Electrocatalytic Reduction of CO₂ to CO with Re-Pyridyl-NHCs: Proton Source Influence on Rates and Product Selectivities

A series of four electron-deficient-substituted Re(I) pyridyl N-heterocyclic carbene (pyNHC) complexes have been synthesized, and their electrocatalytic reduction of CO₂ has been evaluated by cyclic voltammetry and controlled potential electrolysis experiments. All of the catalysts were evaluated by cyclic voltammetry under inert atmosphere and under CO₂ and compared to the known benchmark catalyst Re(bpy)(CO)₃Br. Among the four Re-NHC catalysts, Re(pyNHC-PhCF₃)(CO)₃Br (2) demonstrated the highest catalytic rate (i_cat/i_p)² at the first and second reduction events with a value of 4 at the second reduction potential (TOF = 0.8 s^-1). The rate of catalysis was enhanced through the addition of proton sources (PhOH, TFE, and H₂O; TOF up to 100 s^-1 (i_cat/i_p)² = 700). Controlled potential electrolysis shows Faradaic efficiencies (FE) for CO production and accumulated charge for the Re(pyNHC-PhCF₃)(CO)₃Br catalyst exceed those of the benchmark catalyst in the presence of 2 M H₂O (92%, 13 C at 1 h versus 61%, 3 C for the benchmark catalyst) under analogous experimental conditions. A peak FE of 100% was observed during electrolysis with Re(pyNHC-PhCF₃)(CO)₃Br.