Ion pair complexes and anion binding in the solution of a ditopic receptor

The synthesis and crystal structures with alkali halides of a ditopic benzo-15-crown-5 bis-urea receptor have been presented. In addition, the anion binding properties of and its alkali metal complexes in solution are presented. A comprehensive single-crystal X-ray crystallographic study of , all together 13 crystal structures, including the ion pair complexes with NaCl, NaBr, NaI, KF, KCl, KBr, KI, RbF, RbCl, and RbI, give a detailed view of how behaves in the solid-state with different alkali halides depending on the size of the cation and anion. In the solid-state forms a 1 : 1 complex with a sodium cation and the anion is complexed as a contact (NaCl) or a separate ion pair (NaBr, NaI). With larger potassium and rubidium cations assembles into a 2 : 1 complex and forms a separated ion pair complex with the anion. Reflecting the crystal structures the forms a 1 : 1 complex with Na(+) in solution, and a 2 : 1 complex with K(+), which were verified by Job's plot analysis in 4 : 1 CDCl3/dimethyl sulfoxide. The binding strength of the monomeric [·Na](+) and the dimeric [2·K](+) toward chloride, bromide and iodide anions was studied by (1)H NMR titrations in 4 : 1 CDCl3/DMSO, and a clear turn-on effect of the cation complexation compared to the neutral receptor alone (Ka with for Cl(-), Br(-) and I(-) being 832, 174 and 32 M(-1), respectively) was observed. The monomeric [·Na](+) binds chloride 9, bromide 8, and iodide 12 times stronger than , while for the dimeric [2K](+) the corresponding increase in binding is 51 (Cl(-)), 84 (Br(-)), and 22 (I(-)) times with the same stoichiometric ratios as observed for the ion pair complexes in the solid-state.