Immobilizing palladium on melamine-functionalized magnetic nanoparticles: An efficient and reusable phosphine-free catalyst for Mizoroki–Heck reaction

Article abstract of DOI:10.1002/aoc.6198

A highly efficient and stable heterogeneous catalyst was successfully prepared by anchoring palladium(0) onto melamine-functionalized Fe₃O₄ magnetic nanoparticles (MNPs-Mel-Pd). With the aid of amine functional groups, melamine was covalently bonded on epoxy functionalized magnetic nanoparticles and then Pd(0) was immobilized on this support with high dispersion. The prepared nanocatalyst exhibits excellent catalytic activity for C-C cross coupling (Mizoroki–Heck) of various aryl halides (iodide, bromides, and chrlorides) with olefins under mild reaction condition in relatively short reaction times. The synthesized nanocatalyst was characterized using Fourier transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), vibrating sample magnetometer (VSM), and X-ray photoelectron spectroscopy (XPS) techniques. The loading level of Pd in MNPs-Mel-Pd catalyst was measured to be 1.26 × 10^?3 mol g^?1 by atomic absorption spectroscopy (AAS). In addition, the catalyst can be easily separated and recovered from the reaction mixture by using an external magnet. The heterogeneity of the catalyst was confirmed by the hot filtration test, which was reused for at least six times under the optimized conditions without any significant loss of its activity.

Full text of DOI:10.1002/aoc.6198

Evaluation Only. Created with Aspose.PDF. Copyright 2002-2021 Aspose Pty Ltd.
Received: 27 August 2020
DOI: 10.1002/aoc.6198
Revised: 15 January 2021
Accepted: 29 January 2021
F U L L P A P E R
Immobilizing palladium on melamine-functionalized
magnetic nanoparticles: An efficient and reusable
phosphine-free catalyst for Mizoroki–Heck reaction
Fezzeh Aryanasab1
Henri Vahabi4
| Meisam Shabanian2
| Fouad Laoutid3
|
1Department of Chemistry, Chemistry and
Petrochemistry Research Center, Standard
Research Institute (SRI), Karaj,
31745-139, Iran
2Department of Petrochemistry and
Polymer, Chemistry and Petrochemistry
Research Center, Standard Research
Institute (SRI), Karaj, 31745-139, Iran
3Polymeric and Composite Materials Unit,
Materia Nova Research Center, Mons,
7000, Belgium
4CentraleSupélec, LMOPS, University of
Lorraine, Metz, F-57000, France
A highly efficient and stable heterogeneous catalyst was successfully prepared
by anchoring palladium(0) onto melamine-functionalized Fe3O4 magnetic
nanoparticles (MNPs-Mel-Pd). With the aid of amine functional groups,
melamine was covalently bonded on epoxy functionalized magnetic
nanoparticles and then Pd(0) was immobilized on this support with high dis-
persion. The prepared nanocatalyst exhibits excellent catalytic activity for C C
cross coupling (Mizoroki–Heck) of various aryl halides (iodide, bromides, and
chrlorides) with olefins under mild reaction condition in relatively short reac-
tion times. The synthesized nanocatalyst was characterized using Fourier
transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD), ther-
mogravimetric analysis (TGA), scanning electron microscopy (SEM), vibrating
sample magnetometer (VSM), and X-ray photoelectron spectroscopy (XPS)
techniques. The loading level of Pd in MNPs-Mel-Pd catalyst was measured to
be 1.26 × 10−3 mol g−1 by atomic absorption spectroscopy (AAS). In addition,
the catalyst can be easily separated and recovered from the reaction mixture
by using an external magnet. The heterogeneity of the catalyst was confirmed
by the hot filtration test, which was reused for at least six times under the
optimized conditions without any significant loss of its activity.
Correspondence
Fezzeh Aryanasab, Department of
Chemistry, Chemistry and Petrochemistry
Research Center, Standard Research
Institute (SRI), Karaj 31745-139, Iran.
Email: f_aryanasab@standard.ac.ir;
f_aryanasab@yahoo.com
Funding information
Standard Research Institute (SRI)
K E Y W O R D S
C
C coupling reactions, heterogeneous catalyst, magnetic nanoparticles, supported palladium
catalyst
1 | INTRODUCTION
reactions.[4-6] Among these reactions, the palladium-
catalyzed Mizoroki–Heck reaction of olefins and aryl
halides has turned out to be extremely powerful synthetic
method for the formation of a wide variety of advanced
materials containing C C double bond, such as biologi-
cally active heterocycles, agrochemicals, pharmaceuti-
cals, and natural products.[7]
Transition metals and their complexes have been widely
used as catalyst in various organic transformations such
as carbon–carbon and carbon–heteroatom bond forma-
tion, oxidation, reduction, and reductive amination
reactions.[1–3] Palladium, one of the most important tran-
sition metals in organic synthesis, catalyzes various
carbon–carbon coupling reactions, such as Mizoroki–
Heck, Suzuki–Miyaura, and Sonogashira–Hagihara
Generally, phosphine-based Pd catalysts such as
Pd(PPh3)4 and PdCl2(PPh3)2 are employed as catalyst for
the Heck reaction.[8–10] However, the main drawbacks
Appl Organomet Chem. 2021;35:e6198.
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https://doi.org/10.1002/aoc.6198

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ARYANASAB ET AL.
with these phosphine–Pd catalyst systems are related to
the fact that most of the phosphine derived ligands are
toxic, highly expensive and moisture and air sensitive.[11]
To overcome these limitations, enormous efforts have
been devoted to synthesize various nonphosphine-based
homogeneous catalytic systems with new types of ligands
such as palladacycles,[12,13] N-heterocyclic carbine,[14]
and pincer ligands.[15,16] However, these homogeneous
catalyst systems, in general, suffer from some drawbacks
such as contamination of final products with Pd species
and limited reusability of expensive catalysts that is a
challenging issue in large-scale applications. Fortunately,
the application of heterogeneous catalytic systems, with
improvement in handling and separation, could be an
alternative strategy to dealing with these limitations.
Immobilization of palladium on insoluble solid supports
is a strategy used for the heterogenization of Pd catalysts.
Various inorganic and organic supports including meso-
porous silica,[17] nanostructured materials,[18,19] metal
oxides,[20] graphitic carbon nitride,[21,22] ionic liquids[23]
carbon nanotubes,[24] graphite oxide,[25] graphene,[26]
and various polymers[27–29] have been explored.
Moreover, as a means to combine the advantages of
homogeneous and heterogeneous catalytic processes,
nanoparticles have been widely employed recently due to
their high surface area and facile recovery. Among them,
palladium nanoparticles (Pd NPs) supported on heteroge-
neous nanoparticles, such as mesoporous organosilica
(especially SBA-15),[30] boehmite,[31,32] and hercynite,[33]
are the most relevant type of catalytic systems due to
their high reactivity and selectivity in coupling reactions.
It is worth noting that in these supported catalytic
systems, the nature of the substrate and the strength of
the interaction between the support and the active metal
have great influence on the activity and selectivity of
catalyst. Therefore, it is important to select suitable
supports.
of MNPs was usually modified by organic or inorganic
materials, such as polymers, biomolecules, silica, and
metals.[39,40] Silica contains reactive silanol groups that
can be used to anchor organic functional groups through
silanization, yielding modified surfaces with a variety of
functional groups, such as epoxy, amino, and thiol.[41,42]
These functional groups could also act as coordinating
sites for the metal cations on silica surfaces.
On continuation of our research interest in develop-
ing heterogeneous metal catalysts loaded on MNPs,[43,44]
herein, the synthesis of a new phosphine-free, heteroge-
neous MNP supported Pd catalyst (MNPs-Mel-Pd) and its
activity in Heck C C coupling reaction is reported.
To prepare a Pd-based magnetically separable cata-
lyst, the Fe3O4-MNPs were first synthesized and coated
with silica prior to be modified with epoxy groups. In the
second step, melamine was attached to the surface of the
prepared epoxy-functionalized MNPs through covalent
bonding using epoxy–amine coupling chemistry. The
resulting nitrogen-rich support was capable of adsorbing
large amounts of palladium ions when treated with
Pd(OAc)2.
The palladium was reduced by sodium borohydride to
palladium(0) nanoparticles in the surfaces of MNPs sup-
port (Scheme 1). The catalytic performance and recycling
of this catalyst in Heck C C coupling reaction were
investigated.
2 | EXPERIMENTAL
2.1 | General
All chemicals were purchased from Merck except mela-
mine, which was obtained from Aldrich. All chemicals
were used without further purification as commercially
available. Nuclear magnetic resonance (NMR) spectra
were recorded in ppm in CDCl3 on a Bruker Avance
DPX-500 NMR spectrometer using TMS as the internal
standard at room temperature. Chemical shifts (d) are
quoted in ppm, and coupling constants (J) are measured
in Hertz (Hz). The elemental analyses (carbon, hydrogen,
and nitrogen) of compounds were obtained from a Carlo
ERBA Model EA 1108 analyzer. The content of Pd in the
MNPs-Mel-Pd nanocatalyst was determined by a Varian
model AA-1275 flame atomic absorption spectrometer
(AAS) with a deuterium background corrector. Fourier
transform infrared (FT-IR) spectra were gathered on a
PerkinElmer RXI spectrometer in the range of 400 to
4000 cm−1 with a resolution of 2 cm−1 and 32 coadded
scans, using attenuated total reflection (ATR) mode. The
X-ray diffraction (XRD) patterns of the samples were
recorded using a Rigaku Miniflex II diffractometer with
In spite of various remarkable advancements in these
area, important environmental aspects related to the
physical profile of the supported catalysts are still of con-
cern, including catalytic activity and selectivity, recovery,
and recycling of catalyst and Pd leaching.[34,35] Thus, the
development of supported catalysts that attain the afore-
mentioned features remains a challenging task. In this
regard, the use of magnetic nanoparticles (MNPs) as
support are very promising because this kind of support
allows Pd catalyst to be simply removed from the reaction
medium by magnetic separation, which present a viable
alternative to both filtration and centrifugation, and
enables preventing loss of the expensive Pd species as
well as saving time and energy.[36–38]
However, MNPs are readily aggregated due to the
self-interactions. To prevent the aggregation, the surface

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ARYANASAB ET AL.
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SCHEME 1 Synthesis of MNPs-Mel-Pd nanocatalyst
Cu Kα radiation (λ = 0.15418 nm). The scanning range
was 5ꢀ–70ꢀ with a step size of 0.02ꢀ and step time of 1 s.
The magnetic properties of Fe3O4 and prepared magnetic
nanocatalyst were evaluated by a vibrating sample mag-
netometer (VSM) (AGFM/VSM 3886 Kashan, Iran) with
an applied field between −10,000 and 10,000 Oe at room
temperature. Thermal decomposition behavior of sam-
ples was studied by thermogravimetric analys (TGA)
using a TGA Q50 device from TA Instruments. Morpho-
logical investigation was performed using a FEI Quanta
scanning electron microscopy (SEM) apparatus. X-ray
photoelectron spectroscopy (XPS; Kratos Axis UltraDLD,
Japan) with Al (mono) Kα radiation (E = 1486.7 eV) was
utilized to identify the elements percentage on the sur-
face of magnetic nanocatalyst.
trimethoxysilane (GPTMS) (5 ml) was added to the solu-
tion under N2 atmosphere at 80ꢀC and stirred for 24 h.[47]
The prepared Fe3O4/SiO2/GPTMS was collected with an
external magnet, washed with toluene and ethanol
respectively, and then dried under vacuum at 80ꢀC.
2.3 | Preparation of melamine-
functionalized Fe3O4 MNPs
In a round-bottomed flask (100 ml), melamine (1.26 g,
1 mmol) was dissolved in 30 ml of ethanol, and 1 ml of
1,8-diazabicyclo(5.4.0)undec-7-ene (DBU) was added. To
this solution, 0.5 g of Fe3O4/SiO2/GPTMS was added. The
mixture was well dispersed by ultrasonication (10 min)
and then was stirred under reflux for 6 h at 82ꢀC. The
melamine grafted on Fe3O4/SiO2/GPTMS was then
collected with an external magnet and finally dried under
vacuum at 70ꢀC (MNPs-Mel).
2.2 | Preparation of epoxy-functionalized
Fe3O4 MNPs
First, Fe3O4 MNPs were synthesized in alkali solution of
Fe(III) and Fe(II) (2:1) at 80ꢀC using the chemical
coprecipitation technique,[45] and then the surface of pre-
pared Fe3O4-MNPs was coated with SiO2 using the Stober
method.[46] In a round-bottomed flask (100 ml), as-
prepared Fe3O4/SiO2 MNPs (1 g) were well dispersed in
dry toluene (50 ml) by ultrasonication (10 min) to achieve
2.4 | Immobilization of Pd on the
surface of melamine-functionalized Fe3O4
MNPs: Preparation of nanocatalyst
A solution of Pd(OAc)2 (150 mg, 0.668 mmol) in acetone
(10 ml) was added to a 100 ml round-bottomed flask
charged with MNPs-Mel (0.5 g), and the mixture was
a
uniform dispersion. Then, (3-glycidyloxypropyl)

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ARYANASAB ET AL.
stirred at room temperature for 24 h to yield the dark-yel-
low-colored Pd2+-loaded melamine-functionalized Fe3O4
MNPs (MNPs-Mel-Pd2+). MNPs-Mel-Pd2+ was reduced
with an aqueous solution of sodium borohydride
(0.1 mol L−1
) to yield the MNPs-Mel stabilized
Pd(0) nanoparticles. Finally, magnetic nanocatalyst was
separated with an external magnet and washed with
deionized water to remove the excess Pd(OAc)2 and dried
under vacuum to obtain the final catalyst (MNPs-Mel-
Pd), which was then used in the Mizoroki–Heck coupling
reaction. The content of Pd(0) in MNPs-Mel-Pd catalyst
was determined to be 1.26 × 10−3 mol g−1 by AAS.
2.5 | General procedure for heck
FIGURE 1 Fourier transform infrared (FT-IR) spectra of
reaction
(a) Fe3O4/SiO2, (b) Fe3O4/SiO2/GPTMS, (c) MNPs-Mel, and
(d) MNPs -Mel-Pd
A mixture of aryl halide (1 mmol), alkene (1.2 mmol),
triethylamine (Et3N) (2 mmol), and MNPs-Mel-Pd
(0.01 g) was stirred in DMSO (2 ml) at 100ꢀC for an
appropriate time. The progress of the reaction was moni-
tored by thin layer chromatography (TLC). After comple-
tion of the reaction, the mixture was cooled to room
temperature. The catalyst was separated by using an
external magnet; then the mixture was diluted with
deionized water and extracted with diethyl ether
(3 × 10 ml), and the organic layer was dried over anhy-
drous Na2SO4. Then, the solvent was evaporated, and the
desired product was obtained.
silica, the GPTMS propyl C H stretching vibration band
at 2976 cm−1 was clearly visible.[49] Melamine shows two
bands at 3435–3554 cm−1 arising from the vibrational
stretching of the NH2 groups, which are appeared in
the spectrum of MNPs-Mel (Figure 1c). Furthermore, the
bands at 3168–3354 cm−1 are related to the stretching
vibration of N H in the triazine ring unit.[50] Also, the
NH2 bending vibration at 1670 cm−1, the C N
stretching vibration peak at 1590 cm−1 and the distinct
bands corresponding to the quadrant. (1587 cm−1) and
semicircle stretching C N (1470 cm−1) of triazine ring
are present in the FT-IR spectrum of MNPs-Mel indicat-
ing the successful incorporation of melamine into the
surface of MNPs.[51] On the other hand, the vibration
peak of NH2 in MNPs-Mel spectrum (Figure 1c) was
shifted to lower wavenumbers (3451–3572 cm−1) in the
spectrum of MNPs-Mel-Pd (Figure 1d), indicating that Pd
nanoparticles were successfully attached onto the surface
of the MNPs-Mel through the nitrogen atoms.
Purity and crystal structure of the synthesized nano-
Fe3O4 MNPs, MNPs-Mel, and MNPs-Mel-Pd were investi-
gated using XRD (Figure 2). The characteristic peaks at
diffraction angles 2θ = 30.2ꢀ, 35.5ꢀ, 43.3ꢀ, 53.8ꢀ, 57.2ꢀ,
and 62.8ꢀ, which are indicated by Miller indices in the
spectrum, represent the magnetic cubic structure of
Fe3O4 NPs (JCPDA card no. 65-3107) (Figure 2a).[52] The
XRD patterns of MNPs-Mel and MNPs-Mel-Pd match
well with the characteristic peaks of bare Fe3O4, which
indicate that the surface modification of the MNPs does
not affect its crystallographic phase. In Figure 2c, the
new peaks appeared at 2θ = 40.1ꢀ and 46.9ꢀ, which
corresponding to the (111) and (200) plans, are attributed
to the Pd species indicating that Pd exists in
Pd(0) form.[53,54]
3 | RESULTS AND DISCUSSION
3.1 | Characterization of novel MNPs-
Mel-Pd catalyst
The magnetic nanocatalyst was prepared using a simple
synthetic route, which is shown in Scheme 1. To investi-
gate the structure of the MNPs-Mel-Pd nanocatalyst, FT-
IR, XRD, TGA, SEM, VSM, and XPS measurements
were used.
The FT-IR spectra of (a) Fe3O4/SiO2, (b) Fe3O4/SiO2/
GPTMS, (c) MNPs-Mel, and (d) MNPs-Mel-Pd are shown
in Figure 1. The wide and strong adsorption band at
3450 cm−1 reveals the presence of some free Si OH
bands in the samples. The characteristic bands at
1040–1265 and 820 cm−1 were assigned to the asymmet-
ric and symmetric stretching vibration of Si O and
terminal Si O Si groups.[48] The peaks at around
545–710 cm−1 are attributed to the Fe O vibration of the
magnetite phase. In Figure 1b, although the characteris-
tic adsorption bands of the epoxy group at around
1150 cm−1 overlapped with the strong absorption of the