Mechanistic Insights into the C–H Bond Activation of Hydrocarbons by Chromium(IV) Oxo and Chromium(III) Superoxo Complexes

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, [CrIV(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, [CrIII(OH)­(TMC)­(Cl)]+ (3), was observed as a decomposed product of 2 in the C–H bond activation reaction. The CrIIIOH 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 CrIIIO•– character. The oxidizing power
of 2 was then compared with that of a chromium­(III) superoxo
complex bearing the identical TMC ligand, [CrIII(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.