Pyridine-Quinoline and Biquinoline-Based Ruthenium -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(η--cymene)(L)Cl][PF] (-) and [Ru(η--cymene)(L)Cl][Ru(η--cymene)Cl] (-) were synthesized and fully characterized. These were prepared from the reaction of pyridine-quinoline and biquinoline-based ligands (L) with [Ru(η--cymene)(μ-Cl)Cl], 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 (), 8-methyl-2,2'-pyridyl-quinoline-4-carboxylic acid (), 6'-methyl-2,2'-pyridyl-quinoline-4-carboxylic acid () and 8,6'-dimethyl-2,2'-pyridyl-quinoline-4-carboxylic acid (), affording the desired ligands , and , or by the classical Friedländer condensation, to yield 4,6'-dimethyl-2,2'-pyridyl-quinoline () and 4-methyl-2,2'-pyridyl-quinoline (), respectively. The solid-state structures of complexes -, and were determined showing a distorted octahedral coordination geometry. The unit cell of contains two independent molecules (Ru-), (Ru'-) 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 KOPr. Among them, complexes and bearing methyl groups at the 8 and 4 position of the quinoline moiety, convert acetophenone to 1-phenylethanol quantitatively, within approximately 10 min with final TOFs of 1600 h. The catalytic performance of complexes -, towards the transfer hydrogenation of -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.