Pronounced effect of pore dimension of silica support on Pd-catalyzed Suzuki coupling reaction under ambient conditions

The pore size of the mesoporous silica support is found to have an enormous effect on the reactivity of the Pd-catalysts in the Suzuki coupling reaction. Heterogeneous palladium catalysts, MCM-Pd and SBA-Pd, have been synthesized using two different types of silica supports that differ from each other in their pore dimension. The silica surfaces are grafted with aminopropyl silane groups and functionalized with tris(4-formylphenyl) amine engaging one of the formyl groups while the other two formyl groups are reacted with two 2-aminothiophenol moieties. This results in two Schiff base sites with adjacent -SH groups leading to active metal binding sites in the samples and increases the hydrophobicity of the framework. The materials have been characterized by powder X-ray diffraction, nitrogen sorption studies, transmission electron microscopy, thermal analysis, and different spectroscopic techniques. The samples are used as catalysts for Suzuki cross-coupling reactions of aryl halides with phenylboronic acid under ambient conditions. The products of the reactions are identified and estimated by H-1 NMR and gas chromatography. SBA-Pd performs much better compared to MCM-Pd and a yield of up to 98% is achieved in CH3OH within 24 h of the reactions. This is attributed to the higher pore dimension of SBA-Pd that allows better interaction of the substrate molecules with the active centers on the surface of the pores.