The challenge of cyclic and acyclic schiff bases and related derivatives

The use of planar or tridimensional cyclic and acyclic Schiff bases, their polyamine homologues and related derivatives, for the formation of mononuclear and especially polynuclear complexes, is reported. Attention is devoted to the synthetic pathways and to the structural aspects of the resulting systems, particularly the role of the shape of the coordinating ligands and of the different metal ions in directing the synthesis totally or preferentially towards mono-, di- or poly-nuclear entities. Special care is paid to compartmental ligands, owing to their ability to bind two or more metal ions in close proximity. The insertion of asymmetry into these ligands provides important diversification of the coordinating sites and allows for different and well defined recognition processes involving specific ions and/or molecules at the adjacent coordinating sites. New molecular materials, devices and probes for specific recognition and detection in the solid state or solution take advantage of the presence of different metal ions at appropriate distances from each other in a preordered stereochemical environment with variation of oxidation state, magnetic, optical and redox properties. Such polynuclear systems can exhibit unusual reactivity or catalysis.