Turning on the Protonation-First Pathway for Electrocatalytic C0_2 Reduction by Manganese Bipyridyl Tricarbonyl Complexes

摘要

Electrocatalytic reduction of C0_2 to CO is reported for the complex, {fac-Mn~I([(MeO)_2Ph]_2bpy)-(CO)_3(CH_3CN)}(OTf), containing four pendant methoxy groups, where [(MeO)_2Ph]_2bpy = 6,6'-bis(2,6-dimethoxy-phenyl)-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(mes_2bpy)(CO)_3(CH_3CN)]-(OTf), which prevents Mn~0—Mn~0 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 CO_2 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 CO_2 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(CH_3CN)}(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.