Catalytic oxidation of organic sulfides by new iron-chloro Schiff base complexes: The effect of methoxy substitution and ligand isomerism on the electronic, electrochemical and catalytic performance of the complexes

Article abstract of DOI:10.1016/j.poly.2021.115135

Four new Fe(III)-Chloro Schiff base complexes were synthesized and characterized. The N₂O₂ type tetradentate Schiff base ligands were synthesized from the condensation of meso-1,2-diphenyl-1,2-ethylenediamine with salicylaldehyde (H₂L¹), 3-methoxysalicylaldehyde (H₂L²), 4-methoxysalicylaldehyde (H₂L³) and 5-methoxysalicylaldehyde (H₂L⁴). The corresponding iron-chloro complexes with the general formula [Fe(Lⁿ)Cl] (n = 1–4 for complexes (1–4)) were characterized by FTIR and UV–Vis spectroscopy and elemental analysis. Crystal structure of (1) was obtained by single-crystal X-ray crystallography. Cyclic voltammetry was used to study the electrochemistry of the complexes. The complexes were used as catalysts for the selective oxidation of organic sulfide compounds to sulfoxides. High catalytic performance and selectivity were obtained. The spectroscopic, electrochemical and catalytic data are discussed based on the electronic, steric and electrochemical properties of the complexes.

Full text of DOI:10.1016/j.poly.2021.115135

Polyhedron 200 (2021) 115135
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Catalytic oxidation of organic sulfides by new iron-chloro Schiff base
complexes: The effect of methoxy substitution and ligand isomerism on
the electronic, electrochemical and catalytic performance of the
complexes
a
a
b
b
c
Fatemeh Aghvami , Abolfazl Ghaffari , Monika Kucerakova , Michal Dusek , Rahman Karimi-Nami ,
c,d
a,
⇑
Mojtaba Amini , Mahdi Behzad
a
Faculty of Chemistry, Semnan University, Semnan 35351-1911, Iran
b
c
Institute of Physics ASCR, v.v.i, Na Slovance 2, 182 21 Praha 8, Czech Republic
Department of Chemistry, Faculty of Science, University of Maragheh, Maragheh, Iran
Department of Inorganic Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran
d
a r t i c l e i n f o
a b s t r a c t
Article history:
Four new Fe(III)-Chloro Schiff base complexes were synthesized and characterized. The N O2 type
2
Received 3 January 2021
Accepted 25 February 2021
Available online 5 March 2021
tetradentate Schiff base ligands were synthesized from the condensation of meso-1,2-diphenyl-1,2-
1
2
ethylenediamine with salicylaldehyde (H
2
L ), 3-methoxysalicylaldehyde (H
2
L ), 4-methoxysalicylalde-
3
4
hyde (H
2
L ) and 5-methoxysalicylaldehyde (H
2
L ). The corresponding iron-chloro complexes with the
n
general formula [Fe(L )Cl] (n = 1–4 for complexes (1–4)) were characterized by FTIR and UV–Vis spec-
troscopy and elemental analysis. Crystal structure of (1) was obtained by single-crystal X-ray crystallog-
raphy. Cyclic voltammetry was used to study the electrochemistry of the complexes. The complexes were
used as catalysts for the selective oxidation of organic sulfide compounds to sulfoxides. High catalytic
performance and selectivity were obtained. The spectroscopic, electrochemical and catalytic data are dis-
cussed based on the electronic, steric and electrochemical properties of the complexes.
Ó 2021 Elsevier Ltd. All rights reserved.
Keywords:
Catalysis
Iron-chloro
Schiff base
Sulfoxidation
1
. Introduction
Sulfoxides are great chemicals. They are used as intermediates
Schiff base ligands are nitrogen containing ligands that could be
easily synthesized and their stereo-electronic properties could be
easily tuned by using different amines or aldehydes. Various tran-
sition metal Schiff base complexes have been employed for the
preparative oxidation of sulfides, including Co, Mn, Cu, Ni and Fe
[12–16]. Among them, less attention has been paid to the Fe(III)
Schiff base complexes. Hence, herein we report the synthesis, char-
acterization and electrochemistry of four new Fe(III)-chloro com-
in the synthesis of a variety of important materials and are also
found as the main structure of some drugs [1–3]. Omeprazole,
esomeprazole and armodafinil are just three examples. Selectively
converting sulfides to sulfoxides is an interesting reaction. Over the
past decades, several transition metal complexes have been
employed for the catalytic transformation of organic compounds
to more valuable materials, including the selective transformation
of sulfides to sulfoxides [7–10]. Iron containing enzymes catalyze
several reactions such as oxygen activation and insertion into
organic compounds [4–6]. Selectivity and rapid catalysis are two
most important characteristics of enzymatic reactions. Hence, the
designation and study of new materials with the same characteris-
tics is an interesting and challenging field of study. In this regard,
nitrogen containing ligands have gained great interest [11].
2 2
plexes of tetradentate N O type Schiff base ligands. Crystal
structure of one of the complexes, i.e. (1), was also obtained. The
new complexes were used for catalytic oxidation of sulfides.
Methylphenylsulfide (MPS) and (1) were used as model substrate
and catalyst, and urea hydroperoxide (UHP) was used as the oxi-
dant. Various reaction conditions were optimized. It was proven
that the complexes showed good catalytic activity and sulfoxide
selectivity. Electronic and electrochemical behaviors of the com-
plexes were rationalized based on the presence and location of
the substituted methoxy group.
⇑
Corresponding author.
E-mail address: mbehzad@semnan.ac.ir (M. Behzad).
https://doi.org/10.1016/j.poly.2021.115135
277-5387/Ó 2021 Elsevier Ltd. All rights reserved.
0

F. Aghvami, A. Ghaffari, M. Kucerakova et al.
Polyhedron 200 (2021) 115135
2
. Experimental
mixture was refluxed and stirred for six hours. The progress of
the reaction was monitored by TLC. The resulting brown precipi-
tates were filtered off, washed with ethanol and air-dried. The pre-
2.1. Materials and methods
cipitates were then recrystallized from CHCl
complexes.
3
to obtain pure target
All of the used chemicals and solvents were purchased from
commercial sources and were used as received. Meso-1,2-diphe-
1
1
nyl-1,2-ethylenediamine [17] and the Schiff base ligands H
2
L -
2
.2.1. [Fe(L )Cl]ÁCHCl
3
(1)
4
H L were synthesized as described elsewhere [18]. FT-IR spectra
2
Yield: 0.52 g, 82%. Anal. Calcd. for C28
H
22ClFeN
2
O
2
ÁCHCl
3
: C:
were obtained as KBr plates using a Shimadzu FT-IR instrument.
UV–Vis spectra were obtained on a Shimadzu UV-1650 PC spec-
trophotometer. Elemental analyses were performed on a Perkin-
Elmer 2400II CHNS-O analyzer. The X-ray diffraction measure-
ments were carried out on a SuperNova diffractometer of Rigaku
Oxford Diffraction. A Metrohm 757 VA computerace instrument
was employed to obtain cyclic voltammograms in freshly prepared
acetonitrile solutions at room temperature (25 °C) using 0.1 M
tetra-n-butylammonium hexafluorophosphate (TBAHFP) as sup-
5
5.31; H: 3.65, N: 4.45. Found: C: 55.70; H: 3.94, N: 4.90. Selected
IR (cm ): 1602, 1539, 1434 and 1294. UV–Vis. 10 M solution in
À1
-5
À1
À1
CH
3
CN [kmax nm, (
e
M
cm )]: 236 (39,400), 268 (8900), 326
(
11,500), 494 (3800).
2
2.2.2. Synthesis of [Fe(L )Cl]ÁH
2
O (2)
Yield: 0.54 g, 87%. Anal. Calcd. for C30
H
28ClFeN
2
O
5
: C: 61.29; H:
À1
4
1
.80, N: 4.77. Found: C, 61.70; H, 4.59; N, 4.63. Selected IR (cm ):
602, 1435, and 1255. UV–Vis. 10 M solution in CH
À5
3
CN [kmax nm,
porting electrolyte. The solutions were purged with N
2
for at least
À1
À1
(e
M
cm )]: 232 (49,200), 276 (26,000), 368 (16,000), 454
1
0 min to remove any dissolved O . A platinum working electrode,
2
(
3970), 530 (2300).
a platinum auxiliary electrode and an Ag/AgCl reference electrode
were used to obtain cyclic voltammograms in the range of À1 to
3
2
.2.3. Synthesis of [Fe(L )Cl] (3)
+
1 V. The ferrocenium/ferrocene redox couple was used as the
À1
Yield: 0.53 g, 85%. Selected IR (cm ): 1587, 1525 and 1226.
internal reference for the potential measurements.
À5
À1
À1
UV–Vis. 10 M solution in CH
47,800), 278 (43,800), 386 (7500), 400 (6300), 492 (5400). Anal.
Calcd. For C30 26ClFeN (%): C, 63.01; H, 4.60; N, 4.90. Found:
3
CN [kmax nm, (
e
M
cm )]: 250
(
H
2 4
O
2.2. General synthesis of the complexes
C, 63.22; H, 4.40; N, 5.01.
The complexes were prepared following a general procedure for
similar complexes [19,20]. Typically, 1.00 mmol of each ligand (see
Scheme 1) was dissolved in 40 mL of warm ethanol in a round bot-
4
2.2.4. Synthesis of [Fe(L )Cl] (4)
À1
Yield: 0.50 g, 81%. Selected IR (cm ): 1593, 1535, 1458 and
1288. UV–Vis. 10 M solution in CH
250 (36,500), 274 (25,300), 330 (9300), 396 (6100), 524 (4900).
À5
À1
À1
tom flask. 1.00 mmol of FeCl
ethanol, was then added to the ligand solution and the reaction
3
Á6H
2
O (0.27 g), dissolved in 20 mL of
3
CN [kmax nm, (
e
M
cm )]:
Scheme 1. Reaction pathway for the synthesis of the complexes.
2

F. Aghvami, A. Ghaffari, M. Kucerakova et al.
Polyhedron 200 (2021) 115135
Anal. Calcd. For C30
2
H26ClFeN O
4
(%): C, 63.01; H, 4.60; N, 4.90.
The packing of the complex is governed by hydrogen bond C2–
Found: C, 62.88; H, 4.48; N, 5.15.
H1C3Á Á ÁCl1 [distance DÁ Á ÁA 3.439(2) = Å, angle D–HÁ Á ÁA = 126°];
p p interaction between aromatic rings Cg5 [Cg5 = C4, C8, C22,
Á Á Á
C19, C23, C24, distance Cg5Á Á ÁCg5 = 3.6614(12) Å]; C25–
H1C25Á Á ÁCg5 interaction [distance C25Á Á ÁCg5 = 3.474(2) Å]; C8–
H1C8Á Á ÁCg7 interaction [Cg7 = C11, C13, C29, C27, C20, C15, dis-
tance C8Á Á ÁCg7 = 3.738(3) Å]; and C18–Cl3Á Á ÁCg4 interaction
2.3. X-ray crystallography
Diffraction data from selected single crystal of (1) with dimen-
sions 0.102 Â 0.073 Â 0.033 mm were collected at 95 K on Super-
[
Cg4 = C2, C28, C10, C6, C12, distance ClÁ Á ÁCg4 = 3.5933(12) Å].
Nova (Rigaku Oxford Diffraction) microsource diffractometer
These interactions shown in Fig. 2 connect molecules into a slab
extended along a (Supplemental Fig. S.1). No remarkable interac-
tion was found between the slabs.
equipped with a CCD detector Atlas S2, using CuK
Structure was solved with SHELXT [21] and refined with Jana2006
22]. The process of structure solution and refinement was stan-
dard; hydrogen atoms on carbon were refined as riding atoms.
a radiation.
[
3.2. Spectroscopic characterization
2.4. General procedure for the sulfide oxidation
The complexes were synthesized following a general route for
the synthesis of Fe(III)-chloro Schiff base complexes and were
characterized by different spectroscopic and analytical methods.
The FTIR spectra of the synthesized complexes (see Supplemental
Fig. S.2–5) showed a strong signal at around 1600 cm which is
assignable to the stretching vibration of the imine group m(C@N).
To a solution of sulfide (0.2 mmol), chlorobenzene (0.2 mmol)
as internal standard, and catalyst (0.005 mmol) in a 1:1 mixture
of CH OH/CH Cl (1 mL) as solvent was added 0.4 mmol UHP as
3
2
2
À1
an oxidant. After stirring the mixture at room temperature, the
reaction progress was monitored by GC, and oxidation products
were assigned by matching with authentic samples (Scheme 2).
This is the most characteristic signal in the FTIR spectra of such
complexes and clearly confirms the synthesis of the Schiff bases.
Comparison of this signal with the
m(C@N) of the previously
3
. Results and discussions
reported corresponding ligands [18] showed that this signal is
shifted by about 30 cm to lower wavenumbers in the complexes,
À1
3.1. Description of the crystal structure
which confirms the coordination through the iminic nitrogen
atoms. This shift to lower wavenumbers is due to the
p-back dona-
Fig. 1 shows the molecular structure of the complex (1), i.e. [Fe
tion of electron density from the central metal ion to the
p
* orbitals
1
(
2
L )Cl]ÁCHCl
3
with common atom numbering scheme. Tables 1 and
of the imines which weakens the mentioned bond. Interestingly,
3
summarize the crystallographic data and selected bond lengths
the [Fe(L )Cl] complex (3), in which the OMe is located trans to
and bond angles, respectively. This complex is neutral in which a
the C@N, has the lowest
the presence of OMe trans to C@N results in the transfer of electron
m(C@N). We have previously shown that
doubly deprotonated Schiff base ligand (L2 ) and a chloro (Cl )
ligand are coordinated to a central Fe(III) metal ion. The geometry
around the central metal ion is a square-based pyramid (SBP) with
À
À
density to
p
*(C@N), and causes a decrease in the wavenumber [18].
À1
The signals observed at around 1535 and 1435 cm
could be
À1
N
O
2 2
coordinating atoms of the Schiff base ligand at basal posi-
attributed to
m(C@C) and the signal at around 1250 cm is also
tions, and the chloro ligand at the axial position. The N coordi-
2
O
2
assignable to m(CAO).
nating atoms comprise two iminic nitrogen atoms and two
deprotonated phenolic oxygen atoms. The central metal ion is
located 0.488 Å above the basal plane of the square pyramid. Con-
sequently, Cl–Fe–N and Cl–Fe–O angles are larger than 90°. The
average Fe-N, Fe–O and Fe–Cl bond lengths are in the common
range of the previously reported similar complexes [23–27]. The
The UV–Vis spectra of the four ligands were quiet similar and
have been reported earlier [18]. Two intense signals at around
300 and 250 nm in those ligands had been assigned to the
p ?
p* transitions of the azomethine and the aromatic ring, respec-
tively. In the UV–Vis spectra of the complexes (See Supplemental
Fig. S.6), the former signal is blue-shifted by about 20–30 nm,
which confirms the coordination through the azomethine nitrogen
atoms. The electronic spectra of the complexes exhibited signals
s
5
geometry index calculated from (b-
equation, b and are the two greatest angles around the central
metal ion. The value can range from 0 to 1, where zero stands
a)/60 is equal to 0.25. In this
a
s
above 300 nm with lower intensity than the p ? p* transitions in
for an ideal SBP while 1 indicates ideal trigonal bipyramidal geom-
etry [28]. Thus the coordination polyhedron could be considered as
a distorted SBP.
the same complex, which could be assigned to the LMCT from
the orbital mainly located on the central metal ion to the orbital
mainly located on the azomethine p* [27–30]. In the complex with
Scheme 2. General oxidation pathway.
3

F. Aghvami, A. Ghaffari, M. Kucerakova et al.
Polyhedron 200 (2021) 115135
1
Fig. 1. Molecular structure of complex [Fe(L )Cl]ÁCHCl
3
(1). Thermal ellipsoids are drawn at 50% probability level, and hydrogen atoms are drawn as spheres of arbitrary radii.
Table 1
Table 2
Experimental crystallographic details.
Selected bond lengths and bond angles around the central metal ion.
1
Crystal data
Chemical formula
[Fe(L )Cl]ÁCHCl
3
Bond lengths (Å)
Bond angles (°)
C
29
H
23Cl4Fe
629.2
1 2 2
N O
Fe1–Cl1
Fe1–O1
Fe1–O2
Fe1–N1
Fe1–N2
2.2335 (5)
1.8941 (14)
1.895 (2)
2.0750 (16)
2.091 (2)
Cl1–Fe1–O1
Cl1–Fe1–O2
Cl1–Fe1–N1
Cl1–Fe1–N2
O1–Fe1–O2
O1–Fe1–N1
O1–Fe1–N2
O1–Fe1–N2
O2–Fe1–N2
N1–Fe1–N2
109.22 (4)
104.47 (4)
106.82 (4)
95.88 (5)
95.10 (8)
142.06 (6)
87.49 (8)
87.02 (8)
157.29 (6)
77.39 (8)
M
r
Crystal system, space group
Temperature (K)
a, b, c (Å)
Triclinic, PÀ1
95.00(10)
10.5789(3), 10.7786(2),
4.4673(4)
76.458(2), 72.098(3),
2.746(3)
1386.98(7)
2
1
Α, b,
c (°)
6
V (Å3)
Z
Radiation type
Cu K
8.15
a
À1
l
(mm
)
Crystal size (mm)
0.10 Â 0.07 Â 0.03
Tmin, Tmax
0.697, 1
observed as two split signals in the OMe substituted complexes
but as a very broad band centered at around 500 nm in (1). Again
the lowest energy transition has shown a blue shift due to the loca-
tion of C@N [27]. The obtained results are similar to the previously
reported complexes with similar structures [27,30].
No. of measured, independent and observed
I > 3 (I)] reflections
8289, 5389, 4722
[
r
R
int
0.022
0.030, 0.040, 1.63
5389
343
0.36, À0.30
2
2
2
R[F > 3
r
(F )], wR(F ), S
No. of reflections
No. of parameters
À3
D
q
max
,
D
q
min (e Å
)
3.3. Cyclic voltammetry
n
The electrochemical behavior of the [Fe(L )Cl] complexes
n = 1–4) was studied by means of cyclic voltammetry, and the
results are shown as Supplemental Fig. S.7 and are summarized
in Table 3. The complexes showed a reversible or a quasi-reversible
reduction potential due to Fe(III)/Fe(II) redox couple [29–31]. As
could be seen from Table 3, the results of the CV studies are also
no methoxy substituents, i.e. (1), the higher energy signal in this
region observed at 326 nm but in the complexes with methoxy
substituents, this band is red shifted. In (3) in which the OMe is
located trans to the C@N group, this signal is observed at 386 nm
which shows the highest red shift. This is consistent with the above
mentioned IR result for the same complex. Hence, the location of
OMe has shown a drastic effect on the LMCT bands, which further
confirms our previous study [18]. Other LMCT transitions, which
could be mixed with spin forbidden d ? d transitions, in these
complexes are also observed above 400 nm. These signals are
(
consistent with the results of spectroscopic studies. Again the [Fe
3
(
L )Cl] complex (3) showed the most negative standard reduction
potential which is due to the para location of the OMe to C@N,
which has resulted in the transfer of electron density to Fe(III). This
results in the stabilization of Fe(III) and hence, its lower tendency
4

F. Aghvami, A. Ghaffari, M. Kucerakova et al.
Polyhedron 200 (2021) 115135
Fig. 2. Weak interactions influencing packing of the complex (1). Centroids Cg4, Cg5 and Cg7 as well as and geometric characteristics of the interactions are given in the text.
Atoms and centroids participating in the interactions and atoms of iron are indicated as balls and labeled.
Table 3
À3
The cyclic voltammetry data for the new complexes in CH
3
CN (10 M) containing 0.1 M tetra-n-butylammoniumhexafluorophosphate as supporting electrolyte.
Complex
E0 (V)
Ec (V)
Ea (V)
DE (V)
(1)
(2)
(3)
(4)
À0.656
À0.240
À0.820
À0.280
À0.677
À0.285
À0.909
À0.315
À0.635
À0.196
À0.730
À0.244
0.042
0.089
0.179
0.071
to reduction. Complexes (2) and (4) with OMe substitution in the
meta position of the C@N have almost similar E0 value and more
positive values.
The increase in UHP amount up to 0.4 mmol also affected and
increased the catalytic activity. Reaction time was also optimized,
and the best results were obtained in 60 min reaction time.
The study of the effect of different substituents on the catalytic
transformation of organic compounds is an interesting field of
study [32–34]. Hence, in order to investigate the effect of the pres-
ence of the methoxy substitutions, as well as their position, i.e.
ligand isomerism, on the catalytic activity of the new Schiff base
complexes, the catalytic activity of other complexes was also stud-
ied using the optimized conditions for (1), and the results are col-
lected in Table 5. As could be seen from Table 5, complexes (1) and
3.4. Catalysis
MPS was used as the model substrate for catalytic oxidation of
sulfides using complex (1) as a model catalyst and UHP as oxidant.
Various reaction conditions, including catalyst amount, oxidant
amount, solvent type and reaction time were optimized. The
results are summarized in Table 4. As could be seen from entry 1
of this table, in the absence of the catalyst, trace amount of the pro-
(
4) showed almost higher activity than complexes (2) and (3). This
means that the steric effect of the OMe substituents mainly gov-
erns the catalytic behavior of the complexes. Complex (1) with
no OMe substituents and (4) in which OMes are located far from
the central metal ion showed higher catalytic activity which sup-
ports the steric effect of the OMe substituents. Complex (2) in
which OMe substituents are at the closest location of the central
metal ion showed the least activity. Other stereo-electronic factors
may also be important; for example, it could be seen from Table 3
that the complexes with higher reversibility in cyclic voltammetry
have shown greater catalytic activity. We are currently synthesiz-
ing new complexes to further investigate this idea.
duct was observed, while by increasing the catalyst up to 5 lmol,
the yield of the product was increased. Besides, excellent selectiv-
ity to sulfoxide was also observed. The solvent type also had a dra-
matic effect on catalytic activity. As could be seen from entries 11–
1
5, in the presence of the used single solvents, the conversion was
not high enough, but in the 1:1 CH Cl :CH OH solvent, the conver-
2
2
3
sion was increased. Nonpolar solvents such as dichloromethane
and chloroform are not the preferred solvents for sulfide oxidation,
but the better solubility of the complex in these solvents had
prompted us to test mixed polar-nonpolar solvent CH
2 2 3
Cl :CH OH
for this reaction, and the best results were obtained in this mixture.
5

F. Aghvami, A. Ghaffari, M. Kucerakova et al.
Polyhedron 200 (2021) 115135
Table 4
The effect of various conditions in the oxidation of methylphenylsulfide by (1)/UHP.
Entry
Amount of catalyst (mmol)
Amount of UHP (mmol)
Solvent (1 mL)
Time (min)
Conversion (%)
Selectivity to Sulfoxide (%)a
1
2
3
4
5
6
7
9
0
0.4
0.4
0.4
0.4
0.4
0.1
0.2
0.3
0.5
0.4
0.4
0.4
0.4
0.4
0.4
0.4
CH
CH
CH
CH
CH
CH
CH
CH
CH
CH
2
2
2
2
2
2
2
2
2
2
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
2
2
2
2
2
2
2
2
2
2
:CH
:CH
:CH
:CH
:CH
:CH
:CH
:CH
:CH
3
3
3
3
3
3
3
3
3
OH
OH
OH
OH
OH
OH
OH
OH
OH
60
60
60
60
60
60
60
60
60
60
60
60
60
60
30
45
Trace
13
48
79
75
21
42
65
86
17
15
51
60
58
53
67
–
0.001
0.003
0.005
0.007
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
0.005
100
100
97
96
100
100
98
10
11
12
13
14
15
16
17
88
100
100
96
95
99
CHCl
3
CH
CH
CH
CH
CH
3
3
3
2
2
CN
OH
COCH
3
Cl
Cl
2
:CH
:CH
3
OH
OH
99
97
2
3
Substrate = methylphenylsulfide = 0.2 mmol.
a
Selectivity to sulfoxide = (sulfoxide%/(sulfoxide% + sulfone%)) Â 100.
Table 5
a
Oxidation of sulfides catalyzed by the complexes (1–4)/UHP.
Entry
1
Substrate
Conversion (%)b (Selectivity (%)c)
1
2
3
4
79(97)
65(98)
68(97)
77(98)
2
3
75(98)
71(98)
67(98)
60(97)
69(98)
65(99)
77(97)
68(99)
4
5
81(96)
59(99)
64(98)
66(98)
72(97)
65(98)
75(97)
70(98)
6
7
8
45(100)
39(100)
37(100)
41(100)
35(100)
27(100)
36(100)
39(100)
40(100)
38(100)
36(100)
33(100)
a
2 2 3
The molar ratios for complexes (1–4):substrate:oxidant are 1:20:40. The reactions were performed in (1:1) mixture of CH Cl /CH OH (1 mL) under air at room tem-
perature within 60 min.
b
The GC yield (%) are measured relative to the starting sulfide.
Selectivity to sulfoxide = (sulfoxide%/(sulfoxide% + sulfone%)) Â 100.
c
To demonstrate the superior properties of these prepared com-
plexes, the activity of catalyst 1 was compared with some of the
previously reported iron-containing complexes. The relevant com-
parison is given in Table 6. According to Table 6, Catalyst (1) is
superior to previous works in terms of having the highest selectiv-
ity in reactions (entries 1–5), low reaction time (entries 3–5) and
low solvent consumption in the oxidation reactions (entries 3–5).
According to the previously reported works, a mechanism was
proposed for the sulfide oxidation catalyzed by catalyst (1) [39].
By the addition of UHP, a Fe@O bond is formed on the iron complex
that reaction of organic sulfide to the Fe@O can lead to the produc-
tion of sulfoxide and the primary catalyst (Scheme 3).
4. Conclusion
Four new iron-chloro complexes with tetradentate N O2 Schiff
base ligands were synthesized and used as catalyst for selective
2
oxidation of organic sulfides. The electronic spectroscopic data,
the electrochemical data and the catalytic performance of the com-
plexes were rationalized based on the presence and the position of
methoxy substituents on the Schiff base ligand. The complexes
showed good catalytic potential in the selective oxidation of
organic sulfides to sulfoxides and it was proven that ligand iso-
merism has a considerable effect on electronic, electrochemical
and catalytic properties in such complexes.
6

F. Aghvami, A. Ghaffari, M. Kucerakova et al.
Polyhedron 200 (2021) 115135
Table 6
Recently reported catalytic systems for the sulfides oxidation in the presence of iron-containing complexes.
Entry Catalyst
Condition
Conversion
%)
Selectivity
(%)
Ref.
(
1
The molar ratios for catalyst: substrate: UHP are 1:20:40; (CH
2
2
Cl :CH
3
OH
79
97
Present
work
(
1 mL); 60 min; at room temperature)
2
The molar ratios for catalyst: substrate: UHP are 1:20:40; (CH
1 mL); 15 min; at room temperature).
2
2
Cl :CH
3
OH
85
86
[35]
(
3
The molar ratios for catalyst: substrate are 1:60:50; (anisic acid (10 equiv.) as 85
an additive, CH CN (4 mL); 60 min; at room temperature)
93
[36]
3
4
5
The molar ratios for catalyst: substrate: H
room temperature).
2
O
2
are 1:25:25; (CH
3
CN (5 mL) at
94
–
[37]
[38]
The molar ratios for catalyst: substrate are 1:50; (O
20 min; at 100 °C
2
(2 MPa) PEG-1000 (0.3 g), 100
94
1
Scheme 3. The proposed mechanisms for oxidation of the sulfides catalyzed by (1).
7

F. Aghvami, A. Ghaffari, M. Kucerakova et al.
Polyhedron 200 (2021) 115135
[
16] E. Zamanifar, F. Farzaneh, J. Simpson, M. Maghami, Inorg. Chim. Acta 414
Declaration of Competing Interest
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2014) 63–70.
[
17] M.N.H. Irving, R.M. Parkins, J. Inorg. Nucl. Chem. 27 (1965) 270–271.
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
[18] A. Ghaffari, M. Behzad, M. Pooyan, H.A. Rudbari, G. Bruno, J. Mol. Struct. 1063
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Appendix A. Supplementary data
[
21] G. Sheldrick, SHELXT
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