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• Dyes used in the manufacturing process, such as inks, textiles, leathers, foods and pharmaceuticals, have been the human carcinogens and ecological threat for the preservation of human health and the protection of environment^[1-5]. The removal of dyes from polluted water is greatly significant. Adsorption of them by porous solid materials appears to be an effective approach to remove the organic pollutants^[1]. In comparison with traditional porous solid materials(i.e., zeolite, activated carbon), MOFs(metal-organic frameworks) with modular assembly, intriguing structures, tunable pore sizes and shapes, modifiable pore surfaces and so on, serve as a class of more promising porous materials^[2-4]. In addition, MOFs display a potential application in the photoluminescence^[6-9]. However, the report of MOFs combined with the properties of dye adsorption and photoluminescence is still less. We are interested in the design and construction of functional MOFs aiming at specific applications, i.e., dye adsorption and photoluminescence. Herein, we utilized an oxalamide-functionalized organic ligand of 5, 5′-[oxalylbis(azanediyl)]diisophthalic acid (H₄L), reacting with the salt of Cd(NO₃)₂·4H₂O, affording a 2-fold interpenetrated 3D MOF, {[(CH₃)₂NH₂]₂(CdL)}·x(Solvent)(1). Due to its anionic framework attribute, compound 1 exhibits selective adsorption toward cationic dye of methylene blue(MB) over anionic dye of methyl orange(MO). Additionally, compound 1 also displays interesting photoluminescence emission at visible region.

  1   Experimental

  1.1   Materials and methods

  All reagents were obtained from commercial sources and used without further purification [Cd(NO₃)₂·4H₂O(alfa, AR99%), 5-Aminoisophthalic acid(acros, AR98%) and Oxalyl chioride(Sinopharm Chemical Reagent, CP98%)]. H₄L was prepared according to the previously reported procedure^[10]. Fourier-transform infrared(FT-IR) spectrum(4000~400 cm^-1, KBr pellet) was collected in the solid state on a Bruker EQUINOX-55 FT-IR spectrometer(Germany, Bruker, Co). Powder X-ray diffraction(PXRD)(Germany, Bruker, Co) measurements were performed on a D/Max2550VB+/PC diffractometer with CuKα(λ=0.15406 nm), and the X-ray tube was operated at 40 kV and 40 mA. High resolution thermogravimetric analysis(TGA)(US, TA instrument Co) were performed under a continuous N₂ flow and recorded on a Q600SDT thermal analyzer with a heating rate of 5 ℃ per minute. The photoluminescence and phosphorescence spectra were recorded on a F-4600 fluorescence spectrophotometer(Japan, Hitachi, Ltd).

  1.2   Synthesis of {[(CH₃)₂NH₂]₂ (CdL)}·x(Solvent)(1)

  H₄L(36.19 mg, 0.087 mmol), Cd(NO₃)₂·4H₂O(13.42 mg, 0.0435 mmol), N, N-dimethylformamide(DMF, 1 mL), C₂H₅OH(0.5 mL) and CH₃CN(0.5 mL) were combined in a 20 mL scintillation vial, sealed, and heated to 105 ℃ for 36 h and cooled to room temperature. The block crystals were collected and washed by DMF. IR(KBr), σ/cm^-1:1681, 1642, 1565, 1488, 1372, 786, 728.

  1.3   Crystal structure determination

  Single-crystal X-ray diffraction data was collected using a Bruker D8 Venture diffractometer(CuKα, λ=0.154178 nm) for compound 1. Indexing was performed using APEX2(Difference Vectors method)^[11]. Data integration and reduction were performed using SaintPlus 6.01^[12]. Absorption correction was performed by multi-scan method implemented in SADABS^[13]. Space groups were determined using XPREP implemented in APEX2^[12]. Structures were solved using SHELXS-97(direct methods) and refined using SHELXL-97(full-matrix least-squares on F²) contained in Olex2^[14]. For compound 1, the contribution of heavily disordered solvent molecules was treated as diffuse using Squeeze procedure implemented in Platon program^[15-16]. Crystal data and refinement conditions are shown in Table 1. CCDC:1551383.

  Table 1.  Crystal data and structure refinement for compound 1

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  Identification code	Compound 1
  Empirical formula	C18H8CdN2O10
  Formula mass	524.66
  T/K	293(2)
  Wavelength/nm	0.154 178
  Crystal system	Monoclinic
  Space group	P21/n
  Unit cell dimensions	a=1.024 88(1) nmα=90°
  	b=1.854 8(3) nmβ=108.074(3)°
  	c=1.559 1(2) nmγ=90°
  Volume/nm3	2.817 6(7)
  Z	4
  Density(calculated)/(mg·m-3)	1.237
  Absorption coefficient/mm-1	6.603
  F(000)	1 032
  Crystal size/mm3	0.100×0.060×0.050
  Identification code	Compound 1
  Theta range for data collection/(°)	5.176 to 72.368
  Index ranges	-12≤h≤12, -22≤k≤22, -19≤l≤19
  Reflections collected	26302
  Independent reflections	5 388 [R(int)=0.047 5]
  Completeness to theta=20.879°	96.7%
  Refinement method	Full-matrix least-squares on F2
  Data/restraints/parameters	5 388/0/290
  Goodness-of-fit on F2	1.086
  Final R indices [I > 2sigma(I)]	R1=0.034 3, wR2=0.085 5
  R indices(all data)	R1=0.035 0, wR2=0.085 9
  Largest diff. peak and hole/(e·nm-3)	813 and -907

  2   Results and Discussion

  2.1   Crystal structure of compound 1

  Indeed, solvothermal reaction of Cd(NO₃)₂·4H₂O and H₄L in the presence of DMF/EtOH/acetonitrile afforded high yields of colorless block crystals of compound 1. Crystallographic data reveals that compound 1 crystallizes in monoclinic space group P2₁/n. In compound 1, each Cd²⁺ ion is coordinated by seven oxygen atoms from four different carboxylate groups, meanwhile, each ligand is linked by four Cd²⁺ ions, leading to the formation of a 3D anionic framework, which further takes place the 2-fold interpenetration giving the formation of the double-fold interpenetrated three-dimensional anionic porous compound 1(Fig. 1A, 1B). Along a-and c-axis, compound 1 exhibits the 1D channels with the pore size(determined by the van der Waals diameter of the inserted pseudo atom) of 0.36 and 0.33 nm, respectively. The total potential solvent accessible volumes in desolvated compound 1 calculated by the PLATON/SOLV program is ca. 44.5%.

  图1 (A)Inorganic and organic secondary building units(SBUs) in compound 1; (B)The 2-fold interpenetrated 3D porous anionic framework of compound 1 built from Cd²⁺ ion and H₄L; (C)The topological network of compound 1 with a new point(Schläfli) symbol of {8³}₂{8⁶} constructed from the SBUs in (A). Hydrogen atoms are omitted for clarity Figure1. (A)Inorganic and organic secondary building units(SBUs) in compound 1; (B)The 2-fold interpenetrated 3D porous anionic framework of compound 1 built from Cd²⁺ ion and H₄L; (C)The topological network of compound 1 with a new point(Schläfli) symbol of {8³}₂{8⁶} constructed from the SBUs in (A). Hydrogen atoms are omitted for clarity

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  To better understand the structure, the Cd²⁺ ion is simplified as 4-connected node, and the organic ligand serve as two 3-connected nodes, then compound 1 may be described as a (3, 3, 4)-connected topological network with a new point(Schläfli) symbol of {8³}₂{8⁶} (Fig. 1C).

  2.2   Thermal stabilities and powder X-ray diffraction

  The thermogravimetric analysis(TGA) of as-synthesized compound 1 was performed to confirm its thermal stability. As shown in Fig. 2, a large mass loss is observed before 300 ℃, which indicates the loss of solvent molecules in the pores. Then, a further mass loss is corresponding to the framework collapse around 370 ℃. Moreover, the TGA curves of solvent-exchanged compound 1 exhibit the similar profiles.

  图2 TGA plots of the as-synthesized and solvent-exchanged compound 1 Figure2. TGA plots of the as-synthesized and solvent-exchanged compound 1

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  The powder X-ray diffraction(PXRD) has been measured to confirm the phase purity of the bulk samples(Fig. 3). The pattern of the as-synthesized sample is well coincident with the simulated one derived from the X-ray single crystal data, implying the phase purity of the bulk sample. In addition, the PXRD patterns of solvent-exchanged compound 1 that are also coincident with the as-synthesized one indicate the good stability of compound 1 in the different solvent of acetone, CH₂Cl₂, CH₃CN, methanol and ethanol, respectively.

  图3 PXRD patterns of the calculated, as-synthesized and solvent-exchanged compound 1 Figure3. PXRD patterns of the calculated, as-synthesized and solvent-exchanged compound 1

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  2.3   Selective adsorption of MB over MO

  To evaluate the adsorption capacity of compound 1 for dyes of MB and MO, the UV/Visible absorption spectrum of cationic MB in DMF solution in the presence of compound 1 was monitored at room temperature. As shown in Fig. 4, the main absorption band of MB at 670 nm drops rapidly along with time and completely vanishes after 74 h, which indicates the certain adsorption capacity of MB by compound 1 and simultaneously confirms the inherent anionic framework of compound 1. Meanwhile, the UV/Visible absorption spectrum of anionic MO in DMF solution in the presence of compound 1 was also monitored at room temperature, resulting in negligible change of the main absorption band of MO at 430 nm.

  图4 Sequential UV-Vis spectra changes of MB(A) and MO(B) in DMF solution after the addition of compound 1 Figure4. Sequential UV-Vis spectra changes of MB(A) and MO(B) in DMF solution after the addition of compound 1

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  2.4   Photoluminescence properties

  Motivated by the luminescent emission of many d¹⁰-configuration transition metals-based MOFs, the photoluminescence(PL) properties of compound 1 was measured together with the organic ligand of H₄L(Fig. 5). Upon photo excitation at 270 nm, the ligand displays a broad emission at 400 nm, while besides the luminescent emission at 400 nm, compound 1 also demonstrates an apparent luminescence emission band peaked at 485 nm. The luminescence emission band of compound 1 may be attributed to the ligand-to-metal charge transfer(LMCT)^[17-19].

  图5 Solid state PL spectra of linker H₄L and compound 1 recorded at room temperature Figure5. Solid state PL spectra of linker H₄L and compound 1 recorded at room temperature

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  3   Conclusion

  In conclusion, based upon a tetratopic ligand of 5, 5′-[oxalylbis(azanediyl)]diisophthalic acid, a new oxalamide-functionalized anionic MOF of compound 1 has been successfully synthesized with 2-fold interpenetrated 3D porous framework. Interestingly, compound 1 exhibits a (3, 3, 4)-connected topological network with a new point(Schläfli) symbol of {8³}₂{8⁶}. Moreover, it displays selective adsorption capacity toward methylene blue over methyl orange due to its anionic framework and the photoluminescence emission at visible region.

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• 

  Figure 1  (A)Inorganic and organic secondary building units(SBUs) in compound 1; (B)The 2-fold interpenetrated 3D porous anionic framework of compound 1 built from Cd²⁺ ion and H₄L; (C)The topological network of compound 1 with a new point(Schläfli) symbol of {8³}₂{8⁶} constructed from the SBUs in (A). Hydrogen atoms are omitted for clarity

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  Figure 2  TGA plots of the as-synthesized and solvent-exchanged compound 1

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  Figure 3  PXRD patterns of the calculated, as-synthesized and solvent-exchanged compound 1

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  Figure 4  Sequential UV-Vis spectra changes of MB(A) and MO(B) in DMF solution after the addition of compound 1

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  Figure 5  Solid state PL spectra of linker H₄L and compound 1 recorded at room temperature

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  Table 1.  Crystal data and structure refinement for compound 1

  Identification code	Compound 1
  Empirical formula	C18H8CdN2O10
  Formula mass	524.66
  T/K	293(2)
  Wavelength/nm	0.154 178
  Crystal system	Monoclinic
  Space group	P21/n
  Unit cell dimensions	a=1.024 88(1) nmα=90°
  	b=1.854 8(3) nmβ=108.074(3)°
  	c=1.559 1(2) nmγ=90°
  Volume/nm3	2.817 6(7)
  Z	4
  Density(calculated)/(mg·m-3)	1.237
  Absorption coefficient/mm-1	6.603
  F(000)	1 032
  Crystal size/mm3	0.100×0.060×0.050
  Identification code	Compound 1
  Theta range for data collection/(°)	5.176 to 72.368
  Index ranges	-12≤h≤12, -22≤k≤22, -19≤l≤19
  Reflections collected	26302
  Independent reflections	5 388 [R(int)=0.047 5]
  Completeness to theta=20.879°	96.7%
  Refinement method	Full-matrix least-squares on F2
  Data/restraints/parameters	5 388/0/290
  Goodness-of-fit on F2	1.086
  Final R indices [I > 2sigma(I)]	R1=0.034 3, wR2=0.085 5
  R indices(all data)	R1=0.035 0, wR2=0.085 9
  Largest diff. peak and hole/(e·nm-3)	813 and -907

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