An ion pair receptor showing remarkable enhancement of anion-binding strengths in the presence of alkali metal cations

An ion pair receptor was prepared by coupling of a diazacrown ether and a rigid biindole scaffold bearing hydrogen bond donors of two indole NHs. The former serves as the cation-binding site and the latter functions as the anion-binding site. The anion-binding affinities to the receptor, determined by ¹H NMR titration experiments in 10% (v/v) DMSO-d₆/CD₃CN at 24 ± 1 °C, have been greatly improved when an alkali metal cation binds to the adjacent diazacrown ether. For example, the association constant between chloride and receptor alone is 7 M⁻¹, but the magnitudes increase into 120 M⁻¹, 14,000 M⁻¹, and 6200 M⁻¹ in the presence of lithium, sodium, and potassium ions, respectively. The enhanced binding affinities must be attributed to electrostatic interactions by possibly forming contact ion pairs.