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". 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.

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

Structure and bonding

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.

File:Mo(dith)3.png

Structure of Mo(S₂C₂Ph₂)₃

Because of the unusual redox and intense optical properties of dithiolenes, the 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 their delocalized electronic structure, metal dithiolenes undergo reversible redox reaction. When oxidized, dithiolene complexes have greater 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. The true structure lies somewhere between these resonance structures. 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 oxidation state. This suggestion was generally accepted, and 'dithiolene' is now a universally accepted term.

File:Resonance structures of dithiolene complex.png

Limiting resonance structures of a R₂C₂S₂M ring

Applications and occurrence

Dithiolenes occur widely in nature in the form of the molybdopterin-bound Mo and W-containing enzymes.

File:Activesite.jpg

Active site of the enzyme DMSO reductase features two pyranopterindithiolene ligands.

Commercial applications of 1,2-dithiolene complexes are limited. A few dithiolene complexes have been commercialized as dyes in laser applications. Metal dithiolenes have been discussed in the context of conductivity, magnetism, and nonlinear optics.

Preparation

From alkenedithiolates

Most dithiolene complexes are prepared by reaction of alkali metal salts of 1,2-alkenedithiolates with metal halides. A thiolate is the conjugate base of a thiol, so alkenedithiolate is, formally speaking, the conjugate base of an alkenedithiol. A common alkenedithiolate is mnt²⁻, which forms a very stable Ni(II) complex:

Ni²⁺ + 2 (NC)₂C₂S₂²⁻ → Ni[S₂C₂(CN)₂]₂²⁻

Some alkenedithiolates are generated in situ, often by complex organic reactions:

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

Once generated, these anions are deployed as ligands:

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

Often the initially formed, electron-rich complex undergoes spontaneous air-oxidation:

[Ni(S₂C₂H₂)₂]²⁻ + 2 H⁺ + 1/2 O₂ → Ni(S₂C₂H₂)₂ + H₂O

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 mixture with metal salts. This method is used to prepare Ni[S₂C₂Ar₂]₂ (Ar = aryl).

From dithietes

Although 1,2-dithiones are rare and thus not useful precursors, their valence isomer, the 1,2-dithietes are occasionally used. One of the more common 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 (CF₃)₂C₂S₂ → [(CF₃)₂C₂S₂]₃Mo + 6 CO

Ni(CO)₄ + 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.

History and nomenclature

Early studies on dithiolene ligands, although not called by that name until the 1960s, 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)₂C₂S₂²⁻, and ethylenedithiolene, H₂C₂S₂²⁻.