Catalyst enhancement and recyclability by immobilization of metal complexes onto graphene surface by noncovalent interactions

Article abstract of DOI:10.1021/cs5003959

The immobilization of a homogeneous catalyst onto a solid surface is one of the major challenges in catalysis, because it may facilitate the separation of the catalyst and the reaction products and may also give rise to the reutilization of the catalyst in multiple subsequent cycles. Noncovalent interactions between the catalyst and the support are arising as interesting alternatives to the more widely used covalent interactions, because they avoid the functionalization of both the catalyst and the surface, which may in turn lead to the modification of the inherent properties of the catalyst. However, some other problems may arise, such as leaching. In this work, we have obtained two complexes containing an N-heterocyclic carbene ligand with a pyrene tag, which we immobilized onto the surface of reduced graphene oxide (rGO), by π-stacking. The catalytic properties of the parent molecular complexes and hybrid materials have been studied in the palladium-catalyzed hydrogenation of alkenes and the ruthenium-catalyzed alcohol oxidation. The results show that the catalytic properties are improved in the hybrid materials, compared to the catalytic outcomes provided by the homogeneous analogues. Although the palladium-catalyzed reactions may be due to the formation of Pd nanoparticles, the ruthenium-catalyzed ones are facilitated by the supported molecular catalyst. The catalyst stability was analyzed by means of recyclability studies, hot filtration test, and large-scale experiments. Both hybrid materials have been reused up to 10 times without any decrease in activity, affording quantitative yield of products. The hot filtration experiment reveals that the catalysis is heterogeneous in nature without any detectable leaching or boomerang effect. The work constitutes a clear improvement over other known immobilization methodologies and offers a practical methodology which may inspire future developments of efficient heterogenized catalysts.

Full text of DOI:10.1021/cs5003959

Research Article
pubs.acs.org/acscatalysis
Catalyst Enhancement and Recyclability by Immobilization of Metal
Complexes onto Graphene Surface by Noncovalent Interactions
Sara Sabater, Jose A. Mata,* and Eduardo Peris*
Departamento de Química Inorgan
Spain
́ ́ ́ ́
ica y Organica, Universitat Jaume I, Avda. Sos Baynat s/n, 12071 Castellon de la Plana, Castellon,
*
S Supporting Information
ABSTRACT: The immobilization of a homogeneous catalyst
onto a solid surface is one of the major challenges in catalysis,
because it may facilitate the separation of the catalyst and the
reaction products and may also give rise to the reutilization of
the catalyst in multiple subsequent cycles. Noncovalent
interactions between the catalyst and the support are arising
as interesting alternatives to the more widely used covalent
interactions, because they avoid the functionalization of both
the catalyst and the surface, which may in turn lead to the
modification of the inherent properties of the catalyst.
However, some other problems may arise, such as leaching. In this work, we have obtained two complexes containing an N-
heterocyclic carbene ligand with a pyrene tag, which we immobilized onto the surface of reduced graphene oxide (rGO), by π-
stacking. The catalytic properties of the parent molecular complexes and hybrid materials have been studied in the palladium-
catalyzed hydrogenation of alkenes and the ruthenium-catalyzed alcohol oxidation. The results show that the catalytic properties
are improved in the hybrid materials, compared to the catalytic outcomes provided by the homogeneous analogues. Although the
palladium-catalyzed reactions may be due to the formation of Pd nanoparticles, the ruthenium-catalyzed ones are facilitated by
the supported molecular catalyst. The catalyst stability was analyzed by means of recyclability studies, hot filtration test, and large-
scale experiments. Both hybrid materials have been reused up to 10 times without any decrease in activity, affording quantitative
yield of products. The hot filtration experiment reveals that the catalysis is heterogeneous in nature without any detectable
leaching or boomerang effect. The work constitutes a clear improvement over other known immobilization methodologies and
offers a practical methodology which may inspire future developments of efficient heterogenized catalysts.
KEYWORDS: hydrogenation, alcohol oxidation, graphene, ruthenium, palladium, N-heterocyclic carbenes
INTRODUCTION
bond between the support and the catalyst. As an alternative,
noncovalent interactions are emerging as a powerful tool that
gives rise to supported catalysts without chemical modification
of the homogeneous catalyst, although the stability against
leaching may become a problem that needs to be avoided by
the careful selection of the reaction conditions, immobilization
■
Despite the attractive properties of many homogeneous
catalysts in terms of activity and selectivity, they suffer from
difficulties associated with separating the products from the
1
catalyst, an important drawback that causes industries to be
relatively reluctant to use them for the large-scale production of
materials. Another important drawback for the industrial use of
homogeneous catalysts concerns catalyst recycling, for which an
enormous effort in the immobilization of known catalysts onto
2d
method, nature of the support, and catalytic complex.
Graphene and derivatives have attracted increasing attention3
because of their unique physical and chemical properties,
which make them particularly interesting in the fields of
2
solid supports is currently being carried out. The main goal in
4
nanochemistry and catalysis. Chemically derived graphenes
recycling a catalyst refers to achieving consistent high yields in
repeated runs that confirm the robustness of the supported
catalytic system, a necessary requirement for long-term
applications. Undoubtedly, the most widely used method for
the immobilization of a homogeneous catalyst is the formation
of a covalent bond between the solid support and the ligand.
This general method has two important drawbacks: first, arising
from the need for an additional functionalization of the catalyst
ligand and solid surface (which in turn implies an increase in
the catalyst preparation cost) and second, arising from the
possible changes in chemical reactivity (and therefore in
catalytic activity) derived from the formation of the covalent
(
CDGs) such as graphene oxide (GO) and reduced graphene
oxide (rGO), are ideal candidates for immobilization due to
their inertness, large surface area, stability, and availability. In
fact, graphene derivatives have recently been used as supports5
for the covalent immobilization of homogeneous catalysts.
Together with their inherent properties, graphene and
derivatives offer a unique opportunity to noncovalent
Received: March 25, 2014
Revised: May 13, 2014
©
XXXX American Chemical Society
2038
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Scheme 1. Synthesis of Hybrid Materials
modifications by π-stacking interactions with molecules
Ru(II) complex 2 was obtained starting from the imidazolium
salt A by transmetallating the NHC from the corresponding
Ag-NHC complex, which was generated and used in situ by
6
containing polycyclic aromatic hydrocarbons, from which7
pyrene-functionalized systems constitute promising examples.
Also, and very important is the fact that the immobilization of
polyaromatic hydrocarbons onto rGO does not depend on the
reaction of the imidazolium salt with Ag O. Both the palladium
2
and ruthenium complexes are air-stable in the solid state and in
solution. Complexes 1 and 2 were characterized by NMR
spectroscopy, mass spectrometry, and elemental analysis. The
7
a
pH of the solution. From these basic ideas, it becomes rather
obvious that one potentially effective form for catalyst
heterogenization would be the preparation of pyrene-
containing catalysts, which could be noncovalently supported
onto a graphene surface. Pyrene-containing metal complexes
have been already supported onto graphene derivatives, proving
to be a versatile way of functionalization of nanostructured
1
H NMR spectra of complexes 1 and 2 confirm the
disappearance of the acidic signal of the imidazolium salt at
9
.21 ppm, suggesting a carbene-type bonding of the
imidazolylidene ligand. A more concluding evidence is obtained
13
from the C NMR, which shows the characteristic signals of
8
graphitic materials, and they have even been used in catalysis
the M-Ccarbene at 172.9 ppm for 1 and 174.8 ppm for 2. Positive
9
proving interesting recyclability properties. The noncovalent
ion electrospray mass spectra analysis (ESI-MS) for complexes
interaction between the pyrene and rGO is based on van der
+
1
=
and 2 in MeCN showed an intense peak for [M − X] at m/z
Waals forces (π-stacking), and it is controlled by thermody-
536.3 and m/z = 567.2, respectively, which confirm the
1
0
namics. This type of rGO functionalization is particularly
attractive, because it offers the possibility of attaching metal
complex without affecting the electronic network. For the
choice of the suitable catalyst, N-heterocyclic carbene-based
complexes seem to be good candidates, due to the σ-donor
capacity of NHC ligands and the stability of the metal
complexes derived that make them suitable to operate under
harsh reaction conditions. On the basis of all these arguments,
we now describe the synthesis of an imidazolium salt with a
pyrene tag that allows the preparation of palladium and
ruthenium complexes that have been immobilized onto the
rGO surface by π-stacking interactions. We have studied the
catalytic properties of these new hybrid materials in the
ruthenium-catalyzed oxidation of alcohols and in the palladium-
mediated hydrogenation of a wide set of organic molecules. In
both reactions, we were glad to observe that the supported
catalysts displayed higher activities than the homogeneous
related complexes, and the heterogeneized catalysts were
recycled 10 times without loss of activity.
proposed molecular composition of the complexes based on the
mass/charge relation and the isotopic pattern (see Supporting
Information for details).
The molecular structure of compound 1 was confirmed by
means of X-ray diffractometry. The structure analysis reveals a
palladium center with a bromide, the NHC ligand, and the
chelating bound N,N-dimethylbenzylamine in a square-planar
coordination environment (Figure 1). The C−N orthometa-
lated amine forms a five-membered ring that adopts an
11
envelope conformation. The Pd−C
distance measures
carbene
1
.970(8) Å. The packing diagram of complex 1 shows an
intermolecular slipped π-stacking between pairs of molecules
interplanar distance 3.6 Å) indicating the tendency of the
(
12
pyrene moiety to display this type of noncovalent interaction.
Complexes 1 and 2 were supported onto reduced graphene
oxide (rGO) by mixing the molecular complexes and the rGO
material in dichloromethane in an ultrasound bath for 30 min
and stirring for 10 h. The first evidence that molecular
complexes 1 and 2 were anchored over the surface of rGO
arises from the disappearance of the color of the solution. The
yellow solution of complex 1 in dichloromethane disappears as
the complex is retained on the rGO. The palladium-containing
material, NHC-Pd-rGO (3), was filtered and washed with
RESULTS AND DISCUSSION
■
The imidazolium salt A was obtained by alkylation of methyl-
imidazole using commercially available 1-(bromomethyl)-
pyrene. The reaction of A with the palladacycle dimer
1
dichloromethane. The H NMR spectrum of the filtrate
[
Pd(dmba)OAc] (dmba = cyclo-orthometalated N,N-dimethyl
revealed the absence of any trace of 1 in the solution and
therefore provided us with the first evidence of the quantitative
deposition of 1 onto rGO. The exact palladium content on 3
2
benzylamine) in the presence of KBr affords the corresponding
NHC-Pd(II) complex 1 in 60% yield (Scheme 1). The NHC-
2
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to the weak signal and overlapping of absorption by other
groups present in the support (Figures S3 and S4). On the
other hand, UV−vis measurements (Figures S1 and S2) reveal
that the spectra of the hybrid materials consist of the overlap of
the molecular complexes (1 or 2) with rGO, clearly confirming
the presence of the complex on the rGO, as seen by the pyrene
9
b,14
UV−vis characteristic signals.
The Raman spectrum of rGO
−1
shows the two prominent peaks at 1350 and 1592 cm , which
are attributed to D and G bands, respectively (Figure S5).
The incorporation of the NHC-Pd and NHC-Ru complexes
15
−1
show a slight blue shift of the G band (1594 cm , for both 3
and 4), which may be attributed to the π-stacking of the pyrene
1
6
fragment onto the rGO layer. The analysis of the surface area
of both materials was performed by argon sorption analysis at
2
−1
7
7 K. The BET area for 3 and 4 was 262 and 238 m g ,
respectively, significantly lower than that shown by the rGO
2
−1
raw material (456 m g ).
The elemental mapping by energy-dispersive X-ray spectro-
scopic analysis (EDS) performed by means of HRTEM of 3
and 4, confirmed the homogeneous distribution of palladium
and ruthenium in the two hybrid materials (Figure 2).
In order to study the catalytic activity of the hybrid materials
NHC-Pd-rGO (3) and NHC-Ru-rGO (4), we decided to test
the materials in two benchmark reactions typically catalyzed by
Pd and Ru. The palladium-containing material, 3, was tested in
the hydrogenation of unsaturated organic substrates using
1
7
molecular hydrogen, whereas the ruthenium-containing
18
material, 4, was tested in the dehydrogenation of alcohols.
The palladium-catalyzed reduction of alkenes was performed in
toluene in the presence of Cs CO at 100 °C using H (1 atm)
2
3
2
with a catalyst loading of 1 mol % based on the amount of
metal (ICP-MS analysis). As can be seen by the results shown
in Table 1, the palladium hybrid material 3 was highly effective
for a wide variety of substrates. Because the inherent catalytic
properties of GO and rGO have been studied in a variety of
Figure 1. Molecular (top) and packing (down) diagrams showing the
π-stacking of complex 1. Ellipsoids at 50% probability level. Hydrogens
omitted for clarity. Selected bond lengths (Å) and angles (°): Pd(1)−
C(2) 1.978(3), Pd(1)−C(9) 2.002(3), Pd(1)−Br(19) 2.5435(7),
Pd(1)−N(16) 2.138(3), C(2)−N(7) 1.343(5), N(7)−C(8) 1.471(4),
Br(19)−Pd(1)−C(2) 92.99(8), Br(19)−Pd(1)−N(16) 93.25(7),
N(16)−Pd(1)−C(9) 83.00(11), C(9)−Pd(1)−C(2) 90.77(12),
Br(19)−Pd(1)−C(2)−N(3) 67.5. The π-stacking interplanar distance
measures 3.6 Å.
4a,19
processes,
control experiments using the rGO under exactly
the same reaction conditions were carried out, showing that the
palladium-free material was completely ineffective (entry 1).
For the reaction carried out using complex 1, we obtained a
moderate yield of ethylbenzene, whereas 3 produced a
quantitative yield under the same reaction conditions (compare
entries 2 and 5). These results suggest that the support
provides a significant benefit in the catalytic hydrogenation
reaction, most probably due to the surface area of rGO, which
may facilitate the interaction between the substrates and the
was determined by digestion of the samples in hot HCl/HNO3
followed by ICP-MS analysis. The results obtained accounted
for a 9.3 wt % of 1 in 3. The ruthenium complex 2 was
anchored over the rGO using the same procedure, and the ICP-
MS analysis accounted for a 9.0 wt % of complex in the hybrid
material NHC-Ru-rGO (4).We did not observe desorption of
the compounds 1 or 2 from the rGO when changing the
solvent polarity or by increasing the temperature in the catalytic
experiments carried out (vide infra). In order to confirm that
the pyrene fragment is responsible for the grafting of the
molecular complexes on the graphene surface, we performed a
control experiment, in which we applied the same grafting
20
catalyst. This “abnormal” enhancement in the activity of the
immobilized catalyst onto the graphene material has already
been observed before for the case of the hydrogenation of
cyclohexene facilitated by a rhodium complex covalently
5
supported onto graphene oxide. The NHC-Pd-rGO material,
3, is also very active in the hydrogenation of other terminal and
internal alkenes (entries 6−10), also including nonaromatic
ones (entry 10). The ICP-MS analysis after the experiment
carried out with trans-stilbene as substrate (entry 8) accounts
for 9.0 wt % of palladium, indicating negligible palladium
leaching.
In order to determine if the process is homogeneously
catalyzed by the molecular palladium complex, we performed
the mercury-drop experiment for the reduction of styrene
catalyzed by 1, and we observed that the reaction stopped upon
addition of mercury (compare entries 2, 3, and 4, Table 1). It
has to be taken into account that for this test to be effective, the
addition of mercury has to be done after the catalyst has been
experiment using the pyrene-free complex [RuCl (InBu)(p-
2
1
3
cymene)] (InBu = 1,3-di-n-butylimidazolylidene), and we
observed that, under exactly the same conditions, the molecular
complex quantitatively remained in the solution, thus
confirming that the pyrene fragment is responsible for the
adsorption of the complex to the surface of the solid.
The characterization of the hybrid materials 3 and 4 was
performed using UV−vis, FTIR, Raman, SEM, and HRTEM
(
see Supporting Information for details). The FTIR spectra of
NHC-Pd-rGO (3) and NHC-Ru-rGO (4) displayed hardly
evident additional peaks compared to that shown by rGO, due
2
040
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Figure 2. STEM images of NHC-Pd-rGO (3) (a), NHC-Ru-rGO (4) (c), and EDS elemental mapping image showing the homogeneous
distribution of palladium (b) and ruthenium (d).
activated, and therefore the drop was added after 15 min of
reaction. This result suggests that the reaction may be
heterogeneously rather than homogeneously catalyzed and
should imply that palladium nanoparticles are produced along
The oxidant-free oxidation of alcohols is a highly attractive
process for the synthesis of carbonyl species and for the
generation of molecular hydrogen from easily available
25
alcohols. Metal-based ruthenium catalysts have been widely
26
21
used for this process. The reactions were carried out in
toluene, under aerobic conditions, Cs CO , and a catalyst
the reaction course. In order to confirm this point, we also
analyzed by HRTEM the NHC-Pd-rGO material, 3, after being
used in the reduction of styrene (Table 1, entry 5), and we
observed a homogeneous distribution of palladium along the
sample, together with the formation of small amounts of
palladium nanoparticles. Very recent examples of NHC-based
complexes covalently immobilized onto graphene oxide (GO)
have also resulted in decomposition of the parent original
complexes onto palladium nanoparticles, although the catalytic
performances were maintained, and good recyclability could be
2
3
loading of 2 mol % based on metal amount (Table 3). The
optimization of the reaction was carried out using 1-phenyl
ethanol (entries 1−3) and benzyl alcohol (entries 4−7) as
model substrates. Blank experiments on both substrates using
rGO under the same reaction conditions, revealed that no
oxidation occurs (entries 1 and 4). Oxidation of 1-phenyl
ethanol and benzyl alcohol under homogeneous conditions
using the molecular catalyst 2 afforded low yields of
acetophenone (26%) and moderate yields of benzaldehyde
2
2
achieved.
(
47%) (entries 2 and 5, respectively). When the oxidation
Catalyst 3 was also tested in the reduction of nitroarenes, a
very interesting process due to the applications of function-
alized anilines in the synthesis of pharmaceuticals and fine
reaction was carried out using the hybrid material NHC-Ru-
rGO (4), the acetophenone yield increased up to 60% and
>
99% for the case of benzaldehyde (entries 3 and 7). These
2
3
chemicals. The results show that 3 is also an efficient catalyst
for the reduction of nitroarenes in short times and low
hydrogen pressures (Table 2). For this reaction, we also
observed that rGO is completely inactive (entry 1), and that the
molecular complex 1 was less active than the supported
counterpart (entries 2−3), therefore suggesting that the rGO
plays a positive role in the overall catalytic cycle, as we observed
before. In a very recent work, palladium nanoparticles have also
been supported onto graphene supports, by reduction of Pd(II)
precursors, and the resulting materials have shown excellent
results suggest that immobilization of the molecular catalyst 2
on rGO provides a benefit in the catalytic process as previously
observed in the case of the hydrogenation processes. Catalyst 4
was very effective in the oxidation of a wide set of benzyl
alcohols, affording quantitative yields, regardless of the presence
of electron-accepting or electron-donating substituents in the
substrate (entries 8−11). For this process, we confirmed that
the molecular complex 2 was the effective catalyst in the
oxidation of benzyl alcohol, because the mercury-drop
experiment resulted in no deactivation of the process, therefore
suggesting that the reaction is homogeneously catalyzed
2
4
activities in the reduction of alkenes and nitroarenes.
2
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[
a]
Table 1. Alkene Hydrogenation Using Molecular
Hydrogen
Table 2. Nitro Reduction Using Molecular Hydrogen
[
a]
[b]
entry
catalyst
R
yield (%)
1
2
3
4
5
rGO
H
H
0
50
1
NHC-Pd-rGO (3)
NHC-Pd-rGO (3)
NHC-Pd-rGO (3)
H
100
100
CH3
[c]
CH O
100
3
[
a]
Reactions were carried out with 0.3 mmol of nitro compound,
Cs CO (0.3 mmol), H (1 atm), catalyst (2 mol %), 5 mL of toluene
2
3
2
[
b]
for 2 h at 100 °C. Yields determined by GC analyses using anisole
as internal standard. Isolated yield.
[
c]
[
a]
Table 3. Oxidant-Free Dehydrogenation of Alcohols
[b]
entry
catalyst
R1
R2
yield (%)
1
2
3
4
5
rGO
2
CH3
CH3
CH3
H
H
H
0
26
NHC-Ru-rGO (4)
rGO
H
60
H
0
2
H
H
47
[
c]
6
2
H
H
49
7
8
9
NHC-Ru-rGO (4)
NHC-Ru-rGO (4)
NHC-Ru-rGO (4)
NHC-Ru-rGO (4)
NHC-Ru-rGO (4)
H
H
100
100
100
100
100
H
Cl
H
NO2
OMe
CH3
1
0
1
H
1
H
[
a]
Reactions were carried out with 0.3 mmol of alcohol, Cs CO (0.3
2
3
[
b]
mmol), catalyst (2 mol %), 5 mL of toluene for 12 h at 100 °C.
Yields determined by GC analyses using anisole as internal standard.
[
c]
The reaction was allowed to proceed during 1 h, and then a
mercury drop was added. The final yield is obtained after 11 more
hours of reaction.
(
Figure 3). The reaction conditions used in these experiments
were the same as those described in the hydrogenation
reactions, and the reaction progress was monitored by GC.
After completion of each run (15 min), the reaction mixture
was allowed to reach room temperature, and the solid catalyst
was separated by centrifugation, washed with CH Cl , dried,
[
a]
Reactions were carried out with 0.3 mmol of alkene, Cs CO (0.3
2
3
mmol), H (1 atm), catalyst (1 mol %), 5 mL of toluene for 15 min at
2
[
b]
2
2
1
00 °C. Yields determined by GC analyses using anisole as internal
[
c] [d] [e]
standard. Isolated yield. Data after 30 min of reaction. The
reaction was allowed to proceed during 15 min, and then a mercury
drop was added. The final yield is obtained after 15 more minutes of
reaction.
(
entries 5−6). The HRTEM analysis of the catalyst NHC-Ru-
rGO (4), after being used in the oxidation of benzyl-alcohol,
revealed the absence of Ru-based nanoparticles in the solid,
therefore confirming that the nature of the catalytic species for
this system should be molecular.
In view of the good results obtained with the hybrid materials
in catalysis, we decided to explore the catalyst stability by
exploring their recyclability and activity in large-scale experi-
ments. Recycling experiments of the palladium-containing
catalyst 3 were carried out using styrene as model substrate
Figure 3. Recycling experiment of styrene hydrogenation using NHC-
Pd-rGO (3).
2
042
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2
7
and reused in the subsequent run. Interestingly, the base was
only needed for the first run. This observation may be due to
the transformation of the complex into the active catalytic
species during the first run or simply to the adsorption of
residual Cs CO to the rGO surface, which avoided the need of
further addition in the subsequent experiments. EDS analysis of
NHC-Pd-rGO (3) after the 10th run revealed the presence of
homogeneously distributed Cs, which may suggest that some
base remains attached to the surface of the solid (Figure S13).
Following this methodology, the palladium-containing catalyst
catalyst is saturated with the substrate, and therefore suggests
that in our case there is probably some interaction between the
substrate and the support.
The recycling experiments for the ruthenium-containing
catalyst 4, were performed by using benzyl alcohol as model
substrate (Figure 5). The reactions were carried out using a
2
3
3
was reused up to 10 times without any decrease in activity,
affording ethylbenzene in quantitative yield for each of the 10
runs. The ICP-MS analysis after the recycling experiments
accounts for 8.5 wt % of palladium, indicating a total loss of 0.8
wt %, and thus 8.6% of the palladium is lost after 10 catalytic
cycles. However, this small loss of percent of metal content may
be due to the mass gain during the subsequent cycles due to the
adsorption of substances from the catalyst reactions (we already
mentioned that Cs CO is adsorbed), so it may not be due to
2
3
desorption or leaching of the metal complex. After the 10th
experiment, a SEM image of the NHC-Pd-rGO (3) revealed
that the morphology of rGO is maintained (Figure S6−S7). A
deeper analysis by HRTEM showed that there is a well-
dispersed palladium distribution, although the presence of some
small (<8 nm diameter) palladium nanoparticles (Figure S12−
S14) was detected.
Figure 5. Recycling experiment of benzyl alcohol oxidation using
NHC-Ru-rGO (4).
catalyst load of 2 mol %, at 100 °C and in the presence of
Cs CO (only in the first run). The mixture was allowed to
The high activity of catalyst 3 was further analyzed in a large-
scale experiment. The reaction was carried out using 2 mL of
styrene (17.3 mmol) and a catalyst load of 0.01 mol % based on
2
3
react for 12h for each run. Then the catalyst was separated from
the reaction mixture by centrifugation, washed, dried and
replaced in the reaction vessel to be used in the subsequent run.
As stated above, the fact that the base was only needed in the
first cycle may indicate that either once the precatalyst has been
transformed into its active form and that the active species is
stable for recyclability during the subsequent runs, or that the
base is adsorbed onto the solid support (as indicated by the
detection of homogeneously distributed Cs by EDS analysis of
the solid after the 10th run and therefore no further base is
needed for the subsequent runs (Figure S13). Catalyst 4 was
reused up to ten times without any decrease in activity,
affording quantitative yields of benzaldehyde. The ICP-MS
analysis after the recycling experiments accounts for 7.7 wt % of
ruthenium, indicating that a total loss of 1.3 wt % and thus
−
3
palladium (1.74 × 10 mmol). The reaction achieved
quantitative yield after 20h, therefore achieving a maximum
TON of 10.000. A recyclability study using these conditions
reveals that the system is active at least for three runs without
decrease in activity (Figure 4). The monitoring of the reaction
1
4.4% of ruthenium is lost. However, as mentioned for the
palladium supported catalyst, it is possible that this loss of
percent of metal content is due to the mass gain during the
subsequent cycles, rather than to leaching of the metal complex.
The comparison of the SEM and HRTEM analysis of 4 before
and after the recycling experiments revealed the same well-
dispersed ruthenium distribution, without modification of the
support morphology and the absence of ruthenium nano-
particles (Figures S8 − S9 and S15 − S17).
Figure 4. Recycling experiment of styrene hydrogenation using NHC-
Pd-rGO (3) at a catalyst load of 0.01 mol %.
The large-scale experiment was carried out using 1 mL of
benzyl alcohol (9.7 mmol) and a catalyst loading of 0.03 mol %
based on ruthenium (3.0 × 10− mmol). The reaction yielded
75% of benzaldehyde after 24 h. As can be seen in Figure 6,
NHC-Ru-rGO (4) could be reused up to three times, and the
reaction profiles where identical in the first two cycles, therefore
suggesting that the deactivation of the catalyst is negligible. In
the third cycle, a slight decrease in catalyst activity is observed,
and the reaction afforded a 65% yield in 24 h. This experiment
is very interesting, because the reaction conditions used for this
catalytic experiment show that the catalyst is using its full
potential to achieve a 75% yield in the first cycle, therefore any
3
progress by GC indicates that the activity of catalyst 3 is
maintained along the three consecutive runs (the accumulated
TON is 30.000), therefore revealing that there is no trace of
catalyst deactivation. A very interesting feature that is worth
mentioning after evaluating these experiments, is the zero order
dependence of the rate on the concentration of the substrate
(
styrene), as can be observed from the linear plots shown in
Figure 4. Zero-order dependence on the concentration of the
substrate is usually observed in heterogeneous hydrogenations
of olefins and is interpreted to indicate that the surface of the
2
043
dx.doi.org/10.1021/cs5003959 | ACS Catal. 2014, 4, 2038−2047

ACS Catalysis
Research Article
complexes and hybrid materials have been studied in the
hydrogenation of alkenes and alcohol oxidation. The results
show that the catalytic properties are improved in the hybrid
materials, compared to the catalytic outcomes provided by the
homogeneous analogues. The catalyst stability was analyzed by
recyclability, hot filtration, and large-scale experiments.
Although the activity of the palladium catalyst may be ascribed
to the formation of palladium nanoparticles, we confirmed the
molecular nature of the ruthenium-based catalysts, therefore
constituting a unique example of a molecular catalyst
noncavalently supported onto a graphene derivative. Both
catalysts 3 and 4 were reused up to 10 times without any
decrease in activity affording quantitative yield of products.
Probably more revealing is the fact that the catalysts used in
large-scale experiments could be recycled, with catalyst loadings
of 0.01 and 0.03 mol % for 3 and 4, respectively. Even under
these reaction conditions, the catalysts could be reused up to
three times without loss of activity and display the same
reaction profiles, therefore indicating that catalyst deactivation
is negligible. The hot filtration experiment reveals that the
catalysis is heterogeneous in nature without any detectable
leaching or boomerang effect.
Figure 6. Recycling experiment of benzyl alcohol oxidation using
NHC-Ru-rGO (4) at a catalyst load of 0.03 mol %.
minimum loss of catalyst or any deactivation should have
measurable consequences in the recycling of the catalyst. Again,
as can be observed from the linear plots depicted in Figure 5,
the reaction shows quasi-zero-order dependence of the rate on
the concentration of the substrate (benzyl alcohol), thus
suggesting surface saturation due to the interaction between the
substrate and the support.
We strongly believe that the simplicity of the design of these
heterogeneous catalytic systems, in combination with its great
catalytic performance and recyclability, may inspire future
research in the field of catalyst heterogeneization. To the best
of our knowledge, this is the first example of a supported NHC-
molecular catalyst onto a graphene derivative, and we
demonstrated its great potential in practical applications.
In a parallel experiment, and in order to prove that the
reaction was heterogeneously catalyzed, we performed a hot
filtration experiment. This experiment was carried out using the
general conditions described in Table 3 using benzyl alcohol as
a substrate. After 6 h (GC yield 48%), the catalyst was filtered
off at 100 °C. The filtrate was allowed to stir for 16 h under
identical conditions, but GC analysis indicated that no further
oxidation occurred (GC final yield is maintained in 48%). This
experiment confirmed the absence of catalytic active species in
solution due to ruthenium leaching or desorption of the
EXPERIMENTAL SECTION
■
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 M BRAUN) or purchased from
commercial suppliers and degassed prior to use by purging with
dry nitrogen and kept over molecular sieves. Reduced graphene
oxide and the other reagents were used as received from
commercial suppliers. NMR spectra were recorded on Varian
28
molecular catalyst by the boomerang effect. The boomerang
effect consists of a release-and-return of the molecular complex
from the support, therefore implying that the catalytic active
29
1
species may be homogeneous in nature. This observation
contrasts with previously published results regarding the π-
stacking between pyrene and carbon nanotubes, which is
Innova spectrometers operating at 300 or 500 MHz ( H NMR)
13
and 75 and 125 MHz ( C NMR), respectively. Elemental
analyses were carried out in an EA 1108 CHNS-O Carlo Erba
analyzer. Electrospray mass spectra (ESI-MS) were recorded on
a MicromassQuatro LC instrument, and nitrogen was
employed as drying and nebulizing gas. Raman spectra were
acquired on a JASCO NRS-3100 dispersive spectrometer.
Infrared spectra (FTIR) were recorded on a JASCO FTIR-6200
9
b,30
affected by the polarity of the solvent and by temperature.
For pyrene-containing complexes grafted onto graphene, it has
also been published that desorption is temperature-dependent
9
when polar organic solvents or water are used and that
leaching is reduced when the metal complex is decorated with
8
b
−1
three pyrene tags. In our case, and probably unexpectedly, we
did not observe desorption of the molecular complexes 1 or 2
in polar or nonpolar solvents such us dichloromethane,
acetone, toluene, or i-propanol.
spectrometer with a spectral window of 4000−500 cm .
Scanning electron micrographs were taken with a field emission
gun scanning electron microscope (FEG-SEM) model JEOL
7001F, equipped with a spectrometer of energy dispersion of X-
ray (EDX) from Oxford instruments. High-resolution trans-
mission electron microscopy images (HRTEM) 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 (EDX) operating at 200
kV. The UV−vis spectra were recorded between 200 and 800
nm by a Cary 300 Bio UV−vis Varian spectrophotometer.
The determination of the metal loading was done by 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
CONCLUSIONS
■
In summary, we have prepared palladium and ruthenium
molecular complexes containing an N-heterocyclic carbene
ligand with a pyrene tag. The two complexes have been
anchored on the surface of rGO by noncovalent interactions
under mild reaction conditions yielding the hybrid materials
NHC-Pd-rGO (3) and NHC-Ru-rGO (4). This modular
methodology allows incorporation of well-defined molecular
complexes onto chemical derived graphenes (CDGs), where
the properties of the complex are not altered by the effect of the
immobilization. The catalytic properties of the molecular
2
044
dx.doi.org/10.1021/cs5003959 | ACS Catal. 2014, 4, 2038−2047

ACS Catalysis
Research Article
3
after the recycling experiments were carried out after several
washes with milli-Q water (10 × 10 mL).
Synthesis of Imidazolium Salt A. 1-(Bromomethyl)-
pyrene (1 g, 3.39 mmol) and 1-methyl imidazole (0.28 mL,
9.3 Hz, 1H, CH ), 8.27−8.00 (m, 7H, CH ), 7.70 (d, J
=
H,H
Ar
Ar
3
8.0 Hz, 1H, CH ), 7.04 (d, J
= 1.9 Hz, 1H, CHimidazole),
Ar
H,H
3
6.92 (d, J = 1.8 Hz, 1H, CHimidazole), 6.75−6.33 (broad s,
H,H
3
2H, NCH −), 5.26 (broad s, 2H, CH
), 4.89 (d, J = 5.9
H,H
2
p‑cym
3
.39 mmol) were refluxed under nitrogen in dry THF (10 mL)
Hz, 2H. CH
), 4.11 (s, 3H, NCH ), 2.98−2.76 (m, 1H,
p‑cym
3
3
for 16 h. The resulting suspension was filtered yielding a white
precipitate which was washed with diethyl ether (2 × 10 mL).
Yield: 1.2 g (96%). H NMR (300 MHz, DMSO): δ 9.21 (s,
CHisopp‑cym), 2.02 (s, 3H, CH
), 1.18 (d, J = 6.7 Hz, 6H,
3p‑cym H,H
13
1
CH3isop p‑cym). C{ H} NMR (75 MHz, CDCl ): δ 174.8
(Ccarbene− Ru), [131.3, 131.1, 130.8, 130.8, 128.7, 128.4, 127.8,
127.2, 126.3, 125.6, 125.6, 124.9, 124.7, 124.6, 124.6, 123.8,
123.6, 122.4](CH ,CHimidazole), [108.1, 98.9] (Cqp‑cym), [85.1,
82.4] (CHp‑cym), 52.8 (NCH -), 39.9 (NCH ), 30.7
(CHisopp‑cym), 22.3(CH3p‑cym) 18.7 (CH3isop p‑cym). Anal. Calcd
3
1
3
1
H, NCHN), 8.51−8.08 (m, 9H, CH ), 7.87 (d, J
= 1.7
Ar
H,H
3
Hz, 1H, CH
), 7.73 (d, J = 1.7 Hz, 1H, CHimidazole),
imidazole
H,H
Ar
1
3
1
6
.23 (s, 2H, NCH −), 3.82 (s, 3H, NCH ). C{ H} NMR (75
2
3
2
3
MHz, DMSO): δ 137.1(NCHN), [131.9, 131.1, 130.6, 129.2,
1
29.1, 128.6, 128.4, 127.8, 127.7, 127.1, 126.4, 126.2, 125.6,
for C H N RuCl (620.58): C, 59.99; H, 5.19; N, 4.51.
31
30
2
2
1
24.5, 124.3, 124.1, 123.1, 122.8] (CH , CHimidazole), 50.4
Found: C, 60.38; H, 5.43; N, 4.62. Electrospray MS (Cone 20
Ar
+
(NCH −), 36.3 (NCH ). Anal. Calcd for C H N Br·H O
(395.29): C, 63.80; H, 4.84; N, 7.08. Found: C, 64.23; H, 5.21;
V) (m/z, fragment): 567.2 [M − Cl] . HRMS ESI-TOF-MS
2
3
21 17
2
2
+
(positive mode): [M − Cl] monoisotopic peak 567.1144; calcd
N, 6.94. Electrospray MS (Cone 20 V) (m/z, fragment): 297.4
567.1146, ε 0.4 ppm.
r
+
[
[
M ] . HRMS ESI-TOF-MS (positive mode):
Preparation of NHC-M-rGO. In a round-bottom flask were
introduced 90 mg of rGO and 10 mL of DCM. The suspension
was sonicated for 30 min. Then, 10 mg of the corresponding
metal complex 1 or 2 was added. The suspension was stirred at
room temperature for 10 h until the solution become clear. The
black solid was filtrated and washed with 2 × 15 mL of DCM,
affording the resulting product as a black solid. The filtrates
were combined and evaporated to dryness under reduced
+
M] monoisotopic peak 297.1389; calcd 297.1392, ε 1 ppm.
r
Synthesis of 1. Imidazolium salt A (127 mg, 0.334 mmol),
[
0
Pd(dmba)OAc] (100 mg, 0.167 mmol), and KBr (40 mg,
.334 mmol), were refluxed under nitrogen in 10 mL of
2
acetonitrile for 16 h. The resulting suspension was filtered
through Celite, and the solvent was evaporated under reduced
pressure. The crude solid was purified by column chromatog-
raphy. Elution with a mixture DCM/acetone 9:1 afforded the
separation of a yellowish band containing the desired product.
Precipitation with DCM/hexanes afforded an analytically pure
1
pressure. Unsupported complex 1 or 2 was analyzed by H
NMR using anisole as internal standard. Integration of the
characteristic signal of anisole (−OMe) versus (NCH -Pyr)
2
1
yellow solid. Yield: 128 mg (60%). H NMR (300 MHz,
reveals the amount complex that has been deposited on the
rGO. The exact amount of complex supported was determined
by ICP-MS analysis.
3
CDCl ): δ 8.38 (d, J = 9.3 Hz, 1H, CHArpyrene), 8.19−7.89
3
H,H
(
m, 8H, CHArpyrene), 7.09−7.01 (m, 2H, CH
), 6.87 (td,
), 6.77 (d, J = 1.9 Hz, 1H,
Arphenyl
3
3
JH,H= 7.4, 2.8 Hz, 1H, CH
General Procedure for Alkene Reductions. Molecular
Arphenyl
H,H
3
CH
), 6.43 (d, J = 1.9 Hz, 1H, CH
), 6.31 (d,
hydrogen was added with a balloon filled with 1 atm of H to a
imidazole
H,H
imidazole
2
3
3
JH,H = 14.5 Hz, 1H, NimidazoleCH H −), 6.18 (d, J = 14.5
mixture of alkene (0.3 mmol), Cs CO (0.3 mmol), and NHC-
a
b
H,H
2
3
3
−3
Hz, 1H, NimidazoleCH H −), 6.15 (d, J
CHArphenyl), 4.01 (d, J = 14.0 Hz, 1H, N
= 6,9 Hz, 1H,
CH H −), 3.99
Pd-rGO (3 × 10 mmol, based on metal) in toluene (5 mL).
The system was then evacuated and backfilled with H in cycles
a
b
H,H
3
H,H
amine
a
b
2
3
(
s, 3H, NimidazoleCH ), 3.87 (d, JH,H = 14.0 Hz, 1H,
for three times before putting the reaction vessel in an oil bath
at 100 °C for 15 min. Yields were determined by GC analyses
using anisole (0.3 mmol) as internal standard. Products were
identified according to spectroscopic data of the commercially
available compounds.
Recycling Experiments. The hydrogenation reaction was
carried out under identical reaction conditions as described in
the alkene reduction procedure. After completion of each run
(15 min), the reaction mixture was allowed to reach room
temperature and was centrifuged. The remaining solid was
3
NamineCH H −), 2.95 (s, 3H, N
NamineCH ). C{ H} NMR (75 MHz, CDCl ): δ 172.9
CH ), 2.92 (s, 3H,
a
b
amine
3
1
3
1
3
3
(
1
1
Ccarbene− Pd), [150.3, 148.7, 135.6, 131.8, 131.1, 130.7,
29.7, 128.8, 128.6, 128.1, 127.9, 127.2, 126.1, 125.7, 125.6,
25.5, 124.9, 124.7, 124.5, 124.1, 123.7, 122.5, 122.1, 120.3]
(
CH ,CHimidazole), 72.2 (NamineCH −), 53.7 (N
CH −),
Ar
2
imidazole
2
[
51.2, 50.6] (NamineCH ), 38.7 (NCH ). Anal. Calcd for
3 3
C H N PdBr·H O (634.90): C, 56.75; H, 4.76; N, 6.62.
Found: C, 56.93; H, 5.09; N, 6.35. Electrospray MS (Cone 20
3
0
28
3
2
+
V) (m/z, fragment): 536.3 [M − Br] . HRMS ESI-TOF-MS
washed thoroughly with CH Cl (5 × 10 mL), dried, and
2 2
+
(
5
positive mode): [M − Br] monoisotopic peak 536.1333; calcd
reused in the following run. The base was only added in the
first run.
36.1330, ε 0.6 ppm.
r
Synthesis of 2. In a round-bottom flask were mixed, under
exclusion of light, the imidazolium salt A (124.2 mg, 0.326
Large-Scale Recycling Experiments. Molecular hydrogen
was added with a balloon filled with 1 atm of H to a mixture of
2
mmol) and Ag O (75.5 mg, 0.326 mmol) in 10 mL of
styrene (2 mL, 17.3 mmol), Cs CO (2.8 g, 8.65 mmol),
2
2
3
acetonitrile, and the suspension was refluxed for 5 h. Then
anisole as internal standard (1.9 mL, 17.3 mmol), and NHC-
6
−3
[
RuCl (η p-cymene)] (100 mg, 0,163 mmol) and KCl (243
Pd-rGO (1.74 × 10 mmol, based on metal) in toluene (20
2
2
mg, 3,25 mmol) were added, and the reaction mixture was
stirred at room temperature for 15 h. The resulting suspension
was filtered through Celite, and the solvent was evaporated
under reduced pressure. The crude solid was purified by
column chromatography. Elution with a mixture of DCM/
acetone (9:2) afforded the separation of an orange band
containing the desired product. Precipitation with DCM/
diethyl ether afforded an analytically pure orange solid. Yield:
mL). The system was then evacuated and backfilled with H in
2
cycles for three times before putting the reaction vessel in an oil
bath at 100 °C. Reaction monitoring, yields, and conversions
were determined by GC analyses. After completion of each run
(20 h), the reaction mixture was allowed to reach room
temperature and was centrifuged. The remaining solid was
washed thoroughly with CH Cl (5 × 10 mL), dried, and
2
2
reused in the following run. The base was only added in the
first run.
1
3
1
20 mg, 61%. H NMR (300 MHz, CDCl ): δ 8.38 (d, J
=
3
H,H
2
045
dx.doi.org/10.1021/cs5003959 | ACS Catal. 2014, 4, 2038−2047

ACS Catalysis
Research Article
General Procedure for Nitroarene Reductions. Molec-
ular hydrogen was added with a balloon filled with 1 atm of H2
ACKNOWLEDGMENTS
Dedicated to Prof. Pascual Lahuerta, on the occasion of his
70th birthday. We thank the financial support from the
Ministerio de Ciencia e Innovacion
4055/BQU). We would also like to thank the Generalitat
Valenciana for a fellowship to S.S. The authors are grateful to
the Serveis Centrals d’Instrumentacio Cientifica (SCIC) of the
Universitat Jaume I for providing us with spectroscopic and X-
ray facilities.
■
to a mixture of nitroarene (0.3 mmol), Cs CO (0.3 mmol),
2
3
anisole as internal standard (0.3 mmol), and NHC-Pd-rGO (6
́
of Spain (CTQ2011-
−3
×
10 mmol, based on metal) in toluene (5 mL). The system
2
was then evacuated and backfilled with H in cycles for three
2
times before putting the reaction vessel in an oil bath at 100 °C
for 2h. Yields were determined by GC analyses using anisole
0.3 mmol) as internal standard. Products were identified
́
́
(
according to spectroscopic data of the commercially available
compounds.
ABBREVIATIONS
■
General Procedure for Alcohol Oxidations. In a round-
bottom flask a mixture of the alcohol (0.3 mmol), Cs CO (0.3
rGO, reduced graphene oxide; GO, graphene oxide; dmba,
cyclo-orthometalated N,N-dimethyl benzylamine; NHC, N-
heterocyclic carbene ligand; CDGs, chemically derived
graphenes
2
3
−3
mmol), and the NHC-Ru-rGO (6 × 10 mmol, based on
metal) was refluxed in toluene (4 mL) for 12 h. Yields were
determined by GC analyses using anisole (0.3 mmol) as
internal standard. Products were identified according to
spectroscopic data of the commercially available compounds.
Recycling Experiments. The recycling of benzyl alcohol
oxidation was carried out under identical reaction conditions as
described before. After completion of each run (12 h), the
reaction mixture was allowed to reach room temperature and
was centrifuged. The remaining solid was washed thoroughly
with CH Cl (5 × 10 mL), dried, and reused in the following
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(
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*
S
Supporting Information
Full characterization of molecular complexes and hybrid
materials including high-resolution mass spectroscopy, NMR,
X-ray crystal structure, UV−vis, FTIR, and Raman spectrosco-
py, as well as microscopy SEM and HRTEM images. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(
(
(
2
AUTHOR INFORMATION
■
*
*
(
Corresponding Authors
E-mail: jmata@uji.es.
E-mail: eperis@uji.es.
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The authors declare no competing financial interest.
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dx.doi.org/10.1021/cs5003959 | ACS Catal. 2014, 4, 2038−2047

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