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 andchiral packing modes in solid state

Zinc(II)-assisted self-assembly of a new bis-N,O-bidentate Schiff base liga nd, (N-salicylidene-4,4'-diaminodiphenyl)methane (L-16) with two chelating sites linked by a spacer group (-C6H4CH2C6H4-), afforded in high yield the double-helical dinuclear complex (L-16 : Zn-II = 2 : 2). Single-crystal X-r ay analyses demonstrated clearly that the two Zn-II centers have a distorte d tetrahedral (T-d) coordination sphere with two-wrapped ligands. The analo gous Schiff base, bis[4-(2-pyridylmethyleneaminophenyl]methane (L-17) was a lso designed to self-assemble in the presence of metal ions, leading to a t riple-helical dinuclear (L-17:Zn-II = 3.2) supramolecular motif. Each zinc ion has six-coordinate octahedral geometry with six nitrogens from three li gands. These unprecedented helical motifs in the solid state and solution s eem to be induced by the geometrical preference for octahedral or tetrahedr al coordination mode of the Zn-II ion and the interligand pi-stacking inter actions between the spacer groups of L-16 and L-17. Electrospray mass spect rometry proved a very useful characterisational tool in detecting the distr ibution of supramolecular species in solution. Use of a N,N'-bis(2-pyridylm ethylene)-1,4-diaminobenzene (L-35) with a rigid phenylene spacer in its ce nter resulted in the one-dimensional zigzag polymeric structure ((L-35:Zn-I I)(infinity) = (1 : 1)(infinity)) where the multiple pi-stacking interactio ns 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.