Dispersed Ru nanoclusters transformed from a grafted trinuclear Ru complex on SiO2 for selective alcohol oxidation

Article abstract of DOI:10.1039/c3dt51142a

Ru nanoclusters (average diameter = 1.3 ± 0.3 nm) were successfully prepared by using a Ru₃ cluster Ru₃O(CH ₃COO)₆(H₂O)₃·(CH ₃COO) grafted on a pyridine-functionalized SiO₂ surface. The pyridine moiety dispersed on the SiO₂ surface spread the Ru ₃ cluster, controlling its surface density, and the nanoclusterization of the Ru cluster proceeded on the surface. The structures of the Ru nanoclusters were characterized by means of elemental analysis; thermogravimetric analysis; FT-IR, UV/vis, and solid-state NMR spectroscopy; BET analysis; X-ray photoelectron spectroscopy; X-ray diffraction; transmission electron microscopy; and Ru K-edge X-ray absorption fine structure analysis. It was found that the catalytic activity for the selective oxidation of alcohol to the corresponding aldehyde using O₂ highly depended on the dispersion and structures of the Ru particles, and the Ru nanocluster was found to be efficient in the selective oxidation of a variety of alcohols.

Full text of DOI:10.1039/c3dt51142a

Dalton
Transactions
An international journal of inorganic chemistry
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Volume 42 | Number 35 | 21 September 2013 | Pages 12509–12774
ISSN 1477-9226
COVER ARTICLE
Muratsugu, Tada et al.
Dispersed Ru nanoclusters transformed from a grafted trinuclear Ru complex
on SiO2 for selective alcohol oxidation

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PAPER
Dispersed Ru nanoclusters transformed from a grafted
trinuclear Ru complex on SiO2 for selective alcohol
oxidation†
Cite this: Dalton Trans., 2013, 42, 12611
Satoshi Muratsugu,*a,b,c Min Hwee Lim,a,d Takahiro Itoh,a,c
Wipavee Thumrongpatanaraks,a,e Mio Kondo,a,c Shigeyuki Masaoka,a,c
T. S. Andy Hord,f and Mizuki Tada*a,b,c,g
Ru nanoclusters (average diameter = 1.3 0.3 nm) were successfully prepared by using a Ru3 cluster
Ru3O(CH3COO)6(H2O)3·(CH3COO) grafted on a pyridine-functionalized SiO2 surface. The pyridine moiety
dispersed on the SiO2 surface spread the Ru3 cluster, controlling its surface density, and the nanoclusteri-
zation of the Ru cluster proceeded on the surface. The structures of the Ru nanoclusters were charac-
terized by means of elemental analysis; thermogravimetric analysis; FT-IR, UV/vis, and solid-state NMR
spectroscopy; BET analysis; X-ray photoelectron spectroscopy; X-ray diffraction; transmission electron
microscopy; and Ru K-edge X-ray absorption fine structure analysis. It was found that the catalytic activity
for the selective oxidation of alcohol to the corresponding aldehyde using O2 highly depended on the
dispersion and structures of the Ru particles, and the Ru nanocluster was found to be efficient in the
selective oxidation of a variety of alcohols.
Received 2nd May 2013,
Accepted 14th May 2013
DOI: 10.1039/c3dt51142a
www.rsc.org/dalton
stability of the catalyst due to the site isolation of an active
metal site and due to the formation of a new metal coordi-
1. Introduction
Chemical grafting of a homogeneous metal complex on a solid nation structure generated on a support surface.1–20
surface is a practical method for creating a site-isolated metal
There are several methods to chemically coordinate metal
complex with a regulated metal-coordination structure on a complexes on solid support surfaces, and the grafting of metal
support surface, and unique catalytic properties are often complexes using organic functional groups as ligands is one of
provided.1–8 The advantage of a supported metal-complex cata- the typical ways to attach metal complexes. For example, we
lyst is not only simpler separation of the catalyst from the reac- have prepared a Ru–diamine complex on a SiO2 surface whose
tion medium but also the improvement of the activity and p-styryl-functionalized moiety was used as a grafting unit.14–16
The loading of the supported Ru complex can be regulated by
the surface density of the grafting unit, and further transform-
aInstitute for Molecular Science, 38 Nishigo-Naka, Myodaiji, Okazaki,
Aichi 444-8585, Japan
ations of the supported Ru complex provided unique cata-
lytically active structures for selective oxidation reactions. The
utilization of specific ligand-functionalized supports to graft
metal complexes has been developed in other systems: Stack
et al. prepared a 1,10-phenanthroline-functionalized meso-
porous silica (SBA-15) and a supported Mn bis-phenanthroline
complex using this functionalized support and a Mn ion
source.17 Highly dispersed Mn complexes were obtained and
epoxidation activity was reported to be enhanced. Notestein
et al. reported carboxylic acid-functionalized SiO2 and grafted
a Mn cluster complex with 1,4,7-trimethyl-1,4,7-triazacyclo-
nonane ligands.18–20 A catalytically active Mn cluster for the
oxidation of alkenes was produced on the SiO2 surface through
a simple grafting method.
bDepartment of Chemistry, Graduate School of Science, Nagoya University, Furo-cho,
Chikusa, Nagoya 464-8602, Japan. E-mail: smuratsugu@chem.nagoya-u.ac.jp
cDepartment of Structural Molecular Science, The Graduate University for Advanced
Studies (SOKENDAI), 38 Nishigo-Naka, Myodaiji, Okazaki, Aichi 444-8585, Japan
dDepartment of Chemistry, National University of Singapore, 3 Science Drive 3,
S117543, Singapore
eCenter for Catalysis, Department of Chemistry, Faculty of Science, Mahidol
University, Rama VI Road, Ratchatewi, Bangkok, 10400, Thailand
fInstitute of Materials Research and Engineering, Agency for Science, Technology and
Research, 3 Research Link, S117602, Singapore
gResearch Center for Materials Science, Nagoya University, Furo-cho, Chikusa,
Nagoya 464-8602, Japan. E-mail: mtada@chem.nagoya-u.ac.jp;
Tel: +81-52-788-6200
†Electronic supplementary information (ESI) available: FT-IR, 13C NMR, k3-
weighted Ru K-edge EXAFS oscillations and their curve-fitted data, Ru K-edge
XANES spectra, XRD spectra, TEM images, and reaction profiles. See DOI:
10.1039/c3dt51142a
A Ru3 cluster complex with a Ru3-(μ3-O)-(μ-CH3COO)6 frame-
work has been reported to be an active catalyst for the selective
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oxidation of alcohols to aldehydes in EtOH solutions.21 under N2 for 1 h. After cooling to room temperature, the
Because the Ru3 cluster complex has a high binding constant mixture was filtered and the collected gel was washed with
with pyridine (Py) ligands through ligand exchange with H2O– MeCN under N2 until the filtrate became colorless. The gel was
alcohol ligands,22–31 we have prepared a SiO2 support functio- washed at 353 K for about 2–4 h under N2 with MeCN in a
nalized with a Py moiety for the grafting of Ru3(μ3-O)- Soxhlet extractor. Finally the gel was dried under vacuum for
(CH3COO)6 (H2O)3·(CH3COO) on SiO2. 4-Pyridylethyltriethoxy- at least 24 h and a blue solid was obtained, which is denoted
silane was tethered on a SiO2 surface, whose surface density as C. Ru loading was measured by X-ray fluorescence spectro-
was regulated, and the Ru3 cluster was successfully co- scopy (XRF).
ordinated on the SiO2 surface.
2.3. Preparation of D
In this paper, we also investigated the further structural
transformation of the supported Ru3 cluster to dispersed Ru C was placed in a glass flow cell, which was then attached to a
nanocluster on the SiO2 surface. Ru nanoclusters prepared by vacuum line. After evacuating the cell (ca. 10−2 Pa) it was
conventional impregnation methods of Ru salts (for example, heated to 823 K and held at that temperature for 2 h. After
Ru(OH)x on oxide supports) were reported to be efficient for that, H2 (40 kPa) was passed through the cell for 1 h at 823 K.
the selective alcohol oxidation using O2.32–36 We have suc- Black solid was obtained, which is denoted as D.
ceeded in preparing Ru nanoclusters, whose average diameter
2.4. Preparation of E
was 1.3 0.3 nm, with negligible aggregation of the supported
Ru3 cluster on the SiO2 surface. The sizes and distribution of C was heated at 823 K under air and kept for 2 h at 823 K.
the surface Ru particles highly depended on the preparation After that, it was treated with H2 (40 kPa) for 1 h at 823 K. Gray
conditions, and the supported Ru nanoclusters were found to solid was obtained, which is denoted as E.
be active for the selective oxidation of a variety of alcohols to
2.5. Preparation of F
their corresponding aldehydes using O2. The preparation,
characterization, and alcohol oxidation performances of C was heated at 823 K under air and kept for 2 h at 823 K.
the Ru nanoclusters prepared from supported Ru3(μ3-O)- Bluish black solid was obtained, which is denoted as F.
(CH3COO)6(H2O)3·(CH3COO) on SiO2 are reported.
2.6. Catalyst characterization
2. Experimental section
Elemental analysis and thermogravimetric analysis (TGA).
Chemicals were purchased from commercial sources (Wako Organic residues on C, D, E, and F were analyzed by elemental
Chemicals, TCI, Sigma-Aldrich, and Gelest) and used without analysis (Microorganic Element Analyzer CHN Corder MT-6,
further purification. Unless indicated otherwise, all prep- Yanako) and TGA (TGA2950, TA Instruments). For TGA
arations and manipulations were performed using standard measurements, each sample was kept at 298 K for 10 min
Schlenk techniques in an N2 atmosphere. Ru3(μ3-O)- before measurement and heated under air at a rate of 10 K min−1
.
(CH3COO)6(Py)3·(PF6) (A-Py3) was synthesized according to the
XRF. XRF was measured on
spectrometer.
a
JEOL JSX-3400RII
literature procedure.23
BET analysis. Nitrogen adsorption was performed on a
Micromeritics ASAP-2020 analyzer (Shimadzu) at 77 K. Each
sample (50 mg) was degassed at 368 K for 4 h before an
2.1. Preparation of B: attachment of
4-pyridylethyltriethoxysilane to SiO2
SiO2 (4.0 g, Aerosil 300; Degussa) was calcined at 673 K for 2 h. adsorption measurement, and the dead volume was estimated
After cooling under vacuum, 80 mL of toluene was added. To by using He.
this suspension was added the solution of 4-pyridylethyl-
FT-IR spectroscopy. The FT-IR spectra of A, B, and C were
triethoxysilane (Py-et-Si(OEt)3; 0.19 mL, 7.1 × 10−4 mol, to give recorded on an FT-IR spectrometer in a transmission mode
a surface density of 0.35 nm−2 of Py groups on the SiO2 (4 cm−1 resolution, FT/IR-6100, JASCO).
surface) and toluene (10 mL). The mixture was refluxed under
NMR spectroscopy. The 13C liquid-state NMR spectra of A
N2 for 24 h. After cooling to room temperature, the resulting and A-Py3 were measured on a JNM-LA400 spectrometer
gel was collected by filtration and washed with toluene under (JEOL). The 13C solid-state (SS) magic angle spinning (MAS)
N2, followed by washing under N2 with toluene in a Soxhlet NMR spectra of B and C were recorded on an NMR spectro-
extractor for about 16 to 20 h and next with hexane for about 4 meter (ECX-400, JEOL) using a cross-polarization (CP) method.
to 8 h. Finally, the gel was dried under vacuum and a yellowish The magic angle spinning rates were 10 kHz and 8 kHz for dis-
white solid was obtained, which is denoted as B.
tinguishing genuine signals from side-band signals. The relax-
ation decay, the contact time, and the scan number were 0.5 s,
5 ms, and 2 000 000, respectively. Hexamethylbenzene (13C:
2.2. Preparation of C: coordination of the Ru3 cluster (A) to B
To a suspension of B (1 g, 0.33 Py nm−2) in 4 mL of degassed, 17.17 ppm) was used as an external standard for calibrating
dehydrated acetonitrile (MeCN) was added a solution of chemical shifts.
Ru3(μ3-O)(CH3COO)6(H2O)3·(CH3COO) (A; 0.118 g, 1.50 × 10−4
UV/vis spectroscopy. Transmittance and diffuse-reflectance
mol, for a 1 : 1 ratio of Ru3 and Py) in 1.5 mL of degassed, de- (DR) UV/vis spectra were recorded on a V-550-DS spectrometer
hydrated MeCN–EtOH (2 : 1, v/v). The suspension was refluxed (JASCO) equipped with an integrated sphere.
12612 | Dalton Trans., 2013, 42, 12611–12619
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X-ray diffraction (XRD). Powder XRD patterns of D, E and F correction-of-edge energy (ΔE0). Phase shifts and backscatter-
were recorded at a scan rate of 2° min−1 using a MultiFlex ing amplitudes were calculated with the FEFF8 code39 using
X-ray diffractometer (Rigaku; Cu Kα, λ = 1.5418 Å, 40 kV, 40 mA). the crystal structures of Ru metal, RuO2, and A.40
Transmission electron microscopy (TEM) and energy-disper-
2.7. Catalytic alcohol oxidation
sive X-ray spectrometry (EDS). TEM images of D and E were
taken on a JEM-3200FS transmission electron microscope
(JEOL) equipped EDS analyzer using an accelerating voltage of
300 kV.
X-ray photoelectron spectroscopy (XPS). XPS spectra of D, E,
and F were recorded with an AXIS HSi instrument (Kratos
Analytical) using an Mg Kα X-ray source. Narrow multiplex
scans were recorded with 80 eV pass energy and 0.1 eV step
size. A charge neutralization function was employed to com-
pensate for charge built up on solid samples by X-ray
irradiation. Binding energies were referenced to that of Si 2p
as 103.4 eV.
Ru K-edge X-ray absorption fine structure (XAFS). XAFS
spectra at the Ru K-edge were measured in transmission mode
at the NW10A station of the Photon Factory at KEK-IMSS. The
energy and current of electrons in the storage ring were 6.5
GeV and 60 mA, respectively. X-rays from the storage ring were
monochromatized with a Si(311) double-crystal monochroma-
tor, and ionization chambers filled with pure Ar and Kr gases
were used to monitor the incident and transmitted X-rays,
respectively. All samples were measured at 20 K under vacuum.
Ru K-edge XAFS spectra were analyzed with the ATHENA and
ARTEMIS using IFEFFIT (ver. 1.2.11).37 Threshold energy was
tentatively set at the inflection point of the Ru K-edge, and
background was subtracted by the Autobk method.38 The k3-
weighted extend XAFS (EXAFS) oscillations were Fourier trans-
formed into R-space, and single-scattering curve-fitting ana-
lysis was performed in R-space. Fitting parameters of each
shell were the coordination number (CN), interatomic distance
(R), Debye–Waller factor (σ2: mean-square-displacement), and
A reactant (0.33 mol L−1) and an internal standard (dodecane)
were dissolved in dry toluene, and the solution was heated in an
O2 atmosphere (101.3 kPa) at 353 K for about 5–10 min. The solu-
tion was then added to an oxide-supported catalyst (Ru3: 1.0 ×
10−5 mol) or a homogeneous Ru3 cluster (Ru3: 1.0 × 10−5 mol) in a
Schlenk tube, which was purged and filled with O2 (101.3 kPa) in
advance and pre-heated at 353 K for 2 min. The molar ratio of Ru3
to the reactant and the internal standard was kept at 1 : 100 : 50.
The reaction mixture was stirred at 353 K under O2, and the reac-
tion was monitored at appropriate intervals by FID-GC (GC-14B,
Shimadzu; Chiraldex B-DM column). Leaching of Ru into the reac-
tion solution was estimated by ICP-AES (SPS 7800, Seiko).
3. Results and discussion
3.1. Synthesis of a pyridine-anchored SiO2 support (B)
Ru nanoclusters (D) were prepared in three steps as shown in
Fig. 1: the preparation of a pyridine-anchored SiO2 support,
the immobilization of a Ru3 cluster (A), and the nanoclusteri-
zation of the supported Ru3 cluster (C) on the surface. The
density of the pyridyl moiety was regulated to be 0.3 nm−2 in
order to disperse the Ru3 cluster (A) on the surface. Solid
samples in Fig. 1 were characterized in a step-by-step manner.
The attachment of 4-pyridylethyltriethoxysilane (Py-et-Si-
(OEt)3) on SiO2 (Aerosil 300) yielded Py-functionalized SiO2 (B),
which would serve as a surface functionalized with coordi-
nation ligands for grafting a Ru3 cluster (A). The surface
density of Py on B was calculated to be 0.33 0.05 nm−2, as
Fig. 1 (A) Schematics of the grafting of Ru3(μ3-O)(CH3COO)6(H2O)3·(CH3COO) (A) on the pyridine-functionalized SiO2 surface (B) through coordination interaction
to produce C and the transformation of C to Ru nanoclusters (D) or Ru particles (E and F). (B) A TEM image of D with its particle size distribution.
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was estimated to be 3.3 0.3 wt% (the surface density was esti-
mated to be 0.2 nm−2). In contrast, the loading of A was
limited to at most 1.2 wt% on SiO2 without the Py ligands,
indicating that the Py moiety acted as a coordination site for
the Ru3 cluster A on the SiO2 surface. The density of the Py
ligand was regulated to be 0.3 nm−2 on the surface so that the
Ru3 cluster was suggested to be also dispersed on the surface.
Carbon EA value showed the amount of carbon species (both
supported A and the Py ligand) on C (Table 1), and TGA analy-
sis of C showed the weight loss of the sample, which was
attributed to the combustion of the carboxylate ligands of A
and the Py ligand on C, and the weight loss (4.9 wt%) was con-
sistent with the result of carbon EA (4.2% C) in Table 1.
The coordination structure of the Ru3 clusters on C was
examined by DR-UV/vis, FT-IR, 13C SS NMR, and Ru K-edge
XAFS. Peaks attributed to intracluster charge transfer (ICCT)
transitions (615 nm and 696 nm (shoulder))31 were observed
in the UV/vis spectrum of A (Fig. 2(B)). After the attachment of
A to the SiO2 surface, the ICCT transition peaks were still
observed at 612 nm and 693 nm (shoulder) in C.
Table 1 Characterization data on B, C, D, E, and F (Ru: 3 wt%)
BET
Elemental analysis surface TGA weight
Ru 3p XPS
binding
(observed
area/
loss/wt%
Catalyst (calculated)a)
m2 g−1 (temperature/K) energy/eVb
B
C
D
E
F
C 2.64% (1.57%)
H 0.55% (0.19%)
N 0.22% (0.20%)
C 4.20%
H 0.78%
N 0.38%
C 1.02%
H 0.38%
N 0.20%
C 0.21%
H 0.41%
N 0.00%
C 0.18%
H 0.48%
253
260
294
265
277
—
—
—
4.9 (270, 293)
1.1 (259)
n.d.
483.6
461.3
483.9
461.1
n.d.
484.1
462.4
N 0.00%
a Calculated from the summation of percentages of C, H, and N for Py-
et-Si(OEt)3 assumed to be supported on SiO2 at a surface density
.
of 0.3 nm−2 b XPS binding energies were calibrated against Si 2p at
103.4 eV.
The FT-IR v(CvC) peaks of the Py ligands were sensitive to
metal coordination. The v(CvC) peak of free Py was observed
at 1482 cm−1 and that of A-Py3 (prepared by reacting A with an
excess amount of Py; three Py ligands were coordinated to
Ru)23 was observed at 1488 cm−1, suggesting that the v(CvC)
peak was shifted to higher wavenumber by the coordination to
Ru. On the other hand, the v(CvC) peak of the Py ligand on B
was observed at 1497 cm−1, and it was shifted to 1505 cm−1 on
C (Fig. S1(C)†). No peak was observed within the region of
1480–1510 cm−1 on the FT-IR spectrum of a sample of A-
grafted SiO2 without Py (Fig. S1(C)†). These results suggested
that the surface Py ligand was coordinated to the Ru3 complex
on C.
estimated from its N content determined by EA (Table 1).
FT-IR, UV/vis, and 13C SS MAS NMR spectra suggested the
existence of the Py moiety on B. A peak attributed to the π–π*
transition of the Py ligand (255 nm) was observed in the UV/vis
spectrum of Py-et-Si(OEt)3 in CH2Cl2, and a characteristic peak
due to the π–π* transition was still observed at 255 nm after
the attachment of Py-et-Si(OEt)3 to the SiO2 surface (Fig. 2(A)).
The FT-IR spectra of Py-et-Si(OEt)3 and B were similar to each
other (Fig. S1(A, B)†). In the 13C SS MAS NMR spectrum of B,
seven peaks (154.7, 148.1, 123.6, 58.6, 28.1, 16.3, and
12.3 ppm) were clearly observed that corresponded to the
peaks of Py-et-Si(OEt)3 (152.7, 149.4, 123.3, 57.8, 27.7, 18.1,
and 10.7 ppm in DMSO-d6) (Fig. S2†). Peaks at 58.6 and
16.3 ppm were attributed to OEt of Py-et-Si(OEt)3, indicating
that a part of the three OEt groups remained on B.
The 13C SS MAS NMR of C exhibited the feature of both A
and B (Fig. 3). Peaks attributed to the CH3COO ligand were
observed at 221.8 ppm for A and at 199.0 ppm for A-Py3, indi-
cating the upfield shift of the peak by the Py coordination.
Indeed, when A was mixed with 1 equiv. of Py in CD3OD and
refluxed for 20 min, its 13C NMR spectrum showed several
signals at 221.5 ppm (no ligand coordinated to A), 206.3 ppm
(one Py ligand coordinated to A, A-Py), 205.8 ppm (two Py
ligands coordinated to A, A-Py2), and 196.9 ppm (three Py
ligands coordinated to A, A-Py3) (Fig. 3). We found that the
CH3COO peak of C was observed at 203.4 ppm, which was
between the peaks of A and A-Py3 (Fig. 3); thus it is suggested
that the Ru3 cluster coordinated to one or two Py ligands on
the surface. The peaks of A-Py and A-Py2 were similar to each
other; therefore, the number of the coordinated Py ligands
could not be fixed.
3.2. Preparation of C: grafting of the Ru3 cluster (A) on B
The grafting of the Ru3 cluster A on B was performed through
the coordination of the Py moiety to A. The loading of Ru on C
The local coordination structure of C was almost identical
to that of A, as shown by Ru K-edge EXAFS analysis (Fig. 4, S3†,
and Table 2). A peak attributed to Ru⋯Ru at 0.331 0.001 nm
was observed in A. For C, the contribution of Ru⋯Ru was
fitted at 0.332 0.001 nm with the coordination number (CN)
of 1.9 0.7. The fitting was completed together with the contri-
butions of Ru–O (μ3-O) at 0.189 0.003 nm (CN = 0.9 0.2),
Fig. 2 (A) A UV/vis spectrum of Py-et-Si(OEt)3 in CH2Cl2 and a DR-UV/vis spec-
trum of B. (B) A UV/vis spectrum of A in CH3OH and a DR-UV/vis spectrum of C.
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These parameters suggested that the framework of Ru3-(μ3-O)-
(μ-CH3COO)6 was maintained on the SiO2 surface. Although it
is not possible to determine the exact number of the co-
ordinated Py ligands, FT-IR and 13C NMR analyses indicate
that at least one Py ligand was coordinated to the Ru3 cluster
on C. Identical features in the Ru K-edge X-ray absorption near
edge structure (XANES) also supported the similarity in the
structure and oxidation state between A and C (Fig. S4†). The
concentration of the Py ligand was regulated to be as small as
0.3 nm−2 on the surface; hence, the major species of C would
be A-Py/SiO2 as presented in Fig. 1.
3.3. Transformation to Ru nanocluster and particles (C → D,
E, and F)
We performed the transformation of the supported Ru3 cluster
C to Ru nanoclusters D by a treatment at 823 K for 2 h under
vacuum (ca. 10−2 Pa) and a subsequent treatment with H2
(40 kPa) at 823 K for 1 h. E and F were prepared under different
treatment conditions: E was prepared by treatment at 823 K in
air before the subsequent treatment with H2, and F was pre-
pared by the treatment in air without the final reduction with
H2. These three catalysts had similar BET surface areas (294,
265, and 277 m2 g−1 for D, E, and F, respectively), which were
comparable to that of the support (Table 1).
Fig. 3 13C liquid-state NMR spectra for A (in CD3OD), A-Py3 (in CD3CN), and A
with pyridine after refluxing for 20 min (in CD3OD), and 13C SS MAS NMR
spectra for B and C.
The XRD pattern of D did not show a diffraction peak of
either Ru or RuO2, indicating the absence of large Ru or RuO2
particles (Fig. S5†). It only showed a broad peak due to amor-
phous SiO2 at 2θ around 21°, and no significant peak of
carbon species was observed. In contrast, the XRD patterns of
E and F clearly showed the existence of Ru and RuO2 particles,
respectively. The XRD pattern of E exhibited the characteristic
peaks of Ru metal at 2θ of 38.4° (100), 42.2° (002), and 44.0°
(101), while that of F showed the characteristics peaks of RuO2
at 2θ of 28.1° (110) and 35.1° (101) (Fig. S5†).
The TEM image of D exhibited obscure dark spots with the
average diameter of 1.3 0.3 nm (Fig. 1 and S6(A)†). The EDS
analysis of D showed the presence of Ru, suggesting that the
spots in the TEM images were a kind of Ru nanocluster. A
large aggregation of Ru particles was not observed. On the
other hand, the TEM image of E showed the existence of hetero-
geneous aggregated Ru particles greater than 10 nm in size,
and the interplanar distance of 0.203 nm was clearly observed
as a Ru (101) plane (Fig. S6(B)†). The Ru 3p1/2 and 3p3/2 XPS
peak intensities of E and F, which were normalized by the O 1s
peak, were much smaller than that of D (Fig. 5), reflecting the
dispersion of the Ru nanoclusters on D.
Ru K-edge EXAFS was analyzed for the local coordination
structure of the Ru species of D. In comparison with C, Fourier
transforms of the EXAFS spectrum of D exhibited a new peak
at 0.2–0.3 nm, together with a strong peak at 0.16 nm (Fig. 4).
The new peak was assigned to a combination of Ru–Ru bonds
at 0.266 0.003 nm (CN = 2.0 1.1) and Ru⋯Ru contribution
at 0.305 0.004 nm (CN = 1.3 0.8), suggesting the presence
of both metallic and oxidized Ru species. The strong peak was
Fig. 4 k3-Weighted Ru K-edge EXAFS Fourier transforms for A, C, D, E, and F
measured at 20 K (Ru: 3 wt%).
Ru–O/N (CH3COO, Py, H2O) at 0.204 0.001 nm (CN = 5.1
0.2), Ru⋯C (CH3COO) at 0.301 0.003 nm (CN = 3.7 0.9), fitted with Ru–O interaction at 0.202 0.001 nm (CN = 3.9
and Ru⋯O (CH3COO) at 0.331 0.006 nm (CN = 3.7 0.9). 0.5) (Table 2). For E and F, metallic Ru and RuO2 were
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Table 2 Structural parameters determined by curve-fitting analysis of Ru
K-edge EXAFS measured at 20 K
σ2/×105
Shell
CN
Distance/nm ΔE0
0.331 0.001 −2
nm2
A: [Ru3O(CH3COO)6(H2O)3] CH3COOa
Ru⋯Ru
Ru–O (μ-O)
Ru–O (CH3COO, H2O)
Ru⋯C (CH3COO)
Ru⋯O (CH3COO)
C:b
2
1
5
4
4
4
2
2
2
2
3
4
4
4
4
1
1
1
1
1
0.190 0.003
0.204 0.001
0.300 0.003
0.331 0.004
0
0
0
0
Ru⋯Ru
1.9 0.7 0.332 0.001 −2
4
2
2
2
2
3
4
4
4
4
1
1
1
1
1
Ru–O (μ-O)
0.9 0.2 0.189 0.003
1
1
1
1
Ru–O/N (CH3COO, Py, H2O) 5.1 0.2 0.204 0.001
Ru⋯C (CH3COO)
Ru⋯O (CH3COO)
D (Ru: 3 wt%)c
Ru–Ru
3.7 0.9 0.301 0.003
3.7 0.9 0.331 0.006
2.0 1.1 0.266 0.003
1.3 0.8 0.305 0.004
3
3
4
4
2
10
10
8
3
3
1
Ru⋯Ru
Ru–O
3.9 0.5 0.202 0.001 13
E (Ru: 3 wt%)d
Ru–Ru
11.3 0.6 0.268 0.001
6
1
3
1
F (Ru: 3 wt%)e
Ru⋯Ru
1.8 0.3 0.312 0.001
7.1 0.7 0.356 0.001
9
9
1
1
3
2
2
3
1
1
1
Ru⋯Ru
Ru–O
5.4 1.2 0.198 0.001 11
Ru powderf
Ru–Ru
10.9 0.7 0.268 0.001
3
1
5
1
g
RuO2
Fig. 5 Ru 3p XPS spectra of D, D (collected after the benzyl alcohol oxidation),
E, and F. The intensity of the spectrum was normalized by the peak area of O 1s.
Ru⋯Ru
Ru⋯Ru
Ru–O
2.4 0.4 0.312 0.001 10
7.8 0.8 0.355 0.001 10
5.6 1.8 0.198 0.001 12
1
1
2
3
3
3
1
1
1
a k = 30–160 nm−1, R = 0.10–0.33 nm, Rf = 1.6%. b k = 30–160 nm−1, R =
0.10–0.33 nm, Rf = 1.7%. c k = 30–160 nm−1, R = 0.11–0.30 nm, Rf =
1.7%. d k = 30–160 nm−1, R = 0.16–0.30 nm, Rf = 0.2%. e k =
30–160 nm−1, R = 0.10–0.36 nm, Rf = 1.7%. f k = 30–180 nm−1, R =
0.16–0.30 nm, Rf = 0.5%. g k = 30–180 nm−1, R = 0.10–0.36 nm, Rf =
2.5%.
Table 3 Catalytic performances of Ru catalysts for the selective oxidation of
benzyl alcohol to benzaldehydea
Selectivity of
respectively observed (Table 2, Fig. 4 and S3†). Ru K-edge
XANES spectra (Fig. S4†) agreed with the results of EXAFS.
Small Ru nanoclusters were reported by Mizuno et al.: a
highly dispersed Ru(OH)x/TiO2 catalyst for hydrogen-transfer
reaction (CN(Ru⋯Ru) = 0.37 0.17),41 and Fukuoka et al.: a
carbon-supported RuO2·2H2O catalyst for cellulose hydrolysis
(CN(Ru⋯Ru) = 1.0 0.4).42 The Ru nanoclusters on D, which
were different from aggregated Ru species on E and F, were not
metallic and not fully oxidized and they would be a mixture of
metallic Ru and oxidized Ru such as RuO2, RuO2·2H2O, or
Ru(OH)x. Although the detailed structure is still controversial,
Ru nanoclusters whose size was much smaller than those on E
and F were formed on D as characterized by XRD, TEM, XPS,
and Ru K-edge EXAFS.
Entry
Catalyst
TONb
aldehyde %c
1
2
3
4
5
6
7
8
—
A
B
0
57.1
0
—
∼100
—
Ru powder
RuO2
0.4
1.1
6.9
67.0
91.4
75.2
3.1
5.2
n.d.
n.d.
n.d.
84.5
90.4
90.1
n.d.
n.d.
C (Ru: 3 wt%)
D (Ru: 1 wt%)
D (Ru: 3 wt%)
D (Ru: 5 wt%)
E (Ru: 3 wt%)
F (Ru: 3 wt%)
9
10
11
a Reaction conditions: Ru3 = 1.0 × 10−5 mol, Ru3/benzyl alcohol/
dodecane (internal standard) = 1/100/50 (molar ratio), 0.33 mol L−1 of
benzyl alcohol in toluene, 353 K, 101.3 kPa of O2, 6 h. Mass balance
was above 0.9. b TON = (initial mole of reactant − residual mole of
reactant)/mole of Ru3 (at 6 h). c Selectivity of aldehyde (%) = mole of
aldehyde product/(initial mole of reactant − residual mole of reactant)
× 100%.
3.4. Catalytic performances in the selective oxidation of
alcohols using O2
We investigated the selective oxidation of alcohols using O2
using the supported Ru3 cluster C, Ru nanocluster D, and Ru
particles E and F. Ru is known to be active for the oxidation of
alcohols43–46 on homogeneous Ru complexes43,44,47–49 and
heterogeneous Ru catalysts.32–36,45,46,50–66 Table 3 shows the
catalytic performances of the Ru catalysts in the selective
oxidation of benzyl alcohol to benzaldehyde in toluene at
353 K under 101.3 kPa of O2. The oxidation did not proceed on
the Py-functionalized SiO2 support B without Ru (entries 1
and 3), and metallic Ru and RuO2 were almost inactive (TON:
0.4 and 1.1, respectively, after 6 h, entries 4 and 5).
12616 | Dalton Trans., 2013, 42, 12611–12619
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The Ru3 cluster A exhibited substantial catalytic activity in a
homogeneous solution with TON of 57.1 after 6 h with quanti-
tative selectivity for benzaldehyde (entry 2). However, the blue
solution of the Ru3 cluster turned into a black suspension,
suggesting the decomposition of the original Ru3 cluster. In
contrast, catalyst C with the supported Ru3 cluster showed
much less activity: TON was only 6.9 after 6 h, but such a color
change of the catalyst was not observed (entry 6). Thus, it is
suggested that the intrinsic activity of A was low but the
decomposed species of A was active. The immobilization of A
would suppress the aggregation of C on the SiO2 surface.
Note that the Ru nanocluster D was significantly active for
the selective oxidation of benzyl alcohol as shown in Table 3.
The differences in the activity were observed against the
loading of Ru species on the SiO2 surface. D (Ru: 1 wt%) was
less active than D (Ru: 3 wt%). TEM images suggested that its
cluster size was smaller than that of D (Ru: 3 wt%) (Fig. S7†),
indicating the strong interaction between the Ru nanocluster
and the support. Indeed the Ru 3p XPS peaks of D (Ru: 1 wt%)
were observed at 461.5 and 483.9 eV, respectively, which were
slightly larger than those of D (Ru: 3 wt%) (461.3 and 483.6 eV,
Table 1), indicating that the Ru species of D (Ru: 1 wt%) were
slightly oxidized compared to those of D (Ru: 3 wt%). TEM
images (Fig. S7†) suggested that the Ru cluster size increased
with the loading of the Ru3 complex on SiO2, and changes in
the number of surface Ru atoms would lead to a decrease in
the activity of D (Ru: 5 wt%).
Catalysts E and F with aggregated metallic Ru particles and
RuO2 particles gave a similar low activity to those on Ru metal
powder and RuO2 (entries 10 and 11), indicating that the
Ru nanoclusters on D were unique in the high activity. The Ru
K-edge EXAFS of D after the oxidation reaction suggested that
the aggregation of D did not proceed under the oxidation con-
ditions (Fig. S8†). The high stability of the supported Ru
nanocluster D would positively contribute to the greater cata-
lytic activity of D in addition to its intrinsic activity.
The heterogeneity test of the Ru nanocluster catalyst D
suggested that the oxidation reaction occurred on the hetero-
geneous catalyst without leaching of the Ru nanocluster in the
reaction solution (Fig. S9†); the filtrate of the reaction mixture
showed almost no activity and no leaching of Ru was observed
Table 4 Catalytic performances of the Ru nanocluster catalyst D (Ru: 3 wt%)
for the selective oxidation of alcoholsa
Conversion
%
b (24 h)
Selectivity of
aldehyde %c
Entry Reactant
1
Product
∼100 (6 h)
90.4
2
3
4
99.2 (11 h)
92.5
95.0 (12 h)
81.6
95.2
89.6
5
74.3
51.8
76.5
78.5
56.7
85.2
6d
7
8
9
∼100
93.8
97.4
73.8
10
11
12
43.6
88.1
30.3
94.5
96.4
88.7
a Reaction conditions: Ru3 = 1.0 × 10−5 mol, Ru3/alcohol/dodecane
(internal standard) = 1/100/50 (molar ratio), 0.33 mol L−1 of alcohol in
toluene, 353 K, 101.3 kPa of O2. Mass balance was above 0.9.
b Conversion (%) = (initial mole of reactant − residual mole of
reactant)/initial mole of reactant × 100%. c Selectivity of aldehyde (%) =
mole of aldehyde product/(initial mole of reactant − residual mole
of reactant) × 100%. d The reaction temperature was 333 K.
by ICP-AES after the full conversion of benzyl alcohol. Together alcohols with electron donating groups were larger than those
with similar Ru 3p XPS peaks after the catalytic oxidation with electron withdrawing groups.34 It is noteworthy that a
(Fig. 5), it is suggested that the Ru nanoclusters on the SiO2 thiophene-containing benzylic alcohol was also efficiently oxi-
surface of D were responsible for the catalytic reaction.
dized to its corresponding aldehyde (entry 8). As for allylic
The Ru nanocluster catalyst D was also found to be active alcohols, cinnamyl alcohol was efficiently and selectively
for the selective oxidation of a variety of alcohols as summar- oxidized to cinnamaldehyde (entry 4). Catalyst D was also
ized in Table 4. Substituted benzylic alcohols (p-OMe and efficient in the oxidation of secondary benzylic alcohols
p-Cl) were efficiently oxidized to the corresponding aldehydes (entries 9–11), although the activity was lower for secondary
with excellent conversion and selectivity without overoxidation aliphatic alcohol such as 2-octanol (entry 12). Primary alcohols
(entries 2 and 3). The initial reaction rates of the substituted were poor substrates on D, similar to most of the reported
benzylic alcohols were independent of the type of the substi- heterogeneous Ru catalysts (sacrificial reagents are generally
tuted groups, suggesting that active species and reaction mech- required).33–36,51–53,59 1-Octanol was converted to n-octanal in
anism for the alcohol oxidation might be different from the 51.8% conversion and 56.7% selectivity after 24 h (entry 6)
reported heterogeneous Ru nanocluster catalysts; for example, even if the reaction temperature was dropped to 333 K. Major
Ru/Al2O3 exhibited that the catalytic activities for benzyl by-products were the corresponding carboxylic acids.
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Dalton Transactions
12 N. Rendón, R. Berthoud, F. Blanc, D. Gajan, T. Maishal,
J.-M. Basset, C. Copéret, A. Lesage, L. Emsley,
S. C. Marinescu, R. Singh and R. R. Schrock, Chem.–Eur. J.,
2009, 15, 5083–5089.
4. Conclusions
Dispersed Ru nanoclusters with the average diameter of 1.3
0.3 nm were successfully transformed from the Ru cluster
Ru3O(CH3COO)6(H2O)3·(CH3COO) grafted on a pyridine-func- 13 S. Muratsugu, Z. Weng, H. Nakai, K. Isobe, Y. Kushida,
tionalized SiO2 surface. The sizes and distribution of the Ru
species highly depended on the treatment conditions of the
T. Sasaki and M. Tada, Phys. Chem. Chem. Phys., 2012, 14,
16023–16031.
supported Ru3 cluster, and the nanoclusterization of the Ru3 14 M. Tada, R. Coquet, J. Yoshida, M. Kinoshita and
cluster successfully proceeded when the loading of the pyri- Y. Iwasawa, Angew. Chem., Int. Ed., 2007, 46, 7220–7223.
dine moiety and the Ru3 cluster was regulated, which was eval- 15 M. Tada, S. Muratsugu, M. Kinoshita, T. Sasaki and
uated by TEM, NMR, XPS, and XAFS analyses. It was found Y. Iwasawa, J. Am. Chem. Soc., 2010, 132, 713–724.
that significant differences in the catalytic activity for the selec- 16 S. Muratsugu and M. Tada, Acc. Chem. Res., 2013, 46, 300–
tive oxidation of alcohol to corresponding aldehyde were 311.
observed on the Ru nanocluster and Ru particle catalysts, and 17 T. J. Terry and T. D. P. Stack, J. Am. Chem. Soc., 2008, 130,
the Ru nanocluster catalyst exhibited significant activity for 4945–4953.
the selective oxidation of a variety of alcohols using O2. The 18 N. J. Schoenfeldt, A. W. Korinda and J. M. Notestein, Chem.
nanoclusterization is promising in providing highly dispersed
and robust Ru clusters for oxidation catalysis.
Commun., 2010, 46, 1640–1642.
19 N. J. Schoenfeldt, Z. Ni, A. W. Korinda, R. J. Meyer and
J. M. Notestein, J. Am. Chem. Soc., 2011, 133, 18684–18695.
20 N. J. Schoenfeldt and J. M. Notestein, ACS Catal., 2011, 1,
1691–1701.
21 C. Bilgrien, S. Davis and R. S. Drago, J. Am. Chem. Soc.,
1987, 109, 3786–3787.
Acknowledgements
This work was supported by the JSPS Funding Program for
Next Generation World-Leading Researchers (GR090) and 22 A. Spencer and G. Wilkinson, J. Chem. Soc., Dalton Trans.,
MEXT/JSPS KAKENHI (grant no. 22108534 and 23750068) in 1972, 5, 1570–1577.
Japan. XAFS measurements were performed with the approval 23 J. A. Baumann, D. J. Salmon, S. T. Wilson, T. J. Meyer and
of PF-PAC (no. 2010G020 and 2011G177). We also thank the W. E. Hatfield, Inorg. Chem., 1978, 17, 3342–3350.
JASSO JENESYS program in Japan and the international collab- 24 J. A. Baumann, D. J. Salmon, S. T. Wilson and T. J. Meyer,
oration program at the Institute for Molecular Science.
Inorg. Chem., 1979, 18, 2472–2479.
25 J. A. Baumann, S. T. Wilson, D. J. Salmon, P. L. Hood and
T. J. Meyer, J. Am. Chem. Soc., 1979, 101, 2916–2920.
26 Y. Sasaki, A. Tokiwa and T. Ito, J. Am. Chem. Soc., 1987, 109,
6341–6347.
Notes and references
1 R. A. Sheldon, M. Wallau, I. W. C. E. Arends and 27 M. Abe, Y. Sasaki, Y. Yamada, K. Tsukahara, S. Yano,
U. Schuchardt, Acc. Chem. Res., 1998, 31, 485–493.
2 P. McMorn and G. J. Hutchings, Chem. Soc. Rev., 2004, 33,
108–122.
3 J. M. Thomas, R. Raja and D. W. Lewis, Angew. Chem., Int.
Ed., 2005, 44, 6456–6482.
4 J. M. Notestein and A. Katz, Chem.–Eur. J., 2006, 12, 3954–
3965.
5 A. Corma and H. Garcia, Adv. Synth. Catal., 2006, 348,
1391–1412.
T. Yamaguchi, M. Tominaga, I. Taniguchi and T. Ito, Inorg.
Chem., 1996, 35, 6724–6734.
28 T. Ito, T. Hamaguchi, H. Nagino, T. Yamaguchi,
J. Washington and C. P. Kubiak, Science, 1997, 277, 660–663.
29 M. Abe, T. Masuda, T. Kondo, K. Uosaki and Y. Sasaki,
Angew. Chem., Int. Ed., 2005, 44, 416–419.
30 B. J. Lear, S. D. Glover, J. C. Salsman, C. H. Londergan and
C. P. Kubiak, J. Am. Chem. Soc., 2007, 129, 12772–12779.
31 A. Inatomi, M. Abe and Y. Hisaeda, Aust. J. Chem., 2012, 65,
1599–1607.
6 M. Tada and Y. Iwasawa, Chem. Commun., 2006, 2833–2844.
7 C. Copéret and J.-M. Basset, Adv. Synth. Catal., 2007, 349, 32 K. Yamaguchi, K. Mori, T. Mizugaki, K. Ebitani and
78–92. K. Kaneda, J. Am. Chem. Soc., 2000, 122, 7144–7145.
8 S. L. Wegener, T. J. Marks and P. C. Stair, Acc. Chem. Res., 33 K. Yamaguchi and N. Mizuno, Angew. Chem., Int. Ed., 2002,
2012, 45, 206–214. 41, 4538–4542.
9 M. Tada, M. Shimamoto, T. Sasaki and Y. Iwasawa, Chem. 34 K. Yamaguchi and N. Mizuno, Chem.–Eur. J., 2003, 9, 4353–
Commun., 2004, 2562–2563. 4361.
10 K. Yamaguchi, C. Yoshida, S. Uchida and N. Mizuno, J. Am. 35 K. Yamaguchi, J. W. Kim, J. He and N. Mizuno, J. Catal.,
Chem. Soc., 2005, 127, 530–531. 2009, 268, 343–349.
11 E. Le Roux, M. Chabanas, A. Baudouin, A. de Mallmann, 36 S. Venkatesan, A. S. Kumar, J.-F. Lee, T.-S. Chan and
C. Copéret, E. A. Quadrelli, J. Thivolle-Cazat, J.-M. Basset, J.-M. Zen, Chem. Commun., 2009, 1912–1914.
W. Lukens, A. Lesage, L. Emsley and G. J. Sunley, J. Am. 37 M. Newville, B. Ravel, D. Haskel, J. J. Rehr, E. A. Stern and
Chem. Soc., 2004, 126, 13391–13399.
Y. Yacoby, Physica B, 1995, 208–209, 154–156.
12618 | Dalton Trans., 2013, 42, 12611–12619
This journal is © The Royal Society of Chemistry 2013

View Article Online
Dalton Transactions
Paper
38 B. Ravel and M. Newville, J. Synchrotron Radiat., 2005, 12, 53 Z. Opre, J.-D. Grunwaldt, M. Maciejewski, D. Ferri,
537–541. T. Mallat and A. Baiker, J. Catal., 2005, 230, 406–419.
39 A. L. Ankudinov, B. Ravel, J. J. Rehr and S. D. Conradson, 54 Z. Opre, D. Ferri, F. Krumeich, T. Mallat and A. Baiker, J.
Phys. Rev. B: Condens. Matter, 1998, 58, 7565–7576. Catal., 2006, 241, 287–295.
40 G. Powell, D. T. Richens and A. Bino, Inorg. Chem. Acta, 55 K. Ebitani, K. Motokura, T. Mizugaki and K. Kaneda,
1995, 232, 167–170. Angew. Chem., Int. Ed., 2005, 44, 3423–3426.
41 K. Yamaguchi, T. Koike, J. W. Kim, Y. Ogasawara and 56 D. V. Bavykin, A. A. Lapkin, P. K. Plucinski, J. M. Friedrich
N. Mizuno, Chem.–Eur. J., 2008, 14, 11480–11487. and F. C. Walsh, J. Catal., 2005, 235, 10–17.
42 T. Komanoya, H. Kobayashi, K. Hara, W.-J. Chun and 57 J. Liu, F. Wang, K. Sun and X. Xu, Adv. Synth. Catal., 2007,
A. Fukuoka, Appl. Catal., A, 2011, 407, 188–194. 349, 2439–2444.
43 Modern Oxidation Methods, ed. J.-E. Bäckvall, Wiley-VCH, 58 M. L. Kantam, U. Pal, B. Sreedhar, S. Bhargava, Y. Iwasawa,
Weinheim, 2004, ch. 4, and references therein.
M. Tada and B. M. Choudary, Adv. Synth. Catal., 2008, 350,
1225–1229.
59 S. Mori, M. Takubo, K. Makida, T. Yanase, S. Aoyagi,
T. Maegawa, Y. Monguchi and H. Sajiki, Chem. Commun.,
2009, 5159–5161.
60 V. V. Costa, M. J. Jacinto, L. M. Rossi, R. Landers and
E. V. Gusevskaya, J. Catal., 2011, 282, 209–214.
61 Y. Zhang, J. Wang, J. Yin, K. Zhao, C. Jin, Y. Huang, Z. Jiang
and T. Zhang, J. Phys. Chem. C., 2010, 114, 16443–16450.
44 Ruthenium in Organic Synthesis, ed. S.-I. Murahashi, Wiley-
VCH, Weinheim, 2004, ch. 3, and references therein.
45 T. Mallat and A. Baiker, Chem. Rev., 2004, 104, 3037–3058.
46 T. Matsumoto, M. Ueno, N. Wang and S. Kobayashi,
Chem.–Asian. J., 2008, 3, 196–214.
47 P. A. Shapley, N. Zhang, J. L. Allen, D. H. Pool and
H.-C. Liang, J. Am. Chem. Soc., 2000, 122, 1079–1091.
48 A. Dijksman, A. Marino-González, A. M. I. Payeras, I. W. C.
E. Arends and R. A. Sheldon, J. Am. Chem. Soc., 2001, 123, 62 Y. Zhang, J. Wang and T. Zhang, Chem. Commun., 2011, 47,
6826–6833. 5307–5309.
49 N. Komiya, T. Nakae, H. Sato and T. Naota, Chem. 63 Y. Y. Gorbanev, S. Kegnæs, C. W. Hanning, T. W. Hansen
Commun., 2006, 4829–4831. and A. Riisager, ACS Catal., 2012, 2, 604–612.
50 B.-Z. Zhan, M. A. White, T.-K. Sham, J. A. Pincock, 64 S. Kobayashi, H. Miyamura, R. Akiyama and T. Ishida,
R. J. Doucet, K. V. R. Rao, K. N. Robertson and
T. S. Cameron, J. Am. Chem. Soc., 2003, 125, 2195–2199.
51 M. Musawir, P. N. Davey, G. Kelly and I. V. Kozhevnikov,
Chem. Commun., 2003, 1414–1415.
J. Am. Chem. Soc., 2005, 127, 9251–9254.
65 T. Matsumoto, M. Ueno, J. Kobayashi, H. Miyamura,
Y. Mori and S. Kobayashi, Adv. Synth. Catal., 2007, 349,
531–534.
52 T. L. Stuchinskaya, M. Musawir, E. F. Kozhevnikova and 66 T. Matsumoto, M. Ueno, N. Wang and S. Kobayashi,
I. V. Kozhevnikov, J. Catal., 2005, 231, 41–47.
Chem.–Asian J., 2008, 3, 239–243.
This journal is © The Royal Society of Chemistry 2013
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