Imidazolium chloride-Co(iii) complex immobilized on Fe3O4@SiO2 as a highly active bifunctional nanocatalyst for the copper-, phosphine-, and base-free Heck and Sonogashira reactions

Article abstract of DOI:10.1039/c8gc03919d

A heterogeneous, magnetically recoverable Fe₃O₄@SiO₂@Im[Cl]Co(iii)-melamine nanocomposite was prepared by immobilization of a novel Co(iii) Schiff base complex on Fe₃O₄@SiO₂ nanoparticles followed by treatment with melamine, and was found to be an efficient catalyst for the Heck and Sonogashira reactions. The reactions were performed in the presence of the catalyst (0.5 mol% Co) along with Mn additive in the absence of any base, phosphine ligand, or Cu/co-catalyst in ethanol under reflux conditions. The nanocatalyst was well studied by FTIR, CHN, XRD, XPS, TGA, EDX, VSM, ICP, NMR, FE-SEM, TEM, BET, CV, and DLS analyses. The catalyst was compatible with a variety of substrates, with which all the Heck and Sonogashira coupling products were obtained in high to excellent yields. Also, protocols such as hot filtration, three-phase testing, and mercury poisoning provided a complete insight into the nature of the heterogeneous catalyst. The recycling and reuse of the catalyst were studied for both coupling reactions several times. Moreover, the mechanism of the coupling reactions was entirely investigated.

Full text of DOI:10.1039/c8gc03919d

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Imidazolium chloride-Co(III) complex immobilized
on Fe3O4@SiO2 as a highly active bifunctional
nanocatalyst for the copper-, phosphine-, and
base-free Heck and Sonogashira reactions†
Cite this: DOI: 10.1039/c8gc03919d
Milad Kazemnejadi, *a Seyyedeh Ameneh Alavi G.,a Zinat Rezazadeh,a
Mohammad Ali Nasseri,a Ali Allahresania and Mohsen Esmaeilpourb
A heterogeneous, magnetically recoverable Fe3O4@SiO2@Im[Cl]Co(III)-melamine nanocomposite was
prepared by immobilization of a novel Co(III) Schiff base complex on Fe3O4@SiO2 nanoparticles followed
by treatment with melamine, and was found to be an efficient catalyst for the Heck and Sonogashira
reactions. The reactions were performed in the presence of the catalyst (0.5 mol% Co) along with Mn
additive in the absence of any base, phosphine ligand, or Cu/co-catalyst in ethanol under reflux con-
ditions. The nanocatalyst was well studied by FTIR, CHN, XRD, XPS, TGA, EDX, VSM, ICP, NMR, FE-SEM,
TEM, BET, CV, and DLS analyses. The catalyst was compatible with a variety of substrates, with which all
the Heck and Sonogashira coupling products were obtained in high to excellent yields. Also, protocols
such as hot filtration, three-phase testing, and mercury poisoning provided a complete insight into the
nature of the heterogeneous catalyst. The recycling and reuse of the catalyst were studied for both
coupling reactions several times. Moreover, the mechanism of the coupling reactions was entirely
investigated.
Received 14th December 2018,
Accepted 8th February 2019
DOI: 10.1039/c8gc03919d
rsc.li/greenchem
Csp).8 The traditional Sonogashira reaction utilizes CuI as a co-
catalyst, which converts the alkyne into a copper acetylide
Introduction
In the early 1970s, palladium-catalyzed reactions were intro- in situ. However, it also induces homo-coupling reactions and
duced by Negishi, Kumada, and Suzuki, and subsequently subsequently produces symmetrical bis-aryl ethynes by-pro-
various homogeneous and heterogeneous Pd-catalyzed systems ducts. Therefore, research is ongoing toward the introduction
were developed.1–3 The Heck cross-coupling reaction has of copper-free protocols.
become one of the most applicable for Csp2–Csp2 bond for-
On the other hand, the high cost and toxicity of palladium
mation involving C(sp2) centers in the presence of palladium complexes along with the need for expensive and sensitive
catalysts in organic synthesis,4 and allows the manufacture of phosphine ligands are some of the limitations that have per-
complex molecules from simple precursors. The Heck reaction suaded scientists to seek to find efficient alternative method-
is widely used in the agrochemical, pharmaceutical, and fine ologies with less expensive metals. To solve these problems,
chemical industries.5–7
the discovery and development of different methodologies are
The Sonogashira reaction also belongs to the family of pal- necessary. In the past decade, many studies have been
ladium-catalyzed carbon–carbon coupling reactions, i.e., cross- reported about the use of alternative transition metals, such as
coupling of a vinyl or aryl halide and a terminal alkyne (Csp2
–
Ni,9 Cu,10 Fe,11 and Co,12 as more convenient, sustainable,
cost-effective, and safe approaches for cross-coupling
reactions.13
Among these, due to their high catalytic activity, low cost,
and easy preparation with readily accessible materials, Co com-
plexes have received considerable attention. Cobalt-catalyzed
coupling reactions are of interest in organic synthesis because
of the notable properties of cobalt metal, including it being
readily available, non-toxic, low-cost, stable, and having power-
ful catalytic properties.8,12 Various attempts have been devel-
aDepartment of Chemistry, Faculty of Science, University of Birjand,
P. O. Box 97175-615, Birjand, Iran. E-mail: miladkazemnejad@yahoo.com,
miladkazemnejadi@birjand.ac.ir
bDepartment of Chemistry, College of Science, Shiraz University, Shiraz 7194684795,
Iran
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c8gc03919d
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oped to extend Co-based catalysts to make a suitable readily factors, we developed an efficient protocol for the C–C cross-
available alternative catalytic system compared with Pd cata- coupling reaction of aryl halides with olefins and acetylenes by
lytic systems. Co-Catalyzed C–C coupling reactions efficiently introducing a new magnetically recoverable Fe3O4@SiO2@ Im
eliminate the β-H elimination that accompanies the processes [Cl]Co(III)-melamine nanocomposite in a highly efficient and
in Pd- and Ni-catalyzed systems as a by-product. Furthermore, selective manner under mild conditions. The present method
they have also demonstrated advantages in the coupling reac- is phosphine-/Pd-/Cu-/base-free and contains an imidazolium
tion of alkyl halides,6 which is problematic and difficult to moiety that plays the role of a base to overcome the above-men-
achieve with other transition metals.
tioned limitations regarding Pd complexes or the obstacles
Previously, Gosmini et al. and Cahiez et al. individually regarding homogeneous catalytic systems.
reviewed a wide range of Co-catalyzed cross-coupling reac-
tions.13,14 In the last decade, various Co-catalyzed systems have
been reported for C–C cross-coupling reactions, such as
CoCl2(dpph)/Me3SiCH2MgCl for the alkenylation of alkyl
Results and discussion
Catalyst characterization
halides, intramolecular Heck-type reactions and the Heck-type
reaction of alkyl halides with styrenes,5,15,16 Co-methyl
salicylate@MNPs/chitosan for Heck and Sonogashira coup-
lings,3 CoBr2/Zn for the arylzincation of alkynes,12 CoBr2/Bpy/
Mn for the vinylation of functionalized aryl halides,17 Co NPs
for the Heck reaction,18 Fe3O4@Boehmite-NH2-Co(II) for
Suzuki and Heck cross-coupling reactions,19 and Co complex/
blue LEDs for the cyclization coupling of alkyl iodides with
alkenes.20 Recently, Maity et al.4 reported a Co-catalyzed dehy-
drogenative Heck reaction of allylic alkenylation with internal
aliphatic olefins.
The elemental analysis results for the samples is tabulated in
Table 1. The results are in good agreement with theoretical cal-
culations and confirmed the preparation of compounds 1, 2, 3,
4. Also, the results are in accordance with the proposed struc-
ture sketched in Scheme 7, as well as the previously reported
structure in the literature.26
Fig. 1 shows the FTIR spectra of 1, 2, 3, 4, 5, 6, 7, 8, and 9
compounds. The FTIR absorption spectrum of 1 shows two
characteristic peaks at 1481 cm−1 and 725 cm−1, corres-
ponding to the methylene groups (–CH2–) in the meta-substi-
However, most of these approaches incur high cost and
energy and also involve harsh reaction conditions, such as high
temperature and long reaction times. So, finding a novel Co-
based catalyst that can overcome to these limitations is desired.
Nowadays, the utilization of efficient nanoparticles for cata-
lytic goals is a smart strategy, and they can significantly
promote catalytic activity due to their high surface to volume
ratio. In this way, they are usually coated with various
polymers, especially silicate, to avoid agglomeration.21,22
This simple strategy allows the efficient functionalization of
nanoparticles for a specific goal. Unfortunately, catalytic
systems based on NPs suffer from tedious work-up, agglomera-
tion, contamination, etc. For this purpose, magnetic nano-
particles were introduced as a heterogeneous magnetically
recoverable solid support in organic synthesis22–25 as they
could completely resolve the catalyst separation problems. The
most prominent and applicable of these magnetic NPs is
Fe3O4 NPs.
Herein, in accordance with green chemistry considerations
as well as an insight into the economic and environmental
Fig. 1 FTIR spectra of (a) 1, (b) 2, (c) 3, (d) 4, (e) 5, (f) 6, (g) 7, (h) 8, and
(i) 9.
Table 1 Results of the elemental analysis and physical properties of the compounds
Elemental analysis
%C
Compounds
Appearance
%H
%N
2-Hydroxy-5-chloromethyl benzaldehyde (1)
2-Hydroxy-5-imidazole benzaldehyde (2)
4-((1H-Imidazol-1-yl)methyl)-2-((allylimino)methyl)phenol (3)
Co(III) Schiff base complex (4)
Purple powder
56.30 (56.82)a
65.74 (65.34)
69.47 (69.69)
64.80 (64.34)
4.14 (4.25)
5.09 (4.98)
6.22 (6.27)
5.02 (5.23)
—
Dark brown powder
Pale yellow powder
Brown powder
13.97 (13.85)
17.64 (17.41)
15.32 (15.58)
a Theoretically calculated.
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tution and the stretching vibration of C–Cl, respectively
(Fig. 1a).25 The FTIR spectrum of 2 represents a series of peaks
at 2490–2780 cm−1 related to the imidazole moiety, which are
in agreement with compounds containing imidazole moieties
reported in the literature (Fig. 1b).27 Moreover, elimination of
the C–Cl bond at 725 cm−1 in the spectrum of 1 corroborates
the imidazole functionalization of 5-chloromethylene salicylal-
dehyde. Stretching vibration related to the N–H of imidazole
moiety appeared at 3413 cm−1. Imine bond formation in 3 was
confirmed by the presence of a CvN bond at 1611 cm−1
(Fig. 1c).28 This absorption was shifted to 1565 cm−1 due to Co
complex formation. This shift, along with Co–O and Co–N
stretching vibrations at 504 cm−1 and 615 cm−1, respectively,
confirmed the successful coordination of Co ions through N
and O in 4 (Fig. 1d). Fe3O4 NPs showed a single characteristic
peak at 570 cm−1 related to Fe–O bond29 (Fig. 1e). The pres-
ence of three main peaks at 779, 1072, and 3433 cm−1 corres-
pond to the stretching vibrations of Fe–O, Si–O–Si (symm.),
and Si–O–Si (asymm.), respectively, strongly confirming the
coating of the Fe3O4 NPs by silica shells (Fig. 1f).30 Fig. 1g
demonstrates the vibrations attributed to C–H stretching.,
–CH2– bending, and C–Cl stretching at ∼2920, 1458, and
633 cm−1, respectively, which strongly support the preparation
of Fe3O4@SiO2-Cl (Fig. 1g). The linkage of complex 4 to
Fe3O4@SiO2-Cl was proved by the presence of CvN, Si–O–Si,
Fe–O, and CvC bonds correspond to the wavenumbers at
1560, 1018, 405, and 1414 cm−1, respectively (Fig. 1h). Fig. 1i
shows the spectrum of the catalyst 9 prepared from the interca-
Fig. 2 XRD patterns of (a) Fe3O4, (b) Fe3O4@SiO2, (c) Fe3O4@SiO2-Cl,
(d) Fe3O4@SiO2@Im[Cl]Co(III)-melamine nanocomposite.
lation of melamine to the Fe3O4@SiO2@Im[Cl]Co(III)-Schiff melamine nanocomposite (Fig. 2d),29 clearly manifesting that
base complex (8). The elimination of CvC stretching bonds at the surface modification of the Fe3O4 NPs does not lead to
1407 cm−1 in the FTIR spectrum of 9 (Fig. 1i) represents the their phase change or structure. The broadening of the peaks
intercalation of melamine into the catalyst framework. The shows that Fe3O4@SiO2 NPs were functionalized with the
peaks at 1445 and 1540 cm−1 could be attributed to the quad- amorphous Co(III) Schiff base complex and revealed the suc-
rant stretching of the 1,3,5-s-triazine ring, and semicircle cessful preparation of the nanocomposite. Also, the broad
stretching of the 1,3,5-s-triazine ring, respectively.31 Also, the peak at 2θ = 20° was assigned to the amorphous Co(III) Schiff
absence of the peaks associated with the N–H (primary) base complex moiety (Fig. 2d). Beside the other analyses, the
stretching and bending in the catalyst spectrum was further size of the nanoparticles was calculated using Scherrer’s
evidence of the insertion of melamine into the catalyst equation: D = Kγ/β cos θ, where K is a constant (K = 0.9 for
framework.
Cu–Kα), D is the average diameter in Å, β is the broadening of
XRD spectra for the synthesized nanoparticles are shown in the diffraction line measured at half of its maximum intensity
Fig. 2. As presented in Fig. 2, Fe3O4 nanoparticles have a in radians, γ is the wavelength of the X-rays, and θ is the Bragg
highly crystalline cubic inverse spinel structure, which is well diffraction angle.
in agreement with the standard Fe3O4 (cubic phase) XRD spec-
The electrochemical activity of the catalyst was studied at
trum (PDF#88-0866, reference JCPDS card no. 19-629).32 The room temperature in BR buffer solution in the potential range
characteristic diffraction peaks at 2θ = 30.1°, 35.4°, 43.1°, of −2.0 to 2.0 V.33 As shown in Fig. 3A, the Co(III) complex
53.4°, 57°, and 62.6°, and could be marked, respectively, by demonstrated two reversible peaks corresponding to CoI/CoII
their indices to the (220), (311), (400), (422), (511), and (440) and CoII/CoIII couples.34–36 The first redox peak was assigned
planes, demonstrating the Fe3O4 structure.28,29
to the CoIII/CoII, with Epa1 = 0.5 V, Epc1 = 0.32 V, and E1/2
=
These peaks were also observed for Fe3O4@SiO2 and 0.09 V. The second redox peak corresponded to the CoII/CoI
Fe3O4@SiO2-Cl, which confirmed the inverse spinel Fe3O4 NPs couples with Epa2 = −0.97 V, Epc2 = −1.23 V, and the average
in their structures. The XRD patterns of Fe3O4@SiO2 and formal potential equal to E1/2 = −1.1 V. These results suggest a
Fe3O4@SiO2-Cl showed an obvious diffusion peak at 2θ = two-step redox reaction with a reversible mechanism.
15–25°, representing the coated amorphous silica on the Fe3O4
The elements in the catalyst were probed by EDX analysis
NPs (Fig. 2b and c).32 The presence of peaks corresponding to (Fig. 3B). Regarding Fig. 3B, the EDX spectrum confirmed all
the Fe3O4 structure as well as the amorphous silica peak the elements present in catalyst 9, including iron, carbon,
appeared in the XRD pattern of the Fe3O4@SiO2@Im[Cl]Co(III)- nitrogen, silicon, chlorine, cobalt, and oxygen.
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Fig. 3 (A) Cyclic voltammogram of catalyst 9 in 0.1 mol L−1 Britton–Robinson (BR) buffer solution (pH 7.0) with a scan rate of 100 mV s−1 at room
temperature; (B) EDX spectrum of catalyst 9; (C) magnetization curves for (a) Fe3O4, (b) Fe3O4@SiO2 (c) Fe3O4@SiO2@Im[Cl]Co(III)-Schiff base
complex (8), and (d) Fe3O4@SiO2@Im[Cl]Co(III)-melamine nanocomposite (9), at room temperature; (D) TGA curves of (a) Co(III) Schiff base complex
4, (b) Fe3O4@SiO2@Co(III) Schiff base complex (8), and (c) catalyst 9.
The results of the magnetic properties of the samples are nanocomposite. As shown in Fig. 3C, c, catalyst 9 exhibited a
shown in Fig. 3C. These NPs exhibited a high permeability in much better thermal stability than 8. Under a nitrogen atmo-
magnetization that was sufficient to be able to separate them sphere, they showed
a 41% weight loss, including the
with an external magnetic field. The magnetization and decomposition of water and organic moieties on the surface of
demagnetization curves were coincident and no hysteresis the core–shell. Thermal analysis of the catalyst initially showed
phenomenon was found.
a negligible weight loss (2.2%) in the region of 90–275 °C,
As shown in the figure, the remnant magnetization for all which showed the loss of hydrogen-bonded water molecules
NPs is equal to zero. Magnetic measurements showed satur- present at the surface of the Fe3O4 core–shell. The second
ation magnetization (Ms) values of 70.0, 39.8, 35.9, and 33.1 small weight loss (3.0%) occurred between 280 °C and 480 °C
emu g−1 for Fe3O4, Fe3O4@SiO2, and the Fe3O4@SiO2@Im[Cl] and was associated with the removal of water trapped from the
Co(III)-Schiff base complex (8), and the Fe3O4@SiO2@Im[Cl]Co lattice.38 The weight loss of about 35% at the temperature
(III)-melamine nanocomposite (9), respectively (Fig. 3C, a, b range of 475–750 °C was assigned to the decomposition of
and c). The results demonstrated that the magnetization of organic moieties on the surface of the Fe3O4@SiO2 core–shell
Fe3O4 was considerably decreased due to the coated silica-shell nanoparticles.
or Co(III)-Schiff base complex on its surface (Fig. 3C, b and c).
XPS analysis was applied in order to elucidate the chemical
Nevertheless, the Fe3O4@SiO2@Im[Cl]Co(III)-melamine nano- elements of the catalyst. The low-resolution (wide scan) XPS
composite exhibited superparamagnetic characteristics with spectra for the Fe3O4@SiO2@Im[Cl]Co(III)-melamine nano-
high magnetization values, which meant it could be readily composite is shown in Fig. 4(a). Fig. 4a confirms the presence
separated from the mixture by the use of a simple external of Fe, Si, O, Co, and Cl elements in the catalyst. Also, the pres-
magnet. As shown in Fig. 3C, c and d, treatment of the ence of Cl− ions as the imidazolium counter ion was detected
Fe3O4@SiO2@Im[Cl]Co(III)-Schiff base complex (8) with mela- as a sharp peak at 287 eV,39,40 indicating the presence of the
mine caused a drop in magnetization from 35.9 to 33.1 emu imidazolium moiety in the catalyst. Fig. 4b represents the Co
g−1 for 9. The thermal behavior of the samples was studied by 2p XPS spectrum for the Fe3O4@SiO2@Im[Cl]Co(III)-melamine
TGA (Fig. 3D). The TGA curve of the Co-Schiff base complex 4 nanocomposite. The Co 2p1/2 and Co 2p3/2 lines appeared at
exhibited only one weight loss step in the temperature range of 797 and 782.3 eV, respectively. These results indicated that
98–345 °C with a continuous weight loss (Fig. 3C, a). This Co(III) is the only component in the catalyst and the cobalt
weight loss may arise from the decomposition of the Co-Schiff ions are in +3 oxidation state. Also, the energy band gap of
base ligand to cobalt oxides as well as to the CO and CO2 14.7 eV is in good agreement with the literature reports on the
species produced by thermal decomposition of the oxygenated Co3+ XPS spectra. The results were completely fitted with the
carbon surface groups (Fig. 3C, b).37 The incorporation of mel- reported Co(III) 2p XPS spectrum.41–43
amine to 9 clearly revealed the effect of melamine on the
The specific surface area and porosity of the samples were
thermal behavior of the Fe3O4@SiO2@Im[Cl]Co(III)-melamine investigated by N2 adsorption–desorption isotherm analysis.
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Fig. 4 (a) Overall survey and (b) high-resolution XPS Co 2p spectrum of catalyst 9.
Table 2 Surface characteristics of Fe3O4, the Fe3O4@SiO2 Fe3O4@SiO2-Cl, Fe3O4@SiO2@Im[Cl]Co(III)-Schiff base complex (8), and the
Fe3O4@SiO2@Im[Cl]Co(III)-melamine nanocomposite (9)
Entry
Sample
Specific surface area (m2 g−1
)
Pore volume (cm3 g−1
)
Average pore radius (nm)
1
2
3
4
5
Fe3O4
Fe3O4@SiO2
Fe3O4@SiO2-Cl
Fe3O4@SiO2@Im[Cl]Co(III)-Schiff base complex
Catalyst 9
480
455
400
380
372
0.803
0.785
0.755
0.700
0.720
1.254
1.787
1.802
1.814
1.820
The specific surface area values, the average pore diameter still maintained a spherical shape with an average diameter of
(according to the BJH method), and the total pore volume are ∼77 nm (Fig. 5d).
tabulated in Table 2. According to the BET isotherm, the
The size distribution of these NPs was centered at 12, 20,
active surface area of Fe3O4, Fe3O4@SiO2, Fe3O4@SiO2-Cl, and 75 nm for Fe3O4, Fe3O4@SiO2, and the Fe3O4@SiO2@Im
Fe3O4@SiO2@Im[Cl]Co(III)-Schiff base complex (8), and the [Cl]Co(III)-melamine nanocomposite, respectively (Fig. 5d–f).
Fe3O4@SiO2@Im[Cl]Co(III)-melamine nanocomposite (the cata-
lyst) were determined as 480, 455, 400, 380, and 372 m2 g−1
The morphology and structure of the Fe3O4@SiO2@Im[Cl]
Co(III)-melamine nanocomposite were also observed using
,
respectively (Table 2, entries 1–4). According to the results, TEM (Fig. 6). The TEM image revealed that the catalyst had
the specific surface area decreased after intercalation of the well-shaped spherical or ellipsoidal particles. The diameter of
melamine. However, on the other hand, the pore volume and the nanoparticles was found to be approximately 75 nm
average pore radius increased when melamine was added. (Fig. 6).
These results propose that the intercalation of melamine leads
to the formation of a more porous network structure.
Optimization of the reaction parameters
The loading amount of Co on the catalyst was measured by A highlight point in this article is that there was no need for
atomic absorption spectroscopy (AAS) and also inductively any base in these reactions. As will be discussed in the
coupled plasma (ICP) instruments in order to obtain more cer- “mechanism studies” section, it is proposed that manganese
tainly about the results. The results for the two analyses were powder was found to be a reducing agent for Co(III) species
close to each other with only a small difference (0.005). The (manganese was pre-activated by traces of acid). Stoichiometric
experiments indicated that 0.5 mmol of Co metal per gram of equivalents of manganese (0.5 mol%) relative to Co species
the catalyst was loaded on the catalyst 9 framework.
present in the catalyst 9 was sufficient to afford the coupling
According to the FE-SEM images, the synthesized Fe3O4 product in the highest possible efficiency. The need for the
NPs and Fe3O4@SiO2 were nearly spherical in shape and were reaction to have a reducing agent was elucidated by obser-
well dispersed; although in some areas, bigger structures with vation of only an 11% reaction progress (GC) for the Heck
a non-spherical morphology were observed (Fig. 5a and b). The coupling reaction of iodobenzene with n-butyl acrylate cata-
Fe3O4 and Fe3O4@SiO2 nanoparticles had an average diameter lyzed by the Fe3O4@SiO2@Im[Cl]Co(III)-melamine nano-
of 10–12 nm and 20–22 nm (Fig. 5a and b), which is fully com- composite (0.5 mol%) in EtOH/reflux.
patible with their DLS analyses (Fig. 5d and e). Fig. 5c showing
To optimize the reaction conditions, we chose the cross-
the FE-SEM image of the Fe3O4@SiO2@Im[Cl]Co(III)-melamine coupling of iodobenzene with n-butyl acrylate as a model reac-
nanocomposite with a spherical morphology. After successive tion (Scheme 1). Here, first, the effect of different amounts of
coating with the Schiff base Co complex, the nanocomposites Fe3O4@SiO2@Im[Cl]Co(III)-melamine nanocomposite was
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Fig. 5 FE-SEM images and DLS results of Fe3O4 (a, d), Fe3O4@SiO2 (b, e), and the Fe3O4@SiO2@Im[Cl]Co(III)-melamine nanocomposite (c, f).
Fig. 7 Effect of the catalyst amount (blue curve) and temperature
(black curve) on the reaction conversion.
Fig. 6 TEM image of Fe3O4@SiO2@Im[Cl]Co(III)-melamine nanocomposite.
results were achieved under reflux conditions. The reaction
afforded a 40% yield at room temperature. With increasing the
temperature from 25 °C to 79 °C, and the conversion ranged
from 40% to 95% (Fig. 7).
Effect of solvent
Scheme 1 Heck model reaction of iodobenzene with n-butyl acrylate
catalyzed by the Fe3O4@SiO2@Im[Cl]Co(III)-melamine nanocomposite.
It is well known that ionic moieties in a composite or a
polymer promote their solubility in a suitable solvent.41 The
soluble parts of a catalyst, i.e., that part containing a Schiff
base complex moiety, improve the solubility of this part in the
investigated (Fig. 7). The best amount was found to be
0.5 mol% containing 0.005 mmol Co. A further increase in the
catalyst amount did not result in a change in efficiency.
medium and cause an increase in the efficiency.42,44,45
Solvents such as EtOH, MeOH, DMF, and DMSO gave high
conversions, ranging from 95–97% (Fig. 8). It seems that polar,
protic, or polar–protic solvents could afford high efficiency.
However, due to environmental issues, we chose ethanol for
Effect of temperature
The temperature effect was screened by fulfilling the model the reactions, although it only had a negligible difference with
reaction at different temperatures in EtOH, and the best MeOH, DMF, or DMSO. This little difference can be ignored
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electron withdrawing). Though, with a general look in Table 3,
it can be understood that aryl halide-bearing electron-with-
drawing groups provide the highest efficiency (12e, 12g, 12h,
12j, 12l, 12m, 12q, and 12r). However, this was expected for a
typical Pd-catalyzed Heck coupling reaction with a default
mechanism (oxidative addition and reductive elimination).1
On the other hand, although electron-withdrawing groups
delayed the reaction, they did not have a significant effect on
the reaction efficiency as much as their effect on the time for
the reaction. Also, iodide as a leaving group facilitates the
Heck reaction, and indeed the reactivity of halides was in the
order of I > Br ≫ Cl (Table 3, entries 1–3 and 22–24). This reac-
tivity provides for a high selectivity for the preparation of com-
pounds 12f, 12g, and 12r involving iodine substitution in com-
petition with bromide or chloride (Table 3, entries 8, 9, and
28). In general, styrene demonstrates better performance than
n-butyl acrylate for Heck coupling under the reaction
conditions.
Encouraged by the obtained results from the Heck reac-
tions, we then examined the versatility and generality of the
present method toward the Sonogashira coupling reaction. As
shown in Table 4, a variety of acetylenes and aryl halides
bearing electron-donating or electron-withdrawing group
afforded the desired Sonogashira products in high to
excellent yields under EtOH/reflux in the presence of the
Fe3O4@SiO2@Im[Cl]Co(III)-melamine nanocomposite (0.5 mol%)
and Mn dust (0.5 mol%) (Table 4). Same as for the Heck coup-
ling reaction, the coupling reaction of phenylacetylene with
both electron-donating and electron-withdrawing groups gave
the desired products in high to excellent yields (Table 4). Also,
the method was completely selective toward iodide as a leaving
group compared to bromide, with the coupling occurring
exclusively in the iodide position at 14f and 14g (Table 4).
Also, in order to demonstrate the effect of melamine in
catalyst 9, the reaction of iodobenzene with n-butyl acrylate
(Table 3, entry 1) was carried out in the presence of compound
8 (10 mg) under identical reaction conditions used for 9
(Table 1, entry 1). The reaction gave an 83% isolated yield for
80 min. However, the yield of the reaction reached 89% after
4 h. The intercalation of melamine to this system provided a
robust-rigid network structure with an enhancement in
thermal stability (Fig. 3D), porosity (Table 1), and low metal
leaching (ESI, Fig. S1†). These enhanced properties are respon-
sible for the higher efficiency of 9 than 8.
Fig. 8 Solvent effect on the model Heck reaction catalyzed by the
Fe3O4@SiO2@Im[Cl]Co(III)-melamine nanocomposite.
given the environmental aspects, cheapness, availability, lower
toxicity, and easier work-up. Water showed only a 68% yield
under reflux conditions. This may be due to the formation of a
biphasic mixture that brings about problems such as mass
transfer. The reaction in solvent-free conditions did not give a
satisfactory yield (78%, Fig. 8).
It is noteworthy that, under the optimized reaction con-
ditions for the Heck reaction, high efficiency was obtained for
the Sonogashira reaction (the reaction of phenyl acetylene and
iodobenzene). In this case, also, the reaction was performed in
the absence of any base.
To survey the catalytic activity of the Fe3O4@SiO2@Im[Cl]
Co(III)-melamine nanocomposite, the Heck–Mizoroki cross-
coupling reaction was conducted by a variety of aryl halides
and alkenes (Scheme 2). It was found that the yields were
generally moderate to excellent for all substrates (Table 3,
65–95%), which reflected that the reaction was relatively insen-
sitive to the nature of the substitution (electron donating or
Mechanism study
A plausible mechanism for the Fe3O4@SiO2@Im[Cl]Co(III)-mel-
amine nanocomposite-catalyzed Heck reaction is depicted in
Scheme 3. In general, according to our observations and the
reported mechanisms,46,47 a plausible mechanism is proposed
that involves four main steps: (i) oxidative addition, (ii)
π-complex formation, (iii) migration, and (iv) reductive elimin-
ation. The possibility of a radical mechanism for this system
was also investigated. For this purpose, the Heck model reac-
tion was performed in the presence of hydroquinone as an
electron capture process. In this test, after 45 min of reaction
Scheme 2 Catalytic activity of the Fe3O4@SiO2@Im[Cl]Co(III)-melamine
nanocomposite toward the Heck and Sonogashira reactions.
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Table 3 Heck reaction of aryl halides with n-butyl acrylate or styrene catalyzed by the Fe3O4@SiO2@Im[Cl]Co(III)-melamine nanocompositea
Entry
Product 12
X
R
W
Time (min)
Yieldb (%)
TON
TOF (h−1
)
1
2
3
4
5
6
7
8
12a
I
H
H
H
4-OMe
4-OH
4-Me
4-CHO
4-Br
–CO2Bun
–CO2Bun
–CO2Bun
–CO2Bun
–CO2Bun
–CO2Bun
–CO2Bun
–CO2Bun
–CO2Bun
–CO2Bun
–CO2Bun
–CO2Bun
–CO2Bun
–CO2Bun
–CO2Bun
–CO2Bun
–CO2Bun
–CO2Bun
–CO2Bun
–CO2Bun
–CO2Bun
–Ph
–Ph
–Ph
–Ph
–Ph
–Ph
–Ph
–Ph
–Ph
–Ph
–Ph
–Ph
–Ph
80
100
218
120
110
75
60
70
86
65
70
63
135
80
73
135
200
193
193
245
200
68
91
86
82
78
80
80
84
85
88
94
90
92
75
77
82
84
65
72
74
66
76
88
77
72
80
82
95
91
75
75
80
66
77
77
182
172
164
156
160
160
168
170
176
188
180
180
150
154
164
168
130
144
148
132
152
176
154
144
160
164
190
182
150
150
160
132
154
154
136
102
45
78
87
128
168
142
122
174
155
174
67
115
137
75
38
45
46
33
48
Br
Cl
I
I
I
I
I
I
I
12b
12c
12d
12e
12f
12g
12h
12i
9
4-Cl
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
4-NO2
3-Me
2-NO2
4-OMe
4-Me
4-NO2
2-Me
4-Me
4-NO2
4-CN
4-OMe
3-NO2
H
I
I
12j
12b
12d
12h
12k
12d
12h
12l
12b
12m
12n
Br
Br
Br
Br
Cl
Cl
Cl
Cl
Cl
I
Br
Cl
I
I
I
154
103
43
H
H
90
200
110
75
60
70
125
70
60
280
200
190
12o
12p
12q
12r
12o
12s
12q
12o
12s
12q
4-OMe
4-Me
4-NO2
4-Br
4-OMe
4-CN
4-NO2
4-OMe
4-CN
4-NO2
87
131
190
157
75
129
160
29
I
Br
Br
Br
Cl
Cl
Cl
46
49
a Reaction conditions: Alkene (1.3 mmol), Ar–X (1.0 mmol), catalyst 9 (10 mg, 0.5 mol% Co), Mn (0.005 mmol), EtOH (3.0 mL), reflux. b Isolated yield.
progress (GC), hydroquinone was added to the reaction be approved by the advent of peaks at 641.4 eV and 653 eV,
mixture. The reaction did not show any change compared to corresponding to the binding energies of Mn 2p3/2 and Mn
the normal conditions (in the absence of hydroquinone), 2p1/2, respectively, in the XPS spectrum of the reaction mixture
which confirmed the absence of any radical in the mixture (Fig. 9a). A peak appeared at 638.6 eV was assigned to the Mn
(ESI, Fig. S2†). The understanding regarding the valence state metal used as a reducing agent.
of Co species responsible in the reaction mechanism was
An oxidative addition took place between highly active low-
addressed by XPS analysis of the above-mentioned mixture valent cobalt and aryl halide. In the following process, it
after 45 min (64% conversion, Fig. 9a) of the reaction time. formed a π-complex through coordination to olefin. In the next
The results indicated that the all of the cobalt species present step, a migratory insertion of the olefin afforded the Co-σ
in the mixture were in a +3 oxidation state, corresponding to intermediate, which underwent β-H elimination to give a
the binding energies of 797 eV (2p1/2) and 782.5 eV (2p3/2), in π-complex Co(III) species. The newly formed Co(III)-olefin
agreement with the literature.41 No Co(II) species were detected π-complex furnished the desired Heck coupling product as
in the spectrum. Also, due to fast consumption of Co(I) well as H–Co(III)–X species. The fact there was no need for a
species, they did not appear in the spectrum. In this viewpoint, base in this reaction can be explained by the proposed mecha-
like the palladium process, the Heck reaction begins when the nism. As shown in Scheme 3, Cl− as a counter ion of the imi-
Co(III) species is reduced by manganese powder, present as an dazolium moiety could attack the cobalt hydride intermediate,
additive in the reaction mixture.46,48–50 In the absence of Mn, with the leaving group taking chlorine as a pair of ions for
the reaction progress was found to be 11% for 3 h. This con- imidazole. The highly active Co(I) species is regenerated and
firmed the reductive role of Mn for Co(III) ions. Also, this can returned to the cycle.
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Table 4 Sonogashira reaction of aryl halides with phenyl acetylenes catalyzed by the Fe3O4@SiO2@Im[Cl]Co(III)-melamine nanocompositea
Paper
Entry
Product 14
X
R
R
Time (min)
Yieldb (%)
TON
TOF (h−1
)
1
2
3
4
5
6
7
8
14a
I
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
Ph
C6H13
C6H13
C6H13
Ph
Ph
Ph
120
180
120
230
140
165
120
130
140
100
230
250
250
90
94
85
76
88
80
88
96
90
90
94
88
75
66
98
88
82
96
88
85
86
96
90
88
77
80
188
170
152
176
160
176
192
180
180
188
176
150
132
196
176
164
192
176
170
172
192
180
176
154
160
94
57
76
46
67
64
96
83
77
113
46
36
32
131
77
55
120
66
43
47
96
55
50
37
38
Br
Cl
I
I
I
I
I
I
I
14b
14c
14d
14e
14f
14g
14h
14i
4-NH2-ph
4-CN-ph
4-Me-ph
4-COMe
Br-ph
Cl-ph
3-Pyridine
Ph
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
I
Br
Cl
I
I
Br
I
I
I
I
I
Ph
Ph
14j
14k
4-NO2-ph
1-Naphthalene
1-Naphthalene
5-Pyrimidine
2-Me-ph
2-OMe-ph
4-CN-ph
3-Thiophene
Ph
140
180
95
14l
Ph
Ph
Ph
C6H13
14m
14n
14o
14p
14q
160
240
220
120
200
210
250
250
Ph
I
2-Cyclohexanol
2-Cyclohexanol
2-Cyclohexanol
C6H13
Br
Cl
I
Ph
Ph
4-Me-ph
14r
a Reaction conditions: Acetylene (1.3 mmol), Ar-X (1.0 mmol), catalyst 9 (10 mg, 0.5 mol% Co), Mn (0.005 mmol, 0.5 mol%), EtOH (3.0 mL),
reflux. b Isolated yield.
to some observations (Scheme 4), namely: (i) the reaction pro-
gress was 17% in the absence of Mn dust, whereupon after its
addition, the reaction proceeded efficiently; (ii) it was shown
that the reaction proceeded without the need for any base, in
which the imidazolium counter ion played the role of the base;
(iii) in situ XPS analysis after 60 min of the reaction mixture
demonstrated the presence of Co(III) as well as Co(I) species
(Fig. 9b). Here, the presence of peaks appearing at 653 eV and
641/637 eV related to Mn and Mn(2+), respectively, completely
confirmed the reduction of Co(III) species (Fig. 9b); (iv) the
reaction was not affected by the addition of hydroquinone as a
radical scavenger over the coupling reaction of phenyl acety-
lene and iodobenzene in the presence of 9 (0.5 mol% Co), Mn
(0.5 mol%), EtOH/reflux; which withdrew the formation of any
radicals through the reaction and subsequently the radical
mechanism (ESI, Fig. S2†).
So, in accordance with our observations as well as the
reported mechanisms for the Co-catalyzed coupling reactions,
it could be concluded that the reaction proceeds via oxidative
Scheme 3 Postulated mechanism for the Fe3O4@SiO2@Im[Cl]Co(III)-
addition and reductive elimination steps. In the first step of
this proposed mechanism, the reduction of Co(III) to Co(I)
takes place using Mn dust. Then, a π-complex was formed
melamine nanocomposite catalyzed Heck coupling reaction.
It is worth noting that a few mechanism studies have been between the catalyst and acetylene, followed by the formation
conducted with the cobalt-catalyzed Sonogashira reaction.6,51 of a Co-acetylide intermediate with interference of imidazo-
In this study, a plausible mechanism was suggested according lium counter ions and the generation of a HCl molecule. In
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Fig. 9 In situ normalized high-resolution XPS spectrum of: (a) reaction mixture of iodobenzene, phenyl acetylene, n-butyl acrylate; and (b) reaction
mixture of iodobenzene, n-butyl acrylate in EtOH/reflux catalyzed by the Fe3O4@SiO2@Im[Cl]Co(III)-melamine nanocomposite.
Scheme 5 Chemical structure of 15.
complex 4 provided the desired coupling product within 1 h
with a 94% yield.
Based on the presented mechanism, the catalytic active site
can be calculated based on the generated HX. Hence, the gen-
erated HX provides a good estimate of the catalytic active sites.
In addition to the fact that the production of HX confirms the
mechanism, in order to measure the active sites, a typical reac-
Scheme 4
A plausible reaction mechanism for the Co-catalyzed
tion was carried out with iodobenzene, n-butyl acrylate, in the
presence of catalyst 9. The reaction was allowed to proceed
completely. After that, the catalyst was separated from the reac-
tion mixture by an external magnet and the residue was
titrated by NaOH 0.1 N in the presence of a drop of litmus
(azolitmin) solution as an indicator. The obtained amount
according to the consumed base was found to be
0.0044 mmol. It was shown that 88% of the metal sites present
in the mixture (0.005 mmol) play a catalytic role as active sites.
Sonogashira reaction.
the next step, oxidative addition occurs between the C–X bond
in aryl halide and the Co-acetylide intermediate. Finally, reduc-
tive elimination leads to the desire Sonogashira product and
catalyst 9 is regenerated for the next reaction cycle. To support
our claim regarding the base functionality of the imidazoluim
part of 9, we prepared the Co-complex 15, that is a free imid-
azole homolog of 4, by the treatment of salicylaldehyde
Recoverability
and allylamine followed by Co complexation with CoCl2 The reusability of a catalyst is an important property from
according to the procedure described by Khorshidifard et al. the viewpoint of green chemistry.52 Therefore, reusability of
(Scheme 5).26 The Heck coupling model reaction was per- the Fe3O4@SiO2@Im[Cl]Co(III)-melamine nanocomposite was
formed in the presence of 15, and the results were compared studied under optimized conditions for the Heck model reac-
with the Co(III) Schiff base complex (4). The reactions were per- tion. As exhibited in Fig. 10, the catalyst was recovered and
formed under optimized conditions for the model reaction reused for at least seven consecutive runs. The yield of 12a
(Scheme 1). The reaction in the presence of 15 did not show reached 93% for the 7th run, which means that only a 2%
any noticeable progress for 8 h of the reaction. The reaction drop in efficiency was observed compared to the corres-
proceeded upon the addition of Et3N. Interestingly, the Co ponding fresh catalyst (95%). Also, the amount of Co leached
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Fig. 10 Recyclability of the catalyst (a) in the Heck–Mizoroki, and (b) Sonogashira reactions.
into the solution for the Heck coupling reaction of iodoben-
zene and n-butyl acrylate after each cycle was measured by ICP.
The catalyst showed very slight leaching in each cycle, which
0.6% metal leaching detected after the 7th run, thus reflecting
its stability under the reaction conditions (Fig. 10). Also, in
order to clarify the effect of melamine in the nanocomposite 9,
these experiments were also carried out in the presence of
compound 8, and similarly, the leaching amount was
measured in 7 successive cycles (ESI, Fig. S1†). According to
the results, the metal leaching in compound 8 was noticeably
larger than that with catalyst 9. This reduction in the amount
of leaching and the increase in stability may be attributed to
the presence of melamine in the catalyst. The amount of leach-
ing was 0.35% in the first run (ESI, Fig. S1†).
Fig. 11 Reaction kinetics, Hg(0) poisoning, and hot filtration studies for
the Heck coupling reaction of iodobenzene with n-butyl acrylate cata-
lyzed by Fe3O4@SiO2@Im[Cl]Co(III)-melamine nanocomposite in EtOH/
reflux.
Moreover, a vast investigation was conducted in order to
elucidate the heterogeneous nature of the catalyst. First, we
tried the hot filtration test over the Heck model reaction under
premium conditions (Table 3), in which the catalyst was mag-
netically removed in situ after 64% (t = 45 min) conversion
(GC) and the reactants were allowed to undergo further reac- through the reaction of Wang resin-bound aryl iodide with
tion.28 The results indicated that, after removal of the hetero- n-butyl acrylate in EtOH/reflux in the presence of 9. As shown
geneous catalyst, the free catalyst residue was weakly active, in Scheme 6 (1), the coupling reaction did not take place
and the conversion reached 66% after 2.5 h of reaction. This under these conditions, and at the end of the reaction the cata-
confirmed that the catalyst acted heterogeneously in the reac- lyst could be magnetically separated. Hydrolysis of the organic
tion and consequently only slight leaching occurred during the molecules on the Wang resin led to 4-iodo benzoic acid. In
reaction. Second, in order to completely ensure the hetero- another test, the reaction of Wang resin-bound aryl iodide
geneous role of the catalyst, the mercury poisoning test was with n-butyl acrylate and iodobenzene was performed. In this
also carried out. Mercury (0) was absorbed as a metal (or by case, the desired coupling product was obtained, but same as
amalgamation) and significantly deactivated the metal cata- the previous test, no coupling product was detected with
lyst53 (active site) and thus suppressed the catalyst activity. hydrolysis of the Wang resin. The obtained results from the
This test was evidence of the heterogeneous catalyst.53 The test three-phase test illustrated that the catalyst is truly hetero-
was conducted with the aforementioned model reaction (opti- geneous, by which the Heck reaction did not occur in the
mized conditions). After 45 min of the reaction, 320 molar liquid phase. The negative results from all of the heterogeneity
equivalents of mercury vs. Co catalyst was added to the reac- tests (Hg(0) poisoning, hot filtration, and three-phase test)
tion mixture. The reaction medium was stirred for more than suggested that the solid catalyst was truly heterogeneous and
2.5 h, while the reaction conversion was screened by TLC no detectable Co leaching occurred over the Heck reaction.
(Fig. 11). As shown in Fig. 11, after 4 h of reaction, the addition
Finally, in order to ensure the structure of the recovered
of Hg(0) almost suppressed further catalysis. No more conver- catalyst was retained, we studied it after the 7th run over the
sion was observed after 1.5 h of the catalyst being poisoned. A Heck model reaction of iodobenzene and n-butyl acrylate
kinetics plot of the reaction in the absence of Hg(0) is depicted under the obtained premium conditions by some analyses
in Fig. 11. Finally, the three-phase test (Scheme 6) was applied (Fig. 12). FTIR analysis of the recovered catalyst corroborated
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Scheme 6 The three-phase test of 4-iodebenzoate, polymer bound form, and n-butyl acrylate in the presence of 9.
Fig. 12 (a) FTIR, (b) VSM, (c) EDX, (d) XPS spectra, (e) FE-SEM, and (f) TEM images of the recovered catalyst 9 after the 7th run for the Heck coupling
reaction of iodobenzene with n-butyl acrylate.
that the structure of the catalyst remained completely intact peak. This was completely compatible with ion exchange of
during recycling (Fig. 12). The EDX spectrum of the recovered the imidazolium Cl− ion, which reduces the peak intensity
catalyst revealed the presence of the expected elements, thus (Fig. 12).
confirming the catalyst’s stability during the reactions. An
XPS analysis revealed that the Co species present in the
interesting point was there was an intensity reduction in the Cl catalyst remained intact after the 7th run and their oxidation
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Table 5 Comparison of the catalytic activity of the Fe3O4@SiO2@Im[Cl]Co(III)-melamine nanocomposite with literature examples for the cross-
coupling reaction of iodobenzene with styrene (Heck) and phenyl acetylene (Sonogashira)
Entry
Reaction
Heck
Catalyst/conditions
Base
Time (h)
Yielda (%)
Ref.
1
2
3
4
5
6
PdNPs@CNCs/H2O : CH3CN (1 : 1)/100 °Cb
Agarose@Pd NPs/solvent-free/100 °C
Hydrazone-Pd@ZnO (0.26 mol%)/DMF/130 °C
Co–B (5 mol% Co)/water/DMF (1/1 v/v)/130 °C
MNPs@Cs-MS-Co (0.55 mol%)/DMSO/140 °Cd
Fe3O4@SiO2@Im[Cl]Co(III)-melamine nanocomposite (0.5 mol%)/
Mn(0.5 mol%)/EtOH/reflux
K2CO3
Et3N
Et3N
K2CO3
KOH
—
24
2
1
12
1
1
75
90
98
98c
75
88
54
55
56
57
6
Present study
7
8
9
10
11
12
Sonogashira
CuI (0.2 mol%)/PPh3 (4 mol%)/H2O/140 °C
K2CO3
KOH
K2CO3
K2CO3
KOH
—
24
2
8
8
10
2
43
96
71
82
72
94
58
59
60
61
Cu(I)-PANI@MWCNT/DMF/135 °Ce
NHC precursor (20 mol%)/CuSO4·5H2O (20 mol%)/DMF/125 °Cf
Co(C9H9NO2)3/DMF/ethylene glycol/visible light/r.t.
MNPs@Cs-MS-Co (0.55 mol%)/DMSO/140 °Cd
Fe3O4@SiO2@Im[Cl]Co(III)-melamine nanocomposite (0.5 mol%)/
Mn(0.5 mol%)/EtOH/reflux
6
Present study
a Isolated yield. b NPs: nanoparticles; CNCs: colloidal cellulose nanocrystallites. c GC yield. d MNPs@Cs-MS-Co: cobalt tagged on MNPs-chitosan
functionalized with methyl salicylate. e Cu(I)-PANI@MWCNT: polyaniline functionalized multiwall carbon nanotube-supported Cu(I). f NHC:
N-heterocyclic carbine.
state was similar to that of the fresh catalyst. This was also in The superparamagnetic nature of the Fe3O4@SiO2@Im[Cl]
agreement with the proposed catalytic cycles in the mecha- Co(III)-melamine nanocomposite, in particular, simplified the
nism studies. No other oxidation states were found in the cata- easy separation of the catalyst from the reaction mixture by
lyst (Fig. 12d). The recovered catalyst responded to the external employing a simple external magnetic field. In that way, the
magnetic field and could be readily collected from the reaction catalyst was stable under the reaction conditions and main-
mixture same as the fresh catalyst. The FE-SEM image showed tained its catalytic activity and selectivity for at least seven suc-
that the nanoparticles were still approximately spherical in cessive runs without the need for any work for its re-activation,
shape even after the 7th cycle. It was worth noting that the which addresses economic and environmental issues. Also,
nanocatalyst did not show any morphological changes as hot filtration, mercury poisoning, and three-phase testing cer-
proved by the TEM images obtained from the recovered cata- tainly confirmed the heterogeneous nature of the catalyst with
lyst (Fig. 12f).
insignificant metal leaching. We characterized the catalyst
To show its novelty, uniqueness, and to further evaluate the from various aspects by: FTIR, CHNS, UV-Vis, TGA, VSM,
present method, the catalytic activities of the Fe3O4@SiO2@Im NMR, EDX, XRD, CV, ICP, DLS, BET, and XPS analyses. The
[Cl]-Co(III)-melamine nanocomposite were compared with present methodology was found to be highly efficient and a
those of the reported Co-catalyzed Heck and Sonogashira wide variety of functional groups could be tolerated in these
coupling reactions in terms of the reaction time, base, and conditions in high to excellent yields. The results indicated
yield. As shown in Table 5, this method was obviously superior that 88% of the metal sites play a catalytic role as active
to the most supported Pd, Cu, or Co catalysts. The coupling sites. It is believed that the Heck reaction goes through a
reaction proceeded efficiently in the presence of only 0.5 mol% four-step mechanism, namely oxidation addition, insertion,
Co in the absence of any base. Also, the Fe3O4@SiO2@Im[Cl]- β-elimination, and reductive elimination. This catalytic system
Co(III)-melamine nanocomposite is a more versatile approach is bi-functional, in which the imidazolium moiety participates
for Sonogashira and Heck coupling reactions in terms of in the reaction and plays the role of a base, while on the other
energy consumption, time consumption, catalyst preparation, hand, the Co(III) complex moiety catalyzes the reaction. This
and in eco- and economic friendly aspects.
methodology is proposed as an affordable alternative to the
previous use of transition metal catalysts, especially Pd-based
catalytic systems.
Conclusions
In conclusion, we developed a new non-toxic, low-cost, and
stable catalytic system for the selective Heck–Mizoroki and
Sonogashira cross-coupling reactions using a Schiff base Co(III)
Experimental
Materials and apparatus
complex decorated on Fe3O4@SiO2 NPs as a magnetically reco- All the chemicals were obtained from Sigma and Fluca suppli-
verable inorgano-nanocatalyst (Fe3O4@SiO2@Im[Cl]Co(III)-mel- ers and used without further purification. Wang resin
amine nanocomposite). The reactions were performed under (polymer-bound 4-iodobenzene) was purchased from
ethanol as a solvent and base-free conditions, which are suit- NovaBiochem containing 0.64–1.1 mmol g−1 loading,
able from a green- and sustainable chemistry point of view. 100–300 mesh, and cross-linked with 1% divinylbenzene. All
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the solvents were distilled and dried before use. All the other with a surface area and pore size analyzer (Micromeritics ASAP
reagents were of analytical grade. Reactions’ progress was 2000 instrument) using the BET method.
monitored by thin layer chromatography (TLC) on silica gel or
Preparation of Co(III) Schiff base complex (4)
gas chromatography (GC) using a Shimadzu-14B gas chromato-
graph equipped with an HP-1 capillary column and N2 as a First, 2-hydroxy-5-chloromethyl benzaldehyde was synthesized
carrier gas. Anisole was used as the internal standard. FTIR and purified according to a procedure described elsewhere23
spectra were obtained using a JASCO FT/IR 4600 spectrophoto- (purple solid, 98% yield). 2-Hydroxy-5-chloromethyl benz-
1
meter using KBr pellet. The H NMR (250 MHz) and 13CNMR aldehyde (1) (2.0 mmol, 0.34 g) was dissolved in 20 mL of pro-
(62.9 MHz) spectra were recorded on a Bruker Avance DPX-250 pylene carbonate and then imidazole (10.0 mmol) was added
spectrometer in CDCl3 and DMSO-d6 as the solvents and TMS to the mixture. The mixture was stirred under a N2 atmosphere
as an internal standard. Electrochemical measurement (cyclic at 90 °C for 48 h. After refluxing, the reaction was stopped,
voltammetry) for the Co catalyst was performed on a CHI cooled to room temperature, and then 10 mL of water was added
1210A electrochemical workstation (CH Instrument, China) to the mixture; then the product was extracted into ethyl acetate
with a three-electrode system consisting of a standard Ag/AgCl (15 mL). The solvent was removed under reduced pressure and
as a reference electrode, a platinum wire electrode and a modi- the product 2 was washed with dilute NaOH and deionized
fied glassy carbon with the catalyst22 as an auxiliary and water and then dried into an oven at 50 °C (dark brown powder,
working electrode, respectively. The CVs were recorded at 96% yield). The transformation of 2-hydroxy-5-imidazole benz-
room temperature under argon atmosphere in the potential aldehyde to 3 was done according to the procedure described by
range from −2.0 to 2.0 V after 300 s accumulation under stir- Khorshidifard et al.26 (Pale yellow powder, 98% yield). 4-((1H-
ring with a scan rate of 100 mV s−1. The cell temperature was Imidazol-1-yl)methyl)-2-((allylimino)methyl)phenol (3, 1.0 mmol,
maintained at 25.0 0.1 °C by means of a HAAKE D8 recircu- 0.24 g) was complexed to Co ions by its dissolution to 15 mL of
lating bath. Elemental analyses were performed on ethanol followed by the addition of CoCl2·6H2O (0.5 mmol,
PerkinElmer-2004 instrument. XPS studies were conducted 0.12 g). The resulting mixture was stirred for 12 h at room temp-
using an XR3E2 (VG Microtech) twin anode X-ray source with erature under a N2 atmosphere (Scheme 7). The product 4 was
AlKα = 1486.6 eV. Field emission scanning electron microscopy simply filtered, washed with cooled ethanol and diethyl ether,
(FE-SEM) images were obtained on a HITACHI S-4160. EDX then dried at room temperature and isolated as a dark brown
spectroscopy was performed using a field emission scanning powder (95% yield, isolated weight: 0.5 g) (Scheme 7).
electron microscope (FESEM, JEOL 7600F), equipped with a
Preparation of Fe3O4@SiO2 core–shell nanoparticles (7)
spectrometer for energy dispersion of X-rays from Oxford
instruments. Transmission electron microscopy (TEM) was Core–shell Fe3O4@SiO2 nanospheres were prepared according
performed on a Philips EM208 microscope and was operated to the previously described procedure (Scheme 8).20
at 100 kV. The size distribution of the nanoparticles was
Preparation of Fe3O4@SiO2@Im[Cl]Co(III)-melamine nano-
composite (9)
measured by dynamic light scattering (DLS) analysis on a
HORIBA-LB550 instrument. TGA of the samples was per-
formed on a NETZSCH STA 409 PC/PG in nitrogen atmosphere Fe3O4@SiO2@Im[Cl]Co(III)-melamine nanocomposite was syn-
with a heating rate of 10 °C min−1 in the temperature range of thesized according to the following procedure: Fe3O4@SiO2-Cl
25–850 °C. The magnetic behavior of the samples was con- (1 g) was added to the solution of Co(III)–Im-Schiff base
ducted on a Lake Shore vibrating sample magnetometer (VSM) complex (4, 1.0 mmol, 0.54 g) in 20 mL propylene carbonate
at room temperature. ICP experiments were accomplished and the resultant mixture was stirred at 90 °C for 24 h under a
using a VARIAN VISTA-PRO CCD simultaneous ICP-OES instru- N2 atmosphere. Then, the mixture was cooled to room temp-
ment. The surface area, pore volume, and pore diameter of the erature, and the product was filtered by an external magnet,
obtained NPs were determined by N2 physisorption at −196 °C and washed with EtOH and propylene carbonate to remove any
Scheme 7 Preparation of Co(III) Schiff base complex 4.
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Scheme 8 Illustration of the preparation of the Fe3O4@SiO2@Im[Cl]Co(III)-melamine nanocomposite.
unreacted species and then dried at 70 °C for
6 h. (1.0 mmol), n-butyl acrylate (1.3 mmol), manganese
Immobilized Co(III)–Im-Schiff base complex (8) along with a (0.005 mmol), and the nanocatalyst 9 (0.5 mol%) in ethanol at
weighted amount of melamine (0.5 mmol) were transferred to 78 °C as a model reaction. Also, these experiments were
a 25 mL oven-dried round-bottom flask equipped with a magnet repeated for the cross-coupling reaction of iodobenzene
and a condenser. A solution of trimethylamine (4.0 mmol) in (1.0 mmol) with phenyl acetylene (1.3 mmol), catalyst 9
absolute MeOH (5.0 mL) and AlCl3·2H2O (4.0 mmol) were (10 mg, 0.5 mol% Co), Mn (0.005 mmol, 0.5 mol%), and EtOH
added to the mentioned mixture. The mixture was refluxed in a (3.0 mL) under reflux conditions.
N2 atmosphere for 6 h. The product, Fe3O4@SiO2@Im[Cl]Co(III)-
The catalyst was collected and separated by an external
melamine nanocomposite (9) was collected using an external magnetic field for each cycle, washed with EtOH and deionized
magnetic field, washed with water (2 × 10 mL) and EtOH (2 × water, and stored at room temperature under an air atmo-
10 mL), and dried in a vacuum oven (6 h, 60 °C). Scheme 8 sphere for the next experiment. For each cycle, the residue was
shows the complete route for the preparation of 9.
analyzed by ICP to measure the Co species leached from the
catalyst to the solution. Also, hot filtration, mercury poisoning,
and three-phase tests were employed for investigation of the
heterogeneous nature of the catalyst.
General procedure for the catalytic Heck and Sonogashira
reactions catalyzed by Fe3O4@SiO2@Im[Cl]Co(III)-melamine
nanocomposite
A mixture of alkene or phenyl acetylene (1.3 mmol), Ar–X
(1.0 mmol), Fe3O4@SiO2@Im[Cl]Co(III)-melamine nano-
composite (10 mg, 0.5 mol% Co), Mn (0.005 mmol), and EtOH
(3.0 mL) was refluxed in an oil bath. The reaction progress was
screened by TLC at different time intervals. After completion
of the reaction, the catalyst was separated by an external mag-
netic field, washed with EtOAc, dried in the oven (60 °C), and
stored at room temperature for the next run. Then, the solvent
was removed under reduced pressure and 3.0 mL distilled
water was added to the residue for dissolution of the Mn(II)
salts produced during the reaction;16,43 then the resultant
mixture was extracted with Et2O (2 × 3.0 mL). All the manga-
nese was transferred to the aqueous phase and separated from
the organic phase. The combined organic layers were dried
over anhydrous Na2SO4, then filtered, and the solvent was
removed under reduced pressure and finally the crude product
was purified by column or flash chromatography.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors are grateful to the University of Birjand for finan-
cial support.
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