Hyper-crosslinked polyphosphines as microporous macroligands for catalytic CO2 hydrogenation – a systematic study on structure-activity relations

Hyper-crosslinked polymers (HCPs) enable the tailored synthesis of functionalized materials and open up a versatile design strategy for porous macroligands. Based on the prototypical triphenylphosphine (PPh3) monomer, we investigate the roles of the involved crosslinking reagents on the formation of polyphosphines and evaluate structure-activity-relations for application in the catalytic CO2 hydrogenation: namely by varying the Friedel-Crafts catalyst, the crosslinker unit and the degree of crosslinking. The examination of the monomeric reactivities states the insufficient activation of the phosphine through iron chloride catalyzed crosslinking and the requirement for the stronger aluminum chloride to ensure PPh3 incorporation. Applying aromatic crosslinker 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 over consecutive recycling runs. The partial structural degradation of the frameworks during catalysis is addressed by adjusting higher degrees of crosslinking, leading to an improved stabilization of the catalyst. Overall, this study highlights crosslinking strategies for the tailored crafting of phosphine-based HCPs and the design of stable macroligands under catalytic conditions.