Stereospecific Alkane Hydroxylation by Non-Heme Iron Catalysts: Mechanistic Evidence for an FeVO Active Species

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Abstract
High-valent iron−oxo species have frequently been invoked in the oxidation of hydrocarbons by both heme and non-heme enzymes. Although a formally FeVO species, that is, [(Por•)FeIVO]+, has been widely accepted as the key oxidant in stereospecific alkane hydroxylation by heme systems, it is not established that such a high-valent state can be accessed by a non-heme ligand environment. Herein we report a systematic study on alkane oxidations with H2O2 catalyzed by a group of non-heme iron complexes, that is, [FeII(TPA)(CH3CN)2]2+ (1, TPA = tris(2-pyridylmethyl)amine) and its α- and β-substituted analogues. The reactivity patterns of this family of FeII(TPA) catalysts can be modulated by the electronic and steric properties of the ligand environment, which affects the spin states of a common FeIII−OOH intermediate. Such an FeIII−peroxo species is high-spin when the TPA ligand has two or three α-substituents and is proposed to be directly responsible for the selective C−H bond cleavage of the alkane substrate. The thus-generated alkyl radicals, however, have relatively long lifetimes and are susceptible to radical epimerization and trapping by O2. On the other hand, 1 and the β-substituted FeII(TPA) complexes catalyze stereospecific alkane hydroxylation by a mechanism involving both a low-spin FeIII−OOH intermediate and an FeVO species derived from O−O bond heterolysis. We propose that the heterolysis pathway is promoted by two factors: (a) the low-spin iron(III) center which weakens the O−O bond and (b) the binding of an adjacent water ligand that can hydrogen bond to the terminal oxygen of the hydroperoxo group and facilitate the departure of the hydroxide. Evidence for the FeVO species comes from isotope-labeling studies showing incorporation of 18O from H218O into the alcohol products. 18O-incorporation occurs by H218O binding to the low-spin FeIII−OOH intermediate, its conversion to a cis-H18O−FeVO species, and then oxo−hydroxo tautomerization. The relative contributions of the two pathways of this dual-oxidant mechanism are affected by both the electron donating ability of the TPA ligand and the strength of the C−H bond to be broken. These studies thus serve as a synthetic precedent for an FeVO species in the oxygen activation mechanisms postulated for non-heme iron enzymes such as methane monooxygenase and Rieske dioxygenases.