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 H-1 NMR titration experiments in 10% (v/v) DMSO-d(6)/CD3CN at 24 +/- 1 degrees 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. (c) 2007 Elsevier Ltd. All rights reserved.