A kinetic electron spin resonance study of the reaction of some oxy radicals with vanadyl acetylacetonate

Reactions of phenoxyl, iminoxyl, nitroxyl, peroxyl, and alkoxyl radicals with VO(acac)2 in solution have been studied by kinetic esr spectroscopy. Rates of reaction are first-order with respect to each reactant and radical reactivity increases in the order galvinoxyl <2,4,6-tri-tert-butylphenoxyl <di(1-adamantyl)methylene-iminoxyl <tert-butylperoxyl. Rates of reaction are retarded by pyridine and accelerated by methanol and there is an isotope effect on the rate when the hydroxyl proton of CH3OH is replaced by deuterium. Vanadium-51 nmr spectroscopy has demonstrated that several vanadium(V) products are formed while conventional product studies have indicated that, in the case of phenoxyl and alkoxyl, the radical is reduced almost exclusively to the parent phenol and alcohol and that VO(acac)2 is eventually regenerated in substantial yields. The net reaction is, therefore, catalytic reduction of the radical. Variable temperature esr studies have shown that phenoxyl and iminoxyl radicals and VO(acac)2 exist in equilibrium with diamagnetic reaction products.