Molybdenum(VI) complexes with tridentate Schiff base ligands derived from isoniazid as catalysts for the oxidation of sulfides: synthesis, X-ray crystal structure determination and spectral characterization

Article abstract of DOI:10.1007/s13738-021-02355-0

Two new complexes, including [MoO₂(L¹)(CH₃OH)] and [MoO₂(L²)(CH₃OH)], with tridentate ONO–donor Schiff base ligands (H₂L¹: (E)-N'-(2-hydroxy-3-methoxybenzylidene)isoni

Full text of DOI:10.1007/s13738-021-02355-0

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-021-02355-0
ORIGINAL PAPER
Molybdenum(VI) complexes with tridentate Schif base ligands
derived from isoniazid as catalysts for the oxidation of sulfdes:
synthesis, X‑ray crystal structure determination and spectral
characterization
Hadi Kargar1
Received: 26 May 2021 / Accepted: 20 July 2021
© Iranian Chemical Society 2021
Abstract
Two new complexes, including [MoO2(L1)(CH3OH)] and [MoO2(L2)(CH3OH)], with tridentate ONO–donor Schif base
ligands (H2L1: (E)-N'-(2-hydroxy-3-methoxybenzylidene)isonicotinohydrazide and H2L2: (E)-N'-(5-chloro-2-hydroxyben-
1
zylidene)isonicotinohydrazide) have been synthesized and characterized spectroscopically through FT-IR and H-NMR
and by elemental analyses. Their solid-state structures were also confrmed by single-crystal X-ray difraction (SC-XRD)
technique. These tridentate Schif base ligands coordinated to the metal ion via phenolate oxygen, imine nitrogen and enolic
oxygen atoms. In the complexes, the molybdenum center adapts a slightly distorted octahedral geometry, by using three
O,N,O-donor atoms of the tridentate ligands, methanol and an oxo group employing the axial positions, while another oxo
group makes the equatorial plane. Moreover, the complexes were utilized in oxidizing the diferent sulfdes as an efcient
homogeneous catalyst in the presence of tert-butyl hydroperoxide (TBHP) as an oxidizing agent in 1,2-dichloroethane (DCE)
as a solvent under refuxed conditions. This method has numerous ascendancies such as high yield, short reaction time and
excellent selectivity to produce corresponding sulfoxides without overoxidation to sulfones.
Keywords Molybdenum · Crystal structure · ONO-tridentate Schif base · Homogeneous catalyst
Introduction
subsequently be reacted with metal salts to give Schif base
metal complexes [12]. Hence, the introduction of azome-
thine group is responsible for the exhibition of diverse kinds
of biological activities due to enhancement of therapeutic
efectiveness [13–15].
Isoniazid (INH) is the N-containing heterocyclic compound,
which has the main constituents of natural products and has
remarkable medicinal applications [1–3]. INH got much
more attention due to the exhibition of various kinds of bio-
logical activities like antibacterial, anti-analgesic, antiviral,
antitumor, antifungal and anti-convulsant [4–9]. INH is also
used as an antitubercular drug since more than 65 years [10].
One of the promising properties of INH is its coordination
with the metal cations through pyridinic nitrogen and/or
carbonylic oxygen, and nitrogen atoms of the hydrazine
part [11]. Another point of interest is that it can be treated
with several alkanals to generate Schif bases, which can
There are various challenges in the feld of chemical
sciences, and the top most is the enhancement of catalytic
activity, reproducibility and selectivity of the catalytic spe-
cies. The catalytic activity and selectivity rely on the rea-
gents used and the conditions provided [16]. It is necessary
to attain the maximum rate of conversion along with the
increased yield and to minimize the waste products gener-
ated during chemical synthesis [17]. The parameters that
have strong infuence for the selective catalytic conversions
include the type of oxidant, amount of the catalyst and tem-
perature of reaction [18].
* Hadi Kargar
There are many traditional processes, which are com-
monly used for the oxidation of sulfdes to sulfoxides by
employing nitric acid, ozone, dinitrogen tetroxide, hydro-
gen peroxide, peracids, tert-butyl hydroperoxide, etc., as a
h.kargar@ardakan.ac.ir; hadi_kargar@yahoo.com
1
Department of Chemical Engineering, Faculty
of Engineering, Ardakan University, P.O. Box 184, Ardakan,
Iran
Vol.:(0123456789)
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Journal of the Iranian Chemical Society
source of oxygen [19]. Sulfoxides have a lot of applications
in pharmacy as well as in engineering for the production
of polymers [20–22]. The direct route for the production
of sulfoxides is selective oxidation [23]. A wide variety of
supported metal oxides and homogeneous transition metal
complexes have been documented in the literature for the
oxidation of sulfoxides [24–26]. However, none of these
methods give an ideal combination of simplicity as well as
selectivity. The selective oxidation of organic moieties by
the metal complexes becomes a turning point not only for
the synthesis of organic compounds but also for bioinorganic
modelling of the oxygen transfer metalloenzymes.
500-MHz spectrophotometer was used for recording NMR
spectra of the synthesized compounds by using DMSO-d6 as
a deuterated solvent at ambient temperature. Tetramethylsilane
(TMS) was used an internal standard for the comparison of
chemical shift values (δ), which are presented in ppm. The
Fourier transform infrared spectra were recorded from 4000
to 400 cm−1 in the form of KBr pellets using an IRPrestige-21
spectrophotometer (Shimadzu). The elemental analysis (CHN)
of the synthesized compounds was taken down on a LECO
CHNS-932 elemental analyzer.
General procedure for the synthesis of the Schif
Molybdenum is a very promising transition metal of
the second row of transition series, which is involved in
life-sustaining processes as it is an important component
of enzymes involved in redox reactions [27–29]. The bio-
chemical potential of molybdenum is due to its capability to
form complexes with a variety of ligands having O, N and
O donor sites [30, 31]. In addition to the biological applica-
tions, molybdenum is also equally important from a catalytic
perspective. Mononuclear dioxomolybdenum(VI) complexes
have played an auspicious role for numerous chemical reac-
tions like hydrodesulfurization, olefn metathesis/epoxida-
tion, sulfoxidation and oxidation of benzylic alcohols [32,
33].
base ligands (H2Ln)(n = 1,2)
ONO-type tridentate Schif base ligands, H2Ln, were synthe-
sized by mixing equimolar amounts of isoniazid and the cor-
responding salicylaldehyde (3-methoxysalicylaldehyde=H2L1
and 5-chlorosalicylaldehyde=H2L2) in absolute MeOH. The
mixed contents were refuxed for about 3 h to get a clear yellow
colored solution, which were then left as such for a period of
fve days until the products were crystallized out in pure form.
The elemental analyses and the spectroscopic data clearly con-
frm the composition of the targeted compounds. The overall
preparation of the subjected ligands is shown in Scheme 1.
H2L1; (E)-N'-(2-hydroxy-3-methoxybenzylidene)isoni-
cotinohydrazide, Yield: 87%. Calculated for C14H13N3O3:
C 61.99, H 4.83, N 15.49%. Analysis found: C 61.82, H
4.88, N 15.56%. FT-IR (KBr, cm−1): 3200 (ꢀ N–H); 1689 (ꢀ
C=O); 1602 (ꢀ C=N); 1566 (ꢀ C=C); 1246 (ꢀ C–O(phenolic)); 1064
Among the transition metal catalysts, the molybdenum-
based complexes proved themselves to be highly active for
the selective oxidation reactions by using peroxides as a
source of oxygen. Hydrazones obtained by the condensation
of isonicotinic acid hydrazides with o-hydroxybenzaldehyde
derivatives are an important class of Schif base ligands,
which have great tendency to form transition metal com-
plexes [34]. These tridentate ONO-donor-type ligands can
stabilize the metal in high oxidation states, which make them
stable to bear the drastic catalytic reaction conditions [35].
In the view of excellent catalytic potential of dioxomo-
lybdenum complexes for sulfoxidation and in continuation
of our previous works in oxidation reactions [36–39], we
have aimed to synthesize and characterize isoniazid-based
molybdenum complexes to explore its catalytic efciency.
1
(ꢀ N–N). H NMR (500 MHz, DMSO-d6, ppm): 3.81 [s, 3
H, (-OCH3)], 6.82 [1 H, (H-C4), t, 3 J=7.9 Hz], 7.00 [1 H,
(H-C3), dd, 3 J=7.9 Hz, 4 J=0.9 Hz], 7.17 [1 H, (H-C5), dd,
3 J=7.9 Hz, 4 J=0.9 Hz], 7.84 [2 H, (H-C10, H-C13), dd,
3 J=4.6 Hz, 4 J=1.4 Hz], 8.69 [1 H, s, (-CH=N)], 8.79 [2
H, (H-C11, H-C12), d, 3 J=4.6 Hz], 11.72 [1 H, s, (-NH)],
12.27 [1 H, s, (-OH)].
H2L2; (E)-N'-(5-chloro-2-hydroxybenzylidene)isonico-
tinohydrazide, Yield: 91%. Calculated for C13H10ClN3O2:
C 56.64, H 3.66, N 12.86%. Analysis found: C 56.78, H
3.71, N 12.77%. FT-IR (KBr, cm−1): 3171 (ꢀ N–H); 1680 (ꢀ
C=O); 1620 (ꢀ C=N); 1548 (ꢀ C=C); 1284 (ꢀ C–O(phenolic)); 1066
(ꢀ N–N). 1H NMR (500 MHz, DMSO-d6, ppm): 6.89 [1 H,
(H-C2), d, 3 J =8.7 Hz], 7.42 [1 H, (H-C3), dd, 3 J =8.7 Hz,
4 J=2.4 Hz], 7.81 [1 H, (H-C5), d, 4 J=2.4 Hz], 7.83 [2 H,
(H-C10, H-C13), d, 3 J=5.4 Hz], 8.64 [1 H, s, (-CH=N)],
8.78 [2 H, (H-C11, H-C12), d, 3 J=5.4 Hz], 11.12 [1 H, s,
(-NH)], 12.34 [1 H, s, (-OH)].
Experimental
Materials and methods
Aldehydes and isonicotinic hydrazide were acquired from
Sigma-Aldrich and Merck, respectively. The dioxomolybde-
num salt, [MoO2(acac)2], was synthesized by using ammo-
nium molybdate and acetylacetone as described previously
[40]. All solvents and other reagents employed in the cur-
rent work were of HPLC grade and used as obtained without
adapting any further purifcation step. BRUKER AVANCE
General procedure for the preparation
of the complexes
The dioxomolybdenum(VI) complexes of the type MoO2(Ln)
(CH3OH) were synthesized by suspending the equimolar
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Journal of the Iranian Chemical Society
Scheme 1 The synthetic
pathways of H2Ln ligands and
MoO2(Ln)(CH3OH) complexes
amounts of the respective ligands, H2Ln (1 mmol), and
MoO2(acac)2 (1 mmol, 0.330 g) in 100 mL of methanol in
a round-bottom fask with a provision of magnetic bar for
steady stirring to attain the uniformity. The resultant suspen-
sion was refuxed for approximately 3 h, and then, 2/3rd of
the solvent was evaporated to concentrate the mixture, and
the remaining contents were cooled down over an ice bath
to generate the red-colored crystals appropriate for SC-XRD
studies. These crystals were separated by the process of fl-
tration and then washed carefully with H2O, MeOH, and
(C2H5)2O separately and fnally dried out in vacuo.
[MoO2(L2)(CH3OH)] (2) Yield: 78%. Calculated for
C13H8ClMoN3O4: C, 38.88; H, 2.01; N, 10.46%. Analysis
found: C, 38.96; H, 1.97; N, 10.39%. FT-IR (KBr, cm−1):
3431 (ꢀ O–H) (coordinated methanol); 1604 (ꢀ C=N); 1544
(ꢀ C=C); 1525 (ꢀ C=N–N=C); 1344, 1259 (ꢀ C-O); 1014 (ꢀ
N–N); 935 (ꢀ O=Mo=O) asym; 904 (ꢀ O=Mo=O) sym; 601 (ꢀ
Mo–O); 462 (ꢀ Mo–N). 1H NMR (500 MHz, DMSO-d6, ppm):
3
8.99 [s, 1 H, (-CH = N)], 8.75 [d, J(H,H) = 9.6 Hz, 2 H,
(H-C11, H-C12)], 7.89 [d, 4J(H,H)=4.5 Hz,1 H, (H-C5)],
7.86 [d, 3J(H,H)=9.6 Hz, 2 H, (H-C10, H-C13)], 7.58 [dd,
3J(H,H)=14.7 Hz, 4J(H,H)=4.5 Hz, 1 H, (H-C3)], 7.01 [d,
3J(H,H)=14.7 Hz, 1 H, (H-C2)], 4.10 [q, 3J(H,H)=8.7 Hz,
1 H, -OH (Methanol)], 3.15 [d, 3J(H,H)=8.7 Hz, 3 H, -CH3
(Methanol)].
[MoO2(L1)(CH3OH)] (1) Yield: 83%. Calculated for
C15H15MoN3O6: C, 41.97; H, 3.52; N, 9.79%. Analysis
found: C, 42.11; H, 3.47; N, 9.64%. FT-IR (KBr, cm−1):
3431 (ꢀ O–H) (coordinated methanol); 1600 (ꢀ C=N); 1560
(ꢀ C=C); 1525 (ꢀ C=N–N=C); 1342, 1263 (ꢀ C-O); 1020 (ꢀ N–N);
931 (ꢀ O=Mo=O) asym; 906 (ꢀ O=Mo=O) sym; 601 (ꢀ Mo–O); 462
(ꢀ Mo–N). 1H NMR (500 MHz, DMSO-d6, ppm): 9.00 [s, 1 H,
(-CH=N)], 8.75 [d, 3J(H,H)=8.9 Hz, 2 H, (H-C11, H-C12)],
7.86 [d, 3J(H,H)=8.9 Hz, 2 H, (H-C10, H-C13)], 7.33 [d,
3J(H,H)=13.2 Hz,1 H, (H-C5)], 7.26 [d, 3J(H,H)=13.2 Hz,
1 H, (H-C3)], 7.04 [t, 3J(H,H)=13.2 Hz, 1 H, (H-C4)], 4.10
[q, 3J(H,H)= 8.7 Hz, 1 H, -OH (Methanol)], 3.80 [s, 3 H,
Crystallographic data of complexes
The X-ray difraction data of the complexes 1 and 2 were
collected at 296(2) K on STOE IPDS II difractometer with
MoKα radiation. The unit cell parameters refnement, data
reduction and correction for Lp and decay were performed
using X-AREA [41] software. Absorption corrections were
applied using MULABS [42] routine in PLATON [43]. The
structures were solved by direct methods and refned by the
least squares method on F2 using the SHELXTL program
3
-OCH3, (H-C14)], 3.15 [d, J(H,H) = 8.7 Hz, 3 H, -CH3
(Methanol)].
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Journal of the Iranian Chemical Society
package [44]. All of the calculations were done by using
the PLATON. The non-hydrogen atoms were refned ani-
sotropically, and the carbon-bonded hydrogen atoms were
positioned geometrically and refned with a riding model
approximation with their parameters constrained to the
parent atom with Uiso (H)=1.2 or 1.5 Ueq (C). The hydro-
gen atoms bonded with the oxygen atoms in the ligand and
complexes were located from the diference Fourier map
and constrained to refne with the parent atoms with Uiso
(H)=1.5 Ueq (O). The crystallographic data and the refne-
ment parameters of 1 and 2 are shown in Table 1.
catalytic conversion was scrutinized by thin-layer chroma-
tography by employing n-hexane and ethyl acetate in 5:2
ratio. On the accomplishment of sulfoxidation, the products
were separated by column chromatography using over silica
gel with n-hexane and ethyl acetate (70:30) as an eluent.
All of the products were known and also further confrmed
by comparison of their physicochemical characteristics with
those of authentic samples.
Results and discussion
General procedure of catalytic sulfoxidation
Synthesis
To a solution of sulfde (1 mmol) in DCE (5 mL), tert-
BuOOH (TBHP) (2 mmol) and Mo complex (0.01 mmol)
were added, and the contents were stirred under refuxed
conditions for the specifed times. The continuation of the
Hydrazone-based ONO-tridentate Schif base ligands, H2Ln,
were prepared by treating isoniazid with the corresponding
salicylaldehyde in nearly 87–91% yield in methanol. The
treatment of H2Ln with MoO2(acac)2 in equimolar amounts
Table 1 Crystallographic
Identifcation code
1
2
data and structure refnement
parameters for 1 and 2
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
C15H15MoN3O6
429.27
293(2)
0.71069
Monoclinic
P21/n
6.816(5)
30.411(5)
7.946(5)
90
93.477(5)
90
1644.0(16)
4
1.734
0.836
C14H12ClMoN3O5
433.66
293(2)
0.71069
Triclinic
P-1
6.542(5)
10.732(5)
12.913(5)
68.053(5)
83.918(5)
75.409(5)
813.7(8)
2
b (Å)
c (Å)
α (°)
β (°)
γ (°)
Volume (Å3)
Z
Calculated density (Mg/m3)
Absorption coefcient (mm−1
F(000)
1.770
1.000
432
)
864
Crystal size (mm)
θ range for data collection (°)
Limiting indices
0.25×0.12×0.08
2.65 to 29.17
0.20×0.14×0.12
2.10 to 29.00
− 8≤h≤9
− 8≤h≤8
− 41≤k≤34
− 10≤l≤10
− 14≤k≤14
− 14≤l≤17
Refections collected
11,916
8224
Independent refections
Data/restraints/parameters
Goodness of ft on F2
4394 [R(int)=0.022]
4394 / 0 / 228
1.044
4209 [R(int)=0.032]
4209 / 0 / 218
1.043
Final R indices [I>2σ(I)]
R1 =0.0260
R1 =0.0324
wR2 =0.0600
wR2 =0.0733
R indices (all data)
R1 =0.0363
R1 =0.0448
wR2 =0.0627
wR2 =0.0768
Largest dif. peak and hole (e.Å−3
)
0.398 and -0.494
0.490 and -0.498
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Journal of the Iranian Chemical Society
Table 2 Characteristic 1H
NMR chemical shift values
(ppm), peak multiplicity and
coupling constant J (Hz) for the
synthesized ligands and their
corresponding Mo complexes
recorded in DMSO-d6
Ligands
Protons
H2L1
H2L2
3.81 (s, 3H)
-
-CH3
H2
-
6.89 (d, 1H), [3J = 8.7 Hz]
-
6.82 (t, 1H), [3J = 7.9 Hz]
7.00 [dd, 1H), [3J = 7.9 Hz, 4J = 0.9 Hz]
7.17 (dd, 1H), [3J = 7.9 Hz, 4J = 0.9 Hz]
7.84 (dd, 2H), [3J = 4.6 Hz, 4J = 1.4 Hz]
8.69 (s, 1H)
H4
7.42 [dd, 1H), [3J = 8.7 Hz, 4J = 2.4 Hz]
7.81 (d, 1H), [4J = 2.4 Hz]
7.83 (d, 2H), [3J = 5.4 Hz]
8.64 (s, 1H)
H3
H5
H10 & H13
H7
8.79 (d, 2H), [3J = 4.6 Hz]
11.72 (s, 1H)
8.78 (d, 2H), [3J = 5.4 Hz]
11.12 (s, 1H)
H11 & H12
-NH
12.27 (s, 1H)
12.34 (s, 1H)
-OH
Complexes
Protons
MoO2L1(CH3OH)
MoO2L2(CH3OH)
3.80 (s, 3H)
-
-CH3
3.15 (d, 3H), [3J = 8.7 Hz]
4.10 (q, 1H), [3J = 8.7 Hz]
-
3.15 (d, 3H), [3J = 8.7 Hz]
4.10 (q, 1H), [3J = 8.7 Hz]
7.01 (d, 1H), [3J = 14.7 Hz]
-
-CH3 (MeOH)
-OH (MeOH)
H2
7.04 (t, 1H), [3J = 13.2 Hz]
7.26 (d, 1H), [3J = 13.2 Hz]
7.33 (d, 1H), [3J = 13.2 Hz]
7.86 (d, 2H), [3J = 8.9 Hz]
8.75 (d, 2H), [3J = 8.9 Hz]
9.00 (s, 1H)
H4
7.58 (dd, 1H), [3J = 14.7 Hz, 4J = 4.5 Hz]
7.89 (d, 1H), [4J = 4.5 Hz]
7.86 (d, 2H), [3J = 9.6 Hz]
8.75 (d, 2H), [3J = 9.6 Hz]
8.99 (s, 1H)
H3
H5
H10 & H13
H11 & H12
H7
Table 3 FTIR spectral data of the ligands and their Mo complexes
(cm−1
NMR spectra
)
The 1H NMR spectral data of the ligands H2Ln and their cor-
responding dioxomolybdenum(VI) complexes in DMSO-d6
are furnished in the experimental part, and the spectra are
presented in Figures S1–S4. The two important peaks in
the spectrum of ligand H2L1 at δ 12.27 and δ 11.72 ppm are
assigned to the –OH and –NH protons, respectively. These
peaks disappeared on complexation, which is an indication
of points of attachment of ligand with metal ion, as well as
the occurrence of keto-enol tautomerization on complex for-
mation. In addition to this, there is also a downfeld shifting
of signal for the azomethine proton (HC=N) of the ligand
from δ 8.69 ppm to δ 9.00 ppm in the spectrum of the com-
plex on coordination with metal atom. This is because of
deshielding of azomethine proton due to the reduction of
electronic density at azomethinic bond due to coordination.
This azomethine linkage is also in accordance with the
results obtained from the FTIR spectral investigations of
MoO2(L1)(CH3OH), where the HC=N signal shifts at lower
wavenumber as compared to the related free ligand. Hence,
in addition to –OH and –NH the nitrogen of HC=N behaves
as third site of coordination with metal ion. Hence, the
ligand is monobasic and tridentate in nature. The signals for
Assignment
H2L1 MoO2L1(CH3OH) H2L2 MoO2L2(CH3OH)
ꢀ O–H(MeOH)
ꢀ N–H
ꢀ C=O
–
3200
1689
3431
–
–
3171
1680
3431
–
ꢀ C=N
ꢀ C=C
ꢀ C=N–N=C
ꢀ C–O(phenolic)
ꢀ C–O
1602 1600
1566 1560
1620 1604
1548 1544
–
1525
1246 1342
1263
1064 1020
–
1525
1284 1344
1259
1066 1014
–
–
ꢀ N–N
ꢀ O=Mo=O(asym)
ꢀ O=Mo=O(sym)
ꢀ Mo–O
–
–
–
–
931
906
601
462
–
–
–
–
935
904
601
462
ꢀ Mo–N
under refux conditions leads to the synthesis of targeted
Mo(VI) complexes. Spectroscopic characterization con-
frms the proposed molecular formula of the synthesized
compounds. Scheme 1 represents the adapted procedure for
production of novel Mo(VI) complexes.
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Journal of the Iranian Chemical Society
Fig. 1 The molecular struc-
tures of complexes 1 (up) and
2 (down) with displacement
ellipsoids at 50% probability
and atom numbering scheme
aromatic protons of the ligands as well as complex come into
sight within the expected range. There is no considerable
change in the chemical shift values of the aliphatic protons
upon complexation. The same kind of pattern is observed
for other two ligands and their respective dioxomolybdenum
complexes.
which vanish in the complex spectrum. This disappearance
is reconcilable by the enolization of the amide functional
group due to the replacement of proton with the metal ion
[45]. This kind of coordination pattern is further supported
by the SC-XRD analysis of the complex. The attachment of
MeOH molecule trans to the oxo group to occupy the sixth
coordination site to stabilize the octahedral geometry is also
very common in these types of ligands, especially when the
preparation and recrystallization of the complexes are per-
formed in the presence of respective solvents [45, 46].
The emergence of new bands in the region of 1263 cm−1
is attributed to the enolic (C-O) part. The determining of
specifc peaks for carbon–carbon double bond of aromatic
rings and azomethine in the spectrum of the ligand could
not be possible because of the complexity of the spectrum
around 1600 cm−1. The confrmation of oxo groups in the
complex was carried out by appearance of the bands at 931
and 906 cm−1, which are ascribed for the asymmetric and
symmetric stretching of the cis-Mo(O)2 moiety [47]. Simi-
larly, the presented bands at 601 and 462 cm−1 are attributed
to Mo–O and Mo–N vibrations, respectively, which are in
accordance with the previously reported compounds [48].
The coordinated methanol molecule also shows its pres-
ence by giving its respective peak at 3431 cm−1. The FTIR
The solvent molecule (MeOH) coordinated with the metal
atom also shows its presence by giving a quartet for -OH
proton at δ 4.10 ppm [3 J (H,H)=8.7 Hz] and a doublet for
-CH3 protons at δ 3.15 ppm [3 J (H,H)=8.7 Hz]. The details
of chemical shift values of the ligands and their respective
dioxomolybdenum(VI) complexes are presented in Table 2.
FTIR spectral studies
The FTIR spectra of the ligands and their respective Mo
complexes are displayed in Figures S5–S8, and the spectral
details are presented in the experimental portion. The indica-
tion of binding sites of the ligand from magnetic spectra is
further investigated by a conscientious assessment of FTIR
spectra of the H2L1 ligand and its corresponding MoO2(L1)
(CH3OH) complex. The two fagship regions of interest, i.e.,
3200 and 1689 cm−1, are there in the spectrum of the ligand.
These regions are attributed to the (NH) and (C=O) groups,
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Journal of the Iranian Chemical Society
Table 4 The selected bond lengths (Ǻ) and bond angles (°) in 1 and
2 complexes
vibrational data for all of the ligands and their corresponding
dioxomolybdenum complexes are given in Table 3.
1
2
Illustration of single‑crystal analysis of complexes
Bond lengths
Mo(1)–O(1)
Mo(1)–O(2)
Mo(1)–O(3)
Mo(1)–O(4)
Mo(1)–O(5)
Mo(1)–O(6)
Mo(1)–N(1)
N(1)–C(7)
N(1)–N(2)
C(1)–O(3)
C(8)–O(4)
Bond angles
O(1)–Mo(1)–O(2)
O(1)–Mo(1)–O(3)
O(1)–Mo(1)–O(4)
O(2)–Mo(1)–O(3)
O(2)–Mo(1)–O(4)
O(3)–Mo(1)–O(4)
O(1)–Mo(1)–N(1)
O(2)–Mo(1)–N(1)
O(3)–Mo(1)–N(1)
O(4)–Mo(1)–N(1)
1.7096(17)
1.6907(17)
1.9354(15)
2.0219(14)
–
2.3659(18)
2.222(2)
1.289(3)
1.393(2)
1.341(2)
1.320(3)
1.687(2)
1.705(2)
1.9338(19)
2.0161(18)
2.316(2)
–
2.242(2)
1.279(3)
1.396(3)
1.295(3)
1.324(3)
The simple molecular structures of the Mo complexes are
presented in Fig. 1. The complex 1 crystallized out in mono-
clinic, while complex 2 was appeared in triclinic systems.
The space groups found in single-crystal X-ray difraction
analysis are P21/n and P-1 for 1 and 2, respectively. A tri-
dentate (ONO) Schif base ligand, two oxo groups at cis
positions and a coordinated methanol molecule are responsi-
ble for the establishment of a distorted octahedral geometry
around the molybdenum center. This coordination sphere is
further validated by the bond angles associated with the cen-
tral molybdenum atom, which are given in Table 4. The bond
lengths for Mo–N and Mo–O in the complexes are almost
analogous to the similar hydrazone-based ligands already
reported in the literature [49–51].
105.07(9)
102.02(8)
97.62(8)
98.66(8)
96.68(8)
150.90(6)
157.25(7)
96.38(7)
81.77(6)
72.04(6)
105.02(11)
99.01(11)
96.68(10)
102.90(10)
98.14(10)
149.39(8)
90.94(9)
162.41(10)
81.39(7)
72.21(7)
The iminic N1–C7 [1.277(6) Ǻ] and N1–N2 [1.408(5)
Ǻ] bond lengths were increased after coordination with
Mo atom [50]. As commonly revealed from structurally
similar complexes, the elongation Mo1–O6 [2.316(3)
Ǻ] bond length trans to oxo O1 group is an indication of
weak coordination of methanol at the axial position, which
is due to the strong π-donor character of the oxo group
opposite to coordinated methanol [50]. The two Mo = O
bond distances and the subtended O(1) = Mo(1) = O(2)
are also in accordance with literature values [49–51]. The
methoxy-substituted phenyl ring and pyridine ring are
Fig. 2 Packing diagram of 1
showing formation of zigzag
chain of molecules through
H-bonding. Only selected
H-atoms are shown for clarity
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Journal of the Iranian Chemical Society
dimers via O–H…N bonding to form R2(18) loop; N-atom
Table 5 Hydrogen-bond geometry (Å, º) for 1 and 2
2
of pyridine ring acts as acceptor as shown in Fig. 2 and
presented in Table 5. The dimers are interlinked by com-
paratively weak C–H…O bonding, where one of the oxo
groups acts as H-bond acceptor. By the combination of
O–H…N and C–H…O bonding, C16 zigzag chain of mol-
ecules is formed along [010] direction. Crystal packing of
complex 2 is also mainly stabilized by O–H…N bonding
to form R2(18) dimeric loop as shown in Fig. 3 and fur-
ther stabil2ized through the comparatively weak C–H…O
bonding, likewise ofset π…π stacking interaction is found
between rings for the further strengthening of the crystal
packing but diference between crystal packing of complex
1 and 2 is that in 1 inter-centroid distance for ofset π…π
stacking interaction between pyridine ring and chelating
ring is less than inter-centroid distance between similar
rings in complex 2.
D—H···A
D—H H···A D···A
D—H···A °
1
O6—H6···N3i
0.75
0.93
1.98
2.38
2.64
2.61
2.66
1.98
1.92
2.56
2.33
2.60
2.50
2.724 (3) 171
C7—H7A···O2ii
3.291 (4) 166
3.148 (4) 115
3.455 (4) 151
3.467 (4) 143
2.724 (3) 171
2.691 (4) 165
3.257 (4) 132
3.181 (4) 151
3.273 (4) 130
3.417 (5) 167
C11—H11A···O2iii 0.93
C12—H12A···O1iv 0.93
C14—H14B···O1v
O6—H6···N3i
0.96
0.75
0.79
0.93
0.93
0.93
2
O5—H5···N3vi
C5—H5A···O1vii
C7—H7A···O1viii
C7—H7A···O1vii
C12—H12A···O2ix 0.93
Symmetry codes: (i)−x+1,−y,−z+1; (ii) x+1, y, z;
(iii)−x+1,−y,−z+2; (iv)−x,−y,−z+1; (v) x+1/2,−y
+1/2, z+1/2. (vi)−x,−y+1,−z; (vii)−x,−y+1,−z+1;
(viii) x−1, y, z; (ix)−x+1,−y+1,−z. (x) x,−y+3/2, z−1/2;
(xi)−x, y−1/2,−z−1/2; (xii)−x+1,−y+1,−z+1; (xiii)−x+1, y
−1/2,−z+1/2
Catalytic oxidation results
Dioxomolybdenum complexes have widely been utilized as
catalysts in various organic transformations, especially in
oxidation reactions [52]. The catalytic activity of the pre-
pared Mo complexes was studied in the selective sulfoxi-
dation of several aryl and alkyl sulfdes. To optimize the
reaction conditions, the oxidation of diphenyl sulfde in the
presence of MoO2(L1)(CH3OH) complex was chosen as a
model, and the efect of diferent factors afected on the reac-
tion was investigated.
roughly planar with respective root-mean-square devia-
tion of 0.0192 Å and 0.0116 Å in complex 1. In complex 2,
chloro-substituted phenyl ring and pyridine ring are planar
with respective r.m.s deviation of 0.0179 Å and 0.0113 Å.
The dihedral angle between methoxy-substituted phenyl
ring and pyridine ring is 8.98 (1)° in complex 1. While the
dihedral angle between chloro-substituted phenyl ring and
pyridine ring is 11.96 (1)° in complex 2. The molecules
of complex 1 are connected with each other in the form of
Fig. 3 Packing diagram of com-
plex 2. Only selected H-atoms
are shown for clear representa-
tion
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Journal of the Iranian Chemical Society
Table 6 The efect of solvent on the oxidation of diphenyl sulfde
The reaction was examined in various polar and nonpolar
organic solvents in the presence of TBHP as the oxidant and
MoO2(L1)(CH3OH) complex as the catalyst (Table 6). Based
on the results, it was found that 1,2-dichloroethane (DCE)
is the best solvent for the reaction due to its higher reactiv-
ity and selectivity. By changing the reaction temperature in
DCE, it can be found that the rate of reaction increases with
the increase of temperature and the best result was observed
under refux conditions.
with TBHP catalyzed by MoO2(L1)(CH3OH).a
Entry
Solvent
Conditions
Conversion (%)b
1
2
3
4
5
6
7
8
9
Toluene
n-Hexane
Acetonitrile
Acetone
Ethanol
DCE
Refux
Refux
Refux
Refux
Refux
Refux
r.t
70
75
85
45
80
100
Trace
35
A summary of the efect of diferent oxidants like NaIO4,
H2O2, urea H2O2 (UHP), (Bu)4NIO4 and TBHP on the
oxidation of diphenyl sulfde in the presence of MoO2(L1)
(CH3OH) is presented in Table 7. The reaction did not per-
form in the absence of oxidant and the best results obtained
using TBHP as the source of oxygen. The investigation of
the reaction in the presence of diferent amounts of oxidant
showed that the conversion did not complete in the presence
of amounts less than 2 mmol of oxidant. The best amount of
oxidant for the oxidation to diphenyl sulfoxide was 2 mmol,
which completed the reaction and produced sulfoxide as the
only product. Higher amount of oxidant did not have much
efect on improving the results.
DCE
DCE
DCE
50 ºC
70 ºC
75
aReaction conditions: diphenyl sulfde (1 mmol), TBHP (2 mmol),
catalyst (1 mol%), solvent (5 mL), refux, 1 h
bIsolated yields
Table 7 The efect of type and amount of oxidant on the oxidation of
diphenyl sulfde catalyzed by MoO2(L1)(CH3OH)a
Entry
Oxidant
Oxidant amount
(mmol)
Conver-
sion
(%)b
To obtain the optimum amount of the catalyst, the model
reaction was performed in the presence of diferent amounts
of MoO2(L1)(CH3OH). It was observed that the reaction did
not complete in the absence of the catalyst even in higher
times. The conversion rate increased with increasing the cat-
alyst amount, and when 1 mol % of the catalyst was applied,
the transformation was completed (Table 8).
1
2
3
4
5
6
7
8
9
No oxidant
NaIO4
H2O2
0
2
2
2
2
2
1
0
85
75
65
10
100
35
70
100
UHP
Bu4NIO4
TBHP
TBHP
TBHP
TBHP
After optimization of the reaction conditions, oxidation
of diferent sulfdes in the presence of MoO2(Ln)(CH3OH)
complexes was evaluated under these optimized reaction
conditions (Table 9). It is obvious from the table that all
studied sulfdes were oxidized under this catalytic system
and aforded the corresponding sulfoxides in 100% selectiv-
ity. It is noteworthy that there is no evidence of overoxida-
tion to produce sulfone in any substrate.
1.5
3
aReaction conditions: diphenyl sulfde (1 mmol), oxidant, catalyst
(1 mol%), DCE (5 mL), refux, 1 h
bIsolated yields
Conclusion
Table 8 The efect of amount
Entry Catalyst
amount
Conver-
sion
of catalyst on the oxidation of
diphenyl sulfde catalyzed by
MoO2(L1)(CH3OH)a
In the present work, we have described the synthesis of
[MoO2(Ln)(CH3OH)] complexes with hydrazone-based
Schif bases derived by the condensation of substituted
salicylaldehydes and isoniazid. The synthesized products
were characterized spectroscopically and analytically by
(mol%)
(%)b
1
2
3
4
5
No catalyst
0
0.2
0.5
0.8
1
50
70
90
100
1
FTIR, H NMR and elemental analysis (CHN), and the
solid-state structures of the complexes were confirmed
by single-crystal X-ray difraction studies. The tridentate
(ONO) Schif base ligands coordinate to Mo through the
phenolic and enolic O atoms and the iminic N atom to dis-
play a distorted octahedral geometry around molybdenum
center. Moreover, the catalytic efciencies of the synthesized
aReaction conditions: diphe-
nyl sulfde (1 mmol), TBHP
(2 mmol), catalyst, DCE
(5 mL), refux, 1 h
bIsolated yields
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Journal of the Iranian Chemical Society
Table 9 Selective oxidation of sulfdes with TBHP catalyzed by MoO2(Ln)(CH3OH) complexes.a
Entry
Sulfde
Sulfoxideb
Time (min.)/
Yield (%)c
1
2
1
2
3
4
20/92
20/93
60/91
30/91
30/92
60/93
30/93
30/90
5
6
7
8
30/86
20/92
20/88
20/91
30/87
20/93
20/89
20/90
aReaction conditions: sulfde (1 mmol), TBHP (2 mmol), catalyst (1 mol%), DCE (5 mL), refux conditions
bAll products were identifed by comparison of their physical and spectral data with those of authentic samples
cIsolated yield
dioxomolybdenum(VI) were evaluated for the selective sul-
foxidation of sulfdes using TBHP in 1,2-dichloroethane
under refux conditions. There are numerous advantages
of the selected catalytic process like high yield, reduced
time for the catalytic conversion and excellent selectivity
to produce corresponding sulfoxides without overoxidation
to sulfones.
Declarations
Conflict of interest The authors declare that they have no confict of
interest.
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