Nickel and palladium nanocomposite carbon aerogels as recyclable catalysts for Suzuki-Miyaura reaction under aerobic and phosphine-free conditions in water

Article abstract of DOI:10.1016/j.tet.2012.05.099

Recyclable and readily prepared Ni and Pd nanocomposite carbon aerogels have been successfully used in the Suzuki-Miyaura reaction in water medium and aerobic conditions. In these ligand-free and environmentally-friendly conditions the Ni-carbon aerogel shows better recyclability than the corresponding palladium material. No appreciable leaching of Nickel could be detected after six reaction cycles. Interestingly enough, we have demonstrated for the first time that metal embedded carbon aerogels can be also used in water medium.

Full text of DOI:10.1016/j.tet.2012.05.099

Tetrahedron 68 (2012) 6517e6520
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Tetrahedron
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Nickel and palladium nanocomposite carbon aerogels as recyclable catalysts for
SuzukieMiyaura reaction under aerobic and phosphine-free conditions in water
,
,
a,
Laura Martín a b, Elies Molins b , Adelina Vallribera
*
*
a
ꢀ
ꢀ
Departament de Química, Universitat Autonoma de Barcelona, Cerdanyola del Valles, 08193 Barcelona, Spain
Institut de Ciencia de Materials (ICMAB-CSIC), Campus de la UAB, Cerdanyola del Valles, 08193 Barcelona, Spain
b
ꢀ
ꢀ
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 10 April 2012
Received in revised form 18 May 2012
Accepted 22 May 2012
Available online 28 May 2012
Recyclable and readily prepared Ni and Pd nanocomposite carbon aerogels have been successfully used
in the SuzukieMiyaura reaction in water medium and aerobic conditions. In these ligand-free and
environmentally-friendly conditions the Niecarbon aerogel shows better recyclability than the
corresponding palladium material. No appreciable leaching of Nickel could be detected after six reaction
cycles. Interestingly enough, we have demonstrated for the first time that metal embedded carbon
aerogels can be also used in water medium.
Keywords:
Water medium
Suzuki reaction
Nickel
Ó 2012 Elsevier Ltd. All rights reserved.
Aerogel
1. Introduction
tosylates, nevertheless the reactions were carried out in the pres-
ence of a phosphine ligand, in dioxane as solvent and catalyst
The SuzukieMiyaura cross-coupling reaction is a powerful
synthetic tool for the synthesis of unsymmetrical biaryls and his
great industrial significance has been recognized by the Nobel Prize
2010. Many efforts have been maid to solve some shortcomings of
this palladium catalyzed cross-coupling of organoboranes.1 In this
context, the development of ligand-free, reusable catalysts and
environmentally-friendly procedures with water as reaction me-
dium is desirable.2 Remarkable results have been obtained using an
oximeecarbapalladacycle complex covalently anchored to silica3a
or PdeEDTA immobilized in an ionic liquid brush in water.3b
Moreover, finding suitable alternatives to organopalladium chem-
istry4 for this valued cross-coupling would suppose a large im-
provement. As far as we know only a few examples of the use of
recyclable Ni instead of Pd based materials in Suzuki reactions have
been described. Nickel ion-containing ionic liquid immobilized in
a silica surface showed high activity as catalyst for the cross-
coupling of arylchlorides however, the reusability of this catalyst
was limited (the yield was reduced on the second cycle) and ad-
dition of triphenylphosphine was necessary.5 Additionally, im-
pregnated nickel(II) onto graphite was activated through reduction
and then used as catalyst for the reaction of arylchlorides and
recycling was demonstrated by only two successive cycles.6
Aerogels form a new class of solids showing potentialities for
a range of applications.7 The open mesoporosity and high surface
area made them excellent candidates to support metal nano-
particles for catalysis applications.8 The most common aerogels are
inorganic, usually derived from the gel polymerization of tetraal-
koxysilanes. In 2003, we described the synthesis and use of silica
aerogeleiron oxide nanocomposites as recoverable catalysts in
conjugate additions and in the Biginelli reaction.9 Some years later
we demonstrated that a palladium silica aerogel could be an ex-
cellent catalyst, however not reusable, for the MizorokieHeck re-
action.10 Moreover, polycondensation of certain organic monomers
can be used to prepare organic aerogels, which, once pyrolyzed,
give rise to carbon aerogels. For both type of aerogels, silica and
carbon, the drying of the gels was carried out by supercritical sol-
vent evaporation in a computer controlled plant. We have dem-
onstrated that this carbon aerogel can be doped with metallic
palladium nanoparticles and used as an effective recoverable cat-
alyst for the hydroxycarbonylation of aryl halides,11 the Mizor-
okieHeck10 and the Sonogashira12 cross-coupling reactions. In all
the cases the reactions were carried in organic solvents, under
aerobic conditions and without any particular precaution.
Herein we describe the preparation and use of nickel and palla-
dium carbon aerogel nanocomposites as efficient recoverable cata-
lysts for the SuzukieMiyaura reaction under aerobic, phosphine-
free conditions in water.
* Corresponding authors. Tel.: +34 93 5801853; fax: +34 93 5805729 (E.M.);
tel.: þ34 93 581 3045; fax: þ34 93 581 2477 (A.V.); e-mail addresses: elies@
icmab.es (E. Molins), adelina.vallribera@uab.es (A. Vallribera).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2012.05.099

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L. Martín et al. / Tetrahedron 68 (2012) 6517e6520
2. Results and discussion
collaborators describes the use of a Pdeorganic aerogel prepared
from a Feetricarboxylate based Pd-gel after supercritical CO2 ex-
traction. The BET surface area was very low (22 m2 ꢀ1), although
no pyrolitic process has been carried out. The aerogel exhibited
moderate catalytic activity (36% yield for the reaction of bromo-
benzene and phenylboronic acid in MeOH). The best yield was
obtained for the reaction of 4-bromoanisole and phenylboronic
acid in refluxing DMF (62%). The recycling of this material failed.17
Carbon aerogels were obtained through the solegel poly-
condensation of the potassium salt of 2,4-dihydroxybenzoic acid
and formaldehyde to produce organic gels. Then the potassium ions
were replaced with the desired metal with an ion-exchange pro-
cess. Thus nickel and palladium loaded organic gels were prepared
from Ni(NO3)2$6H2O and Pd(OAc)2, respectively. The gels were then
dried under supercritical conditions of the solvent, otherwise
a xerogel will be obtained, and carbonized to generate a metal-
doped carbon aerogel. No reduction step (H2 at high tempera-
ture) is required to obtain metallic nanoparticles. The metal parti-
cles were formed by reduction of the M2þ ions during pyrolysis of
the organic aerogels.13
g
A
variety of aryliodides and boronic acids incorporating
electron-rich and electron-poor aromatic rings were converted into
the corresponding biaryl compounds using water as solvent (Table
2). For all the reactions 1 mol % of metal with respect to the limiting
reagent, and K2CO3 were used. When a mixture of K2CO3/KF (1:1)
was tested the yields dropped down (compare entries 3 and 4 with
1 and 2). Niecarbon aerogel exhibited a slightly higher catalytic
activity than the Pd embedded aerogels. Just compare entries 1 and
2 (97 and 92% yield for Ni and Pd, respectively) for the reaction of 4-
iodobenzoic acid with phenylboronic acid, and entries 8 and 9 for
the reaction of 4-acetoxyphenylbromide with phenylboronic acid
(65 and 50% yield for Ni and Pd, respectively). Interestingly enough,
we have demonstrated for the first time that metal embedded
carbon aerogels can be also used in water medium.
Carbon aerogels containing nickel nanoparticles contained
13 wt % of metal and possessed a BET surface area of 673 m2 gꢀ1
(Fig. 1 and Table 1). Its powder X-ray diffraction pattern (PXRD) was
characteristic of metallic fcc-Ni, however a small amount of fcc-NiO
phase was also observed. The transmission electron microscopy
(TEM) image of Niecarbon aerogel showed well-defined spherical
particles regularly dispersed in the carbon matrix. The mean di-
ameter determined by TEM was about 51 ꢁ 26 nm. Pdecarbon
aerogels (420e459 m2 ꢀ1) containing 36e46 wt % of palladium
g
were obtained by the same method. The PXRD pattern obtained for
Pdecarbon aerogels was characteristic of the metallic fcc-Pd phase
(for features of Pdecarbon aerogels see Supplementary data).
Table 2
SuzukieMiyaura reactions catalyzed by Pd and Niecarbon aerogels (Scheme 1)a
Entry M-carbon
aerogel
Ar1eX
Ar2B(OH)2
T (h) Product
yieldb (%)
1 Niecarbon
2 Pdecarbon (1) 4-HO2CC6H4eI
3 Niecarbon 4-HO2CC6H4eI
4 Pdecarbon (1) 4-HO2CC6H4eI
5 Pdecarbon (1) 4-HO2CC6H4eI
6 Pdecarbon (1) 4-HO2CC6H4eI
7 Pdecarbon (1) PheI
8 Pdecarbon (1) 4-MeOC6H4eI
9 Niecarbon
10 Pdecarbon (2) 4-MeC(O)C6H4eBr PhB(OH)2
4-HO2CC6H4eI
PhB(OH)2
PhB(OH)2
PhB(OH)2
PhB(OH)2
4-MeOC6H4B(OH)2
4-CF3C6H4B(OH)2
4-MeOC6H4B(OH)2 94
2
2
2
2
1
3a14 97(92)
3a 92
3a 80c
3a 76c
3b14 94(89)
3c14 89(86)
3d15 87(78)
3d 9
48
PhB(OH)2
72
72
70
4-MeC(O)C6H4eBr PhB(OH)2
3e16 65
3e 50
a
Reactions were carried out in 20 mL of water in the presence of air, using
10 mmol of aryl halide, 15 mmol of arylboronic acid, 20 mmol of K2CO3, and
0.1 mmol (1 mol %) of M-carbon aerogel.
b
Yield was determined by 1H NMR. Yield in parenthesis correspond to isolated
compound.
c
K2CO3/KF (1:1) were used as bases.
Scheme 1. SuzukieMiyaura cross-coupling reaction.
Fig. 1. Features of Niecarbon (a) TEM image; (b) particle size distribution; (c) electron
diffraction; (d) PXRD pattern.
To obtain information about the presence of leached metal
species in our reaction we carried out some hot filtration experi-
ments. The reaction of p-iodobenzoic acid with phenylboronic acid
was interrupted at a low conversion (10 min of reaction) and then
continued after catalyst removal. In both cases (Ni and Pd) the
conversion was complete in 2 h. Curiously enough, if the reaction
was stopped at 5 min no further conversion in the catalyst-free
solution phase is detected. Probably, in 5 min no leaching of ac-
tive metal species can be achieved. This results points out to the fact
that the nanoparticles can act as the source of the active catalysts
species in solution through metal leaching, being at least in part,
responsible for the catalytic activity.
Table 1
Characteristics of metal nanocomposite carbon aerogels
Materiala
Metal (%) Surface area Pore Volume Mean particle size
(m2 gꢀ1
)
(cm3 gꢀ1
)
(nm) PXRD phase
Carbon
Niecarbon
Pdecarbon (1) 46
Pdecarbon (2) 36
d
514
673
420
459
1.5
1.5
0.9
1.3
Amorphous
13
51ꢁ26 fcc-Ni, fcc NiO
21ꢁ10 fcc-Pd
17ꢁ7 fcc-Pd
a
The numbers in parentheses indicate different batches.
Encouraged by the good catalytic activity we examined its
recyclability in the reaction of 4-iodobenzoic acid with phenyl-
boronic acid. The catalyst system proved to be readily recyclable
(Table 3). In the case of the Niecarbon aerogel the last two cycles
were carried out one year after showing the robustness of the
These Ni(0) and Pd(0)-containing materials were then tested as
catalysts in SuzukieMiyaura reactions. To the best of our knowl-
edge it is the first time that a M-carbon aerogel is used as catalyst of
SuzukieMiyaura reactions. Only one manuscript from J. Zhang and

L. Martín et al. / Tetrahedron 68 (2012) 6517e6520
6519
Table 3
3. Experimental section
3.1. Preparative procedures
Recycling studies for the SuzukieMiyaura reactiona
Catalyst
Time (h)
Yield (%)b
TON
TOF (hꢀ1
)
Niecarbon
Pdecarbon (1)
2
2
95/96/97/97/71c/90(81)c
92/87(84)/80/89/81/85
546
514
273
257
Ni and Pd nanocomposite carbon aerogels: A suspension of
2,4-dihydroxybenzoic acid (2.9 g, 18.8 mmol) in distilled water
(100 mL) was treated with K2CO3 (1.3 g, 9.4 mmol) under vigorous
stirring, in the presence of air. The suspension became clear after
0.5 h, when all the acid was neutralized. Formaldehyde (3.0 g,
37 wt %, 37 mmol) was then added to the solution, followed by the
catalyst K2CO3 (26 mg, 0.19 mmol). The clear solution was poured
into glass moulds that were then sealed hermetically. The mixture
was allowed to cure for 24 h at room temperature, and 96 h at 70 ꢂC.
Next, the gels that will be nanocomposite with Ni or Pd were
washed with water or acetone, respectively, and they were soaked
in a 0.1 M aqueous solution of Ni(NO3)2$6H2O or Pd(OAc)2 in ace-
tone for 24 h. The procedure was repeated three times. The doped
gels were then washed with acetone and dried with supercritical
CO2 inside the autoclave (following the next procedure: (1) ex-
change of acetone by liquid CO2 at rt and 100 bar, over 2 h; (2)
drying with supercritical CO2 at 42 ꢂC, 120 bar, over 2 h), obtaining
the Pd and Ni organic aerogels. Finally, the material was pyrolyzed
at 1050 ꢂC under N2 in a tubular furnace to afford the Ni and Pd
nanocomposite carbon aerogels.
a
Reactions were carried out in the presence of air, in 20 mL of water, using
10 mmol of p-iodobenzoic acid, 15 mmol of phenylboronic acid, 20 mmol of K2CO3
and 0.1 mol % M-carbon aerogel at 110 ꢂC.
b
Yields were determined by 1H NMR analysis. Yield in parenthesis correspond to
the isolated compound.
c
Last two cycles were carried out after one year.
material. The TEM image of the recovered catalysts Nie (Fig. 2) and
Pdecarbon (Supplementary data) aerogels after 6 cycles showed
well dispersed spherical nanoparticles with practically the same
mean diameter, thus no aggregation processes has taken place and
from PXRD and ED studies no changes were observed in the me-
tallic phase after the recycling process.
Typical procedure for SuzukieMiyaura reaction in water:
The Ni and Pd carbon aerogels were always sinked in water 24 h
before their use as catalysts and kept in the same solvent.
Catalytic reactions: In a 100 mL three necked round-bottom
flask, arylboronic acid (15 mmol), aryl halide (10 mmol) and
K2CO3 (2.76 g, 20 mmol) were dissolved in 20 mL of H2O. Then, Ni or
Pd carbon aerogel (0.1 mmol, 1 mol %) was added to the mixture
and the reaction was carried out under reflux (110 ꢂC), in the
presence of air and mechanical stirring. Periodic sampling of the
reaction media was made to analyze the reaction evolution by GC
and 1H NMR measurements. The liquid phase was decanted and the
carbon aerogel was washed with water. This water extracts and the
reactive solution were mixed together and acidified until pH 1 to
cause the precipitation of the final product. The solid was filtrated,
washed with water and dried. The pieces of aerogel were washed
with AcOEt, with water and were kept submerged in this solvent
before reused.
Fig. 2. Features of Niecarbon after 6 reaction cycles (a) TEM image; (b) particle size
distribution; (c) electron diffraction; (d) PXRD pattern.
Acknowledgements
Financial support from the MICINN of Spain (CTQ2008-05409,
CTQ2011-22649, CTQ2010-20387, CTQ2009-12520-C03-03 and
Consolider Ingenio 2010 CSD2007-00006, CSD2007-00041) and
the DURSI-Generalitat de Catalunya (2009SGR-1441, 2009SGR-203)
are acknowledged. L.M. wants to thank the MICINN of Spain for
a predoctoral grant.
On the bases of the elemental analysis of the recovered
Niecarbon aerogel after 6 reaction cycles (11ꢁ2%) no appreciable
leaching has occurred if we take into account the intrinsic error of
the analysis. Moreover no metal leaching could be observed in the
analysis of the reaction crude. This suggest that the homogeneous
Ni species released from the immobilized nickel systems, being at
least in part responsible of the catalysis, would be then redeposited
on the aerogel.18 Opposite to that, in the case of the recycled
Pdecarbon aerogel the content of Pd decreased from 46ꢁ1% to
30ꢁ2%, which implies that a large amount of Pd was lost through
the cycles.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tet.2012.05.099.
In conclusion, we have demonstrated that recyclable and readily
prepared Ni and Pdecarbon aerogels can be successfully used in the
SuzukieMiyaura reaction in water medium and in the presence of
air. To the best of our knowledge metal carbon aerogels nano-
composites have never been used as catalyst in water. Moreover, in
these ligand-free conditions no leaching of Ni could be observed
after 6 cycles. The advantages of using a recyclable supported Ni
instead of more expensive metals, and using water as a reaction
medium in the presence of air makes this methodology really in-
teresting from an industrial point of view.
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