Novel tricarbonyl Mn catalysts based on N-heterocyclic Carbene ligands for efficient electrochemical CO2 reduction

In the last decade, Mn fac-tricarbonyl derivatives with polypyridyl ligands have been shown to be competent molecular catalysts for selective electrochemical conversion of CO2 to CO in the presence of Brønsted or Lewis acids. However, their further improvement is mainly limited to the functionalization of bipyridyl or pyridyl moieties, whereas reported examples of catalysts with non-pyridyl diimine units are rare and usually inefficient. We report here the first purely organometallic fac-[Mn(CO)3(bis-MeNHC)Br] (1) complex with unprecedented activity for selective electrocatalytic CO2-to-CO reduction. It revealed to highly outperform the corresponding single NHC-containing Mn catalyst, namely [Mn(CO)3(py-MeNHC)I] (2), in terms of selectivity (faradaic yield = 95%), catalytic turnovers and maximum TOF (TOFmax~103 s-1) under CO2 in anhydrous CH3CN. Moreover, the addition of water (0.56 M) produces an impressive TOFmax increase of 2 orders of magnitude (~320000 s-1). Complementary FTIR-Spectroelectrochemistry (SEC) and DFT calculations suggest that the strongly nucleophilic character of the Mn atom is likely responsible for the positive effect on catalysis.