Palladium imine-pyridine-imine complex immobilized on graphene oxide as a recyclable catalyst for the carbonylative homo-coupling of aryl halides

Article abstract of DOI:10.1080/00958972.2020.1861257

A heterogeneous 3-N,N,N-(II) Pd(OAc)₂ catalyst was prepared from the reaction of Pd(OAc)₂ with Si-Prn-N = C-Py-C = N-Prn-Si immobilized on graphene oxide (GO-Si-Prn-N = C-Py-C = N-Prn-Si-GO). The prepared catalyst was characterized by inductively coupled plasma optical emission spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, Raman spectroscopy, energy dispersive X-ray spectroscopy, X-ray diffraction, UV-vis spectroscopy, BET surface area, scanning electron microscopy, transmission electron microscopy, and thermogravimetric analysis. The catalyst was employed as a heterogeneous catalyst for carbonylative homo-coupling of aryl iodides and bromides under carbon monoxide gas-free condition. Cr(CO)₆ was used as the carbon monoxide source and the desired symmetrical diaryl ketones were achieved in good to excellent yields. Moreover, the catalyst was reused up to five consecutive cycles without significant loss of activity.

Full text of DOI:10.1080/00958972.2020.1861257

Journal of Coordination Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Palladium imine-pyridine-imine complex
immobilized on graphene oxide as a recyclable
catalyst for the carbonylative homo-coupling of
aryl halides
Mahsa Niakan , Zahra Asadi & Fatemeh Khosrozadeh
To cite this article: Mahsa Niakan , Zahra Asadi & Fatemeh Khosrozadeh (2020): Palladium
imine-pyridine-imine complex immobilized on graphene oxide as a recyclable catalyst for
the carbonylative homo-coupling of aryl halides, Journal of Coordination Chemistry, DOI:
10.1080/00958972.2020.1861257
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JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2020.1861257
Palladium imine-pyridine-imine complex immobilized
on graphene oxide as a recyclable catalyst for the
carbonylative homo-coupling of aryl halides
Mahsa Niakan, Zahra Asadi and Fatemeh Khosrozadeh
Department of Chemistry, School of Sciences, Shiraz University, Shiraz, Iran
ABSTRACT
ARTICLE HISTORY
Received 16 June 2020
Accepted 26 October 2020
A heterogeneous 3-N,N,N-(II) Pd(OAc)2 catalyst was prepared from
the reaction of Pd(OAc)2 with Si-Prn-N¼ C-Py-C¼ N-Prn-Si immo-
bilized on graphene oxide (GO-Si-Prn-N ¼ C-Py-C ¼ N-Prn-Si-GO).
The prepared catalyst was characterized by inductively coupled
plasma optical emission spectroscopy, Fourier transform infrared
(FT-IR) spectroscopy, Raman spectroscopy, energy dispersive X-ray
spectroscopy, X-ray diffraction, UV-vis spectroscopy, BET surface
area, scanning electron microscopy, transmission electron micros-
copy, and thermogravimetric analysis. The catalyst was employed
as a heterogeneous catalyst for carbonylative homo-coupling of
aryl iodides and bromides under carbon monoxide gas-free condi-
tion. Cr(CO)6 was used as the carbon monoxide source and the
desired symmetrical diaryl ketones were achieved in good to
excellent yields. Moreover, the catalyst was reused up to five con-
secutive cycles without significant loss of activity.
KEYWORDS
Carbonylative homo-
coupling reaction; Cr(CO)6;
graphene oxide; palladium
catalyst; symmetrical
diaryl ketones
1. Introduction
Pd-catalyzed carbonylative reactions are valuable strategies in organic synthesis
because of their applicability in the preparation of a variety of carbonylated functional-
ities including aldehydes, carboxylic acids, ketones, esters, and amides [1–8]. Such
CONTACT Zahra Asadi
zasadi@shirazu.ac.ir
Department of Chemistry, School of Sciences, Shiraz University,
Shiraz 71454, Iran
Supplemental data for this article is available online at https://doi.org/10.1080/00958972.2020.1861257.
ß 2020 Informa UK Limited, trading as Taylor & Francis Group

2
M. NIAKAN ET AL.
compounds have attracted enormous interest since they are multipurpose building
blocks for many biologically active molecules, natural products, and pharmaceuticals
[9,10]. The carbonylative reactions generally proceed in the presence of gaseous CO as
the carbon monoxide source. However, gaseous CO is toxic, odorless, flammable, and
difficult to handle, which make these protocols difficult to use in practical applications
[11,12]. As a result, recently, efforts have been made to use other carbonyl substitu-
ents such as chloroform, formic acid, N-formylsaccharin, aldehydes, transition-metal
carbonyls, oxalic acid, and DMF as the CO source in these transformations [13–15].
Among them, transition-metal carbonyls are much more desirable for their easy han-
dling and in-situ generation of CO upon heating [16,17].
One of the most important Pd-catalyzed carbonylative reactions is the carbonylative
homo-coupling of aryl halides, which is a versatile and straightforward method for the
synthesis of valuable symmetrical diaryl ketones. Researchers have developed different
gaseous CO-free Pd-catalyst systems for this important transformation. For example, in
2003, Larhed et al. used Co2(CO)8 as the CO source for the synthesis of symmetrical
diaryl ketones from aryl iodides [18]. In 2016, Iranpoor’s group synthesized symmetrical
diaryl ketones via carbonylative homo-coupling of aryl iodides in the presence of
Cr(CO)6 as the CO source [19]. Very recently, Wu and co-workers showed an efficient
protocol for the synthesis of symmetrical diaryl ketones starting from aryl iodides with
the use of formic acid as the CO source [20]. In spite of their high efficiency, all of
these systems require homogeneous Pd catalysts.
The recovery and recycling of the expensive Pd catalysts used in coupling reactions are
important from economic and environmental perspectives [21–23]. Therefore, many
researchers have been trying to develop efficient heterogeneous Pd-catalyst systems for
these reactions by immobilization of homogeneous Pd complexes onto an array of solid
support materials [24–31]. Among the solid supports, graphene oxide (GO) has attracted
particular attention as catalyst support owing to its outstanding properties like 2D struc-
ture, high surface area, excellent mechanical strength, intrinsic low mass, and high water
dispersibility [32–39]. Various oxygen-containing functional groups present on basal
planes and edges of GO promote its easy surface modification and subsequently the
immobilization of homogeneous catalysts through covalent bond to the support [40–43].
In coordination chemistry and catalysis, nitrogen donor pincer ligands are important
N-based ligands, as they offer high stability towards heat, air, and moisture.
Furthermore, metal complexes possessing nitrogen donor pincer ligands present high
stability owing to the formation of two five-membered metallacycles with a tridentate
nitrogen ligand which provides additional stabilization of the nitrogen-metal bond
[15]. Especially, bis(imino)pyridines with a central nitrogen donor and flanking imine
arms which are known as NNN tridentate pincer ligands can coordinate easily to tran-
sition metals [44]. In the present article, we decided to prepare a NNN pincer Pd com-
plex supported on GO and illustrated its application as catalyst for the carbonylative
homo-coupling reactions of aryl iodides and bromides. The catalyst was efficient in
the carbonylative homo-coupling reactions of different aryl iodides and bromides in
ethanol at 75 ꢀC with Cr(CO)6 as the CO source. It is important to note that the pre-
pared heterogeneous catalyst could be recovered and reused for up to five runs with-
out major loss in activity.

JOURNAL OF COORDINATION CHEMISTRY
3
2. Experimental
2.1. General
All chemicals and solvents were purchased from commercial sources and used without
purifications.
Inductively coupled plasma optical emission spectroscopy (ICP-OES, Varian, Vista-
pro) was used to measure the Pd content in the prepared catalyst. Fourier transform
infrared (FT-IR) spectra were recorded on a Shimadzu FT-IR-8300 spectrophotometer
using KBr pellets. Raman spectrum of the catalyst was collected using a Horiba (Lab
RAM HR Evolution) Raman spectrometer. The X-ray diffraction (XRD) patterns of the
samples were collected on a Panalytical X Pert-Pro using Cu-Ka radiation. UV-vis dif-
fuse reflectance absorption spectrum of the catalyst was measured on a 2550
Shimadzu UV-vis spectrophotometer. Nitrogen adsorption-desorption analysis was car-
ried out using a Belsorp-mini (II) gas analyzer at 77 K. The samples were degassed in
vacuum at 150 ꢀC for 12 h before the measurement. The Brunauer-Emmett-Teller (BET)
method was used to analyze surface area. Scanning electron microscopy (SEM) images
with energy dispersive X-ray (EDX) spectroscopy pattern were collected using a
TESCAN-Vega3 electron microscope at an acceleration voltage of 15 kV. Transmission
electron microscopy (TEM) micrographs were taken with a Zeiss-EM10C instrument at
an acceleration voltage of 200 kV. Thermogravimetric analysis (TGA) was conducted on
a Mettler TGA/STTA 851 instrument under an Ar flow with heating rate of 20 ꢀC minꢁ1
from room temperature to 800 ꢀC. The reaction monitoring was accomplished by thin
1
layer chromatography (TLC) on silica gel Polygrams SILG/UV254 plates. The H NMR
spectra were recorded on a Bruker Avance III spectrometer (400 MHz) in CDCl3 for
checking the final products. Melting points were determined with a Buchi 510 instru-
ment in open capillary tubes and are uncorrected.
2.2. Synthesis of the catalyst
2.2.1. Immobilization of 3-aminopropyltriethoxysilane on GO
GO was synthesized according to the improved Hummers method [45]. A mixture of
the as-prepared GO (1 g) and 3-aminopropyltriethoxysilane (3-APTES, 1 mL) in 50 mL of
dry toluene was stirred at reflux under nitrogen for 24 h. The obtained solid was col-
lected by filtration, washed with ethanol and purified by Soxhlet extraction with a mix-
ture of ethanol and dichloromethane (1:1) for 12 h. Finally, the product was dried in a
vacuum at 80 ꢀC for 12 h to obtain GO-Si-Prn-NH2 (1).
2.2.2. Reaction of 1 with pyridine-2,6-dicarbaldehyde to form GO-Si-Prn-N 5 C-Py-
C 5 N-Prn-Si-GO
A solution of pyridine-2,6-dicarbaldehyde (3 mmol, 0.4 g) in 20 mL of methanol was
added to a suspension of 1 (1 g) in 30 mL of methanol. The reaction mixture was
refluxed at 60 ꢀC for 12 h. Then, the resultant solid was filtered, washed thoroughly
with methanol and dried in a vacuum at 80 ꢀC for 12 h to form GO-Si-Prn-N ¼ C-Py-
C ¼ N-Prn-Si-GO (2).

4
M. NIAKAN ET AL.
2.2.3. Synthesis of 3-N,N,N-(II)Pd(OAc)2 catalyst (3)
1 g of 2 and 0.1 g of Pd(OAc)2 (0.5 mmol) in 30 mL of acetone was stirred at room tem-
perature for 24 h. The obtained 3-N,N,N-(II)Pd(OAc)2 catalyst (3) was separated from
the mixture by filtration, washed several times with acetone and dried in a vacuum at
80 ꢀC for 12 h.
2.3. Synthesis of Si-Prn-N ¼ C-Py-C ¼ N-Prn-Si
To a vigorously stirred solution of pyridine-2,6-dicarbaldehyde (1 mmol, 0.14 g) in
10 mL of ethanol, a stoichiometric amount of 3-APTES (2 mmol, 0.47 mL) was added
dropwise. A red solution, which was obtained due to the imine formation, was
refluxed for 6 h. The resulting pure precipitate was extracted from oily liquid by n-hex-
ane and ether and then dried in a vacuum. FT-IR (Supplementary information Figure
S1a, KBr pellets, cmꢁ1): 3021 (C–H stretching), 1624 (C ¼ N), 1563 (C ¼ C Aromatic),
1318 (C–N). Elemental analysis: Anal Calcd (%) for C25H47N3O6Si2: C, 55.42; H, 8.74; N,
7.76. Found: C, 55.53; H, 8.79; N, 7.81.
2.4. Synthesis of [Si-Prn-N ¼ C-Py-C ¼ N-Prn-Si]Pd(OAc)2
1 mmol of palladium acetate (0.2 g) was dissolved in 10 mL of absolute ethanol and
poured into 10 mL of hot ethanolic solution of Si-Prn-N ¼ C-Py-C ¼ N-Prn-Si (1 mmol,
0.51 g). The mixture was stirred and refluxed for 5 h under nitrogen. During the reaction,
a dark red solid formed. The resulting solid was filtered, washed with hot ethanol and
diethyl ether in alternating steps and then dried in a vacuum. FT-IR (Supplementary
information Figure S1b, KBr pellets, cmꢁ1): 3038 (C–H stretching), 1605 (C ¼ N), 1569
(C ¼ C Aromatic), 1354 (C–N), 742 (Pd–N). Elemental analysis: Anal Calcd (%) for
C30H55N3O10Si2Pd: C, 46.17; H, 7.10; N, 5.38. Found: C, 46.32; H, 7.17; N, 5.45.
2.5. General procedure for the carbonylative homo-coupling of aryl halides
Aryl halide (2 mmol), K2CO3 (2 mmol, 0.276 g), Cr(CO)6 (1 mmol, 0.22 g) and 3 (2 mol%,
0.08 g) were taken in a round-bottomed flask. To the reaction mixture, EtOH (3 mL)
was added and stirred at 75 ꢀC in an oil bath for the desired time. After completion of
the reaction which was monitored by TLC, the reaction mixture was cooled to room
temperature and the catalyst was separated by simple filtration. Afterwards, the filtrate
was extracted with water and ethyl acetate. The combined organic fractions were
dried over Na2SO4 and concentrated by reduced pressure. The residue was purified by
column chromatography on silica gel using n-hexane/ethyl acetate gradient (20/1) to
obtain the desired symmetrical diaryl ketones.
3. Results and discussion
3.1. Synthesis and characterization of the catalyst
The catalyst was synthesized according to the synthetic route in Scheme 1. Treatment
of GO with 3-APTES in refluxing dry toluene for 24 h provided the amino-

JOURNAL OF COORDINATION CHEMISTRY
5
Scheme 1. The synthetic route for 3.
functionalized GO (1). Immobilization of bis(imino)pyridine was performed through
condensation of pyridine-2,6-dicarbaldehyde with the amino-functionalized GO to
form supported bis(imino)pyridine (2). Then, the catalyst (3) was prepared by complex-
ation of the supported bis(imino)pyridine ligands with Pd(OAc)2 precursor in acetone
at room temperature for 24 h. From the ICP-OES analysis, the exact amount of Pd in
the prepared catalyst was 0.25 mmol gꢁ1
.
Fourier transform infrared (FT-IR) spectra of the pristine GO and the modified mate-
rials are presented in Figure 1. The FT-IR spectrum of GO (Figure 1a) exhibited bands
located at 3416, 1723, 1607, 1225, and 1052 cmꢁ1, which were attributed to the
stretching modes of O-H, C ¼ O, C ¼ C, C–O, and C–O–C bonds, respectively [45,46].
Compared with the spectrum of GO, there were new broad bands at 2870–2930 cmꢁ1

6
M. NIAKAN ET AL.
Figure 1. FT-IR spectra of GO (a), 1 (b), 2 (c) and 3 (d).
clearly appearing in the spectrum of 1 (Figure 1b), due to the aliphatic stretching
vibrations of propyl chains. This proved that 3-APTES was grafted onto the GO surface.
Moreover, immobilization of bis(imino)pyridine on the surface of 1 was confirmed by
the observed band at 1642 cmꢁ1 in the spectrum of 2, which could be attributed to
the C ¼ N stretching vibrations of imine groups (Figure 1c). Upon immobilization of
Pd(OAc)2 on 2, the C ¼ N band shifted to lower wavenumber (1631 cmꢁ1) (Figure 1d),
implying strong metal-ligand interactions [47]. Moreover, Raman spectrum of 3
(Supplementary information Figure S2) showed two characteristic peaks at 1316 and
1601 cmꢁ1 corresponding to the D and G bands of GO, respectively. The value of ID/IG
ratio of the catalyst (1.7) was higher than that of GO (reported elsewhere), due to
increasing disorder in the GO lattice after immobilization of NNN pincer Pd com-
plex [37].
Energy dispersive X-ray (EDX) spectroscopy analysis performed on the surface of 3
(Figure 2) demonstrated the existence of C, N, O, Si, and Pd, which was a clear indica-
tion of the immobilization of NNN pincer Pd complex on the surface of GO.
Additionally, the EDX elemental mapping of the catalyst was carried out to obtain
elemental distribution of C, N, O, Si, and Pd in its structure. As presented in
Supplementary information Figure S3, all of the elements were uniformly distributed
in the catalyst texture.
The X-ray diffraction (XRD) patterns of GO and 3 are shown in Figure 3. The XRD
pattern of GO in Figure 3a revealed a strong diffraction peak at 2h ¼ 11.3ꢀ with a cor-
responding d-spacing of 0.78 nm related to the (001) reflection plane, which was
owing to the various oxygen containing functional groups in the basal plane of GO
causing increase in the interlayer distance [34, 37, 48]. After the surface functionaliza-
tion of GO with 3-APTES, immobilization of bis(imino)pyridine and finally the synthesis
of 3, the peak indexed to the (001) diffraction disappeared and a new weak and broad
peak at 2h ¼ 22.4ꢀ corresponding to the (002) plane of the reduced GO appeared

JOURNAL OF COORDINATION CHEMISTRY
7
Figure 2. The EDX analysis of 3.
Figure 3. XRD patterns of GO (a) and 3 (b).
(Figure 3b), demonstrating that the oxygen containing groups of GO were functional-
ized and the catalyst was prepared [49]. No characteristic peaks of Pd were observed
in the XRD pattern of the catalyst, suggesting the fine dispersion of Pd and the
absence of metallic Pd in the catalyst. Also, the presence of the peak at 410–420 nm
in the UV-vis spectrum of the catalyst (Supplementary information Figure S4), which
was assigned to the d-d transition of Pd, confirmed the oxidation state þ2 for
Pd center.
The nitrogen adsorption-desorption isotherms of GO and 3 are plotted in Figure 4.
Both samples showed typical nitrogen adsorption-desorption type IV profile with a
hysteresis loop, indicative of the presence of mesoporosity [26]. The pristine GO was
found to have a BET surface area of 237 m2 gꢁ1. For 3, the BET surface area decreased
to 22 m2 gꢁ1. This significant decrease in surface area from GO to 3 confirmed the
immobilization of NNN pincer Pd complex on the surface of GO nanosheets.

8
M. NIAKAN ET AL.
Figure 4. Nitrogen adsorption-desorption isotherms of GO (a) and 3 (b).
From the SEM images shown in Supplementary information Figures S5a and S5b, it
can be seen that pristine GO and 3 both possessed stacked layered morphology. TEM
analysis was employed to obtain more information about their structures and mor-
phologies. The TEM images of the pristine GO and 3 indicated that both samples had
sheet-like and layered structures (Supplementary information Figures S5c and S5d).
However, some agglomeration and crumpling of the nanolayers were observed in the
TEM image of the catalyst, which was from functionalization of the surface of GO with
3-APTES, bis(imino)pyridine and Pd complex. These results demonstrated that the
structure of GO was preserved during the synthesis of the catalyst.

JOURNAL OF COORDINATION CHEMISTRY
9
Table 1. Optimization of the carbonylative homo-coupling reaction of 4-iodoanisole cata-
lyzed by 3.
Temperature Time
Entry
Catalyst (mol)%
Base
CO source Solvent
(ꢀC)
(h)
2a %a 3a %b
1
2
3
4
5
6
7
8
2
3
1
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
KOAc
Cr(CO)6
Cr(CO)6
Cr(CO)6
Cr(CO)6
Cr(CO)6
Cr(CO)6
Fe(CO)5
W(CO)6
Mo(CO)6
Cr(CO)6
Cr(CO)6
Cr(CO)6
Cr(CO)6
Cr(CO)6
Cr(CO)6
Cr(CO)6
Cr(CO)6
Cr(CO)6
Cr(CO)6
Cr(CO)6
Cr(CO)6
DMF
DMF
DMF
DMF
DMF
Cr(CO)6
DMF
DMF
75
75
75
75
75
75
75
75
75
75
75
75
75
60
90
75
75
75
75
75
75
6
6
6
12
12
12
6
6
6
6
13
8
18
10
6
6
6
6
6
6
5
84
84
67
0
0
0
7
10
7
0
0
None
GO (80 mg)
2 (80 mg)
0
2
2
2
2
2
2
2
2
2
2
2
2
2
2
24
56
71
86
53
78
32
81
86
79
71
66
75
43
93
15
10
<5
<5
13
20
<5
<5
11
<5
10
10
28
36
<5
9
DMF
10
11
12
13
14
15
16
17
18
19
20
21
EtOH
toluene
DMSO
H2O
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
Na2CO3
NaOAc
t-BuONa
Et3N
[Si-Prn-N ¼ C-Py-C ¼ N-Prn-Si]
K2CO3
Pd(OAc)2 (2)
aReaction conditions: 4-iodoanisole (2 mmol), base (2 mmol), CO source (1 mmol), and solvent (3 mL).
bIsolated yields.
Thermogravimetric analysis (TGA) of the catalyst was conducted. A representative
TGA curve of 3 is shown in Supplementary information Figure S6. The catalyst
revealed a weight loss to 150 ꢀC due to removal of physically adsorbed water. A sig-
nificant weight loss was observed from 150 to 800 ꢀC, attributed to the thermal
decomposition of organic moieties on the surface of the catalyst. The TGA of 3 indi-
cated that it has high thermal stability.
3.2. Catalytic activity
After full characterization of 3, it was used as a heterogeneous catalyst for the carbon-
ylative homo-coupling reaction.
The carbonylative homo-coupling reaction of 4-iodoanisole was selected as model
reaction in order to determine the optimal reaction conditions, and the results are
shown in Table 1. Initially, we tested the reaction of 4-iodoanisole in DMF at 75 ꢀC
using K2CO3 as the base and 2 mol% of the catalyst, in the presence of Cr(CO)6 as the
CO source. Under this condition, 4,4-dimethoxy benzophenone product was achieved
in 84% yield after 6 h (Table 1, entry 1). As shown in entries 2 and 3 in Table 1, with
increasing the amount of catalyst (3 mol%), the desired product was obtained in a
similar yield whereas decreasing the catalyst loading to 1 mol% decreased the product

10
M. NIAKAN ET AL.
Table 2. Comparison of the results for the synthesis of 4,4-dimethyl benzophenone using GO-
NNN pincer-Pd catalyst with those obtained by the reported catalysts.
Entry
Catalyst
CO Source
Solvent
T (ꢀC)
Time (h)
Yield (%)
Ref.
1
2
3
4
5
[(cinnamyl)PdCl]2
Pd(Cl)2
NiBr2
HCO2H
Cr(CO)6
Mo(CO)6
DMF
THF
PEG
1,4-dioxane
DMF
EtOH
120
90
120
100
75
24
3
6
14
6
60
94
78
88
81
[20]
[19]
[13]
NiBr2ꢂdiglyme
[50]
GO-NNN pincer-Pd
Cr(CO)6
[this work]
yield remarkably. The reaction was carried out without the catalyst and in the pres-
ence of GO and 2, but the corresponding product was not obtained (Table 1, entries
4-6). The influence of the CO source was also checked. While various other metal car-
bonyls including Fe(CO)5, W(CO)6, and Mo(CO)6 could perform this reaction, their effi-
ciencies were lower than that of Cr(CO)6 (Table 1, entries 7–9). We then scanned
various solvents (Table 1, entries 10–13) and found that both DMF and EtOH provided
the best yields. Since EtOH is a green and inexpensive solvent, it was chosen for con-
tinuing the optimization process. After choosing EtOH as the solvent, we examined
other reaction temperatures. When the reaction was carried out at 60 ꢀC, the product
yield did not change but the reaction times increased significantly (Table 1, entry 14).
However, increasing the temperature to 90 ꢀC did not affect the resulting yield and
time of the reaction (Table 1, entry 15). Consequently, 75 ꢀC was chosen as the opti-
mized reaction temperature. Under the optimized reaction solvent and temperature, a
variety of organic and inorganic bases such as K2CO3, KOAc, Na2CO3, NaOAc, t-BuONa,
and Et3N were screened (Table 1, entries 16–20). Among them, K2CO3 was the best.
Finally, the described conditions in Table 1, entry 10, were chosen as the optimum
conditions for further experiments.
For comparison, NNN pincer Pd complex without GO ([Si-Prn-N ¼ C-Py-C ¼ N-Prn-
Si]Pd(OAc)2) was prepared by direct condensation of 3-aminopropyltriethoxysilane and
pyridine-2,6-dicarbaldehyde, followed by treatment with Pd(OAc)2. The model reaction
was also performed in the presence of [Si-Prn-N ¼ C-Py-C ¼ N-Prn-Si]Pd(OAc)2 under
the optimized reaction conditions. Interestingly, high yield of product (93%) was
achieved after 5 h (Table 2, entry 21). Comparison of the results clearly showed that
the reaction with [Si-Prn-N ¼ C-Py-C ¼ N-Prn-Si]Pd(OAc)2 proceeded much better than
with 3. However, it is difficult to separate the product from the catalyst and reuse
complex. Considering this intrinsic limitation of homogeneous catalysts, in addition to
environmental and economical concerns (Pd is an expensive metal), we decided to
apply 3 as catalyst for exploring the scope and limitation of the reaction.
With the optimized conditions, we explored the scope of the reaction with a variety
of aryl iodides and bromides. The results are shown in Scheme 2. As illustrated in
Scheme 2, the reactions of aryl iodides and bromides bearing both electron-withdraw-
ing and electron-donating groups at para position provided the corresponding sym-
metrical diaryl ketones in good to excellent yields. Generally, electron-deficient
substrates (Scheme 2, entries 2b–2d) gave higher yields than electron-rich ones
(Scheme 2, entries 2e and 2f). Further studies indicated that ortho-substituted aryl hal-
ides, which are sterically hindered, offered good yields (Scheme 2, entries 2g–2i).
However, they required longer reaction times than their para-substituted aryl halides.
Consequently, both steric and electronic effects of the benzene ring had influence on

JOURNAL OF COORDINATION CHEMISTRY
11
Scheme 2. Substrate scope of the carbonylative homo-coupling reaction of aryl halides. Reaction
conditions: aryl halide (2 mmol), K2CO3 (2 mmol), Cr(CO)6 (1 mmol), catalyst (3) (2 mol%, 0.08 g),
EtOH (3 mL), 75 ꢀC. Yields were isolated.
the reaction yield and time. Heterocyclic aryl iodide was also tolerant in this reaction
and exhibited good yield as exemplified by entry 2j in Scheme 2. As can be seen from
Scheme 2, the competing reaction of aryl bromides was slower than aryl iodides.
Traces (2–5%) of byproducts from the direct homo-coupling reactions of aryl iodides
were detected in all of the tested experiments.
We compared the results obtained in this work with those reported elsewhere for
synthesis of 4,4-dimethyl benzophenone. The results are summarized in Table 2. The
present catalyst showed good catalytic activity for the synthesis of 4,4-dimethyl benzo-
phenone in terms of yield and reaction conditions. Although some of them also

12
M. NIAKAN ET AL.
Figure 5. The reusability of 3 for the carbonylative homo-coupling of 4-iodoanisole under the opti-
mized conditions.
obtained high yields, they are unfortunately associated with many obstacles in prac-
tical applications, in terms of catalyst separation and recycling.
3.3. Reusability and stability of the catalyst
Reusability and stability of the catalysts are two crucial parameters for their potential
applications in industry. Therefore, the recyclability of 3 was examined by performing
the carbonylative homo-coupling of 4-iodoanisole under the optimized reaction condi-
tions (Table 1, entry 8). At the end of each catalytic cycle, the catalyst was recovered
from the reaction mixture through filtration, washed thoroughly with ethanol and
water, and then dried for the next run. Little loss in catalytic activity could be
observed for the catalyst after five cycles (Figure 5). To further examine the stability of
the catalyst, it was characterized by FT-IR spectroscopy and ICP-OES analysis after five
consecutive times. The FT-IR spectrum of the used catalyst (Supplementary informa-
tion Figure S7) implied that its structure did not change after recycling. Moreover, ICP-
OES analysis result demonstrated that only minimal leaching of Pd from the catalyst
occurred after five runs (less than 1.5%).
4. Conclusion
A nanohybride material was prepared by immobilization of a NNN pincer ligand on
the organo-functionalized surface of GO and subsequent complexation with Pd(OAc)2.
The prepared nanohybride was a highly active and stable heterogeneous catalyst for
carbonylative homo-coupling of aryl iodides and bromides using Cr(CO)6 as the carbon
monoxide source. To see viability and scope of the present methodology, a variety of
aryl iodides and bromides were used and in all cases the desired symmetrical diaryl
ketones were obtained in good to excellent yields. The catalyst also revealed outstand-
ing stability and recyclability, could be readily recovered by filtration and reused for
five cycles without decrease of its catalytic efficiency.

JOURNAL OF COORDINATION CHEMISTRY
13
Acknowledgements
We gratefully acknowledge the financial support received from the Research Council of
Shiraz University.
Disclosure statement
No potential conflict of interest was reported by the authors.
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