Characterization of σ and π reaction channels in hydrogen atom transfer reactions

Abstract

C(sp³)–H bond activation mechanisms typically involve σ- and π-channel pathways, as characterized by FeOH (or FeOC) angles of ca. 180° and 120°, respectively. It is well known that the preference for either the σ- or π-channel depends on the spin state, but doubts exist on what would be characteristic values for the FeOX (X = H or C) angles. Here we study the oxidation of methane and ethane mediated by an Fe(IV)oxo model complex through density functional theory. A systematic comparison of dispersion-corrected B3LYP (B3LYP-D2, B3LYP-D3, B3LYP-D3BJ, B3LYP-D4) and the uncorrected counterpart (B3LYP) was conducted to evaluate the role of dispersion interactions in both gas and solvent phases. Our results reveal that dispersion corrections significantly influence barriers at transition states (TSs), particularly in the solvent phase, where dispersion contributions enhance stabilization of TS structures. The σ-channel pathway dominates for high spin (S = 2), while intermediate spin (S = 1) states favor the π-channel. Dispersion effects were found to be more pronounced for ethane, where larger non-covalent interactions between the substrate and Fe(IV)oxo complex arise. The FeOX angles vary substantially depending on the choice of dispersion correction, and between gas phase and solution phase. Indeed, for the reaction with ethane the FeOX values of the σ-channel approach values that are typically associated with the π-channel. Fortunately, the Spin-Resolved Charge Displacement Function provides a clear visual tool to distinguish the two channels. These insights advance the understanding of hydrocarbon functionalization by high-valent iron-oxo species, with implications for synthetic catalyst design in homogeneous and enzymatic catalysis.