Influence of weak Bronsted acids on electrocatalytic CO₂ reduction by manganese and rhenium bipyridine catalysts

[Re(bpy)(CO)₃]^- and [Mn(bpy)(CO)₃]^- are homogeneous electrocatalysts for the reduction of CO₂ to CO. Their turnover frequencies depend on the type of Bronsted acid used, with the Mn catalyst exhibiting no catalytic turnover without added Bronsted acid. In this work, we use density functional theory together with continuum solvation and microkinetics simulations to understand these differences. The computed turnover frequencies reproduce the experimental trends. In absolute numbers, the computed turnover frequencies differ from the experimental ones by about an order of magnitude. We find that some of the experimentally used acids are too weak to protonate CO₂ or to stabilize CO₂ binding. Catalysis with these acids requires more negative applied potentials or higher acid concentrations compared to catalysis with stronger acids. This trend is more pronounced for the Mn catalyst than for the Re catalyst, the latter working at maximum turnover with acids that produce submaximum turnover with the Mn catalyst. In the absence of Bronsted acids, the first catalytic steps are driven by the solvent acetonitrile, which can act as proton donor for protonation of CO₂ in the case of the Re catalyst. For the Mn catalyst, the endergonic CO₂ binding free energy prevents protonation by acetonitrile. C-O bond cleavage, however, cannot be assisted by acetonitrile for either catalyst. Electrolyte-assisted C-O bond cleavage via Hofmann degradation is also predicted to be strongly disfavored kinetically. Water produced during catalysis might be responsible for completing the reaction cycle. (Chemical Equation Presented).