Homochiral Metal–Organic Frameworks with Tunable Nanoscale

May 24, 2017 - As a new material, porous metal–organic frameworks (MOFs) have shown their potential application in this field because their structur...
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Homochiral Metal−Organic Frameworks with Tunable Nanoscale Channel Array and Their Enantioseparation Performance against Chiral Diols Chao Zhuo,†,‡ Yuehong Wen,† Shengmin Hu,† Tianlu Sheng,† Ruibiao Fu,† Zhenzhen Xue,†,‡,§ Hao Zhang,†,‡,∥ Haoran Li,†,‡ Jigang Yuan,†,‡ Xi Chen,†,‡ and Xintao Wu*,† †

State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, People’s Republic of China ‡ University of Chinese Academy of Sciences, Beijing 100049, People’s Republic of China S Supporting Information *

ABSTRACT: Enantioseparation is an integral process in the pharmaceutical industry, considering the ever-increasing demand for chiral medicine products. As a new material, porous metal−organic frameworks (MOFs) have shown their potential application in this field because their structures are easy to adjust and control. Though chiral recognition between racemic substrates and frameworks has made preliminary progress, discussions of their size-matching effects are rare. Herein with the help of channel-tunable homochiral MOFs (HMOFs), diols of different sizes have been separated in good enantiomeric excess (ee%). In addition, the ee% reaches 67.4% for the first time for diols as large as 1,1,2-triphenyl-1,2-ethanediol, which turns out to be the most effective value so far.

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INTRODUCTION Currently, numerous drugs used in the pharmaceutical field are chiral.1 Most of the time, only one enantiomer is helpful, while the other can be useless or even detrimental under specific conditions. Thus, it is essential to implement the production of enantiopure medicines. Before sophisticated methods of asymmetric catalysis are developed, enantioseparation is always an alternative to choose, considering the ever-increasing medical demands. However, in most cases, catalysis products still need enantioseparation even when the enantiomeric excess values (ee%) reach as high as 99%. In addition, the technique is also needed for various analysis requests. Focusing on enantioseparation, several classic methods have been developed, such as crystallization resolution, salification resolution, inclusion resolution, combinatorial resolution, and chromatography resolution.2−4 In addition to those, new techniques still come up every now and then.5,6 In recent years, porous metal−organic frameworks (MOFs), which have experienced rapid development over the past two decades,7−12 have shown their potential application in this field.13−18 Under the combination of chemical interaction and steric effects, MOFs can serve as a heterogeneous separation material without any extra species being brought into the substrates. According to reviews, the ee% values of racemic substrates separated through MOFs are distributed over a large range.19,20 In addition, the substrates contain amino acids,21 sulfoxides,22−25 alcohols,26−33 phenols,34 and even some complexes.35 It is assumed that the enantioseparation performance of MOFs, © 2017 American Chemical Society

more precisely homochiral MOFs (HMOFs), is related to both chiral recognition and pore size. Though recognition between substrates and frameworks has made preliminary progress, discussions about their size-matching effects are rare, because small-molecule substrates have been greatly preferred in previous work.36 To investigate the size-matching effects, HMOFs with large accessible pore sizes are needed. Therefore, it is necessary to do research into the regulation and control of HMOF structures.37−39 In general, it is effective and convenient to construct HMOFs when chiral components are introduced into the self-assembling process.40−42 On the basis of that strategy, we have succeeded here in preparing two pairs of layered-structure HMOFs with amino acids and pyridines. The frameworks contain two chiral sources, helix and chiral sites, which were induced by inheritance from the chiral amino acid ligands. The first Co pair with smaller channels was the initial HMOFs obtained in our experiment. In addition, their ladderlike structure inspired us to see if the pore size could be broadened to carry out the investigation expected above. It turned out that lengths of both linkers used were successfully modified to provide larger channels, assembling to the second Cd pair, where extra benzene rings were embedded into the structure (Figure 1). Different from predecessors’ work, the design was more flexible, because the rectangle in the ladder could stretch along two directions, making it easier to construct Received: February 8, 2017 Published: May 24, 2017 6275

DOI: 10.1021/acs.inorgchem.7b00352 Inorg. Chem. 2017, 56, 6275−6280

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hygroscopic, even more so than proline, while BPBP is quite stable in air. Therefore, PBP must be diluted in aqueous solution before use. The L and D configurations totally depend on the proline used. 4,4′-Bipyridine (BPY) is commercially available. 1,4-Bis[2-(4pyridyl)ethenyl]benzene (BPEB) was prepared as reported previously.47−50 Synthesis of HMOFs. The two shorter linkers PBP and BPY were reacted with Co(NO3)2 in aqueous solution. After reflux, red crystals were obtained through volatilization in 62.32% yield. L-PBP led to Co2[(L-PBP)(BPY)3(OH)2(H2O)2]·BPY (1-L), while D-PBP led to the D configuration (1-D). Similarly, Cd2[(L-BPBP)2(BPEB)(H2O)] (2-L) and the opposite hand 2-D was constructed with the corresponding BPBP, using the longer linkers. They were obtained in a solution mixture of N,N′-dimethylformamide and water by a solvothermal reaction at 85 °C. Under those conditions, yellow crystals were obtained in 32.51% yield. X-ray Crystal Structure Analysis. X-ray single-crystal structure determinations (SC-XRDs) revealed that both 1-L and 1-D belong to the monoclinic system with space group C2.51 Here, only a detailed structural description of 1-L (deposition number CCDC 1527927) is presented (Figure 2). The asymmetric unit consists of two Co2+ ions, one L-PBP, four BPYs, two hydroxyl ions, and two coordinated water molecules. Both Co2+ ions have an octahedral geometry, coordinated by three N atoms from three BPYs, one O atom from L-PBP, and two O atoms from two water molecules or hydroxyl ions, respectively. The whole framework is a stack of ladderlike layers which have two different kinds of channels throughout the structure along the b axis. The arms of the ladder are made up of Co2+ and BPY, while the bars are BPY and L-PBP. If the bars are ignored, we will get two planes. All arms in the same plane are parallel, but they are interlaced fitly between the planes. To make the picture clear, its simplified topology is shown with topological type 4L11 (4̂3.6̂3) (Figure 2). The arms are highlighted. It is interesting that there are dissociative BPYs between layers, which have π···π interactions with BPYs in the arm planes. Other channels are filled with disordered water molecules. 2-L and 2-D crystallize in space group C2 as well. With the single-crystal structure determination of 2-L (deposition

Figure 1. Structure of linkers and their extension modes.

large pores. Also, owing to the flexibility of peptide bonds, another ladderlike structure similar to that of the Co pair was obtained, whose pore size was as large as 1 × 1.7 nm. In addition, it was interesting that those channels in the second Cd pair arranged in a staggered A−B−A form to create a nanoscale channel array. With those HMOFs, diols of different sizes were chosen as substrates to study. Though separation of diols has been reported before, investigations have been limited to substrates with less than seven carbon atoms, except for those found by Maspoch’s group.43−45 However, in this experiment, the ee% reached 67.4% for the first time for diols as large as 1,1,2triphenyl-1,2-ethanediol.

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RESULTS AND DISCUSSION Synthesis of Linkers. Phenyl-4,4′-bis[carbonyl-N-(proline)] (PBP) and biphenyl-4,4′-bis[carbonyl-N-(proline)] (BPBP) have been prepared in a modified way through an amidation reaction according to the literature.46 In that case, proline is treated in an alkaline aqueous solution. Then 1,4dichloroformylbenzene or 4,4′-biphenyldicarbonyl chloride is added and the mixture was stirred overnight. Because of the chemical similarity of proline and the target products, purification is needed in postprocessing. White frothy products are obtained after vacuum drying in yields of 55.97% and 57.76%, respectively. It is notable that PBP is highly

Figure 2. (a) Coordination environment of Co1 and Co2 in 1-L. (b) Crystal-packing diagram of 1-L along the b axis. (c, d) Simplified topology of the 1-L network. The arms of the ladder are shown in bold to emphasize the crossed structure. 6276

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Figure 3. (a) Coordination environment of Cd1 and Cd2 in 2-L. (b) Crystal-packing diagram of 2-L along the b axis. (c, d) Simplified topology of the 2-L network. The arms of the ladder are shown in bold to emphasize the crossed structure.

Figure 4. (a) Solid-state CD spectra of bulk samples of 1-L and 1-D. (b) Solid-state CD spectra of bulk samples of 2-L and 2-D. (c) P-XRD patterns of 1-L and 1-D and a simulated profile. (d) P-XRD patterns of 2-L and 2-D and a simulated profile.

homochirality. 1-L and 1-D give opposite CD signals at 230 and 285 nm. The same phenomenon occurs for 2-L and 2-D at 268 and 395 nm (Figure 4). Thermal gravimetric analysis (TGA) indicates that 1-L loses nearly 15% of its weight before it begins to collapse, which matches well with the elemental analysis (Figure S5 in the Supporting Information). This means that there are many water molecules in its channels. Without those guests, the framework can be ruined easily. However, 2-L with larger channels possesses less water guests in it. If we examine the topological structures of 1-L and 2-L carefully, we can find that there are more bridges between layers in 2-L than in 1-L. This is why 2-L can be stable in that situation. In addition, it is stable until 200 °C (Figure S7 in the Supporting Information). Channel Tuning and Enantioselective Separation. 1-L and 1-D were the initial HMOFs obtained in our experiment. The channels through the structure were thought to play a key role in enantioseparation. Unfortunately, they were not stable enough in conventional solvents such as methanol and

number CCDC 1529067) as an example, the asymmetric unit consists of two Cd2+ ions, two L-BPBPs, one BPEB, and one coordinated water (Figure 3). The two Cd2+ ions have different coordination modes. Cd1 has a distorted-octahedral geometry. Four O atoms in the equatorial plane are from three carboxyl groups. The axial sites are occupied by one N from BPEB and one O from water. In addition, Cd2 has a distorted-squarepyramidal geometry. There are three O atoms from two carboxyl groups and one N aton from BPEP in the bottom surface and one O atom from a carboxyl group in the conic node. The framework is also a ladderlike layer. However, the arms are made of L-BPBP and BPEB alternatively, while the bars are only L-BPBP. The point symbol for the net is {6∧3}{6∧5.8} (Figure 3). Similarly, water exists in the channels. Other Characterization Data. According to X-ray powder determination (P-XRD) measurements, the two pairs share the same structure and match well with the simulated spectrum, respectively (Figure 4). Furthermore, their solid-state circular dichroism (CD) measurements demonstrate their enantiotive 6277

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Inorganic Chemistry dichloromethane. This is probably because of the dissociative BPYs between layers. When other solvent molecules got into the layers, the π···π interaction maintained by the dissociative BPYs was weakened, leading to the isolation of layers, which presented crystal expansion and flocculation from a macroscopic point of view. To make crystals stable in solvents, it was necessary to keep away guests that could induce strong interaction between layers from the very start of crystallization. Therefore, linkers were changed before thermal synthesis to eliminate the interruption of those BPYs. At the same time, the lengths of both linkers increased in comparable size to maintain the ladder structure. Though the ratio of mixed linkers changed, a new ladderlike structure was obtained as expected. Thus, 2-L and 2-D were obtained, which were capable of being investigated for enantioseparation ability. In comparison to other HMOFs reported in this field, 2-L and 2-D have larger channels, which have a size of 1 × 1.7 nm. It is obvious that their enantioseparation toward small-molecule alcohols will give low ee% values, according to an assumption of the predecessors. Therefore, larger substrates such as 1-phenyl1,2-ethanediol, hydrobenzoin, and 1,1,2-triphenyl-1,2-ethanediol were chosen as substitutes to study. The separation was carried out by guest exchange in solution. This means that 2-L/2-D must be immersed into a solution of racemic substrates to absorb them in a certain proportion and then into a fresh solution to release them. It is important that they are stable during the process. The P-XRD spectrum of 2-L indicates that the framework stays nearly the same even when it has been through all the infusions (Figure 5).

Table 1. Enantioselective Separation for Racemates at Room Temperaturea

a

Determined by HPLC at room temperature.

is feasible for HMOFs to have a good potential in the pharmaceutical industry.

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CONCLUSIONS To conclude, we have succeeded in preparing two pairs of HMOFs. On structure adjustment where the linkers constituting both edges of the ladder were lengthened, the pore size of the framework was enlarged dramatically while the ladderlike framework self-assembled in a similar way. In addition, one of the HMOFs showed high performance in the enantioseparation of chiral diols, better than ever reported. Also, the closer the sizes of the substrates and channels are with each other, the better the enantioseparation performance they will give. In a word, this indicates that HMOFs are qualified for large diols, even other kinds, as long as they have the proper pore size. Nevertheless, accurate matching of size, accompanied by chiral recognition, may be the topic of further research in the future.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00352. Experimental details, spectral data, and crystal data and refinement parameters (PDF)

Figure 5. P-XRD of 2-L, as-synthesized and after separation.

Accession Codes

CCDC 1527926, 1527927, and 1529067 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

All of the substrates are shown in Table 1; 2-L and 2-D give various results for aromatic alcohols of different sizes, according to high-performance liquid chromatography (HPLC) analysis. This analysis reveals that ee% values of those substrates released from 2-L and 2-D are at nearly the same level, which again demonstrates their same structural characterization. It is obvious that the ee% values increased when the structures of substrates possess more phenyl groups, especially in a huge range between one and two phenyl groups. However, ee% values increase at in a lower rate when the number of groups goes to three. In addition, the presence of one or two chiral sites of substrates here did not make any remarkable difference. The result is mainly size-directed. It is estimated that the ee% value will increase but at a slower and slower rate if the substrates are larger, considering the variation trend of data we obtained. In a word, the good enantioseparation performance of HMOFs toward large diols is confirmed. This proved that it

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AUTHOR INFORMATION

Corresponding Author

*E-mail for X.W.: [email protected]. ORCID

Tianlu Sheng: 0000-0003-4679-8989 Xintao Wu: 0000-0002-8624-166X Present Addresses §

Z.X.: College of Chemistry and Chemical Engineering, Collaborative Innovation Center for Marine Biomass Fiber

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Materials and Textiles, Qingdao University, Shangdong 266071, People’s Republic of China. ∥ H.Z.: Department of Physical Chemistry, School of China Pharmaceutical University, Nanjing 211198, People’s Republic of China. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation of China (21233009), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB20010200) and the 973 Program (2014CB845603) for financial support.

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ABBREVIATIONS HMOFs, homochiral metal−organic frameworks; ee%, enantiomeric excess value; PBP, phenyl-4,4′-bis[carbonyl-N-(proline)]; BPBP, biphenyl-4,4′-bis[carbonyl-N-(proline)]; BPY, 4,4′bipyridine; BPEB, bis[2-(4-pyridyl)ethenyl]benzene; SC-XRD, X-ray single-crystal structure determination; P-XRD, X-ray powder determination; CD, circular dichroism; TGA, thermal gravimetric analysis; HPLC, high performance liquid chromatography

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DOI: 10.1021/acs.inorgchem.7b00352 Inorg. Chem. 2017, 56, 6275−6280