Dual Heterogeneous Catalyst Pd-Au@Mn(II)-MOF for One-Pot Tandem Synthesis of Imines from Alcohols and Amines

Article abstract of DOI:10.1021/acs.inorgchem.6b02592

A new Mn(II) metal-organic framework (MOF) 1 was synthesized by the combination of 4,4,4-trifluoro-1-(4-(pyridin-4-yl)phenyl)butane-1,3-dione (L) and Mn(OAc)₂ in solution. 1 features a threefold-interpenetrating NbO net containing honeycomb-like channels, in which the opposite Mn(II)···Mn(II) distance is 23.5075(10) ?. Furthermore, 1 can be an ideal platform to support Pd-Au bimetallic alloy nanoparticles to generate a composite catalytic system of Pd-Au@Mn(II)-MOF (2). 2 can be a highly active bifunctional heterogeneous catalyst for the one-pot tandem synthesis of imines from benzyl alcohols and anilines and from benzyl alcohols and benzylamines.

Full text of DOI:10.1021/acs.inorgchem.6b02592

Article
pubs.acs.org/IC
Dual Heterogeneous Catalyst Pd−Au@Mn(II)-MOF for One-Pot
Tandem Synthesis of Imines from Alcohols and Amines
Gong-Jun Chen,* Hui-Chao Ma, Wen-Ling Xin, Xiao-Bo Li, Fa-Zheng Jin, Jing-Si Wang, Ming-Yang Liu,
and Yu-Bin Dong*
College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center 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 new Mn(II) metal−organic framework (MOF) 1 was
synthesized by the combination of 4,4,4-trifluoro-1-(4-(pyridin-4-yl)-
phenyl)butane-1,3-dione (L) and Mn(OAc)2 in solution. 1 features a
threefold-interpenetrating NbO net containing honeycomb-like channels,
in which the opposite Mn(II)···Mn(II) distance is 23.5075(10) Å.
Furthermore, 1 can be an ideal platform to support Pd−Au bimetallic
alloy nanoparticles to generate a composite catalytic system of Pd−Au@
Mn(II)-MOF (2). 2 can be a highly active bifunctional heterogeneous
catalyst for the one-pot tandem synthesis of imines from benzyl alcohols
and anilines and from benzyl alcohols and benzylamines.
hand, because of the synergistic effects between different metal
NPs, heterometallic alloy NPs have proven superior to
homometallic NP catalysts in some clean and energy-efficient
chemical processes. Examples include vinyl acetate synthesis
over a Pd−Au/SiO2 composite catalyst and other catalytic
reactions with Pd−Au@MOFs.10
Herein we report a new porous Mn(II) MOF, 1, which can
be a platform to support Pd−Au bimetallic alloy nanoparticles
to produce a composite catalytic system. The obtained Pd−
Au@Mn(II)-MOF (2) can efficiently promote the synthesis of
imines from alcohols and amines. 2 is heterogeneous and can
be reused at least five times with retention of its high catalytic
performance.
INTRODUCTION
■
It is well-known that imines are very important class of
intermediates in the synthesis of various pharmaceutical,
agricultural, and biological compounds. As an electrophile,
the imine group is widely used in many types of organic
reactions such as reduction, addition, condensation, and
cycloaddition and also in multicomponent reactions.1 Imines
are conventionally synthesized by the condensation of
aldehydes or ketones with amines.2
The direct synthesis of imines from alcohols and amines via a
one-pot tandem reaction, however, is more advantageous.
Compared with aldehydes and ketones, alcohols are much
more stable and readily available starting materials.3 On the
other hand, tandem synthesis features cost and time savings,
fewer chemicals, less energy consumption, and simplified
purification processes,4 so the imine synthesis via tandem
catalysis is more eco-friendly and economical. To date, some
useful approaches have been developed for the direct synthesis
of imines, including oxidation of secondary amines,5 self-
condensation of primary amines upon oxidation,6 and oxidative
coupling of alcohols and amines.7 The catalysts for the reported
direct synthesis of imines, however, are mainly homogeneous
ones. Compared with homogeneous catalysts, heterogeneous
catalysts are more preponderant because of their inherent
merits of stability, durability, reusability, and separability.8
Very recently, heterogeneous metal−organic framework
(MOF)-supported metal nanoparticle (NP) catalytic systems
have been demonstrated to be highly efficient in promoting a
wide variety of organic reactions.9 In comparison with MOF-
supported monometallic NPs, MOF-supported bimetallic metal
alloy NPs have received much less attention. On the other
EXPERIMENTAL SECTION
■
Materials and Chemicals. The reagents and solvents are
commercially available and were used without further purification.
The ligand 4,4,4-trifluoro-1-(4-(pyridin-4-yl)phenyl)butane-1,3-dione
(L) was synthesized according to our reported procedure.11
Instrumentation. Powder X-ray diffraction (PXRD) patterns were
collected on a D8 ADVANCE X-ray diffractometer with Cu Kα
radiation (λ = 1.5405 Å). The total surface areas of the catalysts were
measured by the Brunauer−Emmett−Teller (BET) method using
carbon dioxide adsorption at 195 K on a Micromeritics ASAP 2000
sorption/desorption analyzer. ICP-LC was performed on an IRIS
InterpidII XSP and NU AttoM instrument. High-resolution trans-
mission electron microscopy (HRTEM) analysis was performed on a
JEOL 2100 electron microscope at an operating voltage of 200 kV.
Scanning electron microscopy (SEM) images and energy-dispersive X-
ray (EDX) spectra were taken on a SUB010 scanning electron
Received: October 26, 2016
© XXXX American Chemical Society
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Figure 1. Syntheses of 1 and 2.
microscope with an acceleration voltage of 20 kV. Gas chromatog-
raphy (GC) analysis was performed on an Agilent 7890B gas
chromatograph. Electron spin resonance (ESR) spectra were obtained
on a Bruker A300-10/12/S-LC spectrometer. X-ray photoelectron
spectroscopy (XPS) was performed using a PHI Versaprobe II
spectrometer.
the reaction system was heated at 110 °C for 24 h to afford the desired
N-benzylidenebenzylamine product (95% conversion, 99% selectivity,
monitored by GC).
N-Benzylideneaniline Formation from Alcohols and Amines.
A toluene solution (2 mL) of benzyl alcohol (52 μL, 0.52 mmol),
aniline (66 μL, 0.62 mmol), KOH (2.8 mg, 0.05 mmol), and 2 (2.0
mg, 0.75 mol %) was heated at 110 °C for 30 h to afford N-
benzylideneaniline with excellent conversion (99%) and selectivity
(99%) as monitored by GC.
Regeneration of 2. 2 was recovered by centrifugation, washed
with ethanol (3.0 mL) and dichloromethane (3.0 mL) (three times),
and dried at 90 °C in vacuum for the next run under the same reaction
conditions.
Leaching Test. 2 was separated from the solution by
centrifugation, and the reaction solution was transferred to another
reaction vial under the same conditions.
X-ray Crystallography. Diffraction data for the complex were
collected at 293(2) K on an Agilent SuperNova single-crystal X-ray
diffractometer using Cu Kα radiation (λ = 1.54178 Å) with the ω−2θ
scan technique. An empirical absorption correction was applied to the
raw intensities.12 The structure was solved by direct methods
(SHELXS-2014) and refined by the full-matrix least-squares technique
on F2 using the SHELXS-2014.13 The hydrogen atoms were added
theoretically, riding on the concerned atoms and refined with fixed
thermal factors. The details of crystallographic data and structure
refinement parameters are summarized in Table S1. The selected
bonds lengths and angles for 1 are summarized in Table S2. CCDC
1486541 contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/
cif.
Synthesis of Mn(II)-MOF ((MnL2)·2CH3OH, 1). A solution of
Mn(OAc)2·4H2O (2.4 mg, 0.01 mmol) in MeOH (1 mL) was layered
onto a solution of L (4.8 mg, 0.02 mmol) in CH2Cl2 (2 mL). The
solutions were left for about 3 days at room temperature, and
compound 1 was obtained as light-brown crystals. Yield: 84%. IR (KBr
pellet, cm−1): 3068 (w), 1607 (s), 1569 (s), 1536 (s), 1458 (m), 1299
(m), 1250 (m), 1193 (s), 1145 (s), 1073 (m), 787 (s), 701 (s).
Elemental analysis (%) calcd for C30H18F6MnN2O4 (desolvated
sample): C 56.35, H 2.84, N 4.38. Found: C 56.12, H 2.88, N 4.18.
Synthesis of Pd−Au@Mn(II)-MOF (2). 1 (10 mg, 0.002 mmol)
was added to a CH3OH (2 mL) solution of chloroauric acid (12 mg,
0.12 mmol) and palladium nitrate (6.7 mg, 0.12 mmol). The mixture
was stirred for 1 h at room temperature. The resulting solid was
isolated by centrifugation and washed with CH3CN. The obtained
green-yellow crystalline solid was mixed with NaBH4 (25 mg, 0.65
mmol) in water (2 mL), and the mixture was stirred for additional 5 h
to afford 2 as dark-brown crystalline solids. The obtained crystalline
solids were washed with CH3CN and EtOH and dried in air. ICP
measurement indicated that the encapsulated amount of Pd−Au NPs
in 2 is 27.7 wt % (13.4% Pd and 14.3% Au).
RESULTS AND DISCUSSION
■
Structural Analysis of 1 and 2. Mn(II)-MOF (1) was
obtained as light-brown crystals by the combination of
Mn(OAc)2 and L in a mixed solvent system. Single-crystal
analysis indicated that 1 crystallizes in the hexagonal space
group P3. Each Mn(II) center adopts a 4 + 2 octahedral
̅
coordination sphere (Figure 1) in which two coplanar chelating
β-diketone units form the square (Mn−O distances of
2.131(4)−2.144(4) Å) and the axial positions are occupied
by two pyridyl N donors (Mn−N distance of 2.244(5) Å). The
solid-state packing pattern revealed that 1 is a threefold-
interpenetrating NbO framework, similar to its Cu(II)
analogue.11 As indicated in Figure 1, large hexagonal channels
along the crystallographic c axis exist in the structure, and the
opposite Mn···Mn distance in the channel is 23.5075(10) Å
(Figure S1). 1 is stable up to 150 °C, and the encapsulated
MeOH molecules can be removed by heating, which is well-
demonstrated by thermogravimetric analysis (TGA) and PXRD
(Figure S2), indicating that 1 can be a thermally stable support
to upload metal NPs and facilitate organic reactions at relatively
higher temperature.
Pd−Au-encapsulated Pd−Au@Mn(II)-MOF (2) was pre-
pared by impregnation of 1 with equimolar Pd(NO3)2 and
HAuCl4 in MeOH for 1 h followed by a NaBH4 reduction
process (H2O, 5 h). As shown in Figure 1, the color of 1
N-Benzylidenebenzylamine Formation from Alcohols and
Amines. A toluene solution (2 mL) of benzyl alcohol (52 μL, 0.52
mmol), benzylamine (66 μL, 0.62 mmol), and 2 (2.0 mg, 0.75 mol %)
was stirred in vacuum. After injection of air (50 mL, 0.44 mmol of O2),
B
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changed from light brown to dark after impregnation and
reduction. The total uploaded Pd−Au amount, as determined
by inductively coupled plasma (ICP) measurement, was up to
27.7 wt % (13.4% Pd, 14.3% Au). Thus, the formula of 2 can be
described as Pd1.6Au1.0@Mn(II)-MOF.
As shown in Figure 3, the one-pot synthesis of N-
benzylidenebenzylamine from benzyl alcohol and benzylamine
The oxidation state of the encapsulated Au and Pd after
reduction was determined by X-ray photoelectron spectroscopy
(XPS). The observation of Au 4f7/2 and 4f5/2 peaks at 83.86 and
87.46 eV and Pd d5/2 and d3/2 peaks at 335 and 341 eV
demonstrated the existence of Au(0) and Pd(0) (Figures S3
and S4) in 2.14 In addition, the XPS spectra show that no
change in valence state of the Mn(II) in 2 occurred upon
treatment with NaBH4 in aqueous solution. The Mn(II) 2p1/2
and 2p3/2 peaks appeared at 653.9 and 642.3 eV with a splitting
of 11.6 eV, indicating that the manganese in 2 is bivalent
(Figure S5).15
Moreover, we used high-resolution transmission electron
microscopy (HRTEM) to investigate the dispersion and size
distribution of the Pd−Au NPs in Mn(II)-MOF. As indicated
in Figure 2, the Pd−Au NPs in 2 were highly dispersed with an
Figure 3. (left) Relation of conversion/selectivity to air injection
volume. (right) Reaction time examination and leaching test for the
synthesis of N-benzylidenebenzylamine from alcohols and amines
catalyzed by 2. Reaction conditions: 2 (2.0 mg, 0.75 wt % Pd−Au),
benzyl alcohol (0.52 mmol), benzylamine (66 μL, 0.62 mmol), toluene
(2 mL), and air (50 mL) at 110 °C. The solid catalyst was filtered from
the reaction solution after 12 h, whereas the filtrate was transferred to
a new vial and the reaction was carried out under the same conditions
for an additional 24 h.
was chosen as the model reaction to examine the catalytic
activity of 2. When the reaction was carried out in toluene at
110 °C in vacuum, no expected N-benzylidenebenzylamine was
obtained because no alcohol oxidation could occur without
oxygen. On the other hand, the reaction also failed to afford the
expected imine product once it was performed in toluene at
110 °C in air. This could have been caused by the poor stability
of the oxidizable benzylamine under the reaction conditions.
Thus, the oxygen amount could be the key factor for this one-
pot tandem synthesis. A series of control experiments with
different amounts of air were performed (air volumes: 20, 30,
40, 50, and 60 mL). As shown in Figure 3, the best conversion
(>95%) and selectivity (∼100%) were observed when 50 mL of
air (0.62 mmol of O2) was used for the reaction under the
reaction conditions (110 °C, toluene; Figure S7).
Having determined the optimal amount of air, we also
examined the reaction time. Figure 3 shows that the initial
conversion of N-benzylidenebenzylamine continuously in-
creased as time went on, and the maximum yield (>95%)
appeared at 24 h (Figure S8). The conversion and especially
the selectivity for the desired product began to decrease after 24
h. The turnover number (TON) and turnover frequency
(TOF) at 24 h for N-benzylidenebenzylamine formation from
alcohols and amines were 126.7 and 5.28 h−1, respectively,
under the optimized conditions (Figure 3). In order to gain
insight into the heterogeneous nature of 2, a hot leaching test
was carried out. As indicated in Figure 3, no further reaction
took place without 2 after ignition of the reaction at 12 h,
indicating that 2 exhibits a typical heterogeneous catalyst nature
in this reaction.
Figure 2. HRTEM and SEM-EDS images of 2 and PXRD patterns
simulated for 1 and measured for 1 and 2.
average particle size of 2−5 nm. Atomic lattice fringes with a
spacing of 0.231 nm were observed, corresponding to (111)
planes.16 SEM-EDS elemental mapping of 2 further supported
the above observation (Figure 2). The PXRD pattern of 2 is in
good agreement with that of pristine 1, demonstrating that the
crystallinity and structural integrity of 1 is well-maintained
during the Pd−Au loading and reducing processes.
In addition, the porosity before and after Pd−Au loading was
confirmed by gas adsorption−desorption experiments. CO2
adsorption by 1 and 2 at 195 K revealed absorption amounts
of 56.7 and 53.6 m3/g by Mn(II)-MOF and Pd1.6Au1.0@
Mn(II)-MOF, respectively. The pore size distribution curve,
calculated from Barrett−Joyner−Halenda analysis, shows that
the narrow pore diameter distributions for 1 and 2 have widths
of 1.80 and 0.63 nm, respectively (Figure S6).
N-Benzylidenebenzylamine Formation from Alcohols
and Amines. On the basis of above observations, we next
examined the catalytic behavior of 2 for the one-pot synthesis
of imine species, an important class of compounds in organic
synthesis, biomedicine, and materials science, from amines and
alcohols. The direct synthesis of −RCN− from amines and
alcohols by a one-pot tandem reaction is challenging because
the optimal reaction conditions for the first aerobic alcohol
oxidation step and the following anaerobic condensation step
largely differ from each other.
The recyclability of 2 was also examined. After each run, the
solid catalyst was collected by centrifugation, washed with
acetonitrile, dried at 90 °C, and reused in the next run under
the same conditions. The conversion was still at 90% even after
five catalytic cycles (Figure S9). The size of the Pd−Au NPs in
2 slightly increased (5−20 nm), and the atomic lattice fringes
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a
Table 1. Summary of the Tandem Reactions Based on Substituted Benzyl Alcohols and Benzylamines
a
The reactions were carried out under the optimized conditions: toluene, 110 °C, 2 (0.75% Pa−Au alloy), 24 h.
(0.231 nm) were still clearly observed after five runs (Figure
S10). No valence change for Pd and Au species was detected by
XPS (Figures S11 and S12). The PXRD patterns of 2 after
reuse for five cycles indicated that the structural integrity of 2
was well-preserved (Figure S13). After five catalytic runs, the
filtrate was examined by ICP-AES. The results showed the total
amount of Pd and Au was 25.4 wt % (10.6 wt % Pd, 14.8 wt %
Au), indicating that only tiny amounts of Au (2.8 wt %) and Pd
(0.5 wt %) were lost during the catalytic cycles. Thus, Mn(II)-
MOF is an ideal platform to support Pd−Au alloy NPs for this
tandem reaction.
It is noteworthy that the yield of the benzyl alcohol oxidation
step is 71% (Figure S14). It was expected that amine addition
would significantly shift the reaction equilibrium to the right
and thus that more N-benzylidenebenzylamine would be
formed. A control experiment indicated that the yield of N-
benzylidenebenzylamine in the presence of Mn(II)-MOF 1 (5
mol %) was only 2%. In addition, the tandem reaction catalyzed
by monometallic Au@Mn(II)-MOF or Pd@Mn(II)-MOF
resulted in much lower conversion (48%; Figure S15).
After that, we examined the substrate scope of the tandem
reaction (Figure S16). As shown in Table 1, compared with
alcohols bearing electron-donating groups (48−75%), benzyl
alcohols with electron-withdrawing groups led to imines in
much higher conversions (93−99%). On the other hand, amine
substrates with electron-withdrawing groups resulted in
excellent yields (93−99%) that were significantly better than
those for amines possessing electron-donating groups (59−
93%).
N-Benzylideneaniline Formation from Alcohols and
Amines. To further investigate the catalytic efficiency of 2, the
tandem reaction was expanded to benzyl alcohols and aromatic
amines. As shown in Figure 4, benzyl alcohol and phenylamine
were chosen as model substrates, and the reaction was
performed in air under alkaline conditions (Figure 4). To our
delight, the corresponding imine was obtained in 99% yield
Figure 4. (top left) Leaching test. Reaction conditions: 2 (2.0 mg, 0.75
wt % Pd−Au), toluene solution (2 mL), benzyl alcohol (52 μL, 0.52
mmol), phenylamine (66 μL, 0.62 mmol), and KOH (2.8 mg, 0.05
mmol) at 110 °C for 30 h. The solid catalyst was filtered from the
reaction solution after 15 h, whereas the filtrate was transferred to a
new vial and the reaction was carried out under the same conditions
for an additional 15 h. (top right) Catalytic cycles. (bottom left)
PXRD patterns of 2 after each catalytic run. (bottom right) TEM
images of 2 after five catalytic runs.
after 30 h (Figures 4 and S17). The TON and TOF values for
N-benzylideneaniline formation from alcohols and phenylamine
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a
Table 2. Summary of the Tandem Reactions Based on Substituted Benzyl Alcohols and Phenylamines
a
The reactions were carried out under the optimized conditions: toluene, 110 °C, 2 (0.75% Pd−Au alloy), 30 h.
Figure 5. (left) N-Benzylidenebenzylamine formation from benzaldehyde−benzylamine condensation. Reaction conditions: benzaldehyde (0.52
mmol), benzylamine (0.62 mmol), toluene (2 mL), air (50 mL), 2 (2.0 mg, 0.75 wt % Pd−Au), 1 (4 mg), T = 110 °C. (right) N-Benzylideneaniline
formation from benzaldehyde−phenylamine condensation. Reaction conditions: benzaldehyde (0.52 mmol), phenylamine (0.50 mmol), toluene (2
mL), 2 (2.0 mg, 0.75 wt % Pd−Au), 1 (4 mg), T = 110 °C.
were 132 and 4.4 h−1, respectively. The hot leaching test
confirmed that 2 in this reaction is the heterogeneous catalyst,
and it could be reused for five catalytic cycles without
significant loss its activity (yields of 93% over five catalytic
cycles) (Figures 4 and S18). The PXRD patterns of 2 after
reuse for five cycles indicated that the structural integrity of 2
was well-preserved (Figure 4). The Pd−Au NPs were slightly
agglomerated, and the size of the Pd−Au NPs in 2 increased
from 2−5 nm to 5−20 nm after the fifth run; the atomic lattice
fringes for the Pd−Au NPs, however, were still clearly observed
(Figure 4), and their good dispersity was further confirmed by
SEM-EDS measurements (Figure S19). In addition, ICP-AES
analysis indicated that the Pd−Au amount decreased to 15.8 wt
% (5.9% Pd, 9.9% Au) which might have resulted from the
presence of strong base. The zero-valent oxidation states of Au
and Pd species in 2 after 5 runs were still maintained (Figures
S20 and S21). In the absence of base, the desired imine product
was obtained even with an enhanced amount of 2.
Again, the scope of this tandem reaction was explored by the
use of various substituted benzyl alcohols and phenylamines. As
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indicated in Table 2, benzyl alcohols with electron-donating
groups (e.g., CH3, OCH3) afforded the desired products in
higher yields (87−98%) than those with electron-withdrawing
groups (49−81%). On the other hand, aromatic amines with
electron-withdrawing groups (such as halogen and NO2)
provided excellent yields (99%) compared with those bearing
electron-donating groups (Figure S22).
2013CB933800), and the Taishan Scholar’s Construction
Project.
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CONCLUSION
■
We successfully synthesized a Mn(II)-MOF (1)-supported
Pd−Au alloy NP catalyst that was demonstrated to be a highly
efficient heterogeneous catalyst for the one-pot tandem
synthesis of imines from benzyl alcohols and anilines and
from benzyl alcohols and benzylamines. We expect this
approach to be viable for the construction of many more new
and interesting MOF-supported metal alloy NPs catalysts, and
studies toward the preparation of new mixed-metal alloy-loaded
MOF catalytic systems are underway.
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ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b02592.
Single-crystal data of 1 and its thermal stability,
additional characterization of 2, GC results for the one-
pot syntheses of N-benzylidenebenzylamine and N-
benzylideneaniline, and singlet oxygen detection (PDF)
Crystallographic data for 1 (CIF)
(9) Falcaro, P.; Ricco, R.; Yazdi, A.; Imaz, I.; Furukawa, S.; Maspoch,
D.; Ameloot, R.; Evans, J. D.; Doonan, C. J. Application of metal and
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AUTHOR INFORMATION
■
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Corresponding Authors
*E-mail: gongjchen@126.com.
*E-mail: yubindong@sdnu.edu.cn.
ORCID
Yu-Bin Dong: 0000-0002-9698-8863
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the National Natural
Science Foundation of China (Grants 21671122, 21475078,
21301109, and 21271120), the 973 Program (Grant
F
DOI: 10.1021/acs.inorgchem.6b02592
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DOI: 10.1021/acs.inorgchem.6b02592
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