Synthesis and binding properties of cation-templated anion receptors

Abstract

Acyclic crown ether derivatives containing urea/thiourea moieties, 5a, 5b and 5c, were synthesized. Complexation studies of ligand 5a with various ions were carried out by ¹H-NMR titrations. Ligand 5a were able to form 1:1 complexes with various ions in the order of H₂PO₄⁻ > AcO- > NO₃⁻ ~ BzO⁻ > C1⁻ > Br⁻ > Na⁺> I-> K⁺. Complexation studies of ligand 5b with alkali metal ions by UV-visible spectrophotometry could not give stability constants. Nevertheless, anion binding abilities of ligand 5b were calculated and varied as follows: NO₃⁻ > Br⁻~ C1⁻ as 1:1 complexes, and BzO⁻ > H₂PO₄⁻ > I⁻ as 1:2 complexes. In allosteric studies of alkali metal ions or various anions for both ligands, the anion or alkali metal ion binding abilities depended on structures of complexes between ligands and alkali metal ions and anions as well as the ion-pair formation between alkali metal ions and anions. The electrochemical studies of ligand Sb by cyclic and square wave voltammetry confirm the occurring of ion-pair formations.