Phosphonium Ionic Imine Cage with an Uncommon Tri ₂ ⁴ Di ⁶ Topology for Selective Capture of Propyne in Propyne/Propylene Separation

The incorporation of phosphine units into porous organic cages (POCs) imparts the rich chemistry of phosphorus atoms to the POC system and enables a range of relevant applications. However, phosphine-containing POCs remain underexplored. In this work, quaternary phosphonium ionic cages were successfully constructed via the integration of phosphine groups, where premethylation and anion exchange of the triphenylphosphine (PPh₃)-aldehyde precursor were employed before the imine condensation reaction. The counterions of the PPh₃-aldehyde precursor were found to play a crucial role in dictating the topology of the resulting cage product. While the precursor bearing an iodide (I^-) anion yielded a soluble [2 + 3] cage <b>P-cage-I</b>, the bis(trifluoromethane)sulfonimide (NTf₂^-) and hexafluorophosphate (PF₆^-) analogues (<b>P-cage-NTf</b>_<b>2</b> and <b>P-cage-PF</b>_<b>6</b>) formed [4 + 6] assemblies with the rare Tri₂⁴Di⁶ (nomenclature for porous organic cage topology classification). Benefiting from the synergistic effects of a favorable topology, flexible windows, and guest-affinitive anions in the Tri₂⁴Di⁶ cage structure, <b>P-cage-PF</b>_<b>6</b> demonstrated excellent propyne/propylene separation performance, as confirmed by breakthrough experiments.