Experimental and theoretical studies of new dioxomolybdenum complex: Synthesis, characterization and application as an efficient homogeneous catalyst for the selective sulfoxidation

Article abstract of DOI:10.1016/j.ica.2021.120568

A new dioxomolybdenum(VI) complex with tridentate ONO-donor Schiff base ligand derived from isoniazid has been synthesized and characterized by elemental analyses, FT-IR and ¹H NMR, and its solid state structure was confirmed by single crystal

Full text of DOI:10.1016/j.ica.2021.120568

Inorganica Chimica Acta 527 (2021) 120568
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Inorganica Chimica Acta
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Experimental and theoretical studies of new dioxomolybdenum complex:
Synthesis, characterization and application as an efficient homogeneous
catalyst for the selective sulfoxidation
,
Hadi Kargara *, Mehdi Fallah-Mehrjardib, Reza Behjatmanesh-Ardakanib,
,
Khurram Shahzad Munawarc d, Muhammad Ashfaqe,f, Muhammad Nawaz Tahire
a Department of Chemical Engineering, Faculty of Engineering, Ardakan University, P.O. Box 184, Ardakan, Iran
b Department of Chemistry, Payame Noor University, 19395-3697 Tehran, Iran
c Department of Chemistry, University of Mianwali, Mianwali, Pakistan
d Department of Chemistry, University of Sargodha, Punjab, Pakistan
e Department of Physics, University of Sargodha, Punjab, Pakistan
f Department of Physics, University of Mianwali, Mianwali, Pakistan
A R T I C L E I N F O
A B S T R A C T
Keywords:
Isoniazid
Schiff base
SC-XRD
A new dioxomolybdenum(VI) complex with tridentate ONO-donor Schiff base ligand derived from isoniazid has
been synthesized and characterized by elemental analyses, FT-IR and 1H NMR, and its solid state structure was
confirmed by single crystal X-ray diffraction (SC-XRD) technique. In the complex, the molybdenum center adopts
a slightly distorted octahedral geometry, by using three O,N,O-donor atoms of the tridentate ligand, an oxo group
makes the equatorial plane, while a methanol and another oxo group employ the axial positions. QTAIM, MEP,
NCI calculations and Hirshfeld surface analysis were performed to investigate the nature and types of non-
covalent linkages present among the sample molecules. The theoretical calculations, performed by DFT using
the B3LYP/Def2-TZVP level of theory, direct that the intended outcomes are in compliance with the actual
consequences. In addition, the catalytic performance of the complex was studied in oxidizing the different sul-
fides with H2O2 as an oxidizing agent in ethanol as a solvent under refluxed conditions. The main edge of the
present catalytic work is the accomplishment of reaction in a short period of time, high percentage yield and full
selectivity to sulfoxides without any formation of sulfones.
HSA
Theoretical
Sulfoxidation
1. Introduction
species to form hydrazone-based Schiff bases with the characteristics
–
–
–
HC
– – – –
N NH linkages. The carbonyl group next to the NH part in
The metal complexes with Schiff bases containing oxygen and ni-
trogen donor sites are of considerable interest due to their unique con-
figurations, structural flexibility and sensitivity to the molecular
environments [1–3]. Schiff bases can support a variety of transition
metals in diverse coordination modes, enabling the effective synthesis of
metal complexes with a wide range of stereochemistry [4]. Isoniazid is
considered as one of the most effective therapeutic agents and first-
generation standard drugs for the treatment and prevention of tuber-
culosis by WHO [5–8]. Isoniazid is analogous to nicotinamide and
consists of a pyridine ring and a hydrazide moiety [9]. The presence of
free amino groups in the hydrazide species is responsible for the estab-
lishment of condensation reactions with several carbonyl-bearing
hydrazine generates the possibility of tautomerism [10]. In addition to
this, the presence of an alcoholic (OH) functional group at a suitable
–
position with respect to azomethine nitrogen (HC N) gives an oppor-
–
tunity for the development of various kinds of inter and intra molecular
interactions [11–14]. The imine group is also responsible for the
development of a wide variety of biological potentials in this particular
class of compounds [15]. Hydrazones are very suitable candidates for
drug development, a very promising ligand for metal complexes, a very
good candidate for catalysis for the synthesis of heterocyclic compounds
and very effective biocidal agents [16]. Hydrazones can coordinate with
the metal centers either by deprotonated or by non-deprotonated forms
[17,18]. Hence, the greater lipophilicity of hydrazones-based Schiff
* Corresponding author.
E-mail address: h.kargar@ardakan.ac.ir (H. Kargar).
https://doi.org/10.1016/j.ica.2021.120568
Received 15 June 2021; Received in revised form 23 July 2021; Accepted 8 August 2021
Available online 13 August 2021
0020-1693/© 2021 Elsevier B.V. All rights reserved.

H. Kargar et al.
Inorganica Chimica Acta 527 (2021) 120568
dioxomolybdenum(VI) complexes with a variety of ligands with various
donor sites having cis-MoO22+ moiety are commonly employed to carry
out a variety of organic synthetic reactions [34].
Table 1
Crystallographic data and structure refinement parameters for the MoO2L
(CH3OH) complex.
The aforesaid points and knowledge of our previous findings [35–38]
are responsible for developing our research interest in sorting out the
new routes for the exploration of catalytic routes for sulfoxidation based
on the dioxomolybdenum(VI) complex with the Schiff base ligand
derived from isoniazid.
Identification code
MoO2L(CH3OH)
Empirical formula
Formula weight
Temperature (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
C15H15MoN3O6
429.24
293(2)
0.71073
Monoclinic
P21/c
2. Experimental
17.989(4)
12.518(3)
7.4050 (2)
90
b (Å)
c (Å)
2.1. Chemicals
α
(◦)
β (◦)
99.70(3)
All chemical materials were purchased from Sigma-Aldrich and
Merck companies. The dioxomolybdenum salt, [MoO2(acac)2], was
synthesized by using ammonium molybdate and acetylacetone as
described previously [39]. All solvents and other reagents employed in
the current work were of HPLC grade and used as obtained without
adapting any further purification step.
γ (◦)
90
Volume (Å3)
1643.7(6)
4
Z
Calculated Density (Mg/m3)
Absorption coefficient (mmꢀ 1
F(000)
1.735
)
0.836
864
Crystal size (mm)
θ range for data collection (◦)
Limiting indices
0.38 × 0.08 × 0.08
1.99 to 26.00
ꢀ 22 ≤ h ≤ 22
ꢀ 15 ≤ k ≤ 13
ꢀ 7 ≤ l ≤ 9
8384
2.2. Instruments
A BRUKER AVANCE 500 MHz spectrophotometer was used for
recording NMR spectra of the synthesized compounds by using
DMSO‑d6 as a deuterated solvent at ambient temperature. Tetrame-
thylsilane (TMS) was used as an internal standard for the comparison of
chemical shift values (ppm). The infrared spectra were recorded from
4000 to 400 cmꢀ 1 using the KBr pellets with the help of an IRPrestige-21
spectrophotometer (Shimadzu). The elemental analysis (CHN) of the
prepared compounds was taken on a LECO CHNS-932 elemental
analyzer.
Reflections collected
Independent reflections
3139 [R(int) = 0.0715]
3139 / 1 / 231
0.810
Data / restraints / parameters
Goodness-of-fit on F2
Final R indices [I > 2
σ
(I)]
R1 = 0.0290
wR2 = 0.0640
R1 = 0.0513
wR2 = 0.0668
0.413 and ꢀ 0.616
R indices (all data)
Largest diff. peak and hole (e.Åꢀ 3
)
bases derived from isoniazid is responsible for their enhanced thera-
peutic efficiencies like antibacterial, antifungal, antituberculosis, anal-
gesic, antiviral, and anticonvulsant activities [19–21].
2.3. Syntheses
2.3.1. Preparation of the Schiff base ligand
Sulfoxides are termed as one of the most prevalent and valued in-
termediates due to their wide variety of applications in organic synthetic
processes [22,23]. Sulfoxides have played an important role in the
pharmaceutical industry for the last two decades in the production of
many antibiotics like Modafinil, Fipronil, Omeprazole, Lansoprazole,
Fipronil, etc., [24–26]. Therefore, due to enormous applications, the
demand for the production of sulfoxides increases day by day. This has
created great inspiration among organic researchers to develop new
promising methods or catalytic protocols for the preparation of organic
sulfoxides. The most familiar and effective method for the production of
sulfoxides is the oxidation of sulfides. There are a large number of
conventional and catalytic methods have been developed by using
various transition metals for the oxidation of sulfides, but the most
auspicious challenge is selectivity [27,28].
ONO type tridentate Schiff base ligand, H2L, was synthesized by
mixing equimolar amounts of isoniazid (10 mmol, 1.37 g) and 5-
methoxysalicylaldehyde (10 mmol, 1.52 g) in absolute MeOH. The
mixed contents were refluxed for about 3 h to get a clear, yellow-colored
solution, which was then left as such for a period of five days until the
product crystallized in the pure form. The elemental analysis and the
spectroscopic data clearly confirm the composition of the targeted
compound.
H2L; (E)-N’-(2-hydroxy-5-methoxybenzylidene)isonicotinohydrazide,
Yield: 83 %. Calculated for C14H13N3O3: C 61.99, H 4.83, N 15.49 %.
Analysis found: C 62.13, H 4.76, N 15.37 %. FT-IR (KBr, cmꢀ 1): 3292
–
–
–
(
νN H); 1659 (νC O); 1612 (νC N); 1579 (νC C); 1271 (
ν
–
C
O(phenolic));
–
–
–
–
1
–
1045 (νN–N). H NMR (500 MHz, DMSO‑d6, ppm): 3.76 [s, 3H, ( OCH3)],
6.32 [1H, (H-C5), d, 4J = 2.2 Hz], 6.35 [1H, (H-C3), dd, 3J = 8.4 Hz, 4J =
2.2 Hz], 7.35 [1H, (H-C2), d, 3J = 8.4 Hz], 7.84 [2H, (H-C10, H-C13), dd,
Molybdenum is a very important element present in biological sys-
tems with a variety of stable oxidation states and coordination numbers
ranging from + 4 to + 8. It is an important part of the active sites of
many enzymes, like aldehyde oxidase, nitrogenase, xanthine oxidase,
nitrate reductase, xanthine dehydrogenase, and sulfite oxidase [29,30].
Molybdenum-based transition metal complexes are very effective cata-
lysts and have been employed in organic synthetic reactions performing
many oxidation reactions, especially the oxidation of sulfides to sulf-
oxides, since the last decade [31]. In some earlier reports by Chand,
Bahrami and their coworkers developed protocols for sulfoxidation
using a molybdenum(VI) salt (MoO3, MoO2Cl2) and H2O2 at room
temperature [32,33]. The main features of these methods were high
efficiency, selectivity, simplicity, inexpensive nature of oxidants, easy
work-up for purification and high yields. The dominance of Mo(VI) is
3
4
–
–
–
–
J = 4.7 Hz, J = 1.4 Hz], 8.53 [1H, s, ( CH N)], 8.77 [2H, (H-C11, H-
3
–
C12), d, J = 4.7 Hz], 11.25 [1H, s, ( NH)], 12.10 [1H, s, ( OH)].
2.4. Preparation of the molybdenum complex
The dioxomolybdenum(VI) complex, MoO2L(CH3OH), was synthe-
sized by suspending the equimolar amounts of the respective ligand, H2L
(2 mmol, 0.542 g) and MoO2(acac)2 (2 mmol, 0.660 g) in methanol (150
mL). The stuffing of the suspension was refluxed for roughly three hours.
After that, it was concentrated with the help of a rotary evaporator by
creating an environment of reduced pressure to facilitate the evapora-
tion of excess solvent. At the end, the flask was ice-cooled to procure the
red-colored precipitates, which were then filtered and cleansed metic-
ulously with diethyl ether and water, and then dehydrated in a desic-
cator under an inert atmosphere.
usually attributed to the oxo ligand, which is a very good δ and
π donor
and can stabilize the metal in its highest oxidation states by making a
formal double bond between the metal and the ligand. Hence,
MoO2L(CH3OH); Yield: 73 %. Calculated for C14H11MoN3O5: C,
2

H. Kargar et al.
Inorganica Chimica Acta 527 (2021) 120568
Scheme 1. The synthetic pathway of the H2L ligand and MoO2L(CH3OH) complex.
42.33; H, 2.79; N, 10.58 %. Analysis found: C, 42.44; H, 2.85; N, 10.49
2.6. Computational details
%. FT-IR (KBr, cmꢀ 1): 3433 (νO H) (coordinated methanol); 1600
–
–
–
–
–
(
(
νC N); 1554 (νC C); 1527 (νC
C); 1271 (
N
–
ν
O(phenolic)); 1169
The Gaussian (09 version package) software [46] is used for density
functional theory (DFT) calculations by employing the B3LYP level of
theory [47] along with the Def2-TZVP basis set [48]. By considering the
solvent, a solution phase was modeled by employing the IEFPCM [49].
Optimization of the geometry was verified by frequency analysis to
make sure that all of the atoms are at their local minima on the mo-
lecular potential energy surface (PES). The absence of imaginary fre-
quency was also revealed from the obtained results. The solution phase
1H & 13C NMR magnetic isotropic shielding tensors were calculated with
the help of the standard Gauge-Independent Atomic Orbital (GIAO)
method [50]. The chemical shifts (δ) of H2L and its corresponding mo-
lybdenum complex were proposed with the help of the B3LYP method
on the Def2-TZVP basis set and the IEFPCM model as an implicit model
of the solvent and equated with the experimental findings in deuterated
DMSO‑d6. All IEFPCM calculations for the prepared compounds were
done by using the same solvent. The chemical shift values were
computed by subtracting the suitable isotropic part of the shielding
–
–
N
) asym; 9C06 (
ν
) sym;
O–Mo–O
–
–
–
– –
– –
ν
O(enolic)); 1012 (νN N); 937 (
ν
–
C
–
O–Mo–O
596 (νMo ); 474 (νMo N). 1H NMR (500 MHz, DMSO‑d6, ppm): 3.15 [d,
–
O
–
3
–
–
J(H,H) = 8.7 Hz, 3H, CH3 (Methanol)], 3.76 [s, 3H, OCH3, (H-
3
3
–
C14)], 4.10 [q, J(H,H) = 8.7 Hz, 1H, OH (Methanol)], 6.91 [d, J(H,
H) = 15.0 Hz, 1H, (H-C2)], 7.16 [dd, 3J(H,H) = 15.0 Hz, 4J(H,H) = 5.2
Hz, 1H, (H-C3)], 7.37 [d, 4J(H,H) = 5.2 Hz, 1H, (H-C5)], 7.85 [d, 3J(H,
H) = 9.1 Hz, 2H, (H-C10, H-C13)], 8.74 [br, 2H, (H-C11, H-12)], 8.96 [s,
–
–
–
1H, ( CH N)].
2.5. Crystallographic data
The X-ray diffraction data of the complex was collected at 296(2) K
on a Bruker Kappa Apex-II diffractometer with Mo Kα radiation. Ab-
sorption corrections were applied using SADABS [40] software. The
structure was solved by direct methods on SHELXS-97 software [41] and
refined by the least squares method on F2 using the SHELXTL program
package [42]. The non-hydrogen atoms were refined anisotropically and
the carbon bonded hydrogen atoms were positioned geometrically and
refined with a riding model approximation with their parameters con-
strained to the parent atom with Uiso (H) = 1.2 or 1.5 Ueq (C). The
hydrogen atoms bonded with the oxygen atoms in the ligand and com-
plexes were located from the difference Fourier map and constrained to
refine with the parent atoms with Uiso (H) = 1.5 Ueq (O). For graphical
purposes, ORTEP [43], PLATON [44] and Mercury 4.0 [45] were uti-
lized. The crystallographic data and the refinement parameters of the
complex are shown in Table 1.
tensor from that of tetramethylsilane δi = σTMS
ꢀ σi. The solution phase
isotropic shielding constants for tetramethylsilane determined by
B3LYP/Def2-TZVP level of theory were equal to 184.52 ppm for the 13C
and 31.92 ppm for the 1H nuclei. The contour plots of the lowest un-
occupied molecular orbital (LUMO) and highest occupied molecular
orbital (HOMO) were produced by using the Chemissian program [51].
Molecular electrostatic potential (MEP) data was deliberated with the
Multiwfn program [52] and visualized with the help of VMD [53]. All
types of non-covalent linkages were explored by the NCIPLOT [54]
program.
3

H. Kargar et al.
Inorganica Chimica Acta 527 (2021) 120568
Fig. 1. The molecular structure of the MoO2L(CH3OH) complex with displacement ellipsoids at 50 % probability and atom numbering scheme.
Table 2
The selected experimental and calculated bond lengths (Ǻ) and bond angles (◦) of MoO2L(CH3OH) complex.
Bond lengths
Experimental
Calculated
Bond angles
Experimental
Calculated
Mo1–O1
Mo1–O2
Mo1–O3
Mo1–O4
Mo1–O6
Mo1–N1
N1–C7
1.716(2)
1.690(3)
1.920(2)
2.042(2)
2.316(3)
2.227(3)
1.286(4)
1.409(3)
1.365(4)
1.310(4)
1.704
1.687
1.937
2.009
2.631
2.281
1.290
1.375
1.338
1.312
O1–Mo1–O2
O1–Mo1–O3
O1–Mo1–O4
O2–Mo1–O3
O2–Mo1–O4
O3–Mo1–O4
O1–Mo1–N1
O2–Mo1–N1
O3–Mo1–N1
O4–Mo1–N1
105.51(14)
104.81(12)
96.26(12)
98.60(11)
96.89(11)
149.33(11)
161.16(12)
90.77(12)
81.50(9)
108.20
101.92
96.32
101.66
100.30
145.26
151.24
99.42
N1–N2
C1–O3
79.62
C8–O4
71.96(9)
70.46
Fig. 2. Graphical representation of voids in the crystal packing for MoO2L(CH3OH) complex along (a) b-axis, (b) c-axis.
2.7. General procedure of catalytic sulfoxidation
with the standard samples.
3. Results and discussion
3.1. Preparation
The MoO2L(CH3OH) complex (1 mol %) was mixed with a 1 mmol
solution of sulfide and a 2 mmol, 30 % aqueous solution of H2O2 in 5 mL
of EtOH, and the contents of the reaction were continuously stirred
under reflux for a definite period of time. The catalytic conversion is
continuously observed by employing thin layer chromatography by
using a 50:20 mixture of n-hexane and diethyl ether as the eluent. On
complete conversion, the resultant mixture was filtered off and then the
solvent evaporated slowly to get the desired refined products, which
were then further refined with column chromatography by employing
silica gel. The products gained after the completion of the catalytic cycle
were characterized spectroscopically and then the data was compared
Hydrazone-based ONO-tridentate Schiff base ligand was prepared by
treating isoniazid with 5-methoxysalicylaldehyde in 83 % yield in
MeOH. The treatment of MoO2(acac)2 with H2L in equimolar amounts in
refluxed MeOH leads to the synthesis of the targeted Mo(VI) complex.
Scheme 1 represents the adopted procedure for the production of the Mo
(VI) complex.
4

H. Kargar et al.
Inorganica Chimica Acta 527 (2021) 120568
Fig. 3. Optimized structures of H2L ligand and MoO2L(CH3OH) complex.
Fig. 4. Frontier molecular orbitals of the H2L ligand (keto and enol) and MoO2L(CH3OH) complex.
5

H. Kargar et al.
Inorganica Chimica Acta 527 (2021) 120568
Table 3
Frontier molecular orbitals energies and their gaps for the H2L ligand and
MoO2L(CH3OH) complex.
E (eV)
H2L (keto)
H2L (enol)
MoO2L(CH3OH)
EHOMO
ꢀ 5.861
ꢀ 2.288
3.573
ꢀ 5.933
ꢀ 2.506
3.427
ꢀ 6.204
ꢀ 3.032
3.172
ELUMO
*Energy gap
*Eg = ELUMO ꢀ EHOMO
Scheme 2. Selective oxidation of substituted sulfides catalysed by MoO2L
(CH3OH) complex.
Fig. 5. The non-covalent interaction index surface gradient for the self-assembled dimeric unit of MoO2L(CH3OH) with isosurfaces of 0.5 a.u. The surfaces are
colored on a blue-green–red scale according to values of sign (λ2)ρ.
Table 4
Optimization of the reaction conditions on the sulfoxidation of diphenyl sulfide.
Entry
Oxidant
Catalyst (amount)
Solvent
Condition
Time (h)
Conversion (%)
1
H2O2
No catalyst
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
MeOH
MeCN
1,2-DCE
Acetone
CHCl3
CCl4
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
Reflux
r.t.
2
2
2
1
1
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
2
Trace
Trace
30
2
H2O2
H2L (2 mol %)
3
H2O2
MoO2(acac)2 (2 mol %)
4
H2O2
MoO2L(CH3OH) (2 mol %)
MoO2L(CH3OH) (1 mol %)
MoO2L(CH3OH) (0.5 mol %)
MoO2L(CH3OH) (1 mol %)
MoO2L(CH3OH) (1 mol %)
MoO2L(CH3OH) (1 mol %)
MoO2L(CH3OH) (1 mol %)
MoO2L(CH3OH) (1 mol %)
MoO2L(CH3OH) (1 mol %)
MoO2L(CH3OH) (1 mol %)
MoO2L(CH3OH) (1 mol %)
MoO2L(CH3OH) (1 mol %)
MoO2L(CH3OH) (1 mol %)
MoO2L(CH3OH) (1 mol %)
MoO2L(CH3OH) (1 mol %)
MoO2L(CH3OH) (1 mol %)
MoO2L(CH3OH) (1 mol %)
MoO2L(CH3OH) (1 mol %)
100
100
70
5
H2O2
6
H2O2
7
H2O2
70
8
H2O2
90
9
H2O2
80
10
11
12
13
14
15
16
17
18
19
20
21
H2O2
Trace
20
H2O2
H2O2
10
H2O2
CH2Cl2
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
20
H2O2
Trace
40
H2O2
50 ◦
70 ◦
C
C
H2O2
90
No oxidant
NaIO4
(Bu)4NIO4
tert-BuOOH
UHP
Reflux
Reflux
Reflux
Reflux
Reflux
0
65
15
25
90
aReaction conditions: diphenyl sulfide (1 mmol), oxidant (2 mmol), catalyst, solvent (5 mL) under different temperatures
6

H. Kargar et al.
Inorganica Chimica Acta 527 (2021) 120568
Table 5
Selective oxidation of sulfides with H2O2 catalyzed by MoO2L(CH3OH) complex.a
Entry
Sulfide
Sulfoxideb
Time (min.)
Yield (%)c
1
20
90
2
3
4
60
30
30
90
92
92
5
6
7
8
30
20
20
20
85
91
86
91
a
Reaction conditions: sulfide (1 mmol), H2O2 (2 mmol), catalyst (1 mol %), EtOH (5 mL), reflux conditions
All products were identified by comparison of their physical and spectral data with those of authentic samples
Isolated yield
b
c
3.2. Illustration of single crystal analysis of complex
The equatorial plane A (N1/O1/O3/O4), methoxy substituted phenyl
ring B (C1-C6/C14/O5) and pyridine ring C (C9-C13/N3) are planar
with respective root mean square deviation of 0.0187, 0.0152 and
0.0016 Å. The dihedral angle between the methoxy substituted phenyl
ring and the pyridine ring is 8.96 (2)◦ which indicates that these rings
are almost parallel to each other. The molecules are connected with each
The Mo complex (Fig. 1, Table 1) crystallized in a monoclinic crystal
system with space group P21/c. A tridentate (ONO) Schiff base ligand,
two oxo groups at cis positions and a coordinated methanol molecule are
responsible for the establishment of a distorted octahedral geometry
around the molybdenum center. The coordination geometry is validated
by the bond lengths and bond angles associated with the central mo-
lybdenum atom, which are given in Table 2. The bond lengths for Mo-O
and Mo-N in the complex are almost analogous to the similar hydrazone-
based Schiff base ligands already reported in the literature [55–57]. The
bond lengths of the N1–N2 [1.408(5) Ǻ] and iminic N1–C7 [1.277(6) Ǻ]
groups were increased upon coordination with the molybdenum center
[54]. As commonly revealed by structurally similar complexes, the
elongation of the Mo1–O6 [2.316(3) Ǻ] bond length trans to the oxo O1
group is an indication of weak coordination of methanol at the axial
–
–
other through strong O H… N as well as comparatively weak C H… O
H-bonding as displayed in Figure S1 and given in Table 2. The molecules
–
are connected to form a C17 infinite chain through O H… N bonding
that runs along the b crystallographic axis, where the pyridine N-atom
acts as H-bond acceptor as displayed in Figure S2. Comparatively weak
–
C
H… O also interlinks the molecules to form infinite C5 chains.
Through C2-H2A… O5 and C7-H7A… O2 bonding, separate C5 chains
are formed, but the difference between these C5 chains is that an infinite
chain is formed by C2-H2A… O5 bonding runs along the b crystallo-
graphic axis, whereas an infinite chain is formed by C7-H7A… O2
bonding runs along the c crystallographic axis. Moreover, C12-H12A…
O1 bonding connects the molecules in the form of dimers to form R22(16)
position, which is due to the strong π-donor character of the oxo group
opposite to coordinated methanol. In the coordination sphere, one of the
oxo groups (O2) and the O-atom of the coordinated methoxy group
(C15/O6) occupy axial sites, whereas the imine N-atom (N1), phenolic
O-atoms (O3/O4) and one of the oxo groups (O1) occupy equatorial
sites. The Mo-atom is deviated by 0.2933 (2)◦ from the equatorial plane
towards the O-atom of the oxo group. A six-membered ring (C1/C6/C7/
N1/O3/Mo1) and a five-membered ring (C8/N1/N2/O4/Mo1) are
formed by the chelation of tridentate ligand. Ring puckering investiga-
tion infers that the six-membered ring adopts to a half-chair configura-
tion with puckering parameters (Q(2) = 0.280 (2) Å, θ = 58.1 (6), φ =
21.9 (7)◦). The five-membered ring adopts an envelope configuration on
Mo-atom with puckering parameters (Q(2) = 0.082 (2) Å, φ = 187(2)◦.
–
loop. Dimers are interlinked through O H… N bonding (Table S1).
Further stabilization in crystal packing is due to the presence of offset
π… π stacking interaction between the molecules. The pyridine rings of
symmetry-related molecules (x, 3/2 ꢀ y, 1/2 + z) are involved in off-set
stacking interaction with an inter-centroid separation of 3.76 Å and
π… π
a ring off-set value of 1.319 Å as displayed in Figure S3. By literature
review, we found two very closely related structures (1 and 2) to our
structure. One close related structure 1 has a chloro-substituted phenyl
ring instead of a 4-methoxy substituted phenyl ring, while another
compound 2 is a very closely related compound that has a 2-methoxy
substituted phenyl ring instead of a 4-methoxy substituted phenyl ring
7

H. Kargar et al.
Inorganica Chimica Acta 527 (2021) 120568
Scheme 3. Plausible mechanism for the oxidation of sulfides with H2O2 catalyzed by MoO2L(CH3OH) complex.
[58]. The crystal packing in the titled compound is the same as the
crystal packing in compounds 1 and 2 in terms of strong H-bonding.
Besides strong H-bonding, we have explored the crystal packing by
considering the role of comparatively weak intermolecular interactions
the methoxy substituted phenyl ring. The C-atom attached to the imine
N-atom and the CH from the pyridine ring indicate that these atoms are
–
involved in comparatively weak C H… O bonding. O-atoms of oxo
–
groups and CH from the phenyl ring (C1-C6) are also involved in C H…
–
as well, like C H… O bonding and off-set
π…
π
stacking interaction, but
O bonding, but no faint red spots are visible around these atoms due to
the authors of compound 1 and 2 did not explore the weak intermo-
lecular interactions.
graphical limitations. π… π stacking interactions between the molecules
can be displayed by the utilization of HS plotted over the shape index.
The presence of consecutive red and blue regions of triangular shape on
Hirshfeld Surface Analysis can be utilized to further explore the non-
covalent interactions that are responsible for the crystal packing by
using software known as Crystal Explorer version 17.5 [59,60]. The
Hirshfeld surface of a molecule is a region where electronic promo-
lecular density dominantes over electronic procrystal density. The
Hirshfeld surface can be plotted over different properties like dnorm
(normalized distance), shape index and electrostatic potential. These
surfaces provide unique information about non-covalent interactions. H-
bonding interactions can be displayed by colors on HS plotted over
dnorm. The red, white and blue colors on this HS describe the inter-
atomic contacts for which the distance between interacting atoms is less
than, equal to, and greater than the sum of the Van der Waals radii of the
atoms. Deep red spots are found on the HS around the N-atom of the
pyridine ring and the hydroxyl group of the coordinating methanol in-
the HS around the aromatic rings, indicating that π… π is present in the
crystal packing. Such regions are present around the phenyl rings in-
dicates that π… π stacking interaction is present in the crystal packing
(Figure S4b). H-bond donors and H-bond acceptors can be classified by
plotting the HS on the property named as electrostatic potential. Blue
and red spots on the HS, respectively (Figure S4c), display H-bond do-
nors and H-bond acceptors.
For the quantization of interatomic contacts, 2D fingerprint plots are
employed [61–63]. The quantization of interatomic contacts in terms of
the percentage contribution of each interatomic contact is important to
compute because it helps in further exploration of crystal packing. In 2D
plots, de stands for the distance from the HS to the nearest nucleus
outside the HS, whereas di stands for the distance from the HS to the
nearest nucleus inside the HS. In computing the percentage contribution
of an interatomic contact, the reciprocal contact is also included.
–
dicates that these atoms are involved in O H… N bonding (Figure S4a).
Comparatively faint red spots are found on the HS around the O-atom of
8

H. Kargar et al.
Inorganica Chimica Acta 527 (2021) 120568
3.3. Geometrical parameters
Table 6
Comparison of the efficiency of MoO2L(CH3OH) complex with some other re-
ported catalysts for the oxidation of methylphenyl sulfide.
The optimized geometrical parameters of the Schiff base ligand and
its corresponding dioxomolybdenum complex in the gas phase are
shown in Fig. 3. The bond angles and bond lengths for the MoO2L
(CH3OH) complex obtained from DFT calculations using the B3LYP/
Def2-TZVP method are given in Table 2 along with its X-ray diffrac-
tion data. It can be concluded from Table 2 that the computational
particulates are fairly parallel with the actual experimental outcomes.
The slight difference may be justified by the fact that experimental re-
sults were obtained from the sample in a solid state while the theoretical
findings were based on considering the single molecule in an isolated
gaseous form. Just like the diffraction data derived from X-ray analysis,
the computational results also confirm the same distorted octahedral
geometry around the molybdenum center due to the coordination of
oxygen and nitrogen atoms.
Entry
Catalyst
Conditions
Time
Yield
(%)
Ref.
(min)
5-
1
2
3
4
[PZnMo2W9O39
]
H2O2, CH3CN, r.
t.
30
92
85
76
95
[79]
[80]
[80]
[81]
IL(R = t-Bu)-V(IV)
IL(R = NO2)-V(IV)
VO2F(dmpz)2
H2O2, CH2Cl2,
240
240
300
20 ◦
C
H2O2, CH2Cl2,
20 ◦
C
H2O2, MeCN,
0–5 ◦
C
5
6
V(IV)O-MCM-41
VO(acac)
UHP, EtOH, r.t.
TBHP, DCE,
480
60
95
[82]
[83]
100
@Sb@SMNPs
Fe3O4/salen of Cu
(II)
85 ◦
C
7
H2O2, EtOH,
180
75
75
10
10
20
83
85
93
70
70
90
[84]
[85]
[85]
[38]
[86]
60 ◦
C
The Schiff base ligand H2L deprotonates to L2- and plays the role of a
tridentate dianionic moiety in which one N and two O- atoms are co-
ordinated to the central metal atom. The calculated N1–C7 (1.290 Å) and
8
PMo12 ⊂ Mo72Fe30
H2O2, CH2Cl2/
EtOH, r.t.
H2O2, CH2Cl2/
EtOH, r.t.
H2O2, EtOH,
reflux
9
SiMo12 ⊂ Mo72Fe30
MoO2(L)(CH3OH)
MoO2L
–
N –C (1.299 Å) bond lengths are in close approximation to the HC
N
–
10
11
12
2
bond8length (1.25 Å). The calculated C1–O3 and C8–O4 bond lengths are
H2O2, EtOH,
reflux
1.338 and 1.312 Å, respectively, which are somewhat intermediate to
–
–
the C O and C O bond lengths, 1.43 and 1.21 Å. The bond lengths of
–
MoO2L(CH3OH)
H2O2, EtOH,
reflux
This
C6–C7, C1–C6 and C8–C9 are 1.438, 1.415 and 1.476 Å, respectively,
which are between carbon–carbon single and double bond (1.54 and
1.34 Å). It can be concluded from the values that electronic clouds may
be delocalized over the azomethine moiety, carbonyl group, aromatic
rings, and the C1–O3 bond.
work
Figure S5a shows the 2D plot for the overall interactions. The presence
of the central triangular region of sky blue color indicates the presence
of π… π stacking interaction is present in the crystal packing. The most
important and vital interatomic contact is H… H as this contact plays the
most prominent role in the crystal packing with a percentage contri-
bution of 34.2 % (Figure S5b) in the crystal packing. The contribution of
the H… H contact in the crystal packing is almost equal to the percentage
contribution of the O… H contact in the crystal packing, which is 34 %
(Figure S5c). The other important contributors to the crystal packing are
C… H, N… H, C… C, C… N and O… N with percentage contributions of
13.8 %, 8.3 %, 4.1 %, 3.3 % and 1.6 %, respectively (Figure S5d-h). The
C… C and N… C interatomic contacts have an equal contribution to
crystal packing of 3.2 % (Figure S6f-g). The contacts with a negligibly
small contribution to crystal packing are O… N (0.5 % Figure S5i), O… O
(0.2 % Figure S5j) and N… N (0.1 % Figure S5k).
3.4. FT-IR spectral studies
The FT-IR spectral details of the H2L ligand and its respective MoO2L
(CH3OH) complex are presented in the experimental portion and the
spectra are displayed in Figure S7. The two flagship regions of interest, i.
e., 3294 and 1659 cmꢀ 1, are there in the spectrum of the ligand. These
–
–
regions are attributed to the (NH) and (C O) groups, which vanish in
the complex spectrum. This disappearance is reconcilable with the
amide functional group enolization due to the replacement of proton
with the metal ion [68]. 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 tridentate ligands, especially when the complexes are synthe-
sized and recrystallized in the presence of respective solvents [68,69].
The emergence of a new band at 1271 cmꢀ 1 is attributed to the enolic
For further exploration of the packing of molecules, the interaction of
atoms present inside HS with all the atoms located in the surroundings of
HS is determined [64,65]. This inspection infers that the H-ALL inter-
action is the strongest interaction with a percentage contribution of 59.1
%. The other interactions of such kind are O-ALL, C-ALL and N-ALL with
percentage contributions of 19.9 %, 14.6 % and 6.4 %, respectively, as
shown in Figure S6a. The interaction of all the atoms present inside HS
with an atom located in the surroundings of HS is also explored. The
outcome of this study is that the ALL-H interaction is the strongest
interaction with a percentage contribution of 65.3 %. Other interactions
of this kind are ALL-O, ALL-C and ALL-N with percentage contributions
of 16.5 %, 12.3 % and 5.9 %, respectively, as shown in Figure S6b.
The mechanical strength of a single crystal depends on the crystal
packing. If crystal packing is strong, then the single crystal can bear the
applied force of a significant magnitude. If the molecules are not
strongly packed i.e. if they have large empty spaces in the crystal
packing, then the crystal will break by applying a small amount of
external force [66,67]. To check the strength of crystal packing, void
analysis is performed, based on an assumption that sums up the electron
densities of all the atoms and all the atoms are spherically symmetric.
The void volume in the unit cell is found to be 154.27 Å3. The percentage
of space occupied by the molecules in the unit cell is 92.2 % indicates
that the molecules are strongly packed and there is no large cavity in the
crystal packing (Fig. 2).
–
–
–
(C O) part. The absorption vibration of HC N in the spectra of H L
2
ligand and Mo complex appeared at 1612 and 1601 cmꢀ 1, respectively.
The confirmation of oxo groups in the complexes was carried out by the
appearance of two bands at 937 and 907 cmꢀ 1, which are ascribed to the
asymmetric and symmetric stretching of the cis-Mo(O)2 moiety [70].
Similarly, the bands for Mo-O and Mo-N vibrations came up at 679 and
474 cmꢀ 1, respectively, which are in accordance with the previously
reported compounds [71]. The coordinated methanol molecule also
shows its presence by giving its respective peak at 3449 cmꢀ 1. The
experimental and calculated vibrational parameters of H2L and MoO2L
(CH3OH) are given in Table S2. The percentage difference between both
types of values is less than 5 %, which is due to the implementation of
the harmonic approximation for the calculations accomplished in the
vacuum.
3.5. NMR spectra
The 1H NMR spectral detalis of the H2L ligand and its dioxomo-
lybdenum(VI) complex in deuterated DMSO is furnished in the experi-
mental part and the spectra are presented in Figures S8 and S9. In the
ligand spectrum, there are two important signals at δ 12.10 and δ 11.25
9

H. Kargar et al.
Inorganica Chimica Acta 527 (2021) 120568
–
–
ppm related 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 and confirms the occurrence of
keto-enol tautomerism on complexation [72].
3.8. Non-covalent associations
Non-bonded Van der Waals interactions, including π⋯π interactions,
CH⋯ interactions, and dihydrogen bonds, are important in crystal and
π
There is also a downfield shifting of signal for the azomethinic proton
of the ligand from δ 8.53 ppm to δ 8.96 ppm in the spectrum of the
complex on coordination with metal atom. This is because of the
deshielding of azomethine proton and the reduction of electronic den-
sity at the azomethinic moiety due to coordination. This azomethine
linkage is also in accordance with the results obtained from the infrared
multidimensional structure engineering [77,78]. Experimental
geometrical data of MoO2L(CH3OH) shows that this compound is self-
assembled in its conventional unit cell. To characterize the nature of
non-bonded Van der Waals interactions, the wavefunction of MoO2L
(CH3OH) was calculated by using its crystal information file (CIF), and
then using the wavefunction quantum theory of atoms in molecules
(QTAIM) and Non-Covalent Interactions (NCI) analyses were done.
QTAIM data showed that there are ten non-covalent interactions be-
tween two complexes in the unit cell. The lowest and highest charge
density for these non-bonded interactions are 0.0017 a.u. (for
–
spectral investigations of MoO L(CH OH), where the HC N signal
–
2
3
shifts at a lower wavenumber as compared to the free ligand [73].
–
–
–
Hence, in addition to OH and NH the nitrogen of HC N behaves as
–
a third site of coordination with metal ion. The signals for aromatic
protons of the ligand 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 [74].
–
–
CH⋯O CH3 hydrogen bond) and 0.0092 a.u. (for CH⋯O Mo
hydrogen bond). The geometric coordinates of these bond critical points
are shown in Figure S11. This topological graph shows that non-bonded
Van der Waals interactions contain two dihydrogen bonds, three
The solvent molecule (MeOH) coordinated with the metal atom also
shows its presence by giving a doublet at δ 3.15 ppm and a quartet at δ
hydrogen bonds, one
π⋯N, two π⋯O, one CH⋯π and one π⋯π in-
–
–
4.10 ppm for CH3 and OH protons, respectively, with the same
coupling constant (3J (H,H) = 8.7 Hz) [75]. The experimentally
observed and computationally calculated 1H NMR data of the prepared
compounds are in close association with each other (Table S3).
teractions between two complexes.
In addition to the QTAIM, a three-dimensional colored graph of non-
covalent interaction was shown in Fig. 5. Non-bonded Van der Waals
interactions are highlighted by a green cloud between two self-
assembled dimeric complexes. The cloud thickness is high between
two benzene rings, which is a sign of good π⋯π and CH⋯π interactions
between the two complexes. In general, the NCI graph is in line with the
previously reported QTAIM.
3.6. Mulliken atomic charges
Table S4 presents the atomic charge on the active centres of the
ligand and its attributed complex. There is a significant reduction in
charge density on the molybdenum atom as compared to its formal
charge (6+), which is due to the shifting of charge density from the
ligand to the metal centre. Furthermore, a maximum change in charge
density is noticed for the carbon atoms, C1 and C8, bonded to O3 and O4
after coordination with the metal centre.
3.9. Catalytic activity
After successful synthesis of the MoO2L(CH3OH) complex and its full
characterization by various spectroscopic techniques, to evaluate the
catalytic efficiency of the complex, the oxidation reaction of diphenyl
sulfide was carried out by using H2O2 as a safe oxidant in ethanol as a
green solvent. After preliminary experiments (Table 4), it was found that
the sulfide was efficiently oxidized to the corresponding sulfoxide as the
only product in the presence of 1 mol % of the catalyst and 2 mmol of
H2O2 in refluxed ethanol (Table 4, Entry 5). Subsequently, various alkyl
and aryl sulfides were reacted under this catalytic system and the cor-
responding sulfoxides were obtained in high yields (Scheme 2, Table 5).
A probable mechanism for the sulfoxidation of sulfides by using
H2O2 catalyzed by MoO2L(CH3OH) is proposed in Scheme 3. It seems
that the metal atom of the catalyst can be attacked by nucleophiles.
Therefore, on the treatment of the molybdenum complex with H2O2, a
peroxomolybdenum moiety is formed. This intermediate is vital for the
oxidation of sulfides to sulfoxides through a single-step oxygen-transfer
reaction. The sulfur atom attacks electrophilically the oxygen of
3.7. Electronic properties
The LUMO and HOMO energies of the frontier molecular orbitals and
the difference in energy of these orbitals (ΔE = ELUMO ꢀ EHOMO) for the
prepared compounds (H2L ligand & MoO2L(CH3OH) complex) were
calculated by the B3LYP/Def2-TZVP method (Fig. 4, Table 3). As it is
evident from Table 7, the ΔE for H2L (keto), H2L (enol) and MoO2L
(CH3OH) are 3.573, 3.427 and 3.172 eV, respectively. It is obvious from
the data that the ligand is slightly more stable in keto form rather than in
enol form due to the higher ΔE value of 0.146 eV (3.4 kcal.molꢀ 1).
Therefore, the ligand in its enol form is softer than that in its keto form
and acts in a tridentate fashion upon interaction with the molybdenum
ion to produce the corresponding Mo(VI) complex. The Schiff base
ligand is a main contributor to HOMO, whereas the engagement of metal
atom in this molecular orbital is insignificant. In contrast to this, the Mo
atom contributes significantly to LUMO due to the greater proclivity of
metal to take electronic density.
–
peroxide followed by the breaking of the O O bond, simultaneously,
which causes the transfer of oxygen to sulfide to convert it into
sulfoxide.
To demonstrate the efficiency of MoO2L(CH3OH) complex as a ho-
mogeneous catalyst over the reported catalysts in the literature for
oxidation of sulfides, the oxidation reaction of methylphenyl sulfide was
considered as a representative example and conducted in the presence of
different catalysts (Table 6). The results of this comparison study
revealed that MoO2L(CH3OH) catalyst accelerates the rate of reaction
and provides a high yield of the desired product.
Molecular electrostatic potential is used to obtain more information
about the molecules’ reactive sites [76]. To predict the reactive sites of
the compounds toward electrophilic and nucleophilic attack, the MEP
was calculated using the hybrid method of B3LYP in conjunction with
the triple-zeta Def2-TZVP basis set. The colored shape of MEP was
depicted in Figure S10 to simply show the electrophilic and nucleophilic
sites by using blue and red for positive and negative regions, respec-
tively. The electrostatic potential values for the H2L in keto and enol
4. Conclusion
In conclusion, this study describes the synthesis of a tridentate
(ONO) Schiff base ligand and its dioxomolybdenum complex. The pre-
pared compounds were characterized by various spectroanalytical
techniques. It was concluded from the results that the ligand behaves in
a dianionic manner and exists in an enol form upon coordination. The
complex molecular structure was determined by SC-XRD. The
forms and the MoO2L(CH3OH) complex are 59, 33, and 38 kcal.molꢀ 1
,
respectively. The maximum negative regions of the ligand are located
around the electronegative atoms. The oxygen and nitrogen atoms are
theoretically potential active sites to coordinate with positively charged
metal.
10

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Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
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