Electrodeposited palladium on MWCNTs as 'semi-soluble heterogeneous' catalyst for cross-coupling reactions

Article abstract of DOI:10.1016/j.tetlet.2015.05.019

Abstract Platinum group metals (PGM) catalyze many important chemical processes including hydrogenation; electro-catalysis applied to fuel cell and battery technologies and one of the most prominent the C_sp2-C_sp2 cross-coupling reactions such as Sonogashira, Suzuki-Miyaura, and C_sp2-N_sp3 cross-coupling as for the Buchwald-Hartwig type reactions. The main concern of the catalytic reaction is the recovery and recyclability of used catalytic entities. Recent efforts in the catalyst fabrication focus on combining a nano-sized metal with various high surface area supports like resins or carbon-based materials that allow recovering the catalyst after each reaction in a simple and convenient way. This work aims to investigate the catalytic activity of the carbon-supported (MWCNTs) electrodeposited metallic nanoparticles (Pd) and their applications in several cross-coupling reactions. We propose a simple and affordable synthesis of the high surface area Pd-C composite that can be re-used until deactivated. The intramolecular interactions between carbon and metallic fractions studied by X-ray photoelectron spectroscopy are discussed with respect to the catalytic efficiencies.

Full text of DOI:10.1016/j.tetlet.2015.05.019

Tetrahedron Letters xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Electrodeposited palladium on MWCNTs as ‘semi-soluble
heterogeneous’ catalyst for cross-coupling reactions
Mariusz Radtke , Steffi Stumpf , Bernd Schröter , Stephanie Höppener b,d, Ulrich Sigmar Schubert b,d
Anna Ignaszak
a
a
b,d
c
,
a,
⇑
Institute of Organic and Macromolecular Chemistry, Friedrich-Schiller-University, Lessingstrasse 12, 07743 Jena, Germany
Institute of Organic and Macromolecular Chemistry, Friedrich-Schiller-University, Philosophenweg 7, 07743 Jena, Germany
Institute of Solid State Physics, Friedrich-Schiller University, Philosophenweg 7, 07743 Jena, Germany
b
c
d
Jena Center of Soft Mater (JCSM), Friedrich-Schiller University, Philosophenweg 7, 07743 Jena, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Platinum group metals (PGM) catalyze many important chemical processes including hydrogenation;
electro-catalysis applied to fuel cell and battery technologies and one of the most prominent the
Received 9 April 2015
Revised 1 May 2015
Accepted 6 May 2015
Available online xxxx
Csp2–Csp2 cross-coupling reactions such as Sonogashira, Suzuki–Miyaura, and Csp2–Nsp3 cross-coupling
as for the Buchwald–Hartwig type reactions.
The main concern of the catalytic reaction is the recovery and recyclability of used catalytic entities.
Recent efforts in the catalyst fabrication focus on combining a nano-sized metal with various high surface
area supports like resins or carbon-based materials that allow recovering the catalyst after each reaction
in a simple and convenient way. This work aims to investigate the catalytic activity of the carbon-
supported (MWCNTs) electrodeposited metallic nanoparticles (Pd) and their applications in several
cross-coupling reactions. We propose a simple and affordable synthesis of the high surface area Pd–C
composite that can be re-used until deactivated. The intramolecular interactions between carbon and
metallic fractions studied by X-ray photoelectron spectroscopy are discussed with respect to the catalytic
efficiencies.
Keywords:
Electrodeposited nanoparticles
Cross coupling
Suzuki–Miyaura reaction
Sonogashira reaction
Buchwald–Hartwig reaction
Ó 2015 Elsevier Ltd. All rights reserved.
Introduction
production (e.g., to minimize the contamination by Pd and ligands
that are difficult to separate from the synthesis mixture). In addi-
Pd catalysis is very versatile and has broad application in the
synthesis of natural products, agrochemicals, pharmaceuticals,
and polymers that involve the cross coupling chemistry including
tion, the catalyst utilization while maintaining or improving the
catalytic efficiency is an important task. This is usually accom-
plished by generating the nano-sized catalyst particles or by dis-
persing the nanocatalyst on a high surface area support, such as
1
2
the Suzuki–Miyaura, a copper-free Sonogashira reaction, and
sp2–Nsp3 coupling Buchwald–Hartwig reactions.3 There are still
graphene oxide. This combination generates a very large catalytic
7
C
several challenges behind, one is the cost and availability of Pd pre-
cursors and the second is a recovery of the Pd catalyst and the
problem of contamination by a residual Pd and ligands that are dif-
interphase that significantly enhances the process efficacy at lower
overall cost of the catalyst in comparison to the commercial
7
2
Pd-complex type catalysts such as Pd(OAc) .
4
ficult to separate from the product. As the solution, the promising
One of the most promising, affordable, and non-toxic support is
carbon. Regardless of the price, carbon is chemically inert and can
be synthesized with desired porosity and surface functionalization.
For this particular application (organic cross-coupling), carbon
nanotubes are of special interest since their morphology promotes
approach is to immobilize the catalyst onto various supports from
5
which the carbon-based materials are the most suitable. Such
reusable heterogeneous Pd–carbon systems allow recovering the
catalyst in a convenient way after each reaction, as for the
6
8
Pd/C-catalyzed Suzuki–Miyaura couplings. Regarding the environ-
a fast mass transport to the catalytic active sites. The disadvantage
mental aspect, this will contribute to the minimization of chemical
waste release/disposal that is always a serious issue in a large-scale
of carbon is its low dispersibility. In order to overcome this limita-
tion, various surface functionalizations are offered to control the
hydrophilic/hydrophobic balance, depending on the application
requirement. One of the biggest challenge for the heterogeneous
catalyst (also called ‘semi-soluble’ due to the presence of soluble
9
⇑
Corresponding author. Tel./fax: +49 (0)36 41 94 8290.
E-mail address: anna.ignaszak@uni-jena.de (A. Ignaszak).
http://dx.doi.org/10.1016/j.tetlet.2015.05.019
040-4039/Ó 2015 Elsevier Ltd. All rights reserved.
0
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2
M. Radtke et al. / Tetrahedron Letters xxx (2015) xxx–xxx
Pd and insoluble carbon) is to maintain (or exceed) the solubility
that should be comparable to the homogenous analog (Pd) or bet-
ter. In this work we will introduce soluble Pd nanoparticles into
the carbon matrix in order to deliver a long-life and active system
with an expanded catalytic interface. This will be accomplished by
homogenous dispersion of metal nano-objects onto high surface
area carbon resulting in continuous release of Pd (and
re-deposition).
product but necessitate using toxic chemicals or high
1
3b
temperature,
followed by a time-consuming washing process
9
(usually viscous solvents/surfactants are involved). To address
these, the electrodeposition of metal seems to be a good
1
4
alternative. One of the biggest advantages of this approach—
1
5
often classified as a ‘green chemistry method’ —is processing at
ambient conditions while using the aqueous inorganic precursors.
This electro-synthesis is very fast and easy to scale up at low cost.
There are several modes of Pd plating, some involve scanning the
With respect to the synthesis methods of the active metal clus-
ter supported (or not) on carbon, a well-known one is the conven-
tional reduction of the metal cation by a strong reducing agent
1
0
potential in the required range, the galvanostatic mode is carried
1
4
15
out at constant current or at constant potential from a solution
of Pd inorganic salts (e.g., PdCl ). An interesting one is the simulta-
1
0
11
such as borohydride, ascorbic acid or polyol methods in organic
2
1
2
phase, also in the presence of organic surfactants (e.g., PVP,
neous deposition of carbon and metal carried out by applying the
charge or potential to the conductive surface (working electrode)
immersed in the solution composed of the metal salt, carbon,
and electrolyte (e.g., 1 M KCl). The working electrode (glassy car-
bon plate, Ni mesh) acts as the scavenger of the carbon-reinforced
1
3a
CTAB).
All listed fabrication pathways offer a nano-size
1
6,17
metal particles.
The particle size and the metal load on carbon is usually con-
trolled by the concentration of the deposition solution18 or the
electrochemical parameters (time, deposition potential, or
1
5
current).
In this study we aim to generate the Pd–carbon nanotube
semi-soluble catalyst using electrodeposition methods at ambient
conditions. The intramolecular metal–carbon interactions will be
examined by X-ray photoelectron spectroscopy and discussed with
respect to the catalyst efficiencies for selected cross-check
coupling reactions: Sonogashira, Suzuki–Miyaura, and Buchwald–
Hartwig type reactions.
Results and discussion
Figure 1 represents cyclic voltammogram of the Pd electrodepo-
sition from the aqueous solution of PdCl
electrode. The potential scan reveals the Pd metal deposition at
2
onto the glassy carbon
Figure 1. Cyclic voltammogram of Pd electrodeposition on multi walled carbon
nanotubes from PdCl aqueous solution.
2
2
+
2+
À
0
.23 V via an electrochemical reduction of the Pd (Pd + 2e ?
0
Pd ) followed by the electrochemical dissolution (anodic stripping)
0
2+
À 16
of the metal deposit at 0.93 V (Pd ? Pd + 2e ). The deposition
and the stripping peaks of Pd are slightly smaller for the first CV
scan when comparing to the final voltammogram, which is associ-
ated with the nucleation process and the dissolution rate of the
Figure 2. SEM image of the as-prepared Pd@MWCNT catalyst.
Figure 3. XPS Pd 3d narrow scan of Pd@MWCNT catalyst.
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M. Radtke et al. / Tetrahedron Letters xxx (2015) xxx–xxx
3
Table 1
stripping peak is larger that the corresponding reduction counter-
part and is correlated with the formation of palladium com-
Yields of the cross-coupling reactions catalyzed by Pd@MWCNTs
1
6
Yieldsa (after 1 /2nd/3rd
use
st
)
Reaction type
plexes. The peak separation (
DEa–c >60 mV) indicates that the
redox system is irreversible due to the complexity (multistep
1
6
deposition ) and a residual metal crystallization on the electrode
Suzuki–Miyaurab
84/62/31
10
surface.
Figure 2 shows the SEM image at low (A) and high magnifica-
tion (B) as-prepared Pd@MWCNTs catalysts. The metal particles
with uniform spherical shape and size in the range from 30 to
8
7/50/41
Sonogashirab
71/42/12
8/51/10
7
0 nm are evenly distributed on carbon. Figure 2B reveals a very
homogenous metal deposit generated during a three-dimensional
Pd growth, resulting in interconnected structures with a superior
surface area. The Pd-to-carbon weight ratio was 1:263 as esti-
mated electrochemically according to the Faraday law (details of
this analysis are provided in Supporting information).
7
Buchwald–
Hartwigc
9/0/0
Figure 3 represents XPS narrow scan of Pd 3d signal of the
Pd@MWCNTs revealing the most important chemical states of
the metal. The core-level Pd 3d spectrum is composed of three
doublets (3d5/2 signals at lower BE and 3d3/2 at higher BE,
respectively). The binding energies of 337.6 eV and 342.4 eV
0
/0/0
a
b
c
With respect to the phenyl iodide/4-nitro iodobenzene.
Experimental conditions: aerobic atmosphere, reflux, 2.5 h, MeOH/H
Experimental conditions: nitrogen atmosphere, 110 °C, 16 h, MeOH/H
2
O 3:1.
O 3:1.
0
2+
2
represent Pd , at 337.9 and 343.3 eV are assigned to Pd species
e.g., oxidized metal, PdO), a high-BE doublet with peaks at
(
deposited metal. Pd seeds generated in first scan act as nucleation
centers for the following deposition, resulting in slightly higher
peak current at 0.23 V. Also, an interesting observation is that the
338.1 and 343.8 eV represents palladium complexes (can be
generated during the reaction with supporting electrolyte:
2 2 4
PdCl + 2KCl ? K PdCl
) or Pd species at higher oxidation state.19
Figure 4. Proposed mechanism for Suzuki reaction based on Pd@MWCNT catalyst.
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4
M. Radtke et al. / Tetrahedron Letters xxx (2015) xxx–xxx
Pd0 signals are slightly shifted toward higher binding energy,
which is associated with strong electronic interactions between
Buchwald–Hartwig amination was very poor, revealing that at
operating conditions the electron-rich reactive ligands were not
generated. The catalyst recovered after the (homo)cross-coupling
reaction was repeatedly used until deactivated due to the partial
dissolution of Pd or phase separation in Pd@MWCNTs. The stronger
catalyst-support interaction can be the way of improvement of the
heterogeneous catalyst reusability—this can be done by adapting
new carbon allotropes and will be the subject of future work.
The scope of this study will be extended by application of the
heterogeneous catalyst in the synthesis of bioactive natural prod-
ucts with cooperation of FSU IOMC Jena.
2
0
the metal and MWCNTs. Such an effect originates from the elec-
tron rearrangement between the C and Pd atoms, resulting in the
formation of interface states (this is also associated with the deple-
tion of electrons at the metal–carbon and outer metal surface. This
electron shift at the Pd-CNTs contact is even stronger for a highly
functionalized carbon surface, as it is in our case.21 The XPS C 1s
narrow scan (Fig. S1, Supporting information) shows the high level
of MWCNTs functionalization that contributes in the observed BE
shift for Pd lines and is included in Supplementary materials.
Acknowledgments
Suzuki–Miyuara, Sonogashira, Buchwald–Hartwig coupling
We thank Carl-Zeiss Foundation Germany for the financial sup-
port. Cryo-TEM investigations were performed at the cryo-TEM
facilities of the Jena Center for Soft Matter (JCSM). TEM facilities
In order to validate the catalytic activity of Pd@MWCNTs we
carried out several cross-coupling reactions with the Pd/MWCNT
system. Table 1 summarizes efficiencies of the coupling reactions
obtained in this work.
Figure 4 represents an example of the proposed mechanism for
synthetic cycle of Suzuki reaction catalyzed by Pd@MWCNTs. In
this process the following standard steps for homo/cross-coupling
are projected: in the first phase an oxidative addition between aryl
halide and aryl boronic followed by the sub-sequential first trans-
metalation with potassium carbonate leading to transmetalation
with aryl borate. The coupling product is generated in the last step
via reductive elimination. It is important to notice that dehalo-
genated products were not identified.
were funded by
a grant of the DFG (German Research
Foundation) and the EFRE (European Fund for Regional
Development).
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2015.05.
019.
With respect to yields accomplished for the Suzuki–Miyaura,
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