Suzuki- and Heck-type cross-coupling with palladium nanoparticles immobilized on spherical polyelectrolyte brushes

Article abstract of DOI:10.1002/adsc.200700422

We report on a systematic study of the use of palladium nanoparticlesimmo bilized on spherical polyelectrolyte brushes - Pd@SPB - for Heck- and Suzuki-type coupling reactions. The spherical polyelectrolyte brush particles serving as carriers for the palladium nanoparticles consist of a solid polystyrene core with a radiusof 46 nm onto which long chains of cationic polyelectrolytesare grafted. The palladium nanoparticles have directly been generated within this brush layer and the stabilization of the nanoparticlesis effected by the colloidal carriers, no further surface stabilization is necessary. We demonstrate that these composite particles present robust catalysts for the Heck- and Suzuki-type coupling reactions. This was shown by carrying out the Suzuki- and Heck-type coupling reactionsat relatively low temperatures (Suzuki reaction: 50°C, Heck reaction: 70°C). We demonstrate that the catalytic composite particles are not changed by these reaction conditions and retain their full activity for at least four runs. The yields obtained for both reactions are good to excellent. The mild operation conditions of the palladium nanoparticles are traced back to the absence of surface stabilization. Further mechanistic implications are discussed.

Full text of DOI:10.1002/adsc.200700422

FULL PAPERS
DOI: 10.1002/adsc.200700422
Suzuki- and Heck-Type Cross-Couplingwith Palladium
Nanoparticles Immobilized on Spherical Polyelectrolyte Brushes
S. Proch,a Y. Mei,b J. M. Rivera Villanueva,a Y. Lu,b A. Karpov,c M. Ballauff,b,*
and R. Kempea,*
a
Anorganische Chemie II, Universität Bayreuth, 95440 Bayreuth, Germany; e-mail: kempe@uni-bayreuth.de
Physikalische Chemie I, Universität Bayreuth, 95440 Bayreuth, Germany
b
Fax : (+49)-921-552-780; e-mail: matthias.ballauff@uni-bayreuth.de
Catalysis Research, BASF AG, 67056 Ludwigshafen, Germany
c
Received: August 28, 2007; Revised: December 4, 2007; Published online: February 8, 2008
Abstract: We report on a systematic study of the use and Heck-type coupling reactionsat relatively low
of palladium nanoparticlesimmobilized on ps herical
temperatures(Suzuki reaction: 50 8C, Heck reaction:
polyelectrolyte brushes – Pd@SPB – for Heck- and 708C). We demonstrate that the catalytic composite
Suzuki-type coupling reactions. The spherical poly- particlesare not changed by thees reaction condi-
electrolyte brush particles serving as carriers for the tionsand retain their full activity for at leats four
palladium nanoparticles consist of a solid polystyrene runs. The yields obtained for both reactions are good
core with a radiusof 46 nm onto which long chains
of cationic polyelectrolytesare grafted. The palladi-
um nanoparticleshave directly been generated
to excellent. The mild operation conditionsof the
palladium nanoparticlesare traced back to the ab-
sence of surface stabilization. Further mechanistic
within this brush layer and the stabilization of the implications are discussed.
nanoparticlesiseffected by the colloidal carrier,s no
further surface stabilization is necessary. We demon-
strate that these composite particles present robust Keywords: catalysis; Heck-type cross-coupling; palla-
catalysts for the Heck- and Suzuki-type coupling re- dium nanoparticles; spherical polyelectrolyte brush;
actions. This was shown by carrying out the Suzuki- Suzuki-type cross-coupling
Introduction
conditionsapplied. Moreover, no tsabilizing agent
should be induced that may alter or block the surface
of the nanoparticles. Such molecules could also be re-
Metal nanoparticlesshow different propertiesin com-
parison to their bulk materials and to isolated moved during the work-up procedure. The carrier sys-
atoms.[1,2] In principle, the high surface area per tems should also be sufficiently stable during the recy-
volume rendersmetal nanoparticlesideal candidates
cling of the catalyst. In efforts to develop a heteroge-
for catalysis.[3] The palladium-catalyzed Heck[4] and neous catalyst system for different applications, a vari-
Suzuki[5] reaction (cobalt nanospheres are also able to ety of solids, particularly, active carbon,[10] mesopo-
catalyze these reactions[6]) between aryl halidesand
rousislica,
inorganic oxides,[12] molecular sieves,[13]
[11]
alkenes or boronic acids, respectively, are well-estab- polymers,[14] capsules[15] and basic metal phosphates
lished tools for C C bond formation in organic syn- (apatites)[16] have been used as carrier systems.
À
thesis. Thus, a convenient route for cross-coupling re-
actionsinvolvesreusable palladium nanoparticlesthat
In particular, recent work hasdemonstrated that Pd
nanoparticlesmay lead to excellent yieldsin a variety
promote these reactions in organic solvents or in of catalytic reactions.[17–21] An issue of great impor-
water.[7–9] The handling of the nanoparticles, however, tance isthe leaching of Pd during the catalytic
may impose problems during work-up unless the par- cycle,[22–24] especially since recent work has given evi-
ticlesare immobilized on suitable carriers. Such a car-
dence that metal ionsare the catalytically active spe-
rier system should allow separation, for instance, via ciesin the Heck reaction at elevated temperatures
filtration, have a long-term stability, should be easy to (see the discussion of this problem in Refs.[20,22–25]).
handle, and should prevent the metallic nanoparticles
Recently, we showed that well-defined gold, plati-
from coagulating. For a re-use of the catalyst nanopar- num and palladium nanoparticlescan be generated in
ticles the support should be inert towards the reaction
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S. Proch et al.
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water). Colloidal stability is brought about mainly by
the special microenvironment provided by the organic
carrier particles.
Here we show that Pd nanoparticles immobilized in
spherical polyelectrolyte brushes (SPB) present a
composite system that can be used as an efficient cat-
alyst for the Heck and Suzuki reactions. We shall
demonstrate that both reactions can be carried out
under mild conditionsand low temperatures(Suzuki
reaction: 508C; Heck reaction: 708C). Special empha-
sis is laid on a possible degradation of the particles
during these reactions and their catalytic activity after
several runs.
Results and Discussion
Figure 1. Scheme of the spherical polyelectrolytes used as
carrier particlesand the composite particlesPd@SPB. Long
chainsof poly[(2-methylpropenyloxyethyl)trimethylammoni-
um chloride] are grafted to polystyrene cores of ca. 100 nm
Synthesis and Characterization of the Pd@SPB
Composite Particles
2À
The synthesis of the carrier particles has been done
by photo-emulsion polymerization[33] asdecsribed re-
cently.[27,28] Table 1 summarizes the pertinent parame-
ters of the composite particles. Figure 2 displays a mi-
diameter. PdCl4 ionsare introduced ascounterionsand
subsequent reduction by NaBH4 in aqueousoslution leads
to Pd nanoparticlesof 2.6 Æ0.5 nm immobilized on the sur-
face of the carrier particles.
spherical polyelectrolyte brushes (SPB)[26] and used as
catalysts.[27–31]
Table 1. Characterization of the Pd@SPB composite particle
used in this study.[a]
Figure 1 shows the structure of the spherical poly-
electrolyte brushes (SPB) in a schematic manner:
Long polyelectrolyte chainswith quaternary amino
groupsare chemically grafted to colloidal polymer
particlesof ca. 100 nm in diameter. The layer of poly-
electrolyte chainsattached to the surface of the carri-
er particlesisvery denes, that i,s the contour length
Lc of the chainsismuch higher than their average dis-
tance on the surface of the carrier particle. In this
way a polyelectrolyte “brush” results in a system of
strongly interacting polymer chains inserted densely
to a curved surface.[26,32] Since the polyelectrolyte
chainsare grafted to the surface of the core particles
by covalent bonding, a very stable support system re-
sults. The counterions neutralizing the charge of the
polyelectrolyte chainsare nearly fully confined in the
brush layer.[26] Metal ionussch a, s for example,
R
Lc
Mw
[gmolÀ1]
s
D
G
d
[nm]
[nm] [nm]
G
A
46
182
150800
0.019
8.2
2.6Æ0.5
[a]
Lc: contour length of grafted chainsdetermined from
Mw; s: graft density on surface of core particles; D: the
average distance between two neighbouring points onto
which the polyelectrolyte chainsare grafted to; d: aver-
age size of the Pd nanoparticles.
2À
PdCl4 can be immobilized in thisway and reduced
to well-defined metal nanoparticles.[27–29,31] Work done
by cryogenic transmission electron microscopy (cryo-
TEM) hasdemontsrated that thees metal nanoparti-
clesare located near the surface of the core particles
(see Figure 1 and Refs.[28,31]). It hasbeen demonstrat-
ed that platinum nanoparticlesimmobilized on SPB
can be re-used many times for catalytic hydrogenation
reactionswithout any detectable losof catalytic ac-
tivity.[29] It should be noted that the metal nanoparti-
clesgenerated in thisway are not tsabilized by any
Figure 2. Cryo-TEM imagesof Pd@SPB composite particles
before (left) and after (right) catalytic application in the
Suzuki coupling. No noticeable change wasobesrved after
catalysis, the palladium nanoparticles were embedded firmly
surface group or other stabilizing agents (beside on the surface of SPB particles.
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Suzuki- and Heck-Type Cross-Coupling with Palladium Nanoparticles
FULL PAPERS
crograph of the particlesobtained by cryo-TEM. It
in water in the presence of Pd@SPB (see Scheme 1)
should be noted that the contrast of the polyelectro- was studied to investigate if the described catalyst
lyte chains in this system is not high enough so that system is suitable for all three halides. All catalytic
they cannot be visualized in these micro- experiments(ese Table 2 and Figure 3) were carried
graphs.[26,29,31,34]
out in sealable pressure tubes with Teflon stoppers in
air (Suzuki: 508C, Heck: 708C) for 24 h. Pd loadings
of 0.09 mol% (Suzuki) and 0.029 mol% (Heck) were
used. In order to address the problem of homocou-
pling, runswere carried out in which we only added
either the aryl halide or the boronic acid. It wasob-
Suzuki-Type Cross-Couplingof Aryl Halides using
Pd@SPB as Catalyst
The Suzuki-type cross-coupling reaction between served that the boronic acid gave rise to homocou-
iodo-, bromo-, chlorobenzene and phenylboronic acid pling productsin 14% yield under the above-men-
tioned mild reaction conditions. Heterocoupling was
observed for bromides (conversions of 80–90%,
Figure 3) and iodides(around 70%). Chloridesgave
only low yields( ca. 6%). Substituents in the ortho-
and meta-positions resulted in lower yields (due to
steric hindrance) than para-substituted arenes. Select-
ed results are listed in Table 2.
Bromobenzene and phenylboronic acid were used
to determine reproducibility of palladium-SPB com-
posite catalyst. The flask filled with defined amounts
of palladium-SPB composite catalyst, reactants, PTC
(phase-transfer catalyst: tetrabutylammonium bro-
Scheme 1. Test reaction for Suzuki-type cross-coupling using
palladium nanoparticlestsabilized in SPB particlesascata-
lyst.
Table 2. Suzuki-type cross-coupling using palladium nanoparticles stabilized in SPB particles as catalyst, yields were determined
by GC, hcp=homocoupling product (biphenyl).
Halide
Product
GC conversion
83%
Homocoupling
4%
79%
15%
5%
6%
22%
50%
1%
1%
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S. Proch et al.
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Table 2. (Continued)
Halide
Product
GC conversion
94%
Homocoupling
1%
89%
19%
1%
17%
6%
1%
2%
1%
74%
78%
2%
1%
69%
83%
87%
80%
2%
2%
0%
0%
71%
0%
mide; without PTC the conversion of the reaction of 24 h. After the reaction time the productswere re-
iodobenzene and phenylboronic acid dropsto 15% moved by ether and new eductsand THF were
compared to 70%) and base was stirred at 508C for added, thisproceswasrepeated four time.s The con-
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Suzuki- and Heck-Type Cross-Coupling with Palladium Nanoparticles
FULL PAPERS
bromobenzene (and Heck-type coupling involving sty-
rene and iodobenzene), in both cases no cross-cou-
pling reaction could be observed.
Hence, the present catalyst system could be easily
recycled and re-used (Figure 3). The good reproduci-
bility can be understood by the robustness of the cata-
lyst system during catalysis and work-up (see the dis-
cussion of Figure 2). No change of the Pd nanoparti-
cles or the entire composite system can be found. A
check of the content of Pd in the crude product re-
sulting from the cross-coupling of phenylboronic acid
with bromobenzene demonstrated that the content of
Pd waslesthan 6 ppm ( cf. Refs.[16,35,36]).
Figure 3. Reproducibility of the cross-coupling catalyzed by
Pd@SPB.
Analysis of the Yields: Hammett Plots
A catalytic reaction expanded to different substrates
version of each run was checked by GC using dodec- can be regarded as a system of parallel reactions.[37]
ane as an internal standard. Palladium-SPB composite Hence, the ratiosof the rate constantsof the parallel
particlesafter catalytic application were purified and
separated by an ultrafiltration appliance with cellu- approximation. Therefore the Hammett equation –
lose membrane (100 nm pore size supplied by ln(k/kH)=s·1 – can be applied and a Hammett plot
Schleicher & Schuell). The results are shown in can be used to discuss the different reactivity arising
reactionsare equal to the ratiosof the yieldsin good
AHCTREUNG
Figure 3. There isno noticeable change in conversion,
which indicatesthe catalytic activity of the palladium
from different substituents in the same position of the
benzene ring. To avoid complicationsby steric effects,
nanoparticles remained stable at least within the four Hammett plotsare done best for reactivitiesobtained
accomplished runs. Similarly, no noticeable difference with different substituents in the para-position. The
wasfound in cryo-TEM imagesbefore and after cata-
Hammett analysis of the effect of para-substitution in
lytic reaction (see Figure 2). Palladium nanoparticles the Suzuki reaction catalyzed by Pd@SPB hasbeen
were still embedded firmly on the surface of the SPB compared to Pd
particles. Also, the filtrate of a freshly prepared Table 3).
Pd@SPB solution was used as a catalyst in the
C
Suzuki-type coupling between phenylboronic acid and aswell asthe Heck reaction involvesan oxidative ad-
Figure 4. A: Hammett-plots of the Suzuki-type cross-coupling under the same reaction conditions (1 mmol aryl bromide,
1 mmol phenylboronic acid, 1.5 mmol KO-t-Bu, 1 mmol NBu4Br asPTC, 2 mL H O, 2 mL THF, 0.09 mol% Pd) using two
2
different catalyst systems; grey circles: Pd@SPB; black squares: Pd
pling compiled from the literature. Black squares: PS-3.3, Pd (Pd immobilized on polystyrene-b-poly-4-vinylpyridine block
copolymer);[9] grey triangles: Pd(OAc)2.[24] The figures show the relative rate constants plotted vs. sp (measure of total polar
effect exerted by substituent X relative to H as substituent in the para-position to the reaction centre).
A
AHCTREUNG
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Table 3. Hammett analysis of the Suzuki-type cross-cou-
pling.
Scheme 2. Heck-type cross-coupling using palladium nano-
particles stabilized in SPB as catalyst.
R
sp
Pd
A
Pd@SPB
Table 4. Heck reaction promoted by Pd@SPB in aqueous
OMe
Me
H
CF3
NO2
À0.15
À0.14
0
0.64
0.77
3%
2%
15%
>99%
>99%
79%
83%
80%
89%
94%
media.[a]
Iodide
Product
GC Conversion
97%
dition of a palladium species into the carbon-halide
bond. Therefore the substitution should have a pro-
found impact on the conversions.
The conversions for both the Suzuki and the Heck
reactionsare analyzed in Figure 4. A marked depend-
ence of the rate on the substituent can be observed
for the Heck-type coupling at elevated temperatures.
Thus, for the data of Klingelhofer et al. (Pd immobi-
98%
96%
lized on a polystyrene-b-poly-4-vinylpyridine block
[9]
copolymer ascatalyts)
the Hammett plot shows a
slope of 1=5Æ1 and the data of Kçhler et al. [Pd-
98%
96%
[24]
A
gives 1=3Æ1. Also, in case of
the Suzuki-type cross-coupling, e.g., Pd@Smopex-
111[36] yieldsfor the coupling of activated p-bromo-
acetophenone a conversion of 97% using 2.5 wt% Pd
in 2.5 h, while p-methoxybenzene (deactivated) needs
for the same conversion 5 wt% Pd and 3 h. In this in-
vestigation we find a strong dependency on the para-
substitution only if Pd(OAc)2 is used as a catalyst
G
[a]
1 mL of catalyst solution leads to a loading of 0.029 mol
% Pd; The catalytic reactionswere carried out at 70 8C
for 24 h.
(1=3Æ1). In contrast to this, the heterogeneous
system Pd@SPB gives 1=0.18Æ0.03 (see Figure 4).
Such a value isfar below the valuesobtained from
[38]
ionic reactionsin oslution.
Certainly more work
needsto be done to further elucidate the mechanism
of the coupling catalyzed by Pd@SPB.
cles as catalyst of Heck-type reactions was also found
to be very good. In four runsthe productswere re-
moved by ether and new starting materials were
added to the water phase.[18] Again, we found that
Pd@SPB could be used repeatedly without loss of ac-
tivity (Figure 3). After these four cycles the nanopar-
Heck-Type Cross-Coupling
The Heck-type reaction (see Scheme 2 and Table 4) ticleswere filtered off and invetsigated by TEM in
using palladium nanoparticles as catalyst was investi- order to detect possible changes of the number and
gated using 8 different aryl halides. With a catalyst morphology of the nanoparticles. We found that the
loading of 0.029 mol% Pd a variety of aryl iodides nanoparticles are still embedded in the SPB support.
show almost complete conversion, aryl bromides do
not react under the conditionsemployed. Under such
mild conditionsin water it isnot poisble to expand
the scope of the Heck reaction to bromides or chlor-
ides.
Conclusions
In conclusion, we present a novel type of composite
As already shown above for the Suzuki-type cross- particles that can be used as a filterable and robust
coupling, the reproducibility of palladium nanoparti- catalyst for cross-coupling reactions. The conditions
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After that the catalyst was added: 10 mL of Pd catalyst solu-
tion (solid content 0.125%, 2.4% Pd@SPB) and NBu4Br
(3220 mg) was used as phase-transfer reagent. The mixture
wastsirred for 24 h. The organic productswere extracted
with 60 mL of ether. The conversion was checked by GC
with dodecane (850 mg) asinternal tsandard. After the ex-
traction of the productsthe water phase containing the cata-
lyst and base was reused for catalysis for three times (alto-
gether four runs). After four cycles the particles were fil-
tered off via ultrafiltration and investigated by cryo-TEM.
Since iodides tend to be light-sensitive (UV light induced
radical reactions), all catalytic reactions involving iodides
were carried out with the exclusion of light.
for both the Suzuki aswell asfor the Heck coupling
are mild and environmentally benign. Neither mois-
ture nor air hasto be avoided. The catalyts works
under mild conditionsand can be recycled with no
significant loss of activity. The carrier particles seem
to ensure colloidal stability. We were not able to re-
produce ultrahigh TONs(turnover number) for the
Heck reaction of bromides using drastic conditions
(elevated temperatures, organic solvents and Pd
salts), but we found almost no dependency of the re-
action rate on the para-substitution.
Experimental Section
Suzuki-Type Cross-Coupling
Aryl halides(ACROS, except bromobenzene [Alfa Aesar]),
phenylboronic acid (ACROS), styrene (ACROS), NBu4Br
tetrabutylammonium bromide (Fluka), potassium tert-butox-
ide (ACROS) and potassium carbonate (Merck) were used
asreceived. Styrene (BASF) wasdetsabilized on an Al 2O3
column and stored in the refrigerator. Sodium tetrachloro-
palladate (Na2PdCl4; Aldrich) and reducing agent sodium
borohydride (NaBH4; Aldrich) were used as commercially
available. Solventswere uesd without further purification.
Water was purified using reverse osmosis (MilliRO; Milli-
pore) and ion exchange (MilliQ; Millipore).
All Suzuki-type catalytic experimentswere carried out in
pressure tubes sealable with a Teflon stopper at 508C for
24 h. At first, distilled water (2 mL) and THF (2 mL) were
filled in the tubes. To this solution the following substrates
were added: phenylboronic acid (122 mg), an aryl halide, for
example, bromobenzene (157 mg), and KO-t-Bu (168 mg).
Third, the catalyst and the phase-transfer reagent were
added: 1 mL of Pd@SPB composite (solid content 0.66%,
1.5% Pd@SPB) and NBu4Br (322 mg). The tube wassealed
and the mixture stirred for 24 h. The organic products were
extracted with 6 mL of ether. The conversion was checked
by GC with dodecane (85 mg) asinternal standard. Since io-
dides tend to be light-sensitive (UV light induced radical re-
actions), any catalytic reactions involving iodides were car-
ried out with the exclusion of light.
The synthesis of the SPB and the Pd@SPB system has
been presented recently.[28–30] Table 1 summarizes the charac-
terization of the carrier particles. Immobilization of palladi-
[28,30]
um nanoparticleswasdecrsibed recently.
At first,
Na2PdCl4 was added to the spherical polyelectrolyte brush,
2À
Experimentsfor the Hammett-plot using Pd
alyst were carried out in the same way as using Pd@SPB,
only that 0.1 mL of a 0.9M Pd(OAc)2 solution in THF was
(OAc)2 ascat-
then NaBH4 wasused to reduce PdCl
anionswhich were
4
confined in the polyelectrolyte brush layer. Nearly monodis-
perse palladium nanoparticles were generated on the surface
of the brush particles, and color of the mixture became dark
gray. The SPB-palladium composite particles after synthesis
were cleaned by ultrafiltration for further catalytic applica-
tion. According to TEM measurements palladium nanopar-
ticleshave a size of 2.6 Æ0.5 nm.
AHCTREUNG
used. By adding the catalyst palladium black precipitated
immediately. Results are summarized in Table 3; no homo-
coupling wasobserved.
Reproducibility experimentswere carried out uisng 100-
mL flasks equipped with a reflux condenser at 508C and
24 h. First, distilled water (20 mL) and THF (20 mL) were
filled into the flasks. To this solution the following substrates
were added: phenylboronic acid (1220 mg), bromobenzene
(1570 mg) and KO-t-Bu (1680 mg). Third, the catalyst was
added: 10 mL of Pd catalyst solution (solid content 0.66%,
1.5% Pd@SPB). Last, NBu4Br (322 mg) wasadded and used
as phase-transfer reagent. The mixture was stirred for 24 h.
The organic productswere extracted with 60 mL of ether.
The conversion was checked by GC with dodecane (850 mg)
asinternal standard. After the extraction of the productsthe
water phase containing the catalyst and base was reused for
catalysis for three times (altogether four runs), after four ac-
complished cycles the particles were filtered off via ultrafil-
tration and investigated by cryo-TEM.
The analysis of the Pd content in the product of a Suzuki
coupling was done by inductively coupled plasma mass spec-
trometer (ICP-MS; Fa. Agilent Technologies, HP 7500; sen-
sitivity range 0.1–10 ppm, error of margin 0.2 ppm). 1 g of
the sample was digested at first with 10 mL of 1.84 gLÀ1
H2SO4 at 3608C and then with 8 mL of a mixture of
1.41 gLÀ1 HNO3/1.84 gLÀ1 H2SO4/1.67 gLÀ1 HClO4 =2:1:1
vol% at 1608C.
Heck-Type Cross-Coupling
All Heck-type catalytic experimentswere carried out in
pressure tubes sealed by a Teflon stopper at 708C for 24 h.
First, distilled water (2 mL) was filled in the tube. Second,
substrates were added: styrene (104 mg) and an aryl halide,
for example, iodobenzene (204 mg). Third, K2CO3 (829 mg)
wasadded. Fourth, the catalyst wasadded: 1 mL of Pd cata-
lyst solution (solid content 0.125%, 2.4% Pd@SPB), and fi-
nally NBu4Br (322 mg) was added and used as phase-trans-
fer reagent. The tube was sealed and the mixture stirred for
24 h. The organic productswere extracted with 6 mL of
ether. The conversion was checked by GC with dodecane
(85 mg) asinternal tsandard. Gaschromatography wascar-
ried out on an Agilent 6890N with FID equipped with Agi-
lent 19091 J-413, HP-5 column.
Reproducibility experimentswere carried out uisng 200-
mL flasks equipped with a reflux condenser at 708C for
24 h. At first, distilled water (100 mL) was filled into the
flask. To this mixture the substrates were added: styrene
(1040 mg), iodobenzene (2040 mg), and K2CO3 (8290 mg).
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Acknowledgements
Financial support by the Deutsche Forschungsgemeinschaft,
Sonderforschungsbereich 481, Bayreuth is gratefully ac-
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