Ferromagnetic Ordering in Bisthiaselenazolyl Radicals: Variations on a Tetragonal Theme

A series of five isostructural bisthiaselenazolyl radicals 2 have been prepared and characterized by X-ray crystallography. The crystal structures, all belonging to the tetragonal space group P (4) over bar2(1)m, consist of slipped pi-stack arrays of undimerized radicals packed about (4) over bar centers running along the z-direction, an arrangement which gives rise to a complex lattice-wide network of close intermolecular Se---Se' contacts. Variations in R, (Et, Pr, CH2CF3) with R-2 = Cl lead to significant changes in the degree of slippage of the pi-stacks and hence the proximity of the Se---Se' interactions. By contrast, variations in R-2 (Cl, Br, Me) with R, = Et induce very little change in either the degree of slippage or the intermolecular contacts. Variable-temperature conductivity (sigma) measurements show relatively constant values for the conductivity sigma(300 K) (10(-5)-10(-4) S cm(-1)) and thermal activation energy E-act (0.27-0.31 eV). Variable-temperature magnetic susceptibility measurements indicate that radicals 2b and 2c (R-1 = Pr, CH2CF3; R-2 = Cl) behave as weakly antiferromagnetically coupled Curie-Weiss paramagnets, but in 2a, 2d and 2e (R-1 = Et; R-2 = Cl, Me, Br) ferromagnetic ordering is observed, with T-c values of 12.8 (R-2 = Cl), 13.6 (R-2 = Me), and 14.1 K (R-2 = Br). The origin of the dramatically different magnetic behavior across the series has been explored in terms of a direct through-space mechanism by means of DFT calculations on individual pairwise exchange energies. These indicate that antiferromagnetic exchange between radicals along the g-stacks increases with pi-stack slippage.