Electronic Structure of Tetragonal Fe-Oxo Complexes Governs HAT vs PCET Mechanisms in C-H Oxidation

Discerning hydrogen atom transfer (HAT) from proton-coupled electron transfer (PCET) mechanisms in C-H oxidation by biomimetic Fe-oxo complexes remains a longstanding challenge, with a central question being how the electronic structure and spin state of the iron-oxo core dictate this mechanistic preference. To address this, we conducted detailed electronic structure analyses of the hydrogen atom abstraction (HAA) reaction of 1,4-cyclohexadiene by the tetragonal iron(IV)-oxo complex, [Fe^IVN4Py(O)]²⁺ (1) and its one-electron-reduced congener, [Fe^IIIN4Py(O)]⁺ (2) using DFT, CASSCF, NEVPT2, and IBO methods. Electronic structure analyses at varying Fe-O distances reveal that the emergence of "oxyl" character promotes HAT, while its absence favors PCET. Notably, the PCET pathway in the high-spin (S = 5/2) Fe(III)-oxo complex shifts to HAT in the intermediate-spin (S = 3/2) state, underscoring the pivotal role of electronic configuration and spin state. The electronic nature of the HAT or PCET transition states for complexes 1 (⁵TS1_σ) and 2 (⁶TS2_π and ⁴TS2_σ) was verified using multiconfigurational CASSCF calculations, complemented by IBO analysis along the intrinsic reaction coordinate. Overall, this study clarifies the distinction between HAT and PCET mechanisms and highlights the iron-oxo core's decisive role in shaping the reactivity of synthetic Fe-oxo species.