Synthesis, spectroscopy, electrochemistry and thermal study of vanadyl unsymmetrical Schiff base complexes

Article abstract of DOI:10.1016/j.ica.2010.08.041

The new tetradentate unsymmetrical N₂O₂ Schiff base ligands and VO(IV) complexes were synthesised and characterized by using IR, UV-Vis and elemental analysis. The electrochemical properties of the vanadyl complexes were investigated

Full text of DOI:10.1016/j.ica.2010.08.041

Inorganica Chimica Acta 365 (2011) 108–112
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Synthesis, spectroscopy, electrochemistry and thermal study of vanadyl
unsymmetrical Schiff base complexes
a
a
b
b
Ali Hossein Kianfar a, , Vida Sobhani , Morteza Dostani , Mojtaba Shamsipur , Mahmoud Roushani
⇑
a Department of Chemistry, Yasouj University, Daneshjoo Street, 75914-353 Yasouj, Iran
b Department of Chemistry, Razi University, Kermanshah, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 16 June 2010
Received in revised form 13 August 2010
Accepted 21 August 2010
The new tetradentate unsymmetrical N2O2 Schiff base ligands and VO(IV) complexes were synthesised
and characterized by using IR, UV–Vis and elemental analysis. The electrochemical properties of the van-
adyl complexes were investigated by means of cyclic voltammetry. The oxidation potentials are increased
by increasing the electron-withdrawing properties of functional groups of the Schiff base ligands accord-
ing to the trend of MeO < H < Br < NO2. The thermogravimetry (TG) and differential thermoanalysis (DTA)
of the VO(IV) complexes were carried out in the range of 20–700 °C. The complexes were decomposed in
two stages. Also decomposition of synthesised complexes is related to the Schiff base characteristics. The
thermal decomposition of the studied reactions was first order.
Keywords:
Vanadyl complexes
Schiff base complexes
Thermogravimetry
Electrochemistry
Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction
parameters were calculated using Coats and Redfern [13]
method.
The high stability potential of Schiff base complexes with differ-
ent oxidation states extended the application of these compounds
in a wide range. The complexes studied were known as catalysts in
organic redox and electrochemical reduction reactions [1–4].
Knowledge of electronic and steric effects to control the redox
chemistry of these compounds may prove to be critical in design-
ing new catalysts. The vanadyl Schiff base complexes play an
important rule in this area [5]. Vanadium complexes are very inter-
esting model compounds to clarify several biochemical processes
[6,7].
Thermogravimetry (TG) and differential thermoanalysis (DTA)
are valuable technics for studying the thermal behaviour of
chemical compounds [8–12]. Also the electrochemical methods
can be envisioned to provide valuable information on the cata-
lytic processes since catalytic conversions are frequently accom-
panied with the changes in the structure of complexes and metal
oxidation states. The present study describes the electronic influ-
ence of salophen derivatives on thermal and electrochemical
properties of vanadyl(IV) Schiff base complexes (Fig. 1). The elec-
trochemical properties of the vanadyl complexes were studied by
cyclic voltammetry in DMF solvent. Kinetics and thermodynamic
2. Experimental
2.1. Chemicals and apparatus
All of the chemicals and solvents used for synthesis and electro-
chemistry were of commercially available reagent grade and used
without purification. The elemental analyses were determined by
CHN–O–Heraeus elemental analyzer. Infrared spectra were
recorded by FT-IR JASCO-680 spectrophotometer in the 4000–
400 cmꢀ1. UV–Vis spectra were recorded by means of JASCO
V-570 spectrophotometer in the range of 190–900 nm. The 1H
NMR spectra were recorded in DMSO-d6 solvent by DPX-
400 MHz FT-NMR. Thermogravimetry (TG) and differential
thermoanalysis (DTA) were carried out by using a PL-1500. The
measurements were performed in air atmosphere. The heating rate
was held at 10 °C minꢀ1
.
Cyclic voltammograms were performed by using an autolab
modelar electrochemical system (ECO Chemie, Ultrecht, The Neth-
erlands) equipped with a PSTA 20 module and driven by GPES (ECO
Chemie) in conjunction with a three-electrode system and a PC for
data processing. An Ag/AgCl (saturated KCl)/3 M KCl reference
electrode, a Pt wire as counter electrode and a glassy carbon elec-
trode as working electrode (metrom glassy carbon, 0.0314 cm2)
were used for the electrochemical studies. Voltammetric measure-
ments were performed at room temperature in DMF solution with
⇑
Corresponding author. Tel./fax: +98 741 2223048.
E-mail addresses: akianfar@mail.yu.ac.ir, asarvestani@yahoo.com (A.H. Kianfar).
0020-1693/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2010.08.041

A.H. Kianfar et al. / Inorganica Chimica Acta 365 (2011) 108–112
109
H3C
O
Methanol
+
H3C
N
NH2
OH
NH2
NH2
OH
HL
O
HL
+
H3C
Methanol
N
N
HO
X
OH
HO
X
H2L
X = H,
L1,
5-Br, 5-NO2,
5-MeO, 4-MeO,
L4, L5,
3-MeO
L6
L2,
L3,
CH3
Methanol
+
O
V
H2L
VO(acac)2
N
O
N
O
X
Fig. 1. The structure of Schiff bases and their complexes.
0.1 M tetrabutylammonium perchlorate as the supporting
electrolyte.
1609 (C@N), 1324 (NO2), 991 (V@O). UV–Vis, kmax (nm) (e,
L molꢀ1 cmꢀ1) (ethanol): 372 (40 000), 311 (48 000).
VOL4 yield (85%): Anal. Calc. for C22H18N2O4V: C, 62.11; H, 4.23;
N, 6.58. Found: C, 62.68; H, 4.32; N, 6.87%. FT-IR (KBr cmꢀ1
1614 (C@N), 1273 (C–O), 980 (V@O). UV–Vis, kmax (nm) (
L molꢀ1 cmꢀ1
(ethanol): 469(sh) (19 000), 397 (29 000), 322
) mmax
2.2. Synthesis
e
,
The tetradentate Schiff base ligands, L1–L4, the new L5 and L6
were prepared according to the literature [14]. The vanadyl com-
plexes were synthesised by refluxing a methanolic solution of the
tetradentate Schiff base ligands and vanadylacetylacetonate. The
reaction was continued for 2 h until a green precipitate was ob-
tained. It was filtered, washed with methanol and dried in vacuum.
)
(45 000), 247 (84 000).
VOL5 yield (80%): Anal. Calc. for for C22H18N2O4V: C, 62.11; H,
4.23; N, 6.58. Found: C, 62.46; H, 3.25; N, 6.65%. FT-IR (KBr
cmꢀ1
(nm) (
(37 000).
VOL6 yield (75%): Anal. Calc. for for C22H18N2O4V: C, 62.11; H,
4.23; N, 6.58. Found: C, 62.93; H, 4.28; N, 6.77%. FT-IR (KBr
cmꢀ1
mmax 1605 (C@N), 1249 (C–O), 986 (V@O) . UV–Vis, kmax
(nm) (
, L molꢀ1 cmꢀ1) (ethanol): 401 (4200), 342 (65 000), 310
)
mmax 1614 (C@N), 1242 (C–O), 978 (V@O) . UV–Vis, kmax
e
, L molꢀ1 cmꢀ1) (ethanol): 387 (26 000), 325 (21 000), 243
H2L5 yield (80%): (C21H18N2O2), FT-IR (KBr cmꢀ1
) mmax 1635
(C@N), 1504 (C@C), 1222 (C–O). UV–Vis, kmax (nm)
(e,
L molꢀ1 cm-1) (CHCl3): 313 (3400), 278 (7400). 1H NMR (DMSO-
d6, 400 MHz) d = 2.50 (s, 3H, CH3), d = 3.80 (s, 3H, O–CH3),
d = 6.20–7.80 (m, 11H), d = 9.00 (s, 1H, HC@N), d = 12.90 (s, 1H,
OH), d = 14.30 (s, 1H, OH).
)
e
(56 000), 241 (93 000).
H2L6 yield (85%): (C21H18N2O2), FT-IR (KBr cmꢀ1
)
mmax 1643
(C@N), 1461 (C@C), 1257 (C–O). UV–Vis, kmax (nm)
L molꢀ1 cmꢀ1 (CHCl3): 343 (14 000), 265 (42 000). 1H NMR
(e,
)
3. Results and discussion
(DMSO-d6, 400 MHz) d = 2.50 (s, 3H, CH3), d = 3.60 (s, 3H, O–CH3),
d = 6.10–7.50 (m, 11H), d = 9.80 (s, 1H, HC@N), d = 15.40 (s, 2H,
OH).
3.1. IR characteristics
VOL1 yield (80%): Anal. Calc. for C21H16N2O3V: C, 63.79; H, 4.05;
The IR spectra of the free Schiff base ligands and the complexes
show several bands in the 400–4000 cm-1 region. The OH stretch-
ing frequency of the ligands is observed in the region of 2500–
3100 cmꢀ1 due to the internal hydrogen bonding vibration (O–
Hꢁ ꢁ ꢁN). This band disappeared in the spectra of the complexes
[11,12,15].
The free ligands have a characteristic C@N bond in 1611–
1643 cmꢀ1 region. For the Schiff base complexes C@N was ob-
served in 1600–1614 cmꢀ1. The C@N stretching band of the schiff
base complexes is generally shifted to a lower frequency, indicat-
ing a decrease in the C@N bond order due to the coordinate bond
N, 7.08. Found: C, 63.95; H, 4.09; N, 7.12%. FT-IR (KBr cmꢀ1
) mmax
1610 (C@N), 976 (V@O). UV–Vis, kmax (nm) (e
, L molꢀ1 cmꢀ1) (eth-
anol): 393 (24 500), 320 (27 000), 244 (58 000).
VOL2 yield (80%): Anal. Calc. for C21H15BrN2O3V: C, 52.95; H,
3.16; N, 5.90. Found: C, 53.57; H, 3.21; N, 5.98%. FT-IR (KBr
cmꢀ1
L molꢀ1 cmꢀ1
(19 000), 249 (32 000).
VOL3 yield (85%): Anal. Calc. for C21H15N3O5V: C, 57.27; H, 3.40;
N, 9.54. Found: C, 57.89; H, 3.47; N, 9.83%. FT-IR (KBr cmꢀ1
)
mmax 1600 (C@N), 981 (V@O). UV–Vis, kmax (nm)
(e,
)
(ethanol): 389 (11 500), 320 (13 000), 277
)
mmax

110
A.H. Kianfar et al. / Inorganica Chimica Acta 365 (2011) 108–112
Table 1
formation between the metal and the imine nitrogen lone pair
[11,12].
Redox potential data of vanadyl complexes in DMF solution.
The band which appeared at 976–991 cmꢀ1 is assigned to
Compounds
Epa (V ? IV)
Epc (IV ? V)
E1/2 (IV M V)
m
(V@O). This band is observed as a new peak for the complexes
VOL1
VOL2
VOL3
VOL4
VOL5
VOL6
0.657
0.684
0.774
0.612
0.693
0.666
0.405
0.495
0.603
0.396
0.450
0.414
0.531
0.589
0.688
0.504
0.571
0.540
and is not observed in the spectrum of free ligands [16,17].
3.2. Electronic spectra
In solution, the electronic spectra of the Schiff base ligands con-
sist of a relatively intense band in the 250–350 nm region, involv-
ing p?p transition [14]. The vanadyl complexes show an intense
CT transition in 380–401 nm regions. Also the d–d transition in this
type of complexes appears in the range of 550–900 nm. However,
this band does not appear due to the low intensity of the d–d tran-
sitions and low solubility of the studied complexes in the above
range [18].
*
state while the electro-donating groups have a reverse effect
[19–23]. Hammett type relationship has been found between the
Epa values and the appropriate para-substituent parameters [24],
which reflect the variation of the electrode potential as a function
of the electron-withdrawing ability of the substituent at positions
five (Fig. 3).
To investigate the effect of functional group on different
positions, the electrochemical property of complexes with 5-
MeO, 3-MeO and 4-MeO functional groups were studied (Fig. 4).
The anodic peak potentials increased in the trend of 5-MeO < 3-
MeO < 4-MeO. The substituted functional groups of 5-MeO and
3-MeO are electron donors, while the meta-substituted group (4-
MeO) is as an electron acceptor by induction effect. Also the meth-
oxy group on the ortho situation has shown a steric hinderance.
3.3. The electrochemical study of vanadyl complexes
The cyclic voltammetry of VOL complexes were carried out in
DMF solution at room temperature. A typical cyclic voltammogram
of VOL1 complex in the potential range from 0.0 to 1.0 V (versus
Ag/AgCl) is shown in Fig. 2b. An oxidation peak is observed at
about Ca. 0.657 V. VOL1 is oxidized to [VOL1(solvent)]+ in a fully
reversible one electron step [19]. The electron is removed from
the nonbonding orbitals and the V(V) complex is formed. Upon
reversal of the scan direction, the V(V) complex is reduced to
V(IV) at lower potentials. Multiple scans which resulted in nearly
superposable cyclic voltammograms show that the five coordinate
geometry is stable in both oxidation states, at least on the cyclic
voltammetry time scale. These results revealed that the redox pro-
cess of all studied vanadyl Schiff base complexes are the one – elec-
tron transfer reactions. The oxidation potentials for the different
complexes are shown in Table 1. The formal potentials (E1/2 (IV
V)) of the V(IV/V) redox couple were calculated as the average of
the cathodic (Epc) and anodic (Epa) potentials peak of this process.
To study the effect of functional groups of the Schiff base li-
gands on the oxidation potentials of [VOL], a series of the vanadyl
Schiff base complexes were studied by the cyclic voltammetry
method. The results show that the anodic peak potential (Epa) var-
ies as can be expected from the electronic effects of the substitu-
ents at positions five. Thus, Epa becomes more positive according
to the sequence MeO < H < Br < NO2. Similar results have been re-
ported for analogous vanadyl(IV), copper(II), nickel(II) and cobal-
t(III) systems, and have been interpreted assuming that the
strong electron-withdrawing effects stabilize the lower oxidation
3.4. Thermal analysis
The thermal decomposition of the studied complexes presented
pathways which depend on the nature of the ligands. These path-
ways are observed in the TG/DTA curves (Fig. 5). All TG and DTA
figures of the other compounds have the same trend. The absence
of weight loss up to 80 °C indicates that the crystalline solids have
no water molecule. Also, in the studied complexes, the TG showed
no wight loss up to 150 °C indicating the absence of water mole-
cule coordinated to complexes [11,12]. All the complexes were
decomposed in two steps (Figs. 5 and 6). The temperature range
and the percent of loss weight calculated and found for two steps
are presented in Table 2. For all the complexes, the first step occurs
between 280 and 370 °C. The second step of the thermal decompo-
sition which results in the formation of the V2O5 occurs in the
range 370–450 °C [8].
3.5. Kinetics aspects
All the well-defined stages were selected to study the decompo-
sition kinetics of the complexes. The kinetics parameters (the acti-
Fig. 2. Cyclic voltammograms of VOL, in DMF at room temperature. Scan rate: 100 mV/s. a = L4, b = L1, c = L2, d = L3.

A.H. Kianfar et al. / Inorganica Chimica Acta 365 (2011) 108–112
111
10
0
3
1
0.5
0
2
-10
-20
-30
-40
-50
-60
-70
-80
-90
1
0
-1
-2
-3
-4
-5
-6
R2 = 0.9975
-0.5
0.6
0.65
0.7
(V)
0.75
0.8
0
100
200
300
400
500
600
700
T(ºC)
E
pa
Fig. 3. Correlation between the Epa values for Schiff bases (L1–L4) and the respective
rpa substituent.
Fig. 5. The TG and DTA of VOL6 complex.
vation energy Ea and the pre-exponential factor A) were calculated
using the Coats-Redfem equation [13]
VOL1
T = 323oC
-C7H4N
T = 438oC
V2O5
C15H12N2O3V
C15H12N2O3V
ꢀ
ꢁ
ꢀ
ꢁ
C21H16N2O3V
VOL2
gð
a
Þ
AR
/E
2RT
Ea
Ea
-C14H12NO0.5
log
¼ log
1 ꢀ
ꢀ
ð1Þ
T2
2:303RT
T = 332oC
T = 439oC
where g(
pletion of the reaction, W is the mass loss up to temperature T, R is
the gas constant, Ea is the activation energy and is heating rate.
In the present case, a plot of left hand-side (L.H.S) of this equation
versus 1/T gives straight line (Fig. 7) whose slope and intercept are
used to calculate the kinetics parameters by the least square meth-
od. The goodness of fit was checked by calculating the correlation
coefficient. The other systems and their steps show the same trend.
The entropy of activation S– was calculated using the equation
a
) = [(Wf)/(Wf ꢀ W)], Wf is the mass loss upon the com-
V2O5
C21H15BrN2O3V
-C7H3BrN
-C14H12NO0.5
u
VOL3
T = 336oC
T = 406oC
C21H15N3O5V
V2O5
C15H12N2O3V
C15H11N2O3V
C15H11N2O3V
-C7H3N2O2
-C14H12NO0.5
VOL4
T = 308oC
-CH3O
T = 431oC
V2O5
C22H18N2O4V
–
kTs
S
R
A ¼
e
ð2Þ
-C21H15N2O0.5
T = 425oC
h
VOL5
where k, h and Ts are the Boltzmann’s constant, the Planck’s con-
stant and the peak temperature, respectively. The enthalpy and free
energy of activation were calculated using equations
T = 320oC
-C7H7O
C22H18N2O4V
V2O5
-C15H12N2O0.5
Ea ¼ H– þ RT;
G– ¼ H– ꢀ TS–
ð3Þ
ð4Þ
VOL6
T = 299oC
-C7H7O
T = 412oC
V2O5
C22H18N2O4V
C15H11N2O3V
-C15H12N2O0.5
The various kinetics parameters calculated are given in Table 2. The
activation energy (Ea) in the different stages is in the range of 119–
376 kJ molꢀ1. The respective values of the pre-exponential factor
Fig. 6. The TG decomposition pathway the vanadyl Schiff base complexes.
Fig. 4. Cyclic voltammograms of VOL, in DMF at room temperature. Scan rate: 100 mV/s. a = L4, b = L6, c = L5.

112
A.H. Kianfar et al. / Inorganica Chimica Acta 365 (2011) 108–112
Table 2
Thermal and kinetics parameters for vanadyl complexes.
Compounds
VOL1
D
T (°C)a
Percent (%)b
Ea (kJ/mol)
A (sꢀ1
)
S– (kJ molꢀ1 Kꢀ1
)
H– (kJ/mol)
G– (kJ/mol)
300–360
380–450
26(25)
51(51)
253.58
213.41
4.47 ꢂ 1019
127
41
249.99
207.11
178.81
176.19
2.32.1015
VOL2
VOL3
VOL4
VOL5
VOL6
290–370
385–430
38(40)
42(45)
292.54
119.57
3.44 ꢂ 1022
108
ꢀ90
171
ꢀ48
98
10
287.95
113.27
188.09
182.57
3.03 ꢂ 1008
300–365
375–455
33(32)
45(45)
282.80
151.50
1.01 ꢂ 1022
4.80 ꢂ 1010
278.21
145.20
184.21
181.74
280–350
370–430
7.5(9)
68(65)
231.24
259.15
164 ꢂ 1018
226.15
252.84
172.24
170.09
8.14 ꢂ 1018
280–360
380–430
25(27)
53(55)
244.56
185.12
1.83 ꢂ 1019
3.49 ꢂ 1013
119
6
239.97
178.82
173.92
174.52
280–355
370–450
25(26)
53(47)
376.26
264.91
2.67 ꢂ 1031
352
125
371.67
258.61
178.42
163.85
5.17 ꢂ 1019
a
The temperature range of decomposition pathways.
Present of weight lose calculated (found).
b
was smaller than that of the first one, and occured at the later
stages.
6.7
6.2
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1.62
1.67
103.T-1(0C)
1.72
W
f
Fig. 7. Coats-Redfern plots of VOL2 complex, step 2, A = logW
.
ꢀW
f
T2
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(A) vary from 3.03 ꢂ 108 to 2.67 ꢂ 1031 sꢀ1. The corresponding
values of the entropy of activation (S–) were in the range of ꢀ90
to +352 J molꢀ1. The enthalpy of activation (H–) values were in
the range of 113–371 kJ molꢀ1. The corresponding values of the free
energy of activation (G–) were in the range of 163–188 kJ molꢀ1
.
There was no definite trend in the values of activation parameters
among the different stages in the present series. However, due to
the unstable intermediate the activation energy of the second stage