Synthesis and applications of polymeric N-heterocyclic carbene palladium complex-grafted silica as a novel recyclable nano-catalyst for Heck and Sonogashira coupling reactions

A new catalytic system based on palladium nanoparticles supported on polymeric N-heterocyclic carbene-grafted silica (Si-PNHC-Pd) is introduced. Aminopropylsilica was reacted with benzoylchloride to form acrylamidopropylsilica. Onto this functionalized silica, vinylimidazole monomer was copolymerized by free radical polymerization. Poly(vinylimidazole)-grafted silica was treated with methyl iodide to form a quaternary salt of poly(vinylimidazole). Si-PNHC-Pd was obtained by subsequent treatment of imidazolium salt with PdCl2. Determination of Pd content was performed using an inductively coupled plasma (ICP) analyzer and the crystalline character was determined by an XRD technique. The topography and particle size of the catalyst was studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and atomic force microscopy (AFM). TEM images showed that palladium was dispersed through the polymer surface in nano particle size. This catalytic system exhibited excellent activity in Heck and Sonogashira coupling reactions with various aryl halides. The catalyst was successfully re-used up to 12 runs without appreciable change in its activity. High efficiency of the catalyst along with high yields of products, easy purification, large scale synthesis and high TON and TOF are among the advantages of this heterogeneous catalyst.