Evidence for an Alternative to the Oxygen Rebound Mechanism in C–H Bond Activation by Non-Heme Fe^IVO Complexes

The hydroxylation of alkanes by heme Fe^IVO species occurs via the hydrogen abstraction/oxygen rebound mechanism. It has been assumed that non-heme Fe^IVO species follow the heme Fe^IVO paradigm in C–H bond activation reactions. Herein we report theoretical and experimental evidence that C–H bond activation of alkanes by synthetic non-heme Fe^IVO complexes follows an alternative mechanism. Theoretical calculations predicted that dissociation of the substrate radical formed via hydrogen abstraction from the alkane is more favorable than the oxygen rebound and desaturation processes. This theoretical prediction was verified by experimental results obtained by analyzing iron and organic products formed in the C–H bond activation of substrates by non-heme Fe^IVO complexes. The difference in the behaviors of heme and non-heme Fe^IVO species is ascribed to differences in structural preference and exchange-enhanced reactivity. Thus, the general consensus that C–H bond activation by high-valent metal–oxo species, including non-heme Fe^IVO, occurs via the conventional hydrogen abstraction/oxygen rebound mechanism should be viewed with caution

Determination of Spin Inversion Probability, H‑Tunneling Correction, and Regioselectivity in the Two-State Reactivity of Nonheme Iron(IV)-Oxo Complexes

We show by experiments that nonheme Fe^IVO species react with cyclohexene to yield selective hydrogen atom transfer (HAT) reactions with virtually no CC epoxidation. Straightforward DFT calculations reveal, however, that CC epoxidation on the <i>S</i> = 2 state possesses a low-energy barrier and should contribute substantially to the oxidation of cyclohexene by the nonheme Fe^IVO species. By modeling the selectivity of this two-site reactivity, we show that an interplay of tunneling and spin inversion probability (SIP) reverses the apparent barriers and prefers exclusive <i>S</i> = 1 HAT over mixed HAT and CC epoxidation on <i>S</i> = 2. The model enables us to derive a SIP value by combining experimental and theoretical results