Sustainable and recyclable magnetic nanocatalyst of 1,10-phenanthroline Pd(0) complex in green synthesis of biaryls and tetrazoles using arylboronic acids as versatile substrates

Article abstract of DOI:10.1016/j.mcat.2021.111489

A magnetic nanocatalyst was purveyed as a heterogeneous recoverable palladium-based catalyst anchored on green, sustainable and phosphine free support. Resulted Fe₃O₄@SiO₂-Phen-Pd(0) nanocatalyst bearing powerful phenanthroline ligand was thoroughly characterized by physicochemical approaches like UV–vis, FT-IR, EDX, XRD, TGA, ICP, VSM, DLS, FESEM, and TEM analyses. After finding trustable data, the obtained magnetic catalyst was considered to be applied in the Suzuki-Miyaura type C-C couplings and getting corresponding tetrazoles using arylboronic acid derivatives as alternate precursors of aromatic halides and stupendous data were observed.

Full text of DOI:10.1016/j.mcat.2021.111489

Molecular Catalysis 504 (2021) 111489
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Sustainable and recyclable magnetic nanocatalyst of 1,10-phenanthroline
Pd(0) complex in green synthesis of biaryls and tetrazoles using arylboronic
acids as versatile substrates
Nastaran Bagherzadeh, Ali Reza Sardarian*, Hassan Eslahi
Chemistry Department, College of Sciences, Shiraz University, Shiraz, 7194684759, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords:
A magnetic nanocatalyst was purveyed as a heterogeneous recoverable palladium-based catalyst anchored on
green, sustainable and phosphine free support. Resulted Fe3O4@SiO2-Phen-Pd(0) nanocatalyst bearing powerful
phenanthroline ligand was thoroughly characterized by physicochemical approaches like UV–vis, FT-IR, EDX,
XRD, TGA, ICP, VSM, DLS, FESEM, and TEM analyses. After finding trustable data, the obtained magnetic
catalyst was considered to be applied in the Suzuki-Miyaura type C-C couplings and getting corresponding tet-
razoles using arylboronic acid derivatives as alternate precursors of aromatic halides and stupendous data were
observed.
1, 10- Phenanthroline
Palladium catalysis
Magnetic core-shell
Arylboronic acids
Suzuki-Miyaura
Tetrazol
1. Introduction
To stabilize the Fe3O4 nanoparticles and enhance their distribution,
the utilization of which using silica (SiO2) shell is of the most interests
The human health promotion and maintenance of the environment
are “better to prevent than to cure”. The concept of “sustainable
development” was articulated from UN Commission on Environment
and Development in 1987 (Brundtland Commission) to consider green
chemistry in the fields of organic syntheses, processing, and using eco-
friendly alternative chemicals [1]. To reach such context, nanotech-
nology has been reaped notable attentions in varies research fields
including chemistry, environmental remediation, biomedical engineer-
ing, and biotechnology because of introducing unique properties of
nanoparticles (NPs) belongings [2].
due to its stability, non-toxicity, and biocompatibility. The resultant
magnetic core-shells have both advantages of magnetic core and sus-
tainable shell [14,15]. As mentioned previously, this unique scaffold can
be applied in the synthesis of magnetic catalyst to be performed in
different organic processes especially in the synthesis of transition
metal-based catalysts. The reactions such as Suzuki-Miyaura C-C cou-
plings as so popular Csp2-Csp2 bond formation reactions as well as
getting tetrazole derivatives are attractive strategies in organic synthesis
that have comprehensively applied palladium (Pd) based magnetic
nanocatalysts [16–18].
In the green chemistry point of view, nanotechnology is being
considered to design more environmentally and economically sustain-
able and safe nano products with no creation health risk [3]. Among
important chemicals in nanotechnology, the Fe3O4-based nanoparticles
have been found remarkable dignity due to their impressive super-
paramagnetic, simple synthesis and biochemical properties such as
biodegradability, non-toxicity, and convenient separation from the re-
action [2–4]. The deployment of such nanoparticles present extensive
industrial and laboratory applications in drug delivery [4], hyperther-
mia [5], sensors [6], magnetic resonance imaging (MRI) [7,8], nano-
composites [9], bioseparations [10], therapeutic and medical usages
[11], magnetic storage [12], and catalysts [13].
These methodologies can introduce a vast library of pharmaceuticals
including anticancer [19,20], antifungal [21,22], antibacterial [23,24],
antitubercular [25,26], and antimalarial [27,28] compounds. Further-
more, nitrogen-rich heterocycles specifically tetrazoles are being
attended as photosensitive reagents [29–35], ligands in coordination
chemistry, constructing agents in metal-organic frameworks (MOFs)
[36,37], valuable ligands in chiral catalysts in asymmetric reactions
[38]. Some bioactive structures resulting from these strategies are
shown in Fig. 1.
–
The synthesis of tetrazoles and the formation of C C bonds by
Suzuki-Miyaura C-C coupling reactions have recently attracted much
attention and various preparative protocols have been introduced.
* Corresponding author.
E-mail address: sardarian@shirazu.ac.ir (A.R. Sardarian).
https://doi.org/10.1016/j.mcat.2021.111489
Received 20 November 2020; Received in revised form 15 February 2021; Accepted 16 February 2021
Available online 2 March 2021
2468-8231/© 2021 Elsevier B.V. All rights reserved.

N. Bagherzadeh et al.
Molecular Catalysis 504 (2021) 111489
Nevertheless, these protocols have some drawbacks such as harsh re-
action conditions, using strong acidic catalysts, having tedious work-ups
attributed to homogeneous catalyst, and applying non-reusable catalysts
[39–42]. More importantly, some reports in the synthesis of Suzuki
products and tetrazole derivatives considered catalysts bearing toxic
phosphorous ligands [43,44] and it is the main motivation to accomplish
such procedure using non-toxic catalysts free of phosphine ligands. For
the first time and only approach in the literature, Vignesh et al. syn-
thesized some tetrazoles directly from arylboronic acids using ONO
pincer type palladium complex as a catalyst [44]. Arylboronic acids are
of superior sources of aryl groups in Suzuki type C-C couplings as well as
finding tetrazole derivatives owing to their stability, availability, high
tolerance against various functional groups, and commonly, require-
ment of mild reaction conditions [45].
2. Experimental section
2.1. Chemical and instrumentation
All chemicals with high quality were prepared from Sigma-Aldrich
and Merck. Determination of the monitoring the progress of reactions
and purity of the substrate was accomplished by thin layer chromatog-
raphy on silica gel polygram SILG/UV 254 plates. NMR spectra were
recorded in CDCl3 and DMSO on Bruker Avance DPX250, DPX300, and
DPX400 spectrometers, at 250, 300 and 400 MHz, respectively. FTIR
spectrometer model Shimadzu FT-IR 8300 were considered to find FTIR
measurements using KBr pellet. Melting points were measured by a
micro melting point apparatus (electrothermal, BUCHI 510). The
elemental analyses (C, H, N) were obtained using a Thermo Finnigan
Flash EA-1112 CHNSO rapid elemental analyzer. X-ray diffraction
(XRD) patterns were recorded on a Bruker AXS D8-Advance X-ray
Noteworthy, the transition metal-catalyzed functionalization of ar-
–
omatic C H bonds obtains an efficient replacement to the cyanation
reactions with metallic precursors of NaCN, KCN, CuCN, and ZnCN
[46–48]. Hence, the development of the easy and accessible methodol-
ogy for the cyanation of arylboronic acid compounds using NaCN is
much desirable due to its robust properties, being cheap, water solubi-
lity, and unlike CuCN and TMSCN, it does not produce any undesirable
waste [49].
diffractometer using CuKα radiation (λ = 1.5418). Dynamic Light
Scattering (DLS) using a HORIBA-LB550. The Brunauer-Emmett-Teller
(BET) surface areas were obtained by using the nitrogen adsorption-
desorption isotherms determined at the temperature of liquid nitrogen
on an automatic analyzer (Autosorb-iQ-MP, Quantachrome, USA). Field
emission scanning electron microscopy (FE-SEM) characterization was
operated on a Hitachi S-4160. Transmission Electron Microscopy (TEM)
is performed on the Philips EM208 microscope, which operates at an
accelerator voltage of 100 kV. Vibrating sample magnetometer (VSM) of
magnetic nanocomposites were obtained using an instrument model
BHV-55. The loading amount of palladium was measured by inductively
coupled plasma (ICP, Varian,Vista- pro). A PerkinElmer device was used
to get thermal gravimetric analysis (TGA) data. UV–vis spectra were
obtained on a PerkinElmer Lambda 25 UV–vis spectrometer. The ele-
ments in the catalyst were investigated using an energy dispersive X-ray
(EDX) spectroscopy accessory to the Philips scanning electron micro-
scopy (SEM). Finally, the melting points and CHNS of products and their
An increasing demand for utilization environmentally benign pro-
cess convinced us to design and develop a novel stable super-
paramagnetic heterogeneous phosphine ligand-free Pd complex of 1,10-
phenanthroline (Phen) as a bidentate nitrogen ligand, which has been
widely used for preparation of various metal complexes [50]. Therefore,
in ongoing of our research group, it was decided to prepare magnetic
nanocatalyst of Fe3O4@SiO2-Phen-Pd(0) to be checked out its catalytic
activity against one-pot multicomponent transformation of arylboronic
acids to 5-substituted 1H-tetrazoles and biaryls through Suzuki cross
coupling reaction, Scheme 1.
Fig. 1. Some bioactive scaffolds have been obtained through Suzuki-Miyaura coupling reactions and the preparation of 5-substituted 1H- tetrazole derivative.
2

N. Bagherzadeh et al.
Molecular Catalysis 504 (2021) 111489
Scheme 1. Application of Fe3O4@SiO2- Phen-Pd(0) in Suzuki-Miyaura couplings and tetrazole formation reactions.
1H NMR spectra were compared with literature values.
EtOAc /8:2) in 81 % yield. The approach of finding L4 is illustrated in
Scheme 2.
2.2. General procedure
2.2.4. Preparation of Fe3O4 NPs
As it has been reported previously [54], to a solution of 1 g of
polyvinyl alcohol (PVA 15000) in 30 mL of deionized water, it was
added 0.9 g of FeCl2 (4.5 mmol) and 1.3 g of FeCl3.6H2O (4.8 mmol) at
80 ◦C, respectively. After stirring for 0.5 h, 1.0 mol/L of hexamethy-
lenetetramine (HMTA) was poured into the solution dropwise to
adjusted pH of the reaction medium on 10, and then, the mixture was
heated for more 2 h at 60 ◦C to provide magnetic Fe3O4 NPs as a black
solid residue. In the final step, Fe3O4 nanoparticles were regularly
collected by a magnet, washed with H2O and dried at 60 ◦C for 10 h.
2.2.1. The synthesis of 1, 10-phenanthroline-5, 6-dione (L2)
On the base of the literature report [51], a 200 mL double-neck
round bottom flask was charged by 90 mL of H2SO4. The container
was transferred to liquid nitrogen and simultaneously, it was maintained
under inert nitrogen atmosphere. Then, 20 mmol of 1, 10-phenanthro-
line (L1) (3.65 g) and 208.8 mmol of KBr (24.85 g) were added to the
vessel and allowed to the mixture to reach room temperature. After the
appearance of orange solution, the concentrated HNO3 (50 mL) was
carefully added dropwise until the solution color turns to red. Then, it
was heated at 90 ◦C for 5 h under continuously mild stirring. After that,
the crude was poured slowly in to 1 L of cooled water followed by
neutralization by Na2CO3. The reaction mixture was extracted using
CH2Cl2 to find white yellow organic layer. The obtained organic layer
was dried over MgSO4. The orange-yellow precipitate was observed
after the evaporation of solvent under vacuum. Finally, the yield of pure
1, 10-phenanthroline-5,6-dione was calculated to be 91 % after recrys-
tallization from EtOH.
2.2.5. Preparation of Fe3O4@SiO2 core-shell
Routinely, Fe3O4 NPs (0.5 g) was dispersed in 5 mL of deionized H2O
with stirring at room temperature. Afterwards, this crude was treated by
a solution of 0.2 mL of TEOS in ethanol (50 mL). Then, NaOH solution
(5.0 mL, 10 wt%) was slowly added to the latter drop by drop and the
resulting was let to be stirred for extra 30 min. Finally, Fe3O4@SiO2
core-shells were accumulated magnetically, washed three times with
H2O/EtOH (1:1, 30 mL), and dried at 80 ◦C [55].
2.2.2. The synthesis of 3a, 1H-imidazo [4,5-f][1,10]phenanthroline (L3)
Based on reported procedure in the literature [52], 5 mmol of 1,
10-phenanthroline-5,6-dione (1.05 g), 5.5 mmol of formaldehyde (37
wt.%), 16 ml glacial acetic acid, and 103.78 mmol of ammonium acetate
(8 g) were surcharged to a flask and the resultant was let to be stirred
under the reflux condition for 1 h. The mixture was then cooled down to
room temperature, diluted with H2O (10 mL), and neutralized by
ammonia solution to attain cream residue. In the next, the precipitate
was washed three times by acetone and water, respectively, and trans-
ferred to a desiccator to be dried. The recrystallization process from
EtOH presented pure compound 3 in 88 % yield.
2.2.6. Preparation of Fe3O4@SiO2-Cl
1.0 g of as prepared Fe3O4@SiO2 NPs were dispersed ultrasonically
in 20 mL of ethanol and the resulting mixture was treated by (3-chlor-
opropyl)triethoxysilane (3 mmol, 0.7 mL). In continuous, the obtained
solution was refluxed with stirring for 12 h to graft (3-chloropropyl)
triethoxysilane on the surface of Fe3O4@SiO2 NPs. The product,
Fe3O4@SiO2-Cl, was entirely collected by magnetic separation process,
washed with a mixture of water and ethanol (3 × 30 mL) to rectify
unreacted precursors and dried at 70 ◦C [54].
2.2.7. Preparation of Fe3O4@SiO2-Phen (L5)
Subject to a simple method, 1 g of the synthesized Fe3O4@SiO2-Cl
particles were interspersed in 20 mL of THF. The presented solution was
augmented by 3 mmol of ligand L4 and the final mixture
was stirred for 12 h under the nitrogen atmosphere. The obtained L5
scaffold was purified as the same as former step.
2.2.3. The synthesis of 1-butyl-3a, 11b-dihydro-1H-imidazo[4, 5-f] [1,10]
phenanthroline (L4)
Accordingly [53], 0.67 g of as prepared structure L3 (3 mmol) was
firstly dissolved in acetone and then, was treated by KOH (0.2 g, 3.6
mmol). Next, the resulted crude was stirred under the reflux conditions
and after 1 h, 3 mmol of butyl iodide (0.34 mL) was subjoined to ob-
tained mixture. This crude was kept on the reflux condition for 48 h with
somehow vigorous stirring. The reaction progress was monitored by TLC
and after the completion of reaction, the solvent was vaporized and the
obtained residue was extra purified by dissolving in DCM followed by
filtering off. Eventually, light brown precipitate (81 % yield calculated
by GC) was observed by column chromatography (eluent: n-hexane:
2.2.8. Preparation of Fe3O4@SiO2-Phen-Pd (0) (L5-Pd)
Eventually, the magnetic nanocatalyst L5-Pd was synthesized by the
addition of 5 mmol of Pd(OAc)2 to the mixture of dispersed L5 (1 g) in
EtOH (20 mL) and the crude was stirred drastically for 6 h under the
reflux condition. This ultimate magnetic nanocatalyst L5-Pd was wholly
detached from the reaction medium, washed with H2O: EtOH three
3

N. Bagherzadeh et al.
Molecular Catalysis 504 (2021) 111489
Scheme 2. The synthesis steps of the Fe3O4@SiO2-Phen-Pd (0) Nanocatalyst.
times, and finally, dried at 70 ◦C. The synthetic pathway for synthesizing
L5-Pd is illustrated in Scheme 2.
transform infrared), powder XRD (X-ray diffraction), DLS (Dynamic
light scattering), BET (Brunauer ꢀ Emmett ꢀ Teller), FE-SEM (Field
emission scanning electron microscopy), TEM (Transmission electron
microscopy), TGA (Thermogravimetric analysis), ICP (Inductively
coupled plasma), VSM (Vibrating sample magnetometer), EDX (Energy
dispersive X-ray), and UV–vis (Ultraviolet–visible spectroscopy). The
FTIR was the first investigation for the characterization of Fe3O4,
Fe3O4@SiO2, 1-butyl-1H-imidazo[4,5f][1,10] phenanthroline (Phen),
Fe3O4@SiO2-Cl, Fe3O4@SiO2-Phen, and Fe3O4@SiO2-Phen-Pd(0) mag-
netic NPs, respectively (Fig. 2a-f). The peak at 581 cmꢀ 1 can be depicted
to the presence of Fe-O bonds in Fe3O4 and is clearly observed in the
other spectrum that clarifies the stability of Fe3O4 MNPs in all synthetic
steps (Fig. 2a, b, d–f). Moreover, the broad peak at 3424 cmꢀ 1 is
attributed to OH groups on Fe3O4 particles. Fe3O4@SiO2 scaffolds were
characterized by two well-known broad and intense bands at around
1080ꢀ 1100 cmꢀ 1 assigned to the presence of Si-O-Si asymmetric and
symmetric vibrations in the silica shell (Fig. 2b,d–f). The successful
preparation of 1-butyl-1H-imidazo[4,5-f][1,10] phenanthroline(Phen)
is confirmed by the presence of specific absorbed peaks at 1455 cmꢀ 1
2.2.9. General procedure for Suzuki-Miyaura coupling reaction
Aryl halide (1 mmol), 1.1 mmol of arylboronic acid and 2 mmol of
K2CO3 were transferred to 10 mL round bottom flask as in addition to the
addition of Fe3O4@SiO2-Phen-Pd(0) (5 mg, 0.17 mol% Pd) in 4 mL of
EtOH/H2O (1:1). The mixture was stirred at 50 ◦C for appropriate time
and the progress of the reaction was checked out by TLC analysis. After
finishing the reaction, the magnetic nanocatalyst was segregated from
the reaction and the obtained crude was extracted using H2O:DCM so-
lution. Continuously, the organic layer was dried
over MgSO4 and filtered off. The filtrate was concentrated by evap-
oration under vacuum and the residue was finally purified using flash
chromatography by n-hexane / DCM (10:2) as eluent to detect desire
product.
2.2.10. General procedure for synthesizing of 5-substituted 1H-tetrazole
derivatives
(Bending vibrations of aliphatic C H), 1673 cmꢀ 1 (aromatic stretching
–
To the same vessel containing 4 mL EtOH/H2O (1:1), there were
added all ingredients including arylboronic acid (1 mmol), KCN (3
mmol), NaN3 (3 mmol), K2CO3 (2 mmol) and 4 mg of Fe3O4@SiO2-Phen-
Pd(0) (0.14 mol% Pd). This multicomponent one-pot reaction was
refluxed under nitrogen in appropriate times. The reaction progress was
monitored by TLC analysis and eventually, the inorganic impurities
were taken away from the mixture after extracting with H2O:DCM (1:1)
mixture (3 × 50 mL). The organic layer was dried over anhydrous
Na2SO4 and then, the filtrate was concentrated under vacuum to get
crude residue. The pure outcomes were achieved in good to excellent
yields using column chromatography with petroleum ether:DCM (10:2)
as eluent.
ꢀ 1
–
–
–
of C C), 1681 cm (stretching vibrations of C N), and 2870-2970
–
cmꢀ 1 (CH bonds stretching peaks) (Fig. 2c). The existence of vibra-
tions at 610 cmꢀ 1 in Fig. 2d along with almost the same peaks in com-
parison with Fig. 2 a and b assured us from successful production of
Fe3O4@SiO2-Cl. In continues, the interaction between phenanthroline
derivative and magnetic Fe3O4@SiO2-Cl was supported by occurring
(Fig. 2e). The characteristic peaks at 1461, 1671, 1689, 2874–2938, and
–
–
3405 cmꢀ 1, which are allocated to bending vibrations of aliphatic C
H
––
––
bonds, aromatic CC, CN
bond stretching vibrations, stretching peaks
–
of C H bonds, respectively, and also the absence of C-Cl absorption
(Fig. 2e). At the end, the presence of the peak at 696 cmꢀ 1 in the
Fe3O4@SiO2-Phen-pd(0) NPs spectrum is attributed to formation of Pd-
N bonds [56] as in addition to the other repetitious absorptions showed
that magnetic nanocatalyst of Fe3O4@SiO2-Phen-Pd(0), has been syn-
thesized (Fig. 2f).
3. Results and discussion
The 1, 10-phenanthroline complex of palladium (0) was anchored on
Fe3O4@SiO2 nanospheres via the multistep procedure exhibited in
Scheme 2.
The crystalline structure of Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2-
Phen-Pd(0) MNPs was investigated by XRD analyses (Fig. 3a-c). In the
XRD patterns, the peaks at 2θ = 30.7◦, 35.6◦, 43.3◦, 53.1◦, 56.9◦ and
The prepared catalyst was then examined by FT-IR (Fourier
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N. Bagherzadeh et al.
Molecular Catalysis 504 (2021) 111489
62.3◦ with the Bragg reflections of (220), (311), (400), (422), (511), and
(440), respectively, revealed the cubic spinel structure of Fe3O4 MNPs
which agrees with the standard magnetite XRD spectra (JCPDS card no.
19-629) (Fig. 3a). Additionally, a decrease in the intensity of all
diffraction peaks was observed as in addition to appearance of broad
peak between 2θ = 10◦-20◦ owing to the presence of amorphous silica
shell around magnetic particles in Fe3O4@SiO2 NPs (Fig. 3b). The XRD
pattern of Fe3O4@SiO2-Phen-Pd(0) contains somehow the same re-
flections but in shorter peaks due to the presence of organic compounds
beside amorphous silica(Fig. 3c). These data exhibit the excellent
coating of Fe3O4 magnetic particles. Moreover, the mean crystal size of
Fe3O4 particles (D) was calculated to be approximately 19 nm using the
Debye-Scherrer equation (D= Kλ/βcosθ).
The N2 adsorption-desorption isotherms were taken to examine the
porous structure and surface area of the NPs. The measured specific
surface areas were 473.2, 423.7 and 389.3 m2/g for Fe3O4, Fe3O4@SiO2
and Fe3O4@SiO2-Phen-Pd(0), respectively, and the mean pore diameter
was determined by BET analysis to be 20.1 nm (Table 1). In addition,
Scherrer equation was applied to get the mean particle sizes of synthe-
sized nanoparticles in each step. The sizes were obtained to be 13.7,
16.3, and 19 nm for Fe3O4, Fe3O4@SiO2, and Fe3O4@SiO2-Phen-Pd(0),
respectively (Table 1).
The morphology and size of synthesized magnetic nanoparticles are
of the main goals in the synthesis of such particles. As illustrated in
Fig. 4, the FESEM (Fig. 4a-c) and DLS analyses of nanoparticles
(Fig. 4d–f) present the abiding shape of these nanoparticles indicating
the stability of synthesized particles in such conditions. The size of
nanoparticles is in good harmony with the results had been provided
from XRD investigations. The increasing trend in the particle size can
also be followed by DLS measurements in which the mean particle size
was increased in each step (Fig. 4d–f).
TEM (Fig. 5a-c) images of Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2-
Phen-Pd(0)
Fig. 2. FT-IR spectra of (a) Fe3O4, (b) Fe3O4@SiO2, (c) Phen, (d) Fe3O4@SiO2-
Cl, (e) Fe3O4@ SiO2-Phen, and (f) Fe3O4@SiO2-Phen-Pd(0) NPs.
MNPs exhibit spherical shape and the clear core-shell structure of
nanoparticles (Fig. 5c).
The stability and magnetic capability of final nanomagnetic catalyst
were pursued using TGA (between 0ꢀ 600 ◦C) and VSM analyses (Fig. 6).
As it can be observed, the first weight loss was occurred at around 110 ◦C
in TGA curve resulting from the elimination of absorbed water and some
solvents (Fig. 6A) [57]. In continue, the other weigh loses observed were
related to the organic constructing compounds of magnetic nanocatalyst
up to 600 ◦C, which are totally equal to only 29 % weight loss that is
significantly snazzy. This information presents the sustainable structure
of synthesized nanocatalyst and its applicability in higher temperatures
which are suitable for industrial processes.
Based on the magnetization curves, no hysteresis in samples was
obtained with the saturation magnetization values of 70.7, 49.6, and
24.8 emu/g for Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2-Phen-Pd(0)
respectively, which disclose the superparamagnetic character of mag-
netic particles. The reductions in magnetization properties of Fe3O4@-
SiO2 and Fe3O4@SiO2-Phen-Pd (0) nanoparticles attributes to silica shell
and the organic layers. However, the magnetic strength of final nano-
catalysts is enough to be entirely separated from the reaction media
using an external magnet (Fig. 6Ba-c).
Table 1
Some properties of Fe3O4, Fe3O4@SiO2 and Fe3O4@SiO2-Phen-Pd(0) NPs.
Sample
Fe3O4 crystal
structure
Specific
Magnetite
surfacearea (m2/
g)a
particlesize (nm)b
Fe3O4
Cubic spinel
Cubic spinel
Cubic spinel
473.2
423.7
389.3
13.7
16.3
19
Fe3O4@SiO2
Fe3O4@SiO2-
Phen-Pd(0)
Fig. 3. XRD diffraction pattern of (a) Fe3O4, (b) Fe3O4@SiO2, and (c)
Fe3O4@SiO2-Phen-Pd(0) NPs.
a
b
Calculated using the BJH method.
Calculated using the Scherrer equation based on XRD patterns.
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Fig. 4. FE-SEM images of (a) Fe3O4, (b) Fe3O4@SiO2, and (c) Fe3O4@SiO2-Phen-Pd (0); and size distributions of (d) Fe3O4, (e) Fe3O4@SiO2, and (f) Fe3O4@SiO2-
Phen-Pd (0) NPs.
Fig. 5. TEM images of (a) Fe3O4, (b) Fe3O4@SiO2, and (c) Fe3O4@SiO2-Phen-Pd (0) NPs.
Fig. 6. (A) TGA spectrum of Fe3O4@SiO2-Phen-Pd(0) NPs;(B) magnetic hysteresis curves of (a) Fe3O4, (b) Fe3O4 @SiO2 and (c) Fe3O4@SiO2-Phen-Pd(0) NPs; (C)
catalyst ability for effective recovery at the end of reactions by an external magnetic field.
Inductively coupled plasma atomic emission spectrometry (ICP-MS)
analysis was taken to evaluate the exact content of decorated Pd parti-
cles in the Fe3O4@SiO2-Phen-Pd(0) magnetic nanocatalyst structure. As
a result, the anchored Pd particles were measured to be 0.33 mmol in 1 g
of MNPs.
UV–vis measurement has been used to prove the incorporated of Pd
onto Fe3O4@SiO2-Phen nanocomposite particles (Fig. 7A). The UV–Vis
spectra of Fe3O4, Pd (OAc)2, Fe3O4@SiO2–Phen and
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N. Bagherzadeh et al.
Molecular Catalysis 504 (2021) 111489
Fig. 7. (A) UV–Vis spectra (B) EDX spectrum of Fe3O4@SiO2-Phen -Pd (0).
Fe3O4@SiO2–Phen–Pd(0) NPs were studied and shown in Fig. 7A. The
presence of two absorption bands in the range of 225ꢀ 305 nm are
Table 2
Evaluation of reaction parameters in the Suzuki-Miyaura reaction between
attributed to π→π*transitions Phen ring [58a]. The disappearance of the
phenylboronic acid and bromobenzene.a
peak at around 400ꢀ 430 nm which belongs to Pd (II) species, demon-
strates the complete conversion of Pd (II) to Pd(0) [58b]. Energy
dispersive spectroscopy (EDX) is a useful method to characterize the
elemental profile of compounds. Therefore, the EDX analysis, which can
obtain further indication for anchoring palladium complex on the sur-
face of Fe3O4@SiO2, was performed. The EDX spectrum of Fe3O4@-
SiO2–Phen–Pd(0) shows the presence of Fe, Si, Pd, Cl, C, N, and O
elements, illustrating the successful synthesis of Fe3O4@SiO2-Phen-Pd
(0) MNPs (Fig. 7B).
entry
catalyst (mg/
mol% of Pd)
solvent
base
temp.
(◦C)
time
(h)
yield
(%)b
1
2
3
4
10/0.33
10/0.33
10/0.33
10/0.33
H2O
K2CO3
K2CO3
K2CO3
K2CO3
50
50
50
50
2
2
2
2
66
86
91
90
EtOH
DMF
EtOH/H2O
(1:1)
5
10/0.33
DMF/H2O
(1:1)
K2CO3
50
2
89
With the full characterization data in hand, the catalytic ability of
magnetic nanocatalyst was considered in Suzuki-Miyaura C-C coupling
reaction and the synthesis of variety of different tetrazoles. Firstly, the
optimized reaction parameters were found for Suzuki-Miyaura cou-
plings in the reaction between bromobenzene and phenylboronic acid as
a test reaction. As summarized in Table 2, a library of solvents, bases,
various temperatures, reaction times, and catalyst amounts were
employed. Different protic and aprotic solvents were applied (Table 2,
entries 1–7) and the best efficiency was observed in EtOH/H2O as sol-
vent (Table 2, entry 4). In ongoing, varies amounts of the synthesized
magnetic nanocatalyst were applied (Table 2, entries 8–11) and sur-
prisingly, only 5 mg of which (0.17 mol% of pd) triggered the reaction
(Table 2, entry 10). Noteworthy, no more yield was obtained with more
amounts of catalyst. Among the bases applied, K2CO3 exhibited highest
efficiency (Table 2, entry 10). The Suzuki-Miyaura reaction is somehow
temperature sensitive in which different temperatures exhibit different
results. So, the model reaction was explored in diverse temperatures to
find the best one and as it cited in (Table 2, entries 10, 17–19), not better
data were achieved in higher and lower temperatures than 50 ◦C
(Table 2, entries 10, 18, and19). Finally, the reaction time was subjected
to study be investigated and checking out the reaction progress revealed
that 1.5 h is enough to get the best result (Table 2, entry 22). Thereupon,
the optimized conditions were found to be using EtOH/H2O as solvent,
in the presence of 5 mg of catalyst (0.17 mol% of pd), K2CO3, 50 ◦C, and
1.5 h reaction time.
6
7
8
10/0.33
10/0.33
–
DMSO
K2CO3
K2CO3
K2CO3
50
2
90
57
–
Toluene
EtOH/H2O
(1:1)
50
2
Reflux
24
9
3/0.1
EtOH/H2O
(1:1)
K2CO3
K2CO3
K2CO3
–
50
2
60
94
86
Trace
71
80
82
84
61
77
90
55
96
92
10
11
12
13
14
15
16
17
18
19
20
21
22
5/0.17
7/0.23
5/0.17
5/0.17
5/0.17
5/0.17
5/0.17
5/0.17
5/0.17
5/0.17
5/0.17
5/0.17
5/0.17
EtOH/H2O
(1:1)
50
2
EtOH/H2O
(1:1)
50
2
EtOH/H2O
(1:1)
Reflux
50
24
2
EtOH/H2O
(1:1)
Na2CO3
K3PO4
NaOAc
Et3N
EtOH/H2O
(1:1)
50
2
EtOH/H2O
(1:1)
50
2
EtOH/H2O
(1:1)
50
2
EtOH/H2O
(1:1)
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
r.t
24
24
3
EtOH/H2O
(1:1)
40
EtOH/H2O
(1:1)
60
EtOH/H2O
(1:1)
50
1
EtOH/H2O
(1:1)
50
50
1.5
2.5
The next step is using perceived optimized conditions to scrutiny the
reaction scope in which a wide spread various of aromatic alides (I, Br
and Cl) and arylboronic acids have been adverted (Table 3). It was
observed that almost all purified products were achieved in excellent
yields in TOFs point of view, but aryl iodides are more reactive than aryl
bromides and the latter are more reactive that aryl chlorides (Table 3).
Interestingly, both electron-releasing groups and electron-withdrawing
groups on arylboronic acids structure presented good to excellent
yield with slightly better result for electron-withdrawing groups
EtOH/H2O
(1:1)
a
Reaction conditions: phenylboronic acid (1.1 mmol), bromobenzene (1
mmol), Fe3O4@SiO2-Phen -Pd (0), solvent (4 mL), base (2 mmol).
b
Isolated yield.
7

N. Bagherzadeh et al.
Molecular Catalysis 504 (2021) 111489
Table 3
Suzuki coupling reaction of arylboronic acids with aromatic halides in the presence of Fe3O4@SiO2-Phen-Pd (0) NPs.a
(3a)
(3b)
(3c)
X = I: 0.7 h, 97 %d, TOF = 815.1 (hꢀ 1
)
)
X = I: 1.4 h, 92 % d, TOF = 386. (hꢀ 1
)
X = I: 1.3 h, 92 %d, TOF = 416.3(hꢀ 1
)
Br: 1.5 h, 96 % d, TOF = 376.5 (hꢀ 1
)
Br: 2.8 h, 89 % d, TOF = 187 (hꢀ 1) Cl: 15 h,
Br: 2.5 h, 90 % d, TOF = 211.8 (hꢀ 1
)
Cl: 12 h, 66 % dtof = 32.4 (hꢀ 1
)
63 % d, TOF = 24.7 (hꢀ 1
)
Cl: 12 h, 65 % d, TOF = 32 (hꢀ 1
)
(3d)
(3c)
(3f)
X = I: 0.5 h, 98 % d, TOF = 1153 (hꢀ 1
X = I: 1.3 h, 92 %d, TOF = 416.3 (hꢀ 1
)
X = I: 1.5 h, 94 % d, TOF = 368.6 (hꢀ 1
)
Br: 1.2 h, 95 % d, TOF = 465.7 (hꢀ 1
)
Br: 2.5 h, 90 % d, TOF = 211.8 (hꢀ 1
)
Br: 3 h, 90 % d, TOF = 176.5 (hꢀ 1
)
Cl: 10 h, 83 % d, TOF = 48.8 (hꢀ 1
)
Cl: 12 h, 65 % d, TOF = 32 (hꢀ 1
)
Cl: 12 h, 75 % d, TOF = 36.8 (hꢀ 1
)
(3 g)
(3 h)
(3i)
X = I: 0.6 h, 96 % d, TOF = 941.2 (hꢀ 1
)
X = I: 2 h, 90 % d, TOF = 267.7 (hꢀ 1
)
X = I: 3 h, 88 % d, TOF = 172.6 (hꢀ 1
)
Br: 1.5 h, 91 % d, TOF = 357 (hꢀ 1
)
Br: 3.5 h, 83 % d, TOF = 139.5 (hꢀ 1
)
Br: 4 h, 80 % d, TOF = 117.6 (hꢀ 1
)
Cl: 11 h, 77 % d, TOF = 41.2 (hꢀ 1
)
Cl: 13 h, 70 % d, TOF = 31.7 (hꢀ 1
)
Cl: 15 h, 68 % d, TOF = 26.7 (hꢀ 1
)
(3 j)
(3k)
(3 l)
X = I: 3.1 h, 88 % d, TOF = 167 (hꢀ 1
)
)
X = I: 3.5 h, 85 % d, TOF = 144.5 (hꢀ 1
)
X = I: 3.5 h, 84 % d, TOF = 141.2 (hꢀ 1
)
)
Br: 4.2 h, 81 % d, TOF = 113.4 (hꢀ 1
)
Br: 4.5 h, 78 % d, TOF = 102 (hꢀ 1
)
Br: 4.3 h, 77 % d, TOF = 105.3 (hꢀ 1
)
Cl: 15 h, 69 % d, TOF = 27 (hꢀ 1
)
Cl: 15 h, 64 % d, TOF = 25.1 (hꢀ 1
)
Cl: 14 h, 62 % d, TOF = 26 (hꢀ 1
)
(3 m)
(3n)
(3o)
X = I: 4 h, 80 % d, TOF = 117.6 (hꢀ 1
X = I: 1.5 h, 92 %d, TOF = 360.8 (hꢀ 1
)
X = I: 1.7 h, 90 % d, TOF = 311.4 (hꢀ 1
Br: 5 h, 72 % d, TOF = 84.7 (hꢀ 1
)
Br: 2.2 h, 89 % d, TOF = 238 (hꢀ 1
Cl: 11 h, 66 % d, TOF = 36.3 (hꢀ )
)
)
Br: 2.6 h, 84 % d, TOF = 190 (hꢀ 1
)
Cl: 16 h, 60 % d, TOF = 22.1 (hꢀ 1
)
Cl: 13 h, 61 % d, TOF = 27.6 (hꢀ 1
)
(3p)
(3q)
(3 r)
X = I: 1.6 h, 91 % d, TOF = 344.6 (hꢀ 1
)
X = I: 3.2 h, 91 % d, TOF = 167.3 (hꢀ 1
)
)
X = I: 3.5 h, 90 % d, TOF = 151.3(hꢀ 1
)
Br: 2.3 h, 82 % d, TOF = 210 (hꢀ 1
)
Br: 4 h, 88 % d, TOF = 129.4 (hꢀ 1
Br: 4.2 h, 87 % d, TOF = 122 (hꢀ 1
)
Cl: 12 h, 60 % d, TOF = 29.4 (hꢀ 1
)
(3 s)
(3 t)
(3 u)
X = I: 3 h, 89 % d, TOF = 174.5 (hꢀ 1
)
X = I: 1.8 h, 90 % d, TOF = 294.1 (hꢀ 1
Xb= I: 3 h, 84%d, TOF = 82.4 (hꢀ 1
)
Br: 4.2 h, 83 % d, TOF = 116.2 (hꢀ 1
)
Br: 2.1 h, 86 % d, TOF = 241 (hꢀ 1
)
Br: 4 h, 75 %d, TOF = 55.2 (hꢀ 1
)
(3v)
(3 w)
(3x)
Xb= I: 2.4 h, 88%d, TOF = 108(hꢀ 1
)
Xc= Cl: 5 h, 80%d, TOF = 31.4 (hꢀ 1
)
Xc= Cl: 5 h, 83%d, TOF = 32.6 (h1)
Br: 3.2 h, 83 %d, TOF = 76.3 (hꢀ 1
)
a
Reaction conditions: arylboronic acid (1.1 mmol), aromatic halide (1 mmol), Fe3O4@SiO2-Phen-Pd(0) (5 mg, 0.1 mol% Pd), K2CO3 (2 mmol), EtOH/H2O (1:1, 4
mL), 50 ◦C.
b
Reaction conditions: 2.2 mmol of arylboronic acid, 1 mmol of aromatic halide, Fe3O4@SiO2-Phen-Pd(0) (10 mg, 0.34 mol% Pd), 4 mmol of K2CO3, EtOH/H2O (1:1,
10 mL), and 60 ◦C.
c
Reaction conditions: 3.5 mmol of arylboronic acid, 1 mmol of aromatic halide, Fe3O4@SiO2-Phen-Pd(0) (15 mg, 0.51 mol% Pd), 7 mmol of K2CO3, EtOH/H2O (1:1,
20 mL), and 60 ◦C.
d
Isolated yield.
8

N. Bagherzadeh et al.
Molecular Catalysis 504 (2021) 111489
Table 4
Table 5
Comparison of performance of the some reported Pd-based catalysts in the
Evaluation of the reaction parameters for the synthesis of 5-phenyl 1H-
Suzuki
ꢀ Miyaura coupling reaction between phenylboronic acid and
tetrazole.a
bromobenzene.
Entry
1
Catalyst
Condition
Time
(h)
Yield
(%)
TOF
(hꢀ 1
Ref.
)
Entry
Molar ratio:
Catalyst
Reaction
Time
(h)
Yield
(%)b
Pd-Fe3O4 (0.1 mol
%)
DME/H2O
(3:1)/
48
92
19.2
[59]
Phenylboronic acid
/ NaN3/KCN
amount (mg/
mol% of Pd
(0))
conditions
K2CO3/
reflux
2
C/Co@PNIPAM-
PPh2-Pd(3 mol%)
Toluene/
H2O (2:1)/
K2CO3/85
◦C
16
99
2.1
[60]
1
2
3
1:3:3
1:3:3
1:3:3
5/0.17
5/0.17
5/0.17
MeOH/
3
3
3
45
35
43
Reflux
EtOH/
Reflux
3
4
Fe3O4-Bpy-Pd
Toluene/
K2CO3/80
◦C
6
2
99
96
33
44
[61]
[62]
CH3CN/
Reflux
(OAc)2 (0.5 mol%)
4
5
1:3:3
1:3:3
5/0.17
5/0.17
H2O/Reflux
EtOH/H2O
(1:1)/Reflux
Dioxane/
Reflux
3
3
47
90
Co/C-ROMPgel
immobilized Pd-
complex (1.1 mol
%)
THF/H2O
(1:2)/
Na2CO3/65
6
1:3:3
5/0.17
8
30
◦C
5
Fe@FexOy/Pd
(0.5 mol%)
EtOH/H2O
(1:1)/
2
2
6
4
2
5
1
5
98
99
85
92
95
75
90
84
98
[63]
[64]
[65]
[66]
[67]
[68]
[69]
[70]
7
8
1:3:3
1:3:3
5/0.17
5/0.17
PEG/100 ◦
PEG/H2O
C
3
3
78
83
K2CO3/r.t
CH3OH/
Na2CO3/60
◦C
(1:1)/90 ◦
C
6
Xerogel g1-MNPs
(1 mol%)
49.5
14
9
1:3:3
1:3:3
5/0.17
5/0.17
DMF/110 ◦
DMF/H2O
(1:1)/100 ◦
C
3
3
90
89
10
C
7
Pd@Mag-MSN (1
mol%)
CH2Cl2/
K2CO3/80
◦C
11
12
1:3:3
1:3:3
5/0.17
5/0.17
NMP/120 ◦C
DMSO/110
◦C
4
3
63
65
8
Pd(0)-ZnFe2O4
(4.62 mol%)
EtOH/
5
13
1:3:3
5/0.17
Ethylene
1.5
89
K2CO3/
Reflux
glycol/90 ◦
C
14
15
1:3:3
1:3:3
5/0.17
5/0.17
THF/Reflux
Solvent-
8
8
40
20
9
Pd–MWCNT (0.3
MeOH,
NaOAc/
Reflux
158
15
mol%)
free/110 ◦
C
16
17
18
19
20
21
22
23
24
25
1:1:1
1:2:2
1:4:4
1:3:3
1:3:3
1:3:3
1:3:3
1:3:3
1:3:3
1:3:3
5/0.17
5/0.17
5/0.17
None
EtOH/H2O
(1:1)/Reflux
EtOH/H2O
(1:1)/Reflux
EtOH/H2O
(1:1)/Reflux
EtOH/H2O
(1:1)/Reflux
EtOH/H2O
(1:1)/Reflux
EtOH/H2O
(1:1)/Reflux
EtOH/H2O
(1:1)/Reflux
EtOH/H2O
(1:1)/Reflux
EtOH/H2O
(1:1)/Reflux
EtOH/H2O
(1:1)/
3
48
10
11
12
Pd–Fe3O4@IM (1
H2O/
mol%)
K2CO3/70
3
76
◦C
Fe3O4@MCM-
41@Pd-SPATB
(1.26 mol%)
Pd0-AmP-SMC
(0.2 mol%)
PEG/
142.8
98.8
3
84
K2CO3/100
◦C
24
8
Trace
47
EtOH/H2O
(1:1)/
2/0.07
4/0.14
6/0.2
K2CO3/90
◦C
3
93
13
Fe3O4@SiO2-
Phen-Pd(0) (0.17
mol% Pd)
EtOH/H2O
(1:1)/
1.5
96
376.5
This
work
5
88
K2CO3/50
◦C
4/0.14
4/0.14
4/0.14
1
55
1.5
2
87
97
Reflux
26
a
1:3:3
4/0.14
EtOH/H2O
(1:1)/Reflux
2.5
95
Reaction conditions: Phenylboronic acid (1.0 mmol), K2CO3 (2 mmol), NaN3
(3 mmol), KCN (3 mmol), solvent (4 mL).
b
Isolated yield.
(Table 3, entries 3n-3o). The existences of aromatic halides with
electron-withdrawing groups were let to the better result in comparison
with ones with electron-releasing groups (Table 3, entries 3d-3 g). For
expanding of the applicability of this approach, heteroaryl halides were
also employed and all of them undergo efficiently the Suzuki reactions
and provided high to excellent yields but in higher temperature (Table 3,
entries3p and 3 u-3x).
To ascertainment the capability of applied nanocatalyst, the appli-
cation of that to accomplish the Suzuki coupling between bromobenzene
and phenylboronic was selected to be compared with the literature re-
ports (Table 4). As it can be observed in Table 4, the most of catalysts
published catalyze the C-C cross coupling reaction to prepare biphenyl
Scheme 3. Plausible mechanism of Suzuki-Miyaura coupling reaction.
9

N. Bagherzadeh et al.
Molecular Catalysis 504 (2021) 111489
Table 6
Table 7
Catalyzed one-pot synthesis of 5-substituted 1H-tetrazoles with Fe3O4@SiO2-
Catalytic performance of two Pd-based catalysts in the one-pot reaction for
conversion of 4- methoxy phenylboronic acid to 5-(4-methoxyphenyl)-1H-
tetrazole.
Phen-Pd (0) in water/ethanol.a
Entry
1
Catalyst
Condition
Time
(h)
Yield
(%)a
TOF
(hꢀ 1
Ref.
)
Pd(II)-ONO
DMSO/H2O
(1:1)/KOH/90
◦C, NaN3,
6
86
143
[44]
Pincer(PPh3)
(0.1 mol% Pd)
(4a)
(4b)
(4c)
Time =2 h, Yield = 97%,b
Time =1.7 h, Yield =
Time =1.5 h, Yield =
91%, b
92%, b
NH4HCO3
TOF =346.4 h-1
TOF =382.4 hꢀ 1
TOF =438 hꢀ 1
2
a
Fe3O4@SiO2-
Phen-Pd(0)
EtOH/H2O
(1:1)/K2CO3/
Reflux, NaN3,
KCN
1.5
92
438
This
work
(0.14 mol% Pd)
Isolated yield.
(4d)
(4e)
(4 F)
Time =1.7 h, Yield = 88
Time =2 h, Yield = 86%,
Time =1.5 h, Yield =
%,b
95%, b
b
nanocatalyst will be produced to take part in consecutive runs.
TOF =369.7 hꢀ 1
TOF =245.7 hꢀ 1
TOF =452.4 hꢀ 1
4. The synthesis of tetrazoles
–
Attractive results associated to the successful C C bond formation
using Suzuki-Miyaura coupling reaction by Fe3O4@SiO2-Phen-Pd(0)
magnetic nanocatalyst caused strong tendency to check out the appli-
cability of such nanocatalyst in the production of a library of tetrazoles
starting from arylboronic acids, as novel alternatives precursors.
Inspiring from reported data in the literature [44], the reaction among
phenylboronic acid, NaN3, and KCN as a model reaction, was approved
to find optimized reaction conditions (Table 5). As shown in Table 5, a
group of solvents were firstly perused (Table 5, entries 1–15) and using
EtOH/H2O and DMF displayed the best results (Table 5, entries 5 and 9).
(4 g)
(4 h)
(4i)
Time =1.5 h,Yield = 97
Time =3 h, Yield = 86 %,
Time =3 h, Yield = 94 %,
%, b
b
b
TOF =462 hꢀ 1
TOF =204.8 hꢀ 1
TOF =223.8 hꢀ 1
(4 j)
(4k)
(4 l)
Time =3.5 h, Yield = 96
Time =4 h, Yield = 93 %,
Time =4 h, Yield = 92 %,
%, b
b
b
TOF =196 hꢀ 1
TOF =166 hꢀ 1
TOF =164.3 hꢀ 1
(4 m)
(4n)
(4O)
Time =3 h, Yield = 96 %,
Time =4 h, Yield = 91 %,
Time =6 h, Yield = 88 %,
b
b
b
TOF =228.6 hꢀ 1
TOF =162.5 hꢀ 1
TOF =104.8 hꢀ 1
(4p)
(4q)
(4 r)
Time =5.5 h, Yield = 95
Time =5 h, Yield = 96 %,
Time =6 h, Yield = 90 %,
%,b
b
b
TOF =123.4 hꢀ 1
TOF =137.1 hꢀ
TOF =107.1 hꢀ 1
(4 s)
Time =6 h, Yield = 84 %, b
TOF =100 hꢀ 1
a
Reaction conditions: arylboronic acid (1 mmol), K2CO3 (2 mmol), KCN (3
mmol), NaN3 (3 mmol), Fe3O4@SiO2-Phen-Pd(0) (4 mg, 0.14 mol% Pd,), EtOH/
H2O (1:1, 4 mL), Reflux.
b
Isolated yield.
at higher temperatures, in toxic solvents, and using larger amounts of
catalysts after longer reaction times in lower yields with significantly
low TOFs (Table 4, entries 1–12).
In consonance with, the plausible mechanism is proposed in Scheme
3. The oxidative addition is the first step, which can be carried out on the
catalyst A to reach the intermediate B. After getting unstable structure C
in situ, resulting from the treatment of arylboronic acid with hydroxyl
ions, the transmetalation step is happened and both intermediated E and
F would be apperceived. The final product G is obtained after passing
reductive elimination step and eventually, Pd (0)-based magnetic
Scheme 4. Proposed mechanism of the direct transformation of arylboronic
acids to the corresponding tetrazoles.
10

N. Bagherzadeh et al.
Molecular Catalysis 504 (2021) 111489
However, in green point of view, the mixture of EtOH/H2O (1:1) was
selected to be utilized as a green solvent for the rest of more in-
vestigations (Table 5, entry 5). The effect of the molar ratios of KCN and
NaN3 relative to phenylboronic acid was also investigated (Table 5,
entries 16–18). The best yield of 5-phenyl 1H-tetrazole was produced
with a 1:3:3 M ratio of phenylboronic acid relative to KCN and NaN3
(Table 5, entry 5). Then, the catalyst amount was optimized (Table 5,
entries 19–22) and maximum efficiency was achieved when 4 mg of
nanocatalyst (0.14 mol% of Pd) was loaded into the reaction mixture
(Table 5, entry 21). Accordingly, the yield of the desired product,
5-phenyl 1H-tetrazole, was declined in the presence of lower and higher
amount than 4 mg of the magnetic nanocatalyst (Table 5, entries 20 and
22). Finally, the reaction time effect was considered on the preparation
of 5-phenyl 1H-tetrazole and best catalyst efficiency was obtained after
2 h (Table 5, entry 25). Consequently, the best conditions were found
using 4 mg of nanocatalyst (0.14 mol% of Pd) in EtOH/H2O (1:1) as
solvent under reflux was chosen as the best conditions for the rest of
study and 2 h of reaction time.
tetrazoles in the presence of Fe3O4@SiO2-Phen-Pd(0) magnetic nano-
catalyst as indexed in Table 6. Variety of arylboronic acids with electron-
releasing groups and electron-withdrawing groups carried out the
preparation of 5-substituted 1H-tetrazoles in high to excellent yields in
range of 1.5–6 h (Table 6) under reflux condition in EtOH/H2O (1:1). In
general, arylboronic acids with electron releasing substitution are more
reactive than electron-withdrawing one. Ortho isomers of arylboronic
acids undergo usually the reaction in longer reaction times and in lower
yields (Table 6, entries 4 and 8) probably due to ortho steric effect.
Phenethyl boronic acid as an aliphatic boronic acid provides the related
tetrazole in lower yield at longer time than arylboronic acids (Table 6,
entry 16). Polynuclear arylboronic acids are transformed to the desired
tetrazole in comparable yields to arylboronic acids but in longer reaction
time (Table 6, entries 14–16).
Finally, we compared the capability of the synthesized nanocatalyst
in the synthesis of 5-(4-methoxyphenyl)-1H-tetrazole with the only
report published in the literature using arylboronic acids (Table 7). As
can be seen, the ONO pincer type Pd(II) complex bearing toxic ligand of
triphenylphosphine catalyzes preparation of 5-(4-methoxy phenyl)-1H-
tetrazole in aqueous solution of DMSO as toxic solvent at higher tem-
peratures and longer reaction time in lower yield with significantly low
As the same as the previous survey on the application of arylboronic
acids in Suzuki type coupling reactions, we have pursued the applica-
bility of such starting materials in the preparation of 5-substituted 1H-
Fig. 8. A) Recyclability of the catalyst in the synthesis of a) 1,1’-biphenyl and b) 5-phenyl-1H-tetrazole; B), C) DLS and FE- SEM images of Fe3O4@SiO2-Phen -Pd (0)
nanoparticles after eight reaction cycles in the synthesis of 1,1’-biphenyl in model reaction.
11

N. Bagherzadeh et al.
Molecular Catalysis 504 (2021) 111489
mainly, the supported catalytic Pd(0) is responsible in the process of
getting products. The ICP analysis was finally studied and it demon-
strates that the leaching amounts of Pd particles are only around 1.1 %
and 0.92 % after passing 8th run in tetrazole and Suzuki reactions,
respectively. All achievements proposed the efficient and impressive
coverage of Pd(0) by magnetic support and the negligible leaching
amount of active palladium particles.
5. Conclusions
In summary, green, unflinching and sustainable nanocatalyst
Fe3O4@SiO2-Phen-Pd(0) was synthesized and entirely characterized in
this reports. This excellent recoverable catalyst was applied in the both
Suzuki-Miyaura and tetrazole reactions starting from arylboronic acids
as superior sources of aryl groups and significant results were observed
in TOFs and recyclability points of views which are mostly important in
the laboratory and industrial applications. The nanocatalyst was sepa-
rated 8 times and reused in subsequent cycles with no diminution in its
activity.
Fig. 9. EDX spectrum of the reused catalyst.
TOF. It is noteworthy, there are significant differences between the two
catalysts in terms of the recovery and reusability, previous report is
recovered by centrifuge and reused just for four runs while the present
nanomagnetic catalyst is separated from the media reaction by an
external magnet and reused for eight cycles.
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
In Scheme 4 it is illustrated how the products are produced using
arylboronic acid precursors inspiring from the reported data in the
literature [44]. Firstly, the Pd-based nanocatalyst A is treated with
activated boronic acid B, potassium trihydroxy(pheny)borate, to give
the sigma complex of palladium(II) C through transmetalation reaction.
In the next step, the scaffold D, which is produced from the reaction KCN
with C, is converted to F via the reaction of E with NaN3. An intra-
molecular 1,3-dipolar cycloaddition reaction transform F to G which, in
turn, is hydrolyzed to the 5-substituted-1H-tetrazole H and regenerate
magnetic nanocatalyst A to begin another cycle of reaction.
CRediT authorship contribution statement
Nastaran Bagherzadeh: Investigation, Visualization, Methodology,
Writing - original draft, Validation, Data curation. Ali Reza Sardarian:
Conceptualization, Resources, Project administration, Funding acquisi-
tion, Writing - review & editing. Hassan Eslahi: Methodology, Writing -
original draft, Validation, Data curation, Resources, Supervision.
Recyclability is a very important aspect of catalyst applications,
economically and environmentally, in laboratory and industry. There-
fore, recyclability of Fe3O4@SiO2-Phen-Pd(0) nanocatalyst was tested
on the both model reactions. Due to supermagnetic behavior of the
catalyst, in a typical approach, the catalyst was taken away from the
reaction media, washed with the mixture of EtOH:H2O (1:1) to remove
unreacted chemical reagents, and reused directly in the next run without
further purification. It was interestingly observed that this potent cata-
lyst presents catalytic properties at least for 8 consecutive cycles
(Fig. 8A).
Acknowledgements
Authors gratefully acknowledge the financial support of this work by
the Research Council of University of Shiraz and the Council of Iran
National Science Foundation.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2021.111489.
Fig. 8B and C exhibit DLS and FE-SEM analyses of the isolated
nanocatalyst after the 8th run for the synthesis of 1,1’-biphenyl in the
model reaction in which the spherical morphology as well as the size
distributions of nanoparticles confirm the stability of catalyst in shape
and applicability. Substantially, the hydrodynamic diameter of
Fe3O4@SiO2-Phen-Pd (0) after eight runs is illustrated in Fig. 8B, the
mean diameter of Fe3O4@SiO2-Phen-Pd 0) and the centered particle size
was calculated to be 41 nm, which is somehow larger than the fresh
catalyst particles. The EDX analysis was also taken on the recovered
catalyst after eight runs. This analysis displayed clearly the existence of
the main catalyst components of the fresh catalyst such as Fe, Pd, Si, O,
N, C and Cl (Fig. 9).
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