Carbon monoxide release catalysed by electron transfer: electrochemical and spectroscopic investigations of [Re(bpy-R)(CO)4](OTf) complexes relevant to CO2 reduction

Abstract

[Re(bpy-tBu)(CO)₄](OTf) (bpy-tBu = 4,4′-di-tert-butyl-2,2′-bipyridine, OTf = trifluoromethanesulfonate) (1) and [Re(bpy)(CO)₄](OTf) (bpy = 2,2′-bipyridine) (2) were synthesized and studied as proposed intermediates in the electrocatalytic reduction of carbon dioxide (CO₂) by Re(bpy-R)(CO)₃X. Both compounds demonstrated increased current responses in cyclic voltammograms under CO₂. Complex 1 was also characterized by X-ray crystallography. Infrared-spectroelectrochemistry (IR-SEC) of 1 and 2 indicated that upon exposure of the cationic tetracarbonyl compounds to a reducing potential, a CO ligand is labilised and [Re(bpy-R)(CO)₃(CH₃CN)]⁺ species are formed. This is proposed to occur via an electron-transfer-catalysed process wherein a catalytic amount of reduced species propagates a ligand exchange reaction. Addition of a catalytic amount of potassium intercalated graphite (KC₈), a chemical reductant, to a solution of 1 or 2 also yielded quantitative formation of [Re(bpy-R)(CO)₃(CH₃CN)]⁺, which indicates that the CO loss is catalysed by electron transfer, and not the electrode itself.