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) (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 (Eox = 2.20 V vs SCE), was oxidized by (N4Py)Fe(IV)(O) in the presence of HClO4 in acetonitrile (MeCN) to yield a stoichiometric amount of benzyl alcohol, in which (N4Py)Fe(IV)(O) was reduced to (N4Py)Fe(III)(OH2). The second-order rate constant (kobs) of the oxidation of toluene derivatives by (N4Py)Fe(IV)(O) 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) 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)) 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) through formation of strong precursor complexes between toluene derivatives and (N4Py)Fe(IV)(OH).