Synthesis and Applications of Poly(N-vinylimidazole) Grafted Silica-containing Palladium Nanoparticles as a New Re-cyclable Catalyst for Heck, Sonogashira and Suzuki Coupling Reactions

A novel and environmentally friendly catalytic system based on palladium nano-particles supported on poly (N-vinylimidazole) (PVI) grafted silica was prepared. The advantages of this system lie not only in its easy separation from the prod- ucts, recycling and preventing the loss of metal, it has also excellent chemical and thermal stability, good accessibility and the dispersion of catalytic sites. The com- plexation of (PVI)-grafted silica with PdCl2 was carried out to obtain the heterogeneous catalytic system. X-ray diffraction (XRD) technique and transmission electron microscopy (TEM) image showed that palladium dispersed through the support in nanometer size and there was no change in the size and morphology of the palladium nanoparticles after coupling reactions. This catalytic system exhibited excellent activity in cross-coupling reactions of aryl iodides, bromides and also chlorides in Heck, Sonogashira and Suzuki reactions in short reaction times and high yields. The use of aryl chlorides in cross-coupling reactions is usually hardly successful, but excellent results were gained in the presence of tertabutylammonium bromide (TBAB) as an additive. Elemental analysis of Pd by inductively coupled plasma (ICP) technique and hot filtration test showed low leaching of the metal into solution from the supported catalyst and that the catalyst behaves as a heterogeneous system. The catalyst can be reused several times in repeating coupling reactions cycles without considerable loss in its activity.