Acid p K_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

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 S = 1/2 Fe^III-OOH species [(BnTPEN)Fe^III(OOH)]²⁺ (2, BnTPEN = N-benzyl-N,N′,N′-tris(2-pyridylmethyl)-1,2-diaminoethane). Addition of acids having pK_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 pK_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.