English

• 1. INTRODUCTION

  Design and synthesis of metal-organic frameworks (MOFs) are an important branch of MOF's research, because the potential applications, such as photoluminescence property, catalysis, gas storage and separation, have attracted much attention^[1-4]. Although more and more attention has focused on the investigation of MOFs, predicting and controlling architectures and properties exactly is still a challenge. Significantly the structures and properties depend on synthetic factors and the nature of building blocks. For example, solvents, temperatures and pH values are all common influence factor in the process of synthesis, while the characteristic of ligands, such as rigidity/flexibility, geometric configuration and symmetry, also play crucial role in building blocks^[5-10]. With the development of MOFs, design and construct new MOFs with desired structures is possible, particularly when the ligand is rigid. Compared with rigid ligands, it is a greater challenge to predict the final structures for flexible ligands. The reason may be that flexible ligands may adopt several kinds of conformation when they coordinate to metal ions^[11-13].

  Recently, some groups have become interested in utilizing flexible carboxylate ligands to construct MOFs. The effect of conformation of flexible carboxylate ligands on the structures has also been studied. It is evident that flexible ligands in the construction of CPs may generate novel complexes with interesting topologies and attractive properties, because flexible ligands have variable coordination modes and a variety of conformations^[14-18]. As a continuation of our previous studies^[19-23], we introduce 2, 2´-(1, 4-phenylenebis(methylene))bis(sulfanediyl)dinicotinic acid (H₂L¹) and 2, 2´-(1, 2-phenyl-enebis(methylene))bis(sulfanediyl)dinicotinic acid (H₂L²) as ligands with CuSO₄·5H₂O to investigate the effect of the position of substituent groups of benzene on the structures of MOFs (Scheme 1). In this work, two new MOFs [Cu₄(L¹)₄(DMF)₈·2DMF]_n (DZ-1) and [Cu(L²)(DMF)]_n (DZ-2) were synthesized under solvothermal conditions. They were structurally characterized by single-crystal X-ray diffraction analyses, elemental analyses, IR spectroscopy and thermogravimetric analyses (TGA). In addition, the sorption properties of DZ-1 were investigated.

  Scheme 1

  

  Scheme 1.  Ligands used in the work

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  2. EXPERIMENTAL

  2.1 General experimental section

  All commercially available chemicals and solvents were of reagent grade and used as received without further purification. Ligand H₂L¹ and H₂L² were prepared according to the procedure reported in the literature^{[24, 25]}. Elemental analyses for C, H, and N were performed on a Perkin-Elmer 240C Elemental Analyzer at the analysis center of Nanjing University. Thermogravimetric analyses (TGA) were carried out on a simultaneous SDT 2960 thermal analyzer under nitrogen with a heating rate of 10 ℃ min^-1. FT-IR spectra were recorded in the range of 400~4000 cm^-1 on a Bruker Vector 22 FT-IR spectrophotometer using KBr pellets. Sorption experiments were carried out on a Belsorp-max volumetric gas sorption instrument.

  2.2 Synthesis of DZ-1

  A mixture of CuSO₄·5H₂O (25.50 mg, 0.1 mmol) and H₂L¹ (20.1 mg, 0.05 mmol) was stirred in DMF (10 mL). The resultant solution was sealed in a 20 mL bottle and heated at 90℃ for four days. After cooling to room temperature, blue block-shaped crystals of DZ-1 were obtained in 55% yield. Anal. Calcd. for C₉₈H₉₈O₂₂N₁₄S₈Cu₄: C, 50.39; H, 4.19; N, 8.39; S, 10.97%. Found: C, 50.02; H, 3.95; N, 8.51; S, 10.56%. IR (KBr): 3379 (s), 1644 (m), 1597 (m), 1549 (w), 1388 (s), 1183 (w), 1110 (w), 1056 (m), 861 (w), 814 (m), 782 (m).

  2.3 Synthesis of DZ-2

  A mixture of CuSO₄·5H₂O (25.50 mg, 0.1 mmol) and H₂L² (20.1 mg, 0.05 mmol) was stirred in DMF/H₂O (1:1) (10 mL). The resultant solution was sealed in a 20 mL bottle and heated at 90℃ for four days. After cooling to room temperature, blue block-shaped crystals of DZ-2 were obtained in 45% yield. Anal. Calcd. for C₂₃H₂₀O₅N₃S₂Cu: C, 50.59; H, 3.69; N, 7.69; S, 11.74%. Found: C, 50.32; H, 3.75; N, 7.78; S, 11.65%. IR (KBr): 3400 (s), 1656 (m), 1603 (m), 1534 (m), 1393 (s), 1097 (w), 1018 (w), 863 (w), 818 (m), 786 (m).

  2.4 Structure determination

  The single crystals of complexes DZ-1 and DZ-2 with appropriate dimensions were chosen under an optical microscope and coated with high vacuum grease (Dow Corning Corporation) quickly before being mounted on a glass fiber for data collection. For DZ-1, a crystal with dimensions of 0.20mm × 0.19mm × 0.16mm was mounted on a Bruker Apex Ⅱ CCD Image Plate single-crystal diffractometer in an ω scan mode at 293(2) K with graphite-monochromated MoKα radiation source (λ = 0.71073 Å). The θ range for data collection is from 2.32 to 26.12º with the following index ranges: –20≤h≤15, –20≤k≤17 and –13≤l≤22. A total of 24248 reflections were collected and 9011 were independent (R_int = 0.0620), of which 6243 with I > 2σ(I) were observed. For DZ-2, a crystal with dimensions of 0.22 mm × 0.18 mm × 0.17 mm was mounted on a Bruker Apex Ⅱ CCD Image Plate single-crystal diffractometer in an ω scan mode at 293(2) K with graphite-monochromated MoKα radiation source (λ = 0.71073 Å). The θ range for data collection is from 2.23 to 23.06º with the following index ranges: –12≤h≤14, –22≤k≤20 and –13≤l≤14. A total of 13221 reflections were collected and 4227 were independent (R_int = 0.0505), of which 3251 with I > 2σ(I) were observed. For DZ-1 and DZ-2, absorption correction was applied by the correction of symmetry-equivalent reflections using the Multi-scan program. The diffraction data were integrated by using the SAINT program^[26], which was also used for the intensity corrections for the Lorentz and polarization effects. Semi-empirical absorption correction was applied using SADABS program^[27]. The structure of DZ-1 and DZ-2 were solved by direct methods using SHELXS and all the non-hydrogen atoms were refined on F² by full-matrix least-squares procedures with SHELXL^{[28, 29]}. Hydrogen atoms were generated geometrically and refined isotropically using the riding model. For DZ-1, the final refinement converged at R = 0.0620, wR = 0.1287 (w = 1/[σ²(F_o²) + (0.0493P)² + 2.3227P], where P = (F_o² + 2F_c²)/3), S = 1.030, (Δρ)_max = 0.806 and (Δρ)_min = –0.610 e·Å^-3. For DZ-2, the final refinement converged at R = 0.0502, wR = 0.1300 (w = 1/[σ²(F_o²) + (0.0493P)² + 2.3227P], where P = (F_o² + 2F_c²)/3), S = 1.165, (Δρ)_max = 1.666 and (Δρ)_min = –0.548 e·Å^-3. Crystal structure pictures were obtained using Diamond. The selected bond lengths and bond angles for DZ-1 and DZ-2 are summarized in Table 1.

  Table 1

  

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) for DZ-1 and DZ-2

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  Complex DZ-1
  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Cu(1)–O(1)	1.966(3)		Cu(1)–O(8)#1	1.966(4)		Cu(1)–O(5)	1.967(4)
  Cu(1)–O(4)#1	1.971(3)		Cu(1)–O(9)	2.129(3)		Cu(2)–O(2)	1.957(3)
  Cu(2)–O(3)#1	1.982(3)		Cu(2)–O(7)#1	1.985(3)		Cu(2)–O(6)	1.988(3)
  Cu(2)–O(10)	2.130(3)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–Cu(1)–O(8)#1	91.67(15)		O(1)–Cu(1)–O(5)	90.28(16)		O(8)#1–Cu(1)–O(5)	169.85(15)
  O(1)–Cu(1)–O(4)#1	167.74(15)		O(8)#1–Cu(1)–O(4)#1	88.54(15)		O(5)–Cu(1)–O(4)#1	87.45(16)
  O(1)–Cu(1)–O(9)	97.22(14)		O(8)#1–Cu(1)–O(9)	96.58(15)		O(5)–Cu(1)–O(9)	93.05(15)
  O(4)#1–Cu(1)–O(9)	94.93(14)		O(2)–Cu(2)–O(6)	89.58(14)		O(3)#1–Cu(2)–O(6)	89.18(14)
  O(7)#1–Cu(2)–O(6)	166.56(14)		O(2)–Cu(2)–O(10)	97.54(14)		O(3)#1–Cu(2)–O(10)	93.83(14)
  O(7)#1–Cu(2)–O(10)	93.70(14)		O(6)–Cu(2)–O(10)	99.54(14)
  Complex DZ-2
  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Cu(1)–O(2)	1.924(3)		Cu(1)–O(3)#1	1.973(3)		Cu(1)–O(1)#2	1.976(3)
  Cu(1)–O(4)#3	1.982(3)		Cu(1)–O(5)	2.147(3)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(2)–Cu(1)–O(3)#1	88.60(15)		O(2)–Cu(1)–O(1)#2	167.04(14)		O(3)#1–Cu(1)–O(1)#2	90.29(14)
  O(2)–Cu(1)–O(4)#3	89.54(15)		O(3)#1–Cu(1)–O(4)#3	167.27(12)		O(1)#2–Cu(1)–O(4)#3	88.71(14)
  O(2)–Cu(1)–O(5)	100.21(14)		O(3)#1–Cu(1)–O(5)	101.65(12)		O(1)#2–Cu(1)–O(5)	92.67(13)
  O(4)#3–Cu(1)–O(5)	91.07(13)
  Symmetry transformations used to generate the equivalent atoms: #1: –x+1, –y+1, –z+1 for DZ–1; #1: –x+3/2, y+1/2, –z+3/2; #2: –x+1, –y+2, –z+2; #3: x–1/2, –y+3/2, z+1/2 for DZ–2

  3. RESULTS AND DISCUSSION

  3.1 Structural description of DZ-1

  Crystal structure determination reveals that complex DZ-1 crystallizes in monoclinic space group P2₁/n and is a 0D framework. The asymmetric unit of DZ-1 contains two Cu(Ⅱ) ions, two L¹ ligands, two coordinated DMF molecules and one free DMF molecule (Fig. 1a). Two crystallographically independent Cu(Ⅱ) ions, Cu(1) and Cu(2), have similar coordination environments. They are both in a five-coordinated mode CuO₅. Cu(1) and Cu(2) atoms with four carboxylate groups formed a [Cu₂(CO₂)₄] paddlewheel SBU, and at the same time two DMF molecules were coordinated in the axis. Two such [Cu₂(CO₂)₄] paddlewheel SBUs are further linked by four L¹ ligands to result in the formation of one metal-organic coordination cage (Fig. 1b). The binuclear SBUs can be considered as the vertices, while the coordinated L¹ ligands as the edges of the cage and the remaining coordination positions were occupied by DMF molecules. The structure of DZ-1 is similar to the previously reported MOCC-3, which is a manganese-organic coordination cage based on the flexible bended dicarboxylate ligand 2, 2΄-(2, 4, 6-trimethyl-1, 3-phenylene)bis(methylene)-bis(sulfanediyl)dibenzoic acid^[19]. The vertical distance between two vertices of the cage is 11.532 Å measured by the software of mercury, which is bigger than MOCC-3 (9.817 Å). In addition, there are two types of interchain π⋅⋅⋅π stacking interactions operative between the adjacent rings with center-to-center separation distances (dc-c) of 3.842 and 4.000 Å in the packing diagram (Fig. 1c)^{[30, 31]}. In the structure, all L¹ ligands adopted syn-conformation and every carboxylate group chelated two different copper ions. In addition, according to a calculation of PLATON, the resulting effective free volume is 14.7% of the total crystal volume in DZ-1, after the removal of free solvent molecules^[32].

  Figure 1

  

  Figure 1.  (a) Asymmetric unit of DZ-1 with ellipsoids drawn at 50% probability level (#1: –x+1, –y+1, –z+1). (b) Cage structure of DZ-1 with two coordinated molecules inside. (c) Schematic illustration of π⋅⋅⋅π interaction in DZ-1 from 0D to 3D (black: 3.842 Å and purple: 4.000 Å)

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  3.2 Structural description of DZ-2

  The crystal structure of DZ-2 was determined by single-crystal X-ray diffraction analysis, which crystallizes in monoclinic space group P2₁/n with an asymmetric unit that contains one Cu(Ⅱ) ion, one L² ligand and DMF molecule. As shown in Fig. 2a, Cu(1) is five-coordinated by five O atoms. Two adjacent Cu(1) atoms and four carboxylate groups form a [Cu₂(CO₂)₄] paddlewheel SBU, while DMF molecules were coordinated in the axis. Thus, every [Cu₂(CO₂)₄] SBU links four L² ligands with anti-conformation to generate a 2D sheet with (4, 4) net (Fig. 2b). The structural analyses of DZ-2 show that every carboxylate group of L² ligand chelated two different copper ions, which is the same to L¹ ligand in DZ-1. However, all L² ligands adopted anti-conformation in the structure of DZ-2, which is different from DZ-1. Different conformations may be formed at different positions of substituents of L¹ and L² ligands. Because of the steric hindrance, it is difficult for the ortho-substituents of L² ligand to form syn-conformation.

  Figure 2

  

  Figure 2.  (a) Asymmetric unit of DZ-2 with ellipsoids drawn at 50% probability level (#1: x–1/2, –y+3/2, z+1/2; #2: –x+1, –y+2, –z+2; #3: –x+3/2, y+1/2, –z+3/2). (b) Structure of DZ-2 with (4, 4)-net

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  3.3 Thermogravimetric analysis (TGA)

  The stability of coordination polymers is important for MOFs' practical applications. Thence, TGA of DZ-1 and DZ-2 was examined in the N₂ atmosphere from 30 to 800 ℃. As shown in Fig. 3, complex DZ-1 shows a weight loss of 6.95% in the temperature range of 30~90 ℃, corresponding to the departure of free DMF molecules (Calcd.: 6.25%). Above 90 ℃, there is one obvious weightlessness step. Until about 230 ℃, the weight loss is 18.33% in total, which corresponds to the sum of the loss of free and coordinated DMF molecules (Calcd.: 18.76%). For coordinated DMF molecules, there are half coordinated DMF molecules in cages and half outside, so the linearity presents one step near 160 ℃. After 160 ℃, the curve does not show any obvious step before the decomposition at about 230 ℃, which is in agreement with the result of crystal structure. For the curve of DZ-2, there is no obvious step in the lower temperature. The phenomenon coincides with the fact of no free solvent molecule in the structure. Until 230 ℃, the total weight loss is 12.81%, which fits in with the removal of coordinated DMF molecules (Calcd.: 13.34%). When the temeparature is higher than 230 ℃, the structure begins to decompose.

  Figure 3

  

  Figure 3.  TGA curves of DZ-1 and DZ-2

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  3.4 Adsorption property

  The resulting effective free volume of DZ-1 is 14.7% calculated by PLATON after the removal of free solvent molecules. To investigate the permanent porosities of DZ-1, the gas isotherms were measured. The result shows that the maximum value of N₂ adsorption is 37.8 cm³·g^-1 at 77 K and 0.99 atm (Fig. 4). What's more, CO₂ and N₂ adsorption isotherms were also measured at 195 K. Selective sorption of CO₂ over N₂ was found. Besides, the maximum uptake of CO₂ at around 1 atm is 58.1 cm³·g^-1, while almost no sorption was observed for N₂ at 195 K (Fig. 4). The selective CO₂ uptake over N₂ observed may be partially attributed to the molecular size, given the smaller kinetic diameter of CO₂ (3.30 Å) compared to that of N₂ (3.64 Å).

  Figure 4

  

  Figure 4.  CO₂ and N₂ gas adsorption isotherms of DZ-1 (filled shape, adsorption; open shape, desorption)

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  4. CONCLUSION

  In this paper, we have made a successful attempt to synthesize two new copper-organic frameworks DZ-1 and DZ-2. The two complexes have been constructed from two flexible dicarboxylic ligands with the different position of substituent groups. The result of structural analyses shows that all L¹ ligands adopted syn-conformation and all L² ligands adopted anti-conformation, which is maybe the influence of para- and ortho-substituents of ligands. DZ-1 with a copper-organic coordination cage shows certain adsorptive ability of CO₂ over N₂ at 195 K, implying a possible application in selective adsorption and separation.

  
  
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• 

  Scheme 1  Ligands used in the work

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  Figure 1  (a) Asymmetric unit of DZ-1 with ellipsoids drawn at 50% probability level (#1: –x+1, –y+1, –z+1). (b) Cage structure of DZ-1 with two coordinated molecules inside. (c) Schematic illustration of π⋅⋅⋅π interaction in DZ-1 from 0D to 3D (black: 3.842 Å and purple: 4.000 Å)

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  Figure 2  (a) Asymmetric unit of DZ-2 with ellipsoids drawn at 50% probability level (#1: x–1/2, –y+3/2, z+1/2; #2: –x+1, –y+2, –z+2; #3: –x+3/2, y+1/2, –z+3/2). (b) Structure of DZ-2 with (4, 4)-net

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  Figure 3  TGA curves of DZ-1 and DZ-2

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  Figure 4  CO₂ and N₂ gas adsorption isotherms of DZ-1 (filled shape, adsorption; open shape, desorption)

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  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°) for DZ-1 and DZ-2

  Complex DZ-1
  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Cu(1)–O(1)	1.966(3)		Cu(1)–O(8)#1	1.966(4)		Cu(1)–O(5)	1.967(4)
  Cu(1)–O(4)#1	1.971(3)		Cu(1)–O(9)	2.129(3)		Cu(2)–O(2)	1.957(3)
  Cu(2)–O(3)#1	1.982(3)		Cu(2)–O(7)#1	1.985(3)		Cu(2)–O(6)	1.988(3)
  Cu(2)–O(10)	2.130(3)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(1)–Cu(1)–O(8)#1	91.67(15)		O(1)–Cu(1)–O(5)	90.28(16)		O(8)#1–Cu(1)–O(5)	169.85(15)
  O(1)–Cu(1)–O(4)#1	167.74(15)		O(8)#1–Cu(1)–O(4)#1	88.54(15)		O(5)–Cu(1)–O(4)#1	87.45(16)
  O(1)–Cu(1)–O(9)	97.22(14)		O(8)#1–Cu(1)–O(9)	96.58(15)		O(5)–Cu(1)–O(9)	93.05(15)
  O(4)#1–Cu(1)–O(9)	94.93(14)		O(2)–Cu(2)–O(6)	89.58(14)		O(3)#1–Cu(2)–O(6)	89.18(14)
  O(7)#1–Cu(2)–O(6)	166.56(14)		O(2)–Cu(2)–O(10)	97.54(14)		O(3)#1–Cu(2)–O(10)	93.83(14)
  O(7)#1–Cu(2)–O(10)	93.70(14)		O(6)–Cu(2)–O(10)	99.54(14)
  Complex DZ-2
  Bond	Dist.		Bond	Dist.		Bond	Dist.
  Cu(1)–O(2)	1.924(3)		Cu(1)–O(3)#1	1.973(3)		Cu(1)–O(1)#2	1.976(3)
  Cu(1)–O(4)#3	1.982(3)		Cu(1)–O(5)	2.147(3)
  Angle	(°)		Angle	(°)		Angle	(°)
  O(2)–Cu(1)–O(3)#1	88.60(15)		O(2)–Cu(1)–O(1)#2	167.04(14)		O(3)#1–Cu(1)–O(1)#2	90.29(14)
  O(2)–Cu(1)–O(4)#3	89.54(15)		O(3)#1–Cu(1)–O(4)#3	167.27(12)		O(1)#2–Cu(1)–O(4)#3	88.71(14)
  O(2)–Cu(1)–O(5)	100.21(14)		O(3)#1–Cu(1)–O(5)	101.65(12)		O(1)#2–Cu(1)–O(5)	92.67(13)
  O(4)#3–Cu(1)–O(5)	91.07(13)
  Symmetry transformations used to generate the equivalent atoms: #1: –x+1, –y+1, –z+1 for DZ–1; #1: –x+3/2, y+1/2, –z+3/2; #2: –x+1, –y+2, –z+2; #3: x–1/2, –y+3/2, z+1/2 for DZ–2

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