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

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 H-1 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-d(6) in CDCl3. In the resulting complex, 1 & BULL;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.