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 [Fe^V(NTs)(TAML^+•)] (<b>1</b>; NTs = tosylimido), an iron(V)-imido complex [Fe^V(NTs)(TAML)]^- (<b>2</b>), and an iron(V)-oxo complex [Fe^V(O)(TAML)]^- (<b>3</b>). The one-electron reduction potential (<i>E</i>_red vs SCE) of <b>1</b> was determined to be 0.86 V, which is much more positive than that of <b>2</b> (0.30 V), but the <i>E</i>_red of <b>3</b> is the most positive (1.04 V). The rate constants of ET of <b>1</b>-<b>3</b> were analyzed in light of the Marcus theory of adiabatic outer-sphere ET to determine the reorganization energies (λ) of ET reactions with <b>1</b>-<b>3</b>; the λ of <b>1</b> (1.00 eV) is significantly smaller than those of <b>2</b> (1.98 eV) and <b>3</b> (2.25 eV) because of the ligand-centered ET reduction of <b>1</b> as compared to the metal-centered ET reduction of <b>2</b> and <b>3</b>. In oxidation reactions, reactivities of <b>1</b>-<b>3</b> 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 <b>1</b>-<b>3</b> 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.