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.
Through the high dilution technique, we obtained macrocyclic ion pair receptors R1 and R2, anion receptor R3, as well as fluorescent sensor R4 using a combination of particular members of...
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