Preparation of Recoverable Pd Catalysts for Carbonylative Cross-Coupling and Hydrogenation Reactions

Article abstract of DOI:10.1002/cctc.201200129

We report on the synthesis, characterization and catalytic performance of new, supported Pd^II and Pd⁰ catalysts. The catalysts are characterized by TEM, XRD, FTIR, X-ray photoelectron spectroscopy, and vibrating sample magnetometry. The catalysts are found to be active in both forms, Pd^II and Pd⁰, for the carbonylative cross-coupling reaction of aryl iodides with arylboronic acids, and for the hydrogenation of aromatic nitro- and unsaturated compounds. The newly developed catalysts are prepared by a synthetic strategy that is similar to the one used for other supported catalysts but are easier to recover-they can be recycled by magnetic separation from liquid phase reactions-and can be used for at least 5 consecutive trials without any decrease in activity.

Full text of DOI:10.1002/cctc.201200129

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DOI: 10.1002/cctc.201200129
Preparation of Recoverable Pd Catalysts for Carbonylative
Cross-Coupling and Hydrogenation Reactions
Jian-Rui Niu, Xing Huo, Feng-Wei Zhang, Hai-Bo Wang, Ping Zhao, Wu-Quan Hu,
[a]
Jiantai Ma,* and Rong Li*
We report on the synthesis, characterization and catalytic per-
and unsaturated compounds. The newly developed catalysts
are prepared by a synthetic strategy that is similar to the one
used for other supported catalysts but are easier to recover—
they can be recycled by magnetic separation from liquid phase
reactions—and can be used for at least 5 consecutive trials
without any decrease in activity.
II
0
formance of new, supported Pd and Pd catalysts. The cata-
lysts are characterized by TEM, XRD, FTIR, X-ray photoelectron
spectroscopy, and vibrating sample magnetometry. The cata-
II
0
lysts are found to be active in both forms, Pd and Pd , for the
carbonylative cross-coupling reaction of aryl iodides with aryl-
boronic acids, and for the hydrogenation of aromatic nitro-
Introduction
[
10]
Biaryl ketones are important moieties in many biologically
active molecules, natural products, and pharmaceuticals. A va-
py. In this study, Pd complexes were immobilized on this
type of support to create heterogeneous catalysts because
they are easily handled, recyclable, and the process is
[1]
riety of methods for their preparation have been reported.
[11]
The Suzuki carbonylative coupling reaction is one of the most
promising methods for the direct synthesis of biaryl ketones
from carbon monoxide, aryl halides, and arylboronic acids,
owing to the wide variety of functionalities that can be tolerat-
ed on either partner and the fact that arylboronic acids are
“green”. To enhance the stability, we used coated (MNPs) for
the metal immobilization.
A number of functionalized MNPs have been employed in
a range of organic transformations, and show excellent catalyt-
[12]
[13]
[14]
ic activities in CÀC coupling, hydrogenation, oxidation,
amination, and nitrile hydration reactions. Inspired by the
[2]
[15]
[16]
generally nontoxic and thermally-, air-, and moisture-stable.
[17]
[18]
As a result of improvements in selectivity and activity, the in-
terest in homogeneous catalysts has grown dramatically. Nev-
ertheless, one key challenge for the commercial development
and practical use of homogeneous catalysts is the separation
of product from the catalytic media. This process is often com-
plicated and usually accomplished by means of complex work-
work of Fryxell et al. and Pinnavaia and Mercier, who dem-
onstrated that thiol-modified mesoporous materials are re-
[19–20]
markable scavengers for mercury, and by Kang et al.,
who
showed that SBA-15-SH has a higher affinity for Pd and Pt
compared with other metals, we began a study into the immo-
bilization of Pd on a mercaptopropyl-modified silica-coated
magnetic support with great catalytic and separation proper-
ties. We hoped that the functionalized magnetic catalyst
would act as both a support, and as a scavenger for any small
[
3]
up procedures. Attempts to improve catalyst recovery and re-
[
4]
cycling, include the use of biphasic systems or the immobili-
[
5]
zation of catalysts on solids. In such circumstances, systems
that use silica-coated magnetic nanoparticles (MNPs) have
drawn attention. These systems, which are comprised of mag-
netically recoverable solid supports, can be easily and quickly
removed from the reaction medium, and are used in fields
[21]
amounts of residual Pd that escaped from the surface. The
II
supported Pd catalyst is active towards selective conversion in
the carbonylative Suzuki coupling reaction and the minimiza-
tion of waste generation, which is highly relevant to the devel-
opment of “green” chemical processes. If the metal is properly
[
6]
[7]
such as drug delivery, magnetic resonance imaging, biomo-
[
8]
[9]
0
lecular sensors, bioseparations, and magneto-thermal thera-
reduced, Pd nanoparticles immobilized on the magnetic sup-
port surface are obtained, and the catalyst becomes active for
hydrogenation reactions. We present here a reusable Pd cata-
lyst supported on a magnetic material for carbonylative cross-
[
a] J.-R. Niu, X. Huo, F.-W. Zhang, H.-B. Wang, P. Zhao, W.-Q. Hu, Prof. J. Ma,
Prof. R. Li
Key Laboratory of Nonferrous Metals Chemistry and Resources Utilization
College of Chemistry and Chemical Engineering
Lanzhou University
[22]
coupling and hydrogenation reactions.
2
22 Tianshui South Road, Lanzhou 730000 (P.R. China)
Fax: (+86)0931-891-2582 (J.M.)
+86)0931-891-2311 (R.L.)
Results and Discussion
(
E-mail: majiantai@lzu.edu.cn
liyirong@lzu.edu.cn
Catalyst preparation and characterization
II
The preparation of the Fe O @SiO -SH-Pd catalyst follows the
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201200219.
3
4
2
steps described in Scheme 1. Firstly, the silica-coated MNPs
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Unambiguous evidence of Pd
particles on Fe O @SiO -SH is
3
4
2
provided by XRD. The XRD pat-
tern of Fe O @SiO shows char-
3
4
2
acteristic peaks of magnetite
nanoparticles and the sharp,
strong peaks confirm the prod-
ucts are well-crystallized. The
XRD results reveal their high
crystallinity, which is in agree-
Scheme 1. Preparation of the catalysts.
[23]
were prepared by a reverse microemulsion. Then, the silica-
coated MNPs were functionalized with mercaptopropyl trime-
ment with the results reported by Li et al. As shown in Fig-
ure 2b and c, the new peaks that appeared are attributed to
thoxysilane. Finally, under reflux conditions, PdCl was support-
2
ed on the surface of ligand-modified MNPs.
The typical TEM image of Fe O @SiO -SH nanoparticles pre-
3
4
2
pared by a reverse microemulsion procedure is shown in Fig-
ure 1a. As can be seen from the image, the average diameter
of the as-synthesized spherical particles was about 35 nm and
0
nearly monodisperse. The Fe O @SiO -SH-Pd catalyst didn’t
3
4
2
change considerably after attachment of the Pd onto the sur-
face of the MNPs (Figure 1c). It can also be concluded that the
Pd particle size distribution was centered at 5 nm, which, un-
fortunately, indicates that agglomeration occurred (Figure 1c).
II
Figure 2. XRD patterns of (a) Fe
3
O
4
@SiO
2
-SH, (b) Fe
3
O
4
@SiO
2
-SH-Pd , and
0
(
3 4 2
c) Fe O @SiO -SH-Pd .
the Pd species. The results from XRD imply that the Pd nano-
particles have been successfully immobilized on the surface of
MNPs.
The XPS elemental survey scans of the surface of the solid
catalyst are shown in Figure 3. The peaks corresponding to C,
II
Figure 1. TEM images of (a) Fe
3
O
4
@SiO
2
-SH, (b) Fe
3
O
4
@SiO
2
-SH-Pd , and
0
II
(c) Fe
3
O
4
@SiO
2
-SH-Pd .
Figure 3. XPS spectrum of the elemental survey scan of Fe
3
O
4
@SiO
2
-SH-Pd .
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and could be completely and quickly separated from the aque-
ous solution by placing a magnet near the reaction vessels.
Catalyst testing for the hydrogenation reaction
The hydrogenation results of aromatic nitro- and unsaturated
compounds with different substituent groups as catalyzed by
Fe O @SiO -SH-Pd are shown in Table 1. The hydrogenation re-
3
4
2
actions were performed in ethanol, at room temperature and
under a 1 atm pressure of H . The catalyst was found to be ef-
2
fective for the hydrogenations of these compounds under mild
conditions. As shown in Table 1, the hydrogenation reactions
reached completion within 50 min, except for 1-nitronaphtha-
lene and trans-stilbene, which required 90 min. Further experi-
ments with the substrate nitrobenzene were performed to
verify the catalyst recyclability. After each hydrogenation reac-
tion, the catalyst was easily separable by the application of
a magnetic field, the product could be removed with a syringe,
and new portions of the substrate were added to the reactor.
The new Pd magnetically-separable catalyst could be reused
for up to eight successive hydrogenation reactions without
a change in substrate conversion after 50 min (100% conver-
sion). In contrast, commercial 10% Pd/C was used to catalyze
the hydrogenation of nitrobenzene, and the reaction took 2 h
to reach completion, whereas the reaction with Fe O @SiO -
3
4
2
SH-Pd required no more than 1 h with a relatively low Pd load-
ing under similar experimental conditions.
Owing to the use of the magnetic property, the time interval
for both the catalyst separation and product removal is insig-
nificant compared with the time consumed during the reac-
tion. Such a procedure also avoids unnecessary use of other
separation techniques such as filtration, decantation, and cen-
trifugation, which greatly simplifies the workup procedure
Moreover, the Pd content (3.81 wt%) of the recovered
Fe O @SiO -SH-Pd catalyst remained approximately the same
0
II
Figure 4. XPS spectrum of (a) Fe
3 4 2 3 4 2
O @SiO -SH-Pd and (b) Fe O @SiO -SH-Pd ,
which shows the Pd 3d5/2 and Pd 3d3/2 binding energies.
Cl, O, Pd, S, and Si are clearly observed. The Pd 3d5/2 and Pd
II
3
d3/2 binding energies of Fe O @SiO -SH-Pd were determined
3 4 2
to be 343.8 and 338.4 eV, respectively (Figure 4). These values
3
4
2
II
agree with the Pd binding energy of the complex. Unfortu-
as that of the fresh catalyst, which indicates that Pd leaching
was negligible.
nately, the Pd 3d5/2 and Pd 3d3/2 binding energies changed to
0
3
36 and 341 eV, which corresponds to the Pd bind-
0
ing energy. The XPS results showed that Pd was
formed during the catalyst preparation. This is proba-
II
bly attributable to a small amount of Pd ions that
were reduced by the mercaptopropyl group.
Magnetic measurements were performed by using
a vibrating sample magnetometer at room tempera-
ture. The magnetization curves for Fe O @SiO ,
3
4
2
II
Fe O @SiO -SH, and Fe O @SiO -SH-Pd are compared
3
4
2
3
4
2
in Figure 5. There was no hysteresis in the magnetiza-
tion for the three types of nanoparticles. Neither co-
ercivity nor remanence was observed, which suggests
that the three nanoparticles are superparamagnetic.
The observed saturation magnetizations were
À1
À1
1
3.83 emug for Fe O @SiO , 11.7 emug , 14.5 for
3 4 2
À1
Fe O @SiO -SH, and 10.6 emug for Fe O @SiO -SH-
3
4
2
3
4
2
II
Pd .The decrease in saturation magnetization might
be responsible for the increased mass of mercapto-
propyl on the surface of the MNPs. However, the
3 4 2 3 4 2
Figure 5. The magnetization curves of (a) Fe O @SiO , (b) Fe O @SiO -SH, and
II
3 4 2
nanomaterials still retained good magnetic properties (c) Fe O @SiO -SH-Pd .
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system was observed. For example, aryl iodides containing
electron-withdrawing groups, including 4-CH CO and 2-NO ,
0
[a]
3 4 2
Table 1. Hydrogenation of various substrates by Fe O @SiO -SH-Pd .
3
2
Entry Substrate
Product
Reaction Yield
time
produced desired ketones in excellent yields (entries 15–20).
Notably, up to a 99.7% yield of unsymmetrical biaryl ketone
was obtained for 4-chlorobenzeneboronic acid with 4-iodoni-
trobenzene (Table 2, entry 17). In addition, both 4-methylben-
zeneboronic acid and 4-chlorophenylboronic acid reacted
smoothly with aryl iodides to give unsymmetrical biaryl ke-
tones in high yields (Table 2, entries 4, 5, 7, 8, 10, 11, 13, 14, 16,
17, 19, and 20).
[min]
[%]
1
50
>99
[
[
b]
c]
5
20
0
>99
>99
1
2
3
4
50
100
96
II
Recyclability of Fe O @SiO -SH-Pd
3
4
2
After the first cycle of the reaction, the catalyst was recovered
with the help of a magnet, successively washed with distilled
water (to remove an excess of base) and ethanol, and then
dried at room temperature ready for the next cycle. The car-
bonylative cross-coupling reactions of 4-iodoacetophenone
with phenylboronic acid catalyzed by recycled catalyst are dis-
played in Table 3.
50
45
[
d]
92
5
6
7
50
50
50
97
>99
97
The hydrogenation reaction of 4-bromonitrobenzene was
also catalyzed by Fe O -SH-Pd and Fe O -NH -Pd (Table 4).
3
4
3
4
2
Again, the catalyst showed good activity, and Pd leaching was
not detected (entry 1). Interestingly, although the amine-func-
tionalized catalyst was also active, Pd leaching was substantial,
0
.017% (entry 2), which illustrated the importance of the thiol-
modified surface for retaining Pd, which was determined by in-
ductively coupled plasma mass spectrometry.
8
9
50
98
Conclusions
50
90
99
99
We have developed a novel, practical and economic heteroge-
neous catalytic system for the Suzuki carbonylative cross-cou-
pling reaction by using mercaptopropyl-modified MNPs that
are supported on Pd as the catalyst in anisole under an atmos-
pheric pressure of carbon monoxide. This novel Pd catalyst can
be conveniently prepared by the general method, it exhibits
higher activity and selectivity for the Suzuki carbonylative cou-
pling reaction and hydrogenation reactions and could be
reused five times without significant loss in catalytic activity
and selectivity. Our catalyst also avoids the use of phosphine li-
gands, which renders it more environmentally friendly.
1
0
1
1
90
92
[a] Reaction conditions: 20 mg catalyst,
H
2
atmosphere (p=1 atm),
1
mmol substrate in 5 mL ethanol. [b] Yield after 5 runs. [c] Hydrogena-
3 4 2
tion with 10% Pd/C. [d] Hydrogenation with 20 mg 8.43% Fe O -NH -Pd.
Carbonylative Suzuki coupling reaction
The activity of Pd complexes immobilized on Fe O @SiO was Experimental Section
3
4
2
tested in the Suzuki carbonylative coupling reactions of aryl
halides, namely the reactivity of aryl halides with arylboronic
acids with K CO as a base under balloon pressure of CO in
Synthesis of the hydrophobic magnetite nanoparticles
Fe O NPs)
(
3
4
2
3
anisole. Firstly, the Suzuki carbonylative reaction of iodoben-
zene with phenylboronic acid was chosen as the model reac-
tion to evaluate the reaction time. The catalyst was active with
nearly 92% conversion in 8 h, and the products of the diphen-
yl ketone were highly selective (Table 2, entries 1–3). Further
investigation of the substrate scope was performed under op-
timized reaction conditions. As shown in Table 2, various aryl
iodides and arylboronic acids were examined and good func-
The Fe O NPs were prepared by using a published method with
3 4
[24]
a slight modification.
Firstly, FeCl ·6H O (4.8 g), FeCl ·4H O
3 2 2 2
(
(
2.0 g), and oleic acid (1.0 mL)were added to deionized water
40 mL) under vigorous stirring. Secondly, the mixture solution was
purged with nitrogen gas for 30 min. The mixture solution was
then heated to 908C. Finally, ammonium hydroxide (10 mL,
2
8 wt%) was added rapidly to the solution, and it immediately
turned black. The reaction was kept at 908C for 2.5 h. The black
precipitate was obtained with the help of a magnet and resus-
pended in cyclohexane for further processing.
II
tional group tolerance of the Fe O @SiO -SH-Pd catalytic
3
4
2
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Synthesis of silica-coated MNPs
(Fe O @SiO )
Table 2. Suzuki carbonylative coupling reaction of various aryl iodides with arylboronic acids in the presence
of the Fe O @SiO -SH-Pd catalyst.
3 4 2
II
[a]
3
4
2
The Fe O @SiO nanoparticles were
3
4
2
synthesized by a reverse microe-
mulsion procedure developed by
[25]
Rossi et al.
Triton X-100 (50 g)
[
b]
was dispersed in cyclohexane
(500 mL) by sonification. Then,
Entry
R
1
R
2
Reaction time
Yield
[%]
[h]
1
2
Fe O4 NPs (200 mg, 5.0 mL from
a 40 mgmL solution in cyclohex-
ane), and ammonium hydroxide
3
À1
1
2
3
4
5
6
7
8
9
H
H
H
H
H
H
H
H
4
6
8
8
8
8
8
8
8
8
8
12
12
12
6
6
6
72
87
91
81
85
93
91
93
92
94
99
89
88
92
97
97
99
91
96
86
28
13
9
19
15
6
(10 mL, 28 wt%) were added, and
4-CH
4-Cl
H
4-CH
4-Cl
H
4-CH
4-Cl
H
4-CH
4-Cl
H
4-CH
4-Cl
H
4-CH
4-Cl
3
3
3
3
3
3
the mixture was stirred violently at
room temperature. After 30 min,
tetraethyl orthosilicate (6.0 mL,
TEOS) was added drop by drop
and the resulting solution was
kept under mechanical agitation
for 24 h. The resulting products
were precipitated with methanol
and collected by centrifugation
and then washed three times with
ethanol. Finally, the product was
dried under vacuum at 508C
overnight.
4-CH
3
4-CH
4-CH
3
7
7
8
5
3
4-CH
4-CH
4-CH
3
3
3
O
O
O
10
1
1
0.3
12
13
14
15
16
17
18
19
20
2-NH
2-NH
2-NH
2-NO
2-NO
2-NO
2
2
2
2
2
2
11
11
7
0.4
2
<0.1
9
4-CH
4-CH
4-CH
3
3
3
CO
CO
CO
6
6
6
2
13
Synthesis of mercaptopropyl-
modified silica-coated MNPs
[
(
2 3
a] The reactions were performed with aryl iodide (0.5 mmol), arylboronic acid (0.6 mmol), CO (1 atm), K CO
1.5 mmol), and Pd catalyst (1 mol%) in anisole (5 mL) at 808C. [b] Determined by GC.
(
Fe O @SiO -SH)
3 4 2
The functionalization of the
Fe O @SiO nanoparticles with eth-
3
4
2
ylenediamine groups was per-
formed by adding mercaptopropyl
trimethoxysilane (0.5 mL) to dry
toluene (200 mL) that contained
Fe O @SiO nanoparticles (500 mg).
Table 3. The carbonylative cross-coupling reaction of 4-iodoacetophenone with phenylboronic acid catalyzed
by recycled catalyst.
[
a]
3
4
2
The resulting solution was heated
at reflux for 24 h and then washed
with toluene and acetone. The ob-
tained solid material was dried
under vacuum at 508C for 24 h.
Recycle
Catalyst cycle
Yield
%]
[
1
5
1
5
91
89
[
(
2 2 3
a] The reactions were performed with 4-iodoacetophenone (0.5 mmol), PhB(OH) (0.6 mmol), CO (1 atm), K CO
Loading of Pd on mercapto-
propyl-modified silica coated
1.5 mmol), and Pd catalyst (1 mol%) in anisole (5 mL) at 808C.
II
MNPs Fe O @SiO SH-Pd
3
4
2
The
above-synthesized
Fe O @SiO -SH (2.7 g) was mixed
3
4
2
with PdCl (0.226 g, 1.27 mmol) in dry acetone (150 mL). The mix-
2
3 4
Table 4. Fe O -modified-Pd catalysts for the hydrogenation reaction.
ture was heated at reflux for 24 h under an argon atmosphere. The
solid product was filtered by suction, washed three times with ace-
tone, distilled water and acetone successively, and dried at 508C
Entry Substrate
Catalyst
loading)
Reaction Yield Pd leaching
time
(
under vacuum for 24 h to give the palladium complex Fe O @SiO -
[min]
[%] [%]
3
4
2
II
SH-Pd . The weight percentage of Pd in the catalyst, as determined
by atomic absorption spectroscopic analysis, was 3.81 wt%.
Fe
reused 5 times
3.81 %)
3 4
O -SH-Pd
1
2
50
45
96
92
n.d.
(
0
Fe -NH
reused 5 times
8.43 %)
3
O
4
2
-Pd
Preparation of the Pd catalyst
0.017
0
II
The Pd catalyst was prepared by reduction of a Pd precursor,
(
which was obtained by using NaBH in ethanol solution. Typically,
4
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II
Fe O @SiO -SH-Pd (100 mg) was added to ethanol (20 mL) that
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contained NaBH4 (10 mg). Thereafter, the material was washed
twice with distilled water and acetone and dried under vacuum.
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8
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Typical procedure for the carbonylative Suzuki coupling
A mixture of aryl iodide (0.5 mmol), arylboronic acid (0.6 mmol),
K CO (1.5 mmol), and 1 mol% Pd catalyst in anisole (5 mL) were
2
3
[
stirred at 808C under a 1 atm pressure of CO. After the reaction,
the mixture was cooled to room temperature, separated by mag-
netic decantation, and the resultant residual mixture was diluted
with H O (10 mL); this was followed by extraction twice (2ꢁ10 mL)
2
with ethyl acetate. The organic fraction was dried over MgSO , the
4
solvents evaporated under reduced pressure and the residue redis-
solved in ethanol (5 mL). An aliquot was taken with a syringe and
subjected to GC analysis. Yields were calculated against consump-
tion of the aryl iodides.
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Keywords: CÀC coupling · green chemistry · heterogeneous
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