Substituent Effect on the Catalytic Activity of Ruthenium(II) Complexes Bearing a Pyridyl-Supported Pyrazolyl-Imidazolyl Ligand for Transfer Hydrogenation of Ketones

Air- and moisture-stable ruthenium­(II) complexes bearing a multisubstituted pyrazolyl-imidazolyl-pyridine ligand were synthesized and structurally characterized by NMR and X-ray single-crystal crystallographic analyses. The substituents on the imidazolyl moiety of the NNN ligand exhibited a remarkable impact on the catalytic activity of the corresponding Ru­(II) complexes for transfer hydrogenation of ketones in refluxing 2-propanol, following the order NHTs > Me > H > NO₂, to tune the catalytic activity. The highest final TOF value of 345 600 h^–1 was reached by means of 0.05 mol % of the Ru­(II)-NHTs-substituted NNN complex as the catalyst. The corresponding structurally confirmed RuH complexes are proposed as the catalytically active species.