New oxovanadium and dioxomolybdenum complexes as catalysts for sulfoxidation: experimental and theoretical investigations of E and Z isomers of ONO tridentate Schiff base ligand

Article abstract of DOI:10.1080/17415993.2021.1941020

A new ONO-tridentate Schiff base ligand (H₂L) derived by the condensation of nicotinic hydrazide with 5-chlorosalicylaldehyde has been prepared and characterized by combustion analysis (CHN), FT-IR and multinuclear (¹H and ¹³C) NMR spectroscopy. The crystalline nature and molecular structure of the ligand were confirmed by single-crystal X-ray diffraction analysis. Furthermore, the optimized structural parameters of the four possible configurations of the ligand including Z and E stereoisomers each containing two tautomeric forms (enol and keto) have also been investigated. The theoretical parameters were calculated by performing the DFT method using the B3LYP/Def2-TZVP level of theory. In addition to this, dioxomolybdenum(VI) (MoO₂L) and oxovanadium(V) (VOL) complexes with the entitled Schiff base ligand have also been prepared and characterized by different techniques. Then, the catalytic efficiencies of synthesized VOL and MoO₂L complexes were also explored for the oxidation of sulfides using 30% aqueous H₂O₂ as a source of oxygen. These homogeneous catalysts showed excellent catalytic activities in the oxidation of both aromatic and aliphatic sulfides.

Full text of DOI:10.1080/17415993.2021.1941020

Journal of Sulfur Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/gsrp20
New oxovanadium and dioxomolybdenum
complexes as catalysts for sulfoxidation:
experimental and theoretical investigations of E
and Z isomers of ONO tridentate Schiff base ligand
Hadi Kargar, Atefeh Moghimi, Mehdi Fallah-Mehrjardi, Reza Behjatmanesh-
Ardakani, Hadi Amiri Rudbari & Khurram Shahzad Munawar
To cite this article: Hadi Kargar, Atefeh Moghimi, Mehdi Fallah-Mehrjardi, Reza Behjatmanesh-
Ardakani, Hadi Amiri Rudbari & Khurram Shahzad Munawar (2021): New oxovanadium and
dioxomolybdenum complexes as catalysts for sulfoxidation: experimental and theoretical
investigations of E and Z isomers of ONO tridentate Schiff base ligand, Journal of Sulfur Chemistry,
DOI: 10.1080/17415993.2021.1941020
To link to this article: https://doi.org/10.1080/17415993.2021.1941020
Published online: 16 Jun 2021.
Article views: 29
View supplementary material
Submit your article to this journal
View related articles
View Crossmark data
Full Terms & Conditions of access and use can be found at
https://www.tandfonline.com/action/journalInformation?journalCode=gsrp20

JOURNAL OF SULFUR CHEMISTRY
https://doi.org/10.1080/17415993.2021.1941020
New oxovanadium and dioxomolybdenum complexes as
catalysts for sulfoxidation: experimental and theoretical
investigations of E and Z isomers of ONO tridentate Schiff
base ligand
b
Hadi Kargar a, Atefeh Moghimi b, Mehdi Fallah-Mehrjardi
Reza Behjatmanesh-Ardakani b, Hadi Amiri Rudbari c and
,
d,e
Khurram Shahzad Munawar
aDepartment of Chemical Engineering, Faculty of Engineering, Ardakan University, Ardakan, Iran;
bDepartment of Chemistry, Payame Noor University Tehran, Iran; cDepartment of Chemistry, University of
Isfahan, Isfahan, Iran; dDepartment of Chemistry, University of Sargodha, Pakistan; eDepartment of
Chemistry, University of Mianwali, Pakistan
ABSTRACT
ARTICLE HISTORY
Received 8 February 2021
Accepted 4 June 2021
A new ONO-tridentate Schiff base ligand (H2L) derived by the con-
densation of nicotinic hydrazide with 5-chlorosalicylaldehyde has
been prepared and characterized by combustion analysis (CHN), FT-
IR and multinuclear (1H and 13C) NMR spectroscopy. The crystalline
nature and molecular structure of the ligand were confirmed by
single-crystal X-ray diffraction analysis. Furthermore, the optimized
structural parameters of the four possible configurations of the lig-
and including Z and E stereoisomers each containing two tautomeric
forms (enol and keto) have also been investigated. The theoretical
parameters were calculated by performing the DFT method using
the B3LYP/Def2-TZVP level of theory. In addition to this, dioxomolyb-
denum(VI) (MoO2L) and oxovanadium(V) (VOL) complexes with the
entitled Schiff base ligand have also been prepared and charac-
terized by different techniques. Then, the catalytic efficiencies of
synthesized VOL and MoO2L complexes were also explored for the
oxidation of sulfides using 30% aqueous H2O2 as a source of oxygen.
These homogeneous catalysts showed excellent catalytic activities in
the oxidation of both aromatic and aliphatic sulfides.
KEYWORDS
Vanadium(V);
molybdenum(VI); Schiff base;
homogeneous catalyst;
sulfoxidation
CONTACT Hadi Kargar
h.kargar@ardakan.ac.ir
Department of Chemical Engineering, Faculty of Engineering,
Ardakan University, P.O. Box 184, Ardakan, Iran
Supplemental data for this article can be accessed here. https://doi.org/10.1080/17415993.2021.1941020
© 2021 Informa UK Limited, trading as Taylor & Francis Group

2
H. KARGAR ET AL.
1. Introduction
Hydrazone, having –C(O)–NH–N=CH– moiety, constitutes a distinctive class of Schiff
bases due to its particular modes of coordination and prodigious applications [1]. The
aroylhydrazones possessing –OH group ortho to C=N group have the ability to manifest
keto–enol tautomerism [2,3]. On the basis of this tautomeric interconversion, aroylhydra-
zones can behave like a neutral, monobasic, dibasic bidentate or tridentate ONO ligand
and have the potential to bind with one or more than one metal centers. Usually, upon
coordination with metal atoms, the deprotonation from OH to NH groups takes place
[4–8].
The coordination chemistry of vanadium and molybdenum acquired considerable
attention during the past few years due to their biochemical significance [9–12]. In spite of
the key role of oxovanadium(V) [VO(V)] and dioxomolybdenum(VI) [MoO2(VI)] com-
plexes with aroylhydrazones in biological fields, their high catalytic activity made them
more glamorous [13–16]. These vanadium and molybdenum based metal complexes have
appreciable utilization in the field of organic chemistry, specifically for the oxidation of var-
ious organic species. These compounds have been extensively studied as oxidation catalysts
for hydroxylation of benzene and phenols [17–19], epoxidation of olefins [20], oxidation
of alcohols [19] and particularly sulfoxidation [21–24–25].
Sulfoxides are of prime importance due to their contribution in biological along with
industrial asymmetric synthetic procedures. Sulfoxides are served as intermediates in for-
mulations and processing of drugs, Swern oxidation [26,27] coupling reactions for the
formation of C–C bond [28,29] and in the Diels–Alder reaction [30]. Numerous kinds
of transition metals were employed in the literature for the sulfoxidation reactions such as
iron, cobalt, titanium and manganese [31,32]. But oxovanadium complexes got an advan-
tage over the rest of the metals because their cyclic voltammetry curves are capable of being
reversed [33]. On the other hand, sulfoxidation by dioxomolybdenum complexes has the

JOURNAL OF SULFUR CHEMISTRY
3
advantages like excellent results under mild reaction conditions and the ability to interact
with various kinds of substrates having diverse functional groups [34].
Many inorganic and organic species are employed for the oxidation processes and the
most common oxygen-transfer reagents include hypochlorite (NaClO), peroxycarboxylic
acids, periodic acid, dioxygen and hydrogen peroxide [35,36]. Among these oxidizing
agents, H2O2 is considered an ideal oxidant because it is clean, non-toxic, cheap and
environmentally friendly [37,38]. Moreover, it is frequently miscible with H2O and many
organic solvents, and produces only water as a by-product during the oxidation process.
So, aqueous H2O2 along with a catalytic amount of various transition metals, specifically
VO(V) and MoO2(VI) complexes, are most frequently used [34,36,39–41].
In the current study, we are describing the synthesis of an ONO-donor aroylhydrazone-
based azomethine compound by condensing nicotinic hydrazide and 5-chlorosalicylal-
dehyde and its vanadium and molybdenum complexes. On the basis of our interest
in oxidation reactions [42–46] and keeping in mind the requirements of new effective
catalytic systems, we have undertaken the exploration of new VO(V) and MoO2(VI)
complexes for the oxidation of sulfides by employing 30% aqueous H2O2.
2. Results and discussion
2.1. Synthesis
An ONO donor Schiff base ligand (H2L) has been synthesized in an alcoholic environment
via condensation reaction by employing equimolar quantities of nicotinic hydrazide and
5-chlorosalicylaldehyde. The synthesized ligand was then treated separately with acety-
acetonate salts of vanadium [VO(acac)2] and molybdenum [MoO2(acac)2] in MeOH
to generate the desired metal complexes, as given in Scheme 1. The synthesized metal
Scheme 1. Synthetic route of H2L Schiff base ligand, VOL and MoO2L complexes.

4
H. KARGAR ET AL.
Scheme 2. Oxidation of sulfides with H2O2 carried out by homogeneous catalysts, VOL and MoO2L.
complexes were then engaged in sulfoxidation to investigate their catalytic capabilities in
the presence of aqueous hydrogen peroxide as presented in Scheme 2.
2.2. Structure description
A graphical representation of ligand (H2L) is shown in the form of Figure 1. The exact
molecular structure of the ligand is determined and investigated with the help of the DFT
method by utilizing the B3LYP/Def2-TZVP level of theory both in gaseous and in solution
forms. The geometrically optimized shapes of four possible configurations of the ligand
including Z and E stereoisomers each containing two tautomeric forms, enol and keto, are
shown in Figure S1. The summation of the electronic and the zero-point energy (EZPE),
Gibbs free energy (G) and enthalpy (H) of four possible geometrical structures is given in
Table S1. It was revealed from the data that the keto form of the stereoisomer E is more sta-
ble in its both gaseous and solution forms. The inferred results about the stability were
further supported by the results obtained from single-crystal analysis. Table S2 gives a
brief illustration of the actual (experimental) and theoretically calculated bond angles and
bond lengths of the prepared ligand (H2L). The slight variation between the experimental
and computational parameters may be ascribed on the basis of the fact that a solid crys-
talline sample is used for the collection of experimental data while computational details
are collected by taking into account an isolated single gaseous molecule.
2.3. FT-IR study
Infrared spectral studies give a brief account of points of attachment of ligand with the
metal atoms. Figure S2 shows a spectral representation of the ligand and its corresponding
Figure 1. ORTEP view of the asymmetric unit of H2L ligand. Thermal ellipsoids are drawn at the 50%
probability level, while the size of the hydrogen is shown arbitrarily.

JOURNAL OF SULFUR CHEMISTRY
5
metal complexes. The absorption spectra of the ligand show two prominent peaks i.e. 1676
and 3277 cm−1, which are assigned to the ν(C = O) and ν(NH) moieties. The aforesaid
peaks disappear in the FT-IR spectra of the oxovanadium and dioxomolybdenum com-
plexes supporting the process of enolization of the amide group and a loss of proton to get
attached with the metal atoms. The next point of attachment of ligand H2L with the metal
center is Schiff base linkage which is evident by transferring of its signal from 1616 cm−1
.
The emergence of new peaks close to the fingerprint region at 1247 and 1292 cm−1 is
assigned to the enolic ν(C–O) group in oxovanadium and dioxomolybdenum compounds,
accordingly. Furthermore, the dioxomolybdenum moiety [cis-Mo(O)2] shows its particu-
lar peaks at 916 and 933 cm−1, which are strongly supported by the previously outlined
analogous structures from the literature [47,48]. Likewise, V=O peak, which is the charac-
teristic for the oxovanadium complexes observed at 952 cm−1, is also comparable with the
structurally equivalent compounds delineated earlier [49–51]. Coordination of ligand with
metals is further assisted by the emergence of metal–oxygen and metal–nitrogen peaks at
572 and 478 cm−1 for dioxomolybdenum compound and at 580 and 484 cm−1 for oxo-
vanadium compound, accordingly [49–52]. A comparative comprehensive analysis of the
actual and theoretically calculated stretching vibrational peaks of the ligand is given in
Table S3. It is revealed from the data that the results obtained from both kinds of parameters
are closely related to each other.
2.4. 1H and 13C NMR studies
The nuclear magnetic studies are further helpful in confirming the mode of attachment
of ligand upon complexation. The prepared compounds were easily soluble in dimethyl
sulfoxide; hence, its deuterated form (DMSO-d6) was selected to measure the nuclear mag-
netic resonances (Figures S3–S8). The two very significant signals visible at δ = 11.37 and
1
12.32 ppm in the H NMR spectra of H2L were allocated to the iminic (NH) and phe-
nolic (OH) protons, correspondingly. The disappearance of these signals on reacting with
metal centers is the confirmation of the fact that phenolic and enolic oxygens are involved
in bonding with metal atoms. It also gives an indication of keto-imine tautomerism dur-
ing complex formation. Furthermore, a downfield shifting of signal for azomethine proton
was spotted from δ = 8.63 ppm to δ = 8.75 in VOL and δ = 8.98 ppm in MoO2L, which
is due to the reduction of electronic density at this site due to the involvement azomethine
nitrogen in the establishment of bond with the metal atom. The aromatic protons gave
their peaks in the anticipated area ranging from δ = 6.96 to 9.10 ppm. The influence of
coordination on these aromatic protons is negligible as there is an insignificant change in
chemical shift values upon complexation.
In 13C NMR spectra, the characteristic peaks for carbonyl (C=O), phenolic (C–OH)
and methine ( =C–) carbons in the oxovanadium(V) complex were detected at δ = 167.2,
158.7 and 155.4 ppm, accordingly. The same kinds of peaks in the dioxomolybdenum com-
plex were also noticed in a nearly comparable range with the oxovanadium complex at
δ = 167.7, 158.1 and 155.7 ppm, correspondingly. There is an appreciable change in the
NMR signals of the carbon atoms, existing in the close territory of coordinated atoms i.e.
C7, C1 and C8, which gives a confirmation of change in the electronic cloud due to the
involvement of these moieties in complexation. The appearance of peaks for aromatic car-
bon atoms of all of the synthesized compounds in their corresponding portions confirms

6
H. KARGAR ET AL.
the earlier documented reports. All of the chemicals shift values whether experimental or
calculated of the prepared compounds are listed in Table S4. It is revealed from the data
that both kinds of investigations are supportive to each other and hence strengthens the
concept of modes of coordination of ligand with metals.
2.5. Mulliken atomic charges
From Mulliken’s charge analysis of H2L (Table S5), it is evident that the negative charge
densities in the ligand are mostly located on phenolic and carbonyl oxygens, and the carbon
atoms connected with them (C1 and C8) have the highest charge densities.
2.6. Electronic properties studies
The elaboration of chemical reactivities of the molecules and their affiliations with physi-
cal and chemical properties can be investigated by using molecular electrostatic potential
(MEP). The MEP studies are helpful for the location of electrophilic and nucleophilic sites
of the molecules on the basis of their electrostatic potentials. These sites are denoted either
by blue (electrophilic) or by red (nucleophilic) colors. The molecular electrostatic poten-
tial diagrams and the data of electrostatic potentials of four possible configurations of
the ligand are presented in Figure S9. As it can be seen from the figure that in the keto
form of E stereoisomer, the negative site with an electrostatic value of ∼55 kcal mol−1
,
present between the oxygen atoms of the carbonyl (C=O) and phenolic (–OH) groups, is
marked by red color. Therefore, these atoms are the best candidates for nucleophilic attack
on the metal atom to form a complex. The obtained data are completely compatible with
Mulliken’s atomic charges distribution and are given in Table S5.
To get the information about the electronic distribution of the frontier molecular
orbitals, the highest occupied molecular orbitals (HOMO) and the lowest unoccupied
molecular orbitals (LUMO) energy levels of the H2L were calculated by the B3LYP/Def2-
TZVP level of theory. The HOMO and LUMO energies for four possible configurations
of the ligand and their energy gaps (ꢀE) are presented in Figure S10. In the keto form
of E stereoisomer, the HOMO is localized mostly around the aryl ring, azomethine and
carbonyl groups, while the LUMO is distributed over almost all of the atoms.
2.7. Catalytic activity studies
The catalytic efficiencies of the oxovanadium and dioxomolybdenum complexes were eval-
uated through the sulfoxidation reactions. The reaction conditions were optimized by
selecting a model reaction involving the oxidation of 1 mmol of diphenyl sulfide [(C6H5)2]
by using 2 mmol of H2O2.
2.7.1. Effect of solvent
Several different kinds of solvents, like C2H5OH, CH3OH, CH3CN, (CH3)2CO, 1,2-
dichloroethane, CH3Cl, CCl4 and dichloromethane (DCM) were examined by using H2O2
as an oxidant, and VOL and MoO2L as catalysts, to sort out the most suitable solvent.
Although it was revealed from the data that the reaction was carried out well in methanol

JOURNAL OF SULFUR CHEMISTRY
7
and acetonitrile, the ecofriendly nature of ethanol persuaded us to choose it as the reaction
solvent (Figure S11).
2.7.2. Effect of temperature
A series of experiments, performed in ethanol at different temperatures, indicated that the
reaction was completed only under reflux conditions for both catalysts (Figure S12).
2.7.3. Effect of oxidant
The effect of various kinds of oxidants like NaIO4, H2O2, urea H2O2 (UHP), (Bu)4NIO4
and tert-BuOOH was explored for the oxidation of (C6H5)2 by using VOL and MoO2L
as catalysts. It was inferred from the results obtained that H2O2 can be regarded as the
best source for the provision of oxygen. Furthermore, the sulfoxidation can’t be carried
out without the use of oxidant (Figure S13).
Although diphenyl sulfoxide was obtained as the only product by using <2 mmol of
oxidant, the conversion could not be completed even at a longer period of time. Therefore,
at least 2 mmol of H2O2 was selected as the optimized amount for oxidizing 1 mmol of
sulfide.
2.7.4. Optimization of catalytic amount
The optimization of the amount of catalyst is also very important for developing a catalytic
cycle; in this regard, the effect of various amounts of catalysts was explored and the data
are presented in Figure S14. It was inferred from the results that the reaction could not be
proceeded without the use of a catalyst. The amount of catalyst has a direct relationship
with the rate of reaction until the optimum catalytic amount is involved, i.e. 0.004 mmol
for VOL and 0.006 mmol for MoO2L, for sulfoxidation of 1 mmol of (C6H5)2S.
2.7.5. Oxidation of sulfides
To explore the generalization of the developed procedure, a variety of aliphatic and aro-
matic sulfides were oxidized under optimal conditions to produce respective sulfoxides
and sulfones (Table 1). The data in the table directed us to conclude that the majority of
the sulfides were entirely transformed into their respective products, and in many cases,
two different reaction times have been reported in which the time taken by VOL com-
plex was less than that of MoO2L complex. Also, oxovanadium catalyst showed higher
selectivity for the sulfoxide formation over the sulfone. It was found that in these reaction
conditions, sulfoxides were produced first which were gradually converted into sulfones
with the increase in reaction time.
2.7.6. Mechanism
Based on previously reported mechanisms [53], for the oxidation carried out by oxometal
complexes, a probable mechanism for the oxidation of sulfides to the respective sulfox-
ides and sulfones with H2O2 catalyzed by MoO2L is proposed in Scheme 3. It seems that
the metal atom of the catalyst, MoO2L, can be attacked by nucleophiles. Therefore, on
the treatment of molybdenum complexes with H2O2, a peroxomolybdenum moiety is
formed. This intermediate is vital for the oxidation of sulfides to sulfoxides via a single-
step oxygen-transfer reaction. The sulfur atom attacks electrophilically on the oxygen of

8
H. KARGAR ET AL.
Table 1. Oxidation of sulfides with H2O2 catalyzed by VOL and MoO2L complexes.a
VOL complex
MoO2L complex
Entry
1
Sulfide
Time (min) Sulfoxide (%)b,c Sulfone (%)b,c Time (min) Sulfoxide (%)b,c Sulfone (%)b,c
30
100
0
30
95
5
2
3
60
5
70
100
30
0
70
5
80
95
20
5
4
5
6
10
15
15
100
100
90
0
0
10
50
15
70
80
95
30
20
5
10
7
8
30
90
0
10
0
35
75
50
25
50
120
210
aReaction conditions: sulfide (1 mmol), H2O2 (2 mmol), EtOH (10 mL), catalyst (VOL = 0.004 mmol or MoO2L =
0.006 mmol).
bGC (entries 1–4) or isolated yield (entries 5–8).
cAll products were identified by comparison of their physical and spectral data with those of authentic samples.
Scheme 3. Plausible mechanism for the oxidation of sulfides with H2O2 catalyzed by VOL complex.
peroxide followed by the breaking the O–O bond, simultaneously, which causes the trans-
fer of oxygen to sulfide to convert it into sulfoxide. Further oxidation, according to the
same mechanism, leads to the formation of sulfone. The same kind of mechanism is also
proposed for the VOL complex.

JOURNAL OF SULFUR CHEMISTRY
9
3. Conclusion
The current work is based on the synthesis of an ONO-donor tridentate Schiff base and its
oxovanadium and dioxomolybdenum complexes, and their characterization by using ele-
mental and various spectroscopic techniques. Only ligand was isolated in the form of single
crystal; hence, its theoretical calculations were performed by employing DFT method with
the B3LYP/Def2-TZVP level of theory. The computational data for four possible config-
urations of the ligand including Z and E stereoisomers were also calculated. The exact
molecular structure of H2L ligand determined by single-crystal X-ray crystallography is
in agreement with the theoretical results for the keto form of E stereoisomer. Moreover,
after preparation and characterization of the VOL and MoO2L complexes, their catalytic
activities were investigated for the oxidation of sulfides by using H2O2 in ethanol under
reflux conditions. The metal centers of the catalysts are attacked by the hydrogen perox-
ide to form intermediates which are further ambushed by the sulfur to give sulfoxides
by breaking O–O bond. The results showed that oxovanadium catalyst has higher selec-
tivity for the sulfoxide formation over the sulfone. In the future, these catalysts can be
employed industrially for hydroxylation, epoxidation and selective oxidation of benzylic
alcohols.
4. Experimental
4.1. Materials and methods
All of the chemicals, solvents and reagents mentioned in the current research work were of
laboratory/analytical grades and used without adapting any additional purification meth-
ods. Percentage composition of C, H and N was calculated by combustion analysis by
operating Heraeus CHN-O-FLASH EA 1112 instrument. Nuclear magnetic studies of 13
C
and 1H were performed by BRUKER AVANCE 400 MHz spectrometer by taking tetram-
ethylsilane (TMS) as a standard reference material to be used internally. All chemical
shift values (δ), assigned with respect to TMS, are mentioned in ppm. Infrared spec-
tra were recorded by making the pellets of samples by mixing them with KBr with the
assistance of an FT-IR Prestige21 spectrophotometer. The data of X-ray diffraction anal-
ysis were recorded on a STOE IPDS-II diffractometer, which generates Mo-Kα radiations
monochromated by the graphite.
4.2. Synthesis
4.2.1. Synthesis of a tridentate (ONO) Schiff base ligand (H2L)
To a 15 mL of hot methanolic solution of 5-chlorosalicylaldehyde (10 mmol, 1.57 g), a
15 mL methanolic solution of Nicotinic hydrazide (10 mmol, 1.37 g) was mixed dropwise.
The resultant solution was then refluxed by using a water bath for ∼3 h by using a mag-
netically equipped hot plate. After that, the contents of the mixture were left as such to
procure the room temperature. The precipitates of the anticipated Schiff base product
(H2L) appeared which were then separated by filtration and washed in triplicate with cold
MeOH for the purpose of purification. The obtained product was further dissolved in hot
MeOH to prepare a saturated solution for the growth of single crystals appropriate for the
X-ray single-crystal diffraction analysis.

10
H. KARGAR ET AL.
H2L: Yield 73%. Anal. Calc. for C13H10ClN3O2: C, 56.64, H, 3.66; N, 15.24, Found: C,
56.75; H, 3.71; N, 15.15%. FT-IR (KBr, cm−1); 3277 (νN–H); 1676 (νC=O); 1616 (νC=N);
1591, 1477 (νC=C); 1186 (νC–O); 1026 (νN–N).1H NMR (400 MHz, DMSO-d6, ppm):
6.96–9.10 [7 H, m, Ar-H], 8.63 [1 H, s, (CH =N)], 11.17 [1 H, s, (–NH)], 12.32 [1 H, s,
(–OH)]. 13C NMR (100 MHz, DMSO-d6, ppm): 118.2, 120.6, 123.0, 123.6, 127.4, 128.6,
130.9, 135.4, 146.1, 148.6, 152.5, 156.0, 161.5.
4.2.2. Synthesis of VOL complex
The oxovanadium complex VOL was synthesized by treating equimolar quantities of
[VIVO(acac)2] (0.5 mmol, 0.132 g, acac = acetylacetonate) with H2L (0.5 mmol, 0.138 g)
after dissolving them in hot CH3OH (25 mL), separately. The mixed stuff was kept under
the refluxed condition at least for 3 h until the oxovanadium(V) complex was settled down
at the bottom which was then filtered off and washed sensibly with H2O, CH3OH and
(C2H5)2O and eventually dried in a desiccator by employing anhydrous CaCl2.
VOL: Yield 66%. Anal. Calc. for C14H11ClN3O4V: C, 45.25; H, 2.98; N, 11.31, Found:
C, 45.34; H, 3.03; N, 11.22%. FT-IR (KBr, cm−1); 1600 (νC=N); 1352 (νC=N−N=C); 1456,
1544 (νC=C); 1292 (νC–O); 1014 (νN−N); 952(νV=O); 580 (νV−O); 484 (νV−N).1H NMR
(400 MHz, DMSO-d6, ppm): 3.19 [3 H, s, (–OCH3)], 7.01–9.16 [7 H, m, Ar-H], 8.76 [1
H, s, (CH=N)]. 13C NMR (100 MHz, DMSO-d6, ppm): 65.7, 119.1, 120.7, 121.7, 123.9,
125.7, 126.2, 135.5, 147.7, 148.8, 149.5, 155.4, 158.7, 167.2.
4.2.3. Synthesis of MoO2L complex
The same procedure was repeated for the preparation of dioxomolybdenum(VI) complex
(MoO2L), as described above just by replacing the vanadium precursor with molybdenum
salt [MoVIO2(acac)2]. In this case, the product was obtained by employing an ice bath for
the purpose of cooling.
MoO2L: Yield 72%. Anal. Calc. for C14H12ClMoN3O5: C, 38.77; H, 2.79; N, 9.69, Found:
C, 38.96; H, 2.86; N, 9.53%. FT-IR (KBr, cm−1); 1614 (νC=N); 1427 (νC=N−N=C); 1469,
1546 (νC=C); 1247 (νC–O); 1035 (νN–N); 933 (νO=Mo=O) asym; 916 (νO=Mo=O) sym; 572
(νMo−O); 478 (νMo−N).1H NMR (400 MHz, DMSO-d6, ppm): 3.37 [3 H, d, (–OCH3)], 4.07
[1 H, q, (–OH)], 7.01–9.15 [7 H, m, Ar-H], 8.98 [1 H, s, (CH=N)]. 13C NMR (100 MHz,
DMSO-d6, ppm): 65.5, 120.6, 121.5, 124.0, 124.9, 125.9, 133.0, 134.4, 135.5, 148.8, 152.5,
155.7, 158.1, 167.7.
4.3. X-ray crystallographic data collection H2L ligand
The diffraction analysis of the ligand H2L was performed with the help of STOE IPDS-
II diffractometer, which employs graphite-based monochromator to generate Mo-Kα
radiations.
The details of diffractions were obtained at 298(2) K in a series of ω-scans in 1° oscil-
lations and solved with the help of Stöe X-AREA [54] software package. A numerical
absorption correction was applied using the X-RED [55] and X-SHAPE [56] software for
the compound under investigation. The results were amended for Lorentz and polariz-
ing effects. The molecular structure of the ligand was resolved by direct procedures using
SIR2004 [57]. All of the atoms except hydrogen atoms were refined anisotropically by the

JOURNAL OF SULFUR CHEMISTRY
11
full-matrix least-squares method on F2 using SHELXL [58]. Table S6 gives a brief overview
of diffraction data collected from H2L.
4.4. Computational details
The calculations based on density functional theory (DFT) were accomplished by employ-
ing Gaussian 09 package[59] at the B3LYP level of theory [60] with the help of the
Def2-TZVP basis set [61]. The solution phase modulation was processed with the help
of IEFPCM by considering the solvent [62]. The optimization of geometrical parameters
was analyzed by frequency analysis to make sure that they are at the local minima on
the molecular potential energy surface (PES). It was revealed from the data that there is
no imaginary frequency. The multinuclear (1H and 13C) NMR magnetic isotropic shield-
ing tensors were obtained by the standard Gauge-Independent Atomic Orbital (GIAO)
approach in the solution phase [63]. The chemical shifts of the ligand were calculated with
the help of the B3LYP/Def2-TZVP level and IEFPCM model as an implicit model of sol-
vent and equated with the actual experimental results collected in deuterated dimethyl
sulfoxide. The same solvent was employed for all IEFPCM calculations for H2L and tetram-
ethylsilane. The chemical shift values (δ) were calculated by subtracting the appropriate
isotropic part of the shielding tensor from that of tetramethylsilane δi = σ
– σi. The
TMS
isotropic shielding constants for tetramethylsilane calculated in the solution phase at the
B3LYP/Def2-TZVP level of theory were equal to 31.92 and 184.52 ppm for the 1H nuclei
and the 13C nuclei, accordingly. The Chemissian program was utilized for the prepara-
tion of contour plots of the highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) [64].
4.5. Catalysis experiments
In a typical experiment, to a 1 millimolar solution of diphenyl sulfide and VOL
(0.004 mmol) or MoO2L (0.006 mmol) in EtOH (10 mL), 30% aqueous H2O2 (2 mmol)
was mixed with the reacting stuff and refluxed along with vigorous stirring for a definite
period of time and the results are given in Table 1. The monitoring of the catalytic cycle
was ensured by TLC (eluent, n-hexane:ethyl acetate, 5:2) while the percentage yield was
determined with the help of gas chromatography. The products were then purified with
the help of liquid chromatography by using silica gel as a stationary phase and a mixture of
n-hexane and ethyl acetate (70:30) as a mobile phase. The spectroscopic data of the purified
products were then compared with the available data bank of standard samples for their
characterization.
Acknowledgments
We gratefully acknowledge the practical support of this study by Ardakan University and Payame
Noor University.
Disclosure statement
No potential conflict of interest was reported by the author(s).

12
H. KARGAR ET AL.
Supplementary materials
CCDC 2056759 contains the supplementary crystallographic data of the Schiff base ligand.
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.
ORCID
Hadi Kargar http://orcid.org/0000-0002-7817-0937
Atefeh Moghimi http://orcid.org/0000-0003-3784-6205
Mehdi Fallah-Mehrjardi http://orcid.org/0000-0001-9780-6235
Reza Behjatmanesh-Ardakani http://orcid.org/0000-0001-9163-9333
Hadi Amiri Rudbari http://orcid.org/0000-0002-3020-8596
Khurram Shahzad Munawar http://orcid.org/0000-0001-9055-2519
References
[1] Zhang XZ, Li LX, Li HH, et al. Synthesis, characterization and crystal structures of oxovana-
dium (V) complexes derived from similar aroylhydrazone ligands. Bull Chem Soc Ethiop.
2015;29:423–430.
[2] Hossain SM, Lakma A, Pradhan RN, et al. Valence directed binding mode of [2×2] iron grids
of an unsymmetrical picolinic hydrazone based ligand. Dalton Trans. 2017;46:12612–12618.
[3] Galić N, Rubčić M, Magdić K, et al. Solution and solid-state studies of complexation of transi-
tion metal cations and Al(III) by aroylhydrazones derived from nicotinic acid hydrazide. Inorg
Chim Acta. 2011;366:98–104.
[4] Mudi PK, Bandopadhyay N, Joshi M, et al. Schiff base triggering synthesis of copper(II) com-
plex and its catalytic fate towards mimics of phenoxazinone synthase activity. Inorg Chim Acta.
2020;505:119468.
[5] Pal CK, Mahato S, Joshi M, et al. Transesterification activity by a zinc(II)-Schiff base complex
with theoretical interpretation. Inorg Chim Acta. 2020;506:119541.
[6] Mahato S, Meheta N, Kotakonda M, et al. Ligand directed synthesis of a unprecedented tetrag-
onalbipyramidal copper (II) complex and its antibacterial activity and catalytic role in oxidative
dimerisation of 2-aminophenol. Appl Organomet Chem. 2020;34:e5935.
[7] Roy S, Paul P, Karar M, et al. Cascade detection of fluoride and bisulphate ions by newly
developed hydrazine functionalised Schiff bases. J Mol Liq. 2021;326:115293.
[8] Mahato S, Meheta N, Kotakonda M, et al. Synthesis, structure, polyphenol oxidase mimicking
and bactericidal activity of a zinc-Schiff base complex. Polyhedron. 2021;194:114933.
[9] Kumar A, Dixit A, Sahoo S, et al. Crystal structure, DNA crosslinking and photo-induced
cytotoxicity of oxovanadium (IV) conjugates of boron-dipyrromethene. J Inorg Biochem.
2020;202:110817.
[10] Munawar KS, Ali S, Tahir MN, et al. Synthesis, spectroscopic characterization, X-ray crystal
structure, antimicrobial, DNA-binding, alkaline phosphatase and insulin-mimetic studies of
oxidovanadium (IV) complexes of azomethine precursors. J Coord Chem. 2020;73:2275–2300.
[11] Munawar KS, Ali S, Tahir MN, et al. Investigation of derivatized Schiff base ligands of 1,2,4-
triazole amine and their oxovanadium (IV) complexes: synthesis, structure, DNA binding,
alkaline phosphatase inhibition, biological screening, and insulin mimetic properties. Russ J
Gen Chem. 2015;85:2183–2197.
[12] Adam MS, Ahmed MS, El-Hady OM, et al. Bis-dioxomolybdenum (VI) oxalyldihydrazone
complexes: synthesis, characterization, DFT studies, catalytic epoxidation potential, molecular
modeling and biological evaluations. Appl Organomet Chem. 2020;34:e5573.

JOURNAL OF SULFUR CHEMISTRY
13
[13] Nair RS, Kuriakose M, Somasundaram V, et al. The molecular response of vanadium complexes
of nicotinoyl hydrazone in cervical cancers—a possible interference with HPV oncogenic
markers. Life Sci. 2014;116:90–97.
[14] Li AM. Vanadium(V) complexes with hydrazone and benzohydroxamate ligands: synthesis,
structures and catalytic epoxidation. J Coord Chem. 2014;67:2076–2085.
[15] Liu HY, Zang LQ, Lv JL. Synthesis, structure, and catalytic oxidation of a molybdenum(VI)
complex [Moo2(CH3OH)L]. Russ J Coord Chem. 2015;41:451–455.
[16] Liang M, Zou DH. A dioxidomolybdenum (VI) complex of tridentate ONO aroylhy-
drazone ligand: synthesis, crystal structure and catalytic activity. Inorg Nano-Met Chem.
2017;47:110–115.
[17] Maurya MR, Kumar M, Titinchi SJJ, et al. Oxovanadium(IV) Schiff base complexes encap-
sulated in zeolite-Y as catalysts for the liquid-phase hydroxylation of phenol. Catal Lett.
2003;86:97–105.
[18] Alsalim TA, Hadi JS, Al-Nasir EA, et al. Hydroxylation of phenol catalyzed by oxo-
vanadium(IV) of salen-type schiff base complexes with hydrogen peroxide. Catal Lett.
2010;136:228–233.
[19] Maurya MR, Agarwal S, Bader C, et al. Synthesis, characterisation and catalytic potential of
hydrazonatovanadium(v) model complexes with [VO]3+ and [VO2]+ cores. Dalton Trans.
2005: 537–544.
[20] Walmsley RS, Tshentu ZR. Imidazole-based vanadium complexes as haloperoxidase models
for oxidation reactions. S Afr J Chem. 2010;63:95–104.
[21] Bunce S, Cross RJ, Farrugia LJ, et al. Chiral Schiff base complexes of copper (II), vanadium
(IV) and nickel (II) as oxidation catalysts. X-ray crystal structures of [Cu(R-salpn)(OH2)] and
[Cu( -busalcx)]. Polyhedron. 1998;17:4179–4187.
[22] Pramanik M, Bhaumik A. Self-assembled hybrid molybdenum phosphonate porous nano-
materials and their catalytic activity for the synthesis of benzimidazoles. Chem Cat Chem.
2014;6:2577–2586.
[23] Moradi-Shoeili Z, Zare M, Bagherzadeh M, et al. Synthesis, structural characterization, and
catalytic reactivity of a new molybdenum(VI) complex containing 1,3,4-thiadiazole derivative
as a tridentate NNO donor ligand. J Coord Chem. 2015;68:548–559.
[24] Baskaran T, Kumaravel R, Christopher J, et al. An environmentally friendly route for grafting of
molybdenum carbonyl onto a diaminosilane-modified SBA-15 molecular sieve and its catalytic
behaviour in olefin epoxidation. New J Chem. 2015;39:3758–3764.
[25] de Azevedo Marques AP, Dockal ER, Skrobot FC, et al. Synthesis, characterization and cat-
alytic study of [N,Nꢀ-bis(3-ethoxysalicylidene)-mxylylenediamine]Oxovanadium(IV) com-
plex. Inorg Chem Commun. 2007;10:255–261.
[26] Ferńandez I, Khiar N. Recent developments in the synthesis and utilization of chiral sulfoxides.
Chem Rev. 2003;103:3651–3705.
[27] Khenkin AM, Neumann R. Oxygen transfer from sulfoxides: oxidation of alkylarenes catalyzed
by a polyoxomolybdate, [PMo12o40]3−. J Am Chem Soc. 2002;124:4198–4199.
[28] Baig N, Madduluri VK, Sah AK. Selective oxidation of organic sulfides to sulfoxides using
sugar derived cis-dioxo molybdenum (vi) complexes: kinetic and mechanistic studies. RSC
Adv. 2016;6:28015–28022.
[29] Hua DH. In: V Snieckus, editor. Advances in carbanion chemistry. London: JAI Press; 1992.
[30] Renaud P, Gerster M. Use of lewis acids in free radical reactions. Angew Chem, Int Ed.
1998;37:2562–2579.
[31] Wang YZ, Wang P, Li C, et al. Synthesis, crystal structure and catalytic property of
an oxovanadium(v) complex derived from Nꢀ-(3,5-ditertbutyl-2-hydroxybenzylidene)−4-
methoxybenzohydrazide. Synth React Inorg, Met-Org, Nano-Met Chem. 2016;46:927–930.
[32] Suresh P, Srimurugan S, Babu B, et al. Asymmetric sulfoxidation of prochiral sulfides using
aminoalcohol derived chiral C-3-symmetric trinuclear vanadium Schiff base complexes. Tetra-
hedron Asymmetry. 2007;18:2820–2827.
[33] Zabardasti A, Shangaie SA. Vanadium(IV) complexes with Schiff base ligands derived from
2, 3-diaminopyridine as catalyst for the oxidation of sulfides to sulfoxides with H2O2. J Iran
Chem Soc. 2019;16:57–64.

14
H. KARGAR ET AL.
[34] Chakravarthy RD, Suresh K, Ramkumar V, et al. New chiral molybdenum complex catalyzed
sulfide oxidation with hydrogen peroxide. Inorg Chim Acta. 2011;376:57–63.
[35] Fomenko IS, Gushchin AL, Abramov PA, et al. New oxidovanadium(IV) complexes with 2,2ꢀ-
bipyridine and 1,10-phenathroline ligands: synthesis, structure and high catalytic activity in
oxidations of alkanes and alcohols with peroxides. Catalysts. 2019;9:217–238.
[36] Srour H, Maux PL, Chevance S, et al. Metal-catalyzed asymmetric sulfoxidation, epoxidation
and hydroxylation by hydrogen peroxide. Coord Chem Rev. 2013;257:3030–3050.
[37] Boruah JJ, Das SP. Solventless, selective and catalytic oxidation of primary, secondary and
benzylic alcohols by a Merrifield resin supported molybdenum(vi) complex with H2O2 as an
oxidant. RSC Adv. 2018;8:34491–34504.
[38] Veisi H, Rashtiani A, Rostami A, et al. Chemo-selective oxidation of sulfide to sulfox-
ides with H2O2 catalyzed by oxo-vanadium/Schiff-base complex immobilized on modified
magnetic Fe3O4 nanoparticles as a heterogeneous and recyclable nanocatalyst. Polyhedron.
2019;157:358–366.
[39] Hadigavabar AD, Tabatabaeian K, Zanjanchi MA, et al. Efficient molybdenum catalyzed
chemoselective, solvent-free oxidation of sulfides to sulfones at room temperature. J Chil Chem
Soc. 2018;63:3829–3833.
[40] Cantwell KE, Fanwick PE, Abu-Omar MM. Mild, selective sulfoxidation with molybde-
num(VI) cis-dioxo catalysts. ACS Omega. 2017;2:1778–1785.
[41] Langeslay RR, Kaphan DM, Marshall CL, et al. Catalytic applications of vanadium: a mecha-
nistic perspective. Chem Rev. 2019;119:2128–2191.
[42] Hatefi M, Moghadam M, Sheikhshoaei I, et al. Ru(salophen)Cl supported on polystyrene-
bound imidazole: an efficient and robust heterogeneous catalyst for epoxidation of alkenes with
sodium periodate. Appl Catal A Gen. 2009;370:66–71.
[43] Moghadam M, Mirkhani V, Tangestaninejad S, et al. Highly efficient epoxidation of alkenes
with sodium periodate catalyzed by reusable polystyrene-bound ruthenium(III)salophen. J
Iran Chem Soc. 2011;8:1019–1029.
[44] Kargar H. Rapid and efficient biomimetic oxidation of 2-imidazolines to their corre-
sponding imidazoles with NaIO4 catalyzed by Mn(salophen)Cl. Inorg Chem Commun.
2011;14:863–866.
[45] Kargar H. Synthesis, characterization and crystal structure of a manganese(III) Schiff base
complex and investigation of its catalytic activity in the oxidation of benzylic alcohols. Transit
Met Chem. 2014;39:811–817.
[46] Javadi M M, Moghadam M, Mohammadpoor-Baltork I, et al. Oxidation of alkenes and sulfides
catalyzed by a new binuclear molybdenum bis-oxazoline complex. Polyhedron. 2014;72:19–26.
[47] Vrdoljak V, Prugovecki B, Calogovic DM, et al. Supramolecular hexagon and chain coordina-
2+
tion polymer containing the MoO2 core: structural transformation in the solid state. Cryst
Growth Des. 2011;11:1244–1252.
[48] Kia R, Kargar H. Synthesis, spectral characterization and crystal structure studies of
a new hydrazone Schiff base and its dioxomolybdenum(VI) complex. J Coord Chem.
2015;68:1441–1451.
[49] Kurbah SD, Asthana M, Syiemlieh I, et al. New dioxido-vanadium(V) complexes contain-
ing hydrazone ligands: syntheses, crystal structure and their catalytic application toward C–H
bond functionalization. J Organomet Chem. 2018;876:10–16.
[50] Li Y, Xu L, Duan M, et al. An acetohydroxamate-coordinated oxidovanadium(V) complex
derived from pyridinohydrazone ligand with urease inhibitory activity. Inorg Chem Commun.
2019;105:212–216.
[51] Gao S, Zhang X-F, Huo L-H, et al. (Methanol-κO)[3-methoxysalicylaldehyde (4-methoxyben
zoyl)hydrazonato(2–)-κ3O,Oꢀ,N]Dioxomolybdenum(VI). Acta Crystallogr. 2004;E60:m1731–
m1733.
[52] Rana A, Dinda R, Sengupta P, et al. Synthesis, characterisation and crystal structure of cis-
dioxomolybdenum(VI) complexes of some potentially pentadentate but functionally tridentate
(ONS) donor ligands. Polyhedron. 2002;21:1023–1030.
[53] Ruff F, Fabian A, Farkas O, et al. Mechanism for the oxidation of sulfides and sulfoxides with
periodates: reactivity of the oxidizing species. Eur J Org Chem. 2009: 2102–2111.

JOURNAL OF SULFUR CHEMISTRY
15
[54] X-AREA. Version 1.30, program for the acquisition and analysis of data. Darmstadt: Stoe &
Cie GmbH; 2005.
[55] X-RED. Version 1.28b, program for data reduction and absorption correction. Darmstadt: Stoe
& Cie GmbH; 2005.
[56] X-SHAPE. Version 2.05, program for crystal optimization for numerical absorption correction.
Darmstadt: Stoe & Cie GmbH; 2004.
[57] Burla MC, Caliandro R, Camalli M, et al. An improved tool for crystal structure determination
and refinement. J Appl Crystallogr. 2004;2005(38):381–388.
[58] Sheldrick GM. A short history of SHELX. Acta Crystallogr. 2008;A64:112–122.
[59] Frisch MJ, Trucks GW, Schlegel HB, et al. GAUSSIAN 09 (revision D.01). Wallingford (CT):
Gaussian, Inc.; 2013.
[60] Becke AD. Density-functional thermochemistry. III. The role of exact exchange. J Chem Phys.
1993;98:5648–5652.
[61] Weigend F, Ahlrichs R. Balanced basis sets of split valence, triple zeta valence and quadruple
zeta valence quality for H to Rn: design and assessment of accuracy. Phys Chem Chem Phys.
2005;7:3297–3305.
[62] Tomasi J, Mennucci B, Cammi R. Quantum mechanical continuum solvation models. Chem
Rev. 2005;105:2999–3093.
[63] Gauss J. Effects of electron correlation in the calculation of nuclear magnetic resonance
chemical shifts. J Chem Phys. 1993;99:3629–3643.
[64] http://www.chemissian.com