Palladium(II) tetrasulfophthalocyanine covalently immobilized on keratin protein grafted graphene oxide nanosheets as a new high-performance catalyst for C-C coupling reactions

Article abstract of DOI:10.1039/c6ra18814a

Wool keratin protein, as an amphiphilic biomaterial, was extracted from natural wool and used to modify the surface of graphene oxide nanosheets. An aqua soluble palladium(ii) complex with a phthalocyanine ligand possessing the ability to covalently bind on keratin-protein-grafted graphene oxide nanosheets was used to chemically attach palladium(ii) complex to modified graphene oxide. The covalently supported palladium(ii) tetrasulfophthalocyanine complex revealed efficient catalytic reactivity by controlled temporary release of active species for Heck and Sonogashira coupling reactions in aqueous solution. Heck coupling of styrene and Sonogashira coupling of phenylacetylene with different aryl halides were successfully catalyzed by the synthesized palladium(ii) tetrasulfophthalocyanine covalently supported on keratin-protein-grafted graphene oxide nanosheets as a novel catalyst precursor. The controlled release of palladium active species prevents the formation of inactive palladium agglomerates under harsh conditions and consequently this controlled release leads to high catalytic performances.

Full text of DOI:10.1039/c6ra18814a

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Palladium(II) tetrasulfophthalocyanine covalently
immobilized on keratin protein grafted graphene
oxide nanosheets as a new high-performance
catalyst for C–C coupling reactions
Cite this: RSC Adv., 2016, 6, 98956
*
Zeinab Hezarkhani and Ahmad Shaabani
Wool keratin protein, as an amphiphilic biomaterial, was extracted from natural wool and used to modify the
surface of graphene oxide nanosheets. An aqua soluble palladium(II) complex with a phthalocyanine ligand
possessing the ability to covalently bind on keratin-protein-grafted graphene oxide nanosheets was used to
chemically attach palladium(II) complex to modified graphene oxide. The covalently supported palladium(II)
tetrasulfophthalocyanine complex revealed efficient catalytic reactivity by controlled temporary release of
active species for Heck and Sonogashira coupling reactions in aqueous solution. Heck coupling of styrene
and Sonogashira coupling of phenylacetylene with different aryl halides were successfully catalyzed by the
synthesized palladium(II) tetrasulfophthalocyanine covalently supported on keratin-protein-grafted
graphene oxide nanosheets as a novel catalyst precursor. The controlled release of palladium active
species prevents the formation of inactive palladium agglomerates under harsh conditions and
consequently this controlled release leads to high catalytic performances.
Received 24th July 2016
Accepted 10th October 2016
DOI: 10.1039/c6ra18814a
www.rsc.org/advances
The immobilization of a homogeneous palladium complex
catalyst onto a solid facilitates the separation of the catalyst and
Introduction
improves its catalytic activity. Recently, the covalent immobili-
zation of palladium complexes onto graphene oxide using
different ligands such as N-heterocyclic carbenes,14–16 N-ami-
noguanidine,17 aminosilane ligand spacer,18 nitrogen bidentate
ligand,19 and ring-opening metathesis polymerization-derived
polymers20 were reported for the various C–C coupling reac-
tions. These systems were showed improved catalytic activity
and high recyclability without any decrease in activity.
Palladium phthalocyanines (PdPcs), as precursors for the
Pd(II) oxidation state, exhibit very high thermal and chemical
stability. The central palladium atom in phthalocyanine
ligand of PdPcs is completely coordinatively saturated. Since
PdPcs possess an extraordinarily high complex stability, the
release of Pd under coupling reaction conditions would be very
limited. Due to the very rigid structure of phthalocyanine
ligand in PdPcs, they cannot dissociate partially and thus Pd
released by them would be “completely unsaturated”. Because
the release of palladium from its phthalocyanine complex is
reversible,13 the phthalocyanine ligand has mainly the task of
avoiding the deactivation of the released palladium species.
Thus, PdPcs can be very applicable systems for C–C coupling
reactions and the released palladium species that are formed
in situ from the PdPc precursor have the highest activity.
Recently, it was reported that organometallic compounds with
Palladium-catalyzed cross-coupling reactions as extraordinary
powerful and widespread methods are found in all areas of
chemistry which construct carbon–carbon and carbon–hetero-
atom bonds.1–8 The palladium-catalyzed formation of C–C
bonds is typically associated with “ligandless” palladium
species released from homogeneous and heterogeneous cata-
lysts in the reaction solution. Since irreversible precipitation of
palladium in the reaction media affects its catalytic activities, to
settle the matter, a controlled and reversible release of palla-
dium from macrocyclic complexes, as a novel method, was
proposed.9–13 The interplay between the release and recoordi-
nation of Pd in the presented new concept is the reason for the
high catalytic performances of macrocyclic Pd complexes as
precatalysts in C–C coupling reactions. Kostas et al. have re-
ported the use of a water-soluble Pd porphyrin precatalyst for
the Suzuki reaction in water.9 Wan et al. have used a Pd
porphyrin as a catalyst precursor for the Heck reaction in ionic
liquids as the reaction medium.10 The use of a macrocyclic
Robson-type complex of Pd as the precatalyst in the Heck
11
¨
¨
¨
reaction was reported by Rohlich and Kohler. Also, Rohlich
¨
and Kohler reported that homeopathic Pd concentration in
solution released from macrocyclic Pd complexes leads to high
C–C coupling activity in Heck and Suzuki reactions.12
a
p-electron system like metal phthalocyanine can be
adsorbed/intercalated (onto/into graphene layers),21,22 and/or
Faculty of Chemistry, Shahid Beheshti University, G. C., P. O. Box 19396-4716, Tehran,
Iran. E-mail: a-shaabani@sbu.ac.ir
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chemically attached23,24 to graphene-based materials to as high thermal and mechanical stability, high water dis-
pointedly enhance their performance. persibility, high surface area, easy surface modications, and
Natural biopolymers have highly specialized properties such oxygen-carrying functionalities. These properties make gra-
as inspiring morphologies, complex three-dimensional struc- phene oxide a promising material for the immobilization of
tures, biocompatibility, and biodegradability. Because of the catalysts.37–39 Recently, graphene oxide nanosheets have been
attractive features of natural biopolymers, these materials have modied with various natural polymers such as chitosan,40
superior performances as a choice for nanocomposites.25–28 starch,41 cellulose,42 regenerated silk broin,25 and keratin
Wool is a brous protein that composed of approximately 95 obtained from chicken feathers,43 providing them with inter-
wt% pure keratin protein.29 The keratin protein has different esting new properties. Functionalization of graphene to
kinds of the amino acids that between them the cysteine can be manipulate its interfacial property is of signicant importance
found in high concentration. The high amount of the cysteine in catalytic chemistry.44 Functionalization of graphene can
and the presence of amino acids like serine, threonine, argi- improve the graing or loading of supported catalysts. Natural
nine, and lysine makes it possible to attach different proteins, as amphiphilic biomaterials, are well-known as
compounds chemically, by using the nucleophilic substitution “adhesive” to link hydrophobic and hydrophilic substrates.44
reactions because of the presence of the –OH and –NH2 func- The keratin protein has –OH and –NH2 functional groups that
tional groups in the mentioned amino acids. Due to the pres- can be used to chemically attach different compounds using
ence of amino acid chains in the keratin protein, the mentioned the nucleophilic substitution reaction. Therefore, because of
protein can react with functional complexes of metals and chemical inertness of graphene oxide to form catalyst–gra-
suitable supports; therefore, keratin protein can be used as phene interactions, we used keratin protein of natural wool as
a linker for the attaching of catalysts to the supports. So, a modier to covalently link graphene oxide and water-soluble
extraction of keratin protein from natural wool would be palladium(II) tetrasulfophthalocyanine (PdTSPc). By immobi-
a useful method for the synthesis of catalytic systems. Because lization of PdTSPc onto modied graphene oxide, the recy-
of the presence of strong disulde bonds, inter- and intra- clable catalyst can be obtained with improved catalytic utility
molecular bonding of polar and nonpolar amino acids, and a- and activity.
helices structure of polypeptide in wool keratin, this protein
During the course of our studies toward the synthesis of
shows resistance to degradation in common solvents and it is phthalocyanines45 and introduction of new catalysts for the
insoluble in water, dilute acids, alkalis, and organic solvents. organic transformations,23,24,46–48 herein, palladium(II) tetra-
Solubilisation of wool proteins to produce useful materials can sulfophthalocyanine supported on keratin-protein-graed gra-
be achieved using reduction,30–32 oxidation,32–34 sultolysis of phene oxide nanosheets (PdTSPc@KP–GO) was synthesized and
the disulde bonds,30,35,36 and dissolution techniques by ionic its catalytic activity was investigated in Heck coupling of styrene
liquids and deep eutectic solvents.29
and Sonogashira coupling of phenylacetylene with different aryl
Interest in graphene oxide (GO) in the elds of nano- halides in water as reaction media. PdTSPc@KP–GO is prepared
chemistry and catalysis stems from its unique properties such as Scheme 1.
Scheme 1 Preparation of PdTSPc@KP–GO.
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(1.0 g) was added to a round-bottom ask containing a mixture
Materials and methods
of K2S2O8 (1.0 g) and P2O5 (1.0 g) in concentrated H2SO4 (6.0
1
General
ꢀ
mL). The mixture of the reaction was stirred for 6 h at 80 C.
Deionized water (DI water, 200 mL) was added to the cooled
mixture and stirred overnight. Aer ltration of the mixture, the
residue was washed with DI water and dried overnight to give
the preoxidized graphite. To a round-bottom ask containing
concentrated H2SO4 (50 mL) at 0 ꢀC, the as-prepared preoxi-
dized graphite powder was added and stirred for 10 min.
KMnO4 (6.0 g) was added to the mixture and the mixture of
reaction stirred at room temperature. Aer 4 h, the reaction
ask was placed in an ice bath and DI water (400 mL) in two
steps (at rst 100 mL and aer 2 h, 300 mL) was added to the
mixture. Aer that, by dropwise addition of 30% H2O2 to the
mixture of reaction, the color of the reaction mixture changed
into yellow. The mixture of the reaction was ltered and the
residue was washed with 0.1 M HCl and 300 mL DI water,
respectively, and dried in the air.
Natural white wool (sheep of Kangavar/Iran) was washed with
NaOH solution, distilled water, and methanol, and then cut into
very short pieces. All reagents were obtained from Aldrich or
Merck and used without further purication. Palladium determi-
nation was carried out on a FAAS (Shimadzu model AA-680 ame
atomic absorption spectrometer with a Pd hollow cathode lamp at
244.8 nm, using an air-acetylene ame) and inductively coupled
plasma optical emission spectrometer (ICP-OES) Varian Vista PRO
Radial. UV-vis spectra were recorded employing an Analytik Jena
Specord S600 Diode Array spectrometer. FT-IR spectra were
recorded on a Shimadzu IR-470 spectrometer. X-ray diffraction
(XRD) pattern of catalyst was recorded on a STOE STADI P with
scintillation detector, secondary monochromator using Cu Ka
radiation (l ¼ 0.1540 nm). Thermogravimetric analysis (TGA) was
ꢀ
carried out using STA 1500 instrument at a heating rate of 10 C
minꢁ1 in the air. Scanning electron microscopy (SEM) observa-
tions were carried out on a scanning electron microscope (TESCAN
Vega Model). All samples were sputtered with gold before obser-
vation. Transmission Electron Microscopy (TEM) characterization
was performed using a transmission microscope Philips CM-30
with an accelerating voltage of 150 kV. X-ray photoelectron spec-
troscopy (XPS) analysis was performed using a Gammadata-
scienta ESCA 200 hemispherical analyzer equipped with an Al
Ka (1486.6 eV) X-ray source. Raman spectra were recorded on
a Bruker SENTERR (2009) with an excitation beam wavelength at
785 nm. Microwave irradiation experiment was carried out in
a Microsynth Milestone SRL microwave apparatus. Substances
were identied and quantied by a Varian 3900 GC.
2.3 Preparation of keratin protein (KP) of natural wool. The
dissolution of natural wool bers was done in 1-butyl-3-
methylimidazolium chloride ([BMIM]Cl) at 130 ꢀC for 12
hours under an N2 atmosphere based on the previously reported
method.29 Briey, to a magnetically stirred solution of 2-mer-
captoethanol (15 mg, 0.2 mmol) in [BMIM]Cl (5.0 g), the natural
wool bres (75 mg) were added during 1 h at 130 ꢀC. The stirring
was continued for 12 h until the bres completely dissolved.
2.4 Synthesis of keratin protein graed graphene oxide
nanosheets (KP–GO). The prepared GO (40 mg) was added to
the solution of dry DMF (1.0 mL) in SOCl2 (30 mL). The mixture
of theꢀreaction was continuously stirred with a magnetic stirrer
ꢀ
at 70 C for 24 h. Aer evaporation of the solvent (100 C); the
obtained acylated graphene oxide product (GO–COCl) was
washed with dry THF. The product was added to the as-prꢀepared
solution of wool keratin protein in the [BMIM]Cl at 80 C and
the mixture was stirred for 12 h. The product was ltered and
the residue was washed successively with DI water and ethanol
2
Experimental: synthesis of palladium(II)
tetrasulfophthalocyanine supported on keratin protein
graed graphene oxide nanosheets (PdTSPc@KP–GO)
Pd(II) tetrasulfophthalocyanine (PdTSPc) supported on keratin- and nally dried at 50 ꢀC under vacuum to give keratin-protein-
protein-graed graphene oxide nanosheets (KP–GO) is graed graphene oxide nanosheets (KP–GO).
prepared as Scheme 1.
2.5 Synthesis of PdTSPc@KP–GO. The as-prepared KP–GO
2.1 Synthesis of palladium(II) tetrasulfophthalocyanine (0.2 g) was added to dry DMF (30 mL) and sonicated until
(PdTSPc). The synthesis of PdTSPc was done inspired by previ- a dispersed mixture was obtained. To a magnetically stirred
ously reported procedures for synthesis of metal(II) phthalocy- solution of palladium trisulfophthalocyanine sulfonyl chloride
anines (MPcs)45,49 and the sulfonation of the MPcs,50 as (0.3 mmol) in 50 mL of dry DMF at room temperature, the
following: the palladium(II) phthalocyanine (PdPc) was synthe- dispersed KP–GO mixture was added and the temperature is
sized by controlled microwave heating of homogeneous powder raised to 100 ꢀC. The stirring of reaction media is continued at this
containing of phthalic anhydride (0.88 g, 6.0 mmol), urea (1.6 g, temperature for about 24 h until the PdTSPc is substantially
26 mmol), Pd(II) chloride (0.32 g, 1.8 mmol), and ammonium supported on the surface of KP–GO. Finally, the mixture was
heptamolybdate (0.02 g, 0.02 mmol) that were grounded centrifuged and the catalyst was washed with H2O (3 ꢂ 20 mL) and
together. The obtained mixture was irradiated at 130 C for 10 EtOH (3 ꢂ 20 mL) in sequence and dried under vacuum at 80 ꢀC.
ꢀ
min. The crude product was puried using the method reported
in the mentioned references and the nal PdPc dried in vacuum
at 80 ꢀC. Palladium trisulfophthalocyanine sulfonyl chloride
was produced by heating the synthesized PdPc (1.0 mmol) with
3
General procedures for catalytic tests
3.1 Heck coupling. Styrene (0.1 g, 1.0 mmol) and an aryl
chlorosulfonic acid (7.0 mmol) in 1,2,4-trichlorobenzene halide (1.5 mmol) were added to a round-bottomed ask con-
ꢀ
(50 mL) at 180–190 C for 2 h.
taining K2CO3 (0.3 g, 2.0 mmol) and dispersed PdTSPc@KP–GO
2.2 Synthesis of graphene oxide (GO). GO was synthesized (98 mg) in H2O (10 mL). The reaction mixture was reuxed with
by a modied Hummers method.51,52 The graphite powder vigorous stirring for the respective reaction time. Aer
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completion, the mixture of the reaction was cooled and ltered which proves the presence of nitrogen, sulfur, and palladium in
to separate PdTSPc@KP–GO as precatalyst. The solvent of the the mentioned precatalyst (Fig. 2).
ltrate was evaporated (under vacuum) and the residue was
The cysteine, serine, threonine, arginine, and lysine of the
analyzed by GC method (1-dodecane as an internal standard).
keratin protein, which have –OH and –NH2 functional groups
3.2 Sonogashira coupling. A mixture of phenylacetylene can participate in the nucleophilic substitution reaction with the
(0.1 g, 1.0 mmol), an aryl halide (2.0 mmol), and dispersed –COCl group of the acylated graphene oxide (GO–COCl) and the
PdTSPc@KP–GO (123 mg) in 5.0 mL of H2O and a solution of –SO2Cl group of the palladium trisulfophthalocyanine sulfonyl
K2CO3 (0.3 g, 2.0 mmol) and CuI (4.0 mol%) in 5.0 mL of H2O chloride to produce KP–GO and PdTSPc@KP–GO, respectively.
were prepared, separately. In continue, the solution of K2CO3 The covalent interactions between GO, keratin protein, and
and CuI were added to the mixture of phenylacetylene, aryl palladium(II) complex in PdTSPc@KP–GO were indicated by FT-
halide, and PdTSPc@KP–GO. The mixture of the reaction was IR method (Fig. 3). FT-IR spectrum of GO reveals characteristic
stirred under reux conditions for the indicated time in Table 2. bands at 3375, 1724, 1569, 1384, 1208, and 1049 cmꢁ1, which
The mixture was cooled to room temperature. Aer separation exhibit the successful synthesis of graphene oxide nanosheets
of PdTSPc@KP–GO precatalyst and evaporation of the ltrate and the presence of oxygenated functional groups on the gra-
solvent under vacuum, the residue was analyzed by GC method phene skeleton: –OH (n ¼ 3375 cmꢁ1), carboxyl –C]O and –C–O
(1-dodecane as an internal standard).
(n ¼ 1724 cmꢁ1 and 1384 cmꢁ1 respectively), aromatic –C]C– (n
¼ 1569 cmꢁ1), epoxy –C–O (n ¼ 1208 cmꢁ1) and alkoxy –C–O (n ¼
1049 cmꢁ1).53 The chemical attachment of the keratin protein to
the GO is conrmed by the presence of the new absorption
bands at 2958, 2920, and 2850 cmꢁ1 in the IR spectra that are
related to the C–H of the CH2 groups of keratin protein. The
elimination of the adsorption band at 1724 cmꢁ1 (–C]O of
–CO2H on the graphene oxide skeleton) and formation of the
new adsorption band at the 1662 cmꢁ1 (–C]O of –CONH–) is
conrming the formation of the amide bonds from the reaction
between the –NH2 group of keratin protein and –COCl group of
acylated GO. Also, the removal of the absorption band at the
1384 cmꢁ1 and formation of the new band at the 1446 cmꢁ1 that
are related to the –C–O of –CO2H skeleton of the GO and –C–N of
–CONH–, respectively is approving the chemically attachment of
the keratin protein to GO.
The covalent attachment of the PdTSPc to the KP–GO is
conrmed by the observing new adsorption band at the 1375
cmꢁ1, related to the S]O of sulfonamide functional group via
the reaction between the –NH2 group of the KP–GO and the
–SO2Cl group of the palladium trisulfophthalocyanine sulfonyl
chloride.
The thermal stability of PdTSPc@KP–GO precatalyst was
examined by thermogravimetric analysis (TGA). The thermo-
grams of GO and PdTSPc@KP–GO are shown in Fig. 4. The main
Results and discussion
Aer preparation of GO and palladium trisulfophthalocyanine
sulfonyl chloride, extraction of keratin protein from natural
wool, and subsequently surface modication of GO by keratin
protein of wool, PdTSPc@KP–GO was synthesized by simple
mixing of the palladium trisulfophthalocyanine sulfonyl chlo-
ride with keratin-protein-graed graphene oxide. The Pd
content of the PdTSPc@KP–GO precatalyst was determined
using ame atomic absorption spectrometry (FAAS) method.
The amount of Pd in the PdTSPc@KP–GO precatalyst was ob-
tained 0.86%. Inductively coupled plasma optical emission
spectrometry (ICP-OES) corroborates the amount of Pd in the
PdTSPc@KP–GO determined using FAAS method. In addition,
the amount of PdTSPc in the PdTSPc@KP–GO was measured
using UV-vis method (Fig. 1). To do that, PdTSPc@KP–GO was
dissolved in H2SO4 (5.0 mL) with sonication and then the total
volume of the solution made up to about 50 mL with H2O. In
continue, the concentration of Pd(II) tetrasulfophthalocyanine
was determined using calibration curves of prepared PdTSPc
solution standards. The obtained result using UV-vis method
was consistent with the result of FAAS and ICP-OES methods.
The energy dispersive spectroscopy (EDS) analysis was used
to determine the chemical composition of PdTSPc@KP–GO,
Fig. 1 UV-vis spectrums of GO, KP–GO, and PdTSPc@KP–GO in
H2O : H2SO4 (9 : 1).
Fig. 2 EDS result for PdTSPc@KP–GO.
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Fig. 4 TG curves of GO and PdTSPc@KP–GO in air.
of graphene oxide nanosheets shows the diffraction peak at
2q ¼ 11.3ꢀ that indicates graphite is oxidized completely. Two
broad peaks of natural wool bers are observed around 2q of 9ꢀ
and 21ꢀ whereas the wool keratin protein graed graphene
oxide nanosheets manifests peaks at around 2q ¼ 9.9ꢀ and
19.8ꢀ. Compared to characteristic diffraction peaks of the
natural wool, the XRD pattern for the wool–keratin-protein
graed graphene oxide nanosheets clearly shows the decrease
of the peak at about 9ꢀ and the enlargement of the peak at about
19.8ꢀ; because, the a-helix structure of keratin protein is
destroyed during the dissolving process and is not restored.55
The XRD pattern of PdTSPc indicates the characteristic
diffraction peaks at 2q values of 4.7ꢀ, 8.2ꢀ, 9.6ꢀ, 26.6ꢀ, 31.5ꢀ,
45.2ꢀ, 56.2ꢀ, 66.0ꢀ, and 74.9ꢀ. In the XRD pattern of
PdTSPc@KP–GO, the characteristic diffraction peaks at around
2q ¼ 9.8ꢀ and 19.4ꢀ are related to the KP–GO. Also, the
diffraction peaks of PdTSPc are observable at 2q values of 26.6ꢀ,
31.5ꢀ, and 45.2ꢀ in the mentioned catalyst. At around 2q ¼ 9ꢀ the
diffraction peak of PdTSPc@KP–GO related to the KP–GO are
observable; the mentioned peak is stronger than the diffraction
peaks of PdTSPc at the same region, therefore, the peaks related
to the PdTSPc are not obviously evident. It should be mentioned
that in the XRD patterns of GO, natural wool, KP–GO, PdTSPc,
Fig. 3 FT-IR spectra of GO, natural wool, KP–GO, PdTSPc, and
PdTSPc@KP–GO.
ꢀ
mass loss of GO takes place above 170 C, which it should be
related to CO, CO2, and steam release from the thermal
decomposition of the most labile oxygen-containing functional
groups.54 The result related to TGA thermogram of
PdTSPc@KP–GO revealed the high thermal stability of it. The
main decomposition process of PdTSPc@KP–GO occurs in the
temperature range of 155–527 ꢀC owing to the decomposition of
labile oxygen functional groups of GO support, graed-keratin-
protein, and PdTSPc. PdTSPc@KP–GO becomes thermally more
stable than GO during the heating process due to the func-
tionalization of labile oxygen functional groups by keratin
protein.
Further characterization of the structure of the catalyst was
done using X-ray diffraction (XRD) patterns of GO, natural wool,
Fig. 5 XRD patterns of GO, natural wool, KP–GO, PdTSPc, and
KP–GO, PdTSPc, and PdTSPc@KP–GO (Fig. 5). The XRD pattern PdTSPc@KP–GO.
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and PdTSPc@KP–GO the diffraction peaks at 2q ¼ 44ꢀ and 64ꢀ it is observed that two different types of nitrogen are present in
are related to the aluminum sample holder.56
the PdTSPc@KP–GO nanocomposite. The N 1s spectra could be
The keratin protein was used for the chemical modication divided into two peaks, one peak at 399.09 eV ascribed to sp2
of the graphene oxide to make it suitable for the attaching the nitrogen in the phthalocyanine ring of the PdTSPc complex and
PdTSPc chemically. To study the structure, morphology, and another peak at 400.09 eV attributed to the aliphatic nitrogen of
surface of the prepared material, the scanning electron the keratin protein.58,59 The S 2p spectrum of PdTSPc@KP–GO is
microscopy (SEM) and transmission electron microscopy (TEM) shown in Fig. 8c. There are two kinds of S containing group,
methods were used (Fig. 6). The SEM image in Fig. 6a shows –SO3H and –S–S–, in PdTSPc@KP–GO, and their S 2p binding
that the graphene oxide nanosheets are uniform throughout the energies are different. The S 2p signal for PdTSPc@KP–GO
surface. Although, some wrinkles were detected on the surface nanocomposite includes two components. The rst component
of graphene oxide because of the randomly aggregated and located at 167.70 eV is attributable to –S–S– groups of the
crumpled thin nanosheets. As it is seen in representative SEM keratin protein.59 The second one situated at 168.74 eV is
image of KP–GO (Fig. 6c), it remains as a nanosheet-like attributed to –SO3H groups of PdTSPc.59 The peaks at 338.09
structure, but the wrinkles of the KP–GO are more in compar- and 343.35 eV are associated with palladium in the oxidation
ison with the GO due to the presence of the keratin protein. The state of II in the PdTSPc@KP–GO nanocomposite which
wrinkles of the nal prepared PdTSPc@KP–GO catalyst is much conrmed the +2 oxidation state of Pd in phthalocyanine ligand
higher (Fig. 6e) than parent KP–GO, because of the addition of (Fig. 8d).58
the PdTSPc to the system.
For examination of the catalytic activity of the prepared
The TEM image in the Fig. 6b shows thin layers of GO. In the PdTSPc@KP–GO precatalyst in C–C coupling reactions, Heck
case of the KP–GO, Fig. 6d, an increase in the layer thickness is coupling of bromobenzene with styrene and Sonogashira
observable because of the addition of the keratin protein layer coupling of bromobenzene with phenylacetylene were selected
to the GO. By adding the PdTSPc to the KP–GO, the thickness of as model reactions. To optimize the required base and the
the prepared catalyst was increased as it is observable in the amount of catalyst, the mentioned reactions were performed in
Fig. 6f.
H2O as green reaction media and the results are summarized in
Phthalocyanine molecules are prone to aggregation, which Table 1. The comparison of the results related to the effect of the
greatly decreases the active sites of the catalyst. Therefore, by various amounts of PdTSPc@KP–GO precatalyst revealed that
attaching phthalocyanine molecules to large planar graphene the 0.8 and 1.0 mol% of PdTSPc@KP–GO are providing the best
oxide sheets, aggregation between the phthalocyanine mole- result for Heck coupling of bromobenzene with styrene and
cules is prevented during the reaction processes, maintaining Sonogashira coupling of bromobenzene with phenylacetylene,
isolation between catalytically active sites.57 Due to the unique respectively. PdTSPc@KP–GO was found to give good yield with
structure of keratin-protein-graed graphene oxide nanosheets Heck coupling of bromobenzene with styrene (entry 1) and to be
(KP–GO), use of KP–GO as support for covalent immobilization effective for Sonogashira coupling of bromobenzene with phe-
of PdTSPc prevents the aggregation of PdTSPc. Preventing of nylacetylene (entry 8). Palladium tetrasulfophthalocyanine as
aggregation of PdTSPc is providing more active sites for the a preference for the Pd(II) oxidation state has complete coordi-
catalytic application. To prove this claim, the SEM and TEM native saturation of the central Pd atom and high chemical and
images of prepared PdTSPc@KP–GO catalyst were provided. thermal stability. The very rigid structure and extraordinarily
The SEM and TEM images of prepared PdTSPc@KP–GO catalyst high complex stability of PdTSPc lead to the very limited release
in comparison with GO and KP–GO did not show any aggrega- of Pd when heated under coupling reaction conditions. Since Pd
tion which conrmed the monomeric molecular dispersion of released by palladium tetrasulfophthalocyanine would be
PdTSPc on the surface of KP–GO.
limited and completely unsaturated, PdTSPc is a very applicable
Raman spectroscopy, as a useful tool for investigating the model system to catalyze C–C coupling reactions.12 On the other
electronic and phonon structure of graphene-based materials, hand, PdTSPc remains thermally and chemically stable even
was used to study the synthesized GO and PdTSPc@KP–GO. The under the harsh reaction conditions needed for the activation
Raman spectra of prepared GO and PdTSPc@KP–GO are shown and conversion of demanding substrates.
in Fig. 7. Raman spectrum of GO shows two prominent peaks at
GO shows good dispersity in water. Since graphene oxide is
1578 and 1305 cmꢁ1 related to G-band and D-band, respectively. inert for chemically PdTSPc–graphene interactions, wool
The G-band is the result of the rst-order scattering of the E2g keratin protein as amphiphilic biomaterial with hydrophobic
mode of sp2 carbon domains and D-band is characteristic of and hydrophilic segments in backbone or side groups was
a breathing mode for k-point phonons of A1g. The characteristic extracted from natural wool and used as “adhesive” to chemi-
D and G bands of PdTSPc@KP–GO are observed around 1312 cally link PdTSPc and graphene oxide. The keratin protein was
and 1589 cmꢁ1, respectively, which indicate the D-band and G- added to the GO to make it suitable support to chemically
band of GO.
immobilize the PdTSPc. The high dispersity of the catalytic
The electronic property of PdTSPc@KP–GO nanocomposite system is essential for its high efficiency; therefore, the dis-
was explored by X-ray photoelectron spectroscopy (XPS) anal- persity of the prepared catalytic system was studied in water (as
ysis. As shown in Fig. 8, the peaks corresponding to C 1s, N 1s, O reaction media) to evaluate its stability and dispersibility. The
1s, S 2s and 2p, and Pd 3s, 3p and 3d was clearly observed in the results showed that not only the dispersity of the prepared
XPS survey spectrum. From the N 1s XPS scan shown in Fig. 8b, catalyst is not decreased but also, it was increased to some
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Fig. 6 SEM and TEM images of GO (a, b), KP–GO (c, d), and PdTSPc@KP–GO (e, f).
extent. Both of GO and PdTSPc@KP–GO showed good dispersity aqua soluble palladium tetrasulfophthalocyanine increased the
and stability in water, but PdTSPc@KP–GO is more dispersible hydrophilic properties of the PdTSPc@KP–GO, which conse-
and stable in long times (3 weeks). Use of keratin protein and quently the stability and dispersity of the precatalyst were
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accomplished using PdTSPc and PdTSPc@KP–GO under the
same reaction conditions. The obtained results which are
shown in Table 1 (entries 1, 7, 8, 14) showed that PdTSPc@KP–
GO gave better conversions and it has higher catalytic activity
than unsupported PdTSPc. It could be concluded that KP–GO
enhance the catalytic activity of the prepared system in Heck
and Sonogashira coupling reactions. Use of KP–GO as a support
for covalent immobilization of PdTSPc can promote the
adsorption of aromatic reactants because of the p–p interaction
between graphene support and benzene skeleton of the reac-
tants. Also, the good dispersity of PdTSPc@KP–GO in the
aqueous solution as an important factor seems to enhance the
efficiency of prepared precatalyst.
Fig. 7 Raman spectra of GO(I) and PdTSPc@KP–GO(II).
To examine the controlled temporary release of active
species from PdTSPc@KP–GO in Heck coupling of styrene and
bromobenzene, the catalyst was separated from reaction media
aer 9 h in two different conditions. Once immediately from the
improved in aqueous solution (Fig. 9). In addition, by immo-
bilization of the water soluble PdTSPc on the insoluble keratin-
protein-graed graphene oxide nanosheets via covalent
attachment, the separation process of the homogeneous
PdTSPc is simplied.
To show the role of the support in C–C coupling reactions,
Heck coupling of bromobenzene with styrene and Sonogashira
coupling of bromobenzene with phenylacetylene were
ꢀ
hot reaction media (100 C) and another one aer cooling the
reaction mixture. Flame atomic absorption spectrometry (FAAS)
method was used to determine the palladium content in the
both of the solutions. The analysis of the hot and cooled residue
solution was showed that the Pd content in the reaction media
is 2.1 ppm and below 0.1 ppm, respectively, which it could be
Fig. 8 (a) Full-range XPS spectrum of PdTSPc@KP–GO nanocomposite. (b) N 1s, (c) S 2p, (d) Pd 3d core-level region XPS spectra of PdTSPc@KP–
GO nanocomposite.
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Table 1 Optimization of the reaction conditions for the Heck coupling of bromobenzene with styrene and Sonogashira coupling of bromo-
benzene with phenylacetylene by PdTSPc@KP–GO precatalysta,b
Entry
Amount of catalyst (Pd content/mol%)
Base
Yieldc
1
2
3
4
98 mg (0.8)
86 mg (0.7)
74 mg (0.6)
98 mg (0.8)
98 mg (0.8)
98 mg (0.8)
8.0 mg (0.8)
123 mg (1.0)
111 mg (0.9)
98 mg (0.8)
123 mg (1.0)
123 mg (1.0)
123 mg (1.0)
10 mg (1.0)
K2CO3
K2CO3
K2CO3
Na2CO3
NaOAc
Et3N
K2CO3
K2CO3
K2CO3
K2CO3
Na2CO3
NaOAc
Et3N
87
71
42
41
72
69
53
85
64
49
32
78
76
45
5
6
7d
8
9
10
11
12
13
14d
K2CO3
a
Reaction conditions for Heck coupling of bromobenzene with styrene (entries 1–7): bromobenzene (1.5 mmol), styrene (1.0 mmol), base (2.0
b
mmol), H2O (10 mL), reux, 9 h. Reaction conditions for Sonogashira coupling of bromobenzene with phenylacetylene (entries 8–14):
bromobenzene (2.0 mmol), phenylacetylene (1.0 mmol), CuI (4.0 mol%), base (2.0 mmol), H2O (10 mL), reux, 9 h. GC yield, n-dodecane was
used as an internal standard. PdTSPc was used as precatalyst.
c
d
presence of 98 and 123 mg of PdTSPc@KP–GO as preferred
amounts of precatalyst, respectively (Table 2). As expected, the
catalytic activity of PdTSPc@KP–GO depended on the halide in
the order ArCl < ArBr < ArI (entries 1–3 and 8–10). On the other
hand, electron-withdrawing groups on the aryl halide ring
increased the rate of reaction. As indicated in Table 2, aer 9 h
reaction time for Heck coupling of styrene with different bro-
mobenzenes, the yield of the coupling product was found to be
80% for deactivated 4-bromoanisole (entry 4), 87% for non-
activated bromobenzene (entry 2), and 99% for activated 4-
bromonitrobenzene (entry 6).
To examine the leaching of PdTSPc from PdTSPc@KP–GO, in
Heck coupling of styrene and bromobenzene, the catalyst was
Fig. 9 Dispersity of GO and PdTSPc@KP–GO after 3 weeks.
removed by ltration aer ꢃ30% conversion. The residue was
applied to further treatment under similar reaction conditions
concluded that release of Pd2+ active species from PdTSPc@KP–
for 24 h. Aer determination of the conversion by GC method,
the obtained results showed that the reaction did not proceed
and no coupling product was detected. Also, FAAS shows that
the palladium content in the residue solution is below 0.1 ppm
which leaching in the PdTSPc@KP–GO catalyst is negligible.
With respect to the mentioned results, it could be concluded
that excellent heterogeneity is displayed by PdTSPc@KP–GO.
In Table 3, the results of our investigations have been
compared to previous reports of graphene-based palladium
complex catalysts.14–20,63 The main advantages of the present
system in comparison with the previous graphene-based palla-
dium complex catalysts are the controlled and reversible release
of the Pd from the precatalyst. The reversible release of the Pd
GO is reversible by cooling of reaction media. These observa-
tions are conrming the release of ligandless Pd2+ species in the
hot reaction media. Because the equilibrium constant of the
metal release from the complex is related to the temperature
and the release of metal will be increased by increasing the
temperature.60–62 Therefore, the obtained results can be attrib-
uted to the increase of metal release from the complex by
increasing the temperature.
Since, the optimized reaction conditions were achieved, the
scope and limitation of the mentioned method were investi-
gated for the Heck coupling of styrene and Sonogashira
coupling of phenylacetylene with different aryl halides in the
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Table 2 The Heck coupling of aryl halides with styrene and Sonogashira coupling of aryl halides with phenylacetylene by PdTSPc@KP–GO
precatalysta,b
Entry
R/X
Substrate
Yieldc
TON/TOF (hꢁ1
)
1d
2
3
4
5
H/Cl
H/Br
H/I
4-CH3/Br
4-CH3/I
4-NO2/Br
4-NO2/I
H/Cl
H/Br
H/I
4-CH3/Br
4-CH3/I
4-NO2/Br
4-NO2/I
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Styrene
Phenylacetylene
Phenylacetylene
Phenylacetylene
Phenylacetylene
Phenylacetylene
Phenylacetylene
Phenylacetylene
25
87
89
80
85
99
99
33
85
88
79
83
95
94
31.2 (3.47)
108.7 (12.08)
111.2 (12.36)
100.0 (11.11)
106.2 (11.80)
123.7 (13.75)
123.7 (13.75)
33.0 (3.66)
85.0 (9.44)
88.0 (9.77)
79.0 (8.77)
83.0 (9.22)
6
7
8d
9
10
11
12
13
14
95.0 (10.55)
94.0 (10.44)
a
Reaction conditions for Heck coupling of aryl halides with styrene: aryl halide (1.5 mmol), styrene (1.0 mmol), PdTSPc@KP–GO (98 mg), K2CO3
(2.0 mmol), H2O (10 mL), reux, 9 h. b Reaction conditions for Sonogashira coupling of aryl halides with phenylacetylene: aryl halide (2.0 mmol),
phenylacetylene (1.0 mmol), PdTSPc@KP–GO (123 mg), K2CO3 (2.0 mmol), CuI (4.0 mol%), H2O (10 mL), reux, 9 h. c GC yield, n-dodecane was used
as an internal standard. d Reaction time: 48 h.
makes it possible to reuse the catalyst. Also, irreversible
Recyclability of the PdTSPc@KP–GO precatalyst was exam-
precipitation of palladium in the reaction media, which affected ined in the Heck coupling of styrene and bromobenzene.
its catalytic activities, is prevented by controlled and reversible Accordingly, PdTSPc@KP–GO catalyst was separated from
ꢀ
release of palladium from macrocyclic phthalocyanine complex. reaction media by centrifuging, dried under vacuum at 60 C,
The interplay between the release and recoordination of Pd in and reused for ve repetitive cycles in the mentioned reaction.
the phthalocyanine ligand is the reason for high catalytic As indicated in Fig. 10, the results show that only minor
performances of PdTSPc@KP–GO as precatalyst in C–C decreases in the reaction yields are observable; it means that the
coupling reactions. Furthermore, the use of green keratin activity and efficiency of the catalyst were saved during succes-
protein to chemically attaching of the PdTSPc is providing the sive uses due to the high stability of PdTSPc@KP–GO under the
green and recyclable catalyst.
reaction conditions.
Table 3 Comparison of the results obtained from graphene-based palladium complex catalysts for the C–C coupling reactions
Molar ratio
Entry Reaction
Catalyst
Conditions
substrate to catalyst Recyclability Ref.
1
2
3
4
5
6
7
8
9
Suzuki reaction
GO–NHC–Pd
DMF–H2O/Cs2CO3/50 ꢀC/1 h
EtOH–H2O/K2CO3/80 ꢀC/1–24 h
EtOH–H2O/K2CO3/60 ꢀC/2.5 h
EtOH–H2O/K2CO3/r.t./2.5 h
H2O/K2CO3/r.t./2.5 h
1 : 0.0100
1 : 0.0025
1 : 0.0010
1 : 0.0090
1 : 0.0090
5 runs
6 runs
5 runs
20 runs
20 runs
4 runs
4 runs
6 runs
8 runs
7 runs
14
15
16
17
17
63
63
18
19
20
Suzuki reaction
Suzuki reaction
Heck reaction
Suzuki reaction
Heck reaction
Suzuki reaction
Suzuki reaction
Heck reaction
Suzuki reaction
GO–NHC–Pd2+
NHC–Pd/GO–IL
Pd@AGu@MGO
Pd@AGu@MGO
Pd(II)–FRGO
DMF/Et3N/100–120 ꢀC/10 or 20 h 1 : 0.0100
Pd(II)–FRGO
GO–2N–Pd(II)
TRGO–NPy–Pd
EtOH–H2O/Na2CO3/r.t./8 h
EtOH/K2CO3/80 ꢀC/0.5–20 h
DMF/Na2CO3/140 ꢀC/1.5–21 h
1 : 0.0025
1 : 0.0050
1 : 0.0030
1 : 0.0110
10
Co/C–ROMPgel immobilized THF–H2O/Na2CO3/65 ꢀC/2–12 h
Pd-complex
11
12
Heck reaction
Sonogashira reaction PdTSPc@KP–GO
PdTSPc@KP–GO
H2O/K2CO3/ref./9 h
H2O/K2CO3/ref./9 h
1 : 0.0080
1 : 0100
5 runs
—
This work
This work
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Conclusion
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¨
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