5,5′-Bis(2,4,6-trinitrophenyl)-2,2′-bi(1,3,4-oxadiazole) (TKX-55): Thermally Stable Explosive with Outstanding Properties

Article abstract of DOI:10.1002/cplu.201600078

The novel, thermally stable explosive 5,5′-bis(2,4,6-trinitro-phenyl)-2,2′-bi(1,3,4-oxadiazole) (TKX-55) is reported. This compound can be prepared by means of a facile synthetic procedure and shows outstanding properties (detonation velocity, detonation pressure, sensitivity toward mechanical stimuli, and temperature of decomposition). TKX-55 was isolated and characterized by means of mass spectrometry, multinuclear (¹H, ¹³C) NMR spectroscopy, and vibrational spectroscopy (IR and Raman). The structure in the crystalline state was determined by low-temperature single-crystal X-ray diffraction. From the calculated standard molar enthalpy of formation (CBS-4M) and the densities, the Chapman-Jouguet detonation properties were predicted by using the EXPLO5 V6.01 thermochemical computer code. The sensitivity of TKX-55 towards impact, friction, and electrostatic discharge was determined. The shock reactivity (explosiveness) of TKX-55 was measured by applying the small-scale shock reactivity test.

Full text of DOI:10.1002/cplu.201600078

DOI: 10.1002/cplu.201600078
Communications
5,5’-Bis(2,4,6-trinitrophenyl)-2,2’-bi(1,3,4-oxadiazole)
(TKX-55): Thermally Stable Explosive with Outstanding
Properties
Thomas M. Klapçtke* and Tomasz G. Witkowski[a]
The novel, thermally stable explosive 5,5’-bis(2,4,6-trinitro-
phenyl)-2,2’-bi(1,3,4-oxadiazole) (TKX-55) is reported. This com-
pound can be prepared by means of a facile synthetic proce-
dure and shows outstanding properties (detonation velocity,
detonation pressure, sensitivity toward mechanical stimuli, and
temperature of decomposition). TKX-55 was isolated and char-
acterized by means of mass spectrometry, multinuclear (1H,
13C) NMR spectroscopy, and vibrational spectroscopy (IR and
Raman). The structure in the crystalline state was determined
by low-temperature single-crystal X-ray diffraction. From the
calculated standard molar enthalpy of formation (CBS-4M) and
the densities, the Chapman–Jouguet detonation properties
were predicted by using the EXPLO5 V6.01 thermochemical
computer code. The sensitivity of TKX-55 towards impact, fric-
tion, and electrostatic discharge was determined. The shock re-
activity (explosiveness) of TKX-55 was measured by applying
the small-scale shock reactivity test.
Dacons and Sitzmann,[5] who reported the synthesis of 2,5-bis-
(2,4,6-trinitrophenyl)-1,3,4-oxadiazole (DPO), which had a de-
composition temperature much higher than that of HNBP.
Herein, the synthesis and investigation into the novel, thermal-
ly stable compound 5,5’-bis(2,4,6-trinitrophenyl)-2,2’-bi(1,3,4-
oxadiazole) (TKX-55 (5)) is presented for the first time. In TKX-
55, two 2,4,6-trinitrophenyl groups are connected by a di(1,3,4-
oxadiazole) bridge. The synthesis of TKX-55 is achieved in four
steps from commercially available 2,4,6-trinitrotoluene
(Scheme 1). The synthetic route presented herein results in
high yields of TKX-55 and selectivity of the desired compound.
The intermediates and final product all precipitated from the
reaction mixture without involving costly and time-consuming
purification processes. It is worth noting that the process for
synthesizing TKX-55 starts from 2,4,6-trinitrotoluene, which is
Covalent compounds that contain a conjugated system with
attached explosophore nitro are of particular interest in the de-
velopment of new explosives which are stable at high temper-
atures. These explosives must also possess targeted per-
formance properties (detonation velocity, specific energy), but,
at the same time, show safety in handling and practicability for
deep sea oil and gas exploitation as well as space explora-
tion.[1] The known compound 2,2’,4,4’,6,6’-hexanitrobiphenyl
(HNBP) consists of two directly connected trinitrophenyl moiet-
ies and shows a moderate decomposition temperature and
good detonation parameters.[2] Inclusion of different moieties
between the two picryl groups is one of the methodologies
used in research in attempts to find heat-resistant explosives.
The most prominent examples are 2,2’,4,4’,6,6’-hexanitrostil-
bene (HNS)[3] and 2,6-bis(picrylamino)-3,5-dinitropyridine
(PYX).[1c,4] The insertion of endothermic azole functionalities
into the conjugated system improves the heat of formation of
the final species. This also has a positive impact on the detona-
tion parameters of the explosive. The connection of two picryl
moieties through a 1,3,4-oxadiazole bridge was reported by
[a] Prof. Dr. T. M. Klapçtke, T. G. Witkowski
Department of Chemistry
Ludwig Maximilian University
Butenandtstrasse 5–13 (D), 81377 Munich (Germany)
E-mail: tmk@cup.uni-muenchen.de
Homepage: http://www.hedm.cup.uni-muenchen.de
Supporting information for this article can be found under http://
dx.doi.org/10.1002/cplu.201600078.
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Scheme 1. Synthesis of TKX-55.
also used as a starting material for the synthesis of HNS (the
most commonly used heat-resistant explosive).
The first step in the synthesis of TKX-55 is oxidation of 2,4,6-
nitrotoluene to form 2,4,6-trinitrobenzoic acid (1) by using
a mixture of NaClO3/HNO3 (yield 88%).[6] The second step in-
volves chlorination of 1 with POCl3 (yield 89%).[7] The next step
involves the reaction of 2,4,6-trinitrobenzoyl chloride (3) with
oxalyldihydrazide (which can be obtained in essentially quanti-
tative yields through the reaction of diethyl oxalate with hy-
drazinium) in tetrahydrofuran (yield 83%). In the final step,
bis(2,4,6-trinitrobenzoyl)oxalohydrazide (4) is added to oleum
(20%) and stirred at room temperature before the reaction
mixture is poured onto crushed ice to yield TKX-55 (yield
93%). Compounds 4 and TKX-55 are pale, off-white solids that
are practically insoluble in commonly used organic solvents.
The structure of TKX-55 in the crystalline state was deter-
mined by means of low-temperature single-crystal X-ray dif-
fraction.[8] Selected crystallographic data, measurement param-
eters, and refinement details are given in the Supporting Infor-
mation. TKX-55 crystallizes from hot 1,4-dioxane with three sol-
vent molecules per TKX-55 molecule in the triclinic space
group PÀ1, with a density of 1.625 gcmÀ3 at 227 K and one
molecule in the unit cell. The molecular structure of TKX-55 is
shown in Figure 1.
Figure 1. Molecular structure of TKX-55 shown with thermal ellipsoids at the
50% probability level. Cocrystallized solvent molecules are removed for clari-
ty.
benzene ring plane (aC3-C4-N3-O3, 1.957(3)8; aC5-C6-N4-O4
5.667(3)8), whereas one ortho-nitro group is significantly bent
(aC3-C8-N5-O7, 36.061(3)8).
TKX-55 shows the desired properties required for a heat-re-
sistant explosive: good detonation parameters, low sensitivity
values, and a high decomposition temperature. The thermal
stability of TKX-55 was measured by using differential scanning
calorimetry (DSC), which showed that it decomposed above
3358C. The measured density of the material is 1.837 gcmÀ3
(obtained from a pycnometer measurement at 298 K), which is
higher than the densities of both HNS and PYX.[3e] The calculat-
ed standard molar enthalpy of formation for TKX-55 is more
than 2.5 times higher than that of HNS, and more than
4.5 times higher than that of PYX. The high density and high
heat of formation result in very good detonation parameters
for TKX-55. The Chapman–Jouguet (C–J) characteristics: deto-
nation velocity (VC–J =8.030 msÀ1) and detonation pressure (PC–
J =27.3 GPa) are both higher than the corresponding values for
The carbon–carbon bond connecting the 1,3,4-oxadiazole
rings is involved in the conjugated p-electron system, and
therefore, its length is shorter (1.449 Š) than that of a carbon–
carbon single bond. 1,3,4-Oxadiazole rings are coplanar and
twisted out of the plane of the 2,4,6-trinitrophenyl functionali-
ties by 70.347(3)8. 2,4,6-Trinitrophenyl moieties are also parallel
to each other. Two nitro groups are slightly twisted out of the
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PYX and HNS.[2c–e] The nitrogen content of TKX-55 is enhanced
owing to the inclusion of the oxadiazole moieties. The impact
sensitivity of TKX-55 is 5 J, which is equal to the value reported
for HNS, and the friction sensitivity of TKX-55 is higher than
the measuring range of the friction tester apparatus (according
to the STANAG regulations).[9] TKX-55 is also less sensitive
toward electrostatic discharge (typical values for the human
body are within the range 0.005–0.02 J) than PYX and HNS
(Table 1).[1f] Moreover, TKX-55 is practically insoluble in water,
which simplifies its isolation and purification. The combination
of these properties makes TKX-55 unique among thermally
stable explosives. Details on the measurements and computa-
tions are given in the Supporting Information.
Figure 2. Details of the SSRT setup: schematic drawing (A); photograph of
the test setup (B).
Table 1. Physicochemical properties of PYX, HNS, and TKX-55.
PYX
HNS
TKX-55
formula
IS[a] [J]
C17H7N11O16
10
C14H6N6O12
5[1f,3e]
C16H4N10O14
5
>360
1.0
25.00
À57.11
335
1.837
197.6
FS[b] [N]
ESD[c] [J]
N[d] [%]
W[e] [%]
360
0.5
240[1f,3e]
0.8
24.80
À55.36
360[4a]
1.757[o]
43.7
18.67
À67.6
318[1f,3e]
1.74[3e]
78.2
[f]
Figure 3. The SSRT results for TKX-55: aluminum block and steel block filled
with TKX-55 (A); dented aluminum blocks after initiation of the explosive
with a commercial detonator (B).
Tdec [8C]
1[g] [gcmÀ3
]
DfH8[h] [kJmolÀ1
]
EXPLO5 V6.01
ÀDEU8[i] [kJkgÀ1
]
4870
3609
25.1
7757
633
5142
3677
24.3
7612
602
4961
3681
27.3
8030
604
TC–J[j] [K]
Table 2. Results of the SSRT for PYX, HNS, and TKX-55.
PC–J[k] [GPa]
VC–J[L] [msÀ1
]
PYX
HNS
TKX-55
gas V[m] [dm3 kgÀ1
f[n] [kJkgÀ1
]
]
1008
1008
1004
mE[a] [mg]
474
637
469
672
496
641
[b]
mSiO [mg]
2
[a] IS: impact sensitivity (BAM drop hammer, method 1 of 6). [b] FS: fric-
tion sensitivity (BAM drop hammer, method 1 of 6). [c] ESD: electrostatic
discharge device (OZM research). [d] Nitrogen content. [e] Oxygen bal-
ance. [f] Temperature of decomposition. [g] Density at 298 K. [h] Standard
molar enthalpy of formation. [i] Heat of detonation. [j] Detonation tem-
perature. [k] Detonation pressure. [l] Detonation velocity. [m] Volume of
detonation gases at standard temperature and pressure conditions.
[n] Specific energy. [o] X-ray analysis.
[a] Mass of the explosive: mE =Vs10.95 (Vs =284 mm3). [b] Mass of SiO2.
gen content, detonation velocity, and detonation pressure
values of TKX-55 are higher than those of the currently used
heat-resistant explosives HNS and PYX. TKX-55 is insensitive to
friction and its sensitivity to impact and electrostatic discharge
are comparable with those of HNS. TKX-55 is practically insolu-
ble in water (which avoids toxicity problems) and can be pre-
pared by using a facile synthetic route. This combination of
properties makes TKX-55 unique among thermally stable ex-
plosives.
To evaluate the explosive performance of TKX-55 on a small
scale, a small-scale shock reactivity test (SSRT) was undertaken.
The SSRT measures the shock reactivity (explosiveness) of po-
tential energetic materials, often below the critical diameter,
without requiring a transition to detonation.[10] TKX-55 was
pressed into a perforated steel block. Initiation of the tested
explosive was performed by using a commercially available
detonator (Figures 2 and 3). The dent sizes were measured by
filling them with finely powdered SiO2 and measuring the re-
sulting weight. For comparison, the corresponding values for
PYX and HNS are also given (Table 2).
Experimental Section
Bis(2,4,6-trinitrobenzoyl)oxalohydrazide (4)
Oxalyldihydrazide (10 mmol, 0.59 g) was added in one portion to
a solution of 3 (5 mmol, 2.76 g) in tetrahydrofuran (THF; 25 mL).
The mixture was stirred for 72 h at ambient temperature. The pre-
cipitate was filtered off and washed with THF, acetone, and diethyl
ether (2.48 g, 83%). DSC (58C minÀ1): 3078C (dec); 1H NMR
(400.18 MHz, [D6]DMSO, 268C): d=11.47 (s, 2H; NH), 11.42 (s, 2H;
NH); 9.13 ppm (s, 4H; CH); 13C{1H} NMR (100.0 MHz, [D6]DMSO,
268C): d=158.0, 157.5, 147.92, 147.91, 129.2, 124.1 ppm; IR (ATR):
v˜ =3339 (m), 3295 (m), 3108 (w), 1714 (vs), 1671 (s), 1606 (m),
From measuring the volumes of the dents (HNS>TKX-55>
PYX), it can be concluded that the performance of TKX-55 on
a small scale is equivalent to the currently used heat-resistant
explosives PYX and HNS.
It can be concluded that TKX-55 is one of the most thermal-
ly stable explosives. The density, enthalpy of formation, nitro-
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1552(s), 1536 (vs), 1484 (m), 1452 (w), 1344 (vs), 1293 (m), 1230
(m), 1181 (w), 1076 (w), 923 (s), 830 (w), 819 (w), 784 (w), 736 (s),
725 (s), 686 cmÀ1 (m); MS (DEI+): m/z: 596.1 [M]+; elemental analy-
sis calcd (%) for C16H8N10O16 (596.29): C 32.23, H 1.35, N 23.49;
found: C 32.22, H 1.61, N 22.87.
Keywords: aromaticity · energetic materials · oxadiazoles ·
structure elucidation · thermochemistry
´
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analysis (DTA; 58CminÀ1): 3358C (dec); 1H NMR (399.78 MHz,
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Acknowledgements
Financial support of this study by the Ludwig-Maximilian Univer-
sity of Munich (LMU) and the Office of Naval Research (ONR)
under grant no. ONR.N00014-16-1-2062, and the Bundeswehr–
Wehrtechnische Dienststelle für Waffen und Munition (WTD 91)
under grant no. E/E91S/FC015/CF049 is gratefully acknowledged.
The X-ray team around Prof. Dr. Konstantin Karaghiosoff is grate-
fully thanked for the single-crystal measurement. We thank
Stefan Huber for accompanying the sensitivity testing and helpful
suggestions and discussions. T.W. also thanks the DAAD for
a PhD scholarship.
Manuscript received: February 24, 2016
Accepted Article published: February 25, 2016
Final Article published: March 8, 2016
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