Control of C–H Bond Activation by Mo-Oxo Complexes: pKa or Bond Dissociation Free Energy (BDFE)?

A density functional theory (DFT) study (BMK/6-31+G­(d)) was initiated to investigate the activation of benzylic carbon–hydrogen bonds by a molybdenum-oxo complex with a potentially redox noninnocent supporting liganda simple mimic of the active species of the enzyme ethylbenzene dehydrogenase (EBDH)through deprotonation (C–H bond heterolysis) or hydrogen atom abstraction (C–H bond homolysis) routes. Activation free-energy barriers for neutral and anionic Mo-oxo complexes were high, but lower for anionic complexes than neutral complexes. Interesting trends as a function of substituents were observed that indicated significant Hδ+ character in the transition states (TS), which was further supported by the preference for [2 + 2] addition over HAA for most complexes. Hence, it was hypothesized that C–H activation by these EBDH mimics is controlled more by the pKa than by the bond dissociation free energy of the C–H bond being activated. Therefore, the results suggest promising pathways for designing more efficient and selective catalysts for hydrocarbon oxidation based on EBDH active-site mimics.