Cyclopentazole anion

Chemists have long sought to identify stable all-nitrogen chemical species. N₂, of course, is ubiquitous; and N₃^–, the azide anion, is well characterized. But what about N₄ and higher?

Several studies, mostly theoretical, have been done in search of N₄. In 1996, Mikhail N. Glukhovtsev at the University of Sydney and Sergei Laiter at the University of North Carolina (Chapel Hill) calculated that the energies of the three most likely N₄ isomers increase in the order open-chain N₄ triplet diradical < tetrazetetetrahedrane < planar tetrazete. The researchers attributed the higher energies of tetrahedral and planar N₄ to ring strain and antiaromaticity, respectively. Note the reversal in the energies of the cyclic N₄s versus those of the isoelectronic carbon systems tetrahedrane and cyclobutadiene.

In 2000, J. George Radziszewski and coauthors at the Colorado School of Mines and the National Renewable Energy Laboratory (both in Golden, CO), detected N₄ in a nitrogen plasma and assigned its structure to tetrazetetetrahedrane.

In another 1996 theoretical study, Glukhovtsev, Haijun Jiao, and Paul von Ragué Schleyer at the University of Erlangen-Nuremberg (Germany) and Rostov University (Rostov on Don, Russia) found that the most stable N_n compounds contain N₅, or cyclopentazole, rings. An example is N₈, azidopentazole. They attributed the stability to the aromaticity of the N₅ rings.

This past year, the quest for a stable N₅ species came to fruition. Yehuda Haas and colleagues at the Hebrew University of Jerusalem prepared the cyclopentazole anion, N₅^–, by reducing phenylpentazole with sodium metal. The sodium salt of the anion in tetrahydrofuran solution is indefinitely stable below –40 ºC and has a half-life of minutes at ambient temperature.