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Wendt and I. Persson, Catal. Sci. Technol., 2019, DOI: 10.1039/C8CY02430H.
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Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
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In situ XAS study of the local structure and oxidation state
evolutions of palladium in a reduced graphene oxide supported
Pd(II) carbene complex during an undirected C−H acetoxylation
reaction†
Ning Yuan,*†a,b Maitham H. Majeed,†c Éva G. Bajnóczi,a Axel R. Persson,c,d L. Reine Wallenberg,c,d
A. Ken Inge,b Niclas Heidenreich,e,f Norbert Stock,e Xiaodong Zou,b Ola F. Wendt*c and Ingmar
Persson*a
In situ X-ray absorption spectroscopy (XAS) investigations have been performed to provide insights into the reaction
mechanism of a palladium(II) catalyzed undirected C–H acetoxylation reaction in the presence of an oxidant. A Pd(II) Nheterocyclic carbene complex -stacked onto reduced graphene oxide (rGO) was used as catalyst. The Pd speciation during
the catalytic process was examined by XAS, which revealed a possible mechanism over the course of the reaction. Pd(II)
complexes in the as-synthesized catalyst first go through a gradual ligand substitution where chloride ions bound to Pd(II)
are replaced by other ligands with a bond distance to Pd corresponding to carbon, nitrogen and/or oxygen (L). Parallel to
this the mean oxidation state of Pd increases indicating the formation of Pd(IV) species. At a later stage, a fraction of the
Pd complexes start to slowly transform into Pd nanoclusters. The mean average oxidation state of Pd decreases to the
initial state at the end of the experiment which means that comparable amounts of Pd(0) and Pd(IV) are present. These
observations from heterogeneous catalysis are in good agreement with its homogeneous analog and they support a Pd(II)Pd(IV)-Pd(II)
reaction
mechanism.
Introduction
The formation of carbon–oxygen (C–O) bonds directly from
carbon–hydrogen (C–H) bonds is an attractive synthetic
pathway to many important organic compounds such as
pharmaceuticals and agrochemicals.1,2 Such transformations
are usually catalyzed by transition metals (TMs) and most
examples include homogeneous catalysts, but heterogeneous
TM catalysts are in demand, not only because these supported
systems are recyclable and easy to separate from the reaction
mixture, but also for their potential to introduce novel
chemical reactivity.3,4,5 Recently, reduced graphene oxide
(rGO) was successfully applied as a supporting material for
a. Department
of Molecular Sciences, Swedish University of Agricultural Sciences,
P.O. Box 7015, SE-750 07 Uppsala, Sweden. E-mail: ning.yuan@mmk.su.se
of Materials and Environmental Chemistry, Stockholm University, SE106 91 Stockholm, Sweden. E-mail: ingmar.persson@slu.se
c. Centre for Analysis and Synthesis, Department of Chemistry, Lund University, P.O.
Box 124, SE-221 00 Lund, Sweden. E-mail: ola.wendt@chem.lu.se
d. National Center for High Resolution Electron Microscopy and NanoLund, Lund
University, Box 124, SE-221 00 Lund, Sweden
e. Institut für Anorganische Chemie, Christian-Albrechts-Universität zu Kiel, DE24118 Kiel, Germany
f. Deutsches-Elektronen-Synchrotron DESY, DE-22607 Hamburg, Germany
† These authors contributed equally to this work.
† Electronic Supplementary Information (ESI) available: Details about XAS data
collection and analysis, XANES spectra of the complex under study, free and bound
to rGO, and EXAFS data treatment of data collected in in situ mode See
DOI: 10.1039/x0xx00000x
b. Department
different types of transition metal N-heterocyclic carbene
(TM–NHC) complexes and applied for many transformations
including C−H acetoxylation.6–10 Recently, we developed an
anthracene-tagged Pd(II)–NHC complex, 1, supported on rGO
through -stacking (here labeled 1@rGO) according to an
adaptation of the procedure proposed by Peris and coworkers.8 The preparation procedure is depicted in Scheme
1.11 It was shown that 1@rGO works as an active
heterogeneous catalyst in undirected C−H acetoxylation of
benzene and its catalytic efficiency is slightly higher than its
homogenous analog.11 This, together with our previous work,4
shows that Pd(II)–NHC complexes are effective as supported
catalysts for arene acetoxylation. Ex situ studies indicate that
the catalyst is not reduced during catalysis but there is little
information on the mechanism.4,11 It can be noted that there
are also systems based on Pd(0) pre-catalysts but these were
applied for directed C–H activation.3
Although a previous study by Tato et al. has shed light on
our understanding of the reaction mechanism of C−H
acetoxylation catalyzed by Pd(II)–NHC complexes,12 such
efforts were limited to homogenous reaction conditions due to
the complexity of heterogeneous systems. Also, the oxidation
state evolution during directed catalysis with Pd nanoparticles
has been studied.13 However, there are few, if any,
experimental studies on the nature and particularly oxidation
state of the Pd catalytic species during undirected C–H
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activation. Many of the traditional techniques used for
characterization of catalysts are either difficult to conduct
under reaction conditions, such as X-ray photoelectron
spectroscopy (XPS), or unable to probe the catalytic centres
directly, such as Fourier-transform infrared (FTIR)
spectroscopy.
To meet the demand of studying the catalytic species in a
direct manner on atomic level, X-ray absorption spectroscopy
(XAS) has become a suitable experimental approach, as it is
element specific with high sensitivity to local structure and
valence state of the absorbing element.14 XAS can be applied
on all states of aggregation as well as their mixtures, and only
millimolar concentrations are required for successful structure
analysis around the absorbing atom. Furthermore, a scanning
rate of minutes allows in situ XAS measurements to be
conducted under various chemical conditions.15–17 In the
Scheme 1. Immobilization of 1 on rGO.
recent years of in situ XAS research on catalysis, major efforts
have been focussed on solid-gas heterogeneous catalytic
systems due to the manageable design and construction of in
situ reactors.18–23 For solid-liquid heterogeneous reactions or
liquid homogenous reactions, it becomes more challenging to
build up a suitable sample cell because of more demanding
reaction conditions and interference from the reaction
mixture. The growing interest in in situ XAS measurements on
solid-liquid and solution phase reactions has promoted the
development of the reactors.24,25 With improved control of the
chemical conditions several successful examples have been
reported.26–29 These studies have proven that in situ XAS is a
powerful tool to study the catalytic species in solution and
solution-solid reaction mixtures.
In the current study we aim to elucidate the palladium
species and their oxidation states over the course of the
undirected C−H acetoxylation of benzene catalyzed by 1@rGO
using in situ XAS spectroscopy and gain insights into the
reaction mechanism. The focus is on the heterogeneous
system and the homogeneous analog is used for comparison.
Experimental section
Materials. 1@rGO (2.5 wt% Pd loading) and 1 were used as
catalysts and synthesized according to procedures reported
previously.8,11 All reagents and solvents used in the
experiments were purchased from commercial suppliers
without further purification.
Catalytic reactions for in situ XAS experiments. Benzene
(1.75 g, 22.44 mmol, 21.77 equiv.), (Diacetoxyiodo)benzene
(oxidant, 0.33 g, 1.03 mmol, 1 equiv.), 1@rGO (0.10 g, 2.3
mol% with respect to the oxidant) and 1 (0.07 g, 9.2
with
Viewmol%
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10.1039/C8CY02430H
respect to the oxidant), glacial acetic acid
(1.50
mL), and acetic
anhydride (0.17 mL) were transferred into a reaction vessel
which was sealed in the preparation room and transported
immediately to the hutch for measurements. The reaction
mixture was stirred at 92 °C during the in situ XAS
measurements.
Reactor for in situ experiments. In situ XAS data were collected
using a custom-made reactor designed for investigations under
solvothermal conditions.24 The remotely controlled reactor
was used to heat and stir the reaction mixture in a sealed 5mL
glass vial with 1 mm thick walls, allowing the synchrotron
radiation to penetrate the sample in transmission mode. The
target temperature is typically achieved within two minutes,
and a thermocouple was immersed into the reaction mixture
to monitor the temperature throughout the experiment.
Figure S1 in Section S1 shows the exploded-view illustration of
the reactor.
XAS experiments. All XAS data were collected at beamline
P64 at Petra III Extension, Deutsches Elektronen-Synchrotron
(DESY), Hamburg, Germany. The XAS measurements were
conducted in transmission mode at the Pd K-edge (24.349 keV)
with an energy range from 24.00 to 25.15 keV for dry samples
or 25.00 keV for in situ measurement.30 A palladium metal foil
was measured simultaneously and its first inflection point on the
absorption edge was used to individually calibrate all the XAS
spectra during the measurements. As-synthesized 1@rGO, 1, and
recycled 1@rGO were measured using a standard sample holder for
powders. For the in situ measurements, each XAS scan was set to
ca. 6 min with considerations of both time resolution and data
quality. More technical details and data analysis are described in
Section S1.
Results and discussion
The successful preparation of 1@rGO is confirmed by both
X-ray absorption near edge structure (XANES) and extended Xray absorption fine structure (EXAFS) spectra of the assynthesized 1 and 1@rGO (Figure S2-a in Section S2 and Figure
S3 in Section S3). The structure of 1 was described in the
previous work where each Pd binds two chloride ions (Cl−), one
carbene and one pyridine ligand. The oxidation state of Pd was
determined to +II.11 This information is consistent with the
analysis of the XANES spectra of 1 and its structure determined
by EXAFS (Table 1). The XANES and EXAFS spectra of 1@rGO
are similar to those of 1 indicating a successful immobilization
of 1 onto rGO. EXAFS refinement of 1@rGO reveals one Pd–C
bond with a length of 1.95 Å which corresponds to the bond
between Pd and the NHC ligand. Another bond distance to Pd
with a relatively large error, 0.08 Å, is observed at 2.18 Å. This
bond length is close to the distance between Pd and pyridine
ligand in 1. However, our previous results based on XPS show
that the pyridine ligand dissociates from Pd and is stacked on
the support.11 One possibility is that Pd atoms are pi-bound to
a C=C double bond and 2.18 Å is a reasonable bond length in
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such configuration.31–33 Scheme 1 describes the immobilization
process based on our understanding.
In the previous study, we have shown that 1@rGO is an
effective catalyst in the acetoxylation of benzene and a few
other arenes. The reaction conditions were optimized and
used for the in situ XAS measurement as described in Scheme
2. Benzene was chosen as a model substrate with excess
amount of oxidant PhI(OAc)2.
form of Pd(III) species is dimers.37,38 Interestingly,
the
first
in
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10.1039/C8CY02430H
situ XANES spectrum displayed changesDOI:
in comparison
to assynthesized 1@rGO, and thereafter the observed changes slow
down significantly. This implies a process of pre-catalyst
activation in the beginning of the reaction which has been
realized as an important procedure for the catalyst which will
be further discussed in the EXAFS analysis.29,39
Scheme 2. 1@rGO catalyzed undirected C–H acetoxylation reaction of benzene for in
situ measurement.
1@rGO was added to the reaction mixture and the XAS
data collection started when the reaction was initiated with a
time resolution of ca. 6 min. Due to the low concentration of
Pd (ca. 7 mM) and low signal/noise (S/N) ratio, groups of XAS
spectra with identical features were averaged to improve the
statistics. Figure 1 displays the selected Pd K-edge XANES
spectra during the in situ measurement which is divided into
two parts. The first part of the measurement is from the
beginning of the reaction to ca. 80 min as presented in
Figure 1a. The main changes of the XANES spectra captured
here are the increased white line intensity, especially during
the first six minutes of the reaction, and a continuous shift of
edge position towards higher energies. The increase in white
line intensity has been proposed to be caused by an increase in
the oxidation state of palladium in molecular complexes.34,35
Regarding the edge shift observation, theoretically, it can be
caused either by a partial oxidation of Pd(II) to Pd(IV), or a
change of bound ligands from covalently bound to more
electrostatically bound ones. In this system the only strong
electron-pair donor to Pd in 1@rGO is the NHC ligand and the
Pd–NHC bond is very stable with a calculated bond energy of
45-50 kcal·mol-1.36 Therefore, it is expected to remain bound to
Pd during the reaction. Furthermore, the 1@rGO catalyst
showed good recyclability and could be reused up to four runs
with only slight decrease in the activity which attests to the
stable Pd–C(NHC) bond. Hence, a partial oxidation of Pd(II)
complexes seems to take place by the shift towards higher
energy, together with the increased white line intensity. Even
though the edge shift is small, ca. 1 eV, and corresponds to a
mean oxidation state change less than one, it is clearly
observable. However, XAS spectra display a mean value of the
radiated absorbing species, and a shift of ca. 1 eV, ca. 25% of
an expected shift between Pd(II) and Pd(IV), implies a
reasonable scenario where a minor fraction of the Pd(II)
complexes have been oxidized to Pd(IV), considering the
relatively short lifetime of Pd(IV) in the catalytic redox cycle.
The argument to propose Pd(IV) instead of Pd(III) is that no
back Fourier transformed EXAFS spectra indicating Pd---Pd
dimers with short Pd---Pd bonds were observed; the common
Figure 1. In situ Pd K-edge XANES spectra of 1@rGO catalyzed C–H acetoxylation
reaction. (a) First part of measurement when edge position shifts towards higher
energy. (b) Second part of measurement when edge position shifts towards lower
energy. The edge shifts are magnified as inset.
Selected XANES spectra of the second part of the in situ
measurement, 80 min - 24 h, are given in Figure 1b. As the
measurement proceeds, the edge position shifts backward to
lower energies. The shift continues until the end of the
measurement at 24 h. The XANES spectrum of the recycled
1@rGO has an edge position and shape after the edge
identical to the catalyst at 24 h. This indicates that the
evolution of the Pd species ceases after 24 h, and that the
recycling procedure does not introduce further changes of the
Pd species. It is noted that the edge position of Pd at the end
of the measurement returns to a position very close to the one
of as-synthesized 1@rGO (Figure 1 and Figure S2-b in Section
S2). This means that the mean oxidation state of Pd at the end
of the measurement is +II. Meanwhile, the white line intensity
has a general trend of declining even though the S/N ratio is
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low. The shape of the absorption edge appears to more
resemble that of metallic Pd even though it is damped due to
the small fraction.
Figure 2. Fourier transformed k3-weighted EXAFS data as a function of measurement
time showing the coordination environment of Pd in 1@rGO and 1. The spectra are not
phase correct and the k range used is fixed at 2-9 Å-1 for comparison. The arrows
indicate the stepwise dissociation of Cl- ligands.
These observations indicate that the mean oxidation state of
the Pd species changes in the opposite direction in the second
part as compared to the first one, and that the Pd(IV) species is
mostly reduced to Pd(II) and to a small extent to Pd(0) as the
reaction proceeds to the end. Interestingly, a small fraction of
Pd(IV) species should remain considering the mean oxidation
state of +II. Kim and co-workers have shown the co-existence
of Pd(0), Pd(II) and Pd(IV) species by oxidizing Pd nanoparticles
at controlled conditions where Pd(IV) formed polymeric
species with enhanced stability.13 Although such Pd(IV) signals
are not observed in the EXAFS of our material, it can be
attributed to its low content.
The specific coordination environment of the Pd species
in 1@rGO during the reaction were also studied by EXAFS and
the Fourier transformed spectra are presented in Figure 2a.
The k range of all the EXAFS spectra is 2-9 Å−1 for easier
comparison of the data at different reaction stages. The EXAFS
spectra in full treatable range and their refinements are
reported in Table 1 and Figure S4 in Section S3. From the assynthesized 1@rGO to 6 min an obvious shift of the main peak
towards shorter distance is observed indicating a change in the
coordination environment of Pd, Figure 2a. The refinement of
the EXAFS spectrum of the catalyst at 6 min shows that on
average each Pd centre is bound to three L ligands (L ligands:
C, N and/or O ligands) at a mean distance of ca. 2.01 Å and one
Cl− ligand at ca. 2.32 Å. Because of the average nature of EXAFS
the refinement parameters can be also interpreted as half of
the palladium centres have four coordinated L ligands and no
Cl− ligands, while the other half retain the coordination
structure of the as-synthesized catalyst. However,
the
second
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scenario does not seem reasonable DOI:
considering
the local
structural change of Pd slows down immediately after 6 min
and certain transformations should be completed. As the
changes of the XANES spectra after 6 min in Figure 1a are less
pronounced, the differences of EXAFS spectra from 6 to 80 min
are also very small and cannot be resolved by refinements at
the current resolution. This implies that the local structure of
the palladium centre remains the same after activation while a
fraction of them have been further oxidized and bound to
ligands with a Pd–L bond length of ca. 2.0 Å.
Another change occurs from 80 min to 3.5 h where the
main peak moves further towards shorter distances. The
refinement reveals that the remaining Cl− is replaced by
another L ligand resulting in Pd complexes with four L ligands
after 3.5 h. At the same time, the introduction of a Pd–Pd
bond distance of 2.7 Å corresponding to Pd nanoclusters with
a mean coordination number of ca. 0.8 is necessary to achieve
a proper fit of the EXAFS spectrum. This signal can be observed
as the relatively small peak at ca. 2.3 Å (without phase
correction) after the main peak. It strongly implies that a small
fraction of the Pd complexes have been reduced to metallic Pd
nanoclusters; this could be caused by a reductive elimination
of ligands. Interestingly, the contribution of Pd–Pd bond
distances is not rapidly growing between 6 and 8 hours, and
the local environment of Pd seems to be stable. This
observation is in contrast to a previously investigated system
where the transformation of mononuclear Pd complexes to Pd
nanoclusters is much faster.29 Although the local structure of
the Pd appears to be the same during this process, the nature
of the L ligands bound to Pd seems to change as the edge
position shifts towards lower energies. At 24 h the peak at
2.7 Å (after phase correction) became more pronounced
indicating an increased amount of metallic Pd nanoclusters.
Meanwhile, the first main peak shifts towards longer distance
at 24 h. It is known that the Pd–Pd bond has a satellite peak
with similar appearance as a Pd–Cl bond and it becomes more
pronounced when the applied k range is shortened.29 Due to
limited EXAFS data quality for the catalyst at 24 h, the satellite
peak of Pd–Pd bond distances and the Pd–L peak merge
causing the abnormal peak position. The spectrum of the
recycled catalyst is similar to the one at 24 h and ca. 40% of Pd
is estimated to be present as Pd nanoclusters based on linear
combination fit of the XANES spectrum (Figure S6a, Section
S4), while the remaining palladium is in the form of Pd(II)
complexes with a small amount of Pd(IV) complexes. The Pd
species in the recycled 1@rGO are further proven by
transmission electron microscopy (TEM) and X-ray energy
dispersive spectroscopy (EDS) showing the presence of Pd well
dispersed over the whole observing area of the catalyst,
including Pd aggregates (Section S5). The oxidant is thermally
degraded over time which means that towards the end of the
reaction there is no or little oxidant left.4 In the absence of
oxidant Pd may be reduced which could lead to the formation
of metallic Pd nanoclusters although these were not observed
in the previous ex situ investigation.11
4 | J. Name., 2012, 00, 1-3
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Previously, it was also found that 1@rGO mostly retained
its activity for at least four runs in the un-directed
acetoxylation of benzene. However, the GC yield of
acetoxybenzene showed a slow decreasing trend from 50%
(first cycle) to 46% (fourth cycle). The solutions separated from
the catalyst after each cycle contained less than 0.011 ppm.11
Since there is little Pd leaching, we propose that the slight
decrease of productivity using recovered catalyst could be be
related to the formation of Pd nanoclusters which should be
an inactive species under the current reaction conditions.
Although the oxidant is in excess in the in situ measurement,
its relative amount is lower than in the reported catalysis
conditions, meaning that the formation of Pd nanoclusters in
this work is expected to be more extensive.
The homogenous analog of 1@rGO was also investigated
to compare with the observations and conclusions from
1@rGO. 1 was used as catalyst and the in situ experiment was
repeated under the same condition as for 1@rGO except that
the concentration of Pd in the homogenous reaction mixture
was increased to ca. 35 mM to improve the absorbance and
the S/N ratio of the individual in situ XAS scans. Figure 3 shows
selected Pd K-edge XANES spectra of 1 during the reaction. The
same strategy as for 1@rGO was used where the
measurement is divided into two parts. The first part is from
the beginning of the reaction to 78 min. It is noted that the
white line intensity increases continuously until 52 min, while
the edge position is shifting towards higher energy, a shift that
also ceases at 52 min. There is no additional change from 52 to
78 min. From 78 min to 33 h after the start of the reaction, the
white line intensity and the edge position return to the starting
point. These observations are in good agreement with the
observations using 1@rGO. The oxidation state of Pd in 1 also
increases at first, followed by a decrease at the later stage of
the measurement.
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Figure 3. In situ Pd K-edge XANES spectra of 1 catalyzed C–H acetoxylation reaction. (a)
First part of measurement when edge position shifts towards higher energy. (b) Second
part of measurement when edge position shifts towards lower energy. The edge shifts
are magnified as insets.
Table 1. Refined distances (d/Å), and mean number of distances (N) and Debye-Waller factor (σ2/Å2) in selected scans using 1@rGO and 1 as catalysts. Single
scattering at outer shells and multiple scatterings are not shown in the table.
Catalyst
As-synth. 1@rGO
6 min (22, 80min)
3.5 h
8 h (6 h)
Recycled (24 h)
d(Pd-N/C/O)
1.953(6)
2.18(8)
2.013(6)
2.020(4)
2.015(7)
1.966(4)
Nb
1.0
1.0
3.0
4.0
3.5
2.0
σ2
0.0017(7)
0.007(8)
0.009(1)
0.006(1)
0.005(2)
0.004(2)
d(Pd-Cl)
2.293(9)
N
2.0
σ2
0.0045(9)
2.320(6)
1.0
0.007(2)
d(Pd-Pd)
2.68(2)
2.70(4)
2.652(4)
N
σ2
0.8
1.0
3.5
0.007(2)
0.005(4)
0.010(4)
As-synth. 1
1.95(1)
1.0
0.003(2)
2.301(6)
2.0
0.0015(9)
2.17(8)
1.0
0.005(9)
6 min
2.00(1)
2.5
0.009(1)
2.293(8)
1.5
0.0023(7)
40 min
2.017(7)
3.0
0.003(1)
2.29(3)
1.0
0.003(1)
78 min
2.034(3)
4.0
0.0036(2)
2.65(2)
0.5
0.005(1)
5.5 h
2.032(2)
3.5
0.0043(4)
2.709(7)
1.0
0.010(1)
33 h
2.025(9)
3.0
0.007(2)
2.69(1)
3.0
0.012(2)
aThe standard deviations in parentheses were obtained from k3 weighted least-squares refinements of the EXAFS function χ(k) and do not include systematic
errors of the measurement. b The estimated error of the N values is ca. 25%. Underscored parameters have been optimized and fixed in the refinement.
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The corresponding EXAFS spectra of 1 catalyzed reaction
were analyzed as well, and their Fourier transformed spectra
are displayed in Figure 2b. The k range used for 1 and 1@rGO
is the same for proper comparison. Due to the better S/N ratio
each individual in situ scan could be analyzed and they are
presented as function of reaction time. In general, the change
in local structure around Pd in 1 is the same as in 1@rGO. The
first main peak gradually shifts towards shorter bond lengths
corresponding to a stepwise dissociation of Cl− ligands, after
which the peak corresponding to Pd–Pd single scattering
appears and grows at a later stage of the measurement
indicating the formation of metallic Pd nanoclusters. It is also
noted that the intensity of the white line in Figure 3a increases
more smoothly than in the case of 1@rGO. This might be a
consequence of the higher Pd concentration in the 1 catalyzed
reaction. However, the earlier catalysis study showed that
1@rGO lead to higher yield of acetoxybenzene compared to 1
and this observation might also be correlated to the slower
activation rate of 1 in the beginning of the reaction.11 The
results of the refinements of the EXAFS spectra are
summarized in Table 1, and the fits are shown in Figure S5 in
Section S3. The fraction of the metallic Pd nanoclusters at the
end of the measurement (33 h) is estimated to be ca. 25%
from the linear combination fit of its XANES spectrum (Figure
S6b, Section S4). As outlined above this fraction is probably
higher in the in situ measurements than in the actual catalysis
because of the higher mol% of the catalyst with respect to the
oxidant loading in the former. The fraction of Pd nanoclusters
in 1 at 33 h is noted to be lower than in the recycled 1@rGO,
which is also reflected by its lower mean number of Pd–Pd
bonds shown in Table 1.
remains, Pd(0) nanoclusters are slowly formed resulting in a
relatively stable mixture of mononuclear Pd complexes and Pd
metal nano-sized aggregates. During this process the mean
oxidation state of Pd returns to the starting state resulting in a
mixture dominated by the Pd(II) species, and smaller amounts
of Pd(0) nanoclusters and Pd(IV) species. Overall, our
observations provide evidence for a mechanism including
oxidation state changes Pd(II)-Pd(IV)-Pd(II) during the
undirected C−H acetoxylation reaction as proposed through
computational methods.40,41
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work has been supported by the MATsynCELL project
through Röntgen-Ångström Cluster, the Swedish Research
Council (VR) and the German Federal Ministry of Education
and Research (BMBF). We are also thankful to the Berzelii
Center EXSELENT on Porous Materials, the Swedish Research
Council (VR) and the Project Management Organization at
DESY (Deutsches Elektronen-Synchrotron). The allocation of
beamtime at P64, Petra III Extension, DESY is gratefully
acknowledged. We thank the staff at beamline P64,
particularly Dr. Vadim Murzin and Dr. Wolfgang Caliebe for the
assistance during the data collection.
To further improve the understanding of the activation of
the pre-catalyst, the XAS spectrum of 1 in the reaction mixture
at room temperature was collected and it is identical to the assynthesized 1. This demonstrates that the induced heat
triggers the exchange of the ligands bound to Pd leading to the
activation of the catalyst.
2
3
Conclusions
4
In conclusion, we have used in situ XAS to probe the
speciation and the mean oxidation state of palladium over the
course of an undirected C−H acetoxylation reaction. The
heterogeneous catalyst 1@rGO and its homogenous analog 1
were examined. In both cases, a stepwise exchange of Cl− ions
by L ligands (L: carbon, nitrogen and/or oxygen) is observed.
Meanwhile a small fraction of Pd(II) is oxidized to Pd(IV). At a
later stage of the reaction, when less or no oxidant (PhI(OAc)2)
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In situ XAS is used to reveal the evolution of palladium species during an undirected C H DOI:
acetoxylation
10.1039/C8CY02430H
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Catalysis Science & Technology Accepted Manuscript