Impact of Linking Groups in Chelating Bis-carbene Iridium Catalysts for Transfer Hydrogenation of Inorganic Carbonates with Glycerol

Reduction of carbonate salts by transfer hydrogenation, utilizing glycerol as sacrificial hydrogen donor, to generate formate and lactate is an attractive reaction to produce value added products from chemical wastes. Iridium complexes have emerged as highly active catalysts for this transformation. Herein, we report the synthesis of a series of iridium(I) bis-carbonyl complexes, supported by neutral chelating bis-N-heterocyclic carbene (bis-NHC) ligands which define 7-membered ring metallacycles. A rigid (ortho-phenylene-bis(N-methylimidazol-2-ylidene, Ph-bis-mim) and a flexible ethylene-bis(N-methylimidazol-2-ylidene ligand, C2H4-bis-mim) were utilized as chelating ligands. We performed a comparative study with the analogue complex bearing a bis-NHC with an imidazolium bridging group (1,3-dimethyl-imidazolium-4,5-bis(N-methylimidazol-2-ylidene), Im-bis-mim), and found that this positively charged ligand enables high selectivity towards the generation of formate, and a high activity a low catalyst loading. Our study reveals general design principles for iridium bis-N-heterocyclic carbene catalysts which can guide further designs for fast and selective carbonate transfer hydrogenation with glycerol a low catalyst concentration.