Substituent effect of diimino-palladium (II) pincer complexes on the catalysis of Sonogashira coupling reaction

Three NCN diimine ligands 4-6 were synthesized after condensation of isophthalaldehydes 1-3 and ani-lines. Treatment of 4-6 with Pd-2(dba)(3) in toluene resulted in the corresponding Pd-II-NCN-Bu-t (7), Pd-II-NCN-H (8) and Pd-II-NCN-NO2 (9) pincer complexes, respectively with high yields. Palladium pincers 7-9 and their precursors 4-6 were fully characterized by elemental analysis, IR, H-1 NMR and C-13 NMR spectroscopy. The molecular structures of 7b and 9b were also determined by X-ray single crystal diffraction. Sonogashira coupling of phenyl acetylene and 3-nitrobenzene catalyzed by 7-9 show that 9 exhibits the highest catalytic activity, suggesting that the electron withdrawing groups at the position-4 of palladium atom in palladium pincers will enhance their catalytic activity. (C) 2015 Elsevier Ltd. All rights reserved.