A density functional study on a biomimetic non-heme iron catalyst:: Insights into alkane hydroxylation by a formally HO-FeV=O oxidant

The reactivity of [HO(tpa)Fe-V=O] (TPA = tris(2-pyridylmethyl)amine), derived from O-O bond heterolysis of its [H2O-(tpa)Fe-III-OOH] precursor, was explored by means of hybrid density functional theory. The mechanism for alkane hydroxylation by the high-valent iron-oxo species invoked as an intermediate in Fe(tpa)/H2O2 catalysis was investigated. Hydroxylation of methane and propane by HO-Fe-V=O was studied by following the rebound mechanism associated with the heme center of cytochrome P450, and it is demonstrated that this species is capable of stereospecific alkane hydroxylation. The mechanism proposed for alkane hydroxylation by HO-Fe-V=O accounts for the experimentally observed incorporation of solvent water into the products. An investigation of the possible hydroxylation of acetonitrile (i.e., the solvent used in the experiments) shows that the activation energy for hydrogen-atom abstraction by HO-Fe-V=O is rather high and, in fact, rather similar to that of methane, despite the similarity of the H-CH2CN bond strength to that of the secondary C-H bond in propane. This result indicates that the kinetics of hydrogen-atom abstraction are strongly affected by the cyano group and rationalizes the lack of experimental evidence for solvent hydroxylation in competition with that of substrates such as cyclohexane.