Controlled Exchange of Achiral Linkers with Chiral Linkers in Zr-Based UiO-68 Metal-Organic Framework

Article abstract of DOI:10.1021/jacs.8b09606

The development of highly robust heterogeneous catalysts for broad asymmetric reactions has always been a subject of interest, but it remains a synthetic challenge. Here we demonstrated that highly stable metal-organic frameworks (MOFs) with potentially acid-labile chiral catalysts can be synthesized via postsynthetic exchange. Through a one- or two-step ligand exchange, a series of asymmetric metallosalen catalysts with the same or different metal centers are incorporated into a Zr-based UiO-68 MOF to form single- and mixed-M(salen) linker crystals, which cannot be accomplished by direct solvothermal synthesis. The resulting MOFs have been characterized by a variety of techniques including single-crystal X-ray diffraction, N₂ sorption, CD, and SEM/TEM-EDS mapping. The single-M(salen) linker MOFs are active and efficient catalysts for asymmetric cyanosilylation of aldehydes, ring-opening of epoxides, oxidative kinetic resolution of secondary alcohols, and aminolysis of stilbene oxide, and the mixed-M(salen) linker variants are active for sequential asymmetric alkene epoxidation/epoxide ring-opening reactions. The chiral MOF catalysts are highly enantioselective and completely heterogeneous and recyclable, making them attractive catalysts for eco-friendly synthesis of fine chemicals. This work not only advances UiO-type MOFs as a new platform for heterogeneous asymmetric catalysis in a variety of syntheses but also provides an attractive strategy for designing robust and versatile heterogeneous catalysts.

Full text of DOI:10.1021/jacs.8b09606

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Article
Controlled Exchange of Achiral Linkers with Chiral
Linkers in Zr-Based UiO-68 Metal-Organic Framework
Chunxia Tan, Xing Han, Zijian Li, Yan Liu, and Yong Cui
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b09606 • Publication Date (Web): 03 Nov 2018
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Controlled Exchange of Achiral Linkers with Chiral Linkers in
Zr-Based UiO-68 Metal-Organic Framework
Chunxia Tan,† Xing Han,† Zijian Li,† 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
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ABSTRACT: The development of highly robust heterogeneous catalysts for broad asymmetric reactions has always been a subject of
interest, but it remains a synthetic challenge. Here we demonstrated that highly stable metal−organic frameworks (MOFs) with potentially
acid-labile chiral catalysts can be synthesized via postsynthetic exchange. Through a one- or two-step ligand exchange, a series of
asymmetric metallosalen catalysts with the same or different metal centers are incorporated into a Zr-based UiO-68 MOF to form single-
and mixed-M(salen) linker crystals, which cannot be accomplished by direct solvothermal synthesis. The resulting MOFs have been
characterized by a variety of techniques including single crystal X-ray diffraction, N2 sorption, CD and SEM/TEM-EDS mapping. The
single-M(salen) linker MOFs are active and efficient catalysts for asymmetric cyanosilylation of aldehydes, ring-opening of epoxides,
oxidative kinetic resolution of secondary alcohols and aminolysis of stilbene oxide and the mixed-M(salen) linker variants are active for
sequential asymmetric alkene epoxidation/epoxide ring-opening reactions. The chiral MOF catalysts are highly enantioselective and
completely heterogeneous and recyclable, making them attractive catalysts for eco-friendly synthesis of fine chemicals. This work not only
advances UiO-type MOFs as a new platform for heterogeneous asymmetric catalysis in a variety of syntheses but also provides an
attractive strategy for designing robust and versatile heterogeneous catalysts.
reaction conditions. Despite the increased use of this synthetic
INTRODUCTION
method, there is no report on the use of PSE to introduce chiral
organic linkers into such robust MOFs.15,16 Chiral salen ligands
such as (R,R)-1,2-cyclohexanediamino-N,N′-bis(3-tert-butyl-sali-
cyliene) are important privileged ligands for asymmetric
catalysis.20 M(salen) complexes have, for example, been shown to
be highly effective in catalyzing asymmetric epoxidation of
olefins (M = Mn and Fe),21a,b cyanation of aldehydes (M = V)21c
and ring opening of epoxides (M = Co and Cr).21d,e Herein we
reported that PSE allowed for incorporation of potential acid-
labile chiral metallosalen catalysts into the superior stable Zr(IV)-
based UiO-68 MOF to prepare single- and mixed-M(salen) linker
crystals, one of which has been characterized by single-crystal X-
ray diffraction. The afforded UiO-type CMOFs exhibit drastically
enhanced chemical stability and recyclability compared to non-
zirconium MOFs with the same M(salen) linkers. These CMOFs
were shown to be active and efficient heterogeneous asymmetric
catalysts for the archetypical M(salen)-catalyzed a variety of
asymmetric organic transformations and related sequential
reactions with high enantioselectivities and completely
recyclability.
As an emerging class of crystalline porous materials, metal-
organic frameworks (MOFs) have received tremendous attention
because of their potential applications in many areas such as
molecular storage and separation,1 sensing,2 catalysis3 and drug
delivery.4 In particular, MOFs provide a highly tunable platform
to engineer heterogeneous catalysts with site-isolated catalytically
active centers for many important reactions,5 e.g., asymmetric
organic reactions that cannot be achieved with traditional porous
inorganic materials.6 Moreover, the abundant choice of porous
structures can impose size- and shape-selective restrictions
through well-defined channels and pores.7 For example, size-,
regio- and enantioselective reactions can be realized by
introducing stereoselective catalysts into MOFs or confining
chiral substrates within the pores of frameworks.6,7a Chiral ligands,
such as BINOL,8 BINAP,9 biphenol,10 metallosalen11 and L-
proline,12 have been proved to be useful catalysts in MOFs for
catalyzing asymmetric organic reactions. However, with one
exception of a Zr6-MOF with BINAP,9 these chiral MOF (CMOF)
catalysts typically displayed low chemical stability to water and
harsh acidic or alkaline reaction conditions.8,9-12 The synthesis of
highly stable CMOFs has been underexplored,13 mainly because
the solvothermal procedures for MOF synthesis require
enantiopure organic linkers that are thermally and acidically
robust. In this work, we demonstrated that highly stable Zr-based
UiO (University of Oslo) MOFs14 with potentially acid-labile
asymmetric catalysts can be synthesized via postsynthetic
exchange (PSE).15
RESULTS AND DISCUSSION
Synthesis of UiO-68 Type CMOFs via PSE. We began this
study by first attempting to directly synthesize isoreticular analogs
of UiO-68-M using various linear dicarboxylate linkers (H2LM)
derived from metallosalen. UiO-68, which adopts the highly open
fcu net and consists of Zr6O4(OH)4 clusters periodically linked
with 4,4′-terphenyldicarboxylate (TPDC), stands out among
MOFs due to its exceptional chemical, mechanical, and thermal
stabilities.14 Despite many attempts, at this stage we were unable
to prepare the isoreticular analogs of UiO-68 containing chiral
metallosalen motifs. This prompted us to explore ligand exchange
as a strategy for accessing chiral analogues of UiO-68. The UiO-
68-Me consisting of Zr(IV)-based secondary building units and 2-
methyl-terphenyl dicarboxylate ligand (H2TPDC-Me) was chosen
for incorporation of M(salen) because of its exceptionally high
structural stability with respect to water and acids while the
similar length of H2TPDC-Me (15.3 Å) to that of H2LM (14.9 Å)
Postsynthetic methods, especially PSE (also termed solvent-
assisted linker exchange, SALE) has recently proven a powerful
tool to fabricate single-site MOF catalysts, due to their limited
accessibility via direct solvothermal synthesis.15,16 Specially,
Cohen and Hupp et al. have reported on the PSE phenomenon in
highly stable MOFs, such as MIL (Material Institute Lavoisier),17
ZIF (Zeolitic Imidazolate Framework)18 and UiO materials.19
These structures are an attractive platform for incorporation of
molecular catalysts due to their stability under a range of harsh
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Scheme 1. a) Schematic representation of the PSE approach. The as-synthesized achiral UiO-68-Me was converted to chiral UiO-68-M
and UiO-68-M-M’ via PSE of H2LM. b) Microscope images of the MOFs after PSE.
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(Tables S1-S3).14 Highly crystalline UiO-68-Me was synthesized
under solvothermal conditions using ZrCl4, H2TPDC-Me, and
trifluoroacetic acid in DMF at 120 C for 72 h in 63% yield,
followed by washing with DMF and THF, and then activation
under vacuum. Field-emission scanning electron microscopy (FE-
SEM) shows an octahedral morphology of the resulting UiO-68-
Me crystals (Figure S11a).
Taking advantages of the similar length and same connectivity
of H2TPDC-Me and H2LM ligand, we employed PSE as a mild
functionalization approach to introduce M(salen) into UiO-68-Me,
as shown in Scheme 1a. Optimization of the PSE conditions
o
o
revealed that the use of DMF at 100 C for 10 days produced the
best exchange results. After washing thoroughly with fresh DMF
and Soxhlet extraction with THF, the filtered products were dried
in vacuum at 100 oC for 2 hours. The mixed-M(salen) linker UiO-
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68-Mn-Cr and UiO-68-Mn-V were obtained using the newly
fabricated UiO-68-Mn as a parent material in a similar procedure.
in the colors of the crystals, which were consistent with the colors
reported for the molecular metallosalen analogs (Scheme 1b).
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The ligand exchanges were clearly suggested by dramatic changes
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Figure 1. a) Kinetic results for the PSE process of UiO-68-Cr. b) 1H NMR spectroscopy of the digested UiO-68-Cr during PSE (The
peaks due to the methyl groups of H2TPDC-Me and the tert-butyl groups of H2LM are highlighted by green and purple, respectively).
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Figure 2. (a) and (b) Structures of the tetrahedral and octahedral cages in UiO-68-Cu, respectively. c) The 3D structure of UiO-68-Cu. Zr,
blue polyhedral; Cu, green; N, royal; O, red; C, gray.
Framework postsynthetic modification of UiO-68-Me using
H2LM was monitored by 1H NMR. After digestion of the MOF in
HF/DMSO-d6, the metallosalen ligand was decomposed into 3-
(tert-butyl)-5-formyl-4-hydroxybenzoic acid and 1,2-cyclohexane
diamine. Integration of the proton resonances for the tert-butyl
groups and the free H2TPDC-Me ligands confirmed the degree of
PSE (Figures 1b and S12), which is tunable by varying the
modification time. For example, UiO-68-Me was combined with
5 equiv. of H2LCr in DMF at 100 °C for 24, 96, 168 and 240 h to
produce the desired UiO-68-M with 23.1%, 57.3%, 80.6% and
92.6% overall LCr loadings. The composition of the solution
of Zr/M/M’are 1:0.48:0.45 for UiO-68-Mn-Cr and 1:0.29:0.64
for UiO-68-Mn-V. These ratios indicated 83-95% of TPDC-Me
in UiO-68-Me were exchanged for H2LM. In all cases, the amount
of LM included can be controlled between 23.1% to 95% by
varying the PSE time from 24 to 240 h. SEM micrographs
revealed that average particle sizes of the CMOFs ranged from
51.13 to 51.77 m, slightly smaller than that of UiO-68-Me
(53.64 m). A nearly identical octahedral particle morphology
similar to UiO-68-Me, as shown in Figure S11a, suggested that
the present PSE occurred in a single-crystal to single-crystal
mechanism rather than a dissolution-recrystallization process.
Structural Characterization. Single crystals of UiO-68-Cu
suitable for X-ray diffraction were obtained by soaking crystals of
UiO-68-Me in 10 equiv. of H2LCu in DMF at 100 °C for over two
months. UiO-68-Cu crystallized in the chiral cubic F432 space
group, which is different from the parent UiO-68-Me that
crystallized in achiral cubic Fm3m. Consistent with the shorter
lengths of the LM linkers (14.9 Å) than that of TPDC-Me (15.3
Å), the unit cell parameter of UiO-68-Cu [a = 32.0319(7) Å and
V = 32866(2) Å3] is slightly smaller than that of UiO-68-Me [a =
32.6205(16) Å and V = 34711 (3) Å3]. As expected, the chiral
structure contains an inner Zr6O4(OH)4 core in which the
triangular faces of the Zr6-octahedron capped by four µ3-O and
1
during PSE was also monitored by H NMR. It is found that the
molar percentage of H2TPDC-Me decreased over time, and after
8 days, <0.16% H2TPDC-Me was detected (Figure S12i). The
decreased molar amount of H2TPDC-Me was almost the same as
the increased amount of H2LM in the MOFs, indicative of the
gradual exchange of H2TPDC-Me by H2LM. This finding thus
eliminated the possibility of MOF dissolution or trapping of
metallosalen ligands in the cavities during PSE. ICP-OES
(inductively coupled plasma optical emission spectrometry)
showed that molar ratios of Zr/M in the final frameworks are
1:0.95 for UiO-68-Cu, 1:0.85 for UiO-68-V, 1:0.88 for UiO-68-
Mn, and 1:0.93 for UiO-68-Cr, 1:0.83 for UiO-68-Fe, and ratios
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four µ3-OH groups (Figure 2). The polyhedral edges of the
inner diameter for small tetrahedral pores and 2.0 nm for large
octahedral pores. The solvent-accessible void space was
calculated to be 54% using PLATON,22 which is obviously
smaller than that of UiO-68-Me (75%), consistent with the
introduction of bulky steric tert-butyl
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Zr6O4(OH)4 cluster are bridged by twelve bidentate carboxylate
groups from twelve linear dicarboxylate ligand LM, leading to a
3D network of UiO-type structure with fcu topology. The
framework structure possesses two kinds of cages with 1.4 nm
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b)
Figure 3. a) PXRD patterns of UiO-68-Me and UiO-68-M/M-M’. b) PXRD patterns of UiO-68-Cu after treatment in different solutions
for 24 h. c-f) SEM images and EDS mappings of single crystals of UiO-68-V/Cr/Cu/Mn-V. g-h) TEM images and EDS mappings of
UiO-68-Cu/Mn-V
and cyclohexane groups on the LM linkers. The resulting materials
are among the rare examples of UiO-type CMOFs.
consequences of the incorporation of the bulky LM ligands.
Elemental mappings for UiO-68-M and UiO-68-M-M’ at the
microstructural level by SEM with energy dispersive spectra
(EDS) shows uniform distribution of M throughout the crystal. To
test the depth range distribution of LM at more microscale, we
applied high resolution transmission electron microscopy-energy
dispersive spectra (TEM-EDS) analysis on the CMOFs. In all
tested samples, both M and Zr were distributed uniformly at the
scale of 80 nm. Besides, the cross-sectional SEM-EDS images for
partially exchanged samples by H2LCr (for 24 h, 72 h, 120 h and
168 h) were also collected. Both Cr and Zr were found
homogenously distributed throughout the crystal even at a short
PSE exposure time of 24 h, indicating that the H2TPDC-Me
ligand was exchanged homogeneously during PSE (Figures 3c-3h
and S11b-11d). The solid-state circular dichroism (CD) spectra of
UiO-68-M and UiO-68-M-M’ exchanged from (R)- or (S)-
enantiomers of H2LM are mirror images of each other,
demonstrating their enantiomeric nature (Figure S7).
Powder X-ray diffraction (PXRD) revealed that all UiO-68-
type CMOFs were isostructural with each other (Figure 3a). They
showed similar PXRD patterns to that of the parent UiO-68-Me
with some shifting of the peak positions and slight changes of the
peak intensities. After the incorporation of LM, the peak
intensities at 5.5, 11.0and 22.2 degrees decreased for UiO-68-M-
M’, while the peaks at 8.98 and 9.40 degrees were slightly shifted
to 9.14 and 9.57 degrees, respectively, corresponding to a unit cell
reduction, which may result from the replacement of the longer
TPDC-Me linker with the shorter LM linkers. PXRD analysis on
partially exchanged crystals by H2LCr revealed that no mixed-
phases MOFs were formed during PSE and the two low-angle
peaks were gradually shifted to the high-angle peaks, further
indicating that the PSE process followed a single-crystal to single-
crystal transformation mechanism (Figure S5f).
All CMOFs are stable in air and common organic solvents.
TGA indicated that these frameworks start to decompose around
400 C (Figure S8). The permanent porosity of these activated
CMOFs has been confirmed by N2 adsorption experiments at 77
K. All of them displayed typical type-I gas sorption isotherms,
with BET (Brunauer-Emmett-Teller) surface areas ranging from
2566 to 2758 m2/g (Figure 4a). These values are smaller than the
BET surface area of the parent UiO-68-Me (3738 m2/g), as a
To investigate the chemical stability of UiO-68-type CMOFs,
we dispersed the MOFs samples for 24 h in boiling water, 3 M
HCl and pH = 12 NaOH solutions, respectively. In all cases, the
solids displayed similar PXRD patterns to the pristine MOFs with
slight changes of the peak intensities at 5.5 and/or 22.2 degrees,
indicative of their structural stability in a wider pH range (Figures
o
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Figure 4. a) and b) N2 adsorption (filled symbols) and desorption (open symbols) isotherms at 77 K.
3b and S5a-5e). After the acid and base treatments, the residual
weight percentages were found to 95/92/92% and 84/82/82% for
UiO-68-V/Cu/Mn, respectively. These residual values are close
to those observed for UiIO-68-Me (96% and 85%), but are high
than those for H2LM (72% and 62%) (Figure S6), suggesting the
rigid UiO-68 type framework capable of enhancing the chemical
stability of metallosalen ligands.
To further confirm the chemical stability, we conducted N2
sorption measurements after different treatments for UIO-68-
Cu/Cr. The as-treated UiO-68-Cu has a BET surface area of
2469, 2012 and 1461 m2g−1, respectively, and UiO-68-Cr has a
surface area of 2417, 2012 and 1301 m2g−1 (Figures 4b and S10).
A decrease in the surface area indicated the partial structural
collapse upon treatment.
.
Scheme 2. Asymmetric organic transformations catalyzed by UiO-68-M and UiO-68-M-M’
80-85% conv., 80% ee (Ar = Ph)
87% ee (Ar = 4-OMePh), 96% ee (Ar = 4-ClPh)
MnCl + CrCl
+
80-89% conv.
85-90% conv.
80% ee (Ar = Ph)
99% ee (Ar = 2-Me-6-EtPh)
97% ee (Ar = 4-IPh)
81% ee (R = Me)
84% ee (R = OMe)
80% ee (R = Br)
VO
CrCl
50-60% conv.
MnCl/Fe(OAc)
MnCl
99.7% ee (R1/R2 = CF3/Me)
97% ee (R1/R2 = H/Me)
99.7% ee (R1/R2 = Br/Me)
80-91% conv.
88/86% ee (R= H)
83/84% ee (R = Me)
96/93% ee (R = NO2)
.
Heterogeneous Catalysis. With successful incorporation of
catalytically active M(salen) complexes into a robust UiO-68
MOF, we sought to investigate their catalytic ability for
heterogeneous asymmetric catalysis. As shown in Scheme 2, the
family of chiral MOFs UiO-68-M and UiO-68-M-M’
demonstrated excellent catalytic activity and enantioselectivity for
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several meaningful organic transformations. We first examined
green catalyst for a range of asymmetric organic reactions
involving sequential asymmetric transformations. Complete
heterogeneity and recyclability of these CMOF catalysts may
overcome the problem of metal leaching from other MOFs and
hybrid solid catalysts, making them potentially find application in
practical synthesis of fine chemicals. This work thus provides an
attractive approach to achieve high catalytic activity,
enantioselectivity, chemical stability and recyclability in MOFs
and will promote the design of CMOFs from functional
enantiopure molecules that will display novel chirality properties.
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the catalytic activity of (R)-UiO-68-Mn. It was found to be an
effective catalyst for both asymmetric epoxidation of alkenes with
sPhIO [2-(tertbutylsulfonyl)iodo sylbenzene] as the oxidant and
oxidative kinetic resolution of racemic secondary alcohols with
PhI(OAc)2, and afforded up to 98% ee of the epoxides (Table S5)
and 99.7% ee of the resolved alcohols(Table S6), respectively.
Interesting, (R)-UiO-68-Fe was also capable of smoothly
catalyzing epoxidation of alkenes to epoxides with up to 96% ee
(Scheme 2 and Table S7), but it cannot enantioselectively promote
oxidative kinetic resolution of racemic alcohols. After oxidation
of V(IV) to V(V) with m-chloroperoxybenzoic acid (m-CPBA), (R)-
UiO-68-V was shown to be an active catalyst for cyanosilylation
of aldehydes with Me3SiCN, producing the cyanohydrins silyl
ethers with 80-87% ee (Table S8). (R)-UiO-68-Cr can promote
aminolysis of trans-stilbene oxide with different anilines, and the
amino alcohols were obtained with up to 99% ee (Table S9).
When UiO-68-Me was used as a catalyst for catalyzing
aminolysis of trans-stilbene oxide under identical conditions, no
target products were detected by 1H NMR even after 48 h,
indicating that the Zr6 clusters in the CMOF catalysts cannot
promote the transformation (Table S9). This level of ee values
observed for UiO-68-M MOFs demonstrated excellent activity
and enantioselectivity rivals those of the monomeric metallosalen
catalyst. The UiO-68-M catalysts gave stereoselectivities
comparable to those of related homogeneous controls and other
hybrid solid catalysts derived from M(salen) motifs for the tested
reactions.11,23,24
Considering that UiO-68-Mn and UiO-68-Cr can catalyze
epoxidation of alkenes and ring opening of epoxides, respectively,
we combined the different catalytic metal centers in UiO-68-Mn-
Cr to promote sequential reactions initiated by epoxidation of
alkene followed by ring opening reactions of epoxide to afford the
targeted products. In the presence of UiO-68-Mn-Cr, epoxidation
of 2,2-dimethyl-2H-chromene (DMCH) with sPhIO and
subsequent ring-opening reactions with different anilines were
performed in CHCl3 at r.t. The transformation proceeded
efficiently to produce the desired amino alcohol in 80-85% yields
with 80-99.5% ee (Table S10). Thus, the UiO-68 CMOF-
mediated sequential reactions may provide a new approach for the
synthesis of complex molecules with high enantiocontrols.
9
ASSOCIATED CONTENT
Supporting Information.
Details on experimental procedures, X-ray crystallographic data,
supporting figures, reaction procedures and spectra. This material
is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*yongcui@sjtu.edu.cn
*liuy@sjtu.edu.cn
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was financially supported by the National Science
Foundation of China (Grants 21431004, 21522104 and
21620102001), the National Key Basic Research Program of
China (Grants 2014CB932102 and 2016YFA0203400), Key
Project of Basic Research of Shanghai (17JC1403100 and
18JC1413200), and the Shanghai “Eastern Scholar” Program. We
thank the staffs from BL17B beamline of National Facility for
Protein Science at Shanghai Synchrotron Radiation Facility for
assistance during data collection.
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All UiO-68 CMOF catalysts can be recycled after extracting
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CONCLUSIONS
We have demonstrated that the PSE method was an efficient
and mild approach to fabricate UiO-68-type MOF containing one
or two types of chiral M(salen) species, which appear to be
difficult to be obtained via direct solvothermal synthesis. The as-
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M(salen) linker CMOFs have been proved to be an efficient and
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