The effects of halogen substituents on the catalytic oxidation of benzyl-alcohols in the presence of dinuclear oxidovanadium(IV) complex

Article abstract of DOI:10.1016/j.ica.2017.05.049

A new dinuclear complex of oxidovanadium(IV), namely [(VO)₂(HL)(μ-O)] (1), has been synthesized by the reaction of VO(acac)₂ with the heptadentate N₄O₃-donor Schiff base ligand, 2-(5-Bromo-2-hydroxyphenyl)-1,3-bis(2-(5-bromo-2-hydroxybenzylideneamino)ethyl)imidazolidine (H₃L). The complex has been characterized by elemental analysis, spectroscopic methods and single-crystal X-ray diffraction. The latter technique revealed that the vanadium ions have distorted octahedral geometry and are connected together by oxido and phenolic oxygen atoms. The bridging oxido ligand shares the equatorial positions of the two metal centers while the oxygen atom of the bridging phenol group connects the axial positions. The catalytic activity of this complex has been tested for the oxidation of some benzyl alcohol derivatives by using H₂O₂ as a green oxidant. In order to maximize the yields, the effects of various influential parameters in catalytic reactions such as the oxidant-to-substrate molar ratio, the temperature and the solvent, were studied. Moreover, the electronic and steric effects of halogen substituents on the phenyl group of the substrate were also explored by analyzing the oxidation of benzyl alcohol derivatives with F, Cl and Br atoms in the relative para-position (electronic effect), and of another set of substrates with a Cl substituent in relative ortho-, meta, and para-positions (steric effect). The results of these catalytic studies show that complex 1 catalyzes the oxidation of benzyl alcohol derivatives to the corresponding benzaldehydes with little amounts of the benzoic acid being detectable in the reaction mixture. Both the reaction conditions and the substituents on the phenyl group of the benzyl alcohols affect the selectivity and the activity of this catalytic system.

Full text of DOI:10.1016/j.ica.2017.05.049

Accepted Manuscript
Research paper
The effects of halogen substituents on the catalytic oxidation of benzyl-alcohols
in the presence of dinuclear oxidovanadium(IV) complex
Rahman Bikas, Elaheh Shahmoradi, Nader Noshiranzadeh, Marzieh Emami,
Santiago Reinoso
PII:
S0020-1693(17)30055-5
http://dx.doi.org/10.1016/j.ica.2017.05.049
ICA 17619
DOI:
Reference:
Inorganica Chimica Acta
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17 May 2017
19 May 2017
Please cite this article as: R. Bikas, E. Shahmoradi, N. Noshiranzadeh, M. Emami, S. Reinoso, The effects of halogen
substituents on the catalytic oxidation of benzyl-alcohols in the presence of dinuclear oxidovanadium(IV) complex,
Inorganica Chimica Acta (2017), doi: http://dx.doi.org/10.1016/j.ica.2017.05.049
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The effects of halogen substituents on the catalytic oxidation
of benzyl-alcohols in the presence of dinuclear
oxidovanadium(IV) complex
Rahman Bikas,a Elaheh Shahmoradi,a Nader Noshiranzadeh,a,* Marzieh
Emami,a and Santiago Reinosob
a
Department of Chemistry, Faculty of Science, University of Zanjan 45371-38791,
Zanjan, Iran
b
Institute for Advanced Materials (InaMat), Universidad Pública de Navarra,
Edificio Jerónimo de Ayanz, Campus de Arrosadia, 31006 Pamplona, Spain
Abstract
A new dinuclear complex of oxidovanadium(IV), namely [(VO)2(HL)(µ-O)] (1), has
been synthesized by the reaction of VO(acac)2 with the heptadentate N4O3-donor
Schiff
base
ligand,
2-(5-Bromo-2-hydroxyphenyl)-1,3-bis(2-(5-bromo-2-
hydroxybenzylideneamino)ethyl)imidazolidine (H3L). The complex has been
characterized by elemental analysis, spectroscopic methods and single-crystal X-ray
diffraction. The latter technique revealed that the vanadium ions have distorted
octahedral geometry and are connected together by oxido and phenolic oxygen atoms.
The bridging oxido ligand shares the equatorial positions of the two metal centers
while the oxygen atom of the bridging phenol group connects the axial positions. The
catalytic activity of this complex has been tested for the oxidation of some benzyl
alcohol derivatives by using H2O2 as a green oxidant. In order to maximize the yields,
the effects of various influential parameters in catalytic reactions such as the oxidant-
*
Corresponding author. Tel.: +98 243 3052576; fax: +98 243 2283203. E-mail
addresses: nadernoshiranzadeh@yahoo.com, nadernoshiranzadeh @znu.ac.ir (N.
Noshiranzadeh).
1

to-substrate molar ratio, the temperature and the solvent, were studied. Moreover, the
electronic and steric effects of halogen substituents on the phenyl group of the
substrate were also explored by analyzing the oxidation of benzyl alcohol derivatives
with F, Cl and Br atoms in the relative para- position (electronic effect), and of
another set of substrates with a Cl substituent in relative ortho-, meta, and para-
positions (steric effect). The results of these catalytic studies show that complex 1
catalyzes the oxidation of benzyl alcohol derivatives to the corresponding
benzaldehydes with little amounts of the benzoic acid being detectable in the reaction
mixture. Both the reaction conditions and the substituents on the phenyl group of the
benzyl alcohols affect the selectivity and the activity of this catalytic system.
Keywords: Dinuclear vanadium complex; Crystal structure; Oxidation of alcohols;
Catalyst; Imidazolidine ligand
1. Introduction
The chemistry of vanadium and its complexes is gathering increasing attention in
recent years because these species have important roles in biological systems, as well
as in medical applications as an essential cofactor in certain haloperoxidases and
nitrogenases [1]. Vanadium compounds can heterogeneously or homogeneously
catalyze several oxidation reactions [2]. They have been used as effective catalysts in
reactions such as the oxidation of alcohols [3], epoxidation of olefins [4], oxidative
brominations [5], oxidation of sulfides [6] and oxidative coupling of naphthols [7].
The small size of the vanadium metal ion [8], the fact that it can present several
oxidation states up to +V [9] (where V(IV) and V(V) are the most stable ones under
aerobic conditions [10]), and its flexible coordination sphere has led to a wide variety
of vanadium complexes. Those containing the metal ion in high-oxidation states have
2

a potential interest in catalytic oxidation reactions [11]. Therefore, the development of
new oxidation catalysts based on vanadium(IV or V) is one of the appealing fields in
modern chemical research [12].
Selective oxidation of alcohols to their corresponding carbonyl compounds is one of
the important and fundamental reactions in organic synthesis [13]. From the
viewpoint of the green chemistry, this type of reaction using catalytic amounts of
metal complexes and molecular oxygen (O2), H2O2 or air as oxidants has attracted
much attention in recent years [14]. Several complexes of transition metals such as Ru
[15], Cu [16], Mo [17], Pd [18], V [3,19], etc. have been used as (pre)catalysts for the
oxidation of alcohols in the presence of various oxidants. Although several of such
catalytic systems have been reported to date, the development of new catalysts and
methods is gaining much attention [20]. Vanadium complexes could be amongst the
most suitable catalysts for the oxidation of alcoholic compounds in milder conditions
due to their higher oxidative catalytic activity compared with other transition metal
complexes [21]. However, vanadium complexes have primarily been applied in
epoxidation reactions and the research articles about their catalytic activity in the
oxidation of alcohols are still relatively rare. As a result, the influence of different
reaction parameters such as the oxidant-to-substrate molar ratio, the temperature, the
solvent, or the electronic/steric effects has not been yet analyzed in detail for this type
of catalytic systems [22].
We present herein the preparation, spectroscopic characterization, electrochemical
behavior and single-crystal X-ray structure of a new dinuclear vanadium (IV)
complex, namely [(VO)2(HL)(µ-O)] (1), derived from the heptadentate Schiff-base
ligand,
2-(5-Bromo-2-hydroxyphenyl)-1,3-bis(2-(5-bromo-2-
hydroxybenzylideneamino)ethyl)imidazolidine (H3L). We demonstrate that complex
3

1 constitutes an efficient catalytic system for the oxidation of benzyl alcohols to their
corresponding aldehydes in the presence of H2O2 with the purpose of developing a
new oxidation system in organic chemistry. Moreover, we also explore the effects of
halogen substitutes on the phenyl group of the benzyl alcohol based substrates over
the selectivity and activity of the title oxidovanadium(IV) complex.
2. Experimental
2.1. Materials and instrumentations
Vanadyl(IV)-bis(acetylacetonate) (VO(acac)2), 5-bromo-2-hydroxybenzaldehyde,
triethylentetramine and other starting materials were purchased from Acros and used
as received. Solvents of the highest grade commercially available (Merck) were used
without further purification. The Schiff-base ligand was synthesized according to our
1
13
previous report [23]. The H- and C-nuclear magnetic resonance (NMR) spectra of
the ligand in CDCl3 solution were measured on a Bruker 250 and 62.9 MHz
spectrometer, respectively, and the chemical shifts are indicated in ppm relative to
tetramethylsilane (TMS). Infrared (IR) spectra were recorded as KBr disks using a
Bruker FT–IR spectrophotometer. UV–Vis spectra of methanolic solutions of the
ligand and the title complex were recorded on a thermo spectronic Helios Alpha
spectrophotometer. The products of the oxidation reactions were determined and
analyzed by gas chromatography (GC) using a HP Agilent 6890 gas chromatograph
equipped with an HP-5 capillary column (phenyl methyl siloxane 30 m × 320 µm ×
0.25 µm ), as well as by gas chromatography-mass spectrometry (GC-MS) using a
Hewlett–Packard 5973 Series MS-HP gas chromatograph with a mass-selective
detector. Elemental analyses were carried out using a Perkin–Elmer 240 elemental
analyzer. The vanadium content of the complex was measured by atomic absorption
analysis by using a Varian Spectra AA-220 equipment. Electron paramagnetic
4

resonance (EPR) spectra were recorded on powdered crystalline samples using a
Bruker EMX continuous wave spectrometer equipped with an ER-510-QT resonator,
and they were simulated with the program WINEPR-Simfonia, v. 1.5 (Bruker
Analytische Messtechnik GmbH).
2.2. Synthesis of complex [(VO)2(HL)(µ-O)] (1)
For synthesis of complex 1, the ligand H3L (0.69 g, 1.00 mmol) and VO(acac)2 (0.53
g, 2.00 mmol) were placed in the main arm of a branched tube. Ethanol (96%) was
carefully added to fill the arms. The tube was sealed and the arm that contained the
reagents was immersed in an oil bath at 60 °C, while the other arm was kept at
ambient temperature. After 2 days, black crystals appeared in the coolest arm, and
they were separated and washed with ethanol. Yield: 78.87% (0.665 g). Anal. Calc.
for C27H25Br3N4O6V2 (MW = 843.11): C, 38.46; H, 2.99; N, 6.65; V, 12.08. Found:
C, 38.51; H, 3.02; N, 6.59; V, 12.13%. FT-IR (KBr, cm-1): 3431 (m, br), 2953 (m),
2921 (s), 2853 (m), 1633 (vs),1590 (m), 1558 (w), 1538 (w), 1526 (w), 1480 (m),
1457 (s), 1422 (w), 1381 (m), 1342 (w), 1305 (m), 1295 (m), 1198 (w), 1172 (w),
1093 (w), 1079 (w), 1066 (w), 1038 (w), 1017 (w), 971 (w), 956 (s), 973 (m), 873
(w), 840 (w), 820 (m), 803 (w), 775 (w), 761 (w), 748 (w), 733 (w), 684 (m), 668 (w),
650 (w), 566 (w), 546 (w), 517 (m),503 (m), 468 (w), 450 (w), 422 (m). UV-Vis
(CH3OH, c = 2.5×10-5 M, λmax [nm] with ε [M-1 cm-1]): 228 (71200), 283(20000), 342
nm (7440).
2.3. Single-crystal X-ray diffraction
A summary of the crystal data and refinement details for complex 1 is given in Table
1. The intensity data collection was performed on an Agilent Technologies
SuperNova diffractometer equipped with mirror-monochromated Cu Kα radiation (λ
5

= 1.54184 Å) and Atlas CCD detector. The measurement was carried out using the ω-
scan method at a controlled temperature of 100(2) K that was set with an Oxford
Cryosystems open-flow nitrogen cryostat. Data collection, unit cell refinement, data
reduction and corrections for Lorentz and polarization effects were performed with
the CrysAlisPro software package [24]. This software was also used to apply an
analytical absorption correction with face indexing to the data. The structures were
solved by direct methods using OLEX2 [25], and refined by a full-matrix least
squares technique on F2 with SHELXL-97 [26]. The refinement included anisotropic
thermal parameters for all non-H atoms. All H-atoms were found in difference Fourier
maps. In the final refinement cycles, the C-bonded H atoms were repositioned in their
calculated positions and refined using a riding model with C–H bond lengths
restrained to the standard values of 0.95(2) Å (aromatic C–H), 0.99(2) Å (methylenic
C–H) or 1.00(2) Å (methinic C–H). The thermal parameters were Uiso(H) =
1.2Ueq(C) in all cases. The O-bonded phenolic H atom was refined with the O–H
bond length restrained to 0.84(2) Å and a thermal parameter of Uiso(H) = 1.5Ueq(O).
Table 1.
2.4. General procedure for catalytic oxidation of alcohols
Oxidation reactions were carried out under aerial conditions using 3 mL of solvent
and aqueous H2O2 (30 wt%) as oxidant. In a typical experiment, complex 1 (1.186
µmol, 0.001 g), the solvent (3 mL) and the substrate (1 mmol) were placed in a 25 mL
round bottomed glass flask. The mixture was heated to the desired temperature, and
after addition of H2O2, the resulting solution was kept stable at the selected
temperature to continue the reaction. At appropriate intervals, aliquots were taken and
6

analyzed immediately by GC. The oxidation products were identified by comparison
of their retention times with those of authentic samples.
2.5. Electrochemical Properties
Voltammetric experiments were performed with a using EmStat3+ device from the
PalmSens Company (The Netherlands). A conventional three-electrode system was
used with a Nafion/complex 1/FTO electrode as working electrode, a platinum wire as
counter electrode and an aqueous Ag/AgCl saturated electrode as reference electrode.
The electrolytic medium consisted of a 0.1 mol·L–1 lithium perchlorate (LiClO4)
solution in deionized water as supporting electrolyte. The solution was freshly
prepared before use and purged with nitrogen saturated with solvent for ca. 10 min
prior to performing the measurements in order to remove any dissolved O2. The
voltammograms were recorded at room temperature in the range from –1.0 to 1.5 V
vs. Ag/AgCl.
3. Results and discussion
3.1. Syntheses and spectroscopy
The heptadentate ligand, H3L (Scheme 1), was synthesized by refluxing a mixture of
5-bromo-2-hydroxybenzaldehyde (3 mmol) with triethylentetramine (1 mmol) in
methanol. The ligand was characterized by spectroscopic methods and elemental
analysis as described in our previous report [23]. Complex 1 was obtained from the
reaction of VO(acac)2 and H3L in 2:1 molar ratio in ethanolic medium. It is worth
highlighting that this reaction seems to be solvent-dependent, as another closely
related complex, namely [(VO)2(L)(µ-OCH3)]·CH3OH, is obtained when methanol is
used as solvent instead of ethanol [23]. The latter complex features a fully
7

deprotonated, trianionic Schiff-base ligand and a bridging methoxy group instead of
the monoprotonated, dianionic ligand and bridging oxido group of complex 1.
Comparing the infrared spectra of the complex 1 with that of the free ligand, it was
possible to identify the presence of oxidovanadium groups on the basis of the strong
band at 956 cm-1, which was assigned to the stretching ν(V=O) vibration [27]. This
band appeared as an intrinsic new signal for the complex and was not present in the
spectrum of the free ligand. In addition, complex 1 exhibits a band at 820 cm-1 due to
the ν(V–O–V) vibration, which is in the normal range for oxido-bridged dinuclear
oxidovanadium compounds [28]. The signal corresponding to the C=N stretching in
the complex (1633 cm-1) was shifted to lower frequency relative to the free Schiff
base ligand, indicating coordination of the azomethine nitrogen atom to the metal ion
[29].
Scheme 1.
The electronic absorption spectra of H3L and complex 1 were recorded in methanolic
solution (see Fig. 1). Complex 1 is shiny and nearly black in the solid state, but its
methanolic solution is light-brown in color. The spectra of the Schiff base ligand and
its complex showed two bands in the visible region and an intense band in the near-
UV region. In the UV-Vis spectrum of complex 1, the absorption maxima at 228 and
283 nm were assigned as due to intraligand transitions. All of the bands shift in the
oxidovanadium(IV) complex relative to the free ligand indicates the coordination of
the ligand to the metal ion. The lowest energy transition around 342 nm was assigned
to a ligand-to-metal charge-transfer (LMCT) transition of the type O(phenolic) →V4+
[30].
Fig. 1.
8

3.2. Description of the structure
Single crystals of 1 suitable for X-ray diffraction structural analysis were obtained by
slow evaporation of ethanolic solution of the title complex during two days. The
molecular structure of 1 and the atom labeling scheme are shown in Fig. 2. Selected
bond lengths, bond angles and interatomic distances around the metal centers are
collected in Table 2.
Fig. 2.
Table 2.
Single-crystal X-ray diffraction studies confirmed that compound 1 is a neutral
dinuclear complex of oxidovanadium(IV) in which the Schiff base acts as a
heptadentate compartmental ligand that provides two tetradentate N2O2 cavities for
allocating two oxidovanadium, (V=O)2+, units. It is worth noting that the hydrogen
atom of central phenolic ring in the free H3L Schiff base is retained upon
complexation, and therefore, the ligand coordinates the oxidovanadium units in its
−
dianionic form (HL)2 . The two metal centers are connected together by the oxygen
atom of this central phenolic group, as well as by an exogenous bridging oxido ligand.
Coordination of phenolic oxygens to two metal ions without deprotonation has
previously been observed in several transition metal complexes [31] including a
copper(II) dimer with similar type of heptadentate Schiff base ligand [32]. However,
complex 1 constitutes the first example reported for such type of coordination in
vanadium complexes.
The title compound crystallizes in the orthorhombic Pnma space group with the
complex located at a mirror plane that contains both the bridging oxido and phenolic
groups. The crystallographically related vanadium centers are six-coordinated and
show a distorted octahedral geometry with one terminal oxygen atom from an external
9

phenoxido group (O1), one azomethine nitrogen atom (N1) and one aminic nitrogen
atom (N2) in equatorial positions. The exogenous µ2-oxido group (O1B) links the
remaining equatorial positions of the two vanadium atoms. The axial positions of the
octahedral coordination spheres are completed with one terminal oxido group on each
metal center (O1T) and the oxygen atom of the central phenolic group (O2B), which
provides an additional axial–axial linkage between both metal centers. The bridging
mode of the O1B and O2B atoms yields a syn-coplanar geometry for the dinuclear
complex. The resulting V2O2 metallacycle is nearly planar with an intramolecular
V···V distance of 3.091(2) Å and V–O–V angles of 114.6(3)° for O1B and 89.4(2)°
−O1T bond length (1.618(4) Å) is the shortest around the metal
for O2B. The axial V1
ion in good agreement with its double bond character. On the contrary, the trans-
−O
related V
bond is the longest around the metal center (2.197(4) Å) as a result
phenol
of the trans influence of the V=O moiety, and consequently, the vanadium atom
undergoes a displacement by 0.2688(11) Å from the mean plane defined by the
equatorial donor atoms towards the V=O bond.
The crystal packing of 1 is mainly dominated by Br···Br and C−H···O interactions.
The Br2 atoms of the central phenol rings are embedded in the cleft formed by the
phenoxido rings of neighboring complexes and interact with the Br1 atoms
(Br2···Br1i = Br·2··Br1ii = 4.0262(10) Å; i) 1/2+x, y, 3/2-z, ii) 1/2+x, 3/2-y, 3/2-z). As
shown in Fig. 3, zigzagging rows of complexes held together by these Br···Br
interactions are formed along the crystallographic a axis. These rows arrange in layers
−H···O
parallel to the ac plane through C
interactions that involve (a) the bridging
oxido ligand O2B and the methinic carbon atom C11 (C11···O2Biii = 3.117(10) Å,
<C11−H11−O2Biii> = 176°; iii) 1/2+x, 3/2-y, 3/2-z) and (b) the terminal oxido ligand
O1T and the methylenic carbon atom C9 (C9···O1TiV = 3.275(8) Å,
10

<C9−H9B−O1Tiv> = 161°; iv) 1/2+x, y, 1/2-z). The stacking of these layers along the
[010] direction is stabilized by additional Br···Br interactions involving the Br1 atoms
(Br1···Br1v = 3.8329(13) Å; v) 1-x, 1-y, 2-z).
Fig. 3.
The X-band EPR spectrum of complex 1 at room temperature shows a broad and
quasi-isotropic signal centered at ca. 3350 Gauss (Fig. 4). The spin Hamiltonian
parameters calculated from the comparison of the experimental spectrum with that
obtained from computer simulation are the following: g|| = 1.955 and g⊥ = 1.985 (<g>
= 1.975). These values are typical of oxidovanadium(IV) complexes involved in
nearly axial ligand fields with small anisotropy [33]. The signal becomes less intense
as the temperature decreases and this fact is indicative of the presence of
antiferromagnetic interactions. This behavior is consistent with the structural features
of complex 1 because the equatorial–equatorial exchange pathway between the
oxidovanadium(IV) units is expected to result in antiferromagnetic coupling
according to the bridging V1–O1B–V1i angle of 114.6(3)°.
Fig. 4.
3.3. Catalytic activity of complex 1
Complex 1 was used as catalyst precursor for the oxidation of alcohols. The catalytic
activity of this complex was tested by studying the oxidation of benzyl alcohol
derivatives as model substrates in the presence of hydrogen peroxide as a selective
and environmentally benign oxidant (Scheme 2). Control experiments (in the absence
of complex or oxidant) revealed that the presence of both complex 1 and oxidant
(H2O2) is essential for the oxidation of benzylic alcohols to proceed. The oxidation of
benzyl alcohol did not take place in the absence of H2O2 or in the blank reaction
without complex, but the reaction proceeded when both complex 1 and hydrogen
11

peroxide were added to the reaction flask. Although the details of the catalytic
mechanism and possible intermediates are not studied in the present work, the
oxidation of the substrate can be predicted to occur by oxidoperoxidovanadium-Schiff
base species formed in the reaction media on the basis of the electronic absorption
spectroscopic analyses (Fig. S1), and also by considering some other previous studies
[27,34]. Electronic absorption spectroscopy indicates that the solution of complex 1 is
sensitive towards the addition of H2O2. By increasing the amount of diluted H2O2
added to the solution of complex 1, the intensity of the absorption band below 300 nm
increases while the intensity of the band at about 350 nm decreases. These changes
can be attributed to the formation of peroxidovanadium-Schiff base species upon
addition of H2O2 to the solution of complex 1, which can in turn oxidize benzyl
alcohol to benzaldehyde.
In order to obtain the most suitable reaction conditions and to maximize the yields in
the oxidation of alcohols, some parameters such as the concentration of oxidant
(moles of oxidant per moles of substrate), solvent and temperature of the reaction
were investigated. These reactions were carried out by using benzyl alcohol as the
model substrate and the results of these studies are summarized in Table 3.
Table 3.
Scheme 2.
The effect of substrate-to-oxidant ratio is one of the key parameters influencing the
results of this type of oxidation reaction. Thus, oxidations of benzyl alcohol with
different H2O2 : substrate molar ratios (1 : 1, 2: 1, 3 : 1 and 4 : 1) were carried out at
70 °C in the presence of complex 1 and 3 mL of acetonitrile as the solvent. GC
analyses showed that benzaldehyde was the main product of the catalytic reaction, but
nevertheless, some amounts of benzoic acid were also detected as the side product of
12

the oxidation. This side product is most likely formed by over-oxidation of
benzaldehyde as its amount increased with increasing the reaction time. The time
dependence of the catalytic oxidation shows that the selectivity toward the main
product changes with increasing the reaction time. The benzaldehyde : benzoic acid
ratio remained relatively constant up to 4 hours, beyond which the amount of benzoic
acid considerably increased. Moreover, the amount of oxidant has also significant
effect on the formation of benzoic acid in the reaction media. The amount of acid
increased with increasing the amount of oxidant and this fact shows that the excess of
H2O2 is directly involved in the over-oxidation of benzaldehyde. Therefore, the lowest
selectivity toward benzaldehyde was observed with an oxidant : substrate molar ratio
of 4 : 1 (Table 3, entry 4). The maximum conversion of benzyl alcohol was obtained
at 70 1 °C when the H2O2 : benzyl alcohol ratio was 2 : 1 (Fig. 5). Hence, this ratio
was selected as the optimum amount of oxidant in the reaction.
Temperature is another parameter with strong influence on the progress of catalytic
reactions. Thus, the oxidation of benzyl alcohol was studied at 25 (room temperature),
40, 60, 70 and 80 °C. The results are shown in Table 3 and Fig. 6 and they indicate
that the oxidation of benzyl alcohol at room temperature is very low. Increasing the
reaction temperature from 25 to 70 °C resulted in higher conversions of benzyl
alcohol and a yield of 87% was obtained at 70 °C. The results listed in Table 3 also
show that the selectivity of the reaction toward benzaldehyde decreases with
increasing the temperature and the largest amount of benzoic acid is indeed obtained
at 80 °C. Considering that the higher selectivity toward benzaldehyde is observed at
60 °C, this temperature can be identified as the most suitable for performing the
oxidation reactions of benzyl alcohols.
13

Solvent is often an additional parameter with a crucial role in the conversion and
product distribution of the oxidation reactions. In order to study its effect on this
catalytic system, the experiments were carried out using various solvents including
acetonitrile, toluene, chloroform, dichloromethane, acetone, methanol, DMSO, THF
and ethyl acetate. The selected solvent should possess certain criteria, such as: (i) it
should dissolve all the materials (substrate, catalyst and oxidant); and (ii) it should be
stable during the reaction process. The better solubility of substrate and oxidant in the
solvent of choice would facilitate that they could easily approach the active sites of
the catalyst. As shown in Table 3, the highest conversion was observed in acetonitrile
(entry 2) and the catalytic activity of complex 1 decreased in the following order:
acetonitrile > THF > ethyl acetate > acetone > methanol > chloroform > toluene >
dichloromethane > DMSO (Fig. 7). The higher catalytic activity in acetonitrile is in
agreement with other studies on oxidation reactions catalyzed by vanadium
complexes [27,3c] and is attributed to the high dielectric constant and low donor
number of this solvent [35].
Fig. 5.
Fig. 6.
Fig. 7.
It must also be pointed out that increasing the amount of complex added to the
reaction mixture led to higher conversions of benzyl alcohol. For example, the use of
0.0005 g, 0.0010 g, 0.0015 g and 0.0020 g of complex 1 at 60 °C resulted in
conversions of 44%, 85%, 88% and 92%, respectively. Changing the amount of
complex did not have any effect on the selectivity towards the aldehyde as the
obtained selectivity was constant in all of the cases.
14

3.4. Oxidation of various primary alcohols
In order to explore further the potential of complex 1 in catalyzing the oxidation of
alcohols, various primary benzyl alcohol derivatives were selected as substrates and
their oxidation was performed under the optimized reaction conditions determined for
benzyl alcohol. More specifically, we were particularly interested in investigating the
electronic and steric effects of halogen substituents on the phenyl ring, and hence we
selected one set of benzyl alcohol derivatives with F, Cl and Br atoms in the relative
para- position (electronic effect) and another set of substrates with Cl in relative
ortho-, meta, and para- positions (steric effect). The summary of these studies is
collected in Table 4. The profile for the H2O2-based oxidation of benzyl alcohol
catalyzed by complex 1 is depicted in Scheme 2, which shows that the reaction
proceeds via two steps: (1) oxidation of benzyl alcohol to benzaldehyde and (2)
oxidation of benzaldehyde to benzoic acid. According to Table 4, substituted benzyl
alcohols turned out to be readily oxidized into aldehydes in good yields, whereas their
over-oxidation into the acidic derivative proved to be inefficient as the acid was only
obtained as a minor side-product. Both the selectivity and the efficiency of this
catalytic system appear to be dependent on the electronic and steric demands of the
substrate. The halogen substituents have two electronic effects on the phenyl ring: 1)
resonance donation, and 2) inductive withdrawal. In the aromatic alcohols, halogen
substituents in the relative ortho and para positions act as electron-donor groups and
provide major resonance contribution due to the availability of their lone pairs. The
position of the substituent in the phenyl ring has also certain steric effects on the
reactivity of the benzyl alcohol derivatives. The substrates with chloride or fluoride
substituents in the ortho position showed poor reactivity in comparison with the
corresponding meta- and para-substituted benzyl alcohols, and this fact can be
15

attributed to the higher steric hindrance in the former case. Also, the substrate with the
chloride substituent in the meta-position had low conversion in comparison with the
para-substituted derivative, and this fact can be attributed to the higher electron-
withdrawing effect of the Cl group at this position toward the -CH2OH moiety. As
shown in Table 4, the phenyl rings bearing fluoride substituents had the lowest
conversion in comparison with the substrates containing other halogens (Cl and Br).
This is related to the electronegativity of fluorine being higher than those of other
halogen atoms. All of these findings show that electron-withdrawing groups decrease
the conversion in the oxidation of benzyl alcohols with H2O2 catalyzed by complex 1,
but it turn increase the selectivity of the reaction toward the aldehyde product.
Table 4.
3-5 Electrochemical Studies
Electrochemical cyclic voltammetry measurements were carried out to probe the
redox behavior of complex 1. The cyclic voltammogram of the complex displays two
quasi-reversible redox peaks due to two VO2+–VO3+ couples (Fig. 8). Two anodic
peaks appear at potentials of –0.05 and +0.40 V versus Ag/AgCl and they can be
attributed to the oxidation of two vanadium(IV) cores in the structure of complex 1.
During the reverse scan, reduction waves appear at –0.7 and +0.25 V, which can be
attributed to the reduction of two V(V) centers to V(IV).
Fig 8.
Conclusion
In summary, a new dinuclear oxidovanadium(IV) complex with a N4O3-heptadentate
Schiff-base ligand was synthesized and characterized by elemental analysis,
spectroscopic methods and single crystal X-ray diffraction. The latter technique
revealed that one exogenous oxido group and one phenolic oxygen atom of the Schiff-
base ligand bridge the axial and equatorial positions of the two vanadium centers,
16

respectively. The catalytic activity of this complex was studied in the oxidation of
benzyl alcohol derivatives with H2O2 as oxidant. The effect of various critical
parameters in catalytic reactions was studied to determine the optimum conditions for
this catalytic system. The results showed that reaction conditions such as the oxidant-
to-substrate molar ratio, the temperature, or the solvent, together with the electronic
and steric effects of the substrate are key parameters affecting the activity and
selectivity of this catalytic system.
Acknowledgment
The authors are grateful to the University of Zanjan for financial support of this study.
S.R. acknowledges financial support from the program “Ayudas para la Captación del
Talento Adscritas a los Institutos de Investigación de la Universidad Pública de
Navarra” funded by a collaboration agreement with Obra Social la Caixa and
Fundación Caja Navarra. Technical and human support provided by SGIker
(Universidad del País Vasco UPV/EHU) is also gratefully acknowledged. The authors
thank Luis Lezama (UPV/EHU) for the EPR measurements.
Appendix A. Supplementary data
CCDC 1477659 contains the supplementary crystallographic data for complex 1.
These data can be obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/structures.
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21

Tables/Schemes/Figures caption
Table 1. Crystallographic data of 1
Table 2. Selected bond lengths, bond angles and interatomic distances for complex 1
Table 3. Comparison of the catalytic activities of complex 1 in the oxidation of
benzyl alcohol with H2O2 under different conditions.
Table 4. Oxidation of various primary benzyl alcohol derivatives catalyzed by 1.
Scheme 1. Synthetic pathways of the ligand H3L and the complex 1
Scheme 2. Oxidation of benzyl alcohol with H2O2 catalyzed by complex 1
Fig. 1. Electronic spectra of the H3L ligand (red) and complex 1 (brown) in methanol.
Fig. 2. (a) Molecular structure of complex 1 with atom labeling (symmetry code: i) x,
3/2−y, z); (b) Detail of the coordination environment around the V(IV) atoms
highlighting the syn-coplanar conformation of the terminal oxido ligands (O1T and
O1Ti).
Fig. 3. Projection of a layer of complexes on the crystallographic ac plane of 1 and
detail of the embedding of the Br2 atom of one complex in the cleft in between the
phenoxido rings of a neighboring complex (Br···Br and C−H···O interactions
depicted as dashed olive green and red lines, respectively).
Fig. 4. Thermal dependence of the X-band EPR spectra of 1 (left) and fitting of the
spectrum recorded at room temperature (right).
Fig. 5. Effect of the oxidant (H2O2) concentration on the oxidation of benzyl alcohol
in acetonitrile catalyzed by complex 1. Reaction conditions: catalyst: 186 µmol (0.001
g); solvent: 2 mL; substrate: 1 mmol; temperature: 70 2 °C.
Fig. 6. Effect of the temperature on the oxidation of benzyl alcohol with H2O2 in
acetonitrile catalyzed by complex 1. Reaction conditions: catalyst: 186 µmol (0.001
g); solvent: 2 mL; substrate: 1 mmol; oxidant: 3 mmol.
Fig. 7. Effect of the solvent on the oxidation of benzyl alcohol with H2O2 catalyzed
by complex 1. Reaction conditions: catalyst: 1.186 µmol (0.001 g); substrate: 1 mmol;
oxidant: 2 mmol; temperature: 60 2 °C.
Fig. 8. Cyclic voltammograms of complex 1 with LiClO4 (0.1 mol L-1) as the
supporting electrolyte; scan rate: 30, 50, 100, 200, 300, 400 and 500 m Vs-1.
22

Table 1. Crystallographic data of 1
Complex 1
Formula
C27H25Br3N4O6V2
843.11
0.33 × 0.15 ×0.13
Mr/g mol−1
crystal size/mm3
crystal shape, color
T/K
block, black
100(2)
radiation
diffractometer
Cu Kα (λ = 1.54184 Å)
SuperNova
crystal system
space group
orthorhombic
Pnma
a/Å
12.3885(5)
b/Å
18.3281(7)
c/Å
12.7570(4)
90
α/°
90
β/°
90
γ/°
V/Å3
2896.57(18)
4
Z
ρcalc/g cm−3
1.933
10.683
µ/mm−1
1656
F(000)
4.2–68.5
θ range/°
(h,k,l)
Rint
(−14, −22, −15)→(14, 22, 13)
0.091
0.063
0.176
R(Fobs)
wR(F2)
1.048
S
absorption correction
hydrogen refinement
collected reflections
independent reflections
reflections with I > 2σ(I)
parameters
Analytical
Mixed
23340
2758
2204
207
restraints
1
max electron density/e Å−3
min electron density/e Å−3
2.092 (at 0.98 Å from Br2)
−0.782 (at 0.79 Å from Br2)
23

Table 2. Selected bond lengths, bond angles and interatomic distances for complex 1
Bond Lengths (Å)
Bond Angles (°)
V1–O1T
V1–O1B
V1–O1
V1–N1
V1–N2
V1–O2B
1.618(4)
1.837(3)
1.922(4)
2.096(5)
2.189(5)
2.197(4)
O1T–V1–O1B
O1T–V1–O1
O1T–V1–N1
O1T–V1–N2
O1T–V1–O2B
O1B–V1–O1
O1B–V1–N1
O1B–V1–N2
O1B–V1–O2B
O1–V1–N1
102.7(2)
101.0(2)
96.6(2)
89.3(2)
171.9(2)
99.5(2)
157.4(2)
90.2(2)
77.08(18)
88.1(2)
Interatomic Distances (Å)
V1···V1i
3.091(2)
O1–V1–N2
O1–V1–O2B
N1–V1–N2
N1–V1–O2B
N2–V1–O2B
V1–O1B–V1i
V1–O2B–V1i
163.90(19)
87.02(19)
78.40(19)
82.15(17)
82.59(19)
114.6(3)
89.4(2)
−
Symmetry code: i) x, 3/2 y, z.
Table 3. Comparison of the catalytic activity of complex 1 in the oxidation of benzyl
alcohol with H2O2 under different conditions.
Conversion(%)[a] TON[b] Selectivity(%)[c]
Aldehyde/acid ratio
Entry H2O2 (mmol) Solvent
T (°C)
24

1
2
3
4
5
6
7
8
1
2
3
4
2
2
2
2
2
2
2
2
2
2
2
2
2[d]
CH3CN 70
CH3CN 70
CH3CN 70
CH3CN 70
CH3CN 25
CH3CN 40
CH3CN 60
CH3CN 80
49
87
78
76
15
48
85
89
45
65
30
78
70
38
51
6
263.4
467.7
419.3
408.6
80.6
258
457
478.5
241.9
349.5
161.3
419.4
376.3
204.3
274.2
32.3
89.7
89.6
82
44/5
78/9
64/14
60/16
15/0
45/3
80/5
78/11
38/7
60/5
30/0
68/10
63/7
35/3
49/2
5/1
79
100
93.7
94.1
87.6
84.4
92.3
100
87
90
92.1
96
83.3
-
9
CHCl3
60
10
11
12
13
14
15
16
17
Acetone 60
CH2Cl2 60
THF
60
EtOAc 60
Toluene 60
CH3OH 60
DMSO 60
CH3CN 60
<0.1
-
-
Reaction conditions: solvent: 3 mL; catalyst: 1.186 µmol (0.001 g); time: 4 h
[a] Conversion and yield were determined by GC analysis (see Scheme 2).
[b] (mmol of product)/(mmol of catalyst)
mol Aldehyde
[c] Selectivity =
×100
mol Aldehyde+ mol Acid
[d] Blank tests (in the absence of any catalyst precursor)
Table 4. Oxidation of various primary benzyl alcohol derivatives catalyzed by 1.
Entry Substrate Conversion (%); Products (%)
(Selectivity)
Aldehyde
Acid
25

1
2
85; (94.1)
77; (95)
(80)
(5)
3
4
61; (98)
O
63; (100)
H
–
Cl
(63)
5
80; (94)
58; (100)
45; (100)
6
7
–
–
Reaction conditions: catalyst: 1.186 µmol, 0.001 g; solvent: CH3CN; oxidant: H2O2 (2
mmol); Temperature: 60 °C.
26

N
N
N
N
Br
H
Br
O
CH3OH
-3 H2O
N
NH2
OH
Br
H2N
N
+ 3
HO
H
OH
OH
Br
H3L
CH3CH2OH/H2O
H3L + 2[VO(acac)2]
[(VO)2(HL)( O)] + 4Hacac
Complex 1
Scheme 1. Synthetic pathways of the ligand H3L and the complex 1
Scheme 2. Oxidation of benzyl alcohol with H2O2 catalyzed by complex 1
Fig. 1. Electronic spectra of the H3L ligand (red) and complex 1 (brown) in methanol.
27

Fig. 2. (a) Molecular structure of complex 1 with atom labeling (symmetry code: i) x,
−
3/2 y, z); (b) Detail of the coordination environment around the V(IV) atoms
highlighting the syn-coplanar conformation of the terminal oxido ligands (O1T and
O1Ti).
28

Fig. 3. Projection of a layer of complexes on the crystallographic ac plane of 1 and
detail of the embedding of the Br2 atom of one complex in the cleft in between the
phenoxido rings of a neighboring complex (Br···Br and C−H···O interactions
depicted as dashed olive green and red lines, respectively).
29