Selective transport of polyfunctional cations through bulk liquid membranes assisted by macrocyclic polyethers

In the past decade, the role of complementary size, shape, and functionality in the molecular recognition of neutral molecules has been pointed out. Artificial receptor molecules have been designed with multiple recognition sites and a well-defined geometry, capable of strong and selective binding of the appropriate substrates by hydrogen bonding, ionic interaction, and/or hydrophobic interactions. ^2-8 Rebek et al.² have used molecular clefts with
acidic functions for the complexation of diamines, such as imidazole. Our group has shown that additional hydrogen-bond donors⁹ or electrophilic metal centers,¹⁰ incorporated in the cavities of macrocyclic polyethers, improve
strongly the complexation of urea. This type of cocomplexation
of a neutral guest by a host molecule either by hydrogen bonding or by coordination with a metal ion, is frequently observed in (metal1o)enzymes.