− ligands in mediocre yields (50,59–64).

2. Carbon Disulfide

The most widely encountered ligand in dithiolene chemistry is 1,2-dicyanoethene-1,2-dithiolate abbreviated (mnt)2−, and so-named from the cis-orientation of the cyanide substituents found in maleonitrile. The disodium salt of maleonitriledithiolate is easily prepared from the combination of sodium cyanide and carbon disulfide in DMF to form [S2CCN]1− (Eq. 1). Two equivalents of this adduct decompose to give Na2mnt and elemental sulfur (65).

Despite the noted virtues of the thiophosphorester synthetic approach, it is restricted to just a handful of commercially obtainable acyloins. Modifying the ligand appendages to alter solubility, electronics, or sterics, relies on a different tactic, and the methods providing the most variety are syntheses of functionalized 1,3-dithiole-2-ones or vinylene dithiocarbamates (Scheme 4).

Scheme 4. Synthetic route to 1,3-dithiole-2-one.

These are produced by combining an α-bromoketone with the sulfiding agent potassium o-isopropyl xanthate, K(S2COi-Pr) (66), forged in the reaction of isopropyl alcohol, potassium hydroxide, and carbon disulfide (67). This procedure leads to phenylethene-1,2-dithiolate, (sdt)2−, and its analogues tolylethene-1,2-dithiolate, (toldt)2−, anisylethene-1,2-dithiolate, (adt)2−, p-chlorophenylethene-1,2-dithiolate, (csdt)2−, and p-bromophenylethene-1,2-dithiolate, (bsdt)2− (68); bis(3-thienyl)ethene-1,2-dithiolate, (thdt)2− (69); several substituted 1,2-diphenyl-1,2-dithiolates: 1,2-ditolylethene-1,2-dithiolate, (dtdt)2−, 1,2-dianylethene-1,2-dithiolate, (dadt)2−, 1-tolyl-2-phenylethene-1,2-dithiolate, (tpdt)2−, 1-anisyl-2-phenylethene-1,2-dithiolate, (apdt)2−, and 1-anilyl-2-phenylethene-1,2-dithiolate, (anpdt)2−, (68,70); and a more tractable form of 1,2-dimethylethene-1,2-dithiolate, (mdt)2− (71). The latter was crystallographically characterized with average S—C and C—C distances within the {S2C2} unit of 1.754(1) Å and 1.340(2) Å, respectively (72), offering baseline bond lengths in the “free” ligand (Fig. 7). Base hydrolysis cleaves the ketyl protecting group affording the dianionic dithiolate ligand poised for complexation.

Figure 7. Molecular structure of 4,5-dimethyl-1,3-dithiole-2-one.

Dithiolene ligands saturated with sulfur atoms find favor in electrically conducting salts and charge-transfer complexes (15,73,74). The motivation for ligands of this type followed the discovery of heterocyclic TTF (C6H4S4) described as an “organic metal” (75), and bearing a striking resemblance to bis(dithiolene) complexes (Fig. 8), and therein the anionic coordination complexes provide an ideal complement to the TTF radical cation (15,73).

Figure 8. Structure of (a) TTF and (b) a generic bis(dithiolene) complex.

The analogy motivated a new synthetic direction in the preparation of heterocyclic dithiolene ligands to design coordination complexes with enhanced electronic, photonic, and magnetic properties (15,77). The progenitor of this sulfur-rich collection of ligands is 1,3-dithiole-2-thione-4,5-dithiolate abbreviated (dmit)2− from its original name dimercaptoisotrithione (78). The preparation is exceedingly simple: Carbon disulfide is reacted with an alkali metal (Na or K) in DMF (79,80). Importantly, the formation of a carbon disulfide adduct (a thioxanthate) has provided a simple route to multi-gram amounts of the ligand. The ligand is stabilized when ZnSO4 is added in the final step to generate [Zn(dmit)2]2− salts (Scheme 5) (81), an efficient ligand delivery reagent (78,82). The ligand has also been structurally characterized as an air-sensitive NMe4+ salt where again the S—C and C—C distances of 1.724(6) and 1.371(8) Å, respectively (83), offer baseline intraligand bond lengths (Fig. 9).

Scheme 5. Derivatives of (dmit)2−.

Figure 9. Molecular structure of (NMe4)2dmit.

A variety of ligands are generated from (dmit)2−, such as 2-oxo-1,3-dithiole-4,5-dithiolate, (dmid)2− (84), and 1,2-dithiole-3-thione-4,5-dithiolate, (dmt)2− (85,86), by protecting the thiolates with benzoyl groups (81,86,87), or alkyl-substituted 1,4-dithiin-2,3-dithiolates by alkylating the dithiolate, followed by converting and cleaving the thione (Scheme 5) (79,88,89). The TTF based dithiolates have also been prepared from P(OEt)3 promoted coupling of protected (dmit)2− and (dmid)2− moieties (Scheme 5) (90,91).

3. Phosphorus Pentasulfide

Schrauzer and Finck (92) discovered in their one-pot synthesis of [Ni(pdt)2] that dithiobenzil cannot be isolated; rather it is stabilized by forming covalent bonds with itself or a transition metal ion. The scale of the reaction was improved by heating the α-hydroxybenzoin with phosphorus pentasulfide in xylene or dioxane (Scheme 6) (9,93,94). The dithiolene is stabilized as a thiophosphorester that readily relinquishes the ligand to a waiting metal center (94). The poor yields are offset by the multi-gram scale of the reaction and inexpensive reagents (95). A variety of substituted α-hydroxyketones (acyloins) can be used to generate a range of dithiolene substitution patterns, for example acetoin to form complexes with (mdt)2− (96).

Scheme 6. Preparation of thiophosphoresters and their alkyl stabilized derivatives.

Recently, Donahue and co-workers (95) confirmed the constitution of these thiophosphoresters by adding an alkylating agent to the amber reaction mixture (Scheme 6). This reaction affords more tractable thiophosphoryl thiolates, (R2C2S2)P(S)(SR′) [R′ = Me, Bz (benzyl)], as precursors to 1,2-diphenylethene-1,2-dithiolate, (pdt)2−, and (dadt)2−. Several have been structurally characterized and the average S—C (1.773 Å) and C—C (1.342 Å) bond lengths serve as useful benchmarks of the intraligand distances of alkene-1,2-dithiolates in the absence of a metal ion (Fig. 10). The dithiolene ligand can be liberated from the thiophosphoryl thiolate by straightforward base hydrolysis. The benefit is that well-defined stoichiometric amounts of ligand can be added to metals circumventing bis- and tris(dithiolene) thermodynamic dead ends (22,23,95).

Figure 10. Crystallographic structures of (a) (dadt)P(S)(SMe) and (b) (pdt)P(S)(SMe).

4. Other Sulfur Sources

The simplest of all dithiolenes, ethene-1,2-dithiolate, (edt)2−, is derived from the combination of cis-1,2-dichloroethylene, benzoyl chloride, and thiourea (Scheme 7) (29). The benzoyl-protected thioether is cleaved via base hydrolysis to give multi-gram quantities of (Li/Na)2edt (97). The diethyl-substituted analogue is formed via a Pd catalyzed cross-coupling of bis(triisopropylsilyl)disulfide and hex-3-yne to give 3,4-bis(triisopropylsilanylsulfanyl)hex-3-ene (98). The silyl groups are jettisoned during the reaction with the metal precursor. This method has been used to assemble mono(dithiolene) analogues of the active sites of pyranopterin-containing Mo and W enzymes (99,100). To date, only 1,2-diethylethene-1,2-dithiolate, (etdt)2−, has been complexed with transition metal ions, but the versatility of this approach has been demonstrated with numerous substituted alkynes, and the products can be further trapped by including methyl iodide to form 1,2-alkylthioolefins as precursors to alkene-1,2-dithiolates (101). The ligand has also been prepared as a thiophosphoryl dithiolene (95).

Scheme 7. Synthesis of Na2edt.

Dipotassium salts of 1,2-dithiocroconate, K2dtcr (102,103), and 1,2-dithiosquarate, K2dtsq (104), are prepared by treating the corresponding dimethylcroconate and diethylsquarate molecules with potassium hydrogen sulfide (Scheme 8). Modification of the five-membered croconate ring is accomplished postcomplexation. For instance, malononitrile displaces a ketyl group to give tris(dithiolene) complexes with 4-dicyanomethyl-1,2-dithiocroconate, (dcmdtcr)2−, ligands (Scheme 8) (103).

Scheme 8. Preparation of (dtcr)2− and (dcmdtcr)2− complexes with M = Cr(III), Fe(III), and Co(III).

C. Dithiones

Collectively known as dithioxamides, these dithiones are the only known ligands of this type found in dithiolene chemistry (Fig. 11). The entry level compound is rubeanic acid, (SCNH2)2, first identified two centuries ago (105). It is prepared by bubbling H2S through an aqueous solution of KCN and [Cu(NH3)4]2+ (106), though today it is readily acquired from chemical suppliers. Infrared (IR) spectral data confirmed the dithione structure for the molecule (107). Dthiooxamide has found wide ranging use as a metal deactivator in petroleum products, inhibitor of certain bacteria and dehydrogenases, accelerator of vulcanization, dichroic stain in light polarizing films, and to detect the presence of cuprous ions (108,109). These molecules have a long history in coordination chemistry as they can either bind through the sulfur or nitrogen atoms, or both, depending on the preference of the targeted metal ion (108–113). In alkaline media, deprotonation of the amine groups generates the dianionic form, [S2C2(NH)2]2−, which has a propensity to form insoluble polymeric substances with metals (110). N-Alkyl-substituted variants are prepared by reacting the parent dithiooxamide with a primary amine, or treating the corresponding N,N′-dialkyloxamide with phosphorus pentasulfide (114,115), depending on the desired substitution pattern. Tetra-alkyl substituted dithiooxamides lack amine protons (Fig. 11), and therefore bulky groups favor sulfur coordination of metal ions, a conclusion based on electronic and IR spectroscopy (116–118). However, the existence of {MS6} polyhedra is entirely speculative in the absence of structural evidence to prove three chelating dithiones.

Figure 11. 1,2-Dithione ligands and their abbreviations. Dithiooxamide = dto, methyldithiooxamide =mtdo, dimethyldithiooxamide = dmdto, tetramethyldithiooxamide = tmdto, tetraethyldithiooxamide = tedto, 1,4-Dimethylpiperazine-2,3-dithione = Me2pipdt.

The preparation of Me2pipdt (Fig. 11), commences with the cyclocondensation of N,N′-dimethyl-1,2-diaminoethane with dimethyl oxalate in refluxing toluene to form the N,N′-dimethyloxamide (119). This compound is converted to the corresponding dithiooxamide with p-methoxyphenylthioxophosphine. Several variants are known with different alkyl groups and these have been structurally characterized (120,121). The short S—C length of 1.668(2) Å in Me2pipdt is synonymous with a double bond and the long C—C distance of 1.523(2) Å is consistent with a single bond (Fig. 12). The short C—N distance of 1.352(2) Å and the planar nature of the amide-like units point to a degree of electron delocalization over the SCN atoms, which stabilizes the dithione form and dissuades dimerization with a neighboring molecule.

Figure 12. Crystal structure of Me2pipdt.

Neutral square-planar compounds of the type [M(L2−)(L0)], containing a dianionic dithiolate and a neutral dithione, have found application as near-infrared (NIR) dyes and nonlinear optical (NLO) materials (122,123). These “push–pull” complexes are so-named because the electron-donating dithione pushes and the electron-withdrawing dithiolate pulls. Complexes with two or three dithione ligands are the only known cationic species in transition metal dithiolene chemistry. A single tris(dithiolene) complex is known, this being [Fe(Me2pipdt)3]2+ (124). Dithiones are considerably weaker donor ligands than their dithiolate counterparts and struggle to chelate early transition metal ions.

III. Complexes

Attaching the aforementioned dithiolene ligands to metal ions is rather trivial in comparison to the synthesis of the ligands themselves. The vast majority of tris(dithiolene) complexes are prepared from combining the correct stoichiometric ratio of free dithiol or dialkali dithiolate with an appropriate metal reagent, principally metal chlorides. Alternative methods include transmetalation and alkyne reduction by a metal sulfide.

A. Metathesis

The metathetical approach works for most arene-1,2-dithiolates [bdt, tdt, xdt, qdt, tbbdt (3,5-di-tert-butylbenzene-1,2-dithiolate), bdtF4 (3,4,5,6-tetrafluorobenzene-1,2-dithiolate), bdtCl4 (3,4,5,6-tetrachlorobenzene-1,2-dithiolate), bdtCl2, tms, bdt-crown ethers, bn-bdt3 [(1,3,5-tris(amidomethylbenzenedithiolate)benzene], dbddto], Na2mnt, deprotected 1,3-dithiole-2-ones, (Na/K)2dmit, Na2dmt, dipotassium 5,6-dihydro-1,4-dithiin-2,3-dithiolate, K2dddt, and K2dtcr. The assembled tris(dithiolene) complexes are typically anionic and charge balanced by alkali metals that are replaced by larger ammonium, phosphonium, and arsonium cations to facilitate crystallization. Neutral complexes are formed by exposing the reaction mixture to the atmosphere. Group 4 (IV B) and 5 (V B) complexes with (bdt)2− and (tdt)2− ligands are innovatively prepared from metal amide rather than chloride precursors (Eq. 2) (125,126).

The procedure requires both the free dithiol and its singly deprotonated form, (bdtH)1−, achieved by including a base (e.g., sodium cyclopentadienide or n-butyllithium). The alkali metal is replaced with a bulkier countercation. The dianionic [Sn(bdt)3]2− and [Sn(tdt)3]2− analogues were produced from the reaction of [Sn(NMe2)2] with 3 equiv of dithiol (127). Switching to the alkoxide, [Ti(i-PrO)4], affords the same result as for the amido precursor (128), and remains the reagent of choice for the elegant array of supramolecular constructs by Hahn and co-workers (129–133).

The tris(dithiolene) complexes [Fe(mnt)3]3− (134), [Fe(mnt)3]2− (134,135), [Co(mnt)3]3− (135–137), and [Fe(bdtCl4)3]2− (43), are formed during the preparation of the corresponding bis(dithiolene) complexes: [Fe(mnt)2]22− [Co(mnt)2]2−, and [Fe(bdtCl4)2]n, respectively. These two metals mark the point along the first-row, where homoleptic complexes transition from tris(dithiolene) species to bis(dithiolene) ones (135). An extra dose of ligand (Na2mnt or bdtCl4H2) generates the tris(dithiolene) complex. A plethora of related Lewis base adducts has been documented (138).

Thiophosphoresters formed in the reaction of an acyloin and a three- to sixfold excess of P4S10 are added to metal chlorides or high-valent oxides of V, Mo, W, and Re (94,96). The simultaneous addition of dilute HCl cleaves the ester and generates the dithiol form of the ligand, which is in turn stabilized by complexation. Neutral complexes, [M(pdt)3], prevail for Mo, W, and Re, as the reaction is stirred in air. The analogous complexes [M(mdt)3] (M = Mo, W, Re) (96,139,140), [W(dtdt)3] (96,139), [W(dadt)3] (96,139), and [W(andt)3], where (andt)2− is 1,2-dianilylethene-1,2-dithiolate (141), have been prepared by this method. Hydrazine added to the thiophosphorester solution prior to the addition of [VO(acac)2], where (acac)1− is acetylacetonate, and NEt4Br results in the monoanionic complex, NEt4[V(pdt)3] (96). Neutral [V(dtdt)3] and [V(dadt)3] stem from oxidation of the corresponding monoanions (139). Davison et al. (142) used anhydrous VCl3 in combination with dilute NaOH to isolate this compound. Hydrazine or hydroxide can reduce the neutral species to its monoanionic complex. As mentioned above, the thiophosphorester can be alkylated by introducing an appropriate reagent after removal of excess P4S10. These isolable and structurally characterized thiophoshoryl dithiolenes can be activated for metal chelation following standard base hydrolysis conditions (95).

High-valent oxides of V ([VO(acac)2], [VO]SO4·xH2O), Mo (Na2[MoO4]·2H2O, (NH4)6[Mo7O24]·4H2O, [MoO2(acac)2]), Tc(K[TcO4]), W (Na2[WO4]·2H2O), and Re([Re2O7]), are alternative reagents for the preparation of tris(dithiolene) complexes with (tdt)2− (31,143,144), (bn-bdt3)6−, where bn = benzene (145), (tr–bdt3)6−, where tr = triazole (145), (edt)2− (96), (sdt)2− (146), and (pdt)2− (94,96). The reaction uses HCl to labilize the oxo ligands.

Heteroleptic complexes are sparingly encountered in tris(dithiolene) chemistry. All known examples are prepared by metathetical procedures, where the dithiol or dithiolate were added to a metallo-bis(dithiolene) unit with one or two labile ligands. The earliest reported complexes are [Mo(pdt)2(mnt)] and [Mo(pdt)2(edt)] formed by acidification of a mixture of [Mo(pdt)2(CO)2] and Na2mnt or Na2edt, respectively (147). Katakis and co-workers (148) combined equimolar amounts of 4-(4-methoxyphenyl)-1,3-dithiole-2-one and 4-(4-bromophenyl)-1,3-dithiole-2-one with [WBr4(MeCN)2] to form four different complexes. Each was separated by column chromatography: [W(adt)3] with 1:9 benzene/cyclohexane as eluent; [W(adt)2(bsdt)] with 3:7 benzene/cyclohexane; [W(adt)(bsdt)2] with 3:2 benzene/cyclohexane; and [W(bsdt)3] with neat benzene.

A truly elegant series of tris(dithiolene) complexes, [Mo(tfd)x(bdt)3−x] (x =0–3), where tfd = bis(trifluoromethyl)ethene-1,2-dithiolate, was recently prepared by Fekl and co-workers (149). The heteroleptic combinations are described in Scheme 9, where an oxo ligand is replaced by (bdt)2− to give [Mo(tfd)2(bdt)]1− prior to oxidation by [Mo(tfd)3], converting it to the neutral form. Labile phosphine ligands are displaced by bis(trifluoromethyl)dithiete to give neutral [Mo(tfd)(bdt)2]. Addition of ethylene to this species generates a complex adduct [Mo(tfd)(bdt){bdt(CH2CH2)}], where the alkene adds across the SS unit of (bdt)2− to form 2,3-dihydro-1,4-benzodithiin, bdt(CH2CH2). Such nucleophilic addition reactions have been performed previously (150), and the weakly bound thioether ligand was displaced by (mnt)2− to give [Mo(tfd)(bdt)(mnt)]2−, the only known complex with three different dithiolene ligands (149).

Scheme 9. Reaction sequences for preparation of heteroleptic complexes, (a) [Mo(tfd)2(bdt)], and (b) [Mo(tfd)(bdt)2] and [Mo(tfd)(bdt)(mnt)]2−.

B. Redox

The first compound of this type, [Mo(tfd)3], was formed from the reaction of bis(trifluoromethyl)dithiete and molybdenum hexacarbonyl (1). The tungsten analogue was also prepared, but necessitated longer reaction times (72 h) due to the inherent reluctance of [W(CO)6] to surrender its ligands. The reaction with [Cr(CO)6] proceeds to completion in <5 h to form [Cr(tfd)3] (10,65). Davison et al. (134) claimed to have formed [Fe(tfd)3] via the same procedure, however, this has been revised as dimeric [Fe2(tfd)4] (152,153), following structural characterization of [Co2(tfd)4] (154). The conversion to the tris(dithiolene) complex is carried out in the high boiling point solvents methyl or ethyl cyclohexane at 100–130 °C wherein the zero-valent metal is oxidized to formally a +VI ion by three dithietes. The corresponding vanadium complex, [V(tfd)3]1−, is oxidized to a formally +V ion (142). The complex dianions [M(tfd)3]2− (M = Mo, W) have been isolated from the reaction of [MS4]2− and 3 equiv of hexafluorobut-2-ene (50). Here, the high-valent tetrathiometalate reduces the alkyne to an alkene via induced internal electron transfer. Similar reactions with activated alkynes, principally the aforementioned dialkylacetylene dicarboxylates, led to tris(dithiolene) dianions of V, Mo, and W from the corresponding tetrathiometalates or [MoS(S4)2]2− (53–58). Following the successful production of [Ni(pdt)2] (3,58), Schrauzer et al. (93) isolated the [M(pdt)3] (M = Cr, Mo, W) from the combination of zero-valent hexacarbonyl, sulfur, and diphenylacetylene. Rauchfuss and co-workers (155) utilized the same reagents in the reaction with 3 equiv of tetrathiapentalenedione, which gave [M(dmid)3]2− (M = Mo, W). This esoteric reaction generates 3 equiv of COS; neither CO2 nor CS2 were detected. The peripheral ketones on each of the (dmid)2− ligands can then be hydrolyzed and alkylated to give three 1,4-dithiin-2,3-dithiolates bound to the metal ion; an example exists with Mo (155).

Photolysis of a metal hexacarbonyl in the presence of a dithiete or a dithione provides an alternative method to forming tris(dithiolene) complexes, following the same procedure for quinones (156,157). Carbon monoxide can be liberated from 1,3-dithiole-2-ones using ultraviolet (UV) light, and the ensuing dithione is captured by a transition metal ion. The dark green neutral complexes [Mo(pdt)3] (56), [Mo(andt)3] (56), [Mo(thdt)3] (69) [Mo(cydt)3] (57), and [Mo(mtdt)3] (mtdt2− = 1,2-bis(methylthio)ethene-1,2-dithiolate) (158), have all been prepared in this manner.

Access to different members of each tris(dithiolene) electron-transfer series is accomplished by using an appropriate oxidizing/reducing agent. Many complexes are synthesized in reactions open to the atmosphere and therein the most air-stable form prevails; relevant examples being neutral complexes of Mo, W, and Re following Schrauzer's thiophosphorester synthetic approach (8,96). These complexes are readily transformed by mild reducing agents (e.g., hydroxide) to the corresponding monoanions. More potent reagents (e.g., hydrazine, n-butyllithium, elemental sodium, and cobaltocene) have been used to generate more reduced species. Selecting the appropriate reducing agent is dependent on both the reduction potential for the parent compound and the solvent in which to perform the conversion in order to eliminate counterproductive side reactions. Alternatively, reduced tris(dithiolene) complexes isolated from anaerobic reactions, such as Li2[Mo(bdt)3] (159), can be sequentially oxidized. Many oxidizing agents have been used from simple ferrocenium salts and halogens, to neutral complexes [Ni(tfd)2] and [Mo(tfd)3] (160). Complete or partial oxidation with radical cation salts of TTF leads to paramagnetic materials with attractive magnetic and conductive properties (15,73,77,161).

C. Transmetalation

An efficient and high-yielding approach to various tris(dmit) complexes involves combining the metal chloride with [Zn(dmit)2]2− (78,81). This complex is highly soluble in a wide range of solvents and the ligand stability is greatly enhanced when coordinated to Zn(II) (162). The combination of (NBu4)2[Zn(dmit)2] with anhydrous VCl3 and ReCl5 led to the clean isolation of (NBu4)2[V(dmit)3] and (NBu4)2[Re(dmit)3], respectively (161,163). Using Na2dmit mainly gave [VO(dmit)2]2− and [ReO(dmit)2]1− (161). Analogues with TTF based dithiolene ligands have been prepared from the reaction of their Zn(II) salt and VCl3 to create a series, [V(R2TTFdt)3]2− (R = Et, n-Bu; R2 = —CH2CH2CH2—) (91). Dithiolate salts of Zn obtained from a commercial source were used to prepare [W(tdt)3] and [Re(tdt)3] from acidified aqueous solutions of Na2[WO4] and NH4[ReO4], respectively (144).

Transmetalation with dithiolenes dates to experiments in the mid-1960s by Schrauzer et al. (147), who monitored the transference of ligands from [Ni(pdt)2] to [M(CO)6] (M = Cr, Mo, W). The major product was the thermodynamically favored [M(pdt)3] (M = Cr, Mo, W), although rather forcing conditions were employed. The desired heteroleptic carbonyl–dithiolene complexes were preferably isolated via UV irradiation of mixtures of [Ni(pdt)2] and [M(CO)6] (M = Cr, Mo, W) forming products that retained CO ligands. Holm and co-workers (22,23) used transmetalation from bis(dithiolene)nickel complexes to generate Mo and W small molecule analogues of oxotransferase enzyme active sites. They frequently encountered the facile formation of tris(dithiolene) species of Mo and W. These reaction sinks are conveniently separated from the product mixture by column chromatography.

The isolation of mixed dithiolene–carbonyl complexes of Mo and W has been elegantly performed using Sn protected dithiolenes (72). The process involves base hydrolysis of 4,5-dimethyl-1,3-dithiole-2-one to form (mdt)2−, which is readily scavenged by [SnCl2(n-Bu)2] giving a colorless precipitate. Purification by column chromatography successfully led to isolation of [Sn(mdt)(n-Bu)2] in modest yields, but on a multi-gram scale. The Sn complex was crystallographically characterized exhibiting S—C distances 1.778(6) and 1.776(3) Å, and a C—C bond length of 1.338(8) Å (Fig. 13).

Figure 13. Molecular structure of [Sn(mdt)(n-Bu)2].

These distances are synonymous with a dianionic dithiolate ligand, and the 0.3 Å increase reflects the loss of a complete set of π bonds around the 1,3,2-dithiastannisole ring system compared to the 1,3-dithiole-2-one (71). Tin-dithiolene complexes have been structurally characterized with (mnt)2− and (dmit)2− ligands (164). This ligand chaperone substantially improved the yield of [Ni(mdt)2] in comparison to the original Schrauzer method (166) negating the need for further purification. The technique was applied to the synthesis of [W(mdt)3] in 80% yield (Scheme 10), with [SnCl2(n-Bu)2] re-formed in the process. This product was the first tris(dithiolene) complex synthesized using a Sn based dithiolene-transfer reagent despite being successful in the preparation of other dithiolene complexes (167–169).

Scheme 10. Synthesis of [W(mdt)3].

Although not a metal, silyl-protected (etdt)2− cleanly transfers to a variety of metals without a separate deprotection step (98). Akin to the aforementioned Sn chemistry, the reaction is driven by the formation of strong M—S in the tris(dithiolene) complexes and Si—F/Cl/O bonds in the byproducts.

IV. Structures

Since the most recent structural update in this Forum (170), the number of reported tris(dithiolene) crystal structures found in the Cambridge Structural Data Centre has trebled (24). Figure 14 displays the distribution of structurally characterized complexes across the periodic table along with a comparison to the 2004 compilation published by Stiefel and co-workers (172). For the most part, the increase in population number stems from repeats of existing structures particularly in the case of Ti and Sn. On the other hand, the first structures containing Mn and f block elements Nd and U are welcomed into the family.

Figure 14. Distribution and frequency of tris(dithiolene) complexes in 2004 and 2013.

Intrigue in transition metal dithiolene compounds stemmed from their vibrant colors and rich redox chemistry. It is perhaps not surprising that the dawn of dithiolene chemistry and related coordination complexes coincided with the rise of single-crystal X-ray diffractometry. Although an established technique, it was not until the 1950s that automated diffractometers become available and accessible to chemists. The impact on the field was profound, especially in regard to the unusual non-octahedral geometry adopted by tris(dithiolene) compounds (24,25). Prior to X-ray crystallography, structures and formulas were derived from spectroscopic data [electronic absorption and electron paramagnetic resonance (EPR)] and accurate elemental analysis. At the same time, the concept of ligand field theory (LFT) was developed representing an amalgamation of crystal field and molecular orbital (MO) theory. It seemed appropriate to apply the benefits of LFT to these new dithiolene complexes, and this required their molecular structure to be defined.

A. Beginnings

1. Neutral Complexes

Eisenberg, armed with crystals from Schrauzer, undertook the structure determination of [Re(pdt)3] at the Brookhaven National Laboratory with Ibers (24,25). This research followed from the prior year's successful characterization of square-planar (NMe4)2[Ni(mnt)2] (5,171). After laboriously sifting through diffraction data and manually estimating their intensities, the structure was defined as a trigonal prismatic (TP) array of sulfur atoms about a central Re atom (Fig. 15a) (172,173). This structure was the first example of a coordination complex bearing TP geometry that was considered implausible for six-coordination complexes as an octahedral arrangement minimizes interligand repulsion. Each {ReS2C2} metallodithiolene ring was planar and the polyhedron adopted D3h point symmetry remarkably similar to the Mo site in molybdenite (174). Almost simultaneously, a second neutral tris(dithiolene) complex was crystallographically characterized (Fig. 15b), namely, [Mo(edt)3], with the same TP polyhedron (175). This compound exhibited a previously unseen structural distortion that lowered the molecule to C3h point symmetry by virtue of a pronounced bend along the SS vector of each ligand.

Figure 15. Diagrams showing (a) the molecular structure of [Re(pdt)3] and its TP coordination geometry as originally determined in 1964 compared with the 2006 redetermination shown as a thermal ellipsoid plot. [Adapted from (170,171,174)]; (b) The molecular structure of [Mo(edt)3] showing TP geometry and folded dithiolene ligands [adapted from (173)]. (c) Comparison of the original and modern molecular structure of [V(pdt)3] from the 1966 and 2010 determinations. [Adapted from (177–179)].

The periodic diagonal was completed in 1966 with the structural characterization of [V(pdt)3] by Eisenberg et al. (177,178) exhibiting TP geometry (Fig. 15c). The maxim that six-coordinate equated to octahedral was shattered by these structure determinations, and tris(dithiolene) complexes distinguished themselves with this unique geometry. Moreover, it was clear that these were not anomalous results imposed by lattice forces, as electronic spectra revealed the geometry persisted in solution (177,180,181). In the absence of diffraction quality single crystals, X-ray powder diffraction was sought to ascertain how widespread this motif was across tris(dithiolene) complexes. Using [Re(pdt)3] as a calibrant, and adhering to the general idea that isomorphous materials are isostructural, powder patterns were recorded for a vast number of compounds with conflicting results. Neutral complexes [W(pdt)3], [W(bdt)3], and [W(tdt)3] showed similar patterns to the corresponding Re species, and were diagnosed as TP (172,180). Similar powder patterns were obtained for [V(pdt)3]1−, [V(pdt)3], [Cr(pdt)3], and [Mo(pdt)3], and these were deemed TP despite differing from their third-row analogues (172,173). The subsequent structural report of [V(pdt)3] consolidated this conclusion (177,178). Schrauzer and co-workers (175) reported the powder patterns of [M(edt)3] (M = V, Mo, W, Re), concluding that the Mo and W species were definitely different from the Re compound, and the V species was decidedly different from the others. This finding disagreed with the conclusions drawn by the team at Columbia University stating that [W(pdt)3] is isomorphous with isoelectronic [V(pdt)3]1−, [Cr(pdt)3], and [Mo(pdt)3], but not [Re(pdt)3]. Powder diffraction studies of [Ru(pdt)3] and [Os(pdt)3] gave distinctly different patterns to their group 6 (VI B) and 7 (VII B) analogues, and were even distinct from each other (175). The veracity of these speculations mandated single-crystal diffraction studies, which in most cases, were not forthcoming for several decades.

The prism dimensions in these three structures are strikingly similar (177). The M—S distances are invariant across the series despite the increase in the ionic radius descending the d block (Table I) (182,183). The original text quoted radii from Paulings seminal work The Nature of the Chemical Bond, however, the values have been refined and reinterpreted over the years. This text utilizes corrected ionic radii by Shannon (182), which for all purposes are identical to those catalogued by Pauling. A recent update of covalent radii has been reported and lists the values for V, Mo, and Re as 153, 154, and 151 pm, respectively, which indicates that the M—S bond length and size of the prism should be identical in all three structures. However, this was not known at the time and has been omitted from the main text so as not to undermine the argument made by proponents of interligand bonding. The size of the prism, where the length is given by the intraligand distance (SSintra) and width by the interligand distance (SSinter) of the trigonal SSS face, were only marginally smaller than the same motif characterized for the infinitely extended molybdenite lattice with the empirical formula MoS2 (Table I) (174). This observation led researchers to hypothesize the unusual geometry is stabilized by an SS bonding interaction as the interligand distance of ∼3 Å lies well within the sum of the van der Waals radii (Fig. 16) (172,173,177,178,184). The same interligand distance was observed in bis(dithiolene) complexes (171,184–187). The bonding interaction was suggested to arise from partial oxidation of the ligand system. Neutral [V(pdt)3] exemplifies this point, as the formal oxidation state of the metal is +VI if the ligands are regarded as dianionic dithiolates (181). Interestingly, ab initio theoretical studies on the sulfur analogue of superoxide, (S2•)1−, produced S—S bond distances ranging from 2.8–2.9 Å that are marginally shorter than the interligand contacts in these TP structures (188,189). The envelope distortion of the dithiolene ligands in [Mo(edt)3] attenuates the interligand interaction resulting in a slightly larger prism (Fig. 15b). The structure of a diselenolene analogue, [Mo{Se2C2(CF3)23], also exhibited this distortion to C3h symmetry (190). The likelihood of oxidized ligands was further evidenced by the intraligand bond distances in these neutral complexes. Removal of electrons from a dithiolate leads to a shortening of the S—C bonds and lengthening of the C—C bond as they attain more double- and single-bond character, respectively (Scheme 1). The average S—C and C—C bond distances listed in Table I express a degree of ligand oxidation as these values are distinct from the dithiolate dianions characterized in the structures of [Ni(mnt)2]2− (171), [Co(mnt)2]2− (186), and [Cu(mnt)2]2− (187) (1.73 and 1.32 Å, respectively). However, the low quality of the data undermined the usefulness of these ligand metrics.

Table I Comparison of Metric Parameters for Three Tris(dithiolene) Complexes and Molybdenite, MoS2

Complex

Ionic Radius

a

Θ

b

M—S

c

S

S

intra

d

S

S

inter

e

S—C

c

C—C

c

[V(pdt)

3

]

68

4.3

2.34

3.06

3.06

1.69

1.41

[Mo(edt)

3

]

75

0

2.33

3.10

3.11

1.70

1.34

[Re(pdt)

3

]

72

3.4

2.32

3.03

3.05

1.69

1.34

MoS

2

f

79

0

2.41

3.17

3.13

a

Ionic radii in picometers (pm) were taken from (182) with the metal ion in the +V oxidation state in each complex and +IV oxidation state for MoS

2

.

b

Mean twist angle, Θ = (Θ

1

+ Θ

2

+ Θ

3

)/3.

c

Average distance in angstroms (Å).

d

Average distance between S atoms within the dithiolene ligand, in angstroms (Å).

e

Average distance between adjacent S atoms related by the threefold axis, in angstroms (Å).

f

The S

S values define the dimensions of the TP array of sulfur atoms in the structure presented in (174).

Figure 16. Pictorial representation of the sulfur bonding interaction between trigonal prismatically arranged dithiolene ligands.

Several conventions have been developed to describe the topology in these TP complexes. The trigonal twist angle (Θ) is defined as the dihedral angle between the S atoms in the two-dimensional projection along the threefold axis (Fig. 17) (191–195). In six-coordinate complexes, Θ ranges from 0° in a perfect TP to 60° in a regular octahedron (or trigonal antiprism). However, chelate ligands by their very composition possess a maximum twist angle as a function of the M—L bond length and ligand bite (L—M—L) angle (194–197). For example, Θ is limited to ∼48° in (bdt)2− complexes of early transition metals (198). Therefore, the trigonal twist represents a D3h → D3 symmetry reduction. The trigonal twist angles for [V(pdt)3], [Mo(edt)3], and [Re(pdt)3] are listed in Table I, and show each to be a (nearly) perfect prism. Other parameters used to gauge the complex geometry include the trans SMS angle (136° for TP; 180° for octahedral) and the dihedral angle between the MS2 and trigonal SSS planes (90° for TP; ∼55° for octahedral) (170). The folding of the dithiolene ligands in [Mo(edt)3] represents a D3h → C3h symmetry reduction, and is defined by the dihedral angle, α (Fig. 18). Together these two parameters (Θ and α) describe the symmetry of any tris(dithiolene) complex.

Figure 17. Depiction of trigonal twist angle, Θ, as viewed along (a) the C3 axis and (b) the C2′ axis in D3 point symmetry.

Figure 18. Depiction of the dithiolene folding angles, α1, α2, α3 viewed along the threefold axis.

2. Reduced Complexes

A largely antitrigonal prismatic geometry was reported for the crystal structure of (NMe4)2[V(mnt)3] reported in 1967 (Fig. 19) (199). With Θ = 38° and the SSinter stretched to 3.15 Å, it was postulated that the twist away from TP alleviated the increased ligand repulsion generated by two extra valence electrons compared to [V(pdt)3]. The ligands were considered dianionic dithiolates, and the intraligand S—C distance averaging 1.72 Å and C—C bond length of 1.34 Å correlate with the reduced dithiolene level. Therefore, the twisted structure was regarded as a compromise between residual SS bonding and interligand repulsion.

Figure 19. Depiction of the structure of (NMe4)2[V(mnt)3]. [Adapted from (199).]

X-ray powder diffraction patterns were collected for first-row [M(mnt)3]2− (M = Ti, V, Cr, Mn, Fe) complexes (199,200). All were described as isostructural with distorted octahedral structures and +IV metal ions coordination by three (mnt)2− ligands. The crystal structure of (AsPh4)2[Fe(mnt)3] solved in 1973 cemented this conclusion, with Θ = 49° (201). The average Fe—S distance at 2.266 Å is appreciably shorter than the corresponding distance of 2.36 Å in [V(mnt)3]2− (199), which can be ascribed to the difference in the covalent radius of the metal ion. The consequence were shorter interligand contacts at 3.10 Å, similar in magnitude to the three neutral complexes described above. The increased Fe—S covalency overrides ligand repulsion in this reduced complex, as potential SS bonding is greatly perturbed by the twist to octahedral and subsequent misalignment of adjacent S p orbitals (Fig. 16). A similar outcome was observed for (AsPh4)2[M(mnt)3