Whether it’s actually true or not, most of us can’t help being charmed by the snake-biting-its-own-tail story of how Kekule figured out the structure of benzene (as well as the associated Ouroboros symbolism). And I’d be willing to bet that if you sat down with the the latest issue of your favourite chemistry journal (although who does that these days?!) the vast majority of the chemical structures drawn out in figures and schemes would contain at least one ring. Chemists seem to have a thing for rings, and I’m no exception.
Why do I bring this up? Well, I thought that now is as good a time as any to explain what the image at the top of this blog is all about. I’ve always been fascinated by molecules that have unusual topologies – particularly molecules that are made up of two or more components that are held together because they are mechanically interlocked rather than being covalently bonded to one another.
Perhaps one the simplest types of interlocked molecule is a catenane – shown schematically as structure A below. Molecules with this sort of topology are relatively easy to make. You start by making one of the rings (let’s say the red one) and then you thread a linear precursor of the blue ring through the red ring and then tie the ends of the blue thread together to make the blue ring. By using the principles of host–guest chemistry and designing your system so that the red ring and blue thread exhibit mutual molecular recognition, it is possible to make catenanes in very high yield. If you string five rings together like this, you get a compound that looks a bit like something we’ll be seeing a lot more of next year – especially here in the UK.
Another interesting thing about structure A is that it has a non-planar graph. What that means is that no matter how much you stretch or bend either of the rings, you cannot draw the structure in two dimensions without one line crossing another. Most molecules do have a planar graph; sure, you’ll need to bend and elongate bonds to chemically meaningless degrees, but you can draw a net in which none of the bonds cross one another (the nodes are the atoms and the lines that join them are the bonds). Hey, even C60 has a planar graph!
So, if catenanes are fairly straightforward, how about another — slightly more intricate — topology. Structure B above is also known as the Borromean rings, and you can find out a lot more about them and their history here and here. Now, to make these in molecular form is quite a challenge; it’s not just a case of threading a linear molecule through a macrocyle and tying up a few loose ends. And why is it so much harder? Let’s take a closer look at the topology.
Look again at B. Those three rings are interlocked, you can’t separate them just by pulling or twisting on them – and they definitely have a non-planar graph. Now, imagine taking a pair of scissors and snipping the red ring. Pull on one end of what is now a red thread (rather than a ring) and it can be removed from the ensemble. Notice what you have left? A green ring and a blue ring that are not interlocked in any way; they can be simply pulled apart. Start over and do the same thought experiment with your imaginary scissors, but snip either the blue or the green ring. You’ll soon realise that it doesn’t matter which ring you snip, breaking just one of the rings leads to all three unravelling.
What this means, is that no pair of rings in B is catenated (like they are in A), so the strategies applied to making a molecular version of A won’t work for making B. Anyway, this post is getting long and so I won’t go into the gory details of how we did it, but suffice to say that I was part of a team that made a molecular version of the Borromean rings. We weren’t the first, but we made the smallest! If you want to know more (and have access), the original paper is here and a handy review article explaining the synthetic strategy is here.
The X-ray crystal structure of our molecular Borromean rings is shown in C, from which the banner at the top of this page is taken. Just a side note (and probably the topic for a future post), I’m a big fan of making molecules just to make molecules — particularly challenging and funky ones like these — and this project didn’t receive any funding. It was one of those fun side projects in the lab.
And by the way, the molecular Borromean rings aren’t hard to make. We optimised the synthesis on a gram-scale for an undergraduate laboratory course (if you have access, the J. Chem. Educ. paper is here).