AUTOXIDATION OF TRANSITION-METAL COMPLEXES - REACTION OF A 1-1 COBALT MOLECULAR OXYGEN COMPLEX WITH ACIDS TO YIELD HYDROGEN-PEROXIDE - KINETICS AND MECHANISM

Dioxygen (pyridine)-N,N’-ethylenebis(acetylacetoniminato)cobalt(II) reacts with acids in organic solvents containing excess pyridine to give 0.5 mol of molecular oxygen, 0.5 mol of hydrogen peroxide, and 1 mol of dipyridine N,N’-ethylenebis(acetylacetoniminato)cobalt(III) ion as an ion pair with the anion of the acid. In pyridine below 0°C the reaction proceeds quantitatively within experimental uncertainty, and no detectable buildup of intermediates is observed. Kinetic studies were done with aCetic acid in pyridine at -10.7 °C by monitoring the evolution of oxygen at constant pressure. The empirical rate law is found to be second order in the cobalt oxygen complex, inverse first order in oxygen pressure, inverse one-half order in cobalt(III) acetate product, and between second and third order in acetic acid. This law holds throughout the course of the reaction, strongly indicating that only one mechanism is in operation. The effect of added bromide and acetate salts on the rate and the demonstrated occurrence of the homoconjugation equilibrium for acetate, OAc-+HOAc⥭H(OAc)2-, lead to the conclusion that the high empirical order in acetic acid is equivalent to a first-order dependence each on acetic acid and free, dissociated pyridinium ion. The proposed mechanism involves protonation, and subsequent dissociation as hydrogen peroxide, of the bridging dioxygen ligand from a binuclear complex intermediate, pyCo(acacen)02Co(acacen)py, which exists in rapid equilibrium with the starting mononuclear dioxygen complex. The involvement of both acid species in the transition state is discussed. The absence of free-radical intermediates was strongly indicated by the results of experiments carried out in the presence of organic radical scavengers and by ESR spectroscopy. The reaction is discussed in terms of autoxidation of transition-metal complexes. © 1979, American Chemical Society. All rights reserved.