Energetic Di-and trinitromethylpyridines: Synthesis and characterization

Article abstract of DOI:10.3390/molecules23010002

Pyridine derivatives based on the addition of trinitromethyl functional groups were synthesized by the reaction of N₂O₄ with the corresponding pyridinecarboxaldoximes, then they were converted into dinitromethylide hydrazinium salts. These energetic compounds were fully characterized by IR and NMR spectroscopy, elemental analysis, differential scanning calorimetry (DSC), and X-ray crystallography. These pyridine derivatives have good densities, positive enthalpies of formation, and acceptable sensitivity values. Theoretical calculations carried out using Gaussian 03 and EXPLO5 programs demonstrated good to excellent detonation velocities and pressures. Each of these compounds is superior in performance to TNT, while 2,6-bis(trinitromethyl)pyridine (D = 8700 m·s^?1, P = 33.2 GPa) shows comparable detonation performance to that of RDX, but its thermal stability is too low, making it inferior to RDX.

Full text of DOI:10.3390/molecules23010002

molecules
Article
Energetic Di- and Trinitromethylpyridines: Synthesis
and Characterization
ID
Yiying Zhang, Xiaoyu Sun, Shannan Yu, Lingxiang Bao, Chenghui Sun *
and Siping Pang *
School of Material Science & Engineering, Beijing Institute of Technology, Beijing 100081, China;
zhangyy989312@126.com (Y.Z.); sxy1204254800@126.com (X.S.); nandyu@163.com (S.Y.); blx823@126.com (L.B.)
*
Correspondence: sunch@bit.edu.cn (C.S.); pangsp@bit.edu.cn (S.P.); Tel.: +86-010-6891-8049 (C.S.);
86-010-6891-3038 (S.P.)
+
Received: 6 November 2017; Accepted: 18 December 2017; Published: 21 December 2017
Abstract: Pyridine derivatives based on the addition of trinitromethyl functional groups were
synthesized by the reaction of N O with the corresponding pyridinecarboxaldoximes, then they
2
4
were converted into dinitromethylide hydrazinium salts. These energetic compounds were fully
characterized by IR and NMR spectroscopy, elemental analysis, differential scanning calorimetry
(
DSC), and X-ray crystallography. These pyridine derivatives have good densities, positive enthalpies
of formation, and acceptable sensitivity values. Theoretical calculations carried out using Gaussian
3 and EXPLO5 programs demonstrated good to excellent detonation velocities and pressures.
Each of these compounds is superior in performance to TNT, while 2,6-bis(trinitromethyl)pyridine
0
−
1
(
D = 8700 m·s , P = 33.2 GPa) shows comparable detonation performance to that of RDX, but its
thermal stability is too low, making it inferior to RDX.
Keywords: energetic materials; trinitromethyl; dinitromethyl; pyridine; detonation properties
1
. Introduction
There are currently ongoing efforts to synthesize novel energetic materials with improved
performance and decreased sensitivity [1–6]. However, these requirements are quite often mutually
exclusive, such that the desired combination of high detonation performance with maximum possible
chemical stability is a major challenge. Over the last few decades, several strategies for the design of
energetic materials that have a high energy content in conjunction with good thermal and mechanical
stabilities have been developed [
heterocycles as fundamental components of molecular structures [
ring, nitrogen-containing heterocycles typically possess a greater enthalpy of formation as well as a
higher density and more positive oxygen balance [10 11], all of which are desirable characteristics
7,
8
], one of which is to replace benzene rings with nitrogen-containing
2,9]. Compared with the benzene
,
for new energetic compounds. In recent years, the application of azine as a new energetic skeleton
has attracted considerable interest, and many polynitro-azine derivatives have been designed and
synthesized [12]. However, due to the electron deficient character of the azine ring, it is difficult to
perform electrophilic aromatic substitution on the ring, so the introduction of nitro groups into azine
rings remains a challenging task in practice.
Nitro-based moieties represent the most common energetic functional group, and contribute
markedly to the overall performance of an energetic compound by enhancing both the oxygen balance
(
OB) and density [13
content, and its introduction to a molecule is essentially equivalent to adding two nitro groups
since one of the nitro groups is necessary for the complete oxidation of the carbon atom in the methyl
–17]. Among these groups, the trinitromethyl group has the highest oxygen
(
group). The incorporation of trinitromethyl moieties into an azine ring is thus a potential means of
increasing the OB so as to improve the detonation performance, and some trinitromethylazines have
been synthesized using various methods. As an example, 2,4,6-tris(trinitromethyl)-1,3,5-triazine
Molecules 2018, 23, 2; doi:10.3390/molecules23010002
www.mdpi.com/journal/molecules

Molecules 2018, 23, 2
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was obtained by the destructive nitration of the carboxyl groups to trinitromethyl entities in
2
,4,6-tris(dicarboxymethylene)-1,3,5-triazine by action of concentrated HNO [18]. This compound is
3
3
an oxidant and has a high density (2.00 g/cm ), but it is unstable in the air and very easy to hydrolyze.
Trinitromethyl-substituted pyrazine was also obtained as an unexpected product by the treatment
of 2-alkoxy-3-methylpyrazine with an HNO /H SO mixture [19]. The attachment of trinitromethyl
3
2
4
groups with a pyridine ring has also been attempted by different techniques. Gakh and Khutoretskii
reported the synthesis of 2-trinitromethylpyridine by the reaction of N-fluoropyridinium fluoride with
trinitromethane [20]. Pyridine with methyl groups in the 2, 3, and 4 positions behaved in the same
way and the corresponding trinitromethyl-substituted methylpyridines were obtained. Subsequently,
they also studied the behavior of this series of trinitromethylpyridines upon electron impact [21].
Another synthesis method for 2-trinitromethylpyridine was reported in 1993 by the ozone-mediated
C-nitration of methylpyridines with nitrogen dioxide [22]. More recently, Katritzky et al. obtained
5-methyl-2-trinitromethylpyridine on the nitration of 2,5-lutidine with nitric acid in trifluoroacetic
anhydride [23]. However, these methods shared an obvious disadvantage in that the yields were rather
low (below 20%).
In contrast, the synthesis route of 2-trinitromethylpyridine through the reaction of N O
2
4
with pyridine-2-carboxaldoxime, which was developed at the N.D. Zelinsky Institute of Organic
Chemistry, appeared to be a favorable method for high yield (more than 60%) [24]. In fact, a series
of trinitromethyl-substituted arenes and hetarenes have been prepared through the interaction of
N O with corresponding carboxaldehyde oximes [25,26]. Although tris(trinitromethyl)benzene
2
4
has been synthesized by the reaction [25], to the best of our knowledge, heterocyclic compounds
with more than one trinitromethyl group synthesized in this way have not yet been reported.
Hence, introducing more trinitromethyl groups into the pyridine ring becomes an attractive but
challenging objective, as it requires overcoming electronic and stereo-hindrance effects (Figure 1).
At the same time, in consideration of the well-known instability of the trinitromethyl moiety at
higher temperatures, the ionization of trinitromethyl-substituted pyridines is also worth exploring, as
ionic compounds usually display better stabilities than their nonionic precursors [27]. An interesting
denitration reaction of trinitromethyl groups into dinitromethyl groups in heterocycle substrates
has been studied. For example, The denitration of 2-trinitromethylpyridine with potassium iodide
in methanol allowed for the obtainment of the potassium salt of 2-(dinitromethyl)pyridine [24].
The reaction of mono(trinitromethyl)-1,3,5-triazines with potassium iodide in methanol resulted
in dinitromethyl-1,3,5-triazine potassium salts [28]. Additionally, the conversion of trinitromethyl
groups attached to imidazoles, triazoles, and tetrazoles into the corresponding dinitromethylide salts
following treatment with hydrazine has been reported [4,29,30]. Based on these successful examples,
the trinitromethyl-substituted pyridines in our research were subject to attempted denitration
using hydrazine.
Figure 1. Representative trinitromethylazines/trinitromethylbenzenes and bis(trinitromethyl)-
substituted pyridine.
In our continuing search for new high-performance polynitro energetic materials, we are
interested in developing novel trinitromethyl-substituted heterocyclic compounds. Thus, in the
present study, we attempted to introduce more trinitromethyl groups onto the pyridine ring to increase

Molecules 2018, 23, 2
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energetic performance. These trinitromethyl-substituted pyridines were subsequently converted into
dinitromethylide hydrazinium salts. Each of these compounds was fully characterized and their
energetic properties were determined either empirically or theoretically.
2
. Results and Discussion
2
.1. Syntheses
As was stated in the introduction, 2-trinitromethylpyridine (1) was synthesized through the
reaction of N O with pyridine-2-carboxaldoxime (Scheme 1). Based on a method reported in literature,
2
4
a simpler procedure was employed. N O was added into a solution of pyridine-2-carboxaldoxime
2
4
at room temperature and the purification of products was carried out by column chromatography
in a Biotage Isolera One apparatus. The reaction mechanism has been given in the literature earlier
(Scheme 2) [25
,31]. The preparation of bis(trinitromethyl)-substituted pyridine was somewhat more
challenging. Firstly, compared with the preparation of 1, the reaction of pyridine-2,6-dicarboxaldehyde
◦
◦
dioxime with N O required a higher reaction temperature (60 C rather than 40 C). In addition,
2
4
X-ray diffraction (XRD) data indicated that the reaction yielded two main products: the desired
,6-bis(trinitromethyl)pyridine ( ) and the byproduct 2-cyano-6-(trinitromethyl)pyridine ( ). It was
found that increasing the N O4 concentration (within the range of 0.5-2.5 mL N O /mmol
2
2
3
2
2
4
pyridine-2,6-dicarboxaldehyde dioxime) increased the relative proportion of 2 in the product mixture.
The formation of the by-product 3 is also explained based on literature reports (Scheme 3) [32]. Firstly,
the intermediate nitrolic acid underwent the loss of a molecule of nitrous acid and the nitrile oxide
was obtained. In the second step, nitrogen monoxide removed an oxygen atom with the formation of
nitrile and nitrogen dioxide.
Scheme 1. The synthetic routes of 1–3.
Scheme 2. The reaction mechanism for the synthesis of 2-trinitromethylpyridine.

Molecules 2018, 23, 2
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Scheme 3. The reaction mechanism for the formation of the cyano-compound.
Following the preparation of the above compounds, the ionization of 1 and 2 was attempted.
Based on reports, the analogous denitration of trinitromethyl-substituted pyridines using hydrazine
was examined in our research. The trinitromethyl-substituted pyridine 1 was first reacted with 80%
aqueous hydrazine, and the light yellow hydrazinium salt of 2-dinitromethylpyridine (
in high yield (Scheme 4). Subsequently, 2,6-bis(trinitromethyl)pyridine was also successfully converted
into the dihydrazinium salt of 2,6-bis(dinitromethyl)pyridine ( ). To the best of our knowledge, this is
4) was obtained
5
the first report of the preparation of a dihydrazinium salt in this manner.
Scheme 4. The syntheses of dinitromethylpyridine salts 4 and 5.
All the compounds prepared in this work were stable in air and could be stored for extended
periods of time. Their structures were confirmed by infrared (IR) and nuclear magnetic resonance
(NMR) spectroscopy, mass spectrometry (MS), and elemental analysis. Compounds 1, 2, 3, 4 and 5
were also characterized by X-ray crystallography. Selected data and X-ray structural parameters are
provided in the Extended Supplemental Information.
2
.2. X-ray Crystallography
Single crystals of
of dichloromethane and n-hexane solutions of these compounds at room temperature, whereas crystals
of and were obtained from methanol and water, respectively. All crystals were stable at room
1, 2 and 3 suitable for single-crystal XRD were obtained by the slow evaporation
4
5
temperature and were found not to be hygroscopic. Their molecular structures are shown in Figures 2–6
and the crystallographic data are summarized in Table 1.
(
a)
(b)
Figure 2. (a) Molecular structure of 1; (b) packing diagram of 1 viewed down the a-axis.

Molecules 2018, 23, 2
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(
a)
(b)
Figure 3. (a) Molecular structure of 2; (b) packing diagram of 2 viewed down the a-axis.
(
a)
(b)
Figure 4. (a) Molecular structure of 3; (b) packing diagram of 3 viewed down the b-axis.
(
a)
(b)
Figure 5. (a) Molecular structure of 4; (b) packing diagram of 4 viewed down the b-axis.
(
a)
(b)
Figure 6.
(a) Molecular structure of 5. The disordered hydrazinium cations were partly deleted for
clarity; (b) packing diagram of 5 viewed down the c-axis.

Molecules 2018, 23, 2
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Table 1. Crystallographic data for 1, 2, 3, 4 and 5.
1
2
3
4
5
CCDC
1,582,435
1,583,118
1,582,533
1,582,458
1,583,040
Empirical
formula
C H N O
C H N O
C H N O
C H N O
C H N O
7 13 9
6
4
4
6
7
3
7
12
7
3
5
6
6
9
5
4
8
Formula mass
Temperature/K
Crystal system
Space group
a/Å
228.12
153(2)
377.14
105
253.13
126
215.17
153(2)
351.26
153(2)
monoclinic
triclinic
monoclinic
monoclinic
orthorhombic
Pbcn
P2 /c
1
P1
P2 /c
1
P2 /n
1
8.0672(16)
10.496(2)
11.234(2)
90.00
7.5655(3)
11.7552(4)
31.9329(11)
79.435(3)
87.936(3)
78.473(3)
8
11.2611(5)
10.6182(4)
8.2766(3)
90.00
11.672(2)
6.5976(13)
12.193(2)
90.00
27.413(6)
7.2092(14)
6.6259(13)
90.00
b/Å
c/Å
◦
α/
◦
β/
109.66(3)
90.00
95.406(3)
90.00
109.73(3)
90.00
90.00
◦
γ/
90.00
Z
4
4
4
4
Volume/Å3
895.7(3)
1.692
2735.47(17)
1.832
985.26(7)
1.707
883.8(3)
1.617
1309.4(4)
1.782
−
3
$
/g·cm
calc
−
1
µ/mm
0.154
0.178
0.152
0.137
0.160
F(000)
464
1520
512
448
728
Crystal
size/mm
0
.13 × 0.12 × 0.08 0.50 × 0.15 × 0.14 0.40 × 0.35 × 0.30 0.15 × 0.13 × 0.10 0.16 × 0.14 × 0.03
3
◦
θ range/
3.31–27.49
3.04–26.00
3.13–26.00
2.97–27.50
2.92–27.48
−
10 ≤ h ≤ 10;
−9 ≤ h ≤ 9;
−14 ≤ k ≤ 14;
−32 ≤ l ≤ 39
−13 ≤ h ≤ 9;
−13 ≤ k ≤ 12;
−10 ≤ l ≤ 10
−14 ≤ h ≤ 15;
−8 ≤ k ≤ 8;
−15 ≤ l ≤ 15
−35 ≤ h ≤ 35;
−9 ≤ k ≤ 6;
−7 ≤ l ≤ 8
Index ranges
−13 ≤ k ≤ 13;
−
13 ≤ l ≤ 14
Reflections
collected
6
328
24085
10754
1.077
4005
1927
1.073
5836
2007
1.122
5938
1498
1.157
Independent
reflections
2
028
Goodness-of-fit
1
.203
2
on F
Final R indexes
R1 = 0.0556;
wR2 = 0.1121
R1 = 0.0552;
wR2 = 0.1422
R1 = 0.0339;
wR2 = 0.0785
R1 = 0.0456;
wR2 = 0.1158
R1 = 0.0865;
wR2 = 0.2420
[
I > 2σ(I) i.e., Fo
>
4σ(Fo)]
Final R indexes
all data]
R1 = 0.0633;
wR2 = 0.1171
R1 = 0.0607;
wR2 = 0.1481
R1 = 0.0397;
wR2 = 0.0820
R1 = 0.0477;
wR2 = 0.1175
R1 = 0.0920;
wR2 = 0.2500
[
Compound
and has a calculated density of 1.692 g
1 crystallizes in the monoclinic space group P2 /c with four molecules per unit cell
1
−
3
·
cm at 153 K (Figure 2). The bond lengths and bond angles in
the pyridine ring of this compound are in the typical ranges [33], although the C6–N2, C6–N3, and
C6–N4 bond lengths in the trinitromethyl group are 1.528, 1.538, and 1.524 Å, respectively, all of which
are longer than a standard C–N single bond (1.460 Å) [34]. The C6 atom is in the plane of the pyridine
ring and is associated with the torsion angles C1–N1–C5–C6 =
The pyridine ring and three nitro groups are attached to C6 in a tetragonal configuration.
Compound crystallizes in the triclinic space group P with eight molecules per unit cell and
a calculated density of 1.832 g cm at 105 K (Figure 3). Compared to , its density is significantly
increased because of the introduction of the second trinitromethyl group. Similar to , the C–N bond
◦
◦
179.68 .
−
179.47 and C3–C4–C5–C6 =
−
2
1
−
3
·
1
1
lengths in the trinitromethyl groups are longer than normal C–N single bonds. The large volume
of the two trinitromethyl groups makes the region beside the pyridine N atom quite crowded and
they stretch in an optimal conformation, trying to decrease the repulsive strain. The N1, N2, N6

Molecules 2018, 23, 2
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and N7 atoms are distributed in an approximately symmetric manner on both sides of the pyridine
ring. The N5 atom and the pyridine ring are almost coplanar, as can be seen from the N4–C6–C7–N5
◦
◦
torsion angle (
torsion angle (
−
3.65 ), while the N3 is slightly twisted out of this plane based on the N4–C2–C1–N3
−
15.7 ). However, it is evident that a repulsive strain is still present, as demonstrated
◦
◦
by a comparison of bond angles; the N4–C2–C1 and N4–C6–C7 bond angles are 114.7 and 113.8 ,
respectively, while the corresponding bond angle in compound 1 is 111.9 .
◦
Compound 4 crystallizes in a monoclinic crystal system with space group P2 /n and a density
1
−
3
of 1.617 g
·
cm at 153 K (Figure 5). The C–N and C–C bond lengths of the pyridine ring are in the
, the C1–C2 bond length (1.484 Å) is slightly shorter. In addition, the
expected ranges. Compared to
1
C1–N1 and C1–N2 bond lengths of the dinitromethyl group are 1.362 Å and 1.391 Å, respectively, and
thus are both much shorter than the analogous bonds in 1 and normal C–N single bonds. This can be
explained by the change in the electron density distribution after the removal of a nitro substituent of
the trinitromethyl group. Compared with the earlier reported crystal structures of several zwitterionic
dinitromethylazine derivatives in which the protonation occurred at one of the nitrogens of the azine
+
ring [35
,
36], the only nitrogen atom in the pyridine ring was not protonated, while the N H cation
2
5
was formed, which was the same situation with the analogical reactions in earlier literature [
4,29,30].
Interestingly, the remaining atoms of the dinitromethyl group are inclined to form a plane with the
◦
◦
torsion angles O4–N2–C1–N1 = 171.24 and O2–N1–C1–N2 = 3.7 . The two planes defined by the
◦
dinitromethyl group and the pyridine ring are almost perpendicular (N3–C2–C1–N2 = 94.53 and
◦
C3–C2–C1–N1 = 95.42 ).
Compound
density of 1.782 g
cations was observed in the crystal. Similar to compound
dinitromethyl groups as well as the C–NO bonds become shorter than its parent compound
5 crystallizes in an orthorhombic crystal systemspace group Pbcn with a calculated
−
3
·
cm at 153 K and four molecules in a unit cell (Figure 6). Disorder of hydrazinium
, the bonds connecting the pyridine and
and the
protonation sites are also located in the nitrogen atoms of hydrazine. However, the remaining atoms
of each dinitromethyl group in compound are not inclined to be coplanar with the torsion angles
O1–N1–C1–N2 and O8–N5–C7–N5 = 160.70 ; O3–N2–C1–N1 and O6–N4–C7–N5 = 9.07 ), which is
different from compound 4.
4
2
2
5
◦
◦
(
−
2
.3. Physicochemical and Energetic Properties
The physicochemical and energetic properties of each of the compounds are summarized in
Table 2.
Table 2. Physical properties of trinitromethyl derivatives of pyridine.
c
d
D e
P f
IS g
FS h
OB i
Compd
Tm a
Td b
$
4 HM
f
1
2
3
70
96
68
-
114
101
106
181
125
170
230
295
◦
1.62/1.69 j
1.77/1.83
1.62/1.71
1.56/1.62
160.65
258.96
314.22
151.32
150.80
-
92.6
−59.4
7678
8700
7683
7438
8383
8369
8795
7303
24.2
33.2
23.9
19.6
27.7
-
16
9
27
>360
192
>360
>360
>360
160
−56.11
−14.85
−60.04
−92.94
−56.94
−27.8
j
j
j
4
5
14
j
-
1.71/1.78
11.5
1.5–3.0
7.4
k
TNPyO
RDX
TNT
170
204
81
1.86
1.80
1.65
l
34.9
21.3
120
>353
−21.62
−74.0
l
15
a
b
◦
c
Melting point (onset) ( C); Decomposition temperature (onset) ( C); Density measured by a gas pycnometer
◦
−3
g
d
−1
e
−1
f
at 25 C (g
·
cm ); Calculated enthalpy of formation (KJ
·
mol ); Detonation velocity (m
·
s
); Detonation
pressure (GPa); Impact sensitivity (J); h Friction sensitivity (N); Oxygen balance assuming the formation of CO
i
2
j
at combustion (for C
a
H O
c
N , OB = 1600 (c
−
2a
−
b/2)/MW, MW = molecular weight of compound) (%); Crystal
b
d
−
4
density at 298 K, recalculated from low-temperature X-ray densities, $298K
=
$T/(1 + αv(298
−
T
0
)) αv = 1.5
×
10
,
T0 is the crystal testing temperature (g·cm−3); Ref. [37]; l Ref. [38].
k
In this work, the thermal stabilities of all compounds were determined by differential scanning
◦
−1
calorimetry (DSC) measurements scanning at 5 C
·
min under dry, oxygen-free nitrogen over the
◦
◦
temperature range 25–500 C. The melting points of compounds
1-
3
were in the range 68–96 C,

Molecules 2018, 23, 2
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◦
while their decomposition temperatures ranged from 101 to 114 C. Both
1
and
3
were found to
have lower melting points than TNT. These two compounds were also observed to have very similar
melting points, but the latter had a significantly lower decomposition point, demonstrating that the
introduction of a cyano group has negative effects on the thermal behavior of 1. The introduction of the
second trinitromethyl in the case of
stability becomes worse with a decomposition temperature of 101 C. The relatively low decomposition
2
significantly increases the melting temperature, but its thermal
◦
points of
1, 2 and 3 can be attributed to the well-known instability of the trinitromethyl group at higher
temperatures. However, when the trinitromethyl group underwent the loss of a nitro group and was
converted to a dinitromethylide salt, salts
parent compounds, with decomposition temperatures of 181 and 125 C, respectively.
4
and
5
displayed much better thermal stability than their
◦
The sensitivities of these compounds towards impact (IS) and friction (FS) were determined
according to the standard BAM methods [39]. Compound
1 (IS = 16 J, FS > 360 N) has impact and
friction sensitivities similar to those of TNT. As a result of the introduction of the second trinitromethyl
group, 2 (IS = 9 J, FS = 192 N) is more sensitive than 1 but still less sensitive than RDX (IS = 7.4 J,
FS = 120 N). As noted, the formation of salts is often an effective means of reducing sensitivity [40].
However, salt 4 possesses similar sensitivity to its parent compound
1, and is even slightly more
sensitive regarding impact sensitivity. In contrast, salt
with regard to FS.
5
is significantly less sensitive than , especially
2
OB plays an important role in determining the detonation properties of energetic
compounds. Comparing the OB values of has the highest value of 14.85%, exceeding
that of 2,4,6-trinitropyridine-1-oxide (TNPyO), which has the best OB ( 27.8%) among the
previously-reported energetic pyridine derivatives. The OB values of the hydrazinium salts are
1-
5,
2
−
−
lower due to the addition of cations and the loss of nitro groups, although salt 5 still has a good OB
comparable to that of 1.
Density is another important physical property of energetic materials. The densities of all the
◦
compounds were measured using a gas pycnometer at 25 C, and were found to be in the range of 1.56
−
3
(4
)–1.77 g
packing, the densities of these compounds increase in proportion to the accumulation of trinitromethyl
groups, such that possesses a significantly improved density compared to . The enthalpies of
formation were computed based on the Gaussian 03 program package [41] and each compound
exhibited a positive enthalpy of formation. Interestingly, the byproduct had the highest value
mol ), most likely due to the high bond energy of the cyanogroup. Based on the calculated
·
cm
(2). Although the bulky trinitromethyl group usually has a negative effect on crystal
2
1
3
−
1
(314.22 kJ
·
enthalpies of formation and the measured densities at ambient temperature, the detonation properties
of all the compounds were determined using the EXPLO5 (v6.01) program [42]. The calculated
detonation velocities (D) and detonation pressures (P) were in the range of 7438–8700 m
9.6–33.2 GPa, respectively. Compound exceeds TNT in terms of detonation properties, while
exhibits superior performance to . The performance of the former also exceeds that of presently-known
−
1
·
s
and
1
1
2
1
energetic pyridine derivatives and is only slightly lower than that of RDX. The detonation properties
of salts 4 and 5 are lower than those of their parent compounds.
In conclusion, compound
2 possesses superior detonation properties and acceptable sensitivity
values comparable to those of RDX, but its thermal stability is too low for practical applications.
Through the denitration reaction and formation of hydrazinium salts, compounds
significantly improved thermal stability.
4 and 5 display
3
. Materials and Methods
Caution
Although none of the compounds described herein exploded or detonated in the course of this research, these
energetic materials should be handled with extreme care using best safety practices (including the use of personal
protective equipment such as leather gloves, face shields, and ear plugs).

Molecules 2018, 23, 2
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3
.1. General Information
All starting materials were commercially available and were used as-received. IR spectra were
obtained from KBr pellets using a Nicolet Magna IR 560 spectrophotometer (Madison, WI, USA) over
−
1
1
13
the range of 400–4000 cm . H-NMR and C-NMR spectra were recorded on a Bruker ARX-400
instrument (Zurich, Switzerland). Chemical shifts are reported in ppm relative to tetramethylsilane.
Elemental analyses (C, H and N) were performed using an Elementar Vario EL (Bremen, Germany).
Liquid chromatography (LC)-MS electrospray ionization (ESI) data were acquired with an Agilent 6120
LC-MS spectrometer (Santa Clara, CA, USA). Crystal structures were determined on a Rigaku RAXIS
IP diffractometer (Rigaku Corporation, Tokyo, Japan) with the SHELXTL crystallographic software
package of molecular structure. CCDC 1,582,435, 1,583,118, 1,582,533, 1,582,458 and 1,583,040 contain
the supplementary crystallographic data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via http://www.ccdc.cam.ac.uk/data_request/cif.
The melting and decomposition points of the compounds were determined using a TA-DSC Q2000
◦
−1
differential scanning calorimeter (New Castle, DE, USA) at a scanning rate of 10 C min . Densities
were measured at 25 C using a Micromeritics Accupyc II 1340 gaspycnometer (Norcross, GA, USA).
◦
The impact and friction sensitivity measurements were carried out with a BAM fall hammer apparatus
(BFH-10) and a BAM friction apparatus (FSKM-10) (San Diego, CA, USA), respectively.
3
.2. Theoretical Studies
Calculations were performed using the Gaussian 03 suite of programs [41]. The geometric
optimization of the structures and frequency analyses employed the density functional theory
DFT) B3LYP method with the 6-311+G** basis set and single-point energies were calculated at the
MP2/6-311+G** level. Each optimized structure was characterized to determine the true local energy
minima on the potential energy surface without imaginary frequencies. The heats of formation ( H ) of
the neutral molecules and the corresponding anions were computed using the isodesmic reaction and
protonation reaction [43 44]. The enthalpy of each isodesmic reaction was obtained by combining the
(
∆
f
,
MP2/6-311+G** energy difference for the reaction, the scaled zero-point energies (B3LYP/6-311+G**),
and other thermal factors (B3LYP/6-311+G**). Thus, the heats of formation of the cations and anions
being investigated could be readily extracted. Finally, the heat of formation of each salt was obtained
by combining the enthalpies of the cation and anion with the lattice energy of the salt according to the
Born-Haber cycle. Using these heats of formation and densities, detonation velocities and detonation
pressures were calculated using the EXPLO5 v6.01 program (Brodarski Institute, Zagreb, Croatia)
according to the Kamlet-Jacobs equations [45–47].
3
.3. Experiment Procedures
-trinitromethylpyridine ( ): Pyridine-2-carboxaldoxime (488 mg, 4 mmol) was dissolved in acetonitrile
20 mL) at room temperature, after which N O (5 mL) was added dropwise with vigorous stirring.
2
(
1
2
4
◦
The reaction mixture was subsequently heated to 40 C and maintained at this temperature for 2 h.
Then the mixture was cooled to room temperature and stirred vigorously to remove residual N O .
2
4
The solvent was evaporated under vacuum and the residue was purified by column chromatography in
a Biotage Isolera One apparatus using a Flash Silica-CS column (25 g) and eluting with n-hexane and
dichloromethane. The product 1 was isolated with satisfactory purity as a white solid (620 mg, 68%).
◦
1
R = 0.7 (silica gel, 33% EtOAc/hexanes); T = 114 C; H-NMR (CDCl ):
δ
8.76 (d, J = 4.7 Hz, 1H), 7.96
f
d
3
13
(
(
m, 2H), 7.62 (m, 1H); C-NMR (CDCl3):
δ
150.32 (s), 142.72 (s), 138.09 (s), 128.10 (s), 125.19 (s); MS
1] ; The other characterization data including the melting point, IR spectra, and
elemental analysis were consistent with the literature report [24].
,6-bis(trinitromethyl)pyridine ( ) and 2-cyano-6-trinitromethylpyridine (
dropwise to a suspension of pyridine-2,6-dicarboxaldehyde dioxime (660 mg, 4 mmol) in acetonitrile
40 mL) with vigorous stirring. The solid was observed to gradually dissolve and the reaction mixture
−
ESI) m/z: 227 [M
−
2
2
3): N O (10 mL) was added
2 4
(

Molecules 2018, 23, 2
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◦
eventually became dark brown. The mixture was subsequently heated to 60 C and stirred at
this temperature for 2 h, then cooled to room temperature and stirred vigorously to remove any
residual N O . The solvent was evaporated under vacuum and the residue was purified by column
2
4
chromatography, using a Biotage Isolera One apparatus (Uppsala, Sweden) with a Flash Silica-CS
column (25 g), eluting with n-hexane and dichloromethane. Both and were isolated with satisfactory
purity as a white solid (536 mg, 35.5%) and an orange solid (148 mg, 14.6%), respectively.
2
3
◦
◦
1
Compound 2: R = 0.5 (silica gel, 33% EtOAc/hexanes); m.p. 96–98 C; T = 101 C; H-NMR (CDCl ):
f
d
3
δ
8.43 (s, 1H); 13C-NMR(CDCl3):
δ
142.78 (s), 140.78 (s), 129.55 (s); IR (KBr): 3095, 2922, 2853, 1622, 1589,
450, 1349, 1291, 1170, 1078, 999, 856, 804, 785 cm ; MS (ESI) m/z: 376 [M-1] , 331 [M-46] ; Elemental
−
1
−
−
1
analysis for C H N O : calculated C 22.29, H 0.80, N 26.00%; measured C 22.21, H 1.01, N 25.68%.
7
3
7
12
◦
◦
1
Compound 3: R = 0.4 (silica gel, 33% EtOAc/hexanes); m.p. 68–72 C; T = 106 C; H-NMR (CDCl ):
8.25 (m, 2H), 8.06 (dd, J = 6.3, 2.4 Hz, 1H); C-NMR (CDCl3): δ 144.13 (s), 139.89 (s), 134.43 (s),
f
d
3
13
δ
1
9
32.26 (s), 128.42 (s), 115.12 (s); IR (KBr): 3086, 2921, 1630, 1594, 1447, 1347, 1293, 1208, 1169, 1097, 1042,
−
1
−
−
46] ; Elemental analysis for C H N O :
7 3 5 6
93, 798, 735 cm ; MS (ESI) m/z: 252 [M
−
1] , 207 [M
−
calculated C 33.21, H 1.19, N 27.67%; measured C 33.09, H 1.26, N 27.48%.
Hydrazinium 2-dinitromethylpyridine ( ): An 80% solution of hydrazine hydrate (3 mmol) was added
slowly to a solution of (1 mmol) in methanol (5 mL). The resulting mixture was stirred at room
4
1
temperature for 4 h, during which time a light-yellow precipitate was formed. The precipitate was
filtered off, washed with cold ethanol, and recrystallized from methanol:water = 1:1 (v/v) to give the
target product. (173 mg, 80.5%).
◦
1
R = 0.7 (silica gel, 80% EtOAc/MeOH); T = 181 C (decomposition without melting); H-NMR
f
d
(
DMSO-d ):
δ
8.51 (d, J = 4.4 Hz, 1H), 7.77 (dd, J = 10.9, 4.4 Hz, 1H), 7.50 (d, J = 7.9 Hz, 1H), 7.18 (m,
6
13
1
H); C-NMR (DMSO-d6):
333, 3125, 1587, 1489, 1462, 1376, 1329, 1303, 1243, 1128, 1091, 1054, 951, 797, 748, 696 cm ; Elemental
analysis for C H N O : calculated C 33.49, H 4.22, N 32.55%; measured C 33.32, H 4.31, N 32.62%.
δ 151.70 (s), 148.54 (s), 136.39 (s), 134.20 (s), 125.26 (s), 121.62 (s); IR (KBr):
−
1
3
6
9
5
4
Dihydrazinium 2,6-bis(dinitromethyl)pyridine (
5): An 80% hydrazine hydrate solution (6 mmol) was
added slowly to a solution of (1 mmol) in methanol (5 mL). The resulting mixture was stirred at room
2
temperature for 4 h, during which time a yellow precipitate was formed. The precipitate was filtered
off, washed with cold ethanol, and recrystallized from methanol:water = 1:1(v/v) to give the target
product. (267 mg, 76.1%).
◦
1
R = 0.3 (silica gel, 33% EtOAc/MeOH); T = 125 C (decomposition without melting); H-NMR
(
f
d
13
DMSO-d6): δ 7.71 (t, J = 7.5 Hz, 1H), 7.31 (d, J = 7.7 Hz, 2H); C-NMR (DMSO-d6): δ 151.30 (s), 136.43
(
1
s), 134.60 (s), 123.32 (s); IR (KBr): 3362, 3113, 1586, 1491, 1447, 1415, 1335, 1278, 1224, 1171, 1128, 1096,
−
1
070, 927, 815, 752, 720 cm ; Elemental analysis for C H N O : calculated C 23.94, H 3.73, N 35.89%;
7
13
9
8
measured C 23.85, H 3.59, N 36.02%.
4
. Conclusions
The mono and bis(trinitromethyl)-substituted pyridines
1
and 2 were synthesized in moderate
to excellent yields via the reaction of the corresponding pyridinecarboxaldoximes with N O .
2
4
The by-product
3 was also separated and identified. Compounds 1 and 2 reacted with aqueous
hydrazine to give dinitromethyl hydrazinium salts. All the compounds were fully characterized and
their energetic properties were both measured and calculated. Due to the successful introduction of
two trinitromethyl groups onto the pyridine ring,
2
possesses the best oxygen balance (
−
14.85%) and
−
superior detonation properties (D = 8700 m s
1
·
, P = 33.2 GPa) compared to other energetic pyridine
derivatives, in conjunction with moderate sensitivity values (IS = 9 J, FS = 192 N) comparable to those
of RDX, but its thermal stability is too low for practical applications making it inferior to RDX. The
formation of salt
5 significantly improves the thermal stability of 2 while maintaining much of the
detonation performance.
Supplementary Materials: Supplementary materials can be accessed online.

Molecules 2018, 23, 2
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Acknowledgments: The authors acknowledge financial support from the National Natural Science Foundation
of China (21576026 and U153062).
Author Contributions: Chenghui Sun and Siping Pang conceived and designed the experiments; Yiying Zhang,
Shannan Yu, and Lingxiang Bao performed the experiments; Yiying Zhang, Xiaoyu Sun, and Chenghui Sun
analyzed the data; Yiying Zhang, Chenghui Sun, and Siping Pang wrote the paper.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are available from the authors.
©
2017 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).