Schiff base-functionalized boehmite nanoparticle-supported molybdenum and vanadium complexes: Efficient catalysts for the epoxidation of alkenes

Article abstract of DOI:10.1002/aoc.3335

Boehmite nanoparticles were prepared using aluminium 2-butoxide. They were covalently functionalized with 3-(trimethoxysilyl)propylamine, after which all the terminal amine groups were changed to Schiff base by refluxing with salicylaldehyde in order to support vanadium oxosulfate and molybdenum hexacarbonyl complexes. These catalysts were applied in the epoxidation of olefins with tert-BuOOH in CCl₄. The catalytic procedures for both catalysts were optimized for various parameters such as solvent, oxidant, temperature and time.

Full text of DOI:10.1002/aoc.3335

Full Paper
Received: 22 February 2015
Revised: 26 April 2015
Accepted: 26 April 2015
Published online in Wiley Online Library: 1 June 2015
(wileyonlinelibrary.com) DOI 10.1002/aoc.3335
Schiff base-functionalized boehmite
nanoparticle-supported molybdenum and
vanadium complexes: efficient catalysts for the
epoxidation of alkenes
Mahdi Mirzaee*, Bahram Bahramian and Adonis Amoli
Boehmite nanoparticles, with high surface area and high degree of surface hydroxyl groups, were prepared via hydrothermal-assisted
sol–gel processing of aluminium 2-butoxide. The produced powder was covalently functionalized with 3-(trimethoxysilyl)propylamine,
and then, in order to support vanadium oxosulfate and molybdenum hexacarbonyl complexes, all the terminal amine groups were
changed to Schiff bases by refluxing with salicylaldehyde. These catalysts were applied in the epoxidation of cis-cyclooctene and
other olefins with tert-BuOOH in CCl . The catalytic procedures for both catalysts were optimized for various parameters such as
4
solvent and oxidant. Recycling experiments revealed that these heterogeneous nano-catalysts could be repeatedly applied for the
epoxidation of alkenes. Copyright © 2015 John Wiley & Sons, Ltd.
Keywords: boehmite nanoparticles; heterogeneous Schiff base catalysts; molybdenum hexacarbonyl; vanadium oxosulfate; olefin
epoxidation
Additionally, the surfaces of some metal oxides and metal hy-
droxides are covered with hydroxyl groups. These surface hydroxyl
Introduction
Epoxides are known as some of the most valuable building blocks
for producing commercially important chemicals such as chiral
pharmaceuticals, pesticides, detergents, epoxy paints, agrochemi-
cals, various polymers, surfactants, antistatic agents, corrosion
protection agents, lubricating oils, textiles, sweeteners, cosmetics
groups provide valuable sites which can be grafted with various
functionalities in order to produce active and stable catalysts for
liquid-phase organic reactions. Boehmite (γ-AlOOH) is one of these
inorganic insoluble supports which contain extra hydroxyl groups
on its surface. It has been used as an important precursor for the
preparation of advanced catalysts, alumina and alumina-derived
[1–4]
and perfumes.
complexes of Cr,
In the last decade, several soluble Schiff base
[4,5]
[4,6,7]
[8]
[9]
[10,11]
[12,13] [14–17]
[38]
Mo,
Re, Ni, Co,
Mn,
V,
ceramics.
[18]
[19]
[20]
Cu, Ru and Ti have been employed as active homogeneous
catalysts in the epoxidation of alkenes. However, most of these cat-
alysts are expensive and contaminate the environment. In that re-
spect, the development of efficient methods for recovering and
reusing them is very important from environmental and economic
points of view.
The research reported here used boehmite nanoparticles (BNPs)
grafted with 3-(trimethoxysilyl)propylamine (MSPA) to anchor
Schiff base and to support two complexes of molybdenum and va-
nadium. Then, these supported catalysts were used in the epoxida-
tion of various olefin substrates.
One way to reach this goal is to immobilize the homogeneous
catalysts on insoluble supports. This can also efficiently increase
the reactivity of some catalysts by preventing the formation of
oxo-bridged dimeric or polymeric complexes which are deleterious
Experimental
Materials and Methods
[21,22]
for catalytic activity.
This has been the subject of numerous
Solvents and reagents were purchased from Merck or Fluka and
used without any further purification. IR spectra were recorded with
publications and reviews which addressed the application of sev-
eral Schiff base-functionalized supports for various catalytic organic
reactions. A variety of organic and inorganic materials can be used
as insoluble supports. Organic insoluble supports include polymers
ꢀ
1
a Bomem MB series FT-IR instrument at 4 cm resolution, using KBr
1
6
pellets. H NMR spectra were obtained in DMSO-d using a Bruker
[23,24]
Avance 300 MHz spectrometer. Powder X-ray diffraction (PXRD)
patterns were collected with a Philips PW-1800 or STOE diffractom-
eter with Cu Kα radiation. Thermogravimetric (TG) analyses were
such as polystyrene.
as zirconium poly(styrene phenylvinylphosphonate)phosphate
have also been used for this purpose. Activated carbon, multi-
Hybrid organic–inorganic materials such
[
25]
[26]
[
21,27]
wall carbon nanotubes,
nanostructured ordered carbon
[
21,28]
[29,30]
[31,32]
[21,33,34]
material,
zeolites
porous silica,
and magnetic nanoparticles have also been used
MCM-41,
SBA-15,
[
1,21,35,36]
[37]
* Correspondence to: Mahdi Mirzaee, Department of Chemistry, University of
Shahrood, Shahrood, Iran. E-mail: mmirzaee@shahroodut.ac.ir
as insoluble supports. Inorganic supports generally have higher
chemical stability and higher surface area than the organic ones.
Department of Chemistry, University of Shahrood, Shahrood, Iran
Appl. Organometal. Chem. 2015, 29, 593–600
Copyright © 2015 John Wiley & Sons, Ltd.

M. Mirzaee et al.
carried out with a Rheometric Scientific STA-1500 or BAHR
Mo(CO)
6
and VO(SO
4
)ꢁH
2
O complexes were used for supporting
ꢀ
1
Thermoanalyse GmbH with a heating rate of 10°C min in air. A
00 W Hg lamp was used for activation of hexacarbonyl molybde-
6
on IFBNPs (Scheme 1). For this purpose, Mo(CO) (0.276 g, 1.05
4
mmol) was activated under ultraviolet light in THF (20.0 ml) for 30
min. Then this activated molybdenum complex was added to dry
IFBNPs and the mixture was refluxed for 6 h. The mixture was
cooled, filtered and the precipitate was washed several times with
THF and dried at 100°C overnight. In a similar procedure, VO(SO
H O (0.200 g, 1.05 mmol) was refluxed with IFBNPs in ethanol
2
num complex. Elemental analyses were carried out with a Hereans
CHN-O-Rapid analyser. Electron microscopy was performed with a
JEOL JSM-6360LV transmission electron microscope. Gas–liquid
chromatography (GLC) was carried out with a Shimadzu GC-16A
instrument using a 2 m column, packed with silicon DC-200.
4
)ꢁ
(20 ml) for 7 h and the powder produced was washed several times
with ethanol and dried at 100°C overnight.
Preparation of BNPs
Aluminium 2-butoxide was synthesized from aluminium and
Catalytic Epoxidation of Alkenes
2
-butanol according to the general preparation procedure of
[
39]
In a typical process, a 25 ml round-bottom flask, equipped with a
magnetic stirrer bar and a condenser, was charged with alkene
aluminium alkoxides.
vacuum-distilled in order to obtain a high-quality aluminium alkox-
ide precursor. For hydrothermal-assisted hydrolysis, according to
The prepared aluminium alkoxide was
(0.7 mmol), oxidant, catalyst (Mo-IFBNPs or V-IFBNPs) and solvent,
[
40]
and the mixture was refluxed. The reaction progress was monitored
using GLC. Since different alkenes have different reactivity towards
oxidation, these reactions were continued until no further progress
was observed. The details of the reaction conditions are summa-
rized in Tables 1 and 2.
our previous report,
aluminium 2-butoxide was diluted in
2-butanol to 50% and was loaded in a glass container. The glass
container was placed in a 300 ml stainless steel autoclave. The
gap between the autoclave chamber and the glass container was
covered with 50 ml of distilled water and then the autoclave was
heated at 100°C. Aluminium 2-butoxide was hydrolysed by diffu-
sion of water vapour into solution in the hydrothermal chamber.
All manipulations of aluminium alkoxide were carried out in a
Schlenk line. The transference into the autoclave was performed
in a moisture-free atmosphere in order to prevent hydrolysis of
aluminium alkoxide before introducing it into the hydrothermal
chamber. After 5 h, the autoclave was cooled and the produced
powder was filtered off and dried at 100°C overnight. The BNPs thus
produced were used as catalytic support.
Catalyst Recycling
In a typical experiment, after recovering the catalyst from the reac-
tion mixture at optimum conditions, it was washed several times
with CCl , and then used in the same procedure under the same
4
conditions. The detailed procedures of the recycling experiment
are summarized in Table 3.
Results and Discussion
Preparation of Molybdenum and Vanadium Complexes
Supported on Functionalized BNPs
There are various methods for the preparation of BNPs. Among
them, a hydrothermal-assisted sol–gel process with aluminium alk-
[
40]
oxide was used here. It has some advantages, such as prepara-
tion in a one-pot process and processing at low temperature.
Then BNPs were grafted with MSPA, and their amine group was
converted to Schiff base in order to support two different com-
plexes of molybdenum and vanadium. These supported catalysts
were used in the epoxidation of cis-cyclooctene. The catalytic pro-
cedures for both catalysts were optimized for different parameters
such as catalyst, solvent and oxidant. Then both catalysts were used
for the catalytic epoxidation of different olefins under optimum
conditions. In addition, recycling experiments were done to reveal
the reusability of these catalysts.
As the first step in the preparation of functionalized BNPs, 1.00 g of
BNPs was refluxed with MSPA (61 μl, 0.35 mmol) in dry toluene
(50.0 ml) for 24 h (Scheme 1). The amine-functionalized BNPs
(AFBNPs) as a white solid was filtered off, washed three times with
dry toluene and dried at 100°C overnight. Then as the second step,
1.00 g of AFBNPs was refluxed with an equivalent amount of
2-hydroxybenzaldehyde (36.5 μl, 0.35 mmol) in ethanol (20.0 ml)
for 12 h (Scheme 1). Then the resulting imine-functionalized BNPs
(IFBNPs) were filtered off, washed three times with hot ethanol
and dried at 100°C overnight.
Preparation and Characterization of Molybdenum and Vana-
dium Complexes Supported on Functionalized BNPs
BNPs were prepared by hydrothermal-assisted sol–gel processing
of aluminium 2-butoxide solution in 2-butanol. The most promising
property of these hydrothermally processed BNPs is the formation
[
40]
of crystalline single-phase product with no organic residue. This
is confirmed by the lack of C–H stretching vibration in the IR spec-
trum and also the lack of organic residue ignition in TG/differential
scanning calorimetry (DSC) thermograms in addition to the XRD
pattern of this sample. The IR spectrum of BNPs (Fig. 1(a)) only
[
40]
shows the characteristic bands of boehmite. The PXRD pattern
of BNPs (Fig. 2(a)) also confirms crystallization of single-phase
[
40]
boehmite.
In the TG/DSC curve of BNPs (Fig. 3(a)), there are
two endothermic peaks below 100°C and around 450°C. The first
one is due to the elimination of physically adsorbed water, and
Scheme 1. Preparation of AFBNPs, IFBNPs, Mo-IFBNPs and V-IFBNPs.
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Copyright © 2015 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2015, 29, 593–600

Schiff base-functionalized BNPs for catalytic epoxidation of alkenes
a,b
Table 1. Epoxidation of cis-cylooctene under reflux conditions
c
e
Entry
Catalyst (mg)
Solvent
Oxidant
Catalyst loading
Oxidant/substrate
molar ratio
Epoxide yield (%)
d
(
mol%)
1
2
3
4
5
6
7
8
9
Mo-IFBNPs (40)
Mo-IFBNPs (40)
Mo-IFBNPs (40)
Mo-IFBNPs (40)
Mo-IFBNPs (40)
Mo-IFBNPs (40)
Mo-IFBNPs (40)
Mo-IFBNPs (40)
Mo-IFBNPs (40)
Mo-IFBNPs (40)
Mo-IFBNPs (40)
Mo-IFBNPs (40)
Mo-IFBNPs (40)
Mo-IFBNPs (40)
Mo-IFBNPs (40)
Mo-IFBNPs (30)
Mo-IFBNPs (20)
Mo-IFBNPs (15)
Mo-IFBNPs (10)
Mo-IFBNPs (5)
Mo-IFBNPs (15)
Mo-IFBNPs (15)
Mo-IFBNPs (15)
Mo-IFBNPs (15)
Mo-IFBNPs (15)
V-IFBNPs (20)
V-IFBNPs (20)
V-IFBNPs (20)
V-IFBNPs (20)
V-IFBNPs (20)
V-IFBNPs (20)
V-IFBNPs (20)
V-IFBNPs (20)
V-IFBNPs (20)
V-IFBNPs (20)
V-IFBNPs (20)
V-IFBNPs (20)
V-IFBNPs (20)
V-IFBNPs (5)
CH
CH
(CH
3
CN
OH
TBHP
2.0
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1
15
3
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
2.0
No reaction
3
)
2
CO
CH
Cl
CHCl
2.0
No reaction
6
C H
5
3
2.0
25
CH
2
2
2.0
5
3
2.0
35
CCl
CCl
4
2.0
73
4
H
H
H
H
2
2
2
2
O
O
O
O
2
2
2
2
2.0
3
CH
3
CN
2.0
2
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
40
41
42
43
44
45
46
47
48
CCl
4
ꢁCON
ꢁCON
2
H
H
4
2.0
5
CH
3
CN
2
4
2.0
2
f
CCl
4
NaIO
NaIO
4
2.0
No reaction
CH
3
CN
4
2.0
No reaction
CCl
CCl
CCl
CCl
CCl
CCl
CCl
CCl
CCl
CCl
CCl
CCl
4
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
4.0
76
4
4
4
4
4
4
4
4
4
4
4
1.3
50
1.5
73
1.0
73
0.75
0.50
0.25
0.75
0.75
0.75
0.75
0.75
1.0
73
65
54
45
2
80
2.5
2
75
85
2
95
CH
CH
(CH
3
CN
OH
CO
CH
Cl
CHCl
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1
8
3
1.0
No reaction
3
)
2
1.0
No reaction
6
C H
5
3
1.0
27
CH
2
2
1.0
12
3
1.0
21
CCl
CCl
4
1.0
45
4
H
H
H
H
2
2
2
2
O
O
O
O
2
2
2
2
1.0
4
CH
3
CN
1.0
5
CCl
4
ꢁCON
ꢁCON
2
H
H
4
1.0
5
CH
3
CN
2
4
1.0
2
f
CCl
4
NaIO
NaIO
4
1.0
No reaction
CH
CCl
CCl
CCl
CCl
CCl
CCl
CCl
CCl
CCl
CCl
3
CN
4
1.0
No reaction
4
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
0.25
0.50
0.75
1.5
20
35
45
45
45
28
50
52
56
67
V-IFBNPs (10)
V-IFBNPs (15)
V-IFBNPs (30)
V-IFBNPs (40)
V-IFBNPs (15)
V-IFBNPs (15)
V-IFBNPs (15)
V-IFBNPs (15)
V-IFBNPs (15)
4
4
4
4
4
4
4
4
4
2.0
0.75
0.75
0.75
0.75
0.75
2
2.5
2
2
a
Ratio of pendant Schiff base group (mmol)/BNPs (g) is 0.35, except entry 8 (0.70) and entry 9 (0.23).
b
Times of reactions are 150 min for Mo-IFBNPs and 90 min for V-IFBNPs.
c
Amount of solvent is 3 ml except entries 24 and 47 (2 ml) and entries 25 and 48 (1 ml).
d
Calculated for 0.7 mmol cyclooctene.
e
GLC yields are based on the starting cyclooctene.
f
Tetrabutylphosphonium bromide was used as phase transfer reagent.
Appl. Organometal. Chem. 2015, 29, 593–600
Copyright © 2015 John Wiley & Sons, Ltd.
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M. Mirzaee et al.
a
Table 2. Epoxidation of various olefins with TBHP catalysed by Mo-IFBNPs or V-IFBNPs under refluxing condition
b
b
Entry
Alkene
Mo-IFBNPs catalyst
V-IFBNPs catalyst
Conversion (%)
96
Epoxide (%)
Time (h)
Conversion (%)
92
Epoxide (%)
Time (h)
1
2
3
4
5
6
7
8
9
Cyclooctene
Cyclohexene
1-Octene
96
89
70
71
80
89
72
2
2
6
4
6
2
4
4
3
92
84
61
65
58
85
71
3.5
3.5
12
c
c
98
92
70
71
61
65
1-Hexene
12
d
d
Styrene
86
90
12
e
e
α-Methyl styrene
α-Pinene
trans-Stilbene
cis-Stilbene
98
94
3.5
3.5
72
71
f
g
f
g
96
96 (trans)
92
92 (trans)
12
f
h
f
h
94
81 (cis) and 13 (trans)
91
79 (cis) and 12 (trans)
6
a
Reaction conditions: alkene (0.7 mmol), TBHP (1.4 mmol), catalyst (15 mg), CCl
4
(1 ml).
b
GLC yield based on the starting alkenes.
c
By-product is 9% for Mo-IFBNPs and 8% for V-IFBNPs.
d
e
By-product is 6% for Mo-IFBNPs and 32% for V-IFBNPs.
By-product is 9% for both catalysts.
f
1
Both H NMR and GLC data confirm the reported yields.
g
1
Trans isomer is the only product according to H NMR data.
h
1
Cis and trans isomers are both produced according to H NMR data.
BNPs themselves have promising catalytic properties for the
multi-component synthesis of highly substituted imidazoles which
Table 3. Epoxidation of cis-cylooctene with TBHP under reflux condi-
tions using recycled catalysts
[
41]
was done in our research group and reported elsewhere. In ad-
dition, it was proved that for efficient epoxidation of olefins under
mild conditions, a catalyst that contains an active metal site is vital.
An oxidant could coordinate to this site and be activated for the
epoxidation process. But BNPs show low reactivity in epoxidation
of olefins as Choudhary and co-workers reported. Consequently
we have changed our strategy and tried to functionalize BNPs with
a Schiff base in two steps. During the first step, by refluxing a mix-
Cycle number
a
a
Mo-IFBNPs catalyst
V-IFBNPs catalyst
ꢀ1
ꢀ1
Epoxide yield TOF (h
)
Epoxide yield TOF (h
)
b
b
(%)
(%)
[
42]
1
2
3
4
5
6
7
8
9
1
95
95
94
94
93
91
90
88
85
80
44
44
44
44
43
42
42
41
39
37
92
92
92
90
89
87
85
83
80
75
29
29
29
28
28
27
26
26
25
23
ture of BNPs and MSPA in dry toluene for 24 h, –O
group is grafted on the hydroxyl-covered surface of BNPs
Scheme 1) and AFBNPs are thus produced. The IR spectrum of
3 2 3 2
Si(CH ) NH
(
ꢀ
1
AFBNPs (Fig. 1(b)) shows a new band at 1553 cm which is
assigned to the N–H stretching vibrations of grafted amine groups
in addition to the BNP characteristic bands. Elemental analysis
shows that the nitrogen content of AFBNPs is 0.46%, which means
that 0.33 mmol of pendant amine groups are covalently bound to
the surface of 1.00 g of BNPs. ICP analysis also shows there are no
molybdenum or vanadium contaminants in AFBNPs and they shows
no reactivity in catalytic epoxidation of olefins as predictable accord-
ing to the above discussion. In the second step, pendant amine
groups of AFBNPs are changed to Schiff base by refluxing a mixture
of AFBNPs and 2-hydroxybenzaldehyde in ethanol for 12 h which
produces IFBNPs (Scheme 1). In addition to the BNP characteristic
bands, the IR spectrum of IFBNPs (Fig. 1(c)) shows a sharp band at
0
a
Reaction condition: 0.7 mmol cyclooctene, 1.4 mmol TBHP, 15 mg
, 120 min for Mo-IFBNPs and 180 min for V-IFBNPs.
catalyst, 1 ml CCl
b
4
GLC yields are based on the starting cyclooctene.
the other is attributed to dehydroxylation of boehmite and crys-
tallization of γ-alumina. The absence of C–H stretching vibrations
in the IR spectrum and also the absence of an exothermic peak at
around 250°C in the TG/DSC curve of BNPs confirm loss of all or-
ganic residue in the hydrothermally processed BNPs. Inductively
coupled plasma (ICP) and elemental analyses also confirm the
lack of molybdenum or vanadium and carbon and nitrogen con-
tent in BNPs, respectively. The calculation of particle size from the
PXRD pattern according to the Scherer equation shows 10 nm
particles for BNPs. This is confirmed by the transmission electron
microscopy image of BNPs (Fig. 4). In this image, needle-shaped
BNPs are seen over 50 nm long and up to 10 nm in width.
ꢀ
1
1663 cm which is assigned to the C¼N stretching vibration of
azomethine (imine) group. Elemental analysis shows that the nitro-
gen content of IFBNPs is 0.46%, which means all the pendant amine
groups are changed to imine by this reaction. ICP analysis also
confirms the lack of molybdenum or vanadium content in IFBNPs
and they show no reactivity in catalytic epoxidation of olefins.
The IFBNPs were then used for supporting molybdenum and va-
nadium complexes. For this purpose, Mo(CO)
were used. But Mo(CO) is not labile in ligand substitution reaction
so its solution in THF was activated by irradiation with UV light. This
process resulted in the formation of Mo(CO) (THF) complex which
6
4 2
and VO(SO )ꢁH O
6
2
ꢀ1
Effective surface area of the BNPs is 326 m g according to
BET experiments.
5
substituted its THF ligand with coordinating atoms of imine group
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Schiff base-functionalized BNPs for catalytic epoxidation of alkenes
Figure 2. XRD patterns of (a) BNPs, (b) Mo-IFBNPs and (c) V-IFBNPs.
peak in the DTA curve due to the dehydroxylation of boehmite
and the crystallization of γ-alumina similar to BNPs. The calculation
of particle size from the PXRD pattern of Mo-IFBNPs, according to
the Scherer equation, shows no change in particle size in compari-
son to BNPs.
The IR spectrum of V-IFBNPs (Fig. 1(e)) shows a new band due to
ꢀ
1
the V¼O stretching vibration of VO(SO
4
) complex at 970 cm ,
confirming the coordination of vanadium complex to the function-
alized BNPs. ICP analysis also shows that over 90% of Schiff base
sites on IFBNPs are used for anchoring 0.31 mmol of vanadium
complex on 1.00 g of V-IFBNPs, which is comparable to the metal
loading in Mo-IFBNPs. The PXRD pattern of V-IFBNPs (Fig. 2(c)) also
confirms the retention of the crystalline structure after metal load-
ing. In the TG/DTA curve of V-IFBNPs (Fig. 3(c)) there is an endother-
mic peak below 100°C with 9% weight loss in the TG curve due to
the elimination of physically adsorbed water. Similar to the molyb-
denum counterpart, there is an exothermic peak at around 300°C
with 7% weight loss in the TG curve of V-IFBNPs due to the ignition
of organic parts of IFBNPs. Additionally, analogous to BNPs and
Mo-AFBNPs, this thermogram shows 12% weight loss in the range
400–550°C which is accompanied by an endothermic peak in the
DTA curve and assigned similarly. The calculation of particle size
from the PXRD pattern of V-IFBNPs, according to the Scherer
equation and similar to the molybdenum counterpart, shows no
change in particle size in comparison to BNPs.
Figure 1. FT-IR spectra of (a) BNPs, (b) AFBNPs, (c) IFBNPs, (d) Mo-IFBNPs
and (e) V-IFBNPs.
on BNPs, after refluxing for 6 h (Mo-IFBNPs). VO(SO
was also refluxed with IFBNPs in ethanol for 7 h to support the va-
nadium complex on the IFBNPs (V-IFBNPs).
4
)ꢁH
2
O complex
The IR spectrum of Mo-IFBNPs (Fig. 1(d)) shows some new weak
ꢀ
1
bands at around 1800–1900 cm due to the carbonyl stretching
vibration of coordinated molybdenum complex. On the one hand,
the lower symmetry of the molybdenum complex as a result of co-
ordination is evidenced by the redshift and splitting of carbonyl
[43]
stretching vibrations ; on the other hand, the low metal content
of the catalyst is evidenced by the weakness of them. ICP analysis
shows that approximately 90% of Schiff base sites on IFBNPs are
used for anchoring 0.3 mmol of molybdenum complex on 1.00 g
of Mo-IFBNPs. The PXRD pattern of Mo-IFBNPs (Fig. 2(b)) confirms
the retention of the crystalline structure after functionalization
and metal loading. In the TG/DTA curve of Mo-IFBNPs (Fig. 3(b))
there is an endothermic peak below 100°C with 7% weight loss in
the TG curve due to the elimination of physically adsorbed water.
There is also an exothermic peak at around 350°C with 13% weight
loss in the TG curve due to the ignition of organic parts of pendant
groups on BNPs. This thermogram also shows 12% weight loss in
the range 400–500°C which is accompanied by an endothermic
Alkene Epoxidation with tert-Butylhydroperoxide Catalysed by
Mo-IFBNPs and V-IFBNPs
With the purpose of achieving optimized conditions for the title re-
action, epoxidation of cyclooctene was investigated by changing
various parameters including solvent, oxidant, catalyst and reaction
time. The experiments show that with these catalysts, tert-
butylhydroperoxide (TBHP) is the only oxidizing system that can
4
achieve the epoxidation of cyclooctene in CCl . Other oxygen
Appl. Organometal. Chem. 2015, 29, 593–600
Copyright © 2015 John Wiley & Sons, Ltd.
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M. Mirzaee et al.
sources such as hydrogen peroxide, sodium periodate (in the pres-
ence of 0.01 g of tetrabutyl phosphonium bromide as phase trans-
fer reagent) and urea–hydrogen peroxide in CCl or CH CN do not
4 3
show any activity (Table 1, entries 1, 7–13, 26, 32–38). In addition,
under mild conditions, the epoxidation of cyclooctene using TBHP
does not proceed in the absence of catalyst.
In a first step, the effect of solvent was investigated; conse-
quently, protic and aprotic solvents with various polarity and coor-
dinating ability were used in the epoxidation of cyclooctene with
TBHP. The observed trend (Table 1, entries 1–7, 26–32) is the same
as that of literature reports on homogeneous alkene epoxidation by
[44]
Mo(CO)6 catalyst.
Coordinating solvents such as CH CN and
3
CH OH compete with TBHP to occupy the coordination sites on
3
the transition metal catalyst. Therefore in the presence of these sol-
vents, the observed yields are low. Among other solvents, CCl4
leads to the highest yield of epoxide product for both Mo-IFBNPs
and V-IFBNPs catalysts. This could be attributed to the higher boil-
ing point of CCl in comparison to the other solvents investigated.
4
In the next step, the amount of pendant Schiff base groups an-
chored on BNPs (Table 1, entries 7, 14 and 15) was changed. For this
purpose, 0.35 (entry 7), 0.7 (entry 14) and 0.23 (entry 15) mmol of
MSPA were used in order to anchor to 1.00 g of BNPs and convert
6
them to Schiff base. After supporting Mo(CO) on the functionalized
BNPs under the same conditions, these catalysts were used for the
epoxidation of 0.7 mmol of cyclooctene with 1.05 mmol of TBHP.
The results show that after 2.5 h, the conversions for entries 7 and
1
0
4 are about the same and higher than that for entry 15; therefore
.35 mmol of pendant Schiff base group per gram of BNPs was se-
lected for other investigations.
As another parameter, the amount of catalysts was changed in
the epoxidation of cyclooctene with TBHP. The results (Table 1,
entries 7, 16–20, 32, 39–43) show that by increasing the amount
of both Mo-IFBNPs and V-IFBNPs catalysts up to 15 mg, the yield
of cyclooctene epoxide increases.
The ratio of oxidant to substrate is one of the most crucial param-
eters in catalytic epoxidation processes. The results (Table 1, entries
18, 21–23, 41, 44–46) for Mo-IFBNPs and V-IFBNPs show that the op-
timum ratio of oxidant to substrate for both catalysts is 2. Although
increasing this ratio to 2.5 does not change the yields significantly,
decreasing the ratio to 1 reduces the yields.
As the last parameter, the amount of solvent was investigated.
The results (Table 1, entries 22, 24, 25, 45, 47, 48) show that by de-
creasing the amount of solvent for both catalysts from 3 to 1 ml, the
yield of cyclooctene epoxide increases.
Figure 3. Thermal analyses curves of (a) BNPs, (b) Mo-IFBNPs and (c) V-
IFBNPs.
Figure 5 shows the effect of reaction time on the epoxidation
of cyclooctene with TBHP. Comparing the reactivity of Mo-IFBNPs
and V-IFBNPs catalysts reveals that the former is more reactive,
and after 2 h the conversion reaches a maximum and becomes
constant. But the supported vanadium complex needs 3.5 h to
reach this point.
These new catalysts, Mo-IFBNPs and V-IFBNPs, can also be used
for the epoxidation of a wide range of substituted alkenes (Table 2).
Blank experiments, performed without catalysts or BNPs, show a
very low conversion even after 5 h reflux. However, in the presence
of these catalysts, a wide range of both cyclic and linear alkenes can
efficiently and selectively be converted to epoxides. Based on the
[
45]
epoxidation mechanism suggested earlier, higher electron do-
nating ability of olefin double bond is expected to show greater
epoxidation reactivity. Therefore, cyclooctene and cyclohexene
with inner double bonds should exhibit greater activity in compar-
ison to 1-hexene and 1-octene which contain terminal double
bonds. trans-Stilbene is also completely converted to epoxide using
Figure 4. Transmission electron microscopy image of BNPs.
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Copyright © 2015 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2015, 29, 593–600

Schiff base-functionalized BNPs for catalytic epoxidation of alkenes
1
00
9
8
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
0
0
Mo-AFBNPs
V-AFBNPs
9
5
92
3
0
45
60
90
120
150
180
210
240
Time (min)
Figure 5. Epoxidation of 0.7 mmol of cis-cyclooctene with 1.4 mmol of
TBHP catalysed by 15 mg of Mo-IFBNPs or V-IFBNPs in 3 ml of CCl
4
under
reflux conditions.
Mo-IFBNPs and V-IFBNPs catalysts. Surprisingly, trans-Stilbene gives
only trans-epoxide. This reflects the high selectivity of these cata-
lysts in the epoxidation of alkenes.
Scheme 3. Proposed mechanism for the epoxidation of olefin with TBHP
catalysed by V-IFBNPs.
[
45]
On the basis of Sobczak’s ideas and other experimental and
[44,46–48]
Lewis acidic metal centres. Then, the olefin substrate coordinates
to the metal centre and, as a nucleophile, inserts into the metal–
oxygen bond of coordinated peroxide electrophile anion. This
mechanism easily explains the faster reaction of electron-rich
olefins in comparison to electron-poor olefins. In the next step,
epoxide product is formed and tert-butylperoxide anion is con-
verted into tert-butoxide anion. Then the peroxide product is re-
leased and the catalytic cycle continued by substitution of a new
tert-butylperoxide instead of tert-butoxide anion.
theoretical reports,
probable mechanisms are proposed
for the epoxidation of cyclooctene with TBHP catalysed by
Mo-IFBNPs and V-IFBNPs (Schemes 2 and 3). According to our ex-
perimental results, reaction rates are of first order depending on
catalyst and substrate concentrations. There are also some crucial
stages in these processes. V-IFBNPs catalyst initially contains oxo
ligand at its metal centre, but for Mo-IFNBPs, in the first step, pen-
dant molybdenum complex is oxidized and an oxo-molybdenum
complex is formed. The next step for both catalysts involves trans-
ferring TBHP proton to a terminal oxygen atom of these oxo com-
plexes which results in coordination of tert-butylperoxide anion to
Catalyst Recovery and Reuse
As discussed earlier, the reusability of solid supported catalysts is
one of their most important benefits. Therefore, the reusability of
Mo-IFBNPs and V-IFBNPs catalysts was monitored by means of mul-
tiple sequential epoxidations of cyclooctene with TBHP (Table 3).
After each cycle, the reaction mixture was centrifuged and the su-
pernatant solutions were analysed using ICP. This analysis shows
that there is no metal contaminant in the supernatant solution. This
also confirms that metal ions do not leach from the heterogeneous
catalysts and the reactions proceed entirely under heterogeneous
conditions. But after ten cycles of recovering both catalysts, they
lose 15–17% of their activities in the epoxidation of olefins. The rea-
son for this phenomenon is not clear now and needs more investi-
gation in the future.
Conclusions
We have successfully prepared new hybrid heterogeneous cata-
lysts by anchoring oxosulfate vanadium(IV) and hexacarbonyl mo-
lybdenum complexes to BNPs which are functionalized with
Schiff base. These catalysts show good catalytic activities in the
epoxidation of various important olefin substrates under entirely
heterogeneous conditions. The results show that solvents have a
crucial effect on the catalytic reactions, and maximum conversion
is observed in CCl4.
Acknowledgment
The authors thank the Vice-President’s Office for Research Affairs in
Scheme 2. Proposed mechanism for the epoxidation of olefin with TBHP
catalysed by Mo-IFBNPs.
the University of Shahrood for supporting this work.
Appl. Organometal. Chem. 2015, 29, 593–600
Copyright © 2015 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

M. Mirzaee et al.
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