Researchers have developed a stable “antiaromatic” compound that can be switched between the aromatic and anti-aromatic states and a first ever found intermediate state.
This research has been published online in the journal Nature Chemistry.
Aromatic compounds are the class of organic chemical compounds that have one or more rings of carbon atoms and undergo chemical reactions characteristic of benzene. It was found that nearly half of all organic compounds are aromatic compounds. Ring-like structure in those compounds cause the electrons to be shared between the atoms resulting in high degree of stability and no change in structure under different conditions in which the molecules would react.
“That’s one of the reasons why they’re so useful in industry,” Jonathan Sessler, the Rowland Pettit Centennial Chair in Chemistry in the College of Natural Sciences, said in a statement. “It’s also why they tend to be pro-carcinogenic. They’re very hard for us to metabolize or catabolize, and the results of that are usually not benign. One of the first class of tumors ever observed was testicular cancer. It was highly prevalent among 18th century chimney sweeps, who were exposed to aromatic compounds found in coal tar.”
On the other hand, antiaromatics are the compounds that have two additional or two fewer electrons than aromatic compounds. Antiaromatics are as unstable and highly reactive as the aromatics are stable in nature.
“They don’t want to exist in a planar form without giving up or adding the two electrons that distinguish them from their aromatic analogues,” said Sessler, “so they tend to twist around, to a lower energy state. That destroys their antiaromaticity. The net result is that bona fide antiaromatic compounds are elusive. What we have done, by rational design, is put big buttressing groups around the compounds, basically clamping them into place.”
When the two electrons moved away from the antiaromatics the resulting compound was something in between the aromatics and anti-aromatics with stability of aromatics and nature of antiaromatics. This intermediate compound has been seen for the first time and therefore it has no common name yet.
“When you have to struggle for the words to describe what’s being done, you know that it’s cutting edge,” said Christian Brueckner, a fellow porphyrin chemist and a professor at The University of Connecticut. “Twenty years ago when I was a graduate student I was told simply that you can’t make large antiaromatics like this. Later the idea was that you can make them but you can’t do much with them. Now you can do it, and it can switch between states, and it can exist in the intermediate state. It’s just a beautiful progression of scholarship, a beautiful example of how the ability of chemists to manipulate matter is advancing.”
Ishida, M., Kim, S., Preihs, C., Ohkubo, K., Lim, J., Lee, B., Park, J., Lynch, V., Roznyatovskiy, V., Sarma, T., Panda, P., Lee, C., Fukuzumi, S., Kim, D., & Sessler, J. (2012). Protonation-coupled redox reactions in planar antiaromatic meso-pentafluorophenyl-substituted o-phenylene-bridged annulated rosarins Nature Chemistry DOI: 10.1038/nchem.1501