Hydrogen Atom Transfer Barriers by High-Valent Iron(IV)-Oxo Complexes: A DFT and Multireference Ab Initio Study

Hydrogen atom transfer (HAT) from strong C-H bonds to high-valent iron-oxo species is central to enzymatic and synthetic oxidation, yet accurate barrier prediction remains difficult due to the open-shell nature of iron-oxo and competing pathways. Here, we investigate methane HAT by ten Fe^IVO complexes using density functional theory and multireference ab initio calculations (CASPT2) to obtain reference activation free energies for methane activation. Across the series, we find that the activation free energy correlates strongly with key distances along the H-transfer coordinate but only weakly with oxo radical character. Among the complexes studied, [Fe^IVO(^tBu₃tacn)]²⁺, an iron(IV)-oxo species with a low coordination number and an S = 2 ground state, is predicted to be a promising methane-activation catalyst at room temperature. Benchmarking common DFT functionals against multireference barriers reveals strong pathway dependence: several functionals are accurate for the top pathway but systematically underestimate side-pathway barriers. We further identify a strong linear relationship between DFT mean absolute error and the fraction of Hartree-Fock exchange, indicating that high-%HF functionals yield larger barrier errors. These findings emphasize the need for multireference data and motivate expansion to larger data sets for data-driven prediction of Fe^IVO HAT reactivity.