English

• 1. INTRODUCTION

  Research in the field of energetic materials is nowadays directed toward the synthesis of simple molecules with high energy, high density, high heat resistance, and low sensitivity^[1]. Recently, considerable attention has been paid in the study of triazole and its derivatives as ligands to metals due to their varied structures and energetic properties^[2-5]. As one of the derivatives of triazole, 3, 5-bis(3-pyridyl)-1H-1, 2, 4-triazole (3, 3΄-Hbpt) serves as a N, N´-donor ligand and acts as a bridging ligand, thus mediating the exchange coupling^[6]. Furthermore, the prototropy and conjugation between the 1H-1, 2, 4-triazole and pyridyl groups alter the electron density in different sections of the molecules, making the ligand more flexible^[7]. Many coordination compounds about Hbpt are extensively studied for their fascinating architectures and potential applications in magnetism, photocatalysis and photoluminescence^[8-12]. However, the potential applications in energetic materials based on Hbpt are rarely reported^[13-15].

  Ammonium perchlorate (AP) is one of the common oxidizers in composite solid propellants and the thermal deposition characteristics of AP directly influence the combustion behavior of such solid propellants^[16]. The reaction rate and pyrolysis temperature of the thermal decomposition of AP are closely related to the combustion rate of the solid propellants^[17]. In order to study the effects of compounds containing Hbpt on the thermal decomposition of AP, we choose cobalt(II) ion as the coordination center and trimesic acid (H₃tm) as an auxiliary ligand on the basis of the following considerations: (1) The formation of cobalt(II)/cobalt oxide at the molecular level shows good catalytic performance for the decomposition of propellants^[18]; (2) The cobalt(II) is more environmentally friendly compared with the toxic heavy metal cations such as lead(II) and mercury(II)^[19]; (3) H₃tm can improve the oxygen balance of propellant components^[20].

  In this paper, we report the synthesis and structure of a new cobalt(II) coordination compound with 3, 3΄-Hbpt and H₃tm, namely, [Co(3, 3΄-Hbpt)(Htm)]·H₂O, which is confirmed by single-crystal X-ray diffraction and characterized by EA, IR, TGA and XPRD.

  Furthermore, the catalytic kinetic performance of the title complex in the thermal decomposition of AP is explored, which indicates that it is a good candidate for a promoter of the thermal decomposition of ammonium perchlorate.

  2. EXPERIMENTAL

  2.1. Materials and measurements

  Commercially available reagents were used as received without further purification. Elemental analyses (C, H and N) were performed on a Vario EL III analyzer. Infrared spectra were obtained using KBr pellets on a BEQ VZNDX 550 FTIR instrument within the 400～4000 cm^-1 region. ¹H NMR spectra were recorded on a Varian Inova 400 instrument using tetramethylsilane (TMS) as an internal standard.

  Thermal analyses were performed on a NETZSCH STA 449C instrument under an atmosphere of hydrostatic air at a heating rate of 10 ℃·min^-1. X-ray powder diffraction (XPRD) was received on a Bruker Advance-D8 instrument at room temperature. Differential scanning calorimetry (DSC) experiments were performed on a Perkin-Elmer Pyris DSC 4000 thermal analyzer (calibrated using pure indium and zinc as standards) from 50 to 490 ℃.

  2.2. Synthesis of 3, 3´-Hbpt

  3, 3΄-Hbpt was synthesized according to literature^[21]. Anal. Calcd. for C12H9N5 (%): C, 64.56; H, 4.06; N, 31.37. Found (%): C, 64.61; H, 4.10; N, 31.42. m.p: 232～233 ℃. IR (cm^-1, KBr): 3327(s), 3156(s), 3048(m), 2843(m), 1620(s), 1594(s), 1482(w), 1406(s), 1308(s), 1196(w), 1051(m), 821(m), 709(m), 625(w). ¹HNMR (400 MHz, DMSO-d6, δ ppm): 7.534～7.552 (2H, m, J = 7.2 Hz), 8.408～8.427 (2H, d, J = 7.6 Hz), 8.723～ 8.743 (2H, dd, J = 8.0 Hz), 9.272(2H, s), 14.926 (1H, s, triazole-H).

  2.3. Synthesis of [Co(3, 3´-Hbpt) (Htm)]∙H₂O (1)

  A mixture containing Co(OAc)₂·4H₂O (14.9 mg, 0.06 mmol), 3, 3΄-Hbpt (11.2 mg, 0.05 mmol), H₃tm (10.5 mg, 0.05 mmol) and water (7 mL) was sealed in a 15 mL Teflon-lined stainless steel vessel and heated at 140 ℃ for 3 days, and then cooled to room temperature at a rate of 5 ℃·h^-1. Purple prism crystals of 1 were collected in a yield of 42% (based on 3, 3΄-Hbpt). Anal. Calcd. for 1 (C21H15CoN5O7) (%): C, 49.62; H, 2.97; N, 13.78. Found (%): C, 49.73; H, 3.06; N, 13.94. IR (cm^-1, KBr): 3415(s), 1624(s), 1586(m), 1543(m), 1475(w), 1438(m), 1417(m), 1372(s), 1135(s), 1049(m), 990(m), 818(s), 764(m), 751(m), 738(m), 699(s), 638(m), 616(m), 534(w), 411(m).

  2.4. X-ray crystallography

  All diffraction data for compound 1 were collected on a Bruker SMART APEX II CCD diffractometer equipped with graphite monochromated Mo-Kα radiation (λ = 0.71073 Å) at 293(2) K. Absorption corrections were applied using SADABS. All structures were solved by direct methods using SHELXS-97 and refined with full-matrix leastsquares refinements based on F² using SHELXL-97. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed in the geometrically calculated positions. For 1, a total of 6218 reflections were collected in the range of 2.49≤θ≤25.00°, of which 3429 were independent (Rint = 0.0589). The final R = 0.0523 and wR = 0.0935 for observed reflection with I > 2σ(I), R = 0.0788 and wR = 0.1101 for all data with (Δρ)max = 0.400 and (Δρ)min = -0.445 e·Å-3. Selected bond lengths and bond angles for 1 are summarized in Table 1. Hydrogen bonding geometry for the title complex is collected in Table 2.

  Table 1

  

  Table 1.  Selected Bond Lengths and Bond Angles for Complex 1

  DownLoad: CSV

  Bond	Dist.	Bond	Dist.
  Co(1)–O(2)i	2.009(3)	Co(1)–O(3)ii	2.143(3)
  Co(1)–O(1)	2.029(3)	Co(1)–N(5)	2.162(4)
  Co(1)–N(4)	2.189(4)	Co(1)–O(4)ii	2.317(3)
  Angle	(°)	Angle	(°)
  O(2)i–Co(1)–O(1)	124.43(11)	O(3)ii–Co(1)–N(5)	92.85(12)
  O(2)i–Co(1)–O(3)ii	86.55(11)	O(2) i–Co(1)–N(4)	89.46(13)
  O(1)–Co(1)–O(3)ii	148.67(11)	O(1)–Co(1)–N(4)	87.89(12)
  O(2)i–Co(1–N(5)	89.01(13)	O(3)ii–Co(1)–N(4)	87.81(12)
  O(1)–Co(1)–N(5)	92.35(12)	N(5)–Co(1)–N(4)	178.29(12)
  C(16)–N(4)–Co(1)	125.7(3)	C(17)–N(5)–Co(1)	116.2(3)
  C(12)–N(4)–Co(1)	117.2(3)	C(21)–N(5)–Co(1)	126.5(3)
  C(17)–N(5)–C(21)	117.3(4)	C(1)–O(1)–Co(1)	121.8(3)
  O(2)i–Co(1)–O(4)ii	144.94(11)	N(4)–Co(1)–O(4)ii	90.98(11)
  O(1)–Co(1)–O(4)ii	90.61(10)	C(1)–O(2)–Co(1)i	164.9(3)
  O(3)ii–Co(1)–O(4)ii	58.46(10)	C(2)–O(3)–Co(1)iii	94.7(2)
  N(5)–Co(1)–O(4)ii	90.71(11)	C(2)–O(4)–Co(1)iii	86.5(2)
  Symmetry codes: i: -x+1, -y+1, -z+1; ii: x+1, y, z; iii: x-1, y, z

  Table 2

  

  Table 2.  Hydrogen Bond Len gths (Å) and Bond Angles (°)

  DownLoad: CSV

  D–H∙∙∙A	d(D–H)	d(H∙∙∙A)	d(D∙∙∙A)	∠DHA
  O(7)–H(7D) ∙∙∙O(4)iv	0.85	2.04	2.882(4)	169.5
  O(7)–H(7C)∙∙∙O(4)v	0.85	2.08	2.916(4)	169.6
  O(6)–H(6)∙∙∙N(3)vi	0.82	2.06	2.875(4)	171.3
  N(2)–H(2)∙∙∙O(7)vii	0.86	1.87	2.721(5)	167.7

  3. RESULTS AND DISCUSSION

  3.1. Structural description

  Asymmetric unit of 1 comprises one crystallographically independent Co(II) ion, one 3, 3´-Hbpt, one Htm²-and one lattice water molecule. Each Co(II) ion is bound to four O atoms from three Htm²-in the equatorial plane and two N atoms from two 3, 3΄-Hbpt ligands at the axial position, displaying a slightly distorted octahedral geometry (Fig. 1). The Co-O bond lengths range from 2.009(3) to 2.317(3) Å and Co-N from 2.162(4) to 2.189(4) Å, which are all in normal ranges^{[22, 23]}. The two deprotonated carboxylic groups in the Htm²-ligands show different coordination modes. One bidentately bridges two Co(II) ions to form an 8-membered ring and the other adopts a chelating-bridging mode linking two Co(II) ions to form a 25-membered ring. Then, the adjacent 8-and 25-membered rings are connected into 1D ladder-like chains along the [010] direction (Fig. 2). Furthermore, the resulting 1D chains are linked by 3, 3΄-Hbpt at a nearly vertical position to form 1D cages (Fig. 3). Afterwards, the molecular packing is further interconnected by O(7)- H(7D)···O(4)^iv and O(7)-H(7C)···O(4)^v to constitute 2D plains, and these plains are cross-linked by O(6)-H(6)···N(3)^vi and N(2)-H(2)···O(7)^vii (iv = -x, -y+1, -z+1; v = x+1, y+1, z; vi = x-1, y-1, z; vii = x+1, y, z+1) to create the ultimate 3D supramolecular architecture (Fig. 4).

  Figure 1

  

  Figure 1.  Coordination environment of Co²⁺ in 1 (Lattice water and H atoms are omitted for clarity). Symmetry codes: i = -x+1, -y+1, -z+1

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  Figure 2

  

  Figure 2.  1D Co ladder-like chains along the [010] direction in 1

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  Figure 3

  

  Figure 3.  1D cage-like structure of 1

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  Figure 4

  

  Figure 4.  Hydrogen bonds forming a 3D framework

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  3.2. IR analysis of compound 1

  As shown in Fig. 5, the IR spectrum of 1 shows a broad absorption band at 3415 cm^-1, corresponding to the O-H stretching of the lattice water molecule in the complex^[23]. The C=C and C-O absorption bands can be observed at 1624 and 1372 cm^-1, respectively. The characteristic bands of asymmetric stretching vibrations of carboxylate group appear at 1586 and 1543 cm^-1, and the symmetric ones at 1438 and 1417 cm^{-1[24, 25]}. The IR spectrum is in agreement with the crystal structure of compound 1.

  Figure 5

  

  Figure 5.  IR of complex 1

  DownLoad: Full-Size Img PowerPoint

  3.3. Thermal decomposition

  process of the complex Thermogravimetric analyses (TGA) were conducted from 30 to 900 ℃ at a heating rate of 10 ℃·min^-1 in air atmosphere. As shown in Fig. 6, the complex exhibits two main steps of weight loss. The first weight loss of 3.43% in the range of 45～223℃ corresponds to the expulsion of the lattice water molecule (calcd. 3.54%). The second step of 238～ 849 ℃ is due to the release of 3, 3´-Hbpt and H₃tm, giving cobalt oxides as the final decomposition product with residue percent of 15.05% (calcd.14.74%). In addition, to confirm the stable framework of complex 1, the original sample and processed samples with the lattice water molecule removed were characterized by X-ray powder diffraction (XRPD) at room temperature. As shown in Fig. 7, the processed samples resulted in a slightly broadened XRPD pattern with similar peak positions to that of the original sample, which shows that the crystallinity of 1 is retained. Fig. 8 shows XRPD patterns of the residual. It is seen that all diffraction peaks are in good agreement with the standard diffraction data for CoO (JCPDS card file No.1-1227), while no diffractions of the impurities Co₂O₃ or Co₃O₄ are observed.

  Figure 6

  

  Figure 6.  TGA curve of complex 1

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  Figure 7

  

  Figure 7.  Comparison of XRPD patterns of the original sample and the desolvated phases

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  Figure 8

  

  Figure 8.  Comparison of the XRPD patterns of the standard data for CoO and experimental data for the residual

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  3.4. Effects on thermal decomposition of AP

  The performance of complex 1 on the thermal decomposition of AP (complex 1 and AP were mixed at a mass ratio of 1:3) was investigated by DSC measurement at a heating rate of 10 ℃·min^-1 in N₂ atmosphere from 50 to 490 ℃ with Al₂O₃ as reference. The total sample mass used was less than 1.0 mg for all runs. Fig. 9 shows the DSC curves of both AP and the mixture of AP with the compound. The endothermic peak of AP at 245 ℃ is due to the crystal transformation of AP from orthorhombic to cubic phase^[26]. The exothermic peaks at 288 and 448 ℃ are attributed to partial decomposition of AP to form some intermediate products and then complete decomposition to volatile products corresponding to the heat of 0.735 and 0.787 kJ·g^-1, respectively.

  Figure 9

  

  Figure 9.  DSC curves for AP and AP with complex 1 at a heating rate of 10 ℃·min⁻¹

  DownLoad: Full-Size Img PowerPoint

  After adding the mixture of AP with 1, there are no significant impacts on the phase transition of AP. While noticeable change can be seen in the exothermic phase. The exothermic peak appearing in the region 250～490 ℃ for pure AP becomes narrowed which appear in the region 288～375 ℃, which indicates that AP decomposes in a short time with the presence of complex 1 at same heating rate. What’s more, the decomposition heat changes to 2.531 kJ·g^-1, significantly higher than pure AP. Obviously, AP is completely decomposed in a shorter time and released much heat in presence of the title complex. It can be inferred that the title complex decomposes and releases heat itself which enhances the total heat of the mixture, as well as the formation of metal at the molecular level on the propellant surface which may contribute toward the catalytic effect ^{[27, 28]}.

  It is known that the decomposition temperature is related with the heating rate and the relationship can be described by Kissinger's correlation, as shown in Eq. 1. In this work, the Kissinger's method is used to investigate the activation energy (Ea) and pre-exponential factor (A) of thermal decomposition for AP with complex 1 by DSC measurement at four different heat rates of 5, 10, 15 and 20 ℃·min^-1 (Fig. 10).

  1 $\text{ln }\frac{\beta }{T_{p}^{2}}\text{ }=\text{ ln }\frac{AR}{{{E}_{a}}}\text{ }-\text{ }\frac{{{E}_{a}}}{R{{T}_{p}}}$ $

  Figure 10

  

  Figure 10.  DSC curves of AP with additives at various heating rates

  DownLoad: Full-Size Img PowerPoint

  where β is the heating rate, R the gas constant and T_p the peak temperature. As shown in Table 2, the calculated activation energy Ea of pure AP is 74.65 kJ·mol⁻¹. With the presence of the complex, Ea is changed to 81.83 kJ·mol⁻¹. It is contrary to the generally observed trend of lowering Ea for a reaction whose rate is increased by a catalyst. This discrepancy may be attributed to the facts like strong interaction and binding between AP and the title complex, and secondly along with Ea the pre-exponential factor (A) also showed variations. In a reaction catalyzed by a catalyst, there is an increase in reactant concentration at the catalyst surface, as confirmed earlier by observing high heat release^[29]. Thus, the reaction gets acceleration through a relatively higher A in the presence of 1 (ln(A) = 7.36) compared to pure AP (ln(A) = 5.71) (Table 3). As a result, a direct correlation of Ea with the reaction rate becomes difficult since both Ea and A are altered in the reaction. Thus the pre-exponential factor plays an important role in increasing the reaction rate of the thermal decomposition of AP.

  Furthermore, the increase in activation energy and the corresponding increase in the A value are due to the kinetic compensation effect as reported^[30]. The ratio of Ea/ln(A) could be used to describe the reactivity^[31]. Usually, a bigger ratio means a greater stability of the reactant. The ratios of Ea/ln(A) are 13.07 and 11.05 for pure AP and the mixture, respectively. The title complex shows good catalytic activity toward AP decomposition, which indicates the potential application in solid propellants.

  Table 3

  

  Table 3.  Kinetic Parameters of Thermal Decomposition for AP and AP with Complex 1 (ΔH Were Given at the Heating Rate of 10 ℃·min^-1 )

  DownLoad: CSV

  	ΔH/kJ·g-1	Ea/kJ·mol-1	lnA	Ea/lnA
  AP	0.735	74.65	5.71	13.07
  AP+1	2.531	81.83	7.36	11.05

  4. CONCLUSION

  In summary, the title complex has been successfully synthesized, and the structure was determined by single-crystal X-ray diffraction. Complex 1 exhibits a 1D cage structure. The catalytic kinetic performance reveals that the complex accelerates the thermal decomposition of AP. The present study projects the probable application of the title complex in a solid propellant field.

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  Figure 1  Coordination environment of Co²⁺ in 1 (Lattice water and H atoms are omitted for clarity). Symmetry codes: i = -x+1, -y+1, -z+1

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  Figure 2  1D Co ladder-like chains along the [010] direction in 1

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  Figure 3  1D cage-like structure of 1

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  Figure 4  Hydrogen bonds forming a 3D framework

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  Figure 5  IR of complex 1

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  Figure 6  TGA curve of complex 1

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  Figure 7  Comparison of XRPD patterns of the original sample and the desolvated phases

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  Figure 8  Comparison of the XRPD patterns of the standard data for CoO and experimental data for the residual

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  Figure 9  DSC curves for AP and AP with complex 1 at a heating rate of 10 ℃·min⁻¹

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  Figure 10  DSC curves of AP with additives at various heating rates

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  Table 1.  Selected Bond Lengths and Bond Angles for Complex 1

  Bond	Dist.	Bond	Dist.
  Co(1)–O(2)i	2.009(3)	Co(1)–O(3)ii	2.143(3)
  Co(1)–O(1)	2.029(3)	Co(1)–N(5)	2.162(4)
  Co(1)–N(4)	2.189(4)	Co(1)–O(4)ii	2.317(3)
  Angle	(°)	Angle	(°)
  O(2)i–Co(1)–O(1)	124.43(11)	O(3)ii–Co(1)–N(5)	92.85(12)
  O(2)i–Co(1)–O(3)ii	86.55(11)	O(2) i–Co(1)–N(4)	89.46(13)
  O(1)–Co(1)–O(3)ii	148.67(11)	O(1)–Co(1)–N(4)	87.89(12)
  O(2)i–Co(1–N(5)	89.01(13)	O(3)ii–Co(1)–N(4)	87.81(12)
  O(1)–Co(1)–N(5)	92.35(12)	N(5)–Co(1)–N(4)	178.29(12)
  C(16)–N(4)–Co(1)	125.7(3)	C(17)–N(5)–Co(1)	116.2(3)
  C(12)–N(4)–Co(1)	117.2(3)	C(21)–N(5)–Co(1)	126.5(3)
  C(17)–N(5)–C(21)	117.3(4)	C(1)–O(1)–Co(1)	121.8(3)
  O(2)i–Co(1)–O(4)ii	144.94(11)	N(4)–Co(1)–O(4)ii	90.98(11)
  O(1)–Co(1)–O(4)ii	90.61(10)	C(1)–O(2)–Co(1)i	164.9(3)
  O(3)ii–Co(1)–O(4)ii	58.46(10)	C(2)–O(3)–Co(1)iii	94.7(2)
  N(5)–Co(1)–O(4)ii	90.71(11)	C(2)–O(4)–Co(1)iii	86.5(2)
  Symmetry codes: i: -x+1, -y+1, -z+1; ii: x+1, y, z; iii: x-1, y, z

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  Table 2.  Hydrogen Bond Len gths (Å) and Bond Angles (°)

  D–H∙∙∙A	d(D–H)	d(H∙∙∙A)	d(D∙∙∙A)	∠DHA
  O(7)–H(7D) ∙∙∙O(4)iv	0.85	2.04	2.882(4)	169.5
  O(7)–H(7C)∙∙∙O(4)v	0.85	2.08	2.916(4)	169.6
  O(6)–H(6)∙∙∙N(3)vi	0.82	2.06	2.875(4)	171.3
  N(2)–H(2)∙∙∙O(7)vii	0.86	1.87	2.721(5)	167.7

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  Table 3.  Kinetic Parameters of Thermal Decomposition for AP and AP with Complex 1 (ΔH Were Given at the Heating Rate of 10 ℃·min^-1 )

  	ΔH/kJ·g-1	Ea/kJ·mol-1	lnA	Ea/lnA
  AP	0.735	74.65	5.71	13.07
  AP+1	2.531	81.83	7.36	11.05

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