Surfactant-free synthesis of palladium nanoclusters for their use in catalytic cross-coupling reactions

Article abstract of DOI:10.1039/c1cc11487e

Surfactant-free Pd nanoclusters (Pd NCs) (size: 1-1.5 nm) showed high catalytic activity in the Suzuki-Miyaura cross-coupling and Mizoroki-Heck reactions. The Pd NCs had a high turnover number, up to 6.0 × 10 ⁸, which can be recycled at least five times without loss of catalytic activity.

Full text of DOI:10.1039/c1cc11487e

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COMMUNICATION
Surfactant-free synthesis of palladium nanoclusters for their use in
catalytic cross-coupling reactionsw
Megumi Hyotanishi, Yuto Isomura, Hiroko Yamamoto, Hideya Kawasaki and
Yasushi Obora*
Received 15th March 2011, Accepted 5th April 2011
DOI: 10.1039/c1cc11487e
Surfactant-free Pd nanoclusters (Pd NCs) (size: 1–1.5 nm)
showed high catalytic activity in the Suzuki–Miyaura cross-
coupling and Mizoroki–Heck reactions. The Pd NCs had a high
NCs; the maximum emission wavelengths were around 460 nm
at a UV excitation of 350 nm (Fig. 1, right). This photo-
luminescence may have originated from the size-dependent effect
of the metal clusters when the free electrons are confined to the
8
turnover number, up to 6.0 Â 10 , which can be recycled at least
10
five times without loss of catalytic activity.
Fermi wavelength in the cluster conduction band. The high-
resolution transmission electron microscope (HRTEM) image of
the Pd NCs shows numerous nanoparticles of size approximately
1–1.5 nm (Fig. 1, left), and these small nanoparticles
were confirmed to be Pd species by transmission electron
microscopy-energy dispersive X-ray (TEM-EDX) analysis
(Fig. S2, ESIw).
Small colloidal metal nanoclusters (NCs) (o2 nm) have attracted
much interest in catalytic applications because of their high
surface-to-volume ratio and their high surface energy that makes
1
their surface atoms very active, which have different properties
from metal nanoparticles with larger sizes (3–100 nm). To date,
various methods for the preparation of colloidal metal nano-
2
particles (NPs) have been reported, but they additionally suffer
The Pd NCs prepared above were evaluated as catalysts for
11 12
Suzuki–Miyaura and Mizoroki–Heck cross-coupling reactions,
from significant corrosion and stability problems. Metal NPs are
which are versatile methods for carbon–carbon bond formation
in organic synthesis. The results for the Suzuki–Miyaura cross-
coupling reaction are shown in Table 1. For instance, the
reaction of iodotoluene (1a: 0.5 mmol) with phenylboronic acid
(2a: 0.75 mmol) was performed in the presence of a Pd NCs
prone to loss of reactivity because they precipitate or aggregate as
3
bulk metals, so stabilizers such as functionalized polymers,
4
5
dendrimers, inorganic solids (e.g., carbon, metal oxides, sol–gel
6
clays, and zeolites), ligands (e.g., pincer ligands), or ionic
7
surfactants are generally used in their preparation, and utilized
2 3
catalyst (0.2 mol% based on 1a used) and K CO (0.5 mmol) in
3,4a–d,5b,6,7a,8
as catalysts in cross-coupling reactions.
Therefore,
NMP/H O (1 : 1, 2 mL) at 100 1C for 5 h, giving 3a in 86% yield
2
we need to develop an innovative preparation method for
surfactant-free and stable metal nanoclusters with minimum
surface deactivation for the highly active catalyst. Here, we
report the first example of a solution synthesis of DMF-protected
Pd NCs with sizes of 1–1.5 nm using a surfactant-free DMF
(entry 1, Table 1). When the catalyst loading was further
decreased to 0.002 mol%, the reaction proceeded efficiently with
81% yield (entry 2, Table 1). Next, the solvent was evaporated
under vacuum from Pd NCs (1 mM) synthesized in DMF,
and the residue was redissolved in selected solvents such as
N-methylpyrrolidone (NMP), methanol (MeOH), and tetra-
hydrofuran (THF). When Pd NCs using NMP as the solvent
were used, the reaction was promoted to give 3a in quantitative
yield and 95% isolated yield (entry 3, Table 1). The reaction
using MeOH was also efficient, giving 92% yield (entry 4,
Table 1). In contrast, THF was less effective in this reaction,
9
reduction method. We found substantially enhanced catalytic
activity of the Pd NCs in Suzuki–Miyaura, and Mizoroki–Heck
cross-coupling reactions, and the Pd NCs had a high turnover
5
8
number, up to 4.5 Â 10 and 6.0 Â 10 , respectively. The Pd NCs
catalysts can be recycled at least five times without loss of
catalytic activity in the Suzuki–Miyaura cross-coupling reaction.
The preparation of Pd NCs is as follows. A solution of 150 mL
2
of 0.1 M aqueous PdCl was added to 15 mL of DMF at 140 1C;
the DMF solution was heated under reflux at 140 1C for 6 h.
The resulting yellow solution contained photoluminescent Pd
Department of Chemistry and Materials Engineering, Faculty of
Chemistry, Materials and Bioengineering, Kansai University, Suita,
Osaka 564-8680, Japan. E-mail: obora@kansai-u.ac.jp;
Fax: +81-6-6339-4026; Tel: +81-6-6368-0876
w Electronic supplementary information (ESI) available: TEM image,
UV-visible absorption spectra and photoluminescence emission spectra;
experimental and characterization data and original H and C NMR
Fig. 1 Left: HRTEM image of DMF-protected Pd NCs. Bar length:
1
13
2
nm (Â4 500 000); right: a photograph of colloidal Pd NCs (1 mM in
spectra for products 3a–3i and 5a–5h. See DOI: 10.1039/c1cc11487e
DMF) under UV light.
5
750 Chem. Commun., 2011, 47, 5750–5752
This journal is c The Royal Society of Chemistry 2011

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Table 1 Pd-NCs-catalyzed Suzuki–Miyaura cross-coupling reaction
of iodotoluene (1a) with phenylboronic acid (2a)
Table 2 Pd-NCs-catalyzed Suzuki–Miyaura cross-coupling reaction
of aryl iodides (1) with various arylboronic acids (2)
a
a
1
Entry 1 (R =)
2
2 (R =)
b
Yield (%)
c
4
b
Yield 3a (%) TON
c
TON (Â10 )
3b >5.0
Entry Pd NCs (mol%) Solvent
À1
2
4
4
4
4
5
4
1
2
3
p-CH
p-CF
p-NH
3
O
1b
1c
1d
H
H
H
2a >99 (93)
2a 97 (94)
2a >99 (87)
1
2
3
4
5
6
7
2 Â 10
2 Â 10
2 Â 10
2 Â 10
2 Â 10
2 Â 10
2 Â 10
DMF/H
DMF/H
NMP/H
MeOH/H
THF/H
NMP/H
NMP/H
2
O
O
O
86
81
>99 (95)
4.3 Â 10
4.1 Â 10
>5.0 Â 10
4.6 Â 10
2.8 Â 10
4.5 Â 10
4.9 Â 10
À3
À3
À3
À3
À4
À3
3
3c 4.8
3d >5.0
2
2
2
2
O
92
56
89
86
2
O
O
O
4
5
1e
1f
H
H
2a
87 (75)
3e
4.6
2
d
2
2a
2b
82 (69)
87 (79)
2c >99 (88)
75 (73)
3f
4.7
4.5
3h >5.0
3i 3.8
a
Conditions: 1a (0.5 mmol) was allowed to react with 2a (0.75 mmol)
in the presence of 1 mL of Pd NCs catalyst solution (1 mM–1 mM) and
6
7
8
p-CH
p-CH
p-CH
3
1a p-Cl
1a p-CH
1a m-NO
3g
3
3
O
K
2
CO
3
(0.5 mmol) in solvent (1 mL)/H
2
O (1 mL) at 100 1C for
3
2
2d
b
h. GC yields based on 1a used. The number in parenthesis shows
5
the isolated yield. TON = 3a (mol)/Pd NCs (mol). Bromotoluene
was used instead of iodotoluene.
a
b
Conditions: same as given in entry 3, Table 1. GC yields based on 1
c
d
c
used. The numbers in parentheses show the isolated yields. TON = 3
mol)/Pd NCs (mol).
(
and a 56% yield was obtained (entry 5, Table 1). The Pd NCs
showed efficient catalytic activity, and the highest turnover
5
number (TON, 4.5 Â 10 ) was achieved in the presence of
À4
2
 10 mol% of the Pd NCs catalyst (entry 6, Table 1). When
bromotoluene was used instead of iodotoluene, the reaction
4
generated 3a in 86% yield and the TON was 4.9 Â 10
(
entry 7, Table 1).
Fig. 2 Depiction of the catalyst-recycling sequence in the Suzuki–
Miyaura cross-coupling reaction under the reaction conditions given
in entry 4, Table 1. Step 1: reaction mixture containing 1a, 2a, Pd NCs,
methanol, and water before the Suzuki–Miyaura cross-coupling
reaction began; Step 2: reaction mixture when the reaction was
finished (first time); Step 3: reaction mixture when n-hexane
(8 mL) was added and stirred (upper: organic layer containing 3a,
bottom: aqueous layer containing Pd NCs); Step 4: aqueous layer
containing Pd NCs after removal of the organic layer; this layer can be
used for the next catalytic sequence.
When the reaction time course was examined, the reaction rate
for Pd NCs in NMP was faster than that for Pd NCs in DMF.
5
The highest turnover frequency achieved was 1.7 Â 10 for 1 h.
To expand the scope of the reaction substrate, various aryl
iodides and different arylboronic acids were used in the
Suzuki–Miyaura cross-coupling reaction, under the same
conditions as for entry 3 in Table 1, as shown in Table 2.
The reactions of aryl iodides bearing both electron-donating
and electron-withdrawing substituents, such as –OMe, –CF
and –NH , with phenylboronic acid gave rise to the corres-
3
2
ponding biphenyls, 3b–3d, in excellent yields (entries 1–3,
Table 2). 1-Iodonaphthalene and 2-iodothiophene also reacted
with 2a to produce the aryl derivatives 3e and 3f in good yields
To further examine the efficacy of the Pd NCs catalyst, the
Mizoroki–Heck reaction of iodobenzene (1g) with ethyl
acrylate (4a) was chosen as a model reaction, and carried
out under various conditions (Table 3). For example, the
reaction of 1g (1 mmol) with 4a (1.2 mmol) was performed
in the presence of the Pd NCs catalyst (0.1 mol% based on 1g
(
entries 4 and 5, Table 2). The Suzuki–Miyaura cross-coupling
reaction of 1a with various substituted phenylboronic acids
proceeded efficiently (entries 6–8).
To investigate the economy of the reaction, we performed
recycling experiments for the Pd NCs catalyst in the
Suzuki–Miyaura cross-coupling reaction. In the first cycle, the
coupling product 3a was obtained in 92% yield under
the conditions given in entry 4, Table 1 (Step 1 to Step 2, Fig. 2).
After performing the reaction under the conditions given in
entry 4, Table 1 (Step 2), the reaction mixture was extracted
with hexane (8 mL Â four times) (Step 3), and the organic
layer containing 3a and unreacted 1a was separated (Step 4).
The resulting aqueous layer containing Pd NCs can be used
for the next catalytic sequence (see the ESIw for the detailed
experimental procedure). The catalyst-recycling methodology
was successfully applied to a second reaction cycle and
afforded 3a in 90% yield. The Pd NCs catalyst could be
recycled at least five times without loss of catalytic activity
to afford 3a in high yields (Fig. 3).
used) and NEt (1 mmol) in DMF (2 mL) at 140 1C for 15 h,
3
giving 5a in quantitative yield (entry 1, Table 3). A TON of
3
over 10 was achieved. When the catalyst loading was further
À3
À5
decreased to 10
mol% and 10
mol%, the reaction
proceeded efficiently with 97% and quantitative yields,
respectively (entries 2 and 3, Table 3).
Next, a DMF solution of Pd NCs was evaporated and the
residue was redissolved in selected solvents such as NMP and
THF. The reaction using NMP as a solvent was not effective
and gave 5a in 30% yield (entry 4, Table 3). In the reaction
using THF, no 5a was produced (Table 3, entry 5).
It is noteworthy that the Pd NCs showed efficient catalytic
8
activity and that the highest TON (6.0 Â 10 ) was achieved in
À7
the presence of 10 mol% of the Pd NCs catalyst (entry 6,
Table 3). The catalytic activity of surfactant-free Pd NCs is
better than that of Pd NCs stabilized with polymer micelles
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 5750–5752 5751

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underwent coupling and afforded the corresponding product
in excellent yields in the presence of a Pd NCs catalyst.
In conclusion, we have demonstrated a method for solution
synthesis of Pd NCs by DMF reduction. The photolumines-
cent Pd NCs can be prepared by a simple, non-supported, and
surfactant-free method, which showed high catalytic activity
for two important carbon–carbon bond forming reactions,
namely the Suzuki–Miyaura and the Mizoroki–Heck cross-
coupling reaction, and excellent TONs were achieved (TONs
5
8
Fig. 3 Multiple catalyst-recycling in the Suzuki–Miyaura cross-
up to 4.5 Â 10 and 6.0 Â 10 , respectively). We have
developed a method for recycling the catalyst at least five
times in the Suzuki–Miyaura cross-coupling reaction.
coupling reaction. Conditions: as given in entry 4, Table 1.
Table 3 Pd-NCs-catalyzed Mizoroki–Heck reaction of iodobenzene
(
This work was supported by the Strategic Project to Support
the Formation of Research Bases at Private Universities
a
1g) with ethyl acrylate (4a) under various conditions
(2010–2014), matching fund subsidy from the Ministry of
Education, Culture, Sports, Science and Technology, Japan.
Notes and references
b
Yield 5a (%)
c
Entry
Pd NCs (mol%)
Solvent
TON
1 (a) Metal Nanoclusters in Catalysis and Materials Science: The
Issue of Size Control, ed. B. Corain, G. Schmid and N. Toshima,
Elsevier, 2007; (b) S. Vajda, M. J. Pellin, J. P. Greeley,
C. L. Marshall, L. A. Curtiss, G. A. Ballentine, J. W. Elam,
S. Catillon-Mucherie, P. C. Redfern, F. Mehmood and P. Zapol,
Nat. Mater., 2009, 8, 213; (c) A. Roucoux, J. Schulz and H. Patin,
Chem. Rev., 2002, 102, 3757, and references therein.
À1
À3
3
5
7
6
1
2
3
4
5
6
10
10
DMF
DMF
DMF
NMP
THF
>99
>1.0 Â 10
>1.0 Â 10
>1.0 Â 10
3.0 Â 10
—
97
>99 (85)
30
À5
À5
10
10
À5
À7
À1
10
10
n.d.
60
23
8
2
DMF
DMF
6.0 Â 10
2
Nanoparticles and Catalysis, ed. D. Astruc, Wiley-VCH,
Weinheim, Germany, 2008.
d
7
10
2.3 Â 10
a
Conditions: 1g (1 mmol) was allowed to react with 4a (1.2 mmol) in
the presence of 1 mL of Pd NCs catalyst solution (1 mM–1 nM) and
3 (a) K. Okamoto, R. Akiyama, H. Yoshida, T. Yoshida and
S. Kobayashi, J. Am. Chem. Soc., 2005, 127, 2125;
(b) H. Oyamada, R. Akiyama, H. Hagio, T. Naito and
S. Kobayashi, Chem. Commun., 2006, 4297; (c) R. Xing, Y. Liu,
H. Wu, X. Li, M. He and P. Wu, Chem. Commun., 2008, 6297;
b
NEt
3
(1 mmol) in DMF (1 mL) at 140 1C for 15 h. GC yields based
on 1g used. The number in parenthesis shows the isolated yield.
c
d
TON = 5a (mol)/Pd NCs (mol). Bromobenzene was used instead
(d) G. Wei, W. Zhang, F. Wen, Y. Wang and M. Zhang, J. Phys.
Chem. C, 2008, 112, 10827.
of iodobenzene.
4
(a) E. H. Rahim, F. S. Kamounah, J. Frederiksen and
J. B. Christensen, Nano Lett., 2001, 1, 499; (b) K. R. Gopidas,
J. K. Whitesell and M. Anne Fox, Nano Lett., 2003, 3, 1757;
Table 4 Pd-NCs-catalyzed Mizoroki–Heck reaction of aryl iodides
1) with olefins (4)
a
(
(
c) T. Mizugaki, T. Kibata, K. Ota, T. Mitudome, K. Ebitani,
K. Jitsukawa and K. Kaneda, Chem. Lett., 2009, 1118;
d) M. Pittelkow, K. Moth-Poulsen, U. Boas and
(
J. B. Christensen, Langmuir, 2003, 19, 7682; (e) K. Yamamoto,
M. Higuchi, S. Shiki, M. Tsuruta and H. Chiba, Nature, 2002, 415,
5
09; (f) C. Ornelas, J. R. Aranzaes, L. Salmon and D. Astruc,
2
4 (R =)
b
Yield 5 (%)
c
Chem.–Eur. J., 2008, 14, 50; (g) G. R. Newkome, E. He and
C. N. Moorefield, Chem. Rev., 1999, 99, 1689.
Entry
1
TON
5
4
5
4
4
4
5
4
5 (a) T. Mitsudome, K. Nose, K. Mori, T. Mizugaki, K. Ebitani,
K. Jitsukawa and K. Kaneda, Angew. Chem., Int. Ed., 2007, 46,
3288; (b) Z. Zhang and Z. Wang, J. Org. Chem., 2006, 71, 7485;
(c) B. Yoon and C. M. Wai, J. Am. Chem. Soc., 2005, 127, 17174.
6 J. L. Bolliger, O. Blacque and C. M. Frech, Chem.–Eur. J., 2008,
14, 7969.
7 (a) G. Zhang, H. Zhou, J. Hu, M. Liu and Y. Kuang, Green Chem.,
2009, 11, 1428; (b) J. Huang, T. Jiang, H. Gao, B. Han, Z. Liu, W. Wu,
Y. Chang and G. Zhao, Angew. Chem., Int. Ed., 2004, 43, 1397.
M. B. Thathager, J. E. Ten Elshof and G. Rothenberg, Angew.
Chem., Int. Ed., 2006, 45, 2886.
9 (a) H. Kawasaki, H. Yamamoto, H. Fujimori, R. Arakawa,
Y. Iwasaki and M. Inada, Langmuir, 2010, 26, 5926;
1
2
3
4
5
1a
1b
1c
1e
1f
1g
1g
1g
CO
CO
CO
CO
CO
CO
CO
Ph
2
2
2
2
2
2
2
Et
Et
Et
Et
Et
4a
4a
4a
4a
4a
4b
4c
4d
>99 (83)
97 (94)
>99 (95)
82 (80)
51 (50)
72 (57)
>99 (92)
87 (70)
5b
5c
5d
5e
5f
5g
5h
5i
>1.0 Â 10
9.7 Â 10
>1.0 Â 10
8.3 Â 10
d
d
5.1 Â 10
t
6
7
8
Bu
Cy
7.7 Â 10
>1.0 Â 10
e
9.7 Â 10
a
b
Conditions: same as given in entry 2, Table 3. GC yields based on 1
8
c
used. The numbers in parentheses show the isolated yields. TON = 5
d
mol)/Pd NCs (mol). NEt
e
(2 mmol) was used. A 9 : 1 mixture of
(
3
5
i and 1,1-diphenylethylene.
(
b) H. Kawasaki, H. Yamamoto, H. Fujimori, R. Arakawa,
M. Inada and Y. Iwasaki, Chem. Commun., 2010, 46, 3759.
10 (a) J. Zheng, J. T. Petty and R. M. Dickson, J. Am. Chem. Soc.,
2003, 125, 7780; (b) S. Link, A. Beeby, S. Fitzgerald,
M. A. Elsayed, T. G. Schaaff and R. L. Whetten, J. Phys. Chem.
B, 2002, 106, 3410; (c) Y. Negishi, Y. Takasugi, S. Sato, H. Yao,
K. Kimura and T. Tsukuda, J. Am. Chem. Soc., 2004, 126, 6518;
reported by Kobayashi and co-workers, which showed highest
5
3a
TON up to 2.8 Â 10 in the reaction of 1g and 4a. When
bromotoluene was used instead of iodotoluene, the reaction
proceeded and 5a was obtained in 23% yield (entry 7, Table 3).
Table 4 shows data for the Mizoroki–Heck reactions
of various aryl iodides (1) with olefins (4) under the same
conditions given in entry 2, Table 3. As in the Suzuki–Miyaura
cross-coupling reaction, various aryl iodides and olefins
(d) A. Dass, J. Am. Chem. Soc., 2009, 131, 11666; (e) C. M. Aikens,
J. Phys. Chem. C, 2008, 112, 19797.
1
1
1 N. Miyaura and A. Suzuki, Chem. Rev., 1995, 95, 2457.
2 The Mizoroki-Heck Reaction, ed. M. Oestreich, Wiley-VCH,
Weinheim, Germany, 2009.
5
752 Chem. Commun., 2011, 47, 5750–5752
This journal is c The Royal Society of Chemistry 2011