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

摘要

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-d_6/CD3CN at 24 ± 1℃,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.