Kinetics of the Superoxide Radical Oxidation of (2+). A Flash Photolytic Study

Article abstract of DOI:10.1021/ja00364a025

The postulated formation of the superoxide radical anion, O2^-, as an intermediate in the reaction of Co(sep)²⁺ (sep = sepulchrate) with molecular oxygen has now been confirmed by a trapping reaction with Cu²⁺.In the absence of Cu²⁺, O2^- oxidizes a second Co(sep)²⁺ to Co(sep)³⁺.The latter reaction, studied directly by use of the flash photolytic technique, has at 25 deg C a rate constant of (4.6 +/- 1.1) * 10⁷ M^-1 s^-1, independent of pH in the range 11.3-12.6.Nitrogen perdeuteration yields d(N)⁶-Co(sep)²⁺, which reacts with O2 at the same rate but with O2^- 2.1 times more slowly.The proposed mechanism of the O2^- reaction consists of the hydrogen atom abstraction from a N-H bond of Co(sep)²⁺⁾ by O2^-, followed by the rapid protonation of the products, Co^III(sep-H)²⁺ and HO2^-, to form Co(sep)³⁺ and H2O2.In contrast, the reaction between Co(sep)²⁺ and O2, which shows no kinetic isotope effect, occurs by outer-sphere electron-transfer.