Chemists Discover Molecule Considered Too Unstable To Exist

"I’ve said this molecule is unstable and doesn’t exist dozens of times in organic chemistry class, but, as it turns out, the molecule had different ideas," said Joseph B. Lambert, Clare Hamilton Hall Professor of Chemistry at Northwestern. He and graduate student Lijun Lin discovered that the cation (a positively charged ion) is stable in the solid state for weeks at room temperature and in solution.

The preparation of the cation and the solving of its X-ray structure are reported in the April 15 issue of the chemistry journal Angewandte Chemie.

The cyclopentadienyl cation is a common textbook example of an antiaromatic molecule, a molecule so electronically unstable and, therefore, extremely reactive that it should not exist for any length of time. Lambert believes the cation now should be described as nonaromatic.

The last example of the synthesis of a simple, stable molecule with the electronic configuration of antiaromaticity — cyclooctatetraene — was in 1913.

Lambert and Lin discovered the molecule when trying to figure out how to make stable organic cations in the laboratory. When Lin came to Lambert with the crystal structure of a molecule other than the one expected, Lambert quickly recognized the structure as an example of the elusive cyclopentadienyl cation of textbook fame.

They found that the cation was stable in the open atmosphere at room temperature. The cation achieves this stability by avoiding interactions among electrons. This condition is known as localized bonding. Normally molecules are stabilized by delocalization of electrons, whereby they may be located in more than one part of the molecule. For unsaturated, cyclic molecules, delocalization usually results in heightened stability. Such molecules have been termed aromatic.

For certain electron configurations, however, delocalization lessens stability, and such molecules have been called antiaromatic. It was thought that the cyclopentadienyl cation ought to be antiaromatic and hence unstable, because its electronic configuration corresponds to that predicted by theory to be antiaromatic.

"We didn’t realize there would be this localized alternative," said Lambert. "Now we have to rethink the properties of antiaromaticity."

Lambert and Lin currently are studying the chemistry of the pentamethylcyclopentadienyl cation by carrying out reactions with the cation to learn more about its properties.

The third author on the paper is Vitaly Rassolov, of the University of South Carolina, Columbia, who carried out theoretical calculations. The research was supported by the National Science Foundation.