Chemistry Reference
In-Depth Information
Metals in
double helicate
Product after cyclisation
2
3
1
(Trefoil) knot
3
4 Crossing [2]Catenane
4
5
1
knot
5
6 Crossing [2]Catenane
Fig. 8 Double-stranded helicates (with metal ion centres depicted by
filled circles
) and the mole-
cular knots and catenanes derived from them by appropriate joining of the ends in the helical strands.
Half turns give multiply crossed [2]catenanes whereas an integral number of turns leads to knots
their interest in the context of interlocking molecular structures has been discussed
[
29
]. In particular, if synthesis is to be done, the helices must be formed efficiently
and must be stable to the conditions of the covalent bond-forming reactions. As we
shall see, the advances in softer reactions for covalent bond forming are proving
extremely useful for the preparation of complex molecular forms.
The synthesis of trefoil knots based on copper(I) coordination by phenanthroline
ligands as the template is a reliable and flexible route to knots in which the metal
ion can be removed to liberate the free “knotand” (a knot that behaves as a ligand).
The route is conceptually simple: Linking the termini of strands in a double helical
strand gives the knot as its copper complex, which upon removal of the templating
metal ion leaves the knotand. The route shown in Fig.
9
illustrates the approach
elegantly. It relies on linking the strand termini on the same side of the double helix,
rather than at the ends, which leads to the macrocycle.
Many attempts with various linkers were carried out before it was found that
1,10-phenanthroline moieties, connected via their 2-positions by a butyl chain, form
a double helix when complexed with two copper(I) ions. In addition, by introducing
appropriate functions at the 9-positions, the strategy of Fig.
9
could be followed
to achieve the synthesis of a molecular knot. The route to the knot, showing the
