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

Mechanistic understanding of electro- and photocatalytic CO₂ reduction is crucial to develop strategies to overcome catalytic bottlenecks. In this regard, herein it is presented a new CO₂-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^Metacn)(OTf)₂] (1) (py^Metacn = 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 CO₂, ¹³CO₂ and CO with DFT disclosed that 1^(I) directly reacts with CO₂ 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 CO₂-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^N4Co⁰CO] (L^N4 = py^Metacn), [L^N4Co^II(CO₂)CO] and [L^N4Co^ICO)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 CO₂ reduction of molecular catalysts, via alleviating bottlenecks, such as the CO poisoning.