Pentazole is an

aromatic In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satur ...

molecule consisting of a five-membered ring with all

nitrogen Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...

atoms, one of which is bonded to a

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

atom. It has the

molecular formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...

. Although strictly speaking a homocyclic, inorganic compound, pentazole has historically been classed as the last in a series of heterocyclic

azole Azoles are a class of five-membered heterocyclic compounds containing a nitrogen atom and at least one other non-carbon atom (i.e. nitrogen, sulfur, or oxygen) as part of the ring. Their names originate from the Hantzsch–Widman nomenclature. T ...

compounds containing one to five nitrogen atoms. This set contains pyrrole,

imidazole Imidazole (ImH) is an organic compound with the formula C3N2H4. It is a white or colourless solid that is soluble in water, producing a mildly alkaline solution. In chemistry, it is an aromatic heterocycle, classified as a diazole, and has non-a ...

pyrazole Pyrazole is an organic compound with the formula C3H3N2H. It is a heterocycle characterized by a 5-membered ring of three carbon atoms and two adjacent nitrogen atoms, which are in ortho-substitution. Pyrazole is a weak base, with p''K''b 11.5 ( ...

, triazoles,

tetrazole Tetrazoles are a class of synthetic organic heterocyclic compound, consisting of a 5-member ring of four nitrogen atoms and one carbon atom. The name tetrazole also refers to the parent compound with formula CH2N4, of which three isomers can be fo ...

, and pentazole.

Derivatives

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 In organic chemistry, an aryl is any functional group or substituent derived from an aromaticity, aromatic ring, usually an aromatic hydrocarbon, such as phenyl and naphthyl. "Aryl" is used for the sake of abbreviation or generalization, and "Ar ...

pentazole compounds.

Ions

The cyclic pentazolium cation () is not known due to its probable antiaromatic character; whereas the open-chained

pentazenium In chemistry, the pentazenium cation (also known as pentanitrogen) is a positively-charged polyatomic ion with the chemical formula and structure . Together with solid nitrogen polymers and the azide anion, it is one of only three poly-nitrogen ...

cation () is known. Butler ''et al.'' first demonstrated the presence of the cyclic in solution through the decomposition of substituted aryl pentazoles at low temperature. The presence of and (held in solution through the interaction with zinc ions) was proven primarily using ¹⁵N NMR techniques of the decomposition products. These results were initially challenged by some authors, but subsequent experiments involving the detailed analysis of the decomposition products, complemented by computational studies, bore out the initial conclusion. 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 , which should be highly potent propellants for space travel. In 2002, the

pentazolate In chemistry, a pentazolate is a compound that contains a ''cyclo''-N5− ion, the anion of pentazole. In 2017, researchers prepared the first salt (N5)6(H3O)3(NH4)4Cl containing pentazolate anion starting a substituted phenylpentazole, ''m''-CPB ...

anion was first detected with electrospray ionization mass spectrometry In 2016, the ion was also detected in solution. In 2017, white cubic crystals of the pentazolate salt, (N₅)₆(H₃O)₃(NH₄)₄Cl were announced. In this salt, the 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. When heated, the salt is stable up to 117 °C, and over this temperature it decomposes to ammonium azide. 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. 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.

References

{{Reflist, 30em Nitrogen hydrides Explosive chemicals Simple aromatic rings