Synergistic NHC-Phosphine Palladium Complexes Embedded in Hyper-Cross-Linked Polymers as High-Performance Heterogeneous Catalysts for Coupling Reactions

Abstract

The susceptibility of highly active phosphine ligands to oxidation limits their utility in developing robust, recyclable heterogeneous catalysts capable of multistep coupling reactions. Herein, we address this by immobilizing synergistic, well-defined N-heterocyclic carbene (NHC)-phosphine palladium complexes into a rigid hyper-cross-linked porous polymer (HCP) network via one-pot knitting polymerization. The resulting material, IPr–Pd–PPh3@HCP, exhibits a high surface area (1260 m2 g–1) and a micro/mesoporous structure, ensuring efficient accessibility of reactants. The central innovation stems from the deliberate pairing of a sterically encumbered NHC ligand, which stabilizes the palladium center and lowers the barrier for oxidative addition, with a comparatively π-accepting phosphine (PPh3) ligand that electronically modulates the metal center to accelerate reductive elimination and overall catalytic turnover. This dual-ligand architecture establishes a finely tuned electronic and steric environment around the Pd center, thereby enabling the efficient activation of challenging, less reactive aryl bromides and even aryl chlorides under mild conditions. The IPr–Pd–PPh3@HCP catalyst demonstrated excellent performance in Mizoroki–Heck, Sonogashira, and Carbonylative Sonogashira couplings, achieving yields up to 99% with a low Pd loading of 0.15 mol %. Furthermore, it exhibited remarkable recyclability, maintaining over 90% yield across ten consecutive cycles. The use of a safe CO surrogate (dicyclohexylcarbodiimide/formic acid) in carbonylative reactions enhances the practical utility of this system. The combination of a sterically demanding NHC ligand and a comparatively π-accepting phosphine ligand creates a synergistic environment that suppresses phosphine oxidation. The rigid HCP network additionally imparts mechanical robustness and spatial confinement, protecting the stabilized active sites from deactivation pathways such as aggregation or leaching, thereby ensuring long-term catalytic stability and making IPr–Pd–PPh3@HCP a versatile and sustainable catalyst for industrially relevant Pd-catalyzed cross-coupling reactions.