Chemistry Reference
In-Depth Information
Fig. 7 Depiction of the
Mobius strip approach
to a trefoil knot
Macrocyclisation
Rung removal
system that leads to two strands whose planes are held at a large angle to one
another [
23
-
25
].
A very suggestive route to knots is through multiply ravelled species in the form
of helices, the double helix for example. The double helix itself attracts great
interest in itself as a beautiful object. In the age of genetics, we are almost con-
stantly bombarded with images of the DNA double helix. Indeed, many authors
refer to this intertwined structure when describing their work on helices, simply
because it is a beautiful object that demonstrates intricate function with relative
simplicity of structure.
Hypothetical joining of the termini of helices with covalent bonds [
26
] (Fig.
8
)-
joined in the right way of course thanks to the correct positioning of reactive
ends - gives a route to a series of knots and multiply interlocked catenanes. The
double helical coordination complexes, which are relatively frequent [
27
], are ideal
precursors for this purpose, where transition metal ions are surrounded by winding
coordinating strands to give the double helix. Connecting the termini of the strands
in the correct way gives knots and multiply ravelled [2]catenanes.
The preparation of double helices from different bis-chelate ligands and transi-
tion metals in all likelihood occurred long ago, yet it is only relatively recently that
the first such system was recognised and characterised. Apart from their beauty,
their scientific relevance was not at all apparent. One of the earliest dinuclear
helical complexes was discovered by Fuhrhop and co-workers in 1976 [
28
]. Since
then, several double helical complexes have been created and characterised, and
