Superconductivity 1,2-dithiolene complexes

Let us now pass to metallo-dithiolene complexes, which constitute a relatively old class of compounds,68 which are a matter of great renewed interest in the field of superconduction just because of their extended redox propensity.69... 

As with most molecule-based solids, most of the dithiolene complex-based solids are insulators, or, at best, semiconductors, which can hardly be considered as a remarkable property. Therefore, in the following sections we focus on systems exhibiting metal-like or superconducting behavior. 

Metal bis-dithiolene complexes exhibiting conducting or superconducting properties can be neutral or in a fractional oxidation state. In FOSCs, the associated cation is of the closed-shell type, while in DA compounds, the metal bis-dithiolene is an acceptor and the donor is an open-shell cation. As already mentioned in Section 4.2.2.3, the most encountered... 

Ten Ai5(l,2-dithiolene) complexes exhibiting superconducting properties have been reported to date, namely (TTF)[Ni(dmit)2], a-(EDT-TTF)[Ni... 

Abstract Metal complexes including the dmit2- (1,3-dithio-2-thione-4,5-dithiolato) ligand are the only class of metal bis-dithiolenes to give rise to superconductive molecular materials. This chapter first focuses on the description of these superconductive phases. Further sections describe the association of M(dmit)2 moieties with three types of magnetic molecules, i.e., metalloceniums, radical cations, and spin crossover complexes. 

M(dmit)2 complexes are the only metal bis-dithiolenes known to lead to superconductive phases. Twelve compounds have been isolated up to now (Table 1). 

Despite the large number of [Ni(mnt)2]" units, the most investigated Ni dithiolene unit is [Nildmifh]" with 89 examples. The discovery of the Ni-dmit based molecular superconducting material spurred the investigation of metal complexes of dmit and dmit derivatives such as those listed in Table IIA-C [see Chapter 8 in this volume (318)]. 

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). 

It is the only series of dithiolene-based compounds (and the only series of molecular metal complex-based compounds) exhibiting superconducting behavior. 

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