Covalently cyclopalladium(II) complex/reduced-graphene oxide as the effective catalyst for the Suzuki–Miyaura reaction at room temperature

Article abstract of DOI:10.1016/j.jorganchem.2016.11.016

In this study, cyclometallated palladium complexes, [Pd(C?N)Cl(L)], in which C?N = N(1), C(2′)-chelated deprotonated 2-phenylpyridine, ppy, and L = dmso (dimethylsulfoxide) (complex 1) or PPh₃(complex 2), have been synthesized. Also, complex 1

Full text of DOI:10.1016/j.jorganchem.2016.11.016

Accepted Manuscript
Covalently cyclopalladium(II) complex/reduced-graphene oxide as the effective
catalyst for the Suzuki–Miyaura reaction at room temperature
Roghayeh Hashemi Fath, S. Jafar Hoseini
PII:
S0022-328X(16)30520-4
10.1016/j.jorganchem.2016.11.016
JOM 19701
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 17 July 2016
Revised Date: 7 November 2016
Accepted Date: 8 November 2016
Please cite this article as: R.H. Fath, S.J. Hoseini, Covalently cyclopalladium(II) complex/reduced-
graphene oxide as the effective catalyst for the Suzuki–Miyaura reaction at room temperature, Journal of
Organometallic Chemistry (2016), doi: 10.1016/j.jorganchem.2016.11.016.
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ACCEPTED MANUSCRIPT
Covalently cyclopalladium(II) complex / reduced-graphene oxide as the
effective catalyst for the Suzuki–Miyaura reaction at room temperature
∗
Roghayeh Hashemi Fath, S. Jafar Hoseini
Department of Chemistry, Faculty of Sciences, Yasouj University, Yasouj 7591874831, Iran
A novel cyclometalated organopalladium(II) complex covalently functionalized graphene oxide
nanohybrid has been successfully synthesized and was used as efficient catalyst in the Suzuki–
Miyaura coupling reaction.
∗
Corresponding author: E-mail: jhosseini@ yu.ac.ir, sjhoseini54@yahoo.com, Fax: +98 74 33342172; Tel: +98 74
3
3223048.

ACCEPTED MANUSCRIPT
Covalently cyclopalladium(II) complex / reduced-graphene oxide as the
effective catalyst for the Suzuki–Miyaura reaction at room temperature
∗
Roghayeh Hashemi Fath, S. Jafar Hoseini
Department of Chemistry, Faculty of Sciences, Yasouj University, Yasouj 7591874831, Iran
ABSTRACT
In this study, cyclometallated palladium complexes, [Pd(C^N)Cl(L)], in which C^N = N(1),
C(2′)-chelated deprotonated 2-phenylpyridine, ppy, and L = dmso (dimethylsulfoxide) (complex
1
) or PPh (complex 2), have been synthesized. Also, complex 1 attached to amine functionalized
3
graphene oxide (1-f-GO) has been synthesized by covalently bonding of complex 1 onto the
graphene oxide surface through coordination interaction with 3-(aminomethyl)pyridine ligand
spacers. The supported catalyst showed enhanced catalytic performance towards Suzuki–
Miyaura reaction of aryl halides and phenylboronic acid at room temperature compared with the
homogeneous analogues, and they could be readily recycled and reused several times without
discernible loss of their activity.
Keywords: Graphene oxide; Palladium nanoparticle; Organopalladium complex; Catalyst;
Suzuki–Miyaura reaction
1
. Introduction
The Suzuki–Miyaura reaction is routinely used in the pharmaceutical industry for the
assembly of biaryl synthetic targets, and this transformation is overwhelmingly the most
common Pd-catalyzed coupling for the large scale synthesis of drug candidates [1]. Palladacycle
compounds identified as important intermediates in palladium mediated organic synthesis [2,3]
and more recently due to their unique physical properties, these compounds have experienced a
renaissance that has been fundamental in the recent development of homogeneous catalysis. This
is particularly true in the case of C−C cross coupling reactions [3-5]. In general, these
compounds can be synthesized in a very facile manner, making it possible to modulate both their
steric and electronic properties [1, 3-5]. However, the practical applications of these effective
∗
Corresponding author: E-mail: jhosseini@ yu.ac.ir, sjhoseini54@yahoo.com, Fax: +98 74 33342172; Tel: +98 74
3
3223048.
1

ACCEPTED MANUSCRIPT
catalysts are seriously limited, because separating them from the homogeneous reaction mixture
remains a difficulty. So the recovery of these expensive and toxic complexes is crucial for both
economic and environmental reasons. To overcome this separation problem, many research
groups have focused on immobilization of a catalyst onto a solid surface that can be efficiently
recovered from the reaction mixture by filtration or centrifugation and reused [6-12].
Graphene oxide (GO) platelets have chemically reactive oxygen functionality, such as
carboxylic acid groups at their edges, and epoxy and hydroxyl groups on the basal planes. GO
has very often been selected as the starting material for the formation of graphene derivatives
through the covalent attachment of organic groups on its surface due to the rich content of
hydroxyl, carboxyl, and epoxy groups [13]. One of the advantages of the GO is its easy
dispersibility in water and other organic solvents. An ideal approach to the chemical
modification of GO would utilize orthogonal reactions of these groups to selectively
functionalize one site over another [14-16]. Its two-dimensional structure with a huge surface
area makes it a promising support for different nanoparticles (NPs) and complexes [17-21]. In
particular, the two-dimensional structure may allow reactive species on it readily accessed with
limited mass transfer resistance. Based on these considerations, we predict that GO may be used
as a desirable support for anchoring palladacycle compounds.
Numerous reports have been published in the last five years with respect to the excellent
catalytic properties including recyclability of different palladium complexes immobilized on
different supports [22-27]. Recently, Wang et al, have reported the immobilization of
palladium(II) on GO using N-heterocyclic carbene ligand-spacer [24]. El Shall et al have
reported the cross-coupling catalytic activity of Pd nanoclusters on graphene with high turnover
number (TON) and turnover frequency (TOF) and without the need for the ligand support system
[25]. Peris et al describe the synthesis of an imidazolium salt with a pyrene tag that allows the
preparation of palladium and ruthenium complexes that have been immobilized onto the reduced
graphene oxide (RGO) surface by π-stacking interactions [26]. The supported Pd catalyst was
prepared by the in situ coordination between Pd(II) and aminosilane ligand that was covalently
immobilized on GO via the one-pot silylation, have been studied by Bai et al [27]. In this study,
we demonstrate an effective method for the synthesis of a supported palladacycle catalyst
covalently anchored onto GO.
2

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The amine-functionalized GO (f-GO) was prepared by reaction between GO and 3-
aminomethyl)pyridine (3-ampy). Two palladium complexes [Pd(C^N)Cl(dmso)], 1, and
(
[
Pd(C^N)Cl(PPh )], 2, in which C^N = N(1), C(2′)-chelated deprotonated 2-phenylpyridine and
3
dmso = dimethylsulfoxide, were synthesized and the complex 1 was covalently anchored onto
the GO surface through coordination interaction by employing 3-ampy ligand spacers. The
catalytic performance of the complex 1, complex 2 and the supported complex 1 on GO (shown
as 1–f–GO) were investigated in Suzuki–Miyaura reaction under mild conditions (room
temperature (r.t.)) (Scheme 1).
<
<Scheme 1>>
2
. Experimental section
All of the chemical compounds were purchased from Merck and Sigma-Aldrich companies.
The powder X-ray diffraction (XRD) patterns were obtained by a Bruker AXS (D8, Advance)
instrument employing the reflection Bragg–Brentano geometry with Cu-Kα radiation. The
qualitative and quantitative elemental composition of the treated samples were acquired by
means of energy dispersive X-ray (EDAX, model Seron AIS 2300, Korea). Transmission
electron microscopy (TEM) images were taken with a Philips CM-10 microscope operated at
-1
-1
1
00 kV. Raman spectra were recorded from 4250 cm to 100 cm on a high resolution
dispersive Raman Thermonicolet (LabRAM HR UV/Vis/NIR, Electrooptics). The surface atomic
concentration and chemical composition of the samples were investigated by X-ray
photoelectron spectroscopy (XPS) equipped with an Al Kαx-ray source at energy of 1486.6 eV
−
9
in an ultrahigh vacuum (UHV) system with a base pressure lower than 2×10 Torr. The loading
amount of palladium was determined using an inductively coupled plasma (ICP) analyzer
(Varian, Vista-Pro). FT-IR spectra were taken with a Jasco FT/IR-680 plus spectrometer. The
microanalysis was performed using a Thermofinnigan Eager 300 CHN-O elemental analyzer.
NMR spectra were taken with a Bruker 400 MHz ultra-shield spectrometer using CDCl as the
3
2 2
solvent and TMS as the internal standard. cis-[PdCl (dmso) ] was prepared according to the
literature methods [28].
3

ACCEPTED MANUSCRIPT
2
.1. Preparation of [Pd(C^N)Cl(dmso)], 1
-phenylpyridine (70.0 µl, 0.48 mmol), HC^N, and a solution of sodium acetate (39.2 mg,
.48 mmol) in methanol (3 ml) were added to a solution of cis-[PdCl (dmso) ] (160.1 mg, 0.48
2
0
2
2
mmol) in dried toluene (50 ml). This mixture was refluxed at 90 °C for 10 h under argon
atmosphere. The obtained green precipitate was filtered, washed with dry toluene and dried
under vacuum. Yield: 147.28 mg (82%), m.p ≈ 326°C (dec). Anal. Calcd. for C13
H14ClNSOPd:
1
C, 41.73; H, 3.77; N, 3.74. Found: C, 41.68; H, 3.72; N, 3.68. NMR in DMSO-d6. 1: δ ( H) =
3
.34 [s, 6H, 2 Me groups of the dmso ligand], 7.00 [s, 1H, Ar H], 7.06 [s, 1H, Ar H], 7.35 [s, 1H,
6
Ar H], 7.61 [s, 1H, Ar H], 7.82 [s, 1H, Ar H], 8.00 [s, 2H, Ar H], 9.15 [d, 1H, H of pyridine
group of C^N ligand].
2
.2. Preparation of [Pd(C^N)Cl(PPh )], 2
3
To a solution of [Pd(C^N)Cl(dmso)] (green color), 1, (40.0 mg, 0.107 mmol) in acetone (10
ml) was added PPh (28.0 mg, 0.107 mmol). After a few min a white precipitate was formed.
3
The mixture was stirred at RT for 4 h. The obtained white precipitate was filtered, washed with
acetone and dried under vacuum. Yield: 51.9 mg (87%), m.p ≈ 282°C (dec). Anal. Calcd. for
C H ClNPPd: C, 62.38; H, 4.15; N, 2.51. Found: C, 62.31; H, 3.85; N, 2.46. NMR in CDCl . 2:
29
23
3
1
δ ( H) = 6.53 [m, 2H, Ar H], 6.95[t, 1H, Ar H], 7.23 [t, 1H, Ar H], 7.32-7.54 [m, 10H, 1H of
C^N ligand and 9H of PPh ligand], 7.73-7.84 [m, 8H, 2H of C^N ligand and 6H of of PPh
3
3
3
6
5
6
31
ligand], 9.64[d, JH H = 8 Hz, 1H, H of pyridine group of C^N ligand]; δ ( P) = 43.35 [s, 1P of
PPh3].
2
.3. Preparation of f–GO
GO was prepared from natural graphite flakes by a modified Hummers method [17-19, 29,
0]. 0.01 g of GO was dispersed in distilled water and placed for 10 min under ultrasonic waves.
3
Then, a mixture contains 3-ampy (0.01 mmol) and 0.01 g NaOH in water was added. Finally, the
reaction mixture was heated under a reflux condenser at 80 °C for 24 h after that the mixture was
centrifuged and the black solid separated, washed with water and air-dried [29].
4

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2
.4. Preparation of 1–f–GO nanohybrid
.040 g of f-GO was dispersed in 50 mL of dry toluene and placed for 15 min under
ultrasonic waves, then 0.015 g of complex 1 was added slowly. The suspension was sonicated for
5 min and was heated under a reflux condenser at 70 °C for 12 h, then the mixture was stirred
0
1
for 12 h at room temperature after that the mixture was centrifuged and the solid separated,
washed with dry toluene and air-dried.
2
.5. General procedure for Suzuki–Miyaura carbon–carbon coupling reaction
Aryl halide (0.5 mmol) and phenylboronic acid (0.75 mmol) were added to a flask containing
the catalyst and K CO (1 mmol) in 5 mL of toluene at room temperature. After completion of
2
3
the reaction (monitored by TLC), the catalyst was separated and the reaction solution evaporated
1
under vacuum to give the organic product, then analyzed by FT–IR and H NMR spectra with
those of authentic sample and the details are given in the supporting information. The TON (=
mol of product per mol of catalyst) and TOF (= TON per time (h)) were calculated on the basis
of the amount of biaryl product formed.
2
.6. Isolation of the catalyst during Suzuki−Miyaura coupling
A mixture of Pd(II) complex 1 (or complex 2) (0.107 mmol), bromobenzene (3.5 mmol),
phenylboronic acid (5.25 mmol), and K
at room temperature. The black residue (palladium particles) were centrifuged after adding
O:EtOH (1:4), and the solid separated, washed three times with ethanol and air-dried. The
catalyst residue was characterized by XRD.
2 3
CO (7.0 mmol) in toluene (20.0 ml) was stirred for 1h
H
2
3
. Results and discussion
3
.1. Structure characterization
The synthetic procedures are summarized in Scheme 2. The starting complex
[Pd(C^N)Cl(dmso)], 1, in which C^N = deprotonated 2-phenylpyridine, ppy, was synthesized by
the reaction of cis-[PdCl (dmso) ] with 1 equiv of ppy. The structure of complex 1 was readily
2
2
1
deduced from its H NMR spectrum. The protons of methyl groups on dmso ligand were
observed at δ =3.34 as a singlet signal. The hydrogens of ppy were seen in 7.00–9.00 ppm
6
region. A singlet signal at δ =9.15 was assigned to H proton of ppy ligand (Fig. 1).
5

ACCEPTED MANUSCRIPT
Complex palladacycle 1 readily underwent ligand substitution reaction with 1 equiv of PPh
3
1
31
to give the complex 2 and was characterized by its H and P NMR spectra. The hydrogens of
6
ppy and PPh were seen in 6.53-7.84 ppm region. A singlet signal at δ =9.64 was assigned to H
3
3
1
proton of ppy ligand. The appearance of only one signal around 43.35 ppm in the P NMR
spectrum for complex 2 (Fig. 2).
<
<Scheme 2>>
<
<
<Fig. 1>>
<Fig. 2>>
The 1–f–GO nanohybrid synthesized according to procedure titled in the experimental
section and graphically summarized in Scheme 1. FT-IR spectra of GO, f-GO, complex 1 and 1–
f–GO nanohybrid are shown in Fig. 3. The results from FT-IR spectroscopy revealed that the
-1
characteristic strong band at ca. 1729 cm attributes to stretching vibration of C=O in carboxylic
-1
acid group and the band at ca.1226 cm assigned to the C–O epoxy groups (Fig. 3a) [31]. The
−
1
CH vibrations of the 3-ampy in f-GO appear at 2865 and 2927 cm which corresponds to
2
s 2 2
symmetric ν CH and asymmetric νas CH , respectively (Fig. 3b). As the GO sheet contains
reactive epoxy groups, its exposure to amine groups would lead to a ring opening reaction of the
reactive three-membered epoxide ring, creating new C–N bonds and –OH functional groups. The
-1
increase of the peak intensities at 1575 and 1199 cm corresponds to a stretching of the new NH
bending and C–N vibration, respectively. Amine groups did not reacted with carboxylic acid
groups of GO through the amidation process because the amide ν NH vibration expected at 3300
−
1
−1
cm is not present and the 1650–1750 cm region does not show a strong ν C=O band.
Conditions used for GO-epoxide ring opening by amino group do not fit for amide bonds
formation. Nevertheless NH vibration bands are present in the spectrum as the secondary amino
−
1
groups attribute. The absorption band at around 1700 cm is attributed to the carboxyl groups.
However, the corresponding absorption (Fig. 3b) at this range is not observable, likely due to the
overlapping of absorption of graphene sheets in this region [32,33]. The FT-IR result in Fig. 3d
shows that 1–f–GO nanohybrid is formed via a substitution reaction between dimethylsulfoxide
6

ACCEPTED MANUSCRIPT
−
1
groups of complex 1 and 3-ampy moieties of f-GO. The absorption bands around 2927 cm
show the existence of a stretching vibration of C−H of the methyl groups and the peak at 1020
−
1
cm is assigned to the stretching vibration of S=O, indicating the existence of dimethylsulfoxide
group in the complex 1 (Fig. 3c). The FT-IR result in Fig. 3d shows no absorption at this range
due to removed the dimethylsulfoxide group. All of these indicate that complex 1 moiety has
been successfully grafted on the surface of f-GO.
<
<Fig. 3>>
Raman spectroscopy has been widely used to probe structural and electronic characteristics
of graphite materials, providing useful information on the defects (D band) and in plane vibration
2
of sp carbon atoms (G band). Fig. 4 shows the Raman spectra of GO, f-GO and 1–f–GO
nanohybrid. The prominent D Peak is from the structural imperfections created by the attachment
of hydroxyl and epoxide groups on the carbon basal plane [34]. The Raman spectrum in Fig. 4a
-1
shows the characteristic peaks located at 1341 and 1583 cm corresponding to the D and G
bands of GO, respectively. The Raman spectrum of f-GO also exhibited fundamental vibrations
-1
similar to GO at 1356 and 1598 cm , with a D/G ratio of 1.45 (Fig. 4b). There is a blue shift of
-1
the G peak at around 15 cm for the f-GO, corresponding to a possible interaction of the GO and
-ampy molecules and indicating a better exfoliation of graphene layers [35]. In addition,
3
−
1
compared with the G-band position in f-GO, which is centered at 1598 cm , the G-band of 1–f–
−
1
GO nanohybrid shift to 1585 cm (Fig. 4c). Both f-GO and 1–f–GO nanohybrid show increased
D/G ratios (1.45 and 1.13 respectively) relative to GO. The increased D/G ratio is consistent with
the functionalization of GO through covalent bonding [36,37].
<
<Fig. 4>>
The composition of the 1–f–GO nanohybrid was further examined by EDAX where the Pd,
Cl, N, O and C elements were observed (Fig. 5).
<
<Fig. 5>>
7

ACCEPTED MANUSCRIPT
XPS was used to analyze the elemental composition of the 1–f–GO nanohybrid surface. The
survey spectrum of 1–f-GO nanohybrid (Fig. 6a) gives rise to the spectroscopic signature of
carbon, oxygen, nitrogen, chlorine and palladium. C1s XPS spectrum of the 1–f–GO nanohybrid
the first main peak at a binding energy of 284.51 eV is assigned to the bonding energy of C-C in
the graphene sheet (Fig. 6b). As shown in Fig. 6b, the intensity of the C–O (epoxy) groups in 1–
f–GO nanohybrid at 286.09 eV was significantly lower compared to the C–O (epoxy) groups in
GO [20,38], a new component corresponding to C-N groups was observed at the same energy
peak. This means that the NH groups in 3-ampy form the new C-N bonds by means of the
2
additional ring opening reaction [39]. For the N1s peaks of 1–f–GO nanohybrid, C=N, C-N, and
Pd-N were observed at 398.35, 399.55 and 400.72 eV, respectively (Fig. 6c). Also, the O1s XPS
can be divided into three components, including a C=O bond at 531.18 eV, a O-H bond at 532.58
eV, and a C–O bond at 534.01 eV (Fig. 6d). The Pd 3d spectrum exhibited the presence of Pd(II)
and Pd(0) species (Fig. 6e). The XPS spectrum was characterized by two pairs of Pd3d peaks:
the binding energies of 337.77 and 343.20 eV were assigned to Pd(II) and the binding energies of
3
36.66, 342.30 eV were assigned to Pd(0), for 3d5/2 and 3d3/2, respectively (Fig. 6e). The
contents of Pd(II) and Pd(0) are 77.7 % and 22.3%, respectively. The existence of a minor Pd(0)
component (22.3%) in the sample of 1-f-GO after Pd(II) complex immobilization is due to the
presence of reducing agent such as GO, done by oxygen functionalities of the initial GO sheets.
Apparently, at the initial state of precursor decomposition Pd(II) NPs chemically interact with
the oxygen functional groups of GO by forming Pd-O linkages, facilitated by the highly
electrophilic character of Pd(II). These linkages then provide heterogeneous nucleation sites on
which the Pd(0) NPs form upon reduction. This may explain the efficient and firm anchoring of
the Pd NPs on RGO, in a similar way as recently described for Pd NPs supported on GO [40].
<
<Fig. 6>>
In order to further probe the morphology and dispersion of materials, TEM measurement was
performed (Fig. 7). As shown in Fig. 7a, wrinkles on GO nanosheets at several places are
3
originated by various defects and functional groups carrying sp hybridized carbon atoms, which
are introduced during the oxidation process [31]. TEM images of f-GO nanosheet are shown in
Fig. 7b and 7c. After the functionalization reaction of GO, the TEM spectrum of f-GO exhibits
8

ACCEPTED MANUSCRIPT
more bends and wrinkles compared to GO nanosheets, which implies the decreased size of the
2
sp domains upon the localized chemical reduction of GO by 3-ampy [41]. The immobilization
of Pd complex was further studied using TEM to obtain the distribution of the Pd complex on the
functionalized GO (Fig. 7d and 7e). Spherical Pd(0) NPs with a size of around 2-3 nm are well-
dispersed on the support (Fig. 7f). Also, it should be mentioned that XPS analysis showed the
sample content of 22.3% Pd(0) and 77.7% Pd(II).
<
<Fig. 7>>
3
.2. Suzuki−Miyaura carbon–carbon coupling reaction
Reaction conditions such as solvents and reaction temperature have been optimized. In the
presence of heterogeneous catalysts, the cross-coupling reaction of bromobenzene with
phenylboronic acid was investigated in commonly by using solvents in air (Table 1). The
heterogeneous catalysts are used in the present project. When the reaction was tested at room
temperature, the nonpolar solvent toluene gave an excellent yield [42]. The solvents such as
2 2
THF, 1,4-dioxane, EtOH, H O, and THF/H O=2:1 also afforded low yields of the coupling
product. While the reactions were performed at high temperature (80 °C), all solvents displayed
high efficiency and the coupling product was obtained in a high yields. These data show that
least yield was obtained when 1,4-dioxane was used as solvent.
<
<Table 1>>
As shown in Table 2, the catalyst / K
2
CO system was effective for the coupling of various
3
aryl halides with phenylboronic acid in toluene at room temperature. As expected, electron-
withdrawing groups on the aryl ring increased the reaction rate. For the non-activated
chlorobenzene and the activated 1-chloro-4-nitrobenzene, the reaction proceeded with yields
ranging from 60 to 80%. For the deactivated 4-bromotoluene and the activated bromobenzene,
the reaction proceeded with yields ranging from 67 to 90%. The used catalyst was analyzed by
ICP and no discernible leaching of Pd was observed, attributed to the strong coordination bonds.
<
<Table 2>>
9

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In the course of Suzuki–Miyaura reactions catalyzed with 1 and 2, black particles appear
immediately at the start of the reaction. Therefore, it is possible that 1 and 2 are precatalysts and
during catalytic coupling reactions black particles that form are Pd-containing species, which
have a role in the catalytic process [5, 43, 44]. Thus the black precipitates obtained from 1 and 2
in the course of catalysis of the coupling reaction of bromobenzene with phenylboronic acid
under optimum reaction conditions have been isolated (details in the Experimental section). The
XRD (Fig. 8) of the black precipitate suggest that the strong diffraction peaks of (111), (200),
(220), (311) and (222) are attributable to Pd(0) [17,45].
<
<Fig. 8>>
As shown in Table 2, for the reactions of aryl halides with boronic acid in the presence of 1-
f-GO catalyst, TOF or TON factors are higher than that for the complex 1 or the complex 2 and
shorter times are seen. The most activity of 1-f-GO is attributed to nanostructure of the catalyst
and the precense of graphene sheets that these properties increases the surface to volume ratio of
the catalyst to accelerate the reactions. Also, higher TOF or TON is seen for complex 1 catalyst
in comparison to the complex 2 in different reactions. The presence of ligands of dmso or PPh3
on complexes shows influence on the decomposition rate of palladcycle, which in turn affects the
catalytic efficiency. As for the phosphine-substituted complex 2, the decomposition was much
slower than that of complex 1, likely due to the stabilization of the coordinating phosphine.
Several observations confirm that the Suzuki-Miyaura coupling catalyzed by palladcyclic
complexes is via the Pd(0) [5, 44]. Bedford et al. [46] show that the phenylation and reductive
elimination steps outlined occur as a key step in such in the catalytic cycles of Suzuki–Miyaura
reactions catalysed by imine palladacycles and that the substantial quantities of palladium(0)
species generated during this process act as the active catalysts. For the precatalysts detailed, in
2
this work, the reaction color changes occur rapidly on mixing the PhB(OH) , base and catalyst in
distilled toluene at ambient temperature (Fig. 9 shows the appearance of the initially colorless
solutions after 20 min of stirring at ambient temperature). But the color of PhB(OH) , catalyst
2
2 3
and toluene mixture or K CO , catalyst and toluene mixture was not changed. In the presence of
base, transmetallation between palladacyclic complexes and PhB(OH) occurs to give
2
transmetallation products of A, as shown in Scheme 3. Then, the isomerization A to B appears to
1
0

ACCEPTED MANUSCRIPT
occur easily, followed by the loss of the C^N ligand via reductive elimination with the phenyl
0
group, affording Pd (Scheme 3).
<
<Fig. 9>>
<
<Scheme 3>>
To further evaluate the stability of the used catalyst, ICP was also employed to determine the
palladium content before and after the Suzuki-Miyaura reaction. We found that 1–f–GO after
eight cycles showed no discernible difference in the palladium content with the fresh one (Table
3
).
<
<Table 3>>
4
. Conclusions
In summary, a novel cyclometalated organopalladium(II) complex, covalently functionalized
graphene oxide nanohybrid, has been successfully synthesized and characterized by TEM,
EDAX, XPS, FT-IR and Raman spectroscopy. We successfully developed an efficient method
for the synthesis of a GO supported palladium complex catalyst. The 1–f–GO nanohybrid was
used as efficient catalyst in the Suzuki–Miyaura coupling reaction, with the lowest loading of
0
.0012 mol%, the reaction was completed after 20 min at room temperature. These results
demonstrate the remarkable catalytic activity of 1–f–GO nanohybrid with a TON of 80000 and
-1
TOF of 240000 h (Table 2, entry 1). To our knowledge, this is one of the highest TOF observed
in Suzuki-Miyaura reaction by a heterogeneous catalyst [47-50]. The supported catalyst can be
readily recycled from the reaction mixture and reused without discernible loss of its catalytic
activity. The above results demonstrate that GO is a suitable and promising support for
anchoring of homogeneous transition-metal complexes as remarkable catalysts. This new
strategy opens new avenues to coordination catalysis with graphene materials. Also, the used
catalyst in this study behaved as a very good catalyst toward aryl chloride derivatives.
1
1

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Acknowledgements
We thank the Iran National Science Foundation (Grant no. 94001880) for their support. We
would also like to thank the Yasouj University Research Council and the Iranian
Nanotechnology Initiative Council for their partial support.
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Figure Captions:
Scheme 1. Schematic illustration of the preparation of 1–f–GO nanohybrid.
Scheme 2.
1
Fig. 1. H NMR spectrum (400 MHz), in DMSO-d6, of [Pd(C^N)Cl(dmso)], 1.
1
31
Fig. 2. H and P NMR spectra (400 MHz), in CDCl , of [Pd(C^N)Cl(PPh )], 2.
3
3
Fig. 3. FT-IR spectra of (a) GO, (b) f-GO, (c) complex 1, and (d) 1–f–GO nanohybrid.
Fig. 4. Raman spectera of (a) GO, (b) f-GO, and (c) 1–f–GO nanohybrid.
Fig. 5. EDAX spectrum of 1–f–GO nanohybrid.
Fig. 6. XPS spectra of 1–f–GO nanohybrid, (a) the survey curve, (b) C 1s, (c) N 1s, (d) O 1s, and
(e) Pd 3d.
Fig. 7. TEM images of (a) GO, (b and c) f-GO, (d and e) 1–f–GO nanohybride, and (f) histogram
of particle size distribution.
Fig. 8. XRD pattern of Pd particles generated from the coupling reaction catalyzed by complex
1
.
Fig. 9. Color evolution of the catalyst 1, K CO and PhB(OH) in toluene.
2
3
2
Scheme 3. Mechanism for degradation of 1 and 2.
Table Captions:
Table 1 Solvent and temperature effects on the Suzuki–Miyaura cross-coupling reaction of
a
bromobenzene with phenylboronic acid .
Table 2 Suzuki–Miyaura cross-coupling reactions catalyzed by palladacycles.
Table 3 Recycling result for the coupling reaction of bromobenzene with phenylboronic acid.
1
5

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Table Captions:
Table 1 Solvent and temperature effects on the Suzuki–Miyaura cross-coupling reaction of
a
bromobenzene with phenylboronic acid .
Table 2 Suzuki–Miyaura cross-coupling reactions catalyzed by palladacycles.
Table 3 Recycling result for the coupling reaction of bromobenzene with phenylboronic acid.
1

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Table 1
Solvent and temperature effects on the Suzuki–Miyaura cross-coupling reaction of
a
bromobenzene with phenylboronic acid .
b
Entry
Solvent
Yield (%)
r.t. °C
80 °C
64%
55
95
95
95
81
85
95
95
95
67
65
c
1
2
3
4
5
6
7
8
1,4-Dioxane
1,4-Dioxane
EtOH
10%
trace
60
56
60
37
47
90
87
c
c
d
EtOH
EtOH
e
c
THF/H
2
O=2:1
c
THF
c
Toluene
Toluene
Toluene
d
9
e
c
1
1
1
0
1
2
90
20
20
H O
2
d
H
2
O
a
Reaction conditions: 0.50 mmol of bromobenzene, 0.75 mmol of
phenylboronic acid, 1.00 mmol of K CO , 5.0 mL of solvent, reaction
2
3
b
c
d
time 1 h. Isolated yield. complex 1 (0.002 mol% of Pd). complex 2
(
e
0.002 mol% of Pd). 1–f–GO nanohybrid (0.0012 mol% of Pd).
2

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Table 2
Suzuki–Miyaura cross-coupling reactions catalyzed by palladacycles.
a
b
b
-1
Entry Substrate
Product Cat.
Time (min) Yields (%) TON TOF (h )
1
30
35
94
93
96
88
86
90
70
67
75
65
60
67
75
70
80
75
70
81
47000 94000
46500 79714
80000 240000
44000 58667
43000 51600
75000 140625
35000 14000
33500 11167
62500 31250
32500 15000
30000 12000
55833 74444
37500 25000
35000 19091
66667 100000.5
37500 7031.25
35000 4375
1
2
3
4
5
6
C H I
1a
1a
2
1
1
2
1
1
2
1
1
2
1
1
2
1
1
2
1
6
5
–f–GO 20
45
C
6
5
H Br
50
–f–GO 32
150
4-MeC H Br 1b
180
–f–GO 120
130
150
–f–GO 45
90
110
–f–GO 40
320
480
–f–GO 240
6
4
C
6
5
H Cl
1a
4-NO C H Cl 1c
2
6
4
C
5
H
4
NCl
1d
67500 16875
a
Isolated yields.
b
See Experimental section for the calculation of TONs and TOFs.
3

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Table 3
Recycling result for the coupling reaction of bromobenzene with phenylboronic acid.
Run
1st
85
2nd
83
3rd
86
4th
81
5th
84
7th
84
8th
81
Yield (%)
-1
Final TOF (h ) 132812 129687 134375 126562 131250 131250 126562
4

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Figure Captions:
Scheme 1. Schematic illustration of the preparation of 1–f–GO nanohybrid.
Scheme 2.
1
Fig. 1. H NMR spectrum (400 MHz), in DMSO-d6, of [Pd(C^N)Cl(dmso)], 1.
1
31
Fig. 2. H and P NMR spectra (400 MHz), in CDCl , of [Pd(C^N)Cl(PPh )], 2.
3
3
Fig. 3. FT-IR spectra of (a) GO, (b) f-GO, (c) complex 1, and (d) 1–f–GO nanohybrid.
Fig. 4. Raman spectera of (a) GO, (b) f-GO, and (c) 1–f–GO nanohybrid.
Fig. 5. EDAX spectrum of 1–f–GO nanohybrid.
Fig. 6. XPS spectra of 1–f–GO nanohybrid, (a) the survey curve, (b) C 1s, (c) N 1s, (d) O 1s, and
(e) Pd 3d.
Fig. 7. TEM images of (a) GO, (b and c) f-GO, (d and e) 1–f–GO nanohybride, and (f) histogram
of particle size distribution.
Fig. 8. XRD pattern of Pd particles generated from the coupling reaction catalyzed by complex
1
.
Fig. 9. Color evolution of the catalyst 1, K CO and PhB(OH) in toluene.
2
3
2
Scheme 3. Mechanism for degradation of 1 and 2.
1

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Scheme 1. Schematic illustration of the preparation of 1–f–GO nanohybrid.
2

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Scheme 2.
3

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1
Fig. 1. H NMR spectrum (400 MHz), in DMSO-d6, of [Pd(C^N)Cl(dmso)], 1.
4

ACCEPTED MANUSCRIPT
1
31
Fig. 2. H and P NMR spectra (400 MHz), in CDCl , of [Pd(C^N)Cl(PPh )], 2.
3
3
5

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Fig. 3. FT-IR spectra of (a) GO, (b) f-GO, (c) complex 1, and (d) 1–f–GO nanohybrid.
6

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Fig. 4. Raman spectera of (a) GO, (b) f-GO, and (c) 1–f–GO nanohybrid.
7

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Fig. 5. EDAX spectrum of 1–f–GO nanohybrid.
8

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Fig. 6. XPS spectra of 1–f–GO nanohybrid, (a) the survey curve, (b) C 1s, (c) N 1s, (d) O 1s, and
(e) Pd 3d.
9