Hydrogen generation from alcohols (α-hydroxy carboxylic acids) and alcohol-ammonia coupling in aqueous media catalysed by water-soluble bipyridine-Cp*Ir (Rh or Os) catalyst: a computational mechanism insight

Density functional theory (DFT) calculations were performed to elucidate the mechanism of the dehydrogenative oxidation of various primary alcohols (or alpha-hydroxy carboxylic acids) and the dehydrogenative coupling of alcohols with ammonia catalysed by the same water-soluble (CpIr)-Ir-* complex bearing a 2-pyridonate-based ligand (A-Ir). Another two new catalysts A-Rh and A-Os are computationally designed for the dehydrogenative oxidation of alcohols. The plausible pathway for alcohol dehydrogenation includes three steps: alcohol oxidation to aldehyde (step I); the generation of dihydrogen in the metal coordination sphere (step II); and the liberation of dihydrogen accompanied with the regeneration of active catalyst A (step III). Among them, the step I follows bifunctional concerted double hydrogen transfer mechanism rather than the beta-H elimination. For step II, the energy barriers involving the addition of one or two water molecules are higher than in absence of water. Our results also confirm that A-Ir can be applied in the dehydrogenation of various alpha-hydroxy carboxylic acids by the similar mechanism. Remarkably, A-Ir is also found to be efficient for the coupling reactions of various primary benzyl alcohols with ammonia to afford amides.