Palladium-coated nickel nanoclusters: New Hiyama cross-coupling catalysts

Article abstract of DOI:10.1039/b513587g

The advantages of bimetallic nanoparticles as C-C coupling catalysts are discussed, and a simple, bottom-up synthesis method of core-shell Ni-Pd clusters is presented. This method combines electrochemical and 'wet chemical' techniques, and enables the preparation of highly monodispersed structured bimetallic nanoclusters. The double-anode electrochemical cell is described in detail. The core-shell Ni-Pd clusters were then applied as catalysts in the Hiyama cross-coupling reaction between phenyltrimethoxysilane and various haloaryls. Good product yields were obtained with a variety of iodo- and bromoaryls. We found that, for a fixed amount of Pd atoms, the core-shell clusters outperform both the monometallic Pd clusters and the alloy bimetallic Ni-Pd ones. THF is an excellent solvent for this process, with less than 2% homocoupling by-product. The roles of the stabiliser and the solvent are discussed. the Owner Societies 2006.

Full text of DOI:10.1039/b513587g

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PAPER
www.rsc.org/pccp | Physical Chemistry Chemical Physics
Palladium-coated nickel nanoclusters: new Hiyama cross-coupling
catalysts
ˇ
Laura Duran Pachon, Mehul B. Thathagar, Frantisek Hartl and Gadi Rothenberg*
´
´
Received 26th September 2005, Accepted 4th November 2005
First published as an Advance Article on the web 17th November 2005
DOI: 10.1039/b513587g
The advantages of bimetallic nanoparticles as C–C coupling catalysts are discussed, and a simple,
bottom-up synthesis method of core–shell Ni–Pd clusters is presented. This method combines
electrochemical and ‘wet chemical’ techniques, and enables the preparation of highly
monodispersed structured bimetallic nanoclusters. The double-anode electrochemical cell is
described in detail. The core–shell Ni–Pd clusters were then applied as catalysts in the Hiyama
cross-coupling reaction between phenyltrimethoxysilane and various haloaryls. Good product
yields were obtained with a variety of iodo- and bromoaryls. We found that, for a fixed amount
of Pd atoms, the core–shell clusters outperform both the monometallic Pd clusters and the alloy
bimetallic Ni–Pd ones. THF is an excellent solvent for this process, with less than 2%
homocoupling by-product. The roles of the stabiliser and the solvent are discussed.
larly interesting, as catalysis occurs on the shell surface, so one
Introduction
can envisage a cluster where the core is an inexpensive, inactive
metal, and the shell made from an active (noble) metal.25 In
this paper, we report the first combined chemical/electroche-
mical cluster synthesis approach to make core–shell Ni–Pd
nanoparticles. We then show that not only these clusters are
active in Hiyama cross-coupling, but they are also superior to
the monometallic and bimetallic alloy structures. The pros and
cons of the synthesis method and the possible applications of
such clusters in other catalytic systems are discussed.
Assembling functional nano-sized objects is one of the key
objectives of nanotechnology, with applications ranging from
controlled drug release to improved television screens.1–3 Due
to their size, there is no clear-cut argument for building nano-
structures using either a top-down or a bottom-up approach.
On one hand, top-down methods (such as chemical etching or
machining), usually give more precise and controllable results.
On the other hand, bottom-up protocols, especially those
based on self-assembly, are attractive because they can enable,
in the long run, the inexpensive manufacturing that is needed
for technological applications.4–7
One exciting area of application for metal nanoparticles is
catalysis.8 Owing to their ‘intermediate’ size, too big to be
‘homogeneous catalysts’ but too small to be ‘bulk metals’,
nanoparticles (referred to also as nanoclusters9,10) can some-
times catalyse reactions that are not accessible to their homo-
geneous or heterogeneous counterparts, as we and others
recently showed in the case of Suzuki11–14 and Sonogashira15
cross-coupling. Moreover, the large surface area can give a
catalytic advantage over conventional systems.13,16,17 Bimetal-
lic catalysts18,19 are especially interesting for several reasons:
combining two metals may provide control over the catalytic
activity, selectivity and stability, and some combinations may
exhibit synergistic effects.20–22 Moreover, by controlling the
type of cluster synthesised, one can improve the ‘‘catalyst atom
economy’’.23,24
Results and discussion
Preparation and characterization of metal clusters
Homometallic cluster suspensions of Ni and Pd were electro-
chemically prepared in the presence of TOAB (tetraoctyl
ammonium bromide) as surfactant to avoid particle agglom-
eration. Dark brown suspensions were obtained for both Ni
clusters and Pd clusters, as observed previously in the litera-
ture.20 The Pd clusters were stable for long periods (several
months) when kept under nitrogen, without observing any
agglomeration. In contrast, the Ni clusters remained stable
only for ca. two weeks, and only under a dry and non-
oxidising atmosphere.
The bimetallic alloy Ni–Pd clusters were prepared in a
similar way by simultaneous electrolyses of the corresponding
metal wire electrodes. Good yields and high stabilities were
obtained. This observation already points to one advantage of
the alloy preparation: it enables the study and application of
Ni-containing clusters in a stable configuration.
Using bottom-up synthesis, one can envisage three types of
cluster mixtures (Fig. 1): alloy particles, core–shell particles,
and segregated particles. The core–shell species are particu-
To synthesise the structured core–shell Ni–Pd particles, we
combined electrochemical and ‘wet chemical’ methods.26 First,
homometallic Ni particles were prepared electrochemically as
Van ‘t Hoff Institute of Molecular Sciences, University of Amsterdam,
Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands.
E-mail: gadi@science.uva.nl
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Fig. 1 Different compositions and structures of metal nanoclusters.
above. Then, the Pd shells were grown on these Ni cores from
solution, using chemical reduction (see below). This method
leads to spherical bimetallic nanoparticles with a narrow size
distribution and no aggregation, with a mean diameter of
4.9 nm (see Fig. 2).
UV–visible spectroscopy was also used to determine the
presence of Pd2+ in the core–shell Ni–Pd clusters. Fig. 3 shows
that the UV absorption of PdCl2 (lmax B 370 nm) is not
observed in the cluster solution. Thus, we assume that all of
the Pd(II) is reduced to Pd0 atoms. Control experiments
confirmed that Ni0 was not UV–visible active. Separate mea-
surements yielded the expected spectra for the alloy Ni–Pd
clusters and for Pd0 clusters (all in DMF).27
Fig.
2
Transmission electron micrograph (top, magnification
ꢁ200 000) and corresponding size distribution (bottom, based on
Regarding the formation of the Ni cores, the mechanism
of electrochemical cluster synthesis has been described pre-
viously by Reetz and co-workers.20 It involves formation of
adatoms in the vicinity of the cathode, followed by cluster
formation and stabilization. The self-organisation of the am-
monium salt around the metal core is essential for preventing
undesired metal powder formation. Having examined this
system, we observed differences in the nickel cluster stability,
depending on the solvent polarity and the stabiliser concen-
tration.
100 particles counted) of the core–shell Ni–Pd nanoparticles.
(ET = 37.4 and ET = 30, respectively).28,29 We saw that
aggregation was faster in more polar solvents, in good agree-
ment with the results of Helbig.30 This may be due to a
reduced electrostatic interaction between the metal nanopar-
ticles and the TOAB in the more polar surrounding. In a less
polar solvent, the stabilizing shell would shield the nanopar-
ticles more effectively, preventing further growth.
To study the influence of solvent polarity, we compared
acetonitrile (MeCN), tetrahydrofuran (THF) and dimethyl-
formamide (DMF). In each run, all the conditions were the
same as in the standard procedure. The cluster stability
increased in the order MeCN o THF o DMF. When MeCN
was used, the clusters aggregated already after a few hours.
Conversely, in THF, we were able to keep the dispersion stable
for several days, and in DMF the clusters remained stable and
did not aggregate even after four weeks.
To examine the effects of the stabilizer concentration, we
repeated the process using 0.01 M, 0.1 M, and 1.0 M TOAB
under otherwise identical conditions. Significantly, we ob-
served that with increasing TOAB concentrations the solution
was stable for a longer period. In the case of 0.01 M TOAB,
the nickel particles precipitated immediately after the electro-
lysis was stopped. We also observed a green layer on the
platinum cathode, assumed to be a nickel(II) species (possibly
nickel oxide particles formed by reaction with traces of water).
MeCN is a very polar solvent (polarity parameter ET
=
45.6), whereas THF and DMF are considerably less polar with
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Fig. 3 UV-Vis spectra showing the absorbance of PdCl2, core–shell
Ni–Pd clusters and alloy Ni–Pd clusters (all solutions are in DMF).
Fig. 4 Comparison of catalytic activity for six different systems in the
Hiyama cross-coupling of iodotoluene 1b and trimethoxyphenylsilane
2: Ni(OAc)2, Ni(0) clusters, Pd(OAc)2, Pd(0) clusters, Ni(0)/Pd(0)
alloy clusters and Ni(0)/Pd(0) core–shell clusters. Reactions were
carried out for 24 h in THF at 65 1C.
This indicates that the concentration of ammonium ions near
the cathode was not enough to stabilize all the adatoms.
Accordingly, this green layer was not observed when 0.1 M
or 1.0 M TOAB was used. With 1.0 M stabilizer, the Ni cluster
suspension did not aggregate even after four weeks! However,
this suspension was less active in catalysis, perhaps because the
cluster surface was covered by ammonium ions that impeded
the access of the reactants. Both the solvent polarity and
concentration of stabiliser are therefore important factors in
optimising the cluster stability and activity.
oxyphenylsilane 2 (eqn (3)) as model substrates. The rationale
behind using core–shell cluster catalysts is that in cluster
catalysis one expects the reaction to occur at the cluster
surface. The metal in the core is therefore not used. We
envisaged that core–shell Ni–Pd clusters would maximise the
efficiency of the Pd species (one needs a high Pd surface area,
but there is no need to ‘waste Pd’ in the cluster core).
The deposition of the Pd shells on the Ni cores is driven by
the different standard reduction potentials (eqns (1) and (2))
allowing the reduction of Pd(II) by Ni(0). As shown by Miyake
and co-workers,23 this process can be catalysed by the nickel
nanoparticles themselves.
Pd2+ + 2eꢂ - Pd0 E0 = +0.83 V
Ni2+ + 2eꢂ - Ni0 E0 = ꢂ0.23 V
(1)
(2)
Hiyama cross-coupling catalysis
Cross-coupling is a versatile method for forming carbon–
carbon bonds in a catalytic or stoichiometric manner.31 In
particular, biaryl formation is sought after because biaryl--
containing compounds are key intermediates for preparing
biologically active molecules, organic semiconductors and
liquid crystals.32 The Stille reaction (coupling of aryl halides
with organostannanes)33 and the Suzuki–Miyaura reaction
(coupling of aryl halides with organoboranes)34 are two
practical routes to biaryls, though the former suffers from
the toxicity of the tin by-product.35 An alternative protocol is
the Hiyama cross-coupling of aryl halides and organosilico-
nates.36–39 The usual catalysts for Hiyama cross-coupling are
{ligand-Pd(II)} complexes. Typically, 2.5–4 mol% catalyst is
used, with a large excess (3–4 equivalents) of base.31
ð3Þ
Experiments carried out in DMF yielded only 30% of the
cross-coupling product and ca. 40% of the undesired silane
homocoupling by-product. However, when we switched to
THF, complete conversion of iodotoluene was observed, with
only 1–2% of the homocoupling by-product (Fig. 4, far right).
Note that the Ni–Pd clusters were first synthesized in DMF
and then redispersed in THF.
We then compared the catalytic activity of Ni–Pd alloy
clusters, core–shell Ni–Pd clusters, and segregated Pd and Ni
clusters, as shown in Fig. 5. Control experiments confirmed that
no conversion occurred without catalyst added. To further rule
out the possibility that the non-reduced Pd(II) may be the active
species, we also tested Pd(OAc)2 and Ni(OAc)2 separately
under the same conditions.40,41 Note also that the amount of
the metal catalyst was the same in each case (1 mol% Pd or Ni,
depending on the experiment, relative to aryl halide).
Recently, we showed that transition metal clusters can
catalyse ligand-free Huisgen-type cycloadditions17 as well as
Suzuki11,12 and Sonogashira15 reactions. Here, we applied
core–shell Ni–Pd clusters as alternative, ligand-free catalysts
for the Hiyama reaction, using iodotoluene 1b and trimeth-
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Table 1 Hiyama cross-coupling of C6H5Si(OMe)3 with various ha-
loarylsa
Entry
1
Alkyne
Product
Yield(%)b
>99
2
3
4
5
6
7
>99
>99
>99
>99
>99
Fig. 5 Photograph and schematic drawing of the home-made elec-
trochemical cell used for the nanocluster synthesis. In the schematic,
the two wire anodes are shown as solid bars and the supporting glass
rods are omitted for clarity.
Pure Pd clusters gave a lower yield (44%) compared to the
alloy Ni–Pd ones (63%), that in turn were less active than the
Ni–Pd core–shell clusters. As all these catalysts contain the
same amount of palladium, this indicates that the core–shell
structure results in more Pd atoms on the surface. This means
more accessible catalytic sites per mole of palladium, and is
reflected by the higher catalytic activity. EXAFS studies by
Toshima and co-workers have shown that the total co-ordina-
tion number around Ni atoms in bimetallic clusters is usually
higher than that around Pd.21 This also suggests that the Pd
atoms are located preferentially on the surface. The tendency
of Pd to go to the surface may explain the difference in activity
between the Pd clusters and the alloy Ni–Pd clusters. No
reaction took place when the Ni clusters or Ni(II) alone were
used. Therefore, it is likely that only Pd is responsible for the
catalysis in the case of the alloy and the core–shell clusters.
The most important finding is that by combining palladium
with another, non-reactive metal (in this case Ni), we can
increase the activity per Pd atom (segregated Pd clusters o
alloy Ni–Pd clusters o core–shell Ni–Pd clusters).
62
6
8
9
86
74
To examine the scope of this catalytic system, we tested the
core–shell Ni–Pd clusters using a variety of substrates (eqn
(4)). Aryl iodides with either electron-donating or electron-
withdrawing substituents gave practically quantitative yields
(Table 1, entries 1–6). For aryl bromides we observed good
yields for electron-neutral and electron-withdrawing substitu-
ents, but much less conversion for the electron-donating
bromoanisole. Further studies implementing these new cata-
lysts in other reactions, as well as supporting the clusters on
solid supports for easier separation will be the subject of future
research in our laboratory.
10
a
Reaction conditions: 1.0 mmol substrate, 1.5 mmol phenyltri-
methoxysilane, 1.5 mmol TBAF, 1.0 mol% catalyst (core–shell
Ni–Pd clusters containing 0.1 mmol Ni and 0.01 mmol Pd), 5.0 mL
THF, N2 atmosphere, 65 1C, 24 h. Reaction time is not optimised.
b
Yields are based on GC analysis, corrected for the presence of an
internal standard. Side-products account for 2% or less of the aryl
halides.
Conclusions
These methods yield stable and highly monodispersed
particles. These bimetallic nanoparticles catalyze the Hiyama
cross-coupling reaction. The catalytic activity studies show
that the core–shell Ni–Pd clusters are superior to alloy
Core–shell Ni–Pd nanoparticles can be synthesised easily
by combining electrochemical and ‘wet chemical’ methods,
at room temperature in the presence of surfactant stabilisers.
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bimetallic clusters and monometallic clusters, indicating an
efficient use of the surface Pd atoms. We believe that this type
of structuring can open new pathways in bimetallic and
ligand-free catalysis.
mm). GC conditions: isotherm at 105 1C (2 min); ramp at
30 1C minꢂ1 to 280 1C; isotherm at 280 1C (5 min). Pentade-
cane was used as internal standard. Sets of 16 reactions
for catalytic tests were performed using a Chemspeed Smart-
start 16-parallel reactor block, modified in-house for efficient
reflux and stirring. All chemicals were purchased in their
dry pure form or dried using the standard drying procedure
prior to use. All reactions were carried out under N2 atmo-
sphere. All products are known compounds and were identi-
fied by comparison of their spectral properties to those of
authentic samples.
Experimental section
Materials and instrumentation
Electrochemical experiments were performed using a special
home-made cell coupled to a dual current supply
Electrochemical setup
All the reactions were carried out under dry N2 atmosphere
and at 25 1C in a single-compartment 100 mL cell constructed
in-house (see Fig. 5), a space-divided three-electrode modifica-
tion of the two-electrode design published by Reetz and
Helbig.42 The cell was equipped with special glass cover,
bearing a tubular glass ‘finger’ supporting the coiled Pt
cathode. Holes drilled in this tubular ‘finger’ ensured free
movement of ions between the helical turns, enabling efficient
mixing throughout the cell, while still separating the cathodic
and anodic spaces. The cover also included two outlets for the
anodes, a pin contact to the Pt counter electrode and inert gas
inlet/outlet. Pd and/or Ni wires (3 ꢁ 0.5 cm2 surface area, 0.5
mm thickness and 99.99% purity) served as sacrificial anodes,
coiled on glass rods for support. Pt wire (3 ꢁ 3 cm2 surface
area, 0.5 mm thickness and 99.99% purity) was used as the
inert cathode. Before use, electrodes were cleaned with scour-
ing powder and washed with H2O, acetone and toluene. The
supporting electrolyte was dry 0.1 M tetraoctylammonium
bromide (TOAB) in dimethylformamide (DMF), degassed
for 30 min prior to use. The yield for the cluster electrosynth-
esis was calculated as the current efficiency (the ratio of the
theoretical charge required for the amount of product ob-
tained to the total charge passed through the electrolysis cell
during the duration of the electrolysis).43
ð4Þ
(bigalvanostat) with a maximum applied output of 10 V/40
mA and three terminal connections (two working electrodes
1
and a single counter electrode). H NMR was measured on a
Synthesis of monometallic clusters
Varian Mercury vx300 NMR spectrometer at 25 1C. Chemical
shifts are referenced to Me4Si and internal solvent resonances.
UV–visible spectra were measured using a Hewlett Packard
8453 spectrophotometer with a diode array detector. The
quartz cuvette (Hellma Benelux, 3.5 mL, path length 1 cm)
was mounted on a sample holder with controlled heating
(Neslab constant temperature bath) and stirring (compressed
air was used to turn the magnet stirrer under the cuvette). The
wavelength range was 190–1100 nm, at 1 nm resolution.
Transmission electron microscopy (TEM) images for nano-
particles characterization were obtained with a JEOL-TEM
1200 EXII instrument, operated at an accelerating voltage of
120 kV. Samples for TEM were prepared by placing 150 mL of
cluster suspension on carbon-coated copper grids. These were
then placed in vacuum oven at 50 1C at 250 mm of Hg to
evaporate the solvent. GC analysis was performed using an
Interscience GC-8000 gas chromatograph with a 100% di-
methylpolysiloxane capillary column (DB-1, 30 m ꢁ 0.325
Pd nanoclusters. Pd wire as the anode and Pt wire as the
inert cathode were immersed in 60 mL of the supporting
electrolyte solution (12 m mol, 3.3 g, 0.1 M of (C8H17)4N+Brꢂ
in 60 mL dry DMF). A constant current density of 20 mA
cmꢂ2 was applied and the charge consumed was 2 F molꢂ1
.
The solution in the cell was stirred for 5 h. A dark brown
suspension was obtained, transferred to a Schlenk tube and
kept under nitrogen to be tested in the catalytic studies. The
current yield was 75–85%.
Pd clusters in tetrahydrofuran (THF) were similarly pre-
pared first in DMF, followed by drying the suspension under
vacuum (0.4 atm, 60 1C) for 5 h and redispersing the resulting
powder in dry THF.
Ni nanoclusters. These were prepared as above, using a Ni
wire anode. The current yield was 60–70%. CAUTION! In the
case of nickel, extra care should be taken to keep the system
dry and oxygen-free, as these clusters readily oxidise.
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Synthesis of alloy Ni–Pd clusters
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Ni and Pd wires as the anodes and Pt wire as the inert cathode
were immersed in 60 mL of the supporting electrolyte solution
(12 mmol, 3.3 g, 0.1 M (C8H17)4N+Brꢂ in 60 mL dry DMF).
A constant current density of 20 mA cmꢂ2 was applied and the
charge consumed was 2 F molꢂ1. The solution in the electro-
lysis cell was stirred for 5 h. A dark brown suspension was
obtained, transferred to a Schlenk tube and kept under nitro-
gen. The current yield was 60–80%. The suspension was dried
under vacuum (0.4 atm, 60 1C) for 5 h, and the resulting solid
was redispersed in dry THF.
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Synthesis of core–shell Ni–Pd clusters
The Ni wire anode and the Pt wire (the inert cathode) were
immersed in 60 mL of the supporting electrolyte solution (12
mmol, 3.3 g, 0.1 M (C8H17)4N+Brꢂ in 60 mL of dry DMF). A
constant current density of 20 mA cmꢂ2 was applied. The
solution in the electrolysis vessel was stirred for 2 h. A light
brown suspension (the Ni cores) was obtained. Then, Pd(OAc)2
(0.16 g, 0.7 mmol, 10 mol% relative to Ni) was added to the
suspension. The mixture was stirred for 2 h. A dark brown
suspension was obtained, transferred to a Schlenk tube, dried
under vacuum (0.4 atm, 60 1C) for 5 h, and redispersed in dry
THF. The resulting suspension was kept under an inert atmo-
sphere for characterization and catalytic studies.
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Example: 4-trifluoromethyl-biphenyl (3f). A solution of phenyl-
trimethoxysilane 2 (1.5 mmol, 0.29 g), 4-bromobenzotrifluoride
1f (0.5 M, 2.0 mL, 1.0 mmol) and tetra-n-butylammonium
fluoride (TBAF, 1.5 mmol, 0.47 g) in 5 mL THF was stirred
in the reactor (magnetic stirring, 5 min). A suspension of core–
shell Ni–Pd clusters with a 10 : 1 Ni–Pd ratio (10.0 mM, 5.0
mL, 1.0 mol% total metal, equivalent to 0.1 mol% of Pd
relative to 1) was then added in one portion, and the reaction
mixture was stirred at 65 1C for 24 h. Reaction progress was
monitored by GC. The product was extracted by washing the
reaction mixture three times with diethyl ether/water (1 : 1, 20
mL). The phases were separated and the ether phase was
evaporated under vacuum to obtain 0.82 g (82 mol% based
on 1) as a yellowish crystalline solid. 1H NMR (ppm, Me4Si): d
7.72 (s, 4 H), 7.58 (d, 2 H), 7.46 (t, 2 H), 7.38 (t, 1 H). A good
agreement was found with the literature values.44
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Acknowledgements
We thank Bert van Groen and Paul F. Collignon (both
University of Amsterdam) for excellent technical assistance
in building the electrochemical cell and the bi-galvanostat, and
Anil V. Gaikwad for measuring the UV–visible spectra.
38 M. E. Mowery and P. DeShong, J. Org. Chem., 1999, 64,
1684.
39 S. K. Kang and W. Y. Kim, Synth. Commun., 1998, 28,
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