Tetraamidoindolyl calix[4]arene as a selective ion pair receptor for LiCl

Abstract

A calix[4]arene (1) functionalized with amidoindole groups on the upper rim and with propyl groups on the lower rim has been synthesized. It was revealed by ¹H NMR spectroscopic analyses and single crystal X-ray diffraction analysis that compound 1 functions as an ion pair receptor capable of binding the lithium cation and the chloride anion concurrently in 10% DMSO-d₆ in CDCl₃. In the resulting complex, 1·LiCl, the lithium cation is coordinated to the amide carbonyl oxygen atoms of receptor 1 while the chloride anion is hydrogen-bonded to the indole NHs forming a contact ion pair with the co-bound lithium cation. With the lithium cation pre-complexed, the affinity of receptor 1 for Cl⁻ (K_a = 139 M⁻¹) and Br⁻ (K_a = 31 M⁻¹) was enhanced by 1.7-fold (K_a = 242 M⁻¹) and 2.1-fold (K_a = 65 M⁻¹), respectively, relative to its ion-free form. Likewise, the chloride complex of receptor 1 binds the lithium cation with 1.5-fold improved affinity (K_a = 353 M⁻¹) relative to 1 alone. Receptor 1 was also found to extract and solubilize the otherwise insoluble salts LiCl and LiBr selectively into chloroform.