Experimental and Multireference ab Initio Investigations of Hydrogen-Atom-Transfer Reactivity of a Mononuclear MnIV-oxo Complex

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 B-4(1), and E-4 states. The B-4(1) state is the ground state in the absence of substrate, and the E-4 state is related to the ground state by a one-electron Mn-IV e(d(xz),3d(yz)) to Mn-IV b(1)(d(x)(-y)(2)(2)) 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 E-4 state by the CASSCF/NEVPT2 method is within 2 kcal mol(-1) of the experimental value. The E-4 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 B-4(1) and E-4 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.