Control of C-H Bond Activation by Mo-Oxo Complexes: pK_a 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 H^δ+ 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 pK_a 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.