A PdII Carbene Complex with Anthracene Side-Arms for π-Stacking on Reduced Graphene Oxide (rGO): Activity towards Undirected C–H Oxygenation of Arenes

Article abstract of DOI:10.1002/ejic.201800978

An N-heterocyclic carbene palladium(II) complex containing two anthracene side arms was immobilized on the surface of reduced graphene oxide (rGO) by π-stacking. The activity of the homogeneous analogue and the supported complex in undirected C–H acetoxylation reaction of arenes was studied. The results show that the catalytic efficiency in acetoxylation of benzene is improved in the immobilized materials compared to the homogeneous analogue. According to XPS analysis, the immobilized catalyst maintains the original oxidation state of Pd^II after the catalytic reaction.

Full text of DOI:10.1002/ejic.201800978

A Journal of
Accepted Article
Title: A Pd(II) carbene complex with anthracene side arms for π-
stacking on reduced graphene oxide (rGO): activity towards
undirected C–H oxygenation of arenes
Authors: Maitham H. Majeed, Payam Shayesteh, Axel R. Persson, L.
Reine Wallenberg, Joachim Schnadt, and Ola F. Wendt
This manuscript has been accepted after peer review and appears as an
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201800978
Link to VoR: http://dx.doi.org/10.1002/ejic.201800978

10.1002/ejic.201800978
European Journal of Inorganic Chemistry
FULL PAPER
A Pd(II) carbene complex with anthracene side arms for π-stacking on reduced
graphene oxide (rGO): activity towards undirected C–H oxygenation of arenes
Maitham H. Majeed,[a] Payam Shayesteh,[b] Axel R. Persson,[a,c] L. Reine Wallenberg,[a,c] Joachim
Schnadt[b] and Ola F. Wendt*[a]
†
†
aCentre for Analysis and Synthesis, Department of Chemistry, Lund University, Box 124, SE-221 00 Lund, Sweden. bDivision of Synchrotron
Radiation Research, Department of Physics, Lund University, Box 118, SE-221 00 Lund, Sweden. cNational Center for High Resolution
Electron Microscopy and NanoLund, Lund University, Box 124, SE-221 00 Lund, Sweden. †These authors contributed equally to this work.
Abstract: An N-heterocyclic carbene palladium(II) complex
containing two anthracene side arms was immobilized on the
surface of reduced graphene oxide (rGO) by π-stacking. The activity
of the homogeneous analogue and the supported complex in
undirected C–H acetoxylation reaction of arenes was studied. The
results show that the catalytic efficiency in acetoxylation of benzene
is improved in the immobilized materials compared to the
homogeneous analogue. According to XPS analysis, the
immobilized catalyst maintains the original oxidation state of Pd(II)
after the catalytic reaction.
recently demonstrated that immobilization of N-heterocyclic
carbene Pd(II) or Ru(II) complexes onto a graphene surface by
noncovalent interactions results in efficient and highly recyclable
catalysts for many chemical transformations.23-25 We have
previously used a polymerizable Pd(II) NHC complex for the
formation of
a
heterogeneous C–H activation catalyst.20
Encouraged by the results of Peris, we set out to modify our
Pd(II) system to include a π-stacking capability by installing two
anthracene moieties in the backbone of the NHC ligand to give
the new Pd complex, 1 (Scheme 1). This modified complex was
supported on reduced graphene oxide (rGO) and the so
obtained system was applied in the undirected C–H
acetoxylation of arenes. The reactivity and regioselectivity of the
heterogeneous system was evaluated and shows good activity
for the formation of acetoxylated products in many examples.
Introduction
Considerable efforts have been spent on heterogenizing
homogeneous catalysts on an insoluble support as an attempt to
aid catalyst separation and to allow recovery and recycling of the
catalyst several times; this is crucial both from an economic and
sustainability standpoint.1 One group of reactions for which a
significant interest in a switch-over from homogeneous to
heterogeneous catalysts has been shown is the undirected
metal-catalysed oxidation of simple arenes. The methodology in
itself is attractive for obtaining valuable chemicals with important
Results and Discussion
NHC–PdII complex synthesis
The synthesis of 2 was reported previously.26 The carbene
transfer agent 3 was prepared by the silver oxide route in
moderate yield.27,28 The nature of 3 was confirmed by 1H and 13
C
applications in many areas.2 As
a
specific example, the
NMR spectroscopy, which showed a clear downfield shift of the
carbene carbon around 183.1 ppm. Transmetallation between 3
and PdCl2(PhCN)2 followed by chloropyridine addition gave 1 in
good yield (Scheme 1). The 13C NMR spectrum of 1 exhibited a
typical signal at 147.4 ppm, which was assigned to the carbenic
carbon.
palladium-catalysed C–H acetoxylation of organic molecules
without directing groups has been described in many reports,3-6
and the vast majority of these examples employs homogeneous
catalysts, particularly Pd(OAc)2. In many cases the reaction,
however, suffers from Pd black formation and difficult catalyst
separation.6-9 To improve on the situation palladium N-
heterocyclic carbene catalysts have been used to stabilize the
higher oxidation state, and as
a further step also the
heterogenization has been targeted. It turns out that these new
heterogeneous catalyst materials exhibit a remarkable catalytic
activity and selectivity in C–O, C–halogen, and C–C bond
formation under both batch and flow conditions.10-21 There are,
however, still difficulties to tackle: in many cases the additional
functionalization of the ligand that aid the anchoring or
immobilization step leads to high costs of preparation and
sometimes lead to changes in the original reactivity of the
catalyst.22 Furthermore, the supports are often expensive, and
new, efficient and inexpensive methods to heterogenize and
modify homogeneous catalysts are greatly needed by the
catalysis community. In light of this, there has been a recent
development of non-covalent support methodologies via π-
interactions, which minimizes the need for chemical modification
of the homogeneous catalyst or the support. The group of Peris
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10.1002/ejic.201800978
European Journal of Inorganic Chemistry
FULL PAPER
oxidation state. Possibly, there is even a small remaining
component at the original binding energy. This overall shift to
lower binding energy must be associated with a change in the
Pd environment and can probably be explained by the
replacement of the chloropyridine ligand by a noncovalent π-
interaction of the polycyclic aromatic system of rGO. The Cl 2p3/2
XP spectrum of 1 exhibits a low-energy peak (ca. 197.9 eV),
which is assigned to the palladium chlorine atom, and a high-
energy peak (ca. 198.7 eV) assigned to the chloropyridine
chlorine (Supporting Information Figure S1a).31 The ratio
between the palladium and chloropyridine chlorine peaks is 2:1
as expected. The Cl 2p spectrum of 1@rGO shows a significant
broadening in the spectral features and shifts by 1.5 eV toward
higher binding energies compared to the spectrum of complex 1
(Supporting Information Figure S1a). Such a shift could be due
to a transfer of chloropyridine from palladium to the rGO surface
during the support reaction. The spectrum could also contain
more components, providing a more complicated scenario for
the dissociated chloropyridine ligand (for more details see
Supporting Information Figure S1c). In order to investigate the
fate of the chloropyridine and understand the change in Cl 2p
binding energy, we performed an XPS analysis of chloropyridine
deposited onto rGO (chloropyridine@rGO) (see the Supporting
Information for details). In the Cl 2p3/2 XP spectrum of
chloropyridine@rGO only one peak at a higher energy of around
200.28 eV is found, matching the shifted peak in 1@rGO (Figure
S1a). All in all, these results corroborate that there is no
reduction of palladium during the formation of 1@rGO, and the
chloropyridine ligand is displaced by the π-interaction with the
support. This is schematically shown in Scheme 1. The N 1s
spectra for 1 are shown in Figure S1b (Supporting Information).
N
N
N
N
Ag2O/ MeOH
Ag
Cl
Cl
3
2
i) PdCl2(PhCN)2/
MeCN
Cl
Pd
Cl
N
N
N
ii)
Cl
N
Cl
N
N
Cl
rGO/DCM
N
N Pd
Cl
Pd Cl
N
Cl
N
Ultrasound bath
Cl
Cl
1
1@rGO
Scheme 1. Synthesis and heterogenization of double anthracene-tagged
Pd(II) carbene complex.
π-Stacked catalyst synthesis and characterization
Complex 1 was supported onto reduced graphene oxide (rGO)
by mixing the complex and rGO in dichloromethane in an
ultrasound bath for 40 min and stirring the mixture for 1 day
(Scheme S1). Notably, the colour of the solution turned from
intense yellow to pale yellow, indicating that complex 1 was
retained on the rGO material. The resulting material containing
the NHC–PdII complex π-stacked onto rGO (1@rGO) was
separated by centrifugation and washed extensively with
dichloromethane to remove unsupported 1. The 1H NMR
spectrum of the supernatant revealed the presence of 1 in the
solution; thus, not all of the complex had been deposited onto
rGO during the immobilization step. The palladium content of
1@rGO was determined by an inductively coupled plasma-
optical emission spectroscopy (ICP-OES) analysis. The results
showed a 2.5 wt.% Pd loading corresponding to a deposit
efficiency of ca 50%. Figure 1a shows a high resolution
transmission electron microscope (HRTEM) image of the native
rGO support. The rGO sheet shows an interlayer distance
around 3.8-4.3 Å in areas where the sheets can be viewed
edge-on.29 Figure 1b shows a HRTEM image of the 1@rGO. No
indication of metallic Pd particles from the process of attaching
the Pd-containing molecules could be found. X-ray energy-
dispersive spectrometry (XEDS) data confirmed the presence of
Pd and Cl at the 1@rGO sheets, whereas these elements were
absent in the native rGO support (Figure 1c). The XEDS
The high energy peak (ca. 401.1 eV) is related to the imidazole
32
nitrogen,24,
and the lower energy peak (ca. 400.4 eV) is
assigned to the chloropyridine nitrogen; the ratio between the
peaks is 2:1 as expected. Peak broadening towards lower and
higher binding energies is observed in the N 1s spectrum of
1@rGO. This significant broadening can be assigned to the
variation of the N chemical environment as a consequence of
the formation of pyrazole at the edges of rGO during the
synthesis of the rGO support33 and the dissociation of
chloropyridine, which opens up different interaction possibilities
as shown in Scheme S1 in the Supporting Information.
Thermogravimetric analysis (TGA) shows that 1@rGO
possesses approximately the same thermal stability as rGO with
no substantial decomposition under N2 below 500 °C (see Figure
S2).
mapping (Figure 1d) of 1@rGO shows
a homogeneous
dispersion of Pd and Cl on the graphene sheets.
X-ray photoelectron spectroscopy (XPS) analysis addresses the
oxidation state of Pd in 1@rGO and results are given in Figure 2.
The X-ray photoelectron (XP) spectrum of complex 1 shows the
Pd 3d5/2 and Pd 3d3/2 peaks at approximately 337.9 and 343.4
eV binding energy in the range of the expected energies for a
Pd(II) species.20, 30 For 1@rGO, i.e. after supporting 1 on RGO,
the peaks are shifted to lower binding, and the Pd 3d5/2 peaks
appears at around 337.6 eV, still within the range of a Pd(II)
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10.1002/ejic.201800978
European Journal of Inorganic Chemistry
FULL PAPER
Figure 1. (a) and (b) HRTEM images of rGO and 1@rGO, respectively,
showing resolved individual graphene-like sheets with insets enhancing the
lattice from which distances are measured. Spots are marked at which the
measurements in (c) were carried out. (c) EDX spectra, averaged for three
sites of of rGO (blue line) and three sites of 1@rGO (blue line). (d) High angle
annular dark-field (HAADF) STEM image and XEDS-mapping of 1@rGO. The
red box in the STEM image marks the areas for which the elemental mapping
was carried out. Note that there is no indication of Pd or Cl on the amorphous
carbon film from the TEM support grid (upper part of the image).
Activity in non-directed C–H acetoxylation
Benzene was chosen as a model substrate for non-directed
acetoxylation and it was used in excess over the oxidant
PhI(OAc)2 (PIDA) (5:1). The catalyst (1@rGO) loading was 0.3
mol % (based on measured Pd content), and, initially, different
solvent systems were screened as summarized in Table S1
(Supporting Information). The best combination was HFIP/Ac2O
giving over 50 turnovers, but, gratifyingly, the inexpensive
GAcOH/Ac2O solvent system also worked well so we set out to
Figure 2. Pd 3d XP spectra of the indicated samples
Lower concentrations of PIDA (0.6 M) gave a slightly lower yield
of 46% (Supporting Information Figure S3b). The concentration
dependence of PIDA may be coupled to its limited thermal
stability.20 After catalysis, 1@rGO was analyzed by XPS. The Pd
3d5/2 and Pd 3d3/2 peaks after acetoxylation reaction showed a
broadening towards lower and higher binding energies
compared to fresh 1@rGO (Figure 2). The identified broadening
is probably related to an increased variation in the chemical
environments of the Pd atoms after catalysis and to ligand
exchange between chloride and acetoxy group, similarly to what
was observed previously.20 Undoubtedly, though, the XPS
results show that the palladium largely remains in oxidation state
(II) after the reaction.
optimize conditions for this.
The optimization of the
concentration of the oxidant (PIDA) is shown in Supporting
Information Figure S3a. The results show a significant increase
in the yield up to 40% of Ph–OAc product upon increasing
[PIDA] to 0.6 M. Using an excess of benzene also further
increased the yield with the best result of 50% found at 30 eq. of
benzene and [PIDA] = 1.0 M.
Recyclability test
Catalyst 1@rGO was evaluated for recyclability. It can be easily
separated from the reaction mixture by a simple centrifugation
process. The recovered 1@rGO was shaken with 5 ml of
chloropyridine/ CH2Cl2 solution for 15 min after each cycle and
then washed with CH2Cl2 to remove the remaining non-bonded
chloropyridine. After each cycle the Pd content of the solution
was tested and found to be ≤ 0.011 ppm. The so-treated solid
catalysts were dried and used directly for the next reaction cycle.
1@rGO was found to be effective in four cycles (probably in
more, but this was not tested) of the acetoxylation reaction. A
slight decrease in the productivity of the recovered catalysts was
observed as shown in Figure 3.
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10.1002/ejic.201800978
European Journal of Inorganic Chemistry
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Oxidation of other arenes
The scope of C–H acetoxylation for other arenes is shown in
Figure 4 and Figure S4, where the specific conditions are given.
For comparison the corresponding scope for the previously
reported polymer supported catalyst20 is also given Figure 4. In
the acetoxylation of benzene 1@rGO showed an increase in the
[1@rGO]
(a)
[mol. catalyst/ mol. oxidant]= 0.3%
1 equiv. [PhI(OAc)2]
OAc
GAcOH:Ac2O (9:1)
92oC, 24h
5 equiv.
70
60
50
40
30
20
10
0
(b)
50
48
47
46
1
2
3
4
Recycle run
Figure 3. (a) C–H acetoxylation reaction of benzene over 1@rGO catalyst
using PhI(OAc)2 as an oxidant. Yields were determined by GC using a
calibration curve based on decane as an internal standard. (b) Reusability test
of 1@rGO.
Figure 4. Acetoxylation using a polymer supported catalyst (ref. 20, 104 °C) or
1@rGO (92°C). Yields were determined by GC using a calibration curve based
on decane as an internal standard.
yield of acetoxybenzene compared to homogeneous catalyst 1
(Supporting Information Figure S3b and Figure S4). Furthermore,
1@rGO and 1 showed similar GC yields for acetoxylation of
substrates biphenyl and toluene. Without catalyst, no formation
of product was observed according to GC. The regioselectivity in
the C–H acetoxylation of biphenyl using 1@rGO is 39:47 for
para- and meta-substituted positions, while there was 14%
ortho-substituted product. No significant change in the
regioselectivity was observed using 1 as a catalyst for the same
reaction. The regioselectivity in the C–H acetoxylation of toluene
using 1@rGO is very low with 38:33:29 for para-, meta-and
ortho-substituted products, respectively. Also here, 1 showed
approximately the same regioselectivity. For benzene 1@rGO is
superior to the polymer supported catalyst in ref 20, but for the
other arenes the opposite is true.
Conclusions
A new heterogeneous catalytic system for undirected C–H
oxidation reaction of arenes is described. It is based on
depositing an NHC Pd system with anthracene side arms on a
reduced graphene oxide surface. Characterization shows that
the complex retains the Pd(II) oxidation state upon deposition
and that the supporting pyridine ligand is displaced and
deposited on the surface. The supported catalyst is effective in
the acetoxylation of benzene, toluene and biphenyl. The
immobilized catalyst displays a catalytic efficiency that is better
or equal compared to the homogeneous analogue.
Experimental Section
Procedure for acetoxylation of benzene catalysed by 1@rGO
catalysts
The catalytic reactions were performed in a glass vial with a magnetic stir
bar. The vial was loaded with benzene (0.60 g, 7.7 mmol, 30 equiv.),
PhI(OAc)2 (0.083 g, 0.25 mmol, 1 equiv.), catalyst (0.3 mol% with respect
to the oxidant), glacial acetic acid (230 µl), and acetic anhydride (25.7 µl)
and sealed tightly by using a screw cap. The mixture was heated to
92 °C using a heating block. At the end of the reaction the vessel was
cooled to room temperature, and the catalyst was separated by
centrifugation. The mixture was diluted with Et2O (5 ml), shaken for 2 min,
and extracted with a saturated aqueous solution of K2CO3 (9M in water,
3×2ml). The organic layer was then separated and the solvent was
removed by evaporation. CH2Cl2 (1.2 ml) and decane (20 µl, as an
internal standard for quantitative GC analysis) were added to the
resulting solids and analysed by GC.
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10.1002/ejic.201800978
European Journal of Inorganic Chemistry
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Acknowledgements
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Financial support from the Swedish Research Council (contract
no. 621–2012–5815), the Knut and Alice Wallenberg Foundation,
and the Royal Physiographic Society is gratefully acknowl-
edged.
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unpublished.
Keywords: C–H activation
·
N-heterocyclic carbenes
·
palladium(II) · reduced graphene oxide supported Pd(II) ·
undirected acetoxylation
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