English

• 1. INTRODUCTION

  Discrete coordination containers featuring well-defined hollow structures have attracted significant attention in view of their well-organized polyhedral architectures, convenient coordination-driven self-assembly process, as well as promising applications such as gas storage and separation, supramolecular catalysis, molecule recognition, and so on^[1-10]. We devote oneself to the development of a new class of coordination containers named metal-organic supercontainers (MOSCs)^[10-18], which were built from sulfonylcalix[4]arenes, divalent metal ions, and suitable carboxylate linkers. They feature unique multi-pore architecture containing an endo cavity defined by metal ions-connected carboxylate linkers and sulfonylcalix[4]arenes, and multiple exo cavities from the upper rim of sulfonylcalix[4]arenes. The exo cavity is tunable by chemical modification on the para substituent group of the sulfonylcalix[4]arenes, giving rise to unique selective gas adsorption properties through modulation on the solid-state porosity^{[10, 19]}. On the other hand, by judicious selection of carboxylate linkers, various molecular topologies of MOSCs have been readily achieved. For example, typical MOSCs with the geometries of face-directed octahedron (type I MOSCs)^[11], edge-directed octahedron (type II MOSCs)^[10], barrel-shaped box (type III MOSCs)^[12], and cylindrical structure (type IVMOSCs)^[13] were readily achieved using trigonal, linear, angular-planar, and angular-nonplanar carboxylate ligand as linkers, respectively. It has been well studied that elongating the tricarboxylate linkers successfully resulted in remarkable extension of the endo cavity in the face-directed octahedral type I MOSCs^{[11, 20-22]}. In contrast, investigations on elongated edge-directed octahedral type II MOSCs are limited^[23].

  Figure 1

  

  Figure 1.  Structural representations of (a) type I, (b) type II, (c) type III, and (d) type IV MOSCs^[10-13]. The yellow sphere serves to guide the eyes

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  Type II MOSCs with the 1, 4-benzenedicarboxylate (BDC) ligand as linker exhibited remarkable O₂/N₂ adsorption selectivity depending on different degree of porosity collapse, which is closely related to different molecular sizes of MOSCs manipulating by para substituent modification on the sulfonylcalix[4]arene precursor^[10]. Herein, we present the assembly of a new MOSC using 2, 6-naphthalenedicarboxylate (NDC) ligand as linear ditopic linkers. Replacing the BDC ligand with elongated NDC linker significantly enlarges the molecular size of resulted MOSC, and hence effectively modulates the solid-state porosity and gas adsorption properties.

  2. EXPERIMENTAL

  2.1 General methods

  Unless otherwise noted, starting materials and solvents were purchased from commercial suppliers and used without further purification. p-tert-Butylsulfonylcalix[4]arene (H₄TBSC)^[24] was prepared according to the reported procedures. Powder X-ray diffraction results were measured on Rigaku Miniflex with CuKα radiation of λ = 1.5405 Å operated at 30 kV at the scan rate of 2 degree/min. Thermogravimetric analysis (TGA) was obtained from a Netzsch thermal analyzer (model-STA449C) at a heating rate of 5 ℃/min under a constant nitrogen flow. Gas adsorption isotherms were measured using a Micromeritics ASAP3020 instrument based on a volumetric method. Samples were typically washed with methanol and pre-dried on a Schlenk line at 120 ℃ for 8 h before being transferred to pre-weighed analysis tubes which were then capped with seal frits. The samples were degassed under dynamic vacuum (< 6 µmHg) at 120 ℃ for about 24~48 h until the outgas rates were lower than 5 µmHg/min. The analysis tubes containing the evacuated samples were weighed again to determine the sample weights before being transferred back to the analysis port of the instrument. The N₂ isotherms were measured at 77 K in a liquid N₂ bath using ultra high pure (UHP) grade gas (99.99%), the CH₄, C₂H₂, C₂H₄ and C₂H₆ isotherms were measured at 298 K in a dry water bath using ultra high pure (UHP) grade gases (99.99%).

  2.2 Synthesis of container 1

  Co(NO₃)₂∙6H₂O (119.2 mg, 0.4 mmol), 2, 6-naphthalenedicarboxylic acid (43.24 mg, 0.2 mmol) and H₄TBSC (84.9 mg, 0.1 mmol) were dissolved in a mixture solvent of dimethylacetamide (DMA) (5 mL) and methanol (1 mL) in a scintillation vial (20 mL capacity). The vial was placed in a sand bath, which was transferred to a programmable oven and heated at a rate of 0.5 ℃/min from 30 to 100 ℃. The temperature was held at 100 ℃ for 24 h before the oven was cooled at a rate of 0.2 ℃/min to the final temperature of 30 ℃. Pink crystals of 1 were formed after one week and isolated by filtration. The crystals were then vacuum dried at 120 ℃ to give rise to 24.67 mg of the activated material (yield: 10%).

  2.3 Structure determination

  X-ray single-crystal diffraction data were collected on a Bruker D8 Venture diffractometer using IμS 3.0 microfocus source Mo-Kα radiation (λ = 0.71073 Å) and PHOTON II CPAD detector. Frames were integrated with the Bruker SAINT software package (V8.38A) using a SAINT algorithm. Data were corrected for absorption effects using the multi-scan method (SADABS)^[25]. The structure was solved and refined using the Bruker SHELXTL Software Package (SHELXT 2014/5), a computer program for automatic solution of crystal structures, and refined by the full-matrix least-squares method with ShelXle Version 4.8.6, a Qt graphical user interface for the SHELXL-2016/6^{[26, 27]}. Hydrogen atoms are obtained by theoretical calculations, and all non-hydrogen atoms were generated by Fourier synthesis and correction by difference electron density function. The electron count due to disordered solvent molecules in the void space of the crystals was calculated using the program SQUEEZE in PLATON software package and refined further using the data generated^[28]. SQUEEZE gives 5142 electrons/unit cell for the voids, which are occupied by the solvents (DMA, 48 e^– and/or CH₃OH, 18 e^–). Taking into account of the TGA data, container 1 can be formulated as [Co₄(μ₄-H₂O)(TBSC)]₆(NDC)₁₂·(DMA)₂₅·(CH₃OH)₇₆.

  A total of 59011 reflections were collected in the range of 2.28≤θ≤25.00º, of which 18433 were unique (R_int = 0.1203). The empirical formula of container 1 is C₃₈₄H₃₄₈Co₂₄O₁₂₆S₂₄, tetragonal system, space group I4/m, with a = b = 30.9119(19) Å, c = 43.565(3) Å, V = 41628(6) ų, D_c = 0.731 g/cm³, M_r = 9162.36, Z = 2, F(000) = 9384, μ = 0.566 mm^–1, the final GOOF = 1.027, R = 0.0987 and wR = 0.2474 for 7240 observed reflections with I > 2σ(I).

  3. RESULTS AND DISCUSSION

  3.1 Synthesis and characterizations

  The new coordination container 1 was prepared from the reaction of p-tert-butylsulfonylcalix[4]arene (TBSC), 2, 6-naphthalenedicarboxylic acid (NDC), and Co(NO₃)₂∙6H₂O in the mixture solvent of DMA and MeOH at 100 ℃ for 24 h (Fig. 2), wherein red block crystals suitable for single-crystal X-ray diffraction (SCXRD) analysis were obtained after one week. As shown in Fig. 3a, the powder X-ray diffraction (PXRD) studies indicated the phase purity and crystallinity of the as-synthesized material of 1, however, it lost its crystallinity after removal of solvent molecules, plausibly attributing to partial structural collapse of the activated sample^[10]. The thermogravimetric analysis (TGA) (Fig. 3b) revealed the excellent thermal stability of compound 1 with the decomposition temperature higher than 420 ℃. The ca. 25% weight loss observed from the starting point to 300 ℃ was ascribed to the escape of solvent molecules (DMA and/or MeOH).

  Figure 2

  

  Figure 2.  Synthetic scheme of container 1. The yellow sphere serves to guide the eyes

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  Figure 3

  

  Figure 3.  (a) Experimental measured PXRD patterns of the as-synthesized (black) and activated (red) sample of 1 in comparison with the simulated patterns (blue) from single-crystal structures; (b) TGA of as-synthesized (black) and activated (red) sample of 1

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  3.2. Crystal structure

  The SCXRD study revealed that container 1 crystalized in the tetragonal crystal system and I4/m space group as same as the BDC analogue of MOSC-II-tBu-Co^[10]. The molecular structure of container 1 adopted an edge-directed octahedral architecture with C_4h symmetry, consisting of six TBSC-capped tetranuclear cluster subunits occupying the vertices and twelve NDC bridging ligands locating in edges (Fig. 2 and 4a). In the TBSC-capped tetranuclear unit, the TBSC serves as a tetrakis tridentate ligand to coordinate with the square Co₄ cluster through four phenoxo and four sulfonyl O atoms. Each Co(II) ion is octahedrally coordinated with eight oxygen atoms from TBSC (two μ-phenoxo and one sulfonyl O atoms), carboxylate ligand (two oxygen atoms from two carboxylate units), and µ₄-H₂O. Noticeably, two TBSC-capped tetranuclear units of compound 1 located in the axial position (Fig. 4b) slightly rotated along the C₄ axis as compared to the corresponding BDC-based MOSC-II-tBu-Co (Fig. 4c) due to the obvious dislocation of two carboxylate groups in NDC ligand. Additionally, four NDC linkers occupied in the equatorial position are disordered over two positions. With the elongated NDC dicarboxylate linker, the inner and outer dimension of compound 1 (ca. 2.0 and 3.6 nm, respectively) are slightly longer than those of the BDC-based analogue (ca. 1.7 and 3.3 nm, respectively)^[10]. The hydrophobic interactions between tert-butyl groups from six adjacent container molecules (Fig. 5) resulted in a pseudo body-centered cubic (bcc) crystal packing in 3D dimension.

  Figure 4

  

  Figure 4.  Crystal structure of 1 along (a) b axis and (b) c axis, and (c) MOSC-II-tBu-Co^[10] along c axis

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  Figure 5

  

  Figure 5.  (a) 2D and (b) 3D packing of 1 through hydrophobic interactions

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  3.2 Gas adsorption properties

  The solid porosity was confirmed by the N₂ adsorption experiment using activated sample of compound 1. As depicted in Fig. 6a, the adsorption isotherm of N₂ at 77 K displayed a "pseudo" type I model indicating the micro porous structure of 1 similar to the MOSC-II-tBu-Co^[10]. However, the Brunauer-Emmett-Teller (BET) surface area of 1 (205 cm²/g) was calculated to be only half of MOSC-II-tBu-Co (423 cm²/g) due to more severe degree of porosity collapse after enlarging the molecular size^[10]. Gas adsorption experiments of CH₄, C₂H₂, C₂H₄ and C₂H₆ were further investigated at 298 K (Fig. 6b). The C₂H₂ adsorption capacity for 1 is higher than its C₂H₄ and C₂H₆ uptake capacities and much higher than its CH₄ uptake. At 298 K and 100 kPa, it shows the highest uptake of 22.2 cm³/g for C₂H₂, which is nearly six times its CH₄ adsorption capacity (3.8 cm³/g). Meanwhile, it adsorbs 12.4 cm³/g of C₂H₄ and 11.6 cm³/g of C₂H₆ at 298 K and 100 kPa, which are three times its CH₄ uptake capacity. The results indicated its potentially relevant application in separation and purification of light hydrocarbons.

  Figure 6

  

  Figure 6.  (a) N₂ isotherm for 1 at 77 K; (b) CH₄, C₂H₂, C₂H₄ and C₂H₆ adsorption isotherms of 1 at 298 K

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  4. CONCLUSION

  In conclusion, a new metal-organic supercontainer adopting edge-direct octahedral architecture has been successfully obtained by using p-tert-butylsulfonylcalix[4]arene as the capping ligand and 2, 6-naphthalenedicarboxylate as the bridging ligand. Gas adsorption studies indicated that the new supercontainer is porous and show good adsorption selectivity of C₂ hydrocarbons over CH₄, suggesting the potential applications of this material for light hydrocarbons adsorption and separation.

  
  
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  Figure 1  Structural representations of (a) type I, (b) type II, (c) type III, and (d) type IV MOSCs^[10-13]. The yellow sphere serves to guide the eyes

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  Figure 2  Synthetic scheme of container 1. The yellow sphere serves to guide the eyes

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  Figure 3  (a) Experimental measured PXRD patterns of the as-synthesized (black) and activated (red) sample of 1 in comparison with the simulated patterns (blue) from single-crystal structures; (b) TGA of as-synthesized (black) and activated (red) sample of 1

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  Figure 4  Crystal structure of 1 along (a) b axis and (b) c axis, and (c) MOSC-II-tBu-Co^[10] along c axis

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  Figure 5  (a) 2D and (b) 3D packing of 1 through hydrophobic interactions

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  Figure 6  (a) N₂ isotherm for 1 at 77 K; (b) CH₄, C₂H₂, C₂H₄ and C₂H₆ adsorption isotherms of 1 at 298 K

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