Supramolecular motifs in metal complexes of Schiff bases. Part 5. Zinc(II)-assisted self-assembly of some bis-N,N- and N,O-bidentate Schiff bases and chiral packing modes in solid state

Abstract

Zinc(II)-assisted self-assembly of a new bis-N,O-bidentate Schiff base ligand, (N-salicylidene-4,4′-diaminodiphenyl)methane (L¹⁶) with two chelating sites linked by a spacer group (-C₆H₄CH₂C₆H₄-), afforded in high yield the double-helical dinuclear complex (L¹⁶ : Zn^II = 2∶2). Single-crystal X-ray analyses demonstrated clearly that the two Zn^II centers have a distorted tetrahedral (T_d) coordination sphere with two-wrapped ligands. The analogous Schiff base, bis[4-(2-pyridylmethyleneaminophenyl]methane (L¹⁷) was also designed to self-assemble in the presence of metal ions, leading to a triple-helical dinuclear (L¹⁷∶Zn^II = 3∶2) supramolecular motif. Each zinc ion has six-coordinate octahedral geometry with six nitrogens from three ligands. These unprecedented helical motifs in the solid state and solution seem to be induced by the geometrical preference for octahedral or tetrahedral coordination mode of the Zn^II ion and the interligand π-stacking interactions between the spacer groups of L¹⁶ and L¹⁷. Electrospray mass spectrometry proved a very useful characterisational tool in detecting the distribution of supramolecular species in solution. Use of a N,N′-bis(2-pyridylmethylene)-1,4-diaminobenzene (L³⁵) with a rigid phenylene spacer in its center resulted in the one-dimensional zigzag polymeric structure ((L³⁵∶Zn^II)_∞ = (1∶1)_∞) where the multiple π-stacking interactions operate between the aromatic rings of linking ligands. Each zinc ion is octahedrally coordinated by two N,N-bidentate arms of two different ligands and two cis oxygens from N,N-dimethylformamide as coordinating solvent.