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. 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.