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 B-3(u) diradical triplet. Cyclic voltammetry on DC-TAF reveals two reversible oxidation waves at 0.80 and 1.25 V (in CH3CN, reference SCE). The ESR signal (g = 2.0117) of the radical cation [DC-TAF](+) (in SO2(1)) 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- = BF4-, ClO4-, FSO3-). The crystal structures of these salts are isomorphous, monoclinic space group P2(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 Angstrom. Variable-temperature conductivity and magnetic measurements on [DC-TAF][ClO4] indicate Mott insulator behavior, with a measured band gap of 0.30 eV.