Synthesis, spectral, thermal and thermodynamic studies of oxovanadium(IV) complexes of Schiff bases derived from 3,4-diaminobenzoic acid with salicylaldehyde derivatives

Article abstract of DOI:10.1016/j.saa.2013.01.035

Synthesis and evaluation of three new oxovanadium(IV) complexes, formed by the interaction of vanadyl acetylacetonate and the Schiff bases: 3,4-bis((E)-2-hydroxybenzylideneamino)benzoic acid (L¹), 3,4-bis-((E)-2-hydroxy-3-methoxybenzylideneamin

Full text of DOI:10.1016/j.saa.2013.01.035

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 107 (2013) 145–150
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Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, spectral, thermal and thermodynamic studies of oxovanadium(IV)
complexes of Schiff bases derived from 3,4-diaminobenzoic acid with
salicylaldehyde derivatives
⇑
⇑
Khosro Mohammadi , Mahmood Niad , Amene Irandoost
Chemistry Department, Faculty of Sciences, Persian Gulf University, Bushehr 75169, Islamic Republic of Iran
g r a p h i c a l a b s t r a c t
h i g h l i g h t s
The synthesis and evaluation of three new oxovanadium(IV) complexes, formed by the interaction of van-
adyl acetylacetonate and the Schiff bases, Khosro Mohammadi, Mahmood Niad and Amene Irandoost.
"
"
"
Three new oxovanadium(IV)
complexes have been synthesized
and characterized.
From TG curves, the order of thermal
stability for the complexes is
VOL3 > VOL1 > VOL2.
The trend of formation constants of
the complexes are as follow:
VOL3 > VOL2 > VOL1.
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthesis and evaluation of three new oxovanadium(IV) complexes, formed by the interaction of vanadyl
acetylacetonate and the Schiff bases: 3,4-bis((E)-2-hydroxybenzylideneamino)benzoic acid (L1), 3,4-bis-
((E)-2-hydroxy-3-methoxybenzylideneamino)benzoic acid (L2) and 3,4-bis((E)-2,4-dihydroxybenzylide-
neamino)benzoic acid (L3) in methanol. The complexes have been characterized and studied by IR spec-
tra, UV–Vis spectroscopy and thermogravimetry in order to evaluate their thermal stability and thermal
decomposition. According to the results discussed from TG curves, the order of thermal stability for the
complexes is VOL3 > VOL1 > VOL2. Their formation constants (Kf) were obtained by UV–Vis spectroscopic
titration at 15, 25, 35 and 45 °C in methanol by SQUAD software. The trend of formation constants of the
complexes as follows: VOL3 > VOL2 > VOL1.
Received 1 September 2012
Received in revised form 28 December 2012
Accepted 11 January 2013
Available online 23 January 2013
Keywords:
Oxovanadium(IV)
Schiff base
Thermogravimetry
Formation constant
Ó 2013 Elsevier B.V. All rights reserved.
Introduction
groups which may be variously substituted. These compounds are
also known as anils, imines or azomethines. Several studies [1]
Schiff bases are the condensation products of primary amines
with carbonyl compounds. The common structural feature of these
compounds is the azomethine group with a general formula
RHC@NAR0 where R and R0 are alkyl, aryl, cyclo alkyl or heterocyclic
showed that the presence of a lone pair of electrons in an sp2 hybrid-
ized orbital of nitrogen atom of the azomethine group is of consider-
able chemical and biological importance. Because of the relative
easiness of preparation, synthetic flexibility, and the special proper-
ties of C@N group, Schiff bases are generally excellent chelating
agents [2,3] especially when a functional group like AOH or ASH is
present close to the azomethine group so as to form a five or six
member ring with the metal ion. Versatility of Schiff base ligands
⇑
Corresponding authors. Tel.: +98 7714222340; fax: +98 7714541494.
E-mail addresses: Khmohammadi@pgu.ac.ir, khmohammadi@yahoo.com (K.
Mohammadi).
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2013.01.035

146
K. Mohammadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 107 (2013) 145–150
and biological, analytical and industrial applications of their com-
plexes make further investigations in this area highly desirable.
Transition metal complexes of vanadium with Schiff bases have
been amongst the most widely studies of coordination compounds
in the past few years, since they are found to be of importance as
analytical and antimicrobial agents [4,5]. Considerable efforts have
been made to study the behavior and functions of vanadium in bio-
logical system [6,7] as well as in catalytic and pharmaceutical
applications [8,9].
Vanadium is recognized as an essential trace element for differ-
ent organism [10]. Vanadium complexes are also of major concern
because of their adverse effect on the hydroprocessing catalysts
used in the refining crude oil [11,12].
3,4-bis((E)-2-hydroxy-3-methoxybenzylideneamino)benzoic acid
oxovanadium(IV), [VOL2]
Color: green; yield: 65%, m.p.: >250 °C. FT-IR (KBr, cmꢀ1): 2400–
3700(mOAH(acid)), 1680(mCAO), 1600(mCAN), 1480, 1540(mCAC),
1250(
m
CAO), 978(mVAO), 550(mCANAM), 450(mCAOAM). UV–Vis: (kmax,
nm, MeOH): 305, 374. Mass spectra (ESI): m/z(%) = 485 [M]+, 425,
381, 303, and 99.
3,4-bis((E)-2,4-dihydroxybenzylideneamino)benzoic
acid)oxovanadium(IV), [VOL3]
Color: green; yield: 60%, m.p.: >250 °C. FT-IR (KBr, cmꢀ1): 2400–
3700(mOAH(acid)), 1610(mCAO), 1600(mCAN), 1460, 1490(mCAC),
1250(
m
CAO), 990 (mVAO), 530(mCANAM), 460(mCAOAM). UV–Vis: (kmax,
nm, MeOH): 262, 345. Mass spectra (ESI): m/z(%) = 457 [M]+, 413,
In this study, we have described synthesis, spectroscopic and
formation constant measurements of three new oxovanadium(IV)
complexes, formed by the interaction of vanadyl acetylacetonate and
the Schiff bases: 3,4-bis((E)-2-hydroxybenzylideneamino)benzoic
acid (L1), 3,4-bis((E)-2-hydroxy-3-methoxybenzylideneamino)benzoic
335, and 99.
Results and discussion
acid
(L2)
and
3,4-bis((E)-2,4-dihydroxybenzylideneamino)
IR spectra
benzoic acid (L3) which have been characterized by IR, Mass,
UV–Vis spectroscopy, thermogravimetry and kinetic studies.
In the FT-IR spectral data of complexes, a band at 2400–
3700 cmꢀ1 related to OAH(acid) group. The strong bands around
1600 cmꢀ1 is assigned to azomethine (C@N) groups [15]. The ring
skeletal vibrations (C@C) were in the region of 1400–1600 cmꢀ1
[16], and the bands at 1220–1250 cmꢀ1 range can be related to
the phenolic (CAO) group vibrations [15]. IR spectra of vanadyl
Schiff base complexes show two kinds of V@O stretching bands
around (960–990 cmꢀ1) for the monomeric forms and (850–
880 cmꢀ1) for the polymeric forms [17,18]. In this study, the strong
bands at 970–990 cmꢀ1 can be related to the (V@O) group [19].
Weak bands in the 530–555 cmꢀ1 and 450–460 cmꢀ1 ranges can
Experimental
Materials and reagents
All chemicals used were of the analytical reagent grade (AR), and
of highest purity available. They included vanadyl acetylacetonate,
3,4-diaminobenzoic acid, 2-hydroxybenzylaldehyde, 2-hydroxy-
3-methoxybenzylaldehyde and 2,4-dihydroxybenzylaldehyde. All
reagents and solvents were commercially obtained from Merck, Al-
drich or Fluka. Spectrograde solvents were used for spectral
measurements.
be attributed to mCANAM and mCAOAM, respectively [20]. IR spectrum
of [VOL1] is shown in Fig. 1.
Physical measurements
Mass spectra
The mass spectrum of [VOL1] gives isotropic peak of m/z at 425
[M+] which is corresponding to the composition [VOL1] (molecular
mass = 425) (see Fig. 1-sup). Moreover, another observed peaks are
in 381(C20H14N2VO3), 303(C14H8VO3), and 99(VO3). The mass spec-
trum of [VOL2] gives isotropic peak of m/z at 485 [M+] which is cor-
responding to the composition [VOL2] (molecular mass = 485).
Moreover, another observed peaks are in 425(C21H14N2VO5),
381(C20H14N2VO3), 303(C14H8VO3), and 99(VO3). The mass spec-
trum of [VOL3] gives isotropic peak of m/z at 457 [M+] which is cor-
responding to the composition [VOL3] (molecular mass = 457).
Moreover, another observed peaks are in 413(C20H14N2VO5),
335(C14H8N2VO5), and 99(VO3).
UV–Vis measurements were carried out in Perkin–Elmer Lamb-
da 25 UV–Vis spectrophotometer. The NMR spectra were recorded
by a Bruker Avance DPX-500 spectrometer in [D6]-DMSO and
CDCl3 solvent using TMS as an internal standard at 500 MHz. IR
spectra were measured from 4000 to 400 cmꢀ1 as KBr pellets on
a Shimadzu FTIR-8300 spectrophotometer. Mass spectra were ob-
tained with AGILENT 5973 instrument and thermogravimetry were
carried out by a PL-1500. The measurements were performed in air
atmosphere and the heating rate was held at 10 °C minꢀ1
.
Synthesis of vanadyl Schiff base complexes
All of the ligands were prepared by the reaction of 3,4-diamino-
benzoic acid and salicylaldehyde derivatives [13,14] (Scheme 1-
sup). The complexes were prepared by the reaction of Schiff base
ligands with vanadyl acetylacetonate in methanol. To a methanolic
solution (50 ml) of vanadyl acetylacetonate (1 mmol, 0.267 gr) was
added to the ligand (1 mmol in 30 ml methanol) and the mixture
was stirred for 3 h at 50 °C. The resulting precipitate was collected
by filtration, washed with H2O and dried in vacuum (Scheme 2-
sup).
The electronic spectra
Electronic absorption spectral data of the complexes are very
similar to each other because of their structural identity. The
strong or a shoulder absorption band observed below 300 nm is as-
signed to intraligand
p ?
pꢁ transitions of the phenolic chromoph-
ores. The absorption band observed in all ligand spectra within the
range of 260–340 nm is most probably due to the transition of
n ? pꢁ of imine group [21].
Charge-transfer bands are expected for vanadium complexes,
which can be seen in the region of 345–405 nm [22].
3,4-bis((E)-2-hydroxybenzylideneamino)benzoic acid
oxovanadium(IV), [VOL1]
Color: black green; yield: 60%, m.p.: >250 °C. FT-IR (KBr, cmꢀ1):
2400–3700(
m
OAH(acid)), 1607(
m
C@O), 1580(
m
C@N), 1440, 1465(
m
C@C),
Thermal gravimetry
1200(
m
CAO), 975(
m
V@O), 555(
m
CANAM), 450(mCAOAM). UV–Vis: (kmax,
nm, MeOH): 273, 404. Mass spectra (ESI): m/z(%) = 425 [M]+, 381,
The thermal decomposition of the studied complexes presented
pathways. All TG and DTA figures of the compounds had the same
303, and 99.

K. Mohammadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 107 (2013) 145–150
147
Fig. 1. IR spectrum of [VOL1].
trend. All the complexes were decomposed in two or three steps
[23,24]. The TG and DTA thermograms of [VOL1] complex are
shown in Fig. 2. At 700 °C, the stable metal oxide V2O5 was formed
[25].
In the thermogram of [VOL3], the lattice water molecule
decomposed between 90 and 110 °C. The first mass loss occurs
between 330 and 350 °C that corresponding to the decomposi-
tion of 25% (calculated: 26.3%) and the second step occurs be-
tween 390 and 430 °C that corresponding to the decomposition
of 54% (calculated: 53.8%) in TG and finally the stable metal
oxide V2O5 was formed.
In the thermogram of [VOL1], two steps were observed, the first
mass loss occurs between 310 and 340 °C that corresponding to the
decomposition of 17% (calculated: 18.4%) in TG. The second mass
loss occurs between 390 and 420 °C that corresponding to the
decomposition of 57% (calculated: 60.2%). At 650–700 °C, the sta-
ble metal oxide, V2O5 was formed.
ꢂC
390—430 ꢂC
!
C21H14N2O7V 330—350
!
C
14
H10N2O5V
V2O5
ꢀC7H4O2
ꢀC14H10N2O5=2
ꢂC
390—420 ꢂC
!
C21H14N2O5V 310—340
!
C
H8N2O5V
V2O5
15
Kinetic studies
ꢀC6H6
ꢀC15H8N2O5=2
Broido has suggested a simple and sensitive graphical method
for the treatment of TGA data [27]. According to this method, the
weight at any time, t(Wt) is related to the fraction of initial mole-
cules not yet decomposed (y) by the equation:
In the thermogram of [VOL2], the lattice water molecule decom-
posed between 90 and 110 °C [19,26]. Then three consecutive steps
were observed during the thermal decomposition. The first mass
loss (TG = 10%, calculated: 9.1%) occurs between 150 and 200 °C.
The second mass loss (TG = 15%, calculated: 15.6%) occurs between
270 and 300 °C. The last mass loss (TG = 53%, calculated: 56.5%) oc-
curs between 390 and 450 °C with formation V2O5 (TG = 22%, cal-
culated: 18.8%).
N
ðWt ꢀ W1Þ
y ¼
¼
ð1Þ
N0 ðW0 ꢀ W1Þ
where W0 is the initial weight of the materials and W is the weight
1
of residue at the end of decomposition.
If the pyrolysis is carried out isothermally, the rate of the reac-
tion is given by:
ꢂC
ꢂC
C23H18N2O7V 150—200
!
C
H18N2O5V 270—300
!
ꢀC6H4
C H14N2O5V
16
22
ꢀCO2
dy
dt
¼ kyn
ð2Þ
330—350 ꢂC
C
16H14N2O5V
!
V2O5
ꢀC16H14N2O5=2
Fig. 2. The TG and DTA thermograms of VOL1 complex.

148
K. Mohammadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 107 (2013) 145–150
ꢀ
ꢁ
Z
2
where n is the reaction order. The rate constant, k, changes with
absolute temperature according to the equation of Arrhenius.
1
y
A
u
Tm
T
E
RT
ln
¼
eꢀ dT
ð10Þ
k ¼ A ꢃ eꢀE=RT
and if T is linear function of time, t, therefore:
ð3Þ
ð4Þ
ð5Þ
changing variable has
ꢀ
ꢁ
1
T
1
T2
1
x2
x ¼ ) dx ¼ dT ) dT ¼ ꢀ
dx
ð11Þ
ð12Þ
T ¼ T0 þ u ꢃ t
ꢀ
ꢁ
Z
u is the heating rate. The first derivative of this equation is:
1
y
A
u
1
x2
E
R
ln
ln
ln
¼
¼
¼
T2m x2eꢀ X
ꢀ
dx
dT ¼ u ꢃ dt
This rearranges to:
1
y
A ꢃ R ꢃ T2m
E ꢃ u
E
R
ꢀ ꢁ
ꢃ eꢀ X
ð13Þ
ð14Þ
dy
yn
A
u
E
RT
¼ ꢀ
ꢃ eꢀ dt
ð6Þ
1
y
A ꢃ R ꢃ T2m
E ꢃ u
E
1
The thermogravimetric analysis curve for this reaction repre-
sents the last equation integrated from a temperature T0, where
y = 1, to a temperature for another value of y.
ꢃ eꢀ
RðTÞ
This equation can be represented by:
Z
Z
Z
1
T
T0
ꢀ
ꢁ
ꢀ ꢁ
dy
yn
A
u
A
u
E
E
eꢀ dT ¼
eꢀ dT
ð7Þ
RT
RT
1
y
E
R
1
T
¼ ꢀ
ln ln
¼ ꢀ
þ const:
ð15Þ
y
T0
T
The main consideration of this method is that the reaction is of
the first order. With this supposition, the left side of the reaction
can be resolved.
Eq. (15) is a straight line. The gradient of the graph ln[ln(1/y)]
vs. (1/T) is the activation energy E#a and the intercept is frequency
factor, A.
ꢀ ꢁ
Z
Z
1
1
dy
yn
dy
y
1
y
ꢀE#a ¼ slope ꢅ 2:303 ꢅ R
where E#a is activation energy and R is the gas constant [29]. Appli-
cation of this method used to determine the kinetic parameters for
complexes.
ð16Þ
¼
¼ ꢀ ln y ¼ ln
ð8Þ
y
y
There are various methods for resolving the right side of the
equation. Broido’s method is based on approximations done by
other authors. We have utilized the approximations introduced
by Horowitz and Metsger [28].
The entropy of activation
equation:
D
S# was calculated using the
ꢀ
ꢁ
2
E
m
Tm
T
E
RT
#
eꢀ
ꢄ
eꢀ
ð9Þ
kT
A ¼ m eD
h
S
R
RT
ð17Þ
Tm is the temperature at which the maximum reaction rate oc-
curs. Introducing this approximation has
where k, h and Tm are the Boltzmann’s constant, the Planck’s con-
stant and the peak temperature, respectively.
Fig. 3 shows Broido plots of [VOL1] complex.
The enthalpy and free energy of activation were calculated
using equations:
D
D
H# ¼ E#a ꢀ RT
ð18Þ
G#
¼
D
H# ꢀ T
D
S#
ð19Þ
The results were reported in Table 1.
The positive values of
D
H# mean that the decomposition pro-
cesses are endothermic and high values of the activation energies
reflect the thermal stability of the complexes.
Thermodynamic interpretation
The thermodynamic parameters are useful tools for studying
the interactions between the donor and the acceptor species. The
first step in this study is the determination of formation constants
Fig. 3. Broido plots of [VOL1] complex.
Table 1
Thermal study of oxovanadium(IV) complexes.
Compounds
VOL1
Step
D
T (°C)
D
H# (kJ molꢀ1
)
D
S#(JKꢀ1 molꢀ1
)
D )
G# (kJ molꢀ1
E#a (kJ molꢀ1
)
I
II
310–340
390–420
497.8
80.4
492.6
74.6
60.6
454.8
242.2
ꢀ241.8
VOL2
VOL3
I
II
III
150–200
270–300
390–450
17.2
42.1
76.6
13.3
37.3
70.6
ꢀ300.9
ꢀ277.7
ꢀ256.9
125.5
ꢀ183.9
155.6
196.4
256.3
I
II
330–350
390–430
605.1
176.2
599.1
170.4
520.9
299.7

K. Mohammadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 107 (2013) 145–150
149
Fig. 4. The variation of the electronic spectra of L1 titrated with various concentrations of VO2+ in constant ionic strength (I = 0.10 M NaClO4) and at 25 °C in MeOH.
Table 2
The trends in the formation constants of the given metal with
these Schiff base ligands are as follow:
The logKf for vanadyl ion with the tetradentate Schiff base ligands in MeOH at 15, 25,
35 and 45 °C at I = 0.10 M NaClO4.
L3ðOHÞ > L2ðOMeÞ > L1ðHÞ
Compounds
LogKf
15 °C
25 °C
35 °C
45 °C
which is according to electron-donor properties of functional
groups of the Schiff base ligands [13].
L1(H)
2.90 ( 0.05)
3.34 ( 0.02)
3.96 ( 0.01)
3.61 ( 0.09)
4.00 ( 0.09)
4.29 ( 0.03)
3.79 ( 0.01)
4.12 ( 0.03)
4.37 ( 0.04)
4.08 ( 0.01)
4.65 ( 0.03)
5.22 ( 0.03)
L2(OMe)
L3(OH)
This is in contrast to the electronic spectra that the charge
donation from VO2+ ion to Schiff base’s pꢁ molecular orbital is in-
creased by electron-withdrawing groups on the Schiff base ligands
[31]. OH in L3 is a richer donor than OMe and H. Therefore, the li-
gand with the H group has the smallest formation constant while
the ligand with OH has the highest [32]. Also, the formation con-
stants are increased by increasing the temperature:
Table 3
The thermodynamic parameter values,
in MeOH.
D
H°,
D
S° and
D
G°, for the complex formation
G° (kJ molꢀ1
a
Compounds
D
H° (kJ molꢀ1
)
D
S° (JKꢀ1 molꢀ1
)
D
)
L1
L2
L3
ꢀ28.1
ꢀ30.6
ꢀ29.2
122.8
134.6
133.8
ꢀ2.6
ꢀ2.9
ꢀ3.4
45 ꢂC > 35 ꢂC > 25 ꢂC > 15 ꢂC
a
D
G° = ꢀRTlnK and T = 298 K.
Conclusions
in various temperatures, which make help a better understanding
of relative stability of adducts.
In this study, the oxovanadium(IV) complexes have been syn-
thesized and characterized by several spectroscopic methods.
These complexes form a green monomeric structure with square-
pyramidal coordination geometry around the metal center.
According to the results discussed from TG curves, the order of
thermal stability for the complexes is VOL3 > VOL1 > VOL2. The
number of decomposition steps of the ligands depends on the sub-
stituent group leading to differences in thermal process for the
complexes. By considering the results of thermodynamic study,
the formation constants (of the complexes for the given metal with
different ligands) which is according to the donor properties of the
substituent:
The formation constants measurements were carried out by
spectrophotometric titration at constant ionic strength 0.1 M
(NaClO4) at 15, 25, 35 and 45 °C ( 0.1 °C). In a typical measure-
ment, 2.5 ml solution of the ligands (5 ꢅ 10ꢀ5 M) in MeOH was ti-
trated with various concentrations of the vanadyl acetylacetonate
(6 ꢅ 10ꢀ3–18 ꢅ 10–2) in MeOH. UV–Vis spectra were recorded in
the range 200–700 nm about 5 min after each addition. The formed
product showed different absorption from the free ligand, while
the metal ion solution shows no absorption at those wavelengths.
As an example, the variation of the electronic spectra for L1 titrated
with various concentrations of VO2+ at 25 °C in MeOH is shown in
Fig. 4. The same changes are valid for other systems.
L3ðOHÞ > L2ðOMeÞ > L1ðHÞ
VO2þ þ H2LX¡VOLX þ 2Hþ Kf
ð20Þ
Depending on the method used for solution treatment (potenti-
ometric, spectrophotometric, calorimetric, and so on) there are dif-
ferent computer programs for determining the formation constant.
For determination of formation constant, we chose SQUAD
program [30]. SQUAD program is designed to calculate the best
values for the formation constants of the proposed equilibrium
model by employing a nonlinear, least-squares approach. Other
thermodynamic parameters were calculated by using the well-
known van’t Hoff equation. The equilibrium constants and the
thermodynamic parameters data are collected in Tables 2 and 3.
Acknowledgement
The authors thank Persian Gulf University of Bushehr Research
Councils for financial support of this work.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.saa.2013.01.035.

150
K. Mohammadi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 107 (2013) 145–150
[16] G.G. Mohamed, Z.H.A.E. Wahab, Spectrochim. Acta A 61 (2005) 1059.
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