Biodeposited Pd/Au bimetallic nanoparticles as novel Suzuki catalysts

Article abstract of DOI:10.1016/j.tetlet.2012.01.030

The use of two nanoparticulate palladium based catalysts in the Suzuki reaction is described. One monometallic (Pd) and one bimetallic (Pd/Au) catalyst were prepared by the environmentally benign method of bioreductive precipitation by Shewanella oneidensis. Both catalysts successfully mediated the Suzuki coupling, however, the Au doped catalyst was shown to deliver more reproducible results with a broader reaction scope.

Full text of DOI:10.1016/j.tetlet.2012.01.030

Tetrahedron Letters 53 (2012) 1410–1412
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Biodeposited Pd/Au bimetallic nanoparticles as novel Suzuki catalysts
Thomas S.A. Heugebaert a,1, Simon De Corte b,1, Tom Sabbe a,b, Tom Hennebel b, Willy Verstraete b,
Nico Boon b, Christian V. Stevens a,
⇑
a Research Group SynBioC, Department of Sustainable Organic Chemistry and Technology, Faculty of Bioscience Engineering, Ghent University, Coupure links 653,
B-9000 Gent, Belgium
b Laboratory of Microbial Ecology and Technology (LabMET), Department of Biochemical and Microbial Technology, Faculty of Bioscience Engineering, Ghent University,
Coupure Links 653, B-9000 Gent, Belgium
a r t i c l e i n f o
a b s t r a c t
Article history:
The use of two nanoparticulate palladium based catalysts in the Suzuki reaction is described. One mono-
metallic (Pd) and one bimetallic (Pd/Au) catalyst were prepared by the environmentally benign method
of bioreductive precipitation by Shewanella oneidensis. Both catalysts successfully mediated the Suzuki
coupling, however, the Au doped catalyst was shown to deliver more reproducible results with a broader
reaction scope.
Received 6 December 2011
Revised 4 January 2012
Accepted 6 January 2012
Available online 14 January 2012
Ó 2012 Elsevier Ltd. All rights reserved.
Keywords:
Suzuki coupling
Nanoparticles
Palladium
Gold
Bimetallic
Bioreductive precipitation
Palladium (Pd) is able to catalyze a wide variety of chemical
reactions and therefore it is one of the most widely used metal cat-
alysts.1 It can be applied both in homogeneous and heterogeneous
phases. Heterogeneous catalysts, however, allow a more easy sep-
aration from the reaction medium. Due to a higher available cata-
lytic surface, heterogeneous catalysts are more and more used in
the form of nanoparticles. However, these small-sized products re-
quire more complex production methods which often imply the
use of toxic and expensive chemicals. Therefore, environmentally
benign production methods of Pd nanocatalysts are very desirable.
A possible option is to exploit the metal-reducing capacities of mi-
cro-organisms.2 A widely used organism in this context is the me-
tal-respiring bacterium Shewanella oneidensis. It has been shown to
be able to precipitate Pd3 and Au4 in nanoparticulate form on its
cell wall structures. The catalytic properties of this biologically pre-
cipitated Pd by S. oneidensis (‘bio-Pd’) have already been repeatedly
demonstrated.3,5,6 Furthermore, we have recently shown that co-
precipitation of Pd(0) with Au(0) by S. oneidensis (bio-Pd/Au) cre-
ated unique bimetallic structures which display a considerable
contraction of the crystal lattice when compared to the monome-
tallic catalysts. These bimetallic nanoparticles showed a greatly
improved catalytic activity in dehalogenation reactions compared
to the monometallic bio-Pd catalyst.7
Dehalogenations of environmental contaminants catalyzed by
biologically synthesized Pd catalysts are the best studied reactions
for this type of biogenic catalysts.8,9 However, more recently, the
application of these biogenic metal nanocatalysts in synthetic or-
ganic chemistry, for example in carbon–carbon coupling reactions,
has become the subject of several studies.10,11 The aim of this study
was to test the applicability of bio-Pd produced by S. oneidensis in
the Suzuki coupling reaction and more importantly, to see if bio-
Pd/Au showed an advantage in terms of reactivity and/or selectiv-
ity for these coupling reactions compared to monometallic bio-Pd.
In this Letter, two catalyst types were used: bio-Pd, and a Au
doped bio-Pd, obtained by co-precipitation of Pd and Au, denoted
bio-Pd/Au.12 Previous work showed that doping of bio-Pd with
2 wt % Au showed the highest activity for the dechlorination of dic-
lofenac and was therefore chosen as the reference bimetallic cata-
lyst in this study. These catalysts were characterized extensively in
previous studies.3,7
In order to determine the reactivity profile of both catalysts,
iodobenzene and phenylboronic acid were used as model sub-
strates. The reactions were run at 70 °C in a 2/1 mixture of EtOH/
H2O, using three equivalents of K2CO3 as a base and 2.35 molar
percentage of palladium.13 As a reference, Pd/C and a biodeposited
gold catalyst, denoted bio-Au were used. 1.27 molar percentage of
gold was used for reaction run with the bio-Au catalyst.
⇑
Corresponding author. Tel.: +32 (0) 9 264 5957; fax: +32 (0) 9 264 6221.
E-mail address: Chris.Stevens@UGent.be (C.V. Stevens).
As can be seen from Figure 1, both catalysts mediate the Suzuki
coupling to completion within 24 h under the chosen conditions.
1
These authors contributed equally to this work.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2012.01.030

T. S. A. Heugebaert et al. / Tetrahedron Letters 53 (2012) 1410–1412
1411
Figure 1. Comparison of the conversion rate of the model substrates using Pd/C,
bio-Pd, bio-Pd/Au and bio-Au as a catalyst.
Figure 2. Comparison of conversion rates of different phenyl halides (X = I, Br, Cl)
using the bio-Pd and bio-Pd/Au catalyst.
The reaction rate however is increased considerably by gold dop-
ing. This effect is caused by structural modifications of the Pd sur-
face, as pure Au nanoparticles do not catalyze the Suzuki coupling.
Furthermore, as reactions were repeated, it became clear that the
reproducibility of results obtained with the Pd catalyst was rather
poor. Despite intensive efforts to standardize the production pro-
cess of the catalyst, one batch would give good results, while the
next would give no conversion at all. Presumably the biological
variability induces the presence of varying amounts of sulfur at
or near the nanoparticle surface, causing catalyst poisoning. It
has indeed been shown that the presence of Au salts during palla-
dium reduction not only improves the metal conversion yield,14
but also sequesters sulfur containing compounds from the Pd sur-
faces, inhibiting the formation of Pd4S. Gold doping in this way acts
as a detoxification agent.15 In our case, this postulate seems to be
confirmed, as results obtained with the Pd/Au catalyst are highly
reproducible and similar conversion percentages were observed
with every single batch of catalyst produced.
These improvements in catalyst efficiency are reflected in the re-
sults obtained from the screening of different substrates (Table 1).
Reactions using the palladium/gold alloyed catalyst continue to
completion, whereas some substrates are not completely converted
within the 24 h time frame by the pure palladium catalyst.
Regarding the reaction of substituted aryliodides with phenyl-
boronic acid (entries 1–7), it can be clearly seen that the bio-Pd
catalyst struggles early on when electron donating substituents
are present (entries 4 and 5). The Pd/Au catalyst has a broader
scope, however, strong electron donating groups are not tolerated
by either one (entries 2 and 3).
As for the boronic acids, both catalysts tolerate electron donat-
ing and electron withdrawing substituents. Only the very electron
poor fluorophenylboronic acid (entry 11) shows a lower conver-
sion when using the non doped bio-Pd catalyst.
Table 1
When using very electron rich boronates, however, (entries 8
and 14) homo-coupling of the boronate can be observed with the
bio-Pd/Au catalyst, causing a significant drop in the reaction yield.
This type of conversion was not observed with the Pd catalyst, and
as such, bio-Pd may be the better choice in these cases.
Scope study of the Suzuki coupling of different aryliodides and arylboronic acids using
both bio-Pd and bio-Pd/Au as a palladium source
As can be seen from entries 12 to 15, ortho substitution is well
tolerated on both the arylic iodide and the boronic acids, for both
catalysts. Only the ortho iodoaniline shows poor reactivity. This
is due to the strong electron donating capacity, rather than due
to steric or ligating16 effects (compare entries 2 and 12).
When moving on to more challenging arylic halides (Fig. 2), the
usual reactivity profile of I > Br ꢀ Cl is observed, however the con-
version delivered by the Pd catalyst drops dramatically. The gold
doped catalyst on the other hand preserves it activity upon the
use of for example bromobenzene. This substantial difference in
reactivity shows that not only reproducibility is higher, but the cat-
alyst activity is improved by Au doping. Due to the bimetallic
structure the oxidative insertion, the rate determining step, may
be facilitated giving the catalyst a broader scope.
In conclusion, two types of biogenic nanocatalysts containing,
respectively, Pd and Pd/Au, produced by the environmentally be-
nign process of bioreductive precipitation were described, using
Shewanella oneidensis as the carrier bacterium. Their use as cata-
lysts for the Suzuki reaction was evaluated, showing the benefits
of bimetallic catalysts. Both reproducibility and reaction scope
were significantly improved by the introduction of 2 wt % gold
Entry
R1
R2
Conversiona (%)
Bio-Pdb
Bio-Pd/Auc
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
H
H
H
H
H
H
H
H
OMe
Me
CN
F
H
H
98
1
3
0
4
87
100
93
90
98
58
0
100
98
94
100
5
7
88
100
95
100
77d
94d
100
100
6
100
36d
100
NH2
OMe
Ph
Me
COCH3
CN
H
H
H
H
o-NH2
o-F
H
o-NH2
o-F
H
a
b
c
Determined by HPLC.
Average of two successful trials (see discussion on reproducibility).
Average of two trials.
d
Conversion was complete, but product yield is lowered by the formation of
homo-coupling side products.

1412
T. S. A. Heugebaert et al. / Tetrahedron Letters 53 (2012) 1410–1412
7. De Corte, S.; Hennebel, T.; Fitts, J. P.; Sabbe, T.; Bliznuk, V.; Verschuere, S.; van
der Lelie, D.; Verstraete, W.; Boon, N. Environ. Sci. Technol. 2011, 45, 8506–8513.
8. Baxter-Plant, V.; Mikheenko, I. P.; Macaskie, L. E. Biodegradation 2003, 14, 83–
90.
9. Deplanche, K.; Snape, T. J.; Hazrati, S.; Harrad, S.; Macaskie, L. E. Environ.
Technol. 2009, 30, 681–692.
10. Sobjerg, L. S.; Gauthier, D.; Lindhardt, A. T.; Bunge, M.; Finster, K.; Meyer, R. L.;
Skrydstrup, T. Green Chem. 2009, 11, 2041–2046.
11. Gauthier, D.; Sobjerg, L. S.; Jensen, K. M.; Lindhardt, A. T.; Bunge, M.; Finster, K.;
Meyer, R. L.; Skrydstrup, T. ChemSusChem 2010, 3, 1036–1039.
within the palladium nanoparticle. Biosupported catalysts can
have several advantages over chemically produced catalysts. They
can for example be prepared more sustainably. The bacterial cells
serve both as reducing and as stabilizing agents for the nanoparti-
cles, implicating a lower need of chemicals during the production
process of the nanoparticles. Moreover, the bacterial cell is a sup-
port material with an extremely high specific surface area, which
is relatively simple to produce. The bacterial carrier can also have
high affinity for the reacting substances and thus allowing a better
contact between the reagents and the catalyst.2 Nevertheless, the
long term stability of biosupported nanocatalysts needs to be fur-
ther investigated. Especially the leaching of nanoparticles to the
reaction medium and to the environment needs to be prevented
since the adverse effects of nanoparticles are still of great uncer-
tainty.17 Furthermore, it needs to be mentioned that the highest
reaction rates using Pd/Au catalysts have been obtained using
bimetallic core–shell structures, consisting of an Au core with a
Pd shell.18 It has not yet been possible to design a similar core–
shell structure on a bacterial cell wall.7,19 However, we strongly be-
lieve that the production process of biosupported bimetallic Pd/Au
catalysts can be further optimized, which will make these catalysts
more competitive with chemically produced catalysts.
12. Preparation of the catalysts was performed as follows. Shewanella oneidensis
MR-1 was grown overnight in Luria–Bertani medium (10 g LÀ1 peptone, 5 g LÀ1
yeast extract, 5 g LÀ1 NaCl). Cells were harvested by centrifugation at 4100g.
Bio-Pd was produced according to De Windt et al.3 Briefly, cells were washed
and resuspended in M9 medium.20 Formate was added to final concentration
of 50 mM. Finally, Na2PdCl4 (Sigma–Aldrich, Germany) was added to a final
concentration of 50 mg Pd LÀ1. Bio-Pd/Au (50/1 weight ratio) was produced
according to a similar procedure, according to De Corte et al.7 Briefly, cells were
washed and resuspended in aqua dest. Na2PdCl4 and HAuCl4 (Sigma–Aldrich,
Germany) were added to final concentrations of 50 mg Pd LÀ1 and 1 mg Au LÀ1
.
H2 was supplied as electron donor in the headspace of the flasks. Incubation
was continued for 48 h in order to allow complete reduction of the metals. Bio-
Au was produced similarly. Briefly, cells were washed and resuspended in
distilled water. HAuCl4 (Sigma–Aldrich, Germany) was added to
a final
concentration of 50 mg Au LÀ1
.
H2 was supplied as electron donor in the
headspace of the flasks. Incubation was continued for 48 h in order to allow
complete reduction of Au. Precipitation efficiencies were determined using a
AA-6300 atomic absorption spectroscope (Shimadzu, Japan). Detection limits
of Au and Pd were 0,1 mg LÀ1. The Pd precipitation efficiency in the case of bio-
Pd was 94.8 0.2%. In the case of bio-Pd/Au, Au was precipitated for >90% and
Pd for >99.8%.
Acknowledgments
13. Suzuki cross-coupling reactions were performed as follows. The prepared
catalyst solution (50 ml) containing 0.0235 mmol palladium (bio-Pd),
T. Heugebaert and S. De Corte are financially supported by the
Fund of Scientific Research Flanders (FWO-Vlaanderen). Tom
Hennebel is supported by Ghent University Multidisciplinary Re-
search Partnership (MRP) – Biotechnology for a sustainable econ-
omy (01 MRA 510 W).
0.0235 mmol Pd and 0.25 lmol Au (bio-Pd/Au), or 0.0127 mmol Au (bio-Au)
was centrifuged at 4100 g and the cells were resuspended in approximately
25 ml of a 2/1 mixture EtOH/water. The flask was equipped with an air
condenser and placed under inert atmosphere (N2). 1.1 mmol of boronic acid,
3 mmol of K2CO3 and 1 mmol of aryl halide were added and the resulting
solution was heated to 70 °C and stirred for a period of 24 h. Reaction mixtures
were analysed by means of HPLC, conversions were determined by means of
integration of UV signals at 235 nm. UV integrations were corrected for the
respective response factors, determined from the commercially available
compounds (Sigma–Aldrich, Germany and Acros, Belgium). All standard curves
contained four concentration measurements ranging from 0 to 100 mg/l and
displayed a correlation coefficient above 0.99. Product identities were shown
by their retention time on HPLC and by their ESI-MS ionisation pattern after GC
analysis.
References and notes
1. For recent reviews on palladium catalysis see: (a) Lamblin, M.; Nassar-Hardy,
L.; Hierso, J.-C.; Fouquet, E.; Felpin, F.-X. Adv. Synth. Catal. 2010, 325, 33–79; (b)
Narayanan, R. Molecules 2010, 15, 2124–2138; (c) Molnar, A. Chem. Rev. 2011,
111, 2251–2320; (d) Selander, N.; Szabo, K. J. Chem. Rev. 2011, 111, 2048–2076;
(e) Xiu, L.; Lin, Z. Chem. Soc. Rev. 2010, 35, 1692–1705.
14. Zheng, Z.; Li, H.; Liu, T.; Cao, R. J. Catal. 2010, 270, 268–274.
15. Menegazzo, F.; Canton, P.; Pinna, F.; Pernicone, N. Catal. Commun. 2008, 9,
2353–2356.
2. Hennebel, T.; De Gusseme, B.; Boon, N.; Verstraete, W. Trends Biotechnol. 2009,
27, 90–98.
16. Patil, S.; Weng, C.; Huang, P.; Hong, F. Tetrahedron 2009, 65, 2889–2897.
17. De Corte, S.; Hennebel, T.; De Gusseme, B.; Verstraete, W.; Boon, N. Microb.
Biotechnol. 2012, 5, 5–17.
18. Nutt, M. O.; Heck, K. N.; Alvarez, P.; Wong, M. S. Appl. Catal. B Environ. 2006, 69,
115–125.
19. Hosseinkhani, B.; Sobjerg, L. S.; Rotaru, A. E.; Emtiazi, G.; Skrydstrup, T.; Meyer,
R. L. Biotechnol. Bioeng. 2012, 109, 45–52.
20. Sambrook, J.; Fritsch, E. F.; Maniatis, T. Molecular Cloning: A Laboratory Manual;
Cold Spring Harbor Laboratory: New York, 1989.
3. De Windt, W.; Aelterman, P.; Verstraete, W. Environ. Microbiol. 2005, 7, 314–
325.
4. De Corte, S.; Hennebel, T.; Verschuere, S.; Cuvelier, C.; Verstraete, W.; Boon, N. J.
Chem. Technol. Biotechnol. 2011, 86, 547–553.
5. Hennebel, T.; Simoen, H.; De Windt, W.; Verloo, M.; Boon, N.; Verstraete, W.
Biotechnol. Bioeng. 2009, 102, 995–1002.
6. Hennebel, T.; De Corte, S.; Vanhaecke, L.; Vanherck, K.; Forrez, I.; De Gusseme,
B.; Verhagen, P.; Verbeken, K.; Van der Bruggen, B.; Vankelecom, I.; Boon, N.;
Verstraete, W. Water Res. 2010, 44, 1498–1506.