Immobilization of Pyrene-Tagged Palladium and Ruthenium Complexes onto Reduced Graphene Oxide: An Efficient and Highly Recyclable Catalyst for Hydrodefluorination

Article abstract of DOI:10.1021/om501040x

(Graph Presented). The co-immobilization of palladium and ruthenium complexes with pyrene-tagged N-heterocyclic carbene ligands onto reduced graphene oxide allows the formation of a highly efficient catalyst for the hydrodefluorination of a series of fluoroarenes. This procedure constitutes an easy one-pot preparation of materials with homogeneously distributed polymetallic catalysts. The catalytic system can be recycled up to 12 times without measurable loss of activity. The activity of the catalyst is attributed to the synergistic action of the two metals.

Full text of DOI:10.1021/om501040x

Article
pubs.acs.org/Organometallics
Immobilization of Pyrene-Tagged Palladium and Ruthenium
Complexes onto Reduced Graphene Oxide: An Efficient and Highly
Recyclable Catalyst for Hydrodefluorination
Sara Sabater, Jose A. Mata,* and Eduardo Peris*
́
́ ́ ́
Departamento de Química Inorganica y Organica, Universitat Jaume I, Avenida Sos Baynat s/n, 12071, Castellon, Spain
S
* Supporting Information
ABSTRACT: The co-immobilization of palladium and ruthenium complexes with pyrene-tagged N-heterocyclic carbene ligands
onto reduced graphene oxide allows the formation of a highly efficient catalyst for the hydrodefluorination of a series of
fluoroarenes. This procedure constitutes an easy one-pot preparation of materials with homogeneously distributed polymetallic
catalysts. The catalytic system can be recycled up to 12 times without measurable loss of activity. The activity of the catalyst is
attributed to the synergistic action of the two metals.
wide set of fluoro-organic molecules.6 Our studies showed that
INTRODUCTION
■
the overall process implies that the palladium fragment
facilitates the C−F activation, while the ruthenium center
allows the reduction of the substrate via transfer hydrogenation
from iPrOH/tBuONa, in a clear example of synergistic
behavior.
The immobilization of a homogeneous catalyst onto a solid
surface is one of the major challenges in catalysis, because it
allows the separation of the catalyst and the reaction products,
and may facilitate the reutilization of the catalyst in multiple
subsequent cycles.1 Carbon surfaces have recently gained
attention as convenient supports for molecular attachment.
Over the more widely used covalent immobilization strategies,
which often require nontrivial pretreatment of the solid surface,
noncovalent methods that use strong π interactions between
polyaromatic hydrocarbons and graphitized surfaces have
attracted great interest because they offer a convenient,
nondestructive approach to catalyst immobilization.1e Due to
the inherent capability of pyrene to afford noncovalent
interactions with graphitic surfaces,2 pyrene-containing metal
complexes have been supported on graphitized surfaces3 and
have also been used in catalysis, proving interesting recyclability
properties.4 We recently obtained two complexes of palladium
and ruthenium containing an N-heterocyclic carbene ligand
with a pyrene tag, which were used for the hydrogenation of
alkenes (palladium) and oxidation of alcohols (ruthenium).5
We proved not only that the catalytic efficiencies of the
catalysts were enhanced upon immobilization but also that the
catalysts could be recycled 10 times without apparent loss of
activity. Precisely, the combination of these two metals into a
single-frame heterodimetallic complex allowed us to recently
obtain a highly efficient catalyst for the hydrodefluorination of a
Hydrodefluorination (HDF) is the simplest C−F activation
process, but it features a surprising mechanistic diversity.7 The
reaction formally implies the activation of a carbon−fluorine
bond followed by the introduction of hydrogen to form the
final hydrodefluorinated molecule. The reaction constitutes a
great challenge in catalysis, because carbon−fluorine bonds lie
among the most unreactive functionalities in chemistry.8
Carbon−fluorine activation has been an active research field
during the last three decades, but HDF has only flourished
recently, probably triggered by the need for effective catalysts
that allow the activation and disposal of the chlorofluor-
ocarbons (CFCs), considered as “supergreenhouse gases”.9
By combining our experience in the immobilization of
pyrene-functionalized palladium and ruthenium complexes
onto a reduced graphene oxide surface (rGO), with our
experience in using mixed palladium/ruthenium complexes in
catalytic HDF, we now report the immobilization of two
complexes of palladium and ruthenium onto rGO and report a
Received: October 15, 2014
© XXXX American Chemical Society
A
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Scheme 1
Figure 1. STEM image of rGO-Pd-Ru, 3 (left), and EDS elemental mapping images showing the homogeneous distribution of ruthenium (middle)
and palladium (right).
study on the use of the resulting heterogenized catalyst in the
HDF of a series of aromatic fluorocarbons.
material (373 m2 g−1). The elemental mapping by energy-
dispersive X-ray spectroscopic analysis (EDS) performed by
means of HRTEM of 3 confirmed the homogeneous
distribution of palladium and ruthenium in the hybrid material
(Figure 1).
RESULTS AND DISCUSSION
■
An equimolecular mixture of complexes 1 (Pd) and 2 (Ru) was
supported onto reduced graphene oxide by mixing the
molecular complexes and rGO in dichloromethane in an
ultrasound bath for 30 min and stirring for 10 h (Scheme 1).
The first indication that the immobilization has taken place is
the loss of the color of the solution. The resulting rGO
containing the two catalysts was filtered and washed with
dichloromethane. The 1H NMR spectrum of the filtrate
revealed the absence of signals due to 1 or 2 and therefore
constitutes indirect evidence that the two complexes have been
effectively grafted on the solid surface. This methodology
allows the simultaneous immobilization of different metal
complexes onto the surface of rGO. The materials obtained
show a homogeneous distribution of metals.
In order to evaluate the activity of the palladium−ruthenium
hybrid material 3 in HDF, we first tested its activity toward a
series of fluoroarenes. The reactions were carried out in iPrOH
at 80 °C, over 3 h and in the presence of tBuONa. The
reactions were performed using 2 mol % of the catalyst, based
on the amount of metal (in the case of the solid material 3, the
catalyst loading was 1 mol % of Ru and 1 mol % of Pd). For
comparative purposes, we also tested the activity of rGO and
the previously reported materials rGO-Pd and rGO-Ru, which
contain only one of the two metal complexes grafted onto the
solid surface.5 We noticed that the supported catalysts needed
to be activated prior to their use in the catalytic reactions. The
activation of the catalysts was carried out by refluxing the solid
in iPrOH in the presence of tBuONa, for 24 h. As can be seen
from the results shown in Table 1, for the hydrodefluorination
of fluorobenzene, it becomes clear that both rGO and rGO-Ru
are completely ineffective, while the palladium-containing solid,
rGO-Pd, produces moderate amounts (50%) of the defluori-
nated product (compare entries 1, 2, and 3). When complexes
1 and 2 are mixed together and used for the homogeneously
catalyzed hydrodefluorination of fluorobenzene, we did not
observe the formation of benzene. This result is rather
surprising, especially if we compare it with the result obtained
for the use of the heterogenized catalyst 3, which afforded full
conversion to benzene (compare entries 4 and 5). This
“abnormal” enhancement in the activity of the immobilized
catalyst onto the graphene material has already been observed
before for catalysts supported on graphene oxide,5,11 but this
new example that we report here is far more striking because it
The exact palladium and ruthenium content in the final solid
rGO-Pd-Ru, 3, was determined by ICP-MS analysis after
digestion of the solid in hot HCl/HNO3. The results indicated
a 5.19 and 4.97 wt % for palladium and ruthenium, respectively.
The characterization of 3 was carried out by using UV/vis,
FTIR, Ar-sorption experiments, AFM, and HRTEM (See
Supporting Information for details). The UV/vis measurements
(Figure S1) reveal that the spectrum of 3 consists of the overlap
of the molecular complexes (1 or 2) with rGO, thus confirming
the presence of the complexes on rGO, as seen by the
characteristic pyrene UV/vis signals.4b,10 For the determination
of the surface area of the solid, the argon adsorption−
desorption isotherms at 77 K were measured (most commonly
referred to as BET surface area, where BET stands for
Brunauer−Emmet−Teller). The BET area for 3 was 165 m2
g−1, significantly lower than that shown by the rGO raw
B
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a
palladium). This result verifies our initial hypothesis that the
synergistic role of the two metals is playing an important role in
the catalytic performance of our hybrid catalyst 3. The high-
resolution images of transmission electron microscopy
(HRTEM) of 3 after being activated showed the formation
of well-dispersed small nanoparticles (<5 nm diameter). The
analysis by energy-dispersive X-ray spectroscopy (EDS)
performed by means of HRTEM of 3 did not provide any
definitive information about whether these nanoparticles were
formed by Pd, Ru, or a mixture of these two metals (see Figure
S9 in the SI for details). In order to determine the nature of
these nanoparticles, we performed a control experiment by
independently incubating the palladium- and ruthenium-
supported catalysts (rGO-Pd and rGO-Ru) in iPrOH at 80
°C in the presence of tBuONa during 24 h. The HTREM
images of the incubated samples showed that only the rGO-Pd
material developed nanoparticles (Figure S12 in the SI). The
palladium content of the nanoparticles was further confirmed
by the EDS spectrum of the sample (Figure S13 in the SI). For
the rGO-Ru sample, the energy-dispersive X-ray spectroscopic
analysis performed by means of scanning transmission electron
microscopy (STEM) confirmed the homogeneous distribution
of ruthenium in the solid (Figure S11 in SI), and this time we
did not detect the presence of nanoparticles. This result
indicates that the small nanoparticles formed when 3 is used are
formed by palladium, which has a higher tendency to aggregate
than ruthenium under the reaction conditions used, as we
previously demonstrated.5 This finding should explain the
differences found in the catalytic activity of the solid material 3,
compared to the inactive catalytic system formed by the
mixture of the two homogeneous catalysts 1 and 2. Also, the
mechanism for this process is different from the mechanism for
our previously described homogeneous Ru−Pd catalyst.6 In this
case, we believe that the C−F activation should be promoted by
the palladium nanoparticles, while the introduction of the
hydrogen by transfer hydrogenation should be facilitated by the
ruthenium heterogenized complex, and therefore the synergistic
action between the two metals is crucial for explaining the
superior activity of the catalyst.
We were interested in determining if we could recycle
catalyst 3 for several catalytic cycles without loss of its catalytic
activity. The recycling experiments were performed using
fluorobenzene as model substrate. The reaction conditions used
were the same as those shown in Table 1, and the reaction
progress was determined by GC. After completion of each run
(3 h), the reaction mixture was allowed to reach room
temperature, and the solid catalyst was separated by
centrifugation, washed with CH2Cl2, dried, and reused in the
subsequent cycle. Each cycle required the addition of one
equivalent of tBuONa with respect to fluorobenzene. By
following this procedure, catalyst 3 could be recycled for up to
12 runs, without any observable decrease of activity. Only
during the 13th run did we observe a decrease in activity to
85%, but we believe that this result may be due to the
accumulated loss of catalyst during the successive filtrations.
The ICP-MS analysis after the first run accounted for 4.87 wt %
of ruthenium and 4.99 wt % of palladium, indicating a negligible
loss of metals. When the ICP-MS analysis was carried out after
the final 13 runs, we did observe that the total amount of metals
found was 3.94 wt % for ruthenium and 3.85 wt % for
palladium, indicating an approximate 20% loss of metal with
respect to the initial content. However, this loss of percent of
metal content may be due to the mass gain during the
Table 1. Hydrodefluorination of Fluoroarenes
a
Reactions were carried out with 0.3 mmol of fluoroarene, tBuONa
(0.3 mmol), preactivated catalyst (1 mol % base on each metal), 2 mL
of iPrOH for 3 h at 80 °C. Yields determined by GC analyses using
anisole as internal standard. GC yields after 20 h. Catalyst loading 2
mol %. Two equivalents of tBuONa (0.6 mmol). Yields in
b
c
d
e
f
parentheses are after 6 h. Three equivalents of tBuONa (0.9 mmol).
shows that the catalyst is active only when it is immobilized
onto the graphene surface. Catalyst 3 was also very active in the
hydrodefluorination of 4-fluoroaniline and 4-fluoro-
trifluoromethyltoluene, and showed moderate activity for the
C−F activation of 4-fluorotoluene, 1,4-difluorobenzene, and
1,3,5-trifluorobenzene, although the conversions increased
upon increasing the reaction times (see entries 9 and 10).
In order to test if the activity of the catalysts could be
maintained upon catalyst recovery, we performed a second run
by recycling the catalyst from the reaction mixture. As can be
seen from the results shown, the activity of the palladium-
supported catalyst, rGO-Pd, improved with respect to the first
cycle (entry 3, first run 50%, second run 75%). This result is
probably due to the activation of the catalyst along the reaction
course and may be ascribed to the slow formation of palladium
nanoparticles. In the case of the heterometallic solid material, 3,
we also observed that the activity was maintained for the case of
the hydrodefluorination of fluorobenzene (entry 5). For the
hydrodefluorination of the rest of the substrates that we tested,
we observed a slight enhancement of the catalytic activity, again
probably as a consequence of the formation of active palladium
nanoparticles (see below for further analysis and discussion). As
can be seen from the results shown in the table, despite the fact
that the palladium-containing material, rGO-Pd, is active in the
hydrodefluorination of fluorobenzene, its activity is much lower
than that shown by the Ru−Pd material, 3, which affords better
results even using a lower loading of palladium (rGO-Pd
contains 2 mol % of palladium, while 3 contains 1 mol % of
C
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subsequent cycles due to the adsorption of substances from the
catalyst reactions (we believe that a mass gain due to tBuONa
adsorption may have occurred, as we previously observed),5 so
it may not be ascribed to desorption or leaching of the metal
complex. The atomic force microscopy measurements (AFM)
of 3 before and after its activation revealed that the height of
the rGO sheets is about 2 and 1.5 nm, respectively, both within
the accepted range of few-layer graphenes.12 Interestingly, the
BET surface area of 3 is 164 and 221 m2/g for the unactivated
and activated materials, respectively. These data are in
agreement with the data coming from the AFM analysis,
which suggests a partial exfoliation of the catalyst upon
activation.
corresponding to the first run has a sigmoidal form, indicating
that the rate of formation of benzene is slowly increased with
the reaction time, as a consequence of the gradual activation of
the catalyst. Run 2 needs 12 h to complete, and runs 3 to 8
display quasi-identical profiles, achieving completion in about 3
h. The similarities of the reaction profiles displayed for runs 3−
8 are a very good indication that the catalyst deactivation is
negligible.
CONCLUSIONS
■
In summary, we have obtained a very effective catalyst for the
hydrodefluorination of aromatic fluoroarenes. The catalyst is
formed by a mixture of palladium and ruthenium complexes
with a pyrene-NHC ligand, which are grafted on the surface of
reduced-graphene oxide, by π-stacking interactions. The
analysis of the catalyst before and after being used in the
catalytic reaction indicates that small nanoparticles presumably
formed by palladium aggregates, which together with the
ruthenium-heterogenized complex may form the active catalytic
system. The catalyst can be recycled up to 12 runs, without any
decrease of activity and affording the quantitative hydro-
defluorination of fluorobenzene. The reaction profiles of the
process clearly show that the rate of hydrodefluorination slowly
increases with the reaction time during the first run, in a clear
indication that the catalyst needs an incubation time for its full
activation. As inferred by the analysis of the solids after being
used in the catalytic reaction, the activation consists of the
formation of palladium nanoparticles; thus the catalytic hybrid
system consists of a combination of palladium nanoparticles
and the ruthenium molecular compound. The resulting catalyst
can be recycled up to 12 times in the hydrodefluorination of
fluorobenzene, with minimum loss of activity, thus showing
great potential practical applications. We strongly believe that
the simplicity of the design of this heterogeneous catalytic
system, in combination with its great catalytic performance and
recyclability, may inspire future research in the field of
hydrodefluorination.
Figure 2. Recycling experiments of hydrodefluorination of fluo-
robenzene in the presence of 3. The reactions were carried out in
iPrOH in the presence of tBuONa, at 80 °C. After each run (3 h), the
catalyst was filtered, washed with CH2Cl2, and dried. Run number 13
was carried out under air and using technical grade iPrOH.
We also performed an analysis of the reaction profiles of the
hydrodefluorination of fluorobenzene using catalyst 3. The
time-course reaction profiles are shown in Figure 3 and are
displayed for eight successive cycles. The reactions were
monitored by 19F NMR spectroscopy. We were interested in
observing the activation process of the catalyst, and thus we
started the experiments using the unactivated version of 3. As
can be seen from the profiles shown in Figure 3, the curve
EXPERIMENTAL SECTION
■
General Procedures. All manipulations were carried out under
nitrogen using standard Schlenk techniques and high vacuum, unless
otherwise stated. Anhydrous solvents were obtained using a solvent
purification system (SPS MBraun) or purchased from commercial
suppliers and degassed prior to use by purging with dry nitrogen and
kept over molecular sieves. Complexes 1 and 2 were obtained
according to published methods.5 Graphitic oxide (GO) was prepared
from graphite powder (natural, universal grade, 200 mesh, 99.9995%)
by the Hummers method,13 and reduced graphene oxide was prepared
by reduction of a colloidal suspension of exfoliated GO sheets in water
with hydrazine hydrate.14 All other reagents were used as received
from commercial suppliers. NMR spectra were recorded on Varian
Innova spectrometers operating at 300 or 500 MHz (1H NMR) and
75 and 125 MHz (13C NMR), respectively. Elemental analyses were
carried out in an EA 1108 CHNS-O Carlo Erba analyzer. Infrared
spectra (FTIR) were recorded on a JASCO FTIR-6200 spectrometer
with a spectral window of 4000−500 cm−1. Atomic force microscopy
measurements were carried out with a JSPM-5200 JEOL scanning
probe microscope using contact mode. High-resolution transmission
electron microscopy images and high-angle annular dark-field
(HAADF)-STEM images of the samples were obtained using a Jem-
2100 LaB6 (JEOL) transmission electron microscope coupled with an
INCA Energy TEM 200 (Oxford) energy dispersive X-ray
spectrometer operating at 200 kV. The UV/vis spectra were recorded
between 200 and 600 nm by a Cary 300 Bio UV/vis Varian
spectrophotometer. The determination of the metal content in the
Figure 3. Reaction profiles for eight successive cycles of the
hydrodefluorination of fluorobenzene with 3. The reactions were
carried out in iPrOH in the presence of tBuONa, at 80 °C. After each
run, the catalyst was filtered, washed with CH2Cl2, and dried. The
monitoring of the reactions was carried out by 19F NMR spectroscopy.
D
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solids was done by an ICP-MS Agilent 7500 CX. The digestion of the
samples was carried out under reflux of a mixture of concentrated
nitric and hydrochloric acids (3:1) for 12 h. The digestion of the
samples after the recycling experiments was carried out after several
washes with milli-Q water (10 × 10 mL).
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introduced 270 mg of rGO, 30 mg of an equimolecular mixture of
metal complexes 1 (15.2 mg, 0.025 mmol) and 2 (14.8 mg, 0.025
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1
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experiment the catalyst (1 mol % based on each metal) and tBuONa
(0.3 mmol) were placed together in a thick-walled Schlenk tube with a
Teflon cap. The tube was then evacuated and filled with nitrogen three
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times. 2-Propanol (2 mL) and fluoroarene (0.3 mmol) were added.
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following run.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Full characterization of hybrid materials including UV/vis and
FTIR spectroscopy, HRTEM, and AFM images. This material
is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
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Corresponding Authors
*E-mail: jmata@uji.es. Fax: (+34) 964387522. Tel: (+34)
964387516.
(14) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.;
Kleinhammes, A.; Jia, Y.; Wu, Y.; Nguyen, S. T.; Ruoff, R. S. Carbon
2007, 45, 1558−1565.
*E-mail: eperis@uji.es.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the financial support from the Ministerio de Ciencia
e Innovacion
́
of Spain (CTQ2011-24055/BQU). We would
also like to thank the “Generalitat Valenciana” for a fellowship
(S.S.). The authors are grateful to the “Serveis Centrals
d’Instrumentacio
for providing spectroscopic X-ray facilities.
́
́
Cientifica (SCIC)” of the Universitat Jaume I
E
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