N-heterocyclic carbene Pd(II) complex supported on Fe3O4@SiO2: Highly active, reusable and magnetically separable catalyst for Suzuki-Miyaura cross-coupling reactions in aqueous media

Article abstract of DOI:10.1016/j.jorganchem.2021.121823

A new type magnetic nano Fe₃O₄@SiO₂@NHC@Pd-MNPs heterogeneous catalyst was fabricated and characterized by Fourier Transform Infrared (FTIR) spectroscopy, Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Energy Disperse X-ray analysis (EDX), Thermogravimetric Analysis (TGA), Differential Thermal Analysis (DTA), and Scanning Electron Microscopy (SEM). The loading amount of Palladium (Pd) to magnetic nano Fe₃O₄@SiO₂@NHC@Pd-MNPs was measured by Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) analysis. The catalytic activity of magnetic nano Fe₃O₄@SiO₂@NHC@Pd-MNPs heterogeneous catalyst was examined on Suzuki-Miyaura cross-coupling reactions of aryl halides with different substituted arylboronic acid derivatives. All coupling reactions yielded excellent results and high TOF (up to 76528 h^?1) in the presence of 2 mg of Fe₃O₄@SiO₂@NHC@Pd-MNPs catalyst (0.0197 mmolg^?1, 0.00394 mmol%Pd) at 80 °C in 2-propanol/H₂O (1:2). In addition, the magnetic nano Fe₃O₄@SiO₂@NHC@Pd-MNPs catalyst was easily recovered by using an external Nd-magnet and reused for the Suzuki cross-coupling reactions. The catalyst showed strong structural and chemical stability and was reused six times without losing its catalytic activity substantially.

Full text of DOI:10.1016/j.jorganchem.2021.121823

Journal of Organometallic Chemistry 943 (2021) 121823
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
N-heterocyclic carbene Pd(II) complex supported on Fe3O4@SiO2:
Highly active, reusable and magnetically separable catalyst for
Suzuki-Miyaura cross-coupling reactions in aqueous media
c
Mitat Akkoça,∗, Nesrin Bug˘dayb, Serdar Altınc, Nadir Kirazd, Sedat Yas¸ arb,∗, Ismail Özdemir
˙
a Malatya Turgut Özal University, Hekimhan Vocational College, Department of Property Protection and Security, Hekimhan, Malatya, Turkey
b
˙
Inönü University, Faculty of Science and Arts, Department of Chemistry, 44280 Malatya, Turkey
c
˙
Inönü University, Faculty of Science and Arts, Department of Physics, 44280 Malatya, Turkey
d Akdeniz University, Faculty of Science and Arts, Department of Chemistry, Antalya, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
A new type magnetic nano Fe3O4@SiO2@NHC@Pd-MNPs heterogeneous catalyst was fabricated and char-
acterized by Fourier Transform Infrared (FTIR) spectroscopy, Transmission Electron Microscopy (TEM),
X-Ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS), Energy Disperse X-ray analysis (EDX),
Thermogravimetric Analysis (TGA), Differential Thermal Analysis (DTA), and Scanning Electron Microscopy
(SEM). The loading amount of Palladium (Pd) to magnetic nano Fe3O4@SiO2@NHC@Pd-MNPs was mea-
sured by Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) analysis. The catalytic ac-
tivity of magnetic nano Fe3O4@SiO2@NHC@Pd-MNPs heterogeneous catalyst was examined on Suzuki-
Miyaura cross-coupling reactions of aryl halides with different substituted arylboronic acid derivatives.
All coupling reactions yielded excellent results and high TOF (up to 76528 h−1) in the presence of 2 mg
of Fe3O4@SiO2@NHC@Pd-MNPs catalyst (0.0197 mmolg−1, 0.00394 mmol%Pd) at 80 °C in 2-propanol/H2O
(1:2). In addition, the magnetic nano Fe3O4@SiO2@NHC@Pd-MNPs catalyst was easily recovered by using
an external Nd-magnet and reused for the Suzuki cross-coupling reactions. The catalyst showed strong
structural and chemical stability and was reused six times without losing its catalytic activity substan-
tially.
Received 3 February 2021
Revised 11 March 2021
Accepted 7 April 2021
Available online 16 April 2021
Keywords:
Magnetic palladium catalyst
Pd(II)-N-heterocyclic carbene complex
Suzuki-Miyaura reaction
Green chemistry
Magnetite
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
(ZIFs) and silica derivatives are mostly used as support mate-
rials for transition-metal catalysts and organo-catalysts. Among
Magnetic nanoparticles (MNPs) have gained a lot of attention
in recent years due to their superior properties and wide range
of applications in the areas such as electronics, sensors, memory
devices and medical applications [1]. Compare to other fields, the
applications of MNPs in the medical field are quite high because
of their superparamagnetic properties, low toxicity, biocompatibil-
ity and have high magnetic susceptibility [2–12]. In recent years,
there have been significant challenges in the development of ma-
terials with such superior properties. However, due to their chemi-
cal activity, robustness and reusability, the MNPs are used as either
support materials or active components in catalysis.
them, silica-supported transition-metal catalysts have been pre-
ferred more than other support materials in terms of prepara-
tion, utilization and applications. However, silica is used as a pro-
tective layer. Metal oxides can be protected by silica to prepare
MxOy@SiO2 structures. For this reason, Fe3O4 core and silica shell
have recently gained great interest. Fe3O4@SiO2 has unique chemi-
cal and physical properties such as chemically functionable surface
and magnetic response. Thus, Fe3O4@SiO2 is a strong support ma-
terial for the preparation of heterogeneous catalysts [13–19]. The
superiority of the Fe3O4@SiO2 material supported catalysts is that
the catalyst can be easily separated from the reaction medium by
applying an external magnet [20].
Carbon nanomaterials, polymer materials, metal oxides, Metal-
Organic Frameworks (MOFs), Zeolitic Imidazolate Frameworks
Due to their superior characteristics, the N-heterocyclic car-
benes (NHC) have recently gained substantial interest as an al-
ternative to phosphine ligand [21]. NHCs are one of the most
known versatile ligands for transition metals because of their
strong σ-donor property and the ease of structural modification
in the design of homogeneous and heterogeneous catalysts. The
∗
Corresponding authors.
E-mail addresses: mitat.akkoc@ozal.edu.tr (M. Akkoç), sedat.yasar@inonu.edu.tr
(S. Yas¸ ar).
https://doi.org/10.1016/j.jorganchem.2021.121823
0022-328X/© 2021 Elsevier B.V. All rights reserved.

M. Akkoç, N. Bug˘day, S. Altın et al.
Journal of Organometallic Chemistry 943 (2021) 121823
σ-donor capacities of NHCs ligands differ depend on the type
of NHC. Szostak et. al. found the sigma donor properties of dif-
ferent types of NHC ligands different. In this study, diaminocar-
bene and cyclic (alkyl) (amino) carbene show the strongest
sigma donor capacity [22]. Furthermore, 6-membered NHCs (IPr
and IMes) are significantly stronger sigma-donors than their 5-
membered parent IPr and IMes analogues. In the same study, it
was stated that the N-alkyl-benzimidazolin-2-ylidene NHC pre-
cursor has better sigma donor property than N-alkyl-imidazol-
hertz. NMR multiplicities are abbreviated as follows: s = singlet,
d = doublet, t = triplet, m = multiplet signal. All reactions were
monitored on a Shimadzu GC2010 Plus system by GC-FID with an
HP-5 column of 30 m length, 0.32 mm diameter, and 0.25 mm
film thickness. Some of the catalytic products were previously re-
ported. New compounds were characterized by different spectro-
scopic techniques. See Supporting Information section. The GC-MS
analysis of compounds was performed on a Shimadzu GC-MS 2010
plus using a TRX-5 column on positive ESI mode. The FTIR anal-
ysis was performed with a PerkinElmer Spectrum 100 GladiATR
FT/IR spectrometer. The crystal structure analysis of the synthe-
sized powder was performed by an X-ray diffractometer labeled
1
2-ylidenes [22]. In this study, the JCH coupling constant of 1-
(2,3,4,5,6-pentamethylbenzyl)-3-(ethanol)-benzimidazolium iodine,
which we used as the NHC ligand precursor, is found as 219,16 Hz.
This value found is in agreement with the relevant literature and
shows that similar σ-donor capacities than N-alkyl-imidazol-2-
ylidenes analogues.
Rigaku Rint 2000 which powered by Cu K radiation was used
α
for characterization. The scan rate was chosen as 2°/min between
2° and 80°/min. The microstructural properties and the surface
morphology of the synthesized powders were investigated by Leo
EVO-40 VPX scanning electron microscope (SEM). For XPS analyse:
XPS instrument: PHI 5000 VersaProbe II, X rays: hn = 1486,6 eV
monochromatic, spot 200 μm and Energy resolution (hi-resolution
spectra): 0.8 eV were used. The TEM analyses were performed
by FEI TECNAI OSIRIS operating at 200 kV, Morphological analy-
sis with HAADF-STEM imaging mode, Chemical analysis with the
Super-X EDX detector system (4 SDD detectors).
NHCs bind to metal centers stronger than phosphines, com-
plexes formed are more stable toward air, moisture and heat in
many applications [23–25]. This may help the stability of Metal-
NHC complexes under catalytical and biological conditions. The
combination of NHCs and MNPs is very important for the synthesis
of new types of MNPs catalysts. To date, several complexes immo-
bilized on the MNPs have been successfully synthesized and used
as catalysts for C-C bond formation reactions [26–40]. Dong et al.
synthesized a supported Pd(II) complex on magnetite core-shell of
Fe3O4@SiO2 for Suzuki-Miyaura cross-coupling reaction [40]. They
claimed that their catalyst’s reusability is high and can catalyze the
Suzuki reaction six times without a significant decrease of the cat-
alyst performance at 80 °C in EtOH. Khazaei et al. prepared nano-
Fe3O4@SiO2 supported Pd(0) nanocatalyst and used as a catalyst
on Suzuki coupling reaction of aryl iodide and aryl bromide with
aryl boronic acid derivatives in the presence of CaO as a base at
85 °C in aqueous media [41]. This catalyst can also show high
catalytic activity and reusability without loss on catalytic activity
after five runs. Fafiee et al. also prepared Fe3O4@SiO2 supported
Vitamin B1-Pd complex and investigated its catalytic activity on
Suzuki coupling of aryl halides at 60 °C in EtOH [28]. They re-
ported good to high yields of coupled products of aryl halides
with substituted phenylboronic acid derivatives. Also, catalyst re-
ported showed five reusabilities with slight loss of its activity. Tao
et al. synthesized magnetic nanoparticle-supported N-heterocyclic
carbenepalladacycle (SMNP@NHC-Pd) and investigated its catalytic
activity on Suzuki-Miyaura cross-coupling reaction of heterocyclic
9-chloroacridine with diverse boronic acids. They reported that
2.2. General procedure for Suzuki-Miyaura cross-coupling reaction
Aryl halide (0.5 mmol), aryl boronic acid (0.6 mmol), potas-
sium carbonate (1 mmol), and palladium nano-catalyst (2 mg,
0.00394 mmol% Pd) was placed in a flask under air and 3 ml of
2-propanol/water mixture (1:2, v:v) was added to flask and stirred
in a preheated oil bath (80 °C) for an appropriate time. After com-
pletion reaction, the reaction was cooled to room temperature and
the palladium nano-catalyst was removed by an external magnet
from the reaction mixture. Then, the reaction was diluted with 3
ml of water and 3 ml of ethyl acetate. The organic phase was sep-
arated by extraction and dried over MgSO4, and filtrated and the
filtrate evaporated under reduced pressure to obtain crude prod-
uct. The pure products were purified by column chromatography
using ethyl acetate/hexane as the eluent (1:30).
2.3. Synthesis
2.3.1. Synthesis of NHC salt
(1-(2,3,4,5,6-pentamethylbenzyl)-3-(etanole)-benzimidazolium iodine)
The NHC salt was synthesized by the following re-
ported procedure [42a,b]. 2-iyodoethanole (1.8 g; 110 mmol)
was added to 10 mL n-BuOH solution of 1-(2,3,4,5,6-
pentamethylbenzyl)benzimidazole (2.78 g; 100 mmol). Then,
the reaction solution was refluxed for 4 h. After the reaction
cooled to room temperature, yellow crystals were obtained. Crys-
tals were removed from the solution by filtration, washed with
diethyl ether (5 × 5 mL) and dried under vacuum. Yellow crystals
were characterized by 1H and 13C NMR spectroscopy.
0.5-1 mol% catalyst efficiently catalyse the reactions and resulted
in high yields at 100 °C in 24 h.
Herein, we reported synthesis and characterization of
Fe3O4@SiO2-based MNPs catalysts and investigated the cat-
alytic activity of it on Suzuki-Miyaura cross-coupling reaction
under mild reaction conditions in aqueous media.
2. Experimental section
Yield: 75%, 1H NMR (400 MHz, DMSO-d6) δ
=
8.97 (s,
2.1. General considerations
1H, NCHN), 8.19 and 8.12 (d, J 28 Hz, 4H, C6H4), 5.72
=
(s, 2H, CH2C6(CH3)5-2,3,4,5,6), 5.02 (s, 1H, CH2CH2OH), 4.54 (t,
j = 4.6 Hz, 2H, CH2CH2OH), 3.74 (m, 2H, CH2CH2OH), 2.27 (s,
3H, CH2C6(CH3)5-2,3,4,5,6), 2.24 (s, 6H, CH2C6(CH3)5-2,3,4,5,6),
2.21 (s, 6H, CH2C6(CH3)5-2,3,4,5,6).13C NMR (100 MHz, DMSO-
d6) δ = 141.8, 137.8, 134.4, 133.4, 132.1, 131.9, 127.1, 127.0, 125.9,
114.6, 114.3, 59.1, 49.8, 46.8, 17.5, 17.2, 16.9. Elemental analysis for
C21H27N2OI: %Calc. C: 56.01, H: 6.04, N: 6.22; %found C: 56.12, H:
6.14, N: 6.38.
Otherwise stated, all synthesis was carried out in aerobic con-
ditions. All chemicals and solvents purchased from sigma Aldrich,
Fluorochem and BLDpharm chemical companies and used as ob-
tained. For NMR analysis, a Bruker Avance III 400 MHz NMR spec-
trometer was used at room temperature with the decoupled nu-
cleus, using CDCl3 and DMSO-d6 as a solvent and referenced ver-
sus TMS as standard. Coupling constants (J values) are given in
2

M. Akkoç, N. Bug˘day, S. Altın et al.
Journal of Organometallic Chemistry 943 (2021) 121823
Scheme 1. Synthesis pathway of the Fe3O4@SiO2@NHC@Pd0-MNPs catalyst, 3.
2.4. Synthesis of silica-coated magnetic nanoparticle NHC-Pd
catalyst, Fe3O4@SiO2@NHC@Pd0-MNPs
Fe3O4 was synthesized via the reported co-precipitation
method with little modifications using FeCl3.6H2O and FeCl2.4H2O
[43]. The synthesized Fe3O4 nanoparticles were coated with silica
by tetraethylorthosilicate (TEOS) to fabricate Fe3O4@SiO2 nanopar-
ticles. This nanoparticle functionalized with 1-ethanol-3-(2,3,4,5,6-
pentamethylbenzyl)benzimidazolidinium hydroiodine in methanol
under reflux conditions to obtain Fe3O4@SiO2@NHC. Finally, the
interaction of Pd(OAc)2 with Fe3O4@SiO2@NHC in 50 mL ace-
tonitrile under reflux conditions for 24 hours resulted in the
formation of Fe3O4@SiO2@NHC@Pd0-MNPs catalyst labeled as 3
(Scheme 1). As a result of the ICP-OES analysis, it was determined
that our Fe3O4@SiO2@NHC@ Pd0-MNPs catalyst contains 0.21 wt%
Pd (0.0197 mmol/g).
Fig. 2. XRD pattern of Fe3O4(Magnetite)@SiO2@NHC@Pd-MNPs.
of benzimidazole) to 1394 cm−1 is suggesting the formation of
Pd-C bond (Pd-CNC stretching of benzimidazole) after reaction
with Pd(OAc)2.
3. Result and discussions
3.2. XRD analysis
3.1. FTIR analysis
The second characterization of the catalyst was done by XRD
analysis. The powder XRD pattern of the of Fe3O4@SiO2@NHC@Pd-
MNPs presented in Fig. 2 gives information related to three phases
as magnetite (Fe3O4), SiO2, and organic structure. In this prepared
Fe3O4@SiO2@NHC@Pd-MNPs, the Fe3O4 as a central component
and SiO2 layer for protect magnetite and NHC-Pd attached to SiO2
over this structure. The XRD peaks at 30.08, 40.02, 48.64 and 62.83
are due to (110), (011), (120), and (220) reflection planes of the
SiO2 phase (JCPDF number: 45-1374). Similarly, the peaks of 35.42,
43.05, 53.40, and 56.9 are related to (311), (400), (422) and (511)
reflection planes of magnetite (JCPDF number: 19-0629). The wide
halo around 8-28o may be due to the amorphous form of SiO2 on
magnetite and organic ligands in the structure. It also may be due
to the nano-structured formation of Fe3O4@SiO2@NHC@Pd-MNPs
as seen in reported literature [26].
The Fe3O4@SiO2@NHC@Pd and other chemical compo-
nents used in the catalyst were first analyzed by FTIR spec-
troscopy. The FTIR spectrum of NHC, Fe3O4, Fe3O4@SiO2 and
Fe3O4@SiO2@NHC@Pd as shown in Fig 1. The band at 584 cm−1
−1
–
is belonged to Fe O bonds and the peak at 3421 cm
associated
–
with the O H groups [40,41]. The absorption bands at 1091,
799 and 464 cm−1 were attributed to Si O Si antisymmetric
–
–
–
–
– –
stretching vibrations, Si O Si symmetric stretching and Si O Si
–
–
or O Si O, respectively. According to FT-IR analysis, it shows that
we have coated Fe3O4 with a silica layer with success. The FT-IR
spectra of NHC give bands at 3506 (OH stretching), 2970 (CH
(CbH2) stretching, 2937 [CH(CH2-CH2) stretching] and 1600, 1400,
1300 cm−1. The characteristic spectral for NHC is the intense peak
at 3506 cm−1, due to the supposition of the OH bending vibration.
After connection of NHC to Fe3O4@SiO2 and Fe3O4@SiO2@NHC@Pd
the bands at 3506 cm−1 were disappeared and these results indi-
cate that functionalization of Fe3O4@SiO2 with NHC successfully
done. The shift of absorption band at 1560 cm−1 (CNC stretching
3.3. SEM and TEM and EDX analysis
The characterization of the catalyst was also done by
SEM and TEM techniques. The morphology and size of the
Fig. 1. FT-IR spectra of Fe3O4@SiO2@NHC@Pd-MNPs catalyst and its constituent components.
3

M. Akkoç, N. Bug˘day, S. Altın et al.
Journal of Organometallic Chemistry 943 (2021) 121823
Fig. 3. (a) SEM, (b) TEM images of Fe3O4@SiO2@NHC@Pd, (c) EDX-dot mapping and (d), (e), (f) and (g) elemental distribution of Fe, O, Si and Pd, respectively.
Fig. 4. a) and b) The TEM image of Fe3O4@SiO2@NHC@Pd-MNPs c) SEM-EDX spectrum of Fe3O4@SiO2@NHC@Pd-MNPs.
Fe3O4@SiO2@NHC@Pd-MNPs were obtained by SEM and TEM anal-
ysis. Fig. 3a shows the SEM images of the powders at 60kX magni-
fication. It is seen that there are spherical type grain formation and
the size of the particles are in nm range size with a cluster-type
formation in the solid form. To see the particle size in more de-
tail, the HAADF-STEM images of the Fe3O4@SiO2@NHC@Pd-MNPs
powders were investigated as seen in Fig. 3b. The particle size
was measured as below 10 nm and some of them are around 40-
60 nm (Fig. 3). According to the TEM images (Fig. 4a), the Pd metal
species are seem well dispersed and composed in nano-sized par-
ticles between 20-35 nm range.
The elemental distribution of ions in the powder and chemical
analysis based on the elements are presented in Fig. 3c-g. It is ob-
served that the distribution of Si, Fe, and O shows homogeneous
Fig. 5. XPS spectra of Fe3O4@SiO2@NHC@Pd-MNPs.
behavior which is the expected result for the study and it was
proven by EDX. It should be noted that the observation of well-
dispersed Pd particles is seen in the figures which is an expected
XPS spectrum was investigated by the aspect of all layers in
the Fe3O4@SiO2@NHC@Pd. The XPS peaks of SiO2 were given in
Figs. 5 and 6a-b. The Si2s, Si2p, C1s and O1s electrons were ob-
served at 152, 102, 285 and 527 eV, respectively (Fig. 6a-d). The
behavior for catalytic studies.
The elemental composition of the Fe3O4@SiO2@NHC@Pd-MNPs
was also analyzed by Energy Dispersive X-ray spectroscopy (EDX)
spectrum. The EDX results reveal the presence of the elemen-
tal composition of NHC (C, Si, O, N) and Pd elements in the
Fe3O4@SiO2@NHC@Pd-MNPs catalyst surface (Fig. 4b). Also, ele-
mental mapping of Fe3O4@SiO2@NHC@Pd shows the dispersion of
Pd on the texture. As a result of the ICP-OES analysis, the exact
amount of Pd in our Fe3O4@SiO2@NHC@Pd-MNPs was found 0.21
wt% Pd (0.0197 mmolg−1).
=
energy level of C1s in Fig. 6c is related to C-C/C C bonds in the
structure [44]. The N1s electron peak at 398 eV in Fig. 6e is due
to imidazolate-N atom in the structure [45]. These data show the
presence of the atoms mentioned in the structure of the synthe-
sized catalyst [46]. The another peak at 531 eV for O1s belongs
to the O atom in the NHC ligand which has a -1 oxidation state
[47]. It shows that Fe3p demonstrate the presence of Fe ions and
Fe2p split into two energy level at ~708 eV for 2p1/2 and ~721 eV
for 2p3/2 (Fig. 6e) and it can be explained by the formation of
Fe2+ and Fe3+ in the structure which is an expected result for
magnetite [48]. Fig. 6g exhibits the XPS spectra of the Pd element
and gives two peaks at 333.2 and 338.4 eV which correspond to
3d5/2 and 3d3/2 orbitals of metallic state Pd0 rather than Pd(II) [49].
3.4. XPS analysis
The structural characterization of the catalyst was further clar-
ified by using the X-ray photoelectron spectroscopy (XPS) analy-
sis method. The XPS spectra of Fe3O4@SiO2@NHC@Pd-MNPs and
each ion spectra present in Figs.
5 and 6, respectively. The
4

M. Akkoç, N. Bug˘day, S. Altın et al.
Journal of Organometallic Chemistry 943 (2021) 121823
Table 1
Atomic concentration values of Fe3O4@SiO2@NHC@Pd-MNPs catalyst determined by XPS analysis.
Sample
N 1s (%.at)
0.57
C 1s (%.at)
12.57
O 1s (%.at)
62.89
Si 2p (%.at)
15.22
Fe 2p (%.at)
8.23
Pd 3d3/2 (%.at)
0.52
Fe3O4@SiO2@NHC@Pd-MNPs
Fig. 7. DTA and TGA curve of Fe3O4@SiO2@NHC@Pd-MNPs catalyst.
Fig. 6. XPS spectra of (a) Si2s, (b) Si2p, (c) C1s, (d) O1s, (e) N1s, (f) Fe2p and (g)
Pd3d.
Fig. 8. Magnetization of the Fe3O4@SiO2@NHC@Pd-MNPs catalyst in the i-PrOH
when an external magnetic field is applied.
Unlike the literature [28], these results show that Pd+2 is reduced
to metallic palladium during the synthesis process due to the heat
applied. This reduction process may be promoted by the weak σ-
donor ability of NHC ligand. Szostak et al., have calculated the
sigma donor capability of the different NHC ligands and they indi-
cated that the sigma donor capability can be estimating from the
3.5. DTA and TGA analysis
The thermal stability of Fe3O4@SiO2@NHC@Pd-MNPs catalyst
was evaluated by thermal gravimetric analysis-differential thermal
analysis (DTA and TGA) and presented in Fig. 7.
coupling constants of JCH in13C NMR [22]. CH J-coupling value can
1
DTA and TG analysis shows that there are three regions for both
data which are related to endothermic activity in the samples. The
first reaction at 238 °C should be due to the decomposition of
the Fe3O4@SiO2@NHC@Pd-MNPs which corresponds to the high-
est temperature for the stability of samples. The other two reac-
tions are the transition should be the formation of a new phase
with losing some oxygen and nitrogen ions from the structure. So,
it should be noted that the Fe3O4@SiO2@NHC@Pd-MNPs can stable
without any decomposition and it can be used in catalytic reac-
tions of temperature up to 238 °C.
in principle be calculated both the1H-coupled 13C NMR spectrum
and from the13C satellites of the1H NMR spectrum [50,51] Based
1
on this information, the JCH coupling constant of the NHC pre-
cursor we used in this study is calculated 219.16 Hz from the13
C
satellites of the1H NMR spectrum. This value is the average value
when compared to the values obtained from other NHC precursors
[22]. This sigma donor property of NHC may have played a role
in reducing Pd2+ to Pd0. The reduction of Pd (II) to metallic pal-
ladium from the reaction medium without any specific reduction
process and the presence of only one type of Pd in the medium is
a desired situation for synthesis of Pd nanomaterials without using
extra reducing agents.
The catalyst has a very high magnetization due to the magnetite
in the structure of the catalyst support material. Fig. 8 shows that
when an external magnetic field is applied, the catalyst shows high
magnetization in a short time.
The atomic concentration of Fe3O4@SiO2@NHC@Pd-MNPs cat-
alyst was determined by XPS analysis presented in Table 1. Ac-
cording to XPS result, Fe3O4@SiO2@NHC@Pd-MNPs catalyst con-
tains 0.52 wt%, 0.0488 mmol/g Pd0. The difference between the
ICP-OES and XPS results may be due to the XPS analysis deter-
mining the amount of Pd0 metal present on the surface. However,
ICP-OES analysis determines the amount of Pd in the catalyst with
higher accuracy. Thus, we used ISP-OES to analyze results in the
calculation of Pd loadings.
3.6. Catalytic studies
We examined the catalytic activity of Fe3O4@SiO2@NHC@Pd-
MNPs catalyst, 3, for the Suzuki-Miyaura cross-coupling reaction.
First, we conducted a series of experiments to find out the op-
timum conditions for the catalytic system. A series experiments
5

M. Akkoç, N. Bug˘day, S. Altın et al.
Journal of Organometallic Chemistry 943 (2021) 121823
Table 2
Optimization of the conditions of phenylboronic acid and 4-bromoacetophenone catalyzed
by Fe3O4@SiO2@NHC@Pd-MNPs, 3.
Entry
[Cat]
Base
Solvent
Temp./Time
Yield %
1
[3], 1 mg
K2CO3
K2CO3
K2CO3
K2CO3
NaOH
Na2CO3
Cs2CO3
KtOBu
KOH
H2O/i-PrOH
H2O/i-PrOH
H2O/i-PrOH
H2O/i-PrOH
H2O/i-PrOH
H2O/i-PrOH
H2O/i-PrOH
H2O/i-PrOH
H2O/i-PrOH
H2O
80 °C, 5 min.
50 °C, 5 min.
25 °C, 5 min.
80 °C, 5 min.
80 °C, 5 min.
80 °C, 5 min.
80 °C, 5 min.
80 °C, 5 min.
80 °C, 5 min.
80 °C, 5 min.
80 °C, 5 min.
80 °C, 5 min.
80 °C, 5 min.
80 °C, 5 min.
80 °C, 5 min.
80 °C, 5 min.
80 °C, 5 min.
80 °C, 5 min.
80 °C, 6 h.
92
22
8
2
[3], 1 mg
3
[3], 1 mg
4
[3], 2 mg
99
42
60
45
38
25
22
5
5
[3], 2 mg
6
[3], 2 mg
7
[3], 2 mg
8
[3], 2 mg
9
[3], 2 mg
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
[3], 2 mg
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
[3], 2 mg
i-PrOH
[3], 2 mg
EtOH
3
[3], 2 mg
DMF
20
12
55
88
65
[3], 2 mg
Dioxane
[3], 2 mg
DMF/H2O
H2O/EtOH
H2O/Dioxane
H2O/i-PrOH
H2O/i-PrOH
H2O/i-PrOH
H2O/i-PrOH
H2O/i-PrOH
H2O/i-PrOH
H2O/i-PrOH
[3], 2 mg
[3], 2 mg
b
[3], 2 mg
-
[3], 2 mg
20b
Pd(OAc)2, 0,0088 mg
PdCl2, 0,007 mg
Pd(OAc)2, 0,0088 mg
PdCl2, 0,007 mg
-
80 °C, 5 min.
80 °C, 5 min.
80 °C, 5 min.
80 °C, 5 min.
80 °C, 5 min.
53
45
b
-
b
-
trace
aReaction conditions:
bromoacetophenone (0.5 mmol), Base (1.0 mmol), solvent 3 ml.
3 (2 mg, 0.00394 mmol%Pd), phenylboronic acid (0.6 mmol), 4-
b
4-Chloroacetophenone
was used. GC yields determine according to 4-bromoacetophenone or 4-chloroacetophenone
using (trifluoromethyl)benzene as an internal standard.
were performed to find out the effective amount of catalyst, the
suitable solvent or solvent mixtures and base. In these reactions,
the 4-bromacetophenone and phenylboronic acid were selected as
a substrates. To find the effective amount of catalyst and reac-
tion time we tried catalytic reaction catalyzed by different cat-
alyst loading at 25 °C, 50 °C and 80 °C (Table 2, entry 1-
3). According to the results, 2 mg catalyst loading give highest
yield at 80 °C in short time. After determining the specific cat-
alyst loading and reaction temperature, the next step is find out
the appropriate base (Table 2, entry 4-9). Different polar solvents
were also tested solely or as a solvent mixture (Table 2, entry 10-
17). According to our previously reported results [52–60], DMF, 2-
propanol, EtOH, Dioxane, DMF-water (1:1), 2-propanol-water (1:2)
were selected as a solvent. The carbonate-derived bases such as
Na2CO3, K2CO3, Cs2CO3 were tried first due to their good sol-
ubility in in aqueous media. The best catalytic activity was ob-
tained with 2-propanol-water (1:2) and K2CO3 solvent-base couple.
Other solvents and bases also give moderate to good yield. Than,
4-chloroacetophenone was used as a substrate to understand the
effectiveness of the catalyst or to show the applicability of the cat-
alytic system to chlorinated aryl halides (Table 1, entries 18-19). No
product formation was observed in the reaction performed under
the same reaction conditions with the 4-chloroacetophenone sub-
strate (Table 1, entry 18). In the reaction for 6 h, a very low yield
of product formation was observed (Table 1, entry 19). For com-
parison, different commercially available palladium sources such
as PdCl2 and Pd(OAc)2 were tested at the same concentration in
the catalytic system we created. From the results we obtained,
it was seen that the catalyst we synthesized had much better
activity than commercially available palladium sources (Table 1,
(0.0394 mmol%Pd), 1 mmol K2CO3, 2-propanol-water (1:2, w/w) at
80 °C.
The optimized reaction condition on hand, Suzuki-Miyaura
cross-coupling of different aryl bromides with different phenyl-
boronic acid derivatives were examined (Table 3) First, differ-
ent p-substituted aryl bromides contain CN, NO2, COCH3, CH3,
OCH3, CF3 with phenylboronic acid, 4-tert-buthylboronic acid, 2-
methoxyboronic acid and 3-formyl-4-methoxyboronic acid were
investigated. The results clearly showed that the reaction can be
completed in 5-30 min with moderate to excellent yields for both
electron-donating and electron-withdrawing substituted aryl bro-
mides and phenylboronic acids. It has been observed that the
Suzuki coupling of aryl bromides with electron-donating groups
and phenylboronic acids take a longer time when compared to
aryl bromides containing electron-withdrawing groups. It may be
due to the electronic effect of electron-donating substituents. How-
ever, o-substituted aryl bromides and boronic acid derivatives need
more reaction time due to steric hindrance of the substituted
group. Moreover, heteroaryl bromides were also tried with these
phenyboronic acid derivatives and the result were satisfied with
high mono or diarylated selectivity (Table 3). The products contain
heteroaryl groups may be important for biologically active applica-
tions.
3.7. Reusability studies
The reusability of the catalyst was investigated with a set of
experiments via a used catalyst with 4-bromoacetophenone under
optimized reaction condition for the Suzuki coupling reaction. Af-
ter the first run, the catalyst was recovered from reaction media
with the external magnet and washed with a sufficient amount
of ethanol and water. After final washing with ethanol, the cata-
lyst was dried at 80 °C for 12 h and used for the next reaction.
For each new reaction, new substrates were used. These processes
were tried 6 times consecutively and the results are presented in
Fig. 9. According to the results, there was no significant decrease
entries 20-23). As
a result of the reaction without catalyst,
trace amount of product formation was observed (Table 1, entry
24).
As
optimum conditions for the catalytic system described here as
follows: 0.5 mmol Ar-Br, 0.6 mmol boronic acid, mg of
a result of all these different reactions conditions, the
2
3
6

M. Akkoç, N. Bug˘day, S. Altın et al.
Journal of Organometallic Chemistry 943 (2021) 121823
Table 3
Suzuki-Miyaura cross-coupling of aryl bromides with substituted phenylboronic acids derivatives catalyzed by 3.
(continued on next page)
7

M. Akkoç, N. Bug˘day, S. Altın et al.
Journal of Organometallic Chemistry 943 (2021) 121823
Table 3 (continued)
a
Reaction conditions: 3 (0.00394 mmol% Pd), phenylboronic acid (0.6 mmol), aryl bromide (0.5 mmol), Base (1.0 mmol),
i-PrOH/water (1:2, 3 mL), 80 °C. GC yields determine according to aryl bromide using (trifluoromethyl)benzene as an internal
standard.
b
50 °C.
c
Room temperature.
Table 4
Comparison results of Fe3O4@SiO2@NHC@Pd-MNPs with reported catalyst on the Suzuki-Miyaura reactions for 4-
bromobenzonitrile with phenylboronic acid.
Ref.
Cat. (mol%), Solvent, Temperature
Time(min)
Yield (%)
55
CMC-NHC-Pd (0.8 mol%), EtOH/H2O (1:1), 60 °C, X=Br, X♣=Cl
γ -Fe2O3-acetamidine-Pd, DMF, 100 °C, X=Br
180, 300♣
120, 30∗
120
94, 52♣
92,96∗
99
56
57
Cell-NHC-Pd (0.75 mol%), DMF/H2O (1:1), 80 °C, X=Br
Pd/C-NHC-P(OR)3 (0.03 mol%), EtOH, 80 °C, X=Br
58
60
98
28
Fe3O4@SiO2@VB1-Pd NPs (0.022 mol%), EtOH, 60 °C, X=Br
Fe3O4@SiO2-NHC-Pd(II) (0.37 mol%), H2O, 60 °C, X=I
Pd-TEG, (0.005 mol%) H2O, 25 °C, X=Br
20, 20∗
60, 90∗
840
8∗
5, 10∗
98, 95∗
92, 93∗
91
95∗
90, 99∗
26
59
60
(PdII-NHC)n@SiO2 (0.27 mol%), DMF/H2O (2:1), 60 °C, X=Br
Fe3O4@SiO2@NHC@Pd-MNPs (0.00394 mmol%, i-PrOH/H2O (1:2), 80 °C, X=Br
This work
∗
♣
Yield obtained for 4-bromobenzene substrate.
Yield obtained for 4-chlorobenzene substrate.
3.8. Hot filtration test study
Furthermore, to understand the palladium leaching, a hot fil-
tration test was performed. There was no increase in the amount
of the yield after GC analysis while the reaction continued after
the catalyst was removed from the reaction medium. The concen-
trations of recovered catalyst after 6 runs were found by ICP-OES
analysis as 0.20 wt%, which indicated that the leaching of Pd from
the support material is very limited and thus does not affect cat-
alyst performance significantly. Herein, the M-NHC strong covalent
bond strength may play a role in the stability of catalyst. To see the
changes in the structure of the used catalyst, SEM and EDX anal-
ysis was performed during each time. SEM and EDX results shows
that there are very limited structural changes and agglomeration
on the catalyst (Figure S3).
Fig. 9. Recyclability of the catalyst 3 for the Suzuki-Miyaura cross-coupling reac-
tion.
in yield and catalytic performance of the catalyst after 6 trials. Al-
though, it is not entirely possible to transfer the entire catalyst
amount to the next reaction with an externally applied magnetic
field. This small problem can cause a slight drop in product yields
(5% reduction in yield after 6 runs).
After many catalytic reactions, our Fe3O4@SiO2@NHC@Pd-MNPs
catalyst has showed high catalytic activity on the Suzuki-Miyaura
cross-coupling reaction for a variety of boronic acids and different
aryl bromides.
8

M. Akkoç, N. Bug˘day, S. Altın et al.
Journal of Organometallic Chemistry 943 (2021) 121823
3.9. Comparison with the literature
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Fe3O4 composite nanospheres for in vivo imaging and hyperthermia, Adv.
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Comparing the activities of our catalyst and similar types of
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lyst has better catalytic activity and application utility (in terms of
separation of the catalyst from the reaction medium, catalyst load-
ing and reusability) than many reported catalyst (Table 4). As far as
we know, the amount of the Pd in the catalyst used has not been
reported in the literature so far. Considering the amount of cata-
lyst used and the yield obtained, it appears that the catalyst has
an extraordinary activity. Although our catalytic system positively
differentiated from reported catalytic systems, the disadvantage of
the catalytic system in this study is almost ineffective with aryl
chloride substrates under same reaction conditions. From these re-
sults, we hope that this catalyst can be used as a catalyst for other
organic transformation reactions where palladium catalysts use.
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In summary, an efficient and green Fe3O4@SiO2@NHC@Pd-
MNPs catalyst was successfully synthesized and applied for the
Suzuki-Miyaura cross-coupling reaction. to synthesize fine substi-
tuted and biological important biaryls. This novel catalyst showed
very high catalytic activity in Suzuki-Miyaura cross-coupling reac-
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high strong magnetic and disperse properties. Due to these prop-
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from reaction media. Stability towards air and moisture, low cat-
alyst loading, easy handling and recyclability of this catalyst are
some of its unique characteristics. The applicability of this catalyst
in other palladium-catalyzed catalytic systems is under research in
our lab and we expect that it will provide high catalytic activity
with reusability in other reactions as well.
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Conflict of interest: The author declared no conflict of interest.
Declaration of Competing Interest
[17] X.Q. Xu, C.H. Deng, M.X. Gao, W.J. Yu, P.Y. Yang, M.X. Zhang, Synthesis of mag-
netic microspheres with immobilized metal ions for enrichment and direct
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The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
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magnetization microspheres with an Fe3O4@SiO2 core and perpendicularly
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Acknowledgments
Authors would like to thanks to NFFA for XPS and TEM studies
which performed in France (CEA LETI/DTSI/SCMC Grenoble Cedex 9
FRANCE and thanks to N. GAMBACORTI for organization of XPS and
TEM analysis) within the framework of the NFFA-Europe Transna-
tional Access Activity (NFFA Project ID: 723).
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