The effect of the axial ligand on distinct reaction tunneling for methane hydroxylation by nonheme iron(IV)–oxo complexes

Abstract

Comprehensive density functional theory computations on substrate hydroxylation by a range of nonheme iron(IV)–oxo model systems [Fe^IV(O)(NH₃)₄L]⁺ (where L = CF₃CO₂⁻, F⁻, Cl⁻, N₃⁻, NCS⁻, NC⁻, OH⁻) have been investigated to establish the effects of axial ligands with different degrees of electron donor ability on the reactivity of the distinct reaction channels. The results show that the electron-pushing capability of the axial ligand can exert a considerable influence on the different reaction channels. The σ-pathway reactivity decreases as the electron-donating ability of the axial ligand strengthens, while the π-pathway reactivity follows an opposite trend. Moreover, the apparently antielectrophilic trend observed for the energy gap between the triplet π- and quintet σ-channel (ΔG(T–Q)) stems from the fact that the reaction reactivity can be fine-controlled by the interplay between the exchange-stabilization benefiting from the ⁵TS_H relative to the ³TS_H by most nonheme enzymes and the destabilization effect of the orbital by the anionic axial ligand. When the former counteracts the latter, the quintet σ-pathway will be more effective than the other alternatives. Nevertheless, when the dramatic destabilization effect of the orbital by a strong binding axial σ-donor ligand like OH⁻ counteracts but does not override the exchange-stabilization, the barrier in the quintet σ-pathway will remain identical to the triplet π-pathway barrier. Indeed, the axial ligand does not change the intrinsic reaction mechanism in its respective pathway; however, it can affect the energy barriers of different reaction channels for C–H activation. As such, the tuning of the reactivity of the different reaction channels can be realised by increasing/decreasing the electron pushing ability.