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 P4̅21m, consist of slipped π-stack arrays of undimerized radicals packed about 4̅ 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 R1 (Et, Pr, CH2CF3) with R2 = Cl lead to significant changes in the degree of slippage of the π-stacks and hence the proximity of the Se---Se′ interactions. By contrast, variations in R2 (Cl, Br, Me) with R1 = Et induce very little change in either the degree of slippage or the intermolecular contacts. Variable-temperature conductivity (σ) measurements show relatively constant values for the conductivity σ(300 K) (10−5−10−4 S cm−1) and thermal activation energy Eact (0.27−0.31 eV). Variable-temperature magnetic susceptibility measurements indicate that radicals 2b and 2c (R1 = Pr, CH2CF3; R2 = Cl) behave as weakly antiferromagnetically coupled Curie−Weiss paramagnets, but in 2a, 2d and 2e (R1 = Et; R2 = Cl, Me, Br) ferromagnetic ordering is observed, with Tc values of 12.8 (R2 = Cl), 13.6 (R2 = Me), and 14.1 K (R2 = 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 π-stacks increases with π-stack slippage.