A Unified Electro- and Photocatalytic CO2 to CO Reduction Mechanism with Aminopyridine Cobalt Complexes

Mechanistic understanding of electro- and photocatalytic CO 2 reduction is crucial to develop strategies to overcome catalytic bottlenecks. In this regard, herein it is presented a new CO 2 -to-CO reduction cobalt aminopyridine catalyst, a detailed experimental and theoretical mechanistic study toward the identification of bottlenecks and potential strategies to alleviate them. The combination of electrochemical and in-situ spectroelectrochemical (FTIR/UV-Vis SEC) studies together with spectroscopic techniques (NMR, EXAFS) lead us to identify elusive key electrocatalytic intermediates derived from complex [Co(py Me tacn)(OTf) 2 ] ( 1 ) (py Me tacn = 1-[2-pyridylmethyl]-4,7-dimethyl-1,4,7-triazacyclononane) such as a highly reactive cobalt (I) ( 1 (I) ) and cobalt (I) carbonyl ( 1 (I) -CO ) species. 1 (I) was obtained by electrochemical reduction of 1 (II) , and characterized by NMR, EXAFS and FTIR/UV-Vis SEC. The combination of spectroelectrochemical studies under CO 2 , 13 CO 2 and CO with DFT disclosed that 1 (I) directly reacts with CO 2 to form the pivotal 1 (I) -CO intermediate at the 1 (II/I) redox potential. At this redox potential the theoretical energy barrier for the C-O bond cleavage was found to be as low as 12.2 kcal·mol -1 . However, the catalytic process does not proceed at the 1 (II/I) redox potential, due to the formation of 1 (I) -CO, which is a thermodynamic sink and the CO release restricts the electrocatalysis. In agreement with the experimental observed CO 2 -to-CO electrocatalysis at the 1 (I/0) redox potential, computational studies suggested that the productive electrocatalytic cycle involves striking metal carbonyl intermediates such as [L N4 Co 0 CO] (L N4 = py Me tacn), [L N4 Co II (CO 2 )CO] and [L N4 Co I CO) 2 ]. In contrast, under photochemical conditions, the catalytic process smoothly proceeds at the 1 (II/I) redox potential. Under the latter conditions, it is proposed that the electron transfer rate is under diffusion control and then the CO release from 1 (II) -CO is kinetically favored, facilitating the catalysis. Finally, we have found that visible light irradiation has a positive impact under electrocatalytic conditions. We envision that light irradiation can serve as an effective strategy to improve the CO 2 reduction of molecular catalysts, via alleviating bottlenecks, such as the CO poisoning.