Chiral 3D Covalent Organic Frameworks for High Performance Liquid

Jan 5, 2018 - The service life of the COF column is at least 2 months (Figure S12). Chiral HPLC using stereoregular synthetic or natural polymers as C...
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Chiral 3D Covalent Organic Frameworks for High Performance Liquid Chromatographic Enantioseparation Xing Han, Jinjing Huang, Chen Yuan, 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 S Supporting Information *

permanent porosity as well as exhibits enhanced chemical stability (Scheme 1).

ABSTRACT: In spite of their great promise for enantioselective processes due to the rich host−guest chemistry, it remains a challenge to construct covalent organic frameworks (COFs) with chiral three-dimensional (3D) structures. Here we report bottom-up synthesis of the first example of 3D chiral COFs by imine condensation of an enantiopure 2-fold symmetric TADDOL-derived tetraaldehyde with a tetrahedral tetra(4-anilyl)methane. After postsynthetic oxidation of imine linkages, the framework is transformed into an amide-linked COF with retention of crystallinity and permanent porosity as well as enhanced chemical stability. The resultant isostructural COFs feature a 4-fold interpenetrated diamondoid open framework with tubular channels decorated with chiral dihydroxy auxiliaries. Both COFs can be used as chiral stationary phases for high performance liquid chromatography to enantioseparate racemic alcohols, and the oxidized COF shows superior separation performance compared to the pristine framework.

Scheme 1. Synthesis of the CCOFs

C

ovalent organic frameworks (COFs) have emerged as a new class of crystalline porous materials by integrating organic building blocks into predetermined network structures through covalent bonds.1−3 Given their high porosity with functionalized internal surface and ability to tune structural and physical parameters, COFs have shown promise for applications such as molecule storage and separation,4,5 optoelectronics,6 energy storage,7 drug delivery,8 sensing9 and catalysis.10,11 It has become prevalent to construct two-dimensional (2D) COFs, which rely on van der Waals and other noncovalent interactions in addition to covalent interactions to form layered stacking structure. It remains a challenge to design and synthesize threedimensional (3D) COFs by organizing building blocks that rely only on covalent bonds.12 From a supramolecular point of view, 3D COFs warrant exploration owing to their large surface areas, high density of open sites and fascinating confinement effects.13 In this work, we report a bottom-up strategy for the construction of a rare 3D chiral COF (CCOF) by cocondensation of a tetrahedral tetra(4-anilyl)methane (TAM) with a tetraaldehyde derived from the privileged chiral Tetraaryl1,3-dioxolane-4,5-dimethanols (TADDOL) backbone.11a Oxidation of imine linkages of the resultant CCOF14 affords an amide-linked framework, which retains crystallinity and © XXXX American Chemical Society

Despite their potential utilization in asymmetric catalysis, enantiopure separation and chiral optics,12 crystalline chiral porous solids based on COF networks are quite rare. This can be presumably attributed to the synthetic challenge of simultaneously achieving the balance and conflict of asymmetry and crystallinity. Taking advantage of the mild conditions that are typically used for COF synthesis, we and others have shown that 2D CCOFs for asymmetric catalysis could be prepared by the bottom-up assembly of judiciously selected chiral building blocks or postmodification of achiral COFs with chiral functionalities.11 However, with one exception,5a COFs have not been explored for chiral separation, which represents one of the most difficult preparative and analytical separations because of the identical physical and chemical properties for enantiomers. Here we report for the first time use of CCOFs as chiral stationary phases (CSPs) for high performance liquid chromatography (HPLC) to separate racemic alcohols, sulfoxides, carboxylic acids and esters. Received: November 15, 2017 Published: January 5, 2018 A

DOI: 10.1021/jacs.7b12110 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society CCOF 5 was prepared by solvothermal reactions of enantiopure teraaldehyde TTA (0.05 mmol) and tetraamine TAM (0.05 mmol) in 1,4-dioxane (1.5 mL) in the presence of 6 M acetic acid catalyst (0.2 mL) at 120 °C for 72 h, which afforded white microcrystalline solid in ∼70% yield. CCOF 6 was synthesized from 5 according to the procedure reported in literature.13 Both CCOFs 5 and 6 are not soluble in water and common organic solvents such as MeOH, DMF and THF. The COFs were characterized by spectroscopic techniques. The Fourier transform infrared (FT-IR) spectrum of CCOF 5 showed disappearance of the characteristic CO vibration band (1702 cm−1) and appearance of the strong CN stretching band (1627 cm−1), indicating successful polymerization (Figure S1). For the oxidized product 6, the FT-IR spectrum showed emergence of the characteristic CO amide stretch at 1657 cm−1 and disappearance of the CN imine stretch at 1627 cm−1. These characteristic imine and amide stretches were corroborated by the spectra obtained for a molecular model (Figure S1). The 13C cross-polarization magic angle spinning (CP-MAS) NMR spectrum shows the characteristic signal for the CN group at ∼159 ppm for 5 (Figure S2). The aldehyde carbon peaks were barely observed. The chemical shifts of other fragments are in good agreement with those of the monomers. Conversion of the materials from imine to amide was also confirmed by 13C CP-MAS NMR spectroscopy. In the 13C CPMAS NMR spectrum of 6 at natural abundance, the imine carbon peak at 159 ppm was no longer present after oxidation. Indeed, a new peak, associated with the amide carbonyl, was observed at 166 ppm (Figure S2). In addition, small shifts in the frequencies of the aromatic carbon resonances between 5 and 6 suggests the oxidation of the imine functionality to the amide. Circular dichroism (CD) spectra of the COFs based on the opposite enantiomers of TADDOL monomers were mirror images of each other, suggesting their enantiomeric nature (Figure S3). Thermal gravimetric analysis (TGA) reveals they are stable up to 350 °C (Figure S5). Scanning electron microscopy (SEM) images showed both possess a uniform granular morphology (Figure S6). The crystalline structures of the CCOFs were determined by powder X-ray diffraction (PXRD) analysis with Cu Kα radiation (Figure 1). In light of the orthorhombic phase of 5, a lowersymmetry model was built with space group P2 adapting the unaltered dia-C4-net interpenetrated along the c-axis (Figure 2a,b), which is coherent with the observed PXRD pattern. The lattice modeling and Pawley refinement (Materials Studio, version 7.0) gave optimized parameters of a = 16.97 Å, b = 13.51 Å, c = 16.33 Å, α = 90.0°, β = 108.87° and γ = 90.0° for the unit

Figure 2. Structural representations of CCOF 5. (a) Adamantine-like cage in the diamond net. (b) Structure viewed along a-axis showing 4fold interpenetration of a diamond net. (c) Two kinds of 1D channels along c-axis. C gray; N blue; H white; O red.

cell with the space group of P2, which provided two good agreement factors (Rp = 4.55% and Rwp = 5.07%). The experimental PXRD pattern (Figure 1a, black curve) shows the main diffraction peaks at 6.59°, 8.48°, 11.11°, 12.60°, 13.17°, 14.12°, 15.60°, 17.40°, corresponding to the (010), (110), (200), (210), (012), (102), (112) and (022) facets of space group P2 (No. 3), respectively. Several types of other possible 3D structures were generated for them, and non-, 2-fold- and 3fold-interpenetrated diamond nets for 5 were constructed from the space groups P21212, I222 and P4,̅ respectively (Figure S7). However, the simulated PXRD patterns from these hypothetical nets did not match the experimental data. The structure of CCOF 6 was also confirmed by PXRD using the same space group (Figure S8). This analysis indicates the symmetry of 5 (space group P2) is conserved, implying retention of the diamond configuration (Figure 1b). The lattice modeling and Pawley refinement gave optimized parameters of a = 16.98 Å, b = 13.47 Å, c = 16.40 Å, α = 90.0°, β = 108.9° and γ = 90.0° for the unit cell with acceptable residuals (Rp = 4.32% and Rwp = 5.66%). A slight change in unit cell parameters was found for the amide CCOF compared with the imine one. The TADDOL units in combination with TAM thus allowed the formation of a diamond network with 1D open channels of 0.58 and 0.71 nm for 5 and 0.51 and 0.75 nm for 6 (Figure 1, inset). The porosity of the materials was examined by measuring N2 sorption isotherms at 77 K on the fully activated samples. The adsorption curves of the CCOFs exhibited the type-I isotherm (Figure 3), which is the characteristic of microporous

Figure 1. PXRD patterns of CCOFs 5 (a) and 6 (b) with the experimental profiles in black, Pawley refined in red, calculated in blue and the difference between the experimental and refined PXRD patterns, in dark cyan (inset: views of ball−stick models along the caxis).

Figure 3. N2 adsorption−desorption isotherms (77 K) and pore size distribution profiles (insert) of CCOFs 5 (red) and 6 (blue). B

DOI: 10.1021/jacs.7b12110 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Communication

Journal of the American Chemical Society

Figure 4. (a−d) HPLC separation of racemic 1-phenyl-2-propanol, 1-phenyl-1-pentanol, 1-phenyl-1-propanol and 1-(4-bromophenyl)ethanol on the CCOF 5 (blue line) and 6 (red line) packed columns, respectively, using hexane/isopropyl alcohol (v/v = 99:1) as mobile phase at a flow rate of 0.2 mL/min.

Both CCOFs 5 and 6 possessed chiral tubular channels with the maximum aperture sizes of ∼0.71 and 0.75 nm, respectively. The sizes cover the minimum diameters of all tested racemates (∼6.0 Å). The resolution abilities of the CCOFs are likely to come from a combination of the chiral channels with amphiphilic channel interior lined with chiral hydroxyl groups, which may govern host−alcohol interactions favoring enantioselectivity during the adsorption process.15 To prove this point, 1-(1-naphthyl)-ethanol with a minimum diameter of 0.85 nm being larger than the maximum channel sizes of the CCOFs was selected as an analyte. The elution appeared with less retention time than those resolved enantiomers, indicating no separation for this large molecule (Figure S11). The oxidation of imine linkages in 5 to amide linkages allows us to finely tune the pore interior and affinity to maximize their stereoselective effects and thus to perform highly enantioselective resolution.16 We also fabricated monodispersed amorphous COF@SiO2 and (R,R)TTA/SiO2 hybrid microspheres and evaluated their chiral recognition ability as CSPs. The results showed they cannot separate racemic alcohols (Figure S15), further highlighting the key role of crystalline 3D structure of the COF in chiral recognition. To gain further insight into the separation of chiral molecules, an increase in 1-phenyl-2-propanol mass in hexane was injected into the 6-based column for separation. The results showed a few decrease in the retention time for the first eluted enantiomer, while the resolution for the analyte slightly decreased as the mass increased. In addition, the chromatographic peak area of each single antipode rose linearly with the increase in the injected mass. The service life of the COF column is at least 2 months (Figure S12). Chiral HPLC using stereoregular synthetic or natural polymers as CSPs has become one of the most popular ways for chiral separation.17 To develop new types of chromate-graphic materials, metal−organic frameworks have been explored as CSPs for HPLC separation,18,19 but they in general suffer from the disadvantages of low chemical stability. For the separation of racemic alcohols, the resolution and selectivity factors observed for COF 6 are comparable to or higher than those of reported for MOF-based CSPs (Table S4).19 In summary, we have demonstrated direct synthesis of the first 3D CCOF by imine condensation of tetrahedral tetraamine and chiral tetraaldehyde building blocks as well as postsynthetic oxidation of the framework to create an isostructural amidelinked CCOF with enhanced chemical stability. PXRD and modeling studies, together with pore size distribution analysis demonstrated both CCOFs are 4-fold interpenetrated microporous frameworks with tubular channels lined with chiral dihydroxy groups. Both can work as CSPs of HPLC for

materials. Brunauer−Emmett−Teller (BET) surface areas of 655 and 613 m2 g−1 were estimated for 5 and 6, respectively. Their total pore volumes were 0.51 and 0.42 cm3 g−1 at P/P0 = 0.99, respectively. The nonlocal density functional theory (NLDFT) gave rise to a narrow pore size distribution with an average pore width 0.62 and 0.74 nm for 5, and 0.59 and 0.74 nm for 6, corresponding to their simulated values (Figure 3, inset). The chemical stability of the CCOFs was examined by PXRD and sorption after 1 day treatment in boiling water, 12 M HCl (aq) and 1 M NaOH (aq) (Figures S9 and S10). Both CCOFs are stable in boiling water, but 5 showed decreased crystallinity and surface area. The difference between the stabilities of the imine and amide materials is striking in the case of acidic and alkaline conditions, where the amide CCOF retained its crystallinity while the imine one was nearly dissolved and the remaining material was rendered amorphous. The as-treated samples of 6 have a BET surface area of 580 and 477 m2 g−1, respectively. A small decrease in signal-to-noise ratio and a decrease of 6% or 22% in surface areas of the amide materials indicate partial structural collapse upon treatment. The two CCOFs packed columns for HPLC were prepared by loading the mixture of the crystalline samples (an average size of ∼0.3 μm) and silica (an average size of ∼5 μm) in EtOH into a 20 cm-long 2.3 mm internal diameter (i.d.) stainless steel column. The performance of the column packed with 5 was first evaluated by resolution of 1-phenyl-2-propanol. After optimizing mobile phase composition and its flow rate, racemic 1phenyl-2-propanol was successfully baseline separated on the CSP with hexane/isopropanol (optimized v/v = 99:1) as the mobile phase at a flow rate of 0.2 mL/min. The separation with a good selectivity factor (α = 1.19) and chromatographic resolution (Rs = 1.52) was achieved within 40 min, and the elution sequence was the S-enantiomer followed by the Renantiomer. However, as shown in Figure 4, racemic 1-phenyl-1pentanol, 1-phenyl-1-propanol and 1-(4-bromophenyl)-ethanol cannot be baseline separated, affording α/Rs = 1.20/0.92, 1.21/ 1.26 and 1.17/1.44, respectively. The column packed with 6 can baseline resolve all racemates of 1-phenyl-2-propanol, 1-phenyl1-pentanol, 1-phenyl-1-propanol and 1-(4-bromophenyl)ethanol, affording α/Rs = 1.29/1.78, 1.21/1.58, 1.33/2.47 and 1.24/1.54, respectively (Table S3). The elution order was also the S-enantiomer followed by the R-enantiomer. Other types of racemates such as sulfoxides, carboxylic acids and esters can also be completely or partly resolved on the COF-based CSPs (Figure S16). It appears, in all cases, 6 showed improved resolution ability related to 5 under similar conditions. PXRD indicated that the recovered CCOFs after HPLC measurement remained crystalline and structurally intact (Figure S13). C

DOI: 10.1021/jacs.7b12110 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

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Journal of the American Chemical Society

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separation of racemic alcohols with excellent repeatability and reproducibility, whereas the oxidized CCOF showed superior resolution performance compared with the pristine framework. This work advances COFs as a platform for chiral resolution and will expand the scope of materials design and engineering to make new types of COFs with unique enantioselective functions.

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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/jacs.7b12110. Experimental procedures and characterization (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] ORCID

Xing Han: 0000-0002-7921-8197 Yan Liu: 0000-0002-7560-519X Yong Cui: 0000-0003-1977-0470 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the financial support of the NSFC (21431004, 21522104 and 21620102001), the “973” Program (2014CB932102 and 2016YFA0203400), Key Project of Basic Research of Shanghai (17JC1403100) and the Shanghai Eastern Scholar Program.

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DOI: 10.1021/jacs.7b12110 J. Am. Chem. Soc. XXXX, XXX, XXX−XXX