Encapsulated Metal Ions

This paper reviews some of the more interesting and recent phenomena associated with the metal ion cage chemistry involving hexaazamacropolycycles. Synthetic aspects are addressed in relation to making cages with smaller and larger hole sizes in order to modulate redox potentials and electron transfer rates. The introduction of substituents is also examined in relation to these factors along with the effect of ligand geometry and strain. Relevant redox potentials, spectroscopic and photochemical properties of the different metal ion cages are displayed. Aspects of the synthesis of dimer complexes and some properties of the dimer complexes are outlined. The route whereby the metal is extruded from the cage in acid conditions is examined in some detail for the Cusar2 ion. The properties of an extraordinarily stable zinc alkyl cage are examined and finally various prospects for the use of the metal ion cages are reviewed. Multidentate ligand chemistry has given us a plethora of interesting phenomena over the years, limited only by our ability to make these fascinating molecules. The results are manifest in very stable complexes, static and dynamic selectivity phenomena, stabilization of oxidation states and spin states, solubilization of ions in organic solvents, transport of ions through biological membranes, oxidation of the ligand through a high oxidation state at the metal centre, coupling of chelates through radicals generated on the ligand periphery and antifungal and antibacterial activity to mention some of the more interesting facets. The ligands themselves are now too numerous to review but in the last 10—15 years large molecules which have a capacity -to encapsulate other molecules or ions have become a prominent subclass in this interesting research area (Ref. 1). This lecture addresses that subclass and especially the synthesis and properties of sexidentate ligands of the macropolycyclic type shown in Fig. 1 involving N atom donors. These cage molecules have