Dicopper double-strand helicate complex

Figure 2.16 The redox-driven disassembling of a dicopper(I) double-strand helicate complex to give two mononuclear copper(II) complexes, in which each strand behaves as a quadridentate ligand. On subsequent reduction, the two mononuclear complexes reassemble to give the helicate. The illustrated process fits well the behavior of copper complexes of 16 in a MeCN solution.

However, it has been recently demonstrated by Pallavicini et al. that the lifetime of the dicopper(II) double-strand helicate [ 2 (16)]4 + can be significantly increased by introducing hindering substituents on the framework of 16. In particular, this was shown to occur with the copper complexes of the bis-bidentate ligand 17.21... 

Preparation of the double-stranded helical precursor with copper(l) using the 1,3-phenylene linker turned ont to be qnantitative. Reaction of this tetraphenolic donble helix with two eqnivalents of the di-iodo derivative of hexaethyleneglycol, in the presence of caesinm carbonate, afforded a single isolable copper(I) complex the dicopper(l) knot was isolated in 29% yield after chromatography. H-NMR spectroscopy data indicated that the knot contained a compact helical core. This was fully confirmed by its subsequent X-ray strnctnre determination. The spectacularly improved yield can be increased even further by optimizing the covalent-bond-forming reactions, whose conditions can harm the helical precnrsors. 

Double-strand dicopper helicate complexes are interesting systems in that they may show hysteresis (as observed with ligand 16), thus giving rise to a rare example... 

Reports of double-helical complexes have appeared in the literature since the sixties. Despite the early interest, it is only more recently that emphasis has been given to the use of metal template synthesis for obtaining a wide range of doubly-and triply-stranded systems. In part, this attention has had its origins in an early report by Lehn et al. in which the spontaneous assembly of a dicopper(I)-containing double helix was described. 

---