Application of enaminone ruthenium(II) complexes as catalysts in the transfer hydrogenation of ketones

This study reported the synthesis of two new ligands (1,2) and their Ru(II)-enaminone complexes (3,4). The Ru(II) complexes were obtained from the reaction of the [RuCl₂(p-cymene)]₂ dimers with the new enaminone derivative ligands (1,2). The use of standard spectroscopy techniques entirely characterized the compounds. Single crystal studies investigated the crystal structure of ligand (1) and complexes (3,4). X-ray diffraction data analysis was used to confirm the enaminone form of 1. Also, 3 and 4 exhibited the classical three-legged piano stool structure with Ru(II) coordinated by the nitrogen and oxygen atoms of 1 and a chloride ligand as the legs and the η⁶-π-bound p-cymene ligand occupies the seat of the piano stool. The Ru(II) complexes have been studied as catalysts in the transfer hydrogenation of acetophenone and other various ketones in the presence of KO^tBu. The catalytic conditions were optimized using different substrate/catalyst and base/catalyst ratios. We found that the complexes show good activities and up to 100 % selectivity. The best turnover frequency (1667 h⁻¹) was found for 3 when using acetophenone as the substrate.