Thiophene Methanimine–Palladium Schiff Base Complex Anchored on Magnetic Nanoparticles: A Novel, Highly Efficient and Recoverable Nanocatalyst for Cross-Coupling Reactions in Mild and Aqueous Media: Γ-Fe2O3/AEPH2-TC-Pd Catalyzed Suzuki–Miyaura and Heck–Mizoroki Reactions

Article abstract of DOI:10.1007/s10562-017-2170-x

Abstract: In this study, a novel thiophene methanimine–palladium Schiff base complex anchored on decorated γ-Fe₂O₃ with 2-aminoethyl dihydrogen phosphate (γ-Fe₂O₃/AEPH₂-TC-Pd) was synthesized as a new magnetically separable nanocatalyst. Characterization of the new designed nanocatalyst was performed successfully using different techniques such as FT-IR, XRD, XPS, TEM, TGA, VSM, ICP and elemental analysis. This nanocatalyst presented a superb catalytic activity for Suzuki–Miyaura and Heck–Mizoroki cross-coupling reactions. The most important features of the prepared catalytic system which makes the current protocol more beneficial from both industrial and environmental viewpoints are its ease of recovery and reusability up to nine cycles without appreciable loss of the catalytic performance, as well as accomplishing the reactions under mild conditions in aqueous media which is a great challenge in some cross-coupling reactions. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-017-2170-x

Catal Lett
DOI 10.1007/s10562-017-2170-x
Thiophene Methanimine–Palladium Schiff Base Complex
Anchored on Magnetic Nanoparticles: A Novel, Highly Efficient
and Recoverable Nanocatalyst for Cross-Coupling Reactions
in Mild and Aqueous Media
γ-Fe2O3/AEPH2-TC-Pd Catalyzed Suzuki–Miyaura and Heck–Mizoroki Reactions
Roya Jahanshahi1 · Batool Akhlaghinia1
Received: 8 June 2017 / Accepted: 14 August 2017
© Springer Science+Business Media, LLC 2017
Abstract In this study, a novel thiophene methanimine–
palladium Schiff base complex anchored on decorated
γ-Fe2O3 with 2-aminoethyl dihydrogen phosphate (γ-Fe2O3/
AEPH2-TC-Pd) was synthesized as a new magnetically sep-
arable nanocatalyst. Characterization of the new designed
nanocatalyst was performed successfully using different
techniques such as FT-IR, XRD, XPS, TEM, TGA, VSM,
ICP and elemental analysis. This nanocatalyst presented a
superb catalytic activity for Suzuki–Miyaura and Heck–Miz-
oroki cross-coupling reactions. The most important features
of the prepared catalytic system which makes the current
protocol more beneficial from both industrial and environ-
mental viewpoints are its ease of recovery and reusability
up to nine cycles without appreciable loss of the catalytic
performance, as well as accomplishing the reactions under
mild conditions in aqueous media which is a great challenge
in some cross-coupling reactions.
Graphical Abstract
Keywords γ-Fe2O3/AEPH2-TC-Pd · Sustainable
chemistry · Magnetically separable nanocatalyst ·
Palladium–Schiff base complex · Suzuki–Miyaura · Heck–
Mizoroki
1 Introduction
Nowadays, novel approaches focus on development of more
environmentally sustainable chemical processes in both aca-
demic and industrial viewpoints [1]. One of the important
criteria that support a chemical transformation in a move-
ment toward the green chemistry, is utilizing the heterogene-
ous catalytic systems instead of their homogeneous coun-
terparts [2, 3]. Although homogeneous catalysts provide
excellent activity and selectivity, they are encounter with the
problems associated with catalyst separation, catalyst recy-
cling and the contamination of the ligand or metal residue
in the final products [4]. On the other hand, in spite of hav-
ing substantial merits in terms of stability and reusability,
heterogeneous catalysts have lower activity and selectivity
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-017-2170-x) contains supplementary
material, which is available to authorized users.
* Batool Akhlaghinia
akhlaghinia@um.ac.ir
1
Department of Chemistry, Faculty of Sciences, Ferdowsi
University of Mashhad, Mashhad 9177948974, Iran
Vol.:(0123456789)
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R. Jahanshahi, B. Akhlaghinia
compared with homogeneous catalytic systems, which is due
to their lower available surface area [5]. Interestingly, the
combination of nanotechnology and heterogeneous catalytic
systems through the immobilization of the stable ligands,
complexes and other catalytic species onto the nanostruc-
tured supports, could overcome the mentioned drawbacks [5,
6]. In the other words, nano-chemical technology has closed
the homogeneous and heterogeneous catalytic systems from
activity and selectivity outlooks.
2-Aminoethyl dihydrogen phosphate (AEPH2) is a type of an
almost unknown biodegradable phospholipidic compound,
which has very recently attracted an increasing attention as
a surface modification ligand [5, 42–47].
In spite of a great deal of heterogeneous catalytic systems
suggested for Suzuki–Miyaura and Heck–Mizoroki cross-
coupling reactions to date, furnishing such a reactions in
aqueous and also mild conditions is still an essential require-
ment, especially for more challenging partners due to the
performance of side reactions in Suzuki–Miyaura cross-
coupling reactions and inherent difficulty of Heck–Mizoroki
cross-coupling reactions under mild conditions.
Among various nanostructured supports, magnetic nano-
particles (MNPs) have emerged as one of the most popular
ones [7]. The main advantage of MNPs is that they can be
easily separated from the reaction media by applying a sim-
ple external magnetic field without using traditional filtra-
tion or centrifugation approaches. Additionally, MNPs have
superlative physical properties such as excellent chemical
and thermal stability, high surface area and the surface mod-
ification ability [8].
Considering the above facts and as a part of our ongoing
research interest in developing new heterogeneous catalysts
[43–54], herein, we have synthesized a novel thiophene
methanimine-palladium Schiff base complex anchored on
decorated γ-Fe2O3 with 2-aminoethyl dihydrogen phosphate
(γ-Fe2O3/AEPH2-TC-Pd) as a highly efficient magnetically
separable nanocatalyst (Scheme 1). We hoped to conduct
these coupling reactions upon mild and aqueous media by
means of the existence of nitrogen and sulfur containing
ligand in the catalyst structure, which can strongly stabilized
the palladium species. The catalytic activity of γ-Fe2O3/
AEPH2-TC-Pd was subsequently investigated towards the
Suzuki–Miyaura and Heck–Mizoroki cross-coupling reac-
tions, in green media (see Scheme 2 and 3). The proposed
catalytic system has privileged features such as high stabil-
ity, moisture and air insensitiveness as well as satisfying
magnetic separability along with simple catalytic prepara-
tion procedure.
Suzuki–Miyaura and Heck–Mizoroki cross-coupling
reactions are one of the most potent transformations for
the effective and straightforward C–C bond formation. The
resulting coupling products have extensive applications as
fundamental building blocks in the synthesis of numerous
structurally diverse molecules in several fields of the chem-
istry such as pharmaceuticals, agrochemicals and material
sciences [9–11]. General driving force to furnish such a
beneficial transformation is the transition metal-catalyzed
cross-coupling reactions [12, 13]. Among various transi-
tion metals [14], palladium has been the first choice and is
still considered as one of the most commonly investigated
metals for cross-coupling reactions [15–18]. Tradition-
ally, Suzuki–Miyaura and Heck–Mizoroki cross-coupling
reactions were performed under homogeneous palladium
catalytic systems comprising various ligands such as N-het-
erocyclic carbenes [19], dibenzylideneacetone (dba) [20],
phosphines [9, 21, 22], thiols [23], phthalocyanines [24],
palladacycles [25], porphyrins [26], imidazolium [27] and
Schiff bases [28]. Although, each of these approaches has
their own advantages, but they suffer from previously men-
tioned limitations related to the homogeneous catalytic sys-
tems. By consideration the above facts, designing the hetero-
geneous Pd-based catalysts via the immobilization of related
palladium containing catalysts on different supports [29–36]
has recently gathered a great deal of attention.
2 Experimental
2.1 Materials and Instruments
All chemical reagents and solvents were purchased from
Merck and Sigma-Aldrich chemical companies and were
used as received without further purification. γ-Fe2O3 nano-
particles were prepared by the previously reported chemical
co-precipitation technique in literature [55].
The purity determinations of the products and the pro-
gress of the reactions were accomplished by TLC on silica
gel polygram STL G/UV 254 plates. The melting points
of the products were determined with an Electrothermal
Type 9100 melting point apparatus. The FT-IR spectra
were recorded on pressed KBr pellets using an AVATAR
370 FT-IR spectrometer (Therma Nicolet spectrometer,
USA) at room temperature in the range between 4000 and
400 cm−1 with a resolution of 4 cm−1. The NMR spectra
were provided by Brucker Avance 300, 400 and 500 MHz
instruments in CDCl3 in the presence of tetramethylsilane as
the internal standard and the coupling constants (J values)
A well-known strategy for preparing a heterogeneous
catalyst is anchoring a suitable ligand to a support surface
for further immobilization of a transition metal [37, 38].
The selected ligand for this purpose has a vital influence
on tuning the catalyst activity and catalyst stability at room
temperature in air or in aqueous solution [39]. Using a
proper support or supported ligand for Pd heterogenization
would offer a stable and reusable catalytic system with the
minimized metal leaching and aggregation issues [40, 41].
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Thiophene Methanimine–Palladium Schiff Base Complex Anchored on Magnetic Nanoparticles:…
Scheme 1 An overview on the synthesis of γ-Fe2O3/AEPH2-TC-Pd nanocatalyst
Scheme 2 Suzuki–Miyaura cross-coupling reaction in the presence of γ-Fe2O3/AEPH2-TC-Pd nanocatalyst
are given in Hz. Elemental analyses were performed using
a Thermo Finnigan Flash EA 1112 Series instrument (fur-
of 25–900 °C at a heating rate of 10 °C min−1, under air
atmosphere. Transmission electron microscopy (TEM) was
performed with a Leo 912 AB microscope (Zeiss, Germany)
with an accelerating voltage of 120 kV. Room temperature
magnetization isotherms were obtained using a vibrat-
ing sample magnetometer (VSM) (Magnetic DaneshPajoh
Inst.). Inductively coupled plasma optical emission spectros-
copy (ICP-OES) was carried out on a 76004555 SPECTRO
nace: 900 °C, oven: 65 °C, flow carrier: 140 mL min−1
,
flow reference: 100 mL min−1). Mass spectra were recorded
with a CH7A Varianmat Bremem instrument at 70 eV elec-
tron impact ionization, in m/z (rel%). Thermogravimetric
analysis (TGA) was carried out using a Shimadzu Ther-
mogravimetric Analyzer (TG-50) in the temperature range
1 3

R. Jahanshahi, B. Akhlaghinia
Scheme 3 Heck–Mizoroki cross-coupling reaction in the presence of γ-Fe2O3/AEPH2-TC-Pd nanocatalyst
ARCOS ICP-OES analyzer. The crystal structure of the cata-
lyst was analyzed by XRD using a D8 ADVANCE-Bruker
diffractometer operated at 40 kV and 30 mA, utilizing Cu
Kα radiation (λ=0.154 Å), at a step size of 0.040° and step
time of 1.5 s. X-ray photoelectron spectroscopy (XPS) was
performed using the Thermo Scientifi, ESCALAB 250 Xi
Mg X-ray resource. All yields refer to isolated products after
purification by thin layer chromatography. In addition, all of
the products were known compounds and they were char-
(γ-Fe2O3/AEPH2) (II) was separated by an external magnet,
washed with distilled water repeatedly and air-dried.
2.4 Preparation of γ-Fe2O3/AEPH2-TC (III)
1 g of γ-Fe2O3/AEPH2 (II) was dispersed in dried methanol
(20 mL), by placing in an ultrasonic bath for 20 min. There-
upon, thiophene-2-carbaldehyde (TC) (0.8 g) was added to
the resulting suspension, followed by refluxing for 4 h. In
the last step, the obtained nanoparticles were separated by
magnetic decantation and repeatedly washed with hot etha-
nol and finally dried at ambient temperature overnight.
1
acterized by H NMR, 13C NMR spectroscopy, and mass
spectrometry and comparison of their melting points with
known compounds.
2.2 Preparation of γ-Fe2O3 Nanoparticles (I)
2.5 Preparation of γ-Fe2O3/AEPH2-TC-Pd (IV)
FeCl2·4H2O (0.01 mol, 1.99 g) and FeCl3·6H2O (0.01 mol,
3.25 g) with a mole ratio of 1:1, were dissolved in deionized
water (30 mL) under argon atmosphere at room temperature.
A NH4OH solution (0.6 M, 200 mL) was then added drop
by drop (drop rate=1 mL min−1) into the stirring mixture
at room temperature. When the reaction pH was reached to
11, addition of NH4OH solution was stopped. The resulting
black dispersion was vigorously stirred for 1 h at room tem-
perature, followed by heating to reflux temperature for 1 h
to yield a brown dispersion. The resultant magnetic nano-
particles were next separated using a magnetic bar, washed
with deionized water (5× 20 mL) and then dried at 40 °C
for 24 h. Thereafter, the obtained nanoparticles were heated
at 2 °C min−1 up to 220 °C, and then kept in the furnace for
3 h to give a reddish-brown powder.
The prepared γ-Fe2O3/AEPH2-TC (1 g) was sonicated in eth-
anol (30 mL) for 30 min. After that, a solution of Pd(OAc)2
(0.34 mmol, 0.076 g) in ethanol (10 mL) was added to the
dispersed suspension and continuous stirring was conducted
at reflux condition for 12 h. After cooling down to room
temperature, the solid nanocatalyst was separated from the
reaction medium using an external magnet. The nanocatalyst
preparation process was completed by washing the resultant
material with ethanol (3×15 mL) before being dried under
vacuum at 50 °C.
2.6 Typical Procedure for Suzuki–Miyaura
Cross-Coupling Reaction
Typically, a 10-mL round-bottom flask was charged with
iodobenzene (1.0 mmol, 0.204 g), phenylboronic acid
(1.1 mmol, 0.134 g), K2CO3 (1.2 mmol, 0.165 g), TBAB
(0.2 mmol, 0.064 g), H2O (3 mL) and 0.0015 g of γ-Fe2O3/
AEPH2-TC-Pd (0.1 mol%). The resultant mixture was heated
under stirring at 40 °C for 10 min. After completion of the
reaction (as judged by TLC), the reaction mixture was
allowed to cool down to room temperature and the nanocata-
lyst was easily separated by using a proper magnetic field,
washed with EtOH and vacuum-dried at 50 °C to be ready
2.3 Preparation of γ-Fe2O3 NPs Functionalized
with AEPH2 (γ-Fe2O3/AEPH2) (II)
γ-Fe2O3 nanoparticles (1 g) were dispersed in 20 mL deion-
ized water by sonication for 20 min. Then, AEPH2 (0.7 g,
5 mmol) was added portion by potion to the resulting sus-
pension and the obtained mixture was vigorously stirred
at room temperature. After 12 h, the afforded material
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Thiophene Methanimine–Palladium Schiff Base Complex Anchored on Magnetic Nanoparticles:…
for utilizing in successive runs. Subsequently, the reaction
mixture was extracted with ethyl acetate (5×5 mL). Com-
bined organic phase was dried over anhydrous Na2SO4, and
solvent was next removed on a rotary evaporator. Thereafter,
the obtained crude product was chromatographed on silica
gel (eluted with n-hexane:ethyl acetate; 50:1), to afford the
corresponding pure product (0.148 g, % 98 yield).
2.7 Typical Procedure for Heck–Mizoroki Coupling
Reaction
Typically, a 10-mL round-bottom flask was charged
with iodobenzene (1.0 mmol, 0.204 g), n-butyl acrylate
(1.1 mmol, 0.140 g), K2CO3 (1.2 mmol, 0.165 g), TBAB
(0.2 mmol, 0.064 g), H2O (3 mL) and 0.0015 g of γ-Fe2O3/
AEPH2-TC-Pd (0.1 mol%). The resulting mixture was heated
under stirring at 65 °C till the reaction was judged to be
Fig. 1 FT-IR spectra of a γ-Fe2O3; b γ-Fe2O3/AEPH2; c γ-Fe2O3/
complete by TLC (10 min). Then, the reaction mixture was
allowed to cool down to room temperature and the nanocata-
lyst was easily separated by using a proper magnetic field,
washed with EtOH and vacuum-dried at 50 °C to be ready
for utilizing in the next run. Thereupon, the reaction mix-
ture was extracted with ethyl acetate (5×5 mL). Combined
organic phase was dried over anhydrous Na2SO4, and solvent
was then removed on a rotary evaporator. Afterwards, the
obtained crude product was chromatographed on silica gel
(eluted with n-hexane:ethyl acetate; 50:1), to afford the cor-
responding pure product (0.148 g, % 98 yield).
AEPH2-TC; d γ-Fe2O3/AEPH2-TC-Pd and e 9th recovered γ-Fe2O3/
AEPH2-TC-Pd
hydroxyl groups and adsorbed water molecules [58, 59].
In the FT-IR spectrum of γ-Fe2O3/AEPH2 (Fig. 1b), the
appearance of the broad absorption band in the region of
about 3600–3000 cm−1, could be related to the overlapping
of uncoated O–H and NH2 stretching modes [5]. Moreover,
the absorption bands visualized at 2960 and 2882 cm−1
,
could be in turn assigned to the asymmetric and symmet-
ric methylene C–H stretching vibrational frequencies [60].
Also, the stretching vibrations of P=O and Fe–O–P bonds
which were observed at 1150 and 1030 cm−1, respectively
[5], would confirm the successful chemically attachment
of AEPH2 onto the MNPs surface. Schiff base formation
was verified by the appearance of characteristic absorption
band of imine group (C=N) at 1656 cm−1 [61], as well as
the intensity diminution of NH2 stretching band (Fig. 1c). It
is interesting to note that upon metallation with palladium
acetate, the diagnostic absorption band of C=N bond at
1656 cm−1 was shifted to a lower wave number (1640 cm−1)
[62]. These results confirmed the strong metal–ligand inter-
action (Fig. 1d).
3 Results and Discussion
γ-Fe2O3/AEPH2-TC-Pd as a new heterogeneous nanocata-
lyst was synthesized according to the pathway shown in
Scheme 1.
It is important to note that the stability of the phosphonate
linker which has utilized for fabrication of the respective
nanocatalyst, as well as the simplicity of its tethering on the
magnetic γ-Fe2O3 support [56], besides its ability to act as
stabilizing palladium agent against agglomeration and pal-
ladium black formation, makes the presented protocol more
fascinating.
Furthermore, XRD analysis was conducted to perceive
the crystalline nature of the novel synthesized catalyst
(Fig. 2). As it is obvious in the XRD pattern of γ-Fe2O3/
AEPH2-TC-Pd, the presence of characteristic peaks appeared
at angles attributing to (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5
1 1) and (4 4 0) reflection planes are in a good agreement
with the face-centered cubic structure of γ-Fe2O3 (JCPDS
file no. 01-078-6916) [63, 64]. Importantly, three diffrac-
tion peaks appeared at 2θ =40.00, 46.40 and 68.06 corre-
sponding to (1 1 1), (2 0 0) and (2 2 0) crystallographic
planes, respectively, could be assigned to the existence of
the face-centered cubic structure of Pd(0) (JCPDS file no.
3.1 Characterization of γ-Fe2O3/AEPH2-TC-Pd
FT-IR spectroscopy was conducted for every step of the
nanocatalyst synthesis to verify the successful function-
alization of the MNPs surface (Fig. 1). As it is exhibited
in Fig. 1a, the absorption bands at around 698–571 cm−1
are attributed to the vibration of Fe–O bonds in the crys-
talline lattice of γ-Fe2O3 [57]. The characteristic bands at
3436 and 1620 cm−1 are respectively ascribed to the stretch-
ing and bending vibration modes of the surface attached
1 3

R. Jahanshahi, B. Akhlaghinia
with the average particle size observed in the TEM analysis
(See Figs. 4, 5).
X-ray photoelectron spectroscopy (XPS) is a powerful
technique to evaluate the electron properties of the species
anchored on the surface, such as oxidation state, electron
environment and the binding energy of the core electron of
the metal. As can be seen in the XPS spectra of both fresh γ
Fe2O3/AEPH2-TC-Pd and ninth reused nanocatalyst (Fig. 3a,
b), the presence of binding energies at 335.0 and 335.1 eV
(due to Pd 3d5/2) alongside the binding energies at 340.2 and
340.3 eV (due to Pd 3d3/2) relating in turn to the fresh and
ninth reused nanocatalyst, might be assigned to the metallic
Pd with zero oxidation state [67, 68]. Moreover, the peaks
corresponding to P, C, N, O, Fe and Pd atoms are also clearly
observed in the XPS elemental survey of both fresh and
ninth reused nanocatalyst (Fig. 3c, d).
Fig. 2 XRD pattern of γ-Fe2O3/AEPH2-TC-Pd
The XPS analysis in agreement with XRD pattern, con-
tributes to the presence of metallic Pd nanoparticles in
the prepared catalyst, while we used no reducing agent
during the immobilization of Pd species. Construction of
metallic Pd nanoparticles in our designed catalyst could
be attributed to the presence of thiophene rings and some
05-0681) in the nanocatalyst structure [65, 66]. The average
crystallite size of γ-Fe2O3 nanoparticles (15 nm), which was
calculated using Debye–Scherrer equation, is well matched
Fig. 3 XPS spectra of the fresh and 9th recovered γ-Fe2O3/AEPH2-TC-Pd showing Pd 3d5/2 and Pd 3d3/2 binding energies (a, b) and XPS ele-
mental survey scan of the fresh and 9th recovered nanocatalyst (c, d)
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Thiophene Methanimine–Palladium Schiff Base Complex Anchored on Magnetic Nanoparticles:…
Fig. 4 TEM images of γ-Fe2O3/
AEPH2-TC-Pd
Fig. 5 Particle size distribution histogram of γ-Fe2O3/AEPH2-TC-Pd
existing heteroatoms on the structure of the catalyst, which
can increase the electron density of the support surface.
On the other hand, ethanol also can operate as a reducing
agent during the synthetic procedure.
TEM images of the new nanocatalyst were recorded
and shown in Fig. 4. It can be easily deduced from TEM
Fig. 6 TGA thermogram of γ-Fe2O3/AEPH2-TC-Pd
images that the as-synthesized catalyst has spherical mor-
phology with a very satisfying monodispersity. Particle
size distribution of γ-Fe2O3/AEPH2-TC-Pd which was
calculated according to the TEM analysis displayed that
the average diameter of the nanoparticles are about 15 nm
(Fig. 5).
AEPH2-TC-Pd represented that 0.87 mmol of palladium was
anchored on 1.00 g of the nanocatalyst.
Magnetic behavior of the novel synthesized nanocata-
lyst in comparison with bare γ-Fe2O3 MNPs was evaluated,
using VSM analysis (Fig. 7). As it is evident from the result-
ing magnetization curves, the value of saturation magnetic
moment of the prepared nanocatalyst was diminished from
76 to 60 emu g−1. This reduction in the saturation magnetic
moment arises from the surface modification of γ-Fe2O3
MNPs with non-magnetic materials during the catalyst syn-
thetic pathway. By the way, having the superparamagnetic
property, guarantees very facile and efficient separation of
the nanocatalyst from the reaction media.
TGA was carried out to determine the thermal stability
and organic content of the new nanocatalyst (Fig. 6). The
obtained results revealed two significant mass changes
at different temperature ranges. The first mass change of
about 1.9% below 200 °C, could be assigned to remove of
the trapped physically adsorbed water and probably ethanol.
The second and indeed the significant mass change, which
was started at around 200 °C and continued up to 670 °C can
be attributed to the elimination of organic functional groups
incorporated in the surface of γ-Fe2O3 MNPs (15.5%).
According to the TGA, the amount of organic motif sup-
ported on γ-Fe2O3 MNPs was estimated to be 0.85 mmol g−1.
These findings were in a very good agreement with the
obtained elemental analysis data (N=1.2% and C=5.62%)
and ICP-OES, as well. The ICP-OES analysis of γ-Fe2O3/
3.2 Heterogeneity Tests
To ascertain whether the catalyst is actually functioning in a
heterogeneous pathway or not, a variety of different control
1 3

R. Jahanshahi, B. Akhlaghinia
Fig. 8 Reaction progress as a function of time on the Suzuki–
Miyaura cross-coupling reaction of 4-methoxy bromobenzen with
phenylboronic acid by using γ-Fe2O3/AEPH2-TC-Pd catalyst in water
under: normal conditions (red triangle); in the presence of 400 equiv-
alents Hg(0) (blue square)
Fig. 7 Magnetization curves of a γ-Fe2O3 and b γ-Fe2O3/AEPH2-TC-
Pd
experiments have been performed. Firstly, we conducted a
hot-filtration test for the model Suzuki–Miyaura coupling
reaction under the optimal conditions. In this line, precisely
10 min after the reaction starting (before complete con-
sumption of all substrates), the catalyst was separated from
the reaction mixture using an external magnet. In this step,
only 45% conversion was achieved. Afterwards, the reaction
was permitted to continue for further 3 h. No considerable
reaction progress hereon, well suggested that no leaching of
palladium happened during the reaction process. Neverthe-
less, a negative hot-filtration test could not be a conclusive
indication for the actual catalytic activity attributed solely
to heterogeneity [69]. This is due to the fact that in many
circumstances the leached and soluble Pd species could be
redepositing back on the insoluble support during the hot
filtration. Hence, while the negative hot-filtration test might
point out that the catalyst could be to some extent recyclable,
it should not be considered as a unique evidence for hetero-
geneity of the solid catalysts [69].
place during the reaction and that the catalyst is purely
heterogeneous in nature.
However, it is very essential to note that a negative
result from the poisoning test by considering the final yield
alone can sometimes consequence in an erroneous conclu-
sion [70]. So, to further elucidate the heterogeneous nature
of the designed nanocatalyst, we decided to set out some
additional experiments to study the reaction kinetics, both
in the absence and in the presence of the poison [Hg(0)
(Hg/Pd, 400:1)]. The kinetic experiments were conducted
using 4-methoxy bromobenzene and phenylboronic acid as
coupling partners under the optimized reaction conditions
in the presence of γ-Fe2O3/AEPH2-TC-Pd (Fig. 8). In this
study, the reaction progress was evaluated in 10-min inter-
vals for both mentioned experiments. As it is obvious from
data in Fig. 8, the reaction advancement was almost the
same either in the absence or presence of the poison. So,
based on the kinetic studies it could be nicely certified that
the catalytic activity of the nanocatalyst derives from the
leached out Pd. By considering all of the above investiga-
tions, we have speculated that the respective nanocatalyst
most likely operated in a heterogeneous manner.
To further clarify whether the γ-Fe2O3/AEPH2-TC-
Pd-catalyzed cross-coupling reaction we encountered
in this study is truly heterogeneous in nature or not, we
also performed the typical poisoning test for the model
Suzuki–Miyaura coupling reaction under the optimized
conditions, using an excess amount of Hg(0) (Hg/Pd,
400:1), as an effective palladium scavenger which selec-
tively coordinates and deactivates the leached out palla-
dium. In this regard, if the coupling reaction was proceeds
through the leached out palladium species, the cessation
of the reaction would be expected. However, affording the
corresponding biphenyl in excellent isolated yield without
any detectable induction period in this investigation, inter-
estingly documented that no leaching of palladium takes
The heterogeneous pathway observed using this system
might be attributed to the presence of sulfur-containing
ligand on the surface of the catalyst. Since sulfur based
ligands are strong Pd scavengers, we believed that the
thiophene ligand on the structure of our catalyst is tightly
reserved the Pd nanoparticles on the support surface and
prevents their leaching to solution to act via possible
homogeneous pathway.
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Thiophene Methanimine–Palladium Schiff Base Complex Anchored on Magnetic Nanoparticles:…
3.3 Catalytic Activity of γ-Fe2O3/AEPH2-TC-Pd
in Suzuki–Miyaura Cross-Coupling Reaction
in the catalytic process of such a coupling reaction is high-
lighted. In the next step, to survey the influence of the sol-
vent on the reaction progress, several solvents including
DMF, CH3CN, Toluene, EtOH, H2O and H2O/EtOH [1/1
(v/v)] were tested under the same conditions in the presence
of 0.1 mol% of the catalyst (Table 1, entries 2–7). These
experiments led us to found that H2O and DMF were supe-
rior to all of the examined solvents. By the way, owing to the
awful environmental impact of DMF, the conclusive choice
for the reaction medium would be H2O, especially regarding
to the concepts of the green chemistry. Then, to assess the
base effect on the reaction rate, a series of available bases
were screened (Table 1, entries 8–12). As can be readily
perceived from the data in Table 1, inorganic bases such
as potassium carbonate (K2CO3), sodium hydrogen carbon-
ate (NaHCO3) and potassium hydroxide (KOH) were more
effectual vs. the organic bases triethylamine, sodium acetate
Following the successful preparation and characterization of
γ-Fe2O3/AEPH2-TC-Pd, to explore the potential activity of
such a new heterogeneous nanocatalyst, its catalytic appli-
cability was initially evaluated against the Suzuki–Miyaura
cross-coupling reaction. In the preliminary assessment, the
reaction conditions were optimized by varying the solvent,
base, temperature and catalyst amount toward the model
reaction of iodobenzene (1 mmol) with phenylboronic
acid (1.1 mmol) (Table 1). When a blank experiment in
the absence of catalyst was conducted by carrying out the
model reaction in DMF at 100 °C, using 1:1.5 molar ratio
of iodobenzene/K2CO3, only a trace amount of the product
was acquired even after a prolonged time (Table 1, entry 1).
Accordingly, the essential role of γ-Fe2O3/AEPH2-TC-Pd
Table 1 Suzuki–Miyaura cross-coupling reaction of iodobenzene with phenylboronic acid in the presence of γ-Fe2O3/AEPH2-TC-Pd nanocata-
lyst, under different reaction conditions
Entry Catalyst (mol%) Molar ratios of
iodobenzene:base
Base
Solvent
Additive (mmol) Temp. (°C) Time (min) Isolated yield (%)
1
2
3
4
5
6
7
8
–
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.5
1:1.2
1:1.1
1:2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Et3N
DMF
DMF
100
100
Reflux
Reflux
Reflux
100
Reflux
100
100
100
100
100
80
80
80
80
80
80
60
40
40
18 h
20
60
18 h
20
20
35
30
20
60
20
4 h
25
25
45
25
60
10
10
10
15
15
35
2 h
Trace
93
25
15
82
91
90
75
86
36
81
25
85
85
79
85
82
98
98
98
64
35
87
15
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.09
0.1
0.1
0.1
0.1
0.1
0.1
0.1
CH3CN
Toluene
EtOH
H2O
H2O/EtOH 1/1
H2O
H2O
H2O
H2O
9
NaHCO3
NaOAc
KOH
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24d
n-Butylamine H2O
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
TBABa (0.2)
TBAB (0.2)
TBAB (0.2)
CTABb (0.2)
SDSc (0.2)
40
40
40
TBAB (0.1)
TBAB (0.2)
Reaction conditions: iodobenzene (1 mmol), phenylboronic acid (1.1 mmol) and solvent (3 mL)
aTetrabutylammonium bromide
bCetyl trimethyl ammonium bromide
cSodium dodecylsulfate
dReaction was performed in the presence of Pd(OAc)2 as catalyst
1 3

R. Jahanshahi, B. Akhlaghinia
Table 2 Substrate scope for Suzuki–Miyaura cross-coupling reaction using γ-Fe2O3/AEPH2-TC-Pd nanocatalyst
Entry
R1
R2
X
Product
Time (min)
Isolated yield (%)
1
2
3
4
5
6
7
8
H
4-NO2
4-Cl
H
H
H
H
H
H
H
3-NO2
3-NO2
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
I
I
I
I
I
I
I
I
3a
3b
3c
3d
3e
3f
3g
3h
3i
3a
3b
3j
3c
3k
3l
3e
3a
3b
3k
3e
3m
3f
10
10
20
30
55
25
50
35
55
25
25
25
30
30
45
90
2 h/18 h
2 h/18 h
3 h/24 h
3 h/24 h
3 h/24 h
2 h/24 h
5 h/24 h
24 h
98
98
94
91
90
88
92
90
84
95
97
93
91
89
89
80
20/80
25/82
20/78
15/65
0/Trace
Trace/30
20/20
0
4-Me
4-OMe
2-Me-4-NO2
2-Thiophene
H
4-OMe
H
4-NO2
4-CN
4-Cl
4-CHO
3-CHO
4-OMe
H
4-NO2
4-CHO
4-OMe
4-NH2
2-Me-4-NO2
2-Thiophene
4-CHO
9
I
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
Cl
Cl
F
3g
3k
Reaction conditions: aryl halide (1 mmol), boronic acid (1.1 mmol), K2CO3 (1.2 mmol), TBAB (0.2 mmol), γ-Fe2O3/AEPH2-TC-Pd (0.1 mol%),
in H2O (3 mL) at 40 °C
and n-butylamine. This might be due to the well solubil-
ity of inorganic bases in the aqueous media. Thus, K2CO3
was clearly authenticated as the best option, from the both
economic and reaction progress standpoint. By diminishing
temperature to 80 °C, the reaction progress became a little
sluggish (Table 1, entry 6 vs. 13). During our optimization
studies, the effect of using different molar ratios of iodoben-
zene/K2CO3 was also investigated. As observed, by decreas-
ing the amount of K2CO3 to 1.2 mmol, the reaction pro-
gress was just as good as when the amount of 1.5 mmol was
used (Table 1, entry 14). However, no valuable yield of the
product was acquired by further reducing the base amount
to 1.1 mmol (Table 1, entry 15). On the other hand, upon
increasing the amount of K2CO3 to 2 mmol, no improvement
was observed in the reaction rate (Table 1, entry 15). In
continuation of our efforts to seek more optimal conditions,
the effect of catalyst amount was next evaluated. As demon-
strated in Table 1, when the desired coupling reaction was
conducted in the presence of 0.09 mol% of the nanocatalyst,
the product yield was not exceed 82% even after 60 min
(Table 1, entry 17). Then, taking advantage of the fact that
using a phase-transfer agent would usually promote an
organic transformation in aqueous media, the effect of add-
ing 0.2 mmol of tetrabutylammonium bromide (TBAB) to
the reaction medium under the previously described condi-
tions was also studied. In this context, the reaction proceeds
very surprisingly even at lower temperatures (60 and 40 °C),
to furnish a well-yielded product within a short reaction time
1 3

Thiophene Methanimine–Palladium Schiff Base Complex Anchored on Magnetic Nanoparticles:…
the presence of 0.1 mol% of γ-Fe2O3/AEPH2-TC-Pd and
0.2 mmol of TBAB in H2O was selected as the best opti-
mized condition (Table 1, entry 20).
With the optimized reaction conditions in hand, the scope
of the Suzuki–Miyaura cross-coupling reaction was extended
to various aryl iodides/bromides/chlorides with phenylbo-
ronic acids (Table 2). As clearly understood from the results,
the designed catalytic system was equally applicable for
both aryl iodides and aryl bromides bearing either electron-
withdrawing or electron-donating substituents and delivered
the corresponding coupled products in good to excellent
yields (Table 2, entries 1–16). However, the required times
for completion the reactions of related aryl bromides with
phenylboronic acids were somewhat longer. Importantly, in
a challenging experiment, it was found that the coupling
reactions for a number of aryl chlorides with phenylboronic
acid were also accompanied with satisfying results, although
prolonged times were required due to the stronger strength of
carbon-halogen bond in these cases (Table 2, entries 17–20).
However, this catalytic system did not support the coupling
reactions of phenylboronic acid with some substrates such
as 4-chloroaniline, 1-chloro-2-methyl-4-nitrobenzene and
2-chlorothiophene (Table 2, entries 21–23). Also, the reac-
tion of 4-fluorobenzaldehyde with phenylboronic acid was
associated with no coupling product (Table 2, entry 24).
Interestingly, the prepared catalyst showed improved activ-
ity in the Suzuki–Miyaura cross-coupling reaction of highly
challenging heterocyclic substrates such as 2-iodothiophene
using very low Pd loading (0.1 mol%) to afford the corre-
sponding cross-coupled product in excellent yield (Table 2,
entry 7).
Scheme 4 Plausible mechanism for the Suzuki–Miyaura cross-cou-
pling reaction
(Table 1, entries 18–20). Cetyl trimethyl ammonium bro-
mide (CTAB) and sodium dodecylsulfate (SDS) were also
examined as phase-transfer agents, but the results repre-
sented their less efficiency (Table 1, entries 21, 22). However
by decreasing the amount of TBAB, the reaction rate was
dropped (Table 1, entry 23). In addition, by keeping other
conditions the same, the effect of using Pd(OAc)2 as catalyst
was also investigated. Nevertheless, the result was far from
satisfactory in this context (Table 1, entry 24). Ultimately,
using 1:1.2 molar ratio of iodobenzene/K2CO3 at 40 °C in
Table 3 Heck–Mizoroki cross-coupling reaction of iodobenzene with n-butyl acrylate in the presence of γ-Fe2O3/AEPH2-TC-Pd nanocatalyst,
under different reaction conditions
Entry
Catalyst (mol%)
Molar ratios of
Base
Solvent
Additive (mmol)
TBABa (0.2)
Temp. (ºC)
Time (min)
Isolated yield
(%)
iodobenzene:base
1
2
3
5
6
7
8
9
10
11
12
13
14
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.09
0.2
0.1
0.1
0.1
–
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.2
1:1.1
1:1.2
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Et3N
H2O
EtOH
DMF
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
40
40
40
55
65
90
65
65
65
65
65
65
65
45
75
40
35
10
10
45
25
10
35
20
20
18 h
79
64
81
90
98
98
89
73
98
81
88
75
15
TBAB (0.2)
TBAB (0.2)
TBAB (0.2)
TBAB (0.1)
TBAB (0.2)
TBAB (0.2)
TBAB (0.2)
TBAB (0.2)
TBAB (0.2)
TBAB (0.2)
NaHCO3
K2CO3
K2CO3
Reaction conditions: iodobenzene (1 mmol), n-buthyl acrylate (2 mmol) and solvent (3 mL)
aTetrabutylammonium bromide
1 3

R. Jahanshahi, B. Akhlaghinia
Table 4 Substrate scope for Heck–Mizoroki cross-coupling reaction using γ-Fe2O3/AEPH2-TC-Pd nanocatalyst
Entry
R1
R2
X
Product
Time (min)
Isolated yield (%)
1
2
3
4
5
6
7
8
H
4-NO2
4-Cl
Bun
Bun
Bun
Bun
Bun
Bun
Bun
Me
Me
Me
Me
Me
Me
Bun
Bun
Bun
Me
Me
Me
Me
Me
Me
Bun
Bun
Bun
Bun
Bun
I
I
I
I
I
I
I
I
I
I
I
I
5a
5b
5c
5d
5e
5f
5g
5h
5i
5j
5k
5l
5m
5a
5b
5e
5h
5i
5n
5j
5l
5h
5b
5e
5o
5f
10
10
20
30
40
1 h/15 h
55
20
15
20
35
45
60
35
35
60
90
90
90
90
120
2 h/24 h
2 h/24 h
2 h/24 h
5 h/24 h
4 h/24 h
4 h/24 h
98
98
95
92
91
Trace/45
87
96
96
92
90
90
84
89
93
84
82
91
89
88
79
Trace/79
20/81
Trace/60
15/15
0/0
4-Me
4-OMe
2-Me-4-NO2
2-Thiophene
H
4-NO2
4-Cl
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
4-Me
4-OMe
2-Thiophene
H
4-NO2
4-OMe
H
4-NO2
4-CN
4-Cl
4-OMe
H
4-NO2
4-OMe
4-NH2
2-Me-4-NO2
2-Thiophene
I
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
Cl
5g
Trace/35
Reaction conditions: aryl halide (1 mmol), acrylate (1.1 mmol), K2CO3 (1.2 mmol), TBAB (0.2 mmol), γ-Fe2O3/AEPH2-TC-Pd (0.1 mol%), in
H2O (3 mL) at 65 °C
3.4 Proposed Catalytic Mechanism for the Suzuki–
Miyaura Cross-Coupling Reaction
facilitates the catalytic cycle by regenerating the active Pd(0)
species, while the cross coupled product is produced (IV).
By analogy with the previous reports in literature [71], as
well as our investigations, the following mechanism can be
proposed for the Suzuki–Miyaura cross coupling reaction
(Scheme 4). This catalytic pathway involves three sequen-
tial steps: oxidative addition, transmetalation and reductive
elimination. At first, Pd(0) species undergoes the oxidative
addition with aryl halide to deliver the Pd(II) σ-aryl complex
(I). Then, in the transmetalation step, the aryl group of the
activated arylboronic acid (II) exchanges with the halide
on the Pd(II) σ-aryl complex (I) to furnish the Pd(II) dia-
ryl complex (III). Finally, the reductive elimination of III,
3.5 Catalytic Activity of γ-Fe2O3/AEPH2-TC-Pd
in Heck–Mizoroki Cross-Coupling Reaction
Encouraged by the promising results obtained with the
Suzuki–Miyaura cross-coupling reaction, the catalytic
activity of γ-Fe2O3/AEPH2-TC-Pd nanocatalyst was next
explored towards the Heck–Mizoroki cross-coupling reac-
tion (Table 3). At the outset, the model reaction of iodoben-
zene (1 mmol) with n-buthyl acrylate (1.1 mmol) was imple-
mented under the optimized conditions for Suzuki–Miyaura
reaction. Unfortunately, the result was not as satisfactory as
1 3

Thiophene Methanimine–Palladium Schiff Base Complex Anchored on Magnetic Nanoparticles:…
aryl iodides with n-butyl/methyl acrylates (Table 4, entries
1–13). However, in the case of the electron-releasing aryl
iodides, the completion of the reaction is slightly slower.
Gratifyingly, the current catalytic system was also effective
for the coupling reactions of substituted aryl bromides and
also more challenging aryl chlorides, as well. It is worthy
of note that the higher the dissociation energies of the C-X
bond, the longer the coupling reaction times. Accordingly,
the reaction of bromo and chloro derivatives with acrylates
needs longer times, respectively, to gain the admissible prod-
uct yields. Particularly, in the case of the aryl chlorides, the
products that were generated at the earliest times of the reac-
tions were unsatisfied.
More importantly, the as-prepared catalyst displayed
remarkable activity toward the highly challenging hetero-
cyclic substrates such as 2-iodothiophene and afforded the
corresponding products in high yields (Table 4, entries 7
and 13).
During our experiments regarding these coupling trans-
formation, we have met a number of unsuccessful examples
which were included in Table 4 to clarify the limitations of
the current protocol (Table 4, entries 6 and 25–27). Fur-
thermore, to investigate whether the Heck–Mizoroki reac-
tion was compatible with more diverse substrates, different
aryl and heteroaryl olefins such as styrene and 2-vinylpyri-
dine were subjected to react with aryl iodide through the
Heck–Mizoroki reaction. The obtained results in these
regards were far from satisfactory and poor yields of the cou-
pled products were obtained after 1 h (Yields=35 and 10%
for the reaction of aryl iodide with styrene and 2-vinylpyri-
dine, respectively). However, in the case of the styrene, the
product yield was reached to 60% after 24 h, whereas the
reaction of the aryl iodide with 2-vinylpyridine did not pro-
ceed any more, even after 24 h. These observations demon-
strated that the current catalytic system has less efficiency
towards the coupling reactions of such substrates (data not
given in Table 4).
Scheme 5 Plausible mechanism for the Mizoroki–Heck cross-cou-
pling reaction
expected and the desired product was obtained in only 79%
within 45 min (Table 3, entry 1). In an effort to develop
more efficient conditions, other solvents such as EtOH and
DMF were also examined under the same condition. With
regard to the results in Table 3, EtOH was not a pleasing
solvent for such a coupling reaction. However, albeit the
reaction was speed up to some extent by using DMF as sol-
vent, its usage was not recommended due to the increasing
environmental concerns. To our great delight, significant
improvement in the product yield was achieved when the
reaction temperature increased to 55 and 65 °C (Table 3,
entries 5, 6). More elevated temperatures (90 °C) did not
have any positive effect on the reaction progress (Table 3,
entry 7). Evaluation of the amount of TBAB and γ-Fe2O3/
AEPH2-TC-Pd nanocatalyst clearly documented that the best
results were obtained in the presence of 0.2 mmol TBAB and
0.1 mol% nanocatalyst (Table 3, entries 8–10). Less impact
of other bases such as Et3N and NaHCO3, stipulated that
K2CO3 is undoubtedly the best one (Table 3, entries 11, 12
vs. entry 6). It is also noteworthy that the molar ratio of
iodobenzene:K2CO3 has a great influence on this transforma-
tion, since the reaction rate declined markedly by decreasing
the mentioned ratio (Table 3, entry 13). Notably, a negligible
yield of the coupled product was detected in the absence of
nanocatalyst, even after 18 h (Table 3, entry 14).
3.6 Proposed Catalytic Mechanism for the Heck–
Mizoroki Cross-Coupling Reaction
According to the literature precedents reports [71], and
the results of our researches, a conceivable mechanism for
the Heck-Mizoroki cross-coupling reaction is suggested in
Scheme 5. Based on the presented mechanism, initially, the
active Pd(0) species converts to Pd(II), through the oxida-
tive addition of aryl halide. Thereafter, the syn addition of
the resulting palladium intermediate I on olefin compound
results to the formation of σ-complex II. In the third step,
after the β-hydride elimination, the desired product III is
formed. Finally, the reductive elimination by K2CO3 on the
Pd(II) intermediate IV, provides the Pd(0) species, which
can re-enters the catalytic cycle, as well (Scheme 4).
With the optimized conditions in hand, the scope of the
γ-Fe2O3/AEPH2-TC-Pd-catalyzed Heck–Mizoroki reac-
tions was investigated by employing various substituted
aryl halides to react with n-butyl/methyl acrylate (Table 4).
Excellent yields of the products were achieved in the cou-
pling reactions of both electron-rich and electron-deficient
1 3

R. Jahanshahi, B. Akhlaghinia
Table 5 Comparison of the catalytic activity of γ-Fe2O3/AEPH2-TC-Pd with some literature precedents for Suzuki–Miyaura and Heck–Mizo-
roki cross-coupling reactions
Entry Catalyst (mol%)
Reaction conditions
Time (h) Yield (%) References
1a
2a
3a
4a
Fe3O4@SiO2@C22-Pd(II) (0.5)
Et3N, DMF/H2O (1:1), 75 °C
K2CO3, TBAB, H2O, 80 °C
K3PO4.3H2O, H2O, TBAB, 40 °C
K2CO3, H2O, 100 °C
1
3
5
3
92
94
98
97
[72]
[73]
[74]
[75]
Fe3O4@EDTA–PdCl2 (20 mg)
NHC-Pd(II) complexc (0.2)
N,N′-bis(2-pyridinecarboxamide)-1,2-benzene
palladium complex (1.0)
5a
6a
7a
8a
9a
PdCl2 (0.05)
PANI-Pd (2.2)
Au/Pd NPs (4.0)
Pd(II)–NHC complex (0.01 mmol)
Cs2CO3, DMF, 130 °C
2
4
24
24
l
95
91
88
99
97
[76]
[77]
[78]
[79]
[80]
K2CO3, 1,4-dioxane: H2O (1:1), 95 °C
K2CO3, EtOH/H2O (2:1), N2 atm, 80 °C
Cs2CO3, DMA, 100 °C
Pd NPs synthesized by solar irradiation
KOH, H2O, 100 °C
(0.0005 mmol)
10a
11a
12a
13b
ZrO2@AEPH2-PPh2-Pd(0)
ZrO2@ECP-Pd
γ-Fe2O3/AEPH2-TC-Pd (0.1)
K2CO3, H2O, 80 °C
K2CO3, H2O, 90 °C
K2CO3,TBAB, H2O, 40 °C
DABCO, Solvent-free, 140 °C
20
15
95
98
[45]
[52]
Present work
[81]
10 min 98
2
SMNPs-supported 4,5-diazafluroen-9-one-Pd
92
(0.2)
14b
15b
16b
Pd/DIAION HP20d (0.2)
Pd-SMF (1)
Bu3N, DMA, 100 °C
Et3N, CH3CN, 100 °C
4
20
4.5
95
92
82
[82]
[83]
[84]
PdCl2 (0.02 mmol)
Na2CO3, TBAB, H2O, Ultrasonic Irradiation at
25 °C
17b
18b
19b
20b
21b
Pd(0)-Arg-boehmite (2.7)
MPPI-Pde (0.0007)
Pd–ZnFe2O4 MNPs (4.62)
HMMS-NH2-Pd (4)
PdCl2/TiO2 (0.5)
K2CO3, DMF, 120 °C
Et3N, TBAB, H2O, 100 °C
Et3N, DMF, 120 °C
50 min 98
17
3
8
5
[71]
[85]
[86]
[87]
[88]
96
90
98
94*
K2CO3, NMP, 130 °C
Et3N, DMF, under 400 W (mercury vapor lamp)
UVvisible irradiation 45 3 °C
22b
23b
24b
ZrO2@ECP-Pd
ZrO2@AEPH2-sPPh2-Pd(0)
γ-Fe2O3/AEPH2-TC-Pd (0.1)
[bmim]PF6, NEt3, 100 °C
K2CO3, PEG 600, 100 °C
K2CO3,TBAB, H2O, 65 °C
30
20
95
95
[52]
[45]
Present work
10 min 98
PANI polyaniline, SMNPs silica-coated magnetite nanoparticles, HMMS monodispersed hollow magnetic spheres
*Conversion
aSuzuki–Miyaura cross-coupling reaction of iodobenzene with phenylboronic acid
bHeck–Mizoroki cross-coupling reaction of iodobenzene with n-butyl acrylate
cNHC-Pd(II): [1,1-(hexylene-1,6-diyl)bis(3-n-butylbenzimidazol-2-ylidene)][PdCl2(CH3CN)]2
dDIAION HP20 is a commercial synthetic adsorbent
eConvoluted polymeric palladium catalyst
To further elucidate the merits of the proposed cata-
lytic system over several literature reported catalysts in the
Suzuki–Miyaura and Heck–Mizoroki cross-coupling reac-
tions, the comparing efficacy results of some previously
reported Pd catalysts against γ-Fe2O3/AEPH2-TC-Pd, was
tabulated in Table 5. Even though, each of these catalytic
systems has their own advantages, it can be apparently seen
that the current catalyst is much superior to almost all of the
reported literature precedents not only with respect to the
mildness of the reaction conditions, shorter reaction times
and reaction high-yields, but also in terms of the convenient
catalyst recovery (by using an external magnet), reasonable
reusability and high stability.
3.7 Reusability of the γ-Fe2O3/AEPH2-TC-Pd
Nanocatalyst
The recycling and recovery of the heterogeneous cata-
lytic systems are very crucial aspects from the economi-
cal, industrial and environmental standpoints. In this line,
the nanocatalyst reusability was next explored in the model
Suzuki–Miyaura and Heck–Mizoroki cross-coupling
reactions. At the end of each run, the catalyst was simply
separated by an external magnet, washed with ethanol
and vacuum-dried at 50 °C to be ready for re-employing
directly in another fresh reaction mixture of both catalytic
systems. As can be seen in Fig. 9, a negligible decrease in
1 3

Thiophene Methanimine–Palladium Schiff Base Complex Anchored on Magnetic Nanoparticles:…
Fig. 9 Recyclability of
γ-Fe2O3/AEPH2-TC-Pd nano-
catalyst in the Suzuki–Miyaura
and Heck–Mizoroki model
reactions, under the optimized
conditions
the nanocatalyst activity after successive nine runs, clearly
points out that the nanocatalyst was not only very active, but
also very stable during the reaction cycles.
techniques including FT-IR, XRD, TEM, TGA, ICP-OES,
XPS, VSM and elemental analysis. The efficiency of the
newly fabricated catalyst was monitored in the aqueous
Suzuki–Miyaura and Heck–Mizoroki cross-coupling reac-
tions. Interestingly, the prepared catalytic system displayed
improved activity in the cross-coupling reactions of a broad
range of aryl iodides, aryl bromides and most importantly
the highly challenging aryl chlorides, using very low Pd
loading amount. The obtained results based on the hot fil-
tration test, controlled poisoning experiments and ICP-OES
analysis clearly confirmed that the respective catalyst oper-
ates in a truly heterogeneous manner. This catalyst could be
readily recovered (by using a magnetic bar) and reused at
least nine times without significant decrease in its catalytic
activity. It is worthwhile to mention that the high activity,
durability and recyclability of the present catalytic system
could be directly attributed to the nature of the applied
ligand, which is not only an air and moisture stable ligand
intrinsically but also acts as stabilizing Pd NPs agent to
protect the palladium species against agglomeration and
palladium black formation. Good to excellent yields of the
coupled products and short reaction times as well as using
H2O as the reaction media are another advantages that sup-
port the current protocol towards the green chemistry.
The amount of leached Pd into the model Suzuki–Miyaura
reaction was also tested, using ICP-OES technique. While,
the measured palladium content of the freshly prepared
γ-Fe2O3/AEPH2-TC-Pd was estimated to be 0.87 mmol of
Pd per 1.00 g of the nanocatalyst, ICP-OES analysis of the
9th recycled catalyst revealed that the recovered nanocatalyst
contains 0.85 mmol of Pd per 1.00 g of the nanocatalyst. In
the other words, only <1 ppm of the total amount of the orig-
inal Pd species was lost during the course of the reaction.
Excellent catalytic performance and stability of the afore-
mentioned catalytic system most likely originated from the
capability of the sulfur-based ligand for high stabilization
of the palladium nanoparticles, which is concerned to the
strong interaction of the palladium species and the ligand
sulfur atoms [89].
It is noteworthy that the FT-IR spectrum of 9th recov-
ered catalyst (Fig. 1e), revealed the entire preservation of
the shape, position and relative intensity of the character-
istic absorption bands. These results clearly proved that no
substantial changes occurred at the chemical structure of
functional groups and the hydrogen bonding network of the
designed nanocatalyst.
Acknowledgements The authors gratefully acknowledge the partial
support of this study by Ferdowsi University of Mashhad Research
Council (Grant No. p/3/39490).
Furthermore, the XPS spectrum of the 9th recycled
γ-Fe2O3/AEPH2-TC-Pd clearly indicated the existence of
peaks at 335.1 and 340.3 eV, corresponding to 3d5/2 and
3d3/2 for Pd(0) species, respectively (Fig. 3b, d). These find-
ings offered that the oxidation state of the immobilized Pd
was preserved even after repeated reactions.
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