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

TL;DR

Scientists created molecular cages that detect neutral anions using anion–π interactions, with binding affinity influenced by substituents on the cage structure.

Contribution

The study introduces neutral tetrahedral cages with tunable anion-binding properties based on substituent effects and anion–π interactions.

Findings

Electron-withdrawing substituents enhance anion binding, while electron-donating ones reduce it.

Fluorine substituents near the binding pocket unexpectedly weaken anion binding due to repulsive field effects.

Encapsulation of anions alters intramolecular CH–π interactions, detectable via NMR signatures.

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.

A series of tetrahedral molecular cages were synthesized via imine condensation reactions. Each face features a central triazine core, which imparts an electron-deficient cavity that facilitates anion encapsulation through anion–π interactions.

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