H2O2 Oxidation by Fe-III-OOH Intermediates and Its Effect on Catalytic Efficiency

The oxidation of the C-H and C=C bonds of hydrocarbons with H2O2 catalyzed by non-heme iron complexes with pentadentate ligands is widely accepted as involving a reactive Fe-IV=O species such as [(N4Py)Fe-IV=O](2+) formed by homolytic cleavage of the O-O bond of an Fe-III-OOH intermediate (where N4Py is 1,1-bis(pyridin-2-yl)-N,N-bis(pyridin-2-ylmethyl)methanamine). We show here that at low H2O2 concentrations the Fe-IV=O species formed is detectable in methanol. Furthermore, we show that the decomposition of H2O2 to water and O-2 is an important competing pathway that limits efficiency in the terminal oxidant and indeed dominates reactivity except where only sub-/near-stoichiometric amounts of H2O2 are present. Although independently prepared [(N4Py)Fe-IV=O](2+) oxidizes stoichiometric H2O2 rapidly, the rate of formation of Fe-IV=O from the Fe-III-OOH intermediate is too low to account for the rate of H2O2 decomposition observed under catalytic conditions. Indeed, with excess H2O2, disproportionation to O-2 and H2O is due to reaction with the Fe-III-OOH intermediate and thereby prevents formation of the Fe-IV=O species. These data rationalize that the activity of these catalysts with respect to hydrocarbon/alkene oxidation is maximized by maintaining sub-/near-stoichiometric steady-state concentrations of H2O2, which ensure that the rate of the H2O2 oxidation by the Fe-III-OOH intermediate is less than the rate of the O-O bond homolysis and the subsequent reaction of the Fe-IV=O species with a substrate.