Volume 94 Issue 49 | p. 13 | Concentrates
Issue Date: December 19, 2016

Six bonds to carbon: Confirmed

Chemists obtain elusive crystal structure of hexamethylbenzene dication, confirming its nonclassical structure with a six-coordinate carbon atom
Department: Science & Technology
News Channels: Organic SCENE, Analytical SCENE
Keywords: chemical bonding, carbocation, aromaticity, magic acid, norbornyl cation
Pyramidal C6(CH3)62+ contains the first confirmed example of six-coordinate carbon; carbon = gray, hydrogen = white.
Credit: Angew. Chem. Int. Ed.
Structure of C6(CH3)62+.
Pyramidal C6(CH3)62+ contains the first confirmed example of six-coordinate carbon; carbon = gray, hydrogen = white.
Credit: Angew. Chem. Int. Ed.

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, O2- is pulled off to generate C6(CH3)6(SbF6)2•HSO3F. Structural and computational analysis revealed that the molecule retains its aromaticity.

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Russell N. Grimes (Wed Dec 28 15:13:19 EST 2016)
Very interesting and important structure. However, 6-coordinate carbon atoms have been known for 60 years, in the form of thousands of carboranes, such as the icosahedral C2B10H12 isomers which feature two 6-coordinate carbons. It should also be recognized that the C6Me6(2+) dication completes the isoelectronic and isostructural series of pentagonal pyramidal molecules B6H10, CB5H9, C2B4H8, C3B3H7, C4B2H6, C5BH7(+), C6H6(2+), the middle four members of which are carboranes (all known as either parent or alkylated species).

Gareth R. Eaton (Wed Dec 28 17:54:05 EST 2016)
As Russell Grimes said, this is an important structure that fits well in the carborane series (in an email to me, he corrected one formula - "One formula in my comment is incorrect: C5BH7(+) should be C5BH6(+)").
In addition, there are other areas of chemistry in which 6-coordinate C is well-known. 6-coordinate C is known in the center of an octahedron of Au (J. AM. CHEM. SOC. 2009, 131, 16634–16635), and in a C2O4 environment (J. AM. CHEM. SOC. 2009, 131, 16634–16635), just to select two examples.
Our perspective on C should not be limited to the examples in introductory organic chemistry textbooks.
HAIBO XIE (Wed Jan 04 14:44:16 EST 2017)
There is another example of 6-coordinate C which exist in natural enzyme-nitrogenases.FeMo cofactor cluster,the C is coordinated by six Fe.(One Atom Makes All the Difference, Science 18 Nov 2011: Vol. 334, Issue 6058, pp. 914-915 DOI: 10.1126/science.1215283)
David Dodds (Wed Dec 28 16:57:11 EST 2016)
Does this allow any decisions or new insights to be made regarding the two mecahnisms (paring and side-chain methylation) proposed for MTO catalysts?

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