5,6-Di(2-fluoro-2,2-dinitroethoxy)furazano[3,4-: B] pyrazine: A high performance melt-cast energetic material and its polycrystalline properties

Article abstract of DOI:10.1039/c7ra07035g

5,6-Di(2-fluoro-2,2-dinitroethoxy)furazano[3,4-b]pyrazine was synthesized in a three-step process starting from 3,4-diaminofurazan (DAF) including a significant nucleophilic substitution reaction under the catalytic effect of trisodium phosphate dodecahydrate. Characterization of this molecule indicates that it possesses a higher crystal density than that of 2,4,6-trinitrotoluene (TNT) at ambient temperature with acceptable melting-point and energetic properties approaching those of 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX) but with a higher thermal stability and lower sensitivity towards impact and friction. To investigate its polycrystalline properties, differential scanning calorimetry (DSC), powder-X-ray-diffraction (PXRD) and ab initio calculation using the Viena Ab initio simulation package (VASP) were employed.

Full text of DOI:10.1039/c7ra07035g

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5,6-Di(2-fluoro-2,2-dinitroethoxy)furazano[3,4-b]
pyrazine: a high performance melt-cast energetic
material and its polycrystalline properties†
Cite this: RSC Adv., 2017, 7, 38844
ab
b
b
b
a
b
* *
Zhipeng Lu, Longyu Liao, Jinglun Huang, Dabin Liu, Jinshan Li
Qing Ma,
and Guijuan Fan
b
*
5,6-Di(2-fluoro-2,2-dinitroethoxy)furazano[3,4-b]pyrazine was synthesized in a three-step process starting
from 3,4-diaminofurazan (DAF) including a significant nucleophilic substitution reaction under the catalytic
effect of trisodium phosphate dodecahydrate. Characterization of this molecule indicates that it possesses
a higher crystal density than that of 2,4,6-trinitrotoluene (TNT) at ambient temperature with acceptable
melting-point and energetic properties approaching those of 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX)
but with a higher thermal stability and lower sensitivity towards impact and friction. To investigate its
polycrystalline properties, differential scanning calorimetry (DSC), powder-X-ray-diffraction (PXRD) and
ab initio calculation using the Viena Ab initio simulation package (VASP) were employed.
Received 24th June 2017
Accepted 1st August 2017
DOI: 10.1039/c7ra07035g
rsc.li/rsc-advances
To date, plenty of uorine containing polynitro-aryl-triazoles
and nitrate ester or nitramine derivatives of uoro derivatives
were developed as melt cast explosives.6 Fluorodinitro- (FDN-)
moiety is an attractive decorating explosophore group, which
can improve the thermal stability and decrease the mechanical
Introduction
Melt-cast energetic materials have been used in civil mining,
aero-propellants and military ammo for decades since 2,4,6-
trinitrotoluene (TNT) was synthesized by J. Willbrand in 1893.1
However, the underlying issues of TNT restricted its manufac-
ture and applications, such as cracks, exudation, voids, irre-
versible growth, etc.2 Aer the proposal of the “Green energetic
materials” concept, research on new candidates to replace TNT
has been ongoing for the past several years. As shown in Fig. 1,
polynitroazoles featuring methyl groups were developed at rst
since the presence of methyl groups similar to that in TNT
helped to decrease the melting point, such as 1-methyl-2,4,5-
trinitroimidazole
(MTNI),
1-methyl-3,4,5-trinitropyrazole
(MTNP) and 1-methyl-3,5-dinitro-1,2,4-triazole (DNMT).3 As for
the methyl group, nitrofurazan-compounds were also discov-
ered because of their low melting point and high energy, such as
3,4-bis(3-nitrofurazan-4-yl)furoxan (DNTF), 3-(4-aminofurazan-
3-yl)-4-(4-nitrofurazan-3-yl)furazan (ANTF) and 4,400-dinitro-
[3,30,40,300]-tris-[1,2,5]-oxadiazole (BNTF).4 However, the safety of
a scalable process for these compounds was still problematic.
The search for TNT candidates has never stagnated. Recently,
the nitrazapropane moiety was incorporated into the synthetic
decoration of energetic materials with low melting-point.5
aSchool of Chemical Engineering, Nanjing University of Science and Technology,
Xiaolinwei 200, Nanjing 210094, P. R. China. E-mail: dabinl63@vip.sina.com
bInstitute of Chemical Materials, China Academy of Engineering Physics, P.O.Box 919-
311, Mianyang 621900, P. R. China. E-mail: lijinshan@caep.cn; fanguijuan@caep.cn
† Electronic supplementary information (ESI) available. CCDC 1535226–1525228
and 1535921. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c7ra07035g
Fig. 1 A variety of molecular design and synthesis strategies for
obtaining energetic compounds with low melting-point.
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sensitivity comparing with trinitromethyl moieties. For the past 2-uoro-2,2-dinitroethanol (FDNE). The second route (b) is
several years, FDN-moiety was introduced into the design and so-called esterication through the nucleophilic substitution of
synthesis of energetic materials based on different frameworks chlorine atoms with FDNE. The third route (c) is the uorina-
such as 1,4-dioxane, 1,3,5-tirazine, carbamate, nitramine, bis- tion reaction by exchanging potassium or ammonium in
tetrazolyl-dihydroborate, bi(1,2,4-oxadiazoles), 2H-tetrazole gem-dinitro carbon with uorine atoms in selectuor™ or XeF2.
and azofurazan.7 Among them, bis(2-uoro-2,2-dinitroethyl) Among them, uorination is relatively easy in most cases but
formal (FEFO) is a commercial energetic plasticizer because of usually synthesizing the precursor is complicated; Mannich
its good mechanical, aging and moulding properties, which has reaction has close relationship with the selectivity and reactivity
been applied in rocket propellants as an important formula but of substrates; esterication needs catalyzed bases to perform
its safety performance still existed.8 Other energetic plasticizers nucleophilic substitution.
bearing FDN-moieties such as bis(1-uoro-1,1-dinitromethyl)
Chavez et al. reported a two-step esterication of 3,6-
difurazalyl ether (FOF-13), 1,3-bis(2,2,2-uoro-dinitroethyoxy)- dichloro-1,2,4,5-tetrazine by rst introduction of 2,4,6-collidine
2,2-bis(diuoramino)propane (SYEP) and uorodinitroethyl in dichloromethane (DCE) with FDNE, followed by 4-(N,N-
boron esters were studied for their analogous performance to dimethylamino)pyridine (DMAP) with FDNE to obtain a bis-
FEFO.9 To date, the similar structures as FEFO in energetic substituted FDN-based tetrazine compound.10 Though this
heterocyclic compounds were less released in literatures. method is very effective for synthesizing above-mentioned
Recently, uorodinitroethyl ethers was successfully synthesized compounds, it is still complicated and difficult for further
based on 1,2,4,5-tetrazines, which brought the possibilities for purication. As shown in Scheme 2, we rstly synthesized 5,6-
synthesizing other heterocyclic frameworks bearing FDN- dihydroxyfurazano[3,4-b]pyrazine (3) from a cyclization of 3,4-
ethers.10
diaminofurazan and oxalic acid.14 Subsequently, 5,6-dichlor-
In a continuing effort to seek more FDN-based energetic ofurazano[3,4-b]pyrazine (4) was synthesized by a mixture of 3
materials with high performance, we designed a new structure and SOCl2 in DMF at 75 ꢀC. The nucleophilic substitution of 5,6-
by using fused furazano[3,4-b]pyrazine to replace the methylene di(2-uoro-2,2-dinitroethoxy)furazano[3,4-b]pyrazine (6) was
bridge in FEFO (Fig. 1). The aims of this strategy were to: (a) realized by a mixture of 4 and 2-uoro-2,2-dinitroethanol (5) in
improve the heat of formation (HOF) and chemical energy acetone under the catalytic effect of trisodium phosphate
because of their relatively higher HOF of furazan and pyrazine11 dodecahydrate at 25 ꢀC for 8 h. We also tried to synthesize 6 by
in N-heterocycles than that in carbon chains; (b) cause positive using sodium carbonate or sodium hydroxide instead of triso-
effect of ring strain energy12 in fused heterocyclic frameworks dium phosphate dodecahydrate, but the substitution reaction
on detonation performance and thermal stability of the target was not complete or there was a decomposition of FDNE
molecule; (c) produce intramolecular conjugated, coplanar and occurred in sodium hydroxide solution. The whole reaction
intermolecular p–p stacking effect13 on the crystal packing and process was monitored by TLC (ethyl acetate : petroleum ether
structural stability, etc. Herein we report on a eco-friendly and ¼ 1 : 3). Compound 6 was very stable upon storage under
easy scale-up method of synthesizing 5,6-di(2-uoro-2,2- ambient conditions and was soluble in high-polar solvent such
dinitroethoxy) furazano[3,4-b]pyrazine within three steps from as ethanol, acetone and methanol. We also attempted to
the starting material 3,4-diaminofurazan (DAF). Likewise, its synthesize its N-oxide derivatives 7 and 8 by using previously
special polycrystalline properties and physicochemical perfor- effective N-oxidation condition in tetrazine system (triuoro-
mance are further investigated in detail.
acetic acid (TFA) or triuoroacetic acid anhydride (TFAA) with
50% hydrogen peroxide),15 and just conrmed a trace of 7 under
Results and discussion
Synthesis
Three pathways for obtained FDN-based energetic compounds
were proposed as shown in Scheme 1. The rst route (a) is the
Mannich reaction with the assistance of Lewis bases and
Scheme 1 Three pathways for synthesizing FDN-based compounds
(R represents different organic frameworks).
Scheme 2 Synthesis of 6 starting from DAF.
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Table 1 Crystal data and structure refinement details for 6 re-crys-
tallized from acetone (6-A) and methanol (6-M)
the analysis of mass spectrum but still could not obtain the
target compounds. We supposed that the chemical reactivity of
nitrogen atoms in pyrazine ring was not comparable with those
in tetrazine ring.
6-A
6-M
Formula
Mw [g molꢁ1
T [K]
C8H4F2N8O11
426.19
130
0.10 ꢂ 0.05 ꢂ 0.03
Orthorhombic
Fdd2
10.833(2)
14.932(2)
18.906(4)
90
C8H4F2N8O11
426.19
130
0.18 ꢂ 0.10 ꢂ 0.02
Orthorhombic
Pca21
10.9222(18)
13.057(2)
10.5202(17)
90
90
90
1500.3(3)
4
0.71073
1.887
0.190
856
1.560–30.584
14 547/4524
ꢁ15 # h # 15
ꢁ15 # k # 18
ꢁ15 # l # 14
0.0288
]
Multinuclear NMR spectroscopy
Compound 6 was characterized by H, 13C, 15N and 19F NMR
1
Crystal size [mm3]
Crystal system
Space group
spectroscopy. In the 1H NMR spectrum, chemical shis of CH2
show coupling constant of 5.95 ppm (JH–F ¼ 15.0 Hz). The 13C
signals for CH2, CF(NO2)2, NCN and OCN are observed at
65.1 ppm, 119.6 ppm, 150.3 ppm and 153.9 ppm, respectively
(as shown in the ESI†). The signal of CH2 splits into a doublet
due to JC–F coupling with a coupling constant of 20.1 Hz. For
carbon atom in CF(NO2)2, the signal also splits into a doublet
because of JC–F coupling with a coupling constant of 299.4 Hz.
The carbon atoms in fused rings are normal, which show single
signals. As shown in Fig. 2, the 19F NMR spectrum shows the
uorine shi at ꢁ110.5 ppm as a doublet due to JF–N coupling
with a coupling constant of 4.7 Hz. In the 15N NMR spectra the
nitrogen atom in furazan is observed at ꢁ22.6 ppm and that in
pyrazine is at ꢁ152.9 ppm, while for gem-dinitro the signal
shows a doublet because of its JN–F coupling with a coupling
constant of 15.2 Hz.
˚
a [A]
˚
b [A]
˚
c [A]
a [ꢀ]
b [ꢀ]
g [ꢀ]
90
90
3
˚
V [A ]
3058.2(11)
8
0.71073
1.851
0.186
Z
˚
l [A]
rcalc [g cmꢁ3
]
m [mmꢁ1
F (000)
]
1712
q Range [ꢀ]
2.561–31.188
7542/2354
ꢁ15 # h # 15
ꢁ17 # k # 21
ꢁ27 # l # 27
0.0535
Reections collected
Index ranges
Rint
Data/restraints/parameters
Final R index [I > 2s(I)]
2354/1/132
R1 ¼ 0.0396
wR2 ¼ 0.0705
R1 ¼ 0.0888
wR2 ¼ 0.0860
0.955
4524/1/262
R1 ¼ 0.0322
wR2 ¼ 0.0795
R1 ¼ 0.0383
wR2 ¼ 0.0828
1.055
Crystal structure
The single-crystal of compound 4 was obtained by slow evapo-
ration of methanol, while 6-A and 6-M suitable for single-crystal
X-ray diffraction were obtained by slow evaporation of 6 in
anhydrous acetone and methanol at ambient temperature,
Final R index [all data]
GOF on F2
CCDC number
1535226
1535228
respectively. The crystal and structure data of 6-A and 6-M at 130
K are listed in Table 1, respectively. The crystal and structure
data of 4 at 293(2) K and 6-M at 293(2) K, selected bond lengths,
angles, torsion angles and hydrogen bonds are listed in the
ESI.†
5,6-Dichlorofurazano[3,4-b]pyrazine (4) crystallizes in the
orthorhombic space group Pbca with eight formulas per unit
cell and a density of 1.961 g cmꢁ3 at 293(2) K (molecular
structure and crystal packing are displayed in the ESI†). The
torsion angles of O1–N1–C1–N3 (179.50(16)ꢀ) and N3–C1–C2–
N2 (ꢁ179.89(17)ꢀ) reveal that the furazan ring are coplanar with
neighboring fused pyrazine ring.
The crystal structure of 6 is depicted in Fig. 3. To our
surprise, single crystals of 6 grew from acetone and methanol
show different polymorph. We also crystallized 6 in other
solvents such as ethyl acetate and dichloromethane, but just
obtained the same crystalline forms as that in methanol. 6-A
crystallizes in the orthorhombic space group Fdd2 with eight
formulas per unit cell and a density of 1.851 g cmꢁ3 at 130 K,
while 6-M crystallizes in the orthorhombic space group Pca21
with four formulas per unit cell and a density of 1.887 g cmꢁ3 at
130 K. Aer substituted by FDN moiety, the fused furazano[3,4-
b]pyrazine ring system still shows a planar arrangement, which
is reected in the dihedral angels of almost 180ꢀ (O1–N1–C1–
N3, ꢁ178.4(6)ꢀ, N1–C1–C2–N4, 179.7(6)ꢀ, O1–N1–C1–C2,
Fig. 2 19F and 15N NMR spectroscopy of 6 in d6-DMSO.
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from Fig. 4 wherein the crystal packing of 6-A shows wavelike
stacking form while 6-M shows a “bowl” like trapeziform
stacking. The later one is unusually seen in those of previously
reported LSHEs. This buffer structure with bottom may help
reduce the formation of so-called “hot spots” when the
mechanical stimuli is introduced, and also have positive effect
on decreasing the sensitivities and improving stability.
Herein, to describe quantitatively interactions in poly-
morphism of compound 6, we used the Bader's QTAIM
method18 and ab initio calculations to analyze the real-space
quantum–mechanical interactions in periodic solids based on
the electron densities and related scalar elds. This method was
rstly and successfully used to analyze chemical bonding
interactions in b-phase of the 1,3,5,7-tetranitro-1,3,5,6-
tetraazacyclooctane (HMX) by A. A. Pinkerton et al.19 With
respect to the XB network in the crystal, the forms of XBs
surrounding 6-A and 6-M are different due to their different
polymorph (Fig. 5). The (3, ꢁ1) bond critical points (BCPs) of the
intermolecular XBs, together with their related electronic and
energy data of 6-A and 6-M at 130 K are listed in Table S20† (the
computational details can be found in the ESI†). There are six
types of XBs found in 6-A and 6-M, respectively. Although one
type of hydrogen bond can be found in 6-A, the distances of
Fig. 3 ORTEP representations of single-crystal X-ray structure of 6
shown at the 50% probability level.
˚
hydrogen bonds lie in the range of 2.469–2.643 A which suggest
0.1(8)ꢀ). Additionally, the torsion angels of C2–N4–C3–O3
(ꢁ177.2(5)ꢀ) and C7–O3–C3–C4 (ꢁ176.3(5)ꢀ) indicate that OCH2
is nearly in the same plane with fused ring system. The C–F
˚
bond lengths in 6-A and 6-M are in the range of 1.315–1.332 A,
which is normal in most of energetic compounds featuring
uorodinitro-groups.8
Polycrystalline properties
Crystal packing and intermolecular interactions such as
hydrogen-bonds and p–p interaction in energetic compounds
have drawn much attention by experimental studies. Quantum
chemical methods were employed for interpreting those
contacts between molecules may contribute to their enhanced
thermal, impact and friction stabilities.16 In the previous work,17
four types of crystal packing have been discovered in low-
sensitivity and high-energy explosives (LSHEs). It can be seen
Fig. 5 XBs surrounding 6-A (a) and 6-M (b) with labelled atoms. For
Fig. 4 Crystal packing of single-crystal X-ray structure for 6-A and 6- clarity, the crystalline cell was transferred into primitive cell from
M with different types of packing form.
conventional cell due to its high symmetry of 6-A crystalline cell.
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Fig. 7 DSC plot of 6, 6-A and 6-M at the heating rate of 5 ꢀC minꢁ1
.
Fig. 6 PXRD spectra of 6, 6-A and 6-M.
these are strong. For 6-M, there are two types of hydrogen bonds
Table 2 Energetic and physical properties of 6
˚
˚
found, with their distances ranging from 2.503 A to 2.873 A. In
6-A, the strongest type of XB is C–H/O (EXB ¼ 5.52158 kJ
6
TNT26
RDX26
molꢁ1), while for 6-M the strongest types are C–H/O (EXB
¼
r (g cmꢁ3 a
)
1.83
1.65
1.80
6.56344 kJ molꢁ1) and C–H/N (EXB ¼ 6.44252 kJ molꢁ1).
Comparing with strong hydrogen bond, halogen bond energies
basically lie in the weak range of 0.82226–5.02857 kJ molꢁ1 and
OB (%)b
ꢁ26.3
ꢁ74
ꢁ21.6
D (m sꢁ1 c
)
8512
6881
8795
P (GPa)d
32.4
ꢁ166.1/(ꢁ0.39)
19.5
ꢁ67.0/(ꢁ0.30)
34.9
92.6/(0.42)
DHf (kJ molꢁ1)/
˚
˚
˚
distances ranging from 2.975 A to 3.363 A in 6-M (3.004–3.472 A
in 6-A) which indicate that halogen bonding interaction is weak
and also in accord with the tendency to form strong XBs (I > Br >
Cl > F).20 In addition, it is worth noting that the total EXB of 6-M
is higher than those of insensitive energetic compounds such as
FOX-7 (60.452 kJ molꢁ1) and LLM-105 (50.362 kJ molꢁ1).21
To further study the effect of polymorphism on crystalline
properties experimentally, powder X-ray diffraction (PXRD) was
employed. It is obvious from Fig. 6 that the powder sample of 6
at 2q is basically similar with those of 6-M, while 6-A shows
different peaks at 2q ranging from 15ꢀ to 20ꢀ, 25ꢀ to 30ꢀ, 33ꢀ to
36ꢀ and 40ꢀ to 50ꢀ. The above-mentioned PXRD patterns
conrm the presence of different polymorph and even a variety
of conformation, just like those of FOX-7 or CL-20.22
(kJ gꢁ1 e
)
Tm (ꢀC)f
Tdec (ꢀC)g
IS (J)h
117
226
17
252
0.13
258
81
295
15
240
0.37 (ref. 27)
209
204
210
7.4
120
0.20 (ref. 27)
258
FS (N)i
ESD (J)j
Isp (s)k
a
Density measured by single-crystal X-ray diffraction at 25 ꢀC or
adopted from the literatures.
detonation velocity.
Onset melting point measured by DSC.
decomposition temperature measured by DSC. Impact sensitivity
measured by BAM drop-hammer test. Friction sensitivity measured
by a BAM friction tester. Electrostatic discharge device. Specic
b
c
Oxygen balance.
Calculated detonation pressure.
Calculated
d
e
Heat of
f
g
formation.
Onset
h
i
j
k
impulse.
The onset melting point and onset decomposition temper-
ature of powder sample 6 as well as its crystals were obtained by
using differential scanning calorimetry (DSC) measurement
and shown in the Fig. 7. Powder 6 shows a onset melting point
at 117 ꢀC, which is slightly higher than that of TNT but still
meets the requirement of melt-cast process in energetic mate-
Energetic and physical properties
The energetic and physical properties are summarized in
Table 2. The heats of formation were calculated using the
Gaussian 09 (Revision D.01) suite of programs based on the
isodesmic reaction.23 6 gives a negative heat of formation of
ꢁ166.1 kJ molꢁ1 (ꢁ0.39 kJ gꢁ1), which has been enhanced
compared with similar compound featuring bis-FDN-moiety,7f
but still lower than those with mono-FDN-moiety.7g Based on
the heat of formation and its corresponding density at ambient
temperature, the detonation and combustion parameters were
performed by using EXPLO5 (version 6.02) code.24 All calculated
results are also listed in Table 2. The computed detonation
velocity of 6 (8512 m sꢁ1) is comparable with that of RDX (8795
m sꢁ1), which is higher than the famous melt-cast explosive
rials.5 It shows a onset decomposition temperature at 226 C,
ꢀ
which is better than that of RDX but lower than that of TNT. For
6-A and 6-M, it is worth noting that the onset meltꢀing point of 6-
A and 6-M all move up, especially 6-A shows 18 C ahead of 6
ꢀ
while 6-M shows only 5 C in advance. Though the decompo-
sition peaks of three samples do not change basically, the onset
decomposition temperature of 6-A and 6-M clearly move up and
ꢀ
show nearly 34 C ahead of 6.
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TNT (6881 m sꢁ1). Its detonation pressure (32.4 GPa) is also CCl3F as external standards. The melting point and decompo-
comparable with that of RDX (34.9 GPa). sition temperature were recorded on a differential scanning
To evaluate its safety properties, the sensitivities towards calorimeter-thermal gravity (TGA/DSC1, METTLER TOLEDO LF/
impact and friction were determined by using a standard BAM 1100) at a heating rate of 5 ꢀC minꢁ1. Infrared (IR) spectra were
Fallhammer and a BAM friction tester. Compound 6 show measured on Thermosher Nicolet 800 FT-IR spectrometer in
impact sensitivity (IS) of 17 J, suggesting it is comparable with the range of 4000–400 cmꢁ1 as KBr pellets at 20 C. Elemental
ꢀ
that of TNT (15 J) and more insensitive than that of RDX (7.4 J). analyses (C, H, N) were carried out on a elemental analyzer
The measured friction sensitivity (FS) of 6 (252 N) also reveals its (Vario EL Cube, Germany). The phases of the samples were
insensitivity as that of TNT (240 N) and exceeds that of RDX (120 obtained via powder X-ray diffraction (PXRD, X'Pert Pro, PAN-
N). The electrostatic discharge test (ESD) was performed to anlytical, Netherlands).
determine the ignitable possibility of energetic material, which
show that 6 is more sensitive (0.13 J) compared with TNT (0.37 J)
and RDX (0.20 J) but still in the scope of insensitivity towards
Computational methodology
electrostatic discharge (for human body, the values are within Intermolecular interaction in polycrystalline systems was
the range of 0.005–0.02 J).25
analyzing by using VASP (Vienna Ab initio Simulation Package)28
with rst-principles pseudopotentials constructed by projected
augmented waves (PAW), representing an all-electron DFT
technique with a frozen-core approximation.29,30 The PAW
potentials indicate the numerical advantages of pseudopotential
calculations while retaining the physics of all-electron calcula-
tions, involving the correct nodal behavior of the valence elec-
tron wave functions and the ability to include upper core states
in addition to valence states in the self-consistent iterations. The
H1s, C2s2p, N2s2p, O2s2p and F2s5p orbitals were taken as
valence states, and the exchange-correlation functional was
treated with the generalized gradient approximation (GGA)
following the Perdew, Burke and Ernzerhof (PBE) formulation.31
The static calculations and structure optimization were
performed using the conjugate gradient method with 3 ꢂ 3 ꢂ 3
Monkhorst–Pack k-point sampling in reciprocal space and
a plane-wave basis set with an energy cutoff of 850 eV. The DFT-
D2 method of Grimme32 was used to correct long-range
dispersion interactions. In DFT-D2 method, dispersion correc-
tions are calculated not only for forces acting on the atoms, but
also for the stresses on the unit cell, and thus a simultaneous
optimization of all degrees of freedom is permitted. The self-
consistent convergence criteria of energy were set to 1 ꢂ 10ꢁ7
for electronic relaxation. The intra- and intermolecular
hydrogen bonds and halogen bonds (XBs) in the crystal were
analyzed by Bader's QTAIM method and ab initio calculations,
with the Critic2, a code for determining the real-space
quantum–mechanical interactions in periodic solids based on
the electron densities and other related scalar elds.33,34 The
core and valence electron densities of a crystal were acquired
from ab initio (density functional theory, DFT) calculations
implemented in VASP code. The XB energy or the bond disso-
ciation energy of XB (EXB) was predicted using an empirical
equation EXB ¼ ꢁ(1/2)v35 and used to assess XB strength in
terms of electron densities (r) at the bond critical points, aer
which the potential energy densities (v) was obtained.
Conclusions
In this study we have developed a new and convenient synthetic
route for 5,6-di(2-uoro-2,2-dinitroethoxy)furazano[3,4-b]pyr-
azine (6) by nucleophilic substitution directly of 5,6-dichlor-
ofurazano[3,4-b]pyrazine (4) with 2-uoro-2,2-dinitroethanol
(5). Experimental results show that 6 has realizable melt-
castable properties, high decomposition temperature
ꢀ
exceeding 200 C and exhibits better detonation properties (D:
8512 m sꢁ1, P: 32.4 GPa) than those of TNT. Worth noting that it
also indicates slightly better insensitivities (IS: 17 J, FS: 252 N)
than those of TNT. For the polycrystalline properties, PXRD and
DSC reveal that different polymorph has signicant effect on
their phase structure and thermal properties, respectively.
Moreover, the ab initio simulations of unit cell in crystal indi-
cate that hydrogen and halogen bonding interaction in 6-M is
stronger than that of 6-A. The above-mentioned performance
not only suggest that the incorporation of FDN moiety into
framework with high HOF is helpful for enhancing the ener-
getic properties but also show that 6 has the potential as an
ingredient in melt-cast energetic materials.
Experimental section
Safe cautions
Although none of the above-mentioned energetic materials have
exploded or detonated in the procedure of this research, small
scale and safety training are strongly encouraged. Mechanical
actions such as scratching and scraping must be avoided.
Manipulations should be performed behind a polymethyl
methacrylate (PMMA) guard with at least 12 mm thickness. Face
shield, eye protection and leather gloves must be strictly put on.
General methods
The heat of formation was computed by Gaussian 09 code.
All reagents were purchased from Sigma Aldrich or J&K in First, the geometric optimization, frequency analysis and
analytical grade and were used as received. 1H, 13C, 15N and 19
single-point energies of compound 6 were accomplished by
F
NMR spectra were recorded on a 300 MHz (Bruker AVANCE 300) using the M06-2X with the 6-311+G** basis set.36 Then, the
or 500 MHz (Bruker AVANCE 500) nuclear magnetic resonance atomization energies were obtained by employing the G2 ab
spectrometer. Chemical shis in the 1H and 13C spectra are initio method or from NIST Chemistry Webbook37 and solid
reported relative to Me4Si, 15N NMR to MeNO2 and 19F NMR to phase heat of formation was computed via isodesmic reactions.
This journal is © The Royal Society of Chemistry 2017
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Paper
The solid-state heat of formation can be estimated by sub- 69.5 g, yield 90.3%).14 5–10 ꢀC, 3 (8 g, 52 mmol) was suspended
tracting the heats of formation from gas-phase heats of in SOCl2. DMF (8 mL) was added dropwise to the above mixture
ꢀ
formation. The heat of sublimation can be estimated following with a temperature below 15 C. Then the mixture was stirred
the Trouton's rule,38 which is shown in the eqn (1):
for 0.5–1 h at 20 C and for 1–2 h at 75 C. Under 75 C, the
mixture became a clear-yellow solution. Aer cooling to the
ambient temperature, the solution was poured slowly into ice
water (500 mL), the precipitate was ltered and dried in air to
give the product (4) (white solid, 4.7 g, yield 48.0%). 13C{1H}
NMR (DMSO-d6, 125.78 MHz): d ¼ 155.8, 151.4 ppm.
ꢀ
ꢀ
ꢀ
DHsub ¼ 188/J molꢁ1 Kꢁ1
T
(1)
Here, T represents either the melting point or the decom-
position temperature when no melting occurs prior to decom-
position. Detonation parameters were carried out in the
EXPLO5 (V6.02) program using X-ray densities which were
converted to room temperature values via the following
equation:39
2-Fluoro-2,2-dinitroethanol (5)
5 was synthesized by an improved method according to the
literature (colorless oil, 5.8 g, yield 49.1%).9c 1H NMR (CDCl3,
300.06 MHz): d ¼ 2.62 ppm (t, JH–F ¼ 7.5 Hz, 1H, OH), 4.66 ppm
(dd, JH–F ¼ 15.0 Hz, 2H, OCH2); 13C NMR (CDCl3, 75.46 MHz):
r298 K ¼ rT/[1 + an(298ꢁT0)
(2)
where the coefficient of volume expansion an is 1.5 ꢂ 10ꢁ4 Kꢁ1
,
rT and T0 are the crystal density and the relative temperature,
d ¼ 61.8 ppm (d, JC–F ¼ 21.2 Hz, OCH2), 121.1 ppm (d, JC–F
¼
respectively.
289.7 Hz, CF(NO2)2); 19F(CDCl3, 282.30 MHz): ꢁ111.7 ppm.
HPLC purity >99.7%.
X-ray diffraction
A colorless prismatic crystal of dimension 0.20 ꢂ 0.17 ꢂ 0.12
mm3 (4), colorless block crystal of dimension 0.10 ꢂ 0.05 ꢂ 0.03
mm3 (6-A), colorless prismatic crystal of dimension 0.20 ꢂ 0.17
ꢂ 0.10 mm3 (6-M), and colorless plate crystal of dimension 0.18
ꢂ 0.10 ꢂ 0.02 mm3 (6-M) were mounted on a MiteGen Micro-
Mesh using a small amount of Cargille Immersion Oil. Data
were collected on a Bruker three-circle platform diffractometer
equipped with a SMART APEX II CCD detector. A kryo-Flex low-
temperature device was used to keep the crystals at 293(2) K and
130 K during data collection. Data collection was performed and
the unit cell was initially rened using APEX2. Data reduction
was carried out using SAINT and XPREP. Corrections were
applied for Lorentz, polarization, and absorption effects using
SADABS. The structures were further solved and rened with the
aid of the programs using direct methods and least-squares
minimization by SHELXS-97 and SHELXL-97 code.40 The full-
matrix least-squares renement on F2 involved atomic coordi-
nates and anisotropic thermal parameters for all non-H atoms.
The H atoms were included using a riding model. The non-H
atoms were rened anisotropically. The nalized CIF les
were checked with checkCIF, and deposited at the Cambridge
Crystallographic Data Centre as supplementary publications 4
(293 K), 6-A (130 K), 6-M (293 K) and 6-M (130 K) (the crystalline
parameters were listed in the ESI†). Intra- or intermolecular
hydrogen-bonding interactions were analyzed with Diamond
soware (version 3.2 K) as well as the illustrations of molecular
structures.
5,6-Di(2-uoro-2,2-dinitroethoxy)furazano[3,4-b]pyrazine (6)
To a solution of 4 (0.96 g, 5 mmol) and 5 (1.7 g, 11 mmol) in
acetone (20 mL) was added trisodium phosphate dodecahydrate
(1.8 g, 5 mmol) in portions. The mixture was stirred at ambient
temperature for 8 h. The precipitate was ltered off and then the
ltrate was evaporated under vacuum, recrystallized in ethanol
and water to yield 6 (white solid, 1.92 g, yield 89.3%), m.p. 115–
117 ꢀC. 1H NMR (DMSO-d6, 500.20 MHz): d ¼ 5.95 ppm (d, JH–F
¼ 15.0 Hz, 4H, OCH2); 13C NMR (DMSO-d6, 125.78 MHz): d ¼
65.1 ppm (d, JC–F ¼ 20.1 Hz, OCH2), 119.6 ppm (d, JC–F
¼
299.4 Hz, CF(NO2)2), 150.3 ppm (N]C–N), 153.9 ppm (N]C–O);
15N (DMSO-d6, 50.68 MHz): ꢁ22.6 ppm, ꢁ25.0 ppm (d, JN–F
¼
15.2 Hz), ꢁ152.9 ppm; 19F(DMSO-d6, 470.66 MHz): ꢁ110.5 ppm
(d, JF–N ¼ 4.7 Hz). IR (KBr pellet): 3442 (m), 1614 (s), 1556 (w),
1508 (w), 1436 (w), 1402 (w), 1332 (w), 1301 (w), 1260 (w), 1226
(w), 1020 (w), 997 (w), 867 (w), 850 (w), 794 (w), 592 (w), 491 (w)
cmꢁ1. Elemental analysis for C8H4F2N8O11 (426.19): C 22.55, H
0.95, N 26.29%; found: C23.17, H 1.02, N 25.15%.
Acknowledgements
We are indebted to the Science and Technology Development
Foundation of China Academy of Engineering Physics (Grant
No. 2015B0302055), the National Natural Science Foundation of
China (Grant No. 11402237), and NSAF Foundation of National
5,6-Dichlorofurazano[3,4-b]pyrazine (4)
3,4-Diaminofurazan (DAF) (50 g, 0.5 mol) and anhydrous oxalic Natural Science Foundation of China and China Academy of
acid (49.5 g, 0.55 mol) were suspended in 10% HCl (80 mL) at Engineering Physics (Grant No. U1530262) for funding of this
ambient temperature, and the above mixture was reuxed for work. Mr Yinshuang Sun and Mrs Min Zhong are gratefully
another 3 h. Under the reaction temperature, the mixture acknowledged for the assistance in performing the BAM and
became clear solution and then white precipitate appeared. The ESD sensitivity tests. National Supercomputer Centre in
precipitate was ltered, washed with cold water and dried in air Guangzhou (NSCCGZ) is also thanked for the support with the
to give 5,6-dihydoxyfurazano[3,4-b]pyrazine (3) (white solid, quantum chemical calculations.
38850 | RSC Adv., 2017, 7, 38844–38852
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38852 | RSC Adv., 2017, 7, 38844–38852
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