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Self-assembled energetic coordination polymers based on multidentate pentazole cyclo-N5

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  • ReceivedMar 1, 2018
  • AcceptedMar 30, 2018
  • PublishedApr 27, 2018

Abstract

Coordination to form polymer is emerging as a new technology for modifying or enhancing the properties of the existed energetic substances in energetic materials area. In this work, guanidine cation CN3H6+ (Gu) and 3-amino-1,2,4-triazole C2H4N4 (ATz) were crystallized into NaN5 and two novel energetic coordination polymers (CPs), (NaN5)5[(CH6-N3)N5](N5)3 (1) and (NaN5)2(C2H4N4) (2) were prepared respectively via a self-assembly process. The crystal structure reveals the co-existence of the chelating pentazole anion and organic component in the solid state. In polymer 1, Na+ and N5 were coordinated to form a cage structure in which guanidine cation [C(NH2)3]+ was trapped; for polymer 2, a mixed-ligand system was observed; N5 and ATz coordinate separately with Na+ and form two independent but interweaved nets. In this way, coordination polymer has been successfully utilized to modify specific properties of energetic materials through crystallization. Benefiting from the coordination and weak interactions, the decomposition temperatures of both polymers increase from 111°C (1D structure [Na(H2O)(N5)]∙2H2O) to 118.4 and 126.5°C respectively. Moreover, no crystallized H2O was generated in products to afford the anhydrous compounds of pentazole salts with high heats of formation (>800 kJ mol–1). Compared to traditional energetic materials, the advantage in heats of formation is still obvious for the cyclo-N5 based CPs, which highlights cyclo-N5 as a promising energetic precursor for high energy density materials (HEDMs).


Funded by

the National Natural Science Foundation of China(11702141,21771108,U1530101)


Acknowledgment

This work was financially supported by the National Natural Science Foundation of China (11702141, 21771108, and U1530101). The authors gratefully acknowledge Dongxue Li (College of Chemical Engineering, Nanjing Tech University) for her tests of the Raman spectra.


Interest statement

The authors declare no conflict of interest.


Contributions statement

Wang PC, Lin QH and Lu M conceived the idea, designed the experiments and co-wrote the manuscript. Wang PC and Xu YG conducted synthetic experiments, calculations, and performed the DSC, TGA, PXRD measurements. Wang Q and Shao YL assisted the synthetic experiments. Wang PC assisted the calculations. Xu YG performed the single-crystal XRD measurements. All authors discussed the results and commented on the manuscript.


Author information

Pengcheng Wang obtained his BSc in 2008 and PhD in 2013 at Nanjing University of Science and Technology (NJUST), and was a postdoc at the National Institute of Advanced Industrial Science and Technology (AIST) from 2013 to 2014. He joined NJUST in 2014. His current research interest is in the synthesis and crystal engineering of energetic materials.


Yuangang Xu was born in 1990. He is a PhD candidate of applied chemistry in NJUST. His research focuses on the synthesis of nitrogen-rich energetic materials.


Qiuhan Lin obtained his BSc in 2008 and PhD in 2013 at Beijing Institute of Technology. He joined NJUST as an associate professor in 2014. His current research interest is in the synthesis and crystal engineering of energetic salts.


Ming Lu obtained his BSc in 1984, MSc in 1989 and PhD in 1999 at NJUST. He then became a professor in 2001. His current research interest focuses on the synthesis and crystal engineering of energetic materials, pharmaceutical intermediates and green chemistry.


Supplement

Supplementary information

Experimental details and supporting data are available in the online version of the paper.


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  • Figure 1

    Crystal structure of 1. (a) The coordination mode of Na+ in CP 1. (b) Perspective view of the coordination mode along c axis. (c) A topological net of simplified unit cell. (d) Unit cell stacking diagram view along b axis (4×4×4). (e) Unit cell stacking diagram view along c axis (4×4×4). Carbon, hydrogen, nitrogen, oxygen, and sodium atoms are shown in gray, white, blue, red, and purple respectively.

  • Figure 2

    Crystal structure of 2. (a) The coordination mode of Na+ in CP 2. (b) A topological net of simplified unit cell. (c) Two-helical structure of simplified unit cell. (d) Unit cell stacking diagram view along a axis (4×4×4). (e) Unit cell stacking diagram view along c axis (4×4×4). Carbon, hydrogen, nitrogen, oxygen, and sodium atoms are shown in gray, white, blue, red, and purple respectively.

  • Figure 3

    Geometric parameter, FT-IR and Raman spectra of CPs 1 and 2. (a) Bond length of tri- quadri-, and pentadentate-cyclo-N5 in CPs 1 and 2. (b) FT-IR spectra of CPs 1 and 2, Gu (guanidine hydrochloride), and ATz (4-amino-1,2,4-triazole). (c) Raman spectra of CPs 1 and 2, Gu, and ATz.

  • Figure 4

    Differential scanning calorimetry spectra of CPs 1 and 2. The point of decomposition temperature is onset temperature.

  • Table 1   Crystal data and structure refinement details of CPs and

    Sample

    1

    2

    CCDC

    1544794

    1575478

    Empirical formula

    CH6N48Na5

    CH2N7Na

    Temperature (K)

    170

    173

    Crystal system

    Hexagonal

    Trigonal

    Space group

    P63/mmc

    P3221

    a (Å)

    13.5635(11)

    9.687(4)

    b (Å)

    13.5635(11)

    9.687(4)

    c (Å)

    13.8295(12)

    9.585(4)

    α (°)

    90.00

    90.00

    β (°)

    90.00

    90.00

    γ (°)

    120.00

    120.00

    Volume (Å3)

    2203.3(4)

    779.0(6)

    Z

    1.99992

    6

    ρcalc(g cm−3)

    1.214

    1.728

    μ (mm−1)

    0.139

    0.205

    F(000)

    806.0

    408.0

    Crystal size (mm3)

    0.28×0.17×0.12

    0.28×0.15×0.12

    2θ (°)

    3.468 to 54.93

    4.856 to 54.766

    Reflections collected

    11891

    1948

    Rint/Rsigma

    0.0564/0.0276

    0.0503/0.0809

    Data/restraints/

    parameters

    975/0/54

    1160/6/85

    GOF on F2

    1.048

    1.046

    R1/wR2 [I >= 2σ (I)]

    0.0339/0.0821

    0.0485/0.1105

    R1/wR2 [all data]

    0.0457/0.0879

    0.0607/0.1240

    Largest diff.

    peak/hole (e Å−3)

    0.25/−0.23

    0.27/−0.26

  • Table 2   Properties of and and comparison with TNT, RDX, and HMX

    ρa

    (g cm−3)

    Nb

    (%)

    Ωc

    (%)

    ΔHfd

    (kJ mol−1)

    Tde

    (oC)

    Qf

    (kcal g−1)

    Dg

    (m s−1)

    Ph

    (GPa)

    1

    1.19

    83.48

    −14.9

    1015

    118.4

    0.329

    4407

    6.5

    2

    1.70

    72.59

    −41.5

    902

    126.5

    1.650

    7863

    26.4

    HMX

    1.90

    37.84

    −21.6

    105

    279

    1.320

    8900

    38.4

    RDX

    1.81

    37.84

    −21.6

    80

    210

    1.386

    8600

    33.9

    TNT

    1.65

    18.50

    −74.0

    −67

    290

    0.897

    7178

    20.5

    Recalculated from low-temperature X-ray densities (ρ = ρT /(1 + αV(298 – T0)); αV=1.5×10−4 K−1). b) Nitrogen content. c) Oxygen balance. d) Calculated heat of formation. e) Thermal decomposition temperature (onset) under nitrogen gas (DSC, 5°C min−1). f) The heat of detonation. g) Detonation velocity. h) Detonation pressure.

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