Octanitropyrazolopyrazole: A gem-trinitromethyl based green high-density energetic oxidizer

Article abstract of DOI:10.1039/d0cc05704e

Environmental concerns demand the replacement of ammonium perchlorate (AP) by a green oxidizer in composite propellants. Herein, we report the synthesis and characterization of a novel green high-density energetic oxidizer octanitropyrazolopyrazole (ONPP). With its high specific impulse (256 s), high density (1.997 g cm-3) and good thermal stability (160 °C), ONPP can potentially replace AP. This journal is

Full text of DOI:10.1039/d0cc05704e

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Octanitropyrazolopyrazole: A Gem-trinitromethyl Based Green
High-Density Energetic Oxidizer
Mohammad Khaja,a Vikranth Thaltiri,a Nagarjuna Kommu,*a and Anuj A. Vargeese*b
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
The environmental concerns demand the replacement of functionalized heterocyclic compounds with trinitromethyl
ammonium perchlorate (AP) by a green oxidizer in composite groups have emerged as promising candidates for high-density
propellants. Herein, we report the synthesis and characterization of energetic oxidizers.6
O2N
a
novel
green
high-density
energetic
oxidizer
NO2
NO2
octanitropyrazolopyrazole (ONPP). With its high specific impulse
(256 s), high density (1.997 g cm-3) and good thermal stability (160
°C), ONPP can potentially replace AP.
N
N
N
O2N
O2N
O2N
N
NO2
NO2
NO2
O2N
O2N
N
O
N
NO2
O2N
OH
II
NO2
I
O2N
NO2
III]
Development of a new energetic oxidizer for solid propellant
needs an environmental friendly molecule with a high oxygen
content, high density, and high heat of formation. Composite
solid propellants are majorly comprised of an oxidizer molecule
(60–80%) with stoichiometrically excess oxygen.1 Ammonium
perchlorate (AP) has long been utilized as an oxidizer for rocket
propellants because of its beneficial properties such as high
thermal stability, good specific impulse, and low cost.1-3
However, the perchlorate released into the environment easily
contaminates the groundwater and soil, leading to severe
chronic diseases.4 In addition, the chlorine-containing species
generated during the combustion of AP causes ozone depletion,
and acid rain.4 Hence, intensive efforts are being made to
replace it with a green high-density energetic oxidizer.
Two important chlorine-free compounds, ammonium
dinitramide (ADN) and hydrazinium nitroformate (HNF), have
been developed a few decades ago as possible alternatives to
AP.5 Unfortunately, both of them suffer from several drawbacks
such as low thermal and chemical stability, extreme
hygroscopicity, incompatibility, and industrially unviable
synthetic routes, which limit their application in practical
systems. In the last decade, many CHNO-based organic oxidizers
have been developed (Figure 1).1c,3,6 In particular, polynitro-
O2N
NO2
NO2
NO2
NO2
O2N
NO2
NO2
NO2
NO2
O2N
O2N
O2N
O
O
O2N
N
N
N
N
N
N
N
N
N
N
NO2
N
N
N
N
NO2
NO2
O2N
O2N
O2N
NO2
O
VI
O2N
O2N
IV
NO2
V
Figure 1. Energetic oxidizers based on trinitromethyl substitution [I3a, II6b
,
III6c, IV3c, V6d, VI1c]
However, most of the developed oxidizers suffer from either
low density, poor thermal stability, and/or low specific impulse.
For example, the recently reported gem-trinitromethyl bearing
tetranitro bis-pyrazole (V) offers excellent density (2.02 g cm-3),
but has a low thermal stability (125 °C) (Scheme 1).6d Therefore,
it is necessary to continue the efforts to develop new stable
high-performance organic oxidizers.
Compared to open-ring heterocyclic compounds, molecular
entries with fused heterocyclic backbones offers better co-
planarity and large conjugation leading to higher densities.7 In
addition, fused heterocycles offer the advantage of better
thermal stability and heat of formation. This motivated us to
design a fused high-energy-density oxidizer to achieve the ideal
combination of high energetic performance and good thermal
a. Advanced Center of Research in High Energy Materials, University of Hyderabad,
Hyderabad 500046 (India) Email: nagarjunakommu@gmail.com (NK)
b. Laboratory for Energetic and Energy Materials Research , Department of
Chemistry, National Institute of Technology Calicut, Calicut 673601 (India)
E-mail: aav@nitc.ac.in
Electronic Supplementary Information (ESI) available: [General experimental
details, Spectral and Analytical data, and Computational details]. See
DOI: 10.1039/x0xx00000x
stability. 3,6-dinitropyrazolo[4,3-c]pyrazole (DNPP), is
a
promising energetic precursor for developing energetic
materials with high thermal stability.8 Using DNPP,
multipurpose energetic materials have been reported through
N-functionalization strategies,7,9 however, N-NO2- and N-
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gemdinitro-substituted DNPP derivatives are unstable.10 On the
other hand, the gem-trinitromethyl-substituted molecules
offer improved density, better oxygen balance and higher
thermal stability. This encouraged us to synthesize DNPP based
View Article Online
DOI: 10.1039/D0CC05704E
gem-trinitromethyl-substituted
energetic
oxidizers.
In
continuing our research efforts, we report the synthesis and
characterization of a novel fused high-energy-density green
oxidizer, octanitropyrazolopyrazole (ONPP, Scheme 1).
N-gemtrinitro bearing
bispyrazole
(b)
a) Previous work
Oxidizer
Key properties
(a)
NO2
NO2
O2N
NO2
Density: 2.02 g cm-3
OB: +10.5%
O2N
N
Topology
based on
bispyrazole
Td: 125 o
C
N
HOF: +0.85 kJ g-1
Isp: 254 s
N
N
NO2
NO2
NO2
O2N
O2N
N2 : 40%
CO2 : 43%
(V)
N-gemtrinitro bearing
fusedpyrazole
b) Current work
Oxidizer
Key properties
O2N
NO2
Density: 1.997 g cm-3
OB: +12.9%
O2N
NO2
N
Topology
based on
fusedpyrazole
Td: 160 o
C
N
N
HOF: +1.17 kJ g-1
Isp: 256 s
N
(c)
O2N
NO2
NO2
ONPP (3)
N2 : 42%
CO2 : 40%
O2N
Figure 2. Molecular structure of ONPP (a) with thermal ellipsoids drawn at the
30% probability level; (b) side view of ONPP; Ball and stick diagrams of
ONPP(c) viewed along the c axis with O...O and N…O interactions.
Scheme 1. Physico-chemical properties of ten-nitro bearing bipyrazole and
octanitro bearing fused pyrazole.
15N NMR spectroscopy is a potential tool for distinguishing
various types of nitrogens in a molecule. The 15N NMR spectrum
of ONPP (3) was recorded in acetone-d6 (Figure 3) and the peaks
were assigned based on the GIAO NMR calculations using
Gaussian 09 (ESI).12 The chemical shifts in the 15N NMR spectra
(Figure 3) are given with respect to the chemical shift of Liq. NH3
as an external standard (15N NMR spectra with nitromethane as
standard is provided as ESI).13 In 15N NMR spectrum of ONPP
four different kinds of nitrogens were observed. In which,
nitrogens of C3-NO2 resonate at 341.14 (N1), C(NO2)3 resonate
at 335.23 (N4) ppm and the pyrazole nitrogens N2 and N3
resonate at 319.95 and 152.46 ppm respectively.
In general, the introduction of trinitromethyl functionality into
heterocylces was achieved by the destructive nitration on N-
heteryl substituted propanones/butanones.11 Using this
protocol, at first DNPP (1) was reacted with methylvinlyketone
in presence of trimethylamine in aqueous medium at RT for 2 h
to
produce
4,4-(3,6-dinitropyrazolo[4,3-c]pyrazole-1,4-
diyl)bis(butane-2-one) in 58 % yield (Scheme 2). Later, nitration
of intermediate (2) using 98% H2SO4 and 100% HNO3 at RT for
18 h delivered octanitropyrazolopyrazole (3) in 18 % yield
(Scheme 2). The precursor, DNPP, was synthesized by the
modified procedure of Shreeve et.al.8
O
O2N
NO2
O2N
NO2
N
NO2
N
O
NO2
N
H
N
N
N
98% H2SO4
N
N
N
TEA, H2O
RT, 2 h
58%
100% HNO3
RT, 18 h
18%
N
N
N
H
O2N
O2N
O2N
NO2
NO2
DNPP
O2N
3
2
1
O
Scheme 2. Synthesis of octanitropyrazolopyrazole (ONPP)
Single crystal of ONPP was grown by the slow evaporation of
methanol at RT and ambient pressure. The structure of ONPP
was confirmed by X-ray diffraction (Figure 2) and it crystallizes
in monoclinic space group P 21/c with the cell volume of
825.38(9) Å. The crystal of ONPP has two formula units in the
unit cell, and exhibits an outstanding crystal density of 1.997 g
cm-3 (T = 298 K) (ESI).
Figure 3.15N spectrum of ONPP (compound 3)
2 | J. Name., 2012, 00, 1-3
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Table 1. Physical and Chemical Properties of ONPP, AP, ADN, Compound I-VI
ONPP (3)
C6N12O16
1.997
160
+12.9
256
+1.17
7
40
AP[a]
NH4ClO4
1.95
>200
+34
157
-2.52
15
>360
6368
15.8
54
ADN[a]
NH4N(NO2)2
1.81
I[b]
C2HN5O9
1.84
137
+30.1
209
-0.56
19
20
II[c]
C4N8O13
1.92
102
+21.7
225
0.08
4
240
III[d]
C4HN9O12
1.90
157
+15.3
238
0.06
4
120
IV[e]
C6N10O14
1.94
124
+7.3
246
+0.14
10
80
V[f]
C8N14O20
2.02
125
+10.5
254[j]
+0.85
9
215
9320
40
52
84
VI[g]
C7N14O17
1.94
164
+8.7
219
-1.13
9
240
Formula
ρ[h] (g cm-3)
Td[i] (oC)
159
+25.8
202
-1.13
3-5
64-72
7860
23.6
OB[j] (%)
Isp[k] (s)
ΔHf[l]( kJg-1)
IS[m] (J)
FS[n] (N)
Dv[o] (ms-1)
P[p] (GPa)
O[q] (%)
9182
37.8
51
7503
23
60
8229
29.2
56
8434
30.7
52
8814
34.5
51
8252
28.6
49
45
97
N+O[r] (%)
85
65
89
86
87
83
84
[a] Reference 1(c); [b] Reference 3(a); [c] Reference 6(b); [d] Reference 6(c); [e] Reference 3(c); [f] Reference 6(d); [g] Reference 1(c); [h] Density determined by X-ray
analysis (at 298 K); [i] Thermal decomposition temperature (10 °C min-1); [j] Oxygen balance based on CO2; [k] Specific Impulse; [l] Heat of formation; [m] Impact sensitivity
(BAM drophammer, method 1 of 6); [n] Friction sensitivity (BAM friction tester, method 1 of 6); [o] Detonation velocity [p] Detonation pressure; [q] Oxygen content; [r]
Combined nitrogen and oxygen content ([k], [o], and [p] calculated with Explo5 v6.03, other details are provided in ESI)
To understand the structure-property relationships, Hirshfeld
surface graph and 2D fingerprint spectra of ONPP generated
using Crystalexplorer17.5 software14 (ESI) were analysed. In
Figure 4a and 4b, strong intermolecular(N···O and O···N)
interactions are revealed in the ONPP. While O···O interactions
specified on the bottom left in Figure 4c, indicates strong
intermolecular interaction between O atoms in ONPP.15 As per
Figure 4d, 75.3% of O···O interactions were observed in ONPP
indicating higher density and more sensitivity than molecules
having fewer nitro groups. Higher relative frequencies of close
O···O contacts, arising from oxygen atoms in trinitromethyl
groups, indicate the greater number of exposed nitro groups on
the surface of the molecule possibly makes it more sensitive.16
The crystal density of ONPP is one of the highest among the
reported heterocyclice oxidizers and energetic materials. It is
also thermally stable and the DSC analysis shows that the
molecule begins to decompose above 160 °C. Its positive
oxygen balance (+12.9%) signifies that it can be used as a green
energetic oxidizer in solid rocket propellants. ONPP was
optimized using the Gaussian 09 software with the B3LYP6-
311G++(d,p) basis set. Its heat of formation was calculated
using the isodesmic approach12 (ESI), and it was found to have
a high positive enthalpy of formation (ΔHf = +1.17 kJ g-1). Its
combustion and energetic parameters were computed using
the Explo5 v6.03 software17 (ESI) by utilizing its density and
enthalpy of formation. The specific impulse (Isp), total impulse
per unit weight of propellant, is an important figure of merit of
the performance of a rocket propulsion system and is greatly
influenced by the efficiency of oxidizer. ONPP demonstrates a
(a)
(b)
(c)
(d)
Figure 4. Hirshfeld surface graph of ONPP and 2D fingerprint plots in the
crystal structure. (a) Hirshfeld surface graph with proximity of close contacts
around ONPP molecule (b) fingerprint plots in crystal packing found in ONPP.
(c) the O···O atomic contact percentage contribution to the Hirshfeld surface
for ONPP. (d) the percentage contributions of the individual atomic contacts
to the Hirshfeld surface for ONPP.
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3
4
a) T. T. Vo, D. A. Parrish, J. M. Shreeve, J. Am. Chem. Soc. 2014,
higher specific impulse (256 s) compared to AP (157 s), ADN
(202 s), and other reported oxidizers (Table 1 and ESI, Table SIII).
The table 1 shows the properties of recently developed CHNO
and CNO oxidizers. The theoretical estimation of Isp of
propellant formulations (Table SIII) also displays the potential of
the molecule with commonly used binder Hydroxyl terminated
polybutadiene (HTPB) and advanced binder Glycidyl azide
polymer (GAP). In comparison, ONPP possess competitive
properties for an ideal oxidizer. Interestingly, ONPP exhibited
good explosive properties with a high detonation velocity (Dv =
9182 m s-1) and detonation pressure (P = 37.8 GPa).
The impact and friction sensitivities of the ONPP were carefully
determined using a BAM Fall Hammer apparatus (BFH-10 by
OZM) with a 2 kg drop weight on 15–20 mg samples and a BAM
friction apparatus (FSKM-10 by OZM). The impact sensitivity of
ONPP was 7 J and the friction sensitivity was 40 N. As shown in
Figure 4, the sensitivity data of ONPP is correlated with the
Hirshfeld surface graphs. Generally, the sensitivity of the
molecule towards impact increases with the percentage of O···O
contacts in the crystal packing.15b
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136, 11934-11937; b) T. M. Klapotke, E. Gokcinar, Turk. J.
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In conclusion, a simple and efficient synthetic methodology was
developed for constructing a novel oxidizer molecule, ONPP,
using DNPP. The structure of ONPP was confirmed by single-
crystal X-ray diffraction. It has a remarkably high density of
1.997 g cm-3 (at 298 K), which lies in the range of that observed
for
hexanitrohexaazaisowurtzitane
(2.04
g
cm-3),18
10 a) Y. Li, Y. Shu, B. Wang, S. Zhang, L. Zhai, RSC Adv. 2016, 6,
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Am. Chem. Soc. 2015, 137, 1697−1704.
17 M. Suceska, EXPLO5 version6.03, 2014.
octanitrocubane (1.98 g cm-3),19 and hexanitroethane (2.05 cm-
3 20
)
and is very high for a hydrogen-free and neutral CNO
compound. Importantly, ONPP shows a positive oxygen balance
(+12.9%), positive heat of formation (+1.17 kJ g-1), and high
thermal stability (160 °C). In addition, ONPP has a high specific
impulse (256 s) and high energetic performance (Dv = 9182 m s-1
and P = 37.8 GPa). Hirshfeld surface graphs and fingerprint plot
analysis indicated the crystal packing comprised 75.3% of O···O
contacts. ONPP has an experimental impact sensitivity of 7 J and
friction sensitivity of 40 N. Hence, because of the excellent
energetic properties and a positive oxygen balance, ONPP can
replace AP in solid rocket propellant applications.
18 A. T. Nielsen, A. P. Chafin, S. L. Christian, D. W. Moore, M. P.
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Conflicts of interest
There are no conflicts of interest to declare.
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Chem. Solids. 1986, 47, 1129-1137.
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