Oxidovanadium(V) complexes containing hydrazone based O,N,O-donor ligands: Synthesis, structure, catalytic properties and theoretical calculations

Article abstract of DOI:10.1016/j.poly.2013.11.020

Two new mono oxidovanadium(V) complexes, [VOL¹(OEt)] (1) and [VOL²(OMe)] (2), of the tridentate Schiff base hydrazone-type O,N,O-donor ligands H₂L¹ and H₂L², obtained by monocondensation of 3-hydroxy-2-naphthohydrazide with 5-bromo-2-hydroxybenzaldehyde and benzoylacetone, respectively, have been synthesized starting from VO(acac)₂ [H₂L¹ = (E)-N′-(5-bromo-2-hydroxybenzylidene)-3-hydroxy-2-naphthohydrazide; H ₂L² = (E)-3-hydroxy-N′-((Z)-4-hydroxy-4-phenylbut-3- en-2-ylidene)-2-naphthohydrazide]. Single-crystal X-ray structure analysis revealed for both complexes a slightly distorted square-based pyramidal NO ₄ coordination environment around the metal centre, with the aroylhydrazone Schiff bases acting as O,N,O-tridentate, dinegative ligands. The complexes were also characterized by spectroscopic methods in the solid state (IR) and in solution (UV-Vis, ¹H NMR) and by cyclic voltammetric experiments in DMSO, and their properties were interpreted by means of DFT theoretical calculations. The catalytic potential of these complexes has been tested for the oxidation of thioanisol using H₂O₂ as the terminal oxidant. The effects of various parameters, including the molar ratio of oxidant to substrate, the temperature and the solvent, have been studied. Both complexes showed superabundant catalytic activity in the oxidation of thioanisol at room temperature with excellent conversions.

Full text of DOI:10.1016/j.poly.2013.11.020

Polyhedron 69 (2014) 90–102
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Oxidovanadium(V) complexes containing hydrazone based O,N,O-donor
ligands: Synthesis, structure, catalytic properties and theoretical
calculations
a
b
Hassan Hosseini-Monfared a, Rahman Bikas a, , Parisa Mahboubi-Anarjan , Alexander J. Blake ,
⇑
Vito Lippolis c, N. Burcu Arslan d, Canan Kazak d
a Department of Chemistry, Faculty of Science, University of Zanjan, 45371-38791 Zanjan, Iran
b School of Chemistry, The University of Nottingham, University Park, NG7 2RD Nottingham, UK
c Dipartimento di Scienze Chimiche e Geologiche, Università degli Studi di Cagliari, S.S. 554 Bivio per Sestu, 09042 Monserrato (CA), Italy
d Department of Physics, Faculty of Arts and Sciences, Ondokuz Mayis University, 55019 Kurupelit, Samsun, Turkey
a r t i c l e i n f o
a b s t r a c t
Two new mono oxidovanadium(V) complexes, [VOL1(OEt)] (1) and [VOL2(OMe)] (2), of the tridentate
Schiff base hydrazone-type O,N,O-donor ligands H2L1 and H2L2, obtained by monocondensation of 3-
hydroxy-2-naphthohydrazide with 5-bromo-2-hydroxybenzaldehyde and benzoylacetone, respectively,
have been synthesized starting from VO(acac)2 [H2L1 = (E)-N0-(5-bromo-2-hydroxybenzylidene)-3-
hydroxy-2-naphthohydrazide; H2L2 = (E)-3-hydroxy-N0-((Z)-4-hydroxy-4-phenylbut-3-en-2-ylidene)-2-
naphthohydrazide]. Single-crystal X-ray structure analysis revealed for both complexes a slightly dis-
torted square-based pyramidal NO4 coordination environment around the metal centre, with the aro-
ylhydrazone Schiff bases acting as O,N,O-tridentate, dinegative ligands. The complexes were also
characterized by spectroscopic methods in the solid state (IR) and in solution (UV–Vis, 1H NMR) and
by cyclic voltammetric experiments in DMSO, and their properties were interpreted by means of DFT the-
oretical calculations.
Article history:
Received 3 October 2013
Accepted 12 November 2013
Available online 27 November 2013
Keywords:
Oxidovanadium(V)
Molecular structure
Cyclic voltammetry
Spectroscopy
DFT calculation
Catalyst
The catalytic potential of these complexes has been tested for the oxidation of thioanisol using H2O2 as
the terminal oxidant. The effects of various parameters, including the molar ratio of oxidant to substrate,
the temperature and the solvent, have been studied. Both complexes showed superabundant catalytic
activity in the oxidation of thioanisol at room temperature with excellent conversions.
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
with vanadium in a trigonal bipyramidal coordination environ-
ment [11–14]. Some VO3+ complexes are being considered as po-
There is continuous interest in the chemistry of vanadium(IV/V)
complexes due to their biological relevance and catalytic proper-
ties [1,2]. In most cases, the active site contains either VO3+ or
VO2+ coordinated by O,N-donor ligands. The discoveries of several
medicinal properties of vanadium(IV/V) complexes, i.e. their insu-
lin-mimetic [3–5], anticancer [6], antitumour [7] and antibacterial
activities [8], have further stimulated research in this area. In par-
ticular, the presence of vanadium in the prosthetic group of certain
nitrogen fixing nitrogenases and haloperoxidases involved in the
insertion of halogens into organic substrates has also been re-
ported. Vanadium haloperoxidases are enzymes that catalyze dif-
ferent oxidation reactions [9,10]. The active site structure of the
vanadium-dependent haloperoxidases contains vanadate cova-
lently bound by histidine and stabilized by an H-bonding network,
tential functional models of vanadium-dependent haloperoxidases.
Schiff bases are widely employed as ligands in coordination
chemistry. These ligands are readily available, versatile and,
depending on the nature of the starting materials employed for their
preparation, can exhibit various denticities and functionalities.
[15,16]. Hydrazones are a special group of compounds in the Schiff
base family. They are characterized by the presence of the RR0C@N–
NH–C(@O)R00 moiety, where R, R0 and R00 can be H, alkyl or aroyl
groups. These ligands, due to their facile keto–enol tautomerization
and the availability of several potential donor sites depending on
the nature of the substituents attached to the hydrazone unit, rep-
resent good polydentate chelating agents with interesting modes
of ligation for a variety of metal ions [17–20]. Aroylhydrazone com-
plexes, on the other hand, are good candidates for catalytic oxida-
tion studies because of their ability to resist oxidation. Keto–enol
tautomerism occurs readily in hydrazones and has an important
role in determining the overall charge on the ligands coordinating
⇑
Corresponding author. Tel.: +98 241 5152576; fax: +98 241 5283203.
E-mail addresses: bikas@znu.ac.ir, bikas_r@yahoo.com (R. Bikas).
0277-5387/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.poly.2013.11.020

H. Hosseini-Monfared et al. / Polyhedron 69 (2014) 90–102
91
the metal ions. The @N–NH–C(@O)R moiety in hydrazone based
ligands can lose its amidic hydrogen and coordinate in the enol form
to metal ions as a mono-negative ligand [21]. In addition, this
moiety in its keto form can coordinate to metal centres as a neutral
ligand [22]. The lengths of the NC and C@O bonds are very important
indicators of the type of coordination. When hydrazone based
ligands coordinate to metal ions in the keto form, the lengths of
the HN–C and C@O bonds are around 1.34 and 1.27 Å, respectively
[23], while their bond lengths in the enol form are about 1.28 and
1.34 Å, respectively [24]. From a spectroscopic point of view, FT-IR
is very suitable for detecting the keto/enol form of coordinated li-
gands. There is a strong band between 1630 and 1670 cmꢀ1 in the
FT-IR spectra of complexes containing the hydrazone ligand coordi-
nated in the keto form, which is attributed to the C@O moiety [25],
while this band is not observed for complexes having ligands coor-
dinated in the enol form [26]. However, keto–enol tautomerism in
hydrazones depends on other parameters, such as pH, the nature
of the metal, etc. In different types of compounds, hydrogen bonding
plays a crucial role in the keto–enol tautomerism and it can shift the
keto–enol equilibrium in one of the two directions [27]. It was re-
cently observed in hydrazone ligands based on 3-hydroxy-2-naph-
thoic acid hydrazide that intramolecular hydrogen bonding affects
the keto–enol tautomerism and despite the elimination of the NH
hydrogen, it prevents tautomerism and the ligand coordinates in
the keto form [28]. To continue exploring the structural and elec-
tronic effects of the ortho and para substituents in the aryloxy ring
of hydrazone ligands on vanadium in [VVO(ONO)(O)] complexes ap-
plied to oxidation catalysis [29], we report herein the synthesis,
spectroscopic characterization and catalytic reactivity of two new
oxidovanadium complexes with hydrazone ligands based on 3-hy-
droxy-2-naphthoic acid hydrazide (Scheme 1). In addition, with
the aim of explaining the origin of the properties of the complexes
and to understand the details of intramolecular hydrogen bonding
effects on the keto–enol tautomerism, we carried out geometry
optimizations, electronic and vibrational spectra calculations along
with density functional theory (DFT) [30] for both the ligands and
their complexes. The prepared complexes were successfully used
as catalysts in the oxidation of thioanisol using H2O2 as the terminal
oxidant.
benzoylacetone and methyl phenyl sulfide (thioanisol) were pur-
chased from Merck and used as received. (E)-N0-(5-bromo-2-
hydroxybenzylidene)-3-hydroxy-2-naphthohydrazide (H2L1) was
synthesized according to the literature [29]. Solvents of the highest
grade commercially available (Merck) were used without further
purification. IR spectra were recorded as KBr discs using a Bruker
FT-IR spectrophotometer. UV–Vis solution spectra were recorded
using a thermo-spectronic Helios Alpha spectrometer. 1H and 13C
NMR spectra of the ligands and complexes in DMSO-d6 solution
were recorded on a Bruker 250 MHz spectrometer and chemical
shifts were indicated in ppm relative to tetramethylsilane. Cyclo-
voltammetric experiments were performed using a Metrohm com-
putrace voltammetric analyzer model 757 VA.
The reaction products of the oxidation were determined and
analyzed using an HP Agilent 6890 gas chromatograph equipped
with an HP-5 capillary column (phenyl methyl siloxane
30
l
m ꢁ 320
l
m ꢁ 0.25
lm) and gas chromatograph-mass spec-
trometry (Hewlett–Packard 5973 Series MS-HP gas chromatograph
with a mass-selective detector). The elemental analyses (carbon,
hydrogen and nitrogen) of the compounds were obtained from a
Carlo ERBA Model EA 1108 analyzer. The vanadium content of
the complexes was measured by atomic absorption on a Varian
AAS-110 spectrometer.
2.2. Synthesis of (E)-3-hydroxy-N0-((Z)-4-hydroxy-4-phenylbut-3-en-
2-ylidene)-2-naphthohydrazide (H2L2)
This ligand was synthesized by refluxing a mixture of 3-hydro-
xy-2-naphthoic acid hydrazide (1.5 mmol, 303 mg) and benzoyl-
acetone (1.5 mmol, 243 mg) in 20 mL methanol for 5 h. The
solution volume was decreased to 5 mL by removing the solvent
under reduced pressure and then cooled to room temperature.
The obtained solids were separated and filtered off, washed with
cooled methanol and then dried in air. Yield 92% (480 mg). Anal.
Calc. for C21H18N2O3 (MW = 346.38): C, 72.82; H, 5.24; N, 8.09.
Found: C, 72.57; H, 5.21; N, 8.16%. FT-IR (KBr, cmꢀ1): 3473 (w,
br), 3270 (m), 1644 (vs), 1620 (vs), 1557 (s), 1537 (s), 1492 (m),
1361 (m), 1295 (m), 1224 (m), 1208 (m), 1082 (s), 1026 (m), 920
(s), 877 (s), 850 (s), 787 (S), 752 (vs), 730 (s), 686 (w), 666 (s),
612 (s), 481 (m), 443 (m). 1H NMR (250.13 MHz, DMSO-d6, 25 °C,
TMS, ppm) d: 12.58 (s, 1H, N–H), 11.10 (s, 2H, O–H), 8.4 (s, 1H,
C–Hazomethine), 8.02 (d, J = 7.25 Hz, 1H), 7.75 (d, J = 8.25 Hz, 2H),
7.46 (m, 4H), 7.31 (m, 3H), 6.01 (s, 1H), 2.15 (s, 3H). 1H NMR
(250.13 MHz, DMSO-d6 + D2O 25 °C, TMS, ppm) d: 8.37 (s, 1H, C–
Hazomethine), 7.17–7.95 (m, 11H, aromatic), 5.96 (s, 1H), 2.10 (s,
3H). 13C NMR (62.90 MHz, DMSO-d6, ppm) d: 185.0, 165.9, 164.6,
2. Experimental details
2.1. Materials and instrumentation
Vanadyl(IV) bis(acetylacetonate), VO(acac)2, 3-hydroxy-2-
naphthoic acid hydrazide, 5-bromo-2-hydroxybenzaldehyde,
Scheme 1. Ligands discussed in this paper in their neutral and dianionic forms present in the corresponding VO3+ isolated complexes.

92
H. Hosseini-Monfared et al. / Polyhedron 69 (2014) 90–102
153.9, 139.2, 136.32, 135.0, 131.4, 131.2, 129.2, 128.9, 128.7, 127.2,
use and purged with N2 for ca. 15 min prior to taking measure-
ments in order to remove dissolved O2. The voltammograms were
recorded under various scan rates in the range ꢀ0.5–1.0 V versus
Ag/AgCl.
126.3, 124.3, 120.5, 111.0, 92.3, 18.5. UV/Vis (CH3OH): kmax, nm (e,
Mꢀ1 cmꢀ1): 237 (41700), 410 (32600).
2.3. Synthesis of the complex [VO(L1)(OEt)] (1)
2.6. X-ray diffraction data collection and refinement
Single crystals of [VO(L1)(OCH2CH3)] (1) were obtained by the
thermal gradient method. VO(acac)2 (1.0 mmol, 265 mg) and
H2L1 (1.0 mmol, 385 mg) were placed in the main arm of a
branched tube. Ethanol was carefully added to fill the arms, the
tube was sealed and the reagents containing arm was immersed
in an oil bath at 60 °C while the other arm was kept at ambient
temperature. After 3 days, crystals were deposited in the cooler
arm, which were filtered off and air dried. Yield 76% (375 mg).
Anal. Calc. for C20H16BrN2O5V (MW = 495.20): C, 48.51; H, 3.26;
N, 5.66; V, 10.29. Found: C, 48.59; H, 3.22; N, 5.72; V, 10.36%.
FT-IR (KBr, cmꢀ1): 3460 (vs, br), 2927 (m), 2865 (m), 1637 (vs),
1615 (vs), 1584 (m), 1541 (m), 1520 (s), 1464 (s), 1444 (m),
1384 (s), 1374 (m), 1339 (m), 1299 (m), 1269 (m), 1240 (m),
1218 (m), 1199 (m), 1171 (m), 1148 (m), 1124 (m), 1087 (s),
1032 (s), 1001 (s), 964 (s), 905 (m), 876 (m), 829 (m), 818 (w),
798 (m), 761 (m), 746 (m), 700 (m), 670 (s), 634 (s), 580 (s), 528
(m), 506 (m), 481 (m), 471 (m), 468 (w), 435 (w), 402 (m). 1H
NMR (250.13 MHz, DMSO-d6, 25 °C, TMS, ppm) d: 11.45 (s, 1H,
O–Hnaphthol), 9.05 (s, 1H, CH = N), 6.88–8.61 (m, 9H, aromatic),
3.42 (q, J = 6.75 Hz, 2 H, OCH2), 1.036 (t, J = 6.75 Hz, 3H, CH3). 13C
NMR (62.90 MHz, DMSO-d6, ppm) d: 18.8 (CH3), 84.67 (OCH2),
110.9, 111.2, 119.4, 122.3, 124.1, 126.4, 126.6, 127.3, 128.9,
129.3, 131.2, 135.1, 136.8, 137.1, 152.7 (C–Ophenol), 154.8 (C–Onaph-
A summary of the crystal data and refinement details for the
compounds discussed in this paper are given in Table 1. Only spe-
cial features of the analyses are mentioned here. Single crystal data
collection for [VO(L1)(OEt)] (1) and [VO(L2)(OMe)] (2) was per-
formed on a Bruker SMART APEX CCD area detector and a Stoe IPDS
II diffractometer, respectively, equipped with an Oxford Cryosys-
tems open-flow nitrogen cryostat, using
x scans and graphite-
monochromated Mo K (k = 0.71073 Å) radiation. All data sets
a
were corrected for Lorentz, polarization and absorption effects
(semi-empirical based on multi-scan methods). The structures
were solved by direct methods using SHELXS97 [31] and SIR92 [32]
for 1 and 2, respectively, completed with difference Fourier synthesis
and refined with full-matrix least-square procedures based on F2
using SHELXL97. All non-hydrogen atoms were refined with aniso-
tropic displacement parameters. The hydrogen atoms were placed
in idealized positions and constrained to ride on their parent atoms.
2.7. Computational methods
The ground state geometries in the gas phase of the ligands H2L1
and H2L2 and of the complexes [VO(L1)(OEt)] (1) and [VO(L2)(-
OMe)] (2) were fully optimized using the restricted HF/6-31G,
B3LYP/6-31G and B3LYP/6-31G(dp) methods, starting from the
crystallographic structural models [33,34]. The functional B3LYP,
used throughout this study, consisted of a hybrid exchange func-
tional as defined by Beckes’s three parameter equation and the
non-local Lee–Yang–Parr correlation functional. The electronic
spectra of the compounds were calculated using the time depen-
dent density functional theory (TDDFT) method [35] in a methano-
lic solution environment employing the Polarizable Continuum
Model (PCM). The vibrational frequencies were obtained at the
thol), 162.5 (C@N), 170.8 (N@C–Oamide). UV/Vis (CH3CH2OH) kmax
,
nm (
e
, Mꢀ1 cmꢀ1): 229 (26800), 263 (20180), 326 (14460), 408
(6220).
2.4. Synthesis of the complex [VO(L2)(OMe)] (2)
Single crystals of [VO(L2)(OCH3)] (2) were obtained following
the same procedure as for complex 1, using VO(acac)2 (1.0 mmol,
265 mg) with H2L2 (1.0 mmol, 346 mg) in the branched tube and
methanol as the solvent. After 2 days, the obtained dark brown
crystals were filtered off and air dried. Yield 85% (376 mg). Anal.
Calc. for C22H19N2O5V (MW = 442.34): C, 59.74; H, 4.33; N, 6.33;
V, 11.52. Found: C, 59.68; H, 4.36; N, 6.29; V, 11.67%. FT-IR (KBr,
cmꢀ1): 3430 (m), 1639 (s), 1588 (s), 1572 (s), 1540 (vs), 1518 (s),
1485 (s), 1466 (vs), 1427 (s), 1385 (s), 1366 (s), 1305 (s), 1214
(m), 1099 (m), 1068 (m), 1046 (m), 992 (vs), 875 (m), 767 (s),
752 (s), 745 (s), 702 (vs), 687 (s), 632 (vs), 524 (m), 469 (m). 1H
NMR (250.13 MHz, DMSO-d6, 25 °C, TMS, ppm) d: 11.41 (s, 1H,
O–Hnaphthol), 8.85 (s, 1H, CH@N), 7.40–8.53 (m, 11H, aromatic),
6.09 (s, 1H), 3.46 (s, 3H, OCH3), 2.39 (s, 3H, CH3). 13C NMR
(62.90 MHz, DMSO-d6, ppm) d: 18.6 (CH3), 43.1 (OCH3), 65.9,
111.3, 119.5, 123.6, 124.7, 126.4, 127.2, 128.6, 129.1, 131.8,
134.1, 136.3, 137.5, 139.9, 154.9 (C–Onaphthol), 164.5 (C@N), 167.8
Table 1
Crystal data for [VO(L1)(OEt)] (1) and [VO(L2)(OMe)] (2).
[VO(L1)(OEt)] (1)
[VO(L2)(OMe)] (2)
Empirical formula
Formula weight
T (K)
Crystal system
Space group
Crystal size (mm)
a (Å)
C
20H16BrN2O5V
C22H19N2O5V
442.33
293 (2)
triclinic
P1
0.3 ꢁ 0.3 ꢁ 0.3
9.9860(6)
14.1137(8)
16.1355(8)
109.075(5)
99.902(5)
103.126(5)
2016.84(22)
4
495.20
150 (2)
triclinic
ꢀ
ꢀ
P1
0.08 ꢁ 0.03 ꢁ 0.02
6.8538(12)
11.386(2)
12.460(2)
89.067(3)
82.650(3)
79.057 (3)
946.8 (3)
2
1.737
2.670
2.45–27.22
8225
b (Å)
c (Å)
(N@CO), 183.2 (C–Oenol). UV/Vis (CH3CH2OH) kmax, (e
, Mꢀ1 cmꢀ1):
a
(°)
234 (32000), 265 (27300), 370 (11600), 410sh (8900).
b (°)
c
(°)
V (Å3)
2.5. Cyclic voltammetry
Z
Dcalc(g cmꢀ3
)
1.457
0.529
3.21–26.37
16280
(mmꢀ1
)
For the cyclic voltammetry studies, a conventional three-elec-
trode system was used with a polished glassy carbon electrode
(area 3.14 mm2) as the working electrode and a platinum wire
counter electrode. The reference electrode was an aqueous Ag/AgCl
saturated electrode, separated from the bulk of the solution by a
bridge with solvent and a supporting electrolyte. The electrolytic
medium consisted of 0.1 mol/L tetrabutylammonium perchlorate
(TBAP) in dimethyl sulfoxide; experiments were carried out at
room temperature. The solutions were freshly prepared before
l
h (o)
Reflections collected
Unique reflections (Rint
)
4169, 0.015
3514
1.04
multi-scan
0.654, 0.746
0.0292, 0.0757
0.36, 0.55
8242, 0.027
6349
1.055
multi-scan
0.757, 1.000
0.0551, 0.1434
0.930, 0.789
Observed reflections [I > 2
Goodness-of-fit (GOF)
Absorp. Correction
Tmin, Tmax
r(I)]
Final R1, wR2 [I > 2
r(I)]
Largest diff peak and hole (e Åꢀ3
)

H. Hosseini-Monfared et al. / Polyhedron 69 (2014) 90–102
93
B3LYP/6-31G level for the validation of the optimized geometries.
All of the calculations were performed with the GAUSSIAN-03 pro-
gram [36]. Topological analyses were carried out on 1 and 2 to cal-
Selected IR spectroscopic data are presented in Table 2. A com-
parison of the spectra of the complexes with those of the ligands
(Fig. S2) provides evidence for the coordination mode adopted by
the ligands in 1 and 2. There is a band at 3189 and 3270 cmꢀ1 in
the IR spectra of H2L1 and H2L2, respectively, which is due to the
N–H stretching vibration. In IR spectra of the ligands a very strong
band also appears at 1647 (in H2L1) and 1644 cmꢀ1 (in H2L2) due to
the C@O stretching vibration. The absence of the band due to the
NH stretching in the IR spectra of the complexes indicates enoliza-
tion/deprotonation of the amide functionality upon coordination to
the metal centre. However, the presence of a strong band at 1638
(in 1) and 1639 cmꢀ1 (in 2) in the IR spectra of the complexes,
which can be assigned to the C@O stretching vibration, indicates
that keto–enol tautomerism has not occurred and the deproto-
nated ligands coordinate in the keto form.
The amide deprotonation without enolization is in contrast
with reported studies of aroylhydrazones complexes, but is in good
agreement with our previous observation in copper(II) and manga-
nese complexes using the 2-hydroxy-3-naphthohydrazone based
ligand. The infrared spectra of 1 and 2 also display a band at
1584 and 1588 cmꢀ1, respectively, which can be assigned to the
C@N stretching frequency of the coordinated hydrazone ligand
and this band is red shifted with respect to the free ligand. There-
fore, the absence of an NH stretching band and the red shift in the
azomethine (C@N) stretching band [38] indicate a tridentate coor-
dination of the dianionic ligands (Scheme 1). Furthermore, in the IR
spectra of the two complexes a very broad band at around
3460 cmꢀ1 is observed, which can be assigned to the stretching
vibration of the naphtholic –OH group involved in intramolecular
hydrogen bonding. The strong band at 987 and 992 cmꢀ1 in the
culate the charge density (
q) and its second derivative Laplacian
(r
2 q) at the bond critical points (BCP) using Bader’s Atoms in Mol-
ecules (AIM) theory with the program AIM2000 [37]. According to
QTAIM, any bonded atoms are connected by a single line, ‘‘bond
path’’, of locally maximum electron density,
a bond critical point (BCP), which represents a minimum of
along the bond path. The sign of the Laplacian,
2q, of the electron
density at a BCP indicates two limit situations; (1) a negative value
of
2q indicates a local charge concentration, which means that
the interaction is a covalent bond; (2) a positive value of
2q indi-
cates charge depletion, which means a closed-shell interaction, as
found in ionic bonds. It is also possible to find positive
2q and rel-
ative high (r) at the BCP, which indicates a covalent polar bond.
The absolute value of the Laplacian and the bond critical point va-
q(r), that originates
q
(r)
r
r
r
r
q
lue for
nuclei.
q(r) relate to the bond order between two neighboring
2.8. Catalytic procedure
Liquid phase catalytic oxidations of thionaisol were carried out
under air (atmospheric pressure) in a 25 mL round bottom flask
equipped with a magnetic stirrer. In a typical experiment, H2O2
was added to a flask containing the catalyst (2 ꢁ 10ꢀ3 mmol) and
thioanisol (1.0 mmol) in a solvent (3 mL). The course of the reac-
tion was monitored using a gas chromatograph equipped with a
capillary column and a flame ionization detector. The oxidation
products were identified by comparing their retention times with
those of authentic samples or alternatively by 1H NMR and GC–
Mass analyses. Control reactions were carried out in the absence
of catalyst, but otherwise under the same conditions as the cata-
lytic runs.
IR spectra of 1 and 2, respectively, can be assigned to
m(V@O)
stretching.
Methanol solutions of the complexes are of a brown-yellow col-
our and have been used to record their electronic spectra. The
hydrazone ligands show the most intense band at around
230 nm and other less intense bands in the range 300–400 nm
3. Results and discussion
(Fig. S3). These bands may be assigned to
p ?
p⁄ and n ? p⁄ tran-
sitions, respectively. All the bands shift in the complexes, indicat-
ing the coordination of the ligands to the metal ion [39]. The
oxidovanadium complexes also show a band at around 265 nm,
which corresponds to the O ? V ligand to metal charge transfer
(LMCT) transition of the V@O moiety, which it is seen at 274 nm
for [VO(acac)2]. The lowest energy transition at about 410 nm for
the two complexes corresponds to L ? V LMCT transitions [40–42].
3.1. Synthesis of the ligands and complexes, and spectroscopic
characterization
H2L2 was synthesized by the reaction of 3-hydroxy-2-naph-
thohydrazide with 2-benzoyilacetone in methanol to afford the de-
sired dissymmetric tridentate O,N,O-donor Schiff base ligand in
excellent yield and purity. The oxidovanadium(V) complexes
[VO(L1)(OEt)] (1) and [VO(L2)(OMe)] (2) were prepared in high
yields by treating an ethanol or methanol solution of the appropri-
ate ligand with an equimolar amount of VO(acac)2. During the syn-
thesis, V(IV) is oxidized to V(V) by aerial oxygen (dissolved in the
solvent) according to Eq (1).
3.2. Description of the structures
In order to define conclusively the coordination sphere of the
metal centre in 1 and 2, a single-crystal X-ray diffraction study
was undertaken. X-ray analysis indicates both 1 and 2 are neutral
mononuclear complexes of V(V). The vanadium centre in both
complexes has a square pyramidal coordinated geometry in an
NO4 donor environment with one nitrogen and two oxygen donor
1
2H2L1ꢀ2 þ 2VIVOðacacÞ2 þ 2ROH þ O2
2
! 2½VVOðL1ꢀ2ÞðORÞꢂ þ 4Hacac þ H2O
ð1Þ
Table 2
Selected IR bands (cmꢀ1) of the ligands H2L1 [29] and H2L2, and the complexes [VO(L1)(OEt)] (1) and [VO(L2)(OMe)] (2)a.
(C@N)b + d(NH)
(C@O)
(NH)
m
(OH)
m
(V@O)
(V–O)
m
~
m
~
m
~
m
~
~
~
m
~
(V–N)
H2L1
H2L2
1
1625(m), 1552(s)
1620(vs), 1537(s)
1584(m)
1647(vs)
1644(vs)
1638(vs)
1639(s)
3189(s)
3270(m)
–
–
3662(w, br)
3473(w, br)
3460
987(vs)
992(vs)
634(s)
632(vs)
471(m)
469(m)
2
1588(s)
3430(m)
a
Scaled IR spectra for 1 and 2 were also calculated at the DFT/B3LYP level (see Experimental) and assignments of the vibration modes were made with a very good
correlation coefficient between the experimental and calculated values (see Tables S1 and S2, and Fig. S1).
b
~
m
~
m(C@C) stretching bands.
The bands assigned to the
(C@N) (azomethyne stretching vibration) are not pure as they contain significant contributions from the aromatic

94
H. Hosseini-Monfared et al. / Polyhedron 69 (2014) 90–102
Table 3
atoms provided by the Schiff base ligands, an oxygen atom from a
coordinated alkoxy group (ethoxy in complex 1 and methoxy in
complex 2) and an oxido ligand (see Figs. 1 and 2, and Table 3
for selected interatomic distances). The terminal oxido ligand
occupies the axial position at a distance of 1.5775(16) (1) and
1.588(2)/1.569(3) Å (2) (Table 3).
Selected bond lengths (Å) and angles (°) in the crystal structure of complexes 1 and 2.a
1
2a
2b
V1—N2
V1—O2
V1—O3
V1—O4
V1—O5
C11—O2
C11—N1
C12—N2
2.1318(18)
1.9152(16)
1.8245(15)
1.5775(16)
1.7558(16)
1.312(3)
2.067(2)
2.079 (3)
2.079 (3)
1.9126(18)
1.8501(18)
1.588(2)
1.768(2)
1.321(3)
1.842 (2)
1.569 (3)
1.742 (3)
1.324 (4)
1.295 (4)
1.309 (4)
In both complexes, the vanadium to oxygen bond lengths follow
the order V@Ooxido < V–Oalkoxo < V–Ophenolate < V–Ohydrazone
. The
metal centre is displaced by 0.457 (1) and 0.474/0.491 Å (2) from
the mean plane defined by the donor atoms N2, O2, O3 and O5 to-
wards the V@O moiety. The tridentate O,N,O donor ligands form
one six-membered and one five-membered chelate ring with bite
angles of about 83° (O3–V1–N2) and 75° (N2–V1–O2) (Table 3).
The structures of the complex units are stabilized by an intramo-
lecular OHꢃ ꢃ ꢃN hydrogen bond, with the nitrogen atom of the
amide group (–N@C–O) acting as the hydrogen bond acceptor
(see Figs. 1 and 2).
1.305(3)
1.289(3)
1.293(4)
1.324(3)
N1—N2
1.390(2)
1.391(3)
1.394 (4)
109.7 (2)
O4—V1—O5
O4—V1—O3
O5—V1—O3
O4—V1—O2
O5—V1—O2
O3—V1—O2
O4—V1—N2
O5—V1—N2
O3—V1—N2
O2—V1—N2
106.42(8)
107.65(8)
98.95(7)
104.56(8)
91.09(7)
141.77(7)
95.77(8)
155.96(7)
82.58(7)
105.88(11)
104.75(10)
97.11(9)
107.23(10)
89.65(9)
144.04(9)
99.85(10)
153.16(10)
83.41(8)
104.52 (13)
97.43 (14)
104.00 (13)
88.39 (11)
146.91 (10)
101.21 (16)
147.71 (15)
83.21 (10)
75.11 (10)
The crystal packing of complex 1 is mainly determined by four
p-p stacking interactions [43]; two of them are shown in Fig. 3a.
74.16(6)
75.36(8)
One of these interactions is between the naphthalene and salicylid-
ene rings (centroid–centroid distance = 3.743 ÅA) and the other
a
0
In the crystal structure of 2, two crystallographically independent molecules of
[VO(L2)(OMe)] (described as 2a and 2b) are present in the asymmetric unit (see
interaction is between the second naphthalene ring and the six
Fig. S4).
0
membered chelate ring (centroid–centroid distance = 3.565 ÅA).
These two interactions cause the pairing of two [VO(L1)(OEt)] com-
with each other via an extensive network of hydrogen bonds
(Fig. S6).
plex units (Fig. 3a). The remaining two
p–p stacking interactions
are between two naphthalene moities belonging to symmetry re-
lated dimeric associations of [VO(L1)(OEt)] complex units, as de-
0
scribed above, with
(Fig. S5), which create a lamellar stacking of complex units in the
crystal lattice (Fig. 3b).
a
centroid–centroid distance of 3.725 ÅA
3.3. Geometry optimization, charge distribution and electronic
structure
Dimeric associations, similar to those observed between the
complex units in 1, are also formed in 2 between pairs of [VO(L2)(-
OMe)] molecules (2A or 2B, see note a in Table 3); these interact
The geometries of the ligands and oxidovanadium complexes 1
and 2 were optimized in the singlet state by means of the HF and
DFT/B3LYP methods using the 6-31G and 6-31G(dp) basis sets. In
general, the predicted bond lengths and angles are in good agree-
ment with the values based upon X-ray crystal structural data,
with a maximum difference in bond length of about 3% at the
B3LYP/6-31G level. Therefore, the B3LYP/6-31G basis set was used
for all further calculations. Selected optimized and experimental
values of geometrical parameters for complexes 1 and 2 are listed
in Tables S3 and S4, respectively. The slight differences between
the theoretical and experimental values may be related to the fact
that the experimental data belong to the solid state, while the cal-
culated values refer to a single molecule in the gas phase. The gen-
eral trends observed in the experimental data are well reproduced
in the calculated ones. In Table 4, the changes observed upon coor-
dination in some selected bond distances and angles of the two li-
gands are compared. The experimental differences and calculated
ones at the DFT level are similar for each selected geometrical
parameter. The maximum variation is observed/computed for the
C–Ohydrazone bond length, with a lengthening upon coordination
to the vanadium centre.
Fig. 1. View of the asymmetric unit in [VO(L1)(OEt)] (1) with the atom numbering
scheme adopted. Displacement ellipsoids are drawn at the 30% probability level.
H1ꢃ ꢃ ꢃN1 1.91, O1ꢃ ꢃ ꢃN1 2.632(6) Å; <O1–H1ꢃ ꢃ ꢃN1 143.10°.
The Mulliken atomic charges for both complexes are summa-
rized in Table 5. The calculated Mulliken charges on the vanadium
atom in both complexes are considerably lower than the formal
charge of 5+, confirming a significant charge donation from the li-
gands. The charges on the terminal oxido oxygen, alkoxo oxygen
and oxygen atoms from the Schiff base ligands are significantly
smaller than ꢀ2 and ꢀ1, respectively. The terminal oxido oxygen
atoms are less negative in comparison with the oxygen atoms from
the Schiff-base ligands. This indicates a higher electron density
delocalization from the terminal oxido ligands towards the vana-
dium centers and agrees with the observed differences in bond
lengths: V@O4oxido < V–O5alkoxo < V–O3phenolate < V–O2hydrazone
.
Interestingly, while the hydrogen atom of the naphthol group
(OH) in the free ligands is oriented towards the outer part of the
molecules and takes part in intermolecular hydrogen bonding
Fig. 2. View of one of the two asymmetric units (2A) in [VO(L2)(OMe)] (2) with the
atom numbering scheme adopted. Displacement ellipsoids are drawn at the 30%
probability level. H1Aꢃ ꢃ ꢃN1A 1.91, O1Aꢃ ꢃ ꢃN1A 2.632(6) Å; <O1AAH1Aꢃ ꢃ ꢃN1A 146°.

H. Hosseini-Monfared et al. / Polyhedron 69 (2014) 90–102
95
Fig. 3. (a) The p–p
stacking interactions between two symmetry-related [VO(L1)(OEt)] (1) complex units involving the naphthalene, salicylidene and six membered chelate
rings; (b) partial view of the lamellar stacking of [VO(L1)(OEt)] (1) complex units.
Table 4
Selected optimized geometrical parameters (bond lengths, Å; bond angles, °) for the ligands H2L1 and H2L2, and their changes upon metal coordination.
B3LYP/6-31G
H2L1
B3LYP/6-31G
DComplex-Ligand [DFT (Experimental)]
H2L2
Complex 1
Complex 2
Complex 1
Complex 2
C11—O2
C11—C10
C11—N1
N2—N1
1.2492
1.4992
1.3835
1.3672
1.3010
116.778
121.899
118.917
120.249
1.2521
1.5020
1.3763
1.3762
1.3195
116.708
122.315
118.932
120.249
1.3123
1.4657
1.3188
1.3728
1.2999
120.313
120.398
109.581
117.271
1.3156
1.4654
1.3129
1.3762
1.3222
120.552
120.044
109.815
117.220
0.0631 (0.077)
ꢀ0.0335 (ꢀ0.029)
ꢀ0.0647 (ꢀ0.033)
0.0056 (0.012)
ꢀ0.0011 (0.005)
3.535 (4.04)
ꢀ1.501 (ꢀ1.16)
ꢀ9.336 (ꢀ12.58)
ꢀ2.978 (2.95)
0.0635 (0.087)
ꢀ0.0366 (ꢀ0.025)
ꢀ0.0634 (ꢀ0.045)
0 (0.013)
0.0027 (0.042)
3.844 (4.30)
ꢀ2.271 (ꢀ1.94)
ꢀ9.117 (ꢀ12.06)
ꢀ3.029 (2.83)
N2—C12
N1—C11—C10
N1—C11—O2
C11—N1—N2
C12—N2—N1
Experimental data for the ligands are taken from Ref. [44].
[44,45], in both complexes it prefers an endo orientation and
participates in intramolecular hydrogen bonding (see Scheme 1,
Figs. 1 and 2). This different behavior can be attributed to the pres-
ence of the NH group of the hydrazone moiety in the free ligands,
which is eliminated upon complexation and thus provides enough
space for the rotation of atom of the naphthol group towards the
inner part of the ligand molecules. The intramolecular hydrogen
bonding established by the naphtholic OH group of both ligands
facilitates their keto–enol tautomerisation upon coordination and
stabilizes the keto-form while the amidic hydrogen atom is elimi-
nated. In Fig. 4, the bond lengths and the Mulliken charges calcu-
lated for the hydrazone part of complexes 1 and 2 are compared
with those calculated for model complexes featuring a naphtha-
lene moiety instead of the naphtholic group.

96
H. Hosseini-Monfared et al. / Polyhedron 69 (2014) 90–102
Table 5
After complexation, a decrease of the electron density is observed
Selected calculated atomic charges for complexes 1 and 2.
on the benzene rings, phenolic oxygens and carbonyl moieties of
the ligands, in keeping with metal coordination of the oxygen
atoms and, therefore, with a charge transfer from the O donors to-
wards the vanadium(V) metal centre. Interestingly, the naphtholic
–OH group, upon coordination, passes to a negative MESP region of
the complex molecule. The O4 oxygen atom of the V@O moiety is
located in a more negative region than those of the other oxygen
atoms in the complexes.
Figs. 6 and 7 show some calculated frontier molecular orbitals
(Kohn–Sham MOs) and the corresponding energies in eV for com-
plexes 1 and 2, respectively. Frontier Kohn–Sham MOs for H2L1 and
H2L2 are represented in Figs. S7 and S8, respectively.
In both complexes, the HOMO is mainly localized on the napht-
holic oxygen atom and on the naphthalene ring of the Schiff base
ligands. The five d-type orbitals of the vanadium centre mainly
contribute to unoccupied MOs, in agreement with a d0 configura-
tion for the metal centre in these complexes. On the other hand,
the LUMO to LUMO+2 MOs in both complexes are mainly localized
on the vanadium center and coordinated atoms from the ligands.
The HOMO–LUMO gap for complexes 1 and 2 equals 2.595 and
2.558 eV, respectively. This value of this gap for H2L1 and H2L2 is
3.701 and 3.401 eV, respectively.
Atom
Mulliken charge
Complex 1
Complex 2
V1
O2
O3
O4
O5
N2
+1.238
ꢀ0.615
ꢀ0.666
ꢀ0.371
ꢀ0.564
ꢀ0.426
+1.665
ꢀ0.653
ꢀ0.692
ꢀ0.451
ꢀ0.655
ꢀ0.585
Clearly, the presence of intramolecular hydrogen bonding be-
tween the naphtholic –OH group and the deprotonated amidic
nitrogen atom stabilizes the keto form of the coordinated ligands
and the increased amount of negative charge on the amidic
nitrogen.
A molecular electrostatic potential (MESP) map was also com-
puted for the two ligands and the two complexes to better identify
the most probable nucleophilic and electrophilic regions on the
molecules [46–48], as shown in Fig. 5. Blue colors indicate positive
MESP regions and red colors indicate negative MESP regions. The
MESP maps show that for the free ligands the negative regions
are mainly located on the benzene ring, phenolic oxygens and
the carbonyl group of the hydrazone moiety. The oxygen of the
naphthol group in the free ligands is included in a positive region.
TDDFT/B3LYP-6-31G calculations using the PCM model (metha-
nol was selected as the solvent) were performed for both ligands
and complexes. Selected calculated states, together with their
Fig. 4. Bond lengths and the Mulliken charges calculated for the hydrazone part of complexes 1 (top left) and 2 (bottom left) and for the analogous model complexes (right)
featuring a naphthalene moiety instead of the naphtholic moiety.
Fig. 5. B3LYP/6-31G calculated 3D molecular electrostatic potential (MESP) maps of the complexes (above) and ligands (below) in various orientations. The considered charge
range for the ligands is the same as that for the corresponding complexes.

H. Hosseini-Monfared et al. / Polyhedron 69 (2014) 90–102
97
Fig. 6. Frontier molecular orbitals of complex 1 with the energy eigenvalues in eV.
Fig. 7. Frontier molecular orbitals of complex 2 with the energy eigenvalues in eV.
vertical excitation wavelengths, scaled by 0.7 for the complexes
and 0.95 for the ligands [49], and oscillator strengths are displayed
in Table 6. For both complexes, the lowest energy band (408 nm in
1) corresponds to the HOMO ? LUMO monoelectronic excitation,
which results from a LMCT (ligand to metal charge transfer) from
the Schiff base ligand to the vanadium(V) metal centre.
The broad absorption band observed at 326 nm for 1 falls in the
same region where n ? p⁄ and p⁄ ligand transitions are pres-
ent (Table S5). This band arises from the overlap of several bands
originating from various transitions, all LMCT in nature. In complex
1, the highest energy band observed at 263 nm is assigned to a
HOMO ? LUMO+1 monoelectronic transition which is LMCT in
p
?

98
H. Hosseini-Monfared et al. / Polyhedron 69 (2014) 90–102
Table 6
Calculated and observed kmax values for the principal singlet electronic transitions of complexes 1 and 2.
Electronic transition
Calculated k (nm)a
fb
Observed k (nm)
Character
Complex 1
1:
HOMO ? LUMO
HOMOꢀ1 ? LUMO
2:
HOMOꢀ2 ? LUMO
HOMOꢀ1 ? LUMO
3:
378.5
366.5
317.4
0.0043
0.0211
0.0074
408
326
263
L ? V (LMCT)
L ? V (LMCT), n ? p⁄(L)
L ? V (LMCT),
p
?
p⁄(L)
HOMO ? LUMO+1
Complex 2
L ? V (LMCT),
p
?
p⁄(L)
1:
HOMO ? LUMO
2:
HOMO ? LUMO
HOMOꢀ1 ? LUMO
3:
430.7
372.2
0.0227
0.0006
418
370
L ? V (LMCT)
L ? V (LMCT)
L ? V (LMCT), n ? p⁄(L),
p ?
p⁄(L)
HOMOꢀ1 ? LUMO+1
318.4
0.3103
234, 265
L ? V (LMCT),
L ? V (LMCT),
p
p
?
?
p⁄(L)
p⁄(L)
HOMOꢀ1 ? LUMO+3
a
Scaling factor: 0.7.
Oscillator strength.
b
nature. Also in this case intra-ligand
p
?
p⁄ transitions can con-
V–N2 bond is the lowest (0.066 and 0.075 au for 1 and 2, respec-
tively). The order of bond lengths on the basis of the (r) values
at the BCPs, V–O4oxido < V–O5alkoxo < V–O3phenolate < V–O2hydrazone
is in good agreement with the experimental findings. Interestingly,
the low values calculated for the ellipticity, , at the BCP of these
tribute to the absorption band. For complex 2 the highest energy
is assigned to the HOMOꢀ1 ? LUMO+1 monoelectronic transition,
which is LMCT in nature.
q
,
e
bonds (Table 7) indicates an axial symmetry for them. The param-
eters calculated at the O1–H1 and N1–H1 BCPs (Table 7) agree with
the formation of an intramolecular O1–H1ꢃ ꢃ ꢃN1 hydrogen bond.
A comparison of the electron density at the RCPs shows that the
electron density at the RCP of the chelating ring containing the
vanadium metal centre (RCP5) is of the same magnitude of that
3.4. Atoms in molecules theory (QTAIM)
The quantum theory of atom in molecules (QTAIM) has been
shown to provide valuable information about many different bond-
ing chemical systems by an analysis of the molecular electron den-
sity distribution. Several excellent reviews [50–52] have been
published on this theory developed by Bader. The values computed
at the RCPs of the aromatic rings, thereby confirming the
p–p
for
r
2q and
q(r) at the bond critical points (BCP) of some selected
stacking interactions observed in the crystal packing of complexes
1 and 2.
bonds in 1 and 2 are collected in Table 7. The bond critical points
(BCP) and ring critical points (RCP) of complexes 1 and 2 were
schematized with a small red symbol for BCPs and a yellow symbol
for RCPs (Figs. S9 and S10, respectively). These figures show the
positions of the maxima in electron density (nuclei) and bond
3.5. Electrochemical studies
Electrochemical cyclic voltammetry measurements were car-
ried out to probe the redox properties of the complexes in solution
[53]. The cyclic voltammograms of the complexes in dimethylsulf-
oxide (DMSO) displayed a quasi-reversible redox peak due to the
VO3+–VO2+ couple. Cyclic voltammetry data are collected in Table 8
and Fig. 8 displays the cyclic voltammograms of the complexes at
paths connecting the nuclei, as well as the positions of the BCPs
and RCPs. With regard to the Laplacian of density (
2
r
q) values,
the bonds between the vanadium and the coordinated donor atoms
can be classified as ionic or highly polar covalent because
2 q for
these bonds has a positive value (
2q > 0). The electron charge
r
r
different scan rates, from 10 to 700 mVsꢀ1. The
value is 458 mV for complex 1 and 0.571 mV for complex 2. Ip.a
DEp = (Epa – Epc)
density at the BCP of the V–O4 bond is the highest (0.255 and
/
0.263 au for 1 and 2, respectively) and that at the BCP of the
Ip.c is 0.715 and 0.744 for complexes 1 and 2, respectively. There
is a linear relationship between the cathodic and anodic peak cur-
rents and the square root of the scan rate (m
1/2) in the range 10–
Table 7
Calculated AIM parameters for some important bonds of complexes 1 and 2.
700 mVsꢀ1 (Fig. 9). This behavior is diagnostic of a diffusion con-
trolled electron transfer process.
Complex 1
(r) [au]
Complex 2
(r) [au]
q
r
1.035
2q [au] Ellipticity
q
r
0.968
2q [au] Ellipticity
3.6. Catalytic reactivity of the complexes
V1–O4 0.255
V1–O5 0.153
V1–O3 0.119
V1–O2 0.099
V1–N2 0.066
C1–O1 0.269
O1–H1 0.305
N1–H1 0.042
RCP1
RCP2
RCP3
RCP4
RCP5
RCP6
0.0041
0.1109
0.0774
0.0827
0.0565
0.0080
0.0171
0.0258
ꢀ1.1742
ꢀ1.1836
ꢀ1.2299
ꢀ1.6678
ꢀ1.2807
ꢀ1.1754
0.263
0.155
0.118
0.105
0.075
0.267
0.312
0.049
0.019
0.019
0.016
0.031
0.016
0.020
0.0031
0.1049
0.0617
0.0807
0.0807
0.883
0.692
0.545
0.292
ꢀ0.504
ꢀ1.389
0.1454
0.151
0.147
0.097
0.174
0.091
0.155
0.759
0.568
0.465
0.261
ꢀ0.494
ꢀ1.768
0.138
0.152
0.149
0.097
0.177
0.098
0.160
Since oxidovanadium complexes are active catalysts for the oxi-
dation of sulfides [54] and hydrocarbons [55], the catalytic oxida-
tion of thioanisol (Scheme 2) as a representative substrate with
0.0057
0.01841
0.02817
ꢀ1.1829
ꢀ1.1905
ꢀ1.2441
ꢀ1.6859
ꢀ1.2773
ꢀ1.1854
Table 8
0.019
0.018
0.015
0.030
0.015
0.019
Electrochemical data of the complexes 1 and 2 in DMSO solution at 298 K.
Ep.a(V) Ip.a
Ep.c.(V) Ip.c
DEP(V) Ip.a/Ip.c
Complex 1 0.545
Complex 2 0.587
6.07 ꢁ 10ꢀ7 0.087
ꢀ8.49 ꢁ 10ꢀ7 0.458
0.715
0.744
5.15 ꢁ 10ꢀ7 0.0154 ꢀ6.65 ꢁ 10ꢀ7 0.571

H. Hosseini-Monfared et al. / Polyhedron 69 (2014) 90–102
99
Fig. 8. Cyclic voltammograms of complexes 1 and 2 (103 mol Lꢀ1) in DMSO and with TBAP as the supporting electrolyte (0.1 mol Lꢀ1); scan rate 10, 20, 30, 40, 60, 80, 100, 150,
200, 300, 350, 400, 450, 500, 600 and 700 mV sꢀ1
.
Fig. 9. Plot of the cathodic and anodic currents versus the square root of sweep rate (m
1/2) for the complexes 1 and 2.
O
Table 9
S
Comparison of the catalytic activities of 1 and 2 in the oxidation of thioanisol with
hydrogen peroxide in different solvents.
S
H2O2/Catalyst
R.T.
Entry Catalyst H2O2 (mmol) Solvent
Temp. (°C) Yield (%)a
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
CH3CN
25
25
25
25
25
98
95
85
93
61
45
24
19
97
92
87
90
55
48
27
16
CH3OH
CH3CH2OH
CHCl3
CH2Cl2
petroleum ether 25
Scheme 2. Catalytic oxidation of thioanisol in the presence of the oxidovanadium
complexes 1 or 2.
DMF
25
25
25
25
25
25
25
hydrogen peroxide was studied in the presence of complexes 1 and
2. The results of control experiments revealed that the presence of
the catalyst and oxidant (H2O2) is essential for the oxidation. The
results of the studies are summarized in Table 9. Thioanisol was
converted to the corresponding sulfoxide with 100% selectivity
by both complexes at room temperature, the activity of 1 being a
little higher than that of 2. Other reaction conditions which can
be manipulated to achieve the maximum oxidation of thioanisol,
such as the oxidant concentration (moles of oxidant per moles of
thioanisol), solvent and temperature of the reaction were investi-
gated. The effects of oxidant concentration on the oxidation of thi-
DMSO
CH3CN
CH3OH
CH3CH2OH
CHCl3
CH2Cl2
petroleum ether 25
DMF
DMSO
25
25
Reaction conditions: catalyst, 2 lmol; thioanisol, 1 mmol; solvent, 3 mL; time,
15 min.
a
Yields are based on the starting thioanisol.

100
H. Hosseini-Monfared et al. / Polyhedron 69 (2014) 90–102
Fig. 10. Time dependence of thioanisole conversion to the corresponding sulfoxide in the presence of [VO(L1)(OEt)] (1) (2
T = 25 °C.
lmol), CH3CN (3 mL), thioanisol (1 mmol), at
Fig. 11. Time dependence of thioanisole conversion to the corresponding sulfoxide in the presence of [VO(L2)(OMe)] (2), (2
T = 25 °C.
lmol), CH3CN (3 mL), thioanisol (1 mmol), at
oanisol by these complexes are illustrated in Figs. 10 and 11. Dif-
ferent oxidant/thioanisol molar ratios (1:1, 2:1, 2.5:1 and 3:1) were
considered, while the ratio of thioanisol (1.0 mmol) to catalyst
of the catalysts decreased in the order acetonitrile > metha-
nol > chloroform > ethanol > dichloromethane > petroleum ether >
DMF > DMSO. Overall, the reactivity of these catalysts in other sol-
vents was very much lower than acetonitrile. From Table 9, one
could see that solvents with moderate donation ability and high
polarities seemed to favor the oxidation reaction. Solvents with a
high coordinating property, such as DMF and DMSO, inhibited
the reaction. The selectivity of sulfoxide decreased when the reac-
tion was carried out at high temperatures (40 and 60 °C) (Table 10).
At high temperatures methylsulfonylbenzene (sulfone) was ob-
tained by further oxidation of methylsulfinylbenzene (sulfoxide).
The mechanism of the oxidation reactions by oxidovanadium com-
plexes of naphthohydrazone ligands have been studied in detail
and it is predicted that the active site in this catalytic reaction is
a peroxo-vanadium-hydrazone Schiff base species, formed in the
reaction mixture in the presence of hydrogen peroxide [29].
(2 lmol) in 3 mL of acetonitrile at room temperature was kept con-
stant. The conversion of thioanisol increased with increasing
amounts of hydrogen peroxide in the reaction mixture. When the
H2O2/substrate mole ratio was 2.5/1, a maximum conversion of
98% was achieved. Aqueous hydrogen peroxide was selected by
considering its high selectivity, atom economy and environmen-
tally friendly properties. Table 9 illustrates the influence of the sol-
vent nature in the catalytic oxidation of thioanisol by these
complexes. Methanol, ethanol, dichloromethane, chloroform,
petroleum ether, DMF, DMSO and acetonitrile were used as sol-
vents, and acetonitrile was found to be the most suitable solvent
for this reaction. The highest conversion of 98% was obtained in
acetonitrile after 15 min. It was observed that the catalytic activity

H. Hosseini-Monfared et al. / Polyhedron 69 (2014) 90–102
101
Table 10
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2.
Entry
Catalyst
Temp. (°C)
Conversion (%)a
Sulfoxide
Sulfone
1
2
3
1
2
2
60
40
60
100
100
100
61
83
52
39
17
48
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2.5 mmol.
a
Yields are based on the starting thioanisol.
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H2L2, were fully synthesized and characterized by spectroscopic
and single crystal X-ray analyses, as well as DFT calculations. Elec-
trochemical studies by the cyclic voltammetry technique indicated
that these complexes behave as quasi-reversible electroactive spe-
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oxidation of thioanisol to the corresponding sulfoxide in acetoni-
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for financial support of this study.
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