Ethylene cyclization to benzene confirmed

New evidence that a niobium anion can dehydrogenate and cyclize three ethylene molecules into benzene has been reported by Doreen Geller Leopold and coworkers of the University of Minnesota, Minneapolis (J. Am. Chem. Soc. 2007, 129, 3482). This gas-phase cyclization reaction of one of the simplest unsaturated hydrocarbons has been inferred previously by mass spectrometry. But direct spectroscopic proof for a metal-bound benzene product had not been reported until now. Leopold's group isolated NbC₆H₆^- ions from a flow-tube reactor and probed the structure of the compound by photoelectron spectroscopy and additionally by density functional calculations. The researchers show that the spectrum matches that of NbC₆H₆^- formed by reacting benzene with niobium atoms. They propose that a neutral niobium atom reacts initially with ethylene to produce NbC₂H₂^- with a loss of H₂, and that NbC₂H₂^- adds ethylene molecules sequentially to form the metal-coordinated benzene. The research suggests that "even the smallest of metal species may possess powerful catalytic potential," Leopold notes.