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

Abstract

Tripodal 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. H-1 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. H-1 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.