Synthesis, structure, catalytic and magnetic properties of a pyrazole based five coordinated di-nuclear cobalt(II) complex

Article abstract of DOI:10.1016/j.poly.2015.12.062

The reaction of 4-methyl-2,6-bis(3,5-dimethyl-1H-pyrazol-1-yl)methyl)phenol and Co(NO₃)₂·6H₂O in MeCN gives rise to [Co₂(L)₂(N₃)₂] (1), a dinuclear five coordinated complex of Co(II), in satisfactory yield. The complex has been characterized by C, H and N microanalyses, FT-IR and UV-Vis spectral measurements. A single crystal X-ray diffraction study of 1 reveals that each Co(II) atom is in a distorted square-pyramidal geometry. Electrochemical studies in CH₃CN showed that the Co(II)-Co(III)/Co(III)-Co(III) redox couple appears to be quasi-reversible (E_1/2 = 0.409 V), while the corresponding Co(II)-Co(III)/Co(II)-Co(II) couple is mostly irreversible (E_1/2 = -0.447 V). This complex displays modest catalytic activity toward the oxidation of various allylic compounds using TBHP as an oxidant in MeCN under mild conditions. A radical trapped experiment carried out in the presence of 4-tert-butylphenol clearly reveals that the epoxidation reactions occur purely through a radical pathway rather than a concerted one. Low temperature magnetic studies showed that the present Co(II) dimeric complex possesses a high positive magnetic anisotropy (D = +29.8 cm^-1) and a weak ferromagnetic exchange interaction (2J = +10.3 cm^-1).

Full text of DOI:10.1016/j.poly.2015.12.062

Accepted Manuscript
Synthesis, structure, catalytic and magnetic properties of a pyrazole based five
coordinated di-nuclear cobalt(II) complex
Rabiul Alam, Kaberi Pal, Bikash Kumar Shaw, Malay Dolai, Nabanita Pal,
Shyamal Kumar Saha, Mahammad Ali
PII:
S0277-5387(16)00003-6
DOI:
http://dx.doi.org/10.1016/j.poly.2015.12.062
POLY 11761
Reference:
To appear in:
Polyhedron
Received Date:
Accepted Date:
11 November 2015
20 December 2015
Please cite this article as: R. Alam, K. Pal, B.K. Shaw, M. Dolai, N. Pal, S.K. Saha, M. Ali, Synthesis, structure,
catalytic and magnetic properties of a pyrazole based five coordinated di-nuclear cobalt(II) complex, Polyhedron
(
2016), doi: http://dx.doi.org/10.1016/j.poly.2015.12.062
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1
Synthesis, structure, catalytic and magnetic properties of a pyrazole
based five coordinated di-nuclear cobalt(II) complex
a
Rabiul Alam , Kaberi Pal , Bikash Kumar Shaw , Malay Dolai , Nabanita Pal , Shyamal
a
b
a
a
b
Kumar Saha and Mahammad Ali *
a
a
Department of Chemistry, Jadavpur University, Kolkata 700 032, India.
E-mail: mali@chemistry.jdvu.ac.in
b
Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur,
Kolkata 700032, India.
Abstract
The
reaction
of
4-methyl-2,6-bis(3,5-dimethyl-1H-pyrazol-1-yl)methyl)phenol
and
Co(NO
3
)
2
.6H O in MeCN gives rise to [Co
2
2
(L) (N ] (1), a dinuclear five coordinated complex
2
3 2
)
of Co(II), in satisfactory yield. The complex has been characterized by C, H and N
microanalyses, FT-IR and UV-Vis spectral measurements. A single crystal X-ray diffraction
study of 1 reveals that each Co(II) atom is in a distorted square-pyramidal geometry.
Electrochemical studies in CH
appears to be quasi-reversible (E
Co(II) couple is mostly irreversible (E
3
CN showed that the Co(II)-Co(III)/Co(III)-Co(III) redox couple
= 0.409 V), while the corresponding Co(II)-Co(III)/Co(II)-
= -0.447 V). This complex displays modest catalytic
½
½
activity towards the oxidation of various allylic compounds using TBHP as an oxidant in MeCN
under mild conditions. A radical trapped experiment carried out in the presence of 4-tert-
butylphenol clearly reveals that the epoxidation reactions occur purely through a radical pathway
rather than a concerted one. Low temperature magnetic studies showed that the present Co(II)
-1
dimeric complex possesses a high positive magnetic anisotropy (D = + 29.8 cm ) and a weak
-1
ferromagnetic exchange interaction (2J = + 10.3 cm ).
Keywords: di-nuclear cobalt(II) complex; synthesis; structure; catalytic studies; magnetic
properties.

2
Introduction
Unique physical properties that correlate to novel modes of chemical reactivity are often
displayed when a metal ion is in an unusual coordination geometry. In metalloenzymes such
subtle structure/function relationships are exploited by intimately tuning the local stereochemistry
and ligand-field of a protein active site to achieve specific catalytic transformations [1]. An
appreciation of such specific ligand-to-metal interactions in protein active sites is therefore highly
dependent on our basic understanding of the elementary stereochemical and ligand-field
relationships in coordination chemistry [2-5]. These elementary principles are often employed to
rationally design catalysts selective for specific transformations [6,7].
Since the discovery that the active site structure of methionine aminopeptidase (MAP, EC
3
.4.11.18) contain a bis(m-carboxylato)dicobalt(II) core [8], the chemistry of dinuclear cobalt
complexes has attracted major attention from coordination chemists to elucidate the relationship
between dinuclear structures and their properties and functions.
7
Additionally, complexes with a high spin d configuration are of great interest owing to
their interesting magnetic properties. The general approach is to use multidentate and bulky
ligands that favor metal aggregation, keeping the resulting clusters well isolated from their
neighbours [9]. Our interest lies in exploring the intrinsic properties that influence the electronic
configuration of dinuclear high spin Co(II) complexes. Towards this end, a number of reports on
dinuclear high-spin Co(II) complexes, viz. [Co
2
(tmpc)Cl
2
][CoCl
4
], [Co
2
(tmpc)Cl
2
][PF
6
2
] ,
-
[
Co
2
(tmpc)(NO
3
)
2
] [NO
3
]
2
.MeOH, [Co
2
(baib)(Me-CO
2
)
3
]BPh and [Co
4
2
(baib)(PhCO
2
)
3
]BPh
4
, are
available in the literature [10]. We are especially interested in the magnetism of dinuclear high
spin cobalt(II) complexes since di- or polynuclear complexes with high magnetic spin and
anisotropy may exhibit single-molecule magnetic (SMMs) behavior [11-14].
The cobalt(II) ion and its coordination complexes are well-known catalysts for the
selective oxidation of alkanes and alkylbenzenes with molecular oxygen [15]. Cobalt(II/III) Schiff
base complexes have also been found to be efficient catalysts for the epoxidation of olefins by
molecular oxygen [16-20]. As an example, cobalt(II)-salen complexes were reported to show
catalytic activity for the epoxidation of alkenes with O in the presence of a sacrificial co-
2
reductant like isobutyraldehyde, 2-ethylbutyraldehyde etc [19-23]. Recently, the aerobic oxidation
of alkenes over Co(II)-compounds with and without a co-reductant in DMF have also been

3
studied [24-26]. Here, DMF was regarded as a ‘‘sacrificial’’ solvent that functions as a co-
reductant in the epoxidation reaction. Although there are number of reports where mononuclear
cobalt complexes are used as catalysts in many organic transformation reactions [16-26], the use
of the corresponding di- or polynuclear complexes for such purposes are rare. Herein, we report
the synthesis, characterization through a single crystal X-ray diffraction study and magnetic
susceptibility measurements, along with the catalytic activities towards the oxidation of olefins,
namely cyclohexene, cyclooctene and styrene, in the presence of tert-butyl hydroperoxide
II
(
TBHP)of a homodinuclear complex (LCo
2
).
Experimental section
Materials and reagents.
All chemicals were of analytical reagent grade and were used without further purification. Cobalt
nitrate was purchased from Sigma-Aldrich.
Physical measurements
1
Elemental analyses were carried out using a Perkin-Elmer 240 elemental analyzer. H NMR
spectra were recorded in CDCl
tetramethylsilane (δ = 0) as an internal standard. Electronic spectra were recorded on an Agilent-
453 diode array UV-Vis spectrophotometer. The melting point was determined by an electro-
3
on a Bruker 300 MHz NMR spectrophotometer using
8
thermal digital melting point apparatus (SUMSIM India) and is uncorrected. FTIR spectra (KBr
disc) [(vs) = very strong, (s) = strong, (m) = medium and (w) = weak] were recorded with a
Nicolet Magna-IR spectrophotometer (Series II). Mass spectra were recorded on an XEVO
G2QTof spectrometer (Waters) with an electron spray ionization source. Electrochemical
measurements were carried out using a computer controlled AUTOLAB (Model: AUTOLAB
3
02) cyclic voltammeter with a platinum (Pt) working electrode, a platinum-wire auxiliary
electrode and an Ag/AgCl reference electrode. The supporting electrolyte is n-Bu
M).
4
NClO (0.1
4
Synthesis of 4-methyl-2,6-bis(3,5-dimethyl-1H-pyrazol-1-yl)methyl)-phenol (HL):

4
4
-Methyl-2,6-bis(3,5-dimethyl-1H-pyrazol-1-yl)methyl phenol was prepared using a reported
method [27]. In a typical procedure 4-methyl-2,6-di(chloromethyl)phenol (1.68 g, 8.2 mmol)
was dissolved in 15 ml dry THF in a round bottom flask. 1.57 g (16.4 mmol) 3,5-dimethyl
pyrazole and 1.659 g (16.4 mmol) triethylamine (Et
separately. The later solution was then added dropwise to the previous one with constant stirring.
An instant precipitation of Et NHCl was observed and the colour of the solution turned bright
3
N) were dissolved in 5-10 ml dry THF
3
yellow. After 24 hours of stirring the precipitate was filtered off. From the filtrate THF was
removed under reduced pressure. Silicagel column chromatography was performed using DCM
(
dichloromethane) and methanol (7:3, v/v) as the eluent. The removal of solvent under reduced
1
pressure afforded a light yellow solid in the pure form in 60% yield. H NMR (CDCl
3
, 300 MHz)
), 5.83 (s, 2H, -CH),
.60 (2H, s, -ArH), 10.36 (1H, brs, -ArOH), (Fig. S1) IR (cm ): 1640 (C=N), 3400 (-OH).
Fig.S2)
δ (ppm): 2.07-2.20 (12H, s, -CH
3
), 2.48 (3H, s, Ar-CH
3
), 5.08 (4H, s, -CH
2
-1
6
(
Synthesis of complex (1)
Co (L) (N ]: 4-Methyl-2,6-bis(3,5-dimethyl-1H-pyrazol-1-yl)methyl) phenol (0.324 g, 1
mmol) was dissolved in MeCN, to which triethylamine (0.101 g, 1 mmol) was added dropwise
with constant stirring. Co(NO .6H O (0.528 g, 2mmol) in 30 ml MeCN was then added
dropwise to the above solution. After stirring for 30 min, NaN (0.13 g, 2 mmol) was added,
[
2
2
3 2
)
3
)
2
2
3
stirring was continued for another 3 h and finally the resulting solution was filtered off and the
filtrate was kept in a 100 ml beaker undisturbed. Slow evaporation afforded rod-shaped violet
coloured crystals, suitable for X-ray studies, within 3 days. Anal. Cal. for C38
848.75): C, 53.78; H, 5.46; N,23.10; Found: C, 52.78; H, 5.49; N, 23.11. IR (cm ) 1642
C=N), 2045 (-N ) (Fig. S3).
H46Co
2
N
14
2
O (MW
-1
=
(
3
Table 1.

5
X-ray crystallography
Single crystal X-ray data of complex 1 were collected on a Bruker SMART APEX-II CCD
diffractometer using graphite monochromated Mo-K radiation (λ = 0.71073 Å). Data collection,
α
reduction, structure solution and refinement were performed using the Bruker APEX-II suite
program (v 2.0-2). All available reflections to 2θmax were harvested and corrected for Lorentz and
polarization factors with Bruker SAINT plus. Reflections were then corrected for absorption,
inter frame scaling and other systematic errors with SADABS [28]. The structure was solved by
2
direct methods and refined by means of the full matrix least-square technique based on F with
the SHELX-97 software package [29]. All the non-hydrogen atoms were refined with anisotropic
thermal parameters. All the hydrogen atoms belonging to carbon and nitrogen atoms were placed
in their geometrically idealized positions. Drawings of the molecules were generated with the
program PLATON v-1.16. The crystallographic data for 1 are given in Table 1.
Magnetic susceptibility measurements.
The temperature dependent magnetization was acquired with an MPMS XL-5 Quantum Design
SQUID magnetometer on a polycrystalline sample in the range T = 2 to 300 K, at B = 0.01 T,
and the isothermal magnetization was measured at T = 2.0, 4.0, 7.0 and 10 K up to a magnetic
field B = 5 T. The experimental data were corrected for the diamagnetism of the constituent
atoms using Pascal’s constants and for the diamagnetism of the sample holder.
Catalytic study
Allylic oxidation of various olefins was carried out using this material as a catalyst and tert-
butylhydroperoxide (TBHP) as an oxidant. The overall reaction was performed in a two-necked
round-bottomed flask fitted with a condenser and a temperature-controlled oil bath on a magnetic
stirrer. In a typical procedure, 0.25 g of the substrate and TBHP oxidant (6 M in decane, Sigma-
Aldrich) were taken (2:1 molar ratio) in 2.0 ml MeCN containing required amount (0.05 g, 0.059
mmol) of the catalyst. The temperature of the reaction was maintained at 50-55 °C and the
products were collected at regular intervals. The progress of the reaction was monitored by an
Agilent 7890D gas chromatograph (FID detector) fitted with a capillary column. The products
were identified by a comparison with known standards.

6
Results and Discussion
Synthesis and structural description
2
,6-Bis-(chloromethyl)-4-methyl-phenol was synthesized in a simple way by chlorination of the
alcoholic group of 2,6-bis(hydroxymethyl)-4-methyl-phenol without protecting the phenolic OH
group, as reported earlier, in a modest yield. This, on subsequent treatment with 3,5-dimethyl
pyrazole in dry THF in presence of triethylamine (TEA) as a base, yields the novel ligand 2,6-
bis-(3,5-dimethyl-pyrazol-1-ylmethyl)-4-methylphenol (HL) in ~90% yield. When it was treated
with Co(NO
shaped violet crystals suitable for X-ray diffraction were obtained on slow evaporation within 2-
days.
3
)
2
.6H O in MeCN in presence of TEA and 2 equivalents of sodium azide, rod-
2
3
Scheme 1
The results of single crystal X-ray diffraction analysis indicate that complex 1, with a neutral
II
dinuclear [Co
2
(L)
2
3
(N )
2
] core, crystallizes in the monoclinic system with the space group C2/c.
II
The coordination environment around the Co ion is shown in Fig. 1, where the two cobalt atoms
-
are centrosymmetric and coordinated by two tridentate (N
2
O) L ligands and two azide ions, each
having a distorted square pyramidal geometry. Co1 is coordinated by two N atoms (N1 and N3)
from two 3,5-dimethyl pyrazole units belonging to two different ligands, one azide ion (N5) and
one phenoxo O-atom (O1), which is further coordinated to second Co1a atom in a µ -O fashion.
2

7
Fig. 1.
The Co1-Nx distances (Table 2) (x = 1, 3 and 5) are in the range 2.016-2.156 Å, while Co1-
O1/O1a = 1.98 and 2.19 Å, which clearly indicate that the two Co1 atoms are in a highly
distorted square pyramidal geometry. The geometry of a pentacoordinated metal centre can be
measured by the Addison parameter (τ), which is calculated to be 0.88 (τ = [(α - β)/60)]x100,
where α and β are the two largest ligand-metal-ligand angles of the coordination sphere)
suggesting a distorted trigonal-bipyramidal geometry (τ = 0 for a perfect square pyramid and τ =
1
for a perfect trigonal bipyramid). This distortion may arise to avoid steric hindrance caused by
the two methyl groups on two co-facially disposed 3,5-dimethyl pyrazole groups. The inter-
metallic Co1•••Co2 distance is found to be 3.198 Å. There are a number of intermolecular
bifurcated H-bonding interactions between the N atom of the coordinated azide ion of one
dinuclear unit and a methylinic-H atom of the pyrazole group and CH proton of the benzene ring
belonging to a neighboring dinuclear complex, as portrayed in Fig. 2 and Table 3. The
intramolecular π•••π interaction between two co-facial pyrazole rings strengthen the stability of
the complex. All these interactions lead to a 1D supramolecular structure. The H-bonded close
packing diagram of complex 1 is shown in Fig. 3.
Fig. 2
Table 2
Fig. 3.
Bond Valence Sum (BVS) calculations.
Bond Valence Sum (BVS) [30] is a method very frequently used to examine the nature of the
coordination and oxidation state of a central metal atom using crystallographic data for
coordination complexes as well as for many biological molecules. The most successfully used
empirical equation is:
Σsij = Σexp[(R -Rij)/0.37]
o
where R is a parameter characteristic of the bond and Rij are the crystallographically obtained
o

8
bond distances. Σsij represents the charge on the central metal atom. From the calculations (Table
S1) it can be seen that the oxidation state of the cobalt is 2.13 (Co1) and 2.12 (Co1a), which
unequivocally indicate the +2 oxidation state for the Co atoms in complex 1.
UV-vis spectrum.
3
-1
-1
The UV spectrum of complex 1 in MeCN showed bands at 663 (ε = 224 dm mol cm ) and 583
3
-1
-1
3
-1
nm (ε = 280 dm mol cm ), with a non-well resolved shoulder at 515 nm (ε = 230 dm mol
-1
4
4
4
4
4
4
cm ) (Fig. 4), which may be assigned to T1g ꢀ T2g (F), T1g ꢀ A2g (F) and T1g ꢀ T1g (P)
transitions, respectively. A comparison with other high spin, five-coordinated Co(II) spectra
reveals a similarity to the known trigonal bipyramidal complexes, as expected on the basis of the
crystal structure and magnetic moment [31-37].
Fig. 4
Fig. 5.
Electrochemical studies
Complex 1 remains as a dinuclear entity, as evidenced from its HRMS spectrum. Complex 1 is
electroactive in MeCN solution vs. Ag/AgCl. The two anodic peaks at -0.282 and 0.548 V
correspond to the Co(II)-Co(II)ꢀCo(II)-Co(III) and Co(II)-Co(III)ꢀCo(III)-Co(III) oxidations,
while the cathodic peaks at 0.270 and -0.611 V correspond to Co(III)-Co(III)ꢀCo(II)-Co(III)
(
E
½
= 0.409 V) and Co(II)-Co(III)ꢀCo(II)-Co(II) (E = -0.447 V) reductions. While the Co(II)-
½
Co(III)/Co(III)-Co(III) redox couple appears to be quasi-reversible, the corresponding Co(II)-
Co(III)/Co(II)-Co(II) couple is mostly irreversible (Fig. 5).
Magnetic study
To gain an insight into the magnetic property of this dimeric Co(II) complex, variable-
temperature dc magnetic susceptibility measurements were performed between 2 and 300 K for a
crystalline sample at a magnetic field of 100 Oe (Fig. 6). The thermal variation of the χMT
product is plotted after correcting for the diamagnetic contribution, as given in standard
3
–1
literature. This shows a room temperature value of around 8.158 cm K mol , which is relatively

9
higher than the result for two uncoupled Co(II) ion spins (Fig. 7) [39]. The decrease in χ
M
T
T
behavior down to 55 K is due to a temperature independent paramagnetic effect. Fitting of χ
M
data, considering the temperature independent paramagnetic contribution (TIP), provides a small
–
4
3
-1
amount of TIP species (10 cm mol ). The extracted g parameter (Lande factor) is 2.39, which
is also reasonable compared to the expected value obtained for an S = 3/2 Co(II) system. As the
3
-1
temperature decreases, the χ
M
T product slightly falls to 7.256 cm K mol at 55 K and then rises
3
-1
to a high value of 12.26 cm K mol at 10 K. This implies the population of a triplet state on
3
–1
lowering the temperature. A sudden fall of the χ
below 10 K. This low temperature fall of the χ
M
T product to 4.745 cm K mol has been seen
M
T value reveals the depopulation of the triplet
state at low thermal energy due to presence of magnetic anisotropy (zero field splitting) in the
Co(II) ion. This is also in agreement with the field-cooled-zero-field-cooled protocol (100 Oe)
which shows a transition peak at around 10 K (Fig. 6). The best fit is obtained for the above χ T
M
behavior considering a bilinear exchange term 2J associated with single ion anisotropy (D), by
the following spin Hamiltonian (1).
2
2
∧
∧
∧
∧
∧
∧
∧
∧
∧
∧
∧



















1
1
H = g µ BS
1
+ g µ BS − 2J
S
1
.S
2
+ D11
S
1z
−
3 S
1
S
1
+1 + D22

S
2z
−
3 S
2
S
2
+1

1
B
2
B
2
Co-Co

2
2
2
2
∧
∧
∧
∧






+E11 
S
1x − S1y + E22
S
2x − S2y ...............(1)





where the specific terms have their usual meanings.
Table 3.
The same values of the Lande g factors (g
for the two oxo-bridged Co(II) ions (S and S
fitted by fixing the g values obtained from the χ
fits of χ
1
and g
2
) and the ZFS terms (D
1
and D
2
) are assumed
1
2
). The field dependent magnetization data were
M
T fit ((g x,y = 2.91; g
z
= 2.18)) (Fig. 7). Both the
M
T and the magnetization data provide ferromagnetic exchange coupling (2J = + 10.3
−
1
-1
-1
cm ) and high single-ion magnetic anisotropy (D = + 29.8 cm , E = + 1.6 cm ) values. Good
2
-4
agreement factors (R ) in the order of 10 are obtained in all the least squares fitting processes.
The high positive zero-field splitting originates from the distortion of the coordination geometry
from the ideal square pyramidal geometry. A ferromagnetic exchange interaction is also
observed from the experimental magnetization data as it shows saturation in magnetization (4.9

10
/
Nβ) at high field with a coercivity of 200 Oe (2K), as shown in Fig. S4. The small intermetallic
Co(II)•••Co(II) distance (3.198 Å) and the borderline bridge angle (99.3°) favours a suitable
overlap for the ferromagnetic exchange coupling in the dimeric moiety. The complex exhibits
intermolecular π•••π ordering (offset, 4.086 Å) between the two co-facial pyrazole moieties,
which might also mediate the weak ferromagnetic exchange interaction between the two
paramagnetic Co(II) ions. In conclusion, the present Co(II) dimeric complex possesses a high
-1
positive magnetic anisotropy (D = + 29.8 cm ) and a weak ferromagnetic exchange interaction
-1
(
J = + 10.3 cm ). It shows complete magnetic hysteresis curves at several temperatures (Fig. 8).
The inset shows the appearance of a coercivity of 200 Oe at 2K, an indication of a soft
ferromagnet.
AmAmPy
The two reported pentacoordinated Co(II) dimers [{Co[O
2
NN′]
}
2
] and [Co
2
(TPA) (µ-
2
tp)](ClO
4
)
2
.2H
2
O [38.39] exhibited ferro- and antiferromagnetic coupling with 2J values of >0.2
-1
-1
and -0.8 cm , respectively (Table 3). The ZFS D parameter for the latter is only 7.3 cm .
However, in the present investigation the dimeric neutral Co(II) complex [Co
2
(L)
2
(N )] exhibits
3
comparatively higher 2J and D values than the so far reported pentacoordinated Co(II) dimers.
Fig. 6
Fig. 7
Fig. 8
EPR spectra
EPR spectroscopy is a powerful technique which is sensitive towards the electronic structure of
paramagnetic species. The X-band (9.5 GHz) EPR spectra (Fig. 9) of the sample (solid phase)
containing the Co(II) ion in a distorted square-pyramidal geometry was run at 77 K in a liquid
nitrogen finger dewar. The experiment shows two derivative signals of S = 3/2 species with
resonance at g ≈ 2.141 (g
value is evidenced from the low field splitting at 2.951 and the corresponding high field splitting
is responsible for the axial splitting (g ). The appearance of a high anisotropic g value is
z
) and 2.951 (gx,y). The appearance of a high anisotropic rhombic gx,y
z
z
evidenced from the low field splitting at 2.951. From the spectra, it is seen that the experimental
Lande splitting factor is in close agreement with the theoretical values obtained from magnetic

11
fitting procedures.
Fig. 9
Catalytic activity
A keen study of the catalytic activity of the neutral cobalt(II) dimer [Co
2
(L)
2
(N ) ] (1) towards
3
2
the oxidation of different common olefins, like styrene, cyclohexene and cyclooctene, under
homogeneous conditions has been carried out in a small volume of MeCN using TBHP as the
oxygen donor and the corresponding results are presented in Table 4.
Scheme 2: Various oxidation products of styrene using TBHP and the Co complex.
Table 4.
The oxidation of styrene using TBHP initially produces styrene oxide, which is converted to the
more stable benzaldehyde product, and after prolonged oxidation it produces a small amount of
benzoic acid. In another parallel reaction some styrene oxide isomerizes to generate small
amount of 2-phenyl acetaldehyde, as outlined in Scheme 2. Our catalyst shows an appreciable
conversion of styrene to give a high yield of benzaldehyde
Scheme 3: Oxidation products of cyclohexene in the presence of TBHP.

12
Scheme 4: Oxidation of cyclooctene in the presence of TBHP and the Co catalyst.
Similarly, several products are formed from cyclohexene oxidation, which is presented in
Scheme 3. Among these products cyclohexene-1-one is the major product obtained, with above
8
0% selectivity (Entry 2, Table 4). On the other hand, cyclooctene, on oxidation under similar
conditions, exhibits a good conversion to offer a reasonable amount of cyclooctene oxide
Scheme 4, Entry 3, Table 4). The time based conversion of all these alkenes and the formation
(
of the various products are graphically shown in (Fig. S5). From Fig. 10 it is observed that
although benzaldehyde is the only product (100% selectivity) up to 8 h, the total yield is not so
high (ca. 36%). After 24 h the conversion of styrene is increased appreciably, but due to the
formation of other by-products, like styrene oxide, benzoic acid etc., the selectivity or % yield of
benzaldehyde decreases to 75%. In the case of cyclohexene, initially the main product is formed
in a small amount owing to formation of epoxide, alcohol and a TBHP derivative. As the time of
reaction is increased the amount of those products goes down, enhancing the selectivity of the
ketone (major, about 80%). Without the aid of a catalyst almost no conversion is observed for
cyclohexene, despite using the same amount of TBHP (Entry 5, Table 4).
Fig. 10.
Fig. 11
In order to optimize the effect of the TBHP amount on this oxidation reaction we have also
carried out the reaction with a cyclohexene:TBHP molar ratio of 1:1 and it is observed that
although the conversion after 24 h is slightly higher than that with a 2:1 ratio, the selectivity
towards the main product decreases drastically to ca. 64% (Entry 4) with a subsequent increase
in the amounts of the by-products. The amounts of the different products after 24 h conversion
with both molar ratios (2:1 and 1:1) is clearly shown in Fig. 11. Thus it can be concluded that 2:1
is the best substrate:oxidant ratio for this catalytic reaction condition. We have also tried to carry

13
out the same catalytic reaction using H
there is no catalytic conversion of olefins. A reasonable explanation may be furnished by
considering the fact that the intermediate that is formed by the reaction of the complex with H
is quite stable and stops the catalytic cycle here. We have performed HRMS studies which
2
O
2
as an oxidant, and it was astonishingly observed that
2
O
2
suggests the formation of [[Co
2
(L)
2
(HO ) ] species where the two azide ions are replaced by two
2
2
-
hydroperoxide (HOO ) species (Figure S8).
The catalytic oxidation of olefins may occur either through a radical or concerted
pathway [40]. In order to get a clear picture of the mechanistic detail we have carried out the
catalytic reactions in the presence of a radical scavenger, like 2,4-di-tert-butylphenol. After 24 h
it is observed that only 7 and 11% conversions are observed in the case of styrene and
cyclooctene, respectively, clearly supporting the operation of a radical pathway over the
concerted one. Here, in Scheme 5 (as a representative example for cyclooctene), the first step
II
•
involves the reductive cleavage of the peroxide bond of t-BuOOH by LCo to form t-BuO and
III
•
LCo (OH). The t-BuO radical in turn rapidly abstracts a hydrogen atom from t-BuOOH to form
•
the thermodynamically more stable t-BuOO [41]. Now the one electron oxidation of t-BuOOH
III
•
II
by LCo (OH) leads to the formation of t-BuOO and H O, and regenerates LCo , thereby
2
completing one catalytic cycle. Thus, in one complete catalytic cycle three mole of t-BuOOH are
•
used up to generate one mole of t-BuOH, one mole of water and two moles t-BuOO . The t-
•
BuOO radical thereby generated now reacts with cyclooctene to generate cyclooctene-oxide.
The same mechanism was found to be operative for the other olefins.
Scheme 5

14
Conclusions
The present study demonstrates an interesting coordination compound obtained from a tridentate
ligand, 2,6-bis-(3,5-dimethyl-pyrazol-1-ylmethyl)-4-methyl-phenol (HL), where two ligands are
coordinated to two cobalt atoms. X-ray crystallographic studies revealed that in the complex both
the cobalt atoms are in the +2 oxidation state, as supported by BVS calculations. Low
temperature magnetic studies showed that the present Co(II) dimeric complex, devoid of any
-
single molecule magnetic behavior, possesses high positive magnetic anisotropy (D = +29.8 cm
1
-1
)
and a weak ferromagnetic exchange interaction (J = + 10.3 cm ). It shows the complete
magnetic hysteresis curves at several temperatures, with the appearance of coercivity of 200 Oe
at 2K, an indication of a soft ferromagnet. The catalytic epoxidation of cyclohexene, styrene and
cyclooctene by t-BuOOH has been explored in MeCN, which appeared to proceed through a
radical pathway with modest TONs and quite an impressive selectivity towards epoxides.
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,
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,
D.D.M.
,

17
Table 1. Crystallographic and structure refinement data for complex 1.
Formula
C
38
H46Co
2
N
14
O
2
(
CCDC 1062604)
Formula Weight
Crystal System
Space group
848.75
Monoclinic
C2/c (No. 15)
a [Å]
b [Å]
c [Å]
α [º]
β [º]
γ [º]
20.0837(6)
11.6348(3)
17.1398(6)
90
108.024(4)
90
3
V [Å ]
3808.5(2)
4
Z
3
D(calc) [g/cm ]
1.480
0.927
-1
α
Mu(MoK ) [mm ]
F(000)
1768
Temperature [K]
150
Radiation [Å] MoK
α
0.71073
θ Min-Max [º]
3.0, 25.1
Dataset
-23: 23; -13: 13; -20: 20
17358, 3391, 0.035
2814
Tot., Uniq. Data, R(int)
Observed data [I > 2.0
sigma(I)]
Nref, Npar
3391, 266
R, wR
2
, S
0.0310, 0.0762, 1.02

18
Table 2. Selected bond distance and bond angles of 1
Bond distance (Å)
Co1−O1
Co1−N1
Co1−N3
Co1−N5
Co1−O1a
2.1929(15) O1−Co1−O1a 80.47(5)
2.0495(18) N1−Co1−N3 92.41(7)
2.1563(19) N1−Co1−N5 125.31(7)
2.0165(19)
O1a
108.15(6)
−Co1−N1
1.9792(14) N3−Co1−N5 90.65(8)
Bond Angles (˚)
O1a
100.90(6)
−Co1−N3
O1−Co1−N1 86.21(6)
Symmetry a =1-x,y,1/2-z
O1a
Co1−N5
124.73(7)
−

19
Table 3. Magnetic data of some relevant dinuclear cobalt(II) complexes.
−
1
−1
Complex
2J (cm )
<0.2
D (cm )
Ref.
[38]
[39]
AmAmPy
a
(
[
[
[{Co[O NN′]
} ]
2
------
2
a
Co (TPA) (µ-tp)](ClO ) .2H O
-0.8
7.3
2
2
4 2
2
a
b
b
Co (L) (N ) ]
10.3
This work
2
2
3 2
+ 29.8
[
[
[
[
[
[
Co (H O)(PhCO ) (py) ].0.5(PhCO H).1.5(MePh)
-1.90
-0.70
-2.43
-0.90
-1.63
-1.12
None
-1.65
+92.4
+50.7
+99.6
+68.7
+79.9
+77.5
+94.1
2
2
2 4
4
2
Co (H O)(PhCO ) (Mepy) ]
2
2
2 4
4
Co (H O)(PhCO ) (iqu) ].iqu
2
2
2 4
4
c
Co (H O)(PhCO ) (fupy) ]
[40(e)]
2
2
2 4
4
Co (H O)(PhCO ) (Mefupy) ]
2
2
2 4
4
Co (H O)(PhCO ) (Me fupy) ]
2
2
2 4
2
4
trans-[Co(bz) (H O) (nca) ]
2
2
2
2
c
+
67.2
54.7
[
[
Co (µ-PhCOO) (qu) ]
[40(f)]
2
4
2
+
-
1.51
Co (µ-bz) (bz) (inca) ]
3
4
2
6
5
a
CoL
------
+1.4
+48.4
+151
[40(g)]
a
[
CoL Cl ]
[40(h)]
3
2
1
CoCl
2
L
-0.075
- 0.026
+ 71.7
+ 46.8
2
[
CoCl
2
L ]
[
40(i)]
a
b
c
pentacoordinated cobalt(II); 0.1 T; hexacoordinated
Table 4. Oxidation of olefins (0.25 g) over the Co-pyrazole complex (1).
Entry
Substrate
Conversion
%)
Main product
Selectivity (%)
(
1
2
3
Styrene
58.50
94.18
78.13
Benzaldehyde
cyclohexene-1-one
cyclooctene-oxide
75.47
80.45
66.81
Cyclohexene
Cyclooctene

20
a
4
5
Cyclohexene
Cyclohexene
98.86
0.33
cyclohexene-1-one
63.93
-
b
-
Reaction conditions: substrate:TBHP = 2:1 molar ratio, MeCN = 2 ml, catalyst = 0.05 g, temperature = 50-55 °C,
a
b
time = maximum 24 h. reaction carried out with substrate:TBHP = 1:1 molar ratio, reaction was performed without
catalyst.
Figure Captions
Fig. 1 Molecular view with the atom numbering of 1, symmetry a = 1-x, y, 1/2-z. All H-atoms
are omitted for clarity.
Fig. 2 π···π bonding as well as H-bonding interactions of complex 1 resulting in a 1D chain.
Fig. 3 Packing diagram of complex 1 showing the hydrogen bonded 3D-layers.
-3
Fig. 4 UV-Vis spectra of L and complex 1 in CH CN with [c] = 1.0 x 10 M.
3
Fig. 5 CV of complex 1 in MeCN solvent at 25°C, [c] = 1.0 mM; [TBAPC] = 0.10 M, scan
-1
rate = 100 mVs
Fig. 6
The temperature dependence of the magnetic susceptibility measured at 100 Oe using a
field-cooled-zero-field-cooled protocol. Inset reveals the splitting of the ZFC and FC
curves at low temperature.
Fig. 7
Fig. 8
Fig. 9
The variation of χ T with temperature for the Co(II) dimeric complex. Solid line
M
represents the theoretical curve and the points show the experimental data.
The complete magnetic hysteresis curves at several temperatures. Inset shows the
appearance of coercivity of 200 Oe at 2K, an indication of a soft ferromagnet.
EPR spectrum of the S = 3/2 Co(II) system showing a high anisotropy axial signal at g
≈
2.951 for solid phase.
Fig. 10 Time dependent conversion of styrene and formation of benzaldehyde.
Fig. 11 Effect of substrate:TBHP ratio on the yield of different products of cyclohexene.

21
Fig. 1
Fig. 2

22
Fig. 3
Fig. 4

23
Fig. 5
Fig. 6
Fig. 7

24
Fig. 8
Fig. 9

25
Fig. 10
Fig. 11

Synthesis, structure, catalytic and magnetic properties of a pyrazole
based five coordinated di-nuclear cobalt(II) complex
Rabiul Alama, Kaberi Pala, Bikash Kumar Shawb, Malay Dolaia, Nabanita Pala, Shyamal Kumar Sahab and
Mahammad Alia*
A five coordinated dinuclear cobalt(II) complex, [Co (L) (N ) ] (1), displayed modest catalytic activity towards the oxidation of
2
2
3 2
-
1
various allylic compounds. Low temperature magnetic studies gives high positive magnetic anisotropy (D = + 29.8 cm ) and a
-
1
weak ferromagnetic exchange interaction (J = + 10.3 cm ).

Synthesis, structure, catalytic and magnetic properties of a pyrazole
based five coordinated di-nuclear cobalt(II) complex
Rabiul Alama, Kaberi Pala, Bikash Kumar Shawb, Malay Dolaia, Nabanita Pala, Shyamal Kumar Sahab and
Mahammad Alia*
A five coordinated dinuclear cobalt(II) complex, [Co (L) (N ) ] (1), displayed modest catalytic activity towards the oxidation of
2
2
3 2
-
1
various allylic compounds. Low temperature magnetic studies gives high positive magnetic anisotropy (D = + 29.8 cm ) and a
-
1
weak ferromagnetic exchange interaction (J = + 10.3 cm ).