Direct Detection of Key Intermediates during the Product Release in Rhenium Bipyridine-Catalyzed CO<sub>2</sub> Reduction Reaction
Samir Chattopadhyay, Mun Hon Cheah, Reiner Lomoth, Leif Hammarström
Abstract
High Resolution Image Download MS PowerPoint Slide Rhenium bipyridine tricarbonyl complexes, fac -[Re(bpy)(CO) 3 X] n +, are highly effective in selectively converting CO 2 to CO under electrochemical and photochemical conditions. Despite numerous mechanistic studies aimed at understanding its CO 2 reduction reaction (CO 2 RR) 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 CO 2 RR. Furthermore, we directly observed the release of the product (CO) from this intermediate. Additionally, we detected the accumulation of [Re(bpy)(CO) 3 (CH 3 CN)] + 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 CO 2 RR efficiency of this class of catalysts under photochemical conditions or during electrocatalysis at mild overpotentials.