Highly active, selective, and reusable RuO2/SWCNT catalyst for heck olefination of aryl halides

Article abstract of DOI:10.1021/cs500460m

Very fine RuO₂ nanoparticles (RuO₂NPs) with a mean diameter of about 0.9 nm were decorated on single-walled carbon nanotubes (SWCNTs) by a straightforward "dry synthesis" method. TEM images and the Raman spectrum of the resultant material (RuO₂/SWCNT) revealed excellent adhesion and homogeneous dispersion of the RuO₂NPs on anchoring sites of the SWCNTs. The surface area of RuO₂/SWCNT was found to be 416 m² g^-1. The SEM-EDS results showed that the weight percentage of Ru in RuO₂/SWCNT was 13.8%. The oxidation state of Ru in RuO₂/SWCNT was +4, as confirmed by XPS and XRD analyses. After the complete characterization, a 0.9 mol % loading of RuO ₂/SWCNT was used as a nanocatalyst for the Heck olefination of a wide range of aryl halides to yield products in excellent yields with good turnover numbers and turnover frequencies. Less reactive bromo-and chloroarenes were also used for the formation of coupled products in good yields. RuO ₂/SWCNT is regioselective, chemoselective, heterogeneous in nature, and reusable. The stability of RuO₂/SWCNT was also studied by means of TEM, ICP-MS, SEM-EDS, and XPS analyses.

Full text of DOI:10.1021/cs500460m

Research Article
pubs.acs.org/acscatalysis
Highly Active, Selective, and Reusable RuO2/SWCNT Catalyst for Heck
Olefination of Aryl Halides
Mayakrishnan Gopiraman,† Ramasamy Karvembu,*,‡ and Ick Soo Kim*,†,§
†Nano Fusion Technology Research Lab, Interdisciplinary Graduate School of Science and Technology, Shinshu University, Ueda,
Nagano 386 8567, Japan
‡Department of Chemistry, National Institute of Technology, Tiruchirappalli 620 015, India
§Division of Frontier Fibers, Institute for Fiber Engineering (IFES), Interdisciplinary Cluster for Cutting Edge Research (ICCER),
National University Corporation, Shinshu University, Ueda, Nagano 386 8567, Japan
S
* Supporting Information
ABSTRACT: Very fine RuO2 nanoparticles (RuO2NPs) with
a mean diameter of about 0.9 nm were decorated on single-
walled carbon nanotubes (SWCNTs) by a straightforward “dry
synthesis” method. TEM images and the Raman spectrum of
the resultant material (RuO2/SWCNT) revealed excellent
adhesion and homogeneous dispersion of the RuO2NPs on
anchoring sites of the SWCNTs. The surface area of RuO2/
SWCNT was found to be 416 m2 g−1. The SEM−EDS results
showed that the weight percentage of Ru in RuO2/SWCNT
was 13.8%. The oxidation state of Ru in RuO2/SWCNT was
+4, as confirmed by XPS and XRD analyses. After the
complete characterization, a 0.9 mol % loading of RuO2/SWCNT was used as a nanocatalyst for the Heck olefination of a wide
range of aryl halides to yield products in excellent yields with good turnover numbers and turnover frequencies. Less reactive
bromo- and chloroarenes were also used for the formation of coupled products in good yields. RuO2/SWCNT is regioselective,
chemoselective, heterogeneous in nature, and reusable. The stability of RuO2/SWCNT was also studied by means of TEM, ICP-
MS, SEM−EDS, and XPS analyses.
KEYWORDS: single-walled carbon nanotubes, ruthenium dioxide nanoparticles, heterogeneous nanocatalyst, Heck olefination,
regioselectivity, reusability
use of NPs formed from other metals such as Cu,6 Ni,7 Fe,8
1. INTRODUCTION
Rh,9 and Ir10 have been reported for the Heck-type olefination
The transition-metal-catalyzed C−C cross-coupling reaction is
of aryl halides. In fact, from an economical point of view, these
a key step in the synthesis of organic building blocks, natural
products, pharmaceuticals, and agricultural derivatives.1 Cer-
metal NPs (excluding Rh and Ir) are much cheaper and more
reusable than supported PdNPs. For instance, Houdayer et
tainly, Pd-catalyzed olefination of aryl halides (the Heck−
al.11a reported an inexpensive and highly efficient polyaniline/
Mizoroki reaction) is one of the most powerful tools for the
construction of C−C bonds.2 In fact, the Pd-based catalytic
NiNP nanocatalyst for the Heck coupling reaction. Recently,
Miao and co-workers11b found that composites consisting of
systems are highly efficient, and they generally offer excellent
product yields with good selectivity.3 To date, several Pd-based
RhNPs and PtNPs supported on polymeric hollow latex
spheres are efficient for coupling reactions. In spite of their
homogeneous and heterogeneous catalytic systems have been
good catalytic activity in Heck coupling reactions, the scope
proposed for the Heck coupling reaction. Among them,
and functional group tolerance of these catalytic systems are
because of their high activity, easy separation, stability, and
generally limited. More importantly, they show less effective-
ness in the Heck olefination of less-active bromo- and
reusability, Pd nanoparticles (PdNPs), particularly supported
PdNPs, have gained vast importance.4 Mehnert et al.5a prepared
chloroarenes. In addition, most of the catalytic systems suffer
from the use of higher metal loadings (typically 5−10 mol %)
and poor reusability. Therefore, the development of an efficient
catalytic system for Heck-type olefination is a challenging task.
a PdNP-grafted mesoporous MCM-41 material (Pd-TMS11)
by a vapor deposition method and used it as a heterogeneous
catalyst for Heck reactions. They concluded that the Pd-
TMS11 catalyst is highly active, easily accessible, and
exceptionally stable. Similarly, Ioni et al.5b found that PdNPs
decorated on a carbon support such as graphene oxide showed
higher activity for the Heck−Mizoroki reaction. Although the
PdNP-based catalytic systems are highly dominant, recently the
Received: December 11, 2013
Revised: May 19, 2014
© XXXX American Chemical Society
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Figure 1. Schematic illustration of the preparation of RuO2/SWCNT.
Because of its wide range of oxidation states (−2 to +8) and
tunable properties, Ru metal has shown the ability to catalyze a
remarkable range of organic transformations.12 Particularly, in
the past few years RuNP-catalyzed cross-coupling reactions
were found to be an effective tool for the construction of C−C
bonds.13 Na et al.14 employed a Ru/Al2O3 catalyst for both
Heck olefination and Suzuki coupling reactions. They found
that the Ru/Al2O3 catalyst is highly effective and reusable but
that bromo- and chloroarenes are less reactive. Hence, there is a
continuous exploration for a better heterogeneous Ru-based
catalytic system for the Heck coupling reaction. According to
Joo et al.,15a the activity of a supported metal NPs catalyst is
dependent on three main factors: (i) the nature of the support,
(ii) the metal−support interactions, and (iii) the particle size.
Indeed, single-walled carbon nanotubes (SWCNTs) are one of
the promising supports for active metal catalysts in
heterogeneous catalysis, and they are trustable because of
their astounding properties such as high specific surface area
and chemical as well as electrochemical inertness.16 Recently,
Krasheninnikov et al.17 demonstrated that inert SWCNTs can
be transformed to a very active catalyst through interactions
between active metal clusters and carbon vacancies. We
presumed that the decoration of very fine RuO2NPs could
transform SWCNTs to a very active catalyst for the Heck
olefination reaction. In this study, ultrafine RuO2NPs were
decorated over SWCNTs by a simple “dry synthesis” method,
and the resulting material, termed RuO2/SWCNT, was used as
a nanocatalyst for the Heck olefination of aryl halides. The
regioselectivity and chemoselectivity of the RuO2/SWCNT-
catalyzed Heck olefination reaction were investigated, and the
heterogeneity, reusability, and stability of RuO2/SWCNT were
also examined.
diffraction (XRD) experiments were performed at room
temperature using a Rotaflex RTP300 diffractometer (Rigaku
Co., Tokyo, Japan) operated at 50 kV and 200 mA. Nickel-
filtered Cu Kα radiation (5° < 2θ < 80°) was used for XRD
measurements. X-ray photoelectron spectroscopy (XPS) was
performed on a Kratos Axis-Ultra DLD spectrometer (Kratos
Analytical Ltd., Manchester, U.K.) to confirm the oxidation
state of Ru in RuO2/SWCNT. During the XPS analysis, the
sample was irradiated with a Mg Kα X-ray source. Fourier
transform infrared (FT-IR) spectra were recorded on KBr
pellets at room temperature using a NEXUS 670 FT-IR
spectrophotometer (Thermo Nicolet Co., Madison, WI, USA)
in the range of 500−4000 cm−1. The conversions of the
reactants and the yields of coupled products were determined
using a Shimadzu GC-2014 gas chromatograph. The
heterogeneity of the RuO2/SWCNT was tested using
inductively coupled plasma-mass spectrometry (ICP-MS)
(Agilent 7500CS). NMR spectra were recorded on a Bruker
400 MHz NMR spectrometer using tetramethylsilane (TMS)
as a standard.
2.2. Dry Synthesis of RuO2/SWCNT. Functional groups
such as CO, COOH, C−OH, and C−O−C are very
important anchoring sites for metal NPs that assist in
homogeneous decoration and good adhesion of metal NPs
on SWCNTs.18a Hence, at first, SWCNTs were treated with
acid. In a typical procedure, 1.0 g of SWCNTs was chemically
treated with a 3:1 mixture of conc. H2SO4 (75 mL) and HNO3
(25 mL), and then the mixture was sonicated at 40 °C for 3 h
in an ultrasonic bath. After cooling to 21 °C, the solution
mixture was diluted with 1000 mL of deionized water and then
vacuum-filtered through filter paper of 0.65 μm porosity. The
resultant solid (f-SWCNTs) was washed with deionized water
until the pH became neutral and then dried in vacuo at 60 °C.
After that, 0.13 g of Ru(acac)3 was added to 0.5 g of f-SWCNTs
and mixed well using a mortar and pestle. A homogeneous
mixture of f-SWCNTs and Ru(acac)3 was obtained in 13−15
min. Finally, the mixture was calcined under a nitrogen
atmosphere at 350 °C for 3 h in a muffle furnace. Figure 1
shows a schematic illustration of the procedure for the
preparation of RuO2/SWCNT.
2. EXPERIMENTAL SECTION
2.1. Materials and Characterization. SWCNTs (purity
>90% with >70% of the carbon as SWCNTs) with diameters
ranging from 0.7 to 1.3 nm were purchased from Sigma-Aldrich
and used as received. All other chemicals were purchased from
Sigma-Aldrich or Wako Pure Chemicals (Osaka, Japan).
The surface morphology of RuO2/SWCNT was investigated
by transmission electron microscopy (TEM) on a JEOL JEM-
2100F high-resolution transmission electron microscope at an
accelerating voltage of 200 kV. To quantify the weight
percentage of Ru in RuO2/SWCNT, scanning electron
microscopy−energy-dispersive X-ray spectroscopy (SEM−
EDS) was performed using a Hitachi 3000H scanning electron
microscope. The same field of view was then scanned using an
EDS spectrometer to acquire a set of X-ray maps for Ru, C, and
O using 1 ms point acquisition for approximately 1 million
counts. A Raman spectrometer (Hololab 5000, Kaiser Optical
Systems, Inc., Ann Arbor, MI, USA) was applied to examine the
interaction between RuO2NPs and SWCNTs. The Ar laser was
operated at 532 nm with a Kaiser holographic edge filter. X-ray
2.3. Procedure for the Heck Olefination Reaction. In a
typical procedure, RuO2/SWCNT (5 mg, 0.9 mol %) was
added to a mixture of styrene (343 μL, 3.0 mmol), iodobenzene
(111 μL, 1.0 mmol), and (CH3)3COK (224 mg, 2.0 mmol) in
DMF, and the resulting mixture was stirred at 100 °C for 10
min. The progress of the reaction was monitored by thin-layer
chromatography (TLC) and gas chromatography (GC). After
the completion of reaction, the RuO2/SWCNT was separated
from the reaction mixture via centrifugation, and then the
separated nanocatalyst was washed well with ethyl acetate
followed by diethyl ether and dried in an oven at 60 °C for 5 h.
On the other hand, the centrifugate was partitioned between 15
mL of ethyl acetate and 10 mL of saturated aqueous sodium
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Figure 2. (i, ii) Low- and (iii−v) high-magnification HRTEM images of RuO2/SWCNT and (vi) the particle size distribution of RuO2NPs in RuO2/
SWCNT.
Figure 3. (i) SEM image and (ii) EDS spectrum of RuO2/SWCNT and corresponding EDS mappings of (iii) C, (iv) Ru, and (v) O.
hydrogen carbonate. Subsequently, the organic layer was
separated out and dried over anhydrous magnesium sulfate.
The yield of olefinated product was determined by GC. Finally,
the organic layer was concentrated to obtain the coupled
as the carrier gas. The initial column temperature was increased
from 60 to 150 °C at a rate of 10 °C/min and then to 280 °C at
a rate of 40 °C/min. During the product analyses, the
temperatures of the FID and injection port were kept constant
at 150 and 280 °C, respectively.
1
product. The products were confirmed by H and 13C NMR
spectra.
3. RESULTS AND DISCUSSION
2.4. Product Analyses. In order to confirm the formation
of the products, samples of both reactants and products were
dissolved in ethyl acetate and then analyzed by GC. The gas
chromatograph was equipped with a Restek 5% diphenyl/95%
dimethylsiloxane capillary column (0.32 mm diameter, 60 m in
length) and a flame ionization detector (FID). He gas was used
3.1. Characterization of RuO2/SWCNT. Figure 2 shows
the high-resolution TEM (HRTEM) images of RuO2/SWCNT
and the particle size distribution histogram of RuO2NPs in
RuO2/SWCNT. As shown in Figure 2i−v, very fine and
homogeneously dispersed RuO2NPs were externally attached
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Figure 4. (center) Raman spectra of (a) pure SWCNTs, (b) f-SWCNTs, and (c) RuO2/SWCNT. (left) Magnified D band region. (right) Magnified
G band region.
Figure 5. (i) C 1s peak of f-SWCNTs. (ii) O 1s peak of f-SWCNTs. (iii) C 1s peaks and (inset) O 1s peaks of (a) SWCNTs, (b) f-SWCNTs, and
(c) RuO2/SWCNT. (iv) Main Ru 3p peaks of RuO2/SWCNT.
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on anchoring sites of the SWCNTs (for more details, refer to
the TEM images shown in Figure S1 in the Supporting
Information). The homogeneous dispersion of RuO2NPs in
RuO2/SWCNT was also confirmed by HRTEM elemental
mapping of Ru (see Figure S2 in the Supporting Information).
The size distribution histogram of RuO2NPs revealed that the
diameter of the RuO2NPs ranged from 0.1 to 3.0 nm with a
mean diameter of 0.9 nm (Figure 2vi). It is worth mentioning
that no free RuO2NPs were observed in the background of the
TEM images, which shows complete utilization of the
RuO2NPs by the SWCNTs. RuO2/SWCNT has a Brunauer−
Emmett−Teller (BET) surface area of 415.74 m2 g−1 with a
pore volume of 0.6541 cm3 g−1 and a Barrett−Joyner−Halenda
(BJH) desorption average pore diameter of 1.2 nm. These
values are slightly lower than those of f-SWCNTs (BET surface
area = 461.51 m2 g−1, pore volume = 0.8194 cm3 g−1, and BJH
average pore diameter = 1.5 nm) as a result of the
incorporation of the RuO2NPs on the SWCNTs. Moreover,
the theoretical specific surface area (i.e., the surface area per
unit mass) of RuO2NPs was calculated to be S = 956.5 m2 g−1
using the equation Scalcd = 6000/(ρd),15b where d is the mean
diameter and ρ is the density of RuO2 (6.97 g cm−3). According
to Bartholomew et al.,4j maximizing the S value of catalytic NPs
obviously enhances the number of active sites per unit mass
upon which catalytic reactions can occur via chemisorption of
the substrates. The weight percentage of Ru in RuO2/SWCNT
was 13.79 wt %, as determined by SEM−EDS analysis (Figure
3i,ii). Figure 3iv,v shows the homogeneous distribution of
RuO2NPs in RuO2/SWCNT. RuO2/SWCNT contains only
the elements C, Ru, and O, as shown by EDS analysis (Figure
3iii−v), which indicates the reliability of the proposed method.
Since a trace amount (5−15 ppm) of metal impurities such as
Pd may also catalyze the Heck reaction,3 the possible presence
of metal impurities (mainly Pd impurities in RuO2/SWCNT)
was investigated by ICP-MS analysis. RuO2/SWCNT was
found to be free from metal impurities, including Pd (0 ppb).
The detection limit of Pd species by ICP-MS is ∼5 ppt.18b
To investigate the interaction of RuO2NPs on SWCNTs,
Raman spectra were recorded for pure SWCNTs, f-SWCNTs,
and RuO2/SWCNT over the Raman shift interval of 200−4000
cm−1 (spectra a−c, respectively, in Figure 4). All three samples
showed two characteristic peaks at 1345 and 1592 cm−1
corresponding to the sp3- and sp2-hybridized carbons,
respectively, confirming the presence of disordered graphite
(D band) and ordered-state graphite (G band) in the
SWCNTs.19 Since the ratio of the intensities of the D and G
bands (ID/IG) is often used as a diagnostic tool to measure the
defect concentration in SWCNTs, the ID/IG ratio was
calculated for all three samples, and the values are given in
Figure 4.19 It was found that the ID/IG ratio of pure SWCNTs
was 0.5267, whereas after acid treatment (f-SWCNTs) the ID/
IG value increased to 0.7317, confirming the successful
functionalization of SWCNTs. In addition, positive shifts
were observed in the D band (1342 to 1351 cm−1) and the
G band (1587 to 1591 cm−1) of the f-SWCNTs. This is due to
shortening of the SWCNTs as well as the creation of oxygen
functional groups on the SWCNTs during the oxidative
treatment, affording more anchoring sites for RuO2NPs.20 In
fact, the oxygen functional groups make the SWCNTs
hydrophilic and support homogeneous decoration and good
adhesion of RuO2NPs.21 The calculated ID/IG ratio for RuO2/
SWCNT (0.1875) was low compared with that for f-SWCNTs
(0.7317). This may be due to the reduction of the oxygen
functional groups during the calcination process.19 Mainly,
significant reduction of C−O−C, CO, and COOH groups
might lead to the reformation of SWCNTs from the f-
SWCNTs, which is a probable reason for the lower ID/IG ratio
of RuO2/SWCNT (0.1875).19c Negative and positive shifts
were also observed in the D band (1342 to 1331 cm−1) and G
band (1587 to 1593 cm−1), respectively. The results revealed
that the RuO2NPs were strongly attached to the anchoring sites
in the defect structure as well as in the prefect structure of
SWCNTs.20 This phenomenon might be due to the good
dispersion of Ru(acac)3 on f-SWCNTs during the grinding
process.
XPS spectra were recorded for SWCNTs, f-SWCNTs, and
RuO2/SWCNT, and the results are shown in Figure 5. As
expected, all three samples displayed C 1s and O 1s peaks at
284.4 and 532.5 eV, respectively (see Figure S3 in the
Supporting Information).21 Figure 5i,ii shows the deconvoluted
C 1s and O 1s XPS spectra of f-SWCNTs, respectively. In the C
1s spectrum, the binding energies (BEs) of C−C/CC, C−
OH, C−O−C, CO, and COOH groups are assigned as
284.3, 285.0, 285.8, 286.7, and 288.3 eV, respectively.21
Likewise, deconvolution of the O 1s spectrum of f-SWCNTs
resulted in five peaks located at 529.8, 530.7, 531.5, 532.2, and
533.5 eV, which were assigned to CO, COOH, C−OH, C−
O−C, and H2O, respectively.21 Moreover, in comparison with
the C 1s spectrum of SWCNTs, the p → p* shakeup satellite
peak at 291.5 eV completely disappeared in the spectrum of f-
SWCNTs (inset of Figure 5i). FT-IR spectra (see Figure S4 in
the Supporting Information) were also recorded for (a)
SWCNTs and (a) f-SWCNTs to prove the creation of oxygen
functional groups on the SWCNTs. Both samples showed a
broad band at 3450 cm−1, which corresponds to the stretching
of the −OH group. In pure SWCNTs (a), the peaks observed
at 2950 and 2830 cm−1 are ascribed to the asymmetric (νas
CH2) and symmetric (νs CH2) stretching of the C−H bonds,
and the typical peaks observed in the region from 1600 to 1350
cm−1 are due to the aromatic rings.22 The intense band at 1725
cm−1 in the FT-IR spectrum of f-SWCNTs (b) can be assigned
to the CO stretching of carboxyl or carbonyl groups, and the
peak appearing at 1210 cm−1 corresponds to the C−O−C
stretching vibration. In addition, the band at 1570 cm−1 may be
ascribed to the stretching of the carbon nanotube backbone.22
In comparison with that for the pure SWCNTs, the intensity of
the peak at 3450 cm−1 for the f-SWCNTs dramatically
increased, which supports the formation of −COOH groups.
These results confirm the successful creation of the oxygen
functional groups on the SWCNTs. According to Fuller et al.,23
the reactivity of SWCNTs is directly decided by the
concentration of functional groups. The functional groups,
mainly COOH, assist in creating good dispersion and adhesion
of RuO2NPs by replacement of the proton. In addition, the
presence of functional groups facilitates exfoliation of the
SWCNT bundles;21 therefore, very high dispersion of
RuO2NPs on SWCNTs is obtained.
The XPS spectrum of RuO2/SWCNT in the Ru 3p region
(Figure 5iv) showed BE peaks for Ru 3p3/2 at 462.5 eV and Ru
3p1/2 at 485.2 eV, which are attributed to the photoemission
from RuO2 (Ru4+).24 Likewise, in the FT-IR spectrum of
RuO2/SWCNT (see Figure S5 in the Supporting Information),
new peaks at around 700 cm−1 were observed compared with f-
SWCNTs, which confirms the presence of Ru oxo species
(most probably Ru4+ in RuO2).25,19b Furthermore, the weight
percentage of the elements present in the RuO2/SWCNT was
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a
Table 1. Screening for Optimal Reaction Conditions
b
c
d
entry
solvent
base
amount of base (mmol) amount of catalyst (mol %) temp. (°C) time (min) yield (%)
TON/TOF (h−1
)
1
2
DMSO
toluene
DMF
DMAc
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
(CH3)3COK
(CH3)3COK
(CH3)3COK
(CH3)3COK
KOH
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
1.5
2.5
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
2.0
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.45
1.35
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
100
100
100
100
100
100
100
100
100
100
100
100
60
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
5
47
32
91
57
19
39
55
14
69
90
71
91
32
56
89
78
91
91
90
91
52/306
36/212
101/594
63/371
21/124
43/253
61/359
16/94
3
4
5
6
NaOH
7
K2CO3
8
NaOAc
9
(CH3)3COK
(CH3)3COK
(CH3)3COK
(CH3)3COK
(CH3)3COK
(CH3)3COK
(CH3)3COK
(CH3)3COK
(CH3)3COK
(CH3)3COK
(CH3)3COK
(CH3)3COK
77/453
100/529
158/929
67/394
36/212
62/365
99/582
87/975
101/404
101/306
100/238
101/202
10
11
12
13
14
15
16
17
18
19
20
80
120
100
100
100
100
100
15
20
25
30
a
b
c
d
Reaction conditions: 1a (3.0 mmol), 2a (1.0 mmol), air atmosphere. A 5 mL aliquot of solvent was used in all of the reactions. GC yields. The
turnover number (TON) is defined as TON = (molar amount of product)/(molar amount of active sites). The turnover frequency (TOF) was
calculated as TOF = TON/(time in h).
measured by XPS analysis. It is worth mentioning that only
three elements (C, O, and Ru) were detected on the surface of
RuO2/SWCNT, and their weight percentages were 71.22 (C),
15.20 (O), and 13.58 (Ru). This result is in good agreement
with the results of the SEM−EDS and ICP-MS analyses. The
intensities of the peaks in both the O 1s and C 1s spectra of
RuO2/SWCNT dramatically decreased (Figure 5iii), indicating
virtually complete reduction of the functional groups, as also
revealed from the Raman spectra. Interestingly, the strong
interaction of RuO2NPs with SWCNTs was also confirmed by
the positive shift in the C 1s peak of RuO2/SWCNT compared
with that of f-SWCNTs (Figure 5iii). The crystalline structure
of the Ru species in RuO2/SWCNT was also confirmed by
XRD (see Figure S6 in the Supporting Information). The very
weak XRD peaks observed at 27.5°, 34.9°, 39.9°, and 57.5°
correspond to the typical crystal faces (110), (101), (200), and
(220), respectively, of RuO2 (JCPDS no. 21-1172), confirming
the nanocrystalline nature of the RuO2.26
3.2. Screening for Optimal Reaction Conditions.
Initially, the reaction of styrene (1a) with iodobenzene (2a)
was chosen as the model reaction to screen the reaction
conditions (Table 1). In this screening, reaction variables such
as solvent, base, amount of base, amount of catalyst,
temperature, and time were optimized to find the most
effective reaction conditions. Among the various solvents
tested, DMF was found to be most effective (Table 1, entries
1−4). It was found that (CH3)3COK was the most efficient
base (Table 1, entries 3 and 5−8). The amount of base plays a
crucial role in the efficiency of the present catalytic system, and
2.0 mmol of (CH3)3COK was found to be optimal (Table 1,
entry 3). When the amount of base was decreased to 1.5 mmol,
the reaction was very slow (Table 1, entry 9). Unlike PdNP-
based catalytic systems,3,4 the drawback of the present catalytic
system is the limitation on the use of weak and cheap bases
such as KOH, NaOAc, and NaOH. The typical base K2CO3
gave the coupled product in moderate yield of 55%.
Subsequently, the amount of catalyst was optimized (Table 1,
entries 3, 11, and 12). A 5.0 mg loading of RuO2/SWCNT (0.9
mol % Ru) was found to be the optimal amount of catalyst,
affording an excellent yield of 91% (Table 1, entry 3). To our
delight, this is the lowest loading of Ru catalyst (0.9 mol %)
used to perform the Heck olefination of aryl halides. However,
a further decrease in the catalyst loading to 0.45 mol % reduced
the yield of the product to 71%; this might be due to an
inadequate number of catalytically active species (Ru).21d In the
temperature optimization (Table 1, entries 3 and 13−15), an
excellent yield of 91% was obtained when the reaction mixture
was stirred at 100 °C (Table 1, entry 3). To the best of our
knowledge, among the Ru-based catalytic systems for Heck
olefination of aryl halides, the present one requires the lowest
reaction temperature. In the time optimization (Table 1, entries
3 and 16−20), a 91% yield of the product was obtained after 10
min. Further increases in the reaction time did not enhance the
yield of the product. Moreover, under the optimized reaction
conditions, the present catalytic system achieved a good yield of
91% with a good turnover number (TON) of 101 and turnover
frequency (TOF) of 594 h−1 (Table 1, entry 3). The optimal
reaction conditions were adopted to extend the scope of the
Heck olefination of aryl halides.
3.3. Substrate Scope. As shown in Table 2, a wide range
of aryl halides were effectively coupled to give the olefinated
products in moderate to good yields. The product yield was
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a
Table 2. Substrate Scope of the RuO2/SWCNT-Catalyzed Heck-Type Olefination of Aryl Halides
a
Reaction conditions: 1 (3.0 mmol), 2 (1.0 mmol), RuO2/SWCNT (0.9 mol %), (CH3)3COK (2.0 mmol), DMF (5.0 mL), air atmosphere, 10−50
b
c
d
min, 100 °C. GC yield. Isolated yield. TON/TOF.
fairly affected by various substituents on the aromatic ring of
the aryl halide. Aryl iodides gave slightly higher yields than aryl
bromides and aryl chlorides. In the olefination of iodobenzene
with styrene, the present catalytic system gave a better yield of
3a (91%) than the silylated Pd−NHC system.27 The same
reaction conditions were adopted to investigate the catalytic
activity of our previously reported RuO2-based catalyst (RuO2/
GNP).26 The RuO2/GNP catalyst gave a moderate yield of
67%, which may be due to the lower surface area of RuO2/
GNP (65.17 m2 g−1) compared with the present RuO2/
SWCNT catalyst (415.77 m2 g−1). Interestingly, aryl iodides
containing electron-donating groups such as CH(CH3)2 and
OCH3 at the para position were also effectively coupled with
styrene to give the corresponding olefinated products 3b and
3d in excellent yields, whereas the same substrates exhibited
lower yields with ethyl acrylate (3c). In the olefination of 1-
iodo-3-nitrobenzene with styrene, an excellent 92% yield of 3e
was achieved. Similarly, the present system afforded a 92% yield
of ethyl 3-(3-nitrophenyl)acrylate (3f) from the reaction of 1-
iodo-3-nitrobenzene with ethyl acrylate. It was found that a
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Scheme 1. Regioselectivity of the RuO2/SWCNT-Catalyzed Heck Olefination Reaction
Scheme 2. Chemoselectivity of the RuO2/SWCNT-Catalyzed Heck Olefination Reaction
moderate 64% yield of 3g was obtained in the coupling reaction
of 1-iodo-4-benzoic acid with ethyl acrylate. The good yields
obtained from aryl iodides are due to the better leaving ability
of the iodo group.
none with styrene gave a better yield of 3m (71%) compared
with Pd-catalyzed coupling.28 However, the olefination of the
aryl bromide containing CH3 at the para position afforded a
lower yield of 3n (28%). In the reactions of aryl halides
containing a CHO group at the meta position to afford 3o(i)
and 3o(ii), the GC analyses showed the presence of side
products: bromo- or chlorobenzoic acid and styrylbenzoic acid.
Importantly, for the Ru-catalyzed Heck coupling reactions, the
present TONs (31−101) and TOFs (94−600 h−1) are the
highest reported to date. The excellent catalytic activity of
RuO2/SWCNT is due to three most important reasons: (i) the
ultrafine nature of the RuO2NPs, (ii) high specific surface area
of RuO2/SWCNT, and (iii) effective dispersion of RuO2/
SWCNT in the reaction medium.
3.4. Regio- and Chemoselectivity. To study the
regioselectivity of the present catalytic system, the coupling
between styrene and 1-bromo-4-iodobenzene was carried out
under the optimized reaction conditions (Scheme 1). As
expected, 1-bromo-4-styrylbenzene was selectively formed in
67% yield with a higher TON (74) and TOF (435 h−1),
whereas only a 17% yield of 1-iodo-4-styrylbenzene was
obtained. This can be explained by the better leaving ability
of the iodo group compared with the bromo group.
The present catalytic system also worked well for the
olefination of less reactive chloro- and bromoarenes. As shown
in Table 2, a wide range of chloro- and bromoarenes were
olefinated (3h−o). In particular, bromoarenes reacted faster
than chloroarenes. For example, in the coupling of 4-
bromobenzonitrile with styrene, the product 3h(i) was
obtained in 78% yield after just 10 min, whereas the coupling
of 4-chlorobenzonitrile with styrene gave a 79% yield of the
desired product 3h(ii) only after 20 min. In the same way, an
excellent 87% yield of ethyl 3-(4-cyanophenyl)acrylate [3i(i)]
was achieved from the coupling of 4-bromobenzonitrile with
ethyl acrylate in 15 min, but the similar olefination of 4-
chlorobenzonitrile yielded only an 81% yield of 3i(ii) after 30
min. The present RuO2/SWCNT system gave a moderate 61%
yield of 3j(i) or 3j(ii) in the coupling of 4-bromobenzaldehyde
or 4-chlorobenzaldehyde with styrene. The present catalytic
system required only 20 min to afford a 61% yield of the
desired product 3k from the coupling of 2-bromobenzonitrile
with styrene, whereas the CuO/aluminosilicate system yielded
the same product after 20 h.6b Interestingly, a good 86% yield
of 3l was obtained from the olefination of 2-bromobenzonitrile
with ethyl acrylate. These results show the effectiveness of the
present catalytic system toward the olefination of bromo- and
chloroarenes. With a 0.9 mol % loading of RuO2/SWCNT
under optimal conditions, the olefination of 4-bromoacetophe-
When a mixture of styrene (3 mmol), ethyl acrylate (3.0
mmol), and either 4-bromobenzonitrile (Scheme 2a) or 4-
chlorobenzonitrile (Scheme 2b) (1.0 mmol) was allowed to stir
under the optimized reaction conditions, the aryl halide
selectively coupled with styrene to give 4-styrylbenzonitrile in
good yield with higher TON (97/70) and TOF (571/412 h−1)
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Figure 6. (i) Heterogeneity and recyclability test of RuO2/SWCNT [styrene (343 μL, 3.0 mmol), iodobenzene (111 μL, 1.0 mmol), (CH3)3COK
(224 mg, 2.0 mmol) and RuO2/SWCNT (5 mg, 0.9 mol % of Ru) at 100 °C in 5 mL of DMF], (ii) TEM image, (iii) Ru 3p XPS spectrum, (iv)
SEM and corresponding EDS spectrum of u-RuO2/SWCNT, and (v) corresponding EDS mapping of Ru.
in the presence of ethyl acrylate. These results prove the
chemoselective nature of the RuO2/SWCNT catalytic system.
3.5. Heterogeneity, Reusability, and Stability of RuO2/
SWCNT. It is well-known that several heterogeneous catalysts,
particularly in Heck-type olefination reactions, suffer from
leaching of the active species from the support, and therefore,
the stability and reusability of the catalysts are highly limited.
To check whether the RuO2 active species leached out of the
SWCNT support during the reaction, a heterogeneity test was
performed. In a typical test, a mixture of styrene (343 μL, 3.0
mmol) and iodobenzene (111 μL, 1.0 mmol) was stirred under
the optimized reaction conditions. After the reaction was
completed, the solid RuO2/SWCNT was separated from the
reaction mixture by centrifugation and then the filtrate was
analyzed by ICP-MS (Figure 6i); a very low content of Ru
(∼10 ppb) confirmed that the leaching of active species
(RuO2) from the SWCNT support was negligible. This result
inspired a subsequent investigation of the reusability of the
catalyst. After the first use, the catalyst was separated by
centrifugation, washed, dried at 60 °C, and then reused. The
catalyst was reused eight times, and the yields of product are
shown in Figure 6i. The merit of the proposed catalytic system
can be realized from the reusability of RuO2/SWCNT, which
showed an excellent yield (86%), TON (96), and TOF (565
h−1) in the eighth cycle. Moreover, to confirm the stability of
the catalyst, after the first use, the used RuO2/SWCNT (u-
RuO2/SWCNT) was analyzed by TEM, XPS, and SEM−EDS
(Figure 6ii−v). It can be seen that no significant change in the
morphology of u-RuO2/SWCNT was found compared with the
fresh RuO2/SWCNT. The oxidation state and weight
percentage of Ru in u-RuO2/SWCNT were found to be +4
and 13.01 wt %, respectively. These results indicate that RuO2/
SWCNT is physically as well as chemically stable. However, the
TEM image of the catalyst after the eighth cycle (see Figure S7
in the Supporting Information) shows a significant change in
the morphology compared with the fresh RuO2/SWCNT. In
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Figure 7. Proposed mechanism for the Heck olefination of iodobenzene with styrene.
particular, the mean diameter of the RuO2NPs increased (from
0.9 to 6.1 nm) with aggregation of the RuO2NPs. On the
contrary, no significant changes in the oxidation state and
weight percentage of Ru were found. This might be the reason
for the slight decrease in the yield of the coupled product (from
91 to 86%) in the eighth cycle.
Ru(IV) to Ru(VI) by an addition reaction of aryl halide species
with RuO2NPs supported on SWCNTs. The XPS spectrum of
u-RuO2/SWCNT (see Figure S10 in the Supporting
Information) confirmed the formation of K-tOBu·HI (I 3p
and K 2p XPS spectrum) during the reaction. Moreover, after
the reaction, the organic layer was separated out and analyzed
by ICP-MS; a negligible amount of Ru (∼10 ppb) leached from
the catalyst during the reaction.
On the basis of the results obtained from the XPS, ICP-MS
and FT-IR analyses, we concluded that the catalytic reaction
might take place on the RuO2NPs surface via oxidative addition
followed by reductive elimination (Figure 7). In step 1, the aryl
halide is oxidatively added to the Ru(IV) of the RuO2NPs
surface to form a Ru(VI) complex. In step 2, alkyl insertion into
the Ru(VI)−π complex takes place, forming the Ru(VI)−σ
complex intermediate (step 3). Subsequently, in step 4 the
intermediate is converted into the product by β-hydride
elimination. Finally, RuO2/SWCNT is regenerated after the
HI elimination in the presence of K-tOBu (step 5).
3.6. Proposed Mechanism. In order to understand the
mechanism of the RuO2/SWCNT-catalyzed Heck olefination
of aryl halides, FT-IR and XPS spectra (see Figures S8−S10 in
the Supporting Information) were recorded for RuO2/SWCNT
(the pure nanocatalyst), h-RuO2/SWCNT (the catalyst after
stirring with iodobenzene in 5 mL of DMF at 100 °C for 5
min), and u-RuO2/SWCNT (the used nanocatalyst). In
comparison with pure RuO2/SWCNT, h-RuO2/SWCNT
showed extra peaks in both the FT-IR and C 1s XPS spectra,
which may be attributed to adsorption of the aryl halide on the
active sites of RuO2NPs. Furthermore, the XPS spectrum of h-
RuO2/SWCNT showed new peaks in the I 3p region (see
Figure S9ii in the Supporting Information).29a A positive shift
in the Ru 2p3/2 peak (see Figure S9iii in the Supporting
Information) was also observed. Recently, in their experimental
investigation of the redox properties of RuO2NPs decorated on
single-layer graphene, Soin and co-workers29b confirmed the
formation of high-valent Ru species such as Ru(VI) and
Ru(VIII) on the RuO2NPs surface by XPS analysis. Likewise, in
the present case, the positive shift in the Ru 2p3/2 peak might be
due to a change in the oxidation state of RuO2NPs from
4. CONCLUSIONS
In conclusion, a highly efficient SWCNT-supported RuO2NP-
based catalytic system for the Heck olefination of aryl halides
has been developed. The reaction could be efficiently carried
out with as low as a 0.9 mol % loading of the supported RuO2
catalyst over a wide range of substrates in moderate to excellent
yields with good TON and TOF values. In addition to the
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iodoarenes, less reactive bromo- and chloroarenes could also be
effectively olefinated using the present catalytic system. To the
best of our knowledge, RuO2/SWCNT is the most active Ru-
based heterogeneous catalyst for the Heck olefination of aryl
halides among those reported to date. RuO2/SWCNT is highly
regioselective and chemoselective for the Heck olefination
reaction. The heterogeneity and reusability of RuO2/SWCNT
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excellent activity make RuO2/SWCNT an alternative to the
existing Ru-based catalysts for Heck coupling reactions.
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ASSOCIATED CONTENT
* Supporting Information
■
S
TEM and corresponding elemental mapping images, an XRD
pattern, FT-IR and XPS spectra, and NMR data for selected
products. This material is available free of charge via the
Internet at http://pubs.acs.org.
(7) (a) Lipshutz, B. H.; Sclafani, J. A.; Blomgren, P. A. Tetrahedron
2000, 56, 2139−2144. (b) Saito, S.; Oh-tani, S.; Miyaura, N. J. Org.
Chem. 1997, 62, 8024−8030.
(8) Furstner, A.; Leitner, A.; Mendez, M.; Krause, H. J. Am. Chem.
Soc. 2002, 124, 13856−13863.
AUTHOR INFORMATION
Corresponding Authors
*Phone: +91 431 2503636. E-mail: kar@nitt.edu.
*Phone: +81 268 21 5139. E-mail: kim@shinshu-u.ac.jp.
■
(9) (a) Lautens, M.; Roy, A.; Fukuoka, K.; Fagnou, K.; Martın-
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Honma, Y.; Inoue, Y. Org. Lett. 2002, 4, 667−669.
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Chem., Int. Ed. 2003, 42, 89−92. (b) Iyer, S. J. Organomet. Chem. 1995,
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Notes
The authors declare no competing financial interest.
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Lambert, J. Synth. Met. 2005, 151, 165−174. (b) Miao, S.; Zhang, C.;
Liu, Z.; Han, B.; Xie, Y.; Ding, S.; Yang, Z. J. Phys. Chem. C 2008, 112,
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ACKNOWLEDGMENTS
■
This work was supported by a Grant-in-Aid for a Global COE
Program by the Ministry of Education, Culture, Sports, Science,
and Technology, Japan. R.K. thanks DST, Ministry of Science
and Technology, Government of India, for financial support.
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