Conjugated Energetic Salts Based on Fused Rings: Insensitive and

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Conjugated Energetic Salts Based on Fused Rings: Insensitive and Highly Dense Materials Lu Hu, Ping Yin, Gang Zhao, Chunlin He, Gregory H. Imler, Damon A. Parrish, Haixiang Gao, and Jean'ne M. Shreeve J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b09519 • Publication Date (Web): 17 Oct 2018 Downloaded from http://pubs.acs.org on October 17, 2018

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Journal of the American Chemical Society

Conjugated Energetic Salts Based on Fused Rings: Insensitive and Highly Dense Materials Lu Hu,1,2 Ping Yin,1 Gang Zhao,1 Chunlin He,1 Gregory H. Imler,3 Damon A. Parrish,3 Haixiang Gao,2* and Jean’ne M. Shreeve1* 1Department

of Chemistry, University of Idaho, Moscow, Idaho 83844-2343, United States of Applied Chemistry, China Agricultural University, Beijing, China 100193 3Naval Research Laboratory, 4555 Overlook Avenue, Washington, D.C. 20375, United States 2Department

Supporting Information Placeholder ABSTRACT: Nitroamino-functionalized 1,2,4-triazolo[4,3-b][1,2,4,5]tetrazine (1) when combined with intermolecular hydrogen bonds (HBs), and strong noncovalent interactions between layers, results, for example, in an interlayer distance of 2.9 Å for dihydroxylammonium 3, 6-dinitramino-1,2,4-triazolo[4,3-b][1,2,4,5] tetrazine (2c) with a packing coefficient of 0.805. For dihydroxylammonium 6,6’-dinitramino-3,3’-azo-1,2,4-triazolo [4,3-b] [1,2,4,5]tetrazine (3b), two fused rings were linked by an azo group, which expands the conjugated system resulting in an even shorter interlayer distance of 2.7 Å and a higher packing coefficient of 0.807. These values appear to be the shortest interlayer distances and the highest packing coefficients reported for tetrazine energetic materials. With high packing coefficients, both possess high densities of 1.92 g cm-3 and 1.99 g cm-3 at 293 K, respectively. Compared with its precursor, the hydroxylammonium moiety serves as a buffer chain (H-N-O-H), connecting the anion and cation through hydrogen bonds, giving rise to more favorable stacking, and resulting in higher density and lower sensitivity. The sensitivities of all the hydroxylammonium salts are lower than that of their neutral precursors, such as compound 2 (3 J, >5 N) and compound 2c (25 J, >360 N). The detonation properties of 2c (detonation pressure P = 43 GPa; detonation velocity vD = 9712 m s-1) and 3b (vD = 10233 m s-1; P = 49 GPa), exceed those of present high explosive benchmarks, such as octahydro1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX) and hexanitrohexaazaisowurzitane (CL-20). The molecular structures of several of these new energetic materials were confirmed by single crystal X-ray diffraction measurements. Using calculated and experimental results, the fused ring with a planar large π-conjugated system results in a compromise between desirable stabilities and high detonation properties, thus enhancing future utilization in the design of energetic materials.

INTRODUCTION New high energy density materials (HEDMs) were designed to address the ever increasing requirements in performance, such as higher density, better detonation properties, lower impact and friction sensitivities, and higher thermal stability. Research on the new members of low sensitivity and high energy density energetic materials with improved properties continues to grow worldwide.1 Conjugated energetic materials, a class of compounds with high nitrogen content that contain planar, conjugated structures, possess both enhanced safety and energetic performance properties and are ideal for HEDMs.2 Planar conjugated molecules like TATB and LLM105 are dense (1.94 g cm-3 and 1.92 g cm-3) and exhibit low sensitivities (Figure 1).3 Chavez reported the fused ring of tetrazolo 1,2,4,5-tetrazine N-oxides, which are large conjugated systems, with a 1.93 g cm-3 density and lower sensitivity (6 J, 109 N).4 Another fused ring, 4-amino-3,7dinitro-[1,2,4]triazolo[5,1-c] [1,2,4]triazine, has a density of 1.86 g cm-3 and low sensitivity (29 J, >360 N).1 Fused nitrogen-rich heterocycles with coplanar molecular structures which show low sensitivity and high thermal stability toward destructive stimuli are among the most sought structural motifs.5

Tetrazine has become useful structure in the search of the new generation of energetic materials because of the high O

NH2 O 2N

a

NO2

H 2N

H 2N

NH2

O 2N

O

N

N N N N

NH2

N

NO2

N

NO2 TATB

b

1.92 g cm 29 J, >360 N

O 2N N

N NO2 N N N N

1)

O 2N 2)

N O 2N O 2N

3)

N N

O 2N

N N

N O N N

N N

NH2

N

NO2

N

N

H N N N

N NO2 N N

N

O 2N N NO2

N

N

N N

N O

NO2 O 2N NO2

N N

N N N N N

N NO 2 N

O

NO2

O N

N N N N N O 2N

1.86 g cm-3 29 J, >360 N

6 J, 109 N

N

N O N

NH2

N

1.93 g cm-3

-3

1.94 g cm >60 J, >360 N

N

O

LLM-105 -3

O 2N N

N N

N

O 2N O 2N

N N N

NO2 N NO2 N N

NO2

O 2N

NO2

Figure 1. a, Compounds with conjugated structures; b, Three structure types which have two nitroamine functionalities.

nitrogen content and conjugated system which helps to improve the detonation performance and stabilize the structure.6 Tetrazolo 1,2,4,5-tetrazine and its derivatives with high nitrogen content are quite dense.4 Although tetrazolo

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tetrazine possesses higher nitrogen content, it has only one position for modification and thus there is no opportunity to introduce additional energetic groups to enhance the possibility of its application on modification to more HEDMs. The planar conjugated structure of another fused ring based on tetrazine, 1,2,4-triazolo[4,3-b][1,2,4,5]tetrazine, has two positions where substituents may well be introduced. Energy-rich functional groups such as nitro (−NO2), nitrato (−ONO2), nitroamine (−NNO2), and azido (−N3), which enhance density, nitrogen content, and oxygen balance, are often found in HEDMs.7 Among them, only nitroamine can serve as an anion to form energetic salts, which provides a promising structural modification to achieve HEDMs with better performance.8 Many high energy materials utilizing dinitroamine-based compounds often demonstrate higher detonation performance. Three kinds of structures can possess two nitroamines (Figure 1), e.g., 1) two nitroamine groups bonded to a single ring, such as triazole,9 tetrazole,10 tetrazine,11 or furazan;12 2) two nitroamine groups separately on two joined single rings13 or bridged with azo,14 methylene15 or other rings.16 Many compounds with this structure provide a route to highly energetic materials. An azo linkage is a particularly good and a most efficient way to connect two rings. By introducing more rings, higher heats of formation can be obtained, which result in better detonation properties; 3) two nitroamines located on fused rings.17 Here a fused ring can be formed by different single rings, which means more structures can be designed to build dinitroamine-based highly energetic materials. However, few dinitroamine-based fused rings have been reported. Azo linked dinitroamino-based fused rings which contain large π-conjugated systems with extremely high heats of formation are unknown. Given this background, our synthetic interest focused on nitroamino-functionalized 1,2,4-triazolo[4,3b][1,2,4,5]tetrazine fused rings including 6-nitramino-1,2,4triazolo[4,3-b][1,2,4,5] tetrazine (1), 3, 6-dinitramino-1,2,4triazolo[4,3-b][1,2,4,5]tetrazine (2) and 6,6’-dinitramino-3,3’azo- 1,2,4-triazolo[4,3-b][1,2,4,5]tetrazine (3) and their salts. Most of them show high detonation velocity and pressure, high density, and good decomposition temperature and reasonable sensitivity. Compared with traditional HEDMs, multiple routes were employed for the design of modern energetic materials, including crystal engineering, synthetic methodology, and computational science. Among them, absolutely necessary in acquiring structural information is crystal engineering of HEDMs.18 The crystals of those HEDMs were examined by studying the correlation between the large conjugated system structures and density and impact sensitivities of explosives. RESULTS AND DISCUSSION Synthesis and Crystal Structures. The intermediate compounds 1’ and 2’ were found in the literature (Scheme 1).19 The azo compound was obtained by using bleach, and compound 3’ was prepared by a method resembling that used for 1’ and 2’. The nitration of 1’, 2’ and 3’ was realized with fuming nitric acid. Due to good solubility in water, 1 was obtained by air drying used to remove all the nitric acid. Compound 2 was decomposed when all the nitric acid was removed leaving a viscous liquid. Therefore, only half of the nitric acid was removed, trifluoroacetic acid anhydride was mixed, and the solution stirred for several hours resulting in an orange precipitate. Nitration of 2 was also tried by using a combination of fuming nitric acid with trifluoroacetic acid anhydride, but decomposition occurred immediately.

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Compound 3 can be obtained by pouring the solution directly into an ice/water mixture to give a precipitate. All the salts resulted from reactions with respective bases (Scheme 1). Scheme 1. Preparation of 1, 2 and 3 and their energetic salts.

Figure 2. Planar structures of parent frameworks. Main structure of salts of A, 6-nitramino-1,2,4-triazolo[4,3-b] [1,2,4,5] tetrazine; B, 6,6’-dinitramino-1,2,4-triazolo [4,3-b][1,2,4,5]-tetrazine; C, 6,6’-dinitramino-3,3’-azo-1,2,4-triazolo[4,3-b][1,2,4,5]tetrazine.

The new compounds were characterized fully by multinuclear NMR spectroscopy and infrared and elemental analysis. The parent compounds 1 and 2, the hydroxylammonium salt of all three neutral compounds (1c, 2c, 3b) and the ammonium salt of compound 2 (2a) were characterized by structural analysis with single crystal X-ray. In the case of 3b, only very thin plates were formed that tended to stack on top of one another giving a structure which was of insufficient quality to permit observation of the hydrogen atoms. The preliminary structure of 3b is given in the Supporting Information (Figures S22-S26). The formation of energetic salts increased the impact and friction stability, but lowered the density and detonation performance when compared with the parent framework. However, the hydroxylammonium salts exhibit greater energetic performance than their precursors. All of the crystals were


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Journal of the American Chemical Society grown from water, acetonitrile, ether, methanol or their mixtures. With the large conjugated systems, the structure in the neutral compounds or their anions are all nearly planar (Figure 2). Density of Energetic Materials. The energy of HEDMs is normally represented by detonation velocity, which is directly proportional to the packing density making it the most important parameter in implying the highest energy.20 A high packing coefficient correlates with a high density. The packing coefficient is influenced by the interlayer distance and the packing arrangement. Usually π-conjugated energetic materials result in close packing because of the strong interlayer interactions.8 The packing in a crystal greatly affects the physicochemical and detonation properties.21 The reduced density gradient is a basic dimensionless quantity in DFT, which is used to describe the deviation from a homogeneous electron distribution, assuming very small values that approach zero, for areas of both noncovalent interactions and covalent bonding.22 Based on the approach of Yang et al.,23 the noncovalent interaction (NCI) plots of 1c, 2c and 3b unit cells were determined in real space to study the influence on crystal packing (Figure 3).22 The differences between van der Waals interactions, hydrogen bonds, and repulsive steric clashes can be detected and observed by analysis of the relationship between the reduced density gradient (s = 1/2(3π2)1/3)|∇ρ|/ρ4/3) and the quantum-mechanical electron density (ρ). Based on this, face-to-face π-π interactions are readily seen between two layers. In compound 1c, the NCI domains exist only on the parallel triazole rings, resulting in an interlayer distance of 3.1 Å, which is shorter than that found for most energetic materials. For compound 2c, the introduction of an additional nitroamine moiety aids in forming a larger conjugated system (the C−N distance (1.37 Å) in the C-NNO2 group lies between double and single C−N bonds, which suggests an obvious conjugation effect. The NCI domains are abundant and larger between the parallel offset face-to-face fused rings in 2c, which results in an interlayer distance of 2.9 Å. With the introduction of N=N, two fused rings connect to form the largest conjugated system, abundant and larger NCI domains corresponding to the structure are seen in compound 3b which results in the shortest interlayer distance of 2.7 Å (Figure S26). As shown in Table 1, a shorter interlayer distance results in a higher packing coefficient, and thus higher density. As shown in Figure 3, except for the π-π interaction of the conjugated system, hydrogen bonding is observed by small, round blue shapes indicating a large buildup of electron density. With respect to electron density in the NCI domain, hydrogen bonding has the strongest interaction. This kind of directional hydrogen-bonding interaction and directional cation−anion contact in the hydroxylammonium salt is suggested as part of the explanation for closer packing. The hydroxylammonium cation in these crystals serves as a buffer chain, which helps to create a better stacking arrangement. As seen in Figure 4, 1 and 2 are found in cross stacking arrangements; however, modification caused by the hydroxylammonium cation results in wave-like stacking for 1c and face-to-face stacking for 2c. The H−O−N−H chain in the hydroxylammonium cation adjusts the direction of the hydrogen bond, in order to form the best organization of those molecules. In Figure 5, four protons in the hydroxylammonium moiety form hydrogen bonds with different anions. The three protons bonded to nitrogen form three different H−O−N−H chains (torsion angles: 83.14, 156.8, 36.9 for 1c; 51.2, 171, 69 for 2c) to connect to

those anions. In 1c, two anion structures are in a layer, one in another parallel layer, one on a layer in another direction, which forms wavelike stacking. In 2c, three anion structures are in one layer, another one is on a parallel

Figure 3. NIC plots of gradient isosurfaces (s = 0.5 au) for 1c, 2c, and 3b unit cells. Surfaces are colored on a blue-green-red scale which indicates strong attractive interactions, weak attractive, and strong non bonded overlap, respectively. (The geometries of the unit cell were optimized at the BLYPD3/def2-QZVPP method with ORCA 3.0.)

Figure 4. Cross stacking for 1 and 2, wave-like stacking for 1c, face-to-face stacking for 2c.

Figure 5. The hydroxylammonium cation controls the stacking style by hydrogen bonding.

layer, which forms face to face stacking. In acting as a buffer chain, the H−O−N−H moiety forms hydrogen bonds behaving as a connecting spot which regulates the anion structures to form perfect stacking types. With the introduction of the hydroxylammonium cation, the packing coefficient increases from 0.7 for 1 to 0.76 for 1c, 0.71 for 2 to 0.8 for 2c (Table 1). Energetic compound 1 is found in the monoclinic space group with P21/n (Z = 4) symmetry, with a calculated density of 1.74 g cm-3 at 293 K (Figure 6). Compound 1c occurs in the monoclinic space group with P21/c symmetry with a density of 1.83 g cm-3 at 293 K and four molecules in each lattice cell (Z = 4). Compound 2·H2O·CH3CN crystallizes in an


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Table 1. Properties of Crystals Compounds

Stacking style

Layer distance (Å)

Packing coefficient

Density (g cm-3 )

Impact sensitivity (J)

1 1c 2·H2O·CH3CN 2a·H2O 2c·H2O 3b

crossing wavelike crossing face to face face to face wavelike

3.1 3.1 2.9 3.0 2.9 2.7

0.70 0.76 0.71 0.76 0.80 0.81

1.76 1.82 1.91 1.84 1.92 1.99

20 28 3 32 25 14

orthorhombic crystal system with the space group of P212121. Due to the presence of one water molecule and one acetonitrile molecule, the crystal density is 1.641 g cm-3 at 293 K. Compound 2c·H2O appears in the monoclinic space group with I2/a (Z = 8), contains one molecule of water, with a crystal density of 1.82 g cm-3 (293 K). The densities, measured with a gas pycnometer, of the anhydrous compounds (25 °C) are 1.91 g cm-3 and 1.92 g cm-3 for 2 and 2c, respectively, which are larger than most currently used energetics. For the azo compound, the introduction of the hydroxylammonium species increases the density from 1.85 g cm-3 (3) to 1.99 g cm-3 (3b). These new nitroamine derivatives exhibit good to excellent densities in the range of 1.70 g cm-3 to 1.99 g cm-3. Among these new materials, 3b with an experimental density of 1.99 g cm-3 is superior to HMX (1.91 g cm-3). Heats of Formation for Energetic Materials. Besides density, enthalpies also have a major influence on the detonation properties of HEDMs. Energetic material heats of formation are most often determined using relatively highprecision theoretical methods because of the uncertainties of calorimetric measurements. In this work, the computation was carried out by using the Gaussian03 (Revision D.01) suite of programs.24 As shown in Table 2, all compounds are endothermic with heats of formation (positive) falling between 1.97 kJ g-1 to 5.22 kJ g-1 which, e.g., exceed that of CL-20 (397.8 kJ mol-1 /0.90 kJ g-1).25 With the presence of a large number of N−N or C N bonds in the fused rings, especially two fused rings linked by an azo moiety have the highest heats of formation spanning 2014.7 to 2099.3 kJ mol-1 (4.60 5.22 kJ g-1). Detonation Parameters for Energetic Materials. With the calculated heats of formation and measured density , the detonation properties of all energetic compounds were obtained by EXPLO5 (v6.01) program.25 As is shown in Table 2, all new energetic compounds exhibit good or excellent calculated detonation pressures (28.4-48.6 GPa) and velocities (8621-10233 m s-1), which are comparable to RDX and HMX. The hydroxylammonium salts 3b and 2c exhibit the best detonation performances, with donation velocities of 9712 and 10233 m s-1, respectively, which exceed those of CL-20. Overall, the fused rings with dinitroamine groups show better detonation properties than those which contain only one nitroamine, but less than the dinitroamine-based fused ring compounds with an azo link. Stabilities of Energetic Materials. Most energetic materials with excellent detonation properties are accompanied by high sensitivities which often limit their potential applications to replace current explosives or propellants. Therefore, it is a significant challenge to achieve a fine balance between low sensitivity and high detonation performance.26 Energetic compounds with hydrogen bondaided π-π stacking often result in low-sensitivity/high-energy explosives (LSHEs), like TATB, and FOX-7.20 In this study,

the strong interlayer intermolecular interactions produced by large π-conjugated fused rings present in the structure of the crystal, and the hydrogen bonds resulting from the cation favor formation of LSHEs.

Figure 6. Single-crystal X-ray structures of 1, 1c, 2 and 2c.

Thermal stability is one of the key properties because practical applications are strictly limited by impractically low decomposition temperature. Differential scanning calorimetry (DSC) was used to measure the thermal stabilities of the nitroamino-functionalized 1,2,4-triazolo[4,3-b][1,2,4,5] tetrazines. As shown in Table 2, the onset decomposition temperatures of all compounds lie between 133 C and 248 °C. Compounds 1f (see Scheme 1) and 2e exhibit the highest decomposition temperatures at 244 °C and 248 ᵒC, respectively, as the ammonium salts; decomposing at 193 °C, 213 °C, 240 °C are 1a, 2a, 3a respectively. Most of the ionic derivatives show enhanced thermal stabilities relative to their neutral nitroamino compounds (1, 148 °C; 2, 138 °C; 3, 170 °C). The bond dissociation enthalpies (BDEs) in these neutral compounds for the N−NO2 bonds which were found to be the trigger bonds27 were calculated to be 135.4 kJ mol−1, 106.6 kJ mol-1 and 139.3 kJ mol-1 for 1, 2 and 3, respectively. This agrees with the experimental observation of thermal stabilities. Compound 2 has the lowest thermal stability because the NNO2 bonded to the triazole ring is less stable than that of the tetrazine. Friction (FS) and impact (IS) sensitivity measurements were made by using friction tester and standard BAM drop hammer techniques, respectively.28 As shown in Table 2, all of the salts are less sensitive to friction and impact than the neutral molecules because of the negative charge delocalization on the nitroamino moieties, and the hydrogen bonds with the anions. Among them, the azo compound 3b is the most impact and


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Journal of the American Chemical Society

Table 2. Physical Properties of nitroamino-functionalized 1,2,4-triazolo[4,3-b] [1,2,4,5]tetrazine fused rings.

79.1

Td[a] (ᵒC) 148

d[b] (g cm-3) 1.76

ΔfH[c] (KJ mol-1/kJ g-1) 675.1/3.71

vD[d] (m s-1) 8621

P[e] (GPa) 30.4

IS[f] (J) 20

FS[g] (N) >240

Compound

N+O (%)

1 1a

79.4

193

1.76

649.1/3.26

8937

31.6

20

>240

1b

80.4

133

1.77

802.1/3.75

9216

33.9

12

240

1c

80.9

156

1.82

698.1/3.24

9276

36.1

28

>240

1d

79.7

152

1.79

959.5/3.35

8748

28.4

38

>360

1e

76.2

155

1.76

1233.6/3.67

9085

32.8

17

>360

1f

80.8

248

1.80

1176.4/4.17

8916

30.9

>40

>360

2

84.3

138

1.91

740.9/3.06

9301

38.3

3

>5

2a

84.0

213

1.84

543.5/1.97

9301

35.4

32

>360

2b

84.9

154

1.82

871.8/2.85

9625

37.9

13

>240

2c

85.7

154

1.92

638.9/2.07

9712

42.9

25

>360

2d

82.7

150

1.70

1301.6/2.89

9061

30.8

30

>360

2e

76.2

244

1.76

1780.9/3.24

8792

29.8

>40

>360

3

81.0

170

1.85

2037.8/5.22

9500

39.8

14

40

3b

82.4

185

1.99

2099.3/4.60

10233

48.6

14

>10

HMX

81.1

280

1.90

105/0.36

9320

39.5

7

120 94

CL-20 38.4 210 2.03 397.8/0.90 9406 44.6 4 [a] Decomposition temperature (onset temperature - heating rate of 5 °C min-1). [b] Density, measured - gas pycnometer (25 °C). [c] Heat of formation (calculated). [d] Detonation velocity (calculated - EXPLO5 V6.01). [e] Detonation pressure – (calculated - EXPLO5 V6.01). [f] Impact sensitivity. [g] Friction sensitivity.

Figure 7. The 2D fingerprint plots in crystal stacking for (a) 1 and (b) 1c. Images (c) and (d) show the Hirshfeld surfaces for 1 and 1c molecules (white, distance d = the van der Waals distance; blue, d > the van der Waals distance; red, d < van der Waals distance). In images e, the individual atomic contacts percentage in the bar graphs for 1 and 1c, respectively.

friction sensitive, which mainly arises from the short interlayer distance for the crystal packing. The interlayer distance of 3b is 2.7 Å so that the molecules easily contact each other resulting in an explosion (Figure S26). For the neutral compound 2, the friction and impact sensitivities are measured to be 5 N and 3 J, but for the hydroxylammonium salt the values of 360 N and 25 J, which are less than those of HMX (7 J, 120 N).29 The influences of the crystal packing features of theenergetic materials are explored on sensitivity. The crystal packing is effectively reflected by the Hirshfeld surface.30 In Figures 7 and S1, in crystal stacking for 1 and 1c, and 2a•H2O and 2c•H2O, the 2D fingerprint plots are present. Because of

planar conjugated molecular structures, all the surfaces look like plate shapes. Furthermore, the red dots (predominant intermolecular interactions) exist on the edges of each plate-like surface indicating that the layers are supported by π-π stacking. These two Hirshfeld surface features are characteristic of lowsensitivity and high-energy materials.31 The red dots mainly denote the intermolecular H···O, O···H, N···H and H···N interactions, whereas the blue parts normally belong to π-π stacking, such as C−N, O−C, and C−C interactions, which is in agreement with the structure of interlayer intermolecular hydrogen bonds supporting the layers to give π-π stacking.


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This is also observed from the two-dimensional fingerprint plots in Figure 7a and 7b. A pair of unusual spikes on the bottom left denote the hydrogen bonds among cation and anion. The thicker spikes indicate that many hydrogen bonds are observed.30 From the near contact populations in Figure 7e, the N···H and O···H possess 35.3% of the total weak interactions for 1 and 58.7% for 1c, suggesting that hydrogen bonding is an important characteristic of the hydroxylammonium salt. For nitrogencontaining fused rings, present as π-π stacking are N=N, N−C and C−N interactions. Both 1 and 1c have high percentages of 29.7% and 19.8 %. Based on these rules, the Hirshfeld surfaces agree with the measured IS and FS values, both 1 and 1c possess low sensitivity. However, 1c is more stable than 1 because of additional hydrogen bonding. Both 2a•H2O and 2c•H2O possess similar distributions of weak interactions. Therefore, there is no major difference in their sensitivities. It is reported that the stacking style can affect the impact sensitivity. Different stacking leads to different steric hindrances where interlayer sliding occurs.32 As shown in Figure 4, the face-to-face stacking of 2a and 2c will allow the most ready slide, as it occurs along the molecular plane, which results in very low impact sensitivity (25 J, 32 J). However, with the wave-like stacking of 1c (28 J) and 3b (14 J) (Figure S26), the sliding is limited to a single line, with fewer sliding orientations with respect to the face-to face type. The cross stacking of 1 (20 J) and 2 (3 J) involves two cases where the sliding is effectively precluded and difficultly allowed, respectively. Compared with the parent molecules, the energetic salts tend to form better stacking formats which suggest lower impact sensitivity. CONCLUSION A family of new nitroamino-functionalized 1,2,4triazolo[4,3-b][1,2,4,5]tetrazine fused rings was prepared. All the new compounds were completely characterized by IR, and NMR spectroscopy as well as elemental analysis. Because of the large conjugated structures, the main structures in a neutral compound or its anions are all nearly planar. The strong interlayer noncovalent interactions shorten the layer distance, resulting in higher packing coefficients. Compared with its precursor, the hydroxylammonium anion not only forms more intermolecular hydrogen bonds, but also behaves as a buffer chain (H-N-O-H) to adjust the molecule to form a better stacking type, favoring higher density and lower impact sensitivity. Compound 2c and 3b have an interlayer distance of 2.9 Å and 2.7 Å resulting in a packing coefficient more than 0.8, which is infrequently found in usual energetic materials. The experimental measurements and theoretical calculations confirm that the hydroxylammonium salt has greater detonation performance, higher density, lower sensitivities, and higher decomposition temperature than its neutral precursor. Compound 3b, in which the fused rings are linked by an azo moiety, shows high density (1.99 g cm-3), and heat of formation (2077 kJ mol-1), and with a calculated velocity of detonation of 10233 m s-1 indicating that energetic materials with large conjugated structures provide a promising pathway to design low-sensitivity high-energy materials.

ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org Experimental procedures, characterization data (infrared, NMR and DSC).

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X-ray crystallographic files in CIF format for 1 CIF) X-ray crystallographic files in CIF format for 1c (CIF) X-ray crystallographic files in CIF format for 2 (CIF) X-ray crystallographic files in CIF format for 2a (CIF) X-ray crystallographic files in CIF format for 2c (CIF)

AUTHOR INFORMATION Corresponding Author *[email protected]

ACKNOWLEDGMENT This work was supported by the Office of Naval Research (N00014-16-1-2089), and the Defense Threat Reduction Agency (HDTRA 1-15-1-0028). We are also grateful to the M. J. Murdock Charitable Trust, Reference No.: 2014120: MNL:11/20/2014 for funds supporting the purchase of a 500 MHz NMR spectrometer.

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Conjugated Energetic Salts Based on Fused Rings: Insensitive and

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