Insensitive energetic 5-nitroaminotetrazolate ionic liquids

Abstract

Five energetic ionic liquids of 5-nitroaminotetrazolate anion (NAT) combined with 1,3-dimethylimidazolium (1), 1-ethyl-3-methylimidazolium (2), 1-butyl-3-methylimidazolium (3), 1-hexyl-3-methylimidazolium (4), and 1-methyl-3-octylimidazolium (5) cations were synthesized in high yields and fully characterized by IR, NMR and elemental analysis. Colorless block crystals of 1 were isolated in methanol/ethanol and crystallized in the orthorhombic system Fdd2(43) (a = 49.337(3) Å, b = 20.9073(12) Å, c = 3.6993(2) Å, V = 3815.84(38) ų, Z = 16). The ionic liquids 1–5 are thermally stable at temperatures higher than 200 °C. Among them, 2–5 are found to be room temperature ionic liquids. The heats of formation of 1–5 obtained by both experimental and theoretical methods are all positive. 1 possesses the highest value of 194.6 kJ mol⁻¹ and 0.86 kJ g⁻¹. These novel NAT energetic ionic liquids contain only C, H, N and O elements. The CHNO type ionic liquids 1–5 are insensitive towards impact (>40 J) and friction (>360 N). They showed good combustion characteristics after being ignited by a flame. They are of interest as liquid energetic materials with modestly high energy, high thermal stability, and good insensitivity to impact and friction, as well as environmentally friendly decomposition gases.

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1. Introduction

In recent years, ionic liquids have been shown to be distinctive modifiable soft materials in a wide range of applications as solvents, catalysts, electrolytes, lubricants, surfactants, magnetic fluids, and optical fluids, etc.¹ Originating from the feature of entire ions, ionic liquids have the advantages of enhanced thermal stability, negligible vapor pressure, large liquidus range, little or no vapor toxicity, and improved safety.² Some energetic ionic liquids have been found to be potential green liquid propellants and explosives for applications in energetic materials in certain condition to generate considerable amounts of energy and gaseous products.³ Energetic ionic liquids usually exhibit the uniform property of liquid materials unlike that of solid energetic materials highly depends on particle size and shape.⁴ Many energetic groups including –N₃, –NO₂, –CN and –NH₂ have been introduced to construct energetic ionic liquids.⁵ However, the very important energetic group, nitroamino group (–NHNO₂), never appeared in any known energetic room temperature ionic liquid. Moreover, the search for novel and improved energetic ionic liquids carrying higher energy density suitable for application as explosives, propellants or pyrotechnics, is a continuing challenge.

Five-membered nitrogen-rich heterocycles, and in particular tetrazoles, are important frameworks as potential replacements for traditional energetic materials (e.g. 2,4,6-trinitrotoluene (TNT), 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), and 1,3,5,7-tetranitro-1,3,5,7-tetraazacyclooctane (HMX)), due to their high nitrogen content and their thermal and chemical stability that results from the pseudoaromatic character of the heterocyclic ring.⁶ The large number of N–N and C–N bonds in energetic nitrogen-rich tetrazoles results directly in high heats of formation and high densities. Their low carbon and hydrogen content also gives rise to good oxygen balance. The decomposition products of these CHNO-type energetic compounds results predominantly in the generation of CO₂, H₂O and N₂ which give rise to very promising candidates for applications requiring environmentally-friendly energetic materials.

Nitroaminotetrazoles are of special interest because they combine both energetic nitroamino group and nitrogen-rich backbone in one molecule. The nitroamino group is the crucial component in the high energetic density materials (HEDMs) with the best performance (RDX, HMX), and also supposed to be the “trigger spot” in the energetic materials and affect their sensitivities/stabilities. As a simple nitrogen-rich heterocyclic compound, 5-nitroaminotetrazole (NAT), or 5-nitroiminotetrazole in another resonance form, has attracted considerable interest in recent years, because of its high nitrogen content, high heat of formation, and high oxygen balance. Although neutral 5-nitroaminotetrazole is not stable, some salts of NAT, including guanidinium salts, triazolium salts, and salts of NAT derivates such as 1-methyl-5-nitriminotetrazole, 2-methyl-5-nitroaminotetrazole have already been investigated.⁷ However, liquid NAT energetic salts have never been reported. Herein, we report five energetic NAT ionic liquids, 1,3-dimethylimidazolium (1), 1-ethyl-3-methylimidazolium (2), 1-butyl-3-methylimidazolium (3), 1-hexyl-3-methylimidazolium (4), and 1-methyl-3-octylimidazolium (5) nitroaminotetrazolate. The CHNO-type compounds 1–5 were fully characterized. To illustrate the structural information of these ionic liquids, single crystal of 1 was prepared for X-ray diffraction analysis. In addition, their thermal and chemical stabilities, sensitivities along with combustible characteristics were also determined.

2. Experimental section

Caution We have not experienced any problems in handling these compounds. Although they are impact and friction insensitive materials, we strongly suggest they should be handled with extreme care using all of the standard safety precautions such as leather gloves, leather coat, face shields and ear plugs.

2.1. General methods

All chemicals of analytical grade were obtained commercially and used as received. The 5-nitroaminotetrazole (5-NAT) was synthesized by a nitration reaction of 5-aminotetrazole, following the literature procedure.⁸ Silver 5-nitroaminotetrazolate (AgNAT) was prepared from the mixing of AgNO₃ aqueous solution with 5-NAT aqueous solution as white precipitate. Infrared (IR) spectra were recorded using KBr plates on a NEXUS 670 FT-IR spectrometer. ¹H and ¹³C NMR spectra were recorded on a Bruker 400 MHz nuclear magnetic resonance spectrometer operating at 400 (¹H) and 100 (¹³C) MHz, respectively, with d₆-DMSO as locking solvent unless otherwise stated. ¹H and ¹³C chemical shifts are reported in ppm relative to TMS. Melting points were determined by differential scanning calorimetry (DSC) using a NETZSCH DSC 200 PC calorimeter, and calibrated with pure indium. Measurements were performed at a heating rate of 10 °C min⁻¹ in sealed aluminium pans with a nitrogen flow rate of 20 mL min⁻¹. Thermogravimetric analysis (TGA) measurements were carried out on NETZSCH TG 209F1 by heating samples at 10 °C min⁻¹ from 25 to 400 °C. The densities were measured at 25 °C using a pycnometer. Elemental analyses (H, C, N) were performed on an Elementar Vario MICRO CUBE Elemental Analyzer.

2.2. X-ray crystallography

Single crystals of 1 were removed from the flask; a suitable crystal was selected, attached to a glass fiber; and data were collected at 143.00(10) K using a Xcalibur, Eos diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71069 Å). Using Olex2,⁹ the structure was solved with the ShelXS¹⁰ structure solution program using direct methods and refined with the ShelXL¹⁰ refinement package using Least Squares minimisation. The structure was solved in the space group Fdd2(43) by analysis of systematic absences. All non-hydrogen atoms were refined anisotropically, and hydrogen atoms were located and refined. No decomposition was observed during data collection. Details of the data collection and refinement are given in Tables 1–3. ESI.†

Table 1 Crystal data and structure refinement for 1

a R 1 = ∑||F0| − |Fc||/∑|F0|. b wR2 = [∑w(F0^2 − Fc^2)^2/∑w(Fo^2)^2]^1/2.
Formula	C6H10N8O2
FW [g mol^−1]	226.22
T [K]	143(2)
Crystal system	Orthorhombic
Space group	Fdd2
a [Å]	49.337(3)
b [Å]	20.9073(12)
c [Å]	3.6993(2)
α [°]	90.00
β [°]	90.00
γ [°]	90.00
V [Å^3]	3815.8(4)
Z	16
ρ [g m^−3]	1.575
μ [mm^−1]	0.125
F(000)	1888.0
λ MoKα [Å]	0.71073
Reflns	5571
R int	0.0185
Params	147
S on F^2	1.063
R 1 (I > 2σ(I))^a	0.0306
wR2 (I > 2σ(I))^b	0.0820
R 1 (all data)^a	0.0327
wR2 (all data)^b	0.0845
Δρmin and max [e Å^−3]	0.22 and −0.19

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2.3. Computational methods

Computations were performed by the Gaussian 03 (Revision E.01) suites of programs.¹¹ The geometric optimization and the frequency analyses were carried out using B3LYP functional analyses with the 6-31+G** basis set.¹² Single-point energies were calculated at the MP2/6-311++G** level.¹³ All of the optimized structures were characterized to be true local energy minima on the potential energy surface without imaginary frequencies. The gas-phase heats of formation for the separate ions were calculated by the MP2/6-311++G**//B3LYP/6-31+G**.¹⁴

The theoretical heats of formation of the cations and anions were computed using the method of isodesmic reactions (seeing ESI†).¹⁵ The sources of the energies of the parent ions in the isodesmic reactions were calculated from protonation reactions (Δ_fH°(H⁺) = 1528.085 kJ mol⁻¹).¹⁵ The standard heat of formation of NH₃ is used directly from the literature (Δ_fH°(NH₃, g) = −45.9 kJ mol⁻¹).¹⁶ The enthalpies of reaction (Δ_rH°, 298.15 K) were obtained by combining the MP2/6-311++G** energy difference for the reaction, the scaled zero point energies, and other thermal factors.

The heats of formation of 1–5 can be estimated from the calculated based on a Born–Haber energy cycle (seeing ESI†), and can be simplified by the expression:

Δ_fH^°₂₉₈ (ILs) = ∑Δ_fH^°₂₉₈ (cation) + ∑Δ_fH^°₂₉₈ (anion) − ΔH_L (1)

where ΔH_L is the lattice energy of the ionic liquid. For NAT salts 1–5, ΔH_L (kJ mol⁻¹) can be predicted by the formula suggested by Jenkins et al.¹⁷

ΔH_L = U_POT + [p(n_M/2 − 2) + q(n_X/2 − 2)]RT (2)

where n_M and n_X depend on the nature of the ions M_p⁺ and X_q⁻, respectively, and have a value of 6 for nonlinear polyatomic ions. This equation simply assumes that the vibrational degrees of freedom are equally excited in both the crystal and the gaseous ions while applying corrections for rotational degrees of freedom possessed by the product gaseous ions. The equation for lattice potential energy U_POT (eqn (3)) has the form:

U_POT (kJ mol⁻¹) = 1981.2(ρ_m/M_m)^1/3 + 103.8 (3)

where ρ_m is density, g cm⁻³, M_m is the chemical formula mass of the ionic material, g mol⁻¹.¹⁷

2.4. Bomb calorimetry

The calorimetric measurement was performed on a Parr 6725 bomb calorimeter (static jacket) equipped with a Parr 207A oxygen bomb. The samples were carefully dropped into the platinum crucible, which were subsequently burned in a 3.05 MPa atmosphere of pure oxygen and the Parr 45C10 alloy fuse wire was used for ignition. The experimental heat of combustion was obtained from the average of three separated measurements with standard deviations calculated as a measure of experimental uncertainty. The bomb was examined for no evidence of unburned carbon after each run.

The experimentally determined constant-volume energies of combustion (Δ_cU) of the ionic liquids 1–5 were measured using oxygen bomb calorimetry. The heat of combustion, Δ_cH°, was calculated from Δ_cU and a correction for change in gas volume during combustion was included: Δ_cH_m = Δ_cU_m + ΔnRT, where Δn is the difference in the number of moles of gases between the products and the reactants. The standard heats of formation of 1–5, Δ_fH°, were back calculated from the heats of combustion on the basis of combustion equations, Hess's Law as applied in thermochemical equations.¹⁸ The combustion equations of 1–5 can be expressed as in Scheme S3.† From the experimentally determined heats of combustion and the known heats of formation of CO₂ (−393.51 kJ mol⁻¹), H₂O (−285.83 kJ mol⁻¹),¹⁶ the heats of formation of 1–5 were calculated.

1,3-Dimethylimidazolium 5-nitroaminotetrazolate (1). 1,3-Dimethylimidazolium iodide (448 mg, 2.0 mmol) was dissolved in methanol (20 mL), and then AgNAT (711 mg, 3.0 mmol) was added. The resulting mixture was stirred in dark at 40 °C for 24 h. Insoluble AgNAT along with the by-product, silver iodide, were removed as precipitate. The filtrate was collected and dried to yield 1 as white solid (425 mg, 94%). Recrystallization of 1 from methanol/ethanol gives clear colorless block crystals suitable for X-ray structure determination. ¹H NMR: δ = 14.98 (s, 1H), 9.07 (s, 1H), 7.68 (s, 2H), 3.86 (s, 6H) ppm. ¹³C NMR: δ = 163.67, 137.63, 122.41, 35.13, 9.55 ppm. IR (KBr): ν_max = 3427, 3114, 3062, 1630, 1576, 1543, 1448, 1400, 1327, 1227, 1173, 1063, 1038, 1029, 872, 760, 621, 420 cm⁻¹. Anal. calcd for C₆H₁₀N₈O₂ (226.20): C, 31.86; H, 4.42; N, 49.56; found: C, 31.76; H, 4.49; N, 49.81.

1-Ethyl-3-methylimidazolium 5-nitroaminotetrazolate (2). The similar procedure was followed as that described above for 1. 1-Ethyl-3-methylimidazolium bromide (382 mg, 2.0 mmol) and AgNAT (711 mg, 3.0 mmol) were reacted in methanol to obtain a colorless liquid 2 (460 mg, 96%). ¹H NMR: δ = 9.25 (s, 1H), 7.78 (s, 1H), 7.71 (s, 1H), 4.22 (q, 2H), 3.88 (s, 3H), 1.38 (t, 3H) ppm. ¹³C NMR: δ = 160.22, 136.73, 123.67, 122.05, 44.41, 35.77, 15.25 ppm. IR (film): ν_max = 3149, 3101, 2987, 2860, 1649, 1570, 1522, 1446, 1367, 1313, 1171, 1113, 1014, 960, 764, 620 cm⁻¹. Anal. calcd for C₇H₁₂N₈O₂ (240.22): C, 35.00; H, 5.00; N, 46.67; found: C, 34.80; H, 5.11; N, 46.33.

1-Butyl-3-methylimidazolium 5-nitroaminotetrazolate (3). The similar procedure was followed as that described above for 1. 1-Butyl-3-methylimidazolium bromide (438 mg, 2.0 mmol) and AgNAT (711 mg, 3.0 mmol) were reacted in methanol to obtain a colorless liquid 3 (493 mg, 96%). ¹H NMR: δ = 9.35 (s, 1H), 7.80 (s, 1H), 7.73 (s, 1H), 4.18 (t, 2H), 3.87 (s, 3H), 1.74 (m, 2H), 1.22 (m, 2H), 0.86 (t, 3H) ppm. ¹³C NMR: δ = 160.39, 137.10, 123.78, 122.42, 48.64, 35.86, 31.64, 18.97, 13.47 ppm. IR (film): ν_max = 3146, 3087, 2962, 2872, 1633, 1570, 1520, 1464, 1383, 1344, 1234, 1169, 1111, 1084, 1022, 831, 758, 621 cm⁻¹. Anal. calcd for C₉H₁₆N₈O₂ (268.28): C, 40.29; H, 6.30; N, 41.77; found: C, 40.25; H, 6.08; N, 42.02.

1-Hexyl-3-methylimidazolium 5-nitroaminotetrazolate (4). The similar procedure was followed as that described above for 1. 1-Hexyl-3-methylimidazolium bromide (494 mg, 2.0 mmol) and AgNAT (711 mg, 3.0 mmol) were reacted in methanol to obtain a pale yellow liquid 4 (550 mg, 93%). ¹H NMR: δ = 9.35 (s, 1H), 7.78 (s, 1H), 7.71 (s, 1H), 4.17 (t, 2H), 3.85 (s, 3H), 1.74 (m, 2H), 1.22 (m, 6H), 0.81 (t, 3H) ppm. ¹³C NMR: δ = 162.85, 136.70, 123.68, 122.35, 48.98, 35.79, 29.55, 25.29, 22.04, 13.99 ppm. IR (film): ν_max = 3149, 3111, 3095, 2956, 2933, 2864, 1635, 1570, 1522, 1467, 1419, 1319, 1232, 1167, 1082, 1028, 918, 833, 756, 663, 621 cm⁻¹. Anal. calcd for C₁₁H₂₀N₈O₂ (296.33): C, 44.54; H, 6.75; N, 37.80; found: C, 44.39; H, 6.84; N, 37.54.

1-Methyl-3-octylimidazolium 5-nitroaminotetrazolate (5). The similar procedure was followed as that described above for 1. 1-Methyl-3-octylimidazolium bromide (548 mg, 2.0 mmol) and AgNAT (711 mg, 3.0 mmol) were reacted in methanol to obtain a yellow liquid 5 (611 mg, 94%). ¹H NMR: δ = 9.35 (s, 1H), 7.77 (s, 1H), 7.71 (s, 1H), 4.16 (t, 2H), 3.85 (s, 3H), 1.75 (m, 2H), 1.22 (m, 10H), 0.84 (t, 3H) ppm. ¹³C NMR: δ = 163.91, 137.00, 124.05, 122.72, 49.21, 36.21, 31.61, 29.85, 28.92, 28.79, 25.95, 22.50, 14.39 ppm. IR (film): ν_max = 3147, 3109, 2927, 2856, 1670, 1639, 1570, 1522, 1464, 1419, 1317, 1232, 1169, 1082, 1030, 916, 831, 756, 663, 621 cm⁻¹. Anal. Calcd for C₁₃H₂₄N₈O₂ (324.38): C, 48.13; H, 7.40; N, 34.54; found: C, 48.33; H, 7.49; N, 34.81.

3. Results and discussion

3.1. Synthesis

NAT anion has a five member-ring aromatic system, with all nitrogen, carbon and oxygen atoms conjugated. The rigidity of heterocyclic ring may increase the melting point. On the other hand, the larger ionic radius and the lower anion symmetry (only a symmetry plane) of NAT anion likely result in the decrease of melting point of salts by reducing Coulomb interaction by delocalization of negative charge. Thus, NAT anion may be a good candidate for room temperature ionic liquid with good fluidity and stability by introducing suitable cations. Imidazolium cation is the most commonly used cation to form ionic liquids, because of its low symmetry.¹⁹ Salts of imidazolium cation usually have lower melting points than pyridinium salts, undelocalized quaternary ammonium salts, guanidinium salts etc.

The 5-nitroaminotetrazole (5-NAT) was synthesized by a nitration reaction of 5-aminotetrazole, following the literature procedure (Scheme 1).⁸ The mixing of AgNO₃ aqueous solution with 5-NAT aqueous solution immediately produces silver 5-nitroiminotetrazolate (AgNAT) as white precipitate.

Scheme 1 Synthesis of 5-nitroiminotetrazole (5-NAT) and its silver complex (AgNAT).

The general syntheses of NAT salts 1–5 were described in Scheme 2. The reaction of slightly excess AgNAT with N-substituted imidazolium halides¹⁹ in methanol gave NAT salts 1–5 along with AgBr or AgI precipitate. The excess AgNAT mixed with the by-product AgBr or AgI was removed by filtration. The filtrate was concentrated to yield the desired products 1–5. 1–5 were characterized by IR, NMR and elemental analysis. ¹H NMR spectra showed the typical peaks attributed to the imidazolium framework (7.71, 7.78 and 9.36 ppm). In the ¹³C NMR, a signal at 160–165 ppm was found for the NAT anion in each NAT salt.

Scheme 2 Synthesis routes of 5-NAT salts 1–5.

3.2. X-ray crystallography

Crystals of 1 suitable for X-ray diffraction were obtained as colorless blocks by slow recrystallization from methanol/ethanol solution. The structure is shown in Fig. 1–3 and crystallographic data are summarized in Table 1. Selected bond distances and angles are given in Table 2. 1 crystallizes in the orthorhombic space group Fdd2(43) with sixteen molecular moieties in each unit cell, a density of 1.575 g cm⁻³ and a unit cell volume of 3815.8(4) ų (Fig. 1). No coordinated water molecule is found.

Fig. 1 Molecular structure of 1. Hydrogen atoms are shown as open spheres of arbitrary radius and are unlabelled for clarity.

Table 2 Selected bond lengths [Å] and bond angles [°] in crystal for 1

Bond length	[Å]	Bond angle	[°]
N1–C1	1.330(2)	C1–N1–N2	105.95(13)
N1–N2	1.3558(19)	N1–N2–N3	111.13(13)
N2–N3	1.296(2)	N2–N3–N4	106.17(13)
N3–N4	1.3500(18)	N3–N4–C1	108.83(13)
N4–C1	1.343(2)	C1–N5–N6	116.59(12)
N5–C1	1.378(2)	N5–N6–O1	123.13(12)
N5–N6	1.3165(18)	N5–N6–O2	116.09(12)
N6–O1	1.2615(17)	C3–N7–C2	108.50(13)
N6–O2	1.2512(17)	C3–N7–C5	125.93(14)
N7–C5	1.466(2)	C4–N8–C2	108.58(13)
N7–C3	1.376(2)	C4–N8–C6	125.94(14)
N7–C2	1.326(2)	C2–N8–C6	125.48(14)
N8–C2	1.325(2)	N1–C1–N4	107.91(14)
N8–C6	1.465(2)	N1–C1–N5	119.46(13)
N8–C4	1.381(2)	N4–C1–N5	132.63(14)
C3–C4	1.347(2)	N7–C2–N8	108.81(13)
		C4–C3–N7	107.28(14)
		C3–C4–N8	106.82(14)

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The evidence of delocalization of 5-NAT anion is confirmed by the X-ray single structure of 1. The C, H, N and O atoms in the NAT anion are essentially planar. Three N–N bonds (N1–N2, N2–N3, N3–N4) in tetrazolate ring are shorter than general N–N bond (1.47 Å), but significant longer than N=N bond (1.24 Å). Similar phenomenon is observed for C–N bonds. The lengths of C–N bonds (C1–N1, 1.330(2) Å; C1–N4, 1.343(2) Å; C1–N5, 1.378(2) Å) are close to the value of C=N bond (1.316 Å). This is commonly seen in other 5-NAT anions.⁷ The 1,3-dimethylimidazolium cation is also coplanar, with a bonding pattern in accord with X-ray diffraction data of other 1,3-dimethylimidazolium containing salts.²⁰ The bond lengths are very similar in each cation with strong delocalization, with bond-length harmonization and loss of pyramidalization at the nitrogen atoms.

The packing structure of 1 is built up by hydrogen bonds (Fig. 2). The details of hydrogen bonds are summarized in Table 3. The entire molecular charge delocalization is accomplished by extensive hydrogen bonding in the extended structure, which may lead to the enhanced decrease of C–N bond lengths in 1. Both interionic and intraionic hydrogen bonds are observed in 1. The resulting hydrogen bonding in 1 is much different in contrast to the structures measured in the NAT congeners.⁷ Each NAT anion moiety in 1 acts as both acceptor and donor to form strong hydrogen bonds. The intraionic hydrogen bonding scheme starts from N(4) of tetrazolate ring directly linked to O(1) of the terminal nitro group (N(4)–H(4)⋯O(1), 2.5804(17) Å). The donor–acceptor distances are presented, with an essentially nonlinear DHA angle of 113.2°. The interanionic hydrogen bonding scheme starts from the N atom directly linked to the oxygen atom of the adjacent NAT anion (N(4)–H(4)⋯O(1)#1, 2.8825(18) Å, 1 − X,1/2 − Y,−1/2 + Z symmetry transformation). There is weak hydrogen bonding between the unlike ions with longer than 2.2 Å day(H⋯A). The ions are stacked into three-dimensional infinite the crystal structure by interionic hydrogen bond.

Fig. 2 Packing diagram of 1 viewed down the c-axis. Unit cell is indicated and dashed lines represent hydrogen bond.

Table 3 Hydrogen bonds [Å] for 1^a

D–H⋯A	d(D–H)	d(H⋯A)	d(D⋯A)	<(DHA)
a Symmetry transformations used to generate equivalent atoms: #1 +X,1/2 + Y,−3/2 + Z; #2 3/4 − X,1/4 + Y,−5/4 + Z; #3 1 − X,1/2 − Y,−1/2 + Z.
C(2)–H(2)⋯O(2) #1	0.95	2.28	3.158(2)	153.6
C(2)–H(2)⋯N(5) #1	0.95	2.53	3.355(2)	145.0
C(3)–H(3)⋯N(3)	0.95	2.50	3.327(2)	145.3
C(6)–H(6)B⋯N(2) #2	0.98	2.55	3.521(2)	170.8
N(4)–H(4)⋯O(1)	0.8800	2.10	2.5804(17)	113.2
N(4)–H(4)⋯O(1)#3	0.8800	2.10	2.8819(18)	146.9

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3.3. Thermal property

The thermal stability is a significant feature of energetic materials. Compounds 1–5 exhibit high stability at room temperature, without any decomposition even stored for months. Their thermal performances are evaluated by thermalgravimetric analysis (TGA). All the ionic liquids 1–5 decomposed at the temperature ranging from 206 °C to 225 °C. No water release was observed in the thermogravimetric analysis (TGA) curves. The hydrogen bonding could play the predominant role in enhancing their thermal stability. Measurements of 1–5 for their melting points and glass transition temperatures were assessed using differential scanning calorimetry (DSC) analysis from the first heating cycle. The melting point of 1 is 76 °C. For 2–5, an endothermal step was found, with their glass transition temperature (T_g) varying from −74 to −65 °C. Salt 3 gives the lowest value of −74 °C. A symmetrical structure and the rigidity of 1,3-dimethylimidazolium cation trend to produce 1 with a melting point. In contrast, the asymmetric structure and relatively large imidazolium cations radius lead to low glass transition temperature instead of melting point for 2–5.

3.4. Heat of formation

The theoretical heats of formation of 1–5 are summarized in Table 4. The enthalpy criteria of energetic materials are governed by their molecular structures. Nitrogen-rich azoles with large number of N–N and N–C bonds give high heats of formation. 5-NAT, a tetrazole with nitroamino group, has nitrogen content of 64.61%. Thus, 5-NAT anion (N% = 65.12%) may be a good candidate to produce compounds with high heat of formation. The theoretical heats of formation of 1–5 are between +88.7 kJ mol⁻¹ and +194.6 kJ mol⁻¹. 1 possesses the highest energy, with values of +194.6 kJ mol⁻¹ and +0.86 kJ g⁻¹. In Table 4, it is seen that NAT salts are highly endothermic compounds. Their heats of formation decrease when the longer alkyl group was introduced into the structure of imidazolium cation.

Table 4 Physicochemical properties of 5-NAT compounds 1–5 and reference compounds

	1	2	3	4	5	UDMH^n	NG^o	TNT^p
a Molecular weight. b Melting point/phase transition temperature. c Decomposition temperature/boiling point. d Nitrogen content. e Oxygen balance of CO2. f Density, 25 °C. g Molar heat of formation, kJ mol^−1, calculated/experimental. h Specific heat of formation, kJ g^−1, calculated/experimental. i Molar heat of combustion, kJ mol^−1. j Detonation pressure. k Detonation velocity. l Impact sensitivity. m Friction sensitivity. n 1,1-Dimethylhydrazine, ref. 21. o Nitroglycerine, ref. 21. p Trinitrotoluene, ref. 21.
Formula	C6H10N8O2	C7H12N8O2	C9H16N8O2	C11H20N8O2	C13H24N8O2	C2H8N2	C3H5N3O9	C7H5N3O6
MW^a	226.20	240.22	268.28	296.33	324.38	60.10	227.09	227.13
T m/Tg^b [°C]	76	/−70	/−74	/−65	/−68	−58	13.5	81
T d/Tb^c [°C]	225	218	221	209	206	/64	/93	300
N ^d [%]	49.56	46.33	42.26	37.54	34.81	46.61	18.50	18.50
Ω ^e [%]	−106	−120	−143	−162	−178	−213	3.5	−74
ρ ^f [g cm^−3]	1.55	1.51	1.46	1.39	1.32	0.79	1.59	1.65
ΔfH°^g [kJ mol^−1]	194.6/200.7	172.4/156.0	139.8/133.8	114.5/109.6	88.7/91.6	49.8	−370.7	−67.0
ΔfH°^h [kJ g^−1]	0.86/0.89	0.72/0.65	0.52/0.50	0.39/0.37	0.27/0.28	0.83	−1.63	−0.30
−ΔcH°^i [kJ mol^−1]	3984.8 (7.3)	4642.0 (36.8)	5968.0 (50.2)	7301.4 (22.8)	8634.3 (11)	1980.1	1524.4	3402.1
P ^j (GPa)	19.04	17.27	15.86	13.94	12.23			19.53
D ^k (m s^−1)	6917	6645	6441	6144	5862		7700	6881
IS^l [J]	>40	>40	>40	>40	>40		0.2	15
FS^m [N]	>360	>360	>360	>360	>360		>353	>353

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The experimental measurements for their heats of formation are assessed. The values of 1–5 are found to be well agreed with the theoretical data. All the NAT ionic liquids exhibit positive heats of formation, illustrating these ionic liquids may be high energetic to release. Their high heats of formation are derivate from their large amount of N–N, N–C, and even N–O bonds in the nitrogen-rich NAT anion. Therefore, these ionic liquids should lead to increased values for the heats of formation. Salt 1 possesses the highest values of +200.7 kJ mol⁻¹ and +0.89 kJ g⁻¹, associated to its highest nitrogen content. These values are substantially much higher than that of NG (−370.7 kJ mol⁻¹, −1.63 kJ g⁻¹) and TNT (−67 kJ mol⁻¹, −0.29 kJ g⁻¹).²¹ Such data indicated that 1–5 are even comparable with that of UDMH (49.8 kJ mol⁻¹, 0.83 kJ g⁻¹).²¹ which is an outstanding molecule for high energy storage. Compared with other CHNO-type 1-butyl-3-methylimidazolium cation-based energetic ionic liquids, the heat of formation of 3 is Δ_fH° = +133.8 kJ mol⁻¹ (+0.50 kJ g⁻¹), which is comparable to that of [bmim][NCA] (1-butyl-3-methylimidazolium nitrocyanamide, +128.5 kJ mol⁻¹), and much higher than the value for [bmim][DNM] (1-butyl-3-methylimidazolium dinitromethanide, –72.1 kJ mol⁻¹).²⁰ Therefore, the NAT anion is a good candidate for producing energetic compounds with high heats of formation.

3.5. Detonation property

The detonation parameters, detonation velocity (D) and detonation pressure (P) are important parameters of scaling the detonation characteristics of energetic materials. For the CHNO-type explosives, detonation velocities and pressures can be calculated by the Kamlet–Jacobs equation:²²

D = Φ^0.5(1.011 + 1.312ρ)

Φ = NM^0.5Q^0.5

P = 1.558Φρ²

where P is the detonation pressure (GPa), D is the detonation velocity (km s⁻¹), ρ is the packed density (g cm⁻³), Φ is the characteristic value of explosives, N are the moles of gas produced per gram of explosives, M is an average molar weight of detonation products, and Q is the estimated heat of detonation.

The calculated detonation velocities (D) of 1–5 are from 5862 to 6917 m s⁻¹. Ionic liquid 1 has the highest detonation pressure 19.04 GPa and the highest detonation velocity 6917 m s⁻¹. Its detonation properties are comparable with those of the secondary explosive TNT (19.53 GPa, 6881 m s⁻¹).²¹

3.6. Combustion test

In a salt system, cation with positive charge is more resistant toward oxidation.²³ Electron- and fuel-rich anion is more likely to oxidize under specific conditions to arouse combustion. NAT anion is an electron-rich anion to hold the potential to construct combustible ionic liquids. Thus, their combustion behaviour is of interest. Ignition tests of 1–5 were examined by white fuming nitric acid (WFNA, 100% HNO₃) oxidizer. However, no flame was observed. Such phenomenon indicates that there is no hypergolic process occurred, through the reaction of the NAT ionic liquids and WFNA. Consequently, WFNA is unsuitable to form hypergolic ignition with the NAT ionic liquids. The combustible performance of 1–5 in air was assessed and their reactivity was monitored (Fig. 3). All these NAT ionic liquids can be ignited by flame in a very short time. After the burner was removed, combustion sustained until the sample has been burned out in air. During the combustion, no smoke was generated and no explosion occurred, due to their elements of only C, H, N and O. The energy release process of these NAT ionic liquids are environmental friendly. 1–5 could exhibit good combustible characteristics. Such feature is in well agreement with the combustible characteristic of other energetic ionic liquids derived from NO₃⁻, BTA and AT anion.⁵

Fig. 3 Combustion behaviour of NAT ionic liquids.

3.7. Impact and friction sensitivity

Initial safety testing of compounds 1–5 towards impact and friction were performed using a BAM method.²⁴ The data collected are also summarized in Table 4. All NAT ionic liquids 1–5 are insensitive towards impact (>40 J). Similar trends are recorded in the friction sensitivity results, with the values in excess of 360 N. The combination imidazolium cation and NAT anion contributes to the insensitivity of 1–5. Based on the UN standard,²⁵ compounds 1–5 are impact and friction insensitive compounds, and can be handled in safe. Such NAT compounds 1–5 are confirmed to be much more insensitive than their homologues with other nitrogen-rich cations.⁷ The presence of 1,3-dialkylimidazolium cations with relatively low energy, further decreases the energy of the whole molecule, and may results in more stable compounds with higher stability than the nitrogen-rich cations. Although nitroamino group in NAT anion is a typical “trigger spot”, the large number of N–N and N–C bonds in 1–5 limit its sensitive trigger to form more safe energetic materials.

4. Conclusions

Five energetic 5-NAT ionic liquids 1–5 were prepared in high yield. Crystals of 1 were recrystallized from methanol/ethanol suitable for X-ray diffraction. Their densities are in the range of 1.32–1.55 g cm⁻³. Compounds 2–5 are room temperature ionic liquids along with desirable thermal stability. Their heats of formation were evaluated using both experimental and calculated methods. All 5-NAT ionic liquids exhibit positive heats of formation from between +91.6 to +200.7 kJ mol⁻¹ (0.28 to 0.89 kJ g⁻¹), and signify high energy to release. 1 has the highest detonation pressure 19.04 GPa and the highest detonation velocity 6917 m s⁻¹. Confirmed by BAM standards, these 5-NAT salts are impact (>40 J) and friction (>360 N) insensitive materials. 1–5 can be ignited by the flame in air without smoke and residue. In summary, NAT anion is promising anion toward design and synthesis of room temperature energetic ionic liquid materials with desired properties of low sensitivity to heat, shock and friction and high heat of formation.

Acknowledgements

The authors gratefully acknowledge the support of NSFC (no. 21303108, J1210004), FRFCU (2013SCU04A12), and SSTIC (no. ZDSY20130331145131323). We thank the Comprehensive training platform of specialized laboratory, College of chemistry, Sichuan University for instrumental measurement.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1032258. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c5ra07415k

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