Investigating Anion-π Interactions In Ion-Pair Receptors Based On 3,5-Dinitrobenzoic Acid.

The design, synthesis, and characterization of a series of supramolecular receptors based on electron-deficient aromatic systems capable of engaging in anion-π interactions are reported. Receptors 1 and 3 combine an electron-poor aromatic scaffold with a cation-binding crown ether unit. Binding studies monitored by ¹H NMR titrations in acetonitrile revealed that these receptors exhibit enhanced affinity for bromide anions in the presence of sodium cations, indicating cooperative ion-pair recognition. Receptor 1, incorporating both nitro-substituted aromatic rings and a macrocyclic cation-binding site, demonstrated the most significant anion-π binding enhancement. In contrast, control receptor 2, lacking electron-withdrawing groups, exhibited negligible anion affinity, supporting the role of π-acidity in anion binding. Quantum chemical calculations and electrostatic potential maps further confirmed the contribution of anion-π interactions in receptor function. The incorporation of amide functionalities in receptors 3 and 4 improved binding affinity, highlighting the synergistic effect of multiple binding domains. These findings highlight the potential for developing advanced ion-pair receptors that harness anion-π interactions alongside classical noncovalent binding motifs.