Nickel complexes dithiolenes

Fig. 10. Representation of the mechanism of redox driven K + transport using an electron and a cation carrier. (59-Ni°) and (59-Ni ) are the oxidized and reduced form of the electron carrier, the nickel bis-dithiolene complex 59 [] and [K+] are dicyclohexyl-18-crown-6 and its K+ complex. (Cited from Ref. 59>)...

Several transition metal ions form stable complexes with aliphatic 1,2-dithiols, which absorb in the near-lR. Known as dithiolenes, their nickel complexes in particular have been found to have valuable properties. The physical properties of dithiolenes can be readily tailored by variations on the substituents attached to the dithiols, see (4.13). Although they have low molar absorption coefQcients, when compared to cyanines etc., they do have one big advantage in that they show very little absorption in the visible region." Stracturally analogous dyes can be made from aromatic dithiols and oxothiols (4.14), and the much more bathochromic naphthalene derivatives (4.15), but they are much weaker absorbers. 

The chemistry of bis(l,2-dithiolene) complexes of nickel has stimulated considerable investigations over the past 20 years, owing to the peculiar and unusual electronic and electrochemical properties which the complexes exhibit both in the solid state and in solution. A number of articles cover the early reports up to 19701923 1925 1927 2108 on nickel complexes having general formulas (287) and (288). The formulas of representative complexes together with die synthetic routes and some physicochemical properties are summarized in Table 91. 

Dithiolenes have been found to stabilize nickel(III) and a number of structural investigations have been performed on nickel(III) dithiolene complexes. Structural data and physical properties of selected compounds are collected in Table 120. The EPR spectra of the [NiS4] unit have been extensively studied in order to decide whether the unpaired electron resides mainly on the metal or on the ligand3202,3203,3210,3212-3217 giving rise to a true nickel(III) complex (422) or to a nickel(II)-stabilized ligand radical complex (423). 

The more recent work includes two detailed studies on tetramethylphenylenediamine (TMPD) complexes of Ni- and Pt-mnt180,181 and an example of a complex in which both the donor and the acceptor are nickel complexes.182 However, most recent activity has been devoted to materials in which the strict one-dimensionality of the dithiolene salts and complexes has given way to interstack coupled systems with lesser degrees of anisotropy. 

The group 8 (VIII) metals comprise almost 80% of the structurally characterized homoleptic bis(dithiolene) units. Nickel complexes alone account for... 

Neutral nickel bis(dithiolene) complexes undergo ligand substitution reactions with a variety of amines to form mixed-ligand dithiolene complexes (52, 60). Dance and Miller (60) pointed out that reduction of electron-poor bis(dithiolene) complexes ( i/2 > 0 V vs. SCE) by halides or pseudo-halides, unhindered amines, or common weakly Lewis basic solvents (such as acetonitrile)... 

The mechanism was proposed to involve reduction of the neutral nickel bis(dithiolene) complex by methyl viologen radical cation, followed by dimerization to form the active catalyst Ni(SS)212 2 - as shown in Eqs. 2-6 ... 

In one seminal work on dithiolene compounds, Schrauzer and Mayweg (37) considered three possible structures for neutral dithiolene nickel complexes in which the oxidation state of nickel may be Ni(0), Ni(II), or Ni(IV) [Scheme 8 ... 

Some homoleptic unsymmetrical (dmit/mnt, dmit/tdas) dithiolene nickel complex-based D-A compounds with D = TTF and EDT-TTF also exhibit metal-like conductivity (see Table I) (101). Their molecular structure is shown in Scheme 3. The unsymmetrical tetraalkylammonium salts [MLjLJ- (M = Ni, Pd, Pt) have been prepared by ligand exchange reaction between tetraalkylammonium salts of MLj and ML21 (128, 129) and the D-A compounds have been synthesized by electrooxidation. Among these complexes, only the Ni derivatives exhibit metallic-like properties, namely, TTF[Ni(dmit)(mnt)] (metallic down to --30 K), a-EDT-TTF[Ni(dmit)(mnt)] (metallic down to 30 K), TTF[Ni(dmit)(tdas)] (metallic down to 4.2 K), and EDT-TTF[Ni(dmit)(tdas)] (metallic down to --50 K) (see Table I). The complex ot-EDT-TTF-[Ni(dmit)(mnt)J is isostructural (130) to a-EDT-TTF[Ni(dmit)2)] [ambient pressure superconductor, Section II.B.2 (124)]. Under pressure, conductivity measurements up to 18 kbar show a monotonous decrease of the resistivity but do not reveal any superconducting transition (101). 

These reactions demonstrated the selective alkylation of sulfur donors in dithiolene ligands. However, they appeared to be confined to nickel triad dithiolene complexes, and the resulting multidentate sulfur ligand could not be decoordinated. 

Redox non-innocent ligands have also been employed in other kinds of processes. For example, a nickel-based system has been used in the purification of ethene gas streams [41]. The two forms (reduced and oxidized) of the dithiolene complex have different affinities for olefin, leading to separation of ethene from gas mixtures (Scheme 14). Intermediate 47 is obtained after electrochemical oxidation of the anionic nickel complex 46. The oxidized complex 47 reacts selectively with ethylene to form the adduct 48, thus the non-olefinic contamination of the multi-component stream... 

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