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

Mononuclear nonheme iron(V)-oxo complexes bearing a tetraamido-macrocyclic (TAML) ligand ([Fe^V(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, [Fe^V(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 [Fe^V(O)(L)]⁽ⁿ⁺¹⁾⁺ complexes were produced by electron transfer from [Fe^IV(O)(L)]ⁿ⁺ to the triplet excited state of 2,3-dichloro-5,6-dicyano-p-benzoquinone (³DDQ*), which was generated by laser excitation. [Fe^V(O)(L)]⁽ⁿ⁺¹⁾⁺ reacts with H₂O to produce [Fe^III(OOH)(L)]ⁿ⁺, which was further oxidized by DDQ to evolve O₂, accompanied by regeneration of [Fe^II(L)]ⁿ⁺. The decay rates of [Fe^V(O)(L)]⁽ⁿ⁺¹⁾⁺ became faster in the presence of substrates via oxidation of substrates by [Fe^V(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 outer-sphere electron transfer to afford the one-electron reduction potentials and reorganization energies of [Fe^V(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 [Fe^IV(O)(L)]ⁿ⁺ indicate that electron-transfer oxidation of [Fe^IV(O)(L)]ⁿ⁺ occurs at the metal center to produce [Fe^V(O)(L)]⁽ⁿ⁺¹⁾⁺ rather than [Fe^IV(O)(L^•+)]⁽ⁿ⁺¹⁾⁺.