Self-Assembled Cationic Cages for Anion Recognition in Aqueous Solution

Two types of supramolecular cages were synthesized in high yields via a self-assembled dynamic organic reaction: tricationic cages, PT³⁺/MT³⁺, and hexacationic cages, PH⁶⁺/MH⁶⁺. The formar imine cages are only stable in their in situ self-assembly solutions and showed limited anion recognition. In contrast, the latter hydrazone cages are kinetically inert and isolable as solid-state compounds via counterion exchange. The hexacationic cages exhibited significantly enhanced anion binding properties. For example, both PH⁶⁺ and MH⁶⁺ with chloride counterions can recognize Br^-, NO₃^- and SO₄^2- in aqueous media, where these anions are considered highly hydrated.