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.

A Heterobimetallic Superoxide Complex formed through O2 Activation between Chromium(II) and a Lithium Cation

“…The results are also in agreement with the reported magnetic ground state of S = 1 for I. [19] While to date only two other chromium(III) superoxide compounds have been structurally characterized, from which 2 clearly differs through the oxo environment around the chromium center, another unique feature of 2 is its M-(cis-m-1,2-OOC À )-M' moiety. Since the first reports by Nam and coworkers, [20] the stabilization of reactive metal/oxygen species by redox-inert, Lewis acidic cations (M') and the influence of these cations on reactivity has become a subject of intense research.…”

Efficient C–H bond activations via O2cleavage by a dianionic cobalt(ii) complex

“…Firstly, metal superoxo complexes often exhibit much higher k H / k D values in intermolecular C–H bond activations (a range of k H / k D = 6.3 to 50 is typical). 15 The substrate competition experiments are consistent with initial proton abstraction from a C–H bond, but the p K a of hydrogen superoxide (p K a = 12 in DMF) 16,17 suggests that such species should not be basic enough to accomplish such deprotonations. Furthermore, the observed reaction chemistry for O 2 closely parallels that observed with oxo-transfer reagents PhIO and IO 4 – , suggesting a cobalt( iv ) oxo species as a common intermediate.…”

Hydrolytic and redox transformations of chromium(III) bis-oxalato complexes with glutaminic acid and glutamine: a kinetic, UV–Vis and EPR, study

“…Very recent works on biological and medicinal aspects of chromium coordination chemistry include: (1) separation and speciation of chromium in chromium yeast [13], (2) syntheses, characteristics and biological activity of metforming drug for diabetes patients with chromium(III) [14] and (3) kinetic and mechanistic studies on Cr(V) ? Cr(III) transformations [15][16][17]. As it was established, chromium(III) complexes being to the left of the ''oxo wall'', vide [18], can be involved not only in four-electron reduction of the oxygen, but also can act as selective catalysts in two-electron transformation of O 2 ?…”