Tris(pyridin‐2‐ylmethyl)amine‐Based Ion Pair Receptors for Selective Lithium Salt Recognition

Abstract

AbstractTripodal ion pair receptors 1 and 2 consisting of tris(pyridine‐2‐ylmethyl)amine as a cation binding site linked with nitrophenyl urea or amidonitroindole groups as anion binding motifs have been synthesized. 1H NMR spectroscopic analyses carried out in 10 % DMSO in acetonitrile revealed that both ion pair receptors 1 and 2 are able to bind the LiCl ion pair with high selectivity over NaCl, KCl, RbCl, and CsCl. 1H NMR spectroscopic analyses in combination with molecular mechanics calculations proved that the lithium cation is bound to the tris(pyridin‐2‐ylmethyl)amine subunit while the chloride anion interacts with the NHs of urea or amido indole groups via hydrogen bonds. The addition of fluoride to the lithium cation complexes of receptors 1 and 2 leads to the release of the lithium cation from the receptors whereas chloride and bromide are co‐bound with lithium forming LiCl and LiBr ion pair complexes, respectively. The affinity of receptor 1 for the lithium cation was found to be improved by 5.0‐fold when the counter anion of Cl− is co‐bound within the receptor.