Deciphering the Mechanistic Details of Manganese-Catalyzed Formic Acid Dehydrogenation: Insights from DFT Calculations.

A comprehensive density functional theory investigation has been carried out to unravel the complete mechanistic landscape of aqueous-phase formic acid dehydrogenation (FAD) catalyzed by a pyridyl-imidazoline-based Mn(I) catalyst [Mn(PY-NHIM)(CO)3Br], which was recently reported by Beller and co-workers. The computed free energy profiles show that for the production of a Mn-formate intermediate [Mn(HCO2-)], a stepwise mechanism is both kinetically and thermodynamically favorable compared to the concerted mechanism. This stepwise mechanism involves the dissociation of a Br- ion from a Mn-bromide complex [Mn(Br)] to create a vacant site and coordination of water solvent to this vacant site, followed by the dissociative exchange of the aqua ligand with the formate ion to form Mn(HCO2-). Non-covalent interaction analysis revealed that the steric hindrance at the transition state is the cardinal reason for the preference to a stepwise mechanism. The β-hydride elimination process was estimated to be the rate-determining step with a barrier of 19.0 kcal/mol. This confirms the experimental observation. The generation of a dihydrogen-bound complex was found to occur through the protonation of Mn-hydride by a hydronium ion instead of formic acid. The mechanistic details and insights presented in this work would promote future catalytic designing and exploration of earth-abundant Mn-based catalytic systems for potential applications toward FAD.