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Oct 26, 2016 - Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, Shaanxi Key Laboratory of...
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Structural Modulation from 1D Chain to 3D Framework: Improved Thermostability, Insensitivity, and Energies of Two Nitrogen-Rich Energetic Coordination Polymers Zhaoqi Guo,*,†,‡ Yunlong Wu,§ Chongqing Deng,† Guoping Yang,*,§ Jiangong Zhang,† Zhihua Sun,† Haixia Ma,‡ Chao Gao,† and Zhongwei An† †

State Key Laboratory of Fluorine & Nitrogen Chemicals, Xi’an Modern Chemistry Research Institute, Xi’an, 710065 Shaanxi, China Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of the Ministry of Education, Shaanxi Key Laboratory of Physico-Inorganic Chemistry, College of Chemistry & Materials Science, Northwest University, Xi’an, 710127 Shaanxi, China ‡ School of Chemical Engineering, Northwest University, Xi’an, 710069 Shaanxi, China §

S Supporting Information *

ABSTRACT: Two new energetic coordination polymers (CPs) [Pb(BT)(H2O)3]n (1) and [Pb3(DOBT)3(H2O)2]n· (4H2O)n (2) with 1D and 3D structures were synthesized by employing two rational designed ligands, 1H,1′H-5,5′bitetrazole (H2BT) and 1H,1′H-[5,5′-bitetrazole]-1,1′-diol ligands (DHBT), respectively. Thermal analyses and sensitivity tests show that the 3D architecture reinforces the network of 2 which has higher thermal stability and lower sensitivity than that of 1. Through oxygen-bomb combustion calorimetry the molar enthalpy of formation of 2 is derived to be much higher than that of 1 as well as the reported CPs. Herein, more importantly, the heats of detonation (ΔHdet) were calculated according to the decomposition products of TG-DSC-MS-FTIR simultaneous analyses for the first time. The calculated results show that ΔHdet of 2 is 23% higher than that of 1. This research demonstrates that 3D energetic CP with outstanding energetic properties can be obtained through efficient and reasonable design.

1. INTRODUCTION The design and preparation of molecular solids with specific structures and functions is the main concern of crystal engineering. Since the physical properties of a crystal are related to its molecular structure and crystal packing, rational design of the expected crystal is of great significance and challenging for chemists. Coordination polymers (CPs) represent a fascinating class of crystalline materials with one-, two-, or three-dimensional (1D, 2D, or 3D) architectures that can be readily prepared from organic ligands and metal salts. Thanks to the diversities of ligands, metals, as well as ordered pores and channels within crystal, CPs have proved as a kind of material with tremendous applications in gas storage, separation, heterogeneous catalysis, sensing, luminescence, and so on.1−5 The advantages of physical properties, such as favorable thermal stability, high density, and excellent mechanical strength, have compelled researchers to develop various CPs possessing new functions. Recently, CPs have emerged as outstanding high-energy-density materials (HEDMs) with low sensitivity.6−9 As is well known, high energy and low sensitivity are always contradictory in the nature of energetic materials, which has greatly restricted development of conventional energetic materials. 2,4,6-Trinitrotoluene (TNT), 1,3,5-trinitro-1,3,5© XXXX American Chemical Society

triazacyclohexane (RDX), and 1,3,5,7-tetranitro-1,3,5,7-tetrazacyclooctane (HMX), etc., were discovered more than 60 years ago and are still used as the major explosives today. Recently, research of energetic CPs has revealed a new concept for the insensitive HEDMs, that is, the nitrogen-rich CPs, formed by metal ions with organic ligands such as azide,10 furazan,11 triazoles,12 and tetrazoles13 via coordination bonds, have emerged as promising candidates for primary or secondary explosives. This new generation of energetic CPs has presented potential advantages over conventional organic explosives such as high density, low sensitivity, and high thermal stability, which are the most important properties of energetic materials and can be modulated through rational selection of metal ions and design of organic ligands. Besides, the abundance and diversities of energetic ligands provide plentiful opportunities to construct novel energetic CPs with high performance. Among the reported energetic CPs, 1D structures show high energy but are highly sensitive. The complexes [Ni(N2H4)3(NO3)2] (NHN), [Ni(N2H4)5(ClO4)2] (NHP), and [Co(N2H4)5(ClO4)2] (CHP)10a have comparative heats of detonation to that of CL-20 (hexanitrohexaazaisowurzitane). Received: July 15, 2016

A

DOI: 10.1021/acs.inorgchem.6b01630 Inorg. Chem. XXXX, XXX, XXX−XXX

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Materials and Instruments. 1H,1′H-5,5′-bitetrazole (H2BT)18 and 1H,1′H-[5,5′-bitetrazole]-1,1′-diol (DHBT)19 were prepared according to literature methods. Organic solvents and Pb(NO3)2 were commercially available and used without further treatments. IR spectra were recorded on a Nicolet Nexus 870 FT-IR spectrophotometer (KBr pellets). Elemental analysis was measured on an Elementar Vario EL III (Germany). DSC measurements were carried out with a TA Q2000 instrument (TA Instruments, New Castle, DE, USA). The DSC conditions were as follows: sample mass, about 1 mg; heating rate, 10 °C min−1; atmosphere, dynamic atmosphere of nitrogen with a flow rate of 50 mL min−1 and pressure of 0.1 MPa. TG/DTG measurements were taken on a TA4000 instrument (TA Instruments, New Castle, DE, USA). The measurement conditions were as follows: sample mass, about 1 mg; heating rate, 10 °C min−1; atmosphere, dynamic atmosphere of nitrogen with a flow rate of 50 mL min−1 and pressure of 0.1 MPa. A TG-DSC-MS-FTIR coupling system composed of a Netzsch STA449C (Selb, Germany), Netzsch QMS403C (Selb, Germany), and Nicolet 6700 FTIR (Madison, WI) was used for thermal analysis. The ionizing electron energy of Netzsch-QMS403C is 70 eV. The pressure injection of the quartz capillary gas connector is 1 bar, and the capillary temperature is 200 °C. About 1 mg of a sample was placed in an alumina crucible with a pinhole on the lid and heated from 30 to 500 °C. α-Al2O3 was used as the reference sample. High-purity argon was used as protecting gas with a gas flow rate of 20 mL min−1. The data were recorded with a heating rate of 10 °C min−1. The constant-volume combustion energies of the complexes were determined with a precise rotatingbomb calorimeter (RBC-type II). Impact sensitivity (IS) and friction sensitivity (FS) were measured according to the UN Recommendations on the Transport of Dangerous goods, Manual of Tests and Criteria.20 The particle sizes of the samples are in the range of 200− 500 μm. Synthesis. [Pb(BT)(H2O)3]n (1): H2BT (5 mmol, 0.691 g) and Pb(NO3)2 (5 mmol, 1.655 g) were dissolved in 20 mL of hot deionized water. The resulting solution was filtered and kept at 30−40 °C for a few days. Colorless single crystals suitable for X-ray diffraction were obtained. Yield: 63%. Anal. Calcd: C, 6.05; H, 1.52; N, 28.20. Found: C, 6.02; H, 1.54; N, 28.26. IR (KBr pellets, λ, cm−1): 3473, 1621, 1593, 1335, 1303, 1171, 1010, 733. [Pb3(DOBT)3(H2O)2]n·(4H2O)n (2): A solution of Pb(NO3)2 (5 mmol, 1.655 g) in 20 mL of deionized water was added to the bottom of a glass tube. A solution of DHBT (5 mmol, 0.851 g) in hot acetonitile (20 mL) was layered on top of the water solution. Colorless single crystals suitable for X-ray diffraction were obtained after several days. Yield: 86%. Anal. Calcd. C, 5.84; H, 0.98; N, 27.24. Found: C, 5.80; H, 1.00; N, 27.01. IR (KBr pellets, λ, cm−1): 3444, 1627, 1596, 1507, 1408, 1346, 1232, 1170, 992, 730. X-ray Structure Determination. The diffraction data were collected at 296(2) K with a Bruker-AXS SMART CCD area detector diffractometer using ω rotation scans with a scan width of 0.3° and Mo Kα radiation (λ = 0.71073 Å). Absorption corrections were carried out utilizing the SADABS routine. The structure was solved by direct methods and refined by full-matrix least-squares refinements based on F2. All non-hydrogen atoms were refined anisotropically with the hydrogen atoms added to their geometrically ideal positions and refined isotropically except that the hydrogen atoms on the coordinated water molecules were located from the electron density and then refined using a riding model. The void volume of each framework was estimated by the PLATON program. The final formula was determined by combining single-crystal structures, elemental analyses, and TGA. The details of crystal data, data collection parameters, and refinement statistics are given in Table 1. Selected bond lengths and bond angles of complexes 1 and 2 are listed in Table S1 (Supporting Information). Hydrogen bonding parameters of 1 are listed in Table S2. CCDC 1473830 and 1473840 for complexes 1 and 2 can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk.

However, they are so sensitive to spark and mechanical stimuli that cannot be used in military and civil equipment. Comparing with 1D CPs, 2D-layered structures such as [Co 2 (N 2 H 4 ) 4 (N 2 H 3 CO 2 ) 2 ][ClO 4 ] 2 ·H 2 O (CHHP) and [Zn2(N2H4)3(N2H3CO2)2][ClO4]2·H2O (ZnHHP)14 have lower sensitivity while maintaining comparative energy. On employing ligands with multiple chelate points, 3D energetic CPs have also been synthesized. For example, complexes [Cu(atrz)3(NO3)2]n and [Ag(atrz)1.5(NO3)]n are reported, exhibiting high thermal stability and extraordinary insensitivity and energy content.12a Another 3D CP [Co9(bta)10(Hbta)2(H2O)10]n·[22(H2O)]n with higher nitrogen content also achieves unity of high energy and low sensitivity.13a The sophisticated 3D architecture reinforces the mechanical stability of the crystal, leading to high thermal stability and low sensitivity.6,12a,13a Despite the tremendous achievements in exploitation of energetic CPs, the strategy of designing a 3D energetic CP with excellent performance and the relationships between the structures and the properties of CPs with different dimensions are unclear,8 which are of great importance for guiding the rational design and synthesis of energetic CPs. In view of the above considerations, we focus our attention on the manipulation of energetic CPs with different dimensionalities through rational selection of ligands and metal ions and study their structure−property relationships. There is no exception that the reported energetic CPs employ nitrogen-rich ligands such as triazoles and tetrazoles because they are energetic compounds and possess multichelating sites. We herein disclose the design and synthesis of energetic CPs with two similar organic skeletons. The 1H,1′H-5,5′-bitetrazole (H2BT, Scheme 1) was first chosen as a functional ligand to Scheme 1. Structures of H2BT and DHBT Ligands

study the structures and properties. Then the 1H,1′H-[5,5′bitetrazole]-1,1′-diol (DHBT) with additional two hydroxyl groups was designed to examine the structural modulation upon introducing oxygen chelate sites. Since lead salts are used as explosives12b,15 and additives in double-based propellants,16,17 which is thus chosen as knot ion to construct CPs with different dimensionalities for it has different coordination preferences to nitrogen and oxygen atoms. On the basis of the above-discussed strategies, two energetic CPs 1D [Pb(BT)(H2O)3]n (1) and 3D [Pb3(DOBT)3(H2O)2]n·(4H2O)n (2) have been yielded successfully. Also, their crystal structures, thermal stability, sensitivity, constant-volume combustion energies, thermal decomposition, as well as calculated heats of detonation are well investigated carefully.

2. EXPERIMENTAL SECTION Caution! H2BT, DHBT, and the corresponding coordination compounds are explosive hazardous materials. Wearing safety glasses and face shields is strongly recommended when handling these materials. The use of metal spatulas is strictly forbidden. Fire and static electricity discharge should be avoided. TGA, DSC, and TG-DSC-MS-IR analyses should use no more than 5 mg of sample to avoid instrument damage. B

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than to the nitrogen atom, which is a harder base. Therefore, the DHBT ligands adopts two different coordination fashions with lead(II) ions in 2 (Scheme S1b and S1c), which should be ascribed to the additional oxygen atoms in the DHBT ligands. As a result, a 3D porous framework is formed, where the lattice and coordinated water molecules are located in these 1D channels viewing along the c axis (Figure 2b). The densities of complexes 1 and 2 were determined to be 3.01 and 2.72 g·cm−3, respectively. Density is one of the most important physical properties of energetic materials. Due to the loading volume restriction of a bomb or rocket, higher loading density represents higher total energy of the ammunition.21 The density of a compound is mainly determined by its spatial molecular packing.22 Due to the different structures of the ligands, 1 and 2 exhibit different behaviors in crystal stacking. A closely packed 1D chain structure as well as a 3D supramolecular network generated by hydrogen bonding interactions between coordinated water molecules and ligands would be responsible for the high density of 1. In contrast, the open channels are observed in 2, which make it less dense than 1. Sensitivity Test. The contradiction of high energy and low sensitivity has restricted applications of plenty of energetic materials. The sensitivity toward impact, friction, and heat is of major concern for stability, which closely relates to the safety in manufacturing, storage, transportation, and use of energetic materials. A BAM fall hammer and BAM friction apparatus were employed to measure impact sensitivity (IS) and friction sensitivity (FS).20 The IS and FS of 1 were measured to be 6 J and 120 N, respectively, which in accordance with the reported results shows that 1D energetic CPs are usually sensitive to mechanical stimuli. Owing to the 3D crystal structure, 2 is less mechanically sensitive with an IS value of 14 J and friction sensitivity of 360 N. It is worth mentioning that the insensitivity of friction is vital to safely process the material with or without other energetic components since friction is usually considered an important source of accidental ignition. Therefore, 2 is competent for actual use. Thermostability. The differential scanning calorimetry (DSC) and thermogravimetry/derivative thermogravimetry (TG/DTG) measurements of bulk crystals of 1 and 2 were taken to assess their thermal stability. As shown in Figure S2a, 1 is thermally stable up to 210 °C. The onset decomposition temperature is at 213 °C (peak at 229 °C), which is assigned to loss of one coordination water molecule as result of weight loss of 4.53% at 234 °C on the TG curve. This is an endothermic process as demonstrated on the DSC curve. The gradually exothermic decomposition of 1 is observed until it explodes at 383 °C. An endothermic weight loss of 2 starts at 70 °C, which is related to the removal of physically adsorbed water molecules

Table 1. Crystal Data and Structure Refinements for 1 and 2 CCDC formula formula mass cryst syst space group a [Å] b [Å] c [Å] α [deg] β [deg] γ [deg] V [Å3] Z Dcalcd.[g·cm−3] μ [mm−1] F [000] θ [deg] no. of reflns collected GOOF Ra [I > 2σ(I)] a

1

2

1473830 C2H6PbN8O3 397.34 monoclinic P21/n 6.740(3) 13.805(6) 9.742(4) 90 104.937(6) 90 875.8(6) 4 3.013 19.265 720 2.62−25.00 4132/1514 1.257 R1 = 0.0347 wR2 = 0.1125

1473840 C6H4Pb3N24O8 1161.90 monoclinic C2/c 13.8912(18) 24.074(3) 8.5487(11) 90 96.655(2) 90 2839.5(6) 4 2.718 17.820 2072 1.69−24.99 6672/2487 1.072 R1 = 0.0428 wR2 = 0.1316

R1 = ∑∥Fo| − |Fc∥/∑|Fo|, wR2 = [∑w(Fo2 − Fc2)2/ ∑w(Fo2)2]1/2.

3. RESULTS AND DISCUSSION Crystal Structure of 1. Complex 1 crystallizes in the monoclinic P21/n (Z = 4) space group and exhibits a 1D chainlike structure. The asymmetric building unit of 1 contains one lead(II) ion, one deprotonated BT ligand, and three coordinated water molecules (Figure 1a). In 1, each BT ligand is coordinated with three lead(II) ions as the repeat units to form a 1D motif (Scheme S1a and Figure 1b), and the three aqua molecules, as the terminal ligands, linked with lead(II) ion to satisfy the requirement of the coordination geometry. The bond lengths of Pb−N and Pb−O are in the normal range of Pb(II)-based complexes (Table S1). Because of the existence of abundant uncoordinated nitrogen atoms in BT ligands and coordinated water molecules, these 1D chains are further packed into a 3D supramolecular network via hydrogen bonding interactions (Figure S1, Table S2). Crystal Structure of 2. Complex 2 is a 3D porous framework with C2/c space group. There exist three independent lead(II) ions (one Pb1 and two Pb2 ions), three deprotonated DHBT ligands, two coordinated water molecules, and four guest water molecules in the asymmetric unit (Figure 2a). It is well known that lead(II) ion is a relatively softer acid and much more easily coordinates to the soft base oxygen atom

Figure 1. (a) Coordination environment of Pb(II) ion in 1. Symmetry codes: #1, 0.5 + x, 1.5 − y, −0.5 + z; #2, 0.5 + x, 1.5 − y, 0.5 + z. (b) 1D chain of 1 extended by the BT ligands with Pb(II) ions. C

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Figure 2. (a) Coordination environments of Pb(II) ions in 2. Symmetry codes: #1, 1 − x, y, 1.5 − z; #2, x, 1 − y, 0.5 + z; #3, 1 − x, 1 − y, 2 − z; #4, 0.5 − x, 1.5 − y, 1 − z; #5, 0.5 + x, 1.5 − y, 0.5 + z; #6, −x, y, 1.5 − z; #7, 0.5 − x, 1.5 − y, 2 − z. (b) 3D porous network of 2.

rotating-oxygen-bomb calorimetry. To ensure safe combustion, 200 mg of the sample was pressed with 800 mg of benzoic acid to form a tablet before measurement. The recorded data are the average of six single measurements. The calorimeter was calibrated by combustion of certified benzoic acid (Standard Reference Material) in an oxygen atmosphere at a pressure of 30.5 bar. The combustion energies of 1 and 2 were determined to be −9175.5 and −6993.6 J·g−1, respectively. The standard molar combustion enthalpies of 1 and 2 are calculated according to eqs 2, 3, and 4 at 298.15 K and 101.325 kPa.

in the open channel of 3D crystal (Figure S2b). The loss of all guest molecules is found at 230 °C (weight loss of 5.84%) on the TG curve, indicating the backbone of 2 is preserved until this temperature. The onset decomposition temperature of the backbone of 2 is found at 253 °C, followed by two intense exothermic peaks at 288 and 304 °C, respectively. The results show that both 1 and 2 are thermally stable for military and civilian use, and the 3D structure endows 2 higher thermal stability. Nonisothermal Kinetics Analyses. Differential scanning calorimetry was measured at four different heating rates of 5, 10, 15, and 20 °C/min to determine the apparent activation energies Ea. On the basis of the Kissinger equation, eq 1 ln

1

+ 3H 2O(l) + 2CO2 (g)

E β AR = ln − a 2 Ea RT T

(1)

2

where T is the peak temperature (K), A is the pre-exponential factor (s−1), R is the gas constant (J·mol−1·K−1), and β is the linear heating rate (K·s−1). The calculated parameters are listed in Table 2. The peak temperatures of the exothermic decomposition curves shift to Table 2. Nonisothermal Kinetics Parameters T (β)

Ea log A r

C2H6N8O3Pb(s) + 2.5O2 (g) → PbO(s) + 4N2(g)

1

2 552.8 (5) 560.6 (10) 564.2 (15) 568.3 (20) 229.3 19.5 0.998

C6H4N24O8Pb3(s) + 4.5O2 (g) → 3PbO(s) + 12N2(g) + 2H 2O(l) + 6CO2 (g)

ΔcHmθ = Q v + ΔnRT

(3) (4)

where Qv represents the constant volume combustion energy, Δn is the change in the number of gas constituents in the reaction process, R = 8.314 J·mol−1·K−1, and T = 298.15 K. The calculated enthalpies of combustion (ΔcH) are −3637.2 kJ· mol−1 for 1 and −8092.4 kJ·mol−1 for 2. These values are comparable to the most reported energetic CPs. The standard molar enthalpies of formation are calculated according to Hess’s law

a

606.7 (5) 617.7 (10) 622.0 (15) 626.8 (20) 210.4 15.9 0.997

(2)

1

Δf Hmθ (C2H6N8O3Pb, s) = Δf Hmθ (PbO, s) + 3Δf Hmθ (H 2O, l) + 2Δf Hmθ (CO2 , g)

β, heating rate (°C/min). T, peak temperature of the exothermic stage (K). Ea, apparent activation energy (kJ/mol). A, pre-exponential factor (s−1). r, linear correlation coefficient. a

− ΔcHmθ (C2H6N8O3Pb, s) = 2187.70 kJ ·mol−1 2

Δf Hmθ (C6H4N24O8Pb3 , s) = 3Δf Hmθ (PbO, s) + 2Δf Hmθ (H 2O, l) + 6Δf Hmθ (CO2 , g)

higher temperature as the heating rate increases. The good linear correlation indicates that the measurements are reliable. The calculated Ea values are 210.4 and 229.3 kJ·mol−1 for 1 and 2, respectively. Higher apparent activation energy means better thermal stability. The nonisothermal kinetics analyses proved that 2 is more thermally stable than 1. Oxygen-Bomb Calorimetry. To study the heats of combustion and enthalpies of formation of 1 and 2, the constant-volume combustion energies (Qv) were measured by

− ΔcHmθ (C6H4N24O8Pb3 , s) = 5436.68 kJ ·mol−1

where ΔfHθm (PbO, s) = 63.0 kJ mol−1, ΔfHθm (H2O, l) = −241.83 kJ·mol−1, and ΔfHθm (CO2, g)= −393.51 kJ·mol−1 (data were obtained from the literature23). The enthalpy of formation of 1 is close to other reported CPs. However, the enthalpy of formation of 2 is much higher than that of RDX, HMX, and reported energetic CPs.10b Due D

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Figure 3. TG-DSC-QMID curves of 1 (a) and 2 (b). Some of the QMID curves are not pointed out because they are weak in intensity and stacked together.

Figure 4. (a) Infrared absorption intensity of gaseous decomposition products of 2 detected from TG-DSC-MS-FTIR simultaneous analyses. (b) Temperature-dependent FT-IR spectra of gaseous decomposition products of 2 detected from TG-DSC-MS-FTIR simultaneous analyses.

to the small differences between ligands H2BT and DHBT, the significant difference of enthalpies of formation between 1 and 2 should be ascribed to the different coordination frameworks. TG-DSC-MS-FTIR Simultaneous Analyses. Thermal behavior analysis is a helpful technique for characterization of energetic materials. Analysis of the decomposition products can offer information to understand the decomposition mechanism and help to conclude the combustion products for calculating the energy of detonation. To date, the detonation products of the reported CPs have always been assumed rather than experimentally determined when computing the heat of detonation, which may overestimate the energy of CPs. Herein,

the thermogravimetry-differential scanning calorimetry-mass spectrometry-Fourier transform infrared spectroscopy (TGDSC-MS-FTIR) simultaneous analyses of powdered samples were performed for the first time. Figure 3a shows TG-DSC curves of 1 along with quasimultiple ion detection (QMID) curves of m/z 12, 14, 16, 17, 18, 26, 27, 28, 30, 44, and 46 which are assigned to C+, N+, O+, NH3+, H2O+, CN−, HCN+, CO+ and/or N2+, NO+, CO2+, and NO2+, respectively. It is necessary to note that the DSC data usually changes with crystal size for a certain compound.24 The TG curves of bulk crystal and ground sample of 1 are similar except for the difference of residue weight. Due to the explosive E

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Inorganic Chemistry decomposition of bulk crystal, part of the residue splashes out of the container so that the final residue weight percentage is less than that of lead (52.1%). The DSC curve shows that the exothermic decomposition of powdered sample does not show the explosion feature. There exist four exothermic peaks at 256, 297, 338, and 387 °C on the DSC curve. Correspondingly, four peaks at 256, 300, 343, and 389 °C appear on QMID curves of m/z 14 and 28 representing ion signals of N+ and N2+. The other ion signals are quite weak, indicating the major gaseous decomposition products are nitrogen. This result is confirmed by FT-IR spectra that there is no clear gaseous absorption detected (Figure S3), which means the QMID curve of m/z 28 is not contributed by CO+. N2 molecules do not absorb IR because symmetric bond stretching does not change the dipole moment, and bending cannot occur with only two atoms in the molecule. Upon heating the coordinated water molecules will release from the crystal. Therefore, it could be concluded that the decomposition products of 1 are N2, H2O, C, and Pb due to the deficiency of oxygen. TG-DSC curves as well as QMID curves of a ground sample of 2 are also given in Figure 3b. A weak endothermic peak attributed to release of guest water molecules is found at 80 °C, which appears earlier than that of bulk crystal (108 °C). However, the release of guest molecules is proved a slow process derived from the TG curve that started from the beginning of heating until 190 °C. The maximum exothermic peak of the ground sample appears at 285 °C, and the shoulder peak shifts to 309 °C. A strong peak at 317 °C is observed on both QMID curves of m/z 28 and 44, respectively. A strong gaseous absorption is found on FT-IR spectra maximum at 326 °C, which means the gaseous decomposition products are not only N2 (Figure 4a). Infrated absorption peaks of CO and CO2 were detected at regions of 2170−2250 and 2320−2380 cm−1, respectively (Figure 4b). Therefore, it could be concluded that the decomposition products of 2 are N2, CO, CO2, H2O, C, and Pb. Ion currents of all gaseous products appear at the same temperature region of 280−400 °C, which coincides with the exothermic process on the DSC curve as well as the main weight loss temperature region on the TG curve. This phenomeon proves that decomposition of the CP backbone is a cooperative rather than successive procedure so that all gaseous products are detected simultaneously. Simultaneous thermal analysis of both CPs have evidenced a clear decomposition mechanism. Heat of Detonation. The heat of detonation is one of the most important measures of the performance of explosives. It can be determined from the heat of formation of the explosive and that of the detonation products through eq 5 according to Kamlet and Jacobs25

2

With the above experimentally determined enthalpies of formation and the known heats of formation of H2O, CO, CO2, C, and Pb,21 the heats of detonation of 1 and 2 were calculated according to eq 5. As listed in Table 3, the heat of Table 3. Physicochemical Properties of 1 and 2 Tda

dcb

ISc

FSd

Qv e

ΔfHf

ΔHdetg

1 2

213 253

3.01 2.72

6 14

120 360

9175.5 6993.6

2187.7 5436.7

3.23 4.23

detonation of 2 is 4.23 kJ·g−1, which is 23% higher than that of 1. Given that the only structural difference between two ligands H2BT and DHBT are two −OH groups, the increasing oxygen balance and 3D structure endows 2 higher energy.

4. CONCLUSIONS In summary, to demonstrate the design and synthesis of 3D energetic CPs and also reveal the structure−property relationships, two new energetic CPs with 1D and 3D structures have been synthesized by employing two similar skeleton ligands H2BT and DHBT, respectively. Although the 1D CP 1 has higher crystal density, the 3D CP 2 shows improved thermostability, insensitivity, and energy content. The superiority of 3D energetic CPs makes this kind of materials have great application potential. More importantly, the heats of detonation of the two CPs were calculated based on oxygenbomb combustion calorimetry and TG-DSC-MS-FTIR simultaneous analyses for the first time. This research demonstrates that 3D energetic CPs with low sensitivity and high energy can be synthesized through rational design. Besides, this work reveals the relationships of structures and properties of energetic CPs, which may supply important guidance for designing new energetic materials.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01630. Selected bond lengths and bond angles for 1 and 2, hydrogen bonding distances and bond angles for 1, coordination fashions of deprotonated H2BT and DHBT ligands in 1 and 2, 3D supramolecular network of 1 via hydrogen bonding interactions, TG/DTG-DSC curves of bulk crystal of 1 and 2, infrared absorption intensity of decomposition products of 1 detected from TG-DSCMS-FTIR simultaneous analyses, DSC curves of 1 and 2 at four different heating rates, detonation properties of 1 and 2 (PDF) (CIF) (CIF)

∑ ΔHf (det onation products) − ΔHf (exp losive) formula weight of exp losive

The TG-DSC-MS-FTIR simultaneous analyses have offered clear information on decomposition products of 1 and 2. The complete detonation reactions considered for both CPs are described by eqs 6 and 7 C2H6N8O3Pb → Pb + 4N2 + 3H 2O + 2C

entry

a Decomposition temperature (DSC, °C). bDensity from X-ray diffraction analysis (g·cm−3). cImpact sensitivity (J). dFriction sensitivity (N). eConstant-volume combustion energy (J·g−1). fThe enthalpy of formation calculated from Qv (kJ·mol−1). gHeat of detonation (kJ·g−1).

(5)

1

(7)

+ 2CO + 2C

ΔHdet =

C6H4N24O8Pb3 → 3Pb + 12N2 + 2H 2O + 2CO2

(6) F

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Inorganic Chemistry

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant Nos. 21504067 and 21201139), Postdoctoral Science Foundation of Northwest University (Grant Nos. 334100024 and 334100049) and State Administration of Science, Technology and Industry for National Defense.

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DOI: 10.1021/acs.inorgchem.6b01630 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.6b01630 Inorg. Chem. XXXX, XXX, XXX−XXX