Diruthenium(II)-NNN pincer complex catalysts for transfer hydrogenation of ketones

Dinuclear ruthenium(II)-NNN pincer complexes bearing a pi linker-supported bis(pyrazolyl-imidazolyl-pyridine) ligand were synthesized and structurally characterized, and they exhibited excellent catalytic activity for the transfer hydrogenation of ketones in refluxing isopropanol, reaching TOF values up to 1.3 x 10(6) h(-1). Compared with the corresponding mononuclear Ru(II)-NNN pincer complexes, the bimetallic complexes could be applied at concentrations as low as 0.03 mol% Ru and demonstrated remarkably enhanced catalytic activity in the transfer hydrogenation reactions of ketones. The high catalytic activity of the diruthenium(II) complexes is attributed to the excellent stability and possible cooperativity of the two coordinated Ru(II) metal centers through the p linker. The present synthetic methodology has established an applicable strategy to construct highly active bimetallic NNN pincer complex catalysts