Pendent trifluoroethanol reveals solvation dependent mechanisms of electrocatalytic CO₂ reduction by rhenium(I) bipyridine fac-tricarbonyl

Abstract

Local proton sources capable of interacting with catalytic intermediates have been shown to affect proton-dependent reactions. Herein we report the synthesis of a Re(diimine)(CO)₃Cl catalyst analog containing a trifluoroethanol-appended bipyridine ligand (bpy-CF₃OH), and the corresponding methyl-ether (bpy-CF₃OMe) to study the role of the pendent proton donor in electrocatalytic CO₂ reduction. Compared to the parent catalyst, Re(bpy)(CO)₃Cl, and Re(bpy-CF₃OMe)(CO)₃Cl, the alcohol analog revealed additional electrocatalytic features and a 200 mV anodic shift in catalytic onset potential. We also show evidence of proton-coupled electron transfer or hydrogen bond-assisted catalysis in all analogs, detailed by a progressive anodic shift in the main catalytic wave with increasing [H₂O] which displays slopes of ~120 mV log[H₂O]⁻¹. Multiple catalytic mechanisms appeared to occur simultaneously in all the analogs, and attempts were made to differentiate their dependence on [H₂O] and [CO₂]. The effect of a local proton source is significant under conditions where [H₂O] < 1 M but becomes negligible at high [H₂O]. These results suggest that the appended alcohol affects the solvation of the molecule, and may act directly as a proton source or as part of a proton shuttle.