Nanorings Synthesized Simply

A circular templating approach yields 4.7-nm-diameter macrocycles from small, easily prepared templates, reports a group of British researchers (Nature, DOI: 10.1038/nature09683).

The method is based on Vernier complexes, which are precise assemblies generated by the coming together of components with different numbers of binding sites—for example, different four- and six-membered building blocks. Past experiments with Vernier systems, however, have all involved linear complexes. In the new work, a group led by Harry L. Anderson, a chemistry professor at Oxford University, experimented with radial templates and linear building blocks to produce 12-membered porphyrin nanorings.

Realizing that Vernier complexes can be circular “is a significant step forward in the science of template-directed synthesis,” says J. Fraser Stoddart, a chemistry professor at Northwestern University. The technique should enable and ease the preparation of macromolecules with consistent size and shape, he adds.

Anderson and coworkers used a circular hexapyridyl template with six binding sites. They combined the template with four-membered zinc porphyrin chains, with the porphyrins linked by butadiyne groups. Three porphyrin tetramers can bind around and occupy all of the binding sites on two of the templates, linking the two in the process. The researchers then oxidatively coupled the template-bound porphyrin tetramers to form a figure-eight compound. Dissociating the porphyrins from the templates produced a 12-membered, π-conjugated porphyrin nanoring, among the largest of similar macrocycles ever synthesized.

The researchers were able to prepare the same ring using a single dodecapyridyl template, but they note that the larger template required a 10-step, low-yielding synthesis, while the smaller template can be made in two steps. It was also more difficult to purify rings synthesized from the larger template.

The new Vernier approach could lead to even larger cyclic structures. Mathematically, if a template has n binding sites and the building block has m binding sites, the number of interactions in the resulting Vernier complex should be equal to the lowest common multiple of n and m. Combining the six-site template with porphyrin pentamers, for example, should produce a 30-membered ring.

The method put forth by Anderson and colleagues “shows how the careful use of templates allows for the control of reactivity and the formation of elegant and complex structures whose synthesis lies at the frontier of our current conception,” says Jonathan L. Sessler, a chemistry professor at the University of Texas, Austin. “The novelty, elegance, and intellectual power of the Vernier templating approach, as well as the apparent versatility of the chemistry in question and the pure aesthetic appeal of the product systems that Anderson and coworkers have obtained, are sure to fire the imagination of chemists everywhere.”