Contrasting effects of axial ligands on electron-transfer versus proton-coupled electron-transfer reactions of nonheme oxoiron(IV) complexes

The effects of axial ligands on electron-transfer and proton-coupled electron-transfer reactions of mononuclear nonheme oxoiron(IV) complexes were investigated by using [Fe^IV(O)(tmc)(X)]ⁿ⁺ (1-X) with various axial ligands, in which tmc is 1,4,8,11-tetramethyl-1,4,8,11- tetraazacyclotetradecane and X is CH₃CN (1-NCCH₃), CF ₃COO^- (1-OOCCF₃), or N₃^- (1-N₃), and ferrocene derivatives as electron donors. As the binding strength of the axial ligands increases, the one-electron reduction potentials of 1-X (E_red, V vs. saturated calomel electrode (SCE)) are more negatively shifted by the binding of the more electron-donating axial ligands in the order of 1-NCCH₃ (0.39) > 1-OOCCF₃ (0.13) > 1-N₃ (-0.05 V). Rate constants of electron transfer from ferrocene derivatives to 1-X were analyzed in light of the Marcus theory of electron transfer to determine reorganization energies (λ) of electron transfer. The λ values decrease in the order of 1-NCCH₃ (2.37) > 1-OOCCF₃ (2.12) > 1-N₃ (1.97 eV). Thus, the electron-transfer reduction becomes less favorable thermodynamically but more favorable kinetically with increasing donor ability of the axial ligands. The net effect of the axial ligands is the deceleration of the electron-transfer rate in the order of 1-NCCH₃ > 1-OOCCF₃ > 1-N ₃. In sharp contrast to this, the rates of the proton-coupled electron-transfer reactions of 1-X are markedly accelerated in the presence of an acid in the opposite order: 1-NCCH₃ < 1-OOCCF₃ < 1-N₃. Such contrasting effects of the axial ligands on the electron-transfer and proton-coupled electron-transfer reactions of nonheme oxoiron(IV) complexes are discussed in light of the counterintuitive reactivity patterns observed in the oxo transfer and hydrogen-atom abstraction reactions by nonheme oxoiron(IV) complexes (Sastri etal. Proc. Natl. Acad. Sci. U.S.A. 2007, 104, 19181-19186).