Henry Rzepa's Blog

Science is rarely about a totally new observation or rationalisation, it is much more about making connections between known facts, and perhaps using these connections to extrapolate to new areas (building on the shoulders of giants, etc). So here I chart one example of such connectivity over a period of six years.

The story starts with this article[1], a preview talk about which (Hypervalent Carbon Atom: “Freezing” the S_N2 Transition State) I actually saw at an ACS conference a year or so earlier. When the article was published, Steve Bachrach blogged about it, noting the claim for pentavalent carbon. The semantics of a valency vs a coordination are subtle, and I was not convinced that this frozen transition state deserved its elevation from penta-coordinate to pentavalent. After some discussion on Steve’s blog, I built upon these ideas with a few thoughts of my own on the present blog and then wondered whether they could be finally distilled into a more formal publication (testing the precedent in some ways of whether collaborative and public discussions of ideas could be published formally, or whether they would be rejected as having been already “published”). Well, these final distilled thoughts were indeed published in 2010[2], including their genesis in Steve’s blog (I wanted to put blogs more firmly into the acceptable scientific circle). This article included one species (numbered 5 in that article in 2010[2]) and pointed out an analogy to replacing CH²⁺ by e.g the isoelectronic BH¹⁺, in as much as an example of the latter is indeed known as a stable crystalline compound.[3]. Iso-electronics is a very fruitful source of connections in chemistry!

Matters rested there until yesterday, when I spotted this on Steve’s blog where he discusses this recent article on the structure of the benzene dication.[4] Hey presto, there is that molecule again, but now there is firm experimental evidence of its existence! It was I think rather too much to expect the authors of this article to have spotted the connection to mine (although as it happens, both address the issue of complexes to He). The relationship between CH²⁺ and BH¹⁺ is a little more subtle. From my point of view, it is always worth trawling through the crystal structure database in favour of evidence for hypothetical species (or their isoelectronic substitutions), and so it proved in this case!

There are other connections possible. Thus the dication of benzene has a (higher energy) isomer which is in fact a 4π antiaromatic species which avoids this antiaromaticity by a geometric distortion, with two C-H bonds bending above and below the ring. Such avoided antiaromaticity has been noted elsewhere here.

There is one final connection for me to make. My 2010 article[2] contained one of my interactive tables containing the data for the various structures (yes, although its data, you will need to have a subscription to the journal to access it). As it happens, last year we wished to reprise this style of publication, but as I blogged at the time, the journal had changed its production processes, and they could no longer offer me that opportunity. Some quick thinking came up with a replacement, which we now use extensively.[5] So the chain of connections resulting from that original talk some six years ago continues.

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p>As for that chain, it arose distressingly randomly. I do not routinely read the entire ToC of JACS and so would not have discovered[4] the connection by that route. Fortunately, Steve Bachrach does and helped me make that connection to the molecule shown above. Although I did spend a few minutes thinking to myself “does that structure ring any bells?”. Fortunately, one did (eventually) ring. But for every connection made in this wonderfully human manner, I cannot help but think how many are not! However, if connections were much easier to make, could we as humans cope with the overwhelming deluge of new ideas?

References

1. S. Pierrefixe, S. van Stralen, J. van Stralen, C. Fonseca Guerra, and F. Bickelhaupt, "Hypervalent Carbon Atom: “Freezing” the S_N2 Transition State", Angewandte Chemie International Edition, vol. 48, pp. 6469-6471, 2009. http://dx.doi.org/10.1002/anie.200902125
2. H.S. Rzepa, "The rational design of helium bonds", Nature Chemistry, vol. 2, pp. 390-393, 2010. http://dx.doi.org/10.1038/nchem.596
3. C. Dohmeier, R. Köppe, C. Robl, and H. Schnöckel, "Kristallstruktur von [Cp★BBr][AlBr4]", Journal of Organometallic Chemistry, vol. 487, pp. 127-130, 1995. http://dx.doi.org/10.1016/0022-328X(94)05089-T
4. J. Jašík, D. Gerlich, and J. Roithová, "Probing Isomers of the Benzene Dication in a Low-Temperature Trap", Journal of the American Chemical Society, vol. 136, pp. 2960-2962, 2014. http://dx.doi.org/10.1021/ja412109h
5. A. Armstrong, R.A. Boto, P. Dingwall, J. Contreras-García, M.J. Harvey, N.J. Mason, and H.S. Rzepa, "The Houk–List transition states for organocatalytic mechanisms revisited", Chem. Sci., vol. 5, pp. 2057-2071, 2014. http://dx.doi.org/10.1039/C3SC53416B