Construction of a Thermally Stable and Highly Energetic Metal-Organic Framework as Lead-Free Primary Explosives

Article abstract of DOI:10.1021/acs.cgd.8b00053

Two energetic compounds, 4,8-dinitraminodifurazano[3,4-b,e]pyrazine (1) and its potassium-based energetic metal-organic framework (E-MOF) (2), were prepared, and their crystal structures were confirmed by single-crystal X-ray diffraction analysis. Compound 1 cocrystallizes with water molecules and shows a three-dimensional (3D) sandwich-like supramolecular structure, which is rare in the known energetic organic compounds. Compound 2 has a pillared layered structure with a pcu topology. The layered structure in the 3D framework featuring sql topology was constructed from inorganic chains {K₂O} and nitroamine groups. The crystal density of 2 is up to 2.114 g cm^-3. This potassium-based E-MOF shows high thermal stability, high detonation velocity, and high impact and friction sensitivities, which make it a potential high-performing primary explosive.

Full text of DOI:10.1021/acs.cgd.8b00053

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Article
Construction of a Thermally Stable and Highly Energetic
Metal-Organic Framework as Lead-Free Primary Explosives
Wei Li, Kangcai Wang, Xiu-Juan Qi, Yunhe Jin, and Qinghua Zhang
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00053 • Publication Date (Web): 26 Feb 2018
Downloaded from http://pubs.acs.org on February 27, 2018
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Construction of a Thermally Stable and Highly
Energetic MetalꢀOrganic Framework as Leadꢀ
Free Primary Explosives
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†
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Wei Li , Kangcai Wang , Xiujuan Qi , Yunhe Jin and Qinghua Zhang*
†
Research Center of Energetic Material Genome Science, Institute of Chemical Materials, China
Academy of Engineering Physics (CAEP), Mianyang, 621900, P. R. China
‡
Sichuan CoꢀInnovation Centre for New Energetic Materials, Southwest University of Science
and Technology, Mianyang, China
ABSTRACT: Two energetic compounds, i.e., 4,8ꢀdinitraminodifurazano[3,4ꢀb,e]pyrazine (1)
and its potassiumꢀbased energetic metalꢀorganic framework (EꢀMOF) (2), were prepared, and
their crystal structures were confirmed by single crystal Xꢀray diffraction analysis. Compound 1
coꢀcrystallizes with water molecules and shows a threeꢀdimensional (3D) sandwichꢀlike
supramolecular structure, which is rare in the known energetic organic compounds. Compound 2
has a pillared layered structure with a pcu topology. The layered structure in the 3D framework
featuring sql topology was constructed from inorganic chains {K O} and nitroamine groups. The
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crystal density of 2 is up to 2.114 g cm . This potassiumꢀbased EꢀMOF shows high thermal
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stability, high detonation velocity, and high impact and friction sensitivities, which make it a
potential highꢀperforming primary explosive.
1
. INTRODUCTION
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In recent years, the research on leadꢀfree primary explosives has aroused increasing attention
since the most widely used leadꢀbased primary explosives (e.g., lead azide (LA) and lead
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ꢀ3
styphnate (LS)) have high toxicity to human beings and environment. In order to offset these
drawbacks, two kinds of metalꢀfree energetic materials, namely, cyanuric triazide (CTA) and 2ꢀ
diazoꢀ4,6ꢀdinitrophenol (DDNP), have been prepared and tried to be used as potential primary
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ꢀ5
explosives. In spite of high detonation performance and high sensitivities, both compounds
showed poor thermal stability. The design of novel energetic materials as leadꢀfree primary
explosives should consider their chemical composition, thermal stability, energetic performance,
sensitivities and some other related properties. The diverse requirements of primary explosives
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make the development of environmentally friendly primary explosives a great challenge.
Metalꢀorganic frameworks (MOFs) have attracted extensive interests in various research
areas including catalysis, gas adsorption and sensors, mainly due to their tunable structures and
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functional properties.
In recent years, using explosive ligands to construct energetic MOFs
(i.e., EꢀMOFs) has been actively pursued since their energy levels and sensitivities can be
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controlled by reasonable assembly of metal ions and energetic ligands.
For example, Pang
and coꢀworkers synthesized a 3D EꢀMOF using Cu(II) as metal centers and 4,4’ꢀazoꢀ1,2,4ꢀ
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triazole (atrz) as energetic ligand. This EꢀMOF has the high calculated detonation velocity and
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low impact and friction sensitivities. The low mechanical sensitivity may be due to the strong
coordination interactions between metal ions and energetic ligands. Chen and coꢀworkers
prepared a EꢀMOF using Cu(I) as metal center and 3ꢀ(1Hꢀtetrazolꢀ5ꢀyl)ꢀ1Hꢀtriazole (H tztr) as
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energetic ligand, which exhibited high thermal stability (Tdec = 355 C) and excellent detonation
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property (detonation velocity: 10400 m s ). Shreeve and coꢀworkers reported an interesting 3D
EꢀMOF, i.e., potassium 4,4’ꢀbis(dinitromethyl)ꢀ3,3’ꢀazofurazanate, which showed promise for
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primary explosive applications (Scheme 1). Previous studies have demonstrated that those Eꢀ
MOFs composed of transition metal ions and high nitrogen ligands may have good thermal
stability, while those EꢀMOFs composed of main group metal ions and energetic ligands may
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have high calculated denotation velocity and high sensitivity.
Scheme 1. Molecular Structures of Some Reported PotassiumꢀBased EꢀMOFs.
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Nowadays, the research of developing leadꢀfree primary explosives focused on the design
and construction of novel highꢀperforming EꢀMOFs. Most EꢀMOFs adopted the potassium ion as
the metal centers, and some representative examples included potassium 1,5ꢀ(dinitramino)ꢀ
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tetrazolate, potassium 1,1′ꢀdinitraminoꢀ5,5′ꢀbisꢀ(tetrazolate) (K DNABT), potassium 4,5ꢀ
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bis(dinitromethyl)furoxanate, potassium 3ꢀdinitromethylꢀ4ꢀnitraminofurazan, and potassium
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,4ꢀbis(3ꢀdinitromethylfurazanꢀ4ꢀoxy)furazan (K BDFOF) (Scheme 1). As compared to the LA
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and LS, these potassiumꢀbased energetic salts as primary explosives have the advantages of low
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toxicity, high detonation capacity and good thermal stability. As a result, the energetic ligands
play significant roles in the construction of highꢀperforming EꢀMOFs. In this context, a number
of nitrogenꢀrich energetic ligands (e.g., triazole, tetrazole, and tetrazine) have been widely used
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to develop potential EꢀMOFs with different structures and promising properties.
Compared
to these nitrogenꢀrich energetic ligands, furazan has a relatively higher oxygen and nitrogen
content which facilitates the high oxygen balance of EꢀMOFs. Also, the high heat of formation
24ꢀ25
of furazan can enhance the detonation performance of EꢀMOFs.
In this work, we designed and synthesized a 3D EꢀMOF by using potassium ions as the
metal centers and nitroamine furazans as energetic ligands. A molecular fragment of 4,8ꢀ
dihydrodifurazano[3,4ꢀb,e]pyrazine (DFP) in the energetic ligand contains two furazan rings and
many energetic chemical bonds (i.e., C–N, N–O and C=N bonds), which can enhance the energy
level of final EꢀMOFs. Recently, theoretical research on 4,8ꢀdinitraminodifurazano[3,4ꢀ
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b,e]pyrazine was reported, which showed excellent detonation performances of this compound
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D = 9320 m s , P = 40.4 GPa). The synthesis and characterization of 4,8ꢀ
dinitraminodifurazano[3,4ꢀb,e]pyrazine is helpful for the development of novel EꢀMOF due to
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the excellent calculated performances. Furthermore, the symmetric planar structure of DFP
skeleton is beneficial to the construction of new EꢀMOFs with regular structures. These
predictions were further confirmed through the asꢀsynthesized EꢀMOF in this work. The
structure, thermal stability, energetic performance, and sensitivities of this EꢀMOF were also
studied in detail.
Scheme 2. Synthesis Route of the Energetic Ligand 4,8ꢀDinitraminodifurazanoꢀ[3,4ꢀb,e]pyrazine
and Its Potassium Salt
2
. EXPERIMENTAL SECTION
Materials and Methods. All of the reagents were obtained from commercial resource and used
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as received. NMR spectra were measured on a Bruker AV IIꢀ600 MHz spectrometer ( H NMR at
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600 MHz and C NMR at 150 MHz). The H NMR chemical shifts were measured relative to
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DMSOꢀd as the internal reference (DMSOꢀd : δ = 2.50 ppm). The C NMR chemical shifts
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were given using DMSOꢀd as the internal standard (DMSOꢀd : δ = 39.52 ppm). C, H, N
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elemental analyses were carried out on a Euro EA3000 analyzer. FIꢀIR (KBr pellets) spectra
were recorded on a Nicolet impact 410 FTIR spectrometer.
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Caution: Compounds
1 and 2 must be handled by using eye protection and leather
gloves. These energetic compounds must be synthesized only in small amounts, and
mechanical actions involving scraping or scratching must be avoided in the synthesis
process.
Synthesis of 4,8ꢀDinitraminodifurazano[3,4ꢀb,e]pyrazine (1). The energetic ligand was
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synthesized according the route shown in Scheme 2.
The key step of synthesizing 1 is the
nitration reaction of 6, because the aminoꢀgroups contained in 6 were easy to lose when HNO3
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1ꢀ32
(100%) or NO BF was used as the nitrating reagent.
Compound 1 was obtained via the
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nitration of 6 with a mixed acid of nitric acid and sulfuric acid (1:5, v/v). Notably, compound 1 is
prone to decompose in the reaction system. Fortunately, the purified 1 are stable at room
temperature. The purified 1 was obtained by column chromatography as soon as the nitration
reaction finished.
In a typical experiment, compound 6 (98 mg, 0.5 mmol) was added in portions to
concentrated H SO (2.5 mL) with stirring. The mixture was stirred for 20 min at room
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the above solution, the mixture was stirred at –15 C for 0.5 h. Then the reaction mixture was
poured into ice water (20 mL), and extracted with diethyl ether (25 mL × 2). The combined
organic phases were dried over anhydrous MgSO and filtered. The filtrate was evaporated and
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the residue was purification by column chromatography on silica gel (petroleum ether/ethyl
acetate = 3/1 to 1/3, v/v) afforded a compound 1 (17 mg, 12% yield). IR (KBr): 3629, 3159,
3
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052, 2841, 1659, 1637, 1593, 1433, 1403, 1385, 1368, 1313, 1130, 1100, 963, 932, 848, 835,
–1
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20, 764, 721, 697, 585 cm . H NMR (600 MHz, DMSOꢀd ): δ = 7.62 ppm. C NMR (150
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MHz, DMSOꢀd ): δ = 147.0 ppm. Elemental analysis, calcd (%) for C H N O (286.02): C:
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16.79, H: 0.70, N: 48.95; Found, C: 17.42, H: 0.98, N: 48.60.
Synthesis of Potassium 4,8ꢀDinitraminodifurazano[3,4ꢀb,e]pyrazine (2). Excessive KOH was
added to the combined organic phases, which were obtained by the same method given above.
And then stirring at room temperature for 10 min, the mixture was filtered and washed with
small amount of cold water. The solid was crystallized from H O afforded compound 2 (32 mg,
2
1
8% yield). IR (KBr): 1650, 1595, 1567, 1400, 1358, 1299, 1107, 992, 932, 877, 820, 819, 778,
–1 13
716, 535 cm . C NMR (150 MHz, DMSOꢀd ): δ = 146.7 ppm. Elemental analysis, calcd (%)
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for C K N O (361.93): C: 13.26, H: 0.00, N: 38.66, Found, C: 13.44, H: 0.22, N: 38.69.
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3. RESULTS AND DISCUSSION
Structural Description. The single crystals of 1·4H O were collected by slow
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evaporation of water at room temperature. Single crystal Xꢀray diffraction revealed that
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compound
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coꢀcrystallized with water molecules in monoclinic P2 /c (No.14) space
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group with a density of 1.793 g cm at 170 K. The asymmetric unit of 1·4H O contained
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a half energetic molecule and two water molecules (Figure S1). As shown in Figure 1, in
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the energetic molecule of
1, N2, N2A and the DFP moiety stayed in a plane, while the
nitro groups were hanging on the sides of the plane with a torsional angle as 112.77(2)°
(
N1–N2–N3) and a dihedral angle as 78.02(1)° (N1–N2–N3–C1).
Figure 1. The molecular structure of 1·4H O.
2
In the crystal of 1·4H O, each energetic molecule connected to four adjacent water
2
molecules through hydrogen bonds. The distance of N···O is in the range of 2.719(2)–
.038(2) Å. Interestingly, in 1·4H O, each water molecule interacted with three adjacent
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water molecules to form 2D layer structures with two types of windows, i.e., 8ꢀ and 16ꢀ
ring windows (Figure S2), through hydrogen bonds. If we considered the water molecules
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as 3ꢀconnect nodes, hydrogen atoms as 2ꢀconnected linkers, the water layers revealed 4·8
topology (Figure 2a). The distance of O···O is from 2.768(2) Å to 2.835(2) Å,
demonstrating the existence of strong hydrogen bonds between the nearby water
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molecules. On the other hand, as shown in Figure 2b, a series of energetic molecules
stack in a “T” type mode to form an energetic layer. Furthermore, the energetic layers
interacted with water layers through hydrogen bonds to form a sandwichꢀlike 3D
supramolecular structure (Figure 2c). This sandwichꢀlike supramolecular structure is
rarely observed in energetic materials. In most case, water molecules occupied the voids
among the energetic molecule and each water molecule may interacted with several
energetic molecule via hydrogen bonds.
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Figure 2. (a) The 4·8 layer structures constructed by water molecules contained in 1·4H O view
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along the a axis. (b) The energetic layer contained in 1·4H O view along the a axis. (c) The
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sandwichꢀlike 3D supramolecular structure of 1·4H O view along the b axis.
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Colorless crystals of
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temperature. The single crystal Xꢀray diffraction displayed that compound
in the monoclinic 2 /
K, as calculated from single crystal data. The asymmetric unit of
potassium ion and a half of energetic ligand (Figure S3a). The structure of deprotonated
energetic ligand in was similar to that in 1·4H O, whereas the torsional angle of N1–
2
crystallizes
–3
P
c (No.14) space group with a high density of 2.114 g cm at 293
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contains one
2
2
N2–N3 decreased from 112.78(1)° to 109.77(2)° and the dihedral angle of N1–N2–N3–
C2 changed from 78.02(1)° to 77.71(1)°.
Figure 3. The coordination environment of the energetic ligand in compound 2.
In the structure of 2, each deprotonated energetic ligand is connected with eight
+
adjacent K ions through K–O and K–N bonds (Figure 3). The bond lengths of K–O and
K–N are in the range of 2.724(3)–2.781(3) and 2.842(3)–3.016(3) Å, respectively. Each
+
K ion is coordinated with six adjacent atoms (three oxygen atoms and three nitrogen
atoms) from six different energetic ligands (Figure S3b). Therefore, the 3D framework of
2
was constructed through a 6, 8ꢀconnected mode. The 3D framework of 2 can also be
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looked as a pillared layered structure (Figure 4a). As shown in Figure 4b, the nearby K
ions are connected each other via oxygen and nitrogen atom to form infinite inorganic
chains (K–O–K–O), and then the inorganic chains are further connected by the nitro
groups of energetic ligands to build layered structures. Then the parallel layers are
1
1
1
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pillared by DFP groups to construct the 3D framework of
2
. Two different types of
channel structures were observed in the framework of
were not big enough for further study (Figure S4).
2, but the windows of the channel
Figure 4. (a) The 3D structure of 6 view along the a axis. (b) The inorganic layer contained in 6
views along the b axis. (c) The sql network of the inorganic layer. (d) The pcu topologic structure
of 6.
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To gain more insights into the complicated 3D network, the topology analysis was
33
+
performed. If we consider each two adjacent K ions as four connected nodes, the layer
structure can be considered as a sql network (Figure 4c). In the whole structure, the sql
networks are further pillared by DFP groups. If the DFPs are looked as two connected
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linkers, compound
compound was confirmed by powder XRD. As shown in Figure S5, the main peak
positions of are consistent with the simulated one, demonstrating the phase purity of
To gain more insights of the interactions among the molecules contained in 1·4H O and 2,
2 has a pcu topologic structure (Figure 4d). The phase purity of
2
2
2.
2
3
4ꢀ36
Hirshfeld surfaces associated with twoꢀdimensional fingerprint spectra were analyzed.
The
red and blue spots on the Hirshfeld surface denote the high and low close contact populations,
respectively. In the 1·4H O, the red spots represent the strong hydrogen bonds of O–H···N and
2
N–H···O. As shown in Figure 5a, most σꢀπ interactions, such as N–O, O–O, and N–C
interactions, existing on the plate area of the Hirshfeld surface, may explain the “T” type packing
mode of energetic molecules in 1·4H O. The two characteristics of Hirshfeld surface are
2
accordance well with the molecular packing modes of 1·4H O. From fingerprint spectra of
2
1·4H O (Figure 5c), it was observed that two sharp spikes were existent in the left bottom of the
2
spectra, denoting the interactions of O···H and N···H, respectively. Interestingly, The
interactions of N···H (8.7%) was much lower than that of O···H (28.4%), demonstrating the
main interactions between energetic molecules and water molecules were O···H interactions.
This is also matched well with the supramolecular structure of 1·4H O. Furthermore, high ratio
2
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of σꢀπ interactions (59.4% of the total weak interactions) further explained the existence of
energetic layers in 1·4H O.
2
1
1
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Figure 5. (a) & (b) Hirshfeld surface of 1·4H O and compound 2, respectively. (c)&(d)
2
Fingerprint spectra of 1·4H O and compound 2, respectively, inset picture: the content of
2
different interactions contributed to the Hirshfeld surface.
In the case of 2, much more red spots were observed on the Hirshfeld surface of energetic
ligand implied relatively stronger interactions among the adjacent molecules than that contained
in 1·4H O (Figure 5b). The red spots on the edges of the Hirshfeld surface denoted the
2
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interactions of K···O, K···N, N···O, and N···N, respectively. The regular plate feature of
Hirshfeld surface (Figure 5b) may be due to the parallel arrangement of energetic ligands in 2
along the bꢀaxis. The fingerprint spectra of 2 is much different to that of 1·4H O (Figure 5d).
2
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Most weak interactions in compounds 2 are the N···O interactions (29.8% of the total weak
interactions), demonstrated the high content of πꢀπ stacking interaction of energetic ligands
which was confirmed by the pillared layered structure of 2.
Thermal Stability. Both thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC) were employed to evaluate the thermal stability of compound 1 and 2. TGA
o
ꢀ1
and DSC analyses were performed at a heating rate of 10 C min under a N flow. From TGA
2
and DSC results (Figure S6 and Figure 6), it can be found that two compounds have much
different thermal stabilities. Compound 1 displayed very poor thermal stability and it began to
o
decompose when the temperature reached 74 C. While from the TGA curve of compound 2
o
(
Figure 6), no obvious weight loss was observed from 50 to 290 C, indicating good thermal
o
stability of 2 until 290 C. Upon further heating, a sharp weight loss peak appeared and
o
simultaneously all the weight of 2 was lost, demonstrating that 2 exploded at 290 C. The
thermal stability of 2 was also confirmed by DSC, and no any heat flow peak was observed in
o
o
the DSC curve before 290 C, which was consistent well with the TGA result. At 292 C, a sharp
heat flow peak was observed, indicating that 2 had exploded at this temperature.
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Figure 6. The TGA and DSC curves of compound 2.
Energetic Properties and Sensitivity. Energetic properties of
1
and
have high nitrogen and
oxygen contents, which are up to 82.5% and 65.2%, respectively. The COꢀbased oxygen
balances of and are up to 5.6 and 4.4, respectively. Gaussian 09 program was used for
calculating their heats of formation. Both and have the positive heats of formation
750.3 kJ mol and 277.1 kJ mol , respectively). The high heats of formation and high
2 were evaluated by
37ꢀ39
theoretical methods.
As shown in Table 1, both 1 and 2
1
2
1
2
–1
–1
(
–3
density (2.114 g·cm ) of
2 implied its excellent energetic properties (e.g., detonation
velocity and pressure). According to the calculated results by EXPLO5 v6.02, the
–1
detonation velocities of
1 and 2 are both greater than 9000 m s . Notably, the detonation
–1
velocity of is up to 9660 m s , which is much superior to most typical high explosives
2
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–
1
–1
–1
including RDX (8795 m s ), HMX (9144 m s ), and even CLꢀ20 (9455 m s ).
Moreover, the impact and friction sensitivities of
1 and 2 were further evaluated by
40
experimental methods. The BAM dropꢀhammer test showed that the impact sensitivity
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and friction sensitivity of
2 are 2 J and 1 N, respectively, demonstrating its very high
mechanical sensitivities. As compared to the typical primary explosive Pb(N ) , it is clear
3
2
that
towards the impact and friction.
Table 1. Physicochemical and energetic properties of compounds 1 and 2.
2 exhibited the higher energetic performance and comparable mechanical sensitivities
Compounds
Formula
1
2
Pb(N3)2
PbN6
291.3
315
C H N O
C K N O
6
4
2
10
6
4
2
10
–
1
M [g mol ]
286.0
74
361.9
292
o
a
T [ C]
dec
b
ꢁCO [%]
5.6
4.4
–11.0
28.9
c
N [%]
49.0
82.5
38.7
65.2
d
N + O [%]
28.9
–
3
e
f
ρ [g cm ]
1.803
750.3
9153
36.5
1
2.114
277.1
9660
36.8
2
4.800
450.1
5877
33.4
–
1 g
∆
H [kJ mol ]
f
–1 h
D [m s ]
i
P [GPa]
j
IS [J]
2.5–4.0
0.1–1
k
FS [N]
24
1
a
o
–1
b
Decomposition temperature (onset temperature at a heating rate of 10 C min ). Oxygen
c
d
balance for C H O N , 1600(cꢀaꢀb/2)/MW (based on CO). Nitrogen content. Nitrogen and
a
b
c
d
e
f
oxygen content. Density measured by using a gas pycnometer at room temperature. Crystal
g
density at 293 K. Heat of formation (calculated with Gaussian 09 program package).
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h
i
Detonation velocity calculated with EXPLO5 v6.02. Detonation pressure calculated with
j
k
EXPLO5 v6.02. Impact sensitivity. Friction sensitivity.
4
. CONCLUSION
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
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In summary, two energetic compounds, i.e., 1·4H O and the EꢀMOF 2 were designed and
2
synthesized. In the crystal structure of 1·4H O, the energetic ligand molecules interact with
2
adjacent energetic ligands via σꢀπ interactions to form an energetic layer, while each water
molecule interacts with nearby water molecules via hydrogen bonds to form a water layer. The
regular water layers observed in 1·4H O are very rare in the energetic organic molecules. The
2
energetic layers and water layers of 1·4H O are further packing to construct a sandwichꢀlike 3D
2
supramolecular structure. In compound 2, potassium ions and energetic ligands are connected
each other through a 6,8ꢀconnection mode to form a 3D framework with pcu topology.
–
3
o
Compound 2 has the high crystal density (2.114 g·cm ), high thermal stability (T : 292 C), high
d
–1
detonation velocity (9660 m s ), and the high impact and friction sensitivities (2 J and 1 N,
respectively). The excellent detonation performances, high thermal stability associated with high
mechanical sensitivities make compound 2 a potential highꢀperforming primary explosive.
ASSOCIATED CONTENT
Supporting Information. NMR, additional structural figures, and computational methods are
available free of charge via the Internet at http://pubs.acs.org.
Xꢀray data in CIF format (CIF)
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AUTHOR INFORMATION
Corresponding Author
*
Eꢀmail: qinghuazhang@caep.cn
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Author Contributions
§ These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
This work was supported by NSAF (U1530262), China Postdoctoral Science Foundation
(2016M602714), the National Natural Science Foundation of China (21602211, 21702196), and
the PRC’s 1000 Plan Recruitment Program for Yong Talents.
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38) Parr, R. G.; Yang, W. Density Functional Theory of Atoms and Molecules, Oxford
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40) Tests were conducted according to the UN Recommendations on the
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Construction of a Thermally Stable and Highly Energetic Metalꢀ
Organic Framework as LeadꢀFree Primary Explosives
A 3D energetic metalꢀorganic framework featuring pillared layered structure and pcu topology
was prepared. This compound exhibited high crystal density, high thermal stability, high
detonation velocity, and high sensitivities, which makes it a potential highꢀperforming primary
explosive.
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