A Phosphinonitronyl Nitroxide

Nitronyl nitroxides are widely used in chemistry, physics, and materials science because of their inherently high reactivity and magnetic activity, but the synthesis of C(2)-organoelement derivatives is still a challenge. This paper describes effective synthesis and properties of the first nitronyl nitroxide carrying an additional redox-active phosphine group, namely 1,3,5,7-tetramethyl-8-(4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl-2-yl)-2,4,6-trioxa-8-phosphaadamantane. This phosphinonitronyl nitroxide was prepared by reacting lithiated nitronyl nitroxide with 8-bromo-1,3,5,7-tetramethyl-2,4,6-trioxa-8-phosphaadamantane. The paramagnet was characterized by cyclic voltammetry and electron spin resonance (ESR), infrared, and UV/vis spectroscopy supported by quantum chemical calculations. Molecular and crystal structure of the phosphinonitronyl nitroxide was established by X-ray structural analysis. ESR spectroscopy and density functional theory (DFT) calculations proved spin delocalization onto the phosphorus atom. According to the cyclic-voltammetry data, oxidation of the phosphinonitronyl nitroxide takes place quasi-reversibly at E_1/2 = 0.76 V, and its reduction occurs chemically and electrochemically reversibly at E_1/2 = −0.95 V (rel. Ag/AgCl).