Dense energetic salts of N,N′-dinitrourea (DNU)

Article abstract of DOI:10.1039/b712417a

The structure of dinitrourea (DNU) as determined by X-ray single-crystal diffraction is orthorhombic; space group: Fdd2; a = 12.0015(9), b = 17.6425(13), c = 4.5555(4); V = 964.57 A³, Z = 8, T = 90 K; D_c = 2.067 g cm^-3. The heat of formation (Δ_fH° ₂₉₈) of DNU in the gas phase was calculated to be 24.88 kJ mol ^-1 by using the G3MP2 method based on isodesmic reactions. Eleven mono-organic salts of DNU were prepared in acetonitrile and characterized via NMR spectra, elemental analyses and DSC. Derivatives of 1,2,4-triazolium salts of DNU exhibit densities ranging from 1.75 to 1.86 g cm^-3 and detonation properties comparable with those of RDX and HMX. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/b712417a

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www.rsc.org/njc | New Journal of Chemistry
Dense energetic salts of N,N0-dinitrourea (DNU)w
Chengfeng Ye, Haixiang Gao, Brendan Twamley and Jean’ne M. Shreeve*
Received (in Durham, UK) 13th August 2007, Accepted 18th September 2007
First published as an Advance Article on the web 1st October 2007
DOI: 10.1039/b712417a
The structure of dinitrourea (DNU) as determined by X-ray single-crystal diffraction is
orthorhombic; space group: Fdd2; a = 12.0015(9), b = 17.6425(13), c = 4.5555(4); V = 964.57
A3, Z = 8, T = 90 K; Dc = 2.067 g cmꢀ3. The heat of formation (DfH1298) of DNU in the gas
phase was calculated to be 24.88 kJ molꢀ1 by using the G3MP2 method based on isodesmic
reactions. Eleven mono-organic salts of DNU were prepared in acetonitrile and characterized via
NMR spectra, elemental analyses and DSC. Derivatives of 1,2,4-triazolium salts of DNU exhibit
densities ranging from 1.75 to 1.86 g cmꢀ3 and detonation properties comparable with those of
RDX and HMX.
more energy via detonation resulting in better detonation
Introduction
properties. To date, knowledge about DNU is limited to its
density, thermal behavior and reactions with formaldehyde4 as
well as its metal and ammonium salts.5 No other energetic
salts of DNU are available. In this paper, we report the crystal
structure of DNU, its calculated detonation properties, and
reactions with nitrogen heterocycles to form energetic salts
(Scheme 1).
As an important family of high energy density materials
(HEDM), dinitrourea (DNU)1–5 and its derivatives have
attracted considerable interest due to their relatively high
densities. For example, DNU has a density of 1.98 g cmꢀ3
,
while other high-explosive containing the dinitrourea fragment
such as K-6,6 K-56,7 TNGU,8 TNCB,9 HHTDD10 (Fig. 1) are
very dense ranging from 1.93 to 2.07 g cmꢀ3. These materials
were expected to exhibit superior detonation properties since
density is a critical factor that, according to a semi-empirical
equation suggested by Kamlet and Jacobs,11 affects detonation
performance, viz., detonation pressure (P) is dependent on the
square of the density, and the detonation velocity (D) is
proportional to the density.
Discussion
Dinitrourea (DNU) (1) was synthesized according to the
literature,3 i.e., after nitration of urea with mixed acid, the
desired product precipitated, was washed with trifluoroacetic
acid, and dried under vacuum. This colorless solid was readily
soluble in CH3CN, diethyl ether, ethyl acetate, benzene, THF,
CHCl3, CH2Cl2, etc. In contrast with a previous report,3 we
found that DNU was immediately decomposed in DMSO with
concomitant gas evolution, and it gradually decomposed in
water. In CD3CN as solvent, the proton NMR spectrum
contains a broad peak at 11.0 ppm which corresponds to a
typical N-acid proton. The mass spectrum (MS-EI solid
DNU was also found to be a good precursor to nitroamine
(H2NNO2)12 and a facile starting material for preparation of
K-6.13,14 However, dinitrourea was initially reported to under-
go decomposition at room temperature which could lead to
spontaneous ignition.1 Later, this drawback was overcome by
washing with trifluoroacetic acid to remove the trace amount
of acid contamination introduced during the manufacturing
process.3 Another hurdle for the practical utility of DNU is its
instability to heat and friction which could be overcome by
converting to an organic salt. Our previous experiences have
demonstrated that when a strong N-acid was transformed into
a corresponding organic salt, the thermal stability was im-
proved by nearly 50 1C. For example, 5-nitroaminotetrazole
decomposed at 128 1C, but 4-amino-1,2,4-triazolium-5-nitro-
aminotetrazolate was stable up to 184 1C.15
Most recently, ionic liquids/solids have been prepared and
characterized as propellants where nitrate, perchlorate,
dinitramide, azide, 3,5-dinitrotriazolate, 5-nitrotetrazolate,
5-nitroaminotetrazolate, 2,4,5-trinitroimidazolate, or azote-
trazolate, etc. served as anion.16–22 Considering that dinitro-
urea has a positive oxygen balance, its salts should release
Department of Chemistry, University of Idaho, Moscow, Idaho,
83844-2343, USA. E-mail: jshreeve@uidaho.edu
w Electronic supplementary information (ESI) available: Computation
details. See DOI: 10.1039/b712417a
Fig. 1 Structures of some explosive candidates containing a DNU
fragment.
ꢁc
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structurally determined congeners, however, K-6,6 K-56,7a
DNDAGU,7b 1,3-dinitro-1,3-diazacyclopentanone7c and
a
few derivatives7d of the latter are known. These all display
structural features similar to the DNU framework although
the bond lengths in K-56 are surprisingly asymmetric com-
pared to DNU. The packing of DNU displays stacks of
hydrogen bonded units (via the amino N–Hꢂ ꢂ ꢂOC ketone
synthon) parallel to the c-axis. The packing is efficient and
results in a high density of 2.067 g cmꢀ3 at 90 K.
DNU is a fairly strong N-acid, which readily reacts with
organic bases. In a previous report,5 its alkali and alkaline
earth metal salts were successfully prepared in water. Reac-
tions of DNU with hydrazine or hydroxylamine give a sub-
stitution product rather than simple salts. Ethanol is not
recommended as a solvent for DNU since the reaction may
form N-nitrocarbamates. In our hands, acetonitrile was the
most suitable solvent for reactions of DNU to form organic
salts. Although some organic bases, e.g. 3,4,5-triamino-1,2,4-
triazole, are slightly soluble in CH3CN, reaction of DNU in
CH3CN solution for an extended time normally gave the
desired mono salt as a precipitate with good purity based on
elemental analysis. Although, under all conditions tried, we
were not successful in synthesizing bianionic salts even when a
two-fold excess of base was used, this work is continuing.
Some of the mono salts can be prepared in water, but in lower
yields than in CH3CN as a solvent. For example, when DNU
and 1,2,4-triazole or 3-amino-2,4-triazole were mixed in 1 : 1
or 1 : 2 ratio in water, the mono salts were immediately
crystallized. The structure of 2a (triazolium dinitrourea) is
shown in Fig. 3(a). This compound is also chiral and again the
absolute configuration could not be determined reliably. The
deprotonation of the DNU has led to an asymmetry in the
bond lengths with a contraction of N8–N9 to 1.321 (2) A
(1.394(2) A in neutral DNU) and a 0.02–0.05 A lengthening of
the N–O bonds indicating delocalization of the anionic charge.
The addition of the cation leads to significant hydrogen
bonding in the extended structure shown in Fig. 3(b)
(N2–H2. . .O11 synthon, N4–H4. . .O6 synthon N12–H12ꢂ ꢂ ꢂO6
synthon as well as cation–cation interactions e.g.,
N2–H2ꢂ ꢂ ꢂN1). This leads to a structure that is almost as
closely packed as the neutral DNU with a relatively high
density 1.868 g cmꢀ3 at 90 K.
Scheme 1 Synthesis of DNU salts.
probe) clearly supports the existence of the molecule with a
molecular ion peak 150 (M+, 0.1), and reasonable fragments
such as 107 ((NO2)2NH, 0.2), 89 (M+ ꢀ NHNO2, 0.4), 62
(NO2NH2, 3.2), 46 (NO2+, 100).
DNU was crystallized from a diethyl ether–hexane mixture
yielding clear colorless needles suitable for crystallographic
analysis (Table 1). The complete molecule, symmetry gener-
ated in the chiral orthorhombic space group Fdd2, is shown in
Fig. 2(a). The absolute configuration could not be determined
reliably. There is a spiral twist to the molecule and the torsion
angle O1–N1–N2–C1 is ꢀ11.21. There are relatively few
Table 1 Crystal data for 1 and 2a
Empirical formula
Mr
CH2N4O5
150.07
C3H5N7O5
219.14
Crystal system
Space group
a/A
b/A
Orthorhombic
Fdd2
12.0015(9)
17.6425(13)
4.5555(4)
90
964.57(13)
8
90(2)
2.067
0.210
4.11–30.03
3712
389 (0.0191)
389/1/47
1.097
Monoclinic
P21
Physical properties of DNU salts
7.6156(3)
4.9837(2)
10.3119(4)
95.530(1)
389.55(3)
2
90(2)
1.868
0.173
1.98–29.00
5716
1151 (0.0196)
1151/1/136
1.091
0.0255
0.0655
As observed previously, DNU itself began to lose weight at
about 90 1C, and completely decomposes at 110 1C.3 After
forming salts, the thermal stability was greatly improved. For
example, the DNU salts of 2e and 2g were thermally stable to
180 1C. Density is one of the most important properties of
energetic materials. From Table 2, it can be seen that triazo-
lium based DNU salts exhibit higher densities than other
species. Normally, when a dense compound acts as an anion,
the density of the corresponding salt will decrease by 0.2 g
cmꢀ3 or more. Most recently, we established volume para-
meters to predict the density of energetic salts.23 Based on
these parameters, the volume of DNU anion was estimated at
137 A3. The predicted densities are also listed in Table 2 which
are reasonably close to the experimental values providing that
c/A
b/1
V/A3
Z
T/K
Dc/Mg mꢀ3
m/mmꢀ1
y Range for data collection/1
Reflections collected
Indep. reflections (Rint
)
Data/restraints/parameters
GOF
a
R1 [I 4 2s(I)]
0.0236
0.0617
a
wR2 ([I 4 2s(I)])
P
P
P
a
2
R1
=
||Fo|
.
ꢀ
|Fc||/ |Fo|; wR2
=
{
[
w(Fo
ꢀ
Fc2)2]/
P
[w(Fo2)2]}1/2
ꢁc
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318 | New J. Chem., 2008, 32, 317–322

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Fig. 2 (a) Displacement ellipsoid (30%) plot of DNU (1) with hydrogen atoms (arbitrary spheres) shown. (b) Ball-and-stick packing diagram
showing alternating hydrogen bonded stacks parallel to the c axis indicated by dashed lines.
the volumes of triazolium salts were corrected by ꢀ8 A3 to
account for strong hydrogen bonds. Normally materials with
strong hydrogen bonds have low solubility in common sol-
vents; this is also true for DNU salts. As a matter of fact, most
of the DNU salts are slightly soluble in water due to strong
hydrogen bonding. As demonstrated, some triazolium salts
did have unexpectedly high densities, but very few imidazo-
lium salt or guanidinium salts would have much higher
densities than the predicted values based on our volume
parameters. In other words, triazolium could be the candidate
cation of choice to form high density salts with oxygen-rich
anions.
ꢀ45.94 ꢃ 0.35 kJ molꢀ1, 111.78 ꢃ 0.42 kJ molꢀ1, which are
taken from NIST webbook.26 Accordingly, DfH1298 of DNU,
DNUꢀ and DNU2ꢀ in the gas phase were calculated to be
24.88, ꢀ198.46 and 31.38 kJ molꢀ1, respectively. Using 20 kcal
molꢀ1 as the widely-accepted heat of sublimation for DNU,
the estimated DfH1298 of DNU in the solid state is ꢀ58.8 kJ
molꢀ1. The detonation pressure and detonation velocity of
DNU was thus calculated using Cheetah 4.0 as 36.1 GPa
and 8861 m sꢀ1, which compare favorably with RDX
(P = 34.4 GPa, D = 8750 m sꢀ1).
The heats of formation of the triazolium cations were also
27
computed with isodesmic reactions using G3
or G3MP2
Heats of formation (DfH1298) of DNU and DNU anions
were computed using the G3MP2 method24 in the Gaussian03
program suite,25 based on the isodesmic reactions below
(Scheme 2):
method with 1,2,4-triazolium as reference whose heat of
formation in the gas phase is known as 836.78 kJ molꢀ1
.
The heats of formation of 3-amino-1,2,4-triazolium, 4-ami-
no-1,2,4-triazolium, 3,5-diamino-1,2,4-triazolium and 3,4,5-
triamino-1,2,4-triazolium were calculated to be 799.41,
942.70, 763.99, 877.55 kJ molꢀ1, respectively. The incremental
contribution of each additional N–NH2 is 110 kJ molꢀ1, while
Gas phase heats of formation (DfH1298) for the refeꢀrence
molecules, e.g., urea, HNO2, NH2NH2 and NH3, NH2 are
ꢀ235.5 ꢃ 1.2 kJ molꢀ1, ꢀ76.73 kJ molꢀ1, 95.35 kJ molꢀ1
,
Fig. 3 (a) Displacement ellipsoid plot of 2a with hydrogen atoms shown. (b) Ball and stick packing diagram of 2a viewed down the b axis.
Hydrogen bonding indicated by dashed lines.
ꢁc
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Table 2 Melting points, densities, heat of formation (DfH1298) and detonation properties of DNU saltsa,b,c
Density/g cmꢀ3
Compd
Mp/1C
Exptl.
Calc.
DfH1298 (cation)
DfH1298 (anion)
DHL
DfH1298
P/GPa
D/m sꢀ1
1
90
122
148
110
157
182
105
179
148
128
151
110
105
135
1.98
1.83
1.80
1.86
1.85
1.75
1.68
1.73
1.67
1.73
1.65
1.73
1.75j
1.70j
1.98
1.80d
1.79d
1.81d
1.78d
1.79d
1.68
1.73
1.69
1.71
1.68
1.71
1.76
1.61
b
—
—
—
510.72
ꢀ58.8
127.6
101.17
240.16
70.25
36.1
33.8
32.2
36.4
35.1
31.0
23.8
28.2
26.2
29.8
21.3
28.85
32.3
31.1
8861
8648
8568
8905
8881
8609
7681
8244
8240
8676
7715
8507
9051
9016
2a
2b
2c
2d
2e
2f
2g
2h
2i
836.78e
799.41f
942.70f
763.99f
877.55g
720.18e
1903.55h
563.74f
660.31f
799.10i
1630.5f
630.54e
630.54e
ꢀ198.46
ꢀ198.46
ꢀ198.46
ꢀ198.46
ꢀ198.46
ꢀ198.46
ꢀ198.46
ꢀ198.46
ꢀ198.46
ꢀ198.46
ꢀ198.46
ꢀ198.46
31.38
499.78
504.08
495.28
480.85
500.01
1398.48
510.30
506.24
462.35
1514.11
542.24
1585.77
c
198.24
21.71
108.15
ꢀ145.02
ꢀ44.39
138.29
ꢀ280.53
ꢀ110.16
ꢀ293.31
2j
2k
2l
2mk
a
DfH1298, DHL (lattice energy) kJ molꢀ1
.
Lattice energy (DHL)—estimated according to ref. 27. Detonation pressure (P), detonation velocity
d
e
(D)—calculated using Cheetah 4.0. Volume of the salts were corrected by ꢀ8 A, for strong hydrogen bonds, see ref. 23. NIST web book,
f
ref. 26. Calculated value using G3 method. Calculated value using G3MP2 method. From ref. 18k. From ref. 18i. From
g
h
i
j
ref. 5. (NH4+)2DNU2ꢀ
.
k
each additional C–NH2 accounts for a decrease in heat of
formation by 36 kJ molꢀ1. Salts 2c and 2d show detonation
properties similar to those of DNU. It is notable that the
mono ammonium salt of DNU (2l) (9051 m sꢀ1) exhibits the
highest detonation velocity among the current salts tested,
which is similar to that of HMX (9100 m sꢀ1). This is similar
to the bis-ammonium salt of DNU (2m), although 2m has a
lower heat of formation and a more negative oxygen balance.
This apparently benefits from its higher density than predicted
(Scheme 3).
obtained on a differential scanning calorimeter and a thermo-
gravimetric analyzer at a scan rate of 10 1C minꢀ1, respec-
tively. Densities of solid salts were obtained at room
temperature by employing a Micromeritics Accupyc1330 gas
pycnometer. Elemental analyses were determined using an
Exeter CE-440 elemental analyzer.
X-Ray crystallography
Crystals of compounds DNU, 1, (2a) were removed from the
flask, a suitable crystal was selected and attached to a glass
fiber, and data were collected at 90(2) K using a Bruker/
Siemens SMART APEX instrument Bruker/Siemens SMART
APEX instrument (Mo Ka radiation, l = 0.71073 A)
equipped with a Cryocool NeverIce low temperature device.
Experimental
CAUTION: Although we experienced no difficulties in hand-
ling these energetic materials, small scale and best safety
practices (leather gloves, face shield) are strongly encouraged!
General methods
1H and 13C NMR spectra were recorded on a 300 MHz
nuclear magnetic resonance spectrometer operating at 300.13
and 75.48 MHz, respectively, using DMSO-d6 as solvent
unless otherwise indicated. Chemical shifts are reported rela-
tive to Me4Si. The melting and decomposition points were
Scheme 2 Isodesmic reactions for DNU and DNU anions.
Scheme 3 Isodesmic reactions for triazolium cations.
ꢁc
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Data were measured using omega scans 0.5(0.3)1 per frame for
30 s for both. A total of 1464 (2400) frames were collected with
a final resolution of 0.71 (0.83) A. Cell parameters were
retrieved using APEX2 (SMART) software28 and refined using
the INTEGRATE module (SAINT Plus)29 on all observed
reflections. Data reduction and correction for Lp and decay
were performed using the SAINT Plus software. Absorption
corrections were applied using multi-scan absorption correc-
tion in the SCALE module in APEX2 (SADABS).30 The
structure was solved by direct methods and refined by least
squares method on F2 using the SHELXTL program pack-
age.31 Structure were solved in the space group Fdd2 for 1 and
P21 for 2a by analysis of systematic absences. In this all-light-
atom structure the value of the Flack parameter did not allow
the direction of the polar axis to be determined and Friedel
reflections were then merged for the final refinement. All non-
hydrogen atoms were refined anisotropically. No decomposi-
tion was observed during data collection. Details of the data
collection and refinement are given in Table 1.
2e. 1H NMR d: 5.55 (s, 2H), 7.00 (s, 4H). 13C NMR d 150.0,
153.9. Calc. for C3H8N10O5: C, 13.64; H, 3.05; N, 53.02.
Found: C, 13.79; H, 2.88; N, 52.42%.
2f. 1H NMR d: 9.02 (s, 1H), 7.63 (s, 2H). 13C NMR d 119.5,
134.5, 154.0. Calc. for C4H6N6O5: C, 22.03; H, 2.77; N, 38.53.
Found: C, 22.09; H, 2.56; N, 38.25%.
1
2g. H NMR d: 8.15 (s). 13C NMR d 153.7, 154.2, 158.5.
Calc. for C6H12N18O10: C, 14.52; H, 2.44; N, 50.80. Found: C,
14.79; H, 2.26; N, 50.31%.
2h. 1H NMR d: 7.00 (s). 13C NMR d 156.7, 158.2. Calc. for
C2H7N7O5: C, 11.49; H, 3.37; N, 46.89. Found: C, 11.76; H,
3.30; N, 46.71%.
2i. 1H NMR d: 5.54 (br s, 2 H), 6.76 (br s, 2 H), 7.14 (br s, 2
H), 8.55 (br s, 1 H), 13C NMR d 154.1, 158.9. Calc. for
C2H8N8O5: C, 10.72; H, 3.60; N, 49.99. Found: C, 10.84; H,
3.53; N, 49.33%.
CCDC reference numbers 654504 and 654505.
2j. 1H NMR d: 4.81 (s, 12H). 13C NMR d 71.4, 153.9. Calc.
for C7H14N8O5 ꢂ 0.5H2O: C, 28.10; H, 5.05; N, 37.45. Found:
C, 28.04; H, 4.66; N, 37.50%.
For crystallographic data in CIF or other electronic format
see DOI: 10.1039/b712417a
2k. H NMR d: 3.06 (s, 4H), 9.16 (br s, 6H). 13C NMR d
36.7, 154.1. Calc. for C4H12N10O10: C, 13.34; H, 3.36; N,
1
Synthesis of dinitrourea (DNU, 1)
DNU was synthesized according to the literature.3 Urea (3.0 g)
was added in portions to a mixture of 6 mL H2SO4 (98%) and
8 mL HNO3 (100%, prepared from H2SO4 and NaNO3). After
stirring for 3 h at room temperature, a white precipitate was
collected and washed with trifluoroacetic acid (20 mL ꢄ 4),
and dried under vacuum. 2.5 g, yield 40%. 1H NMR (CD3CN)
d: 11.48 (br s, 2H). 13C NMR (CD3CN) d 142.3. MS (EI-solid
probe): 150 (M+).
38.89. Found: C, 13.33; H, 3.25; N, 39.13%.
Conclusions
In conclusion, a variety of new energetic salts of DNU were
synthesized and characterized. The structure of DNU was
established using X-ray single-crystal diffraction. DNU and
derivatives of 1,2,4-triazolium show high density (1.75–1.86 g
cmꢀ3) as result of strong hydrogen bonding. Most of the DNU
salts exhibit detonation pressures (430 GPa) and detonation
velocities comparable to RDX. To design salts with the most
attractive energetic properties, the cation and anion should be
carefully selected to deliberately form strong inter- and/or
intra-molecular hydrogen bonds which in turn enhances the
density and concomitantly the detonation characteristics.
General synthesis procedure for DNU salts
To a DNU (1.2 mmol) solution in 5 mL CH3CN, was added
l mmol of triazole, 3-aminotriazole, 4-aminotriazole, 3,5-dia-
minotriazole, 3,4,5-triaminotriazole, imidazole, guanidinium
bicarbonate or hexamethylenetetrazine or 0.5 mmol of 3,6-
diguanidine-1,2,4,5-tetrazine, aminoguanidinium carbonate,
or ethylenediamine (0.5 mmol), respectively. After stirring at
room temperature for 3–12 h, the white precipitate was filtered
off and washed with CH3CN (3 mL ꢄ 3), diethyl ether
(3 mL ꢄ 3) and dried under vacuum.
Acknowledgements
The authors gratefully acknowledge the support of HDTRA1-
07-1-0024), NSF (CHE-0315275), and ONR (N00014-06-1-
1032). The Bruker (Siemens) SMART APEX diffraction
facility was established at the University of Idaho with the
assistance of the NSF-EPSCoR program and the
M. J. Murdock Charitable Trust, Vancouver, WA.
2a. 1H NMR d: 9.24 (s, 2H). 13C NMR d 143.7, 153.2. Calc.
for C3H5N7O5: C, 16.44; H, 2.30; N, 44.75. Found: C, 16.33;
H, 2.16; N, 44.37%.
2b. 1H NMR d: 8.23 (s, 1H). 13C NMR d 139.6, 151.1, 153.9.
Calc. for C3H6N8O5: C, 15.39; H, 2.58; N, 47.86. Found: C,
15.41; H, 2.38; N, 47.76%.
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5 S. G. Il’yasov, A. A. Lobanova, N. I. Popov and R. R. Sataev,
Russ. J. Org. Chem., 2002, 38, 1731–1738.
2d. 13C NMR d 144.1, 151.5, 154.0. Calc. for C3H7N9O5: C,
14.46; H, 2.83; N, 50.60. Found: C, 14.40; H, 2.60; N, 50.45%.
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322 | New J. Chem., 2008, 32, 317–322