Dithiolene

A 1,2-dithiolene is an unsaturated bidentate ligand wherein the two donor atoms are sulfur. These ligands readily form complexes by co-ordinating to metal centres via the sulfur atoms; such complexes are often referred to as "metallodithiolenes" or "dithiolene complexes".^[1]

Dithiolenes have been intensely studied since the 1960's, when they were first popularized by G. N. Schrauzer, who prepared Ni(S₂C₂Ph₂)₂ by the reaction of nickel sulfide and diphenylacetylene.^[2] A large number of such complexes have been described with regards to their structural, spectroscopic, and electrochemical properties. Research on dithiolenes intensified with the discovery that most molybedenum-containing and tungsten-containing proteins contain dithiolene ligands, the so-called molybdopterin cofactor.^[3]

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Background and nomenclature

Early studies on dithiolene ligands, although not called by that name until the the 1960's,^[4] focused on the quinoxaline-2,3-dithiolates and 3,4-toluenedithiolates, which form brightly colored precipitates with several metal centres. Such species were originally of interest in analytical chemistry. Dithiolenes lacking benzene backbones represented an important development of the area, especially maleonitrile-2,3-ditholate ("mnt"), (NC)₂C₂S₂^2-, and ethylenedithiolene, H₂C₂S₂^2-.

Electronic structure

The electronic structure of dithiolene complexes has been the subject of intense study because of their unusual redox properties. Dithiolene ligands are often thought to exist in two oxidation states separated by a two-electron redox; the dianionic "ene-1,2-dithiolate" and the neutral "1,2-dithioketone," which possess different electronic characteristics. 1,2-dithiones have not been characterized crystallographically. Dithioketones, more so than thioketones, are expected to be unstable with respect to self-condensation to give oligomers.

The two substituents on the backbone of the dithiolene ligand, R and R', affect the properties of the resulting metal complex in the expected way. Long chains confer solubility in less polar solvents. Electron acceptors (e.g. CN, CO₂Me) stabilize reduced and anionic complexes. Derivatives are known where the substituents are the same, symmetrical dithiolenes (R = R') are more common than unsymmetrical.

Due to the delocalized nature of dithiolenes, metal dithiolenes often exist in multiple oxidation states. In oxidized dithiolene complexes have relatively more 1,2-dithioketone character. In reduced complexes, the ligand assumes more ene-1,2-dithiolate character. These descriptions are evaluated by examination of differences in C-C and C-S bond distances.

These limiting structures do not represent a true description of the actual structure of the complex, the true structure lies somewhere between. Reflecting the impossiblility to provide an unequivocal description of the structure, McCleverty introduced the term 'dithiolene' to give a general name for the ligand that does not specify a particular oxidations state. This suggestion was generally accepted, and 'dithiolene' is now a universally accepted term.

Preparation

From alkenedithiolates

Most dithiolene complexes are prepared by reaction of alkali metal salts of 1,2-alkenedithiolates with metal halides. A common alkenedithiolate is mnt^2-, which forms a very stable Ni(II) complex:^[5]

Ni²⁺ + 2 (NC)₂C₂S₂^2- → Ni[S₂C₂(CN)₂]₂^2-

This red-colored complex is often used as a standard for electrochemical studies.

For more electron-rich alkenedithiolates, the dianion is generated in situ, treated with the metal salt, and the product is often air-oxidized:

cis-H₂C₂(SCH₂Ph)₂ + 4 Na → cis-H₂C₂(SNa)₂ + 2 NaCH₂Ph

NiCl₂ + 2 cis-H₂C₂(SNa)₂ → Na₂[Ni(S₂C₂H₂)₂] + 2 NaCl

[Ni(S₂C₂H₂)₂]^2- → Ni(S₂C₂H₂)₂ + 2 e^-

From acyloins

An early and still powerful method for the synthesis of dithiolenes entails the reaction of α-hydroxyketones, acyloins, with P₄S₁₀ followed by hydrolysis and treatment of the ill-defined mixture with metal salts. This method is used to prepare Ni[S₂C₂Ar₂]₂ (Ar = aryl).

From dithietes

Although 1,2-dithiones are extremely rare and thus not useful precursors, their valence isomer, the dithietes are occasionally used. Dithietes have even been characterized crystallographically.^[6] Probably the most important dithiete is (CF₃)₂C₂S₂, prepared from the reaction of elemental sulfur and hexafluoro-2-butyne. This electrophilic reagent oxidatively adds to many low valent metals to give bis- and tris(dithiolene) complexes.

Mo(CO)₆ + 3 1,2-(CF₃)₂C₂S₂ → [(CF₃)₂C₂S₂]₃Mo + 6 CO

Ni(CO)₄ + 2 1,2-(CF₃)₂C₂S₂ → [(CF₃)₂C₂S₂]₂Ni + 4 CO

By reactions of metal sulfides with alkynes

Species of the type Ni[S₂C₂Ar₂]₂ were first prepared by reactions of nickel sulfides with diphenylacetylene. More modern versions of this method entail the reaction of electrophilic acetylenes with well defined polysulfido complexes.

Structural characteristics

Dithiolene complexes can be found where the metal centre is coordinated by one, two, or three dithiolene ligands. The tris(dithiolene) complexes were the first examples of trigonal prismatic geometry in coordination chemistry. One example is Mo(S₂C₂Ph₂)₃. Similar structures have been observed for several other metals.^[7]  

Applications

Applications of 1,2-dithiolene complexes have been disccussed in the context of conductivity, magnetism, and nonlinear optics. Relatively few dithiolene complexes have been commercialized for use as dyes in laser applications.

References

1. ^ Karlin, K. D.; Stiefel, E. I., Eds. “Progress in Inorganic Chemistry, Dithiolene Chemistry: Synthesis, Properties, and Applications” Wiley-Interscience: New York, 2003. ISBN 0-471-37829-1
2. ^ Schrauzer, G. N.; Mayweg, V. "Reaction of Piphenylacetylene with Ni Sulfides" Journal of the American Chemical Society 1962, 84 3221. DOI: 10.1021/ja00875a061
3. ^ Romão, M. J.; Archer, M.; Moura, I.; Moura, J. J. G.; Legall, J.; Engh, R.; Schneider, M.; Hof, P. and Huber, R., "Crystal Structure of the Xanthine Oxidase-Related Aldehyde Oxido-Reductase from D-Gigas", Science, 1995, volume 270, pages 1170-1176
4. ^ McCleverty, J. A., "Metal 1,2-Dithiolene and Related Complexes", Progress Inorganic Chemistry, 1968, volume 10, 49-221.
5. ^ R. H. Holm, A. Davison "Metal Complexes Derived from cis-1,2-Dicyano-1,2-Ethylenedithiolate and Bis(trifluoromethyl)-1,2-Dithiete" Inorganic Syntheses 1967, volume X, pp.8-26.
6. ^ Donahue, J. P. and Holm, R. H., "3,4-Bis(1-Adamantyl)-1,2-Dithiete: the First Structurally Characterized Dithiete Unsupported by a Ring or Benzenoid Frame", Acta Crystallographica, Section C: Crystal Structure Communications, 1998, volume C54, 1175-1178.
7. ^ Eisenberg, R. and Gray, H. B., "Trigonal-prismatic coordination. Crystal and Molecular Structure of Tris (cis-1,2-diphenylethylene-1,2-dithiolato)vanadium", Inorganic Chemistry, 1967, volume 6, pp. 1844-9. DOI: 10.1021/ic50056a018