Hollow nanoshell-sphere Fe@Fe/Pd reactors: a magnetically recoverable catalyst for the Csp–S cross-coupling reactions in water

Article abstract of DOI:10.1007/s13738-018-1349-4

The hollow Pd–PVP–Fe nanosphere and Fe–PVP nanoparticle catalysts were synthesized by thermal method. Mixing of two metallic nanocatalysts was applied in the C_sp–S cross-coupling reactions between diphenyl disulfide and phenylacetylene under mi

Full text of DOI:10.1007/s13738-018-1349-4

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-018-1349-4
ORIGINAL PAPER
Hollow nanoshell‑sphere Fe@Fe/Pd reactors: a magnetically
recoverable catalyst for the Csp–S cross‑coupling reactions in water
Parvaneh Ghaderi‑Shekhi Abadi2 · Ezzat Rafee1,2 · Mohammad Joshaghani1,2
Received: 25 October 2017 / Accepted: 19 March 2018
© Iranian Chemical Society 2018
Abstract
The hollow Pd–PVP–Fe nanosphere and Fe–PVP nanoparticle catalysts were synthesized by thermal method. Mixing of two
metallic nanocatalysts was applied in the Csp–S cross-coupling reactions between diphenyl disulfde and phenylacetylene
under mild conditions in water. Results show that bi-catalytic system has higher catalytic efciencies than their monocatalytic
systems due to synergy between two catalysts. Order of adding two metallic catalysts were adjusted into the coupling reac-
tion medium. Therefore, various bi-catalytic systems were obtained and characterized by XRD, SEM, EBSD, EDX, UV–Vis
spectra, and particle size analyzer. Under special order of adding, the obtained hollow nanoshell-sphere Fe@Fe/Pd reactor
showed higher catalytic activity in the coupling reaction compared to other bi-catalytic systems. The Csp–S coupling products
obtained of various diaryl disulfdes and phenylacetylene at presence Fe@Fe/Pd (only 7.3×10−5 mmol Pd) catalyst with
moderate to high yields in water solvent and mild reaction conditions. After the reaction, the catalyst/product(s) separation
could be easily achieved with an external magnet and more than 95% of catalyst could be recovered. The recovered catalyst
was characterized by XRD, SEM, EBSD, EDX, and UV–Vis spectra. The Fe@Fe/Pd was reused at least six repeating cycles
without any loss of its high catalytic activity. Tuning morphology and chemical composition of bi-catalytic system are key
mainstays of high activity of Fe@Fe/Pd in repeating cycles of cross-coupling reactions.
Keywords Palladium/iron · Heterometallic nanocatalyst · Hollow nanoshell-sphere reactors · Magnetically recoverable
catalyst · Csp–S cross-coupling reactions
Introduction
particular, the use of Ni, Fe, and Co in combination with
noble metals can decrease the content of expensive noble
metals, thereby afording low-cost catalysts. Also nanomet-
ric dispersed noble metal embedded in other metal substrates
exhibit enhanced catalytic properties due to the synergistic
efects of the metallic species [5–8].
Recovery and reuse of the precious metallic catalysts (Au,
Ag, Pd, Pt, Ru, and Rh) have become signifcant from eco-
nomical concern and ecological perspective view due to
growing environmental cognizance [1–3]. The strategy of
magnetic separation is typically more efective than decan-
tation, fltration or centrifugation as it prevents loss of the
metallic catalyst [4]. The magnetic compounds, due to non-
toxicological feature, ease and various preparations process,
commercially accessible, and not time-consuming separation
of them from reaction mixtures, have attracted wide spread
attention as catalyst and co-catalyst in the past decades. In
Nanostructured metals assembled from several small
nanometals have emerged as one of key mainstays of mod-
ern nanotechnology [9]. Many efcient techniques were
developed for detecting quantify cluster multimetallic com-
position and structure, mainly including coupled plasmon
resonances [10], optical microscopy [11], and confocal
microscopy [12], but none are fully satisfactory for nano-
material with nanoscale size. Electronic microscopy (EM)
[13] and dynamic light scattering (DLS) [14] can overcome
many of these limitations and provide rapid rate of aggrega-
tion of cluster composition at nanoscale size. Phase content
and distribution and grain boundary character distribution
of material were characterized using scanning electron
* Mohammad Joshaghani
mjoshaghani@razi.ac.ir; joshaghanim@yahoo.com
1
Department of Inorganic Chemistry, Faculty of Chemistry,
Razi University, Kermanshah 67149, Iran
2
Institute of Nanoscience and Nanotechnology, Razi
University, Kermanshah 67149, Iran
Vol.:(0123456789)
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Journal of the Iranian Chemical Society
microscopy-electron backscatter difraction/energy-disper-
sive X-ray (SEM-EBSD/EDX) [15, 16].
dimethyl sulfoxide (DMSO), ethyl acetate (EtOAc),
n-heptane, diethyl ether, absolute ethanol, K2CO3, NaOAc,
NaOtBu, N(Me)3, and N(Butyl)3 were purchased from
Merck, Germany, with analytical grades. These were used
without further purifcation.
Alkynyl sulfdes are interesting class of compounds hav-
ing a variety of uses in chemical purposes [17]. Although
interesting results have already been reported for the Csp–S
coupling reactions, the coupling products were usually
achieved under drastic reaction conditions (for example,
using toxic substrate, catalyst, solvent, or high-temperature
conditions during coupling reaction) [18–21]. Another prob-
lem is the formation of bis- and tris-chalcogen alkenes in
this coupling reaction (Scheme 1) [22–25].
Preparation of the metallic nanocatalysts
The studied catalysts were synthesized by the thermal
method [31]. The preparation conditions for catalysts and
their abbreviations are summarized in Table 1.
In this article, we demonstrate in situ synthesis of metallic
patterns of Pd0 and Fe0 nanoparticles under various order
of adding them into the Csp–S cross-coupling reaction for
surface engineering of heterometallic nanocatalysts. Thus,
diference assemble-controlled synthesis in various ordered
patterns of metallic nanoparticles were achieved. We used
XRD and SEM-EBSD/EDX analyses to determine the phase
and sort form of these metallic sites in the polymer matrix
in the obtained bimetallic catalytic systems. Meanwhile, the
hollow nanoshell-sphere Fe@Fe/Pd reactors, obtained from
special order of adding, exhibited excellent catalytic perfor-
mance with extraordinary high order and stability structure
for at least six repeating cycles of the Csp–S cross-coupling
reactions. The Csp–S coupling products obtained of various
diaryl disulfdes with phenylacetylene. According to green
chemistry, we used of water as a low-cost and green sol-
vent with greater chemoslectivity in the coupling reaction
[26–30].
Methods
Atomic absorption spectrometer (AAS)
The Pd2+ and Fe2+ contents in metallic catalysts were
determined by PerkinElmer 1100-AAS flame atomic
absorption spectrometer, England, with a deuterium back-
ground corrector and Palladium and Iron hollow cathode
lamps, respectively.
Table 1 Experimental parameters for the preparation various metallic
nanocatalysts and metals weight percent of thema
No. Symbol
[Fe] (mmol) [Pd] (mmol) Fe (wt%)b Pd (wt%)b
1
2
3
4
Pd–PVP–Fe 0.075
0.0063
0.0063
–
59.0
–
60.7
53.6
7.8
8.1
–
Pd–PVP
Fe–PVP
Pd–Fe
–
Experimental
0.075
0.075
0.0063
6.3
Materials
aReaction conditions: PVP 0.0029 mmol, distilled water: DMF
(1:5 V/V), T=110 °C, Time=16 h
PdCl2, FeCl2·4H2O, PVP (M.W. = 40,000), phenylacety-
lene, diaryl disulfdes, N, N-dimethylformamide (DMF),
bAAS data
Scheme 1 Mono-, Bis-, and
Tris-chalcogenation products
for the cross-coupling reaction
of sulfde source and phenyla-
cetylene
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Dynamic light scattering (DLS)
spectroscopy. The spectral data of known compounds are in
agreement with that previously reported.
The size distribution of the metallic catalysts were
obtained using a Malvern, UK laser analyzer, HPPS5001
model equipped with a green laser at 532 nm at distilled
water.
Reusability of the bi‑catalytic system of the Csp–S
cross‑coupling reactions
The cross-coupling between diphenyl disulfde and phenyla-
cetylene was selected for reusability experiment in the Csp–S
cross-coupling reactions. The bi-catalytic system was sepa-
rated from the coupling reaction solution using an external
magnet. For recycle experiment, the recovered bi-catalyst
(hollow nanoshell-sphere Fe@Fe/Pd reactors) was washed
with diethyl ether (4 mL×3), absolute ethanol (4 mL×3),
and distilled water (4 mL×3) and dried in vacuum for 24 h
before the re-usage. The bi-catalytic system (57×10−3 mmol
Fe, 7.3×10−5 mmol Pd) was reused for at least six repeating
cycles. Bi-catalytic system after frst reusing cycle was char-
acterized by XRD, SEM-EBSD/EDX, UV–Vis, and particle
size analyzer without pretreatment. More than 95 wt% of the
catalyst could usually be recovered from each run.
Electron microscopy (EM)
Scanning electron–electron backscatter difraction micros-
copy (SEM-EBSD) has been performed using an SU3500
microscope with scanning range from 0 to 20 keV, Japan.
Electron-dispersive X-ray spectroscopy (EDX) measure-
ments were made with an IXRF model 550i attached to
SEM. SEM-EBSD/EDX samples were prepared by coating
of solid particles into a conductive layer.
UV–Vis spectra
UV–Vis spectra of samples at ultrapure water solution were
recorded on CECIL UV/Vis 2021 model spectrophotometer,
England, using quartz cells of 1 cm optical path.
Results and discussion
We initially conducted the reaction by adding diphenyl
disulfde and phenylacetylene to an aqueous basic solution
at 70 °C with various catalytic systems (Table 1).
In the absence of catalyst, the Csp–S coupling product
was not observed while acetophenone and thiophenol which
are the expected by-products in basic solution were detected
(Table 2, entry 1). The reduction in sulfur agents and genera-
tion of thiophenol is prospected in the metal-free solution
and air [32].
X‑ray Difraction (XRD)
XRD pattern was recorded by an Inel French, EQUINOX
3000 model X-ray difractometer using CuKα radiation in
the 2θ range of 20–80 with step width 2θ = 0.05 and time
per step=1 s.
The Pd-free Fe–PVP showed no activity in the cross-
coupling reaction (Table 2, entry 2). The Fe-free Pd–PVP
was active but showed less selectivity toward 1a (Scheme 1,
Table 2, entry 3). The selectivity was improved using
Pd–PVP–Fe (Table 2, entry 4). Therefore, it seems that
both Pd and Fe sites are vital to achieve desired activity and
selectivity.
Typical experimental procedure for the Csp–S
cross‑coupling reaction
In a typical reaction, a suspension of Fe–PVP (0.01 g,
57 × 10−3 mmol Fe), base (1.0 mmol), phenylacetylene
(1.0 mmol), diaryl disulfde (0.5 mmol), distilled water
(1.0 mL), and Pd–PVP–Fe (0.1 mg, 0.5 × 10−5 mmol Fe,
7.3 × 10−5 mmol Pd) were mixed in a reaction fask. The
reaction mixture was stirred at a given temperature and for
an appropriate time. Progress of the reaction was monitored
by TLC (thin-layer chromatography on precoated silica gel,
0.2 mm, Fluorescent 254 nm on aluminum plates). Upon
completion of the reaction, the reaction mixture was cooled
to room temperature. The bi-catalytic system was separated
from the product solution using an external magnet. Sol-
vent of the reaction mixture was evaporated to generate the
crude product. The product was concentrated and purifed
by column chromatography on silica gel using EtOAc/n-
heptane as eluent. The product was characterized by 1HNMR
The size, shape, and purity of chemical of metallic nano-
particles are critical factors for the use of them in catalytic
systems. These parameters determine the activation status
of the metal component. We decided to investigate the mix-
ing of metallic bi-catalytic systems in detail. To an aqueous
basic solution of 0.005 g Fe–PVP, diphenyl disulfde and
phenylacetylene were added and after 5 min, 0.1× 10−3
g
of the studied Pd-containing catalyst was added [26, 31].
This order was fxed as the model order (path I) in the Csp–S
cross-coupling reaction.
PdCl2 and Pd–Fe showed low yield and selectivity
(Table 2, entries 5, 6). For both of them, the FT-IR and
UV–Vis spectra indicated that the palladium was in the +2
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Table 2 Comparison of the
Entry
Catalytic system
Time (min)
Yield 1a (%)b
Yield 2a–c
and 3a (%)b
TONc/TOF (h−1 d
)
diferent catalytic systems in the
Csp–S cross-coupling reactiona
1
2
None
Fe–PVP
300
120
–
> 5
–
–
–/–
–/–
3
4
5
6
7
8
9
10
11
12e
13f
Pd–PVP
Pd–PVP–Fe
15 (100)
15 (90)
15
15
15
20 (80)
30 (85)
34
44
84
12 (12)
10,879/8159
11,596/7537
4522/18,088
6790/27,160
10,751/43,005
13,097/52,387
7094/4729
6139/4093
4093/2046
11,323/13,588
10,232/12,278
–
–
–
–
–
–
–
–
–
–
Fe–PVP+PdCl2
Fe–PVP+Pd–Fe
Fe–PVP+Pd–PVP
Fe–PVP+Pd–PVP–Fe
FeCl2·4H2O+Pd–PVP–Fe
FeCl3·6H2O+Pd–PVP–Fe
Fe3O4 +Pd–PVP–Fe
Fe–PVP/Pd–PVP–Fe
Pd–PVP–Fe+Fe–PVP
15
96
15 (90)
15 (90)
15 (120)
15 (50)
15 (50)
6 (52)
5 (45)
– (30)
21 (83)
16 (75)
aReaction conditions: Path I=Fe-containing catalyst (0.005 g); K2CO3 (1.0 mmol); diphenyl disulfde
(0.5 mmol); phenylacetylene (1.0 mmol); at 1.0 mL distilled water and 70 °C; Pd-containing catalyst
(0.1×10−3 g)
bIsolated yield
cTON=mmol of product/mmol of Pd catalyst
dTOF=TON/time (TON and TOF were calculated for maximum yield of 1a coupling product for Pd sites
in the catalytic system)
eThe order of adding reactants: Path II=Pd and Fe-containing catalyst; K2CO3; diphenyl disulfde; pheny-
lacetylene at distilled water and 70 °C
fThe order of adding reactants: Path III=Pd-containing catalyst; K2CO3; diphenyl disulfde; phenylacety-
lene at distilled water and 70 °C; Fe-containing catalyst
oxidation state [26, 31]. In addition, AAS data indicated that
Pd–Fe has less Pd content (Table 1).
The best Csp–S cross-coupling yield was obtained using
Fe–PVP+Pd–PVP and Fe–PVP+Pd–PVP–Fe as catalytic
systems (Table 2, entries 7, 8). In contrast to Pd–Fe, these
catalysts consist of Pd(0) sites which may be due to the
electrosteric and steric stabilization efect of PVP [26, 31,
33–35].
In the next step, we changed the source of iron. Chloride
salts of iron as well as Fe3O4 showed low activity (Table 2,
entries 9–11). In addition, oxidation by-products were
obtained using Fe3O4.
Fig. 1 Progress of the Csp–S cross-coupling reactions. Reaction
conditions: a path I: Fe–PVP, diphenyl disulfde, phenylacetylene,
K2CO3, distilled water and 70 °C, Pd–PVP–Fe, b path II: Fe–PVP/
Pd–PVP–Fe, diphenyl disulfde, phenylacetylene, K2CO3, distilled
water, at 70 °C and c path III: Pd–PVP–Fe, diphenyl disulfde, pheny-
lacetylene, K2CO3, distilled water and 70 °C, Fe–PVP
Knowing the binary mixture of Fe–PVP + Pd–PVP–Fe
as the optimal catalyst (Table 2, entry 8), we studied the
order of adding of metallic catalysts due to efect on yield
of the product in organic reactions. Simultaneous adding
of the mixing of catalysts at the start (path II) or adding
Pd–PVP–Fe followed by Fe–PVP (path III) to the basic solu-
tion of substrates resulted in the same selectivity toward 1a
(Scheme 1) but lower activity with delay time in reaction
rate compared to order of adding of catalysts with path I
(Table 2, entries 8, 12, 13 and Fig. 1a–c).
Therefore, we are interested to investigate reasons of this
phenomena by analyzing the recovered bi-catalytic systems
of paths I, II, and III of the Csp–S cross-coupling reaction
medium with SEM-EBSD/EDX and DLS analyses.
According to above results, the order of adding of metallic
catalysts afects on yield and reaction rate of the Csp–S cross-
coupling product (Table 2, entries 8, 12, 13 and Fig. 1a–c).
XRD patterns of fresh Pd–PVP–Fe and fresh Fe–PVP
are shown in Fig. 2. The difraction peaks observed at
fresh Pd–PVP–Fe correspond to cubic structure of Pd0 and
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Journal of the Iranian Chemical Society
Fig. 2 XRD patterns of a fresh Pd–PVP–Fe and b fresh Fe–PVP
Fe0 with JCPDS 00-087-0643 and JCPDS 00-088-2324,
respectively (Fig. 2a) [31]. Also, Fig. 2b shows cubic
structure of Fe0 for fresh Fe–PVP with JCPDS 00-088-
2324 [35].
Figure 5 shows particle size distribution of the recovered
bi-catalytic systems (Fe–PVP and Pd–PVP–Fe) of paths I,
II, and III after the frst run of the Csp–S cross-coupling
reaction. Particle size profle of the recovered catalysts from
paths II and III are observed dual distribution (Fig. 5b, c)
compared to the recovered catalyst from path I (Fig. 5a). The
recovered catalyst of path I (Fig. 5a) has the stable structure
than the recovered catalysts of paths II and III (Fig. 5b, c)
in solution phase during DLS measurements. It seems that
unsuitable and unstable connections between two metallic
catalysts in solution phase are basic reason of low catalytic
activity of these recovered bi-catalytic systems (Table 2,
entries 12, 13 and Fig. 1).
SEM images in Fig. 3a, e were revealed that fresh
Pd–PVP–Fe and Fe–PVP display uniformly hollow spheres
and nanoparticles, respectively. According to EBSD and
EDX images of fresh Pd–PVP–Fe, Pd nanoparticles were
assembled on the surface of the hollow Fe microsphere
(Fig. 3b–d) [26, 31]. Also EBSD and EDX images of the
fresh Fe–PVP nanoparticles are shown in Fig. 3f, g.
The EBSD images of the bi-catalytic systems
(Fe–PVP + Pd–PVP–Fe) recovered from paths I, II, and
III are shown in Fig. 4a–c, respectively, after the frst run
of the Csp–S cross-coupling reaction. The bi-catalytic sys-
tem recovered of path I shows that crude Fe–PVP nano-
particles assembled on the surface of hollow Pd–PVP–Fe
nanospheres. Therefore, hollow nanoshell-sphere Fe@Fe/
Pd rectors appear under in situ modifcation of the Csp–S
cross-coupling reaction with this path (Fig. 4a). For the bi-
catalytic system recovered of path II, separated phases of
aggregation nanoparticles and hollow nanospheres were
obtained without any connection between both metallic
catalysts (Fig. 4b). According to results of coupling reaction
(Table 2, entry 12, Fig. 1b), this separated metallic phases
in metallic catalytic systems are not suitable conformation
for organic reactions. EBSD image of the bi-catalytic sys-
tem recovered of path III shows that the shape of rocksalt
bi-catalytic system evolves during in situ modifcation of
the Csp–S cross-coupling reaction. According to Table 2,
entries 8, 12, 13 and Figs. 1 and 4, the formation of hollow
nanoshell-sphere Fe@Fe/Pd reactors via self-assembly of
Fe–PVP nanoparticles into hollow Pd–PVP–Fe nanospheres
of path I is the suitable confguration to catalyze the Csp–S
cross-coupling reaction.
Using the order of adding of path I as suitable reaction
conditions and the recovered catalyst by this path, a series
of parameters including temperature, catalyst loading, base,
and solvent were optimized (Table 3).
The selectivity toward 1a (Scheme 1) and reaction time
remain constant with decreasing the temperature till 50 °C
(Table 3, entries 1, 2). Further decreasing temperature till
room temperature resulted in dramatic decreasing of yield in
higher reaction time (Table 3, entry 3). In this temperature
conditions, the oxidation by-products were obtained [22].
Therefore, further investigations were carried out at 50 °C. In
DMF and DMSO besides 1a (Scheme 1), amounts of vinylic
sulfde and oxidation by-products were formed, respectively
(Table 3, entries 4, 5) [23, 36]. It seems that the formation of
oxidation by-products due to oxidant properties of DMSO
in the reaction medium [23]. Therefore, H2O was selected
as a solvent of choice. Using inorganic and organic bases
failed to produce the desire product efciently (Table 3,
entries 6–9). However, vinylic sulfde or other by-products
were obtained under base-free condition (Table 3, entry
10) [22, 36]. Increasing the amount of Pd–PVP–Fe did not
change the selectivity, yield, and reaction time signifcantly
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Journal of the Iranian Chemical Society
Fig. 3 a SEM image, b EBSD image, and c, d EDX maps of fresh Pd–PVP–Fe; e SEM image, f EBSD image, and g EDX map of fresh Fe–PVP
(Table 3, entry 11), but increasing the amount of Fe–PVP
resulted in improving the reaction time (Table 3, entry 12).
Therefore, the binary mixture containing 0.01 g Fe–PVP
and 0.1×10−3 g Pd–PVP–Fe were selected as the optimal
catalyst contained only 0.073 mmol% Pd.
Reusability of the hollow nanoshell-sphere Fe@Fe/Pd
reactors was examined at least six times in the Csp–S cross-
coupling reaction (Table 4). To an aqueous basic solution
of Fe@Fe/Pd (57 × 10−3 mmol Fe and 7.3 × 10−5 mmol
Pd), diphenyl disulfde and phenylacetylene were added.
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Journal of the Iranian Chemical Society
Fig. 4 EBSD images of the bi-
catalytic systems (Fe–PVP and
Pd–PVP–Fe) recovered of paths
a I, b II, and c III during the
Csp–S cross-coupling reaction
No signifcant decrease in the yield was observed dur-
ing the bi-catalytic system reusing study. Metals leach-
ing of the Fe@Fe/Pd catalyst were further investigated
after repeated cycles by AAS (Table 4). The results indi-
cated that only a trace amount of the Pd and Fe metals are
leached out from the bi-catalyst surface after each catalytic
cycle. This means that nature of the hollow nanoshell-
sphere Fe@Fe/Pd reactors did not change after being used
repetitively at least for six times.
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Fig. 5 Particle size distribution
of the recovered bi-catalytic
systems (Fe–PVP and Pd–PVP–
Fe) of paths a I, b II, and c III
during the Csp–S cross-coupling
reaction
Figure 6 shows the XRD patterns of the recovered Fe@
Fe/Pd catalyst after frst reusing cycle. It seems that metallic
phases (Pd0 and Fe0 sites) at Fe@Fe/Pd are stable during the
Csp–S cross-coupling reaction and recovery process [31, 37].
Figure 7a shows morphology of the recovered Fe@Fe/Pd
after frst reusing cycle. The SEM image reveals that Fe@
Fe/Pd displays uniformly the hollow nanoshell-sphere reac-
tors with an average diameter around 10 μm. This reactors
constitute a shell with diameter about 2–2.5 μm (Fig. 7a).
According to the EBSD micrographs (Fig. 4a) and the EDX
elemental mapping images (Fig. 7b1), hollow sphere con-
tained iron phase (Fe K). Also EBSD images are showed that
there is clear contrast diference between dispersed nanopar-
ticles on the surface of hollow sphere (Fig. 4a). According to
the EDX elemental mapping images, dual-phase structure of
nanoparticles contained iron (Fe L, Fig. 7b2) and palladium
phases (Pd L, Fig. 7b3). According to above results, Pd0
(Pd L) and Fe0 (Fe L) sites were assembled and arranged on
the surface of hollow iron nanosphere (Fe K) with uniform
distribution (the hollow nanoshell-sphere Fe@Fe/Pd reac-
tors). Table 2, entry 8 and Fig. 1a show that this orientation
is very suitable at catalytic applications.
Figure 7c shows the UV–Vis spectra of the recovered
Fe@Fe/Pd after frst reusing cycle. It seems that metallic
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Table 3 Optimization of the Csp–S cross-coupling reaction conditionsa
Ph
S
S
Catalyst
1/2
+
H
Ph
2
Entry
Catalytic system (Fe–
PVP+Pd–PVP–Fe, g)
Solvent
Temp.(°C)
Base
Time (min)
Yield 1a (%)b
Yield 2a–c
and 3a (%)b
1
2
3
4
5
6
7
8
0.005+0.1×10−3
0.005+0.1×10−3
0.005+0.1×10−3
0.005+0.1×10−3
0.005+0.1×10−3
0.005+0.1×10−3
0.005+0.1×10−3
0.005+0.1×10−3
0.005+0.1×10−3
0.005+0.1×10−3
0.005+0.15×10−3
0.01+0.1×10−3
H2O
H2O
H2O
DMF
DMSO
H2O
H2O
H2O
H2O
H2O
H2O
H2O
70
50
r.t.
50
50
50
50
50
50
50
50
50
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
NaOAc
NaOtBu
N(Me)3
N(Butyl)3
–
15
20
120
50
120
45
120
60
96
92
52
89
64
51
44
55
41
19
94
92
–
–
25
10
–
–
–
–
–
9
120
120
20
10
11
12
33
–
–
K2CO3
K2CO3
15
aReaction conditions: Path I=Fe–PVP; base (1.0 mmol); diphenyl disulfde (0.5 mmol); phenylacetylene (1.0 mmol) at 1.0 mL solvent and tem-
perature; Pd–PVP–Fe
bIsolated yield
Table 4 Reusing test of the hollow nanoshell-sphere Fe@Fe/Pd reac-
tors at the Csp–S cross-coupling reaction (reaction condition is listed
in Table 3, entry 12)
Run Pd (wt%)a Fe (wt%)a Yield (%)b TONc
TOF (h−1 d
)
1
2
3
4
5
6
7.8
7.8
7.5
7.1
7.1
7.1
77.2
77.2
77.2
77.0
77.0
77.0
90 2.0
88 2.5
84 3.2
81 2.3
81 2.3
81 1.1
12,276 49,104
12,003 48,012
11,458 45,832
11,048 44,192
11,048 44,192
11,048 44,192
aAAS data
bData are presented as the mean SD of three independent experi-
ments performed in triplicate
cTON=mmol of product/mmol of Pd catalyst
dTOF=TON (15 min)/time
sites at the hollow nanoshell-sphere Fe@Fe/Pd reactors are
stable during the Csp–S cross-coupling reaction and recovery
process [26, 31].
Plausible mechanism of the hollow nanoshell-sphere
Fe/Fe@Pd reactors catalyzed the Csp–S cross-coupling
Fig. 6 XRD patterns of the recovered Fe@Fe/Pd catalyst after frst
run of the Csp–S cross-coupling reaction
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Journal of the Iranian Chemical Society
Fig. 7 a SEM image, b EDX elemental mapping data images of the
recovered Fe@Fe/Pd catalyst after the frst run of the Csp–S cross-
coupling reaction, and c UV–Vis spectra of the (1) fresh Pd–PVP–Fe,
(2) fresh Fe–PVP, and (3) recovered Fe@Fe/Pd after frst run of the
Csp–S cross-coupling reaction
reactions which is shown in Scheme 2. It seems that most
plausible mechanism involves several steps:
ii. Deprotonation of acidic hydrogen in phenylacetylene.
iii. Coordination of phenylacetylene (as stable alkynyl-
phenol) to Pd sites. Alkenyl–Pd cluster species [33]
as strong complex agent prevent polymer synthesis by
homo-coupling terminal alkynes [48].
i. Coordination of sulfde to Fe sites [7, 28, 38]. Accord-
ing to Table 2, entries 2, 4, 8 and Figs. 1, 2, 3, 4, 5,
6 and 7, the Fe–PVP nanoparticles as co-catalyst at
bi-catalytic system not only enhance the dispersion
of Pd nanoparticles as active sites on the hollow Fe
microsphere [29, 39] to provide adequate promoted
morphology property of the as-obtained bi-catalytic
system at the coupling reactions [31, 40, 41] but also
act as an electrophilic bufer to protect the palladium
centers from attack of the thiols [7, 26, 38, 42–47].
iv. Phase-transfer catalysis for inorganic base/substrate/
product phase (coupling reaction) [49].
v. Elimination of the Csp–S coupling product.
To evaluate the scope and generality of this new proto-
col, a series of diaryl disulfdes was studied (Table 5). In
the cases containing electron donating substituents, the
corresponding Csp–S coupling products were obtained in
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Journal of the Iranian Chemical Society
Scheme 2 Plausible mechanism of the Csp–S cross-coupling reactions in the presence of the hollow nanoshell-sphere Fe@Fe/Pd reactors
excellent yields and low reaction time (Table 5, entries 2,
3). It seems that suitable conditions exist in the coordination
step at presence electrophilic substrates. Electron withdraw-
ing substituents slow down the reaction and decrease the
coupling products yields (Table 5, entries 4, 5).
Conclusion
In summary, the Csp–S cross-coupling reactions could
be catalyzed very selective, air stable, and using very
small amount of Pd (7.3 × 10−5 mmol) in catalytic system
(Fe–PVP+Pd–PVP–Fe) in aqueous media as a green solvent
at short reaction times. The most important advantages of
this bi-catalytic system compared to previous reports are
excellent, stable, and controllable morphology of the used
bi-catalytic system (the hollow nanoshell-sphere Fe@Fe/Pd
reactors) by adjusting the order of adding of the two metallic
catalysts into the coupling reaction medium. The surface of
hollow sphere reactors contains metallic sites (Pd0 and Fe0
nanoparticles). Due to the higher contact between the Pd and
Fe nanoparticles on surface of the hollow Fe microsphere,
potential synergistic efects between metallic nanoparticles
Several catalytic and reaction systems have to be applied
for the Csp–S cross-coupling reactions (Table 6). However,
toxic solvents, strong conditions, longer reaction times, etc.,
in some reported systems are not benefcial to the industrial
and synthetic applications. The comparison study of our
results with other catalytic systems illustrates that the Fe@
Fe/Pd catalyst gives higher yield of the coupling product
in very low catalyst loading, short reaction time, and green
conditions (solvent and atmosphere).
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Journal of the Iranian Chemical Society
Table 5 Yields of the Csp–S cross-coupling reactions of phenylacetylene with various diaryl disulfdesa
R
S
Ph
S
R
Fe-PVP + Pd-PVP-Fe
1/2
+
H
Ph
O
H2O, K2CO3, 50 C
2
Entry
1
Product
Time (min)
15
Yield (%)b
92
TON/TOF (h−1 c
)
12,551/50,205
Ph
S
S
S
2
3
4
5
15
98
12,824/32,060
Ph
Ph
Ph
Ph
15
89
12,142/30,355
11,733/17,599
10,095/13,461
OCH3
Br
15 (40)
15 (45)
35 (86)
20 (74)
S
S
Cl
aReaction conditions: path I=Fe–PVP (0.01 g); K2CO3 (1.0 mmol); diaryl disulfde (0.5 mmol); phenylacetylene (1.0 mmol) at 1.0 mL distilled
water and 50 °C; Pd–PVP–Fe (0.1×10−3 g)
bIsolated yield; selectivity for all entries was 100%; all products are known and identifed by comparing of their spectral data with those of the
authentic samples [50]
cTON and TOF were calculated for maximum yield of the coupling product for Pd sites in the catalytic system
Table 6 Comparison of the activity of the Fe@Fe/Pd catalyst over
other heterogeneous and homogeneous catalysts in the Csp–S cross-
coupling reactions
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