Azidotetrazolate

Azidotetrazolate (CN7−) is an anion which forms a highly explosive series of salts. The ion is made by removing a proton from 5-azido-1H-tetrazole. The molecular structure contains a five-membered ring with four nitrogen atoms, and an azido side chain connected to the carbon atom. Several salts exist, but they are unstable and spontaneously explode.[1] Rubidium azidotetrazolate was so unstable that it explodes while crystallizing.[1] The potassium and caesium salt also spontaneously explode when dry.[1]

Azidotetrazolates are under investigation for use as bullet propellant, rocket propellants and high explosives. The nitrogen base compounds have the advantage of being "green explosives", meaning that their waste products are safe.[1] Amongst the tetrazolates, they have the highest nitrogen fraction.[2]

Properties

Different stimuli can set off explosions. Possibilities include impact, friction, sparks or heat. Guanidinium azidotetrazolate could be melted to an ionic liquid at 100°C, but would decompose smoothly at 159°. Sodium azidotetrazolate explodes when the temperature reaches 155°C.[1]

The CNNNN ring in azidotetrazolate is planar and aromatic. The NNN side chain is attached to the carbon atom, and bent at the first nitrogen so that it is roughly parallel to the ring.[3]

The base acids, 5-azido-1H-tetrazole and 5-azido-2H-tetrazole both can exist. The 1H version has a hydrogen atom bonded to a nitrogen atom next to the carbon. It is the stable form in solids and solutions in polar liquids. This is explained by a favourable hydrogen bond, and a greater dipole moment. The 2H version with hydrogen attached in the second nitrogen from the carbon is purportedly more stable in the gas phase.[4]

Formation

The azidotetrazolate ion can be made from cyanogen bromide and sodium azide in water:[1]

BrCN + 2NaN3 → N4CN3− + Na+ + NaBr

Salts
formula[1] name molecular weight density structure unit cell dimension Å cell volume formula per cell Specific impulse melt °C decompose °C CAS[5]
NH4CN7 Ammonium azidotetrazolate 128.12 1.608 monoclinic a=3.9103 b=6.9140 c=9.9127 β=99.177° 264.57 2 248 157 35038-47-2
N2H5CN7 Hydrazinium azidotetrazolate 143.14 1.568 monoclinic a=10.8114 b=7.4641 c=7.6676 β=101.437° 606.47 4 265 136
CN3H6CN7 guanidinium azidotetrazolate semihydrate 358.35 1.546 monoclinic a=9.5374 b=15.3120 c=10.5731 β=94.361 1539.6 4 100 159
CN4H7CN7 aminoguanidinium azidotetrazolate 185.19 1.524 triclinic a=9.7652 b=9.7803 c=10.0434 α=71.327° β=74.399° γ=63.962° 807.2 4 228
NH3OHCN7 Hydroxylammonium azidotetrazolate[6] 144.12 1.649 monoclinic a=3.677 b=20.885 c=7.641 β=98.367 580.5 4
LiCN7•H2O Lithium azidotetrazolate monohydrate 135.03 1.683 monoclinic a=8.9207 b=4.6663 c=12.8648 β=95.608 532.96 4
NaCN7•H2O sodium azidotetrazolate monohydrate 151.09 1.743 monoclinic a=11.203 b=7.138 c=14.409 β=91.630° 1151.8 8 explode 155 35038-45-0
KCN7 Potassium azidotetrazolate 149.18 1.917 monoclinic a=9.7759 b=6.2990 c=8.4533 β=96.720° 516.96 4
RbCN7 Rubidium azidotetrazolate 195.12 Unknown; all attempts at isolation terminated by detonation of solution. <25
CsCN7 Caesium azidotetrazolate 242.99 2.810 orthorhombic a=4.3082 b=7.1345 c=18.6869 574.38 4
3Ca(CN7)2•16H2O Calcium azidotetrazolate 1068.98 1.657 monoclinic a=24.448 b=6.616 c=28.057 β=109.246° 4285 4
AgCN7 silver azidotetrazolate
HCN7 5-Azido-1H-tetrazole 111.07 1.72 75 165 35038-46-1

Chemicals related to the azidotetrazolate anion include 1-diazidocarbamoyl-5-azidotetrazole with the same percentage of nitrogen.

References
  1. Klapötke, Thomas M.; Stierstorfer, Jörg (28 January 2009). "The CN7- Anion". Journal of the American Chemical Society. 131 (3): 1122–1134. doi:10.1021/ja8077522. PMID 19115947.
  2. Petkewich, Rachel A. "High-Nitrogen Anion Exceeds The Power Of RDX | January 12, 2009 Issue - Vol. 87 Issue 2 | Chemical & Engineering News". cen.acs.org.
  3. Zhaoxu, Chen; Jianfen, Fan; Heming, Xiao (January 1999). "Theoretical study on tetrazole and its derivatives. Part 7: ab initio MO and thermodynamic calculations on azido derivatives of tetrazole". Journal of Molecular Structure: THEOCHEM. 458 (3): 249–256. doi:10.1016/S0166-1280(98)00249-8.
  4. Stierstorfer, Jörg; Klapötke, Thomas M.; Hammerl, Anton; Chapman, Robert D. (June 2008). "5-Azido-1H-tetrazole – Improved Synthesis, Crystal Structure and Sensitivity Data". Zeitschrift für anorganische und allgemeine Chemie. 634 (6–7): 1051–1057. doi:10.1002/zaac.200800003.
  5. Marsh, F. D. (September 1972). "Cyanogen azide". The Journal of Organic Chemistry. 37 (19): 2966–2969. doi:10.1021/jo00984a012.
  6. Klapötke, Thomas M.; Piercey, Davin G.; Stierstorfer, Jörg (11 November 2011). "The Taming of CN7−: The Azidotetrazolate 2-Oxide Anion". Chemistry - A European Journal. 17 (46): 13068–13077. doi:10.1002/chem.201102064. PMID 21971954.
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