Pentazole

Substituted analogs of pentazole are collectively known as pentazoles. As a class, they are unstable and often highly explosive compounds. The first pentazole synthesized was phenylpentazole, where the pentazole ring is highly stabilized by conjugation with the phenyl ring. The derivative 4-dimethylaminophenylpentazole is among the most stable pentazole compounds known, although it still decomposes at temperatures over 50 °C. It is known that electron-donating groups stabilize aryl pentazole compounds.^[2]

The cyclic pentazolium cation (N⁺
₅) is not known due to its probable antiaromatic character; whereas the open-chained pentazenium cation (N⁺
₅) is known. Butler et al. first demonstrated the presence of the cyclic N⁻
₅ in solution through the decomposition of substituted aryl pentazoles at low temperature. The presence of N
₅H and N⁻
₅ (held in solution through the interaction with zinc ions) was proven primarily using ¹⁵N NMR techniques of the decomposition products.^[3] These results were initially challenged by some authors,^[4] but subsequent experiments involving the detailed analysis of the decomposition products, complemented by computational studies, bore out the initial conclusion.^[5][6][7] The pentazolide anion is not expected to last longer than a few seconds in aqueous solution without the aid of complexing agents. The discovery of pentazoles spurred attempts to create all-nitrogen salts such as N⁺
₅N⁻
₅, which should be highly potent propellants for space travel.

In 2002, the pentazolate anion was first detected with electrospray ionization mass spectrometry^[8] In 2016, the ion was also detected in solution.^[9] In 2017, white cubic crystals of the pentazolate salt, (N₅)₆(H₃O)₃(NH₄)₄Cl were announced. In this salt, the N⁻
₅ rings are planar. The bond lengths in the ring are 1.309 Å, 1.310 Å, 1.310 Å, 1.324 Å, and 1.324 Å.^[10] When heated, the salt is stable up to 117 °C, and over this temperature it decomposes to ammonium azide.^[10] Under extreme pressure conditions, the pentazolate ion was also synthesized. It was first obtained in 2016 in the form of the CsN₅ salt by compressing and laser-heating a mixture of CsN₃ embedded in molecular N₂ at 60 GPa. Following the pressure release, it was found metastable down to 18 GPa.^[11] In 2018, another team reported the high pressure synthesis of LiN₅ above 45 GPa from a pure lithium surrounded by molecular nitrogen. This compound could be retained down to ambient conditions after the complete release of pressure.^[12]