From mononuclear to polynuclear macrocyclic or macroacyclic complexes

The design and synthesis of polydentate Schiff bases and their properties and potential in the selective coordination of metal ions is reviewed. The self-condensation reaction of appropriate formyl or keto precursors with suitable polyamines can give rise to well-defined planar or tridimensional macrocyclic or macroacyclic Schiff bases, but different reaction pathways can also occur. Illustrative examples of [1 + 1], [2 + 2], [2 + 3] condensation products and of unexpected compounds are included. Moreover, the role of metal ions as templating agents and the capability of several Schiff base complexes to undergo transmetallation reactions are reported. Ring expansion and ring contraction processes of the macrocyclic cavity of Schiff bases occur and the experimental conditions for the obtainment or the stabilization of one isomer with respect to the others, in the presence or in the absence of suitable metal ions, are discussed. The preparation of functionalized ligands, containing pendant arms, capable of promoting dynamic complexation-decomplexation processes and their use in selective metal ion transportation and separation are also evaluated. The formation of polydentate, particularly macrocyclic, Schiff base complexes, followed by reductive demetallation with NaBH4 is also discussed especially for the obtention of more stable, flexible and versatile ligands useful in metal transfer studies. The synthetic strategies for the preparation of macrocyclic and/or macroacyclic mononuclear and polynuclear complexes from these Schiff bases are outlined. For mononuclear and dinuclear complexes 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 arrangement. Different synthetic strategies for the assembly of polynuclear molecular arrays are studied and the synthesis of trinuclear, tetranuclear, hexanuclear, dodecanuclear etc. complexes together with their properties (especially the structural and magnetic aspects) are discussed 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 predetermined properties are considered in detail. Finally some related systems, such as polyaza macrocycles, compartmental ligands etc., also capable of forming polynuclear systems, are briefly discussed.