Pillar[5]arene-Based Molecular Recognition Induced Crystal-to-Crystal

May 22, 2019 - Single-crystal-to-single-crystal (SCSC) transformations are of wide interest in the research field of crystal engineering, which are wi...
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Pillar[5]arene-Based Molecular Recognition Induced Crystal-to-Crystal Transformation and Its Application in Adsorption of Adiponitrile in Water Bingbing Shi,† Liqing Shangguan,† Huanhuan Wang,‡ Huangtianzhi Zhu,† Hao Xing,† Peiren Liu,† Yuezhou Liu,† Jiyong Liu,† and Feihe Huang*,† †

State Key Laboratory of Chemical Engineering, Center for Chemistry of High-Performance & Novel Materials, Department of Chemistry, Zhejiang University, Hangzhou, Zhejiang 310027, P. R. China ‡ MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, P. R. China S Supporting Information *

ABSTRACT: Single-crystal-to-single-crystal (SCSC) transformations are of wide interest in the research field of crystal engineering, which are widely used for the precise study of the relationship between structure and property. Here, we report one SCSC transformation triggered by pillararene-based molecular recognition. One pillar[5]arene-based linear supramolecular polymer forms in the solid state initially and, then, converts into side-chain polypseudorotaxane induced by molecular recognition between a naphthalene-containing pillar[5]arene NP5 and adiponitrile. Moreover, the SCSC transformation is further applied in the efficient removal of adiponitrile from water.

S

Pillar[n]arenes (n = 5−15) are a new class of macrocyclic hosts with unique pillar-shaped architectures and well-defined cavities.15 The host−guest properties of pillar[n]arenes have been widely applied in the fabrication of various supramolecular functional materials in solution, as well as in the solid state, including molecular machines,16 interlocked structures,17 supramolecular amphiphiles,18−20 supramolecular polymers,21,22 artificial transmembrane channels,23 and adsorption and separation materials.24,25 During our curiositydriven study on pillar[n]arene-based crystal engineering,26 we thought that the design of pillar[n]arene-based host−guest interactions induced SCSC transformations would have remarkable impacts on the application of pillar[n]arene-based functional crystalline materials. Another important issue in this work is the fact that adiponitrile, an important raw chemical that is widely used as a

ince Desiraju and co-workers initiated the basic concept of crystal engineering in the late 1980s based on small organic molecules with geometrical simplicity,1,2 more complex single crystals with tailored functions were obtained by employing organic molecules that have further structural complexity.3−7 In this field, the research of single-crystal-tosingle-crystal (SCSC) transformations arouses great interests from scientists because of their wide use for the precise investigation of the relationship between structure and property.8−10 The single-crystal structures can be compared before and after the crystal structural change using singlecrystal X-ray analysis, which provides a molecular level understanding of the phase transformations and solid-state reactions. The studies of these relationships will be helpful for the design of functional switchable materials. In particular, supramolecular systems showing SCSC transformations accompanied by chemical and physical property changes have vital implications for the design and application of many functional materials.11,12 However, the reported examples of these types are quite rare.13,14 © 2019 American Chemical Society

Received: May 13, 2019 Accepted: May 22, 2019 Published: May 22, 2019 111

DOI: 10.1021/acsmaterialslett.9b00163 ACS Materials Lett. 2019, 1, 111−115

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Cite This: ACS Materials Lett. 2019, 1, 111−115

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ACS Materials Letters precursor in synthesizing chemical products, such as nylon and hexanediamine,27 has emerged as a health and environmental crisis in many communities all over the world.28,29 For example, it has been classified as an extremely hazardous substance in the United States.30 Therefore, the development of efficient water-treatment materials towards adiponitrile is important. Recently, pillar[n]arene-based crystals were successfully used for adsorption and separation of carbohydrates.25,26 Inspired by this, we wondered whether we could use pillar[n]arene-based crystals to effectively adsorb and remove adiponitrile micropollutant from water. Herein, we present the details of supramolecular SCSC transformation which was induced by pillar[5]arene-based molecular recognition between a naphthalene containing pillar[5]arene (NP5) and adiponitrile (Scheme 1). Specifically,

Figure 1. (a) Photo of NP5 single crystals in acetonitrile. (b) Optical microscopy image of NP5 single crystals in acetonitrile. (c) Photo of NP5/adiponitrile single crystals in aqueous solution of adiponitrile. (d) Optical microscopy image of NP5/adiponitrile single crystals in aqueous solution of adiponitrile.

Scheme 1. Chemical Structures and Single-Crystal Structures of Pillar[5]arene NP5, Adiponitrile, and the NP5/Adiponitrile Host−Guest Complexa

Figure 2. Crystal structures: (a, b) linear supramolecular polymer obtained by crystallization of NP5 in acetonitrile and (c, d) linear side-chain polypseudorotaxane obtained from NP5 after adsorption of adiponitrile. Hydrogen atoms in Figures 2−5 are omitted for clarity.

a

Hydrogen atoms are omitted for clarity.

a linear supramolecular polymer first formed in the solid state by crystallization of NP5 in acetonitrile. Then, after immersion of the single crystals in an aqueous solution of adiponitrile for 5 days, the linear supramolecular polymer converted into sidechain polypseudorotaxane in the solid state induced by the host−guest interactions between NP5 and adiponitrile. The crystal-to-crystal conversion was evidenced by single crystal Xray diffraction studies before and after the transformation. The result realizes the host−guest complexation induced SCSC transformation. Moreover, this pillar[5]arene-based SCSC transformation was further applied in efficient removal of adiponitrile micropollutant from water. Pillar[5]arene derivative NP5 was synthesized from a previously reported pillar[5]arene derivative 1 in one step (Scheme S1).31 Light yellow single crystals were obtained by slow evaporation of an acetonitrile solution of NP5 (Figure 1). As shown in Figures 2 and 3, the single crystal has a triclinic structure that contains infinite chains of NP5 molecules connected by π−π stacking interaction (3.373 Å) between the naphthalene groups on NP5 (Figure S6). One C−H···O interaction with H···O distance of 2.635 Å and one C−H···π interaction with distance of 2.800 Å are also observed (Figure S6), which further helps the formation of the assembled

Figure 3. (a, b) Packing structures of NP5. (c, d) Packing structures of NP5/adiponitrile.

structure. These lengths determined here are shorter than the van-der-Waals radius sum of the interacting atoms.32 Each linear supramolecular polymer is also stabilized by adjacent linear supramolecular polymers via C−H···π interaction. These results indicated that two linear supramolecular polymers successfully formed driven by C−H···π, π−π stacking, and C− H···O interaction in the solid state. After we successfully constructed the linear supramolecular polymer, a linear side-chain supramolecular polypseudorotaxane33,34 in the solid state was also obtained via SCSC transformation. The pseudorotaxane35−38 formation between NP5 and adiponitrile was first investigated by 1H NMR in acetonitrile. On the proton NMR spectrum of an equimolar (5.00 mM) solution of NP5 and adiponitrile in acetonitrile, 112

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The existence of NP5/adiponitrile phases was successfully confirmed by the comparison of the PXRD patterns recorded before and after adsorption of adiponitrile with the simulated profile (Figure S11). Thermogravimetric analysis (TGA) results also confirmed the stable storage and the quantitative adsorption of adiponitrile by NP5 crystals. As shown in Figure S12, TGA results indicated the weight loss of NP5 crystals started at about 350 °C, while the weight loss of NP5/ adiponitrile crystals started at about 150 °C, strongly supporting the adsorption of adiponitrile by NP5 single crystals. As a comparison, the adiponitrile adsorption behavior of amorphous solid of NP5 was also investigated by using the same adsorption method as NP5 crystals. The result showed that amorphous solid of NP5 removed adiponitrile from water with the corresponding uptake up to 83% (Figure S13), which is lower than that of NP5 crystals. This is mainly due to the highly ordered structure of NP5 crystals. An important feature of adsorbent materials is their reusability.40,41 In contrast to degradative and energyconsuming regeneration processes of many traditional adsorbent materials,42 the NP5 crystals showed good recyclability with a simple and energy-saving regeneration procedure. As shown in Figure 5, the adiponitrile-adsorbed

peaks related to protons on adiponitrile became broad and shifted upfield (Figure S7). Furthermore, the relative spatial positions of NP5 and adiponitrile in their host−guest complex was investigated by 2D NOESY NMR in acetonitrile39 (Figure S8). Correlation signals between the protons on adiponitrile and the bridging methylene protons and the aromatic protons on NP5 were observed, indicating that adiponitrile was threaded into the macrocyclic cavity of NP5 to form a pseudorotaxane in acetonitrile. Having established the NP5/adiponitrile recognition motif in acetonitrile, single crystals of a supramolecular polypseudorotaxane were obtained by immersing single crystals of NP5 into an aqueous solution of adiponitrile. X-ray analysis was performed to produce the single crystal structure of the linear side-chain supramolecular polypseudorotaxane. As shown in Figures 2 and 3, the resultant solid-state structure shows remarkably ordered 3-component assemblies. The adiponitrile molecule is threaded into the NP5 cavity to form a pseudorotaxane unit (Figure S9). Three hydrogen atoms on the included adiponitrile molecule with C−H···π distances of 2.693−2.895 Å are observed in the NP5/adiponitrile crystal structure. As shown in Figure S10, the pseudorotaxane units of NP5/adiponitrile single crystal are connected by C−H···π and C−H···O interactions with the distances of 2.860 and 2.687 Å to form linear side-chain polypseudorotaxanes throughout the entire NP5/adiponitrile crystal structure. Here the formation of the polypseudorotaxane and the crystal structure conversions were driven by the host−guest complexation between adiponitrile and NP5. From the SCSC transformations alone, it might be expected that the single crystal of NP5 has potential to adsorb adiponitrile from water because of its ideal cavity size. This proposal was tested by solid adsorption experiments. Quantitative 1H NMR was used to investigate the adiponitrile adsorption behavior of NP5 single crystal. As shown in Figure 4, after the immersion of 100 mg of the single crystals of NP5

Figure 5. Schematic representations of recycling NP5 from NP5/ adiponitrile single crystals: (I) dissolved in methylene chloride, (II) addition of methanol, (III) filtration, and (IV) crystallization.

crystals were first dissolved in methylene chloride. After the addition of methanol to the solution of NP5 and adiponitrile, NP5 precipitated from the solution. The resulting NP5 powder was collected by filtration and then dried under vacuum. Then NP5 single crystals were regained by crystallization of the NP5 powder in acetonitrile. Comparison of the 1H NMR data recorded after regeneration of NP5 with the initial profile of NP5 suggests the good reusability of NP5 (Figure S14). In conclusion, a SCSC transformation was successfully realized by taking advantage of pillar[5]arene-based molecular recognition. Specifically, a pillar[5]arene-based linear supramolecular polymer driven by C−H···π, π−π stacking, and C− H···O interactions formed in the solid state by the crystallization of pillar[5]arene NP5 in acetonitrile. The linear supramolecular polymer converted into side-chain polypseudorotaxane in the solid state triggered by the molecular recognition between NP5 and adiponitrile after immersing the single crystals in an aqueous solution of adiponitrile for 5 days. The SCSC transformation was evidenced by X-ray diffraction study. Moreover, the SCSC transformation was further applied in the efficient removal of adiponitrile micropollutant from water. The results in this work highlight the importance of the

Figure 4. Partial 1H NMR (600 MHz, D2O, 298 K) spectra: (a) adiponitrile in stock (c = 1.00 mmol/L) and (b) adiponitrile after adsorption by NP5 single crystals. Acetic acid was used as internal standard (c = 1.00 mmol/L). (c) The percentage of the micropollutant removal efficiency of adiponitrile from water by NP5 single crystals determined after a contact time of 5 days. (d) Schematic representations of crystal structure transformations upon adsorption of adiponitrile by NP5 crystals.

in an aqueous solution of adiponitrile (1.00 mmol/L, 10.0 mL) for 5 days, the NP5 crystals effectively removed the organic micropollutant adiponitrile from water with large uptake. Specifically, our results showed that NP5 crystals removed adiponitrile from water with the corresponding uptake up to 98% (Figure 4). Powder X-ray diffraction (PXRD) was then employed to monitor the adsorption processes of adiponitrile. 113

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SCSC transformation in the research area of supramolecular chemistry and expand the possibilities for the development of complicated supramolecular structures and their further applications.



ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmaterialslett.9b00163. Experimental details, NMR spectra, crystal data, and other materials (PDF) Crystallographic information file for NP5 (CIF) Crystallographic information file for NP5/adiponitrile (CIF)



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Bingbing Shi: 0000-0001-9523-5758 Huangtianzhi Zhu: 0000-0002-1271-6415 Hao Xing: 0000-0002-5643-3433 Feihe Huang: 0000-0003-3177-6744 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (21434005, 91527301). REFERENCES

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