Synthesis, crystal structure, spectral properties and catalytic activity of a binuclear copper(II) complex containing a Schiff base ligand

Article abstract of DOI:10.1016/j.poly.2013.04.034

The newly designed multidentate ligand HL, derived from 2-(benzylthio)aniline and 2-hydroxy-5-methylbenzene-1,3-dialdehyde, upon reaction with Cu(II) perchlorate in methanol yielded the binuclear Cu(II) complex [Cu₂(L)(μ-OH)](ClO₄)₂ (1), which was authenticated by single crystal X-ray diffraction. The diffraction analysis revealed that the ligand binds each of the two Cu(II) centers in an (O, N, S) fashion in a distorted square pyramidal geometry where the two copper centers are bridged by μ-phenoxo and μ-hydroxo oxygen atoms. The apical position of one copper center is coordinated by one perchlorate ion and the other center by a water molecule. The packing of the molecule is stabilized through OH ₂a??O hydrogen bonds, mediated through solvent water and perchlorate anions. The emission and redox properties of both the ligand and the complex were examined. The electronic spectra and redox properties of the complex have been explained with DFT computations. The complex [Cu ₂(L)(μ-OH)](ClO₄)₂ shows very good catalytic activities towards the oxidation of benzyl alcohol to benzaldehyde and organic thioethers to the corresponding sulfoxide and sulfones using H₂O ₂ as the oxidant.

Full text of DOI:10.1016/j.poly.2013.04.034

Polyhedron 59 (2013) 23–28
Contents lists available at SciVerse ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, crystal structure, spectral properties and catalytic activity
of a binuclear copper(II) complex containing a Schiff base ligand
b,
c
d
d
d
Poulami Pattanayak a, , Jahar Lal Pratihar , Debprasad Patra , Paula Brandão , Dasarath Mal , Vitor Felix
⇑
⇑
a Department of Chemistry, University of Kalyani, Kalyani 741235, India
b Department of Chemistry, Kandi Raj College, Murshidabad 742137, India
c Department of Chemistry, East Calcutta Girls’ College, Kolkata 700089, India
d Department of Chemistry/CICECO, University of Aveiro, 3810-193 Aveiro, Portugal
a r t i c l e i n f o
a b s t r a c t
Article history:
The newly designed multidentate ligand HL, derived from 2-(benzylthio)aniline and 2-hydroxy-5-meth-
ylbenzene-1,3-dialdehyde, upon reaction with Cu(II) perchlorate in methanol yielded the binuclear Cu(II)
Received 21 February 2013
Accepted 9 April 2013
Available online 25 April 2013
complex [Cu2(L)(
fraction analysis revealed that the ligand binds each of the two Cu(II) centers in an (O, N, S) fashion in a
distorted square pyramidal geometry where the two copper centers are bridged by -phenoxo and l-
l-OH)](ClO4)2 (1), which was authenticated by single crystal X-ray diffraction. The dif-
l
Keywords:
Copper(II)
Crystal structure
Redox
Emission
hydroxo oxygen atoms. The apical position of one copper center is coordinated by one perchlorate ion
and the other center by a water molecule. The packing of the molecule is stabilized through OH2ꢀ ꢀ ꢀO
hydrogen bonds, mediated through solvent water and perchlorate anions. The emission and redox prop-
erties of both the ligand and the complex were examined. The electronic spectra and redox properties of
the complex have been explained with DFT computations. The complex [Cu2(L)(l-OH)](ClO4)2 shows
Oxidation reactions
very good catalytic activities towards the oxidation of benzyl alcohol to benzaldehyde and organic thio-
ethers to the corresponding sulfoxide and sulfones using H2O2 as the oxidant.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
hols and thioethers to their corresponding oxidised compounds,
which are important in synthetic organic chemistry. Moreover,
Copper(II) complexes have been studied widely for their inter-
esting coordination geometries, magnetic, EPR and electrochemical
properties, oxidative catalysis and also as potential models for
binuclear copper metalloproteins [1–16]. Copper complex medi-
ated activation of dioxygen, i.e. the binding of O2 to copper in
chemical systems that involve redox activity or oxidative transfor-
mations, is of considerable interest [1–12]. A lot of important bio-
logical functions of binuclear copper species include dioxygen
transport, mild and selective oxygenation and dehydrogenation
of substrates, combined with the transformation of O2 to either
H2O2 or water [12–16]. Karlin and coworkers extensively investi-
gated a series of dinuclear copper complexes incorporating super-
oxo, peroxo, hydroperoxo or hydroxo species and their structural,
spectroelectrochemical, kinetic/thermodynamic study and DNA
interactions [5–8,16]. These types of oxo-species are generated
through dioxygen activation [5–8,16]. The present work originates
from our interest in designing a new multidentate ligand and its
binuclear copper(II) complex that stabilize oxo-species and to
study the reactivity of such species toward the oxidation of alco-
the conventional methods for the synthesis of aldehydes from alco-
hol generally involve the use of stoichiometric amounts of inor-
ganic oxidants, which generate large quantities of waste [17,18].
Therefore, the development of effective and environmentally be-
nign catalyzed oxidation of alcohols is an important challenge.
Cu(II) complexes incorporating various organic ligands have been
used as catalysts for alcohol oxidation using air, tertiary butyl
hydrogen peroxide (TBHP), sodium perchlorite or hydrogen perox-
ide as the oxidant [18–22]. The use of H2O2 as the oxidant has
many advantages over the others because dilute aqueous H2O2
solution is safe, non-toxic and low cost, and water is the only ex-
pected side product to be dealt with after the reaction.
Herein, we describe the synthesis of the newly designed multid-
entate ligand HL and its binuclear copper complex [Cu2(L)(l-
OH)](ClO4)2 (1). The ligand and complex have been characterized
by spectroscopic data. The crystal structure of complex 1 has been
determined, confirming the molecular structure. The fluorescence
and redox properties of both the ligand and the complex have been
studied. Plausible descriptions of redox orbitals and electronic
spectra have been ascribed on the basis of DFT calculations. The
catalytic oxidation of benzyl alcohol to benzaldehyde and organic
thioethers to sulfoxides and sulfones has been investigated using
H2O2.
⇑
Corresponding authors. Fax: +91 33 25828282.
E-mail addresses: pparka@rediffmail.com (P. Pattanayak), jlpr@rediffmail.com
(J.L. Pratihar).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.04.034

24
P. Pattanayak et al. / Polyhedron 59 (2013) 23–28
After slow evaporation of the solvent at 25 °C, a dark coloured crys-
2. Experimental
2.1. Materials
tal was obtained, washed with cold methanol, and dried in a vac-
uum. Dark green crystals suitable for X-ray analysis were
obtained after two weeks by slow evaporation of a methanol solu-
tion. Yield: 0.137 g (85%). Anal. Calc. for Cu2C35H30N2O2S2ꢀ(ClO4)2:
C, 46.62; H, 3.33; N, 3.10. Found: C, 46.27; H, 3.14; N, 3.25%. UV–
The solvents used in the reactions were of reagent grade
(E. Marck, Kolkata, India) and were purified and dried by general
procedures [23]. Copper perchlorate hexahydrate, benzyl chloride,
salicylaldehyde, benzyl alcohol were purchased from Emark, India.
2-Aminothiophenol was purchased from Aldrich. 2-(Benzyl-
thio)aniline and 2-hydroxy-5-methylbenzene-1,3-dialdehyde were
prepared following reported procedures [24,25].
Vis (kmax/nm (
(18148); 230 (26610). IR (KBr, cmꢁ1): 1620 and 1598
1343 (O–C), 1080 (Cl–O), 762 (C–S).
e
/dm2 molꢁ1), dichloromethane): 415 (8880); 321
(C@N),
m
m
m
m
2.4. General procedure for the catalytic oxygenation of thioethers
Two catalytic reactions were performed by a similar procedure.
2.2. Physical measurements
To a solution of the catalyst [Cu2(L)(l-OH)](ClO4)2 (0.0125 mmol)
in methanol–dichloromethane (1:9) mixed solvent, 2.8 mmol of
sulfide and 1 mL of 50% H2O2 were added at 0 °C. The reaction mix-
ture was stirred for 2 h. The solution was then dried in vacuum.
The products were separated and purified by preparative TLC using
benzene–acetonitrile (95:5) mixed solvent. The products were
characterized by IR and 1H NMR spectra.
Microanalysis (C, H, N) was performed using a Perkin–Elmer
2400 C, H, N, S/O series II elemental analyzer. Infrared spectra were
recorded on a Perkin-Elmer L120-00A FT-IR spectrometer with the
samples prepared as KBr pellets. Electronic spectra were recorded
on a Shimadzu UV-1800 PC spectrophotometer. 1H NMR spectra
were obtained on Brucker 400 NMR spectrometers in CDCl3 using
TMS as the internal standard. Emission spectra were recorded with
a Perkin Elmer LS-55 Luminescence spectrometer. Electrochemical
measurements were made under dinitrogen atmosphere using a
CH instruments model 600D potentiostat. A platinum disc working
electrode, a platinum wire auxiliary electrode and an aqueous sat-
urated calomel reference electrode (SCE) were used in a three-elec-
trode configuration. All electrochemical data were collected at
298 K and are uncorrected for junction potentials.
2.5. Procedure for the catalytic oxidation of benzyl alcohol
To a solution of benzyl alcohol (5 mmol), the catalyst [Cu2(L)(l-
OH)](ClO4)2 (0.0125 mmol) 4 mol% and 30% H2O2 5 mL were added
and the mixture was vigorously stirred at 70 °C for 1 h. After, the
mixture was poured into water and the product was extracted with
dichloromethane and dried over Na2SO4. The solvent was removed
and the product was purified by column chromatography. The
product was characterized by IR and 1H NMR spectra.
2.3. DFT calculations
Using the X-ray coordinates of the [Cu2(L)(l-OH)] complex,
2.6. Crystallography
ground state electronic structure calculations have been carried
out using DFT [26] methods with the Orca 2.7 program [26].
Becke’s hybrid function [27] with the Lee–Yang–Parr (LYP) correla-
tion function [28] was used throughout the study. B3LYP valence
and effective core potential functions were used. All energy calcu-
lations were performed using the self-consistent field ‘‘tight’’ op-
tion of the Orca 2.7 program to ensure sufficiently well
converged values for the state energies [29,30].
Single crystals of C35H30N2O2S2Cu2.(ClO4)2 were grown by slow
evaporation of a methanol solution of complex 1 at 298 K. Data
were collected by the
fractometer with graphite monochromatic Mo K
x
-scan technique on a Bruker Smart CCD dif-
radiation. The
a
structure solution was determined by the direct method with the
SHELXS-97 program [31–32]. Full matrix least square refinements
on F2 were performed using the SHELXL-97 program [31,32]. All
non-hydrogen atoms were refined anisotropically using reflections
2.3.1. Synthesis of ligand
Ethanol solution (30 mL) containing 4-methyl-2,6-diformyl
phenol (0.5 g, 3.04 mmol) was slowly added to an ethanol solution
(20 mL) of 2-(benzylthio)aniline (1.31 g, 6.09 mmol). After com-
plete addition, the reaction mixture was stirred at 25 °C for 6 h
and kept at room temperature overnight. The product that formed
was separated as yellowish-orange crystals, which were collected
by filtration and washed with ethanol and diethyl ether. The fil-
trate on standing for an additional 24 h provides a second crop
of product, and the combined yield was 1.61 g (95%) or more. Anal.
Calc. for C35H30N2OS2: C, 75.17; H, 5.36; N, 5.01. Found: C, 75.23; H,
Table 1
Crystallographic data for 1.
Chemical formula
Formula weight
Crystal system
space group
a (Å)
C35H32ClCu2N2O7S2, ClO4, H2O
936.78
monoclinic
P21/c
16.7800(6)
16.8622(6)
13.6837(4)
90
103.2070(10)
90
0.71073
3769.4(2)
1912
b (Å)
c (Å)
a
(°)
b (°)
(°)
5.20; N, 5.18%. UV–Vis (kmax/nm (
385 (12144); 260 (33211). IR (KBr, cmꢁ1): 3343 (
(C@N), 1254
e
/dm2 molꢁ1), dichloromethane):
c
k (Å)
m
OꢁH), 2914
V (Å3)
F(000)
Z
m
(CꢁH), 1621 and 1592
m
m
(OꢁC), 744
m
(CꢁS). 1H
NMR (400 MHz, CDCl3): 2.35 (CH3, s, 3H), 4.14 (CH2, s, 4H), 7.10-
4
7.36 (ArH, m, 20H), 8.53 (CH@N, s, 2H), 13.65 (Ar–OH, s, 1H).
T (K)
150
1.651
1.447
1.3, 27.9
28668
8919 (0.030)
6504
0.0354
0.0940
1.00
D (Mg/mꢁ3
)
l
(mmꢁ1
)
2.3.2. Synthesis of the complex
hmin.-max. (°)
Reflections measured
Unique reflections (Rint
No. of reflections used [I > 2
R1 [I > 2 (I)]
wR2 (all data)
Goodness-of-fit
A methanolic solution of copper(II) perchlorate hexahydrate
(0.132 g, 0.35 mmol) (10 mL) was added to a solution of the ligand
HL (0.1 g, 0.17 mmol) in methanol (10 mL) (2:1, v/v). The colour of
the solution immediately changed to olive green, but stirring was
continued for 5 h at room temperature. The resulting green solu-
tion was then filtered and allowed to stand at room temperature.
)
r(I)]
r

P. Pattanayak et al. / Polyhedron 59 (2013) 23–28
25
with I > 2r(I). The C-bound hydrogen atoms were included in cal-
culated positions and refined as riding atoms. Data collection
parameters and relevant crystal data are collected in Table 1.
3. Results and discussion
3.1. Synthesis
The ligand HL used in this work was synthesized by the conden-
sation of 2-(benzylthio)aniline and 2-hydroxy-5-methylbenzene-
1,3-dialdehyde (2:1) in ethanol (Scheme 1). The preformed ligand
HL upon reaction with copper(II) perchlorate hexahydrate (1:2)
in methanol afforded a binuclear
l-hydroxo Cu(II) complex of
the composition [Cu2(L)( -OH)](ClO4)2 (1), where the ligand offers
l
an (O, N, S) coordination mode to each of the two Cu(II) centers.
Fig. 1. UV–Vis spectra of the ligand HL (---) and complex 1 (—) in dichloromethane.
3.2. Spectral characterization and solution structures
The free Schiff base ligand HL has been characterized by its 1H
NMR spectrum. In the 1H NMR spectrum of the ligand HL the phe-
nolic –OH appeared as a sharp singlet at 13.65 ppm, and this disap-
peared upon D2O exchange. A broad band at 8.53 ppm for two
equivalent protons was observed for the azomethine (–HC@N–)
protons. A sharp singlet at 4.14 ppm is assigned to the benzylic
protons (–CH2–) of the ligand. All the aromatic protons appeared
in the range 7.10-7.36 ppm, which could not be assigned accu-
rately, but the proton counts matched well with the composition
of the ligand in solution. A sharp singlet at 2.35 ppm was assigned
to the CH3 signal.
The ligand HL and complex 1 are soluble in common organic
solvents, and are furnished with yellow and green colours respec-
tively. The UV–Vis spectra of the ligand and complex were re-
corded in dichloromethane solutions (Fig. 1). The high energy
transitions at 385 nm for the ligand are attributed to intraligand
p
p⁄ transitions [33,34]. A characteristic low energy absorption
ꢁ
band for [Cu2(L)(OH)](ClO4)2 at 415 nm is assigned as a mixed me-
tal ligand charge transfer (see DFT results).
The characteristic stretching frequencies of the azomethine
m
(HC@N) groups in the free ligand (1621 and 1592 cmꢁ1) are
shifted to 1620 and 1598 cmꢁ1 in the complex due to coordination
to the metal centre [35]. The (O–H) band in the ligand is observed
m
3.3. X-ray structure determination of 1
as a broad band at 3343 cmꢁ1 due to O–Hꢀ ꢀ ꢀN hydrogen bonding
[35]. The phenolic C–O band in the free ligand (1254 cmꢁ1) is
shifted to a higher frequency in the complex (1343 cmꢁ1), indicat-
ing coordination through the phenolic oxygen atom [34]. A very
strong and split broad band near 1080 cmꢁ1 and a split strong band
near 622 cmꢁ1 are observed in the IR spectrum of the dicopper(II)
complex, which could be due to the antisymmetric stretching and
bending of perchlorate ions, respectively [34]. The splitting pattern
of the bands indicates the presence of two non-equivalent perchlo-
rate ions, one of which is coordinated very weakly to the Cu(II)
The structure of 1 is shown in Fig. 2 along with the atom labels.
The hydrogen atoms, uncoordinated perchlorate ion and water
molecule are omitted for clarity. Selected bond distances and an-
gles are collected in Table 2. The coordination geometry around
each of the two copper centers can be best described as distorted
square pyramidal, where the apical position of one copper center
is coordinated very weakly by a perchlorate ion and a water mol-
ecule for the other center. The two copper centers are bridged by
l-phenoxide and l-hydroxo oxygen atoms, and the CuNSO2 plane
center. The m
(C–S) band near 744 cmꢁ1 in the ligand appeared at
is completed by the imine nitrogen and thioether sulfur. The ligand
Lꢁ adopts roughly a planar structure, excepting the two benzyl
fragments. The Cu(1)–N(1), Cu(1)–S(1), Cu(1)–O(1) and Cu(1)–
O(2) bond lengths, 1.941(2), 2.3188(8), 1.9714(17) and
1.9072(19) Å respectively, are within the normal range [5–
8,24,36]. The axial Cu(2)–O(4) (perchlorate) and Cu(1)–OW(1)
762 cmꢁ1 in the complex, indicating the coordination of the thio-
benzyl group [36].
HC
N
CH
N
OH
NH2
S
EtOH
Stir
+
S
S
CHO
OHC
OH
Cu(ClO4)2. 6H2O
HL
MeOH
Stir
HC
N
CH
N
O
Cu
Cu
(ClO4)2
O
S
S
H
[Cu2 (L)(OH)](ClO4)2
Scheme 1. Synthesis of the ligand HL and complex 1.
1
Fig. 2. Molecular structure of [Cu2(L)(
ing scheme. Hydrogen atoms except O2 and OW1 are omitted for clarity.
l-OH)](ClO4)(2H2O) with the atom number-

26
P. Pattanayak et al. / Polyhedron 59 (2013) 23–28
Table 2
Selected bond distances (Å) and angles (deg) for compound 1.
Distances
Cu(1)–S1(1) 2.3188(8)
Cu(1)–O(1) 1.9714(17)
Cu(1)–O(2) 1.9072(19)
Cu(1)–N1(1) 1.941(2)
Cu(2)–S(2) 2.2862(7)
Cu(2)–O(1) 1.9414(18)
Cu(2)–O(2) 1.8954(17)
Cu(2)–N(2) 1.943(2)
N(2)–C(23) 1.429(3)
Cu(1)–OW(1) 2.216(3)
S(1)–C(16) 1.842(3)
S(1)–C(15) 1.776(3)
S(2)–C(29) 1.853(3)
S(2)–C(28) 1.775(3)
O(1)–C(1) 1.317(3)
N(1)–C(8) 1.294(3)
N(1)–C(10) 1.428(3)
N(2)–C(9) 1.292(3)
Cu(1)–Cu(2) 3.002
Cu(2)–O(4) 2.502(2)
Angles
S(1)–Cu(1)–O(2) 100.07(6)
S(1)–Cu(1)–N(1) 87.93(7)
O(1)–Cu(1)–O(2) 77.25(8)
O(1)–Cu(1)–N(1) 91.95(8)
O(1)–Cu(1)–N(1) 168.63(9)
S(2)–Cu(2)–O(1) 165.72(6)
Cu(2)–O(1)–C(2) 129.56(16)
S(2)–Cu(2)–O(2) 99.74(6)
Cu(1)–O(1)–Cu(2) 100.20(8)
Cu(1)–O(1)–C(1) 130.03(16)
S(2)–Cu(2)–N(2) 88.72(6)
Cu(1)–O(2)–Cu(2) 104.26(9)
O(1)–Cu(2)–O(2) 78.26(8)
O(1)–Cu(2)–N(2) 93.15(8)
O(2)–Cu(2)–N(2) 171.40(9)
Cu(1)–N(1)–Cu(8) 124.85(19)
Cu(1)–N(1)–Cu(10) 115.91(17)
Cu(2)–N(2)–C(9) 123.90(18)
Fig. 4. Emission spectra of ligand HL (
complex 1 ( ) in dichloromethane.
), upon addition of Cu2+
(
) and
Fig. 5. CV of the ligand HL (---) and the complex [Cu2(L)(
dichloromethane acetonitrile mixed solvent.
l-OH)](ClO4)2 (ꢁ) in
Fig. 3. Dimeric packing structure of the complex [Cu2(L)(l-OH)](ClO4)2, 2H2O
viewed down the c-axis. Dashed lines represent hydrogen bonds.
1.30 V, respectively Eq. (1). The identical nature of the cyclic volta-
mograms indicates that the oxidation may be ligand centered,
which is in good agreement with the DFT results.
(water) bonds (2.502 and 2.216 Å, respectively) are considerably
elongated due to the Jahn-Teller effect of copper(II) [24,37,38].
The distance between the Cu(1)–Cu(2) centers is found to be
3.002 Å [5–8]. The packing (Fig. 3) of the molecule is stabilized
through three-dimensional OH2ꢀ ꢀ ꢀO hydrogen bonds, mediated
through solvent water and perchlorate anions.
3þ
½CuIIðLÞꢂ2þ ꢀ ½CuIIðLÞꢂ
ð1Þ
According to the DFT results, the composition of HOMOꢁ1,
HOMO, LUMO and LUMO+1 (Fig. 6(a–d)) of [Cu2(L)( -OH)] are
l
mainly ligand centered, indicating the oxidation to be ligand cen-
tric. The HOMO ? LUMO transition is mixed metal ligand to metal
ligand, consistent with the assignment of the UV–Vis spectrum.
3.4. Fluorescence
The fluorescence properties of the ligand HL and the [Cu2(L)(l-
3.6. Oxidation of thioethers
OH)](ClO4)2 complex were investigated in acetonitrile chloroform
mixed solvent (Fig. 4). The ligand HL exhibited two low intensity
emissions bands at 440 and 575 nm upon excitation at 385 nm.
Upon addition of H+ or Lewis acid (Cu2+) to the ligand solution,
the emission intensity sharply increased, whereas a very low
Catalytic oxygenation of organic thioethers to sulfoxides and
sulfones is an important area in current chemical research to
search for suitable catalysts [41–44]. Use of H2O2 as an oxidant
has been studied extensively for this purpose [44,45]. The complex
intensity emission was observed for the isolated [Cu2(L)(
OH)](ClO4)2 complex. The quenching of emission intensity of [Cu2
(L)( -OH)](ClO4)2 may be due to the ‘‘heavy atom effect’’ [39,40].
l-
[Cu2(L)(l-OH)](ClO4)2 showed a pronounced catalytic activity to-
ward H2O2 induced oxidation of phenyl methyl sulfide and phenyl
benzyl sulfide under atmospheric conditions Eq. (2). The conver-
sion was calculated on the basis of the isolated yields (Table 3).
The oxidation of the organic thioether did not occur in the absence
of catalyst.
l
3.5. Electrochemistry
The electrochemical behavior of the ligand HL and the [Cu2(-
O
O
O
L)(l-OH)](ClO4)2 complex were investigated in dichloromethane
H2O2
+
S
S
..
(2)
S
acetonitrile mixed solvent (0.1 M TBAP) by cyclic voltammetry
R1
R2
Cat. 1
with a scan rate of 50 mV sꢁ1 (Fig. 5) versus SCE. The cyclic voltam-
R1
R2
R1
R2
mograms of both HL and [Cu2(L)(
identical one electron irreversible oxidative responses at 1.45 and
l-OH)](ClO4)2 complex displayed
ð2Þ

P. Pattanayak et al. / Polyhedron 59 (2013) 23–28
27
Fig. 6. Surface plot of [Cu2(L)(l-OH)].
4. Conclusion
Table 3
Oxidation of sulfides catalyzed by [Cu2(L)(
anol (9:1) mixed solvent.
l-OH)](ClO4)2 in dichloromethane meth-
In conclusion, the newly synthesized multidentate ligand offers
the (O, N, S) coordination mode to each of the two copper centers
of a binuclear complex, where the two copper atoms are bridged
Entry
Equiv
Time
Yield (%)
H2O2 (50%) (h)
Sulfoxide Sulfone
through l-phenoxo and l-hydroxo oxygen atoms. The redox prop-
5
5
2
25
50
S
erties and emission behavior of both the ligand and complex were
examined. The activity of the binuclear complex as a catalyst to-
wards the oxidation of benzyl alcohol to benzaldehyde and organic
thioethers to sulfoxides and sulfones was tested in the presence of
H2O2 as an oxidant under atmospheric conditions.
2
30
48
S
Acknowledgments
Special thanks are given to Professor Surajit Chattopadhyay,
Department of Chemistry, University of Kalyani, for his valuable
suggestions throughout this work. The author PP acknowledges a
DST, New Delhi under WOS-A Scheme (Sanction No. SR/WOS-A/
CS-140/2011) and JP acknowledges a University Grants Commis-
sion (under Minor Research Project, Sanction No. F.PSW/067/
2011-12).
3.7. Oxidation of benzyl alcohol
The oxidation of alcohols to the corresponding carbonyl com-
pounds is one of the most fundamental and important reactions
in synthetic organic chemistry [18–22]. The complex [Cu2(L)(l-
OH)](ClO4)2 has been tested for the liquid biphasic peroxidative
oxidation of benzyl alcohol with H2O2 (30%) as the oxidant Eq.
(3). The oxidation reaction has been carried out under atmospheric
pressure and in the absence of any additive other than the sub-
strate, benzyl alcohol, H2O2 and the catalyst. The yield has been
optimized by varying the relative proportions of hydrogen perox-
ide with respect to the catalysts and also by varying the reaction
time and temperature. The best yield of benzaldehyde (90%) was
obtained during the oxidation of benzyl alcohol (5 mmol), with
catalyst 4 mol% and 5 mL 30% H2O2 at 70 °C for 1 h. The conversion
was calculated on the basis of the isolated yield.
Appendix A. Supplementary data
CCDC 906100 contains the supplementary crystallographic data
for 1. These data can be obtained free of charge via http://
www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
UK; fax: +44 1223 336 033; or e-mail: deposit@ccdc.cam.ac.uk.
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2013.04.034.
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