CN7 - An Anion that Goes Boom

How could I pass up reading this article when the abstract had a picture of an explosion? But seriously, the molecule is cool and has an interesting structure.

The tetrazole ring looks like it ought to be aromatic. It resembles pyrrole: a five-membered ring with two double bonds and a lone pair on the NH nitrogen so you would think it would be aromatic. In addition, the tetrazolate anion is typically written with an aromatic symbolism having a circle inside the ring rather than showing double and single bonds.

The tetrazole ring itself is used in biological chemistry because it is acidic like a carboxylate group but it will not do the same reactions as a carboxylate, like form amide or ester bonds. In designing drugs, a carboxylate can be replaced with a tetrazole group. It is roughly the same size and will have a negative charge like a carboxylate, but since it cannot react chemically like a carboxylate it can get stuck without reacting and thus serve as an inhibitor.

Of course that is not what the present authors are interested in. They like the fact that it blows up. Be reassured, not all tetrazole compounds are explosive, especially not those used in the pharmaceutical industry.

In this paper, the authors react CHN7 with different bases to prepare a number of different salts of the CN7 anion paired with different cations. All of these compounds are explosive. The authors' interests are in developing "green explosives," such as explosives which do not contain toxic elements like lead (found in many primers) or perchlorate (used in rocket fuel.)

They prepared CN7 salts with both metal counter ions (Li, Na, K, Cs and Ca) and ammonium-type ions (NH4, N2H5, CH6N3 and CH7N4). The nitrogen salts were relatively stable, but the metal salts required a LOT of care given the description in the paper. The lithium and sodium salts were "relatively" stable - quotes in the original. Of the potassium salt, the paper says that even small amounts explode violently. They managed to isolate three crystals of the cesium salt and "a few hours later the whole preparation exploded spontaneously." These compounds are not to be taken lightly.

The next time a student asks if we can make something that explodes, I may just share with them the caution from this paper:

5-Azido-1H-tetrazole as well as its salts ... are extremely energetic compounds with increased sensitivities towards various stimuli. Therefore proper protective measures(safety glasses, face shield, leather coat, earthened equipment and shoes, Kevlar gloves and ear plugs) should be used at all time during work .... All compounds should be stored in explosive cases since they can explode spontaneously.

Once they safely obtained crystals of the salts, they determined their crystal structures. Hydrogen bonding seems to play a role in stabilizing these compounds. All of the nitrogen-cation salts were fairly stable, and their crystal structures show a fair amount of hydrogen bonding between the CN7 ions and the counter ions. The Li and Na salts crystallized with one water of hydration. The presence of water in these crystals seems to stabilize them through hydrogen bonds. The most reactive salts, K and Cs, have no waters of hydration and make no hydrogen bonds in the crystal. The other metal salt studied was Calcium. The Ca salt was also relatively stable, and co-crystallized with about 4.3 waters of hydration per Ca so there will be a lot of hydrogen bonding here as well.

The authors measured a number of properties of these compounds which had not been done previously. However, in the end they concluded that they are probably too sensitive for practical applications.

Thomas M. Klapötke, Jörg Stierstorfer (2009). The CN Anion Journal of the American Chemical Society, 131 (3), 1122-1134 DOI: 10.1021/ja8077522