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(1 H ‐imidazol‐1‐yloxyl) ( 1 ), 4,5‐dihydro‐2,4,4,5,5‐pentamethyl‐3‐oxido(1 H ‐imidazol‐1‐yloxyl) ( 2 ), 2‐(4‐chlorophenyl)‐4,5‐dihydro‐4,4,5,5‐tetramethyl‐3‐oxido(1 H ‐imidazol‐1‐yloxyl) ( 3 ), and 4,5‐dihydro‐2‐(2‐hydroxy‐5‐nitrophenyl)‐4,4,5,5‐tetramethyl‐3‐oxido(1 H ‐imidazol‐1‐yloxyl) ( 4 ) are reported. Compound 1 has two polymorphic forms: the α phase is monoclinic ( P 2 1 /n space group), with a single molecule in the asymmetric unit, and the β phase is monoclinic ( P 2 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 CH⋅⋅⋅ON. 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 90°. Magnetic measurements showed that both α and β polymorphs of 1 exhibit antiferromagnetic coupling. The best fit to the experimental data was obtained using BleanyBower 's singlet‐triplet model (H=−2 JS a S b): J =−11.2 K for the α phase and J =−15.0 K for the β phase. Compounds 3 and 4 show no evidence for spin coupling.