Agglomeration of Pd0 nanoparticles causing different catalytic activities of Suzuki carbonylative cross-coupling reactions catalyzed by PdII and Pd0 immobilized on dopamine-functionalized magnetite nanoparticles

Article abstract of DOI:10.1039/c4nj02285h

Solvent-dispersible magnetite nanoparticles (Fe₃O₄) end-functionalized with amino groups were successfully prepared by a facile one-pot template-free method to immobilize Pd^II and Pd⁰ using a metal adsorption and reduction procedure. They were characterized by TEM, XRD, XPS, FT-IR and VSM. Interestingly, the Pd^II catalyst exhibited better catalytic activity for carbonylative cross-coupling reactions than the Pd⁰ catalyst. According to the catalytic activities of a variety of arylboronic acids and aryl iodides catalyzed by two kinds of Pd catalysts, the proposed reaction mechanism of Suzuki carbonylative cross-coupling reactions using the Pd catalyst was also inferred. More importantly, agglomeration of Pd⁰ nanoparticles was obviously observed in the TEM images of the catalysts after reactions. Therefore, agglomeration of Pd⁰ nanoparticles should be considered as a significant reason for different catalytic activities of the reactions catalyzed by immobilized Pd^II and Pd⁰ catalysts. Furthermore, the Pd^II catalyst revealed high efficiency and stability during recycling stages.

Full text of DOI:10.1039/c4nj02285h

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ARTICLE TYPE
Agglomeration of Pd0 nanoparticles causing different catalytic activities
Ⅱ
of Suzuki carbonylative cross-coupling reactions catalyzed by Pd and
Pd0 immobilized on dopamine-functionalized magnetite nanoparticles
Yu Long, Kun Liang, Jianrui Niu, Xin Tong, Bing Yuan, Jiantai Ma*
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
Solvent-dispersible magnetite nanoparticles (Fe3O4) end-functionalized with amino groups were
Ⅱ
successfully prepared by a facile one-pot template-free method to immobilize Pd and Pd0 with a metal
adsorption and reduction procedure. They were characterized by TEM, XRD, XPS, FT-IR and VSM.
Ⅱ
10 Interestingly, Pd catalyst exhibited better catalytic activity for carbonylative cross-coupling reactions
than Pd0 catalyst. According to the catalytic activities of a variety of arylboronic acids and aryl iodides
catalyzed by two kinds of Pd catalysts, proposed reaction mechanism of Suzuki carbonylative cross-
coupling reactions by Pd catalyst were also inferred. More importantly, agglomeration of Pd0
nanoparticles was obviously observed in the TEM images of the catalysts after reactions. Therefore,
15 agglomeration of Pd0 nanoparticles should be considered as a significant reason for different catalytic
Ⅱ
Ⅱ
activities of the reactions catalyzed by immobilized Pd and Pd0 catalysts. Furthermore, the Pd catalyst
revealed high efficiency and stability during recycling stages.
groups may serve as functional groups suitably for the
Introduction
immobilization of Pd. Thus, interest in preparation of shell coated
magnetic Fe3O4 particles functionalized with hydroxyl, carboxyl
50 and primary amino groups is increasing dramatically.9
Unfortunately, conventional synthetic processes of core-shell
particles usually require tedious synthetic steps, long reaction
time and toxic solvent. Therefore, developing a simple and facile
approach to obtain functionalized magnetic materials is
55 desperately needed.
Synthesis of biaryl and heteroaryl carbonyl compounds has
20 attracted considerable interest, since these compounds are
important moieties in many biologically active molecules, natural
products, and pharmaceuticals.1 A variety of methods have been
reported for their preparation.2 Among them, Suzuki
carbonylative coupling reaction is one of the most promising
25 routes for the direct synthesis of biaryl ketones from carbon
monoxide, as aryl halides and arylboronic acids with various
functionalities can be tolerated on either partner and arylboronic
acids are generally nontoxic and thermally-, air- and moisture-
stable.3 In recent years, many palladium-based homogeneous
30 catalysts possess many merits, such as high reaction rate, high
turnover number and efficient selectivity.4 However, one of the
greatest drawbacks of such catalysis is that the products might be
contaminated by metalleaching.5 Additionally, Pd is not common
and cheap enough for widespread applications. So it is a big
35 challenge to avoid Pd leach, improve the efficiency and
increase recycle rate. A heterogeneous Pd-catalyst can efficiently
solve these problems since heterogeneous system is facile to be
handled and recovered. Therefore, supported Pd-catalysts can
offer high activity and selectivity in the carbonylative coupling
40 reactions.6
In the past few years, dopamine (DA) had been used for surface
modification of unfunctionalized Fe3O4 nanoparticles in the
5b
literature.10,
Among them, one-pot solvothermal synthesis
dopamine-functionalized magnetite nanoparticles are promising
60 as catalyst carrier due to their favorable solvent-dispersibility and
10a
facile template-free preparation method.5b,
DA with amino
groups was used as a surfactant as well as an interparticle linker
instead of using the traditional surfactants. In this paper, we
Ⅱ
stabilized Pd and Pd0 on Fe3O4/DA with a metal adsorption and
65 reduction procedure due to the covalent bonding between Pd and
amine groups on the surface of Fe3O4/DA act as a robust anchor
and avoid Pd leaching. Furthermore, the dopamine improved the
Ⅱ
dispersibility of the nanoparticles in aromatic solution. The Pd
and Pd0 catalysts were both used to catalyze Suzuki carbonylative
70 coupling reaction under mild conditions. According to the
catalytic activities of the two kinds of Pd catalysts, proposed
reaction mechanism and influence factors of Suzuki
carbonylative cross-coupling reactions by Pd catalyst were also
inferred.
In recent years, there has been an increasing trend toward the use
of magnetically retrievable materials in a variety of areas.7 Fe3O4
as a superior magnetic material, has been used in efficient green
chemical synthesis and biomedical sciences.8 Nevertheless it was
45 difficult to be used directly, so surface modification with active
groups was indispensable for further applications. Primary amino
75 Experimental
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Characterization
K2CO3 (1.5 mmol), and 2 mol% palladium catalyst in anisole (5
mL) was stirred at 80 ℃ under 1 atm pressure of CO. After the
reaction, the mixture was cooled down to room temperature,
separated by magnetic decantation, and the resultant residual
60 mixture was diluted with 10 mL of H2O, followed by extraction
with ethyl acetate (2×10 mL). The organic fraction was dried
with MgSO4, the solvents were evaporated under reduced
pressure and the residue was redissolved in 5 mL of ethanol. An
aliquot was taken using a syringe and subjected to GC analysis.
65 Yields were calculated against the consumption of the aryl
iodides.
These magnetic materials were characterized inductively coupled
plasma (ICP), powder X-ray diffraction (XRD), transmission
electron microscopy (TEM), X-ray photoelectron spectroscopy
(XPS), fourier transform-infrared (FT-IR) and vibrating sample
magnetometry (VSM). XRD measurement was performed on a
Rigaku D/max-2400 diffrctometer using Cu-Kα radiation as the
X-ray source in the 2θ range of 10–80°. The size and morphology
of the magnetic nanoparticles were observed by a Tecnai G2 F30
5
10 transmission electron microscopy and samples were obtained by
placing a drop of a colloidal solution onto a copper grid and
evaporated in air at room temperature. Magnetic measurement of
Ⅱ
Fe3O4, Fe3O4/DA, Fe3O4/DA-Pd and Fe3O4/DA-Pd0 was
Result and Discussion
investigated with
a
Quantum Design vibrating sample
Catalyst preparation and characterization
15 magnetometer (VSM) at room temperature in an applied
magnetic field sweeping from -15 to 15 kOe. FTIR of the MNCs
was examined by using a 170SX spectrometer in the range of
400–4000 cm-1. XPS was recorded on a PHI-5702 instrument and
the C1s line at 284.8 eV was used as the binding energy reference.
20 Synthesis of the DA modified magnetic nanoparticles
(Fe3O4/DA)
Typically5b, 10a, FeCl3 (0.973 g) was dissolved in ethylene glycol
(30mL) to form a clear solution, followed by the addition of
NaOAc (1.97 g) and DA (56.9 mg). The mixture was stirred
25 vigorously for 30 min and then sealed in a Teflon-lined stainless-
steel autoclave (50 mL capacity). Then the autoclave was heated
to and maintained at 200 ℃ for 12 h, and allowed to cool to room
temperature. The black products were washed several times with
ethanol and dried at 50℃ for 24 h.
Ⅱ
Scheme 1 Preparation of Fe3O4/DA-Pd and Fe3O4/DA-Pd0 catalysts
70
Ⅱ
The process for the preparation of Fe3O4/DA-Pd and Fe3O4/DA-
Ⅱ
30 Loading of Pd on DA functionalized magnetic nanoparticles
Ⅱ
(Fe3O4/DA-Pd )
Pd0 catalysts is schematically described in Scheme 1. Briefly,
Fe3O4/DA magnetite nanoparticles were first prepared by a facile
200 mg of as-synthesised Fe3O4/DA nanoparticles were first
dispersed in a 100mL ethanol solution under ultrasonication for
0.5 h. The formed black suspension was ultrasonically mixed
35 with a 10 ml PdCl2 (0.01 M) solution for 1 h, and then the mixed
solution (0.01 M) was vigorously stirred for 2 h. The products
were obtained with the help of a magnet, washed thoroughly with
deionized water and ethanol then dried in a vacuum at room
one-pot solvothermal synthetic strategy and then immobilized
Ⅱ
75 Pd and Pd0 by the amine groups on the surface of nanoparticles
acting as a robust anchor and avoid Pd leaching.
Ⅱ
temperature overnight. The loading level of Pd in Fe3O4/DA-Pd
40 catalyst was measured to be 3.42 % by AAS.
Loading of Pd0 on DA functionalized magnetic nanoparticles
(Fe3O4/DA-Pd0)
200 mg of as-synthesised Fe3O4/DA nanoparticles were first
dispersed in a 100mL ethanol solution under ultrasonication for
45 0.5 h. The formed black suspension was ultrasonically mixed
with a 10 ml PdCl2 (0.01 M) solution for 1 h, and then an excess
100 ml NaBH4 solution (0.01 M) was slowly dropped into the
above mixture with vigorous stirring. After 6 h of reduction, the
products were obtained with the help of a magnet, washed
50 thoroughly with deionized water and ethanol then dried in a
vacuum at room temperature overnight. The loading level of Pd
in Fe3O4/DA-Pd0 catalyst was measured to be 4.17 % by AAS.
General procedure for Suzuki carbonylative coupling
reactions
Ⅱ
Fig. 1 TEM images of (a, b) Fe3O4/DA, (c, d) Fe3O4/DA-Pd and (e, f)
55 A mixture of aryl iodide (0.5 mmol), arylboronic acid (0.6 mmol),
Fe3O4/DA-Pd0
2
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Figures 1a and 1b show the typical TEM image of Fe3O4/DA
nanoparticles prepared by the versatile solvothermal reaction. It
could be indicated that the Fe3O4 modified with dopamine was
highly dispersed with a spherical shape and a nearly uniform size
of approximate 350 nm in diameter. The TEM image exhibited
5
Ⅱ
that the Fe3O4/DA-Pd (Fig. 1c, d,) and Fe3O4/DA-Pd0 (Fig. 1e, f),
catalyst didn’t change considerably after attachment of the
palladium onto the surface of the magnetic nanoparticles. From
Figure 1c, d, it could be revealed that after fastening PdCl2, no
10 changes of the morphology of these nanoparticles had occurred,
and none of Pd or PdOx nanoparticles were observed in Fig. 1d.
As shown in Figure 1e, f, it could also be concluded that the
palladium particle size was cantered at about 8 nm. In order to
Ⅱ
Ⅱ
Fig. 3 XRD patterns of (a) Fe3O4/DA, (b) Fe3O4/DA-Pd and (c)
give a powerful evidence of the existence of Pd and Pd0, EDX
Fe3O4/DA-Pd0
Ⅱ
15 spectra of Fe3O4/DA-Pd and Fe3O4/DA-Pd0 was presented in Fig.
2. The peak of copper (Cu) arised from Cu grid in TEM
analysis. The peaks corresponding to Pd were expressly found in
The compositions of Fe3O4/DA-Pd composite were confirmed by
40 FT-IR (Fig. 4). For comparison, FT-IR of Fe3O4/DA was also
shown in Figure 4a. The bands at 1628, 1473 and 872 cm-1 were
associated with amine, indicating that plenty of DA molecules
were immobilized on the surface of the nanoparticles. The FT-IR
Ⅱ
both EDX spectra of Fe3O4/DA-Pd and Fe3O4/DA-Pd0, and Pd
content was 3.35% and 4.26%, respectively, which was almost
20 the same with the content measured by AAS.
Ⅱ
spectrum of the Fe3O4/DA-Pd and Fe3O4/DA-Pd0 catalyst (in
45 Fig. 4b, c) demonstrated that almost no change occured after
immobilization of palladium on the magnetite nanoparticle
surface. Stretching vibrations took place blue shift or red shift,
which exhibited PdCl2 and Pd nanoparticles were fastened on the
supporter.
50
Ⅱ
Fig. 4 FT-IR spectra of (a) Fe3O4/DA, (b) Fe3O4/DA-Pd and (c)
Fe3O4/DA-Pd0
Figure 5 presented XPS elemental survey scans of the surface of
the Fe3O4/DA-Pd
Ⅱ
Fig. 2 EDX spectra of (a) Fe3O4/DA-Pd and (b) Fe3O4/DA-Pd0
Ⅱ
and Fe3O4/DA-Pd0 catalyst. Peaks
55 corresponding to oxygen, carbon, nitrogen, palladium and iron
were clearly observed. To establish the oxidation state of Pd in
The crystalline structures of the resulting products were
investigated by XRD. In Figure 3, the main peaks of the
Ⅱ
Fe3O4/DA-Pd and Fe3O4/DA-Pd0, XPS analysis was conducted.
Ⅱ
25 Fe3O4/DA-Pd and Fe3O4/DA-Pd0 composites were similar to the
Ⅱ
The Pd 3d5/2 and Pd 3d3/2 binding energies of Fe3O4/DA-Pd
Fe3O4/DA. The characteristic diffraction peaks in the samples at
2θ of 30.1º, 35.5º, 43.3º, 53.8º, 57.1º, and 62.8º were
corresponded to the diffraction of (220), (311), (400), (422),
(511) and (440) of the Fe3O4. All the diffraction peaks matched
30 with the magnetic cubic structure of Fe3O4 (JCPDS 65-3107)11
and the sharp and strong peaks confirmed the products were well
crystallized. Figure 3c showed that apart from the original peaks,
the appearance of the new peaks at 2θ= 39.8, 46.1 and 67.8 were
attributed to the Pd0 species, implying that the Pd nanoparticles
35 had been successfully immobilized onto the surface of Fe3O4/DA.
were determined to be 338.8 and 344 eV, respectively (Fig. 5b).
60 These values agreed with the PdII binding energy of the
composite. The binding energy of Pd 3d5/2 and 3d3/2 for the
Fe3O4/DA-Pd0 was found to be 334.8 and 340 eV (Fig. 5d),
indicating that the loaded Pd nanoparticles were in its 0 state.
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15
Fig. 6 Room temperature magnetization curves of (a) Fe3O4/DA, (b)
Ⅱ
Fe3O4/DA-Pd and (c) Fe3O4/DA-Pd0
Catalyst testing for the Suzuki carbonylative cross-coupling
reactions
In order to explore the optimal reaction conditions, including
20 temperature, solvent and base, the carbonylative cross-coupling
reaction of iodobenzene with phenylboronic acid under an
atmospheric pressure of carbon monoxide was chosen as the
model reaction. As shown in Table 1, the yields of the target
Ⅱ
product catalyzed by both Fe3O4/DA-Pd and Fe3O4/DA-Pd0
25 were low at 40℃ and 60℃ (entry 1-4). When the temperature
was increased to 80 ℃, high yields of the target product (93%
and 79%) were obtained (Table 1, entry 5, 6). However, the
yields of the desired product (91% and 77%) did not increase at
100℃ (Table 1, entry 7, 8). Therefore, 80 ℃ was chosen as the
30 optimum reaction temperature. According to that both less polar
solvents (anisole, dioxane and toluene) and polar solvent (DMF)
obtained lower yields (Table 1, entry 9-14), non-polar solvent
anisole was found to be the best choice of solvent. Under the
optimized reaction temperature and solvent, several bases were
35 examined, such as NaOAc, K3PO4, Cs2CO3 and K2CO3 (Table 1,
entry 2, 15-20), due to the inorganic bases used in the
carbonylative Suzuki coupling reaction could affect the
selectivity of the products. As a result, the most used base K2CO3
was the most efficient base to produce carbonylative coupling
Ⅱ
40 product. Therefore, whether using Fe3O4/DA-Pd or Fe3O4/DA-
Pd0 as catalyst, the optimized reaction conditions are: 80 ℃,
anisole, and K2CO3. Interestingly, it was obviously revealed that
Ⅱ
Fe3O4/DA-Pd always had a higher yield than Fe3O4/DA-Pd0 for
Ⅱ
Fig. 5 XPS spectra of (a, b) Fe3O4/DA-Pd and (c, d) Fe3O4/DA-Pd0
the carbonylative cross-coupling reaction of iodobenzene with
45 phenylboronic acid under any reaction conditions.
Ⅱ
The magnetic properties of Fe3O4/DA, Fe3O4/DA-Pd and
Ⅱ
In addition, the prepared Fe3O4/DA-Pd and Fe3O4/DA-Pd0
Fe3O4/DA-Pd0 were investigated with
a VSM at room
catalysts revealed similar catalytic activities compared with
literature datas on application of the amino-functionalized
magnetite nanoparticles supported palladium catalysts as shown
5
temperature. As shown in Figure 6, magnetization curves
revealed the superparamagnetic behaviour of the magnetic
nanoparticles and the magnetic saturations of these were 81.6,
74.1 and 72.5 emu/g, respectively. The decrease of the saturation
Ⅱ
50 in Table 1, entry 21-24. It could be concluded that supported Pd
catalysts exhibited higher catalytic activities for the carbonylative
cross-coupling reaction than supported Pd0 catalysts, so were our
prepared catalysts.
Ⅱ
magnetization suggested the presence of Pd or Pd0 on the
10 surface of the magnetic support. Even with this reduction in the
saturation magnetization, the nanomaterials still could be
efficiently separated from the solution with a magnet near the
vessels (shown in the insert in Fig. 6).
4
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Ⅱ
In order to investigate catalytic acivities of Fe3O4/DA-Pd and
Table 1 The carbonylative cross-coupling of iodobenzene with
phenylboronic acid under different conditionsa.
Fe3O4/DA-Pd0 for carbonylative cross-coupling reaction,
a
5
variety of arylboronic acids and aryl iodides was tested under
optimized reaction conditions, and the results were shown in
Ⅱ
Table 2. As expected, Fe3O4/DA-Pd always had a higher
Entry Solvent
Base
Catalyst
Yieldb (%)
9
T/℃
40
40
60
60
80
80
100
100
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
conversion and target yield than Fe3O4/DA-Pd0 for the
carbonylative cross-coupling reaction of iodobenzene with
Ⅱ
1
2
Anisole
Anisole
Anisole
Anisole
Anisole
Anisole
Anisole
Anisole
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Fe3O4/DA-Pd
10 phenylboronic acid whether using arylboronic acids and aryl
iodides containing either electron-withdrawing groups or
electron-donating groups (Table 2, entry 1–30). And the
carbonylative cross-coupling reactions could proceed smoothly
under mild conditions to afford the corresponding carbonylative
Fe3O4/DA-Pd0
3
Ⅱ
3
46
Fe3O4/DA-Pd
4
Fe3O4/DA-Pd0
35
Ⅱ
15 coupling products in high yields by Pd catalyst. Furthermore, it
Ⅱ
5
93
Fe3O4/DA-Pd
was important to receive that aryl iodides substituted with
electron-withdrawing groups, such as 4-CH3CO and 2-NO2
(Table 2, entry 7–18), were found to be the most active reagents,
since highest TOF was obtained. And since nitryl has stronger
20 electron-withdrawing ability than acetyl, aryl iodides substituted
with 4-CH3CO had a higher TOF than aryl iodides substituted
(Table 2, entry 7–18), were found to be the most active reagents,
since highest TOF was obtained. And since nitryl has stronger
electron-withdrawing ability than acetyl, aryl iodides substituted
25 with 4-CH3CO had a higher TOF than aryl iodides substituted
with 2-NO2 (Table 2, entry 7–18). In the contrast, aryl iodides
substituted with electron-donating groups, such as 4-CH3 and 4-
CH3O (Table 2, entry 19–30), exhibited relatively inferior
properties. On the other hand, arylboronic acids with different
30 substituents also afforded different yields of target products,
especially, had distinct effect for the generation of by-products
(Table 2, entry 1–30). It could be concluded that aryl boronic acid
containing the electron-attracting group, -Cl, produced the
highest yeilds of target products as well as the unacceptable direct
35 coupling products (by-product). Meanwhile, arylboronic acid
containing the electron-donating group, –CH3, also showed the
lowest yields of target products as well as the unacceptable by-
product.
6
Fe3O4/DA-Pd0
79
Ⅱ
7
91
Fe3O4/DA-Pd
8
Fe3O4/DA-Pd0
77
Ⅱ
9
Toluene K2CO3
Toluene K2CO3
Dioxane K2CO3
Dioxane K2CO3
51
Fe3O4/DA-Pd
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Fe3O4/DA-Pd0
42
Ⅱ
76
Fe3O4/DA-Pd
Fe3O4/DA-Pd0
69
Ⅱ
DMF
DMF
K2CO3
K2CO3
55
Fe3O4/DA-Pd
Fe3O4/DA-Pd0
52
Ⅱ
Anisole NaOAc
Anisole NaOAc
Anisole Cs2CO3
Anisole Cs2CO3
<1
<1
56
Fe3O4/DA-Pd
Fe3O4/DA-Pd0
Ⅱ
Fe3O4/DA-Pd
Fe3O4/DA-Pd0
41
Ⅱ
Anisole
Anisole
Anisole
Anisole
Anisole
Anisole
K3PO4
K3PO4
K2CO3
K2CO3
K2CO3
59
Fe3O4/DA-Pd
Fe3O4/DA-Pd0
Fe3O4/PPy–PdII
49
92c
91d
97e
90f
Fe3O4@SiO2-SH-PdII
Fe3O4@PNAI-PdII
K2CO3 hollow Fe3O4-NH2-Pd0 90
a The reaction was carried out with 0.5 mmol of iodobenzene (0.5 mmol),
arylboronic acid (0.6 mmol), CO (1 atm), base (1.5 mmol), solvent (5
mL), 2 mol% palladium catalyst and reaction time: 8h.
b Determined by GC or GC-MS.
c [Ref. 9e] The reaction was carried out with 0.5 mmol of iodobenzene
(0.5 mmol), arylboronic acid (0.6 mmol), CO (1 atm), base (1.5 mmol),
solvent (5 mL), 2 mol% palladium catalyst and reaction time: 8h.
d [Ref. 3e] The reaction was carried out with 0.5 mmol of iodobenzene
(0.5 mmol), arylboronic acid (0.6 mmol), CO (1 atm), base (1.5 mmol),
Pd catalyst (1 mol %) in anisole (5 mL) and reaction time: 8h.
e [Ref. 1d] The reaction was carried out with 0.5 mmol of iodobenzene
(0.5 mmol), arylboronic acid (0.6 mmol), CO (1 atm), base (1.5 mmol),
Pd catalyst (1 mol %) in anisole (5 mL) and reaction time: 8h.
f [Ref. 12] The reaction was carried out with 0.5 mmol of iodobenzene
(0.5 mmol), arylboronic acid (0.6 mmol), CO (1 atm), base (1.5 mmol),
Pd catalyst (1 mol %) in anisole (5 mL) and reaction time: 12h.
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Table 2 The carbonylative cross-coupling reactions of various aryl
iodides with arylboronic acids in the presence of the catalystsa.
Yieldb (%)
Time
(h)
Conversion TOFc
Entry
R1
R2
Catalyst
(%)
(h-1)
1
2
5
Ⅱ
d
1
2
3
4
5
6
7
8
9
H
H
H
8
8
8
8
8
8
6
6
6
6
6
6
6
6
6
6
6
6
93
79
89
75
93
77
94
80
90
78
91
79
96
83
92
78
95
85
98
87
92
82
99
88
97
88
93
84
95
89
97
90
93
83
97
91
5.81
4.94
5.56
4.69
5.81
4.81
7.83
6.67
7.50
6.50
7.58
6.58
8.0
Pd
H
Pd0 e
8
Ⅱ
H
H
4-CH3
4-CH3
4-Cl
4-Cl
H
3
Pd
Pd0
7
Scheme 2 The proposed reaction mechanisms for catalytic cycles of
5
carbonylative cross-coupling reaction
Ⅱ
H
6
Pd
In order to account for this result, according to the reaction
mechanisms proposed in the literature1d,6c,6e,9e and combining
with the results in Table 2, the proposed reaction mechanism was
inferred in Scheme 2. This mechanism scheme was similar with
H
Pd0
11
3
Ⅱ
4-CH3CO
4-CH3CO
4-CH3CO
Pd
H
Pd0
8
10 previous, however we came up with a more particular and
distinguishing exposition as followed. Firstly, in the step 1, Pd0
Ⅱ
4-CH3
4-CH3
4-Cl
4-Cl
H
3
Pd
Ⅱ
was inserted to form Pd between -ph and -I. Perhaps, the
10 4-CH3CO
11 4-CH3CO
12 4-CH3CO
Pd0
6
existence of electron-withdrawing groups could reduce electron
cloud density of the bond between ph- and I-, resulting in easier
15 formation of bonding pair between phenyl and palladium, thus
promoted the oxidative addition of Pd (0). On the contrary, the
existence of electron-donating groups went against this step.
Hence, aryl iodides substituted with electron-withdrawing groups
exhibited higher yields than aryl iodides substituted with
20 electron-donating groups (Table 2, entry 1–30). Secondly,
intermediate product was faced with two competition processes,
that was step 2 and step 5 as shown in Scheme 2. This
competition process was mainly determined by the ability (or rate)
of inserting CO and direct coupling with arylboronic acids. Under
25 the same local concentration of CO, arylboronic acids substituted
with electron-withdrawing groups were more likely to form direct
coupling products, due to the relatively easier connection with
phenyl of arylboronic acids caused by electron-withdrawing
inductive effect. So, arylboronic acids substituted with electron-
Ⅱ
4
Pd
Pd0
10
<1
7
Ⅱ
13
14
15
16
17
18
2-NO2
2-NO2
2-NO2
2-NO2
2-NO2
2-NO2
Pd
H
Pd0
6.92
7.67
6.50
7.92
7.08
Ⅱ
4-CH3
4-CH3
4-Cl
4-Cl
<1
5
Pd
Pd0
Ⅱ
2
Pd
Pd0
6
Ⅱ
19
20
21
22
23
24
25
26
27
28
4-CH3
4-CH3
H
H
8
8
8
8
8
8
8
8
8
8
89
74
84
68
88
72
86
71
79
62
3
6
92
80
86
73
94
82
89
76
81
68
5.56
4.63
5.25
Pd
Pd0
Ⅱ
4-CH3
4-CH3
4-CH3
4-Cl
4-Cl
H
2
Pd
4-CH3
Pd0
5
4.25 30 withdrawing groups exhibited higher yields of direct coupling
products than aryl iodides substituted with electron-donating
groups, while the aryl iodides substituted with electron-
withdrawing groups was diametrical (Table 2, entry 1–30).
Thirdly, in step 3, -I was replaced by the phenyl of arylboronic
35 acids to form carbonylative coupling intermediate product with
the aid of K2CO3. The substituent effect of this step was found to
be similar with the step 5. Finally, in step 4 and 6, Pd0 catalyst
separated from the complex to form target product and by-
product, respectively.
Ⅱ
4-CH3
6
5.50
Pd
4-CH3
Pd0
10
3
4.50
Ⅱ
4-CH3O
4-CH3O
4-CH3O
4-CH3O
5.38
4.44
4.94
3.88
Pd
H
Pd0
5
Ⅱ
4-CH3
4-CH3
2
Pd
Pd0
6
Ⅱ
29
30
4-CH3O
4-CH3O
4-Cl
4-Cl
8
8
87
65
5
92
78
5.44
4.40
Pd
40 As shown in Scheme 2, catalytic cycles of Suzuki carbonylative
cross-coupling reactions were started from the formation of Pd0.
Unfortunately, it could be apparently found that Fe3O4/DA-Pd0
with Pd0 exhibited much more inferior catalytic performance than
Pd0
13
a The reactions were carried out with iodobenzene (0.5 mmol),
arylboronic acid (0.6 mmol), CO (1 atm), K2CO3 (1.5 mmol), anisole (5
mL) and 2 mol% palladium catalyst.
Ⅱ
Ⅱ
Fe3O4/DA-Pd with Pd (Table 2, entry 1–30). To our best
45 knowledge, this peculiar catalytic property could be ascribed to
that Pd0 nanoparticles are prone to conglomerate in reactions. In
b Determined by GC or GC-MS.
c TOF: tmoles of 1-compound yield per mole of Pd per hour.
Ⅱ
order to prove our conjecture, TEM images of Fe3O4/DA-Pd and
Ⅱ
d Fe3O4/DA-Pd . e Fe3O4/DA-Pd0.
Fe3O4/DA-Pd0 after Suzuki carbonylative cross-coupling reaction
6
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Page 7 of 9
New Journal of Chemistry
were shown in Figure 7 (a, b). Compared with Pd0 nanoparticles
of Fe3O4/DA-Pd0 before the reaction (Fig. 2e, 2f), Pd0
nanoparticles of Fe3O4/DA-Pd0 after the first cycle of Suzuki
carbonylative cross-coupling reaction occurred obvious
agglomeration (Fig. 7b). And after 5 cycles of the reaction, the
agglomeration of Pd0 nanoparticles became more severe (Fig. 7d).
5
Ⅱ
To the contrary, morphology of Fe3O4/DA-Pd could be basically
considered to maintain invariable after the reactions. It is well
known that agglomeration of nanoparticles will significantly
10 reduce the utilization of palladium, thus the relatively inferior
catalytic performance of Fe3O4/DA-Pd0 is acceptant.
Ⅱ
Fig. 8 XPS spectra of Fe3O4/DA-Pd after 5 cycles of the reaction
35
On the other hand, in the catalytic cycles of Suzuki carbonylative
Ⅱ
Ⅱ
cross-coupling reactions catalyzed by Pd catalyst, Pd had to be
firstly reduced to Pd0 by CO to activate this cycle. In our opinion,
this step may be a relatively fast process comparing with steps 1-
Ⅱ
Fig. 7 TEM images of (a) Fe3O4/DA-Pd and (b) Fe3O4/DA-Pd0 after the
first cycle of Suzuki carbonylative cross-coupling reaction, (c) Fe3O4/DA-
Ⅱ
Pd and (d) Fe3O4/DA-Pd0 after 5 cycles of the reaction
40 6, rather than a rate determining step for the catalytic cycle.
Importantly, compared with Pd0, the extra process of reducing
15
Ⅱ
As shown in Fig. 7c, some kind of nanoparticles could be
Pd certainly led large amount of CO to be absorbed onto the
Ⅱ
observed faintly in the TEM image of Fe3O4/DA-Pd after 5
cycles of the reaction, which might be Pd(0) nanoparticles as
Pd(Ⅱ) should be reduced to Pd(0) to start the carbonylative
surface of catalyst, resulting in the increase of local concentration
of CO. And, high local concentration of CO might be beneficial
45 to obtain target product. Therefore, as shown in Table 2, the TOF
Ⅱ
20 cross-coupling reaction. In order to ascertain the state of the
and yields of target products catalyzed by Fe3O4/DA-Pd were
Ⅱ
invariably higher than that by Fe3O4/DA-Pd0 no matter what
substrates were tested.
palladium in Fe3O4/DA-Pd after 5 cycles of the reaction, the
recycled catalyst’s XPS spectra had been conducted as shown in
Fig. 8. It was interesting to observe three peaks assigned to Pd3d
with binding energies of 334.8, 339.2 and 344 eV, respectively
25 (Fig. 8b). The XPS result proved the existence of both Pd(Ⅱ) and
Furthermore, we investigated the recyclability of the Fe3O4/DA-
Ⅱ
50 Pd and Fe3O4/DA-Pd0. After the first cycle of the reaction, the
catalyst was recovered with the help of a magnet, successively
washed with distilled water (to remove excess of base), ethanol,
and dried at room temperature ready for the next cycle. The
carbonylative cross-coupling reaction of iodobenzene with
55 phenylboronic acid catalyzed by recycled catalyst was displayed
Ⅱ
Pd(0) in the Fe3O4/DA-Pd after 5 cycles of the reaction. This
could testify the reliability of the proposed reaction mechanism in
Scheme 2. On the other hand, since only a part of Pd(Ⅱ) was
reduced to Pd(0), the amount of Pd0 nanoparticles in Fe3O4/DA-
Ⅱ
30 Pd catalyst was much less than Pd0 nanoparticles in Fe3O4/DA-
Ⅱ
in Figure 9. After five times cycles, Fe3O4/DA-Pd still
Pd0, which might be unable to reunite. Therefore, the
maintained high activity with an acceptable decrease. The weight
Ⅱ
Ⅱ
and Fe3O4/DA-Pd0,
agglomeration of Pd(0) nanoparticles in Fe3O4/DA-Pd catalyst
after 5 cycles of the reaction is not obvious.
percentage of Pd in Fe3O4/DA-Pd
determined by AAS analysis, was 3.15 % and 3.72 %, affording
60 Pd losses of 7.8% and 10.8%, respectively. Meanwhile, after 5
cycles of the reaction, the agglomeration of Pd0 nanoparticles was
Ⅱ
very severe (Fig. 7d). It could be concluded that the Pd of
Ⅱ
Fe3O4/DA-Pd was more stable than the Pd0 of Fe3O4/DA-Pd0.
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DOI: 10.1039/C4NJ02P28a5gHe 8 of 9
Notes
45 aState Key Laboratory of Applied Organic Chemistry, College of
Chemistry and Chemical Engineering, Lanzhou University, Lanzhou,
Gansu 730000, P. R. China. Fax: +86-931-891258; Tel: +86-931-
8912577; E-mail: majiantai@lzu.edu.cn
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Ⅱ
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Ⅱ
10 aromatic solution. Contrary to expectation, Pd catalyst exhibited
better catalytic activity for carbonylative cross-coupling reactions
than Pd0 catalyst. And substituent group played an important role
in the reaction. For instance, both aryl iodides and arylboronic
acids substituted with electron-withdrawing groups were
15 beneficial to improve yield of target product, unfortunately, also
increase yield of direct coupling product. According to the
catalytic activities of a variety of arylboronic acids and aryl
iodides catalyzed by the two kinds of Pd catalysts, we came up
with a particular and distinguishing exposition about the proposed
20 reaction mechanism of Suzuki carbonylative cross-coupling
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Pd0 nanoparticles was obviously observed in the TEM images of
the catalysts after the reaction. And after 5 cycles of the reaction,
the agglomeration of Pd0 nanoparticles was very severe.
25 Therefore, agglomeration of Pd0 nanoparticles should be
considered as a significant reason for different catalytic activities
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80
85
Ⅱ
of the reactions catalyzed by immobilized Pd and Pd0 catalysts.
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90
Ⅱ
reducing Pd to Pd0 certainly led large amount of CO to be
30 absorbed onto the surface of catalyst, resulting in the increase of
local concentration of CO. Thus, high local concentration of CO
Ⅱ
95
might be beneficial to obtain target product. Furthermore, the Pd
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recycling stages. We envisaged that our work demonstrated an
35 acceptable reason for the difference of catalytic activities
Ⅱ
catalyzed by supported Pd and Pd0. It could enable further
100
implications in a number of other heterogeneous catalytic systems
as well.
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