Did you know that carbon was allegedly found to engage in six bonds? The strange hexamethylbenzene dication has attracted significant attention due to structure featuring a hyper-coordinated carbon. But can carbon be hypervalent?

Interactive 3D model of the hexamethylbenzene dication

To add more confusion, this system is apparently like a transition-metal complex – though no metal can be found. If you want to make this at home, you just need some equally cursed Dewar benzene, one of the strongest super-acids we know, some hydrofluoric acid for good measure – and the utmost care to not turn your reaction into black tar. Here, we explain if this cursed molecule is an exception to the octet rule.

Octet rule definition

To refresh our memory, elements in the second period typically form compounds that satisfy the Octet rule which states that atoms strive to surround themselves with 8 valence electrons or 4 bond equivalents. However, once we go to lower periods, larger elements start to misbehave. For example, the halogen fluorine can’t expand its octet, but hypervalent iodine compounds like the Dess-Martin periodinane oxidant are common. Similarly, nitrogen is limited by the octet rule but it’s heavier and strange cousin antimony can form more than the four permissible bonds.

But what about carbon, the favorite element of organic chemists? Does carbon follow the octet rule?

We have previously discussed several exotic carbon species, like carbon 2+. This compound is an exception to the Octet rule but it’s hypovalent, not hypervalent. It’s made by oxidizing an already electron-deficient carbene, leaving it with only four instead of the desired eight valence electrons. To pick another example, we’ve also talked about carbones when looking at a dual Diazo-Wittig reagent. This stabilized bis-ylide looks very strange but is not out of order if you count its valence electrons.

CH5+ (Methanium) and 2-Norbornyl cations

To understand other compounds where carbon appears to have more than four bonds, we need to watch out for non-classical bonding. The simplest case, if you can call it simple, is protonated methane or CH5+. On first sight, you might think this is a pentavalent carbon – but beware, as carbon only has four available orbitals. The solution is that instead of having 5 2-center 2-electron bonds, two hydrogens are bound as H2 and partially share their sigma bond with carbon in a 3-center 2-electron bond. This means CH5+ looks more like H2 coordinated to CH3+.

Because of their elusive nature, such non-classical cations were typically not isolated and just observed with methods like spectroscopy. One major breakthrough was in 2013, when a team of chemists finally managed to isolate the 2-norbonyl cation. It was long debated whether this was a rapidly equilibrating classical cation or rather non-classical, penta-coordinated carbon. The chemists cleverly used bromide abstraction from this precursor to generate the cation.

The byproduct bromoaluminate anion nicely stabilizes this species, allowing the growth of, as they put it, slightly brownish but nearly colourless crystals. These were extremely labile towards normal atmosphere and room temperature, so the x-ray analysis had to be highly controlled at low temperatures of just 120 and even 40K.

Still, they somehow made it work and confirmed the non-classical, symmetrical structure. The C-C double bond interacts with the primary carbocation through a 3-center 2-electron bond. As you would expect, this interaction is longer than a normal C-C single bond.

The Hexamethylbenzene dication

So compared to the disputed norbornyl cation, the hexamethylbenzene dication received much less attention. The cursed structure was however directly proposed upon its first synthesis 50 years ago, which I find quite impressive, based on just  NMR and some ancient computational methods. Still, it wasn’t clear which   structure was present – and we still need to answer if the carbon is truly hypervalent here.

Fast forward to 2017, chemists finally rose to the challenge. Similar to the norbornyl cation, the key task was to stabilize the cation in a crystalline matrix that allowed characterization via x-ray analysis.

Synthesis of a hexa-coordinated carbon

You might think that the simplest way of generating the cation is to take hexamethylbenzene and oxidize the living hell out of it. This would be too easy for such an epic compound, so it doesn’t work. Instead, the strained, high-energy Dewar benzene isomer is more susceptible to the intended manipulations.

It looks strange itself but can be made by bicyclo-trimerization of alkynes through [2+2] additions with aluminium trichloride as a Lewis acid. Oxidizing one of the double bonds is pretty simple, with one equivalent of perbenzoic acid delivering the epoxide. If we count the atoms, we simply need to remove O2- to get to the correct sum formula.

Magic Acid for O2- abstraction

This calls for magic acid. This is a mixture of fluorosulfuric acid, a super acid with a pKa equivalent of -10, and the hypervalent antimony pentafluoride. This Lewis acid forms very stable adducts with anything that has electron pairs. This leads fluorosulfuric acid to react with itself, generating an acid so devilish it can protonate the C-H bond in methane to give CH5+, the compound we’ve talked about earlier.

The reaction is performed at extremely low temperatures given the dication instability. It requires masterful rinsing of frozen dewar benzene epoxide with the super acid mixture. If you swirl too fast – whatever that means – your precious reactants will turn into black tar. You know what we are missing? Of course, after forming the dication, we apparently need to add an excess of anhydrous HF to crystallize it out after some days at -80 °C. I like how the authors wrote ‘in the aftermath’, indicating this reaction is a wicked battle that will require attempts to get it just right.

But how does this work mechanistically? It’s just guessing, but likely, the protonation of the epoxide triggers a rearrangement from the Dewar benzene framework into the bridged five-membered ring. The positive charge ends up at the top of the pyramid, stabilized by a 3 center 2 electron bond, reminiscent of the 2-norbornyl example we covered.

This intramolecular rearrangement is very fast, but remember we are still in a soup of excess super-acid. Thus, another protonation finally gets rid of the oxygen, giving another carbocation. In similar fashion, we again oxidize the top carbon and create a cyclopentadiene at the bottom of the pyramid. This dication crystallizes with an entourage of two SbF6 anions and one molecule of fluorosulfuric acid.

Crystal Structure of the Hexamethylbenzene dication

The apical carbon binds the methyl group on the top and seems to interact with all five carbons of the pyramid base at a distance of roughly 1.7A. This is quite a bit longer than normal C-C single bonds, so you should not be surprised that these are not fully-fledged, classical two-electron bonds.

Based on orbital computations, the multi-center p-electron interactions can be seen as having bond orders of roughly one half each. In total, the apical carbon experiences just below four bond equivalents. This means that just because it’s hexa-coordinated does not mean its hexavalent.

By the way, the carbon NMR spectrum is pretty interesting – we see the ring carbons, the apical carbon, the five methyl groups at the base and at a negative chemical shift, the top methyl group which is extremely shielded. The chemists initially proposed that the system corresponds to a cyclopentadiene cation interacting with an ethyl-ylidene cation through a total of six electrons in the pyramid.

However, subsequent research showed that there might be more than meets the eye here. Based on some fancy computations of effective oxidation states, they believe the cyclopentadienyl ligand is not cationic. Instead, it’s an aromatic cyclopentadiene donor anion and the top group is actually a formal methyl cation . This means the central carbon behaves like a transition metal with two modes of coordination. On one hand, it binds a donor anion as a Lewis acid, and on the other it donates an electron pair to the acceptor cation.

Overall, this is an awesome system that remarkably, is stable enough for crystallization and x-ray analysis. It definitely adds to the list of strange carbon species.

Thanks for reading!

References

• Crystal Structure Determination of the Pentagonal-Pyramidal Hexamethylbenzene Dication C6(CH3)62+ | Angew. Chem. Int. Ed. 2017, 56, 368
• The Pentagonal-Pyramidal Hexamethylbenzene Dication: Many Shades of Coordination Chemistry at Carbon | Chem. Eur. J. 2018, 24, 12340
• On the Aromaticity and 13C-NMR Pattern of Pentagonal-Pyramidal Hexamethylbenzene Dication [C6(CH3)6]2+: A {C5(CH3)5}−–{CCH3}3+ Aggregate | Chemistry 2021, 3, 1363
• Crystal Structure Determination of the Nonclassical 2-Norbornyl Cation | Science 2013, 341, 62
• Structure and charge distribution of the (CH)62+ dication | Tetrahedron Letters 1973, 14, 1671
• Direct observation of a remarkably stable dication of unusual structure: (CCH3)62+ | Tetrahedron Letters 1973, 14, 1665