Synthesis of Functionalized Tetrazenes as Energetic Compounds Johannes Heppekausen, Thomas M. Klapo¨tke,* and Stefan M. Sproll Department of Chemistry and Biochemistry, UniVersity of Munich (LMU), Butenandtstr. 5-13, D-81377, Germany
[email protected] ReceiVed December 16, 2008
1,4-Bis[1-(2-hydroxyethyl)-1H-tetrazol-5-yl]-1,4-dimethyl-2-tetrazene (12a), 1,4-bis[1-isopropoxycarbonylmethyl-1H-tetrazol-5-yl]-1,4-dimethyl-2-tetrazene (12b), and 1,4-bis[1-carboxymethyl-1H-tetrazol-5yl]-1,4-dimethyl-2-tetrazene (13) have been synthesized as new nitrogen-rich compounds. The tetrazenes were obtained by oxidation of the corresponding tetrazolylhydrazines using bromine. Moreover, a new method to prepare tetrazolylhydrazines in high yield using 5-bromotetrazoles has been developed. 12a, 12b, and 13 were characterized using vibrational spectroscopy (IR, Raman), mass spectrometry, and multinuclear NMR spectroscopy. The crystal structures of 12a, 12b, and 13 were determined using single crystal X-ray diffraction. Furthermore, the energetic properties of 12a, 12b, and 13 have been investigated using DSC and bomb calorimetric measurements. The sensitivity data toward impact and friction has been determined using BAM methods.
Introduction Tetrazoles are nitrogen-rich compounds, covering a wide range of applications. They are used as pharmaceuticals,1 for biomedical applications,2 as membranes,3 as antifoggants in photographic materials,4 or as energetic materials.5 The advantages of tetrazoles include their high nitrogen content along with a sufficiently high thermal stability,3 rendering them as unique nitrogen building blocks. Beside the usage for military devices, (1) (a) Makino, E.; Iwasaki, N.; Yagi, N.; Kato, H.; Ito, Y.; Azuma, H. Chem. Pharm. Bull. 1990, 38, 201. (b) Reddy, R. P.; Ravindrath, A.; Ramaiah, T. S.; Rao, M. V. Ind. J. Chem. Sci. 1987, 1, 45. (c) Alabaster, C. T.; Bell, A. S.; Campbell, S. F.; Ellis, P.; Henderson, C. G.; Morris, D. S.; Roberts, D. A.; Ruddock, K. S.; Samuels, G. M. R.; Stefaniak, M. H. J. Med. Chem. 1989, 32, 575. (d) Uchida, M.; Komatsu, M.; Morita, S.; Kanbe, T.; Nagakawa, K. Chem. Parm. Bull. 1989, 37, 322. (2) (a) Matyjaszewski, K.; Sumerlin, B. S.; Tsarevsky, N.; Spanswick, J. Int. Patent 2005087818, 2005. (b) Mwale, F.; Girard-Lauriault, P.-L.; Wang, H. T.; Lerouge, S.; Antoniou, J.; Wertheimer, M. R. Tissue Eng. 2006, 12, 2639. (c) Truica-Marasescu, F.; Wertheimer, M. R. Plasma Process. Polym. 2008, 5, 44. (3) Gaponik, P. N.; Ivashkevich, O. A.; Karavai, V. P.; Lesnikovich, A.; Chernavina, N.; Sukhanov, G. T.; Gareev, G. A. Angew. Makromol. Chem. 1994, 219, 77. (4) Tuites, R. C.; Whiteley, T. E.; Minsk, L. M. U.S. Patent 1,245,614, 1971. (5) (a) Singh, R. P.; Verma, R. D.; Meshri, D. T.; Shreeve, J. M. Angew. Chem., Int. Ed. 2006, 45 (22), 3584. (b) Klapo¨tke, T. M. Structure and Bonding: High Energy Density Compounds; Springer: Berlin/Heidelberg, 2007; Vol. 125.
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energetic materials based on nitrogen-rich compounds like tetrazoles can be used as gas generating agents for civil applications.6 To obtain even higher contents of nitrogen, tetrazoles were used as anions to form salts with nitrogencontaining cations of guanidine, aminoguanidine, diaminoguanidine, or triaminoguanidine.7 Another method to build nitrogen-rich tetrazole-containing molecules is the usage of moieties like tetrazine,8 nitramine,9 or azide.10 The disadvantages of these compounds lie in their low physical and thermal stability and high explosion temperature. In particular, with regard for possible application as air bags in vehicles, a low heat of explosion is desired to protect the person from burns. Moreover, a reduced heat of explosion lessens the erosion of guns. Therefore, the investigation and (6) (a) Ganta, S. R.; Williams, G. K.; Miller, C. G. Int. Patent WO2008054531, 2008. (b) Khandhadia, P. S.; Mizuno, H. U.S. Patent US2008078486, 2008. (7) (a) Takacs, E. A. U.S. Patent US3354172, 1967; (b) Klapoetke, T. M.; Stierstorfer, J.; Wallek, A. Chem. Mater. 2008, 20, 4519. (8) (a) Butler, R. N.; Ni Bhradaigh, E. P.; McArdle, P.; Cunningham, D. J. Chem. Res 1995, 6, 224. (b) Sauer, J.; Pabst, G.; Holland, U.; Kim, H.-S.; Loebbecke, S. Eur. J. Org. Chem. 2001, 4, 697. (9) (a) Herbst, R. M.; Garrison, J. A. J. Org. Chem. 1953, 18, 941. (b) Gao, A.; Rheingold, A. L.; Brill, T. B. Propellants, Explos., Pyrotech. 1991, 16, 97. (10) Marsh, F.; Thatcher, D. N. U.S. Patent US3374188, 1968.
10.1021/jo802738c CCC: $40.75 2009 American Chemical Society Published on Web 02/17/2009
Synthesis of Functionalized Tetrazenes SCHEME 1.
Synthesis of Tetrazolylhydrazines from Thiosemicarbazides
development of new nitrogen-rich compounds with a low heat of explosion is of great interest. A suitable class of nitrogen-rich compounds are 1,4-bistetrazolyltetrazenes. 1,4-Bis[1-methyltetrazol-5-yl]-1,4-dimethyl2-tetrazene was first prepared in 2004 by nitrosylation of N,1dimethyl-1H-tetrazol-5-amine and reduction to the corresponding 1-methyl-1-(1-methyl-1H-tetrazol-5-yl)hydrazine. The tetrazolylhydrazine was then oxidized to form the 1,4-bis[1methyltetrazol-5-yl]-1,4-dimethyl-2-tetrazene using bromine.11 In this work, we present a general route for the preparation of tetrazenes in high yields. The synthesis of bromotetrazoles12 was generalized and used to establish a new route to form the corresponding methyltetrazolylhydrazines in high yields.
SCHEME 2.
Synthesis of 5-Bromotetrazoles
SCHEME 3.
Synthesis of the Tetrazolylhydrazines
Results and Discussion Tetrazenes can be prepared by oxidation of hydrazines.13 As a consequence, the tetrazolyltetrazenes presented in this work have been obtained via the corresponding tetrazolylhydrazines. Therefore, efficient routes for the synthesis of tetrazolylhydrazines were necessary. A possible pathway reported in literature starts from thiosemicarbazides, which are converted to the corresponding S-methylthiosemicarbazides. The methylated sulfur atom is subsequently substituted by sodium azide, followed by an electrocyclic ring closure yielding the tetrazolylhydrazine14 (Scheme 1). Taking into account that a purification of tetrazole 5 by column chromatography is necessary, the reaction proves not to be efficient for upscaling. To improve the synthesis, a new route using 5-bromotetrazoles as intermediates has been developed. Despite being useful starting materials for 5-substituted tetrazoles, only a small number of 5-bromotetrazoles have been reported in the literature.15 The synthesis of 2-(5-bromo-tetrazol-1-yl)-ethanol (8a) reported as a patent by Bayes16 was generalized and used to prepare the (5-bromo-tetrazol-1-yl)-acetic acid isopropyl ester (8b), with 2-aminoethyl acetate and isopropyl 2-aminoacetate as suitable starting materials. The experiments showed that in case of the aminoacetate derivative (to form 8b) only the (11) Klapoetke, T. M.; Mayer, P.; Schulz, A.; Weigand, J. J. Propellants, Explos., Pyrotech. 2004, 29, 325. (12) (a) Lim, G. M. F.; Endo, M. U.S. Patent US4374994, 1983. (b) Bayes, D. E. European Patent EP0117368A1, 1982. (13) (a) Back, T. G. J. Chem. Soc., Chem. Commun. 1981, 11, 530. (b) Bae, D. H.; Shine, H. J. J. Org. Chem. 1981, 46, 4700. (c) Klapo¨tke, T. M.; Mayer, P.; Schulz, A.; Weigand, J. J. Propellants, Explos., Pyrotech. 2004, 29, 325. (14) (a) Atherton, R. A.; Lambert, R. W. Tetrahedron 1983, 39, 2599. (b) Weigand, J. J. Dissertation, University of Munich (LMU), 2005. (c) Banert, K.; Klapo¨tke, T. M.; Sproll, S. M. Eur. J. Org. Chem. 2009, 2, 275. (15) (a) Collibee, W. L.; Nakajima, M.; Anselme, J.-P. J. Org. Chem. 1995, 60, 468. (b) Bayes, D. E. European Patent EP0117368A1, 1982. (c) Bruns, R.; Uhr, H.; Vogl, E. International Patent WO2006161228, 2006. (16) Bayes, D. E. European Patent EP0117368A1, 1982.
isopropyl ester proved to be a stable protecting group against an intramolecular attack of the monomethylhydrazine moiety later inserted, in contrast to the ethyl or methyl ester. To form the tetrazoles, the amine group was converted into the 5Htetrazole using standard reactions of triethyl orthoformate and sodium azide in acetic acid.17 In the following step, the 5-position of the tetrazole was brominated using elemental bromine (Scheme 2). In the case of 8a, the resulting 5-bromotetrazole was deprotected by hydrochloric acid. A purification by column chromatography was not possible because of decomposition during the process. Crude 8a and 8b were converted into the corresponding tetrazolylhydrazines by substitution of the bromine atom by monomethylhydrazine. The experiments showed that isopropanol was a suitable solvent, whereas the yields dropped using alcohols, acetonitrile, or other organic solvents. When the reaction was carried out, neither the 2-methyl isomer nor side products arising from a hydrolysis of the bromotetrazole were observed (Scheme 3). The tetrazolylhydrazines were obtained as slightly impure oil. The crude products were used for further reactions. A deprotection of 9b using potassium hydroxide in water yielded the bicycle 11 as crystalline solid along with several other compounds, which were not identified. The formed carbon acid undergoes immediately an intra- or intermolecular condensation with the hydrazine moiety, even in alkaline medium, yielding either 11 or polymeric side products (Scheme 4). (17) (a) Justiz, O. H.; Fernandez-Lafuente, R.; Guisan, J. M.; Negri, P.; Pregnolato, M.; Terreni, M. J. Org. Chem. 1997, 62, 9099. (b) van Koningsbruggen, J. P.; Garcia, Y.; Bravic, G.; Chasseau, D.; Kahn, O. Inorg. Chim. Acta 2001, 326, 101. (c) Potewar, T. M.; Shafi, A.; Lahoti, R. J.; Srinivasan, K. V. Tetrahedron Lett. 2007, 48, 1721.
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Heppekausen et al. SCHEME 4.
Formation of 11
SCHEME 5.
Synthesis of the Tetrazolyltetrazenes
Bicycle 11 crystallizes in the orthorhombic space group Pcnn with eight molecular formulas per unit cell. Two hydrogen bonds are present in the crystal structure. The donor atom N6 forms one intermolecular hydrogen bond to O1, leading to dimers. The tetrazolylhydrazines 9a and 9b were converted into the corresponding tetrazenes using saturated bromine water at 0 °C. The product precipitates from the clear reaction mixture and was filtered off. The yields varied between 50% and 60% (Scheme 5). Compound 12b was deprotected using boiling concentrated sodium hydroxide solution. The corresponding carbon acid 13 was recovered by neutralizing the alkaline solution using hydrochloric acid. Compound 12a crystallizes in the orthorhombic space group Pbca with four molecular formulas per unit cell. The bond distances and angles of the tetrazole moiety and the alkyl substituent are in accordance with values reported in the literature.18 The bond distance between N6 and N7 (125.5 pm) equates to common values of double bonds (125 pm).19 The bond distance of N5-N6 (136.6 pm) lies between a N-N single bond (146 pm) and a double bond (125 pm). Furthermore the C1-N5 bond (136.6 pm) is shortened by 12.6 pm, compared to C-N single bonds (147 pm) commonly observed. The crystal structure is stabilized by an intermolecular hydrogen bond between the donor atom O1 and the acceptor atom N4. Thereby, the tetrazole moieties are connected by two hydrogen bonds between each other. As a result, three molecules are connected by four hydrogen bonds. Compound 12b crystallizes in the triclinic space group P-1 with one molecular formula per unit cell. The bond distances and angles are in correspondence with 12a. As a result of the protected carbon acid no inter- or intramolecular interactions are observed. Compound 13 crystallizes in the monoclinic spacegroup P21/c with four molecular formulas per unit cell (Figure 1). The bond distances and angles are in accordance with those of compounds (18) (a) Lyakhov, A. S.; Voitekhovich, S. V.; Gaponik, P. N.; Ivashkevich, L. S. Acta Crystallogr. 2004, C60, o293. (b) Klapoetke, T. M.; Kuffer, C.; Mayer, P.; Polborn, K.; Schulz, A.; Weigand, J. J. Inorg. Chem. 2005, 44, 5949. (19) Holleman, A. F.; Wiberg, E.; Wiberg, N. Lehrbuch der Anorganischen Chemie, 101 Auflage; Walter de Gruyter Verlag: Berlin, New York, 1995; p 1842.
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FIGURE 1. Molecular structure and intermolecular hydrogen bonds of 13. Thermal ellipsoids are drawn at the 50% probability level. TABLE 1.
12a 13
Hydrogen Bonds of 12a and 13
donor atom
acceptor atom
D-H [pm]
H· · ·A [pm]
D· · ·A [pm]
(D-H · · · A)
O1 O2 O4
N4 N4 N12
88.8 94.1 93.9
201.0 181.0 179.1
289.5 273.7 271.4
173.8 167.8 166.8
12a and 12b. The crystal structure is stabilized by four intermolecular hydrogen bonds, one between the donor atom O2 and the acceptor atom N4 and the other between the donor atom O4 and the acceptor atom N12. In contrast to the structure of 12a, five molecules are connected to each other by the four hydrogen bonds. A comparison between the hydrogen bonds of 12a and 13 is given in Table 1. Compounds 12a, 12b, and 13 were identified using vibrational IR spectroscopy. Selected values are given in Table 2. The calculated values given in Table 2 were used to assign the frequencies. A comparison between the 15N spectra of 13 and 12a is given in Figure 2. The assignment of the nitrogen atoms was carried out in analogy to values reported in literature.20 In the case of 12a, N2 shows a clear triplet, resulting of a 3J-coupling (3J ) 2.0 Hz) with the CH2 group. N1 forms a muliplet by the 2Jand 3J-coupling with the hydroxyethyl group. In contrast, 13 only shows the triplet of N2 (3J ) 1.7 Hz). N1 forms a broad signal without showing any clear 2J-coupling with the CH2group. An explanation for the missing 2J-coupling could be the much lower concentration, due to the low solubility of 13 in (20) Klapo¨tke, T. M.; Minar, N.; Stierstorfer, J. Polyhedron 2009, 28, 13.
Synthesis of Functionalized Tetrazenes TABLE 2. Comparison between Selected Vibrations of 12a, 12b, and 13 with the Calculated Values of 13 using DFT BLYP/6-31G* 12a 12b 13 theory × exptl exptl exptl 0.9940 theory 3347 3436 2972 2999 3012 2944 2984 2972 2885 2944 2945 1754 1747 1738 1572 1569 1565 1470 1470 1478 1444 1450 1447 1417 1433 1423 1414 1413 1394 1400 1360 1348 1351 1337 1332 1295 1278 1295 1261 1262 1224 1218 1207 1106 1106 1100
3502 3036 3007 2977 1778 1777 1527 1479 1436 1421 1417 1416 1397 1360 1312 1282 1250 1190 1100
3524 3055 3026 2995 1789 1788 1537 1488 1445 1430 1426 1425 1406 1369 1320 1290 1258 1198 1107
1098 1083 1075 1015 1007 1020
1060 1023
1067 1030
993 965
980 972
986 978
848
854 798
866 778
872 783
658
656
654
658
978 959 857
vibration νs/νas(OH) νas(CH3) νs(CH2) νs(CH3) νas(COOH) νas(COOH) νas(tetrazol) δs(CH3) νs(tetrazene)/δs(CH3) δs(CH3)/δs(CH2) νs(tetrazene)/δs(CH3) δs(CH3)/δs(CH2) νs(tetrazole)/δs(CH3)/δs(CH2) νs(CH2COOH) νs(tetrazene)/δas(CH2) νs(CH2COOH) νs(tetrazol) νs(tetrazole)/δas(tetrazole)/δas(CH2) νas(COOH)/νas(tetrazole)/ νas(tetrazene)/νs(N-CH3) νas(tetrazole)/δas(tetrazole)/νas(tetrazene) νas(CH2COOH)/νas(tetrazole)/ νas(tetrazene)/δs(N-CH3) νs(tetrazole)/δs(N-CH3)/νas(tetrazene) νas(tetrazole)/δs(N-CH3)/νas(tetrazene)/ γ(COCH2) νas(tetrazole)/γ(COCH2) νs(CH2COOH)/νas(tetrazole)/ νas(tetrazene)/δs(N-CH3) γs(COOH)
FIGURE 2.
15
N-spectra of 13 and 12a in d6-DMSO.
FIGURE 3. Combustion reactions of 12a, 12b, and 13.
DMSO. In both cases neither the 2J- nor the 3J-coupling with the methyl moiety was obvious. Since 12a, 12b, and 13 are energetic materials, determination of the energetic properties was or interest of us. Therefore, the thermal behavior was investigated by DSC measurements, the heats of combustion by bomb calorimetric measurements, and the sensitivity toward impact and friction by BAM methods.21,22 The tetrazenes show no melting points but sharp decomposition points. Comparing the DSC data of the tetrazenes, compound 12a and 12b possess similar points of decomposition of 167 and 160 °C. In contrast, the point of decomposition of 13 (207 °C) is about 40 °C higher than those of 12a and 12b. This effect can be rationalized by strong intermolecular interactions of the molecules in the crystal structure, as already shown in Figure 1, resulting in a stabilization of the molecule. Taking the crystallographic data of 12a, 12b, and 13 into account, the points of decomposition can be correlated with the amount of intermolecular interactions. Compound 12b shows no intermolecular interactions at all and possesses a decomposition point of 160 °C. Compound 12b forms four hydrogen bonds between a hydroxyl group and the tetrazole moiety connecting three molecules (decomposition point 167 °C). The structure of 13 stabilized most by four hydrogen bonds between the carboxylic acid and the tetrazole moiety shows the highest decomposition point (207 °C). Electronic effects of the different moieties of the tetrazoles 12b and 13 can be neglected. The difference between 12b and 13 is the esterfication of the carbon acid. (21) (a) Suceska, M. Test Methods for ExplosiVes; Springer: New York, 1995; pp 21 (impact); 27 (friction). (b) http://www.bam.de/. (22) Impact: insensitive > 40 J, less sensitive > 35 J, sensitive > 4 J, very sensitive < 4 J. Friction: insensitive > 360 N, less sensitive ) 360 N, 80 N < sensitive
