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.