Cu-Schiff base complex grafted onto graphene oxide nanocomposite: Synthesis, crystal structure, electrochemical properties and catalytic activity in oxidation of olefins

Article abstract of DOI:10.1016/j.ica.2017.07.013

Copper(II) Schiff base complex has been immobilized onto graphene oxide (GO) via covalent bonding. The nanocomposite was characterized using powder X-ray diffraction (XRD), Fourier transform infrared (FT-IR) and UV–visible spectroscopies, transmission electron microscopy (TEM), scanning electron microscopy (SEM), thermo-gravimetric analysis (TGA) and the energy-dispersive X-ray spectroscopy (EDX). The amount of catalyst loading on the solid support was determined by atomic absorption spectroscopy. The thermogravimetric analysis (TGA) demonstrated that the nanocatalyst was stable up to almost 220?°C, exhibiting high thermostability. Highly active and selective catalyst was found with excellent turnover numbers (up to 47,000 for oxidation of styrene) for the epoxidation of olefins in ethanol using aqueous hydrogen peroxide as oxidant. It could be readily reused for successive ten times without discernible activity and selectivity deterioration, which displays potential for practical applications.

Full text of DOI:10.1016/j.ica.2017.07.013

Accepted Manuscript
Research paper
Cu-Schiff base complex grafted onto graphene oxide nanocomposite: Synthesis,
crystal structure, electrochemical properties and catalytic activity in oxidation
of olefins
Saeed Rayati, Elham Khodaei, Saeedeh Shokoohi, Majid Jafarian, Bahareh
Elmi, Andrzej Wojtczak
PII:
S0020-1693(17)30605-9
http://dx.doi.org/10.1016/j.ica.2017.07.013
ICA 17736
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
18 April 2017
27 May 2017
6 July 2017
Please cite this article as: S. Rayati, E. Khodaei, S. Shokoohi, M. Jafarian, B. Elmi, A. Wojtczak, Cu-Schiff base
complex grafted onto graphene oxide nanocomposite: Synthesis, crystal structure, electrochemical properties and
catalytic activity in oxidation of olefins, Inorganica Chimica Acta (2017), doi: http://dx.doi.org/10.1016/j.ica.
2
017.07.013
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Cu-Schiff base complex grafted onto graphene oxide
nanocomposite: Synthesis, crystal structure,
electrochemical properties and catalytic activity in
oxidation of olefins
a,
a
a
a
a
Saeed Rayati, * Elham Khodaei, Saeedeh Shokoohi, Majid Jafarian, Bahareh Elmi, Andrzej
b
Wojtczak
a
Department of Chemistry, K.N. Toosi University of Technology, P.O. Box 16315-1618, Tehran
1
5418, Iran
b
Faculty of Chemistry, N. Copernicus University, Gagarina 7, 87-100 Torun, Poland
ABSTRACT
Copper(II) Schiff base complex has been immobilized onto reduced graphene oxide (RGO) via
covalent bonding. The nanocomposite was characterized using powder X-ray diffraction (XRD),
Fourier transform infrared (FT-IR) and UV–visible spectroscopies, transmission electron
microscopy (TEM), scanning electron microscopy (SEM), thermo-gravimetric analysis (TGA)
and the energy-dispersive X-ray spectroscopy (EDX). The amount of catalyst loading on the
solid support was determined by atomic absorption spectroscopy. The thermogravimetric
analysis (TGA) demonstrated that the nanocatalyst was stable up to almost 220 °C, exhibiting
high thermostability. Highly active and selective catalyst was found with excellent turnover
numbers (up to 47000 for oxidation of styrene) for the epoxidation of olefins in ethanol using
aqueous hydrogen peroxide as oxidant. It could be readily reused for successive ten times
1

without discernible activity and selectivity deterioration, which displays potential for practical
applications.
Keywords: Graphene oxide, Schiff base, olefin, oxidation, Crystal structure
*
Corresponding author. Tel.: +98 21 22850266; Fax: +98 21 22853650.
E-mail addresses: rayati@kntu.ac.ir, srayati@yahoo.com (S. Rayati).
1. Introduction
Since epoxides are very important building blocks for preparation of biological molecules,
epoxidation of alkenes have been extensively studied [1-7]. Schiff-base complexes are among
transition metal compounds which have been considered as catalysts for the epoxidation of
olefins [8-10]. Also copper complexes have been used as catalysts in various catalytic reactions
[
11, 12]. In order to increase catalyst stability and allow for catalyst recycling and product
separation, heterodenization of the homogeneous catalysts on various solid supports include
organosilica materials [13, 15], carbon materials [16, 17], silica materials [18–20], zeolites [21,
2
2] and layered compounds [23] has been studied. Recently, reduced graphene oxide (RGO)
with interesting two-dimensional structure and unique properties [24−27] has been demonstrated
as ideal support for a variety of catalytic species [28−32] due to unique nanostructure, high
chemical, mechanical and thermal stability with the large specific surface area allowing excellent
dispersion of the catalytic species. Therefore, remarkable catalytic performance of RGO-
supported catalysts has been observed. At the moment only few studies have been published on
the application of graphene oxide and other chemically modified graphene-based materials as
heterogeneous catalysts [33-35]. For this purpose, we studied the catalytic activity and
2

electrochemical behavior of copper Schiff base complex supported on graphene oxide in the
oxidation of various olefins.
2. Experimental
2.1. Instruments and reagents
Infrared spectra were recorded as KBr pellets using Unicam Matson 1000 FT-IR. A Varian
(
AA 240) atomic absorption spectrometer was used for copper determination. X-ray powder
diffraction (XRD) was carried out on a Philips PW 3710 diffractometer with CuKαradiation.
Scanning electron microscopy (SEM) images and the energy-dispersive X-ray spectroscopy
(
EDX) were obtained on a VEGA3 TESCAN field emission scanning electron microscope using
HV voltage of 20 kV. Transmission electron microscopy (TEM) image were obtained on a Zeiss
EM-1030 transmission electron microscopy device using voltage of 100 kV. Thermogravimetric
analysis was performed using a Mettler Toledo (TGA-DSC). The oxidation products were
analyzed by HP Agilent 7890 gas chromatograph equipped with a HP-5 capillary column
(
phenyl methyl siloxane 30 m × 320 µm× 0.25µm) and a flame ionization detector. UV-Vis
spectra were obtained on Unico 4802 double beam spectrometer. Electrochemical studies of
catalysts were carried out by Solartron model SI 1287 electrochemical interface. A glassy carbon
(
GC) disc electrode and graphite electrode were used as working and counter electrode,
respectively. The potentials are relative to the aqueous Ag/AgCl placed in separate compartment
containing 0.1M tetrabutylammonium hexafluorophosphate (TBAH) in anhydrous acetonitrile.
1
,2-Propanediamine, bis(2,4-dihydroxyacetophenone), copper(II) nitrate trihydrate, hydrogen
peroxide (solution 30% in water) were used as received from commercial suppliers. Solvents
were purchased from Merck or Fluka chemical companies.
2
.2. Methods
3

2
.2.1. Preparation of the Schiff base ligand (H
2
L)
To a stirred ethanolic solution (20 mL) of 1,2-propanediamine (0.074 g, 1 mmol), bis(2,4-
dihydroxyacetophenone (0.304 g, 2 mmol) was added. The mixture was stirred and heated to
reflux for 1 h. A yellow precipitate was filtered and washed with cold ethanol. Yield (94%),
°
1
Mp.= 180 C. H NMR (δ): 1.2 66 , 1.28 (d, 3H, NCH CH(CH
3
3
)N), 2.2–2.49 (s, 6H, (CH )C=N),
2
3
6
5
.3–3.7 (m, 1H, NCH CH(CH )N) CH N), 4.18–4.19 (m, 2H, NCH CH(CH )N), 6.01–7.41 (m,
2
2
3
2
2
3
1
3
H, ArH), 16.52–16.67 (s, 4H, OH). C NMR (δ): 14.34 (NCH CH(CH
3
)N), 19.61 (CH
3
)C=N),
2
2.85 (NCH
2
CH(CH )N), 54.06 (NCH CH(CH )N), 130–170 (ArC), 172.36 (CH )C=N).
3
2
3
3
2
.2.2. Preparation of copper (II) complex (CuL)
The copper complex was prepared as follows: the Schiff base ligand, H L (0.34 g, 1 mmol)
2
was dissolved in 20 mL of ethanol. An ethanolic solution of copper(II) nitrate (0.31 g, 1.2 mmol)
was added to the solution of the ligand and the reaction mixture was refluxed for 2 h. The
solution was concentrated to half volume and dark brown powders were obtained. The products
washed with cold ethanol (CuL: Yield (83%), Mp 265 °C). The complex (CuL) (0.15 g, o.34
mmol) was dissolved in 15 mL of hot ethanol then the solution was kept for slow evaporation at
3
°C. After 5 days, well-shaped dark brown X-ray quality single crystals deposited at the bottom
of the vessel. Figure 1 shows the crystal structure of CuL.
4

Fig. 1. Crystal structure of copper(II) Schiff base complex
.2.3. X-ray structure of the complex (CuL)
The diffraction data for CuL were collected with an Oxford Sapphire CCD diffractometer
2
using MoKαradiation λ=0.71073 Å, at 293(2) K, by ω-2θmethod. The structure was solved by
2
Patterson method and refined with the full-matrix least-squares method on F with the use of
SHELX2014 [36] program package. The analytical absorption correction was applied with the
maximum and minimum transmission of 0.82669 and 0.76360 (Table 1) [37]. Positions of
hydrogen atom were found from the electron density maps, and hydrogen atoms were
constrained in the refinement with the appropriate riding model as implemented in SHELX. The
X-ray experimental data and structure refinement for the reported structure are summarized in
Table 1. The structural data have been deposited with the Cambridge Crystallographic Data
Centre, the deposition number CCDC 1537879. The data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/structures.
5

Table 1
Crystal data and structure refinement for CuL.
Empirical formula
Formula weight
Temperature; K
Wavelength; Å
Crystal system
C19 H24 Cu N2 O6
439.94
293(2)
0.71073
Triclinic
P-1
Space group
Unit cell dimensions; Å, °.
a = 8.3404(4)
b = 10.7798(6)
c = 11.8474(6)
α= 103.880(4)
β= 94.438(4)
γ= 108.979(5)
963.74(9)
3
Volume; Å
Z
2
3
1.516
Density (calculated); Mg/m
-
1
1.172
Absorption coefficient; mm
F(000)
458
Crystal size; mm
0.299 x 0.255 x 0.200
2.620 to 28.434°.
-10<=h<=11, -14<=k<=13, -15<=l<=12
6622
Theta range for data collection
Index ranges
Reflections collected
Independent reflections
Completeness to theta
Max. and min. transmission
Absorption correction
Refinement method
4280 [R(int) = 0.0248]
25.242° 99.9 %
0.82669 and 0.76360
Analytical
2
Full-matrix least-squares on F
Data / restraints / parameters
4280 / 0 / 253
0.949
2
RGOodness-of-fit on F
Final R indices [I>2sigma(I)]
R indices (all data)
R1 = 0.0441, wR2 = 0.1003
R1 = 0.0702, wR2 = 0.1058
0.368 and -0.360
-3
Largest diff. peak and hole; e.Å
6

2
.2.4. Preparation of graphene oxide
Graphene oxide was synthesized from graphite by using KMnO and a 9:1 mixture of
4
H SO /H PO as oxidizing agents according to the method reported by Marcano and co-workers
2
4
3
4
[
(
38]. Firstly, 3 g of graphite was added to 400 mL mixture of sulfuric acid and phosphoric acid
volume ratio 9:1). After stirring for 10 min, potassium permanganate (18 g) was added slowly
into the mixture. Then the mixture was heated to 50 °C and stirred for 12 h. After the reaction
was completed, the mixture was cooled to room temperature, and 400 mL of ice water was added
and stirred continuously for 30 min. Then, 30% hydrogen peroxide was added dropwise into the
reaction mixture. The bright yellow precipitate was obtained. The mixture was kept overnight
and then the supernatant was discarded. The solid was washed with 1 L hydrochloric acid (5–
1
0%) three times and with 1 L of deionized water five times to remove metal ions, sulfate and
chloride ions. Finally, the solid was dried and then placed in deionized water for preparation of 1
mg/mL water solution. Later it was ultra-sonicated for 1 h. When oxidized graphene was
dissociated into graphene oxide, the solution was centrifuged for 30 min at 1000 rpm and then
the supernatant was removed. The sediment was then dried at 50 °C and the graphene oxide
powder was obtained.
2.2.5. Immobilization of CuL on graphene oxide
The covalent bonding between the Schiff base complex and the graphene oxide surface was
carried out by ester bond formation between Schiff base hydroxyl groups and the carboxylic acid
of the graphene oxide using TBTU (2-(1H-benzotriazole-1- yl)-1,1,3,3-tetramethyluronium
tetrafluoroborate) as a highly effective uronium salt that has been used as an activation agent for
carboxylic acids for the preparation of esters [39,40] in the presence of DIPEA (N,N-
diisopropylamine) [41]. RGO (1 g) was added to CuL (0.44 g, 1 mmol) dissolved in 70 mL DMF
in the presence of (0.4 g, 1.1 mmol) TBTU and DIPEA (0.3 g, 2 mmol). The resulted mixture
7

was stirred for 24 h and the resulting materials were filtered and washed by CHCl
0 °C. The Cu-content of the supported catalyst determined by atomic absorption spectroscopy
revealed that the metal content of the catalyst is 0.63 mmol per gram of the catalyst.
3
and dried in
8
3. Result and discussion
-1
In the IR spectrum of the ligand H L a sharp band at 1623 cm which is assigned to the
2
-1
azomethine (C=N) group, shifted to the lower frequency (1605 cm ) in the CuL complex which
indicated the coordination of imine nitrogen to copper center (Table 2). Also phenolic-OH band
has disappeared in CuL spectrum due to the coordination of ligand to copper [42].
Table 2
Important IR spectra bands of the ligand H L and its complex (CuL)
2
Compounds υ O-H υ C=N υ C=C υ C-O
H L
3422 1623 1537 1042
- 1605 1520 1046
2
CuL
Electronic spectral data of the ligand and complex in EtOH are summarized in Table 3. The
−3
absorption spectra for 0.083 × 10 M solution of H L in absolute ethanol gives two intense bands
2
−3
at 227, 312 whereas the spectrum of CuL (0.083×10 M solution in absolute ethanol) shows
∗
∗
three bands at 239, 298 and 344 nm. These bands can be attributed to π-π and n-π transitions
within the Schiff base while more intense band in 344 nm is attributed to metal-ligand charge
transfer (Table 3).
Table 3
Electronic spectral data of the ligand and copper (II) complex
−
1
3
−1
Compound
λ (nm) (ε, mol dm cm )
Assignment
max
8

H L
227 (3373)
12 (2881)
239 (21686)
π•π*
n•π*
π•π*
2
3
CuL
2
3
5
98 (15420)
44 (9363)
43 (722)
n•π*
MLCT
d-d
3
.1. X-ray structure of CuL
The asymmetric part of the structure (CuL) consists of a single CuL molecule and 2 water
molecules (Fig 1). The Schiff base acts as a four-dentate ligand, with pair of phenolate O atoms
and pair of imine N atoms occupying cis positions in the coordination sphere of Cu(II). The
coordination sphere CuN O has a square-planar geometry, with the mean deviation of atoms
2
2
from the N O coordination plane being 0.067 Å. The Cu1-O2 and Cu1-O1 bonds being
2
2
1
.8694(19) and 1.8988(18) Å, respectively, are significantly shorter than Cu-N bonds (Table 4).
Slight deformation of the square-planar geometry results in the angles between atoms positioned
cis ranging from N1-Cu1-N2 87.03(10) to O2-Cu1-N2 93.27(10)º. The angles between bonds in
trans positions are O2-Cu1-N1 174.56(11) and O1-Cu1-N2 177.11(10)º.The valence geometry of
the L ligand is typical for such systems. The C7-N1 and N2-C10 imine bond distances are
1
.290(4) and 1.298(4) Å, respectively. The O1-C1 and O2-C16 are relatively short with the
distances of 1.320(3) and 1.325(3) Å, respectively. Due to the Cu-O distances shorter than the
Cu-N ones, the Schiff base ligand is not planar, with the dihedral angle between its phenyl rings
being 35.44(16)º. In the detected conformation, the torsion angle N1-C8-C9-N2 is 43.3(3)º. The
C18 methyl substituent is positioned on the convex side of the CuL catalyst, creating the
hydrophobic wall limiting the access to the metal center. The C7-N1-C8-C18 torsion angle
describing its position is -95.9(4)º. On the other hand, the methyl substituents of both imine C
atoms extend the flat surface of the ligand ring system. The Cu1...C intermolecular contacts are
observed with the phenolate ring C11, C12, and imine C10 atoms, the Cu...C[1-x,3-y,1-z]
distances ranging from 3.452 to 3.740 Å. These contacts might mimic the catalyst – substrate
9

interactions positioning the substrate within the active complex. Both hydroxyl substituents of
the phenolate rings are donors in the H-bonds involving water O atoms. Also all water OH
groups are donors in the H-bonds with phenolate O1, O2 or hydroxyl O3 as acceptors (see
supplemental).
Table 4
Selected bond lengths [Å] and angles [º] for [CuL].
Bond distances
Cu1-O2
Cu1-O1
Cu1-N1
Cu1-N2
Angles
1.8694(19)
1.8988(18)
1.933(2)
1.936(2)
1.320(3)
O2-Cu1-O1
O2-Cu1-N1
O1-Cu1-N1
O2-Cu1-N2
O1-Cu1-N2
N1-Cu1-N2
88.15(8)
174.56(11)
91.78(9)
93.27(10)
177.11(10)
87.03(10)
O1-C1
3
.2. Characterization of the heterogeneous catalyst (RGO-CuL)
CuL is anchored onto graphene oxide through an ester bond (scheme 1).
O
O
Me
O
N
Me
DIPEA
TBTU
Cu
N
O
Me
HO
Scheme 1. Schematic representation of the preparation of RGO-CuL
The FT-IR spectrum of pure RGO support exhibits various vibrational peaks of functional
-1
-1
-1
groups such as C–OH (1227 cm ), C=O (1707 cm ), C=C (1578 cm ) and C-O-C (1052)
revealing the presence of hydroxyl, carboxyl and epoxy groups [43]. The broad peak ranging
-1
from 3000 to 3600 cm can be attributed to the stretching modes of O–H bonds in RGO (Fig 2).
1
0

The shifting of these absorption bands to lower wave numbers confirmed the bonding of Cu(II)
−1
Schiff base complex to graphene oxide. An enhanced sharp band appearing near 1526.52 cm
−1
was associated with stretching vibrations of C=N. The new bands at 664.94 and 580 cm were
attributed to the stretching modes Cu–N and Cu–O bonds, respectively. These results confirm
that the CuL complex is attached to the carboxylic group of the RRGO support via its hydroxyl
group.
GO
GO-CuL
Fig. 2. IR spectra of RRGO and RRGO-CuL
The scanning electron micrograph (SEM) images of the RRGO precursor and RRGO-CuL
catalyst were recorded to examine the morphological changes that have occurred (Fig 3a and b).
The SEM images of RRGO-CuL exhibited nanosheets in disordered phase with wrinkles and
3
crumpling features. These folded edges owing to sp -carbon in RRGO, bear the Cu immobilized
Schiff base complex on both sides of the RRGO nanosheets. These sites are ideal for reactant
molecules to attain the desired transformation [44].
(
a)
(b)
1
1

Fig. 3. SEM Photographs of (a) RRGO and (b) RRGO-CuL
The TEM image of RRGO-CuL (Fig 4) revealed the sheet like structure with more crumpling
features which may be induced by the chemical modifications.
Fig. 4. TEM image of RRGO-CuL
The TGA curves of RGO and RGO-CuL are presented in Fig 5. The TGA of RGO shows two
o
distinct steps of weight loss. The 10% weight loss near 100 C was due to evaporation of water
~
molecules present in the lattice of RGO. The second significant weight loss was observed in the
o
range of 170–300 C due to thermal decomposition of oxygen carrying functionalities [45]. The
TGA curves for RGO-CuL shows three-step weight losses from 20 to 800 °C under nitrogen
atmosphere [46]. Weight loss below 100 °C corresponds to physically adsorbed water. The
second weight loss is in the range 140–240 °C, is attributed to the thermal decomposition of
oxygen-carrying functionalities which have not participated in further chemical fictionalization.
The third major weight loss is in the range 220–600 °C due to the decomposition of the Schiff
base [47].
1
2

(
b)
(
a)
Fig. 5. (a) TGA curves of the RGO and (b) RGO-CuL
EDX elemental mapping was performed to understand the distribution of the Cu-Schiff base
complex on RGO (Fig 6).
GO
GO-CuL
Fig. 6. The energy-dispersive X-ray spectroscopy (EDX) spectrum of RGO and CuL-RGO
The obtained RGO and RGO-CuL samples were identified using XRD. As shown in Fig 7, the
XRD pattern of RGO shows peaks at 2θ= 19° and 25°, which are due to the introduction oo f
oxygenic functional groups and trapped water molecules between the graphite layers [48]. After
immobilization of the metal Schiff base complex on the RGO surface, the diffraction peaks at
around 19° and 25° do not disappear, suggesting that the structure of RGO is not destroyed
during the preparation procedure of RGO-CuL. However, it is clear from the XRD pattern that
the line intensity of RGO is higher than for RGO-CuL. This difference shows the presence of the
Schiff base complex on the surface of RGO.
1
3

(
a)
(b)
Fig. 7. XRD patterns of (a) GO and (b) GO-CuL
The UV–visible spectra of RGO and RGO-CuL are shown in Fig 8. RGO shows two
absorption peaks due to the π•π ** transi tt ion of g rr aphiti cc carbon dd ouble bonds at 210 nm and the
n•π* transition of carbonyl groups at around 248 nm [49]. After loading Schiff base onto RGO,
the π–π* and n–π* tra nn sitio nn s of RGO-CuL show a red shift. The red shift might be related to the
involvement of carbonyl/C–OH groups of RGO in binding the copper Schiff base complex.
Fig. 8. UV-visible spectrum of RGO and RGO-CuL
3.3. Catalytic activity of the synthesized catalyst
The catalytic activity of the RGO-CuL for oxidation of olefins with hydrogen peroxide were
investigated.
1
4

3
.3.1. Oxidation of alkenes
Blank experiments show that in the oxidation of cyclooctene with hydrogen peroxide
proceeded sluggishly and gave only 8 % yield of cyclooctene oxide after 1 h at 50 ᵒC. Also we
performed a blank experiment in the presence of neat RGO at 50 °C, which afforded no
cyclooctene oxide after 1 h. Catalytic performance of the immobilized catalyst has been
evaluated through the epoxidation of olefins using H O . The heterogeneous catalyst (RGO-CuL)
2
2
was examined for epoxidation of cyclooctene in ethanol. Oxidation of cyclooctene with TBHP,
UHP and H O in the presence of catalytic amounts of RGO-CuL led to the formation of 47, 38
2
2
and 41% cyclooctene oxide as the sole product respectively (Table 5, No. 5,10 and 15). In this
study H O was selected as a green oxidant for further investigation. The effect of temperature
2
2
(
25˚C, 35˚C, and 50˚C) on the heterogeneous oxidation of cyclooctene by RGO-CuL in ethanol
has been studied (Table 5, No. 20,21 and 22). The highest oxidation product was achieved at
5
0˚C.
Table 5
a,b,c,d.
The effect of various conditions on the oxidation of cyclooctene catalyzed by RGO-CuL.
No
Time
T(˚C)
OX
alkene/OX
Conversion %
Solvent
cat. (g)
1
2
3
4
5
6
7
8
9
4h
4h
4h
4h
4h
4h
4h
4h
4h
4h
4h
4h
4h
4h
4h
4h
4h
4h
4h
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
UHP
UHP
UHP
UHP
UHP
1:1
1:2
1:3
1:4
1:5
1:1
1:2
1:3
1:4
1:5
1:1
1:2
1:3
1:4
1:5
1:10
1:10
1:10
1:10
13
19
23
32
38
16
27
33
39
47
15
24
29
36
41
100
23
44
39
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
CHCl3
0.009
0.009
0.009
0.009
0.009
0.009
0.009
0.009
0.009
0.009
0.009
0.009
0.009
0.009
0.009
0.009
0.009
0.009
0.009
TBHP
TBHP
TBHP
TBHP
TBHP
H O
1
1
1
1
1
1
1
1
1
1
0
1
2
3
4
5
6
7
8
9
2
2
2
2
2
2
2
2
2
2
H O
2
H O
2
H O
2
H O
2
H O
2
H O
2
H O
CH CN
2
3
H O
CH Cl2
2
2
1
5

2
2
2
2
2
0
1
2
3
4
5
1h
1h
1h
1h
1h
1h
25
35
50
50
50
50
H O2
1:10
1:10
1:10
1:10
1:10
1:10
41
70
100
59
30
8
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
0.009
0.009
0.009
0.006
0.003
0
2
H O2
2
H O2
2
H O2
2
H O2
2
2
H O2
2
a
Entry 1-16: Reaction conditions: catalyst (mmol): cyclooctene: oxidant: is 1:50:X at room temperature for 4h
b
reaction time. Entry 16-19: Reaction conditions: catalyst (mmol): cyclooctene: H O : is 1:50:500 in different
solvents at room temperature for 4h reaction time. Entry 20-22: Reaction conditions: catalyst (mmol): cyclooctene:
H O : is 1:50:500 at different temperature for 60 min reaction time. Entry 22-25: Reaction conditions: catalyst
2
2
c
d
2
2
(
mmol): cyclooctene: H O : is 1:50:500 at different amount of catalyst for 60 min reaction time.
2
2
The highest conversion of RGO-CuL (100%) with respect to the neat (homogeneous system)
CuL (93%) in the oxidation of cyclooctene in the optimal condition (catalyst (mmol): alkene:
H O is 1:50:500 at 50 °C for a 1h reaction time) illustrate the advantage of the heterogeneous
2
2
catalyst over homogeneous catalyst. After the reaction, the solid catalyst could be easily
recovered from the reaction mixture by direct filtration and ready for reuse after washing with
chloroform and ethanol. The intrinsic stability of RGO nanosheet, allows the catalyst to be
recovered efficiently at least eight times without any significant loss of the catalytic activity
during its reuse in the oxidation of cyclooctene (Fig 9).
100
8
6
4
2
0
0
0
0
0
Run
1
2
3
4
5
6
7
Conversion % 100
100
100
100
100
100
100
1
6

Fig. 9. Results of catalyst reuse experiments in the epoxidation of cyclooctene. Reaction
conditions: catalyst (mmol): alkene: H O is 1:50:500 at 50 °C for a 1 h reaction time.
2
2
In order to investigate the universality of the heterogeneous catalyst, further studies on a range
of olefins were performed (Table 6). Excellent conversion and selectivity to the epoxide were
obtained within 1 h reaction time. Complete conversion of cyclooctene, cyclohexene, styrene, 4-
methyl styrene, α-methyl styrene, indene and stilbene was achieved. 4-Chloro styrene showed
lower conversion ratio, with a conversion of 91% whereas the chain aliphatic 1-octene gave 87%
conversion. The relatively lower conversion of the long-chain olefin is most likely attributed to
the increased steric hindrance raised from chain structure. These catalytic results revealed that
RGO-CuL was an efficient catalyst for the epoxidation of olefins.
In order to estimate the longevity of the heterogeneous catalytic system, total turnover number
(
TON) has been studied. The catalyst exhibited excellent activities for oxidation of styrene in
ethanol solution under mild conditions (Cat.:Styrene:H O is 1:50000:500000) with a TON of
2
2
4
7000 after 24 h which is one of the highest values reported to date for a heterogeneous catalytic
system.
Table 6
1
7

a
The effect of various conditions on the oxidation of cyclooctene catalyzed by RGO-CuL
f
Substrate
products
Conversion %
Selectivity %
TON
(Epoxide)
1
1
1
00
00
00
100
50
50
50
O
100
O
O
b
97(3)
c
O
1
00
99(1)
50
H CO
3
H CO
3
O
9
1
100
45.5
50
Cl
Cl
O
d
1
00
89(11)
O
1
1
00
00
100
50
50
O
e
94(6)
H C
3
H C
3
O
5
87
100
100
43.5
50
5
O
1
00
a
The molar ratio of catalyst:alkene:H O was 1:50:500 in EtOH at 50°C for a 1 h reaction time.
2
2
b
c
d
e
Benzaldehyde is the by-product. 4-Methoxybenzaldehyde is the by-product. Acetophenone is the by-product.
-Methylbenzaldehyde is the by-product.
f
4
1
8

3
.4. Electrochemical studies
The electrochemical properties of the copper(II) complex as heterogeneous catalyst for the
oxidation of cyclooctene have been investigated in the potential range of -3 to 3 V, in CH CN
3
containing 0.1 M tetrabutylammonium hexafluorophosphate (TBAH) as the supporting
-1
electrode, at scan rate of 100 mVs (Fig. 10). An increase in the current intensity in the presence
of cyclooctene and RGO-CuL (Fig. 10a) compared to the blank experiment (oxidation of
cyclooctene with H O in the absence of catalyst) clearly indicates the catalytic activity of the
2
2
catalysts. Also cyclic voltammograms of (Glassy Carbon)/RGO-CuL in the presence of 50 mmol
cyclooctene at various potential sweep rates and the proportionality of anodic and cathodic peak
-1
currents to the square root of sweep rates in a range of 100 to 800 mVs are illustrated in Fig
1
0b. The oxidation or reduction peaks became broader with increasing scan rate. In addition, in
higher scan rates, the oxidation peak became bigger at a much more positive potential. This
phenomenon indicates that the electro-oxidation of copper species to higher valance state is
much faster than the catalytic oxidation of substrates. Meanwhile, the anodic peak currents that
are linearly proportional to the square root of scan rate suggest that the overall oxidation of
cyclooctene at this electrode is controlled by the diffusion of alkene to the surface redox sites
[
50].
Fig. 10. (a) The voltammetric behavior of RGO-CuL with blank at 100 mV/s (b) Cyclic voltammograms of RGO-
CuL in different scan rates (100, 200, 400, 600, 800 ( m V/s)). Reaction conditions: catalyst (mmol): alkene: H O : is
2
2
1
:50:500 at room temperature time between -3 and 3 V vs. reference electrode. in acetonitrile solution containing 0.1
M TBAH as supporting electrolyte.
1
9

Chronoamperometry of the acetonitrile solution of the RGO-CuL (red line) and blank (blue
line) in the presence of cyclooctene (Fig. 11) was used to confirm the conclusions derived from
cyclic voltammograms of the catalysts. The current intensity at the oxidation and reduction
potentials indicated the higher catalytic activity of the RGO-CuL system when compared to the
blank test.
Fig. 11. Chronoamperograms of RGO-CuL and blank in (a) oxidation potential (0.88 v) and (b) reduction potential
(
-1.64 v) in acetonitrile solution containing. 0.1 M TBAH as the supporting electrolyte for oxidation of cyclooctene.
(
Reaction conditions: catalyst (mmol): alkene: H O : is 1:50:500.
2
2
4. Conclusion
In this study, a tetradentate Schiff base (N O ) ligand and its corresponding copper(II) complex
2
2
CuL were synthesized and characterized. CuL was supported on the surface of graphene oxide
nanosheet witch led to the formation of RGO-CuL. The potential catalytic activity of RGO-CuL
as heterogeneous catalyst for oxidation of alkenes was examined by H O as a green oxidant in
2
2
ethanol by approaching to the green chemistry. The heterogeneous catalyst was reused multiple
consecutive times for oxidation of cyclooctene without any noticeable loss in activity or
selectivity of nanocatalyst. The synthesized RGO-CuL catalyst system exhibited excellent
2
0

activities for oxidation of styrene in ethanol solution with a TON of 47000 after 24 h which is
one of the highest values reported to date for a heterogeneous catalytic system.
Acknowledgements
The financial support of this work by K.N. Toosi University of Technology research council is
acknowledged.
Reference
[
[
[
1] G. Barf, R. Sheldon, J. Mol. Catal. A: Chem. 102 (1995) 23-39.
2] J.R. Monnier, Appl. Catal. A: Gen. 221 (2001) 73-91.
3] Y. Shen, P. Jiang, J. Zhang, G. Bian, P. Zhang, Y. Dong, W. Zhang, Mol. Catal.,
4
33 (2017) 212-223.
4] S. Rayati, N. Torabi, A. Ghaemi, S. Mohebbi, A. Wojtczak, A. Kozakiewicz, Inorg.
Chim. Acta, 361 (2008) 1239-1245.
[
[
[
[
[
[
5] S. Rayati, N. Rafiee, A. Wojtczak, Inorg. Chim. Acta, 386 (2012) 27-35.
6] S. Rayati, S. Zakavi, V. Noroozi, S. H. Motlagh, Catal. Commun., 10 (2008) 221-226.
7] S. Zakavi, F. Heidarizadi, S. Rayati, Inorg. Chem. Commun., 14 (2011) 1010-1013.
8] S. Rayati, A. Ghaemi, N. Sadeghzadeh, Catal. Commun., 11 (2010) 792-796.
9] M. Sedighipoor, A. H. Kianfar, W. A. K. Mahmood, M. H. Azarian, Inorg. Chim.
Acta, 457 (2017) 116-121.
[
10] M. R. Maurya, L. Rana, F. Avecilla, Inorg. Chim. Acta, (429) (2015) 138-147.
11] S. Rayati, S. Zakavi, M. Koliaei, A. Wojtczak, A. Kozakiewicz, Inorg. Chem.
[
Commun., 13 (2010) 203-207.
2
1

[12] F. Heshmatpour, S. Rayati, M. A. Hajiabbas, P. Abdolalian, B. Neumüller,
Polyhedron, 31 (2012) 443-450.
[
[
[
13] N. Pal and A. Bhaumik, RSC Adv. 5 (2015) 24363-24391.
14] M. Ferré, R. Pleixats, M. W. Manand, X. Cattoën, Green Chem. 18 (2016) 881-922.
15] X. Wang, S. Wu, Z. Li, X. Yang, H. Su, J. Hu, Q. Huo, J. Guan, Q. Kan, Micropor.
Mesopor. Mater., 221 (2016) 58–66.
[
[
[
16] Edmond Lam and John H.T. Luong, ACS Catal. 4 (2014) 3393–3410.
17] T. Krawczyk, K. Jasiak, A. Kokolus, Catal Lett. 146 (2016) 1163.
18] X. Z. Shu, S. C. Nguyen, Y. He, F. Oba, Q. Zhang, C. Canlas, G. A. Somorjai, A. P.
Alivisatos, and F. D.Toste, J. Am. Chem. Soc. 137 (2015) 7083−7086.
[
19] M. Nandi, P. Roy, H. Uyama, A. Bhaumik, Dalton Trans. 40 (2011) 12510-12518.
20] T. Ennaert, J. V. Aelst, J. Dijkmans, R. D. Clercq, W. Schutyser, M. Dusselier, D.
[
Verboekend and B. F. Sels, Chem. Soc. Rev. 45 (2016) 584-611.
[21] Y. Hou, N. Wang, J. Zhang and W. Qian, RSC Adv. 7 (2017) 14309-14313.
[22] S. C. Oh, T. Nguyendo, Y. He, A. Filie, Y. Wu, D. T. Tran, I. C. Lee and D. Liu,
Catal. Sci. Technol. 7 (2017) 1153-1166.
[23] B. Tang, W. C. Song, E. C. Yang and X. J. Zhao, RSC Adv.7 (2017) 1531.
[24] K. J. Dunst, B. Scheibe, G. Nowaczyk, S. Jurga and P. Jasiński, Meas. Sci. Technol,
2
8 (2017) 1133-1139.
25] S. H. Dave, C. RGOng, A. W. Robertson, J. H. Warner and J. C. Grossman, ACS
Nano. 10 (2016) 7515−7522.
26] W. Meng, E. Gall, F. Ke, Z. Zeng, B. Kopchick, R. Timsina, X. Qiu, J. Phys. Chem.
C. 119 (2015) 21135–21140.
[
[
2
2

[
27] Z. Li, S. Wu, D. Zheng, H. Ding, X. Wang, X. Yang, Q. Huo, J. Guan, Q. Kan,
ChemPlusChem, 79(5) (2014) 716–724.
28] M. D. Meganathan, S. Mao, T. Huang, G. Sun, J. Mater. Chem. A. 5 (2017) 2972-
980.
[
2
[29] Y. Wang, Z. Wen, H. Zhang, G. Cao, Q. Sun, J. Cao , Catalysts. 6 (2016) 214.
[30] S. Kumari, A. Shekharand D. D. Pathak, New J. Chem. 40 (2016) 5053-5060.
[31] M. R. Acocella, C. E. Corcione, A. Giuri, M. Maggio, A. Maffezzoli and G. Guerra,
RSC Adv. 6 (2016) 23858-23865.
32] A. Primo, M. Puche, O. D. Pavel, B. Cojocaru, A. Tirsoaga, V. Parvulescu and
H.García, Chem. Commun. 52 (2016) 1839-1842.
[
[
[
[
33] S. Kumari, A. Shekhar and D.evendra D. Pathak, RSC Adv. 6 (2016) 15340-15344.
34] S. M. Aila, S. Kaul and S. L. Jain, RSC Adv.4 (2014) 30598-30604.
35] Q. Zhao, C. Bai, W. Zhang, Y. Li, G. Zhang, F. Zhang, and X. Fan, Ind. Eng. Chem.
Res. 53 (2014) 4232–4238.
[
[
[
36] G.M. Sheldrick, Acta Crystallographica A. 64 (2008) 112-122.
37] CrysAlisPro 1.171.38.43 package of programs, Rigaku OD, 2015.
38] L. Wu, L. Yu, X. Ding, P. Li, X. Dai, X. Chen, H. Zhou, Y. Bai, J. Ding, Food
Chem, 217 (2017) 320–325.
[39] S. Balalaie, M. Mahdidust, R.E. Najafabadi. Chin. J. Chem. 26 (2008) 1141.
[40] R. A. Nyquist, R.O. Kagel, Infrared Spectra of Inorganic Compounds, Academic
Press, New York, London, 1971.
2
3

[41] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination
Compounds, Part B: Applications in Coordination, Organometallic and Bioinorganic
Chemistry, Ed., Wiley., New York, 2009.
[42] A. A. Adel, E. Azza, A. A. Abou-Hussein, Spectrochim. Acta Part A. 64 (2006)
1010-1024.
[
[
[
(
[
43] D. Chen, H. Feng, J. Li, Chem. Rev. 112 (2012) 6027.
44] S. Choudhary, H. P. Mungse, O. P. Khatri, J. Mater. Chem. 22 (2012) 21032.
45] H. P. Mungse, S. Verma, N. Kumar, B. Sain and O. P. Khatri, J. Mater. Chem., 22
2012) 5427.
46] Y. Zhu, Y. Shi, Z. Huang, L. Duan, Y. Hu and X. RGOng., RSC Adv. 6 (2016)
9
0018.
47] L. Stobinski, B. Lesiak, A. Malolepszy, M. Mazurkiewicz, B. Mierzwa, J. Zemek, P.
Jiricek, I. Bieloshapka, J. Electron. Spectrosc. Relat. Phenom. 195 (2014) 145–154.
[
[
48] A. Ourari, D. AgRGOun, L. Ouahab, Inorg. Chem. Commun. 33 (2013) 118-124.
49] S. Sakthinathan, S. Kubendhiran, S. M. Chen, F. M. A. Al-Hemaid, W. C. Liao, P.
[
Tamizhdurai, S. Sivasanker, M. A. Alic and A. A. Hatamlehc, RSC Adv. 6 (2016)
07416.
50] R. S Nicholson, I. Shain. Anal. Chem. 4 (1964) 36.
1
[
2
4

•
•
•
•
•
Cu(III) Schiff base complex grafted onto graphene oxide
Excellent activity of the heterogeneous catalyst for oxidation of olefins
Reusability of the catalyst with negligible leaching of metal ion
Investigation of the electrochemical properties of the Cu-complex
excellent turnover numbers for the green epoxidation of styrene
2
5

Cu-Schiff base complex grafted onto graphene oxide nanocomposite:
Synthesis, crystal structure, electrochemical properties and catalytic activity
in green oxidation of olefins
Saeed Rayati, Elham Khodaei, Saeedeh Shokoohi, Majid Jafarian,Bahareh Elmi, Andrzej
Wojtczak
O
C
C
C
C
C
The synthesis, crystal structure, electrochemical behaviour and catalytic performance of a Cu-
Schiff base complex grafted onto graphene oxide as a separable heterogeneous catalyst in the
selective and green oxidation of olefins in ethanol are described.
2
6