Acid p<i>K</i>_a Dependence in O–O Bond Heterolysis of a Nonheme Fe^III–OOH Intermediate To Form a Potent Fe^V═O Oxidant with Heme Compound I-Like Reactivity

Abstract

Protons play essential roles in natural systems in controlling O-O bond cleavage of peroxoiron(III) species to give rise to the high-valent iron oxidants that carry out the desired transformations. Herein, we report kinetic and mechanistic evidence that acids can control the mode of O-O bond cleavage for a nonheme <i>S</i> = 1/2 Fe^III-OOH species [(BnTPEN)Fe^III(OOH)]²⁺ (<b>2</b>, BnTPEN = <i>N</i>-benzyl-<i>N,N',N'</i>-tris(2-pyridylmethyl)-1,2-diaminoethane). Addition of acids having p<i>K</i>_a values of >8.5 in CH₃CN results in O-O bond homolysis, leading to the formation of hydroxyl radicals that give rise to alcohol/ketone (A/K) ratios of around 1 in the oxidation of cyclohexane. However, the introduction of acids with p<i>K</i>_a values of <8.5 elicits a different outcome, namely the achievement of A/K ratios of as high as 9, the observation of rapid and catalytic hydroxylation of cyclohexane, and a million-fold acceleration in the decay rate of the Fe^III-OOH intermediate at -40 °C. These results implicate the generation of a highly reactive Fe^V═O species via proton-assisted O-O bond heterolysis of the Fe^III-OOH intermediate, which is unprecedented for nonheme iron complexes supported by neutral pentadentate ligands and serves as a nonheme analogue for heme enzyme compounds I.