Selective Formation of S4- and T-Symmetric Supramolecular Tetrahedral Cages and Helicates in Polar Media Assembled via Cooperative Action of Coordination and Hydrogen Bonds

We report on the synthesis and self-assembly study of novel supramolecular monomers encompassing quadruple hydrogen-bonding motifs and metal-coordinating 2,2′-bipyridine units. When mixed with metal ions such as Fe²⁺ or Zn²⁺, the tetrahedron cage complexes are formed in quantitative yields and full diastereoselectivity, even in highly polar acetonitrile or methanol solvents. The symmetry of the complexes obtained has been shown to depend critically on the flexibility of the ligand. Restriction of the rotation of the hydrogen-bonding unit with respect to the metal-coordinating site results in a T-symmetric cage, whereas introducing flexibility either through a methylene linker or rotating benzene ring allows the formation of S₄-symmetric cages with self-filled interior. In addition, the possibility to select between tetrahedral cages or helicates and to control the dimensions of the aggregate has been demonstrated with a three-component assembly using external hydrogen-bonding molecular inserts or by varying the radius of the metal ion (Hg²⁺ vs Fe²⁺). Self-sorting studies of individual Fe²⁺ complexes with ligands of different sizes revealed their inertness toward ligand scrambling.