Ralf
Haiges
* and
Karl O.
Christe
Loker Hydrocarbon Research Institute, University of Southern California, Los Angeles, CA 90089-1661, USA. E-mail: haiges@usc.edu; Tel: +1-213-7403197
First published on 18th March 2015
5-(Fluorodinitromethyl)-2H-tetrazole (HFDNTz) has been prepared by the cycloaddition reaction of HN3 with F(NO2)2CCN, which in turn was prepared by aqueous fluorination of sodium dinitrocyanomethanide. HFDNTz was converted into the ammonium, silver and tetraphenylphosphonium 5-(fluorodinitromethyl)tetrazolates. While the reaction of trinitroacetonitrile with HBr, followed by the treatment with NaOH, resulted in the formation of sodium dinitrocyanomethanide, the reaction of trinitroacetonitrile with aqueous ammonia produced ammonium dinitrocyanomethanide. Hydrazinium dinitromethanide was obtained from trinitroacetonitrile and hydrazine hydrate. All compounds were fully characterized by multinuclear NMR spectroscopy, IR spectroscopy and X-ray crystal structure determinations. Initial safety testing (impact and friction sensitivity) and thermal stability measurements (DTA) were also carried out.
In recent years, much effort in energetic materials research was dedicated to the synthesis of novel energetic highly over-oxidized compounds that can be used to replace ammonium perchlorate as high-oxygen carrier in solid rocket propellant formulations. An expression that is often being used in order to indicate the degree of oxidation of a compound is the oxygen balance (OB). In its original form, the OB was defined as percentage of oxygen required for complete conversion of a molecule to carbon dioxide, water and metal oxide.16 Because if the usually high combustion temperatures in rocket engines, it is more useful to calculate the OB of a rocket propellant based on combustion to carbon monoxide instead of carbon monoxide. While energetic, tetrazoles are usually notoriously under-oxidized. 5-Nitrotetrazolate12 has an OB of 14.0% for combustion to CO but the recently investigated 5-(trinitromethyl)tetrazolate5 has an OB of 29.4%. The general disadvantage of most 5-(trinitromethyl)tetrazolates are their high impact (IS) and friction sensitivities (FS) (e.g. IS = 0.5 J and FS < 1 N for the rubidium and cesium salts).5
The instability of the trinitromethyl group toward catastrophic decomposition is well established. It has been found that the fluorodinitromethyl group –CF(NO2)2 is generally more stable than the trinitromethyl group –C(NO2)3 without incurring too much of a performance penalty. The 5-(fluorodinitromethyl)tetrazolate anion has an OB of 20.9% for combustion to N2, CO and CF4. Fluorine-containing compounds such as Teflon or Viton are common ingredients of metallized formulations for flares and pyrotechnics (e.g. MTV: Mg/Teflon/Viton). The highly exothermic formation of metal fluorides during combustion increases the flame temperature which in turn improves the performance of the formulation.
The synthesis of 5-(fluorodinitromethyl)-2H-tetrazole (HFDNTz) by reaction of fluorodinitroacetonitrile with sodium azide or trimethylsilyl azide has been reported but the obtained tetrazole was not isolated and directly converted into the sodium or ammonium salt. The tetrazole and the salts were identified only by IR and 19F NMR spectra and have not been well characterized.17,18
In this manuscript we report the synthesis and full characterization of 5-(fluorodinitromethyl)-2H-tetrazole and three salts with the energetic 5-(fluorodinitromethyl)tetrazolate anion.
Neat trinitroacetonitrile was obtained as a colourless solid by removing the solvent from a cold dichloromethane solution (−40 °C) on a vacuum line.
NMR (CDCl3) δ(ppm): 13C (100.54 MHz) 103.4 (s, CN), 112.1 (sept, 1J(13C14N) = 9.5 Hz, C(NO2)3); 14N (36.14 MHz) −45.4 (s, ν½ = 20 Hz, NO2), −272 (s, ν½ = 350 Hz, CN); Raman (25 °C, 50 mW): 2265 (7.8), 1628 (1.9), 1623 (2.1), 1350 (1.4), 1344 (1.4), 1276 (3.1), 1164 (1.5), 943 (4.6), 856 (6.8), 801 (0.8), 657 (0.9), 477 (2.2), 444 (2.5), 380 (6.6), 360 (10.0), 200 (3.8), 151 (6.5), 110 (7.6) cm−1.
DTA: 150 °C (exotherm); NMR (acetone-d6) δ(ppm): 13C (100.54 MHz) 109.1 (s, CN), 157.0 (s, C(NO2)2); 14N (36.14 MHz) −20.6 (s, ν½ = 40 Hz, NO2), −88 (s, ν½ = 320 Hz, CN).
Raman (25 °C, 20 mW): 2244 (7.7), 2226 (4.4), 1493 (1.0), 1442 (1.5), 1433 (2.1), 1377 (8.8), 1295 (1.3), 1260 (5.7), 1250 (5.7), 1230 (10.0), 1159 (2.6), 1153 (1.8), 1070 (0.6), 1026 (0.2), 1004 (0.2), 921 (0.3), 883 (1.4), 865 (7.1), 858 (8.5), 778 (0.5), 769 (0.8), 753 (0.3), 731 (0.3), 577 (0.9), 515 (0.7), 505 (0.8), 469 (1.1), 440 (0.5), 408 (1.1), 273 (2.0), 217 (3.4), 212 (3.3), 151 (4.2), 117 (7.4), 98 (7.3) cm−1; IR (ATR): 3583 (m), 3511 (m), 2230 (ms), 1631 (m), 1611 (m), 1498 (s), 1481 (s), 1424 (m), 1376 (w), 1224 (vs), 1150 (s), 861 (w), 855 (w), 792 (vw), 773 (w), 744 (m), 569 (vw), 497 (vw), 417 (vw) cm−1.
DTA: 240 °C (exotherm); NMR (D2O) δ(ppm): 1H (599.80 MHz) 7.2 (s br, NH4), δ(ppm): 13C (100.54 MHz) 111.8 (s, CN), 152.4 (s, C(NO2)2); 14N (36.14 MHz) −23.1 (s, ν½ = 50 Hz, NO2), −89 (s, ν½ = 300 Hz, CN), −363.4 (quint, 1J(1H14N) = 56.9 Hz, NH4); IR (ATR): 3500–2800 (br, s), 2224 (ms), 1791 (w), 1661 (m), 1610 (w sh), 1523 (w sh), 1477 (m sh), 1396 (s), 1341 (m sh), 1207 (vs), 1144 (s sh), 1110 (s), 852 (w), 827 (w), 788 (w), 772 (m), 744 (s), 566 (m), 501 (m), 433 (w) cm−1.
DTA: 134 °C (exotherm); NMR (DMSO-d6) δ(ppm): 1H (599.80 MHz) 5.6 (s br, N2H5), 8.3 (s, (NO2)2CH); 13C (150.84 MHz) 165.6 (s, CH(NO2)2); 14N (36.14 MHz) −24.1 (s, ν½ = 100 Hz, NO2), −336 (s, ν½ = 600 Hz, N2H5); IR (DTA): 3600–2800 (br, m), 1604 (w), 1585 (w), 1499 (vw), 1483 (w), 1459 (vw), 1436 (m), 1341 (w), 1298 (w), 1251 (vw sh), 1240 (vw sh), 1192 (m), 1105 (s), 1071 (m), 997 (m), 948 (w), 758 (m), 719 (s), 687 (s), 615 (vw), 568 (vw), 522 (vs), 503 (m sh), 470 (m sh), 436 (w sh), 411 (vw) cm−1.
NMR (CDCl3) δ(ppm): 13C (100.54 MHz) 104.8 (d, 2J(13C19F = 35.3 Hz, CN), 105.5 (d, quint, 1J(13C19F) = 298.1 Hz, 1J(13C14N) = 3.0 Hz, CF(NO2)2); 14N (36.14 MHz) −36.7 (d, 2J(14N19F) = 11.2 Hz, ν½ = 5 Hz, NO2), −95 (s, ν½ = 320 Hz, CN); 19F (470.55 MHz) −90.8 (quint, 2J(14N19F) = 11.3 Hz, CF(NO2)2); IR (gas-phase, 10 Torr): 2914 (vw), 2261 (m), 1632 (vs), 1297 (s), 1093 (m), 1022 (vw), 842 (w), 801 (s), 794 (s sh), 655 (w) cm−1.
DTA: 110 °C (explosion); NMR (CDCl3) δ(ppm): 1H (599.80 MHz) 14.0 (s, CN4H); 13C (150.84 MHz) 115.2 (d, 1J(13C19F), = 290.1 Hz CF(NO2)2), 153.9 (d, 2J(13C19F = 25.9 Hz, CN4); 14N (36.14 MHz) −27.3 (s, ν½ = 70 Hz, NO2), −50 (s, ν½ = 350 Hz, CN4); 19F (564.33 MHz) −98.1 (s, CF(NO2)2); Raman (25 °C, 20 mW): 3100–2900 (3.9), 1701 (2.1), 1693 (2.0), 1611 (2.4), 1482 (5.7), 1424 (2.4), 1359 (4.2), 1313 (2.2), 1239 (1.6), 1210 (2.7), 1188 (2.0), 1104 (2.9), 1081 (2.0), 1057 (1.6), 1034 (1.5), 982 (6.4), 956 (3.4), 837 (10.0), 801 (4.2), 544 (2.4), 539 (2.3), 425 (3.1), 397 (3.7), 371 (8.6), 300 (2.6), 282 (2.8), 196 (3.3) cm−1; IR (KBr): 3072 (w), 3069 (vw), 3012 (w), 2911 (w), 2774 (w), 2682 (vw), 2593 (vw), 1694 (s), 1611 (vs), 1478 (w), 1422 (w), 1362 (m), 1307 (m), 1240 (s), 1211 (m), 1173 (m), 1093 (vw), 1078 (w), 1054 (m), 1031 (s), 980 (m), 883 (m), 849 (m), 836 (s), 800 (s), 745 (w), 695 (vw), 663 (vw), 594 (m), 582 (m), 549 (w), 459 (vw), 403 (vw) cm−1.
DTA: 140 °C (explosion); NMR (D2O) δ(ppm): 1H (599.80 MHz) 7.04 (s, NH4); 13C (150.84 MHz) 117.9 (d, 1J(13C19F) = 290.1 Hz CF(NO2)2), 150.8 (d, 2J(13C19F) = 23.2 Hz, CN4); 14N (36.14 MHz) −38.2 (s, ν½ = 60 Hz, NO2), −52 (s, ν½ = 350 Hz, CN4), −362.3 (quint, 1J(1H14N) = 58.7 Hz, NH4); 19F (564.33 MHz) −98.5 (s, CF(NO2)2); Raman (25 °C, 40 mW): 3200–2800 (3.4), 1685 (1.3), 1609 (2.8), 1472 (8.9), 1364 (4.1), 1326 (1.8), 1320 (1.9), 1319 (1.9), 1313 (1.9), 1309 (1.8), 1191 (5.4), 1161 (1.7), 1153 (1.7), 1102 (4.4), 1071 (2.9), 1067 (3.0), 985 (6.9), 949 (7.8), 839 (9.1), 805 (1.7), 707 (1.3), 652 (2.2), 542 (1.9), 537 (2.1), 533 (2.2), 443 (3.6), 437 (3.3), 401 (3.7), 398 (3.7), 373 (10.0), 303 (3.2), 292 (2.9), 204 (5.8), 170 (6.3), 166 (6.3) cm−1; IR (ATR): 3600–3050 (s), 2977 (m), 2931 (w), 2901 (w), 1655 (w sh), 1606 (m), 1455 (w sh), 1401 (vs), 1320 (w), 1275 (vw), 1184 (vw), 1089 (s), 1049 (vs), 980 (w), 880 (m), 836 (w), 801 (w), 643 (vw), 615 (vw), 452 (w) cm−1.
DTA: 185 °C (exotherm); IR (ATR): 1644 (s), 1604 (vs), 1467 (vw), 1375 (vw), 1305 (w), 1255 (w), 1201 (w), 1114 (vw), 1042 (vw), 1004 (vw), 973 (w), 833 (w), 796 (w) cm−1.
DTA: 165 °C (endotherm, loss of NH3), 180 °C (exotherm); IR (ATR): 3363 (m), 3201 (m), 2339 (vw), 2261 (w), 2166 (m), 1665 (s), 1594 (vs), 1498 (vw), 1465 (w), 1356 (s), 1308 (m), 1243 (m), 1195 (w), 1112 (m), 1048 (vw), 979 (m), 834 (ms), 799 (ms), 617 (m), 538 (w) cm−1.
DTA: 175 °C (exotherm); NMR (CDCl3) δ(ppm): 1H (599.80 MHz) 7.5–7.9 (m, PPh4); 13C (150.84 MHz) 117.9 (d, 1J(13C19F), = 290.1 Hz CF(NO2)2), 117.5 (d, 1J(13C31P = 88.0 Hz, PPh4), 120.6 (d, 1J(13C19F = 282.0 Hz, CF(NO2)2), 130.8 (d, 2J(13C31P = 15.6 Hz, PPh4), 134.4 (d, 2J(13C31P = 10.5 Hz, PPh4), 135.9 (d, 3J(13C31P = 3.2 Hz, PPh4), 150.8 (d, 1J(13C19F = 88.0 Hz, CN4); 150.8 (d, 2J(13C19F = 23.2 Hz, CN4); 14N (36.14 MHz) −39.1 (s, ν½ = 50 Hz, NO2), −52 (s, ν½ = 380 Hz, CN4); 19F (564.33 MHz) −94.0 (s, CF(NO2)2); 31P (242.82 MHz) 23.14 (s, PPh4); IR (ATR): 3088 (vw), 3071 (vw), 2956 (vw), 2892 (vw), 1593 (s), 1480 (m), 1435 (s), 1395 (vw), 1358 (w), 1339 (vw), 1314 (m), 1215 (m), 1185 (w), 1161 (m), 1106 (s), 1073 (m), 1025 (w), 996 (m), 970 (m), 937 (vw), 835 (m), 795 (m), 754 (m), 721 (s), 688 (s), 645 (vw sh), 615 (w), 523 (vs), 447 (w), 436 (vw sh), 404 (vw) cm−1.
Nitration of cyanoacetamide with fuming nitric acid in 20% oleum under anhydrous conditions resulted in the formation of trinitroacetonitrile.5,29 Due to the compound's reported sensitivity and high reactivity, trinitroacetonitrile was recommended to be handled only in solution.27 As a result, trinitroacetonitrile had not been fully characterized. We were able to isolate the compound by careful evaporation of the solvent from dichloromethane solutions in a vacuum at −40° C as a moisture sensitive, wax-like colourless solid. It is a very noxious lachrymator that slowly vaporizes in the air at ambient temperature.
Treatment of a trinitroacetonitrile solution in methanol with a 48% aqueous HBr solution resulted in gas evolution and the slow formation of elemental bromine. When the resulting reaction mixture was neutralized with sodium hydroxide, sodium dinitrocyanomethanide could be isolated in up to 53% yield. While this reaction had already been described in the literature, its mechanism is unknown but was assumed to involve the formation of N2O.27 Single crystals of Na[(NO2)2CCN]·H2O were obtained from an aqueous solution by slow evaporation of the solvent.
When trinitroacetonitrile was reacted with N2H4·H2O in water, a vigorous gas evolution was observed. However, no evidence for the formation of the dinitrocyanomethanide anion could be obtained. Instead, yellow crystals of [N2H5][(NO2)2CH] were formed when the resulting bright yellow solution was taken to dryness (Scheme 2). The mechanism for the formation of the dinitromethanide anion is unknown. However, it should be noted that a mixture of pure trinitroacetonitrile and neat N2H4 is hypergolic. The reaction of the two compounds is highly exothermic. On several occasions, flames and/or explosions were observed when neat hydrazine was mixed with solid trinitroacetonitrile.
When a clear colourless solution of trinitroacetonitrile in dichloromethane was treated with an aqueous ammonia solution at ambient temperature, a yellow effervescing mixture was obtained. Yellow crystals of [NH4][(NO2)2CCN] were isolated in quantitative yield when the solvent was removed from the reaction mixture after 12 hours of stirring at ambient temperature. The evolved gas was identified by IR spectroscopy as N2O (Scheme 3).
Fluorodinitroacetonitrile, F(NO2)2CCN, was obtained by aqueous fluorination of sodium dinitrocyanomethanide with 10% F2 in N2 (Scheme 1). The off-gas of the fluorination reaction was passed trough a series of cold traps at −78 °C. Lower cold trap temperatures were not used to avoid trapping of fluorine nitrate, a possible fluorination side product. While Wiesboeck and Ruff reported only moderate yields of 29% of F(NO2)2CCN and the formation of the hydrolysis products acetonitrile and fluorodinitroacetamide when aqueous solutions with more than 3% Na[(NO2)2CCN] were fluorinated,28 we did not observe the formation of appreciable amounts on hydrolysis products even in the case of reaction mixtures containing more than 10% of Na[(NO2)2CCN]. However, we did observe a contamination of the crude F(NO2)2CCN with up to 10% fluorotrinitromethane, FC(NO2)3, and also trace amounts of CO2. Although not necessary for this work, crude F(NO2)2CCN could be purified by fractional condensation at −31 °C, −78 °C and −196 °C. The F(NO2)2CCN stopped in the −78 °C trap and was isolated as a colourless liquid with a vapour pressure of 56 Torr at 23 °C.
The 1,3-dipolar cycloaddition reaction of F(NO2)2CCN with HN3, followed by extraction with dichloromethane resulted in the isolation of HFDNTz as a colourless, hygroscopic solid in approximately 85% yield. Single crystals suitable for an X-ray crystal structure determination were obtained from a dichloromethane solution by slow evaporation of the solvent in vacuo.
HFDNTz is acidic and forms ammonium 5-(fluorodinitromethyl)tetrazolate in quantitative yield when treated with aqueous ammonia (Scheme 4).
Silver 5-(fluorodinitromethyl)tetrazolate was obtained as a white amorphous precipitate in quantitative yield when an aqueous solution of silver nitrate was added to a solution of HFDNTz in water. Colourless crystals of the ammonia adduct [Ag][FDNTz]·½NH3 were obtained by recrystallization of the amorphous precipitate from an aqueous ammonia solution. While the nitrotetrazoles of this work are energetic and must be treated with great care, both silver salts are especially treacherous and can explode upon provocation (heat or mechanical shock). The tetraphenylphosphonium (PPh4) salt of FDNTz was precipitated from an aqueous solution of HFDNTz by the addition of an aqueous PPh4Cl solution. Crystalline [PPh4][FDNTz] was obtained by recrystallization from acetone.
Na[(NO2)2CCN]·H2O | [NH4][(NO2)2CCN] | [PPh4][(NO2)2CCN] | [N2H5][(NO2)2CH] | |
---|---|---|---|---|
Formula | C2H2N3NaO5 | C2H4N4O4 | C26H20N3O4P | CH6N4O4 |
mol wt [g mol−1] | 171.06 | 148.09 | 469.42 | 138.10 |
Temp [K] | 100(2) | 100(2) | 104(2) | 100(2) |
Crystal system | Orthorhombic | Monoclinic | Monoclinic | monoclinic |
Space group | P212121 | P21/n | P21/n | P21/n |
a [Å] | 7.5138(6) | 8.1144(12) | 11.6601(8) | 3.6434(4) |
b [Å] | 8.1663(6) | 4.8009(8) | 14.2600(9) | 13.7827(13) |
c [Å] | 19.9151(14) | 14.218(2) | 13.4402(9) | 10.6457(10) |
α [°] | 90 | 90 | 90 | 90 |
β [°] | 90 | 96.914(3) | 90.099 (1) | 98.230(2) |
γ [°] | 90 | 90 | 90 | 90 |
V [Å3] | 1221.99(16) | 549.85(15) | 2234.7(3) | 529.08(9) |
Z | 8 | 4 | 4 | 4 |
λ [Å] | 0.71073 | 0.71073 | 0.71073 | 0.71073 |
ρ calc [g cm−3] | 1.860 | 1.789 | 1.395 | 1.734 |
μ [mm−1] | 0.237 | 0.170 | 0.163 | 0.169 |
F(000) | 688 | 304 | 976 | 288 |
Reflns collected | 30![]() |
12![]() |
55![]() |
12![]() |
Ind reflns | 3734 | 1681 | 6814 | 1614 |
R int | 0.0355 | 0.0265 | 0.0385 | 0.2311 |
No. of parameters | 201 | 103 | 307 | 100 |
R 1 [I > 2σ(I)] | 0.0253 | 0.0292 | 0.0359 | 0.0255 |
wR2 [I > 2σ(I)] | 0.0651 | 0.795 | 0.0922 | 0.0769 |
GOF | 1.060 | 1.053 | 1.035 | 1.087 |
HFDNTz | [NH4][FDNTz] | Ag[FDNTz]·½NH3 | [PPh4][FDNTz] | |
---|---|---|---|---|
Formula | C2HFN6O4 | C2H4FN7O4 | C24H18Ag12F12N78O48 | C26H20FN6O4P |
Mol wt [g mol−1] | 192.09 | 209.12 | 3689.60 | 530.45 |
Temp. [K] | 100(2) | 101(2) | 100(2) | 101(2) |
Crystal system | Monoclinic | Monoclinic | Triclinic | Monoclinic |
Space group | C2/c | P21/c |
P![]() |
P21 |
a [Å] | 28.8030(7) | 13.9642(7) | 15.4270(10) | 11.8077(14) |
b [Å] | 5.3703(1) | 8.9672(5) | 15.4526(10) | 7.1276(8) |
c [Å] | 28.897(1) | 13.9957(7) | 20.5225(13) | 14.7721(18) |
α [°] | 90 | 90 | 82.7270(10) | 90 |
β [°] | 119.723(2) | 115.4716(1) | 88.1560(10) | 95.585(2) |
γ [°] | 90 | 90 | 82.6690(10) | 90 |
V [Å3] | 3881.71(19) | 1582.19(14) | 4812.6(5) | 1237.3(3) |
Z | 24 | 8 | 2 | 2 |
λ [Å] | 0.71073 | 0.71073 | 0.71073 | 0.71073 |
ρ calc [g cm−3] | 1.972 | 1.756 | 2.546 | 1.424 |
μ [mm−1] | 0.200 | 0.174 | 2.540 | 0.165 |
F(000) | 2304 | 848 | 3528 | 548 |
Reflns collected | 46![]() |
37![]() |
22![]() |
26![]() |
Ind reflns | 5932 | 4778 | 22![]() |
7495 |
R int | 0.0774 | 0.0384 | 0.0939 | 0.0451 |
No. of parameters | 361 | 277 | 1576 | 343 |
R 1 [I > 2σ(I)] | 0.0414 | 0.0329 | 0.0584 | 0.0435 |
wR2 [I > 2σ(I)] | 0.0830 | 0.0786 | 0.0977 | 0.0916 |
GOF | 1.035 | 1.026 | 0.992 | 1.023 |
From an aqueous solution, sodium dinitrocyanomethanide crystallizes as a monohydrate in the orthorhombic space group P212121 with the unit cell parameters a = 7.5138(6) Å, b = 8.1663(6) Å and c = 19.9151(14) Å. The solid-state structure of Na[(NO2)2CCN]·H2O does not consist of isolated ions but is dominated by cation–anion interactions that result in a polymeric structure. The asymmetric unit of the structure contains two formula units (Z′ = 2). One CN nitrogen atom as well as several oxygen atoms of the anion coordinate each sodium ion, which in turn is bridged to another sodium ion through a water molecule (Fig. 1).
The ammonium salt of the dinitrocyanomethanide anion crystallizes with four formula units per unit cell (Z = 4) in the monoclinic space group P21/n. Not surprisingly, the solid-state structure of [NH4][(NO2)2CCN] contains hydrogen bonds between the ammonium ions and the oxygen atoms as well as the CN nitrogen atom of the anions (Fig. 2). The observed C–N and CN bond distances of 1.385(2)/1.390(2) Å and 1.152(2) Å, respectively, in the anion in [NH4][(NO2)2CCN] are virtually identical to the ones observed for Na[(NO2)2CCN]·H2O (1.385(2)/1.390(2) Å and 1.147(2) Å).
Single crystals of [PPh4][(NO2)2CCN] suitable for structure determination were obtained from an acetone solution by slow evaporation of the solvent. The compound crystallizes in the monoclinic space group P21/n with four formula units in the unit cell (Z = 4). The solid state structure consists of isolated PPh4+ cations and [(NO2)2CCN]− anions (Fig. 3). The closest cation–anion interactions are 3.122(2) Å (O2⋯C19) and 3.187(2) Å (O1⋯C10). The observed C–N and CN bond distances of 1.3966(15)/1.4150(7) Å and 1.1513(17) Å, respectively, in the [(NO2)2CCN] anion are in good agreement with the ones observed for the Na+ and NH4+ salts.
Single crystals of [N2H5][(NO2)2CH] were obtained from an aqueous solution by slow evaporation. The compound crystallizes without crystal water in the monoclinic space group P21/n (Z = 4) with the unit cell parameters a = 3.6434(4) Å, b = 13.7827(13) Å, and c = 10.6457(10) Å. The solid-state structure consists of N2H5+ cations and [(NO2)2CH]− anions (Fig. 4) that are linked through hydrogen bonds between the cation and the oxygen atoms of the anion. The observed C–N bond distances in the [(NO2)2CH]− anion of 1.368(1) Å and 1.365(1) Å are noticeable shorter than the ones observed for the [(NO2)2CCN]− anion in the PPh4+ salt (1.3966(15) and 1.4150(7) Å).
Single crystals of 5-(fluorodinitromethyl)-2H-tetrazole (HFDNTz) were obtained by slow evaporation of a dichloromethane solution in vacuo at a temperature of −20 °C. The compound crystallizes in the monoclinic space group C2/c with a unit cell volume of 3881.71(19) Å3 (Z = 24). Further crystallographic details for the compound are listed in Table 2. The structure of HDNTz is depicted in Fig. 5, while the bond lengths and angles of the tetrazole ring in this compound are summarized in Table 3 together with the ones for the related tetrazolates of this work.
HFDNTza | [NH4][FDNTz]a | Ag[FDNTz]·½NH3![]() |
[PPh4][FDNTz] | |
---|---|---|---|---|
a Values given for only one of the different molecules in the asymmetric unit. | ||||
C1–C2 | 1.490(2) | 1.4850(14) | 1.463(10) | 1.476(4) |
C1–N1 | 1.325(2) | 1.3298(13) | 1.324(9) | 1.335(4) |
C1–N4 | 1.350(2) | 1.3357(13) | 1.325(8) | 1.319(4) |
N1–N2 | 1.324(2) | 1.3438(12) | 1.343(8) | 1.347(3) |
N2–N3 | 1.319(2) | 1.3224(13) | 1.327(8) | 1.311(4) |
N3–N4 | 1.325(2) | 1.3383(12) | 1.348(7) | 1.347(3) |
C2–F | 1.318(2) | 1.3328(11) | 1.313(8) | 1.339(4) |
C2–N5 | 1.540(2) | 1.5466(14) | 1.540(9) | 1.543(4) |
C2–N6 | 1.539(2) | 1.5381(14) | 1.545(9) | 1.534(4) |
C1–N1–N2 | 100.36(14) | 103.48(8) | 104.0(6) | 102.9(2) |
C1–N4–N3 | 105.48(14) | 103.42(8) | 104.4(5) | 103.5(2) |
N1–N2–N3 | 115.32(14) | 109.64(8) | 109.8(6) | 110.0(2) |
N2–N3–N4 | 105.32(13) | 109.91(8) | 108.6(6) | 109.7(2) |
N1–C1–N4 | 113.51(15) | 113.55(9) | 113.2(6) | 113.9(2) |
N1–C1–C2 | 122.49(15) | 124.33(9) | 125.0(7) | 119.9(2) |
C1–C2–N5 | 109.68(13) | 113.60(8) | 113.1(6) | 114.7(2) |
C1–C2–N6 | 113.00(13) | 112.04(8) | 108.7(6) | 110.9(2) |
N5–C2–N6 | 104.60(12) | 103.55(8) | 103.9(5) | 102.7(2) |
The geometry of the five-membered ring in HFDNTz is essentially identical to the one of 5-(trinitromethyl)-2H-tetrazole (HTNTz).5 Due to the strong electron withdrawing effect of the fluorodinitromethyl group, the hydrogen atom of the tetrazole moiety is exclusively located in the 2-position (N2) of the five-membered ring. Similar to HTNTz and atypical for alkyl-substituted tetrazoles, the distance between C1 and N1 (1.325(2) Å) is shorter than the one between C1 and N4 (1.350(2) Å). In addition, the three N–N distances in the five-membered ring of HFDNTz can be considered identical within their margins of error. This is in good agreement with the observed geometry of HTNTz but is unlike the distance pattern of a regular alkyl-substituted tetrazole.5 The asymmetric unit of the 5-(fluorodinitromethyl)-2H-tetrazole solid-state structure consists of three HFDNTz molecules (Z′ = 3) that are arranged in a triangular fashion with the fluorine atoms of the –CF(NO2)2 groups facing each other (Fig. 6). The three fluorine atoms form the corners of a slightly disordered regular triangle with F–F distances of 2.886(2) and 2.939(2) Å and F–F–F angles of 58.82(4) and 60.59(4)°. In addition, the HFDNTz molecules are associated through N(2)–H⋯N(4) hydrogen bonds.
The solid-state structure of ammonium 5-(fluorodinitromethyl)tetrazolate consists of ammonium cations and FDNTz anions that are associated through hydrogen bonding (Fig. 7). Further crystallographic details of the structure are listed in Table 2, the observed bond lengths and angles for the FDNTz− anion are summarized in Table 3.
All attempts to grow single crystals of silver 5-(fluorodinitromethyl)tetrazolate (AgFDNTz) suitable for X-ray structure determination were unsuccessful. The crystallization of an amorphous sample of AgFDNTz from an aqueous ammonia solution resulted in crystals of the ammonia adduct Ag[FDNTz]·½NH3 instead. Selected crystallographic data of the compound are listed in Table 2. The solid-state structure of the silver salt contains [Ag(NH3)2]+ cations and polymeric anion chains. The anion chains are made up from [Ag4(FDNTz)4] units in which two silver atoms are linked in a 1,2-fashion by always two bridging FDNTz− anions. Every tetrazolate anion is coordinated to three different Ag atoms. The units are linked together by Ag(NH3) and Ag(NH3)2 units, resulting in a complex polymeric anion chain. The resulting overall structure can be described as [Ag(NH3)2]3[Ag21(NH3)6(FDNTz)24]. Part of a polymeric anion chain of the structure is depicted in Fig. 8. Further crystallographic details of the structure are listed in Table 2, the observed bond lengths and angles for the FDNTz− anion are summarized in Table 3.
The tetraphenylphosphonium salt [PPh4][FDNTz] crystallizes in the monoclinic space group P21 with two formula units per unit cell (Z = 2). The solid-state structure consists of isolated and well-separated PPh4+ cations and FDNTz− anions (Fig. 9). The closest cation–anion distance is 2.989(3) Å (C18⋯O4). Further crystallographic details of the structure are listed in Table 2, the observed bond lengths and angles for the FDNTz− anion are summarized in Table 3. In going from the neutral tetrazole HFDNTz to the weakly coordinated tetrazolate anion in [PPh4][FDNTz], the geometry of the five-membered ring changes. While the C1–N1 distance increases only slightly (0.01 Å), the second C–N distance (C1–N4) shortens by over 0.03 Å).
It is interesting to note that the N1–N2 and N3–N4 distances in the anion are longer by about 0.02 Å than the ones in the parent tetrazole. The third N–N distance (N2–N3) remains essentially unchanged within the error margins.
Compound |
T
d![]() |
FS [N] | IS [J] | OB [%] |
---|---|---|---|---|
a T d: decomposition temperature, FS: friction sensitivity, IS: impact sensitivity, OB: oxygen balance. b DTA onset. c Sample evaporates. d Endotherm at 180 °C (melting). e Explosion. f Endotherm at 165 °C (loss of NH3). | ||||
RDX | 220 | 120 | 7.5 | −21 |
(NO2)3CCN | —c | 112 | 12 | 17.4 |
Na[(NO2)2CCN] | 150 | >360 | 80 | −5.2 |
[NH4][(NO2)2CCN] | 240 | >360 | 75 | −21.6 |
[PPh4][(NO2)2CCN] | 240d | >360 | >100 | −206.2 |
[N2H5][(NO2)2CH] | 134 | 230 | 90 | −11.6 |
HFDNTz | 110e | 40 | 3.5 | −2.1 |
[NH4][FDNTz] | 140e | 50 | 4 | −13.4 |
[Ag][FDNTz] | 185 | <2 | 2 | −1.3 |
[Ag][FDNTz]·½NH3 | 180f | <2 | 2 | −5.2 |
[PPh4][FDNTz] | 175 | >360 | >100 | −181.7 |
With the exception of trinitroacetonitrile, all investigated compounds are under-oxidized and have a negative oxygen balance. It is not surprising that based on the impact and friction sensitivities, the two most stable compounds, [PPh4][(NO2)2CCN] and [PPh4][FDNTz], are the ones with the lowest oxygen balances. Explosion upon heating were observed only in the case of the free tetrazole HFDNTz as well as the corresponding ammonium salt [NH4][FDNTz]. All other investigated compounds showed smooth thermal decompositions. In the case of (NO2)3CCN, it was not possible to determine a decomposition temperature because the sample evaporated completely upon heating with a nitrogen purge before its decomposition. The thermally least stable compounds are HFDNTz, [N2H5][NO2)2CH], [NH4][FDNTz], and Na[(NO2)2CCN] with decomposition temperatures of 110 °C, 134 °C, 140 °C, and 150 °C, respectively. It is interesting that [NH4][(NO2)2CCN)] shows a much higher thermal stability of 240 °C than the closely related Na[(NO2)2CCN)] (150 °C). This might be related to the different oxygen balances as well as the presence of stabilizing hydrogen bonding in the case of the ammonium salt. With the exception of the PPh4+ salt, all 5-(fluorodinitromethyl)tetrazolates and the parent 5-(fluorodinitromethyl)-2H-tetrazole are sensitive compounds. With impact sensitivities of less than 5 J and friction sensitivities of 50 Nm or less, these compounds must be considered explosion hazards that have to be handled with great care while using proper safety precautions. Both silver salts, [Ag][FDNTz] and [Ag][FDNTz]·½NH3, are especially treacherous with friction sensitivities of fewer than 2 Nm and impact sensitivities of about 2 J.
Footnote |
† Electronic supplementary information (ESI) available: Crystallographic reports including packing diagrams. CCDC 1044179–1044181. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5dt00291e |
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