A green synthesis of biaryls in water catalyzed by palladium nanoparticles immobilized on N-amidinoglycine-functionalized iron oxide nanoparticles

Article abstract of DOI:10.1007/s11243-018-0215-7

Fe₃O₄ nanoparticles were prepared by co-precipitation and coated with SiO₂ following the St?ber process. N-Amidinoglycine amino acid was then covalently connected to provide an excellent ligand for the immobilization of Pd nanoparticles. The resulting material was characterized by FE-SEM, TEM, EDX, XRD, VSM and ICP-AES analysis. The Fe₃O₄@SiO₂@N-amidinoglycine@Pd⁰ proved to be a highly active catalyst for the Suzuki coupling reactions of various aryl halides with substituted phenylboronic acids in water, giving the desired products in excellent yields for short reaction times. Moreover, this catalyst can be easily recovered by using an external magnet and directly reused for several times without significant loss of activity.

Full text of DOI:10.1007/s11243-018-0215-7

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Transition Metal Chemistry
https://doi.org/10.1007/s11243-018-0215-7
A green synthesis of biaryls in water catalyzed by palladium
nanoparticles immobilized on N‑amidinoglycine‑functionalized iron
oxide nanoparticles
Fatemeh Rafee1 · Nasrin Mehdizadeh1
Received: 9 December 2017 / Accepted: 2 February 2018
© Springer International Publishing AG, part of Springer Nature 2018
Abstract
Fe3O4 nanoparticles were prepared by co-precipitation and coated with SiO2 following the Stöber process. N-Amidinoglycine
amino acid was then covalently connected to provide an excellent ligand for the immobilization of Pd nanoparticles. The
resulting material was characterized by FE-SEM, TEM, EDX, XRD, VSM and ICP-AES analysis. The Fe3O4@SiO2@N-
amidinoglycine@Pd0 proved to be a highly active catalyst for the Suzuki coupling reactions of various aryl halides with
substituted phenylboronic acids in water, giving the desired products in excellent yields for short reaction times. Moreover,
this catalyst can be easily recovered by using an external magnet and directly reused for several times without signifcant
loss of activity.
Introduction
employed as coating layer over the surface of magnetic nan-
oparticles. This surface modifcation can weaken the parti-
cle–particle magnetic bipolar interactions and protect the
nanoparticles from aggregation and oxidation processes.
Moreover, silica coating not only stabilizes the nanopar-
ticles, but can also be readily functionalized with amines,
carboxyls, thiols and biological species [12, 13]. The Stöber
method and sol–gel processes are the preferred choices for
the coating of magnetic nanoparticles with silica [14–16].
The low cost, good biocompatibility and range of avail-
able functional groups of amino acids as capping ligands
for magnetic nanoparticles make them an attractive candi-
date for both coating and surface functionalization. Amino
acids interact with the nanoparticle’s surface through their
carboxyl groups, while their side chains containing various
surfaced-exposed functional groups are available. Available
amino acid functional groups include guanidine, thiol and
phenolic hydroxyl groups, which are potential reaction sites
for further modifcation [17–19].
Magnetic nanoparticles are of great interest in the feld
of catalysis [1–3], biotechnology and biomedical applica-
tions such as drug delivery, MRI contrast agents and cancer
therapy [4–8]. Magnetic nanoparticles with good stability
and ready availability can efectively improve the loading
and catalytic efciency of immobilized catalysts, due to
their high surface-to-volume ratios. In terms of recycling
expensive catalysts, immobilization of these active species
on magnetic nanoparticles leads to facile and quick magnetic
separation without the need for fltration, centrifugation or
other workup processes [9, 10]. However, surface oxida-
tion and agglomeration of chemically highly active naked
magnetic nanoparticles can result in a signifcant decrease
in their magnetic properties, dispersibility, surface area and
catalytic activity [11]. Given the strong surface afnity of
magnetic nanoparticles toward silica as a chemically inert
material with high surface area and good stability, it is often
The palladium-catalyzed Suzuki cross-coupling of
organic halides with boronic acids is one of the most versa-
tile methods for the synthesis of biaryls [20]. Biaryls are of
signifcant current interest for the synthesis of biologically
active compounds, natural products, macrocycles, liquid-
crystal materials and asymmetric catalysts [21–24]. The
increasing need for greener synthetic methodologies has led
to a growing interest in the development of catalysis that
can operate in aqueous medium. The stability of boronic
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11243-018-0215-7) contains
supplementary material, which is available to authorized users.
* Fatemeh Rafee
f.rafee@alzahra.ac.ir
1
Department of Chemistry, Faculty of Physic-Chemistry,
Alzahra University, Vanak, Tehran, Iran
Vol.:(0123456789)
1 3

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Transition Metal Chemistry
acids in aqueous solvent is viewed as an advantage of Suzuki
coupling compared to other cross-coupling reactions. Recent
reviews have covered the literature on palladium-catalyzed
Suzuki cross-coupling reactions in water [25, 26].
In this paper, we report the synthesis and characterization
of a palladium nanocatalyst based on Fe3O4@SiO2 function-
alized with N-amidinoglycine amino acid, and its activity in
Suzuki cross-coupling reactions.
supported on this functionalized support, and pure N-amid-
inoglycine. In the spectrum of Fe3O4 nanoparticles, a band
at 584 cm−1 is assigned to the Fe–O bond stretches, and a
peak at 3421 cm−1 is attributed to the –OH groups. The
spectrum of Fe3O4@SiO2 showed a strong absorption band
at 1091 cm−1, related to Si–O–Si antisymmetric stretching
vibrations; peaks at 799 and 464 cm−1 are assigned to the
Si–O–Si symmetric stretching and Si–O–Si and or O–Si–O
bending modes, respectively. These data confrm the suc-
cessful coating of silica layers on the Fe3O4. Comparisons of
the spectra of Fe3O4@SiO2@N-amidinoglycine with those
of pure amidinoglycine (3384 cm−1 (νN–H of the guani-
dino group), 3303 and 3172 cm−1 (νas and νs of primary
amines), 1670 cm−1 (νC=N of the guanidino group), 1581
and 1411 cm−1 (νas and νs of the carboxylate group)) and
Fe3O4@SiO2 all indicate successful functionalization of the
surface of Fe3O4@SiO2 nanoparticles with N-amidinogly-
cine (Fig. S1d).
Results and discussion
Preparation and characterization
of the nanocatalyst
The magnetic nanoparticle-supported Pd catalyst designated
as Fe3O4@SiO2@N-amidinoglycine@Pd0 was synthesized
via multiple steps. The frst step was to prepare Fe3O4 nano-
particles from FeCl3·6H2O to FeCl2·4H2O [27]. The mag-
netic nanoparticles (MNPs) so obtained were subsequently
coated with silica (Fe3O4@SiO2) through the well-known
Stöber method, using tetraethylorthosilicate (TEOS) [14].
The Fe3O4@SiO2 core/shell structure was then sequentially
treated with N-amidinoglycine to form Fe3O4@SiO2@N-
amidinoglycine. Finally, Pd nanoparticles were immobilized
on this support by reaction with Pd(OAc)2 in ethanol, such
that PdII was reduced to Pd0 without the addition of any
external reducing agent (Scheme 1).
In the powder XRD pattern of the catalyst (Fig. S2b),
the difraction peaks observed at 2θ = 30.14, 35.50, 43.07,
53.03, 57.11 and 62.74° correspond, respectively, to the
crystalline planes (220), (311), (400), (422), (511) and (440)
of Fe3O4, with a cubic spinel structure. The XRD pattern
of the nanocatalyst also showed additional peaks at 39.10,
44.57 and 66.29° which are indexed, respectively, to the
(111), (200) and (220) crystalline planes of Pd nanocrystals,
suggesting the formation of metallic Pd NPs (Pd0). Compari-
son of the spectrum of Fe3O4@SiO2@N-amidinoglycine@
Pd0 with that of pure N-amidinoglycine (Fig S2a) indicates
successful immobilization of this amino acid derivative on
the surface of the Fe3O4@SiO2 nanoparticles.
The catalyst was subjected to microanalysis, and the pro-
portions of carbon, hydrogen and nitrogen by weight were
found to be 1.90, 0.73 and 0.79%, respectively. From the N
content, the loading of N-amidinoglycine was calculated as
0.19 mmol/g. The amount of palladium in the catalyst, as
determined by inductively coupled plasma atomic emission
spectroscopy (ICP-AES), was 1.7 mmol/g.
To evaluate the chemical oxidation state of palladium
in the catalyst, X-ray photoelectron spectroscopy (XPS)
was performed (Fig. S3). The resulting spectrum showed
an intense doublet at a binding energy (BE) of 335.7 and
340.8 eV related to Pd0, assigned to the Pd 3d5/2 and Pd
3d3/2 peaks, respectively. This observation confrmed the
successful reduction of PdII to Pd0 in the process of catalyst
preparation. An XPS elemental survey scan of the surface
of the nanocatalyst also revealed the presence of Fe (Fe 2p
at binding energies of 711.08 and 724.08 eV), O (O 1s at
binding energies of 530.58 and 532.98 eV), Si (Si 2p at bind-
ing energy of 103.08 eV and Si 2s at 154.08 eV), N (N 1s at
399.88 eV) and C (C 1s at 284.78 and 288.48 eV).
Figure S1 shows the FT IR spectra of Fe3O4, Fe3O4@
SiO2, Fe3O4@SiO2@N-amidinoglycine, palladium
The magnetic properties of the nanocatalyst were inves-
tigated with a vibrating sample magnetometer (VSM) at
room temperature in an applied magnetic feld sweeping
from − 10 to 10 kOe (Fig. S4). The magnetization curves of
the prepared catalyst exhibit no hysteresis loop, indicating
superparamagnetic behavior. The saturation magnetization
(Ms) value for the Fe3O4@SiO2@N-amidinoglycine@Pd0
catalyst (23.35 emu g−1) is lower than that of bare Fe3O4 due
to the successful grafting of SiO2 and N-amidinoglycine@
Scheme 1 Procedure for preparation of the Fe3O4@SiO2@N-amidi-
noglycine@Pd0 Catalyst
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Transition Metal Chemistry
Pd0 on the surface of the Fe3O4 nanoparticles. The magnetic
properties of the silica coated magnetic samples depend on
the Fe3O4 content. The non-magnetic coating layer on the
surface of magnetic core changed the uniformity or magni-
tude of magnetization due to quenching of surface moments.
However, the magnetization of the catalyst was still suf-
cient to respond to an external magnetic feld.
(0.17 mmol% palladium) at 90 °C. As this catalyst is not
sensitive to oxygen, the reactions were carried out under
air (Table 1).
The scope and efficiency of this approach were then
investigated for the synthesis of a wide variety of biaryl
derivatives under the optimized conditions. The results are
summarized in Table 2. Various aryl halides were tested,
including either electron-donating or electron-withdrawing
substituents such as Me, OMe, CN, NO2, CHO, COMe
and Cl. Also, both electron-rich and electron-defcient aryl
boronic acids with Me, OMe, COMe and F substituents
were utilized. As evident from Table 2, all the examined
aryl iodides and bromides were successfully coupled with
various phenylboronic acids, afording the corresponding
products with good to excellent yields in short reaction
times. Prolonged reaction times were required for the less
active aryl chlorides to obtain the desired products in mod-
erate yields.
The morphology and size of the nanoparticles were
obtained from feld emission scanning electron microscopy
(FE-SEM) and transmission electron microscopy (TEM)
analysis (Fig. S5). The images obtained at diferent mag-
nifcations demonstrated that the prepared magnetic nano-
particles are spherical in shape, with slight agglomeration.
In the EDX spectrum of the nanocatalyst (Fig. S6), the
presence of Fe, Si and O signals indicates that the Fe3O4
nanoparticles are loaded with silica. Figure S6 also reveals
the presence of the elemental composition of N-amidino-
glycine (C, N and O) and palladium on the surface of the
nanocatalyst.
A hot fltration test was also carried out, in order to dem-
onstrate the absence of homogeneous catalytic species which
is produced via leaching phenomena during the reaction.
An mixture of 0.001 g of the Fe3O4@SiO2@N-amidinogly-
cine@Pd0 catalyst and NaHCO3 (2 mmol) in 3 mL water was
prepared and left to stir at 90 °C for 30 min, and then, the
catalyst was magnetically removed. The fltrate so obtained
showed negligible catalytic activity for Suzuki cross-cou-
pling under the optimized reaction conditions (Table 3).
To study the recyclability of this palladium catalyst,
we carried out a set of experiments using the recycled
catalyst for the coupling of 4-bromoacetophenone with
Catalysis of Suzuki cross‑coupling
In order to explore the catalytic activity of the present Pd
nanocatalyst in the Suzuki reaction, we employed the cou-
pling of 4-bromoacetophenone with phenylboronic acid
as a model system to optimize the reaction conditions.
The best results were obtained using water as the solvent
and NaHCO3 as the base, with 0.001 g of nanocatalyst
Table 1 Optimization of
reaction conditions in Suzuki
cross-coupling reactiona
Entry
Solvent
Base
Time (min)
T (°C)
Catalyst (g)
Conversion (%)b
1
2
3
4
5
6
7
8
H2O
H2O
EtOH
PEG
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
Na2CO3
Na2CO3
Na2CO3
Na2CO3
K2CO3
KHCO3
NaHCO3
NaOAc
10
10
30
10
10
10
10
10
10
5
5
5
5
5
r.t
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.01
10
100
40
10
100
85
100
50
85
90
95
100
80
100
100
100
90
60
100
90
90
90
90
90
90
90
90
90
90
90
90
9
KOH
K2CO3
10
11
12
13
14
15
16
Na2CO3
NaHCO3
NaHCO3(1 eq)
NaHCO3
NaHCO3
NaHCO3
5
5
0.003
0.001
aReaction conditions: 4-bromoacetophenone (1 mmol), phenylboronic acid (1.2 mmol), base (2 mmol),
Fe3O4@SiO2@N-amidinoglycine@Pd0 catalyst, solvent (3 mL)
bGC (n-hexane as internal standard) and TLC(n-hexane or n-hexane:ethylacetate 9:1) conversion
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Transition Metal Chemistry
Table 2 Suzuki cross-coupling
reactions using Fe3O4@
SiO2@N-amidinoglycine@Pd0
nanocatalyst
Entry
Ar-X
Ar′B(OH)2
Ar–Ar′
Time min
Yielda (%)
1
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
PhBr
PhB(OH)2
PhB(OH)2
PhB(OH)2
PhB(OH)2
Ph–Ph
p-OHC-Ph–Ph
p-MeOC-Ph–Ph
p-NC-Ph–Ph
p-MeO-Ph–Ph
p-Me-Ph–Ph
p-Me-Ph–Ph-p-CHO
p-Me-Ph–Ph–p-COMe
p-Me-Ph–Ph-p-CN
p-Me-Ph–Ph-m-NO2
p-MeOC-Ph–Ph-p-COMe
p-MeOC-Ph–Ph-p-CHO
p-MeOC-Ph–Ph-p-OMe
p-MeOC-Ph–Ph-p-Me
m-MeO-Ph--Ph-p-OMe
m-MeO-Ph–Ph-m-NO2
m-MeO-Ph–Ph-p-COMe
3,4,5-F-Ph–Ph-p-COMe
3,4,5-F-Ph–Ph-p-CHO
p-Me-Ph–Ph-p-CN
p-NC-Ph–Ph
10
2
5
96
98
98
98
97
97
98
98
96
98
95
95
96
98
97
93
96
98
93
50
35
p-OHC-PhBr
p-MeOC-PhBr
p-NC-PhBr
p-MeOPhI
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
90
90
PhB(OH)2
PhBr
p-Me-PhB(OH)2
p-Me-PhB(OH)2
p-Me-PhB(OH)2
p-Me-PhB(OH)2
p-Me-PhB(OH)2
p-MeOC-PhB(OH)2
p-MeOC-PhB(OH)2
p-MeOC-PhB(OH)2
p-MeOC-PhB(OH)2
m-MeO-PhB(OH)2
m-MeO-PhB(OH)2
m-MeO-PhB(OH)2
3,4,5-F-PhB(OH)2
3,4,5-F-PhB(OH)2
p-Me-PhB(OH)2
PhB(OH)2
p-OHC-PhBr
p-MeOC-PhBr
p-NC-PhBr
m-O2N-PhI
p-MeOC-PhBr
p-OHC-PhBr
p-MeO-PhI
p-Me-PhI
p-MeO-PhI
m-O2N-PhI
p-MeOC-PhBr
p-MeOC-PhBr
p-OHC-PhBr
p-NC-PhCl
p-NC-PhCl
Reactions conditions: aryl halide (1 mmol), arylboronic acid (1 mmol), NaHCO3 (2 mmol), Fe3O4@
SiO2@N-Amidinoglycine@Pd0 nanocatalyst (0.17 mmol%), H2O (3.0 mL), 90 °C, 10 min
aIsolated yields
giving better yields in shorter times with lower loading of
palladium catalyst under mild reaction conditions.
Table 3 Reusability experiment
of Fe3O4@SiO2@N-
Run Time (min) Yield (%)
amidinoglycine@Pd0 catalyst
1
2
3
4
5
10
10
15
98
95
90
90
Experimental
All reagents and solvents were purchased from commercial
suppliers (Acros, Merck and Aldrich) and used without any
additional purifcation. FT IR spectra were recorded using a
Bruker Tensor 27 spectrometer within the 400–4000 cm−1
1
range using KBr disks. H and 13C NMR spectra were
recorded with a Bruker Avance 400 and 100 MHz spec-
trometer using CDCl3 as solvent and TMS as standard. XRD
patterns were recorded with a PW 3710 X-ray difractometer
(Philips) at room temperature using monochromatic Cu Kα
radiation with a wavelength of λ = 0.15418 nm. The peak
positions and intensities were obtained between 5° and 80°
with a rate of 0.04° s−1. The morphology of the catalyst
was observed with FE-SEM (HITACHI (S4160) Japan) and
TEM (PHILIPS CM30) instruments. EDX patterns were
recorded using a TESCAN, VEGA 3 LMU instrument.
Magnetic susceptibility measurements were taken using a
vibrating sample magnetometer (VSM, MDK Co. Ltd, Iran)
with a magnetic feld range of + 10,000 to − 10,000 Oe at
room temperature. Elemental analyses were obtained on an
phenylboronic acid under the optimized conditions.
After the completion of the first reaction, the catalyst
was recovered magnetically, washed with ethanol, and
dried for the next run. Then, a further reaction was car-
ried out with fresh reactants and solvent under the same
conditions. Based on the results, the catalyst retained its
activity quite well over four successive catalytic runs,
albeit with slightly longer reaction times.
We have compared the efficiency of Fe3O4@SiO2@N-
amidinoglycine@Pd0 with previously reported catalysts
for the synthesis of biphenyls to show the merits of the
present work (Table 4). The results obtained with this cat-
alytic system are superior to those for the other systems,
1 3