Threonine stabilizer-controlled well-dispersed small palladium nanoparticles on modified magnetic nanocatalyst for Heck cross-coupling process in water

Article abstract of DOI:10.1002/aoc.4645

We report the synthesis of magnetically separable Fe₃O₄@Silica-Threonine-Pd⁰ magnetic nanoparticles with a core–shell structure. After synthesis of Fe₃O₄@Silica, threonine as an efficient stabilizer/ligand was bonded to the surface of Fe₃O₄@Silica. Then, palladium nanoparticles were generated on the threonine-modified catalyst. The threonine stabilizer helps to generate palladium nanoparticles of small size (less than 4?nm) with high dispersity and uniformity. Magnetically separable Fe₃O₄@Silica-Threonine-Pd⁰ nanocatalyst was fully characterized using various techniques. This nanocatalyst efficiently catalysed the Heck cross-coupling reaction of a variety of substrates in water medium as a green, safe and inexpensive solvent at 80°C. The Fe₃O₄@Silica-Threonine-Pd⁰ catalyst was used for at least eight successful consecutive runs with palladium leaching of only 0.05%.

Full text of DOI:10.1002/aoc.4645

Received: 14 June 2018
DOI: 10.1002/aoc.4645
Revised: 15 September 2018
Accepted: 18 September 2018
S P E C I A L I S S U E
Threonine stabilizer‐controlled well‐dispersed small
palladium nanoparticles on modified magnetic nanocatalyst
for Heck cross‐coupling process in water
Iraj Sarvi | Mostafa Gholizadeh
| Mohammad Izadyar
Department of Chemistry, Faculty of
Science, Ferdowsi University of Mashhad,
Mashhad 91775‐1436, Iran
We report the synthesis of magnetically separable Fe3O4@Silica‐Threonine‐Pd0
magnetic nanoparticles with a core–shell structure. After synthesis of
Fe3O4@Silica, threonine as an efficient stabilizer/ligand was bonded to the sur-
face of Fe3O4@Silica. Then, palladium nanoparticles were generated on the
threonine‐modified catalyst. The threonine stabilizer helps to generate palla-
dium nanoparticles of small size (less than 4 nm) with high dispersity and uni-
formity. Magnetically separable Fe3O4@Silica‐Threonine‐Pd0 nanocatalyst was
fully characterized using various techniques. This nanocatalyst efficiently
catalysed the Heck cross‐coupling reaction of a variety of substrates in water
medium as a green, safe and inexpensive solvent at 80°C. The Fe3O4@Silica‐
Threonine‐Pd0 catalyst was used for at least eight successful consecutive runs
with palladium leaching of only 0.05%.
Correspondence
Mostafa Gholizadeh, Department of
Chemistry, Faculty of Science, Ferdowsi
University of Mashhad, Mashhad 91775‐
1436, Iran.
Email: m_gholizadeh@um.ac.ir
Funding information
Ferdowsi University of Mashhad, Grant/
Award Number: 3/41258
KEYWORDS
Heck cross‐coupling reaction, magnetic catalyst, small palladium nanoparticles, threonine stabilizer
1 | INTRODUCTION
ligands have been developed for the Heck cross‐coupling
reaction.[8–10] Most of them suffer from drawbacks such
Environmentally friendly heterogeneous catalytic systems
have strongly stimulated growth in modern chemical
transformations.[1–3] In this regard, safer, cleaner, simpler
and greener approaches have been considered for devel-
oping sustainable and environmentally friendly methods.
For this purpose, nanoparticles supported on modified
organic–inorganic hybrid surfaces with high selectivity
and recyclability have been investigated.[4,5] The
palladium‐catalysed carbon–carbon bond forming reac-
tion is considered as a powerful synthetic tool and a
major area in catalysis, organic transformations and phar-
maceutical, agrochemical and fine chemical industries.[6]
Among them, Heck–Mizoroki cross‐coupling is one of the
most beneficial reactions for carbon–carbon bond forma-
tion due to its high efficiency and atom economy.[7]
During the last few decades, various homogeneous, het-
erogeneous and organometallic systems with special
as tedious multistep synthesis and work‐up, air‐ and
moisture‐sensitive ligands, expensive, unstable and toxic
ligands such as phosphine, and use of various additives
and harmful solvents.[11,12] Also, a diverse array of
organic and inorganic supports, such as polymers, car-
bon, clay, ordered silicates and zeolites, have been used
as hosts for palladium nanoparticles in cross‐coupling
reactions.[13–17] Magnetic nanoparticles as a readily avail-
able and low‐cost material with high surface area, high
catalyst loading capacity and high stability have also been
used.[18] They can be modified, coated and functionalized
with various ligands/stabilizers and hybrid precursors.
Based on the literature, particle size, crystal structure
and nature of the ligand are crucial in the activity of
nanocatalysts.[19–21] Aggregation/oxidation of metal
nanoparticles, which causes decreased catalytic perfor-
mance,
can be
avoided by
a
well‐chosen
Appl Organometal Chem. 2018;e4645.
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© 2018 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4645

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SARVI ET AL.
ligand/stabilizer. The particle size, shape, magnetic fea-
tures and chemical stability of the hybrid materials can
be controlled with such stabilizers. A stabilizer prevents
the agglomeration of metal species.[22]
respectively. Wide‐angle X‐ray diffraction (XRD) patterns
were recorded using a Bruker D4 X‐ray diffractometer
(Germany) with Ni‐filtered Cu KR radiation (40 kV,
40 mA). The percentage weight loss of shell and arginine
was studied using thermogravimetric analysis (TGA;
Mettler Toledo LF, Switzerland). Fourier transform infra-
red (FT‐IR) spectra were collected with a Nicolet Fourier
spectrophotometer (USA), using KBr pellets. To investi-
gate the magnetic behaviour of the catalyst, vibrating
sample magnetometry (VSM; 7400 Lake Shore, USA)
As a part of our ongoing researches on novel hetero-
geneous catalysis and designing and developing environ-
mentally benign green methods in catalysis,[23–26] our
aim has been to investigate efficient magnetically recover-
able palladium‐based nanocatalysts with high activity for
the Heck cross‐coupling reaction under mild conditions
with minimum by‐products and high yields. In the work
reported herein, we used silica‐coated magnetic nanopar-
ticles (Fe3O4@SiO2) as a recoverable and reusable sup-
port. The surface of Fe3O4@SiO2 was modified with the
amino acid threonine with ‐‐NH2 and ‐‐OH groups serv-
ing as a suitable stabilizer for immobilization of small
palladium nanoparticles without any time‐consuming
steps for the synthesis of the ligand. The use of less toxic
1
was used. H NMR and 13C NMR spectra were measured
(CDCl3) with a Bruker DRX‐300 AVANCE spectrometer
at 300 and 75 MHz, respectively. The contents of palla-
dium on the prepared solid catalysts and palladium
leaching were measured using inductively coupled
plasma optical emission spectrometry (ICP‐OES) with
an Avio 200 ICP instrument.
and
inexpensive
palladium‐based
threonine‐
2.2 | Synthesis of Fe3O4 magnetic
nanoparticles (MNPs)
functionalized Fe3O4@SiO2 catalysts has not been
reported to date for the Heck reaction. A schematic path-
way for preparation of Pd0 immobilized on threonine‐
modified Fe3O4@SiO2 magnetic nanoparticles (FST‐Pd0
MNPs) is illustrated in Scheme 1.
Based on a previously reported procedure, Fe3O4 MNPs
were prepared hydrothermally.[27] At first, FeCl3⋅6H2O
(3 mmol, 0.81 g), NaOAc (24.4 mmol, 2.0 g) and Na3Cit
(2.9 mmol, 0.75 g) were dissolved in 40 ml of ethylene gly-
col and stirred vigorously for 1 h. The obtained yellow
solution was transferred into a Teflon‐lined stainless steel
autoclave (50 ml capacity) and heated at 200°C for 12 h.
After completion of the reaction, the autoclave was
cooled to room temperature. The obtained black Fe3O4
MNPs were collected with a magnet and washed with
water and ethanol. The final precipitate was then dried
under vacuum at 50°C for 4 h.[27]
2 | EXPERIMENTAL
2.1 | Materials and apparatus
Palladium chloride (PdCl2), L‐threonine (C4H9NO3),
iron(III) chloride hexahydrate (FeCl3⋅6H2O), sodium ace-
tate (NaOAc), trisodium citrate (Na3Cit), tetraethyl
orthosilicate (SiC8H20O4) and other compounds were
obtained from Sigma‐Aldrich and Merck of analytical
grade and used without further purification. For mor-
phology and size study of the as‐prepared samples, trans-
mission electron microscopy (TEM) and scanning
electron microscopy (SEM) experiments were conducted
with a Leo 912AB microscope (Germany) operated at
120 kV and a Leo 1450VP microscope (Germany),
2.3 | Synthesis of
Fe3O4@Silica@Threonine magnetic
nanoparticles (FST MNPs)
Fe3O4 powder (0.1 g) was dispersed in HNO3 (5 ml,
0.1 M) solution with ultrasonication for 20 min, and then
SCHEME 1 Schematic representation
of catalyst preparation (FST‐Pd0 MNPs)

SARVI ET AL.
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washed with deionized water. The post‐treated Fe3O4
nanoparticles were dispersed in water (6 ml, 0.05 g) and
then added to 500 ml of absolute ethanol and 130 ml of
deionized water. Ammonia solution (28 wt%, 10 ml) was
added to this mixture under sonication in 30 min. Then,
tetraethyl orthosilicate (8.0 ml, 7.46 g) was added
dropwise under stirring for 12 h. After separation,
Fe3O4@SiO2 was washed with water and ethanol three
times.[23] Finally, 0.1 g of Fe3O4@SiO2 and threonine
(16.8 mmol, 2 g) were dispersed in 40 ml of a mixture of
EtOH and water (1:1) and then sealed in a 50 ml auto-
clave at 150°C for 2 h. The FST MNPs were collected with
a magnet and washed several times with water and etha-
nol and dried at 50°C in an oven.
2.5.2 | Ethyl cinnamate (3b)
1H NMR (CDCl3, 300 MHz, δ, ppm): 1.37 (t, 3H,
J = 6 Hz), 4.30 (q, 2H, J = 6 Hz), 6.47 (d, 1H,
J = 12 Hz), 7.40–7.43 (m, 3H), 7.54–7.57 (m, 2H), 7.72
(d, 1H, J = 12 Hz). 13C NMR (CDCl3, 75 MHz, δ, ppm):
15.01, 30.56, 61.37, 119.13, 128.91, 129.73, 131.07,
135.32, 145.45, 167.88.
2.5.3 | Butyl cinnamate (3c)
1H NMR (CDCl3, 300 MHz, δ, ppm): 1.00 (t, 3H,
J = 6 Hz), 1.48 (sex, 2H, J = 6, 3 Hz), 1.74 (pen, 2H,
J = 9, 3 Hz), 4.25 (t, 2H, J = 6 Hz), 6.47 (d, 1H,
J = 12 Hz), 7.39–7.42 (m, 3H), 7.54–7.56 (m, 2H), 7.72
(d, 1H, J = 12 Hz). 13C NMR (CDCl3, 75 MHz, δ, ppm):
14.62, 20.07, 31.64, 65.28, 119.14, 128.91, 129.72, 131.06,
135.33, 145.41, 167.95.
2.4 | Synthesis of
Fe3O4@SiO2@Threonine‐Pd0 (FST‐Pd0)
nanoparticles
FST MNPs (0.2 g) were dispersed in 15 ml of distilled
water for 10 min. An aqueous solution of PdCl2 (5 ml,
0.1 M) was added to the mixture and stirred for 12 h.
The acquired FST‐PdII MNPs were separated from the
solution and washed with hot water and ethanol five
times. Finally, FST‐PdII MNPs were treated with an aque-
ous solution of NaBH4 (10 ml, 0.05 M) for 6 h in an ice
bath. The FST‐Pd0 thus produced was then collected with
a magnet and washed four times with water and ethanol.
2.5.4 | Methyl‐3‐(p‐tolyl) acrylate (3d)
1H NMR (CDCl3, 300 MHz, δ, ppm): 2.40 (s, 3H), 3.83 (s,
3H), 6.43 (d, 1H, J = 12 Hz), 7.22 (d, 2H, J = 6 Hz), 7.45
(d, 2H, J = 6 Hz), 7.70 (d, 1H, J = 12 Hz). 13C NMR
(CDCl3, 75 MHz, δ, ppm): 22.32, 52.49, 117.54, 128.93,
130.48, 132.50, 141.58, 145.74, 168.50.
2.5 | Typical procedure for Heck
cross‐coupling reaction
2.5.5 | Ethyl‐3‐(p‐tolyl) acrylate (3e)
To a mixture of Et3N (2 mmol, 0.2 g), methyl acrylate
(1.2 mmol, 0.1 g) and bromobenzene (1 mmol, 0.16 g)
in water (1.5 ml, 1.5 g), 0.25 mol% of FST‐Pd0 MNPs
was added and heated at 80°C in an oil bath. After com-
pletion of the reaction (monitored by TLC) the
nanocatalyst was separated using a magnet and washed
with ethyl acetate three times. The reaction mixture was
extracted with ethyl acetate (3 × 5 ml) and the organic
layer was dried over anhydrous MgSO4. Finally, solvent
was evaporated and the crude product purified by column
chromatography (n‐hexane–ethyl cetate, 10:2).
1H NMR (CDCl3, 300 MHz, δ, ppm): 1.36 (t, 3H,
J = 6 Hz), 2.39 (s, 3H), 4.29 (q, 2H, J = 6 Hz), 6.42 (d,
1H, J = 12 Hz), 7.20 (d, 2H, J = 6 Hz), 7.44 (d, 2H,
J = 6 Hz), 7.69 (d, 1H, J = 12 Hz). 13C NMR (CDCl3,
75 MHz, δ, ppm): 15.20, 22.29, 61.24, 118.01, 128.90,
130.46, 132.59, 141.45, 145.44, 168.02.
2.5.6 | Butyl‐3‐(p‐tolyl) acrylate (3f)
1H NMR (CDCl3, 300 MHz, δ, ppm): 1.00 (t, 3H,
J = 6 Hz), 1.48 (sex, 2H, J = 6, 3 Hz), 1.73 (pen, 2H,
J = 9, 6 Hz), 2.39 (s, 3H), 4.24 (t, 2H, J = 6 Hz), 6.43 (d,
1H, J = 12 Hz), 7.21 (d, 2H, J = 6 Hz), 7.45 (d, 2H,
J = 6 Hz), 7.69 (d, 1H, J = 12 Hz). 13C NMR (CDCl3,
75 MHz, δ, ppm): 14.62, 20.08, 22.29, 31.67, 65.17,
118.03, 128.90, 130.45, 132.60, 141.44, 145.40, 168.13.
2.5.1 | Methyl cinnamate (3a)
1H NMR (CDCl3, 300 MHz, δ, ppm): 3.83 (s, 3H), 6.47 (d,
1H, J = 12 Hz), 7.40 (m, 3H), 7.54 (m, 2H), 7.73 (d, 1H,
J = 12 Hz). 13C NMR (CDCl3, 75 MHz, δ, ppm): 52.54,
118.65, 128.94, 129.75, 131.16, 135.23, 145.72, 168.26.

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SARVI ET AL.
75 MHz, δ, ppm): 14.62, 20.07, 31.67, 56.20, 65.11,
115.15, 116.61, 128.06, 130.54, 145.06, 162.17, 168.30.
2.5.7 | Ethyl‐3‐(4‐ethylphenyl) acrylate
(3g)
1H NMR (CDCl3, 300 MHz, δ, ppm): 1.26 (t, 3H,
J = 12 Hz), 1.36 (t, 3H, J = 12 Hz), 2.68 (q, 2H,
J = 6 Hz), 4.28 (q, 2H, J = 6 Hz), 6.43 (d, 1H,
J = 12 Hz), 7.23 (d, 2H, J = 6 Hz), 7.46 (d, 2H,
J = 6 Hz), 7.70 (d, 1H, J = 12 Hz). 13C NMR (CDCl3,
75 MHz, δ, ppm): 15.19, 16.17, 29.64, 61.23, 118.05,
129.00, 129.26, 132.83, 145.46, 147.74, 168.02.
2.5.12 | Methyl‐3‐(4‐formylphenyl) acry-
late (3l)
1H NMR (CDCl3, 300 MHz, δ, ppm): 3.86 (s, 3H), 6.58 (d,
1H, J = 12 Hz), 7.71 (d, 2H, J = 6 Hz), 7.75 (d, 1H,
J = 12 Hz), 7.93 (d, 2H, J = 6 Hz), 10.02 (s, 1H). 13C
NMR (CDCl3, 75 MHz, δ, ppm): 52.85, 121.83, 129.39,
131.04, 138.03, 140.89, 144.00, 167.68, 192.33.
2.5.8 | Butyl‐3‐(4‐ethylphenyl) acrylate
(3h)
2.5.13 | Ethyl‐3‐(4‐formylphenyl) acrylate
(3m)
1H NMR (CDCl3, 300 MHz, δ, ppm): 1.00 (t, 3H,
J = 6 Hz), 1.27 (t, 3H, J = 6 Hz), 1.47 (sex, 2H, J = 6,
3 Hz), 1.73 (pen, 2H, J = 9, 3 Hz), 2.69 (q, 2H,
J = 6 Hz), 4.24 (t, 2H, J = 6 Hz), 6.43 (d, 1H,
J = 12 Hz), 7.24 (d, 2H, J = 6 Hz), 7.48 (d, 2H,
J = 6 Hz), 7.70 (d, 1H, J = 12 Hz). 13C NMR (CDCl3,
75 MHz, δ, ppm): 14.62, 16.18, 20.07, 29.65, 31.66, 65.19,
118.08, 129.00, 129.27, 132.84, 145.43, 147.76, 168.16.
1H NMR (CDCl3, 300 MHz, δ, ppm): 1.34 (t, 3H,
J = 6 Hz), 4.27 (q, 2H, J = 6 Hz), 6.54 (d, 1H,
J = 12 Hz), 6.66 (d, 2H, J = 6 Hz), 7.69 (d, 1H,
J = 12 Hz), 7.88 (d, 2H, J = 6 Hz), 10.01 (s, 1H). 13C
NMR (CDCl3, 75 MHz, δ, ppm): 15.11, 61.65, 122.29,
129.32, 130.98, 137.96, 140.94, 143.63, 167.15, 192.27.
2.5.14 | Butyl‐3‐(4‐formylphenyl) acrylate
(3n)
2.5.9 | Methyl‐3‐(4‐methoxyphenyl) acry-
late (3i)
1H NMR (CDCl3, 300 MHz, δ, ppm): 0.98 (t, 3H,
J = 6 Hz), 1.45 (sex, 2H, J = 12, 6 Hz), 1.72 (pen, 2H,
J = 9, 6 Hz), 4.24 (t, 2H, J = 6 Hz), 6.57 (d, 1H,
J = 12 Hz), 7.69 (d, 2H, J = 6 Hz), 7.72 (d, 1H,
J = 12 Hz), 7.91 (d, 2H, J = 6 Hz), 10.04 (s, 1H). 13C
NMR (CDCl3, 75 MHz, δ, ppm): 14.59, 20.02, 31.55,
65.62, 122.32, 12935, 131.02, 137.95, 140.98, 143.66,
167.33, 192.35.
1H NMR (CDCl3, 300 MHz, δ, ppm): 2.42 (s, 3H), 3.85 (s,
3H), 6.41 (d, 1H, J = 12 Hz), 7.19 (d, 2H, J = 6 Hz), 7.42
(d, 2H, J = 6 Hz), 7.68 (d, 1H, J = 12 Hz). 13C NMR
(CDCl3, 75 MHz, δ, ppm): 22.30, 52.38, 117.68, 128.91,
130.49, 132.52, 141.68, 145.69, 168.49.
2.5.10 | Ethyl‐3‐(4‐methoxyphenyl) acry-
late (3j)
2.5.15 | Methyl‐3‐(4‐cyanophenyl) acrylate
1H NMR (CDCl3, 300 MHz, δ, ppm): 1.34 (t, 3H,
J = 6 Hz), 3.83 (s, 3H), 4.26 (q, 2H, J = 9 Hz), 6.32 (d,
1H, J = 12 Hz), 6.90 (d, 2H, J = 3 Hz), 7.41 (d, 2H,
J = 3 Hz), 7.66 (d, 2H, J = 12 Hz). 13C NMR (CDCl3,
75 MHz, δ, ppm): 15.20, 56.16, 61.14, 115.14, 116.56,
128.01, 130.52, 145.08, 162.18, 168.15.
(3o)
1H NMR (CDCl3, 300 MHz, δ, ppm): 3.84 (s, 3H), 6.54 (d,
1H, J = 12 Hz), 7.62–7.71 (m, 5H). 13C NMR (CDCl3,
75 MHz, δ, ppm): 52.88, 114.27, 119.20, 122.23, 129.26,
133.51, 139.49, 143.27, 167.43.
2.5.11 | Butyl‐3‐(4‐methoxyphenyl) acry-
late (3k)
2.5.16 | Ethyl‐3‐(4‐cyanophenyl) acrylate
(3p)
1H NMR (CDCl3, 300 MHz, δ, ppm): 0.99 (t, 3H,
J = 6 Hz), 1.46 (sex, 2H, J = 6, 3 Hz), 1.70 (pen, 2H,
J = 9, 3 Hz), 3.85 (s, 3H), 4.23 (t, 2H, J = 12 Hz), 6.34
(d, 1H, J = 12 Hz), 6.93 (d, 2H, J = 6 Hz), 7.50 (d, 2H,
J = 6 Hz), 7.66 (d, 1H, J = 12 Hz). 13C NMR (CDCl3,
1H NMR (CDCl3, 300 MHz, δ, ppm): 1.37 (t, 3H,
J = 6 Hz), 4.31 (q, 2H, J = Hz), 6.54 (d, 1H, J = 12 Hz),
7.62–7.71 (m, 5H). 13C NMR (CDCl3, 75 MHz, δ, ppm):
15.12, 61.80, 114.20, 119.23, 122.73, 129.23, 133.50,
139.60, 142.98, 167.00.

SARVI ET AL.
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bending of CH2 and CH3 groups, respectively (Figure 1c).
Also, two bands at 1574 and 1535 cm−1 in Figure 1c can
be attributed to the stretching vibration of CO and bend-
ing vibration of OH groups. In the FT‐IR spectrum of
serine‐modified FS MNPs (Figure 1d), characteristic
peaks of threonine, Fe‐‐O stretching vibration of mag-
netic core and silica shell can be observed that confirm
the functionalization of the FS MNPs with threonine
ligands and the magnetic nature of the catalyst.[28]
2.5.17 | Butyl‐3‐(4‐cyanophenyl) acrylate
(3q)
1H NMR (CDCl3, 300 MHz, δ, ppm): 0.96 (t, 3H,
J = 6 Hz), 1.43 (sex, 2H, J = 9, 6 Hz), 1.69 (pen, 2H,
J = 9, 6 Hz), 4.22 (t, 2H, J = 6 Hz), 6.52 (d, 1H,
J = 12 Hz), 7.60–7.69 (m, 5H). 13C NMR (CDCl3,
75 MHz, δ, ppm): 14.56, 19.99, 31.52, 65.64, 114.14,
119.20, 122.71, 129.23, 133.46, 139.58, 142.92, 167.04.
For thermal stability study of the nanocatalyst and
calculation of the amount of organic moieties on the sur-
face of the nanocatalyst, TGA was carried out in a static
nitrogen atmosphere (Figure 2). The FST‐Pd MNPs
revealed different weight loss in the temperature range
30–800°C. Initial weight loss up to 150°C was probably
due to the removal of surface hydroxyls and surface
adsorbed water, while the weight loss at 200–600°C was
attributed mainly to the decomposition of organic groups
from the catalyst surface. These results clearly corrobo-
rated that organic functional groups were incorporated
on the surface of FST‐Pd0 MNPs.
The crystallographic structures of FS MNPs and FST‐
Pd0 MNPs were investigated using the XRD technique as
shown in Figure 3. Six diffraction peaks located at 30.35°
(220), 35.44° (311), 43.31° (400), 53.14° (422), 56.87° (511)
and 62.51° (440) in the XRD patterns of FS MNPs and
FST‐Pd0 MNPs can be assigned to diffraction peaks of
the Fe3O4 crystalline phase, according to JCPDS card
no. 19‐0629. Based on these results, the crystalline struc-
ture of the magnetic core remained unchanged after the
chemical modification and reduction processes. No other
diffraction peaks can be seen in Figure 3a, which con-
firms the purity of the Fe3O4 crystalline phase. Also, the
Fe3O4 core exhibits a cluster‐like nanostructure with an
average crystalline size of 15 nm calculated using the
Debye–Scherrer equation from the broadness of the
(311) peak. As shown in Figure 3b, the intensities of
Fe3O4 diffraction peaks were decreased due to the heavy
atom effect of Pd nanoparticles. Three new diffraction
3 | RESULTS AND DISCUSSION
In the first step, Fe3O4 MNPs were prepared via a hydro-
thermal process and then coated with a silica shell using
the Stöber method[23] to achieve the Fe3O4@SiO2 MNPs
as a support. In the next step, threonine as an efficient
ligand/stabilizer was bonded on the surface of FS MNPs.
Finally, PdCl2 was added to threonine‐modified FS
MNPs. After chemical reduction of loaded PdII ions with
NaBH4, small Pd0 nanoparticles were generated and dis-
persed uniformly on the surface of the nanocatalyst. The
chemical structure of the catalyst was studied with FT‐
IR spectroscopy (Figure 1). As shown in Figure 1a, the
characteristic peak at 565 cm−1 was attributed to the
stretching vibration of the Fe‐‐O bond of Fe3O4 MNPs.
After surface coating with silica shell, the intensity of
the Fe‐‐O stretching band was significantly decreased
and a strong absorption peak at 1100 cm−1 and weak
absorption peak at 950 cm−1 appeared in the FT‐IR spec-
trum of FS MNPs for asymmetric vibration of the Si‐‐O‐‐
Si bond and symmetric stretching of the Si‐‐OH bond,
respectively (Figure 1b in comparison with Figure 1a)
which confirm that magnetic core was successfully coated
with the silica shell. The FT‐IR spectrum of threonine
revealed characteristic peaks at 3155, 1678, 2928 and
1464 cm−1, corresponding to NH2 stretching, NH2 out‐
of‐plane bending, asymmetric stretching and asymmetric
FIGURE 1 FT‐IR spectra of (a) Fe3O4 MNPs, (b) FS MNPs, (c)
threonine and (d) FST MNPs
FIGURE 2 TGA curve of FST‐Pd0 MNPs

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SARVI ET AL.
a). After the silica coating process, the TEM image of FS
MNPs (Figure 5b) showed that the magnetic core was
well encapsulated by the silica shell and a clear boundary
between the silica shell and magnetic core was observed
in the core–shell morphology (Figure 5b). After PdII ions
were loaded on the surface ‐‐NH2 and ‐‐OH groups of FST
MNPs and the reduction process, uniformly dispersed
small Pd0 nanoparticles on the siliceous shell of the cata-
lyst were observed with particle sizes in the range 5–
20 nm in the TEM image (Figure 5c). Based on this obser-
vation, Pd nanoparticle size and distribution have been
controlled in the presence of the threonine
stabilizer/ligand. The SEM image of FST‐Pd0 MNPs
(Figure 5d) showed spherical morphology of the
nanocatalyst. Also, aggregation of nanoparticles was
observed due to the magnetic nature of the catalyst. The
energy‐dispersive X‐ray analysis result (Figure 5e) indi-
cated that the elemental composition of FST‐Pd0 MNPs
is Fe (21.2%), O (45.8%), Si (31.5%) and Pd (1.5%), which
can be attributed to the existence of Fe3O4 core, silica
shell and dispersed Pd nanoparticles. The content of Pd
nanoparticles in the sample was confirmed using ICP‐
OES. The amount of Pd nanoparticles loaded onto the
FCA was 8.5 wt%.
FIGURE 3 XRD patterns of (a) Fe3O4 MNPs and (b) FST‐Pd0
MNPs
peaks located at 39.79° (111), 46.10° (200) and 67.15°
(220) were appeared in Figure 3a which were assigned
to a face‐centred cubic structure of Pd according to
JCPDS card no. 46‐1043. The average size of Pd nanopar-
ticles was calculated as 2.5 nm using the broadness of the
(111) peak.
The magnetic properties of Fe3O4 MNPs and FST‐Pd0
MNPs were investigated using VSM. As illustrated in
Figure 4, both samples have a ferromagnetic nature. Sat-
uration magnetization values of Fe3O4 MNPs and FST‐
Pd0 MNPs were 59.50 and 38.75 emu g−1, respectively.
Due to the greater mass and size of the silica shell,
organic ligand and metallic species, the saturation mag-
netization value of FST‐Pd0 MNPs (Figure 4b) was
decreased in comparison with that of Fe3O4 MNPs
(Figure 4a). The saturation magnetization of the
nanocatalyst was high enough to provide easy and quick
separation from a reaction mixture with an external
magnet.
After successful synthesis and characterization of the
designed nanocatalyst, its efficiency was evaluated in
C‐‐C bond formation via the Heck cross‐coupling
reaction.
The catalytic activity of FST‐Pd0 MNPs was investi-
gated in the Heck cross‐coupling of bromobenzene
(1 mmol) and methyl acrylate (1.2 mmol) as a model
reaction. For this purpose, the effects of various parame-
ters such as solvent, base, reaction temperature and cata-
lyst amount were examined in the model reaction
(Table 1).
The size and morphology of the nanocatalyst were
studied using SEM and TEM (Figure 5). Based on the
TEM image of Fe3O4 MNPs in Figure 5a, magnetic nano-
particles are approximately spherical particles (Figure 5
The effect of various solvents on the model reaction
was investigated as a first step (Table 1, entries 1–8). Polar
and non‐polar solvents such as dimethylformamide
(DMF), dimethylsulfoxide (DMSO), tetrahydrofuran
(THF), n‐hexane, toluene, 1,4‐dioxane, ethanol and water
were examined in the presence of 1 mol% of catalyst,
among which water can be used as a green, non‐toxic
and inexpensive solvent for the model reaction. In the fol-
lowing step, several common bases were examined in the
model reaction. The results revealed that the presence of
base has a crucial effect in the Heck cross‐coupling reac-
tion progress (Table 1, entries 8–12). Et3N afforded higher
yield and shorter reaction time than the other bases
(Table 1, entry 12). Also, reaction temperature was evalu-
ated in the model reaction. The best result was achieved
when the model reaction was carried out at 80°C
(Table 1, entries 13–15). The amount of catalyst was also
optimized. Higher yield of the desired product in shorter
FIGURE 4 Magnetization hysteresis loops of (a) Fe3O4 MNPs
and (b) FST‐Pd0 MNPs

SARVI ET AL.
7 of 10
FIGURE 5 TEM images of (a) Fe3O4
MNPs, (b) FS MNPs and (c) FST‐Pd0
MNPs. (d) SEM image of FST‐Pd0 MNPs.
(e) Energy‐dispersive X‐ray analysis of
FST‐Pd0 MNPs

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SARVI ET AL.
TABLE 1 Optimization of model reaction catalysed by FST‐Pd0 MNPs
Entry
Catalyst (mol%)
Base
Solvent
DMF
Temp. (°C)
Time (h)
Yield of 3 (%)
1
1
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
—
110
110
110
110
110
110
110
110
110
110
110
110
80
0.8
1.25
20
85
72
35
27
40
53
72
90
0
2
1
DMSO
THF
3
1
4
1
n‐Hexane
Toluene
1,4‐Dioxane
EtOH
H2O
24
5
1
5
6
1
10
7
1
1
8
1
0.75
24
9
1
H2O
10
11
12
13
14
15
16
17
18
19
1
K3PO4
KOH
H2O
2
75
60
98
97
97
96
80
5
1
H2O
1.25
0.6
0.66
0.66
0.66
1.25
24
1
Et3N
H2O
1
Et3N
H2O
0.5
Et3N
H2O
80
0.25
Et3N
H2O
80
0.1
Et3N
H2O
100
—
0.25
Et3N
H2O
—
Et3N
H2O
80
24
—
—
FST (0.2 g)
Et3N
H2O
80
24
Reaction conditions: 1 mmol of bromobenzene, 1.2 mmol of methyl acrylate, 2 mmol of base, 1.5 ml of solvent.
bIsolated yield.
reaction time was achieved with 0.25 mol% of FST‐Pd0
MNPs (Table 1, entry 15). Finally, the model reaction
was conducted in the absence of any catalyst and also
with FST MNPs. Results showed that under these condi-
tions the reaction did not proceed (Table 1, entries 18
and 19). According to this study, water as solvent, Et3N
as base and 0.25 mol% of catalyst at 80°C were chosen
as optimized reaction conditions.
The scope and adaptability of the catalysed Heck
cross‐coupling reaction were also investigated. The opti-
mized reaction conditions were applied for the reaction
of a wide range of aryl halides with electron‐donating
and electron‐withdrawing groups with various olefins.
The results (Table 2) revealed that cross‐coupling reac-
tions of various substituted aryl iodides and aryl bromides
with methyl acrylate, ethyl acrylate and n‐butyl acrylate
proceeded to give high yields in short reaction time
(Table 2, entries 1–17). Also, aryl halides with electron‐
withdrawing substituents react with olefins more quickly
than those with electron‐donating substituents. In the
case of aryl chloride, a yield of only 40% of final cross‐
coupled product was produced under the same reaction
condition after 10 h (Table 2, entry 18).
All of the products were known and isolated as oil or
solid products which were characterized using NMR
spectroscopy (see supporting information). The reaction
progress was monitored by TLC and disappearance of
starting substrates confirmed the completion of the reac-
tion. For practical applications of heterogeneous catalysts
and from a green chemistry point of view, the recovery
and reusability of catalysts is an important factor. Fur-
thermore, a set of experiments was conducted to recover
and reutilize the FST‐Pd0 MNPs in the model Heck
cross‐coupling reaction. After the first run, the FST‐Pd0
nanocatalyst was easily separated from the reaction mix-
ture using an external magnet and then washed with eth-
anol and ethyl acetate (3 × 5 ml). After drying the catalyst
at 50°C in an oven, next model reaction was started using
the recovered catalyst and fresh substrates. This process
was repeated for eight consecutive runs (Figure 6).
Results of hot filtration test for FST‐Pd0 in the middle of
the reactions show a low leaching of Pd after eight runs.

SARVI ET AL.
9 of 10
TABLE 2 Heck cross‐coupling reaction of various substrates
Entry
1
2
X
Time (min)
3
Yield (%)b
1
I/Br
25/40
3a
96/96
2
3
I/Br
I/Br
30/40
35/45
3b
3c
96/90
96/94
4
5
6
I/Br
I/Br
I/Br
35/40
30/45
35/50
3d
3e
3f
94/90
92/92
90/88
7
8
I/Br
I/Br
35/45
45/55
3g
93/90
90/89
3h
9
I/Br
I/Br
Br
25/30
25/30
35
3i
94/92
93/92
91
10
11
3j
3k
12
13
14
Br
35
3l
95
I/Br
I/Br
40/35
35/35
3m
3n
91/90
85/85
15
16
17
I/Br
I
25/25
25
3o
3p
3q
94/90
95
I/Br
30/40
93/90
18
Cl
600
3a
40
Conditions: 1 (1 mmol), 2 (1.2 mmol), base (2 mmol), catalyst (0.25 mol%), H2O (1.5 ml), 80°C.
bIsolated yield.
The content of Pd on the recycled solid catalyst was mea-
sured using ICP‐OES. Comparison between the amount
of Pd in fresh and recycled catalyst revealed that only
0.05% of Pd species was leached after eight cycles.
4 | CONCLUSIONS
FST‐Pd0 MNPs were synthesized and characterized as an
environmentally friendly magnetic catalyst for Heck
cross‐coupling reaction of substrates with various
electron‐withdrawing and electron‐donating groups on
aromatic rings. Due to the high activity and efficiency of
FIGURE 6 Recyclability of catalyst for model reaction

10 of 10
SARVI ET AL.
S. Rostamnia, K. Lamei, F. Pourhassan, RSC Adv. 2014,
4(103), 59626.
this nanocatalyst, high yields in short reaction times were
achieved. Probably, the presence of threonine with ‐‐NH2
and ‐‐OH groups on the surface of the support led to well‐
dispersed Pd nanoparticles of small size. Uniformly dis-
persed small Pd nanoparticles resulted in high catalytic
activity for the Heck reaction in water as a green, non‐
toxic solvent. The FST‐Pd0 nanocatalyst after easy separa-
tion from the reaction mixture using an external magnet
was reused for eight successful runs.
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[19] V. G. Ramu, A. Bordoloi, T. C. Nagaiah, W. Schuhmann, M.
Muhler, C. Cabrele, Appl. Catal. A 2012, 431, 88.
The authors gratefully acknowledged for partially finan-
cial support of this study by Ferdowsi University of
Mashhad Research Council (Grant No: 3/41258).
[20] O. V. Kharissova, B. I. Kharisov, V. M. Jiménez‐Pérez, B. M.
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ORCID
[22] F. Nador, M. A. Volpe, F. Alonso, A. Feldhoff, A. Kirschning,
G. Radivoy, Appl. Catal. A 2013, 455, 39.
Mostafa Gholizadeh http://orcid.org/0000-0002-9947-2248
Mohammad Izadyar
9982
http://orcid.org/0000-0002-3795-
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