Supramolecular Chemistry literally means “chemistry beyond the molecule” or alternatively it may be described as “the chemistry of the intermolecular bond, covering the structures and functions of the entities formed by the association of two or more chemical species“.1
Supramolecular chemistry therefore relies on the use of non-covalent interactions (including, but not limited to, electrostatic interactions, hydrogen bonding, pi-pi stacking and halogen bonding).
As a discipline, supramolecular chemistry may be divided into two: (a) Host-Guest Recognition where a receptor (host) forms a complex with a substrate (guest) and (b) Self-Assembly which involves the association of multiple components to construct some higher structure.
For inspiration the supramolecular chemist may identify with receptors (e.g. enzyme active sites) and self-assembled structures (e.g. the double helix of DNA) that arise in the natural world, but the possibilities are only truly limited by the imagination and the synthetic capability of the chemist themselves.
To achieve efficient (i.e. strong and selective) binding of the target substrate, the receptor should be complementary in size and shape to the substrate, to maximise the attractive non-covalent interactions between the two entities.
Synthetic receptors may range from relatively simple molecules to more complex structures including those constructed using self-assembly. Target substrates may either be charged ions or neutral molecules. Applications of host-guest recognition include sensing, (ionic) transportation and catalysis.
Self-Assembly and Interlocked Molecules (Catenanes and Rotaxanes)
Catenanes are molecules consisting of two or more interlocked rings. Rotaxanes are molecules consisting of one or more rings threaded over an axle component, with the presence of bulky stopper groups on the axle component preventing dethreading. If these stopper groups are absent, the species is a (non-interlocked) pseudorotaxane.
The vast majority of catenanes and rotaxanes are prepared by self-assembly. A template, or templating interaction, is used to arrange the molecular precursors, then this is usually followed by covalent bond formation to irreversibly capture the interlocked structure.
A catenane may be prepared by clipping of a pseudorotaxane or the double clipping of an orthogonal complex. A rotaxane may be prepared by clipping of the ring around a stoppered axle or stoppering of a pseudorotaxane.
The most celebrated application of catenanes and rotaxanes is their use as molecular machines due to the possibility of controlled motion of their interlocked components. However, they also represent excellent 3D molecular scaffolds, that might be exploited in a range of chemical applications – including host-guest recognition.
- J.-M. Lehn, Angew. Chem. Int. Ed. Engl. 1988, 27, 89-112.
Suggested Further Reading