Immobilization of Pyrene-Adorned N-Heterocyclic Carbene Complexes of Rhodium(I) on Reduced Graphene Oxide and Study of their Catalytic Activity

Article abstract of DOI:10.1002/cctc.201701277

Two pyrene-tagged N-heterocyclic carbene (NHC) complexes of rhodium(I) were obtained and characterized. The two complexes were supported onto reduced graphene oxide (rGO), generating two new materials in which the molecular complexes are immobilized by π–

Full text of DOI:10.1002/cctc.201701277

Accepted Article
Title: Immobilization of pyrene-adorned N-heterocyclic carbene
complexes of rhodium (I) on reduced graphene oxide and study
of catalytic activity
Authors: Eduardo Victor Peris and Sheila Ruiz-Botella
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Immobilization of pyrene-adorned N-heterocyclic carbene
complexes of rhodium (I) on reduced graphene oxide and study
of catalytic activity
[
b]
[a]
Sheila Ruiz-Botella, and Eduardo Peris*
Abstract: Two pyrene-tagged N-heterocyclic carbene complexes of
rhodium (I) were obtained and characterized. The two complexes
were supported onto reduced-graphene oxide (rGO), generating two
new materials in which the molecular complexes are immobilized by
have gained interest because they offer a much simpler approach
to catalyst immobilization,[1e] although they may suffer from the
disadvantage that the link between the catalyst and the solid is
sometimes not too strong. Carbon surfaces are interesting
materials for catalyst immobilization, because they offer excellent
thermal and mechanical stability, high surface area and a rich
surface chemistry.[ Due to the inherent capability of pyrene to
afford -stacking interactions with graphitic surfaces,[ pyrene-
tagged metal complexes have been efficiently supported onto
graphitized solids,[4] and some have also been used in catalysis
proving interesting recyclability properties.[5]
-stacking interactions onto the surface of the solid. The catalytic
activity of both complexes and solid hybrid materials were studied in
the 1,4-addition of phenylboronic acid to cyclohex-2-one, and in the
hydrosilylation of terminal alkynes.The studies showed that for both
reactions the dimetallic complex displayed better catalytic
performances than the monometallic one. This accounted for both,
the reactions carried out in homogeneous conditions and for the
reactions carried out with the solid. In the case of the addition of
phenyboronic acid to cyclohexanone, the solid containing the
dimetallic catalyst could be effectively recycled up to five times, with
negligible loss of activity, while the monometallic catalyst rapidly got
inactive activity. In the hydrosilylation of terminal alkynes, the
selectivity towards the (Z)-vinylsilane was improved when the
immobilized dimetallic catalyst was used, although the catalyst started
to loss activity after the second run.
2]
3]
Due to their easy accessibility and high tunability, N-heterocyclic
carbenes (NHCs) have been found to be excellent ligands for
catalyst immobilization.[6] We recently contributed to the field, by
preparing a series of pyrene-tagged NHC-based catalysts, which
were supported onto reduced graphene oxide (rGO), and afforded
excellent recyclability in reactions such as the hydrodefluorination
of aromatic fluorocarbenes,[7] hydrogenation of alkenes and
alcohol oxidation,[8] and in the -alkylation of secondary alcohols
with primary alcohols.[ In most of these processes, we found that
the presence of the pyrene-tags bound to the NHC ligands,
allowed the effective non-covalent immobilization of the
homogeneous catalyst onto the graphene derivative surface, but
also induced significant modifications on the properties of the
homogeneous catalyst, which we related to -stacking
9]
Introduction
While homogeneous catalysts usually show the advantages of
high activity and high selectivity, their wide application in the
industrial large-scale production of materials remains limited due
to the difficulties encountered when trying to separate the reaction
interactions established between the aromatic substrates and the
pyrene functionalities.[9-10] Based on these precedents, we now
prepared to new pyrene-tagged rhodium NHC complexes, which
we used as catalysts in the addition of arylboronic acids to
cyclohexanone, and in the hydrosilylation of terminal alkynes.
These two catalysts were immobilized onto reduced graphene
oxide (rGO), and the activities and recyclability properties of the
resulting heterogeneized catalysts were studied for the same two
processes.
products from the catalyst and from the reaction solvent.[1]
A
strategy to combine the best attributes from homogenous (high
activity and selectivity) and heterogeneous (catalyst recovery and
recyclability) catalysts is to use supported metal-based catalysts.
The most widely used strategy for catalyst immobilization is the
formation of a covalent bond between the solid and one of the
ligands of the homogeneous catalyst. This requires that the
catalyst has an additional functionalization, but this often raises
the catalyst preparation cost and may modify its catalytic
properties. Non-covalent methods for catalyst immobilization
Results and Discussion
The imidazolium and bis-azolium salts A and B were prepared
according to the literature procedures.[11] The coordination to
rhodium, was performed according to the method depicted in
Scheme 1. The reaction of the imidazolium or bisazolium salts A
[
a]
Institute of Advanced Materials (INAM)
Universitat Jaume I
Avda. Sos Baynat. E-12071-Castellón. Spain
Fax: (+) 34 964387522
or B with [RhCl(COD)]
2
in a mixture of THF/DMF at 80ºC in the
E-mail: eperis@uji.es
presence of K CO gave complexes 1 and 2. KBr was added to
2
3
Supporting information for this article is given via a link at the
end of the document.
the reaction mixture in order to facilitate the formation of the
bromide containing complexes and avoid the presence of
mixtures of halides in the reaction products. The resulting
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ChemCatChem
10.1002/cctc.201701277
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products, 1 and 2, were obtained in moderate-high yields (56-
3
digestion of the samples in hot HCl/HNO followed ICP-MS
65 %) after workup.
analysis, and accounted for a 0.9 and 1.11 wt% of rhodium in
rGO-1and rGO-2, respectively. The elemental mapping by
energy-dispersive X-ray spectroscopic analysis (EDS) performed
by means of High Resolution Transmission Electron Microscopy
(
HRTEM) of rGO-2 confirmed the homogeneous distribution of
rhodium in the hybrid material (Figure 1 shows the HRTEM and
EDS elemental mapping images obtained for rGO-2).
Scheme 1. Preparation of compounds 1 and 2
Figure 1. STEM image (left), and EDS (HRTEM) elemental mapping image of
rGO-2 (right; green dots indicate rhodium atoms).
Both complexes were characterized by means of NMR
spectroscopy and Mass Spectrometry, and gave satisfactory
elemental analysis. The 13C NMR of complexes 1 and 2, show the
distinctive doublets due to the metallated carbene carbons at
The catalytic activity of complexes 1 and 2 was tested in two
reactions typically catalyzed by rhodium (I) complexes, namely
the 1,4-addition of arylboronic acids to cyclohexen-2-one, and in
the hydrosilylation of terminal alkynes. The two ligands in 1 and 2
induce quasi-identical stereoelectronic properties, and therefore
the comparison of the activities of these two complexes offer an
excellent opportunity to compare the activity of the dimetallic
complex with respect to its monometallic analogue. The activity of
the heterogeneized materials rGO-1 and rGO-2 will also be
compared with the one shown by their homogeneous counter-
parts.
1
1
1
83.6 ( JRh-C = 50 Hz), and 183.4 ( JRh-C = 50 Hz) ppm. Both
1
H and 13C NMR spectra of complex 2 are in agreement with its
twofold symmetry. The ESI-mass spectrometry of the complexes
shows representative peaks at m/z values of 583.2 (assigned to
+
+
[
1-Br] ), complexes at 1169.2 (assigned to[2-Br] ).
The addition of arylboronic acids to ,-unsaturated ketones,[12]I
is a process for which some Rh(I)-NHC complexes have afforded
excellent activities and chemoselectivities.[12a, 13] As a model
reaction, we studied the addition of phenyl boronic acid to
cyclohexen-2-one. The reactions were carried out using
equimolecular amounts of phenylboronic acid and cyclohexanone,
during 6 hours in toluene at 100ºC in the presence of KOH. The
catalyst loading was 0.2 mol% for 1 and 0.1 mol% for 2, in order
to study the reactions using the same amount of rhodium load.
Under these reaction conditions, catalyst 1 yielded 85% of the
final product, while the dimetallic catalyst 2 afforded almost
quantitative yields (98 %). This result contrasts with our previous
finding that a trimetallic NHC-based rhodium catalyst afforded
quasi-identical activities as its related monometallic analogue for
this same catalytic reaction.[13d] In this case, we believe that the
presence of the pyrene functionalities in the rhodium catalysts
may have some influence in the activity of the catalysts due to
possible -stacking interactions with the aromatic substrates, and
this may induce substantial differences with the activities shown
by related rhodium catalysts not containing pyrene tags.[9] Next,
we studied the activities of the hybrid materials rGO-1 and rGO-2
in the same reaction. Each individual run was carried out using a
Scheme 2. Immobilization of catalysts 1 and 2
Complexes 1 and 2 were grafted onto reduced graphene oxide
(
rGO) by mixing complexes 1 or 2 and rGO in dichloromethane in
an ultrasound bath for 20 min. Then, the suspension was stirred
during 12h (Scheme 2). The first visual evidence of the anchoring
of the catalysts onto the solid is the disappearance of the yellow
color of the solution. The resulting black solids were filtered and
washed with methylene chloride. The 1H NMR spectra of the
filtrate confirmed the absence of signals due to 1 or 2 in the
solution, thus constituting an evidence of the effective
immobilization of the molecular complexes on the solid. The exact
rhodium content in solids rGO-1 and rGO-2 was determined by
0.2 mol % based on rhodium load (the amount of solid was
calculated based on the amount of rhodium determined by ICP-
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ChemCatChem
10.1002/cctc.201701277
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MS). After completion of each run (6 hours), the solid was filtered,
washed with CH Cl and reused in the following run.
2 2
maintained. The morphology of graphene does not show
significant changes compared to the original one, apart from an
increase in the number of wrinkles. Taking all these results
together, we believe that the loss of activity of the catalysts may
be due to partial loss of bulk hybrid catalyst during the filtration
process after each run, combined with the partial poisoning of the
surface of the catalyst by the addition of KOH for each of the
cycles of the experiment.
1
00
Next we studied the activity of the rhodium complexes 1 and 2
and their hybrid heterogeneized analogues, rGO-1 and rGO-2, in
the hydrosilylation of terminal alkynes. This reaction constitutes a
good challenge for study, because the search for an effective
catalyst requires that the catalyst not only affords a high activity,
but also high selectivity toward one of the three possible isomers
that can be obtained. Several rhodium complexes have provided
good activities and selectivities in this reaction with important
practical applications in synthetic organic chemistry.[14]
8
6
4
2
0
0
0
0
0
rGO-1
rGO-2
Table 1. Hydrosilylation of terminal alkynes using catalysts 1 and 2
Figure 2. Recycling experiments of the 1,4-addition of phenyl boronic acid to
cyclohexen-2-one. Reaction conditions: 2-cyclohexen-1-one (0.5 mmol),
arylboronic acid (0.6 mmol), KOH (0.09 mmol) and rGO-1 or rGO-2 (0.2 mol %
based on the metal) toluene (1.5 mL) at 100ºC for 6 h. Yields determined by GC
using anisole as internal standard.
Entr
y
Catalys
t
β(Z)
αa
β(E)
Conversion
alkyne
hexyne
a
a
a
As can be observed from the results shown in Figure 2, both
catalysts rGO-1 and rGO-2 showed similar catalytic activities as
their homogeneous counter-parts in the first cycle. Catalyst rGO-
1
1
2
56
69
6
27
17
89
86
2
hexyne
2
1
gradually diminished its activity during five cycles (from 90% to
phenylacetylene
*
0.
8
3b
1
2
17
56
14
15
46
86
73% yield), and then the activity abruptly ceased. The activity of
rGO-2 was maintained higher than that shown by rGO-1 all along
the recyclability experiments. During the first four cycles rGO-2
produced quantitative yields of the final product, and then the
phenylacetylene
*
2.
3
4b
activity gradually decreased until the complete deactivation of the
catalyst after the 9th run. Considering the accumulated turnover
Reaction conditions: 0.077 mml of alkyne and 0.085 mmol of HSiMe
mL of CDCl , at room temperature during 6h. 2 mol% of catalyst loading, based
on the rhodium content.a Yields and conversions determined by H NMR
spectroscopy, using anisole as internal standard. A small amount of styrene
2
Ph in 0.5
3
numbers of these recyclability experiments, the results shown
here constitute a clear enhancement of the catalytic activity of the
hybrid materials compared to the results shown in homogeneous
conditions. In particular, catalyst rGO-2 accumulated 3500
turnovers. The fact that the dimetallic complex with the two-
pyrene tags affords higher activity and better recyclability
properties than the catalyst with only one pyrene tag is in
accordance with previous studies, which suggest that leaching is
significantly reduced when the metal complex is anchored to the
surface of the solid by more than one pyrene tag.[4b, 9]
The solids resulting from the recycling experiments were
analyzed by means of HRTEM spectroscopy and ICP-MS. The
ICP-MS analysis of the solids revealed that the rhodium content
for rGO-1 and rGO-2 was 0.73% and 1.11 %, respectively,
therefore there was a 30 % loss of rhodium after all five cycles in
the case of rGO-1, while the amount of metal was maintained all
over the 9 cycles in the case of rGO-2. As previously mentioned,
this is in agreement with our previous findings regarding the more
effective immobilization of catalysts containing two pyrene-tags
compared to those containing only one pyrene-tag.[9] The HRTEM
images (see SI for details), did not show any traces of formation
of metal nanoparticles, and the EDS analysis revealed that the
homogeneous distribution of rhodium along the solid surface was
1
b
was observed together with all other reaction products.
We first studied the homogeneously catalyzed reactions between
1-hexyne and phenylacetylene with HSiMe
2
Ph. The reactions
were performed in CDCl at room temperature using catalyst
3
loadings of 2 mol% (based on rhodium concentration). Under
these reaction conditions, both catalysts showed good activity in
the hydrosililation of 1-hexyne, with a clear preference for the
formation of the anti-Markovnikov anti-addition (Z)vinylsilane
product, which reaches 69% yield when the dimetallic catalyst 2
was used (Table 1, entry 2). This observation is very interesting,
because the (Z) is the less frequent product in this reaction, and
thus a very elusive target.[
14c, d, 15]
Catalyst 2 is more active than
Ph,
and also more selective in the production of the -(Z) vinylsilane
56 % yield, entry 4). For this reaction, we observed that the
catalyst 1 in the hydrosilylation of phenylacetylene with HSiMe
2
(
conversion was slightly higher than the sum of the product yields
when catalyst 2 was used. This is because together with the
formation of the three vinylsilanes, we observed the formation of
styrene, a clear indication that the hydrosilylation of the terminal
alkyne was accompanied by the dehydrogenative silylation, with
styrene formed as hydrogen-trapping agent. The product resulting
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ChemCatChem
10.1002/cctc.201701277
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from the dehydrogenative silylation (in our case PhC≡CSiMe
2
Ph)
We next studied the activity of the hybrid materials rGO-1 and
rGO-2 in the hydrosilylation of 1-hexyne and phenylacetylene.
The study of these reactions under heterogeneous conditions
allows to compare the activity and selectivity of the solid materials
with the ones provided by the homogeneous catalysts, and to
determine the recyclability properties of the heterogeneized
catalysts. The studies of the heterogeneization and reutilization of
alkyne hydrosilylation catalysts are very rare, although Messerle
and co-workers recently described an interesting example in
which a Rh(I)-NHC based complex was covalently immobilized
onto graphene, and afforded excellent activities and high
reusability in the hydrosilylation of diphenylacetylene.[18] We
1
is easy to miss when the reaction is followed by H NMR, and the
formation of styrene is the clearest indication that this process is
occurring.[14g, 16]
Because we wanted to have a complete picture of the product
formation along the course of the reaction, we studied the time-
dependent reaction profiles of the hydrosilylation reactions. As
can be seen from the graphics shown in Figure 3, the ratio
between the three isomers formed in the process is maintained all
along the reaction course. This discards that any of the reaction
products are formed via the metal-assisted isomerization of any
of the other vinylsilanes.[14e, 17] Interestingly, the reaction profiles
arising from the evolution of the reactions of 1-hexyne with
3
performed the reactions in CDCl using a 2 mol % of catalyst
loading. While we observed that the reactions at room
temperature did not show any significant product formation after
2
HSiMe Ph show the curved profiles typical for a process that
follows a first order reaction with respect to the concentration of
the alkyne. A different situation arises from the analysis of the
plots referred to the evolution of the reaction of phenylacetylene
with the silane. For these processes, the reaction course is
represented by a straight line, indicating that the reaction is
pseudo-zeroth order with respect to the concentration of the
substrate, and therefore the reaction rate does not depend on the
concentration of phenylacetylene. This type of situation normally
arises when only a small portion of molecules of the substrate are
in a location where they are able to react, and this fraction is
continually replenished from the larger pool. We previously found
that this situation is produced when using pyrene-tagged catalysts
and aromatic substrates. We attributed the effect to the non-
covalent interaction between the aromatic substrate and the
pyrene-tag of the catalyst, which saturates the catalyst, and
8
6
h, the reactions at 60ºC produced conversions above 80 % after
h. Figure 4 shows the results for the reactions performed
2
between phenylacetylene and HSiMe Ph using catalysts rGO-1
and rGO-2. The reaction performed in the presence of rGO-1
afforded 85% conversion, with the (Z) vinylsilane being the major
product formed (61 % yield). Both (E) and  isomers were
obtained with yields below 20 %. In terms of selectivity, this result
is in perfect accordance with the results provided by the
homogeneous catalyst 2. After the first run finished, the solid was
2 2
filtered, washed with CH Cl and reused for a second cycle. In the
second run, the activity of rGO-1 dropped down to only 17 %
conversion, with the same relative proportion of vinylsilanes
formed. The activity of catalyst rGO-2 (87% conversion, 61 %
yield of (Z)) was very similar to that shown by rGO-1 during the
first run. However, this catalyst could be reused a second time
without loss of activity, showing the same relative formation of
isomers as in the first cycle. The activity of the catalyst was
gradually reduced during the third (58% conversion) and fourth
cycles (32 % conversion), and was negligible in the fifth run.
renders
a reaction rate that is non-dependent on the
concentration of the substrate.[9]
a) Substrates: 1-hexyne + HSiMe
Catalyst: 1
2
Ph
b) Substrates: 1-hexyne+ HSiMe
Catalyst: 2
2
Ph
100
a) Substrates: phenylacetylene + HSiMe Ph
2
β-Z
100
β-Z
Catalyst: rGO-1
1
00
80
60
40
20
0
80
60
40
20
0
β-E
β-E
8
6
4
0
conversion
α
α
0
0
conversion
β-Z
β-E
α
conversion
20
0
0
2
4
6
8
t(h)
0
2
4
6
8
t(h)
c) Substrates: phenylacetylene + HSiMe
Catalyst: 1
2
Ph
d) Substrates: phenylacetylene + HSiMe
Catalyst: 2
2
Ph
run 1
run 2
8
0
100
80
conversion
β E
conversion
b) Substrates: phenylacetylene + HSiMe
Catalyst: rGO-2
2
Ph
β E
β Z
α
60
40
20
0
100
β Z
conversion
60
40
20
0
80
β-Z
β-E
60
α
t(h)
0
2
4
6
8
4
0
0
0
Styrene
0
2
4
6
8
t(h)
2
run 1
run2
run 3
run 4
Figure 3. Time-dependent reaction profiles of the reactions of 1-hexyne and
phenylacetylene with HSiMe Ph using catalysts 1 and 2. Reaction
conditions: 0.077 mml of alkyne and 0.085 mmol of HSiMe Ph in 0.5 mL of
CDCl , at room temperature with 2 mol% of catalyst loading (based on
rhodium content). a) 1-hexyne + HSiMe Ph using catalyst1, b) 1-hexyne +
HSiMe Ph using catalyst 2, c) phenylacetylene + HSiMe Ph using catalyst 1
and, d) phenylacetylene + HSiMe Ph using catalyst 2.
2
Figure 4.Recycling experiments of the reaction of phenylacetylene with
HSiMe Ph using hybrid catalysts rGO-1 (a) and rGO-2 (b). Reaction conditions:
alkyne (0.077 mmol), HSiMe Ph (0.085 mmol), and rGO-1 or rGO-2 (2 mol %
(0.5 mL) at 60ºC during 8h. Conversions and yields
2
2
3
2
2
based on rhodium) in CDCl
3
2
2
1
determined by H NMR spectroscopy using anisole as standard.
2
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Given the higher activity and reusability properties of catalyst Conclusions
rGO-2, we decided to study the activity of this catalyst in the
hydrosilylation of 1-hexyne. The results are shown in Figure 5.
The analysis of the results allowed extracting two interesting
conclusions. First, the catalyst showed good activity during the
first cycle (71% conversion), and then the activity was gradually
In summary, we prepared two rhodium(I) catalysts with N-
heterocyclic carbene ligands decorated with pyrene functionalities.
The two catalysts were immobilized on the surface of reduced
graphene oxide (rGO), and the catalytic activity of the
homogeneous catalysts and the solids were compared, in two
organic transformations: the 1,4-addition of phenylboronic acid to
cyclohexan-4-one, and in the hydrosilylation of terminal alkynes.
In both catalytic reactions, the dimetallic complex showed better
catalytic performances than the monometallic one. For the
reactions carried out with the heterogeneized catalysts, we
observed that the dimetallic catalyst with the two pyrene
functionalities displayed higher catalytic activity than the material
with the catalyst containing only one pyrene tag. The solid
containing the dimetallic catalyst could be effectively recycled for
five times without measurable loss of activity in the addition of
phenylboronicacid to cyclohexanone. In the case of the
hydrosilylation of terminal alkynes, the solid containing the
dimetallic catalyst afforded a clear improvement on the selectivity
of the process, with the reaction being stereoselective in the
production of the (Z)-vinylsilane for the case of the
hydrosilylation of 1-hexyne. This result is very interesting,
because it indicates that the immobilization of the catalyst onto
rGO not only can afford catalysts that can be effectively reused,
but also that the selectivity may be improved.
th
reduced until the 6 run (6% conversion). Second, the selectivity
of the catalyst toward the production of the (Z)-vinylsilane was
clearly enhanced compared to the homogeneous catalyst 2, as
can be observed from the very little differences shown between
the conversion values and the yields of production of this isomer.
In fact, we did not detect the formation of any other vinylsilanes
during any of the cycles performed in this series of experiments.
Given that the (Z)-vinylsilane is a very elusive and pursued target
in the hydroslylation of terminal alkynes, we believe that this result
is of major importance.
80
70
60
50
40
30
20
10
0
Conversion
β-Z
run 1
run 2
run 3
run 4
run 5
run 6
The analysis of the solids after the recycling experiments,
indicated that leaching was negligible for the catalysts with two
pyrene fragments, while there was an important loss of rhodium
content for the material containing the catalyst with only one
pyrene tag. These results illustrate how the effectiveness of the
immobilization is clearly dependent on the number of anchoring
pyrene sites on the catalyst.
Figure 5.Recycling experiments of the reaction of 1-hexyne with HSiM
hybrid catalyst rGO-2. Reaction conditions: alkyne (0.077 mmol), HSiMe
2
Ph using
Ph
3
0.085 mmol), and rGO-2 (1 mol %) in CDCl (0.5 mL) at 60ºC during 8h.
2
(
Conversions and yields determined by 1H NMR spectroscopy using anisole as
standard.
The solids resulting from the recycling experiments were
analyzed by means of HRTEM spectroscopy and ICP-MS. The
ICP-MS analysis of the rhodium contentin rGO-2 after the
recycling experiments was 1.08 and 1.10 %, for the
hydrosilylations of phenylacetylene and 1-hexyne, respectively.
These results indicate that the dimetallic catalyst is effectively
supported onto the solid, and that for rGO-2 leaching is negligible
after all recycling experiments. On the contrary, for the
monometallic catalyst, with only one pyrene functionality, the final
rhodium content was 0.8 %, thus indicating a 15% loss of rhodium
after only two recycling experiments. This result is in agreement
with the fact that the immobilization of the monometallic catalyst
is much less efficient than for the catalyst containing two pyrene
functionalities.
The HRTEM images (see SI for details), did not show traces of
formation of nanoparticles. The EDS analysis revealed that the
homogeneous distribution of rhodium along the solid surface was
maintained. The morphology of graphene does not show
significant changes compared to the original one. Taking all these
result together, we believe that the loss of activity of the catalysts
may be due the partial poisoning of the surface of the catalyst by
the reagents.
Experimental Section
General comments. All manipulations were carried out under nitrogen
using standard Schlenk techniques and high vacuum. Anhydrous solvents
were either distilled from appropriate drying agents (SPS) and degassed
prior to use by purging with dry nitrogen and kept over molecular sieves.
The azolium salts 2, 3 and 4 were obtained according to the procedures
reported in the literature.[11] All other reagents were used as received from
commercial suppliers. NMR spectra were recorded on Varian
spectrometers. Electrospray mass spectra (ESI-MS) were recorded on
MicromassQuatro LC instrument, and nitrogen was employed as drying
and nebulizing gas. A gas chromatograph GC-2010 (Shimadzu) equipped
with a FID and Technokroma (TRB-5MS, 30 m x 0.25 mm x 0.25 µm)
column and a gas chromatograph/mass spectrometer GCMS- QP2010
(Shimadzu) equipped with a Technokroma (TRB-5MS, 30 m x 0.25 mm x
0.25 µm) column were used. Elemental analyses were carried out in an
EA 1108 CHNS-O Carlo Erbaanalyzer. Electrospray mass spectra (ESI-
MS) were recorded on a MicromassQuatro LC instrument, and nitrogen
was employed as drying and nebulizing gas. High-resolution transmission
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. Varian spectrophotometer. The determination of the
metal loading was done by ICP−MS Agilent 7500 CX.
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ChemCatChem
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Synthesis and characterization of rhodium complexes
20 mg of compound 6 was added. The suspension was stirred at room
temperature for 12 h until the solution become clear. The black solid was
2 2
filtrated and washed with 2 × 25 mL of CH Cl , affording the resulting
Synthesis of 1. A mixture of salt A (70.0 mg, 0.16 mmol), [RhCl(COD)]
2
product as a black solid. The filtrates were combined and evaporated to
dryness under reduced pressure. Unsupported compound 2 was analyzed
by 1H NMR using anisole as internal standard. Integration of the
(
39.4 mg, 0.08 mmol), K
THF/DMF (7:2 mL) was stirred at 75 ºC for 8 h. The reaction was carried
under N . The resulting suspension was cooled to room temperature and
2 3
CO (67.0 mg, 0.48 mmol) and KBr (70.0 mg) in
2
2
characteristic signal of anisole (−OMe) versus (CH -pyrene) reveals the
the solvent was removed under vacuum. The pure compound 1 was
amount complex that has been deposited on the rGO. The exact amount
of complex supported was determined by ICP-MS analysis. (1.11% wt Rh)
precipitated in a mixture of dichloromethane/hexane to give a yellow solid.
1
) δ 8.53 (d, 3JHH = 9.3 Hz,
Yield: 56% (60.0 mg). H NMR (300 MHz, CDCl
H, CHpyrene), 8.32 – 8.14 (m, 4H, CHpyrene), 8.06 (m, 4H, CHpyrene), 7.37
m, 5H, CHPh), 6.69 (d, 2JHH = 14.8 Hz, 1H, CH ), 6.59 (d, 2JHH = 1.9 Hz,
H, CHimidazole), 6.46 (d, 2JHH= 2.0 Hz, 1H, CHimidazole), 6.33 (d, 2JHH = 14.8
), 5.87 (dd, 2
HH= 2.6 Hz, 2H,CH ), 5.23 (s, 2H, CH,CHCOD),
.58 (m, 1H, CHCOD), 3.48 (m, 1H, CHCOD), 2.47 – 2.12 (m, 4H, CH2,COD),
.89 (m, 4H, CH2,COD). 13C NMR (75 MHz, CDCl ) δ 183. 4 (d, 1J=50Hz,
3
1
(
2
General procedure for the1,4-addition of arylboronic acids to
cyclohex-2-one. In a 10 mL high-pressure Schlenk tube, catalyst (0.2
mol %), 2-cyclohexen-1-one (0.5 mmol), arylboronic acid (0.6 mmol), KOH
(0.09 mmol) ,and dry toluene (2 mL) were placed. The mixture was stirred
and heated to 100 °C for 6 h. The reaction yields were calculated by GC,
using anisole as internal standard.
1
Hz, 1H, CH
2
J
2
3
1
3
Rh-Ccarbene), 136.6 (Pyr), 131.8 (Pyr), 131.4 (Pyr), 131.0 (Pyr), 129.6 (Pyr),
1
1
1
29.0 (Pyr), 128.8 (Pyr), 128.1 (Pyr), 127.6 (Pyr), 127.5 (Pyr), 126.4 (Pyr),
25.8 (CPh), 125.1 (CPh), 124.7 (CHPh), 123.3 (CHPh), 121.3 (CHimidazole),
20.7 (CHimidazole), 98.83 (CHCOD), 98.6 (CHCOD), 76.7, 69.7 (CHCOD), 69.6
Recycling Experiments. In a round-bottom flask, a mixture of 2-
cyclohexen-1-one (0.5 mmol), arylboronic acid (0.6 mmol), KOH (0.09
mmol) and rGO-1 or rGO-2 (0.2 mol % based on the metal) was refluxed
in toluene (1.5 mL) for 6 h. The monitoring of the reaction, yields, and
conversions were determined by GC analyses using anisole as an internal
standard. After completion of each run (6 h), the reaction mixture was
allowed to reach room temperature and was filtered. The remaining solid
(
(
C
CHCOD), 69.4 (CHCOD), 54.60 (CH
CH2,COD ), 29.2 (CH2,COD ), 29.1 (CH2,COD ). Anal.Calcd.for
BrRh·CHCl (782.8291): C, 55.23; H, 4.24; N, 3.57. Found: C,
2 2
), 52.8 (CH ), 33.0 (CH2,COD ), 32.8
H N
35 32 2
3
5
5
5.27; H, 4.15; N, 3.29. Electrospray MS (Cone 20V) (m/z, fragment):
83.2 [M-Br]+.
was washed thoroughly with CH
reused in the following run.
2
Cl , dried under reduced pressure and
2
Synthesis of 2. A mixture of B (80.0 mg, 0.10 mmol), [RhCl(COD)]
mg, 0.10 mmol), K CO (82.9 mg, 0.6 mmol) and KBr (70.0 mg) in
THF/DMF (10:3 mL) was stirred at 75ºC for 8 h. The reaction was carried
under N . The resulting suspension was cooled to room temperature and
2
(49.3
2
3
General procedure for the Hydrosilylation of terminal alkynes. In a
nmr tube, a mixture of the alkyne (0.077 mmol), HSi(Me) Ph(0.085 mmol),
and a catalytic amount of 1 or 2 (2 mol % based on the metal ) were
2
the solvent was removed under vacuum. The crude product was purified
by column chromatography. The pure compound 2 was eluted with
dichloromethane: ethyl acetate (8:2) and precipitated in a mixture of
dissolved in CDCl
3
(0.5 mL). The mixture was stirred at room temperature.
1
The progress of the reaction was monitored by H NMR spectroscopy. The
identity of the products formed was assessed from the literature.
1
dichloromethane/hexane to give a yellow solid. Yield: 65% (90.2 mg). H
NMR (300 MHz, CDCl
m, 16H, CHpyr), 7.47 (s, 4H, CHPh), 6.67 (d, J = 14.7 Hz, 2H, CH), 6.58 (d,
) δ 8.53 (d, 3JHH = 9.3 Hz, 2H, CHpyr), 8.24 – 8.08
3
(
Recycling Experiments. In a 2 mL vial, a mixture of the alkyne (0.077
mmol), HSi(Me) Ph(0.085 mmol), and the rGO-1 or rGO-2 (2 mol % based
on the metal) were dissolved in CDCl
J = 2.0 Hz, 1H, CH), 6.57 (d, J = 1.9 Hz, 1H, CH), 6.47 (d, J = 1.8 Hz, 1H,
CH), 6.44 (d, J = 1.9 Hz, 1H, CH), 6.34 (d, J = 14.7 Hz, 1H, CH), 6.32 (d,
J = 14.7 Hz, 1H, CH), 6.01 (d, J = 14.9 Hz, 1H, CH), 5.95 (d, J = 14.9 Hz,
3
(0.5 mL). The mixture was stirred at
60ºC for the required time. The monitoring of the reaction, yields, and
1
(
H, CH), 5.78 (d, J = 14.9 Hz, 1H, CH), 5.72 (d, J = 14.9 Hz, 1H, CH), 5.22
m, 4H, CHCOD), 3.56 (m, 2H, CHCOD), 3.47 (m, 2H, CHCOD), 2.25 (m, 2H,
CHCOD), 2.00 – 1.71 (m, 4H, CH2,COD), 1.56 (m, 4H, CH2,COD).13C NMR (75
MHz, CDCl ) δ 180.8 (Ccarbene-Ir), 136.3 (Pyr), 131.6 (Pyr), 131.2 (Pyr),
30.7 (Pyr), 129.4 (Pyr), 128.7 (Pyr), 128.4 (Pyr), 127.9 (Pyr), 127.57 (Pyr),
conversions were determined by GC analyses using anisole as an internal
standard. After completion of each run (6 h), the reaction mixture was
allowed to reach room temperature and was filtered. The remaining solid
was washed thoroughly with CH
reused for the following run.
3
2
Cl , dried under reduced pressure and
2
1
1
1
27.23 (Pyr), 126.2 (CPhenylene), 125.6 (CHPhenylene), 124.8(CHimidazole),
24.5, (Pyr), 123.0 (CHimidazole), 120.6(Pyr), 120.1 (Pyr), 84.8 (CHCOD), 84.6
(
(
CHCOD), 54.0 (CH
CH2,COD), 29.7(CH2,COD). Anal.Calcd.for C64
2
), 53.0 (CH2,COD), 52.6 (CH2,COD), 52.2 (CH
2
), 33.3
H
58
N
4
Br Rh ·2H O(1284.13):
2
2
2
Acknowledgements
C, 59.8; H, 4.9; N, 4.3. Found: C, 59.2; H, 4.9; N, 4.3. Electrospray MS
Cone 20V) (m/z, fragment): 1169.2 [M-Br]+.
(
We gratefully acknowledge financial support from MINECO of
Spain (CTQ2014-51999-P) and the Universitat Jaume
P11B2014-02). We are grateful to the Serveis Centrals
I
Preparation of rGO-1. In a round-bottom flask were introduced 150 mg of
rGO and 10 mL of CH Cl . The suspension was sonicated for 30 min. Then,
0 mg of compound6was added. The suspension was stirred at room
temperature for 12 h until the solution become clear. The black solid was
filtrated and washed with 2 × 25 mL of CH Cl , affording the resulting
(
2
2
d’Instrumentació Científica (SCIC-UJI) for providing with
spectroscopic facilities. We would also like to thank the
Generalitat Valenciana for a fellowship (S-R.-B.).
4
2
2
product as a black solid. The filtrates were combined and evaporated to
dryness under reduced pressure. Unsupported compound 1 was analyzed
by 1H NMR using anisole as internal standard. Integration of the
Keywords: Rhodium • graphene • catalyst immobilization •
catalyst recyclability • N-heterocyclic carbene
2
characteristic signal of anisole (−OMe) versus (CH -pyrene) reveals the
amount complex that has been deposited on the rGO. The exact amount
of complex supported was determined by ICP-MS analysis. (0,9% wt Rh)
References
Preparation of rGO-2. In a round-bottom flask were introduced 150 mg of
rGO and 10 mL of CH Cl . The suspension was sonicated for 30 min. Then,
2 2
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ChemCatChem
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Entry for the Table of Contents
FULL PAPER
Sheila Ruiz-Botella, Eduardo Peris*
Page No. – Page No.
Title
Two NHC complexes of rhodium (I) adonerd with pyrene functionalities were
supported onto the surface of reduced graphene oxide. The dimetallic complex with
two pyrene tags, affords higher catalytic activity, better recyclability and improved
selectivity compared to the related monometallic complex with only one pyrene
functionality.
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