HO Oxidation by Fe-OOH Intermediates and Its Effect on Catalytic Efficiency.

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