Azo1,3,4-oxadiazole as a novel building block to design high

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Azo1,3,4-oxadiazole as a novel building block to design high-performance energetic materials Qian Wang, Yanli Shao, and Ming Lu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b01404 • Publication Date (Web): 04 Jan 2019 Downloaded from http://pubs.acs.org on January 6, 2019

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Crystal Growth & Design

Azo1,3,4-oxadiazole as a novel building block to design high-performance energetic materials Qian Wang, Yanli Shao, Ming Lu*

School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China.

ABSTRACT: In this study, the azo1,3,4-oxadiazole energetic fragment was first introduced into the energetic materials using a simple synthetic strategy, yielding two symmetrical covalent compounds 4 and 5. All new compounds (35) were well characterized by IR spectroscopy, NMR spectroscopy, thermal analysis, and single crystal X-ray diffraction analysis. As supported by DSC data, compounds 4 and 5 possess excellent decomposition temperatures as high as 248 °C and 278 °C, respectively. To the best of our knowledge, 278 °C ranks highest in all 1,3,4-oxadiazole-based energetic compounds. Their energetic performances were evaluated with EXPLO5. Both 4 and 5 show good detonation velocities (D) of 8409 m s-1 and 8800 m s-1, and detonation

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pressures (P) of 29.3 GPa and 35.1 GPa, comparable to RDX (D: 8795 m s-1, P: 34.9 GPa). Furthermore, based on the single crystal data, quantum chemical calculations were employed to better understand their intrinsic structure-property relationship.

All these positive results indicate the superior potential of the

azo1,3,4-oxadiazole

backbone

for

designing

next

generation

of

energetic

materials.

Introduction Over the past few decades, benefiting from the efforts of synthetic and theoretical researchers, the development of energetic materials has been rapid.13

Several strategies, such as energetic salts and energetic cocrystals, are

proposed

to

investigate

next-generation

energetic

materials.4-6

However,

preparation of new covalent compounds is still the most fundamental area of research.

In general, combining various functional units with different backbones is one of the most significant methods to access novel energetic molecules.7-9 Furazan is an attractive energetic moiety owing to its high combined nitrogen and


2

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oxygen contents.10-15 Further, introduction of the nitramino group or the nitro group into the furazan ring could result in a favorable performance, for example, 3,3’-dinitramino-4,4’-bifurazane (A in Scheme 1) features a detonation velocity of 9086 m s-1.16 Another example, 3,3’-dinitroxazafurazan (B in Scheme 1) which includes two nitro-furazan fragments, has an excellent detonation performance (D: 9390 m s-1, P: 40.5 GPa).17 However, these furazan-based molecules show undesirably low thermal stabilities. Compounds A and B demonstrate relatively low decomposition temperatures of 80 °C and 100 °C, respectively. These greatly limit their practical application.

To maintain detonation performance while enhancing molecular stability, some strategies isomers

have of

been

furazan,

implemented. were

combined

1,3,4-Oxadiazole with

furazan

and rings

1,2,4-oxadiazole, to

form

bicyclic

compounds C and D.18-19 Unfortunately, their thermal behaviors (C: Td = 115 °C; D: Td = 98 °C) were not improved satisfactorily. Meanwhile, their detonation velocities dropped to around 8200 m s-1. Given this background, such a scientific topic should be challenging and of particular interest.


3


Scheme

1.

Energetic

compounds

with

Page 4 of 25

nitramino-furazan

or

nitro-furazan

fragments.

Our approach involved the possibility of using a novel structural core to link and stabilize nitramino-furazan or nitro-furazan fragments. Thus, two symmetric covalent compounds, 4 and 5 were easily prepared. All new compounds in this study

were

magnetic

well

characterized

resonance

(NMR)

by

infrared

spectroscopy,

(IR)

and

elemental

multinuclear analysis,

nuclear

differential

scanning calorimetry (DSC) and single crystal X-ray diffraction. Further, quantum chemical

calculations

were

employed

to

better

understand

their

intrinsic

structure-property relationship.


4

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Scheme 2.

Synthesis of compounds 3-5.

Synthesis

The synthetic pathway is shown in Scheme 2. Compounds 1 and 2 were synthesized according to the reported procedures.17 Nitro-based compound 3 was obtained by treating 1 with 50%hydrogen peroxide, 98%H2SO4, and Na2WO4. Through the oxidative coupling reaction of 2 and 3, tetracyclic compounds 4 and 5 were easily obtained. New energetic compounds 3-5 were air-stable and were generated in high yield (80-83%).

Crystal structure analysis


5


Page 6 of 25

To acquire more structural information, new compounds 3-5 have been characterized using single crystal X-ray diffraction. Crystals were obtained through slow evaporation in related solutions (3 in ethyl acetate, 4-5 in ethanol). CCDC numbers of these crystals are 1851330-1851332 and details of crystallographic data are given in the Supporting Information.

Bicyclic compound 3 crystallizes in the monoclinic space group P 21/c with a calculated density of 1.780 g cm-3 at 173 K (Figure 1). All atoms of 3 is nearly coplanar, confirming by the dihedral angles of N3a-C1a-C2a-C4a (-5.39°) and C2a-C4a-C5a-O4a (178.60°). The bond lengths of C-N range from 1.291 to 1.453 Å. Additionally, there are five intermolecular hydrogen bonds between molecules (Figure 1b). All hydrogen atoms of the molecule are involved.


6

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Figure 1. a) Single-crystal X-ray structure of 3 with labelling. b) Crystal packing of 3 viewing down a-axis (dotted lines represent hydrogen bonds).

Tetracyclic compound 4 crystallizes in the monoclinic space P 21/c with a moderate density of 1.768 g cm-3 at 296K (Figure 2a). Four oxadiazole rings are almost coplanar, which is evident from the C3-C4-C2-O1 (170.89°) and N3N3-C1-N1 (-178.38°). The nitro group twists out of the furazan plane with a high torsion angle C3-C4-N6-N7 (57.37°). Moreover, there are three pairs of hydrogen bonds were found in the packing system in hydrogen bonding.

Nitro-functionalized compound 5 crystallizes in the same space as 4 while exhibits a higher density of 1.825 g cm-3 at 296 K, resulting from its better molecular planarity (N6-C4-N4-N3 179.97°; N5-C1-N1-O1 179.69°) and more compact crystal packing (Figure 2b). The crystal packing of 4 features a crossing stacking, whereas 5 demonstrates a basic face-to-face stacking. Previous studies show that the face-to-face style has a higher density and is more stable to mechanical stimuli because of free interlayer sliding.20 Hence,


7


Page 8 of 25

this guideline can also be used to explain their differences in mechanical sensitivity. In contrast to 4 (IS: 3 J, FS: 48 N), 5 (IS: 15 J, FS: 220 N) is less sensitive.

Figure 2. a) Crystal structure crystal and packing diagrams of 4 (dotted lines represent hydrogen bonds). b) Crystal structure crystal and packing diagrams of 5.

Energetic properties

To investigate the application prospect of these energetic compounds, some necessary physical properties were determined in Table 1. The decomposition temperatures of bicyclic compounds 2 and 3 are observed at 120 °C and 139


8

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°C, whereas their coupling products 4 and 5 decompose as high as 248 °C and 278 °C, respectively. 278 °C, to the best of our knowledge, is the highest known value for all energetic compounds based on nitramino furazan or nitro furazan. Meanwhile, the thermal stabilities of compounds 4-5 are comparable to some thermally stable oxadiazolefunctionalized energetic compounds, such as compounds E (233 °C) and F (309 °C) (Scheme 3).21

Table 1. Physiochemical properties of 3-5, TNT, and RDX. Compound

3

4

5

TNTk

RDXl

Tma [°C]

139

181

157

81

–

Tdb [°C]

155

248

278

295

204

ρc [g cm-3]

1.747

1.767

1.825

1.65

1.80

N+Od [%]

74.7

76.8

75.5

60.8

81.1

OBcoe [%]

-8.08

-3.79

0

-18.6

0

ΔHf f [kJ mol-1]

219.4

786.1

854.2

-59.4

70.3

Dg [m s-1]

8102

8409

8800

7303

8795

Ph [GPa]

26.9

29.3

35.1

21.3

34.9

ISi [J]

35

3

15

15

7

FSj [N]

360

48

220

>353

120

[a] Melting point. [b] Thermal decomposition temperature (onset) under nitrogen (determined by the DSC exothermal peak, 5 °C min-1). [c] X-ray densities at 296K and the corresponding RT Values using the equation (ρ298k=ρT/(1+av(298-T0)); av=1.5×10-4 K-1). [d] Content of nitrogen and


9


Page 10 of 25

oxygen. [e] Oxygen balance [%] Based on CO for CaHbNcOd: OB[%]=1600×(d-a-b/2)/MW. [f] Calculated molar enthalpy of formation in solid state. [g] Detonation velocity. [h] Detonation pressure. [i] Impact sensitivity. [j] Friction sensitivity. [k] Ref. [23]. [l] Ref. [9].

Scheme 3. Selected oxadiazole-functioned energetic compounds.

Interestingly, comparing the two azo compounds, although 4 has extensive hydrogen-bonding interactions, the decomposition temperature of 5 is 30 °C more than that of 4. The single crystal data provides two possible explanations, trigger bond dissociation enthalpy (TBDE) and noncovalent interaction analysis (NCI).22-23 The TBDE of 4 is observed at 156.2 kJ/mol, much smaller than that in 5 (272.5 kJ/mol). However, both of them outperform RDX (152.8 kJ/mol), thereby their thermal performance is better than RDX (Td = 204 °C).


10

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Electrostatic potential surfaces (ESP) analysis for 4 and 5 was employed here to better understand their bond stabilities. It is shown in Figure 3, in contrast to 4, compound 5 demonstrates lower maximum ESP values and more negative charge accumulation in the C-nitro bonds, thereby increasing the corresponding BDE, and consequently, enhancing the stability.

Figure 3. Electrostatic potential surfaces of 4 (a) and 5 (b). The minimum and maximum of ESP are marked as blue and red points, respectively. The maximum values of ESP are labelled by red texts, the units are in kcal mol-1.

Their results of noncovalent interaction analysis are presented in Figure 4. Compounds 4 and 5 possess edge-to-face π-π stackings. However, compared with 4, the isosurfaces for edge-to-face π-π stacking around the azo linkage in


11


Page 12 of 25

5 are larger, suggesting that such interactions are more extensive in 5 than in 4, thereby increasing the density and the molecular stability.

Figure 4. Noncovalent interaction analysis, including HB (hydrogen bonds) and π-π interactions for 4 (a) and 5 (b) (blue, strong attraction; green, weak interaction; and red, strong repulsion).

The densities of compounds 3-5 were measured by using single crystal densities and converted into the corresponding RT values using the equation (ρ298k=ρT/(1+av(298-T0)); av=1.5×10-4 K-1).24 The bicyclic compound 3 exhibits the lowest density at 1.747 g cm-3, while its coupling product 5 shows the highest density at 1.825 g cm-3, superior to the benchmark explosive RDX (1.800 g cm3).

In contrast to 5, 4 possesses a lower value at 1.767 g cm-3 as its weaker


12

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noncovalent interaction and less compact crystal packing. However, it is still greater than its coupling substrate 2 (1.751 g cm-3).

Higher heats of formation always lead to better detonation performance. The enthalpies of 3-5 were calculated by using Gaussian09 program (more details in the Supporting Information).25 Due to the presence of high-energy N-N and N-O bonds, tetracyclic compounds 4 and 5 show remarkable positive enthalpies ranging from 1.86 kJ g-1 to 2.18 kJ g-1, much higher than those of RDX (0.32 kJ g-1). With calculated enthalpies and crystal densities in hand, their detonation performance (detonation velocity and detonation pressure) were determined by using EXPLO5.26 The detonation velocities fall in the range from 8102 to 8800 m s-1, while detonation pressures are from 26.9 to 35.1 Gpa. Nitro-based compound

5

with

the

zero-oxygen

balance

exhibits

the

best

detonation

performance (D: 8800 m s-1, P: 35.1 GPa), comparable to RDX (D: 8795 m s-1,

P: 34.9 GPa).


13


Page 14 of 25

The IS and FS of these materials were also measured to evaluate their safety.27 Bicyclic compound 3 shows excellent mechanical stability (IS: 35 J, FS: 360 N). Tetracyclic compound 4 is very sensitive (IS: 3 J, FS: 48 N), whereas 5 has an enhanced mechanical stability (IS: 15 J, FS: 220 N). These values of 5 are much higher than those of RDX (IS: 7 J, FS: 120 N), which further suggests its promising application potential.

To better understand their differences in mechanical sensitivity, the twodimensional

(2D)-fingerprint

of

crystals

4-5

and

the

associated

Hirshfeld

surfaces were employed to show their intermolecular interactions (Figure 5). The red and blue regions on the Hirshfeld surfaces represent high and low close

contact

populations,

respectively.

In

general,

block-shaped

surfaces

contributes more efficiently to face-to-face π–π stacking and result in better mechanical stability.28 Compared with 4, nitro-based compound 5 is more platelike. Moreover, for energetic materials, π-π stacking is often present as C-O and O-C interactions, in that case, the value of 5 (12.4%) is twice that of 4 (5.1%). Another index about sensitivities is that the ratio of O-O interactions in


14

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4 is as high as 19.9%, which is higher than that in 5 (17.8%). Recent studies indicate that relative frequencies of close O-O contacts can increase the possibility of unexpected explosion.29 According to these results, it is not surprising that compound 5 is more stable than 4.

Figure 5. a) Hirshfeld surfaces calculation (white, distance d equals the van der Waals distance; blue, d exceeds the van der Waals distance; red,

d

is

less

than

van

der

Waals

distance)

and

two-dimensional

fingerprint plots of 4. b) Hirshfeld surfaces calculation (white, distance d equals the van der Waals distance; blue, d exceeds the van der Waals


15


distance;

red,

d

is

less

than

van

der

Page 16 of 25

Waals

distance)

and

two-

dimensional fingerprint plots of 5.

Conclusion

In conclusion, a comprehensive study of the chemistry of 1,3,4-oxadiazole was conducted. Incorporating azo1,3,4-oxadiazole with nitramino furazan and nitro

furazan,

led

to

two

covalent

compounds

4

and

5

which

showed

remarkable thermal stabilities. The structural details for 4 and 5 were further investigated by X-ray diffraction as well as TBDE, NCI, ESP and Hirshfeld surfaces analysis. Compound 4 is very sensitive (IS: 3 J, FS: 48 N) and features good detonation properties (D: 8409 m s-1, P: 29.3 GPa), while 5 possesses the lower mechanical sensitivity (IS: 15 J, FS: 220 N) and an enhanced detonation performance (D: 8800 m s-1, P: 35.1 GPa). These encouraging discoveries in the 1,3,4-oxadiazole chemistry are expected to provide a new strategy for the construction of thermal stable oxadiazolefunctionalized molecules.


16

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ASSOCIATED CONTENT

CCDC 1851330-1851332

AUTHOR INFORMATION

Corresponding Author

*Ming Lu, Email: [email protected].

Present Addresses

†If an author’s address is different than the one given in the affiliation line, this information may be included here.

Author Contributions

Q. Wang and Y. Shao contributed equally to this work. Q. Wang and M. Lu designed the study. Q. Wang performed most of the reactions. Y. Shao synthesized part of the precursor. Y. Shao finished the DFT calculations. Q. Wang and Y. Shao prepared the manuscript.

ACKNOWLEDGMENT


17


Page 18 of 25

This work is supported by the National Natural Science Foundation of China (NSAF, NO. U1530101).

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Title: Azo1,3,4-oxadiazole as a novel building block to design high-performance energetic materials Author: Qian Wang, Yanli Shao, Ming Lu*

The azo1,3,4-oxadiazole energetic fragment was first introduced into the design of energetic materials, yielding two symmetrical covalent compounds.


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Azo1,3,4-oxadiazole as a novel building block to design high

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