Rapid transfer hydrogenation of acetophenone using ruthenium catalysts bearing commercially available and readily accessible nitrogen and phosphorous donor ligands

Abstract

The screening, synthesis and testing of Ru complexes generated from commercially available ligands or ligands that can be synthesised in one step, is described. The catalysts were tested for activity in the transfer hydrogenation of acetophenone by isopropanol, a probe reaction for hydrogen transfer processes between oxygenated species, often found in applications such as biomass upgrading and fine and specialty chemical synthesis. Ligand screening was conducted by in situ catalyst generation and examined NPN and NNN pincer type ligands bearing NH or CN functional groups. The most active transfer hydrogenation catalysts were found to be those bearing NH functionality, either as amino groups or as benzimidazole groups. Well-defined catalyst precursors were subsequently synthesised, including the novel complex [Ru(1)PPh3(Cl)2] (where (1) = bis(3-aminopropyl)phenylphosphine), the first reported Ru complex for this NPN ligand. Established (PN)2 and PP/NN ketone hydrogenation catalysts were also screened for transfer hydrogen capability, of which [Ru(PhPN)2Cl2] (where PhPN = 2-(diphenylphosphino)ethylamine) was the most active. Subsequently, [Ru(1)PPh3(Cl)2], [Ru(PhPN)2Cl2] and [Ru(4)(PPh3)2Cl][Cl] (where (4) = 2,6-bis(2-benzimidazolyl)pyridine) were investigated more closely to compare rate constants (determined by reaction profile regression analysis) as a more accurate measure of catalyst activity over commonly reported turn over frequencies (TOF). The effect of the reaction products on the catalyst activity was evaluated using feed spiking experiments. Catalyst deactivation was shown to be prevalent and subsequently incorporated into a simple kinetic model which enabled more accurate reaction profile fitting and provided rate constants for both the transfer hydrogenation step and deactivation reaction.