Improving stability of the metal-free primary energetic cyanuric triazide (CTA) through cocrystallization

Article abstract of DOI:10.1039/c9cc09465b

Cyanuric triazide (CTA) and benzotrifuroxan (BTF) form a metal-free primary energetic cocrystal with suppressed volatility and improved thermal properties relative to CTA. Though electrostatic potential maps of the most stable conformations do not predict favorable interactions, a higher energy conformer has appropriate electrostatics and is selected by BTF in the cocrystal.

Full text of DOI:10.1039/c9cc09465b

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Improving stability of the metal-free primary
energetic cyanuric triazide (CTA) through
Cite this:DOI: 10.1039/c9cc09465b
cocrystallization†
Received 5th December 2019,
Accepted 15th January 2020
a
a
a
Leila M. Foroughi,
Adam J. Matzger
Ren A. Wiscons,
Derek R. Du Bois
and
ab
*
DOI: 10.1039/c9cc09465b
rsc.li/chemcomm
Cyanuric triazide (CTA) and benzotrifuroxan (BTF) form a metal-free CTA (Chart 1) is a promising candidate as a green primary
primary energetic cocrystal with suppressed volatility and improved energetic and is more powerful than lead azide;13 CTA can be
thermal properties relative to CTA. Though electrostatic potential synthesized in nearly quantitative yield in a single chemical
maps of the most stable conformations do not predict favorable transformation from inexpensive commercially available starting
interactions, a higher energy conformer has appropriate electro- materials.3,13 However, concerns about the thermal/impact
statics and is selected by BTF in the cocrystal.
sensitivity of CTA1,6,14 and volatility under ambient conditions,13,15
have limited its broad application.
The environmental and health concerns associated with the
Cocrystallization has recently been used to alter the thermal
continued use of heavy metal containing impact sensitive stability, sensitivity, corrosivity, and volatility of energetic
(primary) explosives motivate development of ‘‘green ener- species.16–24 However, cocrystal formation between two species
getics’’ that lack toxic metals such as lead and mercury.1–3 is rarely guaranteed and identifying energetic compounds
The use of lead in energetics has become highly regulated with that can cocrystallize is particularly challenging because
restrictions enacted to reduce environmental release.1,2,4 As a energetics often lack functional groups that participate in
result it is a priority to identify replacements for lead azide directional and complementary intermolecular interactions
(Chart 1), a commonly used primary, that are easily synthesized, (such as hydrogen bonding) that encourage cocrystal formation.
inexpensive, thermally stable, and show competitive, if not For this reason, electrostatic potential maps have become a
improved, explosive performance compared to current heavy valuable tool for identifying molecules and/or functional groups
metal containing formulations.1,5,6 Primary energetics are likely to form favourable electrostatic interactions in systems
generally used to initiate the detonation of secondary energetics, where interaction modes between two distinct species are not
more powerful and less sensitive materials (e.g. HMX, RDX, TNT), evident.25,26
and must show sensitivity to outside stimuli such as impact or
heat. The vast majority of alternatives for lead azide have been
hampered by limitations in thermal stability,7,8 complicated
synthetic procedures, or safety concerns.3,9 Among newly
developed primaries are a copper-containing nitro-tetrazolate
salt (DBX-110,11), and the potassium salt dinitraminobistetrazolate
(K2DNABT),12 both shown in Chart 1. Potential hygroscopicity and
hydrate formation often plague salt-based energetics. A neutral
energetic compound identified as a potential replacement primary
is cyanuric triazide (CTA), also known as 2,4,6-triazidotriazine.
a Department of Chemistry, Macromolecular Science and Engineering Program,
University of Michigan, 930 North University Avenue, Ann Arbor,
MI 48109-1055, USA. E-mail: matzger@umich.edu
b Macromolecular Science and Engineering Program, University of Michigan,
930 North University Avenue, Ann Arbor, MI 48109-1055, USA
† Electronic supplementary information (ESI) available: Synthesis and character-
Chart 1 Molecular structures of some primary energetics and the secondary
energetic coformer benzotrifuroxan.
ization of the co-crystal. CCDC 1969834 and 1969835. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c9cc09465b
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Herein, electrostatic potential maps are used to identify an conformers, and not exclusively the equilibrium geometry, when
energetic coformer for CTA that leads to a CTA-based primary calculating electrostatic potential maps as guides to cocrystal design.
energetic cocrystal showing suppressed volatility and improved
Initial attempts to cocrystallize CTA and BTF were performed
detonation performance while maintaining high sensitivity to by fusion of the CTA melt in the presence of solid BTF, producing
impact. The hydrogen free secondary energetic benzotrifuroxan blade-like crystals that show a unique Raman spectrum from
(BTF) was selected as a cocrystallization candidate due to those of the pure coformers. The cocrystal is also accessible
previous reports that the material has relatively high sensitivity via solvent-mediated slurry conversion of a 1 : 1 mixture of CTA
to impact compared to other secondary energetics27,28 and has and BTF in a 50/50 by volume isooctane/toluene solution at room
previously been used to successfully form energetic cocrystals temperature, resulting in a bulk phase that is 2 : 1 CTA/BTF
with other coformers.22,29
cocrystal (excess BTF remains in the mother liquor). The room
In order to evaluate the likelihood that a cocrystal could temperature crystal structure was obtained using a crystal grown
form between these two compounds, electrostatic potential by fusion and a crystal grown from slurry was used to obtain the
maps were calculated and the regions of maximum electrostatic low temperature crystal structure.
potential (Vs,max) and minimum electrostatic potential (Vs,min
)
The cocrystal formed between CTA and BTF was analysed
were compared for BTF (Fig. 1a) and two conformers of CTA using single crystal X-ray diffraction at room temperature and
(Fig. 1b), the C3h conformation, exhibited in the crystal structure 100 K (ESI†). The cocrystal solves in the C2/c space group with one
of pure CTA,30,31 and the theoretical Cs conformer examined by molecule of CTA and a half molecule of BTF in the asymmetric
Rocha and coworkers.32 Both the Vs,min and Vs,max are calculated unit. CTA adopts the Cs conformer in contrast to the C3h conformer
to be greater in magnitude for the Cs conformer (À158.7 kJ molÀ1 present in the crystal structure of pure CTA. Each molecule of BTF
and 120.7 kJ molÀ1) than for the C3h conformer (À115.8 kJ molÀ1 is flanked by two molecules of CTA, interacting with opposite faces
and 114.7 kJ molÀ1), indicating that the CTA Cs is more of BTF through T-shaped interactions and forming trimers,
polarized and capable of forming more favourable intermole- consistent with electrostatic potential map analysis. The two
cular interactions. The calculated Vs,min and Vs,max values for T-shaped p-interactions for each molecule of BTF are related by
BTF are À87.6 kJ molÀ1 and 197.3 kJ molÀ1, suggesting that the a 2-fold axis that bisects BTF with a closest interaction distance
strongest intermolecular interactions would be between of 2.959 Å between an aromatic nitrogen of the triazine ring and
the most electron-rich site on CTA Cs and the most electron the centroid of the BTF molecule (Fig. 2a). The 2-fold axis also
deficient site on BTF. Additionally, an energy profile was forces disorder of BTF over two positions, which is well-defined
calculated to evaluate the accessibility of CTA Cs and to identify at low temperature and shows thermal ellipsoids elongated in
the transition state energy. Optimized geometries for CTA C3h
,
the plane of the BTF molecule at room temperature, suggesting
the transition state, and Cs were used to determine the rotational additional thermal motion. In both orientations of BTF in the
barrier and relative distribution between CTA C3h and Cs conformers. low temperature cocrystal structure, multiple close contacts are
The Cs conformer is 0.3 kcal molÀ1 higher in energy than the formed between the BTF furoxan rings and the CTA azido
C3h conformer and the interconversion barrier is approximately groups of neighbouring trimers (Fig. 2b) albeit at distances
11.7 kcal molÀ1, a barrier traversable at room temperature and that cannot be precisely defined due to disorder.
consistent with that determined by Rocha and coworkers.32 These
To determine the impact sensitivity of CTA and the 2 : 1 CTA/BTF
results emphasize the importance of considering all accessible cocrystal, small scale impact testing was deployed. In this method,
Fig. 1 (a) Electrostatic potential maps calculated using B3LYP/6-311G** for CTA conformers C3h and Cs, and the coformer BTF, with a shared scale
representing the surface potential in kJ molÀ1. (b) An energy profile was calculated using (B3LYP/6-311G(d)) for rotation around the C–N3 bond relating
the C3h and Cs conformers to identify the transition state geometry. The geometries for CTA C3h, the transition state, and Cs were optimized using
(MP2/6-311G(d,p)) and the rotational barrier was determined using those energies.
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Fig. 2 Packing of CTA and BTF in the 2 : 1 crystal structure. For clarity, the
disorder of BTF is not shown and only one occupancy is present. (a) View
of the Cs conformer of CTA interacting with the centroid in BTF from the
crystal structure. (b) View of a tape of BTF connected by interactions
between the furazan ring and the nitrogen atoms of the azide group.
2 mg of each sample were placed in a non-hermetic DSC pan, and a
5 lb weight was dropped onto the pan from premeasured distances
using our in-house apparatus (see ESI†).33 Over forty drops were
performed to determine the height at which there is a 50%
probability of detonation (Dh50).34 The drop height values were
compared to Pentaerythritol tetranitrate (PETN), a secondary
energetic that is commonly used to differentiate the impact
sensitivities of primary (Dh50 heights below PETN) and secondary
energetics (Dh50 heights above PETN). The impact sensitivity of
the cocrystal is 19 cm, less sensitive than CTA (Dh50 = 12 cm), yet
more sensitive than PETN (Dh50 = 21 cm);35 based on impact
sensitivity, CTA/BTF is a primary energetic‡ that is less sensitive
than CTA. This decrease in sensitivity is advantageous, because
the high sensitivity of CTA has been previously cited as a concern
in using CTA as a replacement for lead azide.14 The detonation
performance of the CTA/BTF cocrystal was evaluated and
compared to its constituents (Fig. 3 and ESI†). In addition to
having a detonation velocity and detonation pressure slightly
higher than CTA, the cocrystal shows a slight increase in density
relative to CTA (cocrystal: 1.737 g cmÀ3; CTA: 1.723 g cmÀ3 36).
The thermal behaviour of CTA/BTF was characterized using
differential scanning calorimetry (DSC) and thermogravimetric
analysis (TGA). The DSC traces indicate melting onset temperatures
of 94, 143, and 196 1C for CTA, the cocrystal, and BTF, respectively
(Fig. 4a and ESI†). The increased onset temperature for the
CTA/BTF melt event relative to CTA demonstrates the stabilizing
Fig. 4 (a) DSC thermograms showing the melting point and decomposition
behaviour of BTF, the cocrystal, and CTA (b) TGA traces of BTF, cocrystal, and
CTA. (c) Relative weight loss from sublimation of CTA and the cocrystal
measured using TGA (see ESI†).
impact of interactions present in the cocrystal; two decomposition
events are observed (B178 1C and 250 1C) which are similar to
those of pure CTA (B175 1C) and BTF (B243 1C) and not shifted
to significantly higher temperatures as has been recently observed
for some cocrystals.18,37 The TGA traces of CTA, the cocrystal, and
BTF also show weight loss in two events, the first one starting at a
temperature approximately 25 1C higher than that of CTA and the
second event occurring at a very similar temperature to BTF
(Fig. 4b). However, the steps do not correspond to clean loss of
individual components as evidenced by the observation of vibra-
tional bands associated with CTA in samples heated to 180 1C.
The propensity of CTA to sublime has hampered its application
and this property is evident in the TGA traces (open pan, Fig. 4b)
which show significant mass loss prior to the decomposition
temperature of CTA (closed pan DSC, Fig. 4a). To investigate
whether cocrystallization leads to a decrease in CTA volatility,
the rate of CTA mass loss was quantified through isothermal
TGA experiments with CTA and the CTA/BTF cocrystal. These
experiments were carried out in a hermetically-sealed aluminium
DSC pan with a hole (r D 330 mm) in the lid. The rate of mass loss
as a function of time, dm/dt, was determined at 85, 100, and 115 1C
(see ESI†). The rate of mass loss for the cocrystal relative to CTA
was 7.6, 7.1, and 5.3 times lower at 85, 100, and 115 1C respectively.
Thus, the volatility of CTA is suppressed significantly through
cocrystallization.
In summary, we present a primary energetic cocrystal
formed between CTA and BTF. Electrostatic potential maps
and an energy profile of both dominant conformers of CTA led
to the prediction that the electron rich area of CTA Cs would be
able to interact favourably with the electron deficient p region
of BTF. The resulting cocrystal has a much higher melting
point, significantly lower volatility, and less impact sensitivity
(while still qualifying as a primary) relative to CTA. This work
Fig. 3 Graph comparing the calculated detonation velocity (blue) and
detonation pressure (red) of CTA, BTF, and the cocrystal. Values were
predicted using the thermochemical code Cheetah 7.0.
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