Three energetic salts based on oxy-bridged bis(gem-dinitro) furazan: Syntheses, structures and thermal behaviors

Inorganica Chimica Acta 423 (2014) 256–262

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Three energetic salts based on oxy-bridged bis(gem-dinitro) furazan: Syntheses, structures and thermal behaviors Hui Li a, Feng-Qi Zhao a,⇑, Hong-Xu Gao a, Jun-Feng Tong b, Bo-Zhou Wang a, Lian-Jie Zhai a, Huan Huo a a b

Science and Technology on Combustion and Explosion Laboratory, Xi’an Modern Chemistry Research Institute, Xi’an, Shaanxi 710065, China Key Laboratory of Opto-Electronic Technology and Intelligent Control, Lanzhou Jiaotong University, Lanzhou, Gansu 730070, China

a r t i c l e

i n f o

Article history: Received 26 June 2014 Received in revised form 13 August 2014 Accepted 14 August 2014 Available online 2 September 2014 Keywords: Energetic salt Crystal structure Thermal behavior

a b s t r a c t Three high-energy salts (3, 4 and 5) based on oxy-bridged bis(gem-dinitro) furazan with potassium and nitrogen-rich cations were reported. Their structures were fully characterized and confirmed by NMR (1H, 13 C), IR spectroscopy, elemental analysis and single crystal X-ray diffraction. The thermal decomposition behaviors were studied by DSC and TG methods. And, the decomposition process of 5 was studied by the thermolysis in-situ rapid-scan FTIR. The non-isothermal kinetics parameters were obtained by Kissinger’s and Ozawa’s methods, respectively. The activation entropy (DS–), activation enthalpy (DH–), activation Gibbs free energy (DG–) and the critical temperature of thermal explosion were calculated. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction In the pursuit of high performance energetic materials, nitrogenrich compounds have attracted considerable attentions [1]. Unlike the traditional energetic materials, such as RDX (1,3, 5-trinitro-1,3,5-triazacyclohexane) and TNT (2,4,6-trinitrotoluene), whose energy relied on the oxidation of carbon backbone, the energy of nitrogen-rich energetic materials originate from high heat of formation which due to their high N–N and C–N bonds contents [2]. Moreover, nitrogen-rich energetic materials are considered as green energetic materials as their main combustion product is environment-friendly nitrogen gas [3]. Furthermore, nitrogen-rich energetic salts which could be considered as derivates of nitrogen-rich compounds often showed high performance and good stability toward heat or stimulation at the same time as a result of intra- and/or intermolecular hydrogen bonds [4]. As one of the main challenges in energetic materials science is to find materials both have high performance and good stability, nitrogen-rich energetic salts have shown promising prospect. Furazan and its derivates are a kind of nitrogen-rich compounds with excellent performance. And if one or more acidic energetic groups is/are introduced, they could serve as an anion for energetic salts. For example, energetic salts based on nitramine [5] or tetrazole-functionalized [6] furazan have been investigated and shown promising performance. The gem-dinitro group was another excellent acidic energetic group which could bring high density ⇑ Corresponding author. Tel.: +86 29 88291663. E-mail address: [email protected] (F.-Q. Zhao). http://dx.doi.org/10.1016/j.ica.2014.08.019 0020-1693/Ó 2014 Elsevier B.V. All rights reserved.

contribution to the energetic materials (its contribution is 1.759 g cm3) [7]. However, there are no energetic salts based on gem-dinitro substituted Furazan reported until now to our knowledge. Potassium salts were usually used as flame suppressor to inhibit the afterburning combustion [8]. And, the traditional flame suppressors, such as KCl, K2SO4, KNO3 and K3AlF6, are non-energetic materials which have negative effect on the energy level of the propellant formulation. So Energetic potassium salts were always the prior pursuits in energetic material science [9]. Under the above considerations, oxy-bridged bis(gem-dinitro) furazan (OBNF, 1) was chosen as the parent molecule to synthesize the mono-potassium energetic salts with one nitrogen-rich cation. They are expected to have good thermal stability and reduced sensitivity. Herein we report the syntheses, structures and thermal behaviors of three high-energy potassium salts, which could be used as flame suppressors in solid rocket propellant. 2. Experimental 2.1. Material and equipment All chemicals were analytical-grade commercial products. OBNF was prepared according to Ref. [10]. 1 H NMR and 13C NMR were obtained on a Bruker AV500 NMR spectrometer. Infrared spectra were obtained on a Nicolet NEXUS870 Infrared spectrometer in the range of 4000–400 cm1. Elemental analyses (C, H and N) were performed on a VARI-El-3 elemental analyzer. The DSC curves under a flowing nitrogen gas

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were obtained by a NETZSCH DSC200 F3 apparatus. The heating rates were 5.0, 7.5, 10.0 and 12.5 °C min1. The TG-DTG experiment was performed on a SDT-Q600 apparatus (TA, USA) under nitrogen atmosphere at flow rate of 100 mL min1. The heating rate was 10.0 °C min1. The sensitivities towards impact and friction were determined according to BAM standards, using a ZBL-B impact sensitivity instrument (Nachen Co., China) and a MGD036 friction sensitivity apparatus (Qingke Co., China), respectively. 2.2. Synthesis 2.2.1. Dipotassium oxy-bridged bis (gem-dinitro) furazan (2) KOH (0.224 g, 4 mmol) was added to a solution of 1 (0.724 g, 2 mmol) in 10 mL methanol. After stirring for 1 h at room temperature, 0.809 g (92.3%) yellow precipitate was filtered off. 13C NMR (DMSO-d6, 125 MHz): d = 160.77, 142.31, 118.67; IR (KBr, cm1): 1479, 1239, 1070, 1589, 1526, 997; Anal. Calc. for C6N8O11K2: C, 16.44; N, 25.57. Found: C, 16.15; N, 24.96%. 2.2.2. General procedure for synthesis of the salts 3–5 A solution of AgNO3 (0.169 g, 1 mmol) in 10 mL water was added to a solution of 2 (0.438 g, 1 mmol) in 10 mL water. After stirring for 30 min at room temperature, chloride salt (1.0 mmol) was added. The resulting mixture was stirred at 60 °C for an additional 1 h. After removal of AgCl, the solution was evaporated in vacuo and the target product was obtained. 2.2.3. Potassium hydrazidinium oxy-bridged bis (gem-dinitro) furazan (3) White solid (327 mg, 75.8%). 1H NMR (500 MHz, DMSO-d6): d = 8.62(br, 3H, NH3), 4.64 (br, 2H, NH2) ppm. 13C NMR (125 MHz, DMSO-d6): d = 160.8, 142.3, 118.6 ppm. IR (KBr, cm1): 3339, 3294, 3133, 1579, 1535, 1497, 478, 1279, 1147, 1075, 1002, 941, 876, 821, 751. Elemental Anal. Calc. for C6H5KN10O11: C, 16.67, H, 1.17, N, 32.40. Found: C, 16.55; H, 1.19; N, 32.28%.

1539, 1481, 1392, 1327, 1297, 1239, 1142, 998, 962, 873, 826, 747. Elemental Anal. Calc. for C7H7KN12O11: C, 17.73; H, 1.49; N, 35.44. Found: C, 17.69; H, 1.47; N, 35.35%. 2.2.5. Potassium diaminoguanidnium oxy-bridged bis (gem-dinitro) furazan (5) Yellow crystal (393 mg, 63.5%). 1H NMR (500 MHz, DMSO-d6): d = 8.54(s. 2H, NH), 7.13(s, 2H, NH2), 4.58 (s, 4H, NH2) ppm. 13C NMR (125 MHz, DMSO-d6): d = 160.8, 159.7, 142.3, 118.6 ppm. IR (KBr, cm1): 3464, 3369, 3321, 1671, 1540, 1479, 1389, 1325, 1238, 1141, 999, 963, 868, 823, 747. Elemental Anal. Calc. for C7H8KN13O11: C, 17.18; H, 1.65; N, 37.21. Found: C, 17.16; H, 1.67; N, 37.05%. 2.3. X-ray crystallography Single crystals of 3, 4 and 5 suitable for X-ray diffraction studies were obtained by slowly evaporating corresponding aqueous solution at room temperature. For all compounds, a Bruker SMART APE II CCD X-ray diffractometer was employed for data collection using Mo Ka radiation (k = 0.71073 Å). The structure was solved by direct methods using SHELXS program of the SHELXL-97 package and refined with SHELXL package [11]. The final refinement was performed by full-matrix least-squares method with anisotropic thermal parameters on F2 for the non-hydrogen atoms. Crystal data and refinement results are summarized in Table 1. Crystallographic data for the structure reported here have been deposited with the Cambridge Crystallographic Data Centre. The data can be obtained via www.ccdc.cam.ac.uk/deposit (or from the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK; Fax: +441223 336033; E-mail: [email protected]). 3. Results and discussion 3.1. Synthetic reactions

2.2.4. Potassium aminoguanidnium oxy-bridged bis (gem-dinitro) furazan (4) Yellow crystal (288 mg, 60.8%). 1H NMR (500 MHz, DMSO-d6): d = 8.54 (s, 1H, NH), 7.24 (s, 2H, NH2), 6.70 (s, 2H, NH2), 4.68 (s, 2H, NH2) ppm. 13C NMR (125 MHz, DMSO-d6): d = 160.8, 158.7, 142.3, 118.7 ppm. IR (KBr, cm1): 3482, 3444, 3363, 1666, 1582,

The synthetic pathways to 2–5 were depicted in Scheme 1. Compound 1 was prepared according to Ref. [10]. And after treated with 2 eq. KOH, the di-potassium salt (2) was obtained with high yield. As to the synthesis of mono-potassium salt 3–5, the reaction of 2 with corresponding halide salts in water was tried firstly, but

Table 1 Crystal data and structure refinement parameters for 3, 4 and 5. Chemical formula

C6H5KN10O11

C7H7KN12O11

C7H8KN13O11

Formula weight (g/mol) T (K) Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc (g cm3) l (mm1) F(0 0 0) h (°) Index ranges Goodness-of-fit (GOF) on F2 Final R indices [l > 2r(l)] R indices (all data) Max, min Dq (e Å3) CCDC No.

432.30 296(2) triclinic  P1 8.745(1) 8.760(1) 10.528(2) 88.015(2) 72.462(2) 72.292(2) 731.2(2) 2 1.964 0.458 436 2.03–25.09 6X 6 h 6 10, 9 6 k 6 10, 12 6 l 6 12 1.046 R1 = 0.0390, wR2 = 0.1281 R1 = 0.0439, wR2 = 0.1460 0.32 and 0.45 988555

474.35 296(2) monoclinic P21/n 8.013(3) 12.577(5) 16.998(7) 90 94.209(6) 90 1708.6(12) 4 1.844 0.404 960 2.40–25.10 9 6 h 6 9, 15 6 k 6 12, 20 6 l 6 17 1.055 R1 = 0.0372, wR2 = 0.1073 R1 = 0.0426, wR2 = 0.1130 0.45 and 0.46 995017

489.36 296(2) monoclinic P21/c 8.315(7) 12.777(1) 17.699(1) 90 112.10(3) 90 1742(2) 4 1.866 0.401 992 1.24–25.10 9 6 h 6 9, 11 6 k < =15, 17 6 l 6 19 1.017 R1 = 0.0422, wR2 = 0.1297 R1 = 0.0510, wR2 = 0.1488 0.54 and 0.49 995018

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failed. An alternative method based on the metathesis reaction of halide salts with the potassium-silver salts of 1, which can easily be transformed from 2, was applied. All the salts were stable in air and could be stored for days. Their structures were investigated fully and confirmed by 1H NMR, 13C NMR, IR spectroscopy and elemental analyses. It is notable that all the OBNF anions have similar chemical shifts in 13C NMR spectra (around 160.8, 142.3 and 118.6 ppm), which indicated that the anion in the salts exists isolated in the solution.

N

O N

N O N O

O 2N

NO2 O 2N

N

KOH

O N

N

O 2N

NO 2

K

NO2 O 2N

K

NO 2

2. MCl

K

2 N O N

NO2 O 2N

M

NH 2

NH2 NH 3

M=

O O 2N

1. AgNO3

O

1 O N

N O N

H2 N

NO 2

3

NH 2

NHNH2 H 2 NHN 4

NHNH2 5

Scheme 1.

The 13C NMR signals of gem-dinitro group carbon (118.6 ppm) appeared at higher field compared to the ones in literature (129 to 132 ppm) [12], which resulted from the conjugation of negative charge throughout the aromatic rings. 3.2. Crystal structure  with calCompound 3 crystallizes in the triclinic space group P1 culated density of 1.964 g cm–3. The asymmetric unit contains one crystallographically independent OBNF2 anion, one hydrazidinium (NH2NH+3) cation and one potassium cation, which could be observed in Fig. 1(a). As shown in Fig. 1(b), potassium ion is hepta-coordinated and connects with six adjacent OBNF2 anions through six K–O coordination bonds [K(1)–O(11)#1; K(1)–O(4)#1; K(1)–O(4)#2; K(1)–O(2); K(1)–O(7)#3; K (1)–O(9)#4, 2.740– 3.116 Å] and one K–N bonds [K(1)–N(5)#5; 3.128 Å] (Table 2). The atoms involved coordination formed a distorted decahedron with potassium ion as coordinate center, atom O11#1 and O4#1 in the axis position and other five atoms at the equatorial plane. Furthermore, the weak K–N interaction (3.226 and 3.346 Å), K–K interaction [K(1)–K(1)#6, 4.356 Å], intra- and/or intermolecular N–H  O hydrogen bonding (Table 3) and p–p (3.672 Å) interactions between the neighboring molecules can be observed.

Fig. 1. (a) Displacement ellipsoid plot of 3 (30%). (b) Coordination environment of K+ ion in 3.

Table 2 Selected bond distances (Å) for complexes. Complex 3 K(1)–O(11)#1 K(1)–O(4)#1 K(1)–O(4)#2 K(1)–O(2) K(1)–O(7)#3 K(1)–O(9)#4 K(1)–N(5)#5 K(1)–N(4)#5 K(1)–N(3) K(1)–K(1)#6

Complex 4 2.740(2) 2.850(2) 2.865 (2) 3.013(2) 3.061 (2) 3.116(2) 3.128(2) 3.226(2) 3.346(3) 4.356(1)

K(1)–O(4)#i K(1)–O(8)#ii K(1)–O(10)#i K(1)–N(5) #ii K(1)–O(9) K(1)–O(3)#i K(1)–N(4) K(1)–N(3)#iii K(1)–N(8)#ii K(1)–O(11)#i K(1)–N(2)#i

Complex 5 2.868(2) 2.883(2) 2.893(2) 2.942(2) 2.956(2) 3.037(2) 3.040(2) 3.117(2) 3.300(2) 3.311(2) 3.358(2)

K(1)–O(4)#a K(1)–N(12) K(1)–O(11)#b K(1)–O(10) K(1)–O(9) K(1)–N(3) K(1)–O(3) K(1)–N(13) K(1)–O(8)#a K(1)–N(17) K(1)–N(8)#b K(1)–N(2)#a

2.806(3) 2.908(3) 2.920(3) 2.940(3) 3.042(3) 3.064(3) 3.094(3) 3.103(3) 3.164(3) 3.266(4) 3.302(3) 3.365(3)

K(2)–O(19)#c K(2)–N(5)#d K(2)–O(13) K(2)–O(12) K(2)–O(14)#c K(2)–N(14)#e K(2)–O(20)#c K(2)–N(4)#d K(2)–O(15)#c K(2)–N(26)#f K(2)–N(9) K(2)–N(15)#c

2.812(3) 2.910(3) 2.926(3) 2.947(3) 3.036(3) 3.054(3) 3.105(3) 3.106(3) 3.158(3) 3.259(4) 3.310(3) 3.372(3)

Symmetry codes: (#1) x, y  1, z; (#2) x + 1, y + 1, z; (#3) x + 1, y  1, z; (#4) x, y + 1, z + 1; (#5) x, y + 1, z; (#6) –x + 1, y, z; (#i) x  1/2, y + 3/2, z  1/2; (#ii) x + 1/2, y + 3/2, z  1/2; (#iii) x + 1/2, y  1/2, z + 1/2; (#a) x + 1,y,z; (#b) x,y,z; (#c) x + 1,y,z; (#d) x + 1,y,z + 1; (#e) x + 1,y + 1,z + 1; (#f) x + 2,y,z + 1.

H. Li et al. / Inorganica Chimica Acta 423 (2014) 256–262 Table 3 Hydrogen bonding distances (Å) and bond angles (°) for compound 3, 4 and 5. D–H  A

D–H

H  A

D  A

D–H  A

Compound 3 N9–H9A  O10#1 N10–H10A  O8#2 N10–H10B  O1 N10–H10C  O1#3 N10–H10C. . .O3#3

0.86 0.83 0.84 0.82 0.82

2.23 2.06 2.06 2.34 2.06

2.984(4) 2.837(3) 2.894(3) 2.993(3) 2.785(3)

147 157 171 138 148

Compound 4 N9–H9A  N11 N9–H9B  O8#i N9–H9B  O11#ii N10–H10  O8#i N11–H11A  O9#iii N11–H11B  N9 N12–H12A  O10#iii N12–H12BO2#iv

0.86 0.86 0.86 0.86 0.86 0.86 0.86 0.86

2.34 2.56 2.52 2.43 2.35 2.29 2.29 2.23

2.637(3) 2.973(3) 3.049(3) 2.900(3) 2.969(3) 2.637(3) 3.002(3) 2.919(3)

101 110 121 115 129 104 140 138

Compound 5 N17–H17A  N19 N18–H18  N21 N19–H19A  N17 N19–H19B  O20#a N19–H19BO21#a N20–H20  O14#a N21–H21A  N18 N22–H22A  N25 N23–H23O9#b N24–H24AN26 N24–H24B  O2#b N24–H24B  O2#b N25–H25N22 N26–H26A  N24 N26–H26B  O15#c

0.86 0.86 0.86 0.86 0.86 0.86 0.86 0.86 0.86 0.86 0.86 0.86 0.86 0.86 0.86

2.37 2.33 2..33 2.17 2.46 2.10 2.36 2.36 2.09 2.33 2.47 2.17 2.32 2.37 2.42

2.665(4) 2.660(5) 2.665(4) 2.900(4) 3.037(5) 2.851(5) 2.660(5) 2.657(5) 2.841(5) 2.665(4) 3.040(5) 2.985(4) 2.657(5) 2.665(4) 3.047(4)

101 103 104 158 125 146 101 101 146 103 125 158 103 101 130

Symmetry codes: (#1) x, 1 + y, z; (#2) x, 2  y, 1  z; (#3) 1  x, 1  y, 1  z; (#i) x, 1  y, 1  z; (#ii) 1  x, 1  y, 1  z; (#iii) 1/2  x,  1/2 + y, 3/2  z; (#iv) x,  1 + y, z; (#a) 1  x, 1  y, 1  z; (#b) 1  x, y, z; (#c) 1 + x, y, z.

As to the anion, the bridged oxygen atom is nearly coplanar with both furazan rings (torsion angle O6–C4–C5–N6 178.307°, O6–C3–C2–N3 179.184°), and the dihedral angle between two furazan rings is 18.604°, which is smaller than those of the reported oxy-bridged furazan structures [6e]. The length of the C–C bond connecting gem-dinitro and furazan ring is 1.46 Å, a value between C–C single bond (1.53 Å) and C@C double bond (1.32 Å) [13]. This might indicate conjugation of negative charge throughout the aromatic rings. Compound 4 crystallizes in the monoclinic space group P21/n with calculated density of 1.844 g cm3. It could be seen from

259

Fig. 2 that the asymmetric unit contains one crystallographically independent OBNF2 anion, one aminoguanidnium cation and one potassium cation. And potassium is octa-coordinated with four adjacent anions through five K–O bonds [K(1)–O(4)#i, K(1)–O(8)#ii, K(1)–O(10)#i, K(1)–O(9)#ii, K(1)–O(3)#i; 2.868–3.037(2) Å] and three K–N bonds [K(1)–N(5), K(1)–N(4), K(1)–N(3)#iii; 2.942– 3.117 Å]. Similar to compound 3, the atoms involved also formed a distorted dodecahedron with atoms O4#i and N4 at the ends of the dodecahedron (Table 2). While, according to the selected bond lengths and bond angles, is a badly distorted dodecahedron. In addition, weak K–N (3.300 and 3.358 Å), K–O (3.311 Å) interactions, associated with N–H  N or N–H  O hydrogen bond interactions were also could be observed (Table 3). It could be seen that the anion in compound 4 have similar structure with the compound 3. Compound 5 crystallizes in the monoclinic space group P21/c with calculated density of 1.866 g cm3. Unlike the above-mentioned two compounds, the asymmetric unit of 5 contains two identical OBNF2 anions, two aminoguanidnium cations and two potassium cations (Fig. 3(a)), and both potassium ions are enneacoordinated with four adjacent OBNF2 anions through six K–O bonds, three K–N coordination bonds (Table 2). The atoms formed a distorted tetrakaidecahedrons. Samely to 3 and 4, weak K–N, N–H  N or N–H  O hydrogen bond interactions were observed (Table 3). The anion in compound 5 have similar structure with the compound 3 and 4, besides the dihedral angle between two furazan rings (16.128°) is slightly smaller than 3 (18.604) and 4 (18.013°). 3.3. Thermal decomposition behavior The thermal decomposition temperatures (Td) of 3, 4 and 5 were determined by DSC at heating rate of 10 °C/min, and no melting point was observed except 5. The peak temperatures of the three energetic salts range from 187.59 (4) to 213.10 °C (3). Typical DSC and TG curves (see Fig. 4) indicate that 3 has one intense exothermic process at the range of 190.81 to 209. 39 °C and the mass loss is about 88.20%, whereas 4 and 5 have an intense exothermic process and a continuous decomposition processes as a shoulder near the main peak. For 4 and 5, the mass loss of the first decomposition processes are 80.31% and 59.14% and the second decomposition processes are 8.59% and 13.28%, respectively. The decomposition process of 5 (as a representative example) was studied by the thermolysis in-situ rapid-scan FTIR. As shown in Fig. 5, the absorption peaks of –NH2 and –NH– decrease at

Fig. 2. (a) Displacement ellipsoid plot of 4 (30%). (b) Coordination environment of K+ ion in 4.

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Fig. 3. (a) Displacement ellipsoid plot of 5 (30%). (b) Coordination environment of K (1)+ ion in 5. (c) Coordination environment of K (2)+ ion in 5.

3 4 5

160

213.10

140

120

159.09 100%

100

174.48 100% 190.81 100%

3 4 5

100

80

80

Weight (%)

Heat flow (mW/mg)

120

60

199.21

40

222.47 40.86%

40

187.59

20

60

182.97 19.69%

243.27 11.10%

20

0

460.81 27.58%

209.39 11.80%

-20 50

100

150

200

Temperature

250

300

350

0 0

100

200

Temperature

Fig. 4. DSC and TG curves of 3, 4 and 5 at a heating rate of 10.0 °C min1.

300

400

500

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H. Li et al. / Inorganica Chimica Acta 423 (2014) 256–262

includes two stages. One is a rapid decomposition process caused by the decomposition of gem-dinitro group and diaminoguanidnium cation, and the other is a slow decomposition process attributed to the decomposition of oxy-bridged bisfurazan fragment.

450

225

Absorbance

3.4. Non-isothermal decomposition kinetics The thermal stabilities of the OBNF-based energetic salts were evaluated by DSC. The heating flow rates were 5, 7.5, 10, and 12.5 °C min1. Fig. 6 shows the DSC curves of 4 as a representative example. The DSC curves show an asymmetric character and the peak temperature (Tp) shift to higher temperature with the increase of heating rate. In order to obtain the kinetic parameters (the apparent activation energy (E) and pre-exponential constant (A)) of the first exothermic decomposition process, a multiple heating method (Kissinger method [14] and Ozawa method [15]) was employed. As shown in Table 4, the apparent activation energy obtained by Kissinger’s method agree well with that obtained by Ozawa’s method, thus resulting in very close linear correlation coefficients (r).

175

25

3500

3000

2500

2000

1500

1000

500

Wavenumber/cm-1 Fig. 5. IR spectra of the condensed products at different temperatures during thermal decomposition process of 5.

3.5. Thermodynamic parameters of activation reaction and the critical temperature of thermal explosion

50 191.53

40

Heat Flow (W/g)

5 K/min 7.5K /min 10 K/min 12.5 K/min

The values of Tp0 in the stage corresponding to b ? 0 obtained by Eq. (1) [16] were also listed in Table 4.

30 187.59

20

186.08

10

181.93

T pi ¼ T ðe0 or p0Þ þ bbi þ cb2i ði ¼ 1; 2; 3; 4Þ

where b and c are coefficients. The activation entropy (DS–), activation enthalpy (DH–) and activation Gibbs free energy (DG–) corresponding to T = Tp0, A = Ak and E = Ek obtained by Eqs. (2–4) [16] are also listed in Table 3.

0 -10

A¼ 50

100

ð1Þ

150

200

250

Temperature (

300

350

)

Fig. 6. The DSC curves of 4 at different heating rates.

kB T DS– =R e h

ð2Þ

DH– ¼ E  RT

ð3Þ

DG– ¼ DH–  T DS–

ð4Þ

where KB is the Boltzmann constant, h is the Planck constant. The critical temperature of thermal explosion (Tb) is an important parameter required to insure safe storage and process operations for energetic materials and then to evaluate the thermal stability. Tb can be obtained by Eq. (5) [17].

175 °C while the other characteristic groups have no obvious change. When the temperature rises to 225 °C, the absorption peaks of furazan ring and C–O–C decrease while the absorption peaks of –NH2, –NH– and –NO2 almost disappear. At 450 °C, the absorption peak of –CN appears in 2165 cm1, which may be caused by the fact that HCN is absorbed by the KBr to form KCN. It can be concluded that the thermal decomposition process of 5

Tb ¼

Eo 

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi E2o ¼ 4Eo RT e0

ð5Þ

2R

Table 4 Thermal decomposition kinetic parameters and thermodynamic parameters.

a b c d e f g h i j

Salt No.

Eka (kJ mol1)

Log A

3 4 5

255.06 164.72 153.80

25.73 16.82 15.13

b

(s1)

rk

c

0.994 0.982 0.996

Eo

d

(kJ mol1)

250.21 163.91 153.69

Apparent activation energy obtained using Kissinger’s method. Pre-exponential constant. Linear correlation coefficient obtained using Kissinger’s method. Apparent activation energy obtained using Ozawa’s method. Linear correlation coefficient obtained using Ozawa’s method. The mean value of the apparent activation energy by two methods. Peak temperature corresponding to b ? 0. Activation entropy. Activation enthalpy. Activation Gibbs free energy.

ro

e

0.995 0.984 0.996

EK

f

252.64 164.32 153.75

Tp0

g

(K)

475.8 448.4 457.1

DS–h (J mol1 K1)

DH– i (kJ mol1)

DG–j (kJ mol1)

235.46 65.38 32.87

251.10 160.99 149.99

139.07 131.68 134.98

262

H. Li et al. / Inorganica Chimica Acta 423 (2014) 256–262

Chemistry Research Institute, China) is acknowledged for his support on the language of the manuscript and valuable suggestions.

Table 5 Sensitivity date of 2–5 (impact and friction test). Salt No.

2

3

4

5

Impact sensitivity (J) Friction sensitivity (N)

<2 96

6 128

15 120

4 160

The values of Tb for 3, 4 and 5 are 483.42, 459.04 and 468.99 K respectively, which indicate that 3 has the best thermal stability. All of above mentioned data provided a basis for the evaluation of the thermal stability. 3.6. Sensitivity The impact and friction sensitivities for 2–5 were determined according to the standard BAM methods and the results are displayed in Table 5. 3, 4 and 5 are classified as sensitive compounds towards impact with 6, 15 and 4 J, respectively.1 Values of 128 N (3), 120 N (4) and 160 N (5) show that 3–5 are sensitive towards friction. Considering that the introduction of nitrogen-rich cations increase intra and intermolecular hydrogen bonds, it is no surprise that 3–5 all are less sensitivity (impact and friction) than parent molecule 2. Furthermore, 4 is the most insensitive one toward impact which possesses impact sensitivity similar with TNT (15 J) [18] and 5 is the most insensitive one toward friction. 4. Conclusions (1) Three high-energy salts with OBNF as anion have been prepared. And corresponding structures were fully investigated and characterized. The X-ray diffraction results showed that the coordination environments around potassium cation were hepta-, octa- and ennea-coordinated for the crystals of 3, 4 and 5, respectively. And there existed abundance of K-N and K-O weak bonds associated with hydrogen bonding interactions. (2) The apparent activation energies of the main exothermic decomposition reaction for 3, 4 and 5 were 252.64, 164.32 and 153.75 kJ mol1, and the pre-exponential constants were 1025.71, 1016.81, and 1015.18 s1, respectively. The critical temperature of thermal explosion for 3, 4 and 5 were 483.42, 459.04 and 468.99 K, respectively. (3) All results indicate that 3 has better thermal stability than 4 and 5, which results from smaller ionic radius of hydrazidinium cation and the stronger non-covalence interaction between OBNF anion and hydrazidinium cation. The decomposition of 5 begins with the gem-dinitro group and diaminoguanidnium cation and then the oxy-bridged bisfurazan fragment. The above-mentioned properties make them potential energetic flame suppressors in propellant.

Acknowledgements This work was supported by the National Natural Science Foundation of China (21373157). Prof. Jian-Hua Bu (Xi’an Modern

1 Impact: Insensitive >40 J, less sensitive P35 J, sensitive P4 J, very sensitive 63 J; friction: insensitive >360 N, less sensitive = 360 N, sensitive <360 N and >80 N, very sensitive 680 N, extremely sensitive 610 N. According to the UN Recommendations on the Transport of Dangerous Goods.

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