Selective oxidation of alcohols and sulfides: Via O2using a Co(ii) salen complex catalyst immobilized on KCC-1: Synthesis and kinetic study

Article abstract of DOI:10.1039/d0ra06863b

The aim of this study was to immobilize a Co(ii) salen complex on KCC-1 as a catalyst that can be recovered (Co(ii) salen complex?KCC-1). Field-emission transmission electron microscopy, FT-IR spectroscopy, thermogravimetric analysis, elemental analysis,

Full text of DOI:10.1039/d0ra06863b

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Selective oxidation of alcohols and sulfides via O2
using a Co(II) salen complex catalyst immobilized
on KCC-1: synthesis and kinetic study
Cite this: RSC Adv., 2020, 10, 37974
*
Ali Allahresani,
Elaheh Naghdi, Mohammad Ali Nasseri
and Kaveh Hemmat
The aim of this study was to immobilize a Co(II) salen complex on KCC-1 as a catalyst that can be recovered
(Co(II) salen complex@KCC-1). Field-emission transmission electron microscopy, FT-IR spectroscopy,
thermogravimetric analysis, elemental analysis, atomic absorption spectroscopy, and XRD were used to
confirm the structure and chemical nature of Co(II) salen complex@KCC-1. The oxidation efficiency was
obtained for an extensive range of sulfides and alcohols using this sustainable catalyst, alongside O2 as
an oxygen source and isobutyraldehyde (IBA) as an oxygen acceptor, with superior selectivity and
conversion for the relevant oxidation products (sulfoxides and ketones or aldehydes) under moderate
conditions. The m-oxo and peroxo groups on the ligands of the Co complex appeared to be responsible
for the superior activity of the catalyst. Essential factors behind the oxidation of alcohol and sulfoxides
were investigated, including the catalyst, solvent, and temperature. In this paper, molecular oxygen (O2)
was used as a green oxidant. Furthermore, kinetic studies were conducted, revealing a first-order
reaction for the oxidation of both benzyl alcohol and sulfide. The reaction progressed at mild
temperature, and the catalyst could be easily recovered and reused for numerous consecutive runs
under the reaction conditions, without any substantial reduction in the functionality of the catalytic system.
Received 9th August 2020
Accepted 14th September 2020
DOI: 10.1039/d0ra06863b
rsc.li/rsc-advances
of advanced functional materials is surface-functionalized
mesoporous materials. Polshettiwar et al.4 reported brous
Introduction
nano-silica (KCC-1), with a large surface area and effortless
access via its bers (unlike the traditional usage of pores). For
catalysts based on noble metals with superb catalytic efficiency
and excellent availability of the active sites, KCC-1 would be an
ideal catalyst support candidate.4
The simple separation and recovery of a catalyst is a signicant
area within green chemistry. It is complicated to separate
reaction products from homogeneous catalysts; their immobi-
lization on various supports is the most common way. There-
fore, their catalytic performance can be improved by xing
metal complexes onto a solid support with a large surface area.
Although some exciting features including the capability to be
reused numerous times and stability against leaching can be
obtained by heterogeneous catalysts,1 procedures such as
ltration, centrifugation and sedimentation are time-
consuming, and separating the catalyst from the reaction
environment can be ineffective. The performance of homoge-
neous catalysts is higher than that of heterogeneous catalysts.
Thus, catalytic systems that exhibit the benets of both
heterogeneous and homogeneous catalysts are extremely
important in modern chemistry.2 The advantages of conven-
tional catalysts include superior activity and improved stability.3
For essential and challenging reactions, anchoring metal
nanoparticles onto a solid support of choice offers an excellent
opportunity for discovering novel, extremely active nano-
catalysts. At the same time, the additional advantage of recy-
clability is also offered. Among the most signicant study areas
Metal nanocatalysts may be immobilized in two ways on the
support. One method involves the deposition of presynthesized
metal nanoparticles onto the support by chemical adsorption.5
The precise control on the shape and size of the metal nano-
particles can be achieved by this approach, but the operation
steps are tedious. During the catalytic reaction, the adsorbed
nanocatalysts can also easily leach from the KCC-1 support. The
other method involves the direct growth of metal nanoparticles
on the KCC-1 support using graed organic functional mole-
cules or polymers as a stabilizer.6 This procedure is superior for
holding the metallic nanoparticles. Recently, chelating ligands
of amine bis(phenol) have performed a signicant role in the
study of transition-metal catalysts,7–9 in attempts to regulate the
catalytic performance of the metal complexes. This exciting
class of ligands contains the donor atoms O and N. Accordingly,
the Lewis acidity of the metal centers can be tuned via various
substituents on the phenol rings of these ligands. There have
been several reports of catalysts containing these ligands
coordinated to transition metals within model complexes of
Co(II) and Co(III).10–12
Department of Chemistry, College of Sciences, University of Birjand, Birjand
97175-615, Iran. E-mail: a_allahresani@birjand.ac.ir
37974 | RSC Adv., 2020, 10, 37974–37981
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
Since the oxidation of alcohols to ketones or aldehydes is
RSC Advances
a basic conversion in organic synthesis, the selective conver-
sion of alcohols to the corresponding carbonyl compounds is
among the most signicant procedures.13,14 To achieve these
selective oxidation reactions, traditional methods include the
usage of a stoichiometric amounts of oxidants such as chro-
mium oxides15 and manganese(IV) oxide,16,17 which involves
the production of vast quantities of poisonous waste. Catalytic
oxidation procedures that use air or molecular oxygen are
exciting and valuable, because of the increasing requirements
for cost-effectiveness and environmental conservation.18 In the
literature, several methods based on metal catalysts and the
aerobic oxidation of alcohols have been reported, including Au
or Au/CuO-hydroxyapatite,19,20 ruthenium,21,22 cobalt,23,24 and
manganese nanoparticles.25,26 The most widely reported
methods involve catalytic systems dependent on NO radicals
such as NHPI and TEMPO in combination with catalysts.27–29
The use of aldehyde as a reducing factor in oxidation reactions
has been broadly reported for the activation of dioxygen, with
catalysts including Co salen complexes30 and metal-
loporphyrins,31 etc. The efficient oxidation of alcohol via iso-
butyraldehyde (IBA) and Cu(II) meso-tetraphenylporphyrin
(TPP) has been reported by Rahimi et al.32 An effective oxida-
tion procedure of alcohol with IBA and O2 using a ruthenium
tetraphenylporphyrin (TPP) has also been reported.33 Using
IBA as a co-substrate, recently, an organic–inorganic hybrid
cobalt phthalocyanine showed great efficiency for the oxida-
tion of alcohols with high selectivity.34 In efforts to improve
catalysts for organic oxidation procedures, metal salen
complexes have been studied intensively. The oxidation of
olens and suldes by Fe3O4@SiO2/Mn(III) chiral salen
complex with H2O2 and TBHP, respectively, as oxidants,35 and
the selective oxidation of alcohols catalyzed via Co(III) salen
complex with molecular oxygen as the oxidant,36 have also
been reported. Moreover, it has recently been reported37–40 that
in the preparation of biological and chemical molecules such
as medications, the selective oxidation of suldes to the cor-
responding sulfoxides via heterogeneous and homogeneous
catalysts, is benecial. In this process, the application of
a green inorganic and organic oxidant are desirable, due to the
waste resulting from most common oxidation reactions, which
can be dangerous.
Scheme
1
Preparation of catalyst (Co(II) salen complex@KCC-1).
Reagents and conditions: (a) TEOS, cyclohexane, 1-pentanol; (b) EtOH,
reflux; (c) Co(OAc)2, EtOH; (d) dry toluene, reflux, 48 h.
procedure using O2. For the oxidation of alcohols and suldes,
the catalyst system is effective, exhibiting high yields for
carbonyl compounds under mild conditions.
Recently, many reactions for the oxidation of alcohols and
suldes have been performed using cobalt salen complexes.
For example, Vargas et al. showed phenol oxidation with
a Co(II) salen complex catalyst supported on nanoporous
materials via air,41 and Nasseri et al. used CoFe2O4@SiO2@-
Co(III) salen complex nanoparticles for the oxidation of benzyl
alcohols by molecular O2.36 Allahresani et al. applied a Co(III)
@Fe3O4@SiO2 salen complex as a highly selective and recov-
erable magnetic nanocatalyst for the oxidation of benzyl
alcohols and suldes.42 The m-oxo and peroxo groups bonded
to the Co in the complex appear to be responsible for the
superior activity of the catalyst. In this work, the aerobic
oxidation of alcohols and suldes catalyzed by Co(II) salen
complex@KCC-1 in the presence of IBA was developed
Fig. 1 FT-IR spectra of KCC-1 (a), salen ligand (b), Co(II) salen complex
(Scheme 1) as an important part of a green oxidation (c) and Co(II) salen complex@KCC-1 (d).
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 37974–37981 | 37975

View Article Online
RSC Advances
Paper
Fig. 4 TEM images of KCC-1 (a) and Co(II) salen complex@KCC-1 (b).
Fig. 2 TGA patterns of KCC-1 (a) and Co(II) salen complex@KCC-1 (b).
Thermal gravimetric analysis (TGA)
To assess the thermal stability of KCC-1 and Co(II) salen
complex@KCC-1, TGA was carried out at temperatures ranging
from ambient temperature to 850 ꢁC (Fig. 2). Fig. 2a shows that
the removal of H2O occurred at temperatures below 130 ꢁC
(KCC-1). In Fig. 2b, water removal is the reason for the 4%
weight loss in the range below 253 ꢁC. The ꢁweight loss of about
10% in the temperature range of 250–450 C was attributed to
the removal of oxygen from the functional groups of the Co(II)
salen complex and the 15% weight loss in the temperature
range of 450–700 ꢁC could be carbon removal. There was no
additional weight loss above 700 ꢁC and the total weight loss of
the organic sections was evaluated to be 29% (see Fig. 2).
Results and discussion
Co(II) salen complex@KCC-1 was characterized using
a vibrating sample magnetometer (VSM), Fourier transform
infrared spectroscopy (FT-IR), transmission electron micros-
copy (TEM), energy dispersive X-ray (EDX) analyses, thermal
gravimetric analysis (TGA), and powder X-ray diffractometry
(XRD).
FT-IR spectroscopy
Fig. 1 presents infrared spectra of all components of Co(II) salen
complex@KCC-1, which was synthesized in our lab. The KCC-1
XRD analysis
spectrum (Fig. 1a) shows peaks at 1091, 915, and 805 cmꢀ1
,
XRD was used to investigate KCC-1 and Co(II) salen
complex@KCC-1 (Fig. 3). KCC-1 is a dendritic nanosilica brous
scaffold, and represents an important discovery due to its
unique brous morphology. The complex was distributed
uniformly in all directions, and the sample consisted of parti-
cles with uniform size (Fig. 3a). In Fig. 3b, the peak at 2q ¼ 20–
30ꢁ is attributed to amorphous silica. The structural similarity
was conrmed by the comparison of the nanostructure pattern
with that of the desired sample of this complex. Due to the
similarity of the peaks, the structures of the complex and the
nanostructure are the same and exist in an amorphous state.
The width of the pattern is different simply because of the small
size of the particles.
attributed to Si–O and Si–OH, respectively. The peak at
3425 cmꢀ1 is attributed to OH. The synthesis of KCC-1 was
conrmed by the above vibrations.7 Fig. 1b presents the infrared
spectrum of the ligand, whose vibrations are in the zones of
1079 and 1633 cmꢀ1, which are attributed to Si and C]N,
respectively.43,44 Fig. 1c presents the infrared spectrum of the
cobalt salen complex, in which the peaks at 1076 and 2923 cmꢀ1
are attributed to Si and the methylene ring, respectively, while
tensile vibrations of C]N are observed at 1620 cmꢀ1
.
The
11,36
peak at 1616 cmꢀ1 appeared in the heterogeneous catalysts
along with the peak 1089 cmꢀ1 of Si–O–Si, conrming the
successful xation of the homogeneous cobalt complex on the
KCC-1 surface (Fig. 1d).
Transmission electron microscopy (TEM)
TEM images of KCC-1 and Co(II) salen complex@KCC-1 are
shown in Fig. 4a and b, respectively. As the images show, the
nanoparticles are spherical.
Fig. 3 XRD pattern of KCC-1 (a) and Co(II) salen complex@KCC-1 (b). Fig. 5 Energy dispersive X-ray (EDX) of Co(II) salen complex@KCC-1.
37976 | RSC Adv., 2020, 10, 37974–37981
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
Table 2 Various benzyl alcohol oxidation reactions with O2/IBA as the
oxygen donor/acceptor in the presence of Co(II) salen complex@KCC-
1a in HOAc. Reaction conditions: benzyl alcohol (1 mmol), IBAb (3
mmol), O2c (5 mL minꢀ1), in 5 mL HOAc at 60 ꢁC, 0.06 g of Co(II) salen
complex@KCC-1a
Energy dispersive X-ray (EDX)
The Si, Co, O, N, and
C composition of Co(II) salen
complex@KCC-1 was conrmed using EDX elemental analysis.
The coordination of cobalt with the Schiff-base of the meso-
porous support is suggested by the peaks shown in Fig. 5.
t
Conversion
(%)
Selectivity
(%)
Entry
Substrate
Product
(min)
Catalytic performance of Co(II) salen complex@KCC-1 for the
1
2
30
30
95
85
99
92
aerobic oxidation of suldes and alcohols
The catalytic performance of Co(II) salen complex@KCC-1 for
benzyl alcohol (BzOH) aerobic oxidation to benzaldehyde (BzH)
and sulde (–S–) to sulfoxide (–SO–) was investigated for the
rst time. The optimum test conditions were determined by
3
4
30
30
68
30
90
98
ꢁ
carrying out reactions at 60 C, with dichloromethane (DCM),
ethanol (EtOH), acetic acid (HOAc), acetonitrile (CH3CN), and
water as solvents, IBA as an oxygen acceptor, and Co(II) salen
complex@KCC-1 as a catalyst (see Table 1, entries 1–7). The
results show that the oxidation in acetic acid (HOAc) solution
was more efficient than other solvents under the same condi-
tions. However, there was no substantial oxidation reaction of
water or ethanol (see Table 1, entries 1 and 2). Conversion by
20% could be obtained aer 100 min of reaction with
dichloromethane (DCM) and chloroform as solvents (see Table
1, entries 5 and 6). Moreover, the oxidation reaction efficiency
5
6
30
30
60
45
98
93
7
8
30
30
80
68
93
98
9
30
90
98
a
Co(II) salen complex@KCC-1 as a catalyst. b IBA as co-substrate. c O2 as
oxidant.
Table 1 The effect of diverse reaction conditions on the oxidation of
benzyl alcohol with O2/IBA as the oxygen donor/acceptor in the
presence of Co(II) salen complex@KCC-1. Reaction conditions: IBA (X
mmol), benzyl alcohol or sulfide (1 mmol), IBAb (3 mmol), O2 (5
c
mL minꢀ1), Co(II) salen complex@KCC-1a (Y g)
was increased in an acidic environment. Acetic acid is a useful
solvent that has been applied in the aerobic oxidation of alco-
hols and suldes. It has previously been reported that acidic
conditions can activate catalysts and increase oxygen
consumption.45
It has been reported that aldehydes (RCHO) should be
utilized to activate O2. This means that IBA is signicant in the
aerobic oxidation method, and without it, the reaction would
instantly cease (see Table 1, entry 14). In the current process,
this suggests that the oxidation procedure takes place through
a free-radical mechanism. Also, in the presence of 2,6-di-tert-
butyl-4-methylphenol as a radical scavenger, a delay in the
oxidation of benzyl alcohol also supports this claim. The opti-
mized quantity of IBA was dened as 3 mmol. Due to the
reduction in the amount of catalyst, signicantly lower
conversions were predictable (see Table 1, entries 8–10). The
optimal result was obtained by using 0.06 g of Co(II) salen
complex@KCC-1 and 3 mmol of IBA in 5 mL of acetic acid at
T
IBAb
Entry Cata (g) (ꢁC) (mmol) (mL)
Solvent
t
Conversion
(min) (%)
1
0.06
0.06
0.06
0.06
0.06
0.06
0.06
—
60
60
60
60
60
60
60
60
60
60
60
RT
80
60
60
60
60
3
3
3
3
3
3
3
3
3
3
3
3
3
—
1
2
4
H2O (5)
90
45
120
30
35
70
85
20
20
95
10
50
55
70
55
45
40
58
64
70
2
3
EtOH (5)
EtOAc (5)
4
5
CH3CN (5) 35
CH2Cl2 (5) 100
6
7
8
9
10
11
12
13
14
15
16
17
CHCl3 (5)
HOAc (5)
HOAc (5)
HOAc (5)
HOAc (5)
HOAc (5)
HOAc (5)
HOAc (5)
HOAc (5)
HOAc (5)
HOAc (5)
HOAc (5)
100
30
180
30
30
30
180
180
30
30
30
0.03
0.04
0.07
0.06
0.06
0.06
0.06
0.06
0.06
ꢁ
60 C (see Table 1, entry 8).
Aerward, the efficacy of Co(II) salen complex@KCC-1 was
determined for other benzylic alcohols (see Table 2).
Then, the efficiency of the catalyst was determined for other
suldes (see Table 3). In the present system, the transformation
of several suldes to the corresponding sulfoxides occurred
with high conversion.
30
Molecular oxygen was utilized as an oxidant in this heter-
ogenous reaction system. In the O2 system, a hot ltration test
a
Co(II) salen complex@KCC-1 as the catalyst. b IBA as co-substrate. c O2
as oxidant.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 37974–37981 | 37977

View Article Online
RSC Advances
Paper
Table 3 Oxidation of various sulfides with O2/IBA as the oxygen
donor/acceptor in the presence of Co(II) salen complex@KCC-1a in
HOAc. Reaction conditions: sulfide (1 mmol), IBAb (3 mmol), O2 (5
c
mL minꢀ1), in 5 mL HOAc at 60 ꢁC, 0.06 g of Co(II) salen com-
plex@KCC-1a
t
Conversion
(%)
Selectivity
(%)
Entry
Substrate
Product
(min)
1
2
30
30
82
90
99
99
3
4
30
30
97
94
98
98
Scheme 3 An acceptable mechanism for Co(II) salen complex@KCC-1
catalyzing the oxidation of alcohol using IBA and O2.
5
30
80
97
salen complex@KCC-1 via a series of radical chains in the
presence of IBA. The production of carbonyl compounds
corresponds to the reaction of Co-oxo species via alcohol, fol-
lowed by the elimination of b-hydride. Based on the explanation
given, in the presence of O2 and IBA, an acceptable mechanism
for benzyl alcohol oxidation catalyzed via Co(II) salen
complex@KCC-1 is presented in Scheme 3.
Firstly, an acyl radical is derived from thermal hydrogen
abstraction at the carbonyl position. Radical propagation
occurs to generate peroxyacid. Then, the smooth interaction
among peroxy acid and Co(II) salen complex (a) should initiate
the reaction to produce Co(II)–OOH (b).47,48 Then, Co-oxo (c) is
formed from the heterolytic cleavage of the O–O bond of species
(b). The production of a carbonyl compound corresponds to the
reaction of the Co-oxo species with alcohol via a transition
species (d), because of the electrophilic character of Co-oxo (c)
followed by b-hydride elimination.49,50
a
Co(II) salen complex@KCC-1 as a catalyst. b IBA as co-substrate. c O2 as
oxidant.
was used to characterize the nature of the heterogeneous cata-
lyst. As a model reaction, the aerobic oxidation reaction of
sulde and benzyl alcohol using Co(II) salen complex@KCC-1 in
acetic acid was performed (Scheme 2). To remove the catalyst,
ltration was used, achieving a conversion of 50%. Aerward,
the ltrate was allowed to carry on reacting under the optimal
reaction conditions. Aer 4 h, there was no substantial increase
in the conversion. In this catalytic system, the high stability of
the catalyst and its heterogeneous nature were conrmed via
the hot ltration approach.
Possible mechanism
Kinetic study
It has been reported that the activation of molecular oxygen via
aldehydes occurs through a procedure involving free radicals.
2,6-Di-tert-butylphenol was utilized as an inhibitor of free-
radical oxidation to verify the free-radical mechanism. The
oxidation reaction was quenched when this inhibitor was
added.
To investigate the kinetic data of the proposed nanocatalyst,
pseudo-rst and second-order models were studied.
The following rst-order model (eqn (1)) was employed with
integration conditions of C0 ¼ 0.2 M to C ¼ Ct at t ¼ 0 to t ¼ t,
expressed as eqn (2).
It has been reported by Beller that oxo ruthenium complexes
are effective catalysts for the asymmetric oxidation of alkenes.46
The mechanism of the reaction can include the contribution
of Co-oxo species resulting in a reaction between O2 and Co(II)
Fig. 6 Alcohol oxidation-pseudo-first order (a1) and sulfide oxidation
Scheme 2 Oxidation of benzyl alcohol and sulfide by Co(II) salen pseudo-first order (a2); alcohol oxidation-pseudo-first order (b1) and
complex@KCC-1.
sulfide oxidation-pseudo-first order (b2).
37978 | RSC Adv., 2020, 10, 37974–37981
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
dC/dt ¼ k1(C0 ꢀ Ct)
ln(C0/Ct) ¼ k1t
(1)
(2)
The rst-order rate constant k1 can be obtained from the slope
of the ln(C0/Ct) versus t plot illustrated in Fig. 6a. k1 was found to
be 0.0873 and 0.1063 minꢀ1 for the oxidation of benzyl alcohols
and suldes, respectively. On the other hand, the pseudo-
second-order model given as follows (eqn (3)), was integrated
for the above boundary conditions, which are simplied in eqn
(4).
dC/dt ¼ k2(C0 ꢀ Ct)2
1/Ct ꢀ 1/C0 ¼ k2t
(3)
(4)
Fig. 8 TEM image of the recycled catalyst.
k2 (mL mmolꢀ1 minꢀ1) is the rate constant of the second-order
model, which can be determined by plotting 1/Ct ꢀ 1/C0 versus t.
k2 values of 1.6722 and 1.3069 were obtained for the oxidation of
benzyl alcohols and suldes, respectively. Also, Fig. 6b shows
that the correlation coefficient values of the pseudo-rst-order
model were higher than those of the pseudo-second-order
model for both benzyl alcohols and suldes, suggesting that
the oxidation procedure using the proposed nanocatalyst
follows rst-order kinetics.
Experimental
Synthesis of KCC-1
KCC-1 was prepared using a microwave-assisted hydrothermal
technique. CTAB (cetyltrimethylammonium bromide, 2 g, 5.48
mmol) and urea (2.4 g, 40 mmol) were stirred (1500 RPM) in
water (300 mL) for 15 min. TEOS (tetraethyl orthosilicate, 10
mL) was taken in cyclohexane (100 mL) and added dropwise to
the above solution, and the solution was stirred further for
15 min, followed by the dropwise addition of 1-pentanol (6 mL,
55.2 mmol). The resultant solution was stirred for 30 min at
room temperature. Then the reaction mixture was transferred
into a Teon-sealed 1 L microwave (MW) reactor, at 120 ꢁC, and
irradiated (800 W power) for 4 h under moderate stirring. Aer
completion of the reaction, the solid product was isolated by
centrifugation, washed several times with water and ethanol,
and dried at 80 ꢁC for 8 h. The aꢀs1-synthesized material was then
calcined at a rate of 2.5 ꢁC min at 550 ꢁC for 6 h in the air to
yield calcined brous nano-silica (KCC-1).
Recycling of the catalyst
The ability to reuse a catalyst is very important both economi-
cally and practically. Thus, the reusability of Co(II) salen
complex@KCC-1 for the alcohol and sulde oxidation reactions
was investigated in the O2 system under the optimized condi-
tions. In each run, centrifugation was applied to recover the
ꢁ
catalyst aer 30 minutes. At 60 C, it was reused aer washing
with ethanol and drying. In the case of the O2 system, Co(II)
salen complex@KCC-1 showed high catalytic performance for at
least ve runs (see Fig. 7 and 8).
Synthesis of salen ligand
Next, we compared the present results with some previously
reported data to evaluate the catalytic efficiency and reusability
of Co(II) salen complex@KCC-1 for aerobic epoxidation of
olens with molecular oxygen (Table 4).
The Schiff base was prepared as follows. 3-Amino-
propyltrimethoxysilane (1.2 mmol) was added to a mixture of 2-
hydroxybenzaldehyde (1.0 mmol) in ethanol (10 mL) and stirred
under reux conditions for 3 h. The color of the reaction solution
changed to yellowish, due to imine formation, and the solvent of
mixture was eliminated at 70 ꢁC, to obtain an oily product.
Synthesis of Co(II) salen complex
Functionalized salen ligand (1.0 mmol) was dissolved in
ethanol (20 mL). Then NaOH (1.67 mmol) and Co(II) acetate
monohydrate (1 mmol) were added and the reaction was
reuxed under argon gas for 3 h. The reaction mixture was
ꢁ
cooled to 0 C and the precipitate was ltered. The precipitate
was washed with methanol, water, and diethyl ether, in
sequence, to purify it. Dark green sediment formed following
ꢁ
drying at 80 C in an oven under vacuum.
Synthesis of Co(II) salen complex@KCC-1
The Co(II) salen complex@KCC-1 nanocatalyst was synthesized
according to the following procedure: KCC-1 (0.25 g) was
Fig. 7 Recyclability of Co(II) salen complex@KCC-1 in the oxidation of
benzyl alcohol and sulfides.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 37974–37981 | 37979

View Article Online
RSC Advances
Paper
Table 4 Comparison of results using Co(II) salen complex@KCC-1 with various catalystsa
Entry
Catalyst
Oxidant
Solvent
Yield (%)
Ref.
1
2
3
4
5
6
7
Ni(II)DPDME
Mn6Ni4
Co–Ni ferrite
Fe (THPP)Cl@MWCNT
Rd2(OAc)4
O2/IBA
O2
CH3CN
Toluene
CH3CN
33
89
18.9
89
88
50
51
52
53
54
55
TBHP/hy
O2/IBA
O2/iPrCHO
O2/IBA
O2/IBA
DMF
Acetone
1,2-Dichloroethane
HOAc
CoO–MCM-41
Co(II) salen complex@KCC-1
90
95
This work
a
c
Reaction conditions: benzyl alcohol (1 mmol), IBAb (3 mmol), O2 (5 mL minꢀ1), in 5 mL HOAc at 60 ꢁC, and 0.06 gram of Co(II) salen
complex@KCC-1.
dispersed in 30 mL of dry toluene for 30 min. Then, 0.7 g of catalyst was reused in alcohol and sulde oxidation reactions
homogeneous Co(II) salen complex was added into the for ve successive cycles.
suspension. Aer reuxing for 20 h, the immobilized catalyst
was ltered and the precipitate was washed 3 times with dry
Conflicts of interest
toluene, 3 times with ethanol, and 3 times with dichloro-
methane. Then, the solid mixture was dried at 80 ꢁC under
vacuum for 8 h.
The authors declare no conict of interest.
Acknowledgements
General procedure for the oxidation of alcohol
The authors are grateful to the University of Birjand for nan-
cial support.
For the aerobic oxidation of alcohols, 0.06 g of Co(II) salen
complex@KCC-1, 1 mmol alcohol, 3 mmol IBA and 5 mL of
acetic acid were added to a 10 mL ask, which was placed under
oxygen gas conditions. At 60 ꢁC, the mixture was stirred for
30 min. Through the reaction, dioxygen was bubbled at a rate of
5 mL minꢀ1. The reaction was monitored using TLC on silica
gel.
References
1 Q. H. Xia, H. Q. Ge, C. P. Ye, Z. M. Liu and K. X. Su, Chem.
Rev., 2005, 105, 1603–1662.
2 Z. X. Li, W. Xue, B. T. Guan, F. B. Shi, Z. J. Shi, H. Jiang and
C. H. Yan, Nanoscale, 2013, 5, 1213–1220.
3 S. M. Sadeghzadeh, Catal. Sci. Technol., 2016, 6, 1435–1441.
4 V. Polshettiwar, D. Cha, X. Zhang and J. M. Basset, Angew.
Chem., Int. Ed., 2010, 49, 9652–9656.
5 J. P. Ge, T. Huynh, Y. P. Hu and Y. D. Yin, Nano Lett., 2008, 8,
931–934.
6 S. M. Sadeghzadeh, R. Zhiani and S. Emrani, RSC Adv., 2017,
7, 24885–24894.
7 A. K. Bowser, A. M. Anderson-Wile, D. H. Johnston and
B. M. Wile, Appl. Organomet. Chem., 2016, 30, 32–39.
8 E. Safaei, L. Hajikhanmirzaei, B. Karimi, A. Wojtczak,
General procedure for the oxidation of sulde
For the aerobic oxidation of sulde, 0.06 g of Co(II) salen
complex@KCC-1, 1 mmol sulde, 3 mmol IBA (IBA) and 5 mL of
acetic acid were added to a 10 mL ask, which was placed under
oxygen gas conditions. At 60 ꢁC, the mixture was stirred for
30 min. Throughout the reaction, dioxygen was bubbled at
a rate of 5 mL minꢀ1. The reaction was monitored using TLC on
silica gel.
Conclusions
ˇ
P. Cotic and Y.-I. Lee, Polyhedron, 2016, 106, 153–162.
In brief, in the presence of molecular oxygen and IBA, Co(II)
salen complex@KCC-1 has been shown to be an excellent
9 S. Yang, K. Nie, Y. Zhang, M. Xue, Y. Yao and Q. Shen, Inorg.
Chem., 2014, 53, 105–115.
catalyst for the oxidation of suldes and alcohols. The chemical 10 M. A. Nasseri, K. Hemmat and A. Allahresani, Appl.
properties and structural stability of the catalyst were Organomet. Chem., 2019, 33, e4743.
conrmed. We demonstrated that the eco-friendly system 11 K. Hemmat, M. A. Nasseri and A. Allahresani,
provides a novel and precise catalyst for the oxidation of a broad ChemistrySelect, 2019, 4, 4339–4346.
range of suldes and alcohols into the corresponding sulfoxides 12 M. A. Nasseri and A. Allahresani, J. Chem. Sci., 2017, 129,
and aldehydes/ketones under green and moderate conditions. 343–352.
Because Co is widely available and inexpensive, this catalyst can 13 R. H. Crabtree, Chem. Rev., 2017, 117, 9228–9246.
be discovered, easily. Also, our catalyst can overcome the 14 H. Su, K. X. Zhang, B. Zhang, H. H. Wang, Q. Y. Yu, X. H. Li,
current problem in the oxidation of suldes: over-oxidation of
sulfoxides to the relevant sulfones. The effects of different
M. Antonietti and J. S. Chen, J. Am. Chem. Soc., 2017, 139,
811–818.
reaction parameters, including catalyst, solvent, and tempera- 15 C. B. Wang, Y. P. Cai and I. E. Wachs, Langmuir, 1999, 15,
ture were evaluated on the selectivity and activity. Moreover, the
1223–1235.
37980 | RSC Adv., 2020, 10, 37974–37981
This journal is © The Royal Society of Chemistry 2020

View Article Online
Paper
RSC Advances
16 O. Delacroix, B. Andriamihaja, S. Picart-Goetgheluck and 36 M. A. Nasseri, K. Hemmat, A. Allahresani and
J. Brocard, Tetrahedron, 2004, 60, 1549–1556.
17 R. Ziessel, P. Nguyen, L. Douce, M. Cesario and C. Estournes,
Org. Lett., 2004, 6, 2865–2868.
E. Hamidi-Hajiabadi, Appl. Organomet. Chem., 2019, 33,
e4809.
37 B. Li, S. Bai, P. Wang, H. Yang, Q. Yang and C. Li, Phys. Chem.
Chem. Phys., 2011, 13, 2504–2511.
18 T. Mallat and A. Baiker, Chem. Rev., 2004, 104, 3037–3058.
19 L. Wang, X. J. Meng, B. Wang, W. Y. Chi and F. S. Xiao, Chem. 38 B. Li, A.-H. Liu, L.-N. He, Z.-Z. Yang, J. Gao and K.-H. Chen,
Commun., 2010, 46, 5003–5005. Green Chem., 2012, 14, 130–135.
20 T. Tian, Y. Liu and X. Zhang, Chin. J. Catal., 2015, 36, 1358– 39 Z. Yang, C. Zhu, Z. Li, Y. Liu, G. Liu and Y. Cui, Chem.
1364. Commun., 2014, 50, 8775–8778.
21 R. Ray, S. Chandra, D. Maiti and G. K. Lahiri, Chem.–Eur. J., 40 W. Zhao, C. Yang, Z. Cheng and Z. Zhang, Green Chem., 2016,
2016, 22, 8814–8822. 18, 995–998.
22 Z. Hao, X. Yan, Z. Li, R. Wu, Z. Ma, S. Li, Z. Han, X. Zheng 41 D. X. M. Vargas, J. R. De La Rosa, S. A. Iyoob, C. J. Lucio-Ortiz,
´
and J. Lin, Transition Met. Chem., 2018, 43, 635–640.
23 Y. B. Kuang, Y. Nabae, T. Hayakawa and M. Kakimoto, Appl.
Catal., A, 2012, 423, 52–58.
24 A. Shaabani, S. Keshipour, M. Hamidzad and S. Shaabani, J.
Mol. Catal. A: Chem., 2014, 395, 494–499.
25 G. Elmaci, D. Ozer and B. Zumreoglu-Karan, Catal.
Commun., 2017, 89, 56–59.
26 G. Wu, Y. Gao, F. W. Ma, B. H. Zheng, L. G. Liu, H. Y. Sun and
W. Wu, Chem. Eng. J., 2015, 271, 14–22.
27 F. Rajabi and B. Karimi, J. Mol. Catal. A: Chem., 2005, 232,
95–99.
28 J. M. Hoover, B. L. Ryland and S. S. Stahl, ACS Catal., 2013, 3,
2599–2605.
29 Y. T. Yan, X. L. Tong, K. X. Wang and X. Q. Bai, Catal.
Commun., 2014, 43, 112–115.
30 Z. F. Li, S. J. Wu, H. Ding, D. F. Zheng, J. Hu, X. Wang,
F. J. C. Cordoba and C. D. Garcia, Appl. Catal., A, 2015, 506,
44–56.
42 A. Allahresani, M. A. Nasseri, A. Nakhaei and S. Aghajani,
Iran. Chem. Commun., 2019, 7, 153–169.
43 K. Hemmat, M. A. Nasseri and A. Allahresani, Appl.
Organomet. Chem., 2019, 33, e4937.
44 E. Vinck, D. M. Murphy, I. A. Fallis, R. R. Strevens and
S. V. Doorslaer, Inorg. Chem., 2010, 49, 2083–2092.
45 R. Malakooti and A. Feghhi, New J. Chem., 2017, 41, 3405–
3413.
46 M. K. Tse, S. Bhor, M. Klawonn, C. Dobler and M. Beller,
Tetrahedron Lett., 2003, 44, 7479–7483.
47 M. Bar, S. Deb, A. Paul and S. Baitalik, Inorg. Chem., 2018, 57,
12010–12024.
48 J. L. Pratihar, P. Pattanayak, J. H. Huang and
S. Chattopadhyay, Inorg. Chim. Acta, 2009, 362, 5170–5174.
Q. S. Huo, J. Q. Guan and Q. B. Kan, New J. Chem., 2013, 49 Q. Han, X. X. Guo, X. T. Zhou and H. B. Ji, Inorg. Chem.
37, 1561–1568. Commun., 2020, 112, 107544.
31 S. Y. Chen, X. T. Zhou and H. B. Ji, Catal. Today, 2016, 264, 50 C. Sun, B. Hu and Z. Liu, Heteroat. Chem., 2012, 23, 295–303.
191–197.
51 Q. Tang, C. Wu, R. Qiao, Y. Chen and Y. Yang, Appl. Catal., A,
2011, 403, 136–141.
52 J. Tong, Q. Zhang, L. Bo, L. Su and Q. Wang, J. Sol-Gel Sci.
Technol., 2015, 76, 19–26.
53 M. J. Beier, W. Kleist, M. T. Wharmby, R. Kissner,
B. Kimmerle, P. A. Wright and A. Baiker, Chem.–Eur. J.,
2012, 18, 887.
32 R. Rahimi, E. Gholamrezapor and M. R. Naimi-jamal, Inorg.
Chem. Commun., 2011, 14, 1561–1568.
33 H. B. Ji, Q. L. Yuan, X. T. Zhou, L. X. Pei and L. F. Wang,
Bioorg. Med. Chem. Lett., 2007, 17, 6364–6368.
34 V. Panwar, P. Kumar, S. S. Ray and S. L. Jain, Tetrahedron
Lett., 2015, 5, 3948–3953.
35 A. Allahresani and M. A. Nasseri, RSC Adv., 2014, 4, 60702– 54 D. Shabashov and M. P. Doyle, Tetrahedron, 2013, 69, 10009–
60710.
10013.
55 D. Dhar, Y. Koltypin, A. Gedanken and S. Chandrasekaran,
Catal. Lett., 2003, 86, 197–200.
This journal is © The Royal Society of Chemistry 2020
RSC Adv., 2020, 10, 37974–37981 | 37981