Palladium/Phosphorus-Doped Porous Organic Polymer as Recyclable Chemoselective and Efficient Hydrogenation Catalyst under Ambient Conditions

Article abstract of DOI:10.1002/adsc.201700374

A new type of phosphorus-doped porous organic polymer (POP) has been readily synthesized through a Heck reaction, which could be used not only as a support but also a ligand for palladium nanoparticles. The dual-functional material supported palladium nanocatalyst was used for the efficient and chemoselective hydrogenation of varieties of nitroarenes and α,β-unsaturated compounds, as well as for the synthesis of indoles from 2-nitrophenylacetonitrile under 1 atm hydrogen in green solvents at room temperature. No obvious aggregation and loss of catalytic activity of the new nanocatalyst were observed after 10 runs in the reaction. (Figure presented.).

Full text of DOI:10.1002/adsc.201700374

Title: Pd/P-doped Porous Organic Polymer as Recyclable
Chemoselective and Highly Efficient Hydrogenation Catalysts
under Ambient Conditions
Authors: Zong-Cang Ding, Cun-Yao Li, Jun-Jia Chen, Jia-Hao Zeng,
Hai-Tao Tang, Yun-Jie Ding, and zhuang-ping zhan
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201700374
Link to VoR: http://dx.doi.org/10.1002/adsc.201700374

10.1002/adsc.201700374
Advanced Synthesis & Catalysis
FULL PAPER
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Palladium/Phosphorus-doped Porous Organic Polymer as
Recyclable Chemoselective and Efficient Hydrogenation
Catalysts under Ambient Conditions
Zong-Cang Ding,a Cun-Yao Li,b Jun-Jia Chen,a Jia-Hao Zeng,a Hai-Tao Tang,a Yun-Jie
Ding, b,* and Zhuang-Ping Zhana,*
a
Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005,
China.
E-mail: zpzhan@xmu.edu.cn
Dalian National Laboratory for Clean Energy, Collaborative Innovation Center of Chemistry for Energy Materials,
b
Dalian Institute of Chemical Physics, Chinese Academy of Science, Dalian, 116023, China.
E-mail: dyj@dicp.ac.cn.
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. A new type of phosphorus-doped porous organic green solvents at room temperature. No obvious
polymer (POP) has been readily synthesized through Heck aggregation and loss of catalytic activity of the new
reaction, which could be used not only as a support but also nanocatalyst were observed after 10 times in the reaction.
a ligand for palladium nanoparticles. The dual-functional
material supported palladium nanocatalyst was used for
efficient and chemoselective hydrogenation of varieties of
nitroarenes and α,β-unsaturated compounds, as well as the
synthesis of indoles from 2-nitrophenylacetonitrile under 1
atm hydrogen in
Keywords: heterogeneous catalysis; palladium
nanoparticles; porous organic polymers; hydrogenation,
nitroarenes; indoles
performed under drastic (for example, T> 100 oC,[4a-g]
or P> 1.0 atm H2[4a-m]) conditions. Furthermore, from
the viewpoint of green chemistry, the direct
hydrogenation of nitroarenes using H2 will be more
favorable, although other hydrogen sources were used
as stoichiometric reducing agents to obtain the high
selectivity had also been reported.[5] For the selective
hydrogenation of C=C bonds, great efforts have been
made to achieve the aimed products, but high H2
pressure, high temperature, toxic solvent, or various
additives like amines, pyridines and diphenylsulfide
are sometimes inevitable.[6]
Introduction
The chemoselective hydrogenation of nitroarenes and
α,β-unsaturated carbonyl compounds when other
sensitive functional groups such as carbonyl,
carboxyl, ester, amide or nitrile groups are appear in
the same substrates represents a cornerstone of
modern industrial chemistry. As the former produces
aniline and aniline derivatives, which are important
feedstocks and key intermediates for the manufacture
of many pharmaceutical, polymer, dye, and natural
products. The latter obtains ketones and has been
extensively applied in the synthesis of functional
materials and fine chemicals. Due to the limitation of
homogeneous catalysis in the difficult separation of
product and catalyst, the heterogeneous catalysts are
desperately in demand for industrial applications
because of the recovering and recycling of the
catalysts.[1] Nevertheless, to develop heterogeneous
catalysts with not only high catalytic activity, but also
selectivity towards the nitro group[2] or C=C bond of
Until now, to obtain the advantages of homogeneous
and heterogeneous catalysts, immobilizing metal
nanoparticles (NPs) onto high surface solids would be
an effective approach. A huge number of microporous
and mesoporous materials were chosen to be the
supports of metal species in hydrogenation reactions,
such as zeolite, silica, titania, carbonaceous materials,
MOFs and POPs.[7] Among above mentioned
materials, the POPs have received special attention
because of their high surface area, tunable pore size
α,β-unsaturated carbonyl compounds[3] is
dominating challenge. For the reduction of nitro
group, the hydrogenation process is sometimes
a
distribution,
excellent
stabilities,
modifiable
functionality, and flexible synthetic strategy. Recently,
kinds of POPs have been used as supports of metal
1
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10.1002/adsc.201700374
Advanced Synthesis & Catalysis
NPs for hydrogenation reactions.[8] For instances,
Wang and coworkers reported several click-based
POP supports of Pd NPs with uniform
nitrophenylacetonitrile over the Pd NPs was also
carried out.
Results and Discussion
To explore the structure-reactivity relationship, the
POL-2 was characterized by thermogravimetry (TG),
N2 adsorption-desorption analysis, and transmission
electron micro graph (TEM). The TG shows that
o
POL-2 remains intact at temperatures up to 400 C,
indicating its good thermal stability (SI, Figure S 1).
Nitrogen adsorption-desorption analysis (Figure 1, a
and b) shows that POL-2 possesses very favorable
hierarchical pore size distributions, according to the
pore size distribution curves, and the pore sizes were
mainly distributed at 0.70, 0.85, 1-10 nm respectively,
which was calculated from the method of non-local
density functional theory (NLDFT). The BET surface
areas and total pore volumes of POL-2 are up to
554.2 m2/g and 0.959 cm3/g respectively. The TEM
image shows that the Pd nanoparticles were highly
dispersed on the surface of POL-2 (Figure 1, c), and
the size mainly range from 2 to 4 nm (Figure 1, d).
For the SEM, XRD, XPS, 13C and 31P MAS data,
please see the supporting information.
Scheme 1. Synthesis of porous organic ligand (POLs) and
preparation of different content Pd nanocatalysts
distributions for hydrogenation of nitroarenes and
olefins.[8a-d] Owing to the coordination of the N atom
to the Pd NPs, the nanocatalysts could be recycled
several times without loss of catalytic activity.
Besides, other N,[8e-f] O,[8g] and S[8h]-doped POPs have
also been designed and applied to the hydrogenation
reactions. Despite these pioneering work, the design
and synthesis of new type of support as well as ligand
for the metal active site to protect it from leaching
and agglomeration is still an exciting yet challenging
task.
As we know, the phosphine ligands are among the
best choice to form the coordination interaction with
the metal ions. Lately, we have reported a porous
organic ligand (POL-Ph3P) (see SI, Scheme S1)
supported palladium for the Suzuki–Miyaura cross-
coupling reactions.[9] To the best of our knowledge, in
spite of so many advantages of these P-doped POPs,
attempts to use them to catalyze hydrogenation
reactions have not been reported. Herein, a new type
of P-doped porous organic polymer POL-1 (Scheme
1, a) was designed and synthesized through Heck
reaction, which could turned to POL-2 after loading
PdCl2 and H2 reduction. The obtained POL-2 could
sever as not only a support but also a ligand for Pd
nanocatalysts (Scheme 1, b) and we demonstrate the
utilization of this new P-doped POP to the room
temperature hydrogenation of both nitroarenes and
C=C bond of α,β-unsaturated carbonyl compounds
with high efficiency, selectivity and recyclability
under atmosphere pressure. What’s more, the
synthesis of indoles by reductive cyclization of 2-
Figure 1. a) N2 adsorption-desorption isotherms, b)
Pore size distribution curves, c) TEM of Pd@ POL-2,
d) Histogram for the Pd particles size distribution of
Cat C before use.
We prepared catalysts with different Pd loadings,
that is, 1 wt%, 4 wt% and 7 wt% Pd@ POL-2, and
the Pd content was determined by ICP-AES.
Interestingly, we found the catalytic activity could be
regulated by loading different contents of Pd onto
POL-2. The reduction of p-nitroacetophenone was
carried out as the model reaction with different
catalysts under 1 atm H2 in an ethanol solvent and the
results were summarized in table 1. Remarkably, Cat
C exhibited a high catalytic activity and selectivity
towards the nitro group of 1a to afford 2a in 91%
yield without the reduction of the carbonyl group
2
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10.1002/adsc.201700374
Advanced Synthesis & Catalysis
Table 1. Pd-Catalyzed hydrogenation of p-
Table 2. Pd NPs Catalyzed hydrogenation of various
nitroacetophenone[a]
nitroarenes.[a,b]
Entry
Catalyst
Pd(PPh3)4
Pd@C[c]
Pd@C[d]
Pd@C[e]
Pd@C[e, f]
Cat A[g]
Cat B[g]
Cat C[g]
t/h
1
Yield (%)[b]
1
2
3
4
5
6
7
8
9
<5
<10
1
1
<20
1
<20
1
68
1
No reaction
15
1
1
91
[h]
Pd@POL PPh3
1
82
[a]
Reaction conditions: 1a (2.0 mmol, 330 mg), catalyst
(Pd, 0.01 mmol), ethanol (5 ml), r.t., 1 atm H2.
[b] Isolated yields.
[c] 10 wt % Pd@C bought from sigma Aldrich.
[d] 10 wt % Pd@C bought from Energy Chemical.
[e] 5 wt % Pd@C bought from Aladdin.
[f] The amount of catalyst is 1 mol %.
[g] Cat A, B, C see Scheme 1, b.
[a] Reaction conditions: 1 (2.0 mmol), Cat C, 15.1 mg (Pd,
0.01 mmol), ethanol (5 ml), r.t., 1 atm H2.
[b] Isolated yields.
[c] Reacted at 60 oC.
[h]
The Pd loading was 7 wt%, the structure of POL-PPh3,
catalyst showed no selectivities towards some
reducible functional groups, such as N-Cbz protective
group, alkene, alkyne and azide, which could be
reduced along with nitro groups, albeit with excellent
yields (Table 2, 2i and 2n). It is worth mentioning
that the reaction still allowed high selectivities when
see SI, Scheme S1.
(Table 1, entry 8). In sharp contrast, homogeneous
Pd(PPh3)4 showed low efficiency (entry 1) and the
commercial Pd/C bought from three different
companies gave unsatisfactory yields (entries 2-5).
Compared with Cat C, Cat A, and Cat B showed
lower activity, either did not lead to any detectable
product formation or brought lower yield (entries 6
and 7), while Pd@POL PPh3 had a moderate yield
(entry 9). Thus, the optimized reaction conditions for
the selective hydrogenation of nitroarenes were
determined to be using 0.5 mol % Cat C, under 1 atm
H2 at room temperature in ethanol (Table 1, entry 8).
o
performed at 60 C (2a, 2c and 2e), and this made it
possible for larger scales, since during which heat
removal caused by the very high exothermicity of the
reaction was difficult.
We next turned our focus on the selective
hydrogenation of α,β-unsaturated compounds
(Scheme 2). Interestingly, It was found that the
optimized conditions used for nitroarenes was not
suitable for this transformation, in which Cat C gave
a 4:1 (4a : 5a) selectivity, albeit with excellent
efficiency. However, Cat B gave almost only 4a in
99% yield, that is, Cat B could realize the
chemoselective reduction of the C=C bond in α,β-
unsaturated compounds under mild reaction
conditions.
The general applicability was demonstrated in
table 2. A number of nitro compounds were converted
into corresponding amines with excellent yields and
selectivities. For example, nitrobenzene and those
substituted with various groups were all tolerated
under the extremely mild conditions (Table 2, 2a-2j,
88-99% yields), and the catalyst showed no effect on
the sensitive groups such as acetyl, cyan, formamide,
-Cl, and p-OBn groups (2a-2e and 2m). In addition,
2k was also achieved from the reduction of 4-nitro-
1H-indole (Table 2, 2k, 99% yield), and
nitronaphthalene was converted to naphthylamine
smoothly (Table 2, 2l, 98% yield). To our regret, the
Scheme 2. The selective hydrogenation of 3a over Cat B
and Cat C
3
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10.1002/adsc.201700374
Advanced Synthesis & Catalysis
Table 3. Pd NPs Catalyzed hydrogenation of various
α,β-unsaturated compound.[a,b]
Table 4. Pd NPs Catalyzed reductive cyclization of
2-nitrophenylacetonitrile to indoles [a,b]
[a] Reaction conditions: 6 (0.3 mmol), Cat C, 45 mg (Pd,
0.03 mmol), methanol (3 ml), r.t., 1 atm H2.
[b] Isolated yields.
[c] 40% of 6e was recovered.
[a] Reaction conditions: 3 (2.0 mmol), Cat B, 26.5 mg (Pd,
0.01 mmol), ethanol (5 ml), r.t., 1 atm H2
[b] Isolated yields
[c] The reaction was performed in CH2Cl2
[d] R3 = N3
the process more simple and realize the
transformation under 1 atm H2 at room temperature,
and there was no need for complicated ligands or
additional acids or bases (Scheme 3, c).
The general scope was tested in the hydrogenation
of varieties of α,β-unsaturated compounds with the
results summarized in table 3. Gratifyingly, in all
cases, we got the full conversion of the substrates
towards the corresponding products (Table 3, 4a-4l,
83-99% yields). Remarkable chemoselectivity was
achieved in the hydrogenation of the C=C bond when
other sensitive groups such as carbonyl, ester, cyan,
carboxyl, amide, aldehyde, benzyl ether and N-Cbz
protective groups are appear in the same substrates,
except that when R3 was azide (Table 3, 4k).
Several representative indoles were synthesized
over the heterogeneous Pd NPs under 1 atm H2 at
room temperature in CH3OH (Table 4), good to
excellent yields were achieved (7a-f, 55-99% yields),
which also gave an obvious evidence for the higher
activity of Pd@POL -2 than commercial Pd/C
catalysts.
Scheme 3. Catalytic synthesis of indoles from 2-
nitrophenylacetonitrile
Next, based on the high catalytic activity towards
hydrogenation of this porous organic polymer
supported Pd NPs, we tried the synthesis of indoles
from 2- nitrophenylacetonitrile, as some efforts have
been made to this reaction. For example, Makosza[10]
et al reported Pd/C catalyzed reductive cyclization of
2-nitrophenylacetonitrile to prepare indole under 3
atm H2 and acetic acid is a necessary additive
(Scheme 3, a). Nagashima[11] also developed the
heterogeneous Pt catalyst [Pt]@SiC6 could furnish
this conversion affording the indole product in 52%
yield, however, the reaction require 10 atm of H2
(Scheme 3, b). Here, we applied our new porous
organic P-doped polymer supported Pd NPs to make
Figure 2. a) Reuse of Pd@POL-2 towards
hydrogenation of 1-Methoxy-4-nitrobenzene. b) and c)
TEM image of Cat C after first use and tenth use.
4
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10.1002/adsc.201700374
Advanced Synthesis & Catalysis
a JEM-2100 with accelerating voltage of 200 kV. Scanning
electron microscopy (SEM) was performed using a
Finally, the reusability of Pd@POL-2 was
examined for the hydrogenation of 1-methoxy-4-
nitrobenzene 1g to obtain 2g (Figure 2). As can be
seen, the Pd@POL-2 was recycled ten times without
loss of activity, (Figure 2, a). In addition, the TEM
images revealed the Pd NPs supported on POL-2[12]
with a mean diameter of about 2.7 nm were formed
after first use (Figure 2, b and SI, Figure S6, left), and
no obvious aggregation of these nanoparticles
occurred up to the 10th run though the mean diameter
incraced to 4 nm (Figure 2, c and SI, Figure S6, right).
These excellent results benefit from the critical
coordination interaction of P atoms and Pd NPs, thus
protecting Pd from leaching and agglomeration.
HITACHI-4800.
spectroscopy(XPS)was conducted using
The
X-ray
photoelectron
Thermo
a
Scientific ESCALAB 250Xi with the Al Kα irradiation at
θ=90o for X-ray sources, and the spectrometer binding
energy was calibrated through the reference C 1s
(284.9eV). The samples were pressed to tablet and were
pasted on sample stage using conducting resin and the
pressure of vacuum chamber was lower than 10-10mbar
when testing while the step size of 0.10 eV was employed.
Thermogravimetric analysis (TGA) was carried out using a
thermal analyzer (NETZSCH STA 449 F3)，the sample
was heated at the rate of 10K•min-1 from room temperature
up to 1073K under a nitrogen atmosphere. Unless
otherwise noted, all reagents and solvents were obtained
commercially and used without further purification. All
reaction mixtures were stirred with a magnetic bar in
flame-dried glassware.
Conclusion
In conclusion, a P-doped porous organic polymer has
been readily designed and synthesized and used as
not only the support but also the ligand for Pd
nanoparticles. By regulating the Pd loading on the
polymer, the nanocatalysts could obtain excellent
efficiency as well as chemoselectivity towards the
hydrogenation reactions. Owing to the coordination
interaction of P atoms with the supported Pd NPs, no
obvious aggregation and loss of catalytic activity of
the new nanocatalyst were observed after 10 times in
the reduction of nitro groups.
Preparation of POL-PPh3
Mg turnings (2.40 g, 100 mmol) were added to a round
bottom flask under nitrogen, and were activated by
treatment with a grain of I2 in THF (80 mL). p-
bromostyrene (14.64 g, 80 mmol) was slowly added to the
flask, and the mixture was stirred for 1 h at room
temperature. Next, PCl3 (3.43 g, 25 mmol) was slowly
added to the solution in the ice water bath over 30 min
followed by stirring at room temperature for 4 h. The
reaction was quenched with aq.NH4Cl, and the mixture
was extracted with ethyl acetate. The organic layer was
washed with H2O twice, dried over NaSO4, filtered and
concentrated. The residue was purified by silica gel
column chromatography (petroleum ether and ethyl
acetate) to afford tris(4-vinylphenyl)phosphane (5.45 g,
64% yield). Under nitrogen, tris(4-vinylphenyl)phosphane
(10.0 g) was dissolved in THF (100 mL), followed by the
addition of AIBN (1.0 g) at room temperature. Next, the
Experimental Section
General materials and methods
The liquid-state NMR was recorded on a 400 MHz
1
spectrometer. Chemical shifts were reported in ppm. H
o
mixture was transferred into an autoclave at 100 C for 24
NMR spectra were referenced to CDCl3 (7.26 ppm), and
13C-NMR spectra were referenced to CDCl3 (77.0 ppm).
All 13C NMR spectra were measured with complete proton
decoupling. Peak multiplicities were designated by the
following abbreviations: s, singlet; d, doublet; t, triplet; m,
multiplet; brs, broad singlet and J, coupling constant in Hz.
The solid-state NMR was recorded on a Bruker AVANCE
III 400 WB spectrometer equipped with a 4 mm standard
bore CP/MAS probehead whose channel was tuned to
400.18 MHz. The samples were packed in the ZrO2 rotor
closed with Kel-F cap which were spun at 12 kHz rate. 13C
CP/MAS spectra were referenced to adamantane (C10H16)
standard (1.63 ppm). 31P CP/MAS were referenced to
adenosine diphosphate (ADP) (0.0 ppm). Nitrogen sorption
isotherms at the temperature of liquid nitrogen were
performed on a Quantachrome Autosorb-1 system, and the
samples were degassed for 10 h at 393 K before the
measurements were obtained. The specific surface areas
were calculated from the adsorption data using Brunauer–
Emmett–Teller (BET) methods. The total pore volume at
P/Po=0.995. The pore size distribution curves were
obtained from the desorption branches using the non-local
density functional theory (NLDFT) method. Transmission
electron microscope (TEM) images were performed using
h. After evaporation of THF under vacuum, a white solid
(9.6 g, 96% yield) was obtained.
Preparation of POL-1 and Cat A
Tris(4-vinylphenyl)phosphane (6.46 g, 19 mmol, 1.0 eq.),
1,3,5-tribromobenzene (6.58 g, 20.9 mmol, 1.1 eq) ,
Pd(PPh3)4 (1.1 g, 0.95 mmol, 0.05eq.) and K2CO3 (18.3 g,
133 mmol, 7.0 eq.) were added to a round bottom flask in
super dry DMF (150 mL) under nitrogen. And the mixture
o
was stirred at 120 C. Gray solid was obtained after 72 h
and the mixture was cooled to room temperature, filtration
and washed by H2O, MeOH, THF, EtOAc and DCM 3
times respectively, then the gray solid was dried under
vacuum to afford POL-1 4.8 g. As a note, during this
process, part of the catalyst Pd(PPh3)4 was loaded to POL-
1, after reduction under 1 atm H2 at 180 oC, the Pd loading
is determined by ICP-AES to be 1 wt% (Cat A).
Preparation of POL-2, Cat B and Cat C
5
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10.1002/adsc.201700374
Advanced Synthesis & Catalysis
Acknowledgements
To a round bottom flask was added POL-1 (1.0 g) , MeCN
15 mL, and PdCl2 (177.3 mg, 1.0 mmol), The reaction
mixture was stirred at room temperature for 5 h, the gray
solid turned to yellow , the yellow solid was washed using
MeOH, EtOAc THF and MeCN 3 times respectively after
We thank the financial support from National Natural Science
Foundation of China (No. 21272190), the strategic priority
research program of the Chinese Academy of Sciences, Grant No
XDB17020400 and NFFTBS (No. J1310024).
o
filtration. Next, the yellow was reduced at 180 C for 5 h
and cooled to room temperature to obtain Cat C (7 wt %
Pd NPs@POL-2), During this process, the C=C double
bonds of POL-1 were reduced to C-C single bonds to
obtain POL-2. Changing PdCl2 loading to 70 mg could
give Cat B (4 wt %NPs@ POL-2). The Pd content was
determined by ICP-AES.
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nitroarenes and the recycling experiment
To a three-necked round bottom flask was added
nitroarenes 1 (2 mmol), Cat C, 15.1 mg (Pd, 0.01 mmol).
The flask was purged with H2 for 5 minutes, and EtOH (5
mL), was added under H2. The reaction mixture was stirred
at room temperature and the reaction progress was
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at room temperature and the reaction progress was
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Advanced Synthesis & Catalysis
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7
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FULL PAPER
Pd/P-doped Porous Organic Polymer as Recyclable
Chemoselective and Highly Efficient
Hydrogenation Catalysts under Ambient
Conditions
Adv. Synth. Catal. Year, Volume, Page – Page
Zong-Cang Ding,a Cun-Yao Li,b Jun-Jia Chen,a Jia-
Hao Zeng,a Hai-Tao Tang,a Yun-Jie Ding, b,* and
Zhuang-Ping Zhana,*
8
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