C–H Bond Oxidation by MnIV–Oxo Complexes: Hydrogen-Atom Tunneling and Multistate Reactivity

The reactivity of six MnIV–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 MnIV–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 MnIV–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 MnIV–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 MnIV–oxo complexes can be well-predicted on the basis of the thermodynamic driving force for these reactions.