Preferential Formation of Homochiral Helical Sandwich-Shaped Architectures through the Metal-Mediated Assembly of Tris(imidazoline) Ligands with a Set of d3-d10 Transition-Metal Ions

A novel type of chiral tris-monodentate imidazolinyl ligands ((S,S,S)-4 and (R,R,R)-4) has been achieved in good yields. The ligands show a strong tendency to induce the generation of the discrete sandwich-shaped M3L2 architectures with programmed helicity through the edge-directed complexation with a series of d(3)-d(10) transition-metal ions, while taking advantage of the steric hindrance of the bulky substituents of the imidazoline rings to avoid the formation of extended metal-organic frameworks (MOFs). In spite of different coordination geometries, monovalent metal ions (e.g. Ag+), divalent metal ions (e.g. Pd2+, Cu2+, Cd2+, Zn2+, Co2+, Mn2+, and Ni2+), and even trivalent metal ions (e.g. Fe3+ and Cr3+) exhibit isostructural coordination. Installation of stereo-centers fused onto the imidazoline rings results in favored handedness of the self-assemblies through the expression of molecular chirality into supramolecular helicity. In the crystal structures of[M-3{(S,S,S)-4}(2)], the self-assembly has to adopt the M form to relax the van der Waals repulsions of the phenyl and isopropyl groups. The replacement of (SSS)-4 with (R,R,R)-4 exclusively affords the opposite helicity (P). These results should provide important insights for the design of chiral helical capsule-like assemblies.