Metal dithiolene complex

Metal dithiolene complexes are complexes containing dithiolene ligands. Dithiolene ligands are unsaturated bidentate ligand wherein the two donor atoms are sulfur. Dithiolenes are often referred to as "metallodithiolenes" or "dithiolene complexes".^[1] Most molybdenum- and tungsten-containing proteins have dithiolene-like moieties at their active sites, which feature the so-called molybdopterin cofactor bound to the Mo or W.^[2]

Metal dithiolenes have been studied since the 1960s when they were first popularized by G. N. Schrauzer, who prepared Ni(S₂C₂Ph₂)₂ by the reaction of nickel sulfide and diphenylacetylene.^[3] The structural, spectroscopic, and electrochemical properties of many related complexes have been described.

Background and nomenclature

Early studies on dithiolene ligands, although not called by that name until the 1960s,^{[4] [5]} 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

Because of the unusual redox and intense optical properties of dithiolenes, their electronic structure of dithiolene complexes has been the subject of intense study. Dithiolene ligands can exist in three oxidation states: the dianionic "ene-1,2-dithiolate", the neutral "1,2-dithioketone," and a monoanionic radical intermediate between these two. When the latter two are complexed to a metal centre, the oxidation state of the ligand (and therefore the metal centre) cannot be easily defined. Such ligands are therefore referred to as non-innocent. The 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.

Limiting resonance structures of a R₂C₂S₂M ring

These limiting structures do not represent a true description of the actual structure of the complex, the true structure lies somewhere between. Reflecting the impossibility 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:^[6]

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. Probably the most important dithiete is the distillable (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 such as dimethyl acetylenedicarboxylate 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]

Structure of Mo(S₂C₂Ph₂)₃

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. ^ Romão, M. J.; Archer, M.; Moura, I.; Moura, J. J. G.; Legall, J.; Engh, R.; Schneider, M.; Hof, P. and Huber, R. (1995). "Crystal Structure of the Xanthine Oxidase-Related Aldehyde Oxido-Reductase from D-Gigas". Science 270 (5239): 1170–1176. doi:10.1126/science.270.5239.1170. PMID 7502041. 
3. ^ Schrauzer, G. N.; Mayweg, V. (1962). "Reaction of Piphenylacetylene with Ni Sulfides". J. Am. Chem. Soc. 84: 3221. doi:10.1021/ja00875a061. 
4. ^ McCleverty, J. A. (1968). "Metal 1,2-Dithiolene and Related Complexes". Progress Inorganic Chemistry 10: 49–221. doi:10.1002/9780470166116.ch2. 
5. ^ Arca,M.; Aragoni, M.C. (2007). "1,2-Dithiolene Ligands and Related Selenium and Tellurium Compounds". Handbook of Chalcogen Chemistry: 797–830. 
6. ^ R. H. Holm, A. Davison (1967). "Metal Complexes Derived from cis-1,2-Dicyano-1,2-Ethylenedithiolate and Bis(trifluoromethyl)-1,2-Dithiete"". Inorg. Synth. 10: 8–26. doi:10.1002/9780470132418.ch3. 
7. ^ Eisenberg, R. and Gray, H. B. (1967). "Trigonal-prismatic coordination. Crystal and Molecular Structure of Tris (cis-1,2-diphenylethylene-1,2-dithiolato)vanadium". Inorg. Chem. 6: 1844–9. doi:10.1021/ic50056a018.