Silica-coated magnetic nanoparticles functionalized cobalt complex: a recyclable and efficient catalyst for the C?C bond formation

Article abstract of DOI:10.1007/s11164-021-04522-7

In this study, the Co-based catalyst was prepared by cobalt immobilization on the surface of functionalized silica-coated magnetic NPs (Fe₃O₄@SiO₂-CT-Co) as a magnetically core–shell nanocatalyst and characterized by FT-IR, TGA, XRD, VSM, SEM, TEM, EDX, EDX mapping, and ICP techniques and appraised in the Suzuki–Miyaura cross-coupling reaction under mild reaction conditions. The results displayed the superparamagnetic behavior of the Fe₃O₄ NPs core encapsulated by SiO₂ shell, and the size of the particles was estimated about 30?nm. Compared with the previously reported catalysts, the engineered Fe₃O₄@SiO₂-CT-Co catalyst provided perfect catalytic performance for the Suzuki–Miyaura cross-coupling reaction in water as a green solvent and it was much cheaper in the comparison with the traditional Pd-based catalysts. Importantly, the durability of magnetic nanocatalyst was studied and observed that it is stable under the reaction conditions and could be easily reused for at least six successive cycles without any significant decrease in its catalytic activity. Graphic abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s11164-021-04522-7

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-021-04522-7
Silica‑coated magnetic nanoparticles functionalized cobalt
complex: a recyclable and efcient catalyst for the C−C
bond formation
Hanif Haqjow1 · Farveh Raouf2
Received: 5 May 2021 / Accepted: 10 June 2021
© The Author(s), under exclusive licence to Springer Nature B.V. 2021
Abstract
In this study, the Co-based catalyst was prepared by cobalt immobilization on the
surface of functionalized silica-coated magnetic NPs (Fe3O4@SiO2-CT-Co) as
a magnetically core–shell nanocatalyst and characterized by FT-IR, TGA, XRD,
VSM, SEM, TEM, EDX, EDX mapping, and ICP techniques and appraised in the
Suzuki–Miyaura cross-coupling reaction under mild reaction conditions. The results
displayed the superparamagnetic behavior of the Fe3O4 NPs core encapsulated by
SiO2 shell, and the size of the particles was estimated about 30 nm. Compared with
the previously reported catalysts, the engineered Fe3O4@SiO2-CT-Co catalyst pro-
vided perfect catalytic performance for the Suzuki–Miyaura cross-coupling reaction
in water as a green solvent and it was much cheaper in the comparison with the
traditional Pd-based catalysts. Importantly, the durability of magnetic nanocatalyst
was studied and observed that it is stable under the reaction conditions and could be
easily reused for at least six successive cycles without any signifcant decrease in its
catalytic activity.
*
Farveh Raouf
fraouf@yahoo.com
1
2
Department of Chemistry, Faculty of Education, Ghor Institute of Higher Education, Firozkoh,
Afghanistan
Department of Chemistry, Ilam Branch, Islamic Azad University, Ilam, Iran
Vol.:(0123456789)
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H. Haqjow, F. Raouf
Graphic abstract
Keywords Cobalt · Cross-coupling reaction · Heterogeneous catalysis · Magnetic
nanoparticles · Suzuki–Miyaura reaction
Introduction
One of the most prominent C(sp2)−C(sp2) coupling reactions that has been con-
sidered as a potent synthetic procedure is a Suzuki–Miyaura cross-coupling reac-
tion that is used to constitute the unsymmetric biaryl derivatives from the reaction
between boronic acids and aryl halides [1–8]. Some reasons that led to broadly
implement Suzuki–Miyaura reaction are the usage of this C–C bond formation reac-
tions in many felds such as pharmaceuticals, agrochemicals, natural products, and
chemical industries [9–13].
During the last decades, miscellaneous transition metal catalysts especially pal-
ladium-based catalysts have been used to construct various organic and bioorganic
molecules via Suzuki–Miyaura cross-coupling reaction. Since palladium catalysts
have some drawbacks like air sensitivity, high cost, and toxicity, eforts have been
highly focused on designing efective transition metal catalysts including cobalt
[14–17], copper [18–20], nickel [21–23], and zinc [24, 25] instead of using catalysts
based on expensive metals like palladium [26–28]. Among all of the transition metal
catalysts used in carbon–carbon coupling reactions, cobalt has gained great attention
due to its precious properties such as eco-friendly, low cost, nontoxicity, and good
availability; therefore, many studies have shown the implementation of catalytic sys-
tems based on cobalt with the usage of various linkers in carbon–carbon coupling
reactions [16, 29, 30].
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Silica‑coated magnetic nanoparticles functionalized cobalt…
Heterogeneous catalysts have played an important role in many catalyst systems
because of their potential applications [31–42]. Among these, magnetite nanoparti-
cles (MNPs) and especially Fe3O4 showed that they possess valuable features such
as easy separation and recycling by using an external magnetic feld to avoid wasting
catalyst after the end of the reaction [43–48]. Typically, magnetic nanoparticles are
sensitive against air moisture, so they have been coated with diferent organic and
inorganic layers like silica, polymers, and noble metals to ameliorate the chemical
stability of nanoparticles. Silica (SiO2) is a useful inorganic layer which is used not
only for coating the surface of the nanoparticles but also for modifying with various
organic ligands [49–54].
As a part of our ongoing work to design the efcient heterogeneous catalyst
with eco-friendly methods [55], we prepared the silica-coated magnetic Fe3O4 NPs
(Fe3O4@SiO2) with excellent magnetic features and inexpensive procedures accord-
ing to the reported literature[19]. In the following step, the creatine amino acid as an
organic ligand was added to the ethanoic mixture of Fe3O4@SiO2 to aford Fe3O4@
SiO2-CT. Finally, we produced the Fe3O4@SiO2-CT-Co by adding aqueous solution
of CoCl2 to the ethanoic mixture of Fe3O4@SiO2-CT to apply as a reusable catalyst
in a Suzuki–Miyaura cross-coupling reaction. Scheme 1 has shown the preparation
strategy of prepared silica-coated magnetic nanoparticles Fe3O4@SiO2-CT-Co.
Experimental
Material
All the materials used in this research have been purchased from Merck or Sigma-
Aldrich without any further purifcation.
Synthesis of catalyst (Fe3O4@SiO2‑CT‑Co)
The magnetic heterogeneous nanocatalyst has been fabricated according to
Scheme 1. For this purpose, initially, the silica-coated MNPs (Fe3O4@SiO2) were
prepared through the method presented in the literature [56], and next, the cre-
atine organic ligand was grafted on the silica-coated MNPs and then followed by
Scheme 1 Synthesis of catalyst (Fe3O4@SiO2-CT-Co)
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H. Haqjow, F. Raouf
immobilization of cobalt complex. On this basis, the Fe3O4@SiO2 (0.1 g) was soni-
cately dispersed in EtOH (40 mL) at room temperature. After 30 min, a solution
of creatine organic ligand (0.2 g) in deionized water (20 mL) was added droop-
ingly to the previously prepared mixture of magnetic mixture, and then, they were
refuxed with stirring for 6 h. Then, the magnetic Fe3O4@SiO2-CT was separated
from the mixture by an external magnet and washed with EtOH and H2O many
times, respectively. Afterward, the prepared modifed MNPs were dried in a vac-
uum oven at 60 °C for 24 h. Finally, in order to synthesis the last catalyst, Fe3O4@
SiO2-CT MNPs (0.25 g) were sonicately dispersed in EtOH (20 mL) for 20 min.
Subsequently, a solution of cobalt acetate (0.5 M) in 5 mL of EtOH was added to
the previous mixture and refuxed. After 6 h, the prepared Co-coordinated catalyst
(Fe3O4@SiO2-CT-Co) was separated from the reaction mixture by an external mag-
net, and then washed successively with EtOH (3×5 mL), and then dried at 60 °C for
24 h to obtain the Fe3O4@SiO2-CT-Co catalyst.
General procedure for the C–C bond formation catalyzed by Fe3O4@SiO2‑CT‑Co
To a solution of phenylboronic acid derivatives (1.2 mmol), aryl halides (1.0 mmol),
KOH (2.5 mmol), and water (3.0 ml), 1.5 mol % of Fe3O4@SiO2-CT-Co was added
at 75 °C with stirring under oil bath condition. After the fnalization of the reac-
tion, the Co-coordinated MNPs catalyst (Fe3O4@SiO2-CT-Co) was separated from
the reaction mixture and washed with EtOH and H2O and, fnally, dried at 60 °C in
a vacuum oven for 12 h. Subsequently, the water as a green solvent was separated
from the reaction mixture by rotary evaporator and the prepared Suzuki products
have been obtained in pure products with good to excellent isolated yields. Some
selected products have been characterized with H and C NMR, and the data have
been added in supporting material section.
Result and discussion
At the preliminary stage, the successful synthesis of Fe3O4@SiO2-CT-Co was
evaluated using some spectroscopic and microscopic techniques including FT-IR,
TGA, XRD, VSM, SEM, TEM, EDX, EDX mapping, and ICP. In the following of
our previous studies and based on the successful preparation of the Fe3O4@SiO2-
CT-Co, the catalytic capability of the aforesaid nanocatalyst was studied in the
Suzuki–Miyaura cross-coupling reaction of diferent aryl halides with various aryl
boronic acids without using any palladium, under green solvent (H2O) conditions.
The FT-IR analysis of Fe3O4 (a), Fe3O4@SiO2 (b), CT (c), and Fe3O4@SiO2-
CT-Co (d) is shown in Fig. 1. As it is evident from Fig. 1a, related to the Fe3O4
MNPs, the broad absorption peak was observed at 3348 cm−1 that can be related
to the O–H stretching bands of hydroxyl functional groups on the surface of
MNPs. The broad and sharp peak at 559 cm−1 is assigned to the Fe–O bonds of
MNPs [57]. As it is evident from Fig. 1b, allocated to the silica-coated Fe3O4
MNPs, the existence of the new bonds at 1101 and 935 cm−1 is attributed to the
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Silica‑coated magnetic nanoparticles functionalized cobalt…
Fig. 1 FT‐IR spectra of a Fe3O4 b Fe3O4@SiO2 c Fe3O4@SiO2-CT d Fe3O4@SiO2-CT-Co
asymmetric and symmetric vibrations of Si–O-Si bond, respectively. The broad
absorption band at 3350 cm−1 approves the attendance of the stretching mode of
the surface-attached hydroxyl groups (ν O–H) on the surface of silica NPs [56].
In the FT-IR spectrum of CT (Fig. 1c), three broad absorption peaks at around
3200, 3000, and 2900 cm−1 are assigned to the stretching vibrations of NH2, OH,
and CH groups, respectively. Besides, the observed absorption peaks at 1418
and 1672 cm−1 are allocated to the bending and stretching vibrations of OH and
C = O groups, respectively [58]. Furthermore, it is worth mentioning that there
are all creatine peaks in the FT-IR spectrum of Fe3O4@SiO2-CT-Co (Fig. 1d)
including the absorption peaks related to NH2, OH, and CH groups at around
2900 to 3200 cm−1. Notably, a blue shift of C = O peak is observed from 1672 to
1655 cm−1 that approves the proper complexation of cobalt on the CT ligand and
successful synthesis of the Fe3O4@SiO2-CT-Co [59].
The phase purity and crystal structure of the bare Fe3O4 (a) and Fe3O4@SiO2-
CT-Co (b) are clearly corroborated via surveying the X-ray powder difraction
(XRD) patterns (Fig. 2). As shown in Fig. 2, the Fe3O4 and Fe3O4@SiO2-CT-Co
have eight distinct peaks at 2Ɵ = 30.27, 35.74, 36.10, 43.27, 53.78, 57.29, 62.87,
and 71.50 that they are assigned to the lattice planes of cubic Fe3O4 MNPs (2
1 3

H. Haqjow, F. Raouf
Fig. 2 XRD pattern of a Fe3O4 and b fnal catalyst (Fe3O4@SiO2-CT-Co)
2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), (4 4 0), and (620) (JCPDS card
No. 98–007-7842) [60]. It is worth noting that in the XRD pattern of Fe3O4@
SiO2-CT-Co, the difraction peaks corresponding to cobalt ions have not been
observed in the fnal catalyst, in agreement with the high cobalt dispersion [61].
The absence of Co difraction peaks indicates the formation of fnely dispersed
cobalt species after immobilization on the surface of modifed SiO2 that are not
detectable by this technique (as cobalt loading is lower than 5 wt% according to
ICP results) [19, 62, 63].
The size and morphology of the Fe3O4@SiO2-CT-Co nanocatalyst were also
studied by the scanning electron microscopy (SEM) technique. As clearly shown
in Fig. 3a, the Fe3O4@SiO2-CT-Co nanocatalyst exhibits highly dispersed uni-
form spherical morphology with an average size of about 35–45 nm. To explore
more details concerning the morphology and structure of the core–shell Fe3O4@
SiO2-CT-Co, transmission electron microscopy (TEM) analysis was employed as
well (Fig. 3b). It was obviously observed in Fig. 3b that the Fe3O4 NPs core is well
encapsulated by the modifed SiO2 coating with a clear boundary between the silica
shell and the Fe3O4 NPs core. As shown in Fig. 3b, the particle sizes of Fe3O4@
SiO2-CT-Co nanocatalyst in irregular geometric shape were approximately 35 nm,
which is very close to the particle size as determined using SEM analysis.
Moreover, to explore the elemental composition uniformity of the Fe3O4@SiO2-
CT-Co, the EDX mapping and EDX analysis are conducted and the results are
shown in Figs. 4 and 5, respectively. As shown in these fgures, the simultaneous
existence of Fe, O, Si, C, N, and Co elements with homogeneous distribution on the
entire surface of the catalyst is well proved.
To investigate the thermal and structural stability and also determine the con-
tent of organic materials of the Fe3O4@SiO2-CT-Co, thermal gravimetric analysis
(TGA) was performed under an air atmosphere at a heating rate of 10 °C min−1 from
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Silica‑coated magnetic nanoparticles functionalized cobalt…
Fig. 3 SEM a and TEM b images of catalyst (Fe3O4@SiO2-CT-Co)
0 to 800 °C (Fig. 6). The as-synthesized catalyst exhibits two mass loss steps. The
frst low mass loss, which eventuated in the temperature range from 50 to 150 °C, is
responsible for the removal of the physically adsorbed H2O and O–H groups on the
surface Fe3O4@SiO2 (about 6% weight loss). The second weight loss (about 20%
weight loss) in the region between 150 and 500 °C can be attributed to the decom-
position of grafted creatine and acetate (related to cobalt acetate) molecules on the
surface of Fe3O4@SiO2 nanoparticles. In comparison with the literature [64], it can
be concluded that creatine showed higher thermal stability due to interaction with
Fe3O4@SiO2 nanoparticles.
Moreover, the ICP analysis revealed that the overall coordinated cobalt content
on the surface of Fe3O4@SiO2-CT MNPs in the freshly prepared catalyst was 4.47%
which confrms the successful coordination of cobalt on the surface of catalyst and
creatine organic ligand.
Next, the magnetic properties of catalyst have been investigated by VSM analysis
at room temperature, as shown in Fig. 7. Based on these studies, the saturation mag-
netization of prepared catalyst Fe3O4 was 80 emu/g that showed superparamagnetic
properties.
After full characterization of Fe3O4@SiO2-CT-Co as a new, reusable, and hetero-
geneous catalyst, its catalytic activities have been studied toward the Suzuki reaction
of various aryl halides with diferent aryl boronic acids (Scheme 2).
Secondly, in order to evaluate the optimum reaction conditions, the reaction
of iodobenzene with phenylboronic acid in the presence of the KOH (2.5 mmol)
and water (as solvent) was chosen as a model reaction by taking into account sev-
eral important parameters such as solvent, base, temperature, and catalyst loading
(Table 1). As expected, the control experiment indicated that no biaryl product
was obtained without the catalyst (at 75 °C), and the reaction did not proceed even
after a long time running the reaction (12 h) (Table 1, entry 1). Then, the infu-
ence of the catalyst loading has been investigated on the selected reaction (Table 1,
1 3

H. Haqjow, F. Raouf
Fig. 4 EDX mapping analysis of Fe3O4@SiO2-CT-Co
entries 2–5). After a few attempts, it was found that 0.02 g (1.5 mol%) of the nano-
catalyst was required to achieve the maximum yield of the desired product (96%
after 30 min) (Table 1, entry 3). A higher amount of the catalyst 0.04 g (3.0 mol%)
did not have any efect on the product yield or the reaction rate (Table 2, entry 2),
whereas a signifcant decrease in the reaction progress was observed by applying
0.01 g (0.75 mol%) of the catalyst (Table 1, entry 2). In the next experiments, the
efect of the common solvents including toluene, DMF, 1,4-dioxane, EtOH, CH3CN,
and solvent-free conditions has been investigated in the model Suzuki reaction.
Among them, the H2O as green solvent was found to be the suitable medium for
obtaining a reasonable isolated yield of the cross-coupled (Table 1, entries 6–11).
To improve the product yield and enhance the reaction rate, the model reaction was
performed in the presence of diferent bases comprising the KOH, NaOAc, Na2CO3,
K2CO3, K3PO4, and Et3N (Table 1, entries 12–16). As can be seen, among the bases
1 3

Silica‑coated magnetic nanoparticles functionalized cobalt…
Fig. 5 EDX analysis of Fe3O4@SiO2-CT-Co
Fig. 6 TGA plot of catalyst (Fe3O4@SiO2-CT-Co)
mentioned, KOH was found to be highly efcient in the preparation of the corre-
sponding coupling product in H2O at 75 ˚C (Table 1, entry 3). After this, the efect
of the content of KOH was surveyed in the model reaction. The cross-coupled prod-
uct was efectively produced when using 2.5 mmol of the base (Table 1, entries
3 and 17–19). Subsequent experiments identifed the efect of temperature in the
Suzuki cross-coupling reaction, which showed that the best yield was obtained at 75
˚C (Table 1, entries 3 and 20–22).
In the next step, a diversity of the substrate has been analyzed in the presence of
the Fe3O4@SiO2-CT-Co under the optimized reaction conditions for the synthesis of
biaryl derivatives. Tables 2 exhibited that the cross-coupling reaction of aryl iodides
1 3

H. Haqjow, F. Raouf
Fig. 7 VSM plot of catalyst
(Fe3O4@SiO2-CT-Co)
Scheme 2 Suzuki cross-coupling reaction in the presence of catalyst (Fe3O4@SiO2-CT-Co)
with phenylboronic acid is faster than that of aryl bromides or aryl chlorides due
to the lower C-I bond strength vs. C–Br and C–Cl bonds (Table 2, 3a-3e). Besides,
diferent electron-poor and electron-rich aryl halides were exploited for analyzing
the Suzuki reaction. Basically, it is crystal clear that the aryl halides containing elec-
tron-withdrawing groups (such as -CHO or -NO2) compared with electron-rich aryl
halides (such as -OMe or -Me) aforded the desired coupled-products with excel-
lent yields in shorter reaction time. Also, heteroaryl halides such as 2-iodopyridine,
2-bromopyridine 2-iodothiophene, and 2-bromothiophene were also good partner
for this coupling reaction (Tables 2, 3j and k). Moreover, the 4-methyl phenylbo-
ronic acid has been examined with diferent aryl halides (Table 2, entries 3 l-n). The
results were similar to the phenylboronic acid, however, the yield of the reaction has
been decreased a little.
According to previously reported Suzuki–Miyaura mechanisms in the literature
[16], and based on our knowledge, a proposed mechanism for the Suzuki–Miyaura
cross-coupling reaction was depicted in Scheme 3. The active form of Co is gener-
ally believed to be CoI in C–C cross-coupling reactions. When CoII precursors are
applied, reduction to CoI by base reagents (as KOH) is observed [65]. According
to the literature reports on the reduction of CoII to CoI by base reagents [65] and
acceleration of this process by ligands that can stabilize the CoI oxidation state, the
following plausible mechanism might be proposed. On this basis, frstly, the CoII
complex reduction is initiated by KOH and then CoI complex is converted to CoIII
intermediate (1) through oxidative addition of the aryl halide. In the following step,
the deprotonation of the phenyl boronic acid under basic conditions by KOH is
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Silica‑coated magnetic nanoparticles functionalized cobalt…
Table 1 Optimization of diferent conditions for the C–C bond formation
Entry Catalyst (g,
mol%)
Aryl iodide/
Base
Base
Condition
Tem. (˚C) Time (min) Yield (%)a
1
2
3
4
5
6
7
8
–
1.0/2.5
1.0/2.5
1.0/2.5
1.0/2.5
1.0/2.5
1.0/2.5
1.0/2.5
1.0/2.5
1.0/2.5
1.0/2.5
1.0/2.5
1.0/2.5
1.0/2.5
1.0/2.5
1.0/2.5
1.0/2.5
1.0/1.0
1.0/2.0
1.0/3.0
1.0/2.5
1.0/2.5
1.0/2.5
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
KOH
NaOAc H2O
Na2CO3 H2O
K2CO3 H2O
K3PO4 H2O
Et3N
KOH
KOH
KOH
KOH
KOH
KOH
H2O
H2O
H2O
H2O
75
75
75
75
75
Refux
12 h
45
30
30
30
60
60
60
60
60
35
30
30
30
30
30
60
30
30
60
30
30
–
0.01 (0.75)
0.02 (1.5)
0.03 (2.25)
0.04 (3.0)
0.02 (1.5)
0.02 (1.5)
0.02 (1.5)
0.02 (1.5)
0.02 (1.5)
0.02 (1.5)
0.02 (1.5)
0.02 (1.5)
0.02 (1.5)
0.02 (1.5)
0.02 (1.5)
0.02 (1.5)
0.02 (1.5)
0.02 (1.5)
0.02 (1.5)
0.02 (1.5)
0.02 (1.5)
65
96
96
96
Trace
66
45
51
76
82
42
65
70
73
44
79
90
96
51
82
96
H2O
Toluene
1,4-Dioxane Refux
DMF
CH3CN
EtOH
Solvent-free 100
Refux
Refux
Refux
9
10
11
12
13
14
15
16
17
18
19
20
21
22
75
75
75
75
75
75
75
75
rt
H2O
H2O
H2O
H2O
H2O
H2O
H2O
50
100
aIsolated Yield
responsible for the formation of intermediate (2) via the insertion of phenyl boronic
acid into intermediate (1). At last, the desired coupled product was obtained through
the reductive elimination reaction of intermediate (2) by regenerating the active CoI
catalyst for the next catalytic cycle.
Considering the environmental and economic factors, and also the principles of
green chemistry, the long-term durability experiment of the prepared catalyst has
been investigated. For this purpose, the model reaction was carried out under the
optimized reaction conditions to test the reusability behavior of the Fe3O4@SiO2-
CT-Co (Fig. 8). Briefy, after the fnalization of each run of the cross-coupling reac-
tion, which was monitored by thin-layer chromatography, the reaction mixture was
cooled to room temperature and the heterogeneous catalyst has been separated using
an external magnet, washed with EtOH and H2O, and dried under vacuum at 60 °C
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H. Haqjow, F. Raouf
Table 2 Synthesis of diferent biaryl derivatives in presence of catalysta
aDiferent aryl halides (1.0 mmol), phenyl boronic acid (1.2 mmol) in presence of (0.02 g, 1.5 mol%) of
catalyst in H2O at 75 °C
bTurn over number (Yield of the reaction/mol%)
cTurn over frequency (TON/time of the reaction)
Table 3 The comparison of our catalyst with other metal-based catalysts for the C–C bond formation
Entry Catalyst (mol%)
Solvent
Temp. (ºC) Time (min) Yield (%) Reference
1
2
3
4
5
6
7
Pd/Ni graphene
Cu/Carbon
Cu nano-colloid
Et2Zn, DMEDA
Co-His@MNPs/Ch
Fe3O4@SiO2-EP–AG–Pd
DMF/H2O 110
30
78
95
62
98
97
99
92
[66]
[67]
[68]
[24]
[69]
[70]
[71]
H2O
DMF
THF
PEG
H2O
50
110
80
80
50
180
360
360
120
120
360
Fe3O4@SiO2/APTES/IMMPh@ DMF
140
Ni
8
9
10
Ni-porphyrin graphene oxide
Fe3O4@GPTMS-AG Cu
Fe3O4@SiO2-CT-Co
Dioxane
Solvent-free 80
H2O 75
80
60
30
30
95
97
96
[72]
[19]
This work
overnight to use in the next run. As shown in Fig. 8, no signifcant decrease in the
yield of the reactions was observed after even six runs. These results confrmed the
high catalytic activity and long-term durability of the Fe3O4@SiO2-CT-Co in the
C–C cross-coupling reaction.
The hot fltration test was another analysis to approve the heterogeneous nature
of the Fe3O4@SiO2-CT-Co in the C–C bond formation reaction. On this basis, the
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Silica‑coated magnetic nanoparticles functionalized cobalt…
Scheme 3 Proposed mechanism for the C–C bond formation in the presence of catalyst
96
96
95
94
92
90
First
Second
Third
Fourth
Fi h
Sixth
Isolated Yield (%)
Fig. 8 Catalyst reusability test
model reaction was studied again under the optimized reaction condition. After
15 min (50% conversion), the Fe3O4@SiO2-CT-Co was removed from the reac-
tion using an external magnet. Afterward, the rest of the reaction was stirred in
the absence of catalyst for a further 15 min. Observation showed that the reaction
did not intend to move forward and confrmed that no leaching of complex and
cobalt ions was happened during the reaction (Fig. 9). Moreover, the amount of
Co in the separated catalyst was also investigated by ICP technique. According
to the ICP results, it was 4.42% that shows a negligible decrease and approves
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H. Haqjow, F. Raouf
Fig. 9 Time-dependent correlation of the product yield in hot fltration test
approximately no leaching of cobalt was happened during the C–C cross-coupling
reaction and it is in agreement with hot fltration test observations.
Finally, to evaluate the performance of the present catalytic activity in the
Suzuki–Miyaura cross-coupling reaction, the current protocol was compared with
some previously reported catalysts in the C–C cross-coupling reaction (Table 3).
Undoubtedly, all of the listed catalysts in Table 3 signifcantly can produce the
desired product in good to excellent yields. But, it can be found that the Fe3O4@
SiO2-CT-Co as a magnetic heterogeneous nanocatalyst is much superior to almost
all of the methods presented in Table 3, in terms of the high temperatures (Table 3,
entries 1, 3, and 7), hazardous solvents (Table 3, entries 1, 3, 4, 7, and 8), long reac-
tion times to obtain the desired product (Table 3, entries 2–7), as well as price and
toxicity are the disadvantages of some of these methods (Table 3, entries 1, 6).
Moreover, the newly synthesized nanocatalyst with high catalytic efciency can be
easily separated from the reaction mixture by using an external magnet.
Conclusion
In the present study, core–shell Fe3O4@SiO2-CT-Co was described as a sustaina-
ble magnetic nanostructured catalyst in a C–C bond formation of aryl halides with
phenyl boronic acid in the water as a green solvent. This method has the privileges
including high yields of coupled-products, environmental acceptability, short reac-
tion time, excellent functional group compatibility, and easy experimental and work-
up procedures. Interestingly, the unique stable nanostructured catalyst could be eas-
ily separated by using an external magnetic from the reaction mixture and reused
at least six times with trace cobalt leaching and without a signifcant reduction in
the yields of the desired products as well. The present work can be considered as
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Silica‑coated magnetic nanoparticles functionalized cobalt…
an attractive alternative to the currently reported methods based on Pd catalysts for
C–C cross-coupling reactions. In our opinion, this efcient method is able to open
a new front for designing and modifcation of environmentally friendly, renewable,
low cost, and high-performance nanostructured catalysts with a well-controlled
morphology for other cross-coupling reactions in mild conditions.
Supplementary Information The online version contains supplementary material available at https://doi.
org/10.1007/s11164-021-04522-7.
Acknowledgements The authors are grateful to the Ghor Institute of Higher Education (Afghanistan) and
Islamic Azad University of Iran, Ilam Branch.
Declarations
Confict of interest The author declared that there is no confict of interest.
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