Pd-bound functionalized mesoporous silica as active catalyst for Suzuki coupling reaction: Effect of OAc^‒, PPh₃ and Cl^‒ ligands on catalytic activity

Highlights: • Schiff-base condensation between 2-hydroxynaphthaldehyde and –NH₂ functionalized silica. • Pd with OAc^‒, PPh₃ and Cl^‒ moieties covalently grafted to functionalized silica. • Heterogeneous Suzuki coupling carried out by these highly active Pd-catalysts. • Mechanism and effect of ligands linked with Pd discussed in details. - Abstract: Three new palladium catalysts, PdCat-I, PdCat-II and PdCat-III, immobilized over heterogeneous silica support have been synthesized using different ligands attached to the palladium precursor. The ligands that have been used in this study are acetate, triphenylphosphine and chloride in PdCat-I, PdCat-II and PdCat-III, respectively. The ligands have different effect on stability of the compounds and impart different oxidation states to the metal center. The materials have been characterized by powder X-ray diffraction, nitrogen adsorption-desorption studies, transmission electron microscopy, thermal analysis, and different spectroscopic techniques. The Pd-content of the samples have been determined by ICP-AES analysis. The materials have been used as catalysts for Suzuki coupling reaction of aryl halides with phenylboronic acid under mild conditions. A comparative study has been carried out to ascertain the effect of the nature of different ligands on the outcome of the catalytic reactions. Products have been identified and estimated by ¹H NMR and gas chromatography. The results show that the best yields are obtained with the catalyst containing triphenylphosphine as the ligand in methanol. Such type of work to study the effect of ligand on Suzuki coupling reaction over functionalized mesoporous silica heterogeneous catalysts have not been carried out so far.