Engineering a homochiral metal-organic framework based on an amino acid for enantioselective separation

Article abstract of DOI:10.1039/d0cc00897d

A chiral metal-organic framework possessing an open amphiphilic channel is constructed from a dicarboxylate ligand derived from an amino acid and is shown to be an efficient and recyclable chiral solid adsorbent, which is capable of separating racemic secondary alcohols, epoxides, and ibuprofen with very high enantioselectivity.

Full text of DOI:10.1039/d0cc00897d

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2020, DOI: 10.1039/D0CC00897D.
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S. J. Connon, M. O. Senge et al.
Conformational control of nonplanar free base porphyrins:
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DOI: 10.1039/D0CC00897D
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Engineering a Homochiral Metal–Organic Framework Based on
Amino Acid for Enantioselective Separation
Haitong Tang,[a+] Keke Yang,[a+] Kun-Yu Wang,[b+] Qi Meng, [a] Fan Wu, [a] Yu fang,[b] Xiang Wu, [a]
Received 00th January 20xx,
Accepted 00th January 20xx
Yougui Li, [a] WenCheng Zhang, [a] Yunfei Luo, [a] Chengfeng Zhu, [a]* and Hong-Cai Zhou[b]*
DOI: 10.1039/x0xx00000x
A chiral metal–organic framework possessing an open amphiphilic examples of separating enantiomers (such as racemic alcohols,
channel is constructed from a dicarboxylate ligand derived from amines, sulfoxides, etc.) by using chiral MOFs have been
amino acid and is shown to be an efficient and recyclable chiral reported, but only
a few exhibit excellent separation
solid adsorbent, which is capable of separating racemic secondary performance.7 In addition, currently the synthesis of versatile
alcohols, epoxides, and ibuprofen with very high enantioselectivity. chiral selectors bearing designed recognition sites remain to be
a challenge.
Selective separation of enantiomers has drawn great attention
in pharmaceutical industries because the individual
enantiomers of a chiral substance often exhibit different
physiological effects and pharmacological actions in a living
organism.1 The separation of enantiomers remains to be a
challenge because they may feature identical physical and
chemical properties in an achiral environment. Although
chromatography based on enantioselective adsorbents such as
cyclodextrins and polysaccharides is viewed as a powerful
approach for separating enantiomers.2 The uncertain
structures, poor stability, high cost, and narrow versatility have
limited their wide applications to some extent.3 Therefore, it is
imperative to develop a new generation of chiral materials to
overcome the shortcomings.
Herein, we present the synthesis of a new chiral MOF
constructed from an amino-acid-derived ligand. Owing to the
intrinsic chirality and open amphiphilic channel, this MOF can
act as a solid chiral adsorbent and feature excellent capability
to separate a variety of chiral aromatic alcohols, epoxides, and
ibuprofen with enantioselectivity up to 99.9%.
Metal–organic frameworks (MOFs) are a novel class of porous
crystalline materials which are built from metal nodes and
organic ligands.4 Because the pore environment and
functionality can be rationally designed according to desired
properties, MOFs have become promising candidates for many
applications including gas storage, sensing, catalysis and
separation.5 In particular, the judicious incorporation of
enantiopure building blocks would confer the resulting chiral
Fig 1. Synthesis of chiral MOF, 1, and view of its structure: a) the connection of Zn4O
clusters with L ligands and 4,4'-bipyridine (bpy) molecules generating a hexagonal
MOFs with tunable pore sizes and functionalities, which offers window on the ab plane. (green, Zn; grey, C; blue, N; red, O; bpy is highlight as purple for
clarity); b) illustration of the space-filling model of the open window along the c-axis; c)
the 1D nanotube highlighted as yellow stick generated from the interlayer hydrogen-
bonding along the b-axis; d) the packing of the 3D supramolecular structure of 1.
unique opportunities to develop novel chiral selectors aiming
for target separations.6 During the past decades, some
The enantiopure ligand with C2 symmetry, (R)-/(S)-H2L, was
a. Anhui Province Key Laboratory of Advanced Catalytic Materials and Reaction
readily synthesized through the condensation of the
Engineering, School of Chemistry and Chemical Engineering, Hefei University of
terephthaloyl chloride and (R)-/(S)-phenylalanine methyl ester,
Technology. Hefei 230009, China. E-mail: ZhuCF@hfut.edu.cn;
b. Department of Chemistry, Texas A&M University, College Station, TX 77843-3255,
USA. E-mail: zhou@chem.tamu.edu;
followed by hydrolysis with a ca. 93% yield. H2L was further
1
characterized by H NMR, ESI-MS, IR spectra (Fig. S1 and S2).
c. Department of Materials Science and Engineering, Texas A&M University, College
Station, Texas 77842, USA.
Reaction of H2L, Zn(OAc)22H2O and 4,4'-bipyridine (bpy) (in a
1:2:1 molar ratio) in a mixture of DMA, EtOH and water at 80 °C
for 24 h afforded colorless crystals in a good yield of 75%. The
products are stable in air as well as solvents such as CH2Cl2,
† These authors contributed equally to this work.
Electronic Supplementary Information (ESI) available: Experimental details,
crystallographic data, additional structural figures, and spectroscopic data. CCDC
1968168. For ESI and crystallographic data in CIF or others electronic format. See
DOI: 10.1039/x0xx00000x
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MeCN and acetone. Based on single-crystal X-ray diffraction the retention time with that of the standard sampVleie.wWArthicelenO(nRlin)e-
DOI: 10.1039/D0CC00897D
(XRD), thermogravimetric analysis (TGA), and IR spectra, the 1 was synthesized following the same procedure as (S)-1 and
chemical formula of the MOF can be depicted as used as the adsorbent, (R)-enantiomer of 1-phenylethanol with
[(Zn4O)2(L)6(bpy)3] (1).
99.9% ee was obtained (Fig. S7), indicating that the chiral nature
Single-crystal X-ray diffraction data analysis confirms that 1 is of the included guest molecule is determined by the chirality of
a porous 2D MOF (Fig. 1), which crystallizes in the chiral trigonal the host framework. Besides, the kinetic study indicated that
P321 space group with one-sixth of one formula unit in the the adsorption of (S)-1 to 1-phenylethanol reach adsorption
asymmetric unit cell. Of the two crystallographically identified equilibrium in 5 h, generating a host-guest complex with a ratio
Zn ions, Zn1 ion (with occupancy of 1/3), locating in a distorted of ca. 3:5 ((S)-1: 1-phenylethanol)(Fig. S8).
tetrahedral coordination polytope, is coordinated with one 3-
OH
R2
OH
R2
O atom and three carboxylate O atoms of three L ligands, while
Zn2 ion (with occupancy of 1) exists in a distorted trigonal
bipyramid polytope and connects with one 3-O atom, one
pyridyl N atom from bpy and three carboxylate O atoms of three
L ligands. All of the lengths of Zn-O and Zn-N are within the
normal range as previous reports (Table S1).8 The basic building
block of 1, therefore, is a tetrameric [Zn4O(O2C)6] unit with a C3
axis passing through Zn1 centre. Each L acts as a tetradentate
ligand binding with four Zn ions from two Zn4O clusters, while
each tetranuclear Zn4O cluster can be viewed as a 3-connected
node linked by six L, affording a 2D network on ab plane. The
network is further strengthened by the coordination of bpy with
Zn1 ions on adjacent Zn4O clusters (Fig. 1a). Besides, the
intermolecular hydrogen bonding (N-HO = 2.92-3.03 Å)
between the amide groups of two adjacent layers direct the
packing of the networks along the b-axis, leading to a 3D
supramolecular structure featuring 1D channels with the largest
diameter around 1.5 nm (Fig. 1b-d). What’s more, the chiral
nanotubular channel in 1 deploys a manifold of functionalized
sites, such as hydrophilic amide groups and hydrophobic
benzene rings pointing inward, providing an ideal platform to
modulate the interactions of the framework with chiral guests
for recognition and discrimination of enantiomers.
The consistence of the observed powder XRD patterns and
simulated ones demonstrates the phase purity of the bulk
sample of 1 (Fig. S3). TGA showed that the framework remained
thermally stable up to ∼320 °C (Fig. S4). The circular dichroism
(CD) spectra of (R)- and (S)-1, which are produced from the two
opposite enantiomers of H2L respectively, are mirrored versions
of each other, confirming their enantiomeric nature (Fig. S5).
The void space in the framework of 1, which is accessible for
guest molecules, occupies about 36.2% of total space calculated
by PLATON program. The porosity of 1 can be further proved by
dye adsorption tests, which showed that 1 could adsorb 1.41
methylene blue (MB, ∼1.25 nm × 0.50 nm × 0.38 nm in size) per
formula unit (Fig. S6).
(S)-1
acetone, rt.
R1
R1
1
1'
OH
OH
OH
Me
OH
Me
Me
1d'
1a'
1e'
1i'
1b'
1c'
99.8% ee
99.7% ee
99.1% ee
OH
99.7% ee
OH
OH
OH
F
Cl
Br
1h'
1f'
1g'
99.9% ee
93.9% ee
93.5% ee
96.9% ee
OH
OH
OH
OH
GO
OH
OG
1l n
.d
1j'
1K'
99.5% ee
99.3% ee
99.9% ee
Fig 2. Enantiosorption of (S)-1 towards 1-phenylethanol and its derivatives (for the bulky
substrate 1l, G = 4-(tert-butyl) benzyl).
With the optimized separation conditions in hand, the
enantioselective adsorption capability of 1 toward a variety of
aromatic alcohols was investigated (Fig. 2). First, the alcohols
with electron-donating substituents on the aromatic ring give
rise to significantly high enantioselectivity, with ee values
ranging from 99.1% to 99.9% (1a’-1h’). Second, after
introducing electron-withdrawing substituents such as -F, -Cl or
-Br, on the para position of the benzene ring of 1-
phenylethanol, 1 also exhibits high enantioselectivities, with
93.9%, 93.5%, and 96.9% ee values for 1-(4-
fluorophenyl)ethanol, 1-(4-chlorophenyl)ethanol, and 1-(4-
bromophenyl)ethanol, respectively (1f’-1h’). Finally, when the
methyl group of 1-phenylethanol was replaced with the ethyl
group, the substrate 1-phenylpropanol was separated by 1 with
an enantioselectivity at 99.5%. Besides, in the adsorption of the
derivative 1-indanol and 1-phenylethane-1,2-diol, 1 exhibits a
high enantioselectivity of 99.3% ee and 99.9% ee, respectively.
These results clearly show that 1 represents one of the best
chiral adsorbents for the separation of racemic alcohols, which
is comparable to some excellent framework adsorbents
reported by Cui and co-workers. 10
Notably, control experiments showed that the optically active
ligand H2L itself could not resolve the enantiomers of the tested
substrates under the identical conditions. This result indicates
that the enantioselective recognition and adsorption process is
manipulated by the well-assembled chiral framework. In
addition, a more bulky aromatic alcohol substrate (1l: ∼2.2 nm
× 1.0 nm in size) was synthesized and subjected to the identical
adsorption condition for 5 h. The 1H NMR result reveals that the
relative contents of this bulky substrate and internal standard
The presence of chiral amphiphilic channel in 1 prompts the
exploration of enantiosorption and separation of chiral alcohols
because enantiopure alcohols are important intermediates in
the pharmaceutical industry.9 During the initial enantioselective
separation, 1-phenylethanol was selected as a model substrate
to optimize conditions. After screening the separation
conditions including solvents and concentrations (Table S3), we
found that 50 mg (S)-1 immersed in a vial containing 5 mL
acetone and 10 mg 1-phenylethanol for 24 h, followed with
washing and extraction procedure, which can afford 99.8% ee
for the (S)-enantiomer. The result was confirmed by comparing
2 | J. Name., 2012, 00, 1-3
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did not change after the adsorption experiment (Fig. S9). Such performed on phenyl glycidyl ether showeVdiew tAhrtiaclte Ontlhinee
a result indicates that the bulky substrate cannot get access to enantioselectivities were 99.5%, 99.3%D9O9I:.140%.1,03999/D.60%CC, 09098.967%D
the chiral channel of 1 due to the size exclusion, which and 99.4% ee, respectively (Fig. S14). The wider substrate
conversely supports that the enantioselective adsorption tolerance and impressive enantioselectivity, compared with the
indeed occurs within the chiral channels for other smaller guest recently reported chiral framework materials for separating
molecules.
racemic epoxides,13 further indicate that 1 is a promising chiral
The crystal structure of 1 reveals that chiral open channels adsorbent. Its excellent separation performance may also
are periodically decorated by amide groups and benzyl groups originate from the amphiphilic channel, which offers a unique
of ligands, which are fairly proximity to the chiral guest chiral environment for recognition and discrimination of
molecules. Such a delicate arrangement of function groups enantiomers of epoxides.
allows the analytes to interreact with the amphiphilic channel
via hydrogen-bonding and π-π stacking interactions, which is
crucial for the enantioselective discrimination of small organic
molecules.11 Based upon the results of Monte Carlo simulation
performed on 1 and (R)-/(S)-1-phenylethanol, the adsorbed
chiral guest molecules tend to be immobilized near a pocket
assembled by flexible benzyl groups, which can mainly be
attributed to the π-π stacking between the guest molecules and
ligands (Fig. S10-13). Besides, simulation demonstrates that
adsorption of (R)-1-phenylethanol is more energy favourable
than the (S)-1-phenylethanol, accounting for the affinity to the
(R)-isomer.
Moreover, to evaluate the durability of the solid adsorbent,
Fig 4. a) Presentative of the enantioseparation of racemic ibuprofen by using glass
(S)-1 column with acetone as the mobile phase; b) Evolution of the enantiomeric
excess (Left Y-axis, shown as a column) and the concentration (Right Y-axis, shown
we investigated the recycling capability of 1 in the separation of
1-phenylethanol. Six consecutive separation experiments
afforded (S)-1-phenylethanol with 99.8%, 99.3%, 99.5%, 99.9%,
99.5% and 99.8% ee, respectively (Fig.S7). Moreover, the PXRD
patterns of the recycled sample revealed that 1 maintained its
crystallinity although a slight distortion in structure occurred
(Fig. S3). This result suggests an excellent recyclability of 1 in
multiple runs for 1-phenylethanol separation.
as a dot ) of each ibuprofen isomer in every 8 mL eluent (red: (S)-ibuprofen, green:
(R)-ibuprofen).
After the finding of the excellent enantioselectivity and
versatility of 1 in the separation of chiral molecules, we decide
to explore its potential as chiral column packing materials for
the enantioseparation of drugs. Ibuprofen is selected as a model
analyte because (S)- ibuprofen enantiomer exhibits a much
stronger inhibitory activity than (R)- ibuprofen when they are
used as nonsteroidal anti-inflammatory drug in relieving pain.
Moreover, ibuprofen possesses an appropriate molecular size
(12.5 × 6.4 × 6.7 Å3) that fits well to the contour of the open
channels. In order to separate racemic ibuprofen, an empty
glass column with an inner diameter of ~0.5 cm was set up as
shown in Fig. 4a (and Fig. S15), packed with solvent-exchanged
crystals of (S)-1 (360 mg), filled with acetone, and then
pressurized under N2 several times to make the MOF particles
packed well. Then, a solution of racemic ibuprofen (6.3 mg, 30
mol), in which 80 mL acetone served as eluent, ran through
the column at near-atmospheric pressure. The resulting eluent
at every 8 mL was collected separately and analysed by HPLC.
The first 8 mL eluent could afford a 31.2% ee with excessive (S)-
enantiomer of ibuprofen, subsequently, the ee value gradually
increased until it reached the highest 94% ee at the fifth 8 ml
eluent (Fig. S15). After this, enantiopure (R)- ibuprofen (ee up
to 99.9%) was obtained in the next 40 mL eluent (Fig. 4b). This
result indicates that enantiopure (S)- ibuprofen leaves the
column first, due to the preferred interaction of (R)- ibuprofen
with (S)-1, resulting in a slower elution of the (R)- ibuprofen. The
separating efficiency of the simple glass column is not as high as
commercial HPLC columns, which should be attributed to the
large sizes of MOF particles, low MOF loading, and insufficient
packing pressure. Nevertheless, the effectiveness of 1 in the
separation of racemic drugs, as the MOFs derived from natural
O
O
O
O
(S)-1
acetone, rt.
R
R
2
2'
O
O
O
O
O
O
O
MeO
2a' 99.9% ee
2b' 99.5% ee
2c' 99.0% ee
O
O
O
O
Me
Cl
2d' 99.7% ee
2e' 99.1% ee
2f' 99.8% ee
Fig 3. Enantiosorption of (S)-1 towards styrene oxide and aryl glycidyl ethers.
The excellent enantioselectivity, size selectivity and recycling
ability of 1 in the separation of racemic aromatic alcohols
encouraged us to examine its potential in the separation of
chiral epoxides, since enantiopure epoxides constitute an
important class of biologically active compounds and
therapeutic drugs, such as antibiotics, antifungal drugs, and
HIV-protease inhibitors.12 Under the identical separation
condition, styrene oxide and a variety of aryl glycidyl ethers with
electron-withdrawing or electron-donating substituents in the
aromatic ring were efficiently resolved by the solid adsorbent
with enantioselectivity ranging from 99.0% to 99.9% as shown
Fig. 3. Moreover, six consecutive separation experiments
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amino acids, made it a promising stationary phase material for
practical enantioseparation processes.14
4
5
The remarkable selectivity of 1 in enantiosorption and
separation of chiral molecules has motivated us to obtain the
single-crystal structures of the inclusion adducts, which
contribute to understanding of the nature of enantioselectivity,
but it was unsuccessful due to the poor crystal quality. In this
case, the excellent performance of 1 may arise from the
combination of chiral channels of appropriate sizes and the
amphiphilic inner surface, which is lined up with hydrophilic
amide groups and hydrophobic benzyl groups. This combination
allows for bioanalogous interaction of the host framework with
adsorbate species during the inclusion process.
In conclusion, we present the synthesis and structure of a
novel chiral MOF constructed from a C2 symmetric enantiopure
ligand, which is readily available from a natural amino acid. The
framework exhibits highly enantioselective abilities to separate
a variety of racemic aromatic alcohols, epoxides, and even chiral
drug by enantiosorption. Besides, the framework adsorbent can
be easily recycled and reused. Further endeavour is aimed at
understanding the enantioselective processes of the amino
acid-based framework and further studying the potential
practical applications in chromatography.
We acknowledge the Fundamental Research Funds for the
Central Universities of China (No. PA2019GDPK0058,
PA2020GDKC0011). We thank the supports of the Postdoctoral
Scientific Research Station of Material Science and Engineering
at Hefei University of Technology and the assistance of the staff
from the BL17B beamline of NFPS at Shanghai Synchrotron
Radiation Facility during crystal data collection. We also thank
the support of the Robert A. Welch Foundation through a Welch
Endowed Chair to H.-C.Z. (A-0030) and Qatar National Research
Fund under Award Number NPRP9-377-1-080.
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Conflicts of interest
There are no conflicts to declare.
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