A Mn Bipyrimidine Catalyst Predicted to Reduce CO₂ at Lower Overpotential

Experimentally, [(L)Mn(CO)₃]^- (where L = bis-alkyl-substituted bipyridine) has been observed to catalyze the electrochemical reduction of CO₂ to CO in the presence of trifluoroethanol (TFEH). Here we report the atomistic level mechanism of complete catalytic cycles for this reaction, on the basis of DFT calculations (B3LYP-D3 with continuum solvation) of the free energies of reaction and activation, as well as reduction potentials for all catalytically relevant elementary steps. The highly exergonic homoconjugation and carbonation of TFE^- play critical roles in reaction thermodynamics and kinetics, the overall half-reaction being 3CO₂ + 2TFEH + 2e^- → CO + H₂O + 2[F₃CCH₂OCO₂]^- (calculated standard reduction potential: -1.49 V vs SCE). In the catalytic cycle for CO formation, CO₂ coordinates to [(L)Mn(CO)₃]^- (1a, L = bpy), and the adduct is then protonated to form [(L)Mn(CO)₃(CO₂H)] (3a). 3a subsequently reacts to form [(L)Mn(CO)₄]⁰ (5a) via one of two pathways: (a) TFEH-mediated dehydroxylation to [(L)Mn(CO)₄]⁺ (4a), followed by one-electron reduction to 5a, or (b) under more reducing potentials, one-electron reduction to [(L)Mn(CO)₃(CO₂H)]^- (3′a), followed by dehydroxylation to 5a. Pathway b has a lower activation energy by 2.2 kcal mol^-1. Consequently, the maximum catalytic turnover frequency (TOF_max) is achieved at ∼-1.75 V vs SCE (∼0.25 V overpotential). For the analogous bipyrimidine compound (not yet studied experimentally), reduction of 3b to 3′b occurs at a potential 0.5 V more positive than that of 3a, and the overpotential required to achieve TOF_max is predicted to be lower by ∼0.25 V. This improvement is, however, achieved at the price of a lower TOF_max, and we predict that 1b has superior TOF at potentials above ∼-1.6 V vs SCE. In addition, the various factors contributing to product selectivity (CO over H₂) are discussed. (Chemical Equation Presented).