This review describes the design and synthesis of hosts based on salen or related N₂O₂ ligands, which are useful for the development of cooperative metal complexation systems. A key to the design of the N₂O₂-based metallohosts is the negative charges of the phenoxo groups, which further coordinate to metal ions in a bridging fashion. In particular, the integration of two or more N₂O₂ coordination sites is effective for the construction of metallohosts. A variety of cyclic and acyclic oligo(N₂O₂) compounds were designed and synthesized. They show unique complexation behavior due to the integrated coordination sites. For examples, cyclic tris(N₂O₂) ligands formed novel cluster complexes and acyclic bis(N₂O₂) ligands bind alkaline earth and rare earth metal ions in the central recognition site via metal exchange. The suitable arrangement of the phenoxo moieties contributes to the preorganization for the cooperative metal complexation, producing metallocyclic or metallohelical structures.

Novel ligands 1,2-bis(salicylideneaminooxy)ethanes were synthesized. They were much more stable against exchange reactions of aldehyde units than the corresponding salen derivatives. They form stable complexes with a copper(II) ion, whose structures were crystallographically determined. Spectroscopic and electrochemical studies indicated that the complexes have lower LUMO levels than salen analogues.