English

• 1. INTRODUCTION

  The coordination-driven supramolecular assembly has proven to be one of the most well-established and powerful bottom-up tools capable of not only constructing well-defined and highly ordered two- or three-dimensional supramolecular architectures with robust and directional metal-ligand coordinative bonding^[1-7], but also, very often, imparting cation- or anion-sensing functionality to the materials due to the involvement of these ions as the bridging modules^[8-13]. Host-guest systems are organized by the type of noncovalent interactions including a wide range of attractive and repulsive forces such as ion-ion interactions, van der Waals forces, ion-dipole interactions, dipole-dipole interactions, hydrogen bonds, halogen bonds, π-π interactions, cation-π, anion-π interactions and so on^[14-16]. In order to form a host-guest complex, it is best to use a flexible host because it is beneficial to overcome energetic (entropy and enthalpy) costs associated with restricting itself to a single conformation which reduces the overall binding free energy^[17].

  With the rapid increase of global demand for energy, nuclear energy has attracted worldwide attention due to its advantages of safety, cleanness, low greenhouse gas emissions and price economy^{[18, 19]}. However, the nuclear fuel processing and some nuclear accidents release large amounts of radioactive iodine isotopes (¹²⁹I and ¹³¹I), which hinders the promotion of nuclear energy and poses severe challenges to modern science and technology^[20]. Among the radioisotopes of iodine, ¹²⁹I is a long-life isotope with a half-life of 1.57 × 10⁷ years. ¹³¹I is a short life isotope with relatively short half-life (8.02 days). Both of them can accumulate biologically through the food chain, thus affecting the metabolism process of human body^{[21, 22]}. The accidents at Chernobyl and Fukushima nuclear power plant prove that we still cannot control nuclear energy safely. Therefore, it has become a hot topic for scientists to deal with radioactive iodine quickly and effectively^[23].

  In previous work, we synthesized a series of metal-organic macrocycles, cages and capsules, which show potential applications in the complexation of inorganic anions, optical sensing and catalysis^[24-28]. In this work, we synthesized four water soluble metal-organic capsules (1·4NO₃^-, 2·4NO₃^-, 3·6NO₃^-, and 4·6NO₃^-) synthesized by using amide based polypyrazole ligands and dipalladium corners. The introduction of amide groups into ¹H-pyrazole-based ligands not only enhances hydrogen bonding between hosts and guests, but also increases the flexibility of the receptor molecules. Moreover, aromatic metal constructs are directional, which can control the configuration of the hosts, enhance the electrostatic interaction, and form a π···π stacking mode. Therefore, all the four supramolecular assemblies have the properties of adsorbing iodine via halogen bonds, electrostatic interactions and other supra-molecular interactions.

  Scheme 1

  

  Scheme 1.  Synthesis of compounds 1·4NO₃^-~4·6NO₃^-

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  2. EXPERIMENTAL

  2.1 Materials and methods

  All the reagents viz. plladium(II) chloride, 2, 2΄-dipyridyl, silver nitrate, 1, 4-dicarboxybenzene, trimesic acid, 3-aminopyrazole and 3-amino-5-methylpyrazole used were obtained from commercial suppliers and used as received. Acetone, thionyl chloride, N, N-dimethylformamide and acetonitrile were applied after redistilling. Deionized water was obtained by distillation using the ultrapure water mechanism of the best series laboratory. ¹H NMR spectra of all compounds were carried out on a Bruker 400 MHz spectrometer in DMSO-d₆ solution with TMS as internal standard at room temperature. Mass spectra were recorded on a JEOL Accu-TOF-4G LC-plus mass spectrometer. Elemental analyses (C, H and N) were carried out using German Elementar Vario Macro cube Analyzer. FT-IR spectra were recorded in a PerkinElmer spectrum two infrared spectrometer in the mid-IR region (4000 to 500 cm^-1). UV-vis spectra were carried out on a UV-2600 UV-vis. spectrophotometer with a 0.5 nm slit width. And thermogravimetric studies (25~800 ℃) were carried out using Labsys Evo Thermal Analyzer at a heating rate of 10 ℃·min^-1.

  2.2 Syntheses

  The ligands H₂L¹, H₂L², H₃L³ and H₄L³, and the cages of 1·4NO₃^-~4·6NO₃^- were synthesized by a same method as our previous work^[29].

  2.2.1   Synthesis of 1·4NO₃^-

  A mixture of H₂L¹ (29.5 mg, 0.1 mmol) and [(bpy)₂Pd₂(NO₃)₂](NO₃)₂ (78 mg, 0.1 mmol) was stirred vigorously in 6 mL H₂O at room temperature for 3 hours, and then 5 mL acetone was added. The mixture was stirred for another hour at room temperature, and the solution became clear. Then, the mixture was removed to the oil-bath of 60 ℃ and continued stirring overnight. ¹H NMR spectroscopy confirmed quantitative formation of {[(bpy)₂Pd₂]₂L₂¹}(NO₃)₄ (1·4NO₃^-). Yield: 90%. ¹H NMR (400 MHz, DMSO-d₆, 300 K): δ 11.06 (s, 2H), 10.91 (s, 2H), 8.73 (d, 4H), 8.57 (d, 4H), 8.48 (t, 4H), 8.39~8.30 (m, 8H), 8.00~7.98 (m, 8H), 7.86~7.83 (m, 12H), 7.71~7.65 (m, 4H), 6.73 (d, 2H), 6.70 (d, 2H). Anal. Calcd. for [C₆₈H₅₂N₂₄O₁₆Pd₄]: C, 43.28; H, 2.78; N, 17.81. Found: C, 43.31; H, 2.75; N, 17.76.

  2.2.2   Synthesis of 2·4NO₃^-

  Compound {[(bpy)₂Pd₂]₂L₂²}(NO₃)₄ (2·4NO₃^-) was synthesized by a similar method as 1·4NO₃^-. Yield: 94%. ¹H NMR (400 MHz, DMSO-d₆, 300K): δ 10.91~10.80 (d, 4H), 8.75~8.72 (m, 4H), 8.56 (t, 4H), 8.48~8.43 (m, 4H), 8.36~8.29 (m, 4H), 8.20 (d, 4H), 8.11 (t, 4H), 7.92 (s, 3H), 7.83 (s, 5H), 7.80~7.76 (m, 4H), 7.69~7.65 (m, 4H), 6.54~6.49 (d, 4H), 2.42 (d, 12H). Anal. Calcd. (%) for [C₇₂H₆₀N₂₄O₁₆Pd₄]: C, 44.51; H, 3.11; N, 17.30. Found (%): C, 44.56; H, 3.05; N, 17.41.

  2.2.3   Synthesis of 3·6NO₃^-

  A mixture of H₃L³ (40.5 mg, 0.1 mmol) and [(bpy)₂Pd₂(NO₃)₂](NO₃)₂ (116 mg, 0.15 mmol) was stirred vigorously in 6 mL H₂O at room temperature for 3 hours, and then 5 mL acetone was added. The mixture was stirred for another hour at room temperature. Then, the mixture was removed to the oil-bath of 60 ℃ and continued stirring overnight. ¹H NMR spectroscopy confirmed quantitative formation of {[(bpy)₂Pd₂]₃L₂³}(NO₃)₆ (3·6NO₃^-). The resulting solution was allowed to stand for several days during which pale yellow crystals of the products along with the co-crystallized solvent water molecules were formed (3·3.5NO₃^-·4O₂). Yield: 90%. ¹H NMR (400 MHz, DMSO-d₆, 300K): δ 10.76 (s, 6H), 8.74 (d, 6H), 8.51~8.44 (m, 18H), 8.38 (d, 6H), 8.03 (d, 6H), 7.93 (d, 6H), 7.84~7.78 (m, 12H), 7.18 (s, 6H), 6.76 (d, 6H). Anal. Calcd. (%) for [C₉₆H₇₂N₃₆O₂₄Pd₆]: C, 41.89; H, 2.64; N, 18.32. Found (%): C, 41.90; H, 2.57; N, 18.28.

  2.2.4   Synthesis of 4·6NO₃^-

  The self-assembly of 4·6NO₃^-: compound {[(bpy)₂Pd₂]₃L₂⁴}(NO₃)₆ (4·6NO₃^-) was synthesized by a similar method as 3·6NO₃^-.

  4·6NO₃^-: yield: 91%. ¹H NMR (400 MHz, DMSO-d₆, 300K): δ 10.66 (s, 6H), 8.71~8.69 (d, 6H), 8.54~8.52 (d, 6H), 8.43 (m, 12H), 8.22~8.21 (d, 6H), 8.17~8.15 (d, 6H), 7.78~7.72 (m, 12H), 7.38 (m, 6H), 6.45 (m, 6H), 2.38 (s, 18H). Anal. Calcd. (%) for [C₁₀₂H₈₄N₃₆O₂₄Pd₆]: C, 43.19; H, 2.98; N, 17.78. Found (%): C, 43.23; H, 2.95; N, 17.69.

  2.3 X-ray data collection and structure refinement

  Data collections were performed at 150 K for 3·3.5NO₃^-·4O₂ on a Bruker Smart APEX II CCD diffractometer equipped with graphite-monochromated MoKα radiation (λ = 0.71073 Å). The absorption correction was performed using SADABS. The structure was solved by direct methods and refined employing full-matrix least-squares on F² by using the SHELXTL (Bruker, 2000) software program and expanded using Fourier techniques. All non-H atoms of the compounds were refined with anisotropic thermal parameters. The hydrogen atoms were included in idealized positions. The final residuals along with unit cell, space group, data collection, and refinement parameters are presented in Tables S1-S2.

  2.4 Host-guest study for compounds 1·4NO₃^-~4·6NO₃^- with iodine molecules

  Pale yellow powders of compounds 1·4NO₃^-~4·6NO₃^- were dried in a programmable oven at 60 ℃ for 24 h. The powder (30~40 mg) with its weight exactly measured was immersed in the n-hexane solutions (5 mL) containing iodine molecules for 24 h at 25 ℃. The process of iodine adsorption by compounds 1·4NO₃^-~4·6NO₃^- was monitored by visual photos and UV-vis. The structures of compounds 1·4NO₃^-, 2·4NO₃^- and 4·6NO₃^- adsorbed iodine were determined by ¹H NMR. The adsorbed amount of iodine into the porous solids was estimated by electrospray ionization mass spectrometry (ESI-MS), thermogravimetric analysis (TGA), infrared spectrum (IR) and UV/vis spectroscopy.

  3. RESULTS AND DISCUSSION

  3.1 Crystal structure of 3·3.5NO₃^-·4O₂

  The structural analysis showed that compound 3·3.5NO₃^-·4O₂ crystallizes in monoclinic space group P2₁/c. It clearly shows that compound 3·3.5NO₃^-·4O₂ consists of two deprotonic ligand L³ and three dipalladium corners, displaying a drum-like cage conformation at the center of compound 3·3.5NO₃^-·4O₂, and the distance between the upper and lower sides of the drum (The distance between the benzene rings of the two ligands) is about 6.07 Å. The opening of the three dipalladium corners faced in the same direction, the arrangement in space formed a propeller shape (Fig. 1), the dihedral angles of the dipalladium corners are 83.74°, 79.10° and 80.65°, respectively, and the distances between Pd···Pd of each dipalladium corner are 3.26, 3.28 and 3.19 Å, respectively, which is in the range of typical Pd···Pd interactions (2.60~3.30 Å), suggesting the presence of weak metal-metal interactions. Interestingly, two nitrate anions are encapsulated in the cage via N–H···O hydrogen bonding interactions. One of the nitrate anions is located between the benzene rings of the two ligands and forms a nearly parallel configuration with the two benzene rings (The dihedral angles between the nitrate and the two benzene rings are 1.77° and 4.68°, respectively) via anion-π interactions. The distances between the nitrate and the two benzene rings are 3.08 and 3.17 Å, respectively. Simultaneously, the distances of N–H···O hydrogen bonds between the nitrate anion and the amide group are 2.15 and 2.09 Å, respectively. While another nitrate anion located in the cavity forms a nearly vertical structure with the benzene rings of the ligands, and the dihedral angles between the nitrate and the two benzene rings are 88.69° and 89.80°, respectively. Meanwhile, the nitrate anion forms an almost parallel configuration with 2, 2΄-bipyridine of a dipalladium corner through anion-π interactions. The dihedral angle between the nitrate and 2, 2΄-bipyridine is 8.44°. Similarly, hydrogen bonds are also built by the nitrate anion and amide NH groups, and the distances between N–H···O are 2.23 and 2.13 Å, respectively. There are a lot of water molecules in the crystal lattice (Fig. S20). The molecule cage of 3 is stacked into an original 1-D infinite chain via b-axis by π···π stacking interactions, while the structure of 3·3.5NO₃^-·4O₂ forms a grid framework via c-axis by π···π stacking interactions, as shown in Fig. 2. The intermolecular π-stacking interaction is shaped by the benzene rings of the ligands and the outside 2, 2΄-bipyridine moieties from the adjacent molecules. The distance between the benzene rings and the adjacent 2, 2΄-bipyridine moieties (π···π distance) is about 3.51 Å, while the π···π distance of the outside 2, 2΄-bipyridine moieties from the adjacent molecules is about 3.30 Å.

  Figure 1

  

  Figure 1.  Crystal structure of compound 3·3.5NO₃^-·4O₂ with hydrogen bonds. (a) Top view and (b) side view of the molecular structure. All the other residual molecules were omitted for clarity (yellow: Pd(II); gray: C; blue: N; red: O; white: H)

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  Figure 2

  

  Figure 2.  Stacking crystal structure of compound 3·3.5NO₃^-·4O₂ (a) along the b-axis and (b) along the c-axis. All the other residual molecules were omitted for clarity (yellow: Pd(II); gray: C; blue: N; red: O; white: H)

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  3.2 Study on the adsorption of iodine molecules by compounds 1·4NO₃^-~4·6NO₃^-

  As compounds 1·4NO₃^-, 2·4NO₃^-, 3·6NO₃^- and 4·6NO₃^- all held stable cage-like structures, an I₂ absorption study has thus been performed on powder samples of these compounds at room temperature. To put 30 mg 1·4NO₃^- powder into the n-hexane solution of iodine, it is very obvious that the color of the solution changed from purplish red to colorlessness which can be easily detected by the naked eye, as shown in Fig. 3. And the color of 1·4NO₃^- powder was changed from light yellow to dark red, which indicates the absorption properties of 1·4NO₃^- and its potential applications in removing radioactive iodine isotopes (¹²⁹I and ¹³¹I) that are liberated during nuclear fuel treatment. To investigate the kinetics of I₂ adsorption of 1·4NO₃^-, in situ UV/vis spectrum was recorded at room temperature. The visible absorption curve of n-hexane decreases with time. During the experiment, no further change was observed after 24 hours. The amount of I₂ loading of I₂@1·4NO₃^- was 13.5% by weighing method. ¹H NMR spectra were analyzed to investigate the iodine loading (Fig. 4a). It clearly showed that no significant change was observed after adsorbing I₂, so the compound 1·4NO₃^- remained undamaged. Importantly, the ¹H NMR spectra of compound I₂@1·4NO₃^- showed that compound 1·4NO₃^- still encapsulated a nitrate anion unsymmetrically after adsorbing iodine. In addition, the powder X-ray diffraction (XRD) spectra of 1·4NO₃^- and I₂@1·4NO₃^- also demonstrated that the structure of compound 1·4NO₃^- after and before adsorption of iodine is consistent with each other (Fig. S1). Therefore, it can be inferred that compound 1·4NO₃^- adsorbs iodine through supramolecular interactions such as halogen bond and electrostatic interaction. In addition, in ESI-MS experiments, the peak present at 881.49 showed the molecular ion peak of [1·2NO₃^-]²⁺. After adsorbing a molecule of iodine, the molecular weight (molecular formula: C₆₈H₅₂N₂₄O₁₆Pd₄I₂) was 2140.78, and ESI-MS peak present at 1008.00 (Fig. 4b) clearly showed that a molecule of iodine is adsorbed by a molecule of 1·4NO₃^-. And thermogravimetric analysis (TGA) was also carried out to study the thermal stability and the incorporation percentage of guest molecules in I₂@1·4NO₃^- (Fig. 4c). These results are in accordance with the above data, which indicated that iodine is adsorbed by intermolecular weak interaction with 1·4NO₃^-, which is further evidenced by infrared data (Fig. 4d). Similarly, 2·4NO₃^-, 3·6NO₃^- and 4·6NO₃^- also have the properties of adsorbing iodine via supramolecular interactions, and the amount of I₂ loading was 11.6%, 8.4%, and 8.2%, respectively (Fig. S2-S19).

  Figure 3

  

  Figure 3.  Photographs showing the color change when 1·4NO₃^- was soaked in n-hexane solution of I₂, and the UV/vis spectra of n-hexane solution of I₂ with 1·4NO₃^- soaked in for the adsorbing process of iodine (Inset shows the photographs of 1·4NO₃^- before and after I₂ adsorption)

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  Figure 4

  

  Figure 4.  (a) ¹H NMR spectra (400 MHz, 300 K, DMSO-d₆) of complex 1·4NO₃⁻ before (blue) and after (red) iodine adsorption. (b) ESI-MS spectra of I₂@1·4NO₃^-. (c) TGA curves of 1·4NO₃^- and I₂@1·4NO₃^-. (d) IR spectra of 1·4NO₃^- and I₂@1·4NO₃^-

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  4. CONCLUSION

  In summary, we have synthesized four metal-organic molecular cages 1·4NO₃^-, 2·4NO₃^-, 3·6NO₃^-, and 4·6NO₃^- from coordination-driven self-assembly of the flexible amide-based polypyrazole ligands and dipalladium corners. These water-soluble compounds were characterized in both solution (NMR, ESI-MS) and solid state (X-ray single-crystal diffraction, TGA and infrared spectrum). Interestingly, all the four supramolecular assemblies are capable of adsorbing iodine through halogen bonds, electrostatic interactions and other supramolecular interactions.

  
  
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  Scheme 1  Synthesis of compounds 1·4NO₃^-~4·6NO₃^-

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  Figure 1  Crystal structure of compound 3·3.5NO₃^-·4O₂ with hydrogen bonds. (a) Top view and (b) side view of the molecular structure. All the other residual molecules were omitted for clarity (yellow: Pd(II); gray: C; blue: N; red: O; white: H)

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  Figure 2  Stacking crystal structure of compound 3·3.5NO₃^-·4O₂ (a) along the b-axis and (b) along the c-axis. All the other residual molecules were omitted for clarity (yellow: Pd(II); gray: C; blue: N; red: O; white: H)

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  Figure 3  Photographs showing the color change when 1·4NO₃^- was soaked in n-hexane solution of I₂, and the UV/vis spectra of n-hexane solution of I₂ with 1·4NO₃^- soaked in for the adsorbing process of iodine (Inset shows the photographs of 1·4NO₃^- before and after I₂ adsorption)

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  Figure 4  (a) ¹H NMR spectra (400 MHz, 300 K, DMSO-d₆) of complex 1·4NO₃⁻ before (blue) and after (red) iodine adsorption. (b) ESI-MS spectra of I₂@1·4NO₃^-. (c) TGA curves of 1·4NO₃^- and I₂@1·4NO₃^-. (d) IR spectra of 1·4NO₃^- and I₂@1·4NO₃^-

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