Pentazole

Pentazole is an aromatic compound consisting of a five-membered ring with all nitrogen atoms, one of which is bonded to a hydrogen atom. It has a molecular formula of HN₅. Its SMILES structure is SMILES|S=n1n [nH] nn1. Although strictly speaking a homocyclic, inorganic compound, pentazole has historically been classed as the last in a series of heterocyclic azole compounds containing one to five nitrogen atoms. This set contains pyrrole, imidazole, pyrazole, triazoles, tetrazoles, and pentazole. One can consider pentazole to be at the interface between inorganic and organic chemistry.

Substituted analogs of pentazole are collectively known as pentazoles. They are highly explosive compounds. The first pentazole synthesized was phenyl pentazole, where the pentazole ring is highly stabilized by conjugation with the phenyl ring. The cyclic pentazolium cation (N₅⁺) is not known due to its probable antiaromatic character; whereas the open chained pentazenium cation (N₅⁺) is known. The mass spectrometer signal for the pentazolide anion (N₅^-) has been detected from the decomposition of substituted phenyl pentazoles. There are reports of the synthesis of pentazole and pentazole anion in solution as evidenced by their ¹⁵N isotope labeled azide product. Pentazole 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, which should be highly potent propellants for space travel.