Living in a cage is a restricted privilege

Abstract

There exist molecules, whose shape is reminiscent of a cage, that are able to include either metal ions or anions or both. In contrast to what happens in the macroscopic world, where a kinetic barrier prevents the escaping of the guest from the cage, the inclusion-extrusion of an ion from a molecular cage is in most cases thermodynamically controlled and the ion can get in or out of the cage at will. This gives the basis for highly selective ion recognition processes by cage-shaped ligands or receptors for metal ions and anions. Nobody in everyday life would say that a cage (for birds or wild animals), even if nicely designed and splendidly decorated, was beautiful and appealing, due to the consciousness of its reprehensible function. This does not happen in chemistry and we admire the ingenuity and skilfulness of synthetic chemists for the design of cage-shaped polycyclic hosts, made for the inclusion of a variety of guests, but also capable of generating in the viewer emotion and gratification of aesthetical origin. We have tried to outline, in this chapter, the development of cages in metal coordination chemistry and in anion coordination chemistry, over the last 50 years. © 2011 Springer-Verlag Berlin Heidelberg.