Chemically modified silica-gel with an azo-schiff ligand and its metal complexes with Cu(II), Co(II), Ni(II) and Mn(II): Applications as catalysts on the oxidation of cyclohexane under microwave power

Article abstract of DOI:10.1080/15533174.2011.611208

A novel 2-hydroxy-5-((2-hydroxyphenyl)diazenyl)benzal- dehyde (L ¹: HPDB) ligand was synthesized and bound to silica gel, which was activated with 3-aminopropyltriethoxysilane. Cu(II), Co(II), Ni(II), and Mn(II) complexes of silica-supported ligand (L²: MDPMP) were synthesized. A buffer (pH = 12) was used for a coupling reaction of diazonium electrophile to salicylaldehyde. The ligand and its complexes were characterized by using NMR, FT-MIR/FAR, elemental analysis, ICP-OES, TGA, and scanning electron microscopy. Catalytic properties of the complexes were investigated for the selective oxidation of cyclohexane under microwave power. Silica-supported L ²-Cu(II) complex showed well selective catalytic activity for the oxidation of cyclohexane to cyclohexanol and cyclohexanone with 8.40% and 3.77% yields with 48.43% conversion. Copyright Taylor & Francis Group, LLC.

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Synthesis and Reactivity in Inorganic, Metal-Organic,
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Chemically Modified Silica-Gel With an Azo-Schiff
Ligand and Its Metal Complexes With Cu(II), Co(II), Ni(II)
and Mn(II): Applications as Catalysts on the Oxidation of
Cyclohexane Under Microwave Power
Serhan Uruş a b , Mecit Özdemir c , Gökhan Ceyhan a & Mehmet Tümer a
a Chemistry Department, Faculty of Science and Letters , Kahramanmaraş Sütçü İmam
University , Kahramanmaraş , Turkey
b Research and Development Centre For University-Industry-Public Relations,
Kahramanmaraş Sütçü İmam University , Kahramanmaraş , Turkey
c Chemistry Department, Faculty of Science and Letters , Kilis 7 Aralık University , Kilis ,
Turkey
Accepted author version posted online: 14 Mar 2012.Published online: 24 Apr 2012.
To cite this article: Serhan Uruş , Mecit Özdemir , Gökhan Ceyhan & Mehmet Tümer (2012) Chemically Modified Silica-
Gel With an Azo-Schiff Ligand and Its Metal Complexes With Cu(II), Co(II), Ni(II) and Mn(II): Applications as Catalysts on the
Oxidation of Cyclohexane Under Microwave Power, Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal
Chemistry, 42:3, 382-391, DOI: 10.1080/15533174.2011.611208
To link to this article: http://dx.doi.org/10.1080/15533174.2011.611208
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Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 42:382–391, 2012
Copyright ꢀ Taylor & Francis Group, LLC
C
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2011.611208
Chemically Modified Silica-Gel With an Azo-Schiff Ligand
and Its Metal Complexes With Cu(II), Co(II), Ni(II) and
Mn(II): Applications as Catalysts on the Oxidation of
Cyclohexane Under Microwave Power
3
1
1
Serhan Urus¸,1,2 Mecit Ozdemir, Go¨khan Ceyhan, and Mehmet Tu¨mer
¨
1
˙
¨
¨
Chemistry Department, Faculty of Science and Letters, Kahramanmaras¸ Sutc¸u Imam University,
Kahramanmaras¸, Turkey
2Research and Development Centre For University-Industry-Public Relations, Kahramanmaras¸
˙
¨
¨
Sutc¸u Imam University, Kahramanmaras¸, Turkey
3Chemistry Department, Faculty of Science and Letters, Kilis 7 Aralık University, Kilis, Turkey
Diazonium compounds can be bound to aromatic rings with
electrophilic substitution reaction.[1–3] Reactions between pri-
mary amines and aldehydes form the products called as Schiff
base ligands that comprise a huge area in inorganic and or-
ganic chemistry. Functionalized Schiff base ligands and their
metal complexes have increasingly become important in cat-
alytic synthesis of organic compounds because of their improved
chemical and physical properties.[4–8] Aryl-aldehydes coupled
with diazo compounds can be bound to silica gel activated
with a primary amine such as 3-aminopropyltriethoxysilane
forming silica-supported azo-Schiff bases. Silica-supported
azo-Schiff base ligands can be loaded with different metal
ions.[1–3]
A
novel 2-hydroxy-5-((2-hydroxyphenyl)diazenyl)benzal-
dehyde (L1: HPDB) ligand was synthesized and bound to silica gel,
which was activated with 3-aminopropyltriethoxysilane. Cu(II),
Co(II), Ni(II), and Mn(II) complexes of silica-supported ligand
(L2: MDPMP) were synthesized. A buffer (pH = 12) was used for
a coupling reaction of diazonium electrophile to salicylaldehyde.
The ligand and its complexes were characterized by using NMR,
FT-MIR/FAR, elemental analysis, ICP-OES, TGA, and scanning
electron microscopy. Catalytic properties of the complexes were
investigated for the selective oxidation of cyclohexane under
microwave power. Silica-supported L2-Cu(II) complex showed
well selective catalytic activity for the oxidation of cyclohexane
to cyclohexanol and cyclohexanone with 8.40% and 3.77% yields
with 48.43% conversion.
The studies on heterogeneous catalysis have replaced with
homogeneous catalysis studies due to the fact that the polymer-
supported metal complexes have high selectivity and they can
be separated easily from the reaction mixture in the oxida-
tion of organic compounds.[6–15] Besides, the catalytic activities
of polymer-supported azo complexes are stable even if in the
presence of moisture and in high-temperature reactions.[9,10–12]
There are numerous metal-azo-Schiff base complexes that are
supported and some polymers exhibit excellent catalytic activ-
ities in oxidation and hydrogenation of various organic sub-
strates.[4,8,9,12,14,15]
Keywords azo ligand, catalysis, cyclohexane, oxidation, silica-
supported, Schiff base
INTRODUCTION
Diazonium salts are the compounds that are shown as R-
N2+X− general formula (R: alkyl or aryl; X: inorganic or or-
ganic anion). Diazonium salts are obtained with the reaction
of arylamines and nitrous acid at the temperature of 0–5◦C.
There are two different approaches to support a complex
to a solid. One is the use of organic polymeric solids such
as polystyrene, another is through inorganic supports such as
silica gel or alumina. Mesoporous solids such as silica gel
have been extensively studied because of their catalytic, ana-
lytical, optical, and electronic properties.[4–9,12,14,15] Activated
silica is synthesized binding an active compound such as 3-
aminopropyltriethoxysilane (APTES).[5–8,12,14–21]
Received 6 July 2011; accepted 3 August 2011.
The authors would like to thank to Prof. Dr. M. Hakkı Alma
˙
¨
¨
˙
(Kahramanmaras¸ Su¨tc¸u¨ Imam University, USKIM: University, Indus-
try, Public Collaboration Development, Application and Research Cen-
ter) for some measurements. The authors also thank Assist. Prof. Dr.
¨
H. Okkes¸ Demir for his laboratory supports. They are grateful to
Su¨tc¸u¨ Imam University and Kilis 7 Aralık University for all labora-
˙
tory supports.
Address correspondence to Serhan Urus¸, Chemistry Department,
Faculty of Science and Letters, Kahramanmaras¸ Su¨tc¸u¨ Imam Uni-
versity, 46100, Kahramanmaras¸, Turkey. E-mail: serhanurus@yahoo.
co.uk
Cyclohexane (CyH) and its derivatives such as cyclohex-
anol (Cy-OH) and cyclohexanone (Cy O) are used as starting
compounds in medicine and in the chemical industry. Cy-OH
˙
382

SILICA-SUPPORTED CATALYST
383
and Cy O are obtained from CyH are used as intermediates Elmer Clarus 600 GC (USA) equipped with MS detector fitted
for pharmaceuticals, plasticizers, rubber chemicals, cyclohexy- with Elite-5 MS and FID detector fitted with BPX5 capillary
lamine, pesticides, and other organic compounds. For example, columns.
cyclohexanol esters are pharmacologically active compounds
and therefore be used as pharmaceuticals, in particular for treat-
Preparation of 2-hydroxy-5-((2-hydroxyphenyl)diazenyl)
benzaldehyde (HPDB) (L1)
ing diabetes and other diseases related to glucose secretion. This
2-aminophenol (0.8630 g, 8 mmol) was mixed with 10 mL
azo-aldehyde ligand was prepared by modifying the process
NaNO2 solution (0.6210 g, 9 mmol), then dropped HCl (37%)
of the literature.[25] Besides, cyclohexane derivatives are also
until pH ∼2. The mixture was stirred for 30 min. Salycilalde-
used as solvents, oil extractants, paints and varnish remover,
hyde (2-hydroxybenzaldehyde) (0.9770 g, 8 mmol) was dis-
dry cleaning material, and in solid fuels. More than 1 billion
solved in 20 mL pH:12 buffer solution containing NaOH (20
tons of cyclohexanone (Cy O) and cyclohexanol (Cy-OH) are
mmol) and Na2CO3 (40 mmol). Diazonium solution was added
produced each year and are generally used for the synthesis of
very slowly to salycilaldehyde solution. While being added, the
Nylon-6 and Nylon-6,6.[22–24] Conaphthenate complex was used
temperature was kept at 0–2◦C and pH was kept at around 8–9.
as a catalyst to initiate the radicalic oxidation of cyclohexane
The reaction mixture was stirred in the ice bath for 2 h. All
using molecular oxygen from the air with the conditions that the
reaction steps were carried out in an ice-water-salt bath at the
temperature is 160◦C and the pressure is 15 bar. After completed
temperature 0–2◦C. The dark orange product was filtered and
the reaction, only 4% of cyclohexane (CyH) was converted to
recrystallized in EtOH:H2O (1:1). Several portions of diethyl
oxidized products and with selectivity towards Cy O and Cy-
ether were used to remove organic impurities from the product.
OH. Microwave technology using green oxidant hydrogen per-
Then the product was dried under vacuum at 60◦C for 12 h.
oxide can reduce the reaction times and energy consumption
Yield: 0.350 g (19%). Anal. Calcd. for [C13H10N2O3]: C 64.46,
with an increase cyclohexane and cyclohexene conversions and
1
H 4.16, N 11.56%. Found: C 64.38, H 4.34, N 11.49%. H-
the selectivity of the desired products.[23,24] Carvalho et al.[24]
NMR (DMSO, δ, ppm): 11.37 [s, 1H, CHO], 7.86–6.92 [m, 7H,
studied the cyclohexane oxidation under microwave conditions
Ar-H], 3.45 [s, 2H, Ar-OH]. FT-IR (υ, cm−1): 3669–3120 (br,
by using Fe(III) complex catalysts, hydrogen peroxide as oxi-
Ar-OH); 3063 (w, Ar-H); 2927 (w, R-H), 2854 (w, CHO), 1613
dant, and acetonitrile as solvent, and Cy-OH, Cy O, and adipic
(s, C O), 1581(s, Ar-C = C), 1463 (s, -N N-), 1109 (w,
acid were obtained as major products with a good CyH conver-
C-H); 1031 (w, C-OH), 751 (s, Ar-H). TG/DTA Data: No water
sion and Cy-OH, Cy O selectivity. In the literature, the metal
or solvent loss was observed below decomposition temperature.
complexes showed good catalytic activities with good selec-
Decomposition began at 150◦C and continued to 900◦C as three
endothermic peaks with a 95.8% loss.
tivities for the oxidation of cyclohexane and cyclohexene with
H2O2 oxidant under microwave power.[23,24]
Preparation of Silica-Supported
3-Aminopropyltriethoxysilane (SiO2-APTES)
EXPERIMENTAL
Silica gel (50 g) was refluxed with excess amount of
1:1 hydrochloric acid for 6 h, then filtered off and washed
with appropriate amount of deionized water until filtrate was
neutral. Activated silica was dried at 150◦C for 12 h. Then,
20 g activated silica was suspended in 100 mL toluene and
20 mL 3-aminopropyltriethoxysilane (APTES) was added to
the suspension. The reaction mixture was refluxed for 72 h.
The suspension was filtered and the filtered solid was washed
with excess amount of toluene, ethanol and diethyl ether,
respectively.[5,7,8,12,14–20] Elemental Analysis: C 8.97, H 2.32, N
3.39%. FT-IR (υ, cm−1): 3696–3319 (br, Si-OH); 3319–3118
(br, -NH2), 2941 (w, R-H); 1056 (br, Si-O). TG/DTA Data:
3.37% water or solvent 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.
General
All reagents and organic solvents were purchased from
commercial sources and used as received, unless noted oth-
erwise. The NMR spectra were recorded at 25◦C in DMSO-d6
and CDCl3 using a Bruker 400 MHz Ultrashield TM NMR
spectrometer. FT-MIR and FAR spectra were obtained by us-
ing Perkin-Elmer Spectrum 400 FT-IR system in the range
4000–30 cm−1. Elemental analyses were performed using
LECO CHNS 932 instrument. Metal contents were determined
by Perkin-Elmer Optima 2100 DV ICP-OES. A Jeol Neoscope
Benchtop scanning electron microscope was used for scanning
electron microscopy (SEM) images. The thermal analyses stud-
ies of the complexes were performed on a Perkin-Elmer Pyris
Diamond DTA/TG thermal system under nitrogen atmosphere at
a heating rate of 10◦C /min in the range 30–900◦C. Kiesegel gel
60 (Merck) silica having particle size of 0.2–0.5 mm was used
as solid support for heterogeneous catalysts. The microwave ex-
periments were carried out in a Berghof MWS3+ (Germany)
Preparation of Silica-Supported Azo Containing Schiff
Base (SiO2-APTES-HPDB: L2)
5 g SiO2-APTES was added to HPDB (1.21 g, 5 mmol)
equipped with pressure and temperature control. Microwave solution in 100 mL EtOH (96%) and then was refluxed at 60◦C
experiments were done in closed DAP60 vessels. The reac- for 12 h. The brown solid was filtered, washed with excess
tion products were characterized and analyzed by using Perkin- amount of EtOH, and dried at 95◦C for 12 h. Yield: 65%. Anal.

384
S. URUS¸ ET AL.
Calcd.: C 15.39, H 2.11, N 2.01%. Found: C 15.08, H 2.37, when pH is lower than 8–9, coupling does not form. The metal
N 2.47%. FT-IR (υ, cm−1): 3665–3098 (br, -OH); 2957 (w, complexes, [Cu2(L2)(H2O)2Cl2] (3), [Co2(L2)(H2O)2Cl2] (4),
Ar-H); 2930 (w, R-H), 1640 (s, CH N), 1490 (s, -N N-), [Ni2(L2)(H2O)2Cl2] (5), and [Mn2(L2)(OAc)2(H2O)2] (6) were
1050 (br, Si-O). TG/DTA Data: 0.68% water or solvent loss prepared using anhydrous CuCl2, CoCl2.6H2O, NiCl2.6H2O,
was observed between 30 and 180◦C. Decomposition began at and Mn(OAc)2 salts. The chemical structures of L1 and L2 and
180◦C and continued to 900◦C as one endothermic peak with a possible structures of the L2-metal complexes are shown in
27.33% loss.
Scheme 1. The colors of L2 and their complexes have brown
tones.
Preparation of the Complexes
The functional groups of the synthesized ligands and their
complexes were assigned by infrared spectra shown in Figures 1
and 2. Ar-OH, Ar-H, and R-H absorptions for ligand L1 (a) were
Silica-supported complexes of L2 were synthesized by the
addition of 1 mmol metal salts (anhydrous CuCl2, CoCl2.6H2O,
NiCl2.6H2O, and Mn(OAc)2) to the 2 g ligand-grafted silica in
EtOH. The mixture was refluxed for 24 h at 50◦C (pH 7–8). Af-
ter stirring, the complexes were filtered and washed with excess
amount of water and finally dried in vacuum at 70◦C. Elemen-
tal analysis (C, H, N%), ICP-OES (Metal% content after mi-
crowave digestion), FT-MIR/FAR, TGA, and SEM techniques
were used for the characterization of the complexes.
observed at about 3669–3120 cm−1, 3063 cm−1, and 2927 cm−1
,
respectively. A –N N– stretch was assigned at 1463 cm−1. In
the spectrum of SiO2-APTES showed a broad band in the range
3696–3319 cm−1 and at 1056 cm−1 can be attributed to silanol-
OH groups stretches and Si-O-Si bonds, respectively.[5–8,11–13]
The continuing band between 3319 and 3118 cm−1 is the stretch
of –NH2 groups of silica-supported APTES. Aliphatic –CH2
vibrations can be attributed at 2941 cm−1 in the SiO2-APTES
spectrum. Absorbance band of –OH groups of L1 begins at 3669
cm−1 continues to the beginning of the aliphatic and aromatic
vibrations at around 2958 cm−1. A band belonging to –NH2
groups in the range of 3319–3118 cm−1 disappeared when L1
Catalytic Oxidation of Cyclohexane under Microwave
Irradiation
The catalytic oxidation of cyclohexane under microwave ir-
radiation was performed as follows: 0.02 mmol synthesized
complex, 2 mmol cyclohexane (Carlo Erba, 99.8%), and 4 mmol
H2O2 (Merck, 35%) were microwaved for 75 min at 400 W (40%
of maximum output power). The catalyst:substrate:oxidant ra-
tio was of 1:100:200. The complexes were individually sus-
pended in 5 mL acetonitrile, and cyclohexane and H2O2 were
added in the microwave vessels, respectively, for each oxida-
tion 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
W microwave power was applied for 75 min. The temperature
was controlled automatically by the microwave instrument at
about 110◦C, however, sometimes it 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
solvent. In order to stop the oxidation before analysis, 1 mL H2O
was added in the vessels. The oxidized organic products except
organic acids were extracted with 10 mL CH2Cl2 and injected
to GC and GC-MS to analyze and characterize. CyH, Cy-OH,
and Cy O amounts were calculated from external calibration
curves that were prepared before analyses.
bound to SiO2-APTES (Figure 1). Nevertheless, –CH
N–
stretch of L2 was observed at 1640 cm−1 as a strong band in the
spectrum. This band prove the Schiff base reaction of the car-
bonyl groups with -NH2 groups of SiO2-APTES.[5,7–9,11–13] The
FT-IR spectra of the silica-supported complexes showed signif-
icant differences from the silica-supported ligand L2. However,
when the metal ions coordinated to L2, –CH
ing frequencies slightly shifted to lower frequencies (1635
N– stretch-
cm−1).[5,7–9,11–13] Additionally, –N
N– frequencies of the
complexes were shifted to about 1506 cm−1. These results show
that L2 coordinates to metal ion with two side of it, one is –CH
N–, –OH groups and other is –N N–, –OH groups[5,11,12]
.
New M-O and M-N absorption bands in the FAR spectra were
observed at about 462 cm−1 and 543 cm−1, respectively (Table
1). In FT-IR spectrum of [Mn(L2)(OAc)(H2O)], the new C
O
stretch of acetate ligand was observed at 1646 cm−1 (Figure 2).
The new M-X (X: Cl− for Cu(II), Co(II), and Ni(II) complexes,
OAc− for Mn(II) complex) stretches were at about 366 cm−1
in FAR spectra. Coordinations of halides or acetate in the
complexes were characterized further with qualitative methods
such as argentometric method for halide coordination.[5,9,11,12]
1H-NMR spectrum of L1 was recorded by using DMSO-
d6 as solvent. The singlet peak of aldehyde hydrogen of L1
was observed at 11.37 ppm. The aromatic protons (7H) gave a
multiple signal at 7.86–6.92 ppm. The singlet signal of aromatic
–OH protons were recorded at 3.45 ppm. These 1H-NMR data
confirm the chemical structure of synthesized ligand:[5,7–13]
Elemental analysis techniques (C, H, N, and metal%) were
used for further characterization of the ligands and their com-
plexes. Silica-supported ligand have 1 mmol ligand:1 g silica
ratio. According to the increase of weight of activated silica
RESULTS AND DISCUSSION
Preparation of Silica-Supported Ligand and Complexes
A novel silica-supported ligand [SiO2-APTES-HPDB]
(L2) was synthesized using a novel ligand [HPDB] (L1) and
activated silica [SiO2-APTES] [5,7,12]. Different reaction
parameters (pH, temperature, time) were tested in order to
determine optimum coupling conditions for the synthesis of
L1, however, maximum 19% yield was obtained when used pH
12 buffer in salicylaldehyde solution. According to our results,

SILICA-SUPPORTED CATALYST
385
O
O
OH
OH
OH
+
Cl-
HO
OH
NH2
N
N
N
N
NaNO2
HCl
pH=12 buffer
0-2 oC
0-2 oC
L1: HPDB
H2
N
H2N
Si
O
Si
C2H5
O
HO
HO
OH
OH
O
O
O
O
OH
OH
O
C2H5O
Si
NH2
+
C2H5O
n
n
APTES
Hydrolized Silica-gel
SiO2-APTES
L1
O
O
HO
N
HO
N
M
N
M
N
N
N
N
N
M2+
O
O
HO
HO
M
N
N
N
M
N
Si
O
Si
Si
Si
O
O
O
O
O
O
O
O
O
O
O
n
n
(L2)
SCH. 1. Synthesis of L1, L2 and the possible coordination of metals to L2. M2+ : Cu2+, Co2+, Ni2+ and Mn2+
gel, nearly 65% of ligand bound to activated silica gel. While C between silica gel and the complexes in SEM images are very
and H percentages increase, N percentage decreased after bind- important evidence of the loading of the complexes both on to
ing L1 to SiO2-APTES. These differences confirm supporting the SiO2 particles and cavities of it as clusters. When L1, which
L1 to SiO2-APTES and Schiff base forming. Metal content of diffuses through the SiO2 channels, reacts with metal ions, it
the complexes were also determined by using ICP-OES. Before is obtained well dispersed complexes through SiO2 channels
ICP analyses, silica-supported complexes were digested with and over SiO2 particles. Well-dispersed silica-supported Cu-L2
the Berghof MWS3+ microwave. Approximately 0.25 g sam- complex showed a good catalytic activity as a result of having a
ple was added in DAP60 vessel and 2 mL HNO3 and 3 mL HCl huge surface on the silica particles and its cavities as an internal
placed into this vessel. After waiting 5 min, vessels were closed and external surface as seen in Figure 3.[6,9,12,13,15]
with their capture and caps and a suitable program (pressure,
Thermal behavior of the ligands and the complexes were
temperature, and microwave power) were applied. Digested so- investigated by using thermogravimetry (TG). TG curves of the
lutions were filtered and diluted appropriate amount of ultrapure ligands and the complexes are shown in Figure 4.
water. According to the metal analyses results, metal absorption
While the ligand (L2) has two mass loss peaks in TG curves,
capability of the silica-supported ligand (L2) is good enough and the complexes show three mass loss peaks. The silica-supported
L2:M ratio is 1:2 (Table 1).
ligand (L2) has a water or solvent loss peak (0.68% loss) be-
The SEM images of SiO2-APTES, L2 and the complexes tween 30 and 180◦C and decomposition starts at 180◦C in
of L2 are shown in Figure 3. The morphological differences TG curves (27.33% loss). When combined the results of the

386

SILICA-SUPPORTED CATALYST
387
FIG. 2. FT-FAR of the complexes (color figure available online).
and 145◦C can be attributed to coordinated water. In addition,
an acetate ligand in Mn(II) complex increased the decompo-
sition percentage between 145 and 660◦C in the TGA curve
(Table 2).
Based on FT-MIR/FAR spectral values, elemental analysis
(C, H, N, and M%) and other chemical characterizations, it
can be concluded that the complexes are tetracoordinated. Two
coordination sites of the metal cations in the complexes have had
two halogen ligands and two aqua ligands for each Cu-L2, Co-
L2, and Ni-L2 complex and two acetate ligands and two water
ligands for the Mn-L2 complex. Having two active functional
regions of the silica-supported ligand L2 allowed 1:2 ratio for
L:M. These probable structures are also accordance with our
previous studies and other literatures.[5,7–9,11–13]
FIG. 1. FT-IR spectra of the ligands and the complexes (color figure available
online)
elemental analyses and TG curves, it can be concluded that L2
has little hydrate or adsorbed/absorbed water. In TG curves of
the complexes, two loss as a result of decomposition of the
supported ligand and the formation of metal oxides could be
Catalytic Oxidation of Cyclohexane Under Microwave
Irradiation
According to the possible oxidation mechanism (Figure 5),
detected between about 145–600◦C.[5,7–9,12,13] 2.28% loss in the first and the slow step is the oxidation of CyH to Cy-
the TGA curve of [Mn(L2)(OAc)(H2O)] complex between 30 OH and after that Cy O and other further oxidized products
TABLE 2
Thermogravimetric data of the ligand and the complexes
Compounds
TG range (◦C)
Mass loss (%)
Assignment
Loss of solvent and water
APTES decomposition
Loss of solvent and water
Ligand decomposition
Loss of solvent and water
Ligand decomposition and metal oxide formation
Loss of solvent and water
Ligand decomposition and metal oxide formation
Loss of solvent and water
SiO2-APTES
30–155
155–900
30–180
180–900
30–170
170–540
30–175
175–685
30–185
185–620
30–145
145–660
3.37
8.64
0.68
27.33
0.17
25.58
0.19
17.47
2.57
22.37
2.28
L2
[Cu2(L2)(H2O)2Cl2]
[Co2(L2)(H2O)2Cl2]
[Ni2(L2)(H2O)2Cl2]
[Mn2(L2)(OAc)2(H2O)2]
Ligand decomposition and metal oxide formation
Loss of solvent and water
Ligand decomposition and metal oxide formation
24.40

388
S. URUS¸ ET AL.
FIG. 3. Scanning electron microscopy images of silica-gel (a) and the complexes; [Cu2(L2)(H2O)2Cl2] (b,f), [Co2(L2)(H2O)2Cl2] (c), [Ni2(L2)(H2O)2Cl2] (d),
[Mn2(L2)(OAc)2(H2O)2] (e). (Accelerating voltage: 10 kV, vacuum: high).
power.[12,13] The optimum oxidation conditions were obtained
as a 1:100:200 ratio for catalyst:substrate:oxidant, respectively,
in acetonitrile under a 400 W microwave power for 75 min. The
temperature and pressure were controlled at about 110◦C and
30 bar, respectively, throughout the experiment. These optimum
parameters were applied for all catalytic experiments. A blank
has been run under the same conditions without any catalyst
(Table 3).
arise. If it is controlled the first step of the oxidation reac-
tion, the desired products Cy-OH and Cy O selectivity can be
increased.
The possible reaction products are shown in Figure 6 using
H2O2 and catalyst under microwave.
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. Reaction time, oxidant amount, catalyst amount,
and microwave oven parameters were tested in order to deter-
mine optimum catalytic oxidation parameters under microwave
The ligands containing N, O atoms, can promote proton-
shift.[26] In the present study, synthesized new ligands has N

SILICA-SUPPORTED CATALYST
389
is very usable, easy, and clean for organic and inorganic syn-
theses. When compared with previous results, in the present
study, new synthesized catalysts are not quite effective on the
oxidation of cyclohexane. On the other hand, these complexes
had good selectivity. It has been shown that Cu(II), Co(II), and
Fe(II) complexes of ligands possess catalytic effect. Cy-OH and
Cy O selectivity of Cu(II) catalyst by using microwave power
is considerable when compared with the classical oxidation of
CyH in the literature.[9,10,12,13,22–27] The best selectivity of the
desired products was obtained with silica-supported Cu(II) com-
plex, whereas the products Cy-OH and Cy O mol percentages
are not good (Figure 6). Ni(II) complex has 38.74% cyclohex-
ane conversion, however, selectivity is not good enough because
of further oxidized products.
The key point in the conversion of cyclohexane to the ox-
idized products is the reduction of Cu(II)–L2 to Cu(I)–L2.
This reduction to Cu(II)–L2 is facilitated the ligands available
around the metal cation. The formation of the oxidation prod-
ucts Cy–OH and Cy O show the preferential attack of the
activated bonds. Tetrahedral geometry of Cu(II) complex may
be the reason of the having higher selectivity rather than the
other complexes.
FIG. 4. TG of the silica-supported ligand (L2) and its complexes (color figure
available online).
and O donor atoms. Classical thermal oxidation studies require
very long time and control of reaction conditions is very diffi-
cult. However, microwave oxidation decreases the reaction time
and increases selectivity.[12,13,22,24,27] Hence, microwave power
FIG. 5. Possible mechanism of catalytic oxidation of cyclohexane. X: Cl-, OAc- or H2O.

390
S. URUS¸ ET AL.
TABLE 3
Catalytic oxidation of cyclohexane (CyH) with H2O2 under microwave irradiationa
Entry
Catalysta
CyH conversion (mol %)
Desired products (mol %)
ol:one By-products (mol %)
1b
2c
3c
4c
5c
Blank
0.86
48.43
6.45
38.74
7.24
Cy-OH (0.54); Cy O (0.32)
Cy-OH (8.40); Cy O (3.77)
Cy-OH (0.48); Cy O (0.06)
Cy-OH (2.95); Cy O (7.15)
Cy-OH (0.54); Cy O (0.51)
1.72
2.23
8.00
0.41
1.06
—
36.26
5.91
28.64
6.19
[Cu2(L2)(H2O)2Cl2]
[Co2(L2)(H2O)2Cl2]
[Ni2(L2)(H2O)2Cl2]
[Mn2(L2)(OAc)2(H2O)2]
a400 W power was applied for 75 min. The reaction temperature and pressure were held at around 110◦C and 30 bar in closed DAP60 vessels.
b2 mmol cyclohexane:4 mmol hydrogen peroxide and 5 mL acetonitrile were used in experiment without catalyst. c0.02 mmol catalyst:2 mmol
cyclohexane:4 mmol hydrogen peroxide (1:100:200) and 5 mL acetonitrile were used for each reaction.
OH
O
OOH
CO2H
CO2H
Catalyst
+
+
+
H2O2 , MeCN
Cy=O
Adipic Acid
CyH
Cy-OOH
Cy-OH
FIG. 6. Catalytic oxidation of cyclohexane under microwave irradiation[12]
.
FIG. 7. Influence of the complexes in cyclohexane oxidation under microwave irradiation. 0.02 mmol complex:2 mmol cyclohexane:4 mmol hydrogene peroxide
(1:100:200) and 5 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 closed DAP60 vessels (color figure available online).

SILICA-SUPPORTED CATALYST
391
CONCLUSION
Cu(II), Co(II), Ni(II) and Mn(II). J. Inorg. Organomet. Polym. 2010, 20,
706–713.
In the present study, the novel silica-supported azo-
containing Schiff base and its Cu(II), Co(II), Ni(II), and Mn(II)
complexes were synthesized. The used characterization tech-
niques have approved the possible structure of the ligand and
the complexes. The clean and green oxidation of cyclohexane to
cyclohexanol and cyclohexanone under microwave irradiation
using H2O2 was also investigated using silica-supported Cu(II),
Co(II), Ni(II), and Mn(II) complexes. The results showed that
the Cu-complex was more selective catalyst for the oxidation
of cyclohexane to cyclohexanol and cyclohexanone when com-
pared with other complexes.
13. Dolaz, M.; McKee, V.; Urus¸, S.; Demir, N.; Sabik, A.E.; Go¨lcu¨, A.; Tu¨mer,
M. Synthesis, structural characterization, catalytic, thermal and electro-
chemical investigations of bidentate schiff base ligand and its metal com-
plexes. Spectrochim. Acta A 2010, 76, 174–181.
14. Urus¸, S.; Keles¸, M.; Serindag˘, O. Catalytic synthesis of 2-methyl-
1,4-naphthoquinone (vitamin K3) over silica-supported aminomethyl
phosphine-Ru(II), Pd(II) and Co(II) complexes. Phosphorus Sulfur Silicon
Relat. Elem. 2010, 185, 1416–1424.
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