Influence of the Pd(II) coordination model on the catalytic performance of Pd-PPh₂-SBA-15 in C-C bond forming reactions

A series of Pd-PPh₂-SBA-15 catalysts were synthesized by coordinating Pd(II) ions with PPh₂ ligands originally bonded to SBA-15 support. The ³¹P CP MAS NMR spectra demonstrated that there were two kinds of coordination models coexisting in the Pd-PPh ₂-SBA-15 catalysts. At high P-content, the Pd nucleus preferentially coordinated with two PPh₂ ligands (referred as Pd-2P model) owing to the strong chelating bond. However, at low P-content, the Pd nucleus mainly coordinated with one PPh₂ ligand (denoted as Pd-1P model) due to long distance between the neighboring PPh₂ ligands. The Pd-PPh ₂-SBA-15-con prepared by co-condensation between PdCl ₂[PPh₂(CH₂)₂Si(OCH ₂CH₃)₃]₂ and TEOS contained the pure Pd-2P model. The Pd-2P coordination model exhibited higher activity and better selectivity than the Pd-1P model in various C-C bond forming reactions including Suzuki, Heck, Barbier, and Sonogashira reactions, which was discussed by considering the electronic state and the coordination configuration of Pd(II) active sites, together with their stability against leaching.