Catalytic comparison of various polar Zn(II)-Schiff base complexes and VO(II)-Schiff base complexes in (ep)oxidation processes of 1,2-cyclohexene and cyclohexane

Article abstract of DOI:10.1007/s11164-019-03855-8

The reactivity of Zn²⁺ and VO²⁺ ions towards pyridinyl Schiff bases, in the absence or presence of a p-sodium sulfonate group (HPSNa and HPS, respectively), provided polar and less polar catalyst complexes, which were characterized b

Full text of DOI:10.1007/s11164-019-03855-8

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-019-03855-8
Catalytic comparison of various polar Zn(II)‑Schif base
complexes and VO(II)‑Schif base complexes in (ep)
oxidation processes of 1,2‑cyclohexene and cyclohexane
Mohamed Shaker S. Adam1,2 · Mohammed A. Al‑Omair1 · Farman Ullah3
Received: 15 November 2018 / Accepted: 15 May 2019
© Springer Nature B.V. 2019
Abstract
The reactivity of Zn2+ and VO2+ ions towards pyridinyl Schif bases, in the absence
or presence of a p-sodium sulfonate group (HPSNa and HPS, respectively), provided
polar and less polar catalyst complexes, which were characterized by various meas-
urements. Stability constants of Zn-complexes [Zn(PS)2 and Zn(PSNa)2], and VO-
complexes [VO(PS)2 and VO(PSNa)2] were measured spectrophotometrically. A
typical (ep)oxidation processes of 1,2-cyclohexene or cyclohexane using 30% aque-
ous H2O2 catalyzed by our synthesized catalyst system, were tested. VO-complexes
were found to be more efective catalysts than Zn-chelates. The polarity of Zn- and
VO-complexes, i.e. the presence of p-SO3Na, displayed an observable infuence on
their catalytic performance chemoselectively. The polar catalyst system, Zn(PSNa)2
and VO(PSNa)2, in polar solvents such as H2O, MeOH, acetonitrile and acetone
exhibited higher catalytic activity towards the (ep)oxidation processes than the less
polar catalysts, Zn(PS)2 and VO(PS)2. In a less polar solvent such as CHCl3, polar
catalysts showed higher conversion, but low chemoselectively, whereas the less
polar catalysts showed relatively higher conversion and chemoselectivity. Under sol-
vent-free conditions, less polar complexes were found to be more efcient catalysts
than the polar chelates.
Keywords Zinc(II) · Vanadyl(II) · Schif base · Catalysis · 1,2-Cyclohexene ·
Cyclohexane · (Ep)oxidation
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s1116
4-019-03855-8) contains supplementary material, which is available to authorized users.
*
Mohamed Shaker S. Adam
madam@kfu.edu.sa; shakeradam61@yahoo.com
1
Department of Chemistry, College of Science, King Faisal University, P.O. Box 380, Al Hufuf,
Al Hassa 31982, Saudi Arabia
2
3
Chemistry Department, Faculty of Science, Sohag University, Sohâg 82534, Egypt
Department of Chemistry, Winnipeg University, 515 Portage Avenue, Winnipeg, MB R3B 2E9,
Canada
Vol.:(0123456789)
1 3

M. S. S. Adam et al.
Introduction
Schif bases have acquired highlights on synthesis and coordination chemistry
with various transition metal ions because of their medicinal chemistry applica-
tions and high demonstrated signifcant biological activities (e.g. anti-apoptotic,
anti-fungal, antibacterial, anti-infammatory and antiviral activities) [1–4]. More-
over, they have recently been applied in sensors [5], organic photovoltaic materi-
als [6] and catalytic organic synthesis [7–9]. Many published works concerned
with substituted Schif bases and their corresponding metal frameworks have
been investigated to measure their sufciency in various catalytic processes [10].
Catalytic (ep)oxidation processes of alkenes and alkanes [11, 12], which are
the subject of developing interest in the production of the corresponding epox-
ides and oxides [13, 14], are the key building blocks for many industrial appli-
cations [15]. The catalytic performance of the inorganic homogeneous catalyst
complexes depends on two main parameters: frst, the type and charge of a central
metal ion [10, 16], and second, the type of the coordinated ligand and its coordi-
nation sphere [17–20].
Central metals with high oxidation states have been used in many catalytic (ep)
oxidation reactions [21, 22]. Consequently, oxide metal complexes with high oxida-
tion states, e.g. oxovanadium (IV) complexes are the most applicable and favored
class for the catalytic oxidation processes [11, 15, 23]. The facile oxidation states
interchange, while the high Lewis acid behavior and the strong bonding of vanadium
ions to oxygen atoms [24–27] enhances its catalytic potential in redox processes in
contrast to the low oxidation state metal complexes [21, 28]. This is done with the
intention to show that the progressed catalytic reports on alkenes and alkanes (ep)
oxidation by zinc complexes (as low valent metal complexes, Zn2+-chelates) in vari-
ous catalytic selective processes are rare and have not been thoroughly investigated
[8, 29, 30]. Moreover, solubility, polarity, electronic and steric properties of the
coordinated Schif base ligands are the keys to control their structural features and to
contribute their potential in most catalytic systems [19, 31–35]. Partial selectivity in
the oxidation reactions of alkenes and alkanes is an essential component within the
highest potential of economic and ecological perspectives of chemistry. So, transi-
tion metal coordination compounds as efective catalysts for saturated and unsatu-
rated hydrocarbons oxidation were probed in detail recently with diverse ligand sys-
tems [9, 14]. A relevant example is the selective oxidation of alkane oxidation [9,
36, 37] with valuable applications in various industrial felds [38, 39]. The polarity
of the catalyst is of great interest in oxidation catalysis, particularly, towards organic
synthesis [40, 41].
p-Sulfonation of salicylaldehyde and its derivatives are of interest to synthesize a
new series of polar and more sustainable Schif base ligands and their correspond-
ing metal complexes [27, 42]. Consequently, the solvent efect on such (ep)oxidation
processes with and without polar derivative in the metal-complex catalyst could be
signifcantly important to future study and could produce valuable results.
Our continued interest in the development of new ligands, complexes and their
application in catalysis [40, 42] has prompted us to investigate the comparison
1 3

Catalytic comparison of various polar Zn(II)‑Schif base…
between commonly applicable catalyst complexes of high valent vanadium ions
(VO2+), and rarely explored catalyst complexes of low valent zinc ions (Zn2+),
which, based on the available literature, has not fully investigated. Herein, we
report the full details for the synthesis of N-pyridiyl Schif base ligands with and
without the substituent, p-sodium sulfonate substituents, and their corresponding
complexes (VO2 and Zn2+). Furthermore, a comparative study in catalytic (ep)
oxidation of 1,2-cyclohexene and cyclohexane between Zn and VO-complex cata-
lyst systems.
Experimental
Reagents, instruments and methodology
Commercially available chemicals and reagents were ordered from Sigma-Aldrich
and Acros and used without any further treatment. A GMBH VarioEl model V2.3
CHNS machine was used for the CHN analyses. Nuclear magnetic resonance spectra
were measured by a multinuclear NMR spectrometer Bruker ARX400 at 400.1 for
a hydrogen proton and 100.6 MHz for carbon nucleus at 25 °C. Chemical shifts (δ)
of 1H and 13C are given in ppm. The J values (coupling constants) between H nuclei
in the NMR are presented by JHH. A JASCO UV–Vis spectrophotometer (V-570)
equipped by a 10 mm silica cells in a thermostatted cell holder was applied to deter-
mine the molecular electronic spectral transitions in a given reference solvent of all
the synthetic compounds. A Shimadzu FTIR-8101 Fourier transform infrared spec-
trophotometer was used for IR spectral scans and were recorded in the region of
4000–400 cm−1. Thermogravimetric analysis was achieved under nitrogen gas fow
(20 cm3 min−1) as carrier gas with a heating rate 10 °C per min from 30 to 400 °C by
a Shimadzu TGA-50H thermal analyzer. The conductivity of the current complexes
in various polar solvents was measured by a Jenway conductivity meter model 4320,
using an epoxy bodied conductivity cell (two electrodes, shiny) with cell constant
calibration from 0.01 to 19.99 at 25 °C and temperature held by using an ultrather-
mostat (HAAKE model F3-k) with 0.2 °C. Mass spectra m/z were measured for
the current compounds on a Waters Qtof Micro YA263 mass spectrometer. Values
of pH was measured on a Metrohm 695 pH/ion meter to 0.005 units with control
of temperature by using an ultrathermostat (HAAKE model F3-k) within range of
errors 0.2 °C. A Thermo Scientifc 9100 machine was applied to measure the melt-
ing point.
Synthesis of Schif base ligands (HPS and HPSNa)
Salicylaldehyde (0.20 mL, 2.0 mmol, in 25 mL methanol) or sodium salicylalde-
hyde-5-sulfonate (0.44 g, 2.0 mmol, in 25 mL water) was mixed with 2-aminopyri-
dine (0.08 g, 2.0 mmol, in 25 mL methanol) at room temperature with continuous
stirring, and after a few minutes of stirring the mixture was heated at 75 °C for 2 h.
1
TLC and H-NMR were used for monitoring the reaction progress. All volatiles
1 3

M. S. S. Adam et al.
were removed in vacuum and a yellow colored crude product was crystallized
from MeOH (HPS) and in (1:1) mixture H2O/MeOH (HPSNa), respectively. Both
ligands were nicely crystalized from the respective solvents providing good yield
1
of the desired compound as a yellow crystalline solid (Table 1). H and 13C NMR
data of HPS ligand were found consistent with previous reported one (Table 2) [43].
Detail NMR data for both ligands are included in the supplementary materials (Figs.
S1-S4).
Synthesis of Zn‑ and VO(PS)2
Zinc acetate dihydrate (0.42 g, 2.0 mmol) or vanadyl acetylacentonate (0.52 g,
2.0 mmol) in 25 mL H2O was mixed with HPS (0.79 g, 4.0 mmol in 40 mL metha-
nol) at room temperature. The resulting mixture was refuxed for 1 h at 100 °C, and
then cooled down to room temperature, and solvent was removed in vacuum. The
desired compound was purifed by recrystallization in methanol [4], afording good
1
yields of both complexes (Table 1). H and 13C NMR data are recorded in Table 2.
NMR data for Zn(PS)2 were found consistent with the previous reported one [4].
Additional technical data are also included in supplementary materials (Figs. S5 and
S6).
Synthesis of Zn‑ and VO(PSNa)2
Zinc acetate dihydrate (0.42 g, 2.0 mmol) or vanadyl acetylacentonate (0.52 g,
2.0 mmol) in 25 mL H2O was mixed with HPSNa (1.20 g, 4.0 mmol) in 50 mL
of H2O at room temperature. The obtained solution was heated to refux for 2 h at
85 °C. The color of the reaction solution was converted gradually to the correspond-
ing color of the complex, dark yellow for Zn(PSNa)2 and dark brown for VO(PSNa)2.
Purifcation of the obtained M-complexes was carried out by solvent removing in
vacuum, followed by recrystallization in binary mixture of water:methanol (1:1)
1
providing good yield, as reported in Table 1. H and 13C NMR of Zn(PSNa)2 are
listed in Table 2. Additional technical data are also included in supplementary mate-
rials (Figs. S7 and S8).
Stability constants of Zn‑ and VO‑complexes
The stability investigation of Zn- and VO-complexes was performed considering
metal–ligand stoichiometries, as 1:2 molar ratios:
M + 2HL ↔ ML
(1)
[ML]
[M][HL]2
Kf =
(2)
Using Beer Lambert relation to determine the absorbance of the coordinated
ligand (HPS or HPSNa) in absence and after addition of diferent concentrations of
1 3

Catalytic comparison of various polar Zn(II)‑Schif base…
1 3

M. S. S. Adam et al.
1 3

Catalytic comparison of various polar Zn(II)‑Schif base…
metal ions could be derived according to the reported method by Beck et al. [44], to
give Eq. 3.
(
)
(
)
1
1
1
=
+
(3)
(ꢀML − ꢀHL)KfCM[Co
]
(ꢀML − ꢀHL)[Co
]
HL
2
2
A − Ao
HL
Equation 3 gives non-linear least squares ftting [45], as shown by Eq. 4, which
verifes the validity of Eq. 4:
(
)
(
)
KM
r [S]
max
1
1
=
+
(4)
ro
r
max
Catalytic studies
(Ep)oxidation of 1,2-cyclohexene (1.0 mmol) or cyclohexane (1.0 mmol) was com-
pleted using H2O2 (30% in water, 3.0 mmol) as a terminal oxidant, catalyzed by
Zn(PS)2, Zn(PSNa)2, VO(PS)2 or VO(PSNa)2 (0.02 mmol) in 10 mL of acetonitrile
or other given solvents at 85 °C for 8 h. The catalytic processes were carried out in a
100 mL three-necked fask connected with a water circulated condenser. The kinetic
control of the catalytic processes was monitored by withdrawing 2 mL of the reac-
tion mixture by pipet samples at exact time intervals. The withdrawn samples were
added to a solid mixture of 20 mg of MnO2 and 20 mg anhydrous MgSO4 in order
to decompose the rest of H2O2 for each catalytic run at the exact time and to extract
water from the samples. By fltration of the resulting slurry on Celite, the fltrate was
diluted by the given solvent (1:3) in vortex and then 1 µL of the sample fltrate was
subjected to GC–MS for analysis.
The (ep)oxidation product percentages of 1,2-cyclohexene or cyclohexane were
detected and analyzed on a Shimadzu Gas Chromatography mass spectrometer
(GC–MS) model QP2010 SE, equipped with an Rxi-5 Sil MS capillary column
(30 m length × 0.25 mm ID × 025 um flm thickness). Injection at room temperature
to the GC–MS, the oven temperature was initiated to 40 °C and held for 1 min. The
temperature rate was increased with 10 °C min−1 up to 200 °C. Operation of the
inlet was carried out in spitless mode at 200 °C mass spectra transfer line tempera-
ture. The carrier gas was helium with high purity up to 99.999%, applied under a
fow rate of 1 mL/min. Lab solution software was applied to analyze the percentages
yields of the obtained products.
Results and discussion
N‑pyridinyl Schif base ligands, Zn‑complexes and VO‑complexes
The condensation of 2-aminopyridine with salicyaldehyde is a well-known reaction
carried out in methanolic solution to aford the non-polar ligand HPS [46]. Mono-
p-sulfonation of salicylaldehyde is reported previously [42], which underwent a
typical condensation similar for HPS, but in an aqueous media to obtain HPSNa
1 3

M. S. S. Adam et al.
Scheme 1 Synthesis of Schif base ligands (HPS and HPSNa)
1
(Scheme 1). Both ligands were characterized by EA, H, 13C-NMR, IR, UV–Vis.
and mass spectra (Tables 1, 2).
Complexation of HPS with Zn(II) and VO(II) ions (Zn(PS)2 and VO(PS)2, respec-
tively) has been studied and reported elsewhere [47, 48]. The novelty here is the
synthesis of Zn(PSNa)2 and VO(PSNa)2 by the reaction of zinc acetate dihydrate or
vanadyl acetylacetonate, respectively, with HPSNa in an aqueous media (Scheme 2).
All complexes were prepared in molar ratios 2:1 of ligand to the metal ion in their
convenient solvent. The tentative molecular structures of Zn- and VO-chelates were
R
N
O
N
CH
N
Zn
.nH2O
HC
N
O
Zn(OAc)2.2H2O
N
H
C
R
R
N
Solvent
2
OH
R
R = H (HPS)
SO3Na (HPSNa)
N
VO(acac)2
O
N
CH
N
V
.nH2O
HC
N
O
O
R
Complex Abbr.
Zn(PS)2
R
H
H
SO3Na
SO3Na
No. of crystalline H2O (n)
Solvent
0
MeOH/H2O
MeOH/H2O
H2O
VO(PS)2
0
2
0.5
Zn(PSNa)2
VO(PSNa)2
H2O
Scheme 2 Synthetic routes for Zn- and VO-Schif base complexes
1 3

Catalytic comparison of various polar Zn(II)‑Schif base…
1
assigned by EA, H-, 13C-NMR, infrared, electronic and mass spectra, TGA, and
conductivity measurements. The stability pH ranges of Zn- and VO-complexes were
found at 1.7–8.6 in the standard universal bufer solutions [42]. HPS is fully solu-
ble in methanol, acetone and ethanol, whereas, its complexes Zn(PS)2 and VO(PS)2
are slightly soluble in those solvents. HPSNa as a novel ligand, and its M-chelates,
Zn(PSNa)2 and VO(PSNa)2, were fairly soluble in an aqueous media and also in
strong coordination behavior solvents like DMF and DMSO, while their solubility in
polar organic solvents like MeOH and CH3CN was sparingly.
Coordination chemical behavior of HPS or HPSNa towards Zn2+ and VO2+ ions
display that both act as a chelating monobasic bidentate ligand within deprotona-
tion of hydroxyl phenolic group. There is no observed role in the complexation
of the N-pyridine ring as detected before [47, 48] and shown in Scheme 2. CHN
analyses of the two ligands and all metal complexes were measured and recorded in
Table 1, which agree with their tentative molecular structure with a small diference
of about 0.5%.
1H and 13C NMR spectra of the synthesized ligands and their corresponding com-
plexes were measured in DMSO-d6 at 25 °C and tabulated in Table 2 and supported
by Figs. S1-S8 in the supplementary materials. The resulting data could help to
estimate the coordination mode of HPS and HPSNa towards Zn2+ ion. The disap-
pearance of the characteristic singlet signal of phenolic-OH in the ligands, 9.48 and
10.24 ppm after complexation with Zn2+ ion Zn(PS)2 and Zn(PSNa)2, respectively,
could prove that HPS and HPSNa coordinate to the central metal ion via deprotona-
tion [35, 47, 48]. There is a strong observed up-feld shift of the singlet broad signal
of azo-methine (–CH=N–) group in HPS and HPSNa from 13.06 and 13.22 ppm,
respectively, to 9.28 and 9.35 ppm after formation of complex with Zn2+ ion. This
could assign the coordination of the nitrogen atom within its lone pair of azo-
methine group to the Zn2+ ion, as shown in Scheme 2. In 13C NMR, the resonating
signals of the attached carbon to the –OH ion is also infuenced by the complexation
to the Zn2+ ion, which shifted from 160.43 and 175.21 ppm in HPS and HPSNa to
162.18 and 173.00 ppm, respectively. Moreover, the carbon signal of the azomethine
group is shifted from 164.92 and 179.95 ppm in HPS and HPSNa to 169.40 and
177.21 ppm in Zn(PS)2 and Zn(PSNa)2, respectively [48].
The most signifcant IR spectral values of HPS, HPSNa and their correspond-
ing Zn- and VO-complexes are listed in Table 3 and given in Fig. S9-S14, in the
supplementary materials. The phenolic –OH infrared spectral band for HPS and
HPSNa could be observed at 3122 and 3166 cm−1, respectively, and disappeared by
complex formation of Zn(PS)2, VO(PS)2, Zn(PSNa)2 and VO(PSNa)2. This disap-
pearance could be related to the deprotonated bonding of the –OH group via oxy-
gen to the central metal ion [47, 48]. The number of crystalline water molecules
could be determined by the characteristic stretching vibrational band, as broad band,
̄ꢀ (O–H) at 3334 and 3281 cm−1 in Zn(PSNa)2 and VO(PSNa)2 [21] (Scheme 2). High
remarkable shifts of the characteristic ̄ꢀ
for HPS and HPSNa from 1699
and 1665 cm−1 to 1618 and 1614 cm−1(afzoo-rmeZthnin(eP)S)2 and Zn(PSNa)2, respectively,
and to 1661 and 1653 cm−1 for VO(PS)2 and VO(PSNa)2, respectively, are resulted
from the coordination of the lone pair of the nitrogen atom of the azo-methine group
to Zn2+ and VO2+ ions. Furthermore, there was no observable shift of the assigned
1 3

M. S. S. Adam et al.
Table 3 Assigned infrared spectral data (̄ꢀ , cm−1) for HPS, HPSNa, and their corresponding Zn- and
VO-complexes. Stability constants of formed Zn and VO-complexes
Compound
HPS
Zn(PS)2
VO(PS)2
HPSNa
Zn(PSNa)2
3334
VO(PSNa)2
3281
Group
O–H(water)
O–H(phenolic)
C–Har
C–O(phenolic)
C=N(azomethine)
C–N(azomethine)
C=N(py)
C–N(py)
S–O−
3122
3050
1424
1699
1142
1581
1040
3166
3095
1480
1665
1101
1588
1026
1429
1165
3058
1395
1618
1152
1616
1037
3061
1373
1661
1148
1599
1041
3093
1499
1614
1106
1566
1027
1386
1165
768
3062
1473
1653
1163
1615
1378
1243
755
S=O
M–O
757
751
V=O
938
934
M–N
550
548
596
509
Kf (L mol−1
Log K
)
–
–
1.1×1010
9.99
3.1×107
8.49
–
–
4.6×109
9.04
8.7×106
7.94
absorption band of the N-pyridine ring ̄ꢀ (C=N py) at 1581 and 1588 cm−1 for HPS and
HPSNa, respectively, to be 1616 and 1566 cm−1 for Zn(PS)2 and Zn(PSNa)2, respec-
tively, and also at 1599 and 1615 cm−1 for VO(PS)2 and VO(PSNa)2, respectively,
is assigned that there is no role of the N-pyridine in the coordination behavior of
HPS and HPSNa for the formation of the M-complexes. However, there are little
shifts of N-pyridine band after complex formation with either Zn2+ or VO2+ ion,
which can be attributed to the infuence of the complexation of M2+ ion to HPS
and HPSNa [35, 49]. The stretching vibrational spectral bands of the S–O− and
S=O bonds of the sodium sulfonate group in HPSNa could be recorded at 1429 and
1165 cm−1, which infuenced by the complexation to Zn2+ and VO2+ ions by unob-
served shift at 1386 and 1165 cm−1 for Zn(PSNa)2 and for VO(PSNa)2 at 1378 and
1243 cm−1, respectively. Additionally, new weal-medium spectral bands appeared at
757, 751, 768 and 755 cm−1 for the new bonding M–O and also new bands at 550,
548, 596 and 509 cm−1 for the M–N bonding in Zn(PS)2, VO(PS)2, Zn(PSNa)2 and
VO(PSNa)2, respectively. Furthermore, a new characteristic band of V=O bonding
has been detected at 938 and 934 cm−1 in VO(PS)2 and VO(PSNa)2 [35].
The UV–visible spectral scans of HPS, HPSNa and their corresponding metal
complexes used to deduce the maximum absorption wavelength, λmax (nm) and the
corresponding molar absorpativity, ε (mol−1 cm−1) in the suitable solvent (metha-
nol and water) (Table 1). In the UV region, both ligands HPS and HPSNa assigned
bands at 240 and 257 nm (for HPS) and at 223 and 259 nm (for and HPS) are attrib-
uted to the intra ligand spin-allowed transition π3 →ꢀ∗ and π3 →ꢀ∗ [43]. The given
bands at 302 and 300 nm in both HPS and HPSNa, 4respectively,5are assigned for
the n→π* transitions. An additional absorption band was observed in the visible
1 3

Catalytic comparison of various polar Zn(II)‑Schif base…
region for HPSNa at 378 nm is attributed to the ligand charge transfer. Zn- and VO-
complexes present diversity of spectral shapes; however, most bands appeared in the
UV region. Zn(PS)2 and Zn(PSNa)2 show a broad band at 388 and 375 nm, respec-
tively, referring to M-LCT (metal to ligand charge transfer). The electronic spectra
of the vanadyl(IV) Schif base complexes (Fig. 1a,b) refers to geometric structure of
VO-chelates. Both VO(PS)2 and VO(PSNa)2 gave three absorption bands without
observable bands in the region from 400 to 600 nm specifcally for the d→d tran-
sition [3]. The fve-coordinate has stereochemical geometry C2ν (2T2g→4T1g for
L-MCT) [47, 48].
To confrm the presence of the crystalline water molecules in Zn- and VO-com-
plexes, theromgravimertic analysis was performed. The number of diagnostic water
molecules in the lattice in Zn(PSNa)2 and VO(PSNa)2 was determined by TGA
(Fig. S9, supplementary materials). Both complexes have an observable decompos-
ing successive step in the range from 100 to 130 °C. The endothermic loss of mass
was detected with Δmrel =5.2 and 1.1% for Zn(PSNa)2 and VO(PSNa)2, respectively,
which is acceptable with the calculated mass loss (Δmrel =5.0 and 1.3%, respec-
tively). Those mass losses refer to the number of water molecule in the crystal lattice
[50], in which Zn(PSNa)2 has two crystalline water for each complex molecule and
VO(PSNa)2 has only half water molecule. In Zn(PS)2 and VO(PS)2, there was no
mass loss in the TGA diagram was remarked in that rang of temperature, further-
more all current complexes have no more decomposition steps at higher tempera-
ture range (at range 180–230 °C for the suggestion of coordinated water molecules)
(Scheme 2). Mass spectra of Zn(PS)2, VO(PS)2, Zn(PSNa)2 and VO(PSNa)2 are
measured and supported by Figs. S10-S13 in the supplementary materials, which
have appropriate base peaks with the tentative complex anion (in the absence of
Na+ ion). It exhibits also an observable peak in the negative mode of the complexes
[M–Na+ +1]. In the positive mode, another peak with the loss of sulfonate group
[M–SO3− +1].
Molar conductivity measurements (Λm) of HPS, Zn(PS)2 and VO(PS)2 were too
low confrming their non-polar nature. The high conductivity values of HPSNa,
4.0
2.0
3.5
2.0
1.8
1.8
(a)
(b)
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
3.0
2.5
2.0
1.5
1.0
340
360
380
400
420
440
330
360
390
420
450
480
, nm
, nm
HPSNa
Zn(PSNa)2
VO(PSNa)2
HPS
Zn(PS)2
VO(PS)2
0.5
0.0
200 250 300 350 400 450 500 550 600
, nm
200
300
400
500
600
700
, nm
Fig. 1 The electronic spectral scans of a HPS, Zn(PS)2 and VO(PS)2 in methanol at concentra-
tion1×10−5 mol dm−3 and room temperature; b HPSNa, Zn(PSNa)2 and VO(PSNa)2 in water at concen-
tration1×10−5 mol dm−3 and room temperature
1 3

M. S. S. Adam et al.
Zn(PSNa)2 and VO(PSNa)2 in DMSO and DMF as recorded in Table 1, interpreted
that HPSNa and its studied complexes are polar compounds because of the existence
of p-sodium sulfonate group which give, particularly, two counter ions in the solu-
−
tion, of –SO3 and Na+ ions in HPSNa and three ions of one disulfonate anion and
two sodium cations in Zn(PSNa)2 and VO(PSNa)2 [50].
Stability constants of M‑complexes
The stability constants of the Zn- and VO-complexes could be determined form the
spectral curves of absorbance versus wavelength at diferent concentration of Zn2+
or VO2+ ions and determined concentration of HPS or HPSNa (Fig. 2a).
Values of stability constants (Kf and log K) are derived from Fig. 2b [45] and
listed in Table 2, in which the absorbance substantially increases with various metal
ion concentrations, referring to the bonding of the central metal ion to the ligand.
At λmax, the repeated spectral scans analysis shows that the complexes of Zn2+ and
VO2+ were formed in molar ratios of 1:2 metal to ligand (HPS or HPSNa) [44].
From the stability constants values, Zn-complexes, Zn(PS)2 and Zn(PSNa)2, are
more stable than VO-species, VO(PS)2 and VO(PSNa)2. Moreover, the presence of
a p-sodium sulfonate group causes reducing in the stability of the formed complexes
as observed for log Kf values. log Kf is reduced from 8.49 or 9.99 for VO(PS)2 or
Zn(PS)2, respectively, to 7.94 or 9.04 for VO(PSNa)2 or Zn(PSNa)2, respectively
(Table 2).
Catalytic processes
To the best of our knowledge, the catalytic (ep)oxidation reactivity of Zn(II)-com-
plexes is not fully investigated [8, 29, 30]. Moreover, due to the high applicability
of mononuclear vanadyl complexes as benefcial catalysts for redox processes under
various reaction conditions [11, 15, 51], were selected for comparative study with
8000
HPSNa in absence of Zn2+ ions
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
(a)
(b)
with 0.2 mL of Zn2+ ions
with 0.4 mL
7000
6000
5000
4000
3000
2000
1000
with 0.6 mL
with 0.8 mL
with 1.0 mL
with 1.2 mL
with 1.6 mL
with 1.8 mL
0
2
4
6
8
10 12 14 16 18
300
350
400
450
, nm
500
550
1/CM
Fig. 2 a Repeated spectral scans of HPSNa (1×10−5 mol dm−3) in the absence and presence of various
volumes addition (diferent concentrations) of Zn acetate dihydreate (1×10−5 mol dm−3) in an aqueous
media. b linear plot of 1/(A–Ao) versus 1/CM (diferent concentrations) of Zn2+ ions
1 3

Catalytic comparison of various polar Zn(II)‑Schif base…
Zn-complexes. The catalytic efciency of the current Zn- and VO-complexes were
scrutinized in the (ep)oxidation of 1,2-cyclohexene and cyclohexane using the envi-
ronmentally favored oxidant, i.e. an aqueous H2O2. Additionally, the polarity of the
M-catalyst complexes, as an interesting search point here, would the main induce-
ment to study their catalytic reactivity in the 1,2-cyclohexene and cyclohexane (ep)
oxidation. Particularly, the chemoselective and oxidation product of 1,2-cyclohexene
is epoxy-1,2-cyclohexane in large amount contaminated with other well documented
side products [49, 52]. On the other hand, oxidation of cyclohexane usually afords
two main products, cyclohexanol and cyclohexanone with other side products [38,
39].
1,2‑Cyclohexene (ep)oxidation
In an essential step to determine the optimized reaction conditions with the highest
amount of the chemoselective product, each catalytic process was tested as a func-
tion of time at the boiling point of the solvent [49].
Acetonitrile (AN) is commonly applied as the standard solvent for such redox
processes due to its high stability against oxidation at high temperatures [53]. For
that, the (ep)oxidation of 1,2-cyclohexene initiated by using an aqueous H2O2 in
acetonitrile catalyzed by either Zn(PS)2, Zn(PSNa)2, VO(PS)2 or VO(PSNa)2. The
conversion and chemoselectivity of the subtracts oxidation are summarized in
Table 4 and plotted in Fig. 3a–d. In absence of the current M-complex catalysts,
there was no remarkable conversion of 1,2-cyclohexene, which refers to that the (ep)
oxidation of 1,2-cyclohexene is considerable sensitive and substantial to the nature
of the catalyst [8].
From Fig. 3a–d and Table 4, the highest amount of the chemoselective product
was obtained in the presence of VO(PS)2 and VO(PSNa)2 (Fig. 3b, d), with short
reaction time, 5 h with 92% of epoxy-1,2-cyclohexane (conversion 99% and selec-
tivity 93%) and 4 h with 94% of epoxy-1,2-cyclohexane (conversion 98% and selec-
tivity 96%), respectively. With Zn(PS)2 or Zn(PSNa)2, the optimized reaction time
was 7 h with 82% of the target product (conversion 98% and selectivity 84%) and
6 h with 85% of epoxy-1,2-cyclohexane (conversion 100% and selectivity 85%),
Table 4 The optimized (ep)oxidation conditions of 1,2-cyclohexene catalyzed by Zn- and VO-complexes
using an aqueous H2O2 in acetonitrile at 85 °C
Complexa
Time (h)
Yield (%)b
Conversion (%)
Selectivity (%)
R
P
Side products
Zn(PS)2
VO(PS)2
Zn(PSNa)2
VO(PSNa)2
7
5
6
4
2
1
0
2
82
92
85
94
16
7
15
98
99
100
84
93
85
96
4
98
aR is 1,2-cyclohexene (1.0 mmol), which oxidized by an aqueous H2O2 (3.00 mmol) catalyzed by current
complexes (0.02 mmol) in 10 mL acetonitrile at 85 °C and at the optimized reaction time
bP is 1,2-epoxy cyclohexane and its percentages determined by GC-Mass
1 3

M. S. S. Adam et al.
Reactant,
Selected product,
Side products
Reactant,
Selected product,
Side products
100
80
60
40
20
0
100
80
60
40
20
0
(a)
(b)
0
1
2
3
4
5
6
7
8
0
1
2
3
4
5
6
7
8
Time, h
Time, h
Reactant,
Selected product,
Side products
100
80
60
40
20
0
Reactant,
Selected product,
Side products
100
80
60
40
20
0
(d)
(c)
0
1
2
3
4
5
6
7
8
0
1
2
3
4
5
6
7
Time, h
Time, h
Fig. 3 Time dependence for the catalytic (ep)oxidation of 1,2-cyclohexene (1.0 mmol) by using H2O2
(3.0 mmol) in 10 mL AN at 85 °C for 8 h catalyzed by a Zn(PS)2; b VO(PS)2; c Zn(PSNa)2 and d
VO(PSNa)2 (0.02 mmol) (Selected product is epoxy cylcohexane)
respectively (Fig. 3a, c, more details in Tables S1-S4, supplementary materials and
summarized in Table 4).
Comparison between Zn-complexes and VO-complexes as efective catalysts
for the 1,2-cyclohexene (ep)oxidation, VO-complexes in the absence or presence
of p-sodium sulfonate group are more efective catalysts than those Zn-catalysts.
Because of the high Lewis acid feature and the high valent of V4+ ion in VO(PS)2
or VO(PSNa)2 are strong factors for their higher catalytic sufciency compared to
Zn(PS)2 and Zn(PSNa)2 of the low oxidation states [28]. Furthermore, the inter-
changeable [11] and the high electrochemical reversible abilities between V4+ and
V5+ ion [8] of the high positive charge in VO(PS)2 and VO(PSNa)2 in the electron
transfer mechanism could be the reason for improving their catalytic potential con-
trasted to that of low and high stable Zn-complexes, i.e. Zn2+ [21, 54].
In other words, the electron transfer process (in the mechanistic pathway) from
the catalyst by the central metal ion in the complex to the oxidant (H2O2) could be
easier for V4+ to V5+ ion and difcult for Zn2+ to give Zn3+ ion [15, 22], which
supports the experimental results of the catalytic potentials of the current Zn- and
1 3

Catalytic comparison of various polar Zn(II)‑Schif base…
VO-complexes. Moreover, the oxygenation reactions or oxygen transfer from the
oxidant to the complex catalyst could be more favored for VO-complexes than that
for Zn-complexes liberating dioxo- and/or peroxo-active intermediate in the cata-
lytic mechanism [23, 26, 27]. The dioxo- and/or peroxo-active intermediate complex
could be responsible for the oxygen transferring to the substrate (e.g. 1,2-cyclohex-
ene) to award the chemoselective product (epoxy cyclohexane), as shown in
Scheme 3 [49].
Polarity and ligand substituents of M‑complex catalysts efects
Polarity of the reaction media depends upon the type of solvent [34, 53] and the
organic nature of the complex catalysts [17, 18, 55], which can be investigated in
various organic solvents of diferent dielectric constants in the catalytic processes.
Scheme 3 Catalytic cycle of the (ep)oxidation of 1,2-cyclohexene catalyzed by Zn (PS)2 and Zn(PSNa)2
complexes
1 3

M. S. S. Adam et al.
From the tentative molecular structure of the M-complex catalysts (Scheme 2),
there are no observable steric and electronic efects of the substituents of the coordi-
nated Schif base ligands, i.e. p-sodium sulfonate group, on their catalytic potential
(Tables 4, 5) [19, 31, 32, 56], due to their position is far away from the central metal
ion. However, this group has an appreciable infuence by its polarity and miscibility
of its corresponding catalysts with the oxidant in the reaction media [55, 57].
The probed solvents in the redox reaction of 1,2-cyclohexene by an aqueous
H2O2, advantage the infuence of the polar group of p-sodium sulfonate group in
Zn(PSNa)2 and VO(PSNa)2 compared to those Zn(PS)2 and VO(PS)2, as the internal
catalysts. In the standard solvent, i.e. acetonitrile AN, the optimized reaction time
for the highest amount of epoxy-1,2-cyclohexane in presence of the polar catalysts,
Zn(PSNa)2 and VO(PSNa)2 is less than that in the presence of the less polar cata-
lysts Zn(PS)2 and VO(PS)2. This phenomenon could be caused by the high dielectric
constant of AN in the catalytic processes [53, 55]. The best solvent for that reac-
tion in presence of non-polar catalysts, Zn(PS)2 and VO(PS)2, is chloroform giving
yield percentages of the target product 87% and 92%, respectively. Similar results
were reported by Grivani et al. [34] in the oxidation of 1, 2-cyclooctene catalyzed
by vanadium (IV) Schif base complexes. The more organic nature of Zn(PS)2 and
Table 5 Various reaction media of 1,2-cyclohexene (ep)oxidation catalyzed by Zn- and VO-complexes
using an aqueous H2O2 at 85 °C
Complexa
Solvent
Yield (%)b
Conversion (%)
Selectivity (%)
R
P
Side products
Zn(PS)2
Acetonitrile
CHCl3
Water
Solvent free
Acetonitrile
CHCl3
2
82
87
62
68
92
92
65
77
85
75
66
76
94
80
59
83
16
5
16
19
7
98
92
78
87
99
95
83
91
100
80
100
82
98
83
100
84
95
79
78
93
97
78
85
85
94
66
93
96
96
59
92
8
22
13
1
5
17
9
0
20
0
18
2
17
0
VO(PS)2
3
Water
18
14
15
5
34
6
4
3
41
7
Solvent free
Acetonitrile
CHCl3
Zn(PSNa)2
VO(PSNa)2
Water
Solvent free
Acetonitrile
CHCl3
Water
Solvent free
10
90
aR is 1,2-cyclohexene (1.0 mmol), which oxidized by an aqueous H2O2 (3.00 mmol) catalyzed by current
complexes (0.02 mmol) in 10 mL solvent (acetonitrile, water or chloroform) at 85 °C and at the opti-
mized reaction time
bP is 1,2-epoxy cyclohexane and its percentages determined by GC-Mass
1 3

Catalytic comparison of various polar Zn(II)‑Schif base…
VO(PS)2 could be the reason for that, as homogeneous catalysts (Scheme 2). Cata-
lytic potential of Zn(PSNa)2 and VO(PSNa)2 was high conversionaly in water but
with less chemoselectivity (Table 5) for the (ep)oxidation of 1,2-cyclohexen prob-
ably due to the aqueous hydrolysis of epoxy-1,2-cyclohexane to other unwanted side
products [41, 49]. However, the catalytic potential of Zn(PSNa)2 and VO(PSNa)2 is
higher than that of Zn(PS)2 and VO(PS)2 in water, due to the string infuence of the
polar p-sodium sulfonate group. The high conversion of catalytic processes in water
(100%) could be related to the strong miscibility between the catalysts, Zn(PSNa)2
and VO(PSNa)2, and oxidant (an aqueous H2O2) in the reaction media [9, 35], which
caused improvement of their catalytic sufciency.
Under solvent-free conditions, and in seeking to be environmentally friendly,
recently, in order to diminish the toxic efects of using organic solvents in homog-
enous processes [54], signifcantly the catalytic reactions were carried out hetero-
geneously, especially for the high polar complexes (Zn(PSNa)2 and VO(PSNa)2),
and were not mixed with the reactant 1,2-cyclohexene. However, the polar nature
of the oxidant, i.e. an aqueous H2O2, may aid to enhance the catalytic sufciency
of Zn(PSNa)2 and VO(PSNa)2 more than the non-polar catalysts, Zn(PS)2 and
VO(PS)2, as seen in Table 5 and as reported previously [21]. Generally, the misci-
bility of the three catalytic oxidation reagents, oxidant, reactant and catalyst, is the
most important parameter for performance of the catalytic processes.
HPS and HPSNa both are typical ligands with only one diference, the presence
of p-sodium sulfonate group in HPSNa, which furnishes additional interesting fea-
tures to the Zn- and VO-chelates, electronic and polarizability [18, 40]. The elec-
tronic efect can be discussed by highlighting the electron-withdrawing efect of
the p-sodium sulfonate group. As reported elsewhere, more electron-withdrawing
substituents attached to the coordinated ligands can diminish the stability of the
M-complex and enhance its catalytic potential [53, 57]. Bertini et al. [31] studied the
infuence of the ligand substituents on the metal electronic character of VO-Schif
base complexes and on their catalytic application. Also, as demonstrated before by
Coletti et al. and by Galloni et al. [58, 59], the oxidation of central metal ions, V4+
to V5+ , is infuenced by the nature of the ligand and the solvent. Signifcantly, the
oxidation potential decreases proportionally with the electron donating character of
the VO-complex catalysts and with the number of substituents on the ligand in the
complex molecules [18, 31]. Consequently, the high electron-withdrawing group,
p-sodium sulfonate, in Zn(PSNa)2 and VO(PSNa)2 progresses their catalytic activity
for 1,2-cyclohexene (ep)oxidation compared to Zn(PS)2 and VO(PS)2.
Cyclohexane oxidation
It has been reported that the catalytic redox processes of alkanes need harsh con-
ditions compared to alkenes or olefns [11, 13, 38, 39]. Moreover, the catalytic
oxidation aforded chemoselective products as observed for that of the unsatu-
rated hydrocarbon (alkenes) [13, 32]. The main target products of the alkane
oxidation are two cycloalkanones and cycloalkanols [60]. Vanadium complexes
1 3

M. S. S. Adam et al.
mediated many oxidation processes as efective catalysts for the selective oxida-
tion of cycloalkanes [11, 15].
Promising the catalytic activity of M-catalyst complexes in the oxidation of
high inert organic reagents, e.g. cyclohexane, was studied by involving VO(PS)2,
Zn(PS)2, VO(PSNa)2 and Zn(PSNa)2 in the oxidation of cyclohexane, as catalysts
[37]. VO(PS)2, Zn(PS)2, VO(PSNa)2 and Zn(PSNa)2 were probed as homogene-
ous catalysts assisted peroxidative (with an aqueous H2O2) in the oxidation of
cyclohexane to cyclohexanol and cyclohexanone. The primary oxidation product
explored was cyclohexyl hydroperoxide, which underwent further decomposition
to the fnal products, cyclohexanol and cyclohexanone (Scheme 4) [32, 61]. The
cyclohexyl hydroperoxide formation of was detected elsewhere using the reported
proposed method by Shul’pin [62].
All current catalysts exhibited good to moderate catalytic potential towards
the oxidation of cyclohexane (Table 6). The optimized reaction conditions were
detected after long time (12 h) to give conversion percentages of the two sig-
nifcant oxidation products (RO and ROH) was 53, 71, 66 and 75% with Zn(PS)2,
VO(PS)2, Zn(PSNa)2 and VO(PSNa)2, respectively. The reaction time was very
long at high temperature (100 °C) due to the need for sufciently high reaction
conditions, as observed elsewhere for the same catalytic process [32]. Saha et al.
[61] reported the suitable time for cyclohexane oxidation was 12 h at 120 °C
under microwave irradiations. The diference in the catalytic potential between
the current catalyst complexes could be assumed to the efect of the ligand sub-
stituents (p-sodium sulfonate group) for Zn(PS)2, VO(PS)2, Zn(PSNa)2 and
VO(PSNa)2, the more electron withdrawing group enhances the catalytic activity
more than that of the electron donating substituents, as observed previously [60,
63]. Połtowicz et al. [60] reported the cycloalkanes oxidation using molecular O2
gas catalyzed by salen metallocomplexes under thermomorphic reaction condi-
tions. The presence of tert-butyl group as electron-donating substituents in the
ligand (Schif base) of the M-salen complex improves the catalytic potential and
solubility of the applied catalysts in the catalytic processes. On the other hand,
the presence of perfuoroalkyl substituents as electron withdrawing substituents
enhances the catalytic activity and reduces the catalysts solubility. So that, the
catalyst solubility depends upon the applied temperature (thermomorphic behav-
ior). Such catalytic results supported our experimental work as listed in Table 6.
OH
O
H2O2
M-catalyst
+
AN
Scheme 4 Cyclohexane oxidation catalyzed by Zn- and VO-complexes using an aqueous H2O2 in ace-
tonitrile
1 3

Catalytic comparison of various polar Zn(II)‑Schif base…
Table 6 Cyclohexane oxidation catalyzed by Zn- and VO-complexes using an aqueous H2O2 in acetoni-
trile at 100 °C
Entrya
Complex
Time (h)
Yield (%)b
Conversion (%)
R
RO
ROH
Side products
1
2
3
4
5
6
7
8
Zn(PS)2
2
4
6
8
12
2
4
6
8
12
2
4
6
8
12
2
4
6
8
12
96
85
80
69
47
93
80
72
58
29
95
86
78
65
34
91
79
70
53
25
3
7
10
15
25
5
10
16
23
42
3
1
5
6
10
18
2
0
3
4
6
10
0
2
2
4
6
0
0
3
5
7
0
3
3
5
8
4
15
20
31
53
7
20
28
42
71
5
14
22
35
66
9
21
30
47
75
VO(PS)2
8
10
15
23
2
5
7
11
22
3
9
10
11
12
13
14
15
16`
17
18
19
Zn(PSNa)2
VO(PSNa)2
9
12
19
37
6
10
17
23
42
8
10
19
25
aThe oxidation of cyclohexane (R) (1.0 mmol) by an aqueous H2O2 (3.00 mmol) catalyzed by Zn- or VO-
complex (0.02 mmol) in 10 mL acetonitrile for 12 h, at 100 °C
bThe percentages derived from GC results, selectivity control of cyclohexanone (RO) and cyclohexanol
(ROH) with presence of other side unknown products
Mechanistic pathway of the (ep)oxidation of 1,2‑cylcohexene
The reactivity of the V4+ central ion towards oxidation in its VO-complexes to V5+
ions is of interest as homogeneous catalysts, but such oxidation of Zn2+ ion is a mys-
terious point [6, 59]. Most of the alkene redox processes were followed by detec-
tion of the chemical changes of the catalyst complexes spectroscopically [48, 58], by
cyclic voltammetry [13, 31] or the electrochemical features of VO-catalysts [19, 24].
Mass spectral [25, 43] and 51V NMR investigations [26] are two of the most efec-
tive tools to study the mechanism of alkenes oxidation processes [64].
Here, the mechanistic pathway of the catalytic redox reactions was initiated by
reaction of H2O2, with the catalyst complex via electron transfer mechanism by oxi-
dation of the central vanadium ion form V4+ to V5+ ion [12, 19]. With another oxi-
dant molecule, the oxidized catalyst VO-catalyst complex could undergo coordina-
tion with a molecule of H2O2 through an oxygen donor atom to the more electron
1 3

M. S. S. Adam et al.
defcient V5+ ion in the oxidized catalyst [35, 40, 50]. This is followed by oxygen
transfer [26, 51] from the coordinated oxidant molecule to the central metal ion
afording an oxide or peroxide intermediate complex [27, 40]. The oxidant behavior
towards VO(PS)2 and VO(PSNa)2 VO-complexes has been monitored previously by
the UV–Vis spectral shifts [47, 58]. Figure 4a assigned observable disappearance
of the π→π* and n→π* bands (226 and 263 nm, respectively) for VO(PSNa)2 in
the reaction media with high energy shift of the characteristic L-MCT band from
303 to 293 nm [27]. Those spectroscopic changes for VO(PSNa)2 just after mixing
with H2O2 can be attributed to the oxidation of the metal ion from V4+ to V5+ with
d0 electronic confguration as strong electrophile [57] and then followed by oxygen
transfer [26] from another oxidant molecule to V5+ ion liberating an active oxide-
peroxide intermediate complex [18, 25–27]. The active intermediate complex inter-
acts with a molecule of the substrate, i.e. 1,2-cyclohexene or cyclohexane, in order
to oxidize the substrate to the chemoselective products [49].
The mechanistic pathway of the catalytic oxidation of 1,2-cyclohexene by VO-
complexes is not convenient for that process catalyzed by Zn-complexes. First, there
were subtle changes in the characteristic electronic spectra of Zn(PSNa)2 after mix-
ing with H2O2 in the catalytic process (Fig. 4b) in the visible region for the L-MCT
band. Both VO(PSNa)2 and Zn(PSNa)2 show similar features in the UV-region with
disappearance of the π→π* and n→π* bands (260 and 301 nm, respectively, for
Zn(PSNa)2). This may be resulted from the coordination of the oxidant molecule
to the metal ion, i.e. Zn2+ ion, without change of its oxidation state, as suggested
elsewhere [40]. The kinetic decay of the characteristic L-MCT band (375 nm)
could improve the probability of coordination of the oxidant molecule to Zn2+ ion.
Scheme 3 exhibits the proposed mechanism for the (ep)oxidation of 1,2-cyclohex-
ene. The mechanistic path in Scheme 3 can be estimated spectroscopically, as shown
in Fig. 4b. The proposed mechanism describes the attack of the oxidant molecule to
the central metal ion Zn2+ and bonds with formation of intermediate I. The second
step demonstrates the approach of the substrate molecule (R=1,2-cyclohexene or
cyclohexane) to the intermediate I to accord the formation of intermediate II, in
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Zn(PSNa) before addition of H O
Zn(PSNa)22 just after addition of2H22O2
VO(PSNa) before addition of H O
VO(PSNa)22 just after addition of2H22O2
(b)
(a)
2.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
2.5
2.0
1.5
1.0
0.5
320
340
360
380
400
420
440
240
260
280
300
320
340
, nm
, nm
250 300 350 400 450 500 550 600 650 700
200
300
400
500
600
700
, nm
, nm
Fig. 4 The electronic spectral changes of a VO(PSNa)2 and b VO(PSNa)2 in the presence of
1,2-cyclohexene in absence and presence of H2O2 to the reaction media in AN at 85 °C with interval
time 15 min for 2.5 h
1 3

Catalytic comparison of various polar Zn(II)‑Schif base…
which the oxidant and substrate molecules are close together by coordination to the
Zn2+ ion. For the second intermediate (II), there is oxygen transfer from the oxidant
to the substrate to give the epoxy product liberating a water molecule (Scheme 3).
The suggested mechanism is compatible with that of the catalytic (ep)oxidation of
olefns by MoO2-complex catalysts [50, 64, 65].
Conclusions
Two pyridinyl Schif base ligands with and without p-sodium sulfonate substitu-
ent (HPSNa and HPS) were synthesized and their complexation towards Zn2+ and
VO2+ ions were formed with molar ratios 1:2 (M:L), respectively. In these com-
plexes deprotonated phenolic –OH group via oxygen and the azomethine within the
nitrogen atom coordinated to the central metal ion. Stability constants of Zn- and
VO-complexes, Zn(PS)2, VO(PS)2 and Zn(PSNa)2, VO(PSNa)2 were determined
spectrophotometrically. Zn-chelates were found to be more stable than those VO-
ones and there was an observable efect of the p-sodium sulfonate substituent on
the stability of the formed Zn(PSNa)2 and VO(PSNa)2 compared to Zn(PS)2 and
VO(PS)2. The catalytic efectivity of the developed Zn- and VO-chelates were stud-
ied in the (ep)oxidation processes of 1,2-cyclohexene or cyclohexane using 30%
aqueous H2O2. VO(PS)2 and VO(PSNa)2 showed higher catalytic potential compare
to Zn(PS)2 and Zn(PSNa)2, which may have attributed to the easily oxidation states
interchange, the high Lewis acid behavior and the strong bonding of vanadium ion
to oxygen. The polarity of Zn- and VO-complexes, i.e. the presence of p-SO3Na, dis-
plays an observable infuence on their catalytic potential chemoselectively. In highly
polar solvents such as water, methanol and acetone, VO(PSNa)2 exhibited higher
catalytic performance than the non-polar catalyst, VO(PS)2. However, in less polar
solvents (chloroform) both polar catalysts showed good potential conversionally, but
low chemoselectively, whereas non-polar catalysts displayed better conversion and
chemoselectivity. A mechanistic pathway was proposed to establish the diference
catalytic behavior between VO-catalysts and Zn-catalysts.
Acknowledgements The authors greatly thank the Vice-President of Graduate Studies and Academic
Research in King Faisal University for their fnancial support and encouragement to produce this work as
a scientifc project (Project No. 17122005).
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