Five high-nitrogen ion salts based on 4,5-Bis(1H-tetrazol-5-yl)-1H-Imidazole: Syntheses,structures and thermal properties

Abstract

Five high-nitrogen ion salts were first synthesized with the nitrogen-rich ligand 4,5-bis(1H-tetrazol-5-yl)-1H-imidazole (H₃BTI) and Guanidine Carbonate (G₂CO₃), Ethylenediamine (EDA), 1,2-Propanediamine (PN), Hydroxylamine Hydrochloride (HN), and were characterized by elemental analysis and Fourier transform infrared (FT-IR) spectroscopy. Their crystal structures were determined by applying X-ray single crystal diffraction. The results showed a large amount of π-π stacking action in their ligand anions. The thermal decomposition curves of five compounds were obtained by differential scanning calorimetry (DSC). The non-isothermal kinetic parameters of the exothermic process were calculated by Kissinger’s and Ozawa’s methods. Five compounds have relatively high exothermic peak temperatures and thermal explosion temperatures (T_b), which indicates excellent thermal stability and safety.

Keywords

High-nitrogen ion salts 4 5-bis(1Htetrazol-5-yl)-1H-imidazole exothermic peak temperature thermal explosion temperatures

1 Introduction

High-nitrogen compounds, featuring in the release of environment-friendly N₂ and good thermal stability, have attracted increasing attention in the area of high energy density materials (HDEMs) [1 –6]. As one class of them, high-nitrogen ion salts not only inherit the novel physical and chemical properties of ionic liquids and salts, but also have a large number of N–N and C–N bonds and therefore possess large positive heats of formation [7, 8].

Five-membered heterocyclic compounds are traditional sources of energetic compounds [9 –11], and considerable attention is currently focused on azoles as high-nitrogen ion salts. Azoles compounds are an attractive, stable, high-nitrogen candidate suitable for practical applications as heat-resistant energetic materials. Table 1 lists five high-nitrogen energetic ion salts that have been reported [12 –16]. They all have relatively high-nitrogen content and good energy characteristics. Among them, the structures of 4-amino-1,2,4-triazole-2,4,6-Trinitro-1,3,5-trihydroxybenzene [(ATZ)(TNPZ)] [12] and the chloride salt of 1,1’-(triaz-1-ene-1,3-diyl)bis(1H-tetrazol-5-amine-5-aminotetrazole) (Cl•TEBA •2ATA) [13] (shown in Fig. 1) are similar to those reported in this paper. They are all connected by ionic bond through high-nitrogen ligands and non-metal cations. However, they all have a relatively low thermal decomposition temperature under 300°C, which could not meet the needs of specific conditions. Therefore, seeking new high-nitrogen ion salts that have better thermal stability is the current research hotspot.

Fig.1

Molecular structure of (ATZ)(TNPG) (a) and Cl•TEBA •2ATA (b).

Fig.2

Synthesis of H₃BTI.

Table 1

Nitrogen content, thermal decomposition temperature and reference number of five known high-nitrogen energetic ion salts that have been reported

Compounds	Nitrogen content/%	Thermal decomposition temperature/°C	Reference number
(ATZ)(TNPG)	28.41	200.1	[12]
Cl•TEBA •2ATA	77.13	230.5	[13]
Copper imidazolates	32.77	200–300	[14]
4-Nitro-5-(5-nitro-1,2,4-triazol-3-yl)-2H-1,2,3-trizaole	52.28	168.45	[15]
5,5^′-bis(trinitromethyl)-3,3^′-bi-1H-1,2,4-triazole	53.84	148	[16]

4,5-Bis(1H-tetrazol-5-yl)-1H-imidazole (H₃BTI) has the advantages of high-nitrogen content of 66%, good stability and a relatively high thermal decomposition temperature of 339°C [17, 18]. Because of the different groups attached to the bis-tetrazole ring, it has a different rigid or flexible structure. By density functional theory (DFT) computing method [19], H₃BTI can lose H⁺ and form H₂BTI^- and HBTI^2-, whose chelating bridge-ligands and free rotation can form more diverse coordination modes and topological structures.

Since H₃BTI is an acidic ligand, we have selected four basic cations: G⁺ (Guanidine), EDA²⁺ (Ethylenediamine), PN²⁺ (1,2-Propanediamine) and HN⁺ (Hydroxylamine) to synthesized five energetic ion salts: G₂(HBTI) (1), EDA(HBTI)•3H₂O (2), PN(HBTI)•H ₂O (3), HN(H₂BTI) (4), EDA(H₂BTI)2•4H₂O (5). The structures were confirmed by the single-crystal analysis and we explored their thermal decomposition properties, non-isothermal kinetics, and corresponding critical temperatures of thermal explosion.

2 Experimental Section

General Caution: 1-5 are energetic compounds and tend to explode under certain conditions. Though we haven’t experienced difficulties in the synthetic process, small-scale and appropriate safety precautions (safety glasses, face shield, leather gloves, and so on) should be taken.

2.1 Reagents

In addition to the ligand (H₃BTI) used, all other reagents and solvents used for the synthesis and analysis were analytical reagent, commercially available and used as received without further purifications.

2.2 Methods

IR spectra were recorded with KBr plates using a Bruker Equinox 55 infrared spectrometer (KBr pellets) in the range of 4000∼400 cm^–1. Elemental analyses were performed with a Flash EA 1112 full automatic trace element analyzer. DSC measurements were performed with a Pyris-1 differential scanning calorimeter in static air atmosphere. The crystal sample was powdered and heated from 50 to 500°C at 5, 10, 15, 20°C•min ^–1, respectively.

2.3 Synthesis

Synthesis of ligand 4, 5-bis(1H-tetrazol-5-yl)-1H-imidazole (H₃BTI). H₃BTI was prepared by reacting 1 equiv 1H-imidazole-4,5-dicarbonitrile (Its concentration is 1 mol·L^-1) with 2.4 equiv sodium azide and 2 equiv zinc chloride in aqueous solution at 120°C and refluxing for 24 h. And then 1 mol·L^-1 HCl was added to the last suspension until a pH value of 1 was reached, continued stirring was taken for 2 h at room temperature. Finally, white powder H₃BTI was filtered, washed and dried. The yield was 89%. Elemental analysis calcd (%) for H₃BTI: C 28.31, H 1.96, N 66.12; found: C 28.54, H 2.03, N 66.04. IR (KBr; cm^-1): 3160, 2875, 1627, 1554, 1488, 1417, 1321, 1245, 1203, 1161, 1078, 1045, 1006, 944, 812, 750.

Synthesis of G₂(HBTI) (1): H₃BTI (0.20 g, 1 mmol) and Guanidine carbonate (G₂CO₃, 0.18 g, 1 mmol) were stirred in 20 mL of an aqueous solution, reacted at 80°C for 30 min to obtain a colorless, transparent solution. It was filtered at room temperature, and the filtrate was allowed to stand for 1 d. The white bulk crystals were filtered, washed and dried. The yield was 93.12%. Elemental analysis calcd (%) for G₂(HBTI): C 26.08; H 4.38; N 69.54; found: C 25.85; H 4.51; N 69.64. IR (KBr; cm^-1): = 3235, 1678, 1635, 1498, 1424, 1094, 1078, 1023.

Synthesis of EDA(HBTI)•3H₂O (2): H₃BTI (0.20 g, 1 mmol) was suspended in 10 ml of water, heated to 80°C, stirred, and Ethylenediamine (C₂H₈N₂, 0.07 g, 1.2 mmol) was diluted in 10 mL of water and slowly added to the suspension. The reaction was conducted at 80°C for 40 min to obtain a colorless transparent solution, which was cooled to room temperature, filtered, and the filtrate was allowed to stand for 1 d. The colorless cubic bulk single crystals were filtered, washed and dried. The yield was 95.33%. Elemental analysis calcd (%) for EDA(HBTI)•3H₂O: C 26.41; H 5.70; N 52.81; found: C 26.23; H 5.77; N 52.67. IR (KBr; cm^-1): = 3743, 1665, 1631, 1492, 1410, 1086, 1053, 1017.

Synthesis of PN(HBTI)•H₂O (3): H₃BTI (0.20 g, 1 mmol) was suspended in 10 mL aqueous solution, heated to 80°C, stirred, and 1,2-Propanediamine (C₃H₁₀N₂, 0.09 g, 1.2 mmol) was diluted in 10 mL water and slowly added to the suspension. The reaction was conducted at 80°C for 30 min to obtain a colorless transparent solution, which was cooled to room temperature, filtered, and the filtrate was allowed to stand for 1 d. The colorless tetragonal columnar crystals were filtered, washed and dried. The yield was 86.51%. Elemental analysis calcd (%) for PN(HBTI)•H ₂O: C 32.42; H 5.44; N 56.73; found: C 32.18; H 5.35; N 56.61. IR (KBr; cm^-1): = 3194, 1653, 1623, 1481, 1405, 1068, 1032, 1001.

Synthesis of HN(H₂BTI) (4): H₃BTI (0.20 g, 1 mmol) was suspended in 5 mL of water, heated to 80°C and stirred. LiOH (0.04 g, 1 mmol) was dissolved in 5 mL of water and slowly added to the suspension. The reaction was conducted at 80°C. for 10 min to obtain a colorless transparent solution. Hydroxylamine hydrochloride (NH₂OH•HCl, 0.07 g, 1 mmol) was dissolved in 10 mL aqueous solution and slowly added to the above solution. The reaction was conducted at 80°C for 30 min to obtain a colorless transparent solution, which was cooled to room temperature, filtered, and the filtrate was allowed to stand for 1 d. The colorless needle-like single crystals were filtered, washed and dried. The yield was 95.52%. Elemental analysis calcd (%) for HN(H₂BTI): C 25.31; H 2.97; N 64.97; found: C 25.12; H 3.12; N 63.54. IR (KBr; cm^-1): = 3530, 3256, 1656, 1503, 1447, 1102, 1084, 1028.

Synthesis of EDA(H₂BTI)₂•4H₂O (5): H₃BTI (0.20 g, 1 mmol) was heated to 80°C and stirred in the water (10 mL).Ethylenediamine (C₂H₈N₂, 0.07 g, 1.2 mmol) was dissolved in water (10 mL) and gradually added to the suspension. After stirring at 80°C for 20 min, a colorless and transparent solution was obtained. When the solution was cooled to room temperature, it was filtered. The yellow block crystals were filtered, washed and dried. The yield was 82.43%. Elemental analysis calcd (%) for EDA(H₂BTI)₂•4H₂O: C 26.66; H 4.48; N 57.02%; found: C 26.43; H 4.32; N 57.30%. IR (KBr; cm^-1): = 3744, 1667, 1636, 1495, 1424, 1079, 1047, 1011.

2.4 Crystal structure determination

The X-ray diffraction data was collected with a Rigaku AFC-10/Saturn724 + CCD diffractometer with graphite monochromated Mo-K_α radiation (λ= 0.071073 nm) with a multiscan mode. The structure was solved using SHELXS-97 program, refined by full-matrix least-squares methods on F² with SHELXL-97 program and finally checked with PLATON software. All non-hydrogen atoms were obtained from the difference Fourier map and refined anisotropically. The hydrogen atoms were obtained geometrically and treated by a constrained refinement.

3 Result and discussion

3.1 Crystal structures

The molecular structure of 1 is shown in Fig. 3(a). The 1 molecule is an ionic salt formed by electrostatic action of one HBTI^2- and two C(NH₂)₃ ⁺ . Under the action of OH^-, the H₃BTI molecule loses the active hydrogen on the two tetrazole rings, thereby forming the anion group HBTI^2-. In HBTI^2-, the atoms on each heterocycle are in their respective planes, and their dihedral angles are all below 1.65°. The three heterocycles are not coplanar, N3-C2-C3-C4 is 37.7°, N6-C2-C3-N2 is 43.53°, N1-C4-C5-N10 is 33.2° and C3-C4-C5-N7 is 29.2°. Packing diagram of 1 is shown in Fig. 3(b), the molecules of 1 cross each other. The heterocyclic rings of HBTI^2- are arranged in parallel, forming a large amount of π-π stacking action. There are four kinds of hydrogen bonds in 1. The H-bonds formed by N-H on G and N on tetrazole ring in HBTI^2-, N11-H11B⋯N8, N12-H12B⋯N5, N13-H13A⋯N6, N13-H13B⋯N9, N14-H14A⋯N4, N14-H14B⋯N4, N15-H15A⋯N10, N15-H15B⋯N5, N16-H16A⋯N and N16-H16B⋯N3, lead 1 to an infinite 3D topological structure.

Fig.3

Molecular structure (a) and packing diagram (b) of 1.

The molecular structure of 2 is shown in Fig. 4(a). The 2 molecule is an ion salt formed by one HBTI^2- and one EDA²⁺ through electrostatic interaction, and contains three crystal waters in the molecule. Under the action of OH^-, the H₃BTI molecule loses the active hydrogen on the two tetrazole rings, thereby forming the anion group HBTI^2-. In HBTI^2-, the atoms on each heterocycle are in their respective planes, and their dihedral angles are all below 1.07°. The imidazole ring is coplanar with its 4-substituted tetrazole ring C2-N1-N2-N3-N4, but is not coplanar with the 5-substituted tetrazole ring C5-N5-N6-N7-N8, N4-C2-C3-C4 is 0.67°, N1-C2-C3-N10 is –0.70°, N9-C4-C5-N8 is -36.34° and C3-C4-C5-N5 is –41.31°. The en²⁺ has a C-shaped configuration, and N11-C6-C7-N12 is –61.33°. Packing diagram of 2 is shown in Fig. 4(b), The unit cell of 2 is approximately layer structure. The heterocyclic rings of HBTI^2- are arranged in parallel, forming a large amount of π-π stacking action. There are six kinds of hydrogen bonds in 2. The presence of H-bonds creates a regular 3D topological structure among 2 molecules. The H-bonds N11-H11B⋯N8 and N12-H12C⋯N5 consist of N5 and N8 on both sides of the tetrazole ring which is not coplanar with the other two heterocycles in HBTI^2-, and N11-H11B and N12-H12C on the adjacent two EDA²⁺, forming a one-dimensional wave structure along the b-axis.

Fig.4

Molecular structure (a) and packing diagram (b) of 2.

The molecular structure of 3 is shown in Fig. 5(a). The 3 molecule is an ion salt formed by electrostatic interaction of one HBTI^2- and one PN²⁺, and contains one crystal water in the molecule. Under the action of OH^-, the H₃BTI molecule loses the active hydrogen on the two tetrazole rings, thereby forming the anion group HBTI^2-. In HBTI^2-, the atoms on each heterocycle are in their respective planes, and their dihedral angles are all below 1.20°. The imidazole ring is substantially coplanar with its 4-substituted tetrazole ring C4-N3-N4-N5-N6, and is not coplanar with the 5-substituted tetrazole ring C5-N7-N8-N9-N10. N1-C2-C4-N3 is 6.5°, C3-C2-C4-N6 is 5.7°, N2-C3-C5-N10 is 37.0°, and C2-C3-C5-N7 is 41.1°. The N11-C7-C8-N12 on PN²⁺ was –49.4°. Packing diagram of 3 is shown in Fig. 5(b), the unit cell of 3 is approximately layer structure. The heterocyclic rings of HBTI^2- are arranged in parallel, forming a large amount of π-π stacking action. There are six kinds of hydrogen bonds in 3. The H-bonds N11-H11A⋯O1, N12-H12B⋯O1 and C8-H8B⋯N8 create a regular 3D topological structure among 3 molecules.

Fig.5

Molecular structure (a) and packing diagram (b) of 3.

The molecular structure of 4 is shown in Fig. 6(a). The 4 molecule is an ionic salt formed by electrostatic action of one H₂BTI^- and one hydroxylamine ion (HN⁺). Under the action of OH^-, the H₃BTI molecule loses the active hydrogen on the 5-substituted tetrazole ring C1-N1-N2-N3-N4, thereby forming the anionic group H₂BTI^-. In H₂BTI^-, the atoms on each heterocycle are in their respective planes, and their dihedral angles are all below 0.51°. The three heterocycles are all in the same plane, N1-C1-C2-N9 is 2.76°, N4-C1-C2-C3 is 2.71°, N10-C3-C4-N8 is –0.73°, and C2-C3-C4-N5 is –2.8°. Packing diagram of 4 is shown in Fig. 6(b). The unit cell of 4 is approximately layer structure. The heterocyclic rings of HBTI^2- are arranged in parallel, forming a large amount of π-π stacking action. There are five kinds of hydrogen bonds in 4. The presence of H-bonds N5-H5⋯N6, N9-H9⋯N1 and N5-H5⋯N4 causes the molecule to exhibit a tricyclic coplanar structure.

Fig.6

Molecular structure (a) and packing diagram (b) of 4.

The molecular structure of 5 is shown in Fig. 7(a). The 5 molecule is an ion salt formed by two H₂BTI^- and one EDA²⁺ via electrostatic interaction, and contains four crystal waters in the molecule. Under the action of OH^-, the H₃BTI molecule loses the active hydrogen on the 5-substituted tetrazole ring C4-N7-N8-N9-N10, thereby forming the anion group H₂BTI^-. In H₂BTI^-, the atoms on each heterocycle are in their respective planes, and their dihedral angles are all below 0.54°. The imidazole ring is coplanar with its 4-substituted tetrazole ring C1-N1-N2-N3-N4, and there is a slight twist angle between the 5-substituted tetrazole ring C4-N7-N8-N9-N10. The 5-substituted tetrazole ring is approximately coplanar with the other two rings, N4-C1-C2-N54 is –0.29°, N1-C1-C2-C3 is 2.15°, C2-C3-C4-N10 is –8.77. °and N6-C3-C4-N7 is –7.05°. The EDA² ⁺ has a Z-shaped configuration with a dihedral angle of –180.00°. Packing diagram of 5 is shown in Fig. 7(b). The unit cell of 5 is approximately layer structure. The heterocyclic rings of HBTI^2- are arranged in parallel, forming a large amount of π-π stacking action. There are seven kinds of hydrogen bonds in 5. The presence of H-bonds N4-H4⋯N5, N6-H6⋯N7, O2-H2A⋯N3, O2-H2B⋯N8, O1-H1A⋯N2 and O1-H1B⋯N9 causes the molecule to exhibit a tricyclic coplanar structure.

Fig.7

Molecular structure (a) and packing diagram (b) of 5.

In 1-5, the length of C-N in ligand anions is 1.3159 Å ∼ 1.3883 Å, between carbon-nitrogen single bond (1.2200 Å) and carbon-nitrogen double bond (1.4700 Å), while the N-N bond length is 1.2997 Å ∼ 1.3671 Å, between nitrogen-nitrogen single bond (1.2000 Å) and nitrogen-nitrogen double bond (1.4800 Å). This indicates a conjugation effect among the atoms on the heterocycle of five compounds.

	1	2	3	4	5	(ATZ)(TNPG)	Cl•TEBA •2ATA
CCDC number	1544239	1544241	1544240	1544242	1544243	880594	889576
empirical formula	C7H14N13	C7H18N12O3	C8H16N12O	C5H7N11O1	C12H24N22O4	C8H7N7O9	C2H6N13, 2(CH3N5), Cl
formula weight	322.34	318.33	296.33	237.22	540.53	345.21	417.82
T	153(2)	153(2)	153(2)	153(2)	153(2)	153(2)	291
Crystal size [mm]	0.38×0.42×0.48	0.28×0.26×0.25	0.15×0.13×0.10	0.03×0.11×0.12	0.05×0.03×0.02	0.41×0.13×0.08
Crystal System	monoclinic	orthorhombic	monoclinic	triclinic	triclinic	monoclinic	monoclinic
Space group	P2 ₁	Pbca	P2₁/c	P1	P1	P2 ₁ /c	P2 ₁ /c
a[Å]	10.149(3)	8.8742(18)	6.7960(14)	6.596(2)	7.0410(14)	11.303(4)	5.1298(12)
b[Å]	7.0598(18)	14.715(3)	23.282(5)	6.8542(18)	7.1386(14)	5.5782(19)	28.857(7)
c[Å]	11.002(3)	21.759(4)	8.5852(17)	10.471(3)	12.617(3)	20.257(8)	6.7151(13)
α[°]	90	90	90	74.271(8)	85.03(3)	90	90
β[°]	116.475(3)	90	93.03(3)	86.241(9)	82.64(3)	101.497(6)	116.751(14)
γ[°]	90	90	90	81.792(11)	61.31(3)	90	90
V[Å³]	705.6(3)	2841.3(10)	1356.5(5)	450.8(2)	551.52(19)	1251.6(8)	887.6(3)
Z	2	8	4	2	1	4	2
ρ_calc[g^.cm^-3]	1.517	1.488	1.451	1.747	1.627	1.832	1.563
μ[mm^-1]	0.114	0.12	0.109	0.138	0.129	0.168
F(000)	336	1344	624	244	282	704
2θ range for data [°]	4.5 to 63.0	3.74 to 55.02	5.06 to 50	6.2 to 63.0	6.5 to 54.92	6.04 to 58.26	4.92 to 54.70
index ranges	–14≤h≤14, –9≤k≤10, –16≤l≤16	–11≤h≤10, –19≤k≤10, –28≤l≤21	–8≤h≤8, –26≤k≤27, –10≤l≤10	–9≤h≤9, –9≤k≤10, –15≤l≤15	–9≤h≤9, –9≤k≤9, –16≤l≤16	–15≤h≤15, –7≤k≤7, –24≤l≤24
reflections collected	6993	10342	7664	6320	7490	10408
R _int	0.0203	0.0452	0.0652	0.0342	0.0473	0.0333
data / restraints / parameters	4084 / 1 / 261	3248 / 0 / 199	2381/ 0 / 191	2923 / 0 / 174	2509 / 0 / 172
goodness-of-fit on F²	0.999	1.165	1.415	1.003	0.999		1.013
final R indexes [I≥2σ (I)]	R₁ = 0.0341, wR₂ = 0.0861	R₁ = 0.0540, wR₂ = 0.1503	R₁ = 0.0754, wR₂ = 0.1944	R₁ = 0.0441, wR₂ = 0.0953	R₁ = 0.0734, wR₂ = 0.2020	R₁ = 0.0452, wR₂ = 0.0995	R₁ = 0.0679, wR₂ = 0.1420
final R indexes [all data]	R₁ = 0.0369, wR₂ = 0.0883	R₁ = 0.0573, wR₂ = 0.1533	R₁ = 0.0899, wR₂ = 0.2137	R₁ = 0.0714, wR₂ = 0.1080	R₁ = 0.0886, wR₂ = 0.2179	R₁ = 0.0690, wR₂ = 0.1101	R₁ = 0.0541, wR₂ = 0.1380
largest diff. peak / hole [e Å^-3]	0.24 / –0.21	0.30 / –0.25	0.35/ –0.28	0.35 / –0.26	0.58 / –0.32	0.325 / –0.261	0.340 / –0.579

Likewise, (ATZ)(TNPG) [12] and Cl•TEBA •2ATA [13] are also energetic salts derived from the combination of high-nitrogen ligands and non-metal cations. But parts of their properties can’t match 1-5. For example, their thermal decomposition temperature are under 300°C, which is lower than that of 1-5, and their corresponding critical temperatures of thermal explosion are also lower than that of 1-5. In Table 2, the crystallographic and the structural refinement data of (ATZ)(TNPG) and Cl•TEBA •2ATA are also listed as a contrast.

3.2 Thermal decomposition and non-isothermal kinetics analysis

The thermal stabilities of 1-5 were evaluated with differential scanning calorimetry (DSC) at a heating rate of β= 10°C·min^–1. The tested samples are the precipitated powder instead of the crystalline solid, which were dried in a water bath oven at 50°C to remove any moisture besides crystal water.

As is shown in Fig. 8, the DSC curves of 1, 4 and 5 contain one endothermic process and one exothermic decomposition process. The DSC curves of 2 and 3 contain two endothermic processes and one exothermic decomposition process. Due to crystal water molecules, 2, 3 and 5 have an endothermic peak before 150°C. 1-4 have an endothermic process of crystal transformation between 180°C and 290°C. In the DSC curve of 1-5, there is only one sharp exothermic process and peak temperatures are between 330 and 340°C, which is close to TATB, higher than (ATZ)(TNPG) [12] and Cl•TEBA •2ATA [13], 200.1°C and 230.5°C, respectively. The rapid exothermic process in a short time demonstrated a potential application in the area of ignition.

Fig.8

DSC curve of 1-5 with a heating rate of 5 °C·min^-1.

The thermal decomposition temperature of 1-5 is close to that of the ligand H₃BTI, which proves that the good thermal properties of 1-5 are mainly derived from H₃BTI. Once the ligand is deactivated, 1-5 will not be stably present and decomposed.

Kissinger’s method [20] and Ozawa’s method [21] are widely used to determine the apparent activation energy (E) and the pre-exponential factor (A), which can be applied to estimate the rate constants of the initial thermal decomposition process. The Kissinger and Ozawa–Doyle equations are as follows, respectively: $ln β / T_{p}^{2} = \ln [RA / E] - E / ({RT}_{p})$ (1) $lg β = lg [AE / RG (α)] - 2.315 - 0.4567 E / ({RT}_{p})$ (2) where T_p is the peak temperature, K; R is the gas constant, 8.314 J·K^-1·mol^-1; β is the linear heating rate, K·min^–1; G(α) is the reaction mechanism function.

For the four heating rates of 5, 10, 15 and 20 °C•min ^-1 tested, the first exothermic peak temperature data of 1-5 is shown in Table 1. The non-isothermal kinetic parameters obtained are shown in Table 2. From Tables 1 and 2, it can be seen that the first exothermic peak temperature of 1-5 are close to the exothermic peak temperature of H₃BTI, indicating it is that the H₂BTI^- and HBTI^2- heterocyclic is broken down. The activation energy of 4 is the highest among the five ionic salts, which indicates that in the process of thermal decomposition, 4 has the largest energy barrier to overcome, and is the most stable one. Meanwhile, 4 has the largest pre-exponential factor, indicating that in the case of thermal decomposition, there are many activated molecules 4 and effective collisions.

Table 1

Peak temperatures of the first main exothermic stage at different heating rates for 1-5

Compound	T_p/°C
	5	10	15	20
1	331.3	336.2	340.5	344.4
2	330.4	337.6	344.7	346.6
3	336.9	347.9	352.9	355.4
4	332.2	336.9	339.5	341.7
5	334.3	341.2	347.6	351.7

Table 2

Non-isothermal kinetic parameters of 1-5

Compound	Kissinger’s method			Ozawa’s method
	E_K/kJ·mol^-1	ln A_K	R _K	E_O/kJ·mol^-1	R _O
1	294.7	23.44	–0.9955	289.9	–0.9958
2	241.4	18.73	–0.9914	239.3	–0.9921
3	221.5	16.73	–0.9923	220.4	–0.9930
4	444.9	36.47	–0.9997	432.7	–0.9997
5	237.5	18.26	–0.9940	235.6	–0.9945

The Arrhenius equation can be expressed by using the calculated E (the average of E_k and E_o) and ln A_k as follows: ln k = 23.44 –292.3×10³/RT (1), ln k = 18.73 –240.4×10³/RT (2), ln k = 16.73 –221.0×10³/RT (3), ln k = 36.47 –438.8×10³/RT (4), ln k = 18.26 –236.5×10³/RT (5). The equations can be used to estimate the rate constant of the initial decomposition process of 1-5 and also determine their thermal decomposition mechanism.

The values of the peak temperatures that heating rate closed to zero (T_p0), the corresponding critical temperatures of thermal explosion(T_b), entropies of activation (ΔS^≠), enthalpies of activation (ΔH^≠), and free energies of activation (ΔG^≠) were obtained by the equations. (Seeing in the Supporting Information.) The physicochemical properties of 1-5 are tabulated in Table 3.

Table 3

Physicochemical properties of 1-5

Compound	T_p0/°C	T_b/°C	ΔS^≠/J·K^-1·mol^-1	ΔH^≠/kJ·mol^-1	ΔG^≠/kJ·mol^-1
1	325.95	329.03	194.26	289.59	226.27
2	319.28	322.89	104.36	237.70	204.37
3	322.52	326.53	66.03	218.27	196.97
4	326.68	328.73	443.40	436.08	291.23
5	325.55	329.36	95.21	233.84	202.85

To estimate the thermal sensitivity of 1-5, the corresponding critical temperatures of thermal explosion (T_b) were correlated with the known T_b for four common energetic materials as shown in Fig. 9. Consequently, the T_b of 1-5 is approximately equal to T_b of TATB, 336.6°C, (ATZ)(TNPG) [12] and Cl•TEBA •2ATA [13], 201.25°C and 224.25°C, respectively.

Fig.9

Representation of the literature corresponding critical temperatures of thermal explosion for TATB, (ATZ)(TNPG), Cl•TEBA •2ATA and 1-5.

4 Conclusion

Five high-nitrogen ion salts (1–5) were prepared and demonstrated by elemental analysis and FT-IR spectroscopy. Their single crystals were obtained for the first time, and the structures were investigated in detail. In the five compounds, the large number of hydrogen bonds contribute to π-π stacking action in their ligand anions and the stable 3D architecture. Thermal analysis indicates that the DSC curve 1, 4 and 5 contain one endothermic process and one exothermic decomposition process, and the DSC curve of 2 and 3 contain two endothermic processes and one exothermic decomposition process. The exothermic peak temperatures of 1-5 are between 330°C and 340°C, which shows excellent thermal stability and safety. Non-isothermal kinetics, the critical temperature of thermal explosion (T_b), entropy of activation (ΔS^≠), enthalpy of activation (ΔH^≠) and free energy of activation (ΔG^≠) of 1-5 were obtained by calculation. The energetic properties and thermal analyses confirmed the potential application of the compounds in the area of energetic materials.

Associated Content

Crystal Data, refinement parameters structure data of compound 1-5 and calculation of the critical temperature of thermal explosion, ΔS^≠, ΔH^≠, and ΔG^≠.

Footnotes

Acknowledgment

We are grateful to acknowledge financial support from the National Natural Science Foundation of China (No. 11672040), the State Key Laboratory of Explosion Science and Technology (No. YB2016-17) and Beijing Institute of Technology Research Fund Program for Young Scholars. We thank the reviewers for their most valuable comments.

Appendix

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