Sulfur modification of Au via treatment with piranha solution provides low-pd releasing and recyclable Pd material, SAPd

Article abstract of DOI:10.1021/ja9100084

We have found in the SR-HXPS measurement of Piranha-treated Au(111)/mica that the gold surface underwent sulfur modification during this treatment, which was believed to have only removed impurities from the gold surface. We also successfully developed a practical Pd material, SAPd, whose Pd was immobilized on sulfur-modified Au. With the lowest Pd-releasing levels and high recyclability, this is one of the best Pd materials thus far developed. Because it leaches extremely low levels of Pd into reaction mixtures, removal of the residual Pd is unnecessary using SAPd, even in syntheses involving pharmaceutical ingredients.

Full text of DOI:10.1021/ja9100084

Published on Web 05/12/2010
Sulfur Modification of Au via Treatment with Piranha Solution Provides
Low-Pd Releasing and Recyclable Pd Material, SAPd
Naoyuki Hoshiya,†,| Masahiko Shimoda,‡ Hideki Yoshikawa,§ Yoshiyuki Yamashita,§ Satoshi Shuto,†
and Mitsuhiro Arisawa*,†
Faculty of Pharmaceutical Sciences, Hokkaido UniVersity, Kita 12, Nishi 6, Kita-ku, Sapporo 060-0812 Japan,
National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047 Japan, and NIMS Beamline
Station at SPring-8, National Institute for Materials Science, 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148 Japan
Received December 2, 2009; E-mail: arisawa@pharm.hokudai.ac.jp
Research involving self-assembled monolayers (SAMs) of alkyl-
thiols (RSH) on metal (especially gold) surfaces has rapidly
expanded due to the potential applications of these materials, the
ease of their preparation, and their facile connection between organic
moieties and metal surfaces.1-5 However, since studies on the
thermal stability of SAMs are limited, and the role of the solvent
in the formation of SAMs is not yet well understood, questions
still remain concerning the mechanism of monolayer formation.6,7
Prior to the emergence of SAMs, Piranha solution (Danger! A
strong oxidizer.) had been traditionally used to clean gold
surfaces.5,8-12
In an effort to create reusuable, Pd-leach-free catalysts, many
heterogeneous catalysts have been developed with Pd immobilized
on supports such as activated carbon, inorganic solids, and
polymers.13,14 Yet few of these immobilized Pd catalysts for C-C
bond formations can claim both high recyclability (>10 times) and
low Pd-leaching (<1 ppm). People purify the product several times
and sometimes they use commercially available Pd scavengers,
which increase the cost.
In this report, we offer new insight into the treatment of Au with
Piranha solution to induce sulfur modification. In addition, we show
the application of this process to the development of novel Pd
materials that exhibited both high recyclability and remarkably low
Pd-leaching in the Suzuki-Miyaura coupling.
Figure 1. Sulfur 1s core-level photoemission spectra. Light blue, Na2SO3;
Pink, PdS; Blue, A after 10 times of Suzuki-Miyaura coupling; Green, A
before Suzuki-Miyaura coupling; Red, Piranha-treated Au(111)/mica.
knowledge, this is the first report that Piranha treatment attaches S
atoms to the gold surface. Since the binding energy of the S 1s
peak for Piranha-treated Au(111)/mica is close to that of Na2SO3,
In the course of our research to develop an environmentally
benign catalyst,15 we unexpectedly found in an SR-HXPS (Syn-
chrotron radiation hard X-ray photoelectron spectroscopy) analysis
that Piranha treatment can place a sulfur atom on the gold surface.
The sulfur is expected to be located under the Pd(dba)2/Pd
overlayers. The usual laboratory-XPS is disadvantageous in observ-
ing the underlying sulfur because of its short observation depth.
SR-HXPS with the long observation depth up to 20 nm enables
the high sensitivity detection of the underlying sulfur.16 SR-HXPS
measurements were performed at the beamline BL15XU in the
synchrotron radiation facility SPring-8.17 We measured the SR-
HXPS spectrum of Piranha-treated Au(111)/mica and found, to our
surprise, that the S 1s peak appeared near 2478 eV in the spectrum
(Figure 1 Red line), although no S 1s peak was observed on
Au(111)/mica before the Piranha treatment. These results indicated
that the surface of Au(111)/mica had been modified by a sulfur
species upon Piranha treatment. Although Piranha treatment has
been frequently used to remove impurities from the gold surface,5,8-12
our results show that this treatment could also be employed for
sulfur modification of the gold surface. To the best of our
† Hokkaido University.
Figure 2. Palladium 2p3/2 core-level photoemission spectra. Red, Pd(meal);
Green, A before Suzuki-Miyaura coupling; Blue, A after 10 times of
Suzuki-Miyaura coupling; Pink, PdS.
‡ National Institute for Materials Science.
§ NIMS Beamline Station at SPring-8.
| Furuya Metal Company Limited, Tokyo, 170-0005 Japan.
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7270 J. AM. CHEM. SOC. 2010, 132, 7270–7272
10.1021/ja9100084  2010 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Yields of the Suzuki-Miyaura Coupling Using Pd Materials A-Da
Yield of 3a (%)b
Entry
Pd material
Support
Pd source
first
second
third
fourth
fifth
sixth
seventh
eighth
ninth
10th
1
2
3
4
A
B
C
Au(111)/mica
Au(foil)
Au(mesh)
Au(mesh)
Pd(dba)2
Pd(dba)2
Pd(dba)2
Pd(OAc)2
96
97
>99
>99
95
96
>99
98
97
92
>99
>99
96
89
>99
>99
98
97
>99
>99
99
97
>99
96
>99
97
>99
>99
96
99
>99
97
96
98
>99
99
99
95
>99
>99
D (SAPd)
b Determined by HPLC. a Conditions of Suzuki-Miyaura coupling: 1a (102 mg, 0.5 mmol), 2a (1.5 equiv), K2CO3 (2 equiv), EtOH (3 mL), Pd
material, 80 °C, 12 h.
Table 2. Amount of Released Pd into Reaction Mixture in the Suzuki-Miyaura Coupling of 1a and 2a
Amount of Releasing Pd (ng)b
Entry
Pd material
Pd (µg)a
first
second
third
fourth
fifth
sixth
seventh
eighth
ninth
10th
Total
1
2
3
B
95
80
90
60
60
80
50
70
50
70
40
30
40
60
30
20
110
50
50
30
30
10
550
480
C
D (SAPd) 38 ( 9c 36 ( 15 26 ( 9 25 ( 11 28 ( 23 24 ( 15 28 ( 14 19 ( 8 30 ( 8 25 ( 12 23 ( 13 260 ( 110
a Amount of absorbed Pd on Au substrate. The standard deviation was calculated from 4 sets of samples. b The whole reaction mixture was directly
subjected to ICP-Mass measurement. The standard deviation was calculated from 4 sets of samples. c Pd (%) based on the amount of starting material is
7.0 × 10-2
.
Table 3. Suzuki-Miyaura Coupling of Various Substrates Using Pd Material D
Substrate [Ar-X]
Ar
Boronic acid
Yield of 3 (%)a,c
Entry
X
Y
first
second
third
fourth
fifth
sixth
seventh
eighth
ninth
10th
1b
1a
C6HM5
I
2a
Cl
Me
H
3a
3b
3c
>99
98
(26)
>99
>99
(25)
92
>99
(28)
97
>99
(24)
99
97
(28)
94
>99
(19)
90
97
(30)
92
99
(25)
>99
>99
(23)
94
(47)
89
(36)
94
2
3
I
I
2b
2c
(55)
92
92
92
90
91
91
85
87
91
(43)
99
96
(1307)
93
97
96
96
(36)
99
>99
(295)
95
95
98
92
(54)
96
93
92
99
(116)
80
89
89
4
5
I
I
2d
2a
Ac
Cl
3d
3e
98
96
>99
98
93
97
>99
99
96
99
94
98
>99
97
99
1b
1c
1d
4-NO2C6H4
4-MeOC6H4
4-NO2C6H4
90
6
7
I
I
I
I
2b
2c
2d
2a
Me
H
Ac
Cl
3f
96
99
>99
97
99
90
89
98
96
>99
87
88
98
95
93
99
96
92
97
94
96
98
95
95
98
98
97
>99
99
94
3g
3h
3i
8
9b
96
96
(641)
>99
99
10
11
12b
13
I
I
I
Br
2b
2c
2d
2a
Me
H
Ac
Cl
3j
3k
3l
>99
99
94
>99
96
99
>99
91
99
95
98
99
>99
>99
95
95
>99
>99
94
>99
98
96
>99
96
97
97
3e
>99
(482)
95
96
96
94
>99
>99
>99
>99
>99
>99
14
15
16
1e
1f
1g
4-Ac-C6H4
2-Me-C6H4
2-HO-C6H4
Br
I
I
2a
2b
2a
Cl
Me
Cl
3m
3n
3o
97
97
97
99
93
93
99
94
94
99
92
92
93
92
92
81
91
91
95
99
99
86
95
95
a Isolated yields. b Yields were determined by HPLC. c Numbers in parentheses indicate the amount of releasing Pd(ng) in the solution.
which was used as a reference sample (Light blue line), the sulfur
on Au appears to be oxidized sulfur.
mica and those in the sample A indicate that the sulfur on Au was
reduced and had chemically bonded with Pd during Pd adsorption.18
In Pd 2p core-level photoemission spectra, the peaks from A appear
close to the metallic palladium peak. These results suggest that, in
the case of A, Pd(dba)2 molecules react with the substrate, yielding
zerovalent molecules or metallic palladium (Figure 2).19
Given that sulfur has a high affinity for Pd, Pd(dba)2 was
adsorbed on this sulfur modified Au(111)/mica in xylene (100 °C,
12 h). After the Pd adsorption, the S 1s peak appears at 2470 eV
(Green line), which is almost identical to the binding energy of the
S 1s peak from the S-modified GaAs.15 Significant changes in S
1s binding energies between those in the Piranha-treated Au(111)/
Single crystal Au(111)/mica is very expensive and not versatile.
Consequently, we used Au foil and mesh instead of Au(111)/mica
9
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C O M M U N I C A T I O N S
to prepare B and C according to the Pd-adsorption procedure for
A. When A, B, or C was subjected to the Suzuki-Miyaura coupling
of iodobenzene and 4-chlorophenylboronic acid, the yields for runs
1 to 10 were excellent to quantitative in all cases (Table 1, entries
1-3). D, prepared from Au(mesh) with Pd(OAc)2 as a Pd source,
was also highly active for the Suzuki-Miyaura coupling (entry 4).
These results showed that the Pd materials A, B, C, and D are
highly recyclable.20
into reaction mixtures, removal of the residual Pd is unnecessary
using SAPd, even in syntheses involving pharmaceutical ingredients.
Acknowledgment. We are grateful to D. Nomoto, Dr. S. Ueda,
and Dr. K. Kobayashi (NIMS, SPring-8), Prof. S. Tsukamoto, Dr.
T. Konishi, K. Tateishi, and T. Tojo (Anan National College of
Technology) as well as Dr. K. Iizuka, M. Yokota, and Y. Furukawa
(Nippon Institute of Technology) for measuring SR-HXPS. This
research was partially supported by Grants-in-Aid from the Ministry
of Education, Culture, Sports, Science and Technology, Japan.
We next measured the amount of Pd adsorbed on the Pd materials
B-D with inductively coupled plasma mass spectrometry (ICP-
MS) analysis. These analyses revealed that B, C, and D included
95, 80, and 38 µg of Pd, respectively.21 We subsequently measured
the amount of Pd released into the reaction mixtures of each
Suzuki-Miyaura coupling run by ICP-MS. Table 2 shows that the
amount of released Pd in each run was extremely low. The amount
of Pd in the reaction mixture using B-D is far lower than the U.S.
government-required value of <5 ppm residual metal in product
streams.22 In particular, in the case of D, the leached Pd into the
reaction mixture was only 76-38 ng for 1 mmol scale preparation
(12.7-6.3 ppb in 3 mL of solvent, 0.2-0.1% of Pd from D), the
average being 26 ng for 10 runs. The Crudden group reported an
excellent Pd material supported on mercaptopropyl-modified me-
soporous silica, which has been recognized as one of the best
catalytic Pd materials from the point of view of leaching: 720-30
ng for 1 mmol scale preparation (72-3 ppb in 2.5 mL of solvent,
0.006-0.13% of Pd from the Pd material), the average being 242
ng for 4 runs.23,24 The amount of the leached Pd in the reaction of
our Pd material D is similar or less, compared with that of
Crudden’s material. Furthermore, since Crudden’s material can only
be recycled 5 times, D would be considered highly recyclable,
making it one of the lowest releasing Pd materials with high
recyclability. Due to its extremely low Pd leaching levels, we
employed D, i.e., SAPd (Sulfur-modified Au supported Pd material)
for further studies.
We then investigated the scope and limitation of SAPd in the
Suzuki-Miyaura coupling using aryl iodides and arylboronic acids.
The corresponding products were obtained in excellent yields as
summarized in Table 3. It is noteworthy that isolated yields (%) of
3i between the first run to fourth run were 54, 46, 8, and 5
respectively, when the control experiment of entry 9 was carried
out using SAPd without Piranha treatment. These control experi-
ments indicate Piranha treatment is necessary and sulfur is needed
to create an active catalyst or retain Pd on the surface.
In summary, we have found in the SR-HXPS measurement of
Piranha-treated Au(111)/mica that the gold surface underwent sulfur
modification during this treatment, which was believed to have only
removed impurities from the gold surface. We also successfully
developed a practical Pd material, SAPd, whose Pd was im-
mobilized on sulfur-modified Au. With the lowest Pd-releasing
levels and high recyclability, this is one of the best Pd materials
thus far developed. Because it leaches extremely low levels of Pd
Supporting Information Available: Experimental procedures and
full characterizations of compounds. This material is available free of
charge via the Internet at http://pubs.acs.org.
References
(1) Review: Ulman, A. An Introduction to Ultrathin Organic Films: From
Langmuir-Blodgett to Self-Assembly; Academic Press: New York, 1991.
(2) Review: Ulman, A. Chem. ReV. 1996, 96, 1533–1554.
(3) Review: Flink, S.; van Veggel, F. C. J. M.; Reinhoudt, D. N. AdV. Mater.
2000, 12, 1315–1328.
(4) Review: Gooding, J. J.; Mearns, F.; Yang, W.; Liu, J. Electroanalysis 2003,
15, 81–96.
(5) Review: Love, J. C.; Estroff, L. A.; Krebel, J. K.; Nuzzo, R. G.; Whitesides,
G. M. Chem. ReV. 2005, 105, 1103–1169.
(6) Nishida, N.; Hara, M.; Sasabe, H.; Knoll, W. Jpn. J. Appl. Phys. 1996, 35,
5866–5872.
(7) Gro¨nbeck, H.; Curioni, A.; Andreoni, W. J. Am. Chem. Soc. 2000, 122,
3839–3842.
(8) Voicu, R.; Ellis, T. H.; Ju, H.; Leech, D. Langmur 1999, 15, 8170–8177.
(9) Chen, I.-W. P.; Chen, C.-C.; Lin, S.-Y.; Chen, C.-H. J. Phys. Chem. B
2004, 108, 17497–17504.
(10) Petrovykh, D. Y.; Suda, H. K.; Opdahl, A.; Richter, L. J.; Tarlov, M. J.;
Whitman, L. J. Langmur 2006, 22, 2578–2587.
(11) Pisarek, M.; Lewandowska, M.; Roguska, A.; Kurzydlowski, K. J.; Janik-
Czachor, M. Mater. Chem. Phys. 2007, 104, 93–97.
(12) Seu, K. J.; Pandey, A. P.; Haque, F.; Proctor, E. A.; Ribbe, A. E.; Hovis,
J. S. Biophys. J. 2007, 92, 2445–2450.
(13) Phan, N. T. S.; Van Der Sluys, M.; Jones, C. W. AdV. Synth. Catal. 2006,
348, 609–679.
(14) Yin, L.; Liebscher, J. Chem. ReV. 2007, 107, 133–173.
(15) For example: Hoshiya, N.; Isomura, N.; Shimoda, M.; Yoshikawa, H.;
Yamashita, Y.; Iizuka, K.; Tsukamoto, S.; Shuto, S.; Arisawa, M.
ChemCatChem 2009, 1, 279–285.
(16) This does not mean a sulfur atom is embedded at an ∼20 nm depth, and
it is unclear whether the sulfur atoms are embedded at the interface between
gold and Pd adlayers or not.
(17) Proposal No. 2007B4600 and No. 2008A4605. For the measurements, the
incident X-ray energy was 5.95 KeV and a total energy resolution of 240
meV was employed.
(18) It should be noted that the sulfur for sample A remained in almost the
same chemical state before (Figure 1, Green line) and after (Figure 1, Blue
line) the Suzuki-Miyaura coupling. Pd-XPS was shown in Figure 2.
(19) We did not use Pd 3d peaks but Pd 2p peaks, since the peaks of Pd 3d5/2
or 3d3/2 overlap with that of Au 4d5/2 of substrates in the energy region.
(20) D could be recycled at least 20 times.
(21) Au(foil) in B is 13 × 11 × 0.127 mm3 (325 mg). Au(mesh) in C and D is
12 × 12 mm3 (100 mesh, 99 mg). D included 14 ( 9 µg of S (n ) 4).
(22) Flahive, E. J.; Ewanicki, B. L.; Sach, N. W.; O’Neill-Slawecki, S. A.;
Stankovic, N. S.; Yu, S.; Guinness, S. M.; Dunn, J. Org. Process Res.
DeV. 2008, 12, 637–645.
(23) Crudden, C. M.; Sateesh, M.; Lewis, R. J. Am. Chem. Soc. 2005, 127,
10045–10050.
(24) Webb, J. D.; MacQuarrie, S.; McLeney, K.; Crudden, C. M. J. Catal. 2007,
252, 97–109.
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