Application of arene ruthenium(II) complexes with pyridine-2-carboxaldimine ligands in the transfer hydrogenation of ketones

Abstract

The reaction of various pyridine-2-carboxaldimine ligands with the [(η6-arene)Ru(μ-Cl)Cl]2 dimer followed by a metathesis reaction with ammonium hexaflourophosphate, yielded the ruthenium(II) arene complex salts [(η6-arene)RuCl(C5H4N-2CHN-Ar]PF6; where (arene=C6H6 (1), p-cymene (2), Ar=3, 5-dimethyl phenyl (a), 2,3-dimethyl phenyl (b), 2,5-dimethyl phenyl (c), 3,4-dimethyl phenyl (d)). The compounds were characterized by elemental analysis, FT- IR, UV–vis and 1H and 13C NMR. Single crystal X-ray structures for compounds 1a, 1d and 2e were also determined and showed that the ruthenium(II) centre has a pseudo-octahedral geometry and the molecule adopted a three legged piano stool geometry in which the arene ring occupies the apex and the nitrogen atoms of the N,N′-bidentate ligand and the chloride atom the base of the stool. The Ru(II) complex salts were active for the catalytic transfer hydrogenation of ketones into alcohols in the presence of NaOH using 2-propanol as the hydrogen source at 82°C. The complexes were suitable for a wide range of aliphatic, cyclic and aromatic ketones giving good turn over numbers.