A high density pyrazolo-triazine explosive (PTX)

Article abstract of DOI:10.1039/c5ta05291b

The fused-ring heterocycle 4-amino-3,7,8-trinitropyrazolo-[5,1-c][1,2,4]triazine (PTX) has promising explosive properties. The Cheetah thermochemical code used its calculated standard enthalpy of formation and its measured crystal density of 1.946 g cm^-3 to predict HMX-like explosive performance, while measurements of its thermal stability, sensitivity to impact, friction, and spark showed greater safety margins.

Full text of DOI:10.1039/c5ta05291b

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A high density pyrazolo-triazine explosive (PTX)†
a
b
a
*
M. C. Schulze, B. L. Scott and D. E. Chavez
Cite this: DOI: 10.1039/c5ta05291b
Received 13th July 2015
Accepted 6th August 2015
DOI: 10.1039/c5ta05291b
www.rsc.org/MaterialsA
The fused-ring heterocycle 4-amino-3,7,8-trinitropyrazolo-[5,1-c] slipping between graphite-like layers in the explosive crystals,
[1,2,4]triazine (PTX) has promising explosive properties. The Cheetah decreasing the likelihood of generating hotspots that inadver-
thermochemical code used its calculated standard enthalpy of tently initiate the material to reactivity and ultimately detona-
formation and its measured crystal density of 1.946 g cmꢀ3 to predict tion.6–9 Higher performing explosives (HMX) have high density
HMX-like explosive performance, while measurements of its thermal and better oxygen balance; in the cases of CL-20 and octani-
stability, sensitivity to impact, friction, and spark showed greater safety trocubane, they have large positive heats-of-formation from
margins.
their strained fused-ring/cage-like structures.10
With those molecular properties as guidelines for the
rational design and discovery of new explosives, we investigated
a fused pyrazolo-triazine explosive (PTX, Scheme 1), a molecule
rst synthesized in seven steps and 10% overall yield by
Dalinger and coworkers.11 The fused-rings produce a at
molecular geometry and p–p interactions. The nitrogen-rich
heterocycles reduce the amount of carbon present, making a
good oxygen balance easier to achieve with fewer exploso-
phores. While density is difficult to predict, the presence of
nitro and amino groups invites hydrogen bonding to increase
molecular density. Herein we report an improved three step
synthesis of PTX in 30% overall yield, its crystal structure, and
its explosive properties.
The discovery and development of high performing and low-
sensitivity explosives is an ongoing effort in the energetic
materials community.1–3 The main challenge to this effort is the
trade-off between sensitivity and explosive performance.
Insensitive high explosives such as 2,4,6-triamino-1,3,5-trini-
trobenzene (TATB), 2,2-dinitroethene-1,1-diamine (FOX-7), 2,6-
diamino-3,5-dinitropyrazine-1-oxide (LLM-105), and 3,30-dia-
mino-4,40-azoxyfurazan (DAAF), lack the explosive power of
more sensitive but higher performing explosives like octahydro-
1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and 2,4,6,8,10,12-
hexanitro-2,4,6,8,10,12-hexaazaisowurtzitane (CL-20).3–5 Under-
standing how molecular features and material properties affect
sensitivity and explosive performance in both high performing
and insensitive explosives is essential in developing the next
generation of explosives. Targeting molecules that combine
features favorable for both explosive performance and stability
may bridge the gap between them.
The penultimate precursor to PTX is 5-amino-3,4-dinitro-
pyrazole (ADNP, Scheme 1) which can be diazotized and
condensed with the sodium salt of nitro-acetonitrile (NaNAN) to
Low sensitivity explosives (TATB, FOX-7, DAAF, and LLM-
105) are oen at molecules that display high thermal stabili-
ties, N–H/O type hydrogen bonding, and p–p interactions.1,6–8
The crystal lattice packing resulting from these features allows
aM-7 High Explosives Sciences and Technology, Los Alamos National Laboratory, Los
Alamos, NM 87544, USA. E-mail: dechavez@lanl.gov
bMPA-11 Materials Synthesis and Integrated Devices, Los Alamos National Laboratory,
Los Alamos, NM 87544, USA
† Electronic supplementary information (ESI) available: Includes experimental
procedures and crystallographic data. CCDC 1412578. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c5ta05291b
Scheme 1 Synthesis of PTX via a VNS route.
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prepare PTX.11 A few synthetic routes to ADNP have been diazonium intermediate has condensed with NaNAN into a
reported12 and involve the reduction of 5-azido-3,4-dinitropyr- soluble species, affording a pure crop of PTX from the ltrate in
azole,13,14 the Hofmann rearrangement of 3,4-dinitropyrazole 56% yield. Solvent free X-ray quality crystals of PTX formed from
carboxamide,11 or substitution of N-protected 3,4,5-trinitropyr- an unstirred aliquot of the ltrate.
azoles.15 We present an improved synthesis to ADNP from
Single crystal X-ray analysis was performed on PTX. Fig. 1b
dinitro-pyrazole (DNP) by utilizing vicarious nucleophilic shows the planarity of the fused-ring structure, with all
substitution (VNS) of the pyrazole C–H16,17 as displayed in torsion angles of N5 and N6 substituents less than 7ꢁ and
Scheme 1. The VNS amination reaction of DNP using trime- those of N7 and N8 substituents between 22–33ꢁ. Nitro group
thylhydrazinium iodide (TMHI) proceeds in 54% yield to (N5) and amino group (N6) are presumably held co-planar
provide ADNP contaminated with 10–15 mol% DNP. We found with the fused-ring by intramolecular hydrogen bonding (H1–
˚
it unnecessary to purify ADNP before converting it into PTX. O1, 2.12 A) (Fig. 1c). Intermolecular hydrogen bonds can also
Insoluble material is ltered from the reaction aer the be seen in Fig. 1c as dashed lines between amino and nitro
˚
groups (H2–O6, 2.29 A). The hydrogen bonds, along with p–p
interactions and a serendipitous packing arrangement give
the crystals an ultra-high density of 2.006 g cmꢀ1 at 100 K.
Fig. 1d shows the p–p interactions that result in molecular
˚
stacking planes with an interplanar distance of 3.45 A. The
presence of four unique molecular stacking plane orienta-
tions results in a mixed molecular stacking seen in Fig. 1d
that prevents interlayer sliding within the crystal lattice.
Interlayer sliding within a crystal lattice is a feature that
contributes to the insensitivity of TATB, FOX-7, DAAF, and
LLM-105, which have graphite-like stacking that allow such
sliding.6–9
The explosive performance of PTX was calculated using the
Cheetah thermochemical code.19 A heat of formation was
calculated by a method described by Rice20 to be 88.44 kcal
molꢀ1. The measured ambient temperature diffraction density
of 1.946 g cmꢀ3 and the calculated heat of formation were used
for the performance predictions. These data along with thermal
stability and sensitivity are compared to those of HMX in Table
1. PTX should exhibit HMX-like performance with lower sensi-
tivity and good vacuum thermal stability. The presence of intra-
and intermolecular hydrogen bonding in PTX could account for
lower sensitivity than HMX. However, the inability for interlayer
sliding to occur within the crystal lattice may be a factor that
prevents PTX from achieving the same insensitivity as mole-
cules like TATB or FOX-7 despite similar intermolecular inter-
actions and at molecular geometries.
Table 1 Explosive performance and sensitivity
PTX
HMX
r (g cmꢀ1
)
1.946@300 K
8998b
1.89a
Vdet (m sꢀ1
PCJ (GPa)
)
9110a
36.04b
39.0a
Hf (kcal molꢀ1
)
88.44b
17.93a
Tonset/Tpeak (ꢁC)c
Impact (cm)d
Friction (N)e
Spark (J)f
246/288
58.4 ꢂ 13.5
324–360
0.0625
265/285
24.9 ꢂ 4.9
150 ꢂ 40
0.025–0.125
a
b
c
From LLNL explosives manual.18
Calculated values.
DSC
decomposition temperature,ꢀo1nset is dened by a 0.01 W per g per ꢁC
d
threshold, with a 10 ꢁC min heating rate. LANL type 12, 50% drop
e
f
Fig. 1 (a and b) Thermal ellipsoids plot (50%) of PTX. (c) Crystal packing
of PTX viewing down unit cell axis a, hydrogen bonds shown in blue. (d)
Crystal packing of PTX showing molecular stacking planes.
height, 2.5 kg. 50% load Bruceton up/down method. ABL spark
3.4% threshold initiation level (TIL).
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7 J. Zhang, Q. Zhang, T. T. Vo, D. A. Parrish and J. M. Shreeve, J.
Am. Chem. Soc., 2015, 137, 1697–1704.
Conclusions
8 C. Zhang, X. Wang and H. Huang, J. Am. Chem. Soc., 2008,
130, 8359–8365.
9 J. M. Veauthier, D. E. Chavez, B. C. Tappan and D. A. Parrish,
J. Energ. Mater., 2010, 28, 229–249.
In summary, we utilized VNS in an improved synthetic route to
PTX. Its notable combination of calculated performance and
decreased sensitivity makes PTX a promising explosive candi-
date that warrants further investigation. Additionally, these
results exemplify the rational design and discovery of new
explosive molecules via the combination of features that
enhance performance (high density and good oxygen balance)
while preserving molecular stability (hydrogen bonding, planar
geometries, and p–p interactions).
¨
10 T. M. Klapotke and T. G. Witkowski, Propellants, Explos.,
Pyrotech., 2015, 40, 366–373.
11 I. L. Dalinger, I. A. Vatsadse, T. K. Shkineva, G. P. Popova,
B. I. Ugrak and S. A. Shevelev, Russ. Chem. Bull., 2010, 59,
1631–1638.
12 G. Herve, Dinitropyrazole Derivatives, US Pat., 0186931,
2009,.
13 I. L. Dalinger, T. I. Cherkasova, G. P. Popova, T. K. Shkineva,
I. A. Vatsadze, S. A. Shevelev and M. I. Kanishchev, Russ.
Chem. Bull., 2009, 58, 410–413.
14 P. Yin, J. Zhang, C. He, D. A. Parrish and J. M. Shreeve, J.
Mater. Chem. A, 2014, 2, 3200–3208.
15 I. L. Dalinger, I. A. Vatsadse, T. K. Shkineva, G. P. Popova and
S. A. Shevelev, Russ. Chem. Bull., 2012, 61, 464–466.
16 P. F. Pagoria, A. R. Mitchell and R. D. Schmidt, J. Org. Chem.,
1996, 61, 2934–2935.
17 P. F. Pagoria, A. R. Mitchell and R. D. Schmidt, US Pat.,
6069277, 2000.
18 B. M. Dobratz and P. C. Crawford, LLNL Explosives Handbook:
Properties of Chemical Explosives and Explosive Simulants,
1985.
Acknowledgements
The DOD/DOE Joint Munitions Technology Development
Program supported this work. The authors would like to thank
Anna Giambra for elemental analyses; Mary Sandstrom and
Jose Archuleta for thermal analyses; and Hongzhao Tian for
sensitivity tests; and Thomas Myers, Phil Leonard, Ken Windler,
and Phil Pagoria for helpful discussions.
Notes and references
1 P. Yin, D. A. Parrish and J. M. Shreeve, J. Am. Chem. Soc.,
2015, 137, 4778–4786.
¨
2 T. M. Klapotke, M. Q. Kurz, P. C. Schmid and J. Stierstorfer, J.
Energ. Mater., 2015, 33, 191–201.
3 M. B. Talawar, R. Sivabalan, M. Anniyappan, G. M. Gore,
S. N. Asthana and B. R. Gandhe, Combust., Explos. Shock
Waves, 2007, 43, 62–72.
4 E. G. Francois, D. E. Chavez and M. M. Sandstrom,
Propellants, Explos., Pyrotech., 2010, 35, 529–534.
5 R. L. Simpson, P. A. Urtiew, D. L. Ornellas, G. L. Moody,
K. J. Scribner and D. M. Hoffman, Propellants, Explos.,
Pyrotech., 1997, 22, 249–255.
19 L. A. Fried, W. M. Howard, S. Bastea, K. Glaesmann,
P. C. Souers, P. A. Vitello and I.-F. Kuo, CHEETAH
Thermochemical Code, Lawrence Livermore National
Laboratory, Livermore, CA, USA, 2007.
20 (a) E. F. Byrd and B. M. Rice, J. Phys. Chem. A, 2006, 110,
1005–1013; (b) E. F. Byrd and B. M. Rice, J. Phys. Chem. A,
2009, 113, 5813.
6 Y. Ma, A. Zhang, C. Zhang, D. Jiang, Y. Zhu and C. Zhang,
Cryst. Growth Des., 2014, 14, 4703–4713.
This journal is © The Royal Society of Chemistry 2015
J. Mater. Chem. A