Catalytic activities of novel silica-supported multifunctional Schiff base ligand & metal complexes under microwave irradiation

Article abstract of DOI:10.1016/j.ica.2014.05.024

In this paper, a novel solid phase supported Schiff base ligand (L ¹: MDPMP) was prepared from 2-hydroxy-5-((2-methoxyphenyl)diazenyl) benzaldehyde (L: HMDB) and silica-gel, activated with 3- aminopropyltriethoxysilane (APTES). Cu (II), Co (II)

Full text of DOI:10.1016/j.ica.2014.05.024

Inorganica Chimica Acta 421 (2014) 1–9
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Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Catalytic activities of novel silica-supported multifunctional Schiff base
ligand & metal complexes under microwave irradiation
⇑
Mecit Ozdemir
Chemistry Department, Faculty of Arts and Science, University of Kilis 7 Aralik, Kilis, Turkey
a r t i c l e i n f o
a b s t r a c t
1
Article history:
In this paper, a novel solid phase supported Schiff base ligand (L : MDPMP) was prepared from
2
Received 27 January 2014
Received in revised form 4 May 2014
Accepted 14 May 2014
-hydroxy-5-((2-methoxyphenyl)diazenyl) benzaldehyde (L: HMDB) and silica-gel, activated with
-aminopropyltriethoxysilane (APTES). Cu (II), Co (II), Ni (II) and Mn (II) complexes of silica-supported
ligand (L : MDPMP) were obtained. The ligand and its four metal complexes were characterized by using
NMR ( H and C), FT-MIR/FIR, elemental analysis, ICP-OES, TGA and Scanning Electron Microscope
SEM). Their catalytic performances in catalytic oxidation of cyclohexane were investigated for the selec-
tive oxidation of cyclohexane under microwave power. Silica-supported L –Cu (II) complex was observed
well selective catalytic properties for the oxidation of cyclohexane to cyclohexanol and cyclohexanone.
Ó 2014 Elsevier B.V. All rights reserved.
3
1
Available online 29 May 2014
1
13
(
Keywords:
Schiff base
Azo-compound
Oxidation
1
Silica-supported
Catalysis
Cyclohexane
1
. Introduction
The condensation reactions of primary amines and aliphatic or
from the oxidation reaction medium [6–15]. The catalytic effects
of the polymer anchored azo-containing complexes demonstrate
quite strong stability. Additionally, they can be used under various
reaction conditions, such as the presence of moisture and high tem-
peratures [9–12]. In the literature, many of the polymer-supported
metal complexes can be used as a catalyst and they exhibit excel-
lent catalytic performance at the hydrogenation and oxidation of
the various organic substrates [4,8,9,12,14,15]. Moreover, silica
gel, as a mesoporous solid, has been extensively studied due to
its catalytic, analytical, optical and electronic properties
[4–9,12,14,15]. Activated silica is easily synthesized binding an
active compound such as 3-aminopropyltriethoxysilane (APTES)
[5–8,12,14–21].
Cyclohexanol (Cy-OH) and cyclohexanone (Cy = O), which are
synthesized by the oxidation reaction of cyclohexane (CyH), are
used as intermediates for pharmaceuticals, plasticizers, rubber
chemicals, cyclohexylamine, pesticides and other organic com-
pounds [22]. For example, cyclohexanol derivative is used as an
intermediate for pharmaceuticals to treat diabetes and other
diseases related to glucose secretion due to its pharmacological
activity. Cyclohexane derivatives are also used in solvents, oil
extractants, paints and varnish removers, dry cleaning materials,
and solid fuels. Each year, over one million tones of cyclohexanone
and cyclohexanol are produced and are usually used for the synthe-
sis of Nylon-6 and Nylon-6,6. Carvalho et al. [25] have worked the
cyclohexane oxidation under microwave conditions by using Fe
aromatic aldehydes to form imine type products, called Schiff base
ligands, are well known and they comprise a broad area of research
in inorganic and organic chemistry. Schiff base ligands are derived
from compounds containing a diazo functional group and their
metal complexes have increasingly become significant for catalysis
reactions based on the synthesis of organic compounds due to their
improved chemical and physical properties [1–5]. Diazonium salts
+
À
with the general formula of R-N X (R: alkyl or aryl; X: inorganic
2
or organic anion) have been generated with the reaction of aryl
amines and nitrous acid at a temperature of 0–5 °C. Diazo com-
pounds have been coupled with aryl-aldehydes under convenient
reaction conditions. These azo-aldehydes can be converted to
primary amines (e.g. 3-aminopropyltriethoxysilane), attached to
2
activated silica-gel (SiO APTES). The new silica-supported multi-
functional azo-containing Schiff base ligands loaded with different
metal ions are useful for obtaining solid phase anchored heteroge-
neous catalysts due to their wide use in catalytic applications [6–8].
In the recent past, researchers have extensively focused on
investigating heterogeneous catalytic reactions. A polymer that
includes a polymeric backbone and a transition metal complex pos-
sesses the advantages of both high selectivity and easy separation
⇑
Tel.: +90 348 0 348 814 26 62; fax: +90 348 814 26 63.
E-mail address: mecitozdemir@kilis.edu.tr
(
III) complex catalysts, hydrogen peroxide as an oxidant and
http://dx.doi.org/10.1016/j.ica.2014.05.024
020-1693/Ó 2014 Elsevier B.V. All rights reserved.
0

2
M. Ozdemir / Inorganica Chimica Acta 421 (2014) 1–9
acetonitrile as a solvent, and Cy–OH, Cy@O and adipic acid were
obtained as major products with a good CyH conversion and
Cy–OH and Cy@O selectivity. In recent progress on catalysis, the
metal-complexes have shown good catalytic activities with good
selectivity for the oxidation of cyclohexane and cyclohexene with
3.57 ppm [s, 2H, OH
163.9 [Ar–C–OH], 156.9 [Ar–C–OCH
133.0–113.8 [Ph–C], 56.4 ppm [O–CH
3173 (w, Ar–OH); 2968 (w, Ar–H); 2941 (w, R–H), 1652 (s, C@O),
1591–1581(s–sh, Ar–C@C), 1487 (s, –N@N–), 1023 (br, C–H);
1023–1043 (s–sh, C–OH), 892 (m, Ar–H). TG/DTA Data: No weight
loss was observed below decomposition temperature. Decomposi-
tion began at 155 °C and two endothermic peaks were between
155–305 °C (82.79% loss) and 305–805 °C (12.97% loss).
2
]. 13C NMR (DMSO-d
6
, 25 °C): d 191.1 [CHO],
3
], 145.8, 141.78 [Ar–C–N–],
À1
3
]. FT-IR (
m
, cm ): 3538–
2
H O
2
oxidant under microwave irradiation [23–25].
In this work, we have prepared a novel azo-containing solid
phase, silica-supported Schiff base ligand and its four metals com-
plexes: Cu(II), Ni(II), Co(II), and Mn(II). NMR, FT-MIR/FIR, elemental
analysis, ICP-OES, TGA and a scanning electron microscope (SEM)
were used for the characterisation of the ligand and its metal com-
2.3. Preparation of silica-supported 3-aminopropyltriethoxysilane
(SiO -APTES)
2
1
plexes. In this study, the synthesis of heterogeneous four L –Cu(II)/
Co(II)/Mn(II)/Ni catalysts and their catalytic activities in the oxida-
tion of cyclohexane to cyclohexanol and cyclohexanone were
reported. Furthermore, the recovery, reusability and, finally, the
recyclability of these novel catalysts were investigated under the
same reaction conditions.
The silica gel (50 g) was refluxed with an excess amount of
hydrochloric acid (6 M) for 8 h, then filtered off and washed with
an appropriate amount of deionized water until the filtrate was
neutral. Activated silica was dried at 150 °C for 12 h. Then, 20 g
of activated silica was suspended in 100 mL toluene, and 20 mL
3
-aminopropyltriethoxysilane (APTES) was added to the suspen-
2
. Experimental
sion. The reaction mixture was refluxed for 72 h. The suspension
was filtered and the filtered solid was washed with an excess
amount of toluene, ethanol and diethyl ether, respectively
2.1. General
All the reagents and organic solvents were purchased from
[
5,7,8,12,14–20]. Elemental Anal.: C, 8.97; H, 2.32; N, 3.3941%.
À1
commercial sources and used as received, unless noted otherwise.
The NMR spectra were recorded at 25 °C in DMSO-d and CDCl
using a Bruker 400 MHz Ultrashield TM NMR spectrometer. The
FT-MIR and FAR spectra were obtained using a Perkin Elmer Spec-
trum 400 FT-IR system in the range of 4000–30 cm . Elemental
FT-IR (
2
m
, cm ): 3696–3319 (br, Si–OH); 3319–3118 (br, –NH
2
),
6
3
941 (w, R–H); 1056 (br, Si–O). TG/DTA Data: % 3.37 water or sol-
vent loss was observed between 30 and 155 °C. Decomposition
began at 155 °C and continued to 900 °C as one endothermic peak
with a 8.64% loss.
À1
analysis was performed using an LECO CHNS 932 instrument.
The metal contents were determined using Perkin Elmer Optima
2
2.4. Preparation of silica-supported azo containing Schiff base (SiO -
1
2
100 DV ICP-OES. A JEOL NeoScope Benchtop scanning electron
APTES-HMDB: L )
microscope (SEM) was used to scan the images. The thermal anal-
ysis studies of the complexes were performed on a PerkinElmer
Pyris Diamond DTA/TG thermal system under nitrogen atmosphere
5
g. SiO
tion in 100 mL EtOH (96%) and then was refluxed at 60 C for
2 h. The brown solid was filtered, washed with excess amount
2
-APTES was added to HMDB:L (1.28 g., 5 mmol) solu-
o
o
at a heating rate of 10 °C/min in the range of 30–900 C. Kieselgel
1
gel 60 (Merck) silica, having a particle size of 0.2–0.5 mm, was used
as solid support for the heterogeneous catalysts. The microwave
experiments, which were conducted in closed DAP60 vessels, were
carried out in a Berghof MWS-3+ (Germany) equipped with pres-
sure and temperature control. The synthesized products were
characterized and analyzed by using a Perkin Elmer Clarus 600
GC (USA) spectrometer equipped with an MS detector fitted with
an Elite-5 MS and FID detector fitted with BPX5 capillary columns.
of EtOH and dried at 95 °C for 12 h. Elemental Anal.. C, 15.08; H,
À1
2
.37; N, 2.47%. FT-IR (
m, cm ): 3660–3186 (br, –OH); 2957 (w,
Ar–H); 2941 (w, R–H), 1622 (s, CH@N), 1487 (s, –N@N–), 1055
(
br, Si–O).). TG/DTA Data: No weight loss was observed below
decomposition temperature. Decomposition began at 180 °C and
two endothermic peaks were between 180 and 380 °C (6.70% loss)
and 380–900 °C (12.83% loss).
2.5. Preparation of the complexes
2.2. Preparation of 2-hydroxy-5-((2-methoxyphenyl)diazenyl)benzalde-
hyde (HMDB:L)
Four metal complexes were synthesized by the addition of
1
mmol metal salts (anhydrous CuCl
2
, CoCl
2
Á6H
2
O, NiCl
2
Á6H
2
O
1
Next, 2-methoxyaniline (o-Anisidine) (0.9852 g, 8 mmol) was
and Mn(OAc)
2
) to the 2 g silica-supported Schiff base ligand (L :
mixed with 10 mL NaNO
37%) was added until the pH reached 2. The mixture was stirred
for 30 min. Salicylaldehyde (2-hydroxybenzaldehyde) (0.9770 g,
mmol) was dissolved in 20 mL pH:12 buffer solution containing
NaOH (20 mmol) and Na CO (40 mmol). The diazonium solution
was added very slowly to the salicylaldehyde solution. While add-
ing, the temperature was kept at 0–2 °C and the pH was kept at
around 7–8. The reaction mixture was stirred in the ice bath for
2
solution (0.6210 g, 9 mmol), then HCl
MDPMP) in EtOH. The mixture was refluxed for 24 h at 50 °C. After
stirring, the complexes were filtered and washed with excess
amount of water, finally dried in vacuum at 70 °C. Elemental anal-
ysis (C, H, N %), ICP-OES (Metal % content after microwave diges-
tion), FT-MIR/FIR, TGA and SEM techniques were used for the
characterization of the complexes.
(
8
2
3
2.6. Preparation of ICP-OES Samples
2
h. All the reaction steps were carried out in an ice-water-salt bath
at a temperature ranging from 0 to 2 °C. The dark orange product
was filtered and recrystallized in EtOH:H O (1:1). Several portions
The complexes were prepared in a Berghof MWS-3+ microwave
before the ICP analyses were conducted. A suitable amount of the
2
of diethyl ether were used to remove the organic impurities from
3
sample, HNO and HCl, was added to the DAP60 vessel and suitable
the product. Then the product was dried under vacuum at 60 °C
oven conditions were applied (pressure, temperature and micro-
wave power). The digested suspensions were filtered and diluted
with an appropriate amount of ultra-pure water. Consequently,
Sample solutions were analyzed by inductively coupled plasma/
optical emission spectrometry (ICP/OES) to determine metal con-
tent (Co, Cu, Ni, Mn).
for 12 h. Yield 2.053 g (78%). [C14
N 10.93%; anal. C 65.38, H 4.64, N 10.43%. H NMR (DMSO-d
5 °C):d 10.38 [s, 1H, CHO], 8.05 and 8.12 ppm [d, 2H, Ar–H(o-/p-
], 7.5 ppm [d, 2H, Ar–H(o-/m-)], 7.19 and 7.05 ppm [t, 2H,
Ar–H(m-/p-)], 7.0 ppm [s, 1H, Ar–H(o-)], 3.96 [s, 3H, Ar–C–OCH ],
12 2 3
H N O ]: calcd C 65.62, H 4.72,
1
6
,
2
)
3

M. Ozdemir / Inorganica Chimica Acta 421 (2014) 1–9
3
2.7. Catalytic Oxidation of Cyclohexane under Microwave Irradiation
the –CH@N– and the –OH groups. To the determine the coordina-
tion sites that might be involved in chelation, the IR spectra of the
1
The catalytic oxidation of cyclohexane under microwave
metal complexes were compared with the free ligand ‘‘L ’’. The
irradiation was performed as follows: 0.02 mmol synthesized com-
plex, 2 mmol cyclohexane (Carlo Erba, 99.8%) and 4 mmol H
Merck, 35%) were microwaved for 75 min at 400 watt (40% of
maximum output power). The catalyst:substrate:oxidant ratio
was 1:100:200. The complexes were individually suspended in
intensities of these peaks were observed to be changed upon che-
À1
2
O
2
lation. The band at around 1635 cm was assigned to the
stretching vibrations of the azomethine of the novel ligand ‘‘L ’’
(Table 1). This band was shifted in range from 13 to 22 cm
m
(C@N)
1
(
À1
a
lower wavenumber which showed the participation of the azome-
1
5
mL of acetonitrile, and cyclohexane and H
2
O
2
were added to
thine group of ligand ‘‘L ’’ in binding to the meta ions. The coordi-
the microwave vessels, respectively, for each of the oxidation
experiments. The vessels were rapidly closed with their captures
and the caps and placed inside the Berghof MWS3+ microwave
oven. For each catalytic oxidation experiment, 400 watt microwave
power was applied for 75 min. The temperature was controlled
automatically by the microwave instrument at about 110 °C; how-
ever, sometimes the temperature increased to 130–140 °C in a
short time during the reaction in the microwave oven and the
pressure also increased to 30–35 bar, due to the evaporation of
the solvent. In order to stop the oxidation before analysis, 1 mL
nation of the halides or the acetate in the complexes was
investigated further using qualitative methods, such as the
argentometric method for halide coordination [5,9,11,12]. The
À1
quite weakbands in the two ranges (678–536 cm ) and (586–
À1
455 cm ) were related to the stretching frequencies of the
2
m(M–N) and m(M–O) bands demonstrating the attached of H L
ligand to the center metal ions through the phenolic oxygen atoms
and the azomethine nitrogen, respectively [26]. Herein, new M–O
and M–N absorption bands in the FIR spectra were respectively
À1
observed at about 449 and 522 cm , which confirmed that the
1
2
H O was added to the vessels. The oxidized organic products were
metal ions bonded to L ligand (Table 1). In addition, the mono-
extracted with 10 mL CH Cl and injected into GC and GC–MS for
2
2
chloro copper (II), nickel (II) and cobalt (II) complexes exhibited a
analysis and characterisation. The amount of CyH, Cy–OH, and
Cy@O were calculated from the external calibration curves that
were prepared before the analyses.
characteristic frequencies
around 365 cm (Fig. 2) [27]. In the FT-IR spectrum of Mn(L )
m
M–Cl(M = C (II), Co (II), and Ni (II))
À1
1
(OAc)(H
2
O), the new carbonyl (C@O) stretch of the acetate ligand
À1
was observed at 1634 cm (Fig. 2).
1
13
The H/ C NMR spectra were evaluated for further character-
isation of HMDB. The singlet peak of the aldehyde hydrogen of L
was observed at 10.38 ppm. The two doublet peaks of L were
observed at 8.12 and 8.05 ppm attributed to para- and meta- posi-
tions hydrogens of salicylaldehyde. The two doublet peaks of L
were observed at 7.5 ppm related to orto- and meta- positions
hydrogens of 2-methoxyaniline. The two triplets of L were
observed at 7.19 and 7.05 ppm assigned to other hydrogens of
2-methoxyaniline. The singlet peak was also seen at 7.0 related
to second orto-position hydrogen of salicylaldehyde. The broad sig-
nal due to water in DMSO was recorded at 3.57 ppm. The aryl-
methoxy protons were recorded at 3.96 ppm as a singlet peak. In
3
. Results and discussion
3.1. Preparation of silica-supported ligand and complexes
1
[
SiO
2
-APTES-HMDB] (L ) novel Schiff base ligand was synthe-
sized using 2-hydroxy-5-((2-methoxyphenyl)diazenyl) benzalde-
hyde [HMDB:L] and activated silica [SiO -APTES] according to
route in the reported studies [5,7,12]. [Cu (L )(H O)Cl] (3), [Co(L )
2
2
1
1
1
1
(
H
2
O)Cl] (4), [Ni(L )(H
complexes were prepared using anhydrous CuCl
NiCl O, and Mn(OAc) salts. The chemical structures of L and
Á6H
and possible structures of the L -metal complexes are shown
2
O)Cl] (5), and [Mn(L )(OAc)(H
2
O)] (6)
Á6H O,
2
, CoCl
2
2
2
2
2
1
1
13
L
order to obtain further information, C NMR shifts were assigned
for HMDB:L. The singlet peak of the –CHO carbon was seen at
194.60 ppm. The multiple aromatic carbons’ peaks were shown
1
in Scheme 1. The colors of L and the complexes have brown tones.
Synthesized ligands and their complexes were characterized by
FT-MIR/FIR infrared spectra, as shown in Figs. 1 and 2. A broad(br)
3
at 163.94 ppm for Ar–C–OH, 156.92 for Ar–C–OCH , 145.80 and
141.80 ppm for Ar–C–N@N–C–Ar and 133.03–113.84 for the other
À1
band in the range 3696–3319 cm
and a sharp(sh) peak at
À1
1
056 cm
can be attributed to the silanol-OH group stretches
-APTES
Ar-carbons. The singlet signal of the aryl-methoxy peak was attrib-
1
13
and the Si-O-Si bonds, respectively, in the spectrum of SiO
2
À1
uted at 56.42 ppm. These H NMR and C NMR data support the
formulation of a synthesized compound ‘‘HMDB:L’’] [5,7–13].
Elemental analysis techniques (percentages of C, H, N and
metal) were also used for further characterisation of the ligands
and the complexes. While the C and H percentages increased, the
[
5–8,11–13]. The sharp band between 3319 and 3118 cm is the
stretch of the –NH
2
group. The aliphatic –CH
2
vibrations can be
À1
attributed at 2941 cm . The absorbance band of the –OH group’s
1
À1
band of ligand L begins at 3670 cm , and finishes at the begin-
ning of the aliphatic and aromatic vibrations at around
N percentage decreased after binding L to SiO
2
-APTES. These differ-
À1
2
968 cm . The Ar–OH, Ar–H and R–H absorptions for ligand L
ences confirm the conversion of L to SiO -APTES and Schiff base
2
À1
À1
(
a) were observed at about 3538–3173 cm , 2968 cm
and
formation. The metal content of the complexes was also deter-
mined using ICP-OES. According to the metal analyses results,
À1
À1
2
1
3
941 cm , respectively. The –N@N– stretch was assigned at
1
487 cm . The –NH
329–2980 disappeared when L was bound to SiO
2
band of the SiO
2
-APTES in the range of
-APTES (Supple-
the metal absorption capability of the silica-supported ligand (L )
1
2
is good enough and the L :M ratio is 1:1 (Table 1).
1
mentary Fig. S5). Nevertheless, the –CH@N– stretch of L was
observed at 1635 cm as a strong(s) band in the infrared spectra.
2
The SEM images of SiO -APTES and the complexes of some
À1
metal ions(Co(II), and Ni(II)) are shown in Fig. 3. The morphological
differences between the silica gel and the complexes in the SEM
images are very important proof of the loading of the complexes,
This band proves the condensation reaction of the carbonyl groups
with the -NH groups of SiO -APTES. The FT-IR spectra of the silica-
2 2
supported complexes show significant differences from the silica-
supported ligand L . The –CH@N– stretching frequencies of the
both onto the SiO
L, which diffuses through the SiO
ions, well-dispersed complexes are obtained through the SiO
channels and over the SiO particles. The well-dispersed silica-sup-
ported Cu-L complex showed good catalytic activity as a result of
having a large surface on the silica particles and also being dis-
persed into the cavities as part of the internal and external surface
clusters, as seen in Fig. 3 [6,9,12,13,15,28].
2
particles and into the cavities as clusters. When
1
2
channels, reacts with the metal
complexes were slightly shifted to lower frequencies compared
2
1
with L (Fig. 1) [5,7–9,11–13]. Due to the fact that the –N@N– fre-
2
1
quencies of the complexes did not change, it can be said that the
azo-groups have not coordinated to any of the metal cations. When
all the characterisation data is combined and evaluated, it can be
concluded that the L1 to the metal ions only coordinated with

4
M. Ozdemir / Inorganica Chimica Acta 421 (2014) 1–9
Scheme 1. Synthesis of HMDB:L, L1 and the possible coordination of metals to L1 (M2+ = Cu2+, Co2+, Ni2+, and Mn2+).
Thermal properties of the ligands and the complexes were
investigated by using thermogravimetry (TG) (Supplementary
Fig. S4).
complexes [5,7–9,12,13]. The 2.28% loss in the TGA curve of the
Mn(L )(OAc)(H O) complex between 30 and 165 °C can be attrib-
2
uted to the coordinated water. In addition, the acetate ligand in
1
According to the TGA curves, L did not have water or solvent loss
before the decomposition temperature was applied. Two endother-
the Mn (II) complex increased the decomposition percentage when
the temperature ranged between 165–800 C in the TGA curve (Sup-
o
mic peaks were formed between 155–305 °C (82.79% loss) and
plementary Table S1). When the thermogravimetric data is evalu-
ated, it can be said that the thermal properties of the complexes
are very similar in character (Table S1).
1
3
05–805 °C (12.83% loss). While ligand (L ) has two mass loss peaks
in the TG curves, the complexes show three mass loss peaks. The sil-
1
ica-supported ligand (L ) did not lose any water molecules between
When the FT-MIR/FIR spectral values, elemental analysis (per-
centages of C, H, N and metal) and other chemical characterisations
are combined, it can be concluded that the complexes are tetra-
coordinated. Two coordination sites of the metal cations in the
complexes had one halogen ligand and one aqua ligand for each
3
0 and 180 °C and decomposition started at 180 °C and continued
to 900 °C in the TG curves. When, the results of the elemental anal-
yses and TG curves are combined, it can be concluded that HMDB(L)
1
and L do not have any hydrated or adsorbed/absorbed water. The
1
1
1
main loss, which was decomposition of the supported ligand and
the formation of the metal oxides, could be detected between
Cu–L , Co–L and Ni–L complex and one acetate ligand and one
1
water ligand for the Mn–L complex. Having a bulk structure of a
silica-supported ligand ‘‘L ’’ does not allow for a 1:2 ratio for the
1
1
50–375 °C and 375–800 °C, respectively, in the TG curves of the

M. Ozdemir / Inorganica Chimica Acta 421 (2014) 1–9
5
1
02,1
00
1
9
9
9
9
9
8
8
8
8
8
7
7
7
7
7
6
6
6
6
8
6
4
2
0
8
6
4
2
0
8
6
4
2
0
8
6
4
2
1
—
—
L
%
T
1
Cu-L1
— Co-L1
— Ni-L
1
—
Mn-L
6
0,3
000,0
4
3600
3200
2800
2400
2000
1800
1600
1400
1200
1000
800
600
450,0
-1
cm
Fig. 1. FT-IR spectra of the ligands and the complexes.
1
10,0
—
—
—
—
Cu-L1
1
1
05
00
Co-L
1
Ni-L
Mn-L1
1
9
5
0
5
0
5
0
5
0
9
8
8
7
7
6
6
%
T
5
5,0
00,0 590 580 570 560 550 540 530 520 510 500 490 480 470 460 450 440 430 420 410 400 390 380 370 360 350 340 330 320 310 300,0
6
-1
cm
Fig. 2. FT-FIR spectra of the complexes.

6
M. Ozdemir / Inorganica Chimica Acta 421 (2014) 1–9
Table 1
Chemical composition and characteristic MIR and FIR peaks of the ligands and the complexes.
a
Compound
C %
H %
N %
Metal loading Metal analysis
m
OH
m
Ar–H
m
R–H
m
CH@N
m
N@N
m
Si–O
m
M–N
m
M–O
À1
À1
(
mmol g
)
(mmol g )
SiO
L
L
2
-APTES
8.97
2.32
3.39
–
–
–
0.50
0.50
0.50
0.50
–
–
–
0.17
0.22
0.21
0.23
3696–3318
3538–3173 2968 2941
3660–3186 2957 2941 1635 1487 1055
3663–3186 2958 2942 1622 1487 1055 522
3662–3186 2958 2941 1613 1487 1055 522
3661–3186 2956 2941 1615 1486 1055 523
3661–3185 2958 2941 1615 1487 1055 523
–
2941
–
–
–
1487
1056
–
–
–
–
–
–
–
449
449
450
449
65.38 (65.62) 4.64 (4.72) 10.43 (10.93)
16.32
13.77
13.61
12.19
1
2.40
2.28
2.26
2.41
2.29
3.65
3.18
3.10
2.77
3.11
1
[
[
[
[
Cu(L )(H
Co(L )(H
Ni(L )(H
2
O)Cl]
O)Cl]
O)Cl]
Mn(L )(OAc)(H
1
2
1
2
1
2
O)] 14.01
a
Metal Analyses were performed by using ICP-OES. Solid-supported complexes were digested with MWS3+ microwave digestion system. Figure in parenthesis indicates
the calculated value.
(
a)
(b)
(
c)
1
1
Fig. 3. Scanning electron microscopy images of silica-gel (a) and the complexes, [Co(L )(H
2
O)Cl] (b), [Ni(L )(H
2
O)Cl] (c). (Accelerating voltage: 10 kV, vacuum: high).
M:L. These probable structures are in accordance with our previous
studies and the findings from other literature [5,7–9,11–13].
are shown in Fig. 4. The catalytic oxidation tests under microwave
irradiation were applied according to our previous studies [12,13].
It has been proposed that the microwave power and the novel
catalysts could affect the selective oxidation of CyH to Cy–OH
and Cy@O. The reaction time, oxidant amount, catalyst amount
and microwave oven parameters were tested in order to determine
the optimum catalytic oxidation parameters under microwave
irradiation [12,13]. The optimum oxidation conditions were
3.2. Catalytic oxidation of cyclohexane under microwave irradiation
The first step, which is also the slowest step, is the oxidation of
CyH to Cy–OH; after that, Cy@O and other further oxidized prod-
ucts arise, according to the possible oxidation mechanism. If the
first step of the oxidation reaction can be controlled, the selectivity
of the desired products, Cy–OH and Cy@O, can be increased. The
possible reaction mechanism and the possible reaction products
obtained as follows:
a
catalyst:substrate:oxidant ratio of
1:100:200 in acetonitrile under 400 watt microwave power for
75 min. The temperature and pressure were controlled at about

M. Ozdemir / Inorganica Chimica Acta 421 (2014) 1–9
7
Fig. 4. Possible mechanism of catalytic oxidation of cyclohexane (X: Cl , OAc- or H
-
O).
2
1
10 °C and 30 bar, by the instrument. These optimum parameters
nanocomposite materials used for cyclohexane oxidation. They
found the conversion (mmol%) of cyclohexane in range from 8.7
to 40.3 [31]. Silva et al. prepared some Schiff base metal complexes
to investigate for cyclohexane oxidation to cyclohexanone and
cyclohexanol. Hence, they used the Fe (III), Fe (II), VO (IV) and Cu
(II) complexes and found significant cyclohexane conversion and
selectivity toward Cy–OH and Cy@O (Yield, 46%) [32]. Shylesh
et al. prepared organic–inorganic hybrid mesoporous silica materi-
als containing chromium and various organo trialkoxysilanes
(chloropropyl, vinyl, methyl) and used them for oxidation reaction
of cyclohexane. Cyclohexane oxidation reactions over various
chromium catalysts were conducted and found the conversion
(mol%) of cyclohexane in range from 3.4 to 10.2 according to the
catalytic activity results of the mesoporous chromium catalysts.
However, they reported high selectivity toward Cy–OH and Cy@O
(29 and 63 mol%) [33]. In this paper, the best selectivity of the
desired products was obtained with a silica-supported Cu (II)
were applied in all the catalytic experiments. A control group has
been run under the same conditions without any catalyst (Table 2).
Classical thermal oxidation studies require a great length of
time and the reaction control is very difficult. However, microwave
oxidation decreases the reaction time and increases selectivity
[
12,13,22–25,28,29]. Thus, microwave power is very easy and
clean to use for organic and inorganic syntheses. When compared
to previous results, in the current study, the new synthesized cat-
alysts are not quite effective on the oxidation of cyclohexane. On
the other hand, these complexes showed good selectivity. It has
been shown that the Cu (II), Co (II), Ni (II), and Mn (II) complexes
of the ligands possess a catalytic effect. Cy–OH and Cy@O selectiv-
ity of the Cu (II) catalyst using microwave power is considerable
when compared with the classical oxidation of CyH in the
literature [9,10,12,13,23–29,30]. Niasari et al. synthesized a novel
host (zeolite-Y)/guest (binuclear transition metal complexes)
Table 2
2 2
Catalytic oxidation of cyclohexane (CyH) with H O
under microwave irradiation.a
Entry
Catalyst
Blank
CyH conversion (mmol%)
Desired products (mmol%)
ol:one
By products (mmol%)
b
1
0.86
42.99
11.85
7.31
Cy–OH (0.54); Cy@O (0.32)
Cy–OH (6.73); Cy@O (4.87)
Cy–OH (0.30); Cy@O (0.59)
Cy–OH (3.43); Cy@O (1.49)
Cy–OH (0.68); Cy@O (0.15)
1.72
1.38
0.51
2.30
4.53
–
c
1
2
[Cu(L )(H
2
O)Cl]
O)Cl]
O)Cl]
31.39
10.96
2.39
4.88
c
1
3
[Co(L )(H
2
c
1
4
[Ni(L )(H
2
c
1
5
[Mn(L )(OAc)(H
2
O)]
5.71
a
400 watt power were applied for 75 min. The reaction temperature and pressure were held at around 110 °C and 30 bar in closed DAP60
vessels.
b
2
0
mmol cyclohexane: 4 mmol hydrogene peroxide and 5 mL acetonitrile were used in experiment without catalyst.
.02 mmol catalyst:2 mmol cyclohexane:4 mmol hydrogene peroxide (1:100:200) and 5 mL acetonitrile were used for each reaction.
c

8
M. Ozdemir / Inorganica Chimica Acta 421 (2014) 1–9
Fig. 5. Influence of the complexes in cyclohexane oxidation under microwave power. 0.02 mmol complex:2 mmol cyclohexane:4 mmol hydrogen peroxide (1:100:200) and
mL acetonitrile were used for each reaction. 400 watt power were applied for 75 min. The reaction temperature and pressure were held at around 110–140 °C and 30 bar in
5
closed DAP60 vessels.
complex, whereas the mmol percentages of the Cy–OH and Cy@O
products are not quite as good (Fig. 5). The Ni (II) complex has a
TGA and a scanning electron microscope (SEM). The characterisa-
tion data confirmed the possible structure of the ligand and the
complexes. To proceed, a clean and green oxidation of cyclohexane
to cyclohexanol and cyclohexanone under microwave irradiation
7
.31% cyclohexane conversion; however, its selectivity is not good
enough because other further oxidized products arise. The catalytic
effects of the Co (II) and Mn (II) complexes are approximately the
same as the catalytic effect of the Ni (II) complex.
2 2
using H O as the oxidant was investigated by virtue of this novel
solid phase-supported ligand containing an azo-group and its four
transition metal complexes. As a result, it was found that the
Cu-complex had selectivity towards the oxidation of cyclohexane
to cyclohexanol and cyclohexanone with quite effective yields
(6.73% and 4.87%) and with 42.99% conversion. In addition to
showing fairly catalytic activity, these catalyst systems are easy
to prepare. Moreover, in comparison to other systems they offer
better advantages, such as the recovery, reusability and recyclabil-
ity of the catalysts. Those characteristics were examined five times
at the oxidation of cyclohexane and no significant changes were
found in the catalytic activity. Finally, we believe that the most
significant thing explained in this study is that it provides a new
strategy for the design of novel azo-containing silica-supported
catalysts.
1
The new ligand ‘‘L ’’ has N and O donor atoms. The N and O atoms
can promote a proton shift [30]. This situation may affect the oxida-
tion reaction. The key point in the conversion of cyclohexane to the
1
1
oxidized products is the reduction of Cu (II)–L to Cu (I)–L . This
1
reduction to Cu (I)–L is facilitated by the ligands that are available
around the metal cations. The formation of the oxidation products,
Cy–OH and Cy@O, show the preferential attack of the activated
bonds. The high flexibility of Cu (I) complexes showing different
geometries allows four-coordinated structures such as tetrahedral
or trigonal-monopyramidal. Cu (II) complexes generally have
square-planar or square-pyramidal geometries [34–36]. The tetra-
hedral flexibility of copper (I) compound facilitating the reduction
to Cu (I) of the square-planar Cu (II) complex may be the reason
why it has a higher selectivity than the other complexes.
4
. Conclusion
Acknowledgements
In recent work, modified silica-supported azo-containing Schiff
I’ve been grateful to Assist Prof. Dr. Serhan URUS for letting me
use instrumental analysis facility at Research and Development
Centre For University–Industry–Public Relations (USTKIM) Sutcu
Imam University, Kahramanmaras, Turkey.
base and its Cu (II), Co (II), Ni (II) and Mn (II) complexes were syn-
thesized, and then their structural determinations were carried out
1
13
using NMR ( H and C), FT-MIR/FIR, elemental analysis, ICP-OES,

M. Ozdemir / Inorganica Chimica Acta 421 (2014) 1–9
9
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Supplementary (Supplementary material data contain the NMR
(1
H and C) spectra of the compound HMDB:L, Scanning electron
13
1004.
microscopy images of the other complexes, TG of the silica-sup-
[
[
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