On the peculiar recycling properties of charcoal-supported palladium oxide nanoparticles in Sonogashira reactions

Article abstract of DOI:10.1016/j.apcata.2014.05.019

The enhanced recycling properties of 5% PdO NPs/C compared to 5% Pd NPs/C catalyst for the Sonogashira reaction has been unveiled. A simple preparation procedure for 5% Pd NPs/C and 5% PdO NPs/C catalysts is described along with their extensive characteri

Full text of DOI:10.1016/j.apcata.2014.05.019

Accepted Manuscript
Title: On the Peculiar Recycling Properties of
Charcoal-Supported Palladium Oxide Nanoparticles in
Sonogashira Reactions
´ ˆ
´
Author: Cybille Rossy Jerome Majimel Mona Treguer
Delapierre Eric Fouquet Franc¸ois-Xavier Felpin
PII:
S0926-860X(14)00353-6
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2014.05.019
APCATA 14839
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
1-3-2014
12-5-2014
19-5-2014
Please cite this article as: C. RossyJ. MajimelM.T. DelapierreE. Fouquet Felpin
On the Peculiar Recycling Properties of Charcoal-Supported Palladium Oxide
Nanoparticles in Sonogashira Reactions, Applied Catalysis A, General (2014),
http://dx.doi.org/10.1016/j.apcata.2014.05.019
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On the Peculiar Recycling Properties of Charcoal-Supported
Palladium Oxide Nanoparticles in Sonogashira Reactions
Cybille Rossy,a,b Jérôme Majimel,c Mona Tréguer Delapierre,c Eric Fouquet,a François-Xavier
Felpin*,b,d
a Université de Bordeaux, UMR CNRS 5255, ISM, 351 cours de la Libération, 33405 Talence
Cedex, France.
b Université de Nantes, UFR Sciences et Techniques, UMR CNRS 6230, CEISAM, 2 rue de la
Houssinière, BP 92208, 44322 Nantes Cedex 3, France.
c Université de Bordeaux, CNRS UPR 9048, 87 Avenue du Dr. Schweitzer, 33608 Pessac
Cedex, France.
d Institut Universitaire de France, 103 blvd St Michel, 75005 Paris Cedex 5, France
Email: fx.felpin@univ-nantes.fr
* Corresponding author
Keywords: Sonogashira reaction / Recycling / Palladium oxide / Charcoal / Palladium
nanoparticles
Abstract
The enhanced recycling properties of 5% PdO NPs/C compared to 5% Pd NPs/C catalyst for
the Sonogashira reaction has been unveiled. A simple preparation procedure for 5% Pd NPs/C
and 5% PdO NPs/C catalysts is described along with their extensive characterizations. The
correlation between the catalyst deactivation and a high content of reduced palladium species
for the alkynylation of aryl iodides has been clearly identified with the monitoring of the
Pd(0)/Pd(II) ratio over the recycling runs through X-ray photoelectron spectroscopy .
1. Introduction
Activated carbon-supported palladium catalysts have found great interest in organic synthesis.
Though Pd/C-catalyzed reductive processes are widely known for decades, the use of Pd/C as
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homogeneous molecular palladium catalyst surrogates for carbon-carbon bond forming
reactions still lacks of recognition.[1-5] Yet, the vast majority of palladium-catalyzed
reactions for C-C bond formation including carbonylation,[6] C-H activation,[7, 8] Suzuki,[9-
11] Heck,[12-14] Sonogashira,[15-17] Stille,[18] Negishi,[19] Larock,[20, 21] Ullmann,[22,
23] Hiyama,[24] Tsuji-Trost[25] and Fukuyama[26] reactions, now considered as classics in
organic synthesis, can be carried out with Pd/C as catalyst. This heterogeneous catalyst has
many advantages over traditional homogeneous catalysts, especially at an industrial scale.[27,
28] Indeed, Pd/C is a relatively inexpensive source of palladium, stable to air and moisture
that allows reactions in aqueous media and under ligand-free conditions. Moreover its
heterogeneous nature in most aqueous and organic solvents allows an easy recovering through
a simple filtration, leaving the solvents and products with low levels of metal contamination.
This feature is of special relevance for the synthesis of biologically active compounds or
ultra-pure materials where a close monitoring of metal contamination is required.
Unfortunately, despite these salient advantages Pd/C still suffers from a poor recyclability
especially for demanding processes such as Heck and Sonogashira reactions.[16, 29-31] This
poor recyclability has been essentially attributed to aggregation phenomena induced by a
leaching/re-precipitation mechanism. Indeed, it is now commonly admitted that most Pd/C-
catalyzed reactions, entailing a C-C bond formation, follow a quasi-homogeneous mechanism
by which active palladium species are leached from the support into the solution and re-
precipitated onto charcoal at the end of the catalytic cycle.[32-39] This boomerang effect
profoundly modified the size of palladium nanoparticles which inexorably aggregate into
large colloids due to the thermodynamic instability of Pd(0) species re-precipitating onto
charcoal. In order to avoid aggregation phenomena, we recently reported an unprecedented
strategy where a co-metal, inactive for a considered reaction, stabilizes Pd species. The proof-
of-concept was described for Pd(0) alloyed with gold atoms and demonstrated its usefulness
for the Pd-Au/C-catalyzed arylation of alkynes.[40] This approach is still currently under
investigation in our laboratory in order to find more recyclable palladium-containing alloys.
Apart from the co-metal concept, we worked on alternative strategies that could be
synthetically more practical for chemists not well versed in the field of heterogeneous
catalysis. We reasoned that in the absence of any stabilizing agents, including ligand,
ammonium and co-metal, the high content of Pd(0) species inexorably enhance the
aggregation phenomenon driven by the lower surface energy of large colloids. We felt that the
unwanted aggregation phenomenon could be dramatically slowed down with a catalyst
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displaying a high content of stable palladium-oxide nanoparticles (PdO NPs) which could act
as a reservoir for active Pd(0) species.
2. Materials and methods
2.1. Preparation of 5% Pd NPs/C
Pd(OAc)2 (110 mg, 0.5 mmol) and charcoal (1 g, 95 wt.-%/Pd) were dispersed in MeOH (100
mL). Hydrogen gas was bubbled through the solution for 5 minutes and the resulting mixture
was stirred for 12 h at 25 °C under H2 (1 atm, balloon). The catalyst was filtered under
Millipore membrane (filters nylon 0.45 µm, 47 mm), washed with MeOH and dried under
vacuum. ICP analyses were performed on the filtrate to verify the final Pd metal loading on
carbon to be 5 wt.-%.
2.2 Preparation of 5% PdO NPs/C
The 5% Pd NPs/C catalyst previously obtained was calcined in air for 2 days at 200 °C.
2.3 General procedure for the Sonogashira reactions
In a sealed tube, aryl iodide (1 mmol, 1 eq), K3PO4 (2 mmol, 2 eq), catalyst (2 mol% Pd) were
suspended in i-PrOH (3 mL) and H2O (3 mL). The acetylene derivative (1.2 mmol, 1.2 eq)
was added and the resulting mixture was stirred at 80 °C for 20 hours. After cooling to room
temperature, EtOAc (20 mL) and H2O (20 mL) were added and the mixture was filtered over
a pad of Celite®. The aqueous layer was extracted twice with EtOAc (2 x 20 mL). The
collected organics extracts were washed by brine (60 mL), dried on MgSO4, filtered and
concentrated under reduced pressure. The crude product was purified by flash
chromatography.
2.4. Recycling experiments
After completion of the reaction, the catalyst was separated and recovered by filtration on a
Millipore membrane (nylon filters, 0.45 µm), washed with H2O (10 mL) and EtOAc (2 x 10
mL) and dried under vacuum for 8 hours.
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3. Results and discussion
With this idea in mind, we prepared charcoal-supported PdO NPs by oxidation of Pd NPs
under very simple conditions. Pd NPs were prepared by reduction of Pd(OAc)2 in MeOH
under a H2 atmosphere at room temperature in the presence of activated charcoal acting as
support and stabilizer for Pd NPs.[41, 42] In the absence of charcoal, only inactive black
palladium, precipitating in the solution, is produced. A 5 wt.% loading of palladium onto the
support was measured by ICP-MS analysis. X-Ray Diffraction (XRD) and Transmission
electron microscopy (TEM) analyses showed the formation of Pd NPs mainly distributed onto
charcoal with a crystallite size in the range of 5-10 nm (Figure 1a-b).
Figure 1. TEM and HRTEM micrographs of 5% Pd NPs/C (a, b) and 5% PdO NPs/C (c, d).
Measured reticular spacings of 0.22 nm and 0.25 nm correspond to (111)Pd and (111)PdO
planes respectively.
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X-ray photoelectron spectroscopy (XPS) experiments revealed Pd(0) as major species (70%)
along with Pd(II) (30%): the Pd 3d3/2 and Pd 3d5/2 bands observed at respectively 340.9 and
335.6 eV accounted for Pd(0) species while the Pd 3d3/2 and Pd 3d5/2 binding energies of
Pd(II) species were recorded at 342.5 and 337.3 eV (Figure 2b). The absence of diffraction
peak on XRD patterns (Figure 2a) and reticular spacing on HRTEM micrographs (Figure 1b)
accounting for PdO, suggests the formation of a thin oxide layer covering Pd(0) particles due
to the oxidation of metallic Pd upon exposure to air (Figure 2b).
In order to obtain the PdO NPs/C material enhancing the thickness of the native PdO layer,
we carried out a further oxidative thermal treatment under air atmosphere at 200 °C. TEM
analyses after this treatment evidenced a coalescence phenomenon starting at only 200 °C, far
below the Tammann temperature, (Figure 1c). High resolution TEM (HRTEM) analysis of an
isolated nanoparticle (Figure 1d) revealed lattice fringes with a 0.25 nm spacing
corresponding to PdO (111) reticular planes on its tetragonal structure (P42/mmc space group,
JCPDS file number 00-043-1024).
Figure 2. XRD patterns and XPS spectra of the Pd 3d region of the 5% Pd NPs/C (a, b) and
5% PdO NPs/C samples (c, d).
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XPS analyses showed that the PdO NPs/C catalyst thus obtained, mainly consisted in Pd(II)
species (>80%) along with a low extent of reduced Pd(0) species (<20%), (Figure 2d). The
residual amount of Pd(0) could be explained by a core-shell arrangement of some particles
most likely with a thicker oxide shell but still with a metallic core. X-ray diffraction pattern
of the PdO NPs/C catalyst (Figure 2c) confirmed the formation of crystalline palladium oxide
in tetragonal phase with the apparition of the diffraction peak at 2θ equal to 34.5° (101), 41.9°
(110), 54.8° (112), 61.2° (103) and 71.3° (211). Residual signals for metallic palladium in
face-centered cubic (fcc) crystalline structure were also identified with peaks at 40.0° (111),
46.5° (200) and 67.9° (220). One can note on Figure 1d that lattice planes go through the
whole particle projected surface with no contrast perturbation around the core region. That
probably means that these particles do not exhibit a core-shell nanostructure and is a pure PdO
arrangement. We next evaluated both the catalytic activity and the recyclability of 5% PdO
NPs/C on a model reaction and compared the results with those obtained with 5% Pd/C
catalyst (Table 1). Importantly, we stress that lower and higher Pd loadings onto the support,
i.e. 1 wt.% and 10 wt.% respectively, gave lower catalytic activity[40], thereby, our further
studies were carried out with 5 wt.% supported Pd catalysts.
Table 1. Recycling behaviour of 5% PdO/C, 5% Pd/C.
Catalyst (2 mol%)
MeOC
I
+
MeOC
K3PO4, i-PrOH/H2O
80 °C, 12 h
2
1
3
100
90
80
70
60
50
40
30
20
10
0
5% Pd NPs/C
5 %PdO NPs/C
1
2
3
4
5
Run
The coupling of 4-iodoacetophenone 1 with phenyl acetylene 2 was conducted in aqueous
iPrOH with K3PO4 as a base at 80 °C for 12 hours.[16] Fresh 5% PdO/C catalyst displayed a
6
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slightly diminished yield compared to the unoxidized 5% Pd/C catalyst (run 1). The first
recycle revealed a drop of the reaction yields for the two catalysts, albeit in a higher extent for
the 5% Pd/C sample. A more obvious divergent behaviour appeared upon the following
reuses. Indeed, the catalytic activity of 5% Pd/C significantly decreased after the third run
reaching a stable behaviour at ca. 50% yield. By contrast, upon the third, fourth and fifth
reuses, the 5% PdO/C catalyst displayed a constant and significantly higher catalytic activity
with yields majored by ca 20% compared to 5% Pd/C. The rapid decrease of the catalytic
activity of the 5% Pd/C catalyst upon ruses cannot be ascribed to Pd leaching since ICP-MS
measurements of residual soluble Pd species into the solution after a single filtration gave 0.2,
0.4, 0.2 and 0.2 ppm after respectively the 1st, 2nd, 3rd and 4th runs. These values of leaching
correspond to less than 3% of the total amount of Pd introduced. Interestingly, the catalytic
activity of the reused 5% Pd/C catalyst could be restored upon a hydrogenation treatment in
MeOH prior each reuses.
In order to explain the behaviours of both catalysts, the evolution of the palladium oxidation
state over the recycles was monitored by XPS analyses of the palladium 3d region (Table 2).
As already stated above, the fresh 5% Pd NPs/C catalyst contains roughly 70% Pd(0) and
30% Pd(II). After the first recycling and over the followings, a high content of Pd(0) (ca. 90-
95%) was observed suggesting that most Pd(II) species have been reduced into Pd(0) after the
first run. With 5% PdO NPs/C catalyst, an increase of the Pd(0)/Pd(II) ratio was also observed
upon recycling experiments but in a much lower extent. Indeed, after the fifth cycle, the PdO
NPs/C catalyst still contained more than 30% of palladium species under their oxidized form.
Table 2. XPS determination of palladium oxidation states on Pd 3d region.
Entry Run
5% PdO/C
5% Pd/C
Pd(0) Pd(II) Pd(0) (Pd(II)
1
2
3
4
5
1
2
3
4
5
20
51
80
49
66
97
34
3
ND
62
ND
38
93
7
ND
90
ND
10
66
34
ND : Not determined.
A rational correlation between the catalytic activity and the Pd(0)/Pd(II) ratio can be drawn at
this stage. Indeed, these results suggest that stable Pd(II) species act as a reservoir for active
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Pd(0) species. In the absence of any stabilizing agent, a high content of Pd(0) is detrimental to
the recycling properties of the catalyst due to thermodynamically instability of small Pd NPs,
having a strong tendency to form less active aggregates of lower surface energy.6 A tick oxide
layer protects unstable but very active Pd NPs from unwanted aggregation phenomena,
thereby, allowing the conservation of the NPs shape upon successive reuses. This claim is
further supported by the TEM picture of Pd NPs/C (Figure 3a) and PdO/C (Figure 3b)
catalysts after respectively 1 cycle and 2 cycles. Indeed, after only one run, the reused Pd
NPs/C catalyst shows the formation of large nanoparticles (~50-100 nm) while this behaviour
is not observed for the PdO/C catalyst even after 2 reuses. A closest analysis of the crystallite
size distribution of the PdO/C catalyst after two successive reuses revealed an average
diameter of 3.5 nm, with >90% of NPs being in the range 2−4 nm.
Figure 3. TEM micrographs of 5% Pd NPs/C (a) after a first run and 5% PdO NPs/C (b) after
the second reuse.
These studies correlate the finding reported by Kölher et al. pointing out the superiority of
Pd/C having a low degree of palladium reduction for Heck and Sonogashira reactions.[29]
However, in their experimental conditions, the catalyst was not recyclable. Djakovitch et al.
also pointed out the high activity of PdO/SiO2 for Heck reactions[43] and tandem Suzuki-
Heck sequences.[44]
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We secured that the 5% PdO NPs/C catalyst was active for a variety of substrates. Our results
depicted in Scheme 1 on a selection of substrates showed that 5% PdO NPs/C allows the
coupling of iodoarenes bearing electron-withdrawing (compounds 3-6, 8) or electron-
releasing (compound 7) substituents. Interestingly, both aromatic (compounds 3-7, 10) and
aliphatic alkynes (compounds 8-9) were tolerated and furnished the coupling products with
good to excellent yields. Interestingly, iodoheteroarenes such as pyridine (compound 9) and
thiophene (compound 10) were smoothly coupled to alkynes, providing an entry to
heterocyclic structures of interest for medicinal or material purposes.
R2
I
5% PdO/C (2 mol%)
R1
R2
R1
+
i
K3PO4, -PrOH/H2O
80 °C, 20 h
OMe
3
, 81%
4, 86%
5, 92%
O2N
O
O
OMe
7
, 70%
8, 58%
NO2
MeO
O
6, 71%
OH
N
S
9
10, 95%
, 67%
Scheme 1. Alkynylation of iodoaryls and iodoheteroaryls catalyzed by 5% PdO/C.
4. Conclusion
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In summary, we have devised a novel and simple strategy significantly increasing the
recycling properties of activated charcoal-supported palladium catalysts for the Sonogashira
reaction. Our strategy relies on the use of stable PdO NPs acting as a reservoir of active Pd(0)
species and limiting the catalyst deactivation. The catalyst deactivation was rationalized
through the increase of the Pd(0)/Pd(II) ratio over the recycling runs with a XPS monitoring.
Further studies directed at the control of the Pd(0)/Pd(II) ratio upon successive reuses are
actively pursued in our laboratory.
Acknowledgments
We gratefully acknowledge the “Université de Bordeaux”, the “Université de Nantes”, the
“Centre National de la Recherche Scientifique (CNRS)”, and the “Agence Nationale de la
Recherche” (ANR JCJC 7141) for the financial support to this project. The authors would like
to thank Dr. Chantal Douchet (Université de Montpellier) for ICP-MS analyses and Julie
Hémez for TEM analyses. F.-X. Felpin is member of the “Institut Universitaire de France
(IUF)”.
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Highlights
Highlights
1. Preparation and characterization of Pd NPs/C and PdONPs/C catalysts
2. Discovery of the peculiar recycling properties of the PdO NPs/C catalyst
3. The work addresses the issue of catalyst deactivation for the Sonogashira reaction
4. Good to high yields of products were obtained with the PdO NPS/C catalyst
5. This work expands the synthetic utility of heterogeneous Pd catalysts
Page 12 of 19

Graphical Abstract
Page 13 of 19

Table
Table 1. Recycling behaviour of 5% PdO/C, 5% Pd/C.
100
90
80
70
60
50
40
30
20
10
0
5% Pd NPs/C
5 %PdO NPs/C
1
2
3
4
5
Run
Page 14 of 19

Table
Table 2. XPS determination of palladium oxidation states on Pd 3d region.
Entry Run 5% PdO/C 5% Pd/C
Pd(0) Pd(II) Pd(0) (Pd(II)
1
2
3
4
5
1
2
3
4
5
20
51
80
49
66
97
34
3
ND
62
ND
38
93
7
ND
90
ND
10
66
34
ND : Not determined.
Page 15 of 19

Figure
Page 16 of 19

Figure
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Figure
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Figure
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