NNN pincer palladium(II) complexes with N-(2-(1H-pyrazol-1-yl)phenyl)-picolinamide ligands: synthesis, characterization, and application to heck coupling reaction

N-(2-(1H-pyrazol-1-yl)phenyl)-picolinamide ligands 3a–c and 5a–f have been easily prepared from commercially available 2-pyridinecarboxylic acid. Reaction of 3a–c and 5a–f with PdCl₂ in toluene in the presence of triethylamine gave the nine NNN pincer Pd(II) complexes 6a–i in 15–91% yields. All the new compounds were characterized by ¹H, ¹³C NMR spectra and HRMS. In addition, the molecular structures of complexes 6a and 6i have also been determined by single-crystal X-ray diffraction. The obtained palladium(II) catalysts showed efficient catalytic activity toward the Heck coupling of aryl bromides with styrenes. Under the condition of 5% catalyst loading, the expected various styrenes were obtained in moderate to high yields (up to 99% yield).