Theory Revealing Unusual Non-Rebound Mechanisms Responsible for the Distinct Reactivities of O=Mn^IV=O and [HO-Mn^IV-OH]²⁺ in C-H Bond Activation

This article uses theory to address the origins of the reactivity differences between Mn^IV=O and Mn^IV-OH complexes, having identical ligand spheres and metal oxidation states, toward 9,10-dihydroanthracene (DHA) under different pH conditions. Theory discovers different non-rebound mechanisms leading to unique products for the two complexes. One of these is a novel mechanism that operates under basic conditions and that rationalizes the formation of anthraquinone through an anthracene radical anion intermediate. In addition, the calculations reveal a rich mechanistic scheme having blended hydrogen atom transfer and proton-coupled electron transfer (HAT/PCET) with both proton transfer/electron transfer (PT/ET) and electron transfer/proton transfer (ET/PT) characters. The distinct nature of the transition states, such as PT/ET and ET/PT, for the second H-abstraction reactions from the substrate radical by the Mn^IV=O and Mn^IV-OH complexes accounts for the observed product distributions for these two species. The formation of an anthracene radical anion, and its participation in a unique non-rebound mechanism, is a testable prediction.