A Unified Electro- and Photocatalytic CO ₂ to CO Reduction Mechanism with Aminopyridine Cobalt Complexes

Abstract

A mechanistic understanding of electro- and photocatalytic CO₂ reduction is crucial to develop strategies to overcome catalytic bottlenecks. In this regard, for a new CO₂-to-CO reduction cobalt aminopyridine catalyst, a detailed experimental and theoretical mechanistic study is herein presented toward the identification of bottlenecks and potential strategies to alleviate them. The combination of electrochemistry and <i>in situ</i> spectroelectrochemistry together with spectroscopic techniques led us to identify elusive key electrocatalytic intermediates derived from complex [L^N4Co(OTf)₂] (<b>1</b>) (L^N4 = 1-[2-pyridylmethyl]-4,7-dimethyl-1,4,7-triazacyclononane) such as a highly reactive cobalt(I) (<b>1</b>^<b>(I)</b>) and a cobalt(I) carbonyl (<b>1</b>^<b>(I)</b><b>-CO</b>) species. The combination of spectroelectrochemical studies under CO₂, ¹³CO₂, and CO with DFT disclosed that <b>1</b>^<b>(I)</b> reacts with CO₂ to form the pivotal <b>1</b>^<b>(I)</b><b>-CO</b> intermediate at the <b>1</b>^{<b>(II/I)</b>} redox potential. However, at this reduction potential, the formation of <b>1</b>^<b>(I)</b><b>-CO</b> restricts the electrocatalysis due to the endergonicity of the CO release step. In agreement with the experimentally observed CO₂-to-CO electrocatalysis at the Co^I/0 redox potential, computational studies suggested that the electrocatalytic cycle involves striking metal carbonyls. In contrast, under photochemical conditions, the catalysis smoothly proceeds at the <b>1</b>^{<b>(II/I)</b>} redox potential. Under the latter conditions, it is proposed that the electron transfer to form <b>1</b>^<b>(I)</b><b>-CO</b> from <b>1</b>^<b>(II)</b><b>-CO</b> is under diffusion control. Then, the CO release from <b>1</b>^<b>(II)</b><b>-CO</b> 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 circumvent the CO poisoning and improve the performance of CO₂ reduction molecular catalysts.