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–PPh2–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–PPh2–SBA-15-con prepared by co-condensation between PdCl2[PPh2(CH2)2Si(OCH2CH3)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.