Enantioselective Separation over a Chiral Biphenol-Based Metal

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Enantioselective Separation over a Chiral Biphenol-Based Metal− Organic Framework Anees Abbas,† Zhao-Xi Wang,†,‡ Zijian Li,† Hong Jiang,† Yan Liu,*,† and Yong Cui*,† †

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School of Chemistry and Chemical Engineering and State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China ‡ Department of Chemistry, Center for Supramolecular Chemistry and Catalysis, Innovative Drug Research Center, Shanghai University, Shanghai 200444, China S Supporting Information *

carboxylate bridging ligand and Cd2+, and the enantioseparation property toward aromatic alcohols and sulfoxides. An investigation of host−guest complex by single-crystal X-ray diffraction technique is conducted to give a straightforward insight and deeper understanding of enantioselective discrimination. The tetracarboxylate bridging ligand H4L was synthesized by bromination of ( S)-6, 6′-dimethyl-5,5′ -di (3 ,5-b is(methoxycarbonyl)phenyl-1,1′-biphenyl-2,2-diol followed by LiOH catalyzed hydrolysis in 78% overall yield. H4L was characterized by 1H and 13C NMR spectroscopy (Figure S1) and ESI-MS (Figure S2). A mixture of H4L and Cd(NO3)2· 4H2O (in a 1:6 molar ratio) in DMF and CH3CN was heated at 100 °C for 12 h to yield colorless crystals in 80% yield. Based on single-crystal X-ray diffraction (XRD), elemental analysis, and thermogravimetric analysis (TGA), the crystal was formulated as [Cd2L(DMF)2(H2O)]·DMF·3H2O (1). Single-crystal XRD study shows that 1 crystallizes in the chiral orthorhombic P212121 space group with one whole formula in the asymmetric unit. There are two crystallographically independent Cd(II) atoms in the structure. Cd1 is octahedrally coordinated by six carboxylate oxygen atoms of four L ligands, whereas Cd2 coordinates to three carboxylate oxygen atoms of three ligands and one water and two DMF molecules, with Cd− O bond lengths ranging from 2.205(4) to 2.380(4) Å. Cd1 and Cd2 are bridged by one μ2−η1,η2 and two μ2−η1,η1 carboxylate groups with Cd1 also coordinated by a chelating carboxylate group and Cd2 coordinated by solvents to form a Cd2(CO2)4 cluster. Each L ligand adopts an exo-heptadentate coordination fashion to bridge seven Cd ions of four Cd2 clusters via four bidentate carboxylate groups. Adjacent dicadmium units are linked through bridging carboxylate groups to form 21 helical chains, which are further connected by the L linkers to generate a 3D framework with 1D channels of ∼0.65 × 1.16 nm2 along the a-axis. If the ligand L and Cd2 cluster are both viewed as 4connecting nodes, the final 3D network can be simplified as a 4-c net of sra topology with the point (Schläfli) symbol (42·63·8) (Figure 1). PLATON calculation shows that 1 contains about 47.5% void space available for guest inclusion.9 TGA showed that the guest molecules could be removed in 50−300 °C. An additional weight loss of around 13% occurring at higher temperatures

ABSTRACT: A chiral porous 3D metal−organic framework (MOF) is constructed from an enantiopure carboxylate ligand of 1,1′-biphenol, which can be utilized as adsorbent for the separation of aromatic alcohols and sulfoxides with enantioselectivity of up to 99.4%. Singlecrystal X-ray diffraction analysis reveals the binding sites and host−guest interactions clearly, providing microscopic insight into the origin of the enantiosorption in the framework.

T

he preparation of chiral compounds in enantiomerically pure form is extremely important in fine chemical and pharmaceutical industries, also a challenging goal in modern organic synthesis.1 Chiral separation is efficient for achieving enantiopure chiral molecules.2,3 However, it is perceived as expensive and inefficient for the production of undesired enantiomer, and chiral selectors with low cost and superior capacity are still rather rare.3 Crystalline porous metal−organic frameworks (MOFs) have emerged as an ideal scaffold to construct the separation materials, whose highly tunable structures can be precisely tailored for adsorption of specific molecules.4 A sensible and discrete incorporation of multiple chiral recognition sites into MOFs can incisively control the orientation of two enantiomers within the confined microenvironment to boost enantiospecific interactions, thus exerting efficient stereocontrol and enantiomeric recognition.5 The separation of enantiomers with chiral MOFs via crystallization, chromatography, and membrane has been reported, and good results have been achieved in the separation of small alcohols, amines, amino acids, sulfoxides, and so on.6 Nevertheless, the resolution of racemic molecules is only evaluated for a relative small number of components with similar properties, and the development of novel MOF-based separators capable of furnishing wider separation ability is still an important and challenging task. Chiral 1,1′-biphenol, a powerful organic backbone for asymmetric induction,7 has been immobilized into structurally porous frameworks for the highly enantioselective separation.8 For instance, chiral Dy−Na−biphenol framework has been explored as adsorbent for separation of mandelate derivatives,8a while Mn−biphenol architecture can resolve racemic aromatic and aliphatic amines.6d In this work, we present the synthesis of a chiral porous MOF constructed from 1,1′-biphenol-derived © XXXX American Chemical Society

Received: April 9, 2018

A

DOI: 10.1021/acs.inorgchem.8b00948 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

meric excess (ee) with the R-enantiomer being in excess. The enantiosorption kinetics was studied by 1H NMR monitoring at different times. As shown in Figure 2, the adsorption of 1 to 1-

Figure 2. Adsorption kinetic profile of (S)-1 toward 1-phenylethanol.

phenylethanol reached equilibrium in 6 h, and the expected formation of kinetically stable 1:1 host−guest complex was also observed by 1H NMR spectroscopy. We then examined different aromatic alcohols under the optimized conditions to evaluate the enantioseparation capability of 1 (Table 1). Various alcohols with electronTable 1. Enantiosorption of (S)-1 Towards Racemic Alcohols and Sulfoxidesa

Figure 1. (a) Construction of MOF 1 from H4L and dimeric [Cd2(CO2)4(DMF)2(H2O)]. (b) The sra topology of 1. (c) The 3D porous structure of 1 viewed along the a-axis (Cd, light green polyhedral; C, gray; O, red; N, light blue; Br, orange; H atoms are omitted for clarity).

(300−400 °C) may be due to the loss of one coordinated water and two coordinated DMF molecules (Figure S3). The phase purity of bulk sample of 1 was established by comparison of its observed and simulated PXRD patterns, though the loss of guest molecules during the test may lead to a little distortion of the framework (Figure S4). The solid-state circular dichroism (CD) spectra of 1 based on the opposite enantiomers of H4L are mirror images of each other, suggesting their enantiomeric nature (Figure S5). The scanning electron microscopy (SEM) image of 1 revealed the formation of rod-like particles with a wide range of sizes in micrometer level (Figure S6). N2 adsorption measurement showed only surface adsorption, probably due to incomplete removal of solvent molecules in the pores as a result of strong host−guest hydrogen-bonding interactions, and/or possible structural distortions upon activation. Compound 1 could adsorb ∼1.4 Rhodamine 6G per formula unit in solution, demonstrating its porous nature and structural integrity in solution. The chiral porous structure and available chiral dihydroxyl groups of 1 have enabled us to examine its potential for enantioselective separation. The performance was first evaluated by enantioseparation of 1-phenylethanol. After initial optimization of the conditions by screening different solvents and temperatures, the inclusion was achieved by cocrystallization of evacuated (S)-1 and 1-phenylethanol in DCM at room temperature. The released alcohol, obtained by soaking the inclusion solids in Et2O was analyzed by chiral high-performance liquid chromatography (HPLC), yielding 99% enantio-

a

For details see the experimental section in Supporting Information. Determined by HPLC (letters in brackets specify the preferable isomer).

b

withdrawing substituents in the aromatic ring could be successfully separated, with 93%, 98%, and 98% ee values for 1-(4-fluorophenyl)ethanol, 1-(4-chlorophenyl)ethanol, and 1(4-bromophenyl)ethanol, respectively. When the methyl group of 1-phenylethanol was replaced by ethyl group, the substrate 1phenylpropanol was resolved with 92% enantioselectivity. The highest sorption ee value observed for alcohols studied was 99.4% for 1-(1-naphthyl)ethanol. These results clearly showed that 1 represents one of the best framework adsorbents for the separation of racemic alcohols reported to date.8b,10 Chiral sulfoxides constitute an important class of biologically active compounds and therapeutic drugs.11 Compound 1 also showed remarkable resolution capability toward sulfoxides. B

DOI: 10.1021/acs.inorgchem.8b00948 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

benzene ring of the ligand (C···O = 3.173 and 2.919 Å). The methyl group of alcohol interacts with the chiral OH group of the biphenol backbone through the bridge of free water guests to form C−H···O interaction and hydrogen bonds (C···O = 2.200 Å; O···O = 2.321 and 2.684 Å) and forms C−H···O interaction (C···O = 2.988 Å) with the carboxyl oxygen atom. The structure is further reinforced by face-to-face intermolecular π−π interaction: each 4-BrPhEtOH unit is engaged in one π−π stacking (plane-to-plane separation = 3.868 Å and dihedral angle = 26°) with one benzene ring of the ligand. In addition, the weak C−H···Br interaction formed between the Br atom of 4BrPhEtOH with the coordinated DMF (C···Br = 3.399 Å) may also have some impact on the enantioselective recognition process. Therefore, the multiple supramolecular interactions between host and guest molecule within the chiral microenvironment thus provide the key driving force and may be responsible for the efficient recognition and separation. In summary, we have presented the assembly of a chiral porous MOF from enantiopure tetracarboxylic acid-functionalized 1,1′-biphenol ligand. The framework can serve as an efficient solid adsorbent for the separation of racemic aromatic alcohols and sulfoxides. Remarkably, crystallographically observed host−guest interactions elucidated the nature of enantioselective recognition and separation. Further research endeavors on resolving other racemic molecules and exploration of MOF-based adsorbents for practical useful enantioseparation processes are underway.

Methyl phenyl sulfoxide, ethyl phenyl sulfoxide, 3-methoxyphenyl methyl sulfoxide, and 4-methoxyphenyl methyl sulfoxide were efficiently resolved by the apohost 1 with enantioselectivity ranging from 80 to 96%. Lansoprazole, a pharmacologically important proton pump inhibitor (PPI),12 could be separated with 91% enantioselectivity. Chiral MOFs have been developed for enantioseparation of sulfoxides,5f,13 and the highest ee reported prior to this work was 62%.13c Importantly, the present MOF represents a new generation of chiral solid separator, which is capable of enantioseparation of racemic alcohols and sulfoxides. It is notable that ligand H4L alone cannot resolve the enantiomers of alcohol and sulfoxide under identical conditions. Furthermore, the adsorbent 1 could be recovered easily through centrifugation and reactivated by ultrasonic washing with DCM for several times. The reactivated material was reused directly in the subsequent run, and the enantioselectivity did not decrease obviously after three cycles. The high angle data of the PXRD pattern gets broader, which is indicative of partial framework collapse. To investigate the nature of enantioselectivity as well as the characteristics of the interactions between host framework and adsorbate molecules, the structure of host−guest complex was carefully examined. Single crystals of 1·4-BrPhEtOH (4BrPhEtOH = 1-(4-bromophenyl)ethanol) were synthesized through the same procedure as that used for 1, except that additional 4-BrPhEtOH and MeOH were added. The structure clearly revealed that the adduct 1·4-BrPhEtOH is isostructural to the original host structure (Figure 3a). Only (S)-4-

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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.8b00948. Experimental details, crystallographic data, and spectral data (PDF) Accession Codes

CCDC 1835511−1835512 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], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zhao-Xi Wang: 0000-0002-2689-7034 Yan Liu: 0000-0002-7560-519X Yong Cui: 0000-0003-1977-0470 Figure 3. (a) View of the structure of host−guest complex 1·4BrPhEtOH. (b) Scheme showing the multiple intermolecular interactions between (S)-1 and (S)-4-BrPhEtOH.

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 (21431004, 21522104, and 21620102001), the National Key Basic Research Program of China (2014CB932102 and 2016YFA0203400), Key Project of Basic Research of Shanghai (17JC1403100 and 18JC1413200), and the Shanghai “Eastern Scholar” Program.

BrPhEtOH was observed in (S)-1, which is consistent with the separation experiment. Each of the trapped alcohols is involved with different supramolecular interactions (Figure 3b). The OH group of alcohol interacts with adjacent carboxylate oxygen to form a hydrogen bond (O···O = 2.401 Å) and forms C−H···O interactions with the coordinated DMF and the C

DOI: 10.1021/acs.inorgchem.8b00948 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.8b00948 Inorg. Chem. XXXX, XXX, XXX−XXX