Electron-Transfer and Redox Reactivity of High-Valent Iron Imido and Oxo Complexes with the Formal Oxidation States of Five and Six

Abstract

We report for the first time electron-transfer (ET) properties of mononuclear nonheme iron-oxo and -imido complexes with the formal oxidation states of five and six, such as an iron(V)-imido TAML cation radical complex, which is formally an iron(VI)-imido complex [FeV(NTs)(TAML+•)] (1; NTs = tosylimido), an iron(V)-imido complex [FeV(NTs)(TAML)]− (2), and an iron(V)-oxo complex [FeV(O)(TAML)]− (3). The one-electron reduction potential (Ered vs SCE) of 1 was determined to be 0.86 V, which is much more positive than that of 2 (0.30 V), but the Ered of 3 is the most positive (1.04 V). The rate constants of ET of 1–3 were analyzed in light of the Marcus theory of adiabatic outer-sphere ET to determine the reorganization energies (λ) of ET reactions with 1–3; the λ of 1 (1.00 eV) is significantly smaller than those of 2 (1.98 eV) and 3 (2.25 eV) because of the ligand-centered ET reduction of 1 as compared to the metal-centered ET reduction of 2 and 3. In oxidation reactions, reactivities of 1–3 toward the nitrene transfer (NT) and oxygen atom transfer (OAT) to thioanisole and its derivatives and the C–H bond activation reactions, such as the hydrogen atom transfer (HAT) of 1,4-cyclohexadiene, were compared experimentally. The differences in the redox reactivity of 1–3 depending on the reaction types, such as NT and OAT versus HAT, were interpreted by performing density functional theory calculations, showing that the ligand-centered reduction seen on ET reactions can switch to metal-centered reduction in NT and HAT.