Synthesis, structural characterization, DFT studies and catalytic properties of dinuclear oxidovanadium(V) complexes derived from adipohydrazone ligands

Article abstract of DOI:10.1016/j.molstruc.2017.04.100

Two new dinuclear oxidovanadium(V) complexes, [V₂O₂(L¹)(OCH₃)₂(CH₃OH)₂]·2(CH₃OH) (1) and [V₂O₂(L²)(OCH₃)₂(CH

Full text of DOI:10.1016/j.molstruc.2017.04.100

Accepted Manuscript
Synthesis, structural characterization, DFT studies and catalytic properties of
dinuclear oxidovanadium(V) complexes derived from adipohydrazone ligands
Hashem Noei-Hootkani, Solmaz Karrari, Hassan Hosseini-Monfared, Peter Mayer,
Behrouz Notash
PII:
S0022-2860(17)30554-9
10.1016/j.molstruc.2017.04.100
MOLSTR 23723
DOI:
Reference:
To appear in: Journal of Molecular Structure
Received Date: 4 February 2017
Revised Date: 19 April 2017
Accepted Date: 25 April 2017
Please cite this article as: H. Noei-Hootkani, S. Karrari, H. Hosseini-Monfared, P. Mayer, B.
Notash, Synthesis, structural characterization, DFT studies and catalytic properties of dinuclear
oxidovanadium(V) complexes derived from adipohydrazone ligands, Journal of Molecular Structure
(2017), doi: 10.1016/j.molstruc.2017.04.100.
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Graphical Abstract

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Synthesis, Structural Characterization, DFT Studies and
Catalytic Properties of Dinuclear Oxidovanadium(V)
Complexes Derived from Adipohydrazone Ligands
a,*
a
a
b
Hashem Noei-Hootkani, Solmaz Karrari, Hassan Hosseini-Monfared, Peter Mayer,
c
Behrouz Notash
a
Department of Chemistry, Faculty of Science, University of Zanjan, 45371-38791,
Zanjan, Iran
b
Fakultät für Chemie und Pharmazie, Ludwig-Maximilians-Universität, München,
Butenandtstr. 5-13, Haus D, D-81377 München, Germany
c
Department of Chemistry, Shahid Beheshti University, G. C., Evin, Tehran
1
983963113, Iran
Abstract
Tow
new
dinuclear
oxidovanadium(V)
complexes,
and
1
[
[
V O (L )(OCH ) (CH OH) ]·2(CH OH)
(1)
2
2
3 2
3
2
3
2
V O (L )(OCH ) (CH OH) ]·2(CH OH) (2), were synthesized from the reaction of
2
2
3 2
3
2
3
1
2
VO(acac) and hexa-dentate N O -donor hydrazone ligands (H L and H L ) in
2
2
4
4
4
methanol solvent. The ligands and complexes were characterized by elemental analysis
and spectroscopic methods. Single crystal X-ray analysis of the brown crystals of
complexes 1 and 2 indicated that these complexes are dinuclear oxidovanadium(V)
complexes which the metal ions have distorted octahedral geometry. There are several
strong and directed hydrogen bonding interactions in the crystal packing of 1 and 2
which stabilize their crystalline format. The catalytic activity of these complexes was
tested in the oxidation of thioanisole using H O . These studies indicated both
2
2
complexes have high catalytic activity in the oxidation of thioanisole. The complexes
were further studied by DFT and TDDFT theoretical calculations.
Keywords: Vanadium complexes; Hydrazone ligands; Catalyst; DFT calculation,
Sulfide oxidation
1

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1
. Introduction
The chemistry of vanadium has received considerable attention due to the
discovery that vanadium in several biological systems and various enzymes [1].
Vanadium complexes, due to their versatile roles and efficient properties, has attracted
scientists for recent decades [2]; nevertheless, there are questions about essential role of
life elements that addressed by vanadium compounds. Due to this, coordination
complexes of dioxovanadium(V) has appealed significant interest for their biorelated
roles in context of diverse enzymic processes; such as their ability to catalytic oxidation
and oxo transfer reactions [3]. Vanadium complexes can show several interesting and
important activities i.e insulin-mimetic [4], antifungal/antibacterial [5], antitumor [6],
and anticancer activities [7].
Structural and functional models for vanadate-dependent haloperoxidases and
other biologically active vanadium compounds have further stimulated vanadium
coordination chemistry [8]. Vanadium complexes are also important catalysts for
several oxidation reactions [9] such as oxidative coupling of phenols [10], epoxidation
of olefins and hydroxylation [11], oxidation of alcohols [12] and sulfides [13]. Schiff
base complexes of vanadium (especially vanadium IV and V), have been classified as
efficient catalysts for the electroreduction of O to H O in acetonitrile [14]. The
2
2
electrochemical reversibility of V–Schiff base complexes enables them to be used as
electron transfer mediator for modification of different electrode materials and the
preparation of chemically modified electrodes. [15].
On the other hand, hydrazone Sciff base ligands, derived from the condensation
of acid hydrazides (R–CO–NH–NH ) with aromatic 2-hydroxy carbonyl compounds,
2
are important tridentate O,N,O-donor ligands [16]. The coordination chemistry and
2

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biochemistry of hydrazones have attracted increasing interest due to their chelating
ability and several biological applications [17]. Hydrazone ligands create environments
similar to biological systems by coordination through oxygen and nitrogen atoms. The
properties of hydrazones are ascribed to the stability of their chelate complexes with
transition metals that catalyze several physiological processes [18].
As part of our research in the study of the coordinating of aroylhydrazones
complexes, here we report synthesis, characterization, crystal structure and catalytic
activity of two new dinuclear oxidovanadium(V) complexes obtained from the reaction
of VO(acac) and hexa-dentate N O -donor hydrazone ligand (Scheme 1). Since
2
2
4
sulfoxides are useful building blocks as chiral auxiliaries in organic synthesis, the
oxidation of sulfides to sulfoxides is one of the important reactions in chemical science
[19]. Therefore, we interested in employing these complexes as catalyst for oxidation
of sulfides in the presence of H O as green oxidant. Moreover, DFT calculations have
2
2
been done to study the molecular orbitals and the nature of electronic transition in the
synthesized complexes.
Scheme 1.
2
. Experimental
2
.1. Materials and instrumentations
Vanadyl bis(acetylacetonate), adipic acid hydrazide, 2-hydroxybenzaldehyde, 5-
bromo-2-hydroxybenzaldehyde and methyl-phenyl sulfide were purchased from Merck
and used as received. Solvents of the highest grade commercially available (Merck)
were used without further purification. FT-IR spectra were recorded in KBr disks with
a Bruker FT-IR spectrophotometer. UV–Vis spectra of solution were recorded on a
1
13
thermo spectronic, Helios Alpha spectrometer. H and C NMR spectra of ligands and
their complexes in DMSO-d solution were recorded on a Bruker 250 and 62.9 MHz
6
3

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spectrometer and chemical shifts are indicated in ppm relative to tetramethylsilane. The
elemental analyses (carbon, hydrogen and nitrogen) were obtained from Carlo ERBA
Model EA 1108 analyzer. The atomic absorption analysis was carried out using Varian
Spectra AA 220 equipment.
2
.2. Synthesis of the ligands
The ligands were prepared in a similar manner by refluxing a mixture of adipic
1
acid hydrazide and 2-hydroxybenzaldehyd (for H L ) or 5-bromo-2-
4
2
hydroxybenzaldehyde (for H L ) with molar ratio of 1:2 in 30 mL methanol. The
4
mixture was refluxed for 7 h. The solution was then evaporated on a steam bath to 5
mL and cooled to room temperature. The obtained solids were separated and filtered
off, washed with 5 ml of cooled methanol and then dried in air. Completion of the
reactions was checked by TLC on silica gel plates.
1
6
1
6
1
2
.2.1. (N' E,N' E)-N' ,N' -bis(2-hydroxybenzylidene)adipohydrazide (H L )
4
Yield: 94%. Anal. Calc. for C H N O (MW = 382.41): C, 62.82; H, 5.80; N,
2
0
22
4
4
-1
1
4.65. Found: C, 62.85; H, 5.82; N, 14.62%. FT-IR (KBr, cm ): 3450 (w, br), 3201 (s),
3
068 (m), 2958 (w), 1682 (vs), 1667 (vs), 1623 (m), 1614 (s), 1559 (vs), 1491 (s), 1461
(m), 1376 (m), 1352 (m), 1277 (vs), 1260 (m), 1140 (s), 1034 (m), 959 (m), 888 (m),
1
7
57 (vs), 696 (m), 658 (w), 560 (m), 492 (w), 474 (w). H NMR (250.13 MHz, DMSO-
d , 25 °C, TMS): δ 1.60 (s, 4H, CH ), 2.23 (s, 2.6H, CO–CH ) + 2.59 (s, 1.4H, CO–
6
H
2
2
CH ), 6.87 (s, 4H), 7.21 (d, 2H, J = 5.5 Hz), 7.45 (s, 2H), 8.24 (s, 0.7H, CH=N) + 8.32
2
(s, 1.3H, CH=N), 10.13 (s, 0.7H, amid N–H) + 11.60 (s, 1.3H, N–H), 11.18 (s, 2H, O–
1
3
H), ppm. C NMR (62.90 MHz, DMSO-d , 25 °C, TMS): δ 25.06, 34.20, 116.75,
6
C
1
19.02, 119.71, 129.86, 131.31, 146.88, 157.71, 168.74 ppm. UV–Vis spectrum in
4

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3
-1
-1
CH OH [λ (ε, dm mol cm )]: 220 (40 800), 280 (48 300), 290 (44 200), 325 nm
3
max
(23 200).
1
6
1
6
2
2
.2.2. (N' E,N' E)-N' ,N' -bis(5-bromo-2-hydroxybenzylidene)adipohydrazide (H L )
4
Yield: 94%. Color: White. Anal. Calc. for C H Br N O (MW = 540.21): C,
20
20
2
4
4
-
1
4
3
1
1
4.47; H, 3.73; N, 10.37. Found: C, 44.50; H, 3.74; N, 10.35%. FT-IR (KBr, cm ):
440 (m, br), 3206 (s), 1659 (m), 1621 (s), 1601 (s), 1570 (m), 1545 (s), 1531 (vs),
502 (vs), 1459 (m), 1307 (m), 1264 (vs), 1218 (m), 1202 (m), 1169 (s), 1130 (vs),
1
104 (s), 1023 (vs), 972 (vs), 950 (s), 801 (s), 786 (vs), 760 (vs), 671 (s), 575 (s). H
NMR (250.13 MHz, DMSO-d , 25 °C, TMS): δ 1.59 (s, 4H, CH ), 2.23 (s, 2.6H, CO–
6
H
2
CH ) + 2.60 (s, 1.4H, CO–CH ), 6.83 (d, 2H, J = 8.25 Hz), 7.35 (d, 2H, J = 8.75 Hz),
2
2
7
1
.70 (s, 2H), 8.18 (s, 0.7H, CH=N) + 8.28 (s, 1.3H, CH=N), 10.35 (s, 0.7H) + 11.68 (s,
13
.3H, N–H amid), 11.21 (s) + 11.27 (s, 2H, O–H) ppm. C NMR (62.90 MHz, DMSO-
d , ppm): δ 25.03 (–CH –), 34.21 (–CH –CO), 110.79, 111.19, 119.01, 121.61,
6
C
2
2
1
23.05, 130.92, 133.77, 138.83, 144.38, 156.70 (–CH=N–), 168.93 (–CO–) ppm. UV–
3
-1
-1
Vis spectrum in CH OH [λmax (ε, dm mol cm )]: 227 (44 000), 282 (49 300), 334 nm
3
(22 500).
2
.3. Synthesis of the complexes
The complexes 1 and 2 were synthesized by the reaction of adipoyil-hydrazone
ligands (1 mmol) and VO(acac) (2 mmol) using the thermal gradient method in a
2
branched tube with methanol as solvent. For this propose, the above mentioned
compounds were placed in the main arm of a branched tube. Methanol was carefully
added to fill the arms, the tube was sealed and the reagents containing arm immersed in
5

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an oil bath at 60 °C while the other arm was kept at ambient temperature. After a week
day, dark brown crystals were deposited in the cooler arm, which were filtered off and
air dried.
1
2
.3.1. Synthesis of [V O (L )(OCH ) (CH OH) ]·2(CH OH) (1)
2
2
3 2
3
2
3
Yield: 80%. Anal. Calc. for C H N O V (MW = 702.50): C, 44.45; H, 5.74;
2
6
40
4
12
2
-1
N, 7.98; V, 14.50. Found: C, 44.51; H, 5.72; N, 8.03; V, 14.45%. FT-IR (KBr, cm ):
060 (w), 1609 (s), 1595 (s), 1566 (m), 1506 (s), 1462 (s), 1435 (m), 1411 (m), 1361
s), 1326 (s), 1308 (m), 1291 (m), 1196 (w), 1149 (m), 1072 (vs), 1053 (s), 1011 (s),
65 (m), 921 (w), 854 (s), 788 (s), 762 (vs), 741 (m), 699 (vs), 683 (s), 655 (s), 600
3
(
9
-1
-1
-5
(
m), 451 (w) 414 (m). UV-Vis spectrum in CH OH [λ , (ε, M cm ), c = 2.5×10 ]:
3 max
2
15 (48 000), 294 (35 400), 387 nm (47 000).
2
2
.3.2. Synthesis of [V O (L )(OCH ) (CH OH) ]·2(CH OH) (2)
2
2
3 2
3
2
3
Yield: 78%. Anal. Calc. for C H Br N O V (MW = 860.29): C, 36.30; H,
2
6
38
2
4
12
2
4
.45; N, 6.51; V, 11.84. Found: C, 36.36; H, 4.47; N, 6.46; V, 11.87%. FT-IR (KBr,
-1
cm ): 3445 (s, vbr, O–H), 3091 (w), 2955 (m), 2864 (w), 1613 (vs), 1544 (vs), 1461
vs), 1414 (m), 1376 (m), 1352 (m), 1274 (s), 1196 (m), 1139 (w), 1054 (m), 1007 (m),
82 (s), 918 (w), 824 (s), 785 (w), 709 (m), 666 (s), 628 (m), 574 (m), 494 (m), 475
(
9
-1
-1
-5
(
m), 449 (w). UV-Vis spectrum in CH OH [λ , (ε, M cm ), c = 2.5×10 ]: 219 (51
3 max
5
00), 275 (38 400), 395 nm (45 000).
2
.4. Catalytic Procedure
The liquid phase catalytic oxidations were carried out under air (atmospheric
pressure) in a round bottom flask equipped with a magnetic stirrer. In a typical
experiment, H O was added to a flask containing catalyst and methyl(phenyl)sulfane
2
2
6

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(1 mmol) in a solvent (3 mL). The course of the reaction 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
1
authentic samples or alternatively by H NMR and GC–Mass analyses.
2
.5. Computational methods
The ground state geometries of the complexes were fully optimized using the
restricted B3LYP/6-31G level of theory in the gas phase, starting from X-ray structure
data [20,21]. All geometry optimizations were followed by frequency calculations at
the same level of theory to confirm the optimized structures to be in true minima on the
potential energy surface. UV–Vis spectra, electronic transitions, absorbance and
oscillator strengths were computed with the time-dependent density functional theory
(
TD-DFT) formalism [22] using the same functional and basis set as for the geometrie
optimizations. All of the calculations were performed with the Gaussian 03 package
23].
[
2
.6. X-ray crystallography data collection and refinement
A dark brown crystal of 1 was investigated by X-ray diffraction at 173 K on a
Nonius KappaCCD diffractometer equipped with a FR591 rotating anode generator
(Mokα λ = 0.7107Å). The structure was solved by Direct Methods with SIR97 [24],
2
and refined with full-matrix least-squares techniques on F with SHELXL-97 [25].All
non hydrogen atoms were refined anisotropically. Hydrogen atoms attached to oxygen
were found in Fourier difference analysis and refined freely whileall other hydrogen
atoms were added in calculated positions. The molecular structure plots were drawn
7

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with ORTEP-3 [26] and DIAMOND [27]. The X-ray diffraction measurements of 2
were made on a STOE IPDS-II diffractometer with graphite monochromated Mo-Kα
radiation. For 2, brown plate shape crystal was chosen using a polarizing microscope
and was mounted on a glass fiber which was used for data collection. Cell constants
and orientation matrices for data collection were obtained by least-squares refinement
of diffraction data from 4551 unique reflections. Data were collected to a maximum 2θ
value of 58.34° in a series of ω scans in 1° oscillations and integrated using the Stoe X-
AREA [28] software package. A numerical absorption correction was applied using X-
RED [29] and X-SHAPE [30] software. The atomic factors were taken from the
International Tables for X-ray Crystallography [31]. All non hydrogen atoms were
refined anisotropically. Hydrogen atoms attached to oxygen were found in Fourier
difference analysis. All other hydrogen atoms were added in calculated positions. All
refinements were performed using the X-STEP32 crystallographic software package
[32]. The crystal data and refinement parameters for 1 and 2 are presented in Table 1.
Table 1.
3
. Results and discussion
3
.1. Synthesis and spectroscopy
The reaction of adipic acid dihydrazide with 2-hydroxyaldehyde or 5-bromo-2-
1
hydroxyaldehyde in methanol gave the desired hexadentate Schiff base ligands (H4L
2
or H L , respectively). The oxidovanadium(V) complexes 1 and 2 with these Schiff
4
base ligands were prepared by treating a methanolic solution of the appropriate ligands
with VO(acac) . These reactions gave dinuclear oxidovanadium(V) complexes in
2
crystalline format. During the synthesis, V(IV) is oxidized to V(V) by aerial oxygen
(dissolved in solvent) according to Eq (1).
8

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1
-2
(IV)
1
(V)
1-2
Eq(1): H4L + 2V O(acac) + 6CH OH + / O → [V O (L )(OCH ) (CH OH) ]
2
3
2
2
2
2
3 2
3
2
·
2(CH OH) + 4Hacac + H O
3 2
1
13
H and C NMR spectral data (Figs. S1-S4) of the ligands in DMSO-d6
1
confirmed the proposed structure of the ligands (Scheme 1). The H NMR spectra of
the ligands indicate that the ligands are in two different forms in solution. The aliphatic
chain of these ligands can rotate around central C−C bonds and this phenomenon
affects on the chemical shifts of two CH groups connected to the -C(=O)- moiety,
2
CH=Nazomethine, N-H and OH groups. This matter can be confirmed by the X-ray
structure of previously reported complexes of adipoyil-hydrazone ligands [33]. As a
result the peaks of these groups appear in two different positions (but very close to each
other). The intensity of these peaks shows that the ratio of two forms is ≈ 65:35. The
signal at δ ≈ 12.2 in the spectra of the ligands is assigned to the common OH group,
concomitant with the observation of a rapid loss of these signals when D O is added to
2
the solution. Also the signals between δ ≈ 10.6 + 11.6 are lost upon addition of D O to
2
the solution. Hence, these signals are assigned to the aromatic N–H groups of two
1
2
different isomers. The resonances at δ 8.24 + 8.32 (for H L ) and 8.18 + 8.28 (for H L )
4
4
are assigned to the azomethine (–CH=N–) group [34].
A list of the important vibrational frequencies of FT-IR spectra the free ligands
and their oxidovanadium complexes, which are useful for determining the mode of
coordination of the ligands, are given in the experimental part (see Figs. S5-S8). A
comparison of the spectra of the complexes with the ligands provides evidence for the
coordination mode of the ligands to the oxidovanadium cores. The adipoyil-hydrazone
1
2
−1
ligands (H L , H L ) exhibit a broad band around 3200 cm due to –NH-vibrations
4
4
-1
[
35] and a broad band centered at about 3400 cm due to the O–H group, probably
9

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involved in intramolecular hydrogen bonding interactions [36]. Also, in FT-IR spectra
1
−1
2
of the ligands very strong bands appear at 1650 (H L ) and 1680 cm (H L ) due to
4
4
C=O-vibration [37]. In addition, the infrared spectra of the complexes display IR
-1
absorption band around 1610 cm which can be assigned to the C=N stretching
frequency of the coordinated hydrazone ligand whereas for the free ligands the same
-1
band is observed around 1600 cm [38]. On complexation the absence of N−H and
C=O bands and shifts of the azomethine (-C=N-) bands of the ligands show
coordination of ligands in enol form (Scheme 1) [39]. In the complexes a very broad
−
1
band around 3450 cm expresses the present of –OH [40] and coordination of
−
1
methanol to the vanadium core. The band at 980 (in 1) and 983 cm (in 2) is assigned
to ν(V=O) [41], this band is observed as a new peak for the complexes and is not
present in the spectra of the free ligands.
Methanolic solutions of ligands and complexes have been used to record the
electronic spectra. These complexes are dark brown in solid state but their methanol
solutions are light brown in color. For the oxidovanadium(V) compounds, no d–d
0
1
bands are expected because of a 3d configuration. H L has three bands in the range
4
2
03-290 nm and a band at 325 nm which are assigned to π → π* and n → π *
2
transitions, respectively. In H L π → π* transitions appear as two peaks at 227 and 282
4
nm while n → π * transition is centered at 343 nm. The oxidovanadium(V) complexes
have bands in the range 210-220 and 370-290 nm which are assigned as intraligand
transitions. The lowest energy transition lying around 380 nm are assigned to LMCT
5
+
transition of the type O → V [42].
1
0

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3
.2. Description of the structures (1 and 2)
In order to define the coordination sphere conclusively, X-ray diffraction
studies were made on single crystals of 1 and 2. A summary of crystallographic data
and refinment of 1 and 2 is given in Table 1. ORTEP diagrams with the atom
numbering scheme of the complexes 1 and 2 are shown in Figs. 1 and 2 and selected
bond lengths and angles are given in Table 2. As shown in Figs. 1 and 2, in both
compounds 1 and 2 the ligands are hexadentate and inversionsymmetric dinuclear
oxidovanadium(V) complexes are formed. In 1 and 2, both vanadium(V) ions have a
six-coordinated environment consisting of five oxygen and one nitrogen donor atom.
The nitrogen atom and the two oxygen atoms are provided by the Schiff base ligands
occupying meridional coordination sites of the distorted VO N octahedrons. The
5
remaining three oxygen atoms are provided by methoxy, methanol and oxo ligands
with the latter two in axial positions. The coordination environments around the
vanadium centers in 1 and 2 are shown in Fig. S9a and Fig. S9b, respectively. In these
compounds the Schiff base ligands form a six-membered [O(1)–V(1)–N(1)] and a five-
membered [O(2)–V(1)–N(1)] chelate ring with bite angles of 83.52(5)º, 83.53(11)º and
7
4.41(5)º, 74.45(10)º for 1 and 2, respectively. The corresponding bond lengths in
complex 1 are O(1)−C(1) 1.3349(19) Å and O(2)−C(8) 1.2947(19) Å, respectively.
Comparison of the C8−N2 and C8−O2 bond lengths with similar bond lengths in free
hydrazone ligands [43] indicates the shortening of the C−N bond length and
lengthening of the C=O bond due to coordination in enol form. These bonds lengths are
very close to the bond lengths observed in complexes containing hydrazone ligands in
the enol form [39]. V−O bond lengths follow the order V−oxo oxygen < V−methoxy
oxygen < V−phenolate oxygen < V−enolate oxygen < V−methanol oxygen (Table 2).
1
1

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Fig. 1.
Fig. 2.
Table 2.
Hydrogen bonding is a common feature for oxidovanadium(IV) and
oxidovanadium(V) compounds in the solid state, if appropriate hydrogen bonding
donors are present [44]. The complexes described in this work contain two major
functionalities which can participate in intermolecular hydrogen bond interactions.
These are the N atom of the hydrazine fragment of the tridentate ligand and the O−H
group of the coordinated methanol. Additionally, two methanol molecules are present
in the crystal packages of both complexes which can contribute hydrogen bonding
donor and acceptor functionality. In complex 1 adjacent molecules of the complex are
linked together intermediated by two solvent methanol molecules by strong
intermolecular O(methanol)−H···N(amide) and O(V-methanol)−H···O(methanol) hydrogen bonds. A
1
D polymeric chain along [110] is formed (Fig. 3). These strong intermolecular
hydrogen bonds stabilize the crystal structure of 1 and 2. A summary of the hydrogen
bond parameters for 1 and 2 are given in Table 3.
Fig. 3:
Table 3.
3
.3. Catalytic reactivity of complexes
The catalytic oxidation of thioanisole (scheme 2) as a representative substrate of
sulfides was studied in the presence of complexes 1 and 2 as catalyst and aqueous
hydrogen peroxide as green oxidant. The results of control experiments revealed that
the presence of both catalyst and oxidant (H O ) is essential for the oxidation. The
2
2
control reactions were carried out with 1 mmol of thioanisole in CH CN and in the
3
1
2

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presence of 2 ꢀmol catalyst or 2 mmol H O at room temperature. The oxidation of
2
2
thioanisole in the absence of H O does not occur and in the absence of catalyst the
2
2
oxidation proceeds only up to 6% after 1 h.
Thioanisole was converted to the corresponding sulfoxide with 100% selectivity by
complexes 1 and 2 at room temperature. Results of the studies are summarized in Table
4
. In order to achieve the maximum oxidation of thioanisole, some important
parameters like the oxidant concentration (moles of oxidant per moles of thioanisole),
solvent and temperature of the reaction were investigated in the presence of complex 1.
The effects of oxidant concentration on the oxidation of thioanisole are illustrated in
Table 4 (entries 3-5). Different oxidant:thioanisole molar ratios (1:1, 2:1 and 3:1) were
considered while the ratio of thioanisole (1.0 mmol) to catalyst (2 ꢀmol) in 3 mL of
acetonitrile was constant. The reactions were carried out at room temperature. The
conversion of thioanisole increased with increasing the amount of hydrogen peroxide
from 1:1 to 3:1. The maximum conversion was obtained when H O :thioanisole mole
2
2
ratio was 3:1 but selectivity towards sulfoxide decreased in this case which can be
attributed to over-oxidation of sulfoxide to sulfone in the presence of excess oxidant.
The reaction was selective towards sulfoxide when the H O /thioanisole mole ratio was
2
2
2
:1. Therefore, the H O :thioanisole mole ratio of 2:1 is selected as the best value of
2 2
oxidant/substrate ratios for oxidation of thioanisole.
The effect of solvent nature was also studied in the catalytic oxidation of thioanisole by
complex 1. For this purpose methanol, ethanol, dichloromethane, chloroform, DMF,
and acetonitrile were used as solvent in the oxidation reaction. The results are collected
in Table 4 (entries 6-10) which indicates acetonitrile was the most suitable solvent for
the reaction. The highest conversion of 100% was obtained in acetonitrile after 30
1
3

ACCEPTED MANUSCRIPT
minutes. Overall, the reactivity of these catalysts in other solvents was much lower than
in acetonitrile which can be attributed to the high solubility of complex in this solvent,
its high dielectric constant and low coordination ability. Low catalytic activity in DMF
can be attributed to its high coordination ability to the metal ions. In the next step, the
effect of reaction temperature was examined in the catalytic activity of complex 1. The
selectivity of sulfoxide decreased when the reaction was carried out at high
temperatures (40 and 60 °C). It seems that high temperatures increase the catalytic
activity of dinuclear oxidovanadium complex but it also facilitates over-oxidation of
sulfoxide to sulfone.
The effect of catalyst amount was also studied in the oxidation of thioanisole. The
oxidation of thioanisole was studied in the presence of 1, 2, 3 and 4 ꢀmol of complex 1.
The conversion of sulfide to sulfoxide increases by increasing the amounts of catalyst.
The effect of increscent from 1 ꢀmol to 2 ꢀmol is quite high but from 2 ꢀmol to 3 ꢀmol
and 4 ꢀmol it is not so considerable. The catalytic activity of complex 2 was studied in
the optimized condition for complex 1. Generally, the catalytic behavior of both 1 and
2
is the same but the activity of complex 1 is a little higher than complex 2. This
difference can be due to the effects of –Br substituent on the benzylidene ring in
complex 2 which can effect on its activity.
Scheme 2.
Table 4.
3
.4. Geometry optimization, charge distribution, frontier molecular orbitals and
electronic structure
The structures of complexes 1 and 2 were optimized in a singlet state by means
of the DFT/B3LYP method along the 6-31(dp) basis set. The absence of negative
1
4

ACCEPTED MANUSCRIPT
normal modes in frequency calculations emphasizes that the optimized structures are in
minimum energies. Selected optimized and experimental values of geometrical
parameters for complexes 1 and 2 are reported in Tables 5. In general, the predicted
bond lengths and angles are in good agreement with the values based on the X-ray
crystal structure data, with a maximum difference in bond length of about 3% at the
mentioned level of theory. Therefore, B3LYP/6-31(dp) level of theory was used for all
further calculations. It is worth to mention that the slight differences between
experimental and theoretical values can be attributed to the fact that the experimental
data at low temperature belong to the solid state in the single crystals of the compounds
while the calculated values refer to a single molecule in the gas phase.
Table 5.
The calculated Mulliken charges for complexes 1 and 2 are shown in Table 6.
The Mulliken charge of vanadium atom is considerably lower than the formal charge of
5
+ which confirms a significant charge donation from the ligands to the metal core.
Moreover, the Mulliken charge on the terminal oxido oxygen is significantly smaller
than 2-. Also, the Mulliken charge of methoxy, phenoxy and enolate oxygen atoms
from the Schiff base ligands are significantly smaller than 1- which can be attributed to
electron density delocalization from these oxygen atoms towards the vanadium centers.
In both complexes the terminal oxido oxygen atoms are less negative in comparison
with the oxygen atoms from the Schiff-base ligands which is related to higher electron
density delocalization from the terminal oxido ligands towards the vanadium centers
and agrees to the observed differences in bond lengths: V–O3oxido < V–O4methoxy < V–
O1phenolate < V–O2enolate and < V–O5methanol
.
Table 6.
1
5

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Frontier Molecular Orbitals (FMOs) have an effective role for providing an
insight into the chemical activities and some of the physical properties of the molecule
[45]. Some of the calculated FMOs and the corresponding energies for complexes 1
and 2 are shown in Figs. 4 and 5, respectively. In both complexes, the HOMO is mainly
localized on the hydrazone moiety (–C(–O)=N–N=CH–) and also the salicylidine part
of the ligands while the LUMO to LUMO+2 MOs are mainly localized on the
vanadium atom. The contribution of five d-type orbitals of vanadium atom to
0
unoccupied MOs is in agreement with the d configuration for the metal center in these
complexes. The HOMO–LUMO gap for complexes 1 and 2 are 2.595 and 2.558 eV,
1
2
respectively. This amount for H L and H L are 3.701 and 3.401 eV, respectively.
2
2
Fig. 4.
Fig. 5.
TDDFT/B3LYP-6-31(dp) was performed for complexes and the selected
calculated states, together with their vertical excitation wavelengths and oscillator
strengths are tabulated in Table 7. For both complexes, the lowest energy band is due to
the HOMO→LUMO monoelectronic excitation, which can be attributed to LMCT
(
ligand to metal) from ligand to vanadium(V) metal centre. The absorption band at
about 270-295 nm for complexes can be attributed to HOMO-1→LUMO and HOMO-
→LUMO transitions. These bands arise from the overlap of several bands originating
2
from various transitions in nature. In complexes 1 and 2, the highest energy band
observed is assigned to contribution of LMCT and intraligand π→π* transitions.
Table 7.
4
. Conclusion
1
6

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In this work two new dinuclear oxidovanadium(V) complexes of hexa-dentate Schiff
base ligands were synthesized and characterized by spectroscopic and single crystal X-
ray analyses. These complexes were used as catalyst in the oxidation of thioanisole in
the presence of H O as green oxidant. The results of catalytic studies indicated that
2
2
these complexes are efficient catalysts for the oxidation of sulfides. The results outlined
by the DFT calculation showed that DFT can be used as a predicative tool to determine
the optimized geometry of V(V) complexes and their UV–Vis spectroscopic properties.
Acknowledgments
The authors are grateful to the University of Zanjan, the Shahid Beheshti University,
the Faculty of Chemistry and Biochemistry of the Ludwig-Maximilians-Universität
München, and the School of Chemistry for financial support of this study.
Supplementary data
Crystallographic data (excluding structure factors) have been deposited at the
Cambridge Crystallographic Data Center (CCDC) as Supplementary Publication
CCDC 1523413 and 1527714 for 1 and 2, respectively. Copies of the data can be
obtained
free
of
charge
via
https://www.ccdc.cam.ac.uk/structures-beta/.
Supplementary data associated with this article can be found, in the online version, at
http://dx.doi.org/.
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1

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1
2
Scheme 1. The structure of ligands (H L and H L ) and the synthesis pathway of
4
4
complexes 1 and 2

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Scheme 2. Catalytic Oxidation of thioanisole in presence oxidovanadium complexes

ACCEPTED MANUSCRIPT
Fig. 1. ORTEP diagram of 1. Thermal ellipsoids are at 30% probability level. Methanol
solvent molecules have been omitted for clarity. Symmetry code: a: 1-x,1-y,-z

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Fig. 2. ORTEP diagram of 2. Thermal ellipsoids are at 30% probability level. Methanol
solvent molecules have been omitted for clarity. Symmetry code: a: -x+1,-y,-z+2.

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Fig. 3. Hydrogen bonds in 1 (red dashed lines). Ellipsoid probability level 50%.

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HOMO-1, -0.21727 eV
HOMO, -0.21614 eV
LUMO+1, -0.09282 eV
LUMO, -0.09302 eV
Fig. 4. Frontier molecular orbitals of complex 1 with the energy eigenvalues in eV.

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HOMO-1, -0.21867 eV
HOMO, -0.21768 Ev
LUMO+1, -0.09874 eV
LUMO, -0.09894 eV
Fig. 5. Frontier molecular orbitals of complex 2 with the energy eigenvalues in eV.