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

Abstract

Density functional theory (DFT) calculations were performed to elucidate the mechanism of the dehydrogenative oxidation of various primary alcohols (or α-hydroxy carboxylic acids) and the dehydrogenative coupling of alcohols with ammonia catalysed by the same water-soluble Cp*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 β-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 α-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.