English

• 1. INTRODUCTION

  With the development of chemistry^[1-3], coordination polymers (CPs) due to their charming structures and wide applications in proton conduction^[4], magnetism^[5], conductivity^[6], heterogeneous catalysis^[7], gas storage and separation^{[8, 9]} and luminescence detection have attracted more attention^{[10, 11]}. For coordination polymers, metal ions and organic components play an essential role. As is well known, the construction of CPs can be affected by the flexibility of ligand. Pyrazole carboxylic acids are widely investigated and result in a large number of coordination polymers due to diverse coordination modes^[12]. The pyrazole carboxylic acid ligands can satisfy the coordination number of metal atoms (such as Co and Zn) using the synthesis of different dimensions of CP. Because pyrazole carboxylic acids can provide sufficient oxygen and nitrogen atoms^[13] and cobalt atoms have a variety of coordination modes, they are easy to form stable complexes.

  In this background, we synthesized two new complexes by using 1-(3, 5-dicarboxybenzyl)-3, 5-pyrazole dicarboxylic acid. In addition, thermal and magnetic analyses of complexes 1 and 2 have also been investigated. During the synthesis of 2, the H₄L ligand was decarboxylated in situ to form H₃L´ (Scheme 1). We suggest that the synergetic effects of both phen and Co(Ⅱ) ions are important factors in controlling the formation of H₃L´ ligand^[14]. Generally, under hydrothermal conditions, some ligands can easily lose partial carboxyl group, which can be proceeded by the cleavage of unstrained C–C bond, and then construct lots of unpredictable complexes. Many reports indicate that metal ions, reaction temperature and pH value play unique catalytic roles in the decarboxylation reaction^{[15, 16]}.

  Scheme 1

  

  Scheme 1.  Hydrothermal decarboxylation of H₄L

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  2. EXPERIMENTAL

  2.1 Materials and methods

  All reagents and solvents in the present work were obtained from commercial sources and used without further purification. Elemental analyses for C, H and N were collected on a PerkinElmer PE-2400 elemental analyzer. The FT-IR spectrum was measured using KBr pellets with a Nicolet 170SX FT-IR spectrophotometer in the range of 4000~400 cm⁻¹. The magnetic susceptibility data were investigated by using Quantum Design MPMS SQUID VSM instrument in the range of 2~300 K.

  2.2 Synthesis

  2.2.1   [Co₂ (L)(4, 4´-bip)(H₂O)₃]_n (1)

  A mixture of Co(NO₃)₂·6H₂O (0.10 mmol, 29.1 mg), 4, 4´-bip (0.05 mmol, 14.3 mg) and H₄L (0.05 mmol, 16.7 mg) in 15 mL H₂O was placed in a 25 mL Teflon-lined autoclave and the temperature was increased from room temperature to 160 ℃ within 2 h, heated to 160 ℃ for 3 days, and then cooled to room temperature at a rate of 10 ℃·h⁻¹. Purple crystals of complex 1 were obtained (yield: 46% based on 4, 4´-bip). Anal. Calcd. for C₃₂H₂₆N₆O₁₁Co₂: C, 48.70; H, 3.30; N, 10.65%. Found: C, 48.69; H, 3.28; N, 10.59%. IR (KBr, cm^-1): 3420 (w), 2364 (w), 2343 (w), 1618 (s), 1542 (s), 1345 (s), 1309 (w), 1124 (m), 1066 (w), 647 (w).

  2.2.2   {[Co(L´) ₂(phen)]·2H₂O}_n (2)

  A mixture of Co(NO₃)₂·6H₂O (0.30 mmol, 87.3 mg), phen (0.6 mmol, 118.9 mg) and H₄L (0.6 mmol, 100.2 mg) in 15 mL H₂O was placed in a 25 mL Teflon-lined autoclave and the temperature was raised from room temperature to 160 ℃ within 2 h, heated to 160 ℃ for 3 days, and cooled to room temperature at a rate of 10 ℃·h⁻¹. Purple crystals of complex 2 were obtained (yield: 72% based on H₄L). Anal. calcd. for C₃₈H₃₀N₆O₁₄Co: C, 53.42; H, 3.51; N, 9.84%. Found: C, 53.43; H, 3.47; N, 9.86%. IR (KBr, cm^-1): 3482 (w), 2963 (w), 2600 (w), 2367 (w), 1705 (s), 1609 (s), 1508 (m), 1425 (s), 1366 (m), 1298 (m), 1202 (w), 1080 (w), 849 (m), 768 (m), 729 (w), 673 (w), 636 (w).

  2.3 Crystal structure determination

  Crystal data for 1 and 2 were collected on a Bruker Smart APEX Ⅱ CCD diffractometer using MoKα radiation (λ = 0.71073 Å) in an ω-scan mode. A semiempirical absorption correction was applied with the SADABS program. By using the SHELXL 2014 programs, the structure was solved by direct methods and refined by full-matrix least-squares on F^{2[17, 18]}. All non-hydrogen atoms were refined anisotropically and all hydrogen atoms were geometrically placed in the calculated positions. The detailed crystallographic data are presented in Table 1. Selected bond lengths and bond angles are listed in Table 2.

  Table 1

  

  Table 1.  Crystal Data and Structure Refinement for Complexes 1 and 2

  DownLoad: CSV

  Complex	1	2
  Empirical formula	C32H26Co2N6O11	C38H30CoN6O14
  Formula weight	788.45	853.61
  Crystal system	Monoclinic	Monoclinic
  Space group	P21/c	I2/a
  Temperature/K	296(2)	296(2)
  Wavelength/Å	0.71073	0.71073
  a/Å	15.759(5)	11.455(3)
  b/Å	13.789(5)	20.722(6)
  c/Å	15.370(5)	16.546(4)
  α/°	90	90
  β/°	105.306(5)	109.915(15)
  γ/°	90	90
  V/Å3	3221.4(18)	3692.7(17)
  Z	4	4
  Dc (g·cm-3)	1.626	1.535
  μ/mm-1	1.103	0.546
  F(000)	1608	1756
  θ range/°	1.994 to 30.960	2.619 to 28.232
  Limiting indices	–22≤h≤11, –15≤k≤19, –19≤l≤19	–15≤h≤9, –25≤k≤27, –18≤l≤22
  No. of reflections collected	20916	11241
  No. of unique rflns.	8563	4459
  No. of parameters	472	275
  Goodness of fit on F2	0.957	0.975
  Final R, indices (I > 2σ(I))	R = 0.0424, wR = 0.1265	R = 0.0513, wR = 0.1217
  R indices (all data)	R = 0.0620, wR = 0.1408	R = 0.0836, wR = 0.1387
  Largest diff. peak, and hole (e·Å-3)	1.636 and –0.449	0.688 and –0.504

  Table 2

  

  Table 2.  Selected Bond Lengths (Å) and Bond Angles (º) for 1 and 2

  DownLoad: CSV

  Complex 1
  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Co(1)–N(5)	2.110(2)		Co(1)–O(2)	2.1340(18)		Co(1)–O(11)	2.1685(19)
  Co(1)–N(1)	2.223(2)		Co(1)–O(3B)	2.0400(19)		Co(1)–O(8C)	2.0761(18)
  Co(2)–O(6A)	2.0380(19)		Co(2)–N(3)	2.082(2)		Co(2)–O(10)	2.103(2)
  Co(2)–O(9)	2.137(2)		Co(2)–O(1)	2.163(2)		Co(2)–O(2)	2.2807(18)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(3B)–Co(1)–O(8C)	85.46(8)		O(3B)–Co(1)–N(5)	94.88(9)		O(8C)–Co(1)–N(5)	91.93(8)
  O(3B)–Co(1)–O(2)	98.84(8)		O(8C)–Co(1)–O(2)	172.52(7)		N(5)–Co(1)–O(2)	93.78(8)
  O(3B)–Co(1)–O(11)	174.16(8)		O(8C)–Co(1)–O(11)	88.73(7)		N(5)–Co(1)–O(11)	85.99(8)
  O(6A)–Co(2)–N(3)	99.76(9)		O(6A)–Co(2)–O(10)	84.81(8)		N(3)–Co(2)–O(10)	96.27(11)
  O(6A)–Co(2)–O(9)	91.70(8)		N(3)–Co(2)–O(9)	87.09(9)		O (10)–Co(2)–O(9)	175.51(9)
  O(6A)–Co(2)–O(1)	106.54(8)		N(3)–Co(2)–O(1)	152.86(9)		O(10)–Co(2)–O(1)	92.72(10)
  Symmetry codes: A –x+1, –y, –z; B –x+1, y+1/2, –z+1/2; C –x+1, –y, –z+1
  Complex 2
  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Co(1)–O(1)	2.1229(17)		Co(1)–O(1A)	2.1230(17)		Co(1)–N(3A)	2.153(2)
  Co(1)–N(3)	2.153(2)		Co(1)–N(1)	2.156(2)		Co(1)–N(1A)	2.156(2)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–Co(1)–O(1A)	101.77(10)		O(1)–Co(1)–N(3A)	90.87(8)		O(1A)–Co(1)–N(3A)	166.62(7)
  O(1)–Co(1)–N(3)	166.62(7)		O(1A)–Co(1)–N(3)	90.87(8)		N(3A)–Co(1)–N(3)	76.91(11)
  O(1)–Co(1)–N(1)	77.13(7)		O(1A)–Co(1)–N(1)	85.85(8)		N(3A)–Co(1)–N(1)	101.41(8)
  Symmetry code: A –x+3/2, y, –z+1

  3. RESULTS AND DISCUSSION

  3.1 Crystal structure of [Co₂(L)(4, 4´-bip)(H₂O)₃]_n (1)

  The result of X-ray single-crystal diffraction analysis reveals that complex 1 crystallizes in monoclinic P2₁/c space group. The asymmetric unit consists of two Co(Ⅱ) ions, one H₄L ligand, one 4, 4´-bip ligand and three coordination water molecules. As shown in Fig. 1a, Co(1) and Co(2) are both six-coordinated in different coordination environments. Co(1) is surrounded by O(2), O(3B) and O(8C) from three H₄L ligands, N(1) from one H₄L ligand, O(11) from one coordination water molecule, and N(5) from one 4, 4´-bip. While Co(2) is coordinated by O(1), O(2), O(6A) from two H₄L ligands, N(3) from one 4, 4´-bip ligand, and O(9), O(10) from two coordination water molecules. The Co–O bond lengths vary from 2.0380(19) to 2.2807(18) Å, and the Co–N distances are within the range of 2.082(2)~2.223(2) Å (Table 2).

  Figure 1

  

  Figure 1.  (a) Coordination environment of the Co(Ⅱ) in 1. Symmetry codes: A: –x+1, –y, –z; B: –x+1, y+1/2, –z+1/2; C: –x+1, –y, –z+1. (b) 1D chain structure of 1. (c) 2D framework of 1. (d) 3D framework of 1

  DownLoad: Full-Size Img PowerPoint

  As shown in Fig. 1c, Co(Ⅱ) ions are connected to L^4- to form a 1D chain. The 1D chain forms a 2D planar structure through the connection of Co(Ⅱ) ions (Fig. 1c) viewed from the b direction. Interestingly, this structure along the b direction is connected to form a 3D structure through 4, 4´-bip (Fig. 1d).

  3.2 Crystal structure of {[Co(L´)₂(phen)]·2H₂O}_n (2)

  The result of X-ray single-crystal diffraction analysis reveals that complex 2 crystallizes in the monoclinic I2/a space group. The asymmetric unit consists of one Co(Ⅱ) ion, two L´^3- ions, one phen ligand and two free water molecules. Co(Ⅱ) in the molecule adopts a 6 coordination mode, forming a twisted octahedral coordination configuration. As show in Fig. 2a, Co(Ⅱ) is surrounded by O(1), O(1A) from two different L´^3- ions, N(3), N(3A) from one phen ligand, and N(1), N(1A) from two different L´^3- ions. The Co–O bond lengths vary from 2.1229(17) to 2.1230(17) Å, and the Co–N distances are within the range of 2.153(2)~2.156(2) Å (Table 2).

  Figure 2

  

  Figure 2.  (a) Coordination environment of the Co(Ⅱ) in 2. Symmetry codes: A: –x+3/2, y, –z+1. (b) 1D chain structure of 2. (c) 3D supramolecular framework of 2

  DownLoad: Full-Size Img PowerPoint

  The molecules of complex 2 form a one-dimensional chain structure through the hydrogen bonding of O(7)– H(7A)…O(2E), O(7)–H(7B)…O(1F), O(4)–H(4)…O(7C) and O(5)–H(5)…O(2D), O(7C) from free water molecules, and O(1F), O(2E), and O(4) from L´^3- ions (Fig. 2c). The detailed information of hydrogen bond is shown in Table 3. The 3D supramolecular structure is shown in Fig. 2c viewed from the a direction.

  Table 3

  

  Table 3.  Hydrogen Bond Lengths (Å) and Bond Angles (º) for Complex 2

  DownLoad: CSV

  D–H…A	d(D–H)	d(H…A)	d(D–A)	∠DHA
  O(4)–H(4)…O(7C)	0.82	1.84	2.658(3)	172
  O(5)–H(5)…O(2D)	0.82	1.86	2.630(3)	155
  O(7)–H(7A)…O(2E)	0.85(3)	2.11(3)	2.927(3)	160(3)
  O(7)–H(7B)…O(1F)	0.86(2)	2.06(3)	2.878(3)	161(3)
  Symmetry codes: C: –1/2+x, –1/2+y, –1/2+z; D: –1+x, –1/2–y, –1/2+z; E: –1+x, –1+y, –1+z; F: –3/2+x, –y, –1+z

  3.3 Thermal analysis

  To explore the stabilities of complexes 1 and 2, their thermogravimetric analyses (TGA) were carried out under a nitrogen atmosphere from room temperature to 900 ℃ (Fig. 3). In 1, the first weight loss of 6.50% is in the range from 25 to 260 ºC corresponding to the removal of three coordinated H₂O (calcd. 6.85%). When further heated to about 300 ℃, the 3D framework begins to decompose. In complex 2, the first weight loss of 4.63% is in the range from 25 to 330 ºC due to the removal of two free water molecules (calcd. 4.21%). When continuing heating to about 350 ℃, the 3D supramolecular framework begins to decompose.

  Figure 3

  

  Figure 3.  Thermogravimetric analyses for 1 and 2

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  Powder X-ray diffraction (PXRD) testing was performed to prove the phase purity of the two complexes. The simulated and experimental peaks of 1 and 2 basically coincide (Fig. 4), indicating that the purity of the sample was good.

  Figure 4

  

  Figure 4.  PXRD of 1 and 2

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  3.4 Magnetic properties

  The magnetic properties of 1 and 2 were measured in a range of 2~300 K at 1000 Oe. The magnetic susceptibility data for 1 are shown in Fig. 5. As observed, the experimental χ_MT value was 3.456 cm³·K·mol^-1 at 300 K, which is lower than the theoretical value (3.75 cm³·K·mol^-1) of two isolated spin-only Co(Ⅱ) ions (S = 3/2). As the temperature gradually decreased, the value of χ_MT decreased slowly and the value of χ_MT decreased to 0.411 cm³·K·mol^-1. When at 2 K, antiferromagnetic interactions are operative. The magnetic susceptibility vs. T of 1 followed the H = –2JS_Co1S_Co2, and the magnetic susceptibility expression for a dinuclear models is given by^[19]:

  $ \chi_{\mathrm{M}}=\frac{2 N g^2 \beta^2}{k T} \frac{e^{2 J /(k T)}+5 e^{6 J /(k T)}+14 e^{12 J /(k T)}}{1+3 e^{2 J /(k T)}+5 e^{6 J /(k T)}+7 e^{12 J /(k T)}} $

  Figure 5

  

  Figure 5.  Thermal variation of χ_M and χ_MT for 1 (Open points are the experimental data, and the solid line represents the best fit obtained from the Hamiltonian given in the text)

  DownLoad: Full-Size Img PowerPoint

  The least-squares fit to the experimental data was carried out with g = 2.10, J = –0.454 cm^-1 and R = 5.249×10^-3. The negative value of J in complex 1 further reveals that the antiferromagnetic interaction exists among the Co(Ⅱ) ion centers.

  The magnetic susceptibility data measured for 2 are shown in Fig. 6. As observed, the experimental χ_MT value was 3.294 cm³·K·mol^-1 at 300 K, which is lower than the theoretical one (3.75 cm³·K·mol^-1) of two isolated spin-only Co(Ⅱ) ions (S = 3/2). As the temperature gradually decreased, the value of χ_MT decreased slowly and that of χ_MT decreased to 2.032 cm³·K·mol^-1. At 2 K, the antiferromagnetic interactions are operative. The magnetic susceptibility vs. T of 2 followed H = –2JS. The least-squares fit to the experimental data was carried out with g = 2.05, J = –0.255 cm^-1 and R = 1.05×10^-3. The negative value of J in 2 further reveals antiferromagnetic interaction existing among the Co(Ⅱ) ion centers.

  Figure 6

  

  Figure 6.  Thermal variation of χ_M and χ_MT for 2

  DownLoad: Full-Size Img PowerPoint

  4. CONCLUSION

  In summary, two new cobalt complexes were successfully synthesized by using 1-(3, 5-dicarboxybenzyl)-3, 5-pyrazole dicarboxylic acid as a ligand under hydrothermal conditions. The structural studies reveal that complexes 1 and 2 show 3D frameworks. Besides, magnetic analysis indicates antiferromagnetic interaction existing among the Co(Ⅱ) ion centers in 1 and 2.

  
  
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• 

  Scheme 1  Hydrothermal decarboxylation of H₄L

  下载: 全尺寸图片 幻灯片
  

  Figure 1  (a) Coordination environment of the Co(Ⅱ) in 1. Symmetry codes: A: –x+1, –y, –z; B: –x+1, y+1/2, –z+1/2; C: –x+1, –y, –z+1. (b) 1D chain structure of 1. (c) 2D framework of 1. (d) 3D framework of 1

  下载: 全尺寸图片 幻灯片
  

  Figure 2  (a) Coordination environment of the Co(Ⅱ) in 2. Symmetry codes: A: –x+3/2, y, –z+1. (b) 1D chain structure of 2. (c) 3D supramolecular framework of 2

  下载: 全尺寸图片 幻灯片
  

  Figure 3  Thermogravimetric analyses for 1 and 2

  下载: 全尺寸图片 幻灯片
  

  Figure 4  PXRD of 1 and 2

  下载: 全尺寸图片 幻灯片
  

  Figure 5  Thermal variation of χ_M and χ_MT for 1 (Open points are the experimental data, and the solid line represents the best fit obtained from the Hamiltonian given in the text)

  下载: 全尺寸图片 幻灯片
  

  Figure 6  Thermal variation of χ_M and χ_MT for 2

  下载: 全尺寸图片 幻灯片

  Table 1.  Crystal Data and Structure Refinement for Complexes 1 and 2

  Complex	1	2
  Empirical formula	C32H26Co2N6O11	C38H30CoN6O14
  Formula weight	788.45	853.61
  Crystal system	Monoclinic	Monoclinic
  Space group	P21/c	I2/a
  Temperature/K	296(2)	296(2)
  Wavelength/Å	0.71073	0.71073
  a/Å	15.759(5)	11.455(3)
  b/Å	13.789(5)	20.722(6)
  c/Å	15.370(5)	16.546(4)
  α/°	90	90
  β/°	105.306(5)	109.915(15)
  γ/°	90	90
  V/Å3	3221.4(18)	3692.7(17)
  Z	4	4
  Dc (g·cm-3)	1.626	1.535
  μ/mm-1	1.103	0.546
  F(000)	1608	1756
  θ range/°	1.994 to 30.960	2.619 to 28.232
  Limiting indices	–22≤h≤11, –15≤k≤19, –19≤l≤19	–15≤h≤9, –25≤k≤27, –18≤l≤22
  No. of reflections collected	20916	11241
  No. of unique rflns.	8563	4459
  No. of parameters	472	275
  Goodness of fit on F2	0.957	0.975
  Final R, indices (I > 2σ(I))	R = 0.0424, wR = 0.1265	R = 0.0513, wR = 0.1217
  R indices (all data)	R = 0.0620, wR = 0.1408	R = 0.0836, wR = 0.1387
  Largest diff. peak, and hole (e·Å-3)	1.636 and –0.449	0.688 and –0.504

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  Table 2.  Selected Bond Lengths (Å) and Bond Angles (º) for 1 and 2

  Complex 1
  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Co(1)–N(5)	2.110(2)		Co(1)–O(2)	2.1340(18)		Co(1)–O(11)	2.1685(19)
  Co(1)–N(1)	2.223(2)		Co(1)–O(3B)	2.0400(19)		Co(1)–O(8C)	2.0761(18)
  Co(2)–O(6A)	2.0380(19)		Co(2)–N(3)	2.082(2)		Co(2)–O(10)	2.103(2)
  Co(2)–O(9)	2.137(2)		Co(2)–O(1)	2.163(2)		Co(2)–O(2)	2.2807(18)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(3B)–Co(1)–O(8C)	85.46(8)		O(3B)–Co(1)–N(5)	94.88(9)		O(8C)–Co(1)–N(5)	91.93(8)
  O(3B)–Co(1)–O(2)	98.84(8)		O(8C)–Co(1)–O(2)	172.52(7)		N(5)–Co(1)–O(2)	93.78(8)
  O(3B)–Co(1)–O(11)	174.16(8)		O(8C)–Co(1)–O(11)	88.73(7)		N(5)–Co(1)–O(11)	85.99(8)
  O(6A)–Co(2)–N(3)	99.76(9)		O(6A)–Co(2)–O(10)	84.81(8)		N(3)–Co(2)–O(10)	96.27(11)
  O(6A)–Co(2)–O(9)	91.70(8)		N(3)–Co(2)–O(9)	87.09(9)		O (10)–Co(2)–O(9)	175.51(9)
  O(6A)–Co(2)–O(1)	106.54(8)		N(3)–Co(2)–O(1)	152.86(9)		O(10)–Co(2)–O(1)	92.72(10)
  Symmetry codes: A –x+1, –y, –z; B –x+1, y+1/2, –z+1/2; C –x+1, –y, –z+1
  Complex 2
  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Co(1)–O(1)	2.1229(17)		Co(1)–O(1A)	2.1230(17)		Co(1)–N(3A)	2.153(2)
  Co(1)–N(3)	2.153(2)		Co(1)–N(1)	2.156(2)		Co(1)–N(1A)	2.156(2)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–Co(1)–O(1A)	101.77(10)		O(1)–Co(1)–N(3A)	90.87(8)		O(1A)–Co(1)–N(3A)	166.62(7)
  O(1)–Co(1)–N(3)	166.62(7)		O(1A)–Co(1)–N(3)	90.87(8)		N(3A)–Co(1)–N(3)	76.91(11)
  O(1)–Co(1)–N(1)	77.13(7)		O(1A)–Co(1)–N(1)	85.85(8)		N(3A)–Co(1)–N(1)	101.41(8)
  Symmetry code: A –x+3/2, y, –z+1

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  Table 3.  Hydrogen Bond Lengths (Å) and Bond Angles (º) for Complex 2

  D–H…A	d(D–H)	d(H…A)	d(D–A)	∠DHA
  O(4)–H(4)…O(7C)	0.82	1.84	2.658(3)	172
  O(5)–H(5)…O(2D)	0.82	1.86	2.630(3)	155
  O(7)–H(7A)…O(2E)	0.85(3)	2.11(3)	2.927(3)	160(3)
  O(7)–H(7B)…O(1F)	0.86(2)	2.06(3)	2.878(3)	161(3)
  Symmetry codes: C: –1/2+x, –1/2+y, –1/2+z; D: –1+x, –1/2–y, –1/2+z; E: –1+x, –1+y, –1+z; F: –3/2+x, –y, –1+z

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