Nonheme Iron Oxidant Formed in the Presence of H2O2 and Acetic Acid Is the Cyclic Ferric Peracetate Complex, Not a Perferryloxo Complex

Abstract

Oxidative C–H bond activation is a transformation of fundamental and practical interest, particularly if it can be carried out with high regio- and enantioselectivity. With nonheme iron oxygenases as inspiration (e.g., the Rieske oxygenases), a family of biomimetic nonheme iron complexes has been found to catalyze hydrocarbon oxidations by H2O2 via a postulated FeV(O)(OH) oxidant. Of particular interest is the Fe(S,S-PDP) catalyst discovered by White that, in the presence of acetic acid as an additive, performs selective C–H bond activation, even in complex organic molecules. The corresponding FeV(O)(OAc) species has been suggested as the key oxidant. We have carried out DFT studies to assess the viability of such an oxidant and discovered an alternative formulation. Theory reveals that the barrier for the formation of the putative FeV(O)(OAc) oxidant is too high for it to be feasible. Instead, a much lower barrier is found for the formation of a [(S,S-PDP)FeIII(κ2-peracetate)] species. In the course of C–H activation, this complex undergoes O–O bond homolysis to become a transient [(S,S-PDP)FeIV(O)(AcO·)] species that performs the efficient hydroxylation of alkanes. Thus, the acetic acid additive alters completely the nature of the high-valent oxidant, which remains disguised in the cyclic structure. This new mechanism can rationalize the many experimental observations associated with the oxidant formed in the presence of acetic acid, including the S = 1/2 EPR signal associated with the oxidant. These results further underscore the rich multioxidant scenario found in the mechanistic landscape for nonheme iron catalysts.