German Edition: DOI: 10.1002/ange.201609267Nitrogen HeterocyclesInternational Edition: DOI: 10.1002/anie.201609267

Synthesis and Investigation of Advanced Energetic Materials Based onBispyrazolylmethanesDennis Fischer, Jennifer L. Gottfried, Thomas M. Klapçtke,* Konstantin Karaghiosoff,Jçrg Stierstorfer, and Tomasz G. Witkowski

Dedicated to Professor Jgrgen Evers on the occasion of his 75th birthday

Abstract: Herein we present the preparation and character-ization of three new bispyrazolyl-based energetic compoundswith great potential as explosive materials. The reaction ofsodium 4-amino-3,5-dinitropyrazolate (5) with dimethyliodide yielded bis(4-amino-3,5-dinitropyrazolyl)methane (6),which is a secondary explosive with high heat resistance (Tdec =

310 88C). The oxidation of this compound afforded bis(3,4,5-trinitropyrazolyl)methane (7), which is a combined nitrogen-and oxygen-rich secondary explosive with very high theoreticaland estimated experimental detonation performance (Vdet

(theor) = 9304 ms@1 versus Vdet(exp) = 9910 ms@1) in therange of that of CL-20. Also, the thermal stability (Tdec =

205 88C) and sensitivities of 7 are auspicious. The reaction of6 with in situ generated nitrous acid yielded the primaryexplosive bis(4-diazo-5-nitro-3-oxopyrazolyl)methane (8),which showed superior properties to those of currently useddiazodinitrophenol (DDNP).

“Safer, greener, stronger …” are the main keywords in thedevelopment of new explosives for civil but also militarypurposes. The new materials must meet many differentrequirements, for example, high performance, insensitivitytoward accidental stimuli, stability, vulnerability, low solubil-ity in water, hydrolytic stability, longevity, compatibility, andenvironmental safety.[1] On the basis of their sensitivities orthe time of their deflagration-to-detonation transition, explo-sives can be classified as secondary and primary explosives.Both groups are divided into subgroups depending on theirapplication. For example, the mining and fracking industryuses heat-resistant explosives, such as hexanitrostilbene(Scheme 1), for deeper drill holes. In contrast, the militaryis undertaking research toward the development of morepowerful and effective explosives (such as CL-20, Scheme 1)for use in future unmanned-vehicle missions. DDNP

(Scheme 1) is an example of a more sensitive energeticmaterial and is used in lead-free priming compositions. It isused as an initiating explosive that is more powerful thanmercury fulminate and slightly less explosive than leadazide.[2] Nevertheless, DDNP is characterized by a relativelylow decomposition temperature.[2]

In the current study, three different explosives, 6–8,belonging to the above-mentioned classes were found toshow very promising performance and sensitivity character-istics. They are all based on methylene-bridged pyrazoles.Nitropyrazoles have been described as powerful energeticmaterials.[3] Furthermore, ethylene-bridged pyrazoles, thesynthesis of which is more trivial, have been described;however, they have been shown to have weaker performancevalues.[3b]

The present synthetic protocol has been optimized partlyon the basis of previous studies. Pyrazole (1) was chlorinatedto give 4-chloropyrazole (2) with chlorine generated in situ(NaClO/HCl; Scheme 2).[4] The nitration of 2 was carried outby the use of a H2SO4 HNO3(100%) mixture.[5] The third stepinvolved the reaction of 4-chloro-3,5-dinitropyrazole (3) withammonia to give 4-amino-3,5-dinitropyrazole (4),[5] followedby reaction with sodium hydroxide to yield sodium 4-amino-3,5-dinitropyrazolide (5) as a monohydrate salt.[6] The sodiumsalt 5 was treated with diiodomethane to give bis(4-amino-3,5-dinitropyrazolyl)methane (6),[6] which could be oxidized tobis(3,4,5-trinitropyrazolyl)methane (7) by stirring in a H2O2

(50 %)/H2SO4 mixture. The final step to provide bis(4-diazo-5-nitro-3-oxopyrazolyl)methane (8) was carried out by diaz-otation and the generation of HNO2 (NaNO2 + H2SO4) at0 88C.

The solid-state structures of compounds 3 and 5–8 weredetermined by XRD. All compounds crystallize in common

Scheme 1. Examples of prominent explosives belonging to differentclasses: heat-resistant explosives (hexanitrostilbene, HNS), high explo-sives (hexanitrohexaazaisowurtzitane, CL-20), and primary explosives(DDNP, diazadinitrophenole).

[*] Dr. D. Fischer, Prof. Dr. T. M. Klapçtke, Prof. Dr. K. Karaghiosoff,Dr. J. Stierstorfer, Dipl.-Ing. T. G. WitkowskiDepartment of Chemistry, University of Munich (LMU)Butenandtstrasse 5–13 (D), 81377 Mfnchen (Germany)E-mail: tmk@cup.uni-muenchen.deHomepage: http://hedm.cup.uni-muenchen.de

Dr. J. L. GottfriedRDRL-WML-B, US Army Research LaboratoryAberdeen Proving Ground, MD (USA)

Supporting information for this article can be found under:http://dx.doi.org/10.1002/anie.201609267.

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space groups (3 : P21/c ; 5 : P-1; 6 : Pbca ; 7: P21/n ; 8 : Pnma).Especially for 6 (1.836 gcm@3@173 K) and 7(1.970 gcm@3@173 K), high densities were observed, asdesired for secondary explosives. The molecular moietiesare shown in Figure 1. Further details can be found in theSupporting Information. In the case of 5 and 6, the 3,5-dinitro-4-aminopyrazole moiety was observed to be nearly planarowing to the intramolecular hydrogen bonds, which are alsofound in TATB (1,3,5-triamino-2,4,6-trinitrobenzene) andFOX-7 (1,1-diamino-2,2-dinitroethylene).[1b] The diazoniumN4@N3 bond length of 1.112(2) c is nearly equal to thosefound in various diazonium inner salts[7] and cations,[8] thusproving the same a triple character of the N4@N3 bond(dN/N = 1.10 c).[9] The bond between the C2 and N3 atomshas a length of 1.320(2) c, which is between the expectedlengths of first- and second-order bonds between carbon andnitrogen atoms (dC@N = 1.47 c; dC=N = 1.28 c).[10] The C3@O3bond length of 1.215(2) c is virtually equal to the expectedvalue for doubly bonded (keto) oxygen atoms (dC=O =

1.20 c[9]) and is in agreement with previously reported datafor this functionality.[7b,d]

Compounds 6–8 are valuable energetic materials withmoderate sensitivities toward impact, friction, and electro-static discharge. The measured sensitivity values (according tothe German Bundesanstalt fgr Materialforschung und -prg-fung (BAM)) are gathered in Table 1. The impact sensitivities(6 : 11, 7: 4 J) are in the range of those of HNS andpentaerythritol tetranitrate (PETN). Compound 8 is moresensitive (1.5 J) and should therefore be classified as a primaryexplosive. Positive enthalpies of formation were calculatedfor 6–8 (6 : 205, 7: 379, 8 : 497 kJmol@1; see the SupportingInformation for details of the computations). With thesevalues, several detonation parameters (Table 1) were calcu-lated by using the EXPLO5 computer code.

The very promising detonation velocity of 7 (Vdet =

9304 ms@1) was corroborated by the estimated detonationvelocity determined in a LASEM experimental setup((9910: 310) ms@1). In LASEM, the laser-induced air shockvelocity is measured by schlieren imaging and correlated tothe laser-induced shock velocities of conventional energeticmaterials to estimate the detonation velocity of the testedenergetic material (see the Supporting Information fora closer description of this method[11]). The average measuredcharacteristic laser-induced shock velocity for 7 is shown inTable 2 (along with 95 % confidence intervals). The LASEMresults suggest that 7 has a higher detonation velocity (at thetheoretical maximum density) than CL-20, and the value isalmost 600 ms@1 higher than that predicted by EXPLO5V6.01. However, LASEM is applied as a screening method,and the obtained data are not definitive, but only serve as anindicator of promising performance. Therefore, further test-ing will be performed to confirm the LASEM results.

The thermal stability of 6–8 at a heating rate of 5 88C min@1

was investigated, and excellent decomposition values werefound (6 : 310, 7: 205, 8 : 226 88C (Tm = 194 88C)). The decom-position temperature of 8 is lower than the value determinedfor 1,2-ethylenebis(4-diazonium-3-nitro-1H-pyrazol-5-olate)[3b] but still higher than that of DDNP (Table 1).

In summary, we have introduced three new methylene-bridged nitropyrazole derivatives with potential use asexplosives for different applications. Bis(4-amino-3,5-dinitro-pyrazolyl)methane (6) is a secondary explosive with a highheat resistance (Tdec = 310 88C), enhanced detonation param-eters in comparison to those of HNS, and lower sensitivity to

Scheme 2. Synthesis of bis(4-amino-3,5-dinitropyrazolyl)methane (6),bis(3,4,5-trinitropyrazolyl)methane (7), and bis(4-diazo-5-nitro-3-oxo-pyrazolyl)methane (8).

Figure 1. Molecular structures of 6–8 with atom labels. Thermal ellipsoids represent the 50% probability level.

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external stimuli. The NH2!NO2 oxidation product bis(3,4,5-trinitropyrazolyl)methane (7) shows excellent detonationvelocity (approximately equal to that of CL-20, the currenthigh-explosive benchmark) according to the theoretical valueand the value estimated by the LASEM method, and isdescribed in a recently submitted patent application. Thereaction of 6 with nitrous acid generated in situ yielded theprimary explosive bis(4-diazo-5-nitro-3-oxopyrazolyl)me-thane (8), the higher performance and better thermal stabilityof which relative to DDNP make this compound a competitivecandidate as a green primary explosive. The syntheticapproach presented herein resulted in three new compoundswhich not only fall into different classes of explosives but alsoshow improved or comparable properties to those of corre-sponding representatives of each group (HNS, CL-20,DDNP). Moreover, all compounds can be synthesizedeconomically.

Experimental SectionGeneral methods, the preparation of 2–5, and more experimental

data on 6–8 can be found in the Supporting Information.Synthesis of 6 : Sodium 3,5-dinitro-4-aminopyrazolate monohy-

drate (4.29 g, 22 mmol) was suspended in N,N-dimethylformamide(15 mL), and diiodomethane (2.68 g, 805 mL, 10 mmol) was added.The mixture was stirred at 90 88C for 16 h and then poured into water

(100 mL). A little sodium thiosulfate solution is added to reduce theprecipitated iodine until the suspension was a clean yellow color. Theprecipitated product was filtered, washed with a little water, and driedin air to give 6 (3.18 g, 89 %) as yellow solid. DSC (5 88Cmin@1): 310 88C(dec.); 1H NMR (400 MHz, [D6]DMSO, 25 88C): d = 7.29, 7.16 ppm;13C NMR{1H} (400 MHz, [D6]DMSO, 25 88C): d = 142.1, 131.7, 130.9,67.2 ppm; 15N NMR (40.6 MHz, [D6]DMSO): d =@23.8 (s, NO2),@29.3 (s, NO2), @73.7 (t, JN,H = 2.7 Hz, Npyr@CH2), @192.8 (t, JN,H =

1.1 Hz, Npyr),@314.1 ppm (t, JN,H = 93.7 Hz, NH2); elemental analysis:calcd (%) for C7H6N10O8 (258.19): C 23.47, H 1.69, N 39.10; found: C23.73, H 1.81, N 38.90; BAM drop hammer: 11 J (< 100 mm); frictiontester: > 360 N (< 100 mm); ESD: > 1 J (< 100 mm).

Synthesis of 7: A solution of 6 (1.00 g, 2.80 mmol) in concentratedH2SO4 (5 mL) was added dropwise to a mixture of 50 % H2O2

(7.5 mL) and H2SO4 (25 mL) at 0 88C. The reaction mixture wasstirred at 0 88C for 3 h and then overnight at room temperature.Subsequently, the reaction mixture was poured onto ice (100 g).Finally, the resulting precipitate was isolated by filtration, washedwith water, and dried in air to give 7 (1.00 g, 86%). DSC (5 88Cmin@1):205 88C (dec.); 1H NMR ([D6]acetone, 25 88C): d = 7.84 ppm; 13C-{1H} NMR ([D6]acetone, 25 88C): d = 144.2, 138.8, 123.8, 67.3 ppm;14N NMR ([D6]acetone, 25 88C): d =@29.1 (NO2), @31.0 (NO2), @33.5(NO2) ppm; elemental analysis: calcd (%) for C7H2N10O12 (418.15): C20.11, H 0.48, N 33.50; found: C 19.91, H 0.82, N 32.55; BAM drophammer: 4 J (< 100 mm); friction tester: 112 N (< 100 mm); ESD:0.6 J (< 100 mm).

Synthesis of 8 (method A): 6 (3 mmol, 1.075 g) was dissolved insulfuric acid (15 mL) and cooled to 0 88C. A solution of sodium nitrite(9 mmol, 0.621 g) in water (2 mL) was then added dropwise. Thereaction mixture was stirred at 0 88C for 2 h and then for 4 h at ambienttemperature, and was then poured onto ice. The yellow precipitatewas filtered and washed with a small amount of cold water. The waterphase was extracted with ethyl acetate (3 X 25 mL), dried overmagnesium sulfate, and evaporated under reduced pressure to give8 (0.84 g, 87%; for method B, please see the Supporting Information).DSC (5 88Cmin@1): 194 (melt., onset), 226 88C (dec., onset); 1H NMR(400 MHz, [D6]DMSO): d = 5.88 ppm (s, 2H, CH2); 13C{1H} NMR(101 MHz, [D6]DMSO): d = 161.3 (CO@), 145.1 (CNO2), 65.6 (CN2

+),53.8 ppm (CH2); 14N NMR (28.9 MHz, [D6]DMSO): d =@24.9(NO2), @139.9 ppm (N+/N); 15N NMR (40.6 MHz, [D6]DMSO):d =@16.0 (N+/N), @28.7 (NO2), @84.1 (t, JN,H = 2.8 Hz, Npyr@CH2),@139.6 (N+/N), @193.0 ppm (Npyr); elemental analysis: calcd (%)for C7H2N10O6 (322.15): C 26.10, H 0.63, N 43.48; found: C 26.03, H

Table 1: Energetic properties and detonation parameters of 6–8 and comparison with state-of-the-art compounds.

6 HNS 7 RDX CL-20 8 DDNPFormula C7H6N10O8 C14H6N6O12 C7H2N10O12 C3H6N6O6 C6H6N12O12 C7H2N10O6 C6H2N4O5

IS [J][a] 11 5 (<100 mm)[b] 4 7.5[b] 3[b] 1.5 1[b]

FS [N][c] >360 >360[b] 144 120[b] 96[b] 40 5[b]

ESD test [J][d] >1.5 1.0[b] 0.1 0.2 0.1 0.10 0.012W [%][e] @40.20 @67.6 @11.48 @21.6 @11.0 @44.70 @60.92Tdec [88C][f ] 310 320[12] 205 210 195 194 (m.p.), 226 (dec.) 157 (dec.)density [g cm-3][g] 1.802 1.745[13] 1.934 1.806[14] 2.035[15] 1.732 1.719[16]

DfH88 [kJmol@1][h] 205 78 379 86 365 497 139EXPLO5 6.01 values@DEU88 [kJkg@1][i] 4942 5146 6147 5853 6130 4862 4987pC–J [GPa][j] 29.6 24.5 39.1 33.8 44.9 26.0 23.8D [ms@1][k] 8332 7629 9304 8803 9673 8016 7651V0 [dm3 kg@1][l] 719 601 720 891 724 700 633

[a] Impact sensitivity (BAM drop hammer, method 1 of 6). [b] Determined at LMU. [c] Friction sensitivity (BAM friction tester, method 1 of 6). [d] OZMelectrostatic discharge device. [e] Oxygen balance (W = xO@2C@1/2H)1600/M). [f ] Decomposition temperature from DSC/DTA (b= 5 88C min@1).[g] Determined by X-ray diffraction and converted into room-temperature values. [h] Calculated (CBS-4M) heat of formation. [i] Explosion temperature.[j] Detonation pressure. [k] Detonation velocity. [l] Volume of detonation gases assuming only gaseous products. RDX=1,3,5-trinitroperhydro-1,3,5-triazine.

Table 2: Measured laser-induced air shock velocity and detonationvelocity (D) estimated by LASEM, along with the theoretically predicteddetonation velocity.

Sample Laser-inducedshock velocity[m s@1]

D (estimated)[kms@1]

D (EXPLO5V6.01)[kms@1]

D (measured)@ TMD[a]

[kms@1]

7 849.54:12.50 9.91:0.31 9.304 –RDX 806.83:7.74 8.85:0.19 8.834 8.833[11b]

CL-20 835.41:9.52 9.56:0.24 9.673 9.57[11b]

[a] TMD is maximum theoretical density.

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0.83, N 43.33; BAM drop hammer: 1.5 J (< 100 mm); friction tester:40 N (< 100 mm); ESD: 0.10 J (< 100 mm).

Acknowledgements

Financial support of this research by the Ludwig MaximilianUniversity of Munich (LMU) and the Office of NavalResearch (ONR.N00014-16-1-2062) is gratefully acknowl-edged. We thank Dr. Burkhard Krumm for NMR measure-ments, Stefan Huber for his help with the sensitivity testing,and Jelena Reinhardt for practical support. T.G.W. alsothanks the DAAD for a PhD scholarship.

Keywords: diazo compounds · energetic materials ·nitrogen heterocycles · pyrazoles · X-ray diffraction

How to cite: Angew. Chem. Int. Ed. 2016, 55, 16132–16135Angew. Chem. 2016, 128, 16366–16369

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Manuscript received: September 21, 2016Final Article published: November 25, 2016

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