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][I]

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 Å and mean interdimer S- - -S contact of 3.761 Å. Cosublimation of the diradical with iodine produces the charge-transfer salt [S₂N₂C-CN₂S₂][I], orthorhombic space group Ccmm, a = 11.909(3) Å, b = 3.271(2) Å, c = 19.860(6) Å, 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 Hückel band calculations.