Crystal structures and magnetic properties of nitronyl nitroxide radicals

The crystal structures and magnetic properties of the nitronyl nitroxide radicals 4,5-dihydro-4,4,5,5-tetramethyl-3-oxido(1H-imidazol-1-yloxyl) (1), 4,5-dihydro-2,4,4,5,5-pentamethyl-3-oxido(1H-imidazol-lyloxyl) (2), 2-(4-chlorophenyl)-4,5-dihydro-4,4,5,5-tetramethyl-3-oxido(1H-imidazol-1-yloxy1) (3), and 4,5dihydro-2-(2-hydroxy-5-nitrophenyl)-4,4,5,5-tetramethyl-3-oxido(1H-imidazol-1-yloxyl) (4) are reported. Compound 1 has two polymorphic forms: the alpha phase is monoclinic (P2(1)/n space group), with a single molecule in the asymmetric unit, and the beta phase is monoclinic (P2(1)/c space group), with four molecules in the asymmetric unit. In the two polymorphs, the molecules are arranged in dimers formed by hydrogen bonds of the type C-(HO)-O-...-N. The crystal structure of 3 contains layers of antiparallel ribbons of molecules. Compound 4 crystallizes with solvent molecules, and an intramolecular hydrogen bond is formed between the 2-OH group of the phenyl ring and the nitroxide O-atom. Compound 4 also loses the two O-atoms of the nitroxide moiety upon heating to 90degrees. Magnetic measurements showed that both alpha and beta polymorphs of 1 exhibit antiferromagnetic coupling. The best fit to the experimental data was obtained using Bleany-Bower's singlet-triplet model (H - 2JSaSb): J = - 11.2 K for the alpha phase and J = - 15.0 K for the beta phase. Compounds 3 and 4 show no evidence for spin coupling.