Theoretical Insights into the Oxidation of Substrates by High-Spin Iron(III)-Acylperoxido Complexes

Abstract

While the oxidation of organic substrates by Fe^IV-oxido complexes has been studied extensively, there are only few highly reactive Fe^III-alkylperoxido complexes. We report here computational mechanistic work of a novel mononuclear nonheme Fe^III-phenylperoxido acetate complex, focusing on the splitting of the peroxide bond, the epoxidation of ethene and the hydroxylation of cyclohexane. Our results reveal that the peroxide bond undergoes homolysis, leading to the formation of an Fe^IV-oxido complex. Spectroscopic evidence supports the presence of an Fe^IV species, aligning with the observed product distribution. Additionally, we have explored a potential sigmatropic rearrangement mechanism. However, based on the activation barriers and initial product energy levels, the Fe^III rather than the Fe^V species is shown to be the active species in the initial step. For both, the epoxidation and hydroxylation reactions, stepwise mechanisms are proposed, involving the Fe^IV-oxido species as the catalytic intermediate. The energy barriers calculated for these pathways are significantly lower than those for concerted mechanisms, involving the Fe^III-peroxido species as direct oxidant. In particular, the calculated activation barrier in the Fe^IV-oxido pathway is 38 kJ/mol for the activation of the C=C bond in ethene, while the analogous step in the Fe^III-peroxido pathway is calculated to be as high as 183 kJ/mol. Our computational results indicate that the Fe^IV-oxido species is the active catalyst in these oxidation reactions.