Constraining Homo- and Heteroanion Dimers in Ultraclose Proximity within a Self-Assembled Hexacationic Cage

A hexacationic cage 3⁶⁺ was synthesized via hydrazone condensation in aqueous acid. Cage 3⁶⁺ bears three biscationic arms, each of which contains four relatively acidic protons, including one NH and three CH protons. These hydrogen bond donors, as well as its intrinsic cationic nature, enable cage 3⁶⁺ to encapsulate two anions concurrently within its cavity. The axial asymmetrical nature of the biscationic arms allow the cage to recognize two different anions in a selective manner, to encompass bound heteroanion dimers, such as Cl^–·NO3^– and Cl^–·Br^–. Single crystal X-ray diffraction analyses reveal that in the solid state the two anions are constrained in ultraclose proximity within the cage; e.g., the Cl^–···Cl^– and Cl^–···Br^– distances are 3.2 and 2.9 Å, respectively, which are shorter than the sum of their van der Waals radii. Evidence consistent with the sequential binding of two identical or disparate anions in CD₃CN is also presented.