I was at a talk by Professor Richard Schrock recently and he put up some very elegant NMR spectra taken by his researchers. Although not an organic chemist, it was an NMR spectrum I measured during my Ph.D. that remains the single result I am most proud of. This post is a diversion into more technical realms of chemistry and posted as part of #realtimechem week.
A pretty (13C) NMR spectrum?
We were trying to distil the ionic liquid 1-octyl-3-methylimidazolium tetrafluoroborate (see the reaction scheme below), which might not seem a big deal. Distilling liquids is something most chemists will do routinely during their career. But ionic liquids are a bit unusual. For a long time it was thought that most ionic liquids simply did not evaporate and would stay as a liquid until they thermally decomposed. At atmospheric pressure and on a standard vacuum line this is still the case, but heat them up at very low pressures and, eventually, they can be forced to evaporate.
We had distilled a number of other ionic liquids with success and were expecting the same with this particular ionic liquid. Sure enough, we distilled over a few mL of liquid in our custom-built ultrahigh vacuum distillation kit. Then, bizarrely, our nice distilled ionic liquid crystallised (see right)!
NMR, mass spectrometry, elemental analysis were all wrong. Things didn’t smell right- in fact the sample smelt, whereas a pure ionic liquid shouldn’t. Most confusingly, the carbon NMR was “missing” a peak corresponding to the NCN carbon of the imidazolium ring. Starting to piece together the data I had and doing a bit of background reading I became convinced the reaction in the scheme below had occurred during distillation.
We expected the ionic liquid on the left to distil unaltered, but instead at a temperature of 250 °C the cation and anion reacted to the give the carbene-borane on the right which distilled instead.
Going back to the NMR technician I asked if there was a way we could try and resolve the missing peak. There were some suggestions in the literature that in carbene-borane compounds similar to the one I thought I had made, fast relaxation processes supress this peak. The technician suggested we tried running another carbon NMR on our most powerful spectrometer and by increasing the time between the pulse and data acquisition. Sure enough, out of the noise came the beautiful multiplet shown in the spectrum at the top of this post. In fact, the peak is a quartet of quartets caused by splitting of the carbon signal with the adjacent boron and through two bonds to the three fluorine atoms.
This result came from a combination of serendipity and planning ending up with a result perfectly matching a hypothesis. That is why I’m probably most pleased with it. Is it an earth-shattering finding? Probably not, but for me it’s a good example of how science really works, with a bit of luck and a lot of hard work!
The work was published in the paper “Borane-substituted imidazol-2-ylidenes: syntheses in vacuo“.
Reference details: A.W. Taylor, K.R.J. Lovelock, R.G. Jones, P. Licence, Dalton Trans., 2011, 40, 1463-1470.
I will write more about ionic liquids in the future, explaining a bit more about why they are interesting. They were recently voted the number one future innovation in the UK.