A highly efficient and air-, thermal-, and moisture-stable nickel-based catalyst with excellent magnetic properties supported on silica-coated magnetic Fe3O4 nanoparticles was successfully synthesized. It was well characterized by Fourier transform infrared spectroscopy, powder X-ray diffraction, transmission electron microscopy, field emission scanning electron microscopy, thermogravimetric analysis, dynamic light scattering (DLS), X-ray photoelectron spectroscopy, vibration sample magnetometry, energy-dispersive X-ray analysis, inductively coupled plasma analysis, and nitrogen adsorption-desorption isotherm analysis. The Suzuki-Miyaura coupling reaction between aryl carbamates and/or sulfamates with arylboronic acids was selected to demonstrate the catalytic activity and efficiency of the as-prepared magnetic nanocatalyst. Using the mentioned heterogeneous nanocatalyst in such reactions generated corresponding products in good to excellent yields in which the catalyst could easily be recovered from the reaction mixture with an external magnetic field to reuse directly for the next several cycles without significant loss of its activity.
A highly efficient and air-, thermal-, and moisture-stable nickel-based catalyst with excellent magnetic properties supported on silica-coated magnetic Fe3O4 nanoparticles was successfully synthesized. It was well characterized by Fourier transform infrared spectroscopy, powder X-ray diffraction, transmission electron microscopy, field emission scanning electron microscopy, thermogravimetric analysis, dynamic light scattering (DLS), X-ray photoelectron spectroscopy, vibration sample magnetometry, energy-dispersive X-ray analysis, inductively coupled plasma analysis, and nitrogen adsorption-desorption isotherm analysis. The Suzuki-Miyaura coupling reaction between aryl carbamates and/or sulfamates with arylboronic acids was selected to demonstrate the catalytic activity and efficiency of the as-prepared magnetic nanocatalyst. Using the mentioned heterogeneous nanocatalyst in such reactions generated corresponding products in good to excellent yields in which the catalyst could easily be recovered from the reaction mixture with an external magnetic field to reuse directly for the next several cycles without significant loss of its activity.
“Nanotechnology
is the sixth truly revolutionary technology
introduced in the modern world”, D. Allan Bromley (1926–2005).[1] Nanotechnology has potentially changed our life
and insights about it. Having many susceptibilities compared with
other approaches, using nanotechnology is one of the main characteristics
of modern scientific notions for the synthesis of novel structures.[1,2] Nanoparticles (NPs) are interesting primary particles for scientists
because of increased activity, modified structure, and surface area
to volume ratio.[3] Moreover, NPs have many
advanced applications in electronic and optical industries, textile
industries, drug delivery, medicine, and cosmetics.[2−4]Magnetic
NPs (MNPs) are the important groups of functional NPs
that have been broadly pursued owing to their interesting magnetic
properties and attractive potential.[5,6] The study of
such nontoxic and biocompatible particles opens new ways to figure
out their great potential in technological or industrial applications
including medical therapeutics and diagnostics, magnetic data storage,
environmental remediation, hyperthermia-based therapy of cancer, magnetic
resonance imaging (MRI), and catalysis.[7] Although these particles especially Fe3O4 NPs
have many advantages; unfortunately, they tends to aggregate quickly
because of anisotropic dipolar attractions and their applications
face a big challenge.[7] Therefore, using
stabilizers as the shell is vital to control their size increases.[8,9]Among various shells, silica is one of the most trustable
coating
layers for Fe3O4 NPs adding to its chemical
and thermal stability, high persistence in wide ranges of pH values,
biocompatibility, and modified surface.[8−11] Moreover, it enables organic
compounds to be covalently attached to a library of intended NPs for
interesting applications as a heterogeneous catalyst.[12] As a result, magnetic core–shell structures offer
a wide range of applications in optics, catalysis, biomedicine, materials,
environmental science, and energy as the leading edge of modern research
and hot topics.[13,14] Besides, having excellent properties
such as versatility, biocompatibility, controllability, stability,
and being inexpensive, they have attracted outstanding interest.[13−15] Furthermore, they can be easily recovered and reused from the reaction
mixture several times by external magnetic fields resulting in a low
cost and green situation for the reaction progress.[8−16]The carbon–carbon bond-formations are the foremost
challenges
in the synthesis of a wide ranges of natural and artificial products
for pharmaceutical and agrochemical applications.[17,18] In recent decades, the transition-metal cross-coupling reactions
have played a major role in the development of designing efficient
reactions.[19,20] In this regard, a number of highly
reactive metal catalysts have been developed and introduced for cross-coupling
reactions.[21] Moreover, the correlation
of these developments with the principles of green chemistry concepts
is very necessary and important.[22,23] Concerns over
contamination of residual toxic transition metals such as palladium
in products and the high price of this catalyst led scientists to
proliferate the use of economical and safer metal catalysts for such
cross-coupling reactions.[24,25] Since the late twentieth
century when the nickel-catalyzed cross-coupling reaction of aryl
halides and arylboronic acids was reported by Miyaura and co-workers
for the first time, this reaction has attracted a lot of attention
in academic and industrial research and has become one of the most
powerful synthetic tools for the C–C bond formations.[26] Therefore, many efforts have recently been made
to improve the efficiency and performance of this protocol.[27−42] One of the most important achievements of these investigations is
using phenol derivatives (aryl mesylates, tosylates, triflates, phosphates,
sulfamates, carbonates, or carbamates) as the proper and effective
alternatives to aryl halides in the C–C bond formation reactions.[29−35] This achievement is so important because: (a) the aryl halides are
generally not environmentally friendly and using such precursors produces
halides as byproducts leading to environmental pollution, (b) the
preparation of aryl halides often involves tedious steps, wasteful
production processes, and harsh reaction conditions, and (c) the phenol
derivatives are safer substrates, more accessible, and usually can
be prepared on an industrial scale.[29−40] However, this reaction still suffers from several limitations and
drawbacks such as the necessity to employ additional quantities of
ligands such as phosphine and carbenes, the high sensitivity of these
ligands toward air and moisture, having expensive and multistep procedures
for the synthesis of ligands, and high catalyst loading.[41,42]Conventionally, because of the high sensitivity of the Ni(0)
complexes
against air and moisture that is the main catalytic limitation in
such reactions,[43,44] these catalysts generally are
being generated from Ni(II) complexes in the presence of reducing
agents via in situ reactions.[45,46] It should be noted
that the use of reducing toxic agents is another disadvantage of these
methods.[47] Fortunately, some effective
methods without the necessity to apply any toxic reducing agent have
been reported in recent years.[48−52]Therefore, based on the aforementioned considerations and
also
the continuation of our efforts for preparing effective magnetic catalytic
systems,[53−56] herein, we have reported the synthesis, characterizations, and employment
of nickel(II) NPs immobilized on EDTA-modified Fe3O4@SiO2 nanospheres (Fe3O4@SiO2–EDTA–Ni(II)) as a reusable and efficient catalyst
for the Suzuki–Miyaura coupling of various aryl carbamates
and/or sulfamates with arylboronic acids under mild conditions (Scheme ).
Scheme 1
Fe3O4@SiO2–EDTA–Ni(II)
as an Efficient and Recyclable Catalyst for the Suzuki Miyaura Cross-Coupling
Reaction
Results and Discussion
The nickel(II) particles were immobilized onn class="Chemical">EDTA-modified Fe3O4@SiO2 nanospheres (Fe3O4@SiO2–EDTA–Ni(II)) via the multistep
procedure according to the procedure described in Scheme .
Scheme 2
Preparation of the
Fe3O4@SiO2-EDTA-Ni(II)
Nanocatalyst
Subsequently, the
prepared catalyst was well-characterized by the
following instrumental techniques: Fourier transform infrared (FT-IR),
powder X-ray diffraction (XRD), transmission electron microscopy (TEM),
field emission scanning electron microscopy (FE-SEM), dynamic light
scattering (DLS), energy dispersive X-ray (EDX), X-ray photoelectron
spectroscopy (XPS), thermogravimetric analysis (TGA), vibrating sample
magnetometer (VSM), Brunauer–Emmett–Teller (BET), inductively
coupled plasma (ICP), and elemental analysisThe successful
functionalization of the MNP surface was confirmed
by examination of the FT-IR spectra. Figure shows the FT-IR spectra of the Fe3O4@SiO2, Fe3O4@SiO2–NH2, Fe3O4@SiO2–TCT, Fe3O4@SiO2–TCT–NH2, Fe3O4@SiO2–EDTA,
and Fe3O4@SiO2–EDTA–Ni(II)
MNPs. These materials showed broad bands around 3400 and 580 cm–1, which are the characteristic of O–H and Fe–O
stretching bands, respectively.[56,57] In the case of Fe3O4@SiO2 NPs, the sharp band at 1090
cm–1 is corresponded to Si–O–Si asymmetric
stretching vibrations indicating the existence of silica in Fe3O4@SiO2 NPs (Figure a).[58] The characteristic
absorption bands at 2810–2986, 1489, 1123, and 576 cm–1 attributed to C-H (stretching vibration), CH2 (bending),
Si–O–Si (stretching vibration), and Fe–O (stretching
vibration) prove the existence of 3-aminopropyl(triethoxy)silane functional
groups on the surface of the Fe3O4@SiO2 NPs. Furthermore, the weak peaks at about 3300–3400 cm–1 can be ascribed to NH2 stretching vibrations
(Figure b). In the
spectrum of Fe3O4@SiO2–TCT
NPs, the characteristic adsorption at 1711, 1564, and 1511 cm–1 are attributed to C=N stretching vibrations
(Figure c). The peak
at 1091 cm–1 was related to the C–Cl groups
of cyanuric chloride, which is overlapped by the stretching vibrations
of Si–O=Si groups in silica shells (Figure c). The FT-IR spectra of Fe3O4@SiO2–TCT–NH2 NPs were characterized by the following absorption bands: stretching
vibrations of C–N arising at 1257 cm–1, CH2 (bending) at 1451 cm–1, and the C–H
(symmetric and asymmetric stretching vibrations) at 2871–3057
cm–1 (Figure d). As can be seen, the typical absorption peak at 3397 cm–1 indicates the overlapped stretching vibrations of
N–H and O–H bonds (Figure d). According to Figure e, the successful Fe3O4@SiO2–TCT–NH2 surface modification
with EDTA moieties was also verified. In the FT-IR spectra of Fe3O4@SiO2–EDTA (Figure e), basic characteristic vibrations
were observed for C–H bands (asymmetric and symmetric stretching)
at 2885–3070 cm–1, Si–O–Si
asymmetric stretching and symmetric stretching at 1095 and 801 cm–1, respectively, and Fe–O (stretching vibration)
at 578 cm–1. Furthermore, the characteristic bands
of carbonyl groups were observed at 1736 cm–1 (C=O
carboxylic acid stretching vibration) and 1629 cm–1 (C=O amide stretching vibration). Eventually, in terms of
Fe3O4@SiO2–EDTA=Ni(II)
(Figure f), a redshift
of the band at 1736 cm–1 is observed (1736 cm–1 → 1724 cm–1), which is probably
the characteristic of the carbonyl group after interaction with the
nickel ions. Thus, the above results indicate that the functional
groups were successfully grafted onto the surface of the magnetic
Fe3O4@SiO2 NPs.
Figure 1
FT-IR spectra of (a)
Fe3O4@SiO2, (b) Fe3O4@SiO2–NH2, (c) Fe3O4@SiO2–TCT, (d)
Fe3O4@SiO2–TCT–NH2, (e) Fe3O4@SiO2–EDTA,
and (f) Fe3O4@SiO2–EDTA–Ni(II)
NPs.
FT-IR spectra of (a)
Fe3O4@n class="Chemical">SiO2, (b) Fe3O4@SiO2–NH2, (c) Fe3O4@SiO2–TCT, (d)
Fe3O4@SiO2–TCT–NH2, (e) Fe3O4@SiO2–EDTA,
and (f) Fe3O4@SiO2–EDTA–Ni(II)
NPs.
The crystalline structure of MNPs
was identified with the XRD technique.
As illustrated in Figure a, the XRD pattern exhibited reflection peaks at around 2θ
= 30.1, 35.4, 43.1, 53.4, 57.0, and 62.6° that can be indexed
to (220), (311), (400), (422), (511), and (440) crystallographic planes
of cubic Fe3O4 (JCPDS 88-0866).[30] However, the peak positions of the composite sample remain
unchanged, revealing that the crystal phase of magnetic cores is well-maintained
after the coating process. However, the crystallinity of the samples
decreases after the coating process and the catalyst went toward an
amorphous structure (Figure b,c). These results provide further evidence for the successful
functionalization of the nanomagnetic surfaces. Furthermore, the XRD
pattern of Fe3O4@SiO2 shows an obvious
diffusion peak between 10 and 20° that appears because of the
presence of amorphous silica (Figure b). For Fe3O4@SiO2–EDTA–Ni(II) MNPs, the broad peak was transferred to
lower angles due to the synergetic effect of amorphous silica and
the dendrimer polymer (Figure c).
Figure 2
XRD diffraction pattern of (a) Fe3O4, (b)
Fe3O4@SiO2, and (c) Fe3O4@SiO2–EDTA–Ni(II) NPs.
XRD diffraction pattern of (a) Fe3O4, (b)
n class="Chemical">Fe3O4@SiO2, and (c) Fe3O4@SiO2–EDTA–Ni(II) NPs.
The morphologies, particle size, and structural
features of the
magnetic nanomaterials and the synthesized catalyst can be observed
directly from the TEM and FE-SEM images (Figure a–c). The obtained magnetite particles
possess a uniformly spherical shape and a mean diameter of ∼20
nm (Figure a,d). Coating
silica over the Fe3O4 NPs was achieved via the
well-known Stöber method. The TEM image of Fe3O4@SiO2 clearly shows the well-defined core–shell
structure with a shell thickness of ∼10 nm (Figure b). Furthermore, the TEM image
presented in Figure c indicates the structure of Fe3O4@SiO2–EDTA–Ni(II) NPs. After being coated with the
organic layer, the particle size of Fe3O4@SiO2–EDTA–Ni(II) NPs was found to be 20 nm in diameter.
Figure 3
TEM images
of (a) Fe3O4, (b) Fe3O4@SiO2, and (c) Fe3O4@SiO2–EDTA–Ni(II); FE-SEM images of (d)
Fe3O4, (e) Fe3O4@SiO2, and (f) Fe3O4@SiO2–EDTA–Ni(II);
and size distributions of (g) Fe3O4, (h) Fe3O4@SiO2, and (i) Fe3O4@SiO2–EDTA–Ni(II) NPs.
TEM images
of (a) Fe3O4, (b) n class="Chemical">Fe3O4@SiO2, and (c) Fe3O4@SiO2–EDTA–Ni(II); FE-SEM images of (d)
Fe3O4, (e) Fe3O4@SiO2, and (f) Fe3O4@SiO2–EDTA–Ni(II);
and size distributions of (g) Fe3O4, (h) Fe3O4@SiO2, and (i) Fe3O4@SiO2–EDTA–Ni(II) NPs.
The FE-SEM photographs demonstrate that the Fe3O4@SiO2 and Fe3O4@SiO2–EDTA–Ni(II) MNPs are in an almost regular spherical
shape (Figure e,f).
Moreover, these results are consistent with the particle-size distribution
histogram of MNPs, which were in a narrow distribution in the range
of 8–16, 16–24, and 23–37 nm and average size
distribution of 12, 20, and 31 nm for Fe3O4,
Fe3O4@SiO2, and Fe3O4@SiO2–EDTA–Ni(II) NPs, respectively
(Figure g–i).The existence of nickel in the Fe3O4@SiO2–EDTA–Ni(II) catalyst was also confirmed by
the EDX detector coupled to the SEM in which the presence of Fe, Si,
C, N, and O can be confirmed clearly (Figure ). The higher intensity of the Si peak compared
with the Fe peaks indicates that the magnetite NPs were trapped by
silica. According to the above analysis, it can be inferred that the
Fe3O4@SiO2–EDTA–Ni(II)
has been successfully synthesized.
Figure 4
EDX spectrum of the Fe3O4@SiO2–EDTA–Ni(II) NPs.
EDX spectrum of the Fe3O4@n class="Chemical">SiO2–EDTA–Ni(II) NPs.
The TGA of the magnetic nanocatalyst was performed over the
temperature
range of 25–700 °C. As shown in Figure A, the first weight loss for all samples,
which occurred below 150 °C, was attributed to the loss of adsorbed
water molecules on the surface of nanostructured materials, and the
second step occurred between about 150 °C and nearly 600 °C
that is attributed to the decomposition of coating organic layers
in the nanocomposite. As shown in Figure A(d), a significant weight loss of nearly
42.7% in the range of 150–600 °C was observed for the
sample catalyst due to the elimination of organic material over Fe3O4@SiO2 NPs.
Figure 5
(A) TGA spectrum of (a)
Fe3O4@SiO2–NH2,
(b) Fe3O4@SiO2–TCT, (c) Fe3O4@SiO2–TCT–NH2, and (d) Fe3O4@SiO2–EDTA–Ni(II)
NPs; (B) magnetic hysteresis loops of (a) Fe3O4 and (b) Fe3O4@SiO2–EDTA–Ni(II)
NPs; (C) good dispersity and easy separation of the catalyst by an
external magnetic field.
(A) TGA spectrum of (a)
Fe3O4@n class="Chemical">SiO2–NH2,
(b) Fe3O4@SiO2–TCT, (c) Fe3O4@SiO2–TCT–NH2, and (d) Fe3O4@SiO2–EDTA–Ni(II)
NPs; (B) magnetic hysteresis loops of (a) Fe3O4 and (b) Fe3O4@SiO2–EDTA–Ni(II)
NPs; (C) good dispersity and easy separation of the catalyst by an
external magnetic field.
To study the magnetic
properties of MNPs, the hysteresis loops
of magnetite and functionalized magnetite NPs at room temperature
were investigated by using a VSM. As shown in Figure B, no hysteresis was observed in the hysteresis
loops of three materials, and the remanence and coercivity were nearly
zero, exhibiting typical superparamagnetic behavior. The magnetizations
of Fe3O4 and Fe3O4@SiO2–EDTA–Ni(II) MNPs were 64.8 and 28.7 emu/g,
respectively (Figure B). The silica shell and other organic compounds cause a reduction
in the magnetic strength of the composite owing to the weight contribution
from the nonmagnetic portion. Nevertheless, Fe3O4@SiO2–EDTA–Ni(II) possesses excellent magnetic
responsibility and suitable magnetization values, which can quickly
respond to the external magnetic field and quickly disperse again
when the external magnetic field is removed (Figure C). These results exhibit the good magnetic
properties of the nanocomposite, which is an advantage in our catalytic
system for separation.N2 adsorption–desorption
isotherms were obtained
to investigate the porous structure and surface area of the NPs. The
measured specific surface areas were 480, 430.3, and 392.6 m2/g for n class="Chemical">Fe3O4, Fe3O4@SiO2, and Fe3O4@SiO2–EDTA–Ni(II),
respectively (Table ). Also, the particle sizes of magnetite calculated using the Scherrer
equation were 11.33, 12.64, and 14.97 nm for Fe3O4, Fe3O4@SiO2, and Fe3O4@SiO2–EDTA–Ni(II), respectively
(Table ).
Table 1
Selected Properties of Fe3O4, Fe3O4@SiO2, and Fe3O4@SiO2–EDTA–Ni(II) NPs
sample
Fe3O4 crystal structure
specific
surface area (m2/g)a
magnetite
particle size (nm)b
Fe3O4
cubic spinel
480
11.33
Fe3O4@SiO2
cubic spinel
430.3
12.64
Fe3O4@SiO2–EDTA–Ni(II)
cubic spinel
371.6
14.97
Calculated by the
BJH method.
Calculated by
the Scherrer equation
based on XRD patterns.
Calculated by the
BJH method.Calculated by
the Scherrer equation
based on XRD patterns.Elemental
analyses for Fe3O4@n class="Chemical">SiO2–NH2, Fe3O4@SiO2–TCT,
Fe3O4@SiO2–TCT–NH2, and Fe3O4@SiO2–EDTA–Ni(II)
were carried out, and the data are tabulated in Table which are in good agreement with the result
obtained from TGA. These obtained results displayed that the contents
of C, H, and N for Fe3O4@SiO2–EDTA–Ni(II)
are 21.61, 2.87, and 9.232%, respectively.
Table 2
TGA and
Elemental Analysis for Fe3O4@SiO2–NH2, Fe3O4@SiO2–TCT, Fe3O4@SiO2–TCT–NH2, and Fe3O4@SiO2–EDTA–Ni(II)
NPs
sample
C (%)
H (%)
N (%)
total (%)a
Fe3O4@SiO2–NH2
TGA (wt %)
6.746
1.501
2.627
10.874
EA (wt %)
6.614
1.475
2.532
10.621
Fe3O4@SiO2–TCT
TGA (wt %)
7.377
0.717
5.736
3.830
EA (wt %)
7.468
0.750
5.627
13.845
Fe3O4@SiO2–TCT–NH2
TGA (wt %)
16.172
2.918
10.564
29.654
EA (wt %)
15.946
2.871
10.377
29.194
Fe3O4@SiO2–EDTA–Ni(II)
TGA (wt %)
22.821
2.853
9.112
34.786
EA (wt %)
21.611
2.871
9.232
33.714
Total (%) = C (%) + H (%) + N (%).
Total (%) = C (%) + H (%) + N (%).Additionally, Ni loading of the
catalyst was confirmed by the ICP
analyzer. For this purpose, the catalyst (1 g) was stirred in aq.
HCl (37%), then the magnetic nanocomposite was separated by an external
magnetic field and the remaining solution was analyzed by ICP to determine
the content of nickel. The amount of Ni on the support was determined
as (0.55) mmol per gram of the catalyst.After the preparation
and detailed characterization of Fe3O4@SiO2–EDTA–Ni(II), the efficiency
of the synthesized nanocatalyst on cross-coupling reactions to form
C–C bonds was evaluated. To find the effective reaction conditions,
the model reaction between phenyl carbamate (A1) with
phenylboronic acid (B1) using the proposed catalytic
system was investigated (Table ). Various factors such as solvents, bases, catalyst loadings,
temperature, and reaction time were tested, and the results have been
depicted in Table . The solvent evaluation for the model reaction showed that using
ethylene glycol (EG) as the solvent presents the highest yield (Table , entry 4). Moreover,
the desired product, C1 was not observed when no base was added (Table , entry 15). This
experiment showed that the presence of a base is crucial to have an
efficient reaction. Therefore, we decided to investigate the series
of organic and inorganic bases that are commonly used in the coupling
reactions (Table ,
entries 4, 7–14). Based on these experiments, the best efficiency
and performance were observed in the presence of EG monosodium salt
(NaOC2H4OH) as the base (Table , entry 4). Also, it seems that the reaction
efficiency depends on the amount of NaOC2H4OH
and the highest yields were observed when its amount was 2 mmol (Table , entry 4). It should
be noted that higher amount of the base did not present better yields
of the desired product (Table , entries 18, 19). To improve the yield of the cross-coupling
reaction, we also checked the catalyst loading (Table S1), temperature (Table S3), and reaction time (Table S3) on the
model reaction and the best results have been obtained using 0.02
g of Fe3O4@SiO2–EDTA–Ni(II)
catalyst (1 mol % Ni(II)) at 100 °C after 6 h (Table , entry 4).
Table 3
Optimization Studies for Cross-Coupling
of Phenyl Carbamate with Phenylboronic Acida
Reaction conditions:
phenyl carbamate
(1 mmol), n class="Chemical">phenylboronic acid (1 mmol), base, Fe3O4@SiO2–EDTA–Ni(II) catalyst(0.018 g, 1 mol
%), solvent, 6 h
Isolated
yield.After getting the
optimized conditions, we tested different phenol-based
electrophiles in Ni-catalyzed Suzuki–Miyaura cross-coupling
reactions to determine the high proficiency and generality of this
catalytic system (Table ). Although the usage of aryl methyl ether did not lead to the production
of the desired product under the optimized reaction conditions (Table , entry 1), moderate
yields were observed when the phenol derivatives such as phenyl acetate,
phenyl pivalate, phenyl mesylate, phenyl tosylate, and phenyl triflate
were used as the electrophile (Table , entries 2–6). Accordingly, phenyl carbonate
furnished the desired product in good yields (Table , entry 7). Finally, it was determined that
the corresponding carbamate and sulfamate substrates introduced the
coupling product in the highest yield (Table , entries 8 and 9). Thus, we elected the
aryl carbamates and sulfamates as the suitable electrophiles for continuing
the research.
Reaction conditions:
phenol-based
electrophiles(1 mmol), n class="Chemical">phenylboronic acid (1 mmol), NaOC2H4OH (2.0 mmol), Fe3O4@SiO2–EDTA–Ni(II) catalyst (0.018 g, 1 mol %), EG (3 mL),
100 °C, 6 h.
Isolated
yield.The optimal reaction
conditions were identified and then, we decided
to investigate the scope, generality, and efficiency of this method
for C–C bond formation. At first, we started from the treatment
of a series of aryl carbamates and sulfamates in the reaction with
phenylboronic acid (Table ). Satisfyingly, both electron-donating and withdrawing groups
on the phenyl ring of aryl carbamates and sulfamates efficiently produced
the desired products in good to excellent yields (Table ). It should be noted that a
lower yield was observed in the case of ortho-substituted aryl carbamates
and sulfamates in comparison to meta- and para-substituted ones, which
might be due to steric factors (Table , entries C4 and C9). It is also noteworthy that the
naphthyl carbamates and sulfamates display high reactivity leading
to the corresponding cross-coupled biaryl products in high yields
(Table , entries C5
and C6). Although pyridinyl and pyrimidinyl derivatives generally
deactivate the transition-metal catalysts, especially in the nickel-based
catalysts, our system was also suitable for these kinds of substrates
and the desired coupling products were produced in significant yields
(Table , entries C16–C18).
Table 5
Substrate Scope of Aryl Carbamates
and Sulfamatesa
Reaction
conditions: aryl carbamates
or sulfamates(1 mmol), phenylboronic acid (1 mmol), NaOC2H4OH (2.0 mmol), Fe3O4@SiO2–EDTA–Ni(II) catalyst (0.018 g, 1 mol %), EG (3 mL),
100 °C, 6 h.
Isolated
yield.
Reaction
conditions: aryl carbamatesn class="Chemical">or sulfamates(1 mmol), phenylboronic acid (1 mmol), NaOC2H4OH (2.0 mmol), Fe3O4@SiO2–EDTA–Ni(II) catalyst (0.018 g, 1 mol %), EG (3 mL),
100 °C, 6 h.
Isolated
yield.Inspired from interesting
results achieved, we also decided to
examine the effects of different substituents on the phenyl ring of
aryl boronic acids under the optimal reaction conditions (Table ). Arylboronic acids
bearing various functional groups were compatible with this reaction
system and the corresponding products were obtained in good to excellent
yields (Table , entries
C19–C33). However, the precursors with an electron-withdrawing
group are somehow less effective than other arylboronic acids under
these reaction conditions (Table , entries C26–C29). It is noteworthy that in
the case of 2-methylphenylboronic acid, lower yield was obtained and
it may be due to steric hindrance (Table , entry C22). Interestingly, we found that
the heterocyclic boronic acids such as thiophen-2-ylboronic acid also
worked efficiently under this catalytic system (Table , entries C30–C33).
Table 6
Substrate Scope of Arylboronic Acidsa
Reaction conditions: aryl carbamates
or sulfamates(1 mmol), aryl boronic acids (1 mmol), NaOC2H4OH (2.0 mmol), Fe3O4@SiO2–EDTA–Ni(II) catalyst (0.018 g, 1 mol %), EG (3 mL),
100 °C, 6 h.
Isolated
yield.
Reaction conditions: aryl carbamatesn class="Chemical">or sulfamates(1 mmol), aryl boronic acids (1 mmol), NaOC2H4OH (2.0 mmol), Fe3O4@SiO2–EDTA–Ni(II) catalyst (0.018 g, 1 mol %), EG (3 mL),
100 °C, 6 h.
Isolated
yield.According to the
reported research in the literature, we proposed
a plausible catalytic mechanism for the Suzuki–Miyaura coupling
of aryl carbamates or aryl sulfamates with arylboronic acids (Scheme ).[59−62] Initially, the process begins
with the reduction of Ni(II) with EG as the reducing agent to provide
the active Ni(0) species. Subsequently, the catalytic cycle starts
with the oxidative addition of Ni(0) to aryl carbamates or aryl sulfamates
to generate the intermediate, (1) in situ. The transmetalation
step occurs by the conversion of intermediate (1) to
a nucleophilic intermediate, (2) in the presence of the
base EG monosodium salt (NaOC2H4OH). This complex
subsequently reacts with an organoboron compound which facilitates
the aryl group transfer to reach the diaryl intermediate, (3). Finally, the formation of a C–C bond through the reductive
elimination step generates the Ni (0) catalyst.
Scheme 3
Proposed Mechanism
of Suzuki–Miyaura Cross-Coupling of Aryl
Carbamates and Aryl Sulfamates
To determine the oxidation state of nickel, high-resolution XPS
spectra of Ni 2p core levels were obtained from the catalyst before
and after the reaction (Figure ). According to the fitted data, the deconvoluted peaks at
the binding energies 855.9 and 861.8 eV are attributed to Ni 2p3/2 and peaks at 872.2 and 879.8 eV are attributed to Ni 2p1/2 for the fresh catalyst that can be indexed to Ni2+ (Figure a).[63] Besides, the XPS patterns of the recovered catalyst
show the peaks of both Ni2+ and Ni(0) (Figure b). The peaks at around 852.4
and 872.2 eV were assigned to the Ni 2p3/2 and Ni 2p1/2 levels in the Ni(0) which are in good agreement with the
literature report.[64] The above phenomena
supported that the reaction proceeded via the traditional Ni2+/Ni(0) cycle mechanism.
Figure 6
XPS spectra of the (a) fresh and (b) reused
catalyst.
XPS spectra of the (a) fresh and (b) reused
catalyst.The recyclability and high stability
of the economically and eco-friendly
catalytic systems are very important in the industry and designing
green and effective synthetic pathways. Therefore, we investigated
these important factors in our catalyst. First, the reusability of
the catalyst in the model reaction was studied and tested (Figure a). After completion
of the model reaction, the catalyst was easily separated by using
an external magnetic field, washed twice with ethanol, and dried in
an oven. Then, the recovered catalyst was used for the next runs with
the same substrates. As shown in Figure a, the recovered Fe3O4@SiO2–EDTA–Ni(II) exhibited almost constant
catalytic activity for at least six runs in the model reaction and
significant reduction in catalytic efficiency was not observed. Additionally,
the TEM micrographs for the MNPs after the sixth cycle are displayed
in Figure b. As revealed
in this figure, almost all Fe3O4@SiO2–EDTA–Ni(II) particles have the morphology and size
the same as the fresh catalyst, indicating that the aggregation of
NPs is venial. Eventually, the hydrodynamic diameter of the catalyst
was investigated by the DLS technique (Figure c) in which this size distribution is centered
at a value of 33 nm.
Figure 7
(a) Reusability of the catalyst for the Suzuki–Miyaura
cross-coupling;
(b) TEM and (c) DLS images of Fe3O4@SiO2–EDTA–Ni(II) after the sixth recycling experiment.
(a) Reusability of the catalyst for the Suzuki–Miyaura
cross-coupling;
(b) TEM and (c) DLS images of Fe3O4@n class="Chemical">SiO2–EDTA–Ni(II) after the sixth recycling experiment.
Additionally, the catalyst after the last run was
investigated
by ICP analysis to determine the amount of nickel leaching. Accordingly,
the amount of loaded nickel on the recovered catalyst was measured
to be 0.54 mmol/g. Propitiously, the ICP analysis after the seventh
run showed less than 1% nickel leaching. Moreover, to determine the
stability of the catalytic systems in the model reaction and the responsibility
of nickel moiety for carrying out the model reaction, a hot filtration
test was performed. When the reaction time of the model reaction reached
the half time of reaction quenching, the catalyst NPs were taken out
from the reaction mixture by an external magnetic field, the residue
was allowed to stir under the reaction conditions. The monitoring
of the reaction mixture by TLC did not show any considerable progress.
These results showed that only a few species of nickel may exist in
the solution phase and the main responsible species that catalyzes
the model reaction is the Fe3O4@SiO2–EDTA–Ni(II) NPs. All of these data confirmed the high
stability and reusability of the catalyst under these reaction conditions.According to the wonderful results obtained, the application of
the presented efficient strategy was also checked out for the preparation
of biphenyl (C1) in different scales (1, 50, and 200
mmol). The obtained results are presented in Table in which the capability of this method was
proved to find efficient large-scale laboratory syntheses with the
yield of trials at small scales.
Table 7
Comparison of Cross-Coupling
of Phenyl
Carbamate or Sulfamate with Phenylboronic Acid at Different Scales
Isolated yield.
Isolated yield.In the end, we decided to compare the capability of the proposed
nanocatalyst in the synthesis of aryl carbamates and sulfamates with
those reported in the literature (Table ). As can be seen, almost all reports have
applied toxic solvents with more reaction times and temperatures.
In addition, they showed less reactivity against such reactions and
also, fewer yields have been observed (Table , entries 1–4, 7–9). Although
there are lower reaction times reported (Table , entries 5, 6), the observed yields are
somehow less than the present work. Moreover, the reaction conditions
for these are harsher. From the point of view of the recovery and
reusability of the catalyst, this catalyst can be recovered and reused
for consequent reactions but other reports applied the catalyst with
no recovery ability.
Table 8
Comparison with Reported
Results for
Suzuki–Miyaura Cross-Coupling Reactions between Phenylboronic
Acid and 4-Methoxyphenyl Carbamates or Sulfamates
In summary, we have
proposed a facile and effective method to prepare
a magnetic nanocatalyst (Fe3O4@SiO2–EDTA–Ni(II)) with characterization by various techniques.
The catalyst demonstrated great performance in the Suzuki–Miyaura
coupling reaction of aryl carbamates and/or sulfamates with arylboronic
acids under mild reaction conditions. The methodology complements
the more established methods without using external ligands or reducing
agents. Also, a wide range of substrate scope was applied, even the
presence of electron-rich substrate. Additionally, the catalyst can
be easily recovered in a very short time using an external magnet
and directly reused for six cycles with no significant loss in catalyst
activity. Considering the improved economic and environmental benefits,
this methodology was presented here to show its attractive features
in using sulfamate and carbamate substrates as highly regarded precursors,
especially in the pharmaceutical industry.
Experimental Section
Chemicals
and Instrumentation
Chemical materials with
high purity were purchased from Fluka and Merck. The reaction monitoring
was accomplished by thin-layer chromatography (TLC) on gel F254 plates.
Melting points were obtained with a micro melting point apparatus
(electrothermal, BUCHI 510). NMR spectra were recorded in CDCl3 on a Bruker Avance DPX 250 spectrometer using TMS as the
internal standard. FT-IR spectra were collected on a Shimadzu FT-IR
8300 spectrometer using KBr pellets. XRD patterns were recorded in
a Bruker AXS D8-advance powder X-ray diffractometer using Cu Kα
radiation. The TEM images were recorded using a Philips EM208 transmission
electron microscope operated at 80 kV accelerating voltage. FE-SEM
characterization was performed on a Hitachi S-4160 operated at a 20
kV accelerating voltage. DLSs were performed using a HORIBA-LB550.
The surface composition was investigated using XPS on XR3E2 (VG Microtech)
spectrometer using a Mg and Al twin anode X-ray gun with a multichannel
detector and a hemispherical analyzer with a resolution of 1.0 eV.
The BET surface area of the material was measured by the nitrogen
adsorption isotherm method (BET; Micromeritics ASAP 2000). The magnetic
properties of Fe3O4 MNPs and Fe3O4@SiO2–EDTA–Ni magnetic nanocomposites
were analyzed using the MDK instrument model 7400 VSM. The TGA was
performed on a NETZSCH STA 409 PC/PG instrument. The nickel amount
on the carriers was measured by ICP–atomic emission spectrometry
(ICP–AES). The elemental analyses (C, H, N) were obtained using
a Flash EA-1112 CHNSO analyzer. Therefore, all products were identified
by comparison of their spectral data and physical properties such
as 1H and 13C NMR, CHNS, and melting points
with those of the authentic sample and all yields refer to isolated
products.
Synthesis of Silica-Coated MNPs (Fe3O4@SiO2NPs)
Magnetic (Fe3O4) NPs were synthesized according to our previous reports.[54−56] In brief, FeCl3·6H2O (1.3 g, 4.8 mmol),
FeCl2·4H2O (0.9 g, 4.5 mmol), and poly(vinyl
alcohol) (PVA 15,000, 1 g) as the surfactant were dissolved in 30
mL of deionized water with vigorous mechanical stirring at 80 °C
for 30 min. Then, hexamethylenetetramine (1.0 mol) was slowly added
to the solution and the solution pH was maintained at 10. The mixture
was held at 80 °C in a water bath for 2 h with constant stirring
and then, the black magnetite was collected by applying an external
magnetic field and was rinsed with ethanol several times and dried
under vacuum at 80 °C for 10 h. In the next step, for the synthesis
of Fe3O4@SiO2, 0.5 g of the synthesized
Fe3O4 NPs were dispersed in a mixture containing
ethanol (50 mL), deionized water (5 mL), and tetraethoxysilane (0.20
mL) in a glass reactor by using ultrasound irradiation.[55−58] Then, NaOH (10 wt %, 5 mL) was added to the mixture and stirred
at room temperature for 30 min. The resulting Fe3O4@SiO2 NPs were collected by an external magnet,
washed with distilled water and ethanol, and dried in a vacuum oven
at 80 °C for 10 h.
Synthesis of Fe3O4@SiO2–NH2NPs[58]
To a suspension
of SiO2-coated magnetite particles (1 g) in ethanol (10
mL), 3-aminopropyl(triethoxy)silane (1 mmol, 0.25 mL) was added. The
mixture was refluxed for 12 h, and then, it was cooled to room temperature
to obtain a brown precipitate. The obtained solid was separated with
an external magnet, washed with water and ethanol (1:1) to remove
unreacted species, and dried under vacuum at 80 °C.
Synthesis of
Fe3O4@SiO2–TCT
NPs
In a typical procedure, 1 g of the as-prepared Fe3O4@SiO2–NH2 NPs was
first dispersed in 10 mL of THF containing 1 mmol (0.185 g) cyanuric
chloride (TCT) and 1 mmol (0.17 mL) of diisopropylethylamine (DIPEA).
Then, the obtained crude was stirred at room temperature for 16 h.
Next, the modified NPs by cyanuric chloride were collected by magnetic
field and washed 3 times with distilled water and ethanol solution.
Finally, the resultant was dried at 60 °C for 4 h.
Synthesis of
Fe3O4@SiO2–TCT–NH2 NPs
In a typical preparation procedure, bis(3-aminopropyl)amine
(2 mmol, 0.25 mL) and DIPEA (2 mmol, 0.35 mL) were added to DMF (5
mL) containing cyanuric chloride-immobilized Fe3O4@SiO2–NH2NPs (1.0 g). After stirring
at 80 °C for 12 h, the precipitate (Fe3O4@SiO2–TCT–NH2) was collected
from the solution using a magnet, washed with water and ethanol several
times, and dried at 70 °C for 4 h.
Synthesis of Fe3O4@SiO2–EDTA–Ni(II)
NPs
First, EDTA (2 mmol) was dispersed inDMSO (15 mL). Then,
the solution of thionyl chloride (SOCl2) (2 mmol) in DMSO
(5 mL) was added dropwise into the solution under vigorous stirring.
Afterward, Fe3O4@SiO2–TCT–NH2 (1.5 g) was quickly added and the mixture was stirred or
2 h at room temperature. The mixture was then separated by an external
magnet, washed with deionized water, sodium carbonate solution (0.1
mol L–1), and acetone and dried at 60 °C to
give Fe3O4@SiO2–EDTA NPs.
To synthesis the Fe3O4@SiO2–EDTA–Ni
NPs, 1.5 g of the as-prepared Fe3O4@SiO2–EDTA nanocomposite and 1 g of nickel(II) acetate tetrahydrate
(4 mmol) were dispersed in 10 mL of THF by ultrasonication and stirred
for 4 h under reflux conditions. Finally, the products (Fe3O4@SiO2–EDTA–Ni NPs) were collected
by a magnet, washed with water and ethanol several times, and dried
at 60 °C overnight.
EG Monosodium Salt
According to
the approach reported
previously,[70] 300 g of EG (4.83 mol) and
176 g of NaOH (4.39 mol) were first mixed in xylenes (3 L). Then,
the mixture was stirred under reflux conditions with a Dean–Stark
trap until H2O separation was finished. Next, the obtained
mixture was cooled to room temperature and the precipitate was filtered,
washed with xylenes (3 × 100 mL) and t-BuOMe
(3 × 125 mL), dried in vacuo, and finally, stored under argon.
General Procedure for Suzuki–Miyaura Reactions Using
Fe3O4@SiO2–EDTA–Ni(II)
NPs
Aryl carbamate and/or aryl sulfamate (1 mmol), arylboronic
acid (1 mmol), sodium monoethylene glycolate (0.17 g, 2 equiv), Fe3O4@SiO2–EDTA–Ni(II) NPs
(0.018 g, 1 mol % Ni(II)), and EG (3 mL) were added to a 10 mL round-bottom
flask. The flask was heated in an oil bath at 100 °C for an appropriate
time with stirring under nitrogen atmosphere. The progress of the
reaction was monitored by TLC analysis. After the completion of the
reaction, the crude was cooled to room temperature and then the catalyst
was separated with an external magnet, washed with ethanol, dried
under vacuum, and used directly for the subsequent reaction runs.
The organic layer was washed with water to get product precipitates
and subsequently, the precipitates were separated by filtration and
washed with distilled water 3 times. For more purification in some
cases, the impure product (checked by TLC) was extra purified by column
chromatography (petroleum ether/ethyl acetate = 10:1 (v/v)) to afford
the pure product.
Scheme 1
Scheme 2
Figure 1
Figure 2
Figure 3
Figure 4
Figure 5
Scheme 3
Figure 6
Figure 7