Defect-Engineered Chiral Metal-Organic Frameworks for Efficient Asymmetric Aldol Reaction

Article abstract of DOI:10.1021/acs.inorgchem.1c00141

By employment of a mixed truncated chiral ligand synthetic strategy, a defect-engineered chiral metal-organic framework with hierarchical micro/mesoporous structure was prepared, and it exhibited efficient heterogeneous catalytic activity and enantioselectivity for asymmetric aldol reaction.

Full text of DOI:10.1021/acs.inorgchem.1c00141

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Defect-Engineered Chiral Metal−Organic Frameworks for Efficient
Asymmetric Aldol Reaction
Zijuan Chen, Xiaodan Yan, Meiyan Li, Shuhua Wang,* and Chao Chen*
Cite This: Inorg. Chem. 2021, 60, 4362−4365
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sı Supporting Information
ABSTRACT: By employment of a mixed truncated chiral ligand synthetic strategy, a defect-engineered chiral metal−organic
framework with hierarchical micro/mesoporous structure was prepared, and it exhibited efficient heterogeneous catalytic activity and
enantioselectivity for asymmetric aldol reaction.
hirality is an important property of nature that is
increasingly considered to be significant in numerous
CMOFs (DCMOFs) with a hierarchical micro/mesoporous
21
C
structure. As shown in Scheme 1, the construction building
fields especially for asymmetric catalysis. Chiral metal−organic
frameworks (CMOFs), constructed by metal nodes or metal
clusters with organic ligands featured in chiral groups, offer great
potential in heterogeneous asymmetric catalysis because of their
periodic porous structure, high surface area, and adjustable
Scheme 1. Synthetic Scheme of DCMOF Using a Mixed
Truncated Chiral Ligand Strategy
1
−4
chiral functionality.
Many CMOFs have been prepared
directly or indirectly by introducing chiral functional groups into
5
−10
the frameworks.
However, most of reported CMOFs to date
are microporous structures with pore sizes below 2 nm, which
not only hinder the fast diffusion and mass transfer of reactants
or products but also inhibit the stereogenic effect for chiral
1
1−16
catalysis.
Recently, a few CMOFs based on mesoporous
MOFs such as UiO-67, UiO-68, and MIL-101 have been
1
7−20
reported.
However, the introduction of chirality to those
parent mesoporous MOFs usually adopts the methods of chiral
preliminary treatment of the ligands or postsynthetic mod-
ification of MOFs. The chiral active molecule suffered from such
covalent or coordinated modification would not only block the
pores of MOFs but also reduce their chiral degree of freedom.
The defect engineering theory of MOFs might provide a new
avenue for the rational synthesis of CMOFs with larger cavities.
By the incorporation of missing linker and missing node defects
into MOFs, some bigger cavities can be generated in the defect
sites without losing long-range-order MOFs, producing a larger
catalytic space. In fact, a homochiral zinc-containing MOF,
MOF-5@imidazolium iodide, has been achieved via the defect
engineering synthetic strategy. However, the defect engineering
approach they adopted to obtain the CMOF is to in situ replace
the terephthalic acid linker and coordinated N,N-dimethylfor-
mamide (DMF) by chiral-functionalized imidazolecarboxylic
acid. The chiral groups block the windows of the cavities, and the
detachment of coordinated DMF leads the partial collapse of the
framework, resulting in a dramatic decrease in the Brunauer−
unit of C -symmetric ligands for parent MOFs is partially
3
replaced by a truncated ligand-functionalized chiral active group.
The truncated ligand would generate a linker defect and produce
mesopores, leading to the formation of defective structures of
the MOFs. Thus, the larger pore volume could maintain the
stereogenic configuration freedom of the larger chiral active
molecules, which is conducive to free chiral transfer from chiral
catalysts to reactants. More importantly, the secondary building
units of the DCMOFs are the same as those of the parent MOFs
with high hydrothermal stability to ensure that they carry out
asymmetric catalytic reactions under harsh conditions. Inspired
by these considerations, CuBTC, a well-known Cu-MOF that is
constructed from a copper(II) paddlewheel cluster and a 1,3,5-
benzenetricarboxylate (BTC) ligand and has been extensively
studied because of its facile synthesis and potential commerci-
22,23
alization, was chosen as the parent MOF.
By the mixing of
chiral (S)-5-[1-(tert-butoxycarbonyl)pyrrolidine-2-
2
−1
Received: January 16, 2021
Published: March 25, 2021
Emmett−Teller (BET) surface area from 4000 m ·g of the
2
−1
parent MOF-5 to 280 m ·g . Therefore, MOF-5@imidazolium
iodide shows no enantioselectivity for asymmetric catalytic
reaction.
Herein, we developed a simple and effective method: a mixed
truncated chiral ligand synthetic strategy to construct defective
©
2021 American Chemical Society
https://doi.org/10.1021/acs.inorgchem.1c00141
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carboxamido]isophthalic acid (IPA-L-Pro-BOC) and BTC
ligands with different ratios into the synthetic system, a series
of defective chiral CuBTC compounds (denoted as DC-
CuBTC) were prepared and exhibited efficient asymmetric
catalytic activities and good recyclability for aldol reactions at
room temperature.
The as-synthesized DC-CuBTC-decorated proline chiral
active group showed green-blue polygonal microcrystals. As
can be seen from Figure 1a, DC-CuBTC-0.17 and DC-CuBTC-
0.17, 0.27, and 0.6 for the feed ratios of 1, 2.5, and 4, respectively.
Therefore, the resulting MOFs were named as DC-CuBTC-
0.17, DC-CuBTC-0.27, and DC-CuBTC-0.6.
The N adsorption experiment at 77 K shows that the porosity
2
of DC-CuBTC depends on the functionalization degree (Figure
1c and Table S1). The parent Cu-BTC has a type I isotherm with
2
−1
a BET specific surface area of 1470 m ·g , indicating its
microporous structure. Additionally, with an increase of the
molar ratio of IPA-L-Pro/BTC from 0.17 to 0.6, the BET surface
areas of the DC-CuBTC samples gradually decrease from 1274
2
−1
to 863 m ·g . In particular, the N sorption data of DC-
2
CuBTC-0.6 apparently displayed a type IV isotherm at relative
P/P , indicating that it has a mesoporous existence. As can be
0
seen from the results of nonlocal density functional theory
(
DFT) calculation (Figure 1d), the pore-size distribution of
CuBTC features two sizes of 8.6 Å (major) and 6.8 Å (minor),
which match quite well with its crystal structure. In addition, the
DC-CuBTC samples exhibit smaller micropore sizes, of ca. 5.6 Å
(
major) and ca. 8 Å (minor), than those of CuBTC possibly
because of the introduction of chiral moieties into the
frameworks. The DFT desorption models reveal that mesopores
with pore sizes from 24 to 40 Å readily developed in the DC-
CuBTC samples, confirming that their hierarchical porous
structures contain both micropores and mesopores, which might
enhance reactant diffusion and mass transport in asymmetric
catalytic reactions. Moreover, through scanning electron
microscopy (SEM; Figure S3), the modified DC-CuBTC
sample morphology did not change significantly.
The optical activity of the as-synthesized MOFs was
confirmed by solid-state circular dichroism (CD) spectroscopy
equipped with an integrating sphere. As shown in Figure 2, there
Figure 1. (a) PXRD patterns of the simulated CuBTC and as-
synthesized DC-CuBTC samples. (b) H NMR spectra of the mixed
ligand DC-CuBTC samples digested in deuterium chloride/dimethyl
1
sulfoxide-d . Blue triangles and black circles stand for the character-
6
ization peaks of the BTC and IPA-L-Pro ligands, respectively. (c) N2
sorption isotherms of CuBTC and DC-CuBTC. (d) Pore-size
distribution data of CuBTC and DC-CuBTC calculated using the
DFT method.
0
.27 exhibit powder X-ray diffraction (PXRD) patterns identical
with that of simulated CuBTC, but the diffraction peak of DC-
CuBTC-0.6 at 5.8° attributed to the 111 crystal plane is very
weak. We presume that axial growth of the crystal along the 111
plane changed with an increase of the chiral ligands, which is also
evidenced by the slightly distorted octahedron and rough surface
of crystal morphology (Figure S5c). However, the other
characteristic diffraction peaks of DC-CuBTC-0.6 are consistent
with the simulated one, indicating that all DC-CuBTC
compounds have a framework connection similar to that of
the parent CuBTC. This could be attributed to the geometry
similarity between IPA-L-Pro and BTC, resulting in maintenance
of the overall CuBTC framework topology even after
incorporation of the IPA-L-Pro moieties. It is noteworthy that
the BOC group is necessary to protect the chiral proline unit of
the IPA-L-Pro-BOC ligand, which prevents its protonation or
coordination to copper(II) under the condition of synthesis of
Figure 2. Solid-state CD spectra of IPA-L-Pro, IPA-D-Pro, CuBTC, DC-
CuBTC-0.6, and DC-CuBTC-D-0.6.
is no dichroism signal in the parent CuBTC because it does not
have a chiral nature. However, the Cotton effect can be observed
in the curve of DC-CuBTC-0.6, which is consistent with the
signal of the powder sample of the IPA-L-Pro ligand. This
demonstrated that IPA-L-Pro was successfully anchored on the
wall of DC-CuBTC and intact during the whole mixed truncated
chiral ligand synthetic procedure. In addition, DC-CuBTC-D-
0.6, the counterpart of DC-CuBTC-0.6, presents nearly mirror
images of DC-CuBTC-0.6, indicating the enantiomeric nature
of the two materials. All of the above characterizations
demonstrated the successful syntheses of DC-CuBTC, further
1
8
DC-CuBTC. Deprotection of the proline ligand can be
triggered by heating at 100 °C to generate the final DC-CuBTC
2
4,25
product.
The successful incorporation of IPA-L-Pro into
1
DC-CuBTC was further confirmed by the H NMR spectra of
the digested MOFs (Figures 1b and S1 and S2). The
incorporated amount of IPA-L-Pro can be adjusted by applying
different feed molar ratios of IPA-L-Pro to BTC. According to
integration of the assigned peaks, the incorporated ratios are
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Communication
proving the possibility of this mixed truncated chiral ligand
synthetic strategy for the synthesis of DCMOFs.
The chiral property with microporous/mesoporous structure
prompted us to explore DC-CuBTC utilization as a heteroge-
neous asymmetric catalyst for aldol reaction. As listed in Table 1,
small pore sizes (their maximum pore diameters are about 8.5,
10, and 11.1 Å, respectively), they only gave 2%, 5%, and 13% ee
values, respectively. For a MIL-101 structure with a larger pore
diameter of 34 Å, 95% conversion with 29% ee can be achieved.
In our work, DC-CuBTC-0.6 has a mesopore with a diameter of
about 40 Å and exhibits the highest ee value of 56.5%. This is
distinct evidence that the pore size of the CMOF is conducive to
the stereogenic effect of aldol reaction. The larger pore volume
could maintain the stereogenic configuration freedom of the
larger chiral active molecules, which is conducive to free chiral
transfer from the chiral catalysts to the reactants.
Table 1. Catalyst Screening for the Asymmetric Aldol
a
Reaction
In order to prove that no leaching occurs, the catalysts were
separated by centrifugation and filtration. In the case of DC-
CuBTC-0.6, the catalyst was filtered off after 24 h of reaction and
the filtrate mixture was then stirred for a duration of up to 72 h.
No significant progress was observed in the results, which
proved the true heterogeneous nature of catalysis over the
catalyst (Figure S3). The synthesized L-proline-functionalized
MOFs were also tested for catalyst recycling over three runs. The
results are illustrated in entries 7 and 8 in Table 1 and Figure S4.
DC-CuBTC-0.6 shows constant conversion and ee values. Even
after the third run, DC-CuBTC-0.6 converts 60.6% of 4-
nitrobenzaldehyde. The selectivity with respect to the addition
product does not change within experimental error. PXRD and
SEM (Figure S5) characterizations also demonstrated that the
recovered catalyst maintained its crystallinity. Finally, the
substrate scope of the direct asymmetric aldol reaction of
various aldehydes with acetone was investigated, and the results
are summarized in Table S2. It was found that the conversion
decreased gradually with an increase of the aldehydes’ size,
pore
time
conv
width
(Å)
b
c
entry
1
catalyst
IPA-L-Pro
(days) (%)
ee (%)
51.1
ref
this
2
2
2
2
2
2
2
2
2
2
98.9
96.7
0
work
this
work
this
work
this
work
9, 19, 24 this
work
9, 19, 24, this
work
this
work
this
work
9, 19, 24, this
40 work
8.5
e
2
3
4
5
6
7
8
9
IPA-D-Pro
−53.9
0
CuBTC
9
DC-CuBTC-0.17
DC-CuBTC-0.27
DC-CuBTC-0.6
21
10.1
20.1
56.5
54.9
53.8
−54.7
2
9, 24
36
66.5
62.9
60.6
59.6
50
4
0
DC-CuBTC-0.6
d
(
second)
DC-CuBTC-0.6
d
(
third)
DC-CuBTC-D-
.6
e
29
indicating the molecular sieving capabilities of DC-CuBTC.
0
In conclusion, we have applied a mixed truncated chiral ligand
synthetic strategy to construct defective chiral MOFs with
mesopores of up to 40 Å. Compared to microporous CMOFs,
DC-MOFs exhibited higher enantioselectivity for asymmetric
reaction because the larger pore volume could maintain the
steregenic configuration freedom of the chiral active molecules,
which is conducive to free chiral transfer from the chiral catalysts
to the reactants. These findings delineated an explicit blueprint
to the synthesis and application of promising defect-engineered
chiral MOFs and other porous materials.
1
0
23-L-Pro/H N-
12
2
MIL-53(Al)
1
1
1
2
UiO-66-LP-120
1
2
62
70
5
13
6, 8, 10
11.1
26
12
27-L-Pro/H-
DUT-5(Al)
1
3
22-L-Pro/H N-
4
95
29
12, 29,
34
27
2
MIL-101(Al)
a
The asymmetric aldol reaction was carried out with 4-nitro-
benzaldehyde (0.66 mmol) and acetone (5 mL) as solvents, a catalyst
28
(
2
25 mol % chiral ligand), and trifluoroacetic acid (additive, 6 μL), at
b c
5 °C for 2 days. Isolated yield of the purified material. The
enantiopurity was determined by high-performance liquid chromatog-
d
e
raphy. The catalyst was DC-CuBTC-0.6. The products are in the S
configuration.
ASSOCIATED CONTENT
■
*
sı Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00141.
the homogeneous catalyst IPA-L-Pro achieved 98.9% conversion
with 51.1% enantiomeric excess (ee) of the R-configured
enantiomer and IPA-D-Pro 96.7% conversion with 53.9% ee of
the S-configured enantiomer. When using DC-CuBTC as the
catalyst, as listed in entries 4−6, their conversion rate and ee
values saw an increase with the addition of chiral ligands. Among
them, DC-CuBTC-0.6 achieved the highest conversion of 66.5%
and especially an ee of 56.5% similar to that of the primitive
chiral ligand IPA-L-Pro. In addition, data analogous to those of
the counterpart configuration appeared on its enantiomers (DC-
CuBTC-D-0.6, entry 9). We summarized the performance of the
reported proline-functionalized CMOFs with only microporous
structure, such as MIL-53(Al), UiO-66, H-DUT-5(Al), and
Experimental details, structural description of DC-
CuBTC, and additional tables and figures (PDF)
AUTHOR INFORMATION
■
Corresponding Authors
Shuhua Wang − Key Laboratory of Jiangxi Province for
Environment and Energy Catalysis, College of Chemistry,
Nanchang University, Nanchang 330031, P. R. China;
Email: chaochen@ncu.edu.cn
Chao Chen − Key Laboratory of Jiangxi Province for
Environment and Energy Catalysis, College of Chemistry,
Nanchang University, Nanchang 330031, P. R. China;
orcid.org/0000-0002-0913-0544; Email: shwang@
ncu.edu.cn
1
2,26,27
MIL-101(Al).
As listed in entries 10−13, although those
microporous CMOFs exhibited acceptable conversion, their ee
values were far below that of DC-CuBTC-0.6. It is noteworthy
that the increase of the ee value seems to be determined by the
pore size of the CMOF. For MIL-53, UiO-66, and DUT-5 with
4
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Authors
(14) Yue, Y.; Fulvio, P. F.; Dai, S. Hierarchical Metal-Organic
Framework Hybrids: Perturbation-Assisted Nanofusion Synthesis. Acc.
Chem. Res. 2015, 48, 3044−3052.
Zijuan Chen − Key Laboratory of Jiangxi Province for
Environment and Energy Catalysis, College of Chemistry,
Nanchang University, Nanchang 330031, P. R. China
Xiaodan Yan − Key Laboratory of Jiangxi Province for
Environment and Energy Catalysis, College of Chemistry,
Nanchang University, Nanchang 330031, P. R. China
Meiyan Li − Key Laboratory of Jiangxi Province for
Environment and Energy Catalysis, College of Chemistry,
Nanchang University, Nanchang 330031, P. R. China
(15) Kabtamu, D. M.; Wu, Y. N.; Li, F. Hierarchically porous metal-
organic frameworks: synthesis strategies, structure(s), and emerging
applications in decontamination. J. Hazard. Mater. 2020, 397, 122765.
(16) Khan, N. A.; Hasan, Z.; Jhung, S. H. Beyond pristine metal-
organic frameworks: Preparation and application of nanostructured,
nanosized, and analogous MOFs. Coord. Chem. Rev. 2018, 376, 20−45.
(17) Fang, Z.; Bueken, B.; De Vos, D. E.; Fischer, R. A. Defect-
Engineered Metal-Organic Frameworks. Angew. Chem., Int. Ed. 2015,
5
(
4, 7234−7254.
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c00141
18) Kutzscher, C.; Nickerl, G.; Senkovska, I.; Bon, V.; Kaskel, S.
Proline Functionalized UiO-67 and UiO-68 Type Metal-Organic
Frameworks Showing Reversed Diastereoselectivity in Aldol Addition
Reactions. Chem. Mater. 2016, 28, 2573−2580.
Author Contributions
Z.C. and X.Y. contributed equally to this work.
Notes
(19) Tan, C.; Han, X.; Li, Z.; Liu, Y.; Cui, Y. Controlled Exchange of
Achiral Linkers with Chiral Linkers in Zr-Based UiO-68 Metal-Organic
Framework. J. Am. Chem. Soc. 2018, 140, 16229−16236.
The authors declare no competing financial interest.
(20) Zhao, N.; Cai, K.; He, H. The synthesis of metal-organic
frameworks with template strategies. Dalton Trans 2020, 49 (33),
11467−11479.
ACKNOWLEDGMENTS
■
This research was financed by the National Natural Science
Foundations of China (Grants 21961021 and 21661020) and
Jiangxi Province (Grant 20202ACB203001).
(21) Hu, Z.; Peng, Y.; Tan, K. M.; Zhao, D. Enhanced catalytic activity
of a hierarchical porous metal−organic framework CuBTC. CrystEng-
Comm 2015, 17, 7124−7129.
(22) Sun, X.; Tao, Y.; Du, Y.; Ding, W.; Chen, C.; Ma, X. Metal
organic framework HKUST-1 modified with carboxymethyl–cyclo-
dextrin for use in improved open tubular capillary electrochromato-
graphic enantioseparation of five basic drugs. Microchim. Acta 2019,
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