Synthesis, Structure, and Characterization of 3, 4′-Bis-1H-1, 2,4-triazolium Picrate Salt: A New High-Energy Density Material

Article abstract of DOI:10.1002/zaac.201500788

The environmentally friendly high-energy density salt (TRTR)(PA) (TRTR = 3, 4′-bis-1, 2,4-1H-triazole, PA = 2, 4,6-trinitrophenol, picric acid) was synthesized and characterized. The X-ray single crystal diffraction results illustrate that the structure of title salt belongs to the monoclinic system, space group P2₁/c. Many parallel relationships exist in the molecule, as well as a strong intramolecular π-π stacking interaction. The DSC result shows only one exothermal decomposition step at 229.1 C. The TG-DTG curve demonstrates a 75.9 % mass loss from 180 C to 300 C at a rate of 3.01 %·K^-1. Experimental data show that the combustion heat approximately equals to TNT (-15.22 MJ·kg^-1) and the enthalpy of formation is +332.2 kJ·mol^-1. Non-isothermal kinetic and thermodynamic parameters were obtained by two methods (Kissinger and Ozawa). Detonation pressure and velocity were calculated to be 23.4 GPa and 7.32 km·s^-1, respectively. Additionally, the sensitivities towards impact and friction were assessed with relevant standard methods.

Full text of DOI:10.1002/zaac.201500788

Journal of Inorganic and General Chemistry
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Zeitschrift für anorganische und allgemeine Chemie
DOI: 10.1002/zaac.201500788
Synthesis, Structure, and Characterization of 3,4Ј-Bis-1H-1,2,4-triazolium
Picrate Salt: A New High-Energy Density Material
Zhenhao Mi,[a] Yan-Gang Bi,[a] Yong-An Feng,[a] and Tonglai Zhang*[a]
Keywords: Energetic salt; 3,4Ј-Bis-1H-1,2,4-triazole; Picric acid; Kinetics; Thermodynamics
Abstract. The environmentally friendly high-energy density salt 75.9% mass loss from 180 °C to 300 °C at a rate of 3.01%·K–1.
(TRTR)(PA) (TRTR = 3,4Ј-bis-1,2,4-1H-triazole, PA = 2,4,6-trinitro-
Experimental data show that the combustion heat approximately equals
phenol, picric acid) was synthesized and characterized. The X-ray to TNT (–15.22 MJ·kg–1) and the enthalpy of formation is
single crystal diffraction results illustrate that the structure of title salt
+332.2 kJ·mol–1. Non–isothermal kinetic and thermodynamic param-
belongs to the monoclinic system, space group P21/c. Many parallel
eters were obtained by two methods (Kissinger and Ozawa). Detona-
relationships exist in the molecule, as well as a strong intramolecular tion pressure and velocity were calculated to be 23.4 GPa and
π–π stacking interaction. The DSC result shows only one exothermal
7.32 km·s–1, respectively. Additionally, the sensitivities towards impact
decomposition step at 229.1 °C. The TG-DTG curve demonstrates a and friction were assessed with relevant standard methods.
azole is alkaline, so it is able to react with a series of acids and
Introduction
obtain corresponding energetic salts. Additionally, the element
Environmentally friendly explosives have gradually become
nitrogen in a triazole ring can be either anion or cation while
a highly regarded concept among researchers all over the
forming azole salts.[22]
world due to their heavy-metal-free property. Long-term use
of mercury fulminate and lead azide as primary explosives for
In our research, we have chosen 3,4-bis-1,2,4-1H-tri-
azole(TRTR) as cation and picric acid (PA) as anion to synthe-
both military and civilian areas has caused lots of environment
size a new energetic salt. PA is a rich oxygen atom acid with
damages.[1–3] Fortunately, they are being replaced by a batch
strong oxidation,[23] which make it very easy to react with sur-
of eco-friendly energetic materials. Lots of novel high-energy-
roundings.[24] This makes picric acid a versatile agent.[25]
density materials (HEDMs), such as CL-20, plastic bonded ex-
TRTR molecule can show different coordination modes, such
plosives various tetrazole-based salts, nitrated cubanes, furox-
as monodentate, bidentate, chelating and bridged, while
ans, and energetic metal organic frameworks (MOFs) were
forming monomeric, dimeric, or polymeric structures.[26]
successfully synthesized in past few decades.[4–8] They were
F. Guédira et al. from France has reported that protonation of
regarded as ideal candidates for a range of current applications
TRTR mainly occurs on one of the pyridine type nitrogen of
including explosives, pyrotechnics, and propellant formula-
the 4Ј-triazolyle groups.[27] The deprotonation involves an in-
tions since their practical performances.[9–15] The study on het-
creasing of potential active sites. The high content of heterocy-
erocyclic energetic salts comprising imidazole, triazole,
cles provides plenty of energy with the aming derivative.
tetrazole, and pentazole as precursors has attracted much atten-
Herein, we reported the synthesis, crystal structure, and ther-
tion in the past few years increasingly due to their high nitro-
mal decomposition of (TRTR)(PA), as well as its practical per-
gen content, high density, and high enthalpy of generation. Be-
formances.
sides, materials comprising structure of azole rings always
show a better oxygen balance property in plenty of practical
test.[14,16–19]
Results and Discussion
Among most of the energetic compounds, 1,2,4-triazole and
Description of the Structure
its derivatives are very promising because of both theoretical
and practical prospect. Shreeve, Klapötke, and others research-
ers have found that they have good physical and chemical per-
formances, as well as excellent environmental compatibility,
compared to other compounds.[20,21] Furthermore, 1,2,4-tri-
The molecular units are shown in Figure 1.
It can be seen that the (TRTR)(PA) molecule is built with a
3,4Ј-bis-1H-1,2,4-triazolium cation and a 2,4,6-trinitrophenol
anion under electrostatic force. The crystal structure the of title
salt belongs to the monoclinic system, space group P21/c with
crystal parameters of the unit cell dimensions a = 12.245(3) Å,
b = 9.215(2) Å, c = 12.892(4) Å, β = 106.770(4)°, V =
1392.8(6) Å3, Z = 4, ρ = 1.742 g·cm–3. It is obvious that a
deprotonation occurs on hydroxyl groups in the PA molecule,
* Prof. Dr. T. Zhang
Fax: +86-10-68911202
E-Mail: ztlbit@bit.edu.cn
[a] State Key Laboratory of Explosion Science and Technology
Beijing Institute of Technology
Beijing, 100081, P. R. China
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rings in each TRTR cation are distorted to 18°, parallel rela-
tionship between every two TRTR cation rings still exists (as
is shown in Figure 3). Plane 1 is parallel to plane 4, whereas
plane 2 is parallel to plane 3. Simultaneously, the plane dis-
tance from plane 2 to plane 3 is 3 Å. Similarly, PA anions in
one unit cell also perform a parallel relationship (Figure 4).
Figure 1. Molecular structure of (TRTR)(PA).
whereas the N2 atom in the triazole ring of the TRTR molecule
gains a proton. It is quite different from the pre-study that the
N4 atom of the tetrazole molecule always gets protonated,
while forming salts.[28] Selected bond lengths and angles of
(TRTR)(PA) molecule are summarized in Table 1. In the
TRTR cation, the C–N bond lengths range from 1.3035(15) Å
[N(1)–C(1)] to 1.456(15) Å [N(7)–C(6)] with an average value
of 1.367 Å, which is 0.097 Å longer than the normal C=N
bond length (1.270 Å) and 0.083 Å shorter than the normal
C–N bond length (1.450 Å). The average N–N bond length is
1.361 Å, longer than the normal N=N bond length of 1.252 Å
and shorter than the normal N–N bond length of 1.470 Å
(Figure 2)
Figure 3. Different viewing directions of TRTR cation.
Figure 2. Packing structure of (TRTR)(PA) along b axis of the unit
cell.
Figure 4. Different viewing directions of PA anion.
It is worth pointing out that there are four (TRTR)(PA) mol-
The distance between two parallel planes is 10 Å, which is
ecules lying in one unit cell in total. Although two triazole too far to make any interaction. However, each PA anion ring
Table 1. Selected bond lengths /Å and bond angles /° for (TRTR)(PA).
Bond
Bond
Bond
O1–C5
O2–N7
O3–N7
O4–N8
1.2611(14)
1.2283(14)
1.2301(14)
1.2222(17)
O5–N8
O6–N9
C5–C10
N1–C1
1.2262(16)
1.2355(15)
1.4404(16)
1.3035(16)
N4–C3
N5–N6
N6–C3
N7–C6
1.3297(15)
1.3563(15)
1.3127(16)
1.4560(14)
Angle
Angle
Angle
O2–N7–O3
O2–N7–C6
O3–N7–C6
O4–N8–O5
O4–N8–C8
O5–N8–C8
123.05(11)
118.95(10)
118.95(10)
123.18(12)
118.06(11)
118.73(11)
O6–N9–O7
O7–N9–C10
N7–C6–C5
C6–C7–C8
N8–C8–C7
N8–C8–C9
123.47(12)
119.03(11)
119.43(9)
118.35(10)
119.32(11)
119.25(11)
C7–C8–C9
C8–C9–C10
N9–C10–C5
C5–C10–C9
121.38(11)
119.27(11)
119.88(10)
124.06(11)
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is closer to another PA ring of the neighboring unit cell. The tioned two curves, (TRTR)(PA) energetic salt could decom-
plane distance between these two rings is as short as 3.5 Å, in pose completely and rapidly, at the same time, acting a high-
which π–π stacking interaction could make great effects so that energy-density performance.
the whole molecule is getting much more stable.
Furthermore, lots of intermolecular and intramolecular as
well as several unclassical C–H–O hydrogen bonds lie in the
molecule, which are extending the structure into a 3D super-
molecule and make an important contribution to enhance the
thermal stability of the salt (Figure 5). The hydrogen bonds
lengths and angles are summarized in Table 2. N2 of TRTR
cation mainly ties two hydrogen bonds to O1 and O7 of PA
anion, which can be described as intramolecular hydrogen
bonds. Simultaneously, N5 ties to O1 and O2 of the outer-
molecular PA anion.
Figure 5. Classical hydrogen bonds diagram of (TRTR)(PA).
Figure 6. DSC and TG-DTG curves in a nitrogen atmosphere with a
heating rate of 10 K·min–1 for (TRTR)(PA).
Table 2. Hydrogen bond lengths /Å and bond angles /° for (TRTR)
(PA).
D–H···A
d(D–H) d(H···A)
d(D···A)
Ͻ(D–H···A)
Heat of Combustion and Enthalpy of Formation
N2–H2N···O1
N2–H2N···O7
N5–H5N···O1
N5–H5N···O2
C1–H1···O2
C2–H2···O2
C4–H4···O4
0.89(2)
0.89(2)
0.89(2)
0.89(2)
0.95(2)
0.95(2)
0.95(2)
1.82(2)
2.34(2)
2.11(2)
2.49(2)
2.44(2)
2.40(2)
2.28(2)
2.6648(14)
2.9004(19)
2.9651(16)
3.0267(16)
3.3544(20)
3.3396(19)
3.1464(20)
157.2(17)
121.0(14)
160.4(18)
119.2(15)
162.7(11)
171.8(12)
150.7(11)
In order to study the combustion heat and the enthalpy of
formation of the title salt, constant-volume combustion heat
(Qv) of (TRTR)(PA) was measured by oxygen bomb calorime-
try, which might yield different results from the DSC studies
because DSC was recorded in a nitrogen atmosphere. The
average result of five times’ attempts was –14.3923 MJ·kg–1.
The combustion equation of (TRTR)(PA) is as follows:
C10H7N9O7 (s) + 33/4O2 (g) Ǟ 10CO2 (g) + 9/2N2 (g) + 7/2H2O (1)
Thermal Decomposition
The energy of combustion is as follows:
To assess the thermal stability of the title salt, the decompo-
sition was investigated by DSC and TGA measurements. The
DSC and TG-DTG curves of (TRTR)(PA) under linear heating
rate of 10 K·min–1 in a nitrogen atmosphere are shown in Fig-
ure 6. As can be seen from the DSC curve, there is only one
exothermic peak with the onset temperature of 213.5 °C and
the peak temperature of 229.1 °C but without any endothermic
progress, illustrating that the title salt has high energy property
and rapid exothermic process. Correspondingly, the TG-DTG
curve reveals that (TRTR)(PA) shows a rapid weight loss pro-
cess with 75.9% between 180–300 °C. It reaches the largest
rate of mass loss percentage of 3.01% per K at 227 °C. After-
ΔH = Qp = QV + ΔnRT = –14.4041 MJ·kg–1 = –5257.5 kJ·mol–1 (2)
The standard enthalpy of formation was calculated from the
combustion heat on the basis of Equation (1) and Hess’s law
as applied in thermochemical Equation (3). The enthalpies of
formation of carbon dioxide and water are –393.5 kJ·mol–1 and
–285.8 kJ·mol–1, respectively.[29] So, the standard enthalpy of
formation of (TRTR)(PA) can be calculated as:
ΔfH°298(s) = 7/2ΔfH°(H2O, l) + 10ΔfH°(CO2, g) – ΔcH°(s) =
+332.2 kJ·mol–1 (3)
Consequently, according to the literature,[30] the standard
wards, it continues losing weight from 300–500 °C but on a enthalpy of formation of (TRTR)(PA) is higher than that of
slower velocity. The final mass residue at 500 °C is only 6.1% TNT (–65.5 kJ·mol–1), RDX (+66.5 kJ·mol–1), and HMX
of the initial. Drawing from in-depth analysis of above-men- (+84.01 kJ·mol–1).
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ΔS϶ = R[lnA – ln(kBT / h)]
ΔG϶ = ΔH϶ – TΔS϶
(9)
Non-Isothermal Kinetic and Thermodynamic Parameters
(10)
The kinetic parameters of (TRTR)(PA) in thermal decompo-
sition were calculated according to the Kissinger and Oz-
awa[31,32] methods, which have been widely used in precedent
studies[32–34] to determine the apparent activation energy (Ea)
and the pre-exponential factor(A). The Kissinger equation and
Ozawa equation are as follows, respectively:
where kB is the Boltzmann constant (1.381ϫ10–23 J·K–1), and
h is the Planck constant (6.626ϫ10–34 J·s). The calculated re-
sults are listed in the Table 4. As is shown, the values of ΔH϶
are greater than zero, implying the activation reaction should
absorb energy from outside. On the contrary, the values of ΔS϶
are under zero, denoting the confusion degrees of the activated
compounds are lower than those of the reactant molecules.
Furthermore, the positive values of ΔG϶ indicate that the ther-
mal decompositions won’t occur spontaneously.
2
lnβ/Tp = ln(RA/Ea) - Ea/(RTp)
(4)
(5)
lgβ = lg[AEa/RG(α)] – 2.315 – 0.4567Ea/(RTp)
where Tp is the peak temperature in K, A is the pre-ex-
ponential factor in s–1, Ea is the apparent activation energy in
kJ·mol–1, R is the gas constant (8.314 J·K–1 mol–1), β is the
linear heating rate in K·min–1, and G(α) is the reaction mecha-
nism function.
Table 4. Thermodynamics of (TRTR)(PA).
Tp0 /°C Tb /°C
143.6 166.9
ΔS϶ /J·K–1·mol–1 ΔH϶ /kJ·mol–1
–206.32 65.545
ΔG϶ /kJ·mol–1
151.53
Both Kissinger’s and Ozawa’s method are applied to calcu-
late the kinetic parameters, which is based on the first exother-
mic peak temperatures measured at four different heating rates
of 5, 10, 15, and 20 K·min–1. Furthermore, the apparent acti-
vation energy Ea, pre-exponential factor A (not necessary in
Ozawa method),[34] linear coefficient R, and standard devia-
tions S were determined by the original experimental data and
is shown in Table 3. The apparent activation energies (Ea) ob-
tained by two methods are in substantial agreement. We take
the average value of two methods as a reasonable result. Con-
sequently, the Arrhenius Equation could be expressed as fol-
lows:
Explosive Properties
For ignition safety testing, the impact and friction sensitivi-
ties[37] were determined by following BAM standard meth-
ods.[38,39] The title compound didn’t explode under the drop
hammer (impact sensitivity of 40 J) and also misfired at the
maximum BAM friction testing condition (360 N). In contrast,
PA itself is a sensitive material both to impact and friction.[40]
So the salt (TRTR)(PA) can be described as “insensitive”
(according to the “UN Recommendations on the Transport of
Dangerous Goods”[41]).
lnk = 4.976 – 71.25ϫ103/(RT)
Since (TRTR)(PA) is a totally C-H-O-N composition ex-
plosive, the detonation parameters can be predicted following
the well-known Kamlet-Jacobs method.[42] The title compound
was calculated to have a detonation pressure of 23.4 GPa and
velocity of 7.32 km·s–1, which is approximately equal to TNT
(7.37 km·s–1, 23.5 GPa).[43]
According to our test, the title salt was insensitive to all of
impact, friction, falling into the group of compounds described
as “low-sensitivity-materials”.
Table 3. Peak temperatures of the exothermal peaks at different heat-
ing rates and the chemical kinetics parameters.
β /K·min–1 Tp /K
Parameter
Kissinger’s
method
Ozawa’s
method
5
206.8
229.1
233.8
244.2
Ea /kJ·mol–1
lgA
R
69.01
4.976
–0.983
0.121
73.48
–
–0.987
0.005
10
15
20
S
The characteristic temperatures of thermal decomposition
are important indicators of the thermal safety of explosive,
such as Tp (the peak temperature of thermal decomposition)
and Tb (the critical temperature of thermal explosion). The cal-
culation methods of Tp and Tb using DSC data are as fol-
lows:[35]
Conclusions
From experimental investigation of the energetic compound
(TRTR)(PA), the following conclusions can be drawn.
An eco-friendly energetic salt, which is lead-free and
mercury-free, and totally composed by C-H-O-N, was synthe-
sized by reaction of the picrate anion and the 3,4Ј-bis-1H-
1,2,4-triazolo cation, and also characterized by single-crystal
X-ray diffraction, as well as IR spectroscopy. The structure of
(TRTR)(PA), which crystallizes in the monoclinic space group
P21/c, was stabilized by a variety of hydrogen bonds, as well
as a great force of intramolecular π–π stacking interaction.
Thermal analysis curves reveal that the decomposition of
(TRTR)(PA) starts at 213 °C with a sharp exothermic peak at
229 °C. The thermal parameters Ea, lnA, R, and S were ob-
tained by two methods and the Arrhenius equation of (TRTR)
(PA) can be expressed as: lnk = 4.976–71.25ϫ103/(RT).
Tp = Tp0 + bβ + cβ2
(6)
(7)
2
E(Tb – T0) – RTb = 0
where b and c are coefficients, Tp0 is the peak temperature
when β tends to zero (K), T0 is the onset temperature from the
non-isothermal DSC curves.
The thermodynamic parameters including the entropy of ac-
tivation (ΔS϶), enthalpy of activation (ΔH϶), and free energy
of activation (ΔG϶) were calculated based on the DSC data
according to the following equations:[36]
ΔH϶ = Ea – RT
(8)
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data collection was performed with a Rigaku AFC-10/Saturn 724+CCD
Furthermore, the critical temperature of thermal explosion,
ΔS, ΔH, and ΔG were calculated out as 166.9 °C,
–206.32 J·K–1·mol–1, 65.545 kJ·mol–1, 151.53 kJ·mol–1,
detector diffractometer with graphite monochromated Mo-Kα radiation
(λ = 0.071073 nm) with φ and ω modes at 103(2) K. The structure was
solved by direct methods of using SHELXS-97[45] and refined by full-
matrix least-squares methods on F2 with SHELXL-97 program.[46] All
non-hydrogen atoms were obtained from the different Fourier maps
and subjected to anisotropic refinement by full-matrix least-squares on
F2. Detailed information concerning crystallographic data collection
and structure refinement are summarized in Table 5.
respectively. The energy of combustion was –14.40 MJ·kg–1
and enthalpy of formation +332.2 kJ·mol–1. The detonation
pressure and velocity were calculated to be 23.4 GPa and
7.32 km·s–1. Besides, the title salt was insensitive to impact
and friction.
(TRTR)(PA) shows quite stable thermal properties due to its
special and compact molecular structure. At the same time,
the large amount of heterocyclic structure and high nitrogen
containing lead to a high-energy performance. The positive
enthalpy of formation and insensitivity make (TRTR)(PA) a
prospective candidate of future energetic materials, whose
main requirements are high energy and low sensitivity.
Table 5. Crystal data and structure of (TRTR)PA.
(TRTR)PA
Empirical formula
Formula weight
Crystal system
Space group
a /Å
C10H7N9O7
365.25
monoclinic
P21/c
12.245(3)
b /Å
9.215(2)
c /Å
β /°
12.892(4)
106.770(4)
1392.8(6)
4
1.742
0.150
744
0.26ϫ0.29ϫ0.41
0.71073
2.8 to 31.5
–17 to 15,–8 to 13,–18 to 18
13367
4584 [Rint = 0.0236,Rsigm a =
0.0290]
Experimental Section
Volume /Å3
Z
Materials and Physical Techniques: All solvents and reagents were
commercially obtained and of analytical grade without further purifica-
tion. The FT-IR spectra were recorded with a Bruker Equinox
55 infrared spectrometer (KBr pellets) in the range of 4000- 400 cm–1
with a resolution of 4 cm–1. DSC and TG-DTG measurements were
performed with Pyris-1 differential scanning calorimeter and Pyris-1
thermogravimetric analyzer (Perkin–Elmer, USA) in a dry nitrogen gas
atmosphere with flowing rate of 20 ml·min–1. The combustion heat
was measured by oxygen bomb calorimeter (Parr 6200, USA) with a
6510 water handling system. The calorimeter was calibrated by the
benzoic acid in an oxygen atmosphere at a pressure of 3.05 MPa. The
testing samples (about 200 mg) were placed in the combustion pots
and burned in a 3.05 MPa pure oxygen atmosphere. The result was
calculated as an average of five single measurements. The impact sen-
sitivity was determined by Fall Hammer Method using BAM FALL
HAMMER BFH-10, and the friction sensitivity was determined with
a BAM FRICTION APPARATUS FSKM-10 by following a BAM
standard.
ρcal /g·cm–3
μ /mm–1
F(000)
Crystal size /mm3
Radiation Mo-Kα (λ)
2θ range /°
h,k,l
Reflections collected
Independent reflections
s
1.002
R, wR2 [I Ն 2σ (I)]a)
R, wR2 [all data]a)
Largest diff. peak/hole /e·Å–3
R1 = 0.0449,wR2 = 0.1205
R1 = 0.0570,wR2 = 0.1294
0.367, –0.236
2
2 2
2
2
2
a) wR2
=
[∑w(Fo –Fc ) /∑w(Fo )]1/2
; P = (Fo +2Fc )/3; w =
2
1/[σ2(Fo ) + (0.0510P)2 + 0.1600P].
Crystallographic data (excluding structure factors) for the structure in
this paper have been deposited with the Cambridge Crystallographic
Data Centre, CCDC, 12 Union Road, Cambridge CB21EZ, UK.
Copies of the data can be obtained free of charge on quoting the de-
pository number CCDC-1016458 (Fax: +44-1223-336-033; E-Mail:
deposit@ccdc.cam.ac.uk, http://www.ccdc.cam.ac.uk).
Synthesis of TRTR: TRTR was prepared according to the method
reported by Bhargavi et al.[44] (Scheme 1).
Acknowledgements
Scheme 1. Synthesis of TRTR.
The authors acknowledge financial support from the State Key
Laboratory of Explosion and Technology (No. QNKT12–02),
Science and Applied Physical Chemistry Laboratory (No.
9140C370303120C37142). The authors are truly grateful to Dr. Sitong
Chen and Ms. Shanshan Qu for many helpful discussions.
Synthesis of (TRTR)(PA): PA (2.29 g, 10 mmol) was added to a solu-
tion, which contained TRTR (12 mmol) in deionized water (100 ml).
The mixture of TRTR and PA was heated to 80 °C and stirred for 60
min. While cooled to room temperature, the product of the reaction
was collected into a beaker by filtration and a yellow solution was
obtained. Yellow crystals of (TRTR)(PA) were formed easily, which
were finally dried in an explosion-proof water-bath dryer. The experi-
mental yield was up to 85%. FT-IR (KBr) ν˜ = 3383 (s), 3483 (s),
3383 (s), 3148 (s), 2933 (w), 1626 (s), 1565 (s), 1453 (w), 1373 (s),
1337 (s), 1296 (s), 1173 (s), 1090 (m), 1056 (s), 971 (m), 876 (w),
791 (m), 704 (m), 630 (m) cm–1.
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Received: November 25, 2015
Published Online: February 2, 2016
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