Assembled Multinuclear Ruthenium(II)–NNNN Complexes: Synthesis, Catalytic Properties, and DFT Calculations

Using a coordinatively unsaturated 16-electron mononuclear ruthenium­(II)-pyrazolyl-imidazolyl-pyridine complex [Ru­(II)–NNN] as the building block and oligopyridines as the polydentate ligands, pincer-type tri- and hexanuclear ruthenium­(II) complexes [Ru­(II)–NNNN]n were efficiently assembled. These complexes were characterized by elemental analyses, NMR, IR, and MALDI-TOF mass spectroscopies. In refluxing 2-propanol, the multinuclear ruthenium­(II)–NNNN complexes exhibited exceptionally high catalytic activity for the transfer hydrogenation of ketones at very low concentrations and reached turnover frequencies (TOFs) up to 7.1 × 106 h–1, featuring a remarkable cooperative effect from the multiple Ru­(II)–NNNN functionalities. DFT calculations have revealed the origin of the high catalytic activities of these Ru­(II)–NNNN complexes.