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

Abstract

The synthesis and crystal structures with alkali halides of a ditopic benzo-15-crown-5 bis-urea receptor L have been presented. In addition, the anion binding properties of L and its alkali metal complexes in solution are presented. A comprehensive single-crystal X-ray crystallographic study of L, 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 L behaves in the solid-state with different alkali halides depending on the size of the cation and anion. In the solid-state L 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 L assembles into a 2 : 1 complex and forms a separated ion pair complex with the anion. Reflecting the crystal structures the L 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 [L·Na]⁺ and the dimeric [2L·K]⁺ toward chloride, bromide and iodide anions was studied by ¹H NMR titrations in 4 : 1 CDCl₃/DMSO, and a clear turn-on effect of the cation complexation compared to the neutral receptor L alone (K_a with L for Cl⁻, Br⁻ and I⁻ being 832, 174 and 32 M⁻¹, respectively) was observed. The monomeric [L·Na]⁺ binds chloride 9, bromide 8, and iodide 12 times stronger than L, while for the dimeric [2L·K]⁺ 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.