High oxidizing reactivity of mononuclear nonheme Iron (V)-oxo complexes

초록

Mononuclear nonheme iron(V)-oxo complexes bearing a tetraamido-macrocyclic (TAML) ligand ([FeV(O) (TAML)]-) are well characterized and the high oxidizing reactivity has been examined. In contrast, the oxidizing reactivity of mononuclear nonheme iron(V)-oxo complexes with neutral ligands, has yet to be studied because of the extreme short lifetimes of the iron(V)-oxo complexes (ca. several microseconds). We report herein systematic studies on the oxidizing reactivity of formal mononuclear nonheme iron(V)-oxo complexes, [FeV(O)(L)](n+1)+ [n = 2 where L is neutral ligand or n = 1 where L is monoanion ligand], by using laser-induced transient absorption measurements. The [FeV(O)(L)](n+1)+ complexes were produced by electron transfer from [FeIV(O)(L)]n+ to the triplet excited state of 2,3-dichloro-5,6-dicyano-p-benzoquinone (3DDQ*), which was generated by laser excitation. [FeV(O)(L)](n+1)+ reacts with H2O to produce [FeIII(OOH)(L)]n+, which was further oxidized by DDQ to evolve O2, accompanied by regeneration of [FeII(L)]n+. The decay rates of [FeV(O)(L)](n+1)+ became faster in the presence of substrates via oxidation of substrates by [FeV(O)(L)](n+1)+. Plots of logarithm of the rate constants vs the one-electron oxidation potentials of donor substrates were evaluated in light of the Marcus theory of outersphere electron transfer to afford the one-electron reduction potentials and reorganization energies of [FeV(O) (L)](n+1)+, which have been compared with those of other iron(V)-oxo complexes. The large reorganization energies of the electron-transfer reduction of the one-electron oxidized species of [FeIV(O)(L)]n+ indicate that electron-transfer oxidation of [FeIV(O)(L)]n+ occurs at the metal center to produce [FeV(O)(L)](n+1)+ rather than

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