Brønsted acid-promoted C-H bond cleavage via electron transfer from toluene derivatives to a protonated nonheme iron(IV)-oxo complex with no kinetic isotope effect

The reactivity of a nonheme iron(IV)-oxo complex, [(N4Py)Fe ^IV(O)]²⁺ (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl) methylamine), was markedly enhanced by perchloric acid (70% HClO₄) in the oxidation of toluene derivatives. Toluene, which has a high one-electron oxidation potential (E_ox = 2.20 V vs SCE), was oxidized by [(N4Py)Fe^IV(O)]²⁺ in the presence of HClO₄ in acetonitrile (MeCN) to yield a stoichiometric amount of benzyl alcohol, in which [(N4Py)Fe^IV(O)]²⁺ was reduced to [(N4Py)Fe ^III(OH₂)]³⁺. The second-order rate constant (k_obs) of the oxidation of toluene derivatives by [(N4Py)Fe ^IV(O)]²⁺ increased with increasing concentration of HClO₄, showing the first-order dependence on [HClO₄]. A significant kinetic isotope effect (KIE) was observed when mesitylene was replaced by mesitylene-d₁₂ in the oxidation with [(N4Py)Fe ^IV(O)]²⁺ in the absence of HClO₄ in MeCN at 298 K. The KIE value drastically decreased from KIE = 31 in the absence of HClO₄ to KIE = 1.0 with increasing concentration of HClO₄, accompanied by the large acceleration of the oxidation rate. The absence of KIE suggests that electron transfer from a toluene derivative to the protonated iron(IV)-oxo complex ([(N4Py)Fe^IV(OH)]³⁺) is the rate-determining step in the acid-promoted oxidation reaction. The detailed kinetic analysis in light of the Marcus theory of electron transfer has revealed that the acid-promoted C-H bond cleavage proceeds via the rate-determining electron transfer from toluene derivatives to [(N4Py)Fe^IV(OH)] ³⁺ through formation of strong precursor complexes between toluene derivatives and [(N4Py)Fe^IV(OH)]³⁺.