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Polyazido High‐Nitrogen Compounds: Hydrazo‐ and Azo‐1,3,5‐triazine

2004; Wiley; Volume: 43; Issue: 37 Linguagem: Inglês

10.1002/anie.200460366

1521-3773

My‐Hang V. Huynh, Michael A. Hiskey, Ernest Hartline, Dennis P. Montoya, R. Gilardi,

Combustion and Detonation Processes

20 nitrogens and six carbons: The compounds 1 and 2, demonstrate that hydrazo and azo linkages can be used to desensitize polyazido high-nitrogen compounds and also decrease their volatility. The compound 2 has the highest experimentally measured heat of formation reported for energetic organic compounds (+2171 kJ mol−1). The performance of a high explosive is measured by its detonation velocity (vD (km sec−1)) and detonation pressure (PCJ (kbar)). These parameters are determined by the oxygen balance (OBCO),1a density (ρ), and heat of formation (ΔHf),1b the higher the oxygen balance, density, and heat of formation, the better the performance. The energy of traditional polynitro compounds (Scheme 1) is primarily derived from the combustion of the carbon backbone using the oxygen carried by the nitro group.2 Traditional energetic polynitro compounds.3a For modern polynitro compounds (Scheme 2), the performance is enhanced not only by an excellent oxygen balance but also by a ring/cage strain which improves both the heat of formation and density.4 Modern energetic polynitro compounds.3b Recently, a new class of energetic compounds containing a large fraction of nitrogen has been investigated.5–8 These "high-nitrogen" compounds form a unique class of energetic materials5a, 9 whose energy is derived from their very high positive heat of formation rather than from the combustion of the carbon backbone or the ring/cage strain (Scheme 3). The high heat of formation is directly attributable to the large number of inherently energetic NN and CN bonds. High-nitrogen compounds.3c High-nitrogen compounds containing polyazides possess even higher heats of formation because their energy content rapidly increases with the number of energetic azido groups in the molecule. However, they are notorious for their extreme sensitivity10a to spark, friction, and impact (H50)10b as well as poor thermal stability,10a, 11, 12 so their applications are very limited. Examples include 3,6-diazido-1,2,4,5-tetrazine13 and cyanuric azide (2,4,6-triazido-1,3,5-triazine;14 Scheme 4). Energetic materials containing azido groups. DSC Exo.=differential scanning calorimetry exotherm There is no literature precedence for high-nitrogen energetic materials containing hydrazo- and azo-1,3,5-triazine backbones. Although Loew and Weis reported the preparations of three inert compounds (4,4′-di(chloro)-6,6′-di(isopropylamino)azo-1,3,5-triazine, 4,4′,6,6′-tetra(dimethylamino)azo-1,3,5-triazine, and 4,4′,6,6′-tetra(chloro)azo-1,3,5-triazine) in 1976, few physical properties and no crystal structures were available.15 We report herein the synthesis and properties of novel 4,4′,6,6′-tetra(azido)hydrazo-1,3,5-triazine (3) and 4,4′,6,6′-tetra(azido)azo-1,3,5-triazine (4), see Scheme 5. The hydrazo and azo linkages not only desensitize but also dramatically increase the melting point of the polyazido products. Remarkably, the heats of formation of these polyazido compounds (Scheme 5) are much higher than those of polynitro and high-nitrogen compounds (Scheme 1–3). Preparation and properties of 3 and 4. Rapid reaction occurs between 4,4′,6,6′-tetra(chloro)hydrazo-1,3,5-triazine (1)15 and an excess of hydrazine monohydrate (H2NNH2⋅H2O) in CH3CN to give 4,4′,6,6′-tetra(hydrazino)hydrazo-1,3,5-triazine (2) which underwent diazotization to yield 3 (Scheme 5). A suspension of 3 in 1:2 (v/v) H2O:CHCl3 solution was oxidized by chlorine gas at room temperature to 4. All products, 2–4, were isolated and fully characterized by elemental analysis, differential scanning calorimetry (DSC), heat of formation, and IR and 1H/13C NMR spectroscopies.16 Compounds 3 and 4 were also characterized by X-ray crystallography, Figure 1–3, 3.17 Compound 3 has only one polymorph (ρ=1.649 g cm−3) in which two 1,3,5-triazine rings are not co-planar but have a central torsion angle of 105° (Figure 1). Compound 4 crystallized in α and β polymorphs, ρ=1.724 g cm−3 and ρ=1.674 g cm−3 (Figure 2). The β polymorph has two conformers whose azido substituents orient in different directions (Figure 3). A) ORTEP diagram (thermal ellipsoids set at 25 % probability) for 3, B) end-on view: the central torsion angle C6-N7-N8-C9 is 105°, and the two halves of the molecule are fairly planar. The α-polymorph of 4: A) ORTEP diagram (thermal ellipsoids set at 25 % probability) of 4, B) an edge-on view of the molecule, showing the "step" in the azo chain that connects the two separate, essentially planar halves of the molecule. ORTEP diagrams (thermal ellipsoids set at 25 % probability) and labeling scheme of the β polymorph crystallized in two conformers for 4. Both molecules sit on a center of symmetry, and neither molecule is completely planar. Reminiscent of 3,6-di(azido)1,2,4,5-tetrazine and cyanuric azide, none of the azido substituents of 3 and 4 tautomerize to form fused tetrazolo rings even though they were heated in polar solvents. The hydrazo linkage in 3 and azo linkage in 4 result in a non-observable melting point up to their fast decomposition at 200 and 202 °C (DSC), respectively. Consequently, the azo and hydrazo linkages have significantly decreased volatility and increased melting point relative to cyanuric azide. Remarkably, the experimentally measured heat of formation for 4 (Scheme 5) is the highest reported for energetic materials.18 As shown by the ΔHf data in ref. 16 and Scheme 5, the replacement of four hydrazino by four azido substituents in the hydrazo-1,3,5-triazine compound (2→3) increases the energy by 1347 kJ mol−1, and 418 kJ mol−1 is gained in the transformation from 3 into 4 (Scheme 6). A) ΔEsubstitution from the hydrazino to azido substituent and B) ΔEtransformation from the hydrazo to azo linkage. For comparison, the explosive properties and sensitivity of reference PETN, (Figure 1), cyanuric azide, 3, and 4 are given in Table 1.10 Compound DSC fast decomp [°C] Impact H50 (Type 12) [cm] Friction (BAM)[Kg] Spark [J] PETN 178 14.5 5.4 >0.36 cyanuric azide 187 6.2 <0.5 <0.36 3 202 18.3 2.9 <0.36 4 200 6.2 2.4 <0.36 Cyanuric azide is extremely sensitive to friction and spark, and its impact is a half less than that of PETN. Compound 3 is spark sensitive, but its impact and friction are three and six times less sensitive than those of cyanuric azide, respectively (Table 1). The impact and spark sensitivity of 4 are comparable to cyanuric azide, but its friction sensitivity is at least five times less than that of cyanuric azide (Table 1). The compounds in this study are novel and important in demonstrating that the hydrazo and azo linkages can be utilized to desensitize and to decrease volatility of polyazido compounds. The compound 4,4′,6′6,-tetra(azido)azo-1,3,5-triazine (4) has the highest measured heat of formation.