Mononuclear oxovanadium(IV) Schiff base complex: Synthesis, spectroscopy, electrochemistry, DFT calculation and catalytic activity

Article abstract of DOI:10.1016/j.jorganchem.2018.04.015

A new complex of oxovanadium, VOL has been synthesized from Schiff base ligand obtained by the condensation of o-phenylenediamine and 5-bromo-salicylaldehyde. All new compounds have been characterized by ¹H NMR, IR spectra, UV–Vis spectroscopy,

Full text of DOI:10.1016/j.jorganchem.2018.04.015

Accepted Manuscript
Mononuclear oxovanadium(IV) Schiff base complex: Synthesis, spectroscopy,
electrochemistry, DFT calculation and catalytic activity
Souad Dekar, Kamel Ouari, Sabrina Bendia, Douniazed Hannachi, Jean Weiss
PII:
S0022-328X(18)30251-1
10.1016/j.jorganchem.2018.04.015
JOM 20407
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 6 November 2017
Revised Date: 10 February 2018
Accepted Date: 11 April 2018
Please cite this article as: S. Dekar, K. Ouari, S. Bendia, D. Hannachi, J. Weiss, Mononuclear
oxovanadium(IV) Schiff base complex: Synthesis, spectroscopy, electrochemistry, DFT calculation and
catalytic activity, Journal of Organometallic Chemistry (2018), doi: 10.1016/j.jorganchem.2018.04.015.
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ACCEPTED MANUSCRIPT
Mononuclear oxovanadium(IV) Schiff base complex: Synthesis, spectroscopy,
electrochemistry, DFT calculation and catalytic activity.
Souad DEKARa, Kamel OUARI a, Sabrina BENDIAa , Douniazed HANNACHIb and Jean WEISSc
*
aLaboratoire d’Electrochimie, d’Ingénierie Moléculaire et de Catalyse Redox (LEIMCR), Faculté de Technologie,
Université Sétif-1, 19000, Sétif, Algeria.
bDépartement de Chimie, Faculté des Sciences, Université Sétif-1, 19000, Sétif, Algeria.
cLaboratoire de Chimie des Ligands à Architecture Contrôlée (CLAC), UMR 7177 CNRS-Université de Strasbourg,
67000 France.
* kamel_ouari@univ-setif.dz
Abstract
A new complex of oxovanadium, VOL has been synthesized from Schiff base ligand
obtained by the condensation of o-phenylenediamine and 5-bromo-salicylaldehyde. All new
1
compounds have been characterized by H NMR, IR spectra, UV–Vis spectroscopy, elemental
analysis and mass spectrometry. The molar conductance data revealed that the metal complex is a
non-electrolyte. Thermal analysis techniques (TGA/DTA) of the ligand and the complex were
performed. The proposed structures for the free ligand and the corresponding vanadium complex
were corroborated by the use of geometry optimization and conformational analysis. The solid state
structure of complex VOL was fully determined by single crystal X-ray diffraction analysis.
Similar electrochemical behavior for both the ligand and the corresponding complex have
been observed by cyclic voltammetry (CV). The complex gave a metal-based one-electron quasi-
reversible redox couple at 606 mV/SCE corresponding to the VOIV/VOV redox process which was
consistent with the hydrodynamic voltammograms. Additional peaks in the CVs that were growing
with the scan rate have been attributed to consecutive one-electron reduction of the ligand. The
catalytic activity of the oxovanadium Schiff base complex VOL was tested in the oxidation reaction
of cyclohexene. The efficiency of the catalyst was influenced by the nature of the oxidant. The
results showed that the complex was highly active and selective for cyclohexenone in optimized
conditions.
Keywords: Oxovanadium complex, Synthesis, Spectroscopy, Crystallography, DFT, Cyclic
Voltammetry, Oxidation.
1. Introduction
In recent decades Schiff bases play an important role in inorganic chemistry as they easily
form stable complexes with most transition metal ions [1-3]. The coordination chemistry of
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transition metal-Schiff base complexes has been extraordinary developed in different aspects [4-7]
because of their high stability and their important properties in different oxidation states.
Recently much research has been focused on the coordination chemistry of vanadium
complexes because of the interesting structural features [8–10], biochemical and pharmacological
efficiency as insulinomimetic profile [11], antiparasitic [12] and antitumor activities [13]. In
medicinal processes, such as haloperoxidation [15], phosphorylation [16], nitrogen fixation [17],
glycogen metabolism [18,19] and insulin mimicking [20,21], spermicidal [22], anti-leukemia [23].
Also, in homogeneous catalysis, vanadium Schiff base complexes formed with chelating ligands
involving hetero atoms are found to be used in a variety of systems [14]. Homogeneous catalysts of
oxovanadium(IV) complexes induce organic reactions in the oxidation of sulfides and sulfones
[24,25], the epoxidation of alkenes [26,27], the hydroxylation of hydrocarbons [28], the oxidation
of alcohols to aldehydes and ketones [29,30], phenols [31] and tertiary amines to N-oxides [32].
These studies are indicative that oxovanadium(IV) complexes are potential catalysts to influence the
yield and selectivity in chemical transformation [33]. In addition, oxovanadium(IV) complexes,
attract considerable attention due to their remarkable catalytic activity and good selectivity for the
oxidation of various substrates using H2O2 as a green oxidant [34], thus responding to the
increasing demand for more efficient homogeneous and heterogeneous catalytic systems [35-38].,
The epoxidation of alkenes is one of the most widely studied reactions in organic chemistry
since epoxides are key starting materials for a wide variety of products and the catalytic epoxidation
of olefins by transition metal complexes is an area of intense research activity [39-40] In the last
decades, the synthesis, the structural study and catalytic activity of oxovanadium Schiff base
complexes in oxidation of different organic substrates have been described in a great number of
publications [41,42]. The central metal oxovanadium is surrounded by N2O2 coordination
environment. Recent advances have shown vanadium complexes to be efficient catalysts for the
activation of peroxides by virtue of vanadium in terms of stereoselectivity, reactivity and specificity
[43]. The ability of these complexes to form metalloperoxo species, which in turn effectively
transfer an oxygen atom to reductants with a high degree of selectivity, render them synthetically
useful for obtaining valuable molecules.
In order to investigate the catalytic activity of oxovanadium Schiff base complexes as
catalysts, we have synthesized in this work a new symmetrical tetradentate Schiff base ligand and
its corresponding oxovanadium complex have been characterized by spectroscopic methods.
Subsequently, the crystal structure of this new Schiff base complex has been determined by X-ray
crystallography. We report hereafter the catalytic epoxidation of cyclohexene in dry solvent using
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hydrogen peroxide and molecular oxygen as oxidants. The electrochemical behavior of the complex
has also been investigated.
2. Experimental
2.1. Materials and Measurements
All chemicals were of reagent grade solvents and all commercially available reagents
(Aldrich or Merck) and used without further purification. The syntheses were carried out under inert
atmosphere (N2) following conventional procedures.
The melting points for the ligand H2L and its complex of VO(IV)L were determined with a
Kofler Banc 7779 apparatus and are uncorrected. Infrared spectra were recorded using KBr pellets
on a Shimadzu FTIR IR Affinity-1 spectrophotometer. High-resolution mass spectrum (HRMS) for
oxovanadium (IV) complex was acquired via electrospray ionization mass spectrometry (ESI-MS).
Electronic spectra were performed on a Shimadzu UV-1800 spectrophotometer using DMF as
1
solvent. Elemental analyses were carried out on an Elementar-Vario EL III CHNSO analyzer. H-
NMR spectra were recorded on a Bruker Avance 500 MHz spectrometer using DMSO-d6 as solvent
and tetramethylsilane (TMS) as internal standard. The molar conductivities of the compounds were
carried out on conductivitymeter MeterLab CDM-210 apparatus. Thermogravimetry (TGA) and
differential thermoanalysis (DTA) were carried out by using a PL-1500 apparatus, the
measurements were performed in nitrogen atmosphere, the heating rate was held at 10 °C min-1.
The electrochemical properties of the structures were investigated at room temperature in
solutions containing Lithium perchlorate (LiClO4), 0.1 M, as supporting electrolyte on a glassy
carbon (GC) working electrode, the counter electrode was a platinum wire and the reference
electrode was a saturated calomel electrode (SCE), the results are given versus SCE. The solvent
was DMF with a 10-3 M concentration for both the ligand and the complex. The scan rate used in all
the voltammograms was of 100 mV/s. All solutions were deoxygenated by passing a stream of pre-
purified nitrogen into the solution for at least 10 min prior to recording the voltammograms. Gas
chromatography (GC) analyses were carried out on a GC-2014 Shimadzu instrument; it is equipped
with a DB5 capillary column of 30 m length, 0.25 mm diameter and flame-ionization detector.
2.2. X-ray crystallography
Single crystals of VO(IV)L complex were grown by slow layer diffusion of DMSO into a
MeOH solution at room temperature. The crystals were placed in oil and a brown single crystal of
dimensions 0.25 × 0.15 × 0.12 mm3 was selected, mounted on a glass fiber and placed in a low-
temperature N2 stream. X-ray diffraction data collection was carried out on a Bruker APEX II DUO
Kappa-CCD diffractometer equipped with an Oxford Cryosystem liquid N2 device, using Mo Kα
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radiation (k = 0.71073 Å). The crystal-detector distance was 38 mm. The cell parameters were
determined (APEX2 software) [44] from reflections taken from tree sets of 12 frames, each at 10 s
exposure. The structure was solved by direct methods using the program SHELXS-97 [45]. The
refinement and all further calculations were carried out using SHELXL-97 [46].
The H-atoms were included in calculated positions and treated as riding atoms using
SHELXL default parameters. The non-H atoms were refined anisotropically, using weighted full-
matrix least-squares on F2. A semi-empirical absorption correction was applied using SADABS in
APEX2 [47]; transmission factors: Tmin/Tmax = 0.580/0.746.
2.3. General procedure of the oxidation reaction
In a typical experiment, 15 mmol of the oxovanadium catalyst were dissolved in 15 mL of
DMF, 15 mmol of cyclohexene and 30 mmol of hydrogen peroxide (H2O2 ) or molecular oxygen
(O2 ) were added. With O2, the reaction is carried out under bubbling with a needle introduced into
the mixture. The reaction mixture was refluxed while being stirred and the reaction progress was
monitored at 1 h intervals. The optimized reaction time was obtained by following the reaction
progress at 1 h intervals up to 24 h; 6 h was the optimized reaction time.
In the absence of the complex, no oxidation products were observed. In gas chromatography,
the retention times for the starting materials and the products were determined by comparison with
authentic samples. The conversion percent (%) are calculated by the following equation, in which
C
and C
are initial and final concentration of the substrate, respectively.
final
initial
ꢀꢇꢂꢇꢉꢇꢊꢋ − ꢀꢌꢇꢂꢊꢋ
%ꢀꢁꢂꢃꢄꢅꢆꢇꢁꢂ = ꢈ
2.4. Synthesis of the ligand
ꢍ × 100
ꢀꢇꢂꢇꢉꢇꢊꢋ
The tetradentate Schiff base ligand, H2L, was synthesized according to the literature method
[48, 49] by condensing of 1 mmol (0.108g) of phenylenediamine with 2 mmol (0.402 g) of 5-
Bromo salicylaldehyde in 15 ml of methanol .The bright orange solution was stirred and heated to
reflux for 2 h under nitrogen atmosphere, scheme 1.
The precipitate was filtered, washed with cold methanol (3x2 ml) and diethyl ether (3x2 ml)
and then dried in vacuum over a night. Suitable single crystals for X-ray crystallography were
obtained from recrystallization using DMSO/MeOH. Yield: 90%; mp = 234-238 °C; ΛM (Ω-1 cm2
mol-1) =1.47; IR, KBr pellets,
ν
cm-1 : 3457 (O-H), 1610 (C=N), 1480 (C=C), 1274 (C-O); UV–
1
Vis: DMF, λ nm, [ε M-1 cm-1 ]: 267.44 [8738], 343.96 [7095]; H NMR: (CDCl3 δ ppm): 13.20 (s,
8.54 (s, CH=N), 6.80 - 8.00 (m, ArH); 13C NMR: (CDCl3 δ ppm): 160.32
(O-H), 162.40
(CH=N), 108–161 (C-Ar).
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2.5. Synthesis of the complex
The title complex, VO(IV)L, was prepared in a 10 ml MeOH by adding dropwise a
solution of 0.5 mmol bis (acetylacetonato) oxovanadium in absolute MeOH to 0.5 mmol of the
ligand H2L. The mixture was refluxed and stirred for 2 h under nitrogen atmosphere, scheme 1. The
green powder was separated from the solution and purified by washing with cold methanol (3x2 ml)
and diethyl ether (3x2 ml). Suitable single crystals for X-ray crystallography were obtained from
recrystallisation using DMSO/MeOH. Yield: 84%; mp > 260 °C. Analysis calculated for
C20H12O2N2Br2VO: C, 44.65%; H, 2.24%; N, 5.20 %; found: C, 44.75%; H, 2.37%; N, 5.30%; ΛM
(Ω-1 cm2 mol-1) = 3.81; IR (KBr pellets, ν cm-1 ): 1595 (C=N), 1428 (C=C), 1172 (C―O), 630
(C―Br), 968 (V=O), 530-555 (N―V), 450-460 (O_V). ; UV–Vis: DMF, λ nm, [ε: M-1 cm-1]: 305.70
[16054], 322.29 [15800], 414.15 [13000]; ESI/MS: [M.+]= 538.76.
CHO
MeOH
Br
OH
+
H2N
2
Reflux
NH2
N
N
Br
OH
Br
HO
MeOH, Reflux
VO(acac)2
N
O
N
V
Br
O
Br
O
Scheme.1. Synthesis of the ligand H2L and the corresponding oxovanadium complex
VO(IV)L.
3. Results and discussion
The Schiff base H2L and oxovanadium complex VO(IV)L are stable at room temperature and
are soluble in organic solvents such as DMF, DMSO, C5H5N and insoluble in inorganic solvents.
3.1. Elemental analysis
Elemental analysis data of the oxovanadium complex show good agreement between found
and calculated values and fitted well with a 1:1 metal to ligand.
3.2. Molar conductivity measurements
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The molar conductivity of ligand and its complex were measured in DMF 10 M as solvent
at room temperature, showed a low conductance value, 1.47 and 3.81 ohm−1 cm2 mol−1,
respectively, suggest a non electrolytic nature and there is no counter ion present outside the
coordination sphere [50,51].
3.3. Electronic spectral studies
The electronic spectral bands of the Schiff base ligand H2L and its corresponding complex
VO(IV)L are presented in Table.1.
In the 200 to 800 nm wavelength range, the Schiff base ligand exhibits two bands at 267 and
344 nm. The first band is attributed to the π_π transitions in the diamine aromatic backbone. The
second one is due to n_π transition of the azomethine group [52, 53].
In the complex, the three bands localized at 306 and 322 nm are assigned to the intra-ligand
π_π* and n_π and 414 nm due to LMCT transitions [54, 55]. The d–d transitions were not observed
for the VO(IV)L complex.
Table.1. Electronic spectra of the H2L and VO(IV)L
UV-Vis
Compounds
Assignement
λmax (nm)
267
(ε, M-1 cm-1)
(8738)
π_π*
n_π
H2L
344
(7095)
306
(16054)
(15800)
(13000)
π_π*
VOL
322
n_π*
414
LMCT
3.4. Spectroscopic characterization
The IR spectra of the free Schiff base ligand and the complex exhibit several bands in 400–
4000 cm-1 region.
The IR spectrum of the ligand, a sharp C=N band was observed at 1621 cm-1 [56]. The ligand
displays intense bands at 3457 and 1274 cm-1 which are assigned to ν(O―H) and ν(C―O)
stretching vibrations of the phenolic moieties. Bands appearing in the range 1473-1561 cm-1 are
related to the aromatic C=C vibrations. Additionally, the band observed at 630 cm-1 is assigned to
the C―Br vibration [57].
In the spectrum of the complex, the vibration bands observed in the Schiff bases were
decreased in intensity and shifted towards a lower frequency for the C=N band, 1604 cm-1[58,59],
the frequency of the C―O band is shifted to a higher value, 1293 cm-1 , indicating that the nitrogen
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atom of the azomethine group and the oxygen of the phenolic moiety are coordinated to the metal
ion [60]. The oxovanadium complex shows a band at 968 cm-1 attributed to V=O frequency [61].
1
The chemical shifts of the different types of protons in the H NMR spectrum of the H2L
ligand was reported in Table 2.
The structure of the synthesized ligand is given by the following scheme:
1
2
7
3
N
N
Br
OH
HO
Br
6
4
5
H2L
Table.2. Chemical shifts (in ppm) of the proton of the ligand in CDCl3
H6 (OH)
H7 (CH=N)
Compound
H2L
H
1-5 (Ar)
13.02
8.54
6.80 - 7.60
The 1H NMR spectra of Schiff bases shows signals in conformity with the structure. The peak
observed at 13.02 ppm in these Schiff bases was assignable to the O-H of the bromophenol form.
The azomethine proton appears at 8.54 ppm. Multiplet signal, due to the aromatic protons, are
observed in the range 6.80-7.60 ppm [49 ]. (see Figure.1)
The figure.2 illustrates the mass spectrum of the complex VO(IV)L. The peak appearing at
m/z = 538.76 is the molecular peak [M.+].
The dibrominated complex mass spectrum presents its expected molecular peak cluster, due to
the relative abundance of the bromine atome isotopes [62, 63].
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Fig.1. 1H NMR spectrum of the Schiff base ligand (H2L) in CDCl3.
Fig.2. The mass spectrum of the oxovanadium complex VO(IV)L.
3.5. X-ray crystal structure
An ORTEP view of VO(IV)L with the atom-numbering scheme is presented in Fig.3. Table 3
shows crystal data, data collection and structure refinement parameters. Most relevant bond lengths
and angles are given in Table.4.
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Fig.3. The molecular structure of the oxovanadium complex. Thermal ellipsoids
are drawn at 50% probability
Table.3. Crystallographic and refinement data for VO(IV)L complex.
Compound
VO(IV)L
Molecular formula moiety
Molecular weight
Temperature (K)
Radiation
Crystal system
Space group
a/Å
b/ Å
c/ Å
C20H12Br2N2O3V
173(2)
Mo-K (0.71073 Å)
Monoclinic
P21/c
11.0545 (8)
18.1170 (12)
9.6547 (7)
90.00
alpha°
Beta°
108.581 (2)
90.00
gamma°
V/Å3
1832.8 (2)
4
Z
1.954
Dcalc (mg m-3)
Crystal size (mm3)
Crystal description
Crystal colour
Absorption coefficient (mm-1)
F(0 0 0)
Reflections collected/unique
Range/indices (h, k, l)
Tetalimit
0.25 × 0.15 × 0.12
Prism
Brown
4.927
1052
28978/3856 [Rint = 0.044]
−15,15; −24,24; −13,8
1.944–29.022
3856
No. of observed data, I > 2Sigma(I)
No. of variables
247
0
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No. of restraints
1.158
Goodness of fit on F 2
4.02 and -1.77
0.0710, 0.1724
0.0901, 0.1815
Largest diff. Peak and hole (e Å-3)
R1, wR2 [I 2Sigma(I)]a
≥
R1, wR2 (all data)a
a R1 = ∑||F0| - |Fc||/∑|F0|, wR2 = [∑w(F0 2 – Fc 2)2/∑w(F0 2 )2]1/2
2
w = 1/[σ2(Fo ) + (0.0612P)2 + 17.1114P] where P = (Fo2 + 2Fc2)/3
The crystal structure shows the presence of one molecule of mononuclear complex in the
asymmetric unit. The VO(IV)L crystallizes in the monoclinic system with space group P21/c and
the cell dimensions a = 11.0545 (8) ; b = 18.1170 (12) ; c = 9.6547 (7) Å; á = ã = 90°; â = 108.581
(2). The oxovanadium atom in this complex is coordinated by the two nitrogen and two oxygen
donor atoms and one oxygen atom to complete the N2O3 coordination sphere. The geometry can be
considered as a distorted square pyramidal coordination with N1, O1, N2 and O2 of the Schiff base
complex. The distances between the coordinated nitrogen and oxygen atoms with the metal center
do not significantly differ (Table 3).
The C=N bond distances are 1.308(7) Å (N1=C7) and 1.300(7) Å (N2=C14), which are
consistent with a slight elongation of the C=N double bond in the corresponding ligand [64, 49].
The V―O bond lengths of 1.888(4) and 1.929(4) Å and the V―N bond lengths of 2.062(5) and
2.058(5) Å are similar to those given in related Schiff base oxovanadium complexes [64-67]. The
V―O bond distances are shorter than V–N, which indicates stronger coordination of the oxygen
atoms.
The β-angle N1―V1―N2 is 78.33(19) and the α-angle O1―V1―O2 is 89.30(19). Thus, the
phenolate oxygens are displaced out of the “equatorial” plane away from the axial oxo ligand.The
oxovanadium V=O bond distance is 1.604(4), which is a typical value for vanadyl compounds
[68,69].
The two phenolate ring moieties of the complex are not coplanar with the above coordination
plane (N1-N2-O1-O2) and bend to the same side. The packing structure of VO(IV)L consists of a
discrete molecule held together by Van Der Waals forces (Fig.4).
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Fig. 4. Schematic drawing of the crystal packing
Table.4. Selected bond lengths (Å) and bond angles (°) for VO(IV)L complex.
Bond lenghts (Å)
Bond angles (°)
N1—V1 2.062 (5)
N2—V1 2.058 (5)
O1—V1 1.888 (4)
O2—V1 1.929 (4)
O3—V1 1.604 (4)
O3—V1—O1 109.8 (2)
O3—V1—O2 108.5 (2)
O1—V1—O2 89.30 (19)
O3—V1—N2 103.4 (2)
O1—V1—N2 145.9 (2)
O2—V1—N2 87.61 (18)
O3—V1—N1 104.4 (2)
O1—V1—N1 85.86 (19)
O2—V1—N1 146.4 (2)
N2—V1—N1 78.33 (19)
3.6. DFT Calculations:
The computational study was carried out starting from the X-ray crystallographic molecular
structures of VO(IV)L complex and H2L ligand
Recent investigations have shown that the Becke's three parameter hybrid functional using the
LYP correlation functional [70] (B3LYP) method [71] is reliable for the description of geometrical
parameters, electronic structure and optical properties of different molecules and complexes [72-
76]. In the present density functional theory (DFT) calculations, the B3LYP functional was adopted
together with one set of two polarization functions (TZ2P) as the basis sets and none frozen cores
in all calculations. Relativistic corrections were taken into account with the use of the relativistic
scalar zero-order-regular approximation (ZORA) method [77, 78]. The numerical integration
parameter was done with the Beckegrid option fixed to the quality very good [79, 80]. The energy
convergence criterions were 1eꢎꢏ au. All quantum chemistry calculations were performed with
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Amsterdam Density Functional (ADF) program developed by Baerends and co-workers [81- 83].
All structures were calculated in their ground state without any symmetry constraints. No imaginary
vibrational frequencies were found for VO(IV)L and H2L optimized geometries.
The important optimized geometrical parameters (distance and angle) of the VO(IV)L and
H2L calculated using the density functionals B3LYP are presented in Fig. 5 and Table 5.
Fig.5. DFT optimized geometries of H2L and VO(IV)L ,with important bond lengths (Å) of ligand.
Experimental data are given in green.
Table .5.Selected Bond Lengths (Å) and Angles (deg) of VO(IV)L complex
Exp
B3LYP/TZ2P
V-N1,2
V-O1,2
V-O3
1.887
2.063
1.604
1.396
0.582
1.893
78
1.945
2.100
1.579
1.407
0.628
1.915
77
C1-C6
Cent*-V
Br-C10,17
N1-V-N2
O1-V-O2
O1-N1-N2-O2
89
0.11
88
0.0
* centroid between O-N-N-O
As seen in Fig.5, the deviations between theoretical result and experimental geometry
parameters of H2L ligand are in satisfactory agreement with experimental results. For the VO(IV)L
complex, the V-O3 distance is significantly shorter than the V-O1,2, as observed in the solid state.
In addition, the dihedral angle O1-N1-N2-O2 and angles N1-V-N2 and O1-V-O2 are same in the
solid state. More generally, the average bond lengths V-O, V-N, Br-C and C-C calculated by
B3LYP/TZ2P method are systematically a little longer than the experimental. Our calculated results
of distance and angles agree well with the experimental data. This agreement, if it was necessary,
clearly supports the ability of B3LYP/TZ2P to accurately reproduce the structure of H2L and
VO(IV)L.
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In Table.6, we display the computed values of the electronic chemical potential ( µ ),
chemical hardness ( η ), global electrophilicity ( ω ) indices and IP of H2L and VO(IV)L.
The global hardness ( η ) measures the stability of a system in terms of resistance to electron
transfer and the chemical potential ( µ ) characterises the escaping tendency of electrons from the
equilibrium system. Each system’s ( η ) and ( µ ) are calculated from the energy of HOMO and
LUMO [84-87]
ꢑ
ꢐ = (ɛꢓ + ɛꢔ)
ꢒ
η
= ɛꢔ−ɛꢓ
The global electrophilicity index (ω) expresses the ability of a molecule to accept electrons
from the surroundings [88]:
μꢒ
ꢕ =
2η
According to the absolute scale of global electrophilicity power ( ω ) reported by Domingo et al.
[89], the H2L ligand can be classified as strong electrophiles (see Table 6).
Table.6. Global reactivity descriptons:
chemical potential ( µ , eV), chemical hardness ( η , eV) and electrophilicity index ( ω ).
H2L
-4.472
4.052
VO(IV)L
-4.65
ꢐ (eV)
3.65
η
(eV)
2.467
2.96
ꢕ
Time dependent density functional theory (TD-DFT) calculations with number of states 30
were performed in order to understand these spectroscopic data. The molecular orbital energy
diagrams of H2L and VO(IV)L are shown in Figures 6 a-b, respectively, and the corresponding
electronic transitions are listed in Table 7.
Table.7. Calculated Lowest Excitations TD- DFT for H2L and VO(IV)L. B3LYP/TZ2P level of
theory
H2L
f
Type
π→π*,
n→π*
π→π*,
n→π*
Composition
λ (nm)
373.5
0.294
HOMO→LUMO
344.52
0.108
HOMO-1→LUMO
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325.44
289.21
271.32
256.101
0.159
0.117
0.280
0.228
π→π*
π→π*
π→π*
π→π*
VO(IV)L
π→π*
n→π*
π→π*
π→π*,
n→π*
π→π*
π→π*
MLCT
HOMO→LUMO+1
HOMO-3→LUMO
HOMO-4→LUMO
HOMO-5→LUMO
315.33
332.20
0.269
0.0012
SOMOβ-2→SUMOβ
SOMOβ-1→SUMOβ+1
SOMOβ→SUMOβ
SOMOα→SUMOα +1
410.49
0.055
0.126
SOMOβ-1→SUMOβ
SOMOα→SUMOα
424.02
SOMOα-2→SUMOα
SOMOα-2→SUMOα+3
SOMOα-2→SUMOα+4
SOMOα-2→SUMOα+1
554.11
589.79
0.0001
0.0003
ICMT
ICMT
MLCT
Fig.6-a Molecular orbital energy diagram for H2L (B3LYP/TZ2P).
14

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Fig.6-b DFT molecular orbital diagrams of SOMO and SUMO of α and β spin of VO(IV)L
complex (the energies are given in eV). B3LYP/TZ2P level of theory.
The lowest energy transitions of H2L, at 373 and 344 nm, involve exclusively 99% the
HOMO and HOMO-1, respectively, localized on the donor part of the ligand and the LUMO,
delocalized on the two aromatic rings, Fig 6-a. Thus, these transitions are classically described as
intramolecular charge transfer (ICT) corresponding to the π →π* with some minor n→π* transition
(HOMO→LUMO). The optical transition centered at 325, 289, 271 and 256 nm can be assigned to
π→π* transition.
15

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On the other hand, the experimental lowest-energy absorptions of the ligand are observed at
267 and 343 nm. From these results, a good agreement is notified between the experimental data
and the excitation calculations.
Our TD-DFT calculations obtained for VO(IV)L complex predict that the transition π→π*
and n →π* are located at{315, 410 and 424 nm} and {332 and 410 nm} are dominated by the {
SOMOβ-2→SUMOβ, SOMOβ→ SUMOβ, SOMOβ-1→ SUMOβ} and {SOMOβ-1→ SUMOβ+1,
SOMOα→ SUMOα1} respectively, Fig.6-b. Furthermore, the electronic transition arises, from the
metal toward the ligand orbitals (metal to ligand charge transfer MLCT), at 424 nm is dominated by
the SOMOα-2→SUMOα and f =0.126, table 3. On the other hand, the excitations at 554 and 589
nm, from the SOMOα-2→SUMOα+3 and SOMOα-2→SUMOα+4, are intra-charge metal transfer
bonds (ICMT character) but the oscillator strengths are zero. These results agree well with
experimental and confirmed that the ICMT (d→d) transition is completely absent in VO(IV)L
complex.
3.7. Thermal analysis
The thermal properties of the Schiff base and the corresponding complex were investigated by
thermo gravimetric analyses TGA and DTA measurements to get information about the thermal
stabilities whether the water molecules are inside or outside the inter coordination sphere of the
central metal ion and to suggest a general scheme for thermal decomposition of this complex.
At a heating rate of 10 °C/min under a nitrogen atmosphere the temperature was increased
from ambient to 950 °C. The absence of weight loss up to 80 °C indicates that there is no water
molecule in the crystalline form [90, 91]. Also, the TGA showed no weight loss up to 150 °C
indicating that the water molecule is not coordinated to complex [92, 93].
The complex was decomposed in two steps, Fig.7. The first step occurs between 343 and 414
°C and was attributed to the release of C6H4N2, as indicated by the mass loss of 19.31% (calculated
18.56%). The second step occurs 414–661°C, a mass loss of 64.15% (calculated 68.26%) due to the
elimination of a C14H8O2Br2 group, with the formation of metal oxide [48].
16

80
70
60
50
40
30
20
10
0
ACCEPTED MANUSCRIPT
100
80
60
40
20
-10
200
400
600
800
Temperature (°C)
Fig.7. TGA and DTA spectrum of the oxovanadium complex VO(IV)L.
3.8. Electrochemical measurements
The electrochemical behavior of the ligand H2L and the complex VO(IV)L was investigated
by cyclic voltammetry with a glassy carbon (GC) electrode in DMF solution containing 10-1 M
LiClO4 as the supporting electrolyte, under nitrogen saturated atmosphere in the potential range -
2200 to 1600 mV/SCE. The Fc/Fc+, redox couple of ferrocene was used as an internal standard. The
cyclic voltammograms of VO(IV)L complex are given in Fig 8c and exhibits a quasi-reversible
waves at E1/2 = 606 mV, attributed to the one-electron VOIV/VOV oxidation couple [94], in which
the VO is coordinated by a chelated ligand containing N-amine and O-phenolate donor atoms.
Which are absent in the CV of the free ligand.
VOIV
VOV + e
VOIV
VOV + e
_
On the other hand, the first irreversible reduction at Ered = 1093 mV/SEC is assigned to the
VOIV/VOIII [95], the second irreversible reduction wave occurred at Ered = -1598 mV/SCE is due to
the reduction of the azomethine groups. [96,97]
17

+
ACCEPTED MANUSCRIPT
Fig.8. Cyclic voltammograms of H2L ligand (a), VOL complex (b) and Fc/Fc (c)
in 0.1 M LiClO4/DMF solution at scan rate of 100 mVs-1.
A typical cyclic voltammogram of complex in the potential range from 200 to 900 mV/SCE is
shown in Fig.9. Multiple scans which resulted in nearly superposable cyclic voltammograms show
that the five coordinates geometry is stable in both oxidation states, at least on the cyclic
voltammetry time scale. The result revealed that the redox process of studied vanadium Schiff base
complex is a one-electron transfer reaction. The results of cyclic voltammetry (CV) measurements
are given in Table 7.
The square root of the scan rate exhibits a linear relation with the current peak Ip (inset of
Fig.9). The Ip/v1/2 value is almost constant for all scan rates. This establishes the electrode process
as diffusion controlled [98].
25
20
18
20
16
14
15
300 mV/s
25 mV/s
12
10
8
Y= 1.0711+ 1.2811X
R2= 0.9986
10
5
6
4
6
8
10
12
14
16
18
V1/2(mV/s)1/2
0
-5
-10
-15
100
200
300
400
500
600
700
800
900
1000
E(mv/SCE)
Fig.9. CV of 10-3 M of VOL in DMF/10-1 M LiClO4 at various scan rates from 25 to 300 mV/s. The
inset shows the anodic peak current (ipa) vs. ν1/2.
The variation of the scan rate between 200 and 900mVs–1 only showed a marginal increase in
∆Ep. At a scan rate of 100 mV/s, the separation between cathodic and anodic peaks (∆Ep ≈ 72 mV)
is suggestive of a one electron- process. The ipa/ipc ratio was closest to 1 indicating chemical
reversibility system.
Table.7. Cyclic voltammetric parameters for the oxovanadium VO(IV)L complex
in different scan rates
ꢖꢗꢘ
ꢖꢗꢙ
ꢚꢖꢗ
ꢖꢛ/ꢜ
ꢝꢗꢘ
ꢝꢗꢙ
v
ipa/ipc
1.37
25
639.8
576.1
63.7
607.9
6,4
4.66
18

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50
642.6
645.1
642.6
647.8
648.7
648.7
650.6
573.3
573.1
570.8
571.7
572.6
569.7
571.7
69.3
72.0
71.8
76.1
76.1
79.0
78.9
607.9
609.1
606.7
609.7
610.6
609.2
611.1
9
6.70
7.79
1.34
1.34
1.34
1.31
1.30
1.27
1.28
75
10,5
12,2
14,49
16,5
18
100
150
200
250
300
9.10
11.00
12.66
14.14
15.4
19,82
3.9. Rotating disk electrode voltammetric studies
In order to determine the diffusionnal character of the complex, a linear sweep
hydrodynamics voltammetry on rotating disk electrode (RDE), using GC working electrode, is
carried out in order to determine the diffusion coefficient D. We have studied the evolution of the
current cathodic as a function of the potential Ic= f(E) at different rotation rate of the disc, from 250
to 3000 rpm, in the interval 300 to 800 mV/ECS for a scan rate of 20 mV/s. The current potential
curves and the Levich plots are shown in Fig.10.
Under the studied conditions, a linear correlation between the limiting current and the square
root of the rotating rate is observed, which leads to the determination of the value of the
proportionality constant using Levich equation for a one electron VOV/VOIV reduction [99].
Diffusion coefficient was calculated from the slope of the ilim vs. ω1/2 plots and its value is 2.10-6
cm²/s [97].
50
35
3000
45
30
40
25
35
20
Y= 0.6237+ 2.3774 X
30
15
R2= 0.9986
25
10
15
20
25
30
35
40
45
50
55
500
250
20
15
10
5
ω1/2
(
rpm)
1/2
0
-5
300
400
500
600
700
800
E(mV/SCE)
19

ACCEPTED MANUSCRIPT
Fig. 10. Voltammograms of VO(IV)L complex at RDE of GC in DMF solution.
Rotation rates: from 250 to 3000 rpm. The inset shows the Levich plot.
4. Catalytic activity
The catalytic activity and selectivity of the VO(IV)L have been studied for the oxidation of
cyclohexene with H2O2 and O2 as oxidants in DMF, under similar experimental conditions [ 100 ].
The oxidation reactions give 2-cyclohexenone as the main product, 2-cyclohexenol and
cyclohexene epoxide in small yields which are evidently identified using gas chromatography by
comparison with standards (Fig.11). In order to obtain maximum conversion of cyclohexene, the
catalytic reactions were carried out using two oxidants.
The results of the catalytic oxidation of cyclohexene by H2O2 and O2 in the presence of
VO(IV)L are shown in table 8, which shows the variation of product yield with the oxidant nature.
Higher yield was obtained for 2-cyclohexenone than 2-cyclohexenol and cyclohexene epoxide for
the H2O2 oxidant. In the case of O2, as oxidant, the yields for cyclohexenol and cyclohexene
epoxide were nearly similar. The yield of the three products increased, but level off with time. This
level off is most likely due to the degradation of the VO(IV)L catalyst by the oxidant with time.
Table .8. Catalytic oxidation of cyclohexene by H2O2 and O2 in the presence of VO(IV)L
%Sélectivity
Catalyst
Oxidant
%Conversion
Epoxide Cyclohexene-ol Cyclohexene-one
H2O2
80
85
-
9.71
3.21
-
26.72
23.62
-
63.56
73.16
-
VO(IV)L
O2
Oxidant Free
The nature and the relative yields of the products, formed by catalytic oxidation of
cyclohexene using oxovanadium Schiff base complex, vary considerably depending with the
oxidant nature. This work shows that the VO(IV)L, as catalyst, in the presence of O2 and H2O2
oxidants, favored the formation of 2-cyclohexenone to the rest of the products (fig. 11).
20

ACCEPTED MANUSCRIPT
Fig. 11. The oxidant effect on the oxidation of cyclohexene.
The results show that the activity of the oxovanadium complex in the oxidation of
cyclohexene in the presence of O2 in DMF is much higher than in the presence of H2O2 in DMF, the
trend of the observed oxidant effect was O2 > H2O2. This may be related to the ability of O2 and
inability of H2O2 to mix with the organic substrate phase [101].
In the proposed mechanism, it seems that active oxidant is oxovanadium(V) initially formed
by oxidation of oxovanadium(IV) in the presence of O2 (Scheme 2) [102]. O2 coordinate to the VVO
species to form VV–O–O- intermediate, which can react with olefin to yield oxide product.
O2
VOIVL
O2
V
-
VOLVO
LV O O
O
OH
+
+
O
Scheme.2. Proposed catalytic cycle for the oxidation of cyclohexene by VO(IV)L
5. Conclusion
New five-coordinate oxovanadium (V) complex derived from Schiff base ligand containing
the N2O2 donor group successfully synthesized with high purity and yield, 90% for H2L and 84%
for VO(IV)L , and characterized with the aid of FT-IR, UV-Vis, and mass spectrometric
21

ACCEPTED MANUSCRIPT
techniques, along with elemental and thermal analyses. The spectroscopic analysis confirmed the
composition and the structure of the newly obtained compound.
The non-electrolytic nature of the VO(IV)L complex was confirmed on the basis of their
molar conductance value.
The thermo gravimetric analysis TGA and differential thermoanalysis DTA, are consistent
with the proposed structures and provided unambiguous evidence for no coordinated water in the
ligand and its complex, and showed that the compound is thermally stable up to 340°C for
VO(IV)L, 220°C for H2L. X-ray analysis reveals that VO(IV)L has a distorted square-pyramid
geometry.
The calculation confirms that calculated geometrical parameters are in agreement with the
experimental data.
Electrochemical studies by the cyclic voltammetry analysis revealed a one-electron quasi-
reversible pair of waves assigned to the VOIV/VOV electroactive species, the half-wave potentials
near to 606 mV.
The catalyst VO(IV)L favored the formation of 2-cyclohexenone (73.16% and 63.56%) to
the rest of the products in the presence of O2 and H2O2 oxidants. The activity of the oxovanadium
complex in the oxidation of cyclohexene in the presence of O2 is much higher than in the presence
of H2O2 (O2 > H2O2).
Supplementary Material
CCDC-1516191 contains the supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif by e-mailing:
data_request@ccdc.cam.ac.uk; or by contacting: The Cambridge Crystallographic Data Centre, 12
Union Road, Cambridge CB2 1EZ, UK; Fax: 44(0)1223-336033.
Acknowledgement
The Authors gratefully acknowledge the financial support from The Algerian Ministry of Higher
Education and Scientific Research. They also acknowledge the help to access the NMR Facility and
Microanalysis Service at GDS 3648 of the University of Strasbourg - CNRS – France.
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Highlights
A novel oxovanadium complex VO(IV)L synthesized with high-yield.
Different spectroscopic analyses confirm the structure.
The complex crystallizes in the monoclinic system; the geometry is confirmed by DFT.
Electrochemical study revealed a quasi-reversible system assigned to VOIV/VOV wave
The catalyst VO(IV)L favored the formation of 2-cyclohexenone.