Catenanes and rotaxanes are both examples of interlocked molecules.
Unlike classical molecular structures, they consist of two or more separate components
which are not connected by chemical (i.e. covalent) bonds. These structures are, however,
true molecules (NOT supramolecular species!), as each component is intrinsically linked to
the other – resulting in a mechanical bond which prevents dissociation without cleavage of
one or more covalent bonds.
What are they?
Catenanes consist of two or more interlocked macrocyclic rings. Figure 1 is a schematic
representation of a [2]catenane. The prefix in square brackets indicates the number of mechanically interlocked
components – in this case just two rings. If both rings are identical (i.e. green = blue in Figure 1) the
molecule is a homocircuit catenane, if the rings are different (i.e. green ≠ blue) it is a heterocircuit catenane.
Figure 1. Cartoon representation of a [2]catenane.
Rotaxanes consist of macrocyclic rings trapped onto a linear unit (the “thread”) by two bulky substituents
(“stoppers”). Figure 2 shows a schematic representation of a [2]rotaxane.
Figure 2. Cartoon representation of a [2]rotaxane.
Catenanes are topological isomers of their un-interlocked components because it does not matter how much you
stretch, twist or deform the rings, they cannot be separated from each other without actually breaking one of the rings.
Rotaxanes, on the other hand, are NOT topological isomers of their un-interlocked components because by infinitely stretching
the ring (obviously this is only mathematically, not physically, possible) it could slip over the stoppers and dissociate it
from the thread (see Figure 3).
Figure 3. Illustrating the principles of chemical topology: a) the common
geometric shapes of a circle, a square and a triangle are all topologically identical, however a triangle with appended line cannot
be formed through deformations of any of the other three – it is a topological isomer of the simpler shapes; b) a catenane is a topological
isomer of its un-interlocked rings; c) a rotaxane is topologically identical with its un-interlocked components.
How are they made?
Up until the early 1980s, catenanes, rotaxanes and other interlocked architectures were considered by most chemists to be academic curiosities with little
potential for practical applications. The existing synthetic routes were low-yielding – relying simply on the chance interlocking of components during their formation or
else on complex and protracted synthetic strategies. With increased interest in the developing field of supramolecular chemistry, however, chemists were able to apply their
new-found knowledge of non-covalent interactions to chemical synthesis, resulting in the various templated synthesis methods which are used to efficiently produce catenanes,
rotaxanes and many other interesting compounds today.1
Figure 4. Two examples of templated synthesis strategies for the construction
of interlocked molecules: a) metal-ligand interactions have been employed by J.-P. Sauvage to construct [2]catenates;2 b) ∏-∏charge transfer interactions are the primary forces involved in templating the formation of this catenane prepared in the group of J. F. Stoddart.3
Templated synthesis employs specific non-covalent interactions between components to hold the molecular precursors in the correct orientation for production of the
interlocked product following a final covalent bond-forming reaction. In the Leigh group, we employ two different types of interactions to construct interlocked molecules: hydrogen
bonds and metal-ligand bonds.
