The fabrication of palladium–pyridyl complex multilayers and their application as a catalyst for the Heck reaction

Abstract

Through layer-by-layer (LbL) technology, (PdCl₂/bpy)_n multilayer films were prepared by alternately immersing quartz slides pre-coated with a PEI layer in PdCl₂ and bpy aqueous solutions (PEI = poly(ethylenimine), bpy = 4,4′-bipyridyl). The (PdCl₂/bpy)_n multilayers were characterized by UV-vis spectroscopy, atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). UV-vis spectra show that the successful fabrication of (PdCl₂/bpy)_n multilayers depends on the coordination interaction between palladium ions and pyridine moieties. AFM images show that the nanostructure aggregates are of Pd form on the surface. With an increase in the number of layers, more particle aggregates are distributed on the surface. When released from the substrate, Pd nanostructure aggregates are responsible for the activity of the catalyst. The XPS analysis demonstrates that the Pd oxidation state is +2. The application of (PdCl₂/bpy)_n multilayers as a catalyst for the Heck reaction was also investigated. Palladium–pyridyl complex loaded slides can be used as a catalyst reservoir. During the Heck reaction process, the desorbed Pd–bpy complexes from the solid slides are soluble molecular Pd species and are the catalytically active species, which show high activity for the Heck reaction of aryl bromides. Catalyst loading is as low as 3.4 × 10⁻⁶ mol% and high turnover numbers (TON) of up to 2.0 × 10⁷ are achieved.