Homochiral Metal-Organic Frameworks with Tunable Nanoscale Channel Array and Their Enantioseparation Performance against Chiral Diols

Article abstract of DOI:10.1021/acs.inorgchem.7b00352

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.

Full text of DOI:10.1021/acs.inorgchem.7b00352

Article
pubs.acs.org/IC
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 pharmaceut-
ical 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.
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
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 sulfox-
ides,22−25 alcohols,26−33 phenols,34 and even some complexes.35
It is assumed that the enantioseparation performance of MOFs,
Received: February 8, 2017
© XXXX American Chemical Society
A
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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-(4-
pyridyl)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
6 2 . 3 2 % y i e l d . L - P B P l e d t o C o 2 [ ( L - P B P ) -
(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.
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 1,1,2-
triphenyl-1,2-ethanediol.
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
RESULTS AND DISCUSSION
■
Synthesis of Linkers. Phenyl-4,4′-bis[carbonyl-N-(pro-
line)] (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 1,4-
dichloroformylbenzene 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
̂
̂
topological type 4L11 (43.63) (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 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.
B
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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.
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-square-
pyramidal 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
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
C
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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 macro-
scopic 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-phenyl-
1,2-ethanediol, hydrobenzoin, and 1,1,2-triphenyl-1,2-ethane-
diol were chosen as substitutes to study.
Table 1. Enantioselective Separation for Racemates at Room
Temperature
a
a
Determined by HPLC at room temperature.
is feasible for HMOFs to have a good potential in the
pharmaceutical industry.
CONCLUSIONS
■
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).
To conclude, we have succeeded in preparing two pairs of
HMOFs. On structure adjustment where the linkers constitut-
ing 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.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b00352.
■
S
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 data_request@ccdc.cam.ac.
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
AUTHOR INFORMATION
Corresponding Author
*E-mail for X.W.: wxt@fjirsm.ac.cn.
■
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
D
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∥H.Z.: Department of Physical Chemistry, School of China
Pharmaceutical University, Nanjing 211198, People’s Republic
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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.
ABBREVIATIONS
■
HMOFs, homochiral metal−organic frameworks; ee%, enantio-
meric 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 chroma-
tography
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DOI: 10.1021/acs.inorgchem.7b00352
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