Experimental and Multireference ab Initio Investigations of Hydrogen-Atom-Transfer Reactivity of a Mononuclear...

A combined experimental-computational study of hydrocarbon oxidation by the Mn^IV-oxo complex of the neutral, pentadentate N4py ligand [N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine] offers support for a complex reaction coordinate involving multiple electronic states. Variable-temperature kinetic investigations of ethylbenzene oxidation by [Mn^IV(O)(N4py)]²⁺ yield experimental activation parameters that were used to evaluate computationally predicted energy barriers. Both density functional theory (DFT) and multireference complete-active-space self-consistent-field (CASSCF) computations with n-electron valence state perturbation theory (NEVPT2) corrections were employed to investigate the hydrogen-atom-transfer reaction barriers for the ⁴B₁ and ⁴E states. The ⁴B₁ state is the ground state in the absence of substrate, and the ⁴E state is related to the ground state by a one-electron Mn^IV e(d,3d) to Mn^IV b₁(d²_-y²) excitation. A comparison of the DFT, CASSCF/NEVPT2, and experimental results shows that the B3LYP-D3 method underestimates the activation barriers of both electronic states by ca. 10 kcal mol^-1. In contrast, the enthalpic barrier predicted for the ⁴E state by the CASSCF/NEVPT2 method is within 2 kcal mol^-1 of the experimental value. The ⁴E state is early, with dominant structural distortions in the Mn-N_equatorial distances and perturbations to Mn═O bonding that lead to strong electronic stabilization of interactions between the Mn^IV-oxo unit and substrate C-H bond. While previous DFT studies were qualitatively correct in their ordering of the ⁴B₁ and ⁴E transition states, this combined use of experimental and CASSCF/NEVPT2 methods provides an ideal means of assessing the two-state reactivity model of Mn^IV-oxo complexes.