Neutral anion-detecting organic cages based on anion-π interactions

A series of neutral tetrahedral molecular cages were self-assembled in relatively high yields by condensing a triamino linker with triangular tris-aldehyde precursors. Each tris-aldehyde features a central triazine core, which imparts an electron-deficient cavity that facilitates anion encapsulation through fourfold anion-π interactions. The anion binding affinity is significantly influenced by substituents on the tris-aldehyde precursors: electron-donating groups (e.g., Ph) diminish binding by compromising the electron-deficient nature of the cage, whereas more electron-withdrawing substituents (e.g., Cl, Br, and CF₃-Ph) enhance it. Interestingly, the strongly electron-withdrawing fluorine (F) substituents, in close proximity to the binding pocket, unexpectedly diminish binding affinity due to a repulsive field effect. Within each corner of the tetrahedral framework, intramolecular CH-π interactions occur between a phenyl proton ortho to the imine bond and an adjacent phenyl plane. The encapsulation of anionic guests within the cavity perturbed or reinforced these CH-π interactions to varying degrees, producing distinct NMR responses that serve as signatures for different anions.