Turning on the Protonation-First Pathway for Electrocatalytic CO₂ Reduction by Manganese Bipyridyl Tricarbonyl...

Electrocatalytic reduction of CO₂ to CO is reported for the complex, {fac-Mn^I([(MeO)₂Ph]₂bpy)(CO)₃(CH₃CN)}(OTf), containing four pendant methoxy groups, where [(MeO)₂Ph]₂bpy = 6,6'-bis(2,6-dimethoxyphenyl)-2,2'-bipyridine. In addition to a steric influence similar to that previously established [Sampson, M. D. et al. J. Am. Chem. Soc. 2014, 136, 5460-5471] for the 6,6'-dimesityl-2,2'-bipyridine ligand in [fac-Mn^I(mes₂bpy)(CO)₃(CH₃CN)](OTf), which prevents Mn⁰-Mn⁰ dimerization, the [(MeO)₂Ph]₂bpy ligand introduces an additional electronic influence combined with a weak allosteric hydrogen-bonding interaction that significantly lowers the activation barrier for C-OH bond cleavage from the metallocarboxylic acid intermediate. This provides access to the thus far elusive protonation-first pathway, minimizing the required overpotential for electrocatalytic CO₂ to CO conversion by Mn(I) polypyridyl catalysts, while concurrently maintaining a respectable turnover frequency. Comprehensive electrochemical and computational studies here confirm the positive influence of the [(MeO)₂Ph]₂bpy ligand framework on electrocatalytic CO₂ reduction and its dependence upon the concentration and pK_a of the external Brønsted acid proton source (water, methanol, trifluoroethanol, and phenol) that is required for this class of manganese catalyst. Linear sweep voltammetry studies show that both phenol and trifluoroethanol as proton sources exhibit the largest protonation-first catalytic currents in combination with {fac-Mn^I([(MeO)₂Ph]₂bpy)(CO)₃(CH₃CN)}(OTf), saving up to 0.55 V in overpotential with respect to the thermodynamically demanding reduction-first pathway, while bulk electrolysis studies confirm a high product selectivity for CO formation. To gain further insight into catalyst activation, time-resolved infrared (TRIR) spectroscopy combined with pulse-radiolysis (PR-TRIR), infrared spectroelectrochemistry, and density functional theory calculations were used to establish the v(CO) stretching frequencies and energetics of key redox intermediates relevant to catalyst activation.