Redox-active ligands dithiolenes

The coordination chemistry of 1,2-dithiolenes and related ligands has been reviewed. 1,2-Dithiolenes are bidentate, redox-active ligands which can coordinate in the hmit as dithiodiketones or dithiolates (Scheme 18). Scheme 19 illustrates the more common dithiolene ligands that have been investigated. Syntheses of these ligands are well established and are usually based upon derivatization of dihalo and ketonic starting materials (Scheme 20). 

Enedithiolate anions are redox-active ligands (see equation 1), capable of coordinating with a variety of metals as neutral dithioketones (1), thioketone radical thiolate monoanions (2) and ene-l,2-dithiolate dianions (3). The complexes are named dithiolenes irrespective of their oxidation state to remind one that they are noninnocent ligands . 

A. Vlcek, Dithiolenes and Non-innocent Redox-active Ligands, Coord. Chem. Rev., 2010, 254(13-14), 1357-1588. 

Many of the problems and misconceptions occurring for dithiolene compounds are related to the fact that the ligands are redox-active and can be oxidized to monoanionic radicals. Typical examples for this phenomenon are the mono and diradical complexes [Fe ( "bdt )( "bdt)(PMe3)] (9) and [Fe ( "bdf)2(PMe3)]-" (10) for which bdt and bdt are tcrt-butyl-dithiolene and its one-electron oxidized form. Originally, these and other bdt derivatives had been described as... 

Coordinated transition metal redox-active macrocycles, 39 108-124 ammonium cation, 39 128-133 crown ether and bis crown ether ligands containing bipyridyl transition metal recognition sites, 39 111 crown ether dithiocarbamate and dithiolene complexes, 39 123-124 metalloporphyrin crown ether compounds, 39 108-109... 

The numerous properties of metallo-dithiolene complexes are a direct consequence of the fantastic redox interplay between the redox-active metal and the dithiolene ligand. A considerable amount of experimental evidence over the last 40 years has pointed to the fact that dithiolenes are highly noninnocent ligands (20, 86, 110, 113, 280-282). That is, they may be viewed in a valence bond description as existing somewhere between the extremes of neutral (dithione/dithiete) and dianionic (dithiolate) forms (Fig. 1). This variable formal... 

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. 

These complexes are redox active. The two one-electron reductions resemble more closely the reduction of the corresponding bis(diimine) complex than those of the corresponding bis(dithiolene) complex. The redox potentials are more sensitive to diimine ligand variation than to dithiolene variation. One-electron oxidation is relatively insensitive to diimine ligand variation. However, the dependence of one-electron oxidation on dithiolene variation has not been assessed directly due to the limited amount of data available on the oxidation of the corresponding dithiolene complexes. It has been proposed that the LUMO of the neutral mixed-dithiolene diimine complexes possesses more diimine than dithiolene character and that the HOMO is mainly metal d orbital in nature (52). 

In summary, bis(dithiolene) complexes are clearly distinct from traditional inorganic or organometallic complexes in which the chemical reactivity is dominated by the metal center. The unique properties of dithiolene ligands such as redox activity, aromaticity, and unsaturation of the metal-ligand chelate rings, in combination with the metal-centered reactivity paths, have generated many unusual reactivity patterns for this class of complexes. 

Other donors that can form mixed-ligand complexes with dithiolene ligands include phosphorous-, nitrogen-, oxygen-, and other sulfur-based ligands. The resultant complexes are abundant and most of them are redox active. However,... 

Among the numerous S-donor chelating ligand complexes, the 1,2-dithiolene derivatives are peculiar since these ligands are redox active and therefore easily and reversibly oxidized and reduced. This makes it often difficult to assign oxidation states to the metal see Dithiolenes and Section 7). ... 

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