Chemistry Reference
In-Depth Information
have very well-defined form when the metal ions are present, holding the phenan-
throline groups tightly in their tetrahedral clutches! No motion in the backbone is
permitted, and the self-wrapped ring might as well be a piece of static jewellery.
The coordination forces that templated the formation of the knot “live on” (to quote
Fraser Stoddart) in the knot after it is formed.
The knotted topology of the first synthetic trefoil was first demonstrated by mass
and NMR spectroscopy, but was later fully confirmed by an X-ray structure
determination showing this situation [
48
] (Fig.
13
).
Of course, static things can be beautiful, but movement can be far more seduc-
tive! Removal of the metal ion from the double helical core of the knots leads to
molecules with no strong non-covalent interactions between the moieties in the
ring. The NMR of this molecule is now far from pretty, though. The slow motions
that result from the tight intertwining on the one hand and the lack of specific
interaction on the other mean that the ring slides through itself at a rate that is slow
on the NMR timescale, with multiple conformations that are poorly defined.
For now, seeing how the knot glides through itself will tease us.
The pattern of crossings in knots means that they can be chiral objects. This is
particularly the case for the three-crossing knot 3
1
(trefoil knot). However, 4
1
, the
four-crossing knot already discussed above, is not chiral, which could be very
Fig. 13 Structure of the first
trefoil knot as it exists in its
crystals, with the copper(I)
ions shown as
clear spheres
in the tetrahedral coordination
sphere of two phenanthroline
ligands
