Efficient reduction of nitro compounds and domino preparation of 1-substituted-1H-1,2,3,4-tetrazoles by Pd(ii)-polysalophen coated magnetite NPs as a robust versatile nanocomposite

Article abstract of DOI:10.1039/d1ra01164b

A new, versatile, and green methodology has been developed for the efficient NaBH₄-reduction of nitroarenes as well as the domino/reduction MCR preparation of 1-substituted-1H-1,2,3,4-tetrazoles using Pd(ii)-polysalophen coated magnetite NPs as an efficient heterogeneous magnetically recyclable nanocatalyst. Polysalophen was firstly prepared based on a triazine framework with a high degree of polymerization, then coordinated to Pd ions and, finally, the resulting hybrid was immobilized on magnetite NPs. The catalyst was characterized by various instrumental and analytical methods, including GPC, DLS, N₂adsorption-desorption, TGA, VSM, TEM, HRTEM, EDX, XPS, XRD, and ICP analyses. The catalyst possesses dual-functionality including the reduction of nitroarenes and the construction of tetrazole rings all in one stepviaa domino protocol. High to excellent yields were obtained for both nitro reduction and the direct preparation of 1-substituted-1H-1,2,3,4-tetrazoles from nitro compounds. Insight into the mechanism was conducted by XPSin situas well as DLSin situalong with several control experiments. Recyclability of the catalyst was studied for 6 consecutive runs along with metal leaching measurements in each cycle.

Full text of DOI:10.1039/d1ra01164b

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PAPER
Efficient reduction of nitro compounds and domino
preparation of 1-substituted-1H-1,2,3,4-tetrazoles
by Pd(II)-polysalophen coated magnetite NPs as
a robust versatile nanocomposite†
Cite this: RSC Adv., 2021, 11, 12484
a
b
c
*
*
DaPeng Xu, Meilu Xiong and Milad Kazemnejadi
A new, versatile, and green methodology has been developed for the efficient NaBH4-reduction of
nitroarenes as well as the domino/reduction MCR preparation of 1-substituted-1H-1,2,3,4-tetrazoles
using Pd(II)-polysalophen coated magnetite NPs as an efficient heterogeneous magnetically recyclable
nanocatalyst. Polysalophen was firstly prepared based on a triazine framework with a high degree of
polymerization, then coordinated to Pd ions and, finally, the resulting hybrid was immobilized on
magnetite NPs. The catalyst was characterized by various instrumental and analytical methods, including
GPC, DLS, N2 adsorption–desorption, TGA, VSM, TEM, HRTEM, EDX, XPS, XRD, and ICP analyses. The
catalyst possesses dual-functionality including the reduction of nitroarenes and the construction of
tetrazole rings all in one step via a domino protocol. High to excellent yields were obtained for both
nitro reduction and the direct preparation of 1-substituted-1H-1,2,3,4-tetrazoles from nitro compounds.
Insight into the mechanism was conducted by XPS in situ as well as DLS in situ along with several
control experiments. Recyclability of the catalyst was studied for 6 consecutive runs along with metal
leaching measurements in each cycle.
Received 11th February 2021
Accepted 15th March 2021
DOI: 10.1039/d1ra01164b
rsc.li/rsc-advances
studied recent methodologies in graphene-based photocatalysts
for nitro reduction in a review paper.5 Mahmoudi et al. recently
Introduction
developed a three-functional redox catalytic system bearing
TEMPO/Co(III)-porphyrin/Ni(II) complex for the efficient
reduction of nitro compounds as well as its direct trans-
formation to 1-substituted-1H-1,2,3,4-tetrazoles.9
Amines and their derivatives are among the most practical raw
materials and intermediates in organic synthesis that are widely
used in the production of chemicals required in pharmaceuti-
cals, biological compounds, dyes, synthetic resins, photog-
raphy, agrochemicals, optical brighteners, rubber, and other
industries.1–3 Moreover, the preparation of azo compounds,
isocyanates, amides, imines, and diazonium salts, are some of
the prominent uses of amines in organic synthesis.4,5 The
complete reduction of nitro compounds to the corresponding
amines is one of the most popular and common processes in
the petrochemical, pharmaceutical, chemical and other indus-
tries.1 Various catalytic systems have been reported for the
complete reduction of nitro compounds to amines, and recently
we could point to: Ag-rGO/g-C3N4/visible light,6 human hair,4
Ni2BH2,7 Fe3O4-MWCNTs@PEI-Ag,2 Fe/TRGO,1 CuFe2O4/
NaBH4,8 and Zn powder/CuSO4.6 Recently, Nasrollahzadeh et al.
a
Amines are one of the most valuable precursors in organic
synthesis, especially MCRs (multicomponent reactions),10 and
one of these reactions is the formation of tetrazole derivatives.
1-Substituted-1H-1,2,3,4-tetrazoles are one of the main
groups of tetrazoles, and the most common method for their
preparation is cyclization between a susceptible compound
such as an amine in the presence of azide ions.1,11–13 Tetrazoles
have prominent biological activities in pharmaceuticals and
medicine, such as antifungal, antibacterial, anti-inammatory,
antihypertensive, antibiotic, anti-HIV, and anticonvulsant
agents.14–16 Tetrazole rings can be found in Sartans as a large
group of commercial pharmaceutical compounds.10,17 More-
over, different applications have been known for tetrazole
derivatives in agriculture, photography, and various other
industries.10,17 Various methodologies have been developed for
the synthesis of 1-substituted-1H-1,2,3,4-tetrazoles,10 and we
can point to heterogeneous catalysts, photocatalysts,
microwave-assisted synthesis, nanoparticles, miscellaneous
methods, etc.10,12
aInstitute of Chemical Technology, Gansu Industry Polytechnic College, TianShui
741000, China
bPublic Foundation College, Gansu Health Vocational College, Lanzhou 730207,
China. E-mail: bearbabyx@sina.com
cDepartment of Chemistry, Faculty of Sciences, University of Birjand, Birjand, Iran.
E-mail: miladkazemnejad@yahoo.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d1ra01164b
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1-Substituted-1H-1,2,3,4-tetrazoles are usually made from
RSC Advances
amines. Direct preparation of these compounds from cheaper
and more available precursors of nitro compounds is very
valuable and noteworthy (compared to amines).9 However, the
use of nitro compounds to prepare tetrazole compounds
requires an additional step of the complete reduction of the
nitro compounds to amines. So, designing and developing
a multifunctional catalytic system for reduction and MCR/
domino processes seems to be the smart/correct strategy.
Domino reactions, also known as cascade or tandem reac-
tions, are chemical processes that involve at least two consec-
utive reactions, making it possible to prepare the desired
products with simpler and more accessible precursors, and the
reaction proceeds through intermediates, each of which alone
can lead to the desired product. For example, Nasseri et al. re-
ported domino protocols for the preparation of benzimidazole
derivatives and Biginelli reactions from alcohols using a Cu–Mn
bimetallic complex immobilized on magnetic NPs.18
Hydroformylation/aldol condensation,19 synthesis of cyclic
amidines,20 one-pot synthesis of vinylsilanes,21 synthesis of
polysubstituted pyrroles and lamellarin R,22 and synthesis of
multisubstituted tetrahydrobenzofuran derivatives23 are some
recent applications of domino reactions in organic synthesis.
Despite the development of various catalytic systems in this
eld, the design and synthesis of a comprehensive catalyst with
high catalytic activity and selectivity is the main concern of
scientists in the eld of synthesis of organic compounds via the
domino protocol. From this viewpoint, the reduction of nitro
compounds generally includes a multi-step reaction with the
formation of various by-products. So, if the applied catalyst does
not act selectively, it produces several by-products that are not
favorable from work-up, product isolation, or cost viewpoints in
either industry or organic synthesis. Scheme 1 shows the six
possible products that could be formed during nitrobenzene
reduction in the presence of a reducing agent. As shown in
Scheme 2, the complete reduction of nitrobenzene gives aniline.
In this way, catalyst selectivity along with its high efficiency are
two inseparable characteristics for a catalyst to be applicable in
the reduction processes.
Scheme 2 Six possible products from the reduction of nitrobenzene.
interesting topic for researchers over the last decade, because it
increases the efficiency of the catalyst and reduces costs. The
polymerization of these compounds is an intelligent strategy to
achieve this goal. However, due to the structure of salen ligands,
its polymerization has many challenges and few reports are
available on the preparation of poly-salen and its derivatives.
Previously, Yao et al.25 and Song et al.26 in two separate studies
developed two novel chiral poly-salen Mn(III) complexes derived
from (R,R)-2-diaminocyclohexane and disalicylaldehydes for the
enantioselective epoxidation of unfunctionalized olens. Wang
et al. reported a new approach for the preparation of poly-salen
based on polyacrylamide and used its Pd complex for a car-
bonylative Sonogashira reaction.27 Wu and Lu synthesized
a series of poly-salens through polycondensation of 1,2-diamine
with a disalicylaldehyde for the dimerization of propylene.28
Recently, a Co(III) salen polymer was prepared via a Sonogashira
Pd cross-coupling reaction between tris(p-ethynyl)triphenyl-
amine (TPA) and bis-bromo salen metal complexes.29
In this work, a polysalophen network structure was prepared
through a polycondensation reaction and then immobilized on
magnetite NPs. The polycondensation reaction was performed on
a salicylaldehyde-treated triazine framework, where the polymeri-
zation occurs from three sides in the presence of o-phenylenedi-
amine. Finally, the resulting polymer was successfully
immobilized on magnetite NPs aer the coordination of Pd ions to
the polysalophen ligand. The resulting nanocomposite showed
noteworthy catalytic activity towards the reduction of nitroarenes
in the presence of NaBH4 and the domino/reduction multicom-
ponent preparation of 1-substituted-1H-1,2,3,4-tetrazoles from
nitroarenes (or aromatic amines) in one step.
Salen and its derivatives are known as among the most
reliable and active ligands in coordination chemistry due to
their rigid 4-dentate structure. The use of salen complexes has
been extended to almost all organic reactions, especially
coupling reactions and MCR reactions.24 The immobilization of
these compounds on heterogeneous supports and their use as
heterogeneous catalysts in organic synthesis has been an
Experimental
Materials and instrumentation
All materials were supplied by Merck (or Sigma) or the Fluka
Corporation and used as received without any further purication.
All solvents (except dimethyl sulfone) were dried before use and
kept under an Ar atmosphere. The dimethyl sulfone used in the
solvent-effect experiments was prepared according to the previ-
ously reported method described by Cheng et al.30 Molecular sieves
Scheme 1 Direct preparation of 1-substituted-1H-1,2,3,4-tetrazole
from nitro compounds in this work, and its comparison with the
traditional approach.
with a 4 A pore diameter, 0.6 (g mLꢀ1) bulk density, 20% w/w
ꢀ
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Synthesis of 4,40,4''-((1,3,5-triazine-2,4,6-triyl)tris(oxy))tris(2-
hydroxybenzaldehyde) (3)
adsorbed water, and 0.5 %w/w attrition were used for the poly-
merization as a water-absorbent agent and catalyst.
In optimization experiments, the conversion of each run was
recorded on a YL 6100 gas chromatograph (GC) system with
a CBP5 column (Shimadzu 30 m ꢁ 0.32 mm ꢁ 0.25 mm). Also,
the conversion and selectivity of the products in the model
reactions were determined qualitatively by GC analysis.
Conversion was calculated by comparing the area of the product
and the starting material. For each run, aer catalyst recovery,
0.2 mL of a sample was injected into the GC instrument, then the
selectivity of aniline production was obtained from the
following eqn (1):31
4,40,400-((1,3,5-Triazine-2,4,6-triyl)tris(oxy))tris(2-
hydroxybenzaldehyde) abbreviated to TA(SA)3 3, was prepared
according to the following procedure: In a dried round bottom
ask, cyanuric chloride (TCT, 1.0 mmol) was dissolved in 25 mL
of dry CH3CN at room temperature. The system was equipped
with an N2 inlet aer the addition of 2,4-dihydroxybezaldehyde
(5.0 mmol) and Et3N (5.0 mmol); these excess amounts were
used to ensure that all chloride sites in TCT were functionalized
by 2,4-dihydroxybezaldehyde. The reaction was performed
under reux conditions for 12 h. The dark yellow solid was
ltered and washed with cold EtOH. Further purication was
achieved by recrystallization (twice) in hot ethanol (to eliminate
any possible mono- and di-substituted products) (Scheme 3).
The product was isolated at 4 ꢂC (0.73 mmol, 73% isolated yield,
MP 145 ꢂC). The structure of TA(SA)3 3 was conrmed by CHN,
1H-NMR, 13C-NMR, and EDX analyses. Elemental analysis: calcd
C: 58.90, H: 3.09, N: 8.59%, found. C: 58.76, H: 3.22, N: 8.60%.
GC peak area of aniline
Aniline selectivity ¼
ꢁ 100
GC peak area of all products
(1)
1H-NMR (400 MHz) and 13C-NMR (100 MHz) spectra were
recorded on a Bruker Ascend NMR 400 MHz in ultra-pure
deuterated solvents. TMS was used as an internal standard
for all analyses. XPS analyses were performed on an XR3E2 (VG
Microtech) twin anode X-ray source with radiation of Al-Ka ¼
1486.6 eV. TEM images were taken with a Philips EM208
microscope, operated at 100 kV. High resolution transmission
electron microscopy (HRTEM) was performed on an FEI Tec-
nai G2 F20 Super Twin TEM microscope with a eld emission
gun at 200 kV. Zeta potentials and DLS (to study the size
distribution of the samples) analyses of the samples were ob-
tained using a zeta potential analyzer from Malvern Instru-
ments Ltd. The thermal behavior of the samples was studied
on a NETZSCH STA 409 PC/PG under N2 atmosphere with
a heating rate of 10 ꢂC minꢀ1 in the temperature range of 25–
850 ꢂC. A Lake Shore vibrating sample magnetometer (VSM)
was used for studying the magnetic properties of the samples.
Metal leaching and Pd contents of the catalyst measurements
were performed on a VARIAN VISTA-PRO CCD simultaneous
ICP-OES instrument. A gel permeation chromatography Kna-
uer Advanced Scientic Instrument (Germany) with an RI
1
EDX analysis (wt%): C: 59.99, H: 3.09, N: 9.62%, O: 30.39. H
NMR (400 MHz, CDCl3) d(ppm) ¼ 6.71 (s, 3H), 7.15 (d, 3H), 7.93
(d, 3H), 9.98 (s, 3H), 11.61 (s, 3H); 13C NMR (100 MHz, CDCl3)
d(ppm) ¼ 109.4, 114.9, 118.7, 132.3, 162.3, 181.9, 195.4.
Synthesis of polysalophen (4) based on TA(SA)3
polycondensation reaction
A polycondensation reaction on TA(SA)3 (3) was performed in
the presence of o-phenylenediamine (o-PDA) based on salophen
preparation. Dissolved TA(SA)3 (2.5 g, 5.0 mmol) in 30 mL of
absolute EtOH was added dropwise to a ethanolic solution of o-
PDA (1.6 g, 15.0 mmol in 35 mL of absolute EtOH) over 1.0 h.
This excess amount was used to avoid dimerization as well as
the formation of short chains. Seven beads of molecular sieves
were added to the mixture. A molecular sieve not only catalyzes
the reaction for the preparation of Schiff base bonds, but also
absorbs the water produced during the polymerization process
and enhances the polymerization reaction.32
3
ꢀ
detector (Smartline 2300) PL gel 10 mm, 10 ꢁ 10 A column
with an injected volume of 20 mL was used for the determi-
nation of the average molecular weight of the polysalophen.
Monodispersed poly(methyl methacrylate) (PMMA) was used
as a standard for calibration. The average molar weight was
measured using Millennium 2010 soware. Energy dispersive
X-ray spectroscopy (EDX) analyses were accomplished on
a eld emission scanning electron microscope, FE-SEM, JEOL
7600F, equipped with an energy dispersive X-ray spectrometer
from Oxford Instruments. The crystal structures of the
samples were performed using a Bruker D8/Advance powder X-
ray diffractometer, and using a HAAKE D8 recirculating bath
the cell temperature was maintained at 25.0 ꢂC. Elemental
analyses of the tetrazole products were obtained on a CHN
ThermoFinnigan Flash EA 1112 instrument. The specic
surface area, pore diameter, and pore volume, of the obtained
Synthesis of Pd(II)-polysalophen complex 5
In order to prepare Pd(II)-polysalophen complex 5, the poly-
salophen ligand (4) (1.0 g) was swelled in 25 mL of DMSO, then
0.01 g of Pd(OAc)2 (anhydrous) was added to the mixture. The
ꢂ
mixture was stirred for 2.0 h at 80 C. The resulting Pd(II)-pol-
ysalophen complex was separated by simple ltration (centri-
fuges can also be used), followed by several washings with cold
EtOH and deionized water. Finally, it was dried in an oven (50
ꢂC) and isolated at room temperature.
Synthesis of Fe3O4@SiO2@Pd(II)-polysalophen 9
Fe3O4@SiO2–Cl was prepared according to a previously reported
procedure.33,34 The immobilization process was accomplished
by the following procedure: Fe3O4@SiO2–Cl was added to 15 mL
of H2O : EtOH mixture (1 : 3, v/v), then sonicated for 15 min at
40 ꢂC. In contrast, Pd(II)-polysalophen was also sonicated in the
same mixture for 15 min at 40 ꢂC, then added slowly to the
ꢂ
NPs were studied by N2 physisorption at ꢀ196 C on a Micro-
meritics ASAP 2000 instrument by the BET method.
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Scheme 3 Preparation of polysalophen 4 and Pd(II)-polysalophen complex 5.
magnetite mixture under ultrasonic conditions. Next, the elemental analysis and MP and comparison with authentic
mixture was stirred for 4 h under reux conditions. The resul- references.
tant Fe3O4@SiO2@Pd(II)-polysalophen was collected by an
external magnetic eld, washed with deionized water and dried
in a vacuum oven for 8 h (Scheme 4).
General procedure for direct transformation of nitroarenes to
1-substituted-1H-1,2,3,4-tetrazoles catalyzed by
Fe3O4@SiO2@Pd(II)-polysalophen 9
Catalytic complete reduction of nitroarenes to the
corresponding amines catalyzed by Fe3O4@SiO2@Pd(II)-
polysalophen 9
In a reaction tube, nitro compound (1.0 mmol), NaBH4 (2.0
mmol), and catalyst 9 (5 mg, 0.3 mol% Pd), were added to
2.0 mL of H2O. The reaction was performed under reux
In a typical run, in a reaction tube, nitrobenzene (1.0 mmol), conditions and monitored by TLC. Aer 30 min of the mixture
NaBH4 (2.0 mmol), and catalyst 9 (5 mg, 0.3 mol% Pd) were reacting, HC(OC2H5)3 (1.2 mmol) and NaN3 (1.5 mmol) were
added to 2.0 mL of H2O. The reaction was performed under added to the mixture. The mixture was stirred for an appro-
reux conditions while being monitored by TLC. Initial conr- priate time under reux conditions. Aer completion of the
mation of product formation was done by comparing pure reaction, the catalyst was ltered magnetically, washed with
amine spots by TLC. Upon completion of the reaction, the EtOH, dried and stored for the next run. Then, acetone (12 mL)
catalyst was ltered magnetically, and the desired amine was added to the residue, and the product was extracted with
product was transferred to the organic phase aer solvent EtOAc (2 ꢁ 15 mL). The solvent was removed under reduced
extraction using EtOAc (2 ꢁ 15 mL). The organic phase was pressure and the desired crude product (1-substituted-1H-
dried over MgSO4, then the solvent was removed under reduced 1,2,3,4-tetrazole) was puried by recrystallization. The products
1
pressure. Finally, the crude product was puried by ash were characterized by H NMR, 13C NMR and compared with
chromatography. The products were conrmed by their CHN authentic ones.
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Scheme 4 Preparation of Fe3O4@SiO2@polysalophen-Pd(II) 9.
the EDX and XPS analyses well indicates the successful immo-
bilization of Pd(II)-polysalophen on the magnetic nanoparticles
in full accordance with expectations and the proposed structure
in Scheme 4. The presence of peaks at binding energies of 342
and 337 eV in the high resolution XPS analysis clearly shows
that the Pd centers have a capacity of +2 (ref. 35) and no
impurities were found arising from the presence of other
species in the catalyst (Fig. 1c). Fig. 1d also shows the overall
survey XPS analysis of polysalophen (5), conrming its structure
with detection of the expected elements of Pd, C, N, O with high
purity.
General procedure for transformation of amines to 1-
substituted-1H-1,2,3,4-tetrazoles catalyzed by
Fe3O4@SiO2@Pd(II)-polysalophen 9
Aryl amine (1.0 mmol), HC(OC2H5)3 (1.2 mmol), catalyst 9
(5 mg, 0.3 mol% Pd), and NaN3 (1.5 mmol) were added to 2.0 mL
of H2O. The mixture was stirred under reux conditions for
a sufficient time, and monitored by TLC according to amine
consumption. Upon completion of the reaction, the mixture
was cooled to room temperature, then extracted with EtOAc (3
ꢁ 15 mL) aer removal of the catalyst by an external magnetic
eld. The organic phases were dried over MgSO4 and concen-
trated under reduced pressure. The crude product was puried
by ash chromatography (EtOAc : hexane) to give the desired 1-
substituted-1H-1,2,3,4-tetrazoles (ESI).†
A study of the thermal behavior of the samples by TGA
analysis provided useful information about the thermal resis-
tance as well as the structure of the compounds, in agreement
with other analyses. As shown in Fig. 2A-a, the thermal
decomposition of the polysalophen ligand begins at a tempera-
tures of about 220 ꢂC, and continues with a gentle slope of up to
Results and discussion
Catalyst characterization
ꢂ
500 C (Fig. 2A-a) and only 2% of the remaining weight is ob-
tained. Coordination of Pd to polysalophen increases the
thermal stability of the compound as expected (Fig. 2A-b). As
shown in the gure, 14% of the remaining weight is generated
The structure of 3 was characterized and conrmed by CHN,
1
EDX, H-NMR, and 13C-NMR analyses (Experimental section).
The average molecular weight for 4 was determined by GPC
analysis, and was found to be ꢃ6000 g molꢀ1 with a satisfactory
polydispersity index of 1.07 (dimensionless). Considering the
mass of 408 g molꢀ1 for each monomeric unit (as shown in
Scheme 3), therefore, the degree of polymerization of lattice
polymer 3 is equal to 13 (considering the two end groups).
ICP analysis from the catalyst revealed that 6 %wt Pd is
present in the polymeric framework. Since 1.0 mmol of reactant
was used in all reactions, the reactions were performed in the
presence of 0.3 mol% Pd. Fig. 1a and b shows the results of EDX
analysis and the XPS overall survey scan, in which the presence
of all expected elements conrms the structure of the catalyst.
XPS analysis of Pd(II)-polysalophen also shows the complex
formation well with the presence of a peak at 320 eV related to
Pd binding energy (Fig. 1b). The presence of chlorine element in
ꢂ
up to 850 C. This value can be attributed to the formation of
PdO nanoparticles at the end of the thermal process, which
conrms the formation of a stable Pd(II)-polysalophen complex.
Immobilization of complex 5 on Fe3O4@SiO2 nanoparticles
caused greater thermal stability of the catalyst. According to the
results, the thermal decomposition of the immobilized groups
on the magnetic nanoparticles starts from a temperature of
ꢂ
around 480 C and continues with a much gentler slope up to
650 ꢂC (Fig. 2A-c). Due to the absence of any degradable groups
in the structure of Fe3O4@SiO2 (according to literature
reports),33,34 the residual nal weight (51.5%) can be directly
attributed to the polysalophen complex immobilized on the
magnetic nanoparticles. The reduction of the initial weight up
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Fig. 1 (a) EDX analysis, (b) overall survey and (c) Pd-3d high-resolution (normalized-energy corrected) XPS spectra of catalyst 9. (d) Overall
survey XPS analysis of Pd(II)-polysalophen 5.
to 120 ꢂC is also related to the loss of water in the network polysalophen on Fe3O4 nanoparticles (Fig. 2B-c). Compared to
structure of the compounds (for all three curves).34,36
the rate of magnetization loss due to silica coating (30 emu gꢀ1),
Fe3O4 and Fe3O4@SiO2–Cl and catalyst 9 magnetic nano- this 12 unit reduction can be directly attributed to the
particles have superparamagnetic properties with coercivity successful immobilization of Pd(II)-polysalophen on magnetic
equal to zero. As shown in Fig. 2B, Fe3O4 and Fe3O4@SiO2–Cl nanoparticles, which by its diamagnetic effect dilutes the
nanoparticles and catalyst 9 have saturation magnetizations of applied magnetic eld and consequently reduces the magneti-
70, 40 and 28 emu gꢀ1, respectively. A signicant drop in zation of the Fe3O4 core.37
magnetization occurred aer immobilization of Pd(II)-
Fig. 2 (A) TGA curves of (a) polysalophen, (b) Pd(II)-polysalophen, and (c) Fe3O4@SiO2@Pd(II)-polysalophen 9. (B) VSM curves of (a) Fe3O4, (b)
Fe3O4@SiO2–Cl, and (c) Fe3O4@SiO2@Pd(II)-polysalophen 9.
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The TEM image of the catalyst shows an average size of
75 nm for the particles (Fig. 3a). As well illustrated in the gure,
the polymer chains appear as lighter regions as the phase is
lighter than the core. In addition, very dark spots also represent
the coordinated Pd particles in the structure of polysalophen.
HRTEM analysis was also undertaken on the catalyst. As
shown in the HRTEM image in Fig. 4, Pd NPs have been
successfully loaded onto the polysalophen ligand with an
average diameter size of 5 nm and an interplanar spacing of
0.223 nm related to the (111) planes of Pd, in agreement with
the literature reports.37–40
DLS analysis also showed that most of the nanoparticles
have an average size of 85 nm (Fig. 3b). This 10 nm difference in
the size of TEM and DLS can be attributed to two factors:
(1) The difference in the nature of the two analyses is that in
DLS, the laser light is deected from the edges of the particles,
somewhere farther from the actual particle size, according to
published reports and (2) in accordance with the structure
designed and proposed for the catalyst, in situ polymerization of
salophen groups creates a phase with a lighter molecular weight
around the magnetite nanoparticle and subsequently causes
premature diversion of laser radiation in the DLS. Therefore,
this difference of about 10 nm can be related to the dense
chains around the nanoparticles, whereas in the TEM image,
the brighter phases are quite clear.
Fig. 4 HRTEM images of Fe3O4@SiO2@polysalophen-Pd(II) 9.
The crystal structure of Pd(II)-polysalophen and the catalyst
(9) were studied by X-ray diffraction analysis (Fig. 5). The three
peaks appearing at angles 2q ¼ 40.2ꢂ, 46.0ꢂ and 68.1ꢂ corre-
spond to planes (111), (200) and (220) in the Pd crystal structure,
respectively, in full accordance with the reported diffraction
patterns (Fig. 5a).27 The presence of peaks related to the crystal
structure of Pd(II)-polysalophen in the X-ray diffraction pattern
of the catalyst at 2q ¼ 40ꢂ, 46ꢂ and 68ꢂ is strong evidence for its
successful immobilization on the magnetic nanoparticles, so
that no phase change or shi in the peak related to the crystal
structure of magnetic nanoparticles occurred. These results
reect the non-alteration of the crystal structure of both phases
of the magnetic nanoparticles as well as Pd(II)-polysalophen
aer immobilization (Fig. 4b).
Surface characteristics of Fe3O4, Fe3O4@SiO2, Fe3O4@SiO2-
Cl, and catalyst 9 were evaluated by N2 adsorption–desorption
isotherm analysis. According to the results, the specic surface
areas for Fe3O4, Fe3O4@SiO2, Fe3O4@SiO2Cl, and catalyst 9
nanoparticles are equal to 478, 450, 398, and 352 m2 gꢀ1 (Table
1). At each stage of functionalization, the specic surface area of
the magnetite nanoparticles is reduced, which reects the
immobilization of different groups on the surface of the nano-
particles. In addition, the porosity volume also decreased with
the reduction of the surface area, at each stage of immobiliza-
tion of various groups. Signicantly, there is a signicant
reduction in the specic surface area and porosity aer
Fig.
3 (a) TEM image and (b) DLS analysis of Fe3O4@SiO2@-
polysalophen-Pd(II) 9.
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such as environmental friendliness, susceptibility, and avail-
ability also led to the selection of water as the premium solvent
for the reactions. Another noteworthy point was the selectivity
above 98% for the reduction of nitrobenzene in water; DMSO,
DMSN and DMF solvents also produced similar selectivity
(above 98%). Due to the high efficiency created by polar solvents
with high dielectric constant, the results suggest a mechanism
involving polar intermediates. As shown in Fig. 6a and b, water
solvent under reux conditions created the highest possible
efficiency with 90% and 94% efficiencies for aniline and 1-
phenyl-1H-1,2,3,4 tetrazole, respectively.
ꢂ
Lowering the temperature to 50 C reduced the efficiencies
by up to 50% and 80% for aniline and 1-phenyl-1H-1,2,3,4-tet-
razole, respectively. No efficiency at room temperature was
observed for any of the products. Another effective parameter
was the amount of catalyst, which in the optimal amount of
5 mg, resulted in the highest efficiency for both reactions.
Higher (up to 10 mg) and lower values were unfavorable for both
reactions. In the absence of the catalyst, only 8% efficiency was
observed for aniline (0% efficiency for tetrazole), which reects
the catalytic activity of Fe3O4@SiO2@polysalophen-Pd(II) (9) to
reduce nitrobenzene and prepare tetrazole (Fig. 6c).
Fig. 5 XRD patterns of (a) Pd(II)-polysalophen and (b) Fe3O4@SiO2@-
Pd(II)-polysalophen 9. Black and red stars represent the planes related
to Pd (in Pd(II)-polysalophen 5) and Fe3O4 crystal structures,
respectively.
In addition, since the loading amount of metal nanoparticles
on the composite support has an inuence on the particle size
as well as their catalytic activity, the effect of the Pd loading
value on the complex was also studied. In order to investigate
the effect of different amounts of nanoparticle loading in the
catalyst, by changing the amount of Pd salt used in the prepa-
ration of the polysalophen complex (Scheme 3), different
percentages of loaded Pd were obtained. Pd complexes of pol-
ysalophen with different amounts of Pd were immobilized on
Fe3O4@SiO2 nanoparticles. The loading of Pd nanoparticles in
the catalyst was measured by EDX analysis (5-point mean) as
well as ICP analyses. DLS analysis was also performed to
determine the size of the catalyst (ESI, Tables S1 and S2†).
Table S1† shows the results of ICP, EDX and DLS analyses.
For a better comparison, the synthesized catalyst in this work
with 6 %wt Pd is also given in Table S1† (entry 2). Increasing the
amount of Pd nanoparticles increased the average particle size
(by DLS analysis). The average nanoparticle size increased from
82 nm with 2.22 %wt Pd to 94 nm with 18 %wt Pd. A noteworthy
point is the high loading capacity of polysalophen complex
compared to Pd, so that different loading percentages of 2.22,
6.06, 10.11, 14.16, and 17.19 wt% were prepared. The catalytic
activity of the catalysts was studied in three reactions: reduction
of nitrobenzene to aniline, direct conversion of nitrobenzene
and conversion of aniline to 1-substituted-1H-1,2,3,4-tetrazoles.
Based on the results of ICP and EDX, catalysts were dened as:
Table 1 Surface characteristics of Fe3O4, Fe3O4@SiO2 Fe3O4@SiO2–
Cl, and Fe3O4@SiO2@polysalophen-Pd(II) (9)
Specic surface Pore volume Average pore
Entry Sample
area (m2 gꢀ1
)
(cm3 gꢀ1
)
radius (nm)
1
2
3
4
Fe3O4
Fe3O4@SiO2
Fe3O4@SiO2–Cl 398
Catalyst 9 352
478
450
0.805
0.782
0.750
0.700
1.255
1.788
1.800
1.865
immobilization of Pd(II)-polysalophen on Fe3O4@SiO2–Cl
nanoparticles, which is a good indication of its successful
immobilization, which is quite comparable to the results ob-
tained from the silica coating on the nanoparticles (Table 1,
entry 4). Subsequently, the porosity increased at each stage of
functionalization, and aer immobilization of the Pd(II)-poly-
salophen, a signicant decrease in agreement with its immo-
bilization was observed (Table 1, entry 4).
Optimization of reaction parameters
The reduction of nitrobenzene and its following transformation Cat. 9/0.12 mol% Pd; Cat. 9/0.3 mol% Pd; Cat. 9/0.6 mol% Pd;
to the corresponding 1-phenyl-1H-1,2,3,4-tetrazole were chosen Cat. 9/0.8 mol% Pd; Cat. 9/1.0 mol% Pd.
as two separate model reactions to optimize temperature,
Increasing the amount of Pd nanoparticles to 0.8 mol% had
catalyst amount, reducing agent, and solvent parameters. The no signicant effect on the reaction efficiency, and only the
best choice was distilled water with 90% and 94% conversion reduction efficiency of nitrobenzene to aniline showed a slight
for the preparation of aniline and 1-phenyl-1H-1,2,3,4-tetrazole, increase (Table S2,† row 4). The yield decreased by decreasing
respectively. Although solvents such as DMSO, DMSN, and the Pd Ps at 5 mg of catalyst, in accordance with the results of
MeOH produced higher efficiencies for aniline, the efficiencies optimizing the amount of catalyst (Fig. 5c), where by reducing
for tetrazole were lower than for water. In addition, parameters the weight of the catalyst (corresponding to the reduction
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Fig. 6 Influence of (a) solvent, (b) temperature, (c) catalyst amount, and (d) NaBH4 amount on the reduction of nitrobenzene and direct
transformation of nitrobenzene to the corresponding 1-phenyl-1H-1,2,3,4-tetrazole. General reaction conditions (except as noted in each
figure): Nitrobenzene (1.0 mmol), NaBH4 (2.0 mmol), H2O (2.0 mL), catalyst 9 (5 mg, 0.3 mol% Pd), reflux, 60 min. Nitrobenzene (1.0 mmol),
triethyl orthoformate (1.2 mmol), NaN3 (1.5 mmol), NaBH4 (2.0 mmol), H2O (2.0 mL), catalyst 9 (5 mg, 0.3 mol% Pd), reflux, 90 min. * 120 ꢂC. **
Dimethyl sulfone, 120 ꢂC. In each experiment, the premium conditions are shown shaded in blue, that was used for the next run.
in mol% Pd), the reaction efficiency decreased signicantly. It
Finally, the stoichiometric amount of NaBH4 as a reducing
seems that increasing the amount of Pd nanoparticles blocks agent was studied in the presence of 5 mg of catalyst and water
some active sites (due to agglomeration of Pd nanoparticles in solvent under reux conditions. The efficiency reaches
the catalyst framework due to increasing their number) and a maximum in a stoichiometric amount of twice that of nitro-
consequently reduces the catalytic activity. Although due to the benzene. As shown in Fig. 6d, stoichiometric values higher and
increase in the amount of nanoparticles in the catalyst, its lower for NaBH4 were consistent with decreased aniline effi-
surface charge increases and repulsion is created, causing ciency and the subsequent preparation of tetrazole compounds.
better dispersion of nanoparticles, but increasing the number Considering the correlation between nitrobenzene reduction
of Pd nanoparticles in the structure of polysalophen blocks efficiency and also the direct conversion of nitrobenzene to
active sites and acts in the opposite direction. Therefore, tetrazole, therefore, the results show good preparation of tet-
increasing the amount of Pd nanoparticles loaded on the razoles through amino intermediates and subsequent domino
magnetic substrate had a similar result to the effect of the reaction. Another proof of this claim is the absence of tetrazole
weight of the catalyst in the reactions. On the other hand, product in the absence of NaBH4 in the direct conversion of
reducing the amount of catalyst also caused a decrease in effi- nitrobenzene to tetrazole (Fig. 6d).
ciency in all 3 reactions with a process similar to the results of
For a better understanding, the results of optimization
reducing the weight of the catalyst in the optimized experiments only in water solvent (as the optimal solvent) are
experiments.
summarized in Table 2.
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Table 2 Summary of the results of optimizing the parameters of the reduction of nitrobenzenea and direct transformation of nitrobenzene to the
corresponding 1-phenyl-1H-1,2,3,4-tetrazole in water solventb
Product
Aniline
1-Phenyl-1H-1,2,3,4-tetrazole
Cat. amount
(mg)
Entry
Solvent
T. (ꢂC)
NaBH4
Time (min)
Yield (%)
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
25c
50
80
5
5
5
5
0
1
3
7
10
5
5
5
5
2
2
2
2
2
2
2
2
2
0
1
2.5
3
60
60
60
60
60
60
60
60
60
60
60
60
60
0
90
90
90
90
90
90
90
90
90
90
90
90
90
0
50
60
90
8
45
75
90
88
0
80
88
94
0
55
80
94
95
0
Reux
Reux
Reux
Reux
Reux
Reux
Reux
Reux
Reux
Reux
9
10
11
12
13
70
92
90
75
90
88
a
Reaction conditions: nitrobenzene (1.0 mmol), NaBH4 (2.0 mmol), H2O (2.0 mL), catalyst 9 (5 mg, 0.3 mol% Pd), reux, 60 min. b Nitrobenzene
(1.0 mmol), triethyl orthoformate (1.2 mmol), NaN3 (1.5 mmol), NaBH4 (2.0 mmol), H2O (2.0 mL), catalyst 9 (5 mg, 0.3 mol% Pd), reux, 90 min.
c
Room temperature.
reduction of a nitrobenzene (Scheme 2). The electronic effects
were also not very signicant and the efficiency for nitroarenes
with electron-donor and electron-withdrawing groups was
almost the same (Table 3). However, nitroarenes containing
electron-withdrawing groups were reduced in a shorter time
than those with electron-donor groups. Nevertheless, high to
excellent selectivity was obtained for all derivatives (Table 3).
Table 4 shows the results of the direct conversion of nitro-
arenes to 1-substituted-1H-1,2,3,4-tetrazoles. Also, in parallel
Catalytic activity
Under optimal conditions, various derivatives of nitro
compounds were reduced to the corresponding amines. Also,
the direct conversion of nitroarenes and also the conversion of
amines to the corresponding tetrazole products were studied
with Fe3O4@SiO2@polysalophen-Pd(II) (9). Scheme 5 shows the
conditions used for each conversion.
Table 2 shows the reduction of a wide range of nitroarenes in
the presence of Fe3O4@SiO2@polysalophen-Pd(II) (9). As shown
in the table, excellent efficiency is obtained for almost all nitro
compounds.
Most importantly, the method has high selectivity, so that
a selectivity of over 95% is achieved in a period of 45–90 minutes
for the nitroarene reduction. This advantage is very important
due to the formation of various products in the resultant
Table 3 Reduction of nitroarenes over Fe3O4@SiO2@polysalophen-
Pd(II) 9 in the presence of NaBH4
a,b
t
S.
(%)
Entry
R
P.
(min)
Y.c (%)
MP (ꢂC)
1
2
3
4
H
4-MeO
4-OH
4-Me
4-Cl
11a
11b
11c
11d
11e
11f
11g
11h
11i
60
80
90
85
80
65
65
50
50
45
70
96
94
76
90
92
92
90
98
95
97
95
99
98
96
99
97
95
99
97
96
96
98
Oil
59–62
184
38–40
70–72
88
5
6d
7
4-CN
2-Pyridyl
3-NO2
4-COMe
4-CO2Me
3-NH2-4-Cl
155
8c,e
9
61–63
104–106
185
Scheme 5 Reduction of nitroarenes and domino preparation of 1-
substituted-1H-1,2,3,4-tetrazoles catalyzed by Fe3O4@SiO2@-
polysalophen-Pd(II) 9. (a) Nitroarene (1.0 mmol), NaBH4 (2.0 mmol),
H2O (2.0 mL), catalyst 9 (5 mg, 0.3 mol% Pd), reflux. (b) amine (1.0
mmol), triethyl orthoformate (1.2 mmol), catalyst 9 (5 mg, 0.3 mol%
Pd), NaN3 (1.5 mmol), H2O (2.0 mL), reflux. (c) Nitroarene (1.0 mmol),
triethyl orthoformate (1.2 mmol), NaN3 (1.5 mmol), NaBH4 (2.0 mmol),
H2O (2.0 mL), catalyst 9 (5 mg, 0.3 mol% Pd), reflux.
10
11
11j
11k
63–65
a
Reaction conditions: see Scheme 5. Nitroarene (1.0 mmol).
b
Denition of the variables: P. ¼ product; t ¼ time; Y. ¼ isolated
c
d
yield; S. ¼ selectivity. Isolated yield. No reduction for cyano group
(preparation of p-phenylenediamine) was observed. e Catalyst: 10 mg.
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Table 4 Domino and traditional preparation of 1-substituted-1H-
1,2,3,4-tetrazoles from nitro and amine, respectively, catalyzed by
Fe3O4@SiO2@polysalophen-Pd(II)a,b
any coordinated transition metal; because Pd(II)-polysalophen
produced 80 and 65% efficiency for aniline and 1-phenyl-1H-
1,2,3,4-tetrazole, respectively (Table 5, entries
4 and 5).
However, the efficiency was lower than that of the catalyst (9),
which could be attributed to the immobilization of Pd(II)-poly-
salophen on the magnetite nanoparticles. Immobilization of
Pd(II)-polysalophen on the magnetite provides a cohesive lattice
structure with a high surface-to-volume ratio (according to the
proposed structure for the catalyst in Scheme 4) that causes the
compounds to penetrate into the catalyst structure in an
aqueous medium, and aer the reaction, they are separated
from the surface of the catalyst, in full agreement with the
suggested mechanisms that water has been used as a solvent in
the organic reactions.41–44
From amine From nitro
In order to investigate this phenomenon, in another control
test, the catalytic activity of a physical mixture of Pd(II)-poly-
salophen and Fe3O4@SiO2 was studied (Table 5, entry 6).
The results clearly provided similar efficiency to Pd(II)-poly-
salophen. The results not only show the successful and effective
immobilization of Pd(II)-polysalophen on magnetic nano-
particles, but also show that the structure of the catalyst
provides a suitable environment for reduction and tetrazole
conversions in an aqueous medium.
t
t
Entry
R
P.
(min) Y.c (%) (min) Y.c (%) MP (ꢂC)[Lit.]
1
2
3
4
5
6
7
8
H
12a 75
12b 60
12c 90
12d 90
12e 110
12f 120
97
96
96
95
95
94
90
95
90
80
98
96
98
96
98
90
92
95
64–65 (ref. 42)
4-MeO
4-OH
4-Me
4-Cl
114–115 (ref. 42)
185–187 (ref. 13)
94–96 (ref. 13)
130
120
150
150
120
160
157–158 (ref. 42)
177–179 (ref. 13)
80–82 (ref. 13)
4-CN
2-Pyridyl 12g 80
3-NO2 12h 140
106–108 (ref. 13)
a
For reaction conditions see Scheme 5. Amine (1.0 mmol),
Reaction mechanism
b
nitrobenzene (1.0 mmol). Denition of the variables: P. ¼ product; t
¼ time; Y. ¼ isolated yield. c Isolated yield.
To evaluate the activity of the catalyst in the reduction of nitro
compounds, the reduction of 4-nitrophenol was selected as
a model reaction. As shown in Scheme 6, the presence of the OH
group increases the length of the conjugate system during the
reduction process and the subsequent red shi to 410 nm. 4-
Aminophenol also has an absorption band at 308 nm.45,46
Therefore, by continuously studying the electronic spectra of
the 4-nitrophenol reduction reaction mixture, the reduction
process can be studied. Fig. 7a shows the results of continuous
electronic spectra from 4-nitrophenol reduction in the presence
of catalyst (9). At the beginning of the reaction, 4-nitrophenol
species are converted to phenolate anions in the presence of
NaBH4, which has an absorption band at 410 nm. Aer reduc-
tion of the nitro group, the absorption decreases and a new
absorption appears at 308 nm related to 4-aminophenol. Fig. 7a
shows the presence of both absorption bands simultaneously in
the last 35 minutes of the reaction. This time is related to the
induction time in the presence of NaBH4 and represents the
reactions to compare the catalytic process, this conversion was
performed from the corresponding amines. As shown in Table
3, there is no signicant difference in conversion reaction times
for amines in comparison with nitroarenes. These results
indicate that the nitro groups participate in the tetrazole ring
formation reaction immediately aer reduction on the catalyst
surface. This also contributes to the further development of the
reduction of nitro compounds and the subsequent preparation
of tetrazole compounds. As with the reduction of nitro
compounds, good to excellent efficiency was achieved for the
direct conversion of nitro compounds as well as the aromatic
conversion of amines to the corresponding tetrazoles.
Control experiments
In order to evaluate and conrm the uniqueness of the catalyst diffusion time required to adsorb 4-nitrophenol on the surface
activity in nitrobenzene reduction as well as the direct prepa- of the catalyst (as shown in the proposed mechanism), and then
ration of 1-phenyl-1H-1,2,3,4-tetrazole from nitrobenzene, the reduction takes place.46,47 As the reaction progresses and the
different control reactions in the presence of different catalyst phenolate intermediate is consumed, its absorption intensity at
components were studied. According to the results summarized 410 nm decreases, and the absorption intensity at 308 nm
in Table 4, the catalyst activity for both products is well increases due to the formation of 4-aminophenol. To conrm
demonstrated. The compounds including Fe3O4, Fe3O4@SiO2 this phenomenon, in a control reaction, 4-nitrophenol reduc-
and polysalophen 3 did not produce any efficiency for tetrazole tion was tested in the absence of the catalyst and in the presence
(Table 5, entries 1–3). Only very small efficiencies of 8% and of NaBH4. The results showed no efficiency for 4-aminophenol
16% were observed for the reduction of nitrobenzene to aniline and only the absorption band at 318 nm for 90 min belonging to
in the presence of Fe3O4 and Fe3O4@SiO2, respectively. The 4-nitrophenol was observed. The results well showed the
absence of catalytic activity of these compounds for the prepa- reduction process of 4-nitrophenol over time. In addition, with
ration of tetrazole can be directly attributed to the absence of these results, the reaction kinetics can be studied
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Table 5 Screening of catalytic activity of the catalyst component toward nitrobenzene reductiona and the preparation of 1-phenyl-1H-1,2,3,4-
tetrazoleb Fe3O4@SiO2@polysalophen-Pd(II) as a control experiment
Product
Aniline
1-Phenyl-1H-1,2,3,4-tetrazole
Entry
Catalyst
Time (min)
Conversion (%)c
Time (min)
Conversion (%)c
1
2
3
4
Fe3O4
Fe3O4@SiO2
60
60
60
60
60
60
8
90
90
90
90
90
90
N.R.
N.R.
N.R.
10
65
65
16
N.R.
5
80
80
Polysalophen 3
Pd(OAc)2
Pd(II)-polysalophen complex
Pd(II)-polysalophen complex +
Fe3O4@SiO2
5
6d
a
Nitrobenzene (1.0 mmol), NaBH4 (2.0 mmol), H2O (2.0 mL), catalyst (5 mg), reux. b Nitrobenzene (1.0 mmol), triethyl orthoformate (1.2 mmol),
NaN3 (1.5 mmol), NaBH4 (2.0 mmol), H2O (2.0 mL), catalyst (5 mg), reux. c GC. d The reaction was performed in the presence of a physical mixture
of Pd(II)-polysalophen complex (0.5 g) and Fe3O4@SiO2 (0.5 g).
spectrophotometrically. According to the results, the induction
time is equal to 25 minutes and therefore the reduction of the
nitro group takes place aer this time (t0 ¼ 25 min).
The results show that the reduction of nitro compounds
follows rst-order kinetics.48,49 In order to investigate the
kinetics of the 4-nitrophenol reduction, ln(A/A0) was plotted in
Fig. 7b in terms of time according to the electronic spectra (UV-
Vis) obtained.
Accordingly, the recorded absorption at 408 nm was used
over a period of 25 to 90 minutes. With constant consideration
of NaBH4, the rate constant for the reaction for 4-nitrophenol
reduction can be calculated with the following eqn (2):46
ꢀ
ꢁ
A
ln
¼ ꢀkt
(2)
A0
where A is the absorption of 4-nitrophenol at time t (sec.), A0 is
the initial absorbance of 4-nitrophenol. According to the
equation, the rate constant for the reduction was calculated as
0.0007 sꢀ1
.
Scheme 7 shows the reduction of nitrobenzene and its
subsequent conversion to tetrazole in two proposed mecha-
nisms. Initially, the nitro and reducing agent (NaBH4) can be
adsorbed on the catalyst surface.45,50 The hydride is then
transferred to the nitro group through four consecutive inter-
mediates (I, II, III, IV). The presence of water and NaBH4 at each
step causes the effective transfer of hydride to the nitro
compound and the removal of the produced hydroxyl group as
a water molecule.51–53
Fig. 7 (a) Consecutive UV-Vis spectra for 4-nitrophenol reduction to
the corresponding 4-aminophenol acquired after t0 ¼ 25 min of
addition of NaBH4. The spectra were recorded every 10 minute until
full reduction. For greater clarity, the recorded spectra before t0 have
been removed. Reaction conditions: NaBH4 (2.0 mmol), H2O (2.0 mL),
catalyst 9 (5 mg, 0.3 mol% Pd), reflux. (b) Plot of ln(A/A0) versus time for
the reduction of 4-nitrophenol for a period of 25–90 minutes (after
induction time).
Scheme 6 A schematic view of the reduction of 4-nitrophenol catalyzed by Fe3O4@SiO2@polysalophen-Pd(II) (9).47
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Scheme 7 Plausible reaction mechanisms for (a) reduction of nitroarenes and (b) their subsequent transformation to 1-substituted-1H-1,2,3,4-
tetrazoles catalyzed by Fe3O4@SiO2@polysalophen-Pd(II).
In order to conrm the proposed mechanism, the catalyst of the reaction has almost the same initial potential equal to
surface charge was recorded by zeta potential analysis at 15, 30 ꢀ30. These results well show the transfer of hydride ions from
and 45 minutes and also at the end of the nitrobenzene the catalyst to nitrobenzene.
reduction reaction. Fig. 8a shows the results of this analysis.
In situ XPS analysis was also performed at 30 min of the
The freshly prepared catalyst has a zeta potential of ꢀ32, which nitrobenzene reduction reaction. The results well show the
is completely consistent with the prepared catalysts based on presence of Pd(0) species in the respective binding energies at
the immobilization of a transition metal complex on magnetic 342.5 eV (Pd 3d3/2) and 335.2 eV (Pd 3d5/2),35 along with Pd2+
nanoparticles.54,55 Aer the reaction, the zeta potentials of the species (Fig. 8b). The results are in complete agreement with the
catalyst reach ꢀ37 and ꢀ40 for 15 and 35 minutes, respectively. proposed mechanism, in which Pd centers play the role of
ꢀ
This amount of negativity is due to the adsorption of BH4 on hydride transfer, thereby converting Pd2+ species to Pd(0).
the surface of the catalyst and the subsequent reduction of
Upon reducing the nitro to amine, in the next step, a domino
nitrobenzene. Aer the reaction and the consumption of reaction is performed to prepare tetrazoles in the presence of
NaBH4, the zeta potential goes to positive values and at the end triethyl orthoformate. As shown in Scheme 7, triethyl
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Fig. 9 Recycling studies and measurement of leaching amount of
heterogeneous catalyst 9 over the preparation of 11a and 12a under
optimized conditions.
dropped to 90% and 92% (equivalent to 6% efficiency drop aer
6 consecutive cycles for both reactions) for 11a and 12a,
respectively. This amount of efficiency drop was correlated with
the amount of metal leaching, so that in each cycle, some metal
leaching was detected in the remaining solution. The amount of
Pd leaching aer the sixth cycle for 11a and 12a was determined
to be 3% and 3.5%, respectively. This amount of leaching is
acceptable and reasonable for heterogeneous catalysts. Also it
has not caused a sharp drop in yield of the products. Another
noteworthy point is that despite the small amount of leaching
observed in the rst cycle, no change in the reaction efficiency
was observed with respect to the freshly prepared catalyst,
which reects the stability and high activity of the catalyst.
The recovered catalyst aer the sixth cycle was studied by
TEM analysis to evaluate its structure and morphology. Fig. 10a
shows the TEM image of the recovered catalyst aer the sixth
cycle, in which no trace of agglomeration or change in particle
morphology can be observed. In addition, compared to the TEM
image of the freshly prepared catalyst, the unchanged particles
have an average diameter of 75 nm. Due to the nature of the
reduction reaction and the presence of NaBH4 as the reduction
agent in the reactions, XPS analysis was also conducted on the
recovered catalyst aer the sixth cycle. As shown in Fig. 10b, the
Pd centers have maintained their oxidation state capacity as +2
as in the freshly prepared catalyst. However, it is possible to
observe low percentages of Pd of zero state at the binding
energies of 335.2 and 340.4 eV, which can be related to the effect
of NaBH4 in the reduction reactions.
Fig. 8 (a) Monitoring the zeta potential of the catalyst in terms of the
reaction time in the reaction mixture of nitrobenzene reduction and (b)
in situ high resolution Pd-3d (normalized, energy-corrected) XPS
analysis of the recovered catalyst 9 after 30 min of the nitrobenzene
reduction reaction.
orthoformate and azide ions (intermediate VI) can be adsorbed
on the catalyst surface. Intermediate VII is formed by the
nucleophilic attacks of the previously produced amine on
triethyl orthoformate, and the subsequent removal of an
ethanol molecule. By a 3 + 2 ring-formation reaction, a tetrazole
ring is formed and by removing another ethanol molecule, the
desired tetrazole product is obtained. The catalyst also returns
to the cycle aer this step.
Reusability
The ability to recover a heterogeneous catalyst is very important
in that (1) some reagents in the reaction mixture have the ability
to inactivate the catalyst (also in the presence of some func-
tional groups) and (2) in catalytic systems containing a transi-
tion metal, there is a possibility of metal leaching and
subsequent reaction progress in the presence of the lost
metal.31,33,34 From this point of view, due to the presence of
NaBH4 in the reaction, metal leaching is very undesirable for
the catalyst. Hence, the recoverability of the catalyst was studied
for 6 consecutive runs in the direct conversion reaction of
nitrobenzene to tetrazole as well as the reduction of nitroben-
zene to aniline. In addition, in each cycle, aer recovery of the
catalyst, the remaining solution was studied to determine the
amount of leaching by ICP analysis. Fig. 9 shows the results of
recovery experiments for both reactions. According to the
results, in each cycle, the amount of efficiency drop for both
reactions was very small, and aer the sixth cycle, the efficiency
A hot ltration test was performed to conrm the effect of
metal leaching on the catalytic activity of the catalyst over the
reactions. For this purpose, the reduction reaction of nitro-
benzene in the presence of Fe3O4@SiO2@polysalophen-Pd(II)
and NaBH4 was performed under optimal conditions and the
catalyst was ltered aer 30 minutes of reaction time. At this
time, the %conversion was 46%, and then the reaction was
allowed to proceed for another 30 minutes (corresponding to
the reaction time for 11a to reach a yield of 96%). No detectable
improvement was observed for product 11a, and aer a further
30 minutes of reaction time, the % conversion was 46%. The
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conditions, an immobilized strong salophen ligand, high
selectivity towards reduction, and stability during consecutive
recycling without any deactivation were some of the advantages
of the present method, which makes it a potential and suitable
alternative method for reduction as well as MCR protocols
compared to other reported methods.
Data availability statement
The data that supports the ndings of this study are available in
the ESI† of this article.
Conflicts of interest
There are no conicts to declare.
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