Prototypal Dithiazolodithiazolyl Radicals:  Synthesis, Structures, and Transport Properties

New synthetic routes to 1,2,3-dithiazolo-1,2,3-dithiazolylium salts, based on double Herz condensations of N-alkylated 2,6-diaminopyridinium salts with sulfur monochloride, have been developed. The two prototypal 1,2,3-dithiazolo-1,2,3-dithiazolyl radicals HBPMe and HBPEt have been prepared and characterized in solution by cyclic voltammetry and EPR spectroscopy. Measured electrochemical cell potentials and computed (B3LYP/6-31G**) gas-phase disproportionation enthalpies favor a low on-site Coulombic repulsion energy <i>U</i> in the solid state. The crystal structures of HBPR (R = Me, Et) have been determined by X-ray crystallography (at 293 K). Both consist of slipped π-stacks of undimerized radicals, with many close intermolecular S- - -S contacts. Magnetic, conductivity, and optical measurements have been perfomed and the results interpreted in light of extended Hückel band calculations. The crystalline materials are paramagnetic above 100 K, with room-temperature conductivities σ<sub>RT</sub> of 10<sup>-5</sup>−10<sup>-6</sup> S cm<sup>-1</sup>; the slightly greater conductivity of the R = Et compound can be associated with a more well developed band structure. We suggest a Mott−Hubbard insulator ground state for these materials, with an on-site Coulomb repulsion energy <i>U</i> of about 1.0 eV.