Electronic structure, electrochemical properties, and mechanism of the autoxidation reaction of cobalt dioxygen complexes containing 2,2'-bipyridine, 1,10-phenanthroline, 2,2':6',2"-terpyridine and several of the dimethyl substituted ligands

Abstract

Autoxidation of cobalt dioxygen complexes containing 2,2'-bipyridine, 1,10-phenanthroline, and 2,2':6',2''-terpyridine in aqueous solution results in the production of simple cobalt(III) chelates and hydrogen peroxide. At neutral or higher pH the subsequent reaction of H(,2)O(,2) to form hydroxyl radicals is shown to be important. Involvement of the mononuclear superoxo cobalt(III) complexes has been demonstrated and the identification of intermediate hydroperoxide complexes has been made. The autoxidation is first-order in dioxygen complex with some non-stoichiometric dependence on the hydrogen ion concentration. Rate constants for the autoxidation reactions at low and high pH are presented. The mechanism at low pH is interpreted in terms of a rapid preequilibrium with hydrogen ions followed by a rate determining water replacement step. Under alkaline conditions the reaction remained first-order in dioxygen complex with larger values of the rate constants. Both the cobalt(II) and cobalt(III) complexes of the aromatic heterocyclic ligands employed possessed catalase-like activity. Electron paramagnetic resonance spectroscopy has been used to identify the formation of spin trapped hydroxyl radicals during the decomposition of hydrogen peroxide. Rate constants for the catalytic reaction are presented. Examination of the electronic absorption spectra of cobalt dioxygen complexes suggests that the electron structure of these complexes suggests that the electron structure of these complexes parallels the simple cobalt(III) species. New assignments for the low energy transitions in the dioxygen complexes, as ligand-to-metal charge-transfer type, have been made. Enhancement of the ligand field bands in the dioxygen complexes is explained in terms of vibronic coupling with a nearby Laporte-allowed transition.