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Amy Hayden
BA/BS Ohio NorthernUniversity
Borromean-styled rings have been of interest for thousands of years in Christian, Norse, and Asian art and iconography.[1] They acquired their name from the wealthy Italian Borromeo family of the Renaissance period. Their family crest bears the symbol of these peculiar interlocking rings.[1] One sees on closer inspection that if one ring is severed, the other rings are no longer held together, serving as a symbol of the importance of strength in unity. From a scientific (and more recent) perspective, interest has been shown in this topology by physicists[2] and by mathematicians investigating knot theory.[3] Even more recently, chemists have embarked on the synthetic journey of making molecular Borromean rings. Seeman4 was the first to have success in the synthesis of such rings, constructing them with the Borromean topology from three different strands of DNA.The recent convergence of supramolecular and dynamic covalent chemistry offers the ability to overcome some current synthetic challenges in more efficient ways.[5] If one combines these concepts with molecular recognition and self-assembly, complex mechanically interlocked molecules can be successfully synthesized in high yields due to molecular ‘proof-reading’ and ‘error-checking’ processes. For example, if an ‘incorrect’ product were made during an equilibrating process, the components would fall apart and return to the equilibrating reaction mixture in order to form eventually the desired, most thermodynamically stable product. Naturally, a system must be designed with tremendous care if one expects to carry out a successful synthesis. Computational chemistry is employed to determine such factors as the correct spacings between recognition sites, an activity which has been done jointly in the Houk and Stoddart research groups at UCLA. The method described above, combining dynamic covalent chemistry, self assembly, and supramolecular chemistry, was utilized recently by the Stoddart group to synthesize molecular Borromean rings using an 18-component template directed synthesis.[6] Six tridentate 2,6-diformylpyridine molecules, six bidentate diamino molecules, and six transition metals ions (in this case zinc(II) derived from zinc acetate) participate in this synthesis. It is apparent that this self-assembly protocol had to be carefully designed. This synthesis is possible due to the coordination of the transition metal ions to the ligands, which provides a template for the three macrocycles to form. Other noncovalent interactions such as hydrogen bonding and π-π stacking also play a cruicial role in forming the three rings with the desired six cross-over points common to many depictions of Borromean rings, molecular or otherwise. One could speculate that a more traditional stepwise approach to a synthesis of this type could be envisioned, but it is highly unlikely that high yields would be obtained from such a stepwise approach. The success of the self-assembly of a nanoparticle of this nature allows chemists to envision the synthesis of other particles with interlocking rings.
There are many other possibilities that could be attempted in addition to what has previously been done. As a joint member of the Houk and Stoddart groups, it will be of interest to see how computational chemistry can aid in the success of the Borromean rings project as well as other active projects in the Stoddart group.
References
1. Cromwell, P.R.; Beltrami, E.; Rampichini, M. Math. Intelligencer 1998, 20(1), 53.
2. Danilin, T.R.; Vaagen, J.S.; Thompson, I.J.; Zhukov, M.V. Phys. Rev. C 2004, 69, 24609.
3. Coxeter, H.S.M. Math. Intelligencer 1994, 16, 25.
4. Seeman, N.C. Angew. Chem., Int. Ed. 1998, 37, 3220.
5. Rowan, S.J.; Cantrill, S.J.; Cousins, G.R.L.; Sanders, J.K.; Stoddart, J.F. Angew. Chem., Int. Ed. 2002, 114, 938.
6. Chichak, K.S.; Cantrill, S.J.; Pease, A.R.; Chiu, S.-H.; Cave, G.W.V.; Atwood, J.L.; Stoddart, J.F. Science 2004, 304, 1308.More About Amy
I am originally from the Pittsburgh area, went to undergrad in Ohio/France.…and am now here in LA pursing my PhD. My current project is involving Borromean rings.
