Chiral Proline-Decorated Bifunctional Pd?NH2-UiO-66 Catalysts for Efficient Sequential Suzuki Coupling/Asymmetric Aldol Reactions

Article abstract of DOI:10.1021/acs.inorgchem.0c00065

The design and development of site-isolating and multifunctional catalysts for multistep sequential reactions at the molecular level is a significant challenge. Herein, we first report bifunctional metal NPs?chiral MOFs catalysts for asymmetric sequential reactions. Pd nanoparticles and chiral proline were successfully added to NH₂-UiO-66 to construct two chiral bifunctional catalysts, in which active Pd nanoparticles were encapsulated into the frameworks via the "bottle-around-ship" method, and chiral proline was introduced into NH₂-UiO-66 by coordination to zirconium nodes and postsynthetic modification (PSM) of the organic linkers. The chiral proline-decorated bifunctional Pd?NH₂-UiO-66 catalysts were applied to sequential Suzuki coupling/asymmetric aldol reactions with excellent coupling performance (yields up to 99.9%) and good enantioselectivities (ee_anti values up to 97%). The heterogeneous catalyst by coordination of proline can be reused, and the reaction activity was not significantly reduced after four cycles.

Full text of DOI:10.1021/acs.inorgchem.0c00065

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Chiral Proline-Decorated Bifunctional Pd@NH2‑UiO-66 Catalysts for
Efficient Sequential Suzuki Coupling/Asymmetric Aldol Reactions
Lin Cheng,* Kaiyuan Zhao, Qingsong Zhang, Yiming Li, Qingchao Zhai, Jinxi Chen, and Yongbing Lou
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ABSTRACT: The design and development of site-isolating and
multifunctional catalysts for multistep sequential reactions at the
molecular level is a significant challenge. Herein, we first report
bifunctional metal NPs@chiral MOFs catalysts for asymmetric
sequential reactions. Pd nanoparticles and chiral proline were
successfully added to NH2-UiO-66 to construct two chiral
bifunctional catalysts, in which active Pd nanoparticles were
encapsulated into the frameworks via the “bottle-around-ship”
method, and chiral proline was introduced into NH2-UiO-66 by
coordination to zirconium nodes and postsynthetic modification
(PSM) of the organic linkers. The chiral proline-decorated
bifunctional Pd@NH2-UiO-66 catalysts were applied to sequential
Suzuki coupling/asymmetric aldol reactions with excellent
coupling performance (yields up to 99.9%) and good enantioselectivities (eeanti values up to 97%). The heterogeneous catalyst
by coordination of proline can be reused, and the reaction activity was not significantly reduced after four cycles.
node (including active open metal centers and catalytic species
coordinated to metal sites), a functionalized organic linker, and
an encapsulated active catalyst (including metal and metal
oxide nanoparticles, enzymes, polymers, and discrete small
molecules) (Figure 1).18,19 The combination of two or three
kinds of such active species makes MOFs promising multi-
functional catalysts for multistep sequential reactions. Mean-
while, postsynthetic modification (PSM) of active function-
alized sites into MOFs has become an efficient and conducive
strategy to functionalize MOFs further and to promote their
application as multifunctional catalysts.20−25
In contrast, metal nanoparticles (NPs) with tiny sizes exhibit
higher catalytic activity but are prone to aggregation and
deterioration because of their high surface energies.26 MOFs
have been well-utilized as ideal host matrixes to provide a
confinement effect for restricting the controllable growth and
further aggregation of metal NPs.27−29 However, compared
with intensive efforts involving the combination of active open
metal centers (usually as Lewis acid sites) and functionalized
organic linkers (such as as Bronsted base sites) as multifunc-
tional MOFs to catalyze multistep sequential reactions, only a
1. INTRODUCTION
Sequential catalytic reactions via multistep processes have
attracted a great deal of interest in sustainable green chemistry
because of their reduced reaction time, decreased catalyst
amount and purification cost, atom economy, and environ-
mental friendliness.1,2 A suitable candidate for a single catalyst
to independently accomplish the whole sequential procedures
usually possesses multifunctionality with two or more active
sites, accompanied by a rapid increase of molecular complex-
ity.3 The spatial distribution of these isolated catalytic sites also
needs to be precisely engineered in order to perform their
different functions in a consecutive manner, especially for
incompatible catalytic groups,4,5 such as acid−base.6−11
Meanwhile, these active sites should be stable under the
catalytic conditions of all the sequential steps.12 Thus, the
design and development of site-isolating and multifunctional
catalysts for multistep sequential catalysis is desired but
remains a significant challenge.
MOFs have become a bridge between homogeneous
catalysts (selectivity and ease of modification) and heteroge-
neous catalysts (reusability and ease of purification) due to
their modular nature, structural diversity, high density of
uniformed catalytic centers, adjustable pore size and porosity,
thermal stability, and crystalline nature for clear elucidation of
structure−function relationships.13−15 In particular, the elab-
orate design and systematic tuning of their structures and
properties at the molecular level enable different active sites to
be introduced to the preprogrammed positions within their
frameworks.16,17 These active sites consist of an active metal
Received: January 8, 2020
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zirconium nodes as a mother MOF to construct two kinds of
chiral bifunctional Pd@MOFs catalysts via the “bottle-around-
ship” method, Pd@NH2-UiO-66(pro)-1 and Pd@NH2-UiO-
66(pro)-2, in which chiral proline with extensive applications
in asymmetric catalysis was introduced into the frameworks of
NH2-UiO-66 by coordination to zirconium nodes and PSM of
the organic linkers (Figure 2). The bifunctional catalysts were
then applied to sequential Suzuki coupling/asymmetric aldol
reactions with excellent coupling performance (yield up to
99.9%) and good enantioselectivities (eeanti up to 97%). To our
knowledge, this is the first report of multifunctional metal
NPs@chiral MOFs catalysts for asymmetric sequential
reactions.
2. EXPERIMENTAL SECTION
2.1. Synthesis of the Catalysts. 2.1.1. Synthesis of Pd NPs. Pd
NPs were obtained by reducing PdCl2 in aqueous methanol solution,
which was a simple and nonpolluting method. Briefly, 50 mg of PdCl2
and 330 mg of PVP (polyvinylpyrrolidone, K = 23−27) were added
into an aqueous methanol solution (450 mL, MeOH/H2O = 8:1).
The mixed solution was stirred at 70 °C for 3.5 h until the solution
turned jet black. The fine black Pd NPs were collected by acetone
sedimentation and washed three times with CHCl3 and n-hexane.
Subsequently, the prepared Pd NPs were evenly distributed in DMF
(5 mL) for further use.
2.1.2. Synthesis of NH2-UiO-66. NH2-UiO-66 was synthesized
according to the previously reported hydrothermal method.53,54
2.1.3. Synthesis of Pd@NH2-UiO-66. Briefly, ZrCl4 (140 mg, 0.6
mmol), 2-amino-terephthalic acid (H2BDC-NH2, 109 mg, 0.6 mmol),
and glacial acetic acid (2 mL) were dissolved in DMF (15 mL). Pd
NPs in DMF were then transferred to the mother liquor, and the
solution was ultrasonicated for 30 min. After that, the mixture was
kept in a 25 mL Teflon-lined autoclave and maintained at 120 °C for
24 h. Finally, the dark powdered product was obtained and cleaned by
DMF and EtOH (3 × 10 mL), then immersed by ethanol for 3 days
and activated under dynamic vacuum at 80 °C for 24 h to obtain
dehydrated Pd@ NH2-UiO-66 for further reactions.
Figure 1. Three kinds of potential active catalytic sites in tunable and
modular MOFs: active metal node, functionalized organic linker, and
encapsulated active catalyst.
limited number of investigations on multifunctional metal
NPs@MOFs catalysts for cascade reactions have been
reported.30−44 There are two main approaches for the
encapsulation of metal NPs into MOFs to construct NPs@
MOFs catalysts: “ship-in-a-bottle” and “bottle-around-ship”.45
In comparison with the “ship-in-a-bottle” deposition of metal
precursors into the pores/channels of MOFs, the “bottle-
around-ship” dispersion of preformed metal NPs with
subsequent MOFs synthesis has been rarely investigated,
though the approach has significant advantages, such as pre-
controlled size and morphology of metal NPs, the prevention
of NPs growth on the surface of MOFs, and the blocking of
MOFs channels by NPs.46−48 Moreover, few studies have been
conducted on chiral multifunctional MOFs to catalyze
asymmetric sequential reactions.49−52
2.1.4. Synthesis of NH2-UiO-66(pro)-1. NH2-UiO-66(pro)-1 was
prepared and purified via the same procedure as that of Pd@NH2-
UiO-66, except that for NH2-UiO-66(pro)-1, the palladium element
was removed and substituted by chiral proline (345 mg, 3 mmol).55
Herein, we selected NH2-UiO-66 with exceptional chemical
stabilities and wonderful modification of amino groups and
Figure 2. Synthetic procedure of catalysts Pd@NH2-UiO-66(pro)-1 and Pd@NH2-UiO-66(pro)-2: (a) solvothermal method, (b) amide reaction
by PSM microwave method, and (c) thermal removal of the Boc groups.
B
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2.1.5. Synthesis of Pd@NH2-UiO-66(pro)-1. In general, Pd@NH2-
UiO-66(pro)-1 was prepared by the same one-pot “bottle-around-
ship” solvothermal method as Pd@NH2-UiO-66, except that chiral
proline (345 mg, 3 mmol) was added to the precursor solution of
Pd@NH2-UiO-66(pro)-1. The as-synthesized sample was designated
as 1.46 wt % Pd@NH2-UiO-66(pro)-1 (where 1.46% stands for the
Pd NPs loading) by the measurement of inductively coupled plasma-
optical emission spectroscopy (ICP-OES).
2.1.6. Synthesis of NH2-UiO-66(pro)-2. NH2-UiO-66(pro)-2 was
synthesized by two steps of PSM of NH2-UiO-66 under microwave
irradiation.56,57 Briefly, BOC-L/D-proline (325 mg, 1.5 mmol),
bromo-tris-pyrrolidino phosphonium hexafluorophosphate (PyBrop,
150 mg, 0.3 mmol), and 4-dimethylaminopyridine (DMAP, 78 mg,
0.6 mmol) were dissolved in 10 mL of anhydrous DCM, and the
degassed NH2-UiO-66 (86 mg) was then suspended in the solution.
The resulting mixture was subjected to reaction under microwave
irradiations for 20 min at 80 °C (300 W). The faint yellow powder
was collected by centrifugation and cleaned by DCM (3 × 10 mL),
and the BOC groups in MOFs were then removed by thermolysis
(DMF, 165 °C, 4 h, 300 W, 10 atm). NH2-UiO-66(pro)-2 was
obtained by decantation and purified by DMF and EtOH,
respectively.
3. RESULTS AND DISCUSSION
3.1. Synthesis and Characterization of the Catalysts.
Pd NPs and chiral proline were introduced into NH2-UiO-66
with exceptional chemical and thermal stabilities to prepro-
gram the bifunctionality of the catalysts. Different from the
traditional reduction procedure of Pd2+@MOFs by hydrogen
or sodium borohydride, our research adopted an in situ “bottle-
around-ship” synthesis method to prepare the targeted Pd@
MOFs, in which Pd NPs were directly introduced into the
frameworks of microporous materials during the growth of
MOFs.59−61 Meanwhile, chiral proline was decorated into the
framework of Pd@NH2-UiO-66 via two different methods. In
Pd@NH2-UiO-66(pro)-1, chiral proline was directly coordi-
nated to zirconium nodes of the MOFs by a one-pot
solvothermal procedure, which was convenient and efficient
without any intermediate treatments. In Pd@NH2-UiO-
66(pro)-2, chiral proline was grafted to the organic linkers of
Pd@NH2-UiO-66 via a two-step PSM method (Figure 2).
As shown in the powder XRD patterns (Figure 3), neither
changes or collapses of the NH2-UiO-66 configuration when
2.1.7. Synthesis of Pd@NH2-UiO-66(pro)-2. Pd@NH2-UiO-66-
(pro)-2 was synthesized by two steps of PSM of Pd@NH2-UiO-66
(Figure 2), similar to the preparation method of NH2-UiO-66(pro)-2.
ICP-OES analysis demonstrated that the Pd NPs loading amount in
Pd@NH2-UiO-66(pro)-2 was 1.36 wt %. Finally, the solid catalyst
was soaked in ethanol for 3 days and activated at 80 °C for 24 h in
vacuo for further reactions. Generally, both Pd@NH2-UiO-66(pro)-1
and Pd@NH2-UiO-66(pro)-2 in catalytic reactions are identified as
the catalysts modified by L-proline, if not specified in this paper.
2.2. General Procedure for Suzuki Coupling Reaction
Catalyzed by Pd@NH2-UiO-66(pro)-1/Pd@NH2-UiO-66(pro)-2.
In a round-bottomed flask, the aryl halide (1 mmol), 4-
formylphenylboronic acid (164.8 mg, 1.1 mmol), Pd@NH2-UiO-
66(pro)-1 (or Pd@NH2-UiO-66(pro)-2) (2 mg), and K2CO3 (14
mg, 0.1 mmol) was added into EtOH−H2O (1.2 mL + 1.2 mL). The
reaction mixture was stirred at the indicated temperature in the
indicated atmosphere. The reaction products were monitored by TLC
to determine whether the substrate was completely consumed.
2.3. General Procedure for Asymmetric Aldol Reaction
Catalyzed by Pd@NH2-UiO-66(pro)-1/Pd@NH2-UiO-66(pro)-2.
A mixture of biphenyl formaldehyde (1 mmol), cyclopentanone (0.88
mL, 10 mmol), H2O (10 μL), and the catalyst Pd@NH2-UiO-
66(pro)-1 (or Pd@NH2-UiO-66(pro)-2) (2 mg) were stirred at the
indicated temperature. The reaction mixture was extracted with DCM
and dried by anhydrous sodium sulfate. The organic phase was
concentrated in vacuo and purified by fast column chromatography.
The ee value of the product was calculated by high-performance
liquid chromatography (HPLC, chiral AD-H or OD-H column, n-
hexane−isopropanol = 90:10, 0.8 mL min−1).58
2.4. General Procedure for Sequential Reactions Catalyzed
by Pd@NH2-UiO-66(pro)-1/Pd@NH2-UiO-66(pro)-2. In EtOH−
H2O (1.2 mL + 1.2 mL) solution, the aryl halide (1 mmol), 4-
formylphenylboronic acid (164.8 mg, 1.1 mmol), Pd@NH2-UiO-
66(pro)-1 (or Pd@NH2-UiO-66(pro)-2) (2 mg), and K2CO3 (14
mg, 0.1 mmol) were reacted under specific conditions. Considering
the influence of K2CO3 and ethanol on asymmetric aldol reactions,
after the coupling procedure was completed, the reaction mixture was
cooled to room temperature and then neutralized with concentrated
hydrochloric acid. HCl (10 μL) and the solvent were evaporated
under reduced pressure. Cyclopentanone (0.88 mL, 10 mmol) and
H2O (10 μL) were added into the above residue, followed by
ultrasonic treatment for 10 min in an ice bath. Then, the mixture was
stirred at the corresponding temperature. The product was collected
by fast column chromatography and analyzed by HPLC spectra
(chiral AD-H or OD-H column, n-hexane−isopropanol = 90:10, 0.8
mL min−1).
Figure 3. Powder XRD patterns of (a) Pd NPs, (b) simulated NH2-
UiO-66, (c) synthesized NH2-UiO-66, (d) NH2-UiO-66(pro)-2, (e)
Pd@NH2-UiO-66(pro)-2, (f) NH2-UiO-66(pro)-1, (g) Pd@NH2-
UiO-66(pro)-1, and (h) recycled Pd@NH2-UiO-66(pro)-1 after 4
cycles.
Pd NPs or chiral proline was introduced. ICP-OES analysis
indicated that the loading amounts of Pd NPs in Pd@NH2-
UiO-66(pro)-1 and Pd@NH2-UiO-66(pro)-2 were 1.46 and
1.38 wt %, respectively. According to the TGA results in Figure
S1, we concluded that Pd@NH2-UiO-66(pro)-1 and Pd@
NH2-UiO-66(pro)-2 exhibited high thermal stability, which
were similar to NH2-UiO-66.62,63 The solid circular dichroism
(CD) spectra of Pd@NH2-UiO-66(pro)-1 (L-proline) and
Pd@NH2-UiO-66(pro)-1 (D-proline) in Figure S2 showed that
these two catalysts were enantio-enriched, in that they
exhibited mirror image signals, which demonstrated the
opposite chirality in two enantioselective heterogeneous
catalysts.
The dosage of enantiotopic chiral proline, incorporated into
56,64
1
UiO-66-NH2, was calculated by H NMR.
As shown in
Figures S3 and S4, the proline group/ligand(H2BDC-NH2)
ratios in Pd@NH2-UiO-66(pro)-1 and Pd@NH2-UiO-66-
(pro)-2 were 1:2 and 1:4, respectively, showing that the
C
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effective content of enantiomers in the one-pot method had
more advantages than that with the PSM method when the
chiral proline with the same quality was used in the precursor.
The permanent porosity of the heterogeneous catalysts was
determined by the N2 adsorption−desorption isotherms at 77
K (Figure 4 and Table S1). The same as pristine NH2-UiO-66,
0.11 cm3 g−1, mainly due to the highly decentralized Pd NPs
occupying the cavities of the porous NH2-UiO-66. Viewing the
N2 adsorption measurements, the introduction of metal
particles into the MOF frameworks had a negative effects on
the surface area and pore volume of catalysts, similar to the
previous report of Chen et al.66 Meanwhile, compared with
catalysts Pd@NH2-UiO-66(pro)-1 and Pd@NH2-UiO-66-
(pro)-2, it was obvious that the catalyst prepared by the
one-pot method possessed more superior adsorption perform-
ance than that prepared via PSM, which may be attributed to
the partial blockage of tunnels in porous materials when chiral
active sites were linked by two-step PSM. Finally, there was a
hysteresis loop in N2 adsorption−desorption isotherms images
of NH2-UiO-66(pro)-1, which was caused by the agglomer-
ation of a few particles (Figure S5).17,19
The FI-IR spectra of the catalysts were shown in Figure S6,
and the peaks at 3450, 3355, and 1575 cm−1 can be attributed
to the asymmetric stretching, symmetric stretching and
scissoring deformation vibrations of the -NH bond, respec-
tively.67,68 Besides, the bonds observed at 1435 and 1260 cm−1
were assigned to the C−N stretching vibrations.69,70 The peaks
obtained at 1655 cm−1 can be ascribed to the stretching
vibrations of −CO.56 Moreover, the peak located at 520
cm−1 demonstrated the existence of Zr−O absorption band of
metal clusters.71
Figure 4. N2 adsorption−desorption isotherms of (a) NH2-UiO-
66(pro)-1, (b) Pd@NH2-UiO-66(pro)-1, (c) NH2-UiO-66(pro)-2,
and (d) Pd@NH2-UiO-66(pro)-2.
Two characteristic absorption peaks were observed in the
UV−vis spectra of Pd@NH2-UiO-66(pro)-1, Pd@NH2-UiO-
66(pro)-2, NH2-UiO-66(pro)-1, and NH2-UiO-66(pro)-2
(Figure S7). The peaks at 260 and 257 nm can be attributed
to the π−π* transition which represented the conjugation
effects of benzene ring system, whereas the peaks at 376 and
366 nm were assigned to the n−π* transition that belonged to
the amino groups.72,73 Notably, compared with the spectra of
Pd@NH2-UiO-66(pro)-1 and NH2-UiO-66(pro)-1, the ab-
sorption bands of Pd@NH2-UiO-66(pro)-2 and NH2-UiO-
66(pro)-2 are blue-shifted owing to the postsynthetic
modification of −NH2 by L-proline.72,73
both Pd@NH2-UiO-66(pro)-1 and Pd@NH2-UiO-66(pro)-2
exhibited the type-I sorption behavior,65 indicating the
presence of micropores inside the catalysts. Meanwhile, the
catalysts with different synthetic methods showed diverse BET
surface areas and total pore volumes. In Pd@NH2-UiO-
66(pro)-1, the BET surface area and the total pore volume are
637 m2 g−1 and 0.38 cm3 g−1, respectively, which were lower
than those of NH2-UiO-66(pro)-1 (759 m2 g−1 and 0.53 cm3
g−1, respectively) because of the incorporation of Pd NPs.
Furthermore, when Pd NPs were doped to catalyst NH2-UiO-
66(pro)-2, the BET surface area decreased significantly from
610 to 159 m2 g−1 and pore volume decreased from 0.33 to
X-ray photoelectron spectroscopy (XPS) tests of bifunc-
tional catalysts Pd@NH2-UiO-66(pro)-1 and Pd@NH2-UiO-
66(pro)-2 were conducted to further analyze the elemental
Figure 5. (a) Transmission electron microscopy (TEM), (b) HR-TEM, and (c) HAADF-STEM images of Pd@NH2-UiO-66(pro)-1. EDX
elemental distribution mapping images of Pd@NH2-UiO-66(pro)-1: (d) mixed elements, (e) C, (f) N, (g) O, (h) Zrm and (i) Pd.
D
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a
Table 1. Study on Reaction Conditions of Suzuki Coupling Reaction in the Model Reaction
d
entry
catalyst
mg
base
T (°C)
t (min)
yield (%)
b
1
Pd@NH2-UiO-66(pro)-1
Pd@NH2-UiO-66(pro)-1
Pd@NH2-UiO-66(pro)-1
Pd@NH2-UiO-66(pro)-1
Pd@NH2-UiO-66(pro)-1
Pd@NH2-UiO-66(pro)-1
Pd@NH2-UiO-66(pro)-1
Pd@NH2-UiO-66(pro)-1
Pd@NH2-UiO-66(pro)-1
Pd@NH2-UiO--66(pro)-2
Pd(PPh3)4
Pd(PPh3)4
Pd NPs
UiO-66-NH2
NH2-UiO-66(pro)-1
none
2
2
2
2
1
2
3
5
2
2
5
5
2
5
5
Na2CO3
Cs2CO3
CsF
50
50
50
50
80
80
80
80
80
80
80
80
80
80
80
80
120
120
120
120
5
5
5
5
10
40
29
12
53
54
91
81
51
>99.9
>99
32
95
33
n.d.
n.d.
n.d.
b
2
b
3
b
4
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
none
b
5
b
6
b
7
b
8
b
9
b
10
11
12
13
14
15
16
10
10
c
c
360
10
360
360
360
b
b
b
be
,
a
Reaction conditions: 4-bromobenzaldehyde (1 mmol), phenylboronic acid (1.1 mmol), base (0.1 mmol), EtOH−H2O (1.2 mL + 1.2 mL), and
b
c
d
e
1
the corresponding catalyst. Reaction in air. Protected by N2 atmosphere. Calculated by H NMR. No catalyst or inorganic base.
composition and valence states on their surfaces. The peaks of
Pd, Zr, C, O, and N were displayed in XPS full spectra (Figure
S8a,b), which indicated the existence of these elements.74 As
described in Figure S8c,d, the peaks of Pd 3d presented an
asymmetric large-wide bonds between 330 and 338 eV owing
to the influence of Zr 3p.75,76 Briefly, two peaks of 347.2 and
333.6 eV were assigned to Zr 3p1/2 and Zr 3p3/2, respectively,
whereas the bonds at 341.2 and 335.9 eV were ascribed to Pd0
3d3/2 and Pd0 3d5/2, respectively, indicating the existence of Pd
NPs in the bifunctional catalysts.60,77 Particularly, the new
peaks at around 343.2 and 337.9 eV in Pd@NH2-UiO-
66(pro)-1 corresponded to the binding energy of the Pd2+
species, in which the unusual high-valence of Pd element was
caused by Pd−N bonds.78 Figure S8e,f showed Zr XPS spectra
at approximately 185.3 and 183.0 eV, which can be attributed
to Zr 3d3/2 and Zr 3d3/2, respectively.74 According to the C 1s
XPS results, three pronounced peaks of chemical environments
C species were presented in Figure S8g,h, which were assigned
to the binding energies of O−CO (288.7 eV), C−N (285.9
eV), and C−C (284.8 eV), respectively.79,80
Transmission electron microscopy (TEM) images of as-
prepared Pd@NH2-UiO-66(pro)-1 and Pd@NH2-UiO-66-
(pro)-2 are shown in Figures 5 and S9, respectively. For
Pd@NH2-UiO-66(pro)-1, the original configuration of the
regular octahedron was kept intact, which was similar to that of
Pd@NH2-UiO-66(pro)-2. Next, the HR-TEM image was
obtained, which demonstrated that Pd NPs held inside the
crystals of Pd@NH2-UiO-66(pro)-1 (Figure 5a). The fringes
observed in Figure 5b were applied to the identification of the
crystallographic spacing of the Pd NPs, which most frequently
correspond to the (111) crystallographic planes of Pd phases.81
Besides, the HAADF-STEM image in Figure 5c established
that Pd NPs existed in Pd@NH2-UiO-66(pro)-1. Moreover,
EDX elemental mapping images of Pd@NH2-UiO-66(pro)-1
in Figure 5d−i indicated that the elements (Zr, C, N, and O)
were evenly distributed and that the palladium elements were
inside the catalysts.
3.2. Evaluation of the Catalytic Activity. 3.2.1. Explora-
tion of Suzuki Coupling Reaction Conditions. In order to
explore whether Pd NPs in NH2-UiO-66 would perform well
as coupling catalysts, phenylboronic acid and 4-bromobenzal-
dehyde were chosen as starting materials in the model reaction
of Suzuki C−C cross-coupling. First, we investigated their
activity in the synthesis of biphenyl-4-carbaldehyde via Pd@
NH2-UiO-66(pro)-1. To assess the catalytic performance and
optimize the reaction conditions, we explored the effects of
inorganic base, reaction temperature, catalyst dosage, and
reaction time on the Suzuki coupling reaction. Pd NPs,
commercial Pd(PPh3)4, NH2-UiO-66, and NH2-UiO-66(pro)
were employed for comparison.
Table 1 showed that Pd@NH2-UiO-66(pro)-1 can be
appointed as an incomparable catalyst for the synthesis of
biphenyl-4-carbaldehyde with an alkaline cocatalyst. Notably,
compared to Na2CO3, Cs2CO3, and CsF, which were
frequently used in coupling reactions, K2CO3 was the most
superior inorganic base (Table 1, entries 1−4). Next, we
investigated the influence of catalyst dosage on the catalytic
system. It is obvious that compared to other catalyst inputs 2
mg of Pd@NH2-UiO-66(pro)-1 gave the highest yield of up to
91% in 5 min (Table 1, entries 5−8). The catalyst additive
amount was fixed at 2 mg for subsequent experiments. As
shown in Table 1 (entries 4 and 9), reaction temperature was
another important indicator that affected the reaction rate.
When the temperature increased from 50 to 80 °C, the
coupling rate increased dramatically, accompanied by a shorter
reaction time and a higher yield. Moreover, the reaction time
greatly affected the reaction rate: As it increased from 5 to 10
min, the yield went up from 91 to 99.9% (Table 1, entries 6
and 9). Besides, catalyst Pd@NH2-UiO-66(pro)-2 showed
coupling activity similar to that of Pd@NH2-UiO-66(pro)-1
(Table 1, entries 9 and 10). It is well-known that the effective
catalytic particles in the two catalysts were Pd NPs, so we tried
to select pure Pd NPs as a reference to inquire the coupling
effect. The results indicated that isolated Pd NPs possessed
much lower catalytic activity than Pd@NH2-UiO-66(pro)-1
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and Pd@NH2−UiO-66(pro)-2 (Table 1, entries 9, 10, and
13), which can be attributed to the fact that during the reaction
process, the unpackaged Pd NPs easily aggregated, leading to
the shielding of the active sites.82
Table 2. Exploration on Solvents of Asymmetric Aldol
Reaction in the Model Reaction
a
The homogeneous catalyst Pd(PPh3)4 was also tested for
comparison. The yield of corresponding coupling product
catalyzed by Pd(PPh3)4 was 32% at 10 min, while the yield
increased to 95% by extending reaction time to 6 h (Table 1,
entries 11 and 12), which was still inferior to the catalytic
activity of Pd@NH2-UiO-66(pro)-1. No percent conversion
could be detected in the reaction system when absolute NH2-
UiO-66 or adorned NH2-UiO-66(pro)-1 was performed under
specific reaction conditions (Table 1, entries 14 and 15).
Moreover, it was found that the catalytic reaction was not
implemented when either palladium composite catalyst or
K2CO3 were absent (Table 1, entry 16).
It is worth noting that Pd@NH2-UiO-66(pro)-1 and Pd@
NH2-UiO-66(pro)-2 can catalyze the Suzuki coupling
reactions under air conditions, different from other Pd
catalysts, such as Pd(OAc)2, Pd(PPh3), and Pd(PPh3)2Cl2
under inert atmosphere, which may be attributed to the
protection of MOF shell to the core of Pd NPs. As a result,
both Pd@NH2-UiO-66(pro)-1 and Pd@NH2-UiO-66(pro)-2
were outstanding catalysts for Suzuki coupling reactions with
an inorganic base K2CO3 at 80 °C, exhibiting extremely high
yield and excellent efficiency.
3.2.2. Exploration of Asymmetric Aldol Reaction Con-
ditions. Considerable attention has been paid to asymmetric
aldol reactions over the years to obtain the desired aldol
products in an enantioselective modality of aldehydes and
ketones by the construction of C−C bonds.83−85 In this
section, we selected biphenyl-4-carbaldehyde (1.0 mmol),
cyclopentanone (0.88 mL, 10 mmol), and Pd@NH2-UiO-
66(pro)-1 (or Pd@NH2-UiO-66(pro)-2) (2 mg) to perform
the model asymmetric aldol reaction. The type of catalysts,
solvents, additives, and reaction temperature were investigated
during the exploratory trials.
First, we explored the effect of different solvents on the
enantioselectivity of aldol product, and the results showed that
the chiral compound was more easily generated in aqueous
solution (Table 2, entries 1−12). Second, the amount of H2O
was taken into account, and we found that the eeanti value of
the aldol product was highest when a low input amount of
H2O (10 μL) was added (Table 2, entries 8, 13, and 14).
Third, several types of bases and acids were tested to inquiry
the effects of different additives on the asymmetric aldol
reaction (Table S2, entries 1−8). It was not found that the
addition of certain substances was conducive to the production
of relatively high enantioselective catalytic activity.
c
b
b
entry
solvent
yield (%)
syn/anti
eeanti (%)
1
2
3
4
5
6
7
8
EtOH
toluene
THF
IPA
DCM
DMF
trace
46
63
40
61
59
35
70
10
46
65
53
76
74
70
58
69
63
58:42
60:40
51:49
61:39
80:20
62:38
51:49
36:64
46:54
45:55
61:39
51:49
37:63
39:61
-
5
12
9
9
3
9
37
2
7
3
24
36
49
68
59
64
−45
1,4-dioxane
H2O
H2O−EtOH
THF−H2O
IPA−H2O
9
10
11
12
13
14
15
16
17
18
free
H2O (100 μL)
H2O (10 μL)
H2O (10 μL)
saturated saline (10 μL)
H2O (10 μL)
H2O (10 μL)
d
d
de
,
52:48
55:45
f
a
Reaction conditions: biphenyl-4-carbaldehyde (1 mmol), cyclo-
pentanone (0.88 mL), solvent (0.88 mL), Pd@NH2-UiO-66(pro)-1
b
(2 mg), room temperature, 5 days. Determined by chiral HPLC
(Chiral AD-H or OD-H column, 0.8 mL min−1, n-hexane−
c
d
isopropanol = 90:10). Isolated yield. Reaction temperature: −20
e
f
°C. Catalyzed by Pd@NH2-UiO-66(pro)-2. Catalyzed by Pd@NH2-
UiO-66(D-pro)-1 (“D-pro” represents D-proline)
reaction, and we found that the catalysts modulated by L-
proline and D-proline showed opposite catalytic activities
(Table 2, entries 14 and 18).
It was always our focus on trials to screen, mine, and develop
particular reaction conditions that were beneficial for the
catalyst to conduct asymmetric aldol reactions with excellent
catalytic activity. Pd@NH2-UiO-66(pro)-1 exhibited catalytic
activity slightly superior to that of Pd@NH2-UiO-66(pro)-2
with 10 μL of H2O at −20 °C.
3.2.3. Exploration of Sequential Reactions Conditions and
Substrate Expansion. The sequential reactions consisted of
two steps: Suzuki coupling reaction (step I) and asymmetric
aldol reaction (step II) (Figure 6). In step I, 4-bromobenzal-
In general, low temperature is beneficial to the formation of
single chiral products. Therefore, we chose a cryogenic reactor
to observe the reaction at −20 °C, and the results showed that
eeanti value increased from 49% (at room temperature) to 68%
(at −20 °C) (Table 2, entries 14 and 15). Meanwhile,
saturated saline (10 μL) was examined because of its low
freezing point, but the enantioselectivity of the product was
not improved compared to that of pure water (Table 2, entry
16). Moreover, proline-modulated Pd@NH2-UiO-66(pro)-1
revealed an eeanti value slightly higher than that of Pd@NH2-
UiO-66(pro)-2 (Table 2, entries 15 and 17), which may be
ascribed to the different grafting mode of L-proline into the
frameworks of NH2-UiO-66. D-Proline-decorated Pd@NH2-
UiO-66(pro)-1 was further used to observe the catalytic
Figure 6. Sequential Suzuki coupling/asymmetric aldol reactions.
dehyde (185 mg, 1 mmol) and phenylboronic acid (134 mg, 1
mmol) in EtOH−H2O (1.2 mL + 1.2 mL) were catalyzed by
Pd@NH2-UiO-66(pro)-1 (simplified as cat. 1, 2 mg) and
K2CO3 (14 mg, 0.1 mmol) at 80 °C for 10 min. It is well-
known that K2CO3, as a base catalyst, can be used for aldol
reactions but can cause racemization of the enantiomers.
Furthermore, EtOH may interfere with the normal aldol
reactions in a reactive manner.58 Considering the terrible
influence of the two above-mentioned substances in step I on
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a
Table 3. Substrate Exploration of Sequential Suzuki Coupling/Asymmetric Aldol Reactions
c
c
d
d
b
b
entry
R
X
I t (min)
II t (days)
RCHO yield (%)
aldol yield (%)
syn/anti
eeanti (%)
1
2
3
4
5
4-CH3
4-CH3O
4-NO2
4-CN
I
I
Br
I
Br
10
10
10
10
10
5
5
5
5
5
>99
95
95
97
94
82
59
71
68
76
29:71
23:77
2:98
64:36
26:74
57
63
64
91
97
2-CN
a
Reaction conditions: step I: aryl halide (1 mmol), (4-formylphenyl) boronic acid (1.1 mmol), ethanol (1.2 mL), water (1.2 mL), K2CO3 (0.1
mmol), Pd@NH2-UiO-66(pro)-1 (2 mg), 80 °C. Step II: (i) HCl neutralization, (ii) solvent removal, (iii) cyclopentanone (0.88 mL, 10 mmoL),
b
H2O (10 μL), (iv) ultrasonic treatment, −20 °C, 5 days. Determined by Chiral HPLC (Chiral AD-H column, 0.8 mL min−1, n-hexane−
c
d
isopropanol= 90:10). Reaction time. Isolate yield.
step II, K2CO3 was neutralized with HCl, and the mixed
solvent (EtOH−H2O) was evaporated under reduced pressure
after the coupling step was completed. Then, cyclopentanone
(0.88 mL, 10 mmol) and H2O (10 μL) were added to the
residue in order to proceed with the aldol procedure at −20 °C
for 5 days.
cally active sites (Pd NPs) in the solid catalyst were coated and
not free in the supernatant solution of the model reaction.
Macrosubstrate experiments were performed to verify
whether the reaction took place inside the pores or on the
surface of solid catalysts. A bulk aldehyde in Table S3 was
examined, whose size (9 Å × 14 Å) is much larger than the
pore size of the catalyst (6 Å).63 After 2 weeks of reaction, we
did not find any microaldol product through HPLC test, while
the aldol product catalyzed by the homogeneous catalyst
obtained 53% yield for 48 h under homogeneous conditions.
The result proved that the reactions occurred inside the
catalyst.
3.4. Recycling Experiments. To explore the reusability of
the bifunctional catalyst, we examined the model sequential
reaction from phenylboronic acid and halogenated benzalde-
hyde catalyzed by Pd@NH2-UiO-66(pro)-1. After the initial
cycle terminated, the solid catalyst was isolated by
centrifugation from the liquid phase, followed by purification
treatment, and then reused in another operation. Test results
proved that the catalytic activity and enantioselectivity of the
product did not show distinct differences within 4 cycles of the
reactions (Figure 7 and Table S4). It was worth mentioning
that the catalyst Pd@NH2-UiO-66(pro)-1 could be recycled at
least 4 times without any loss of activity or construct collapse,
In order to explore further the general catalytic performance
of Pd@NH2-UiO-66(pro)-1, we investigated some substituted
substrates starting from halogenated benzene derivatives and
(4-formylphenyl) boronic acid (Table 3, entries 1−5). First, it
can be seen that regardless of the substituent group Pd@NH2-
UiO-66(pro)-1 exhibited excellent catalytic activity in Suzuki
coupling reactions (yield 94−99%,). However, similar results
did not occur in step II. It was evident that the aryl halides
containing electron-withdrawing groups (−NO2 and −CN)
gave distinct enantiomeric excess (64, 91, and 97%) (Table 2,
entries 3−5), which were higher than those containing
electron-donating groups (−CH3 and −OCH3) (63 and
57%) (Table 2, entries 1 and 2). This phenomenon indicated
that electronic effect of the reactant could affect the
enantioselectivity of the asymmetric aldol reaction by use of
Pd@NH2-UiO-66(pro)-1 as the catalyst.86 Besides, it can be
seen that the enantioselectivity of the ortho-cyano group was
higher than that of the para-cyano one (Table 2, entries 4 and
5), which may be due to the greater steric hindrance of the
ortho position. The aldol product of ortho-cyano group was
difficult to rotate in the pores, which caused the higher ee
value.
3.3. Heterogeneous Test. After the model sequential
reactions were completed, the catalyst-containing solution was
collected by filtration, and the filtrate was detected by ICP-
OES analysis to determine the existence of Pd and Zr.
Palladium and zirconium traces (7.2 and 0.92 ppm,
respectively) were observed in the filtrate, indicating that
there was almost no leaching of palladium or zirconium from
the MOFs after immersion in the reaction system. It can be
concluded that the MOF as a strut can strongly protect the
loading of Pd NPs.
Meanwhile, when the model coupling process was
interrupted and the solid catalyst Pd@NH2-UiO-66(pro)-1
was immediately isolated from the hot solution after 5 min, the
coupling reaction no longer proceeded, and no further increase
in either the conversion of 4-bromobenzaldehyde or the
generation of 4-biphenylcarboxaldehyde was observed during
the following 25 min, while the contrast experiment, in which
the catalyst was not shifted out, proceeded with increase of the
product (Figure S10). This result indicated that the catalyti-
Figure 7. Recycling experiments of sequential Suzuki coupling/
asymmetric aldol reactions.
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Notes
which implied that heterogeneous catalyst Pd@NH2-UiO-
66(pro)-1 had an advantage of recycling. The PXRD spectra of
Pd@NH2-UiO-66(pro)-1 indicated that structure of catalyst
was kept intact after 4 cycles in PXRD patterns (Figure 3).
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors are grateful for financial support from the
National Natural Science Foundation of China (No.
21001024 and 21471031), the Natural Science Foundation
of Jiangsu Province (BK2011587 and BK20131289), the
Teaching and Research Program for the Excellent Young
Teachers of Southeast University (2242015R30026), the
Fundamental Research Funds for the Central Universities,
and the Project Funded by the Priority Academic Program
Development of Jiangsu Higher Education Institutions
(PAPD).
4. CONCLUSIONS
In summary, we have prepared two bifunctional heterogeneous
catalysts, Pd@NH2-UiO-66(pro)-1 and Pd@NH2-UiO-66-
(pro)-2, in which active Pd NPs and chiral proline were
successfully and rationally introduced via a NH2-UiO-66
platform. Pd NPs were encapsulated into the frameworks via
the “bottle-around-ship” method, while chiral proline was
decorated to the zirconium nodes and the organic linkers via
coordination and PSM, respectively. The bifunctional catalysts
were applied to the sequential Suzuki coupling/asymmetric
aldol reactions with excellent coupling performance (yield:
94−99.9%) and good enantioselectivities (eeanti: 57−97%).
Meanwhile, heterogeneous Pd@NH2-UiO-66(pro)-1 can be
reused for 4 cycles without any obvious decline in catalytic
activity. To our knowledge, this is the first report of
multifunctional metal NPs@chiral MOFs catalysts for
asymmetric sequential reactions. We expect our work in this
paper can provide a viable method to construct more chiral
multifunctional MOFs-based catalytic systems for asymmetric
sequential reactions.
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ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00065.
Full instrument details, calculation method of coupling
products in the model reactions, characterization of
racemates, HPLC analysis of asymmetric products and
additional characterizations (TG, SEM, TEM, CD, FT-
IR, UV−vis and XPS spectra) of the bifunctional
catalysts (PDF)
AUTHOR INFORMATION
■
Corresponding Author
Lin Cheng − School of Chemistry and Chemical Engineering,
Southeast University, Nanjing 210009, PR China;
orcid.org/0000-0003-3326-4590; Email: lcheng@
seu.edu.cn
Authors
Kaiyuan Zhao − School of Chemistry and Chemical Engineering,
Southeast University, Nanjing 210009, PR China
Qingsong Zhang − School of Chemistry and Chemical
Engineering, Southeast University, Nanjing 210009, PR China
Yiming Li − School of Chemistry and Chemical Engineering,
Southeast University, Nanjing 210009, PR China
Qingchao Zhai − School of Chemistry and Chemical
Engineering, Southeast University, Nanjing 210009, PR China
Jinxi Chen − School of Chemistry and Chemical Engineering,
Southeast University, Nanjing 210009, PR China;
orcid.org/0000-0002-9531-9705
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Platero-Prats, A. E.; Hafezi, N.; Sarjeant, A. A.; et al. A Hafnium-
Yongbing Lou − School of Chemistry and Chemical Engineering,
Southeast University, Nanjing 210009, PR China;
orcid.org/0000-0002-8224-5057
Complete contact information is available at:
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