A Hexadecacationic Metal–Organic Cage for Anchoring Sulfonates: High-Order Binding of Di- and Trisulfonates and High-Performance Removal of Perfluorosulfonates from Water

Developing synthetic hosts capable of binding sulfonate anions or sequestering perfluorosulfonates is highly important. In this work, the self-assembly of a tripyridinium-tricarboxylate ligand with Cp₂ZrCl₂ (Cp = η⁵-C₅H₅) is reported, yielding a highly cationic tetrahedral cage (1) engineered with acidic C−H protons. The hexadecacationic cage binds a variety of sulfonate anions with unprecedented charge-discriminative phenomena: the cage binds monosulfonates in fast exchange on the NMR timescale, while di- and multisulfonates exhibit slow-exchange binding. In particular, high-capacity complexation of di- and trisulfonates by 1 is achieved, with the pronounced case of 1 accommodating three dianions. Molecular modeling suggests the existence of a “Y”-shaped binding pocket on each edge of tetrahedron 1 (six in total), with each pocket capable of anchoring a guest bearing at least two sulfonate headgroups through electrostatic and hydrogen bonding interactions. Further investigations demonstrate that 1 in solid state is an exceptional adsorbent for perfluorooctane sulfonate (PFOS), with a maximum adsorption capacity of up to 1603 mg g⁻¹ positioning it among the top PFOS adsorbents. Moreover, cage 1 exhibits rapid kinetics, high selectivity, and long-life cycles in PFOS removal, and also effectively adsorbs shorter perfluorosulfonates.