Synthesis and Characterization of cyclo-Pentazolate Salts of NH4+, NH3OH+, N2H5+, C(NH2)3+, and N(CH3)4+

Article abstract of DOI:10.1021/jacs.8b05106

A breakthrough in polynitrogen chemistry was recently achieved by our bulk synthesis of (N₅)₆(H₃O)₃(NH₄)₄Cl in which the cyclo-pentazolate anions were stabilized extensively by hydrogen bridges with the NH₄⁺ and OH₃⁺ cations. Significant efforts have been carried out to replace these nonenergetic cations and the Cl^- anion by more energetic cations. In this paper, the metathetical syntheses of cyclo-pentazolate salts containing the simple nitrogen-rich cations NH₄⁺, NH₃OH⁺, N₂H₅⁺, C(NH₂)₃⁺, and N(CH₃)₄⁺ are reported. These salts were characterized by their crystal structures; vibrational, mass, and multinuclear NMR spectra; thermal stability measurements; sensitivity data; and performance calculations. It is shown that the cyclo-pentazolates are more energetic than the corresponding azides but are thermally less stable decomposing in the range of 80 °C to 105 °C. As explosives, the hydrazinium and hydroxyl ammonium salts are predicted to match the detonation pressure of RDX but exhibit significantly higher detonation velocities than RDX and HMX with comparable impact and friction sensitivities. Although the ammonium salt has a lower detonation pressure than RDX, its detonation velocity also exceeds those of RDX and HMX. As a rocket propellant, the hydrazinium and hydroxyl ammonium salts are predicted to exceed the performances of RDX and HMX. The crystal structures show that the cyclo-pentazolate anions are generally stabilized by hydrogen bonds to the cations, except for the N(CH₃)₄⁺ salt which also exhibits strong cation- interactions. This difference in the anion stabilization is also detectable in the vibrational spectra which show for the N(CH₃)₄⁺ salt a decrease in the cyclo-N₅ stretching vibrations of about 20 cm^-1.

Full text of DOI:10.1021/jacs.8b05106

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Article
Synthesis and Characterization of cyclo-Pentazolate
Salts of NH4+, NH3OH+, N2H5+, C(NH2)3+, and N(CH3)4+
Chen Yang, Chong Zhang, Zhansheng Zheng, Chao Jiang, Jun
Luo, Yang Du, Bingcheng Hu, Chengguo Sun, and Karl O Christe
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b05106 • Publication Date (Web): 05 Nov 2018
Downloaded from http://pubs.acs.org on November 5, 2018
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Synthesis and Characterization of cyclo-Pentazolate Salts of
+
+
+
+
NH4 , NH3OH+, N2H5 , C(NH2)3 , and N(CH3)4
Chen Yang,†, Chong Zhang, †, Zhansheng Zheng,† Chao Jiang,† Jun Luo,† Yang Du† and
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Bingcheng Hu*,† Chengguo Sun,*, †,‡ Karl O. Christe†,§
†School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, China
‡School of Chemical Engineering, University of Science and Technology Liaoning, Anshan, Liaoning 114051, China
§Loker Hydrocarbon Research Institute and Department of Chemistry, University of Southern California, Los
Angeles, California 90089-1661, United States
KEYWORDS: cyclo-Pentazolate anion, Nitrogen-rich compounds, High-energy density materials, Crystal structures,
Vibrational spectra
ABSTRACT:
A
breakthrough in polynitrogen chemistry was recently achieved by our bulk synthesis of
+
(N5)6(H3O)3(NH4)4Cl in which the cyclo-pentazolate anions were stabilized extensively by hydrogen bridges with the NH4
and OH3+ cations. Significant efforts have been carried out to replace these nonenergetic cations and the Clˉ anion by
more energetic cations. In this paper, the metathetical syntheses of cyclo-pentazolate salts containing the simple
nitrogen-rich cations NH4 , NH3OH+, N2H5+, C(NH2)3+, and N(CH3)4+ are reported. These salts were characterized by their
+
crystal structures, vibrational, mass and multinuclear NMR spectra, thermal stability measurements, sensitivity data, and
performance calculations. It is shown that the cyclo-pentazolates are more energetic than the corresponding azides but
are thermally less stable decomposing in the range of 80 °C to 105 °C. As explosives, the hydrazinium and hydroxyl
ammonium salts are predicted to match the detonation pressure of RDX but exhibit significantly higher detonation
velocities than RDX and HMX with comparable impact and friction sensitivities. Although the ammonium salt has a
lower detonation pressure than RDX, its detonation velocity also exceeds those of RDX and HMX. As a rocket propellant,
the hydrazinium and hydroxyl ammonium salts are predicted to exceed the performances of RDX and HMX. The crystal
structures show that the cyclo-pentazolate anions are generally stabilized by hydrogen bonds to the cations, except for
+
the N(CH3)4 salt which also exhibits strong cation-π interactions. This difference in the anion stabilization is also
detectable in the vibrational spectra which show for the N(CH3)4+ salt a decrease in the cyclo-N5ˉ stretching vibrations of
about 20 cm-1.
was made in this direction by the syntheses of
1. INTRODUCTION
[M(H2O)4(N5)2]·4H2O (M = metal cation) and simple
metal cyclo-N5ˉ salts (M = Na, Ag) in which the cyclo-N5ˉ
anions are stabilized by metal-nitrogen bonds.12-19
Recently, the nitrogen-rich pentazolate salt (NH4)+N5ˉ has
been predicted to exist at high pressures above 30 GPa.20
In this paper, we report the syntheses of M+N5ˉ salts in
which M are simple, nitrogen-containing, more energetic
cations. The most common method to accomplish the
exchange of ions in ionic compounds is metathesis. It
requires the starting materials, A+Bˉ and C+Dˉ (Equation
1), to be soluble and one of the desired products, AD or
CB, to be insoluble to permit easy product separation.
High-energy density materials (HEDMs) providing high
energy release and green reaction products has been
pursued for many years without much success.1-6 However,
polynitrogen compounds can meet these requirements
because of their large positive heats of formation and the
huge decomposition-energy release due to the highly
exothermic formation of triply-bonded dinitrogen.7-9 No
neutral room-temperature stable nitrogen allotropes and
only three polynitrogen ions, N3ˉ, N5+ and cyclo-N5ˉ, had
been known.7,10 Whereas bulk syntheses had been
available for N3ˉ and N5+ salts for many years, for cyclo-
N5ˉ a bulk synthesis was discovered only during the past
year by the isolation of (N5)6(H3O)3(NH4)4Cl.11 In this
compound, the cyclo-N5ˉ anions were stabilized by strong
A+B- + C+Dˉ → AD↓ + C+Bˉ
(1)
In this manner, the cyclo-pentazolates of NH4 , NH3OH+,
+
+
N2H5+, C(NH2)3+, and N(CH3)4 were prepared and fully
hydrogen bonds to the NH4 and OH3+ cations. Since
+
characterized.
these cations and the Clˉ anion are nonenergetic,
subsequent work was focused on replacing them with
simpler and/or more energetic cations. Some progress
2. RESULTS AND DISCUSSION
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Syntheses
complete crystal data can be found in the Supporting
Information (Tables S1−S29). ORTEP views of the
asymmetric units and atom labeling of
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Two different metathetical approaches were used in this
study taking advantage of the low solubility of either AgI
or MgSO4 in CH3OH/H2O (Scheme 1). In both cases, the
starting material was [Mg(H2O)6]2+[(N5)2(H2O)4]2ˉ. For the
synthesis of [N(CH3)4]+N5ˉ, the Mg salt was first converted
to the poorly soluble AgN5 salt and then further converted
to the even less soluble AgI salt. For the remaining salts,
the Mg starting material was reacted with the desired
sulfates, and the resulting insoluble MgSO4 was filtered
off. Attempts to isolate the aminoguanidinium cyclo-
pentazolate salt resulted in the corresponding azide due
to the easy loss of N2. In view of this easy loss of N2 for
aminoguanidine, we investigated the effect of other
amines, such as methylamine (CH3NH2), pyridine (C5H5N),
imidazole (C3H4N2), and of triphenylphosphine (PPh3) on
the stability AgN5 (Figure S1). When the amines and
phosphine were added dropwise to an AgN5 suspension in
C2H5OH, the solid dissolved with N2 evolution. The rate
of N2 evolution decreased in the order CH3NH2 > C3H4N2 ≥
PPh3 ≥ C5H5N. A few days after the CH3NH2 addition, the
solution turned black and no signal of cyclo-N5ˉ was
detected by electrospray ionization (ESI) mass
spectrometry (ESI-MS). The other solutions gave colorless
single crystals that were characterized by single-crystal X-
hydroxylammonium cyclo-pentazolate, (NH3OH)+N5ˉ,
and guanidinium cyclo-pentazolate, [C(NH2)3]+N5ˉ, are
given in Figures 1C to D. Hydrogen bonding acts as a
determinant force to sustain the intricate architecture. In
(NH3OH)+N5ˉ, three of the four protons of the (NH3OH)+
cation can also act as three H-bond donor units for
participation in hydrogen bonding with N1, N2 and N3 of
cyclo-N5ˉ (N(6)-H(6B)···N(3)#1, 2.8902(11) Å; N(6)-
H(6C)···N(5)#2, 2.8744(11)Å; O(1)-H(1)···N(1), 2.7401(11) Å).
Figure 2A shows a 2D diagram in which cyclo-N5ˉ units
are linked together to create the regular hydrogen
bonding network. Based on the 2D network, the
crystalline framework in 3D space can be constructed by
extending the 2D layered structures (Figure 3A).
Regarding the structure of [C(NH2)3]+N5ˉ, both the
[C(NH2)3]+ cation and cyclo-N5ˉ anion are completely
planar, and the guanidinium hydrogens were placed in
the idealized positions.21 All hydrogen bonds in
[C(NH2)3]+N5ˉ fall within the hydrogen-bond distance
criterion, ranging in bond length from 3.0 Å to 3.4 Å, but
deviate from linearity, with hydrogen bond angles ranging
from 112.4° to 163.5°. Each individual pair of N-H···N
interactions, shown in Figure 2B, can be characterized as
a dimer formed via three amino protons on three nitrogen
atoms of a single guanidinium ion and three lone electron
pairs on three N atoms of a cyclo-N5ˉ anion. Typical of the
3D framework, progressive increases in dimensionality are
formed by hydrogen bonding (Figure 3B). As revealed by
the unit cell structures, hydrogen bonding provides the
major driving force for crystal packing and the stability of
cyclo-N5ˉ in both [(NH3OH)+N5ˉ] and [C(NH2)3]+N5ˉ
(Figures S32 and S33).
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ray
diffraction
as
(PPh3)2Ag(N3)2Ag(PPh3)2
and
(C3H3N2)Ag(C3H3N2)Ag (Figure 1A, B).
The
tetramethylammonium
cyclo-pentazolate,
[N(CH3)4]+N5ˉ, was also successfully prepared. Its crystal
structure is shown in Figure 1E. It crystallizes in the
monoclinic space group P21/m with two molecules per
unit cell (Figure S34). Most frequently, cyclo-N5ˉ acts as a
hydrogen bond acceptor or prefers to form a coordinative
bond with metal cations when forming stable pentazolate
salts, although it has a conjugated π56 bond. At first glance,
we might suppose that either the weak hydrogen bonding
or C-H···π interaction might be responsible for the
stability of [N(CH3)4]+N5ˉ. To our best knowledge, the C-
H···π interaction is not merely a conventional van der
Waals force but has a hydrogen-bond-like property.22 The
most important geometrical characteristic of hydrogen
bonds is that the distance between the proton and the
acceptor atom is shorter than the sum of their van der
Waals radii.23 However, the C-H···N contacts with a
distance of 2.7992 Å, are appreciably longer than the sum
of the van der Waals radii (2.75) (Figure S35), suggesting
that the hydrogen-bonding might be ignored for
explaining the stability of the tetramethylammonium salt.
Under most circumstances, C-H···π interactions are
weaker than classical chemical bonds, as evidenced by
data that the binding energy of methane with the
benzene ring, primarily attributed to dispersion, is only
Scheme 1. Metathetical syntheses of the different cyclo-pentazolate salts.
The silver atoms in (PPh3)2Ag(N3)2Ag(PPh3)2 are linked
through a double azido (N3ˉ) bridge and each silver atom
is also coordinated to two
P
atoms from two
triphenylphosphine molecules (Figure S29). The crystal
structure of (C3H3N2)Ag(C3H3N2)Ag exhibits linear chains
running along the b axis involving the strong interactions
between the Ag+ cations and the anionic imidazolate
groups (Figure S30). Although we failed to obtain a crystal
structure for the pyridine adduct, the cyclo-N5ˉ was still
detected in the solid by ESI-MS (Figure S31). The azide
ion (N3ˉ) observed in (PPh3)2Ag(N3)2Ag(PPh3)2 suggests
that the PPh3 ligand partially breaks the pentazole ligand,
resulting in the formation of N3ˉ and N2.
Crystal structures of the cyclo-pentazolates
All the structures of the cyclo-pentazolates were
confirmed by single-crystal X-ray diffraction analysis, and
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around 1.4 kcal/mol.24 As a consequence, the lack of
hydrogen bonding and coordination bonds in
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[N(CH3)4]+N5ˉ may enhance the activity of the π system.
Figure 2C shows an illustration of the cation-π interaction
in which the binding of the [N(CH3)4]+ cation to the π-face
of cyclo-N5ˉ is responsible for the overall crystal structure.
The [N(CH3)4]+ cation is located above the ring with an N–
centroid distance of 4.2272(19) Å and a tilt angle of
76.389(162)°, but not exactly over the ring centroid. This
situation is quite different from that observed for K+/Na+-
π interactions, in which the alkali metal ions are usually
located directly over the centroid with a tilt angle of 90°,25
while the distribution occurs in the prevailing models of
anion-π interactions.26 Interestingly, [N(CH3)4]+N5ˉ is the
first example of a known cyclo-pentazolate compound in
which cation-π interaction leads to higher stability.
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In
contrast
to
guanidinium
sulfate,
the
aminoguanidinium cation [(NH2)2(NHNH2)C]+ while
possessing a similar charge delocalization, is projected to
form potentially more hydrogen bonding sites.21 However,
the steric hindrance and asymmetry of the terminal NH2
group causes the decomposition of cyclo-N5ˉ during the
metathesis reaction, resulting in the aminoguanidinium
azide, [(NH2)2(NHNH2)C]+N3ˉ. Figure 1F and Figure 2D
show its crystal structure and the schematic
representation of the hydrogen-bonded network between
[(NH2)2(NHNH2)C]+ and N3ˉ, in which all hydrogens
participate in hydrogen bonding. Each N3ˉ anion is
surrounded by eight hydrogen bonds where six hydrogen
bonds are equally assigned to the terminal nitrogens N1
and N3 of the isolated azide ions, while two hydrogen
bonds involve the N2 atom. Apparently, the N3ˉ anion is
attracted by [(NH2)2(NHNH2)C]+,
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Figure 1. Ellipsoid plots of the synthesized crystals. (A) (PPh3)2Ag(N3)2Ag(PPh3)2; (B) (C3H3N2)Ag(C3H3N2)Ag; (C) (NH3OH)+N5ˉ; (D) [C(NH2)3]+N5ˉ; (E)
[N(CH3)4]+N5ˉ; (F) [(NH2)2(NHNH2)C]+N3ˉ; (G) (NH4)+N5ˉ; (H) (N2H5)+N5ˉ.
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and are perfectly packed together by hydrogen-bonding
(Figure S36). We expected that the replacement of cyclo-
N5ˉ by N3ˉ would disturb the balance of the structure, and
that it would be very hard to match the interaction forces
between the two nearly planar ions due to the steric
hindrance and asymmetry of the terminal NH2 group.
Thus, the forces acting on the ring of cyclo-N5ˉ are not
uniform, and this imbalance causes the decomposition of
cyclo-N5ˉ to N3ˉ. To summarize the observed
phenomenon, we conclude that the counterion used to
pair with cyclo-N5ˉ should be less steric hindering and be
of high symmetry.
caused by heating, indicating that cyclo-N5ˉ cannot
undergo a single-step decomposition to N2 bypassing N3ˉ
as an intermediate. The TG curves demonstrate that these
pentazolate compounds exhibit good thermal stability
with onset decomposition temperatures of more than
85 °C, except for [N(CH3)4]+N5ˉ with an onset
decomposition temperature of 80.8 °C, indicating that the
cation-π interaction in [N(CH3)4]+N5ˉ is weaker than the
hydrogen bonding in the other pentazolate compounds.
Their decomposition processes usually depend on the
counter cation and exhibit distinct weight losses in the
temperature range of 40 °C to 300 °C. Both [C(NH2)3]+N5ˉ
and [N(CH3)4]+N5ˉ underwent two pronounced rapid
drastic weight losses, the first one involving the thermal
decompositions of cyclo-N5ˉ to N3ˉ and the second one
the
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The crystal structures of (NH4)+N5ˉ and (N2H5)+N5ˉ
display interesting and similar molecular packing features
involving strong hydrogen bonds and π-π interactions. In
4
R4
(NH )+N ˉ, an axisymmetric structural motif
(10) was
4
5
generated by hydrogen bonds of N4-H4A···N1 and N4-
H4B···N2 (Figure 1G and Figure 2E). The interesting 3D
hydrogen-bonded framework Figure 3C) is formed by the
R44
interlocked
(10) motifs, which are linked by cyclo-N5ˉ
rings and expand along the [-120] and [120] directions,
and the nearly perpendicular long chains are linked by
+
NH4 . The interpenetrating networks are connected by π-
π
interaction among adjacent cyclo-N5ˉ rings with
centroid-centroid distances of 3.9137(7) Å (Figure 2F, and
Figure S37, S38). By replacing the NH4+ cation with N2H5+,
an extra H-bond donor (NH2) becomes available. The 3D
hydrogen-bonded framework (Figure 3D) can be
interpreted as an interlaced net along the c-axis, that is
R44
interlocked by the
(10) motif (Figure 1H and Figure
2G), and a hydrogen-bonded dimer along the [110]
direction, that is extended into an undulated ribbon
involving a 2 (7) motif (Figure S39). As with the 3D
C2
hydrogen-bonded framework of (NH4)+N5ˉ, there is also
an interpenetrating interlinkage of π–π interactions
between the cyclo-N5ˉ rings in (N2H5)+N5ˉ, and the π–π
stacking provides
a moderate interaction for the
stabilization of the structure (Figure 2H, and Figures S40
and S41).
Thermal Behavior
Thermal stability remains a major concern for cyclo-N5ˉ-
based compounds. The thermal behavior of five cyclo-
pentazolate compounds, (NH3OH)+N5ˉ, [C(NH2)3]+N5ˉ,
[N(CH3)4]+N5ˉ, (NH4)+N5ˉ and (N2H5)+N5ˉ, were evaluated
by thermogravimetry–differential scanning calorimetry–
mass spectrometry–infrared spectroscopy (TG-DSC-MS-
IR) at 5 °C min−1.. The DSC curves in Figure 4 indicate that
they have the common feature that none of these
compounds melts prior to decomposition.
Generally, the 3D plots of the FTIR spectra of evolved
gases give important information on the thermal
degradation of these compounds. As found in the
simultaneously recorded IR spectra (Figure 4F-J), the
evolution curves of the main gaseous products show that
all pentazolate compounds first generated HN3 gas27
Figure 2. Schematic representation of the interionic forces motifs in the
crystal structures. (A) Hydrogen-bonded motifs in (NH3OH)+N5ˉ; (B)
Hydrogen-bonded motifs in [C(NH2)3]+N5ˉ; (C) Cation-π interactions in
[N(CH3)4]+N5ˉ; (D) Hydrogen-bonded motifs in [(NH2)2(NHNH2)C]+N3ˉ;
(E) Hydrogen-bonded motifs in (NH4)+N5ˉ; (F) π-π interactions in
(NH4)+N5ˉ; (G) Hydrogen-bonded motifs in (N2H5)+N5ˉ; (H) π-π
interactions in (N2H5)+N5ˉ.
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A
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C
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Figure 3. 3D crystalline framework. (A) (NH3OH)+N5 ˉ ; (B)
[C(NH2)3]+N5ˉ; (C) (NH4)+N5ˉ; (D) (N2H5)+N5ˉ.
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thermal decompositions of the generated azides. These
results are very similar to our previous findings for
(N5)6(H3O)3(NH4)4Cl and [Co(H2O)4(N5)2]·4H2O.11,12 The
simultaneously recorded IR and MS spectra provide
further evidence for their decomposition phases (Figures
S27 and S28). However, we were most interested in
(NH3OH)+N5ˉ, (NH4)+N5ˉ and (N2H5)+N5ˉ. The thermal
decomposition of these three compounds takes place in
one rapid step between 90 °C and 125 °C with a mass loss
of about 100%. The DSC curves show sharp exothermic
peaks at 112 °C, 101 °C and 100 °C for (NH3OH)+N5ˉ,
(NH4)+N5ˉ and (N2H5)+N5ˉ, respectively, which indicate
extremely fast decomposition and energy releases within
a small temperature range of 30 °C.
Figure 4. Thermal analysis of cyclo-pentazolate compounds under argon
at 5 K/min and the IR spectra of the gaseous products obtained from their
thermal decomposition. (A) TG-DSC curves of [N(CH3)4]+N5ˉ; (B) TG-
DSC curves of [C(NH2)3]+N5ˉ; (C) TG-DSC curves of (NH3OH)+N5ˉ; (D)
TG-DSC curves of (NH4)+N5ˉ; (E) TG-DSC curves of (N2H5)+N5ˉ; (F) IR
spectrum of volatiles from [N(CH3)4]+N5ˉ; (G) IR spectrum of volatiles
from [C(NH2)3]+N5ˉ; (H) IR spectrum of volatiles from (NH3OH)+N5ˉ; (I)
IR spectrum of volatiles from (NH4)+N5ˉ; (J) IR spectrum of volatiles
from (N2H5)+N5ˉ.
Spectral properties
Nuclear magnetic resonance (NMR) studies can provide
detailed structure information. In light of high nitrogen
content of salts and their simple structures, the 15N
unlabeled pentazolate salts were investigated (Figures
S12-S22). The 15N NMR spectra of the cyclo-N5ˉ salts
exhibit a resonance in the −5.40 ppm to −4.98 ppm range
except for [N(CH3)4]+N5ˉ for which no signal was observed
due to its weakness. These signals are in excellent
agreement with the value of −6.5 ppm, predicted by
Bartlett.28 The natural abundance of 15N signal in these
salts show a unanimous chemical shift range of −5 ppm to
−6 ppm for cyclo-N5ˉ. The reliable 15N NMR chemical shift
determination for cyclo-N5ˉ is also relevant with respect
to the previous dispute between the Christe29 and Butler
groups.30
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Figure 5. Normal modes for D5h of cyclo-N5ˉ.
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Table 1. Calculated and observed vibrational spectra of cyclo-N5ˉ in space group D5h
Mode
descript.
calcd freq, cm-1 (intens)a
Obsd freq, cm-1 (rel intens)a
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counter ion
c-N5ˉ in D5h
+
+
B3LYP/aug-cc-p-VDZ
CCSD(T)/aug-cc-
NH3OH
C(NH2)3+
N(CH3)4
1153 vs
N2H5+
NH4
+
pVDZ
1189.7 [38.6]
1243.7 (17.5)
1106.5 [0.3]
1141 [47.8]
1202 (13.6)
1078 [1.8]
1187 vs
1222 vs
1120 vw
1176 vs
1221 vs
1112 vw
1170 vs
1218 s
1176 vs
1221 s
A1’ 1 (RA)
E1’ 2 (IR)
E2’ 3 (RA)
1202 m
1137 vwd
1105 vwd
1108 vw
b
1010 c
-
1006 w
-
1020 vw
-
e
1016.6 [2.4]
769.5
1001 [1.5]
739
1005 vw
-
4 (RA)
-
E2’’ 5 (ia)
a Relative intensities, vs = very strong, s = strong, m = medium, w = weak, vw = very weak; b not observed due to its very low intensity;
c band coincides with the very strong symmetric CN3 stretching mode of the guanidinium skeleton; d doubly degenerate E-mode, split
into its degenerate components; e band is hidden by the strong 976 cm-1 band of the N2H5+ cation.
Table 2. Physical properties of five cyclo-pentazolate compounds and three azide compounds and their comparison with
RDX and HMX
Comp.
da
15N NMRb
(ppm)
Td
Nd
ΔHfe
vD
P g
Sea level
Isp (sec)
ISh
(J)
FSi
c
f
(g
(°C)
(%)
(kJ mol−1)
(km s−1)
(GPa)
(N)
cm−3)
[N(CH3)4]+N5ˉ
[C(NH2)3]+N5ˉ
(NH3OH)+N5ˉ
(NH4)+N5ˉ
(N2H5)+N5ˉ
[N(CH3)4]+N3ˉ
(NH4)+N3ˉ
(N2H5)+N3ˉ
RDX j
1.245
1.515
1.636
1.520
1.620
1.156
1.346
1.40
not obsd
80.8
88.1
104.3
102.0
85.3
-
58.29
86.12
80.70
95.42
95.11
297.2
312.3
371.7
308.1
471.3
222.3
112.1
5.88
7.96
9.93
9.28
10.40
5.50
8.81
10.08
20.14
35.80
27.29
37.00
8.10
197.7
201.5
281.7
238.0
266.3
200.7
242.7
282.2
259.8
258.8
35
>360
>360
60
-5.40
24
-5.03
6
13
-4.98
140
-5.33
6
100
-
-
-
-
-
48.28
93.33
93.33
37.84
37.84
insens
>118
-
insens
-
400
-
21.70
27.85
35.84
41.18
383.4
70.03
75.03
9.63
8.84
9.16
-
1.816
1.905
230
277
7.4(7.5)k
7.4
120(120)k
120
HMX
a Crystal density from crystal structures. b Chemical shift for 15N-unlabeled cyclo-N5ˉ. c Temperature of decomposition by TG (5 °C/min,
onset values).d Nitrogen content. e Heat of formation, all calculations for the cyclo-N5ˉ salts were carried out by Dixon and Vasiliu with
f
g
h
i
j
the G3MP2 method. Detonation velocity. Detonation pressure. Impact sensitivity (this study). Friction sensitivity (this study).
RDX and HMX values calculated with Cheetah 7.0. k This study.
classified as A1’ + E1’ + 2 E2’ + E2’’. Since it has a center of
symmetry, the infrared and Raman bands are mutually
exclusive, with the A1’ and E2’ modes being Raman active,
The pentagonal-planar cyclo-N5ˉ anion possesses D5h
symmetry with (3n – 6) = 9 fundamental vibrations,
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the E1’ mode being infrared active, and the E2’’ mode being contributes an oxygen atom to the combustion process,
1
2
3
4
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7
8
inactive in both the Ra and IR (see Figure 5). Because the
present study provided five high-quality data sets for
while hydrazine by itself is already is a good rocket
propellant. However, the thermal stabilities of these
pentazolates are insuffcient for practical applications as
cyclo-pentazolates
with
relatively
simple
well-
characterized counter ions, it offered an opportunity to
extract the fundamental modes of D5h cyclo-N5ˉ. The
experimental data together with the theoretical
predictions31 are summarized in Table 1 and show
excellent agreement after subtraction of the known cation
energetic
materials.
+
bands (Figure S2-S11). For the vibrational spectra of NH4 ,
9
+
N(CH3)4 , N2H5+, and NH3OH+ see the book by Siebert32
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and for the guanidinium cation the paper by Becker.33
This provides further support for the presence of a highly
symmetrical N5ˉ anion, as shown by the crystal structures
of these compounds.
The infrared spectrum of (N5)6(H3O)3(NH4)4Cl11 was also
recorded, and the presence of the cyclo-N5ˉ anion was
established by a sharp prominent band at 1224 cm-1.
Furthermore, Table 1 shows that the frequencies of the
cyclo-N5ˉ
anion
stretching
modes
in
its
tetramethylammonium salt are red-shifted by about 20
cm-1, when compared to those of the remaining N5ˉ salts.
This shift can be attributed to the fact that the crystal
+
structure of the N(CH3)4 salt significantly differs from
those of the other salts, exhibiting C-H··· interactions.
Physical Properties
To evaluate the properties of the different cyclo-
pentazolate salts, crystal densities, 15N NMR chemical
shifts, decomposition temperature onsets, energetic
performances, and impact and friction sensitivities were
evaluated and are summarized in Table 2. The densities of
the five pentazolates are in the range 1.25−1.64 g/cm3 and
are significantly lower than those of the reported metal
cyclo-pentazolates and cg-N (3.4355 g/cm3) but are almost
the same as that predicted for N8.8 The lower densities of
the pentazolates of this study are mainly due to the
smaller contributions from the cations to the overall
density and the ambient pressure conditions, respectively.
The results of our performance calculations, given in
Table 2 and all carried out with the Cheetah 7.0 program,
show that the cyclo-N5ˉ salts are energetic materials,
outperforming the corresponding azides, and in the case
of the hydrazinium and hydroxylammonium salts match
the detonation pressure of RDX, but significantly exceed
RDX and HMX in terms of detonation velocity due to
their high energetic-nitrogen content. Although the
ammonium salt has a lower detonation pressure than
RDX, its detonation velocity also exceeds those of RDX
and HMX. As rocket propellants, the hydrazinium
compound shows a slightly higher and the hydroxyl
ammonium compound a significantly higher sea-level
specific impulse than RDX and HMX. As expected from
the large nonenergetic cation, the tetramethylammonium
salts are the lowest performers, followed by the
guanidinium salts. The high performances of the
hydrazinium and hydroxyl ammonium
salts are not surprising. The hydroxyl ammonium cation
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material medica, Chinese Academy of Medical Science &
Peking Union Medical College) for expert crystallographic
analysis.
CONCLUSIONS
1
2
3
4
5
6
7
8
The new cyclo-pentazolate salts containing the simple
nitrogen-rich cations NH4 , NH3OH+, N2H5+, C(NH2)3+,
+
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and N(CH3)4 have been prepared by metathetical
reactions. These salts were characterized by their crystal
structures, vibrational, mass and multinuclear NMR
spectra, thermal stability measurements, sensitivity data,
and performance calculations. It is shown that cyclo-
pentazolates are more energetic than the corresponding
azides but are thermally less stable decomposing in the
range of 80 to 105 °C. As explosives, the hydrazinium and
hydroxyl ammonium salts are predicted to match the
detonation pressure of RDX but exhibit significantly
higher detonation velocities with comparable impact and
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D.; Zhang, Q. H. Nat. Commun. 2017, 8, 181.
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friction sensitivities. As
a
rocket propellant, the
hydrazinium salt is predicted to match the performance
of RDX, while the hydroxyl ammonium salt significantly
outperforms RDX. The crystal structures show that the
cyclo-pentazolate anions are generally stabilized by
+
hydrogen bonds to the cations, except for the N(CH3)4
salt which also exhibits strong cation-π interactions. For
practical applications, ways must be found to further
increase the thermal stability of the cyclo-pentazolate
anion.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: XXXXXX
Experimental and characterization data (PDF)
Supporting Figures S1−S41, supporting Tables S1−S29 (PDF)
X-ray crystallographic files (CIF)
M. Nature 2017, 549, 78.
(14) Zhang, W. Q.; Wang, K. C.; Li, J. C.; Lin, Z. E.; Song, S. W.;
Huang, S. L.; Liu, Y.; Nie, F. D.; Zhang, Q. H. Angew. Chem. Int.
Ed. 2018, 130, 2622.
(15) Sun, C. G.; Zhang, C.; Jiang, C.; Yang, C.; Du, Y.; Zhao, Y.;
Hu, B. C.; Zheng, Z. S.; Christe, K. O. Nat. Commun. 2018, 9,
1269.
(16) Steele, B. A.; Stavrou, E.; Crowhurst, J. C.; Zaug, J. M.;
Prakapenka, V. B.; Oleynik, I. I. Chem. Mater. 2016, 29, 735.
(17) Laniel, D.; Weck, G.; Gaiffe, G.; Garbarino, G.; Loubeyre,
P. J. Phys. Chem. Lett. 2018, 9, 1600.
AUTHOR INFORMATION
Corresponding Author
*hubc@njust.edu.cn
*sunyangguo2004@163.com
Author Contributions
C.Y. and C.Z. contributed equally.
Notes
(18) Arhangelskis, M.; Katsenis, A. D.; Morris, A. J.; Friščić ,T.
Chem. Sci. 2018, 9, 3367.
(19) Steele, B. A.; Oleynik I. I. Chem. Phys. Lett. 2016, 643, 21-
26
The authors declare no competing financial interest.
(20) Steele, B. A.; Stavrou, E.; Crowhurst, J. C.; Zaug, J. M.;
Prakapenka, V. B.; Oleynik, I. I. Chem. Mater. 2016, 29, 735.
(21) Dumitrescu, D. Y.; Legrand, M.; Dumitrascu, F.; Barboiu,
M.; Leevan, A. V. D. Cryst. Growth Des. 2012, 12, 4258.
(22) Suezawa, H.; Yoshida, T.; Hirota, M.; Takahashi, H.;
Umezawa, Y.; Honda, K.; Nishio, M. J. Chem. Soc., Perkin Trans 2
2001, 11, 2053.
(23) Taylor, R.; Kennard, O. J. Am. Chem. Soc. 1982, 104, 5063.
(24) Nepal, B.; Scheiner, S. J. Phys. Chem A 2014, 118, 9575.
(25) Hay, B. P.; Custelcean, R. Cryst. Growth Des. 2009, 9,
2539.
(26) Frontera, A.; Gamez, P.; Mascal, M.; Mooibroek, T. J.;
Reedijk, J. Angew. Chem. Int. Ed. 2011, 50, 9564.
(27) Dows, D. A.; Pimentel, G. C. J. Chem. Phys. 1955, 23, 1258.
(28) Perera, S. A.; Gregušová, A.; Bartlett, R. J. J. Phys. Chem A.
2009, 113, 3197.
ACKNOWLEDGMENT
The authors gratefully acknowledge the financial support
provided by the Fundamental Research Funds for the Central
Universities (No. 30917011101), the Priority Academic Program
Development of jiangsu Higher Education Institutions
(PAPD) and the National Natural Science Foundation of
China (No. 21701077). The authors are most grateful to Karl O.
Christe (University of Southern California) for his
professional expertise to help rewriting the paper. All the
theoretical calculation data in the paper are provided by Karl
O. Christe. We are also indebted to and thank Karl O.
Christe and other members of our group for many helpful
and inspired discussions and support of our work. The
authors thank David Dixon (University of Alabama) and
Monica Vasiliu (University of Alabama) for the calculation of
the heats of formation. The authors also thank Yue Zhao
(Nanjing University) and Fengfeng Wang (Institute of
(29) Schroer, T.; Haiges, R.; Schneider, S.; Christe, K. O. Chem.
Comm. 2005, 0, 1607.
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(30) Butler, R. N.; Hanniffy, J. M.; Stephens, J. C.; Burke, L. A.
J. Org. Chem. 2008, 73, 1354.
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(31) a) Dixon, D. A.; Feller, D.; Christe, K. O.; Wilson, W. W.; Vij,
A.; Vij, V.; Jenkins, H. D. B.; Olson, R. M.; Gordon, M. S.; J. Am. Chem.
Soc. 2004, 126, 834; b) Bartlett, R. J. “Structure and Stability of
Polynitrogen Molecules and Their Spectroscopic Characteristics”,
AFOSR-DARPA Report, F49620-98-1-0477, University of Florida,
Gainesville, FL.
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(32) Siebert, H. “Anwedungen der Schwingungsspektroskopie
in der Anorganischen Chemie,” Anorganische und Allgemeine
Chemie in Einzeldarstellungen, VII,Springer Verlag, 1966.
(33) Němec I.; Matulková, I.; Held, P.; Kroupa, J.; Němec, P.; Li,
D. X.; Bohatý, L.; Becker, P. Opt. Mat. 2017, 69, 420.
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Scheme 1. Metathetical syntheses of the different cyclo-pentazolate salts.
Figure 1. Ellipsoid plots of the synthesized crystals. (A) (PPh3)2Ag(N3)2Ag(PPh3)2; (B) (C3H3N2)Ag(C3H3N2)Ag; (C)
(NH3OH)+N5ˉ; (D) [C(NH2)3]+N5ˉ; (E) [N(CH3)4]+N5ˉ; (F) [(NH2)2(NHNH2)C]+N3ˉ; (G) (NH4)+N5ˉ; (H) (N2H5)+N5ˉ.
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Figure 2. Schematic representation of the interionic forces motifs in the crystal structures. (A) Hydrogen-bonded motifs in
(NH3OH)+N5ˉ; (B) Hydrogen-bonded motifs in [C(NH2)3]+N5ˉ; (C) Cation-π interactions in [N(CH3)4]+N5ˉ; (D)
Hydrogen-bonded motifs in [(NH2)2(NHNH2)C]+N3ˉ; (E) Hydrogen-bonded motifs in (NH4)+N5ˉ; (F) π-π interactions in
(NH4)+N5ˉ; (G) Hydrogen-bonded motifs in (N2H5)+N5ˉ; (H) π-π interactions in (N2H5)+N5ˉ.
A
B
D
C
Figure 3. 3D crystalline framework. (A) (NH3OH)+N5ˉ; (B) [C(NH2)3]+N5ˉ; (C) (NH4)+N5ˉ; (D) (N2H5)+N5ˉ.
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Figure 4. Thermal analysis of cyclo-pentazolate compounds under argon at 5 K/min and the IR spectra of the gaseous products
obtained from their thermal decomposition. (A) TG-DSC curves of [N(CH3)4]+N5ˉ; (B) TG-DSC curves of [C(NH2)3]+N5ˉ; (C)
TG-DSC curves of (NH3OH)+N5ˉ; (D) TG-DSC curves of (NH4)+N5ˉ; (E) TG-DSC curves of (N2H5)+N5ˉ; (F) IR spectrum of
volatiles from [N(CH3)4]+N5ˉ; (G) IR spectrum of volatiles from [C(NH2)3]+N5ˉ; (H) IR spectrum of volatiles from (NH3OH)+N5ˉ;
(I) IR spectrum of volatiles from (NH4)+N5ˉ; (J) IR spectrum of volatiles from (N2H5)+N5ˉ.
Figure 5. Normal modes for D5h of cyclo-N5ˉ.
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