Construction of pincer-type symmetrical ruthenium( ii ) complexes bearing pyridyl-2,6-pyrazolyl arms: catalytic behavior in transfer hydrogenation of ketones

Convenient synthesis of four new distorted octahedral ruthenium(II) complexes (1, 2, 3, 4) having general molecular formula [RuCl2LPAr3] (L = pyridine-based tridentate ligands not containing N–H bonds) is described. Their composition and structure were determined by elemental analysis and NMR spectra, and complexes 2 and 4 were also identified by X-ray single-crystal diffraction. All ruthenium(II) complexes exhibited good to excellent catalytic activity in the transfer hydrogenation of ketones. Among them, complex 4 achieved the highest final TOF value of 51 600 h−1 for a high molar ratio of substrate to catalyst (2000 : 1).