Organoselenium-palladium(ii) complex immobilized on functionalized magnetic nanoparticles as a promising retrievable nanocatalyst for the "phosphine-free" Heck-Mizoroki coupling reaction

Article abstract of DOI:10.1039/c8nj02433b

In the present study, for the first time, an air- and moisture-stable organoselenium-palladium complex immobilized on silica-coated magnetic nanoparticles was designed, synthesized and applied as a practical and retrievable catalyst in organic synthesis. The chemical nature and structure of this novel catalytic system were characterized using various techniques such as Fourier transform infrared (FT-IR) spectroscopy, X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), energy dispersive X-ray (EDX) spectroscopy and vibrating sample magnetometry (VSM). Subsequently, the catalytic performance of the synthesized nanocatalyst was investigated in the Heck-Mizoroki cross-coupling reaction and excellent results are obtained. The low catalyst loading, wide substrate scope, high yield, short reaction time, simple separation from the reaction mixture and importantly, the longevity of the nanocatalyst for at least five successive recycles without significant degradation in its activity are the main merits of this protocol. Above all, this work opens up attractive and interesting routes for the use of organoselenium compounds as efficient ligands for the synthesis of heterogeneous catalysts.

Full text of DOI:10.1039/c8nj02433b

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PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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DOI: 10.1039/C8NJ02433B
Journal Name
ARTICLE
Organoselenium-Palladium(II)
Complex
Immobilized
on
Functionalized Magnetic Nanoparticles as a Promising Retrievable
Nanocatalyst for the “Phosphine-Free” Heck-Mizoroki Coupling
Reaction
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Yalda Rangraz, a Firouzeh Nemati*a and Ali Elhampoura
www.rsc.org/
In the present study, for the first time, an air- and moisture-stable organoselenium-palladium complex immobilized on
silica-coated magnetic nanoparticles was designed, synthesized and applied as a practical and retrievable catalyst in
organic synthesis. The chemical nature and the structure of novel catalytic system were authenticated using various
techniques such as Fourier transform infrared (FT-IR) spectroscopy, X-ray powder diffraction (XRD), X-ray photoelectron
spectroscopy (XPS), thermogravimetric analysis (TGA), transmission electron microscopy (TEM), field emission scanning
electron microscopy (FE-SEM), energy dispersive X-ray (EDX) spectroscopy and vibrating sample magnetometer (VSM).
Then, catalytic performance of the synthesized nanocatalyst was investigated in Heck–Mizoroki cross coupling reaction
and excellent results were obtained. The low catalyst loading, wide substrate scope, high yield, short reaction time,
straightforward separation from reaction mixture and more importantly, the longevity of nanocatalyst for at least five
successive times without a significant degradation in its activity are the main merits of this protocol. Above all, this work
opens up attractive and interesting routes for the use of organoselenium compounds as efficient ligands for
heterogeneous
catalysts.
reuse of the catalytic systems and hence the reaction must be
carried out under argon or nitrogen atmosphere to protect
catalyst from deactivation 10-13. These drawbacks restrict their
wide applications in large-scale synthesis. Therefore,
considering both environmental and economic issues, the
design and development of phosphine-free palladium catalysts
continue to be a serious challenge in organic synthesis and
Introduction
The palladium-catalyzed cross coupling reactions (such as
Mizoroki-Heck, Suzuki-Miyaura, Sonogashira-Hagihara, Stille,
Fukuyama and Negishi reactions) are extremely powerful and
straightforward tools for the construction of new carbon-
carbon bonds in modern chemical transformations and
industrial processes because of their regio-stereo selectivities,
functional group tolerances and high selectivities 1-4. Among
them, the olefination of aryl halides, known as Mizoroki- Heck
reaction is one of the most considerable and highly effective
used protocols for the formation of different types of
compounds. The products of this reaction have been widely
applied for the synthesis of pharmaceuticals compounds,
industry 14
.
Chalcogen containing ligands have emerged as viable
alternatives due to their much less insensitivity to air and
moisture, strong electron-donating properties of chalcogen
atoms, solubility in diverse solvents and stability in solution,
which make them currently important and suitable for
designing Pd-complexes for catalyzing of C–C coupling
reactions 15-19. The catalytic activity of organoselenium ligands
not only rivals of their corresponding phosphorus analogs but
vastly outperforms them in many cases. Numerous effective
and selective organoselenium ligands based catalytic systems
have been developed in the past decades, but in spite of these
significant achievements, to date, there are only a few
examples of reports on the use of organoselenium ligands on
solid supports as heterogeneous catalysts 20-23. Homogeneous
catalytic systems suffer from disadvantages such as tedious
isolation from the reaction mixture, lack of catalyst recycling
and reusability, and reduced catalytic activity during the
reaction 24-26. Because of palladium is an expensive and toxic
metal, its recovery is very important from both economic and
green chemistry point of view 27-29. Therefore, designing of
new heterogeneous palladium catalysts using different solid
supports is a suitable way to overcome aforementioned
drawbacks. Among the different solid supports, magnetically
retrievable nanoparticles because of unique physical
3, 5-
natural products, herbicides and fine chemicals technology
8. The most ordinary catalytic system utilized for C–C bond
formation reaction is promoted by homogeneous palladium
complexes. The correct selection of the ligand is the key and
important factor for the synthesis of these complexes 9.
To date, many attempts have been performed to the search
for more effective and practical ligands. In the past few years
phosphine-based ligands have achieved great success but the
majority of them are extremely expensive, toxic, virulent, very
sensitive to air and/or moisture and are easily oxidized to their
corresponding phosphine oxides because of trivalent nature of
phosphorous, which can prevent the simple recovery and
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properties such as thermal and chemical resistance, the large
surface area to volume ratios, low toxicity and low cost, easy
and convenient separation from reaction media with a
permanent magnet without needing tedious workup processes
have received tremendous attention 30-32. Following our
research work on the development of efficient green catalysts
and their application in various reactions 33-42, at the present
paper, we report for the first time the synthesis and
characterization of a new organic and highly efficient
heterogeneous catalyst that contains complex of
organoselenium ligand that is bound covalently to silica-coated
Fe3O4 nanoparticles and then investigate its catalytic
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DOI: 10.1039/C8NJ02433B
performance in the Heck-Mizoroki cross-coupling reactions
between various aryl halides and acrylates.
Results and discussion
Scheme 1. Schematic illustration of the preparation of Fe3O4@SiO2-
Se-T/Pd(II)
Preparation of nano-Fe3O4@SiO2-Se-T/Pd(II)
The schematic procedure for preparing our catalyst,
selenotetrazole Pd(II) complex immobilized on functionalized
magnetic Fe3O4 nanoparticles, that named Fe3O4@SiO2-Se-
T/Pd(II) is illustrated in Scheme 1. In the first step, magnetic
Fe3O4 nanoparticles were prepared using a chemical co-
precipitating technique in accordance with the literature. After
that, the Fe3O4 nanoparticles were covered with a thin layer of
silica by the well-known Stöber method to prevent aggregation
of the magnetic nanoparticles. Then, the silica-coated
magnetic nanoparticles were functionalized by using (3-
aminopropyl) triethoxysilane (APTES) to obtain Fe3O4@SiO2-
APTES. Secondly, for the synthesis of the selenotetrazole
ligand, para aminobenzoic acid was converted to diazonium
salt and this salt was reacted with KSeCN to give
phenylselenocyanate. Subsequently, tetrazole was synthesized
from the reaction of this compound with sodium azide. The
main novelty of our work is the successful grafting of the
selenotetrazole ligand on the surface of MNPs for the first
time. Finally, these nanoparticles having Se and N as chelating
groups on their surface were simply metallated with Pd(OAc)2
in ethanol to obtain the desired Pd complex supported onto
Characterization of the catalyst
FT-IR Spectroscopy
The FT-IR spectra of Fe3O4@SiO2-NH2 (a) and Fe3O4@SiO2-Se-
Figure 1. FT‐IR spectrums of (a) Fe3O4@SiO2-NH2 and (b)
the magnetically retrievable nanoparticles, Fe3O4@SiO2-Se- Fe3O4@SiO2-Se-T/Pd(II)
T/Pd(II).
T/Pd(II) (b) are demonstrated in Fig. 1. The characteristic band
at 568 cm−1 is assigned to the Fe-O stretching vibration of
Fe3O4.
The high-intensity peak of about 1097 cm−1 attributed to
stretching vibration of Si-O-Si bond, proving the coating of
silica on the magnetite nanoparticles. The two weak peaks at
2920 and 2852 cm-1 are allocated to the C-H of the stretching
vibration modes (asymmetric and symmetric) of the
methylene group. Furthermore, the observed absorption band
at 1398 cm−1 belong to stretching vibrations of C-N. The
prominent peaks at 3421 and 3132 cm− 1 can be related to the
OH and NH stretching groups. Therefore, these results confirm
the
successful
Immobilization
of
(3-aminopropyl)
triethoxysilane (APTES) on the surface of the magnetic
Fe3O4@SiO2 NPs (Fig. 1a). The existence of anchored
selenotetrazole ligand is affirmed by the appearance of two
new absorption peaks at 1635 and 1506 cm-1, that can be
associated to the C=O stretching vibration of the amide groups
(–CONH–) and C=C of the aromatic rings, respectively (Fig. 1b).
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Powder X-ray diffraction
ARTICLE
silanization reaction1 . The XPS spectra of C 1 s and N 1 s regions are
The crystalline nature and chemical composition of the displayed in Fig. 3e and 3f. The XPS spectruDmOIe:x1h0i.1b0it3Vs9ie/awCm8ANrtaiJci0lne2Op4ne3l3ianBke
prepared Fe3O4 and Fe3O4@SiO2-Se-T/Pd(II) were investigated with binding energy of 284.38 eV corresponding to C 1 s. As can be
with X-ray powder diffraction (XRD) technique .Fig. 2 display seen in Fig. 3f, there is a peak at binding energies of 399.88 eV for
the wide angle XRD patterns of Fe3O4@SiO2-Se-T/Pd(II) and the the N 1s which can be ascribed to the C-N bond46. In addition,
MNPs. XRD patterns both of these samples exhibit the same successful grafting of the ligand is demonstrated by peak that
diffraction lines in 2θ= 30.86o,36.26o, 43.15o, 53.81o, 57.19o, located in 55.78 eV binding energies, is related to Se 3d (Fig. 3g).
and 63.1o, that related to (220), (311), (400), (422), (511), and
(440) planes, which are in good agreement with the standard
XRD pattern of cubic spinel structure of Fe3O4 MNPs (JCPDS
card no. 85–1436). These results authenticate that the crystal
structure of the magnetic nanoparticles has not been
compromised during the modification. Moreover, the
diffractogram of the final catalyst indicates a broad peak in the
area 2θ= 18-28, correspond to the coating of amorphous silica
around Fe3O4 in the sample. No characteristic diffraction peaks
of metallic palladium can be found in Fig. 2(b), which suggests
the presence of divalent palladium species in the synthesized
nanocatalyst. Also, no peaks corresponding to any other
impurity were observed. In addition, the mean crystal size of
the Fe3O4 NPs, calculated using the Scherrer equation (d =
Kλ/(βcosθ)) is about 16.5 nm.
igure 2. XRD Patterns of (a) Fe3O4 and (b) Fe3O4@SiO2-Se-T/Pd(II)
XPS analysis
X‐ray photoelectron spectroscopy (XPS) is one of powerful and
important surface analysis tools that provides valuable information
regarding the chemical composition, electronic properties and
chemical state of the elements present within the synthesized
catalysts. The XPS elemental survey scan of the surface of the
Fe3O4@SiO2-Se-T/Pd(II) catalyst is presented in Fig. 3. As can be
seen in Fig. 3a, peaks corresponding to iron, oxygen, silica, nitrogen,
carbon, selenium and palladium are clearly detected in the full
range XPS spectrum which are consistent with the EDX results. The
corresponding high-resolution XPS spectrum of Fe 2p region is
shown in Fig. 3b. The peaks appearing at 710 and 724 eV are
assigned to Fe 2p3/2 and Fe 2p1/2.This result confirms that the Fe
ions have a +2 and +3 oxidation state in Fe3O4 MNPs31, 43. Fig. 3c
illustrates O 1s XPS spectrum of the as-synthesized nanocatalyst;
the spectrum is divided into three components at 529.4, 530.7 and
535 eV, can be attributed to binding energies of C=O, C–O and Fe-O
bonds, respectively31, 44, 45. In the Si 2p spectrum (Fig. 2d), the peak
at 102.78 eV is ascribed to siloxane (Si-O-Si), which resulted from
the hydrolysis of the silane curing agent molecules during the
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palladium species) in Fe3O4@SiO2 Se-T/Pd(II), XPSVieawnaArlyticsliesOwnlianes
conducted (Fig.3h). The high‐resolution DPOdI: 103.d103X9P/CS8NsJc0a2n433oBf
Fe3O4@SiO2-Se-T/Pd(II) reveals the doublet peaks at the binding
energy of 337.42 eV and 342.9 eV, which can be indexed to electron
transitions of 3d5/2 and 3d3/2 of Pd, respectively1, 47. This result
confirms that the chemical state of palladium is +2 in Fe3O4@SiO2-
Se-T/Pd(II) and palladium(II) is not reduced during the preparation
of the magnetic nano-catalyst. The shift of Pd(II) 3d5/2 of Pd(OAc)2
binding energy peak from 338.7 eV to 337.42 eV affirms the
formation of Pd(II)-organoselenium complex48.
Thermogravimetric analysis (TGA)
To examine the thermal behavior of Fe3O4@SiO2-Se-T/Pd(II), as well
as to estimate the percent of organic content grafted onto the
surface of magnetic nanoparticles, TGA analysis was carried out at
temperatures ranging from 30 to 800 °C under air flow (Fig. 4). The
thermogram of synthesized nanocatalyst demonstrates a two-step
weight loss. The initial weight loss (~15%) below 200 °C is related to
the loss of water molecules and physically adsorbed solvents, while
the second weight loss (~ 33%) at 200-700 °C could be associated
chiefly with the thermal decomposition of the immobilized organic
moiety on the surface of the MNPs.
Figure 4. TGA plot of Fe3O4@SiO2-Se-T/Pd(II)
Transmission Electron Microscopy
Figure 3. XPS spectrum of as-synthesized Fe3O4@SiO2-Se-T/Pd(II)(a)
full range, (b) Fe 2p, (c) O 1s, (d) Si 2p, (e) C 1s, (f) N 1s, (g) Se 3d, and
(g) Pd 3d
Figure 5. TEM images of Fe3O4@SiO2-Se-T/Pd(II) at different
magnifications
Then, the morphology and core-shell structure of the synthesized
magnetic nanocatalyst were investigated by Transmission Electron
Microscopy (TEM) (Fig. 5). The TEM images of Fe3O4@SiO2-Se-
T/Pd(II) confirmed the successful coating of the amorphous silica
(SiO2) accompanied with organic components (average shell
Additionally, to investigate the oxidation state of Pd (the supported
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thickness ~ 5-10 nm) on the Fe3O4 magnetic nanoparticles, which grafting of diamagnetic silica and other organic segVmieewnAtrsticolenOntlhinee
can be showed as a light region surrounding the dark nano-Fe3O4 magnetic surface. However, the Ms value of the catalyst
DOI: 10.1039/C8NJ02433B
core (core diameter ~ 15-20 nm). Also, TEM images represented Fe3O4@SiO2-Se-T/Pd(II) is still enough to provide facile recovery of
that Fe3O4@SiO2-Se-T/Pd(II) had a nearly spherical morphology with the nanocatalyst from the solution using an external magnetic field.
relatively good monodispersity.
Field emission-scanning electron microscopic analysis
To survey the size, shape and surface morphology of Fe3O4@SiO2-
Se-T/Pd(II) field emission scanning electron microscopy (FE-SEM)
technique was applied. The SEM images in Figure 6 displays that the
synthesized catalyst has a uniform and nearly spherical morphology
and an average diameter of about 15–30 nm.
Figure 6. SEM images of Fe3O4@SiO2-Se-T/Pd(II) at different
magnifications
Figure 8. Magnetization curves of (a) nano-Fe3O4, (b) Fe3O4@SiO2-
Se-T/Pd(II)
Elemental and compositional analysis
Investigation of the catalytic activity of Fe3O4@SiO2-Se-T/Pd(II) in
the Heck coupling reaction
Fe3O4@SiO2-Se-T/Pd(II) catalyst was also examined using Energy‐
dispersive X‐ray spectroscopy (EDS) analysis to determine the type
of elements and chemical composition (Fig. 7). EDX pattern clearly
indicates the presence of Fe, O, Si, N, C and importantly Se and Pd
elements in the catalyst structure witch authenticates the
successful immobilization of the Pd(II) on the surface of magnetic
nanoparticles by selenotetrazole ligand. The amount of palladium
obtained from EDX analysis is around 3.2 (wt%).
Heck–Mizoroki reaction is powerful and important tool for the
synthesis of natural compounds, biologically active products and
advanced materials. Therefore, after the successful characterization
of the catalyst, catalytic performance of the prepared material was
considered in the Heck reaction. For this purpose, the coupling of
iodobenzene with methyl acrylate was chosen as the benchmark
substrates for the optimization of critical parameters such as the
base and used solvent types, temperature and catalyst loading, and
the results of optimization study are shown in Table 1. At first, the
model reaction was carried out in the presence of the various
dosage of catalyst. The best result was afforded by employing 5 mg
of catalyst, whereas increasing the amount of catalyst (10 mg) was
not effective on the rate and yield of reaction. Also, in the absence
of the catalyst no product was detected in DMF solvent. Then, the
model reaction was performed in the existence of a range of bases
such as K2CO3, Cs2CO3, NaOH and Et3N, amongst them Et3N was
found to be the best choice base. While the other bases such as
K2CO3, Cs2CO3 and NaOH were less effective. Subsequently, to
identify a suitable solvent, the same reaction was performed in
different solvents such as DMSO, H2O, DMF and EtOH. Among the
evaluated solvents, DMF was proved to be the best reaction
medium. Afterward, the effect of temperature was examined. As
shown in Table 1, the preferable temperature for Heck coupling
reaction with Fe3O4@SiO2-Se-T/Pd(II) NPs as catalyst was 120 ˚C
(Table 1, entry 3). At lower temperatures the reaction yield was
decreased. The temperature was increased to 150 ˚C but no effect
on the yield of the product was seen. Also, no coupling product at
room temperature was observed. Therefore, the entry 3 of Table 1
was selected as ideal condition for Heck coupling reaction using
Fe3O4@SiO2-Se-T/Pd(II) catalyst.
Figure 7. EDX spectrum of Fe3O4@SiO2-Se-T/Pd(II)
Magnetic measurement
The magnetic properties of Fe3O4 NPs and Fe3O4@SiO2-Se-T/Pd(II)
were thoroughly studied using VSM analysis with a field from -
10000 Oe to +10000 Oe at ambient temperature (Fig. 8). The
saturation magnetization values (Ms) of Fe3O4 and Fe3O4@SiO2-Se-
T/Pd(II)were about 66.27 and 27.71 emu/g, respectively. This
reduction in saturation magnetization is related to the successful
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DOI: 10.1039/C8NJ02433B
Table 1. Optimization of the Heck reaction between iodobenzene
and methyl acrylate using Fe3O4@SiO2-Se-T/Pd(II) as catalysta
Entry
Catalyst Temp. Solvent
(mg)
Base
Time
(min)
Yield
(%)b
(oC)
1
2
3
4
5
7
8
9
-
2.5
5
10
5
5
5
5
5
5
5
5
5
120
120
120
120
150
80
DMF
DMF
DMF
DMF
DMF
DMF
DMF
H2O
EtOH
DMSO
DMF
DMF
DMF
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
10 (h)
45
45
45
45
45
45
45
45
45
45
45
45
-
76
98
97
93
82
-
78
72
88
85
72
78
r.t
reflux
reflux
120
120
120
120
10
11
12
13
14
K2CO3
NaOH
Cs2CO3
aReaction condition: iodobenzene (1.0 mmol), methyl acrylate (1.5
mmol), base (2.0 mmol), solvent (5.0 mL).
bIsolated yield.
Scheme. 2. Plausible mechanism for the cross coupling of aryl
halides with acrylates catalyzed by Fe3O4@SiO2-Se-T/Pd(II)
Scope and generality of reaction
Comparison between catalytic activities of Fe3O4@SiO2-Se-T/Pd(II)
in Heck reaction related to other reported catalysts
Based on the results of aforementioned optimization studies,
the scope and generality of Fe3O4@SiO2-Se-T/Pd(II) was
investigated for different aryl halides and acrylates. As seen in
Table 2, all the substrates containing electron-withdrawing or
electron-donating functional groups were successfully
transformed to the desired cross-coupling products in good to
excellent yields. Reaction of aryl bromides were slower than
aryl iodides, so the reaction times for them were longer in
comparison. This might be because of the different strengths
of the aryl-X bond (C-I < C-Br < C-Cl). Furthermore,
synthetically useful substituents, such as thienyl and naphthyl
were well tolerated during the course of the reaction.
This is the first report wherein a Pd(II) complex of organoselenium
ligand stabilized on magnetic nanoparticles has been used as a
heterogeneous catalyst in the Heck–Mizoroki coupling reaction.
Therefore, to further evaluate the efficiency and capability of
Fe3O4@SiO2-Se-T/Pd(II), the catalytic performance of the present
protocol was compared with some of the previously reported
methods in the Heck–Mizoroki coupling reaction of aryl iodide with
methyl acrylate in the literature (Table 3). However, the reported
catalysts were generate the target product in good to excellent
yield, but the newly synthesized Fe3O4@SiO2-Se-Pd (II) shows
superiority in terms of reaction conditions loading of catalyst
(entries 1-2 and 4), reaction time (entries 1-7 and 9), temperature
(entries 1-5 and entries 7 and 9), as well as recovery and reusability
(entries 1-7 and 9) to most of the listed catalyst systems.
Proposed reaction mechanism
A
plausible mechanism for the Heck-Mizoroki cross-coupling
reaction in the presence of Fe3O4@SiO2-Se-T/Pd(II) is proposed in
Scheme 2 on the basis of the mechanism of the previous reports49-
56. The catalytic cycle begins with the reduction of Pd (II) to the
active Pd (0) species in the presence of the amines or alkenes. Then,
oxidatively addition of the C–X bond of an aryl halide (X = I, Br) to
the palladium atom is generated σ-arylpalladium intermediate (A).
In the third step, the palladium(II) forms a π-complex with acrylate,
and subsequently, the acrylate inserts itself in the palladium–
carbon bond in a syn-addition fashion, which gives complex (B). In
the last step, a new palladium–acrylate π complex is formed by
beta-hydride elimination. Finally, the resultant complex is
destroyed and coupling product is released and the catalytic cycle is
completed by regeneration of the palladium (0) compound by
reductive elimination of the palladium(II) compound in the
presence of the Et3N as base.
Recovery and reusability of the catalyst
From the perspective of economic and ecological demands,
recovery and recyclability of heterogeneous catalysts are two
indispensable factors that need to be investigated. Thus, the
reusability of Fe3O4@SiO2-Se-T/Pd(II) catalyst was examined in Heck
C−C coupling reaction under the optimized conditions using the
model reaction Upon completion of the reaction, the catalyst was
magnetically separated in each run, and washed several times with
hot ethanol for the removal of traces of the previous residual
products and dried in oven at 80 °C. Obviously, the performance of
the recycled catalyst is acceptable for five consecutive runs without
considerable reduction of its catalytic activity (Fig 9).
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Table 2. Heck reaction of different aryl halides and acrylates catalyzed by Fe3O4@SiO2-Se-T/Pd(II) a
Entry
1
Aryl halide
terminal alkene
Product
Time (min)
45
Yieldb(%)
98
2
45
96
3
4
5
6
90
90
92
93
88
83
3(h)
2.5
7
5(h)
78
8
9
45
90
95
93
10
11
6(h)
6(h)
95
80
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3(h)
96
DOI: 10.1039/C8NJ02433B
aReaction conditions: aryl halide (1.0 mmol), acrylate (1.5 mmol), Et3N (2 .0 mmol), Fe3O4@SiO2-Se-T/Pd(II) catalyst (5.0 mg) in DMF (3.0
mL) at 120 ˚C.
bIsolated yield.
Table 3. Comparison of catalytic activity of Fe3O4@SiO2-Se-T/Pd(II) with previously reported procedure for the Heck–Mizoroki
coupling reactions of iodobenzene and methyl acrylate
Entry
Catalyst [ref]
Reaction conditions
Time(h)
Yield(%)
1
2
3
4
5
6
7
8
Pd/MnBDCa (13 mg) 55
Pd/CNCsb(50 mg 3 wt%) 57
Pd/La2O3 (1 mmol%) 58
Fe3O4@SiO2–T/Pdc(20 mg) 36
Co/Al2O3 (0.12 mol%) 59
SiO2-acac-PdNPsd (5 mg) 60
Present catalyst
Et3N, DMA, 90 oC
K2CO3, DMF, 40 oC
NaOAc, DMA, 120 oC
Et3N, DMF, 120 oC
K2CO3, NMP, 150 oC
NaHCO3, NMP, 140 oC
Et3N, DMF, 120 oC
K2CO3, H2O, 90 oC
9
3
10
3
24
97
99
93
95
56
90
98
95
15 (min)
45 (min)
4
h-BNe@Fur@Pd(OAc)2(0.05 mmol) 6
Cellulose supported poly (hydroxamic acid) Pd(II)
(0.03 mol%) 61
9
Et3N, DMF, 130 oC
5
93
aMn-carboxylate metal–organic coordination polymer, Pd/MnBDC, (MnBDC= [Mn3(BDC)3DMF4]n, H2BDC= 1,4
benzendicarboxylic acid
bCarbon nanocoils
cTetrazole-functionalized nanomagnetic Fe3O4 supported palladium
dSilica–acetylacetone‐supported palladium
eHexagonal boron nitride
T/Pd(II). Then, the reaction was stopped at 22.5 min (half the
reaction time), the catalyst was entirely separated from the
reaction mixture with an external magnet and the reaction was
allowed to continue under the same conditions without
Fe3O4@SiO2-Se-T/Pd(II). As shown in Fig. 11, no considerable
increase and progress in the product yield occurred, even after
120 min (monitored by TLC). These results demonstrate the true
heterogeneous nature of the catalyst.
Experimental
Materials and apparatus
All the solvents and chemical reagents in this study were
acquired from Merck and Sigma-Aldrich chemical companies
and utilized as obtained without additional drying or
purification. The progress of the C-C coupling reaction was
monitored using TLC with commercial aluminum-backed plates
of silica gel 60 F254, using UV light. Fourier transform infrared
(FT- IR) spectrums were collected on a Shimadzu 8400 s
spectrometer with a scanning range of 400-4000 cm-1 using KBr
pallet method. Powder X-ray diffraction (XRD) pattern of the as-
prepared nanocatalyst and pure Fe3O4 were obtained using a
Philips diffractometer equipped
Figure 9. Recycling experiment of nano-Fe3O4@SiO2-Se-T/Pd(II)
Interestingly, comparing the XRD pattern of fresh and recovered
Fe3O4@SiO2-Se-T/Pd(II) displayed that the phase composition
and crystalline structure of the prepared nanocatalyst remained
unchanged after using in Heck–Mizoroki cross coupling reaction
and the mean crystallite size calculated using the Scherrer
equation was around 16 nm. The results are shown in Fig. 10.
Moreover, there were no characteristic peaks corresponding to
palladium particles, which is likely to affirm that the palladium
complex stayed unchanged during the reaction.62, 63.Finally, to
explore Pd leaching (the homogeneous or heterogeneous nature
of the catalysis), the model reaction between iodobenzene and
methyl acrylate was performed in the presence Fe3O4@SiO2-Se-
8 | J. Name., 2012, 00, 1-3
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ARTICLE
Synthesis of 4-selenocyanatobenzoic acid and 4-(V(1ieHw-AtretitcrleaOzonlliene-
DOI: 10.1039/C8NJ02433B
5-yl) selanyl) benzoic acid
Initially for the synthesis of 4-selenocyanatobenzoic acid, a
mixture of para-aminobenzoic acid (0.27 g, 2 mmol) and NaBF4
(0.33 g, 3 mmol) was suspended in aqueous HCl solution (3 mL,
2 N) at 0 °C. Next, an aqueous solution of NaNO2 (0.145 g, 2.1
mmol, dissolved in 2 mL of water) was injected drop by drop,
while the mixture was kept at 0 °C. After complete addition of
the reactant, this mixture was stirred for 1 h and then sodium
acetate (ca. 4 mL, saturated) was slowly added until the pH of
mixture was adjusted to 6. Then, the resulting mixture was
successively added into a aqueous solution of KSeCN (0.288 g, 2
mmol) in 4 mL of water and the reaction mixture was
Figure 10. XRD pattern of nano-Fe3O4@SiO2-Se-T/Pd(II) (a) before
use and (b) after reused five runs
mechanically stirred at ambient temperature for
1 h.
Consequently, the product extracted with diethyl ether and the
obtained organic phases dried over MgSO4. The product
obtained was used in the next step without additional
purification67. 4-((1H-tetrazole-5-yl) selanyl) benzoic acid was
prepared in the following fashion 33. Typically, to a mixture of 4-
selenocyanatobenzoic acid (1.0 mmol) and NaN3 (1.5 mmol) in
DMF (2 mL), 30 mg of CuI as the catalyst was added. The
reaction mixture was stirred at 120 °C for the desired time.
Upon the completion of the reaction (monitored by TLC), the
mixture was cooled down to normal temperature and then with
10 mL of ethyl acetate and 20 mL of HCl (5 N) was diluted.
Consequently, the organic layer was isolated, and the aqueous
layer was extracted with 20 mL of ethyl acetate. The separated
organic layer was rinsed with water, dried over MgSO4. Then,
the residue was purified by short column chromatography on
silica gel to afford the target product in excellent yield.
Figure 11. Leaching experiment of nano-Fe3O4@SiO2-Se-T/Pd(II)
Mp: 180–182 ◦C; 1H NMR (DMSO-d6) δH: 7.62 (d, 2H)–7.95 (d,
2H), 12.94 (s, 1H) 68
.
with Cu Kα radiation source with a wavelength of 1.54 Å and 2θ
range of 10-90 degree. X‐ray photoelectron spectroscopy (XPS)
was investigated using a Thermo Scientifi, ESCALAB 250Xi Mg X-
ray resource. Thermogravimetric analysis (TGA) was carried out
using a thermal analyzer (LINSEIS model STS PT 16,000) at a
heating rate of 10 °C min-1, under air flow in the temperature
range of 30–800 °C. Field emission scanning electron
Immobilization of 4-((1H-tetrazole-5-yl) selanyl) benzoic acid
on Fe3O4@SiO2-APTES
A mixture of 4-((1H-tetrazole-5-yl) selanyl) benzoic acid (1.8
mmol), triethylamine (1.8 mmol), N,N'-Dicyclohexylcarbodiimide
(DCC) (1.8 mmol) in degassed dichloromethane was stirred at 0
°C for 30 min under nitrogen. (0.1) g of silanated nanoparticles
(Fe3O4@SiO2-APTES) were suspended in 25 mL dichloromethane
(DCM) and sonicated for 15 min and then added to reaction
mixture. The resulting mixture was stirred for other 1 hour at 0
°C and then was cooled to ambient temperature and stirred
further for 16 h. The mixture was then heated to 40 °C for 3 h.
The heterogeneous ligand was separated using a magnet and
microscopic (FE-SEM) image was recorded by using
a
Tescanvega II XMU digital scanning microscope in order to
acquire information about the size, morphology and shape of
synthesized nanomaterials. Transmission electron microscopy
(TEM) was performed out using
a CM30 300 kV digital
transmission microscope.
washed with dichloromethane and dried at 60 °C 69
.
Besides SEM, Energy dispersive X-ray spectroscopy (EDS)
analysis confirmed the detailed elemental composition of
nanocatalyst. The magnetic properties of the catalyst and Fe3O4
nanoparticles were measured by using vibrating sample
magnetometer (VSM; Lakeshore7407) (Meghnatis Daghigh Kavir
Co., Kashan, Iran) at room temperature from -10000 to +10000
Preparation of magnetite supported palladium nanoparticle
catalyst
Dry nanoparticles from the final step (0.1 g) was dispersed in
ethanol (10 mL) and Pd (OAc)2 (0.004 g, 0.0178 mmol) was
added to the mixture. The reaction mixture was stirred under
reflux condition for 24 h. Next, the resulting product was
isolated via an external magnetic field and washed with ethanol.
After drying in an oven at 80°C, the Fe3O4@SiO2-Se-T/Pd(II)
1
Oe. H NMR spectrum was recorded at 400 MHz in DMSO using
TMS as internal standard.
catalyst as a brown solid was obtained 70
.
Preparation of catalyst
Silanation of the Fe3O4@SiO2 magnetite nanoparticles
General procedure for the Heck-Mizoroki coupling reaction
using the Fe3O4@SiO2-Se-T/Pd(II)
Firstly, magnetic nanoparticles Fe3O4, Fe3O4@SiO2 and
Fe3O4@SiO2-APTES were prepared in accordance with the
procedure previously described in literature 64-66
.
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Page 10 of 12
ARTICLE
Journal Name
A. Kumar, G. K. Rao and A. K. Singh, RSC Adv, 2012,
A mixture of aryl halide (1.0 mmol), acrylate (1.5 mmol),
Fe3O4@SiO2-Se-T/Pd(II) (5 mg), triethylamine (2.0 mmol) , and
DMF (3 ml) was stirred in an oil bath at 120 °C for the
appropriate time. The reaction progress was followed using TLC.
Upon completion of the reaction, the resulting hot reaction
mixture was cooled to room temperature and diluted with hot
ethanol. Then, the catalyst was magnetically separated from the
reaction mixture, washed several times with ethanol, dried in at
80°C and used for the next reaction run without further
purification. After separation of the catalyst, the filtrate was
concentrated and further purification was obtained by short
column chromatography on silica gel by using n-hexane/ ethyl
acetate (9:1) as eluent. Caution: triethylamine is carcinogenic.
5.
6.
View Article Online
2
, 12552-12574.
DOI: 10.1039/C8NJ02433B
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U. Kumar, P. Dubey, V. V. Singh, O. Prakash and A. K.
Conclusions
In conclusion,
a new magnetically retrievable and
sustainable catalyst by immobilizing of the Pd (II) complex
on organoselenium ligand (containing hard N and soft Se
atoms) onto the surface of the amine functionalized silica
coated magnetite nanoparticles, Fe3O4@SiO2-Se-T/Pd(II),
was synthesized for the first time and structurally
characterized using various methods. Then, it was applied
as a novel air and moisture-stable catalyst for Heck C-C
cross coupling reaction. A diverse range of aryl halides were
successfully reacted with acrylates to generate the
corresponding C–C coupling products in good to excellent
yields. More importantly, this heterogeneous palladium
catalyst can conveniently be recovered with an external
magnet and reused five times without remarkable
degradation of activity. Easy recovery and reusability of
catalyst, the broad substrate scope, short reaction times,
good yields and especially use of organoselenium
compound as a moisture-stable ligand are outstanding
advantages of present protocol which make it very
practical. Further studies toward the extension of Pd-
catalysis to other organoselenium ligand as well as the
stabilization of other metal are currently under
investigation in our laboratory.
Singh, RSC Adv, 2014,
K. N. Sharma, A. K. Sharma, H. Joshi and A. K. Singh,
ChemistrySelect, 2016, , 3573-3579.
4, 41659-41665.
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Conflicts of interest
“There are no conflicts to declare”.
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Acknowledgements
The authors gratefully acknowledge the Semnan University
Research Council for the financial support of this work.
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New Journal of Chemistry
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View Article Online
DOI: 10.1039/C8NJ02433B
Graphical Abstract
Organoselenium-Palladium(II) Complex Immobilized on Functionalized Magnetic Nanoparticles as a
Promising Retrievable Nanocatalyst for the “Phosphine-Free” Heck-Mizoroki Coupling Reaction
Yalda Rangraz, a Firouzeh Nemati*a and Ali Elhampoura
An air- and moisture-stable organoselenium-palladium complex immobilized on silica-coated magnetic nanoparticles was
designed, synthesized and applied as a practical and retrievable catalyst in Heck–Mizoroki cross coupling reaction.