The creation of well-known macroscopic objects in molecular form is something that usually leads me to mutter under my breath and roll my eyes. Every now and then, a paper or a press release will herald the synthesis of a molecular ‘X’, where X = pretzel, wheelbarrow, car, tie fighter (yes, really), and so on — I’m not going to link to these, your favourite search engine will do you proud if you really want to know more about these things. In some cases, if you tilt your head to one side and perhaps squint a bit, you can maybe understand the analogy; and admittedly, sometimes the analogy is one of function rather than form. Because of a certain major sporting event that is happening in London at the moment, however, I’m going to make an exception — and be slightly less grumpy about such things.The iconic image associated with the modern Olympic Games is one that many people know well — it consists of five rings linked together to form a chain. I presume it would be OK to reproduce the symbol here on the blog, but I’m not taking any chances, considering the somewhat overzealous manner in which the Olympic family protect their rights… So, I’ve put a bit of a chemistry twist on it: look, hexagons! (And it’s actually easier to draw in ChemDraw that way too…).
Our first competitor (over there on the left) is a molecule that has been named ‘olympicene’ (Wikipedia link here) — and it has received a fair amount of coverage over the past few months, so I’m not going to describe the project in any detail (follow those links for lots of great stories). Particularly striking, however, are some of the AFM images of the actual molecule, which can be found on Flickr. As pointed out by Prof. Martyn Poliakoff of the Periodic Table of Videos, however, it’s fairly obvious that the five six-membered rings in olympicene are not linked together as they are in the Olympic logo. They are fused together to form something that, at first glance, has a passing resemblance to the Olympic rings.
If you look closely at olympicene, you will see that the top-middle ring is fused with all of the other four rings. If you number the individual rings from left to right, the structure has a row of three fused rings along the top (1, 3 and 5) and two fused rings at the bottom (2 and 4). Go back and look at the Olympic rings themselves, and you’ll see that (using the same numbering scheme), ring 1 is only connected to ring 2, ring 2 connects with rings 1 and 3 and so on. Perhaps picene (shown above to the right – Wikipedia link here) would be a better match for the actual Olympic rings?
Alternatively, how about making a molecule that contains five interlocked rings — just like the Olympic symbol? This idea was actually first proposed way back in 1960 by van Gulick (more on this at the end of the post) and the experimental realization was reported in the international edition of Angewandte Chemie in 1994. It came out of the labs of Fraser Stoddart (disclaimer: I did my PhD with Fraser and worked is his lab for many years, so I have a soft spot for interlocked molecules), but the main protagonist stood at the lab bench was David Amabilino, who is currently at the Institut de Ciència de Materials de Barcelona. The title of the paper is simply ‘Olympiadane’ (don’t you love scientific papers with one-word titles?). The structure of this catenane is shown below – Wikipedia link here.
This molecule obviously has the same topology as the Olympic rings, but looks very different on paper (and also in the X-ray crystal structure, which was reported in this JACS paper). Each of the five rings is made up of smaller rings joined together to form closed loops. And three different types of ring make up olympiadane. Rings 2 and 4 (the red ones) are the same, as are rings 1 and 5, but ring 3 (while similar to rings 1 and 5) is different yet again. So, if we’re giving out medals for molecules that best resemble the Olympic rings, which molecule (picene, olympicene or olympiadane) gets the gold, and where do silver and bronze go? As I pointed out above, I’m biased, so I’ll leave it to you lot to fight it out in the comments.
And now back to van Gulick… he wrote a manuscript back in 1960 about ‘theoretical aspects of the linked ring problem’ that was circulated as a preprint, but ultimately rejected from the journal Tetrahedron for ‘not being chemistry’. The paper was finally published in the New Journal of Chemistry in 1993 (I can’t find this online anywhere, but the reference is New J. Chem. 17, 619–625 (1993) if you are interested) and in the same issue, there is a preface written by David Walba explaining the story behind the van Gulick paper (again, I can’t find that either, but it’s page 618). Funny how times change, and what was dismissed as ‘not chemistry’ over 50 years ago, is now being used to promote chemistry in the mainstream media!
Stuart Cantrill (Chief Editor, Nature Chemistry)