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]

by Bryan, C. D.; Cordes, A. W.; Goddard, J. D.; Haddon, R. C.; Hicks, R. G.; MacKinnon, C. D.; Mawhinney, R. C.; Oakley, R. T.; Palstra, T. T. M.; Perel, A. S.

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.

Journal of the American Chemical Society
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1520-5126; 0002-7863