Letters to the editor
Even longer carbon-carbon bonds
I write because I was displeased when I saw your article “The Fight over the Longest Carbon-Carbon Bond Is Redefining What a Bond Is” (C&EN, March 11, page 24). This is an incredible subject that you chose well. Unfortunately, you did not trace all the research done by our group on the subject, reported around 2002–5. In your story, you mention the Xu-Qiong Xiao group reported the synthesis of a series of 1,2-diamino-o-carboranes and remarked on the ultralong carbon-carbon distances (1.931 Å) in those molecules. In this regard, our group pioneered all the research reported (experimental and computational) in different manuscripts, mostly in the period 2002–5. These manuscripts should have been given credit in your article. The longest experimental carbon-carbon bond distance reported is 2.022 Å, in our paper (Inorg. Chem. 2002, DOI: 10.1021/ic011285z). I hope you can find a way to give our former research in this fascinating field the credit it deserves.
In the March 12, 2018, and March 11, 2019, issues of C&EN, 1.806 and 1.931 Å were reported as the longest C–C bond distances. Our 2.9 Å C–C bond (C&EN, May 6, 2013, page 34) present for [TCNE]22– is an example of a longer multicenter two-electron, four-carbon C–C bond (TCNE is tetracyanoethylene). An even longer, 3.14 Å two-electron, eight-carbon C–C bond occurs for [TCNP]22– (TCNP is 1,2,4,5-tetracyanopyrazine) (J. Am. Chem. Soc. 2009, DOI: 10.1021/ja902790q). If one restricts oneself to stretching a C–C bond due to steric effects, then the record is 2.022 Å (from Clara Viñas and Francesc Teixidor: Inorg. Chem. 2002, DOI: 10.1021/ic011285z).
Joel S. Miller
Salt Lake City
Editor’s note: After receiving the letters above, C&EN decided to update its March 2019 article on the competition over carbon-carbon bond distances. The compounds from Clara Viñas and Joel Miller are different from those we highlighted in our original story in that they are charged or, in the case of Miller’s, involve more than two carbons in the ultralong bonds. However, they are impressive—even longer than those we noted—and are important to the field of chemists who are pushing carbon-carbon bonds to their limits. You can see our update at cenm.ag/ccbonds and join the conversation if you know of any other beautifully long bonds we should be aware of.