Palladium(II) anchored on polydopamine coated-magnetic nanoparticles (Fe3O4&at;PDA&at;Pd(II)): A heterogeneous and core–shell nanocatalyst in Buchwald–Hartwig C–N cross coupling reactions

Article abstract of DOI:10.1016/j.poly.2018.09.019

An efficient method was proposed to synthesize polydopamine (PDA)-coated Fe₃O₄ nanoparticles (Fe₃O₄&at;PDA). For the first time, effective deposition of Pd complex is explained by using Fe₃O₄&at;PDA as a core–shell magnetic coordinator and stabilizer agent. In this method, palladium ions were adsorbed on Fe₃O₄&at;PDA surfaces through immersion of the Fe₃O₄&at;PDA into a palladium plating bath. The structure, morphology and physicochemical features of the prepared particles were studied using various analytical methods including high resolution transmission electron microscopy (HR-TEM), field emission scanning electron microscope (FESEM), energy-dispersive X-ray spectroscopy (EDS), vibrating sample magnetometer (VSM), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma (ICP), thermogravimetric analysis (TGA) and FT-IR spectroscopy. Core–shell Fe₃O₄&at;PDA/Pd(II) nanoparticles showed excellent catalytic performance as a reusable nanocatalyst in Pd-catalyzed Buchwald–Hartwig C–N cross coupling reaction. A variety of aryl amines were prepared through reaction of aryl halides (chloride, bromide and iodide) and amines in high yields. The catalyst can be recycled and reapplied up to six cycles with no considerable change in its catalytic activity.

Full text of DOI:10.1016/j.poly.2018.09.019

Accepted Manuscript
Palladium (II) anchored on polydopamine coated-magnetic nanoparticles
(Fe3O4@PDA@Pd(II)): A heterogeneous and core-shell nanocatalyst in Buch-
wald–Hartwig C–N cross coupling reactions
Hojat Veisi, Pegah Sarachegol, Saba Hemmati
PII:
S0277-5387(18)30571-0
https://doi.org/10.1016/j.poly.2018.09.019
POLY 13418
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Revised Date:
Accepted Date:
13 May 2018
6 September 2018
7 September 2018
Please cite this article as: H. Veisi, P. Sarachegol, S. Hemmati, Palladium (II) anchored on polydopamine coated-
magnetic nanoparticles (Fe3O4@PDA@Pd(II)): A heterogeneous and core-shell nanocatalyst in Buchwald–Hartwig
C–N cross coupling reactions, Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.2018.09.019
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Palladium (II) anchored on polydopamine coated-magnetic nanoparticles
(Fe3O4@PDA@Pd(II)): A heterogeneous and core-shell nanocatalyst in
Buchwald–Hartwig C–N cross coupling reactions
Hojat Veisi,* Pegah Sarachegol, Saba Hemmati
Department of Chemistry, Payame Noor University, Tehran, Iran
Email: hojatveisi@yahoo.com
Abstract. An efficient method was proposed to synthesize polydopamine (PDA)-coated Fe3O4
nanoparticles (Fe3O4@PDA). For the first time, effective deposition of Pd complex is explained
by using Fe3O4@PDA as a core-shell magnetic coordinator and stabilizer agent. In this method,
palladium ions were adsorbed on Fe3O4@PDA surfaces through immersion of the Fe3O4@PDA
into a palladium plating bath. The structure, morphology and physicochemical features of the
prepared particles were studied using various analytical methods including high resolution
transmission electron microscopy (HR-TEM), field emission scanning electron microscope
(FESEM), energy-dispersive X-ray spectroscopy (EDS), vibrating sample magnetometer (VSM),
X-ray photoelectron spectroscopy (XPS), inductively coupled plasma (ICP), thermogravimetric
analysis (TGA) and FT-IR spectroscopy. Core-shell Fe3O4@PDA/Pd(II) nanoparticles showed
excellent catalytic performance as a reusable nanocatalyst in Pd-catalyzed Buchwald–Hartwig C–
N cross coupling reaction. A variety of aryl amines were prepared through reaction of aryl halides
(chloride, bromide and iodide) and amines in high yields. The catalyst can be recycled and
reapplied up to six cycles with no considerable change in its catalytic activity.
Keywords: Buchwald-Hartwig reaction, Polydopamine, Magnetic, Palladium, Amination
1. Introduction
Aryl- and heteroarylamines derivatives have a key role as intermediates for agrochemicals,
medicines, natural products [1], dendrimers [2], polymers [3], dyes, pigments [4] as well as
materials with electronic and nonlinear optical features [5]. Buchwald and Hartwig individually
proposed the procedures for the palladium-mediated amination of aryl halides [6]. Numerous
conditions have been improved since then for the Buchwald–Hartwig amination reaction and this
reaction has now arisen as an extremely useful and synthetically vital technique for synthesizing
aryl- and heteroarylamines. Comparing with the other approaches of carbon–nitrogen bond
genration [7] like nucleophilic aromatic substitution, ullmann coupling and nitration followed by
reduction, the Buchwald–Hartwig reaction provides a lower reaction temperature, usually in the
range of 80-100 °C, a variety of substrate scope, a greater selectivity regarding the amines, better
functional group compatibility [8] and the reaction does not contain extremely reactive reactants
[9], which may lead to safety problems.
Many attempts have been done for finding suitable catalyst systems for accomplishing Pd-
catalyzed N-arylation reactions under heterogeneous conditions due to the importance of recovery
of the expensive Pd metal and the interest of industries to produce product without Pd species
contamination. In this case, diverse kinds of heterogeneous Pd catalyst systems have been
proposed for Buchwald–Hartwig C–N cross coupling reaction [10-16]. A number of the proposed
palladium–catalyst systems for N-arylation reaction suffer from key drawbacks including tedious
1

and long work-up procedure, costly process and complexity synthesis of these catalyst systems.
Recyclability of the catalyst and more significantly leaching of Pd as a costly metal, are other
important challenges. Progress of excellent efficiency palladium-catalyzed systems and
application of sustainable and ecologically benign reaction conditions for Buchwald–Hartwig C–
N cross coupling reaction is extremely considered in academic and industrial studies [17-19].
In recent years, magnetite Fe3O4 nanoparticles (NPs) have been arisen extensively in different
field, including biotechnology, medical applications, environmental remediation [20]. The use of
functionalized magnetic nanoparticles (MNPs) has been widely increased for organic synthesis
[21], which have been considered as excellent and ideal supports with important industrial
potential for immobilization since the MNPs supported catalysts are readily accessible, chemically
steady and can be synthesized through simple approaches [22]. Additionally, these MNPs can be
simply recovered from the reaction mixture using a suitable permanent magnet, which provides
easy recovery of the catalysts with no filtration.
The MNPs can efficiently be functionalized based on the formation of relatively stable linker
between hydroxyl groups on the NPs surface and suitable anchoring agents such as phosphonic
acid and dopamine derivatives [23a]. Consequently, PDA as a humic-like substance having a lot
of phenolic hydroxyl and amine functional groups on the framework can be applied to modify
MNP. Additionally, the possibility of complexation of polyphenols or catechol with polyvalent
cations in simple aqueous solutions can progress the surface features and adsorption capacity of
the PDA@Fe3O4 NPs and metal ions stabilization [23b]. Consequently, the nano-adsorbent
(Fe3O4@PDA) can be used as a stabilizer agent for coordination of palladium ions to prepare a
new magnetically separable and recoverable nanocatalyst (Scheme 1). Therefore, in pollution of
our program for developing heterogeneous palladium nanocatalysts and its use in Pd-catalyzed
organic transformations [24], we carried out preparation of palladium(II) immobilized on Fe3O4
NPs coated with polydopamine (Fe3O4@PDA/Pd(II)) as a new core-shell magnetic nanocatalyst
for Buchwald–Hartwig C–N cross coupling reaction catalyzed with Pd (Scheme 1).
2. Experimental Section
2.1. Materials and apparatus
All reagents and chemicals including pure Tris (hydroxy methyl) amino methane
hydrochloride (H2NC (CH2OH)3.HCl or Tris-HCl), dopamine hydrochloride,
ferricchloridehexahydrate (FeCl3.6H2O), ferrouschloridetetrahydrate (FeCl2.4H2O), sodium
hydroxide, deionized water, acetonitrile, palladium chloride, NaCl, ethanol and sulfides, were
prepared from Aldrich and Merck with analytical grade and were utilized with no further
purification. Tris-HCl (pH 8.5, 0.1 M) buffer solution was prepared through dissolving
suitable amount of Tris-HCl in deionized water and pH was set by addition of suitable
amounts of 0.1 M NaOH solution. A Perkin Elmer 65 spectrometer was used to record the
fourier transform infrared (FT-IR) spectra in the range of 400-4000 cm-1. Transmission
electron microscopy (TEM) analysis was carried out on a Phillips CM10 microscope at an
accelerating voltage of 200 kV. A BHV-55 vibrating sample magnetometer (VSM) was
used to measure magnetization. Thermal constancy of the catalyst was studied using
2

Thermogravimetric analysis (TGA, TSTA Type 503) at a heating rate of 10 °C/min under
nitrogen atmosphere.
2.1. Preparation of Fe3O4, Fe3O4@PDA and Fe3O4@PDA/Pd(II) magnetic nanoparticles
The magnetic Fe3O4 NPs were synthesized through the chemical coprecipitation technique [24a],
and the detailed process is explained in following. 2 g of FeCl2·4H2O and 5.2 g of FeCl3·6H2O
were dissolved into 25 mL deoxygenated water followed by addition of 0.85 mL of concentrated
hydrochloric acid. The obtained mixture was poured slowly into 250 mL of 1.5 M NaOH solution
under vigorous stirring and N2 protection at 353 K. The obtained MNPs were isolated from solution
using a strong magnet and washed with 200 mL deionized water three times. Lastly, the Fe3O4
NPs were resulted after drying products at 40 °C.
Fe3O4 NPs (1 g) were spread in 500 mL Tris buffer (10 mM, pH 8.5) following by addition of 1 g
of dopamine, and the resulting solution was mechanically stirred for 24 hours at ambient
temperature. At the end of the reaction, the Fe3O4@PDA NPs were separated using a magnet and
rinsed using deionized water and anhydrous ethanol and dried at 40 °C to result Fe3O4@PDA.
In continue, 500 mg of prepared Fe3O4@PDA was sonically spread in CH3CN (100 mL) for 20
min. Nest, 0.035 g of PdCl2 (0.2 mmol) dissolved in 20 mL acetonitrile was poured into the resulted
solution. The obtained mixture was agitated for 24 hours at ambient temperature. In the next step,
Fe3O4@PDA/Pd(II) was magnetically isolated and rinsed with acetonitrile, distilled water and
acetone, respectively to remove possible unattached substrates. Moreover, the palladium content
of the catalyst was obtained 0.33±0.001 mmol.g-1 using atomic absorption spectroscopy.
2.3. General procedure for Buchwald–Hartwig C–N cross coupling reaction using
Fe3O4@PDA/Pd(II) nanocatalyst
A solution containing aryl halide (1.0 mmol), amine (1.2 mmol), Cs2CO3 (2.0 mmol),
Fe3O4@PDA/Pd(II) (0.015 g, 0.5 mol%), and DMF (3 ml) was agitated for 10 hours at 100 °C
under nitrogen atmosphere. TLC was used for following the reaction. At the end of the reaction,
10 mL of water and 10 mL of ethyl acetate were poured into the reaction solution. Firstly, the
catalyst is separated using an external magnetic field and dried over anhydrous Na2SO4 after
extracting organic solution. The crude product was achieved following the removal of organic
solvent. The chromatography method was applied for more purification. All the products are
known compounds and were compared with authentic samples.
3. Results and discussion
The current research has been aimed to continue our previous study [24] and provide a new hybrid
magnetic nanocomposite with the potency of both coordinator and stabilizer agent for
immobilizing metal ions, e.g. palladium. Firstly, Fe3O4 NPs were synthesized according to our
earlier report [24a]. Next, Fe3O4 NPs were functionalized by polydopamine (PDA), in situ, via
polymerizing dopamine in tris-buffer solution (pH 8.5) under fixed stirring. In the next phase, the
synthesized Fe3O4@PDA nanostructures were used for absorbing palladium ions on their PDA
layers. The coordinating Fe3O4@PDA via PdCl2 leads to the relating Pd(II) complex supported on
polydopamine coated-magnetic Fe3O4 NPs (Fe3O4@PDA/Pd(II)). Finally, an external magnetic
field was utilized for collecting the mixture and the isolated mixture was dried in vacuum
condition. The preparation of the nanocatalyst was showed in Scheme 1. The Palladium (II) level
3

of the nanocomposite was 0.33±0.001 mmol.g-1 as expected from the result of atomic absorption
spectroscopy. FTIR, XPS, FESEM, EDX, HR-TEM, TGA, ICP and VSM were used to
characterize the catalyst.
HO
HO
NH2
Tris, pH = 8.5
Fe3O4
Fe3O4@PDA
HO
H2N
PdCl2
R1
N
H
HO
HO
NH
Pd(II)
Pd(II)
X
N
N
H
HO
Pd(II)
HO
OH HO
OH
Pd(II)
R2
R3
R1
N
H
Pd(II)
Pd(II)
Pd(II)
R3
R2
Fe3O4@PDA/Pd(II)
Scheme 1. Synthesis of Fe3O4@PDA/Pd(II) and its application for Buchwald-Hartwig C–N
cross coupling reaction.
FT-IR is used for studying the surface composition of the prepared Fe3O4, Fe3O4@PDA; and
Fe3O4@PDA/Pd(II) NPs. In the spectrum of simple Fe3O4 (Fig. 1), the strong peak observed at
582 and 632 cm-1 are corresponded to the Fe-O vibration and the peaks appeared at 1625 and 3385
cm-1 are related to the surface-adsorbed water and hydroxyl groups [24a]. The peaks appeared at
3395, 2930, 1520, 1415 cm-1 can be ascribed to O-H stretching, C-H stretching, C=C stretching
and C-N stretching relating with the existence of -OH of phenol and -C=C-NH- in the Fe3O4@PDA
[24a]. However, the peaks at 1632 and 1520 cm-1 can be assigned to the benzene ring skeleton
vibration [24a]. These outcomes offer that PDA polymer is effectively immobilized on the Fe3O4
NPs surface. The signal positioned at 1632 cm-1 of curve 1c for Pd-ligand coordination would
presumably result in shifting of this peak to a lower frequency (1632 to 1628 cm-1). This shift is
evident by comparing curves c (Fig. 1c) and b (Fig. 1b). Meantime, comparison of these curves
verifies successful attachment of PDA layers and coordination of Pd2+ ions within the hybrid
nanocatalyst. Also, it was noticed that intensity of the peaks at 3395 cm-1 has weakened through
bonding interactions between the functional groups of the composites and palladium ions.
4

Fig. 1. FTIR spectra of a) Fe3O4; b) Fe3O4@PDA; and c) Fe3O4@PDA/Pd(II).
Size and morphology of Fe3O4@PDA/Pd(II) were studied using the scanning electron microscopy.
The FESEM image of a) Fe3O4; b) Fe3O4@PDA and c) Fe3O4@PDA/Pd(II) presented that the
synthesized catalyst is nanoscale quasi-spherical with a mean diameter of 70-90±5 nm (Fig. 2).
Furthermore, a continuous polymer layer can be observed on the Fe3O4@PDA and
Fe3O4@PDA/Pd(II) catalyst surface compared with Fe3O4. Moreover, the EDX detector
coupled to the FESEM was applied for proving the existence of PDA shell and palladium
species on the Fe3O4 NPs surface that the outcomes proved the presence of Fe, C, and O in Fig.
3. The presence of the Pd signals in EDX of the nanocatalyst (Fig. 3) indicated the
successful coordination of palladium ions on the Fe3O4@PDA surface
.
5

Fig. 2. FESEM images of a) Fe3O4; b) Fe3O4@PDA; and c) Fe3O4@PDA/Pd(II).
Fig. 3. EDX data for Fe3O4@PDA/Pd(II).
HRTEM images corresponding to Fe3O4@PDA/Pd(II) were shown in Fig. 4 and Fig. 5. It
obviously displays well dispersion of the synthesized catalyst and numerous larger structures,
which can be ascribed to the aggregation/coalescence of each Nps. It can be found that grey
PDA/Pd(II) shells coated dark Fe3O4 cores, approving the successful coating process of
PDA/Pd(II) materials. These images are completely in accordance with FESEM outcomes in terms
of dimension and morphology. The effective coordination of palladium ions on PDA layers could
be intensely confirmed with XPS analysis.
6

Fig. 4. HRTEM images of Fe3O4@PDA/Pd(II).
7

Fig. 5. HRTEM image of Fe3O4@PDA/Pd(II) with different magnification.
X-ray photoelectron spectroscopy (XPS) is a powerful instrument to characterize the electronic
properties of the species formed on the surface, like the electron environment, oxidation state, and
the binding energy corresponding to the core electron of the metal. Fig. 6 shows XPS elemental
survey scans corresponding to the surface of the Fe3O4@PDA/Pd(II) catalyst. The peaks related to
oxygen, carbon, nitrogen, palladium and iron are clearly found in this spectrum. Moreover, XPS
analysis was carried out for determination of the Pd oxidation state. In Fig. 7, the Pd binding energy
of Fe3O4@PDA/Pd(II) displays two peaks centered at 337.81 and 343.08 eV relating with Pd 3d3/2
and Pd 3d5/2, respectively. According to the earlier research [25], These peaks are related with
Pd(II) species, which have coordinated to PDA groups. The local chemical/physical environment
around N 1s often effects on position of its peak whose area presented two type of nitrogen, C-
NH2 (402.0 ev) and N-Pd(II) (399.8 ev), that proved the successful bending of palladium ions to
the surface [26]. Additionally, high resolution XPS outcomes proved existence of carbon through
revealing five peaks at 284.48, 285.91, 287.48, 288.64 and 290.73 eV, in C 1s area. These peaks
corresponding to C-C, C-N, C-O, C-O-C or O-C-O and N-C-C forms of carbon, respectively [27].
The Pd level in the nanocatalyst was 2.09 wt%, which was obtained through XPS analysis.
8

Fig. 6. XPS spectrum related to the elemental survey scan of Fe3O4@PDA/Pd(II).
Fig. 7. XPS spectrum of the Fe3O4@PDA/Pd(II) in Pd 3d region.
Figs. 8a to 8c show the magnetic curves of the Fe3O4, Fe3O4@PDA and Fe3O4@PDA/Pd(II) NPs
as a function of the applied field at room temperature, respectively. Based on these figures,
magnetization saturation equals to 57.3 emu/g for Fe3O4, 37.3 emu/g for Fe3O4@PDA, and 28.5
emu/g for Fe3O4@PDA/Pd(II). It means that coating PDA and loading PdCl2 reduces the
magnetization level of the Fe3O4 NPs. In addition, even though saturation magnetization of
Fe3O4@PDA/Pd(II) NPs is lower comparing with Fe3O4 particles, they can be efficiently isolated
from the aqueous solution using an appropriate magnet (see inset panel of Fig. 8).
9

Fig. 8. Room-temperature magnetization curves of Fe3O4 (a), Fe3O4@PDA (b), and
Fe3O4@PDA/Pd(II) (c).
The TGA analysis was used to study the steadiness of the synthesized Fe3O4@PDA/Pd(II)
nanocatalyst and also percent of organic functional groups on the MNP surface. The TGA curve
corresponding to catalyst (Fig. 9) shows an initial weight loss of 8.3% up to 185 °C which is
because of physically adsorbed solvent and surface hydroxyl groups on the support. The catalyst
thermal degradation happened after 200 °C that showed the high thermo-stability of catalyst. The
second and third weight loss of 51% in the range from 200 to 600 °C was because of the organic
residues degradation.
Fig. 9. TGA curve of Fe3O4@PDA/Pd(II).
10

Following structural characterization of the synthesized metal-polymer composite
(Fe3O4@PDA/Pd(II)), and because of our ongoing interest on the biosynthesis of metal NPs and
heterogeneous catalysts [44], its catalytic activity was systematically studied using Buchwald‐
Hartwig reaction. In order to determine optimum conditions, the reaction between bromobenzene
with morpholine in the existence of Fe3O4@PDA/Pd(II) catalyst was chosen as a model reaction
and the impact of different reaction factors including solvent, base, temperature and the level of
catalyst were investigated in detail. Table 1 shows the outcomes of optimization study. As it could
be expected, no case product could be detected without applying the catalyst (Table 1, entry 16).
Though, introducing the catalyst to the reaction solution quickly improved preparation of the
product in high yields. The reactions were performed in DMF as the optimal solvent. Amongst the
studied bases, Cs2CO3 was the most effective base. Additionally, the influence of catalyst loading
was showed with diverse amounts of the catalyst in the range of 0.2 mol% to 0.5 mol% (Table 1,
entries 7,14 and 15). based on the results, the best yield can be achieved with 0.015 g of the catalyst
which is 0.5 mol% (Table 1, entry 7). Moreover, the optimal reaction temperature was 100 °C
(entry 7 versus entries 10-12). Lower yield of the desired product was obtained at 25 °C reaction
temperature, while enhancing the temperature to 120 °C did not increase product yield, explicitly
(Table 2, entry 13).
Table 1. The optimization of reaction parameters for the Buchwald‐Hartwig amination reaction of
bromobenzene with Morpholine using Fe3O4@PDA/Pd(II).a
Fe3O4@PDA/Pd(II)
Br HN
O
+
N
O
Entry Pd (mol%)
Solvent
DMF
Toluene
EtOH
H2O
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
Base
T (°C) Yield (%)b
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.5
0.4
0.2
0.0
K2CO3
K2CO3
K2CO3
K2CO3
NaOAc
Et3N
100
100
80
75
35
55
30
60
65
96
70
10
75
45
35
96
85
45
0
100
100
100
100
100
100
80
Cs2CO3
NaOH
No base
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
60
25
120
100
100
100
aReaction conditions: Bromobenzene (1.0 mmol), morpholine (1.2 mmol), catalyst, base (2 mmol) and solvent (3 mL)
for 10 h.
bIsolated yield.
11

‐
Fe3O4@PDA/Pd(II)
Mostly, the coupling reaction among morpholine and
aryl iodides/bromides was high yielding without pollutant (Table 2, entries 1, 2, 4, 5, 7-10).
Though, lower yield was attained when to chlorides the leaving group was changed (Table 2,
entries 3, 6). These outcomes ascertained the reactivity arrangement: R-I>R-Br>R-Cl. The
outcomes also show that the catalyst provides a high product yield for both electron‐withdrawing
substituents (-CN) and electron‐donating substituents (-Me, -OMe). In order to
Table 2. N-arylation of aryl halides using Buchwald‐Hartwig reaction in the presence of
Fe3O4@PDA/Pd(II) catalyst.a
Entry
1
2
3
4
5
6
7
8
Aryl halide
C6H5I
C6H5Br
Amine
Yield (%)b Refc
Morpholine
Morpholine
Morpholine
Morpholine
Morpholine
Morpholine
Morpholine
Morpholine
Morpholine
Morpholine
Piperidine
Piperidine
Piperidine
Indole
Indole
Indole
Imidazole
Imidazole
Aniline
Aniline
Pyrrolidine
Pyrrolidine
98
96
75
96
95
65
96
90
92
90
96
90
65
95
92
70
90
85
80
70
95
88
[30]
[30]
[30]
[29]
[29]
[29]
[30]
[30]
[34]
[34]
[31]
[31]
[31]
[29]
[29]
[29]
[35]
[35]
[35]
[35]
[30[
[30]
C6H5Cl
p-NCC6H4I
p-NCC6H4Br
p-NCC6H4Cl
p-CH3C6H4I
p-CH3C6H4Br
p-CH3OC6H4I
p-CH3OC6H4Br
C6H5I
C6H5Br
C6H5Cl
C6H5I
C6H5Br
C6H5Cl
C6H5I
C6H5Br
C6H5I
C6H5Br
C6H5I
C6H5Br
9
10
11
12
13
14
15
16
17
18
19
20
21
22
aReaction conditions: Arylhalide (1.0 mmol), amine (1.2 mmol), Fe3O4@PDA/Pd(II) (0.5 mol%), and Cs2CO3 (2.0
mmol) were stirred in DMF (3.0 mL) at 100 °C for 10 h.
b Isolated yield.
cEarlier reference of the corresponding product.
Recovery of heterogeneous new catalysts is very vital in terms of economic and sustainable
chemistry standpoints. Consequently, we studied the recovery of the nanocatalyst
(Fe3O4@PDA/Pd(II)) for the reaction between bromobenzene and morpholine under the optimal
conditions. For this purpose, after completion of the reaction, catalyst was separated by an external
12

magnet and reused in another batch of the reaction. Results of current research showed 6%
decrease in reaction yields following recycling catalyst for 6 consecutive runs. This outcome
showed the stability and reusability of the synthesized nanocatalyst.
Fig. 10. Recycling of catalyst for the reaction of bromobenzene with Morpholine.
ICP analysis was used to determine the leaching of Pd ions into the reaction solution following
five cycles and it was obtained 1.13 %, which shows the constancy of the catalyst over the reaction.
In order to study the catalyst heterogeneity, we performed a hot filtration experiment for the
Buchwald reaction between bromobenzene with Morpholine by Fe3O4@PDA/Pd(II) under optimal
conditions. The reaction was permitted for processing for 5 hours (yield: 65%), and next the
catalyst was isolated. Though, there is no enhancing in the desired product yield if the reaction
was continued for further 5 h following the magnetically separation of the catalyst, confirming the
heterogeneous nature of the catalyst.
In order to prove the application of Fe3O4@PDA/Pd(II) catalyst in Buchwald‐Hartwig amination
reaction, this catalyst was compared with several other proposed palladium catalysts (Table 3). As
presented in Table 3, our catalyst system is comparable with the proposed homogeneous and
heterogeneous catalysts in terms of efficiency.
Table 3. Comparison efficiency of Fe3O4@PDA/Pd(II) with some reported catalysts for the
Buchwald‐Hartwig amination reaction of bromobenzene with Morpholine.
Entry Reaction conditions
Time Yield TOF Ref.
(h)
12
12
12
%
(h-1)a
4.3
1
2
3
4
5
6
7
8
PFG-Pd (1.7 mol%), NaOtBu, 110 °C
PNP-SSS (0.6 mol%), K2CO3, DMF, 120 °C
GO‐Chit‐Pd(0.5 mol%), DMF, t‐BuOK, 100 ºC
Pd(II) thiosemicarbazone comp., 2-BuOH, K2CO3, 100 ºC 24
88
89
90
89.2
80
94
92
96
[28]
[29]
[30]
12.4
15
18.5
3.3
3.2
3.8
19.2
[31]
[32]
[33]
[34]
[Pd(pp3S4)(dba)] (1 mol %), Isopropanol, Cs2CO3, 80 ºC
Pd-PFMN (1.2 mol% Pd), K2CO3, 120 °C
24
24
24
10
RD Pd NCs (1 mo%), 1,4-Dioxane, KOtBu, 100 °C
Fe3O4@PDA/Pd(II) (0.5 mol%), DMF, Cs2CO3, 100 °C
This work
13

aTOF, turnover frequencies (TOF = (Yield/Time)/Amount of catalyst (mol).
4. Conclusion
In conclusion, the novel magnetic interphase nanocatalyst of palladium (II) anchored on magnetic
Fe3O4 NPs functionalized using polydopamine Fe3O4@PDA/Pd(II) was effectively prepared and
characterized using several analytical methods. It was also indicated that Fe3O4@PDA/Pd(II) can
be used as an environmentally friendly, effective and recoverable heterogeneous magnetic
nanocatalyst for Pd-catalyzed Buchwald–Hartwig C–N cross coupling reaction. A variety of
arylamines were prepared under optimal conditions. The catalyst can be recovered and reused up
to six cycles with no considerable loss in its catalytic performance.
Acknowledgments. We are thankful to Payame Noor University (PNU) for partial support of this
work.
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Graphical Abstract
Palladium (II) anchored on polydopamine coated-magnetic nanoparticles
(Fe3O4@PDA@Pd(II)): A heterogeneous and core-shell nanocatalyst in Buchwald–
Hartwig C-N cross coupling reactions
Hojat Veisi,*Pegah Sarachegol, Saba Hemmati
HO
HO
NH2
HO
H2N
NH
N
H
HO
HO
Tris, pH = 8.5
N
H
HO
HO
OHHO
OH
Fe3O4
Fe3O4@PDA
PdCl2
R1
Pd(II)
Pd(II)
X
N
Pd(II)
Pd(II)
R2
R3
R1
N
Pd(II)
Pd(II)
H
Pd(II)
R3
R2
Fe3O4@PDA/Pd(II)
16