Oxovanadium and dioxomolybdenum complexes: synthesis, crystal structure, spectroscopic characterization and applications as homogeneous catalysts in sulfoxidation

Article abstract of DOI:10.1080/00958972.2021.1915488

New oxovanadium and dioxomolybdenum Schiff base complexes, [VO(L)(OCH₃)] _n and [MoO₂(L)(CH₃OH)], were synthesized by treating an ONO donor Schiff base (H₂L) derived by condensation of 3-ethoxysalicylaldehyde and nicotinic hydrazide with oxo and dioxo acetylacetonate salts of vanadium and molybdenum (VO(acac)₂ and MoO₂(acac)₂), respectively. The synthesized ligand and complexes were characterized by FTIR, multinuclear (¹H, ¹³C) NMR, elemental and single crystal X-ray diffraction analysis. In both complexes, the geometry around the central metal ions was distorted octahedral as revealed by diffraction studies. Theoretical calculations of the synthesized compounds were carried out by DFT at B3LYP/Def2-TZVP level of theory, which showed good correlation with the experimental results. Moreover, the catalytic efficiency of both complexes was investigated by oxidizing aryl and alkyl sulfides in the presence of 30% H₂O₂ in ethanol.

Full text of DOI:10.1080/00958972.2021.1915488

Journal of Coordination Chemistry
ISSN: (Print) (Online) Journal homepage: https://www.tandfonline.com/loi/gcoo20
Oxovanadium and dioxomolybdenum
complexes: synthesis, crystal structure,
spectroscopic characterization and applications as
homogeneous catalysts in sulfoxidation
Hadi Kargar, Azar Kaka-Naeini, Mehdi Fallah-Mehrjardi, Reza Behjatmanesh-
Ardakani, Hadi Amiri Rudbari & Khurram Shahzad Munawar
To cite this article: Hadi Kargar, Azar Kaka-Naeini, Mehdi Fallah-Mehrjardi, Reza Behjatmanesh-
Ardakani, Hadi Amiri Rudbari & Khurram Shahzad Munawar (2021) Oxovanadium and
dioxomolybdenum complexes: synthesis, crystal structure, spectroscopic characterization and
applications as homogeneous catalysts in sulfoxidation, Journal of Coordination Chemistry,
74:9-10, 1563-1583, DOI: 10.1080/00958972.2021.1915488
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JOURNAL OF COORDINATION CHEMISTRY
2021, VOL. 74, NOS. 9–10, 1563–1583
https://doi.org/10.1080/00958972.2021.1915488
Oxovanadium and dioxomolybdenum complexes: synthesis,
crystal structure, spectroscopic characterization and
applications as homogeneous catalysts in sulfoxidation
Hadi Kargara , Azar Kaka-Naeinib, Mehdi Fallah-Mehrjardib, Reza Behjatmanesh-
Ardakanib, Hadi Amiri Rudbaric and Khurram Shahzad Munaward,e
aDepartment of Chemical Engineering, Faculty of Engineering, Ardakan University, Ardakan, Iran;
c
bDepartment of Chemistry, Payame Noor University, Tehran, Iran; Department of Chemistry,
d
University of Isfahan, Isfahan, Iran; Department of Chemistry, University of Sargodha, Punjab,
e
Pakistan; Department of Chemistry, University of Mianwali, Mianwali, Pakistan
ABSTRACT
ARTICLE HISTORY
Received 9 December 2020
Accepted 19 March 2021
New oxovanadium and dioxomolybdenum Schiff base complexes,
[VO(L)(OCH3)]n and [MoO2(L)(CH3OH)], were synthesized by treating an
ONO donor Schiff base (H2L) derived by condensation of 3-ethoxysali-
cylaldehyde and nicotinic hydrazide with oxo and dioxo acetylaceto-
nate salts of vanadium and molybdenum (VO(acac)2 and MoO2(acac)2),
respectively. The synthesized ligand and complexes were characterized
by FTIR, multinuclear (1H, 13C) NMR, elemental and single crystal X-ray
diffraction analysis. In both complexes, the geometry around the cen-
tral metal ions was distorted octahedral as revealed by diffraction stud-
ies. Theoretical calculations of the synthesized compounds were
carried out by DFT at B3LYP/Def2-TZVP level of theory, which showed
good correlation with the experimental results. Moreover, the catalytic
efficiency of both complexes was investigated by oxidizing aryl and
alkyl sulfides in the presence of 30% H2O2 in ethanol.
KEYWORDS
Oxovanadium(V);
dioxomolybdenum(VI);
tridentate Schiff base;
homogeneous catalysis;
sulfoxidation
CONTACT Hadi Kargar
h.kargar@ardakan.ac.ir, hadi_kargar@yahoo.com
Department of Chemical Engineering,
Faculty of Engineering, Ardakan University, P.O.Box 184, Ardakan, Iran
ß 2021 Informa UK Limited, trading as Taylor & Francis Group

1564
H. KARGAR ET AL.
1. Introduction
Aroylhydrazone-based Schiff base ligands drew much attention during the last few
years [1]. These are a new category of tridentate (ONO) ligands which can coordinate
with various transition metals. Schiff bases derived from aroylhydrazones and their
metal complexes have remarkable applications in biological and industrial fields [2, 3].
These compounds behave as potential pharmacophores for a new series of drugs to
be used in many biological processes like inhibition of enzymes, antibacterial, antifun-
gal, antitumor, antileishmanial and antiparasitic activities [4–7]. Among various metal
complexes, the oxovanadium(V) and dioxomolybdenum(VI) complexes with such tri-
dentate ligands have prominent positions in the development and progress of chemis-
try of coordination compounds [8, 9].
The chemistry of vanadium is very rich as it exists in mononuclear (VO3þ, VO2þ) and
dinuclear (V2O34þ) forms. Vanadium and its Schiff base complexes have applications in
biochemical processes like insulin-mimetic, antimicrobial, antifungal, haloperoxidation,
phosphorylation and alkaline phosphatase inhibition [10–17]. Many of the mononuclear
oxovanadium(V) complexes with tridentate ligands having NNO, NNS and NOO donor
sites show excellent catalytic roles such as C-H activation, epoxidation, oxidation of alco-
hols, oxidative halogenation and sulfoxidation by using O2 and H2O2 [18–23].
Molybdenum is an essential element, present in more than 40 different naturally
occurring enzymes involved in redox reactions. It is important for the fixation and
assimilation of atmospheric nitrogen with the help of bacterial nitrogenase and nitrate
reductase [24]. Moreover, molybdoenzymes, sulfite oxidase and aldehyde oxidase are
used for oxidation of sulfite and aldehyde, respectively [25]. In addition to the import-
ance of molybdenum complexes in biological process, they have potential to be used
as effective catalyst in epoxidation of olefins (styrene and cyclohexane), olefin metath-
esis, isomerization of allylic alcohol, oxidation of sulfides to sulfoxides and oxidation of
amines [26–28]. Coordination of molybdenum with the aroylhydrazones usually gener-
ates MoO2L or MoOL complexes which bear one or two open sites that can be used
to enhance the coordination number by binding with substrate molecules. Due to the
presence of these vacant sites molybdenum complexes can be regarded as template
for numerous enzymatic and catalytic sites [29, 30].
Based on previous research [31–34], we are hereby reporting a new ONO-tridentate
ligand (H2L) prepared by condensing 3-ethoxysalicylaldehyde and nicotinic hydrazide
and oxovanadium [VO(L)(OCH3)]n and dioxomolybdenum [MoO2(L)(CH3OH)] complexes.
2. Experimental
2.1. Materials and methods
All the chemicals employed in the current work were 99.9% pure and purchased from
Sigma-Aldrich and Merck. Elemental analyses were carried out by a Heraeus CHN-O-FLASH
1
EA 1112 instrument. H and 13C NMR spectra were measured at ambient temperature
using a BRUKER AVANCE 400 MHz spectrometer employing tetramethylsilane (TMS) as a
reference. Coupling constant (J) and chemical shift (d) values were reported in Hz and

JOURNAL OF COORDINATION CHEMISTRY
1565
ppm, respectively. Fourier transform infrared spectra of the synthesized compounds were
taken with a FTIR spectrometer IRPrestige21 (Shimadzu) as KBr pellets.
2.2. Synthesis
2.2.1. Synthesis of ONO-tridentate Schiff base ligand (H2L)
Nicotinic hydrazide (1.37 g, 10 mmol) and 3-ethoxysalicylaldehyde (1.66 g, 10 mmol)
were dissolved separately in approximately 25 mL of hot methanol. After complete dis-
solution both solutions were mixed dropwise with continuous stirring. The resulting
mixture was refluxed for 3 h and completion of reaction was ensured by monitoring
with TLC. On allowing the reaction mixture to attain room temperature, the product
precipitated leaving impurities in the solvent. Finally, the desired product was col-
lected by filtration and washed thrice with cold methanol to remove impurities.
H2L: Yield 76%. Anal. Calc. for C15H15N3O3: C, 63.15; H, 5.30; N, 14.73, Found: C,
63.04; H, 5.21; N, 14.85%. FTIR (KBr, cmꢀ1); 3570 (mN–H); 1660 (mC¼O); 1606 (mC¼N); 1477,
1
1591 (mC¼C); 1161 (mC-O); 1024 (mN-N). H NMR (400 MHz DMSO-d6, ppm): 1.36 [3 H, (H-
3
3
3
C15), t, J ¼ 6.9 Hz], 4.07 [4 H, (H-C14), q, J ¼ 6.9 Hz], 6.85 [1 H, (H-C4), t, J ¼ 7.9 Hz],
3
3
7.03 [1 H, (H-C3), d, J ¼ 7.9 Hz], 7.18 [1 H, (H-C5), d, J ¼ 7.9 Hz], 7.59 [1 H, (H-C12), dd,
3J ¼ 7.8 Hz, J ¼ 4.8 Hz], 8.29 [1 H, (H-C13), d, J ¼ 7.8 Hz], 8.67 [1 H, s, (-CH ¼ N)], 8.78
4
3
3
4
4
[1 H, (H-C11), dd, J ¼ 4.8 Hz, J ¼ 1.4 Hz], 9.10 [1 H, (H-C10), d, J ¼ 1.4 Hz], 10.79 [1 H, s,
(-NH)], 12.24 [1 H, s, (-OH)]. 13C NMR (100 MHz, DMSO-d6, ppm): 14.69 (C15), 64.09
(C14) 115.23 (C4), 118.94 (C3), 119.09 (C6), 119.14 (C5), 120.73 (C12), 123.62 (C9),
128.67 (C13), 135.43 (C7), 147.05 (C11), 147.42 (C10), 148.58 (C2), 152.41 (C1),
161.37 (C8).
2.2.2. Synthesis of [VVO(L)(OCH3)]n complex
[VO(L)(OCH3)]n, where L ¼ (E)-N’-(3-ethoxy-2-hydroxybenzylidene)nicotinohydrazide
(H2L), was synthesized by treating [VIVO(acac)2] (1 mmol, 0.265 g, acac ¼ acetylaceto-
nate) with H2L (1 mmol, 0.285 g) in methanol (50 mL). The mixture was refluxed for 3 h
to obtain the precipitate which was filtered off, and then, washed thoroughly with
water, methanol and diethyl ether. The precipitate was dried in vacuo, and then, crys-
tallized from CH3OH to obtain green crystals.
[VO(L)(OCH3)]n: Yield 66%. Anal. Calc. for C16H16N3O5V: C, 50.41; H, 4.23; N, 11.02,
Found: C, 50.34; H, 4.17; N, 11.11%. FTIR (KBr, cmꢀ1); 1604 (mC¼N); 1352 (mC¼N-N¼C);
1475, 1552 (mC¼C); 1249 (mC-O); 1039 (mN-N); 993(mV¼O); 588 (mV-O); 472 (mV-N).
2.2.3. Synthesis of [MoVIO2(L)(CH3OH)] complex
Equimolar amounts of H2L (1 mmol, 0.285 g) and MoO2(acac)2 (1 mmol, 0.330 g) were
suspended in 100 mL of methanol in a round bottom flask containing a magnetic bar
for stirring to attain uniformity. The suspension was refluxed for 3 h and then 2/3rd of
the solvent was evaporated and the remaining solution was cooled over an ice bath
giving orange crystals. The crystals were collected by filtration, and then, washed thor-
oughly with water, methanol and diethyl ether and dried in vacuo.
[MoO2(L)(CH3OH)]: Yield 72%. Anal. Calc. for C16H17MoN3O6: C, 43.35; H, 3.87; N,
9.48, Found: C, 43.21; H, 3.78; N, 9.56%. FTIR (KBr, cmꢀ1); 3034 (mO–H) (coordinated

1566
H. KARGAR ET AL.
Table 1. Crystal data and refinement parameters for V and Mo complexes.
Empirical formula
C16H16N3O5V
C16H17MoN3O6
Formula weight
381.26
443.26
Temperature/K
Crystal system
298(2)
Monoclinic
298(2)
Monoclinic
Space group
P21/c
P21/c
a/Å
11.963(2)
8.2916(17)
b/Å
10.797(2)
26.294(5)
90
99.35(3)
16.046(3)
13.541(3)
90
95.16(3)
90
1794.3(6)
c/Å
a/ꢁ
b/ꢁ
c/ꢁ
90
Volume/Å3
Z
3351.2(12)
8
4
qcalc (mg/mm3)
1.511
0.624
1.641
0.769
m (mm–1
)
F(000)
1568
896
h range for data collection (ꢁ)
2.556 to 24.998
–14 ꢂ h ꢂ 14,
ꢀ12 ꢂ k ꢂ 12, ꢀ31 ꢂ l ꢂ 31
19440
2.466 to 24.991
–9 ꢂ h ꢂ 9, ꢀ19 ꢂ k ꢂ 0, ꢀ16 ꢂ l ꢂ 16
Index ranges
Reflections collected
5698
3150 (0.0265)
3150 / 1 / 240
Independent reflections, R(int)
Data / restraints / parameters
Goodness-of-fit on F2
5884 (0.0957)
5884 / 1 / 453
0.705
0.746
Final R indexes [I > 2r (I)]
Final R indexes [all data]
Largest peak and deepest hole (e.Å–3
R1 ¼ 0.0431, wR2 ¼ 0.0807
R1 ¼ 0.1187, wR2 ¼ 0.0916
–0.369, 0.431
R1 ¼ 0.0281, wR2 ¼ 0.0541
R1 ¼ 0.0520, wR2 ¼ 0.0565
–0.395, 0.606
)
methanol); 1610 (mC¼N); 1448 (mC¼N-N¼C); 1448, 1527 (mC¼C); 1257 (mC-O); 1029 (mN-N);
1
937 (mO¼Mo¼O) asym; 918 (mO¼Mo¼O) sym; 597 (mMo-O); 472 (mMo-N). H NMR (400 MHz,
3
3
DMSO-d6, ppm): 1.35 [3 H, (H-C15), t, J ¼ 6.9 Hz,], 4.08 [4 H, (H-C14), q, J ¼ 6.9 Hz], 7.02
[1 H, (H-C4), t, 3J ¼ 7.9 Hz], 7.26 [1 H, (H-C3), d, 3J ¼ 7.9 Hz], 7.33 [1 H, (H-C5), d,
3J ¼ 7.9 Hz], 7.56 [1 H, (H-C12), dd, 3J ¼ 7.9 Hz, 3J ¼ 4.8 Hz], 8.31 [1 H, (H-C13), dt,
4
3
3J ¼ 7.9 Hz, J ¼ 1.7 Hz], 8.76 [1 H, (H-C11), d, J ¼ 4.8 Hz], 8.98 [1 H, s, (CH ¼ N)], 9.15 [s,
1 H, (H-C10)]. 13C NMR (100 MHz, DMSO-d6, ppm): 14.69 (C15), 64.35 (C14), 118.87 (C4),
120.56 (C3), 121.62 (C6), 123.94 (C5), 125.66 (C12), 126.09 (C9), 135.37 (C13), 147.59
(C1), 148.72 (C2), 149.48 (C10), 152.36 (C11), 156.80 (C7), 167.14 (C8).
2.3. X-Ray crystallographic data collection and structure determination
of the complexes
Single crystal X-ray studies of oxovanadium and dioxomolybdenum complexes were
carried out on a STOE IPDS-II diffractometer using Mo-Ka radiations monochromated
by graphite. The data were collected at 298(2) K in a series of x scans in 1ꢁ oscillations
€
and integrated using the Stoe X-AREA [35] software package. A numerical absorption
correction was applied using X-RED [36] and X-SHAPE [37] software for the Mo com-
plex. For the V complex, absorption corrections based on multiscan were applied [38].
The data were corrected for Lorentz and polarization effects. The structures were
solved by direct methods using SIR2004 [39]. Non-hydrogen atoms were refined aniso-
tropically by full-matrix least-squares on F2 using SHELXL [40]. The crystallographic
data of the complexes are provided in Table 1 while selected bond lengths and angles
are listed in Table 2.

JOURNAL OF COORDINATION CHEMISTRY
1567
ꢀ
Table 2. The selected experimental bond lengths (Å) and angles (ꢁ) in V and Mo complexes.
V complex
V(1)–O(1)
V(1)–O(2)
V(1)–O(3)
V(1)–O(4)
V(1)–N(1)
V(1)–N(3)
N(1)–C(7)
N(1)–N(5)
C(1)–O(1)
C(8)–O(2)
V(2)–O(6)
V(2)–O(7)
V(2)–O(8)
V(2)–O(9)
V(2)–N(4)
V(2)–N(7)
1.862(3)
1.974(3)
1.763(3)
1.578(3)
2.112(4)
2.458(3)
1.294(5)
1.415(4)
1.337(5)
1.304(5)
1.853(3)
1.947(3)
1.594(3)
1.765(3)
2.448(4)
2.134(3)
O(4)–V(1)–O(3)
O(4)–V(1)–O(1)
O(3)–V(1)–O(1)
O(4)–V(1)–O(2)
O(3)–V(1)–O(2)
O(1)–V(1)–O(2)
O(4)–V(1)–N(1)
O(3)–V(1)–N(1)
O(1)–V(1)–N(1)
O(2)–V(1)–N(1)
O(6)–V(2)–N(4)
O(7)–V(2)–N(4)
O(8)–V(2)–N(4)
O(9)–V(2)–N(4)
N(4)–V(2)–N(7)
O(8)–V(2)–N(7)
102.86(16)
100.15(14)
104.99(13)
97.70(13)
91.18(13)
152.45(12)
95.12(15)
158.10(14)
83.70(13)
73.92(13)
80.26(13)
73.80(12)
91.99(15)
163.60(15)
82.54(13)
174.39(14)
Mo complex
Mo(1)–O(1)
Mo(1)–O(2)
Mo(1)–O(3)
Mo(1)–O(4)
Mo(1)–O(6)
Mo(1)–N(2)
N(2)–C(7)
2.012(2)
1.9290(19)
1.695(2)
1.695(3)
2.309(3)
2.235(3)
1.297(4)
1.409(4)
1.313(4)
1.360(4)
O(1)–Mo(1)–O(2)
O(1)–Mo(1)–O(3)
O(1)–Mo(1)–O(4)
O(1)–Mo(1)–O(6)
O(3)–Mo(1)–O(6)
O(4)–Mo(1)–O(6)
O(1)–Mo(1)–N(2)
O(2)–Mo(1)–N(2)
O(3)–Mo(1)–N(2)
O(4)–Mo(1)–N(2)
148.73(10)
95.09(11)
98.91(11)
80.10(9)
85.31(12)
168.30(11)
71.69(9)
101.74(13)
160.95(15)
92.16(12)
N(1)–N(2)
C(9)–O(1)
C(1)–O(2)
2.4. General procedure for the oxidation of sulfides catalyzed by the V and
Mo complexes
To a solution of sulfide (1 mmol) and 30% aqueous H2O2 (2 mmol) in EtOH (10 mL),
[VO(L)(OCH3)]n (0.004 mmol) or [MoO2(L)(CH3OH)] (0.006 mmol) was added and the
reaction mixture was refluxed with continuous stirring for intervals of times. The cata-
lytic reaction was scrutinized by TLC (eluent, n-hexane:ethyl acetate, 5:2) and the yield
of products was determined by GC analysis. The products were purified by chromatog-
raphy over silica gel with a 70:30 mixture of n-hexane and ethyl acetate as eluent. All
products were known compounds and identified by comparing their FTIR and NMR
spectra with those of authentic samples.
3. Results and discussion
A tridentate Schiff base (H2L) was prepared by treating nicotinic hydrazide with 3-
ethoxysalicylaldehyde in methanol. Reaction of VO(acac)2 and MoO2(acac)2 with H2L in
methanol under reflux produces the targeted metal complexes (Scheme 1). After syn-
thesis and characterization, the catalytic effectiveness of the complexes were checked
by oxidizing various sulfides using 30% H2O2 (Scheme 2).
1
3.1. H NMR and 13C NMR spectra
1
The H and 13C NMR data of H2L and its Mo complex in DMSO are presented in the
Experimental section and spectra are shown in Figures 1–3. Two signals at d ¼ 12.24

1568
H. KARGAR ET AL.
Scheme 1. Syntheses of H2L, [VO(L)(OCH3)]n and [MoO2(L)(CH3OH)].
Scheme 2. Catalytic oxidation of alkyl sulfides in the presence of H2O2 by [VO(L)(OCH3)]n
and [MoO2(L)(CH3OH)].
1
and 10.79 ppm in the H NMR spectrum of H2L which correspond to OH (phenolic)
and NH protons, respectively, disappear on treatment with Mo salt, demonstrating
that the sites of coordination are phenolato and enolato oxygens of the ligand. This
also ascertains the occurrence of keto-imine tautomerism upon complexation.
Moreover, a singlet of azomethine proton (-HC ¼ N) at d ¼ 8.67 ppm observed in the
spectra of H2L shifted downfield at d ¼ 8.98 ppm showing deshielding due to the
decrease in the electronic density upon the coordination of azomethine nitrogen. This
is in accord with the FTIR spectrum of the complex, where ꢀ(HC ¼ N) appears at higher
wavenumber in comparison with the corresponding ligand. There is a slight shift in
the positions of aromatic protons in the NMR spectra of the ligand upon complex for-
mation. The ethoxy part of the ligand gives a quartet at d ¼ 4.07 ppm [3J (H,H) ¼
6.9 Hz] due to –OCH2 protons and a triplet at d ¼ 1.36 ppm [3J (H,H) ¼ 6.9 Hz] because
of the –CH3 protons coupling with each other. The aliphatic protons failed to exhibit
any noticeable shifts in their positions upon complexation.

JOURNAL OF COORDINATION CHEMISTRY
1569
Figure 1. 1H NMR spectrum of H2L in DMSO-d6.
The 13C NMR spectra of H2L and its Mo complex are shown in Figure 3. The signals
for the carbonyl, phenolic and methine carbon were observed at 161.3, 152.4 and
135.4 ppm, respectively. The chemical shift values of the carbons present close to the
coordinating atoms (i.e. C8, C1 and C7) showed appreciable change in their position
due to coordination-induced shifts, confirming the association of these functionalities
in coordination. The other aromatic carbons of ligand and Mo complex appeared in
their respective regions according to the literature. Hence, 13C NMR spectra also sup-
1
port the conclusions made from H NMR spectral data.
3.2. FTIR spectra
FTIR spectra of H2L and its oxovanadium and dioxomolybdenum complexes are shown
in Figure 4. Comparison of the FTIR spectra of the compounds was carried out to con-
firm the sites of coordination. FTIR spectra of H2L showed bands at 3570 and
1660 cmꢀ1 which correspond to the stretching vibrations of ꢀ(NH) and ꢀ(C ¼ O). These
bands disappeared in spectra of the complexes, in accord with enolization of the
amide functional group and deprotonation for coordination to metal ions. X-ray dif-
fraction data also confirm the mode of coordination of the ligand. The stretching
vibration of azomethine (-HC ¼ N) in H2L appears at 1606 cmꢀ1 and slightly shifts on
complexation. New bands at 1257 and 1249 cmꢀ1 are attributed to enolic ꢀ(C-O) moi-
ety in molybdenum and vanadium complexes, respectively. The appearance of two
new bands at 937 and 918 cmꢀ1 are assigned to symmetric and asymmetric stretching

1570
H. KARGAR ET AL.
Figure 2. 1H NMR spectrum of [MoO2(L)(CH3OH)] in DMSO-d6.
vibrations of the cis-Mo(O)2, in accord with similar structures reported [41, 42].
Similarly, oxovanadium complex also gives its characteristic peak of V ¼ O at 993 cmꢀ1
,
close to the values of reported oxovanadium Schiff base complexes [43–45]. Some
new M-O and M-N peaks are present at 597 and 472 cmꢀ1 for Mo complex and at 588
and 472 cmꢀ1 for
V complex, respectively, in agreement to reported com-
plexes [43–46].
3.3. Crystal structure determination
The molecular structure along with the atom numbering scheme of the vanadium
complex is shown in Figure 5. The complex is a neutral, one-dimensional coordination
polymer of oxovanadium(V). The two crystallographically independent molecules of
the vanadium complex are linked by the nicotinic moiety from the ligand through a
V1-N3 bond to build the repetitive unit. The phenolic oxygen (O1), imino nitrogen
(N1) and enolic oxygen (O2) of the nicotinohydrazone ligand along with methoxy oxy-
gen (O3) make the basal plane while the axial positions are oxygen (O4) of oxido and
nitrogen (N3) of the nicotinohydrazone ligand from the second unit of oxovanadium
complex. Hence, each V(V) center is six coordinate in a distorted octahedron.
The ligand coordinates tridentate (ONO) as a dianion as apparent from N1–C7 and
O2 ꢀ C9 bond lengths. The abnormal bond values indicate the presence of the enolate
form of the amide groups. The V–O, V–N and V ¼ O bonds are within normal ranges
and similar to those observed in similar oxovanadium(V) complexes [5, 47, 48]. Due to

JOURNAL OF COORDINATION CHEMISTRY
1571
Figure 3. 13C NMR spectra of H2L and [MoO2(L)(CH3OH)] in DMSO-d6.
Figure 4. FTIR spectra of H2L and its V and Mo complexes.
the trans effect of the oxo group, the distance between V2 and N3 of the nicotinyl moiety
(2.458(3) Å) is longer than the usual bond length, indicating weak association between
two units to form a polymeric structure. In contrast to this, the V1-O4 bond distance

1572
H. KARGAR ET AL.
Figure 5. ORTEP view of the asymmetric unit of [VO(L)(OCH3)]n (upper) and one-dimensional poly-
meric chain (lower) shown in ball and stick mode. Thermal ellipsoids are drawn at the 50% prob-
ability level, while the hydrogen size is arbitrary.
(1.578(3) Å) is shorter, which indicates a double bond. These polymeric complexes in
which two complex units coordinate through nicotinic nitrogen are few in number.
The molecular structure of the dioxomolybdenum complex is shown in Figure 6.
The coordination geometry around the molybdenum center can be described as dis-
torted octahedral in which two cis positions are occupied by the two oxo groups and
a tridentate dianionic hydrazone Schiff base. The sixth coordination site of molyb-
denum is occupied by the oxygen from the methanol forming the mononuclear com-
plex. The distance between molybdenum and the oxygen of methanol, Mo1-O6 of
2.309(3) Å, represents the largest bond length within the distorted octahedron. The
elongated Mo1–O6 bond length trans to oxo O1 indicates weak coordination of

JOURNAL OF COORDINATION CHEMISTRY
1573
Figure 6. ORTEP view of the asymmetric unit of [MoO2(L)(CH3OH)]. Thermal ellipsoids are drawn at
the 50% probability level, while the hydrogen size is arbitrary.
methanol at the axial position due to the strong p-donor character of the trans oxo,
as commonly observed [49]. The distortion of the octahedral coordination can also be
substantiated from the bond angles related to the Mo ions which are summarized in
Table 2. The Mo–O and Mo–N bond lengths in the complex are comparable to those
observed in other oxomolybdenum complexes with hydrazone ligands [49–51]. As
with previously reported bond lengths of N1–N2 [1.409(4) Å] and iminic N1–C7
[1.297(4) Å] groups increased upon coordination to molybdenum. The two Mo ¼ O
bond distances and the subtended O(1)¼Mo(1)¼O(2) are comparable to those previ-
ously reported for molybdenum complexes [49–51].
3.4. Theoretical results
3.4.1. Optimized structural parameters
Geometry optimizations of the ligand and its complexes are performed by DFT at the
B3LYP/Def2-TZVP level of theory in the gas phase. The optimized structures of the
compounds are shown in Figure 7 and the selected calculated bond lengths and
angles of the V and Mo complexes are listed in Table 3. On comparing the data in
Tables 2 and 3, theoretical data are analogous to the experimental results. The differ-
ences between the theoretical and experimental values are due to the fact that the
experimental data belong to the molecules in the solid state, while the calculated val-
ues correspond to the molecules in the gaseous phase.
By comparing the Gibbs free energy (G) of both enol and keto forms of H2L in the
gas and solution phases, the keto form is more stable than the enol form in both
phases (Table 4).
3.4.2. Electronic properties
The frontier molecular orbital energy levels and their energy gaps (DE ¼ ELUMO
–
EHOMO) are often used to describe the stability and the chemical reactivity of the

1574
H. KARGAR ET AL.
Figure 7. Optimized structures of H2L (keto and enol forms), [VO(L)(OCH3)]n and [MoO2(L)(CH3OH)].
Table 3. The selected calculated bond lengths (Å) and angles (ꢁ) in V and Mo complexes.a
V complex
V(1)–O(1)
V(1)–O(2)
V(1)–O(3)
V(1)–O(4)
V(1)–N(1)
V(1)–N(3)
N(1)–C(7)
N(1)–N(5)
C(1)–O(1)
C(8)–O(2)
V(2)–O(6)
V(2)–O(7)
V(2)–O(8)
V(2)–O(9)
V(2)–N(4)
V(2)–N(7)
1.857
1.959
1.777
1.573
2.160
2.635
1.290
1.372
1.320
1.301
1.850
1.953
1.571
1.780
2.152
2.752
O(4)–V(1)–O(3)
O(4)–V(1)–O(1)
O(3)–V(1)–O(1)
O(4)–V(1)–O(2)
O(3)–V(1)–O(2)
O(1)–V(1)–O(2)
O(4)–V(1)–N(1)
O(3)–V(1)–N(1)
O(1)–V(1)–N(1)
O(2)–V(1)–N(1)
O(6)–V(2)–N(4)
O(7)–V(2)–N(4)
O(8)–V(2)–N(4)
O(9)–V(2)–N(4)
N(4)–V(2)–N(7)
O(8)–V(2)–N(7)
103.19
101.55
103.17
100.21
93.86
148.35
95.01
159.46
82.08
73.36
82.30
73.48
96.10
158.25
81.03
176.65
Mo complex
Mo(1)–O(1)
Mo(1)–O(2)
Mo(1)–O(3)
Mo(1)–O(4)
Mo(1)–O(6)
Mo(1)–N(2)
N(2)–C(7)
2.007
1.950
1.700
1.688
2.579
2.288
1.289
1.373
1.317
1.333
O(1)–Mo(1)–O(2)
O(1)–Mo(1)–O(3)
O(1)–Mo(1)–O(4)
O(1)–Mo(1)–O(6)
O(3)–Mo(1)–O(6)
O(4)–Mo(1)–O(6)
O(1)–Mo(1)–N(2)
O(2)–Mo(1)–N(2)
O(3)–Mo(1)–N(2)
O(4)–Mo(1)–N(2)
143.06
97.04
101.60
78.33
80.57
171.66
70.43
79.21
155.22
96.02
N(1)–N(2)
C(9)–O(1)
C(1)–O(2)
aThe number of atoms is based on the numbering of the X-ray structure.
compounds. The HOMO and LUMO plots of V and Mo complexes as well as the free
ligand drawn by B3LYP/Def2-TZVP level of theory are shown in Figure 8. The energy

JOURNAL OF COORDINATION CHEMISTRY
1575
Table 4. Sum of the electronic and the zero-point energy (EZPE), enthalpy (H) and Gibbs free
energy (G) of the ligand.
H2L (keto)
H2L (enol)
Gas phase
Solution phase
Gas phase
Solution phase
EZPE
H
G
–970.620
–970.560
–970.669
–970.640
–970.614
–970.703
–970.615
–970.596
–970.664
–970.627
–970.601
–970.690
All values are in Hartree units.
1Hartree ¼ 627.5095 kcal.molꢀ1
.
gaps for the H2L (keto), H2L (enol), [VO(L)(OCH3)]n and [MoO2(L)(CH3OH)] were 3.896,
3.748, 3.037 and 3.335 eV, respectively. The energy gap of H2L (keto) is slightly greater
than that of H2L (enol), which can be attributed to higher stability of keto tautomeric
form of the ligand.
3.4.3. Molecular electrostatic potential
Molecular electrostatic potential (MEP) is a very useful property for chemical activities of
the molecules. The three-dimensional MEP diagrams have been specified by the regions
of high electronic density with red (electrophilic reactivity) and electron deficient regions
with blue (nucleophilic reactivity). The MEP diagrams and values of electrostatic potentials
of H2L and its complexes are presented in Figure 9. There is a significant difference
between the values of electrostatic potential for enol and keto forms of H2L. In keto form,
the oxygens of the phenolic and carbonyl groups, with electrostatic potential value of
approximately 57 kcal.molꢀ1, are the most reactive nucleophilic centers for coordination
with metal ions, while in enol form, the two hydroxyl groups, specified by blue color, cor-
respond to the electrophilic sites with a positive value of electrostatic potential, i.e.
approximately 37 kcal.molꢀ1. In the oxovanadium and dioxomolybdenum complexes the
electrostatic potential values are approximately 35 and approximately 42 kcal.molꢀ1
,
respectively. Hence, the metal ion is the center of positive electrostatic potential and rep-
resented by blue color which means it is a good candidate for attack by a nucleophile.
3.4.4. Mulliken atomic charge
The calculated Mulliken charges on the active sites of the oxovanadium and dioxomo-
lybdenum complexes as well as on H2L are presented in Table 5. The Mulliken charge
on the metal ions are þ1.170 and þ1.473 for V and Mo complexes, respectively, which
are considerably less than that of formal charges, þ5 for V and þ6 for Mo. This differ-
ence indicates that a significant amount of charge density is transferred from the lig-
and to the metal atoms. C1, C2 and C8 have the highest positive values of atomic
charges as these C-sites are adjacent to the highly electronegative O-atoms. Also, the
maximum variation in the charge density is shown at O1, O3, N2, C1 and C8, after the
complexation with vanadium and molybdenum.
3.4.5. Theoretical study of FTIR spectra
Some selected calculated and experimental vibrational modes of H2L and its V and Mo
complexes are listed in Table 6 with a good correlation between theoretical and
experimental results. This indicates similarity between the structures of the com-
pounds in the gaseous and solid states.

1576
H. KARGAR ET AL.
Figure 8. DFT-optimized frontier molecular orbitals of H2L (keto and enol), V and Mo complexes.

JOURNAL OF COORDINATION CHEMISTRY
1577
Figure 9. Molecular electrostatic potential (MEP) for H2L (keto and enol), V and Mo complexes with
color range along with scale.
Table 5. The Mulliken atomic charges of H2L and its V and Mo complexes.
Atoms
H2L ligand
V complex
Mo complex
Atoms
H2L ligand
V complex
Mo complex
M1
O1
O2
O3
O4
O5
O6
N1
N2
N3
C1
C2
C3
–
1.170
–0.525
–0.252
–0.519
–0.438
–0.479
–
–0.259
–0.020
–0.203
0.315
1.473
–0.572
–0.251
–0.543
–0.572
–0.539
–0.447
–0.249
–0.063
–0.204
0.287
C4
C5
C6
C7
C8
–0.156
–0.194
0.090
–0.008
0.276
–0.129
–0.225
0.084
–0.040
0.430
–0.126
–0.229
0.115
–0.017
0.419
–0.369
–0.246
–0.323
–
–
–
–0.178
–0.160
–0.199
0.136
0.204
–0.208
C9
0.065
0.019
0.029
C10
C11
C12
C13
C14
C15
C16
–0.070
–0.024
–0.110
–0.179
–0.040
–0.313
–
–0.077
–0.024
–0.095
–0.137
–0.042
–0.316
–0.155
–0.074
–0.024
–0.098
–0.147
–0.046
–0.316
–0.146
0.230
–0.219
0.239
–0.222
The N-H stretch frequency in theoretical (3399 cmꢀ1) and experimental data
(3570 cmꢀ1) are far apart. The same set of patterns is manifested in the illustration of
C ¼ O stretching frequency where theoretical value appeared at 1754 cmꢀ1, which is
away from the experimental data at 1660 cmꢀ1. This huge difference of vibrational fre-
quencies in the values may be due to the involvement of these groups (N–H and
C ¼ O) in hydrogen bonding in the solid state.
1
3.4.6. Theoretical study of H and 13C NMR spectra
1
The experimental and calculated H and 13C NMR chemical shifts of H2L and its com-
plexes are given in Table 7. It is evident from the table that the theoretical data agree
with the experimental findings.

1578
H. KARGAR ET AL.
Table 6. Selected experimental and calculated FTIR vibrational frequencies (cmꢀ1) of H2L and its
V and Mo complexes.
H2L ligand
V complex
Mo complex
Assignment Exp. Calc. Relative error (%)a Exp. Calc. Relative error (%)a Exp. Calc. Relative error (%)a
NH
3570 3399
1660 1754
1606 1671
1591 1551
1477 1492
1246 1261
–4.79
5.66
4.05
–2.51
1.02
1.20
–
C ¼ O
HC ¼ N
C ¼ C
–
–
–
–
–
–
1604 1653
1552 1593
1475 1472
1249 1311
993 1043
3.05
2.64
–0.20
4.96
5.04
1610 1662
1527 1556
1448 1474
1257 1290
937 986
918 915
597 602
472 476
3.23
1.90
1.80
2.63
5.23
–0.33
0.84
0.85
C-O
M ¼ O
–
–
M-O
M-N
–
–
–
–
–
–
588 608
472 484
3.40
2.54
Exp
aRelative error (%) ¼ (XCalc – XExp) 100/X
.
ꢃ
1
Table 7. Experimental and calculated H and 13C-NMR shifts of H2L and its Mo complex (ppm).
H2L ligand
Mo complex
Experimental
Calculated
Experimental
Calculated
1H NMR
OH
NH
12.24
10.79
9.10
8.78
8.67
8.29
7.59
7.18
7.03
6.85
4.07
1.36
11.85
9.06
9.53
9.14
8.51
8.28
7.74
7.23
7.22
7.22
–
–
–
–
CH(10)
CH(11)
HC ¼ N
CH(13)
CH(12)
CH(5)
CH(3)
CH(4)
CH(14)
CH(15)
9.15
8.76
8.98
8.31
7.56
7.33
7.26
7.02
4.08
1.35
9.53
9.10
9.02
8.94
7.74
7.45
7.41
7.43
ꢃ
ꢃ
4.11
1.52
4.10
1.52
ꢃ
ꢃ
13C NMR
CH(8)
CH(1)
CH(2)
CH(10)
CH(11)
CH(7)
CH(13)
CH(9)
CH(12)
CH(5)
CH(6)
CH(3)
CH(4)
CH(14)
CH(15)
ꢃ
161.37
152.41
148.58
147.42
147.05
135.43
128.67
123.62
120.73
119.14
119.09
118.94
115.23
64.09
169.19
158.71
156.79
158.87
161.28
156.34
140.35
136.59
129.68
128.74
124.30
119.21
124.42
69.93
167.14
147.59
148.72
149.48
152.36
156.80
135.37
126.09
125.66
123.94
121.62
120.56
118.87
64.35
177.27
159.62
157.05
157.95
161.32
165.14
143.62
133.08
129.90
131.62
127.22
123.63
128.94
70.56
14.69
17.31
14.69
17.20
Average values obtained from calculations.
3.5. General procedure for oxidation of sulfides catalyzed by [VO(L)(OCH3)]n
and [MoO2(L)(CH3OH)] under reflux
To a 10 mL of ethanolic solution of diphenyl sulfides (1 mmol), [VO(L)(OCH3)]n
(0.004 mmol) or [MoO2(L)(CH3OH)] (0.006 mmol) was added along with 30% solution of
hydrogen peroxide. The obtained mixture was refluxed for periods of time. Progress of
the conversion was continuously monitored by taking samples at regular intervals and
running gas or thin layer chromatograms. The products were purified by gas

JOURNAL OF COORDINATION CHEMISTRY
1579
Table 8. The effect of oxidant on the oxidation of diphenyl sulfide catalyzed by V and Mo com-
plexes in ethanol under reflux.a
V complex Conversion (%)
after 15 minb
Mo complex Conversion (%)
after 60 minb
Oxidant
No oxidant
NaIO4
H2O2
UHP
Bu4NIO4
0
100
100
100
5
0
95
100
95
10
5
tert-BuOOH
5
aReaction conditions: diphenyl sulfide (1 mmol), oxidant (2 mmol), catalyst (V ¼ 0.004 mmol or Mo ¼ 0.006 mmol),
CH3CH2OH (10 mL).
bGC yields.
Table 9. The effect of solvent on the oxidation of diphenyl sulfide with H2O2 catalyzed by V and
Mo complexes under reflux.a
V complex Conversion (%)
after 15 minb
Mo complex Conversion (%)
after 60 minb
Solvent
CH3CH2OH
CH3OH
CH3CN
ClCH2CH2Cl
CH3COCH3
CH2Cl2
100
100
100
100
55
0
100
100
100
100
70
0
CHCl3
0
0
CCl4
0
0
aReaction conditions: diphenyl sulfide (1 mmol), H2O2 (2 mmol), catalyst (V ¼ 0.004 mmol or Mo ¼ 0.006 mmol),
solvent (10 mL).
bGC yields.
Table 10. The effect of amount of catalyst on the oxidation of diphenyl sulfide with H2O2 cata-
lyzed by V and Mo complexes in ethanol under reflux.a
V complex conversion (%)
after 15 minb
Mo complex conversion (%)
after 60 minb
Catalyst amount (mmol)
No catalyst
0.003
0.004
0.005
0.006
0
70
100
100
100
0
55
90
96
100
aReaction conditions: diphenyl sulfide (1 mmol), H2O2 (2 mmol), CH3CH2OH (10 mL).
bGC yields.
chromatography (GC) by packing the column with silica gel as stationary phase and
selecting a 70:30 mixture of n-hexane and ethyl acetate as mobile phase. The products
were identified by comparing their spectral peaks with the library of authentic samples.
Table 8 gives a summary of the effect of oxidants NaIO4, H2O2, urea-hydrogen per-
oxide (UHP), tert-BuOOH and Bu4NIO4 on the oxidation of diphenyl sulfide in the presence
of [VO(L)(OCH3)]n and [MoO2(L)(CH3OH)]. The results showed that H2O2 is the best source
of oxygen and also proved to be inert in the absence of catalyst in ethanol.
The influence of solvent on the rate of catalytic reaction was also studied. For this
purpose, ethanol, methanol, acetonitrile, 1,2-dichloroethane, acetone, dichloromethane,
chloroform and carbon tetrachloride were tested and the results are given in Table 9.
Although the results showed that the reaction performed well in some of these sol-
vents, the ecofriendly nature of ethanol triggered us to use it for our catalytic system.

1580
H. KARGAR ET AL.
Table 11. Oxidation of sulfides with H2O2 catalyzed by V and Mo complexes.a
Sulfoxide (%)b,c
Sulfone (%)b,c
Time (min)
Row
1
Sulfide
V
Mo
V
Mo
V
Mo
70
70
80
95
95
70
30
0
20
5
15
30
5
2
3
4
100
100
100
30
5
0
5
0
30
10
10
5
6
7
100
90
80
95
70
0
20
5
15
15
30
60
15
40
10
10
90
30
8
0
50
0
50
120
240
aReaction conditions: sulfide (1 mmol), H2O2 (2 mmol), CH3CH2OH (10 mL), catalyst (V ¼ 0.004 mmol or Mo ¼
0.006 mmol).
bGC (entries 1–4) or Isolated yield.
cAll products were identified by comparison of their physical and spectral data with those of authentic samples.
Scheme 3. Plausible mechanism for oxidation of sulfides with H2O2 catalyzed by dioxomolybde-
num complex.
Examination of the reaction in the presence of different amounts of catalysts
showed that the optimal amount of [VO(L)(OCH3)]n for oxidation of 1 mmol of
diphenyl sulfide is 0.004 mmol and for [MoO2(L)(CH3OH)] is 0.006 mmol. Diphenyl sul-
fide was not oxidized in the absence of catalyst as shown by GC (Table 10).

JOURNAL OF COORDINATION CHEMISTRY
1581
After optimizing the reaction conditions, various aliphatic and aromatic sulfides were
oxidized by this catalytic system to get corresponding sulfoxides and sulfones. Table 11
shows that most sulfides were completely converted into their products. In many cases,
two different reaction times are reported as time taken by [VO(L)(OCH3)]n was less than
that of [MoO2(L)(CH3OH)]. Also, higher selectivity was observed for vanadium catalyst for
the sulfoxide formation over the sulfone. Initially sulfoxides were produced which were
gradually converted into sulfones on increasing the reaction time.
Based on previously reported mechanisms, a plausible mechanism for oxidation of sul-
fides using H2O2 catalyzed by [MoO2(L)(CH3OH)] is proposed as depicted in Scheme 3.
Oxidation of sulfides to sulfoxides in the presence of peroxides proceeds through a one-
step oxygen-transfer mechanism with simultaneous electrophilic attack of sulfur on one of
the oxygens of peroxide, breaking the O–O bond and formation of the S¼ O bond [52].
Further oxidation, according to the same mechanism, leads to the formation of sulfone. A
similar mechanism can also be drawn for the oxovanadium complex.
4. Conclusion
A new tridentate ONO-donor Schiff base ligand and its V(V) and Mo(VI) complexes
were synthesized and characterized and the molecular structures were determined by
single crystal X-ray crystallography. The coordination geometries around V and Mo
were distorted octahedral. Theoretical calculations of H2L and its V and Mo complexes
were performed using DFT at B3LYP/Def2-TZVP level of theory. The theoretical data
are in consensus with the experimental data. The catalytic activities of the complexes
were performed in oxidation of sulfides with H2O2 in ethanol. The present method
offers high yield of the products and short time for completion of reaction; higher
selectivity was observed for vanadium catalyst for sulfoxide formation over
the sulfone.
Acknowledgement
We gratefully acknowledge practical support of this study by Ardakan University and Payame
Noor University.
Disclosure statement
No potential conflict of interest was reported by the authors.
ORCID
Hadi Kargar
http://orcid.org/0000-0002-7817-0937
Khurram Shahzad Munawar
http://orcid.org/0000-0001-9055-2519
Data availability
Crystallographic data for the structural analysis have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication nos. CCDC 2045556 and 2044074 for
the vanadium and molybdenum complexes, respectively. Copies of this information may be

1582
H. KARGAR ET AL.
obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html or from the
Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (Fax: þ44-1223-336-033; E-mail:
deposit@ccdc.ac.uk).
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