Metal-centered and ligand-centered autoxidations involving u-peroxo-bridged cobalt dioxygen complexes

Abstract

The autoxidation of cobaltous complexes containing tetaethylenepentramine (TETREN), 1,9-bis(4-imidazolyl)-2,5,8-triazanonane (IMDIEN), 2,6-bis(2-(3,6-diazahexyl)) pyridine (EPYDEN), 1,9-bis(2-pyridyl)-2,5,8-triazanonane (PYDIEN), and 1,11-bis(2-pyridyl)-2,6,10-triazaundecane (PYDPT) react through (mu)-peroxo-bridged cobalt(III) dioxygen complexes, resulting in ligand oxidative dehydrogenation or a simple metal-centered oxidation. These reactions are shown to be first order with respect to the concentration of both the (mu)-peroxo-bridged cobalt(III) dioxygen complex and the hydroxide concentration. Reaction products are identified for the simple cobalt(II) oxidation containing TETREN, IMDIEN and EPYDEN as the cobalt(III) chelates and hydrogen peroxide; and for the coordinated ligands PYDIEN and PYDPT, the oxidative dehydrogenation products are imines conjugated to the pyridine ring, water, and cobalt(II). The cobalt(II) complex of the ligand that has undergone oxidative dehydrogenation reoxygenates and further oxidative dehydrogenation of the ligand then takes place. It is proposed that the mechanistic differences in the autoxidation reactions are dependent on the conformation of the aromatic and aliphatic amino groups in the coordination sphere of the (mu)-peroxo-bridged cobalt(III) dioxygen complexes having pyridyl-containing pentadentate polyamines as ligands.