Copper(II)-assisted self-assembly of bis-N,O-bidentate Schiff bases:: new building blocks for a double-helical supramolecular motif

Copper(II)-assisted self-assembly of a new bis-N,O-bidentate Schiff base ligand, bis(N-salicylidene-3,3'-diaminodiphenyl) sulfone (L-1) with two chelating sites linked by a spacer group (-C6H4SO2C6H4-), afforded in high yield the double-helical tetranuclear supramolecular complex with eight components. Single-crystal X-ray analyses demonstrated clearly that two Cu-II centers have a distorted tetrahedral (T-d) coordination sphere, whereas the other two remaining Cu-II have a square-planar coordination sphere. An analogous Schiff base, bis(N-salicylidene-4,4'-diaminodiphenyl) ether (L-6) with a phenyl ether spacer (-C6H4OC6H4-) was also designed to self-assemble in the presence of metal ions, leading to a double-helical dinuclear supramolecular motif. Electrospray mass spectrometry proved a very useful characterizational tool to detect a variety of supramolecular species in solution. These unprecedented double-helical motifs in solid and solution seem to be induced by the geometrical preference for tetrahedral and/or square-planar coordination of copper(II) ion and the interligand aromatic interactions between the bridging groups of L-1 and L-6.