A Mn Bipyrimidine Catalyst Predicted To Reduce CO₂ at Lower Overpotential

Abstract

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 2 O + 2[F 3 CCH 2 OCO 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 2 H)] ( 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 2 H)] − ( 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.