A palladium-nanoparticle and silicon-nanowire-array hybrid: A platform for catalytic heterogeneous reactions

Article abstract of DOI:10.1002/anie.201308541

We report the development of a silicon nanowire array-stabilized palladium nanoparticle catalyst, SiNA-Pd. Its use in the palladium-catalyzed Mizoroki-Heck reaction, the hydrogenation of an alkene, the hydrogenolysis of nitrobenzene, the hydrosilylation of an α,β-unsaturated ketone, and the C-H bond functionalization reactions of thiophenes and indoles achieved a quantitative production with high reusability. The catalytic activity reached several hundred-mol ppb of palladium, reaching a TON of 2 000 000. An array of reactions: A silicon-nanowire-array-stabilized palladium-nanoparticle catalyst, SiNA-Pd, was designed and used in the palladium-catalyzed Mizoroki-Heck reaction. It can catalyze a variety of other reactions and has a high reusability. Copyright

Full text of DOI:10.1002/anie.201308541

Angewandte
Chemie
DOI: 10.1002/anie.201308541
Nanocatalyst
Hot Paper
A Palladium-Nanoparticle and Silicon-Nanowire-Array Hybrid:
A Platform for Catalytic Heterogeneous Reactions**
Yoichi M. A. Yamada,* Yoshinari Yuyama, Takuma Sato, Shigenori Fujikawa, and
Yasuhiro Uozumi
Abstract: We report the development of a silicon nanowire
array-stabilized palladium nanoparticle catalyst, SiNA-Pd. Its
use in the palladium-catalyzed Mizoroki-Heck reaction, the
hydrogenation of an alkene, the hydrogenolysis of nitro-
benzene, the hydrosilylation of an a,b-unsaturated ketone, and
the C-H bond functionalization reactions of thiophenes and
indoles achieved a quantitative production with high reusabil-
ity. The catalytic activity reached several hundred-mol ppb of
palladium, reaching a TON of 2000000.
D
evelopment of highly active and reusable solid catalysts is
among the most important topics not only for organic
syntheses but also for chemical and pharmaceutical process.
Innovative nanodevices for catalytic transformations are
expected to realize instantaneous, selective catalytic reaction
systems.[1] One approach is to downsize the reaction field/
space from macroscopic to microscopic scale (e.g., from flask
size to microreactor size) to achieve high reactivity and novel
selectivity with safety.[2,3] Another is to develop a closed
porous space, such as mesoporous silica materials or metal–
organic frameworks (MOFs), in which heterogeneous cata-
lysts are attached.[4] While acknowledging the pioneering
work in this area, we believe that it remains difficult to create
a novel nanospace/field-mediated catalytic transformation
system exhibiting high catalytic activity, reusability, safety,
and selectivity as well as macroscopic accessibility of a large
Scheme 1. Preparation of the SiNA-stabilized Pd nanoparticle catalyst
(SiNA-Pd).
amount of substrates and reactants under mild and aqueous
conditions.
We envisioned that the development of hybrid catalysts of
palladium nanoparticles[5] and a silicon nanowire array
(SiNA) as a macroscopic and nanoscopic hybrid catalyst
would be promising for this purpose (Scheme 1). The silicon
nanowire array, such as those used in silicon-based optoelec-
tronics, fuel cells, solar cells, and photoelectrodes,[6] was
readily obtained by the metal-assisted chemical etching of
silicon wafers.[7] Copious nanospaces can be provided on the
surface of a silicon wafer whose area is on the order of square
centimeters. The hybrid catalysts should be equipped with
confined nano-size reaction fields surrounded a lot of Pd
nanoparticles (which should entropically drive the organic
transformation in the nanospaces) on the square centimeter
sized silicon wafer (which should afford plenty of reaction
capacity).
Herein, we report a new platform for the catalytic
reactions, a silicon nanowire array-stabilized palladium-nano-
particle catalyst, SiNA-Pd. Its use in the palladium-catalyzed
Mizoroki–Heck reaction, where the quantitative production
of coupling compounds was achieved with 490 molppb
(0.000049 mol%) Pd, is presented. The hybrid catalyst was
readily reused without the loss of catalytic activity. Moreover,
SiNA-Pd promoted the hydrogenation of an alkene, the
hydrogenolysis of nitrobenzene, the hydrosilylation of an a,b-
[*] Dr. Y. M. A. Yamada, Y. Yuyama, Dr. T. Sato, Dr. Y. Uozumi
RIKEN Center for Sustainable Resource Science
Wako, Saitama 351-0198 (Japan)
E-mail: ymayamada@riken.jp
Dr. Y. Uozumi
Institute for Molecular Science (IMS)
Myodaiji, Okazaki, Aichi 444-8787 (Japan)
Dr. S. Fujikawa
Kyushu University, International Institute for Carbon-Neutral Energy
Research (WPI-I2CNER) Fukuoka 819-0395 (Japan)
[**] We are grateful to Prof. Yousoo Kim (RIKEN) for useful discussion
on silicon surface chemistry. We thank Dr. Tetsuo Honma, Dr.
Masafumi Takagaki (JASRI, SPring-8), and Prof. Hikaru Takaya
(Kyoto University) for measurement of XAFS (SPring-8, BL14B2,
2012B1868). We also thank RIKEN for elemental (Dr. Yoshio
Sakaguchi), XPS (Dr. Aiko Nakao), TEM (Tomoka Kikitsu), ICP-AES
(Chieko Kariya) and SEM/EDX (Aya Ohno) analyses, and prepara-
tion of SiNA (Yasuhiro Suzuki). We gratefully acknowledge financial
support from JST ACT-C, JST CREST, JSPS (no 24550126, no
20655035 and no 2105), Takeda Science Foundation, the Naito
Foundation, and RIKEN.
À
unsaturated ketone, and the C H bond functionalization
reactions of thiophenes and indoles.
The silicon nanowire array-stabilized Pd nanoparticle
catalyst was prepared as follows (Scheme 1):[8,9] A p-type
silicon wafer was treated with H2SO4/H2O2, and aqueous HF
À
for cleaning and installation of Si H surface groups (H-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201308541.
termination), respectively.[10] AgNO3 reacted with the H-
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Table 1: Mizoroki-Heck reaction with SiNA-Pd.[a]
Entry
1 or 4 (R1)
2 (R2)
3
Yield [%]
1
1a (H)
1a
1a
1a
1a
2a (CO2Bu)
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2b (Ph)
2b (Ph)
2b (Ph)
2b (Ph)
2b (Ph)
2a
3a
3a
3a
3a
3a
3a
3b
3c
3d
3e
3 f
3g
3h
3i
3j
3k
3l
3a
3c
3d
3e
3 f
3g
98 (99[d])
96 (47[d])
97 (9[d])
97[e]
95
98
99
94
99
88
85
98
72
72
84
83
81
2 (2th use)
3 (4th use)
4 (6th use)
5 (8th use)
6(10th use)
7
8
9
10
11
12
13
14
15
16
17
1a
1b (CH3)
1c (MeO)
1d (CF3)
1e (CH3CO)
1 f (NO2)
1-I-naph 1g
1a
1c
1d
1e
1g
4a (H)
4b (MeO)
4c (CF3)
4d (CH3CO)
4e (NO2)
1-Br-naph 4 f
18[b]
19[b]
20[b,c]
21[b,c]
22[b,c]
23[b]
82
70
87
92
90
89
2a
2a
2a
2a
Figure 1. a) SEM, b) SEM/EDX, c) XPS, d) TEM, e) XANES, and f) FT-
2a
EXAFS of SiNA-Pd.
[a] 1 (1.0 molequiv), 2 (2.0 molequiv), Et3N (3.0 molequiv), TBAA
(0.2 molequiv), SiNA-Pd (0.3 mol%), 1-BuOH, 1008C, 24 h. [b] 4
(1.0 molequiv), 2a (1.3–2.0 molequiv), NaOAc (1.2 molequiv), TBAB
(0.2–1.0 molequiv), SiNA-Pd (0.3 mol%), 1408C, 24–48 h. [c] Using
DMA as a solvent. [d] The yields in parenthesis in entries 1–3 are from
the use of a Pd nanoparticle catalyst on a non-etched flat silicon wafer.
[e] Pd was not detected in the reaction mixture (ICP-AES).
terminated wafer to give an Ag nanoparticle-coated wafer
that was treated with aqueous HF/H2O2, yielding the Ag
nanoparticle-deposited silicon nanowire array. Removal of
Ag nanoparticles and regeneration of Si-H surface of the
silicon nanowire array were carried out with HNO3 and
aqueous HF, respectively. Immobilization of Pd nanoparticles
was performed with K2PdCl4 on the silicon nanowire array to
obtain the silicon nanowire array-stabilized palladium nano-
particle catalyst (SiNA-Pd). An SEM image of the section of
SiNA-Pd (Figure 1) revealed that the length and width of its
nanospace were 5 mm and less than 800 nm, respectively,
where the aspect ratio was approximately 60. BET analysis
indicated that the specific surface area of SiNA-Pd was
30 times larger than the original flat silicon wafer (SBET
117 cm2/cm2 vs. 4 cm2/cm2 (Kr absorption-BET analysis). An
SEM/EDX image (Figure 1b) of the section of SiNA-Pd
showed that the installation of Pd onto the silicon nanowire
array was located mainly on the upper part of the array. A
TEM image of SiNA-Pd (Figure 1d) showed the dispersion of
Pd nanoparticles with a diameter of approximately 5–10 nm.
Pd3d XPS of Pd nanoparticles suggested the formation of
zero-valent Pd species. Pd K-edge XANES (Figure 1e) of
SiNA-Pd, Pd0 foil, and PdIICl2 supported the suggestion that
the Pd species in SiNA-Pd was zero valent. FT-EXAFS of
SiNA-Pd and Pd foil (Figure 1) showed signals corresponding
to Pd–Pd bonds.[11] The peak at 2.5 ꢀ was attributed to the
first neighboring shell of Pd metal. The FT-EXAFS intensity
of SiNA-Pd was lower than that of Pd foil, which suggests the
formation of Pd nanoparticles on SiNA. These results indicate
nanowires on which the resulting Pd0 nanoparticles are
readily immobilized.
We applied SiNA-Pd to the Mizoroki–Heck reaction.[12–16]
The Mizoroki-Heck reaction is among the most important
coupling reactions for synthetic organic and process chemistry
including pharmaceuticals, agricultural chemicals, and sophis-
ticated materials.[17] Thus, the reaction of iodobenzene (1a)
and butyl acrylate (2a) was carried out with SiNA-Pd
(0.3 mol% Pd) with triethylamine and tetrabutylammonium
acetate (TBAA) in 1-butanol at 1008C. The coupling
proceeded smoothly to give butyl cinnamate (3a) with 98%
yield (Table 1, entry 1). Electron-rich and electron-deficient
aromatic iodides 1b–g were readily converted to the corre-
sponding cinnamates 3b–g with 85–99% yield, respectively
(entries 7–12). The coupling of 1a–g and styrene (2b) also
proceeded smoothly to give the corresponding stilbenes 3h–
l with 72–84% yield (entries 13–17). Moreover, aryl bromides
were also suitable substrates for the coupling. When the
reaction of bromobenzene (4a) and 2a was performed with
sodium acetate and tetrabutylammonium bromide (TBAB),
the reaction proceeded to give 3a with 82% yield (entry 18).
The coupling of a variety of aryl bromides 4b–f and 2a led to
the formation of the corresponding cinnamates 3c–g with 70–
92% yield (entries 19–23).
that K2PdCl4 is reduced to Pd0 by Si H on the silicon
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Since SiNA-Pd efficiently promoted the Mizoroki–Heck
reaction, recycling of SiNA-Pd was investigated under similar
conditions. After the reaction in Entry 1 was completed,
SiNA-Pd was recovered, and reused for the identical reac-
tions. SiNA-Pd was reused nine times without loss of catalytic
activity, giving 98% yield of 3a in the 10th use of the catalyst.
Pd leaching was investigated in the reaction mixture in the 6th
use of SiNA-Pd and no leaching of Pd was detected under
ICP-AES analysis (Entry 4).[18] The hot-filtration test showed
the termination of the reaction by use of the filtrate (Fig-
ure S1, Supporting Information). The results strongly sug-
gested that SiNA-Pd worked as a heterogeneous catalyst.[19]
In contrast, when a Pd nanoparticle catalyst on a non-etched
flat silicon wafer was used under similar conditions, the
catalytic activity significantly decreased to give 3a in 99%
(1st use), 47% (2nd use), and 9% (4th use), respectively.
A SEM image of the reused SiNA-Pd showed that the
morphology of the catalyst did not change during the reaction
(Figure S1). These results indicated that silicon nanowire
array stabilized the palladium nanoparticles to provide
a highly active and reusable catalyst.
À
Scheme 3. C H bond functionalization reactions of thiophenes and
indoles.
SiNA-Pd was applied to a variety of organic transforma-
tions. The hydrogenation of an alkene, stilbene (5), proceeded
in the presence of SiNA-Pd (0.3 mol% Pd) in EtOH under
hydrogen (1 atm) to give 1,2-diphenylethane (6) with 99%
yield. The catalyst was readily recovered and reused to afford
6 in 99% (2nd use) and 99% (3rd use) yield. The hydro-
genolysis of nitrobenzene (7) was performed to give aniline
(8) with 99% yield. The hydrosilylation of an a,b-unsaturated
aldehyde 9 and Et3SiH was carried out to give the enolsilyl
ether 10 with 82% yield (Scheme 2).[20]
Scheme 4. Mizoroki–Heck reaction with 490 molppb (0.000049 mol%)
Pd of SiNA-Pd, and the synthesis of Ozagrel, an antiasthmatic agent.
5-phenylthiophene (12a) with 80% yield.[22,23] The C H bond
À
functionalization of 1a with 2-ethylthiophene (11b) afforded
2-ethyl-5-phenylthiophene (12b) with 81% yield. The cou-
pling of 1a with indoles 13a,b also proceeded under similar
conditions to give the corresponding indoles 14 in good
yields.[24]
To attain the highest catalytic activity for the heteroge-
neous catalyst-promoted Mizoroki–Heck reaction, SiNA-Pd
with 490 mol ppb Pd (0.000049 mol% Pd) was used for the
reaction of the 10-gram scale substrate (Scheme 4). When the
reaction of 1a (10.2 g) and 2a was performed with 490 mol
ppb Pd of SiNA-Pd, TBAA, Et3N at 1608C for 48 h, the
desired product 3a was obtained with 95% yield. The
turnover number (TON) and turnover frequency (TOF)
were two million and 40000 hÀ1, respectively. To our knowl-
edge, this is the highest TON for the Mizoroki–Heck reaction
with heterogeneous catalysts.
Scheme 2. Hydrogenation of stilbene, hydrogenolysis of nitrobenzene,
and hydrosilylation of an a,b-unsaturated aldehyde.
À
The development of C H bond functionalization reac-
Ozagrel 17, an important antiasthmatic agent (thrombox-
ane A2 synthesis inhibitor),[25] was synthesized by the
490 molppb Pd SiNA-Pd-catalyzed Mizoroki–Heck reaction.
Thus, the reaction of 4-iodobenzylalcohol (15) (11.7 g) and 2a
was carried out with 490 molppb Pd of SiNA-Pd under similar
conditions to give the cinnamate 16 with 71% yield.
Installation of an imidazole unit, alkaline hydrolysis, and
À
tions (C H bond activation) is an important topic, thus we
À
applied SiNA-Pd for the C H bond functionalization reac-
tions of thiophenes and indoles (Scheme 3).[21] Thus, the
reaction of 1a with 2-methylthiophene (11a) was carried out
with SiNA-Pd (0.3 mol%) and CsOAc in DMF, we were
pleased to find that the reaction proceeded to give 2-methyl-
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acidification provided ozagrel hydrochloride 17 with 50%
yield (3 steps).
In conclusion, we have developed a highly active and
reusable novel catalytic platform SiNA-Pd, a silicon nanowire
array stabilized palladium-nanoparticle catalyst. SiNA-Pd
was readily applied to the Mizoroki–Heck reaction, the
hydrogenation, hydrogenolysis, and hydrosilylation as well as
À
the C H bond functionalization reactions of thiophenes and
indoles. A use of SiNA-Pd in the Mizoroki–Heck reaction
achieved a quantitative production of coupling compounds
with several hundred-mol ppb of palladium, reaching a TON
of 2000000. The hybrid catalyst was readily reused without
the loss of catalytic activity.
Received: October 1, 2013
Published online: November 15, 2013
[11] The fitting of k3-weighted FT-EXAFS of SiNA-Pd with a face-
centered cubic Pd model (K. Baba, U. Miyagawa, K. Watanabe,
Y. Sakamoto, T. B. Flanagan, J. Mater. Sci. 1990, 25, 3910 – 3916)
was performed in the range of 2.0 – 4.8 ꢀ including up to the
third coordination shells. Fitting parameters are as follows: R-
factor= 0.013, S02 = 0.69(6), DE0 = 1.3(9), Rmult = À0.0032(12)
(defined as DR = Rmult ꢄ Reff for all shells), s2(1) = 0.0059(4),
s2(2) = 0.011(2), s2(3) = 0.011(1).
À
Keywords: C H bond functionalization · Mizoroki–
Heck reaction · ozagrel · palladium nanoparticle ·
silicon-nanowire array
.
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