Impact of linking groups in chelating bis-carbene iridium catalysts for transfer hydrogenation of inorganic carbonates with glycerol

Abstract

Reduction of carbonate salts by transfer hydrogenation, utilizing glycerol as a sacrificial hydrogen donor, to generate formate and lactate is an attractive reaction to produce value-added products from chemical waste. 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 (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 high activity at low catalyst loadings. Our study reveals general design principles for iridium bis-N-heterocyclic carbene catalysts that can guide further designs for fast and selective carbonate transfer hydrogenation with glycerol at low catalyst concentrations.