Ruthenium(II) Tris-Pyrazolylmethane Complexes in Transfer Hydrogenation Reactions

While ruthenium(II) arene complexes have been widely investigated for their potential in catalytic transfer hydrogenation, studies on homologous compounds replacing the arene ligand with the six-electron donor tris(1-pyrazolyl)methane (tpm) are almost absent in the literature. The reactions of [RuCl(kappa(3)-tpm)(PPh3)(2)]Cl, 1, with a series of nitrogen ligands (L) proceeded with selective PPh3 mono-substitution, affording the novel complexes [RuCl(kappa(3)-tpm)(PPh3)(L)]Cl (L=NCMe, 2; NCPh, 3; imidazole, 4) in almost quantitative yields. Products 2-4 were fully characterized by IR and multinuclear NMR spectroscopy, moreover the molecular structure of 4 was ascertained by single crystal X-ray diffraction. Compounds 2-4 were evaluated as catalytic precursors in the transfer hydrogenation of a series of ketones with isopropanol as the hydrogen source, and 2 exhibited the highest activity. Extensive NMR experiments and DFT calculations allowed to elucidate the mechanism of the transfer hydrogenation process, suggesting the crucial role played by the tpm ligand, reversibly switching from tri- to bidentate coordination during the catalytic cycle.