English

• 1. INTRODUCTION

  In recent decades,considerable attention has been paid to metal-organic frameworks (MOFs) due to their structural versatility and chemical nature tunability^{[1, 2]}. In particular,owing to their intrig uing architecturesand adjustable pore structures,MOFs have been intensively evaluated as promising porous materials that can be effectively tailored to specific functional applications such as catalysis,sensing,drug delivery,and gas storage and separa-tion^[3-7]. Great efforts have been made on the syste-matic design and synthesisof MOFs. Especially,the mixed-ligand synthetic strategy is a very effective way to synthesize MOFs through rational selection of matching ligands^[8-11]. Thereinto,the O-donor terephthalic acid (H₂bdc) or N-donor 2,4,6-tri(4-pyridinyl)-1,3,5-triazine (tpt) as classic ligands have been widely applied to construct MOFs with aesthetically elegant architectures and various functions,respectively^[12-15]. However,the combina-tion of H₂bdc and tpt ligands to construct MOFs has been less explored in mixed-ligand systems^[16-18].

  In addition,the stability of MOFs towards water or moistureplays an important role in practical applications^{[19, 20]}. However,many MOFs are susceptible to decomposition or loss in crystallinity whenthey are encountered water or even humid air. Thus,the search for water-stable MOFs is still a major challenge. Herein,by employing the mixture of H₂bdc and tpt ligands,a new water-stable copper-organic framework [Cu₂(bdc)₂(tpt)₃]·2H₂O (1) has been obtained by hydrothermal reactions. The crystalline phase purity and thermal/chemical stabi-lities of compound 1 were investigated.

  2. EXPERIMENTAL

  2.1. Materials and instruments

  The ligand 2,4,6-tris-(4-pyridyl)-1,3,5-triazine (tpt) was prepared according to the literature method^[21]. The other solventsand reagents were purchased from commercial sources without any purification. Infrared(IR) spectrum in the 4000～400 cm^-1 regionwas recorded on a Vertex 70 using KBr pallets. Elemental analysesof (C,H and N) were carried out with a Vario EL-Cube elemental analyzer. Thermogravimetric analysis (TGA) per-formed with a TGA/DSC 1 STARe system at a heating rate of 10 ℃/min under nitrogen atmos-phere. Powder X-ray diffraction (PXRD) was performed at room temperature on a Rigaku MiniFlex600 diffractometer using CuKα radiation (λ = 0.154 nm).

  2.2. Synthesis of [Cu₂(bdc)₂(tpt)₃]·2H₂O (1)

  A mixture of Cu(NO₃)2·3H₂O (0.0283 g,0.117 mmol),H₂bdc (0.0336 g,0.202 mmol) and tpt (0.0431 g,0.138 mmol) in 10 mL H₂O was stirred for 30 minutes in open air. The suspension was sealed in a 20 mL Teflon-lined autoclave,kept under autogenous pressure at 160 ºC for 3 days,and then slowly cooled to room temperature for another 3 days. Blue prism crystals of 1 were filtered and washed with distilled water (Yield: 49% based on copper). Anal. Calcd. (%) for C₇₀H₄₈Cu₂N₁₈O₁₀: C, 58.86; H,3.39; N,17.65. Found (%): C,58.84; H, 3.36; N,17.67. IR (KBr,cm⁻¹): 3602 (w),3500 (w),3408 (w),3111 (w),3054 (w),3034 (w),1945 (w), 1610 (vs),1583 (vs),1521 (vs),1373 (vs),1332 (vs),1216 (w),1148 (w),1063 (m),1019 (w),982 (w),873 (m),825 (s),802 (s),753 (s),643 (s),563 (m),512 (s).

  2.3. X-ray crystallographic study

  A blue prism single crystal of 1 with dimensions of 0.80mm × 0.40mm × 0.20mm was selected and mounted on a glass fiber for single-crystal X-ray diffraction. The data collection of 1 was performed on a Mercury CCD diffractometer equipped with graphite-monochromated MoKα radiation (λ = 0.71073 Å) at 293 K. The CrystalClear program was used for absorption correction. The structure was solved by direct methodsand refined on F² by full-matrix least-squares methodsusing the SHELX-97 program package^{[22, 23]}. All non-hydro-genatoms were refinedanisotropically. The hydrogen atoms for tpt and bdc^2- ligands were placed in calculated positions and treated as riding on their parents before the final cycle of refinement,while the hydrogen atoms of water molecules were initially located ina difference Fourier map and included in the final refinement by use of geometrical restraints with the O-H distances fixed at 0.85 Å. A total of 26359 reflections of 1 were collected in the range of 2.41<θ<27.47° (-39≤h≤39,-11 ≤ k ≤ 14,-27 ≤ l ≤ 27) and 7200 were independent with R_int = 0.0336. The final R = 0.0387 and wR= 0.1091 (w=1/[σ2(F_o²) + (0.0563P)² +0.0000P],where P = (F ₀²+ 2F _c²) /3) for 6230 observed reflections with I > 2σ(I). (Δ/σ)_max = 0.001,(Δρ)_max = 0.370 and (Δρ)_min = -0.522 e·Å^-3. The selected bond lengths and bond angles for 1 are listed in Table 1,and the hydrogen bond data in Table 2.

  Table 1

  

  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°)

  DownLoad: CSV

  Angle (°) Angle (°) O(4) -Cu(1) -O(2) a 176.69(5) O(4) -Cu(1) -N(22) 89.56(6)

  Bond	Dist.	Bond	Dist.
  Cu(1) -O(4)	1.9751(14)	Cu(1) -N(36)	1.9940(15)
  Cu(1) -O(2) a	1.9816(14)	Cu(1) -N(22)	2.0174(15)
  O(4) -Cu(1) -N(36)	86.96(6)	O(2) a-Cu(1) -N(22)	93.40(6)
  O(2) a-Cu(1) -N(36)	90.22(6)	N(36) -Cu(1) -N(22)	173.84(6)
  Symmetry transformation: a: x,-y+1,z+1/2

  Table 2

  

  Table 2.  Hydrogen Bond Lengths (Å) and Bond Angles (°)

  DownLoad: CSV

  D-H···A	d(D-H)	d(H···A)	d(D···A)	∠DHA
  O(5) -H(5A)···O(3)	0.85	2.01	2.762(3)	147.8
  O(5) -H(5B)···O(1) a	0.85	2.48	3.225(4)	147.2
  C(21) -H(21A)···O(1) a	0.93	2.59	3.247(3)	128.1
  C(37) -H(37A)···O(2) e	0.93	2.57	3.145(3)	120.1
  C(44) -H(44A)···O(4) f	0.93	2.57	3.217(2)	127.5
  Symmetry codes: (a) x,-y+1,z+1/2; (e) -x+1/2,y-1/2,-z-1/2; (f) -x+1/2,y+1/2,-z-1/2

  3. RESULTS AND DISCUSSION

  3.1. Structure description

  Single-crystal X-ray diffraction studies reveal that 1 crystallizesi n the monoclinic system with space group C2/c,and the asymmetric unit contains one copper atom,one and half tpt ligands,one bdc^2- ligand and one guest water molecule.The Cu(II) atom residesin a square-planar coordination geo-metry defined by two pyridyl nitrogen atoms of two different tpt ligands (Cu(1) -N(36) = 1.9940(15) ,Cu(1) -N(22) = 2.0174(15) Å),two oxygen atoms of two different bdc^2- ligands (Cu(1) -O(4) =1.9751(14) ,Cu(1) -O(2) a = 1.9816(14) Å) (Fig. 1a),and the basal angles ranging from 86.96(6) to 176.69(5) °. All the bond lengths and bond angles arein the normal ranges^[24]. The two crystallogra-phically independent tpt ligands exhibit different coordination modes: one tpt acts as a bidentate ligand to coordinate two Cu(II) atoms through two pyridyl groups; however,the other tpt ligand is coordinated to one Cu(II) atom via one pyridyl group in a monodentate mode (Fig. 1a). Meanwhile,one bidentate tpt ligand and two monodentate tpt ligands are held togetherby two independent Cu(II) atoms to form a half metallacycle [Cu₂(tpt)₃] (Fig. 1a). On the other hand,the bdc^2- ligand links two separate Cu(II)atoms through one oxygen atom ofeach carboxylate group. As a result,the adjacent half metallacycles [Cu₂(tpt)₃] are connected by the bdc^2- ligands to form a 1D bent ladder-like chain (Fig. 2a). Interestingly,when viewed along the [001] direction,there is a nano-sized channel with the size of about 4.43 × 4.43 Å₂ in this bent ladder-like chain,which is similar to the metal-organic tube (Fig. 2b)^{[25, 26]}. Futhermore,the adjacent chains are packed together through intermolecular hydrogen bonds and π-π stacking interactions to form a 3D supramolecular architecture. These hydrogen bonds arefound between carboxylate groups and pyridyl/phenyl rings from the adjacent layers (Table 2,C(37) -H(37A)···O(2) ^e 3.145(3) Å,C(44) -H(44A)···O(4) ^f 3.217(2) Å,symmetry codes: (e) -x+1/2,y-1/2,-z-1/2; (f) -x+1/2,y+1/2,-z-1/2) ; The centroid-centroid distance of π-π stacking interac-tions between pyridyl/triazine rings from the adjacent chains range from 3.792 to 4.479 Å (Fig. 3) . In addition,there aresome hydrogen bonds between thewater O(5) and uncoordinated carboxylate oxygen atoms (O(1) and O(3) ),and the free water molecules are distributed in the voids of the 3D supramolecular architecture (Fig. 4) .

  Figure 1

  

  Figure 1.  View of the half metallacycle [Cu₂(tpt)₃] and the coordination environment of Cu(II),bdc^2- and tpt in compound 1 (Thermal ellipsoids are drawn at the 50%probability level. Symmetry codes: a:x,-y+1,z+1/2; b: x,-y+1,z-1/2; c: -x,y,-z-1/2; d: -x,-y+1,-z-1)

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  Figure 2

  

  Figure 2.  View of the 1Dbent ladder-like chain along the ac plane (a) and caxis (b),respectively

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  Figure 3

  

  Figure 3.  英文标题

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  Figure 4

  

  Figure 4.  View of a 3D supramolecular architecture along the b direction

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  3.2. Infrared spectrum

  The IR spectrum of 1 in the range of 4000～400 cm^-1 is shown in Fig. 5. The very strong absorption bands at 1610 and 1332 cm^-1 are assigned to the asymmetric and symmetric stretching vibrations of the carboxyl groups,respectively. The characteristic bands of the tpt ligand at 1583,1521,1373 and 800 cm^-1 are attributed to ν(C=C),ν(C=N),(ring defor-mation mode C-C) and (out-of-ring bend C-H)

  Figure 5

  

  Figure 5.  IR spectrum of compound 1

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  vibrations,respectively.These results are in accor-dance with the previously reported compounds^{[21, 27]}.

  3.3. Thermogravimetric analysis

  To estimate the thermal stability of 1,the thermalgravimetric analysis (TGA)has been carried out under nitrogen stream from 30 to 800 ℃ at a heating rate of 10 ℃·min^-1. As shown in Fig. 6,the TGA curve of 1 shows that there is the first weight loss step between 30 and 230 ℃,corresponding to the lattice water molecule(calcd.2.52%; found 2.75%),then followsby a relatively steady plateau from230 to 315 ℃. Subsequently,the host framework starts to decompose and does not stop until the heating ends at 800 ℃.

  Figure 6

  

  Figure 6.  TGA curve of compound 1

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  3.3. Powder X-ray diffraction and water stability

  The crystalline phase purity of 1 was comfirmed byagreement between the experimental and simu-lated powder X-ray diffraction (PXRD) patterns (Fig. 7) . In addition,the water stabitity of 1 has been also examined by PXRD patterns. The main diffraction peaks of 1 can be well held in water at 25 ℃ and even boiling water for at least three days,indicating thestructural stability of 1 (Fig. 7) .

  Figure 7

  

  Figure 7.  PXRD patterns forcompound 1 obtained fromas-synthesized samples and after treating in water at 25 and 100 ℃ fordifferent durations

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  4. CONCLUSION

  In summary,a new copper-organic framework has been synthesized by mixed ligands under hydro-thermal reactions. Single-crystal X-ray diffraction analysis reveals that compound 1 possesses a 3D supramolecular architecture composed of packed 1D bent ladder-like chain as analogues of metal-organic nanotube through weak intermolecular hydrogen bonds and π-π stacking interactions. Furthermore,1 showshigh stability in water and even boiling water.

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• 

  Figure 1  View of the half metallacycle [Cu₂(tpt)₃] and the coordination environment of Cu(II),bdc^2- and tpt in compound 1 (Thermal ellipsoids are drawn at the 50%probability level. Symmetry codes: a:x,-y+1,z+1/2; b: x,-y+1,z-1/2; c: -x,y,-z-1/2; d: -x,-y+1,-z-1)

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  Figure 2  View of the 1Dbent ladder-like chain along the ac plane (a) and caxis (b),respectively

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  Figure 3  英文标题

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  Figure 4  View of a 3D supramolecular architecture along the b direction

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  Figure 5  IR spectrum of compound 1

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  Figure 6  TGA curve of compound 1

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  Figure 7  PXRD patterns forcompound 1 obtained fromas-synthesized samples and after treating in water at 25 and 100 ℃ fordifferent durations

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  Table 1.  Selected Bond Lengths (Å) and Bond Angles (°)

  Angle (°) Angle (°) O(4) -Cu(1) -O(2) a 176.69(5) O(4) -Cu(1) -N(22) 89.56(6)

  Bond	Dist.	Bond	Dist.
  Cu(1) -O(4)	1.9751(14)	Cu(1) -N(36)	1.9940(15)
  Cu(1) -O(2) a	1.9816(14)	Cu(1) -N(22)	2.0174(15)
  O(4) -Cu(1) -N(36)	86.96(6)	O(2) a-Cu(1) -N(22)	93.40(6)
  O(2) a-Cu(1) -N(36)	90.22(6)	N(36) -Cu(1) -N(22)	173.84(6)
  Symmetry transformation: a: x,-y+1,z+1/2

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  Table 2.  Hydrogen Bond Lengths (Å) and Bond Angles (°)

  D-H···A	d(D-H)	d(H···A)	d(D···A)	∠DHA
  O(5) -H(5A)···O(3)	0.85	2.01	2.762(3)	147.8
  O(5) -H(5B)···O(1) a	0.85	2.48	3.225(4)	147.2
  C(21) -H(21A)···O(1) a	0.93	2.59	3.247(3)	128.1
  C(37) -H(37A)···O(2) e	0.93	2.57	3.145(3)	120.1
  C(44) -H(44A)···O(4) f	0.93	2.57	3.217(2)	127.5
  Symmetry codes: (a) x,-y+1,z+1/2; (e) -x+1/2,y-1/2,-z-1/2; (f) -x+1/2,y+1/2,-z-1/2

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•