Pyridine–Quinoline and Biquinoline–based Ruthenium <em>p</em>-Cymene Complexes as Efficient Catalysts for Transfer Hydrogenation Studies: Synthesis and Structural Characterization

Searching for new and efficient transfer hydrogenation catalysts, a series of new organometallic ruthenium(II)-arene complexes of the formulae [Ru(η6-p-cymene)(L)Cl][PF6] (1–8) and [Ru(η6-p-cymene)(L)Cl][Ru(η6-p-cymene)Cl3] (9–11) were synthesized and fully characterized. These were prepared from the reaction of pyridine-quinoline and biquinoline-based ligands (L) with [Ru(η6-p-cymene)(μ-Cl)Cl]2, in 1:2 and 1:1, metal (M) to ligand (L) molar ratios. Characterization includes a combination of spectroscopic methods (FT-IR, UV-Vis, multi nuclear NMR), elemental analysis and single-crystal X-ray crystallography. The pyridine-quinoline organic entities encountered, were prepared in high yield either via the thermal decarboxylation of the carboxylic acid congeners, namely 2,2′-pyridyl-quinoline-4-carboxylic acid (pqca), 8-methyl-2,2′-pyridyl-quinoline-4-carboxylic acid (8-Mepqca), 6′-methyl-2,2′-pyridyl-quinoline-4-carboxylic acid (6′-Mepqca) and 8,6′-dimethyl-2,2′-pyridyl- quinoline-4-carboxylic acid (8,6′-Me2pqca), affording the desired ligands pq, 8-Mepq, 6′-Mepq and 8,6′-Me2pq, or by the classical Friedländer condensation, to yield 4,6′-dimethyl-2,2′-pyridyl-quinoline (4,6΄-Me2pq) and 4-methyl-2,2′-pyridyl-quinoline (4-Mepq) respectively. The solid-state structures of complexes 1–4, 6, 8 and 9 were determined showing a distorted octahedral coordination geometry. The unit cell of 3 contains two independent molecules (Ru-3), (Ru′-3) in a 1:1 ratio, due to a slight rotation of the arene ring. All complexes catalyze the transfer hydrogenation of acetophenone, using 2-propanol as a hydrogen donor in the presence of KΟiPr. Among them, complexes 1 and 5 bearing methyl groups at the 8 and 4 position of the quinoline moiety, convert acetophenone to 1-phenylethanol quantitatively, within approximately 10 minutes with final TOFs of 1600 h–1. The catalytic performance of complexes 1–11, towards the transfer hydrogenation of p-substituted acetophenone derivatives and benzophenone, ranges from moderate to excellent. An inner-sphere mechanism has been suggested based on the detection of ruthenium(II) hydride species.