Synthesis and Heterogenization of Siloxane Functionalized Cobalt Complex: Potential Catalyst for Oxidation of Alcohols

Article abstract of DOI:10.1007/s10562-015-1492-9

Abstract Cobalticinium complex containing siloxane moiety on ligand center was first time synthesized, characterized and heterogenized on the surface of SBA-15 molecular sieves. FT-IR and ²⁹Si-MAS-NMR spectrum evidence the successful grafting of the cobalt complex on the surface of SBA-15. N₂ sorption isotherm of cobalticinium complex grafted sample showed the decrease in surface area, pore volume, further confirms the presence of complex inside the channel of mesoporous SBA-15. The heterogenized complex exhibited promising activity for oxidation of alcohols with good selectivity of ketone. Further the catalytic activity remains intact after two recycles. Graphical Abstract: Cobalticinium complex possessing siloxane functionality was prepared first time and heterogenized on SBA-15 surface. The resultant materials showed as promising recyclable heterogeneous catalyst for oxidation of alcohols.[Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-015-1492-9

Catal Lett
DOI 10.1007/s10562-015-1492-9
Synthesis and Heterogenization of Siloxane Functionalized Cobalt
Complex: Potential Catalyst for Oxidation of Alcohols
•
T. Baskaran R. Kumaravel J. Christopher
•
•
•
S. Radhakrishnan A. Sakthivel
Received: 17 December 2014 / Accepted: 30 January 2015
Ó Springer Science+Business Media New York 2015
Abstract Cobalticinium complex containing siloxane
moiety on ligand center was first time synthesized, char-
acterized and heterogenized on the surface of SBA-15
molecular sieves. FT-IR and 29Si-MAS-NMR spectrum
evidence the successful grafting of the cobalt complex on
the surface of SBA-15. N2 sorption isotherm of
cobalticinium complex grafted sample showed the decrease
in surface area, pore volume, further confirms the presence
of complex inside the channel of mesoporous SBA-15. The
heterogenized complex exhibited promising activity for
oxidation of alcohols with good selectivity of ketone.
Further the catalytic activity remains intact after two
recycles.
Graphical Abstract Cobalticinium complex possessing
siloxane functionality was prepared first time and
heterogenized on SBA-15 surface. The resultant materials
showed as promising recyclable heterogeneous catalyst for
oxidation of alcohols.
Keywords Cobalticinium complex Á SBA-15 Á
Oxidation Á Alcohols Á Siloxane function
1 Introduction
In recent years, organometallic complexes containing
siloxane functionality have great interest owing to their
usefulness to bind covalently with hydroxyl group avail-
able on supported materials [1–6]. Several reports de-
scribing the molybdenum complexes with siloxane
functionality present either in ligand center or metal centre,
which were further heterogenized on the surface of silica
has shown promising catalysts for various oxidation reac-
tions [6–13]. Similarly, siloxane functionalized ruthenium
and rhenium complexes are heterogenized on mesoporous
molecular sieve surface has been applied for cyclo-propa-
nation and epoxidation respectively [14, 15]. However,
syntheses of first row transition metal complexes contain-
ing siloxane functional group are rare [3–5]. In addition, it
Electronic supplementary material The online version of this
article (doi:10.1007/s10562-015-1492-9) contains supplementary
material, which is available to authorized users.
T. Baskaran Á R. Kumaravel Á A. Sakthivel (&)
Department of Chemistry, Inorganic Materials and Catalysis
Laboratory, University of Delhi, New Delhi, India
e-mail: sakthiveldu@gmail.com; asakthivel@chemistry.du.ac.in
J. Christopher
R&D Centre, Indian Oil Corporation Ltd, Faridabad 121007,
India
S. Radhakrishnan
NMR Lab, CSIR-Central Electrochemical Research Institute,
Karaikudi 630 006, India
123

T. Baskaran et al.
is well known that these metal complexes are potential
substitution for expensive rare earth and late transition
metal catalysts. Heterogenization of various transition
metal based homogenous catalysis with salen, acac, and
cyclopentadienyl-based ligands are shown very good cat-
alytic ability for several oxidation reactions [16–21]. In this
regard, cobalt based catalysts also shown comparable ac-
tivity towards various organic transformations, such as
hydroformylation, hydrosilylation, Fisher–Tropsch syn-
thesis, hydroxylation, reforming, and various redox pro-
cesses [22–31]. For example, cobalt-polyaniline
nanostructure hosted SBA-15 shown as a capable catalyst
for oxidation process [29]. Similarly, cobalt(II) complex
grafted on SBA-15 has catalytic ability for acetylation of
aldehydes and oxidation of alcohols at elevated tem-
perature and microwave conditions using excess H2O2 [30,
31]. To the best of our knowledge, the structural stability
and direct grafting of siloxane functionalized cobalt based
complexes are not known. In this regard, it is worth men-
tioning here that the cobalticinium complexes are ana-
logues to isoelectronic ferrocene complexes which has
strong resistance under severe reaction conditions [32, 33].
It is interesting to develop stable full sandwich type com-
plex, where the metal ion strongly hold with ligand and
retain their active coordination sphere. It is well established
that the cobalt based full sandwich complexes are highly
stable and will be interesting to introduce siloxane func-
tionality on these types of complexes. In an effort, here-
with, we report the synthesis of siloxane functionalized
cobalticinium complexes and heterogenized on SBA-15
molecular sieves. The resultant host–guest material is ap-
plied for liquid phase oxidation of alcohol at ambient re-
action conditions.
procedure described in literature [34–36]. About 2.42 g
1
(72 %) of the dried ligand 2, was obtained. H NMR (d,
400 MHz, CDCl3): 3.41(s, 3H, CH3), 5.46 (t, 2H, Cp), 6.0
(t, 2H, Cp).
Freshly prepared chlorotris(triphenylphosphine)cobalt
(7.24 g, 8.2 mmol) was added into the solution containing
diphenyl acetylene (3.36 g, 18.9 mmol) dissolved in
toluene (56 ml). The resultant solution was transferred to
the flask containing sodium carbomethoxycyclopentadi-
enide (2) (1.38 g, 9.4 mmol) in THF (14 ml). The final
mixture was refluxed for 5 h and allowed to cool at room
temperature. The solvent was removed under vacuum and
the resultant residue was suspended in petroleum ether
(50 ml) and collected by filtration to give compound 3, as a
yellow crystalline solid (4.06 g, 92 %): mp:222 °C;
1H-NMR (400 MHz, CDCl3) 3.14 (3 H, s, –CH3) 4.7 (2 H,
brs, CpH), 5.12 (2 H, brs, CpH) 7.15–7.22 (12 H, m, m and
p-PhH) 7.37–7.34 (8 H, m, o-PhH):13C (400 MHZ, CDCl3)
51.6 (–CH3), 76.4 (C4Ph4), 84.9 (CpC), 86.8 (CpC), 87.1
(ipso CpC), 127.1(p-PhC), 128.3(PhC), 129.2 (PhC), 135.5
(ipso-PhC), 167.0 (–CO2CH3).
Ester functional group of compound 3, was hydrolysis
using ethanolic potassium hydroxide solution to give rise to
the corresponding acid (4) [34]. The resultant acid was
extracted in ether (3 times, 30 ml) and dried. About 0.6 g
(1.14 mmol) of acid, 4 was dissolved in CH2Cl2 (20 ml).
The oxalyl chloride (0.1 ml, 1.4 mmol) was added to the
above acid solution and the resulting red solution was
stirred at room temperature for 30 min. The excess solvent
and oxalyl chloride were removed under vacuum. The ob-
tained crude acid chloride (5) was dissolved in CH2Cl2
(30 ml) and transferred to a flask containing the solution of
N-(3-Triethoxysilyl) propyl amine (0.24 g, 1.14 mmol) in
triethylamine (0.2 ml) and CH2Cl2 (6 ml). The resulting
orange reaction mixture was stirred at room temperature for
overnight and quenched with water (10 ml). The resultant
complex was extracted with ethyl acetate (EtOAc; 30 ml).
The organic phases were dried using anhydrous MgSO4 and
evaporated under vacuum to get the required product (6) of
yellowish brown viscous oil. The schematic representation
of preparation of complex 6 is shown in Scheme 1. The 1H
NMR spectrum is displayed in Fig. S1 , which evidences
2 Experimental Section
2.1 Solvents and General Procedures
All work up was carried out using standard Schlenk tech-
niques under nitrogen atmosphere. THF and toluene were
freshly distilled in-presence of sodium benzophenone;
hexane was distilled over KOH and dichloromethane over
calcium hydride under nitrogen atmosphere. 1H, 13C, NMR
spectra were obtained using 400-MHz JEOL spectrometer.
1
the formation of complex 6. H NMR (400 MHz, CDCl3)
0.50–0.54 (t, 2 H, Si–CH2), 1.20-1.27 (q, 9 H, CH3),
1.38–1.40 (q, 2H, CH2–CH2–CH3), 2.82–2.84 (q, 2 H,
NCH2), 3.78–3.82 (q, 6 H, O–CH2–CH3), 4.67 (s, 2H,
CpH), 5.02 (s, 2H, CpH), 5.17 (s, 1 H, NH), 7.21–7.24 (12
H, m, m- and p- PhH), 7.43–7.47 (8 H, m, o-PhH) 13C
NMR(400 MHz, CDCl3) 7.7 (Si–CH2), 18.2 (CH3), 23.1
(CH2–CH2-CH2), 41.7 (NCH2), 58.3 (CH3-CH2-O), 77.2
(C4 Ph4), 89.1 (CpC), 92.9 (ipso-CpC), 127.7 (p-PhC),
128.8 (PhC), 129.0 (PhC), 135.0 (ipso-PhC).
2.2 Synthesis of (g5-N-[3-(Triethoxysilyl)
propylcarboxymidecyclopentadienyl)
(g4-tetraphenylcyclobutadiene)cobalt
Chlorotris(triphenylphosphine)cobalt and sodium car-
bomethoxycyclopentadienide (2) were prepared as per the
123

Potential Catalyst for Oxidation of Alcohols
Scheme 1 Synthesis of
complex (6)
2.3 Synthesis of Mesoporous SBA-15
nitrogen atmosphere. About 1 mmol of complex 6 in
toluene (30 ml) was introduced to 1 gm of pre-activated
SBA-15 molecular sieves. The mixture was refluxed for
6 h, and then the solvent was removed under vacuum,
washed with dichloromethane and dried under vacuum.
The heterogenized cobalticinium complex (6) on SBA-15
was designated as SBA-15Co–Si.
and Heterogenization of Complex 6 on SBA-15
Mesoporous silica were synthesized as per the procedure
described earlier [10–12] using pluronic 123 triblock
copolymers (poly(ethylene oxide)-poly(propylene oxide)-
poly(ethylene oxide); (Aldrich)) EOnPO70EOn as templates
with the molar gel composition of the synthesis mixture
was 6.89 9 10-4 P123 triblock copolymers : 0.24 HCl:
0.041 TEOS: 7.88 H2O. The as-synthesized sample was
calcined at 550 °C for 6 h in an atmospheric air.
The complex 6 was heterogenized on calcined SBA-15
molecular sieve as per the following procedure. First, a
SBA-15 molecular sieve was pre-activated at 150 °C for
overnight under vacuum in a Schlenk round bottom flask,
and then it was cooled to room temperature under dry
2.4 Characterization of Materials
The crystallinity of the grafted materials was studied by
powder XRD using a 18 KW Rigaku (2,500 V), Japan with
˚
Cu-Ka radiation (k = 1.54184 A). The diffraction patterns
were recorded in the 2h range of 0.5–10°, with a scan speed
and step size of 1° per minute and 0.02°, respectively. FT-
IR spectra were obtained in the range 400–4,000 cm-1
123

T. Baskaran et al.
using KBr pellets (BRUKER, TENSOR 27 FT-IR Spec-
trometer). The textural properties (BET surface area, BJH
pore volume) of the parent and grafted materials were
derived from N2 sorption analysis, which was carried out at
-196 °C using an automatic micropore physisorption
analyzer (Micromeritics ASAP 2020, USA) after the
samples were degassed at 150 °C for at least 8 h under
10-3 Torr pressure prior to each run. Proton and carbon
NMR spectra of complex were recorded on a JEOL AL-
400 FT NMR instrument using CDCl3 as solvent. The
amount of cobalt loaded on SBA-15 was estimated by
atomic absorption spectrophotometer (AAS, Analytic Jena;
ZEEnit700 P) as per the procedure described below. First,
about 100 mg of catalyst was digested with HCl:HNO3
mixture (3:1), then the resultant solution filtered using
whatman filter paper and made up to 100 ml using double
distilled deionized water. Analysis was carried out using an
air-acetylene flame at wavelength of 240.7 nm wavelength.
The calibration curve was obtained using Co(NO3)2Á6H2O
as standard at different concentration. The electronic
spectra of grafted materials was studied using diffuse re-
flectance ultraviolet–visible (DRUV–Vis; UV-260Shi-
madzu) spectrometer. Solid-state NMR experiments were
carried out on a Bruker AVANCE 400 narrow bore spec-
trometer equipped with a superconducting magnet with a
field of 9.4 T using a 4 mm double resonance cross-po-
larized magic angle spinning (CP-MAS) probe operating at
resonating frequencies of 79.4 and 100.6 MHz for 29Si and
13C respectively. The samples were packed in 4 mm zir-
conia rotors and subjected to a spinning speed of 5 kHz:
single pulse experiment with pulse duration of 4 ls and a
relaxation delay time of 5 s were used for recording 29Si
and 13C MAS NMR pattern with 2000 number of scans.
The chemical shift values are expressed with respect to
TMS for the 29Si and glycine 13C nucleus. Thermo-
gravimetry-analysis (TGA) was carried out using Thermal
Analyzer (Model-2950, TA Instruments, and USA). About
5–10 mg of the sample was taken in the platinum pan
under air atmosphere at the heating rate of 10 °C min-1 up
to 800 °C.
2.5 Catalytic Oxidation of Alcohol Using SBA-15Co–Si
The catalytic activity was studied for oxidation of ben-
zhydrol and other alcohols in liquid phase medium using
TBHP in decane as oxidant (Scheme 2). After the reaction,
catalyst was separated, washed with dichloromethane,
dried in vacuum and reused. The product was collected and
were analyzed using gas chromatography equipped with
FID (Agilent 7890A Series) connected to a HP-5 capillary
column (30 m; HP-5). The catalytic activity of pure SBA-
15 and complex 6 were studied under identical conditions
for comparison.
Scheme 2 Schematic
representation of complex (6)
heterogenization on SBA-15
and its application on alcohol
oxidation
123

Potential Catalyst for Oxidation of Alcohols
3 Results and Discussion
Homogenous complex (6) was obtained as per the modified
procedure described in the literature [34–36] with multi-
1
step route as shown in Scheme 1. The H NMR spectrum
of synthesized complex showed broad signal around
5.17 ppm which is characteristic of amide functional group
along with other signal typical of complex 6 (as shown in
Scheme 1). The cobalt content in the complex 6 was cal-
culated by elemental analysis using AAS shows about
7.13 wt%, which is in agreement with theoretical values.
Cobalticinium complex containing siloxane functionality
was heterogenized on the surface of SBA-15 by refluxing
one mmol of complex with one g of support in toluene for
overnight. Elemental analyses using atomic adsorption
spectrum of the grafted sample revealed the presence of
about 0.98 wt% cobalt on SBA-15Co–Si materials. FT-IR
spectra of calcined SBA-15 and cobalticinium complex
grafted samples (SBA-15Co–Si) are depicted in Fig. 1.
Both the samples showed vibrational bands around 1,220,
1,084 and 804 cm-1 which is ascribed to stretching fre-
quency of mesoporous Si–O–Si framework vibration [10–
15, 33, 34]. SBA-15Co–Si showed additional vibrational
band at 1,542 cm-1 due to amide (–NH–) functionality
derived from cobalticinium complex. In addition, presence
of new bands around 2,929, 2,857, 1,498 and 1,451 cm-1
respectively can be assigned to the stretching and bending
vibrational band of -CH- moiety present in the linker group
of complex [10–15, 33, 34]. The powder XRD pattern of
calcined SBA-15 and complex 6 grafted samples are shown
in Fig. 2. SBA-15 showed three well-resolved peaks
around 2h range of 0.7, 1.4 and 1.7° which is characteristic
of (100), (110) and (200) planes of mesoporous hexagonal
SBA-15 materials [10–15]. Cobalt complex grafted sample
(SBA-15Co–Si) showed the shift in 2h value in higher
Fig. 2 Powder XRD pattern of SBA-15 and cobalticinium complex
grafted SBA-15
angles (2h in the range of 0.9, 1.6 and 1.8°) corresponds to
presence of bulky complex present inside the channel of
SBA-15 [14, 15]. Further, it perceives broadening of peaks
and decrease in the relative intensities owing to grafting of
bulk cobalt complex in the channel of SBA-15 framework.
The decrease in interlayer distance from 5.3 to 4.8 nm for
SBA-15Co–Si compared to parent SBA-15 again supports
the complex 6 located inside the channel of mesoporous
molecular sieve.
Figure 3 depicts the N2 sorption studies on SBA-15 and
SBA-15Co–Si samples. The isotherms are typical of type
IV which is based on IUPAC classification and related to
mesoporous channels. Compared to SBA-15 material, the
SBA-15Co–Si material shows a drastic decrease in pore
volume owing to presence of bulk cobalticinium complex
(6) present inside the channel of SBA-15 mesopores. The
textural properties SBA-15 and SBA-15Co–Si are sum-
marized in Table 1. The BET surface area and total pore
volume of SBA-15Co–Si material were found to be
388 m2 g-1 and 0.60 cm3 g-1 respectively, which are
much lower than the pure SBA-15 material (see Table 1).
Fig. 1 FT-IR spectra of pure SBA-15 and cobalticinium complex
grafted SBA-15 materials
Fig. 3 N2 sorption isotherms of different SBA-15 materials
123

T. Baskaran et al.
Table 1 Textural properties of SBA-15 and SBA-15Co–Si materials
S. no
Sample code
Surface area (m2 g-1
)
Pore volume (cm3 g-1
)
Average BJH desorption
pore diameter (nm)
D(100) (nm)
BET
Micro pore (t-plot)
Total
Micro pore
1
2
3
a
SBA-15
769
388
195
41.9
7.80
–
1.10
0.60
0.41
0.018
0.001
0.000
6.3
6.0
5.2
5.3
4.8
–
SBA-15Co–Si
SBA-15Co–Sia
Used catalyst
Fig. 5 29Si CP-MAS NMR spectra of a parent SBA-15 and b SBA-
15Co–Si
Fig. 4 DR-UV Vis spectrum of cobalticinium complex grafted SBA-
15
the covalent grafting of complex 6 on the surface of SBA-15
molecular sieves. Further, the 13C CP-MAS spectrum of SBA-
15Co–Si (Fig. S2 ) showed peaks around 7, 27, and 44 which
are correspond to different aliphatic functionalities arising
from the siloxane functional group [34, 39]. The peak appears
around 165 ppm corresponds to the carbonyl functionality,
further, the broad peaks present around 72 and 113–123 ppm
characteristic of (C4Ph4) and phenyl group respectively [34,
39]. The solid state NMR studies reveal that the successful
grafting and the structural stability of the complex 6 on the
surface of SBA-15 after heterogenization (Fig. 5).
The decrease in surface area and pore volume confirms that
the complex 6 is located on the internal surface of SBA-15
framework. DR-UV–Vis spectrum (Fig. 4) of SBA-15Co–
Si shows an intense adsorption band below 250 and
275 nm along with weak band around 415 nm. The ab-
sorption band of higher energy in ultraviolet region around
250 nm was assigned to the p ? p* transition which is
mainly due to Cp and the other strong band are due to
Metal–Ligand charge transfer [37, 38]. The above fact
further supports the presence of complex 6 in the channel
of mesoporous SBA-15.
Thoroughly characterized SBA-15Co–Si was studied for
liquid phase oxidation of benzhydrol in presence of TBHP
as oxidant and the results are summarized in Table 2. The
SBA-15Co–Si shows about 87 % conversion with nearly
100 % of benzophenone selectivity. Reaction was also
studied on pure SBA-15 support and without catalyst,
which showed about 18 and 20 % conversion respectively.
The above fact supports the presence of cobalticinium
complex 6 on the surface of SBA-15 is responsible for the
observed catalytic activity. Moreover, the negligible ac-
tivity observed on filtrate solution further support that there
is no leaching of active cobalt species. For comparison, the
catalytic activity of homogeneous complex 6 was also
studied (catalyst: 100 mg, equivalent to 0.007 mmol of
The parent SBA-15, and SBA-15Co–Si samples were ex-
amined using solid-state 29Si CP-MAS NMR spectroscopy.
The parent SBA-15 sample exhibits (Fig. 5) three broad
resonances in the 29Si CP-MAS NMR spectrum at chemical
shift (d) values of -110, -101, and -97 ppm, which can be
assigned to the Q4, Q3, and Q2 species of the silica framework,
respectively [Qn = Si(OSi)n(OH)4-n].11-15 The 29Si CP-
MAS NMR spectrum of the cobalt complex grafted sample
shows a major peak centered around -113 and -106 ppm,
attributed to the Q4 and Q3 species, and also exhibits addi-
tional broad signals between -60.5 and -72 ppm, which can
be assigned to the T2 and T3 organosilica species, respectively
(Tm = RSi(OSi)m(OR)3-m) [10–15], strongly supports that
123

Potential Catalyst for Oxidation of Alcohols
(SBA-15Co–Si-s) was studied by FT-IR analysis (Fig. S3 )
and the results are compared with vibrational band of fresh
SBA-15Co–Si catalyst. The spent catalyst shows identical
vibrational bands as of fresh catalyst (SBA-15Co–Si).
However, the intensity of vibrational band corresponds to
C–H stretching frequencies (2,860, 2,940 cm-1) on SBA-
15Co–Si-s enhanced due to organic reactant molecules
adsorbed on the surface. In order to confirm, there is no
leaching of cobalt species during the reaction, quenching
experiment was carried out by filtering the reaction mixture
after 1 h of reaction time and studied for catalytic activity
of the filtrate [40, 41]. The filtrate does not show any
further conversion, which is strongly supporting that there
is no leaching of cobalt complex 6 during the reaction.
Furthermore, the filtrate does not have any cobalt species,
which is also evident from the AAS analysis. The above
fact confirms that the catalysts are truly heterogeneous in
nature. SBA-15Co–Si catalyst was further studied for
oxidation of various aromatic and aliphatic alcohols and
the results are summarized in Table 3. In all the cases,
excellent conversions with selective formation of carbonyl
compounds were observed and the results are comparable
with literature [30]. In addition, cobalt oxide supported on
silica and alumina was also studied under identical condi-
tions, which yielded only 23 and 12 % conversion with an
exclusive formation of carbonyl compound [23]. Thus, the
presence of cobalt complex on the surface of SBA-15 with
uniform distribution favors better activity. To understand
the mechanistic aspects, the reaction was followed in the
presence of radical scavenger BHT (2,6 di-tert-butyl-p-
cresol) and result is shown in Table 2. It is clear from the
table that the conversion of alcohol reduced from 87 to
44 % in presence of radical scavenger. The above facts
supports that the reaction precedes through radical
mechanism on cobalticinim complex grafted SBA-15,
which is similar to that of the cobalt based catalyst [23, 42].
Fig. 6 TGA and DTA profile of used catalyst
Co), which showed about 78 % conversion with TON of
103 which is much less than heterogeneous catalyst. The
recyclability of SBA-15Co–Si catalyst was studied after
the reaction catalyst washed with dichloromethane (DCM)
and it showed comparable activity with the conversion
level of 70 % was observed even after two recycles. The
observed decrease in catalytic activity is owing to the ad-
sorption of some of organic molecules on the active surface
of mesoporous materials [10–15] and it is supported by
TG–DTA (Fig. 6). The TG–DTA profile of spent catalyst
(Fig. 6) exhibits multiple weight loss, the initial weight loss
around 100 °C is due to physisorbed molecules. Drastic
change in weight loss around 160–400 °C is due to the
decomposition of organic moieties, such as amide func-
tional group and cyclopentadienyl group present on the
surface of SBA-15 grafted materials. The accumulation of
organic reactant molecule inside the channel of SBA-
15Co–Si results in decrease in surface area and pore
volume, which is evident from nitrogen sorption analysis
(Table 1). Further, the nature of spent catalyst
Table 2 Oxidation of benzhydrol using various catalysts
S. no
Catalyst
Conversion (%)
TONc
1
Without catalyst
SBA-15
18
20
78
87
69
70
44
–
–
2
3
Complex 6
103
543
431
437
275
4
SBA-15Co–Si
SBA-15Co–Si
SBA-15Co–Si
SBA-15Co–Si
5a
6b
7d
Reaction conditions: Benzhydrol (10 mmol): TBHP (5.5 M in decane) = 1:1, Catalyst = 0.1 gm, T = 70 °C, t = 6 h, Solvent = 5 ml hexane
a
1st recycle
b
2nd recycle
c
calculated based on Co-content
d
Reaction studied in presence of radical scavenger (2 mmol of BHT)
123

T. Baskaran et al.
Table 3 Oxidation of various
alcohols on SBA-15Co–Si
S. no
1
Substrate
Catalyst
Product
Conversion (%)
TON
543
SBA-15Co–Si
87
91
57
2
3
SBA-15Co–Si
SBA-15Co–Si
568
356
OH
4
5
SBA-15 Co–Si
SBA-15 Co–Si
87
82
543
512
Reaction conditions: alcohol:
TBHP (5.5 M in
decane) = 1:1(10 mmol);
Catalyst = 0.1 g, T = 70 °C,
t = 6 h, Solvent = 5 ml hexane
CH3–(CH2)10–CH2–OH
CH3–(CH2)10–CHO
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4 Conclusion
Siloxane functionalized cobalticinium complexes were first
time prepared and heterogenized on the surface of SBA-15
molecular sieves. The grafting of complex 6 on SBA-15
was evident from FT-IR, XRD, 29Si CP-MAS NMR and N2
sorption studies. Heterogenized catalyst showed promising
activity for oxidation of benzhydrol and other alcohols into
corresponding carbonyl compounds. The catalytic activity
remains same after several recycles. There is no leaching of
cobalt species from the surface, which is evident from
quenching studies.
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Acknowledgments The authors are thankful to CSIR (Project No:
01(2516)/11/EMR-II), University of Delhi for their financial support
and to USIC, University of Delhi, for the support on instrumentation
facility. The authors also wish to express their gratitude for the
equipment grant by Humboldt Foundation on FT-IR.
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