Molecular topology: star-crossed self-assembly.

Progress in the field of metal-templated self-assembly has made it possible to design and build supramolecular architectures with increasingly complex structures and topologies1,2. As well as the synthetic challenges and aesthetic appeal associated with such structures, current activities in this area are also driven by the desire to produce functional systems. This can be achieved in a number of different ways, such as through the incorporation of reactive sites or by building redox activity or photoswitchability into the resulting assemblies. The reversible nature of intermolecular interactions such as hydrogen bonding, π–π-stacking and metal coordination allows for, in principle, very high yields of complex supramolecular architectures from relatively small (and simple) molecular building blocks. Such thermodynamically driven non-covalent approaches are in stark contrast with early syntheses of interlocked structures, which were based on statistically controlled covalent protocols that resulted in poor yields2. Nevertheless, there is a downside to the inherent reversibility of the attractive interactions between the subcomponents of a self-assembled structure in that the stability of a system is not guaranteed when it is taken out of the environment in which it was formed. A simple way to dismantle self-assembled structures that are based on metal cations and organic ligands is to dilute the system, thereby causing the position of the equilibrium to shift towards the side of the reactants rather than the product. Kinetic effects can help to stabilize self-assembled architectures from degradation under these circumstances, but problems arise when removal of the metal cations is desired or the complex is exposed to harsh conditions such as high temperatures, strong acids or bases, competing donors or other metal cations. In order to solve this dilemma, a few strategies have been developed. Fujita and co-workers have shown, for example, that coordination bonds to kinetically stable platinum complexes can be temporarily weakened by slightly acidic solvents3 or photoexcitation4 to allow for reversible metal–ligand selfassembly to occur. Once the thermodynamic product has been obtained, the labilizing stimulus is removed and the product is rendered stable towards disassembly. Another approach is one in which a multicomponent superstructure is fixed through a post-assembly covalentmodification step in which a supramolecular system is converted into a molecular one. Now, writing in Nature Chemistry, David Leigh and co-workers have shown5 that a star-shaped supramolecular assembly comprising six tris-bipyridyl ligands and six Fe(ii) cations can be locked together to form a triply entwined [2]catenane by ruthenium-mediated olefin–metathesis reactions (Fig. 1). The resulting molecular structure resembles a hexagram, a geometric shape that is known in many cultural and religious contexts such as Hinduism, MOLECULAR TOPOLOGY