Recognition and Extraction of Sodium Chloride by a Squaramide-Based Ion Pair Receptor

We synthesized an ion pair receptor 1 consisting of a crown ether cation binding site and a squaramide anion binding domain and compared its binding properties to those of its analogous urea counterpart 2. We studied their salt binding properties using spectrophotometric and spectroscopic measurements in an acetonitrile solution and in acetonitrile/water mixtures. Apart from carboxylate anions, all of the anions tested were found to associate with receptor 1 and 2 more strongly in the presence of sodium cations. A homotopic anion receptor 3, lacking a crown ether unit, was unable to bind sodium salt more strongly than tetrabutylammonium salts. Solution and solid-state X-ray measurements revealed strong sodium chloride coordination to receptor 1, which is able to bind this salt even in the presence of 10% water. In contrast to the urea-based ion pair receptor 2 or anion receptor 3, ditopic receptor 1 is capable of extracting sodium chloride from aqueous media to the organic phase, as was evidenced unambiguously by ¹H nuclear magnetic resonance, mass spectrometry, and atomic absorption spectroscopy analyses.