Long C-C bonds.

In an earlier post, I searched for small C-C-C angles, finding one example that was also accompanied by an apparently exceptionally long C-C bond (2.18Å). But this arose from highly unusual bonding giving rise not to a single bond order but one closer to one half! How long can a “normal” (i.e single) C-C bond get, a question which has long fascinated chemists.

A naive search of the CSD is not as straightforward as it seems. Using the simple sub-structure R₃C-CR₃ as the search query gives LIRPEI, DOI: 10.5517/CCQ043Y[1] an apparently unexceptional molecule with a very exceptional C-C distance of 1.87Å. With long bonds one has to be ultra-careful to look at the crystallographic analysis before drawing any conclusions. One class of molecule where this has been done by many groups is the system shown below (red = long bond), with 47 entries and for which the longest C-C bond emerges with the value of 1.79Å[2]

You can view this structure at DOI: 10.5517/CCS0R6Q[3] and the authors go to some pains to assure us that it is still a closed shell single bond, and not a biradical. That does seem to be the current record holder, but of course we are only talking here about molecules whose crystal structure has been determined.

I will end with an open question; how SHORT could a “single” C-C bond get? Here, a search of the CSD is entirely dominated by crystallographic artefacts, and I am not sure what the value might be. 

References

1. T. Takeda, H. Kawai, R. Herges, E. Mucke, Y. Sawai, K. Murakoshi, K. Fujiwara, and T. Suzuki, "Negligible diradical character for the ultralong C–C bond in 1,1,2,2-tetraarylpyracene derivatives at room temperature", Tetrahedron Letters, vol. 50, pp. 3693-3697, 2009. http://dx.doi.org/10.1016/j.tetlet.2009.03.202