Cucurbit[10]uril-Based Smart Supramolecular Organic Frameworks in

Jun 16, 2017 - Key Laboratory of Macrocyclic and Supramolecular Chemistry of Guizhou Province, Department of Chemistry, Guizhou University, Guiyang 55...
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Cucurbit[10]uril-Based Smart Supramolecular Organic Frameworks in Selective Isolation of Metal Cations Yu-Qing Yao,† Ying-Jie Zhang,‡ Chao Huang,† Qian-Jiang Zhu,† Zhu Tao,† Xin-Long Ni,*,† and Gang Wei*,§ †

Key Laboratory of Macrocyclic and Supramolecular Chemistry of Guizhou Province, Department of Chemistry, Guizhou University, Guiyang 550025, China ‡ Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, New South Wales 2232, Australia § CSIRO Manufacturing, PO Box 218, Lindfield, New South Wales 2070, Australia S Supporting Information *

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orous metal−organic frameworks (MOFs)1 have attracted considerable interest because of their novel structures and potential applications,2 particularly in relation to molecular storage and separation sciences.3,4 For example, the ability to alter the size and shape of the pores within MOFs has led to these architectures being used as shape-selective materials that preferentially retain compounds with specific dimensions.5,6 Surprisingly, there are many metal ions exploited for construction of various functional MOFs, but little attention has been paid to fabricate porous materials for isolation of metal cations.7 As a matter of fact, it is well-known that a significant economic incentive exists for metal ions recovery, not only from ores but also from the waste products of consumer electronics. For example, trivalent cations such as Al3+ and In3+ have been widely used in many industrial fields, including the manufacturing of cars and computers. However, because of their closed shell configuration, it is very difficult to separate them from mixture solutions.8 The most common methods used for metal cations separation include ion exchange, chromatography, liquid−liquid extraction, electrolysis, distillation and coprecipitation,9,10 but they often involve multiple steps, tedious organic solvents, highly poisonous inorganic species, and relatively high cost. Especially, developing green methodologies for metal extraction and recovery is very important from a modern environmental perspective. Herein, we report a simple, lower cost but highly efficient supramolecular approach for the sequence isolation of a number of metal cations such as Ca2+, Cs+, Ga3+, and light lanthanide cations La3+, Ce3+ and Pr3+ by utilizing the different porous size of cucurbit[10]uril11−15-based smart supramolecular organic frameworks (SOFs)16−18 in HCl and HNO3 aqueous solution. Cucurbit[n]urils (Q[n]s or CB[n]s)19,20 are characterized not only by a rigid hydrophobic cavity that can capture various guest molecules, which resulted in the establishment of Q[n]based host−guest chemistry21−36 but also two polar portals rimmed with carbonyl groups that can coordinate to various metal ions, which led to the rapid development of Q[n]-based coordination chemistry.37−39 In particular, our recently studies along with related results from others have revealed that noncovalent interactions, such as hydrogen bonding, as well as ion−dipole interactions involving the electrostatically positive outer surface of Q[n], could serve as driving forces in the © 2017 American Chemical Society

formation of various novel Q[n]-based supramolecular architectures and materials.40 Further studies indicated that the so-called outer-surface interactions of Q[n]s has led to a new area of study in cucurbituril family.41−46 For example, in 2014, Sun and co-workers reported an outersurface interactions derived honeycomb-like Q[6]-based 2D SOFs that exhibited high selectivity for the capture of cesium cations among the common alkali metal ions in a basic medium, and then released these cations under acidic conditions.41 The cesium cations were bound at the portals of Q[6] molecules in triangular branches of the networks. Interestingly, we found that a similar 2D metal-free Q[10]-based SOFs can be observed in a solid compound reported by Isaacs in one of his feature articles in 2009,47 and the outer-surface interaction between different Q[10] molecules could be the driving force responsible for formation of the SOFs (Figure 1a). One can

Figure 1. (a) 2D metal-free Q[10]-based SOFs; (b) three Q[10] molecules in a triangular branch.

also see the triangular branches in the network, and each branch is constructed of three Q[10] molecules, yielding a porous area with a high negatively electrostatic density of portal carbonyl oxygen atoms. Attributed to the larger cavity and portal size of Q[10], it seems that the pore size formed by Q[10] molecules is more suitable to accommodate metal cations compared to that from Q[6]s.41 Essentially, we wonder whether the Q[10]-based SOFs flexible in regards to the pore size as MOFs and whether they can be used for isolation of metal ions (Figure 1b). Received: May 1, 2017 Revised: June 16, 2017 Published: June 16, 2017 5468

DOI: 10.1021/acs.chemmater.7b01751 Chem. Mater. 2017, 29, 5468−5472

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Chemistry of Materials

Q[10]. The linear polymer is constructed from alternating Q[10] molecules and Ba2+ cations through direct coordination. Each Q[10] molecule in the polymer coordinates with four Ba2+ cations, and each Ba2+ cation (Ba1 and Ba2 in green) is coordinated by eight oxygen atoms, four portal carbonyl group from two neighboring Q[10] molecules (in red) and four coordinated water molecules (in light-blue). The Ba−Ocarbonyl distances are in the range 2.726−2.911 Å, (average 2.795 Å) and Ba−Owater distances are in the range 2.786−3.154 Å, (average 2.908 Å) (Figure 2e). Generally, each triangular branch based porous void in the SOFs captures two Ba2+ cations, which directly coordinate to the portal carbonyl oxygen atoms among two of the three Q[10] molecules. Close inspection reveals that the outer surface interaction is mainly derived from the electrostatically positive outer surface of the third Q[10] and the portal carbonyl oxygen atoms of the two coordinated Q[10] molecules in the branch, and the distances between the carbonyl oxygen atoms and the Q[10] methylene, methine and carbonyl carbon atoms are in the range 2.892− 3.515 Å (Figure 2c). The powder X-ray diffraction (PXRD) pattern of 1 matches the corresponding simulated pattern well suggesting the formation of pure phase crystalline product (Figure S2). Moreover, energy-dispersive spectroscopy (EDS) results revealed that the compound 1 contained about 9.6% barium, in accordance with that in the crystal structure (Figure S3). Similar coordination fashions like Ba2+ with Q[10] were observed for La3+ and Ce3+ cations. However, a slightly different coordination behaviors of Gd3+ and Dy3+ within the Q[10]-based porous area were noted in a close inspection (Figure 2b). Namely, each Gd3+ or Dy3+ cation is still coordinated by eight oxygen atoms, but only two portal carbonyl group from two neighboring Q[10] molecules (in red) and six coordinated water molecules (in light-blue) included. The Gd−Ocarbonyl distances are in the range 2.354− 2.382 Å, (average 2.368 Å) and Gd−Owater distances are in the range 2.317−2.423 Å. Similarly, The Dy−Ocarbonyl distances are in the range 2.322−2.351 Å, (average 2.336 Å) and Dy−Owater distances are in the range 2.309−2.394 Å (Figure 2f). This may be ascribed to the shorter ionic radius of Gd3+ and Dy3+ cations (0.94 and 0.91 Å, respectively), which is slightly smaller than the critical radius required to stabilize and balance the coordination force to the four oxygen atoms on the two neighboring carbonyl portals of Q[10]s in the triangular branch. As a result, only two carbonyl atoms on the two Q[10]s were assigned to capture one Gd3+ or Dy3+ cation in the assistance of coordinated water molecules and the outer surface interactions of the third Q[10] molecules (Figure 2d). In an effort to gain more detailed binding information on the free-Q[10] based SOFs on the capture of the target metal cations, a series of structure data including the triangular branch angle in the absence and presence of metal ions were systematically investigated. As depicted in Figure S4, singlecrystal X-ray diffraction analysis revealed that all the Q[10] complexes in the present study appear to have the same crystal system, space group, and similar cell parameters. This result indicated that the free-Q[10] based SOFs is stability in hydrochloric acid even after coordination with metal ions. Furthermore, it is worth noting that the triangular branch angles in the SOFs was obviously tunable by the captured metal ions. In particular, distinguished angle change of the branch after coordination Gd3+ and Dy3+ which bearing shorter ionic radius. For example, the angle of the two neighboring

In the present work, Q[10]-based coordination complexes with a series of metal cations, such as alkali (A+), alkaline earth (AE2+), the third main group metal ions, lanthanides (Ln3+) and so on, have first been studied in aqueous HCl solution. We noted that aqueous HCl solutions containing Q[10] can almost quantitatively yield precipitates of Q[10]−Mn+ complexes when upon addition of various investigated metal ions, respectively. Interestingly, single crystals of Q[10]−Mn+ complexes could be obtained from the microcrystal precipitate in 1−3 days. However, the single crystals were small and the data were only can be collected on the MX beamlines at the Australian Synchrotron. After many tries, crystal structures of Q[10] together with its complexes with Ba2+, La3+, Ce3+, Gd3+ and Dy3+ were successfully determined (Figure 2). At first glance,

Figure 2. Crystal structure of compounds 1 and 5: (a and b) overall view of the Q[10]−Ba2+-based 2D coordination network and the Q[10]−Gd3+-based 2D coordination network along the b-axis; (c and d) detailed interaction between three Q[10] molecules in a triangular branch; (e) detailed interaction between a Q[10] molecule and four Ba2+ cations; (f) detailed interaction between a Q[10] molecule and four Gd3+ cations.

all of the five Q[10]−Mn+ complexes appears to have similar coordination fashions and SOFs assemblies. Salient crystal data and structure refinement details are available in Table S1 in the ESI. Herein, we take the Q[10]−Ba2+-based coordination system as a representative case. Microcrystal precipitation occurred immediately by simply mixing a solution of BaCl2 (0.01 M in 6.0 M HCl) with a saturated Q[10] (ca: 0.01 M) in 6.0 M HCl solution in a 1:1 volume ratio. Single crystals of Q[10]−Ba2+ (1) could be obtained from the microcrystal precipitate in 1−3 days (Figure S1). Figure 2a shows an overview of the Q[10]− Ba2+-based 2D coordination network in compound 1. This network can be viewed as a combination of linear Q[10]−Ba2+based coordination polymer and metal-free Q[10] molecules, assembled mainly through the outer-surface interaction of 5469

DOI: 10.1021/acs.chemmater.7b01751 Chem. Mater. 2017, 29, 5468−5472

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Chemistry of Materials

representative example. The solid of Q[10]−Ba2+ was easily dissolved in a 3 M HNO3 solution, and a white precipitation was quantitatively formed within 1 h. EDS results confirmed that the solid did not contain barium ions (Figure S6), and PXRD results revealed that the solid gave a similar pattern to that of free Q[10] crystallized in HNO3 solution (Figure S7). All the results indicated that the different SOFs of Q[10] formed in HCl and HNO3 could be potentially used to separate metal ions. Similar structures were seen for other compounds obtained from Q[10]−Mn+−HCl systems based on their single-crystal Xray structures or PXRD patterns (Table S1, Figures S8 and S9), and the solids obtained by dissolving these Q[10]−Mn+-based complexes in 3 M HNO3 followed by precipitation also showed similar PXRD patterns to that of compound 2 (Figure S10). Metal ions in the Q[10]−Mn+−HCl systems include A+, AE2+, Ln3+, the third main group metals, but not d-transition metal ions, which generally form polychloride transition metal anions, such as [CdCl4]2− or [ZnCl4]2−, which are well-known structure-directing agents38 in the construction of Q[n]-based coordination polymers and supramolecular assemblies through the outer-surface interactions of Q[n]s. These d-transition metal anions prefer to stay at the outer surface of Q[n]s, rather than at their portals. Thus, the combination of the Q[10]−Mn+−HCl and Q[10]−Mn+−HNO3 systems establishes a reversible process, which could enable possible applications in cation capture and release with implications of potential metal separations. The Q[10]−HCl system captures metal ions to form solid Q[10]− Mn+ complexes, and the Q[10]−Mn+−HNO3 system maintains the Mn+cations in solution, but precipitates the solid Q[10]based supramolecular assembly. This reversible process is shown in Scheme S1. To understand better the practical applications of the Q[10]based SOFs systems in the isolation of metal ions from a mixture, competitive and selective experiments for specific metal cations under certain conditions were carried out. For example, in a 6 M HCl solution, the Q[10] molecule prefers to coordinate common alkali metal ions, such as Na+, Rb+ and Cs+, but most especially K+ cations, and the EDS spectrum of the precipitate showed no signal corresponding to K+ (Figure

coordination Q[10] molecules change from 45.8° to 61.8° and 61.6° upon addition of Gd3+ and Dy3+, respectively. This result suggested that the free-Q[10] based SOFs has a flexible and tunable property toward different metal ions. Interestingly, we have tried different synthetic routes38,39 to prepare Q[10]-metal based coordination complexes or supramolecular assemblies in other acids such as H2SO4, HNO3, HF, CH3COOH and so on, but the experimental results revealed that no precipitates were formed. It was found that nice crystals of 2 can easily be obtained from aqueous HNO3 solution, but they readily disintegrate in air (Figure S5). Consequently, its crystal structure refinement has been very challenging. However, as shown in Figure 3, a porous layer constructed of

Figure 3. Crystal structure of compound 2: (a) a Q[10]-based layer constructed of orthogonal Q[10] molecules; (b) detailed interactions between two orthogonal Q[10] molecules.

Q[10] molecules is observed. Each Q[10] molecule is set almost vertically on a portal of two neighboring Q[10] molecules, meanwhile, two portals of the Q[10] molecule are covered by two respective Q[10] molecules in the same manner, and four orthogonal Q[10] molecules create a square hole. The driving force is undoubtedly the outer-surface interaction of Q[10], which may prevents metal ions from coordinating to its portal carbonyl oxygen atoms in aqueous HNO3 solutions. In other words, metal ions captured by Q[10] in HCl solution as precipitates could be released in HNO3 solutions. Thus, we still take the Q[10]−Ba2+ complex as a

Figure 4. Flow diagram of metal cations recovery process based on Q[10]−SOFs: (a) alkali metal ions; (b) alkaline-earth metal ions; (c) Cs+, Ba2+, Ln3+ system; (d) third main group metal ions. Black arrows and boxes indicate the precipitate of Q[10]−Mn+ complex, blue arrows and boxes indicate metal ions in 6 M HCl solution; red arrows and boxes indicate the isolated metal ions. 5470

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S11); On the other hand, the Q[10]−SOFs system exhibited a specific affinity for Ca2+ cations among the other alkaline-earth metal ions, and no signals of Mg2+, Sr2+ or Ba2+ were detected in the EDS spectrum of the precipitate from the same aqueous HCl solution (Figure S12). Curiously, a Q[10]−HCl solution was added to a solution containing Cs+, Ba2+ and Ln3+ (one or more Ln cations) to yield a precipitate, only the Cs+ cation was found in the EDS spectrum (Figure S13). As expected, the Q[10]−HCl system also showed high affinity for Ga3+ among the metal ions in the third main group elements (Figure S14). Most importantly, according to the coordination fashions of Q[10] with metal ions in the solid state (the molar ratio of Q[10] to metal ions is 3:4 in all the five crystal structures, Figure S4). Our further studies revealed that the Q[10]−SOFs in HCl and HNO3 solutions exhibited sequence selectivity isolation of specific metal cations by controlling the molar ratio between Q[10] and the target metal ions. For example, as shown in Figure 4, upon addition of Q[10] to a mixture in a HCl solution of alkaline-earth metal ions with a molar ratio of 3:4:4:4:4 (Q[10]/Mg2+/Ca2+/Sr2+/Ba2+), it was found that the isolation sequence is following Ca2+, Ba2+, Sr2+, Mg2+ (Figures S12, S16 and S17) where Q[10] was recycled quantitatively. Similar isolation was sequence also observed in Cs+, Ba2+ and Ln3+ mixture system (Figure S13, Figures S18− S20), and the third main group elements (Figure S14, S21 and S22). Accordingly, these observations suggested that the Q[10]−SOFs have possible applications in the selective isolation of metal cations by rearrangement of their SOFs assemblies in different acidic solutions. In summary, we have investigated the interaction of Q[10]− SOFs with a series of metal cations, including A+, AE2+, Ln3+, the third main group metals, in aqueous HCl and HNO3 solutions, respectively. In the HCl medium, Q[10]−SOFs can coordinate to selected metal cations to form 1D coordination polymers, whereas the HNO3 medium can dissociate Q[10] molecules from the coordination polymers to form metal-free Q[10]-based 2D SOFs through the outer-surface interaction of Q[10] molecules. Most interestingly, the Q[10]−SOFs in HCl and HNO3 solutions exhibited sequence selectivity isolation of specific metal cations. This is mainly attributed to the flexible and tunable porous size of Q[10]-based SOFs in different acidic solutions such as HCl and HNO3 aqueous system, and thus led to these architectures being used as metal-selective materials. More detailed investigations on the Q[10]-based SOFs and its functions are currently underway.

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AUTHOR INFORMATION

Corresponding Authors

*X.-L. Ni. E-mail: [email protected]. *G. Wei. E-mail: [email protected]. ORCID

Yu-Qing Yao: 0000-0002-8497-1749 Xin-Long Ni: 0000-0002-5557-1631 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51663005, 21361006), “Chun-Hui” Fund of Chinese Ministry of Education (Z2016011), the Science and Technology Talent Fund of Guizhou Province (20165656), and the Graduate Student’s Fund for innovation of Guizhou University (2017008). The single crystal data were collected on the MX beamlines at the Australian Synchrotron, Victoria, Australia.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b01751. Crystal structure of compound 1 Crystal structure of compound 2 Crystal structure of compound 3 Crystal structure of compound 4 Crystal structure of compound 5 Crystal structure of compound 6 Experimental procedure, Crystal EDS data (PDF)

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(CIF) (CIF) (CIF) (CIF) (CIF) (CIF) data, PXRD patterns,

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Chemistry of Materials

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DOI: 10.1021/acs.chemmater.7b01751 Chem. Mater. 2017, 29, 5468−5472