Kinetics and mechanistic studies for the oxidation of N-Benzylhydroxylamine by a CoIII-bound bridging superoxo complex in perchloric acid medium

Abstract

In aqueous perchloric acid medium (pH = 0.522 – 1.3) , N–benzylhydroxylamine ( two electron reductant)  reduces the  one electron oxidant,  superoxo ligand in [(dien)(en)Co^III(O₂)Co^III(en)(dien)](ClO₄)₅ (1) to the corresponding hydroperoxo complex, [(en)(dien)Co^III(HO₂)Co^III(en)(dien)]⁵⁺ (2) and itself gets oxidised to PhCH₂NO  following both  proton coupled electron transfer (PCET) path and an electron – transfer (ET) reaction. The kinetics, stoichiometry and reaction mechanism clearly indicate that oxidation of PhCH₂NHOH occurs through the formation of an intermediate, benzyl derivative of  aminoxyl radical (PhCH₂NHO^•). In the presence of excess PhCH₂NHOH over 1, the reaction obeys first-order kinetics and rate of the reaction increases with [PhCH₂NHOH]. The reaction rate, however, decreases with increase in [H⁺] and the plot of 1/k_o with [H⁺] is linear with a small but noteworthy intercept. It is also remarked that the reaction rate remarkably decreases with increasing proportion of D₂O replacing H₂O in the solvent. Therefore, an H-atom transfer (HAT) from PhCH₂NHOH to the bridging superoxide in 1 seems reasonable at the rate determining step.