C6N10O4: Thermally Stable Nitrogen-Rich Inner Bis(diazonium) Zwitterions

Article abstract of DOI:10.1021/acs.orglett.9b02875

Two nitrogen-rich inner bis(diazonium) salts, the 4,4′-dinitro-5,5′-diazo-2,2′-bisimidazole (2) and the 4,4′-dinitro-5,5′-diazo-3,3′-bispyrazole (4) zwitterions, are described. Compound 2 was synthesized unexpectedly from the nitration of 4,4′-dinitro-5,5′-diamino-1H,1′H-2,2′-bisimidazole (1) in a mixture of trifluoroacetic anhydride and 100% nitric acid. Both 2 and 4 show good thermal stability, especially 2 has a decomposition temperature of 199 °C, which is the highest one in any of the reported hydrogen-free nitrogen-rich diazonium salts.

Full text of DOI:10.1021/acs.orglett.9b02875

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
C6N10O4: Thermally Stable Nitrogen-Rich Inner Bis(diazonium)
Zwitterions
Yongxing Tang,† Gregory H. Imler,‡ Damon A. Parrish,‡ and Jean’ne M. Shreeve*,†
†Department of Chemistry, University of Idaho, Moscow, Idaho 83844-2343, United States
‡Naval Research Laboratory, 4555 Overlook Avenue, Washington, D.C. 20375, United States
S
* Supporting Information
ABSTRACT: Two nitrogen-rich inner bis(diazonium) salts, the 4,4′-dinitro-5,5′-diazo-2,2′-
bisimidazole (2) and the 4,4′-dinitro-5,5′-diazo-3,3′-bispyrazole (4) zwitterions, are
described. Compound 2 was synthesized unexpectedly from the nitration of 4,4′-dinitro-
5,5′-diamino-1H,1′H-2,2′-bisimidazole (1) in a mixture of trifluoroacetic anhydride and 100%
nitric acid. Both 2 and 4 show good thermal stability, especially 2 has a decomposition
temperature of 199 °C, which is the highest one in any of the reported hydrogen-free
nitrogen-rich diazonium salts.
iazonium compounds have been investigated extensively
it has a low density of 1.73 g cm−3 and a very modest
1
Dsince the first one was reported in 1858. They can be detonation performance (νD, 7780 m s−1; P, 26.7 GPa).
Now two new ternary CNO-inner bis(diazonium) salts, 4,4′-
dinitro-5,5′-diazo-2,2′-bisimidazole zwitterion (2) and 4,4′-
dinitro-5,5′-diazo-3,3′-bispyrazole zwitterion (4) and their X-
ray crystal structures are described. Both compounds have
good thermal stability and excellent detonation performance,
which make them good candidates for the next generation of
green lead-free primary explosives.
used as valuable building blocks or intermediates in a wide
range of organic syntheses.2 In general, the synthesis of a
diazonium salt is initiated by the diazotization of an aromatic
amine with sodium nitrite and a mineral acid or in an organic
solvent using an alkyl nitrite reagent.3−6 Some of them are
prepared by the nitration in a mixed acid.7−9 With the
development of isolation technology and characterization
methods, many more diazonium salts have been isolated and
confirmed by their solid state structures. Nevertheless, the
diazonium salts are most generally used in situ without
isolation due to their extremely dangerous nature rising from
low thermal stability and chemical instability.
The synthetic route to 2 is given in Scheme 1. 4,4′,5,5′-
Tetranitro-2,2′-bisimidazole (TNBI) was prepared from
Scheme 1. Synthesis of Inner Bis(diazonium) Salts 2 and 4
Diazonium moieties are easily initiated by shock and impact,
and therefore, they have been well studied as lead-free primary
explosives. A typical example is 2-diazo-4,6-dinitrophenol
(DDNP), which is now commercially available in many
countries and has extensive application in percussion caps and
in detonators. However, DDNP has several drawbacks
including high sensitivity to mechanical stimuli, photo-
sensitivity, and high toxicity as well as low detonation
performance (detonation velocity, 6900 m s−1; detonation
pressure, 24.2 GPa),10 which encouraged us to search for
environmentally friendly and highly energetic candidates.
Although some other nitrogen-rich diazonium salts, such as
the 4-diazo-3,5-dinitropyrazole zwitterion (DDNPz)11 and
zwitterionic diazonium tetrazolyl-1,2,3-triazolate,7 show prom-
ising performance as potential green primary explosives, due to
their low thermal stability (<160 °C), the most important
criterion for a primary explosive, i.e., stability to least to 180
°C,12 is not realized.
commercially available glyoxal and sodium bisulfite.15 TNBI
was heated at 100 °C for 12 h in aqueous ammonia in a sealed
tube, which resulted in the formation of 4,4′-dinitro-5,5′-
diamino-1H,1′H-2,2′-bisimidazole (1). When 1 was treated
with a mixture of trifluoroacetic anhydride (TFAA) and 100%
nitric acid at 0 °C, the 4,4′-dinitro-5,5′-diazo-2,2′-bisimidazole
zwitterion (2) was isolated and confirmed by single crystal X-
In addition, only one hydrogen-free nitrogen-rich inner
bis(diazonium) salt (inner salt 5,5′-bis(3-diazo-1,2,4-triazole))
has been reported.13,14 However, its decomposition temper-
ature is only 120 °C, which limits its practical application.
Additionally, due to its elemental composition (only C and N),
Received: August 14, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b02875
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
ray diffraction. Since the yield of 2 is low (37%), alternative
synthetic attempts were tried. When 1 was treated with a
mixture of sodium nitrite and dilute sulfuric acid (20%), only a
trace of 2 was isolated, which may be a result of the very low
solubility of 1 in such dilute sulfuric acid. When concentrated
sulfuric acid was used, the yield of 1 was improved to 80%.
The inner bis(diazonium) salt (4) was observed as an
intermediate in our previous study,17 but it was not isolated.
4,4′-Dinitro-5,5′-diamino-2H,2′H-(3,3′-bipyrazole) (3,
Scheme 1) was treated with sodium nitrite in 20% H2SO4 at
0 °C for 30 min. The product was collected by filtration and
washed with water. It is not necessary to add a base to bind the
acid as previous studies have suggested.16 Compound 4 was
also fully characterized by IR, 13C and 15N NMR, and single
crystal X-ray diffraction analysis.
Compound 2 crystallizes in the monoclinic space group P21/
c with two molecules per unit cell and with a crystal density of
1.834 g cm−3 at 296 K. The molecular structure is given in
Figure 1a. The atoms N7, N6, and C5 lie in the same plane
Figure 2. (a) Molecular structure of 4·CH3NO2. (b) Packing diagram
of 4·CH3NO2 along c axis.
molecules in the crystal structure, no hydrogen bonds are
observed. Along the c axis, the nitromethane solvent is in the
“vases” organized by the molecules of 4 through N16···O22
interactions (2.929 Å).
Although the solubility of 1 is low in d6-DMSO, it is
1
sufficient to obtain the H and 13C NMR spectra (Figures S1
and S2, Supporting Information). The 15N NMR spectra of 2
and 4 are given in Figure 3. The assignments were based on
Figure 1. (a) Molecular structure of 2. (b) Packing diagram of 2
along a axis.
with a bond angle of N(7)−N(6)−C(5) = 179.3(2)°. The
bond length of N6−N7 is 1.100(2) Å, which is a typical N−N
triple bond and is comparable to those reported for inner
diazonium salts.7,11 The bond length of C(5)−N(6) is
1.360(2) Å, which is between the bond length of the CN
double bond (1.28 Å) and the C−N single bond (1.47 Å). All
of the atoms are nearly coplanar. The torsion angles of O(1)−
N(3)−C(4)-N(10) and C(5)−N(8)−C(9)-C(9)#1 are
−1.1(3)° and −179.2(2)°, respectively. The packing diagram
shows a zigzag shape and is given in Figure 1b. The molecules
are stacked along the b axis with a distance of 3.23 Å between
layers and at an angle of 75.14°.
Suitable crystals for X-ray diffraction were obtained by
crystallizing in a saturated solution of 4 dissolved in
nitromethane. As a result, the obtained crystals are composed
of a 1:1 molar ratio of 4 and nitromethane. Compound 4·
CH3NO2 crystallizes in the monoclinic space group Cc with
four molecules per unit cell and with a crystal density of 1.721
g cm−3 at 296 K. The molecular structure is given in Figure 2a.
Due to the presence of nitromethane, the crystal structure of 4
is unsymmetric. The parameters including bond lengths and
bond angles show few differences between the two half
pyrazole rings. The bond lengths of N6−N7 and N15−N16
are 1.095(4) and 1.090(3) Å, respectively. The bond length of
C10−C11 is 1.460 Å, which is slightly shorter than that (1.466
Å) in the crystal structure of 3.16 The torsion angle of N(9)−
C(10)−C(11)−C(17) is 120.5(3)°, while the nitro group
(N3/O2/O1) and the diazonium group (N6/N7) are coplanar
with their bonded half pyrazole ring. The torsion angles of
N(6)−C(5)−N(8)−N(9) and O(2)−N(3)−C(4)−C(10) are
−179.7(2)° and 177.7(3)°, respectively. The packing diagram
is shown in Figure 2b. Although there are nitromethane
Figure 3. 15N spectrum of 2 and 4.
comparison with theoretical calculations using Gaussian 09
(MPW1PW91/aug-cc-pVDZ)18 and previously reported struc-
tures.19,20 In the 15N NMR spectrum of 2 (top, Figure 3), the
signal for the nitro group was observed at −25.7 ppm. The
chemical shifts for diazonium nitrogen atoms are at −38.7 and
−146.0 ppm, respectively. In the 15N NMR spectrum of 4
(bottom, Figure 3), all the resonances are also detected. The
signals (N1 and N2) for the diazonium cation were found at
−40.5 and −150.7 ppm. The nitrogen signals (N5 and N4) in
the pyrazole ring were observed at −5.1 and −10.1 ppm,
respectively. In the IR spectrum, the strong bands between
2300 and 2200 cm−1 are assigned to the diazonium groups
(Figure S6 and Figure S7, Supporting Information).
The molecular surface electrostatic potentials for 2 and 4
were calculated (Figure 4). The most positive part
concentrates on the diazonium group indicating its electron-
deficient nature. The negative region surrounds the nitro
B
DOI: 10.1021/acs.orglett.9b02875
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Table 1. Physicochemical and Energetic Properties of 2 and 4 Compared with DDNPz, DDNP and Lead Azide
k
compd
2
4
DDNPz
DDNP
C6H2N4O5
210.1
157
−15.23
26.67
55.25
1.720
1
24.7
Pb(N3)2
formula
C6N10O4
276.13
199
−11.59
50.73
68.12
1.830
1
C6N10O4
276.13
150
−11.59
50.73
68.12
1.850
1
C3N6O4
184.07
154
PbN6
291.3
315
−11.0
28.9
M [g mol−1
]
a
Tdec [°C]
b
ΩCO [%]
8.69
c
N [%]
45.66
71.76
1.810
2.5
d
N + O [%]
28.9
e
ρ [g cm−3
]
4.800
2.5−4
0.1−1
450.1/1.55
5877
f
IS [J]
g
FS [N]
20−40
819.7/2.97
8653
20
<5
h
ΔHf [kJ mol−1/ kJ g−1
]
987.2/3.58
8943
407.8/2.22
9038
321/1.52
6900 (1.6 g cm−3
24.2
i
νD [m s−1
]
)
j
P [GPa]
31.1
34.0
35.0
33.4
a
b
c
Decomposition temperature (onset temperature at a heating rate of 5 °C min−1). Oxygen balance assuming the formation of CO. Oxygen
d
e
f
g
h
content. Nitrogen and oxygen content. Density, measured with a gas pycnometer (25 °C). Impact sensitivity. Friction sensitivity. Heat of
formation. Detonation velocity calculated with EXPLO5 v6.01. Detonation pressure calculated with EXPLO5 v6.01. DDNPz: 4-diazo-3,5-
dinitropyrazole zwitterion (ref 11).
i
j
k
scribed. Compound 2 was prepared unexpectedly in a mixture
of TFAA and 100% nitric acid. It was also obtained by using
sodium nitrite and concentrated sulfuric acid in a yield of 80%.
Both 2 and 4 were fully characterized, and their single crystal
X-ray structures determined. They have good measured
densities and high thermal stabilities, especially 2, which has
the highest decomposition temperature (199 °C) of any of the
reported hydrogen-free nitrogen-rich diazonium salt. Consid-
ering their high detonation performance and sensitivities, they
may be promising environmentally friendly primary explosive
candidates.
ASSOCIATED CONTENT
* Supporting Information
■
S
Figure 4. Molecular surface electrostatic potential (ESP) of (a) 2 and
(b) 4 calculated at the B3LYP/6-31G(d,p) level at the 0.001 e/bohr3
hypersurface. The legend for the color range is given in the middle,
and the range is from −0.035 (red, electron-rich) to 0.035 (blue,
electron-deficient) hartrees.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b02875.
Synthesis, calculation detail, crystal refinements, NMR
spectra, IR spectra, and DSC plots (PDF)
groups and the imidazole rings and the pyrazole rings. Unlike
photosensitive DDNP, neither 2 nor 4 is sensitive to light even
after exposure to ambient light for 4 weeks. Compound 4 has
comparable decomposition temperature (150 °C) with
traditional primary explosive DDNP (157 °C).9 While
compound 2 decomposes at 199 °C (Figure S8, Supporting
Information), which is much higher than 4 and some other
reported proton-free nitrogen-rich diazonium salts.7,9,11 The
heats of formation of 2 and 4 were calculated to be 819.7 and
987.2 kJ mol−1, respectively, which is much higher than that of
DDNPz. The measured density is 1.830 g cm−3 for 2 and 1.850
g cm−3 for 4. With these values in hand, the detonation
performances were calculated using EXPLO 5 v6.01software.21
The results are shown in Table 1. The values for the
detonation velocities (2, 8653 m s−1; 4, 8943 m s−1) and the
detonation pressures (2, 31.1 GPa; 4, 34.0 GPa) are much
higher than those of DDNP and lead azide. The sensitivities
toward impact and friction were also determined. Compounds
2 and 4 show equal impact sensitivities of 1 J. The friction
sensitivity of 2 is 20−40 N, while that of 4 is 20 N, which are
comparable to DDNP and less sensitive than DDNPz.
Accession Codes
CCDC 1915342 and 1936693 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
■
*Fax: (+1) 208-885-6173. E-mail: jshreeve@uidaho.edu.
ORCID
Yongxing Tang: 0000-0002-9549-9195
Jean’ne M. Shreeve: 0000-0001-8622-4897
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
In conclusion, two ternary (C/N/O) inner bis(diazonium)
salts, the 4,4′-dinitro-5,5′-diazo-2,2′-bisimidazole (2) and 4,4′-
dinitro-5,5′-diazo-3,3′-bispyrazole (4) zwitterions, are de-
Financial support of the Office of Naval Research (N00014-16-
1-2089) and the Defense Threat Reduction Agency (HDTRA
1-15-1-0028) is gratefully acknowledged. We are also grateful
C
DOI: 10.1021/acs.orglett.9b02875
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
(TNBP): Synthesis, Characterization and Promising Properties. J.
Mater. Chem. A 2018, 6, 5136−5142.
to the M. J. Murdock Charitable Trust, Reference No.:
2014120:MNL:11/20/2014 for funds supporting the purchase
of a 500 MHz NMR spectrometer.
́
(21) Suceska, M. EXPLO5, Version 6.01; Brodarski Institute:
Zagreb, Croatia, 2013.
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DOI: 10.1021/acs.orglett.9b02875
Org. Lett. XXXX, XXX, XXX−XXX