Graphene anchored palladium complex as efficient and recyclable catalyst in the Heck cross-coupling reaction

Article abstract of DOI:10.1016/j.molcata.2016.02.023

The immobilization of organometallic molecular catalysts on carbon nanomaterials requires an appropriate synthesis and functionalization methodology to yield active heterogeneous systems. In this work, molecular palladium complexes were synthesized on the

Full text of DOI:10.1016/j.molcata.2016.02.023

Accepted Manuscript
Title: Graphene anchored palladium complex as efficient and
recyclable catalyst in the Heck cross-coupling reaction
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Author: Laura Fernandez-Garcıa Matıas Blanco Clara Blanco
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Patricia Alvarez Marcos Granda Ricardo Santamarıa Rosa
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Menendez
PII:
S1381-1169(16)30057-7
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.02.023
MOLCAA 9789
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Journal of Molecular Catalysis A: Chemical
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Accepted date:
3-12-2015
19-2-2016
20-2-2016
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Please cite this article as: Laura Fernandez-Garcia, Matias Blanco, Clara
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Blanco, Patricia Alvarez, Marcos Granda, Ricardo Santamaria, Rosa Menendez,
Graphene anchored palladium complex as efficient and recyclable catalyst in
the Heck cross-coupling reaction, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2016.02.023
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Graphene anchored palladium complex as efficient and
recyclable catalyst in the Heck cross-coupling reaction
Laura Fernández-García, Matías Blanco, Clara Blanco, Patricia Álvarez, Marcos
Granda, Ricardo Santamaría, Rosa Menéndez*
Instituto Nacional del Carbón, INCAR-CSIC. P.O. Box 73, 33080, Oviedo. Spain
*Corresponding author: Prof. Rosa Menéndez; rosmenen@incar.csic.es

GRAPHICAL ABSTRACT
HIGHLIGHTS
 A graphenic material was functionalized with palladium (II) molecular
compounds
 The hybrid system was active in the Heck cross-coupling reaction
 The catalyst converted both vinyl and styril substrates efficiently
 Samples presented excellent recyclability and air stability
2

ABSTRACT
The immobilization of organometallic molecular catalysts on carbon nanomaterials
requires an appropriate synthesis and functionalization methodology to yield active
heterogeneous systems. In this work, molecular palladium complexes were synthesized
on the aromatic network of a graphenic material employing diazonium chemistry and
additional simple modifications to generate a bidentate coordination site for the
palladium atoms. The presence of 0.2 – 0.25 nm palladium (II) species on the surface of
the graphenic layers was confirmed by XPS and HRTEM. The modified palladium-
graphene hybrid was found to be active as catalyst in the cross-coupling Heck reaction
between styrene or n-butyl acrylate with a wide range of aryl bromides substituted with
different electron-acceptor and electron-withdrawing groups. The coupled products
were obtained in good to excellent yields, at initial TOFs as high as 1.1 s-1. In addition,
the catalyst showed excellent cyclability and air stability, and was reused over 8 cycles
without any loss of activity.
Keywords: Graphene, Pd complexes, functionalization, Heck cross-coupling,
recyclability
3

1. INTRODUCTION
Palladium-based homogeneous and heterogeneous catalysts are versatile tools that can
be used to perform a large number of chemical and electrochemical reactions, such as
hydrogenation/dehydrogenation, petroleum cracking or certain fuel-cell processes. [1-5]
In addition, palladium organometallic compounds are known to be valuable catalysts for
forming carbon-carbon bonds in cross coupling processes. [6] Numerous instances of
the use of palladium complexes, [7,8] or even palladium nanoparticles, [9] have been
reported in the literature to catalyze, for instance, Suzuki, Stille, Sonogashira reactions
and Heck couplings. [10,11] All of these have not only contributed to clarifying the
different steps involved in the reaction mechanism [12] but also to achieve total
synthesis, [13] biomolecules, [14] drugs, [15] and even chiral compounds [16] in both at
laboratory and at industrial scale.
Clearly, the high cost of these catalysts, the toxicity of the reaction residues and the
complicated synthesis of the final catalyst precursors make it necessary to develop
palladium catalysts which are easy to recover and that cycle without losing their
catalytic performance and activity. [17] Several studies in the literature deal with the
anchorage of these catalytic species on different kinds of support, such as zeolites, [18]
metal-organic frameworks (MOFs), [19] polymers, [20] activated carbons [21] or even
carbon nanomaterials, i.e. carbon nanotubes and graphene. [22] However, these kinds of
hybrid systems usually involve the use of palladium nanoparticles, and the number of
examples of the covalent anchoring of palladium-based molecular catalysts on a
carbonaceous support is very small. The heterogenization of an active palladium
molecular cross coupling catalyst on graphenic materials could revolutionize the organic
synthesis research due to the transfer of the outstanding properties of graphene (i.e.
electronic transport, thermal conduction, etc.) to the active palladium center. Other
authors have noted synergistic effects between graphenic materials and metallic
nanoparticles in catalyzed chemical and electrochemical processes. [23-25]. Therefore,
graphene can take part, by electron-transfer assistance, in the necessary palladium
oxidations and reductions in which the Heck cross-coupling-reaction mechanism is
based.
The unique properties of graphene and related materials, in relation to electron and
thermal conductivity, [26,27] intrinsic porosity and chemical inertness in most of the
4

media [28] make these materials suitable for use as supports of molecular palladium
catalytic species. Also, long established and well-tried functionalization routes for
decorating the graphene lattice (1,3-dipolar addition, diazonium chemistry, the Bingle
reaction) [29] make it possible to construct the catalyst precursor by applying few steps
based on simple organic reactions as a way to establish accessible binding sites for
immobilizing the palladium center on the surface of the material. In this way, leaching
can be avoided, while, at the same time, a surface effect that enhances activity and also
affords stability and cyclability to the catalytic system can be created. [30]
In this study, the covalent surface decoration of a graphenic material with a nitrogen
bidentate molecular palladium organometallic compound introduced into the aromatic
scaffold by means of a three-step procedure is reported. First, diazonium chemistry is
applied to obtain a nitrogen-based reactive point on the surface of the graphene that can
be modified in a second reaction to achieve a nitrogen bidentate binding site for the
palladium complex. This novel graphene-palladium hybrid system was employed as
catalyst in a Heck cross coupling reaction between aryl bromides of different
characteristics and suitable aliphatic or aromatic vinyl substrates in order to test the
versatility of our system. The cyclability and air stability of the catalyst were also
studied.
2. MATERIALS AND METHODS
2.1 Materials
The solvents were distilled immediately prior to use from appropriate drying agents or
obtained from a Solvent Purification System (Innovative Technologies). All the
chemicals and the graphite were purchased from Aldrich. Reagent grade or better
quality was employed in all of the experiments.
Graphite oxide (GO) was prepared by a modified Hummers method as described
elsewhere. [46] The graphenic material, TRGO, was prepared in a two-steps procedure.
The first step consisted in the simultaneous exfoliation-reduction of GO in a home-
made self-designed stainless-steel reactor by means of thermal treatment of the material
5

at 300 ºC under an inert atmosphere. [47] The graphenic material was then annealed at
2000 ºC for 1 h under an argon atmosphere to yield TRGO which was subsequently
employed in the functionalization steps (Fig. 1).
2.2 Functionalization route
The graphenic material TRGO was modified with a palladium organometallic complex
in a three step procedure. In the first step, 0.15 g of the graphenic material was
suspended in 200 mL of N-methyl-2-pyrrolidone (NMP), subjected to 10 min of
sonication and then mixed with 5.18 g (37.5 mmol) of 4-nitroaniline and 10.1 mL (75
mmol) of isopenthyl nitrite. The reaction mixture was magnetically stirred at 85 ºC for 1
h. Then, the solid product was thoroughly washed with dimethyl formamide (DMF) (3 x
100 mL), methanol (3 x 100 mL), ammonium hydroxide (2 x 200 mL), Milli-Q water (2
x 200 mL), methanol (2 x 100 mL), ethyl acetate (2 x 100 mL) and diethyl ether (1 x 50
mL). The resultant sample was labelled TRGO-NO2. The nitro group (0.125 g of
material obtained in the previous step) was deprotected by reduction with tin (II)
chloride (0.375 g, 2 mmol) in a 200 mL mixture of boiling (1:1) tetrahydrofuran (THF)
– ethanol for 24 h. The reaction crude was washed with a 0.5 M HCl solution (8 x 50
mL), Milli-Q water till neutral pH, a 0.5 M solution of NaOH (8 x 50 mL), Milli-Q
water till neutral pH, THF (4 x 50 mL) and ether (1 x 50 mL). The resulting sample was
labelled TRGO-NH2.
The second step was based on the reaction of the a aniline unit (0.125 g of the material
obtained in the preceding step) with picolinaldehide (0.386 mL, 0.4 mmol) catalyzed by
introducing a drop of formic acid into 200 mL of ethanol at refluxing conditions for 48
h. After the reaction, the sample was washed with 6 x 50 mL of ethanol, 6 x 50 mL of
dichloromethane (DCM) and 1 x 50 mL of ether. The resultant sample was labelled
TRGO-NPy.
Finally, the palladium complex was formed by reacting 0.1 g of TRGO-NPy with 0.05
g (0.13 mmol) of palladium (II) acetate and KCl (0.0193 g, 0.26 mmol) in 100 mL of
acetonitrile. The reaction was held for 5 hours under reflux conditions. The sample was
then washed with 6 x 50 mL of acetonitrile and ether, and labelled TRGO-NPy-Pd.
6

2.3 Catalytic reactions
In a typical experiment, the vinyl substrate (styrene derivatives or butyl acrylate
derivatives, 1 mmol), the aryl bromide (1 mmol), Na2CO3 (2 mmol), solvent (DMF, 4
mL) and the catalyst (0.3 mol%, 15.6 mg of sample) were introduced into a 25 mL
Schlenk round-bottomed flask. The reaction was held at 140 ºC for the desired time
(Table 1). Aliquots were taken and subjected to thin-layer chromatography (TLC) or
NMR spectroscopy in order to follow the progress of the reaction. Once the reaction
was completed, the crude product was washed with water, extracted three times with
ether, dried over anhydrous MgSO4 and concentrated under reduced pressure.
Purification by recrystallization in ethanol or by flash chromatography in silica
(hexane:ethyl acetate 10:1) yielded the final product.
The TRGO-NPy-Pd sample was recovered by centrifugation and washed three times
with 20 mL of acetone. In the E-stilbene synthesis, several catalytic cycles were
performed without adding any fresh catalyst precursor. The last cycle at least was
carried out without an inert atmosphere.
2.4 Characterization methods
Transmission electron microscopy (TEM) images were obtained on a JEOL JEM-2100F
transmission electron microscope, equipped with a field-emission-gun (FEG) operating
at 200 kV and an Oxford Instruments Energy-dispersive X-ray spectroscopy (EDX)
probe. The samples were prepared by casting a few drops of 1 mg mL-1 ethanol
suspensions over carbon grids. Raman spectra were recorded on an Invia Renishaw
microspectrometer (50×) with the laser source set to 532 nm. Fourier-Transformed
Infrared (FTIR) spectra were recorded on a Nicolet 8700 FTIR spectrometer (Thermo
Scientific) fitted with a DFT (deuterated triglycine sulfate) detector, operating in
transmission mode on KBr pellets. The X-ray photoemission spectroscopy (XPS)
spectra were recorded using a SPECS system operating under a pressure of 10-7 Pa with
a Mg Kα X-ray source. Oxygen functional groups within the graphenic material were
quantified by deconvoluting the high resolution C1s XPS peak and employing Gaussian
and Lorentzian functions. [48] Nitrogenized species were analyzed with respect to the
N1s band of the XPS spectra. The amount of palladium present in the samples was
7

determined by means of Inductively Coupled Plasma Mass Spectrometry (ICP-MS) on
an Agilent 7700x instrument. The samples were digested following a method described
elsewhere. [49] Briefly, 30 mg of sample was treated with 5 mL of a mixture of
concentrated nitric and hydrochloric acid (3:1 ratio) at 180 ºC for 3 h under microwave
irradiation.
NMR spectra were recorded on a Bruker Advance 400 spectrometer operating at 400.16
MHz (1H). NMR chemical shifts are reported in ppm relative to tetramethylsilane and
referenced to partially deuterated solvent resonances. Coupling constants (J) are given
in hertz.
3. RESULTS AND DISCUSSION
3.1 Synthesis of the graphenic material
Graphenic materials are prepared by applying thermal flash treatment to a graphite
oxide (GO) synthesized by a modified Hummers method, as described elsewhere.43 The
focus of this study was the reconstruction of the aromatic scaffold on the graphenic
layers. Consequently, a high temperature thermal annealing process was applied after
the thermal treatment of the GO, as a result of which TRGO was obtained (Fig. 1). The
Raman spectrum of TGRO (Fig. 2.a) shows a typical D/G/2D bands pattern which
corroborates the graphenic nature of the material. The shape of the spectrum confirms
the reconstruction of the aromatic lattice, with a very low ID/IG ratio (ID/IG = 0.21), and
a pronounced second order spectrum, typical of a non-defective exfoliated-graphene
material. [31] This was also evidenced by the XPS analysis of the sample, which
showed a C/O ratio of 498 and an amount of Csp2 of 84.0 %, indicating the successful
reconstruction of the aromatic lattice, that is backed up by the deconvolution of the XPS
C1s peak (Appendix A). Residual oxygen groups were mainly hydroxylic functions (6.0
%), while epoxy and carboxylic groups were almost totally absent (0.6 and 0.0 %
respectively). TEM images of the graphenic material used in this study show the
classical wrinkled appearance characteristic of graphenic materials obtained by thermal
reduction (Appendix A).
8

3.2 Decoration of the material with the palladium complex
Reconstruction of the sp2 aromatic lattice of the graphenic material allows
functionalization by diazonium chemistry methods, as reported elsewhere. [32]
Additional chemical modifications of the decorated units can be made in order to obtain
the final catalyst precursor, on the basis of a suitable choice of the aniline. [33] In this
way, the in situ formation of a diazonium salt from the p-nitroaniline amino group,
followed by the thermal release of nitrogen and the entrapment of the nitrobenzene
radical yields a functionalized graphenic layer TRGO-NO2 (Fig. 1). Deprotection of the
nitro group by applying a standard reduction protocol based on tin chloride [20] yields
the amino-functionalized graphenic material TRGO-NH2.
The introduction of the nitrobenzene unit and its subsequent reduction to an amino
group was characterized by Raman and XPS spectroscopy. An increase in the ID/IG ratio
(from 0.21 to 0.32) in the Raman spectra of TRGO-NO2 was detected (Fig. 2.a) as a
consequence of the generation of sp3 carbon due to the formation of a new C-C bond
between the graphenic layer and the diazonium salt. [32] In addition, the ID/IG ratio of
the reduced sample was unchanged, which indicates that reduction took place only in
the nitro group and not in the carbon nanomaterial, as confirmed by the increase in the
nitrogen content of the functionalized samples. The parent graphenic material TRGO
contained a negligible amount of nitrogen (0.0 %), which is increased to 1.2 % in
TRGO-NO2, and this value was maintained in the reduced sample TRGO-NH2 (1.2
%). In addition, nitrogenized species were identified. A shift in the maxima of the N1s
bands occurred (Fig. 2.b), varying from 406.2 eV in TRGO-NO2, corresponding
exclusively to the NO2 group, to lower energies of 399.5 eV in TRGO-NH2. These data
fully agree with the positive reduction from a nitro to an amino group. [34] As can also
be seen in Fig. 2.b, the parent material TRGO only gives a noise curve due to its null
nitrogen content. The presence of the nitro and amino groups in both samples was also
corroborated by FTIR spectroscopy (Appendix A), the characteristic scissors bands at
1600-1400 cm-1 of the N-O bonds had vanished while a broad band at higher
wavenumbers, ~3200 cm-1 appeared corresponding to the N-H bonds. [20]
TRGO-NH2 reacted with picolinaldehide to generate the imino-functionalized material
TRGO-NPy. Finally, individual palladium units were trapped by the two donor nitrogen
9

atoms covalently attached to the graphenic material surface, resulting in the final hybrid
material TRGO-NPy-Pd.
ICP-MS analysis revealed that 2.4 % of palladium was incorporated to the graphenic
material, which is in agreement with the nitrogen content of the previous samples. In
addition, 0.2-0.25 of nm electron-dense regions were found to be attached to the surface
of the graphenic layers in the TEM observations (Fig. 3). Although larger particles were
also detected, possibly due to beam irradiation under high vacuum inside the
microscope chamber, the size of the spots was in agreement with molecular complexes.
Similar observations had been previously reported for molecular iridium complexes
supported on graphene oxide and related thermally reduced graphenic material. [35]
EDX analysis confirmed the presence of palladium species (Appendix A), and the
oxidation state, as determined by XPS, corresponds to palladium (II) species (Fig. 3
inset), while two maxima were obtained at 335.7 and 341.0 eV, [36] evidencing that the
organometallic palladium complex proposed in Fig. 1 anchored onto the graphenic
material surface.
3.3 Catalytic activity
The palladium hybrid material TRGO-NPy-Pd was tested as catalyst precursor in the
C-C cross coupling Heck reaction between aryl bromides and vinyl substrates (Fig. 4).
[37]
Firstly, the catalytic performance of the hybrid material was tested on a model reaction
involving styrene (1 mmol) and bromobenzene (1 mmol), with an excess of Na2CO3 (2
mmol) acting as base in boiling DMF. It is worth noting that in the experimental setup
used, neither phosphine nor any other donor ligand was employed to stabilize the
palladium center. With a catalyst loading of 0.3 mol %, the reaction proceeded
smoothly, yielding the desired product E-Stilbene, in a high isolated yield (97 %) after
18 h of reaction. As expected, the reaction performed without using a palladium source,
employing TRGO-NPy as catalyst, led to a negligible conversion (see Appendix A).
10

Moreover, the heterogeneous system was more active than the homogeneous catalyst
based on palladium acetate test performed under the same conditions for comparative
purposes. In the homogeneous reaction, only 75 % of isolated yield was obtained after
24 h of reaction. Heterogenization of the catalyst, combined with the surface effect
afforded by the graphenic material [38] and the homogeneous dispersion of palladium
centers throughout the graphene layer (see Fig. 3) may have been responsible for
enhancing the catalytic activity.
A kinetic profile was constructed to obtain a better understanding of the system. 0.1 mL
1
aliquots were taken at different reaction times, and analyzed by TLC and H-NMR
spectroscopy. As can be seen in Fig. 5, no induction period was observed following the
thermal equilibration of the reactants. The formation of E-stilbene was detected
immediately, with TRGO-NPy-Pd achieving an initial TOF of 1.08 s-1 (Table 1), which
is higher than that of other palladium systems used for the Heck reaction reported in the
literature, such as metallic nanoparticles supported on phosphates, [39] silica matrixes,
[40] polymers, [41] zeolites [42] and ionic liquids, [43] and even homogenous systems.
[44] Moreover, a conversion of more than 90 % was obtained after 3 h of reaction, and
complete saturation was reached after 5 h. It should also be mentioned that the model
reaction was studied using a substrate (bromobenzene) without any electron-donor or
electron-withdrawing groups that might have modified the electronic density of the
system.
An important feature of a heterogeneous catalyst is its capacity to be reused.
Consequently, the recyclability of the system based on the Heck model reaction was
tested. After each run, the black solids were recovered by centrifugation, washed three
times with 20 mL of acetone and then dried under vacuum. The recovered product was
subjected to 8 catalytic cycles using for every single transformation a fresh reaction
mixture but without adding any fresh catalyst precursor. Recycled TRGO-NPy-Pd
exhibited similar conversions to the fresh catalyst, as shown by the NMR analysis
(Fig. 6). However, a slight decrease in activity was observed after the fifth cycle,
possibly due to a marginal loss of material in each of the recovery steps. Nevertheless,
no leaching of palladium was detected by the ICP measurements. In addition, the
catalyst remained stable under air due to the fact that a similar conversion of E-Stilbene
11

to those of the preceding experiments was achieved in the final cycle without using an
inert atmosphere.
To study the versatility of the catalyst, several reactions were performed employing
electron-donor and electron-withdrawing groups which modify the electronic density of
the aryl bromide, such as –Me, -CHO, -NO2, -OMe, -CF3 and even brominated pyridine.
In addition, the scope of the catalyst was tested using aryl and vinyl substrates with
styryl and acrylic skeletons to preserve the double bond. The results are depicted in
Table 1.
In general, all the reactions with styryl substrates (entries 1-7) were very clean and the
corresponding white (or yellow, entry 6) solids were obtained in good to excellent yields
within 1.5 to 10 h. However, the butyl acrylate reactions produced lower yields and
required longer times than the styrene-based ones. In addition, in the case of the
aliphatic substrate, more byproducts were obtained, which were removed by flash
chromatographic purification instead of using the simple recrystallization procedure
applied to the stilbene derivatives. Interestingly, both pyridine couplings (entries 7, 14)
gave rise to a very smooth reaction, from which a clean final product was obtained in a
very short period of time, evidencing the versatility of the catalyst. In relation to the
chemistry of the reaction, the electron-withdrawing groups required the shortest reaction
times when both substrates were employed. It has frequently been reported in the
literature that the Heck reaction achieves better results when the molecules are aromatic,
and also, when the electron density is withdrawn from the carbon bonded to the halide,
[45] in agreement with our results. In light with these findings it is not surprising that
electron-donating groups required the longest times to complete their reactions.
4. CONCLUSIONS
It has been demonstrated that the preparation of an active palladium-based molecular
catalyst on the basal planes of a high-temperature reduced graphenic material is possible
by applying diazonium chemistry, followed by some straightforward modifications
reactions to yield covalently supported palladium complexes.
12

The modified graphenic material was tested as a catalyst in the Heck cross coupling
reaction, where it showed an excellent activity, a high recyclability and also good air
stability. Furthermore, it was able to perform C-C coupling in a wide range of different
substrates with either electron donor or withdrawing groups, and even with aliphatic and
aromatic compounds. In this study, the heterogeneous catalyst was found to be more
active than the homogenous palladium complex. The good catalytic activity shown by
our catalytic system combined with the fact that it is easy to prepare and the graphenic
material is very stable could make our nanohybrid material an affordable and
competitive alternative to conventional catalysts in organic synthesis research.
ACKNOWLEDGMENTS
The research leading to these results has received funding from the European Union’s
Research Fund for Coal and Steel research program under Grant Agreement number
RFCR-CT-2013-00006. In addition, the authors acknowledge financial support from the
Spanish Economy and Competitiveness Ministry (projects Consolider Ingenio 2010 -
CSD2009-00050- and MAT2013-48107-C3-1-R). M. Blanco thanks the Spanish
Education Ministry for his FPU grant (AP2010-0025).
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in the online version, at
http://dx.doi.org/10.1016/xxx
13

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17

Fig 1. Functionalization route.
18

Fig. 2. a) Raman spectra of the parent and modified samples (inset: ID/IG ratios); b) XPS
N1s curves of the parent and modified sample.
19

Fig. 3. TEM image of TRGO-NPy-Pd. Inset: XPS Pd3d spectrum.
20

Fig. 4. The Heck reaction studied.
21

Fig 5. Kinetic profile of the E-Stilbene synthesis catalyzed by TRGO-NPy-Pd and a
homogeneous palladium based catalyst.
22

Fig. 6. Recycling studies (8th cycle performed under air)
23

Fig. 8. TEM image of TRGO-NPy-Pd sample after the catalytic cycles. Inset: XPS
Pd3d region
24

Table 1. Versatility of the catalyst.
Entry
R1
R2
Yieldb
%
TONc
TOF0
(s-1)d
1.08
0.99
1.00
1.05
1.06
1.07
1.04
0.95
0.67
-
(h)
1
H
97
89
90
94
95
96
93
85
60
-
(5)
323
297
300
313
317
320
310
283
200
-
2
CH3
CF3
OMe
CHO
NO2
Py
(7.5)
(5)
3
4
(10)
(1.5)
(2)
5
6
7
(6)
8
H
(15)
(18)
(19)
(21)
(17)
(18)
(12)
9
CH3
CF3
OMe
CHO
NO2
Py
10
11
12
13
55
70
26
87
183
233
87
0.61
0.78
0.29
0.97
14
290
a
Reaction conditions: vinyl substrate (1 mmol), aryl bromide (1 mmol), Na2CO3 (2
mmol), DMF (4 mL) and the catalyst (0.3 mol% Pd based), 140 ºC. b Determined by 1H-
NMR. c Turnover number. d Turnover frequency calculated after 5 min of reaction
25