Novel ion recognition systems based on cyclic and acyclic oligo(salen)-type ligands

Abstract

This review describes the design of novel ion recognition systems based on salen (H2salen = N,N′-disalicylideneethylenediamine) or related ligands. The phenoxo groups of the salen-based metallohosts play an important role in the ion recognition because the phenoxo groups can further coordinate to metal ions in a bridging fashion. In particular, the integration of two or more salen-type coordination sites in a cyclic fashion is effective for the construction of the metallohosts. They show unique multi-metal complexation behavior and binding selectivities due to the phenoxo-bridged structures. The peripheral salen-type sites are suitable for binding to d-block transition metal ions and the central O6 (or larger) site is for the group 1–3 metals. Acyclic oligo(salen) molecules are also effective for obtaining metallohosts. The metalation of a bis(salen)-type ligand with d-block metals leads to a trinuclear complex with a C-shaped structure, which can selectively recognize Ca2+ and lanthanide(III) ions via a unique metal exchange process. The longer oligo(salen) ligands form a helical structure when they recognize the La3+ or Ba2+ ion in the presence of the zinc(II) ion. The helix inversion behavior of the helical metal complexes due to the labile character of the coordination bonds is successfully utilized for the dynamic helicity control. The transformation of the acyclic ligand into cyclic ones via olefin metathesis significantly changes the binding selectivity.