H2O2 Oxidation by FeIII–OOH Intermediates and Its Effect on Catalytic Efficiency

The oxidation of the C-H and C=C bonds of hydrocarbons with H 2 O 2 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 H 2 O 2 concentrations the Fe IV =O species formed is detectable in methanol. Furthermore, we show that the decomposition of H 2 O 2 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 H 2 O 2 are present. Although independently prepared [(N4Py)Fe IV =O] 2+ oxidizes stoichiometric H 2 O 2 rapidly, the rate of formation of Fe IV =O from the Fe III -OOH intermediate is too low to account for the rate of H 2 O 2 decomposition observed under catalytic conditions. Indeed, with excess H 2 O 2 , disproportionation to O 2 and H 2 O 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 H 2 O 2 , which ensure that the rate of the H 2 O 2 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.