Magnetically recoverable graphene oxide supported Co@Fe3O4/L-dopa for C-C cross-coupling and oxidation reactions in aqueous medium

Article abstract of DOI:10.1016/j.mcat.2019.02.023

We report the synthesis of inexpensive and environmentally benign cobalt(0) nanoparticles on L-3,4-dihydroxyphenylalanine (L-dopa) functionalized and magnetite (Fe₃O₄) grafted graphene oxide (Co@GO/Fe₃O₄/L-dopa) which was fully characterized with different techniques such as Scanning Electron Microscopy (SEM), High Resolution Transmission Electron Microscopy (HR-TEM), X-Ray Photoelectron Spectroscopy (XPS), X-ray Powder Diffraction (XRD), Thermogravimetric analysis (TGA), Fourier Transform Infrared Spectroscopy (FTIR), Vibrating Sample Magnetometer (VSM), Carbon Hydrogen Nitrogen (CHN) analysis, Energy Dispersive X-ray (EDX) and Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). Graphene oxide was used as the core material due to its mechanical, electrical and thermal properties. In order to avoid the cumbersome process of separation, magnetite nanoparticles were coated over the graphene oxide. After the successful preparation of graphene oxide based magnetic nanoparticles, L-dopa was grafted over Fe₃O₄ nanoparticles so as to provide firm anchoring agent for cobalt nanoparticles. Finally, cobalt(0) nanoparticles were immobilized on the developed magnetic support. The catalytic efficiency of the synthesized catalyst was tested for Suzuki cross-coupling and oxidation reactions, usually carried out by precious and expensive second and third row transition metals; products were obtained in good to excellent yields. The synthesized catalyst represents an attractive alternative to conventional catalysts for Suzuki cross-coupling and oxidation reactions, and is recyclable up to five runs.

Full text of DOI:10.1016/j.mcat.2019.02.023

MolecularCatalysis469(2019)27–39
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
Magnetically recoverable graphene oxide supported Co@Fe3O4/L-dopa for
C-C cross-coupling and oxidation reactions in aqueous medium
⁎
Harsha Sharmaa, Shally Sharmaa, Chandan Sharmaa, Satya Paula, , James H. Clarkb
a Department of Chemistry, University of Jammu, Jammu, 180006, India
b Green Chemistry Centre of Excellence, Department of Chemistry, University of York, Heslington, York, YO10 5DD, UK
A R T I C L E I N F O
A B S T R A C T
Keywords:
We report the synthesis of inexpensive and environmentally benign cobalt(0) nanoparticles on L-3,4-dihy-
droxyphenylalanine (L-dopa) functionalized and magnetite (Fe3O4) grafted graphene oxide (Co@GO/Fe3O4/L-
dopa) which was fully characterized with different techniques such as Scanning Electron Microscopy (SEM),
High Resolution Transmission Electron Microscopy (HR-TEM), X-Ray Photoelectron Spectroscopy (XPS), X-ray
Powder Diffraction (XRD), Thermogravimetric analysis (TGA), Fourier Transform Infrared Spectroscopy (FTIR),
Vibrating Sample Magnetometer (VSM), Carbon Hydrogen Nitrogen (CHN) analysis, Energy Dispersive X-ray
(EDX) and Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES). Graphene oxide was used as
the core material due to its mechanical, electrical and thermal properties. In order to avoid the cumbersome
process of separation, magnetite nanoparticles were coated over the graphene oxide. After the successful pre-
paration of graphene oxide based magnetic nanoparticles, L-dopa was grafted over Fe3O4 nanoparticles so as to
provide firm anchoring agent for cobalt nanoparticles. Finally, cobalt(0) nanoparticles were immobilized on the
developed magnetic support. The catalytic efficiency of the synthesized catalyst was tested for Suzuki cross-
coupling and oxidation reactions, usually carried out by precious and expensive second and third row transition
metals; products were obtained in good to excellent yields. The synthesized catalyst represents an attractive
alternative to conventional catalysts for Suzuki cross-coupling and oxidation reactions, and is recyclable up to
five runs.
Graphene oxide
Magnetic nanoparticles
Cobalt nanoparticles
L-dopa
Suzuki cross-coupling
Oxidation reactions
1. Introduction
consequence, there is substantial interest towards the use of hetero-
geneous and recyclable catalysts in synthetic organic chemistry [9].
Transition metals are renowned for catalyzing wide range of im-
portant organic transformations such as CeC, C–N, C–O and C–S cross
coupling reactions, hydrogenations and oxidations [1–4]. But, it has
been observed that expensive and costly second/third row transition
metals are generally used for catalyzing majority of important organic
transformations which makes the overall process costlier [5,6]. In this
context, 3d transition metal based catalytic systems represent an eye-
catching alternative to carry out such reactions because of their natural
abundance and cost-efficiency. As a result, there is a paradigm shift in
the synthetic chemical industries toward the use of inexpensive first-
row transition metal catalysts for carrying out important industrial
transformations. Out of available first-row transition metal catalysts,
cobalt-based catalysts have gained much impetus owing to their natural
abundance and inexpensive nature [7,8]. But their separation from the
reaction mixture is a tiresome task and moreover, it undergoes ag-
Heterogenization of active metal species on different support materials
not only obviates the use of tedious procedures like distillation or ex-
traction, but also leads to greater activity and selectivity [10]. Different
support materials are available such as silica [11,12], clay [12], bio-
material [13], polymers [14,15], etc. which can be used for the het-
erogenization of the active metal species.
Currently, nanocarbon materials have attracted accumulative con-
sideration in the field of catalysis due to their remarkable catalytic,
electronic, mechanical, optical, thermal and magnetic properties; ef-
fortless recovery and large surface area [16,17]. Among the various
well-known nanocarbon materials, graphene has exhibited unique
properties and exceptional tunability in supporting an array of metallic
and bimetallic nanoparticles [18,19]. Graphene possesses a perfect two-
dimensional carbon nanostructure with high surface area, tremendous
stability (mechanical, thermal and chemical) and outstanding accessi-
bility [20]. In addition to this, the occurrence of structural defects in
glomeration in solution which reduces their efficiency. As
a
⁎ Corresponding author.
E-mail addresses: paul7@rediffmail.com (S. Paul), james.clark@york.ac.uk (J.H. Clark).
https://doi.org/10.1016/j.mcat.2019.02.023
Received 8 January 2019; Received in revised form 25 February 2019; Accepted 26 February 2019
2468-8231/©2019ElsevierB.V.Allrightsreserved.

H. Sharma, et al.
MolecularCatalysis469(2019)27–39
Scheme 1. Proposed scheme for the synthesis of Co@GO/Fe3O4/L-dopa.
graphene may be advantageous as they aid in creating new surface
functionalities which enhance the interactions with the anchored active
metal nanoparticles. Moreover, high electron density and extended π-
system in graphene is responsible for the easier reduction of metal ions,
thus helping in designing active metal centers anchored onto the gra-
phene support.
cost-effective and easily separable catalytic systems become the central
focus in industries including pharmaceutical manufacturing.
The oxidation of organic compounds remains the principle method
for functionalizing hydrocarbons [29]. The catalytic oxidation of
benzylic C–H bonds is one of these key oxidative transformations and
has wide applicability in the production of agrochemicals and medi-
cines [30,31]. Generally, benzylic C–H bond oxidation reactions have
been carried out using homogeneous catalysis and hence recyclability
of these expensive catalysts remains one of the major challenge [32].
Recently, heterogeneous noble transition metal catalyst along with
environment friendly oxidants, such as hydrogen peroxide, oxygen or
tert-butyl hydroperoxide (TBHP) have appeared as excellent alter-
natives for the C–H bond oxidation as compared to their homogenous
counterparts due to the synergistic effect [33].
Furthermore, separation of the catalytic system from the reaction
mixture is a laborious task even in the case of heterogeneous catalysis.
In this context, magnetic nanoparticles (MNPs) have attracted a lot of
attention in the field of heterogeneous catalysis owing to their dis-
tinctive physical properties and magnetic nature [21]. Usage of mag-
netic catalyst in heterogeneous catalysis offers several merits viz.
elimination of the necessity of filtration, centrifugation or other pro-
cedures required for the separation of the precious catalysts [22].
Moreover, MNPs may serve both as catalysts and as magnetic carriers
for active metal species. The bare MNPs undergo agglomeration in
extremely acidic or corrosive reaction conditions. However, agglom-
eration of MNPs can be overcome by grafting of organic/inorganic
materials [23,24].
In the present work, we report the synthesis of graphene based
magnetic cobalt nanoparticles and their full characterization using
various techniques such as SEM, HR-TEM, XPS, XRD, TGA, FTIR, VSM,
CHN analysis, EDX and ICP-AES studies. Here, graphene oxide is used
as a support material owing to its high surface area, tremendous sta-
bility and exceptional tunability in supporting an array of active cata-
lytic species. Afterwards, Fe3O4 nanoparticles were immobilized on the
graphene oxide support using ultra sonication to increase stability.
Grafting of Fe3O4 onto the surface of graphene oxide imparted magnetic
character to the synthesized support and this facilitated the easier se-
paration of the catalyst from reaction mixture using external magnet.
Finally, cost-effective first-row transition metal, cobalt(0) nanoparticles
were immobilized on graphene oxide based magnetite nanoparticles
using L-dopa as interparticle linker. L-dopa was used because of its
The Suzuki cross-coupling reaction is the most viable protocol in the
synthetic organic chemistry for the CeC bond formation [25]. These
reactions have been extensively used for the production of substituted
biaryls, which have a wide range of consumption in the chemical,
agriculture, and pharmaceutical industries. Palladium based catalysts
are scrutinized as the best catalysts for CeC cross-coupling reaction,
offering high and fast conversion rate and good selectivity [26–28]. But,
owing to the expensive nature of palladium catalysts and increasing
concerns over the long term availability of the metal, development of
28

H. Sharma, et al.
MolecularCatalysis469(2019)27–39
Fig. 1. SEM images of Co@GO/Fe3O4/L-dopa.
unique functionalized surface [34] which plays an important role in the
strong binding of active metal species, such as cobalt nanoparticles over
heterogeneous surfaces and thereby prevents leaching. Co(0) nano-
particles were chosen to carry out Suzuki cross-coupling reactions
which are usually achieved by precious metals (Pd or Rh). All of the
products were obtained in good yield. We further used our new cata-
lytic materials in the oxidation of hydrocarbons and secondary alcohols
and the corresponding products were obtained in good to excellent
yields. Thus, Co@GO/Fe3O4/L-dopa has appeared as an efficient and
cost-effective catalyst for the Suzuki cross-coupling and oxidation re-
actions as compared to costlier 4d/5d transition metals.
analysis using ARCOS from M/s Spectro, Germany. X-ray diffratograms
(XRD) were recorded in 2 theta range of 10-80° on a Bruker AXSDB X-
ray diffractometer using Cu Kα radiations.
2.2. Synthesis of Co@GO/Fe3O4/L-dopa
For the synthesis of core-shell particles of GO/Fe3O4, Graphene
oxide (2 g) prepared by modified Hummer process [35] was dispersed
in deionized water (50 mL) and ultrasonicated for 30 min to get a stable
ferrofluid. Subsequently, ammonical solution (30 mL, 24 mL water and
6 mL NH4OH) of FeSO4 (2.4 g) and Fe2(SO4)3 (7 g) was added into the
above ferrofluid and again sonicated for 60 min to get GO/Fe3O4 na-
noparticles. GO/Fe3O4 nanoparticles so obtained were filtered, washed
with deionized water (3 × 10 mL), methylene chloride (3 × 10 mL) and
dried under vacuum. For the coating of L-dopa over the surface of GO/
Fe3O4 nanoparticles, the mixture of GO/Fe3O4 (1 g) and L-dopa (0.2 g)
was stirred in deionized water (10 mL) at 120 °C for 2 h. Black pre-
cipitates of GO/Fe3O4/L-dopa were separated magnetically, washed
with deionized water (3 × 10 mL) and dried under vacuum. Lastly, for
the immobilization of Co nanoparticles onto GO/Fe3O4/L-dopa, Co (0)
nanoparticles so prepared by the reduction of the aqueous solution of
Co(acac)2 (0.134 g, 1.0 mmol, 3 mL) using aqueous solution of NaBH4
(1.2 mmol, 5 mL), were added into the dispersed solution of GO/Fe3O4/
L-dopa (1 g) in ethanol (10 mL) and stirred for 3 h at room temperature.
2. Experimental
2.1. General remarks
All the chemicals used were purchased from Aldrich Chemical
Company or Merck. The 1H and 13C NMR spectra were recorded in
CDCl3 on Bruker Avance III, 400 MHz and 100 MHz respectively. The
FTIR spectra were recorded on Perkin-Elmer FTIR spectrophotometer.
High Resolution Transmission Electron Micrographs (HR-TEM) were
recorded on Philips CM-200. SEM images were recorded using FEG SEM
JSM-7600F. EDX analysis was carried out using OXFORD X-MAX JSM-
7600 and the amount of metal in catalyst was determined by ICP-AES
29

H. Sharma, et al.
MolecularCatalysis469(2019)27–39
Fig. 2. HR-TEM images of Co@GO/Fe3O4/L-dopa.
Co@GO/Fe3O4/L-dopa was separated magnetically and washed suc-
cessively with ethanol (3 × 25 mL) and deionized water (3 × 25 mL).
Finally, it was dried under vacuum at room temperature to get Co@GO/
Fe3O4/L-dopa nanoparticles as dark black powder.
pressure or by passing through column of silica gel using EtOAc-pet.
ether as eluting solvent.
2.4. General procedure for the Co@GO/Fe3O4/L-dopa catalyzed benzylic
C–H bond oxidations
2.3. General procedure for the Co@GO/Fe3O4/L-dopa catalyzed Suzuki
cross-coupling
Hydrocarbon or secondary alcohol (1 mmol), Co@GO/Fe3O4/L-
dopa (0.05 g, 1.84 mol% Co) and 70% t-BuOOH (0.25 mL, 1.5 mmol)
were dispersed in deionized water (5 mL) and stirred at 100 °C till the
completion of reaction (Table 5). Afterwards, catalyst was removed
from the reaction mixture via external magnet and the reaction mixture
was diluted with EtOAc (3 × 5 mL) and filtered. The organic fraction
was washed with brine solution (3 × 25 mL) and dried using anhydrous
Na2SO4. Finally, the product was obtained either by the removal of the
solvent under reduced pressure or by passing through column of silica
gel. The removed catalyst was washed with EtOH (3 × 5 mL), deionized
water (3 × 10 mL) and dried under vacuum.
A mixture of aryl halide (1 mmol), phenyl boronic acid (1.2 mmol),
K2CO3 (1.2 eq.) and Co@GO/Fe3O4/L-dopa (0.05 g, 1.84 mol% Co) in
double distilled water (5 mL) was stirred in a round bottom flask
(50 mL) at 100 °C till the completion of reaction (monitored by TLC)
(Table 3). After that, the reaction mixture was cooled to room tem-
perature. The catalyst was removed via external magnet and washed
with EtOAc (3 × 5 mL) followed by deionized water (3 × 10 mL). It
was dried under vacuum for 2 h. The organic fraction was washed with
brine solution and dried over anhydrous Na2SO4. Finally, the product
was obtained either by the exclusion of the solvent under reduced
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MolecularCatalysis469(2019)27–39
3. Results and discussion
3.1. Characterization of Co@GO/Fe3O4/L-dopa
Nanosized, water-dispersible and magnetically recoverable, hetero-
geneous graphene oxide based magnetic Co(0) nanoparticles, Co@GO/
Fe3O4/L-dopa was successfully synthesized. The general scheme for the
preparation of Co@GO/Fe3O4/L-dopa is described in Scheme 1. Firstly,
graphene oxide was synthesized from graphite powder using modified
Hummer’s method [28]. Then, Fe3O4 nanoparticles were grafted over
graphene oxide by co-precipitation of Fe3+ and Fe2+ salts using ultaso-
nication technique. Although, graphene oxide-magnetite composite is
quite stable but the aggregation and leaching of active metal nanoparticles
from its surface remains a drawback. In this context, L-dopa played an
important role in binding of metal nanoparticles and prevents their
leaching. The synthesized GO/Fe3O4 was functionalized with L-dopa and
finally, Co(0) nanoparticles were immobilized onto the support using Co
(acac)2 followed by reduction with aqueous NaBH4 solution. The novel
catalyst, Co@GO/Fe3O4/L-dopa was characterized by various techniques
such as SEM, HR-TEM, XPS, XRD, TGA, FTIR, VSM, CHN analysis, EDX
and ICP-AES studies.
3.1.1. Scanning Electron Microscopy (SEM)
The SEM images of Co@GO/Fe3O4/L-dopa depicted the surface
morphology of the catalyst. From SEM images (Fig. 1), it was clearly
observed that the synthesized catalyst is homogeneous in nature and Co
(0) nanoparticles were uniformly dispersed over the surface of GO/
Fe3O4/L-dopa. Fe3O4/L-dopa and cobalt nanoparticles assume a sphe-
rical shape, which is responsible for the low tendency of the nano-
particles to undergo agglomeration and can also account for their in-
creased catalytic activity.
Fig. 3. XPS spectra of Co@GO/Fe3O4/L-dopa: a) overall survey spectrum; b) Co
2p core level spectrum.
3.1.2. High resolution transmission Electron microscopy (HR-TEM)
HR-TEM was used to characterize the morphology, particle size and
distribution of cobalt(0) nanoparticles over the GO/Fe3O4/L-dopa. In
HRTEM image (Fig. 2a), sheets of graphene oxide of micron size are
clearly visible. Further, HRTEM images also reveals uniform coating of
cobalt(0) nanoparticles over the GO/Fe3O4/L-dopa support (Fig. 2b–c).
Additionally, it also illustrates that uniform covering of L-dopa was
formed over the GO/Fe3O4 support, which is accountable for the better
binding of Co(0) nanoparticles and helps in preventing the leaching of
Co nanoparticles.
3.1.3. X-Ray photoelectron spectroscopy (XPS)
The electronic property of the active species (oxidation state and
binding energy of the core shell electrons of the Co metal) over the
surface of the catalyst was explored using X-ray photoelectron spec-
troscopy (XPS) (Fig. 3). Fig. 3a showed the overall survey spectrum of
Co@GO/Fe3O4/L-dopa in which peaks corresponding to carbon 1 s
(284.9 eV), oxygen 1 s (532.6 eV) and cobalt 2p (778.5 eV) are clearly
observed. Further, Fig. 3b showed typical Co(0) absorptions at 778.5
and 793.4 eV for 2p3/2 and 2p1/2 respectively, which are in accordance
with the values for Co(0) nanoparticles already reported in literature
[36].
3.1.4. X-ray powder diffraction (XRD)
Unambiguous presence of cobalt nanoparticles over the surface of
GO/Fe3O4/L-dopa is also deciphered via X-ray powder diffraction
analysis (XRD). XRD absorption of GO (Fig. 4a) at 2θ = 10.2° having d-
spacing of 8.1 A° signifies the insertion of the hydroxyl and epoxy
groups in between the graphite sheets [37]. Further, XRD of GO/Fe3O4/
L-dopa (4b) showed absorption peaks at 2θ = 30.2, 35.4, 57.3 and
62.3° corresponding to [111], [220], [440] and [511] cubic phase
Fig. 4. XRD of: a) GO; b) GO/Fe3O4/L-dopa; and c) Co@GO/Fe3O4/L-dopa.
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MolecularCatalysis469(2019)27–39
Fig. 5. TGA of Co@GO/Fe3O4/L-dopa.
Fig. 5, which deciphers initial weight loss upto 140 °C attributed to the
exclusion of residual solvent or water trapped onto catalyst, and with
no further appreciable weight loss up to 389 °C. After 389 °C, significant
weight loss has occurred and is due to the chemical decomposition of
adsorbed material i.e., L-dopa from the Co@GO/Fe3O4/L-dopa.
3.1.6. Fourier transform infrared spectroscopy (FTIR)
The presence of different functional groups in Co@GO/Fe3O4/L-
dopa was confirmed by FTIR spectroscopy. FTIR spectra of GO, GO/
Fe3O4/L-dopa and Co@GO/Fe3O4/L-dopa are displayed in Fig. 6. FTIR
spectrum of GO (Fig. 6a) showed bands at 3114, 1721, 1633, 1487,
1007, 870, 609 and 550 cm−1. The band at 3114 cm−1 corresponds to
OeH stretching, whereas bands at 1633 and 1487 cm−1 corresponds to
the carboxylate group of graphene oxide. The bands at 1007 and
870 cm−1 corresponds to C–O symmetrical stretching and C–O-C
asymmetrical stretching of epoxide, respectively. The FTIR spectrum of
GO/Fe3O4/L-dopa (Fig. 6b) showed an absorption band at 612 cm−1
comparable to the Fe-O stretching vibration of Fe3O4, whereas bands at
3202, 1587, 1248, 1071 cm−1 are attributed to OeH symmetrical
stretching, C–H bending vibration and C–O symmetrical stretching of L-
dopa, respectively. Furthermore, FTIR spectrum of Co@GO/Fe3O4/L-
dopa (Fig. 6c) showed no significant absorption bands, but a slight
reduction in the intensity of absorption bands was observed which may
be due to the immobilization of Co nanoparticles over the support.
Thus, FTIR spectrum clearly indicated the formation of protective layer
of L-dopa over the GO/Fe3O4 nanoparticles.
Fig. 6. FTIR spectra of: a) GO/Fe3O4; b) GO/Fe3O4/L-dopa; and c) Co@GO/
Fe3O4/L-dopa.
3.1.7. Vibrating sample magnetometer (VSM)
planes of Fe3O4 lattice [38]. Also XRD spectrum of Co@GO/Fe3O4/L-
dopa (4c) showed 3 new peaks at 44.4, 47.5 and 75.6° corresponding to
[111], [200] and [220] planes of cubic phase of Co nanoparticles [39].
Thus, XRD spectra also helped in providing the information about the
nature of cobalt nanoparticles and successful grafting of Fe3O4 on
graphene oxide.
Vibrating Sample Magnetometry (VSM) was carried out to in-
vestigate the magnetic property of the catalyst at room temperature.
VSM of GO/Fe3O4, GO/Fe3O4/L-dopa and Co@GO/Fe3O4/L-dopa are
presented in Fig. 7. The magnetization curve of GO/Fe3O4 revealed
paramagnetic properties with saturation magnetization of about
33.5 emu/g. After grafting of L-dopa on GO/Fe3O4, the paramagnetic
behavior of GO/Fe3O4/L-dopa has decreased from 33.5 emu/g to
27.8 emu/g. The reduction in saturation value arises due to the coating
of the non-magnetic material (L-dopa) on the surface of magnetic ma-
terial (GO/Fe3O4) in the synthetic pathway of the novel catalyst.
3.1.5. Thermogravimetric analysis (TGA)
The TGA of the catalyst was carried out by heating the sample at the
rate of 10 °C min−1. The TGA of Co@GO/Fe3O4/L-dopa is presented in
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MolecularCatalysis469(2019)27–39
Fig. 7. VSM spectra of: a) GO/Fe3O4; b) GO/Fe3O4/L-dopa and c) Co@GO/Fe3O4/L-dopa.
Fig. 8. CHN analysis of Co@GO/Fe3O4/L-dopa.
Finally, after the successful immobilization of cobalt nanoparticles over
GO/Fe3O4/L-dopa, the magnetic moment was found to be 25.8 emu/g.
It is pertinent to mention that the synthesized catalyst is still magnetic
enough to be separated using magnet and assures facile and efficient
separation of the catalyst from the reaction mixture. Moreover, the
grafting of L-dopa over GO/Fe3O4 protects it from oxidation and fa-
cilitate it to disperse rapidly even in the absence of magnetic field.
elements. This further established the successful grafting of L-dopa over
GO/Fe3O4 nanaoparticles.
3.1.9. Energy dispersive X-ray (EDX)
Energy dispersive X-ray (EDX) spectrum was recorded to analyze
the elemental composition of Co@GO/Fe3O4/L-dopa, which revealed
that the catalyst is composed of C, O, Fe and Co elements (Fig. 9). This
further supports that synthesized catalyst was composed of Co nano-
particles immobilized on L-dopa functionalized magnetic graphene
oxide.
3.1.8. Carbon Hydrogen Nitrogen (CHN) analysis
The coating of L-dopa onto GO/Fe3O4 was further evaluated from
the CHN analysis (Fig. 8) and the result unveiled that catalyst is com-
posed of 1.89% of nitrogen, 1.06% of hydrogen and 10.3% of carbon
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MolecularCatalysis469(2019)27–39
Fig. 9. EDX spectrum of Co@GO/Fe3O4/L-dopa.
results were obtained, and further reactions were carried out using
K2CO3 base, since it is mild, and inexpensive. The test reaction was also
carried out in the presence of different solvents (water, ethanol, acet-
onitrile, methanol) keeping 80 °C as the reaction temperature and found
that water is an excellent solvent (entry 2, Table 1). Increasing the
reaction temperature from 80 to 100 °C, led to a significant increase in
the yield of the desired product (entry 6, Table 1); though no further
improvement was observed above 100 °C (entry 7, Table 1). Different
bases (K2CO3, Cs2CO3, Na2CO3) at 100 °C temperature were also scru-
tinized for the Suzuki coupling using Co@GO/Fe3O4/L-dopa as a cat-
alyst, and the best yield was obtained with K2CO3 at 100 °C (entries
6,8,9, Table 1). Thus, the optimized reaction conditions for the Suzuki
cross-coupling uses water as a green solvent and K2CO3 as a relatively
green base at 100 °C.
Table 1
Optimization of reaction conditions for the Suzuki cross-coupling reaction using
Co@GO/Fe3O4/L-dopa as a catalysta.
S. No.
Solvent
Base
Temperature (oC)
Yieldb (%)
1
2
3
4
5
6
7
8
9
Water
No
80
10
60
45
25
40
81
82
61
63
Water
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Cs2CO3
Na2CO3
80
Ethanol
Acetonitrile
Methanol
Water
80
80
80
100
110
100
100
Water
Water
Water
aReaction conditions: 4-bromoacetophenone (1 mmol), phenyl boronic acid
(1.2 mmol), base (1.2 eq), Co@GO/Fe3O4/L-dopa (0.1 g), solvent (5 mL) at
different temperature.bIsolated yield.
Different test were also performed to study the effect of catalyst
loading on the conversion rate and are summarized in Table 2. From
Table 2, it was concluded that 0.05 g (1.84 mol% Co) of the catalyst was
sufficient to carry out the cross-coupling of 4-bromoacetophenone and
phenylboronic acid smoothly under the optimized conditions.
Further, the scope of the catalytic system was explored for the cross-
coupling reaction between phenyl boronic acid and various substituted
aryl halides using the above optimized reaction conditions. The results
are summarized in the Table 3. It has been observed that the presence of
electron-withdrawing group on aryl halides enhanced the conversion to
biaryls as compared to unsubstituted aryl halides, whereas the presence
of electron-donating group decreases the conversion rate. Both bromo
and chloro substituted aryl halides reacted smoothly with phenyl
boronic acid in the presence of our catalytic system, though, bromo
substituted aryl halides gave somewhat better yields due to their better
leaving nature. With dibromo-benzene, the double substituted product
was obtained, but in the case of bromochloro benzene, inspite of double
substitution, monosubstitution of bromo group by the phenyl group
take place, leaving the chloro group intact. Thus, the catalyst showed
interesting selectivity in case of different halo substituted benzenes.
A plausible mechanism has been suggested for the Co@GO/Fe3O4/
L-dopa catalyzed Suzuki cross-coupling reaction which encompasses
oxidative addition followed by transmetallation and reductive elim-
ination. The coupling process initiated with the oxidative addition of
aryl halide to Co(0) resulted in the formation of intermediate I, which is
prone to react with nucleophiles. This step is followed by transme-
tallation to give intermediate II and reductive elimination leading to
the formation of corresponding biaryl (III) and the catalyst is re-
generated (Fig. 10).
Table 2
Effect of catalyst loading.a.
Entry
Catalyst (mol%)
Yieldb (%)
1
2
3
0.92
1.84
3.69
60
80
81
aReaction conditions: 4-bromoacetophone (1 mmol), phenyl boronic acid
(1.2 mmol), K2CO3 (1.2 eq), water (5 mL) at 100 °C.bIsolated yield.
3.1.10. Inductively coupled plasma atomic emission spectroscopy (ICP-
AES)
The amount of Co nanoparticles loaded over GO/Fe3O4/L-dopa was
estimated using Inductively Coupled Plasma Atomic Emission
Spectroscopy (ICP-AES). The results depicted that the content of Co
nanoparticles loaded onto 0.1 g of GO/Fe3O4/L-dopa was 2.14 wt%
(3.69 mol% Co).
3.2. Catalytic activity of Co@GO/Fe3O4/L-dopa for Suzuki cross-coupling
reaction
In order to evaluate the best possible reaction conditions for Suzuki
cross-coupling, the reaction between 4-bromoacetophenone and phenyl
boronic acid was chosen as the model reaction. Initially, the Suzuki
coupling was carried out with test substrates using Co@GO/Fe3O4/L-
dopa as a catalyst in the absence of base (entry 1, Table 1), but poor
34

H. Sharma, et al.
MolecularCatalysis469(2019)27–39
Table 3
Suzuki cross-coupling reaction of substituted aryl halides with phenyl boronic acida.
Entry
1.
Substrate
Product
Time (h)
6
Yield (%)b
68
2.
3.
7.25
4
62
80
4.
5.5
72
5.
6.
4
4
82c
79c
7.
8.
9.
5
70c
74
4.25
6
70
10.
11.
12.
13.
8
66c
65c
70
7
5
6.5
69c
14.
6
70c
aReaction conditions: aryl halides (1 mmol), phenyl boronic acid (1.2 mmol), K2CO3 (1.2 eq), water (5 mL), Co@GO/Fe3O4/L-dopa (0.05 g, 1.84 mol% Co) at 100 °C.
bIsolated yield; cColumn chromatography yield.
the competence and scope of the Co@GO/Fe3O4/L-dopa for the oxi-
dation of hydrocarbons and secondary alcohols. Effect of oxidants,
solvents and varying temperature conditions on the oxidation of an-
throne was studied and results are summarized in Table 4. Firstly, the
effect of different solvents such as CH2Cl2, CH3CN, EtOH and H2O on
the oxidation of anthrone was tested under similar conditions using
TBHP as oxidant and 80 °C as the reaction temperature. As shown, the
poorest yields were obtained in case of CH2Cl2, CH3CN and EtOH
(entries 1-3, Table 4), while quite satisfactory yield was obtained using
H2O (entry 4, Table 4). Subsequently, the effect of various oxidants i.e.,
O2, H2O2 and TBHP on oxidation of anthrone was also investigated and
the results revealed that TBHP possess better efficiency as an oxidant
(entries 4-6, Table 4). The effect of elevated temperature on the model
reaction was also studied and it was observed that best yield of product
was obtained at 100 °C. It was also noticed that an increase in tem-
perature lead to a decrease in selectivity of product formation (entry 8,
Table 4). Thus, best possible reaction conditions for the oxidation re-
action are TBHP as an oxidant, water as solvent and 100 °C as the re-
action temperature.
Table 4
Optimization of different solvents, oxidants and reaction temperatures for
benzylic C–H bond oxidationa.
Entry
Solvent
Oxidant
Temp (°C)
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
CH2Cl2
CH3CN
EtOH
H2O
TBHP
TBHP
TBHP
TBHP
O2
80
1.5
1
35
48
60
80
55
65
92
89
80
80
1
80
0.5
0.5
0.5
0.5
0.5
H2O
80
H2O
H2O2
TBHP
TBHP
80
H2O
100
120
H2O
aReaction conditions: anthrone (1 mmol), oxidant (1.2 mmol), Co@GO/Fe3O4/
L-dopa (0.05 g, 1.84 mol% Co) and solvent (5 mL). bIsolated yields.
3.3. Catalytic activity of Co@GO/Fe3O4/L-dopa for the oxidation of
hydrocarbons and secondary alcohols
Encouraged by the results of the catalytic system for the Suzuki
cross-coupling reaction, different experiments were performed to assess
In the light of these encouraging results, we tested a series of
35

H. Sharma, et al.
MolecularCatalysis469(2019)27–39
Fig. 10. Proposed mechanism for the Suzuki cross-coupling using Co@GO/Fe3O4/L-dopa.
different hydrocarbons for oxidation to the corresponding carbonyl
compounds using above optimized reaction conditions. As shown in
Table 5, the oxidation of several benzylic C–H substrates to the corre-
sponding products using Co@GO/Fe3O4/L-dopa proceeded smoothly
with good to excellent yields. It is noteworthy to mention that even
relatively unreactive hydrocarbons such as ethyl benzene, p-xylene etc.
were all successfully oxidized to corresponding carbonyl compounds in
the presence of Co@GO/Fe3O4/L-dopa. In case of p-xylene, both methyl
groups were oxidized to aldehydic groups leading to the formation of p-
terephthaldehyde as the major product. Moreover, fluorene and di-
phenylmethane were also efficiently converted into the corresponding
ketone derivatives in good yields. In extending the scope of the opti-
mized conditions for the oxidation of secondary alcohols to ketone
derivatives, it has been found that differently substituted secondary
alcohols were also successfully oxidized to their corresponding ketone
derivatives and products were obtained in good yields. These results
showed that Co@GO/Fe3O4/L-dopa appeared as a highly effective and
efficient catalyst to carry out the oxidation of hydrocarbons and sec-
ondary alcohols.
emerged as an effective substitute which can be effortlessly separated
from the reaction mixture using external magnet. To confirm the re-
cyclable nature of the synthesized catalyst, a fresh reaction between 4-
bromoacetophenone and phenyl boronic acid in the presence of
Co@GO/ Fe3O4/L-dopa using water as solvent and K2CO3 as base at
100 °C was carried out (entry 3, Table 3). After that, catalyst was re-
moved from the reaction mixture via external magnet, washed with
methylene chloride (3 × 10 mL), deionized water (3 × 10 mL) and
dried under vacuum. The organic fraction was washed with brine so-
lution and dried using anhydrous Na2SO4. Finally, the product was
isolated by the exclusion of the solvent under reduced pressure followed
by crystallization. Then, a fresh reaction with same reactants was per-
formed using the catalyst recovered from the previous run and the
process was repeated up to five times. It has been found that up to five
runs, there occurred no significant decrease in the catalytic activity of
catalyst (Fig. 11). Besides, the ICP-AES analysis of the reused Co@GO/
Fe3O4/L-dopa after the 5th run was also carried out to calculate the
cobalt concentration. It was found that the amount of cobalt in the
reused catalyst was reduced from 2.14 wt% (fresh) to 2.08 wt% after
the 5th run. The leaching of small amount of Co may be due to the
microscopic changes that occurred on the surface of the catalyst during
the course of reaction. In addition, heterogeneous nature of the catalyst
was also checked using hot filtration test in case of entry 3, Table 3. It
has been found that after the removal of catalyst from the reaction
mixture, no further conversion takes place. Thus, it strongly excludes
the possibility of the leaching of (active) Co(0) nanoparticles from the
support and confirms its heterogeneous nature. Similarly, recyclability
test for oxidation was also carried out in case of entry 1, Table 5, and
results showed that the catalyst is recyclable up to five runs without
considerable loss of activity (Fig. 11).
3.4. Comparison with other catalyst systems
In order to evaluate the uniqueness and efficiency of the synthesized
catalyst, Co@GO/Fe3O4/L-dopa among the different catalysts reported
in the literature, we have compiled data for Suzuki cross-coupling and
oxidation of hydrocarbons. The results are presented in Tables 6, which
showed the precedence of new catalytic system with others reported
methods in terms of reaction time, product yield and selectivity. Thus,
reported catalyst emerges as an attractive substitute to costlier catalytic
systems and afford a truly green process using mild reaction conditions,
leading to products in good to excellent yields.
4. Conclusion
3.5. Recyclability and heterogeniety
An inexpensive and water dispersible catalytic system, Co@GO/
Fe3O4/L-dopa with enhanced surface area due to the presence of core
particles of graphene oxide has been successfully synthesized. To make
Separation of the catalyst is a laborious task coupled with lot of
tedious workup procedures. In this context, magnetic catalyst has
36

H. Sharma, et al.
MolecularCatalysis469(2019)27–39
Table 5
Oxidation of hydrocarbons and secondary alcohols using Co@GO/Fe3O4/L-dopa as catalyst and TBHP under aqueous medium at 100 °C.a.
Entry
1
Substrate
Product
Time (h)
0.5
Yield (%)b
92
2
1
91
3
4
5
1.25
2
89c
85c
85
2.5
6
7
3
4
88c
80
8
3
83c
9
3.5
3
85c
88
10
11
12
4
80
86
3.5
13
3
85
(continued on next page)
37

H. Sharma, et al.
MolecularCatalysis469(2019)27–39
Table 5 (continued)
Entry
14
Substrate
Product
Time (h)
2.5
Yield (%)b
87
15
3
86c
aReaction conditions: hydrocarbon or secondary alcohol (1 mmol), TBHP (1.2 eq), water (5 mL), Co@GO/Fe3O4/L-dopa (0.05 g, 1.84 mol% Co) at 100 °C. bIsolated
yield; cColumn chromatography yield.
Table 6
Comparison of the catalytic activity of Co@GO/Fe3O4/L-dopa with some reported catalytic systems for the Suzuki cross-coupling of 4-bromoacetophenone with
phenylboronic acid and oxidation of diphenylmethanea.
S.No.
Catalytic
Suzuki
Reaction
Time/Yieldb
(h)/(%)
Catalytic
System
Oxidation
Reaction
Time/Yieldb
(h)/(%)
System
Reaction conditions
Reaction conditions
1
2
3
Cobalt pincer complex
Cu@GO/ Schiff complex
Fe-NCN supported GO
Cs2CO3, CH3CN,
80 °C
16/81 [40]
12/84 [41]
12/76 [42]
SUT–6-Zn
Ag/ZnO
TBHP, CH3CN/AcOH, 65 °C
Solvent-free, H2O2, 80 °C
CH3CN, O2, NHPI, 120 °C
24/94 [44]
7/73[45]
K2CO3, EtOH,
80 °C
Cs2CO3,
CoPcTs/
Zn2Al
12/86 [46]
CH3CN,
82 °C
LDH
4
5
MNPs@SB-Pd
K2CO3, EtOH:H2O,
rt
23/88 [43]
S-CoNC
O2, H2O,
5/31 [47]
0.8 MPa
120 °C
Co@GO/ Fe3O4/
K2CO3, H2O,
100 °C
4/80 [Present work]
Co@GO/
Fe3O4/
L-dopa
TBHP, H2O, 100 °C
1.25/89 [Present work]
L-dopa
aReaction conditions: a) Suzuki cross-coupling: 4-bromoacetophenone (1 mmol), phenyl boronic acid (1.2 mmol), base (1.2 eq), solvent (5 mL), Catalyst at different
temperature. b) oxidation reaction: diphenylmethane (1 mmol), oxidant (1.2 eq), solvent (5 mL), Catalyst at different tempearture; bIsolated yield.
cross-coupling and the oxidation of hydrocarbons and secondary alco-
hols.
Acknowledgements
We thank the Head, ACMS, IIT Kanpur for XPS study; Head, SAIF,
IIT Bombay for SEM, HR-TEM, EDX, ICP-AES, XRD and CHN studies;
and Head, CIF, Guwahati for VSM analysis.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2019.02.023.
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