Challenging the Limits of Nitro Groups Associated with a Tetrazole

Letter Cite This: Org. Lett. 2019, 21, 4684−4688

pubs.acs.org/OrgLett

Challenging the Limits of Nitro Groups Associated with a Tetrazole Ring Qiong Yu,† Gregory H. Imler,‡ Damon A. Parrish,‡ and Jean’ne M. Shreeve*,† †

Department of Chemistry, University of Idaho, Moscow, Idaho 83844-2343, United States Naval Research Laboratory, 4555 Overlook Avenue, Washington, D.C. 20375, United States

‡

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S Supporting Information *

ABSTRACT: To challenge the limits of nitro groups associated with a tetrazole ring, the polynitrotetrazoles, 2,5-bis(dinitromethyl)-2H-tetrazole, and 2-(dinitromethyl)-5-(trinitromethyl)-2H-tetrazole as well as their salts, were designed, synthesized, and characterized. The neutral compounds were also studied by natural bonding orbital (NBO) and Hirshfeld surface calculations. The heats of formation were calculated with Gaussian 03 and combined with experimentally determined densities in order to obtain the detonation properties of the energetic materials with the EXPLO5 program. They exhibit high densities, good oxygen balances, positive heats of formation, and excellent detonation properties.

P

olynitro compounds are of particular interest in the development of high energy density materials (HEDM) since they possess high densities and good oxygen balances.1 Recently, some polynitro compounds of different heterocyclic rings, such as furoxan,2 triazole,3 and 1,3,4-oxadiazole,4 were reported (Figure 1). Compounds i, ii, and iii exhibit high

here have more positive O̧̅ nitro values indicating lower stabilities (Figure 2, Supporting Information). Compound 1 was

Figure 1. Polynitro compounds with different heterocyclic rings.

Figure 2. Average negative nitro charge (O̧̅ nitro) of polynitro compounds based on the number of nitro groups associated with the tetrazole ring.

densities and excellent energetic properties. Tetrazole has been used widely in high-nitrogen energetic materials owing to its high nitrogen content and high heat of formation.5 5(Trinitromethyl)-2H-tetrazole (1) is a combination of tetrazole and a trinitromethyl group and has a density of 1.92 g cm−3.6 In our continuing efforts to introduce as many nitro groups associated with a tetrazole ring as possible, compound 1 was selected as the building block. The molecular stability of polynitro compounds can be evaluated by the average negative nitro charge (-O̧̅ nitro). The more negative the O̧̅ nitro value, the more stable the compound.7 Compared with 1, the structures of the compounds that are under investigation © 2019 American Chemical Society

obtained in one step by nitrating ethyl 2-(2H-tetrazol-5-yl) acetate in good yield. To introduce additional nitro groups associated with the tetrazole, the acidic proton of the tetrazole was replaced by a carbonyl-containing group having an activated methylene moiety which can be nitrated with an acidic nitrating mixture.8 Compound 10 was obtained by Received: May 4, 2019 Published: June 13, 2019 4684

DOI: 10.1021/acs.orglett.9b01565 Org. Lett. 2019, 21, 4684−4688

Letter

Organic Letters neutralizing its potassium salt (9) which was obtained by displacing one nitro group of a trinitromethyl group from 3 under basic conditions. The pure potassium salt was prepared from impure 3 which was made by nitrating 2 with 100% HNO3/98% H2SO4. In order to obtain pure 3, a 90% HNO3/ 90% H2SO4 mixture was used (Table 1). The attempt to Table 1. Nitration of 2 with Mixed Acida entry

nitrateb

productsc

1 2 3 4

70% HNO3 (0.5)/90% H2SO4 (0.5) 80% HNO3 (0.5)/90% H2SO4 (0.5) 90% HNO3 (0.5)/90% H2SO4 (0.5) 100% HNO3 (0.5)/98% H2SO4 (0.5)

2, 3 2, 3 3 3, 4d

Figure 3. TLC result of the reaction of 3 in mixed acid after 3 h at room temperature. The left spot is compound 3. The right column is the extracted products from the reaction.

disappeared after 1−2 min on the TLC plate (Figure 3b). 14N NMR was used to characterize the different chemical environments where nitro groups are found. It is very sensitive to the nitrogen of the nitro group. Both 3 and 4 have two different signals from nitro groups in the 14N NMR spectra. To compare the difference between 3 and 4, the 14N NMR spectrum of pure compound 3 was run in CDCl3 giving peaks at −35.68 ppm and −39.76 ppm (Figure 4a). The extracted

a All reactions were carried out at rt. A 1 mmol amount of 2 was used as substrate in every reaction. Reaction time is 3 h. bVolume of mixed acid is given in parentheses (mL). cProducts were characterized by 1H NMR and 13C NMR. dCompound 4 was observed by TLC and 14N NMR.

identify 4 was more difficult because, although it was observed, it could not be isolated or characterized. As the number of nitro groups increases, the negative O̧̅ nitro values decrease, with the resulting compound being less stable, and the synthesis becoming more difficult. Starting from 1, which is easily obtained from ethyl 2-(2Htetrazol-5-yl)acetate in good yield,6 reaction with bromoacetone under alkaline conditions gave 2 (Scheme 1). Nitration of Scheme 1. Synthesis of 2-(Dinitromethyl)-5(trinitromethyl)-2H-tetrazole (3) and 2,5Bis(trinitromethyl)-2H-tetrazole (4)

Figure 4. (a) 14N spectrum of 3. (b) 14N spectrum of products of the reaction of 3 in mixed acid after 3 h at room temperature. (c) 14N spectrum of sample b which stayed for 3 min.

products in dichloromethane were diluted with CDCl3 in order to run the14N NMR spectrum. As shown in Figure 4b, two peaks at −38.41 ppm and −41.75 ppm corresponding to compound 4 were observed; unfortunately, after 3 min, these two peaks disappeared (Figure 4c). Concomitantly compound 3 was also detected which is much more stable than 4. A series of nitrogen-rich salts 5−9 were obtained by treating 3 with different bases at 10 °C (Scheme 2). The potassium salt 5 and ammonium salt 6 were obtained by reaction of 3 with an excess of potassium bicarbonate and ammonium bicarbonate in acetonitrile. The excess base was removed by filtration. Compound 3 decomposed if the hydroxylamine or hydrazine was dropped directly. Fortunately, these two bases were diluted with acetonitrile and dropped into the reaction to yield the energetic salts, 7 and 8. When 3 was treated with hydroxylamine hydrochloride and potassium hydroxide, the trinitromethyl group attached to carbon was reduced to the dinitromethyl group to form the

2 was attempted by using different concentrations of mixed acids (HNO3/H2SO4) such as 70% HNO3/90% H2SO4, 80% HNO3/90% H2SO4, 90% HNO3/90% H2SO4, or 100% HNO3/98% H2SO4, but interestingly compound 3 was the only product obtained when 90% HNO3/90% H2SO4 was the nitrating reagent (Table 1). When 70% HNO3 or 80% HNO3 was used, the products were a mixture of 2 and 3. When 2 was treated with 100% HNO3/98% H2SO4, the reaction gave 3 and 4 simultaneolusly. Compound 4 was observed using TLC by nitrating 3 with a mixture of pure nitric and concentrated sulfuric acids at room temperature. Dichloromethane was used to extract the products from the reaction mixture. As shown in Figure 3a, compound 4 was observed at first but then 4685

DOI: 10.1021/acs.orglett.9b01565 Org. Lett. 2019, 21, 4684−4688

Letter

Organic Letters Scheme 2. Synthesis of Energetic Salts 5−9

dipotassium (2H-tetrazole-2,5-diyl)bis(dinitromethanide) (9). A similar reaction was reported previously.9 The potassium salt 9 was acidified with sulfuric acid to give 2,5-bis(dinitromethyl)-2H-tetrazole (10) which is a white solid (Scheme 3). The energetic salts of 10 were obtained from the reaction of its silver salt and corresponding hydrochloride salts.

Figure 5. Single crystal X-ray structures of (a) 3, (b) 5·CH2Cl2, (c) 6· CH3CN, (d) 9·H2O, (e) 10, and (f) 12·H2O.

Oxygen balance (OB) is an index of the deficiency or excess of oxygen in a compound.10 All the compounds except 14 exhibit positive oxygen balances ranging from 5.1% to 32.2% (Table 1). It is important to note that the OB value of 4 is 32.2% which is higher than the oxidizer ammonium dinitramide (ADN 25.8%).11 Interestingly, 14 has an oxygen balance of zero. The energetic salts of 2-(dinitromethyl)-5(trinitromethyl)-2H-tetrazole have higher heats of formation than the salts of 2,5-bis(dinitromethyl)-2H-tetrazole arising from the presence of one more nitro group. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) of all compounds were carried out at a heating rate of 5 °C min−1 using dry nitrogen as an atmosphere to determine their thermal behavior. They decomposed between 73 and 128 °C (onset temperature) without melting (Table 2). The densities of these compounds ranged between 1.75 to 2.05 g cm−3 which were obtained from gas pycnometer measurements. The heat of formation is a very important parameter in evaluating the performance of energetic materials. To investigate the energetic properties of these new compounds, the heats of formation (HOF) were computed by using the method of isodesmic reactions with the Gaussian 03 suite of programs (Scheme S1).13 Compound 9 has a negative HOF of −0.98 kJ g−1 while the other compounds have positive HOFs (0.05−1.08 kJ g−1) owing to the high HOF value of the tetrazole ring. With the densities and HOFs in hand, the detonation properties were calculated by using the EXPLO5 6.01 program (Table 2).14 The calculated detonation velocities lie in the range between 7394 and 9590 m s−1, and detonation pressures range from 23.9 to 42.4 GPa, respectively. Among them, compound 13 exhibits the highest detonation velocity and pressure (9590 m s−1, 42.4 GPa) which is higher than those of HMX (9320 m s−1, 39.5 GPa). Compound 3 has a lower detonation velocity and pressure (8511 m s−1, 31.2 GPa) than 9 (9123 m s−1, 37.7 GPa), which indicates that the presence of one more nitro group from the trinitromethyl fragment decreases the energetic power of 3. This decrease is also found for the energetic salts of 2-(dinitromethyl)-5-(trinitromethyl)-2H-tetrazole which have lower detonation veloc-

Scheme 3. Synthesis of 2,5-Bis(dinitromethyl)-2H-tetrazole (10) and Its Energetic Salts, 11−14

All compounds were characterized by IR, 1H NMR, and 13C NMR spectroscopy as well as elemental analysis (Supporting Information). The structures of 3, 5·CH2Cl2, 6·CH3CN, 9· H2O, 10, and 12·H2O were further confirmed by single-crystal X-ray diffraction (Figure 5, Supporting Information). Crystals of 3 and 10 were obtained by recrystallization from dichloromethane with calculated high densities of 1.896 g cm−3 and 1.923 g cm−3 at 296(2) K, respectively. The crystals of 5·CH2Cl2, 6·CH3CN, 9·H2O, and 12·H2O suitable for crystal structure analyses were obtained from a mixture of methanol and dichloromethane, acetonitrile, water, and methanol, respectively with solvent molecules in the crystal structures. As is known, the introduction of nitro groups usually increases compound density; however, compound 3 has a lower density than 10. This apparently arises from the presence of the trinitromethyl group bonded to 3 which leads to greater disorder in the crystal packing compared to that of 10. 4686

DOI: 10.1021/acs.orglett.9b01565 Org. Lett. 2019, 21, 4684−4688

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Organic Letters Table 2. Properties of Energetic Compounds (3, 5−10, and 12·H2O−14) compds

Tda [°C]

ρb [g cm−3]

ΔHfc [kJ g−1]

Dvd [m s−1]

Pe [GPa]

ISf [J]

FSg [N]

OBh [%]

Ispi [s]

3 5 6 7 8 9 10 12·H2O 13 14 ADNj RDXk HMXk

73 118 121 107 97 128 88 114 89 115 159 210 280

1.88 1.98 1.79 1.92 1.81 2.05 1.92 1.75 1.91 1.83 1.81 1.82 1.90

0.88 0.30 0.70 0.78 1.08 −0.98 0.89 0.05 0.30 0.95 − 0.36 0.36

8511 7910 8617 9054 8920 7394 9123 8655 9590 9230 − 8748 9320

31.2 27.6 31.6 36.3 34.6 23.9 37.7 31.8 42.4 37.9 − 34.9 39.5

2 1 1 1 1 4 3 8 5 1 3−5 7.4l 7.4

80 40 120 20 10 80 120 240 80 120 64−72 120 120

32.2 28.0 23.5 27.0 20.3 22.6 23.0 5.1 13.9 0 25.8 0 0

245 219 258 256 266 197 259 262 272 274 202m − −

Decomposition temperature (onset) under nitrogen gas (DSC, 5 °C/min). bDensity measured by gas pycnometer (25 °C). cHeat of formation. Detonation pressure (calculated with Explo5 v6.01). eDetonation velocity (calculated with Explo5 6.01). fImpact sensitivity. gFriction sensitivity. h Oxygen balance (based on CO) for CaHbOcNd, 1600(c − a − b/2)/MW, MW = molecular weight. iSpecific impulse (values obtained from Explo5 v6.01 and calculated at an isobaric pressure of 70 bar and initial temperature of 3300 K). jReference 10. kReference 12. lIdaho exptl value. m Calculated with Explo5 v6.01. a

d

ities and pressures than the salts of 2,5-bis(dinitromethyl)-2Htetrazole except for the potassium salt. The hydrazinium salt 14 also exhibits good detonation velocity of 9230 m s−1 and pressure of 37.9 GPa which are comparable with those of HMX. For initial safety testing, the impact and friction sensitivities of 3, 5−10, and 12·H2O-14 were measured by following BAM standard methods15 and are given in Table 1. Compounds 3, 5, 7, and 8 are very sensitive toward impact and friction. Compounds 6, 10, and 14 are very sensitive toward impact but exhibit moderate sensitivities toward friction. Compound 12·H2O has an impact sensitivity of 8 J and friction sensitivity of 240 N which are comparable with those of RDX and HMX (IS 7.4 J, FS 120 N). The specfic impulse (Isp), a measure of a propellant’s efficiency (in seconds), was also calculated (Explo 5 v6.01) for all compounds. The Isp values range from 197 to 274 s, which, except for 9, are higher than that of ADN (Table 2). As shown in Figure 6, two-dimensional (2D)-fingerprints of crystals 3 and 10 and the associated Hirshfeld surfaces were employed to show their intermolecular interactions.16 The red and blue regions on the Hirshfeld surfaces represent high and low close contact populations, respectively. Both 3 and 10 have the highest ratios of O···O interactions of all kinds of interactions. The trinitromethyl moiety of 3 displays a molecular geometry with a propeller-type orientation of the nitro groups, increasing the chance to form O···O interactions. As a result, the value of 3 (60.5%) is higher than that of 10 (43.9%). It is found that relative frequencies of close O···O contacts can increase the possibility of unexpected explosions.17 Block-shaped surfaces contribute more efficiently to face-to-face π−π stacking and result in better mechanical stability in which π−π stacking is often present as C···O and O···C interactions.18 Unfortunately, only 0.9% and 1% C···O and O···C interactions were observed in 3 and 10, respectively. According to these results, it is not surprising that the compounds reported in this work all have high sensitivities toward impact and friction. It can be seen, from Figure 6c and 6d, a pair of remarkable spikes on the bottom left (O···H and H···O interactions) in the 2D fingerprint plots of both crystals denote the presence of hydrogen bonds among neighboring molecules.19 The thicker spikes of 10 indicate that more

Figure 6. Two-dimensional fingerprint plots in crystal stacking for 3 and 10 as well as the associated Hirshfeld surfaces. Images (a) and (b) showing the Hirshfeld surfaces that use color coding to represent the proximity of close contacts around 3 and 10 molecules (white, distance d equals the van der Waals distance; blue, d exceeds the van der Waals distance; red, d is less than the van der Waals distance). The fingerprint plots in crystal stacking for 3 (c) and 10 (d). In images (e) and (f), the individual atomic contacts percentage contribution to the Hirshfeld surface are shown in the pie graphs for 3 and 10, respectively.

hydrogen bonds are observed. It is also found that the value of O···H and H···O interactions in 10 (20.5%) is almost twice that of 3 (11.0%). Hydrogen bonds are helpful in increasing the density which is in agreement with the fact that 10 (1.92 g cm−3) has a higher density than 3 (1.88 g cm−3). In summary, a family of polynitromethyl functionalized derivatives of 2,5-bis(dinitromethyl)-2H-tetrazole (3) and 2(dinitromethyl)-5-(trinitromethyl)-2H-tetrazole (10) as well as their energetic salts were synthesized and characterized. 2,5Bis(dinitromethyl)-2H-tetrazole (4), which reaches the limit of the number of nitro groups possible associated with one tetrazole ring, was observed by treating 3 with a mixture of 100% nitric acid and concentrated sulfuric acid. Unfortunately, 4687

DOI: 10.1021/acs.orglett.9b01565 Org. Lett. 2019, 21, 4684−4688

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Organic Letters

(5) (a) Zhao, B.; Li, X.; Wang, P.; Ding, Y.; Zhou, Z. New J. Chem. 2018, 42, 14087−14090. (b) Fischer, D.; Klapö tke, T. M.; Stierstorfer, J. Angew. Chem., Int. Ed. 2014, 53, 8172−8175. (6) Haiges, R.; Christe, K. O. Inorg. Chem. 2013, 52, 7249−7260. (7) Zhang, C. J. Hazard. Mater. 2009, 161, 21−28. (8) (a) Semenov, V. V.; Shevelev, S. A.; Bruskin, A. B.; Kanishchev, M. I.; Baryshnikov, A. T. Russ. Chem. Bull. 2009, 58, 2077−2096. (b) Semenov, V. V.; Shevelev, S. A. Mendeleev Commun. 2010, 20, 332−334. (9) (a) Thottempudi, V.; Kim, T. K.; Chung, K.-H.; Kim, J. S. Bull. Korean Chem. Soc. 2009, 30, 2152−2154. (b) Terpigorev, A. N.; Telsinkii, I. V.; Makarevich, A. V.; Frolova, G. M.; Mel’nikov, A. A. Zh. Org. Khim. 1987, 23, 244−254. (c) Thottempudi, V.; Gao, H.; Shreeve, J. M. J. Am. Chem. Soc. 2011, 133, 6464−6471. (10) Göbel, M.; Klapötke, T. M. Adv. Funct. Mater. 2009, 19, 347− 365. (11) Nagamachi, M.; Oliveira, J. I. S.; Kawamoto, A. M.; Dutra, R. C. L. J. Aerosp. Technol. Manag. 2009, 1, 153−160. (12) Zhang, J.; Shreeve, J. M. J. Am. Chem. Soc. 2014, 136, 4437− 4445. (13) Frisch, M. J.; Trucks, G. W.; Schlegel, H. G.; Scuseria, G. E.; Robb, M. A.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, A. L. G.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; AlLaham, M. A.; Peng, C.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision D.01; Gaussian, Inc.: Wallingford, CT, 2004. (14) Sućeska, M. EXPLO5, 6.01; Brodarski Institure: Zagreb, Croatia, 2013. (15) (a) http://www.bam.de. (b) A 20 mg sample was subjected to a drop-hammer test with a 5 or 10 kg dropping weight. The impact sensitivity was characterized according to the UN recommendations (insensitive, >40 J; less sensitive, 35 J; sensitive, 4 J; very sensitive, 3 J). (16) Spackman, M. A.; Jayatilaka, D. CrystEngComm 2009, 11, 19− 32. (17) Zhang, J.; Zhang, Q.; Vo, T. T.; Parrish, D. A.; Shreeve, J. M. J. J. Am. Chem. Soc. 2015, 137, 1697−1704. (18) Tian, B.; Xiong, Y.; Chen, L.; Zhang, C. Relationship between the crystal packing and impact sensitivity of energetic materials. CrystEngComm 2018, 20, 837−848. (19) Zhang, C.; Xue, X.; Cao, Y.; Zhou, Y.; Li, H.; Zhou, J.; Gao, T. CrystEngComm 2013, 15, 6837−6844.

it is too unstable to be isolated or characterized. Compared to 10, 3 has one additional nitro group on the trinitromethyl group but exhibits a lower density, heat of formation, detonation velocity and pressure, decomposition temperature, and higher sensitivities. The energetic salts of 3 also have lower detonation velocities and pressures than those of 10 except for the potassium salt 5. Compound 3 has one of the highest oxygen balances (32.2%) of all tetrazole compounds which is higher than ADN (25.8%) and comparable to ammonium perchlorate (AP 34.0%). Compound 13 has the best detonation properties (9590 m s−1 and 42.4 GPa) which are superior to HMX; however, use will be limited owing to its low decomposition point and high sensitivities. The two-dimensional fingerprint plots in crystal stacking for 3 and 10 as well as the associated Hirshfeld surfaces indicate that the high sensitivities are a result of the high ratios of O···O interactions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.orglett.9b01565. Crystal refinements (PDF) Accession Codes

CCDC 1903374−1903379 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jean’ne M. Shreeve: 0000-0001-8622-4897 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Office of Naval Research (N00014-16-1-2089) and the Defense Threat Reduction Agency (HDTRA 1-15-1-0028). We are also grateful to the Murdock Charitable Trust, Vancouver, WA, Reference No. 2014120:MNL:11/20/2014 for funds supporting the purchase of a 500 MHz nuclear magnetic resonance spectrometer.

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REFERENCES

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DOI: 10.1021/acs.orglett.9b01565 Org. Lett. 2019, 21, 4684−4688