title: 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.
abstract: 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 (Eox = 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 (kobs) 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+).
authors.affiliation: Department of Material and Life Science, Graduate School of Engineering, ALCA, Japan Science and Technology Agency, Osaka University , Suita, Osaka 565-0871, Japan.
authors.forename: Jiyun
authors.initials: J
authors.lastname: Park
authors.forename: Yong-Min
authors.initials: YM
authors.lastname: Lee
authors.forename: Wonwoo
authors.initials: W
authors.lastname: Nam
authors.forename: Shunichi
authors.initials: S
authors.lastname: Fukuzumi
mesh_terms: D004579:Electron Transport; D005290:Ferric Compounds / Q000737:chemistry*; D058085:Iron Compounds / Q000737:chemistry*; D007700:Kinetics; D009942:Organometallic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D010472:Perchlorates / Q000737:chemistry; D011522:Protons; D013696:Temperature; D014050:Toluene / Q000737:chemistry*
chemical_list: D005290:Ferric Compounds; D058085:Iron Compounds; D009942:Organometallic Compounds; D010472:Perchlorates; D011522:Protons; C000499:ferric oxide; D014050:Toluene; C576518:Perchloric Acid