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

Abstract

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.