https://www.science.org/content/blog-post/perfluorocubane-you-would-expect-weird Advertisement * * news * careers * commentary * Journals * Covid-19 Science Science * * * Log in * Become A Member [science] science [sciadv] science advances [sciimmunol] science immunology [scirobotics] science robotics [signaling] science signaling [stm] science translational medicine [spj-cover] science partner journals Quick Search anywhere Enter Search Term[ ] Quick Search in Journals Enter Search Term[ ] Quick Search in Journals Enter Search Term[ ] Quick Search in Journals Enter Search Term[ ] Quick Search in Journals Enter Search Term[ ] Quick Search in Journals Enter Search Term[ ] Quick Search in Journals Enter Search Term[ ] Searching: Anywhere AnywhereScienceScience AdvancesScience ImmunologyScience Robotics Science SignalingScience Translational Medicine Advanced Search Search Trending Terms: * covid-19 * climate * monkeypox * abortion * perovskite Log In Become A Member Quick Search anywhere Enter Search Term[ ] science.org * Custom publishing * collections * videos * podcasts * blogs * visualizations * prizes and awards * authors & reviewers * librarians * advertisers * about * help * * * * * * AAAS Logo * Terms of Service * Privacy Policy * Accessibility * Commentary Home * Opinion * Analysis * Blogs GET OUR E-ALERTS HomeCommentaryBlogsIn the PipelinePerfluorocubane Is (As You Would Expect) Weird Back To In the Pipeline * In the Pipeline Perfluorocubane Is (As You Would Expect) Weird * 23 Aug 2022 * By Derek Lowe * 2 min read * Comments Share: * Twitter * Linked In * Facebook * Reddit * Wechat * Email [octafluorocubane] Here's a paper that comes under the heading of "so odd that it can't be passed over". The authors are looking at octafluorocubane, shown at right, because it is predicted to have the ability to hold a free electron in the middle of that cube (!) That's because of the C-F bond properties - all the electron density is being pulled towards the F atoms, so the center of the cube really has room for one to sit in there without running into a lot of interference. You will note the explanatory style that is characteristic of my long-delayed book, "Quantum Mechanics: A Hand-Waving Approach". If you would like to get a bit more technical, there are eight sigma-star orbitals from those C-F bonds all pointing into the center of the cage and overlapping, and giving you effectively a stabilized vacant orbital there. DFT predicts that the molecule should indeed have a strong electron affinity. But that remained to be proven, because up until now no one had made octafluorocubane, either. It was predicted to be stable, because the F atoms are not large enough to cause steric problems (good luck making the octabromo, is all I can say). But making perfluoro cage compounds is an art form which at the same time involves chemistry that may not be compatible with the classic artistic temperament. As in "get ready for elemental fluorine", which means getting ready for working in either fluoropolymer vessels and/or high-nickel alloys, because the fluorine will violently eat everything else while spitting out clouds of hydrofluoric acid along the way, which is only a slight improvement. Even then, you're going to want to "passivate" the reaction apparatus by introducing small doses of elemental fluorine to form inactivating surface layers before going full speed. In fact, even using these materials and doing everything correctly, you're still faced with the likelihood of the fluorine eating your starting material anyway. Taking cubane itself and trying to photochemically substitute it with fluorine gas has in fact been tried, but (as mentioned in passing as an unpublished result in this paper) it led to ring-opened products. It looks like the highest fluorination level reported is just difluorocubane, so the authors had a bit of a climb awaiting them. A newer method uses fluorinated esters as starting materials with dissolved fluorine gas in a perfluorinated solvent at low temperatures, and had been reported to show much less bond cleavage on sensitive substrates. This method took the synthesis up to the hexafluoro and heptafluoro compounds, and the substitution of the last fluorine was actually easier (it's been reported that fluorinating decarboxylation on perfluoroalkyl substrates is a pretty favorable reaction). You know when you've got it: because of the symmetry, the NMR has one 19F peak and one 13C peak, and the IR has only two peaks total (!) The authors report that they could not get a molecular ion in any mass spec under any conditions, which I guess makes sense, too. It's purified by sublimation out of the reaction mixture (as you might guess - symmetrical compounds generally have a higher vapor pressure), and the solid has a melting point in the 160-170C range, which is a lot higher than I might have imagined. The X-ray nails it down completely - all the bonds are symmetrical, with no odd distortions. Even the C-C distance is almost identical to ordinary cubane - the authors believe that the C-C bond shortening due to electronegative substituents (Bent's rule) is being cancelled out by repulsion between the F atoms themselves. The X-ray packing in notable, too. Each single C-F bond points into the center of a cyclobutane face in the adjacent molecules, in something like a "halogen bond" sigma-hole interaction. Under cyclic voltammetry conditons, the compound does show a reduction in the predicted range, but it's irreversible, indicating that product is decomposing at normal temperatures. At very low temperatures (77K, matrix isolation) in an ESR apparatus, though, you can indeed see the spectrum of the predicted "electron in a cube", split just the way that you would have drawn it out. Weirdly, the spectrum also indicates that the radical anion species is rapidly rotating even at such a low temperature. There are possibilities for new and rather odd functional materials in the perfluorocage space, but for now, this will stand as a win for theoretical predictions of bonding behavior, and as the strangest radical ion in all of chemistry. And we got to read about it without ever having to work with any fluorine ourselves! It's a win-win. About the author Derek Lowe Derek Lowe emailTwitter Derek Lowe, an Arkansan by birth, got his BA from Hendrix College and his PhD in organic chemistry from Duke before spending time in Germany on a Humboldt Fellowship on his post-doc. He's worked for several major pharmaceutical companies since 1989 on drug discovery projects against schizophrenia, Alzheimer's, diabetes, osteoporosis and other diseases. --------------------------------------------------------------------- Comments Please enable JavaScript to view the comments powered by Disqus. IN THE PIPELINE Derek Lowe's commentary on drug discovery and the pharma industry. An editorially independent blog, all content is Derek's own, and he does not in any way speak for his employer. Advertisement YOU MAY ALSO LIKE 25 Jul 2022By * Derek Lowe Faked Beta-Amyloid Data. 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