Preparation and characterization of the disjoint diradical 4,4'-bis(1,2,3,5-dithiadiazolyl) [S2N2C-CN2S2] and its iodine charge transfer salt [S2N2C-CN2S2]

Condensation of oxamidrazone with sulfur dichloride in acetonitrile affords 4,4'-bis(1,2,3,5-dithiadiazolium) dichloride in moderate yield. Reduction of this salt with triphenylantimony yields the diradical 4,4'-bis(1,2,3,5-dithiadiazolyl) [S2N2C-CN2S2], which has been isolated and characterized in the solid state as its dimer [S2N2C-CN2S2](2) The diradical is disjoint, and ab initio molecular orbital methods confirm a very small energy gap (<0.5 kcal/mol) between the triplet and diradical singlet states, regardless of the torsion angle about the central C-C bond. In accord with these theoretical predictions the ESR spectrum of the diradical consists (in CHCl3 at 273 K) of a simple five-line pattern (a(N) = 0.50 mT, g = 2.011), i.e., there is no observable exchange coupling between the two centers. In the solid state, the dimer [S2N2C-CN2S2](2) forms a slipped stack structure, with a mean intradimer S-S distance of 3.078 Angstrom and mean interdimer S---S contact of 3.761 Angstrom. Cosublimation of the diradical with iodine produces the charge-transfer salt [S2N2C-CN2S2][I], orthorhombic space group Ccmm, a = 11.909(3) Angstrom, b = 3.271(2) Angstrom, c = 19.860(6) Angstrom, Z = 4 (at 293 K). In this structure the heterocyclic rings form perfectly superimposed and evenly spaced stacks along the y direction, with channels of disordered iodines. The iodine-doped material is metallic at ambient temperatures, with a single-crystal conductivity of 460 S cm(-1) at 300 K; variable temperature conductivity and magnetic measurements reveal a phase transition near 270 K, with the onset of semiconducting behavior. Transport data for the neutral and doped materials are discussed in the light of Extended Huckel band calculations.