HIGHLY ACTIVE N-HETEROCYCLIC CARBENE MN(I) ELECTROCATALYSTS FOR CO2 REDUCTION

In the last decade, fac-[Mn(CO)3(N^N)X]n complexes (X = Br, n = 0 and X = CH3CN, n = +1; N^N = polypyridyl ligands) have been shown to be competent molecular catalysts for selective electrochemical conversion of carbon dioxide to CO.[1-2] Nevertheless, the development of novel ligand frameworks for Mn-based CO2 reduction electrocatalysts is nowadays mainly restrained to ancillary functionalization of the bipyridyl motif, whose electron storage capability plays an essential role in stabilizing the accumulation of electron density over the metal complex. 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 (>100) 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). Spectroelectrochemistry (SEC) and DFT calculations reveal a strong electron density accumulation over the metal center upon reduction due to the redox-innocent nature of the C^C ligand, which is ascribed as the origin of the enhanced reactivity towards CO2 reduction in comparison with N^N and N^C systems.