Three Rings in an Inseparable Union

A macrocyclic molecule having the topology of Borromean rings has been prepared from 18 components using a “mix the pieces together and shake them all about” approach by chemists at the University of California, Los Angeles, and the University of Missouri, Columbia [Science, 304, 1308 (2004)].

Borromean rings, whose use on the crest of the Italian Borromeo family can be traced back to the 15th century, comprise three mechanically interlinked rings that are inseparably united, although no two rings are linked. When any one of the rings is cut, the other two separate.

“The realization of the Borromean link in a wholly synthetic molecular form has long been regarded as one of the most ambitious and challenging targets in topological chemistry,” comments chemistry professor J. Fraser Stoddart, director of the California NanoSystems Institute at UCLA.

Stoddart carried out the work with a team that included chemistry lecturer and research associate Stuart J. Cantrill and postdoc Kelly S. Chichak at UCLA and chemistry professor Jerry L. Atwood at Missouri.

“The final assembly amounts to a ‘mix-and-heat’ approach that takes less than a day in an alcohol solution,” Cantrill says. “We bring together 18 components—six dialdehydes, six diamines, and six zinc ions—and in the process form 42 bonds in what appears to be an almost quantitative process.”

“We can run this reaction on gram scales using simple and traditional chemistry right up to the last step,” he continues. “In this final step, the six zinc ions template the novel topological architecture, and we employ dynamic covalent chemistry to assemble the final product.”

The authors suggest that their Borromean ring compounds could be exploited as highly organized nanoclusters in spin-electronic materials or for medical imaging.

“We are exploring what we can do with a molecule that can locate precisely six metal ions in an insulating organic framework,” Chichak says. “Although zinc is fairly dull in terms of redox chemistry, copper and cobalt are not, and we are making Borromean rings with these metals.”

In a related development, Volker Böhmer, a chemist at Johannes Gutenberg University in Mainz, Germany, and coworkers report in the same issue of Science (page 1312) the synthesis of a complex interwoven [8]catenane. The multicatenane comprises two belts, each with four rings, attached to the wide rims of two cuplike molecules known as calix[4]arenes. The belts are interwoven in such a way that each ring of one belt penetrates two adjacent rings of the other belt and vice versa.

Jay S. Siegel, a chemistry professor at the University of Zurich, notes in an accompanying Science commentary that topological chemistry is now reaching the stage where chemists can confidently design and synthesize molecules of complex topology.

Last year, Siegel and coworkers reported the synthesis of an orthogonal two-ring polypyridine compound that could be a precursor for a molecular Borromean ring compound [Angew. Chem. Int. Ed., 42, 5702 (2003)]. The threaded two-ring structure has metal-binding sites that are compatible with the threading and knitting together of the third ring.

“The flurry of activity directed toward new topological chemical objects signals that the time is ripe for exploiting their functional potential,” Siegel concludes.