Pyramidal carbon

15 January 2017 - Molecule of the year 2016

pyramidalcarbonMalischewski2016.PNGThanks to the New Scientist article here for reminding us but the discovery of a pyramidal carbon dication by Malischewski and Seppelt (DOI) must be the chemistry event of the last year (work published 26 November, yeah this blog missed it completely). It is common knowledge that carbon will partner in chemical bonds with up to 4 partners but definitely not more. In fact, if a molecule is depicted in any way with 5 bonds going away from it (clearly an error made) the author is met with ridicule and a specific reserved phrase : Texas carbon, a reference to the Texas five-pointed lone star. As it turns out this ridicule is misplaced! It is possible to create Texas carbon molecules after all. Malischewski and Seppelt made theirs starting from the Dewar benzene of hexamethylbenzene (by a cyclotrimerisation of 2-butyne with aluminum chloride). This compound was epoxidized and then reacted with magic acid. This very strong acid basically strips away O2- leaving behind C6(CH3)62+(SbF6-)2. Read the supplementary info to learn the very thin line between reaction success and reaction black tar. The crystal end-product (produced by adding HF and cooling) is stable at -78°C ready for X-ray diffraction. And how to describe the molecule? It is the adduct of the pentamethylcyclopentadienylium cation and the 1-ethylium-1-ylidene cation CH3C+ cation with 6 bonding electrons shared between them. There is of course the issue of precedence, the dication was already observed by NMR in 1973 (DOI) but an actual crystal structure is always a next step.