Approaches to improvement of metal ion selectivity by cryptands

Abstract

Several approaches to improvement of metal ion selectivity by mixed oxygen–nitrogen donor cryptands are discussed. Because of the relatively rigid structures of cryptands, thermodynamic stabilities of the cryptate complexes strongly depend on the match of the cation size and cryptand cavity diameters. Symmetry is an important factor influencing the properties of cryptand complexation. One rule to improve metal ion selectivity by oxygen–nitrogen donor cryptands is to obtain the ligand that has the best size-match cavity with the cation while keeping a symmetric spherical-coordination array. High selectivity for a small cation can be obtained when the cryptand is able to form a number of six-membered chelate rings with the metal ion while the requirements of the high-symmetric donor atom array and size-matched cavity are met. On the other hand, introduction of benzene rings and other groups to cryptands usually decreases metal ion binding and selectivity.