C-H Bond Oxidation by Mn^IV-Oxo Complexes: Hydrogen-Atom Tunneling and Multistate Reactivity

The reactivity of six Mn^IV-oxo complexes in C-H bond oxidation has been examined using a combination of kinetic experiments and computational methods. Variable-temperature studies of the oxidation of 9,10-dihydroanthracene (DHA) and ethylbenzene by these Mn^IV-oxo complexes yielded activation parameters suitable for evaluating electronic structure computations. Complementary kinetic experiments of the oxidation of deuterated DHA provided evidence for hydrogen-atom tunneling in C-H bond oxidation for all Mn^IV-oxo complexes. These results are in accordance with the Bell model, where tunneling occurs near the top of the transition-state barrier. Density functional theory (DFT) and DLPNO-CCSD(T₁) computations were performed for three of the six Mn^IV-oxo complexes to probe a previously predicted multistate reactivity model. The DFT computations predicted a thermal crossing from the ⁴B₁ ground state to a ⁴E state along the C-H bond oxidation reaction coordinate. DLPNO-CCSD(T₁) calculations further confirm that the ⁴E transition state offers a lower energy barrier, reinforcing the multistate reactivity model for these complexes. We discuss how this multistate model can be reconciled with recent computations that revealed that the kinetics of C-H bond oxidation by this set of Mn^IV-oxo complexes can be well-predicted on the basis of the thermodynamic driving force for these reactions.