Cobalt(II), copper(II), zinc(II) and palladium(II) Schiff base complexes: Synthesis, characterization and catalytic performance in selective oxidation of sulfides using hydrogen peroxide under solvent-free conditions

Article abstract of DOI:10.1016/j.poly.2015.03.041

An asymmetric bidentate Schiff-base ligand (HL: 2-tert-butyliminomethyl-phenol) was prepared from the reaction of salicylaldehyde and tert-butylamine. Cobalt(II), copper(II), zinc(II) and Pd(II) complexes, CoL₂, CuL₂, ZnL₂ and PdL₂, were synthesized from the reaction of CoCl₂·6H₂O, CuCl₂·2H₂O, Zn(NO₃)₂·6H₂O and PdCl₂ with the bidentate Schiff base ligand HL in methanol. The ligand and its metal complexes were characterized by elemental analysis (CHN), FT-IR and UV-Vis spectroscopy. In addition, ¹H and ¹³C NMR techniques were employed for characterization of the ligand (HL) and the diamagnetic complexes (ZnL₂ and PdL₂). The molecular structures of all the complexes were determined by the single crystal X-ray diffraction technique. The crystallographic data reveal that in all the complexes the metal centers are four-coordinated by two phenolate oxygen and two imine nitrogen atoms of two Schiff base ligands. The geometry around the metal center in the CoL₂, CuL₂ and ZnL₂ complexes is a distorted tetrahedral and for PdL₂ it is square-planar. The catalytic activity of these complexes has been evaluated for the selective oxidation of sulfides with the green oxidant 35% aqueous H₂O₂ under solvent free conditions. For all the catalysts, using optimized reaction conditions, different sulfides were converted to the corresponding sulfones. ZnL₂ showed a higher catalytic performance for the oxidation of the different sulfides to the corresponding sulfones.

Full text of DOI:10.1016/j.poly.2015.03.041

Polyhedron 95 (2015) 1–13
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Cobalt(II), copper(II), zinc(II) and palladium(II) Schiff base complexes:
Synthesis, characterization and catalytic performance in selective
oxidation of sulfides using hydrogen peroxide under solvent-free
conditions
a
a
b
Mahsa Khorshidifard a, Hadi Amiri Rudbari a, , Banafshe Askari , Mehdi Sahihi , Mostafa Riahi Farsani ,
⇑
Fariba Jalilian a, Giuseppe Bruno c
a Department of Chemistry, University of Isfahan, Isfahan 81746-73441, Iran
b Young Researchers and Elite Club, Islamic Azad University, Shahrekord Branch, Shahrekord, Iran
c Department of Chemical Sciences, University of Messina, Via F. Stagno d’Alcontres 31, 98166 Messina, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
An asymmetric bidentate Schiff-base ligand (HL: 2-tert-butyliminomethyl-phenol) was prepared from
the reaction of salicylaldehyde and tert-butylamine. Cobalt(II), copper(II), zinc(II) and Pd(II) complexes,
CoL2, CuL2, ZnL2 and PdL2, were synthesized from the reaction of CoCl2Á6H2O, CuCl2Á2H2O,
Zn(NO3)2Á6H2O and PdCl2 with the bidentate Schiff base ligand HL in methanol. The ligand and its metal
complexes were characterized by elemental analysis (CHN), FT-IR and UV-Vis spectroscopy. In addition,
1H and 13C NMR techniques were employed for characterization of the ligand (HL) and the diamagnetic
complexes (ZnL2 and PdL2). The molecular structures of all the complexes were determined by the single
crystal X-ray diffraction technique. The crystallographic data reveal that in all the complexes the metal
centers are four-coordinated by two phenolate oxygen and two imine nitrogen atoms of two Schiff base
ligands. The geometry around the metal center in the CoL2, CuL2 and ZnL2 complexes is a distorted tetra-
hedral and for PdL2 it is square-planar.
Received 25 January 2015
Accepted 28 March 2015
Available online 14 April 2015
Keywords:
Schiff base
Crystal structure
Oxidation of sulfide
H2O2
Solvent free
The catalytic activity of these complexes has been evaluated for the selective oxidation of sulfides with
the green oxidant 35% aqueous H2O2 under solvent free conditions. For all the catalysts, using optimized
reaction conditions, different sulfides were converted to the corresponding sulfones. ZnL2 showed a
higher catalytic performance for the oxidation of the different sulfides to the corresponding sulfones.
Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction
by-product [6]. Different catalysts, such as polyoxometallates and
transition-metal Schiff-base complexes, have been used for H2O2-
The selective oxidation of organosulfur compounds has
attracted much attention from the viewpoint of organizing chemi-
cal processes for the preparation of synthetically useful sulfoxides,
and sulfones, which are important and versatile intermediates in
the synthesis of natural products and biologically significant mole-
cules [1,2], ligands in asymmetric catalysis [3] and oxo-transfer
reagents [4,5]. Among a lot of different oxidants, aqueous hydrogen
peroxide has been one of the most noteworthy ‘‘green oxidants’’,
offering the advantages of safety in storage and operation, being
cheap, readily available, of highly effective oxygen content and
environmentally benign, with the formation of water as the only
based oxidation of sulfides [7–20]. However, the oxidation of sul-
fides to sulfones has been much less investigated as compared to
the oxidation of sulfides to sulfoxides. Also, many of these systems
suffer from one or more limitations, such as high cost, the require-
ment of a promoter or a co-catalyst, high temperature, long reac-
tion time, chlorohydrocarbon solvents and excessive H2O2. These
defects are becoming more apparent in view of the developing
environmental concerns in recent years [7,10,11]. In addition, most
of the catalysts are required to use a solvent in the reaction, while
it is highly favorable to expand a process without a solvent from
the viewpoint of green chemistry.
Schiff base ligands have played an important role in the devel-
opment of coordination chemistry, especially their metal com-
plexes exhibit wide applications in biological and industrial
⇑
Corresponding author. Tel.: +98 31 37934918.
E-mail addresses: h.a.rudbari@sci.ui.ac.ir, hamiri1358@gmail.com (H.A. Rudbari).
http://dx.doi.org/10.1016/j.poly.2015.03.041
0277-5387/Ó 2015 Elsevier Ltd. All rights reserved.

2
M. Khorshidifard et al. / Polyhedron 95 (2015) 1–13
OH
crystals were dark green, red, yellow and light red, respectively.
OH
A high-quality piece of crystal was chosen in each case using a
polarizing microscope and they were mounted on a glass fiber,
then used for data collection. Cell constants and an orientation
matrix for data collection were obtained from the least-squares
refinement of diffraction data from 5922 for CuL2, 3108 for CoL2,
3501 for ZnL2 and 2450 for PdL2 unique reflections. Data were col-
CHO
NH2
MeOH or CHCl3
N
+
Ambient temperature
C(CH3)3
HL
Scheme 1. Synthesis of the ligand (HL).
lected at a temperature of 298(2) K in a series of
x scans in 1° oscil-
lations and integrated using the Stöe X-AREA software package [38].
A numerical absorption correction was applied using the X-RED [39]
and X-SHAPE [39] software packages. The data were corrected for
Lorentz and Polarizing effects. The structures were solved by direct
methods using SIR2004 [40]. The non-hydrogen atoms were
refined anisotropically by the full matrix least squares method on
F2 using SHELXL [41]. All hydrogen atoms were added at ideal posi-
tions and constrained to ride on their parent atoms.
systems [21–26]. Furthermore, Schiff bases are very important
tools for inorganic chemists, as they are widely used to design
molecular ferromagnets, in catalysis, in biological modeling appli-
cations, as liquid crystals and as heterogeneous catalysts [27–29].
Unfortunately, application of Schiff base complexes as a catalyst
for the solvent-free oxidation of sulfides with hydrogen peroxide
is very rare [30–33].
So current demand for environmentally friendly processes
requires the synthesis of new catalysts and the development of
green oxidation methods that use clean oxidants, such as hydrogen
peroxide, without any solvent. As part of our research program on
the synthesis, characterization and catalytic activity of Schiff base
ligands and complexes [34–37], in this paper, we synthesized a
bidentate Schiff base ligand, HL, derived from salicylaldehyde
and tert-butylamine (Scheme 1). In order to investigate the coordi-
nation modes of this ligand, cobalt(II), copper(II), zinc(II) and
palladium(II) complexes were synthesized (Scheme 2). After syn-
thesis and characterization of the complexes, we used them in
the no-solvent oxidation of sulfides with hydrogen peroxide as a
green oxidant. This process is clean, safe, selective, simple and
cost-effective for the oxidation of sulfides to the corresponding
sulfones.
In the crystal structures of CoL2 and ZnL2, the C9, C10 and C11
carbon atoms of the tert-butyl group were disordered over two
sites and refined with site occupancy factors 0.67:0.33 and
0.65:0.35 for CoL2 and ZnL2, respectively (details are available in
the archived CIF). Crystallographic data for the complexes are
listed in Table 1. Selected bond distances and angles are summar-
ized in Table 2.
2.3. Synthesis of the Schiff-base ligand (HL: 2-tert-butyliminomethyl-
phenol)
Tert-butylamine (10 mmol) was dissolved in 30 ml of absolute
methanol or chloroform and was added slowly to a stirring solu-
tion of salicylaldehyde (10 mmol) in 30 ml of absolute methanol
or chloroform at ambient temperature. The color immediately
changed to yellow. The mixture was then stirred for 2 h at ambient
temperature before removal of the solvent under vacuum. The
result was a yellow oil. Yield 89%, Anal. Calc. for C11H15ON: C,
74.54; H, 8.53; N, 7.90. Found: C, 74.49; H, 8.56; N, 7.88%. FT-IR
2. Experimental section
2.1. Chemicals and instrumentation
(KBr, cmÀ1): 3480 (
matic), 1628 ( C@N), 1196 (
400 MHz, 298 K): Table 3.
m
O–H, br, vs), 3060–2871 (C–H aliphatic and aro-
m
m
C–O). 1H and 13C NMR (DMSO-d6,
All the chemicals were purchased from Merck Co. and used
without further purification. The FT-IR spectra were recorded on
a JASCO, FT/IR-6300 spectrometer (4000–400 cmÀ1) in KBr pellets.
1H and 13C NMR spectra were recorded on a Bruker Avance 400
spectrometer using CDCl3 (for the palladium(II) complex) and
DMSO-d6 (for the ligand and zinc(II) complex) as solvents. The ele-
mental analysis was performed on Leco, CHNS-932 and Perkin-
Elmer 7300 DV elemental analyzers. UV–Vis spectra were recorded
on a JASCO V-670 UV–Vis spectrophotometer (200–700 nm). The
oxidation products were quantitatively analyzed by gas chro-
matography (GC) on a Shimadzu GC-16A instrument using a 2 m
column packed with silicon DC-200 and an FID detector.
2.4. Preparation of the complexes
General procedures for the preparation of CoL2, CuL2, ZnL2 and
PdL2.
A MeOH solution (20 cm3) of tert-butylamine (2 mmol) was
added dropwise to a MeOH solution (20 cm3) of the salicylalde-
hyde (2 mmol). The yellow solution was stirred for 2 h at ambient
temperature, then a solution of triethylamine (3 mmol) in absolute
MeOH (5 cm3) was added to the solution. The solution turned dark
yellow and was stirred for 10 min, then a solution of the appropri-
ate metal salt (1 mmol) in absolute MeOH (20 cm3) was added
dropwise. The resulting solution was stirred for 12 h at ambient
temperature for the preparation of the CoL2, CuL2 and ZnL2 com-
plexes, while for the palladium(II) complex, PdL2, the solution
was refluxed overnight.
2.2. Single crystal diffraction studies
X-ray data for complexes, CoL2, CuL2, ZnL2 and PdL2, were col-
lected on a STOE IPDS-II diffractometer with graphite monochro-
mated Mo K
a
radiation. For CuL2, CoL2, ZnL2 and PdL2 the
C(CH3)3
C(CH3)3
O
O
N
N
PdCl2, NEt3
Reflux
MX2, NEt3
Pd
(H3C)3C
M
25∞C
N
O
N
O
OH
N
C(CH3)3
(H3C)3C
MX2: Zn(NO3)2
CuCl2
CoCl2
Scheme 2. Synthetic routes for the preparation of the complexes.

M. Khorshidifard et al. / Polyhedron 95 (2015) 1–13
3
Table 1
Crystal data and structure refinement.
CuL2
CoL2
PdL2
ZnL2
Empirical formula
Formula weight
C22H28N2O2Cu
416.00
C22H28N2O2Co
411.39
C22H28N2O2Pd
458.86
C22H28N2O2Zn
417.83
T (K)
298(2)
298(2)
298(2)
298(2)
k (Å)
Crystal system
Space group
0.71073
orthorhombic
P2(1)2(1)2(1)
0.71073
orthorhombic
Pca2(1)
0.71073
monoclinic
C2/c
0.71073
monoclinic
P2(1)
Unit cell dimensions
a (Å)
b (Å)
c (Å)
b (°)
V (Å3)
Z
9.1377(18)
11.192(2)
21.367(4)
20.1258(14)
10.9506(8)
9.8769(9)
19.308(2)
7.7468(7)
14.6967(16)
108.753(9)
2081.6(4)
4
1.464
0.910
944
10.625(2)
9.982(2)
10.821(2)
110.81(3)
1072.8(4)
2
1.294
1.162
440
2185.3(8)
4
1.264
1.017
876
2176.8(3)
4
1.255
0.806
868
Dcalc (Mg mÀ3
)
Absorption coefficient (mmÀ1
F(000)
)
h range for data, collection
Index ranges
2.42–25.00
À12 6 h 6 12
À15 6 k 6 15
0 6 l 6 29
11776
2.75–29.38
0 6 h 6 27
À15 6 k 6 0
0 6 l 6 13
3108
2.86–28.00
À25 6 h 6 24
0 6 k 6 10
0 6 l 6 19
2450
2.87–25.00
À12 6 h 6 12
À10 6 k 6 11
À12 6 l 6 12
9535
Reflections collected
Independent reflections
Data completeness (%)
Refinement method
5922 [R(int) = 0.0277]
99.6
3108 [R(int) = 0.0473]
98.1
2450 [R(int) = 0.0217]
97.5
3501 [R(int) = 0.0456]
99.9
Full-matrix least-squares on Full-matrix least-squares on Full-matrix least-squares on Full-matrix least-squares on
F2
F2
F2
F2
Data/restraints/parameters
5922/0/244
0.945
3108/1/275
0.906
2450/0/128
0.993
3501/1/272
0.954
Goodness-of-fit (GOF) on F2
Final R indices [I > 2
R indices (all data)
r
(I)]
R1 = 0.0319
wR2 = 0.0664
R1 = 0.0522
wR2 = 0.0695
0.172 and À0.463
R1 = 0.0278
wR2 = 0.0687
R1 = 0.589
wR2 = 0.0718
0.169 and À0.316
R1 = 0.0379
wR2 = 0.1204
R1 = 0.0550
wR2 = 0.1243
0.798 and À0.800
R1 = 0.0268
wR2 = 0.0382
R1 = 0.0447
wR2 = 0.0404
0.118 and À0.156
Largest difference in peak and hole
(e ÅÀ3
)
Table 2
Selected bond distances (Å) and angles (°) for CuL2, CoL2, ZnL2 and PdL2.
CuL2
CoL2
ZnL2
PdL2
M(1)–O(1)
M(1)–N(1)
N(1)–C(7)
N(2)–C(18)
1.8957(17)
1.995(2)
1.285(3)
1.286(3)
1.883(3)
2.003(3)
1.272(4)
1.281(4)
1.911(2)
2.0200(19)
1.275(4)
1.272(3)
Pd(1)–O(1)
Pd(1)–N(1)
N(1)–C(7)
1.987(3)
2.066(3)
1.285(5)
O(1)–M(1)–O(2)
O(1)–M(1)–N(1)
O(2)–M(1)–N(1)
N(1)–M(1)–N(2)
O(2)–M(1)–N(2)
137.28(9)
95.88(8)
100.18(7)
145.58(8)
93.22(8)
113.66(12)
97.94(12)
113.45(11)
122.03(12)
96.71(11)
114.19(9)
97.58(11)
96.65(9)
126.95(13)
96.65(9)
O(1)–Pd(1)–O(1A)
O(1)–Pd(1)–N(1)
O(1)–Pd(1)–N(1A)
N(1)–Pd(1)–N(1A)
180.00(17)
88.45(12)
91.55(12)
180.0
2.4.1. Cobalt(II) complex (CoL2)
3036–2870 (w, C–H aliphatic and aromatic), 1615 (
and 1146 ( C–O).
mC@N), 1181
The red solution was slowly evaporated to dryness at room
temperature to yield a red solid which was purified by washing
with 50 ml of diethyl ether several times and the obtained green
precipitate was dried in air. Appropriate single crystals for X-ray
crystallography were obtained directly from the reaction mixture.
The typical yield was 62%. Anal. Calc. for C22H28CoN2O2: C, 64.23; H,
6.86; N, 6.81. Found: C, 64.18; H, 7.00; N: 6.89%. Selected IR data
(KBr, cmÀ1): 3056–2900 (w, C–H aliphatic and aromatic), 1601
m
2.4.3. Zinc(II) complex (ZnL2)
The yellow solution was slowly evaporated to dryness at room
temperature to yield a yellow solid which was purified by washing
with 50 ml of diethyl ether about five times and the obtained
yellow precipitate was dried in air. Appropriate single crystals for
X-ray crystallography were obtained directly from the reaction
mixture. The typical yield was 73%. Anal. Calc. for C22H28N2O2Zn:
C, 63.24; H, 6.75; N, 6.70. Found: C, 63.13; H, 6.67; N, 6.76%.
Selected IR data (KBr, cmÀ1): 3015–2855 (C–H aliphatic and aro-
(mC@N), 1174 and 1145 (mC–O).
2.4.2. Copper(II) complex (CuL2)
matic), 1613 (mC@N), 1174 and 1148 (m
C–O). 1H and 13C NMR
(DMSO-d6, 400 MHz, 298 K): Table 3.
The resultant red solution was allowed to stand overnight. After
concentration at room temperature, a red precipitate was collected
by filtration. Appropriate single crystals for X-ray crystallography
were obtained directly from the reaction mixture. The typical yield
was 87%. Anal. Calc. for C22H28CuN2O2: C, 63.52; H, 6.78; N, 6.73.
Found: C, 63.67; H, 6.81; N, 6.70%. Selected IR data (KBr, cmÀ1):
2.4.4. Palladium(II) complex (PdL2)
A green precipitate was collected by filtration and purified by
washing with 50 ml of diethyl ether several times and then dried

4
M. Khorshidifard et al. / Polyhedron 95 (2015) 1–13
Table 3
Na2SO4. Further purification was achieved by short-column chro-
matography on silica gel with EtOAc/n-hexane as the eluent.
1H and 13C NMR data for the Schiff base ligand (HL), ZnL2 and PdL2 complexes.
H9
C9
3. Results and discussion
C8
OH
C1
N
The Schiff base ligand HL, obtained by the self-condensation
reaction between tert-butylamine and salicylaldehyde in absolute
methanol as solvent at ambient temperature and reported in
Scheme 1, is a yellow oil, stable in air and moderately soluble in
the most common organic solvents. Cobalt(II), copper(II), zinc(II)
and palladium(II) Schiff-base complexes were obtained by treating
CoCl2Á6H2O, CuCl2Á2H2O, Zn(NO3)2Á6H2O and PdCl2 with two molar
equivalents of the ligand in the presence of triethylamine, respec-
tively (Scheme 2). The ligand and all the complexes were
characterized by elemental analysis (CHN) and FT-IR spectroscopy.
Also, the Schiff base ligand and its zinc(II) and palladium(II) com-
plexes were investigated by NMR techniques. The molecular struc-
tures of all the complexes were determined by the single crystal X-
ray diffraction technique.
H2
H3
C7
C2
C3
H7
C6
C5
C4
H4
H5
HL
ZnL2
PdL2
HL
ZnL2
PdL2
1H NMR
13C NMR
H7
H5
H3
H2
H4
H9
8.48(s)
7.44(d of d) 7.40(d)
7.29(d of t)
6.87(d)
6.84(t)
8.52(s)
7.32(s)
C1 161.4 169.3
7.14(d of t) C7 160.9 168.7
7.17(d of t) C3 131.9 137.1
C5 131.8 132.1
6.58(d of t) C4 117.9 122.2
165.0
159.7
134.2
133.5
119.8
125.6
115.4
63.2
7.25(t)
6.63(d)
6.56(t)
1.25(s)
6.80(d)
1.24(s)
1.72(s)
C6 118.4 118.3
C2 116.6 113.7
C8 56.8
C9 29.2
3.1. IR spectra
OH 14.3(s)
59.2
29.9
In the FT-IR spectrum of the ligand, a sharp band appeared at
1628 cmÀ1, which is attributed to the C@N vibrations of the imine
group [42–44]. In the FT-IR spectra of the Schiff base complexes
this band was shifted to a lower wavenumber and appeared at
1601, 1615, 1613 and 1607 cm-1 for the cobalt(II), copper(II),
zinc(II) and palladium(II) complexes, respectively. This shift could
be attributed to a weakening of the M–N bonds on adduct forma-
tion and this can be explained by the donation of electrons from
the nitrogen atom to the empty d-orbitals of the metal atom
[37,45–47].
The spectrum of the free ligand shows a strong band at
1196 cmÀ1, assigned to the phenolic C–O stretching. However,
after complexation of the C–O group via the oxygen atom to the
metal ion, these bands were observed at 1174 and 1145 cmÀ1 for
cobalt(II), 1181 and 1146 cmÀ1 for copper(II), 1174 and
1148 cmÀ1 for zinc(II) and 1180 and 1148 cmÀ1 for the
palladium(II) complex. In these complexes, the absence of the phe-
nolic O–H vibration indicates that ligand is deprotonated.
31.6
s, singlet; d, doublet; t, triplet; m, multiplet.
in air. Recrystallization from methanol yielded single crystals of
PdL2 suitable for X-ray crystallography. The typical yield was
81%. Anal. Calc. for C22H28N2O2Pd: C, 57.58; H, 6.15; N, 6.10.
Found: C, 57.63; H, 6.18; N, 6.19%. Selected IR data (KBr, cmÀ1):
3011–2864 (w, C–H aliphatic and aromatic), 1607 (
mC@N), 1180
and 1148 (
m
C–O). 1H and 13C NMR (CDCl3, 400 MHz, 298 K): Table 3.
2.5. Catalytic activity and optimization of the reaction conditions
2.5.1. Oxidation of thioanisole with H2O2 catalyzed by CoL2, CuL2,
ZnL2 and PdL2 Schiff base complexes
To find the optimized conditions, we investigated the oxidation
of thioanisole as a model substrate using hydrogen peroxide
(Scheme 3). The effect of different reaction parameters (H2O2
amount, temperature and mmol of catalyst) were studied for the
thioanisole oxidation.
3.2. NMR spectra
The Schiff-base ligand, HL, and diamagnetic zinc(II) and
palladium(II) complexes, ZnL2 and PdL2, were studied by 1H and
13C NMR experiments (Table 3). The 1H and 13C NMR spectra of
the ligand and the complexes were run immediately and gave
the expected simple spectra, indicating the integrity of the ligand
and the complexes. The spectra of the complexes obtained after
12, 24 and 120 h were similar to the initial spectra, indicating that
the complexes are stable in solution. The 1H NMR spectra of the
complexes are similar to that of the ligand, but with slight shifts
to lower fields. The signal for the imine proton in the zinc(II) and
palladium(II) complexes appears at 8.52 and 7.32 ppm, respec-
tively and this downfield shift with respect to the corresponding
signal in the free ligand, indicates that the metal–nitrogen bond
is retained in solution. Observing no signal corresponding to
hydroxyl protons at 14.3 ppm, suggests that the hydroxyl groups
are fully deprotonated and the oxygen atoms are most likely
coordinated to the metal ions. The 13C NMR spectra show 9 signals
for the ZnL2 and PdL2 complexes. The peak at 168.7 ppm for ZnL2
and 159.7 ppm for PdL2, assignable to the imine carbon atoms,
confirms the presence of the Schiff base ligand in the complexes
[48].
2.5.2. General procedure for the catalytic oxidation of sulfides under
solvent free conditions
To a mixture of the sulfide (1 mmol) and 30% H2O2 (2 mmol),
the catalyst (1 mmol) was added and the mixture was stirred at
50 °C for a specified time. The progress of the reaction was moni-
tored by TLC (petroleum ether/ethylacetate 8:3) and GC. After
completion of the reaction, the product was extracted with ethyl
acetate and the catalyst was separated by filtration. The combined
organics were washed with brine (5 ml) and dried over anhydrous
O
S
O
S
S
Catalysts
O
+
H2O2, Solvent-free
Scheme 3. Oxidation of thioanisole with hydrogen peroxide in the presence of
CoL2, CuL2, ZnL2 and PdL2 Schiff base complexes as catalysts under solvent-free
conditions.

M. Khorshidifard et al. / Polyhedron 95 (2015) 1–13
5
3.3. Description of the crystal structures
(a)
N
Pd
O
Planar configuration
Step configuration
3.3.1. Structural description of PdL2
In the solid state, PdL2 has a crystallographic center of symme-
try that is right in the middle point of the palladium(II) ion in a
planar-transoid conformation (Fig. 1). The molecular unit is cen-
trosymmetric and is made up of equivalent halves. The tert-butyl
groups show a fixed conformation in the crystal packing. The
molecular structure of PdL2 is shown in Fig. 1. Selected bond dis-
tances and angles are listed in Table 2.
The crystallographic data reveal that the Pd(II) complex has dis-
torted planar configuration and is four-coordinated by two pheno-
late oxygen and two imine nitrogen atoms of two Schiff base
ligands. The ligands coordinated to the Pd(II) center in a trans
geometry with respect to each other.
d
(b)
(c)
N
Pd
O
Bowl configuration
N
Pd
O
The Pd–O distance of 1.987(3) Å and the Pd–N bond length of
2.066(3) Å are similar to those seen in related complexes
[49–51]. For instance, distances of 1.979(1) and 1.982(1) Å for
the Pd–O bonds and 2.013(1) and 2.015(1) Å for the Pd–N bonds
bowl angle: θ
Scheme 4. Planar, step and bowl configurations in bis(salicylidenimine)
palladium(II) complexes. —square of Pd, O, and N atoms; —
arene rings.
chelate rings;
are found in
a
related di-(2-allyliminomethylphenolato)
palladium(II) complex derived from allylamine and salicylaldehyde
[51]. The C@N bond distance is 1.285(5) Å (N1@C7), which is con-
sistent with a slight elongation of the C–N double bond when it
coordinates to a metal center [50].
The O–Pd–N bond angles [O(1)–Pd(1)–N(1) = 88.45(12)° and
O(1)–Pd(1)–N(1A) = 91.55(12)°] are close to 90°. Moreover the
coordination of the two NO bidentate chelate ligands to the
Pd(II) ion results in the formation of two six-membered rings (Pd
(1)/O(1)/C(1)/C(6)/C(7)/N(1) and Pd(1)/O(1A)/C(1A)/C(6A)/C(7A)/
N(1A)).
A search for salicylidenimine palladium(II) complexes in the
Cambridge Structural Database System 2014 showed that except
in one case [52] the N,O-ligand atoms in bis(salicyli-denimine)
palladium(II) complexes, Pd(N,O)2, adopt a trans arrangement in
the square planar Pd(II) configuration. For salen-type complexes,
Pd(ONNO), the N,O-ligand atoms are cis to each other, and they
are excluded from the following analysis. The trans-Pd(N,O)2 com-
plexes can subdivide into three classes (Scheme 4): planar config-
uration (with the distance between the mean planes of the
aromatic rings being less than d = 0.4 Å, see for example Refs.
[53–59]), step configuration (with the distance between the mean
planes of the aromatic rings greater than d = 0.4 Å, see for example
Refs. [60–67]) and bowl configuration (see for example Refs.
[68–73]).
In the Pd(N,O)2 complexes with a planar configuration the Pd
atom lies in the plane of the chelate rings (see for example Refs.
[53–59]). The two six-membered chelate rings and their annulated
phenyl rings are coplanar (Scheme 4a). The complexes have an
inversion symmetry. Frequently, however, a characteristic feature
of Pd(N,O)2 complexes is an envelope configuration of the six-
membered chelate rings, with the Pd atom deviating from the
plane of the other 5 atoms of the chelate rings. A direct conse-
quence is the alternative step or
a
bowl configuration
(Scheme 4b and c). In the step configuration the planes between
the aromatic rings are parallel to each other (see for example
Refs. [60–67]), though complexes still have Ci symmetry. Some
Pd(II) complexes adopt a bowl configuration (see for example
Refs. [68–73]). At best they have C2 symmetry. In the PdL2 com-
plex, the chelate rings have the envelope form and the structure
is the step configuration.
Fig. 1. Perspective view of PdL2 constituted by the asymmetric unit (filled drawings) showing the numbering scheme and the centrosymmetric half part (empty drawings).
Thermal ellipsoids are drawn at the 50% probability level, while the hydrogen size is arbitrary. The suffix A corresponds to the symmetry code -x, -y, -z.

6
M. Khorshidifard et al. / Polyhedron 95 (2015) 1–13
Fig. 2. View of CoL2 in the asymmetric unit showing the numbering scheme. Thermal ellipsoids are drawn at the 50% probability level, while the hydrogen size is arbitrary.
The C9, C10 and C11 carbon atoms of the tert-butyl group are disordered over two sites and refined with site occupancy factors 0.67:0.33. Only the major component of the
disordered tert-butyl group is shown.
Fig. 3. View of CuL2 in the asymmetric unit showing the numbering scheme. Thermal ellipsoids are drawn at the 50% probability level, while the hydrogen size is arbitrary.
3.3.2. Structural description of CoL2, CuL2 and ZnL2
P2(1) space groups for CoL2, CuL2 and ZnL2, respectively. The
ligands coordinate to the metal center in a cis geometry with
respect to each other.
The C@N bond distances are 1.272(4) (N1@C7) and 1.281(4) Å
(N2@C18) for CoL2, 1.285(3) (N1@C7) and 1.286(3) Å (N2@C18)
for CuL2 and 1.275(4) (N1@C7) and 1.272(3) Å (N2@C18) for
ZnL2, which are consistent with a slight elongation of the C–N dou-
ble bond when coordinated to a metal center [74].
An ORTEP view of CoL2, CuL2 and ZnL2 with the atom-number-
ing scheme is presented in Figs. 2–4 and the crystallographic data
and selected bond lengths and angles are collected in Tables 1 and
2. The crystallographic data reveal that the metal center is four-
coordinated by two phenolate oxygen and two imine nitrogen
atoms of two Schiff base ligands. The geometry around the metal
center is distorted tetrahedral, with Pca2(1), P2(1)2(1)2(1) and

M. Khorshidifard et al. / Polyhedron 95 (2015) 1–13
7
Fig. 4. View of ZnL2 in the asymmetric unit showing the numbering scheme. Thermal ellipsoids are drawn at the 50% probability level, while the hydrogen size is arbitrary.
The C9, C10 and C11 carbon atoms of the tert-butyl group are disordered over two sites and refined with site occupancy factors 0.65:0.35. Only the major component of the
disordered tert-butyl group is shown.
dN1-N2
dC8-C19
αN1-M-N2
3.506(5) 4.875(5) 122.03(12)
3.795(4) 5.358(5) 145.58(8)
3.609(3) 5.025(4) 126.95(13)
CoL2
CuL2
ZnL2
Fig. 5. Distances (Å) and angles (°) between the two butyl groups in the CoL2, CuL2 and ZnL2 complexes.
In the crystal structures of all the complexes, the atoms of the
co-workers and showed a large variation in the dihedral angle
between the planes N(1)–Cu(1)–O(1) and N(2)–Cu(1)–O(2) [75].
The results show that bulky R groups induce large Dih values. It
can be seen that packing effects influence the structural features.
phenyl ring plane [C(1)C(2)C(3)C(4)C(5)C(6) and C(12)C(13)C(14)
C(15)C(16)C(17)] and the chelate ring formed by the same ligand
plane [O(1)/M(1)/N(1)/C(7)/C(6)/C(1) and O(2)/M(1)/N(2)/C(18)/
C(17)/C(12)] are nearly coplanar with dihedral angles of
3.82(16)° and 2.17(19)° for CoL2, 2.53 (12)° and 11.13 (11)° for
CuL2 and 5.48 (13)° and 5.00 (11)° for ZnL2, respectively.
Examination of the main metal-ligand distances shows that the
CoÁ Á ÁN, CuÁ Á ÁN and ZnÁ Á ÁN distances are longer than the CoÁ Á ÁO,
CuÁ Á ÁO and ZnÁ Á ÁO distances. The MÁ Á ÁN and MÁ Á ÁO distances are
2.003(3) and 1.883(3) Å for CoL2, 1.995(2) and 1.8957(17) Å for
CuL2 and 2.0200(19) and 1.911(2) Å for ZnL2.
As depicted in Figs. 2–4, all of the complexes have a distorted
tetrahedral geometry, with bidentate coordinated ligands which
have a cis geometry with respect to each other. The N(1)–M–N(2)
angle in CuL2 is larger than that for the CoL2 and ZnL2 complexes
(Fig. 5). With such an angle in CuL2, the distance and steric hin-
drance between the two butyl groups in CuL2 are greater and lower
respectively than in the CoL2 and ZnL2 complexes.
The N–M–O bond angles (bite angle) are 97.94(12) [O(1)–
Co(1)–N(1)] and 96.71(11)° [O(2)–Co(1)–N(2)] in the CoL2
complex, 95.88(8) [O(1)–Cu(1)–N(1)] and 93.22(8)° [O(2)–Cu(1)–
N(2)] in the CuL2 complex and 97.58(11) [O(1)–Zn(1)–N(1)] and
96.65(9)° [O(2)–Zn(1)–N(2)] in the ZnL2 complex.
Thus,
bis[N-(cyclooctyl)-2-oxy-1-naphthaldi-minato]copper(II)
has a bulky c-octyl substituent (c stands for cyclo) that determines
a flat-tetrahedral coordination sphere [76]. Thus, steric hindrance
of the N substituents is not the only factor influencing the coordi-
nation sphere in the solid state.
In our case, CuL2 is more distorted than bis(N-tert-butyl-2-oxy-
1-naphthaldiminato)copper(II), according to their Dih values of
53.6° and 45.4°, respectively [75]. In CoL2 and ZnL2, the dihedral
angle between the planes N(1)–M(1)–O(1) and N(2)–M(1)–O(2)
are 87.1° and 83.1°, respectively, and the two bidentate ligands
in these complexes are almost perpendicular to each other.
3.4. Electronic spectra
Electronic spectra of the free schiff base ligand and the com-
plexes were recorded in a CHCl3 solution. The absorption spectra
for the Schiff base ligand (H2L) and all the complexes are shown
in Fig. 6. The electronic spectrum of the Schiff base ligand exhibits
three main peaks at 255, 316 and 409 nm with a shoulder at
240 nm (Fig. 6). The first and second peaks (240 and 255 nm) are
attributed to benzene
p ? p ?
p⁄ transitions. The azomethine p⁄
Three-dimensional structures of Cu(II) complexes with naph-
thaldiminates and salicylaldiminates found in the Cambridge
Structural Database (CSD) were investigated by Costamagna and
and n ? p⁄ transitions are viewed at 316 and at 409 nm as broad
peaks with low intensity, respectively [77].

8
M. Khorshidifard et al. / Polyhedron 95 (2015) 1–13
Fig. 6. Electronic spectra of the schiff base ligand and the complexes (A: H2L, B: CuL2, C: CoL2, D: PdL2, E: ZnL2).
Table 4
Table 5
Oxidation of thioanisole with different amounts of catalyst in the presence H2O2.a
Oxidation of thioanisole with various amounts of H2O2 in the presence of Co, Ni, Zn
and Pd schiff basea complexes.
Entry
Catalyst
mmol of catalyst
Conversion (%)b
Selectivity (%)c
Entry
Catalyst
mmol H2O2
Conversion (%)b
Selectivity (%)c
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
Co
Co
Co
Co
Cu
Cu
Cu
Cu
Zn
Zn
Zn
Zn
Pd
Pd
Pd
Pd
None
0.001
0.005
0.01
0.05
0.001
0.005
0.01
0.05
0.001
0.005
0.01
0.05
0.001
0.005
0.01
0.05
–
20
23
29
30
48
61
80
83
63
79
93
93
52
60
74
76
16
97
99
99
99
94
99
99
99
98
99
99
99
96
99
99
99
75
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Co
Co
Co
Co
Cu
Cu
Cu
Cu
Zn
Zn
Zn
Zn
Pd
Pd
Pd
Pd
0.5
1
2
4
0.5
1
2
4
0.5
1
2
4
0.5
1
2
4
17
20
29
35
33
51
80
86
37
59
93
96
30
50
74
77
66
81
99
99
58
77
99
99
63
72
99
99
69
80
99
99
a
b
c
a
Reaction conditions: thioanisole (1 mmol) and H2O2 (2 mmol) at 50 °C for 3 h.
Conversion based on sulfide substrates.
Selectivity for sulfone.
Reaction conditions: Thioanisole (1 mmol), Catalyst (0.01 mmol) and H2O2 30%
at 50 °C for 3 h.
Conversion based on sulfide substrates.
b
c
Selectivity for sulfone.
In the complexes CoL2, CuL2, ZnL2 and PdL2, the broad band
that appears in the range 350–450 nm with more intensity than
free Schiff base ligand may be due to MLCT (comprised of a transi-
tion between the metal centered HOMO to the ligand centered
LUMO (C@N(p⁄)), LMCT, d-d and azomethine n ? p⁄ transitions.
This band is in the range usually observed for azomethine
n ? p⁄, d–d and LMCT transitions [36,78]. According to the d10
electronic configuration (+2 formal oxidation state) of zinc, the
Zn(II) complex shows no d–d transition.
solvent-free conditions. The amount of catalyst had a considerable
effect on the conversion of thioanisole to the corresponding sul-
fone. The results are given in Table 4 and show that the reaction
is sensitive to the catalyst amounts for each case. These results
showed that on increasing the catalyst amount, the reaction con-
version enhances without significant changing the selectivity for
all the catalysts. The role of the catalyst is very important in the
reaction because this reaction has low conversion without any
catalyst (Table 4, entry 17). Also, among the four catalysts, ZnL2
showed a better catalytic performance compared to the others.
3.5. Catalytic activity of the different complexes in the solvent free
oxidation of sulfides
3.5.1. Influence of different amounts of catalyst
3.5.2. Influence of different amounts of oxidant
We first studied the oxidation of thioanisole as a model sub-
strate using 30% aqueous hydrogen peroxide as the oxidant under
After optimization of the catalyst amount, we demonstrated the
effect of the amount of oxidant on the oxidation of thioanisole in

M. Khorshidifard et al. / Polyhedron 95 (2015) 1–13
9
Table 6
selectivity to sulfoxide was observed; for example at 85 °C, an
increase in the oxidation activity was observed, which caused the
increased formation of the methyl phenyl sulfone, but selectivity
for sulfoxide decreased for all catalysts.
Oxidation of thioanisole with different catalysts at various temperatures.a
Entry
Catalyst
Temperature (°C)
Conversion (%)b
Selectivity (%)c
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Co
Co
Co
Co
Cu
Cu
Cu
Cu
Zn
Zn
Zn
Zn
Pd
Pd
Pd
Pd
25
50
60
85
25
50
60
85
25
50
60
85
25
50
60
85
16
29
31
43
41
80
82
91
46
93
96
99
37
74
77
84
45
99
99
99
43
99
99
99
47
99
99
99
56
99
99
99
3.5.4. Influence of the geometrical structure of the catalyst
In this work, the catalytic activity of four kinds of metal com-
plexes were examined (Figs. 7–10). Among them, ZnL2 showed
the best catalytic activity for the oxidation of sulfide and CoL2
showed the lowest catalytic performance under these reaction
conditions. In general, there are some different parameters affect-
ing the performance of a catalyst. Here, we studied the influence of
the geometrical structure of the metal complex. As previously
reported [79–81], formation of a metal-peroxo intermediate occurs
in some oxidation reactions that shows the geometrical structure
of the complex plays an important role in its catalytic activity. In
the present study, according to Fig. 5, the CoL2 complex has more
steric hindrance around the metal center due to the geometry of
the ligands. So the bulkiness of the ligands for CoL2 probably lead
to a further lowering of the accessibility of the metal center in the
catalyst and so reduce the catalytic efficiency of the complex (the
slope of the conversion curve for this complex is lower than for
the other ones) (Figs. 7–10).
a
Reaction conditions: thioanisole (1 mmol), H2O2 (3 mmol), 0.01 mmol catalyst
for 3 h.
b
c
Conversion based on sulfide substrates.
Selectivity for sulfone.
the presence of each of the four catalysts. With 0.5 mmol of H2O2
all catalysts showed low conversions (entries 1, 5, 9 and 13,
Table 5); in contrast, on raising the amount of oxidant, higher con-
versions were obtained; among them all, ZnL2 provided the high-
est conversion of 99% and selectivity of 99%. Furthermore, it can
be seen in Table 5 that with an increase in the amount of the
H2O2 with 0.01 mmol catalyst at 50 °C, the conversion of thioani-
sole increased, but selectivity for sulfoxide decreased quickly.
3.6. Oxidation of different sulfides
The oxidation of thioanisole to the corresponding sulfone,
catalyzed by CoL2, CuL2, ZnL2 and PdL2 with 2 mmol of H2O2 at
50 °C, was determined as
a function of the reaction time
(Figs. 7–10). The results indicate that at the beginning of the reac-
tion the sulfoxide is formed selectively, but the amount of sulfone
increases slightly over time. It is remarkable that sulfone can be
obtained with high selectivity with 2 equivalents of 30% H2O2 in
the presence of all the complexes in 3 h under solvent free
conditions.
In an effort to extend the scope of the reaction under the opti-
mized conditions, the CuL2, ZnL2 and PdL2 catalysts were applied
to the oxidation of various types of aliphatic and aromatic sulfides
3.5.3. Influence of reaction temperature
In the present work for optimization of the reaction conditions,
we also studied the effect of temperature on the catalytic perfor-
mance of CoL2, CuL2, ZnL2 and PdL2 (Table 6). Obviously, a sub-
stantially improved conversion of thioanisole to the
corresponding sulfone and sulfoxide was observed with an
increase in the temperature, but in all cases a decrease in
Fig. 7. Reaction profile of the oxidation of thioanisole catalyzed by CoL2 with 2 mmol of H2O2 at 50 °C.

10
M. Khorshidifard et al. / Polyhedron 95 (2015) 1–13
Fig. 8. Reaction profile of the oxidation of thioanisole catalyzed by CuL2 with 2 mmol of H2O2 at 50 °C.
Fig. 9. Reaction profile of the oxidation of thioanisole catalyzed by ZnL2 with 2 mmol of H2O2 at 50 °C.
and the results are summarized in Table 7. CoL2 did not show good
catalytic activity, for this reason we did not use it for oxidation of
different sulfides. The different sulfides were selectively oxidized
to the corresponding sulfones with 2 mmol H2O2 under solvent
free conditions. The oxidation of allylsulfides and hydroxysulfides
proceeded chemoselectively to the corresponding sulfones, with-
out the epoxidation of the double bonds and dehydrogenation of
the hydroxyl groups (entries 6, 7 and 8). Thioanisole was oxidized
under solvent-free conditions to the corresponding sulfone in 93%
yield with 2 mmol H2O2 and ZnL2 as a catalyst (Table 7, entry 2).
The CuL2 catalyst gave sulfone in 80% conversion, while the PdL2
catalyst gave methyl phenyl sulfone in 74% conversion (Table 7,
entry 2). Dibutyl sulfide, thiophene and all the aromatic sulfides,
such as methylphenyl sulfide, diphenyl sulfide and the substituted
diphenyl sulfide (4-nitro), were oxidized to the corresponding sul-
fones with a good conversion in the presence of each of the three

M. Khorshidifard et al. / Polyhedron 95 (2015) 1–13
11
Fig. 10. Reaction profile of the oxidation of thioanisole catalyzed by PdL2 with 2 mmol of H2O2 at 50 °C.
Table 7
The oxidation of various sulfides with CoL2, CuL2, ZnL2, and PdL2 Schiff-base complexes as catalyst using 30% aqueous H2O2.a
Entry
Sulfide
Time (h)
Conversion (%)b
Selectivity(%)c
CuL2
ZnL2
PdL2
1
2
4.5
3
83
96
79
99
99
S
S
S
80
93
74
3
4
5.5
5.3
85
71
97
92
78
77
99
98
Bu
S
Bu
S
NO2
5
6
3.15
5
76
79
97
95
81
76
99
99
S
7
8
7
5
80
76
98
92
79
69
98
99
S
S
OH
9
5.5
22
29
25
99
S
a
b
c
Reaction conditions: catalyst (0.01 mmol), substrate (1.0 mmol), H2O2 (2 mmol), solvent free, at 50 °C.
Conversion based on sulfide substrates.
Selectivity for sulfone.
catalysts (Table 7, entries 1–5). Unfortunately, with this protocol
DBT was oxidized into sulfone (Table 7, entry 9) with a low conver-
sion. Although DBT was oxidized into sulfone with low conversion
under the reaction conditions, these results show the good capabil-
ity of this method for the oxidation of several types of sulfides to
the corresponding sulfones.
4. Conclusion
In conclusion, we have synthesized a new bidentate NO donor
ligand and its Co(II), Cu(II), Zn(II) and Pd(II) complexes. The struc-
tures of the complexes have been established using single crystal
X-ray diffraction analysis. The Co(II), Cu(II), Zn(II) and Pd(II) atoms

12
M. Khorshidifard et al. / Polyhedron 95 (2015) 1–13
[25] P.G. Cozzi, Chem. Soc. Rev. 33 (2004) 410.
are coordinated by two phenolic-O atoms and two azomethine-N
atoms to form a distorted tetrahedral geometry for the CoL2,
CuL2 and ZnL2 complexes and square-planar geometry for PdL2.
We have demonstrated the effectiveness of these complexes as
catalysts for the green oxidation of sulfides to the corresponding
sulfones with hydrogen peroxide. In this system the reactions
can be carried out under solvent-free conditions as a green sustain-
able method using all the catalysts in the presence of H2O2.
Conversions of 20–96%, and selectivities of 98–100% for sulfone
are observed with the four compounds as catalysts. Also, we found
that ZnL2 shows a better catalytic activity for the oxidation of sul-
fide and CoL2 shows a lower catalytic performance with these reac-
tion conditions. Overall, we recommend this green, simple, clean,
and economical procedure for the oxidation of different sulfides
to the corresponding sulfones.
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CCDC 1045369, 1045370, 1045371 and 1045372 contain the
supplementary crystallographic data for CoL2, CuL2, PdL2 and
ZnL2, respectively. 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:
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