A Mn Bipyrimidine Catalyst Predicted To Reduce CO_2 at Lower Overpotential

Experimentally, [(L)Mn(CO)_3]− (where L = bis-alkyl-substituted bipyridine) has been observed to catalyze the electrochemical reduction of CO_2 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 3CO_2 + 2TFEH + 2e– → CO + H_2O + 2[F_3CCH_2OCO_2]− (calculated standard reduction potential: −1.49 V vs SCE). In the catalytic cycle for CO formation, CO_2 coordinates to [(L)Mn(CO)_3]− (1a, L = bpy), and the adduct is then protonated to form [(L)Mn(CO)_3(CO_2H)] (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(CO_2H)]− (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 H_2) are discussed.