NHC-Palladium(II) Mononuclear and Binuclear Complexes Containing Phenylene-Bridged Bis(thione) Ligands: Synthesis, Characterization, and Catalytic Activities

A series of mono- and binuclear Pd(II) complexes with N-heterocyclic carbene (NHC) and phenylene-bridged bis(thione) (SCS) ligands were prepared and characterized by H-1 and C-13 NMR spectroscopy, IR, and mass spectrometry. The molecular structures of 1b, 2a, and 3b have been determined by the single-crystal X-ray diffraction method. The catalytic activities of the synthesized palladium complexes in the regioselective reduction of quinolines to the corresponding 1,2,3,4-tetrahydroquinolines were thoroughly investigated with ammonia-borane under mild reaction conditions. It is observed that the activities of the binuclear Pd(NHC) complexes were higher than those of the corresponding mononuclear complexes under the same conditions.