Redox reactions of dioxygen complexes of cobalt(III) with organic radicals studied by pulse radiolysis

Abstract

Reduction processes of p-superoxo complexes of cobalt(III), [(NH3)4Co(p02,pNH2)C~(NH3)4]4+, 1, [ (ed2Co-(p02,pNH2)Co(en)2]4+, 2 (en = ethylenediamine), [ (CN)&O(~O~)CO(CN)~]~, 3, with the formate radical, the alkyl hydroxy radicals CH20H, CH3CHOH, (CH3)&OH, and (CH3)COH and the solvated electron were investigated. Reduction of these complexes occurs at the dioxygen center and the products of the reduction are determined to be the corresponding p-peroxo complexes. The reduction of complexes 2 and 3 by the solvated electron is studied and the bimolecular rate constant for the reduction of 2 is higher by two orders of magnitude than that for the reduction of 3. The same behavior is observed for the reaction of these p-superoxo complexes with the formate radical, indicating that the effect is due to the presence of the 4+ charge in 1 and 2 and the 5-charge in 3. The isopropyl radical reacts with the p-superoxo cobalt(II1) compelxes faster than .CH20H and C2H40H radicals. The product observed for the reduction of the p-superoxo amine complexes by the isopropyl radical appears to be an isomer of the peroxo complex. It is proposed that the reaction of the isopropyl radical with the superoxo complexes proceeds in part by involving the metal center. IntroductionReversible and irreversible uptake of oxygen by transition metal complexes has been known for some time.3 Oxygenation reactions of transition metal complexes are believed to model the uptake of oxygen in biological syst e m~.~~ Irreversible uptake of oxygen by metal ions results in the formation of superoxo and peroxo complexes with different structures and reactivities.6 The reactions of the oxygen-coordinated complexes have been actively investigated in recent years in order to understand the more complex processes occurring in living cellsS7 Amine complexes of cobalt(I1) and cobalt(II1) ions are known to coordinate with oxygen to give the superoxo (02-) or peroxo (OZ2-) complexes. Reactions of oxygen with cobalt(I1) complexes to give a 1:l adduct are fast at room temperature, and the 1:l adducts formed dimerize to give the peroxo c~mplexes.~ Structural6 and spectroscopic8 studies of these complexes suggest that the coordinated dioxygen exists perdominantly in the superoxo or peroxo form. Electron transfer reactions of the oxo-bridged complexes of cobalt(II1) have been studied with a variety of reducing