The obtained yield of 89% for the second reaction step using 84.7 mmol of 3 could not be improved by using the double amount of 3 (169.4 mmol). The analytical data of 4 agree with the data given in literature.27
Synthesis of 3,3’-bis(1,2,4-oxadiazol-5-one) (5) (BOX)
The second last reaction step comprises a base induced ester cleavage followed by an acid catalyzed ring closure leading to compound 5.
31 To improve the moderate yield of 60%, 5 was treated with 10% sodium hydroxide solution instead of 5% NaOH which results in an amount of only 45%. It is expected that the concentration of sodium hydroxide influenced the ring closure; PhCO2- separates too fast and therefore the ring closure does not occur.
5a and 5b were synthesized with two equivalents KOH or CsOH in water. Analytical data and crystal structures of 5, 5a and 5b are discussed in detail within the PhD thesis of N. Mayr.27 The sensitivity data of both salts are given in Table 8.
Table 8 Sensitivity data of potassium and cesium 3,3’-bis(1,2,4-oxadiazol)-5-one.
5a 5b
IS / J 40 40
FS / N 360 360
ESD / J 1.0 1.0
Synthesis of potassium and cesium 3-nitro-1,2,4-triazole-5(4H)-one (NTO) (7a & 7b)
An already known and commercially available secondary explosive is 3-nitro-1,2,4-triazole- 5(1H,4H)-one (NTO) (7). NTO is discussed as RDX (1,3,5-trinitro-1,2,5-triazacyclohexane) or HMX (1,3,5,7-tetranitro-1,3,5,7,tetraazacyclooctane) replacement in military applications due to comparable performance, less sensitivity and high density of 1.93 g cm-3. NTO is used in automobile airbags as alternative to the toxic primary explosive lead azide.29 Therefore it is an interesting candidate as additive in other pyrotechnic formulations. Because pyrotechnic compositions require compounds with high friction data cesium and potassium NTO were synthesized and tested as possible ingredients for Black Knight formulations.
The synthesis of potassium and cesium 3-nitro-1,2,4-triazole-5(1H,4H)-one is described in Schema 6.29,30
29H. S. Jadhav, M. B. Talawar, Synthesis, characterization and thermolysis of 2,4-dihydro -2,4,5-trinitro-3H-1,2,4-
triazol-3-one (DTNTO): a new derivative of 3-nitro-1,2,4-triazol-5-one (NTO), Indian Journal of Engineering & Material Sciences2005, 12, 467.
32 Schema 6 Synthesis of 3-nitro-1,2,4-triazole-5(1H,4H)-one (7), 7a and 7b with M = K or Cs.
To a hot solution of 85% acetic acid was added semicarbazide. After refluxing the solution for 7 h the acid was evaporated. The solid was treated with water and the solvent was again evaporated. This procedure was repeated two more times. Afterwards the solid was recrystallized from hot water. TO (6) was obtained in moderate yields of 61%. 7 was obtained after adding 6 to an excess of 100% nitric acid under ice-cooling. After 2 h the cooling was removed and the mixture was stirred at room temperature over night. The solid was isolated and washed with water. NTO was obtained as a colorless powder with a yield of 23%. Crystalline NTO was obtained from the cooled solution (4°C) after four days.
1 equivalent of potassium and cesium hydroxide, respectively were used for synthesizing the salts of NTO 7a and 7b. Both were recrystallized from ethanol/water. Potassium NTO (7a) was obtained as a dihydrate and cesium NTO (7b) as a monohydrate.
Analytic and physical-chemical data
The standard analytical data (NMR, EA, mass) for NTO is comparable with the data given in literature.29 Symmetric valence modes of the nitro group in the RAMAN spectra are at 1376 cm-1 (7a) and 1377 cm-1 (7b). The bend mode of the NO2-group is at 484 (7a) and at 488 cm-1 (7b), respectively. The antisymmetric stretch mode for the nitro group in the IR is located at 1587 cm-1 for 7a and 1591 cm-1 for 7b. NH modes in the IR spectra are at 3342 cm-1 for 7a and 3349 cm-1 for 7b. The FAB- mass spectra indicate a single deprotonation at the ring system. Sensitivity data of both salts are given in Table 9.
33 Table 9 Sensitivity data of potassium and cesium 3-nitro-1,2,4-triazol-5(4H)-one.
7a 7b
IS / J 40 40
FS / N 288 288
ESD / J 0.3 0.3
Synthesis of potassium and cesium bis(1-methyl-tetrazole-5-yl)-triazene monohydrate (8a & 8b) (BMTT)
The formation of bis(1-methyl-tetrazole-5-yl)-triazene (8) is illustrated in Schema 7. The compound was obtained as a monohydrate.31
Schema 7 Synthesis of bis(1-methyl-tetrazol-5-yl)-triazene · H2O (8).
Using a half equivalent of sodium nitrite 8 was obtained under acidic conditions by diazotation of 2-methyl-5-aminotetrazole. After stirring the mixture for 24 h under ice- cooling the product was isolated with a yield of 32%. Because of an intense reaction (large amount of fume) and nitrogen release a large reaction flask was used to avoid flooding. Both salts were obtained by solving 8 in ethanol and adding one equivalent of the corresponding base. 8a and 8b were recrystallized from ethanol/water and dried at 50°C. Both were obtained as very fine yellow-greenish powders.
Analytic and physical-chemical data
Standard analytic was carried out for the potassium and the cesium salt of 8. The methyl group in the 1H NMR is detected as a singlet at 3.87 (8a) and 3.84 (8b) ppm. Both ring carbons are located at 162 and both methyl groups at 33 ppm in the 13C NMR spectra. Raman and IR spectra are also comparable with the data given in literature.31
31 T. M. Klapötke, J. Stierstorfer, Investigations of bis(methyltetrazolyl)triazenes as nitrogen-rich ingredients in solid rocket propellants - Synthesis, characterization and properties, Polyhedron2009, 28, 13.
34 Although 8 was obtained as a monohydrate the impact sensitivity is very low in contrast to a friction sensitive greater than 360 N (Table 10). Compared to the neutral compound the impact sensitivity was improved by the formation of salts. 8a and 8b are suitable as possible NIR ingredients but due to very low electrostatic discharge values and very low yields, both compounds are only second quality for pyrotechnic composition studied in this work.
Table 10 Sensitivity data of 8a & 8b.
8 8a 8b
IS / J 3 40 35
FS / N 360 360 288
ESD / J n.d. 0.03 0.03
Synthesis of 3,6-bis-nitroguanidyl-1,2,4,5-tetrazine, bis-potassium and bis-cesium BNGT (12a & 12b)
Potassium and cesium BNGT were synthesized via multiple step synthesis (Schema 8).