Binuclear Palladium Complex Immobilized on Mesoporous SBA-16: Efficient Heterogeneous Catalyst for the Carbonylative Suzuki Coupling Reaction of Aryl Iodides and Arylboronic Acids Using Cr(CO)6 as Carbonyl Source

Article abstract of DOI:10.1007/s10562-019-03087-w

Abstract: In this study, a binuclear palladium complex immobilized on the organo-functionalized SBA-16 was prepared and structurally characterized by routine techniques. Characterizations indicated that the mesostructure of SBA-16 was maintained after the immobilization of palladium complex. Then, the prepared nanomaterial was applied as a heterogeneous catalyst in the carbonylative Suzuki coupling reaction of aryl iodides with arylboronic acids using Cr(CO)₆ as carbonyl source. The catalyst was efficiently promoted the coupling reactions of various aryl iodides and arylboronic acids to give the corresponding diaryl ketones in excellent yields. Moreover, the catalyst was readily recovered by filtration and could be reused for seven cycles without losing its structural integrity and catalytic activity. Graphic Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-019-03087-w

Catalysis Letters
https://doi.org/10.1007/s10562-019-03087-w
Binuclear Palladium Complex Immobilized on Mesoporous SBA‑16:
Efcient Heterogeneous Catalyst for the Carbonylative Suzuki
Coupling Reaction of Aryl Iodides and Arylboronic Acids Using Cr(CO)6
as Carbonyl Source
Mahsa Niakan1 · Zahra Asadi1 · Mohammad Emami1
Received: 12 October 2019 / Accepted: 26 December 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
In this study, a binuclear palladium complex immobilized on the organo-functionalized SBA-16 was prepared and structur-
ally characterized by routine techniques. Characterizations indicated that the mesostructure of SBA-16 was maintained after
the immobilization of palladium complex. Then, the prepared nanomaterial was applied as a heterogeneous catalyst in the
carbonylative Suzuki coupling reaction of aryl iodides with arylboronic acids using Cr(CO)6 as carbonyl source. The catalyst
was efciently promoted the coupling reactions of various aryl iodides and arylboronic acids to give the corresponding diaryl
ketones in excellent yields. Moreover, the catalyst was readily recovered by fltration and could be reused for seven cycles
without losing its structural integrity and catalytic activity.
Graphic Abstract
Keywords Palladium · SBA-16 · Carbonylative suzuki coupling reaction · Cr(CO)6 · Diaryl ketones
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-019-03087-w) contains
supplementary material, which is available to authorized users.
1 Introduction
Diaryl ketones are essential feedstocks and key intermedi-
ates for the production of natural compounds, pharmaceu-
tical drugs, and sunscreen agents. Because of their enor-
mous utility, the development of efective protocols for the
* Zahra Asadi
zasadi@shirazu.ac.ir
1
Department of Chemistry, College of Sciences, Shiraz
University, 71454 Shiraz, Iran
Vol.:(0123456789)
1 3

M. Niakan et al.
preparation of diaryl ketones continues to be an active area
of research [1–5]. There are a number of various synthetic
methods for diaryl ketones, including the Friedel–Crafts
acylation of aromatic compounds, the acylation of aryl-
metal species with functional carboxylic acid derivatives,
oxidation of diaryl methanols, and others [6, 7]. Among all
known methods, the transition metal-catalyzed carbonyla-
tive Suzuki coupling reaction has been studied extensively
to form diaryl ketones from aryl halides, carbon monoxide,
and arylboronic acids due to good functional group com-
patibility and the fact that arylboronic acids are generally
nontoxic, easy available, and stable to air and moisture
[8–10].
been reported to date using SBA-16 as support for palla-
dium-based catalysts.
Inspired by the above considerations, herein we utilized
SBA-16 as a solid support to immobilize a novel binuclear
palladium complex with the aim to prepare a heterogeneous
catalyst. The prepared catalyst showed outstanding catalytic
activity in the carbonylative Suzuki coupling reaction of aryl
iodides with arylboronic acids using Cr(CO)6 as carbonyl
source. Additionally, the recycling potential of the catalyst
was assessed during six successive cycles. As far our aware-
ness reach, the use of Cr(CO)6 as carbonyl source has never
been reported for this transformation.
Despite being a powerful method, the use of gaseous CO
as carbonyl source limits the practical application of carbon-
ylative Suzuki coupling reaction in laboratories and at the
industrial scale since gaseous CO is high toxic, fammable,
odorless, and difcult to handle [11, 12]. This issue trying
to be resolved by using other carbonyl substituents like tran-
sition-metal carbonyls, aldehydes, formic acid, chloroform,
N-formylsaccharin, DMF, and formats as carbonyl source in
this reaction [13, 14]. In this context, the use of transition-
metal carbonyls is often favored because of simplicity in
handling and in-situ generation of CO upon heating [15, 16].
Today, a considerable efort has been devoted to employ
heterogeneous catalysts in the catalytic reactions due to
their easy recovery and reusability [17–20]. The immo-
bilization of homogeneous catalysts on a solid support is
an attractive method for preparing heterogeneous catalysts
with high activities [21, 22]. Especially, palladium-based
catalysts demonstrated remarkably high catalytic activity in
catalyzing the carbonylative Suzuki coupling reaction. Thus,
several types of immobilized palladium catalysts on ionic
liquids [23, 24], polymers [25], silica [26, 27], carbon [28],
graphene oxide [29], mesoporous silica [30, 31], magnetic
nanoparticles [32–36], and metal–organic frameworks [37]
have been developed for this transformation.
2 Experimental
2.1 General
The details of the used materials and instrumentations are
given in the supporting information.
2.2 Synthesis of Mesoporous Silica SBA‑16
Mesoporous silica SBA-16 was prepared in accordance with
the literature method [51]. Typically, 0.63 g of F127 was
mixed with 0.073 g of CTAB and 71.28 g of 2.0 M HCl
solution. Then, 2.083 g of tetraethylorthosilicate (TEOS)
was dropped into the above solution with vigorous stirring.
The mixture was stirred at 40 °C for 6 h followed by an aging
step at 80 °C for 6 h. The obtained white precipitate was
fltered, rinsed several times with deionized water and dried
at 80 °C overnight. Finally, the white powder was calcined
at 550 °C for 6 h to remove the organic surfactant.
2.3 Synthesis of 3‑aminopropyltriethoxysilane
Functionalized SBA‑16 (SBA‑16/NH2)
More recently, mesoporous silica materials as promis-
ing catalyst supports have attracted increasing attention
owing to their regular structure uniform pore size distri-
bution, large specifc surface area, and high thermal sta-
bility [38–42]. Among them, cubic mesoporous SBA-16,
formed by an arrangement of spherical empty cages having
a body-centered cubic symmetry (Im3m), is considered to
be the most attractive mesostructure [43–45]. The isolated
nanocages are three-dimensionally interconnected by pore
entrances. Such mesostructure provides the easy transport of
reactants and products in and out of its mesopores without
pore blockage [46, 47]. The 3D cage-like mesoporous struc-
ture of SBA-16 makes it superior candidate than other 2D
hexagonal mesoporous silicas like MCM-41 and SBA-15 for
preparing of heterogeneous catalysts [48–50]. However, no
studies of the carbonylative Suzuki coupling reaction have
1.0 g of SBA-16 (dried at 120 °C for 12 h in a vacuum oven)
was added to a solution of 3-aminopropyltriethoxysilane
(3-APTES, 1.0 g) in 30.0 mL of dry toluene. The reaction
mixture was heated at refux for 24 h under N2 atmosphere.
The resulting white solid (SBA-16/NH2) was collected by
fltration, rinsed repeatedly with ethanol and dichlorometh-
ane and dried at 80 °C.
2.4 Synthesis of 2,6‑diformyl‑4‑methylphenol
Functionalized SBA‑16 (SBA‑16/DFMP)
2,6-diformyl-4-methylphenol ligand was synthesized accord-
ing to the procedure reported previously [52]. A solution of
2,6-diformyl-4-methylphenol (0.50 mmol, 0.08 g) in abso-
lute ethanol (30.0 mL) was added to the 1.0 g of the above-
synthesized SBA-16/NH2 and refuxed for 24 h under N2
1 3

Binuclear Palladium Complex Immobilized on Mesoporous SBA‑16: Efcient Heterogeneous…
OH
O
O
Si
OH
OH
O
O
O
O
O
Si
Si
NH2
NH2
TEOS
APTES
+
HCl (aq)
Toluene
O
O
O
F127
+
SBA-16/NH2
SBA-16
CTAB
O
HO
EtOH
O
2,6
-dif
orm
yl-
4-m
eth
ylp
hen
ol
O
O
O
O
O
O
N
Si
Si
Si
Si
N
HO
N
Cl
Cl
Pd
Pd
Pd(Cl)2
EtOH
O
N
O
O
O
O
O
O
Cl
SBA-16/DFMP
SBA-16/DFMP-Pd
Scheme 1 Schematic illustration for the preparation of SBA-16/DFMP-Pd
atmosphere. The obtained light yellow solid was removed
from the solvent by fltration, washed thoroughly with etha-
nol and dried at 80 °C (SBA-16/DFMP).
and stirred at room temperature for 24 h. Afterwards, the
resulting pale green–brown powder was fltered, washed 6–8
times with an excess of ethanol and dried at 80 °C (SBA-16/
DFMP-Pd).
2.5 Synthesis of 2,6‑diformyl‑4‑methylphenol
Functionalized SBA‑16 Immobilized Palladium
Complex (SBA‑16/DFMP‑Pd)
To a mixture of SBA-16/DFMP (1.0 g) in absolute ethanol
(30.0 mL), palladium chloride (1.80 g, 1.0 mmol) was added
1 3

M. Niakan et al.
Fig. 1 FT-IR spectra of a SBA-16, b SBA-16/NH2, c SBA-16/DFMP,
d SBA-16/DFMP-Pd
Fig. 3 EDX spectrum of SBA-16/DFMP-Pd
3 Results and Discussion
The overall process of SBA-16/DFMP-Pd preparation is
schematically represented in Scheme 1. To illustrate the
successful synthesis of the as-prepared catalyst, it was well
characterized by diferent analytical techniques, namely,
Fourier transform infrared spectroscopy (FT-IR), thermo-
gravimetric analysis (TGA), energy-dispersive X-ray anal-
ysis (EDX), inductively coupled plasma optical emission
spectroscopy (ICP-OES), X-ray photoelectron spectroscopy
(XPS), nitrogen adsorption–desorption analysis, high resolu-
tion transmission electron microscopy (HRTEM), and pow-
der X-ray difraction (PXRD).
Fig. 2 TGA curves of a SBA-16, b SBA-16/NH2, c SBA-16/DFMP, d
SBA-16/DFMP-Pd
3.1 Characterization of SBA‑16/DFMP‑Pd
2.6 Typical Procedure for the Carbonylative Suzuki
Coupling Reaction in the Presence of SBA‑16/
DFMP‑Pd
Fourier transform infrared (FT-IR) spectroscopy was
employed to characterize the samples collected in each step
of the synthesis of the catalyst, and the profles are shown in
Fig. 1. In Fig. 1a, the bands at approximately 1090, 800, and
460 cm−1 were indexed to the asymmetric stretching vibra-
tion, symmetric stretching vibration, and bending vibration
of Si–O–Si, respectively, demonstrating the presence of con-
densed silica network of SBA-16 [53]. The appearance of the
stretching vibration of the C–H of propyl groups at around
2800–3000 cm−1 for SBA-16/NH2 sample (Fig. 1b) con-
frmed the successful grafting of 3-APTES to the SBA-16
surface [54]. After 2,6-diformyl-4-methylphenol function-
alization, the stretching vibrations of C=C and C=N groups
were shown at 1533 and 1672 cm−1 in the FT-IR spectrum
of SBA-16/DFMP (Fig. 1c), implying the occurrence of
Schif base condensation between amine groups of SBA-16/
Aryl halide (1.0 mmol), arylboronic acid (1.1 mmol),
t-BuONa (2.0 mmol), SBA-16/DFMP-Pd (0.0026 g,
0.2 mol%), and Cr(CO)6 (0.5 mmol, 0.11 g) in DMA
(3.0 mL) was stirred in a fask at 80 °C. The reaction pro-
gress was monitored by thin layer chromatography (TLC).
Upon the reaction’s completion, the mixture was allowed to
cool down to room temperature and then fltered and washed
with H2O and ethyl acetate. The organic fraction was sepa-
rated, dried over anhydrous Na2SO4 and then the solvent was
removed using a rotary evaporator. Further purifcation was
performed on silica gel column chromatography to aford
the pure products.
1 3

Binuclear Palladium Complex Immobilized on Mesoporous SBA‑16: Efcient Heterogeneous…
Fig. 4 EDX elemental mapping of Si, O, C, N, Cl, and Pd
NH2 with the aldehyde groups of 2,6-diformyl-4-methyl-
phenol. Moreover, the absorption peak of C=N was shifted
to a lower wave number in the case of SBA-16/DFMP-Pd
(Fig. 1d), which revealed the coordination of SBA-16/DFMP
through imine nitrogen to Pd (II) metal ions [55, 56].
In order to confrm the successful functionalization of
the surface of SBA-16, thermogravimtric analysis (TGA)
was performed. The TGA curves of SBA-16, SBA-16/NH2,
SBA-16/DFMP, and SBA-16/DFMP-Pd are plotted in Fig. 2.
Except for SBA-16, in all the samples, a signifcant weight
loss was observed in the range of 200–800 °C, which was
evidently attributed to the decomposition of functionalized
organic groups. SBA-16/NH2 was subjected to 5.6% weight
loss in the range of 200–800 °C, which clearly confrmed
the successful anchoring of 3-APTES over SBA-16. Also, a
higher weight loss at higher temperatures for SBA-16/DFMP
(Fig. 2c, 13.71%) and for SBA-16/DFMP-Pd (Fig. 2d,
15.33%) verifed the proper anchoring of the ligand and
metalation of SBA-16/DFMP, respectively. Additionally, the
TGA results led us to conclude that the catalyst had good
thermal stability and it was safe to perform the reactions at
100 °C under heterogeneous conditions.
Si, C, N, O, Cl, and Pd elements and no other peaks of impu-
rities could be observed in the spectrum (Fig. 3). Meanwhile,
EDX-mapping exhibited that C, N, Cl, and Pd elements had
highly uniform and homogeneous distributions over the
mesoporous support (Fig. 4). Also, the concentration of pal-
ladium in SBA-16/DFMP-Pd was measured by inductively
coupled plasma optical emission spectroscopy (ICP-OES)
and a content of 0.76 mmol g−1 was detected.
To further characterize the elemental analysis of SBA-
16/DFMP-Pd, X-ray photoelectron spectroscopy was used.
The XPS survey spectrum of SBA-16/DFMP-Pd is shown
in Fig. 5a, and Si, C, O, N, Cl, and Pd peaks could be
observed in the binding energy region from 0 to 1000 eV.
The high-resolution XPS spectrum of C 1s in Fig. 5b showed
four peaks at binding energies of 284.8, 285.2, 286.1, and
288.8 eV, attributable to C–C, C=C, C=N, and C–N/C–O,
respectively, in accordance with the previous reports [57,
58]. The O 1s spectrum was deconvoluted into two peaks
with binding energies of 531.5 and 532.9 eV (Fig. 5c). The
lower binding energy peak was assigned to O–C and the
higher binding energy peak was ascribed to O–Pd [59]. Also,
the deconvolution XPS spectrum of N 1s illustrated three
peaks at binding energies of 399.8, 400.8, and 401.6 eV, cor-
responding to N–C, N=C, and N–Pd, respectively (Fig. 5d)
[44, 58]. The Cl 2p XPS spectrum in Fig. 5e exhibited
The elemental composition of the catalyst was analyzed
by energy-dispersive X-ray (EDX) analysis. The EDX spec-
trum of SBA-16/DFMP-Pd disclosed only the existence of
1 3

M. Niakan et al.
Fig. 5 XPS elemental survey of SBA-16/DFMP-Pd (a), high resolution XPS spectra of C 1s (b), O 1s (c), N 1s (d), Cl 2p (e), and Pd 3d (f)
doublet peaks at 198.2 and 199.7 eV, which were belong to
Cl 2p3/2 and Cl 2p1/2, respectively. All of these observations
confrmed the successful attachment of palladium complex
on the surface of SBA-16. Another interesting observation
deriving from the XPS data was the determination of the
oxidation state of the catalytic activity site. As can be seen
in Fig. 5f, the high-resolution Pd 3d XPS spectrum exhibited
two peaks at binding energies of 337.7 and 343.1 eV, which
could be ascribed to Pd 3d5/2 and Pd 3d3/2, respectively. This
result clearly highlighted the existence of Pd (II) in the cata-
lyst which coordinated to functionalized ligand [60, 61].
1 3

Binuclear Palladium Complex Immobilized on Mesoporous SBA‑16: Efcient Heterogeneous…
Fig. 6 Nitrogen adsorption–des-
orption isotherms (a, b) and
pore size distribution isotherms
(c, d) of SBA-16 and SBA-16/
DFMP-Pd
Table 1 Textural parameters of SBA-16 and SBA-16/DFMP-Pd
the successful immobilization of palladium complex on
the surface of SBA-16. Moreover, the BJH calculations
showed a narrow pore size distribution, which confrmed
the high regularity of the mesostructure (Fig. 6c, d).
The structures of the synthesized samples were also
inspected by high resolution transmission electron micros-
copy (HRTEM). The HRTEM images of the parent SBA-
16 and SBA-16/DFMP-Pd are presented in Fig. 7. A
high-quality body-centered cubic mesostructure could be
clearly observed for both samples. This result was consist-
ent with the conclusion drawn from the nitrogen adsorp-
tion–desorption analysis and suggested that the structure
of mesoporous SBA-16 was not altered during the prepara-
tion of the catalyst.
Samples
Surface areaa
Pore volumeb
Pore
(m2 g−1)
(cm3 g−1
)
diameterc
(nm)
SBA-16
SBA-16/DFMP-Pd
745
184
0.58
0.208
4.3
3.1
aBET surface area
bTotal pore volume calculated at relative pressure p/p0 of 0.98
cCalculated based on the desorption isotherm using the BJH method
Nitrogen adsorption–desorption analysis was done to
make a detailed analysis on the structural and textural
properties of the synthesized samples. The nitrogen
adsorption–desorption isotherms and the corresponding
BJH pore-size distribution curves of SBA-16 and SBA-16/
DFMP-Pd are revealed in Fig. 6. As can be seen in Fig. 6a,
b, both samples displayed type IV behavior with a H2 hys-
teresis loop [62], indicating the retention of mesoporous
structure of SBA-16 even after the immobilization of pal-
ladium complex. Table 1 summarizes the textural param-
eters of both samples. It could be observed that the spe-
cifc surface area, pore volume, and pore diameter of pure
SBA-16 undertook remarkable decreases upon the modi-
fcation process. These remarkable changes clearly proved
Further structure evaluation of the synthesized samples
was completed with X-ray difraction (XRD) analysis. The
small-angle XRD patterns of SBA-16, SBA-16/NH2, SBA-
16/DFMP, and SBA-16/DFMP-Pd are given in Fig. 8. Each
spectrum illustrated a characteristic (110) refection peak
corresponding to the cubic Im3m mesoporous structure
and thus retained their mesoporous structure throughout
the syntheses. This further confrmed the conclusion from
the nitrogen adsorption–desorption and HRTEM analyses.
A change in the intensity and position of the refection
pattern for the (110) plane was observed upon the modi-
fcation process of SBA-16. Such changes which was also
1 3

M. Niakan et al.
Fig. 7 HRTEM images of a SBA-16 and b SBA-16/DFMP-Pd
difraction peaks related to palladium were detected in the
high-angle XRD pattern of SBA-16/DFMP-Pd (Fig. 9),
implying the formation of highly dispersed palladium spe-
cies in the pores of SBA-16.
3.2 Catalytic Performance
After the fruitful synthesis and characterization of SBA-
16/DFMP-Pd, its catalytic efciency was examined in the
carbonylative Suzuki coupling reaction of aryl iodides with
arylboronic acids. Considering the drawbacks mentioned
above with regard to performing this reaction in the presence
of gaseous CO, we decided to use transition-metal carbon-
yls as carbonyl source. In order to fnd the optimal reaction
conditions, diferent reaction parameters were screened with
iodobenzene and 4-(trifuoromethyl)phenylboronic acid as
model substrates (Table 2).
Fig. 8 Small-angle XRD patterns of a SBA-16, b SBA-16/NH2, c
SBA-16/DFMP, d SBA-16/DFMP-Pd
Initial tests addressed the efect of solvent on the model
reaction, having performed the carbonylative coupling reac-
tion in various solvents at 80 °C in the presence of 0.2 mol%
of the catalyst and using t-BuONa as base and Cr(CO)6 as
carbonyl source. The highest yield of product was observed
in dimethylformamide (DMF) and dimethylacetamide
(DMA), while a moderate yield of product was achieved in
toluene, water, ethanol (EtOH), and tetrahydrofuran (THF)
(Table 2, entries 1–6). Among DMF and DMA, DMA was
herein chosen as the optimal solvent because of the relatively
lower stability of DMF toward the heat [65]. Next, the efect
of bases was tested by employing diferent bases such as
t-BuONa, Et3N, KOAC, K2CO3, KOH, NaOAC, Na2CO3,
and NaOH (Table 2, entries 7–13). The results revealed that
t-BuONa was the best one and aforded the desired product
in 87% yield. Also, the temperature screening indicated that
decreasing the reaction temperature to 30 °C decreased the
product yield (Table 2, entry 14). But when the reaction
temperature was elevated from 50 °C to 80 °C, the product
Fig. 9 High-angle XRD pattern of SBA-16/DFMP-Pd
reported by other researchers [63, 64], could be explained
by a decrease of the degree of ordering because of the
occupation of organic groups and palladium complex in
the pores of SBA-16. It is also worth mentioning that no
1 3

Binuclear Palladium Complex Immobilized on Mesoporous SBA‑16: Efcient Heterogeneous…
Table 2 The carbonylative Suzuki coupling reaction of iodobenzene with 4-(trifuoromethyl)phenylboronic acid under diferent conditions
O
I
(HO)2B
SBA-16/DFMP-Pd, Cr(CO)6
Solvent, Base, Temperature
CF3
+
+
CF3
CF3
1a
2a
3a
4a
Entrya
Catalyst (mol%)
Base
t-BuONa
t-BuONa
t-BuONa
t-BuONa
t-BuONa
t-BuONa
Et3N
KOAC
K2CO3
KOH
NaOAC
Na2CO3
NaOH
t-BuONa
t-BuONa
t-BuONa
t-BuONa
t-BuONa
t-BuONa
t-BuONa
t-BuONa
t-BuONa
Solvent
T (°C)
Time (min)
3a (%)b
4a (%)b
1
2
3
4
5
6
7
8
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.2
0.1
0.3
none
0.2
0.2
0.2
DMA
DMF
toluene
H2O
EtOH
THF
50
50
50
50
50
50
50
50
50
50
50
50
50
30
80
100
80
80
80
80
80
80
15
15
45
60
35
40
15
15
15
15
15
15
15
15
15
15
25
87
89
66
54
71
87
45
57
52
48
73
67
65
80
93
93
92
94
9
<5
<5
<5
9
7
3
25
16
14
21
<5
<5
14
8
<5
12
<5
<5
4
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
DMA
9
10
11
12
13
14
15
16
17
18
19
20c
21d
22e
15
240
240
240
120
8
21
54
22
14
11
aReaction conditions: iodobenzene (1.0 mmol), 4-(trifuoromethyl)phenylboronic acid (1.1 mmol), base (2.0 mmol), Cr(CO)6 (0.5 mmol), and
solvent (3.0 mL)
bIsolated yields
cFe(CO)5 was used
dW(CO)6 was used
eMo(CO)6 was used
yield improved from 87 to 93% (Table 2, entry 15). Further,
no improvement was observed by increasing the temperature
to 100 °C (Table 2, entry 16). Hence, 80 °C was selected to
continue our investigation.
and reaction rate (Table 2, entry 18). However, the product
yield was lower in the presence of 0.1 mol% of the catalyst
(Table 2, entry 17). Note that the reaction process was dif-
cult to perform without the use of the catalyst and just 9% of
the product was obtained even after 240 min (Table 2, entry
19). Also, employing other transition-metal carbonyls such
as Fe(CO)5, W(CO)6, and Mo(CO)6 under the optimized
conditions was not satisfactory (Table 2, entries 20–22).
In the next set of experiments, the model reaction was
optimized with respect to the catalyst amount. It was found
that using 0.2 mol% of the catalyst was enough to exhibit
highest efciency after 0.5 h. Using the highest amounts
of the catalyst did not afect noticeably the product yield
1 3

M. Niakan et al.
due to the steric efect which hindered the reaction. Impor-
tantly, when aryl iodides were substituted with NH2 and OH
functional groups, no product arising from the coupling with
these groups were observed.
The present catalytic system also showed excellent
potential for the synthesis of heterodiaryl ketones. As can
be seen in entries 2r and 2s in Scheme 2, 2-iodothiophene
reacted with phenylboronic acid and 4-(trifuoromethyl)phe-
nylboronic acid and the expected heterodiaryl ketones were
achieved in good yields but with more reaction time. The
latter two products are important components for pharma-
ceutical and biological applications.
Fig. 10 Kinetic profles of SBA-16/DFMP-Pd catalyst in the car-
bonylative Suzuki coupling reaction. Reaction conditions: iodoben-
zene (1.0 mmol), 4-(trifuoromethyl)phenylboronic acid (1.1 mmol),
t-BuONa (2.0 mmol), Cr(CO)6 (0.5 mmol), SBA-16/DFMP-Pd
(0.2 mol%), DMA (3.0 mL), and temperature (80 °C)
Generally, the above results implied that our catalytic sys-
tem was highly efcient for the synthesis of diferent diaryl
ketones through the carbonylative Suzuki coupling reaction
of numerous aryl iodides and arylboronic acids. It is worth
noting that the yields of byproducts from the direct Suzuki
coupling reactions were not more than 5% in all of the exam-
ined reactions.
Also, the reaction kinetics of the carbonylative Suzuki
coupling reaction of iodobenzene with 4-(trifuoromethyl)
phenylboronic acid was studied. As depicted in Fig. 10,
when the reaction time was elevated from 5 to 15 min, the
isolated yield of product increased from 37 to 93%. How-
ever, by further prolonging the reaction time to 40 min,
the product yield did not increase any more, implying that
15 min was the appropriate time for this reaction.
Recyclability is a key parameter for heterogeneous cata-
lysts to be considered for their practical applications in
industry. Therefore, the recyclability experiment for SBA-
16/DFMP-Pd was performed by the model reaction under
the optimized reaction conditions (Table 1, entry 15). The
product yield was measured after 15 min in each cycle and
the catalyst was separated from the reaction mixture by cen-
trifugation. It was washed with ethanol for several times,
dried in oven under vacuum, and then reused immediately
for the next cycle. As illustrated in Fig. 11a, the catalyst
was efciently reused for seven successive catalytic cycles
without marked loss in the product yield and activity.
The stability of SBA-16/DFMP-Pd was verifed by FT-IR
spectroscopy, TGA, and ICP analyses. The FT-IR spectrum
of the catalyst after seven cycles (Fig. 11b) exhibited that all
of the functional groups were stable and no destruction was
observed after recycle tests. The TGA curve of the recycled
catalyst was found to be quite similar with the fresh one
(Fig. 11c). Moreover, the palladium content was determined
as 0.745 mmol g−1 after seven cycles based on ICP analysis,
very close to that in fresh catalyst (0.760 mmol g−1). The
obtained results clearly confrmed that the catalyst had a
long-term durability.
To gain more insight into the scope and limitation of our
method, we investigated the carbonylative Suzuki coupling
reaction of a series of diferent substrates (Scheme 2). As
listed in Scheme 2, under the optimized reaction condi-
tions, a wide range of 4-substituted aryl iodides reacted with
several arylboronic acids to give products in high to excel-
lent yields (Scheme 2, entries 2a–2l). It was observed that
whereas substitutions in the para position of arylboronic acid
rings had no infuence on the product yields, the electronic
nature of the substituents in aryl iodides had a signifcant
infuence on the product yields. Clearly, aryl iodides with
electron-withdrawing groups including NO2, Cl, and CH3CO
provided higher isolated yields compared to those with elec-
tron-donating groups (like CH3 and OCH3).
By virtue of the excellent results that obtained in the cou-
pling reaction of para-substituted aryl iodides with arylbo-
ronic acids, we next explored the applicability of this cata-
lytic system for the reaction of ortho-substituted aryl iodides
with arylboronic acids. It was observed that the ortho-sub-
stituted aryl iodides reacted efciently with phenylboronic
acid and 4-(trifuoromethyl)phenylboronic acid and gave the
corresponding diary ketones in high yields, though longer
reaction times were needed (Scheme 2, entries 2m–2q). The
increase in the reaction time for these substrates might be
To obtain proof that no palladium leaching occurred, a
hot fltration experiment for the model reaction was per-
formed. The catalyst was fltered after 37% of the coupling
product formation and the fltrate was reacted under the opti-
mized conditions for an additional 6 h. No gradual increase
in the product yield was observed after the removal of the
catalyst. Moreover, there was no detectable amount of pal-
ladium in the reaction mixture fltrate, demonstrating that
1 3

Binuclear Palladium Complex Immobilized on Mesoporous SBA‑16: Efcient Heterogeneous…
O
I
(HO)2B
SBA-16/DFMP-Pd (0.2 mol%)
+
t-BuONa , 80oC, DMA
Cr(CO)6
R2
R2
R1
R1
O
O
O
Cl
2a, 15 min, 94%
2c, 15 min, 97%
2b, 15 min, 88%
O
O
O
O
CF3
CF3
CF3
CF3
2e, 15 min, 90%
2d, 15 min, 93%
2f, 15 min, 85%
O
O
O
Cl
O2N
CF3
CF3
2g, 15 min, 97%
O
2h, 15 min, 98%
2i, 15 min, 95%
O
O
O
O
Cl
2j, 15 min, 91%
O
2k, 15 min, 84%
2l, 15 min, 95%
NH2
O
OH O
2m, 40 min, 95%
O
2n, 50 min, 91%
2o, 180 min, 91%
O
O
O
S
CF3
CF3
CF3
2p, 40 min, 96%
2q, 45 min, 97%
2r, 120 min, 94%
O
S
2s, 120 min, 97%
Scheme 2 SBA-16/DFMP-Pd-catalyzed carbonylative Suzuki
coupling reaction of diferent aryl iodides with arylboronic acids.
Reaction conditions: aryl iodides (1.0 mmol), arylboronic acids
(1.1 mmol), t-BuONa (2.0 mmol), Cr(CO)6 (0.5 mmol), SBA-16/
DFMP-Pd (0.2 mol%), and DMA (3.0 mL)
the amount of palladium leaching out was negligible. Con-
sequently, we could conclude that SBA-16/DFMP-Pd was a
heterogeneous catalyst in nature.
In order to demonstrate the merit of this catalytic pro-
tocol, a comparison was made with diferent catalytic sys-
tems reported for the carbonylative Suzuki coupling reac-
tion of iodobenzene with phenylboronic acid (Table 3). An
1 3

M. Niakan et al.
Fig. 11 a Recyclability of SBA-16/DFMP-Pd in the carbonylative Suzuki coupling reaction of iodobenzene with 4-(trifuoromethyl)phenylbo-
ronic acid under the optimized reaction conditions. b, c FT-IR spectrum and TGA curve of SBA-16/DFMP-Pd after seven reaction cycles
Table 3 A comparison of the carbonylative Suzuki coupling reaction using SBA-16/DFMP-Pd catalyst and other reported catalysts
Entry
Catalyst (mol%)
CO source
Base
Solvent
T (°C)
Time (h)
Yield (%)
Refs.
1
2
3
4
5
6
7
8
Fe3O4@SiO2-2P-PdCl2 ) 3 mol%)
Pd(OAc)2/DMAP (2.0 mol%)
P(DVB1-NDIIL1)-Pd (1.0 mol%)
PdNPs/C-SiO2 (0.05 mmol)
Pd@APGO (5.0 mol%)
ImmPd-MNPs (0.14 mol%)
HMMS-SH–PdII (1.5 mol%)
Fe3O4@SiO2-SH-Pd0 (1 mol%)
Fe3O4@PANI–PdII (1 mol%)
hollow Fe3O4-NH2-Pd (1 mol%)
Pd@[Ni(H2BDP-SO3)2] (1.0 mol%)
Pd(OAc)2 (2.0 mol%)
HCOOH
CHCl3
K2CO3
KOH
Toluene
Toluene
toluene
Dioxane
Anisole
DMF
anisole
Anisole
Anisole
Anisole
Anisole
PEG
100
80
120
120
80
80
80
80
80
90
12
12
12
10
8
4
9
8
8
12
5
4
85
75
85
100
93
92
94
91
97
90.4
33
96
94
[2]
[13]
[24]
[26]
[29]
[32]
[33]
[34]
[35]
[36]
[37]
[67]
gaseous CO
Gaseous CO
Gaseous CO
Mo(CO)6
gaseous CO
Gaseous CO
Gaseous CO
Gaseous CO
Gaseous CO
Gaseous CO
Cr(CO)6
K2CO3
Na2CO3
K2CO3
n-Bu3N
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K3PO4
t-BuONa
9
10
11
12
13
105
RT
80
SBA-16/DFMP-Pd (0.2 mol%)
DMA
0.25
[This work]
examination of the data in Table 3 revealed that the catalytic
activity of this catalytic system was better or comparable
than the other reported systems. The reported catalytic sys-
tems required longer reaction time, higher amount of cata-
lyst, and some of them were homogeneous in nature. More
importantly, this catalytic system avoided the use of gaseous
CO as carbonyl source, which made it more environmentally
friendly than others. Finally, a plausible mechanism for the
carbonylative Suzuki coupling reaction, based on previous
reports, was proposed in Scheme 3 [7, 66].
1 3

Binuclear Palladium Complex Immobilized on Mesoporous SBA‑16: Efcient Heterogeneous…
Scheme 3 Proposed mecha-
nism for the carbonylative
N
O
N
(II)
Pd
(II)
Pd
Suzuki coupling reaction in the
presence of SBA-16/DFMP-Pd
catalyst
Cl
Cl
Cl
Preactivation
O
Ar
Ar
N
O
N
(0)
Pd
(0)
Pd
ArX
N
O
N
O
N
N
(II)
Pd
(II)
Pd
(II)
Pd
(II)
Pd
Ar
Ar
C
Ar
X
Ar
X
Ar
Ar
C
O
O
XB(OH)2
CO
ArB(OH)2
+ Base
Heat
Cr(CO)6
N
O
N
(II)
Pd
(II)
Pd
Ar
Ar
C
O
X
C
O
X
4 Conclusions
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