A Melt-cast Explosive 3-Azido-1，3-dinitroazetidine（AzDNAZ）with Gem-azidonitro of Novel Energetic Group：Synthesis and Performance

JIA Si-yuan，ZHANG Hai-hao，ZHANG Jia-rong，LIU Qian，LUAN Jie-yu，BI Fu-qiang，2，WANG Bo-zhou，2

（1.Xi′an Modern Chemistry Research Institute，Xi′an 710065，China；2.State Key Laboratory of Fluorine & Nitrogen Chemicals，Xi′an 710065，China）

Abstract：Starting from（1-tert-butyl-3-nitroazetidin-3-yl）methanol，a melt-cast explosive of 3-azido-1，3-dinitroazetidine（AzDNAZ）with novel gem-azidonitro energetic group was obtained through an improved azidation-salinization-nitration strategy with a total yield of 58.8%. Structures of the intermediates and AzDNAZ were characterized by 1H NMR，13C NMR，IR and element analysis. One of the intermediates，1-t-butyl-3-azido-3-nitro-azetidinium nitrate（compound 2），was obtained for the first time with the single crystal determined by X-ray single crystal diffraction，which reveals that it crystallizes in monoclinic，space group P2（1）/n with a=0.8281（314）nm，b=0.8607（2）nm，c=1.7195（2）nm，α=90°，β=94.983（2）°，γ=90°，V=1.2210（6）nm3，Z=4，Mr=262，Dc=1.427 g·cm-3，μ=0.174 mm-1，F(xiàn)（000）=552，R=0.0418 and ωR=0.1168.Meanwhile，thermal behaviors of AzDNAZ were studied by subsequently differential scanning calorimetry（DSC）and thermal gravimetric analyzer（TG）methods.Based on the measured values of density and heat of formation，the detonation parameters were calculated by Gaussian 09 program and Kamlet-Jacobs equations. Results show that the melting point，decomposition point，density，heat of formation，detonation velocity and detonation pressure are 78.2 ℃，180.7 ℃，1.75 g·cm-3，331.73 kJ·mol-1，8460 m·s-1 and 31.83 GPa，respectively，indicating AzDNAZ can be applied as a promising melt-cast explosive or an energetic plasticizer with satisfactory performances.

Key words：gem-azidonitro energetic compound；3-azido-1，3-dinitroazetidine；melt-cast explosive；oxidative azidation；synthesis；properties

1 Introduction

Azetidine is an important energetic structural moiety with high density，large ring strain and good stability，which is a hotpot in the field of energetic materials［1-5］. Based on azetidine skeleton，1，3，3 -trinitroazetidine （TNAZ）［6-10］，a famous energetic compound with four - membered heterocyclic ring and gem-dinitro energetic group was synthesized by Archibald TG［1］. TNAZ was once regarded as a candidate for melt-cast explosive with good energetic performances（density of 1.84 g·cm-3，melting point of 101 ℃，decomposition temperature of 240 ℃，detonation velocity of 8834 m·s-1，impact sensitivity of 44% and friction sensitivity of 42%）［11］. However，due to the high vapour pressure and the complicated synthetic process，it has been proved impossible to be applied in military equipments. Based on the gemazidonitro energetic group，a similar melt-cast explosive 3-azido-1，3-dinitroazetidine（AzDNAZ）was prepared by Dmitry V. Katorov via oxidative azidation and nitrotion（N2O5/CH3CN）with a total yield of 37.4%［12］. Since both nitro and azide groups were introduced into the azetidine structure，the thermal behaviors and detonation properties of gem-azidonitro compounds attracted great interests［13-16］. We believed that 3-azido-1，3-dinitroazetidine（AzDNAZ）is possible to be a promising melt-cast explosive since it has melting point of 78-80 ℃ and decomposition temperature of 190 ℃. Nevertheless，the physicochemical properties and detonation performances for AzDNAZ，such as density，detonation velocity，impact sensitivity and friction sensitivity，were not fully reported in the Russian literature.

In this report，an improved synthetic strategy for AzDNAZ was designed and studied via oxidative azidation，nitric acid salinization and new nitrolysis system in order to simplify technology and increase the nitration yield. The single crystal of the intermediate 3-azido-1，3-dinitroazetidinium nitrate（compound 2）was firstly obtained and determined by X-ray single crystal diffraction. The physicochemical properties and detonation performances for AzDNAZ，such as density，detonation velocity and sensitivity，were studied in detail by the laboratory test or the theoretical calculation. Meanwhile，the thermal behaviors were also carried out through DSC-TG approaches in order to provide the basic parameters for its subsequent application research.

2 Experimental

2.1 Materials and Instruments

（1-tert-butyl-3-nitroazetidin-3-yl）methanol hydrochloride was prepared according to the published reference［12］. Other chemicals were purchased from commercial sources.

13C NMR and1H NMR spectra were obtained in DMSO-d6with TMS as internal standard on a Bruker 500 MHz spectrometer. Infrared spectra were obtained from Nicolet NEXUS870 Infrared spectrometer in the range of 4000 cm-1to 400 cm-1. Elemental analysis was performed on a VARI-E1-3 elementary analysis instrument. Melting point was measured on a XT4A Melting-Point Apparatus with Microscope and uncorrected. Differential scanning calorimetry（DSC）was carried out on a Q200 apparatus（TA，USA）in the range of 10 ℃and 400 ℃at a heating rate of 10 ℃·min-1.

2.2 Synthesis

Starting from（1-tert-butyl-3-nitroazetidin-3-yl）methanol hydrochloride，the title compound AzDNAZ was synthesized via oxidative azidation，nitric acid salinization and nitration sequence（Scheme 1）.

Scheme 1 The synthesis of AzDNAZ

2.2.1 Synthesis of 3-Azido-1,3-dinitroazetidinium nitrate (2)

To a solution 5.0 g（22.3 mmol）（1-tert-butyl-3-nitroazetidin-3-yl）methanol hydrochloride in 4 mL water， then the solution of NaOH 6.2 g（155.0 mmol）in 45 mL water was added at room temperature with stirring. After dissolution，a solution of NaN37.2 g（111.5 mmol）in 16 mL water was added dropwise. At 20-25 ℃，the reaction mixture was added to a solution of K3［Fe（CN）6］36.6 g（111.5 mmol）in 100 mL water. The mixture was maintained at 20-25 ℃for 5 h. Then，the reaction mixture was extracted with dichloromethane（3×20 mL）. HNO3was added dropwise to adjust the pH to 3-4. White precipitate was formed and filtered out. Drying the filter cake to give 3-azido-1，3-dinitroazetidinium nitrate（4.9 g，84%）. IR（KBr，ν/cm-1）：3050（CH3），2151（—N3），1585（νasNO2），1400（—NO2），1325（—NO2），1255，1040，825；1H NMR（D2O，500 MHz），δ：5.26（d，2HJ=13.6 Hz），4.81（d，2HJ=13.2 Hz），1.641（s 9H）；13C NMR（D2O，125 MHz），δ：22.433，57.512，61.578，90.270.Anal. calcd for C7H14N6O5： C 32.06， H 5.38，N 32.05；found：C 32.16，H 5.28，N 32.25.

2.2.2 Synthesis of 3-Azido-1,3-dinitroazetidine(AzDNAZ)

3 mL 98% HNO3was dropped into 5 mL trifluoroacetic anhydride at -10- -5 ℃，and 6.5 mL acetonitrile was added into the solution，then 1.0 g（5.3 mmol）3- azido-1，3-dinitroazetidinium nitrate was added slowly. The mixture was kept at 0 ℃for 2 h. and poured into ice. White solid was precipitated and filtered，dried to obtain 0.5 g 3-azido-1，3-dinitroazetidine（AzDNAZ）with a yield of 70%，and 98%purity. IR（KBr，ν/cm-1）：2135（N3），1576（N—NO2），1561，1354（C—NO2），1256（N—NO2）；1H NMR（DMSO-d6，500 MHz），δ：4.856（d，2H J=14 Hz），5.039（d，2H J=14 Hz）；13C NMR（DMSO-d6，125 MHz），δ：90.220，66.527.Anal.calcd for C3H4N6O4：C 19.16，H 2.14，N44.68；found：C 19.26，H 2.08，N 44.58.

3 Results and discussion

3.1 X-ray Crystallography

The single crystal for 1-tert-butyl-3-azido-3-nitro-azetidinium nitrate（compound 2）was firstly cultivated by slow evaporation from the mixed solvent of water and ethanol（V∶V=1∶1）at room temperature in order to give satisfactory crystals for X-ray determination. The crystal data and structure refinement parameters for compound 2 are given in Table 1. The displacement ellipsoid plot with atomic numbering scheme and perspective view of the crystal in a unit cell are shown in Fig.1 and Fig.2.

It can be seen from Table 1 and Fig.2 that the crys-tal for compound 2 belongs to monoclinic with space groupP2（1）/ncontaining four molecular moieties in a unit cell witha=0.8281（314）nm，b=0.8607（2）nm，c=1.7195（2）nm，α=90°，β=94.983（2）°，γ=90°，V=1.2210（6）nm3，Z=4，Mr=262，Dc=1.427 g·cm-3，μ=0.174 mm-1，F(xiàn)（000）=552，R=0.0418 andωR=0.1168. Fig.1 and Fig.2 show that the four-membered ring azetidine is basically co-planar. However，the substituent groups including nitro（—NO2），azido（—N3）andt-butyl（—C（CH3）3）are not co-planar. The angulations of nitro（—NO2），azido（—N3）andt-butyl（—C（CH3）3）are main reasons for the loose structure of compound 2 with a density of 1.427 g·cm-3.

Table 1 Crystal data and structure refinement parameters for compound 2

Fig.1 Crystal structure of compound 2

Fig.2 Molecular packing diagram of the unit cell for compound 2

3.2 Thermal Behavior for AzDNAZ

Based on differential scanning calorimeter（DSC）and thermal gravimetric analyzer（TG）measurements，the thermal behavior of AzDNAZ was determined at a heating rate of 10 ℃·min-1from 20 ℃to 400 ℃. The results are shown in Fig.3.

Fig.3 DSC-TG curves ofAzDNAZ ata heating rate of 10 ℃·min-1

From Fig.3，the thermal behavior of AzDNAZ undergoes two stages：first melting and then decomposing. The endothermic peak at 78.2 ℃ and the exothermic peak at 180.7 ℃ are corresponding to the melting point and the decomposition point of AzDNAZ，respectively. From the TG curve，a very small mass loss （nearly 7%） appears before 110.6 ℃. A small amount of mass loss of AzDNAZ can be observed during melting process，which may due to the volatilization of the sample. As the temperature continue to rise，a dramatical mass loss of 83% is observed from 110.6 ℃to 212.9 ℃. With the temperature increasing， AzDNAZ is further decomposed，and remains only 4% at 400 ℃.

3.3 Physicochemical Properties

The detonation pressure and detonation velocity of AzDNAZ were obtained by Kamlet-Jacobs formulas with the help of calculated density and enthalpies of formation［17-18］，and compared with the wellknown energetic compounds of TNAZ［11］and RDX［19］（Table 2）. It was found that AzDNAZ possesses adensity of 1.75 g·cm-3， detonation velocity of 8460 m·s-1and detonation pressure of 31.83 GPa.All of these properties are comparable to those of TNAZ［11］and RDX［19］，which indicates that AzDNAZ is a promising melt-cast energetic compound with low melting temperature and high decomposition point.

Table 2 Physicochemical and energetic properties of AzDNAZ compared with TNAZ and RDX

4 Conclusions

（1）Starting from（1-tert-butyl-3-nitroazetidin-3-yl）methanol，3-azido-1，3-dinitroazetidine（AzDNAZ）was synthesized via oxidative azidation，nitric acid salinization and nitration with a total yield of 58.8%.The structures of the intermediates and AzDNAZ were characterized by1H NMR，13C NMR，IR and element analysis. The single crystal of intermediate compound 2 was firstly determined and the results show that 2 was crystallized in monoclinic with a space group ofP2（1）/n，and the density of 1.427 g·cm-3.

（2） Results show that the density， melting point，heat of formation，detonation velocity and detonation pressure were 1.75 g·cm-3， 78.2 ℃331.73 kJ·mol-1，8460 m·s-1and 31.83 GPa，respectively，indicating AzDNAZ is possible to be applied as a promising melt-cast explosive or an energetic plasticizer with satisfactory performances.