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

Rhenium bipyridine tricarbonyl complexes, fac-[Re(bpy)(CO)₃X]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.