Biomimetic hydrocarbon oxidation catalyzed by nonheme iron(III) complexes with peracids: Evidence for an Fe^v=O species

Mononuclear nonheme iron-(III) complexes of tetradentate ligands containing two deprotonated amide moieties, [Fe(Me₂bpb)Cl(H₂O)] (3a) and [Fe(bpc)Cl(H₂O)] (4a), were prepared by substitution reactions involving the previously synthesized iron(III) complexes [Et₃NH] [Fe(Me₂bpb)Cl₂] (3) and [Et₃NH][Fe(bpc)Cl ₂] (4). Complexes 3 a and 4 a were characterized by IR and elemental analysis, and complex 3 a also by X-ray crystallography. Nonheme iron(III) complexes 3, 3a, 4, and 4 a catalyze olefin epoxidation and alcohol oxidation on treatment with mchloroperbenzoic acid. Pairwise comparisons of the reactivity of these complexes revealed that the nature of the axial ligand (Cl^- versus H₂O) influences the yield of oxidation products, whereas an electronic change in the supporting chelate ligand has little effect. Hydrocarbon oxidation by these catalysts was proposed to involve an iron(V) oxo species which is formed on heterolytic O-O bond cleavage of an iron acylperoxo intermediate (FeOO-C(O)R). Evidence for this iron(V) oxo species was derived from KIE (k_H/k_D) values, H₂¹⁸O exchange experiments, and the use of peroxyphenylacetic acid (PPAA) as the peracid. Our results suggest that an Fe^v=O moiety can form in a system wherein the supporting chelate ligand comprises a mixture of neutral and anionic nitrogen donors. This work is relevant to the chemistry of mononuclear nonheme iron enzymes that are proposed to oxidize organic substrates via reaction pathways involving high-valent iron oxo species.