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 pentazole compounds.

Ions

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

References

1. "1''H''-Pentazole - PubChem Public Chemical Database". National Center for Biotechnology Information.
2. Burke, L. A.. (Dec 2009). "Correlation Analysis of the Interconversion and Nitrogen Loss Reactions of Aryl Pentazenes and Pentazoles Derived From Aryl Diazonium and Azide Ions". International Journal of Quantum Chemistry.
3. (2003). "First generation of pentazole (HN5, pentazolic acid), the final azole, and a zinc pentazolate salt in solution: A new N-dearylation of 1-(p-methoxyphenyl) pyrazoles, a 2-(p-methoxyphenyl) tetrazole and application of the methodology to 1-(p-methoxyphenyl) pentazole". Chemical Communications.
4. (2004-12-31). "The race for the first generation of the pentazolate anion in solution is far from over". Chemical Communications.
5. (2008-01-31). "A Ceric Ammonium Nitrate N-Dearylation of N-p-Anisylazoles Applied to Pyrazole, Triazole, Tetrazole, and Pentazole Rings: Release of Parent Azoles. Generation of Unstable Pentazole, HN5/N5-, in Solution". The Journal of Organic Chemistry.
6. (2009-04-01). "First Calculations of 15N−15N J Values and New Calculations of Chemical Shifts for High Nitrogen Systems: A Comment on the Long Search for HN5 and Its Pentazole Anion". The Journal of Physical Chemistry A.
7. (Aug 13, 2009). "Galway discovery ahead of the world". The Irish Times.
8. (2002). "Experimental Detection of the Pentaazacyclopentadienide (Pentazolate) Anion, cyclo-N₅⁻". Angewandte Chemie International Edition.
9. (2016). "Detection of Cyclo-N₅⁻ in THF Solution". Angewandte Chemie International Edition.
10. (26 January 2017). "Synthesis and characterization of the pentazolate anion ''cyclo''-N5ˉ in (N5)6(H3O)3(NH4)4Cl". Science.
11. (2016-12-06). "High-Pressure Synthesis of a Pentazolate Salt". Chemistry of Materials.
12. (2018-03-13). "High-Pressure Synthesized Lithium Pentazolate Compound Metastable under Ambient Conditions". The Journal of Physical Chemistry Letters.