Novel Energetic Coordination Polymers Based on 1,5-Di(nitramino)tetrazole With High Oxygen Content and Outstanding Properties: Syntheses, Crystal Structures, and Detonation Properties

In this study, a series of novel 1,5-di(nitramino)tetrazole (DNAT)-based bimetallic energetic coordination polymers, MK2(DNAT)2·4H2O [M = Fe, Cu, Ni, Co, and Zn], were designed and synthesized in a simple and convenient self-assembly synthetic process. The obtained compounds were fully characterized by IR spectroscopy, multinuclear NMR spectroscopy, elemental analysis, and differential scanning calorimetry (DSC). Additionally, the structures of target compounds were confirmed by single-crystal X-ray diffraction. Based on the room-temperature X-ray densities (2.095–2.138 g cm−3) and the calculated (CBS–QB3) heats of formation (−41.3 to 170.5 kJ mol−1), the detonation properties such as detonation velocities (8,147.0–8,478.4 m s−1) and detonation pressures (29.7–32.8 GPa) were computed using the EXPLO5 v6.04 program. Their excellent energetic properties indicated that they could serve as promising “green” primary explosives for replacement of lead azide (LA).

To date, LA is the most widely used primary explosive, and other traditional primary explosives include lead styphnate monohydrate (LS) (Ilyushin et al., 2012;Klapötke, 2012) and lead-free primary explosive copper(I) nitrotetrazolate (DBX-1) (Scheme 1) (Klapötke et al., 2013). However, lead contamination in air and soil has attracted people's attention in recent years, which poses a significant threat to personal safety and environmental pollution (Ilyushin et al., 2012;Klapötke, 2012) Furthermore, lead is very difficult to remove once it has been dissolved in the human blood (Matyas and Pachman, 2013). Based on environmental regulations and human health problems, there is a need to develop "green" replacements of lead-based primary explosives (Huynh et al., 2006). "Green" lead-based replacements need to consider the following criteria: the materials (a) must be safe to handle and possess a rapid deflagration to detonation transition; (b) must be thermally stable to >150 • C; (c) also should possess high detonation performance and sensitivity; (d) should have long-term chemical stability; (e) should not contain toxic heavy metals or other known toxins; and (f) should be easy to synthesize and affordable (Ilyushin et al., 2012;Klapötke, 2012;Mehta et al., 2014).
In the process of seeking green candidates for primary explosives, we focused on the use of nitrogen-rich energetic ligands, the main detonation product of which is nontoxic nitrogen gas. Meanwhile, they possess higher heats of formation, which could provide high energy output .
In this work, we focus our attentions on bimetallic energetic coordination polymers with high positive OB and outstanding energetic properties. A new high energetic ligand with a tetrazole ring and nitroamino groups, namely, 1,5-di(nitramino)tetrazole (DNAT), was used to successfully prepare novel high-energydensity compounds. DNAT has great detonation properties (D = 9,967 m s −1 , and P = 43.4 GPa) (Fischer et al., 2015). Herein, we report the design and syntheses of a series of DNAT-based bimetallic energetic coordination polymers, MK 2 (DNAT) 2 ·4H 2 O [M = Fe (1), Cu (2), Ni (3), Co (4), and Zn (5)], all of which contain one potassium and another lead-free metal atom to regulate performances.

X-RAY CRYSTALLOGRAPHY
Room-temperature (296 K) X-ray diffraction was used to determine all of the target compounds 1-5 and to obtain accurate densities for property calculations. Suitable single crystals of compounds 1-5 for X-ray diffraction measurements were grown by slow evaporation from a water solution. The X-ray structures and packing diagrams of single crystals for compounds 1-5 are shown in Figures 1-5, and the crystallographic data for compounds 1-5 are summarized in Tables 1, 2. Compound 1, which crystallizes as yellow prism in the monoclinic space group P2(1)/c with a density of 2.095 g cm −3 and a cell volume of 923.2 Å 3 , contains two molecules of water and consists of two molecules per unit cell. Compound 2 crystallizes in the monoclinic space group P2(1)/c with a density of 2.250 g cm −3 and a cell volume of 817.7 Å 3 . The repeating unit of compound 2 contains two potassium ions, one cupric ion and two K 2 DNAT anions. Compound 3 crystallizes in the SCHEME 1 | Structures and properties of several kinds of primary explosives. SCHEME 2 | Syntheses of 1,5-di(nitramino)tetrazole-based bimetallic energetic coordination polymers.
1.274 and 1.396 Å, which are shorter than the lengths of normal N-N single bond (1.460 Å) and longer than N=N double bond (1.250 Å) (Allen et al., 1987;Tang et al., 2013). The nitrogen atoms N2 and N7 and all the atoms in the tetrazole ring are nearly coplanar except the N-nitramino groups. The nitramino moieties attached to the carbon are almost planar with the tetrazole ring (≮O (2) for compounds 1-5, respectively), and all the N-nitramino groups are twisted out of this plane with the dihedral angle of −74.83(19) • , 95.1(2) • , −77.3(3) • , −76.1(2) • , and −76.0(2) • for target compounds 1-5, respectively. The K ions in all single crystals of the compounds 1-5 are irregularly eight-coordinated pattern by six oxygen atoms from nitramino groups and water molecules and two nitrogen atoms from tetrazolo rings and nitramino groups, whereas the divalent metal ions adopt a six-coordinated pattern by four oxygen atoms from nitramino groups and water molecules and two nitrogen atoms, all from tetrazolo rings. As a result, the dimetallic cations connecting closely with K 2 DNAT dianionic ligands and water molecules generate 3D network crystal packing structures. More detailed information about crystallographic date collection and structure refinement can be found in the Supplementary Material.
The mechanical sensitivities toward impact and friction were determined experimentally according to BAM methods by using a standard BAM Fallhammer and a BAM Friction tester 1,2 (United Nations Publication, 2009). The impact and friction sensitivities of the target compounds 1-5 are between 3 and 6 J and 16 and 26 N, respectively. The impact sensitivities are similar with those of LA. Therefore, they should be considered as primary explosives to be handled with appropriate precautions (Ilyushin et al., 2012;Klapötke, 2012;Mehta et al., 2014).
OB is an important characteristic for energetic materials. All of target compounds 1-5 have positive OBs in the range of 5.4% to 5.8% (based on CO 2 ), which are much higher than those of traditional primary explosives LA (−11.0%) and other reported potassium-based primary explosives based on furazano or tetrazolo energetic ligands (−23.4 to 4.3%, shown in Scheme 1). The OB results indicated that the chemical energy of the compounds could be fully utilized when the detonation reactions were happening, which could improve the detonation properties of energetic materials. In addition, all of the compounds 1-5 possess a higher content of nitrogen and oxygen, which is more than 70.0% (shown in Table 3).
The densities of bimetallic energetic compounds 1-5 were confirmed using single-crystal X-ray diffraction at 296 K, which lies in the range of 2.095 to 2.138 g cm −3 . The heats of formation of compounds 1-5 were calculated to be from −41. programs at the level of theory of DFT with the method of CBS-QB3. In order to explore the properties of bimetallic energetic compounds, several detonation parameters of compounds 1-5 were calculated with the EXPLO5 code (Sucéska, 2017) in its version 6.04 based on the crystal densities and calculated heats of formation. As can be seen in Table 3, compounds 1-5 showed remarkable detonation values, and the calculated detonation velocities (8,147.0-8,478.4 m s −1 ) and detonation pressures (29.7-32.8 GPa) are comparable with those of traditional leadbased primary explosives such as LA (5,877.0 m s −1 , 33.4 GPa). Noteworthy, DNAT-based bimetallic energetic compounds 1-5 displayed excellent overall performance as suitable and non-toxic green replacements for lead-based primary explosives.

MATERIALS AND METHODS
Caution! All of DNAT-based bimetallic energetic coordination polymers are potential primary explosives and may explode under certain conditions. Proper safety precautions should be taken when handling these compounds. Laboratories and personnel should be properly grounded, and safety equipment such as leather gloves, face shield, and ear plugs are recommended.

General Information
All chemical reagents and solvents were used as supplied unless otherwise stated. Elemental analyses (C, H, and N) were performed on a VARI-El-3 elementary analysis instrument. Infrared spectra were obtained from KBr pellets on a Nicolet NEXUS 870 Infrared spectrometer in the range of 400-4,000 cm −1 . 13 C NMR was obtained in D 2 O-d 2 on a Bruker AV 500 NMR spectrometer. The DSC experiments were performed using a DSC-Q 200 apparatus (TA, USA) under a nitrogen atmosphere at a flow rate of 50 ml min −1 . About 0.1-0.5 mg of the samples were sealed in aluminum pans for DSC. Impact and friction sensitivity measurements were determined using a BAM drophammer and a BAM friction tester 1,2 (United Nations Publication, 2009). Energetic properties have been calculated with the EXPLO5 v6.04 program (Sucéska, 2017) code using the X-ray crystal densities at room temperature and calculated solid state heats of formation.

Theoretical Studies
Calculations were performed using the Gaussian 09 suite of programs (Frisch et al., 2009). The geometric optimization of the structures and frequency analyses employed the density functional theory (DFT) B3LYP method with PBE exchange (Perdew et al., 1996) and correlation and plane wave basis set realized by the CASTEP code (Clark et al., 2005). Each optimized structure was characterized to determine the true local energy minima on the potential energy surface without imaginary frequencies. The heats of formation ( H f ) of the target compounds 1-5 was computed using the CBS-QB3 method (Montgomery et al., 1999(Montgomery et al., , 2000. With the use of the roomtemperature X-ray densities and calculated heats of formation, the detonation properties were calculated using the EXPLO5 v6.04 program (Sucéska, 2017) according to the Kamlet-Jacobs equations (Kamlet and Ablard, 1968;Kamlet and Dicknison, 1968;Kamlet and Jacobs, 1968).

Experimental Procedures
General procedure for synthesis of compounds 1-5: Compound K 2 DNAT (0.133 g, 0.5 mmol) was suspended in distilled water (3.0 ml) at room temperature and stirred until the solid was fully dissolved.

DATA AVAILABILITY STATEMENT
All datasets generated for this study are included in the manuscript/Supplementary Files.

AUTHOR CONTRIBUTIONS
YL designed the research and performed the syntheses of target compounds and the drafting of manuscript. TY contributed to the theoretical calculation studies of target compounds. YZ and KX performed the thermal behavior research. TC and JH were involved in compound structure characterization and data analysis. YW participated in drafting the manuscript.