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Synthesis

All-nitrogen ring crystallized

Chemists isolate cyclopentazolate anion as air-stable salt

by Jyllian Kemsley
January 30, 2017 | APPEARED IN VOLUME 95, ISSUE 5

Molecules packed with nitrogen are known for their explosive properties, which are prized for applications such as weapons and propellants. One long-desired synthesis target, an all-nitrogen aromatic ring, was prepared as the cyclopentazolate anion in tetrahydrofuran solution last year (C&EN, July 18, 2016, page 8). Now, researchers have managed to isolate and characterize it in the solid state (Science 2017, DOI: 10.1126/science.aah3840). After “hundreds of experiments,” a team led by Bingcheng Hu of Nanjing University of Science & Technology prepared the compound through oxidative dearylation to cleave the C–N bond in 3,5-dimethyl-4-hydroxyphenylpentazole, yielding cyclo-N5 in the form of (N5)6(H3O)3(NH4)4Cl. The pentazolate anion is stabilized by hydrogen bonds from the ammonium and hydronium cations. The white salt is stable in air at room temperature and doesn’t begin to decompose until 117 °C, although a crystal stored at ambient temperature in ethyl acetate slowly decomposed over six months, forming NH4N3, the researchers report.

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