A Mn Bipyrimidine Catalyst Predicted To Reduce CO<sub>2</sub> at Lower Overpotential

Experimentally, [(L)­Mn­(CO)<sub>3</sub>]<sup>−</sup> (where L = bis-alkyl-substituted bipyridine) has been observed to catalyze the electrochemical reduction of CO<sub>2</sub> 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<sup>–</sup> play critical roles in reaction thermodynamics and kinetics, the overall half-reaction being 3CO<sub>2</sub> + 2TFEH + 2e<sup>–</sup> → CO + H<sub>2</sub>O + 2­[F<sub>3</sub>CCH<sub>2</sub>OCO<sub>2</sub>]<sup>−</sup> (calculated standard reduction potential: −1.49 V vs SCE). In the catalytic cycle for CO formation, CO<sub>2</sub> coordinates to [(L)­Mn­(CO)<sub>3</sub>]<sup>−</sup> (<b>1a</b>, L = bpy), and the adduct is then protonated to form [(L)­Mn­(CO)<sub>3</sub>(CO<sub>2</sub>H)] (<b>3a</b>). <b>3a</b> subsequently reacts to form [(L)­Mn­(CO)<sub>4</sub>]<sup>0</sup> (<b>5a</b>) via one of two pathways: (a) TFEH-mediated dehydroxylation to [(L)­Mn­(CO)<sub>4</sub>]<sup>+</sup> (<b>4a</b>), followed by one-electron reduction to <b>5a</b>, or (b) under more reducing potentials, one-electron reduction to [(L)­Mn­(CO)<sub>3</sub>(CO<sub>2</sub>H)]<sup>−</sup> (<b>3</b>′<b>a</b>), followed by dehydroxylation to <b>5a</b>. Pathway b has a lower activation energy by 2.2 kcal mol<sup>–1</sup>. Consequently, the maximum catalytic turnover frequency (TOF<sub>max</sub>) is achieved at ∼−1.75 V vs SCE (∼0.25 V overpotential). For the analogous bipyrimidine compound (not yet studied experimentally), reduction of <b>3b</b> to <b>3</b>′<b>b</b> occurs at a potential 0.5 V more positive than that of <b>3a</b>, and the overpotential required to achieve TOF<sub>max</sub> is predicted to be lower by ∼0.25 V. This improvement is, however, achieved at the price of a lower TOF<sub>max</sub>, and we predict that <b>1b</b> has superior TOF at potentials above ∼−1.6 V vs SCE. In addition, the various factors contributing to product selectivity (CO over H<sub>2</sub>) are discussed.