trans-4,4‘-Dichloro-1,1‘,2,2‘,3,3‘-tetrathiadiazafulvalene (DC-TAF) and Its 1:1 Radical Cation Salts [DC-TAF][X]: Preparation and Solid-State Properties of BF4-, ClO4-, and FSO3-Derivatives

Reductive coupling of 4,5-dichloro-1,2,3-dithiazolylium chloride yields trans -4,4‘-dichloro-1,1‘,2,2‘,3,3‘-tetrathiadiazafulvalene ( DC-TAF), the first example of this heterofulvalene system. Ab initio molecular orbital (B3LYP/6-31G**) calculations on prototypal TAF confirm that the closed shell 1 A g state lies 22 kcal mol - 1 below the 3 B u diradical triplet. Cyclic voltammetry on DC-TAF reveals two reversible oxidation waves at 0.80 and 1.25 V (in CH 3 CN, reference SCE). The ESR signal ( g = 2.0117) of the radical cation [DC-TAF] + (in SO 2 (l)) exhibits a five-line hyperfine coupling pattern with a N = 0.096 mT. DC-TAF forms a series of 1:1 radical ion salts [DC-TAF][X] by electrooxidation in the presence of tetrahedral counterions (X - = BF 4 -, ClO 4 -, FSO 3 - ). The crystal structures of these salts are isomorphous, monoclinic space group P 2 1 / n, and consist of one-dimensional ladder-like arrays of [DC-TAF] + radical cations bridged by S- - -S contacts ranging from 3.5 to 3.7 Å. Variable-temperature conductivity and magnetic measurements on [DC-TAF][ClO 4 ] indicate Mott insulator behavior, with a measured band gap of 0.30 eV.