Syntheses and Applications of Symmetrical Dinuclear Half-Sandwich Ruthenium(II)–Dipicolinamide Complexes as Catalysts in the Transfer Hydrogenation of Ketones

Abstract

The treatment of [Ru(η6-p-cymene)Cl2]2 with N,N’-(1,2-phenylene)dipicolinamide (H2L1) afforded the double salt complex [{Ru(η6-p-cymene)2-µ-Cl}L1][Ru(η6-p-cymene)Cl3], (Ru1) in moderate yields. Separately, the reactions of ligands (H2L1), N,N’-(4,5 dimethyl-1,2-phenylene)dipicolinamide (H2L2), and N,N’-(4-methoxy-1,2-phenylene)dipicolinamide (H2L3) with the [Ru(η6-p-cymene)Cl2]2 in the presence of KPF6 afforded the respective dinuclear half-sandwich Ru(II) complexes [{(Ru(η6-p-cymene)2--µ-Cl}L1][PF6] (Ru2), [{(Ru(η6-p-cymene)2-µ-Cl}L2][PF6] (Ru3), and [{(Ru(η6-p-cymene)2-µ-Cl}L3][PF6] (Ru4). NMR and FT-IR spectroscopies, ESI-MS spectrometry, and elemental analyses were used to establish the molecular structures of the new dinuclear ruthenium(II) complexes. Single crystal X-ray crystallography was used to confirm the piano-stool geometry of the dinuclear complexes Ru1 and Ru4, as containing N^N chelated ligand and bridging chlorido ligands in each Ru(II) atom. The complexes (Ru1-Ru4) showed good catalytic activities at low catalyst concentrations of 0.005 mol% in the transfer hydrogenation of a wide range of ketone substrates.