N-heterocyclic carbene-palladium(II) complex supported on magnetic mesoporous silica for Heck cross-coupling reaction

Magnetic mesoporous silica was prepared via embedding magnetite nanoparticles between channels of mesoporous silica (SBA-15). The prepared composite (Fe3O4@SiO2-SBA) was then reacted with 3-chloropropyltriethoxysilane, sodium imidazolide and 2-bromopyridine to give 3-(pyridin-2-yl)-1H-imidazol-3-iumpropyl-functionalized Fe3O4@SiO2-SBA as a supported pincer ligand for Pd(II). The functionalized magnetic mesoporous silica was further reacted with [PdCl2(SMe2)(2)] to produce a supported N-heterocyclic carbene-Pd(II) complex. The obtained catalyst was characterized using Fourier transform infrared spectroscopy, scanning electron microscopy, energy-dispersive X-ray analysis, vibrating sample magnetometry, Brunauer-Emmett-Teller surface area measurement and X-ray diffraction. The amount of the loaded complex was 80.3mgg(-1), as calculated through thermogravimetric analysis. The formation of the ordered mesoporous structure of SBA-15 was confirmed using low-angle X-ray diffraction and transmission electron microscopy. Also, X-ray photoelectron spectroscopy confirmed the presence of the Pd(II) complex on the magnetic support. The prepared magnetic catalyst was then effectively used in the coupling reaction of olefins with aryl halides, i.e. the Heck reaction, in the presence of a base. The reaction parameters, such as solvent, base, temperature, amount of catalyst and reactant ratio, were optimized by choosing the coupling reaction of 1-bromonaphthalene and styrene as a model Heck reaction. N-Methylpyrrolidone as solvent, 0.25mol% catalyst, K2CO3 as base, reaction temperature of 120 degrees C and ultrasonication of the catalyst for 10min before use provided the best conditions for the Heck cross-coupling reaction. The best results were observed for aryl bromides and iodides while aryl chlorides were found to be less reactive. The catalyst exhibited noticeable stability and reusability.