Six bonds to carbon: Confirmed

Among chemistry’s most fundamental concepts is that carbon is tetravalent and forms four bonds to other atoms. And when it comes to aromaticity, benzene’s hexagonal planar ring structure is taken as gospel. Chemists have thus been fascinated by finding exceptions to these truths. One example is hexamethylbenzene dication, C₆(CH₃)₆²⁺, a molecule first prepared in 1973 and suspected of not only having a six-coordinate carbon atom but also of being nonplanar. But the structure has only been inferred by spectroscopy. Moritz Malischewski and Konrad Seppelt of Free University of Berlin have now reported the first X-ray crystal structure for C₆(CH₃)₆²⁺, confirming it has a pentagonal pyramidal C₆ framework with a six-coordinate carbon at the apex of the molecule (Angew. Chem. Int. Ed. 2016, DOI: 10.1002/anie.201608795). Like other nonclassical carbocations, such as CH₅⁺ and the norbornyl cation (C₇H₁₁⁺), experimental proof of the C₆(CH₃)₆²⁺ structure proved challenging. The team first prepared an epoxide of hexamethyl Dewar benzene (a bicyclic benzene isomer) and then dissolved the epoxide in magic acid (HSO₃F/SbF₅) and added anhydrous HF at low temperature. Under these superacidic conditions, O^2- is pulled off to generate C₆(CH₃)₆(SbF₆)₂•HSO₃F. Structural and computational analysis revealed that the molecule retains its aromaticity.