Dithiolenes redox properties

The structures and redox properties of these complexes have been extensively reviewed 170,171 of interest here is the presence of an intense NIR transition in the neutral and mono-anionic forms, but not the dianionic forms, i.e., the complexes are polyelectrochromic. The positions of the NIR absorptions are highly sensitive to the substituents on the dithiolene ligands. A large number of substituted dithiolene ligands has been prepared and used to prepare complexes of Ni, Pd, and Pt which show comparable electrochromic properties with absorption maxima at wavelengths up to ca. 1,400 nm and extinction coefficients up to ca. 40,000 dm3 mol-1 cm-1 (see refs. 170,171 for an extensive listing). 

Transition metal complexes of unsaturated 1,2-dithiolates (metal dithiolenes) have attracted much attention because of their interesting structural and redox properties.169 Molybdenum dithiolene complexes have featured prominently170 in these studies and have special significance following the suggestion171,172 that the molybdenum-containing cofactor of the oxomolybdoen-zymes (Section 36.6.7) incorporates a molybdenum complex of an unsymmetrically substituted alkene-1,2-dithiolate. 

In the transition metal complexes of ligands like o-phenylenediamine, we find a distinct analogy to the chemistry of dithiolenes. The full equivalence of their redox chemistry with that of dithiolenes suggests a comparably delocalized structure. The outward structural similarity between this ligand and the benzenedithiols is not reflected in the redox properties of their transition metal complexes... 

Ab initio and semiempirical methods have been applied to the interpretation of many aspects of dithiolene chemistry electronic spectra, ESR, Mossbauer, XPS, charge distributions, redox properties, reaction mechanisms, metal binding in biological systems and ligand-exchange behavior. We shall focus our attention on the theoretical deductions of some representative research groups. For computational details, the reader is referred to the original papers and references therein. 

A few other applications of dithiolenes make use of their redox properties. Kumar et a/.219 proposed the use of dithiolenes as photosensitizers. Umezawa et al.22<> coated a Pt cathode with (Et4N)Ni(mnt)2 and saw a modest degree (1.4 x 10 4 %) of light conversion upon irradiation. On the opposite side, Bradley et al.721 used dithiolenes to stabilize n-type Si anodes against photoanodic decomposition. 

Most of the late transition metals (such as Fe, Co, Rh, Ir, Ni, Pd, Pt, Cu, Au, and Zn) have been found to form bis(dithiolene) complexes. A significant amount of work has been reported on the electronic structures and spectroscopy (32), redox properties (2), as well as the conductivity (33) of bis(dithiolene) complexes. Far less has been reported on their chemical reactivity. 

In summary, all bis(dithiolene) complexes are redox active most of them undergo two or three reversible, one-electron redox reactions. The dithiolene ligand itself is also redox active, which contributes significantly to the redox properties of the metal complex. Molecular orbital pictures derived from quantum mechanical calculations are consistent with the observed redox potential data. 

Compared to the large body of electrochemical data, there have been fewer studies on the chemical reactivity of bis(dithiolene) complexes. In light of the rich redox chemistry of bis(dithiolene) complexes and the redox-active nature of the dithiolene ligands, it is not surprising that much of the reactivity observed is related to the redox properties and is often centered on the dithiolene ligands. 

Although it has already been studied extensively, the redox and redox-related chemistry remains a dominant theme in dithiolene chemistry. Much of the chemical reactivity reported so far is associated with the dithiolene ligand and is, in many cases, related to the redox properties. Clearly, the range of accessible charge levels of dithiolene complexes may be exploited for new reaction... 

A great amount of research has focused on the electronic structure, spectroscopy, redox properties, and conductivity of homoleptic bis(l,2-dithiolene) complexes of c transition metal ions (51). The compounds are often highly... 

The redox properties of tris(quinoxaline-2,3-dithiolato)molybdate(IV), [Mo(qdt)3]2, in the presence of protons provides a clear demonstration of the chemical versatility that is possible for a redox-active metal dithiolene center that involves a pyrazine ring linked to the dithiolene group. In an aprotic solvent, two reversible, Nernstian, waves are observed that (formally) correspond to the Mo(V)/Mo(IV) and Mo(IV)/Mo(III) couples. However, on addition of trifluoroacetic acid (Htfa), the Mo(V)/Mo(IV) couple slightly shifts to a higher potential and becomes non-Nernstian and a new three-electron, quasir-eversible, couple occurs some 900 mV less negative than the original Mo(IV)/ Mo(HI) couple. The latter is attributed to the addition of one electron and one... 

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