Palladium complexes of PNNP type diiminodiphosphine ligands for the Suzuki C[sbnd]C coupling reactions

Abstract

Air and moisture-stable PNNP type tetradentate diiminodiphosphine ligands (L1, L2, and L3) and their cationic palladium(II) complexes (C1, C2, C3) were synthesized and fully characterized. In addition, bimetallic neutral palladium(II) complexes (C4 and C5) were synthesized using L1 and different palladium precursors (Pd(COD)Cl2 and Pd(COD)(CH3)Cl). The molecular structures of ligand L2 and complex C5 were confirmed by X-ray diffraction analysis. The catalytic activities of all the complexes were evaluated on Suzuki CC coupling reactions and high conversions were obtained at low temperature (80 °C) and low catalyst loading (0.02 mol%). Optimization studies were carried out testing the effects of various reaction parameters such as base, solvent, temperature, catalyst ratio and the effects of different aryl bromide derivatives were investigated under these optimized conditions. Also, the results of the catalytic activities were compared on the basis of steric effects around the palladium, and the results showed that the complexes with more steric hindrance gave better results in Suzuki CC coupling reactions.