Synthesis and crystal structures of a series of (μ-thiophenolato)(μ-pyrazolato-N,N′) double bridged dipalladium(II) complexes and their application in Mizoroki–Heck reaction as highly efficient catalysts

Abstract

Three new binucleating S-protected ligand precursors, 2-(N,N-dimethylthiocarbamato)-5-methylisophthalaldehyde di-2′-hydroxy 5′-methylanil (1b), 2-(N,N-dimethylthiocarbamato)-5-tert-butylisophthalaldehyde di-2′-hydroxyanil (2a) and 2-(N,N-dimethylthiocarbamato)-5-tert-butylisophthalaldehyde di-2′-hydroxy 5′-methylanil (2b), have been synthesized. The reaction of these ligand precursors with PdCl2 in the presence of pyrazole under Pd-mediated S–C cleavage yielded a series of binuclear palladium(II) complexes of general formula [LPd2(pz)], where pz is the exogenous bridging pyrazolyl ligand and L3− represents a series of pentadentate thiophenol-based bridging ligands originated from their corresponding ligand precursors. All the compounds were characterized by elemental analysis, IR, 1H NMR and UV–Vis spectroscopies. The binuclear μ-thiophenolato-μ-pyrazolato palladium(II) complexes have also been characterized by single crystal X-ray diffraction analysis. Crystal structure analyses of the complexes show that two PdII centers are located in distorted square-planar environments, arranged in binuclear units with Pd⋯Pd distance of 3.57Å. The catalytic activity of these new binuclear palladium complexes was studied in Mizoroki–Heck C–C coupling reaction of methyl- and n-butyl acrylate with various types of aryl iodides and bromides. All reactions were completed for very short times with very excellent yield. Reactions were stereoselective and only trans isomers were obtained in each case.