Isostructural Bisdithiazolyl and Bisthiaselenazolyl Radicals:  Trends in Bandwidth and Conductivity

Reaction of N-alkylated pyridine-bridged bisdithiazolylium cations [1]+ (R1 = Me, Et; R2 = Ph) with selenium dioxide in acetic acid provides a one-step high-yield synthetic route to bisthiaselenazolylium cations [2]+ (R1 = Me, Et; R2 = 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 (P3121 for R1 = Me and P1̄ for R1 = Et), the two selenium radicals 2 (space groups P3121 for R1 = Me and P3221 for R1 = Et) are isostructural with each other and also with 1 (R1 = Me, R2 = 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 σ(300 K) = 7.4 × 10-6 (R1 = Et) and 3.3 × 10-5 S cm-1 (R1 = Me), with activation energies, Eact, of 0.32 (R1 = Et) and 0.29 eV (R1 = Me). The differences in conductivity within the isostructural series is interpreted in terms of their relative solid-state bandwidths, as estimated from Extended Hückel band-structure calculations.