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)CoIII(O2)CoIII(en)(dien)](ClO4)5 (1) to the corresponding hydroperoxo complex, [(en)(dien)CoIII(HO2)CoIII(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/ko 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.