Enhanced catalytic (ep)oxidation of olefins by VO(II), ZrO(II) and Zn(II)-imine complexes; extensive characterization supported by DFT studies

Article abstract of DOI:10.1016/j.molstruc.2021.130295

Three mononuclear di-valent VO²⁺, ZrO²⁺ and Zn²⁺-complexes (VOL, ZrOL and ZnL, respectively) were prepared from asymmetrical di-basic tetradentate di-imine ligand (6,6′-((1E,1′E)-((4-chloro-1,2-phenylene)bis(azaneylylidene))bis(methaneylylidene))bis(2-ethoxy phenol, H₂L). To confirm the M-complexes compositions, various spectral tools (FT-IR, EI/M and UV-Vis. spectra), molar conductance, thermal, elemental analysis and pXRD analyses were accomplished. Distorted octahedral geometry was confirmed for ZnL and square pyramidal geometry was elucidated for VOL and ZrOL. Their catalytic efficiency was investigated in the epoxidation of 1,2-cyclohexene by H₂O₂. They exhibited moderate to excellent catalytic control. The effect of temperature, time, solvent, type of oxidant and amount of catalysts were studied in order to determine the optimal catalytic atmosphere. The catalysts screening for epoxidation of alternative cyclic and acyclic olefins at optimization was reported. The variation of central metal ions from high to low valents (Zr⁴⁺, V⁴⁺and Zn²⁺ ions) and their capability for oxidation control their catalytic potential are the most effective aspects in the epoxidation reaction. The catalytic oxidation of 2-aminothiophene within VOL, ZrOL and ZnL, as a first trial, by H₂O₂ was examined. Also, QSAR parameters and DFT studies were performed to predict the catalytic properties of VOL, ZrOL and ZnL, to assert on chosen application. Effective surface properties of VO(II) complex were promoted for progressing its catalytic activity, which already happened. The catalytic mechanism was supported by the sequenced stability difference between proposed intermediates based on the difference in their recorded formation energy from the DFT study.

Full text of DOI:10.1016/j.molstruc.2021.130295

Journal of Molecular Structure 1236 (2021) 130295
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
Enhanced catalytic (ep)oxidation of olefins by VO(II), ZrO(II) and
Zn(II)-imine complexes; extensive characterization supported by DFT
studies
Mohamed Shaker S. Adama,b,∗, Laila H. Abdel-Rahmana, Hanan El-Sayed Ahmeda,
M.M. Makhloufc, Mona Alhasanid, Nashwa M. El-Metwalyd,e
a Chemistry Department, Faculty of Science, Sohag University, Sohag-82534, Egypt
b Department of Chemistry, College of Science, King Faisal University, P.O. Box 400, Al Ahsa 31982, Saudi Arabia
c Department of Sciences and Technology, Ranyah University College, Taif University, P.O. Box 11099, Taif 21944, Saudi Arabia
d Chemistry Department, Faculty of Applied Science, Umm Al-Qura University, Makkah, Saudi Arabia
e Chemistry Department, Faculty of Science, Mansoura University, Mansoura, Egypt
a r t i c l e i n f o
a b s t r a c t
Three mononuclear di-valent VO2+
,
ZrO2+ and Zn2+-complexes (VOL, ZrOL and ZnL, respectively)
Article history:
Received 4 November 2020
Revised 27 February 2021
Accepted 11 March 2021
Available online 19 March 2021
ꢀ
ꢀ
were prepared from asymmetrical di-basic tetradentate di-imine ligand (6,6 -((1E,1 E)-((4-chloro-
1,2-phenylene)bis(azaneylylidene))bis(methaneylylidene))bis(2-ethoxy phenol, H2L). To confirm the M-
complexes compositions, various spectral tools (FT-IR, EI/M and UV-Vis. spectra), molar conductance,
thermal, elemental analysis and pXRD analyses were accomplished. Distorted octahedral geometry was
confirmed for ZnL and square pyramidal geometry was elucidated for VOL and ZrOL. Their catalytic effi-
ciency was investigated in the epoxidation of 1,2-cyclohexene by H2O2. They exhibited moderate to ex-
cellent catalytic control. The effect of temperature, time, solvent, type of oxidant and amount of catalysts
were studied in order to determine the optimal catalytic atmosphere. The catalysts screening for epoxida-
tion of alternative cyclic and acyclic olefins at optimization was reported. The variation of central metal
ions from high to low valents (Zr4+, V4+and Zn2+ ions) and their capability for oxidation control their
catalytic potential are the most effective aspects in the epoxidation reaction. The catalytic oxidation of
2-aminothiophene within VOL, ZrOL and ZnL, as a first trial, by H2O2 was examined. Also, QSAR param-
eters and DFT studies were performed to predict the catalytic properties of VOL, ZrOL and ZnL, to assert
on chosen application. Effective surface properties of VO(II) complex were promoted for progressing its
catalytic activity, which already happened. The catalytic mechanism was supported by the sequenced
stability difference between proposed intermediates based on the difference in their recorded formation
energy from the DFT study.
Keywords:
Imine complexes
Studies in solution
DFT
Catalytic (ep)oxidation;1,2-cyclohexene
2-aminothiophene
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
due to their remarkable easy synthesis, high biological poten-
tial [2], variable catalytic performance [3], considerable electro-
Imines are characterized by azo-methine moiety, in which the
imino-group (C=N) could be formed within simple one-pot con-
densation of the substitutes C=O group of aldehyde or ketone
with amines [1]. For more than a century, M-imine complexes
gained a distinguished attentiveness by the inorganic chemists
luminescent properties [4], remarkable fluorescence features [5],
nonlinear optical properties (NLO) [6], applicable sensors [7] and
organic photovoltaic material application [8]. According to their
high tenability, most imines form high stable M-chelating com-
plexes with most of the transition metals [9]. d-Block element
complexes with imines as homogeneous [10] and heterogeneous
[11] catalysts possess fascinating reactivity in various chemical
organic syntheses [12]. Indeed, the epoxidation of C=C dou-
ble bands of unsaturated hydrocarbons, which achieved catalyt-
ically by transition metal complex catalysts, is of interest in
∗
Corresponding author.
E-mail addresses: madam@kfu.edu.sa, shakeradam61@yahoo.com (M.S.S.
Adam), m.makhlouf@tu.edu.sa (M.M. Makhlouf), n_elmetwaly00@yahoo.com,
nmmohamed@uqu.edu.sa (N.M. El-Metwaly).
https://doi.org/10.1016/j.molstruc.2021.130295
0022-2860/© 2021 Elsevier B.V. All rights reserved.

M.S.S. Adam, L.H. Abdel-Rahman, H.E.-S. Ahmed et al.
Journal of Molecular Structure 1236 (2021) 130295
many transmetalations of organic substrates [10-12]. The produced
epoxides of the epoxidation processes, which considered indus-
trial intermediates, are highly applicable in the synthetic organic
technology [10].
2. Experimental
2.1. Chemicals and Solvents
For that purpose, numerous reported works were promoted in
the synthesis and catalytic epoxidation studies of alternative tran-
sition metal complex catalysts to reach the optimization with less
required temperature and time [12]. Oxo and/or dioxo-vanadium
(IV)/(V) imine complexes are appreciated as the most favored
choice and the promising effective catalysts for such redox sys-
tems with different usable oxidants [13,14]. On the other hand,
zinc (II)-complexes were not so the preferable catalysts in the ox-
idation of organic compounds, e.g. olefins, due to their high stable
oxidation state [15]. However, the catalytic reactivity of zinc (II)
complexes, as sufficient catalysts, was studied recently by Nunes
et al. in the epoxidation of olefins using H2O2 [16]. The immo-
bilization of Mn(II)-salen complex on the modified alkoxyl-ZnPS-
PVPA considerably improved the catalytic potential of both Mn(III)
and Zn(II) ions in the asymmetric epoxidation of olefins [17], in
The involving starting materials, reagents and organic solvents,
which benefit in the syntheses and catalytic studies are commer-
cially available with high purity form without specific purification
(Sigma Aldrich, Acros, Fluka and Merck).
2.2. Physical methods
1H and 13C NMR spectra of H2L and ZrOL were scanned by
utilizing a Bruker Advance DPX-500 spectrometer. pXRD powder,
UV–Visible, FT-IR, mass spectra and CHN analyses of the studied
compounds were determined by Bruker D8 Advance, UV-Vis spec-
trophotometer Q5000, a Shimadzu FTIR-8300 spectrophotometer,
DI analysis Shimadzu Qp-2010 plus and a PerkinElmer 240c ele-
mental analyzer, respectively. Thermogravimetric analysis was con-
ducted with a heating rate of 10°C min−1 with DTG 60H Detector.
Magnetic and conductivity measurement was measured utilizing a
Gouy balance and a Jenway 4510 conductivity meter, respectively.
which Zn2+ ions improved the catalytic efficiency of the Mn2+
-
complexes. Additionally, it is so far in the literature to report
such catalytic susceptibility of oxo-zirconium complexes in the
epoxidation of olefins. The high electrophilic character of Zr4+
ion in its reagents motivated its catalytic reactivity toward or-
ganic syntheses, such as ZrCl4 [18]. The catalytic ring-opening
polymerization of lactide was assessed by Zr4+-complex of phos-
phasalen [19], by Zr4+-amino-benzotriazole phenolate complexes
[20] and by some Zr-salen complexes [21], affording high poten-
tiation. This could encourage us here to investigate the catalytic
potential of a new oxo-zirconium complex, as a homogeneous cat-
alyst in some epoxidation protocols. The strong Lewis acidity and
the high valent of V4+/5+ ions in their complex catalysts could pro-
mote remarkably their catalytic potential over those catalysts of
very high stable low valent metal ions, i.e. Zn2+-catalysts [22]. As
well documented, the most favorable oxidizing agent, as a green
reagent, is H2O2. It is classified as a safely stored, high effective,
cheap oxygen source and highly environmentally attractive because
of the green by-products of its oxidation processes, oxygen and
water [23].
2.3. Preparation of H2L ligand and its M-complexes
Depending on the previous synthesis of the cur-
ꢀ ꢀ
rent
H2L
ligand
[27],
6,6 -((1E,1 E)-((4-chloro-1,2-
phenylene)bis(azaneylylidene))bis(methaneylylidene))bis(2-
ethoxyphenol) (H2L) ligand was prepared by heating an ethanol so-
lution (20 mL) of 4-chloro-o-phenylenediamine (0.71 g, 5.0 mmol)
with an ethanol solution (20 mL) of 3-ethoxy-salicylaldehyde (1.66
g, 10 mmol) for 2 h at 70°C. The resulting deep orange solid was
filtered, washed with ether and recrystallized in ethanol.
The obtained H2L ligand (deep orange) gave yield 85% with
m.p. 200°C. FT-IR (KBr, cm−1): 1612 (C=N), 3423 (—OH). 1H NMR
(DMSO-d6, δ, ppm): 12.78-12.71 (s, 2H, 2OH), 8.96 and 8.91 (s,
2H, 2CH=N), 7.60-6.89 (m, 9H, 9CHar), 4.10-4.04 (q, 4J = 1.8 Hz,
3J = 5.9 Hz, 4H, 2OCH2), 1.38-1.33 (t, 3J = 5.4 Hz, 6H, 2CH3).
13C NMR (DMSO-d6, δ, ppm): 15.00 (CH3), 15.23(CH3), 64.32(CH2),
64.62 (CH2), 112.97 (CH), 116.29 (CH), 118.44 (CH), 119.16 (CH),
119.33(CH), 119.46 (CH), 119.75 (CH), 121.17 (CH), 122.94 (CH),
123.55 (CH), 146.42 (CH), 146.99 (CH), 147.50 (Cq), 147.91 (Cq),
148.20 (Cq), 149.07 (Cq), 151.39 (Cq), 153.61 (Cq), 193.06 (CH,
2CH=N). Anal. Calc. for C24H23ClO4N2 (%); N, 6.38; C, 65.60; H,
5.23, found (%): N, 6.30; C, 65.70; H, 5.30.
The coordinated ligands as the organic backbone show a re-
spectable influence in the catalytic activity of their metal com-
plexes [24]. The chelating, electronic and steric effects of the coor-
dinated ligand are of interest in the measuring of the M-complexes
reactivity in catalysis [25]. This could be beneficial here to present
the role of the coordinated imine ligand in the catalytic epoxida-
tion of olefins with various metal ions.
A facile preparation method was employed for metal complex-
ation within mixing of an equimolar ratio (3.0 mmol) of H2L
(1.32 g, 5 mmol) in ethanol (20 mL) with H2O/EtOH solution (20
mL) of 5 mmol of ZrOCl2ꢀ8H2O (0.97 g), VO(acac)2 (0.79 g) or
Zn(acac)2ꢀ2H2O (0.66 g) with heating for at 70°C 2 h. The result-
ing precipitate was filtered, washed with ether and recrystallized
in ethanol. The purity of prepared complexes was monitored by
TLC.
Although, the high applicability of 2-aminothiophene
in biological and industrial proposes [26], its catalytic ox-
idation was not yet reported in the literature. Depend-
ing on those efforts, we report here the preparation of
ꢀ ꢀ
three
imine
complexes
from
6,6 -((1E,1 E)-((4-chloro-1,2-
phenylene)bis(azaneylylidene))bis(methaneylylidene))bis(2-
ethoxyphenol with different metal ions (ZrO2+, VO2+ and Zn2+
ions) with achievement of their physicochemical characteristics.
The catalytic evaluation of the three M-imine complexes was
examined here in redox processes of some unsaturated hydrocar-
bons. The effect of the oxidation state of the central metal ion
(ZrO2+, VO2+ and Zn2+ ions) was also evaluated, comparatively.
Various parameters (temperature, time, oxidant and solvent) were
studied to get the optimization of the catalytic reactions. Inter-
estingly, we present all possible products of 2-aminothiophene
oxidation catalyzed by the current M-complexes. Computerized
studies were done to prove the steric-structures as well as to
develop theoretical controls for the catalytic behavior and confirm
the previously proposed mechanism.
The obtained ZrOL complex (orange) gave yield 74% with m.p.
< 300°C. FT-IR (KBr, cm−1): 1608 (C=N), 434 (M—N), 504 (M—O).
Anal. Calc. for C24H25ClN2O7Zr (%): N, 4.83; C, 49.66; H, 4.31, found
(%): N, 4.90; C, 49.74; H, 4.22. μeff (B.M.): diamagnetic. 1H NMR
(DMSO-d6, δ, ppm): 9.53 and 9.27 (s, 2H, 2CH=N), 7.84-7.46 (m,
9H, 9CHar), 4.88-4.51 (q, 4J = 1.3 Hz, 3J = 5.5 Hz, 4H, 2OCH2), 1.58-
1.42 (t, 3J = 5.1 Hz, 6H, 2CH3).
The obtained VOL complex (olive-green) gave yield 70% with
m.p. < 300°C. FT-IR (KBr, cm−1): 1574 (C=N), 452 (M—N), 532 (M—
O). Anal. Calc. for C24H21ClN2O5V (%): N, 5.55; C, 57.14; H, 4.16,
found (%): N, 5.62; C, 57.06; H, 4.10. μeff (B.M.): 1.76, ꢀm: 3.9 ꢁ−1
mol−1 cm2.
2

M.S.S. Adam, L.H. Abdel-Rahman, H.E.-S. Ahmed et al.
Journal of Molecular Structure 1236 (2021) 130295
The obtained ZnL complex (brown) gave yield 71% with m.p.
< 300°C. FT-IR (KBr, cm−1): 1578 (C=N), 436 (M—N), 519 (M—O).
Anal. Calc. for C24H29ClN2O8Zn (%): N, 4.87; C, 50.17; H, 5.05, found
(%): N, 4.80; C, 50.25; H, 5.12. μeff (B.M.): diamagnetic.
diluted with acetonitrile. The obtained sample (1 μL) was delivered
to the GC/MS apparatus. The chemoselective products were deter-
mined with retention time and also compared with those of au-
thentic stored compounds. For the catalytic reaction in water, the
obtained products were extracted using Et2O (diethyl ether).
The yield percentages of desired products of epoxy-1,2-
cyclohexane for the 1,2-cyclohexene epoxidation were monitored
by GC/MS, Shimadzu (Gas Chromatography mass spectrometer) of
QP2010 SE model with Rxi-5 Sil mass spectroscopy. The characteri-
zation of the capillary column was considered as 0.25 mm ID × 30
m length × 025 um film thickness. The withdrawing samples were
injected into the GC/MS within an auto-sampler. The temperature
of the provided oven was kept at 40°C for 1 min. The oven tem-
perature was increased with a rate of 10°C per 1 min to 200°C. At
200°C, the inlet process was operated with the splitless mode of
the transfer line. The carrier gas was He (Helium) with a purity of
99.999%. Its flow rate was carried out with 1 mL per 1 min. The
high-performance quadrupole mass filter and Shimadzu’s propri-
etary Optdesign simulation program with high-quality mass spec-
tra were supplied. EI is the ionization method for the molecular
weight information. EI with selectively measuring, high sensitivity,
chemical substances and an electron affinity were used in the GC-
MS. The Lab solution software was used to emphasize the amount
of product yield percentage for each sample from the desired prod-
ucts.
2.4. Kinetic and thermodynamic parameters
The thermal decomposition of the studied M-chelates was ex-
amined kinetically by applying Coats–Redfern relation, as shown in
Eq. 1 [28]:
ꢀ
ꢁ
ꢀ
ꢁ
ꢂ
ꢃ
log w / w − w
RA
E∗∅
2RT
E =
2.303R T
(1)
1
(
(
T2
))
∞
∞
log
= log
1 −
−
E =
where, W and W∞ are the mass loss at a defined temperature
T of the studied compounds and the mass loss after their com-
plete decomposition, respectively. R and φ are the general constant
of gases and the heating rate, respectively. The graph was plotted
from the left side of Eq. 1 against 1/T, since (2RT ) ≈ 0. Moreover,
∗
E
the kinetic parameters namely frequency factor (A) and activation
energy (E=) were calculated from the intercept and the slope, re-
spectively. The thermodynamic parameters of the activation en-
thalpy, ꢂH, and the activation entropy, ꢂS, were determined from
the activation energy, E=, using the well knowing Arrhenius rela-
tions [29].
2.7. Computational study
2.5. Studies in solution
All computations were accomplished using the Gaussian09W
software package [30]. The geometry of studied compounds was
fully optimized using DFT/B3LYP method [31] without symme-
try constraints. Using Becke3–Lee–Yang–Parr (B3LYP) exchange-
correlation functional under 6-311G++ basis set. The suitability of
final optimized forms was confirmed by positive values of fre-
quency calculated. The time-dependent DFT (TD-DFT) was con-
ducted by utilizing the polarizable continuum model by apply-
ing an integral equation formalism variant (IEF-PCM) at the same
B3LYP level, to study properties of the ground and excited states
[32]. All computational files (log, chk and fchk) were visualized
over Gauss-View version 5.0.9 [33] to extract all displayed fea-
tures. Among that, the optimized structures, frontier orbitals and
electrostatic potential maps that built over new cubic contours.
Moreover, essential physical parameters were calculated by using
so easy method [34].
The VO, ZrO- and Zn-complexes stoichiometry was studied in
the solution using continuous variation (Jobs method) and molar
ratio methods [28]. Sequenced alternating changes in concentra-
tions between metal and ligand solutions (1 × 10−2 mol dm−3
in DMF) were kept in balance for 2 h. Then, the absorbance was
recorded at λmax for each solution of corresponded M-complex.
The plot was drawn between absorbance and mole fraction of
ligand for each M-complex. The forming constants (Kf) of M-
complexes were gained by utilizing continuous variation curves ac-
cording to Eq. 2:
ꢄ
ꢅ
A
Am
ꢄ
Kf =
(2)
ꢅ
A
Am
(1 −
2C
where, A and Am are arbitrarily chosen from absorbance values
either a part from absorbance peak or maximum absorbance, re-
spectively and C is the primary molar concentration of metal ion
solution. Also, Gibb’s free energy of each M-complex (ꢂG=) was
calculated from this equation: ꢂG= = -RTlnKf.
3. Results and discussion
3.1. Practical characterization for synthesizes
2.6. Catalytic procedures
The synthesis of the current H2L ligand was already reported
elsewhere [27], whereas the VO-, Zn- and ZrO-complexes are the
subject presented in this work. They are characterized by alterna-
tive spectral tools, NMR, IR, MS and UV-Vis., as well as, EA (micro-
analysis), TGA (thermogravimetric analyses) and magnetic features.
Synthesis VOL complex was already reported with studying its bi-
ological reactivity elsewhere [27]. The free ligand H2L and its solid
complexes (ZnL, ZrOL and VOL) are highly stable in air and mois-
ture (Table 1) for a long time. The obtained results of molecular
formula, melting/decomposition points, molar conductivity, mag-
netic moment measurements and elemental analysis are registered
(Table 1). ZnL, ZrOL and VOL are intensely amorphous colored ma-
terial and decomposes above 300°C. The stoichiometry results in-
dicated that H2L behaves as a dibasic tetra-dentate ligand and co-
ordinates with the metals ions through 1: 1 (M: L) stoichiometry
(Scheme 1).
In a round glass flask (100 mL) with two-neck (connected
with a capacity of 100 mL, equipped with fitted glass water
circulated condenser), the 1,2-cyclohexene epoxidation or the 2-
aminothiophene oxidation was accomplished within (0.02 mmol)
M-complex catalyst (VOL, ZrOL or ZnL) in 10mL acetonitrile (stan-
dard solvent or other used solvents), homogeneously, at various re-
action temperature (from 50 to 100°C) with continuous stirring at
the given time. Each catalytic reaction was started by adding the
oxidant (3.0 mmol of 30% H2O2, 1.7 mmol of 70% tBuOOH or 1.7
mmol of solid NaClO4) to the reaction solution. The progress of the
reaction was monitored kinetically by withdrawing ~ 10.0 μL from
reaction contents. The taken samples were mixed with solid MnO2
(0.2 g) destroying the excess amounts of H2O2 or tBuOOH and also
with anhydrous CaCl2 to absorb water from treated samples. Then,
the resulted samples were filtrated through 0.3 g celite and then
3

M.S.S. Adam, L.H. Abdel-Rahman, H.E.-S. Ahmed et al.
Journal of Molecular Structure 1236 (2021) 130295
Scheme 1. Preparation strategy for the H2L ligand and its VO-, ZrO- and Zn-complexes.
3.1.1. Microanalysis, magnetic and conductivity features
ppm for the aryl rings and 193.06 ppm for the imino groups, as
The obtained elemental CHN analyses of the studied com-
pounds are agreed with the calculated ones for their experimental
formula, which suggests 1: 1 molar ratio based on bibasic tetra-
dentate ligand with each metal ion. Magnetic measurements of
complexes agree with a d0 electronic configuration of ZrOL and
ZnL complexes that have a square pyramid and octahedral geome-
tries, respectively. VOL complex is paramagnetic (1.76 B.M.) with d1
electronic configuration and has square pyramidal geometry [13].
Molar conductance of M(II)-complexes (1 × 10−3 M in DMF) re-
veals low values (1.9-8.0 ꢁ−1cm2 mol−1), which indicating non-
electrolytic features of them [35].
presented in Fig. S1b.
3.1.3. Comparative IR-vibrations
To distinguish the characteristic mode of bonding of the lig-
and towards Zn2+, ZrO2+ or VO2+ ion to form the corresponded
M-chelating complexes, the ligand’s IR spectrum of its functional
groups was compared to those of its M-complexes spectra. The
characteristic IR frequencies of the prepared H2L ligand and its
M-imine chelates are showed in Table 1 and displayed in Figs.
S2a,b,c,d. A sharp strong band of the azo-methine groups appears
at 1612 cm−1 in the spectrum of the H2L ligand, in which its
frequency was decreased (1608, 1578 and 1579 cm−1) after com-
plexation with ZrO2+, VO2+and Zn2+ ions, respectively. This in-
dicates the two azo-methine coordination through the two nitro-
gen atoms in the complex formation [36]. Also and regarding the
M-complexes spectra, obscure of broad weak v¯(OH) band, which
appeared for the H2L ligand’s spectrum (3423 cm−1), indicates
the participation of two ionized deprotonated hydroxyl groups to
ZrO2+, VO2+ and Zn2+ ions [36]. This proposed a dibasic tetraden-
tate mode of bonding for imine ligand within all M-complexes. Re-
markably, the water molecules reveal ρr(H2O) and ρw(H2O) bands
at 854.61 and 738.39 cm−1, respectively, besides, v¯(OH) 3315 cm−1
in ZrOL spectrum [37]. In ZnL complex, such bands were appeared
at 823.88, 734.7 and 3418 cm−1, sequentially. New bands that ap-
peared at the lower wavenumber region (532–504 cm−1) were
assigned for v¯(M–O) and v¯(M–N) vibrations and confirm coordinat-
ing NO donors (Table 1) [13]. The phenolic v¯(C–O) band, which
appeared at 1080 cm−1, was shifted to the higher frequency at
1082, 1087 and 1094 cm−1 in ZrOL, VOL and ZnL spectra, respec-
tively, due to coordination after its ionization. Moreover, v¯(V=O) and
v¯(Zr=O) bands appeared at 979 and 995 cm−1 in their correspond-
ing spectra, indicate their vibrational mode in square-pyramidal
geometry [13].
3.1.2. 1H & 13 C-NMR spectra of the ligand
The 1H-NMR spectrum of H2L ligand exhibits singlet signals at
12.78 and 12.71 ppm for two hydroxyl protons, which disappeared
when D2O was added to its deuterated solution (Fig. S1a). For the
1HNMR spectra of ZrOL complex, the two distinguished OH-proton
signals are completely disappeared. The two hydroxy groups coor-
dinated to ZrO2+ ion within the negative charge of the deproto-
nated group. Two azo-methine protons offer two singlet peaks at
8.96 and 8.91 ppm and the aromatic protons present multiple sig-
nals in the area from 7.60 to 6.89 ppm for H2L. Both signals for
the two azo-methine protons were strongly shifted to be located
at 9.53 and 9.27 ppm, referring to their coordination to ZrO2+ ion
through the nitrogen lone pair of electrons, as observed elsewhere
for similar Zr-chelating complexes [20]. Additionally, the two –
OCH2 groups show multiple signals at 4.10-4.04 ppm for H2L and
at 4.88-4.51 ppm for ZrOL complex. The two –CH3 chains of the
ethoxyl groups display multiple signals at 1.38-1.33 ppm for H2L
(Fig. S1a) and at 1.58-1.42 ppm for ZrOL complex. 13C-NMR spec-
trum assigns signals in the aliphatic region for CH3 carbon atoms
at δ 15.00-15.23 ppm and —OCH2 carbon atoms at 64.62-64.93
ppm. The carbon atoms assign 13C-NMR signals at 112.97-153.61
4

M.S.S. Adam, L.H. Abdel-Rahman, H.E.-S. Ahmed et al.
Journal of Molecular Structure 1236 (2021) 130295
Fig. 1. UV–Vis spectra of the ligand H2L, ZrOL, VOL and ZnL complexes dissolved
in methanol with ~ 1 × 10−5 M at the wavelength range 200-800 nm at ambient
temperature.
3.1.4. UV–Vis spectra and optical band gap
UV–Vis spectra are the most effective technique implemented
to study the structural forms of the tested transition metal com-
plexes [37]. The concentration 1 × 10−3 M was prepared from each
compound in methanol and scanned over 200–800 nm range, at
room temperature. Molar absorptivity (εmax) and the maximum
of absorption peaks (λmax), were obtained from UV-Vis spectra
(Fig. 1) and tabulated in Table S1. The ligand spectrum displayed
π→π∗ and n→π∗ transitions, which suffer shift due to the coor-
dination of azomethine and enolized hydroxyl groups. Remarkable
charge transfer bands (L→M) were recorded at 636, 479 and 524
nm in ZrOL, VOL and ZnL complexes, respectively. This feature is
the main cause for the color depicted on such a diamagnetic octa-
hedral complex. Moreover, VOL spectrum displayed a d→d tran-
2
sition band at 570 nm that attributes to 2B2→ B1 transition in
square-pyramidal geometry. Also, square-pyramidal geometry was
proposed for ZrOL complex, as the favorable configuration for oxy-
metals of the d0 system.
The optical band gap (Eg) measures the magnitude of separa-
tion between the ground and excited states under influence of UV-
Vis spectra. Consequently, Eg value is the lowest photon energy
needed to release an electron from the ground to an excited state
and leave a positive hole through the attraction force. Therefore, a
reduced band gap denotes the near the conduction with the optical
band and the excitation is carried out at low energy. Elastic elec-
tronic transition is preferable in different fields, as semiconductors-
like and catalytic proposes [38]. The role of catalyst in the reaction
is mainly focused on lowering the required activation energy as
the preliminary step in the catalytic cycle. Also, the energy of 3d
orbitals, which found in three metal ions, have a great impact on
the activation energy and the catalytic efficiency. The values of Eg
could be estimated from these relations Eqs. 3 and (4);
α = 1/d ln A
(3)
where, d is the width of the cell.
m
αhυ = A hυ − E
(4)
(
)
g
where, α is the absorption coefficient factor and A is a constant
independent of energy. The values of m are 0.5 or 2, which at-
tributed to the direct or indirect transition, respectively. The values
of the absorption coefficient factor (α) were gained from Eq. 1 and
used to calculated (αhυ)2. A relation between (αhυ)2 and hυ
5

M.S.S. Adam, L.H. Abdel-Rahman, H.E.-S. Ahmed et al.
Journal of Molecular Structure 1236 (2021) 130295
Fig. 2. Optical band gap for H2L ligand and its VO-, ZrO- and Zn-complexes.
(Fig. 2) was drawn, a line was extrapolated to interact x-axis in
which (αhυ)2 = 0, this point is the Eg value. The calculated values
of the three complexes were appeared close to each other’s, while
lower than that of their free ligand (H2L). This refers to the closest
optical properties of the three current complexes. So, we couldn’t
differentiate between them in their catalytic feature, but we expect
a slight priority of VOL due to its Eg value affording the lowest one
[39].
at 465-740°C range refers to the C8H5N2Cl loss with 32.5% (calc.
32.6%) leaving behind VO species. The thermogram of ZnL complex
presents lower thermal stability and full degradation reveals five
steps. The first and second steps located at 25-110 and 110–230°C
ranges, respectively, corresponded to the loss of two hydrated and
two coordinated water molecules with 6.3% (calc. 6.2%) mass loss,
respectively. The third step is found at the 230-390°C range, which
is attributed to the C8H8O2 removal with a mass loss of 23.6%
(calc. 23.7%). The fourth step is recorded at 390-545°C range, which
displayed for the C8H8O moiety loss with 21.0% mass loss (calc.
20.9%). The last step is presented in the range 550-710°C, which
considered for the removed C8H5N2Cl part of the complex with a
mass loss of 28.5% (Calc. 28.6%). The residual is ZnO.
3.1.5. TGA and kinetic studies
Thermal analysis is widely used to study the thermophysical
and kinetic properties of the M-complexes. Thermograms have
been executed from 25 to 750°C under nitrogen and heating rate
of 10°C/min, the decomposed mass verify molecular formulae sug-
gested (Fig. S3).
Using Coats-Redfern relation, kinetic and thermodynamic pa-
rameters as, frequency factor (A), the energy of activation (E∗), en-
thalpy energy (࢞H∗), entropy change (࢞S∗) and free energy change
(࢞G∗) were estimated from TGA curves [40] (Table S3a-c). The
positive values of activation energy reflect that the degradation
processes are endothermic. The negative values of entropy con-
sider that the prepared M-complexes are more ordered than the
reactant and the reaction is slow. In the activated state, the po-
larization of bonds and electronic transitions may cause a slow
reaction rate [41]. The high positive values of Gibbs free en-
ergy (࢞G∗) reflect that the decomposition steps are nonsponta-
neous processes, where the free energy of the initial compound
is lower than that of the final residue. The low values of A im-
plied a slow rate of pyrolysis. The great positive values of E∗ indi-
cate the presence of rotational, translational and vibrational states,
TGA of ZrOL chelate shows lower thermal stability and full
degradation reveals four steps. The first one starts at 38°C in agree-
ment with the presence of two hydrated water molecules by 6.3%
mass loss (calc. 6.3%). The second one at 150-300°C range at-
tributes to the removal of C8H8O2 with 23.4% mass loss (calc.
23.5%). The third stage at 300-450°C range refers to the C8H8O2
removal with 23.5% mass loss (calc. 23.4%). The fourth stage at
450-730°C range assigns to the C8H5N2Cl loss with 28.4% mass
loss (calc. 28.3%) leaving behind ZrO. TGA of VOL complex shows
three degradation steps starting at 150°C, which indicates the lack
of water presence with a loss of C8H8O2 by mass loss of 26.9%
(calc. 27.1%). The second step at 240-465°C range displays the re-
moval of C8H8O2 with 27.1% mass loss (calc. 27.0%). The third stage
6

M.S.S. Adam, L.H. Abdel-Rahman, H.E.-S. Ahmed et al.
Journal of Molecular Structure 1236 (2021) 130295
in the solid phase. Consequently, this study takes some importance
to reflect a shadow on the credibility of complexes-stoichiometry
in their solid-state. Using spectrophotometer to execute the con-
tinuous variation and the molar ratio routes for investigation in
solution. The obtained relations (Figs. S6a,b) proposed 1: 1 ratio
for the obtained complexes. This is due to the continuous varia-
tion curves offered maximum absorbance at a mole fraction of lig-
and X = 0.48–0.51 and molar ratio curves assure 1: 1 ratio. The
forming constants (Kf) of complexes are recorded (Table 1) indicat-
ing high stability of 1: 1 ratio of the studied chelates. Moreover,
the negative values of Gibb’s free energy reflect the spontaneous
feature of reactions [40].
3.2.2. Effect of pH on the M-complex stability
As seen, the dissociation curves for metal ion complexes in DMF
(Fig. S7) indicate high stability of VO-, ZrO- and Zn-complexes over
pH = 5-11 range. Therefore and regarding complex implementa-
tions, the appropriate pH range is wide enough for safe applica-
tions.
Fig. 3. Powder X-ray diffraction (pXRD) of the VOL complex.
3.3. Catalytic epoxidation of 1,2-cyclohexene
and the variable mechanical potential energies for the current
M-complexes.
Fundamentally, the catalytic efficiency of VO-, ZrO- and Zn-
complexes was examined in the catalytic system of olefins epoxi-
dation by a convenient oxidizing reagent under aerobic conditions.
1,2-cyclohexene is one of the most widespread standard examples
of olefins for such systems.
3.1.6. Mass spectra of metal complexes
Mass spectra were accomplished for the studied VO-, ZrO- and
Zn-complexes at 250°C and 70 eV within the electron ionization
mode. Consequently, the resulted molecular ion peaks, [M+] were
displayed at m/z = 544 (for ZrOL), m/z = 505/503 (for VOL) and
m/z = 579 a. m. u. (for ZnL). The obtained mass spectra ensure the
chemical structure of the current M-complexes, as shown in Figs.
S5a,c,b, for VOL, ZrOL and ZnL, respectively. Besides, the peaks of
metal isotopes were noticed in spectra at m/z = 50-90 range.
Optimized homogeneous conditions for VOL, ZrOL and ZnL cata-
lysts were estimated as a function of time at various temperatures,
as reported previously [43]. Also, alternative reaction parameters,
which could influence the selectivity and conversion, were stud-
ied. As reported, the percentages of conversion and selectivity of
olefins epoxidation to corresponded epoxy-product, depend mainly
on catalyst type, oxidant nature and reaction atmosphere (solvent,
temperature and time) [12,44].
3.1.7. Powder X-ray diffraction
Powder XRD patterns for H2L ligand and its M-complex were
taken over 5o < 2θ < 60o range using Cu/Kα radiation (1.54060
The catalytic reactions started by injection of 1,2-cyclohexene
(1.0 mmol) by VOL, ZrOL or ZnL catalyst (0.02 mmol) and H2O2
(3.0 mmol) (the oxidant) at different reaction temperatures in ace-
tonitrile (10 mL). Various catalytic control and chemoselectivity for
the conversion of 1,2-cyclohexene were shown in Tables S4-S6. The
epoxidation process took place in a concerted one step with an ex-
planation of the catalytic mechanistic pathway [45]. In the absence
of a significant catalyst, there was no distinguished progress in the
redox process even at high reaction temperatures.
˚
A), as shown in Fig. 3 and Figs. S5a,b,c (in the supplementary
materials). All patterns reflect relatively perfect nano-crystalline
particles with polycrystalline or monoclinic nature where a = b
= c and α = β = 90 = γ . The pattern of H2L ligand showed
characteristic diffraction peaks with a maximum at 2θ = 24.8◦,
which corresponded to inter-planar spacing (d) value of 3.58940
˚
A. The unit cell of H2L ligand has lattice constants of a = 7.15800,
˚
˚
b = 30.88600 and c = 7.37330 A with volume 1542 A. pXRD pat-
tern of VOL complex showed characteristic diffraction peaks with
a maximum at 2θ = 6.9◦, which related to the d-spacing value
3.3.1. Temperature Effect
˚
At various temperatures, 50, 60, 70, 80, 90 or 100°C, the epox-
idation reaction was investigated and percentages of the product
yields were evaluated by GC/MS and listed (Tables S4-S6). At 50°C,
a low temperature, the catalytic potential of all M-complexes was
low even by prolongation of reaction time up to 6 h, as recorded
in conversion and chemoselectivity percentages of the epoxy target
(entries 1-4). The percentages of conversion were increased grad-
ually with VOL (21, 34, 50 and 60%), ZrOL (9, 15, 28 and 40%) and
ZnL (8, 13, 20 and 32%) during the running times, i.e. 1, 2, 4 to
6 h, respectively (Tables S4, S5 and S6). Considerably, the conver-
sion percentages were enhanced gradually within the time for all
catalysts with little reduction of the selectivity at 50°C (Fig. 6a-c).
Similarly, at 60°C, the yield percentages of the welcomed prod-
uct were not highly promoted by time rising up to 6 h with all
M-complexes. Remarkably, the yield percentages were promoted to
66, 62 and 64% within VOL catalyst after 6 h (entry 8, Table S4).
Also, with ZrOL catalyst, the reaction afforded 53 % of the epoxy
product. With ZnL catalyst, it affords 50 % from the selective prod-
uct after 6 h. Hence, the TONs values were incremented gradually
of 13.4007 A. The unit cell of the complex has lattice constants
˚
of a = 12.87200, b = 7.61300 and c = 20.69500 A with volume
˚
2003.41 A, as shown previously for other similar VO-complexes
[42]. pXRD pattern of ZrOL complex showed characteristic diffrac-
tion peaks with a maximum at 2θ = 12.9◦, which exhibited a d-
˚
spacing value of 6.85380 A. Unit cell of the complex has lattice
˚
constants of a = 15.56100, b = 19.63600, c = 13.77600 A with vol-
ume 4188.45 A◦. pXRD pattern of ZnL complex showed character-
istic diffraction peaks with a maximum at 2θ = 6.2◦, which dis-
˚
played a d-spacing value of 12.88640 A. Unit cell of complex has
˚
lattice constants of a = 23.36500, b = 12.74600, c = 17.90400 A
with volume 5224.71 A◦.
3.2. Studies in solution
3.2.1. Stoichiometry of complexes
Significant studies for M-chelating complexes in solution are
completely different from those in solid-state. However, the stoi-
chiometry of the formed complex in solution may agree with that
7

M.S.S. Adam, L.H. Abdel-Rahman, H.E.-S. Ahmed et al.
Journal of Molecular Structure 1236 (2021) 130295
Fig. 4. Frontier orbitals for VO-, ZrO- and Zn-complexes.
with the lessening of TOFs values. Conclusively, the low tempera-
tures, i.e. 50 and 60 °C, are not enough to progress such epoxida-
tion protocols and to optimize the reaction conditions.
uct was low to moderate after 1 h with all M-catalysts. The re-
action showed 68, 38 and 36% with VOL, ZrOL and ZnL (entry 9,
Tables S4, S5 and S6), respectively. The highest percentages were
recorded after 6 h, as 87, 73 and 76% from epoxy-1,2-cyclohexane
with VOL, ZrOL and ZnL (entry 12, Tables S4, S5 and S6), respec-
tively.
Particularly, at 70°C, the yield percentages, conversion and se-
lectivity were increased to high percentages specifically after a
long time (6 h). In the beginning, the amount of the epoxy prod-
8

M.S.S. Adam, L.H. Abdel-Rahman, H.E.-S. Ahmed et al.
Journal of Molecular Structure 1236 (2021) 130295
Fig. 5. Molecular electrostatic maps of VO-, ZrO- and Zn-complexes.
At the favored temperature in acetonitrile (80°C) for most re-
ported epoxidation systems [46], ~100% was the recorded conver-
sion for VOL, ZrOL and ZnL after 4 to 6 h (entries 15 and 16).
Meanwhile, the selectivity was observed as the highest percentage
after 1-2 h (entries 13 and 14, Tables S4-S6), and then it reduced
by running time to 4-6 h (entries 15 and 16). After 1 h, the pre-
sented percentages were acceptable to be 81, 61 and 56% with VOL,
ZrOL and ZnL (entry 10, Tables S4, S5 and S6), respectively. With
2 h, the yield percentages of the desired product was improved
remarkably to be 87 (with VOL), 75 (with ZrOL) and 72% (with
ZnL) (entry 14). Similarly and after 4 h, the yielding percentages of
the welcomed product were also improved gradually as 90 (with
VOL), 82 (with ZrOL) and 81% (with ZnL) (entry 15, Tables S4-
S6). But, after 6 h, the catalytic system gave less product amounts
with VOL (81%), ZrOL (82%) and ZnL (81%), as shown in Fig. 6a-c,
entry 16.
At 100°C, a strong improvement in the yields of unwelcomed
side products [11,12], which minimized the targeted yield of
epoxy-1,2-cyclohexane (entries 21-24) with all M-catalysts, was
neatly recorded. Also, excellent conversion and low selectivity per-
centages were observed at that temperature. Consequently, the
evaporation of reaction components at 100°C, could take place and
hence, could cause reducing in catalytic sufficiency. High temper-
ature with an excess oxidant, could progress further oxidation of
epoxy-1,2-cyclohexane in presence of M-catalysts to give other un-
welcomed side products (see TONs and TOFs values, Tables S4-S6),
as reported elsewhere [15].
Conclusively, catalytic reactivity of VOL, ZrOL and ZnL was pro-
moted remarkably as a function of reaction temperature, Fig. 6a-c.
In which, the best temperature depends on the percentages of con-
version, selectivity and yield [10,15] and is considered as one of the
optimized conditions. Here, VOL showed the optimal temperature
at 80°C with 90% yield from the selective product after 3 h (entry
15, Table S4). On the other hand, ZrOL afforded the optimal tem-
perature after 4 h, with 84 % of the target product at 90°C (entry
19, Table S5). Similarly, the reactions catalyzed by ZnL, awarded
the best yield from the targeted product at 90°C after 4 h (86%),
with very good selectivity (88%), (entry 19, Table S6). Finally, the
catalyzed reactions with all M-complexes could be progressed with
reaction temperatures from 70 to 90°C, referring to the high stabil-
ity of complex catalysts in reaction media.
Moreover, at 90°C more than the boiling point of acetonitrile,
the conversion percentages increased fairly to 100% (entries 19 and
20) by the time. The selectivity was oppositely reduced notably af-
ter 1 h (93%, entry 17) up to 6 h (64%, entry 20) with VOL. Sim-
ilar behavior was detected for ZrO-catalysts and Zn-catalysts (en-
tries 17-20). The yield of the desired product was detected after
4 h with percentages of 84% with ZrOL (Table S5, entry 19), as
shown from the TONs and TOFs values. For ZnL, the yield percent-
ages were 86%, as reported in Table S6, entry 19.
9

M.S.S. Adam, L.H. Abdel-Rahman, H.E.-S. Ahmed et al.
Journal of Molecular Structure 1236 (2021) 130295
Table 2
Solvent type effect on epoxidation of 1,2-cyclohexene by H2O2.
Yield (%)b
Conversion
Selectivity
(%)
Catalysta
VOL
Solvent
(%)
R
P
Side products
AN
0
90
83
50
60
81
84
57
41
58
72
85
70
44
67
77
10
16
28
40
19
16
32
32
40
28
9
100
99
90
84
64
60
81
84
64
56
59
72
90
78
60
67
77
EtOH
H2O
1
22
0
78
DMSO
CHCl3
AN
100
100
100
89
0
ZrOL
ZnL
0
EtOH
H2O
11
27
2
73
DMSO
CHCl3
AN
98
0
100
94
6
EtOH
H2O
10
27
0
20
29
33
23
90
73
DMSO
CHCl3
100
100
0
a
The epoxidation of 1,2-cyclohexene (R) (1.0 mmol) by an aqueous H2O2 (3.00
mmol), catalyst (0.02 mmol) in 10 mL solvent at the optimized reaction time and
temperature.
b
The percentages derived from GC results, selectivity control of P, epoxy-1,2-
cyclohexane, and the other side products.
3.3.2. Time Effect
The effect of time was considered in Figs. 6a-c, which present
plotting of yield percentages for the chemoselective product ver-
sus time. The optimized time catalyzed by each M-catalyst could
be determined by circulating at the best reaction time in Figs. 6a-
c. From Figs. 6a-c, the optimized time was 4 h (90%, with VOL) at
80°C. For ZrOL, it recorded 84% after 4 h and 86% after 4 h with
ZnL at 90°C. Conclusively, the high reactivity of VOL with V4+
-
ions, high-valent oxy-metal ions, could be interpreted with high
reversible electrochemical behavior, more valid oxidation states in-
terchange and strong Lewis acid character. While, Zr4+- and Zn2+
-
catalysts have their high and stable valence and couldn’t display
extra-oxidation [25,45,47]. As well reported that the catalytic re-
activity of the involved M-complex catalyst could be processed
through electron and/or oxygen transfer mechanisms [48]. This
could be taken place easily within VO-catalyst [45].
3.3.3. Solvent Effect
The catalytic potential of VO-, ZrO- and Zn-complex cata-
lysts was influenced markedly by the nature of solvent in 1,2-
cyclohexene epoxidation, as shown pervioulsy [43]. The solvent
effect was studied by acetonitrile, ethanol, chloroform, DMSO
(dimethyl sulfoxide) or water in the epoxidation reactions under
the optimized conditions for each M-catalyst, which presented in
Table 2. The results in Table 2 showed that acetonitrile is the
highest effective solvent in such a catalytic system (90 % cat-
alyzed by VOL, 84% catalyzed by ZrOL and 85 % catalyzed by
ZnL). Although acetonitrile is considered a labile coordinated lig-
and with various metal ions [48,49], it is a highly effective sol-
vent for such epoxidation processes with aqueous H2O2, Table 2.
Commonly, at high temperatures, acetonitrile is strongly stable in
epoxidation processes compared to other applicable solvents with-
out any observable reactivity towards any catalytic component, i.e.
reactant and oxidant [44]. Acetonitrile with a high dielectric con-
stant could progress the catalytic efficiency of H2O2 within elec-
tron and oxygen transfer cycles [50]. In chloroform, the selectivity,
conversion and yielding percentages of the targeted product were
very good referring to the high catalytic reactivity of current M-
catalysts at their optimized conditions. The yielding percentages of
the chemoselective product with VOL, ZrOL and ZnL catalysts were
81, 72 and 77%, respectively. Due to strong organic nature and cur-
rent M-complexes with less charge distribution in their molecules
Fig. 6. The yield percentages of epoxy-1,2-cyclohexane of the epoxidation of 1,2-
cyclohexene by H2O2 catalyzed by (a) VOL, (b) ZrOL and (c) ZnL.
10

M.S.S. Adam, L.H. Abdel-Rahman, H.E.-S. Ahmed et al.
Journal of Molecular Structure 1236 (2021) 130295
Table 3
Oxidant type effect on the epoxidation of 1,2-cyclohexene by H2O2.
Yield (%)b
Conversion
(%)
Selectivity
(%)
Catalysta
VOL
Oxidant
R
P
Side products
H2O2
0
90
94
53
84
88
59
85
89
50
10
6
100
100
90
90
94
59
84
88
75
90
92
58
Scheme 2. The aqueous hydrolysis of the epoxy product in H2O in the catalytic
tBuOOH
NaClO4
H2O2
0
system.
10
0
37
16
12
20
9
ZrOL
ZnL
100
100
79
tBuOOH
NaClO4
H2O2
0
21
6
could be the reason for their less reactivity compared to that in
acetonitrile. So, the less polarity of the reaction media in chloro-
form could reduce their catalytic progress within electron and/or
oxygen transfer [51]. In ethanol, the conversion was close to 100%,
but the selectivity was very good with VOL, ZrOL and ZnL cata-
lyst (85, 60 and 77%, respectively). The reactivity of the catalytic
systems was progressed in ethanol compared to that in chloro-
form. The recorded amount percentages of epoxy-1,2-cyclohexane
were found 83, 57 and 70% using VOL, ZrOl and ZnL catalysts, re-
spectively. Consequently, the catalytic reactions in chloroform and
ethanol awarded close amounts of the chemoselective products
with all M-catalysts.
94
tBuOOH
NaClO4
3
8
97
14
36
86
a
The epoxidation of 1,2-cyclohexene (R) (1.0 mmol) by an aqueous H2O2 (3.00
mmol), tBuOOH (1.7 mmol) or NaClO4 (1.7 mmol), catalyst (0.02 mmol) in 10 mL
acetonitrile at the optimized reaction time and temperature.
b
The percentages derived from GC results, selectivity control of P, epoxy-1,2-
cyclohexane, and the other side products.
In particular, dimethyl sulfoxide, DMSO, which is considered a
high coordinated solvent, could bond to the central metal ion of
the catalyst complex. This caused a high improvement in conver-
sion percentages in catalyzed reactions within all M-catalysts (95-
100%, Table 2). On the other hand, the selectivity and yielding per-
centages were low. Interestingly, the high coordination features of
DMSO diminished also the catalytic efficiency of M-catalysts to-
ward the oxidation selectivity [52] awarding 50, 58 and 67% of the
epoxy product with VOL, ZrOL and ZnL, respectively. The high co-
ordination ability of DMSO towards central metal ion in catalysts
could bond and capsulate the catalyst and, hence, reduce the prob-
ability of a selective catalytic epoxidation [44].
In H2O, the catalytic systems gave a moderate amount of the
selective product (Table 2), 50, 41 and 44% with VOL, ZrOL and
ZnL, respectively. The reactant is not soluble in water, hence, the
reaction contents are not miscible enough in the epoxidation re-
action to being activated. So, under an eco-friendly atmosphere (in
water), the low polarity of M-catalyst and non-polar substrate (1,2-
cyclohexene), may reduce notably their miscibility and then pro-
hibit their catalytic reactivity [11,48]. Also, as well reported that
the aqueous hydrolysis of the reactant and/or epoxy-product gave
high amounts of undesired side products [53]. The aqueous hydrol-
ysis of chemoselective products could improve the yield of the un-
welcome side product (cyclohexane-1,2-diol), Scheme 2.
Fig. 7. The yield percentages of epoxy-1,2-cyclohexane of the epoxidation of 1,2-
cyclohexene by an aqueous H2O2 depending on the loaded amount (0.01, 0.02m
0.05 or 0.1 mmol) of the M-catalyst VOL, ZrOL or ZnL.
the results were listed in Table 3. NaClO4 gave less reactivity com-
pared to those with tBuOOH or H2O2. The yields were found 53,
59 and 50% VOL, ZrOL and ZnL, respectively (Table 3).
3.3.4. Oxidant Effect
It is noted that the processes with aqueous H2O2 consumed
relatively large stoichiometry, i.e. the excess amount of an aque-
ous H2O2 compared to that of tBuOOH, because of a lot of water
amounts inside (70% water). It could relatively reduce the poten-
tiality of aqueous H2O2 in comparison with tBuOOH [55]. The low
potential of NaClO4 could reduce the catalytic reactivity of the VO-,
ZrO- and Zn-catalysts due to less solubility in reaction media [54].
With H2O2, tBuOOH or NaClO4, as an oxygen source, the cat-
alytic reactivity of the current VOL, ZrOL and ZnL catalysts toward
epoxidation of 1,2-cyclohexene was examined and documented
(Table 3). The obtained results illustrated that the 1,2-cyclohexene
epoxidation processes could be controlled by the type of oxidant
at optimized atmospheres in acetonitrile for each M-catalyst. From
Table 3, tBuOOH afforded the best yield of epoxy-1,2-cyclohexane.
tBuOOH is found to be the best oxidant for epoxidation under
the applied catalysts that superior H2O2. The yields of epoxy-1,2-
cyclohexane catalyzed by such catalysts with H2O2 close to those
with tBuOOH. Using tBuOOH, the catalytic processes afforded 94,
88 and 89% of epoxy-1,2-cyclohexane catalyzed by VOL, ZrOL and
ZnL, respectively. With H2O2, the catalytic processes gave 90, 84
and 85% of chemoselective by VOL, ZrOL and ZnL, respectively. It
is documented recently that tBuOOH is a more convenient oxidiz-
ing agent than the aqueous H2O2 especially, in olefins epoxidation
processes, due to its similar organic nature with the substrate in
the reaction media [54]. To establish this behavior, NaClO4 oxidant
with its high polarity was probed at the optimal atmosphere and
3.3.5. Effect of the M-catalyst amount
The catalytic activity of VOL, ZrOl and ZnL catalysts toward the
current epoxidation reaction was examined as a function of the
charged amount of M-catalysts, which listed (Table S7) and plot-
ted (Fig. 7). The effect of VOL, ZrOl and ZnL amounts (0.01, 0.02,
0.05 and 0.1 mmol) was observed by the yield, conversion control
and selectivity percentages (Fig. 7).
From Table S7, when the amount of catalyst was low, i.e. 0.01
mmol, the catalytic reactivity was moderate to good within 75, 78
and 79% using VOL, ZrOL and ZnL, respectively. The reaction sys-
tem gave closed yield percentages when the loaded amount from
11

M.S.S. Adam, L.H. Abdel-Rahman, H.E.-S. Ahmed et al.
Journal of Molecular Structure 1236 (2021) 130295
Table 4
Kinetic parameters for 1,2-cyclohexene epoxida-
tion by H2O2 catalyzed M-complexes at opti-
mized reaction conditions.
Complex
A x 108
Ea, kJ mol−1
ꢂG#
VOL
ZrOL
ZnL
0.97
1.93
2.36
53.0
56.4
57.2
42.6
69.7
70.1
of ZrOL catalyst (56.4 KJmol−1) refers to higher catalytic potential
of ZrOL compared to ZnL (57.2 KJmol−1) with its high electrophilic
character of Zr4+ ion in its complex catalyst over the low valent of
Zn2+ ion in ZnL [12].
3.3.7. Epoxidation of other olefins
Screening of catalytic epoxidation for alternative olefins as
cyclic and acyclic derivatives at the optimized atmosphere was ex-
amined and the obtained percentages of epoxy-target products are
listed in Table 5. VO-, ZrO- and Zn-catalysts were used for such
epoxidation reaction with aqueous H2O2. 1,2-Cyclohexene, 1,2-
cyclooctene and styrene (as cyclic olefins with the strong electron-
donating ability of the C=C group) presented high reactivity more
than those of aliphatic or acyclic olefins (Table 5) [58]. The inner
double-bonded C=C group could improve the reactivity of cyclic
olefins compared to aliphatic olefins with terminal C=C double
bond [58].
Fig. 8. The kinetic plots of the 1,2-cyclohexene epoxidation by H2O2 catalyzed by
ZnL as a function of temperature and time.
homogeneous catalysts was 0.02 and 0.05 mmol, as the optimized
amount to produce the targeted product. The percentages of prod-
uct were 90 and 91% (with 0.02 and 0.05 mmol of VOL), 84 and
83% (with 0.02 and 0.05 mmol of ZrOL) and 85 and 83% (with 0.02
and 0.05 mmol of ZnL), respectively. Conclusively, the best-loaded
amount from homogeneous catalysts could be 0.02 and 0.05 mmol.
However, the less loaded amount of catalysts, i.e. 0.01 mmol, was
not enough to proceed. The overloaded amount of M-catalysts,
i.e. 0.1 mmol, could strongly increase conversion but diminish the
chemoselectivity percentages, as observed elsewhere [56].
3.3.8. The proposed mechanism
Geometrical characteristics of applied M-catalyst species with
tetra-dentate coordinated H2L ligand could help to understand the
mechanistic pathway [25,55]. In the catalytic cycle, VOL with a
square pyramidal geometry has an active vacant site capable of
the coordination of oxidant molecule (H2O2 or tBuOOH). This could
cause an electron transfer process within oxidation of VO2+ to
highly reactive intermediate VO3+ species (V5+, as strong elec-
trophile) [23,59]. Another interaction of a new H2O2 molecule
with the electrophile A formed new active intermediate species
(B) [60]. Then, within an oxygen transfer step from another H2O2
to electrophile V5+ ion in B could give an active oxo- or peroxo-
intermediate (C) with bridged O—O bonding, and liberating H2O or
tBuOH molecule [61]. The substrate molecule, R’ (1,2-cyclohexene),
could attract and bond to a strong electrophile V5+ ion in C to give
a new intermediate (D) [11,12]. Formation of new intermediate
species D caused oxidation for 1,2-cyclohexene to desired epoxy-
1,2-cyclohexane (RO) with liberating and recycling of A (Scheme 3).
The electron and oxygen transfer processes could be established
practically by changing in characteristic VOL color in the reaction
media when H2O2 or tBuOOH was added. The observable repeated
spectral changes of characteristic absorption spectra of VOL in ace-
tonitrile (from 570 to 475 nm) were presented in Fig. 9, which was
assigned for the formation of new intermediates.
3.3.6. Kinetics of epoxidation
The kinetic parameters of epoxidation for 1,2-cyclohexene at
various temperatures (50, 60, 70, 80, 90 and 100°C) catalyzed
by VO-, ZrO- or Zn-complex, as
a pseudo-first-order reaction,
were evaluated. The rate of catalytic reaction was evaluated from
Eq. 5 and plotted in Fig. 8 (for ZnL) and Figs. S12a,b (for VOL and
ZrOl, respectively).
ꢂ
ꢃ
C
−ln
= kt
(5)
Co
where, the initial concentration of 1,2-cyclohexene is Co, the time
is t, the catalytic rate constant is k and the residual concentration
of 1,2-cyclohexene is C. The values of the rate constant were ob-
tained from the slope in Figs. S13a,b,c. Additionally, within the Ar-
rhenius equation, Eq. 6, Ea, the activation energy for catalytic epox-
idation, was obtained by plotting ln k against 1/T, T is the temper-
ature in kelvin (Eq. 6).
The above mechanism could be not convenient for homoge-
neous catalytic systems for ZrOL and ZnL catalysts [47,62]. In par-
ticular, the electron transfer promotion could not be happened for
ZrOL and ZnL due to the high stability of their valence as Zr4+ and
Zn2+ ions (of d0 and d10 electronic configurations, respectively).
This could be proven by UV-Vis spectral data. Spectral scans of
ZrOL catalyst in reaction media, which did not show any significant
changes in characteristic λmax at 418 nm (Fig. 10). Consequently,
another suggestion concerning ZrOL and ZnL catalytic mechanisms
could be proposed [15]. The heterolytic or homolytic cleavage of
oxidizing agent O–O bond could cause direct oxygen transfer to
M2+-catalyst awarding oxo-catalyst without an electron transfer
process. The formed oxo-catalyst could transfer oxygen to the reac-
tants to produce the target oxide-products, as shown in Scheme 4.
Ea
lnk = ln A −
(6)
RT
where, the gas constant is R and the pre-exponential factor is A
[57]. The obtained magnitudes of Ea and A for epoxidation of 1,2-
cyclooctene are listed in Table 4 for 50, 60, 70, 80, 90 and 100°C.
−ꢂG#
kBT
/
RT
k =
e
(7)
h
where, h is Planck’s constant and kB is Boltzmann’s constant. From
Eq. 7, the average Gibb’s free energy for epoxidation processes
could be derived and then listed (Table 4). Hence, the Ea values
assigned that VOL is the most effective catalyst compared to ZrOL
and ZnL with the lowest Ea value (53.0 kJmol−1). Also, the Ea value
12

M.S.S. Adam, L.H. Abdel-Rahman, H.E.-S. Ahmed et al.
Journal of Molecular Structure 1236 (2021) 130295
Table 5
Epoxidation of some cyclic and acyclic alkenes catalyzed.
Scheme 3. The proposed mechanism for the epoxidation of 1,2-cyclohexene by aqueous H2O2 under VO-catalysis, homogeneously.
The proposed mechanism described an attack of the oxidant
molecule to central metal ion Zr4+or Zn2+ and bonds then form
intermediate I. The second step demonstrates the interaction of
oxidant with intermediate I to form intermediate II, with liberat-
ing H2O or tBuOH molecule [18,21]. In which, bridged μ-dioxygen
of the intermediate could be formed within an oxygen transfer
from oxidant to the central metal ion. Then, the substrate molecule
brings close to each bridged μ-dioxygen of the intermediate II by
forming a new intermediate (III), which liberate the epoxy prod-
uct (Scheme 4). The suggested mechanism is compatible with that
of reported catalytic epoxidation of olefins by MoO2-complex cat-
alysts [48,63].
ity as less consumed time and less required temperature compared
to those of ZrOL and ZnL catalysts (Tables S4-S6) [12,15,52].
The catalytic potential of the current M-complex catalysts
(VOL, ZrOL and ZnL) could be estimated compared to other sim-
ilar reported catalysts. VO(II)-dihydroindolone chelate showed re-
spectable catalytic potential in the 1,2-cyclooctene epoxidation
with aqueous H2O2 yielding 87% of epoxy-1,2-cyclooctane after
3 h at 85°C [12]. Similar Zn2+- and VO2+-Schiff base complexes
assigned high catalytic performance in the epoxidation of 1,2–
cyclohexene awarding 82 and 92% of the selective product (in ab-
sence of Na+SO3−-group) after 7 and 5 h at 85°C, respectively,
and also 85 and 94% yields (in the presence of Na+SO3−-group)
after 6 and 4 h at 85°C, respectively [15]. VO-naphthalenylimino-
phenolate complex catalyst displayed high reactivity for the 1,2-
cyclohexene epoxidation with H2O2, which reported 93% yielding
of the target product at 85°C after 1.5 h [38]. Hence, the current
From the mechanistic pathway, VOL catalyst exhibited the high-
est catalytic reactivity compared to those of other studied M-
catalysts (ZrOL and ZnL). The electron transfer process improves
significantly the reactivity of VOL with the highest chemoselectiv-
13

M.S.S. Adam, L.H. Abdel-Rahman, H.E.-S. Ahmed et al.
Journal of Molecular Structure 1236 (2021) 130295
Fig. 10. Electronic spectral scans for ZrOL, as a catalyst for the epoxidation of 1,2-
cyclohexene before and after addition of H2O2 at the optimized conditions with
interval time 10 min.
Scheme 4. Catalytic cycle of the epoxidation of 1,2-cyclohexene catalyzed by ZrO-
and Zn-catalysts.
Fig. 9. Electronic spectral scans for VOL, as a catalyst for the epoxidation of 1,2-
cyclohexene before and after addition of H2O2 at the optimized conditions with
interval time 10 min.
catalysts, specifically VOL, are considered active with convenient
reaction conditions.
3.4. Oxidation of 2-aminothiophene
Scheme 5. All possible products of the oxidation of 2-aminothiophene using H2O2
catalyzed by VOL, ZrOL or ZnL at their optimized reaction conditions.
Oxidation of thiophenes is well documented with an alternative
oxidizing agent under various atmospheres [48] to give two paral-
lel main products, namely thiophene oxide and thiophene dioxide.
Hence, the first attempt to identify all possible products for the
oxidation of 2-aminothiophene (1.0 mmol), which carried out by
using H2O2 (3.0 mmol) and catalyzed by VOL, ZrOL or ZnL (0.02
mmol) under optimum conditions as 1,2-cyclohexene epoxidation.
Types of detectable products and their yielding percentages were
determined by GC/MS and shown in Table 6. Scheme 5 displays all
possible products of 2-aminothiophene oxidation by H2O2 and cat-
alyzed by VOL, ZrOL or ZnL at their optimization. Additionally, the
reaction in absence of a considered catalyst gave low conversion
(15%) without a specific selective product.
The catalytic oxidation of 2-aminothiophene was an unse-
lective reaction, however, it gave some main products depend-
ing on the type of catalysts. With VOL, the highest yield per-
centages was the dioxy-dimeric form (2-((1,1-dioxidothiophen-2-
yl)diazenyl)thiophene-1,1-dioxide) with 26%. On the other hand,
14

M.S.S. Adam, L.H. Abdel-Rahman, H.E.-S. Ahmed et al.
Journal of Molecular Structure 1236 (2021) 130295
Table 6
2-Aminothiophene oxidation catalyzed by VOL, ZrOL or ZnL using an aqueous H2O2.
Scheme 6. Profile for activated intermediates corresponds to the epoxidation of 1,2-cyclohexene by aqueous H2O2 under VO-catalysis, homogeneously.
Table 7
the main product of oxidation reaction with ZrOL and ZnL was 2-
QSAR parameters for VO, ZrO and Zn-complexes.
nitrothiophene-1-oxide. The central metal ion in its catalyst com-
plex played a prominent role in the selectivity of the oxidation
product.
The parameters
Complexes
VOL
ZnL
ZrOL
˚
Surface area (grid) (A)
633.40
1204.04
-12.65
-1.88
642.82
1213.40
-12.88
-3.67
625.78
1185.66
-13.83
1.88
3.5. Supporting computations
˚
Volume(A)
Hydration energy (k cal/mol)
Log p
3.5.1. Quantitative Structural Activity relationships (QSAR)
QSAR indexes were achieved for the current complexes us-
ing HyperChem (8.1) software through successive steps required
for energy minimization. Such steps were, adding H-atoms, setup
of semi-empirical (AM1), setup force-field Molecular Mechanics
(MM+) and then minimization was proceeded. This study was ac-
complished without fixing any parameter and finished over the
Polake-Ribiere conjugated gradient algorithm method [64]. The re-
activity, polarizability, surface area, hydration energy and partition
coefficient (log p), were calculated for the investigated complexes
and reported in Table 7. The factors that reflect surface character-
istics of the M-complexes are what give an impression of the ex-
pected catalytic behavior. Such factors are the reactivity, polariz-
ability and surface area. The superiority of the ZnL complex was
recorded, while VOL and ZrOL appeared with similar and moder-
ate characteristics. Although this superiority may not be useful in
the current catalytic epoxidation systems, due to the lack of zinc’s
ability to oxidize and enter into a catalytic cycle that requires its
oxidation as a first stage.
˚
Reactivity (A)
129.47
46.05
131.19
46.95
129.47
46.04
˚
Polarizability (A)
3.5.2. Global reactivity of optimized geometries
DFT was applied using Becke3–Lee–Yang–Parr (B3LYP)
exchange-correlation functional under 6-311G++ basis set due
to the accuracy, flexibility and consistency. Better performance
was progressed by valence double-zeta polarized basis set (6-
311G++), which adds to 6-31G set five functions known as d-type
Cartesian-Gaussian polarization, on each atom. This computation
aims to demonstrate the best structural forms for VOL, ZrOL
and ZnL complexes and calculate important parameters. Many
characteristics could appear that favor the best for the catalytic
application, as well as, obtaining electrostatic maps to know
the extent of the susceptibility to acquiring electrons from the
external environment. All extracted files were visualized to obtain
15

M.S.S. Adam, L.H. Abdel-Rahman, H.E.-S. Ahmed et al.
Journal of Molecular Structure 1236 (2021) 130295
such requirements and studied to distinguish the main differences
between the M-complexes. The optimized structures (Fig. S8)
showed natural bonds without strain and two central metals (VO
& ZrO) appeared not crowded, which may allow them to add an
extra bond. Also, electrophilicity (ω), absolute-softness (σ), global
softness (S) and electronegativity (χ) indexes were calculated
to set an expectation for closest characteristics for complexes
(Table S3) [65]. High electronegativity and global softness coupled
with low electrophilicity of VOL complex, reflect its inability to
acquire electrons from surrounding acquiring but has high ability
for binding with any donor atom, the small size of vanadium is
a supporter. Dipole moment (Ddebye), formation energy (E, a.u.)
and oscillator strength (ʄ) were taken from log files. Low dipole
moment and formation energy reflect low polarity over the com-
plex surface and high stability recorded with VOL, respectively.
Also, the high oscillator strength value of VOL complex reflects the
labile nature of the d1 electron, which relocated from its orbital
easily (Fig. S9).
(E = -2968.394 a.u., D = 15.066 Debye) displayed with high sta-
bility, which pushed to proceed in the catalytic cycle, further its
polarity is remarkably increased. Conformer C (E = -2892.177 a.u.,
D = 8.806 Debye) appeared with reduced stability and polarity,
which denotes its faster conversion. Conformer D (E = -2881.177
a.u., D = 8.776 Debye) appeared close in the stability and polar-
ity of conformer C, which also has the same ability for faster con-
version to intermediate A, conducting to the original free catalyst.
The success of VOL complex in oxidation-based catalytic reactions
mainly depends on different factors as follows; i) Ease of V4+ ion
oxidation. ii) Free z-axis for extra-bonds and removal of crystal wa-
ter molecules, which happened under the optimal conditions (up
to 80°C). This may evacuate sites inside the molecular building and
then adsorb H2O2 molecule, as an introductory step before bond-
ing.
4. Conclusions
In this work, ZrO(II), VO(II) and Zn(II) complexes have been suc-
cessfully synthesized from an asymmetrical tetra-dentate ligand.
These compounds were characterized by all available tools (analyt-
ical, spectral and theoretical). Distorted octahedral geometry was
proposed for Zn(II) complex, while square pyramidal geometries
were proposed for ZrO(II) and VO(II) complexes. In a catalytic ap-
plication using M-complexes under H2O2, as an oxidant, for the
epoxidation of 1,2-cyclohexene, we found that our three complexes
exhibited moderate to very good catalytic control. Parameters such
as temperature, time, solvent, oxidant and amount of M-catalysts
were changed to show different catalytic effects on both activity
and selectivity of the oxidative process. The catalysts screening for
the epoxidation of various alkenes within cyclic and acyclic chains
at optimization was studied. All M-catalysts were efficiently capa-
ble of catalyzing epoxidation of those alkenes to their correspond-
ing epoxy products by aqueous H2O2. But the superiority of VO(II)
complex in the catalytic role in studied epoxidation reactions, was
clearly noticed. Also, the first attempt for catalytic oxidation of 2-
aminothiophene using H2O2 and catalyzed by VOL, ZrOL and ZnL,
was reported. Various products with high conversion and less se-
lectivity were obtained and defined by GC/MS. Moreover, it is wor-
thy to note that the computational implementations as QSAR and
DFT study, mostly throw a shadow on distinguishing characteris-
tics of VO(II) complex in comparison with the others. The catalytic
pathways proposed were confirmed by DFT studies.
3.5.2.1. Electrostatic potential features. Using fchk file and upon
new cubic contours, HOMO & LUMO, MEP and iso-surface maps
were created, to distinguish electron density distribution on the
whole complexes. HOMO and LUMO levels (Fig. 4) were appeared
extending over the entire complexes and the highly concentrated
around the environment of the central atom. This verifies the im-
pact of metal ions on electronic transitions inside the complex
[66]. The charges on atoms of each M-complex were calculated and
graphed (Fig. S10), to identify the changes on coordinating atoms
(N8, N9, O24, O28 & M) after bonding. Regarding VOL, the charges
on N8, N9, O24 & O28 were enhanced due to M-LCT, while in other
complexes, such atoms suffer minimization in their negativity, due
to L-MCT. This indicates the flexibility of the vanadium electron
cloud, which exerted from the labile electron at the Fermi level.
Molecular electrostatic potential maps (MEP) were created
(Fig. 5) to evaluate electron density features over functional
groups. Electrophilic (electron-poor), nucleophilic or neutral poten-
tial zones were clearly differentiated chromatically by blue, red and
green colors, respectively. Nucleophilicity as the electron-rich zone
was remarked around VO, while ZrOL and ZnL complexes such fea-
ture has slightly appeared with prevailing neutrality one [67]. This
indicates that the vanadium surrounding is electron-rich and could
easily lose electrons. Furthermore, iso-surface maps were created
with the presence of electron distribution array plots (Fig. S11).
This map type was obtained by determining electron density over
multi-points on the surface grid and then connected to build the
electrostatic contour. Yellow lines in the array plot point to the
electron-rich area (inner contour) and the red lines point to the
electrons-poor area (outer contour). The surface boundary of VOL
displayed broad red lines, which denotes its unsaturated surface
compared to that of the others [68].
Declaration of Competing Interest
No.
CRediT authorship contribution statement
Mohamed Shaker S. Adam: Conceptualization, Methodology,
Formal analysis, Investigation, Writing - original draft, Writing -
review & editing, Supervision. Laila H. Abdel-Rahman: Software,
Data curation, Writing - original draft. Hanan El-Sayed Ahmed:
Methodology, Data curation, Validation, Resources, Writing - re-
view & editing. M.M. Makhlouf: Conceptualization, Formal analy-
sis, Project administration, Funding acquisition, Software, Data cu-
ration, Visualization. Mona Alhasani: Software, Writing - original
draft. Nashwa M. El-Metwaly: Methodology, Validation, Resources,
Writing - original draft, Writing - review & editing, Supervision.
3.5.3. DFT studies support catalytic feature
Commonly, the catalytic process oxidation-based systems are
controlled by different features that belong to the electronic con-
figuration and its distribution over the applied catalyst. So, the
computation studies achieve importance in supporting of the sug-
gested mechanism, which based mainly on the logical assump-
tion of how the catalytic process will happen. All suggested in-
termediates faced geometry optimization under 6-31G++ basis set,
to put a suitable view for catalytic behavior. VOL catalyst (E = -
2815.806 a.u., D = 3.37 Debye), which activated initially under op-
timal conditions, raises the energy level of the stabilized catalyst.
An intermediate A (E = -2817.111 a.u., D = 7.51 Debye) appeared
with the closest stability with initial VO(II) form, but has a high
polarity that facilitates its coordination with H2O2. Conformer B
Acknowledgment
Taif University Researchers Supporting Project number (TURSP-
2020/165), Taif University, Taif, Saudi Arabia.
16

M.S.S. Adam, L.H. Abdel-Rahman, H.E.-S. Ahmed et al.
Journal of Molecular Structure 1236 (2021) 130295
Supplementary materials
(b) M. Khorshidifard, H.A. Rudbari, B. Askari, M. Sahihi, M.R. Farsani, F. Jalil-
ian, G. Bruno, Cobalt(II), copper(II), zinc(II) and palladium(II) Schiff base com-
plexes: Synthesis, characterization and catalytic performance in selective oxi-
dation of sulfides using hydrogen peroxide under solvent-free conditions, Poly-
hedron 95 (2015) 1–13.
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.2021.130295.
[19] A.T. Normand, R. Malacea-Kabbara, R. Lapenta, A. Dajnak, P. Richard, H. Cat-
tey, A. Bolley, A. Grassi, S. Milione, A. Auffrant, S. Dagorne, P.Le Gendre, Phos-
phasalen group IV metal complexes: synthesis, characterization and ring open-
ing polymerization of lactide, Dalton Trans 49 (2020) 6989–7004.
[20] C.-J. Yu, C.-Y. Li, C.-Y. Tsai, B.-T. Ko, Titanium, zirconium and hafnium com-
plexes bearing amino-benzotriazole phenolate ligands as efficient catalysts for
ring-opening polymerization of lactides, Inorg. Chem. Commun. 109 (2019)
107561.
References
[1] K.C. Gupta, A.K. Sutar, Catalytic activities of Schiff base transition metal com-
plexes, Coord. Chem. Rev. 252 (2008) 1420–1450.
[2] (a) M. Hajrezaie, M. Paydar, C.Y. Looi, S.Z. Moghadamtousi, P. Hassandarvish,
M.S. Salga, H. Karimian, K. Shams, M. Zahedifard, N.A. Majid, H.M. Ali, M.A. Ab-
dulla, Apoptotic effect of novel Schiff Based CdCl2(C14 H21 N3O2) complex is me-
diated via activation of the mitochondrial pathway in colon cancer cells, Sci.
Rep. 5 (2015) 9097; (b) M.S.S. Adam, H. Elsawy, Biological potential of oxo–
vanadium salicyledieneamino-acid complexes as cytotoxic, antimicrobial, an-
tioxidant and DNA interaction, J. Photochem. Photobio. B 184 (2018) 34–43.
[3] (a) D. Gong, B. Wang, X. Jia, X. Zhang, The enhanced catalytic performance
of cobalt catalysts towards butadiene polymerization by introducing a labile
donor in a salen ligand, Dalton Trans 43 (2014) 4169–4178; (b) M.S.S. Adam,
O.M. El-Hady, F. Ullah, Biological and catalytic potential of sustainable low and
high valent metal-Schiff base sulfonate salicylidene pincer complexes, RSC Adv
9 (2019) 34311–34329.
[21] M. Mandal, V. Ramkumar, D. Chakraborty, Salen complexes of zirconium and
hafnium: synthesis, structural characterization and polymerization studies,
Polym. Chem. 10 (2019) 3444–3460.
[22] M. Sedighipoor, A.H. Kianfar, W.A.K. Mahmood, M.H. Azarian, Epoxidation of
alkenes by an oxidovanadium(IV) tetradentate Schiff base complex as an ef-
ficient catalyst with tert-butyl hydroperoxide, Inorg. Chim. Acta 457 (2017)
116–121.
[23] M. Aschi, M. Crucianelli, A.D. Giuseppe, C.D. Nicola, F. Marchetti, Insights
on the mechanistic features of catalytic oxidations of simple and conjugated
olefins promoted by VO(acac)2/H2O2 system, in acetonitrile: A computational
study, Catal. Today 192 (2012) 56–62.
[24] N. Xing, L.T. Xu, F.Y. Bai, H. Shan, Y.H. Xing, Z. Shi, Synthesis and characteriza-
tion of novel transition metal complexes with indole acetic acid ligands: Eval-
uation of their catalytic activity for the oxidation of cyclohexane, Inorg. Chim.
Acta 409 (2014) 360–366.
[25] W. Wang, D. Agustin, R. Poli, Influence of ligand substitution on molybdenum
catalysts with tridentate Schiff base ligands for the organic solvent-free oxida-
tion of limonene using aqueous TBHP as oxidant, Mol. Catal. 443 (2017) 52–
59.
[4] L. Zhou, C.C. Kwok, G. Cheng, H. Zhang, C.M. Che, Efficient red organic electro-
luminescent devices by doping platinum(II) Schiff base emitter into two host
materials with stepwise energy levels, Opt. Lett. 38 (2013) 2373–2375.
[5] (a) S.A. Lee, G.R. You, Y.W. Choi, H.Y. Jo, A.R. Kim, I. Noh, S.J. Kim, Y. Kim,
C. Kim, A new multifunctional Schiff base as a fluorescence sensor for Al3+ and
a colorimetric sensor for CN− in aqueous media: an application to bioimaging,
Dalton Trans 43 (2014) 6650–6659; (b) A. Ganguly, B.K. Paul, S. Ghosh, S. Kar,
N. Guchhait, Selective fluorescence sensing of Cu(II) and Zn(II) using a new
Schiff base-derived model compound: naked eye detection and spectral deci-
phering of the mechanism of sensory action, Analyst 138 (2013) 6532–6541.
[6] (a) C.R. Nayar, R. Ravikumar, Review: Second order nonlinearities of Schiff
bases derived from salicylaldehyde and their metal complexes, J. Coord. Chem.
67 (2014) 1–16; (b) M.M. Makhlouf, H. Alburaih, M.M. Shehata, M.S.S. Adam,
M.M. Mostafa, A. El-Denglawey, Synthesis, physicochemical and optical char-
acterization of a new isatin hydrazone derivative and its ZnO-complex for po-
tential energy conversion and storage applications, J. Phys. Chem. Solids 151
(2021) 109817.
[26] H. Hartmann, P. Gerstner, D. Rohde, A Simple Route to N-Arylated 2-Aminoth-
iophenes as a New Class of Amorphous Glass Forming Molecules, Org. Lett. 3
(2001) 1673–1675.
[27] (a) R.F.M. Elshaarawy, I.M. Eldeen, E.M. Hassan, Efficient synthesis and eval-
uation of bis-pyridinium/bis-quinolinium metallosalophens as antibiotic and
antitumor candidates, J. Mol. Struct. 1128 (2017) 162–173; (b) R. Hernan-
dez-Molina, A. Mederos, S. Dominguez, P. Gili, C. Ruiz-Perez, A. Castineiras,
X. Solans, F. Lloret, J.A. Real, Different Ground Spin States in Iron(III) Complexes
with Quadridentate Schiff Bases: Synthesis, Crystal Structures, and Magnetic
Properties, Inorg. Chem. 37 (1998) 5102–5108; (c) K. Sampath, C. Jayabalakr-
ishnan, Ruthenium(II) Tetradentate Schiff Base Complexes: Synthesis, Charac-
terization, DNA Binding, and Antioxidant Studies, Synth. React. Inorg. Met.-Org.
Nano-Met. Chem. 45 (2015) 1145–1153.
[7] W. Al Zoubi, N. Al Mohanna, Membrane sensors based on Schiff bases as
chelating ionophores – A review, Spectrochim. Acta A 132 (2014) 854–870.
[8] (a) A.W. Jeevadason, K.K. Murugavel, M.A. Neelakantan, Review on Schiff bases
and their metal complexes as organic photovoltaic materials, Renew. Sustain.
Energy Rev. 36 (2014) 220–227; (b) M.M. Shehata, M.S.S. Adam, K. Abdel-
hady, M.M. Makhlouf, Facile synthesis, characterizations, and impedance spec-
troscopic features of Zn(II)-bis Schiff base complex films towards, J. Solid Stat.
Electrochem. 23 (2019) 2519–2531.
[28] V.R. Chandrasekhar, K.M. Palsamy, R. Lokesh, D.T. Thangadurai, G.N. Indra,
R. Jegathalaprathaban, R. Gurusamy, Biomolecular docking, antimicrobial and
cytotoxic studies on new bidentate schiff base ligand derived metal (II) com-
plexes, Appl. Organomet. Chem. 33 (2019) e4753.
[29] A.S. Munde, A.N. Jagdale, S.M. Jadhav, T.K. Chondhekar, Synthesis, characteriza-
tion and thermal study of some transition metal complexes of an asymmetrical
tetradentate Schiff base ligand, J. Serb. Chem. Soc. 75 (2010) 349–359.
[30] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheese-
man, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Car-
icato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg,
M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishid, T. Nakajima, Y.
Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, J. E. Peralta, F. Ogliaro,
M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, T. Keith, R.
Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar,
J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V.
Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J.
Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V.
G. Zakr-Zewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D.
Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, D. J. Fox, Gaussian,
Inc., Wallingford CT (2010).
[9] R.M. Clarke, T. Storr, The chemistry and applications of multimetallic salen
complexes, Dalton Trans 43 (2014) 9380–9391.
[10] (a) X. Liu, C. Manzur, N. Novoa, S. Celedón, D. Carrillo, J.-R. Hamon, Multi-
dentate unsymmetrically-substituted Schiff bases and their metal complexes:
Synthesis, functional materials properties, and applications to catalysis, Coord.
Chem. Rev. 357 (2018) 144–172; (b) S. Bertini, A. Coletti, B. Floris, V. Conte,
P. Galloni, Investigation of VO-salophen complexes electronic structure, J. In-
org. Biochem. 147 (2015) 44–53.
[11] M.F.I. Al-Hussein, M.S.S. Adam, Catalytic evaluation of copper (II) N-sali-
cylidene-amino acid Schiff base in the various catalytic processes, Appl.
Organomet. Chem. 34 (2020) e5598.
[12] M.S.S. Adam, Catalytic activity of nickel(II), copper(II) and oxovana-
dium(II)-dihydroindolone complexes towards homogeneous oxidation re-
actions, Appl. Organomet. Chem. 32 (2018) e4234.
[13] M. Mandal, V. Nagaraju, G.V. Karunakar, B. Sarma, B.J. Borah, K.K. Bania, Elec-
tronic, Conjugation, and Confinement Effects on Structure, Redox, and Catalytic
Behavior of Oxido-Vanadium(IV) and -(V) Chiral Schiff Base Complexes, J. Phys.
Chem. C 119 (2015) 28854–28870.
[31] (a) B.G. Johnson, M.J. Frisch, Analytic second derivatives of the gradient–
corrected density functional energy. Effect of quadrature weight derivatives,
Chem. Phys. Lett. 216 (1993) 133–140; (b) F. Weigend, R. Ahlrichs, Balanced
basis sets of split valence, triple zeta valence and quadruple zeta valence qual-
ity for H to Rn: Design and assessment of accuracy, Phys. Chem. Chem. Phys.
7 (2005) 3297–3305.
[14] (a) G. Licini, V. Conte, A. Coletti, M. Mba, C. Zonta, Recent advances
in vanadium catalyzed oxygen transfer reactions, Coord. Chem. Rev. 255
(2011) 2345–2357; (b) M. Sutradhar, L.M.D.R.S. Martins, M.F.C.G.da Silva,
A.J.L. Pombeiro, Vanadium complexes: Recent progress in oxidation catalysis,
Coord. Chem. Rev. 301–302 (2015) 200–239.
[32] S.A. Khan, K. Rizwan, S. Shahid, M.A. Noamaan, T. Rasheed, H. Amjad, Synthe-
sis, DFT, computational exploration of chemical reactivity, molecular docking
studies of novel formazan metal complexes and their biological applications,
Appl. Organomet. Chem. 34 (2020) e5444.
[33] R. Dennington II, T. Keith, J. Millam, GaussView, Semichem Inc., Shawnee Mis-
sion, KS, 2007 Version 4.1.2.
[34] R.C. Chikate, S.B. Padhye, Transition metal quinone–thiosemicarbazone com-
plexes 2: Magnetism, ESR and redox behavior of iron (II), iron (III), cobalt (II)
and copper (II) complexes of 2-thiosemicarbazido-1,4-naphthoquinone, Poly-
hedron 24 (2005) 1689–1700.
[15] M.S.S. Adam, M.A. Al-Omair, F. Ullah, Catalytic comparison of various polar
Zn(II)- and VO(II)-Schiff base complexes in (ep)oxidation processes of 1,2-cy-
clohexene and cyclohexane, Res. Chem. Intermediat. 45 (2019) 4653–4675.
[16] C.I. Fernandes, P.D. Vaz, T.G. Nunes, C.D. Nunes, Zinc biomimetic catalysts for
epoxidation of olefins with H2O2, Appl. Clay Sci. 190 (2020) 105562.
[17] (a) J. Huang, J. Xie, J. Cai, Asymmetric epoxidation of unfunctionalized olefins
catalyzed by chiral salen-Mn (III) immobilized on alkoxyl-modified ZnPS-PVPA,
Appl. Organomet. Chem. 33 (2019) e4982; (b) J. Huang, X. Fu, Q. Miao, Cat-
alytic asymmetric epoxidation of unfunctionalized olefins using a series novel
type of layered crystalline organic polymer–inorganic hybrid zinc phospho-
nate–phosphate immobilized aryldiamine modified chiral salen Mn(III) com-
plex, Appl. Catal. A 407 (2011) 163–172.
[35] (a) B. Geeta, K. Shravankumar, P.M. Reddy, E. Ravikrishna, M. Sarangapani,
K.K. Reddy, V. Ravindera, Binuclear cobalt(II), nickel(II), copper(II) and palla-
dium(II) complexes of a new Schiff-base as ligand: Synthesis, structural char-
acterization, and antibacterial activity, Spectrochim. Acta A 77 (2010) 911–915;
(b) M.S.S. Adam, L.H. Abdel-Rahman, A.M. Abu-Dief, N.A. Hashem, Synthesis,
[18] (a) M. Liu, S. Qiu, Y. Ye, G. Yin, Mild and efficient synthesis of isoindigo
derivatives catalyzed by Lewis acid, Tetrahedron Lett 57 (2016) 5856–5858;
17

M.S.S. Adam, L.H. Abdel-Rahman, H.E.-S. Ahmed et al.
Journal of Molecular Structure 1236 (2021) 130295
catalysis, antimicrobial activity and DNA interactions of new Cu(II)-Schiff base
complexes, Inorg. Nano-Met. Chem. 50 (2020) 136–150.
[51] P. Adao, J.C. Pessoa, R.T. Henriques, M.L. Kuznetsov, F. Avecilla, M.R. Maurya,
U. Kumar, I. Correia, Synthesis, Characterization, and Application of Vanadi-
um-Salan Complexes in Oxygen Transfer Reactions, Inorg. Chem. 48 (2009)
3542–3561.
[36] (a) M.S.S. Adam, M.M. Youssef, M.F. Abo Elghar, A.M. Hafez, U. El-Ayaan, Syn-
thesis and characterization of binary and ternary oxovanadium complexes of
N,N’-(2-pyridyl)thiourea and curcumin. Catalytic oxidation potential, antibacte-
rial, antimicrobial, antioxidant and DNA interaction studies, Appl. Organometal.
Chem. 31 (2017) e3650; (b) M.S.S. Adam, Sustainable dipolar homo-dicop-
per (II) dihydrazone complex as a catalyst for Sonogashira cross couplings, J.
Organomet. Chem. 903 (2019) 120985.
[52] M.R. Maurya, B. Mengesha, B. Uprety, N. Jangra, R. Tomara, F. Avecilla, Oxygen
atom transfer between DMSO and benzoin catalyzed by cis-dioxidomolybde-
num (VI) complexes of tetradentate Mannich bases, New J. Chem. 42 (2018)
6225–6235.
[53] T. Joseph, D. Srinivas, C.S. Gopinath, S.B. Halligudi, Spectroscopic and Catalytic
Activity Studies of VO(Saloph) Complexes Encapsulated in Zeolite-Y and Al-M-
CM-41 Molecular Sieves, Catal. Lett. 83 (2002) 209–214.
[37] W.A. Zordok, Synthesis, spectroscopic, structural characterization, thermal
analysis, kinetics, biological evaluation of non-steroidal anti-inflammatory
drug diclofenac zirconium (IV) solvates (L) (L ¼ H2O, DMF, Py and Et3N), J.
Mol. Struct. 1166 (2018) 270–285.
[54] L.M.D.R.S. Martins, A.J.L. Pombeiro, Tris(pyrazol-1-yl)methane metal complexes
for catalytic mild oxidative functionalizations of alkanes, alkenes and ketones,
Coord. Chem. Rev. 265 (2014) 74–88.
[55] S. Huber, M. Cokoja, F.E. Kühn, Historical landmarks of the application of
molecular transition metal catalysts for olefin epoxidation, J. Organomet.
Chem. 751 (2014) 25–32.
[38] M.S.S. Adam, M.M. Makhlouf, F. Ullah, A.D.M. Mohamad, Catalytic and bio-
logical reactivities of mononuclear copper (II) and vanadyl (II) complexes of
naphthalenylimino-phenolate sodium sulfonate, J. Taiwan Inst. Chem. Eng. 118
(2021) 12–28.
[39] M.S.S. Adam, A.D.M. Mohamad, Catalytic (ep)oxidation and corrosion inhibition
potentials of CuII and CoII pyridinylimino phenolate complexes, Polyhedron 151
(2018) 118–130.
[56] J.J. Plata, L.C. Pacheco, E.R. Remesal, M.O. Masa, L. Vega, A.M. Márquez,
J.A. Odriozola, J.F. Sanz, Analysis of the variables that modify the robustness
of Ti-SiO2 catalysts for alkene epoxidation: Role of silylation, deactivation and
potential solution, Mol. Catal. 459 (2018) 55–60.
[57] V. Conte, A. Coletti, B. Floris, G. Licini, C. Zonta, Mechanistic aspects of
vanadium catalysed oxidations with peroxides, Coord. Chem. Rev. 255 (2011)
2165–2177.
[58] A.D.M. Mohamad, E.R. El-Shrkawy, M.F.I. Al-Hussein, M.S.S. Adam, Water–
soluble Cu(II)-complexes of Schiff base amino acid derivatives as biological
reagents and sufficient catalysts for oxidation reactions, J. Taiwan Inst. Chem.
Eng. 113 (2020) 27–45.
[59] N.M.R. Martins, K.T. Mahmudov, M.F.C.G. da Silva, L.M.D.R.S. Martins,
A.J.L. Pombeiro, Copper (II) and iron (III) complexes with arylhydrazone of
ethyl 2-cyanoacetate or formazan ligands as catalysts for oxidation of alcohols,
New J. Chem. 40 (2016) 10071.
[40] L.H. Abdel-Rahman, M.S.S. Adam, A.M. Abu-Dief, H. Moustafa, M.T. Basha,
A.S. Aboraia, B.S. Al-Farhan, H.E. Ahmed, Synthesis, theoretical investigations,
biocidal screening, DNA binding, in vitro cytotoxicity and molecular docking
of novel Cu(II), Pd(II) and Ag(I) chlorobenzylidene Schiff base: Promising an-
tibiotic and anticancer agents, Appl. Organometal. Chem. 32 (2018) e4527.
[41] (a) M. Gaber, Y. El-Sayed, K. El-Baradie, R. Fahmy, Cu(II) complexes of monoba-
sic bi- or tridentate (NO, NNO) azo dye ligands: synthesis, characterization,
and interaction with Cu-nanoparticles, J. Mol. Struct. 1032 (2013) 185–194;
(b) M. Montazerozohori, S. MojahediJahromi, A. Masoudiasl, P. McArdle, Nano
structure zinc (II) Schiff base complexes of a N3-tridentate ligand as new bio-
logical active agents: Spectral, thermal behaviors and crystal structure of zinc
azide complex, Spectrochim. Acta A 5 (2015) 517–528.
[42] H. Katouah, A.M. Hameed, A. Alharbi, F. Alkhatib, R. Shah, S. Alzahrani, R. Zaky,
N.M. El-Metwaly, Green Synthesis Strategy for New Schiff-Base Complexes:
Characterization, Conductometry, In Vitro Assay Confirmed by In Silico Ap-
proach, ChemistrySelect 5 (2020) 10256–10268.
[60] L.H. Abdel-Rahman, A.M. Abu-Dief, M.S.S. Adam, S.K. Hamdan, Some new
nano-sized mononuclear Cu(II) Schiff base complexes: design, characterization,
molecular modeling and catalytic potentials in benzyl alcohol oxidation, Catal.
Lett. 146 (2016) 1373–1396.
[43] (a) M. Kirihara, Aerobic oxidation of organic compounds catalyzed by vana-
dium compounds, Coord. Chem. Rev. 255 (2011) 2281–2302; (b) J.A.L. da Silva,
J.J.R.F. da Silva, A.J.L. Pombeiro, Oxovanadium complexes in catalytic oxidations,
Coord. Chem. Rev. 255 (2011) 2232–2248.
[44] M.S.S. Adam, A.M. Hafez, I. El-Ghamry, Catalytic performance of binary and
ternary oxovanadium complexes of dipyridinyl-urea in (ep)oxidation of cis-cy-
clooctene and 1-octene, Reac. Kinet. Mech. Cat. 124 (2018) 779–805.
[45] N. Mizuno, K. Kamata, Catalytic oxidation of hydrocarbons with hydrogen
peroxide by vanadium-based polyoxometalates, Coord. Chem. Rev. 255 (2011)
2358–2370.
[61] E. Steves, S.S. Stahl, Copper(I)/ABNO-Catalyzed Aerobic Alcohol Oxidation: Alle-
viating Steric and Electronic Constraints of Cu/TEMPO Catalyst Systems, J. Am.
Chem. Soc. 135 (2013) 15742–15745.
[62] M.S.S. Adam, Catalytic potentials of homodioxo-bimetallic dihydrazone com-
plexes of uranium and molybdenum in a homogeneous oxidation of alkenes,
Monatsh. Chem. 146 (2015) 1823–1836.
[63] L.P. de Carvalho, K. F.Silva, L.L. dos Santos, M.V.P. dos Santos, J.A.B. da Silva,
R.L. Longo, Chemoselective oxidation of unsaturated organosulfur, selenium
and phosphorus compounds by molybdenum oxodiperoxo complexes: A com-
putational investigation, Inorg. Chim. Acta 467 (2017) 351–357.
[46] L.S. Yuan, R. Razali, J. Efendi, N.A. Buang, H. Nur, Temperature-controlled se-
lectivity in oxidation of 1-octene by using aqueous hydrogen peroxide in
phase-boundary catalytic system, Appl. Catal. A 460-461 (2013) 21–25.
[47] H. Han, L. Lu, Q. Wang, M. Zhu, C. Yuan, S. Xing, X. Fu, Synthesis and
evaluation of oxovanadium(IV) complexes of Schiff-base condensates from
5-substituted-2-hydroxybenzaldehyde and 2-substituted-benzenamine as, se-
lective inhibitors of protein tyrosine phosphatase 1B, Dalton Trans 41 (2012)
11116–11124.
[48] M.S.S. Adam, M.S.M. Ahmed, O.M. El-Hady, S. Shaaban, Bis-dioxomolybde-
num (VI) oxalyldihydrazone complexes: Synthesis, characterization, DFT stud-
ies, catalytic epoxidation potential, molecular modeling and biological evalua-
tions Appl, Organomet. Chem. 34 (2020) e5573.
[49] (a) G. Grivani, V. Tahmasebi, A.D. Khalaji, K. Fejfarova, M. Duṧek, Synthesis,
characterization and crystal structure determination of a new vanadium(IV)
Schiff base complex (VOL2) and investigation of its catalytic activity in the
epoxidation of cyclooctene, Polyhedron 51 (2013) 54–60; (b) G. Grivani,
A.D. Khalaji, V. Tahmasebi, K. Gotoh, H. Ishida, Synthesis, characterization and
crystal structures of new bidentate Schiff base ligand and its vanadium(IV)
complex: The catalytic activity of vanadyl complex in epoxidation of alkenes,
Polyhedron 31 (2012) 265–271; (c) G. Grivani, G. Bruno, H.A. Rudbari, A.D. Kha-
laji, P.A. Pourteimouri, Synthesis, characterization and crystal structure deter-
mination of a new oxovanadium(IV) Schiff base complex: The catalytic activity
in the epoxidation of cyclooctene, Inorg. Chem. Commun. 18 (2012) 15–20.
[50] C.D. Nunes, P.D. Vaz, V. Felix, L.F. Veiros, T. Moniz, M. Rangel, S. Realista,
A.C. Mourato, Calhorda, Vanadyl cationic complexes as catalysts in olefin oxi-
dation, Dalton Trans 44 (2015) 5125–5138.
[64] M. Morad, T.M Habeebullah, I. Althagafi, B.H Asghar, A.A Bayazeed,
T.M Bawazeer, A.M Al-Solimy, N. El-Metwaly, Copper–acetanilide complexes:
synthesis, characterization, crystal structure, computational analysis and their
application as heterogeneous catalysts for biodiesel synthesis, Res. Chem. In-
termediat. 46 (2020) 4543–4562.
[65] R. Shah, H. Katouah, A.A. Sedayo, M. Abualnaja, M.M. Aljohani, F. Saad, R. Zaky,
N.M. El-Metwaly, Practical and computational studies on novel Schiff base
complexes derived from green synthesis approach: Conductometry as well as
in-vitro screening supported by in-silico study, J. Mol. Liq. 219 (2020) 114116.
[66] A.S. Al-Wasidi, N.M. Al-Jafshar, A.M. Al-Anazi, M.S. Refat, N.M. El-Metwaly,
H.K. Ibrahim, W. Abd El-Fattah, A.M. Naglah, M.A. Al-Omar, A. Kalmouch, In
Methanolic Solvent Synthesis of New MnII, CoII, NiII and CuII Schiff Base of
Aromatic β Amino Acids: Spectroscopic, Thermal, Molecular Docking and An-
timicrobial Studies, Sci. Adv. Mater 12 (2020) 1137–1148.
[67] S.Y. Al-nami, E. Aljuhani, I. Althagafi, H.M. Abumelha, T.M. Bawazeer, A.M. Al-
Solimy, Z.A. Al-Ahmed, F. Al-Zahrani, N. El-Metwaly, Synthesis and Characteri-
zation for New Nanometer Cu(II) Complexes, Conformational Study and Molec-
ular Docking Approach Compatible with Promising in Vitro Screening, Arab. J.
Sci. Eng. DOI (2000), doi:10.1007/s13369-020-04814-x.
[68] F. Alkhatib, A. Hameed, A. Sayqal, A.A Bayazeed, S. Alzahrani, Z.A Al-Ahmed,
R. Zaky, N.M El-Metwaly, Green-synthesis and characterization for new Schif-
f-base complexes; spectroscopy, conductometry, Hirshfeld properties and bio-
logical assay enhanced by in-silico study, Arab. J. Chem. 13 (2020) 6327–6340.
18