Ion-pair Recognition by Ditopic Macrocyclic Receptors

Over the past thirty years, a large number of macrocyclic receptors have been synthesized and evaluated for their abilities to bind cations. More recently, increased attention has been directed towards receptors for anions. Many of these synthetic receptors are uncharged molecules and operate in organic solvents. Under these conditions the target salts exist as associated ion-pairs which can hinder the single-ion recognition process. [1] A strategy to circumvent this problem is to design a single receptor with specific cation and anion binding sites. In other words, a heteroditopic receptor that can simultaneously bind both of the salt ions. The chronological development of salt receptors has been reviewed a number of times in the past few years, [2-6] and so the exercise will not be repeated here. Instead, the chapter will focus on the more specific topic of ion-pair recognition.