Pd(0) immobilized on Fe3O4@AHBA: an efficient magnetically separable heterogeneous nanocatalyst for C–C coupling reactions

Article abstract of DOI:10.1080/00958972.2019.1687890

We have developed a new method to synthesize magnetically separable Fe₃O₄@AHBA@Pd(0) (AHBA = 3-amino-4-hydroxybenzoic acid) heterogeneous nanocatalyst via anchoring of palladium(0) on the surface of AHBA-coated Fe₃O₄ nanoparticles. The synthesized nanocatalyst was characterized by FT-IR, PXRD, ICP, XPS, FESEM, EDX, TEM, TGA, BET, and FT-RAMAN analysis. X-ray photoelectron spectroscopy (XPS) indicated the presence of Pd(0) on the surface of Fe₃O₄@AHBA. BET analysis exhibits high surface area of the prepared nanocatalyst with catalytic activity toward Heck coupling reactions with high turnover frequency (TOF) at 60 °C in water–acetonitrile within 2 h. The nanocatalyst can be easily recovered from the reaction mixture by using an external magnet and recycled six times without significant decrease in its catalytic activity. All the isolated products are obtained as solids or oils, fully characterized by ¹H-NMR spectroscopy.

Full text of DOI:10.1080/00958972.2019.1687890

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Pd(0) immobilized on Fe3O4@AHBA: an efficient
magnetically separable heterogeneous
nanocatalyst for C–C coupling reactions
Tonmoy Chakraborty, Abani Sarkar & Tanmay Chattopadhyay
To cite this article: Tonmoy Chakraborty, Abani Sarkar & Tanmay Chattopadhyay (2019): Pd(0)
immobilized on Fe3O4@AHBA: an efficient magnetically separable heterogeneous nanocatalyst for
C–C coupling reactions, Journal of Coordination Chemistry, DOI: 10.1080/00958972.2019.1687890
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JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2019.1687890
Pd(0) immobilized on Fe3O4@AHBA: an efficient
magnetically separable heterogeneous nanocatalyst for
C–C coupling reactions
Tonmoy Chakrabortya, Abani Sarkara and Tanmay Chattopadhyayb
b
aDepartment of Chemistry, University of Calcutta, Kolkata, India; Department of Chemistry,
Diamond Harbour Women’s University, Sarisha, India
ABSTRACT
ARTICLE HISTORY
Received 22 May 2019
Accepted 24 September 2019
We have developed a new method to synthesize magnetically
separable Fe3O4@AHBA@Pd(0) (AHBA ¼ 3-amino-4-hydroxyben-
zoic acid) heterogeneous nanocatalyst via anchoring of palla-
dium(0) on the surface of AHBA-coated Fe3O4 nanoparticles. The
synthesized nanocatalyst was characterized by FT-IR, PXRD, ICP,
XPS, FESEM, EDX, TEM, TGA, BET, and FT-RAMAN analysis. X-ray
photoelectron spectroscopy (XPS) indicated the presence of Pd(0)
on the surface of Fe3O4@AHBA. BET analysis exhibits high surface
area of the prepared nanocatalyst with catalytic activity toward
Heck coupling reactions with high turnover frequency (TOF) at
60 ꢀC in water–acetonitrile within 2 h. The nanocatalyst can be
easily recovered from the reaction mixture by using an external
magnet and recycled six times without significant decrease in its
catalytic activity. All the isolated products are obtained as solids
KEYWORDS
Fe3O4@AHBA@Pd(0) nano-
catalyst; C–C coupling
reaction; magnetically
separable; six times
reusability
1
or oils, fully characterized by H-NMR spectroscopy.
1. Introduction
Palladium compounds are used as homogeneous catalysts [1, 2] in many coupling
reactions. Electronegativity [3, 4] of palladium leads to relatively strong Pd–H and
Pd–C bonds and also develops a polarized Pd–X bond; it allows easy access of Pd(0)
to Pd(II) oxidation states, essential for oxidative addition, transmetalation, and
CONTACT Tanmay Chattopadhyay
tanmayc2003@gmail.com
Supplemental data for this article can be accessed https://doi.org/10.1080/00958972.2019.1687890.
ß 2019 Informa UK Limited, trading as Taylor & Francis Group

2
T. CHAKRABORTY ET AL.
Scheme 1. Schematic diagram for the preparation of Fe3O4@AHBA@Pd(0).
reductive elimination [5–8]. These homogeneous catalysts have some major drawbacks
such as catalyst separation from the reaction mixture, recycling, and product contam-
ination. Heterogenization of metal catalysts can avoid these problems [9–13]. There
are two major drawbacks preparing nanoparticles; they tend to aggregate and their
recovery is difficult. To prevent aggregation of nanoparticles electrostatic–steric stabil-
izers like polymers, surfactants and organic ligands are used [14, 15]. Magnetic nano-
particles (MNPs) have been extensively used in immobilization of metal nanoparticles
as these catalysts show high activity due to the large surface area and have simple
and efficient recycling from the reaction mixtures by applying an external magnetic
field, simultaneously with simple decantation of the product from the reaction mixture
[16–24]. Although synthesis and applicability of various magnetically separable Pd(0)-
nanocatalysts have been reported, including Fe3O4/DAG/Pd [25], Pd/WO3 [26], Pd/
Fe3O4@C(MFC) [27], Fe3O4@SiO2-EDTA-Pd NPs [28], Pd@ZPGly [29], Pd/TiO2 [30] and
Fe3O4@OA-Pd [31] for the Heck reaction. In most cases, the synthetic procedure for
nanocatalysts requires complicated steps. Furthermore, the use of toxic organic solv-
ent, high temperature, and long reaction time for catalytic reaction (C–C coupling
reactions) are other disadvantages for Pd(0)-nanocatalysts. Therefore, the development
of an alternative general and mild procedure employing a stable and inexpensive
Pd(0)-nanocatalyst is in demand.

JOURNAL OF COORDINATION CHEMISTRY
3
Figure 1. Powder X-ray diffraction spectra of Fe3O4@AHBA and Fe3O4@AHBA@Pd(0).
Hence, in our present work, we sought to address these challenges by developing
a new idea for heterogenizing magnetically separable Pd(0)-nanocatalyst using 3-
amino-4-hydroxybenzoic acid (AHBA) decorated iron oxide and palladium nanopar-
ticles for C–C coupling Heck reactions (Scheme 1). Carboxylate of AHBA modified the
surface of Fe3O4 nanoparticles to form Fe3O4@AHBA and that was connected with
Pd(0) by formation of a five-membered chelate ring of its free amino and hydroxyl
groups enhancing the stability of our synthesized nanocatalyst. The prepared catalyst
was characterized by FESEM, HRTEM, EDX, FT-IR, ICP, XPS, FT-RAMAN, DLS, PXRD, and
BET analyses. Our synthesized catalyst showed better activity and was environmentally
acceptable compared to reported methods. It offered a number of advantages such as
simple process, stability of the catalyst, yield in short reaction time using low tempera-
ture, easy work-up and separation of the catalyst from the reaction mixture by exter-
nal magnet, recyclability with minimal loss of activity along with high yield of product.
2. Experimental
2.1. Materials and physical methods
All reagents and chemicals were purchased from Sigma and used without purification.
Solvents used for spectroscopic studies were purified and dried by standard proce-
dures before use. FeSO4ꢁ7H2O, FeCl3ꢁ6H2O, PdCl2, and sodium borohydride were pur-
chased from Merck and 3-amino-4-hydroxybenzoic acid and CDCl3 were purchased
from Sigma Aldrich. UV–Vis spectra were recorded on a SHIMADZU UV-2450 spectro-
photometer. Infrared spectra (4000–500 cmꢂ1) were recorded at 27 ꢀC using a Perkin-
Elmer RXI FT-IR spectrophotometer with KBr pellets. The Pd content of the catalysts
was determined by ICP analysis (Varian, Vista-pro). Thermal analyses (TG-DTA) were
carried out on a Mettler Toledo (TGA/SDTA851) thermal analyzer in flowing dinitrogen
(flow rate: 30 cm3 minꢂ1). Field Emission Scanning Electron Microscope (FESEM) meas-
urement was performed with a JEOL JSM-6700F. Transmission Electron Microscope
(TEM) measurement was carried out with a JEOL (Japan) JEM2100 high-resolution
transmission electron microscope. X-ray powder diffraction (PXRD) was performed on

4
T. CHAKRABORTY ET AL.
Figure 2. X-ray photoelectron spectroscopy of Fe3O4@AHBA@Pd(0) taken in full range; inset shows
the characteristic band of Pd 3d spectrum.
a XPERT-PRO diffractometer with monochromated Cu-Ka radiation (40.0 kV, 30.0 mA) at
room temperature. The irradiation dependent study was done after different irradi-
ation times with a white light source compiled with a <420 nm cut off filter. A vibrat-
ing sample magnetometer (EV-9, Microsense and ADE) was utilized for obtaining
1
magnetization curves. H-NMR spectra (300 MHz) were recorded in CDCl3 on a Bruker
AV300 Supercon NMR spectrometer.
2.2. Synthesis of heterogeneous catalysts
2.2.1. Synthesis of Fe3O4 NPs
Magnetically separable Fe3O4 NPs were synthesized following the reported proced-
ure [32].
2.2.2. Synthesis of Fe3O4@AHBA (3-amino-4-hydroxybenzoic acid)
3-Amino-4-hydroxybenzoic acid (5 g) was dissolved in 15 mL of ethanol, then ethanolic
suspension solution of 5 g of Fe3O4 was added to the solution and stirred for 24 h at
room temperature. Then the product was allowed to settle and collected using an
external magnet, and washed four times with ethanol. The products were air dried.
2.2.3. Synthesis of Fe3O4@AHBA@Pd(0)
Fe3O4@AHBA (1 g) was dispersed in water (25 mL) by stirring, then PdCl2 (200 mg) was
added slowly to the suspension and the solution was stirred for 24 h at room tempera-
ture. The product was collected by centrifugation, washed with water to produce
Fe3O4@AHBA@PdCl2, then 0.2 g of NaBH4 was added to the solution which was stirred
for 24 h at room temperature. The black precipitate of Fe3O4@AHBA@Pd(0) catalyst
was separated using an external magnet and washed with water several times. The
product was collected and dried under vacuum at 60 ꢀC for 1 h. The Pd content of the
catalyst was determined by ICP to be 0.62 mol%.

JOURNAL OF COORDINATION CHEMISTRY
5
Figure 3. (a) FESEM image; (b) TEM image (zoom out); (c) TEM image (zoom in); and (d) HRTEM
with lattice fringe of Fe3O4@AHBA@Pd(0).
2.2.4. General procedure of coupling reaction
A mixture of alkene (2.5 mmol), aryl halide (2 mmol), Et3N (2 mmol), catalyst (20 mg, Pd:
0.62 mol%) in H2O, and acetonitrile (30 ml) was refluxed at 60 ꢀC for 2h. The catalyst was
separated magnetically and the mixture was filtered and concentrated in vacuo; the resi-
due was diluted with Et2O. The organic layer was washed several times with brine, dried
over MgSO4, filtered, and concentrated in vacuo to give the crude product.
3. Results and discussion
3.1. Synthesis and characterization of Fe3O4@AHBA@Pd(0)
Fe3O4 nanoparticles were prepared following the reported procedure. Then
Fe3O4@AHBA@Pd(0) was prepared by stirring Fe3O4@AHBA with PdCl2 for 24 h in
water and finally NaBH4 was added for reduction of Pdþ2 to Pd0.
Primarily, the reduction of Fe3O4@AHBA@PdCl2 to Fe3O4@AHBA@Pd(0) was con-
firmed by UV–Vis spectroscopy in solution (Figure S1, supplementary material). The
UV–Vis spectra of PdCl2 solution showed a peak at 416 nm but after addition of NaBH4
the reduction of Pdþ2 to Pd0 occurred and the peaks disappeared with a constant
change in color from yellow to black due to the formation of palladium nanoparticles.
FT-IR spectra of Fe3O4@AHBA showed a sharp band at 1600 cmꢂ1 which was the
C¼O stretching frequency of carboxylic acid group and another broad band at
3450 cmꢂ1 due to free O–H and NH2 groups of AHBA. The bands at 620 and 586 cmꢂ1
were due to Fe3O4 NPs, the signature Fe–O vibration [33]; all these peaks were also
present in Fe3O4@AHBA@Pd(0) but all the peaks were broadened and underwent a
low-frequency shift of wavelength for the stretch. FT-IR spectra of Fe3O4@AHBA@Pd(0)
and Fe3O4@AHBA are shown in Figure S2 (supplementary material).

6
T. CHAKRABORTY ET AL.
Figure 4. EDX spectrum Fe3O4@AHBA@Pd(0) and calculated weight percentage of elements.
TGA analysis of Fe3O4 and Fe3O4@AHBA@Pd(0) was examined from 40 ꢀC to 800 ꢀC
(Figure S3, supplementary material). TGA curve of Fe3O4@AHBA@Pd(0) consists of a
weight loss about 18% from 250 to 800 ꢀC whereas for Fe3O4 nanoparticles it was
about 6.5%. The enhanced weight loss is due to increasing amount of attached
organic moiety from Fe3O4 to Fe3O4@AHBA@Pd(0).
The PXRD data of Fe3O4@AHBA (Figure 1) showed the diffraction peaks at
2h ¼ 29.8ꢀ, 35.39ꢀ, 42.6ꢀ, 56.55ꢀ and 62.36ꢀ which can be assigned to the (220), (311),
(400), (422), and (440) planes of Fe3O4 [34]. The same peaks are observed in
Fe3O4@AHBA@Pd(0) with addition of other diffraction peaks at 2h ¼ 40ꢀ, 46.6ꢀ, 68.1ꢀ,
82ꢀ, and 86.8ꢀ which can be assigned to the (111), (200), (220), (311), and (222) planes,
respectively, for Pd(0) [35]. PXRD patterns indicate that the magnetically separable
nanoparticles contain pure Fe3O4 with a spinel structure and Pd(0) [36]. That is why
the grafting process does not induce any phase change of Fe3O4 but only the peak
intensity decreases slightly. The size of the synthesized nanoparticles has been calcu-
lated using the Debye-Scherrer equation, D ¼ kk/bcosh (k ¼ constant, 0.94), where k is
the wavelength of Cu–Ka (1.54 Å), b is the corrected diffraction line full-width at half-
maximum (FWHM), and h is Bragg’s angle. Using this equation, the size of the Pd(0)
nanoparticles at 2h ¼ 41ꢀ is calculated to be 10.57 nm.
The chemical state of palladium in Fe3O4@AHBA@Pd(0) was assessed by XPS
(Figure 2). The binding energies in the XPS spectra are calibrated by using that of C 1s
(284 eV); this signal comes mainly due to instrumental treatment before the XPS test.
Another major peak at 532 eV is due to O 1s spectrum [37]. Two minor peaks at 710
and 730 eV are due to Fe 2p3 and Fe 2p1, respectively [38]. The binding energies in
the XPS spectra showed two peaks at 335.5 and 340.8 eV for Pd 3d5 and Pd 3d3,
attributed to Pd(0) [39]. Figure S4 (supplementary material) showed the XPS spectra of
Fe3O4@AHBA@Pd(0) and free palladium nanoparticles only.
FESEM images of Fe3O4@AHBA@Pd(0) presented in Figure 3a suggest that the Pd(0)
NPs have roughly spherical and hexagonal shaped morphology within the range of
15–20 nm size and the average diameter is 16 nm. TEM images of Fe3O4@AHBA@Pd(0)
(Figure 3(b)) revealed that the diameter of the prepared Fe3O4@AHBA nanoparticles
was 200 nm and each Fe3O4@AHBA surface was uniformly decorated by palladium

JOURNAL OF COORDINATION CHEMISTRY
7
Figure 5. Magnetic curves of Fe3O4, Fe3O4@AHBA and Fe3O4@AHBA@Pd(0); inset shows the pic-
ture of magnetic separation.
Figure 6. Adsorption–desorption isotherm of Fe3O4@AHBA@Pd(0); inset shows pore size distribu-
tion of Fe3O4@AHBA@Pd(0).
Figure 7. FT-RAMAN spectrum of Fe3O4@AHBA and Fe3O4@AHBA@Pd(0).
nanoparticles. Figure 3c shows the higher magnification TEM image of
Fe3O4@AHBA@Pd(0). The spherical morphology of Pd(0) NPs on the surface of

8
T. CHAKRABORTY ET AL.
Table 1. Optimization of Fe3O4@AHBA@Pd(0) for Heck coupling between iodobenzene and styrene.
Entry
Base
Solvent
Time (h)
Yield (%)a
1
2
3
4
5
6
7
8
K2CO3
Na2CO3
NaHCO3
NaOAc
Cs2CO3
H2O/Acetonitrileb
H2O/Acetonitrileb
H2O/Acetonitrileb
H2O/Acetonitrileb
H2O/Acetonitrileb
H2O/Acetonitrileb
H2O/Acetonitrileb
H2O/Acetonitrileb
H2O/Acetonitrileb
H2O/Acetonitrileb
6
6
6
6
6
14
2
14
14
14
68
59
66
69
52
0
97
0
0
No base
Et3N
Et3N (no catalyst)
Et3N (using Fe3O4)
Et3N (using Fe3O4@AHBA)
9
10
0
aReaction conditions: iodobenzene (2 mmol), styrene (2.5 mmol), base (2 mmol), 20 mg, 0.60 mol% catalyst
(Fe3O4@AHBA@Pd(0)).
b1:1.
Fe3O4@AHBA have diameters within the range of 7–12 nm and average diameter
of 11 nm. The size of the Pd(0) nanoparticles calculated from PXRD corroborates
well with the size, obtained from TEM images. Figure 3d shows the lattice fringes
of the palladium nanoparticles as well as the electron diffraction pattern; the inter-
planar spacing was about 0.220 nm, corresponding to the orientation of “111”
atomic planes of palladium nanoparticles.
Energy-dispersive X-ray (EDX) spectrum of Fe3O4@AHBA@Pd(0) is depicted in Figure
4. Here the signals for Fe and Pd are due to the presence of Fe3O4 and palladium
nanoparticles, respectively.
The hydrodynamic size of Fe3O4@AHBA@Pd(0) was represented by dynamic light
scattering (DLS), Figure S6 (supplementary material). The calculated mean diameter
was 220–230 nm measuring the entire particle (Fe3O4 and Pd combined). For TEM
measurements the size estimated was for dried catalyst, whereas DLS measures the
size in the hydrated state of the sample, so the size measured by DLS is a hydro-
dynamic diameter and is comparatively larger than that calculated from TEM.
Figure 5 shows the magnetization behavior of (a) Fe3O4 NPs, (b) Fe3O4@AHBA,
and (c) Fe3O4@AHBA@Pd(0) under applied magnetic field. The curves reveal similar
phenomena as observed earlier [36, 40]. The decrease in the values of the satur-
ation magnetization (Ms) from the Fe3O4 nanoparticles (55.90 emu gꢂ1) to
Fe3O4@AHBA@Pd(0) (40.01 emu gꢂ1) are due to increasing diamagnetic organic
moiety from Fe3O4 NPs to Fe3O4@AHBA@Pd(0). However, the net magnetism of
final nanocatalyst Fe3O4@AHBA@Pd(0) is good for an effective separation from the
solution by external magnetic force.
Figure 6 shows adsorption–desorption isotherm with pore size distribution plot of
Fe3O4@AHBA@Pd(0) investigated by the Brunauer–Emmett–Teller (BET) gas-sorption
measurements carried out at 77 K for the dried powder sample under vacuum. From
the BET analysis, the main pore size of the catalyst was 7.9 nm and surface area was
230 m2gꢂ1. Due to the high surface area of our synthesized nanocatalyst, it provided a
great platform for coupling reactions.

JOURNAL OF COORDINATION CHEMISTRY
9
Table 2. Optimization of catalyst concentration Fe3O4@AHBA@Pd(0) for the reaction of Heck cou-
pling between iodobenzene and styrene.
Entry
Catalyst (mol% Pd)
Solvent
Yield (%)a
1
2
3
4
5
6
7
8
0.700
0.500
0.400
0.600
0.300
0.500
0.600
0.600
0.600
0.600
H2O/ACN
H2O/ACN
H2O/ACN
H2O/ACN
H2O/ACN
H2O/EtOH
H2O/EtOH
H2O/DMF
DMF
78
82
77
92
70
80
86
68
55
82
9
10
H2O/MeOH
aReaction conditions: iodobenzene (2 mmol), styrene (2.5 mmol), base Et3N (2 mmol), time (2 h).
FT-Raman spectra were recorded using a LabRAM HR spectrometer equipped with
an air-cooled argon ion (Arþ) laser of wavelength 488 nm and a Peltier-cooled charge-
coupled-device (CCD) detector, measured between 100–1200 cmꢂ1 at room tempera-
ture (Figure 7) for Fe3O4@AHBA and Fe3O4@AHBA@Pd(0), respectively. The Raman
spectrum of Fe3O4@AHBA showed peaks at 222 (weak), 290 (strong), 405 (weak), 487
(weak), and 613 (weak) cmꢂ1; these values were slightly shifted from Fe3O4 [41] after
coating with AHBA. Also the spectra of Fe3O4@AHBA and Fe3O4@AHBA@Pd(0) are
similar to each other. Thus the surface of catalyst contains palladium as a zero oxida-
tion state (without any PdO) which was further confirmed by XPS study.
To optimize the reaction conditions, a series of experiments were performed using styr-
ene (2.5 mmol) and iodobenzene (2 mmol) with Fe3O4@AHBA@Pd(0) under various reac-
tion conditions (Table 1). The effect of the concentration of the catalyst was studied
(Table 2) using different solvent/mixtures. In absence of catalyst the conversion was low
(Table 1, entry 8). The maximum conversion is observed when the reaction was performed
in the presence of 20 mg, 0.60 mol% of catalyst; Et3N (2 mmol) and catalyst (20 mg,
0.60 mol%) in H2O/acetonitrile (30 ml) was ideal for coupling of styrene with iodobenzene.
After 2 h the conversion of reaction remains constant, the reaction catalyst was separated
magnetically and the mixture was filtered and concentrated in vacuo. The residue was
diluted with Et2O. The organic layer was washed several times with brine, dried over
MgSO4, filtered, and concentrated in vacuo to give the crude product. In the absence of
base or catalyst the reaction failed to produce any product and we also performed in the
presence of only Fe3O4 and Fe3O4@AHBA, also without product (Table 1).
To demonstrate the general reactivity of Fe3O4@AHBA@Pd(0), a series of substrates
were taken using triethylamine as base and water/acetonitrile as solvent (Table 3). The
major drawback related with magnetically separable Pd(0) catalysts when water is the
reaction solvent is the poor interaction of active Pd with organic substrates. To over-
come this drawback, a mixture of organic solvents and water is used as the reaction
medium for magnetically separable Pd catalyzed Heck coupling reaction. All reactions
proceeded smoothly with very good yield.
Electron withdrawing and donating substituent of styrene showed a significant
effect on the substrate reactivity (Table 3) and the reaction with aryl bromide and iod-
ide are equally efficient for coupling. Reactions with aryl chlorides were slow, affording
the desired product in only 30% yield. Table 3 (entries 1–12) display the experimental
results. In all the coupling reactions transformation of reactant to product occurred

10
T. CHAKRABORTY ET AL.
Table 3. Fe3O4@AHBA@Pd(0) Catalyzed Heck coupling reactions with various substrates.
Time
(h)
Conversion
(%)
Yielda,b
(%)
TOFc
Entry
1
Substrate
Product
(hꢂ1
)
2
2
2
97
98
98
94
97
95
7.58
2
3
7.75
7.66
4
5
6
2
2
2
97
98
98
95
86
93
7.66
6.94
7.50
7
8
2
2
97
97
95
94
7.66
7.58
2
2
98
96
92
85
7.42
6.85
9
10
11
12
2
2
97
93
92
90
7.40
7.26
aReaction conditions: alkene (2.5 mmol), aryl halide (2 mmol), Et3N (2 mmol), catalyst (20 mg, 0.62 mol%) in
1:1 H2O–acetonitrile (30 ml), time (2 h).
bIsolated yield.
cTOF was defined as molproduct molꢂ1 hꢂ1
.
Pd

JOURNAL OF COORDINATION CHEMISTRY
11
Table 4. Catalytic performance of different magnetically separable Pd-based nanocatalysts in cou-
pling of iodobenzene and styrene.
Entry
Catalyst
Pd (mol%)
Solvent
Base
Temp (ꢀC) Time (h) Yield (%)
Ref.
1
2
3
4
5
6
7
8
Pd/Fe3O4@Polypyrrole
Pd/N-MCNPs
Pd-biomagnetite
Fe3O4-NH2-Pd
Pd/NiFe2O4
Pd/MFC
Fe3O4/DAG/Pd
Fe3O4@AHBA@Pd(0)
… …
1.2
5
8.83
8.54
0.308
0.280
0.60
DMA
DMAN
DMF
NMP
DMF
DMF
DMF
H2O/ACN
TBA
Et3N
Et3N
K2CO3
TEA
K2CO3
Et3N
Et3N
120
120
120
130
80
120
110
60
3
3
3
10
4
4
97
97.6
100
>99
97
100
98
96
[51]
[52]
[53]
[54]
[55]
[27]
[25]
This work
3
2
DMA: N,N-dimethylacetamide; TBA: tributylamine; Pd/N-MCNPs: Pd/N-dopand magnetic carbon nanoparticles; DMAN:
dimethylacrylonitrile; Et3N: triethylamine; NMP: N-methyl-2-pyrrolidone.
1
with >90% conversion and >85% yield. The H-NMR spectra of the isolated products
are given in the Supporting Information.
We compared the results achieved in the present manuscript with reported mag-
netically separable Pd-based nanocatalysts. In previous work magnetically separable
Pd nanocatalysts such as MgFe2O4@SiO2-PrNH2/Pd/BMO [42], NiFe2O4@TABMA-Pd(0)
[43], Pd@Hal-2N-TCT-EDA [44], Fe3O4@dopa@ML [45], Pd-NHC [46], Fe3O4-supported
Schiff-base complexes [47], alumina-supported Pd catalysts [48], PdCl2(NHC)(PPh3) [49],
and PEPPSI-type palladium NHC complex [50] were prepared for various coupling
reactions by different techniques and using different functionalizing agents like
SiO2-PrNH2, TABMA, Hal-2N-TCT-EDA, alumina, dopa, NHC (N-heterocyclic carbene
complexes) and Schiff base complexes. The functionalizing agent was used to
enhance the stability of nanocatalyst. Most of the catalysts required high tempera-
ture, several steps and cost effective reagents during the preparation of catalyst.
Here we prepared very cheap and economical friendly magnetically separable Pd
nanocatalysts using AHBA (3-amino-4-hydroxybenzoic acid) as a functionalizing
agent. Carboxylate of AHBA modified the surface of Fe3O4 nanoparticles to form
Fe3O4@AHBA and that was connected with Pd(0) by formation of five-membered
chelate rings of its free amino and hydroxyl group. Then the prepared catalyst was
used for C–C (Heck) coupling reactions. For iodobenzene reacting with styrene as a
model substrate, the results are listed in Table 4. The results in this work has advan-
tages: low temperature in short reaction time with high turnover frequency (TOF)
and the solvent (H2O/acetonitrile) used here is much preferred to DMA, TBA and
DMAN used elsewhere.
The recyclability of Fe3O4@AHBA@Pd(0) was examined by removing it from the
reaction media by magnet (external magnetic field), the catalyst was washed with
water-ethanol (50% v/v) solution and regenerated catalyst was reused under the opti-
mum reaction conditions. The regenerated catalyst was further characterized by PXRD,
FESEM, and TEM (Figure 8); after the sixth cycle the catalyst size, shape and morph-
ology were the same. No significant changes were observed in the composition of the
regenerated catalyst. Also ICP analysis indicated leaching of 2.2% of palladium after
the sixth cycles, which suggests the heterogeneous nature of the nanocatalyst.
Reasonable yields of products of the coupling reaction were observed for the sixth
reaction cycle as evident from Figure 9.

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T. CHAKRABORTY ET AL.
Figure 8. (a) PXRD pattern; (b) FESEM image; (c) TEM image of reused catalyst of
Fe3O4@AHBA@Pd(0) toward coupling reaction.
Figure 9. Catalytic activity of Fe3O4@AHBA@Pd(0) as a function of reusability of the catalyst.
4. Conclusion
We have synthesized a simple, economical, environmentally friendly, efficient and sus-
tainable magnetically separable Fe3O4@AHBA@Pd(0) heterogeneous catalyst for cou-
pling reaction of aryl halides and styrene in the presence of triethylamine as a base at
60 ꢀC for 2 h. Our synthesized catalyst showed excellent catalytic properties toward
coupling (yield >85%) reactions. Our catalyst has advantages compared to other
reported catalysts, such as simple process, stability of the catalyst, use of magnetic
nanoparticles as support, effortless recovery, yield in short reaction time using low

JOURNAL OF COORDINATION CHEMISTRY
13
temperature with high turnover frequency (TOF), easy work-up and separation of the
catalyst from the reaction mixture by external magnet, recyclability for at least six
times with minimal loss of activity along with high yield of product. Thus this magnet-
ically separable heterogeneous catalyst could find utilization in several important cata-
lytic processes as well as the common synthetic organic transformations.
Acknowledgements
T. C. is thankful to the UGC-JRF for providing a fellowship [Ref. No: 21/06/2015(i) EU-V]. A. S. is
thankful to the CSIR-JRF for providing a fellowship [sanction number 09/028(1005)/2017-EMR-1].
The authors wish to thank the University of Calcutta for providing the facility of the FESEM,
HRTEM, PXRD, and FT-IR by the DST FIST program.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
Financial support by the Department of Higher Education, Science & Technology and
Biotechnology, Government of West Bengal [Memo No. 21(Sanc.)/ST/P/S&T/15G-16/2017 dt. 12-
06-2018] is gratefully acknowledged by T. C.
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