Hyper-Cross-Linked Polyphosphines as Nanoporous Macroligands–A Systematic Study on Cross-Linking and Their Catalytic Application in the CO₂ Hydrogenation

Hyper-cross-linked polymers (HCPs) enable the tailored synthesis of functionalized materials and provide a versatile design strategy for porous macroligands. Based on the prototypical triphenylphosphine (PPh₃) monomer, we investigate the role of the involved cross-linking reagents in the formation of polyphosphines and evaluate structure-activity relations for application in the catalytic CO₂ hydrogenation: namely by varying the Friedel-Crafts catalyst, the cross-linker unit and the degree of cross-linking. The study of monomeric reactivities shows that phosphines are insufficiently activated by iron chloride catalyzed cross-linking and that the stronger aluminum chloride is required to ensure PPh₃ incorporation. Applying aromatic cross-linker units introduces porosity and promotes the accessibility of ligating centers for the immobilized ruthenium species. The thus formed solid catalysts exhibit excellent performances in the hydrogenation of CO₂ to formic acid in the aqueous phase and are studied in successive recycling runs. The partial structural degradation of the frameworks during catalysis is addressed by adjusting higher degrees of cross-linking, leading to an improved stabilization of the catalyst. Overall, this study highlights cross-linking strategies for the tailoring of phosphine-based HCPs and the design of stable macroligands under catalytic conditions.