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 Ring may refer to: * Ring (jewellery), a round band, usually made of metal, worn as ornamental jewelry * To make a sound with a bell, and the sound made by a bell :(hence) to initiate a telephone connection Arts, entertainment and media Film and ...

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, an ...

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 In chemistry, an inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds, that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as '' inorganic chemist ...

, 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. Th ...

compounds containing one to five nitrogen atoms. This set contains

pyrrole Pyrrole is a heterocyclic aromatic organic compound, a five-membered ring with the formula C4 H4 NH. It is a colorless volatile liquid that darkens readily upon exposure to air. Substituted derivatives are also called pyrroles, e.g., ''N''-meth ...

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 Diazole refers ...

pyrazole Pyrazole is an organic compound with the formula C3H3N2H. It is a heterocycle characterized by a 5-membered ring of three carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent ...

triazole A triazole is a heterocyclic compound featuring a five-membered ring of two carbon atoms and three nitrogen atoms with molecular formula C2H3N3. Triazoles exhibit substantial isomerism, depending on the positioning of the nitrogen atoms within t ...

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 f ...

, 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 Conjugation or conjugate may refer to: Linguistics * Grammatical conjugation, the modification of a verb from its basic form * Emotive conjugation or Russell's conjugation, the use of loaded language Mathematics * Complex conjugation, the chang ...

with the phenyl ring. The derivative

4-dimethylaminophenylpentazole 4-Dimethylaminophenylpentazole is an unstable, explosive compound that contains the rare pentazole ring, which is composed of five nitrogen atoms. The electron donating effect of the 4-dimethylamino substituent on the phenyl ring makes this comp ...

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 aromatic ring, usually an aromatic hydrocarbon, such as phenyl and naphthyl. "Aryl" is used for the sake of abbreviation or generalization, and "Ar" is used as ...

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

propellant A propellant (or propellent) is a mass that is expelled or expanded in such a way as to create a thrust or other motive force in accordance with Newton's third law of motion, and "propel" a vehicle, projectile, or fluid payload. In vehicles, the e ...

s 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 Electrospray ionization (ESI) is a technique used in mass spectrometry to produce ions using an electrospray in which a high voltage is applied to a liquid to create an aerosol. It is especially useful in producing ions from macromolecules bec ...

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 Ammonium azide is the chemical compound with the formula , being the salt of ammonia and hydrazoic acid. Like other inorganic azides, this colourless crystalline salt is a powerful explosive, although it has a remarkably low sensitivity. is phy ...

. 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