Copper(II) Schiff base complex immobilized on graphene nanosheets: a heterogeneous catalyst for epoxidation of olefins

Article abstract of DOI:10.1007/s13738-018-1552-3

The present paper describes the preparation and characterization of Cu(II) Schiff base complex immobilized onto graphene oxide. The structural and morphological characterization of the heterogeneous catalyst was carried out by different techniques such as Fourier transform infrared and diffuse reflectance UV–Vis spectroscopies, X-ray diffraction, thermogravimetric analysis, nitrogen adsorption–desorption, transmission electron microscopy, field emission scanning electron microscopy and inductively coupled plasma atomic emission spectroscopy. The catalytic activity of the heterogeneous catalyst was studied in the epoxidation of various alkenes using tert-butyl hydroperoxide as an oxidant and it showed high selectivity and catalytic reactivity. The graphene-bound copper Schiff base was successfully reused for several runs without significant loss in its catalytic activity.

Full text of DOI:10.1007/s13738-018-1552-3

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-018-1552-3
ORIGINAL PAPER
Copper(II) Schiff base complex immobilized on graphene nanosheets:
a heterogeneous catalyst for epoxidation of olefins
Ali Zarnegaryan2 · Zari Pahlevanneshan1 · Majid Moghadam1 · Shahram Tangestaninejad1 · Valiollah Mirkhani1 ·
Iraj Mohammdpoor‑Baltork1
Received: 15 March 2018 / Accepted: 12 November 2018
© Iranian Chemical Society 2019
Abstract
The present paper describes the preparation and characterization of Cu(II) Schiff base complex immobilized onto graphene
oxide. The structural and morphological characterization of the heterogeneous catalyst was carried out by different techniques
such as Fourier transform infrared and diffuse reflectance UV–Vis spectroscopies, X-ray diffraction, thermogravimetric
analysis, nitrogen adsorption–desorption, transmission electron microscopy, field emission scanning electron microscopy
and inductively coupled plasma atomic emission spectroscopy. The catalytic activity of the heterogeneous catalyst was
studied in the epoxidation of various alkenes using tert-butyl hydroperoxide as an oxidant and it showed high selectivity
and catalytic reactivity. The graphene-bound copper Schiff base was successfully reused for several runs without significant
loss in its catalytic activity.
Keywords Graphene nanosheets · Cu(II) Schiff base · Olefin epoxidation · TBHP · Heterogeneous catalyst
Introduction
support and thus provide an efficient approach to prepare
site-isolated catalysts [5]. Metal Schiff base complexes are
of great importance in the coordination chemistry because of
their stability, structural variability, ease of preparation and
a variety of applications [6, 7]. However, difficult recovery
and non-recyclability of these catalysts make them undesir-
able for large-scale applications. Supported Schiff base com-
plexes have been used for oxidation of olefins and other sub-
stances. Many methods and strategies have been reported to
immobilize the homogeneous catalysts on insoluble supports
such as (i) encapsulation in zeolites, clay or siloxane mem-
branes, (ii) covalent grafting onto inorganic supports such as
silica or MCM-41, (iii) co-polymerization of a functional-
ized salen monomer into an organic polymer, (iv) attachment
or build-up of a salen structure to a performed polymer, (v)
and axial ligation to surface bound ligands [8–18]. Nowa-
days, graphene has attracted much attention due to its novel
and unique properties [19, 20]. Graphene oxide (GO), the
product of oxidation of graphite, possesses abundant oxy-
gen-containing functional groups, which not only render GO
moderate water dispersibility but also offer reactive sites for
further modification [21]. Different chemical reactions have
been reported for modifying and subsequently exfoliating
single and/or few graphene layers from bulk graphite [22,
23], Graphene oxide-based materials have found various
Epoxidation of alkenes catalyzed by metal complexes is an
important reaction in organic synthesis because these com-
pounds serve as useful intermediates that can be used for
synthesis of a wide variety of other compounds [1–3]. In
recent years, heterogenization of homogeneous catalysts
has attracted much attention because they combine the best
properties of both homogeneous and heterogeneous catalysts
[4]. Among the various approaches for immobilizing soluble
catalysts, covalent attachment has been the most frequently
used strategy, as the resulting heterogeneous catalysts have
good stability during the course of catalytic reactions. The
stable covalent bonds reduce the catalyst leaching from the
* Majid Moghadam
moghadamm@sci.ui.ac.ir
* Shahram Tangestaninejad
stanges@sci.ui.ac.ir
* Valiollah Mirkhani
mirkhani@sci.ui.ac.ir
1
Department of Chemistry, Catalysis Division, University
of Isfahan, 81746-73441 Isfahan, Iran
2
Department of Chemistry, Yasouj University,
75918-74831 Yasouj, Iran
Vol.:(0123456789)
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Journal of the Iranian Chemical Society
applications including sensors, solar cells, catalysis, and
gas storage [24–28]. Covalent modification of graphene
involves the reaction of functional molecules and the oxy-
genated groups on the GO surface such as carboxyl groups
at the periphery, and epoxy, hydroxyl, and C=C groups in
the basal plane of GO [29]. For example, the basal surface of
GO can be functionalized by the nucleophilic ring-opening
reaction between the epoxy groups of GO and the amine
groups of 1-(3-aminopropyl)-3-methylimidazolium bromide
(an ionic liquid) and 3-aminopropyltriethoxysilane (APTS)
[30, 31]. GO is one of the most popular supports used in
synthetic organic chemistry because its unique nanostruc-
ture, high specific surface area with intrinsic low mass, high
density of functional groups such as hydroxyl and epoxide
groups on both sides of its surface, ready availability, facile
functionalization, and excellent mechanical strength [32,
33]. These characteristics make GO as the most promising
supports to immobilize various homogeneous materials,
including transition metal complexes as well as metal Schiff
base complexes for olefin epoxidation [34–41]. For instance,
Mungse et al. prepared oxo-vanadium Schiff base covalently
anchored onto graphene oxide sheets for oxidation of alco-
hols [42]. Su et al. reported GO-supported transition metal
Schiff base complexes as efficient and recyclable catalysts
for epoxidation of styrene [43]. Zhao et al. prepared GO-
supported copper(salen) complex for epoxidation of different
alkenes [44]. In this work, we wish to report the design and
characterization of a Cu(II) Schiff base complex supported
on graphene oxide. In this manner, the copper complex of
N,N′-(1,2-ethanediyl)bis(1-(4-pyridyl)methanimine) was
attached to graphene oxide modified with 3-chloropropyltri-
methoxysilane. Also, its catalytic activity was investigated in
the epoxidation of different alkenes using dichloromethane
as solvent and TBHP as oxidant (Scheme 1).
in the range of 400–4000 cm−1 with a JASCO 6300D
instrument. Diffuse reflectance UV–Vis (DR UV–Vis)
spectra were obtained by a JASCO V-670 (190–700 nm)
spectrophotometer. The ICP analyses were performed on
an ICP-Spectrociros CCD instrument. The XRD measure-
ments were carried out using a Bruker D8-advance X-ray
diffractometer with Cu Kα radiation (λ = 1.5406 Å). The
TEM images were taken using a Philips CM10 instrument.
Thermogravimetric analyses (TGA) of the samples were
carried out using a STA (model: 6000 Perkin-Elmer, The
Netherlands). TGA analyses were performed by heating
at a constant rate of 10 °C min−1 from 50 to 900 °C under
argon atmosphere. The yields were determined by a Shi-
madzu GC-16A gas chromatograph (GC) equipped with
a flame ionization detector. In GC experiments, n-decane
was used as internal standard. Scanning electron micros-
copy (SEM) images were obtained using a Hitachi S-4800
field emission scanning electron microscope (FE-SEM)
and iridium (IXRF Systems) software. N2 adsorption des-
orption isotherm, Brunauer–Emmett–Teller (BET) surface
area, pore volume, pore diameter and other surface proper-
ties of samples were obtained by a Micromeritics instru-
ment (Chem. Bet-3000). The copper loading of the cata-
lyst was determined using an inductively coupled plasma
atomic emission spectrometer (ICP-AES). Graphene oxide
(GO) was oxidized from graphite powder by a modified
Hummer method [45].
The functionalized GO, GO-CP, was prepared by func-
tionalization of GO with 3-chloropropyltrimethoxysilane
(CPTMS) as previously reported [44]. The Schiff base
ligand, N,N′-(1,2-ethanediyl)bis(1-(4-pyridyl)methanimine),
was prepared as described in the literature [46].
Synthesis of Schiff base@GO‑CP
Experimental
The GO-CP (500 mg) was dispersed in toluene (100 ml)
with vigorous stirring for 30 min. Then, Schiff base ligand
(1000 mg) was added to this mixture and the resulting
mixture was refluxed for 24 h under nitrogen protection to
execute the reaction between chloro groups of CPTMS moi-
eties and nitrogen atoms Schiff base ligand. After the end of
the reaction, the solid materials were collected by filtration,
Materials and methods
All chemicals were purchased from Merck and Sigma-
Aldrich chemical companies and used without purification.
FT-IR spectra were obtained as potassium bromide pellets
Scheme 1 Alkene epoxida-
tion with TBHP catalyzed by
CuII(Schiff base)Cl2@GO-CP
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Scheme 2 Preparation of the
heterogeneous catalyst
washed with toluene, CH2Cl2 successively to remove the
unreacted Schiff base ligand.
General procedure for catalytic epoxidation
of olefins with TBHP
To a mixture of alkene (1 mmol) and CuII(Schiff base)Cl2@
GO-CP catalyst (45 mg, 0.01 mmol) in dichloromethane
(5 ml), TBHP (2 mmol) was added and stirred at room tem-
perature. Progress of the reaction was monitored by GC.
After completion of the reaction, the catalyst was separated
by centrifugation, washed with dichloromethane, and dried
at 50 °C. The recovered catalyst was used for investigation
of the catalyst reusability.
Immobilization of copper Schiff base on GO‑CP
The Schiff base@GO-CP (600 mg) was dispersed in etha-
nol (50 ml) and refluxed for 12 h in the presence of CuCl2
(200 mg) under a nitrogen atmosphere. After reaction com-
pletion, the product was washed with ethanol (4×10 ml) to
remove the unreacted copper(II) chloride. The final product
was dried in an oven. This black powder, graphene oxide-
immobilized copper Schiff base, CuII(Schiff base)Cl2@
GO-CP, was used as a catalyst for the epoxidation of olefins.
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Journal of the Iranian Chemical Society
content of the catalyst, measured by ICP, was found to be
0.23 mmol g−1.
The FT-IR spectra of GO, GO-CP, Schiff base ligand
and CuII(Schiff base)complex@GO-CP catalyst are shown
in Fig. 1. In the FT-IR spectrum of GO (Fig. 1a), the strong
band at 1726 cm−1 (C=O) represents carboxylic acid, and
the bands at 1051, 1223 and 1618 cm−1 are attributed to the
C–O (epoxy), C–OH and C=C groups in graphene oxide,
respectively [42].
In addition, a broad band at 3429 cm−1 is attributed to
the stretching mode of O–H bonds, revealing the presence
of many hydroxyl groups. After modification of GO with
CPTMS, the bands at 1130 and 1037 cm−1 illustrate the
presence of Si–O–Si and Si–O–C bonds, respectively. Also,
the bands at 2880 and 2920 cm−1 are corresponded to the ali-
phatic C–H bonds. The appearance of these new bands indi-
cates the successful grafting of CPTMS on graphene oxide
nanosheets through chemical bonding (Fig. 1b). The FT-IR
spectra of the ligand Schiff base (Fig. 1c) showed a strong
band at 1644 cm−1 which is assigned to stretching vibration
of C=N group. These observations confirm the successful
grafting of the copper Schiff base complex onto graphene
oxide nanosheets by covalent interaction. This band shifts to
lower frequency (35 cm−1) in Cu(II) complex which proved
the coordination of Cu to Schiff base [50, 51]. The new
stretching vibration bands at 2927, 2876, 1578 and 1230
cm−1 suggest the grafting of Cu(II) complex on the GO-CP
(Fig. 1e). In comparison with the IR spectrum of GO, the
relative intensity for C=O decreased and a shift of peak for
O–H vibration was observed for the Schiff base@GO-CP
heterogeneous catalyst (Fig. 1d). The change in intensity
might be indicated as a possible evidence for carboxylic acid
coordination of Cu (II) ion [52, 53]. The nature of recovered
catalyst was investigated by FT-IR spectroscopy. As can be
seen, the FT-IR spectrum of the reused catalyst is similar to
Fig. 1 FT-IR spectra of a GO, b GO-CP, c Schiff base ligand, d
Schiff base @GO-CP, e heterogeneous catalyst and f recycled of the
heterogeneous catalyst
Results and discussion
Synthesis and characterization of catalyst
3-Chloropropyltrimethoxysilane (CPTMS), which is a
coupling agent to attach the complexes on the surface of
GO [47–49], was used for anchoring the CuII(Schiff base)
complex on GO. First, 3-chloropropyltrimethoxysilane was
grafted on graphene nanosheets by chemical interaction
between methoxy groups of CPTMS and hydroxyl groups
available on graphene oxide. The Schiff base ligand was
attached to GO-CP with via C–N bond. Finally, the copper
complex was prepared by the reaction of GO-CP-supported
Schiff base with copper(II) chloride (Scheme 2). The copper
Fig. 2 UV–Vis spectra of a GO,
b Cu(II)(Shiff-base)@GO-CP
catalyst
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Fig. 3 Nitrogen adsorption–desorption of heterogeneous catalyst
Fig. 4 The TG curves of a GO, b GO-CP, and c heterogeneous cata-
lyst
Fig. 5 FE-SEM images of a GO and b heterogeneous catalyst and
EDX spectrum of: c GO and d heterogeneous catalyst
fresh catalyst which shows the stability of the reused catalyst
during the catalytic cycles (Fig. 1f).
The diffuse reflectance UV–Vis spectra of GO and
CuII(Schiff base)Cl2@GO-CP catalyst are shown in Fig. 2.
The UV–Vis spectrum of GO shows a strong absorption
peak at 224 nm owing to discrete units of sp2-bonded
carbons [54] and an absorption peak shoulder at around
300 nm, which are attributed to n–π* transition of C=O
bonds (Fig. 2a) [42]. In addition, the absorption bands
observed in the 280–330 nm region can be assigned to the n
→ π* and π → π* ligand charge transfer in CuII(Schiff base)
Cl2@GO-CP [55]. The very weak bands at ca. 450 nm may
be attributed to d–d transitions of the centred Cu ion, sup-
porting the immobilization of CuII(Schiff base)Cl2 on GO.
BET surface area, pore diameter and pore of the
CuII(Schiff base)Cl2@GO-CP were measured using nitro-
gen adsorption–desorption isotherms at 77 K (Fig. 3). The
nitrogen adsorption–desorption analysis of heterogeneous
catalyst showed a type V isotherm and H1 hysteresis loop
with intensity lower than its parent (Fig. 3). The BET surface
area and total pore volume of the CuII(Schiff base)@GO
were decreased to 15.79 m2 g−1 and 0.066, cm3 g−1, respec-
tively. The mean pore diameter of the catalyst was also found
to be 16.81 nm according to BJH method [56].
The TGA curves of GO, GO-CP and heterogeneous cata-
lyst in the range of 50–900 °C and a heating rate of 10 °C
min−1 under air atmosphere are shown in Fig. 4. Graphene
oxide shows a low weight loss (3.5 wt%) below 100 °C, evi-
dently owing to evaporation of water molecules which are
held in the material. The second significant weight loss is
observed around 200 °C, due to decomposition of the labile
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Schiff base catalyst illustrates an exothermic major weight
loss over a wide range of temperature (180–450 °C) on
account of slow decomposition of the Schiff base complex
(Fig. 4c) [43].
The morphologies of the surface GO and CuII(Schiff
base)Cl2@GO-CP were studied by field emission scanning
electron microscopy (FE-SEM). As shown in (Fig. 5a),
there are large flakes of GO with macroscopic wrinkling.
Compared to GO sheets, the FE-SEM image of CuII(Schiff
base)Cl2@GO-CP (Fig. 5b) exhibited an agglomerated
layered structure incorporating copper complex moieties
between the sheets. The energy dispersive X-ray spec-
troscopy (EDX) analysis of GO (Fig. 5c) showed that the
major elements are carbon and oxygen. In the EDX spec-
trum of CuII(Schiff base)Cl2@GO-CP (Fig. 5d), the pres-
ence of Cu, N and Cl peaks originates from CuII(Schiff
base)Cl2 which is a good indication in for attachment of
CuII(Schiff base)Cl2 to GO-CP sheets.
The corresponding elemental mapping of the hetero-
geneous catalyst was also performed to understand the
distribution of the copper Schiff base complex on GO-CP.
As can be seen, all carbon, oxygen, silicon, nitrogen and
copper elements have uniform distribution in the catalyst
texture which showed successful immobilization of the
catalyst on GO-CP (Fig. 6).
The texture of the catalyst was also studied by transmis-
sion electron microscopy (TEM). The TEM image of GO
shows transparent stacked sheets while the TEM image of
the Cu catalyst (Fig. 7) shows evenly distributed dark spots,
which are probably due to the attachment of the CuII(Schiff
base)Cl2 onto GO.
Fig. 6 Elemental mapping images of heterogeneous catalyst
oxygen-containing functional groups, yielding CO, CO2, and
steam, and the latter is attributed to pyrolysis of the carbon
skeleton of GO (Fig. 4a) [57, 58]. After chemical function-
alization of GO with CPTMS, the GO-CP shows gradual
weight loss in the temperature range of 140–180 °C and sig-
nificant weight loss around 210 °C. The former can be attrib-
uted mostly to the COOH (undigested oxygen carrying func-
tionalities) groups that are incomplete dearomatized, which
have not participated in interaction with CPTMS (Fig. 4b)
[57]. The thermogram of the graphene-immobilized copper
The XRD patterns are used to study the changes in struc-
ture (Fig. 8). For GO, two peaks at 2θ = 11.3° and 24.26°
correspond to (001) and (002) reflections of GO and graphite
powder, respectively (Fig. 8a) [42]. After immobilization of
CP on GO, a broad peak appeared at around 2θ=21° which
corresponds to amorphous silica (Fig. 8b) [59]. By immobi-
lization of CuII(Schiff base)Cl2, the diffraction peaks become
Fig. 7 TEM images of heteroge-
neous catalyst
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Table 1 Optimization of reaction conditions in the epoxidation of
cyclooctene at room temperature
Entry
Solvent
Catalyst (mol%)
Yield (%)a
1
2
3
4
5
6
7
8
EtOAc
CH3CN
MeOH
THF
EtOH–H2O
C2H4Cl2
H2O
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
1
1
1
1
1
1
1
1
1.25
0.75
0.5
0.25
0
78
88
51
45
32
48
20
99
99
76
54
28
5
9
10
11
12
13
Fig. 8 XRD patterns of a GO; b GO–CP and c heterogeneous catalyst
Reaction conditions: cyclooctene (1 mmol), TBHP (2 mmol), catalyst
aDetermined by GC based on the starting cyclooctene, after 5 h
increased very small from GO to CuII(Schiff base) @GO-CP,
indicating the successful covalent attachment of CuII(Schiff
base)Cl2 complex to the GO support.
Investigation of catalytic activity
of CuII(Schiff base)Cl2@GO‑CP
in the epoxidation of alkenes
The catalytic activity of the CuII(Schiff base)Cl2@GO-CP
was studied in the epoxidation of alkenes with tert-BuOOH
as an oxidant. First, the epoxidation of cyclooctene with tert-
butyl hydroperoxide was chosen to optimize the reaction
parameters such as catalyst amount and solvent. Different
solvents such as dichloromethane, ethyl acetate, acetonitrile,
methanol, ethanol, tetrahydrofuran (THF), ethanol–water,
1,2-dichloroethane and water were studied for optimization
of the solvent in the epoxidation of cyclooctene with TBHP.
As can be seen, dichloromethane is the best reaction medium
(Table 1, entries 1–8). Different amounts of heterogeneous
catalyst (0.25–1.25 mol% based on Cu loading) were also
used in the model reaction. By increasing the amount of cat-
alyst from 0.25 to 1 mol%, the yield was increased from 28
to 99%. Increasing the amount of catalyst did not show any
significant effect on the epoxide yield at room temperature
(Table 1, entry 8–12). It is noteworthy that in the absence
of catalyst, only small amount of epoxide was detected even
after prolonged reaction time (Table 1, entry 13). Under the
optimized reaction conditions, different molar ratio of olefin/
TBHP was used and the best results were obtained with 2
mmole of TBHP.
Fig. 9 Raman spectra of a GO and b CuII(Schiff base)Cl2@GO-CP
catalyst
weak and broad at around 2θ = 11.91°, indicating that the
mentioned catalyst has been successfully prepared (Fig. 8c).
The chemical and structural changes in the obtained
materials can be also studied by the Raman spectroscopy
(Fig. 9). Raman spectra of GO and CuII(Schiff base)Cl2@
GO-CP exhibit the presence of two characteristic G and D
bands. The band at 1354 cm−1 (D) is attributed to the amount
of defects existing in the carbon framework, and the band at
1597 cm−1 (G) corresponds to the E2g mode of hexagonal
sp2 carbon domains, representing the degree of graphitiza-
tion [60, 61]. The intensity ratio between the D and G bands
(ID/IG ratio) indicates the degree of defects, ripples, edge,
and the average size of sp2 domains [62]. At Fig. 9b, the
peaks at 1357 and 1602 cm−1 can be attributed to the D
and G bands of the CuII(Schiff base)Cl2@GO-CP. The ratio
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Table 2 Epoxidation of alkenes with TBHP catalyzed by heterogeneous Cu(II) complex at room temperature
Entry
Olefin
Product Yield (%)a Epoxide Selectivity (%)
TOF (h–1)
O
1
2
99
94
100
19.1
18.2
O
O
97b
91c
3
90
16.4
O
O
4
5
87
80
93d
15.8
15.4
100
6
78
43
100
100
15.1
8.6
O
O
7e
a
Reaction conditions: alkene (1 mmol), TBHP (2 mmol) and heterogeneous Cu(II) complex catalyst (1 mol% based on Cu) in dichloromethane at
room temperature after 5 h
aDetermined by GC based on starting alkene
bThe by-product is cyclohex-2-ene-1-one
cThe by-product is benzaldehyde
dThe by-product is acetophenone
eReaction conditions: alkene (1 mmol), TBHP (2 mmol) and Cu(II) Schiff base complex (1 mol% based on Cu) in dichloromethane at room tem-
perature after 5 h
To expand the scope and generality of this catalytic sys-
tem, different alkenes including cyclic and linear ones were
subjected to epoxidation with TBHP at room temperature in
CH2Cl2. The results are summarized in Table 2. In general,
the cyclic alkenes and the alkenes containing aromatic ring
were more reactive (Table 2, entries 1–4) than linear ones
(Table 2, entries 5 and 6). Excellent conversion and selectiv-
ity to the epoxide were obtained within 5 h (Table 2, entries
1–6). Cyclic olefins such as cyclohexene and cyclooctene
were also epoxidized in high yields and selectivity (Table 2,
entries 1 and 2). While in the case of the terminal C=C dou-
ble bonds conjugated with an aromatic ring, the major prod-
uct was the corresponding epoxide and the related carbonyl
compound was achieved as a by-product resulting from
ring-opening reaction of epoxide (Table 2, entries 3 and 4).
1-Octene and 1-decene as linear alkenes were epoxidized
efficiently by this catalyst in high yields. For comparison, the
catalytic performance of homogeneous Cu(II) Schiff base
complex was also investigated in cyclooctene epoxidation
in the same reaction condition (Table 2, entry 7). As can be
seen, the catalytic activity of the homogeneous Cu(II) Schiff
base catalyst is significantly lower than the heterogeneous
Cu(II) catalyst which confirms that graphene oxide anchor-
ing plays an influential role in catalytic efficiency. Accord-
ing to the related literature [43], a possible mechanism was
suggested for alkene epoxidation with copper(II) complex
using TBHP that we can also consider it in the current study.
Recently, we reported a heterogeneous copper cata-
lyst in the alkene epoxidation [63]. This catalytic system
showed slightly higher catalytic activity compared to previ-
ously reported system. This behaviour can be attributed to
the presence of chloropropyl dangling groups and also the
presence of supported ionic liquid moieties in which the
heterogeneous catalyst acts as a quasi-homogeneous system.
Also, the results obtained using CuII(Schiff base)Cl2@
GO-CP in the epoxidation of cyclohexene were compared
with the results reported for some other copper(II) catalysts.
As can be seen, our reported method is superior in terms
of reaction times, yields or epoxide selectivity (Table 3).
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Journal of the Iranian Chemical Society
Table 3 Comparison of the catalytic efficiency of CuII(Schiff base) complex@GO-CP with other reported copper(II) catalysts for the cyclooc-
tene epoxidation with tert-butyl hydroperoxide
Entry
Catalyst
Time (h)
Conversion (%)
Selectivity (%)
epoxide
TOF (h−1
)
References
1
2
3
4
5
6
7
8
CuII(Schiff base)@GO-CP
[Cu4(O)(L2)2(CH3COO)4]CH3CN
LCuII
5
24
24
24
24
24
24
24
22
12
24
8
99
88
78
74
70
70
64
86
50
52.8
92
97
100
89
60
57
26
24
100
81
50
43.6
93
19.1
4.4
13
12.3
12.8
12.5
0.3
6.8
24
69.6
65.4
17.3
This study
[64]
[65]
[65]
[66]
[66]
[67]
[68]
[69]
LCuICuII
Cu{salnptn(3-OMe)2}
Cu{hnaphnptn}
CuO
[CuL(µ1,1, N3)]n
[Cu2(L)2(L3)]PF6
Cu(salen)-f-GO
Cu-AMM
9
10
11
12
[44]
[70]
[63]
CuII(BAPTE)Cl2@GO
100
Also, in most cases, higher TOFs have been obtained by our
reported method.
the catalyst for five consecutive runs, the epoxide yield was
still 95% (Fig. 10).
Reusability of the catalyst
Conclusions
The reusability of CuII(Schiff base)Cl2@GO-CP was tested
in the repeated epoxidation reactions of cyclooctene. In this
manner, after each reaction, the solid catalyst was easily
recovered from the reaction mixture by simple filtration or
centrifugation, and washed with CH2Cl2 and reused with
fresh reactants. The filtrates were used for determination of
the amount of Cu leached by ICP analysis. No copper was
detected in the filtrates which showed the strong attachment
of the Cu Schiff base to the functionalized GO. After using
In conclusion, a Schiff base ligand was immobilized on GO
nanosheets and complexed with Cu(II) species. This immo-
bilized copper Schiff base complex, CuII(Schiff base)Cl2@
GO-CP, was used as a highly effective and recyclable cata-
lyst for the epoxidation of olefins with TBHP. High yields,
excellent selectivity and reusability of the catalyst are note-
worthy advantages of this catalytic system.
Acknowledgements The authors are grateful to the Research Council
of the University of Isfahan for financial support of this work.
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