Refinement of Copper(II) Azide with 1‐Alkyl‐5H‐tetrazoles: Adaptable Energetic Complexes

Abstract A concept for stabilizing highly sensitive and explosive copper(II) azide with 1‐N‐substituted tetrazoles is described. It was possible to stabilize the system by the use of highly endothermic, nitrogen‐rich ligands. The sensitivities of the resulting energetic copper coordination compounds can be tuned further by variation of the alkyl chain of the ligands and by phlegmatization of the complexes with classical additives during the synthesis. It is demonstrated, using the compound based on 1‐methyl‐5H‐tetrazole ([Cu(N3)2(MTZ)], 1) that this class of complexes can be applied as a potential replacement for both lead azide (LA) and lead styphnate (LS). The complex was extensively investigated according to its chemical (elemental analysis, single‐crystal and powder X‐ray diffraction, IR spectroscopy, scanning electron microscopy) and physico‐chemical properties (differential thermal analysis, sensitivities towards impact, friction, and electrostatic discharge) compared to pure copper(II) azide.

Copper(II) nitrate trihydrate (4.14 mmol, 1000 mg) was dissolved in water (50 mL) and stirred magnetically. An aqueous solution of sodium azide (7.70 mmol, 500 mg, 10 mL) was added dropwise. The gluey, dark brown precipitate was filtered off and washed with water. For purification, the wet azide was brought into an enclosed container of 2-3 % hydrazoic acid (50 mL) and stored for 24 h under HN3. During filtration, the product was washed several times with ethanol and finally with diethyl ether. After drying in air, pure copper(II) azide was obtained as a brown product with a slight reddish shine. Yield: 371 mg (2.51 mmol, 61 %). found: too sensitive for measurement; BAM drop hammer: n.d.; friction tester: < 0.10 N; ESD: < 0.29 mJ (at grain size < 100 µm).
Single crystals growth was achieved by overlaying an aqueous solution (8 mL) of sodium azide and the ligand with an ethanolic solution (8 mL) of copper(II) chloride dihydrate, separated by a mixture (4 mL) of water/ethanol (50/50). After 7 to 14 days crystals suitable for X-ray determination were obtained.

Phlegmatized [Cu(N3)2(MTZ)] + 6 % Dextrin (1a)
The phlegmatized compound was prepared analogous to the synthesis of dextrinated lead azide. [S10] While heating to 60 °C, dextrin from potato starch (120 mg) was added to water (36 mL) under stirring. As soon as the solution became clear, sodium azide was added (17.3 mmol, 1.12 g). An aqueous solution of copper(II) chlorate dihydrate (8.63 mmol, 1.47 g) and MTZ (8.63 mmol, 0.726 g) in water (20 mL) was prepared. To this solution of metal salt and ligand, the dextrinated sodium azide solution was added dropwise over the course of 30 min while stirring and heating at 60 °C continued. After addition, the solution was allowed to cool down and the brown complex filtered, washed with ethanol and air-dried overnight. Yield: 1.72 g (6.91 mmol, 80 %).
After complete addition and precipitation of the complex, it was allowed to stir for another (w); BAM drop hammer: 2 J; friction tester: 0.75 N; ESD: 3.9 mJ (at grain size < 100 µm).

Phlegmatized [Cu(N3)2(MTZ)] + 5 % PVB (1d)
A solution of polyvinyl butyral (25 mg) in methanol was prepared while stirring at room temperature. After the additive was completely dissolved, copper(II) nitrate trihydrate (2.16 mmol, 522 mg) and MTZ (2.16 mmol, 182 mg) were added and dissolved in the PVA solution. An aqueous solution of sodium azide (4.32 mmol, 281 mg) was dripped to the in situ formed nitrate complex. The precipitated complex was allowed to stir for five more minutes and, after filtration, washing with ethanol and air-drying overnight, obtained as brown powder. IR spectroscopy

Scanning electron microscopy
Since all of the prepared compounds differ in their physicochemical properties and appearance, scanning electron microscopy (SEM) was performed to investigate the morphology of pure copper azide as well as the prepared coordination compound with (1a-1d) and without (1) additives.
The pure azide consists of thin fibers with a diameter of less than 1 µm, forming intergrown agglomerates ( Figure S6). This is also represented in the macroscopic scale, as the dry compound does not form a fine powder but keeps the shape of the filter paper and crumbles into large chunks. The coordination compound (1) shows a completely different crystal habit, as small crystallites are homogenously distributed ( Figure S7). Most of them have a plate-like morphology, rounded edges and a size in the range of 0.5-5 µm. This can also be confirmed by the compound's macroscopic appearance, as it precipitates as a fine powder and retains this morphology after drying. Figure S7. SEM images with 500x magnitude (left) and 6500x magnitude (right) of compound 1.
The dextrinated complex (1a) shows a similar overall morphology but with bigger crystallites (up to 30 µm) tending to take on more geometric shapes ( Figure S8).
The complex 1b precipitated from an aqueous solution of polysorbate (Span 80) shows a larger particle size distribution and forms even bigger chunks (up to 80 µm) which are partially intergrown and possess soft edges ( Figure S9). Interestingly, compound 1c, which is prepared using carboxymethyl cellulose as an additive, forms agglomerates consisting of two different crystal morphologies ( Figure S10). Firstly, a plate-like structure (as present in pure complex 1) can be found, as well as a needle-like species.
With no structures being much bigger than 10 µm, the overall morphology seems to be very compact due to space-filling needles between layers of platelets.

Initiation capability tests
As the initiating capability of a compound indicates its suitability as a primary explosive, selected ECC were tested in initiation experiments. The compound to be evaluated was loosely filled on top of a pressed (8 kg weight) main charge (200 mg of PETN or RDX) in a copper shell ( Figure S14). The primary explosive was ignited using an electrical ignitor.

Priming mixtures
In order to test the suitability in priming mixtures (PM), [Cu(N3)2(MTZ)] (1) was applied as a lead styphnate (LS) replacement in a priming composition similar to the so called FA-956. [S20] Instead of using 41 % of primary explosive by mass (37 % LS, 4 % tetrazene), the mixture was tested with 15 % of [Cu(N3)2(MTZ)] as a primary explosive. For obtaining homogenous priming mixtures in lab scale, weighted quantities of all compounds were brought into a sample container and placed into a Heidolph Reax 2 overhead shaker. After several hours at 60 rpm, a homogenous mixture was attained, which was further characterized regarding sensitivities, thermal behavior, the produced flame. Therefore, the PM was filled in commonly used largerifle percussion primer consisting of a brass primer cup (B) covered with a paper disc (C), which is pressed onto the mixture. Lastly an anvil (D) is pressed on top of the paper ( Figure S16). As soon as the firing pin hits the primer cup (A), mechanical stimulus ignites the priming mixture (B), which is confined by the anvil (D) and subjected to impact and friction. Figure S16. Primer processing. A primer filled with the new mixture was pressed into a 7.62 mm cartridge and the priming mixture ignited with the impact of a firing pin. Nitrocellulose was used as propellant. The building up gas pressure was measured and compared to a cartridge with a commercial large rifle primer. The results can be found in Table S3 and Figure S17. The generated gas pressure [bar] is plotted against time [ms].  Figure S17. Generated gas pressure of two mixtures, plotted against time.

9.
Notes on the preparation of copper(II) azide

CAUTION! The pure copper azide is very sensitive toward all external stimuli and shows characteristics of a contact explosive!
Cupric azide was prepared as outlined in the Experimental Section, according to STRAUMANIS and CIRULIS. [S8] After the addition of azide to a solution of copper nitrate, crude copper azide precipitates immediately. The formation of a 3D-polymeric network, which is built up according to the crystal structure, [S21] leads to the appearance of very fine fibers (observable during electron microscopy, Figure S6) and ultimately to an intergrown polymeric mass of product which is hard to filter and process. This crude product is impure, as it contains basic copper azides. It has to be stored under diluted hydrazoic acid for a certain period of time, leading to the destruction of basic byproducts ( Figure S19). That process was monitored by IR spectroscopy, showing a significant reduction of the broad hydroxy band (O-H bond stretching vibration at 3600-3400 cm −1 ) and sharper remaining bands, e.g. azide band at 2130-2070 cm −1 ( Figure S18). Figure S18. IR spectra of copper(II) azide before and after treatment with diluted HN3. Figure S19. Copper azide under HN3 (left) and on a filter paper, after washing with ether. Figure S20. Failed attempt to remove Cu(N3)2 from a filter paper (left), successful attempt (right).