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 U 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 σRT of 10-5−10-6 S cm-1;
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 U of about 1.0 eV.