H 2 O 2 Oxidation by Fe III -OOH Intermediates and Its Effect on Catalytic Efficiency

• Published in: ACS catalysis 2018-10, Vol.8 (10), p.9665-9674
• Main Authors: Chen, Juan, Draksharapu, Apparao, Angelone, Davide, Unjaroen, Duenpen, Padamati, Sandeep K, Hage, Ronald, Swart, Marcel, Duboc, Carole, Browne, Wesley R
• Format: Article
• Language: English
• Subjects: Chemical Sciences
• ISSN: 2155-5435; 2155-5435
• DOI: 10.1021/acscatal.8b02326

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