Heterogeneous catalysts have shown advantages such as high stability, good recyclability and easy separation from reactants over homogeneous catalysts in recent years. In this paper, three pyridine-functionalized N-heterocyclic carbene–palladium complexes (HCP-Pd) were successfully synthesized via a simple external cross-linking reaction. In each catalyst (i.e., complex), palladium (Pd) was immobilized on the hypercrosslinked polymer (HCP) via formation of a six-membered ring by Pd²⁺ and the bidentate ligands of NHC and pyridine. The structure and composition of HCP-Pd were characterized by SEM, TEM, N₂ sorption, FT-IR, TGA and XPS. The catalytic performances of these catalysts in a Suzuki–Miyaura coupling reaction were also studied. The results prove that HCP-Pd is a very effective heterogeneous catalyst for the Suzuki–Miyaura coupling reaction of various aryl halides with aryl boronic acid in an aqueous medium under mild conditions.

Two microporous organic polymer immobilized palladium (MOP-Pd) catalysts were prepared from benzene and 1,10-phenanthroline by Scholl coupling reaction and Friedel–Crafts reaction, respectively. The structure and composition of the catalyst were characterized by FT-IR, TGA, N₂ sorption, SEM, TEM, ICP-AES and XPS. MOP-Pd catalysts were found to possess high specific surface areas, large pore volume and low skeletal bone density. Moreover, the immobilized catalyst also had advantages, such as readily available raw materials, chemical and thermal stability, and low synthetic cost. The Pd catalyst is an effective heterogeneous catalyst for carbon–carbon (C–C) coupling reactions, such as the Heck reaction and Suzuki–Miyaura reaction, affording good to high yields. In these reactions, the catalyst was easily recovered and reused five times without significant activity loss.

A novel hypercrosslinked polymer-palladium catalyst was prepared via external cross-linking reactions and applied in Suzuki–Miyaura reactions as a recyclable catalyst, resulting in TON numbers up to 1.66 × 10⁴ and yields reaching 99%.

Palladium coordinated with N-heterocyclic carbene-functionalized porous organic polymers (Pd@POPs) was successfully prepared via Scholl coupling reaction and a successive immobilization method. The protocol features simple reaction conditions, easy separation, high yield and low cost. The structure and composition of Pd@POPs were characterized by N₂ sorption, TGA, FT-IR, SEM, EDX, TEM, XPS and ICP. Then the obtained heterogeneous catalysts were applied in Suzuki–Miyaura coupling reaction to evaluate their catalytic performance. The Pd@POPs displayed high catalytic activity for the Suzuki–Miyaura coupling reaction in an EtOH/H₂O solvent. A 0.03 mol% Pd loading was sufficient for the reaction with a high turnover number (TON) of 3220. Moreover, the catalyst was easily recovered and reused for at least six consecutive cycles without obvious loss of its initial activity.