Mechanistic Insights of Electrochemical CO2 Reduction Catalyzed by Manganese N-Heterocyclic Carbene Complexes

A fundamental understanding of the electrochemical CO2 reduction reaction (CO2RR) promoted by transition metal-based catalysts at a molecular level is essential to improve the efficiency and selectivity of the process toward the formation of a specific product.[1] In this regard, molecular electrocatalysts, characterized by well-defined active sites, are ideal platforms to investigate the reaction pathways, through the characterization of the main intermediates.[2]-[3] Since the seminal work by Deronzier and co-workers, tricarbonyl Mn complexes with polypyridyl ligands have been extensively studied as homogeneous molecular electrocatalysts for selective CO2 reduction to CO.[4] Recently, bidentate N-heterocyclic carbene (NHC) ligands were found to be suitable alternative platforms to classical diimine moieties, showing excellent performances for selective CO2-to-CO electroreduction in neat non-aqueous media or in the presence of low-to-moderate amounts of water.[5] In this contribution, we shed light on the main mechanistic features of Mn-NHC electrocatalysts for CO2 reduction in both aprotic and protic conditions. While the main catalytic pathway results in a highly efficient and selective CO2-to-CO conversion in aprotic media, a competitive mechanism leading to the catalytic HCOO– production operates in the presence of weak Brønsted acids. By combining organometallic synthetic chemistry and in situ FTIR spectroelectrochemical (SEC) techniques, we unambiguously characterized the key intermediates involved in the electrocatalytic pathways to CO and HCOO–, respectively, establishing a direct correlation between their formation and the observed selectivity. These findings provide new mechanistic insights towards a fundamental understanding of the origin of selectivity in CO2RR catalyzed by earth-abundant transition metal-based molecular systems.