Palladium and Bimetallic Palladium-Nickel Nanoparticles Supported on Multiwalled Carbon Nanotubes: Application to Carbon-Carbon Bond-Forming Reactions in Water

Article abstract of DOI:10.1002/cctc.201500164

Palladium and bimetallic Pd-Ni nanoparticles (NPs) protected by polyvinylpyrrolidone were prepared by the reduction-by-solvent method and deposited on multiwalled carbon nanotubes (MWCNTs). The catalytic activity of these NPs to carbon-carbon bond-forming reactions was studied by using 0.1 mol% Pd loading, at 120°C for 1 h and water as a solvent under ligand-free conditions. The Suzuki-Miyaura reaction took place quantitatively for the cross-coupling of 4-bromoanisole with phenylboronic acid, better than those obtained with potassium phenyltrifluoroborate, with Pd₅₀Ni₅₀/MWCNTs as a catalyst and K₂CO₃ as a base and TBAB as an additive, with good recyclability during 4 cycles with some Ni leaching. The Hiyama reaction of 4-iodoanisole with trimethoxyphenylsilane, under fluoride-free conditions using 50% aqueous NaOH solution, was performed with Pd/MWCNTs as a catalyst in 83% yield with low recyclability. For the Mizoroki-Heck reaction 4-iodoanisole and styrene gave the corresponding 4-methoxystilbene quantitatively with Pd₅₀Ni₅₀/MWCNTs using K₂CO₃ as a base and TBAB as an additive although the recycle failed. In the case of the Sonogashira-Hagihara reaction, Pd/MWCNTs had to be used as a catalyst and pyrrolidine as a base for the coupling of 4-iodoanisole with phenylacetylene under copper-free conditions. The corresponding 4-methoxytolane was quantitatively obtained allowing the recycling of the catalyst during 3 cycles. From black to green: The catalytic activity of palladium and bimetallic Pd-Ni nanoparticles deposited on multiwalled carbon nanotubes as a catalyst for cross-coupling reactions in water under ligandless conditions is disclosed. These reaction conditions represent an environmentally friendly alternative to those previously reported for reactions in organic solvents.

Full text of DOI:10.1002/cctc.201500164

DOI: 10.1002/cctc.201500164
Full Papers
Palladium and Bimetallic Palladium–Nickel Nanoparticles
Supported on Multiwalled Carbon Nanotubes: Application
to CarbonÀCarbon Bond-Forming Reactions in Water
[
a]
[a]
[a]
[b]
Atsushi Ohtaka, JosØ M. Sansano, Carmen Nµjera,* Izaskun Miguel-García,

[b]
[b]
ngel Berenguer-Murcia, and Diego Cazorla-Amorós*
Palladium and bimetallic Pd–Ni nanoparticles (NPs) protected
by polyvinylpyrrolidone were prepared by the reduction-by-
solvent method and deposited on multiwalled carbon nano-
tubes (MWCNTs). The catalytic activity of these NPs to carbon–
carbon bond-forming reactions was studied by using 0.1 mol%
Pd loading, at 1208C for 1 h and water as a solvent under
ligand-free conditions. The Suzuki–Miyaura reaction took place
quantitatively for the cross-coupling of 4-bromoanisole with
phenylboronic acid, better than those obtained with potassium
phenyltrifluoroborate, with Pd Ni /MWCNTs as a catalyst and
of 4-iodoanisole with trimethoxyphenylsilane, under fluoride-
free conditions using 50% aqueous NaOH solution, was per-
formed with Pd/MWCNTs as a catalyst in 83% yield with low
recyclability. For the Mizoroki-Heck reaction 4-iodoanisole and
styrene gave the corresponding 4-methoxystilbene quantita-
tively with Pd Ni /MWCNTs using K CO as a base and TBAB
50
50
2
3
as an additive although the recycle failed. In the case of the
Sonogashira-Hagihara reaction, Pd/MWCNTs had to be used as
a catalyst and pyrrolidine as a base for the coupling of 4-iodoa-
nisole with phenylacetylene under copper-free conditions. The
corresponding 4-methoxytolane was quantitatively obtained
allowing the recycling of the catalyst during 3 cycles.
50
50
K CO as a base and TBAB as an additive, with good recyclabili-
2
3
ty during 4 cycles with some Ni leaching. The Hiyama reaction
Introduction
Carbon nanotubes (CNTs) have excellent electronic properties,
good chemical stability, and large surface areas, leading there-
fore to promising applications as catalyst supports. The attach-
ment of nanoparticles (NPs) on CNTs has attracted great inter-
est, for the nanocomposites not only combine the extraordina-
ry properties of the NPs and CNTs, but also exhibit some new
tested, one of the examples that stands out in the context of
the present manuscript is the deposition of diverse Pd-based
nanoparticles protected by polyvinylpyrrolidone (PVP) synthe-
sized by the reduction-by-solvent method on different carbon
[4]
materials for the selective hydrogenation of phenylacetylene.
Recently, we reported the preparation of Pd and bimetallic Pd–
Ni nanoparticles and their deposition on single-walled carbon
[
1]
properties caused by the interaction between them. Several
research groups have reported the synthesis of CNTs-support-
ed metal NPs and their catalytic activity for various chemical
reactions including Suzuki coupling reaction, Heck reaction,
and hydrogenation. However, most of the reactions were per-
formed in organic solvents or mixed organic/aqueous solvents,
[5]
nanotubes (SWCNTs). On the other hand, multiwalled carbon
nanotubes (MWCNTs) have been found to be superior supports
compared to other carbon materials such as SWCNTs. For ex-
ample, MWCNTs are able to uptake the highest number of NPs
per unit surface area and MWCNTs-supported palladium NPs
[
2,3]
[2c,d]
and neat water is seldom used as a reaction medium.
showed a slightly better reactivity.
We envisaged that these
The enhanced catalytic activity of metal NPs is well-estab-
lished in the literature. Among the applications which we have
Pd and bimetallic Pd–Ni NPs supported on MWCNTs could be
useful catalysts in cross-coupling reactions having two main
advantages a lower amount of palladium and to allow the re-
covery of these supported NPs by simple filtration from the re-
action media. In addition we have chosen water as solvent not
only because of its economical and ecological relevance but
also because Corma et al. have demonstrated that in water Pd
clusters are easily formed from the precursor NPs, showing
higher stability, faster reaction rates, and lower leaching than
in organic solvents in different carbon-carbon bond forming
[
a] Dr. A. Ohtaka, Prof. J. M. Sansano, Prof. C. Nµjera
Dpto. de Química Orgµnica
Centro de Innovación en Química Avanzada
(ORFEO-CINQA)
University of Alicante
Apdo.99, 03080 Alicante (Spain)
Fax: (+34)965903549
E-mail: cnajera@ua.es
[6]
reactions. Herein we report the study about the catalytic ac-
tivity of Pd and bimetallic Pd–Ni nanoparticles stabilized by
[
b] Dr. I. Miguel-García, Dr. . Berenguer-Murcia, Prof. D. Cazorla-Amorós
Dpto. Química Inorgµnica and Instituto Universitario de Materiales
University of Alicante
PVP and supported on MWCNTs for several relevant CÀC bond-
Carretera de San Vicente del Raspeig, s/n
[7]
formation processes in water
such as Suzuki–Miyaura,
0
3690, San Vicente del Raspeig, Alicante (Spain)
[8]
Hiyama, Mizoroki–Heck, and Sonogashira–Hagihara reactions.
Fax: (+34)965903454
E-mail: cazorla@ua.es
ChemCatChem 2015, 7, 1841 – 1847
1841
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
Results and Discussion
Table 2. Catalytic activity of Pd and bimetallic Pd–Ni NPs in the Suzuki–
Miyaura reaction using phenylboronic acid in water.
[a]
To determine the efficiency of the loading step as well as to
assess the particle size and its dispersion in the prepared het-
erogeneous catalysts, inductively coupled plasma optical emis-
sion spectroscopy (ICP–OES) and TEM studies were performed
for all samples. The results of the ICP analysis for the metal
contents of the prepared NPs/MWCNTs samples are summar-
ized in Table 1. It can be observed that the initially targeted
[
b]
Entry
Catalyst
Yield [%]
1
2
3
4
Pd/MWCNTs
54
48
47
63
30
Pd Ni /MWCNTs
7
0
30
1
wt% metal loading was not really achieved. Therefore, the
Pd50Ni50/MWCNTs
Pd30Ni70/MWCNTs
PVP–Pd
rest of the metal was lost during the synthesis procedure,
probably during the washing step, where Pd NPs not bound
strongly enough were washed off the carbon support.
[
c]
5
[
(
[
[
a] Reaction conditions: 4-bromoanisole (0.5 mmol), phenylboronic acid
0.75 mmol), catalyst (0.1 mol% Pd), K CO (1.5 mmol) in H O (0.3 mL).
b] Determined by GC analysis based on 4-bromoanisole consumption.
c] In 0.5 mL of H O.
2
3
2
2
Table 1. TEM and ICP characterization results of the heterogeneous
catalysts prepared.
Catalyst
Metal loadingICP
total wt%]
d
[nm]
TEM
D
TEM
[%]
[
11]
[
Pd(OAc) or an oxime-derived palladacycle (Table 2). Under
2
these reaction conditions, the best 63% yield was obtained
with the bimetallic Pd Ni /MWCNTs (Table 2, entry 4). In all
Pd/MWCNTs
0.75
0.62
0.58
0.70
0.69
2.55Æ0.57
2.39Æ0.42
2.03Æ0.57
2.42Æ0.51
2.71Æ0.54
35.3
37.6
44.3
37.2
33.2
Pd90Ni10/MWCNTs
Pd70Ni30/MWCNTs
Pd50Ni50/MWCNTs
Pd30Ni70/MWCNTs
30
70
these cases, better results were observed by using the catalyst
supported on MWCNTs than with Pd NPs coated with PVP
(
entry 5).
Next the optimization of the reaction conditions was consid-
ered by using the bimetallic Pd Ni /MWCNTs as a precatalyst
30
70
To directly correlate the metal dispersion of NPs/MWCNTs
catalysts, TEM analyses were also performed on the as-synthe-
sized samples. In Figure 1 TEM micrographs of Pd/MWCNTs,
Pd Ni /MWCNTs, and Pd Ni /MWCNTs are shown. Both Pd
(Table 3). If KOH was used as base, a lower yield than with
K CO was obtained (Table 3, entries 1 and 2). If working at
2
3
1008C with K CO as a base, the Pd loading was then increased
2
3
from 0.1 to 0.2 and 0.3 mol%, but the latter two loadings gave
slightly lower yields (entries 2–4). If the reaction time was in-
creased from 1 to 2 h the yield did not increase (61%) and
after 3 h a slightly higher yield
7
0
30
50 50
average particle sizes (d, nm) and metal dispersion (D, %) re-
sults of the three catalysts obtained from TEM analyses are
of 72% was obtained (entries 2,
5
and 6). Increasing the bath
temperature to 1208C had a posi-
tive increment in the yield from
63 to 83% (entry 7). The addition
of additives such as poly(ethy-
lene glycol) 300 (PEG300) as a 1/
5
volume proportion afforded
higher 92% yield (entry 8). If the
amount of phenylboronic acid
was reduced from 1.5 to
Figure 1. TEM images of catalysts a) Pd/MWCNTs, b) Pd70Ni30/MWCNTs, and c) Pd50Ni50/MWCNTs.
1
.2 equivalents and 1 equivalent
of tetra-n-butylammonium bro-
mide (TBAB) was added, which is a well-known NPs stabiliz-
given in Table 1. Remarkably, the particle sizes of the metal
nanoparticles in the catalysts are very similar to the average
particle size of the metal particles of the colloid (2.4Æ
[12]
er, the highest 95% yield was obtained. Under the last reac-
tion conditions, similar yields were obtained with all remaining
supported Pd NPs (entries 10–13), although Pd Ni /MWCNTs
[
9]
0
.5 nm). Thus, not only were the prepared heterogeneous
50
50
catalysts of the desired atomic composition, but also they dis-
played a very small particle size and a narrow size distribution.
Initial studies were performed by using different Pd NPs sup-
seemed to be the best precatalyst (entry 12).
Under the optimum reaction conditions involving the use of
Pd Ni /MWCNTs (0.1 mol% of Pd), 3 equivalents of K CO , and
50
50
2
3
[
10]
ported on MWCNTs in the Suzuki–Miyaura reaction of 4-bro-
moanisole with phenylboronic acid using 0.1 mol% Pd loading,
K CO as a base, and water as a solvent at 1008C for 1 h, reac-
1 equivalent of TBAB at 1208C, the recyclability of the catalyst
was studied (Scheme 1). The cross-coupling reaction of 4-bro-
moanisole with phenylboronic acid (1.2 equiv.) was performed
giving 4-methoxybiphenyl in 99% yield. If the reaction was
2
3
tion conditions which we established previously using
ChemCatChem 2015, 7, 1841 – 1847
www.chemcatchem.org
1842
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
size increased slightly compared
with that of the fresh catalyst
[a]
Table 3. Reaction condition studies for the Suzuki–Miyaura reaction with phenylboronic acid in water.
(
3.65Æ0.95 nm
and
2.42Æ
0
.51 nm, respectively). It appears
that under the conditions de-
scribed the NPs are etched and
their size increases upon consec-
utive reaction cycles.
Entry
Cat.
(mol% Pd)]
Base
KOH
Additive
T
[8C]
t
[h]
Yield
[%]
[b]
[
1
2
3
4
5
6
7
8
Pd30Ni70/MWCNTs (0.1)
Pd30Ni70/MWCNTs (0.1)
Pd30Ni70/MWCNTs (0.2)
Pd30Ni70/MWCNTs (0.3)
Pd30Ni70/MWCNTs (0.1)
Pd30Ni70/MWCNTs (0.1)
Pd30Ni70/MWCNTs (0.1)
Pd30Ni70/MWCNTs (0.1)
Pd30Ni70/MWCNTs (0.1)
Pd/MWCNTs (0.1)
–
–
–
–
–
–
–
100
100
100
100
100
100
120
120
120
120
120
120
120
1
1
1
1
2
3
1
1
1
1
1
1
1
41
63
53
57
61
72
83
92
95
96
94
>99
95
Further applications of these
supported Pd NPs on MWCNTs
in the same Suzuki–Miyaura re-
action using 4-bromoanisole and
potassium phenyltrifluoroborate
were next studied (Table 4).
Working at 1008C with 0.1 mol%
Pd loading using K2CO3 as
a base in water as the solvent,
reaction conditions that we es-
K
2
K
2
K
2
K
2
K
2
K
2
K
2
K
2
K
2
K
2
K
2
K
2
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
3
3
3
3
3
3
[c]
PEG300
TBAB
TBAB
TBAB
[
d]
[
[e]
9
1
d]
d]
[e]
0
[
[e]
1
1
Pd70Ni30/MWCNTs (0.1)
Pd50Ni50/MWCNTs (0.1)
Pd10Ni90/MWCNTs (0.1)
[d]
[e]
1
2
3
TBAB
[
d]
[e]
1
TBAB
tablished
previously
using
[
a] Reaction conditions: 4-bromoanisole (0.5 mmol), phenylboronic acid (0.75 mmol), catalyst, base (1.5 mmol)
in H O (0.3 mL). [b] Determined by GC analysis based on 4-bromoanisole consumption. [c] PEG300/H O (1:5) in
.5 mL volume was used as a solvent. [d] 1.2 equiv. of phenylboronic acid were used. [e] TBAB (0.5 mmol).
Pd(OAc)2 or an oxime-derived
2
2
[13]
palladacycle,
but without ad-
0
ditives (Table 4, entries 1–5), the
highest 64 and 65% yields were
obtained with Pd/MWCNTs and
P Ni /MWCNTs,
respectively
3
0
70
(
entries 1 and 4). By working at 1208C with Pd/MWCNTs either
during 1 or 3 h, similar (81 and 83%) yields were obtained (en-
tries 6 and 7). If 1 equivalent of TBAB was added, a lower 76%
yield was observed (Table 4, entry 8). However, a decrease of
the potassium phenyltrifluoroborate from 1.5 to 1.2 equiva-
lents increased the yield from 83 to 88% (entries 7 and 9), and
in the presence of TBAB or PEG300 to 94 and 95%, respective-
ly (entries 10 and 11). However, the second recycling experi-
ment under the last reaction conditions gave 4-methoxybi-
phenyl in only 21% yield. We can conclude that phenylboronic
acid is a better substrate than potassium phenyltrifluoroborate
for the recycling of the catalyst.
Scheme 1. Recyclability of bimetallic Pd50Ni50/MWCNTs for the Suzuki–
Miyaura reaction with phenylboronic acid in water.
completed, the catalyst was recovered by decantation and suc-
cessively subjected to second through fourth runs of the cou-
pling reaction under the same conditions to afford 4-methoxy-
biphenyl in 89%, 86%, and 87% yields, respectively
[14]
Next, the Hiyama reaction was first evaluated using 4-bro-
moanisole and trimethoxyphenylsilane as a model reaction
with 2.5 equivalents of 50% aqueous NaOH solution in the ab-
sence of a fluoride source, reaction conditions that we estab-
(
Scheme 1). ICP–OES analysis revealed a slight decrease of the
amount of Ni (from 0.24 wt% to 0.12 wt%), and that no metal
species was observed in the product. TEM images (Figure 2) of
the recovered catalyst after the fourth run revealed the particle
lished previously by us using Pd(OAc) or an oxime-derived
2
[15]
palladacycle. In the presence of the different Pd NPs sup-
ported on MWCNTs (0.1 mol% of Pd) at 908C for 3 h afforded
the desired coupling product in low yields (7–32%). Then, the
coupling between 4-iodoanisole and trimethoxyphenylsilane
was performed with 2.5 equivalents of 50% NaOH in the pres-
ence of Pd NPs supported in MWCNTs (0.1 mol% of Pd) at
1
208C (Table 5). In the case of using 1.5 equivalents of trime-
thoxyphenylsilane, all the catalysts gave 4-methoxybiphenyl in
2–96% yield (Table 5, entries 1–4). However, if lowering the
9
amount of siloxane to 1.2 equivalents, the best yield was ob-
tained with Pd NPs supported in MWCNTs (entry 5). The recy-
cling experiments gave very low yields, 40 and 53% in the
second and third cycle, respectively. Notably, after undergoing
3
reaction cycles in the Hiyama reaction, the catalyst Pd/
Figure 2. TEM image of the Pd50Ni50/MWCNTs catalyst after the 4th cycle in
the Suzuki–Miyaura process.
MWCNTs displayed a decrease in the yield of the desired prod-
ChemCatChem 2015, 7, 1841 – 1847
www.chemcatchem.org
1843
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
(
Table 6). The reaction conditions
were studied by using dicyclo-
hexylmethylamine as base
under the previous conditions
Table 4. Reaction conditions studies for the Suzuki–Miyaura reaction with potassium phenyltrifluoroborate in
water.
[a]
a
[17]
developed by our group with
very poor results. However,
better yields were obtained in
the presence of KOH (3 equiv.)
as a base in the presence of
TBAB as an additive or in a mix-
ture of PEG300 and water as
a solvent at 1208C during 1 h re-
action time. In the case of Pd/
Entry
Cat.
(mol% Pd)]
Additive
T
[8C]
t
[h]
Yield
[%]
[b]
[
1
2
3
4
5
6
7
8
Pd/MWCNTs (0.1)
Pd70Ni30/MWCNTs (0.1)
Pd50Ni50/MWCNTs (0.1)
Pd30Ni70/MWCNTs (0.1)
PVP–Pd
Pd/MWCNTs (0.1)
Pd/MWCNTs (0.1)
Pd/MWCNTs (0.1)
Pd/MWCNTs (0.1)
Pd/MWCNTs (0.1)
Pd/MWCNTs (0.1)
–
–
–
–
–
–
–
100
100
100
100
100
120
120
120
120
120
120
1
1
1
1
1
1
3
1
1
1
1
64
30
44
65
38
81
83
76
88
94
95
MWCNTs by using PEG300/H O
2
(
1:1 volume/volume ratio), quan-
[
c]
TBAB
–
TBAB
[
d]
[
titative formation of 4-methoxys-
tilbene was observed (Table 6,
entry 1). By using TBAB (1 equiv.)
as an additive, a 90% yield was
obtained (entry 2). Under these
reaction conditions, Pd Ni /
9
1
d]
d]
[c]
0
[
[e]
11
PEG300
[
(
a] Reaction conditions: 4-bromoanisole (0.5 mmol), potassium phenyltrifluoroborate (0.75 mmol), catalyst, base
1.5 mmol) in H O (0.3 mL). [b] Determined by GC analysis based on 4-bromoanisole consumption. [c] 1 equiv.
2
was added. [d] 1.2 equiv. of potassium phenyltrifluoroborate was used. [e] PEG300/H
was used as a solvent.
2
O (5:1) in 0.5 mL volume
7
0
30
MWCNTs gave the same results
entry 3). Quantitative yields
were also achieved with
Pd Ni /MWCNTs and Pd Ni /MWCNTs catalysts (en-
(
50
50
30 70
Table 5. Reaction conditions studies for the Hiyama reaction using Pd NPs/MWCNTs
in water.
tries 4 and 5). On the other hand, Pd Ni /MWCNTs
10 90
[a]
gave the same yield (90%) as Pd/MWCNTs (entries 2
and 6). If the recycling experiments were performed
with Pd Ni /MWCNTs under the best reaction condi-
50
50
tions the reaction failed in the second run.
For the Sonogashira–Hagihara reaction,
[18]
the
Entry
Cat.
(mol% Pd)]
PhSi(OMe)
[equiv.]
3
t
[h]
Yield
[%]
cross-coupling between 4-iodoanisole and phenyla-
cetylene was studied in the presence of pyrrolidine
as base, at 1208C in water as a solvent, which means
the reaction conditions set up by our group, and
the results are presented in Table 7. In the presence
of Pd/MWCNTs under copper-free conditions, the re-
action needed 5 h to obtain 4-methoxytolane in 74%
[b]
[
1
2
3
4
5
6
7
8
Pd/MWCNTs (0.1)
Pd70Ni30/MWCNTs (0.1)
Pd/MWCNTs (0.1)
Pd70Ni30/MWCNTs (0.1)
Pd/MWCNTs (0.1)
Pd70Ni30/MWCNTs (0.1)
Pd50Ni50/MWCNTs (0.1)
Pd30Ni70/MWCNTs (0.1)
1.5
1.5
1.5
1.5
1.2
1.2
1.2
1.2
3
3
1
1
1
1
1
1
95
96
95
92
85
77
76
82
[19]
yield. However, in the presence of PEG300/H O (1:1,
2
volume/volume ratio) as a solvent, a similar 73%
yield was obtained (Table 7, entry 2). By increasing
the amount of phenylacetylene from 1.2 to 1.5 equiv-
alents it was possible to obtain the corresponding in-
ternal acetylene in 92% yield not only in 5 h but also
in only 1 h reaction time (entries 3–5) . Quantitative
yield could be obtained by using 2.0 equivalents of phenylace-
tylene (entry 5). Under the last reaction conditions, the bimet-
allic Pd–Ni NPs gave lower yields (entries 6–9). Therefore, the
recycling experiments were performed with Pd/MWCNTs under
the reaction conditions indicated in entry 5. The two first
cycles took place in quantitative yields, but in the third one 4-
methoxytolane was obtained in only 64% yield and in the
fourth the reaction failed (Scheme 2).
[
(
a] Reaction conditions: 4-iodoanisole (0.5 mmol), trimethoxyphenylsilane, catalyst
0.1 mol% Pd), 50% aqueous NaOH (2.5 mmol). [b] Determined by GC analysis based
on 4-iodoanisole consumption.
uct from 95% to 53%, indicating a strong deactivation. Post-
reaction ICP–OES and TEM analyses performed on the used
catalyst revealed a marked decrease in the metal loading (from
0
.75%, see Table 1, to 0.2%) and a particle size of 5.2Æ1.5 nm.
This behavior demonstrates that the strongly basic reaction
medium not only leaches the Pd from the catalyst into the
liquid phase, but also provokes a significant increase in the Pd
nanoparticle size, which ultimately results in an impoverished
catalytic activity.
[
16]
For the Mizoroki–Heck reaction in water catalyzed by Pd
NPs supported on MWCNTs (0.1 mol% Pd), 4-methoxyiodoben-
zene and styrene (1.5 equiv.) were chosen as model substrates
ChemCatChem 2015, 7, 1841 – 1847
www.chemcatchem.org
1844
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
Conclusions
Table 6. Reaction conditions studies for the Mizoroki–Heck reaction
using Pd NPs/MWCNTs in water.
[
a]
The catalytic activity of palladium and bimetallic Pd–Ni nano-
particles (NPs) protected by polyvinylpyrrolidone, prepared by
the reduction-by-solvent method and deposited on multiwal-
led carbon nanotubes (MWCNTs), in different carbonÀcarbon-
bond-forming reactions has been evaluated. Bimetallic
Pd Ni /MWCNTs using K CO as a base and TBAB as an addi-
Entry
Cat.
Additive
Yield
[
b]
50 50
2
3
[
%]
tive is the best catalyst for the Suzuki–Miyaura reaction of 4-
bromoanisole with phenylboronic acid and for the Mizoroki–
Heck reaction of 4-iodoanisole with styrene. An effective recy-
cling could be performed only in the former case. On the
other hand, Pd/MWCNTs was the catalyst of choice for Hiyama
reaction of 4-iodoanisole with trimethoxyphenylsilane under
fluoride-free conditions with 50% aqueous NaOH as a base
and for the Sonogashira–Hagihara reaction of 4-iodoanisole
with phenylacetylene under copper-free conditions with pyrro-
lidine as base. The recyclability was moderate only for the
latter reaction. All these processes were performed under
ligand-free conditions at 1208C (bath temperature) during 1 h
reaction time with relative low 0.1 mol% Pd loading in water
as a solvent.
[
c]
1
2
3
4
5
6
Pd/MWNT
Pd/MWNT
Pd70Ni30/MWNT
Pd50Ni50/MWNT
Pd30Ni70/MWNT
Pd10Ni90/MWNT
PEG300
99
90
91
99
99
90
[
[
[
[
[
d]
d]
d]
d]
d]
TBAB
TBAB
TBAB
TBAB
TBAB
[
a] Reaction conditions: 4-iodoanisole (0.5 mmol), styrene (0.75 mmol),
catalyst (0.1 mol% Pd), KOH (1.5 mmol). [b] Determined by GC analysis
based on 4-iodoanisole consumption. [c] PEG300/H
volume was used as a solvent. [d] TBAB (0.5 mmol).
2
O (1:1) in 0.5 mL
Table 7. Reaction conditions studies for the Sonogashira–Hagihara reac-
[a]
tion in water.
Experimental Section
Entry
Cat.
Additive
–
t
Yield
[%]
Synthesis of Pd-based catalysts
[b]
[h]
Colloidal Pd-based nanoparticles were synthesized by following
1
2
Pd/MWCNTs
Pd/MWCNTs
Pd/MWCNTs
Pd/MWCNTs
5
5
5
1
1
1
1
1
1
74
73
92
92
99
74
69
68
60
[20]
[
c]
the procedure described in our previous work, using ethylene
glycol as reducing agent in the so-called reduction-by-solvent
method. The syntheses were performed under an Argon atmos-
phere in a Schlenk system. In a typical synthesis, the palladium pre-
cursor solution was prepared, in a two-necked, round-bottom flask,
by adding palladium(II) acetate (0.2245 g, Sigma–Aldrich,
Ref. 205869, 98% pure) and 1,4-dioxane (50 mL, Sigma–Aldrich,
Ref. 533971, 99% pure) under vigorous stirring for 1 h, resulting in
a dark-orange solution. In another two-necked, round-bottom
PEG300
[
[
[
[
[
[
[
d]
d]
e]
e]
e]
e]
e]
3
4
5
6
7
8
9
–
–
–
–
–
–
–
Pd/MWCNTs
Pd70Ni30/MWCNTs
Pd50Ni50/MWCNTs
Pd30Ni70/MWCNTs
Pd10Ni90/MWCNTs
[
(
a] Reaction conditions: 4-iodoanisole (0.5 mmol), phenylacetylene
0.6 mmol), catalyst (0.1 mol% Pd), pyrrolidine (1 mmol). [b] Determined
flask,
a solution containing poly(n-vinylpyrrolidone) (1.1114 g,
2
by GC analysis based on 4-iodoanisole consumption. [c] PEG300/H O (1:1)
in 0.5 mL volume was used as a solvent. [d] 1.5 equiv. of phenylacetylene
were used. [e] 2 equiv. of phenylacetylene were used.
Sigma–Aldrich, Ref. 234257, Mw 40000) and anhydrous ethylene
glycol (120 mL, Sigma–Aldrich, no. 293237, 99% pure) was pre-
pared. Different amounts of NiSO ·6H O (depending on the final
4
2
colloid composition) were added to the mixture, and the system
was stirred for 3 h at 808C. The solution was light green in color.
This solution was cooled to 08C by means of an ice bath. Both sol-
utions were mixed under stirring to ensure homogenization. Imme-
diately, the pH of the resulting mixture was adjusted to 9–10 by
dropwise addition of a 1m NaOH solution. The final solution was
then heated at 1008C under vigorous stirring. After a few minutes,
the dark brown colloidal solution was formed. The heating was
continued for 2 h, after which the colloidal suspension was cooled
to RT.
[20]
The prepared colloids were purified as reported in the literature.
Scheme 2. Recyclability of Pd NPs/MWCNTs catalyst for the Sonogashira–
Hagihara reaction in water.
An aliquot containing the desired amount of NPs was added to
a large excess of acetone. This treatment produced the extraction
of the protecting polymer to the acetone phase, provoking floccu-
lation of the metallic particles. This made purification possible by
either decantation or centrifugation, depending on the metallic
composition of the nanoparticles. After removal of the acetone
phase, the purified colloids were redispersed in a known amount
ChemCatChem 2015, 7, 1841 – 1847
www.chemcatchem.org
1845
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
of MeOH by very gentle stirring to obtain colloidal suspensions of
perfectly known concentration.
Suzuki–Miyaura cross-coupling reaction with potassium
phenyltrifluoroborate
To a screw-capped vial with a stirring bar were added 4-bromoani-
sole (93.5 mg, 0.5 mmol), potassium phenyltrifluoroborate (110 mg,
Preparation of heterogeneous catalysts
0.6 mmol), Pd/MWCNTs (0.1 mol% of Pd), K2CO3 (207 mg,
1.5 mmol), and PEG300/water (1:1 vol/vol, 0.3 mL). After stirring at
1208C for 1 h, the reaction mixture was cooled to RT. After the
Multiwall Carbon Nanotubes (MWCNTs, NanoBlack) were obtained
from the Columbian Chemicals Co. and used as received. The dif-
ferent colloidal NPs were supported on the MWCNTs using the im-
pregnation method. First, the appropriate volume of the NPs in
methanol dispersion was mixed with the carbon material. All cata-
lysts were prepared so as to have a final metal loading of 1 wt%.
The suspension was then gently stirred at room temperature for
same work-up described above the product was obtained in 83%
1
yield and analyzed by GC and H NMR spectroscopy. The recovered
catalyst was dried in vacuo and reused.
2
d. After this, the suspension was transferred to an oven at 608C
Hiyama cross-coupling reaction
until the methanol was evaporated. The collected solid was
washed with a mixture of ethanol and water (1:1, volume/volume
ratio) several times. Finally, the carbon support containing the de-
posited NPs was dried at 110 8C overnight.
To a screw-capped vial with a stirring bar were added 4-iodoani-
sole (117 mg, 0.5 mmol), trimethoxyphenylsilane (149 mg,
0.75 mmol), Pd/MWCNTs (0.1 mol% of Pd), and 50% NaOH aque-
ous solution (0.1 g, 12.5 mmol). After stirring at 1208C for 1 h, the
reaction mixture was cooled to RT. After separating the catalyst
and the aqueous phase by centrifugation, the aqueous phase was
Catalysts characterization
decanted. Recovered catalyst was washed with H O (5”1.0 mL)
2
and ethyl acetate (5”1.0 mL), which were then added to the aque-
ous phase. The aqueous phase was extracted eight times with
ethyl acetate. The combined organic extracts were dried over
MgSO4 and concentrated under reduced pressure. The product
was analyzed by GC and H NMR spectroscopy. The recovered cata-
lyst was dried in vacuo and reused.
The metal composition in the as-prepared nanoparticles was deter-
mined by ICP–OES, in a PerkinElmer Optima 4300 ICP-OES spec-
trometer. Before analysis, the samples were treated in a HCl/HNO3
mixture for 48 h to dissolve the metallic nanoparticles. The metal
loadings were obtained from the emission intensities by means of
the appropriate calibration curves for Pd and Ni. An average of
three analyses was done to calculate the metal content present in
the samples. The loading of the heterogeneous catalysts was also
performed in the same way both before and after catalytic tests in
order to determine the leaching of the NPs catalysts into the reac-
tion medium.
1
Mizoroki–Heck reaction
To a screw-capped vial with a stirring bar were added 4-iodoani-
sole (117 mg, 0.5 mmol), styrene (78.1 mg, 0.75 mmol), Pd Ni /
5
0
50
The as-prepared metallic colloids and the corresponding heteroge-
neous catalysts were characterized by TEM both before and after
undergoing catalytic tests by using a JEOL JEM-2010 high-tilt in-
strument operating at 200 kV with a structural partial resolution of
MWCNTs (0.1 mol% of Pd), TBAB (161 mg, 0.5 mmol), and
À1
3
.0 molL aqueous KOH solution (0.5 mL). After stirring at 1208C
for 1 h, the reaction mixture was cooled to RT. After separating the
catalyst and the aqueous phase by centrifugation, the aqueous
0
.5 nm. The composition of the nanoparticles was measured by
phase was decanted. Recovered catalyst was washed with H O (5”
2
EDS coupled to the TEM equipment (OXFORD instruments model
INCA Energy TEM100). The spatial resolution of the analytical TEM
used (15 nm) allowed measurement of the composition of groups
of approximately 5 particles, with a sufficient signal-to-noise ratio.
The catalysts particle-size distribution and Pd dispersion were cal-
1
.0 mL) and ethyl acetate (5”1.0 mL), which were then added to
the aqueous phase. The aqueous phase was extracted eight times
with ethyl acetate. The combined organic extracts were dried over
MgSO4 and concentrated under reduced pressure. The product
was analyzed by GC. The recovered catalyst was dried in vacuo
and reused.
[4]
culated as reported elsewhere.
Suzuki–Miyaura cross-coupling reaction with phenylboronic
acid
Sonogashira–Hagihara reaction
To a screw-capped vial with a stirring bar were added 4-iodoani-
sole (117 mg, 0.5 mmol), phenylacetylene (102 mg, 1.0 mmol), Pd/
MWCNTs (0.1 mol% of Pd), pyrrolidine (71 mg, 1.0 mmol), and
water (0.5 mL). After stirring at 1208C for 1 h, the reaction mixture
was cooled to RT. After separating the catalyst and the aqueous
phase by centrifugation, the aqueous phase was decanted. Recov-
To a screw-capped vial with a stirring bar were added 4-bromoani-
sole (93.5 mg, 0.5 mmol), phenylboronic acid (73 mg, 0.6 mmol),
Pd Ni /MWCNTs (0.1 mol% of Pd), TBAB (161 mg, 0.5 mmol),
5
0
50
K2CO3 (207 mg, 1.5 mmol), and water (0.3 mL). After stirring at
208C for 1 h, the reaction mixture was cooled to RT. After separat-
1
ing the catalyst and the aqueous phase by centrifugation, the
aqueous phase was decanted. Recovered catalyst was washed with
ered catalyst was washed with H O (5”1.0 mL) and ethyl acetate
2
(5”1.0 mL), which were then added to the aqueous phase. The
H O (5”1.0 mL) and ethyl acetate (5”1.0 mL), which were then
2
aqueous phase was extracted eight times with ethyl acetate. The
added to the aqueous phase. The aqueous phase was extracted
eight times with ethyl acetate. The combined organic extracts
combined organic extracts were dried over MgSO and concentrat-
4
ed under reduced pressure. The product was analyzed by GC. The
recovered catalyst was dried in vacuo and reused.
were dried over MgSO and concentrated under reduced pressure.
4
The product was obtained in 99% yield and analyzed by GC and
1
H NMR spectroscopy. The recovered catalyst was dried in vacuo
All the products obtained are described in the literature and the
analytical data perfectly fitted with the reported values.
and reused.
ChemCatChem 2015, 7, 1841 – 1847
www.chemcatchem.org
1846
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Papers
[
[
5] J. García-Aguilar, I. Miguel-García, . Berenguer-Murcia, D. Cazorla-
Amorós, Carbon 2014, 66, 599–611.
6] A. Leyva-PØrez, J. Oliver-Messeguer, P. Rubio-MarquØs, A. Corma, Angew.
Chem. Int. Ed. 2013, 52, 11554–11559; Angew. Chem. 2013, 125, 11768–
11773.
Acknowledgements
The Spanish Ministerios de Ciencia e Innovación (MICINN) and
Economía y Competitividad (MINECO) (projects CTQ 2007-62771/
BQU, CTQ2010-20387, CTQ2013-43446-P, Consolider Ingenio 2010,
CSD2007-00006, CTQ2014-51912-REDC, CTQ2012-31762 and RyC-
[
7] Water in Organic Synthesis in Science of Synthesis (Ed.: S. Kobayashi),
George ThiemeVerlag KG, Stuttgart, 2012.
[
8] For recent reviews, see: a) Palladium-Catalyzed Coupling Reactions: Prac-
tical Aspects and Future Developments (Ed.: . Molnµr), Wiley-VCH, Wein-
heim, 2013; b) . Molnµr, Chem. Rev. 2011, 111, 2251–2320; c) R. Jana,
T. P. Pathak, M. S. Sigman, Chem. Rev. 2011, 111, 1417–1492; d) M. Lam-
blin, L. Nassar-Hardy, J. C. Hierso, E. Fouquet, F. X. Felpin, Adv. Synth.
Catal. 2010, 352, 33–79.
2
009-03813 fellowship), FEDER, the Generalitat Valenciana
PROMETEO 2009/039, PROMETEOII/2014/010, and PROMETEOII/
014/017) and the University of Alicante are gratefully acknowl-
(
2
edged for financial support.
[
9] S. Domínguez-Domínguez, . Berenguer-Murcia, D. Cazorla-Amorós, .
Linares-Solano, J. Catal. 2006, 243, 74–81.
Keywords: cross-coupling
palladium · water
· nanoparticles · nanotubes ·
[
10] For recent reviews, see: a) V. Polshettiwar, A. Decottigneies, C. Len, A.
Fihri, ChemSusChem 2010, 3, 502–552; b) D. A. Alonso, C. Nµjera, Chem.
Soc. Rev. 2010, 39, 2891–2902; c) R. Rossi, F. Bellina, M. Lessi, Adv. Synth.
Catal. 2012, 354, 1181–1255.
[
[
1] a) H. Dai, Acc. Chem. Res. 2002, 35, 1035–1044; b) R. H. Baughman, Sci-
ence 2002, 297, 787–792.
2] For recent studies on SWCNT-supported metal NPs for organic reaction,
see: a) B. Movassagh, F. S. Parvis, M. Navidi, Appl. Organomet. Chem.
[
[
11] a) L. Botella, C. Nµjera, Angew. Chem. Int. Ed. 2002, 41, 179–181; Angew.
Chem. 2002, 114, 187–189; b) L. Botella, C. Nµjera, J. Organomet. Chem.
002, 663, 46–57.
12] M. T. Reetz, R. Breinbauer, K. Wanninger, Tetrahedron Lett. 1996, 37,
499–4502.
2
2
2
015, 29, 40–44; b) M. Gopiraman, R. Karvembu, I. S. Kim, ACS Catal.
014, 4, 2118–2129; c) H. Veisi, A. Khazaei, M. Safaei, D. Kordestani, J.
4
[
[
[
[
13] E. Alacid, C. Nµjera, Org. Lett. 2008, 10, 5011–5014.
14] T. Hiyama, J. Organomet. Chem. 2002, 653, 58–61.
15] E. Alacid, C. Nµjera, Adv. Synth. Catal. 2006, 348, 945–952.
Mol. Catal. A 2014, 382, 106–113; d) B. Cornelio, G. A. Rance, M. Lar-
onze-Cochard, A. Fontana, J. Sapi, A. N. Khlobystov, J. Mater. Chem. A
2013, 1, 8737–8744; e) A. R. Siamaki, Y. Lin, K. Woodberry, J. W. Connell,
th
16] The Mirozoki-Heck Reaction, 4 ed. (Ed.: M. Oestrich), Wiley, Chichester,
B. F. Gupton, J. Mater. Chem. A 2013, 1, 12909–12918.
2
009.
[
3] For recent studies on MWCNT-supported metal NPs for organic reac-
tion, see: a) S.-L. Yang, C.-Y. Cao, F.-F. Wei, P.-P. Huang, Y.-B. Sun, W.-G.
Song, ChemCatChem 2014, 6, 1868–1872; b) M. S. Khayoon, A. Abbas,
B. H. Hameed, S. Triwahyono, A. A. Jalil, A. T. Harris, A. I. Minett, Catal.
Lett. 2014, 144, 1009–1015; c) J. Safari, S. Gandomi-Ravandi, RSC Adv.
[
[
17] L. Botella, C. Nµjera, Tetrahedron 2004, 60, 5563–5570.
18] For recent reviews, see: a) R. Chinchilla, C. Nµjera, Chem. Soc. Rev. 2011,
4
2
1
0, 5084–5121; b) N. M. Jenny, M. Mayor, T. R. Eaton, Eur. J. Org. Chem.
011, 4965–4983; c) R. Chinchilla, C. Nµjera, Chem. Rev. 2014, 114,
783–1826.
2014, 4, 11654–11660; d) C. C. Gheorghiu, B. F. Machado, C. Salinas-Mar-
[
[
19] J. Gil-Moltó, C. Nµjera, Eur. J. Org. Chem. 2005, 4073–4081.
20] I. Miguel-García, . Berenguer-Murcia, D. Cazorla-Amorós, Appl. Catal. B
tínez de Lecea, M. Gouygou, M. C. Romµn-Martínez, P. Serp, Dalton
Trans. 2014, 43, 7455–7463; e) C. Jiang, X. Liang, Catal. Commun. 2014,
2
010, 98, 161–170.
46, 41–45.
[4] S. Domínguez-Domínguez, . Berenguer-Murcia, B. K. Pradhan, . Li-
nares-Solano, D. Cazorla-Amorós, J. Phys. Chem. C 2008, 112, 3827–
Received: February 19, 2015
3834.
Published online on May 25, 2015
ChemCatChem 2015, 7, 1841 – 1847
www.chemcatchem.org
1847
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim