Room temperature conductivity, dithiolene

In an attempt to maximize interactions between the metal dithiolene anions, Underhill and Ahmad studied the lithium salt of the [Pt S2C2(CN)2 2] cation.121,122 Slow aerial oxidation of a 50% aqueous acetone solution of H2[Pt S2C2(CN)2 2] and LiCl yielded a black microcrystalline product of small shining black needles and black platelets. Four-probe DC conduction studies on the needle-shaped crystals showed the room temperature conductivity along the needle axis to be 100 Q i cm-1. 

Electrical conductivity measurements have been made on a few of these metal dithiolene systems and show that they are, at best, poor semiconductors, with room-temperature conductivities of less than 10 6Q 1cm 1. This is not surprising as the molecules are in an integral oxidation state. Moreover, the existence of dimers destroys the crystallographic uniformity of the stacks and, as the magnetic studies have shown, the unpaired electrons tend to couple within the dimers rather than delocalize along the stacks. 

Very few dithiolene platinum complex-based compounds have been reported and most of them exhibit poor electrical conductivity (9, 122, 123). The complex (Me4N)[Pt(dmit)2]2 does show metal-like behavior (RT conductivity = 10 S cm 1) but only >220 K (123). Another exception is (NHMe3)[Pt(dmit)2]3. This compound, which has the unusual 1 3 stoichiometry, behaves as a metal down to 180 K, with a room temperature conductivity of 140 S cm-1 (91). The Pt(dmit)2 units form pairs with an eclipsed overlap mode. Unlike the analogue Ni derivatives (see above) substituting one hydrogen atom for one methyl group in the (Me4N)+ cation leads to an improvement in the electrical properties. 

The (BMDT-TTF)JC[Ni(dcit)2] compound (see Scheme 12) involves the (BMDT-TTF) donor CgH4Sg, bis(methylenedithio)tetrathiafulvalene] (187) and the Ni(dcit)2 heterocyclic dithiolene complex (dcit2- = [C4N2S3]2-). Room temperature conductivity was reported to be 20-40 S cm 1 and the... 

Recently, higher conductivities have been reported for several dithiolene complexes (5, 427, 505, 506). For example, (perylene)2[Ni(S2C2(CN)2)2] has a room temperature conductivity of 50i2 cm i with an energy of activation of 0.1 eV. These conductivites have been attributed to conduction through the organic cation and not through the dithiolene anion (5). 

Over the past decade a number of new covalently bonded TTF/ferrocene adducts have been reported [77, 78]. The crystal structure of the l,l -bis(l,3-dithiole-2-ylidine)-substituted ferrocene derivative has been published [77]. In this complex, ferrocene has essentially been incorporated as a molecular spacer between the two l,3-dithole-2-ylidene rings forming a stretched TTF molecule. This adduct, and its methyl-substituted derivative, have been combined with TCNQ to form charge-transfer complexes with room temperature powder conductivities of 0.2 S cm-1. Similar diferrocenyl complexes have been prepared with bis (dithiolene) metal complexes [79, 80]. 

A wide range of metal dithiolene complexes have been prepared and their electrical conduction properties reported.111-114 They include neutral, monoanion and dianion complexes with a variety of substituents on the ligand (R = Ph, Me, CN, H, CF3) and a variety of cations. The choice of cation has often been determined by the desire to obtain easily crystallized products and has resulted in the use of rather bulky substituted ammonium salts. The compounds behave as semiconductors with relatively low conductivities at room temperature. It has been shown that the monoanion complexes are considerably more conducting than either the corresponding neutral complex or the dianion, and Rosseinsky and Malpas have proposed that this is related to the ease of disproportionation.113... 

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