Metal Complexes with Oligo(Salen)‐Type Ligands

Abstract

Various kinds of metal complexes having oligo(salen)-type ligands have been designed, synthesized, and characterized. Convergent arrangement of two or more metallosalen units contributes to the programmed formation of multi-metal structures. The oligo(salen) compounds are classified into three types: macrocyclic, acyclic (helical), and cage-like systems. In the macrocyclic systems, the metalation of the salen moieties significantly enhances the guest ion binding in the cavity. This enables the controlled synthesis of homo- and heterometallic cluster-like structures. In the acyclic systems, the metalation makes the ligand molecules adopt a helical conformation. These helical structures provided a new binding site, at which guest ions are recognized based on metal exchange protocol. The dynamic feature of the helical structures is utilized for stimuli-responsive systems that can change the helicity upon binding with chemical species. Metal complexes obtained from cage-like oligo(salen) ligands have a three-dimensional cavity suitable for size-selective guest recognition. The cooperative metal binding by taking advantage of the convergent oligo(salen) structures discussed in this chapter will be useful to achieve sophisticated responsive functions, which are based on the intrinsic properties and the dynamic structural conversions of the multimetallic core of the complexes. Keywords: salen; host-guest chemistry; ion recognition; metallohost; macrocycle; helical structure; helicity control; molecular cage; cooperative effect; template synthesis