Direct Detection of Key Intermediates during the Product Release in Rhenium Bipyridine-Catalyzed CO2 Reduction Reaction

Rhenium bipyridine tricarbonyl complexes, fac-[Re(bpy)(CO)(3)X](n+), are highly effective in selectively converting CO2 to CO under electrochemical and photochemical conditions. Despite numerous mechanistic studies aimed at understanding its CO2 reduction reaction (CO2RR) pathway, the intermediates further into the catalytic cycle have escaped detection, and the steps leading to product release remained elusive. In this study, employing stopped-flow mixing coupled with time-resolved infrared spectroscopy, we observed, for the first time, the reduced Re-tetracarbonyl species, [Re(bpy)(CO)(4)](0), with a half-life of approximately 55 ms in acetonitrile solvent. This intermediate is proposed to be common in both electrochemical and photochemical CO2RR. Furthermore, we directly observed the release of the product (CO) from this intermediate. Additionally, we detected the accumulation of [Re(bpy)(CO)(3)(CH3CN)](+) as a byproduct following product release, a significant side reaction under conditions with a limited supply of reducing equivalents mirroring photochemical conditions. The process could be unambiguously attributed to an electron transfer-catalyzed ligand substitution reaction involving [Re(bpy)(CO)(4)](0) by simultaneous real-time detection of all involved species. We believe that this side reaction significantly impacts the CO2RR efficiency of this class of catalysts under photochemical conditions or during electrocatalysis at mild overpotentials.