K-Based Energetic Coordination Polymers: Structural

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Light Metal Li/K-Based Energetic Coordination Polymers: Structural Effect on Detonation Performance Lin Sun, wish long, Yutong Zhang, Lianjie Zhai, Qi Yang, Qing Wei, Gang Xie, Bo-Zhou Wang, and Sanping Chen ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00183 • Publication Date (Web): 05 Feb 2018 Downloaded from http://pubs.acs.org on February 8, 2018

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ACS Applied Energy Materials

Light Metal Li/K-Based Energetic Coordination Polymers: Structural Effect on Detonation Performance Lin Sun‡†, Shilong Wei‡†, Yutong Zhang‡†, Lianjie Zhai‡, Qi Yang*†, Qing Wei†, Gang Xie†, Bozhou Wang‡ and Sanping Chen*† †

Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi’an, Shaanxi 710127, China. ‡ Xi'an Modern Chemistry Research Institute, Xi'an Shaanxi 710069, China. ‡These authers contributed equally to this work *S Supporting Information

Corresponding authors Dr. Sanping Chen E−mail address: [email protected]

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ABSTRACT: It is the current development trend to contribute green high-energy coordination polymer (CPs) with light metal to realize effective energy density. In this study, self-assembly of a ligand 3−(tetrazol−5−yl)triazole (H2tztr) with the light metal Li produces Li−CP [Li2(Htztr)2(H2O)4] (1a). Further structure recombination generates the polymorph (1b) in the presence of HNO3, which displays better detonation performance. Nevertheless, the detonation performance is affected by multiple water molecules, which may take some energy away because they evaporate at lower temperatures. Hence, we selected K(I) with the larger radius to replace Li(I), expecting that more ligands participate in coordination, which not only increases energetic units but also avoids a large amount coordinated H2O molecules. Compound [K(Htztr)(H2O)]n (2) with a 2D structure was prepared by facile synthesis, which effectively reduces coordinated H2O molecules. Particularly, 2 possesses outstanding detonation performance with excellent thermostability and low sensitivity (Tdec = 325 °C, ∆Hdet = 4.255 kcal·g−1, D = 9.742 km·s−1, P = 42.659 GPa) among reported energetic CPs. This finding demonstrates a systemic study about calculations of detonation properties and offers an efficient approach to enhancing detonation performance by light metals as nodes to construct ECPs with one-step release of energy. KEYWORDS: high−energy coordination polymer, structural rearrangement, detonation performance, thermostability, sensitivity

1. INTRODUCTION The design and synthesis of new high-energy density energetic materials (HEDMs) that are excellent in terms of explosive performance, thermostability, and sensitivity are a crucial subject of contemporary research considering the central role that such materials act as in a diversity of civilian and military applications.1−4 In particular, the main challenge is how to aggregate the maximum number of energetic groups in the smallest volume while maintaining practical levels of stability and acceptable 2


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sensitivity. The emergence of energy coordination polymers (ECPs) with controllable structure and modifiable property provides a unique architectural platform for developing new HEDMs.5 From azido to azole and its derivatives, from heavy metals to light metals, from 0D structural units and 3D MOFs, acceptable sensitivity and strong explosions are the eternal pursuit of ECPs research. In the last decade, ECPs witnessed rapid development, simultaneously, it represented good balances between low-sensitivity and high detonation performance (high detonation, pressure and velocity).3 In general, nitrogen content and energy density both have an impact on the energetic properties of ECPs. The steric configuration of ligand and the nature of the metal species in ECPs play significant roles in determining the thermal stability, energy density, and sensitivity to external stimuli. Based on this background, nitrogen−rich azoles are expected to be a promising alternative for the construction of advanced ECPs. Combined with high nitrogen content and high heats of formation, most azole-based compounds feature good densities and detonation properties.4,6 Among nitrogen-rich azoles, H2tztr (3−(1H−tetrazol−5−yl)−1H−triazole) as an appropriate candidate possesses a rigid structural framework with a high decomposition temperature (312 °C) and a high nitrogen content (N = 71%), which could improve the insensitivity and thermostability of the anticipated ECPs.7 Meanwhile, a great variety of ECPs7-14 with heavy metal ions as metal nodes, such as Cu2+, Co2+, Zn2+, Pb2+, Cd2+ and Ag+, in which Cu−ECPs7 and Zn−ECPs13 exhibit excellent detonation properties. Whereas, there is no denying the fact that the introduction of heavy metals would inflate the density of ECPs, especially polynuclear ECPs, due to high relative molecular mass of metal, and reduce nitrogen contents. Moreover, the decomposition products will potentially pollute the environment. Based on the development of the new generation EMOFs,15-22 it is imperative to adopt two strategies: one is to remove heavy metal ions; the other is to replace heavy metals with other metals. Therefore, we determined to search a “green” ECP which needs to possess the following properties: a) large energetic density; b) good thermal stability; c) low sensitivity; d) non-toxic; e) synthetic simplicity and 3



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security. In this sense, the light metal, such as Li and K, was optimally selected to construct environmentally friendly ECPs with good performance. Herein,

we

employed

a

nitrogen-rich

heterocyclic

ligand,

3−(1H−tetrazol−5−yl)−1H−triazole (H2tztr) as an energy source. In addition, we use the lightest and environmentally friendly Li(I) ion as the node to construct a Li−ECP ([Li2(Htztr)2(H2O)4] (1a), N% = 54.7%). Fortunately, pH−induced structure rearrangement produces the polymorph 1b (Scheme 1), further enhancing the density (from 1.553 to 1.563 g·cm−3). However, four H2O molecules exist in 1a and 1b blocking the single−step release of energy due to their vaporization at the lower temperature. Owing to the large radius of K(I) ion, it is able to accommodate more coordination ligands around it. Therefore, [K(Htztr)(H2O)]n (2) was obtained with only one bridging H2O molecule. Compound 2 displays presents outstanding energetic performance (Tdec = 325 °C, ∆Hdet = 4.255 kcal·g-1, D = 9.742 km·s-1, P = 42.659 GPa) with low sensitivity.

Scheme 1. Syntheses of compounds 1a and 1b

2. EXRERIMENTAL SECTION 2.1. Materials and instruments All chemical reagents used for the synthesis were purchased commercially and used without further purification. Elemental analyses of C, H and N were 4


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accomplished by a Vario EL III analyzer. Infrared (IR) spectra were recorded on a Bruker FTTR instrument using KBr pellets (4000−400 cm−1). Using a CDR-4P thermal analyzer of Shanghai Balance Instrument factory and a Netzsch STA 449C instrument, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) measurements were realized, respectively, using dry oxygen-free nitrogen as the atmosphere with a flowing rate of 10 mL·min−1. Thermogravimetry-Fourier transform infrared spectrometry of the detonation products were carried out on a TG-209/Vector TM-22 instrument. The sensitivities to impact stimuli were determined by drop hammer device applying standard staircase method using a 2 kg drop weight and the results were reported in terms of height for 50% probability of explosion (h50%).23,24 The friction sensitivities were founded on a Julius Peter’s apparatus by following the BAM method.25 Using an X-ray powder diffraction (XRPD) measurements performed on a Rigaku RU200 diffractometer at 60 kV, 300 mA and Cu Kα radiation (λ = 1.5406 Å), with a scan speed of 5º·min−1 and a step size of 0.02º in 2θ, the purity of the bulk samples can be verified. The constant-volume combustion energy of the compound was determined using a precise rotating-bomb calorimeter (RBC-type II).

2.2 Synthesis of compounds 1a, 1b and 2 2.2.1. Synthesis of [Li2(Htztr)2(H2O)4] (1a). H2tztr was synthesized as described in literature [7]. To a stirred solution of H2tztr (0.1 mmol, 0.014 g) in CH3OH (15mL), LiNO3 (0.1 mmol, 0.007 g) was added (pH = 7). The colourless solution above was kept further stirring for 3 h and filtered. Colourless blocky crystals were collected in two weeks. Yield: 42% (based on Li(I) salt). Anal. Calcd. for C6H12Li2N14O4 (358.18): C, 20.12; H, 3.38; N, 54.75. Found: C, 20.10; H, 3.37; N, 54.74. IR (KBr): 3107(w), 2941(w), 2835(m), 1341(w), 1295 (m), 1284(m), 1181(w), 1153 (m), 1105(m), 1083(w), 998(w), 872(w), 702(m), 659(w), 545(m), 472 cm−1 (w). 2.2.2. Synthesis of [Li2(Htztr)2(H2O)4] (1b). The compound 1b was obtained from the following two ways: 5



(i) The method described above for 1a was used but with the addition HNO3 (65%) to the solution (pH = 5). Colourless needle crystals were obtained after three weeks. Yield: 34% (based on Li(I) salt). Anal. Calcd. for C6H12Li2N14O4 (358.18): C, 20.12; H, 3.38; N, 54.75. Found: C, 20.11; H, 3.36; N, 54.73. IR (KBr): 3109(w), 2951(w), 2842(m), 1352(w), 1289(m), 1279(m), 1193(w), 1148(m), 1097(m), 1076(w), 996(w), 870(w), 701(m), 647(w), 539(m), 469 cm−1 (w); (ii) Compound 1a was dissolved in HNO3 (10 mL), and compound 1b were obtained after 2 weeks. 2.2.3. Synthesis of [K(Htztr)(H2O)] (2). Compound 2 was obtained by a procedure similar to that used for 1a except for using KNO3 instead of LiNO3. Yield: 42% (based on K(I) salt). Anal. Calcd for C3H4KN7O (193.23): C,18.65; H, 2.09; N, 50.75. Found: C, 18.68; H, 2.11; N, 50.79. IR (KBr): 3438s, 3102w, 2914w, 2862w, 1630s, 1511s, 1432s, 1379s, 1309m, 1292w, 1214m, 1180w, 1135m, 1087m, 999m, 865w, 764w, 702m, 671m, 528w, 479w. 2.3. X−ray crystallography The single crystal X-ray experiment was carried out a Bruker Apex II CCD diffractometer using graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å) using ω and φ scan mode. The data integration and reduction were processed with SAINT26 software. Based on multi-scan, absorption correction was carried out using the SADABS27 program. The structures are determined by the direct method using SHELXTL28 and refined by means of full-matrix least-squares procedures on F2 with the programs SHELXL-9729 and SHELXL-2014.30 All non−hydrogen atoms were modified with anisotropic displacement parameters. Hydrogen atoms were situated in their calculated positions and thereafter allowed to ride on their parent atoms. Experimental details of crystal data, data collection parameters and refinement statistics are presented in Table S1 (Supporting Information), whereas the selected bond lengths and angles are shown in Tables S2 and S3 (Supporting Information).

3. RESULTS AND DISCUSSION 6


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3.1 Crystal Structure. Polymorphism is a well-known phenomenon that concerns compounds with more than one crystalline phase. In principle, various crystalline types with distinct density appear accompanying structural rearrangement, and the thermodynamic stability of polymorphs which increases their density.31 The explosives with different crystalline types show diverse detonation properties, such as CL−20, four crystal types (α, β, γ and ε) are stable at ambient temperature and pressure conditions, but the density of

ε−CL−20 is the largest (2.04 g·cm−3).32 ε−CL−20 represents the best thermal stability and the lowest sensitivity, which is applied in the field of weapons. Hence, constructing polymorphs is an effective way to realize the higher density. In this paper, 1a and 1b were obtained by pH control. The coordination of Li(I) with the multidentate nitrogen-rich ligand H2tztr is thermodynamically permissible, while crystallization products are kinetically controlled. Accompanying with addition of HNO3, 1a can transfer into 1b in the solution of HNO3, which suggests 1b needs the higher reaction activation energy. Single-crystal X-ray diffraction analyses reveal that compounds 1a and 1b are polymorphs with the same molecular components crystallizing in the monoclinic space group (P21/c for 1a; P21/n for 1b) but with different packing arrangements. The molecular units and packing arrangements are depicted in Figure1. The lithium ion is coordinated by the nitrogen atom N4 of tetrazole rings and oxygen atoms O2, O1, O1i of the coordinated water molecules, thus completing the LiNO3 coordination sphere. The ligands H2tztr coordinate to Li ions with a new coordination mode. The equivalent Li atoms are doubly bridged by oxygen atoms O1 and O1i from H2O molecules, forming a {Li2O2} dimer (Li−O−Li = 87.3(2)º, Li···Li = 2.757(7) Å for 1a; Li−O−Li =87.5(1)°, Li···Li = 2.761(8) Å for 1b). For 1a and 1b, Li–O distances fall in the range 1.865(4)–2.011(7) Å, and Li–N distances are 2.019(4) and 2.016(6) Å, respectively. In the crystal packing, the structure of the binuclear Li2 molecular in compounds 1a and 1b is further strengthened by hydrogen-bonding interactions and π···π interactions (Figure 1 and Tables S4-S5 (Supporting Information)). 7



Examinations of the crystal packing in compounds1a and 1b reveal that the molecules are stacked in a parallel manner along the a axis, but with distinct packing arrangements along the b and c axes.

Figure 1. Coordination environment of Li(I) ions and packing arrangements along a, b and c axes of 1a and 1b (1a: i −x+2, −y+2, −z+2; 1b: i −x+1, −y+1, −z+1).

The 2D-fingerprint of crystals and the associated Hirshfeld surfaces of 1a and 1b were applied to analysis the intermolecular interactions. On basis of the definition of Hirshfeld surfaces, the red and blue on the surfaces present the high and low close contact populations, respectively.33 As illustrated in Figure 2a, the ligand of the molecule displays a planar structure composed of two π-conjugated five-member rings, which occurs in plate shapes. Most red dots lay on the edges mainly represent the intermolecular HB interactions (N···H: 48.3% for 1a and 46.6% for 1b), while those on the plate faces normally represent weak van der Waals force, (C···N, N···N and H···H) in the 2D fingerprint plots of 1a and 1b (Figure 2d and Figure 2e).

8


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Figure 2. Hirshfeld surfaces calculation of 1a and 1b(a); the individual atomic contact percentage contribution to the Hirshfeld surface for 1a(b) and 1b(c); two-dimensional fingerprint plots in the crystal stacking of 1a(d) and 1b(e).

Figure 3. (a) Molecule structure of compound 2; the H atoms are omitted for clarity. (i x+1, y, z;ii −x+1, y+1/2, −z+1; iii −x+2, y+1/2, −z+1; vii

x+1, y, z+1;

viii

iv

x−1, y, z; v −x+1, y−1/2, −z+1; vi −x+2, y−1/2, −z+1;

x, y, z+1.) (b) Coordination mode of H2tztr ligand in 2; (c) 2D layer of

compound 2 along the c axis; (d) 3D framework of compound 2 along the b axis. The dotted lines represent the hydrogen bonding interactions. 9



The single crystal X-ray analysis reveals that compound 2 crystallizes in the monoclinic P21 space group. Each K(I) ion coordinates to six nitrogen atoms from four Htztr− and two oxygen atoms from two bridging water molecules to form compound 2. The bond lengths of K−N are in range of 2.90-3.26 Å (Table S3, Supporting Information), while the bond angles of N−K−N and N−K−O are in range of 57.06°−151.70° and 55.99°−132.25°, respectively. As shown in Figure 3, the Htztr− anions connect with K(I) ions along two directions: one is bidentate chelating mode along b axis direction (N3 and N4, N1 and N7); the other is bridging a mode along the a axis (N3 and N1). In addition, the adjacent K(I) ions are linked by one bridging O atom simultaneously to form a 1D K···O···K···O chain along the a axis (Figure 3c). Further, K(I) ions are connected through ligands along the b axis, forming the 2D layer. The 3D structure is generated through hydrogen−bonding interactions (Table S6, Supporting Information) between adjacent 2D layers. 3.2. Thermal stability XPRD experiments were performed on the powder samples of 1a and 1b to verify the phase purities of the bulk materials (Figures S1 and S2, Supporting Information). The thermal decomposition behaviours of 1a and 1b have been studied according to means of differential scanning calorimetry (DSC) and thermogravimetry (TG) with a linear heating rate of 10 °C·min−1 in a nitrogen atmosphere (Figures S3 and S4, Supporting Information). As shown in Figure S3, both 1a and 1b display similar thermal decomposition process. We take compound 1b as example to analysis the thermal decomposition process in detail. DSC curve of 1b shows the two endothermic processes appeared at 150−240 °C, which denotes the loss of the water in the molecules, as the mass loss percentage of 20% in the first mass loss stage of TG curves. We observe the one weight loss process before 240 °C in the TG curve. Further, the weight loss process of two types H2O molecules (the axial coordination H2O and the bridged coordination H2O) can be obviously separated in DSC curves. With increasing the heat temperature, there are two exothermic processed happened, 10


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with the peak temperatures of 397 and 441°C, respectively. Mass loss observed from the TG curve is obvious, the largest mass loss percentage reaches at 396°C in the stage of 374–408°C, and 436°C in 424–446°C, which may be due to the loss of the H2tztr ligands. The abrupt curve of weight loss from 374 to 446 °C presents fine thermal stability and the rapid release of energy, which is consistent with the sharp exothermic peaks in the DSC curve. The temperature of beginning decomposition for 1a and 1b is 371 °C and 376 °C, respectively. Although there are two endothermic processes in 1a and 1b in low temperature, a huge amount of energy is released in high temperature. Apparently, compared with 1a, 1b with the higher density performs better stability, and drastic decomposition process of 1b indicates the potential detonation properties. Compound 2 is stable in air and can retain the crystallinity at room temperature for at least several weeks. The DSC curve demonstrates that only one peak in the curve with a peak temperature of 350 °C, indicating the existence of one intense exothermic process (Figure S5, Supporting Information). Accordingly, the abrupt decline appears in about 360 °C, regarding as the collapse of the main frameworks, before but a continuous dehydration process occurs from 85 °C to 350 °C with the ratio of 9.7% in agreement with the loss of bridging water molecules (9.3%) (Figure S6, Supporting Information). Surprisingly, the small heat release of the water molecule is not appeared in the DSC curve. The final product of K2O with residue mass percentage of 24.0% is in good accord with the calculated value 24.3%. In brief, the abrupt curve of weight loss shows the rapid release of energy. 3.3. Oxygen bomb calorimetry The constant−volume combustion energies of energetic compounds were decided with a precise rotating-oxygen bomb calorimeter (RBC-type II).34 Firstly, the average experimental values of six single measurement for the constant−volume combustion energies (Qv) of 1a, 1b and 2 are −21479.562 J·g−1, −23029.355 J·g−1 and −26057.610 J·g−1, respectively. Then, the combustion reaction of the compounds 1a, 1b and 2 is 11



equation (1) and equation (2). The standard molar enthalpy of combustion (∆ܿ‫ )ߠ݉ܪ‬is obtained by equation (3), which be calculated to be −7679.916 kJ·mol−1 for 1a, −8235.021 kJ·mol−1 for 1b and −5366.530 kJ·mol−1 for 2. Finally, the standard molar enthalpy of formation is calculated on basis of the Hess law and the known enthalpies of formation of Li2O (s) (−597.94 kJ·mol−1), K2O (s) (−361.50 kJ·mol−1), CO2 (g) (−393.51 kJ·mol−1) and H2O (l) (−285.83 kJ·mol−1)35 by equations (4)−(5), the standard enthalpy of formation (∆݂‫ )ߠ݉ܪ‬of 1a, 1b and 2 are calculated to be 3022.936 kJ·mol−1, 3561.041 kJ·mol−1 and 3433.590 kJ·mol−1, respectively. C6H12Li2N14O4 (s) + (15/2)O2 (g) → Li2O (s) + 6CO2 (g) + 6H2O (l) + 7N2 (g) (1) C3H4KN7O (s) + (17/4)O2 (g) → (1/2)K2O (s) + 3CO2 (g) + 2H2O (l) + (7/2)N2 (g) (2) ∆ܿ‫ ݒܳ = ݌ܳ = ߠ݉ܪ‬+ ∆ܴ݊݃ܶ (3) where ∆ng is the variation in the number of gas constituents in the reaction process, R = 8.314 J·mol−1·K−1, T = 298.15 K. ∆݂‫( ߠ݉ܪ‬C6H12Li2N14O4, s) = ∆݂‫( ߠ݉ܪ‬Li2O, s) + 6∆݂‫( ߠ݉ܪ‬CO2, g) + 6∆݂‫( ߠ݉ܪ‬H2O, l) − ∆c‫( ߠ݉ܪ‬C6H12Li2N14O4, s) (4) ∆݂‫( ߠ݉ܪ‬C3H4KN7O, s) = (1/2)∆݂‫( ߠ݉ܪ‬K2O, s) + 3∆݂‫( ߠ݉ܪ‬CO2, g) + 2∆݂‫( ߠ݉ܪ‬H2O, l) − ∆c‫( ߠ݉ܪ‬C3H4KN7O, s) (5) 3.4. Detonation property In order to better judge the energetic performances for energetic materials, the heat of detonation (Q), detonation velocity (D) and detonation pressure (P) are significant parameters. In this paper, we provide three methods to evaluate Q, D and P of complexes (see the Supporting Information): (1) Oxygen bomb calorimetry and Kamlet−Jacobs equations36 (Q, D and P for 1a, 1b and 2: 2.962 kcal·g−1, 8.664 km·s−1 and 30.340 GPa; 3.321 kcal·g−1, 8.954 km·s−1 and 32.544 GPa; 4.245 kcal·g−1, 9.112 km·s−1 and 36.498 GPa); (2) Density functional theory and Kamlet−Jacobs equations (Q, D and P for 1a, 1b and 2: 4.923 kcal·g−1, 9.837 km·s−1 and 39.118 GPa; 5.594 kcal·g−1, 10.197 km·s−1 and 42.237 GPa; 4.828 kcal·g−1, 9.332 km·s−1, 38.281 GPa); (3) Oxygen bomb calorimetry and EXPLO5 v6.0137 (Q, D and P for 1a, 1b and 12


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2: 2.973 kcal·g−1, 9.234 km·s−1 and 36.659 GPa; 3.455 kcal·g−1, 9.534 km·s−1 and 38.659 GPa ; 4.255 kcal·g−1, 9.742 km·s−1 and 42.659 GPa). 3.5. Sensitivity Fall Hammer Apparatus is used to measure impact sensitivity. Twenty milligrams of the target compounds are compacted to a copper cap under the press of 39.2 MPa. Then 2 kg drop hammer hits it. The calculated value of h50 displays the drop height of 50% initiation probability. The compounds 1a, 1b and 2 don’t burn at the highest point of 200 cm, indicating that the value of the affected energy is 40 J, while the compound is not sensitive to the effect. The target compound sample with 20mg was measured the friction sensitivity by Julius Peter’s machine. No friction sensitivity is observed up to 36 kg (360 N). 3.6. Discussion for detonation properties Compared with the heat of detonation from three methods, the values are similar, while the Q value from theoretical calculation is slightly larger. Based on the experiment of oxygen bomb calorimetry, the Q values are calculated using the values of heat of formation in the first method and the third method. The discrepant values of D and P from K−J equations and EXPLO5 v6.01 are mainly due to the different final products in the detonation reaction equations. The products of decomposition from EXPLO5 v6.01 are multiple, while these from Pang’s method38 are specified according to rules. The first method and the third method from EXPLO5 v6.01 are both effective and accurate to evaluate Q, D and P. To make a rational comparison on the same level, the detonation parameters of reported energetic CPs and common high explosives from different methods are summarized in Table 1. We analysis and compare the Q, D and P by the third method. First, we compare the detonation properties between the H2tztr ligand and metallic compounds on basis of the ligand. For H2tztr ligand (N% = 71.51%), heat of formation and density are obtained by theoretical calculations. Q, D and P are calculated by the EXPLO5 v6.01 (0.806 kcal·g−1, 7.294 km·s−1 and 18.168 GPa). The 13



values of Q, D and P from 1a, 1b, 2, 37 and 5−88,9,13 are larger than those of the ligands, which indicates ECPs can enhance the detonation performance. Nevertheless, the value of Q from compound 9 9 is less than that of the ligand, likely owing to the low nitrogen content. Next, the performance of the target compounds needs to be analyzed. For the two polymorphs with slightly structure change, the detonation properties are obviously different, where the performance of 1b is superior to those of 1a, which is mainly due to the difference of energy density from distinct packing arrangements. From the view of the second method, the value of Q from 1b (5.594 kcal·g−1) corresponds closely to that reported compound 4 (IFMC−1)13 (5.620 kcal·g−1), which have an advantage over energetic CPs (compounds 2, 3, 5−27) and some common high explosives (RDX,39 HMX,39 TNT39) (Table 1 and Figure 4). In addition, the proportion of H2O molecules is also an important factor to determine D and P. As illustrated in Table 1, we found the values (Q, D and P) of 1a and 1b from three methods are distinct, mainly due to existence of four coordinated H2O molecules in 1a and 1b. The values of Q based the experiment of oxygen bomb calorimetry from 1a and 1b are lower than the values from the second method, likely because of the loss of H2O molecules at the lower temperature. Hence, compound 2 possesses the better detonation performance than 1a and 1b. There is only one H2O molecule in one molecule unit of compound 2, and the decomposition process were affected by the H2O molecules. In addition, traces of H2O molecules can enhance the detonation pressure on the premise of the one-step decomposition. In comparison to other K−ECPs (compounds 16−26), the density of 2 is smaller while 2 shows excellent detonation properties exactly because of the one-step decomposition. In addition, the impact and friction sensitivities of 2 are more than 40 J, and 360 N, respectively, classified as “insensitive”. However, compounds 16−22, 25, 26 are sensitive (Table S10, Supporting Information). In brief, the different packing arrangements make 1b obtain the higher energy density than 1a, thus leading to the better properties. Notably, detonation properties of 2 are significantly better than 1a and 1b and other energetic CPs with low-sensitivity, may be attributed to high heat of detonation and one-step release of energy. 14


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Page 15 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig 4. Bar chart representation of the previously reported ∆Hdet values for the common explosive materials, including TNT, RDX and HMX. The ∆Hdet values for 1a, 1b and 2 are also shown.

Table 1. Physic Chemical properties of 1a and 1b and some energetic materials. ρa

Nb

Ωc

Tdecd

Q

D

P

(g·cm−3)

(%)

(%)

(°C)

(kcal·g−1)

(km·s−1)

(GPa)

2.962e

8.664e

30.340e

2.973f

9.256f

36.621f

4.923g

9.837g

39.118g

3.321e

8.954e

32.544e

3.455f

9.534f

38.569f

5.594g

10.197g

42.237g

4.247e

9.112e

36.498e

4.255f

9.742f

42.659f

4.672g

9.332g

38.281g

3.761e

9.510e

45.080e

3.397f

8.429f

45.020f

3.990g

9.660g

46.440g

1.332g

7.920g

31.990g

ECPs

1a

1.553

54.70

−62.50

150

1b 1.562

54.70

−62.50

155

2 1.773

[Cu(tztr)]n(3) 7

{[Cu(tztr)]·H2O}n(5) 8

2.216

2.316

50.75

49.36

45.23

−62.10

−52.35

−48.00

325

360

325

15



Page 16 of 24

[Cu(Htztr)2(H2O)2]n(6)8

1.892

52.72

−60.24

345

2.218g

8.180g

30.570g

[Cu(Htztr)]n(7) 8

2.435

49.08

−56.09

355

3.958g

10.40g

56.480g

[Pb(Htztr)2(H2O)]n (8) 9

2.519

39.40

−45.03

340

1.359g

7.715g

31.570g

[Pb(H2tztr)(O)]n (9) 9

3.511

27.20

28.86

318

0.255g

8.122g

40.120g

[Cu(atrz)3(NO3)2]n(10)10

1.680

53.35

−58.83

243

3.618g

9.160g

35.680g

[Ag(atrz)1.5(NO3)] n(11)10

2.160

43.76

−49.99

257

1.381g

7.773g

29.700g

{ (AG)2[Cu(btm)2]}n(12)11

1.826

65.40

−

212

5.410g

10.970g

53.920g

[(AG)3(Co(btm)3)] (13)11

1.684

68.70

−

268

4.750g

10.210g

44.450g

{Ag2(DNMAF)(H2O)2}n(14)12

2.545

22.44

−7.69

230

1.950e

9.673e

50.010e

{Ag2(DNMAF)}n(15) 12

2.796

23.81

−8.16

212

2.190e

8.160e

58.300e

CHHP(27) 13

2.000

25.58

−13.05

231

2.218g

8.180g

30.570g

ZnHHP(28)13

2.117

23.61

−49.99

293

2.218g

8.180g

30.570g

K2DNABT (16)15

2.110

50.30

−4.8

200

1.185h

8.330h

31.700h

K2DNMNAF(17) 16

2.174

27.10

0

278

−

7.758h

27.200h

K2DNAF(18) 16

2.187

31.60

0

288

−

7.507h

25.100h

K2BNOD (19) 16

2.100

33.50

−19.10

198

−

8.150h

30.100h

K2NATF(20) 16

1.949

40.90

−23.40

307

−

6.358h

16.000h

KCPT(21)17

1.980

39.00

−24.70

323

1.426h

5.965h

32.500h

K2BDF(22)18

2.130

22.70

+4.30

218

−

7.759h

27.300h

Potassium 5−oxotetrazolate(23) 19

1.872

45.10

−45.10

246

−

−

−

K2NMTO(24)20

2.228

37.85

0

320

0.782h

7.846h

24.00h

K2BDTAF(25)21

2.039

31.10

−7.10

229

−

8.139h

30.10h

KNODP(26) 22

2.130

33.46

−19.10

204

−

8.361h

32.10h

HMX 39

1.950

37.80

−21.60

287

2.218g

8.180g

30.570g

RDX39

1.806

37.80

−21.60

210

2.218g

8.180g

30.570g

TNT39

1.654

18.50

−74.00

244

2.218g

8.180g

30.570g

a

Density measured by gas pycnometer at 25°C or crystal density.

Decomposition temperature.

e

b

Nitrogen content.

c

Oxygen balance.

d

Heat of detonation, detonation velocity, detonation pressure calculated using

RBC−type II and K-J equations; f Heat of detonation, detonation velocity, detonation pressure calculated using

16


Page 17 of 24 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


RBC−type II and EXPLO5 v6.0115; g Heat of detonation, detonation velocity, detonation pressure calculated using DFT and K-J equations; atomization

method

/

h

Heat of detonation, detonation velocity, detonation pressure calculated using the

the

isodesmic

reaction

and

EXPLO5

v6.01;

Abbreviations:

H3dttz

=

4,5−di(1H−tetrazol−5−yl)−2H−1,2,3−triazole; atrz = 4,4′−azo−1,2,4−triazole; H2btm = bis(tetrazole)methane, AG =

aminoguanidinium;

DNMAF2−

3−dinitromethyl−4−nitraminofurazan;

=

4,4’−bis(dinitromethyl)−3,3′−azofurazanate;

DNAF2−

=

3,6−bis(4−nitramino−1,2,5−oxadiazol−3−yl)−1,2,4,5−dioxa−

3,4−dinitraminofurazan; diazine;

DNMNAF2−

=

BNOD2−

=

CPT−

=

4−(5−amino−3−nitro−1H−1,2,4−triazol−1−yl)−3,5−dinitropyrazole; BDF2− = 4,5−bis(dinitromethyl)furoxanate; NMTO2− = 5−Nitriminotetrazole 1−Oxide; BDTAF2− = 4,4’−bis(dinitromethyl)−3,3’−azofurazanate; NODP− = 6− nitro−5−oxidopyrazolo[3,4−c][1,2,5]oxadiazol−4−ide.

4. Conclusions In conclusion, the self-assembly reaction of H2tztr and LiNO3 leads to [Li2(Htztr)2(H2O)4] (1a), further pH-induced structural rearrangement generates the polymorph (1b). Particularly, 1b possesses better detonation performance with low sensitivity (∆Hdet = 3.455 kcal·g−1, D= 9.534 km·s−1, P= 38.569 GPa) among reported ECPs. As is noticed, there are coordinated H2O molecules existing in 1a and 1b, especially for the axial coordinated H2O molecules. The efforts to substitute H2O molecules by suitable energetic coordination groups to achieve much dense Li−ECPs are performing in our laboratory. The bad news is that we obtained 1a or 1b with various reaction conditions. Fortunately, a 2D energetic CP ([K(Htztr)2(H2O)]n) was prepared by replacing Li(I) to K(I). Compound 1a, 1b and 2 were characterized by X−ray diffraction, IR spectroscopy, TG and DSC thermal analyses. Additionally, compound 2 exhibited outstanding detonation properties (Tdec = 325 °C, ∆Hdet = 4.255 kcal·g−1, D = 9.742 km·s−1, P = 42.659 GPa) with the low sensitivity, excellent thermal stability. Compound 2 may be an ideal candidate for green detonation explosive. This finding demonstrates an efficient approach to enhancing detonation

17



performance by light metal as nodes to construct ECPs with one-step release of energy.

ASSOCIATED CONTENT Supplementary information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Crystallographic data for the structures reported in this article have been deposited in the Cambridge Crystallographic Data Center with CCDC reference numbers 1558470, 1558471 and 1582323 for compounds 1a, 1b and 2, respectively. The calculation of detonation properties and PXRD patterns (PDF) X−ray crystallographic file for C6H12Li2N14O4 (CIF) X−ray crystallographic file for C6H12Li2N14O4 (CIF) X−ray crystallographic file for C3H4KN7O (CIF)

ACKNOWLEDGEMENTS We gratefully acknowledge financial support from the National Natural Science Foundation of China (grant no. 21673180, 21727805, 21373162, 21673181 and 21473135).

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22


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For Table of Contents Only

23



This finding demonstrates a systemic study about an efficient approach to enhancing detonation performance by light metal as nodes to construct ECPs and experimental, theoretical determination of detonation properties. 28x20mm (300 x 300 DPI)


Page 24 of 24

K-Based Energetic Coordination Polymers: Structural

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