Schiff base complexes of Mo(VI) immobilized on functionalized graphene oxide nano-sheets for the catalytic epoxidation of alkenes

Article abstract of DOI:10.1080/00958972.2019.1691724

Mo(VI) tetradentate Schiff base complexes were covalently immobilized onto chemically functionalized graphene oxide (GO) using 3-(tri-methoxysilyl) propylamine as a coupler. The resulting heterogeneous catalysts have been characterized by FTIR, DRS, XRD, SEM, TGA, and atomic absorption spectroscopy (AAS). These catalysts were applied in epoxidation of cyclooctene and other olefins. The catalytic procedures with all catalysts were optimized for different parameters such as oxidant, solvent and temperature. Recycling results indicated that the catalysts were highly stable and maintained activity and selectivity even after being used for five cycles.

Full text of DOI:10.1080/00958972.2019.1691724

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Schiff base complexes of Mo(VI) immobilized on
functionalized graphene oxide nano-sheets for the
catalytic epoxidation of alkenes
Behnam Rezazadeh, Ali Reza Pourali, Alireza Banaei & Hossein Behniafar
To cite this article: Behnam Rezazadeh, Ali Reza Pourali, Alireza Banaei & Hossein Behniafar
(2019): Schiff base complexes of Mo(VI) immobilized on functionalized graphene oxide nano-
sheets for the catalytic epoxidation of alkenes, Journal of Coordination Chemistry, DOI:
10.1080/00958972.2019.1691724
To link to this article: https://doi.org/10.1080/00958972.2019.1691724
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JOURNAL OF COORDINATION CHEMISTRY
https://doi.org/10.1080/00958972.2019.1691724
Schiff base complexes of Mo(VI) immobilized
on functionalized graphene oxide nano-sheets
for the catalytic epoxidation of alkenes
Behnam Rezazadeha, Ali Reza Pouralia, Alireza Banaeib and Hossein Behniafara
b
aDepartment of Chemistry, School of Chemistry, Damghan University, Damghan, Iran; Department
of Chemistry, Payame Noor University, Tehran, Iran
ABSTRACT
ARTICLE HISTORY
Received 18 August 2019
Accepted 7 October 2019
Mo(VI) tetradentate Schiff base complexes were covalently immo-
bilized onto chemically functionalized graphene oxide (GO) using
3-(tri-methoxysilyl) propylamine as a coupler. The resulting hetero-
geneous catalysts have been characterized by FTIR, DRS, XRD,
SEM, TGA, and atomic absorption spectroscopy (AAS). These cata-
lysts were applied in epoxidation of cyclooctene and other olefins.
The catalytic procedures with all catalysts were optimized for dif-
ferent parameters such as oxidant, solvent and temperature.
Recycling results indicated that the catalysts were highly stable
and maintained activity and selectivity even after being used for
five cycles.
KEYWORDS
Mo(VI) Schiff base
complexes; graphene oxide;
alkene epoxidation;
heterogeneous catalyst
1. Introduction
Catalytic epoxidation of alkenes is of interest for the synthesis of fine chemicals.
Because of their versatility as intermediates, epoxies have great value in chemical tech-
nology and synthetic organic chemistry [1]. Epoxides react to provide industrially
important products such as antistatic agents, detergents, corrosion protection agents,
CONTACT Behnam Rezazadeh
rezazade1982@yahoo.com; Ali Reza Pourali
pourali@du.ac.ir
School of
Chemistry, Damghan University, Damghan 36716-41167, Iran
ß 2019 Informa UK Limited, trading as Taylor & Francis Group

2
B. REZAZADEH ET AL.
H
R
R
O
O
O
O
O
Br
Br
OH
R
H
H
R=H, L1 . R=Br, L2 . R=OCH3 , L3
Scheme 1. Preparation of L1, L2 and L3 dialdehydes.
surfactants, textiles, lubricating oils and cosmetics [2]. Large epoxides are fine chemi-
cals with interesting chemical properties, e.g. cyclooctene oxides are extensively used
in the manufacturing of pesticides, pharmaceuticals and polyesters [3]. Mo(VI) Schiff
base complexes have been used as oxidation catalysts for organic substrates, because
Mo(VI) complexes offer advantages such as environmental, economic and commer-
cially available [4]. The fundamental role of molybdenum-based catalysts for produc-
tion of both fine chemicals and bulk chemicals has increased attention toward this
metal [5]. Dioxomolybdenum(VI) complexes of tetradentate Schiff bases [6], dimethyl-
dioxo Mo(VI) diazabutadiene and dichloro were reported as active epoxidation cata-
lysts [7].
One of the main problems of homogeneous transition metal complexes as catalysts
is the formation of oxo and peroxo dimeric and other polymeric species [8]. The for-
mation of these compounds can deactivate catalysts irreversibly. This problem may be
solved by supporting of the metal complexes within solid supports so it can separate
complexes from each other [9]. Meanwhile, the separation, recycling, inadequate sta-
bility of homogeneous catalysts and leaching of the active metal into the solvent are
part of serious problems. Heterogenization of homogeneous catalysts onto solid sup-
ports has been studied because of the long catalytic lifetime, thermal stability, easy
separation, easy recyclability and high selectivity [10]. A main method for heterogeniz-
ing of homogenous catalyst is to anchor the soluble catalyst on to large surface area
inorganic supports [11].
Many different solid materials have been tested for heterogenization, such as
mesoporous silica, carbon materials, organic polymers, clays, metal-organic frame-
works, etc. [12, 13]. Graphene shows promise as a future material in composite mate-
rials, sensors, hydrogen storage, drug delivery, solar cells and catalysis due to its
optical, electrical, thermal and mechanical characteristics [14, 15]. Graphene oxide
(GO) possesses advantages, including facile synthesis, high surface area, substantial
solubility, low toxicity, good hydrophilicity, and excellent biocompatibility [16–18].
Owing to the presence of carboxyl, hydroxyl, carbonyl, and epoxy groups, oxygen
functionality modifications can establish sites for anchoring complexes and ions [19].
Therefore, it has been recognized as an ideal candidate for immobilizing various
transition metal complexes.
In this study we report the synthesis and characterization of new Mo(VI) tetraden-
tate Schiff base complexes immobilized onto chemically functionalized graphene oxide
(GO) using 3-(tri-methoxysilyl) propylamine (Schemes 1 and 2) and their application in
epoxidation of different alkene substrates.

JOURNAL OF COORDINATION CHEMISTRY
3
NH2
Si
O
OH
OH
OH
O
O
O
Si
NH2
O
O
GO
GO
R
R
O
O
O
O
H
H
O
O
O
O
O
N
Mo
R
R
R
R
O
N
N
MoO2(acac)2
N
Si
Si
Si
Si
O
O
O
O
O
O
O
O
O
O
O
O
GO
GO
R=H, MoO2L1-GO. R=Br,MoO2L2-GO. R=OCH3, MoO2L3-GO
Scheme 2. Schematic outline of synthesis of MoO2L1-GO, MoO2L2-GO and MoO2L3-GO.
2. Experimental
2.1. Materials and physical measurements
Solvents and reagents were purchased from Merck or Fluka and used without further
1
purification. H-NMR spectra were recorded on a Bruker Asend TM 400 MHz spectrom-
eter in DMSO-d6 and CDCl3 using tetramethylsilane (TMS) as an internal reference. FT-
IR spectra were recorded on a Shimadzu Varian 4300 spectrophotometer in KBr pellets.
Diffuse reflectance spectra (DRS) were registered on a Scinco 4100 from 200-1100 nm
using 6890 series. XRD patterns were recorded by a Rigaku D-Max C III, X-ray diffract-
ometer using Ni-filtered Cu Ka radiation. The metal contents of the catalysts were

4
B. REZAZADEH ET AL.
measured by Atomic Absorption Spectrophotometer (AAS-Perkin–Elmer 4100-1319)
using a flame approach. The purity of the solvents and analysis of the oxidation prod-
ucts were determined by gas chromatography using an Agilent 7890 with a capillary
column and FID detector.
2.2. Synthesis of L1
To a stirred solution of salicylaldehyde (50 mmol) and K2CO3 (3.05 g, 25 mmol) in DMF
(75 mL) was added dropwise 1,2-dibromoethane (9.4 g, 50 mmol) in DMF (25 mL). The
reaction was continued for 6 h in 150 ꢀC and then 1 h at room temperature. After the
addition was completed, 250 mL of distillated water was added and the mixture was
put in the refrigerator; 1 h later the precipitate was filtered and washed with 500 mL
distilled water, then dried in air and recrystallized from ethanol and dried in vacuum.
L1. Yield: 4.99 g (76%), m.p: 130 ꢀC, Color: cream. IR (KBr, m cmꢁ1): 1675 m (C ¼ O),
1
1484, 1468 m (Ar-C ¼ C), 1285, 1235 m (Ar-O), 1173, 1045 m (R-O). H NMR (DMSO-d6,
400 MHz) d (ppm) d 9.86 (s, 2 H, CHO), 7.93 (d, 4 H, ArH), 7.23 (d, 4 H, ArH), 4.40 (s, 4 H,
OCH2). 13C NMR (DMSO-d6, 400 MHz) d 190.5, 162.7, 131.2, 129.1, 114.6, 64.2 ppm.
2.3. Synthesis of L2 and L3
5-Bromo-2-hydroxybenzaldehyde or 5-methoxy-2-hydroxybenzaldehyde (50 mmol) was
dissolved in 75 mL DMF and potassium carbonate (3.05 g, 25 mmol) was added, and
the mixture was stirred at room temperature; 1,2-dibromoethane (9.4 g, 25 mmol) was
added dropwise and then the reaction mixture was stirred under reflux for 6 h. The
mixture was partitioned between water and ethylacetate. The ethyl acetate layer was
collected and concentrated under reduced pressure and then subjected to silica gel
100-200 mesh column chromatography using 1:10 hexane ethylacetate as eluent to
afford compounds in pure form.
L2. Yield: 8.35 g (78%), m.p: 143 ꢀC, Color: cream. IR (KBr, m cmꢁ1): 1678 m (C ¼ O),
1
1491, 1471 m (Ar-C ¼ C), 1289, 1237 m (Ar-O), 1176, 1049 m (R-O). H NMR (DMSO-d6,
400 MHz) d (ppm) 10.20 (s, 2 H, CHO), 7.83 (dd, 2 H, ArH), 7.73 (d, 2 H, ArH), 7.33 (d,
2 H, ArH), 4.58 (s, 4 H, OCH2). 13C NMR (DMSO-d6, 400 MHz) d 195.6, 160.6, 139.6, 136.0,
121.8, 120.1, 111.7, 64.8 ppm.
L3. Yield: 5.78 g (70%), m.p: 118 ꢀC, Color: cream. IR (KBr, m cmꢁ1): 1673 m (C ¼ O),
1
1495, 1477 m (Ar-C ¼ C), 1283, 1234 m (Ar-O), 1170, 1042 m (R-O). H NMR (DMSO-d6,
400 MHz) d 10.52 (s, 2 H, CHO), 7.41 ꢁ 7.45 (m, 2 H, ArH), 7.26 (s, 2H, ArH), 7.14 (d, 2 H,
ArH), 4.50 (s, 4 H, OCH2), 3.88 (s, 3 H, OCH3). 13C NMR (DMSO-d6, 400 MHz) d 189.5,
156.1, 153.4, 125.2, 123.3, 114.4, 110.2, 64.7, 55.6 ppm.
2.4. Synthesis of GO
Graphene oxide (GO) was prepared by oxidation and exfoliation of graphite powder
according to the modified Hummers method [20]. In a typical experiment, graphite
powder (2.0 g) and 46 mL of conc. H2SO4 were mixed in a 500 mL flask. The resulting
mixture was cooled to 0 ꢀC in an ice bath under stirring for 2 h. KMnO4 (6 g) was

JOURNAL OF COORDINATION CHEMISTRY
5
added with vigorous stirring and the flask was kept oxidizing at 0 ꢀC for 2 h.
Subsequently, the reaction mixture was stirred at 35 ꢀC for 30 min. Deionized water
(96 mL) was slowly added into the reaction mixture and then the temperature was
raised to 95 ꢀC for 15 min using an oil bath. The mixture was treated by adding hydro-
gen peroxide (30%) and 240 ml of water. Finally, the solid was filtered off and washed
with 5% HCl and deionized water until the pH reached 6. The resulting solid was dried
at 80 ꢀC for one week to afford GO.
2.5. Synthesis of GO-NH2
GO (1.0 g), APTMS (3 mmol) and toluene (25 mL) were added to a 50 mL two-neck flask
and the mixture was refluxed at 110 ꢀC under N2 for 24 h. Afterwards, the mixture was
filtered and the resultant solid was washed with toluene several times to remove the
residual APTMS. The resulting solid was dried at room temperature to afford GO-NH2.
2.6. Synthesis of Schiff base ligands grafted onto GO
1 g of GO-NH2 in 80 mL of dry ethanol was sonicated for 30 min. Excess dialdehyde lig-
and (L1, L2 or L3) (5 mmol) was added and the reaction mixture was refluxed for 24 h.
The solid was then filtered, washed thoroughly with ethanol, and dried in an oven at
70 ꢀC for 24 h to provide the GO- Schiff base ligand. The resulting materials are
denoted L1-GO, L2-GO and L3-GO, respectively.
2.7. Synthesis of MoO2 Schiff base complexes grafted onto GO
Immobilized Schiff base ligand onto GO (L1-GO, L2-GO or L3-GO) (1 g) was dispersed in
50 mL ethanol and 6 mmol of MoO2(acac)2 in 50 mL ethanol was added to this reaction
mixture and refluxed for 24 h. The solid was filtered and washed with ethanol. To remove
the unreacted MoO2(acac)2, Soxhlet extraction was carried out with ethanol and the
resulting nanocatalyst dried at 80 ꢀC for 5 h. The resulting materials were denoted
MoO2L1-GO, MoO2L2-GO and MoO2L3-GO, respectively. The amount of Mo loadings was
0.375, 0.369 and 0.372 mmolꢂgꢁ1 for MoO2L1-GO, MoO2L2-GO and MoO2L3-GO.
2.8. Catalytic epoxidation of alkenes
In a typical procedure, a 25 mL round bottom flask, equipped with a condenser and a
magnetic stir bar, was charged with an alkene (5 mmol), 10 mmol of TBHP, the catalyst
(MoO2L1-GO, MoO2L2-GO and MoO2L3-GO) (20 mg) and 5 mL 1,2-dichloroethane, and
the mixture was then refluxed. After reaction the solid catalyst was filtered off, washed
with 1,2-dichloroethane and then dried at 80 ꢀC for 12 h. The recovered catalyst was
used to investigate its stability and recyclability. Analysis of the oxidation products
was determined by gas chromatography with a capillary column and FID detector.
Column temperature was programmed between 150 ꢀC and 200 ꢀC (5 ꢀC/min).
Nitrogen was used as carrier gas (40 mL/min) at injection temperature and 1,2-dichlor-
obenzene was used as an internal standard.

6
B. REZAZADEH ET AL.
Figure 1. 1H-NMR of (a) L1 (recorded in DMSO-d6), (b) L2 (recorded in DMSO-d6), (c) L3 (recorded
in CDCl3).
3. Results and discussion
1
3.1. H NMR spectroscopic studies
1H NMR spectra of L1 and L2 were recorded in DMSO-d6 but spectra of L3 were
recorded in CDCl3 (Figure 1). Chemical shifts for aldehyde and OCH2 protons were
found at ꢃ 10.3 and ꢃ 4.5 ppm, respectively. Aromatic protons were set in the region
of 7.09–7.70 ppm for L1, 7.33-7.85 for L2 and 7.14–7.45 ppm for L3. Methoxy protons
for L3 are represented with a peak at 3.88 ppm.

JOURNAL OF COORDINATION CHEMISTRY
7
Figure 2. FT-IR spectra: (a) GO, (b) MoO2L1-GO, (c) MoO2L2-GO, (d) MoO2L3-GO.
3.2. IR spectroscopy
Figure 2 shows the FT-IR spectra of MoO2L1-GO, MoO2L2-GO and MoO2L3-GO. The FT-
IR spectrum of pure GO support exhibits various vibrational peaks of functional groups
such as C – O (1040 cmꢁ1), C ¼ O (1730 cmꢁ1), C ¼ C (1610 cmꢁ1
)
and ¼ C-H
(1382 cmꢁ1), revealing the presence of hydroxyl, carboxyl and epoxy groups [21]. After
immobilization of the molybdenum Schiff base complex on the graphene oxide sur-
face, the bands at 1106 cmꢁ1 and 1030 cmꢁ1 correspond to the stretching vibrations
of Si-O-Si and Si-O-C, giving evidence for silylanization of graphene oxide [22]. The
band at 1620 cmꢁ1 can be assigned to the C ¼ N stretch of the imine group. The
appearance of two adjacent bands at 902 cmꢁ1 and 947 cmꢁ1 in the FT-IR spectra of
MoO2L1-GO, MoO2L2-GO and MoO2L3-GO is characteristic of the presence of cis-MoO2
group [21]. These results confirm that the molybdenum Schiff base complexes are
anchored on the graphene oxide surface.
3.3. XRD analysis
XRD spectra are used to investigate the changes in structure of graphene oxide after
the immobilization of the MoO2 Schiff base complexes (Figure 3). The XRD pattern of
GO (Figure 3a) shows the characteristic diffraction peak at 2h ¼ 11.4ꢀ, corresponding
to (001), which is due to oxidation of graphite powder and trapped water molecules
between the graphite layers [23]. After amino functionalization and immobilization of
MoO2 Schiff base complexes on the GO surface, the diffraction peak at 11.4ꢀ does not
disappear, suggesting that the structure of graphene oxide is not destroyed during
sample preparation [24]. Moreover, the XRD spectra of MoO2L1-GO, MoO2L2-GO and
MoO2L3-GO show a broad peak at 2h ¼ 23.5ꢀ, verifying that the major oxygen-contain-
ing groups of GO have been functionalized.

8
B. REZAZADEH ET AL.
Figure 3. XRD patterns of (a) GO, (b) MoO2L1-GO, (c) MoO2L2-GO, (d) MoO2L3-GO.
3.4. SEM analysis
Scanning electron microscope (SEM) image shown in Figure 4 reveals the nanoscopic
features of GO and GO immobilized MoO2 Schiff base complexes. The similar crumpled
nanosheets are in an agglomerated phase in the four structures. These ridges and pro-
trusions due to sp3 carbon in immobilized GO carry the MoO2 Schiff base complex on

JOURNAL OF COORDINATION CHEMISTRY
9
Figure 4. SEM image of the (a) GO, (b) MoO2L1-GO, (c) MoO2L2-GO, (d) MoO2L3-GO.
both sides of the GO nanosheets, where reactants can easily approach the catalytic
sites. Microstructure of GO is not destroyed through the modification; the small spher-
ical shaped catalyst particles on GO are seen in Figure 4. The size of MoO2 Schiff base
catalysts, mounted on graphene oxide, varies between 50 and 80 nm.
3.5. Solid state UV-Visible study
The diffuse reflectance spectra (DRS) of GO, MoO2L1-GO, MoO2L2-GO and MoO2L3-GO
are shown in Figure 5. As-prepared GO exhibits two absorption peaks due to the
ꢄ
ꢄ
p!p transition of graphitic carbon double bonds at 242 nm and the n!p transition
of carbonyl groups at 304 nm [25]. The peak at ꢃ570 nm for MoO2L1-GO, MoO2L2-GO
and MoO2L3-GO is ascribed to ligand to metal charge-transfer transitions (LMCT) due
to the promotion of an electron from the ligand to the molybdenum empty orbi-
tals [26].
3.6. TGA analysis
The TG curves of GO, MoO2L1-GO, MoO2L2-GO and MoO2L3-GO are depicted in
Figure 6. The samples were heated at a constant rate of 10 ꢀC minꢁ1 from 30 ꢀC to
800 ꢀC under N2 flow. The TG curve of GO shows the first weight loss at 90–180 ꢀC
from evaporation of water molecules trapped in the material. Second degradation

10
B. REZAZADEH ET AL.
Figure 5. DRS spectra of the (a) GO, (b) MoO2L1-GO, (c) MoO2L2-GO, (d) MoO2L3-GO.
Figure 6. TGA patterns of (a) GO, (b) MoO2L1-GO, (c) MoO2L2-GO, (d) MoO2L3-GO.
steps were observed at 200–320 ꢀC which is due to the loss of epoxy and hydroxy
functional groups, and the third degradation step (350–650 ꢀC) is the loss of the
remaining oxygen-containing groups as well as burning of ring carbon. The TGA
curves for MoO2L1-GO, MoO2L2-GO and MoO2L3-GO show three-step weight losses at
temperatures from 30 to 800 ꢀC. The weight loss from 200 to 400 ꢀC is attributed to
the thermal decomposition of undigested oxygen carrying functionalities and APTMS

JOURNAL OF COORDINATION CHEMISTRY
11
Table 1. Epoxidation of cyclooctene under reflux conditions.a,b
Oxidant/Substrate
molar ratio
Epoxide
selectivity (%)
Entry
Catalyst (mg)
Solventc
Oxidant
Conversion (%)
1
2
3
4
5
6
7
8
MoO2L1-GO(20)
MoO2L1-GO(20)
MoO2L1-GO(20)
MoO2L1-GO(20)
MoO2L1-GO(10)
MoO2L1-GO(15)
MoO2L1-GO(25)
MoO2L1-GO(20)
MoO2L1-GO(20)
MoO2L1-GO(20)
MoO2L1-GO(20)
MoO2L1-GO(20)
MoO2L1-GO(20)
MoO2L2-GO(20)
MoO2L2-GO(20)
MoO2L2-GO(20)
MoO2L2-GO(20)
MoO2L2-GO(10)
MoO2L2-GO(15)
MoO2L2-GO(25)
MoO2L2-GO(20)
MoO2L2-GO(20)
MoO2L2-GO(20)
MoO2L2-GO(20)
MoO2L2-GO(20)
MoO2L2-GO(20)
MoO2L3-GO(20)
MoO2L3-GO(20)
MoO2L3-GO(20)
MoO2L3-GO(20)
MoO2L3-GO(10)
MoO2L3-GO(15)
MoO2L3-GO(25)
MoO2L3-GO(20)
MoO2L3-GO(20)
MoO2L3-GO(20)
MoO2L3-GO(20)
MoO2L3-GO(20)
MoO2L3-GO(20)
CH3OH
CH3CN
CH2Cl2
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
UHP
NaIO4
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
UHP
NaIO4
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
UHP
2
2
2
2
2
2
2
2
2
0.5
1
1.5
2.5
2
2
2
2
2
2
2
57.3
72.5
53.3
98
53.4
79
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
CH3OH
CH3CN
98
49.8
31.8
32
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
50.4
69.8
97.8
58.5
75.2
54.7
99.1
53.8
81.4
98.8
51.3
33
CH2Cl2
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
CH3OH
CH3CN
2
2
0.5
1
1.5
2.5
2
2
2
2
2
2
2
2
2
0.5
1
1.5
2.5
31.9
51.5
71
98.1
55.3
71.7
51.6
96.7
52.2
76.8
96
48.4
30.1
30.5
48.3
68.3
95.7
CH2Cl2
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
1,2-DCE
NaIO4
TBHP
TBHP
TBHP
TBHP
aThe amount of metal loading are 0.375(3.60%), 0.369(3.54%), 0.372(3.57%) (mmol)/g catalyst for MoO2L1-GO,
MoO2L2-GO and MoO2L3-GO.
bThe time of reactions are 60 min for MoO2L1-GO, 45 min for MoO2L2-GO and 90 min for MoO2L3-GO.
cThe amount of solvent are 5 mL.
moieties, which have not participated in further chemical functionalization. The last
major mass loss observed from 450–550 ꢀC is related to slow decomposition of the
Schiff base complex.
3.7. Catalytic activity
In order to optimize conditions for the title reaction and also compare the results with
previous reports, epoxidation of cyclooctene was examined by changing parameters
including the catalyst amount, time, oxidant and solvent. Different oxidants such as
tert-butylhydroperoxide, NaIO4 and UHP (urea-hydrogen peroxide) were examined in
epoxidation of cyclooctene in various solvents (Table 1, entries 1–4, 14–17 and 27–30).

12
B. REZAZADEH ET AL.
Figure 7. Effect of time on the epoxidation of cyclooctene with TBHP catalyzed by MoO2L1-GO,
MoO2L2-GO and MoO2L3-GO. Conditions: cyclooctene (5 mmol), TBHP (10mmol), catalyst (20mg),
1,2-DCE (5 mL).
Tert-butylhydroperoxide has some advantages: (i) It has excellent thermal stability; (ii)
It can give high percent of conversion; (iii) environment-friendly.
Solvent plays an important role in oxidation reactions [23]. High epoxide selectivity
and conversion were obtained in 1,2-dichloroethane as solvent for MoO2L1-GO,
MoO2L2-GO and MoO2L3-GO catalysts. Other solvents (CH3OH, CH3CN, and CH2Cl2) did
not show good performance compared to 1,2-dichloroethane (Table 1, entries 1–4,
14–17 and 27–30). Coordinating solvents such as acetonitrile and methanol compete
with TBHP to bind Mo [27]. Therefore, in the presence of these solvents, the observed
yields are low (Table 1, entries 1–4, 14–17 and 27–30). Lower reflux temperature is
important in the lowest conversion in dichloromethane (Table 1, entries 1–4, 14–17
and 27–30).
Catalyst amount also had a role in the reaction. The epoxidation of cyclooctene by
TBHP did not proceed in the absence of catalyst. The results in Table 1 (entries 1, 5–7,
14, 18–20, 27 and 31–33) exhibit that increasing the amount of catalyst up to 20 mg
increased the yield of cyclooctene epoxide.
The ratio of oxidant to substrate is an important parameter in catalytic oxidation
processes. The percent of conversion increased with increasing the molar ratio of [oxi-
dant]:[cyclooctene] from 0.5 to 2 and the conversion of cyclooctene was maximum at
2:1 molar ratio of [oxidant]:[cyclooctene] (Table 1, entries 1–4, 14–17 and 27–30).
Figure 7 reveals the effect of reaction time on epoxidation of cyclooctene with
TBHP. Comparing the reactivity of MoO2L1-GO, MoO2L2-GO and MoO2L3-GO shows
that the first two reaches maximum yield after 60 min and 45 min, respectively, so
they are more reactive than the third, which needs 90 min to reach this point. The
catalytic activity of the complexes is mainly driven by substituent group present on
the phenyl rings [28]. Presence of electron withdrawing group on phenyl rings makes
the complex more active as catalyst for oxidation [29]; electron releasing group

JOURNAL OF COORDINATION CHEMISTRY
13
Table 2. Epoxidation of some alkenes with TBHP catalyzed by MoO2L1-GO, MoO2L2-GO and
MoO2L3-GO under reflux.a
MoO2L1-GO
MoO2L2-GO
Conversion
MoO2L3-GO
Conversion
Conversion
(epoxide)
(%)
(epoxide)
(%)
(epoxide)
(%)
Entry
1
Alkene
Time (min)
60
Time (min)
45
Time (min)
90
88.7 (76)
98 (100)
91 (95)
91.9 (81)
99.1 (100)
94.1 (96)
86 (73)
96.7 (100)
90 (91)
2
3
60
45
90
210
150
240
4
5
6
82.8 (100)
86.6 (100)
70.1 (100)
210
210
420
84 (100)
88.4 (100)
75 (100)
150
150
420
80.2 (100)
82.9 (100)
65.4 (100)
240
240
480
O
7
8
65.1 (100)
60.5 (77)
420
480
68.5 (100)
64.1 (80)
420
420
59.3 (100)
51 (71)
480
480
9
93.8 (97)
180
96 (98)
150
92.4 (93)
240
aReaction conditions: alkene (5 mmol), TBHP (10 mmol), catalyst (20 mg), 1,2-DCE (5 mL).
(-OCH3) attached on the phenyl rings, the complex loses its activity to a great range.
Therefore, the observed trend of activity of the catalysts shows this trend.
These new catalysts, MoO2L1-GO, MoO2L2-GO and MoO2L3-GO, can be used for the
epoxidation of a wide range of substituted alkenes (Table 2). Based on the epoxidation
mechanism suggested earlier [30], higher electron donating ability of olefin double
bond is anticipated to show more epoxidation reactivity. Therefore, cyclohexene and
cyclooctene with inner double bonds should exhibit more activity in comparison to 1-
hexene and 1-octene which contain terminal double bonds. Among the endocyclic
olefins, cyclooctene was the most reactive (Table 2, entry 2). This reactivity arises from
the stability of cyclooctene as well as the high reactivity of its double bond [31]. Steric
hindrance on the C ¼ C bond decreased the conversion of the olefins such as 1-
methoxy-2-methylprop-1-ene (Table 2, entry 5) and 3,3-dimethylhex-1-ene (Table 2,
entry 6) and increased reaction time.
Reusability of solid supported catalysts is an important benefit. Therefore, the reus-
ability of MoO2L1-GO, MoO2L2-GO and MoO2L3-GO was monitored by sequential epoxi-
dations of cyclooctene (Table 3). After the reaction, the solid catalyst could be easily
recovered from the reaction mixture by simple filtration and is ready for reuse after
washing with 1,2-dichloroethane and drying at 100 ꢀC. These catalysts show good
reusability without significant loss of selectivity after five cycles.
On the basis of Sobczak’s ideas [32] and other experimental and theoretical reports
[33–35] a reasonable mechanism can be proposed for epoxidation of cyclooctene with
tert-butylhydroperoxide by MoO2L1-GO, MoO2L2-GO and MoO2L3-GO (Scheme 3).

14
B. REZAZADEH ET AL.
Table 3. Epoxidation of cyclooctene with TBHP using recycled catalysts.
MoO2L1-GOa
MoO2L2-GOb
MoO2L3-GOc
Number
of cycles
Conversion
(epoxide) (%)
Conversion
(epoxide) (%)
Conversion
(epoxide) (%)
TOFd (hꢁ1
)
TOFd (hꢁ1
)
TOFd (hꢁ1
)
1
2
3
4
5
98 (100)
97.2 (100)
96.6 (100)
95.3 (100)
95 (100)
653
648
644
635
633
99.1 (100)
98.7 (100)
98.1 (100)
97.4 (100)
96.6 (100)
895
892
887
880
872
96.7(100)
96.2(100)
95.5(100)
94.7(100)
94.1(100)
433
431
428
424
421
aReaction conditions: 5 mmol cyclooctene, 10 mmol TBHP, 5 mL 1,2-DCE, 20 mg catalyst during 60 min.
bReaction conditions: 5 mmol cyclooctene, 10 mmol TBHP, 5 mL 1,2-DCE, 20 mg catalyst during 45 min.
cReaction conditions: 5 mmol cyclooctene, 10 mmol TBHP, 5 mL 1,2-DCE, 20 mg catalyst during 90 min.
dTOF ¼ (mole of reactant)(yield)/(mole of catalyst)(time).
O
H
O
R
R
O
O
H
O
O
O
N
O
N
Si
Si
O
Si
Si
O
GO
O
O
O
O
Mo
O
Mo
O
GO
O
O
O
O
O
O
O
O
N
N
R
R
O
OH
+
Scheme 3. Proposed mechanism for the epoxidation of olefin with TBHP and MoO2L1-GO, MoO2L2-
GO and MoO2L3-GO.
According to our experimental results, reaction rates are first order depending on sub-
strate and catalyst concentrations. The first step involves transferring tert-butylhydro-
peroxide proton to a terminal oxygen of the oxo complex which resulted in
coordination of tertbutylperoxide to Mo(VI). Then, alkene substrate coordinated to
metal and, as a nucleophile, inserted into the metal oxygen bond of coordinated per-
oxide. This mechanism describes the faster reaction of electron-rich alkenes in com-
parison to electron-poor alkenes.
Many molybdenum complexes have been reported for catalytic epoxidation of
alkenes with tert-butylhydroperoxide. However, the catalytic systems which have been
prepared in this research are superior in terms of turnover frequency (TOF). Very
recently, Yang et al. immobilized a molybdenum Schiff base complex onto SBA-15 and
reported a TOF of 25.6 [36]. Mohammadikish et al. have immobilized the molybdenum
complex onto nano-magnetite and reached a TOF of 19 [37]. Moghadam et al.
achieved a TOF of 84 for the molybdenum complex supported onto multi-walled car-
bon nanotubes [38]. Table 4 compares the efficiency of our catalysts with those in the
literature. All of these reports have lower TOF values than MoO2L1-GO, MoO2L2-GO

JOURNAL OF COORDINATION CHEMISTRY
15
Table 4. Comparison of different Mo-containing catalysts for the epoxidation of cyclooctene.
Catalyst
Conversion (%)
Epoxide (%)
TOF (h–1
)
References
MoO2salen-SBA-15
MoO2–Thio-SCMNPs
Mo(CO)6@DAB-MWCNT
[Mo(O)(O2)2(dmpz)2]
Mo-Im-BNPs
49.1
97
97
100
97
97
49.1
99
100
86
97
97
25.6
19
84
–
126
89
[36]
[37]
[38]
[39]
[40]
Mo-A-BNPs
[40]
Fe3O4@APTMS/CM-SalophMoO(O2)
MoO2L1-GO
MoO2L2-GO
97
98
99.1
96.7
100
100
100
100
133
653
895
433
[41]
Present work
Present work
Present work
MoO2L3-GO
and MoO2L3-GO in the present research. So, the comparison of epoxidation reaction
time of our catalysts (45, 60 and 90 min) with the other reports (18 h) [39], (12 h) [5,
36] and (3.5 h) [40] is significant.
4. Conclusion
We prepared new hybrid heterogeneous catalysts by immobilizing Schiff base di-oxo
molybdenum complexes onto GO which are functionalized with 3-(tri-methoxysilyl)
propylamine. These are active catalysts for the epoxidation of various substituted ole-
fins under heterogeneous conditions. The heterogeneous MoO2L1-GO, MoO2L2-GO and
MoO2L3-GO catalysts are recyclable and retain a high level of catalytic activity during
many cycles.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
The authors acknowledge Damghan University for the support of this work.
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