Pyridylphenylene dendrons immobilized on the surface of chemically modified magnetic silica as efficient stabilizing molecules of Pd species

摘要

Rigid pyridylphenylene dendrons were shown to successfully function as capping molecules for stabilization of both magnetite and Pd nanoparticles (NPs) to form hydrophobic, magnetically recoverable catalysts. However, syntheses in colloidal solutions require large amounts of dendrons and are difficult to scale up. Here, we developed a strategy for the nanocomposite formation by immobilization of the pyridylphenylene dendrons (D) on magnetic silica (Fe3O4-SiO2, MS) surface via the formation of ether or amide bonds, depending on the structure of flexible linkers on the MS surface and dendron focal groups. Both approaches allow attachment of small amounts of the dendrons with high surface coverage and impart amphiphilicity to the final composite. After the binding to the MS surface, the dendron pyridine moieties readily complex with Pd acetate, leading to a "cocktail" of Pd2+ and Pd-0 species (the latter forming Pd NPs) due to partial reduction by composite functional groups. The MS-D-Pd nanocomposites were tested in the model Suzuki-Miyaura cross-coupling reaction of 4-Br-anisole and phenylboronic acid to evaluate their performance in hydrophilic conditions. MS-D-Pd demonstrated excellent performance, even at a very small amount of the catalyst, which is assigned to exceptional stabilization by dendritic ligands, allowing prevention of the metal leaching and preservation of catalytic properties upon magnetic separation. The immobilization of rigid hydrophobic dendrons on the hydrophilic magnetic support may allow one to extend the scope of catalytic reactions due to catalyst amphiphilicity.