Salen–porphyrin-based conjugated microporous polymer supported Pd nanoparticles: highly efficient heterogeneous catalysts for aqueous C–C coupling reactions

Abstract

The salen–porphyrin based conjugated microporous polymer (SP-CMP) was first constructed by polycondensation reaction of a salen-dialdehyde derivative and pyrrole. Due to the outstanding chemical and thermal stability, abundant micropores with a reasonable pore size, and ordered salen–porphyrin arrays in the A₄B₄-type polymer framework, the functional CMP was further applied as a Pd nanoparticle support by the coordinate interactions between the polydentate chelating sites with Pd(OAc)₂ and subsequent reduction with NaBH₄. The as-synthesized composite material (Pd@SP-CMP) was fully characterized by powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and solid-state ¹³C nuclear magnetic resonance (NMR). The porous property of Pd@SP-CMP was also characterized by N₂ adsorption/desorption isotherms and the obtained material exhibited a Brunauer–Emmett–Teller (BET) surface area of 266 m² g⁻¹, together with a pore volume of 0.192 cm³ g⁻¹. The microscopic morphology of Pd@SP-CMP was further evaluated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The Pd@SP-CMP material with highly dispersed Pd nanoparticles exhibited excellent catalytic activity towards Suzuki–Miyaura and Heck–Mizoroki coupling reactions in water or in the dioxane/water mixture. In addition, Pd@SP-CMP also displayed outstanding stability and recyclability, and it can be reused without loss of activity in ten successive reactions. More importantly, the salen–porphyrin based CMPs could be the promising candidates for developing high-performance heterogeneous catalysts.