Application of copper(II) schiff base complex grafted in the silica network as efficient nanocatalyst in oxidation of alcohols

Article abstract of DOI:10.14233/ajchem.2013.13644

Bis[S-allyl-β-N-(benzylidene)dithiocarbazato]copper(II) complex was grafted into SiO₂ matrix. Heterogenized catalyst was characterized by FT-IR, SEM, EDX TEM, N2 absorption and atomic absorption spectroscopy (AAS) and its catalytic performance in the oxidation of alcohols was investigated. The experimental results showed that the prepared heterogeneous nanocatalyst is an efficient and selective catalyst for examined oxidation reaction. Thus, secondary alcohols were oxidized to ketones and primery alcohols converted to aldehydes by hydrogen peroxide in the presence of newly nanocatalyst in mild conditions.

Full text of DOI:10.14233/ajchem.2013.13644

Asian Journal of Chemistry; Vol. 25, No. 6 (2013), 3307-3312
http://dx.doi.org/10.14233/ajchem.2013.13644
Application of Copper(II) Schiff Base Complex Grafted in the
Silica Network as Efficient Nanocatalyst in Oxidation of Alcohols
*
HOSSEIN ESHTIAGH-HOSSEINI , TAYMAZ TABARI and HOSSEIN ESHGHI
Department of Chemistry, Ferdowsi University of Mashhad, Mashhad 91779-1436, Iran
*Corresponding author: Fax: +98 511 8796416; Tel: +98 511 8797022; E-mail: heshtiagh@ferdowsi.um.ac.ir
(Received: 22 February 2012; Accepted: 17 December 2012)
AJC-12573
Bis[S-allyl-β-N-(benzylidene)dithiocarbazato]copper(II) complex was grafted into SiO2 matrix. Heterogenized catalyst was characterized
by FT-IR, SEM, EDX TEM, N2 absorption and atomic absorption spectroscopy (AAS) and its catalytic performance in the oxidation of
alcohols was investigated. The experimental results showed that the prepared heterogeneous nanocatalyst is an efficient and selective
catalyst for examined oxidation reaction. Thus, secondary alcohols were oxidized to ketones and primery alcohols converted to aldehydes
by hydrogen peroxide in the presence of newly nanocatalyst in mild conditions.
Key Words: Alcohols oxidation, Grafted copper Schiff base complex, Aldehyde, Hydrogen peroxide, Nanocatalyst.
salen complexes have been immobilized on activated carbon21
INTRODUCTION
and sol-gel28. However, either leaching or mean selectivity was
Homogeneous metal Schiff base complexes bearing NS
the main problem during their application as catalysts.
Karandikar et al.29 have grafted copper and cobalt salen
macrocyclic ligands have attracted much interest in the field
of catalysis because of their reversible oxygen bonding ability
compounds on the surface of MCM-41 through aminosilane
and high activities in oxidation reactions1-7. However, the
linker by adduct formation. The metal Schiff base complexes
homogeneous catalysts have some drawbacks such as problems
as catalyst were found to be active for epoxidation reactions.
in the catalyst recovery and product separation. In addition,
The stability of the complexes on the surface and the recycla-
bility of the catalysts, however, were not examined29. H2O2
as far as pharmaceutical product is concerned, potential conta-
has also merged as popular oxidant in the past decade30. In the
mination of metal catalyst, which is specifically undesirable
in the cases of homogeneous catalysis, has to be carefully
present study, we have heterogenized copper(II) Schiff base
considered. Furthermore, the use of metal Schiff base comp-
complex (Fig. 1) on the surface of gelatine/siloxane hybrid
lexes as catalyst in homogeneous solution often suffers from
deactivation due to easy formation of dimeric peroxo and
prepared via sol-gel method as nanocatalyst. The test of catalytic
activity of the grafted Schiff base complex, catalyst (1) was
µ-oxo species, which have been demonstrated to be inactive
carried out with various alcohols in the presence of H2O2 as
in different catalytic reactions8-10
.
oxidant.All products characterized by 1H NMR and IR spectro-
scopy. In this work, we present an efficient method for the
catalytic oxidation of primary and secondary alcohols to
corresponding carbonyl compounds using H2O2 as an oxidant
over catalyst (1) (Scheme-1).
In order to overcome these drawbacks, heterogenization
of homogeneous catalysts with comparable activities affords
an appealing methodology to expand the industrial application
of homogeneous catalysts. For this reason, immobilization of
metal Schiff base complexes on organic or inorganic polymers
and inorganic supports has been widely reported11,12.A variety
of copper(II) salts is suitable as catalysts or catalyst pre-
cursors13. In particular, Schiff base complexes of copper(II)
have been used as catalysts in the aziridination and cyclo-
propanation of olefins and also in phenol oxidation14-18. The
heterogenization of transition metal complexes is an area of
growing interest, particularly on zeolites, mesoporous silicate
materials, polymers and activated carbon19-27. Recently, several
OH
O
Cat.1, H2O2 30%
C
C
rt
R1
R2
R1
R2
H
R1,R2: H, Me, Et, Ph
Scheme-I

3308 Eshtiagh-Hosseini et al.
Asian J. Chem.
H
Preparation of heterogenized Cu(II) Schiff base
complex, catalyst (1): Silica composite was activated by
refluxing in concentrated hydrochloric acid and then washed
thoroughly with deionized water and dried before undergoing
chemical surface modification. Hydrated silica was then added
to Schiff base complex solution and the mixture was stirred
over night. The solvent was removed using a rotary evaporator
and the resulting olive green solid dried at 80 ºC overnight.
The product was washed with MeOH and water until the
washings were colourless. Further drying of solid product was
carried out in an oven at 80 ºC for 8 h.
N
S
N
S
Cu
S
N
N
H
S
Fig. 1. Bis[S-allyl-β-N-(benzylidene)dithiocarbazato]-copper(II) complex
Catalytic reaction: The catalytic reaction was undertaken
in a glass batch. Oxidation of alcohols was performed at room
temperature in the presence of catalytic amount of the catalyst
(1) using hydrogen peroxide as the oxidant. The oxidation of
benzhydrol and benzyl alcohol as model substrate carried out
for optimization of the reaction variables such as temperature,
H2O2/alcohol molar ratio, appropriate amount of catalyst and
length of the reaction time. A typical experimental procedure
was as follows: to the catalyst (1) (0.1 mmol) was added the
substrate alcohol (1 mmol), then hydrogen peroxide (10 mmol,
30 %) was slowly added. The reaction mixture was stirred
at room temperature until TLC pointed out the reaction was
complete. The catalyst was separated by a simple filtration
and product was obtained in high purity after evaporation of
solvent. Oxidation took place to afford benzophenone in the
presence of catalyst and H2O2 (10 eq, 30 %) in 55 min (Table-
1, entry 1). Perhaps some of hydrogen peroxide was decom-
posed in the presence of catalyst (1) and O2 was released in
the course of reaction. Determine which of H2O2 or released
O2 plays essential role in the oxidation reaction in the presence
of our catalyst. We examined this reaction by simply bubbling
molecular oxygen through reaction mixture under similar
conditions and only a poor yield of product was obtained
(Table-1, entry 2). So, decomposition played little role in the
oxidation of alcohols. Furthermore, when the oxidation of
benzhydrol was carried out using less catalyst, the reaction
was found to reach completion over much longer time (Table-
1, entry 3). When the silica matrix was used, as catalyst, no
reaction took place and the starting materials were recovered
(Table-1, entry 4). Alternatively, the reaction was carried out
at 50 ºC under the same conditions in which, the oxidation
completed after 0.5 h (Table-1, entry 5). Oxidation of benzyl
EXPERIMENTAL
Bis[S-allyl-β-N-(benzylidene)dithiocarbazato]-copper(II)
complex (Fig. 1) was prepared according to literature procedure31.
Alcohols were obtained from Aldrich company and tetraethyl
orthosilicate (TEOS) was obtained from Merck company. All
materials were used as received without purification. All the
isolated disulfide compounds were identified through 1H NMR.
Preparation of bis[S-allyl-β-N-(benzylidene)dithio-
carbazato]copper(II) complex: Preparation of the complex
is straightforward and is analogous to the procedure described31,
copper(II) nitrate trihydrate (0.242 g; 0.001 mol) in ethanol
(10 mL) was added to the ethanolic solution of ligand S-allyl-
β-N-(benzylidene)dithiocarbazate (0.473 g; 0.002 mol). The
mixture was heated and then left to stand overnight. The
product was filtered off, washed with ethanol, dried in a
vacuum. Yield 69 % (Fig. 1).
Preparation of gelatine/silica composite: The gelatine/
silica was prepared using tetraethyl orthosilicate (TEOS, 99
%, Aldrich), as the silica source, gelatine as organic source,
ethanol (EtOH) as solvent, distilled water and HCl, the
precursor solution was prepared as follows: one mole TEOS
was mixed with 4 mol of ethanol, 4 mol of distilled water and
stirred for ca. 0.5 h, 0.01 mol hydrochloric acid (HCl, 37 wt %
in water) was added into the solution and then the solution
was stirred at room temperature for 1 h. For the organic compo-
nent 12 wt % gelatine solution previously prepared at 45 ºC
with magnetic stirring for ca. 2 h, mixed together and stirred
for 24 h. The xerogel was dried at 70 ºC for 20 h and then
calcinated at 750 ºC for 7 h, the temperature and time prede-
termined for a specific experiment, with a temperature gradient
10 ºC/min.
TABLE-1
OPTIMIZATION OF REACTION CONDITIONS BY OXIDATION OF BENZHYDROL IN CH2Cl2 AT ROOM TEMPERATUREa
Temp.
(ºC)
Catalyst (mol
%)
Reaction
time (min)
Conversion
(%)
Entry
Substrate
Product
Oxidant
Selectivity
1
2
Benzhydrol
Benzhydrol
Benzhydrol
Benzhydrol
Benzophenone
Benzophenone
Benzophenone
Benzophenone
H2O2
O2
H2O2
H2O2
25
25
25
25
Cat. 1 (10%)
Cat. 1 (10%)
Cat. 1 (5%)
Silica
55
55
85
55
100
< 20
100
100
Small
100
3
4b
No reaction
No reaction
composite
Cat. 1 (10%)
–
Cu(Schiff
base) complex
5
6
7
Benzhydrol
Benzyl alcohol
Benzyl alcohol
Benzophenone
Benzoic acid, benzaldehyde
Benzoic acid, benzaldehyde
H2O2
H2O2
H2O2
50
25
25
30
70
35
100
100
100
100
20
80
8
Benzyl alcohol
Benzaldehyde
H2O2
25
Cat. 1 (10 %)
35
100
100
b
aBenzhydrol 1mmol, room temperature, 0.1 mmol of cat. (1), 10 mmol aqueous 30 % H2O2. The reaction carried out with 2 g of gelatin/Silica
composite.

Vol. 25, No. 6 (2013)
Application of Copper(II) Schiff Base Complex Grafted in the Silica Network as Efficient Nanocatalyst 3309
alcohol took place in the absence of catalyst just in presence
of H2O2 (10 eq, 30 %), the main product of this reaction was
benzoic acid with a few amount of benzaldehyde (Table-1,
entry 6). When we used homogeneous Schiff base complex as
catalyst, benzyl alcohol conversion accelerated and selectivity
was improved (Table-1, entry 7). On the other hand, in the
presence of heterogeneous Schiff base catalyst, the catalytic
performance effectively improved. The optimal conversion of
benzyl alcohol reached to 100 % and exclusively benzal-
dehyde was obtained over immobilized catalyst (Table-1,
entry 8).
RESULTS AND DISCUSSION
(a)
(b)
(c)
Immobilization of the copper(II) Schiff base complex:
The metal complex was immobilized in silica matrix by
following ship-in-a-bottle procedure32. Actually, the complex
physically trapped in the cage of the porous silica network.
The complex was anchored over silica matrix by refluxing for
6 h in ethanol solution and then it was extracted with ethanol
in Soxhlet extractor for 6 h. The Cu content was determined
by atomic absorption analysis.
IR spectroscopy: We first prepared the homogeneous
catalyst Cu(II) complex as described byYazdanbakhsh et al.31.
Its structure is shown in Fig. 1. This complex was physically
grafted into the SiO2 matrix. The grafted conditions are mild,
which can ensure the stability of the complex in the process.
The FT-IR spectra of the neat complex and catalyst (1) are
shown the bands at the frequencies of 1630, 1592 and 680 cm-1,
which are assigned to C=C, C=N and vibration of the skeleton
of the phenyl group, respectively. The reason is that the content
of the complex is so low that its absorption bands are masked
by the strong background bands of the SiO2 matrix. Frequencies
of 1100 and 800 cm-1 are assigned to stretching vibration of
Si-O-Si and Si-OH, respectively.
Atomic absorption spectroscopy (AAS): To confirm that
the complex was indeed grafted in the matrix, AAS employed
to analyze the content of Cu in the catalyst. The result showed
the content of Cu is (0.19 %) because the catalyst was extracted
with ethanol thoroughly, no contaminated complex on the
matrix was left and the Cu content should come from the
complex grafted in the matrix.
TEM, SEM test results: SEM and TEM images were
used to evaluate the surface morphology and size distribution
of the nickel complex deposited in the silica network. The
prepared catalyst (1) was analyzed by TEM (Fig. 2c). The
particle size of catalyst (1) was lower than 15 nm, which was
much smaller than the nano-Silica particle size. The SEM
(Fig. 2a) of the silica network shows the average diameter of
115 nm. Hence, catalyst (1) could be attached to the surface
of the nano-Silica particle. A clear change in the morphology
was occurred after the addition of Cu Schiff base complex to
the support Fig. 2b. SEM showed that the number of pores for
the catalyst clearly decreased and the N2 absorption/desorption
confirmed it. It is deduced that more sites will be enable to
increase the catalytic activity of the nanocatalyst (1).
Fig. 2. (a) SEM micrograph of gelatin/siloxane composite, (b) SEM image
of grafted Cu(II) Schiff base complex (c) TEM image of grafted
Cu(II) Schiff base complex
volume in single point (P/P0 = 0.99731) of the catalyst is 0.097
cm3/g (Fig. 3a) compared to 0.231 cm3/g for the support
(Fig. 3b). This indicated the heterogeneous catalyst brings
about a substantial reduction in surface area, possibly due to
pore blockade from the gelatin/siloxane, limiting access to
very strait-entrance pores in the material. Pore size of the
catalyst was 17.8 Å (Fig. 3a), as compared to 13.8 Å for the
support (Fig. 3b); this indicated the total pore volume of catalyst
was reduced. The number of support pores is higher than in
the catalyst, while the catalyst has a larger pore diameter.
BET of substrate and catalyst: Surface analysis showed
the surface area of the catalyst as 167 m2/g (Fig. 3a) and the
gelatinee/siloxane hybrid as 433 m2/g (Fig. 3b). Total pore

3310 Eshtiagh-Hosseini et al.
Asian J. Chem.
TABLE-2
OXIDATION OF ALCOHOLS TO CORRESPONDING CARBOXYLIC GROUP BY CATALYST (1)a
Entry
1
Substrate
OH
Product
O
Yield (%)
Time (min)
Selectivity
100
H
H
85
35
OH
O
H
H
2
3
98
99
30
25
100
100
MeO
MeO
OH
O
MeO
MeO
MeO
MeO
O
OH
4
5
99
99
45
35
100
100
O2N
Br
O2N
Br
O
OH
OH
O
6
7
99
95
35
80
100
100
Cl
Cl
O
OH
H
OH
O
8
Et
Et
95
65
100
O
OH
9
95
92
55
95
100
100
OH
O
Me
Me
10
O2N
O2N
OH
O
11
12
Me
Me
95
96
60
65
100
100
Me
Me
OH
O
Cl
Cl

Vol. 25, No. 6 (2013)
Application of Copper(II) Schiff Base Complex Grafted in the Silica Network as Efficient Nanocatalyst 3311
OH
O
13
98
92
65
70
100
100
Br
Br
OH
O
Me
Me
14
OH
O
Me
Me
15
98
55
100
MeO
MeO
Cl
O
Cl
OH
Me
16
17
Me
90
94
65
100
100
O
OH
110
OH
O
18
19
98
98
150
240
100
100
O
OH
Reaction condition: substrate 1 mmol, room temperature, 0.1 mmol of cat. (1), 10 mmol 30 % aqueous H2O2.
Catalytic reaction:The oxidation of alcohols was carried
out to test the catalytic efficiency of the prepared catalyst. To
show the generality and scope of this procedure, the reaction
was examined with various alcohols such as primary, secondary
and aliphatic alcohols, the results summarized in Table-2.
Efficient oxidation was observed on the benzylic alcohols but
for the oxidation of aliphatic alcohols extra times are required
(Table-2, entries 17, 18). Moreover, the reaction of cyclohexanol
(Table-2, entry19), required the addition of an extra 10 mmol %
of H2O2 after 150 min. The primary benzylic alcohols were
found to be reactive and afforded selectively to their corres-
ponding aldehydes as compared to other aliphatic primary
alcohols. Secondary or benzylic or aliphatic alcohols gave
corresponding ketones in high yields. The yields were
confirmed after the products were isolated by filtration and
purification with ethyl acetate/n-hexane.Although conversion
was observed by TLC, typically, 5-12 % of products were lost
during the work-up procedure. The Cu(II) [Schiff base]/SiO2
also showed almost good catalytic activity in solution of
oxidation reaction. Atomic absorption spectrometric analysis
shows that copper is not leaching out during oxidation
reactions. No copper was detected in the liquid phase of the
reaction mixture, after the completion of reaction. The catalyst
(1) affords selectively only aldehyde (Table-2, entries 1-7)
instead of the two usual product (aldehyde and carboxylic acid)
in oxidation of primary alcohols, in case of the oxidation of
secondary alcohols, the catalyst (1) affords selectively only
ketone instead of two possible product (ketone and carboxylic
ester). After the first use of the catalyst, the reusability of the
catalyst (1) in the oxidation of benzhydrol (Table-1, entry 1)
was carefully examined. The recovered catalyst was success-
fully used in 7 subsequent runs without any significant loss in
catalytic activity under similar experimental conditions (Table-
3). No pre-treatment step was used, although the recovered
catalyst washed with mixed solvent of ethanol/n-hexane by
1:9 molar ratios to remove traces of the previous reaction
mixture and dried before the next cycle.
A
D
Isotherm
65.912
59.320
52.729
46.138
39.547
32.956
26.365
19.773
13.182
6.591
0.000
0.0000 0.1047 0.2095 0.3142 0.4189 0.5237 0.6284 0.7331 0.8379 0.9429 1.0473
Relative pressure, P/P0
(a)

3312 Eshtiagh-Hosseini et al.
Asian J. Chem.
A
D
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ACKNOWLEDGEMENTS
The authors acknowledged the financial support of
Ferdowsi University of Mashhad for this project (P/548).