Palladium catalyst supported on PEGylated imidazolium based phosphinite ionic liquid-modified magnetic silica core-shell nanoparticles: A worthy and highly water-dispersible catalyst for organic reactions in water

Article abstract of DOI:10.1039/c5ra12747e

A highly water-dispersible palladium nanocatalyst was fabricated by the immobilization of Pd onto the surface of PEGylated imidazolium based phosphinite ionic liquid functionalized γ-Fe₂O₃@SiO₂ core-shell nanoparticles. This nanocatalyst (denoted as [Pd@PEGylated ImIL-OPPh₂-γ-Fe₂O₃@SiO₂]) was assessed as a promising Pd catalyst in different organic reactions including Mizoroki-Heck and Sonogashira coupling reactions of aryl halides and the reduction reaction of 4-nitrophenol (4NP) to 4-aminophenol (4AP). From the application point of view, this nanocatalyst showed high thermal versatility, stability, recoverability, and compatibility in aqueous media. The catalyst recovery test revealed that its performance and catalytic activity stayed indefectible during several sequential runs. The loading level of Pd in the [Pd@PEGylated ImIL-OPPh₂-γ-Fe₂O₃@SiO₂] catalyst was 0.087 mmol g^-1. Pivotal properties including high robustness, efficiency and turnover frequency (TOF), mild reaction conditions, utilization of water as a green solvent, simple product work-up, and facile catalyst recovery make this catalyst favourable from the environmental and economic points of view.

Full text of DOI:10.1039/c5ra12747e

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PAPER
Palladium catalyst supported on PEGylated
imidazolium based phosphinite ionic liquid-
modified magnetic silica core–shell nanoparticles:
a worthy and highly water-dispersible catalyst for
organic reactions in water†
Cite this: RSC Adv., 2015, 5, 71297
a
b
a
c
*
*
Saeed Bahadorikhalili, Leila Ma’mani, Hossein Mahdavi and Abbas Shafiee
A highly water-dispersible palladium nanocatalyst was fabricated by the immobilization of Pd onto the
surface of PEGylated imidazolium based phosphinite ionic liquid functionalized g-Fe2O3@SiO2 core–
shell nanoparticles. This nanocatalyst (denoted as [Pd@PEGylated ImIL-OPPh2-g-Fe2O3@SiO2]) was
assessed as a promising Pd catalyst in different organic reactions including Mizoroki–Heck and
Sonogashira coupling reactions of aryl halides and the reduction reaction of 4-nitrophenol (4NP) to 4-
aminophenol (4AP). From the application point of view, this nanocatalyst showed high thermal versatility,
stability, recoverability, and compatibility in aqueous media. The catalyst recovery test revealed that its
performance and catalytic activity stayed indefectible during several sequential runs. The loading level of
Pd in the [Pd@PEGylated ImIL-OPPh2-g-Fe2O3@SiO2] catalyst was 0.087 mmol gꢀ1. Pivotal properties
including high robustness, efficiency and turnover frequency (TOF), mild reaction conditions, utilization
of water as a green solvent, simple product work-up, and facile catalyst recovery make this catalyst
favourable from the environmental and economic points of view.
Received 1st July 2015
Accepted 10th August 2015
DOI: 10.1039/c5ra12747e
www.rsc.org/advances
Mizoroki and the Sonogashira–Hagihara reactions for the
preparation of unsaturated carbon skeletons of organic mole-
cules.4 Apart from the commonly used triarylphosphane
ligands, and the development of methods with non-phosphane
ligands,5 great progress has been achieved in the eld of cata-
lytic reactions and new solid supported phosphane ligands have
been developed.6 In addition, imidazolium-based ionic liquids
are either used as media or ligands in the complexation with the
palladium catalyst in different C–C coupling bond formation
reactions.7
Also the catalyst recovery as well as the recycling of the
expensive palladium catalyst is of great importance from the
economic and environmental perspectives. Therefore, recently,
a number of strategies have been used to immobilize the Pd
catalysts onto solid supports in order to have the advantages of
both homogeneous and heterogeneous catalysts to overcome
the drawbacks of homogeneous Pd catalysts.8 There are
numerous immobilization or heterogenization methodologies
for Pd NPs based on polymers,9 glass–polymer composite
materials,10 ionic liquids,11 and organic–inorganic uorinated
hybrid materials12 as well as common inorganic materials such
as alumina,13 carbon14 or magnetic silica.15 Also, recently several
examples of Pd nanoparticles have been reported as powerful
candidates to facilitate the reduction of 4-nitrophenol (4NP) by
NaBH4 to 4-aminophenol (4AP).16 However, the facile synthesis
Introduction
Magnetic nanoparticles (MNPs) have displayed high applica-
bility in different elds such as chemistry and catalysis and are
known as effective research tools to improve the efficiency and
operation of catalysts.1 Due to the favourable chemical and
physical properties, a silica surface is utilized as the outer layer
in core–shell structures to stabilize the MNPs. Silica-based
nanoparticles have been demonstrated to be a unique class of
nanoparticles that are oen used as nanocarriers for biological
molecules and catalysts.2 Their widespread usage and versatility
make magnetic silica core–shell nanoparticles (such as g-Fe2-
O3@SiO2, Fe3O4@SiO2, etc.) excellent and desirable supporting
materials for providing new and improved green catalysts.3
Several homogeneous and heterogeneous Pd nanocatalytic
systems have been developed in the literature pertaining to the
utility of carbon–carbon coupling reactions such as the Heck–
aDepartment of Chemistry, College of Science, University of Tehran, P. O. Box:
14155-6455, Tehran, Iran
bDepartment of Tissue Engineering, School of Advanced Technologies in Medicine,
Shahid Beheshti University of Medical Sciences, Tehran, Iran
cDepartment of Medical Chemistry, Faculty of Pharmacy and Pharmaceutical Science
Research Center, Tehran, University of Medical Sciences, Tehran, 14176, Iran.
E-mail: shaeea@tums.ac.ir; Fax: +98 21 66461175
† Electronic supplementary information (ESI) available: NMR spectral data and
catalyst characterization data. See DOI: 10.1039/c5ra12747e
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of green and novel Pd–NP catalysts having further improved of HBr (17.34 g, 10 mmol) in 50 mL DW is added drop-wise to
catalytic activities remains a challenge. the latter mixture over 12 h with vigorous stirring at RT and the
Although magnetic silica core–shell nanocatalysts are well- mixture is reuxed overnight. Aerwards, the mixture is cooled
dispersed in many solvents, they cannot provide a suitable to RT and a solution of NaOH 1 N is added to neutralize the
exposure of their catalytically reactive active centres when water solution. The magnetic solid is separated by an external
is used as solvent. To the best of our knowledge, there are only a magnetic device, washed three times with DW and EtOH and
few reports on the application of magnetic Pd-catalysts in neat then dried under vacuum.
water as a reaction medium; however, many of them required
Subsequently, potassium carbonate (1.38 g, 10 mmol),
large amounts of poly ethylene glycol (PEG) and ionic liquids imidazole (0.69 g, 10 mmol) and pyridine (0.79 g, 10 mmol) are
(IL) or n-Bu4NBr (TBAB) as a solvent or phase transfer reagent, added to a mixture of the latter magnetic solid in 50 mL
respectively.16 Therefore, it seems that there is still much desire acetonitrile, and then the mixture is reuxed overnight. Next the
to introduce novel recyclable catalysts displaying improved high mixture is cooled to RT and the solid residue is separated using
catalytic efficiency and solubility in pure water. Recently, some an external magnet and washed with DW and EtOH (3 ꢂ 5 mL)
Pd polymers terminated with triethylene glycol,17a water-soluble to remove the unreacted substrates and give the [PEGylated-
NHC–Pd polymers,17b and impregnated dendrimers bearing Pd ImIL-g-Fe2O3@SiO2].
nanoparticles on a magnetic support8a have been introduced as
recyclable catalysts in Pd catalyzed coupling reactions. Preparation of the [PEGylated ImIL-OPPh2-g-Fe2O3@SiO2]
However, despite the reusability, recycling of the catalyst catalyst
requires a time consuming dialysis technique17b or the reaction
2-chloroethanol (0.4 g, 5 mmol) is poured into a solution of
requires a co-solvent, harsh temperatures and long times.8a
[PEGylated-ImIL-g-Fe2Oꢁ3@SiO2] (2 g) in 50 mL toluene and the
Therefore, we were encouraged to design and manufacture a
more efficient Pd catalyst by emphasizing controlling its high
mixture is stirred at 90 C overnight. Then the solid residue is
magnetically separated, washed with toluene and Et2O, and
water solubility while avoiding time consuming recycling
dried in a vacuum oven at 50 ꢁC for 24 h. The latter dried solid is
processes. Herein, we report
a highly water-dispersible/
poured into a solution of NaBF4 (0.55 g, 5 mmol) in 15 mL dry
acetonitrile and is allowed to stir at RT for 4 days. Aerwards,
the obtained solid is magnetically decanted and washed and
then dried under vacuum. The dried magnetic solid (2 g) and
triethylamine (0.505 g, 5 mmol) are added to a solution of
chlorodiphenylphosphine (IUPAC name, diphenylphosphinous
chloride, 1.1 g, 5 mmol) in dry CH2Cl2. The mixture is stirred
under an argon atmosphere for 12 h at RT. Then the solid
residue is separated by an external magnet, washed with CH2Cl2
magnetically separable Pd catalyst based on a PEG substituted
imidazolium based phosphinite ionic liquid [Pd@PEGylated
ImIL-OPPh2-g-Fe2O3@SiO2] as a robust and magnetically reus-
able catalyst for the Mizoroki–Heck and Sonogashira–Hagihara
reactions, and also for the reduction of 4-nitrophenol (4NP).
Experimental section
Preparation of PEGylated g-Fe2O3@SiO2
ꢁ
and dried under vacuum at 50 C for 12 h to give [PEGylated
PEG600-silane is synthesized from PEG (MW ¼ 600) and 3-
ImIL-OPPh2-g-Fe2O3@SiO2].
(triethoxysilyl)propyl isocyanate (TESPIC) through a hydrogen-
transfer reaction.18 Briey, 0.01 mol of dried PEG600 (at 90 C
ꢁ
Preparation of the [Pd@PEGylated ImIL-OPPh2-g-
for 3 h under vacuum) is dissolved into 50 mL of dry pyridine
with vigorous stirring under an argon atmosphere for 6 h at 70
ꢁC. Then 0.01 mol of TESPIC is added into the latter mixed
solution. Aer 24 h, the solvent is removed by vacuum evapo-
ration. The residue is washed three times with n-hexane, and
then recrystallized from EtOH at 0 ꢁC. The obtained white
PEG600-silane is ltered at 0 ꢁC and then dried at room
temperature (RT) under vacuum.
Fe2O3@SiO2] catalyst
To a mixture of [PEGylated ImIL-OPPh2-g-Fe2O3@SiO2] (1 g) in
50 mL dry CH2Cl2, palladium acetate (2 mmol) is added and
stirred at RT under an argon atmosphere for 24 h. The obtained
solid is magnetically separated and washed with CH2Cl2 (2 ꢂ 10
mL) and Et2O (2 ꢂ 10 mL). Finally, the [Pd@PEGylated ImIL-
OPPh2-g-Fe2O3@SiO2] nanocatalyst is obtained as a dark brown
powder aer drying under vacuum for 12 h.
Then a solution of 0.1 g of PEG600-silane in 30 mL of EtOH is
added drop-wise to a vigorously stirred solution of 25 mg of g-
Fe2O3@SiO2 nanoparticles, which is prepared according to the
previously reported procedure,19 in 30 mL EtOH/H2O (1 : 2) and
HCl (pH ¼ 4). Aer vigorous stirring for 24 h, the solution is
ltered off and washed thoroughly. The white residue is dried at
100 ꢁC under vacuum for 12 h to obtain [PEGylated g-
Fe2O3@SiO2].
General procedure for the Heck coupling reaction
A mixture of aryl halide (1.0 mmol), alkene (1.1 mmol), NaOAc
(1.5 mmol) and a catalytic amount of [Pd@PEGylated ImIL-
OPPh2-g-Fe2O3@SiO2] (5 mg) is taken in a round-bottom ask,
stirred in H2O (3.0 mL) at RT and the reaction progress is
monitored by TLC. At the end of the reaction, the liquid is
isolated by magnetic decantation. The liquid is poured into DW
(10 mL) and the product is extracted with ethyl acetate. The
organic phase is dried over Na2SO4. Then the combined organic
Preparation of PEGylated-ImIL-g-Fe2O3@SiO2
4 mL conc. H2SO4 is added to a solution of [PEGylated g-Fe2- phase is evaporated and the product is puried by column
O3@SiO2] (6 g) in 100 mL deionized water (DW). Then a solution chromatography on silica gel using a mixture of n-hexane/ethyl
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acetate (5 : 1) as the eluent. The recovered catalyst is rinsed with complex by the phosphinite.20a Therefore the imidazolium and
water and EtOH, and nally dried at RT, and used without any phosphinite moieties as two powerful complexing agents
pre-treatment for the next run. The recycling test of the catalyst provide an appropriate condition for the efficient linkage of Pd
is performed in the reaction between bromobenzene and butyl NPs on the surface of PEGylated g-Fe2O3@silica core–shell
acrylate according to the above procedure.
nanoparticles.20b The structure of the prepared nanoparticles
was characterized using diverse techniques including trans-
mission electron microscopy (TEM), vibrating sample magne-
tometry (VSM), thermal gravimetric analysis (TGA), FT-IR, N2
adsorption–desorption analysis, and inductively coupled
plasma-atomic emission spectrometry (ICP-AES) analysis. The
structures of the prepared nanoparticles were characterized
using TEM as shown in Fig. 1.
The magnetic hysteresis measurement was done using VSM
in an applied magnetic eld at RT with the eld sweeping from
ꢀ8000 to +8000 Oe to assay the magnetic behaviour of the [g-
Fe2O3@SiO2] nanoparticles before and aer post modications.
The S-type magnetization hysteresis loop conrmed the super-
paramagnetic nature of the [g-Fe2O3@SiO2] and [Pd@PEGy-
lated ImIL-OPPh2-g-Fe2O3@SiO2] nanoparticles (Fig. 2a). The
saturation magnetization of [g-Fe2O3@SiO2] was 8.23 emu gꢀ1
which, aer functionalization to create the [Pd@PEGylated
ImIL-OPPh2-g-Fe2O3@SiO2] nanocatalyst, was reduced to 4.50
emu gꢀ1. As illustrated in Fig. 2b, the images of aq. solutions of
[Pd@PEGylated ImIL-OPPh2-g-Fe2O3@SiO2] with and without
an external magnet rmly demonstrate excellent and sufficient
magnetization for magnetic separation.
The surface area of the synthesized solids was determined
using the BET (Brunauer–Emmett–Teller) technique. The N2
adsorption–desorption isotherms of [g-Fe2O3@SiO2] and
[PEGylated ImIL-OPPh2-g-Fe2O3@SiO2], are 307.96 m2 gꢀ1 and
31.56 m2 gꢀ1, respectively. The large surface area of the [g-Fe2-
O3@SiO2] nanoparticles was related to the small particle size of
this material as could be observed in the TEM images. The
existence of PEG-ImIL on the surface of the MNPs was proved
using TG analysis. Also TG analysis was applied to reveal the
General procedure for the Sonogashira coupling reaction
A mixture of aryl halide (1.0 mmol), aryl phenylacetylene (1.0
mmol), NaOAc (1.5 mmol) and a catalytic amount of [Pd@PE-
Gylated ImIL-OPPh2-g-Fe2O3@SiO2]. (5 mg) in a round-bottom
ask is stirred in H2O (3.0 mL) at RT and the progress of the
reaction is monitored by TLC. Aer the completion of the
reaction, the catalyst is separated from the reaction mixture by
an external magnet device. Then, the residual mixture is poured
into DW (10 mL) and the product is extracted with ethyl acetate.
The organic phase is washed with brine and dried with anhy-
drous Na2SO4. The organic phase is evaporated and the residue
is puried by column chromatography on silica gel using a
mixture of n-hexane/ethyl acetate (10 : 1) as the eluent to obtain
the corresponding products.
Catalytic reduction of 4NP to 4AP
A mixture of 2.5 mL of fresh aq. solution of NaBH4 (1.2 M), 10
mL aq. solution of 0.34 mmol 4NP and 1 mg of nanocatalyst is
stirred. The colour of the latter solution fades as the reaction
proceeds. The progress of the reaction is monitored using UV-
vis spectra of the supernatant (at short intervals). This proce-
dure is repeated for a blank experiment to show that the reac-
tion does not proceed without the catalyst and only in the
presence of NaBH4.
Results and discussion
Synthesis and characterization of the catalyst
To fabricate the designed Pd catalyst, initially g-Fe2O3 nano- thermal stability and also to determine the amount of loaded
particles were synthesized and conrmed according to a PEGylated imidazolium based phosphinite IL on the surface of
previous report.19 Aerwards, to improve their stability and the [g-Fe2O3@SiO2] nanoparticles. While the TG curve of [g-
biocompatibility, the magnetite nanoparticles were coated with Fe2O3@SiO2] has no weight loss aer 300 ꢁC, for [PEGylated
a silica shell generated by hydrolysis and condensation of TEOS ImIL-OPPh2-g-Fe2O3@SiO2] and [Pd@PEGylated ImIL-OPPh2-g-
to form g-Fe2O3@silica nanoparticles. Then g-Fe2O3@silica was Fe2O3@SiO2] a sharp decrease in weight was observed at 300–
PEGylated by decoration of the MNPs’ surface with PEG chains 700 ꢁC (Fig. 3). Additionally, TGA shows that the amount of the
via covalent graing using PEG600-silane obtained from the loaded PEGylated imidazolium based phosphinite IL moiety is
treatment of PEG600 and TESPIC. The PEGylated g-Fe2O3@silica 0.3 mmol IL gꢀ1. Moreover, the Pd content of the obtained solid
core–shell nanoparticles were treated with imidazole, 2-chlor- catalyst was found to be 0.087 mmol per gram of the [Pd@PE-
oethanol, and chlorodiphenyl-phosphine, to give the decorated Gylated ImIL-OPPh2-g-Fe2O3@SiO2] catalyst using ICP-AES.
imidazolium based phosphinite ionic liquid on the surface of
the MNPs. Finally, the PEGylated imidazolium based phos- characterization peaks for Fe–O–Fe, Si–O–Si and O–H groups
phinite ionic liquid moiety was applied as a dispersible pendant (Fig. 4a). As shown in Fig 4b, aer the reaction between PEG600
FT-IR spectra of the [g-Fe2O3@SiO2] nanoparticles show
-
for the immobilization of Pd nanoparticles on the surface of the silane and [g-Fe2O3@SiO2], the resultant [PEGylated g-Fe2-
g-Fe2O3@silica nanoparticles. [PEGylated ImIL-OPPh2-g-Fe2- O3@SiO2] does not show any vibration at 2000–2500 cmꢀ1
O3@SiO2] was reacted with Pd(OAc)2 to provide the nal catalyst which shows the absence of the isocyanate group. Meanwhile,
(Scheme 1). It is noticeable that the provided phosphinite group the successful immobilization of PEGylated imidazolium based
in the IL performs both as a complexing and reducing agent for phosphinite IL on the surface of the [PEGylated ImIL-OPPh2-g-
Pd(II). It has been established that a Pd(0) complex is sponta- Fe2O3@SiO2] catalyst was conrmed by the observed peaks at
neously formed from the reaction of Pd(II) and phosphinite 1712 cmꢀ1 and 1530 cmꢀ1 which could be assigned to C]N,
ligands by an irreversible intramolecular reduction of a Pd(II) C]C bonds (Fig. 4c).
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Scheme 1 The synthesis of the [Pd@PEGylated ImIL-OPPh2-g-Fe2O3@SiO2] nanocatalyst.
It is believed that the immobilization of the PEGylated ImIL
containing a pincer-like phosphinite pendant on the surface of
the MNPs provides a good distribution of catalytically active Pd
species on the support and accordingly it stabilizes the Pd
nanoparticles during the chemical reaction, and also protects
the Pd nanoparticles from aggregation. In addition, the hydro-
philic character of PEGylated ImIL allows a good dispersion of
MNPs in an aqueous medium, which helps to develop this
system as a semi-homogeneous Pd catalyst for organic reactions
in pure water as a safe and green solvent.
Fig. 1 (a) TEM images of [PEGylated ImIL-OPPh2-g-Fe2O3@SiO2] and
(b) [Pd@PEGylated ImIL-OPPh2-g-Fe2O3@SiO2].
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with the reaction of styrene with bromobenzene to optimize the
reaction conditions. In our rst set of experiments, we tested
the catalytic capability of [Pd@PEGylated ImIL-OPPh2-g-Fe2-
O3@SiO2] by running the reaction with different amounts of the
catalyst from 5 to 15 mg of catalyst. The best results were
observed when 5 mg (0.0435 mol%) of catalyst was used, while
less than this amount led to a signicant decrease in the
conversion yields, and on the other hand, using greater
amounts did not lead to signicant improvements. It should be
noted that no conversion was observed in the absence of the
catalyst. In our next set of experiments, we tested the impact of
different solvents and bases (Table 1). The results obtained
from the array of various bases and solvents for the coupling
reaction of styrene at RT show that the best result is obtained
when sodium acetate and water are used as base and solvent,
respectively (Table 1, entry 1).
The optimal results encouraged us to investigate the effi-
ciency of the catalyst in the Mizoroki–Heck reaction for different
aryl halides. The summarized results in Table 2 clearly show
appreciable product preparation in good to excellent yields for a
wide array of aryl halides (chlorides, bromides, and iodides)
bearing electron decient and electron donating substituents in
relatively short reaction times. Also, heteroaryl halides readily
react with styrene in this protocol (Table 2, entries 15–17). For
aryl halides bearing electron-decient substituents moderate
yields are obtained in longer reaction times (Table 2, entries 9,
11–14). Very good isolated yields were obtained for the reaction
of aryl halides and butyl acrylates in these reaction conditions
(Table 2, entries 18–30).
Fig. 2 (a) The magnetization hysteresis loop of [g-Fe2O3@SiO2] and
[Pd@PEGylated ImIL-OPPh2-g-Fe2O3@SiO2], and (b) high water
dispersion and facile magnetic separation of the catalyst.
Inspired by the high activity and stability of the [Pd@PEGy-
lated ImIL-OPPh2-g-Fe2O3@SiO2] catalyst, the Sonogashira–
Hagihara reaction was subsequently tested. To optimize this
reaction, the coupling reaction between phenylacetylene and p-
bromobenzene was chosen as a model reaction. Of the screened
bases including NaOH, K2CO3, Cs2CO3, NaOAc, and Et3N,
sodium acetate showed the best results and the reaction was
complete aer 1 h. The optimization of the reaction conditions
Fig. 3 TG analysis of the synthesized nanoparticles.
showed
that,
for
a
mixture
of
4-
Table 1 The optimization of the Mizoroki–Heck reaction using the
[Pd@PEGylated ImIL-OPPh2-g-Fe2O3@SiO2] catalysta
Entry
Solvent
Base
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
H2O
H2O
H2O
H2O
H2O
EtOH
PEG
DMSO
DMF
DMA
NaOAc
NaOH
K2CO3
K3PO4
Et3N
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
1
1
1.5
1
3
1
1.5
1
1
1
93
87
76
79
21
89
91
80
82
87
37
45
Fig. 4 FTIR spectra for (a) [g-Fe2O3@SiO2], (b) [PEGylated g-Fe2-
O3@SiO2], and (c) [PEGylated ImIL-OPPh2-g-Fe2O3@SiO2].
9
10
11
12
Catalyst activity in the Heck/Sonogashira coupling reactions
Toluene
Acetonitrile
3
2
To investigate the efficiency of the catalyst, Mizoroki–Heck and
Sonogashira coupling reactions were tested with a wide variety
of substrates bearing different functional groups. We started
a
Condition: styrene (1 mmol) and bromobenzene (1 mmol) in water (3
b
mL), cat. (5 mg), and base (1.5 eq.). GC yield.
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bromobenzene : phenylacetylene : NaOAc,
a
stoichiometric
Table 2 Investigation of the Heck reaction for various aryl halides
using [Pd@PEGylated ImIL-OPPh2-Fe3O4@SiO2] as a catalysta
ratio of 1.0 : 1.0 : 1.5 mmol in the presence of 5 mg (0.0435
mol%) of the [Pd@PEGylated ImIL-OPPh2-g-Fe2O3@SiO2]
catalyst is the suitable and optimal condition. Further
increasing the amount of catalyst showed no signicant
improvement in the conversion. Also, to survey the effect of the
solvent in this reaction, a range of solvents (e.g. H2O, EtOH,
PEG, DMF, DMSO, and toluene) was compared for the model
reaction. This comparison showed that water as a green solvent
provided improved yields under the optimal conditions. It is
notable that no homocoupling product was observed under the
optimal conditions.
To expand the scope of the optimized reaction, several
substrates including a range of more challenging aryl chlorides
were tested. The results of this investigation are given in Table
3. As shown in Table 3, the presented protocol was demon-
strated to be amenable for the successful conversion of a range
of aryl halides (chlorides, bromides, and iodides) to the corre-
sponding coupled products in very good to excellent yields in
short reaction times.
Entry
R
X
I
R0
Product Time (h) Yieldb (%)
1
2
3
3
4
5
6
7
8
9
H
H
H
Ph
3a
3a
3a
3b
3b
3c
3c
3d
3d
3e
1
1.5
6
1.5
2
1
1.5
1.5
2
95
97
80
94
94
95
93
91
90
91
Br Ph
Cl Ph
4-MeO
4-MeO
4-Me
4-Me
2-Me
2-Me
2,4,6-Me
I
Ph
Br Ph
Ph
Br Ph
Ph
I
I
Br Ph
Br Ph
2
9
Br Ph
3f
4
89
Catalyst activity in the reduction of 4-nitrophenol
Owing to the carcinogenic, mutagenic, and cytotoxic and
embryonic toxic effects, 4-nitrophenol (4NP) has been recog-
nized as a priority pollutant by the US Environmental Protection
Agency (EPA). Therefore its efficient and economically and
environmentally benign degradation is considered to be an
urgent need. Herein, to demonstrate the versatility of the
[Pd@PEGylated ImIL-OPPh2-g-Fe2O3@SiO2] catalyst, its poten-
tial was evaluated in environmental applications e.g., the
conversion of 4NP (as an organic pollutant) to the environ-
mentally friendly amino form 4AP. For this purpose NaBH4 was
used as the reducing agent in an aqueous medium. As the
nanocatalyst was added the yellow colour of the 4NP aq. solu-
tion faded and ultimately bleached. This colour change was
accompanied with a rapid decrease of the absorption of 4NP
aqueous solution at 400 nm, and an increase in the intensity of
the new peak at 290 nm in UV-vis spectra, which is assigned to
4AP (Fig. 5).
The aforementioned nanocatalyst accomplished this reac-
tion in only 120 s, even in the presence of as little as 1 mg of
nanocatalyst (0.0087 mol%). The robustness of the catalyst was
conrmed by reusing the catalyst 20 times in this reduction
reaction. The observed high activity and efficiency of the reused
catalyst aer twenty successive cycles, with 100% conversion
within 360 s, guarantees the performance of the [Pd@PEGylated
ImIL-OPPh2-g-Fe2O3@SiO2] catalyst. Notably, the reaction
showed no appreciable conversion in the absence of the cata-
lyst, which proves the pivotal role of the catalyst for this
reduction process. The optimal condition was assessed
considering the amount of catalyst and NaBH4 (Fig. 6).
10
Br Ph
3g
3
91
11
12
13
14
4-Cl
Br Ph
Br Ph
3h
3i
3j
3j
2.5
2.5
3
89
84
88
81
4-CN
4-NO2
4-NO2
I
Ph
Cl Ph
6
15
16
17
Br Ph
3k
3l
4
79
76
95
Br Ph
Br Ph
5
3m
0.5
18
19
20
21
22
23
24
25
26
27
28
H
H
I
CO2Bu 3n
Br CO2Bu 3n
CO2Bu 3o
Br CO2Bu 3o
CO2Bu 3p
1
90
88
93
91
90
89
85
80
89
85
85
0.5
0.5
1
0.5
1
4-MeO
4-MeO
4-Me
4-Me
4-Cl
4-CN
4-NO2
4-NO2
4-COCH3
I
I
Br CO2Bu 3p
Br CO2Bu 3q
Br CO2Bu 3r
1
1.5
1.5
3
I
CO2Bu 3s
Cl CO2Bu 3s
Br CO2Bu 3t
1.5
29
Br CO2Bu 3u
3.5
0.5
77
30
Br CO2Bu 3v
91
In the 4NP reduction reaction, the reduction rate can be
considered to be independent from [NaBH4] because the
concentration of NaBH4, [NaBH4], is more than that of 4NP.
Therefore, this trait allows the second order rate constant of the
4NP reduction to be evaluated using pseudo-rst-order kinetics.
a
Reaction conditions: aryl halide (1.0 mmol), alkene (1.1 mmol), NaOAc
(1.5 mmol), solvent (3.0 mL), and cat. (5 mg). b Isolated yields.
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Table
3 Investigation of the Sonogashira–Hagihara reaction of
various aryl halides using [Pd@PEGylated ImIL-OPPh2-g-Fe2O3@SiO2]
as a catalysta
Entry
R
X
Product
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
9
H
H
H
4-MeO
4-MeO
4-Me
4-Me
2-Me
2-Me
I
5a
5a
5a
5b
5b
5c
5c
5d
5d
1
95
97
80
94
94
95
93
91
90
Br
Cl
I
Br
I
Br
I
Br
1.5
7.5
1.5
2
1
1.5
1.5
2
Fig.
5
Successive UV-vis spectra of the 4NP reduction by the
10
11
Br
Br
5e
5f
4
3
89
91
[Pd@PEGylated ImIL-OPPh2-g-Fe2O3@SiO2] catalyst, condition: cat.
(1 mg), NaBH4 and H2O as reducing agent and solvent, respectively at
RT.
12
13
14
15
4-Cl
Br
Br
I
5g
5h
5i
2.5
2.5
3
89
84
88
81
4-CN
4-NO2
4-NO2
Cl
5i
6
16
17
Br
Br
5j
4
5
79
76
5k
18
Br
Br
5l
0.5
1.5
95
85
Fig. 6 Plots of the concentration of 4NP versus time for the reduction
of 4NP with NaBH4 at different amounts of nanocatalyst.
19
4-COCH3
5m
a
Reaction conditions: aryl halide (1.0 mmol), alkyne (1.1 mmol), NaOAc
(1.5 mmol), solvent (3.0 mL), and cat. (5 mg). b Isolated yields.
For the [Pd@PEGylated ImIL-OPPh2-g-Fe2O3@SiO2] catalyst,
the rate constant (K) is calculated to be 0.025 sꢀ1. Short reaction
times and the ability to perform the reaction at room
As is known, the intensity of the absorbance in the UV-vis
spectra is attributed to the concentration, therefore the ratio
of At to A0 is proportional to the ratio of Ct to C0 and the kinetic
equation for the reduction can be written as
dCt/dt ¼ KappCt or ln Ct/C0 ¼ ln At/A0 ¼ Kapp
t
where the concentration of 4NP at time “t” and t ¼ 0 denoted by
Ct and C0, respectively and the absorbance at time “t” and t ¼
0 is shown by At and A0, respectively. Fig. 7 shows a linear
relationship between ln(Ct/C0) and the reaction time in the
presence of [Pd@PEGylated ImIL-OPPh2-g-Fe2O3@SiO2]. as a
catalyst. The Kapp (apparent rate constant) is extracted from the
slope of the line obtained from plotting ln(Ct/C0) as a function
of the reaction time.
Fig. 7 The plot of ln(Ct/C0) vs. time for the 4NP reduction using
[Pd@PEGylated ImIL-OPPh2-g-Fe2O3@SiO2] as a catalyst.
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temperature are other advantages of the catalyst which makes it coated iron oxide nanoparticles as a robust magnetic and
environmentally favourable and industrially applicable, while a water-dispersed nanomaterial and then it has been evaluated as
number of previous reports suffer a several disadvantages such an effective support for the immobilization of Pd catalysts.
as higher temperatures and longer reaction times.
Indeed the PEGylated imidazolium based phosphinite IL spacer
The recycling studies were then performed. The supported plays a triple role as a potent Pd complexing agent, water
catalyst systems were reused several times without signicant solubility enhancer, and medium via the phosphinite group,
loss of activity (Table 4). [Pd@PEGylated ImIL-OPPh2-g-Fe2- PEG moiety, and IL moiety, respectively. Interestingly, the
O3@SiO2] is revealed as an efficient and economically and nanocatalyst shows an improved catalytic activity in the Miz-
environmentally benign catalyst that shows an extraordinary oroki–Heck and Sonogashira coupling reactions and also the
and unique robustness, durability, and stable catalytic activity reduction of 4-nitrophenol to 4-aminophenol using a very small
in different reactions such as Heck/Sonogashira C–C coupling amount of Pd catalyst and gives excellent yields. The hydro-
reactions and the reduction reaction of 4NP. This catalyst is philic feature of the PEGylated imidazolium based phosphinite
prominent due to the efficient recoverability, high turnover IL spacer improves the robustness, the distribution of Pd
number (TON), and turnover frequency (TOF). The robust nanoparticles on the surface of the support, and the solubility of
catalyst achieved cumulative TONs of about 22 000 over 10 the catalyst which makes the active Pd sites more exposed for
successive runs of Heck/Sonogashira reactions for the model substrates like homogeneous systems. Additionally, the super-
reactions, and about 229 885 over 20 successive runs of the 4NP paramagnetic property provides the possibility of simple sepa-
reduction reaction. Meanwhile, the TOF for every run is ration of the catalyst by applying an external magnetic device
approximately 2200, and 344 828 for the Heck/Sonogashira and means the catalyst can be simply used in consecutive runs
model reactions and 4NP reduction reactions, respectively. without a signicant decline in its activity. This unique water-
TON is calculated using eqn (1). Also, the cumulative TON is solubility and efficacy with negligible leaching introduces the
obtained by the sum of the values for the TONs for all examined [Pd@PEGylated ImIL-OPPh2-g-Fe2O3@SiO2] as an efficient,
runs. Additionally, TOFs are calculated using TON hꢀ1
.
versatile, eco-friendly and economic catalyst for a variety of
crucial reactions under mild conditions. Above all, the perfor-
mance of the reactions in pure water as a green solvent, and at
the same time the easy and rapid recycling protocol via the
magnetic separation technique, makes this protocol a valid and
valuable candidate towards the goal of green chemistry.
No: of mole of substrate converted to product
TON ¼
(1)
mole Pd
To demonstrate the absence of the homogeneous Pd species
which is produced via leaching phenomena during the reaction,
a sample of 10 mg of the [Pd@PEGylated ImIL-OPPh2-g-Fe2-
O3@SiO2] catalyst in 10 mL water was prepared and le to stir at
100 ꢁC for 2 h, and then the catalyst was magnetically removed.
We found that the ltrate showed no catalytic activity in the
Mizoroki–Heck and Sonogashira coupling reactions and the
reduction reaction of 4NP (model reactions) under the optimal
reaction conditions. It is also worth mentioning that the
amount of leached Pd was less than 1 ppm for the separated
liquid (measured by ICP-AES).
Acknowledgements
This research was supported by grants from the research
council of Tehran University of Medical Sciences and INSF (Iran
National Science Foundation).
Notes and references
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