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.
