Schiff base complex of metal ions supported on superparamagnetic Fe 3O4@SiO2 nanoparticles: An efficient, selective and recyclable catalyst for synthesis of 1,1-diacetates from aldehydes under solvent-free conditions

Article abstract of DOI:10.1016/j.apcata.2012.09.010

We report a new multistep method for preparing functionalized superparamagnetic Fe₃O₄@SiO₂ possessing high saturation magnetization. During the first step, Fe₃O ₄@SiO₂ nanosphere core-shell is synthesized using nano-Fe₃O₄ as the core, TEOS as the silica source and PVA as the surfactant. Then, Schiff base complex of metal is synthesized from the reaction between Schiff base and metal acetates [Co(OAc)₂, Mn(OAc)₂, Ni(OAc)₂, Cu(OAc)₂, Hg(OAc) ₂, Cr(OAc)₃ and Cd(OAc)₂] on the Fe ₃O₄@SiO₂ surface. The structural and magnetic properties of functionalized magnetic silica are identified by transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM) instruments. Moreover, functionalized Fe₃O₄@SiO₂ possessed a superparamagnetic characteristic with saturation magnetization value of about 34 emu/g. NMR, FT-IR, elemental analysis and XRD were also used for the identification of these structures. The catalytic ability of Fe ₃O₄@SiO₂/Schiff base complex of metal ions was found to be an efficient nanocatalyst for the conversion of aldehydes to their corresponding 1,1-diacetates compounds under mild and solvent-free conditions at room temperature. This method gives notable advantages such as excellent chemoselectivity, mild reaction condition, short reaction times and excellent yields. Also, the aforementioned nanocatalyst can be easily recovered by a magnetic field and reused for subsequent reactions for at least 5 times without noticeable deterioration in catalytic activity.

Full text of DOI:10.1016/j.apcata.2012.09.010

Applied Catalysis A: General 445–446 (2012) 359–367
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Schiff base complex of metal ions supported on superparamagnetic Fe3O4@SiO2
nanoparticles: An efficient, selective and recyclable catalyst for synthesis of
1,1-diacetates from aldehydes under solvent-free conditions
Mohsen Esmaeilpoura, Ali Reza Sardariana,∗, Jaber Javidib
a Chemistry Department, College of Science, Shiraz University, Shiraz 71454, Iran
b Institute of Nano Science and Nano Technology, University of Kashan, Kashan, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 May 2012
Received in revised form 6 August 2012
Accepted 5 September 2012
Available online 15 September 2012
We report a new multistep method for preparing functionalized superparamagnetic Fe3O4@SiO2 possess-
ing high saturation magnetization. During the first step, Fe3O4@SiO2 nanosphere core–shell is synthesized
using nano-Fe3O4 as the core, TEOS as the silica source and PVA as the surfactant. Then, Schiff base complex
of metal is synthesized from the reaction between Schiff base and metal acetates [Co(OAc)2, Mn(OAc)2,
Ni(OAc)2, Cu(OAc)2, Hg(OAc)2, Cr(OAc)3 and Cd(OAc)2] on the Fe3O4@SiO2 surface. The structural and
magnetic properties of functionalized magnetic silica are identified by transmission electron microscopy
(TEM) and vibrating sample magnetometer (VSM) instruments. Moreover, functionalized Fe3O4@SiO2
possessed a superparamagnetic characteristic with saturation magnetization value of about 34 emu/g.
NMR, FT-IR, elemental analysis and XRD were also used for the identification of these structures. The
catalytic ability of Fe3O4@SiO2/Schiff base complex of metal ions was found to be an efficient nanocat-
alyst for the conversion of aldehydes to their corresponding 1,1-diacetates compounds under mild and
solvent-free conditions at room temperature. This method gives notable advantages such as excellent
chemoselectivity, mild reaction condition, short reaction times and excellent yields. Also, the aforemen-
tioned nanocatalyst can be easily recovered by a magnetic field and reused for subsequent reactions for
at least 5 times without noticeable deterioration in catalytic activity.
Keywords:
Superparamagnetic
Nanocatalyst
1,1-Diacetates
Schiff base complex
Solvent-free
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
Currently, much attention has been focused on the synthesis of
magnetic metal oxide structures by coating a silica shell around
Nanoscale materials have been a subject of particular inter-
est due to properties, which differ from their bulk counterparts
[1]. They have been used extensively in chemistry [2], physics
[3], biology [4] and catalysis [5]. Recently, magnetic core–shell
nano-structures have attracted more attention due to their unique
magnetic properties. In contrast to the difficulty observed in recov-
ering and reusing most solid catalysts, core–shell nanostructure
magnetic catalysts can be easily retrieved under the influence of a
magnetic field and used in subsequent reactions. Due to this prop-
erty, using magnetic core–shell structure composites as catalysts
has been recommended in literature [6,7].
a preformed nanoparticle [12–14].
Schiff base transition metal complexes have been extensively
studied because of their potential use as catalysts in a wide range
of reactions [15,16]. These complexes have been extensively used
for hydrogenation of organic substrates [17], epoxidation of olefins
[18], conversion of epoxides into halohydrines [19,20], asymmet-
ric ring opening of terminal epoxides [21] and oxidation reactions
[22,23].
Obviously, homogeneous catalysts show higher catalytic activ-
ities than their heterogeneous counterparts because of their
solubility in reaction media, which increases catalytic site acces-
sibility for the substrate [24,25]. However, recycling homogeneous
catalysts, is often tedious and time consuming and there is also the
issue of product contamination observed when these catalysts are
used. Moreover homogenous metal Schiff base complex catalysts
are deactivated easily through the formation of dimeric peroxo-
and ␮-oxo species [26,27]. For overcoming the aforenmentioned
drawbacks, metal and organo catalysts are immobilized on silica
coated Fe3O4. Recently, a number of such functionalized Fe3O4
Magnetic metal oxides, in particular magnetite nanoparticles,
were envisioned as magnetic storage media [8], contrast agents
for magnetic resonance imaging (MRI) [9], for cancer treatment
through hyperthermia [10] and a vessel for drug delivery [11].
∗
Corresponding author. Tel.: +98 711 2284822; fax: +98 711 2286008.
E-mail address: asardarian55@yahoo.co.uk (A.R. Sardarian).
0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.09.010

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nanoparticles have been employed in a range of organic transfor-
mations [28–36].
and tetraethoxysilane (TEOS) (0.20 mL), followed by the addition of
5.0 mL of NaOH (10 wt%). This solution was stirred mechanically for
30 min at room temperature. Then the product, Fe3O4@SiO2, was
separated by an external magnet, and was washed with deionized
water and ethanol three times and dried at 80 ◦C for 10 h. FT-IR (KBr
Acylal formation is one of the most useful methods to protect
carbonyl group of aldehydes and ketones and has found wide appli-
cation in multistep syntheses due to acylal stability in neutral and
basic media [37,38]. 1,1-Diacetates were used for the synthesis of
dienes in Diels–Alder reactions [39], 2,2-dichlorovinyl acetate [40]
and 1-acetoxydienes [41]. The conventional method available for
the preparation of 1,1-diacetates involves the reaction of aldehydes
with acetic anhydride using Lewis acids such as FeCl3 [42], Cu(OTf)2
[43], Bi(OTf)3 [44], FeSO4 [45], N-bromosuccinimide [46], montmo-
rillonite clay [47], zeolites [48], H6P2W18O62·24H2O [49] and strong
Brønsted acids such as phosphoric, sulfuric and methanesulfonic
acids [50–52]. Although some of these methods afford good to high
yields of the corresponding 1,1-diacetates, many of them have cer-
tain drawbacks, including long reaction times, high temperatures
and moisture sensitivity. Moreover the catalysts themselves were
often acidic, corrosive, toxic and non-recoverable. So, it is neces-
sary to find new catalysts for the synthesis of 1,1-diacetates from
aldehydes under mild conditions.
pellets, cm−1): 3400 (O H), 1000–1150 (Si
O Si) and 556 (Fe O).
2.1.3. General procedure for the preparation of the Schiff base
ligands, [(OH)-salen]
To the solution of 3-aminopropyl (triethoxy) silane (1 mmol,
0.176 g) in 25 mL ethanol was added dropwise to the stoichiomet-
ric amount of salicylaldehyde (1 mmol, 0.122 g) in ethanol (25 mL).
The yellow solution obtained, due to imine formation, was stirred
at room temperature for 6 h. The resulting Schiff base ligand, as the
bright yellow precipitate, was separated by filtration and washed
with ethanol (5 mL) and then dried in vacuum. The crude prod-
uct was recrystallized from ethanol to obtain the pure product in
98% yield (0.271 g). Anal. found (%): C, 58.36; H, 8.52; N, 4.48. Calc.
for C16H27NO4Si (%): C, 59.04; H, 8.36; N, 4.30. 1H NMR (250 MHz,
CDCl3): ı = 0.7 (t, 2H, CH2, J = 8.5 Hz); 1.22 (t, 9H, CH3, J = 7.0 Hz);
1.82 (m, 2H, CH2); 3.59 (t, 2H, CH2, J = 6.5 Hz); 3.81 (q, 6H, CH2,
J = 7.0 Hz); 6.85–6.96 (m, 2H, CH aromatic); 7.21–7.29 (m, 2H, CH
aromatic); 8.33 (s, 1H, CH); 13.59 (s, 1H, OH). 13C-NMR (63 MHz,
CDCl3): ı = 7.94, 18.32, 24.37, 58.43, 62.05, 117.03, 118.38, 118.81,
131.12, 132.05, 161.40 and 164.78.
Therefore, we report preparation of functionalized magnetic
nanoparticles Fe3O4@SiO2 with Schiff base complex of metal ions
and its application on the transformation of aldehydes to the related
acylals under mild conditions
2. Experimental
2.1.4. General procedure for the preparation of the Schiff base
complex of metal ions
Chemical materials were purchased from the Merck Chemical
Company in high purity. All the solvents were distilled, dried and
purified by standard procedures. Fourier transform infrared (FT-IR)
spectra were obtained using a Shimadzu FT-IR 8300 spectropho-
tometer. The NMR spectra were recorded on a Bruker avance DPX
250 MHz spectrometer in chloroform (CDCl3) using tetramethylsi-
lane (TMS) as an internal reference. X-ray powder diffraction (XRD)
spectra were taken on a Bruker AXS D8-advance X-ray diffrac-
tometer with Cu K␣ radiation (ꢀ = 1.5418). Field emission scanning
electron microscopy (FE-SEM) images were obtained on HITACHI
S-4160 and transmission electron microscopy (TEM) images were
obtained on a Philips EM208 transmission electron microscope
with an accelerating voltage of 100 kV. Magnetic properties were
obtained on a BHV-55 vibrating sample magnetometer (VSM).
Dynamic light scattering (DLS) were recorded on a HORIBA-LB550.
The 1,1-diacetates were characterized by their melting points and
comparison with literature values. The progress of the reaction
was monitored by TLC and purification was achieved by silica gel
column chromatography.
To the solution of the Schiff base ligand (2 mmol) in ethanol
(25 mL) was added metal acetates (1 mmol) and the mixture was
reflux to complete the reaction. The progress of the reaction was
monitored by TLC. After the completion of complex formation, a
color change was observed. Resulting product was filtered and
washed with ethanol. Then the product was purified by recrystal-
lization from ethanol and the resulting pure Schiff base complexes
were obtained.
2.1.5. General procedure for Schiff base complex of metal ions
functionalized magnetite@silica nanoparticles
Fe3O4@SiO2 (2 g) was added to the solution of Schiff complex of
metal ions (1 mmol) in ethanol (10 mL) and the resultant mixture
was under reflux for 12 h. The solvent was removed and the result-
ing solid was dried at 80 ◦C overnight. The product was washed
with ethanol and water to remove no reacted species and dried at
80 ◦C for 6 h. FT-IR spectrum of the catalyst showed the expected
bands, including a distinctive band due to C N stretching, which is
lowered in frequency on complexation to metal ions.
2.1. General procedure
2.1.6. General procedure for the preparation of 1,1-diacetates
To a mixture of aldehyde (1 mmol) and acetic anhydride
(3 mmol) was added 0.035 g Fe3O4@SiO2/Schiff base complex of
metal ion and the mixture was stirred at room temperature. The
progress of the reaction was followed by thin-layer chromatog-
raphy (TLC). After completion of the reaction, and separation of
Fe3O4@SiO2/Schiff base complex of metal ion with using mag-
netic field, the residue was extracted with CH2Cl2 (2× 10 mL). The
resulting solution was successively washed with 10% NaHCO3 (2×
10 mL) solution and water (3× 5 mL) and then dried over anhydrous
Na2SO4. After removal of the solvent in vacuum, the crude prod-
uct was purified by column chromatography on silica gel (ethyl
acetate/hexane, 1:8).
2.1.1. Preparation of Fe3O4 nanoparticles
The mixture of FeCl3·6H2O (1.3 g, 4.8 mmol) in water (15 mL)
was added to the solution of polyvinyl alcohol (PVA 15000) (1 g),
as a surfactant, and FeCl2·4H2O (0.9 g, 4.5 mmol) in water (15 mL),
which was prepared by completely dissolving PVA in water fol-
lowed by addition of FeCl2·4H2O. The resultant solution was left to
be stirred for 0.5 h in 80 ◦C. Then hexamethylenetetramine (HMTA)
(1.0 mol/L) was added drop by drop with vigorous stirring to pro-
duce a black solid product when reaction media reaches pH 10. The
resultant mixture was heated on water bath for 2 h at 60 ◦C and
the black magnetite solid product was filtered and washed with
ethanol three times and was then dried at 80 ◦C for 10 h.
2.1.2. Preparation of Fe3O4@SiO2 core–shell
3. Results and discussion
The core–shell Fe3O4@SiO2 nanospheres were prepared by a
modified Stober method [53]. Briefly, Fe3O4 (0.50 g, 2.1 mmol) was
dispersed in the mixture of ethanol (50 mL), deionized water (5 mL)
The metal ion complex was prepared by refluxing stoichiomet-
ric amounts of Schiff base ligand and metal acetate in ethanol. The

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361
Scheme 1. Preparation process of Schiff base complex of metal ions functionalized Fe3O4@SiO2 nanoparticle.
complexes were insoluble in water but soluble in most organic
solvents (Scheme 1).
base ligand, disappears in the spectra of the complexes
(Fig. 1).
The IR spectra of complexes show important bands from the
free Schiff base ligand. The free Schiff base exhibits a broad band
at 2.450–2.750 cm−1, attributed to hydrogen bonding between the
phenolic hydrogen and azomethine nitrogen. The absence of this
band in the spectrum of the covalently anchored complex indicates
the loss of the hydrogen bond. The free ligand exhibits a ꢁ(C N)
stretch at 1634 cm−1 while in the complexes, this band shifts to
lower frequency and appears at 1617–1631 cm−1 due to the coor-
dination of the nitrogen with the metal. Vibrations in the range of
1480–1600 cm−1 are attributed to the aromatic ring. The presence
of vibration bands in 559–588, 1100 and 3400 cm−1, which are
The crystalline structures of the Fe3O4 particles, Fe3O4@SiO2
and Fe3O4@SiO2/Schiff base Co(II) were determined by powder
X-ray diffraction (XRD). As shown in Fig. 2, it can be seen that
the Fe3O4 obtained has highly crystalline cubic spinel structure
which agrees with the standard Fe3O4 (cubic phase) XRD spectrum
(PDF#88-0866). The patterns indicate a crystallized structure at 2ꢂ:
30.1◦, 35.4◦, 43.1◦, 53.4◦, 57◦ and 62.6◦, which are assigned to the
(2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) crystallographic faces
of magnetite (reference JCPDS card no. 19-629). The XRD pattern
of Fe3O4@SiO2 prepared by the Stöber process, shows an obvious
diffusion peak at 2ꢂ = 15–25◦ that appeared because due to the
existence of amorphous silica. For Fe3O4@SiO2/Schiff base Co(II)
nanoparticles, the broad peak was transferred to lower angles due
to the synergetic effect of amorphous silica and Schiff base com-
plexes of metal (II).
due to Fe O, Si
O Si, and OH respectively, demonstrates the
existence of Fe3O4 and SiO2 components in all of the synthesized
complexes. The presence of several bands with medium inten-
sity in 2750–3000 cm−1 region is allocated to C H stretching of
methylene groups. The presence of two or three bands in the low
frequency region between 420 and 550 cm−1 indicates the
coordination of phenolic oxygen in addition to azome-
The morphology and sizes of (a) Fe3O4 and (b) Fe3O4@SiO2 par-
ticles were observed by transmission electron microscopy (TEM) as
shown in Fig. 3.
thine nitrogen.
centered at 1200–1320 cm−1
ꢁO
found as a medium band at 3410 cm−1 in free Schiff
C
O
stretching vibrations exhibits
a
peak
According to the result calculated by Scherrer equation, it was
found that the diameter of Fe3O4 nanoparticles obtained was
about 12 nm and Fe3O4@SiO2 microspheres were obtained with a
.
The OH stretching vibration,
H

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Fig. 2. XRD patterns of (a) Fe3O4, (b) Fe3O4@SiO2 and (c) Fe3O4@SiO2/Schiff base of
Co(II).
at room temperature. The saturation magnetization values for
Fe3O4, Fe3O4/SiO2 and Schiff base complexes of Co(II) function-
alized Fe3O4@SiO2 were 64.8, 40.3 and 34.0 emu/g, respectively.
These results indicated that the magnetization of Fe3O4 decreased
considerably with the increase of SiO2 and Schiff base complexes
of Co(II). Nevertheless, the metal ion complexes supported on
Fe3O4@SiO2 can still be separated from the solution by using an
external magnetic field on the sidewall of the reactor.
In continuation of our interest in development of novel synthetic
methodology, herein, we would report a simple, mild, efficient and
rapid method for the synthesis of acylals from variety of aldehydes
with acetic anhydride in the presence of catalytic amounts of nano
Fe3O4@SiO2/Schiff base complex of metal ions at room temperature
under solvent free conditions (Scheme 2).
Fig. 1. (1) Fe3O4@SiO2, (2) Schiff base ligand, (3) Schiff base complex of Co(II),
(4) Fe3O4@SiO2/Schiff base of Co(II), (5) Fe3O4@SiO2/Schiff base of Cu(II), (6)
Fe3O4@SiO2/Schiff base of Mn(II), (7) Fe3O4@SiO2/Schiff base of Hg(II), (8)
Fe3O4@SiO2/Schiff base of Ni(II), (9) Fe3O4@SiO2/Schiff base of Cd(II) and (10)
Fe3O4@SiO2/Schiff base of Cr(III).
To find the optimized experimental procedure, we selected p-
chlorobenzaldehyde as model substrate and treated it with Ac2O in
the presence of various Fe3O4@SiO2/Schiff base complex of Cr(III)
at room temperature under different conditions. This reaction was
best carried out using 3 equiv. of Ac2O for 20 min at room temper-
ature in the presence of the nanocatalyst. The results show clearly
that the nanocatalyst is effective for this transformation and fails
to initiate in its absence even after 8 h (Table 1, entry 6). When
0.035 g nanocatalyst was added, higher yield of the respective 1,1-
diacetate was attained after 20 min at room temperature (Table 1,
entry 5).
For optimization of the reaction media, the same reaction was
performed in different solvents and also under solvent-free con-
dition. In all of the solvents studied, the model reaction was
performed and moderate yields of the corresponding 1,1-diacetate
were obtained. When the reaction was performed under solvent-
free condition, higher yields and shorter times were observed
(Table 2, entry 6). Therefore, the solvent-free conditions were
diameter of about 20 nm due to the agglomeration of Fe3O4 inside
nanospheres and surface growth of silica on the shell [54]. Fig. 3(b)
displays the high resolution TEM images of Fe3O4 nanoparticles
coated with silica layers. The mesoporous silica shell on the surface
of Fe3O4 is quite homogeneous and exhibits good monodispersity
with estimated thickness of 8 nm.
The morphology of Fe3O4 and Fe3O4@SiO2 nanoparticle was
also observed by FE-SEM (Fig. 4(a) and (b)). The statistic results
from FE-SEM images clearly show the average product size (inset of
Fig. 4(a)–(c)). Upon coating the Fe3O4 (Fig. 4(b) and (c)), the appear-
ance of agglomerate particles is almost similar to that of the bare
Fe3O4. The FE-SEM images indicate the successful coating of the
magnetic Fe3O4 particles.
The magnetic properties of the samples containing a magnetite
component were studied by a vibrating sample magnetometer
(VSM) at 300 K. Fig. 5 shows the absence of hysteresis phenomenon
and indicates that all of the products have superparamagnetism

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363
Fig. 3. TEM images and the size distributions of (a) Fe3O4, (b) Fe3O4@SiO2 and (c) Fe3O4@SiO2/Schiff base of Co(II).
Fig. 4. FE-SEM images of (a) Fe3O4, (b) Fe3O4@SiO2 and (c) Fe3O4@SiO2/Schiff base of Co(II).

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M. Esmaeilpour et al. / Applied Catalysis A: General 445–446 (2012) 359–367
Scheme 2.
Table 2
The effect of solvent on the conversion of p-chlorobenzaldehyde to the corre-
sponding 1,1-diacetate chlorobenzaldehyde with acetic anhydride catalyzed by
Fe3O4@SiO2/Schiff base complex of Cr(III).a
Entry
Solvent
Time (min)
Yieldb (%)
1
2
3
4
5
6
EtOH
CH2Cl2
CHCl3
CH3CN
CH3CO2Et
Solvent-free
55
65
65
55
60
45
53
44
46
57
52
96
a
Reaction conditions: p-chlorobenzaldehyde (1 mmol), Ac2O (3 mmol), and cat-
alyst (0.035 g) at room temperature.
b
Yields refer to isolated pure product.
(1 mmol) with Ac2O (3 mmol) (Table 3). The catalytic activ-
ity of these various Lewis acids was found to be of the order
Cr(III) > Ni(II) > Zn(II) > Cu(II) > Mn(II) > Co(II) > Cd(II) > Hg(II). There-
fore, we chose Fe3O4@SiO2/Schiff base complex of Cr(III), for the
transformation of aldehydes to acylals (Table 4).
Fig. 5. Magnetization curves at 300 K for (a) Fe3O4, (b)Fe3O4@SiO2 and
(c)Fe3O4@SiO2/Schiff base of Co(II).
As shown in Table 4, various aromatic, heteroaromatic, unsat-
urated and aliphatic aldehydes gave excellent yields of the
corresponding 1,1-diacetates and short reaction times at room tem-
perature (Table 4).
This method was effective even with aldehydes bearing
electron-withdrawing substituents, such as chloro-, bromo- and
nitro-groups on the aromatic ring (Table 4, entries 4–9).
Both activated and deactivated aromatic aldehydes were con-
verted to their corresponding acylals under solvent-free conditions.
used for the conversion of aldehydes to their corresponding 1,1-
diacetates.
Therefore, we employed the optimized conditions (0.035 g
nanocatalyst and solvent-free conditions) for the conversion of var-
ious aldehydes into the corresponding acylals.
In order to study the catalytic activity of various
Fe3O4@SiO2/Schiff base complex of metal ions as catalyst, we
examined their efficiency on the reaction of p-chlorobenzaldehyde
Table 1
Table 3
Optimization of the catalyst amount of Fe3O4@SiO2/Schiff base complex of Cr(III)
in the reaction of p-chlorobenzaldehyde with acetic anhydride under solvent-free
conditions.a
The effect of Fe3O4@SiO2/Schiff base complex of metal ions on the reaction of p-
chlorobenzaldehyde with Ac2O under solvent-free conditions at room temperature
after 45 min.
Entry
Catalyst amount (g)
Time (min)
Yieldb (%)
Entry
Metal ion
Yielda (%)
1
2
3
4
5
6
0.01
0.02
0.03
0.04
0.035
No catalyst
180
120
60
25
20
46
1
2
3
4
5
6
7
8
Co2+
Mn2+
Cu2+
Ni2+
Cr3+
84
91
91
94
97
92
81
76
61
88
96
96
0
480
Zn2+
Cd2+
Hg2+
a
Reaction conditions: p-chlorobenzaldehyde (1 mmol), Ac2O (3 mmol), room
temperature.
b
a
Yields refer to isolated pure product.
Yields refer to isolated pure product.

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365
Table 4
Conversion of aldehydes to the corresponding 1,1-diacetates catalyzed by Fe3O4@SiO2/Schiff base complex of Cr(III).a
Entry
Substrate
Product
Time (min)
Yieldb (%)
96
Mp. (◦C)c (Lit.) Ref.
43–43 (44–45) [41]
CHO
CH(OAc)2
1
15
CH(OAc)2
CHO
CHO
2
23
93
81–83 (81–82) [41]
H3C
H3C
CH(OAc)2
3
4
25
20
94
92
63–65 (64–65) [41]
59–61 (59) [55]
MeO
MeO
CH(OAc)2
CHO
Cl
Cl
CHO
CH(OAc)2
5
6
20
23
91
93
187–189 (189) [56]
94–96 (92–95) [57]
Cl
Cl
Cl
Br
Cl
CH(OAc)2
CHO
Br
CHO
CH(OAc)2
7
20
92
64–66 (65–66) [41]
NO2
NO2
CHO
CHO
CH(OAc)2
CH(OAc)2
8
9
15
20
92
93
123–125 (125–127) [41]
80–82 (82–83) [41]
O2N
O2N
Cl
S
Cl
S
CHO
CHO
CH(OAc)2
CH(OAc)2
10
11
25
30
89
86
65–67 (66–67) [58]
51–53 (52–53) [41]
O
O
CHO
CH(OAc)2
12
13
20
25
94
92
84–86 (84–85) [58]
69–71 (70–71) [59]
CHO
OMe
CH(OAc)2
OMe
OAc
O
14
15
17
94
Oil (oil) [58]
H
OAc
OAc
OAc
O
120
NR
–

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M. Esmaeilpour et al. / Applied Catalysis A: General 445–446 (2012) 359–367
Table 4 (Continued)
Entry
Substrate
Product
Time (min)
120
Yieldb (%)
NR
Mp. (◦C)c (Lit.) Ref.
O
AcO
OAc
CH3
16
–
CH3
O
AcO
OAc
CH3
CH3
–
17
120
NR
O2N
O2N
a
Reaction condition: aromatic and aliphatic aldehydes (1 mmol), acetic anhydride (3 mmol) and Fe3O4@SiO2/Schiff base complex of Cr(III) at room temperature under
solvent-free condition.
b
Yields refer to isolated pure products.
Melting points reported in the parenthesis refer to the literature melting points.
c
Scheme 3. Probable mechanism for conversion of aldehydes to their corresponding 1,1-diacetates catalyzed by Fe3O4@SiO2/Schiff base complex of metal ions.
Ortho substituted benzaldehyde seems to have no effects on
the efficiency of this transformation, shown in entries 4, 5 and
13. The aliphatic aldehydes, heteroaromatic compounds such
as thiophene-2-carbaldehyde, furfural and cinnamaldehyde, ␣,␤-
unsaturated aldehydes, were converted to their corresponding
1,1-diacetates successfully without any side products (Table 4,
entries 10–12 and 14).
the 1,1-diacetate 4 and the chromium catalyst via the complex 3
(Scheme 3).
The possibility of recycling the nanocatalyst was examined
using the reaction of p-chloro benzaldehyde and acetic anhydride
in the presence of Fe3O4@SiO2/Schiff base complex of Cr(III) under
optimized conditions (Table 5).
It is also worth nothing that ketones, such as cyclohexanone,
acetophenone, and 4-nitro acetophenone, did not give any 1,1-
diacetates under the same reaction conditions (Table 4, entries
15–17).
Table 5
Reuse of the nanocatalyst for conversion of p-chloroaldehyde to the corresponding
1,1-diacetates.
It is likely that Fe3O4@SiO2/Schiff base chromium (III) com-
plex (1), as a Lewis acid, increases the electrophilicity of the
carbonyl group on the aldehyde by means of a strong coordi-
nate bond. Then acetic anhydride attacks complex 2 to produce
Cycle
0
First
93
Second
92
Third
92
Fourth
92
Fifth
91
Yielda (%)
93
a
Yields refer to the isolated pure products.

M. Esmaeilpour et al. / Applied Catalysis A: General 445–446 (2012) 359–367
367
The reaction mixture was recovered by a magnetic field and
the remaining solid was washed with dichloromethane (3× 10 mL),
dried and the catalyst reused for subsequent reactions for at least
5 times without any activation process.
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In summary, we developed a novel kind of superparamag-
netic nanocatalysts on based Schiff base complex of metal
ions-functionalized Fe3O4/SiO2. TEM microscopy revealed a very
fine layer of SiO2 on the Fe3O4. Moreover, magnetization
curves demonstrated a high degree of superparamagnetism. Mag-
netic nanocatalyst with saturation magnetization value of about
64.8 emu/g can be readily recovered under an external magnetic
field. The results obtained from the study of these catalysts on
the preparation of acylals indicated a high efficiency. Therefore,
considering the importance of these catalyst, we have shown that
Fe3O4@SiO2/Schiff base complex of metal ions, specially Cr(III), are
efficient catalysts for conversion of aliphatic and aromatic alde-
hydes to their corresponding 1,1-diacetates. The advantages of the
present method are: (a) easy and simple work up, (b) short reaction
times, (c) mild reaction conditions, (d) excellent chemoselectivity
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Acknowledgments
Authors gratefully acknowledge the financial support of this
work by the Research Council of University of Shiraz and the Council
of Iran National Science Foundation.
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