Chiral DHIP-Based Metal–Organic Frameworks for Enantioselective

Communication pubs.acs.org/IC

Chiral DHIP-Based Metal−Organic Frameworks for Enantioselective Recognition and Separation Jie Zhang,† Zijian Li,† Wei Gong,† Xing Han,† Yan Liu,*,† and Yong Cui*,†,‡ †

School of Chemistry and Chemical Engineering and State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China ‡ Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China S Supporting Information *

Scheme 1. Synthesis of the Ligand L and MOFs 1 and 2

ABSTRACT: Two chiral porous 2,3-dihydroimidazo[1,2a]pyridine (DHIP)-based metal−organic frameworks (MOFs) are assembled from an enantiopure dipyridylfunctionalized DHIP bridging ligand. The Zn-DHIP MOF shows a good enantioseparation performance toward aromatic sulfoxides, and the heterogeneous adsorbent can be readily recovered and reused without significant degradation of the separation performance.

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etal−organic frameworks (MOFs) built from metal nodes and multitopic organic linkers have received considerable attention because of their myriad potential applications in gas storage, catalysis, separation, and sensing.1−3 One of the current efforts focuses on the development of MOF materials for the recognition and separation of small molecules, especially when they are chiral.4,5 The judicious incorporation of enantiopure building blocks into the porous framework can precisely control the surfaces, internal pore metrics, and functionalities, which are of significant importance for their highly selective recognition and separation.4b,5 Two enantiomers of a chiral molecule could be effectively oriented within the confined space to promote enantiospecific interactions within the network, thus realizing stereocontrol and chiral separation. During the past years, chiral metal−organic assemblies have been used for enantioselective adsorption/separation, some of which exhibit good to excellent performance.5c,d Nonetheless, reports on the enantioselective recognition and separation behavior of chiral MOFs are scarce because of the difficulty in synthesizing enantioselective selectors with precise synergistic recognition sites. It has been known that chiral 2,3-dihydroimidazo[1,2a]pyridine (DHIP) is a core structure for the design of asymmetric catalysts owing to their ease of preparation and structural diversity including multivariate steric and electronic environments.6 Some examples have been claimed to be active toward enantioselective acyl transfer, kinetic resolution, and cycloaddition.6a,b,d However, with the exception of catalytic assemblies, efficient separation systems derived from DHIP have not been reported. Herein, we report two chiral MOFs constructed from a dipyridyl-functionalized DHIP ligand for the first time, one of which exhibits impressive enantioselective recognition and separation toward aromatic sulfoxides. As shown in Scheme 1, the dipyridyl-functionalized DHIP ligand L was synthesized in 65% yield by a palladium-catalyzed Suzuki coupling reaction between 4-pyridylboronic acid and (R)© XXXX American Chemical Society

5-bromo-2-(4-bromophenyl)-2,3-dihydroimidazo[1,2-a]pyridine, which was obtained from (R)-2-amino-2-(4bromophenyl)ethanol by N-arylation followed by cyclization in 35% overall yield for two steps. The ligand L was characterized by 1 H and 13C NMR spectroscopy and electrospray ionization mass spectrometry. Heating a mixture of L, 1,4-benzenedicarboxylic acid (BDC), and Zn(NO3)2·6H2O or CdI2 (in a 1:1:2 molar ratio) in a MeOH/DMF/MeCN or MeOH/DMF/EtOH mixed solvent at 80 °C for 5 days afforded single crystals of [M3L2(BDC)3]·3H2O [M = Zn (1), Cd (2)] in good yield. The complexes were stable in air and common solvents including water. They were formulated based on single-crystal X-ray diffraction (XRD), microanalysis, and thermogravimetric analysis (TGA). Single-crystal XRD analysis of 1 reveals that it crystallizes in the chiral monoclinic space group C2, and the asymmetric unit contains one whole formula unit. Three crystallographically distinct Zn atoms all lie in a distorted tetrahedral coordination sphere, being coordinated by two carboxylate O atoms of two BDC ligands, one imidazole N atom and one pyridyl N atom for Zn1 and Zn2, or two pyridyl N atoms for Zn3 of two different ligands L, respectively. All of the Zn−O and Zn−N bond lengths and angles are within the normal range. The ligand L employs a twisted conformation with the dihedral angle between the imidazole and phenyl ring in the range of 85.96−88.41°. Received: April 11, 2016

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DOI: 10.1021/acs.inorgchem.6b00894 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

three crystallographically independent Cd atoms are all sixcoordinated by binding to two chelated carboxylate groups of two BDC ligands and two N atoms of two L ligands. Similar to 1, each twisted L ligand bridges three Cd centers to form a righthanded 21 helical chain along the c axis, and adjacent helices associated in parallel are interconnected by BDC to form a 3D network (Figure 1c). The final 3-fold-interpenetrated network was generated by the interpenetration of three independent networks, which displayed a (3,3,4,4,4)-c net with the point symbol (4.8.93.10)(4.8.9)2(4.82.92.10)(82.93.10). 2 possesses smaller open channels than 1, with dimensions of ∼4.1 × 5.1 Å2 along the a axis (Figure 1d). 1 and 2 contain ∼36.8 and 23.1% void spaces, respectively, calculated by PLATON.7 The observed powder XRD patterns are the same as the simulated ones, demonstrating phase purity of the bulk samples. The solid-state circular dichroism spectra of 1 and 2 formed from R and S enantiomers of L showed almost mirror images of each other, indicating their enantiomeric nature. TGA showed that the frameworks are thermally stable up to ∼400 °C for 1 and 2. After the removal of guest solvents by solvent exchange followed by thermal activation under vacuum at 80 °C, 1 and 2 still retained their frameworks indicated by powder XRD. Their permanent porosity was investigated by CO2 adsorption measurements at 273 K, with Brunauer− Emmett−Teller (BET) surface areas of 211 m2 g−1 for 1 and 106 m2 g−1 for 2, respectively. Having the significant importance of chiral sulfoxides as both highly valuable chiral auxiliaries and pharmaceuticals in mind,8 we examined the enantioselective sorption properties toward chiral sulfoxides of the chiral DHIP-based MOFs. The evacuated 1 was immersed in different solutions of racemic sulfoxides for 12 h, filtered, and washed thoroughly with Et2O to remove the sulfoxides on the surface. The adsorbed guest molecules could be obtained by soaking the inclusion solids in Et2O, and the contents of the enantiomers were analyzed by chiral highperformance liquid chromatography (HPLC). The enantioselective separation of 3-methoxyphenyl methyl sulfoxide was selected as a model to optimize separation conditions. After a series of solvents were screened, MeOH was proven to be the most suitable solvent for separation, giving 26% enantiomeric excess (ee) at room temperature. Decreasing the temperature from room temperature to −10 °C led to increased enantioselectivity of 35% ee. The kinetic profile of the adsorption of 3-methoxyphenyl methyl sulfoxide was studied by 1H NMR monitoring at different times. Typically, it took 2 h to reach equilibrium, and the 1H NMR results suggested the formation of a 1:1 host−guest complex for 3-methoxyphenyl methyl sulfoxide adsorption. The Fourier transform infrared spectrum also suggested the complexation of 1 and 3-methoxyphenyl methyl sulfoxide, as shown by the strong SO stretching vibration band at 1050 cm−1. The enantioselective adsorption capability of 1 toward a variety of aromatic sulfoxides was investigated under optimized conditions. As shown in Table 1, the sulfoxides with electrondonating substituents in the aromatic ring gave rise to relatively higher enantioselectivity, with 37.1%, 31.7%, and 34.8% ee values for 3-methoxyphenyl methyl sulfoxide, 4-methoxyphenyl methyl sulfoxide, and 4-methylphenyl methyl sulfoxide, respectively. However, the introduction of electron-withdrawing substituents such as −F or −Cl could reduce the enantioselectivities to 17− 25.1%. The highest ee of 46.8% was obtained for methyl phenyl sulfoxide. Interestingly, when the methyl group of methyl phenyl sulfoxide was replaced by ethyl and isopropyl groups, the

Each L acts as a tridentate ligand connecting to three Zn ions, and each Zn is surrounded by two L to form helical chains running along the b axis, which could be further linked by the BDC ligands to form a 3D net (Figure 1a). Three such

Figure 1. (a) View of one independent network of 1 along the b axis. (b) Scheme showing the 3-fold-interpenetrated nets in 1. (c) View of one independent network of 2 along the c axis. (d) View of the (3,3,4,4,4)-c topology of 2. Color code: Zn, green; Cd, purple; C, gray; O, red; N, blue. The metal centers are drawn as polyhedra in parts a and c.

independent nets interpenetrated with each other, furnishing the final 3D framework with 1D channels of ∼4.7 × 5.4 Å2 along the c axis. The three-fold-interpenetrated framework can be simplified as a (3,3,3,4,4)-c net with the point (Schläfli) symbol (4.72.83)(4.72)3(75.9) if considering the ZnII ions as tetrahedral nodes and the BDC and L ligands as linkers (Figure 1b). 2 also possesses a 3-fold-interpenetrated framework. It crystallizes in the chiral orthorhombic space group P212121, and the asymmetric unit contains one whole formula unit. The B

DOI: 10.1021/acs.inorgchem.6b00894 Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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Table 1. Enantiosorption of 1 toward Racemic Sulfoxidesa

Communication

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00894. Experimental details and spectral data (PDF) Crystallographic file in CIF format (CIF) Crystallographic file in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by NSFC Grants 21371119, 21431004, 21401128, and 21522104, the “973” Program (Grants 2014CB932102 and 2012CB8217), the Shanghai “Eastern Scholar” Program, and Grant SSTC-14YF1401300.

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For details, see experimental procedure for separation in the SI. Determined by HPLC (letters in parentheses specify the preferable isomer).

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sulfoxides were resolved with much lower enantioselectivity as 20.9% and 12.4%, probably as a result of larger stereohindrance. It is worth noting that ligand L alone could not resolve the sulfoxides under otherwise identical conditions, indicating that the enantioselective recognition process is controlled by the wellorganized inner sphere of the chiral framework. The separator could be regenerated and used repeatedly for the next three runs without significant loss of enantioselectivity, and the third recycled sample could provide 41.2% ee for the separation of methyl phenyl sulfoxide. After adsorption, no appreciable difference between the recovered and pristine samples was observed in the powder XRD patterns, and the recovered 1 displayed a slightly decreased BET surface area of 162 m2 g−1, further comfirming the framework robustness. Inductively coupled plasma spectroscopic analysis of the resolution solution indicated almost no loss of the Zn ion (0.0216%) from the structures. Unlike 1, MOF 2 showed almost no enantioselective adsorption ability toward sulfoxides under similar conditions, probably as a result of the smaller pore sizes of its more interpenetrating network. The moderate enantioselectivity of 1 may be due to the lack of efficient chiral host−guest interactions as a result of the fact that the imidazole N atoms proximal to the chiral centers as potential supramolecular bonding sites are coordinated by metal ions. Prior to this work, chiral MOFs built from L-lactic acid or (R)-mandelic acid have been studied for enantioseparation toward sulfoxides by diverse techniques including adsorption, chromatography, and even membrane separation,4b,9 and the enantioselectivity could reach up to ∼62%.9c In conclusion, two novel chiral porous DHIP-based MOFs were constructed, and the enantioseparation ability of a ZnDHIP framework toward racemic sulfoxides was demonstrated. The DHIP MOFs may promise a new heterogeneous system and bring new inspiration to chiral separation in terms of the exploration of advanced adsorbents for varieties of adsorbates. Further efforts will focus on other DHIP-based framework adsorbents as well as chiral stationary phases and membranes with potential actual applications.

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DOI: 10.1021/acs.inorgchem.6b00894 Inorg. Chem. XXXX, XXX, XXX−XXX