Synthesis, structure and properties of neutral energetic materials based on N-functionalization of 3,6-dinitropyrazolo[4,3-c]pyrazole

Abstract

3,6-Dinitropyrazolo[4,3-c]pyrazole (DNPP, 4) was prepared using an efficient modification process. Various neutral energetic derivatives of DNPP were synthesized from N-functionalization of imide (NH) group. All compounds were fully characterized by ¹H and ¹³C nuclear magnetic resonance spectroscopy, infrared spectroscopy, elemental analysis, and differential scanning calorimetry (DSC). The crystal structures of compounds 4·2H₂O, 12 and 15 were confirmed by single-crystal X-ray diffraction, showing extensive hydrogen-bonding. The densities of neutral derivatives ranged from 1.74 to 1.95 g cm⁻³, and all compounds have positive heats of formation in the range of 18.8 to 863 kJ mol⁻¹. Based on the measured densities and calculated heats of formation, theoretical performance calculations, including detonation pressures (27.1–41.5 GPa) and velocities (7819–9364 m s⁻¹), were carried out using the Gaussian 09 program and Kamlet–Jacobs equations, and they compare favorably with those of TNT and RDX. These properties make them potential and competitive for use as new high energy-density materials.

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Introduction

Energetic materials are used extensively for both civilian and military applications. The design and synthesis of modern high-energy density materials (HEDMs) that combine excellent performance and low sensitivities towards shock, friction, heat and electrostatic discharge have attracted significant interest in the field of energetic-materials research over the past decade.¹ Azole-based compounds are a prominent family of novel HEDMs that fulfill many requirements in design and synthesis of new materials, because they are generally highly endothermic with high densities and low sensitivities towards external stimuli.² The majority of these energetic compounds are designed based on versatile heterocyclic rings such as tetrazoles, triazoles, pyrazoles, imidazoles, oxadiazoles, etc.³ These compounds have been studied by worldwide research groups, owing to high positive heats of formation, densities, and oxygen balance resulting from the large number of N–N, N=N, and N–O bonds and the high level of environmental compatibility.^1c,4 In comparison with single heterocyclic ring or coupled heterocyclic ring-based energetic molecules, fused cyclic nitrogen-containing heterocycles have become a major focus in novel energetic materials research field.⁵ Due to the above advantages, many fused-ring-based high-performance explosives (Scheme 1) have been synthesized by our group and others, including furazano-1,2,3,4-tetrazine-1,3-dioxide (FTDO, 1),⁶ benzenetrifuroxan (BTF, 2),⁷ 1,4-dinitrogycoluril (DNGU, 3),⁸ 3,6-dinitropyrazolo[4,3-c]pyrazole (DNPP, 4),⁹ 7-amino-4,6-dinitrobenzofuroxan (ADNBF, 5),¹⁰ 4,8-dihydrodifurazano[3,4-b,e]pyrazine (DFP, 6),¹¹ and 1,2,3-triazolo[4,5-e]furazano[3,4-b]pyrazine 6-oxide (TFPO, 7).^1c,12

Scheme 1 Structures of fused nitrogen-containing heterocycle-based energetic compounds.

In the pursuit of novel energetic materials, to obtain better detonation properties, the most important strategy for the design of new HEDMs is the N-functionalized chemistry of azole-based compounds with ring strain energy. Many energy-functional groups, such as NO₂,¹³ CH₂ONO₂,^11c N–OH,¹⁴ N₃,¹⁵ and NH₂,¹⁶ have been used to modify the azole rings. While NO₂, CH₂ONO₂, and N–OH groups could improve the oxygen content, densities, and detonation properties of energetic compounds, with decreased stabilities. N-Amination and N₃ group with the additional N–N bond(s) have higher positive heats of formation, resulting in improved detonation properties of energetic materials. Moreover, the introduction of an amino group is a more effective method for enhancing stability and lowering sensitivity of energetic compounds, the amino group can undergo further functionalization, such as nitration,^16,17 diazotization,¹⁸ or trinitroethylation to provide versatile energetic materials.¹⁹

In general, fused cyclic compounds with ring strain energy display a higher density and a superior detonation performance. For example, DNPP is thermally stable (T_d = 330 °C) and relatively insensitive to impact (H₅₀ = 68 cm), high crystal density (1.86 g cm⁻³), and has a predictable detonation performance, which is 85% that of 1,3,5,7-tetranitrotetraazacyclooctane (HMX).^9b DNPP was firstly synthesized by Russian researchers, it is attractive because of its high thermal stability and low sensitivity towards mechanical stimuli.^9d Subsequently, the synthesis of DNPP was improved using 2,4-pentanedione as starting material to yield DNPP in 21% overall yield (Scheme 2).^9c As we can see, the synthetic route to DNPP was still rather long and complicated, resulting the low yield of preparing DNPP, which has limited its large-scale and application prospect. Furthermore, because of salt-based energetic materials often possess superior properties, such as lower vapor pressures, lower impact and friction sensitivities, as well as enhanced thermal stabilities. By taking advantage of the reactivity of the acidic N–H group on polynitro-substituted energetic heterocycles, the structures and physico-chemical performance of a series of DNPP nitrogen-rich energetic salts were reported.^1b However, a limited number of DNPP neutral derivatives were mentioned in previous reports, full characterization, detonation parameters and sensitivity tests were not investigated.^9b Thus, compound 4 was chosen as the precursor to be studied thoroughly for the design and synthesis of new neutral energetic derivatives.

Scheme 2 Synthesis route to DNPP.

In this paper, we report the synthesis of various neutral energetic derivatives of DNPP which were realized by N-functionalization reactions of imide (NH) group. Its energetic derivatives include 1,4-diamino-3,6-dinitropyrazolo[4,3-c]pyrazole (12), 1-amino-3,6-dinitropyrazolo[4,3-c]pyrazole (13), 1,4-dinitroxymethyl-3,6-dinitropyrazolo[4,3-c]pyrazole (15), 1,4-diazidomethyl-3,6-dinitro-pyrazolo[4,3-c]pyrazole (17), 1,3,4,6-tetranitropyrazolo[4,3-c]pyrazole (18), and 1,4-dihydroxyl-3,6-dinitropyrazolo[4,3-c]pyrazole (20). All of compounds were fully characterized by IR and multinuclear NMR spectroscopies, elemental analysis, and differential scanning calorimetry (DSC). Their detonation properties were calculated. Furthermore, the crystal structures of 4·2H₂O as well as that of its derivatives, 12 and 15 were determined by single-crystal X-ray diffraction.

Results and discussion

Synthesis of DNPP

Besides the long synthetic route to DNPP, another main restrictive factor was preparing of the thermally unstable intermediate 8, which also makes DNPP less attractive for large-scale manufacture. According to literature methods, compound 8 was obtained by extracting with dichloromethane for several times.^9c In our previous studies, intermediate 8 was obtained by crystallization from reaction solution at −20 °C to −10 °C as a light yellow solid with a yield of 86.1%, which could avoid the extraction step and shorten operation process.^9a The product 8 was sufficiently pure to be used directly in the following reactions. Industrial nitric acid instead of pure nitric acid was used for the nitration of 11. As the above improvement for technological conditions, this strategy allows for easy and safe purification by only washing with ice-water to give analytical pure DNPP product.

Synthesis of energetic derivatives of DNPP

Further N-functionalization reactions of DNPP with the NH₂, CH₂N₃, CH₂ONO₂, NO₂, and OH were continued in order to obtain more neutral energetic derivatives. Various N-functionalized single-ring azole-based compounds have been synthesized by several research groups.^1e,20 N-Amination of fused cyclic nitrogen-containing heterocycle, however, has not been fully explored and fully characterized.^11c N-Amination reaction of DNPP with hydroxylamine-O-sulfonic acid and sodium carbonate could obtain two corresponding N-amino products 12 and 13, respectively. DNPP reacted with formaldehyde solution to produce 1,4-dihydroxymethyl-3,6-dinitropyrazolo[4,3-c]pyrazole (14), followed by nitration with the mixture of nitric acid and acetic anhydride to yield compound 15. The hydroxyl groups in compound 14 were substituted with chlorine atoms by reaction with sulfoxide chloride to produce 1,4-dichloromethyl-3,6-dinitropyrazolo[4,3-c]pyrazole (16). The chlorine atoms in 16 are certainly activated and are reacted with nucleophiles. When 16 was treated with sodium azide, compound 17 was obtained. Additionally, N-nitro compound 18 was prepared readily by nitration of DNPP and dinitrogen pentoxide in dichloromethane solution. Compound 18 decomposed in the solid state at room temperature after a few hours, but it was stable in the solution at lower temperature for several weeks. Reaction of DNPP with ammonium hydroxide formed the ammonium salt diammonium 3,6-dinitropyrazolo[4,3-c]pyrazolate (19) in high yield, either water or methanol can be used as solvent. Treatment of 19 with Oxone® for three days in aqueous solution could prepare compound 20, which could also derive a series of high-nitrogen energetic salts. The synthetic pathways to all energetic derivatives of DNPP are depicted in Scheme 3. All compounds were fully characterized by IR spectroscopy, DSC, ¹H and ¹³C NMR spectroscopies and elemental analysis. The data are listed in the Experimental section.

Scheme 3 The synthetic pathways of neutral energetic derivatives based on DNPP.

Spectroscopy

In the IR spectra, characteristic absorption bands of NH₂, NH, and OH groups were observed for compounds 12, 13, and 20 at 3000–3600 cm⁻¹. The intense absorption bands in the range of 2900 to 3000 cm⁻¹ are assigned to the CH₂ bonds of compounds 15 and 17. Also, strong absorption bands assignable to the nitro groups were observed at 1300–1600 cm⁻¹ for all energetic derivatives. In the ¹H NMR spectra, signals of NH and OH for 13 and 20 were found at 15.09 and 15.02 ppm in d₆-DMSO as the solvent, which could be attributed to their strong acid. The resonance bands for NH₂ in 12 and 13 were observed at 7.22 and 7.27 ppm, respectively. In the ¹³C NMR spectra, two signals (δ ≈ 138 and 132 ppm) were assigned to the chemical shifts of carbon atoms in fused cyclic structures of 13, 15, 17, and 20, which is in generally agreement with compound DNPP.

Sensitivities

To evaluate the stabilities of these neutral energetic compounds, we also studied the impact and friction sensitivities. The sensitivities of all explosives were determined experimentally according to standard BAM methods.²¹ As can be seen in Table 1, the impact and friction sensitivities of DNPP were found to be 15 J and 160 N, respectively. The impact sensitivities of these neutral compounds range from 10 J to 16 J. The value of friction sensitivities are from 160 N to 300 N. All of these compounds are considerably less sensitive toward impact and friction sensitivities than RDX (IS = 7.4 J, FS = 160 N), which suggest that they can serve as promising candidates for safe energetic materials.

Table 1 Physiochemical properties and detonation parameters of the N-functionalized energetic compounds based on DNPP

Compound	4	12	13	15	17	18	20	TNT	RDX
a Thermal decomposition temperature.b Oxygen balance (based on CO2) for CaHbOcNd, 1600(c − 2a − b/2)/MW, MW = molecular weight.c Density measured by gas pycnometer.d Heat of formation.e Detonation velocity.f Detonation pressure.g Impact sensitivity.h Friction sensitivity.i Single crystal density (296 K).
Tdec^a (°C)	336	240	178	208	198	—	296	295	230
OB^b (%)	−40.4	−42.1	−41.3	−18.4	−51.9	0	−20.9	−74.0	−21.6
ρ^c (g cm^−3)	1.85	1.84^i	1.74	1.83^i	1.74	1.95	1.90	1.65	1.82
ΔHf^d (kJ mol^−1)	323	460	356	18.8	863	479	269	−115	70.3
D^e (m s^−1)	8250	8404	7934	8480	7819	9364	8838	6881	8748
P^f (GPa)	27.4	32.3	27.9	32.8	27.1	41.5	36.5	19.5	34.9
IS^g (J)	15	15	14	12	10	—	16	15	7.4
FS^h (N)	160	280	280	160	240	—	300	353	120

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Physiochemical and detonation properties

The thermal stabilities of all these compounds were determined by differential scanning calorimetric (DSC) measurements at a heating rate of 5 °C min⁻¹ (Table 1). With the exception of 18, the decomposition temperatures of DNPP and its energetic derivatives lie in the range between 178 (13) and 336 °C (4), compared with RDX (230 °C). Compound 18 easily decomposed in the solid state at room temperature after a few hours, which could produce DNPP and 1,3,4-tetranitropyrazolo[4,3-c]pyrazole with losing two or one nitro group, but it was stable in the solution at lower temperature for several weeks.

Densities of these energetic compounds which are in the range of 1.74 to 1.95 g cm⁻³ were determined using gas pycnometer or single-crystal X-ray diffraction. It is noteworthy that the densities of compounds 12, 15, 18, and 20 fall in the range designated for new HEDMs (1.8–2.0 g cm⁻³),²² which are higher than RDX (1.82 g cm⁻³). Moreover, the high densities of 18 (1.95 g cm⁻³) and 20 (1.90 g cm⁻³) are even comparable with that of HMX (1.91 g cm⁻³).

Heats of formation are one of the important characteristics for energetic compounds and are directly related to the number of nitrogen–nitrogen bonds in compounds. The standard enthalpies of formation for these derivatives were calculated by using the Gaussian 09 (Revision A. 02)²³ suite of programs and atomization method based on CBS-4M enthalpies.²⁴ All of the optimized structures were characterized to be true local energy minima on the potential-energy surface without imaginary frequencies. As shown in Table 1, the solid phase heats of formation DNPP and its neutral derivatives are highly endothermic compounds. All of the compounds exhibit positive heats of formation ranging between 18.8 and 863 kJ mol⁻³, which are compared with those of TNT (−115 kJ mol⁻³) and RDX (70.3 kJ mol⁻³). Compound 17 exhibits the highest heat of formation (863 kJ mol⁻¹) among them because of azido-functionalization in molecular structure.

Based on the calculated values of heat of formation and the experimental values for the densities of these compounds, the detonation velocities (D) and detonation pressures (P) were calculated using the Kamlet–Jacobs equations.²⁵ The detonation velocities (D) lie between 7819 and 9364 m s⁻¹ (compared with TNT 6881 m s⁻¹, and RDX 8748 m s⁻¹). Detonation pressures of these compounds lie in the range between 27.1 and 41.5 GPa (compared with TNT 19.5 GPa, and RDX 34.9 GPa). Although not all the compounds perform better than RDX by calculations, they can probably find use in certain applications in civilian use or as burn-rate modifiers in military applications.

X-ray crystallography

The molecular structures of the crystalline 4·2H₂O, 12, and 15 are shown in Fig. 1–3, respectively. Relevant data and parameters of the X-ray measurement and refinement are given in Table 2. Crystal 4·2H₂O crystallizes in the triclinic space group P1̄ with two formula units in the crystal cell and a calculated density of 1.734 g cm⁻³. Compound 12 crystallizes in the orthorhombic space group Pbca with four formula units in the crystal cell and a calculated density of 1.835 g cm⁻³. Compound 15 crystallizes in the triclinic space group P1̄ with two formula units in the crystal cell and a calculated density of 1.827 g cm⁻³. Further investigation on hydrogen bonds within the packing arrangement of all crystals reveals that the existence of various hydrogen bonds involving all of the hydrogen atoms, the ring nitrogen, and nitro oxygen atoms could stabilize molecular structures. For crystals 4·2H₂O, 12, and 15, the analytical results indicate that the nitro-substituted fused pyrazole rings are almost coplanar with torsion angles near zero or 180° as a result of conjugation, and the C–nitro bond length is in the range of 1.425(3)–1.436(3) Å. However, in the structure of crystal 15, the nitroxymethyl groups are twisted out of the plane, which can be seen from the torsion angle of N2–N3–C3–O3 (–124.3(6)°).

Fig. 1 (a) X-ray structure of 4·2H₂O with thermal ellipsoids at 30% probability. (b) Packing diagram of 4·2H₂O viewed down the a axis.

Fig. 2 (a) X-ray structure of 12 with thermal ellipsoids at 30% probability. (b) Packing diagram of 12 viewed down the a axis.

Fig. 3 (a) X-ray structure of 15 with thermal ellipsoids at 30% probability. (b) Packing diagram of 15 viewed down the a axis.

Table 2 Crystallographic data for 4·2H₂O, 12, and 15

Compounds	4·2H2O	12	15
Empirical formula	C4H6N6O6	C4H4N8O4	C6H4N8O10
Formula weight	234.15	228.15	348.17
CCDC number	1487948	1434787	1410323
T (K)	296(2)	296(2)	296(2)
Λ (Å)	0.71073	0.71073	0.71073
Crystal system	Triclinic	Orthorhombic	Triclinic
Space group	P1̄	Pbca	P1̄
Unit cell dimensions (Å, °)	a = 5.0574(14), α = 100.110(4)	a = 9.082(2), α = 90	a = 7.743(4), α = 85.500(11)
	b = 9.395(3), β = 97.281(4)	b = 5.4849(13), β = 90	b = 8.302(5), β = 78.807(10)
	c = 9.872(3), γ = 100.102(4)	c = 16.575(4), γ = 90	c = 10.605(6), γ = 71.153(9)
V (Å^3)	448.6(2)	825.7(3)	632.8(6)
Z	2	4	2
Dc (g cm^−3)	1.734	1.835	1.827
Absorption coefficient (mm^−1)	0.162	0.162	0.175
F(000)	240	464	352
Goodness-of-fit on F^2	1.051	1.002	1.038
Final R indices (I > 2σ(I))	R1 = 0.0380, wR2 = 0.0974	R1 = 0.0291, wR2 = 0.0816	R1 = 0.0872, wR2 = 0.2173
Largest diff. peak and hole (e Å^−3)	0.234 and −0.181	0.574 and −0.577	0.454 and −0.437

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Experimental

Caution: although we experienced no problems during the synthesis of the reported compounds, standard safety precautions (leather gloves, face shield and ear plugs) should be used when handling this energetic materials.

General methods

All chemical reagents and solvents (analytical grade) were used as supplied unless otherwise stated. Elemental analyses (C, H and N) were performed on a VARI-El-3 elementary analysis instrument. Infrared spectra were obtained from KBr pellets on a Nicolet NEXUS870 Infrared spectrometer in the range of 4000–400 cm⁻¹. ¹H NMR and ¹³C NMR were obtained in DMSO-d₆ on a Bruker AV500 NMR spectrometer. The DSC experiment was performed using a DSC-Q200 apparatus (TA, USA) under a nitrogen atmosphere at a flow rate of 50 mL min⁻¹. About 0.5 mg of the sample was sealed in aluminium pans for DSC. For all materials, the impact sensitivity were determined with a ZBL-B impact sensitivity instrument, the friction sensitivity were determined with a FSKM 10 friction sensitivity instrument. Energetic properties have been calculated with Gaussian 09 program²³ and Kamlet–Jacobs equations²⁵ using the calculated solid state heat of formation and experimental value of density. These were computed by the atomization method as described in published papers.

X-ray crystallography

Single crystals suitable for X-ray measurement were obtained by slow evaporation of aqueous solution of DNPP, 4, dimethyl formamide solution of 12, and methanol solution of 15 at room temperature. The crystal structures of all compounds were determined by a Bruker SMART APEXII CCD X-ray diffractometer and the SHELXTL crystallographic software package. A single crystal was mounted on a Bruker SMART APEXII CCD X-ray diffractometer equipped with graphite-monochromatized Mo-Kα radiation (0.71073 Å). Data were collected by the ω scan technique. The structure was solved by direct methods using SHELXS-97 program²⁶ and refined against F² by full-matrix least-squares using SHELXL-97 program.²⁷

Theoretical study

All the quantum computations were performed using the Gaussian 09 (Revision A. 02) suite of programs.²³ The optimized structures were characterized to be true local energy minima on the potential-energy surface without imaginary frequencies. The room-temperature gas-phase enthalpies of all explosives were obtained by the atomization method based on CBS-4M calculated electronic enthalpies using NIST²⁸ values as standardized values for the standard heats of formation (Δ_fH°). Often the standard state of the material of interest corresponds to the solid phase. Thus, the solid state enthalpies of formation can be determined using the gas-phase enthalpy of formation and enthalpy of sublimation phase transition according to the Hess' law of constant heat summation as the eqn (1).²⁹

ΔH(solid) = ΔH(gas) − ΔH(sublimation) (1)

Based on the electrostatic potential of a molecule by quantum mechanical prediction, the heat of sublimation can be represented as the eqn (2).³⁰

ΔH(sublimation) = a(SA)² + b(σ_Tot²υ)^1/2 + c (2)

where, (SA) is the molecular surface area for this structure, σ_Tot² is described as an indicator of the variability of the electrostatic potential on the molecular surface, υ is interpreted as showing the degree of balance between the positive and negative potentials on the molecular surface and a, b, and c are the fitting parameters. We further followed the approach of Politzer to predict the heats of sublimation of energetic materials, and then combined these with eqn (1) and (2) to predict the solid enthalpies of formation.

The empirical Kamlet–Jacobs (K–J) equations²⁵ widely employed to evaluate the energy performance of energetic compounds were used to estimate the detonation velocity (D) and detonation pressure (P) of all target compounds. Empirical Kamlet–Jacobs (K–J) equations can be shown in eqn (3)–(5).

P = 1.558ρ²Φ (3)

D = 1.01Φ^1/2(1.011 + 1.312ρ) (4)

Φ = 0.4889N(MQ)^1/2 (5)

where, D is the predicted detonation velocity (m s⁻¹), P is the predicted detonation pressure (GPa), ρ is the density of explosives (g cm⁻³), N is the moles of detonation gases per gram explosive, M is the average molecular weight of these gases, and Q is the chemical energy of detonation (kJ g⁻¹). The densities and the calculated heats of formation were used to compute the D and P values.

1,4-Diamino-3,6-dinitropyrazolo[4,3-c]pyrazole (12). To a solution of sodium carbonate (3.21 g, 30.2 mmol) and 3,6-dinitropyrazolo[4,3-c]pyrazole (DNPP, 4) (2.0 g, 10.1 mmol) in water (43.0 mL) at 70–75 °C, we added a solution of hydroxylamino-O-sulfuric acid (4.57 g, 40.4 mmol) in water (16.0 mL) for 15 min. The pH value was kept at 8–9 by the addition of sodium bicarbonate. The mixture was kept at 70–75 °C for 2 h and was then cooled to 15–20 °C. The precipitate was filtered off, washed with cold water, and dried to give 0.57 g brown solid 12 with a yield of 24.8%. ¹H NMR ([D₆]DMSO, 500 MHz, 25 °C, TMS): δ = 7.26 (s, 4H) ppm. ¹³C NMR ([D₆]DMSO, 125 MHz, 25 °C): δ = 131.9, 128.4 ppm. IR (KBr pellet): ν = 3319, 3272, 3174, 1638, 1532, 1395, 1358, 1331, 1229, 1112, 1019, 861, 776, 750, 583 cm⁻¹. Anal. calcd for C₄H₄N₈O₄: C 21.05, H 1.75, N 49.12%; found C 21.09, H 1.81, N 49.17%. MS (m/z): 228.16 [M⁺].

1-Amino-3,6-dinitropyrazolo[4,3-c]pyrazole (13). When the pH value was in the range of 2–3 during the neutralization of the above alkaline filtrate solution with concentrated hydrochloric acid (36–38%), the mono-N-amino product of DNPP was formed. The precipitate was filtered off, washed with cold water, and dried in air to give 0.71 g brown solid 13 with a yield of 33.0%. ¹H NMR ([D₆]DMSO, 500 MHz, 25 °C, TMS): δ = 15.09 (s, 1H), 7.27 (s, 2H) ppm. ¹³C NMR ([D₆]DMSO, 125 MHz, 25 °C): δ = 144.6, 138.4, 129.7, 111.3 ppm. IR (KBr pellet): ν = 3513, 3318, 3169, 2860, 1626, 1529, 1494, 1392, 1357, 1239, 1218, 1172, 1081, 1033, 964, 883, 836, 805, 750, 719, 650, 592 cm⁻¹. Anal. calcd for C₄H₃N₇O₄: C 22.54, H 1.42, N 46.01%; found C 22.47, H 1.51, N 46.06%. MS (ESI⁻) (m/z): 212.07 [M − H]⁻.

1,4-Dihydroxymethyl-3,6-dinitropyrazolo[4,3-c]pyrazole (14). DNPP (0.5 g, 2.53 mmol) was dissolved in sulfuric acid (10%, 15.0 mL) at room temperature, then ethyl acetate (20.0 mL) and formaldehyde solution (37–40%, 1.64 g) were added dropwise at 0–5 °C, respectively. The mixture was stirred at 20 °C for another 20 h, extracted with ethyl acetate (25.0 mL × 3), dried with magnesium sulfate, evaporated organic solvent under reduced pressure to obtain 0.394 g white solid 14 with a yield of 60.4%. ¹H NMR ([D₆]DMSO, 500 MHz, 25 °C, TMS): δ = 7.40 (t, 2H), 5.84 (d, 4H) ppm. ¹³C NMR ([D₆]DMSO, 125 MHz, 25 °C): δ = 137.5, 130.9, 75.9 ppm. IR (KBr pellet): ν = 3537, 2991, 2925, 1714, 1532, 1450, 1393, 1377, 1365, 1298, 1216, 1181, 1112, 1081, 994, 859, 774, 755, 717, 533 cm⁻¹. Anal. calcd for C₆H₆N₆O₆: C 27.92, H 2.34, N 32.55%; found C 22.87, H 2.37, N 32.60%.

1,4-Dinitroxymethyl-3,6-dinitropyrazolo[4,3-c]pyrazole (15). Acetic anhydride (3.7 mL) was placed in a three-necked round bottom flask and cooled to 0 °C. Nitric acid (98%, 3.7 mL) was added dropwise to acetic anhydride at 0–5 °C. Compound 14 (0.25 g, 0.97 mmol) was slowly added to the cooled mixture acid while maintaining the reaction temperature below 5 °C. After complete addition, the reaction mixture was stirred for 2 h at 16 °C. When the clear solution was poured into ice-water, a white solid precipitated. The solid was filtered off, washed with cold water, and dried in air to give 0.18 g yellowish solid 15 with a yield of 53.4%. ¹H NMR ([D₆]DMSO, 500 MHz, 25 °C, TMS): δ = 6.92 (s, 4H) ppm. ¹³C NMR ([D₆]DMSO, 125 MHz, 25 °C): δ = 139.0, 132.3, 79.1 ppm. IR (KBr pellet): ν = 3046, 3004, 2928, 1756, 1665, 1544, 1427, 1409, 1375, 1338, 1313, 1285, 1206, 1136, 1040, 964, 870, 835, 786, 749, 722, 651, 604, 490 cm⁻¹. Anal. calcd for C₆H₄N₈O₁₀: C 20.70, H 1.16, N 32.19%; found C 20.76, H 1.12, N 32.22%.

1,4-Dichloromethyl-3,6-dinitropyrazolo[4,3-c]pyrazole (16). Compound 14 (1.5 g, 5.81 mmol) was slowly added in batches to sulfoxide chloride (11.0 mL) at room temperature. After complete addition, the reaction mixture was stirred for 5 h at 95 °C. The excessive sulfoxide chloride was removed under reduced pressure, dried with potassium hydroxide to obtain 1.58 g yellow solid 16 with a yield of 92.1%. ¹H NMR ([D₆]DMSO, 500 MHz, 25 °C, TMS): δ = 5.89 (s, 4H) ppm. ¹³C NMR ([D₆]DMSO, 125 MHz, 25 °C): δ = 137.5, 131.0, 75.9 ppm. IR (KBr pellet): ν = 3066, 3002, 2924, 1732, 1711, 1547, 1400, 1286, 1225, 1166, 1132, 1092, 1053, 947, 857, 757, 714, 598 cm⁻¹. Anal. calcd for C₆H₄N₆O₄Cl₂: C 24.43, H 1.37, N 28.48%; found C 24.48, H 1.31, N 28.43%.

1,4-Diazidomethyl-3,6-dinitropyrazolo[4,3-c]pyrazole (17). Compound 16 (1.6 g, 5.42 mmol), acetone (20.0 mL) and sodium azide (1.05 g, 16.15 mmol) were added to a three-necked round bottom flask at room temperature. After complete addition, the reaction mixture was stirred for 48 h at 45 °C. When the reaction mixture was poured into ice-water, a yellow solid precipitated. The solid was filtered off, washed with cold water, and dried in air to give 1.06 g yellow solid 17 with a yield of 63.5%. ¹H NMR ([D₆]DMSO, 500 MHz, 25 °C, TMS): δ = 6.03 (s, 4H) ppm. ¹³C NMR ([D₆]DMSO, 125 MHz, 25 °C): δ = 138.1, 131.6, 66.5 ppm. IR (KBr pellet): ν = 3056, 2115, 1734, 1614, 1547, 1400, 1303, 1223, 1146, 1118, 1054, 906, 859, 764, 601 cm⁻¹. Anal. calcd for C₆H₄N₁₂O₄: C 23.38, H 1.31, N 54.54%; found C 23.46, H 1.26, N 54.57%.

1,3,4,6-Tetranitropyrazolo[4,3-c]pyrazole (18). N₂O₅ (4.0 g, 37.0 mmol) was dissolved in dichloromethane (20.0 mL) at −5 °C. Compound DNPP (1.0 g, 5.05 mmol) was slowly added to the cooled mixture while maintaining the reaction temperature at 0 °C. After complete addition, the reaction mixture was stirred for 2 h at 0 °C. The reaction mixture was poured into ice-water and obtaining white solid. The solid was filtered off, washed with cold water and ethanol, and dried in air to give 0.8 g white solid 18 with a yield of 55.0%. ¹³C NMR ([D₃]CD₃CN, 125 MHz, 25 °C): δ = 133.3, 124.8 ppm. IR (KBr pellet): ν = 1665, 1561, 1457, 1418, 1370, 1282, 1243, 1153, 1084, 847, 805, 780, 728, 718, 585, 550, 520 cm⁻¹. Anal. calcd for C₄N₈O₈: C 16.67, N 38.89%; found C 16.63, N 38.94%. MS (EI) (m/z): 288.13 [M⁺].

Diammonium 3,6-dinitropyrazolo[4,3-c]pyrazolate (19). Compound DNPP (1.87 g, 9.44 mmol) was dissolved in a solution of methanol (45.0 mL) and distilled water (8.0 mL) at room temperature. Then aqueous ammonia (25–28%, 2.5 mL) was added dropwise to the reaction solutions. After complete addition, the reaction mixture was stirred for 6 h at 50 °C. The solid was filtered off, washed with methanol, and dried in air to give 1.9 g yellow solid 19 with a yield of 86.7%. ¹H NMR ([D₆]DMSO, 500 MHz, 25 °C, TMS): δ = 7.21 (s, 8H) ppm. ¹³C NMR ([D₆]DMSO, 125 MHz, 25 °C): δ = 140.3, 138.7 ppm. IR (KBr pellet): ν = 3141, 3068, 2882, 1696, 1619, 1563, 1498, 1485, 1444, 1403, 1377, 1292, 1238, 1175, 1112, 1042, 875, 812, 776, 753, 730, 665, 478 cm⁻¹. Anal. calcd for C₄H₈N₈O₄: C 20.69, H 3.47, N 48.27%; found C 20.77, H 3.42, N 48.32%.

1,4-Dihydroxyl-3,6-dinitropyrazolo[4,3-c]pyrazole (20). Compound 19 (0.4 g, 1.72 mmol) was added to distilled water (10.0 mL) and heated to 55 °C. OXONE® (6.4 g, 20.8 mmol) was added in batches to reaction solution at same temperature. After complete addition, the reaction mixture was stirred for 72 h at 55 °C, then cooled to room temperature. Concentrated sulfuric acid (95–98%, 2.8 mL) was added dropwise to solution under ice-water condition, extracted with diethyl ether (25.0 mL × 6), dried with magnesium sulfate, evaporated organic solvent under reduced pressure to obtain 0.16 g yellow solid 20 with a yield of 40.4%. ¹H NMR ([D₆]DMSO, 500 MHz, 25 °C, TMS): δ = 15.02 (s, 2H) ppm. ¹³C NMR ([D₆]DMSO, 125 MHz, 25 °C): δ = 138.4, 132.0 ppm. IR (KBr pellet): ν = 3596, 3511, 2817, 2763, 1701, 1604, 1515, 1384, 1362, 1245, 1134, 1039, 878, 827, 748, 661, 605 cm⁻¹. Anal. calcd for C₄H₂N₆O₆: C 20.88, H 0.88, N 36.52%; found C 20.83, H 0.92, N 36.57%. MS (APCl⁻) (m/z): 229.17 [M − H]⁻.

Conclusions

A modified and more efficient synthetic process for 3,6-dinitropyrazolo[4,3-c]pyrazole (DNPP, 4) has been described. A new family of neutral energetic derivatives based on 4 were prepared and fully characterized by using NMR and IR spectroscopies, elemental analysis, and single-crystal X-ray diffraction analysis for 4·2H₂O, 12 and 15. These neutral compounds exhibit acceptable thermal stabilities (178–296 °C). Densities for these compounds fall in the range between 1.74 and 1.95 g cm⁻³, which places them in a new class of relatively dense energetic materials. All of the compounds exhibit positive heats of formation ranging between 18.8 and 863 kJ mol⁻³. Their detonation properties were evaluated by Gaussian 09 and Kamlet–Jacobs equations. The calculated detonation velocities (7819–9364 m s⁻¹) and detonation pressures (27.1–41.5 GPa) are comparable to those of explosives such as TNT and RDX. Most of these compounds have reasonable impact sensitivities (10 to 16 J) and friction sensitivities (160 to 300 N), which suggest that they have the potential to be useful energetic materials.

Acknowledgements

Financial support of this work by the National Natural Science Foundation of China (21373157) is gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: (1) X-ray crystallography, (2) ¹H and ¹³C NMR spectra of all compounds, (3) the details for the heat of formation. CCDC 1410323, 1434787 and 1487948. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra19556c

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