Synthesis, characterization and catalytic activities towards epoxidation of olefins of dinuclear copper(II) complexes

Article abstract of DOI:10.1016/j.molstruc.2015.08.017

Two copper(II) complexes, [Cu₂(L¹)Cl₃].2H₂O (1) and [Cu₂(L²)(N₃)Cl₂] (2) where HL¹ = 4-methyl-2,6-bis((2-morpholinoethylimino)methyl)phenol and HL² = 4-methyl-2,6-bis((3-morpholinopropylimino)methyl)phenol have been synthesized and characterized by elemental analysis, various spectroscopic methods, TGA and single crystal X-ray diffraction analysis. Single crystal X-ray diffraction analysis reveals that in both the complexes, two copper atoms are linked by phenoxo oxygen atom and a bridging ligand, namely chloride and azide, respectively. These complexes have been used as catalyst for the epoxidation of cyclohexene, styrene, α-methyl styrene, trans-stilbene and norbornene using tert-butyl hydroperoxide as the oxidant in acetonitrile under mild conditions. All of the substrates undergo conversion to produce respective epoxide as the major product.

Full text of DOI:10.1016/j.molstruc.2015.08.017

Journal of Molecular Structure 1101 (2015) 1e7
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Synthesis, characterization and catalytic activities towards
epoxidation of olefins of dinuclear copper(II) complexes
Shibashis Halder a, Aparajita Mukherjee a, Koushik Ghosh a, Sudipto Dey a,
,
*
Mahasweta Nandi b, Partha Roy a
a Department of Chemistry, Jadavpur University, Jadavpur, Kolkata 700 032, India
b Integrated Science Education and Research Centre, Siksha Bhavana, Visva-Bharati, Santiniketan 731 235, India
a r t i c l e i n f o
a b s t r a c t
Two copper(II) complexes, [Cu2(L1)Cl3].2H2O (1) and [Cu2(L2)(N3)Cl2] (2) where HL1 ¼ 4-methyl-2,6-
bis((2-morpholinoethylimino)methyl)phenol and HL2 ¼ 4-methyl-2,6-bis((3-morpholinopropylimino)
methyl)phenol have been synthesized and characterized by elemental analysis, various spectroscopic
methods, TGA and single crystal X-ray diffraction analysis. Single crystal X-ray diffraction analysis reveals
that in both the complexes, two copper atoms are linked by phenoxo oxygen atom and a bridging ligand,
namely chloride and azide, respectively. These complexes have been used as catalyst for the epoxidation
Article history:
Received 9 July 2015
Received in revised form
4 August 2015
Accepted 5 August 2015
Available online 8 August 2015
of cyclohexene, styrene, a-methyl styrene, trans-stilbene and norbornene using tert-butyl hydroperoxide
as the oxidant in acetonitrile under mild conditions. All of the substrates undergo conversion to produce
respective epoxide as the major product.
Keywords:
Dinuclear
Copper
Crystal structure
Epoxidation
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
Dinuclear complexes can be synthesized by the reaction of
metal ions and suitable ligands with appropriate ‘donor pocket’.
Mono-, di- and polynuclear transition metal complexes with
Schiff-base ligands are of great interest to the researchers from all
over the globe because of their diversity in structural topology and
effective applications in many fields of scientific investigations e.g.
magnetism, catalysis, biochemical reactions, sensing etc. [1e10].
Metal complexes with this type of ligands have become research
hot spot over few decades because Schiff-base ligands could be
easily synthesized by one step condensation reaction and many
properties of the complexes with these ligands could be controlled
by introducing various substitutions (e.g. electron pushing group,
electron withdrawing group, bulky group, etc.) on both the amine
and the aldehyde/ketone groups. Usually, Schiff base ligands pro-
duce stable complexes with a large number of transition metal ions,
providing the platform to study different aspects of such complexes
and their applications. These types of ligands stabilize different
metal ions at their various oxidation states. Therefore, Schiff-base
ligands play a crucial role in controlling the conversions of a
number of organic reactions catalyzed by transition
metal complexes with Schiff-base ligands.
Ligands are designed in such a way that it can accommodate two
metal ions by linking through N,O donor atoms. Acyclic Schiff-base
ligands derived from 4-methyl-2,6-diformylphenol (DFP) are suit-
able to produce dinuclear metal complexes under controlled reac-
tion conditions [11]. Deprotonated phenolic oxygen atom of such
ligand binds two metal centers simultaneously and at the same
time, other O and/or N donor atoms of the same ligand coordinate
to metal ions to form a chelate. Ancillary ligands e.g. hydroxido,
azido, carboxylato, halido etc. may be present in the metal complex
to provide some extra stability to the metal complex.
Many published research articles described syntheses of tran-
sition metal complexes and their applications in different catalytic
reactions. These complexes were found to be active catalyst for
various catalytic reactions and selectivity of the desired product
was excellent. Copper(II) compounds were widely used as catalysts
in catechol oxidation, epoxidation, sulfoxidation, olefin aziridina-
tion, alkane oxidation, atom transfer radical addition (ATRA) [3,4]
etc. in homogeneous as well as heterogeneous media. Epoxida-
tion of olefins is an important reaction in organic chemistry
because epoxides are versatile building blocks for the synthesis of
many bioactive molecules and organic fine chemicals [12,13]. Cu(II)
complexes were employed extensively as efficient catalysts for
epoxidation of various substrates where epoxides were detected as
* Corresponding author.
E-mail address: proy@chemistry.jdvu.ic.in (P. Roy).
http://dx.doi.org/10.1016/j.molstruc.2015.08.017
0022-2860/© 2015 Elsevier B.V. All rights reserved.

2
S. Halder et al. / Journal of Molecular Structure 1101 (2015) 1e7
the major product [14e18].
2.1.4. Synthesis of [Cu2(L2)(N3)Cl2] (2)
In continuation to our interest in CeH activation [19e22], we
report here synthesis, characterization and catalytic properties of
two copper(II) complexes, [Cu2(L1)Cl3].2H2O (1) and [Cu2(L2)(N3)
Cl2] (2) where HL1 ¼ 4-methyl-2,6-bis((2-morpholinoethylimino)
methyl) phenol and HL2 ¼ 4-methyl-2,6-bis((3-morpholinopro-
pylimino)methyl)phenol. These complexes have been character-
ized by elemental analysis, various spectroscopic methods and
single crystal X-ray diffraction analysis. They were found to be
active catalyst for epoxidation of various olefins in acetonitrile in
the presence of tert-butyl hydroperoxide as the oxidant under mild
conditions.
Copper(II) chloride dihydrate (0.136 g, 0.8 mmol) was added into
the Schiff base solution (HL2, ‘0.4 mmol’) in methanol and stirred
for 30 min. Aqueous solution (2.0 mL) of sodium azide (0.026 g,
0.4 mmol) was added dropwise into the mixture with constant
stirring. After that, it was refluxed for 1 h. Then it was cooled to
room temperature. The mixture was filtered and the filtrate was
kept at ambient temperature. From the filtrate, brown colored
crystals of complex 2 suitable for X-ray diffraction were obtained on
evaporation of solvent within two weeks. Yield: 0.117 g, 45%. Anal.
Calc. for C23H35Cl2N7O3Cu2: C, 42.10; H, 5.38; N, 14.95. Found C,
42.03; H, 5.33; N14.99.
Caution! Azide salts are potentially explosive and should be
handled in small quantities with care. No problems were faced with
the complexes reported herein.
2. Experimental section
2.1. Materials and physical methods
2.1.5. X-ray data collection and structure determination
The crystal data of complexes 1 and 2 are summarized in Table 1.
The structures of the complexes were determined by single crystal
X-ray diffraction analysis. Single crystal data collections were per-
formed with an automated Bruker SMART APEX CCD diffractometer
4-(2-Aminoethyl)morpholine and 4-(3-aminopropyl)morpho-
line were purchased from Sigma Aldrich. All other reagents were
obtained from commercial source and used as received. 4-Methyl-
2,6-Diformylphenol (DFP) was synthesized following a published
procedure [23]. Solvents used for spectroscopic studies were pu-
rified and dried by standard procedures before use [24]. FT-IR
spectra were obtained on a RX-1 Perkin Elmer spectrophotometer
with samples prepared as KBr pellets. Elemental analysis was car-
ried out in a 2400 Series-II CHN analyzer, Perkin Elmer, USA. The
ESI-MS was recorded on Qtof Micro YA263 mass spectrometer.
Absorption spectra were recorded on Lambda 25 Perkin Elmer
spectrophotometer. Thermogravimetric analysis was performed on
a Mettler Toledo instrument (Model no.: TGA/SDTA-851e). Gas
chromatography analysis was performed with a Shimadzu made
next generation high speed gas chromatography system (model:
GC-2025 AF) equipped with a fused silica capillary column and a
FID detector. All experiments were carried out at room temperature
in air unless reported otherwise.
using graphite monochromatized Mo K
Reflection data were recorded using the
a
u
radiation (
scan technique. Unit cell
l
¼ 0.71073 Å).
parameters were determined from least-squares refinement of
setting angles with
out of 18,842 collected data 5379 with I > 2
q
in the range 1.62 ꢀ
q
ꢀ 26.50ꢁ. For complex 1,
s (I) were used for
structure solution. For complex 2, out of 40,108 collected data 5697
with I > 2 (I) were used for structure solution. The structures were
s
solved and refined by full-matrix least-squares techniques on F2
using the SHELXS-97 program [25]. The absorption corrections
were done by the multi-scan technique. All data were corrected for
Lorentz and polarization effects, and the non-hydrogen atoms were
refined anisotropically. Hydrogen atoms were generated using
SHELXL-97 [25] and their positions calculated based on the riding
mode with thermal parameters equal to 1.2 times that of the
associated C atoms, and participated in the calculation of the final
R-indices.
2.1.1. Synthesis of 4-methyl-2,6-bis((2-morpholinoethylimino)
methyl) phenol (HL1)
2.1.6. Catalytic activity
To a 20 mL methanolic solution of DFP (0.065 g, 0.4 mmol), 4-(2-
aminoethyl)morpholine (0.104 g, 0.8 mmol) was added. The
resulting mixture was stirred for 15 min and then it was refluxed
for 2 h. This ‘solution of HL1’ was used for subsequent reactions
without isolation and further purification or characterization.
10.0 mmol of the substrate in 5.0 mL of acetonitrile was taken in
a magnetically stirred two necked round-bottomed flask fitted with
a condenser, followed by the addition of 0.03 mmol of the metal
complex. The mixture was heated to 323 K. 20.0 mmol of tert-butyl
hydroperoxide was added to the mixture to initiate the catalytic
reaction. Aliquots from the mixture were collected at regular time
interval. 10.0 mL of diethylether were added for extracting the re-
actants and product(s). The substrate and product(s) were analyzed
by gas chromatography and identified by the comparison with
known standards.
A blank experiment, namely epoxidation of cyclohexene, was
carried out as the representative case without addition of any
catalyst under the same experimental conditions. Another blank
reaction for the epoxidation of cyclohexene was performed in the
presence of copper(II) chloride in place of copper(II) complex with
all other parameters unchanged.
2.1.2. Synthesis of 4-methyl-2,6-bis((3-morpholinopropylimino)
methyl) phenol (HL2)
4-(3-Aminopropyl)morpholine (0.115 g, 0.8 mmol) was added to
methanolic solution (20 mL) of DFP (0.065 g, 0.4 mmol) and the
resulting solution was stirring for 15 min and then refluxed for 2 h.
This ‘solution of HL2’ was used for subsequent reactions without
isolation and further purification or characterization.
2.1.3. Synthesis of [Cu2(L1)Cl3].2H2O (1)
Copper(II) chloride dihydrate (0.136 g, 0.8 mmol) was added into
the Schiff base solution (HL1, ‘0.4 mmol’) in methanol and stirred
for 30 min. Aqueous solution (2.0 mL) of sodium azide (0.026 g,
0.4 mmol) was added drop by drop into the mixture with constant
stirring. After its addition, the mixture was refluxed for 1 h. Then it
was cooled to room temperature. The mixture was filtered and the
filtrate was kept at ambient temperature. From the filtrate, green
colored crystals of complex 1 suitable for single crystal X-ray
diffraction were obtained on evaporation of solvent within few
days. Yield: 0.170 g, 65%. Anal. Calc. for C21H35Cl3N4O5Cu2: C, 38.39;
H, 5.37; N, 8.52. Found C, 38.33; H, 5.30; N, 8.60.
3. Results and discussion
3.1. Synthesis and characterization
Complexes 1 and 2 were synthesized following a reaction
pathway as shown in Scheme 1. The ligand, HL1, was prepared by
refluxing one equivalent of DFP and two equivalents of 4-(2-
aminoethyl)morpholine in methanol for 2 h. Complex 1 was syn-
thesized by in situ reaction between the ligand, copper(II) chloride

S. Halder et al. / Journal of Molecular Structure 1101 (2015) 1e7
3
Table 1
Crystal data for complexes 1 and 2.
Complex 1
Complex 2
Formula
C21 H31Cl3 Cu2N4 O5
652.95
294(2)
C23 H35Cl2 Cu2N7 O3
655.58
293(2)
Formula weight
Temperature (K)
Color
green
green
Crystal system
Space group
a (Å)
b (Å)
c (Å)
triclinic
P-1
7.522(5)
10.600(5)
17.040(5)
100.377(5)
98.547(5)
91.971(5)
1319.0(11)
2
monoclinic
P21/C
14.255(5)
10.519(5)
20.629(5)
90.000(5)
100.104(5)
90.000(5)
3045.6(3)
4
a
(ꢁ)
В (ꢁ)
g
(ꢁ)
V(Å3)
Z
Crystal dimensions (mm)
F(000)
0.14 ꢃ 0.09 ꢃ 0.06
0.15 ꢃ 0.09 ꢃ 0.07
668
1256
D
c (g cmꢂ3
(Mo K
range (ꢁ)
)
1.644
0.71073
27.98e1.96
19,810, 6343
multi-scan
0.0592
1.430
0.71073
25.40e1.45
40,981, 5613
multi-scan
0.0697
l
q
a
) (Å)
Reflection collected/unique
Absorption correction
Rint
Final R indices [I > 2
Goodness-of-fit
s(I)]
R1 ¼ 0.0836, wR2 ¼ 0.2575
R1 ¼ 0.0516, wR2 ¼ 0.1541
1.107
0.915
dihydrate and one equivalent of sodium azide in methanol/water
mixture. No external base was used. However, complex 1 does not
contain any azide ion. So another reaction was performed without
sodium azide to get 1. Even after several attempts, complex 1 could
not be obtained when sodium azide was not used. Here, sodium
azide may act as weak base to deprotonate phenolic proton of HL1.
The ligand, HL2, was prepared by the reaction between one
equivalent of DFP and two equivalents of 4-(3-aminopropyl)mor-
pholine in methanol. Complex 2 was also synthesized by in situ
reaction between the ligand, two equivalents of copper(II) chloride
dihydrate and one equivalent of sodium azide in methanol/water
mixture. It contains one azide ion to bridge two copper centers.
Here also no external base was used.
FT-IR spectra of complexes 1 and 2 were recorded with sample
prepared as KBr pellet. In the spectrum of complex 1, the band
around 1660 cmꢂ1 is due to the presence of C]N moiety. The band
at 3402 cmꢂ1 may be assigned to the nCueO of ligand. The band at
589 cmꢂ1 may be attributed to nCueN [26]. FT-IR spectrum of
complex 1 shows strong nCeH bands at 2800e3000 cmꢂ1 [27]. The
sharp band at 2095 cmꢂ1 indicates the presence of azido moiety in
complex 2. The presence of C]N bond in Schiff-base complex has
been confirmed from the band that appeared at 1638 cmꢂ1. The
band at 3402 cmꢂ1 may be assigned to the nCueO of ligand. The band
at 509 cmꢂ1 may be attributed to nCueN [26]. FT-IR spectrum of
complex 2, like complex 1, also exhibits strong nCeH bands in the
region of 2800e3000 cmꢂ1
Magnetic susceptibilities of the complexes
.
1
and 2 were
measured with powdered sample at 298 K using Guoy balance
method. Both the complexes have been expected to show a similar
behavior. The dinuclear complexes of copper(II) with Schiff-base li-
gands have reported to show antiferromagnatic interactions be-
tween the metal centers [28,29]. The magnetic moment of two
isolated copper(II) ions has been calculated to be 2.83 BM. If the
measured magnetic moment is found to be smaller than the calcu-
lated value, then it indicates antiferromagnetic coupling among the
copper atoms at 298 K. The magnetic moments of complexes 1 and 2
have been determined to be 0.97 and 0.29 BM respectively. These
values are too small compared to 2.83 BM expected for isolated
copper atoms indicating strong antiferromagnetic interactions
Scheme 1. Synthesis of complex 1 and 2.

4
S. Halder et al. / Journal of Molecular Structure 1101 (2015) 1e7
Table 2
amongst copper atoms at 298 K.
Selected coordination bond lengths (Å) and angles (ꢁ) for complex 1.
Cu01eN1
Cu01eO1
1.949(4)
1.979(3)
Cu2eN3
Cu02eO1
1.936(4)
2.006(4)
3.2. Description of crystal structure of complex 1
Cu01eN3
2.072(5)
Cu02eN4
2.082(4)
Cu01eCl3
Cu01eCl1
2.3148(18)
2.5230(2)
91.67(17)
84.70(19)
165.80(17)
156.50(15)
82.87(11)
95.02(14)
95.50(15)
99.37(12)
94.64(14)
107.93(8)
Cu02eCl3
Cu02eCl2
2.3005(17)
2.528(2)
Complex 1 crystallizes in the P-1 space group from methanol. A
perspective view of the molecule is shown in Fig. 1. Selected bond
angles and bond lengths are listed in Table 2. Complex 1 consists of
one deprotonated molecule of ligand, 4-methyl-2,6-bis((3-
morpholinopropylimino)methyl)phenol, two copper atoms and
three chlorine atoms. Two water molecules are present in the unit
cell as solvent of crystallization. Both the copper atoms are in a
pentacoordinated environment. Cu1 is coordinated to one phenolic
oxygen atom (O1), two nitrogen atoms (N1 and N2) from the ligand,
HL1, and two chlorine atoms Cl1 and Cl3. Phenolic oxygen atom O1
and chlorine atom Cl3 act as bridging center. On the other hand,
Cu2 is bonded to one phenolic oxygen atom (O1), two nitrogen
atoms (N3 and N4) from the ligand, and two chlorine atoms Cl2 and
Cl3. Both the copper atoms are in distorted square pyramidal ge-
N1eCu01eO1
N1eCu01eN3
O1eCu01eN3
N1eCu01eCl3
O1eCu01eCl3
N3eCu01eCl3
N1eCu01eCl1
O1eCu01eCl1
N3eCu01eCl1
Cl3eCu01eCl1
N2eCu02eO1
N2eCu02eN4
O1eCu02 N4
N2eCu02eCl3
O1eCu02eCl3
N4eCu02eCl3
N2eCu02eCl2
O1eCu02eCl2
N4eCu02eCl2
Cl3eCu02eCl2
91.35(16)
84.45(18)
163.01(17)
160.28(13)
82.66(10)
95.81(13)
94.35(13)
102.04(12)
94.70(14)
105.26(8)
3.3. Description of crystal structure of complex 2
Complex 2 crystallizes in the P21/C space group from methanol.
A perspective view of the molecule is shown in Fig. 2. Selected bond
angles and bond lengths are given in Table 3. Complex 2 is
composed of one deprotonated molecule of ligand, 4-methyl-2,6-
bis((3-morpholinopropylimino)methyl)phenol, two copper atoms,
two chlorine atoms and one azide moiety. Both the copper atoms
are in pentacoordinated environment. Cu1 is coordinated to one
phenolic oxygen atom (O1), two nitrogen atoms (N1 and N2) from
the ligand, HL2, and one chlorine atom Cl1 and one azide. Phenolic
oxygen atom O1 and one azide (m1,1) act as bridging center. On the
other hand, Cu2 is bonded to one phenolic oxygen atom (O1), two
nitrogen atoms (N3 and N4) from the ligand, one chlorine atom Cl1
and one azide. Both the copper atoms are in distorted square py-
ometry as revealed by trigonal index,
t value. The value of trigonal
index is defined as the difference between the two largest
donoremetaledonor angles divided by 60, a value which is 0 for
the ideal square pyramid and 1 for trigonal bipyramid geometry
[30]. t values for Cu1 and Cu2 have been found to be 0.155 and 0.017
respectively. O1, N1, N2 and Cl3 form the basal plane of square
pyramid while Cl1 occupies the apical position. Cu1echlorine dis-
tances are longer compare to other Cu1edonor distances. Cu1e-
Cl1apical distance is quite longer than the others. Cu1 is out of the
mean plane formed by O1, N1, N2 and Cl3 and towards Cl1 by
0.182 Å. O1, N3, N4 and Cl3 form the basal plane of square pyramid
while Cl2 occupies the apical position. Cu2echlorine distances are
longer compare to other Cu2edonor atom distances. Cu1eCl1apical
distance is quite longer than the others. Cu2 is out of the mean
plane formed by O1, N3, N4 and Cl3 and towards Cl2 by 0.328 Å.
ramidal geometry as revealed by trigonal index, t value. t values for
Cu1 and Cu2 have been calculated to be 0.212 and 0.294 respec-
tively. Cl1, N5, O1 and N1 form the basal plane of square pyramid for
Cu1 while N2 occupies the apical position. Cu1eN2apical distance is
quite longer than the others. Cu1 is out of the mean plane formed
Fig. 1. A perspective view of complex 1. Hydrogen atoms and water molecules were
omitted for clarity.
Fig. 2. A perspective view of complex 2. Hydrogen atoms were omitted for clarity.

S. Halder et al. / Journal of Molecular Structure 1101 (2015) 1e7
5
Table 3
Selected coordination bond lengths (Å) and angles (ꢁ) for complex 2.
3.6. Epoxidation of olefins
Complexes 1 and 2 have been used as catalysts for the oxidation
of cyclohexene, styrene, a-methyl styrene, trans-stilbene and nor-
Cu1eN1
Cu1eN5
Cu1eO1
1.941(4)
1.960(4)
2.026(3)
Cu2eN2
Cu2eN5
Cu2eO1
1.939(4)
1.963(4)
2.024(3)
bornene with tert-butyl hydroperoxide as the oxidant at 323 K.
Respective epoxides have been detected as the major products in
each reaction. The results of catalytic conversions have been given
in Table 4. It is clear from the initial inspection of the table that
complexes 1 and 2 are effective catalysts for such organic trans-
formations. Complex 2 has been found to be the most active cata-
lyst and it catalyzes cyclohexene epoxidation with 86% conversion
and 91% selectivity whereas complex 1 transforms cyclohexene
with 84% conversion and 88% selectivity. 2-Cyclohexen-1-ol and 2-
cyclohexen-1-one have been detected as the minor products.
Epoxidation of styrene draws special attention from industry and
academics. Complexes 1 and 2 can act as catalyst for the epoxida-
tion of styrene with 72% and 75% conversions, respectively. Styrene
oxide has been detected as the major product with the formation of
Cu1eCl1
Cu1eN3
2.2797(15)
2.430(5)
Cu2eCl2
Cu2eN4
2.2705(17)
2.374(4)
N1eCu1eN5
N1eCu1eO1
N5eCu1eO1
N1eCu1eCl1
N5eCu1eCl1
O1eCu1eCl1
N1eCu1eN3
N5eCu1eN3
O1eCu1eN3
Cl1eCu1eN3
165.78(17)
91.39(14)
75.17(14)
99.08(12)
91.28(13)
153.05(11)
83.69(17)
104.61(18)
107.94(16)
97.91(13)
N2eCu2eN5
N2eCu2eO1
N5eCu2eO1
N2eCu2eCl2
N5eCu2eCl2
O1eCu2eCl2
N2eCu2eN4
N5eCu2eN4
O1eCu2eN4
Cl2eCu2eN4
165.65(17)
91.16(14)
75.13(15)
98.51(13)
91.25(13)
148.02(11)
83.96(17)
104.92(17)
112.10(15)
99.25(11)
by Cl1, N5, O1 and N1 towards N2 by 0.286 Å. Cl2, N5, O1 and N3
form the basal plane of square pyramid for Cu1 while N4 occupies
the apical position. Cu1eN4apical distance is quite longer than the
others. Cu1 is out of the mean plane formed by Cl2, N5, O1 and N3,
and towards N4 by 0.650 Å. It is to be noted that for complex 1, Cl
atoms occupy the apical position of square pyramid around Cu1 and
Cu2 whereas for complex 2, morpholinium-N atoms of the ligand
reside in the apical position of square pyramid around both the
copper atoms. This may be possible for complex 2 because of longer
CeC chain length.
small amount of benzaldehyde.
a-Methyl styrene has been con-
verted to its epoxide in the presence of both the metal complexes
with good yield and selectivity. Epoxidation of trans-stilbene re-
veals that conversions of it in the presence of complexes 1 and 2 are
78% and 81%, respectively. Corresponding epoxide formation
selectivity for 1 and 2 is also quite high (87% and 85% respectively).
These metal complexes can also effectively convert bulkier alkene.
In this respect, we have tried with norbornene. Exoepox-
ynorbornane has been identified as the major product in the case of
norbornene epoxidation. Norbornene is converted to 80% and 82%
in the presence of 1 and 2, respectively, with high selectivity to-
wards the corresponding epoxide. All these results for the epoxi-
dation of olefins are comparable to those reactions catalyzed by
different transition metal complexes [17e19,32e35]. Although
there is scope for the improvement of the yield, selectivity and
reaction time, these experiments show that these copper(II) com-
plexes can be used as catalysts for such reactions.
Hydrogen peroxide could have been a better oxidant in com-
parison to tert-butyl hydroperoxide considering the end product(s)
from an environmental view point. Water is obtained at the end
when H2O2 is used while butanol is formed when tert-butyl hy-
droperoxide is employed as the oxidant. However, we have carried
out epoxidation of cyclohexene in the presence of complex 2 with
different oxidants at 323 K to determine the most effective oxidant.
Conversion of cyclohexene has been found to be 86%, 58% and 42%
in the presence of tert-butyl hydroperoxide, hydrogen peroxide and
NaOCl, respectively.
3.4. UVevis spectra
The electronic spectrum of complex 1 was recorded in aceto-
nirtile. It exhibits a broad band at 615 nm (molar extinction coef-
ficient: 3010 Mꢂ1sꢂ1), which may be assigned to ded transition.
Higher intensity charge-transfer transitions are obtained at 370 and
253 nm, with molar extinction coefficients of 6390 Mꢂ1sꢂ1 and
31,200
M , / Cu(II),
ꢂ1sꢂ1 respectively, attributed to PhOꢂ
N(amino) / Cu(II) LMCT and intraligand charge transfer transi-
tions [31].
The electronic spectrum of complex 2 in acetonitrile shows
several absorption bands in the visible region that are assigned to
ded and charge-transfer transitions. It exhibits a broad band at
712 nm (molar extinction coefficient: 337 Mꢂ1sꢂ1) due to ded
transition. The band at 372 nm (molar extinction coefficient:
733
M ) / Cu(II) and
ꢂ1sꢂ1 may be attributed to PhOꢂ
N(amino) / Cu(II) LMCT. Higher intensity charge-transfer transi-
tions at 254 nm, with molar extinction coefficient 3480 Mꢂ1sꢂ1
may be assigned to intraligand charge transfer transitions [31].
Acetonitrile has been selected as solvent in the epoxidation of
Table 4
Epoxidationa of olefins by complexes 1 and 2.
Conversionb
Selectivity
TOFc
3.5. Thermogravimetric analysis (TGA)
Catalysts
Substrate
1
Cyclohexene
Styrene
84
72
74
78
80
86
75
74
81
82
88
85
86
87
82
91
86
83
85
84
11.7
10.0
10.3
10.8
11.1
11.9
10.4
10.3
11.2
11.4
Thermogravimetric analysis of complex 1 has been carried out
with the powdered samples under nitrogen atmosphere in the
range 30e700 ꢁC. Thermal analysis of complex 2 could not be
performed due to the presence of azide ion which could be a po-
tential explosive under heating conditions. Mass vs temperature
plot of complex 1 is given in Supplementary information. Complex
1 shows four step weight loss upto 700 ꢁC. It is clear from the figure
that it is stable about 70 ꢁC. After that it starts to lose water mol-
ecules from it. Within 150 ꢁC, complex 1 lost two water molecules
(2.74% weight loss vs. 2.75% calculated weight loss). After that
complex 1 loses some unidentified organic moiety and mass lost
becomes quite stable near temperature region of 700 ꢁC. Thus, the
presence of water molecule is also indicated by the TGA study.
a-Methyl styrene
trans-Stilbene
Norbornene
Cyclohexene
Styrene
2
a-Methyl styrene
trans-Stilbene
Norbornene
a
Solvent: CH3CN; temperature: 323 K; oxidant: tert-butyl hydroperoxide; cata-
lyst: 0.03 mmol; substrate: 10.0 mmol.
b
Conversions were measured after 24 h of the reaction (average of two mea-
surements carried out under identical reaction conditions).
c
TOF: turnover frequency ¼ moles of substrate converted per mole of metal
complex per hour.

6
S. Halder et al. / Journal of Molecular Structure 1101 (2015) 1e7
the substrates because the catalyst, substrate and product(s) are
soluble in this solvent. It is difficult for acetonitrile to undergo
oxidation under the present reaction conditions.
Catalytic reactions have been carried out at different tempera-
tures to check the effect of temperature on the rate of the reaction.
It has been seen that rate of the reaction increases initially but
decreases after certain enhancement in temperature. Both the
complexes show similar effect of temperature on the rate. The
optimized condition for the best result is obtained when reaction
has been performed at 323 K.
Reaction mixtures were collected at regular time interval and
analyzed by gas chromatography. It has been seen that the con-
version increases with increase in time and reaches its maxima
after 24 h. Plots of conversion vs. time for complexes 1 and 2 have
been given in Fig. 3 and Fig. s7, respectively. It suggests the con-
version follows a sigmoid curve.
566.6757 and 530.7379. These peaks may be obtained due to the
presence of 1‒2Cl, 1‒(2Cl þ H2O) and 1‒(3Cl þ H2O) respectively.
Mass spectrum indicates that two copper atoms in 1 are also pre-
sent in solution. However, mass spectrum of complex 2 is not very
much conclusive. It shows an m/z peak at 417.1884. This peak may
be due to the presence of HL2 ([HL2þH]þ). In previous study [17,18],
it was reported that copper-hydroperoxido or copper-peroxido was
identified as active center for epoxidation of olefin when reaction
passes through formation of tert-butyl alcohol. During analysis of
reaction mixture, tert-butyl alcohol has been identified as the side-
product in our case, thus we expected that catalytic reaction in this
study follows similar reaction pathway involving copper-peroxido
species. The UVevis spectrum of complex in the presence of tert-
butyl hydroperoxide also supports this fact.
4. Conclusions
Main problems associated with homogeneous catalysis are the
recovery and reusability of catalyst. We have attempted to recover
the complexes after completion of catalytic cycle to use these ma-
terials as catalyst again. However, the material recovered after
catalytic reaction is not same as the starting complexes. The ana-
lyses and comparison of FT-IR and UVevis spectra of complexes 1
and 2 with the recovered materials does not support that com-
plexes have been recovered successfully. These materials are found
to be something else other than complexes 1 and 2. However, the
materials recovered could not be identified fully.
We have been able to synthesize and characterize two dinuclear
copper(II) complexes with N4O-donor Schiff-base ligands. These
complexes have been characterized by elemental analysis, different
spectroscopic methods and single crystal X-ray diffraction analyses.
The structural analyses show that complex 1 is Cl-bridged dinuclear
compound whereas in complex 2, two copper atoms are connected
by azide ion. Complex 1 and 2 exhibit catalytic activity for epoxi-
dation of olefins. Epoxides were detected as the major products.
Definite identification of active species in homogeneous catal-
ysis is very complex. It has been reported that copper(II) catalyzed
epoxidation of olefin involves generation of copper-hydroperoxido
or copper-peroxido species in solution. This species has been
identified by recording UVevis spectrum of solution containing
copper(II) complex in the presence of tert-butyl hydroperoxide. The
UVevis spectrum shows peak at 400 nm with a shoulder in the
range of 415e440 nm. This metal-peroxido intermediate can be the
active species for conversion of olefins. Mass spectra of complexes 1
and 2 were recorded in order to get an idea about the number of
copper atom(s) present in the complex in solution state. Spectra of
complexes 1 and 2 are given in Supplementary information (Fig. S3
and S4). Spectrum of complex 1 shows m/z peaks at 584.6467,
Acknowledgments
PR gratefully acknowledges financial support from the Depart-
ment of Science and Technology, New Delhi. SH and SD wishes to
thank CSIR, New Delhi and UGC, New Delhi respectively, for
providing them fellowships. MN wishes to thank the Department of
Science and Technology, New Delhi and Visva-Bharati, Santiniketan
for financial supports. We thank Dr. Pabitra Kumar Paul, Depart-
ment of Physics, Jadavpur University for his support received dur-
ing thermogravimetric analysis.
Appendix A. Supplementary information
CCDC 1058831 and 1058832 contain the supplementary crys-
tallographic data of complexes 1 and 2 respectively for this paper.
These information can be obtained free of charge from the Cam-
bridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif. Supplementary information associated with this article
can be found, in the online version, at http://dx.doi.org/10.1016/j.
molstruc.2015.08.017.
Appendix B. Supplementary information
Supplementary information related to this article can be found
at http://dx.doi.org/10.1016/j.molstruc.2015.08.017.
References
[1] D. Venegas-Yazigi, D. Aravenab, E. Spodine, E. Ruizd, S. Alvarez, Coord. Chem.
Rev. 254 (2010) 2086.
[2] C. Adhikary, S. Koner, Coord. Chem. Rev. 254 (2010) 2933.
[3] K.C. Gupta, A.K. Sutar, C.-C. Lin, Coord. Chem. Rev. 253 (2009) 1926.
[4] K.C. Gupta, A.K. Sutar, Coord. Chem. Rev. 252 (2008) 1420.
[5] R. Vafazadeh, R. Esteghamat-Panah, A.C. Willis, A.F. Hill, Polyhedron 48 (2012)
51.
[6] P. Bhowmik, A. Bhattacharyya, K. Harms, S. Sproules, S. Chattopadhyay,
Polyhedron 85 (2015) 221.
Fig. 3. A plot of conversion vs. time for complex 1. Representation of substrate e black
square box: cyclohexene; red circle: styrene, magenta triangle:a-methyl styrene, blue
[7] P.K. Bhaumik, K. Harms, S. Chattopadhyay, Polyhedron 62 (2013) 179.
ꢀ
[8] P. Chakraborty, J. Adhikary, B. Ghosh, R. Sanyal, S.K. Chattopadhyay, A. Bauza,
triangle:trans-stilbene and Cyan diamond:norbornene.(For interpretation of the ref-
erences to color in this figure legend, the reader is referred to the web version of this
article.)
A. Frontera, E. Zangrando, D. Das, Inorg. Chem. 53 (2014) 8257.
[9] K. Dhara, P. Roy, J. Ratha, M. Manassero, P. Banerjee, Polyhedron 26 (2007)
4509.

S. Halder et al. / Journal of Molecular Structure 1101 (2015) 1e7
7
[10] W. Yu, J. Qiang, J. Yin, S. Kambam, F. Wang, Y. Wang, X. Chen, Org. Lett. 16
(2014) 2220.
[24] D.D. Perrin, W.L.F. Armarego, D.R. Perrin, Purification of Laboratory Chemicals,
Pergamon Press, Oxford, UK, 1980.
[11] P.A. Vigato, V. Peruzzo, S. Tamburini, Coord. Chem. Rev. 256 (2012) 953.
[12] K.A. Jorgensen, Chem. Rev. 89 (1989) 431.
[25] G.M. Sheldrick, SHELXL97: Programs for Crystal Structure Analysis, University
€
€
of Gottingen, Gottingen, Germany, 1997.
[13] T. Mukaiyama, T. Yamada, Bull. Chem. Soc. Jpn. 68 (1995) 17.
[14] C. Adhikary, R. Bera, B. Dutta, S. Jana, G. Bocelli, A. Cantoni, S. Chaudhuri,
S. Koner, Polyhedron 27 (2008) 1556.
[15] R. Bera, C. Adhikary, S. Ianelli, S. Chaudhuri, S. Koner, Polyhedron 29 (2010)
2166.
[26] M.S. Refat, M.Y. El-Sayed, A. Majid, A. Adam, J. Mol. Struct. 1038 (2013) 62.
[27] P. Roy, J. Coord. Chem. 62 (2009) 2003e2011.
[28] N. Novoa, F. Justaud, P. Hamon, T. Roisnel, O. Cador, B.L. Guennic, C. Manzur,
D. Carrillo, J.-R. Hamon, Polyhedron 86 (2015) 81e88.
[29] G. Dutta, R.K. Debnath, A. Kalita, P. Kumar, M. Sarma, R.B. Shankar, B. Mondal,
Polyhedron 30 (2011) 293e298.
[16] D. Saha, T. Maity, T. Dey, S. Koner, Polyhedron 35 (2012) 55.
ꢀ
[17] P. Roy, M. Nandi, M. Manassero, Mauro Ricco, M. Mazzani, A. Bhaumik,
[30] A.W. Addison, T.N. Rao, J. Reedijk, J. Vanrijn, G.C. Verschoor, J. Chem. Soc.
Dalton Trans. (1984) 1349.
P. Banerjee, Dalton Trans. (2009) 9543.
[18] P. Roy, K. Dhara, M. Manassero, P. Banerjee, Inorg. Chem. Commun. 11 (2008)
265.
[31] A.B.P. Lever, in: Inorganic Electronic Spectroscopy, second ed., Elsevier,
Amsterdam, 1984, p. 355.
[19] S. Halder, S. Dey, C. Rizzoli, P. Roy, Polyhedron 78 (2014) 85.
[20] M. Nandi, P. Roy, Indian J. Chem. 52A (2013) 1263.
[21] M. Nandi, P. Roy, H. Uyama, A. Bhaumik, Dalton Trans. 40 (2011) 12510.
[22] P. Roy, M. Manassero, Dalton Trans. 39 (2010) 1539.
[23] R.R. Gagne, C.L. Spiro, T.J. Smith, C.A. Hamann, W.R. Thies, A.K. Schiemke,
J. Am. Chem. Soc. 103 (1981) 4073.
[32] S.K. Maiti, S. Dinda, N. Gharah, R. Bhattacharyya, New J. Chem. 30 (2006) 479.
[33] K. Sato, M. Aoki, M. Ogowa, T. Hasimoto, R. Noyori, J. Org. Chem. 61 (1996)
8310.
[34] R.I. Kureshy, N.-ul H. Khan, S.H.R. Abdi, I. Ahmad, S. Singh, R.V. Jasra, J. Catal.
221 (2004) 234.
[35] W. Nam, S.W. Jin, M.H. Lim, J.Y. Ryu, C. Kim, Inorg. Chem. 41 (2002) 3647.