Solving An Old Bonding Debate

Chemists have used a deep-freeze crystallization method to solve the structure for the 2-norbornyl cation, likely closing an acrimonious 50-year debate on the nature of a fundamental bonding concept. Instead of two electrons shared between two carbon atoms, as in conventional single-bond structures, this cation structure shows nonclassical bonding, with two electrons delocalized over three carbon atoms (Science 2013, DOI: 10.1126/science.1238849).

The structural analysis of the cation, a bridged cyclic hydrocarbon, was led by Ingo Krossing of the University of Freiburg and Karsten Meyer of the University of Erlangen-Nuremberg, both in Germany.

The debate centered on different interpretations of thermodynamic and spectroscopic data for the cation. Last year, University of Richmond chemist and historian Jeffrey I. Seeman characterized the interactions between the two sides as “crude and rude” (C&EN, Nov. 19, 2012, page 44). A crystal structure of the cation likely would have resolved the arguments, but that experimental evidence was elusive. Many other researchers have obtained 2-norbornyl cation crystals, but their molecular disorder stumped scientists, Krossing says.

Meyer, Krossing, and colleagues managed to obtain the structure by stabilizing the 2-norbornyl cation with a bromoaluminate counterion, crystallizing the product for several days at a chilly 245 K. They developed an annealing process of cooling the crystals even further to 40 K, then repeatedly warming and cooling the crystals to freeze out molecular disorder, allowing them to collect good X-ray data.

The structure reveals the two-electron, three-carbon system as having two C–C bonds of 1.80 Å, longer than a standard 1.54-Å C–C single bond. The third bond in the system is 1.39 Å long, similar to the delocalized C–C bonds of benzene.

The work is “a triumph of creative crystallography in taming an uncooperative cation,” says theoretical organic chemist Dean J. Tantillo of the University of California, Davis. Although the existing evidence had largely convinced chemists that the nonclassical structure existed in the gas phase and in solution, the new structure demonstrates that it exists in the solid state as well, Tantillo says.