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

Article abstract of DOI:10.1002/chem.200801154

A novel type of chiral trismonodentate 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 M₃L ₂ architectures with programmed helicity through the edge-directed complexation with a series of d³-d¹⁰ 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⁺, Mn ²⁺, and Ni²⁺), and even trivalent metal ions (e.g. Fe ³⁺ 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 (S,S,S)-A 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.