Azide/thiocyanate incorporated cobalt(III)-Schiff base complexes: Characterizations and catalytic activity in aerobic epoxidation of olefins

Article abstract of DOI:10.1016/j.ica.2014.02.036

Reaction of cobalt(II) perchlorate hexahydrate with a potentially tetradentate Schiff base ligand, HL (2-methoxy-6-[(2-diethylaminoethylimino) methyl]phenol) in presence of sodium azide and sodium thiocyanate yields two complexes [Co(L)(HL)(N₃)

Full text of DOI:10.1016/j.ica.2014.02.036

Inorganica Chimica Acta 415 (2014) 103–110
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Inorganica Chimica Acta
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Azide/thiocyanate incorporated cobalt(III)-Schiff base complexes:
Characterizations and catalytic activity in aerobic epoxidation of olefins
b
c
c
d
Shyamapada Shit a, , Debraj Saha , Dipankar Saha , Tayur N. Guru Row , Corrado Rizzoli
⇑
a Department of Chemistry, Jalpaiguri Government Engineering College, Jalpaiguri 735 102, West Bengal, India
b Department of Chemistry, Jadavpur University, Kolkata 700 032, West Bengal, India
c Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560 012, India
d Università’ degli Studi di Parma, Dipartimento di Chimica, Parco Area delle Scienze 17/A, I-43124 Parma, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 22 July 2013
Received in revised form 23 January 2014
Accepted 22 February 2014
Available online 10 March 2014
Reaction of cobalt(II) perchlorate hexahydrate with a potentially tetradentate Schiff base ligand, HL
(2-methoxy-6-[(2-diethylaminoethylimino)methyl]phenol) in presence of sodium azide and sodium
thiocyanate yields two complexes [Co(L)(HL)(N3)]ꢀClO4 (1) and [Co(L)(HL)(NCS)]ꢀClO4 (2); both being
characterized by different physicochemical methods. Crystal structure of 1 was determined by single
crystal X-ray diffraction while that of 2 was reported earlier. In 1, the central cobalt(III) adopts slightly
distorted octahedral geometry with same donor set to that of 2. Catalytic efficacy of the complexes
towards epoxidation of different alkenes under aerobic condition were investigated in homogeneous
medium which reveals that 1 is better catalyst than 2 with respect to alkene oxidation, reflected from
the turn over frequencies (TOF) measured at an optimum temperature of 60 °C in acetonitrile.
Ó 2014 Published by Elsevier B.V.
Keywords:
Cobalt(III) Schiff base complexes
Spectral and structural characterizations
Homogeneous catalysis
Olefin epoxidation
Aerobic condition
1. Introduction
hydroperoxide (TBHP) [23–25], and H2O2 [26–29] were used which
also appeared as either expensive or dangerous for environment.
Cobalt(III) complexes derived from symmetrical and unsym-
metrical Schiff bases have drawn extensive attention in the domain
of metallo-organic and coordination chemistry due to their syn-
thetic accessibility, fascinating structural features and diverse
range of applications covering organic synthesis, photochemistry
[1], magnetic and electrochromism [2–8], and catalysis [9,10]. Co-
balt(III)-Schiff base complexes also find important biological appli-
cations [11–14].
On the other hand, epoxidation of alkenes remains a thrust area
of research among the various catalytic processes used industrially
as epoxides are important intermediate chemicals for making valu-
able products [15–17], and also used as precursors for many fine
chemicals [18]. Epoxidation of alkenes was performed using differ-
ent single oxygen donor reagents; each of them possesses certain
disadvantages. For example, stoichiometric amount of peracid
[19] was used as conventional alkene epoxidation catalyst but its
use was limited for its high cost and corrosivity, non-selectivity
for the epoxide formed, as well as for the formation of undesirable
products, creating a lot of waste. Similarly, several other single
oxygen donors such as NaIO4 [20], NaOCl [21], PhIO [22], tert-butyl
Manufacturing methods of epoxides by chlorohydrin and Halcon
processes [30] also cause serious environmental pollution. There-
fore from both environmental and economic concern, use of O2
or naturally cheap air over effective catalysts becomes a promising
and attractive method for epoxidation of alkenes [31–34].
Many efforts [35,36] have been made for epoxidation of alkenes
with molecular O2 or air however, among them, Mukaiyama epox-
idation system [37,38] appeared as very effective [39–41]. In this
method, the sacrificial aldehyde, acted as reductant, was trans-
formed to a carboxylperoxo radical in situ, facilitating oxygen
transfer to the olefin and generated carboxylic acid as a stoichiom-
etric side product. Schiff’s base transition metal complexes [42],
metalloporphyrins [43], and meta-cyclam complexes [44] also
demonstrated high catalytic performance for the aerobic oxidation
in the presence of co-reactant aldehyde.
Epoxidation of alkenes with O2 using simple cobalt salts [38,45]
or cobalt(II) Schiff base complexes [31,37,42,46,47] in presence of
different sacrificial co-reductant, e.g. ketone [31,37,42], b-ketoester
[31,42], ethers [38], and aldehyde [31,42,47] has been investigated.
Cobalt(II)-salen derived Jacobsen type complexes were also used as
catalysts for aerobic asymmetric epoxidation [48] though the for-
mation of l-oxo dimers and other polymeric species were the ma-
jor limitations in homogenous solutions which ultimately led to
deactivation of the catalyst. Oxidation of mono-terpenes with O2
⇑
Corresponding author. Tel.: +91 9433261720.
E-mail address: shyamakgcchem@gmail.com (S. Shit).
http://dx.doi.org/10.1016/j.ica.2014.02.036
0020-1693/Ó 2014 Published by Elsevier B.V.

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S. Shit et al. / Inorganica Chimica Acta 415 (2014) 103–110
was also investigated using cobalt(II) chloride and it was observed
that allylic oxidation proceeds predominantly [49]. Recently, aero-
bic oxidation of alkenes over cobalt(II)–catalyst using TBHP initia-
tor in DMF without any co-reductant have been studied [50,51]
and in some cases DMF itself also performed the role of sacrificial
co-reductant [33,52].
3.65 (t, J = 7.5 Hz, 2H4), 3.93 (s, 3H9), 6.97 (t, J = 7.9, 1H7), 7.17 (d,
J = 7.8 Hz, 1H6), 7.21 (d, J = 7.8 Hz, 1H8), 8.53 (s, 1H5), 11.12 (s,
1H10
) d 165.4
ppm (Scheme II). 13C NMR (75 MHz, CDCl3):
(8C = N), 152.9 (1C–OH), 148.6 (2C–OMe), 122.9 (6C–C = N), 118.3
(5CH), 117.4 (4CH), 113.8 (3CH), 57.1 (10CH2), 56.0 (O7CH3), 53.3
(9CH2), 47.4 (11CH2) 12.0 (12CH3) ppm (Scheme II).
In this paper we report the synthesis of a new cobalt(III) com-
plex [Co(HL)(L)(N3)]ꢀClO4 (1) derived from Co(ClO4)2ꢀ6H2O, NaN3
and Schiff base ligand, HL, 2-methoxy-6-[(2-diethylaminoethyli-
mino)methyl] phenol, obtained by 1:1 condensation of o-vanillin
and 2-diethylaminoethylamine in methanol (Scheme I). Title com-
plex is characterized by elemental analyses, FTIR, UV-Vis, EPR,
magnetic, and cyclic voltammetric methods and its crystal struc-
ture was established by single crystal X-ray diffraction and com-
pared to that of the reported 2. Their catalytic potentiality under
homogeneous condition towards aerial olefin epoxidations in pres-
ence of the sacrificial co-reductant, isobutyraldehyde was also
investigated.
2.2.2. Synthesis of [Co(L)(HL)(N3)]ꢀClO4 (1)
To a stirred methanolic solution (20 mL) of cobalt(II) perchlo-
rate hexahydrate (1 mmol, 0.366 g), methanolic solution (10 mL)
of Schiff base (HL) (2 mmol, 0.250 g) was added very slowly with
constant stirring, followed by drop wise addition of an aqueous
solution (10 mL) of sodium azide (1 mmol, 0.065 g). Reaction mix-
ture was stirred for 30 min. and filtered. The filtrate kept for slow
evaporation at room temperature. Deep green crystals of 1 were
separated after 10 days. Anal. Calc. for [C28H43ClCoN7O8]: C,
48.04; H, 6.19; N, 14.01; Found, C, 47.99; H, 6.11; N, 13.95%.
2.3. Physical measurements
2. Experimental
Elemental analyses (carbon, nitrogen and hydrogen) were car-
ried out using a Perkin–Elmer 2400 II elemental analyzer. Infrared
spectra were recorded on a Perkin–Elmer Spectrum RX FTIR instru-
ment in the range of 4000–400 cmꢁ1 as KBr pellets. The UV-Vis spec-
tra in solution were recorded at room temperature on a Perkin Elmer
Lambda 40 UV–Vis spectrophotometer using acetonitrile in 1 cm
quartz cuvettes. 1H NMR and 13C NMR spectra of HL was recorded
on a Bruker 300 MHz FT-NMR spectrometer using tetramethylsilane
as internal standard in CDCl3. Electrochemical measurements were
performed using a PAR VersaStat- potentiostat/ Galvanostat II elec-
trochemical analysis system under a dry argon atmosphere using
conventional three electrode configurations in acetonitrile with tet-
rabutylammonium perchlorate as the supporting electrolyte. Platin-
ised platinum milli electrode and saturated calomel electrode (SCE)
were used as working and reference electrodes, respectively, along
with a platinum counter electrode in cyclic voltammetry performed
2.1. Materials
Caution! Perchlorate salts of metals in presence of organic li-
gands are potentially explosive. Even though we did not encounter
any problems they should be prepared in small amounts and be
handled with caution.
Cobalt(II) perchlorate hexahydrate, o-vanillin, 2-diethylamino-
ethylamine, styrene, 1-heptene, 1-octene, cycloheptene, and cyclo-
octene were purchased from Aldrich Chemical Co. and were used
as received. Sodium azide, sodium thiocyanate and solvents were
purchased from E. Merck (India). The solvents were distilled and
dried before use. Schiff base (HL) and complex 2 were synthesized
following the literature procedure [53].
2.2. Syntheses
at a scan rate of m
= 50 mV secꢁ1. The room temperature X-band EPR
spectra were recorded on an X-band JEOL JES FA200 ESR Spectrom-
eter. Magnetic susceptibilities were measured with a model 155 PAR
vibrating sample magnetometer fitted with a Waker Scientific 175
FBAL magnet using Hg[Co(SCN)4] as the standard. The necessary dia-
magnetic corrections for the ligand were done using Pascal’s table.
The products of the catalytic reactions were identified and quanti-
fied by Agilent HP 6890 series gas chromatograph using a HP-5 GC
column.
2.2.1. Synthesis of the Schiff base ligand (HL)
The Schiff base HL was prepared following the literature proce-
dure [53]. Briefly, o-vanillin (5 mmol, 0.760 g) and 2-diethylamino-
ethylamine (5 mmol, 0.710 g) in 20 mL methanol was refluxed for
half an hour. The resulting yellow solution containing the Schiff
base ligand (HL) was cooled and subjected to TLC which revealed
the presence of some starting materials along with the Schiff base
product. The Schiff base ligand was isolated by column chromatog-
raphy over silica gel (SRL) 60–120 mesh size, using a mixture of
light petroleum and ethyl acetate (v/v, 2:3); subsequent evapora-
tion of this eluent yielded the pure ligand in liquid form. The puri-
fied liquid ligand was then evaporated under reduced pressure to
yield a gummy mass, which was dried and stored in vacuo over
fused CaCl2 for subsequent use. Yield: 0.814 g (65%). Anal. Calc.
for C14H22N2O2 (250.34): C, 67.17; H, 8.86; N, 11.19. Found: C,
67.10; H, 8.76; N, 11.15%. 1H NMR (300 MHz, CDCl3): d 1.29 (t,
J = 7.7 Hz, 6H1), 2.72 (t, J = 7.5 Hz, 2H3), 3.48 (q, J = 7.7 Hz, 4H2),
2.4. X-ray crystallography
A diffraction quality deep brown plate shaped crystal of 1
(dimension; 0.14 ꢂ 0.11 ꢂ 0.03 mm) was mounted on ‘Xcalibur
Eos, Nova’ CCD diffractometer equipped with mirror monochroma-
tized Mo X-ray radiation source (Mo Ka = 0.71073 Å) at 298(2) K.
CrysAlisPro [54] was used for preliminary determination of the cell
constants, data collection strategy, and for data collection.
Scheme I. Synthetic scheme for Schiff base (HL).

S. Shit et al. / Inorganica Chimica Acta 415 (2014) 103–110
105
Table 1
Crystal data and structure refinement parameters for 1.
1
Empirical formula
C
28H43ClCoN7O8
Formula weight
700.07
Crystal dimension (mm)
0.14 ꢂ 0.11 ꢂ 0.03
monoclinic
P21/c
9.620(5)
31.00(2)
11.108(8)
105.87(6)
3186(4)
4
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Scheme II. Proton and carbon numbering scheme of HL.
Following data collection, CrysAlisPro [54] was also used to inte-
grate the reflection intensities. Multi-scan absorption corrections
were applied to the intensity data (Tmax = 0.980, Tmin = 0.911)
empirically using spherical harmonics, implemented in SCALE3
ABSPACK [55] scaling algorithm, followed by cell refinement and
data reduction. The crystal structure was solved by direct methods
using the program SHELXS-97 [56] and refined with full-matrix least-
squares based on F2 using program SHELXL – 97 [56]. All non-hydro-
gen atoms were refined anisotropically. The ethyl groups (atoms
C11–C14) and one carbon atom of the ethylene bridge (atom
C10) bound to the N2 nitrogen atom are disordered over two sets
of orientations (called A and B) with refined occupancy ratio
0.547(4):0.453(4). During the refinement the N—C and C—C bond
lengths involving the disordered atoms were restrained to be
1.50(1) and 1.54(1) Å, respectively. SIMU and EADP restraints
[56] were applied to the disorder components. The perchlorate an-
ion was also found to be rotationally disordered about the Cl1—O8
bond over two orientations (called A and B, with refined occupancy
ratio 0.547(4):0.453(4)) which were refined with a SADI distance
restraint [56], and with the anisotropic displacement ellipsoids of
pairs of disordered oxygen atoms equivalenced by the EADP com-
mand. The amine H atom was located in a difference Fourier map
and refined freely. All other H atoms were placed at calculated
positions and refined using a riding model approximation, with
C—H = 0.93–0.97 Å, and with Uiso(H) = 1.2 Ueq(C) or 1.5 Ueq(C) for
methyl H atoms. A rotating model was applied to the methyl
groups. The publication materials and crystallographic illustrations
for 1 were prepared using PLATON [57] and ORTEP [58] programs.
Crystallographic parameters, details of data collection and refine-
ment procedures for 1 are summarized in Table 1.
b (°)
V (Å3)
Z
T (K)
298(2)
0.71073
1.460
0.682
1472
kMo
(Å)
a
K
Dc (g cmꢁ3
)
l
(mmꢁ1
)
F(000)
h (°)
hkl ranges
Total data
Unique data
Observed data [I > 2
Nref; Npar
2.56ꢁ25.5
ꢁ11 6 h 6 11, ꢁ37 6 k 6 37, ꢁ13 6 l 6 13
25570
5764
r
(I)]
3753
5764; 443
R1 = 0.0675, wR2 = 0.1611
R1 = 0.1083, wR2 = 0.1944
0.083
1.027
0.65
ꢁ0.51
Final R indices [I > 2
R indices (all data)
Rint
r
(I)]
Goodness-of-fit (GOF) on F2
D
D
qmax (e Åꢁ3
qmin (e Åꢁ3
)
)
the free ligand (3445 cmꢁ1) disappeared in the spectra of the com-
plexes suggested deprotonation of this phenolic-OH group during
complexation. Coordination of the phenolic oxygen is further sub-
stantiated by lowering of phenolic C–O stretching vibration of free
ligand (1255 cmꢁ1) which appeared at 1226 and 1224 cmꢁ1 in the
spectra of 1 and 2, respectively. The bands at 2943 for 1 and
2941 cmꢁ1 for 2 may be assigned to alkyl C–H bond stretching of
methoxy group [14]. The band for mN„N for terminal azide in 1
and mC„N of the terminal NCS group in 2 appeared at 2027 and
2081 cmꢁ1. Coordination of nitrogen to cobalt(III) center (mCo–N
)
is also evident by the appearance of weak bands at 492 and
435 cmꢁ1 for 1 and 490 and 431 cmꢁ1 for 2, respectively. A sharp
strong peak appeared at 1087 and 1086 cmꢁ1 in the spectra of 1
and 2 may be assignable to perchlorate stretching vibrations and
this single peak without splitting evidenced the presence of non-
coordinating perchlorate anion [62].
2.5. Catalytic reactions
The catalytic reactions were carried out in a glass batch reactor
according to the following procedure. Substrate (10 mmol), isobu-
tyraldehyde (10 mmol), solvent (20 mL) and catalysts (1.5 mg)
were first mixed. The mixture was then equilibrated to desired
temperature in an oil bath. Reactions were performed in open
air. The products of the epoxidation reactions were collected at dif-
ferent time intervals and were identified and quantified by gas
chromatography. The turnover frequency (TOF) was calculated as
follows:
3.2. UV–Vis spectra
The electronic spectra of the complexes were recorded in
acetronitrile at room temperature and are in good agreement with
octahedral geometries of the complexes. Generally octahedral co-
balt(III) complexes show two strong and broad band around 550
and 360 nm due to spin allowed 1A1g ? 1T1g and 1A1g ? 1T2g tran-
sitions, respectively. In our cases, both complexes show broad band
at ca. 555 nm corresponds to 1A1g ? 1T1g transition. However,
other characteristic d–d transition (1A1g ? 1T2g) was absent. This
high energy band probably was obscured by several ligand to me-
tal charge transfer transition bands [14]. In the high energy region
both complexes show several intense bands observed in the region
340–385 nm, attributed to the charge transfer transition from the
coordinated unsaturated ligands to the respective metal ions
(LMCT) [63]. Two intense high energy bands corresponding to
moles converted
TOF ¼
ꢂ 100
moles of cobaltðactive siteÞtaken for reaction x reaction time
3. Results and discussion
3.1. Fourier transform infrared spectra
The infrared spectra of the complexes are consistent with their
crystal structure. In the spectra of the complexes azomethine
(–CH@N–) stretching frequency is lowered by 14 cmꢁ1 compared
to that of the free Schiff base ligand (1632 cmꢁ1) and appeared at
1618 cmꢁ1, indicating the coordination of imine nitrogen to the
metal center [59–61]. The aromatic –OH stretching vibration of
p
?
p⁄ and n ? p⁄ transitions of the lignad were also observed in
the spectra of 1 and 2 at 239 and 402 nm, and 230 and 385 nm,
respectively.

106
S. Shit et al. / Inorganica Chimica Acta 415 (2014) 103–110
3.3. Electrochemistry
Cyclic voltammograms of 1 and 2 were recorded at room tem-
perature in acetonitrile within the potential range +1.5 to ꢁ1.5 V.
On cathodic scan a reductive response due to Co(III) ? Co(II)
reduction was observed at ꢁ0.75 and ꢁ0.76 V for 1 and 2, respec-
tively. On anodic scan an oxidative response for 1 and 2 was ob-
served at ꢁ0.26 and ꢁ0.25 V correspond to Co(II) ? Co(III)
oxidation. Peak-to-peak separations of 490 and 510 mV for 1 and
2 with Ipc/Ipa > 1 suggest the irreversible nature of these redox pro-
cesses. Upon changing the scan rate peak potentials slightly shifted
towards more anodic potentials and the peak current increased but
Ipc/Ipa always remained greater than one which also confirmed the
irreversible nature of these redox processes. An additional peak at
+0.42 and +0.46 V during anodic scan was also observed for 1 and 2
which may be assigned to the oxidation of the ligand on the elec-
trode surface [1].
3.4. EPR and magnetic studies
The X band EPR spectra of polycrystalline samples 1 and 2 were
recorded at room temperature to get information about the va-
lence and spin state of cobalt ion. The cobalt ion exhibits two oxi-
dation states (II and III), where cobalt(III) with a t62g configuration is
diamagnetic in nature and hence EPR silent, whereas cobalt(II) ex-
ist in both low spin (S = 1/2) and high spin (S = 3/2) states and is
paramagnetic, and hence EPR active. In our complexes they appear
as EPR silent (Supplementary material: Figs. S1 and S2) confirming
the diamagnetic nature (t62g configuration) of the complexes with
the +III oxidation state of the cobalt ion. The diamagnetic behavior
of the complexes was also supported by room temperature mag-
netic moment measurements. The room temperature effective
magnetic moment incorporating the necessary diamagnetic cor-
rections using Pascal’s table for the polycrystalline complexes of
1 and 2 was found to be 0.01 B.M. which indicates the presence
of low spin cobalt(III) ions in these complexes.
Fig. 1. ORTEP plot of 1 with displacement ellipsoids drawn at the 30% probability
level. Only the major components of disorder are shown. Hydrogen atoms are
omitted for clarity.
Table 2
Selected bond lengths (Å) and angles (°) for 1.
Co1–O1
Co1–O3
Co1–N1
1.902(3)
1.908(3)
1.889(4)
Co1–N2
Co1–N3
Co1–N5
2.076(4)
1.950(4)
1.956(5)
3.5. Structure description of [Co(L)(HL)(N3)]ꢀClO4 (1)
O1–Co1–O3
O1–Co1–N1
O1–Co1–N2
O1–Co1–N3
O1–Co1–N5
O3–Co1–N1
O3–Co1–N2
O3–Co1–N3
89.68(14)
94.43(16)
179.77(19)
85.02(15)
88.48(17)
84.59(16)
90.54(16)
94.53(16)
O3–Co1–N5
N1–Co1–N2
N1–Co1–N3
N1–Co1–N5
N2–Co1–N3
N2–Co1–N5
N3–Co1–N5
–
174.31(17)
85.52(18)
178.97(17)
90.19(19)
95.02(17)
91.29(18)
90.67(19)
–
An ORTEP view of 1 is shown in Fig. 1 and selected bond lengths
and angles are listed in Table 2. The structure of 1 consists of
mononuclear [Co(L)(HL)(N3)]+ cation and perchlorate anion. In
the cationic unit two Schiff bases coordinate to the cobalt(III) atom
in tridentate and bidentate modes. The anionic tridentate Schiff
base coordinates to the cobalt(III) center through the phenolato
oxygen (O1), the imine nitrogen (N1), and the amine nitrogen
(N2) atoms while the neutral bidentate Schiff base, which is pro-
tonated at the amine nitrogen atom (N4), coordinates by its pheno-
lato oxygen (O3), and imine nitrogen (N3). The azide ion acts as a
terminal coligand and coordinates through one terminal nitrogen
(N5) atom. Thus the cobalt(III) ion is hexa-coordinated by a N4O2
donor set and its coordination polyhedron is best described as a
slightly distorted octahedron. The best equatorial plane (r.m.s.
deviation = 0.0094) is formed by the imine nitrogen (N1 and N3)
atoms, the azide nitrogen (N5) and the phenolato oxygen (O3) of
the bidentate Schiff base ligand while the two apical positions
are occupied by the phenolato oxygen (O1) and the amine nitrogen
(N2) of the tridentate Schiff base ligand. The metal ion is displaced
form the mean equatorial plane by only 0.0046(8) Å toward N5.
The bond lengths related to cobalt(III) center range from 1.890(4)
to 2.076(4) Å. Cisoid and transoid angles vary in the range
84.58(17)–95.02(17)° and 174.30(18)–179.78(19)°. The N–N bond
distances in the coordinated azide ion are just significantly differ-
ent (N5–N6 = 1.193(6) Å, N6–N7 = 1.166(7) Å) with an almost lin-
ear \N5–N6–N7 angle of 176.3(7)°. The percholarte ion is
disordered and remains uncoordinated to the cobalt(III). However
it is connected to the complex unit through C–Hꢀ ꢀ ꢀO interactions
(Table 3). An intramolecular NꢁHꢀ ꢀ ꢀO hydrogen bond is also ob-
served (Table 3).
Table 3
Hydrogen bonding parameters (Å, °) for 1.
D–Hꢀ ꢀ ꢀA
d(D–H)
d(Hꢀ ꢀ ꢀA)
d(Dꢀ ꢀ ꢀA)
\(D–Hꢀ ꢀ ꢀA)
164(5)
146
138
136
128
152
121
N4ꢁH4Nꢀ ꢀ ꢀO1
0.95(6)
0.93
0.97
0.93
0.97
0.97
0.97
2.21(6)
2.56
2.52
2.56
2.55
2.39
2.57
3.132(7)
3.376(7)
3.301(13)
3.286(16)
3.246(17)
3.28(2)
C8ꢁH8ꢀ ꢀ ꢀO4a
C9ꢁH9Bꢀ ꢀ ꢀO5Ab
C22ꢁH22ꢀ ꢀ ꢀO6Ab
C23ꢁH23Aꢀ ꢀ ꢀO7A
C24ꢁH24Aꢀ ꢀ ꢀO7Ac
C24ꢁH24Aꢀ ꢀ ꢀO6Bc
3.19(2)
a
b
c
Symmetry code: ꢁx, ꢁy, 1 – z.
Symmetry code: x, 0.5 ꢁ y, 0.5 + z.
Symmetry code: x, 0.5 ꢁ y, ꢁ0.5 + z.

S. Shit et al. / Inorganica Chimica Acta 415 (2014) 103–110
107
The complex reported herein may be compared to that of 2,
reported earlier [53]. Both the complexes were derived from
same Schiff base and metal salt with different terminal ligand;
N-3 for 1 while SCNꢁ for 2. In both cases, two coordinating Schiff
bases exhibit similar ligating properties; one in a tridentate fash-
ion through its phenolato oxygen, imine, and amine nitrogen
atoms while the other acts as a bidentate ligand through its phe-
nolato oxygen and imine nitrogen atoms. In both the complexes
pseudohalide ions coordinated through nitrogen atoms in mono-
dentate fashion. As a consequence the cobalt(III) atom is six-
coordinated by the same N4O2 donor set and adopts a slightly
distorted octahedral geometry. Though the degree of distortion
from the ideal geometry is small in both complexes, it originates
from significantly different coordination modes of the pseudoha-
lide ligands. In fact, the Co1–N5–N6 angle in 1 is 122.1(4)° while
in 2 the Co–N5–C29 angle is 157.8(5)°, indicating that azide
leads to a more relevant steric crowding than thiocyanate. The
Co1–O bond (1.908(3) Å) trans to Co1–N5 bond in 1 is slightly
longer than the Co1–O bond (1.880(4) Å) trans to Co1–N5 bond
in 2, indicating higher trans influence of azide ion than that of
thiocyanate [64].
bonds. In the epoxidation of endocyclic alkene such as for cyclooc-
tene, epoxide selectivity was 100% due to its highly active double
bond and relative stability of product, cyclooctene oxide. However,
for cyclohexene, only 49% and 38% epoxide selectivity was ob-
tained for 1 and 2 as their 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 1,2-epoxy alkanes
as an exclusive product. However, the conversion of 1-hexene
(72% for 1 and 61% for 2) was greater than of 1-octene (66% for 1
and 54% for 2) which showed that catalytic activity decreases along
with chain length of olefins. Larger hexyl group of 1-octene con-
nected to double bond sterically hinders its approach to the active
site compared to 1-hexene in which its double bond carries a smal-
ler butyl group. In view of the conjugative effect of benzene ring,
styrene should show lower activity, but surprisingly it gave styrene
oxide with above 54% selectivity; along with this, benzaldehyde
and benzoic acid were also detected. Kureshy et al. studied that
epoxidation of styrene over cobalt(II) chiral Schiff base complexes
in presence of O2 and isobutyraldehyde showed up to 45% conver-
sion [48]. Yield of styrene oxide is maximized to 68% using cobal-
t(II) calix [4] pyrrole complexes in presence 2-ethylbutyraldehyde/
O2 in 24 h [65]. Opre et al. studied that DMF interacts with oxygen,
styrene, and the cobalt-containing catalyst according to a complex
reaction network, resulting in the formation of 49% styrene oxide
with various co-products, such as N-formyl-N-methylformamide,
CO2, and dimethylamine [52] and established that DMF as ‘‘sacrifi-
cial’’ solvent which also functions as co-reductant. We also per-
formed the reaction in DMF under this condition but no
conversion of styrene was detected. Gao et al. obtained 60% styrene
epoxide over cobalt(II) containing zeolite imidazolate framework
under aerobic condition using isobutyraldehyde as sacrificial
reductant [32]. A graphical representation of relative efficacy of 1
and 2 for the epoxidation of various alkenes versus time is shown
in Figs. S3 and S4 (Supplementary material), respectively.
3.6. Catalytic epoxidation reactions
Catalytic activity of 1 and 2 was investigated in the epoxidation
of both cyclic and straight chain alkenes with air. Complete conver-
sion of the substrates such as cyclooctene, cyclohexene, 1-hexene,
1-octene, and styrene required 12 and 18 h, 14 and 22 h, 16 and
24 h, 18 and 26 h, and 13 and 18 h, for 1 and 2, respectively. Bear-
ing in mind both conversion and time, we have studied the cata-
lytic reaction for 6 h only to get satisfactory results. Catalytic
performance of 1 and 2 in the epoxidation of various olefins is
summarized in Table 4. The epoxidation results of various olefins
differed significantly due to the different activity of the double
Table 4
Epoxidation of olefins catalyzed by 1i and 2i.
Substrate
Reaction time (h)
Conversion (wt%)
% Yield of product
Epoxide
TOFii
Others
(hꢁ1
)
a) 6
b) 6
a) 83
b) 70
a) 49
b) 38
a) 22iii, 12iv
b) 19iii, 13iv
a) 660
b) 544
a) 6
b) 6
a) 79
b) 66
a) 79
b) 66
–
a) 628
b) 513
a) 6
b) 6
a) 6
b) 6
a) 6
b) 6
a) 72
b) 61
a) 66
b) 54
a) 78
b) 69
a) 72
b) 61
a) 66
b) 54
a) 43
b) 37
–
–
a) 573
b) 474
a) 525
b) 420
a) 621
b) 537
a) 18v, 17vi
b) 17v, 15vi
(a), (b), corresponds to the catalytic performance of compounds 1 and 2, respectively.
i
Reaction conditions: alkenes (10 mmol); isobutyraldehyde (10 mmol); catalyst (1.5 mg); 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 Varian CP-3800 gas chromatograph equipped
with an FID detector and a CP-Sil 8 CB capillary column.
Turn over frequency (TOF) = moles converted per moles of active site per hour.
2-Cyclohexen-1-ol.
2-Cyclohexene-1-one.
Benzaldehyde.
ii
iii
iv
v
vi
Benzoic acid.

108
S. Shit et al. / Inorganica Chimica Acta 415 (2014) 103–110
Fig. 3. Plot showing cyclooctene conversion with respect to amount of catalyst.
Fig. 2. Plot of % conversion of cyclooctene vs. time for comparison between reaction
with radical scavenger (hydroquinone) and a control reaction without adding
radical scavenger.
Table 7
Epoxidation of olefins catalyzed by cobalt saltsi.
Substrate
Catalyst
Conversion
(wt%)
% yield of
product
TOFii (h-1
)
Table 5
Influence of solvent on 1 and 2 catalyzed epoxidation reactioni.
Epoxide
24
Others
–
Substrate
% of conversion (% of epoxide yield) in different solvent
Co(ac)2ꢀ4H2O
24
25
158
CH3CN
CH3COCH3 CHCl3 C2H5OH
CH3OH
a) 79 (79) a) 58
b) 66 (66) b) 45
a) 49 No conversion No conversion
b) 36
[Co(acac)3]
25
–
164
i
Reaction conditions were the same as given in footnote of Table 4.
i
Reaction conditions were the same as given in footnote of Table 4.
Turn over frequency (TOF) = moles converted per moles of active site per hour.
ii
ac = acetate. acac = acetylacetonate.
Table 6
Temperature effect on 1 and 2 catalyzed epoxidation reactioni.
Substrate % of conversion (% of epoxide yield) at different temperature
solvent. The best performance of the catalyst was observed in ace-
tonitrile (Table 5). The efficiency of catalyst followed the order:
acetonitrile > acetone > chloroform > ethanol/methanol. The cata-
lytic reaction on cyclooctene was also performed at different tem-
perature to study the catalytic efficacy of 1 and 2 at different
temperature (Table 6). We have studied the effect of temperature
up to the boiling point of acetonitrile. Higher temperatures en-
hanced the reaction rate. The epoxide selectivity as well as conver-
sion for cyclooctene increased with increasing temperature, but
above 60 °C, the temperature had little effect on the cyclooctene
conversion. From the view point of energy consumption, it is cost
effective to perform the catalytic reaction at low temperature.
The rate of cyclooctene conversion increases rapidly with the
amount of the catalyst at low concentration of the catalyst
(Fig. 3). But this progress of conversion decreases with the in-
creases in catalyst concentration and thus for full conversion large
amount of catalyst is required. Therefore we have restricted the
use of amount of the catalyst (1.5 mg) which is economically ben-
eficial. A comparison of the catalytic efficiency of 1 and 2 with
other cobalt salts (cobalt(III) acetylacetonate and cobalt(II) acetate)
in homogeneous medium (Table 7) clearly indicates that 1 and 2
perform as a more efficient catalyst than simple cobalt salts.
The catalytic efficacy of the complexes reported herein may be
compared (Table 8) to the closely related Co-salen type of com-
plexes reported earlier [34,51,66,67,68] concerning aerobic epoxi-
dation of styrene. This comparison indicates that epoxide
selectivity (% wt.) increases when cobalt-salen complexes were
30 °C
40 °C
50 °C
60 °C
70 °C
80 °C
a) 42 (42) a) 48 (48) a) 66 (66) a) 79 (79) a) 82 (82) a) 83 (83)
b) 30 (30) b) 35 (35) b) 53 (53) b) 66 (66) b) 70 (70) b) 72 (72)
i
Reaction conditions were the same as given in footnote of Table 4.
Buranaprasertsuk et al. [65] proposed a mechanism for the me-
tal complex-catalyzed oxygenation of substrates by O2 and alde-
hyde. We also propose a similar mechanism, operative in our
case. First, O2 and aldehyde generated the peroxyacid by a radical
mechanism. Then the reaction between metal complex and per-
oxyacid takes place and metal–peroxy adduct is formed. This me-
tal–peroxy adduct then releases peroxy radical and regenerates
metal environment. This is followed by a reaction between peroxy
radical and olefin to form a peroxy species which undergoes migra-
tion of oxygen to form the epoxide product and acid byproduct. To
probe if radicals were involved in our reactions, a radical scaven-
ger, hydroquinone, was added after 1.5 h of the reaction, and the
reaction progress was monitored. It was observed that the reaction
stopped immediately upon addition of the radical scavenger
(Fig. 2), which suggests that the process indeed proceeds via radi-
cal epoxidation.
The catalytic reaction was performed in a variety of solvents on
cyclooctene to study the catalytic efficacy of 1 and 2 in different

S. Shit et al. / Inorganica Chimica Acta 415 (2014) 103–110
109
Table 8
A comparison of catalytic efficacy on aerobic styrene epoxidation of 1 and 2 with other related Co-salen systems.
Catalyst
Solvent
Temperature (°C)
Conversion (wt.%)
Epoxide selectivity (wt.%)
TOF (hꢁ1
)
Refs.
Co-[H4]Salen-SBA-15i
Co-ZSM-5(L1)ii
Co-ZSM-5(L2)iii
Co-ZSM-5(L3)iv
(R,R)-Co(II)Salenv
Co-Salen-graphene oxide
[Co(HLvi)2]NO3ꢀH2O
1
CH3CN
DMF
DMF
80
90
90
90
4
80
60
60
60
78.5
93.9
92.6
83.7
95
70.1
100
78
54.1
89.7
91.1
91.7
30
35.3
46
55
196
–
–
–
–
83.5
659
621
537
[66]
[51]
[51]
[51]
[67]
[68]
[34]
DMF
Toluene
CH3CN
CH3CN
CH3CN
CH3CN
This study
This study
2
69
53.6
i
[H4]Salen-SBA-15 = tetrahydro-salen and salen complex onto amino-functionalized SBA-15.
L1 = salicylaldehyde benzoylhydrazone.
L2 = vanillic aldehyde benzoylhydrazone.
L3 = 4-methyl benzaldehyde benzoylhydrazone.
(R,R)-Salen = (R,R)-(-)-N,N0-bis(3,5-di-tertbutylsalicylidene)-1,2-cyclohexanediamine.
(H2L = 1-(N-3-methoxy-salicylideneimino)-ethane-2-ol).
ii
iii
iv
v
vi
[4] B.S. Snyder, G.S. Patterson, A.J. Abrahamson, R.H. Holm, J. Am. Chem. Soc. 111
(1989) 5214.
[5] C. Fraser, B. Bosnich, Inorg. Chem. 33 (1994) 338.
[6] V.G. Makhankova, O.Y. Vassilyeva, V.N. Kokozay, B.W. Skelton, L. Sorace, D.
Gatteschi, J. Chem. Soc., Dalton Trans. (2002) 4253.
[7] L. Sacconi, M. Ciampolini, G.P. Speroni, Inorg. Chem. 4 (1965) 1116.
[8] R. Dreos, G. Nardin, L. Randaccio, P. Siega, G. Tauzher, V. Vrdoljak, Inorg. Chim.
Acta 349 (2003) 239.
immobilized over ZSM though wt.% conversions in almost all cases
are comparable to that of our complexes except one [34]. When the
catalytic efficacy of the complexes are compared with respect to
their TOF it is observed that our complexes show very high TOF
compared to the reported complexes [66,68] and are comparable
to that of the other reported complex [34].
[9] S. Yamada, Coord. Chem. Rev. 191 (1999) 537.
[10] N.J. Henson, P.J. Hay, A. Redondo, Inorg. Chem. 38 (1999) 1618.
[11] P.K. Mascharak, Coord. Chem. Rev. 225 (2002) 201.
[12] D.P. Kessissoglou, Coord. Chem. Rev. 185 (1999) 837.
[13] H. Chen, D. Han, H. Yan, W. Tang, Y. Yang, H. Wang, Polyhedron 12 (1993)
1097.
[14] S. Thakurta, R.J. Butcher, G. Pilet, S. Mitra, J. Mol. Struct. 929 (2009) 112 (and
references therein).
[15] B.S. Lane, K. Burgess, Chem. Rev. 103 (2003) 2457.
[16] T. Katsuki, Chem. Soc. Rev. 33 (2004) 437.
[17] H.C. Kolb, M.S. VanNieuwenhze, K.B. Sharpless, Chem. Rev. 94 (1994) 2483.
[18] Q.H. Xia, H.Q. Ge, C.P. Ye, Z.M. Liu, K.X. Su, Chem. Rev. 105 (2005) 1603.
[19] D. Swern (Ed.), Organic Peroxide, vol. 2, Wiley-Interscience, New York, 1971.
[20] M. Hatefi, M. Moghadam, V. Mirkhani, I. Sheikhshoaei, Polyhedron 29 (2010)
2953.
[21] N.C. Maity, S.H.R. Abdi, R.I. Kureshy, N.H. Khan, E. Suresh, G.P. Dangi, H.C. Bajaj,
J. Catal. 277 (2011) 123.
[22] A.K. Rahiman, K. Rajesh, K.S. Bharathi, S. Sreedaran, V. Narayanan, Appl. Catal.
A Gen. 314 (2006) 216.
4. Conclusions
Herein we report the synthesis and characterizations of two co-
balt(III)-Schiff base complexes incorporating azide and thiocyanate
as coligands. Structural characterization reveals that in spite of the
same coordination mode of the ligand the relative distortion in the
geometries of the metal centers in the complexes are different
which is due to the relative binding mode of the terminal coligands
(azide in 1 and thiocyanate in 2). Catalytic efficacy of both the com-
plexes towards epoxidation of alkenes in aerobic condition using
isobutyraldehyde as co-reductant has been thoroughly investi-
gated which reveals that 1 shows better catalytic activity than 2
with respect to TOF as well as alkene conversion in acetonitrile
at an optimum temperature of 60 °C.
[23] R. Sen, S. Bhunia, D. Mal, S. Koner, Y. Miyashita, K.-I. Okamoto, Langmuir 25
(2009) 13667.
[24] R. Sen, D. Saha, S. Koner, Catal. Lett. 142 (2012) 124 (and references therein).
[25] J. Zhang, A.V. Biradar, S. Pramanik, T.J. Emge, T. Asefa, J. Li, Chem. Commun. 48
(2012) 6541.
Acknowledgement
[26] J.S. Costa, C.M. Markus, I. Mutikainen, P. Gamez, J. Reedijk, Inorg. Chim. Acta
363 (2010) 2046.
[27] B. Qi, X.-H. Lu, D. Zhou, Q.-H. Xia, Z.-R. Tang, S.-Y. Fang, T. Pang, Y.-L. Dong, J.
Mol. Catal. A: Chem. 322 (2010) 73.
[28] S. Shit, U. Yadava, D. Saha, R. Fröhlich, J. Coord. Chem. 66 (2013) 66.
[29] D. Saha, T. Maity, T. Dey, S. Koner, Polyhedron 35 (2012) 55.
[30] J. Liang, Q. Zhang, H. Wu, G. Meng, Q. Tang, Y. Wang, Catal. Commun. 5 (2004)
665.
The author acknowledges University Grants Commission, New
Delhi, India for financial assistance (Minor Research Project No. F.
PSW-65/12-13 (ERO)). The author is indebted to Prof. Samiran Mi-
tra of Jadavpur University for providing instrumental facilities to
perform spectroscopic measurements.
[31] T. Punniyamurthy, S. Velusamy, J. Iqbal, Chem. Rev. 105 (2005) 2329.
[32] A. Zhang, L. Li, J. Li, Y. Zhang, S. Gao, Catal. Commun. 12 (2011) 1183.
[33] M.J. Beier, W. Kleist, M.T. Wharmby, R. Kissner, B. Kimmerle, P.A. Wright, J.-D.
Grunwaldt, A. Baiker, Chem. Eur. J. 18 (2012) 887.
[34] D. Saha, T. Maity, R. Bera, S. Koner, Polyhedron 56 (2013) 230.
[35] A.K. Sinha, S. Seelan, S. Tsubota, M. Haruta, Angew. Chem., Int. Ed. 43 (2004)
1546.
Appendix A. Supplementary material
CCDC 946732 contains the supplementary crystallographic data
for 1. These data can be obtained free of charge from The Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/da-
ta_request/cif. Supplementary data associated with this article
can be found, in the online version, at http://dx.doi.org/10.1016/
j.ica.2014.02.036.
[36] I. Yamanaka, K. Nakagaki, K. Otsuka, J. Chem. Soc., Chem. Commun. (1995)
1185.
[37] T. Takai, E. Hata, K. Yorozu, T. Mukaiyama, Chem. Lett. (1992) 2077.
[38] T. Mukaiyama, K. Yorozu, Y. Takai, T. Yamada, Chem. Lett. (1993) 439.
[39] F. Loeker, W. Leitner, Chem. Eur. J. 6 (2000) 2011.
[40] E. Angelescu, R. Ionescu, O.D. Pavel, R. Zavoianu, R. Birjega, C.R. Luculescu, M.
Florea, R. Olar, J. Mol. Catal. A: Chem. 315 (2010) 178.
[41] D. Dhar, Y. Koltypin, A. Gedanken, S. Chandrasekaran, Catal. Lett. 86 (2003)
197.
[42] T. Punniyamurthy, B. Bhatia, M.M. Reddy, G.C. Maikap, J. Iqbal, Tetrahedron 53
(1997) 7649.
[43] X.-T. Zhou, Q.-H. Tang, H.-B. Ji, Tetrahedron Lett. 50 (2009) 6601.
[44] W. Nam, S.J. Baek, K.A. Lee, B.T. Ahn, J.G. Muller, C.J. Burrows, J.S. Valentine,
Inorg. Chem. 35 (1996) 6632.
References
[1] S. Thakurta, J. Chakraborty, G. Rosair, R.J. Butcher, S. Mitra, Inorg. Chim. Acta
362 (2009) 2828.
[2] A.C. Rizzi, C.D. Brondino, R. Calvo, R. Baggio, M.T. Garland, R.E. Rapp, Inorg.
Chem. 42 (2003) 4409.
[3] J.S. Miller, M. Drillon (Eds.), Magnetism: Molecules to Materials IV, Wiley–
VCH, Weinheim, Germany, 2003.

110
S. Shit et al. / Inorganica Chimica Acta 415 (2014) 103–110
[45] R.A. Budnik, J.K. Kochi, J. Org. Chem. 41 (1976) 1384.
[56] G.M. Sheldrick, Acta Crystallogr. 64A (2008) 112.
[46] D.E. Hamilton, R.S. Drago, A. Zombeck, J. Am. Chem. Soc. 109 (1987) 374.
[47] B. Rhodes, S. Rowling, P. Tidswell, S. Woodward, S.M. Brown, J. Mol. Catal. A:
Chem. 116 (1997) 375.
[57] A.L. Spek, PLATON – A Multipurpose Crystallographic Tool, Utrecht University,
Utrecht, The Netherlands, 2003.
[58] L.J. Farrugia, J. Appl. Crystallogr. 30 (1997) 565.
[48] R.I. Kureshy, N.H. Khan, S.H.R. Abdi, A.K. Bhatt, P. Iyer, J. Mol. Catal. A 121
(1997) 25.
[49] M.J. da Silva, P. Robles-Dutenhefner, L. Menini, E.V. Gusevskaya, J. Mol. Catal.
A: Chem. 201 (2003) 71.
[50] G. Xu, Q.-H. Xia, X.-H. Lu, H.-J. Zhan, J. Mol. Catal. A: Chem. 266 (2007) 180.
[51] B. Qi, X.-H. Lu, S.-Y. Fang, J. Lei, Y.-L. Dong, D. Zhou, Q.-H. Xia, J. Mol. Catal. A:
Chem. 334 (2011) 44.
[59] S.H. Rahaman, R. Ghosh, T.-H. Lu, B.K. Ghosh, Polyhedron 24 (2005) 1525.
[60] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination
Compounds, Part A and B, fifth ed., John Wiley, New York, 1997.
[61] R.T. Conley, Infrared Spectroscopy, Allyn & Bacon, Boston, 1966.
[62] A.A. Khandar, S.A. Hosseini-Yazdi, Polyhedron 22 (2003) 1481.
[63] A.B.P. Lever, Inorganic Electronic Spectroscopy, second ed., Elsevier, New York,
1984.
[52] Z. Opre, T. Mallat, A. Baiker, J. Catal. 245 (2007) 482.
[53] S.-J. Peng, Z.-L. You, J. Chem. Crystallogr. 37 (2007) 415.
[54] Xcalibur CCD system, CrysAlisPro Software system, Version 1.171.35.21;
Oxford Diffraction Ltd., Abingdon, UK, 2012.
[55] SCALE3 ABSPACK – An Oxford Diffraction program (1.0.4, gui:1.0.3) (C) 2005
Oxford Diffraction Ltd.
[64] N. Mondal, D.K. Dey, S. Mitra, K.M.A. Malik, Polyhedron 19 (2000) 2707.
[65] P. Buranaprasertsuk, Y. Tangsakol, W. Chavasiri, Catal. Commun. 8 (2007) 310.
[66] Y. Yang, Y. Zhang, S. Hao, Q. Kan, Chem. Eng. J. 171 (2011) 1356.
[67] O.A. Kholdeeva, M.P. Vanina, React. Kinet. Catal. Lett. 73 (2001) 83.
[68] Z. Li, S. Wu, H. Ding, D. Zheng, J. Hu, X. Wang, Q. Huo, J. Guan, Q. Kan, New J.
Chem. 37 (2013) 1561.