title: Mechanistic insights into the C-H bond activation of hydrocarbons by chromium(IV) oxo and chromium(III) superoxo complexes.
abstract: The mechanism of the C-H bond activation of hydrocarbons by a nonheme chromium(IV) oxo complex bearing an N-methylated tetraazamacrocyclic cyclam (TMC) ligand, [Cr(IV)(O)(TMC)(Cl)](+) (2), has been investigated experimentally and theoretically. In experimental studies, reaction rates of 2 with substrates having weak C-H bonds were found to depend on the concentration and bond dissociation energies of the substrates. A large kinetic isotope effect value of 60 was determined in the oxidation of dihydroanthracene (DHA) and deuterated DHA by 2. These results led us to propose that the C-H bond activation reaction occurs via a H-atom abstraction mechanism, in which H-atom abstraction of substrates by 2 is the rate-determining step. In addition, formation of a chromium(III) hydroxo complex, [Cr(III)(OH)(TMC)(Cl)](+) (3), was observed as a decomposed product of 2 in the C-H bond activation reaction. The Cr(III)OH product was characterized unambiguously with various spectroscopic methods and X-ray crystallography. Density functional theory (DFT) calculations support the experimental observations that the C-H bond activation by 2 does not occur via the conventional H-atom-abstraction/oxygen-rebound mechanism and that 3 is the product formed in this C-H bond activation reaction. DFT calculations also propose that 2 may have some Cr(III)O(•-) character. The oxidizing power of 2 was then compared with that of a chromium(III) superoxo complex bearing the identical TMC ligand, [Cr(III)(O2)(TMC)(Cl)](+) (1), in the C-H bond activation reaction. By performing reactions of 1 and 2 with substrates under identical conditions, we were able to demonstrate that the reactivity of 2 is slightly greater than that of 1. DFT calculations again support this experimental observation, showing that the rate-limiting barrier for the reaction with 2 is slightly lower than that of 1.
authors.affiliation: Department of Chemistry and Nano Science, Center for Biomimetic Systems, Ewha Womans University , Seoul 120-750, Korea.
authors.forename: Kyung-Bin
authors.initials: KB
authors.lastname: Cho
authors.forename: Hyeona
authors.initials: H
authors.lastname: Kang
authors.forename: Jaeyoung
authors.initials: J
authors.lastname: Woo
authors.forename: Young Jun
authors.initials: YJ
authors.lastname: Park
authors.forename: Mi Sook
authors.initials: MS
authors.lastname: Seo
authors.forename: Jaeheung
authors.initials: J
authors.lastname: Cho
authors.forename: Wonwoo
authors.initials: W
authors.lastname: Nam
mesh_terms: D002857:Chromium / Q000737:chemistry*; D018360:Crystallography, X-Ray; D006838:Hydrocarbons / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D009942:Organometallic Compounds / Q000737:chemistry*; D010100:Oxygen / Q000737:chemistry*; D011789:Quantum Theory
chemical_list: D006838:Hydrocarbons; D009942:Organometallic Compounds; D002857:Chromium; D010100:Oxygen