Supported vanadium Schiff bases complex on nano silica: a heterogeneous catalyst for the selective oxidation of sulfides and alcohols

Article abstract of DOI:10.1007/s13738-016-0840-z

Abstract: A vanadium Schiff base complex on nano silica was prepared by reaction of nano silica NH₂-functionalized, 2,4-dihydroxy benzaldehyde and VO(acac)₂. The VSBC@NS was used as an efficient and reusable heterogeneous catalyst fo

Full text of DOI:10.1007/s13738-016-0840-z

J IRAN CHEM SOC
DOI 10.1007/s13738-016-0840-z
ORIGINAL PAPER
Supported vanadium Schiff bases complex on nano silica: a
heterogeneous catalyst for the selective oxidation of sulfides
and alcohols
Minoo Dabiri1 · Majid Koohshari1 · Fazeleh Shafipour1 · Melika Kasmaei1 ·
Parinaz Salari1 · David MaGee2
Received: 2 October 2015 / Accepted: 14 March 2016
© Iranian Chemical Society 2016
Abstract A vanadium Schiff base complex on nano sil-
ica was prepared by reaction of nano silica NH2-function-
alized, 2,4-dihydroxy benzaldehyde and VO(acac)2. The
VSBC@NS was used as an efficient and reusable heteroge-
neous catalyst for the selective oxidation of sulfides to sul-
foxides and alcohols to aldehydes with H2O2 as an oxidant
in methanol.
reagents, intermediates and catalysts. In particular, sulfox-
ides are important intermediates in the synthesis of natural
and biological materials [1–4].
The first synthesis of sulfoxides was reported by Märker
in 1865, since a large number of methods for the oxidation
of sulfides to sulfoxides have been reported [5]. Because
sulfides are much more nucleophilic than sulfoxides, usu-
ally there is a reasonably good selectivity for the oxidation
of sulfides. However, the selective preparation of sulfox-
ides over sulfones is an important knot in the oxidation of
sulfides [6, 7].
Graphical abstract
Various oxidants are used for the oxidation of sulfides
[8–11]. Because of high cost, production of by-products
and the low content of effective oxygen, most oxidants are
not sufficient for large-scale synthesis. Among all of them
aqueous hydrogen peroxide is the most attractive, as it is
both environmentally benign and economical.
The discovery of new catalysts and their use in organic
reactions is of much current interest in the chemical
research of heterogeneous catalysis, primarily due to
advantages like easy and cheap recovery of catalyst, sim-
ple workup and reduction of waste [12]. In recent years,
selective oxidation of sulfides to sulfoxides has been
reported with heterogeneous systems [13–17], and compe-
tition to find new and efficient processes using a capable
and recyclable catalyst continues. Herein, we report a het-
erogeneous catalytic system of vanadium Schiff base com-
plexed on nano silica. These were prepared by the reaction
of nano silica NH2-functionalized, 2,4-dihydroxyben-
zaldehyde and VO(acac)2, and was used for the selective
oxidation of sulfides to sulfoxides with H2O2 as oxidant
in methanol in quantitative yields at room temperature
(Scheme 1).
Keywords Schiff base · Vanadium · Oxidation ·
Sulfoxide · Nanoparticle
Introduction
Organosulfur compounds such as sulfoxides and sulfones
are very prominent compounds in organic chemistry.
For instance, sulfoxides have found use as final products,
* Minoo Dabiri
m-dabiri@sbu.ac.ir
1
Department of Chemistry, Shahid Beheshti University,
General Campus, Tehran 1983963113, Iran
2
Departement of Chemistry, University of New Brunswick,
P. O. Box 4400, Fredricton, NB E3B 5A3, Canada
1 3

J IRAN CHEM SOC
Scheme 1 Vanadium Schiff
base complexed on nano silica
for the selective oxidation of
sulfides
O
S
nano
O
silica
O
S
HO
OH
VSBC@NS
O
Si
HO
O
V
N
H
2O2
OEt
O
VSBC@NS
and stirring was continued at 40 °C for 1 h. After cool-
ing to room temperature, H2O2 (1 mmol, 0.1 mL, 30 %
aq) was added and the mixture stirred for an additional
30 min. The reaction mixture was filtered, washed with
ethanol and Et2O (3x) and dried under vacuum. The
loading of nitrogen was determined to be 0.5 mmol g−1
from elementary analysis (CHN). The ICP analysis gave
the actual V contents as 0.44 mmol g−1 for VSBC@NS
nanocomposite.
Experimental
Characterization
Scanning electron microscope VSBC@NS was performed
using Hitachi SEM SU3500. The thermal stability of the
VSBC@NS was determined using a thermogravimetric
analyzer (TGA/DTA BAHR: STA 503) under air and a
heating rate of 10 °C min−1. Elemental analysis of VSBC@
NS nanocomposite was performed using an Elementar
Analysensysteme GmbH VarioEL CHNS. Gas chromatog-
raphy was performed on a Trace GC ultra from the Thermo
Company equipped with FID detector and Rtx®-1 capil-
General procedure for the oxidation of sulfide
To a stirred suspension of the selected sulfide (0.5 mmol)
and the heterogeneous catalyst VSBC@NS (0.1 mmol of
V) in methanol (2 mL), H2O2 (0.06 mL, 0.6 mmol, 30 %
aq) was added in one portion. The slurry was stirred at
room temperature for 2 h. The catalyst was filtered off and
washed with acetone (10 mL). The filtrate was diluted with
ethyl acetate (10 mL) and the resulting solution was dried
with anhydrous sodium sulfate, filtered and evaporated in
vacuum to afford the crude product which was purified by
column chromatography on silica gel (10 % EtOAc in hex-
ane) to afford the pure sulfoxide.
1
lary column. H-NMR spectra were recorded on a BRUK-
ERDRX-300AVANCEspectrometer. NMR spectra were
obtained. Melting points of products were measured on an
Electrothermal 9100 apparatus and are uncorrected. The
concentration of vanadium was estimated using an induc-
tively coupled plasma optical emission spectrometer (ICP-
OES) Varian Vista PRO Radial. IR spectra were recorded
on a Bomem MB-Series FT-IR spectrophotometer.
Materials
General procedure for the oxidation of alcohols
Aqueous solutions of hydrogen peroxide (30 %), toluene,
dichloromethane, methanol, ethanol, HCl, 2,4-dihydroxy-
benzaldehyde, (3-aminopropyl) triethoxysilane were pur-
chased from Merck, and used as provided. Silica nanoparti-
cles were purchased from Merck.
To a stirred suspension of the selected alcohol (0.5 mmol)
and the heterogeneous catalyst VSBC@NS (0.1 mmol of
V) in methanol (2 mL), H2O2 (0.06 mL, 0.6 mmol, 30 %
aq) was added in one portion. The slurry was stirred under
reflux condition for 4 h. The catalyst was filtered off and
washed with acetone (10 mL). The filtrate was diluted with
ethyl acetate (10 mL) and the resulting solution was dried
with anhydrous sodium sulfate, filtered and evaporated in
vacuum to afford the crude product which was purified by
column chromatography on silica gel (5 % EtOAc in hex-
ane) to afford the pure aldehyde.
Fictionalization of nano silica (NS)
A suspension of (3-aminopropyl) triethoxysilane (2 mmol,
0.443 g) and activated nano silica (2 g) in toluene (15 mL)
was heated under reflux with stirring under nitrogen atmos-
phere. After heating for 24 h, the powder was filtered,
washed with diethyl ether and dried under vacuum.
All compounds are known compounds and were
reported previously.
Synthesis of VSBC@NS
Methyl phenyl sulfoxide
The mixture of NH2-functionalized nano silica previ-
ously obtained (2 g) and 2,4-dihydroxybenzaldehyde
(1 mmol, 0.14 g) in ethanol (20 mL) was heated at reflux
for 1 h. Then VO (acac)2 (1 mmol, 0.27 g) was added
Mp.: 26–29 °C (Lit [18] Mp.: 28–29 °C),. 1H NMR
(300 MHz,CDCl3) δ: 7.65 (m, 2H), 7.48 (m, 2H), 7.30 (m,
1H), 2.64 (s, 3H).
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J IRAN CHEM SOC
Table 1 Oxidation of methyl phenyl with a–c
Table 2 Capability of catalyst
O
S
S
nano
silica
Si
HO
OH
O
O
O
R
X
O
V
N
S
OEt
Catalyst
Yield (%)a
O
S
Entry
Catalyst
4
a : X = H, R = H
: X = OH, R = H
1
–
10
15
20
90
99
b
c : X = OH, R =
OH
2
SiO2
3b
4c
5d
VSBC
Entry
Catalyst
Yield (%)a
Conversion (%)
VSBC@NS
VSBC@NS
1
2
3
a
b
c
50
97
99
65
99
99
Methyl phenyl sulfide (0.5 mmol), catalyst (20 mol %), H2O2
(1.2 eq), MeOH (2 mL), rt, 2 h
a
GC yield, n-dodecane was used as an internal standard
Methyl phenyl sulfide (0.5 mmol), catalyst (20 mol %), H2O2
(1.2 eq), MeOH (2 mL), rt, 2 h
b
In situ prepared vanadium complex from benzylamine and
a
2,4-dihydroxybenzaldehyde and VO(acac)2
GC yield, n-dodecane was used as an internal standard
c
In situ prepared catalyst
d
Synthesized catalyst in the presence of H2O2
1‑(Ethylsulfinyl)‑4‑methylbenzene
1
Mp.: 140–142 °C (Lit [19] Mp.: 142–143 °C), H NMR
(300 MHz,CDCl3) δ: 7.49 (d, 2H, j = 8.1), 7.31 (d, 2H,
j = 8.1), 2.63–2.93 (m, 2H), 2.40 (s, 3H), 1.15–1.25 (m,
3H).
1‑(Benzylsulfinyl)benzene
Mp.: 120–124 °C (Lit [20] Mp.:122–123 °C), 1HNMR
(300 MHz,CDCl3) δ: 7.38–7.49 (m, 5H), 7.26–7.35 (m,
4H), 6.99 (d, 2H, j = 6), 4.12 (d, 1H, j = 12.6), 4.02 (d,
1H, j = 12.6).
Results and discussion
Vanadium complexes formed from tri-dentate Schiff base
ligands with an ONO donor are widely used as catalyst in the
oxidation of sulfides. Because of outbreak synergic effect of
silica on some catalytic reaction, To explore whether this was
the case, we first attempted an in situ synthesis of a vanadium
Schiff base complex with a bi-dentate ligand (NO donor) by
the mixing of nano silica NH2-functionalized, aldehyde and
VO(acac)2 (Table 1). As shown in Table 1, the presence of a
hydroxyl group at the ortho position had an intensely posi-
tive effect on the yield of reaction (Table 1, entry 2). Also a
2,4-dihydroxy-substituted aromatic was slightly better than a
2-hydroxy-substituted aromatic group with respect to a ben-
eficial effect on the yield of reaction.
Fig. 1 The FT-IR spectra of a nano silica, b nano silica NH2-func-
tionalized and c VSBC@NS
To probe the effect of synthesized catalyst on the sul-
foxidation reaction a series of experiments were per-
formed, Table 2. First, in the absence of catalyst only a
small amount of sulfoxide was formed (Table 2, entry 1).
This did not change dramatically if only SiO2 or vanadium
catalyst in the absence of SiO2 were used (Table 2, entries
2 and 3). However, when the in situ catalyst was prepared
the yield improved dramatically (Table 2, entry 4). When a
pre-synthesized catalyst (entry 5) in the presence of H2O2
(1.2 eq.) was used a modest increase in yield was observed.
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J IRAN CHEM SOC
Fig. 2 SEM images of VSBC@
NS a SE detector, b BSE-3D
detector, and corresponding
quantitative EDS element map-
ping of c V, d N and e C
To try and probe the structure of our catalyst the FT-IR
spectra of nano silica, nano silica NH2-functionalized and
VSBC@NS were taken and are shown in Fig. 1. In the
FT-IR spectroscopy of nano silica, the characteristic bands
at around 3500 cm−1 is attributed to the O–H stretching
absorbance of Si–OH while the peak at around 1100 cm−1
Table 3 Carbon, nitrogen, oxygen, silicon, vanadium in the VSBC@
NS obtained from EDS analysis
Element
Wt%
C
N
O
Si
V
20.01
1.81
29.70
45.66
2.82
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J IRAN CHEM SOC
analysis of element distribution demonstrates that C, N
and V appear to be uniformly distributed on the catalyst
surfaces. The weight percent of C, N, V, O, Si from EDS
analysis is reported in Table 3.
Thermogravimetric analysis (TGA) was further used to
study the thermal behavior and stability of the VSBC@NS.
This nanocomposite showed high thermal stability (Fig. 3).
The total weight lost for VSBC@NS was only 4 % at tem-
peratures 300 °C.
The effects of the catalyst loading (Table 4, entries 1–3),
solvent (Table 4, entries 3–6), temperature (Table 4, entries
6–8), reaction time (Table 4, entries 8–13) and amount of
oxidant (Table 4, entries 13–16) were also evaluated. As
shown in Table 4, methanol as solvent at room temperature
and 1.2 eq H2O2 as oxidant was selected as the optimum
reaction condition.
In an effort to explore the scope of the reaction, a series
of sulfides were used in the oxidation reaction, and the
results are presented in Table 5.
Fig. 3 TGA plot of VSBC@NS
is assigned to the Si–O stretching bond. After NH2-func-
tionalization of nano silica, the peak intensity at 3500 cm−1
decreased greatly. The appearance of new peak at around
1630 cm−1 confirmed the formation of the vanadium com-
plex on nano silica in the VSBC@NS FT-IR spectra.
The density and distribution of the complex on the
nano silica, quantitative energy dispersive X-ray spectros-
copy (EDS) mapping was done. As shown in Fig. 2c–e,
The recyclability and reusability of immobilized cat-
alyst (VSBC@NS) was examined in the oxidation of
methyl phenyl sulfide. After the reaction was complete
the catalyst was filtered off and washed with acetone,
Table 4 Screening of the oxidation of methyl phenyl sulfide conditions
O
S
S
Entry
Solvent
Time (h)
Temp. (°C)
H2O2 (eq)
Catalyst (%)
Conversion (%)
Yield (%)a
Sulfone (%)
1
EtOH
2
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
40
50
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1.2
1
5
10
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
30
65
99
87
94
99
99
99
35
50
95
99
99
85
100
100
68
25
55
91
80
91
99
74
53
25
45
88
99
99
80
53
15
53
–
2
EtOH
2
–
3
EtOH
2
7.5
–
4
Acetone
CH3Cl
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
2
5
2
19
–
6
2
7
2
25
45
–
8
2
9
0.5
1
10
11
12
13
14
15
16
17b
–
1.5
2.5
3.5
2
–
–
–
–
2
1.5
2
45
80
–
2
2
–
Methyl phenyl sulfide (0.5 mmol), solvent (2 mL)
a
GC yield, n-dodecane was used as an internal standard
b
O2 used as an oxidant
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J IRAN CHEM SOC
Table 5 Oxidation of sulfides
O
S
VSBC@NS
R2
S
R1
R2
R1
Entry
R1
R2
Conversion (%)
Yield (%)a
TON
TOF (h−1
)
1
2
3
4
5
6
7
8
H
CH3
Et
99
97
99
90
99
94
80
95
99
83
4.95
4.5
2.48
2.25
2.48
2.35
2.0
H
4-CH3
4-Cl
4-CH3
H
CH3
CH3
Et
99
4.95
4.7
94
98
4.0
Bz
98
4.75
4.95
4.15
2.38
2.48
2.08
4-Cl
Furyl
Ph
100
85
Et
Aryl sulfide (0.5 mmol), VSBC@NS (20 mol %), H2O2 (1.2 eq), MeOH (2 mL), rt, 2 h
a
GC yield, n-dodecane was used as an internal standard
Table 6 Comparison of
Referen
ces
efficiency of various vanadium
based catalysts and the
oxidation of methyl phenyl
sulfide
Entry
Catalyst
Condiꢀon
Yield (%)
H2O2, MeOH, r.t,
20min
1
2
Silica-vanadia (Si-V10-2)
94
96
[21]
[22]
TBHP, CDCl3, 90min
TBHP, CH3CN, 60oC,
N2, 3h
3
4
98
99
[23]
This
VSBC@NS
H2O2, MeOH, r.t, 2h
work
Table 7 Screening of the oxidataion of benzyl alcohol conditions
OH
O
Entry
Solvent
Time (h)
Temp. (°C)
Catalyst (%)
Conversion (%)
Yield (%)a
1
2
3
4
5
CH3CN
CHCl3
MeOH
MeOH
MeOH
4
4
4
4
4
Reflux
Reflux
Reflux
Reflux
r.t.
20
20
20
10
20
73
46
98
57
41
71
41
95
49
38
Benzyl alcohol (0.5 mmol), H2O2 (1.2 eq), solvent (2 mL) 4 h
a
GC yield, n-dodecane was used as an internal standard
1 3

J IRAN CHEM SOC
dried and reused for a subsequent reaction (five runs) by
adding fresh substrate and oxidant under identical reac-
tion procedure. In all cases, reaction times and yield (99,
99, 91, 85, 76 %) of the methyl phenyl sulfoxide slightly
decreased. ICP results of the used VSBC@NS nanocom-
posite catalyst indicate the leaching of 2.3 % of vana-
dium in the oxidation of methyl phenyl sulfide after the
fifth cycle.
Table 6 compares efficiency of VSBC@NS nanocom-
posite (reaction conditions, time, and yield) with efficiency
of other reported heterogeneous vanadium based catalysts
in the oxidation of methyl phenyl sulfide.
Inspired by the high activity and stability of VSBC@
NS nanocomposite the oxidation of sulfides was further
employed as another model reaction to test further the per-
formance of VSBC@NS nanocomposite. We chose the oxi-
dation of benzyl alcohol as an optimum reaction in MeOH
as solvent with a catalyst loading of 20 mol% of V for 4 h
under reflux condition (Table 7, entry 3). Under this condi-
tion, we found that the oxidation proceeds well, affording
the excellent yield (95 %) of the corresponding aldehyde.
To extend the scope of the prepared catalyst, the oxida-
tion of the alcohols to the related carbonyl compounds have
been investigated. The results of the mentioned reactions
Table 8 Oxidation of alcohols
VSBC@NS
R
R
OH
O
Conversion
(%)
Entry
Alcohol
Yield (%)a
TON
TOF (h−1)
1
2
98
93
95
89
4.75
4.45
1.19
1.11
3
4
90
81
88
76
4.4
3.8
1.1
0.95
5
6
84
94
81
91
4.05
4.55
1.01
1.14
7
89
86
4.3
1.08
Alcohol (0.5 mmol), VSBC@NS (20 mol%), H2O2 (1.2 eq), MeOH (2 mL), reflux, 4 h
a
GC yield, n-dodecane was used as an internal standard
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J IRAN CHEM SOC
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In summary a facile, eco-friendly and economical process
for the selective oxidation of sulfides to sulfoxides and
alcohols to aldehydes by introducing a heterogeneous vana-
dium Schiff bases complex supported on nano silica as a
selective oxidative catalyst was developed. Mild reaction
conditions excellent yields and selectivity are other advan-
tages of this protocol. Moreover, the catalyst was chemi-
cally stable and could be recycled for at least five times in
the corresponding reaction without a significant reduction
in the catalytic activity.
Acknowledgments We gratefully acknowledge the financial sup-
port from the Research Council of Shahid Beheshti University.
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