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

Abstract

Copper(II)-assisted self-assembly of a new bis-N,O-bidentate Schiff base ligand, bis(N-salicylidene-3,3′-diaminodiphenyl) sulfone (L¹) 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 (L⁶) 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 L¹ and L⁶.