Effect of substituents and encapsulation on the catalytic activity of copper(II) complexes of two tridentate Schiff base ligands based on thiophene: Benzyl alcohol and phenol oxidation reactions

Article abstract of DOI:10.1007/s11243-014-9817-x

Encapsulation of Cu(II) complexes of methyl-2-(2-hydroxybenzylideneamino)- 4,5,6,7-tetrahydrobenzo[b]-thiophene-3-carboxylate (HL¹) and 2-ethyl-4-methyl 5-(2-hydroxybenzylideneamino)-3-methylthiophene-2,4- dicarboxylate (HL²) in the supercages of zeolite NaY was performed by the flexible ligand method. The neat and encapsulated complexes were characterized by physicochemical and spectroscopic methods and employed as catalysts for oxidation of benzyl alcohol and phenol. The encapsulated complexes were both more reactive and stable as catalysts than the corresponding free complexes. The encapsulated complexes could be reused several times. Complexes of HL² were more reactive than those of HL¹, probably due to the electron-withdrawing effect of the CO₂Et group.

Full text of DOI:10.1007/s11243-014-9817-x

Transition Met Chem (2014) 39:431–442
DOI 10.1007/s11243-014-9817-x
Effect of substituents and encapsulation on the catalytic activity
of copper(II) complexes of two tridentate Schiff base ligands based
on thiophene: benzyl alcohol and phenol oxidation reactions
•
Akbar Mobinikhaledi Mojgan Zendehdel
•
Parvin Safari
Received: 5 January 2014 / Accepted: 11 March 2014 / Published online: 6 April 2014
Ó Springer International Publishing Switzerland 2014
Abstract Encapsulation of Cu(II) complexes of methyl-2-
(2-hydroxybenzylideneamino)-4,5,6,7-tetrahydrobenzo[b]-
thiophene-3-carboxylate (HL1) and 2-ethyl-4-methyl 5-(2-
hydroxybenzylideneamino)-3-methylthiophene-2,4-dicar-
boxylate (HL2) in the supercages of zeolite NaY was per-
formed by the flexible ligand method. The neat and
encapsulated complexes were characterized by physico-
chemical and spectroscopic methods and employed as cat-
alysts for oxidation of benzyl alcohol and phenol. The
encapsulatedcomplexeswerebothmorereactiveand stableas
catalysts than the corresponding free complexes. The encap-
sulated complexes could be reused several times. Complexes
of HL2 were more reactive than those of HL1, probably due to
the electron-withdrawing effect of the CO2Et group.
oxidants such as chromium(VI) and manganese complexes;
as well as forced reaction conditions such as high pressure
or temperature and strong mineral acids [3]. Recently,
some organometallic complexes have been proposed as
environmentally benign catalysts for the oxidation of
organic compounds [4, 5]. These homogenous catalysts can
give high activity and selectivity in oxidation reactions.
However, most of these catalysts have other disadvantages
such as difficulty in product separation and deactivation of
the catalyst during the reaction due to self-aggregation of
active sites [6–8]. In order to solve these problems, such
complexes can be heterogenized by encapsulation or
covalent bonding to insoluble solid supports such as
nanoporous zeolites [9, 10]. Heterogenization increases the
thermal and chemical stability of the complexes by iso-
lating each molecule from the others [11]. Also, the cata-
lytic activity of heterogeneous complexes is usually higher
than their homogeneous equivalents, due to shape selec-
tivity, electrostatics and acid–base properties of the solid
supports [12, 13].
Introduction
Hydroxylation of phenol to catechol and hydroquinone is
an industrially important reaction [1], while benzaldehyde
is an important organic intermediate due to its wide
application in perfumes, dyes, pharmaceuticals, agricul-
tural and flavoring chemicals [2]. The most common
method for preparation of benzaldehyde is selective oxi-
dation of benzyl alcohol. The traditional methods for oxi-
dation of alcohols generally require toxic and expensive
The encapsulation of complexes can be accomplished by
various methods, as follows. When the size of ligand is
smaller than the diameter of the channels, the ligand can be
adsorbed from solution into the solid support and coordi-
nate with a previously exchanged metal atom. The result-
ing complex is enough large to remain in the supercages
(flexible ligand method; FLM) [14]. Alternatively, tem-
plate synthesis (TS) or ship-in-bottle method (SBM) can be
used for ligands whose size is larger than the channel
diameter. In these methods, the ligands are assembled from
smaller species inside the nanopores. Then, the prepared
ligands are coordinated to the resident metal atoms (fixed
by ion exchange) [15]. Finally, when transition metal
complexes are stable under the support synthesis conditions
(generally high temperature and high pH) and they are
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-014-9817-x) contains supplementary
material, which is available to authorized users.
A. Mobinikhaledi Á M. Zendehdel Á P. Safari (&)
Department of Chemistry, Faculty of Science, Arak University,
38156-8-8349 Ara¯k, Iran
e-mail: p.safary@gmail.com
123

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Transition Met Chem (2014) 39:431–442
sufficiently soluble in the synthesis medium to enable
random distribution of the complexes in the prepared
porous materials, the sol–gel synthesis method (SGSM)
can be used [16].
Methyl-2-amino-4,5,6,7-tetrahydrobenzo[b]thiophene-
3-carboxylate (1a): white crystals, m.p. 136–137 °C;
FTIR (KBr, cm-1): 3,414–3,308 (NH2), 2,931 (C–H),
1,656 (C=O), 1,577–1,446–1,492 (C=C), 1,269 (C–O);
1H NMR (300 MHz, DMSO-d6): 7.20 (s, 2H, NH (D2O
exchange)), 3.66 (s, 3H, OCH3), 2.40–2.56 (m, 4H,
H-cyclohexane), 1.65–1.67 (m, 4H, H-cyclohexane) ppm.
2-Ethyl-4-methyl 5-amino-3-methylthiophene-2,4-dicar-
boxylate (1b): white crystals, m.p. 117–119 °C; FTIR (KBr,
cm-1): 3,408–3,310 (NH2), 2,993 (C–H), 1,705–1,659
The objective of the present work was to investigate the
electron-donating and electron-withdrawing effects of
substituents on the benzyl alcohol and phenol oxidation
reactions. For this purpose, we prepared copper complexes
of two tridentate Schiff base ligands based on thiophene,
either with electron-donating or electron-withdrawing sub-
stituents, in free and zeolite NaY-encapsulated modes. The
catalytic activities of these compounds were compared.
1
(C=O), 1,604–1,531–1,444 (C=C), 1,254–1,192 (C–O); H
NMR (300 MHz, DMSO-d6): 7.93 (s, 2H, NH (D2O
exchange)), 4.11 (q; J = 7.1 Hz, 2H, OCH2), 3.73 (s, 3H,
OCH3), 2.58 (s, 3H, CH3), 1.22 (t; J = 7.1 Hz, 3H, CH3
(CO2Et)) ppm.
Experimental
Materials and methods
Preparation of HL1 and HL2
All materials and solvents were purchased from Merck.
Zeolite NaY was synthesized according to the method
reported in the literature [17]. An X-ray diffractometer type
XPERT with radiation Cu-Ka at room temperature was
used for recording XRD patterns. FTIR spectra were
recorded for KBr pellets, using a Galaxy series FTIR 5000
spectrometer, at the range of 500–4,000 cm-1. Also, a
TGA-DSC (PerkinElmer TG/DT 6300 thermogravimetric
analyzer), a BET apparatus (SIBATA, App. 1100-SA with
adsorption of nitrogen at 77°K), a V-670 JASCO spectro-
photometer and elemental analyzer (Vario E1 III) were
used for characterization of the encapsulated complexes.
To determine metal contents, a 990 atomic absorption
spectrophotometer, PG instruments Ltd., was used. EI MS
of the free complexes were recorded using an MSD-Direct
Probe instrument. Finally, the analysis of oxidation pro-
ducts was carried out with a gas chromatograph Perkin-
Elmer 1800 equipped with a packed column OV-17 (1.5 m
in length) and a flame ionization detector (FID).
Concentrated sulfuric acid (2–3 drops) was added to a
solution of salicylaldehyde (1 mmol) and the required
2-aminothiophenes (1a, b) (1 mmol) in ethanol (10 ml).
The mixture was refluxed for 3–4 h, and then cooled in an
ice bath to precipitate the product. The product was filtered
off, washed with water and recrystallized from ethanol.
Methyl-2-(2-hydroxybenzylideneamino)-4,5,6,7-tetra-
hydrobenzo[b]-thiophene-3-carboxylate (HL1): yellow
crystals, m.p. 141.5–143.5 °C, FTIR (KBr, cm-1): 3,074
(C–HAromatic), 2,928 (C–HAliphatic), 1,701 (C=O), 1,601
(C=N), 1,566–1,496–1,448 (C=C), 1,282, 1,213 (C–O);
1H NMR (300 MHz, DMSO-d6): 12.68 (s, 1H, OH), 8.77
(s, 1H, H-imine), 7.40–7.67 (m, 2H, H-Aromatic), 6.94
(t; J = 8.1 Hz, 2H, H-Aromatic), 3.80 (s, 3H, OCH3),
2.69 (m, 4H, H-cyclohexane), 1.74 (m, 4H, H-cyclo-
hexane) ppm.
2-Ethyl-4-methyl 5-(2-hydroxybenzylideneamino)-3-
methylthiophene-2,4-dicarboxylate (HL2): yellow crys-
tals, m.p. 142.5–144 °C; FTIR (KBr, cm-1): 3,055
(C–HAromatic), 2,995- 2,951 (C–HAliphatic), 1,712–1,697
(C=O), 1,599 (C=N), 1,560–1,444–1,384 (C=C),
1,288–1,195 (C–O); 1H NMR (300 MHz, DMSO-d6):
12.35 (s, 1H, OH), 9.00 (s, 1H, H-imine), 7.71 (d,
J = 7.0 Hz, 1H, H-Aromatic), 7.46–7.51 (m, 1H,
H-Aromatic), 6.97–7.02 (m, 2H, H-Aromatic), 4.29 (q,
J = 7.1 Hz, 2H, OCH2), 3.85 (s, 3H, OCH3), 2.61 (s, 3H,
CH3), 1.27 (t, J = 7.1 Hz, 3H, CH3 (CO2Et)) ppm.
Preparation of the Schiff bases
HL1 and HL2 were prepared as reported previously [18], as
detailed below.
Preparation of methyl 2-aminothiophene-3-carboxylate
derivatives
Morpholine (5 ml) was added slowly to a mixture of the
required ketone (0.05 mol of cyclohexanone or ethylace-
toacetate), methylcyanoacetate (0.05 mol) and elemental
sulfur (0.05 mol) in methanol (30 ml) at 35–40 °C with
stirring. The reaction mixture was stirred at 45 °C for 3 h,
and then cooled to room temperature. The precipitate was
filtered off and recrystallized from ethanol.
Preparation of [CuL1(OEt)] and [CuL2(OH)]
In order to obtain the free complexes, the following pro-
cedure was used. A hot ethanolic solution (5 ml) of
Cu(OAc)2Á6H2O (1 mmol) was added dropwise to a hot
ethanolic solution (10 ml) of the required Schiff base (HL1
or HL2, 1 mmol). The mixture was refluxed for 8 h.
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Transition Met Chem (2014) 39:431–442
433
On cooling, the product was precipitated, which was then
collected by filtration, washed with ethanol and air-dried.
Mass spectrum of [CuL1(OEt)]: m/z = 422, 313, 299,
257, 224, 209, 196, 151, 137, 134, 121, 105, 77, 64.
Mass spectrum of [CuL2(OH)]: m/z = 426, 409, 393,
368, 339, 331, 289, 256, 184, 121, 105, 77, 64.
N
O
O
N
O
O
S
S
Cu
O
Cu
O
OEt
EtO2C
OH
[CuL1(OEt)]
Preparation of copper-exchanged zeolite
[CuL2(OH)]
Scheme 1 Proposed structure for free and encapsulated complexes
A solution of CuCl2Á2H2O (1 mmol) in deionized water
(50 ml) was added to a suspension of zeolite NaY (1.0 g)
in deionized water (50 ml), and the mixture was stirred at
room temperature for 24 h. The resulting light blue solid
was filtered off and air-dried. The solid was then stirred in
200 ml of an aqueous solution of sodium chloride (0.01 M)
at room temperature for 8 h to remove copper atoms from
the external surface of the zeolite. The measurement of
copper content by atomic absorption spectroscopy con-
firmed that there was no copper in the used water, indi-
cating the absence of copper atoms on the surface of
zeolite. Finally, the product was filtered off and dried at
120 °C until a constant weight was attained. The copper-
exchanged zeolite is denoted as Cu(II)/Y.
0.025, 0.05, 0.1 or 0.15 mol) were mixed in 2 ml of solvent
(acetonitrile, chloroform, dichloromethane or methanol),
and an appropriate amount (0.013, 0.025, 0.05 or 0.075 g)
of the corresponding catalyst (zeolite NaY, Cu(II)/Y,
[CuL1(OEt)], [CuL2(OH)], [CuL1(OEt)]-Y or [CuL2(OH)]-
Y) was added. The mixture was refluxed with continuous
stirring for the required time. The products were analyzed
by GC.
Results and discussion
Preparation of the encapsulated complexes
The proposed structures of the encapsulated and free
complexes are shown in Scheme 1.
The Schiff base ligands were encapsulated into the zeolite
via the flexible ligand method. A mixture of Cu(II)/Y
(1.0 g) and the required Schiff base (HL1 or HL2) (2.5 g) in
ethanol (100 ml) was refluxed for 24 h with stirring. The
precipitate was then washed with DMF several times till
the used DMF became colorless. Finally, the precipitate
was collected and dried at 120 °C. The resulting encap-
sulated complexes are denoted as [CuL1(OEt)]-Y and
[CuL2(OH)]-Y.
FTIR spectroscopy
FTIR spectra of the free Schiff bases, free complexes, zeolite
NaY and encapsulated complexes are shown in Fig. 1. The
metal coordination sites were identified by carefully comparing
the infrared spectra of the complexes with those of the free
ligands. The spectraof thefree complexes contained absorption
bands at 594–827 cm-1, which can be attributed to Cu–O
stretching vibrations [19]. The mC–O absorption bands of the
ligands are shifted to higher wavenumbers in the spectra of the
complexes (from 1,286–1,288 cm-1 to 1,331–1,365 cm-1),
consistent with coordination of the ligands to copper via oxy-
gen. Also, the mC=N bands of the free ligands at 1,599–
1,601 cm-1 were shifted to 1,570–1,574 cm-1 in the spectra of
the complexes, providing strong evidence that the ligands are
coordinated to copper via the azomethine nitrogen atoms [20].
The mC=O bands of the free ligands were observed at 1,697–
1,699 cm-1. The shifts of these bands to 1,634–1,635 cm-1 in
the spectra of the complexes are in accordance with coordi-
nation of the carbonyl groups to copper [21]. The bands related
to the other carbonyl group of [CuL2(OH)] did not show any
shift in comparison with HL2 (1,712 cm-1), indicating that this
carbonyl group is not involved in the coordination. The band
observed at 3,221 cm-1 in the FTIR spectrum of [CuL2(OH)]
was attributed to an OH stretching vibration, associated with
Catalytic oxidation of benzyl alcohol
Benzyl alcohol (2 mmol) and the appropriate amount of
aqueous tert-butyl hydroperoxide (TBHP; 70 %) (2, 3, 4, 5
or 6 mmol) were mixed in 15 ml of solvent (acetonitrile,
chloroform, dichloromethane or methanol). Then, different
amounts (0.013, 0.025, 0.05, 0.075 or 0.1 g) of the corre-
sponding catalyst (zeolite NaY, Cu(II)/Y, [CuL1(OEt)],
[CuL2(OH)], [CuL1(OEt)]-Y or [CuL2(OH)]-Y) were
added, and the mixture was refluxed. The products were
collected at different time intervals and identified by GC.
Catalytic oxidation of phenol
The neat and encapsulated complexes were also tested as
catalysts for hydroxylation of phenol. In a typical reaction,
phenol (0.05 mol) and aqueous hydrogen peroxide (30 %;
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Transition Met Chem (2014) 39:431–442
Fig. 1 FTIR spectra of zeolite
NaY, [CuL1(OEt)]-Y,
[CuL1(OEt)], HL1;
[CuL2(OH)]-Y, [CuL2(OH)]
and HL2 (KBr pellet, sample:
KBr 1: 6)
the hydroxyl substituents [22]. Furthermore, mass spectros-
copy, elemental and thermal analyses were also consistent with
the proposed formulations of [CuL1(OEt)] and [CuL2(OH)].
In the FTIR spectra of the encapsulated complexes, a
strong and broad band at around 1,026 cm-1 is assigned to
the asymmetric stretching vibration of the Al–O–Si chain
of the zeolite, whereas the bending and symmetric
stretching vibrations of the Al–O–Si framework are
encapsulated complexes, which can be explained by the
overlapping broad zeolite band at around 1,650 cm-1
.
However, the FTIR spectrum of [CuL1(OEt)]-Y exhibited
an absorption band at 1,370 cm-1 due to the C–O
stretching and three bands at 1,400–1,531 cm-1 due to
C=C stretching vibrations. Also, the spectrum of [CuL2-
OH]-Y showed a band at 1,325 cm-1 which was assigned
to the C–O stretching vibration, as well as bands at
1,437–1,550 cm-1 related to the C=C stretching vibrations.
responsible for bands at around 457 and 725 cm-1
,
respectively [23]. Two bands in the regions of 1,639–1,651
and 3,456–3,497 cm-1 can be ascribed to lattice water
molecules and surface hydroxylic groups [24]. No signifi-
cant broadening or shift in the external linkage vibrations
(structure sensitive bands: 457, 725 and 1,026 cm-1) was
observed upon ion exchange and encapsulation of the
complexes. These observations confirmed that there is no
defect of zeolite framework or dealumination and the
matrix remained unchanged during the process, as con-
firmed by XRD analysis [25]. Additional bands in the FTIR
spectra of the encapsulated complexes at the region of
1,200–1,600 cm-1 confirmed interaction between the
ligands and the incorporated copper atoms. However, the
intensity of these peaks in the encapsulated complexes is
weak, due to their low concentration in the zeolite matrix
[23]. The spectra of the encapsulated complexes as well as
the corresponding neat complexes showed similar bands,
confirming that the complexes retained their structures
during the encapsulation, and furthermore that chemical
ligation to the zeolite surface is either non-existent or very
weak [26]. Furthermore, comparison of the spectra of the
encapsulated complexes to those of the respective free
ligands provided evidence for the coordination mode of the
ligands in the encapsulated complexes. The bands related
to mC=N and mC=O were not observed in the IR spectra of the
Electronic and diffuse reflectance spectroscopy
The electronic spectra of the free ligands and free com-
plexes, and the DRS of the encapsulated complexes, are
shown in Fig. 2. The spectra of the free ligands and free
complexes were recorded in DMSO, while the spectra of
the encapsulated complexes were taken in Nujol. The
electronic spectra of the free ligands showed two bands; the
first band at 285 nm (for both ligands) could be attributed
to p ? p* transitions of the phenyl rings, while the second
band at 390–395 nm may be assigned to p ? p* transi-
tions of the azomethine chromophores. In the electronic
spectra of the free complexes, bands centered at
270–275 nm were assigned to p ? p* transitions of the
phenyl rings. Additional bands centered at 445–450 nm
were attributed to p ? p* transitions of the azomethine
chromophores, which were observed at higher wavelength
compared to the free ligands, confirming coordination of
the imine nitrogen atoms to copper. Furthermore, ligand-
to-metal charge transfer (LMCT) bands were observed in
these regions and overlapped with intraligand p ? p*
transitions [27]. The weak bands expected for d ? d
transitions could not be observed, due to overlapping
LMCT bands [28]. The electronic spectra of the
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Transition Met Chem (2014) 39:431–442
435
Fig. 2 Electronic spectra of
free ligands, neat complexes
and encapsulated complexes
encapsulated complexes also showed two bands, whose
wavelengths were similar to those of the free complexes.
This suggests that the geometries of the complexes were
preserved even after encapsulation. Bands at 263–282 nm
corresponded to p ? p* transitions of the phenyl rings,
while bands at 445–450 nm were related to LMCT and
p ? p* transitions of azomethine, as discussed for the neat
complexes.
X-ray powder diffraction study
Figure 3 shows the powder X-ray diffraction patterns of
zeolite NaY and the encapsulated complexes. The XRD
patterns of the encapsulated complexes matched closely
with those of the parent zeolite NaY, except for a slight
change in the intensities of the peaks; no new crystalline
pattern emerged. These observations confirmed that there
was no loss in zeolite crystallinity after encapsulation [29].
No new peaks could be observed for the encapsulated
samples, in accordance with the results in the literature [30,
31], indicating the complexes were well distributed in the
supercages. However, the order of peak intensity was
I331 [ I220 [ I311 for zeolite NaY, while the order became
I331 [ I311 [ I220 for the encapsulated complexes. This
difference indicates that the random distribution of small
extra-framework cations was disturbed upon exchanging
large cations in zeolite NaY [32].
Fig. 3 XRD patterns of zeolite NaY, [CuL1(OEt)]-Y and [CuL2(-
OH)]-Y
water and solvent molecules were in the inner or outer
coordination sphere of the metal atom. Decomposition of
the free complexes occurred in two significant steps. Eth-
oxy (for [CuL1(OEt)]) or hydroxyl (for [CuL2(OH)])
groups were evolved in the first decomposition step of the
free complexes. These steps took place at the ranges of
170–181 °C (for [CuL1(OEt)]) and 160–180 °C (for
[CuL2(OH)]), with endothermic peaks at 175 and 172 °C
and mass losses of 10.0 % (calculated 10.6 %) and 5.0 %
(calculated 4.0 %), respectively. The loss of these groups at
such high temperatures is further evidence for coordination
Thermal analysis
The thermal stabilities of the free and encapsulated com-
plexes were investigated using TGA and DTA analyses.
The results are tabulated in Table 1. Thermogravimetric
analyses of the free complexes helped us to decide whether
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Transition Met Chem (2014) 39:431–442
Table 1 TGA and DTA data
for the free and encapsulated
complexes
Compound
Temperature (°C)
DTA peak (°C)
Mass loss (%)
Decomposition
assignment
Exo
Endo
175
Exp.
Calcld.
[CuL1(OEt)]
[CuL2(OH)]
[CuL1(OEt)]-Y
170–181
181–280
160–180
185–295
Below 200
200–420
420–800
Below 200
210–450
450–800
10.0
70
10.6
70.5
4.0
76.4
–
Ethoxy
240
237
Schiff base ligand
OH
172
120
5. 0
76.8
4.0
Schiff base ligand
H2O, OEt
255
647
7.8
11.4
–
Schiff base ligand
Zeolite
10.8
5.1
[CuL2(OH)]-Y
115
–
H2O, OH
263
625
9.3
12.7
–
Schiff base ligand
Zeolite
9.3
Analytical data
of the ethoxy and hydroxyl ligands to copper [33]. The
weight losses in the ranges 181–280 and 185–295 °C with
exothermic peaks at 240 and 237 °C for [CuL1(OEt)] and
[CuL2(OH)], respectively, roughly matched with complete
decomposition of the ligands. The intermediates could not
be identified owing to continuous weight loss, but the total
mass loss corresponded to one Schiff base ligand. After the
second step, the weight became constant due to formation
of copper oxide [34]. The results of thermal analyses
(Table 1) were in good agreement with the theoretical
formula as suggested from the elemental analyses and mass
spectrometry.
The analytical data for the parent zeolite NaY, Cu(II)/Y,
and the neat and encapsulated complexes are listed in
Table 2. Zeolite NaY has a Si/Al molar ratio of 2.3, which
corresponds to a unit cell formula Na56[(AlO2)56(SiO2)136].
The Si/Al ratio remained unchanged for both Cu(II)/Y and
the encapsulated complexes, confirming the absence of
dealumination during the ion exchange and encapsulation
processes. The unit cell formulae of the copper-exchanged
zeolite indicated a copper dispersion of around 10 mol per
unit cell, (Na37.4Cu9.7[(AlO2)56(SiO2)136]ÁnH2O). The C/N
ratios of the encapsulated complexes were similar to those
of the corresponding free complexes, indicating that the
structures of the complexes remained unchanged in the
cavities of zeolite. The analytical data for Cu, C, H and N
indicated a ligand/metal molar ratio of around 1 for the
encapsulated complexes; however, these values were a
little lower than 1 because the ligands may be unable to
react with all of exchanged copper atoms, especially those
occupying different sites in zeolite NaY [25]. The free
complexes gave a good agreement between the experi-
mental and theoretical values.
In the TG curves of the encapsulated complexes, three
decomposition steps were observed. In the first step, the
endothermic process (weight loss of 4.0 and 5.1 % (for
[CuL1(OEt)]-Y and [CuL2(OH)]-Y, respectively) below
200 °C was related to removal of physically adsorbed and
chemical water. Removal of ethoxy and hydroxyl groups
is also expected in this step. The second step was
ascribable to decomposition of the Schiff base ligands,
and the third step corresponded to change in the structure
of the zeolite framework. In the second step, an exo-
thermic weight loss of 7.8 or 9.3 % took place in the
temperature ranges of 200–420 and 210–450 °C for
[CuL1(OEt)]-Y and [CuL2(OH)]-Y, respectively. The
observed weight losses at this step do not agree with the
theoretical values of 11.4 and 12.7 %, respectively, if we
consider the copper contents as obtained from the ana-
lytical data. This disagreement can be explained by the
presence of unreacted copper atoms in the zeolite voids
[35]. The third step of decomposition started immediately
after the second step and continued up to 800 °C. In this
step, exothermic weight losses of 10.8 and 9.3 % were
observed for [CuL1(OEt)]-Y and [CuL2(OH)]-Y, respec-
tively. The results of thermal analysis indicated that the
thermal stability of the complexes increased after encap-
sulation, due to the zeolite shielding [25].
Mass spectral analysis of the neat complexes
Mass spectra data of [CuL1(OEt)] and [CuL2(OH)] showed
the expected molecular ion fragments, confirming the
proposed structures. The fragmentation mass spectrum of
[CuL2(OH)] is described in Scheme 2. The weak peaks
observed at m/z = 422 and 426 in the mass spectra of
[CuL1(OEt)] and [CuL2(OH)], respectively, can be attrib-
uted to the molecular ions of these compounds. The frag-
mentation mass spectrum of [CuL1(OEt)] and mass spectra
of both compounds are included in Online Resource 1 and
Online Resource 2 (See Supplementary materials) and are
also summarized in the ‘‘Experimental’’ section.
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Transition Met Chem (2014) 39:431–442
437
Table 2 Analytical data for
Catalyst
Color
%Cu
%C
%N
%H
C/N
%Si
%Al
%Na
Si/Al
zeolite NaY, Cu(II)/Y, and the
free and encapsulated
complexes
Zeolite NaY
Cu(II)/Y
[CuL1(OEt)]-Y
[CuL1(OEt)]
[CuL2(OH)]-Y
[CuL2(OH)]
White
–
–
–
–
–
20.5
20.7
20.4
–
8.9
9.0
8.8
–
7.2
3.7
4.1
–
2.3
2.3
2.3
–
Bright blue
Gray
2.5
2.4
15.0
2.5
14.8
–
–
–
–
5.4
54.1
5.9
47.5
0.4
3.8
0.4
3.5
0.5
4.5
0.5
4.4
13.9
14.0
13.1
13.7
Brown
Pink
20.8
–
9.0
–
4.0
–
2.3
–
Brown
N
O
O
S
N
O
N
O
O
Cu
O
H
S
S
Cu
O
Cu
O
EtO2C
O
EtO2C
EtO2C
-H
m/z=393
m/z=409
m/z=426
N
O
O
S
Cu
EtO2C
O
N
O
O
H
S
N
O
S
Cu
H
EtO2C
O
EtO2C
2H
O
m/z=368
m/z=339
m/z=331
N
N
S
S
N
+H
EtO2C
O
S
EtO2C
O
+H
EtO2C
+H
O
m/z=289
m/z=256
m/z=184
+H
N
O
Cu
-2H
Cu
H
O
N
Cu
H
N
O
m/z=184
m/z=121
m/z=105
m/z=64
m/z=77
Scheme 2 Fragments observed in the mass spectra of [CuL2(OH)]
N2 adsorption–desorption isotherms
adsorption–desorption isotherms are typical of micropo-
rous solids for both zeolite NaY and encapsulated com-
plexes. They are type III in the Brunauer classification and
type I in the IUPAC classification [30, 36]. Also, the pre-
sence of a narrow hysteresis loop confirmed the porous
nature of the encapsulated complexes [25]. However, the
presence of a small quantity of macropores (pores larger
N2 adsorption–desorption isotherms, t-plots, BJH, Lang-
muir and BET plots of the encapsulated complexes are
shown in Fig. 4(1a–d, 2a–d). Abbo and Titinchi [37] have
previously reported the N2 adsorption–desorption isotherm
of pure zeolite NaY. As previously reported, the
123

438
Transition Met Chem (2014) 39:431–442
Fig. 4 N2 adsorption–
desorption isotherms (1a, 2a),
BET plots (1b, 2b), BJH plots
(1c, 2c), t-plots (1d, 2d) and
Langmuir plots (1e, 2e) of
[CuL1(OEt)]-Y(1) and
[CuL2(OH)]-Y(2)
˚
than 500 A) in the structures of the encapsulated com-
catalysts showed two linear regions. Surface area (St) and
the external surface area (Sext) of the catalysts were
obtained from the slopes of the first and second linear
regions, respectively, which were multiplied by 15.47.
Furthermore, micropore volume was obtained by extrapo-
lating the second linear region to the y axis. The y-axis
intercept multiplied by the ratio of the gas and liquid
densities of the nitrogen (0.00156) provides the micropore
volume in cm3 per gram of solid [39].
plexes was confirmed by the slightly enhanced uptake of
N2 at P/P0 values greater than 0.9 [38]. The representative
catalysts exhibited narrow BJH pore size distribution
curves, as shown in Fig. 4(1c, 2c) [37].
BET surface area (SBET), total pore volume (Vp), mean
pore diameter derived from BET plots (DBET) and BJH
plots (DBJH), surface area derived from Langmuir plot
(SLang) and t-plot (St), micropore volume (Vmic), the
external surface area of the zeolite crystals derived from
t-plot (Sext) for the encapsulated complexes and also BET
surface area (SBET), total pore volume (Vp) of zeolite NaY
and Cu(II)/Y, are given in Table 3. Both surface area and
pore volume of the copper-exchanged zeolite showed a
reduction when compared to parent zeolite NaY. However,
this reduction was negligible compared to the large
reduction observed after coordination of the ligands. FTIR
spectroscopy and XRD patterns confirmed that the zeolite
framework was not affected by encapsulation, hence
reduction in the surface area and total pore volume upon
introduction of the complex can be assigned to the presence
of complex inside the zeolite nanocages, rather than on the
surface. The correct choice of standard t-curves was sup-
ported by the similarity of SBET and St [25]. T-plots of these
Catalytic activity studies
In order to identify the optimum reaction conditions for
oxidation of benzyl alcohol and phenol, the following
reaction parameters were optimized for these reactions:
effect of substituent and encapsulation on catalytic activity;
catalyst amount; solvent; oxidant concentration; and reac-
tion time.
The oxidation of benzyl alcohol was considered first.
To study on the effect of type and amount of the catalyst
(Table 4), the catalytic oxidation of 2 mmol of benzyl
alcohol using different amounts (0.013, 0.025, 0.05, 0.075 or
0.1 g) of the required catalyst (zeolite NaY, Cu(II)/Y,
[CuL1(OEt)], [CuL1(OEt)]-Y, [CuL2(OH)] or [CuL2(OH)]-
123

Transition Met Chem (2014) 39:431–442
439
Table 3 Surface characteristics
of zeolite NaY, Cu(II)/Y and the
encapsulated complexes
Sample
SBET
(m2/g)
St
(m2/g)
SLang
(m2/g)
Sext
(m2/g)
Vp
(cm3/g)
Vmic
(cm3/g)
DBET
(nm)
DBJH
(nm)
Zeolite NaY
Cu(II)/Y
[C(L1(OEt)]-Y
[CuL2(OH)]-Y
560
543
433
429
n.ma
n.m
502
491
n.m
n.m
512
506
n.m
0.33
0.31
0.30
0.29
n.m
n.m
0.21
0.20
n.m
n.m
2.77
2.70
n.m
n.m
2.41
2.41
n.m
15.43
16.20
Table 4 Influence of type of
solvent, type and amount of
catalyst for the oxidation of
benzyl alcohol
Catalyst
Solvent
Catalyst amount (g)
Conversion (%)
Selectivity
Benzaldehyde
Benzoic acid
Zeolite NaY
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
CH2Cl2
CHCl3
MeOH
0.05
0.05
0.05
0.05
0.05
0.05
0.013
0.025
0.075
0.1
Trace
38
66
52
75
60
50
64
71
66
58
52
46
Trace
31
67
51
70
63
73
75
65
59
77
80
81
Trace
69
33
49
30
37
27
25
35
41
23
20
19
Cu(II)/Y
[CuL1(OEt)]-Y
[CuL1(OEt)]
[CuL2(OH)]-Y
[CuL2(OH)]
[CuL2(OH)]-Y
[CuL2(OH)]-Y
[CuL2(OH)]-Y
[CuL2(OH)]-Y
[CuL2(OH)]-Y
[CuL2(OH)]-Y
[CuL2(OH)]-Y
Reaction conditions: benzyl
alcohol (2 mmol), TBHP (in
water) (4 mmol), solvent
(15 ml), temperature (75 °C)
and reaction time (6 h)
0.05
0.05
0.05
Y) and 4 mmol of aqueous TBHP as oxidant in 15 ml of
CH3CN was carried out. The reaction mixture was refluxed
at 75 °C for 6 h. The parent zeolite NaY showed negligible
conversion, confirming the inability of the zeolite frame-
work to catalyze the oxidation of benzyl alcohol. Further-
more, low conversion was observed over Cu(II)/Y,
indicating the key role of the Schiff base complexes in
catalysis. The neat complexes also showed good activity;
however, the conversion was lower than for the encapsulated
complexes, as expected. Also, one of the major drawbacks
of the neat complexes as catalysts was their irreversible
deactivation during the reaction. The color change of the
reaction solution from very pale to very dark brown indi-
cated degradation of the complexes, due to oxidative self-
destruction and aggregation. This problem can be solved by
isolating the Schiff base complexes from each other by
encapsulation in the supercages of zeolite NaY. These
zeolite-encapsulated complexes did not undergo any color
change during the reaction and could be easily separated and
reused many times. Finally, the results showed that the
complex of HL2 (in both free and encapsulated modes) was
more active than the complex of HL1, probably due to the
existence of an additional electron-withdrawing group
(CO2Et) in the structure of HL2, which would decrease the
electron density of the copper center and thus increased the
Lewis acidity and catalytic activity [6].
As illustrated in Table 4, 0.05 g of [CuL2(OH)]-Y gave
the best conversion and selectivity for the oxidation of
benzyl alcohol; higher catalyst amounts resulted in a
deterioration in these parameters, probably due to adsorp-
tion of substrates on the catalyst surface.
The catalytic efficiency of [CuL2(OH)]-Y was studied in
four different solvents (acetonitrile, dichloromethane, chlo-
roform or methanol). All other reaction parameters such as
benzyl alcohol (2 mmol), solvent (15 ml), TBHP (4 mmol,
in water), [CuL2(OH)]-Y (0.05 g) and reaction time (6 h)
were kept constant, and the reaction was carried out under
reflux conditions. The results are depicted in Table 4.
The best performance of catalyst was observed in acetoni-
trile, possibly due to its dielectric constant (e/e0 = 37.5) that
is higher than the other solvents. The polarity of acetonitrile
would increase the solubility of TBHP, which would facil-
itate the reaction. In contrast, methanol with a relatively high
dielectric constant (e/e0 = 32.7) showed the lowest perfor-
mance. It is likely that its oxygen atom would coordinate to
Lewis acid sites of the catalyst, decreasing its activity [40].
Table 5 shows the effect of TBHP concentration on
benzyl alcohol oxidation. Five different molar ratios of
123

440
Transition Met Chem (2014) 39:431–442
oxidant to benzyl alcohol were applied, and all other
parameters were kept fixed.
at 75 °C with constant stirring. As shown in Table 6, no
appreciable change in conversion was observed upon
increasing the reaction time from 7 to 24 h. This new
heterogeneous catalyst showed higher activity than similar
catalysts (with different ligands) reported in the literature
[41, 42].
The time dependence of the benzyl alcohol oxidation
was studied by reacting benzyl alcohol (2 mmol), TBHP
(4 mmol) and [CuL2(OH)]-Y (0.05 g) in CH3CN (15 ml)
We next studied the catalytic oxidation of phenol. Since
the hydroxyl group of phenol is ortho and para directing,
hydroxylation of phenol is expected to give two main
products, namely catechol and hydroquinone. Gas chro-
matographic analysis confirmed that only these two pro-
ducts were formed as expected.
Table 5 Effect of oxidant concentration on the oxidation of benzyl
alcohol
TBHP: BzOH
Conversion (%)
Selectivity
Benzaldehyde
Benzoic acid
In order to study on the effect of substituent and heter-
ogenization of catalysts in the conversion of phenol,
0.025 g of the required catalyst was used with fixed
amounts of 0.05 mol phenol and 0.05 mol H2O2 in 2 ml
acetonitrile, and the reaction mixture was refluxed at 75 °C
for 8 h. The results are summarized in Table 7. The parent
zeolite NaY showed low activity, while Cu(II)/Y had cat-
alytic activity comparable to the encapsulated complexes.
However, the leaching of copper from Cu(II)/Y is a con-
cern, whereas leaching of the encapsulated complexes was
negligible during the reaction as confirmed by atomic
absorption spectroscopy [43]. The complex of HL2 (both
neat and encapsulated forms) with an additional electron-
withdrawing group was more active than the complex of
HL1, probably due to enhanced Lewis acidity of the copper
center. The neat complexes also showed good activity, but
they were completely destroyed during the first run due to
oxidation and aggregation. Decomposition of the neat
complexes was confirmed by the observed color change of
the reaction solution from light to very dark brown. In
contrast, the zeolite-encapsulated complexes showed no
color change during the reaction and could be easily sep-
arated and reused.
1:1
59
71
75
78
80
79
74
70
63
59
21
26
30
37
41
1.5:1
2:1
2.5:1
3:1
Reaction conditions: benzyl alcohol (2 mmol), [CuL2(OH)]-Y
(0.05 g), acetonitrile (15 ml), temperature (75 °C) and reaction time
(6 h)
Table 6 Effect of time on the oxidation of benzyl alcohol
Time (h)
Conversion (%)
Selectivity
Benzaldehyde
Benzoic acid
2
49
63
69
75
79
81
81
80
77
74
70
71
69
65
20
23
26
30
29
31
35
4
5
6
7
8
24
Reaction conditions: benzyl alcohol (2 mmol), TBHP (4 mmol),
[CuL2(OH)]-Y (0.05 g), acetonitrile (15 ml), temperature (reflux)
Table 7 Influence of type of
Catalyst
Solvent
Catalyst
amount (g)
Conversion (%)
Selectivity
Catechol
solvent, type and amount of
catalyst for the oxidation of
phenol
Hydroquinone
Zeolite NaY
Cu(II)/Y
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
CHCl3
CH2Cl2
MeOH
0.025
0.025
0.025
0.025
0.025
0.025
0.013
0.05
31
73
81
58
89
61
62
75
70
74
71
68
65
55
67
63
72
67
51
65
59
65
67
70
35
45
33
37
28
33
49
35
41
35
33
30
[CuL1(OEt)]-Y
[CuL1(OEt)]
[CuL2(OH)]-Y
[CuL2(OH)]
[CuL2(OH)]-Y
[CuL2(OH)]-Y
[CuL2(OH)]-Y
[CuL2(OH)]-Y
[CuL2(OH)]-Y
[CuL2(OH)]-Y
0.075
0.025
0.025
0.025
Reaction conditions: phenol
(0.05 mol), H2O2 (0.05 mol),
solvent (2 ml), temperature
(reflux) and reaction time (8 h)
123

Transition Met Chem (2014) 39:431–442
441
Next, for phenol (0.05 mol) and H2O2 (0.05 mol) in
acetonitrile (2 ml), four different amounts of [CuL2(OH)]-
Y were used for a series of reactions at 75 °C for 8 h
(Table 7). Lower conversion of phenol with 0.013 g of
catalyst may be attributed to fewer catalytic sites. The
greatest conversion was observed with 0.025 g catalyst, but
there was a reduction in performance when more than
0.025 g of catalyst was employed. This can be explained in
terms of faster decomposition of H2O2 in the presence of
excess catalyst [44, 45]. Furthermore, the conversion might
decrease by incrementing the catalyst amount over 0.025 g
because of adsorption of the substrates on the surface, as
explained previously.
Table 8 Effect of oxidant concentration on the oxidation of phenol
Next, four different solvents (acetonitrile, methanol,
dichloromethane or chloroform) were used to investigate
their effect on the phenol conversion, keeping the other
parameters constant: phenol (0.05 mol), H2O2 (0.05 mol),
solvent volume (2 ml), [CuL2(OH)]-Y (0.025 g), time
(8 h) under reflux conditions (see Table 7). Acetonitrile
was found to be the best solvent for the oxidation of phe-
nol. This can be explained by the high polarity of aceto-
nitrile allowing formation of a completely homogeneous
reaction medium. As with the benzyl alcohol oxidation
discussed above, methanol probably coordinates to the
catalyst, reducing its activity [37].
H2O2: PhOH
Conversion (%)
Selectivity
Catechol
Hydroquinone
0.5:1
1:1
63
89
78
69
76
72
67
69
24
28
33
31
2:1
3:1
Reaction conditions: phenol (0.05 mol), H2O2 (0.025, 0.05, 0.1 or
0.15 mol), [CuL2(OH)]-Y (0.025 g), acetonitrile (2 ml), temperature
(75 °C) and reaction time (8 h)
The effect of H2O2 concentration on oxidation of phenol
using [CuL2(OH)]-Y as catalyst is illustrated in Table 8.
Four different molar ratios of oxidant to phenol were used,
keeping the other parameters constant: specifically phenol
(0.05 mol), acetonitrile (2 ml), [CuL2(OH)]-Y (0.025 g),
time (8 h) and temperature (75 °C). The phenol conversion
increased upon increasing the molar ratio of H2O2 to
phenol from 0.5:1 to 1:1, but decreased with higher ratios.
This may be related to more chemisorption of H2O2 and
less chemisorption of phenol on the active sites [37].
Table 9 gives data for the conversion of phenol with
reaction time, under the following conditions: phenol
(0.05 mol), hydrogen peroxide (0.05 mol), acetonitrile
(2 ml), [CuL2(OH)]-Y (0.025 g) and temperature (75 °C).
The phenol conversion attained a steady state after 8-h
Table 9 Effect of time on the oxidation of phenol
Time (h)
Conversion (%)
Selectivity
Catechol
Hydroquinone
2
55
73
78
82
86
89
89
91
79
77
75
76
74
72
70
65
21
23
25
24
26
28
30
35
4
5
6
7
8
10
24
Reaction conditions: phenol (0.05 mol), H2O2 (0.05 mol), [CuL2
(OH)]-Y (0.025 g), acetonitrile (2 ml), temperature (75 °C)
Table 10 Catalytic activity after successive reuse experiments
Substrate
Run
Conversion (%)
Selectivity
Benzaldehyde
Benzoic acid
Catechol
Hydroquinone
BzOH
BzOH
BzOH
BzOH
PhOH
PhOH
PhOH
PhOH
1
2
3
4
1
2
3
4
80
76
72
68
89
86
82
78
72
69
66
61
–
28
31
34
39
–
–
–
–
–
–
–
–
–
72
70
65
62
28
30
35
38
–
–
–
–
–
–
Reaction conditions: benzyl alcohol (2 mmol), TBHP (in water) (4 mmol), [CuL2(OH)]-Y (0.05 g), acetonitrile (15 ml), temperature (75 °C) and
time (7 h) (for benzyl alcohol); phenol (0.05 mol), H2O2 (0.05 mol), [CuL2(OH)]-Y (0.025 g), acetonitrile (2 ml), temperature (75 °C) and
reaction time (8 h) (for phenol)
123

442
Transition Met Chem (2014) 39:431–442
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phenol was higher than reported previously [37, 44].
Catalysts reuse experiments
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13. Salavati-Niasari M, Shaterian M, Ganjali MR, Norouzi P (2007)
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The potential for reuse of [CuL2(OH)]-Y was investigated
for both oxidation reactions (see Table 10). After the first
run (under the optimized conditions), the catalyst was fil-
tered off, washed with acetonitrile several times, dried at
120 °C and used again under similar reaction conditions.
The catalyst could be reused at least four times without any
significant loss in selectivity and activity, confirming that
the zeolite framework keeps the metal complexes dispersed
and prevented from dimerization. The stability of the
recovered catalysts was confirmed by FTIR spectroscopy,
analytical data and atomic absorption spectroscopy. FTIR
spectra of the recycled catalysts were identical to that of
the fresh catalyst. Also, the analytical data for the recycled
catalysts confirmed the absence of significant depletion of
copper content (\0.3 %). Similarly, analysis of the reaction
mixtures by atomic absorption spectroscopy showed neg-
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copper during the reaction was negligible.
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In summary, the results showed that both HL1 and HL2
could be encapsulated in the nanopores of zeolite NaY by
the flexible ligand method. The encapsulated complexes
were effective catalysts for the oxidations of both phenol
and benzyl alcohol. The complex of HL2 (in both free and
encapsulated modes) showed higher activity than the
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group which decreases the electron density of the copper
centers and so increases their Lewis acidity.
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