Isostructural bisdithiazolyl and bisthiaselenazolyl radicals: Trends in bandwidth and conductivity

Reaction of N-alkylated pyridine-bridged bisdithiazolylium cations [1](+) (R1 = Me, Et; R-2 = Ph) with selenium dioxide in acetic acid provides a one-step high-yield synthetic route to bisthiaselenazolylium cations [2](+) (R-1 = Me, Et; R-2 = Ph). The corresponding radicals 1 and 2 can be prepared by chemical or electrochemical reduction of the cations. Structural analysis of the radicals has been achieved by a combination of single-crystal and powder X-ray diffraction methods. While the two sulfur radicals 1 adopt different space groups (P3(1)21 for R-1 = Me and P1 for R-1 = Et), the two selenium radicals 2 (space groups P3(1)21 for R-1 = Me and P3(2)21 for R-1 = Et) are isostructural with each other and also with 1 (R-1 = Me, R-2 = Ph). Variable-temperature magnetic measurements on all four compounds confirm that they are undimerized S = 1/2 systems, with varying degrees of weak intermolecular antiferromagnetic coupling. Variable-temperature electrical conductivity measurements on the two selenium radicals provide conductivities sigma(300 K) = 7.4 x 10(-6) (R-1 = Et) and 3.3 x 10(-5) S cm(-1) (R-1 = Me), with activation energies, E-act, of 0.32 (R-1 = Et) and 0.29 eV (R-1 = Me). The differences in conductivity within the isostructural series is interpreted in terms of their relative solid-state bandwidths, as estimated from Extended Huckel band-structure calculations.