Structures and Energetic Properties of Two New Salts Comprising the 5,5'-azotetrazolate Dianion

Two new potentially energetic salts comprising the 5,5'-azotetrazolate dianion have been prepared and structurally characterized. The new azotetrazolates are tetraphenylphosphonium-5,5'-azotetrazolate (1) and

A single-crystal X-ray diffraction study of 1 revealed that the compound crystallizes as the tetrahydrate [PPh4]2[C2N10]•4H2O in the triclinic space group Pī. Figure 2 shows the molecular structure of 1 along with selected bond lengths and angles.Table 1 lists the crystallographic data and structure refinement details for the title compounds.All structural parameters in the cations and anions in 1 are unexceptional and in close agreement with those of previously reported azotetrazolates.The central N=N distance in the azotetrazolate dianion is 1.256(4) Å, while the N-N bond lengths in the tetrazole rings fall in the narrow range of 1.326(3)-1.347Å.The tetraphenylphosphonium cations in 1 show only minor deviations from the ideal tetrahedral geometry.C-P-C angles are all in the range of 107.82(11)-112.47(11)°,while the aromatic C-C bond lengths are between 1.378 und 1.402 Å.A comparable range of the C-P-C angles (107.9-110.4°)has been reported for the recently described bis(tetramethylphosphonium)-5,5'-azotetrazolate, [PMe4]2[C2N10] [33].Like virtually all previously reported azotetrazolate salts [19][20][21][22][23][24][25][26][27][28][29][30][31][32][33], the crystal structure of 1 is characterized by an array of hydrogen bonds.Figure 3 shows the unit cell of compound 1.The packing diagram of 1 (Figure 3) shows that hydrogen bonding is an important factor in stabilizing the solid-state structure.However, in contrast to the [PMe4] + cations in the recently reported crystal structure of bis(tetramethylphosphonium)-5,5'-azotetrazolate [33], the phenyl rings of the [PPh4] + cations in 1 do not participate in the hydrogen bonding.The reason for this structural difference can be seen in the fact that 1 crystallizes as the tetrahydrate [PPh4]2[C2N10]•4H2O, whereas [PMe4]2[C2N10] was isolated as an anhydrous material [33].As can be seen in Figure 4, the 5,5'-azotetrazolate dianions are interconnected through hydrogen bonds with the four molecules water of crystallization to give a ladder-like ribbon motif.
Single-crystals of 2 suitable for X-ray diffraction were obtained directly from the original reaction mixture without the need of recrystallization.An X-ray diffraction study of 2 revealed that the compound crystallizes in the monoclinic space group P21/c with no water of crystallization.Figure 7 shows the molecular structure of 2 along with selected bond lengths and angles.Crystallographic data and structure refinement details are found in Table 1.Once again, the structural parameters in the cations and anion in 2 were found to be unexceptional and in close agreement with those of previously reported azotetrazolates.The central N=N distance in the azotetrazolate dianion is 1.2601(18) Å.Like many previously reported azolium azotetrazolate salts [23][24][25][26][27][28][29][30][31][32], the crystal structure of 2 is characterized by layers interconnected through an array of hydrogen bonds.Figure 8 shows a packing diagram of compound 2. Unlike compound 1, the crystal structure of 2 contains no water of crystallization despite the fact that the bulk material was isolated from aqueous solution.Figure 9 illustrates in more detail how the layers are formed through different N-H…N hydrogen bonds not only between cations and anions but also between adjacent cations.Two tetrazole ring nitrogen atoms as well as the N atoms of the central azo group in the 5,5'-azotetrazolate dianion are engaged in the N-H…N hydrogen bonds to the C(NH2)2 amidinium hydrogens of the cations.Interestingly, one amidinium hydrogen of the cationic C(NH2)2 moiety also forms a N-H…N hydrogen bond to a triazole ring-N atom of a neighbouring 1H-1,2,4-triazole-1-carboxamidine cation.Details of the hydrogen bonding in the solid-state structure of 2 are summarized in Table 3.
For initial safety testing, the impact and friction sensitivities of the new azotetrazolates were tested according to established BAM methods [3] using a BAM drophammer and BAM friction tester.Figures 10 and 11 show the devices used in this study.In addition, a simple combustion test was carried out.The results are summarized in Table 4.The tetraphenylphosphonium salt 1 was found to be insensitive towards impact (i > 40 J) and friction (f > 360 N) and showed no deflagration upon combustion.1H-1,2,4-triazole-1-carboxamidine-5,5'-azotetrazolate (2) was also found to be insensitive to friction (> 360 N) but with a value of i = 10 J it was found to be significantly impact-sensitive.It also showed deflagration in the combustion test.In order to assess the energetic properties of the materials described in this study, it is useful to compare the found sensitivity data with those reported for a common secondary explosive such as TNT (trinitrotoluene).With a value of i = 10 J, the 1H-1,2,4-triazole-1-carboxamidine salt 2 is more impact-sensitive than TNT (i = 15 J) and relatively insensitive to friction (TNT: f = 355 N) [3,36].On the other hand, compound 2 is slighty less sensitive to impact than typical primary explosives such as than lead(II) styphnate (i = 2.5-5.0J) or lead(II) azide (i = 3.0-6.5J) [3,36].The reason for the differences in the energetic properties between 1 and 2 can be seen in the fact that 2 is a nitrogen-rich azotetrazolate salt (N-content 73.14%), while compound 1 is not (N-content 16.62%).This can also be described as a "dilution" of the nitrogen-rich azotetrazolate dianions by the bulky tetraphenylphosphonium cations in the crystalline state of 1.In both cases, long-term stability tests over a period of 30 days showed no changes in these sensitivity data.diffraction.Both are characterized by hydrogen bond networks in the solid state.Particularly, the 1H-1,2,4-triazole-1-carboxamidine derivative 2 is easily crystallized and isolated in high yield (86%).Due to its high nitrogen-content of 73.14%, compound 2 is significantly impact-sensitive.

Figure 4 .
Figure 4. Ribbon motif formed through hydrogen bonding between the 5,5'-azotetrazolate dianions and the water of crystallization in the crystal structure of 1.

Figure 5
Figure 5  illustrates in more detail how three tetrazole ring nitrogen atoms as well as the N atoms of central azo group are engaged in the O-H…N hydrogen bonds to the four water molecules.O-H…O hydrogen bonds between two water molecules are also present on both sides of the ribbon.Details of the hydrogen bonding in the solid-state structure of 1 are summarized in Table2.

Figure 5 .
Figure 5. Hydrogen bonding between the 5,5'-azotetrazolate dianions and the water of crystallization in the crystal struture of 1.

Figure 10 .
Figure 10.BAM drophammer setup used in this study.

Figure 11 .
Figure 11.BAM friction test setup used in this study.