Chemistry Reference
In-Depth Information
Living in a Cage Is a Restricted Privilege
Luigi Fabbrizzi
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 reprehensi-
ble 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.
Keywords Anion coordination chemistry Cryptands Ligands Metal
coordination chemistry Molecular recognition Receptors Sarcophagines
Contents
1 Cages in Life and in Art . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
2 From Ammonia to Sarcophagine: Cages for Metals Made by Metals . ................... 129
3 Cryptands and Cryptates: Cages Made to Measure for s-Block Metals .................. 139
4 Cages for Anions . ........................................................................... 145
5 Bis-Tren Cages: Comfortable Shelters for Metal Ions, Anions and Both Metal
Ions and Anions Together ................................................................... 150
