FULL PAPER DOI: 10.1002/asia.201402538
Thermally Stable 3,6-DinitropyrazoloACHTUNGRE[4,3-c]pyrazole-based Energetic Materials Jiaheng Zhang,[a] Damon A. Parrish,[b] and Jean’ne M. Shreeve*[a]
Abstract: 3,6-DinitropyrazoloACHTUNGRE[4,3c]pyrazole was prepared using an efficient modified process. With selected cations, ten nitrogen-rich energetic salts and three metal salts were synthesized in high yield based on the 3,6dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate anion. These compounds were fully characterized by IR and multinuclear NMR spectroscopies, as well as elemental analyses. The structures of the neutral compounds 4 and its salt 16 were con-
firmed by single-crystal X-ray diffraction showing extensive hydrogen-bonding interactions. The neutral pyrazole precursor and its salts are remarkably thermally stable. Based on the calculated heats of formation and measured Keywords: bond dissociation enthalpy · energetic properties · explosives · fused heterocycles · thermal stability
densities, detonation pressures (22.5– 35.4 GPa) and velocities (7948– 9005 m s1) were determined, and they compare favorably with those of TNT and RDX. Their impact and friction sensitivities range from 12 to > 40 J and 80 to 360 N, respectively. These properties make them competitive as insensitive and thermally stable highenergy density materials.
and others,[5] including benzenetrifuroxan (1), trinitrotrisACHTUNGRE(triazolo)benzene (2), furazano-1,2,3,4-tetrazine-1,3-dioxide (FTDO, 3), 3,6-dinitropyrazoloACHTUNGRE[4,3-c]-pyrazole (DNPP, 4), and 1,2,3-triazoloACHTUNGRE[4,5,-e]furazanoACHTUNGRE[3,4,-b]pyrazine 6-oxide (5). DNPP (4), first synthesized by Russian researchers, is attractive because of its high thermal stability and low sensitivity towards mechanical stimuli.[7] Subsequently, the synthesis of 4 was improved using 2,4-pentanedione as starting material to yield DNPP in 21 % overall yield (Scheme 1).[8] However, the synthetic route to DNPP was still rather long and tedious. The synthetic bottleneck described was the extraction of the thermally unstable 4-diazonium-3,5-dimethylpyrazole salt, which also makes DNPP less attractive for large-scale manufacture. In our work, the pretreatment of intermediate 7 was modified by precipitating the crystalline material from solution at 15 8C, which avoids the ex-
Introduction Modern high-energy density materials (HEDM) continue to attract considerable interest because of their military and civilian applications as propellants, explosives, and pyrotechnics.[1] The majority of the energetic compounds are designed based on versatile heterocyclic rings such as tetrazoles, triazoles, pyrazoles, imidazoles, oxadiazoles, etc.[2] These moieties possess a large number of energetic NN, CN, and NO bonds, and exhibit high heats of formation, densities, and oxygen balance.[3] In comparison with single heterocyclic rings or coupled heterocyclic ring-based energetic molecules, fused cyclic nitrogen-containing heterocycles are, at the moment, the ascending stars in the energetic field as they have become a major focus in energetic materials research.[4] Including all these advantages, fused azacyclic compounds exhibit better thermochemical properties, which makes them safe for production, transfer, and storage.[5] Furthermore, the tremendous ring-strain energy stored in these kinds of materials can be used to improve their detonation properties.[6] Many fused-ring-based high-performance explosives (Figure 1) have been developed by our group[4, 6] [a] Dr. J. Zhang, Prof. Dr. J. M. Shreeve Department of Chemistry, University of Idaho Moscow, ID 838344-2343 (USA) Fax: (+ 1) 208-885-9146 E-mail: [email protected] [b] Dr. D. A. Parrish Naval Research Laboratory, Code 6030 Washington, D.C.20375-5001 (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201402538.
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Figure 1. Recently developed fused heterocycle-based high-performance energetics.
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Here, 7 is obtained by precipitation from solution at 15 8C as a light yellow crystalline solid in 84 % yield. With 7 in hand, the fused-pyrazole ring was easily constructed through a rearrangement reaction using acetic acid as a catalyst. Several reagents for the nitration of the pyrazole ring are available; here, 65 % nitric acid and fuming nitric acid were used for the nitration of 8 and 10, respectively. Sodium dichromate was chosen as the oxidizing agent to convert the methyl group of 9 into a carboxy group. As a good method for scaling up production, this strategy allows for easy purification by washing with ice water in the last three steps to give analytical pure compounds. Reactions of 4 with ammonia, hydrazine, hydroxylamine, 3,5-diamino-1,2,4-triazole, and 3,4,5-triamino-1,2,4-triazole resulted in the formation of salts 11–15 (Scheme 2). Other energetic salts, 16–20, were readily synthesized by metathesis reactions of guanidine nitrate, aminoguanidine, diaminoguanidine, triaminoguanidinium, and 2-iminium-5nitriminooctahydroimidazoACHTUNGRE[4,5-d]imidazole hydrochlorides with sodium 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate [Na2 ACHTUNGRE(DNPP)]. Metal salts 21–23 were obtained in a straightforward manner by the reaction of 4 with sodium hydroxide, potassium hydroxide, and silver nitrate, respectively. With the exception of mono-3,5-diamino-triazolium3,6dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate (14), mono-triaminoguanidinium 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate (19), and 2-iminium-5-nitriminooctahydroimidazo [4,5-d]imidazole3,6dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate (20), products were isolated as dicationic salts. The structures of salts 11–23 were determined by IR and multinuclear NMR spectroscopies, as well as elemental analysis. Each of them is non-hygroscopic and stable in air. As a strong acid, the resonance band for the protons in 4 was observed at 15.06 ppm in the 1H NMR spectrum. For other salts, the hydrogen signals of the cations were easily assigned as there is no proton associated with the DNPP anion. In the 13C NMR spectra, except for two signals (d 139 and 133 ppm) assigned to the DNPP anion, the other signals are associated with the nitrogen-rich cations. The proton-decoupled 15N NMR spectrum of 4 was recorded in [D6]acetone, while those of 12 and 16 were measured in [D6]DMSO. The chemical shifts are given with respect to CH3NO2 as an external standard (Figure 2). The 15N NMR spectrum of 4 has three signals at d = 25.60, 59.78, and 211.28 ppm. For all three compounds, the nitrogen signals for C-nitro are observed between 19.60 and 25.60 ppm, which agrees with the values reported in the literature.[10] In the IR spectra, several main absorption bands at around 1595, 1371, 1242, 1126, and 826 cm1 are attributed to the DNPP anion, while the intense absorption bands in the
Scheme 1. Synthesis route to DNPP, 4.
traction step. Additionally, a one-pot reaction was carried out to obtain 6 in a more efficient and shorter procedure. By taking advantage of the reactivity of the acidic NH group(s) on polynitro-substituted energetic heterocycles, the synthesis of high-nitrogen energetic salts has become a recent focus of attention.[9] This is primarily because saltbased energetic materials often possess superior properties relative to non-ionic species because they tend to exhibit lower vapor pressures, lower impact and friction sensitivities, as well as enhanced thermal stabilities.[1b] Recently, a large number of nitrogen-rich salts have been explored by our group and the polynitro-substituted pyrazole-based salts especially show competitive energetic properties.[10] Therefore, it was of interest to study similar salts with nitro-substituted fused pyrazole rings as the anion. Although a limited number of 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazole salts were mentioned in a US patent, full characterization and sensitivity tests were not reported.[11] Compound 4 was chosen as the precursor to be investigated thoroughly for the design and synthesis of new ionic energetic derivatives. We now report the modified synthesis of 4 which is used as a starting material to develop various energetic salts. All compounds were fully characterized by IR and multinuclear NMR spectroscopies, elemental analysis, and differential scanning calorimetry (DSC). The structure of 4 as well as that of its guanidinium salt was further confirmed by singlecrystal X-ray structure analysis. The detonation properties of these compounds were calculated and the values suggest that they are insensitive and thermally stable energetic materials. In addition, quantum chemical investigations were performed to assist in the understanding of the decomposition mechanism and to help identifying the trigger bonds of the selected molecules.
Results and Discussion In this study, DNPP (4) was synthesized using a modified route in which compound 6 was prepared by a one-pot reaction starting with 2,4-pentanedione in yields approaching 90 %.The product was sufficiently pure to be used directly in the following reactions. In previous reports, a pretreatment of 7 was carried out by extracting with dichloromethane.[8]
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range of 3100 to 3600 cm1 are assigned to the NH bonds of the nitrogen-rich cations. Suitable crystals of 4 and 16 for X-ray diffraction were obtained by slow recrystallization from DMSO and aqueous solution at room temperature, respectively. Their structures are shown in Figures 3 and 4, and crystallographic and structural refinement data are given in Table 1. Crystal 4·2 DMSO crystallizes in the monoclinic space group P21/n with a calculated density of 1.563 g cm3. Compound 16 crystallizes in the monoclinic space group P21/ c with a calculated density of 1.684 g cm3. The structure of 16 is stabilized by various Hbonds involving all of the hyScheme 2. Synthesis of 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate salts (G, guanidine; AG, aminoguanidine, DAG, didrogen atoms of the guanidiniaminoguanidine; TAG, triaminoguanidine; INI-Cl, 2-iminium-5-nitriminooctahydroimidazo [4,5-d]imidazole um cation, the ring nitrogen, chloride). and nitro oxygen atoms. The bond lengths of the intermolecular hydrogen bonds are in the range of 2.93 to 3.02 . For crystals 4·2 DMSO and 16, the nitro-substituted fused pyrazole rings are strictly coplanar with torsion angles near zero or 1808, and the Cnitro bond length is in the range of 1.3822–1.4280 . The thermal stabilities of DNPP as well as its salts were determined by differential scanning calorimetric (DSC) measurements at a heating rate of 5 8C min1 (Table 2). As expected, the decomposition temperatures of DNPP and its salts range from 209 (18) to 395 8C (4), respectively. The corresponding DSC curves of selected compounds 4, 12, 16, and 22 are shown in Figure 5. While 4, 13, 15, 18, and 20–23 decompose without melting, the remaining salts melted prior to decomposition between 160 (11) to 318 8C (16). It is worthwhile to point out that DNPP as well as its ammonium (11), hydroxylammonium (12), and guanidinium salts (16) have remarkable high decomposition temperatures of > 300 8C; moreover, its sodium (21) and potassium (22) salts are thermally stable up to 395 8C and 365 8C, respectively, which make them very attractive in special explosive applications under extreme conditions such as deep oil drilling. Various studies have illustrated that the bond dissociation enthalpies (BDEs) for the possible trigger bond, which is the first bond to break in decomposition, is often a key factor when investigating the pyrolysis mechanism for energetic molecules. Therefore, to explore the relationship between bond strength and the relative thermal stability of these interesting molecules, the BDEs of 4, 11, 12, and 13 were selectively calculated. For most of polynitro energetic materials, the Cnitro or Nnitro bond strengths were found to be the trigger bond and related to the thermal staFigure 2. 15N NMR spectra of compounds 4, 12, and 16.
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bilities.[12] Therefore, in this study, the BDEs of Cnitro in selected compounds are listed in Table 3 and compared with Cnitro BDEs of RDX, HMX, and TATB. The calculated BDEs for DNPP and selected salts were found to be > 271 kJ mol1, which are much higher than those of RDX and HMX. However, the Cnitro Figure 3. (a) A view of the thermal ellipsoid plot (30 %) and labeling scheme of 4·2 DMSO. (b) Unit cell view BDE of TATB is about along the a axis. Dashed lines indicate hydrogen bonding. 50 kJ mol1 higher than those of the title compounds, which makes TATB the most thermal stable compound in the competition. These differences also indicate that strong hydrogen bonds play a more important role in increasing the stability of the molecule than the fused ring system. It is interesting to note that the NN BDE of the hydrazinium cation, NH2NH3 + , is 32 kJ mol1 lower than that of Cnitro in compound 13. Figure 4. (a) A view of the thermal ellipsoid plot (30 %) and labeling scheme of 16. (b) Unit cell view along Therefore, it is likely that the the a axis. Dashed lines indicate hydrogen bonding. NN cation bond is the weakest and the trigger bond in this DH f ðsalt, 298 KÞ ¼ DH f ðcation, 298 KÞ molecule; this would also explain the lower decomposition ð2Þ þDH f ðanion, 298 KÞDH L temperature of 13. The heat of formation of the nitrogen-rich cation, DNPP, in which DHL could be predicted by using the formula and the DNPP anion and dianion were calculated by using suggested by Jenkins et al. [Eq. (3)]:[19] the Gaussian 03 (Revision D. 01) suite of programs[13] using atomization and isodesmic reaction approaches (see the Supporting Information). The geometric optimization and frequency analyses of the structures are based on available single-crystal structures and using the B3LYP functional with the 6-31 + G** basis set.[14] Single-point energies were calculated at the MP2/6-311 + + G** level.[15] Atomization energies for cations were obtained by using the G2 ab initio method.[16] All of the optimized structures were characterized without imaginary frequencies. The solid-state enthalpy of formation can be calculated for neutral compounds by subtracting the heat of sublimation from the gas-phase heats of formation. Based on the literature,[17] the heat of sublimation can be corrected with Troutons rule according to Equation (1), where T represents either the melting point or the decomposition temperature when no melting occurs prior to decomposition:[18]
DH L ¼ U pot þ ½pðnM=22Þ þ qðnX=22ÞRT
ð3Þ
In this equation, nM and nX depend on the nature of the ions Mp + and Xq, respectively, and are equal to three for monoatomic ions, five for linear polyatomic ions, and six for nonlinear polyatomic ions. The equation for the lattice potential energy Upot [Eq. (4)] has the form: U pot ½kJ mol1 ¼ gð1m =Mm Þ1=3 þ d
ð4Þ
ð1Þ
in which 1m [g cm3] is the density, Mm is the chemical formula mass of the ionic material, and values for g and the coefficients g (kJ mol1 cm) and d (kJ mol1) are assigned literature values.[3f] The calculated values for the nitrogen-rich cations vary between 575.4 and 883.6 kJ mol1. All of the DNPP salts exhibit positive heats of formation, with 14 and 15 having the highest at 2.76 and 2.30 kJ g1 (RDX: 0.36 kJ g1).
Based on Born–Haber energy cycles (see the Supporting Information), heats of formation of DNPP salts can be simplified by the formula given in Equation (2):
Density is a very important property contributing to the detonation performance of energetic materials. The densities of these new compounds, measured using a gas pycnometer, fall in the range between 1.67 and 1.85 g cm3 for 4 and 11–
DH sub ¼ 188 J mol1 K1 T
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Table 1. Crystallographic data and structure refinement parameters for 4·2 DMSOand 16.
Empirical formula CCDC number[a] Temperature Crystal system Space group a [] b [] c [] a [8] b [8] g [8] V [3] Z Density [mg m3] M [mm1] F (000) Crystal size [mm] q range [8] Index ranges
Reflections collected Independent reflection Goodness-of-fit on F2 Final R indices [I > 2 sigma(I)] R indices (all data) Largest diff. peak and hole
4·2 DMSO
16
C6H2N6O4,2ACHTUNGRE(C2H6OS) 999620 150(2) K Monoclinic P21/n 4.6473(2) 15.1140(7) 10.9532(5) 90 101.7780(10) 90 753.15(6) 4 1.563 0.392 368 0.254 0.261 0.296 2.33 to 26.74 5 < = h < = 5, 19 < = k < = 19, 13 < = l < = 13 9330 1593 [Rint = 0.0111]
C6H12N12O4 983224 150(2) K Monoclinic P21/c 3.5077(3) 10.0767(8) 17.6548(14) 90 91.124(3) 90 623.91(9) 2 1.684 0.142 328 0.61 0.10 0.06 2.31 to 26.51 4 < = h < = 4, 12 < = k < = 12, 21 < = l < = 22 5652 1286 [Rint = 0.0258]
1.083 R1 = 0.0243, wR2 = 0.0679 R1 = 0.0258, wR2 = 0.0691 0.375 and 0.285 e 3
1.092 R1 = 0.0318, wR2 = 0.0820 R1 = 0.0363, wR2 = 0.0847 0.288 and 0.228 e 3
Figure 5. DSC plots of compounds 4, 12, 16, and 22 measured at a heating rate of 5 K min1 (exo up).
ly. With the experimental data found for the densities and the calculated heats of formation, the detonation pressure (P), and velocity (vD) of 4 and 11–20 were calculated using EXPLO5 v6.01.[20] As can be seen in Table 1, the calculated detonation velocities lie between nD = 7948 (16) and 9005 m s1 (12). Compounds 13 (8860 m s1) and 19 (8814 m s1) also exhibit rather high values which are superior to TNT (6881 m s1) and comparable to RDX (8748 m s1). The detonation pressures lie in the range 22.5– 35.4 GPa, which are also higher than that of TNT (19.5 GPa) and essentially equal to RDX (34.9 GPa). Oxygen balance (W) is the index of the excess or deficiency of oxygen percentage in a compound required to convert all C atoms into CO and all H atoms into H2O for a compound with a formula of CaHbOcNd, W (%) = 1600 (cab/2)/Mw (Mw, molecular weight). In this study, the neutral compound 4 has an oxygen balance of 8 %, which is superior to that
[a] These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
20, which is comparable with those of currently used explosives (1.6–1.8 g cm3). The densities of the metal salts 21–23 with heavy cations are 2.14, 2.20, and 3.27 g cm3, respective-
Table 2. Properties of DNPP and its salts compared with TNT and RDX. Comp.
Tm[a] [8C]
Tdec[b] [8C]
W[c] [%]
d[d] ACHTUNGRE[g cm3]
DfH[e] [kJ mol1/ACHTUNGRE(kJ g1)]
vD[f] ACHTUNGRE[m s1]
P[g] ACHTUNGRE[GPa]
IS[h] [J]
FS[i] [N]
Isp[j] [s]
4 11 12 13 14 15 16 17 18 19 20 21 22 23 TNT RDX
– 160 174 – – – 318 213 – 208 – – – – 81 –
336 328 327 247 287 289 324 222 209 215 238 395 365 327 295 230
8 27 12 30 30 41 40 41 42 31 27 0 0 0 27 0
1.85 1.69 1.82 1.72 1.71 1.67 1.68 1.69 1.71 1.76 1.79[k] 2.14[k] 2.20 3.27 1.65 1.82
322.6/1.94 158.5/0.68 274.2/1.04 501.0/1.91 481.9/1.62 963.8/2.30 173.3/0.55 477.0/1.38 679.6/1.80 605.5/2.00 505.6/1.32 – – – 115/0.26 80.0/0.36
8250 8212 9005 8860 8036 8230 7948 8400 8732 8814 8355 – – – 6881 8748
27.4 25.4 35.4 30.3 24.5 24.6 22.5 25.6 28.0 29.9 27.9 – – – 19.5 34.9
15 > 40 29 16 > 40 > 40 > 40 > 40 > 40 12 23 14 > 40 29 15 7.4
160 360 360 160 360 360 360 360 360 80 160 160 160 160 353 120
241 218 260 243 220 225 199 216 225 235 220 – – – 211 258
[a] Melting temperature. [b] Thermal decomposition temperature (onset) under nitrogen gas (DSC, 5 8C min1). [c] Oxygen balance for CaHbOcNd, 1600ACHTUNGRE(c-a-b/2)/Mw; Mw = molecular weight. [d] Gas pycnometer (25 8C). [e] Calculated heat of formation. [f] Detonation velocity. [g] Detonation pressure. [h] Impact sensitivity. [i] Friction sensitivity. [j] Specific impulse for the neat compound. [k] values were corrected by subtracting the volume of a water molecule [VACHTUNGRE(H2O) 25 3].
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(7948–9005 m s1) and detonation pressures (22.5–35.4 GPa) are comparable with those of TNT and RDX. Most of the salts have reasonable impact sensitivities (12 to > 40 J) and friction sensitivities (80 to 360 N), thus making the DNPP salts potentially useful as thermally stable and insensitive energetic materials.
Table 3. Calculated BDEs for 4, 11, 12, and 13 and their decomposition temperatures. Compound
Bond type
Bond length []
BDE(UB3LYP/ 6-31G*/kJ mol1)[a]
Td [8C]
4 11 12 13
C-NO2 C-NO2 C-NO2 C-NO2 NH2-NH3 + C-NO2 C-NO2 C-NO2
1.44 1.43 1.43 1.43 1.44 – – –
288 305 291 303 271 161 166 355
336 328 327 247
RDX[a] HMX[a] TATB[a]
Experimental Section
210 280 340
Caution! Although none of the compounds described herein has exploded or detonated unexpectedly in the course of this research, these materials should be handled with extreme care by using the best safety practices (leather gloves, face shield).
[a] Ref. [13c].
General Methods
of its nitrogen-rich salts (12 to 42 % for 11–20). All metal salts have the highest oxygen balance of 0 which is because of hydrogen-free molecular formulas. This series of salts has an oxygen balance comparable to TNT. Compounds 4, 12, and 13 also exhibit very promising specific impulse values (Isp > 240 s) which are much higher than that of TNT (211 s) and comparable to that of RDX (258 s). For initial safety testing, the impact and friction sensitivities were tested according to Bundesamt fr Materialforschung (BAM) standard methods. As can be seen in Table 1, the impact sensitivity (IS) of DNPP was found to be 15 J. Not surprisingly, improved impact stabilities for DNPP salts were found for 11–18 (16 to > 40 J) except that of 19 which is < 12 J because of the remaining acidic NH. These data show that the presence of organic cations can result in relatively insensitive energetic fused-ring compounds. The metal salts with water(s) of crystallization cause a significant decrease in IS. For example, 22 as a dihydrate has an IS of > 40 J, which is less sensitive than 23 as a monohydrate (29 J) and 21 as an anhydrous analogue (14 J). In terms of friction sensitivity, most of the salts (11, 12, 14–18) could not be initiated even with a force of 360 N, which suggests that they can serve as promising candidates for safe energetic materials.
Analytical grade reagents were purchased from Aldrich and Acros Organics and were used as received. 1H and 13C NMR spectra were recorded on a 300 MHz (Bruker AVANCE 300) NMR spectrometer operating at 300.13 and 75.48 MHz, respectively. A 500 MHz (Bruker AVANCE 500) NMR spectrometer operating at 50.69 MHz was used to obtain 15 N NMR spectra. [D6]DMSO was employed as a locking solvent unless otherwise stated. Chemical shifts in 1H and 13C NMR spectra are reported relative to Me4Si and in 15N NMR spectra relative to MeNO2. The melting and decomposition points were recorded using a differential scanning calorimeter (TA Instruments Co., model Q10) at a scan rate of 5 8C min1. IR spectra were recorded using KBr pellets for solids on a BIORAD model 3000 FTS spectrometer. Densities were measured at room temperature using a Micromeritics AccuPyc 1330 gas pycnometer. Elemental analyses were carried out on an Exeter CE-440 elemental analyzer. X-Ray Crystallography A colorless rod of dimensions 0.25 0.26 0.30 mm3 (4·2 DMSO) and an orange rod of dimensions 0.61 0.10 0.06 mm3 (16) were mounted on a MiteGen MicroMesh using a small amount of Cargille Immersion Oil. Data were collected on a Bruker three-circle platform diffractometer equipped with a SMART APEX II CCD detector. The crystals were irradiated using graphite monochromated MoKa radiation (l = 0.71073). An Oxford Cobra low-temperature device was used to keep the crystals at a constant temperature of 150(2) K during data collection. Data collection was performed and the unit cell was initially refined using APEX2 [v2010.3–0].[21] Data reduction was performed using SAINT [v7.68A],[22] and XPREP [v2008/2].[23] Corrections were applied for Lorentz, polarization, and absorption effects using SADABS [v2008/1].[24] The structures were solved and refined with the aid of the programs in the SHELXTLplus [v2008/4] system of programs.[25] The full-matrix least-squares refinement on F2 included atomic coordinates and anisotropic thermal parameters for all non-H atoms. The H atoms were included using a riding model.
Conclusions A modified, more efficient, and repeatable synthesis process for 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazole (4) was developed. A series of nitrogen-rich energetic salts based on the anion of 4 were prepared and fully characterized. Structures of 4·2 DMSO and 16 were confirmed by single-crystal X-ray diffraction and show extensive hydrogen-bonding interactions. This neutral compound and the corresponding salts exhibit outstanding thermal stabilities (209–395 8C). The quantum chemical investigation assisted in the understanding of the decomposition mechanism and suggests the trigger bond in both cation and anion. Densities for these salts fall in the range between 1.66 and 2.20 g cm3, which places them in a class of relatively dense energetic materials. Their detonation properties were evaluated by Gaussian 03 and EXPLO5 calculations. The calculated detonation velocities
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3,5-Dimethyl-1H-pyrazol-4-amine (6) This compound was prepared according to the reference method.[26] 4-Diazonium-3,5-dimethylpyrazole (7)[27] Compound 6 (28 g) was dissolved in acetic (43 mL) and water (40 mL) at room temperature. After a clear solution was formed, the solution was cooled to 0–5 8C, and sodium nitrite (17.3 g) in water (40 mL) was added dropwise. After complete addition, the mixture was stirred at the same temperature for 5 h. The pH of the mixture was then adjusted to 8 using aqueous ammonia. After blowing air over the mixture for 6 h, the solution was allowed to remain at 15 8C for 2 h, which resulted in formation of yellow crystals. They were collected by filtration and dried under reduced pressure to afford 7 in 84 % yield (25.8 g). 1H NMR: d = 2.38 ppm (6 H, CH3); 13C NMR: d = 12.47, 76.67, 154.93 ppm.
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3,6-DinitropyrazoloACHTUNGRE[4,3-c]pyrazole (4)
was added. The reaction mixtures were stirred with heating (50 8C) for 6 h and then cooled to room temperature. The resulting precipitate was collected by filtration and washed with methanol to give solid products in good yield.
With 7 in hand, intermediates 8, 9, and 10 were prepared according to literature procedures.[27] The synthesis of 4 was accomplished using a slightly modified method. To cooled 100 % nitric acid (9 mL, 0 ~ 5 8C), 10 (1.47 g, 7.5 mmol) was added in portions with stirring over a period of 10 min. The mixture was stirred at 0–5 8C for 30 min and then at 65 8C for 12 h. Subsequently, the reaction mixture was poured onto ice, the precipitate was filtered, washed with cold water, and air dried to yield 4 (1.21 g, 82 %) as an ivory solid; Tdec (onset): 336 8C; IR (KBr): n˜ = 3267, 1548, 1522, 1430, 1369, 1349, 1244, 1146, 1036, 835, 656, 480 cm1; 1H NMR: d = 15.06 ppm (s, 2 H, NH); 13C NMR ([D6]acetone): d = 139.28, 132.70 ppm; elemental anal. calcd (%) for C4H2N6O4 (198.10): C 24.25, H 1.02, N 42.42; found: C 24.17, H 1.07, N 41.96.
Diguanidinium 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate (16) Orange red solid; Tm : 318 8C; Tdec (onset): 324 8C; IR (KBr): n˜ = 3456, 1658,1459, 1412, 1365, 1219, 1089 cm1; 1H NMR: d = 7.47 (s, 8 H, NH2), 5.44 ppm (s, 12 H, NH2); 13C NMR: d = 158.37, 143.60, 141.36 ppm; elemental anal. calcd (%) for C6H12N12O4 (316.24): C 22.79, H 3.82, N 53.15; found: C 23.34, H 3.79, N 52.60. Di(aminoguanidinium) 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate (17) Orange-yellow solid; Tm : 213 8C; Tdec (onset): 222 8C; IR (KBr): n˜ = 3343, 3263, 1690, 1656, 1454, 1419, 1353, 1208, 1071, 820 cm1; 1H NMR: d = 8.60 (s, 2 H, NH), 7.75 ppm (s, 12 H, NH2); 13C NMR: d = 159.01, 143.23, 141.24 ppm; elemental anal. calcd (%) for C6H14N14O4 (346.27): C 20.81, H 4.08, N 56.63; found: C 20.97, H 3.98, N 55.97.
General Procedure for the Synthesis of Salts 11–15 3,6-DinitropyrazoloACHTUNGRE[4,3-c]pyrazole (0.20 g, 1 mmol) was suspended in methanol (3 mL) and water (0.5 mL). Next, aqueous ammonia (0.14 g, 2.1 mmol), hydrazine monohydrate (0.105 g, 2.1 mmol), hydroxylamine (0.139 g, 50 wt % in water, 2.1 mmol), 3,5-diaminotriazole (0.208 g, 2.1 mmol) or 3,4,5-triaminotriazole (0.239 g, 2.1 mmol) was added. The reaction mixtures were stirred with heating (50 8C) for 6 h and then cooled to room temperature. The resulting precipitate was collected by filtration and washed with methanol to give solid products in good yield.
Di(diaminoguanidinium) 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate (18) Yellow solid; Tdec (onset): 209 8C; IR (KBr): n˜ = 3337, 3274, 1675, 1456, 1429, 1346, 1215, 1069, 1018, 818 cm1; 1H NMR: d = 8.63 (br, s, 4 H, NH2 + ), 7.14 (s, 4 H, NH), 4.54 ppm (br s, 8 H, NH2); 13C NMR: d = 159.92, 143.40, 141.34 ppm; elemental anal. calcd (%) for C6H16N16O4 (376.30): C 19.15, H 4.29, N 59.56; found: C 19.28, H 4.16, N 60.06.
Diammonium 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate (11)
Mono-triaminoguanidinium 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate (19)
Orange solid; Tm : 160 8C; Tdec (onset): 328 8C; IR (KBr): n˜ = 3273, 3235, 1686, 1474, 1401, 1350, 1217, 1074, 822 cm1; 1H NMR: d = 6.11 ppm (s, 8 H, NH4 + ); 13C NMR: d = 141.43, 140.52 ppm; elemental anal. calcd (%) for C4H8N8O4 (232.16): C 20.69, H 3.47, N 48.27; found: C 20.76, H 3.44, N 47.70.
Yellow solid; Tm : 208 8C; Tdec (onset): 215 8C; IR (KBr): n˜ = 3345, 1686, 1497, 1375, 1343, 1231, 1108, 1036, 823 cm1; 1H NMR: d = 13.70 (br, 1 H, ring N-H) 8.60 (s, 3 H, NH), 4.49 ppm (s, 6 H, NH2); 13C NMR: d = 159.08, 139.71, 137.85 ppm; elemental anal. calcd (%) for C5H10N12O4 (302.21): C 19.87, H 3.34, N 55.62; found: C 19.85, H 3.31, N 54.93.
Dihydroxylammonium 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate (12) Orange-yellow solid; Tm : 174 8C; Tdec (onset): 327 8C; IR (KBr): n˜ = 3156, 1480, 1439, 1371, 1302, 1242, 1126, 1008, 826 cm1; 1H NMR: d = 8.16 ppm (s, 6 H, NH3 + ); 13C NMR: d = 139.68, 137.50 ppm; elemental anal. calcd (%) for C4H8N8O6 (264.16): C 18.19, H 3.05, N 42.42; found: C 18.56, H 3.03, N 41.62.
Mono-2-iminium-5-nitriminooctahydroimidazo [4,5-d]imidazole3,6dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate monohydrate (20) Yellowish gray solid; Tdec (onset): 238 8C; IR (KBr): n˜ = 3343, 1701, 1581, 1466, 1352, 1285, 1257, 1220, 1075, 892, 822 cm1; 1H NMR: d = 5.71 ppm (s, 2 H); 13C NMR: d = 161.11, 160.04, 139.69, 137.93, 72.23 ppm; elemental anal. calcd (%) for C8H11N13O7 (401.26): C 23.95, H 2.76, N 45.38; found: C 23.96, H 3.06, N 46.41.
Dihydrazinium 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate (13) Yellow solid; Tdec (onset): 247 8C; IR (KBr): n˜ = 3351, 1470, 1439, 1362, 1297, 1233, 1104, 964, 822 cm1; 1H NMR: d = 6.10 ppm (s, 10 H, NH2NH3 + ); 13C NMR: d = 141.11, 139.48 ppm; elemental anal. calcd (%) for C4H10N10O4 (262.19): C 18.32, H 3.84, N 53.42; found: C 18.41, H 3.79, N 52.81.
General Procedure for the Synthesis of Metal Salts 21--23 A solution of 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazole (0.20 g, 1 mmol) in methanol (6 mL) was stirred at room temperature while adding sodium hydroxide (0.084 g, 2.1 mmol in 2 mL H2O), potassium hydroxide(0.118 g, 2.1 mmol in 2 mL H2O), or silver nitrate (0.357 g, 2.1 mmol in 5 mL H2O). The reaction mixtures were stirred for 1 h, and the resulting precipitate was collected by filtration and washed with methanol to give solid products in good yield.
Mono-3,5-Diamino-triazolium 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate (14) Light yellow solid; Tdec (onset): 287 8C; IR (KBr): n˜ = 3447, 3372, 2445, 1832, 1615, 1543, 1414, 1386, 1240, 1145, 1009, 820, 749 cm1; 1H NMR: d = 8.20 ppm (br, 6 H, NH and NH2); 13C NMR: d = 155.27, 138.68, 134.40 ppm; elemental anal. calcd (%) for C6H7N11O4 (297.19): C 24.25, H 2.37, N 51.84; found: C 24.13, H 2.36, N 51.54.
Disodium 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate dihydrate (21) Orange solid; Tdec (onset): 395 8C; IR (KBr): n˜ = 3607, 3544, 1650, 1477, 1440, 1355, 1225, 1086, 816, 752 cm1; elemental anal. calcd (%) for C4H4N6O6Na2 (278.09): C 17.28, H 1.45, N 30.22; found: C 16.84, H 1.39, N 29.87.
Di(3,4,5-triamino-triazolium) 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate (15) Orange solid; Tdec (onset): 289 8C; IR (KBr): n˜ = 3345, 3167,1663, 1449, 1360, 1300, 1234, 1088, 1046, 820 cm1; 1H NMR: d = 6.86 (br, 8 H, NH2), 5.44 ppm (s, 4 H, NH2); 13C NMR: d = 150.48, 139.67, 137.83 ppm; elemental anal. calcd (%) for C8H14N18O4 (426.31): C 22.54, H 3.31, N 59.14; found: C 22.70, H 3.30, N 59.68.
Dipotassium 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate (22) Orange solid; Tdec (onset): 365 8C; IR (KBr): n˜ = 1452, 1428, 1369, 1272, 1211, 1080, 824, 752 cm1; elemental anal. calcd (%) for C4N6O4K2 (274.28): C 17.52, H 0.00, N 30.64; found: C 17.23, H 0.14, N 30.98.
General Procedure for the Synthesis of Salts 16–20
Disilver 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate monohydrate (23)
A solution of 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazole (0.20 g, 1 mmol) in methanol (4 mL) was stirred at room temperature while adding dilute sodium hydroxide solution (5 mL, 0.4 n). To this solution, guanidine nitrate (0.256 g, 2.1 mmol), aminoguanidine monohydrochloride (0.232 g, 2.1 mmol), diaminoguanidine monohydrochloride (0.264 g, 2.1 mmol), triaminoguanidine monohydrochloride (0.155 g, 1.1 mmol) or 2-iminium-5nitriminooctahydroimidazo [4,5- d]imidazole chloride (0.245 g, 1.1 mmol)
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Orange solid; Tdec (onset): 327 8C; IR (KBr): n˜ = 3524, 1501, 1466, 1366, 1245, 1111, 827, 745 cm1; elemental anal. calcd (%) for C4H2N6O5Ag2 (429.83): C 11.18, H 0.47, N 19.55; found: C 10.99, H 0.47, N 19.74.
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Acknowledgements
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We are grateful to Dr. Clifford Bedford, ONR (NOOO14–12–1–0536).
[1] a) R. P. Singh, R. D. Verma, D. T. Meshri, J. M. Shreeve, Angew. Chem. Int. Ed. 2006, 45, 3584 – 3601; Angew. Chem. 2006, 118, 3664 – 3682; b) H. Gao, J. M. Shreeve, Chem. Rev. 2011, 111, 7377 – 7436; c) J. Zhang, C. He, D. A. Parrish, J. M. Shreeve, Chem. Eur. J. 2013, 19, 8929 – 8936. [2] a) G.-H. Tao, B. Twamley, J. M. Shreeve, J. Mater. Chem. 2009, 19, 5850 – 5854; b) Y. Zhang, Y. Huang, D. A. Parrish, J. M. Shreeve, J. Mater. Chem. 2011, 21, 6891 – 6897; c) V. Thottempudi, H. Gao, J. M. Shreeve, J. Am. Chem. Soc. 2011, 133, 6464 – 6471; d) H. Gao, C. Ye, O. D. Gupta, J. Xiao, M. A. Hiskey, B. Twamley, J. M. Shreeve, Chem. Eur. J. 2007, 13, 3853 – 3860; e) Z. Fu, R. Su, Y. Wang, Y. Wang, W. Zeng, N. Xiao, Y. Wu, Z. Zhou, J. Chen, F. Chen, Chem. Eur. J. 2012, 18, 1886 – 1889; f) H. Huang, Z. Zhou, L. Liang, J. Song, K. Wang, D. Cao, W. Sun, C. Bian, M. Xue, Chem. Asian J. 2012, 7, 707 – 714. [3] a) D. E. Chavez, M. A. Hiskey, R. D. Gilardi, Angew. Chem. Int. Ed. 2000, 39, 1791 – 1793; Angew. Chem. 2000, 112, 1861 – 1863; b) D. M. Badgujar, M. B. Talawar, S. N. Asthana, P. P. Mahulikar, J. Hazard. Mater. 2008, 151, 289 – 305; c) Y.-H. Joo, J. M. Shreeve, J. Am. Chem. Soc. 2010, 132, 15081 – 15090; d) F. Boneberg, A. Kirchner, T. M. Klapçtke, D. G. Piercey, M. J. Poller, J. Stierstorfer, Chem. Asian J. 2013, 8, 148 – 159; e) A. A. Dippold, T. M. Klapçtke, Chem. Asian J. 2013, 8, 1463 – 1471; f) J. Zhang, J. M. Shreeve, J. Am. Chem. Soc. 2014, 136, 4437 – 4445. [4] V. Thottempudi, P. Yin, J. Zhang, D. A. Parrish, J. M. Shreeve, Chem. Eur. J. 2014, 20, 542 – 548. [5] J. P. Agrawal, R. D. Hodgson, Organic Chemistry of Explosives, Wiley, New York, 2007. [6] V. Thottempudi, F. Forohor, D. A. Parrish, J. M. Shreeve, Angew. Chem. Int. Ed. 2012, 51, 9881 – 9885; Angew. Chem. 2012, 124, 10019 – 10023. [7] S. A. Shevelev, I. L. Dalinger, T. K. Shkineva, B. I. Ugrak, V. I. Gulevskaya, M. I. Kanishchev, Russ. Chem. Bull. 1993, 42, 1063 – 1068. [8] J. Cutting, J. Forbes, F. Garcia, R. Lee, D. M. Hoffman, A. R. Mitchell, P. F. Pagoria, R. D. Schmidt, R. L. Simpson and R. L. Swansiger, Insensitive Munitions and Energetic Materials Technology Symposium, San Diego, CA, 1998. [9] a) R. Wang, H. Xu, Y. Guo, R. Sa, J. M. Shreeve, J. Am. Chem. Soc. 2010, 132, 11904 – 11905; b) V. Thottempudi, J. M. Shreeve, J. Am. Chem. Soc. 2011, 133, 19982 – 19992. [10] a) Y. Zhang, D. A. Parrish, J. M. Shreeve, J. Mater. Chem. 2012, 22, 12659 – 12665; b) Y. Zhang, D. A. Parrish, J. M. Shreeve, Chem. Eur. J. 2012, 18, 987 – 994; c) K. Wang, D. A. Parrish, J. M. Shreeve, Chem. Eur. J. 2011, 17, 14485 – 14492.
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Received: May 19, 2014 Published online: August 22, 2014
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Thermally Stable 3,6-DinitropyrazoloACHTUNGRE[4,3-c]pyrazole-based Energetic Materials Jiaheng Zhang,[a] Damon A. Parrish,[b] and Jean’ne M. Shreeve*[a]
Abstract: 3,6-DinitropyrazoloACHTUNGRE[4,3c]pyrazole was prepared using an efficient modified process. With selected cations, ten nitrogen-rich energetic salts and three metal salts were synthesized in high yield based on the 3,6dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate anion. These compounds were fully characterized by IR and multinuclear NMR spectroscopies, as well as elemental analyses. The structures of the neutral compounds 4 and its salt 16 were con-
firmed by single-crystal X-ray diffraction showing extensive hydrogen-bonding interactions. The neutral pyrazole precursor and its salts are remarkably thermally stable. Based on the calculated heats of formation and measured Keywords: bond dissociation enthalpy · energetic properties · explosives · fused heterocycles · thermal stability
densities, detonation pressures (22.5– 35.4 GPa) and velocities (7948– 9005 m s1) were determined, and they compare favorably with those of TNT and RDX. Their impact and friction sensitivities range from 12 to > 40 J and 80 to 360 N, respectively. These properties make them competitive as insensitive and thermally stable highenergy density materials.
and others,[5] including benzenetrifuroxan (1), trinitrotrisACHTUNGRE(triazolo)benzene (2), furazano-1,2,3,4-tetrazine-1,3-dioxide (FTDO, 3), 3,6-dinitropyrazoloACHTUNGRE[4,3-c]-pyrazole (DNPP, 4), and 1,2,3-triazoloACHTUNGRE[4,5,-e]furazanoACHTUNGRE[3,4,-b]pyrazine 6-oxide (5). DNPP (4), first synthesized by Russian researchers, is attractive because of its high thermal stability and low sensitivity towards mechanical stimuli.[7] Subsequently, the synthesis of 4 was improved using 2,4-pentanedione as starting material to yield DNPP in 21 % overall yield (Scheme 1).[8] However, the synthetic route to DNPP was still rather long and tedious. The synthetic bottleneck described was the extraction of the thermally unstable 4-diazonium-3,5-dimethylpyrazole salt, which also makes DNPP less attractive for large-scale manufacture. In our work, the pretreatment of intermediate 7 was modified by precipitating the crystalline material from solution at 15 8C, which avoids the ex-
Introduction Modern high-energy density materials (HEDM) continue to attract considerable interest because of their military and civilian applications as propellants, explosives, and pyrotechnics.[1] The majority of the energetic compounds are designed based on versatile heterocyclic rings such as tetrazoles, triazoles, pyrazoles, imidazoles, oxadiazoles, etc.[2] These moieties possess a large number of energetic NN, CN, and NO bonds, and exhibit high heats of formation, densities, and oxygen balance.[3] In comparison with single heterocyclic rings or coupled heterocyclic ring-based energetic molecules, fused cyclic nitrogen-containing heterocycles are, at the moment, the ascending stars in the energetic field as they have become a major focus in energetic materials research.[4] Including all these advantages, fused azacyclic compounds exhibit better thermochemical properties, which makes them safe for production, transfer, and storage.[5] Furthermore, the tremendous ring-strain energy stored in these kinds of materials can be used to improve their detonation properties.[6] Many fused-ring-based high-performance explosives (Figure 1) have been developed by our group[4, 6] [a] Dr. J. Zhang, Prof. Dr. J. M. Shreeve Department of Chemistry, University of Idaho Moscow, ID 838344-2343 (USA) Fax: (+ 1) 208-885-9146 E-mail: [email protected] [b] Dr. D. A. Parrish Naval Research Laboratory, Code 6030 Washington, D.C.20375-5001 (USA) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/asia.201402538.
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Figure 1. Recently developed fused heterocycle-based high-performance energetics.
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Here, 7 is obtained by precipitation from solution at 15 8C as a light yellow crystalline solid in 84 % yield. With 7 in hand, the fused-pyrazole ring was easily constructed through a rearrangement reaction using acetic acid as a catalyst. Several reagents for the nitration of the pyrazole ring are available; here, 65 % nitric acid and fuming nitric acid were used for the nitration of 8 and 10, respectively. Sodium dichromate was chosen as the oxidizing agent to convert the methyl group of 9 into a carboxy group. As a good method for scaling up production, this strategy allows for easy purification by washing with ice water in the last three steps to give analytical pure compounds. Reactions of 4 with ammonia, hydrazine, hydroxylamine, 3,5-diamino-1,2,4-triazole, and 3,4,5-triamino-1,2,4-triazole resulted in the formation of salts 11–15 (Scheme 2). Other energetic salts, 16–20, were readily synthesized by metathesis reactions of guanidine nitrate, aminoguanidine, diaminoguanidine, triaminoguanidinium, and 2-iminium-5nitriminooctahydroimidazoACHTUNGRE[4,5-d]imidazole hydrochlorides with sodium 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate [Na2 ACHTUNGRE(DNPP)]. Metal salts 21–23 were obtained in a straightforward manner by the reaction of 4 with sodium hydroxide, potassium hydroxide, and silver nitrate, respectively. With the exception of mono-3,5-diamino-triazolium3,6dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate (14), mono-triaminoguanidinium 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate (19), and 2-iminium-5-nitriminooctahydroimidazo [4,5-d]imidazole3,6dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate (20), products were isolated as dicationic salts. The structures of salts 11–23 were determined by IR and multinuclear NMR spectroscopies, as well as elemental analysis. Each of them is non-hygroscopic and stable in air. As a strong acid, the resonance band for the protons in 4 was observed at 15.06 ppm in the 1H NMR spectrum. For other salts, the hydrogen signals of the cations were easily assigned as there is no proton associated with the DNPP anion. In the 13C NMR spectra, except for two signals (d 139 and 133 ppm) assigned to the DNPP anion, the other signals are associated with the nitrogen-rich cations. The proton-decoupled 15N NMR spectrum of 4 was recorded in [D6]acetone, while those of 12 and 16 were measured in [D6]DMSO. The chemical shifts are given with respect to CH3NO2 as an external standard (Figure 2). The 15N NMR spectrum of 4 has three signals at d = 25.60, 59.78, and 211.28 ppm. For all three compounds, the nitrogen signals for C-nitro are observed between 19.60 and 25.60 ppm, which agrees with the values reported in the literature.[10] In the IR spectra, several main absorption bands at around 1595, 1371, 1242, 1126, and 826 cm1 are attributed to the DNPP anion, while the intense absorption bands in the
Scheme 1. Synthesis route to DNPP, 4.
traction step. Additionally, a one-pot reaction was carried out to obtain 6 in a more efficient and shorter procedure. By taking advantage of the reactivity of the acidic NH group(s) on polynitro-substituted energetic heterocycles, the synthesis of high-nitrogen energetic salts has become a recent focus of attention.[9] This is primarily because saltbased energetic materials often possess superior properties relative to non-ionic species because they tend to exhibit lower vapor pressures, lower impact and friction sensitivities, as well as enhanced thermal stabilities.[1b] Recently, a large number of nitrogen-rich salts have been explored by our group and the polynitro-substituted pyrazole-based salts especially show competitive energetic properties.[10] Therefore, it was of interest to study similar salts with nitro-substituted fused pyrazole rings as the anion. Although a limited number of 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazole salts were mentioned in a US patent, full characterization and sensitivity tests were not reported.[11] Compound 4 was chosen as the precursor to be investigated thoroughly for the design and synthesis of new ionic energetic derivatives. We now report the modified synthesis of 4 which is used as a starting material to develop various energetic salts. All compounds were fully characterized by IR and multinuclear NMR spectroscopies, elemental analysis, and differential scanning calorimetry (DSC). The structure of 4 as well as that of its guanidinium salt was further confirmed by singlecrystal X-ray structure analysis. The detonation properties of these compounds were calculated and the values suggest that they are insensitive and thermally stable energetic materials. In addition, quantum chemical investigations were performed to assist in the understanding of the decomposition mechanism and to help identifying the trigger bonds of the selected molecules.
Results and Discussion In this study, DNPP (4) was synthesized using a modified route in which compound 6 was prepared by a one-pot reaction starting with 2,4-pentanedione in yields approaching 90 %.The product was sufficiently pure to be used directly in the following reactions. In previous reports, a pretreatment of 7 was carried out by extracting with dichloromethane.[8]
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range of 3100 to 3600 cm1 are assigned to the NH bonds of the nitrogen-rich cations. Suitable crystals of 4 and 16 for X-ray diffraction were obtained by slow recrystallization from DMSO and aqueous solution at room temperature, respectively. Their structures are shown in Figures 3 and 4, and crystallographic and structural refinement data are given in Table 1. Crystal 4·2 DMSO crystallizes in the monoclinic space group P21/n with a calculated density of 1.563 g cm3. Compound 16 crystallizes in the monoclinic space group P21/ c with a calculated density of 1.684 g cm3. The structure of 16 is stabilized by various Hbonds involving all of the hyScheme 2. Synthesis of 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate salts (G, guanidine; AG, aminoguanidine, DAG, didrogen atoms of the guanidiniaminoguanidine; TAG, triaminoguanidine; INI-Cl, 2-iminium-5-nitriminooctahydroimidazo [4,5-d]imidazole um cation, the ring nitrogen, chloride). and nitro oxygen atoms. The bond lengths of the intermolecular hydrogen bonds are in the range of 2.93 to 3.02 . For crystals 4·2 DMSO and 16, the nitro-substituted fused pyrazole rings are strictly coplanar with torsion angles near zero or 1808, and the Cnitro bond length is in the range of 1.3822–1.4280 . The thermal stabilities of DNPP as well as its salts were determined by differential scanning calorimetric (DSC) measurements at a heating rate of 5 8C min1 (Table 2). As expected, the decomposition temperatures of DNPP and its salts range from 209 (18) to 395 8C (4), respectively. The corresponding DSC curves of selected compounds 4, 12, 16, and 22 are shown in Figure 5. While 4, 13, 15, 18, and 20–23 decompose without melting, the remaining salts melted prior to decomposition between 160 (11) to 318 8C (16). It is worthwhile to point out that DNPP as well as its ammonium (11), hydroxylammonium (12), and guanidinium salts (16) have remarkable high decomposition temperatures of > 300 8C; moreover, its sodium (21) and potassium (22) salts are thermally stable up to 395 8C and 365 8C, respectively, which make them very attractive in special explosive applications under extreme conditions such as deep oil drilling. Various studies have illustrated that the bond dissociation enthalpies (BDEs) for the possible trigger bond, which is the first bond to break in decomposition, is often a key factor when investigating the pyrolysis mechanism for energetic molecules. Therefore, to explore the relationship between bond strength and the relative thermal stability of these interesting molecules, the BDEs of 4, 11, 12, and 13 were selectively calculated. For most of polynitro energetic materials, the Cnitro or Nnitro bond strengths were found to be the trigger bond and related to the thermal staFigure 2. 15N NMR spectra of compounds 4, 12, and 16.
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bilities.[12] Therefore, in this study, the BDEs of Cnitro in selected compounds are listed in Table 3 and compared with Cnitro BDEs of RDX, HMX, and TATB. The calculated BDEs for DNPP and selected salts were found to be > 271 kJ mol1, which are much higher than those of RDX and HMX. However, the Cnitro Figure 3. (a) A view of the thermal ellipsoid plot (30 %) and labeling scheme of 4·2 DMSO. (b) Unit cell view BDE of TATB is about along the a axis. Dashed lines indicate hydrogen bonding. 50 kJ mol1 higher than those of the title compounds, which makes TATB the most thermal stable compound in the competition. These differences also indicate that strong hydrogen bonds play a more important role in increasing the stability of the molecule than the fused ring system. It is interesting to note that the NN BDE of the hydrazinium cation, NH2NH3 + , is 32 kJ mol1 lower than that of Cnitro in compound 13. Figure 4. (a) A view of the thermal ellipsoid plot (30 %) and labeling scheme of 16. (b) Unit cell view along Therefore, it is likely that the the a axis. Dashed lines indicate hydrogen bonding. NN cation bond is the weakest and the trigger bond in this DH f ðsalt, 298 KÞ ¼ DH f ðcation, 298 KÞ molecule; this would also explain the lower decomposition ð2Þ þDH f ðanion, 298 KÞDH L temperature of 13. The heat of formation of the nitrogen-rich cation, DNPP, in which DHL could be predicted by using the formula and the DNPP anion and dianion were calculated by using suggested by Jenkins et al. [Eq. (3)]:[19] the Gaussian 03 (Revision D. 01) suite of programs[13] using atomization and isodesmic reaction approaches (see the Supporting Information). The geometric optimization and frequency analyses of the structures are based on available single-crystal structures and using the B3LYP functional with the 6-31 + G** basis set.[14] Single-point energies were calculated at the MP2/6-311 + + G** level.[15] Atomization energies for cations were obtained by using the G2 ab initio method.[16] All of the optimized structures were characterized without imaginary frequencies. The solid-state enthalpy of formation can be calculated for neutral compounds by subtracting the heat of sublimation from the gas-phase heats of formation. Based on the literature,[17] the heat of sublimation can be corrected with Troutons rule according to Equation (1), where T represents either the melting point or the decomposition temperature when no melting occurs prior to decomposition:[18]
DH L ¼ U pot þ ½pðnM=22Þ þ qðnX=22ÞRT
ð3Þ
In this equation, nM and nX depend on the nature of the ions Mp + and Xq, respectively, and are equal to three for monoatomic ions, five for linear polyatomic ions, and six for nonlinear polyatomic ions. The equation for the lattice potential energy Upot [Eq. (4)] has the form: U pot ½kJ mol1 ¼ gð1m =Mm Þ1=3 þ d
ð4Þ
ð1Þ
in which 1m [g cm3] is the density, Mm is the chemical formula mass of the ionic material, and values for g and the coefficients g (kJ mol1 cm) and d (kJ mol1) are assigned literature values.[3f] The calculated values for the nitrogen-rich cations vary between 575.4 and 883.6 kJ mol1. All of the DNPP salts exhibit positive heats of formation, with 14 and 15 having the highest at 2.76 and 2.30 kJ g1 (RDX: 0.36 kJ g1).
Based on Born–Haber energy cycles (see the Supporting Information), heats of formation of DNPP salts can be simplified by the formula given in Equation (2):
Density is a very important property contributing to the detonation performance of energetic materials. The densities of these new compounds, measured using a gas pycnometer, fall in the range between 1.67 and 1.85 g cm3 for 4 and 11–
DH sub ¼ 188 J mol1 K1 T
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Table 1. Crystallographic data and structure refinement parameters for 4·2 DMSOand 16.
Empirical formula CCDC number[a] Temperature Crystal system Space group a [] b [] c [] a [8] b [8] g [8] V [3] Z Density [mg m3] M [mm1] F (000) Crystal size [mm] q range [8] Index ranges
Reflections collected Independent reflection Goodness-of-fit on F2 Final R indices [I > 2 sigma(I)] R indices (all data) Largest diff. peak and hole
4·2 DMSO
16
C6H2N6O4,2ACHTUNGRE(C2H6OS) 999620 150(2) K Monoclinic P21/n 4.6473(2) 15.1140(7) 10.9532(5) 90 101.7780(10) 90 753.15(6) 4 1.563 0.392 368 0.254 0.261 0.296 2.33 to 26.74 5 < = h < = 5, 19 < = k < = 19, 13 < = l < = 13 9330 1593 [Rint = 0.0111]
C6H12N12O4 983224 150(2) K Monoclinic P21/c 3.5077(3) 10.0767(8) 17.6548(14) 90 91.124(3) 90 623.91(9) 2 1.684 0.142 328 0.61 0.10 0.06 2.31 to 26.51 4 < = h < = 4, 12 < = k < = 12, 21 < = l < = 22 5652 1286 [Rint = 0.0258]
1.083 R1 = 0.0243, wR2 = 0.0679 R1 = 0.0258, wR2 = 0.0691 0.375 and 0.285 e 3
1.092 R1 = 0.0318, wR2 = 0.0820 R1 = 0.0363, wR2 = 0.0847 0.288 and 0.228 e 3
Figure 5. DSC plots of compounds 4, 12, 16, and 22 measured at a heating rate of 5 K min1 (exo up).
ly. With the experimental data found for the densities and the calculated heats of formation, the detonation pressure (P), and velocity (vD) of 4 and 11–20 were calculated using EXPLO5 v6.01.[20] As can be seen in Table 1, the calculated detonation velocities lie between nD = 7948 (16) and 9005 m s1 (12). Compounds 13 (8860 m s1) and 19 (8814 m s1) also exhibit rather high values which are superior to TNT (6881 m s1) and comparable to RDX (8748 m s1). The detonation pressures lie in the range 22.5– 35.4 GPa, which are also higher than that of TNT (19.5 GPa) and essentially equal to RDX (34.9 GPa). Oxygen balance (W) is the index of the excess or deficiency of oxygen percentage in a compound required to convert all C atoms into CO and all H atoms into H2O for a compound with a formula of CaHbOcNd, W (%) = 1600 (cab/2)/Mw (Mw, molecular weight). In this study, the neutral compound 4 has an oxygen balance of 8 %, which is superior to that
[a] These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
20, which is comparable with those of currently used explosives (1.6–1.8 g cm3). The densities of the metal salts 21–23 with heavy cations are 2.14, 2.20, and 3.27 g cm3, respective-
Table 2. Properties of DNPP and its salts compared with TNT and RDX. Comp.
Tm[a] [8C]
Tdec[b] [8C]
W[c] [%]
d[d] ACHTUNGRE[g cm3]
DfH[e] [kJ mol1/ACHTUNGRE(kJ g1)]
vD[f] ACHTUNGRE[m s1]
P[g] ACHTUNGRE[GPa]
IS[h] [J]
FS[i] [N]
Isp[j] [s]
4 11 12 13 14 15 16 17 18 19 20 21 22 23 TNT RDX
– 160 174 – – – 318 213 – 208 – – – – 81 –
336 328 327 247 287 289 324 222 209 215 238 395 365 327 295 230
8 27 12 30 30 41 40 41 42 31 27 0 0 0 27 0
1.85 1.69 1.82 1.72 1.71 1.67 1.68 1.69 1.71 1.76 1.79[k] 2.14[k] 2.20 3.27 1.65 1.82
322.6/1.94 158.5/0.68 274.2/1.04 501.0/1.91 481.9/1.62 963.8/2.30 173.3/0.55 477.0/1.38 679.6/1.80 605.5/2.00 505.6/1.32 – – – 115/0.26 80.0/0.36
8250 8212 9005 8860 8036 8230 7948 8400 8732 8814 8355 – – – 6881 8748
27.4 25.4 35.4 30.3 24.5 24.6 22.5 25.6 28.0 29.9 27.9 – – – 19.5 34.9
15 > 40 29 16 > 40 > 40 > 40 > 40 > 40 12 23 14 > 40 29 15 7.4
160 360 360 160 360 360 360 360 360 80 160 160 160 160 353 120
241 218 260 243 220 225 199 216 225 235 220 – – – 211 258
[a] Melting temperature. [b] Thermal decomposition temperature (onset) under nitrogen gas (DSC, 5 8C min1). [c] Oxygen balance for CaHbOcNd, 1600ACHTUNGRE(c-a-b/2)/Mw; Mw = molecular weight. [d] Gas pycnometer (25 8C). [e] Calculated heat of formation. [f] Detonation velocity. [g] Detonation pressure. [h] Impact sensitivity. [i] Friction sensitivity. [j] Specific impulse for the neat compound. [k] values were corrected by subtracting the volume of a water molecule [VACHTUNGRE(H2O) 25 3].
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(7948–9005 m s1) and detonation pressures (22.5–35.4 GPa) are comparable with those of TNT and RDX. Most of the salts have reasonable impact sensitivities (12 to > 40 J) and friction sensitivities (80 to 360 N), thus making the DNPP salts potentially useful as thermally stable and insensitive energetic materials.
Table 3. Calculated BDEs for 4, 11, 12, and 13 and their decomposition temperatures. Compound
Bond type
Bond length []
BDE(UB3LYP/ 6-31G*/kJ mol1)[a]
Td [8C]
4 11 12 13
C-NO2 C-NO2 C-NO2 C-NO2 NH2-NH3 + C-NO2 C-NO2 C-NO2
1.44 1.43 1.43 1.43 1.44 – – –
288 305 291 303 271 161 166 355
336 328 327 247
RDX[a] HMX[a] TATB[a]
Experimental Section
210 280 340
Caution! Although none of the compounds described herein has exploded or detonated unexpectedly in the course of this research, these materials should be handled with extreme care by using the best safety practices (leather gloves, face shield).
[a] Ref. [13c].
General Methods
of its nitrogen-rich salts (12 to 42 % for 11–20). All metal salts have the highest oxygen balance of 0 which is because of hydrogen-free molecular formulas. This series of salts has an oxygen balance comparable to TNT. Compounds 4, 12, and 13 also exhibit very promising specific impulse values (Isp > 240 s) which are much higher than that of TNT (211 s) and comparable to that of RDX (258 s). For initial safety testing, the impact and friction sensitivities were tested according to Bundesamt fr Materialforschung (BAM) standard methods. As can be seen in Table 1, the impact sensitivity (IS) of DNPP was found to be 15 J. Not surprisingly, improved impact stabilities for DNPP salts were found for 11–18 (16 to > 40 J) except that of 19 which is < 12 J because of the remaining acidic NH. These data show that the presence of organic cations can result in relatively insensitive energetic fused-ring compounds. The metal salts with water(s) of crystallization cause a significant decrease in IS. For example, 22 as a dihydrate has an IS of > 40 J, which is less sensitive than 23 as a monohydrate (29 J) and 21 as an anhydrous analogue (14 J). In terms of friction sensitivity, most of the salts (11, 12, 14–18) could not be initiated even with a force of 360 N, which suggests that they can serve as promising candidates for safe energetic materials.
Analytical grade reagents were purchased from Aldrich and Acros Organics and were used as received. 1H and 13C NMR spectra were recorded on a 300 MHz (Bruker AVANCE 300) NMR spectrometer operating at 300.13 and 75.48 MHz, respectively. A 500 MHz (Bruker AVANCE 500) NMR spectrometer operating at 50.69 MHz was used to obtain 15 N NMR spectra. [D6]DMSO was employed as a locking solvent unless otherwise stated. Chemical shifts in 1H and 13C NMR spectra are reported relative to Me4Si and in 15N NMR spectra relative to MeNO2. The melting and decomposition points were recorded using a differential scanning calorimeter (TA Instruments Co., model Q10) at a scan rate of 5 8C min1. IR spectra were recorded using KBr pellets for solids on a BIORAD model 3000 FTS spectrometer. Densities were measured at room temperature using a Micromeritics AccuPyc 1330 gas pycnometer. Elemental analyses were carried out on an Exeter CE-440 elemental analyzer. X-Ray Crystallography A colorless rod of dimensions 0.25 0.26 0.30 mm3 (4·2 DMSO) and an orange rod of dimensions 0.61 0.10 0.06 mm3 (16) were mounted on a MiteGen MicroMesh using a small amount of Cargille Immersion Oil. Data were collected on a Bruker three-circle platform diffractometer equipped with a SMART APEX II CCD detector. The crystals were irradiated using graphite monochromated MoKa radiation (l = 0.71073). An Oxford Cobra low-temperature device was used to keep the crystals at a constant temperature of 150(2) K during data collection. Data collection was performed and the unit cell was initially refined using APEX2 [v2010.3–0].[21] Data reduction was performed using SAINT [v7.68A],[22] and XPREP [v2008/2].[23] Corrections were applied for Lorentz, polarization, and absorption effects using SADABS [v2008/1].[24] The structures were solved and refined with the aid of the programs in the SHELXTLplus [v2008/4] system of programs.[25] The full-matrix least-squares refinement on F2 included atomic coordinates and anisotropic thermal parameters for all non-H atoms. The H atoms were included using a riding model.
Conclusions A modified, more efficient, and repeatable synthesis process for 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazole (4) was developed. A series of nitrogen-rich energetic salts based on the anion of 4 were prepared and fully characterized. Structures of 4·2 DMSO and 16 were confirmed by single-crystal X-ray diffraction and show extensive hydrogen-bonding interactions. This neutral compound and the corresponding salts exhibit outstanding thermal stabilities (209–395 8C). The quantum chemical investigation assisted in the understanding of the decomposition mechanism and suggests the trigger bond in both cation and anion. Densities for these salts fall in the range between 1.66 and 2.20 g cm3, which places them in a class of relatively dense energetic materials. Their detonation properties were evaluated by Gaussian 03 and EXPLO5 calculations. The calculated detonation velocities
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3,5-Dimethyl-1H-pyrazol-4-amine (6) This compound was prepared according to the reference method.[26] 4-Diazonium-3,5-dimethylpyrazole (7)[27] Compound 6 (28 g) was dissolved in acetic (43 mL) and water (40 mL) at room temperature. After a clear solution was formed, the solution was cooled to 0–5 8C, and sodium nitrite (17.3 g) in water (40 mL) was added dropwise. After complete addition, the mixture was stirred at the same temperature for 5 h. The pH of the mixture was then adjusted to 8 using aqueous ammonia. After blowing air over the mixture for 6 h, the solution was allowed to remain at 15 8C for 2 h, which resulted in formation of yellow crystals. They were collected by filtration and dried under reduced pressure to afford 7 in 84 % yield (25.8 g). 1H NMR: d = 2.38 ppm (6 H, CH3); 13C NMR: d = 12.47, 76.67, 154.93 ppm.
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3,6-DinitropyrazoloACHTUNGRE[4,3-c]pyrazole (4)
was added. The reaction mixtures were stirred with heating (50 8C) for 6 h and then cooled to room temperature. The resulting precipitate was collected by filtration and washed with methanol to give solid products in good yield.
With 7 in hand, intermediates 8, 9, and 10 were prepared according to literature procedures.[27] The synthesis of 4 was accomplished using a slightly modified method. To cooled 100 % nitric acid (9 mL, 0 ~ 5 8C), 10 (1.47 g, 7.5 mmol) was added in portions with stirring over a period of 10 min. The mixture was stirred at 0–5 8C for 30 min and then at 65 8C for 12 h. Subsequently, the reaction mixture was poured onto ice, the precipitate was filtered, washed with cold water, and air dried to yield 4 (1.21 g, 82 %) as an ivory solid; Tdec (onset): 336 8C; IR (KBr): n˜ = 3267, 1548, 1522, 1430, 1369, 1349, 1244, 1146, 1036, 835, 656, 480 cm1; 1H NMR: d = 15.06 ppm (s, 2 H, NH); 13C NMR ([D6]acetone): d = 139.28, 132.70 ppm; elemental anal. calcd (%) for C4H2N6O4 (198.10): C 24.25, H 1.02, N 42.42; found: C 24.17, H 1.07, N 41.96.
Diguanidinium 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate (16) Orange red solid; Tm : 318 8C; Tdec (onset): 324 8C; IR (KBr): n˜ = 3456, 1658,1459, 1412, 1365, 1219, 1089 cm1; 1H NMR: d = 7.47 (s, 8 H, NH2), 5.44 ppm (s, 12 H, NH2); 13C NMR: d = 158.37, 143.60, 141.36 ppm; elemental anal. calcd (%) for C6H12N12O4 (316.24): C 22.79, H 3.82, N 53.15; found: C 23.34, H 3.79, N 52.60. Di(aminoguanidinium) 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate (17) Orange-yellow solid; Tm : 213 8C; Tdec (onset): 222 8C; IR (KBr): n˜ = 3343, 3263, 1690, 1656, 1454, 1419, 1353, 1208, 1071, 820 cm1; 1H NMR: d = 8.60 (s, 2 H, NH), 7.75 ppm (s, 12 H, NH2); 13C NMR: d = 159.01, 143.23, 141.24 ppm; elemental anal. calcd (%) for C6H14N14O4 (346.27): C 20.81, H 4.08, N 56.63; found: C 20.97, H 3.98, N 55.97.
General Procedure for the Synthesis of Salts 11–15 3,6-DinitropyrazoloACHTUNGRE[4,3-c]pyrazole (0.20 g, 1 mmol) was suspended in methanol (3 mL) and water (0.5 mL). Next, aqueous ammonia (0.14 g, 2.1 mmol), hydrazine monohydrate (0.105 g, 2.1 mmol), hydroxylamine (0.139 g, 50 wt % in water, 2.1 mmol), 3,5-diaminotriazole (0.208 g, 2.1 mmol) or 3,4,5-triaminotriazole (0.239 g, 2.1 mmol) was added. The reaction mixtures were stirred with heating (50 8C) for 6 h and then cooled to room temperature. The resulting precipitate was collected by filtration and washed with methanol to give solid products in good yield.
Di(diaminoguanidinium) 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate (18) Yellow solid; Tdec (onset): 209 8C; IR (KBr): n˜ = 3337, 3274, 1675, 1456, 1429, 1346, 1215, 1069, 1018, 818 cm1; 1H NMR: d = 8.63 (br, s, 4 H, NH2 + ), 7.14 (s, 4 H, NH), 4.54 ppm (br s, 8 H, NH2); 13C NMR: d = 159.92, 143.40, 141.34 ppm; elemental anal. calcd (%) for C6H16N16O4 (376.30): C 19.15, H 4.29, N 59.56; found: C 19.28, H 4.16, N 60.06.
Diammonium 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate (11)
Mono-triaminoguanidinium 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate (19)
Orange solid; Tm : 160 8C; Tdec (onset): 328 8C; IR (KBr): n˜ = 3273, 3235, 1686, 1474, 1401, 1350, 1217, 1074, 822 cm1; 1H NMR: d = 6.11 ppm (s, 8 H, NH4 + ); 13C NMR: d = 141.43, 140.52 ppm; elemental anal. calcd (%) for C4H8N8O4 (232.16): C 20.69, H 3.47, N 48.27; found: C 20.76, H 3.44, N 47.70.
Yellow solid; Tm : 208 8C; Tdec (onset): 215 8C; IR (KBr): n˜ = 3345, 1686, 1497, 1375, 1343, 1231, 1108, 1036, 823 cm1; 1H NMR: d = 13.70 (br, 1 H, ring N-H) 8.60 (s, 3 H, NH), 4.49 ppm (s, 6 H, NH2); 13C NMR: d = 159.08, 139.71, 137.85 ppm; elemental anal. calcd (%) for C5H10N12O4 (302.21): C 19.87, H 3.34, N 55.62; found: C 19.85, H 3.31, N 54.93.
Dihydroxylammonium 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate (12) Orange-yellow solid; Tm : 174 8C; Tdec (onset): 327 8C; IR (KBr): n˜ = 3156, 1480, 1439, 1371, 1302, 1242, 1126, 1008, 826 cm1; 1H NMR: d = 8.16 ppm (s, 6 H, NH3 + ); 13C NMR: d = 139.68, 137.50 ppm; elemental anal. calcd (%) for C4H8N8O6 (264.16): C 18.19, H 3.05, N 42.42; found: C 18.56, H 3.03, N 41.62.
Mono-2-iminium-5-nitriminooctahydroimidazo [4,5-d]imidazole3,6dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate monohydrate (20) Yellowish gray solid; Tdec (onset): 238 8C; IR (KBr): n˜ = 3343, 1701, 1581, 1466, 1352, 1285, 1257, 1220, 1075, 892, 822 cm1; 1H NMR: d = 5.71 ppm (s, 2 H); 13C NMR: d = 161.11, 160.04, 139.69, 137.93, 72.23 ppm; elemental anal. calcd (%) for C8H11N13O7 (401.26): C 23.95, H 2.76, N 45.38; found: C 23.96, H 3.06, N 46.41.
Dihydrazinium 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate (13) Yellow solid; Tdec (onset): 247 8C; IR (KBr): n˜ = 3351, 1470, 1439, 1362, 1297, 1233, 1104, 964, 822 cm1; 1H NMR: d = 6.10 ppm (s, 10 H, NH2NH3 + ); 13C NMR: d = 141.11, 139.48 ppm; elemental anal. calcd (%) for C4H10N10O4 (262.19): C 18.32, H 3.84, N 53.42; found: C 18.41, H 3.79, N 52.81.
General Procedure for the Synthesis of Metal Salts 21--23 A solution of 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazole (0.20 g, 1 mmol) in methanol (6 mL) was stirred at room temperature while adding sodium hydroxide (0.084 g, 2.1 mmol in 2 mL H2O), potassium hydroxide(0.118 g, 2.1 mmol in 2 mL H2O), or silver nitrate (0.357 g, 2.1 mmol in 5 mL H2O). The reaction mixtures were stirred for 1 h, and the resulting precipitate was collected by filtration and washed with methanol to give solid products in good yield.
Mono-3,5-Diamino-triazolium 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate (14) Light yellow solid; Tdec (onset): 287 8C; IR (KBr): n˜ = 3447, 3372, 2445, 1832, 1615, 1543, 1414, 1386, 1240, 1145, 1009, 820, 749 cm1; 1H NMR: d = 8.20 ppm (br, 6 H, NH and NH2); 13C NMR: d = 155.27, 138.68, 134.40 ppm; elemental anal. calcd (%) for C6H7N11O4 (297.19): C 24.25, H 2.37, N 51.84; found: C 24.13, H 2.36, N 51.54.
Disodium 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate dihydrate (21) Orange solid; Tdec (onset): 395 8C; IR (KBr): n˜ = 3607, 3544, 1650, 1477, 1440, 1355, 1225, 1086, 816, 752 cm1; elemental anal. calcd (%) for C4H4N6O6Na2 (278.09): C 17.28, H 1.45, N 30.22; found: C 16.84, H 1.39, N 29.87.
Di(3,4,5-triamino-triazolium) 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate (15) Orange solid; Tdec (onset): 289 8C; IR (KBr): n˜ = 3345, 3167,1663, 1449, 1360, 1300, 1234, 1088, 1046, 820 cm1; 1H NMR: d = 6.86 (br, 8 H, NH2), 5.44 ppm (s, 4 H, NH2); 13C NMR: d = 150.48, 139.67, 137.83 ppm; elemental anal. calcd (%) for C8H14N18O4 (426.31): C 22.54, H 3.31, N 59.14; found: C 22.70, H 3.30, N 59.68.
Dipotassium 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate (22) Orange solid; Tdec (onset): 365 8C; IR (KBr): n˜ = 1452, 1428, 1369, 1272, 1211, 1080, 824, 752 cm1; elemental anal. calcd (%) for C4N6O4K2 (274.28): C 17.52, H 0.00, N 30.64; found: C 17.23, H 0.14, N 30.98.
General Procedure for the Synthesis of Salts 16–20
Disilver 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazolate monohydrate (23)
A solution of 3,6-dinitropyrazoloACHTUNGRE[4,3-c]pyrazole (0.20 g, 1 mmol) in methanol (4 mL) was stirred at room temperature while adding dilute sodium hydroxide solution (5 mL, 0.4 n). To this solution, guanidine nitrate (0.256 g, 2.1 mmol), aminoguanidine monohydrochloride (0.232 g, 2.1 mmol), diaminoguanidine monohydrochloride (0.264 g, 2.1 mmol), triaminoguanidine monohydrochloride (0.155 g, 1.1 mmol) or 2-iminium-5nitriminooctahydroimidazo [4,5- d]imidazole chloride (0.245 g, 1.1 mmol)
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Orange solid; Tdec (onset): 327 8C; IR (KBr): n˜ = 3524, 1501, 1466, 1366, 1245, 1111, 827, 745 cm1; elemental anal. calcd (%) for C4H2N6O5Ag2 (429.83): C 11.18, H 0.47, N 19.55; found: C 10.99, H 0.47, N 19.74.
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Acknowledgements
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We are grateful to Dr. Clifford Bedford, ONR (NOOO14–12–1–0536).
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Received: May 19, 2014 Published online: August 22, 2014
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