TiO2@UiO-68-CIL: A Metal-Organic-Framework-Based Bifunctional Composite Catalyst for a One-Pot Sequential Asymmetric Morita-Baylis-Hillman Reaction

Article abstract of DOI:10.1021/acs.inorgchem.8b02132

A chiral ionic liquid (CIL) moiety of a l-pyrrolidin-2-ylimidazole-decorated homochiral UiO-68-type metal-organic framework, UiO-68-CIL (1), was successfully prepared by the combination of a new premodified chiral CIL ligand (H₂L-CIL) and ZrCl₄ via a solvothermal method. The TiO₂-loaded TiO₂@UiO-68-CIL (2) was prepared by impregnating 1 in a toluene solution of Ti(OPrⁱ)₄ and sequential in situ hydrolysis. The obtained 2 can be a bifunctional asymmetric heterogeneous catalyst to successfully promote the one-pot Morita-Baylis-Hillman reaction starting from aromatic alcohols in a tandem way.

Full text of DOI:10.1021/acs.inorgchem.8b02132

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
TiO2@UiO-68-CIL: A Metal−Organic-Framework-Based Bifunctional
Composite Catalyst for a One-Pot Sequential Asymmetric Morita−
Baylis−Hillman Reaction
Yu-Hong Hu,† Cong-Xue Liu,† Jian-Cheng Wang,* Xiu-Hui Ren, Xuan Kan, and Yu-Bin Dong*
College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Centre of Functionalized Probes for
Chemical Imaging in Universities of Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of Education, Shandong
Normal University, Jinan 250014, P. R. China
S
* Supporting Information
ABSTRACT: A chiral ionic liquid (CIL) moiety of a L-
pyrrolidin-2-ylimidazole-decorated homochiral UiO-68-type
metal−organic framework, UiO-68-CIL (1), was successfully
prepared by the combination of a new premodified chiral CIL
ligand (H2L-CIL) and ZrCl4 via a solvothermal method. The
TiO2-loaded TiO2@UiO-68-CIL (2) was prepared by impreg-
nating 1 in a toluene solution of Ti(OPri)4 and sequential in situ
hydrolysis. The obtained 2 can be a bifunctional asymmetric
heterogeneous catalyst to successfully promote the one-pot
Morita−Baylis−Hillman reaction starting from aromatic alco-
hols in a tandem way.
We incorporated chiral 1 with TiO2 into a TiO2@MOF
system based on the following reasons: first, TiO2 is known to
be one of the most important photoactive species in
photocatalysis,32−42 especially in photocatalytic oxidation;
second, ILs, as an important class of organocatalysts, have
been proven to be highly efficient in catalyzing a number of
chemical transformations such as acid- or base-mediated
organic reactions;24−27 third, chirality could be readily
achieved during ligand synthesis43−46 via CIL decoration.
Hopefully, the incorporation of photoactive TiO2 and CIL
organocatalytic species along with the chiral function into
MOF platforms would generate a composite multifunctional
asymmetric catalytic system for organic transformations in a
tandem way.
INTRODUCTION
■
Metal−organic frameworks (MOFs), as a promising class of
porous supports, have been widely used in the fabrication of
composite catalytic systems for heterogeneous organic trans-
formations.1−3 To date, various inorganic catalytic active
species such as metal nanoparticles (M NPs) or metal
oxides,4−16 inorganic metal salts,17−21 and even nonmetal
components22,23 have successfully been loaded by MOF
platforms. On the other hand, active organocatalytic functional
moieties, like the functionalized ionic liquid (IL),24−27 can also
be covalently grafted onto MOFs via a direct ligand
modification or a postsynthetic approach to generate MOF-
based heterogeneous organocatalytic systems.19,28−31 In view
of the synthetic methodology, both inorganic and organic
catalytic species, together with chirality, could be simulta-
neously incorporated into MOF platforms; consequently, the
distinctly different types of catalytic species would be
integrated together to enable new functionality.
In this contribution, we report for the first time such a chiral
ionic liquid (CIL)-involved and TiO2-loaded MOF-based
composite bifunctional catalyst, namely, TiO2@UiO-68-CIL
(2). It was prepared by loading TiO2 via hydrolysis of its
precursor Ti(OPri)4, which was preembedded in a chiral UiO-
68-CIL (1) MOF matrix by solution impregnation. The
obtained 2 can be a highly efficient and heterogeneous catalyst
to promote the one-pot sequential asymmetric Morita−
Baylis−Hillman (MBH) reaction starting from alcohol under
ambient conditions.
EXPERIMENTAL SECTION
■
Materials and Instrumentations. All of the chemicals were
obtained from commercial sources (Acros) and used without further
purification. The detailed synthetic procedure for the chiral ligand of
1
H2L-CIL is shown in Supporting Information. H NMR data were
collected on a Bruker Avance-400 spectrometer. Chemical shifts are
reported in δ relative to tetramethylsilane. IR spectra were obtained in
the 400−4000 cm−1 range using a Bruker Alpha Fourier transform
infrared (FT-IR) spectrometer. Elemental analyses were performed on
a PerkinElmer model 2400 analyzer. Inductively coupled plasma
(ICP) measurement was conducted on an IRIS Intrepid (II) XSP and
NU AttoM spectrometer. High-resolution mass spectrometry (MS)
Received: July 28, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b02132
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Scheme 1. Synthesis of H2L-CIL, 1, and 2
Figure 1. (a) PXRD patterns of the simulated UiO-68, 1, and 2. (b) SEM images of 1 and 2 and EDS elemental mapping of 2. (c) TEM images of
1 and 2. (d) CD spectra of 1 (red line) and 2 (black line).
analysis was carried out on a Bruker Maxis ultrahigh-resolution
quadrupole time-of-flight mass spectrometer. Thermogravimetric
analysis (TGA) was performed on a TA Instrument Q5 analyzer
under flowing nitrogen at a heating rate of 10 °C/min. Powder X-ray
diffraction (PXRD) patterns were obtained on a D8 ADVANCE
powder X-ray diffractometer with Cu Kα radiation (λ = 1.5405 Å). A
300 W xenon lamp (PLS-SXE300C, Perfect light Co., Beijing, China)
was used as the light source. Scanning electron microscopy (SEM)
micrographs were recorded on a Gemini Zeiss Supra TM scanning
electron microscope equipped with an energy-dispersive X-ray
detector.
mL) by using ultrasound (30 min). H2L-CIL (124.4 mg, 0.24 mmol)
was added to the obtained clear solution in an equimolar ratio related
to ZrCl4. The tightly capped flasks were kept in an oven at 90 °C
under static conditions. After 24 h, the reaction system was allowed to
cool to room temperature and the generated precipitate was collected
by centrifugation. The solids were suspended in fresh DMF (20 mL).
After the solution was allowed to stand at room temperature for 10 h,
the suspension was centrifuged and the solvent was decanted off. The
obtained MOF micronanoparticles were completely washed with
ethanol (20 mL) several times. Finally, the solids were dried in a
vacuum to generate 1. Yield: 74.1%. IR (KBr pellets, cm−1): 3378(w),
1592(s), 1544(ms), 1417(vs), 1183(w), 1148(w), 1006(w), 836(w),
Synthesis of 1. ZrCl4 (57.6 mg, 0.24 mmol) and 50 equiv of acetic
acid (0.68 mL) were dissolved in N,N-dimethylformamide (DMF; 10
B
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a
780(ms), 712(w), 649(w). Elem anal. Calcd for C174 H160 O32 N18
Zr6: C, 55.18; H, 4.58; N, 6.66. Found: C, 55.08; H, 4.37; N, 6.91.
Synthesis of 2. In a nitrogen atmosphere, 1 (124.3 mg, 0.032
mmol) was added to a Ti(OPri)4 (33.3 μL, 0.109 mmol) solution of
anhydrous toluene (7 mL) at 100 °C. The mixture was stirred at this
temperature for an additional 1 h. The obtained crystalline solids were
consecutively washed with toluene, ethanol, water (H2O), ethanol,
and diethyl ether. Then, the solids were dried at 80 °C for 2 h to
afford 2. IR (KBr pellets, cm−1): 3370(w), 1606(s), 1548(ms),
1417(vs), 1183(w), 1146(w), 1006(w), 836(w), 780(ms), 712(w),
649(w). Elem anal. Calcd for C174H160O39.58N18Zr6Ti3.79: C, 54.02; H,
4.14; N, 6.52. Found: C, 54.29; H, 4.39; N, 6.47. The formula of 2
was described as 3.79TiO2@UiO-68-CIL based on the elemental
analysis and ICP results (Table S1).
General Procedure for Photoinduced Alcohol Oxidation
Catalyzed by 2. A 300 W xenon lamp was used as the visible-light
source in the experiment. In an oxygen atmosphere, a mixture of
ArCH2OH (1 mmol), methanol (MeOH; 5 mL), and 2 (0.02 mmol)
was stirred at room temperature for 24 h [monitored by thin-layer
chromatography (TLC)] to afford the corresponding aldehydes. The
yield was determined by gas chromatography (GC) measurement on
an DB-5 column.
General Procedure for a MBH Reaction Catalyzed by 2. A
mixture of ArCHO (1 mmol), methyl acrylate (1.2 mmol), MeOH (5
mL), and 2 (0.02 mmol) was stirred at room temperature for 48 h
(monitored by TLC) to afford the corresponding products. Yields
were determined by GC measurement on a DB-5 column.
General Procedure for a One-Pot MBH Reaction Catalyzed
by 2. A 300 W xenon lamp was used as the visible-light source in the
experiment. In an oxygen atmosphere, a mixture of ArCH2OH (1
mmol), MeOH (5 mL), and 2 (0.02 mmol) was stirred at room
temperature for 24 h. After the addition of methyl acrylate (1.2
mmol), the light source was removed, and the mixture was stirred at
room temperature for an additional 48 h. After the reaction, 2 was
recovered by centrifugation, and the solvent was removed in a
vacuum. The crude products were purified by a column on silica gel to
afford the corresponding products (Supporting Information).
Table 1. Model Photocatalytic Benzyl Alcohol Oxidation
light
source
time
(h)
yield
b
entry
catalyst
solvent
oxidant
(%)
1
2
3
2
2
2
MeOH
MeOH
MeOH
oxygen
oxygen
oxygen
24
24
24
<1
11
78
sun light
xenon
lamp
4
xenon
lamp
xenon
lamp
xenon
lamp
xenon
lamp
xenon
lamp
xenon
lamp
xenon
lamp
2
2
2
2
1
MeOH
CH3CN
H2O
air
24
24
24
24
24
24
24
21
77
5
oxygen
oxygen
oxygen
oxygen
oxygen
oxygen
6
46
7
THF
68
8
MeOH
MeOH
MeOH
<1
10
9
amorphous
TiO2
1/amorphous
10
<12
TiO2
a
Reaction conditions: alcohol (0.2 M) in MeOH (5 mL), catalyst (1
mmol %), oxygen or air atmosphere (1 atm), room temperature, light
irradiation (300 W xenon lamp; λ > 400 nm; intensity = 75−95 × 103
b
lx). Yields were determined by GC.
Table 2. Photocatalytic Oxidation of Different Benzyl
a
Alcohols Catalyzed by 2
RESULTS AND DISCUSSION
Synthesis and Characterization of H2L-CIL. As shown
in Scheme 1, a chiral ligand precursor of C was prepared in
■
b
entry
R
time (h)
yield (%)
1
2
3
4
5
6
7
8
9
10
H−
24
24
24
24
24
24
24
24
24
24
78
93
90
>99
88
86
93
41
35
45
2-MeO−
3-MeO−
4-MeO−
2-Me−
3-Me−
4-Me−
2-NO2−
3-NO2−
4-NO2−
a
Reaction conditions: alcohol (1.0 mmol), CH3OH (5 mL), catalyst
(2 mol % 2, 7.5 mol % TiO2 equiv), oxygen atmosphere (1 atm),
room temperature, and 300 W xenon lamp (λ > 400 nm; intensity =
b
75−95 × 103 lx). Yields were determined by GC.
corresponding 1H NMR, FT-IR, and MS spectra for A−C and
H2L-CIL are provided in the Supporting Information.
Figure 2. Transient photocurrents of 1 (black line) and 2 (red line).
Synthesis and Characterization of UiO-68-CIL (1) and
TiO2@UiO-68-CIL (2). The chiral 1 was synthesized by a
combination of ZrCl4 and H2L-CIL under solvothermal
conditions (DMF, 90 °C, 24 h) in the presence of acetic
acid in 74% yield (Scheme 1). Notably, the CIL moiety was
stable during the synthesis process of 1 under the given
synthetic conditions (Supporting Information). TiO2-loaded 2
was prepared by immersing 1 in a dry toluene solution of
80% yield by quaternization of the L-pyrrolidineimidazole (A)
with α-bromomethyl and dicarboxylic acid dimethyl ester
substituted p-terophenyl (B) in anhydrous tetrahydrofuran
(THF) under reflux for 48 h. Dicarboxylic ligand H2L-CIL was
obtained via hydrolyzation of C with LiOH in a MeOH/H2O
mixed solvent system at room temperature in 86% yield. The
C
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a
Table 3. Model MBH Reaction Catalyzed by 2
Table 5. One-Pot Sequential MBH Reaction Catalyzed by
2
a
b
b
entry T (°C) catalyst
solvent
time (h) yield (%)
ee (%)
1
2
3
4
5
6
7
8
25
50
60
25
25
25
25
25
25
25
25
2
2
2
2
2
2
2
2
2
1
MeOH
MeOH
MeOH
MeOH
H2O
24
24
24
48
48
48
48
48
48
48
48
44
86
11
<3
83
83
65
80
64
84
83
<1
79
87
96
10
47
66
93
96
96
<5
THF
CH3CN
MeOH
MeOH
MeOH
MeOH
c
d
9
10
11
a
Reaction conditions: benzaldehyde (1.0 mmol), CH3OH (5 mL),
b
catalyst (2 mol %), methyl acrylate (1.0 mmol). Yields and ee values
were determined by GC and chiral high-performance liquid
chromatography (HPLC), respectively. 2 (1.5 mol %). 2 (2.5 mol
c
d
%).
Table 4. MBH Reaction of Acrylic Esters and Various
a
Benzyl Aldehydes Catalyzed by 2
a
Reaction conditions: (1) alcohols (1.0 mmol), CH3OH (5 mL),
b
b
entry
R
yield (%)
ee (%)
catalyst (2.0 mol %), oxygen atmosphere (1 atm), and 300 W xenon
lamp (λ > 400 nm; intensity = 75−95 × 103 lx), room temperature,
24 h; (2) methyl acrylate (1.0 mmol), without light, room
temperature, 48 h. Isolated yield. The ee values were determined
by chiral HPLC (Supporting Information).
1
2
3
4
5
6
7
8
9
2-MeO−
3-MeO−
4-MeO−
2-Me−
3-Me−
4-Me−
2-NO2−
3-NO2−
4-NO2−
88
93
99
68
85
97
44
56
64
90
97
99
85
93
96
87
95
98
b
c
determined by ICP measurement (Supporting Information),
and the Zr/Ti ratio in 2 was 1.57:1.00; therefore, the
corresponding formula of 2 can be described as 3.79TiO2@
UiO-68-CIL, which was further supported by elemental
analysis. The encapsulated TiO2 species in 2 was further
investigated by high-resolution transmission electron micros-
copy (HRTEM). As shown in Figure 1c, the amorphous titania
layers are present in the MOF matrix of 2,51,52 which was not
observed in 1. Moreover, in 1 and 2, the chiral feature of the L-
pyrrolidine species was retained, which was well supported by
the solid circular dichroism (CD) measurement. The CD
spectra of 1 and 2 unambiguously confirmed the enantiomeric
nature of the optical activity. As indicated in Figure 1d, 2
showed positive dichroic signals at 250 and 310 nm in its CD
spectrum, which is basically identical with that of 1. The
framework of 2 was stable up to ca. 160 °C based on TGA and
PXRD measurements (Supporting Information), so it was
stable enough to promote organic transformations under mild
reaction conditions.
a
Reaction conditions: benzyl aldehydes (1.0 mmol), CH3OH (5 mL),
catalyst (2 mol %), methyl acrylate (1.0 mmol), room temperature.
Yields and ee values were determined by GC and chiral HPLC,
b
respectively.
Ti(OPri)4 (100 °C, 1 h), followed by an in situ hydrolysis step
for the uploaded Ti(OPri)4 during the H2O-washing process
(Scheme 1).47
The PXRD patterns indicated that 1 and 2 are highly
crystalline and their structures are identical with that of pristine
UiO-68 (Figure 1a).48,49 So, the loaded TiO2 and L-pyrrolidin-
2-ylimidazole-decorated H2L-CIL did not destroy and
influence formation of the UiO-68 framework under the
reaction conditions. On the other hand, we did not observe the
characteristic peaks for crystalline TiO2 in the PXRD pattern of
2, indicating that the encapsulated TiO2 in the MOF existed in
its amorphous form.50 SEM measurement showed that both 1
and 2 were obtained as micronanoparticles with octahedral
shape (Figure 1b). The energy-dispersive X-ray spectroscopy
(EDS) measurement (Figure 1b) showed that the loaded TiO2
evenly distributed in 2. The uploaded TiO2 amount in 2 was
Through measurement of the nitrogen gas (N2) adsorption
at 77 K, we obtained the permanent porosity of 2. Unlike pure
UiO-68, 2 exhibited typical type IV behavior (Supporting
Information), which should be caused by the introduced CIL
groups and loaded TiO2. There was pore condensation with
pronounced adsorption−desorption hysteresis (the pore
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Figure 3. (a) Reaction time examination (red line) and leaching test (black line) for the one-pot tandem model reaction catalyzed by 2. (b)
Recycling test for the one-pot stepwise reaction based on 4-methoxybenzyl alcohol and methyl acrylate catalyzed by 2. (c) PXRD patterns of as-
synthesized 2 and after five catalytic runs. (d) SEM image and EDS elemental mapping of 2 after five catalytic cycles.
widths were centered at ca. 8.5 Å; Supporting Information).
These results indicated the existence of mesopores in 2 with a
maximum N2 uptake of 592 cm3/g (a Brunauer−Emmett−
Teller surface area of 223 m2/g), but it was drastically smaller
than that of pristine UiO-68.
visible-light-triggered reaction, and basically no benzaldehyde
was obtained in the absence of light irradiation.
In addition, only 11% yield of benzaldehyde was generated
in the presence of 2 under natural sunlight irradiation in
oxygen at ambient temperature (Table 1, entry 2). The best
catalytic result (78% yield) was observed when the reaction
was conducted in oxygen using 300 W xenon lamp (λ > 400
nm) as the visible-light source with aid of 1 mol % 2 (Table 1,
entry 3). In air, the oxidation yield was down to 21% (Table 1,
entry 4), indicating that pure oxygen is important to this
benzyl alcohol oxidation. In addition, the different solvent
systems were also examined (Table 1, entries 5−7), and 44−
77% yields were obtained when the reaction was conducted in
acetonitrile (CH3CN), H2O, and THF, respectively. The
control experiment (Table 1, entry 8) showed that basically no
conversion was observed (less than 1% yield) when 1 was used
instead of 2 to perform the reaction. Furthermore, when free
amorphous TiO2 was employed, the desired oxidation product
was generated in only 10% yield (ca. 90% starting alcohol was
recovered) under the given conditions (Table 1, entry 9). On
the other hand, the simple physical blend of 1 with amorphous
TiO2 also resulted in a very low oxidation yield at <12% (ca.
87% starting alcohol was recovered; Table 1, entry 10), which
is well consistent with the photocurrent measurement given
above.
In addition, the photocurrent responses of 1, 2, and
amorphous TiO2 were studied (Figure 2). A three-electrode
photoelectrochemical cell was built with the photoanode 1 or 2
as the working electrode, Ag/AgCl as the reference electrode,
and platinum wire as the cathode for the visible-light-driven
water splitting. An external bias of 0.7 V (vs NHE) was
applied, and the photocurrents in a 0.1 M Na2SO4 aqueous
solution (pH = 6.4) electrolyte were measured. Obviously, 2
showed a much higher photocurrent density than 1 upon light
irradiation. Under the same conditions, the photocurrent of
the free amorphous TiO2 was also examined, and the results
showed no difference whether it was irradiated or not. This
clearly implied that the synergistic effect of 1 and TiO2 in 2
significantly improved the separation efficiency of photo-
generated electrons and holes.
Alcohol Oxidation Catalyzed by 2. Taking advantage of
the photoactive TiO2@MOF species, the photocatalytic
oxidation activity of 2 was examined. To optimize the reaction
conditions, benzyl alcohol oxidation in MeOH was chosen as
the model reaction. As shown in Table 1, various catalytic
oxidation conditions (Table 1, entries 1−10) were explored in
an oxygen or air atmosphere using a polar solvent as the
reaction medium to furnish the desired product. Table 1 (entry
1) demonstrated that the benzyl alcohol oxidation was a
On the basis of the optimized conditions, the scope and
generality of photocatalytic oxidation by 2 were verified
through different types of substituted benzyl alcohols (Table 2,
entries 1−10). As shown in Table 2, all of the benzyl alcohols
E
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with electron-donating groups such as alkoxy (Table 2, entries
2−4; 90−>99% yields) and alkyl (Table 2, entries 5−7; 86−
93% yields) groups employed under the optimized conditions
furnished good-to-excellent yields within 24 h at room
temperature. In contrast, the electron-withdrawing-group-
attached substrates showed much lower activity for the
oxidation (Table 2, entries 8−10; 35−45% yields). So, the
electronic effect of the substituents appeared to be important
for this 2-catalyzed photocatalytic oxidation. As is known, the
electron-donating substituents could lead to electron-rich
benzylic alcohols, which would facilitate their oxidation,53−55
consequently enhancing the catalytic efficiency of 2.
MBH Reaction Catalyzed by 2. As is known, the MBH
reaction can be catalyzed by the IL-bound quinuclidine in a
homogeneous way.56 So, we wondered if the L-pyrrolidine-2-
ylimidazole-decorated 2 could also promote MBH reaction but
in a heterogeneous manner, moreover, with high enantiose-
lectivity.
only LCuPRO, together with an imidazole cocatalyst, was able
to catalyze asymmetric MBH reactions but with very low ee
values (5−8%).59
One-Pot MBH Reactions Catalyzed by 2. Considering
the above results, we finally explored the possibility of 2-
catalyzed one-pot MBH reactions starting from benzyl alcohols
and acrylic esters (Table 5, entries 1−11). In this context, 4-
methoxybenzyl alcohol and methyl methacrylate were chosen
as the starting substrates for this one-pot MBH reaction (Table
5, entry 4). When the reaction was carried out in an oxygen
atmosphere with visible light (300 W xenon lamp) at room
temperature for 72 h (monitored by TLC), 96% yield of the
desired product was obtained. However, when the reaction was
carried out in a sequential one-pot procedure instead, the
corresponding yield increased to 99%. The slight difference
might demonstrate that the 2-catalyzed benzyl alcohol
oxidation and the following MBH reaction in due succession
could effectively avoid the possible side photoinduced
polymerization of acrylic ester during the process.
For this, the reaction of methyl acrylate with benzaldehyde
was chosen as the model reaction. Optimization of the reaction
was first performed at different temperatures such as 25, 50,
and 60 °C to furnish the desired MBH product under the given
reaction conditions (Table 3, entries 1−3) in the same reaction
time length (24 h). It seems that the higher temperature could
be a benefit for increasing the MBH yields (25 °C, 44%; 50 °C,
79%; 60 °C, 87%), but it led to lower stereoselectivity (25 °C,
86% ee; 50 °C, 11% ee; 60 °C, <3% ee). To our delight, a
facile reaction (Table 3, entry 4) occurred at room
temperature (25 °C) at an extended reaction time (48 h)
but with an excellent yield (96%) and a satisfied ee value
(83%). In addition, when different solvents (cf. THF, H2O,
and CH3CN) were employed in the presence of 2 mol % 2, the
desired product was obtained in low yields (10−66%) with ee
values in the range of 65−83% (Table 3, entries 5−7).
Furthermore, when the reaction was carried out in MeOH with
a lower catalyst loading, 1.5 mol % instead of 2.0 mol %, the
product was isolated in a slightly lower 93% yield, but with the
obviously decreased ee value of 64% (Table 3, entry 8). On the
other hand, the higher catalyst loading (2.5 mol %) did not
affect the results significantly (Table 3, entry 9). Notably, the
involved TiO2 species did not disturb the MBH reaction
between methyl acrylate and benzaldehyde under the given
conditions, which was further supported by the control
experiment in the presence of 1. As shown in Table 3 (entry
10), when 1 was used instead of 2 at the same catalyst loading,
the MBH product was obtained at the same yield (96%) and
ee value (83%). Again, no effective conversion (<5% yield) and
enantiomeric excess (<1% ee) were obtained without the aid of
a catalyst (Table 3, entry 11).
We next explored the reaction scope under the optimized
conditions, and the results are summarized in Table 4. The
benzaldehydes with electron-donating substituents gave higher
yields (Table 4, entries 1−6), while for those substrates
containing electron-withdrawing substituents on the benzene
ring (Table 4, entries 7−9), lower yields were obtained. For all
substituted benzaldehydes, the 2-substituted substrates always
gave relatively lower yields (Table 4, entries 1, 4, and 7), which
clearly resulted from the ortho sterically hindered effect. For
stereoselectivity, all of the substrates, however, exhibited good-
to-excellent ee values (85−99%). So far, MOF-based MBH
catalytic reactions remain quite rare, and a handful MOFs that
can promote MBH reactions have appeared in the
literature.57−59 Among the reported MOF-based catalysts,
The scope of the bifunctional catalytic system was further
investigated by performing the oxidation−MBH reaction for
other various substituted benzyl alcohols in a stepwise way. As
shown in Table 5, the substituent effect on the tandem
reaction was the same as those of the individual reaction steps.
As shown, benzyl alcohols with electron-donating substituents
have relatively higher yields (50−99%; Table 5, entries 1−7).
However, low yields were obtained (12−27%; Table 5, entries
8−10) for those substrates containing electron-withdrawing
substituents on the benzyl alcohols. Again, the steric hindrance
effect still played an important role in the reaction efficiency.
Compared to 2-substituted benzyl alcohols (yields 12−67%;
Table 5, entries 2, 5, and 8), the 3- and 4-substituted ones
displayed more activity and provided higher reaction yields,
16−99% (Table 5, entries, 3, 4, 6, 7, 9, and 10). Notably, all of
the substrates exhibited good-to-excellent stereoselectivities
(81−99%), suggesting that the chiral 1 framework showed an
excellent asymmetric templating effect to arrange the reaction
substrates and induce their contact in the specific spatial
orientations. It is noteworthy that large-sized substrates such as
9-anthracenemethanol did not afford the expected product
because no corresponding 9-anthraldehyde was detected
(Table 5, entry 11). This suggested that internal surface
catalysis should be the dominant process; moreover, the
involved amorphous TiO2 species was indeed located inside
the MOF matrix.
To verify the heterogeneous catalysis nature and stability of
2, a hot leaching test was carried out on the model reaction of
4-methoxybenzyl alcohol with methyl acrylate (Table 5, entry
4). As shown in Figure 3a, no further reaction occurred
without 2 after ignition of the reaction at 48 h, indicating that
no leaching of the catalytically active sites occurred and that 2
featured a typical heterogeneous catalyst nature. In addition,
we also examined the recyclability of 2. After each catalytic run,
the solid catalyst of 2 was collected by centrifugation, washed
with MeOH, dried at 80 °C, and reused in the next run under
the same conditions. Within five catalytic runs, 96−99% yields
were obtained with 99% stereoselectivity (Figure 3b).
The PXRD patterns and SEM image of 2 after five catalytic
runs showed that the structural integrity and morphology of 2
were well-preserved (Figure 3c,d), and the dispersion of the
loaded TiO2 species was still uniform in 2 based on the EDS
elemental mapping (Figure 3d). The TiO2 loss is only 8.2%
after five catalytic runs (determined by ICP atomic emission
F
DOI: 10.1021/acs.inorgchem.8b02132
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
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spectrometry; see the Supporting Information). Thus, 1 could
be considered to be an ideal host platform to load TiO2, and 2
is a satisfied bifunctional chiral heterogeneous catalyst for the
synthesis of asymmetric MBH additive products from alcohols
and methyl acrylate via a one-pot sequential reaction. To our
knowledge, this is the first example for one-pot tandem MBH
reactions promoted by the chiral MOF-based composite
catalyst.
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CONCLUSIONS
(7) Muller, M.; Hermes, S.; Kahler, K.; van den Berg, M. W. E.;
̈
̈
■
Muhler, M.; Fischer, R. A. Loading of MOF-5 with Cu and ZnO
Nanoparticles by Gas-Phase Infiltration with Organometallic
Precursors: Properties of Cu/ZnO@MOF-5 as Catalyst for Methanol
Synthesis. Chem. Mater. 2008, 20, 4576−4587.
In summary, we have demonstrated a facile synthetic method
for the preparation of a MOF-based multifunctional heteroge-
neous catalytic material, in which the chiral organic catalytic
and photoactive TiO2 species were successfully and rationally
combined via a MOF platform. The obtained chiral 2 can be a
highly active bifunctional asymmetric heterogeneous catalyst to
promote both photodriven and thermal-driven reaction in a
stepwise way under mild conditions. In this way, the
asymmetric one-pot MBH reaction starting from benzyl
alcohols was finally realized. We expect this approach to be
viable for the construction of many more new multifunctional
MOF-based catalytic systems for tandem chemical trans-
formation.
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b02132.
■
S
(11) Zeng, T.; Zhang, X.; Wang, S.; Niu, H.; Cai, Y. Spatial
Confinement of a Co3O4 Catalyst in Hollow Metal−Organic
Frameworks as a Nanoreactor for Improved Degradation of Organic
Pollutants. Environ. Sci. Technol. 2015, 49, 2350−2357.
(12) An, B.; Zhang, J.; Cheng, K.; Ji, P.; Wang, C.; Lin, W.
Confinement of Ultrasmall Cu/ZnOx Nanoparticles in Metal−
Organic Frameworks for Selective Methanol Synthesis from Catalytic
Hydrogenation of CO2. J. Am. Chem. Soc. 2017, 139, 3834−3840.
(13) Li, Y.-A.; Yang, S.; Liu, Q.-K.; Chen, G.-J.; Ma, J.-P.; Dong, Y.-
B. Pd(0)@UiO-68-AP: Chelating-Directed Bifunctional Heteroge-
neous Catalyst for Stepwise Organic Transformations. Chem.
Commun. 2016, 52, 6517−6520.
(14) Chen, G.-J.; Ma, H.-C.; Xin, W.-L.; Li, X.-B.; Jin, F.-Z.; Wang,
J.-S.; Liu, M.-Y.; Dong, Y.-B. Dual Heterogeneous Catalyst Pd-Au@
Mn(II)-MOF for One-Pot Tandem Synthesis of Imines from
Alcohols and Amines. Inorg. Chem. 2017, 56, 654−660.
Synthesis and characterization of H2L-CIL, additional
characterization of 2, and MBH product characterization
(PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: wangjiancheng1003@163.com.
*E-mail: yubindong@sdnu.edu.cn.
■
ORCID
Yu-Bin Dong: 0000-0002-9698-8863
(15) Wang, H.-L.; Yeh, H.; Chen, Y.-C.; Lai, Y.-C.; Lin, C.-Y.; Lu,
K.-Y.; Ho, R.-M.; Li, B.-H.; Lin, C.-H.; Tsai, D.-H. Thermal Stability
of Metal−Organic Frameworks and Encapsulation of CuO Nano-
crystals for Highly Active Catalysis. ACS Appl. Mater. Interfaces 2018,
10, 9332−9341.
Author Contributions
†These authors contributed equally.
Notes
The authors declare no competing financial interest.
(16) Noh, H.; Kung, C.-W.; Islamoglu, T.; Peters, A. W.; Liao, Y.; Li,
P.; Garibay, S. J.; Zhang, X.; DeStefano, M. R.; Hupp, J. T.; Farha, O.
K. Room Temperature Synthesis of an 8-Connected Zr-Based Metal−
Organic Framework for Top-Down Nanoparticle Encapsulation.
Chem. Mater. 2018, 30, 2193−2197.
(17) Shultz, A. M.; Sarjeant, A. A.; Farha, O. K.; Hupp, J. T.;
Nguyen, S. T. Post-Synthesis Modification of a Metal−Organic
Framework to Form Metallosalen-Containing MOF Materials. J. Am.
Chem. Soc. 2011, 133, 13252−13255.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the NSFC (Grants
21671122, 21475078, and 21802090), Shandong Provincial
Natural Science Foundation (Grant ZR2018BB006), a Project
of the Shandong Province Higher Educational Science and
Technology Program (J18KA066), and the Taishan Scholar’s
Construction Project.
(18) Madrahimov, S. T.; Gallagher, J. R.; Zhang, G.; Meinhart, Z.;
Garibay, S. J.; Delferro, M.; Miller, J. T.; Farha, O. K.; Hupp, J. T.;
Nguyen, S. T. Gas-Phase Dimerization of Ethylene under Mild
Conditions Catalyzed by MOF Materials Containing (bpy)NiII
Complexes. ACS Catal. 2015, 5, 6713−6718.
(19) Hu, Y.-H.; Wang, J.-C.; Yang, S.; Li, Y.-A.; Dong, Y.-B. CuI@
UiO-67-IM: A MOF-Based Bifunctional Composite Triphase Trans-
fer Catalyst for Sequential One-Pot Azide−Alkyne Cycloaddition in
Water. Inorg. Chem. 2017, 56, 8341−8347.
(20) Wang, J.-C.; Hu, Y.-H.; Chen, G.-J.; Dong, Y.-B. Cu(II)/Cu(0)
@UiO-66-NH2: base metal@MOFs as heterogeneous catalysts for
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