FROM MONONUCLEAR TO POLYNUCLEAR MACROCYCLIC OR MACROACYCLIC COMPLEXES

The design and synthesis of polydentate Schiff bases and their propert ies and potential in the selective coordination of metal ions is revie wed. The self-condensation reaction of appropriate formyl or keto prec ursors with suitable polyamines can give rise to well-defined planar o r tridimensional macrocyclic or macroacyclic Schiff bases, but differe nt reaction pathways can also occur. Illustrative examples of [1 + 1], [2 + 2], [2 + 3] condensation products and of unexpected compounds ar e included. Moreover, the role of metal ions as templating agents and the capability of several Schiff base complexes to undergo transmetall ation reactions are reported. Ring expansion and ring contraction proc esses of the macrocyclic cavity of Schiff bases occur and the experime ntal conditions for the obtainment or the stabilization of one isomer with respect to the others, in the presence or in the absence of suita ble metal ions, are discussed. The preparation of functionalized ligan ds, containing pendant arms, capable of promoting dynamic complexation -decomplexation processes and their use in selective metal ion transpo rtation and separation are also evaluated. The formation of polydentat e, particularly macrocyclic, Schiff base complexes, followed by reduct ive demetallation with NaBH4 is also discussed especially for the obte ntion of more stable, flexible and versatile ligands useful in metal t ransfer studies. The synthetic strategies for the preparation of macro cyclic and/or macroacyclic mononuclear and polynuclear complexes from these Schiff bases are outlined. For mononuclear and dinuclear complex es only the most recent and significant examples are considered, while particular attention is devoted to the preparation of ligands capable of organizing more than two metal centres into a predetermined arrang ement. Different synthetic strategies for the assembly of polynuclear molecular arrays are studied and the synthesis of trinuclear, tetranuc lear, hexanuclear, dodecanuclear etc. complexes together with their pr operties (especially the structural and magnetic aspects) are discusse d in detail. In particular the enlargement of the macrocyclic cavity, the introduction of suitable bridging groups or the use of ''spacers'' between dinuclear entities to obtain polynuclear complexes with prede termined properties are considered in detail. Finally some related sys tems, such as polyaza macrocycles, compartmental ligands etc., also ca pable of forming polynuclear systems, are briefly discussed.