Polypyrrole–palladium nanoparticles composite as efficient catalyst for Suzuki–Miyaura coupling

Synthesis of a new hybrid material (Pd/PPy) composed of polypyrrole globules with uniformly incorporated Pd nanoparticles via direct redox reaction between pyrrole and Pd(NH3)4Cl2 in water has been recently reported (V.A. Zinovyeva, M.A. Vorotyntsev, I. Bezverkhyy, D. Chaumont, J.-C. Hierso, Adv. Funct. Mater. 21 (2011) 1064–1075). In the actual study, this procedure has been extended to synthesize a series of Pd/PPy powders with variable palladium content and morphological parameters. Depending on the monomer-to-oxidant ratio in reaction mixture, average diameters of Pd and PPy particles may change in the ranges of 1.25–1.45 and 27–62 nm, respectively, the Pd concentration being within 33.5–42.0 wt.%. In general terms, decrease of the monomer-to-oxidant ratio led to formation of the Pd/PPy hybrid material with smaller diameters of both components and a higher Pd loading. The Pd/PPy composites have been studied in Suzuki–Miyaura coupling and showed high catalytic efficiency. Aryl iodides, bromides and chlorides are active. The reaction can be performed using arylboronic acids or tetraarylborates, both in organic solvents and in water, thus making the process ecologically friendly. The recycling of the catalyst is possible if its particles are immobilized on the graphite support. The comparison of two samples revealed that Pd/PPy nanocomposite with the diameter of PPy spheres of about 30 nm is more efficient in catalysis, as compared to the sample with bigger PPy spheres (about 60 nm), due to transport limitations for reagents inside the polypyrrole sphere in the latter case. For palladium/polypyrrole nanocomposites with the small diameter of PPy sphere, which are easily penetrable for the reagents and in which all Pd nanoparticles are active, Pd content in polypyrrole spheres does not influence the yield of biaryls: the more is the Pd content in polypyrrole spheres, the less amount of catalyst is necessary to obtain the same yield of biaryl.

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► Direct redox reaction of pyrrole and Pd(NH3)4Cl2 gives Pd–polypyrrole nanocomposites.
► The nanocomposites are highly efficient in Suzuki coupling.
► Morphology of the Pd/PPy hybrid material is dependent on monomer-to-oxidant ratio.
► Catalytic efficiency of the nanocomposite is dependent on the size of PPy globules.
► Pd nanocatalyst is active in water and can be recycled.