The use of metal ions or small molecules to template the synthesis of other compounds is a well-established principle of host–guest chemistry. The templating entity gathers together the required building blocks and organizes them into a specific arrangement such that their covalent attachment to one another results in the formation of a favoured target structure — one that is often difficult to make in the absence of the template. For example, templates are quite useful when it comes to the synthesis of large cyclic compounds — the final ring-closing step is much more likely to occur if the ends of a long linear precursor are held in close proximity on a template rather than waiting for a chance meeting in an untemplated reaction.
Led by Harry Anderson, a team of researchers at the University of Oxford in the UK and Aix Marseille Université in France and have now shown how cyclodextrins decorated with pyridyl groups can be used to template the synthesis of zinc-porphyrin nanorings. Cyclodextrins are naturally occurring macrocyclic compounds made up of six (α), seven (β) or eight (γ) repeating glucose residues. Anderson and co-workers demonstrate that the relatively flexible α-cyclodextrin-based template is almost as effective at orchestrating the synthesis of a hexameric zinc-porphyrin ring around its outer rim (pictured) as a rigid radial oligopyridyl template used in previous studies. A β-cyclodextrin analogue — with its seven repeating glucose units — was found to template the formation of a nanoring made up of seven porphyrins, albeit with a lower yield.
The availability of a molecular scaffold that can present a prime number of coordination sites in a symmetrical arrangement should enable very large porphyrin rings to be made through so-called Vernier templating approaches, in which different building blocks with mismatched numbers of binding sites are combined. Moreover, the use of cyclodextrin-based templates means that the cavities of these well-known macrocyclic molecular hosts could be filled with guest species to modulate the properties of the porphyrin-nanoring–cyclodextrin assemblies.
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Cantrill, S. Running rings around rings. Nature Chem 6, 656 (2014). https://doi.org/10.1038/nchem.2027