Thioether-NHC-Ligated PdII Complex for Crafting a Filtration-Free Magnetically Retrievable Catalyst for Suzuki-Miyaura Coupling in Water

A new benzimidazolium salt, 1-[benzylaminocarbonylmethyl]-3-(2-phenylsulfanylethyl)-1H-benzimidazolium chloride (L), which is a precursor of a novel thioether-functionalized NHC, has been synthesized by subjecting 1H-benzimidazole to a sequence of reactions with 1,2-dichloroethane, sodium thiophenolate, and N-benzyl-2-chloroacetamide, successively. The moisture/air-insensitive complex [Pd(L-HCl)Cl-2] (1) was prepared by the reaction of L with PdCl2. The molecular structure of 1, established by X-ray crystallography, revealed a square-planar geometry around Pd. Complex 1 was screened for Suzuki-Miyaura coupling reactions of various aryl/heteroaryl bromides (yields of up to 94% in 2 h) in water at room temperature. Furthermore, complex 1 was immobilized onto the surface of aminopropyl-functionalized silica-coated magnetite nanoparticles [NPs, Fe3O4@SiO2-(CH2)(3)-NH2] by a stepwise modification strategy to develop the heterogeneous magnetically retrievable catalyst Fe3O4@SiO2-1, in which the amide functionality present in the side-arm of the NHC within the NHC-Pd-II complex serves as linker. This magnetic nanosupport, bearing palladium complexes incorporating a novel thioether-based NHC ligand that functions in aqueous aerobic medium and can be easily separated, renders Fe3O4@SiO2-1 a most desirable catalyst for the Suzuki-Miyaura coupling reaction. It was also observed that the catalyst was effective for up to seven cycles and was easily separated from the reaction medium by the use of an external magnet, further increasing its appeal.