"Click" magnetic nanoparticle-supported palladium catalyst: A phosphine-free, highly efficient and magnetically recoverable catalyst for Suzuki-Miyaura coupling reactions

Article abstract of DOI:10.1039/c2cy20532g

A novel magnetic nanoparticle-supported nano-palladium catalyst was successfully prepared via a "click" route. The as-prepared catalyst was well characterized and evaluated in Suzuki-Miyaura coupling in terms of activity and recyclability in aqueous ethanol. It was found to be highly efficient for the reactions of various aryl bromides with arylboronic acids under phosphine-free and low Pd loading (0.2 mol%) conditions. Moreover, the catalyst could be easily separated from the reaction system by an external magnet and reused several times without a remarkable loss of its activity. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2cy20532g

Catalysis
Science & Technology
View Article Online
View Journal | View Issue
PAPER
‘‘Click’’ magnetic nanoparticle-supported palladium
catalyst: a phosphine-free, highly efficient and
magnetically recoverable catalyst for Suzuki–Miyaura
coupling reactions
Cite this: Catal. Sci. Technol.,
2013, 3, 235
Qiang Zhang, Hong Su, Jun Luo* and Yunyang Wei
A novel magnetic nanoparticle-supported nano-palladium catalyst was successfully prepared via a
‘‘click’’ route. The as-prepared catalyst was well characterized and evaluated in Suzuki–Miyaura coupling
in terms of activity and recyclability in aqueous ethanol. It was found to be highly efficient for the
reactions of various aryl bromides with arylboronic acids under phosphine-free and low Pd loading
(0.2 mol%) conditions. Moreover, the catalyst could be easily separated from the reaction system by an
external magnet and reused several times without a remarkable loss of its activity.
Received 29th July 2012,
Accepted 30th August 2012
DOI: 10.1039/c2cy20532g
www.rsc.org/catalysis
to the diffusion of the reactants through the pores of the
supports. Alternatively, ligand-free Pd catalysts were developed
Introduction
The palladium-catalyzed cross-coupling of aryl halides with
arylboronic acids, known as Suzuki–Miyaura coupling, has
been recognized as one of the most powerful tools for the
construction of unsymmetrical biaryl units,1 and extensively
used in the synthesis of natural products, pharmaceuticals and
functional materials.2 Although, a variety of homogeneous
palladium catalysts, such as palladium–phosphine complexes,3
oxime-carbapalladacycles,4 or palladium–N-heterocyclic carbene
complexes,5 have been proved to be highly active and selective in
this transformation, separation of the catalyst from the product
is often problematic in these homogeneous systems. Moreover,
catalyst recovery is highly desirable, in particular, if expensive
noble metal catalysts are used in large-scale synthesis due to
environmental and economic concerns.
Heterogenization could be an attractive solution to these
problems since the Pd catalyst immobilized on a support could
be easily separated from the product and effectively reused. A
lot of organic and inorganic supports, including microporous
polymers,6 carbon materials,7 amorphous or mesoporous silica,8
metal oxides,9 and molecular sieves,10 have been explored in Pd-
mediated catalysis, in which Pd complexes or nanoparticles are
involved as active species. Although significant efforts have been
made to develop recoverable Pd catalysts, some supported Pd
catalysts suffered from lower efficiency in comparison with their
analogues in homogeneous systems, owing to limitations related
by entrapment or adsorption of Pd salts on porous solids.
Meanwhile, Pd leaching could be found in such systems.11
Nanoparticles (NPs) have recently emerged as robust alter-
natives for the immobilization of homogeneous catalysts due to
their high surface area and low porosity caused by their
nanoscale dimensions.12 This makes the active catalytic sites
on the surface well accessible to the reactants. And the NP-
supported catalysts can be readily dispersed in the reaction
system, where the catalytic sites seem to be soluble, thus the
catalytic NPs can be considered as quasi-homogeneous cata-
lysts. However, they still suffer from the problem of catalyst
separation and recovery due to their colloidal nature. This
could be solved by using magnetic nanoparticles (MNPs),
which can be simply removed from the reaction system by
magnetic separation. Over the past few decades, a series of
MNP-supported Pd catalysts have been designed by anchoring
strategies.13 Recently, Thiel et al. have reported a successful
example of immobilizing a Pd(II)–phosphine complex on
MNPs for Suzuki reaction.14 Li and his co-workers developed
an efficient MNP-supported Pd catalyst for Suzuki and Heck
reactions.15 Wang’s group also prepared another MNP-sup-
ported Pd–phosphine complex for various C–C coupling reac-
tions.16 Among them, phosphines were the most employed
ligands. However, phosphine ligands are often extremely
expensive, air-sensitive, and virulent, which limit their wide
use in large-scale application.17 In addition, high Pd loadings
were generally required for reasonable catalytic results.
Therefore, it is still highly desirable to develop phosphine-free
School of Chemical Engineering, Nanjing University of Science & Technology,
Nanjing, 210094, PR China. E-mail: luojun@njust.edu.cn; Fax: +86-25-84315030
c
This journal is The Royal Society of Chemistry 2013
Catal. Sci. Technol., 2013, 3, 235--243 235

View Article Online
Paper
Catalysis Science & Technology
Scheme 1 Synthesis of magnetic nanoparticle-supported nano-palladium catalyst 6. Reaction conditions: (a) MeOH, pyridine-2-carbaldehyde, reflux, 3 h; (b) K2CO3,
acetone, propargyl bromide, 50 1C, 18 h; (c) NaN3, CH3CN, TBAB, reflux, 24 h; (d) silica-coated Fe3O4 nanoparticles, toluene, reflux, 24 h; (e) 2, CuI, DIPEA, DMF/THF, r.t.,
3 d; (f) Na2Pd2Cl6, MeOH, 60 1C, 24 h, then AcONa, r.t., 1 h.
MNP-supported Pd catalysts with high activity, excellent stability
and low cost.
The copper-catalyzed azide–alkyne cycloaddition (CuAAC)
reaction known as ‘‘click chemistry’’ strategy has been proved
to be a powerful tool for conjugation between azides and
alkynes owing to its high efficiency, simple procedure, mild
conditions and absence of byproducts.18 More recently, the
1,2,3-triazole unit has drawn much attention for catalyst immo-
bilization as a highly stable linker or/and a chelator, and many
metal catalysts, such as Au,19 V,20 and Pd,21 have been succes-
sively explored. Inspired by its attractive features, we tried to
develop a new ‘‘click’’ MNP-supported catalyst incorporating a
1,2,3-triazole moiety for the fusion of linker functionality and a
MNP backbone.
In continuation of our efforts in designing greener supported
catalysts,22 we herein present the preparation of a novel MNP-
Fig. 1 FT-IR spectra of SiO2@Fe3O4 (a), azide-functionalized MNP 4 (b), imino-
supported Pd catalyst (Pdnp@MNP) via the copper-catalyzed
pyridine ligand-functionalized MNP 5 (c).
ligation of an alkynlated imino-pyridine ligand with azide func-
tionalized silica-coated Fe3O4 nanoparticles (Scheme 1), and its
application in Suzuki–Miyaura reactions.
as a typical ‘‘click’’ catalytic system23 led to nearly complete
conversion of all azide groups. FT-IR spectra show a strong
absorption band at 2104 cmꢀ1 (N3 vibrations), which disap-
Results and discussion
pears upon formation of the 1,2,3-triazole linker during the
The magnetic nanoparticle-supported Pd catalyst was prepared ‘‘click’’ reaction. In addition, the typical bands at around
following the procedure shown in Scheme 1. The imino-pyridine 1590 cmꢀ1 (CQN vibration) and 1500 cmꢀ1 (CQC vibration
ligand 1 was firstly synthesized by a simple condensation of of the aryl ring) are observed (Fig. 1). The above results indicate
4-aminophenol with pyridine-2-carbaldehyde, then it underwent that the imino-pyridine ligand was successfully grafted onto
direct propargylation to afford compound 2 in an almost quanti- SiO2@Fe3O4.
tative yield. Secondly, the silica-coated Fe3O4 nanoparticles
Ultimately, the resulting functionalized MNP 5 was treated
(SiO2@Fe3O4) were prepared according to our previous methods22b with a methanolic solution of Na2Pd2Cl6 under reflux condi-
and chosen as the magnetic nano-support. The azide group was tions24 to provide the desired solid catalyst Pdnp@MNP 6. The
then introduced into the SiO2@Fe3O4 through a simple procedure amount of Pd measured by inductively coupled plasma atomic
and verified by the appearance of the N3 band at 2104 cmꢀ1 in emission spectrometry (ICP-AES) was 1.18%. X-Ray reflective
FT-IR spectra (Fig. 1). Meanwhile, the loading of the azide group diffraction (XRD) and transmission electron microscopy (TEM)
was determined to be 0.40 mmol gꢀ1 by elemental analysis of the were then carried out to get detailed information about the
nitrogen content.
structure and morphology of the as-prepared catalyst.
Reaction of the azide-functionalized MNP 4 with an excess of
The high-angle XRD pattern of the novel catalyst exhibits
compound 2 using CuI and N,N-diisopropylethylamine (DIPEA) diffraction peaks corresponding to the standard Fe3O4 sample
c
236 Catal. Sci. Technol., 2013, 3, 235--243
This journal is The Royal Society of Chemistry 2013

View Article Online
Catalysis Science & Technology
Paper
Fig. 2 XRD patterns of SiO2@Fe3O4 (a), MNP-supported Pd catalyst 6 (b). The
bottom row of tick marks indicates the reflection positions for a standard
magnetite pattern (JCPDS no. 19-0629).
Fig. 4 TEM images of SiO2@Fe3O4 (a), MNP-supported Pd catalyst 6 (b), six-times
reused catalyst (c). The arrows indicate some of the Pd nanoparticles.
Fig. 3 Raman spectrum of the as-synthesized magnetic catalyst.
(JCPDS file No. 19-0629, Fig. 2). The broad peak from 2y = 201 to
301 is consistent with an amorphous silica layer. No characteristic
peaks for Pd nanoparticles are observed which proves the excellent
dispersion of the Pd sites on the magnetic nanoparticles.16 In
addition, the average crystal size of the Fe3O4 cores, calculated
using the Scherrer formula, is about 10.5 nm.
As both magnetite Fe3O4 and maghemite g-Fe2O3 have good
magnetic properties and similar XRD patterns,25a Raman
spectroscopy was used to distinguish the different structural
phases of iron oxides. The Raman spectrum of the as-synthesized
magnetic catalyst shows the main feature at 667 cmꢀ1 (A1g)
characteristic of magnetite (Fig. 3).25
Fig. 5 TG-DTG analysis for Pdnp@MNP.
The TEM images of the support SiO2@Fe3O4 and the catalyst
Pdnp@MNP 6 are shown in Fig. 4. The dark nano-Fe3O4 cores
are surrounded by a grey silica shell of about 3–5 nm thick and
the average size of the Fe3O4 cores is about 8–12 nm (Fig. 4a),
which is consistent with that determined using the Scherrer
equation in the XRD pattern. After anchoring of Pd, the Pd
nanoparticles are distinguishable with the difference in their
contrast, as indicated by the arrows in Fig. 4b and c. Some Pd
particles (about 4 nm) are attached to the support particles
(about 25 nm).
c
This journal is The Royal Society of Chemistry 2013
Catal. Sci. Technol., 2013, 3, 235--243 237

View Article Online
Paper
Catalysis Science & Technology
Table 1 Effect of the solvent on Suzuki–Miyaura reactiona
Entry
Solvent
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
MeOH
3
3
6
6
3
6
3
3
3
3
3
54
65
81c
37c
78
96c
94
89
97
92
75
EtOH
iPrOH
H2O
95% EtOH
95% EtOH
MeOH/H2O (1 : 1, v/v)
iPrOH/H2O (1 : 1, v/v)
EtOH/H2O (1 : 1, v/v)
EtOH/H2O (2 : 1, v/v)
EtOH/H2O (1 : 2, v/v)
9
10
11
a
Reaction conditions: 4-bromoacetophenone (1.0 mmol), phenylboronic acid (1.2 mmol), K2CO3 (2.0 mmol), catalyst (0.2 mol% Pd) in 6.0 mL of
b
c
solvent at 60 1C under air. Isolated yield. At 80 1C.
Table 2 Effect of the base and amounts of catalyst on Suzuki–Miyaura reactiona
Entry
Base
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
9
K3PO4
K2CO3
Cs2CO3
Na2CO3
Na3PO4
NaHCO3
NEt3
3
3
3
3
3
3
3
3
3
95
97
94
89
87
46
31
99c
83d
K2CO3
K2CO3
a
Reaction conditions: 4-bromoacetophenone (1.0 mmol), phenylboronic acid (1.2 mmol), base (2.0 mmol), catalyst (0.2 mol% Pd) in 6.0 mL of
b
c
d
EtOH/H2O (1 : 1, v/v) at 60 1C under air. Isolated yield. Catalyst (1.0 mol% Pd) was used. Catalyst (0.1 mol% Pd) was used.
The stability of the catalyst Pdnp@MNP was also investigated of different volume ratios of EtOH/H2O was then tested, and the
by the thermogravimetric (TG) analysis (Fig. 5). The TG curve best one is 1 : 1 (Table 1, entry 9).
indicates an initial weight loss of 1.8% up to 150 1C, which is
Next, a series of bases were taken into consideration for the
owing to the adsorbed water on the support. Thermal degrada- model reaction in EtOH/H2O (1 : 1, v/v). With regard to other
tion of the catalyst occurred after 250 1C, which revealed the
excellent stability. Meanwhile, the DTG curve shows that the
decomposition of the organic structure mainly occurred in one
step from 370 to 560 1C, which is related to a main weight loss
of 13.2%. Therefore, Pdnp@MNP was very stable and could be
used within a broad temperature scale.
The activity of this novel catalyst was initially tested in
Suzuki–Miyaura coupling. The coupling of 4-bromoacetophenone
and phenylboronic acid was chosen as the model reaction. Due to
environmental concerns, only alcohol or water chosen as solvent
were examined, and a significant effect was observed (Table 1).
While the model reaction was carried out in pure alcohol or water,
moderate yield could be obtained (Table 1, entries 1–4). Interest-
ingly, when we adopted the aqueous alcohol as the solvent,
satisfactory results were obtained (Table 1, entries 5–9). The
advantage of the co-solvent is attributed to the good solubility
Fig. 6 Recycling experiment of the MNP-supported Pd catalyst.
of the organic reactants and the inorganic base. The influence
c
238 Catal. Sci. Technol., 2013, 3, 235--243
This journal is The Royal Society of Chemistry 2013

View Article Online
Catalysis Science & Technology
Paper
bases, K2CO3 was found to act as an excellent base (Table 2, excellent yields (Table 3, entries 1–8). And a wide range of
entry 2). K3PO4, Cs2CO3, Na2CO3, and Na3PO4 were also effec- functional groups, such as nitryl, cyano, fluoro, methoxy and
tive (Table 2, entries 1 and 3–5). The organic base NEt3 was also aldehyde, could be well tolerated in the reaction. The ortho-
studied, which afforded an unsatisfactory yield (Table 2, substituted aryl bromide generated the corresponding product
entry 7). Different amounts of catalyst between 0.1 and in lower yield for a longer time because of steric effects (Table 3,
1.0 mol% were further investigated for this reaction, and entry 9). Some substituted arylboronic acids were also tested,
0.2 mol% loading of Pd was found to be optimal. For the higher and they all reacted smoothly with aryl bromides and afforded
amounts of catalyst, the desired product was obtained in a nearly the desired products in high yields (Table 3, entries 13–19).
quantitative yield (Table 2, entry 8). However, the yield of the Strangely, all attempts to carry out the reaction of 4-bromo-
reaction was lower in 0.1 mol% Pd dosage (Table 2, entry 9).
aniline and 4-bromophenol were unsuccessful, only about 20%
The recovery and reuse of catalyst becomes an important yield of the corresponding products were obtained. When
factor due to stringent economical and ecological demands for 4-iodoaniline and 4-iodophenol were chosen as the alternative
sustainability. Therefore, the recyclability of this MNP-sup- substrates, the desired products could be obtained in good yields
ported catalyst was immediately investigated by using the above (Table 3, entries 10–12). Additionally, 4-chloroacetophenone and
model reaction in three different volume ratios of EtOH/H2O 4-chloronitrobenzene were chosen as the challenging substrates,
(1 : 1, 2 : 1, and 19 : 1, respectively). After the completion of the however, the catalytic system was less effective even when using
reaction, the magnetic catalyst could be simply and efficiently higher Pd loading and prolonged reaction time (Table 3, entries
recovered from the reaction mixture with an external magnet, 20 and 21).
washed with water and ethanol, dried under vacuum and
reused in a subsequent reaction. More than 99% of the catalyst
could be recovered from each run. As can be seen from Fig. 6, the
catalyst reusability was obviously influenced by the amount of
Experimental
General
water in the reaction system. Although, some water could make Melting points were determined using a WRS-1B apparatus and
the Suzuki reaction proceed well under milder conditions, the were uncorrected. The IR spectra were recorded on a Nicolet
catalyst lost its activity gradually. A little corrosion on the silica spectrometer (KBr). 1H NMR spectra were recorded on a Bruker
layer of the supported catalyst in aqueous basic reaction medium DRX500 (500 MHz) and 13C NMR spectra on a Bruker DRX500
caused leaching and aggregation of the Pd nanoparticles, which (125 MHz) spectrometer. Elemental analysis was performed on
might be the main reason.13e Interestingly, it was found that the an Elementar Vario EL b recorder. Palladium content of the
catalyst activity could be retained for a few more cycles when a less catalyst was measured by inductively coupled plasma (ICP) on
amount of water was used as the reaction solvent. However, a an L-PAD analyzer (Prodigy). X-ray diffraction (XRD) images
higher temperature and a longer time were necessary. It is worth were obtained from a Bruker XRD D8 Advance instrument with
noting that the catalyst could be reused without a significant loss Cu Ka radiation (l = 0.15418 nm). Transmission electron
of its activity in a test of six cycles, when the reaction was microscopy (TEM) images were obtained using a JEM-2100
performed in 95% EtOH at 80 1C for 6 h.
instrument. The Raman spectra were obtained using a
Pd leaching of the catalyst was further determined. ICP Renishaw inVia Raman Microscope. The 514 nm argon ion
analysis of the clear filtrate showed that the Pd content was laser line was used for excitation and the laser power was kept
less than 0.16 ppm, which revealed that the MNP-supported below 0.1 mW, to avoid sample degradation.
Pd catalyst was very stable and could endure this coupling
Synthesis of imino-pyridine ligand (1)
condition. Meanwhile, the iron leaching was also determined
and analysis of the reaction solution indicated that the Fe Pyridine-2-carbaldehyde (5.35 g, 50.0 mmol) and p-amino-
content was less than 0.23 ppm in the solution, which also phenol (5.46 g, 50.0 mmol) were added into 25 mL of methanol
demonstrated that nearly no corrosion on the silica layer of the and the resulting mixture was stirred under reflux for 3 h. The
SiO2@Fe3O4 supported Pd catalyst occurred in the 95% EtOH mixture was slowly cooled to room temperature and the pure
reaction medium. Moreover, the filtered solution exhibited no solid product 1 (6.44 g, 32.5 mmol) was obtained by filtration
reactivity to a fresh model reaction. The TEM analysis of the immediately (yield: 65%). Yellow crystals, mp: 186–187 1C;
recovered catalyst indicated that the size and morphology of the 1H NMR (500 MHz, DMSO-d6): d 9.64 (s, 1H), 8.63 (s, 1H),
catalyst after the sixth run had no apparent change and 8.67–8.58 (m, 1H), 8.50 (s, 1H), 8.09 (d, J = 8.0 Hz, 1H), 7.92–7.88
agglomeration (Fig. 4c).
(m, 1H), 7.48–7.45 (m, 1H), 7.30–7.26 (m, 2H), 6.82–6.79
With these good results in hand, several representative (m, 2H).
coupling reactions of a variety of aryl halides with arylboronic
Synthesis of compound 2
acids were then explored under two kinds of optimal condi-
tions, in order to survey the versatility of this MNP-supported A round-bottom flask was charged with compound 1 (2.97 g,
Pd catalyst. As shown in Table 3, the coupling between aryl 15.0 mmol), K2CO3 (4.14 g, 30.0 mmol), propargyl bromide
bromides and phenylboronic acid, which contained electron- (1.79 g, 15.2 mmol) and acetone (50 mL). The resulting mixture
donating as well as electron-withdrawing groups, proceeded was stirred at 50 1C under a nitrogen atmosphere for 18 h, and
effectively to afford the corresponding products in good to then cooled and filtered. The solvent was removed by rotary
c
This journal is The Royal Society of Chemistry 2013
Catal. Sci. Technol., 2013, 3, 235--243 239

View Article Online
Paper
Catalysis Science & Technology
Table 3 Suzuki–Miyaura coupling reaction of aryl halide with arylboronic acid
Conditions Aa
Conditions Bb
Entry
1
1
2
3
t (h)
Yieldc (%)
t (h)
Yieldc (%)
3a
3
97
99
95
96
92
97
6
96
2
3
4
5
6
3b
3c
3d
3e
3f
3
3
3
4
4
6
6
6
6
6
97
90
92
88
90
7
8
9
3g
3h
3i
4
4
8
93
90
80
8
8
91
84
72
12
10
11
12
13
3j
7
5
5
3
86
91
90
94
10
8
80
89
87
92
3k
3l
8
3m
6
14
15
3n
3o
3
4
93
86
6
6
89
85
16
17
3p
3q
3
4
95
91
6
6
96
82
c
This journal is The Royal Society of Chemistry 2013
240 Catal. Sci. Technol., 2013, 3, 235--243

View Article Online
Catalysis Science & Technology
Paper
Table 3 (continued )
Conditions Aa
Conditions Bb
Entry
18
1
2
3
t (h)
Yieldc (%)
t (h)
Yieldc (%)
3r
6
89
8
83
19
3s
3p
3s
3
12
12
97
o5
12
6
24
24
94
o5
17
20d
21d
a
Reaction conditions A: aryl halide (1.0 mmol), arylboronic acid (1.2 mmol), K2CO3 (2.0 mmol), catalyst (0.2 mol% Pd) in 6.0 mL of EtOH/H2O
b
(1 : 1, v/v) at 60 1C under air. Reaction conditions B: aryl halide (1.0 mmol), arylboronic acid (1.2 mmol), K2CO3 (2.0 mmol), catalyst (0.2 mol%
Pd) in 6.0 mL of 95% EtOH at 80 1C under air. Isolated yield. Catalyst (1.0 mol% Pd) was used.
c
d
evaporation under reduced pressure, and product 2 was achieved by pre-mixing (ultrasonic) a dispersion of the above
obtained as a brown solid (3.47 g, 98%). 1H NMR (500 MHz, black precipitate (2.0 g) with ethanol (400 mL) for 30 min at
CDCl3): d 8.72 (d, J = 4.2 Hz, 1H), 8.64 (s, 1H), 8.21 (d, J = 7.9 Hz, room temperature. And conc. NH3ꢂH2O (12 mL) and TEOS
1H), 7.82 (td, J = 7.6, 1.3 Hz, 1H), 7.46–7.31 (m, 3H), 7.10–7.00 (4.0 mL) were added successively. After stirring for 24 h, the
(m, 2H), 4.74 (d, J = 2.4 Hz, 2H), 2.56 (t, J = 2.4 Hz, 1H); 13C NMR black precipitate (SiO2@Fe3O4) was collected using a perma-
(125 MHz, CDCl3): d 158.80, 156.81, 154.81, 149.66, 144.60, nent magnet, followed by washing with ethanol three times and
136.62, 124.89, 122.59, 121.70, 115.56, 78.45, 75.66, 56.09.
drying in a vacuum. FT-IR (KBr, cmꢀ1): 3445, 1638, 1097, 580.
Synthesis of 3-azidopropyltriethoxysilane (3)21
Synthesis of azide-functionalized MNP (4)
3-Chloropropyltriethoxysilane (4.82 g, 20.0 mmol) was added to 1.5 g of SiO2-coated Fe3O4 nanoparticles were dispersed in dry
a solution of sodium azide (1.95 g, 30.0 mmol) and tetrabutyl- toluene (75 mL) by sonication for 1 h. 3-Azidopropyltriethoxy-
ammonium bromide (TBAB, 1.29 g, 4.0 mmol) in dry aceto- silane (0.5 g, 2.02 mmol) was then added, and the reaction
nitrile (75 mL) under a nitrogen atmosphere. And the reaction mixture was refluxed for 24 h under nitrogen. After being
mixture was stirred under reflux for 48 h. After the reaction, the cooled to ambient temperature, the azide-functionalized MNP
solvent was removed under reduced pressure. The crude mix- 4 was collected by a permanent magnet and washed thoroughly
ture was then diluted in Et2O (30 mL) and the suspension was with toluene and acetone, and then dried under vacuum over-
filtered and washed with Et2O (2 ꢁ 10 mL). The combined night. The loading of the azide group was determined to be
solvent was removed and product 3 was obtained as a pure and 0.40 mmol gꢀ1 by elemental analysis. According to FT-IR
colorless liquid (4.06 g, 82%). 1H NMR (500 MHz, CDCl3): d 8.82 spectra, the absorption band at 2104 cmꢀ1 clearly indicates
(q, J = 7.0 Hz, 6H), 3.26 (t, J = 7.0 Hz, 2H), 1.74–1.68 (m, 2H), the successful attachment of the azide group onto the surface
1.22 (t, J = 7.0 Hz, 9H), 0.69–0.65 (m, 2H); FT-IR (KBr, cmꢀ1): of SiO2@Fe3O4. FT-IR (KBr, cmꢀ1): 3438, 2941, 2872, 2104,
2979, 2886, 2099, 1443, 1394, 1242, 1072, 952, 776.
1638, 1085, 801, 580.
Synthesis of silica-coated Fe3O4 nanoparticles (SiO2@Fe3O4)22b Preparation of imino-pyridine ligand-functionalized MNP via
click reaction (5)
Magnetite (Fe3O4) nanoparticles were prepared in a chemical
co-precipitation step of ferric and ferrous ions in an alkaline Compound 2 (0.5 g, 2.12 mmol) and azide-functionalized MNP
solution. FeCl3ꢂ6H2O (11.0 g) and FeCl2ꢂ4H2O (4.0 g) were 4 (1 g) were mixed with CuI (17 mg, 0.09 mmol) in DMF/THF
dissolved in 250 mL deionized water under nitrogen with (1 : 1, 10 mL) under nitrogen, to this was injected DIPEA
vigorous stirring at 85 1C. The pH value of the solution was (2 mL), and the reaction mixture was stirred at room tempera-
adjusted to 9 by conc. NH3ꢂH2O. After continuously stirring for ture for 3 d. Then the reaction mixture was subjected to
4 h, the magnetite precipitates were washed by amounts of magnetic separation, and the MNP 5 was washed sequentially
deionized water to reach pH = 7. A black precipitate (Fe3O4) was with Et2O (3 ꢁ 20 mL), H2O (3 ꢁ 20 mL), then acetone (3 ꢁ
collected with a permanent magnet under the reaction flask. 20 mL), and finally dried under vacuum. The loading of an
Coating of a layer of silica on the surface of the nano-Fe3O4 was imino-pyridine ligand was determined to be 0.34 mmol gꢀ1
c
This journal is The Royal Society of Chemistry 2013
Catal. Sci. Technol., 2013, 3, 235--243 241

View Article Online
Paper
Catalysis Science & Technology
by elemental analysis. Notably, the absorption band at 2104 cmꢀ1
3-METHOXYBIPHENYL (3G). Colorless oil; 1H NMR (500 MHz,
disappeared which implied that the ‘‘click’’ process was completed CDCl3): d 7.63 (d, J = 7.0 Hz, 2H), 7.47 (t, J = 7.5 Hz, 2H),
and the imino-pyridine ligand has anchored onto the MNP. FT-IR 7.41–7.37 (m, 2H), 7.22 (d, J = 8.0 Hz, 1H), 7.17 (t, J = 7.0 Hz,
(KBr, cmꢀ1): 3445, 2941, 2872, 1638, 1594, 1506, 1085, 807, 580.
1H), 6.94 (dd, J = 8.0, 2.5 Hz, 1H), 3.89 (s, 3H).
4-METHYLBIPHENYL (3H). White solid, mp: 47–48 1C; 1H NMR
(500 MHz, CDCl3): d 7.57 (d, J = 7.5 Hz, 2H), 7.49 (d, J = 8.0 Hz,
2H), 7.42 (t, J = 7.5 Hz, 2H), 7.32 (t, J = 7.5 Hz, 1H), 7.24 (d, J =
8.0 Hz, 2H).
Preparation of Pdnp@MNP (6)
A 5 mL round-bottom flask was charged with PdCl2 (17.0 mg,
96 mmol) and NaCl (6.6 mg, 113 mmol). MeOH (1 mL) was added
and the mixture was left stirring at room temperature for 24 h,
the mixture was then filtered through a plug of glass wool and
the filtrate diluted with additional 9 mL of MeOH. The MNP 5
(0.5 g) was added to the flask, and the resulting mixture was heated
to 60 1C for 24 h. At the end of this period, NaOAc (56.0 mg,
0.68 mmol) was added, the mixture was stirred at room tempera-
ture for 1 h. Finally, the catalyst Pdnp@MNP 6 was collected by a
permanent magnet, and washed successively with MeOH, water
and acetone, and then dried under vacuum overnight. The content
of Pd was 1.18% as determined by ICP-AES.
2-METHOXYBIPHENYL (3I). Colorless oil; 1H NMR (500 MHz,
CDCl3): d 7.59 (d, J = 8.0 Hz, 2H), 7.47 (t, J = 7.5 Hz, 2H),
7.39–7.36 (m, 3H), 7.09 (t, J = 7.5 Hz, 1H), 7.04 (d, J = 8.5 Hz,
1H), 3.86 (s, 3H).
4-AMINOBIPHENYL (3J). White solid, mp: 52–53 1C; 1H NMR
(500 MHz, CDCl3): d 7.54–7.52 (m, 2H), 7.43–7.37 (m, 4H),
7.28–7.25 (m, 2H), 6.77–6.74 (m, 2H), 3.69 (brs, 2H).
4-HYDROXYBIPHENYL (3K). White solid, mp: 163–164 1C; 1H NMR
(500 MHz, CDCl3): d 7.55 (d, J = 7.0 Hz, 2H), 7.49 (d, J = 9.0 Hz,
2H), 7.43 (t, J = 7.5 Hz, 2H), 7.31 (t, J = 7.5 Hz, 1H), 6.92 (d, J =
7.5 Hz, 2H), 4.90 (s, 1H).
4-HYDROXY-40-METHYLBIPHENYL (3L). White solid, mp: 150–152;
1H NMR (500 MHz, CDCl3): d 7.48–7.43 (m, 4H), 7.23 (d, J =
8.0 Hz, 2H), 6.92–6.88 (m, 2H), 4.91 (s, 1H), 2.39 (s, 3H).
4-CYANO-40-METHYLBIPHENYL (3M). White solid, mp: 109–110;
1H NMR (500 MHz, CDCl3): d 7.73–7.66 (m, 4H), 7.51–7.49
(m, 2H), 7.30 (d, J = 8.0 Hz, 2H), 2.42 (s, 3H).
General procedure for Suzuki–Miyaura coupling reaction
Under an air atmosphere, a round-bottom flask was charged
with aryl halide (1.0 mmol), arylboronic acid (1.2 mmol), Pd
catalyst (18.0 mg, 0.2 mol%), K2CO3 (2.0 mmol) and 6 mL of
aqueous EtOH. The reaction mixture was stirred at 60 1C
(conditions A, EtOH/H2O, 1 : 1) or 80 1C (conditions B, EtOH/
H2O, 19 : 1) for a certain period of time as monitored by GC.
After completion of the reaction, the catalyst was separated by a
permanent magnet, and washed with water and EtOH, and
dried under vacuum for the next run. The aqueous phase was
extracted with ether (2 ꢁ 10 mL) and the combined organic
layers were dried over Na2SO4, filtered, concentrated, and the
residue was purified by recrystallization or flash chromatogra-
phy on silica gel to afford the corresponding products.
40-METHYL-3-BIPHENYLCARBOXALDEHYDE (3N). White solid; 1H NMR
(500 MHz, CDCl3): d 10.09 (s, 1H), 8.35 (s, 1H), 8.08 (d, J =
7.5 Hz, 1H), 7.84 (d, J = 7.5 Hz, 1H), 7.61–7.53 (m, 3H), 7.29
(d, J = 7.5 Hz, 2H), 2.42 (s, 3H).
4-METHOXY-40-METHYLBIPHENYL (3O). White solid, mp: 109–110 1C;
1H NMR (500 MHz, CDCl3): d 7.53 (d, J = 8.5 Hz, 2H), 7.47 (d, J =
8.0 Hz, 2H), 7.25 (d, J = 8.0 Hz, 2H), 6.99 (d, J = 8.5 Hz, 2H), 3.87
(s, 3H), 2.41 (s, 3H).
1-(40-METHOXYBIPHENYL-4-YL)ETHANONE (3P). White solid, mp:
1
154–155 1C; H NMR (500 MHz, CDCl3): d 8.01 (d, J = 8.5 Hz,
Analytical data for the Suzuki–Miyaura coupling products
2H), 7.65 (d, J = 8.5 Hz, 2H), 7.60 (d, J = 8.5 Hz, 2H), 7.02 (d, J =
1-(BIPHENYL-4-YL)ETHANONE (3A). White solid, mp: 120–121 1C; 8.5 Hz, 2H), 3.87 (s, 3H), 2.64 (s, 3H).
1H NMR (500 MHz, CDCl3): d 8.06 (d, J = 8.0 Hz, 2H), 7.71 (d, J =
4-CHLOROBIPHENYL (3Q). White solid, mp: 76–77 1C; 1H NMR
8.0 Hz, 2H), 7.65 (d, J = 7.5 Hz, 2H), 7.50 (t, J = 7.5 Hz, 2H), 7.43 (500 MHz, CDCl3): d 7.56 (d, J = 7.5 Hz, 2H), 7.52 (d, J = 8.5 Hz,
(t, J = 7.5 Hz, 1H), 2.67 (s, 3H).
2H), 7.46–7.40 (m, 4H), 7.36 (t, J = 7.5 Hz, 1H).
4-NITROBIPHENYL (3B). Pale yellow solid, mp: 114–115 1C;
4-CHLORO-40-METHYLBIPHENYL (3R). White solid, mp: 126–127 1C;
1H NMR (500 MHz, CDCl3): d 8.33 (d, J = 8.5 Hz, 2H), 7.77 (d, 1H NMR (500 MHz, CDCl3): d 7.52–7.50 (m, 2H), 7.47–7.45 (m,
J = 8.5 Hz, 2H), 7.65 (d, J = 7.5 Hz, 2H), 7.53 (t, J = 7.5 Hz, 2H), 2H), 7.41–7.38 (m, 2H), 7.26 (d, J = 7.5 Hz, 2H), 2.40 (s, 3H).
7.48 (t, J = 7.5 Hz, 1H).
4-CHLORO-40-NITROBIPHENYL (3S). Pale yellow solid, mp: 144–145 1C;
4-(TRIFLUOROMETHYL)BIPHENYL (3C). White solid, mp: 69–70 1C; 1H NMR (500 MHz, CDCl3): d 8.31 (d, J = 7.5 Hz, 2H), 7.71 (d, J =
1H NMR (500 MHz, CDCl3): d 7.69 (s, 4H), 7.60 (d, J = 7.5 Hz, 7.5 Hz, 2H), 7.57 (d, J = 7.5 Hz, 2H), 7.48 (d, J = 7.5 Hz, 2H).
2H), 7.48 (t, J = 7.5 Hz, 2H), 7.41 (t, J = 7.5 Hz, 1H).
1,10-BIPHENYL-4-CARBONITRILE (3D). White solid, mp: 85–86 1C;
Conclusions
1H NMR (500 MHz, CDCl3): d 7.74–7.68 (m, 4H), 7.59 (d, J =
7.0 Hz, 2H), 7.50–7.43 (m, 3H).
In conclusion, we have successfully prepared a novel MNP-
4-FLUOROBIPHENYL (3E). White solid, mp: 73–74 1C; 1H NMR supported nano-Pd catalyst via a ‘‘click’’ route. The obtained
(500 MHz, CDCl3): d 7.72–7.69 (m, 4H), 7.59 (s, 2H), 7.49–7.43 solid catalyst exhibits high activity towards the Suzuki–Miyaura
(m, 3H).
coupling reactions without any added phosphine ligand under
1,10-BIPHENYL (3F). White solid, mp: 70–71 1C; 1H NMR aerobic conditions. Furthermore, the catalyst could be simply
(500 MHz, CDCl3): d 7.61 (d, J = 7.5 Hz, 4H), 7.44 (t, J = recovered from the reaction system by magnetic separation
7.5 Hz, 4H), 7.35 (t, J = 7.5 Hz, 2H).
and reused several times without a significant loss in activity.
c
242 Catal. Sci. Technol., 2013, 3, 235--243
This journal is The Royal Society of Chemistry 2013

View Article Online
Catalysis Science & Technology
Paper
G. L. Zhang, Green Chem., 2011, 13, 2939; ( f ) P. Dutta and A. Sarkar,
Adv. Synth. Catal., 2011, 353, 2814; (g) H. Gruber-Woelfler,
P. F. Radaschitz, P. W. Feenstra, W. Haas and J. G. Khinast,
J. Catal., 2012, 286, 30; (h) N. Iranpoor, H. Firouzabadi, S. Motevalli
and M. Talebi, J. Organomet. Chem., 2012, 708–709, 118; (i) S. Farsadpour,
The excellent catalytic performance and easy separation make
this catalyst a promising alternative to other supported Pd
catalysts.
¨
L. T. Ghoochany, S. Shylesh, G. Dorr, A. Seifert, S. Ernst and W. R. Thiel,
ChemCatChem, 2012, 4, 401.
Acknowledgements
¨
¨
9 (a) S. S. Soomro, R. Rohlich and K. Kohler, Adv. Synth. Catal., 2011,
353, 767; (b) X. Li, X. Y. Yan, H. H. Chang, L. C. Wang, Y. Zhang,
W. W. Chen, Y. W. Li and W. L. Wei, Org. Biomol. Chem., 2012,
10, 495; (c) Y. Y. Ma, X. B. Ma, Q. Wang and J. Q. Zhou, Catal. Sci.
Technol., 2012, 2, 1879.
We are grateful for the financial support from the Natural Science
Foundation of Jiangsu Province of China (No. BK2010485), and the
Science and Technology Development Fund of Nanjing University
of Science and Technology (No. XKF09066).
10 (a) C. Venkatesan and A. P. Singh, J. Catal., 2004, 227, 148;
(b) M. J. Jin, A. Taher, H. J. Kang, M. Choi and R. Ryoo, Green
Chem., 2009, 11, 309.
References
11 (a) J. S. Chen, A. N. Vasiliev, A. P. Panarello and J. G. Khinast, Appl.
Catal., A, 2007, 325, 76; (b) P. Y. Wang, Z. Y. Wang, J. G. Li and
Y. X. Bai, Microporous Mesoporous Mater., 2008, 116, 400; (c) M. Choi,
D. H. Lee, K. Na, B. W. Yu and R. Ryoo, Angew. Chem., Int. Ed., 2009,
48, 3673; (d) G. Z. Fan, B. Zou, S. Q. Cheng and L. H. Zheng, Ind. Eng.
Chem. Res., 2010, 16, 220; (e) L. Huang, T. P. Ang, Z. Wang, J. Tan,
J. H. Chen and P. K. Wong, Inorg. Chem., 2011, 50, 2094.
12 (a) K. Marubayashi, S. Takizawa, T. Kawakusu, T. Arai and H. Sasai,
Org. Lett., 2003, 5, 4409; (b) S. U. Son, I. K. Park, J. Park and
T. Hyeon, Chem. Commun., 2004, 778; (c) M. K. Schroter,
L. Khodeir, M. W. E. van den Berg, T. Hikov, M. Cokoja, S. Miao,
W. Grunet, M. Muhler and R. A. Fischer, Chem. Commun., 2006,
2498; (d) P. P. Fang, A. Jutand, Z. Q. Tian and C. Amatore, Angew.
Chem., Int. Ed., 2011, 50, 12184; (e) S. Moussa, A. R. Siamaki,
B. F. Gupton and M. S. El-Shall, ACS Catal., 2012, 2, 145;
( f ) A. Fihri, D. Cha, M. Bouhrara, N. Almana and V. Polshettiwar,
ChemSusChem, 2012, 5, 85.
13 (a) A. Saha, J. Leazer and R. S. Varma, Green Chem., 2012, 14, 67;
(b) B. Baruwati, D. Guin and S. V. Manorama, Org. Lett., 2007,
9, 5377; (c) D. Guin, B. Baruwati and S. V. Manorama, Org. Lett.,
2007, 9, 1419; (d) A. J. Amali and R. K. Rana, Green Chem., 2009,
11, 1781; (e) N. J. S. Costa, P. K. Kiyohara, A. L. Monteiro, Y. Copped,
K. Philippot and L. M. Rossi, J. Catal., 2010, 276, 382; ( f ) M. J. Jin
and D. H. Lee, Angew. Chem., Int. Ed., 2010, 49, 1119; (g) H. Q. Yang,
Y. W. Wang, Y. Qin, Y. Chong, Q. Yang, G. Li, L. Zhang and W. Li,
Green Chem., 2011, 13, 1352; (h) S. Zhou, M. Johnson and J. G. C.
Veinot, Chem. Commun., 2010, 46, 2411.
14 S. Shylesh, L. Wang and W. R. Thiel, Adv. Synth. Catal., 2010,
352, 425.
15 Q. W. Du, W. Zhang, H. Ma, J. Zheng, B. Zhou and Y. Q. Li,
Tetrahedron, 2012, 68, 3577.
16 (a) L. Zhang, P. H. Li, H. J. Li and L. Wang, Catal. Sci. Technol., 2012,
2, 1859; (b) P. H. Li, L. Wang, L. Zhang and G. W. Wang, Adv. Synth.
Catal., 2012, 354, 1307.
17 I. P. Beletskaya and A. V. Cheprakov, Chem. Rev., 2000, 100, 3009.
18 (a) F. Amblard, J. H. Cho and R. F. Schinazi, Chem. Rev., 2009,
109, 4207; (b) C. Chu and R. Liu, Chem. Soc. Rev., 2011, 40, 2177.
19 A. K. Ganai, R. Bhardwaj, S. Hotha, S. S. Gupta and B. L. V. Prasad,
New J. Chem., 2010, 34, 2662.
1 For reviews, see: (a) N. Miyaura and A. Suzuki, Chem. Rev.,
1995, 95, 2457; (b) J. Hassan, M. Sevignon, C. Gozzi, E. Schulz and
M. Lemaire, Chem. Rev., 2002, 102, 1359; (c) J. P. Corbet and
G. Mignani, Chem. Rev., 2006, 106, 2651; (d) L. Yin and J. Liebscher,
Chem. Rev., 2007, 107, 133; (e) R. Martin and S. L. Buchwald, Acc.
Chem. Res., 2008, 41, 1461; ( f ) A. Fihri, M. Bouhrara,
B. Nekoueishahraki, J. M. Basset and V. Polshettiwar, Chem. Soc.
Rev., 2011, 40, 5181.
2 (a) R. A. Forsch, S. F. Queener and A. Rosowsky, Bioorg. Med. Chem.
Lett., 2004, 14, 1811; (b) F. Leroux, ChemBioChem, 2004, 5, 644;
(c) S. Lightowler and M. Hird, Chem. Mater., 2005, 17, 5538;
(d) M. C. Kozlowski, B. J. Morgan and E. C. Linton, Chem. Soc. Rev.,
2009, 38, 3193; (e) A. Ogata, C. Furukawa, K. Sakurai, H. Iba, Y. Kitade
and Y. Ueno, Bioorg. Med. Chem. Lett., 2010, 20, 7299;
( f ) S. E. Dickson and C. M. Crudden, Chem. Commun., 2010, 46, 2100.
3 (a) A. F. Littke and G. C. Fu, Angew. Chem., Int. Ed., 2002, 41, 4176;
(b) I. Kondolff, H. Doucet and M. Santelli, Tetrahedron, 2004, 60, 3813;
(c) G. C. Fu, Acc. Chem. Res., 2008, 41, 1555; (d) A. N. Marziale,
D. Jantke, S. H. Faul, T. Reiner, E. Herdtweck and J. Eppinger, Green
Chem., 2011, 13, 169; (e) K. H. Chung, C. M. So, S. M. Wong,
C. H. Luk, Z. Zhou, C. P. Lau and F. Y. Kwong, Chem. Commun.,
2012, 48, 1967; ( f ) A. W. Fraser, J. E. Besaw, L. E. Hull and
M. C. Baird, Organometallics, 2012, 31, 2470.
´
4 (a) L. Botella and C. Najera, Angew. Chem., Int. Ed., 2002, 41, 179;
´
(b) L. Botella and C. Najera, J. Organomet. Chem., 2002, 663, 46;
´
(c) A. Corma, H. Garcıa and A. Leyva, J. Catal., 2006, 240, 87;
´
(d) E. Alacid and C. Najera, Org. Lett., 2008, 10, 5011; (e) E. Alacid
´
and C. Najera, J. Org. Chem., 2009, 74, 8191.
5 (a) O. Navarro, H. Kaur, P. Mahjoor and S. P. Nolan, J. Org. Chem.,
2004, 69, 3173; (b) A. E. Wang, J. H. Xie, L. X. Wang and Q. L. Zhou,
Tetrahedron, 2005, 61, 259; (c) E. A. B. Kantchev, C. J. brien and
M. G. Organ, Angew. Chem., Int. Ed., 2007, 46, 2768; (d) J. W. Ruan,
O. Saidi, J. A. Iggo and J. L. Xiao, J. Am. Chem. Soc., 2008, 130, 10510;
(e) A. Rahimi and A. Schmidt, Synlett, 2010, 1327; ( f ) C. Valente,
S. Çalimsiz, K. H. Hoi, D. Mallik, M. Sayah and M. G. Organ, Angew.
Chem., Int. Ed., 2012, 51, 3314.
6 (a) L. Bai and J. X. Wang, Adv. Synth. Catal., 2008, 350, 315;
´
(b) E. Alacid and C. Najera, J. Organomet. Chem., 2009, 694, 1658;
(c) S. M. Islam, P. Mondal, A. S. Roy, S. Mondal and D. Hossain,
Tetrahedron Lett., 2010, 51, 2067; (d) S. Ogasawara and S. Kato, J. Am.
Chem. Soc., 2010, 132, 4608; (e) J. Yang, P. Li and L. Wang, Synthesis,
2011, 1295; ( f ) R. U. Islam, M. J. Witcomb, M. S. Scurrell, E. van der
Lingen, W. V. Otterlo and K. Mallick, Catal. Sci. Technol., 2011,
1, 308; (g) D. Zhang, C. S. Zhou and R. H. Wang, Catal. Commun.,
2012, 22, 83; (h) M. C. Hong, M. C. Choi, Y. W. Chang, Y. Lee, J. Kim
and H. Rhee, Adv. Synth. Catal., 2012, 354, 1257.
7 (a) Y. Kitamura, A. Sakurai, T. Udzu, T. Maegawa, Y. Monguchi and
H. Sajiki, Tetrahedron, 2007, 63, 10596; (b) G. M. Scheuermann,
L. Rumi, P. Steurer, W. Bannwarth and R. Mu¨lhaupt, J. Am. Chem.
Soc., 2009, 131, 8262; (c) P. P. Zhang, X. X. Zhang, H. X. Sun,
R. H. Liu, B. Wang and Y. H. Lin, Tetrahedron Lett., 2009, 50, 4455.
8 (a) H. Q. Yang, X. J. Han, Z. C. Ma, R. Q. Wang, J. Liu and X. F. Ji,
Green Chem., 2010, 12, 441; (b) B. Karimi, D. Elhamifar, J. H. Clark
and A. J. Hunt, Chem.–Eur. J., 2010, 16, 8047; (c) D. H. Lee, J. Y. Jung
and M. J. Jin, Green Chem., 2010, 12, 2024; (d) W. Chen, P. Li and
L. Wang, Tetrahedron, 2011, 67, 318; (e) G. Li, H. Q. Yang, W. Li and
20 S. L. Jain, B. S. Rana, B. Singh, A. K. Sinha, A. Bhaumik, M. Nandi
and B. Sain, Green Chem., 2010, 12, 374.
21 G. F. Zhang, Y. Wang, X. Wen, C. R. Ding and Y. Li, Chem. Commun.,
2012, 48, 2979.
22 (a) Q. Zhang, J. Luo and Y. Y. Wei, Green Chem., 2010, 12, 2246;
(b) Q. Zhang, H. Su, J. Luo and Y. Y. Wei, Green Chem., 2012, 14, 201.
23 G. H. Lv, W. P. Mai, R. Jin and L. X. Gao, Synlett, 2008, 1418.
´
24 (a) S. Niembro, A. Shafir, A. Vallribera and R. Alibes, Org. Lett., 2008,
10, 3215; (b) R. Bernini, S. Cacchi, G. Fabrizi, G. Forte, F. Petrucci,
A. Prastaro, S. Niembro, A. Shafir and A. Vallribera, Green Chem.,
2010, 12, 150.
25 (a) N. Pinna, S. Grancharov, P. Beato, P. Bonville, M. Antonietti and
M. Niederberger, Chem. Mater., 2005, 17, 3044; (b) D. L. A. de Faria,
ˆ
S. Venancio Silva and M. T. De Oliveira, J. Raman Spectrosc., 1997,
28, 873; (c) L. Slavov, M. V. Abrashev, T. Merodiiska, Ch. Gelev,
R. E. Vandenberghe, I. Markova-Deneva and I. Nedkov, J. Magn.
Magn. Mater., 2010, 322, 1904.
c
This journal is The Royal Society of Chemistry 2013
Catal. Sci. Technol., 2013, 3, 235--243 243