Chemical Communications Blog

Non-covalent interactions dictate the assembly of many of nature’s most elegant structures. Similarly, supramolecular chemists have long been intrigued by the challenge of designing functional structures that spontaneously self-assemble from simpler fragments which mutually recognise each other.

A popular self-assembly approach is to produce coordination compounds from transition metal salts with rigid organic ligands. Directional bonding around transition metal centres allows the production of predictable and controllable shapes. Michael Schmittel’s group at the University of Siegen have been exploring a newer approach. They prepared two assemblies, a 2-component triangle T1 and a 3-component rectangle R1. The transition metal “corner” arrangements in T1 and R1 are disfavoured, so if the two assemblies are combined the components re-shuffle to form a more favourable assembly- the 5-component triangle T2. The transformation occurs at room temperature, and can be completed in just 1 hour in the presence of a catalyst, which accelerates the re-shuffling by labilising the metal-ligand bonds.

Unlike previous examples, the conditions needed for the transformation are very mild. The authors compare the process to gene shuffling, the combination of dissimilar genes to form new genetic material. The strategy could be considered a first step towards the evolution of supramolecular architectures, and a great route to more complex supramolecular assemblies with higher information content.

The full communication can be downloaded here (free to access for a limited period).

Cally Haynes

Researchers in Texas are using rotaxane formation to sterically protect or “insulate” molecular wires.

Molecular wires, in which an unsaturated linker separates two or more redox active metal sites, are of great research interest. These structures allow phenomena such as electron delocalisation or transport between the two redox sites. John Gladysz’s group at Texas A&M University have an ongoing interest in dimetallic polyynediyl complexes, in which two metal centres are linked by conjugated polyynediyl linkers that they now hope to “insulate” to reduce interactions between wires and the external environment. A previous approach used long alkyl bis-phosphine, which wrapped around the wire in a double helix to complex both metal centres. However, this gave two enantiomers, which interconverted rapidly in solution via uncoiling of the protective ligands.

The Gladysz group are now reporting a straightforward solution to this problem. They found that by synthesising their bis-platinum wire in the presence of a 33-membered macrocycle, they could incorporate the wire as the thread of a rotaxane complex. This provides a more robust protection for the wire which is unaffected by dynamic processes.

This work shows a fantastic application of rotaxane chemistry for protection of a molecular wire. What’s more, the synthesis of this rotaxane is adaptable, and the Gladysz group are working on exciting new and improved systems including longer polyynediyl linkers and redox inactive macrocycles to improve the properties of the insulated wires.

The full communication can be downloaded here.

Cally Haynes

A recent communication from Isao Azumaya, Aya Tanatani and colleagues describes their work on some macrocycles based on cyclic triamide 1. The bowl-like structure of these macrocycles is reminiscent of well-known supramolecular building blocks such as calixarenes, and the authors hope that they might be similarly applied to constructing supramolecular architectures. These macrocycles have three possible inter-convertible conformations, of which the most stable syn- forms are enantiomeric.

The group found that a number of triamide-substituted macrocycles such as 2 dimerise into capsule-like structures in the solid state through interactions between the amide groups. Remarkably, 2 formed chiral crystals, as the macrocycles dimerised in the syn-form and only with their own enantiomer. This is the first report of the separation of cyclic triamides without using an additional chiral species, and shows that this particular macrocycle is able to recognise its own enantiomer (in the figure below single enantiomers are shown in the same colour).

In solution, a tricarboxylate-substituted macrocycle was shown to interact with a chiral amine guest. This caused induction of chirality in a host in which, in equilibrium in solution, no particular enantiomer is usually favoured.

These simple macrocycles are easy to synthesise and functionalise and have fascinating dynamic behaviour in solution and in the solid state. They are an exciting prospect for application as molecular building blocks with the added potential for host-guest chemistry.

Find out more – download the communication for free for limited period.