F. Pennetreau, A. L’hoost, I. Nircha, B. Pichon, O. Riant, S. Hermans
Universite catholique de Louvain,
Keywords: immobilization, carbon nanotubes, magnetic decantation, Suzuki cross-coupling, Pd complex
Summary:Carbon nanotubes (CNTs) are excellent candidates for the preparation of supported catalysts, due to their high surface area, high mechanical and thermal resistance. Most of the examples present in the literature concern the anchoring of metallic nanoparticles on their outer walls. Far less examples of supported molecular complexes have been described for applications pertaining to homogeneous catalysis field. In all cases, the drawback is the need for nanofiltration unit to recover the material at the end of the catalytic cycle. Here we propose to make CNTs magnetically recoverable to overcome the filtration issue. We prepared ‘magnetic CNTs’ (Mag-CNTs) either by decoration of the external surface with iron oxide nanoparticles (Fe-CNTs) or by filling the internal cavity of the nanotubes with iron oxide nanoparticles (Fe@CNTs). Pristine carbon nanotubes, Fe-CNTs and Fe@CNTs have been covalently functionalized to anchor Pd(II) complexes on their outer walls. These nanostructured materials are ideal candidates as catalysts for the cross-coupling Suzuki-Miyaura reaction. The functionalization method was devised to avoid harsh conditions that could release the iron oxide nanoparticles which fill or decorate the nanotubes, and was based on the covalent grafting of xanthate compounds in presence of peroxide as radical initiator. CNTs were filled with iron oxide nanoparticles by thermal decomposition of iron stearate in octadecene, while decorated CNTs (Fe-CNTs) were prepared by heating in an autoclave a suspension of CNTs and Fe(acac)3 in triethylene glycol. Characterization by XPS, XRD and TEM confirmed the formation of small particles of magnetite in or on the tubes. A xanthate bearing an activated ester function has been chosen to functionalize the Mag-CNT, to create surface anchoring points. Then, the activated ester moieties have been reacted with the primary amine of a bifunctional ligand. Finally, palladium was coordinated by ligand exchange between a soluble Pd(II) precursor complex and the dipyridylamine function of the ligand. The obtained solids have been characterized by XPS and Pd contents of 0.11 at. % and 0.19 at. % were found for Pd(II)/Fe-CNTs and Pd(II)/Fe@CNTs, respectively. Moreover, the Pd/N ratio and Pd(II) oxidation state were confirmed, indicating that the desired complex had been formed at the CNTs surface. The success was found to highly depend on the Pd precursor complex’s nature. The prepared catalysts were tested in the Suzuki-Miyaura cross-coupling between 4-iodotoluene and phenylboronic acid and were found to be effective with 60-75 % conversion in the tested conditions. Blank tests with Mag-CNT showed no activity. The immobilized Pd(II) homogeneous catalysts could be magnetically separated from the reaction medium at the end of the catalytic reaction.