Armed cyclen receptors: From three dimensional cation recognition to supramolecular architecture

The concept and examples of armed cyclens are described and some of their recent applications in molecular recognition and supramolecular chemistry are explained. Armed cyclens are characterized by a parent tetraamine ring and functionalized sidearms. They can act as three-dimensional ligands for octadentate metal complexation. They exhibit comparable log K values for Na+ complexation to bicyclic cryptands, though they have flexible, open-chained structures. Since the resulting octacoordinate metal complexes have quadruple helicated structures, they can work as unique building blocks having C-4 symmetry for chiral supramolecular architecture. Cholesterol-armed cyclen typically formed a self-aggregate with the integrated chirality in aqueous Solution, in which asymmetrically helicated cyclen complexes were arrayed on a supramolecular scale. The self-aggregate had asymmetrically ordered domains, in which achiral organic anions and racemic metal complexes were nicely accommodated in stereo-controlled fashions. Since the armed cyclens have broad structural variations. they are applicable as specific receptors for cation recognition at the molecular level and also as building blocks for supramolecular architecture.