Syntheses, characterization, and catalytic potential of novel vanadium and molybdenum Schiff base complexes for the preparation of benzimidazoles, benzoxazoles, and benzothiazoles under thermal and ultrasonic conditions

Article abstract of DOI:10.1007/s00706-021-02780-0

A new ONO-tridentate Schiff base ligand (H₂L) derived from 3-methoxysalicylaldehyde and nicotinic hydrazide was synthesized and characterized by elemental analysis, FT-IR, ¹H NMR, ¹³C NMR, UV–Vis, and powder XRD studies. Then, oxovanadium(V) and dioxomolybdenum(VI) Schiff base complexes, VOL and MoO₂L, were also prepared and characterized by different techniques. Moreover, the catalytic activities of both complexes were investigated for the synthesis of benzimidazoles, benzoxazoles, and benzothiazoles under reflux conditions as well as through ultrasonic irradiation. The results revealed several advantages of this procedure, including high product yields, short reaction times, facile work-up procedure, simplicity in operation, eco-friendly reaction conditions, and green aspects by avoiding toxic catalysts and solvents. Graphic abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s00706-021-02780-0

Monatshefte für Chemie - Chemical Monthly (2021) 152:593–605
https://doi.org/10.1007/s00706-021-02780-0
ORIGINAL PAPER
Syntheses, characterization, and catalytic potential of novel vanadium
and molybdenum Schiꢀ base complexes for the preparation
of benzimidazoles, benzoxazoles, and benzothiazoles under thermal
and ultrasonic conditions
Hadi Kargar1 · Khadijeh Kargar · Mehdi Fallah‑Mehrjardi · Khurram Shahzad Munawar
2
2
3,4
Received: 7 February 2021 / Accepted: 29 April 2021 / Published online: 17 June 2021
©
Springer-Verlag GmbH Austria, part of Springer Nature 2021
Abstract
A new ONO-tridentate Schif base ligand (H L) derived ꢀrom 3-methoxysalicylaldehyde and nicotinic hydrazide was syn-
2
1
13
thesized and characterized by elemental analysis, FT-IR, H NMR, C NMR, UV–Vis, and powder XRD studies. Then,
oxovanadium(V) and dioxomolybdenum(VI) Schif base complexes, VOL and MoO L, were also prepared and characterized
2
by diferent techniques. Moreover, the catalytic activities oꢀ both complexes were investigated ꢀor the synthesis oꢀ benzi-
midazoles, benzoxazoles, and benzothiazoles under reꢁux conditions as well as through ultrasonic irradiation. The results
revealed several advantages oꢀ this procedure, including high product yields, short reaction times, ꢀacile work-up procedure,
simplicity in operation, eco-ꢀriendly reaction conditions, and green aspects by avoiding toxic catalysts and solvents.
Graphic abstract
3
0
4
0
t
h
60
n
70
80
o
9
0
l
100
1
0
20
E
5
0
a
Keywords Vanadium (V) · Molybdenum (VI) · Schif base · Homogeneous catalyst · Benzimidazoles · Benzoxazoles ·
Benzothiazoles
Introduction
The heterocyclic species consisting oꢀ nitrogen atoms con-
*
Hadi Kargar
h.kargar@ardakan.ac.ir
stitute the most widespread category oꢀ natural compounds
and played a very important role in metabolic processes tak-
ing place in living organisms [1]. The most salient nitrogen-
containing aromatic compounds are benzimidazoles (BIZs),
benzoxazoles (BOZs), and benzothiazoles (BTZs) which are
available in many natural products. These heteroaromatics are
especially ꢀound in the molecules which are responsible to
carry out the liꢀe cycles in various cells and the best example
1
Department oꢀ Chemical Engineering, Faculty
oꢀ Engineering, Ardakan University, P.O. Box 184, Ardakan,
Iran
2
3
4
Department oꢀ Chemistry, Payame Noor University,
1
9395-3697 Tehran, Iran
Department oꢀ Chemistry, University oꢀ Sargodha, Punjab,
Pakistan
ꢀ
or this is the presence oꢀ methyl-substituted benzimidazole
Department oꢀ Chemistry, University oꢀ Mianwali, Mianwali,
Pakistan
motiꢀs in vitamin B-12 [2, 3]. The BIZs, BOZs, and BTZs
Vol.:(0
1
123456789)
3

594
H. Kargar et al.
are simple aromatic compounds consisting oꢀ a benzene ring
The chemistry oꢀ vanadium and molybdenum has got
special attention due to their recent signiꢂcance in various
catalytic processes [33–35]. The important chemical reac-
tions catalyzed by oxovanadium and dioxomolybdenum
complexes include halo peroxidation, phosphorylation,
epoxidation, sulꢀoxidation, oxygen atom transꢀer reactions
and neutral esteriꢂcation as well [36–41]. In this context,
oxovanadium and dioxomolybdenum complexes with hydra-
zones emerged as ꢀascinating catalysts ꢀor numerous liquid
phase organic transꢀormations, due to the strong Lewis acid
nature under various conditions [1, 41–44].
ꢀused with a ꢂve-membered imidazole/oxazole/thiazole ring
containing heteroatoms at speciꢂed positions.
Thiabendazole, a ꢀamous ꢀungicidal and antiparasitic
agent, is declared as the ꢂrst available BIZ-based drug [4].
Many derivatives oꢀ BIZs, BOZs, and BTZs were developed
which showed a broad spectrum oꢀ pharmacological activi-
ties like antihistamine, anti-ulcerative, antihypertensive,
antitumor, antiꢀungal, antiviral, analgesic, anticancer, HIV,
antihistaminic, and antiemetic efects [3, 5–9].
Although BIZs, BOZs, and BTZs are very important
ꢀ
rom a medicinal point oꢀ view, still there are a very lim-
Hence, in continuation to our eforts to investigate ꢀor
new and efective catalysts [45–49], and to look ꢀor the cata-
ited number oꢀ methods available in the literature to develop
their derivatives. The most conventional synthesis employs
the condensation between 1,2-diaminobenzene or 2-ami-
nophenol or 2-aminothiophenol, and carboxylic acids/alde-
hydes/alcohols or their derivatives under drastic conditions
lytic potential oꢀ VO(V) and MoO (VI) complexes ꢀor the
2
syntheses oꢀ BIZs, BOZs and BTZs, and their derivatives,
in this paper, we intend to report the two new VO(V) and
MoO (VI) complexes with hydrazones. The key point oꢀ the
2
[10–15]. A dehydrogenating regime is required which is usu-
present work is to validate an efortless and environmentally
ally provided with the help oꢀ catalysts that are mostly strong
acids such as polyphosphoric acid, boric acid, and some
mineral acids [16–18]. An alternative route is a two-step
reaction in which the cyclodehydration oꢀ a mono-acylated
motiꢀ is conducted under various conditions [6].
sound ultrasonic procedure ꢀor catalytic synthesis.
Results and discussion
In addition, some new methods were introduced to pre-
pare derivatives oꢀ BIZs, BOZs, and BTZs which include
the use oꢀ erbium(III) triꢁuoromethanesulꢀonate [19], Co(II)
salts [14, 20], magnetic nanoparticles with ionic liquids [21],
ZnO nanoparticles [22, 23], nanoporous aluminosilicates
A tridentate Schif base ligand (H L) was prepared via the
2
reaction oꢀ equimolar amounts oꢀ nicotinic hydrazide with
3-methoxysalicylaldehyde in alcohol. Reaction oꢀ VO(acac)
2
and MoO (acac) with H L in methanol produces the tar-
2
2
2
geted metal complexes (Scheme 1).
[
24], ionic liquids supported over graphene sheets [15, 25],
metallovesicles [26], mesoporous organosilica [27], and
mesoporous TiO -Al O [28]. Anyhow, most oꢀ them so
FT‑IR study
2
2
3
ꢀar reported synthetic routes have plenty oꢀ limitations like
The stacked FT-IR spectra oꢀ the synthesized compounds
are shown in Fig. 1. Careꢀul inspection oꢀ the spectra is
usually accomplished to investigate the site/s oꢀ coordi-
nation oꢀ ligand with metal centers. The two prominent
(
a) rigorous reaction conditions, ((b) employing corrosive
organic solvents, (c) multistep procedures, (d) monotonous
root arch, (e) poor atom economy, and (ꢀ) high temperature.
Moreover, the principal pitꢀall oꢀ numerous existing proto-
cols is the catalyst deactivation at the end oꢀ synthesis [6,
bands in the spectrum oꢀ H L, in the region oꢀ 3545 and
2
−
1
1656 cm , assigned to the stretching vibrations oꢀ the
ν(NH) and ν(C = O), disappeared ꢀrom this speciꢂc region
upon complexation with metals. These particular sites
oꢀ attachment are in accordance with the enolization oꢀ
the amide ꢀunctional group. This enolization is ꢀurther
conꢂrmed by the emergence oꢀ two new bands at 1261
1
8]. Many oꢀ the endeavors have been carried out to work
ꢀ
or the development oꢀ ꢀeasible, cheaper, cleaner, and one-
step methods.
Ultrasonic waves are very well known to play a vital role
in the selective synthesis oꢀ organic compounds. The spec-
tacular efects oꢀ the ultrasonic waves are due to the cavi-
tation phenomena which is associated with some extreme
afects, such as local increase oꢀ temperature, and local
high pressure which leads to the production oꢀ ꢀree radicals
−
1
and 1251 cm which are attributed to the enolic ν(C-O)
moiety in molybdenum and vanadium complexes, respec-
tively. The azomethine linkage (–HC = N) showed its spe-
−
1
ciꢂc peak at 1604 cm which is also shiꢀted to a higher
wavenumber upon attachment with metals. Furthermore,
[
29] and intense liquid microꢁows [30]. Ultrasound can be
−
1
successꢀully utilized to enhance liquid–solid mass transꢀer
and cause considerable physicochemical changes in the
medium [31], which enhances the selectivity as well as the
conversion oꢀ the desired product by enhancing the rate oꢀ
the reaction [32]. Hence, ultrasonic irradiations are usually
employed ꢀor catalytic oxidation reactions.
the surꢀacing oꢀ two new bands at 906 and 937 cm are
assigned to symmetric and asymmetric stretching vibra-
tions oꢀ the cis-Mo(O) moiety which are in accordance
2
with similar structures reported previously in the literature
[50, 51]. Likewise, the oxovanadium complex also gives
−
1
its characteristics peak oꢀ V=O at 991 cm which is very
1
3

Syntheses, characterization, and catalytic potential of novel vanadium and molybdenum Schiꢀ…
595
Scheme 1
V complex, respectively, which are also in accord with the
similar complexes reported ꢀormerly [52–55].
1
13
H and C NMR studies
NH
C=N
1
13
C=O
The H and C NMR spectra oꢀ H L, VOL, and MoO L
2 2
recorded in DMSO-d are presented in the experimental
6
section and are also shown in Figs. 2, 3, 4, 5, 6 and 7. Two
V=O
1
signals appearing at δ = 12.22 and 10.79 ppm in the H
C=N
NMR spectrum oꢀ the ligand, corresponding to OH (phe-
nolic) and NH protons, respectively, disappear on treatment
with V and Mo salts, demonstrating that the phenolate and
enolate oxygens are the sites oꢀ coordination oꢀ ligand with
the metals. This also conꢂrms the occurrence oꢀ keto-imine
tautomerism upon complexation. Moreover, a singlet signal
oꢀ azomethine proton (–HC=N) at 8.68 ppm observed in the
spectrum oꢀ the ligand was shiꢀted downꢂeld at 8.98 ppm
showing the deshielding due to the decrease in the electronic
density upon the coordination oꢀ azomethine nitrogen with
the metal center. This is in accordance with the FT-IR spec-
trum oꢀ the complex, where ν(HC=N) appears at a higher
wavenumber in comparison with the corresponding ꢀree
ligand. All the signals oꢀ aromatic protons were observed in
the expected range oꢀ 6.88–9.09 ppm. There is a slight shiꢀt
in the positions oꢀ aromatic protons signals oꢀ the ligand
C=N
O=Mo=O (asym.)
O=Mo=O (sym.)
-
1
Wavenumber /cm
Fig. 1 FT-IR spectra oꢀ the ligand and its V and Mo complexes
close to the values ꢀor the analogous oxovanadium Schif
base complexes already reported earlier [52–54]. Moreo-
ver, some new M–O and M–N peaks are also visible at 603
and 457 cm− ꢀor Mo complex and at 582 and 470 cm−1 ꢀor
1
1
3

596
H. Kargar et al.
1
Fig. 2 H NMR spectrum oꢀ the
H L ligand in DMSO-d
2
6
13
Fig. 3 C NMR spectrum oꢀ
the H L ligand in DMSO-d
2
6
1
3

Syntheses, characterization, and catalytic potential of novel vanadium and molybdenum Schiꢀ…
597
1
Fig. 4 H NMR spectrum oꢀ the
VOL complex in DMSO-d
6
13
Fig. 5 C NMR spectra oꢀ the
VOL complex in DMSO-d
6
1
3

598
H. Kargar et al.
1
Fig. 6 H NMR spectra oꢀ the
MoO L complex in DMSO-d
2
6
13
Fig. 7 C NMR spectra oꢀ the
MoO L complex in DMSO-d
2
6
upon complex ꢀormation (δ = 7.03–9.15 ppm in VOL and
.05–9.15 ppm in MoO L).
were observed at δ=167.2, 147.7, and 156.9 ppm, respec-
tively, while in the Mo complex these were observed at
167.2, 148.4, and 156.8 ppm, respectively. The chemical
shiꢀts oꢀ the carbons present in the vicinity oꢀ the coordinat-
ing atoms (i.e., C8, C1, and C7) showed appreciable changes
7
2
1
3
The C NMR spectra oꢀ the ligand and its V and Mo
complexes are shown in Figs. 3, 5, and 7. The signals ꢀor the
carbonyl, phenolic, and methine carbon in the V complex
1
3

Syntheses, characterization, and catalytic potential of novel vanadium and molybdenum Schiꢀ…
599
in their positions due to coordination-induced shiꢀts con-
ꢂrming the participation oꢀ these ꢀunctionalities in coor-
dination. The other aromatic carbons oꢀ the ligand and its
complexes appeared in their respective regions according
to the literature.
UV–Vis study
The electronic transitions ꢀor the Schif base ligand and its
complexes were recorded in dimethylꢀormamide (DMF)
medium ꢀrom 270 to 500 nm at room temperature. The
electronic spectra are displayed in Fig. 8. The Schif base,
H L, displayed electronic transitions at 285, 300, 330, and
2
4
00 nm. These electronic bands may be attributed to π→π*
and n → π* electronic transitions. Upon coordination to
vanadium and molybdenum metals, a blue shiꢀt is observed
in the UV–Vis spectra oꢀ the complexes and the UV pat-
tern oꢀ both complexes cross each other at approximately
Fig. 9 PXRD patterns oꢀ the Schif base ligand and its complexes
3
4
00 nm. The emergence oꢀ the new optical band around
00 nm may be assigned as phenoxo to metal ion electronic
9 degree with large intensities ꢀor both complexes seems
a characteristic ꢀor Mo and V bonded with oxygen atoms
transition [56–59].
[
60] Fig. 9.
Powder XRD study
The phase analysis and crystalline structure oꢀ the ligand
and two complexes were ꢀurther conꢂrmed through pow-
der X-ray difraction (PXRD). A scan ꢀrom a low angle and
short-range with 2θ=5–20 degree resulted in distinct PXRD
spectra oꢀ these three compounds. The main diference lies
in terms oꢀ the reduced relative intensities ꢀor complexes in
comparison to that ꢀor ligand. This could be attributed to
the loss oꢀ coordinated solvent molecules during complex
Catalytic activity studies
Aꢀter successꢀul syntheses and characterization oꢀ VOL and
MoO L, their catalytic activities were evaluated ꢀor the syn-
2
thesis oꢀ benzimidazoles, benzoxazoles, and benzothiazoles.
To optimize the reaction conditions, the reaction between
1,2-phenylenediamine and benzaldehyde (1:1 molar ratio)
ꢀor the synthesis oꢀ 2-phenylbenzimidazole was selected as
a model reaction, and the efect oꢀ solvent and the amount oꢀ
catalyst were investigated. As shown in Table 1, water and
various organic solvents such as ethanol, methanol, acetoni-
trile, DMSO, 1,2-dichloroethane, chloroꢀorm, and dichlo-
romethane were examined in the presence oꢀ 10 mol% oꢀ the
catalyst and it was ꢀound that ethanol was the best solvent
ꢀ
ormation and the coordination oꢀ bulky metal atoms. Fur-
ther, the PXRD difractions observed at around 2θ=7 and
ꢀ
or the reaction. Next, the efect oꢀ the catalytic amount on
the time and yield oꢀ the reaction was investigated using
diferent amounts oꢀ VOL and MoO L. The best result was
2
obtained with 10 mol% oꢀ the catalyst. It is noteworthy that
in the absence oꢀ a catalyst, no product was ꢀormed even
aꢀter the period oꢀ 24 h (Table 1, entry 1).
Due to the great popularity oꢀ ultrasound in chemical syn-
theses, which reduces the reaction time and increases the
yield oꢀ products, and ꢀor eco-ꢀriendly reasons, this reaction
was also conducted under ultrasonic irradiation. The results
showed that the reaction was completed in a short time along
with a higher isolated yield in the presence oꢀ both, VOL and
MoO L complexes (Table 1, entry 15).
2
It is noteworthy that in all cases, only 2-phenylbenzi-
Fig. 8 UV–Vis spectra oꢀ the Schif base ligand and its complexes in
DMF medium
midazole was produced. The disubstituted benzimidazole
1
3

600
H. Kargar et al.
Table 1 The Inꢁuence oꢀ solvent and catalytic amount on the synthesis oꢀ 5a
Entry
Amount oꢀ catalyst/mol%
Solvent
Condition
Yield/%a
VOLb
MoO Lc
2
1
2
3
4
5
6
7
8
9
0
EtOH
EtOH
Reꢁux
Reꢁux
Reꢁux
Reꢁux
100 ºC
Reꢁux
Reꢁux
Reꢁux
Reꢁux
Reꢁux
Reꢁux
Reꢁux
r.t
50 ºC
Ultrasound, r.t
Ultrasound, r.t
Ultrasound, r.t
Trace
92
60
40
35
20
15
20
50
62
80
93
60
Trace
90
50
35
40
20
20
25
45
55
75
90
57
10
10
10
10
10
10
10
10
6
MeOH
MeCN
DMSO
1,2-DCE
CHCl3
CH Cl
2
2
H O
2
10
11
12
13
14
15
16
17
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
8
12
10
10
0
10
10
78
73
Trace
Trace
d
e
35
95
22
91
ꢀ
g
3
Reaction conditions: Benzaldehyde (1 mmol), 1,2-phenylenediamine (1 mmol), catalyst, 5 cm solvent
a
Isolated yield
Reaction time: 1 h
Reaction time: 6 h
Catalyst: VO(acac)2
b
c
d
e
Catalyst: MoO (acac)
2
2
ꢀ
Reaction time: 5 min
g
Reaction time: 30 min
(
1-benzyl-2-phenylbenzimidazole) was obtained only iꢀ
oꢀ 1,2-phenylenediamine with benzaldehyde to produce
2-phenyl-1H-benzoimidazole (5a). It can be concluded ꢀrom
Table 3 that many oꢀ the previously reported procedures are
associated with numerous drawbacks such as necessity oꢀ
surplus quantity oꢀ catalyst, elevated temperature, reduced
yield, lengthy reaction time, and employ the use oꢀ volatile
and noxious organic solvents. Thus, the present technique
overcomes the drawbacks oꢀ the previously reported method-
ologies and has dominance due to high yield, short reaction
time, and ꢀacile synthetic conditions.
two equivalents oꢀ benzaldehyde were employed in aque-
ous media under reꢁux conditions.
To explore the generality and versatility oꢀ this procedure,
diferent substituted aryl aldehydes were condensed with
1
,2-phenylenediamine ꢀor the synthesis oꢀ corresponding
benzimidazole derivatives. Aryl aldehydes containing both
electron-donating and electron-withdrawing groups were
used and as expected, they gave well to the excellent yield oꢀ
products (Table 2, entries 1–13). Under similar conditions,
the methodology was extended to the synthesis oꢀ various
benzoxazoles and benzothiazoles ꢀrom 2-aminophenol and
The proposed mechanistic pathway ꢀor the preparation
oꢀ benzimidazoles in the presence oꢀ VOL or MoO L cata-
2
2
-aminothiophenol, respectively (Table 2, entries 14–29).
Table 3 compares the catalytic eꢃciencies oꢀ VOL and
lyst is presented in Scheme 2. Presumably, the Schif base
complex acting as a Lewis acid catalyst, is coordinated
with the carbonyl group oꢀ aldehyde and then the activated
MoO L with some reported catalysts in the condensation
2
1
3

Syntheses, characterization, and catalytic potential of novel vanadium and molybdenum Schiꢀ…
601
Table 2 Synthesis oꢀ various 2-arylsubstituted benzimidazoles, benzoxazoles, and benzothiazoles by VOL or MoO L catalyst
2
Entry
Comp
X
R
Conventional heatinga,b
Yield/%
Ultrasonic irradiationa,c
Yield/%a
Melting point/ºC
Found
VOL
MoO2L
VOL
MoO2L
Reported [Reꢀ.]
1
2
3
4
5
6
7
8
9
5a
5b
5c
5d
5e
5f
5g
5h
5i
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
NH
O
O
O
O
O
O
O
O
O
H
2-Cl
3-Cl
4-Cl
91
85
90
84
91
85
87
85
92
90
83
86
85
93
84
83
92
86
92
89
88
80
86
82
86
85
82
82
79
90
87
85
83
90
84
82
80
88
90
81
82
80
92
87
86
89
82
84
86
85
84
82
80
88
84
78
81
82
93
91
87
89
92
85
88
89
93
92
83
90
82
94
88
92
95
88
93
92
92
89
93
85
89
88
91
88
91
91
90
85
84
91
87
85
85
88
90
85
88
80
94
90
90
93
86
90
88
88
89
91
85
90
85
89
88
92
290–291
230–232
194–196
281–283
278–280
260–262
185–186
307–309
259–261
222–224
278–280
218–220
211–213
98–100
290–292 [28]
230–232 [61]
194–196 [61]
284–286 [28]
279–281 [61]
263–265 [61]
185–187 [61]
302–305 [28]
260–262 [62]
222–224 [28]
275–277 [61]
220–222 [28]
212–214 [61]
99–102 [28]
70–73 [63]
131–133 [63]
146–148 [62]
142–144 [62]
262–264 [62]
113–114 [28]
97–99 [28]
4-Br
2-NO2
3-NO2
4-NO2
4-CH3
4-OCH3
4-N(CH3)2
2,4-Cl2
2,5-(OCH3)2
H
2-Cl
3-Cl
4-Cl
4-Br
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
5j
5k
5l
5m
6a
6b
6c
6d
6e
6f
6g
6h
6i
73–75
130–132
148–150
142–144
261–263
110–112
100–102
120–122
110–112
112–113
180–182
225–227
85–88
4-NO2
4-CH3
4-OCH3
2,4-Cl2
H
119–121 [28]
110–112 [64]
114–116 [64]
177–179 [28]
224–226 [28]
86–88 [28]
7a
7b
7c
7d
7e
7f
S
S
S
S
S
S
S
4-Cl
3-NO2
4-NO2
4-CH3
4-OCH3
2,4-Cl2
120–122
142–144
118–120 [28]
140–143 [28]
7g
Reaction conditions: Aldehyde (1 mmol), 1,2-phenylenediamine or 2-aminophenol or 2-aminothiophenol (1 mmol), catalyst (10 mol%), 5 cm3
EtOH, under reꢁux conditions or under ultrasonic irradiation at r.t.
a
Isolated yield
b
Reaction times in the presence oꢀ VOL and MoO L are 1 and 6 h, respectively
Reaction times in the presence oꢀ VOL and MoO L are 5 and 30 min, respectively
2
c
2
carbonyl group is attacked by 1,2-phenylenediamine to
obtain intermediate I. This intermediate loses water in an
elimination reaction in the presence oꢀ a catalyst to imine II.
Intramolecular nucleophilic addition and then proton trans-
ꢀer causes ring closure and gives compound III, which is
oxidized upon exposure to air and gives the corresponding
1
3

602
H. Kargar et al.
Table 3 Comparison oꢀ the
eꢃciency oꢀ various catalysts in
the synthesis oꢀ 5a
Entry Catalyst (amount)
Solvent/condition
Time/min Yield/% Reꢀ
1
2
3
4
5
6
7
8
9
1
1
1
p-TsOH (20 mol%)
CoO (10 mol%)
DMF, 80 °C
EtOH, reꢁux
EtOH, reꢁux
EtOH, reꢁux
MeOH, r.t
EtOH, reꢁux
EtOH, r.t
10
85
93
96
91
93
88
84
92
91
93
93
91
[65]
[66]
300
240
20
240
100
70
Co(OH) (10 mol%)
[66]
[67]
[68]
[62]
2
Silica sulꢀuric acid (0.1 g)
CuO-NP/SiO (10 mol%)
2
Nano-ZnO (10 mol%)
TiCl OTꢀ (10 mol%)
[69]
3
VOSO (3 mol%)
EtOH, r.t
EtOH, reꢁux
60
20
[64]
[61]
4
Fe O @SiO -SO H (0.04 g)
3
4
2
3
0
1
2
Cu-complex-bipy-MCM-41 (0.03 g) MeOH, ultrasound, 60 °C 90
VOL (10 mol%)
[70]
This work
This work
EtOH, ultrasound, r.t
EtOH, ultrasound, r.t
5
30
MoO L (10 mol%)
2
Scheme 2
benzimidazole. A similar mechanism can be drawn ꢀor the
synthesis oꢀ benzoxazoles and benzothiazoles catalyzed by
ethanol to obtain the pure products. The catalyst was recy-
cled by evaporating the ethanolic phase in air and washing
again with cold methanol. Aꢀter being air dried, the recycled
catalyst could be reused as such in subsequent experiments
under similar reaction conditions. The yields oꢀ the product
remained comparable in all experiments, indicating that the
catalyst can be recycled at least ꢂve times with no consider-
able loss in its activity (Table 4).
VOL and MoO L complexes.
2
The recyclability and reusability oꢀ the V and Mo com-
plexes were studied ꢀor the reaction oꢀ benzaldehyde and
1
,2-phenylenediamine under ultrasonic irradiation at room
temperature. On the completion oꢀ reaction, the reaction
mixture was ꢂltered and the precipitates were washed with
1
3

Syntheses, characterization, and catalytic potential of novel vanadium and molybdenum Schiꢀ…
603
Table 4 Catalyst reusability in
the reaction oꢀ benzaldehyde
and 1,2-phenylenediamine
under ultrasonic irradiation at
room temperature
were collected by suction ꢂltration and washed thrice with
Run
Yield/%
VOL
cold methanol to get the desired product H L. The Schif
2
MoO2L
base ligand was spectroscopically pure and used in the
1
2
3
4
5
93
91
88
88
85
91
88
85
83
83
syntheses oꢀ the corresponding metal complexes. White
solid; yield 87%; FT-IR (KBr): ̄ꢀ= 3545 (ꢀ
), 1656 (ꢀ
C-O
N–H
), 1247 (ꢀ
), 1604 (ꢀ
), 1571, 1475 (ꢀ
), 1076
C=O
C=N
C=C
−
1
1
(ꢀ
) cm ; H NMR (400 MHz, DMSO-d ): δ = 3.83
N–N
6
3
(3H, s, H-C14), 6.88 (1H, H-C4, t, J=7.8 Hz), 7.05 (1H,
3
3
H-C3, d, J = 7.8 Hz), 7.19 (1H, H-C5, d, J = 7.8 Hz),
3
3
7
.59 (1H, H-C12, dd, J=7.8 Hz, J=4.8 Hz), 8.31 (1H,
3
Conclusion
H-C13, dt (br), J=7.8 Hz), 8.68 (1H, s, CH=N), 8.78 (1H,
3
4
H-C11, dd, J=4.7 Hz, J=1.3 Hz), 9.09 (1H, H-C10, d,
4
13
In this research work we have synthesized a new tridentate
ONO-donor Schif base ligand and its oxovanadium and
J=1.6 Hz), 10.79 (1H, NH, s), 12.22 (1H, OH, s) ppm; C
NMR (100 MHz, DMSO-d6): δ = 55.8 (C14), 113.9 (C4),
118.9 (C3), 119.1 (C6), 120.5 (C5), 123.6 (C12), 128.7
(C9), 135.4 (C13), 147.1 (C1), 147.9 (C2), 148.4 (C7), 148.6
(C10), 152.4 (C11), 161.4 (C8) ppm.
dioxomolybdenum complexes, VOL and MoO L, and char-
2
1
13
acterized them by elemental analysis, FT-IR, H NMR,
C
NMR, UV–Vis, and powder XRD studies. Moreover, the
catalytic activity studies oꢀ the complexes have also been
perꢀormed in the synthesis oꢀ benzimidazoles, benzoxazoles,
and benzothiazoles under conventional heating and ultra-
sonic irradiation. The short reaction span, improved yield,
and mild reaction conditions are the main edges oꢀ the ultra-
sonic methodology.
(N‑[(3‑Methoxy‑2‑oxidophenyl)methylidene]pyridine‑3‑
carbohydrazonato)‑bis(oxido)‑methoxo‑oxo‑vanadium(V)
(VOL, C15H14N3O5V) The new VOL complex was prepared
IV
by mixing 0.265 g oꢀ [V O(acac)2] (1 mmol, acac=acet-
3
ylacetonate) with 0.271 g oꢀ H2L (1 mmol) in 50 cm oꢀ
methanol and then keeping the reaction mixture under reꢁux
conditions ꢀor 3 h. The solid product obtained was ꢂltered
of, washed careꢀully with water, methanol, and diethyl ether
and ꢂnally dried in vacuo. Green solid; yield 65%; FT-IR
(KBr): ̄ꢀ= 1604 (ꢀ C=N), 1352 (ꢀ C=N–N=C), 1473, 1550 (ꢀ
C=C), 1251 (ꢀ C-O), 1039 (ꢀ N–N), 991 (ꢀ V=O), 582 (ꢀ V–O),
Experimental
All chemicals were oꢀ reagent grade and used without ꢀur-
ther puriꢂcation. Elemental analyses were perꢀormed with a
1
13
−1 1
Heraeus CHN-O-FLASH EA 1112 instrument. H and
C
470 (ꢀ V–N) cm ; H NMR (400 MHz, DMSO-d6): δ=3.17
NMR spectra were recorded at ambient temperature with
a BRUKER AVANCE 400 MHz spectrometer using tetra-
methylsilane (TMS) as an internal standard reꢀerence. The
chemical shiꢀt values (δ) are given in ppm. Inꢀrared spectra
were recorded using KBr pellets on a IR Prestige-21 spectro-
photometer. The electronic absorption spectra oꢀ the ligand
and its complexes in DMF were recorded by a UV- 1601 PC
spectrophotometer. The XRD patterns were recorded using a
Bruker D8 ADVANCE X–ray difractometer equipped with
nickel monochromatized CuKα radiation (λ=1.5406 Å). A
UP 400S ultrasonic processor equipped with a 3 mm wide
and 140 mm long probe, which was immersed directly into
the reaction mixture, was used ꢀor sonication. The reactions
under ultrasonic irradiation were carried out at room tem-
(3H, -OCH3 coord., s), 3.83 (3H, s, H-C14), 7.03 (1H, H-C4,
3
3
t, J=7.9 Hz), 7.26 (1H, H-C3, d, J=7.9 Hz), 7.34 (1H,
3
3
H-C5, d, J=7.9 Hz), 7.57 (1H, H-C12, dd, J=7.9 Hz,
3
3
4
J=4.8 Hz), 8.32 (1H, H-C13, dt, J=7.9 Hz, J=1.7 Hz),
3
8.76 (1H, H-C11, d, J = 4.8 Hz), 8.98 (1H, CH = N, s),
1
3
9.15 (1H, H-C10, s) ppm; C NMR (100 MHz, DMSO-d6):
δ=55.8 (C14), 64.6 (C, MeOH coord.), 119.0 (C4), 120.7
(C6), 121.7 (C3), 123.9 (C5), 125.7 (C12), 126.2 (C9), 135.5
(C13), 147.7 (C1), 148.8 (C10), 149.5 (C2), 152.5 (C11),
156.9 (C7), 167.2 (C8) ppm.
(N‑[(3‑Methoxy‑2‑oxidophenyl)methylidene]pyridine‑3‑
carbohydrazonato)‑bis(oxido)‑methanol‑molybdenum(VI)
(MoO2L, C15H15MoN3O6) In a round bottom ꢁask equipped
3
perature in a 40 cm glass reactor.
with a magnetic stirrer bar, 0.271 g oꢀ the Schif base H2L
VI
(
1 mmol) and 0.330 g oꢀ [Mo O (acac) ] (1 mmol) were
2
2
3
(
(
E)‑N′(2‑Hydroxy‑3‑methoxybenzylidene)nicotinohydrazide
H L, C H N O ) Nicotinic hydrazide (1.37 g, 10 mmol),
suspended in 50 cm oꢀ methanol. The resulting suspension
was stirred under reꢁux conditions ꢀor 3 h. Aꢀter that, two-
thirds oꢀ the solvent was evaporated and the solution was
cooled over an ice bath to decrease the solubility oꢀ dissolved
contents. The resultant solid product was ꢂltered of and
washed multiple times with water, methanol, diethyl ether,
and eventually dried in vacuo. Orange solid; yield 78%;
2
14 13 3 3
3
dissolved in 25 cm oꢀ hot methanol, was added dropwise to
a solution oꢀ 1.52 g 3-methoxysalicylaldehyde (10 mmol)
3
in 25 cm oꢀ methanol. The reaction mixture was reꢁuxed
ꢀ
or about 3 h using a magnetically equipped hot plate. Upon
cooling to room temperature, the resulting precipitates
1
3

604
H. Kargar et al.
FT-IR (KBr): v
̄
=1620 (ꢀ C=N), 1435 (ꢀ
), 1469, 1564
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(ꢀ
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(
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Acknowledgements We grateꢀully acknowledge the practical support
oꢀ this study by Ardakan University and Payame Noor University.
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