Cobalt(III) Schiff base complex: Synthesis, X-ray structure and aerobic epoxidation of olefins

Article abstract of DOI:10.1016/j.poly.2013.03.050

A new Co(III) Schiff-base complex, [Co(HL)₂]NO ₃·H₂O (1) (H₂L = 1-(N-3-methoxy- salicylideneimino)-ethane-2-ol), has been synthesized and characterized by several physicochemical methods. X-ray crystal structure analysis revealed that complex 1 features a distorted octahedral coordination geometry and the metal center is surrounded by two tridented N,O,O-donor Schiff base ligands (HL ^-). [Co(HL)₂]NO₃·H₂O (1) is capable of activating dioxygen in air to catalyze the epoxidation of various alkenes using isobutraldehyde as a co-reductant under homogeneous conditions. It was observed that the yield of the epoxides and their selectivity is highest in acetonitrile medium.

Full text of DOI:10.1016/j.poly.2013.03.050

Polyhedron 56 (2013) 230–236
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Cobalt(III) Schiff base complex: Synthesis, X-ray structure and aerobic
epoxidation of olefins
⇑
Debraj Saha, Tanmoy Maity, Rajesh Bera, Subratanath Koner
Department of Chemistry, Jadavpur University, Kolkata 700 032, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A new Co(III) Schiff-base complex, [Co(HL) ]NO ꢀH O (1) (H L = 1-(N-3-methoxy-salicylideneimino)-eth-
2
3
2
2
Received 8 September 2012
Accepted 24 March 2013
Available online 4 April 2013
ane-2-ol), has been synthesized and characterized by several physicochemical methods. X-ray crystal
structure analysis revealed that complex 1 features a distorted octahedral coordination geometry and
ꢁ
the metal center is surrounded by two tridented N,O,O-donor Schiff base ligands (HL ). [Co(HL)
(
2
]NO
3 2
ꢀH O
1) is capable of activating dioxygen in air to catalyze the epoxidation of various alkenes using isobutr-
Keywords:
Cobalt(III)
Schiff-base complex
Alkene
aldehyde as a co-reductant under homogeneous conditions. It was observed that the yield of the epoxides
and their selectivity is highest in acetonitrile medium.
Ó 2013 Elsevier Ltd. All rights reserved.
Isobutraldehyde
Aerobic epoxidation
1
. Introduction
transition metal catalysts has been proven to be effective
[
19–21]. The aldehyde is transformed in situ to a carboxylperoxo
The catalytic epoxidation of alkenes is a reaction of great
radical that facilitates oxygen transfer to the olefin. Carboxylic acid
is formed as a stoichiometric side product. Transition metal
complexes, such as Schiff base complexes [22], metalloporphyrins
[23] and metal cyclam complexes [24], have demonstrated good
catalytic performance for the aerobic oxidation in the presence of
the co-reductant aldehyde.
Cobalt salts and cobalt containing coordination complexes are
well-known catalysts for the selective oxidation of alkanes and
2
alkylbenzenes with O [13]. Budnik and Kochi have employed
industrial interest because epoxides are widely used as intermedi-
ate chemicals for making valuable products, such as chiral pharma-
ceuticals, epoxy resins, epoxy paints, surfactants, pesticides,
agrochemicals, perfume materials and sweeteners [1–3]. Epoxides
also have numerous applications as precursors in the production
of fine chemicals [4]. Conventionally the epoxidation of alkenes is
carried out with a stoichiometric amount of a peracid [5]. However,
peracids are very expensive and corrosive, are non-selective for
epoxide formation and also lead to the formation of undesirable
products, creating a lot of waste [6]. Therefore, many alternative
cobalt(III) acetylacetonate (acac) for the epoxidation of alkenes
with molecular oxygen [25]. Oxidation of terminal olefins to their
corresponding products, 2-ketones and 2-alcohols, under molecu-
methodologies using single oxygen donor reagents such as NaIO
7], NaOCl [8], PhIO [9], tert-butyl hydroperoxide (TBHP) [10,11]
and H [12] have been applied for the epoxidation of alkenes.
4
[
2
lar O catalyzed by a cobalt(II) Schiff base complex has been
2
O
2
reported [26]. Bis-(diketonato) cobalt(II) derivatives, e.g. Co(acac) ,
2
Unfortunately, these methods are either expensive or dangerous
for environment. Commercial manufacturing methods of epoxides
are the chlorohydrin and Halcon processes [13], which cause
serious environmental pollution. From both an environmental
are active in the aerobic epoxidation of alkenes in the presence of
propionaldehydediethylacetal as the reducing agent [18]. Cobal-
t(II)-salen complexes were reported to show catalytic activity for
2
the epoxidation of alkenes with O in the presence of a sacrificial
and economic point of view, epoxidation of alkenes with O
2
or air
co-reductant, isobutyraldehyde [27]. Kureshy et al. studied the
epoxidation of styrene over Co(II) salen derived Jacobsen type
complexes in the presence of molecular oxygen using isobutyral-
dehyde, which showed up to 45% conversion [28]. Cobalt (II)
calix[4]pyrrole complexes afforded a maximum yield of 68% of
using effective catalysts is a promising and attractive method [14].
Molecular oxygen has been employed for gas-phase epoxida-
tion of propylene over Au nano-particle dispersed titanosilicate
[
3
15] and EuCl –Zn solid catalyst [16]. The Mukaiyama type epoxi-
dation system [17,18], which facilitates the epoxidation of olefins
with oxygen using a sacrificial aldehyde as a reductant over
2
styrene oxide in the presence of 2-ethylbutyraldehyde/O in 24 h
[29]. Gao and co-workers obtained 60% of styrene epoxide over a
cobalt(II) containing zeolite imidazolate framework under aerobic
conditions using isobutyraldehyde as sacrificial reductant [30]. Iq-
bal and co-workers studied various Schiff base cobalt(II) complexes
⇑
Corresponding author. Tel.: +91 33 2457 2778; fax: +91 33 2414 6414.
E-mail address: snkoner@chemistry.jdvu.ac.in (S. Koner).
0
277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.03.050

D. Saha et al. / Polyhedron 56 (2013) 230–236
231
for the epoxidation of alkenes in the presence of either aliphatic
aldehydes or b-ketoesters [14,22]. CoCl was employed for the
oxidation of mono-terpenes with O , and it was found that
predominantly allylic oxidation proceeded [31]. Recently, the
aerobic oxidation of alkenes over Co(II)-compounds without any
co-reductant in DMF have also been studied [32,33]. Opre et al.
achieved oxidation of styrene catalyzed by cobalt salt in DMF with-
out using any sacrificial aldehyde. In this case 49% of styrene oxide
was obtained along with various co-products, such as N-formyl-N-
1 mmol). The solution was then stirred vigorously for 45 min at
room temperature. The resulting deep brown mixture was filtered
and the filtrate was kept undisturbed at room temperature. On slow
evaporation of the filtrate, dark brown rod shaped crystals appeared
in two days. They were collected by filtration and washed with the
mother liquor. Yield ca. 84% based on the metal. Anal. Calc. for
2
2
C H
20 26
N
3
O
10Co: C, 45.55; H, 4.97; N, 7.97. Found: C, 45.5; H, 4.9,
N, 8.0%. The amount of Co in 1 was determined by atomic absorption
ꢁ
2
spectroscopy. Calc: Co, 11.17%. Found: Co, 11.14% (18.95 ꢂ 10
-
methylformamide, CO
2
and dimethylamine [34]. They proposed
mol%). The phase purity of [Co(HL) ]NO O (1) was confirmed
2
ꢀH
3 2
that DMF should be considered as a ‘‘sacrificial’’ solvent that func-
tions as a co-reductant in the epoxidation reaction.
by powder XRD (see Supplementary material, Fig. S1).
Here we report the synthesis, characterization, X-ray single
crystal structure of a cobalt(III) Schiff base complex and its cata-
lytic efficiency towards aerobic epoxidation of olefins using isobu-
tyraldehyde as a co-reductant.
2.5. X-ray crystallography
X-ray diffraction data for 1 were collected at 293(2) K on a Bruker
SMART APEX CCD X-ray diffractometer using graphite-monochro-
mated Mo Ka radiation (k = 0.71073 Å). Determination of integrated
intensities and cell refinement were performed with the SAINT [36]
software package using a narrow-frame integration algorithm. An
empirical absorption correction [37] (SADABS) was applied. The struc-
ture was solved by direct methods and refined using full-matrix
2
. Experimental
2.1. Materials
2
least-squares technique against F with anisotropic displacement
Ethanolamine, o-vaniline, styrene, 4-methyl styrene, trans-stil-
parameters for non-hydrogen atoms with the programs SHELXS97
and SHELXL97 [38]. Hydrogen atoms for crystallized water were freely
refined. For other atoms, the hydrogen atoms were placed at calcu-
lated positions using suitable riding models with isotropic displace-
ment parameters derived from their carrier atoms. In the final
difference Fourier maps there were no remarkable peaks, except
for ghost peaks surrounding the metal centers.
bene, cyclooctene, cyclohexene, 1-hexene, 1-octene and limonene
were purchased from Sigma Aldrich and were used as received.
Cobalt nitrate hexahydrate and the solvents used were purchased
from Merck (India).
2.2. Physical measurements
Elemental analysis was performed by using a Perkin-Elmer 240C
2
.5.1. X-ray crystallography data of [Co(HL)
10Co, M = 527.37, monoclinic, space group P2
Å) = 15.7359(9), (Å) = 10.8344(7), (Å) = 13.3761(10),
Å ) = 2244.7(3), Z = 4, Final R indices [I > 2 (I)] = 0.0654, Dcalc
2
]NO
3
ꢀH
2
O (1)
elemental analyzer. The metal content of the sample was estimated
C
20
H
26
N
3
O
1
/c, a
1
on a Varian Techtron AA-ABQ atomic absorption spectrometer. H
(
(
(
(
b
c
V
1
3
and C NMR spectra of H
2
L and 1 were recorded on a Bruker Avance
. The chemical
3
r
3
00 instrument at ambient temperature in DMSO-d
6
ꢁ3
ꢁ1
g/cm ) = 1.561, F(000) = 1096,
l
(mm ) = 0.827,
Dq
max
shifts (d) and coupling constants (J) were expressed in ppm and Hz,
respectively. Fourier transformed infrared spectra were measured
on a Perkin-Elmer RX I FT-IR spectrometer as KBr pellets. The
UV–Vis spectral measurement was carried out in HPLC grade etha-
nol solvent using a Shimadzu UV–Vis 1700 spectrophotometer. The
powder X-ray diffraction (XRD) patterns of the sample were
ꢁ3
ꢁ3
e Å ) = 0.540,
Dq
min (e Å ) = ꢁ0.485, Goodness-of-fit on
2
F = 0.861, T = 293(2).
2
.6. Catalytic reactions
Catalytic test reactions were performed in a three-necked flask
50 mL) equipped with a reflux condenser, magnetic stirrer and
recorded on a Scintag XDS-2000 diffractometer using Cu Ka radia-
(
tion at the desired temperature. TG–DTA (Thermogravimetry and
Differential Thermal Analysis) measurements were made using a
PerkinElmer (Singapore) Pyris Diamond TG/DTA unit. The heating
gas inlet. The olefin (10 mmol), isobutyraldehyde (10 mmol) and
acetonitrile (20 ml) were added to the flask and immersed in an
oil bath kept at 60 °C. Bubbling air at atmospheric pressure was
passed through the reaction mixture at a flow rate of ca. 5
ml/min under vigorous stirring. The mixture was then equili-
brated to the desired temperature in an oil bath. The reaction
was started after addition of catalyst (1 mg) into the reaction mix-
ture. The products of the epoxidation reactions were collected at
different time intervals and were identified and quantified by
gas chromatography. The turnover frequency (TOF) was calculated
as follows:
ꢁ1
rate was programmed at 5 °C min with a protecting stream of
ꢁ1
2
N flowing at a rate of 150 ml min . The products of the catalytic
reactions were identified and quantified on a Varian CP-3800 Gas
Chromatograph using a CP-Sil 8 CB capillary column.
2
2
.3. Synthesis of the Schiff base (H L)
The Schiff-base was prepared by following the reported proce-
dure with some modifications [35]. A 10 ml ethanolic solution of
-ethanolamine (0.244 g, 4 mmol) and o-vaniline (0.608 g, 4 mmol)
were mixed together in a flat bottom flask. The mixture was then
refluxed for 30 min. The resulting yellow colored solution was then
cooled to room temperature. Ethanol was then separated almost
completely from the mixture using a rotary evaporator. The dense
yellow liquid thus obtained was used as the ligand. Anal. Calc. for
moles converted
TOF¼
2
moles of cobaltðactive siteÞtaken for reactionꢂreaction time
ꢂ100
3
. Result and discussion
.1. IR spectroscopic study
A strong peak appears at 1655 cm in IR spectrum of 1, which
10 3
C H13NO : C, 61.53; H, 6.71; N, 7.18. Found: C, 61.6; H, 6.6; N, 7.1%.
3
2
.4. Synthesis of the complex [Co(HL)
2
]NO
3
2
ꢀH O (1)
ꢁ1
A 10 mL ethanolic solution of Co(NO
was added dropwise to a 10 mL ethanolic solution of H
3
)
2
ꢀ6H
2
O (0.29 g, 1 mmol)
L (0.179 g,
is attributed to the vibration band for the azomethine (>C@N–)
group. The shift of this band towards lower frequencies compared
2

2
32
D. Saha et al. / Polyhedron 56 (2013) 230–236
for six coordinated Co(III) complexes, 1 shows only one broad
low intensity absorption band at ca. 800 nm, corresponding to
1
1
the
A1g ? T1g transition. The high energy band corresponding
1
1
to the A1g ? T2g transition is obscured by a charge transfer band
ꢁ
Fig. 1. Coordination mode of HL in 1.
appearing in the lower wavelength region. A hump is observed in
⁄
the region ca. 455 nm which can be assigned to the n ?
p
transi-
tion in the ligand [41]. In the higher energy region of the spectrum,
1 exhibits an intense band at ca. 386 nm, which may be attributed
to a ligand to metal charge transfer transition.
Table 1
0
Selected bond lengths ( AÅ ) and angles (°) for 1.
Bond lengths
Bond angles
3
.3. Thermal analysis
Co1–O2
Co1–O3
Co1–O5
Co1–O6
Co1–N1
Co1–N2
Bond angles
O2–Co1–O3
O2–Co1–O5
O2–Co1–O6
O2–Co1–N1
1.869(3)
1.851(3)
1.950(3)
1.973(3)
1.892(4)
1.884(4)
O2–Co1–N2
O3–Co1–O5
O3–Co –O6
O3–Co1–N1
O3–Co1–N2
O5–Co1–O6
O5–Co1–N1
O5–Co1–N2
O6–Co1–N1
O6–Co1–N2
N1–Co1–N2
95.10(16)
178.03(14)
89.36(14)
95.99(19)
88.55(19)
88.68(14)
83.95(18)
91.35(18)
91.45(18)
83.94(18)
173.5(2)
Thermogravimetric analysis confirms that 1 is thermally stable
up to ca. 100 °C (see Supplementary data, Fig. S4). The TG curve
indicates that the mass loss of ca. 3.4% in the temperature range
1
cule. The corresponding DTA curve shows a very small endother-
mic peak at ca. 135 °C. Thereafter the TG curve remains flat up to
ca. 210 °C, indicating no mass loss in this temperature range. On
further heating the TG curve shows a continuous mass loss up to
00–145 °C corresponds to the loss of one crystalline water mole-
92.42(14)
89.55(14)
177.96(14)
89.36(16)
4
50 °C due to decomposition of the complex. The complex decom-
poses steadily in two steps, 210–370 and 370–450 °C, and the DTA
curve shows exothermic peaks at ca. 260, 335 and 398 °C.
1
13
3
.4. H and C NMR spectra
1
The H NMR spectra of H
2
L and 1 are shown in Figs. S5 and S6 in
1
Supplementary data. The H NMR spectrum of the ligand shows a
peak at 8.65 ppm, characteristic of the azomethine proton
1
(
–CH@N–). In H NMR spectrum of 1, the –CH@N– proton peak is
slightly shifted downfield. This difference in the (–CH@N–) peak po-
sition could arise due to the coordination of its neighboring nitrogen
atom, which indicates the ligand coordinates to the metal through
2
the azomethine nitrogen atom. The peak at 5.18 ppm for H L is
assignable to the phenolic proton. The phenolic proton disappears
in the spectrum of 1. This supports the proposal that the phenolic
oxygen atom of the ligand has coordinated with the metal ion in
the deprotonated form. The aromatic ring protons are observed in
the 7.1–6.4 ppm range. The methylene (–CH
hydroxymethyl group resonate in the range 4.5–4.3 ppm. The
methyl (–CH ) protons of the methoxy group and methylene
(–CH –) protons adjacent to the azomethine group appear in the
range 3.5–3.1 ppm and are overlapped with the DMSO proton
peaks. The hydrogens of the hydroxymethyl group and crystallized
2
–) protons of the
Fig. 2. Single crystal X-ray structure of 1.
3
to that of the free Schiff base ligand (ca. 1690 cmꢁ1) indicates the
2
coordination of the imine nitrogen atom to the metal center (see
Supplementary material, Fig. S2). A broad medium intensity band
ꢁ1
water molecule are exchanged with the D
2
O in the DMSO and there-
appears at ca. 3425 cm that can be attributed to the
m(OH) vibra-
fore appear at 2.48 ppm with a low integral value.
tion of the hydroxymethyl group and the crystallized water mole-
1
3
ꢁ
1
The C NMR spectra of H
Figs. S7 and S8) reveal the presence of the expected number of
signals corresponding to different types of carbon atoms. The 13
NMR spectrum of H L shows a peak at 166.56 ppm, assignable to
2
L and 1 (see Supplementary data,
cule. Peaks appearing in the range 3060–2610 cm are due to the
stretching vibrations of methyl (–CH ), methylene (–CH –) and
aromatic CH bonds. Strong peaks appearing in the range of
3
2
C
ꢁ
1
2
1
(
472–1303 cm are due to the stretching vibrations of the nitrate
-
the azomethine carbon (–CH@N–). This peak is slightly shifted
downfield in 1, indicating the involvement of the azomethine
nitrogen atom in coordination.
NO
groups. Bands at 1248–1068 cm
The strong band for the phenolic C–O groups, appearing at
3
) ion and bending vibrations of methyl and methylene
ꢁ1
are attributed to the
m(CO).
ꢁ1
1
232 cm in IR spectrum of the protonated free ligand H
2
L, is
ꢁ1
3.5. Crystal structure of [Co(HL)
2
]NO
3 2
ꢀH O (1)
shifted to a lower frequency (1216 cm
in 1), indicating the
deprotonation and coordination of the phenolic oxygen atoms to
ꢁ
1
The XRD measurement of 1 reveals that the complex crystal-
lizes in the monoclinic space group P2 /c. An ORTEP view with
the atom numbering scheme of compound 1 is shown in Fig. S9
see Supplementary data) and bonding parameters are summa-
rized in Table 1. [Co(HL) ]NO O (1) shows a cationic monomeric
ꢀH
unit, [Co(HL) ], along with one water molecule of crystallization
the metal center. The bands at 470 and 454 cm may be ascribed
to (Co–N) and (Co–O), respectively [39,40]. The coordination
mode of HL in 1 is shown in Fig. 1.
1
m
m
1ꢁ
(
2
3
2
3.2. UV–Vis spectrum
2
and a discrete nitrate anion. The unique metal center (Co1) is
ꢁ
UV–Vis spectrum of an ethanolic solution of 1 was recorded at
surrounded by two Schiff base ligands (HL ), forming a distorted
ꢁ
room temperature (see Supplementary material, Fig. S3). Out of the
octahedron (Fig. 2). HL act as a tridentate ligand with one pheno-
lato-O, one imine-N and one hydroxyl-O as the donor sites. The
two usual d–d transitions, 1
A
1g ? T1g and 1
1
A
1g ? T2g, observed
1

D. Saha et al. / Polyhedron 56 (2013) 230–236
233
Table 2
i
Epoxidation of olefins catalyzed by 1 .
TOF (h 1
ꢁ
)
Substrate
Conversion (wt.%)
% Yield of product
Epoxide
62
Others
25ii
8
7
573
9
2
92
–
606
1
9
1
00
0
100
90
–
659
593
659
–
00
46
54iii
1
00
51
49iv
659
9
8
0
8
79
11v
593
580
88vi
–
i
ꢁ3
Reaction conditions: alkenes (10 mmol); isobutyraldehyde (10 mmol); catalyst (1.0 mg, 1.9 ꢂ 10 mmol); flow rate of air ca. 5 ml/min; solvent (acetonitrile (20 mL));
temperature 60 °C. The products of the epoxidation reactions were collected at different time intervals and were identified and quantified by a Varian CP-3800 gas
chromatograph equipped with an FID detector and a CP-Sil 8 CB capillary column. Here the results of the catalytic reactions are given after 8 h of reaction.
ii
2
-Cyclohexen-1-ol and 2-cyclohexene-1-one.
Benzaldehyde and benzoic acid.
-Methylbenzaldehyde and 4-methylbenzoic acid.
iii
iv
v
4
Benzaldehyde.
Limonene epoxide.
vi
basal plane of the Co(III) center is formed by the two imine nitro-
gen (N1, N2) from the two Schiff base ligands, one phenolato
oxygen (O3) and one hydroxyl oxygen (O5) from one Schiff base
ligand, while the axial positions are occupied by one phenolato
oxygen (O2) and one hydroxyl oxygen (O6) from the other Schiff
base ligand. The axial O2–Co–O6 angle [177.96(14)°] is close to
the ideal 180°. In the equatorial plane, the cis and trans angles vary
from 83.95(18)° to 95.99(19)° and 178.03(14)° to 173.5(2)°, respec-
tively, indicating the nature of the distortion in the coordination
sphere. The bond lengths and bond angles are found to be in the
same range as the corresponding values in other octahedral cobal-
several hydrogen bonding interactions, which play a significant
role in the stabilization of 1. The donor hydrogen atoms H5 and
H6 on the hydroxyl oxygen atoms O5 and O6 are engaged in hydro-
gen bond formation with the acceptor water oxygen atom O7 and
the nitrate oxygen atom O9, respectively (see Supplementary
material, Fig. S10). In addition, the lattice nitrate oxygen atoms
(O8 and O10) interact with H1Z and H2Z, respectively, of the water
oxygen atom O7 through H-bonds, enclosing a twelve-membered
ring motif which can be designated through Etter’s graph set nota-
4
tion as R
2
(12). The corresponding hydrogen bonding parameters
are listed in Table S1 (see Supplementary material). Such
O–Hꢀ ꢀ ꢀO bonding interactions give rise to the formation of an
H-bonded zigzag chain-like structure.
ꢁ
t(III) Schiff base complexes [42]. The hydroxyl groups of HL of
each monomeric unit, nitrate ions and water molecules exhibit

2
34
D. Saha et al. / Polyhedron 56 (2013) 230–236
idation of various olefins is summarized in Table 2. Due to the dif-
ferent activity of the double bonds, the results of various olefins
differed significantly. Epoxidation of an endocyclic alkene, such
as for cylcooctene, 92% conversion and 100% epoxide selectivity
were obtained because of its highly active double bond and the rel-
ative stability of 1,2-epoxycyclooctane. For cyclohexene, only 71%
epoxide selectivity was obtained, since its active allylic site could
also be attacked in the oxidation process. Linear aliphatic olefins,
such as 1-hexene and 1-octene, converted to their corresponding
1
1
,2-epoxyalkanes as the sole product. While the conversion of
-octene was 90%, 1-hexene was converted fully to its epoxide.
This may due to the large hexyl group of 1-octene connected to
double bond which sterically hinders its approach towards the
active site, while 1-hexene with a smaller butyl group can have
access to the active site with some ease [43]. Styrene also has been
converted fully to its oxidized products, giving 46% yield of epox-
ide, while the remaining products were benzaldehyde and benzoic
acid. The yield of epoxide increased to 51% when an electron-
donating group, methyl, was present at the para-position of
styrene; along with the epoxide, 4-methylbenzaldehyde and
Fig. 3. Kinetic plots for the epoxidation of olefins catalyzed by compound 1.
4
-methylbenzoic acid were also formed. The difference in the
conversion between styrene, 4-methylstyrene and trans-stilbene
may be attributed to the electronic effect of the substituents
[
44]. Epoxidation of limonene is catalyzed by complex 1 smoothly
and shows 88% conversion with 100% epoxide selectivity. Da Silva
et al. studied the oxidation of limonene with dioxygen, homoge-
neously catalyzed by CoCl in acetonitrile medium, however in this
2
case, along with epoxide, an equal amount of allylic products also
formed [31]. Profiles of the olefinic epoxidation reactions catalyzed
[
Co(HL)
In a previous study, Buranaprasertsuk et al. proposed a mecha-
nism for the metal complex catalyzed oxygenation of organic
substrates by O and aldehyde [29]. At first a peroxyacid is gener-
ated from O and the aldehyde via a radical mechanism. The metal
2
]NO
3
ꢀH
2
O (1) are given in Fig. 3.
2
2
complex catalyzes the process to release a peroxy radical from the
peroxyacid, which subsequently reacts with olefin to produce
epoxide and other products through oxygen transfer. We propose
a similar mechanism is operative in our case. To probe if radicals
were involved in our reactions, a radical scavenger, hydroquinone,
was added after 2 h of the reaction, and the progress of reaction
was monitored. It was observed that the reaction stopped immedi-
ately upon addition of the radical scavenger (Fig. 4), which
suggests that the process indeed proceeds via radical formation.
The catalytic reaction was performed in a variety of solvents to
study the catalytic efficacy of 1 in different solvent media. The best
performance of the catalyst was observed in the acetonitrile
medium (Table 3). The efficiency of the catalyst followed the order:
acetonitrile > acetone > ethanol > methanol > chloroform > dichlo-
Fig. 4. % Conversion vs. time plot for comparison between reactions with a radical
scavenger and a control reaction without adding a radical scavenger.
3.6. Catalytic activity studies
The catalytic activity of 1 was investigated in the epoxidation of
both cyclic and straight chain alkenes using air as the oxidant in
homogeneous medium. The catalytic performance of 1 in the epox-
Table 3
Influence of solvent on the compound 1 catalyzed epoxidation reaction.i
Substrate
% of conversionii (% of epoxide yield) in different solvent
CH CN CH COCH
3
3
3
C
2
H
5
OH
CH
3
OH
CHCl
3
2 2
CH Cl
9
2 (92)
80 (80)
74 (74)
65 (65)
56 (56)
42 (42)
1
00 (46)
85 (37)
72 (47)
66 (42)
52 (33)
40 (21)
i,ii
Reaction conditions were the same as given in the footnote of Table 2.

D. Saha et al. / Polyhedron 56 (2013) 230–236
235
Table 4
Temperature effect on the compound 1 catalyzed epoxidation reaction.
i
Substrate
% of conversionii (% of epoxide yield) at different temperature
3
4
0 °C
40 °C
50 °C
60 °C
70 °C
80 °C
6 (46)
58 (58)
76 (76)
92 (92)
96 (95)
98 (98)
5
3 (51)
66 (49)
82 (47)
100 (46)
100 (41)
100 (32)
i,ii
Reaction conditions were the same as given in the footnote of Table 2.
Table 5
i
Epoxidation of olefins catalyzed by cobalt salts.
TOF (h 1
ꢁ
)
Substrate
Catalyst
Conversion (wt.%)
% Yield of product
Epoxide
24
Others
–
Co(ac)
Co(ac)
2
ꢀ4H
ꢀ4H
2
O
O
24
58
158
34ii
380
2
2
24
[
[
Co(acac)
3
]
25
60
25
24
–
164
395
Co(acac)
3
]
36ii
i
ꢁ3
Reaction conditions were the same as given in the footnote of Table 2. Amount of the catalyst = 1.9 ꢂ 10 mmol.
ii
Benzaldehyde and benzoic acid. ac = acetate acac = acetylacetonate.
romethane. The catalytic reaction was also performed at different
temperatures to study the temperature dependency of the catalytic
efficacy of 1 (Table 4). In general, the reaction rate was enhanced
with an increase in the reaction temperature. The epoxide selectiv-
ity, as well as conversion for cyclooctene, increased with increasing
temperature, but above 60 °C the temperature had little effect on the
cyclooctene conversion. Conversely, the styrene epoxide selectivity
decreased slowly with increasing temperature up to 60 °C, then for
higher temperatures the selectivity fell abruptly. Considering both
selectivity and conversion, 60 °C was the obvious choice for the cat-
alytic reaction. A comparison of the catalytic efficiency of 1 with
other cobalt salts (cobalt(III) acetylacetonate and cobalt(II) acetate)
in homogeneous medium (Table 5) clearly indicates that complex 1
performs as a more efficient catalyst than simple cobalt salts.
lent catalytic efficacy towards the epoxidation of alkenes, as
reflected in the high turnover frequency of the reactions. The use
of different solvents, temperatures and catalyst to substrate ratios
influence the catalytic reactions. The best catalytic activity of the
complex is observed in acetonitrile at 60 °C. This demonstrates
the choice of an appropriate solvent and temperature is important
in catalytic reactions. The complex shows far better catalytic activ-
ity than simple cobalt salts, such as cobalt(III) acetylacetonate and
cobalt(II) acetate.
Acknowledgments
Financial support from CSIR, New Delhi by a grant (Grant No.
0
1(2542)/11/EMR-II) (to S.K.) is gratefully acknowledged. The
Department of Chemistry, Jadavpur University received funds from
DST under the DST-FIST program to procure a single-crystal XRD
instrument.
4
. Conclusion
In summary, the catalytic efficiency of a newly synthesized and
structurally characterized cobalt(III) Schiff base complex for the
olefin epoxidation reaction under homogeneous conditions with
molecular oxygen as an oxidant and isobutraldehyde as a co-reduc-
tant has been thoroughly investigated. The complex shows excel-
Appendix A. Supplementary data
CCDC 900117 contains the supplementary crystallographic data
for complex 1. These data can be obtained free of charge via http://

2
36
D. Saha et al. / Polyhedron 56 (2013) 230–236
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: +44 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2013.03.050.
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