Cryptands and Spherands

Abstract

The cryptands and spherand are both categories of ligands exhibiting significant degrees of preorganization of their donor atom sets. The design, synthesis, and host–guest binding properties of representative examples of each ligand category are discussed, with emphasis on structure‐function relationships. A characteristic feature of both of these categories is their tendency to exhibit selective complexation behavior toward individual metal cations, especially alkali metal cations. Individual species from each category that bind ammonium and/or alkylammonium cations are also presented. Examples of spherand and cryptand derivatives, including “intermediate” ligand species of the hemispherand and cryptaspherands type, as well as chromoionophoric derivatives of each category, are also discussed. Spherand derivatives in which some or all of the anisyl groups of the “classical” spherands have been replaced by fluoro, cyano, or urea functions are described. Examples of both bicyclic and tricyclic cryptands and their cryptate derivatives, including anion‐binding systems and ones that yield binuclear metal complexes, are also discussed.