Journal of the Korean Chemical Society
2013, Vol. 57, No. 1
Printed in the Republic of Korea
http://dx.doi.org/10.5012/jkcs.2013.57.1.63
Vanadyl Binary Schiff Base Complexes Containing N2O2 Coordination Sphere:
Synthesis, Ab Initio Calculations and Thermodynamic Properties
Mozaffar Asadi*, Mohammad Hadi Ghatee, Susan Torabi, Khosro Mohammadi†, and Fatemeh Moosavi‡
Chemistry Department, College of Sciences, Shiraz University, Shiraz 71454, I.R. Iran.
*
E-mail: mozaffarasadi@yahoo.com; asadi@susc.ac.ir
†
Chemistry Department, Faculty of Sciences, Persian Gulf University, Bushehr 75169, I.R. Iran
‡
Department of Chemistry, Ferdowsi University of Mashhad, Mashhad 91779, Iran
(Received October 17, 2012; Accepted December 5, 2012)
ABSTRACT. Some vanadyl complexes were synthesized by treating a methanolic solution of the appropriate Schiff base
ligand and one equivalent of VO(SO4)2 to yield [(VOL21–14)](L=Salicylaldehyde’s derivatives, Schemes 1, 2). These oxovanadium (IV) complexes were characterized based on their FT-IR, UV-Vis spectroscopy and elemental analysis. The IR spectra
suggest that coordination takes place through azomethine nitrogen and phenolate oxygen. In addition, the formation constants
of the oxovanadium (IV) binary complexes were determined in methanolic medium. The ab initio calculations were also carried out
to determine the structural and the geometrical properties of one of the complexes and its calculated vibrational frequencies
were investigated.
Key words: Oxovanadium (IV), Formation constants, Thermodynamics, Ab initio calculations
the chelation behavior of this class of ligands having C=N
and OH groups, towards VO(IV) metal ion and to evaluate
their thermodynamic properties. Also, the conformational
stability of the bis(5-bromo-salicylideneanilinato) oxovanadium (IV) molecule was investigated through quantum
mechanical calculations. Geometry optimizations of cis
and trans conformers were performed, and the corresponding
relative energies were compared. A complete vibrational
analysis of the complex was performed by infrared data
with quantum mechanical calculations. Infrared spectroscopy is among the traditional methods of analysis, and
particularly powerful for nondestructive characterization
of substances including living and drug materials.31 The
calculated vibrational spectra were analyzed based on
each vibrational mode, which allowed us to obtain a quantitative as well as qualitative interpretation of the infrared
spectrum.
INTRODUCTION
Schiff base ligands are considered “privileged ligands”
because they are easily prepared by condensation between
aldehydes and primary amines. Schiff base ligands are
able to coordinate with many different metals,1−5 and to
stabilize them in various oxidation states. The Schiff base
complexes have been used in catalytic reactions6 and as
models for biological systems.7,8 It has been reported that
the structure of the substituent bonded to the imino nitrogen affects the coordination geometry of the complex.
During the past two decades, considerable attention has
been paid to the chemistry of the metal complexes of
Schiff bases containing nitrogen and other donors.9,10 The
interest in the coordination chemistry of vanadium complexes has grown enormously over the last few decades
due to the role of vanadium in several biological processes,
such as haloperoxidation,11 phosphorylation,12 glycogen
metabolism,13,14 and insulin mimicking.15,16 More recently,
vanadium(IV) coordination compounds have been shown
to catalyze selective oxidation of alkenes by molecular
oxygen.17−21
In view of recent interest in the energetics of the metal
ligand bonding in metal chelates involving N, O donor
ligands,22−30 we tried to synthesize Schiff base complexes
derived from ligands involving N2O2 donor sphere. So,
the aim of the present work was to support and evaluate
EXPERIMENTAL
Materials
All solvents and chemicals were purchased from Merck,
Fluka or Aldrich and used without further purification.
Apparatus and Techniques
The infrared spectra of all ligands and their complexes
were recorded in the range 4000−400 cm−1 using a Shi-63-
64
Mozaffar Asadi, Mohammad Hadi Ghatee, Susan Torabi, Khosro Mohammadi, and Fatemeh Moosavi
madzu FTIR–8300 spectrophotometer applying the KBr
disc technique. The UV-Visible absorption spectra were
recorded using Perkin-Elmer Lambda 2 spectrophotometer at room temperature. The Elemental analysis was carried out by Thermo Finnigan-Flash-1200. The NMR spectra
were recorded by a Bruker Avance DPX 250 MHz spectrometer.
Synthesis of the Ligands
The Schiff bases were prepared by mixing equimolecular amounts of 5-Br-salicylaldehyde or 5-MeO-salicylaldehyde and aniline or substituted anilines in the 3- and
4-positions by methoxy, hydroxy, chloro, bromo, nitro
and cyano groups in 10 mL absolute ethanol in a round
bottomed flask equipped with a condenser. The mixture
was brought into reflux for 4 h. The products obtained
after cooling were filtered off and crystallized from absolute ethanol. The product’s solids were dried under vacuum and kept dry in a desiccator over anhydrous calcium
chloride. Melting points were measured and elemental analysis for the prepared Schiff bases was done. The results
obtained were in good agreement with the calculated values.
Scheme 1. The structural formula of the Schiff bases.
The prepared Schiff bases have the structural formula
shown in Scheme 1.
Synthesis of the Complexes
A methanolic solution (10 mL) of vanadyl sulfate VO(SO4)2
·nH2O (0.25 mmol) was added dropwise to a stirred solution of the Schiff base (0.5 mmol) in a mixture of degassed
MeOH (10 mL) and NEt3 (1 mmol). The reaction mixture
was refluxed for 2 h under N2, and then cooled to room
temperature (RT), affording a greenish-grey solid product
which was isolated by vacuum filtration, washed with
cold methanol, and then dried overnight in vacuo at RT.32
The prepared Schiff base complexes have the structural
formula shown in Scheme 2.
Schiff bases
Bis(5-bromo-salicylideneanilinato)oxovanadium(IV)
[VO(L1)2]a
Bis(5-bromo-salicylidene-4-methoxyanilinato)oxovanadium(IV)
[VO(L2)2]a
[VO(L3)2]a
Bis(5-bromo-salicylidene-4-bromoanilinato)oxovanadium(IV)
Bis(5-bromo-salicylidene-3-chloroanilinato)oxovanadium(IV)
[VO(L4)2]a
Bis(5-bromo-salicylidene-4-chloroanilinato)oxovanadium(IV)
[VO(L5)2]a
[VO(L6)2]a
Bis(5-bromo-salicylidene-3-cyanoanilinato)oxovanadium(IV)
Bis(5-bromo-salicylidene-4-cyanoanilinato)oxovanadium(IV)
[VO(L7)2]a
Bis(5-bromo-salicylidene-3-nitroanilinato)oxovanadium(IV)
[VO(L8)2]a
[VO(L9)2]a
Bis(5-methoxy-salicylidene-4-methoxyanilinato)oxovanadium(IV)
[VO(L10)2]a
Bis(5-methoxy-salicylidene-4-hydroxyanilinato)oxovanadium(IV)
[VO(L11)2]a
Bis(5-methoxy-salicylidene-4-bromoanilinato)oxovanadium(IV)
[VO(L12)2]a
Bis(5-methoxy-salicylidene-4-chloroanilinato)oxovanadium (IV)
[VO(L13)2]a
Bis(5-methoxy-salicylidene-4-cyanoanilinato)oxovanadium(IV)
[VO(L14)2]b
Bis(5-bromo-salicylidenebenzylaminato)oxovanadium(IV)
a
n=0, bn=1
Scheme 2. The structural formula of the Schiff base complexes.
R
H
4-OCH3
4-Br
3-Cl
4-Cl
3-CN
4-CN
3-NO2
4-OCH3
4-OH
4-Br
4-Cl
4- CN
H
X
Br
Br
Br
Br
Br
Br
Br
Br
OCH3
OCH3
OCH3
OCH3
OCH3
Br
Journal of the Korean Chemical Society
Synthesis, Ab initio Calculations and Properties
65
Table 1. The physical properties of the prepared compounds
Compounds
HL1
HL2
HL3
HL4
HL5
HL6
HL7
HL8
HL9
HL10
HL11
HL12
HL13
HL14
[VO(L1)2]
[VO(L2)2]
[VO(L3)2]
[VO(L4)2]
[VO(L5)2]
[VO(L6)2]
[VO(L7)2]
[VO(L8)2]
[VO(L9)2]
[VO(L10)2]
[VO(L11)2]
[VO(L12)2]
[VO(L13)2]
[VO(L14)2]
mp (oC)
125
160
180
130
160
164
190
190
160
167
130
110
160
90
>250
250
250
>250
>250
>250
>250
>250
>250
250
>250
>250
>250
>250
Color
Orange
Yellow
Yellow
Yellow orange
Yellow
Orange
Orange
Pale yellow
Yellow
Orange
Orange
Orange
Yellow
Yellow
Olive green
Olive green
Green
Light green
Green
Light green
Light green
Pale green
Green
Pale green
Olive green
Pale green
Green
Green
Thermodynamic Studies of Complex Formation
The formation constants, Kf, of the VO(IV) complexes
were determined by spectrophotometric titration of a fixed
concentration of the ligands (5×10−5 M) with various concentrations of the metal sulfate (1×10−5−1.7×10−4 M) at 25 oC
and at constant ionic strength (0.1 M NaClO4). The interaction of NaClO4 with the ligands was negligible. In a typical titration, 2.5 mL of the ligand solution was transferred
into the thermostated cell compartment of the UV-Visible
instrument, which was kept at constant temperature (±0.1 oC)
by circulating water, and was titrated by the metal ion solution.
The titration was performed by adding aliquots of the
metal ion with a Hamilton µL syringe to the ligand. The
absorption measurements were carried out at various
wavelengths where the difference in absorption was the
maximum after equilibrium. The formed complex shows
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
2013, Vol. 57, No. 1
Fig. 1. The variation of the electronic spectra of H2L13 titrared
with VO(SO4).nH2O at 25 oC in 96% methanol.
H2L13, titrated with various concentrations of VO (SO4)
·nH2O at 25 oC in MeOH is shown in Fig. 1. The same
procedure was followed for all other systems. The electronic spectra of the formed complexes at the end of titration
were the same as the electronic spectra of the separately
synthesized complexes.
RESULTS AND DISCUSSION
Physico-chemical Characterizations and Geometrical Configuration of the Complexes
VO(IV) salt reacts with Schiff base ligands in 1:2 molar
ratio in alcoholic medium to afford greenish-grey complexes. The ligand and its complexes are stable at room
temperature and are nonhygroscopic. The synthesized ligands
and their complexes were characterized by spectral techniques and elemental analysis. Apart from this, thermodynamic properties of the complexes were studied and the
optimized geometry of one of the newly synthesized compounds has been elucidated by ab initio calculations.
IR analysis
The IR spectra provide valuable information regarding
the nature of functional groups attached to the metal atom.
The ligands and the metal complexes were characterized
mainly using the azomethine band. The main infrared bands
and their assignments are listed in Table 2. The vanadyl
complex shows a band at ~940 cm−1 attributed to V=O frequency.33 In addition the spectra of the ligands show –C=N
band in the region 1608−1620 cm−1, which is shifted to lower
frequencies (1606−1612 cm−1) through complex formation indicating the involvement of the –C=N nitrogen in
the metal ion coordination.34,35 Assignment of the proposed
coordination sites is further supported by the appearance
66
Mozaffar Asadi, Mohammad Hadi Ghatee, Susan Torabi, Khosro Mohammadi, and Fatemeh Moosavi
Table 2. IR Spectral data (cm−1) of the compounds
Compounds
HL1
HL2
HL3
HL4
HL5
HL6
HL7
HL8
HL9
HL10
HL11
HL12
HL13
HL14
[VO(L1)2]
[VO(L2)2].025H2O
[VO(L3)2].01H2O
[VO(L4)2].01H2O
[VO(L5)2].01H2O
[VO(L6)2].0.2H2O
[VO(L7)2]
[VO(L8)2]
[VO(L9)2]
[VO(L10)2].0.1H2O
[VO(L11)2]
[VO(L12)2]
[VO(L13)2]
[VO(L14)2].0.5H2O
νO-H
3463
3463
3441
3433
3433
3433
3444
3440
3456
3242
3427
3436
3427
3440
3413
3421
3444
3444
3444
3332
3345
νC-H
2920
2939
2827
2943
2989
3055
3085
3093
2825
2942
2921
2921
2950
2984
2895
2835
2931
2930
2910
2860
2923
2896
2921
2815
2897
2920
3087
2954
νC=N
1612
1618
1612
1616
1612
1620
1620
1620
1612
1614
1614
1620
1620
1631
1608
1612
1604
1604
1604
1608
1608
1612
1604
1604
1602
1610
1604
1612
of medium bands at 400−450 cm−1 and 450−500 cm−1 which
could be attributed to υ(M-O) and υ(M-N) respectively.36,37
Thus the oxovanadium (IV) complexes have the general
structure which were shown in Scheme 2.
Elemental Analysis
The stoichiometry of the ligands and vanadyl complexes
were confirmed by their elemental analysis. The metal/
ligand ratio was found to be 1:2 has been arrived at by estimating the carbon, hydrogen, and nitrogen contents of the
complexes. Elemental analysis of ligands and their VO (IV)
complexes show good agreement with the proposed structures of the ligands and their complexes (Table 3).
UV-Vis Analysis
The ligands show two absorption bands at UV-Visible
region. A n-π* transition band at 326−410 nm and a π-π*
transition band at 240−297 nm are shown in the ligands.
These absorption bands show a slight shift to higher energy
νC=C
1558
1562
1554
1558
1554
1566
1558
1566
1573
1598
1589
1591
1573
1569
1589
1531
1577
1581
1581
1542
1593
1593
1544
1598
1544
1546
1575
1532
νM-N
νV=O
νC≡N
2225
2221
2223
455
474
435
424
443
416
466
420
480
493
476
478
434
458
983
987
973
987
983
975
975
925
979
956
977
979
987
988
2229
2225
2225
in the complexes that is evident for unalteration of the
structure of ligands upon complexation (Table 3).
All the vanadyl (IV) complexes have a band at 340−470
nm in methanol corresponding to a d-d transition band.
This band is not always observed, being often buried beneath
a high intensity charge transfer band (or more accurately
the low energy tail of that band), and when it is observed it
is generally a shoulder (Table 3). UV-Vis spectra of HL13
and its oxovanadium (IV) are shown in Fig. 2.
1
H NMR
In the 1H NMR spectral data of the salicylideneaniline
ligands, the hydroxy proton is in the range 10−13 ppm. The
spectral data of the ligands show a singlet (1H) signal at
~8.5 ppm which can be assigned to the azomethine proton
group. The signals of the hydrogens of the phenyl group
are appeared at δ = 6.3−8 ppm. The signal appeared as a
singlet at 4.45 ppm. The protons of the methoxy groups
show a signal at ~3.8 ppm.38,39
Journal of the Korean Chemical Society
67
Synthesis, Ab initio Calculations and Properties
Table 3. UV-Visa and elemental analysis data of the compounds
Compounds
HL1
HL2
HL3
HL4
HL5
HL6
HL7
HL8
HL9
HL10
HL11
HL12
HL13
HL14
[VO(L1)2]
[VO(L2)2].025H2O
[VO(L3)2].01H2O
[VO(L4)2].01H2O
[VO(L5)2].01H2O
[VO(L6)2].0.2H2O
[VO(L7)2]
[VO(L8)2]
[VO(L9)2]
[VO(L10)2].0.1H2O
[VO(L11)2]
[VO(L12)2]
[VO(L13)2]
[VO(L14)2].0.5H2O
a
In methanol.
λmax (nm)
226, 268, 349
229, 271, 355
231, 270, 352
231, 270, 353
230, 270, 350
224, 269, 350
230, 275, 354
229, 267, 351
271, 330, 365
272, 335, 365
275, 309, 367
275, 309, 366
245, 284, 375
269, 324, 411
218, 286(sh)
216, 358(sh)
216, 288(sh)
283, 371(sh)
273, 354(sh)
211, 258(sh)
215, 288(sh)
283, 380(sh)
360, 424
365, 429
272, 367
269, 366
211(sh), 273
C
56.55 (56.20)
54.92 (55.21)
43.98 (44.05)
50.28 (49.88)
50.28 (50.64)
55.84 (55.85)
55.84 (56.15)
48.62 (48.37)
70.02 (70.12)
69.12 (69.43)
54.92 (55.03)
64.25 (64.51)
71.42 (71.73)
57.95 (58.05)
50.60 (50.20)
49.33 (49.59)
40.20 (39.83)
45.40 (45.44)
45.40 (45.65)
50.13 (50.52)
50.41 (50.80)
44.16 (44.43)
62.18 (62.40)
60.79 (60.40)
49.66 (50.02)
57.16 (57.29)
63.27 (62.90)
51.40 (51.26)
Elemental analysis (%, Found)
H
3.65 (3.69)
3.95 (4.00)
2.56 (2.30)
2.92 (3.00)
2.92 (2.69)
3.01 (2.87)
3.01 (2.82)
2.82 (2.63)
5.88 (5.55)
5.39 (5.76)
3.95 (3.75)
4.62 (4.62)
4.79 (4.55)
4.17 (4.30)
2.94 (2.74)
3.33(3.13)
2.10 (1.96)
2.37 (2.45)
2.37 (2.19)
2.46 (2.32)
2.42 (2.44)
2.28 (2.05)
4.87 (4.68)
4.41 (4.68)
3.27 (3.15)
3.77 (3.74)
3.89 (4.24)
3.54 (3.93)
N
5.07 (5.14)
4.57 (4.56)
3.95 (3.85)
4.51 (4.61)
4.51 (4.46)
9.30 (9.14)
9.30 (9.25)
8.72 (9.00)
5.44 (5.55)
5.76 (6.02)
4.57 (4.95)
5.35 (5.69)
11.10 (11.02)
4.83 (4.85)
4.54 (4.17)
4.11 (4.08)
3.61 (3.66)
4.07 (4.20)
4.07 (4.09)
8.35 (8.39)
8.40 (8.17)
7.92 (7.96)
4.83 (4.54)
5.06 (4.74)
4.14 (4.50)
4.76 (5.10)
9.84 (9.44)
4.28 (3.93)
Table 4. 1H NMR spectroscopic data of the compounds (δ in ppm)
Fig. 2. UV–Vis spectra of the HL13 ligand (––), VO(L13)2 complex separately synthesized (.......), and the product at the end of
titration (------).
2013, Vol. 57, No. 1
Compounds H−C=N
Ar−H
a
HL1
8.56
6.91−7.52
a
HL2
8.53
6.89−7.49
a
HL3
8.54
6.69−7.67
a
HL4
8.54
6.93−7.67
a
HL5
8.53
6.69−7.61
a
HL6
8.54
6.93−7.65
a
HL7
8.53
6.90−7.62
a
8
HL
8.53
6.69−7.61
a
HL9
8.57
6.87−7.29
b
HL10
8.83
6.79−7.29
a
HL11
8.56
6.90−7.56
a
HL12
8.58
6.86−7.46
a
HL13
8.56
6.90−7.74
a
HL14
8.41
6.32−7.74
a
In CDCl3. bIn d6-DMSO.
OH
13.28
13.44
10.94
10.93
10.94
10.93
10.94
10.94
10.42
12.75
10.65
10.65
10.65
11.39
−CH2−
OCH3
3.81
3.51, 4.12
3.72
3.83
3.86
3.83
4.45
68
Mozaffar Asadi, Mohammad Hadi Ghatee, Susan Torabi, Khosro Mohammadi, and Fatemeh Moosavi
Thermodynamic Studies
To study the effect of the steric and the electronic parameters of the ligands on the formation constants and the
thermodynamic free energy of complexation, the interaction of the ligands as donors and VO (IV) as acceptor was
carried out. The formation constants, Kf, were calculated
using SQUAD computer program,40,41 designed to calculate the best values for the formation constants of the proposed reaction model (reaction 1) by employing a nonlinear, least-squares approach. The free energy change
∆Go values of the formed complexes were calculated from
∆Go = −RT lnKf at 25 oC (See Table 5).
2 HLx + VO(SO4)·nH2O
F [VO(Lx)2]·nH2O + SO42− + 2 H+
(1)
As the results show, in the para substituted Schiff base
ligands, the formation constants (logKf) varies as can be
expected from the electronic effects of the substituents at
positions 4,4. Thus, the formation constants decrease according to the sequence OH > OCH3 > H > Br > Cl > CN. In fact,
Table 5. The formation constants, log Kf, for the complexes at
25 oC in methanol
Ligand
[VO(HL1)2]
[VO(HL2)2]
[VO(HL3)2]
[VO(HL4)2]
[VO(HL5)2]
[VO(HL6)2]
[VO(HL7)2]
[VO(HL8)2]
[VO(HL9)2]
[VO(HL10)2]
[VO(HL11)2]
[VO(HL12)2]
[VO(HL13)2]
log Kf
8.04 ± 0.06
8.19 ± 0.06
7.50 ± 0.09
6.97 ± 0.25
7.11 ± 0.57
6.72 ± 0.46
7.02 ± 0.13
6.05 ± 0.05
9.62 ± 0.45
10.47 ± 0.11
8.98 ± 0.51
8.74 ± 0.09
7.93 ± 0.30
∆Go (kJmol−1)
−45.88 ± 0.15
−46.71 ± 0.15
−42.77 ± 0.22
−39.75 ± 0.62
−40.55 ± 0.43
−38.33 ± 0.41
−40.03 ± 0.32
−34.50 ± 0.12
−54.86 ± 0.41
−59.71 ± 0.27
−51.21 ± 0.47
−49.84 ± 0.22
−45.22 ± 0.74
σp
0
−0.268
0.232
0.227
0.660
−0.268
−0.370
0.232
0.227
0.660
Fig. 3. Linear correlation between the para substituted constants,
σp, and logKf for the substituted salicylideneaniline Schiff bases
with VO(SO4).nH2O in methanol at 25 ºC.
for the selected Schiff bases, Hammett type relationships
were found between the logKf values and σp, the para-substituent constant.42 Such correlations are shown in Fig. 3.
COMPUTATIONAL DETAILS
The electronic structure and the optimized geometries
of the stable conformers of the complex bis (5-bromo-salicylideneanilinato)oxovanadium (IV) were computed by the
Hartree-Fock (HF) method using the Gaussian 03 program43
employing 6-311G basis set. The infrared absorption intensity was calculated in the harmonic approximation, at the
same level of theory as in the geometry optimization to
verify the adequacy of method and the basis set.
Optimized Geometries and Energies
Geometry optimization was performed by HF/6-311G
method on the two conformers of [VO(L1)2·H2O] complexes. Comparison of the energies shows that the trans
conformer is the more stable molecule. Careful examination of the conformer’s structure indicates that the target complex does not involve water molecule coordinated
to vanadium. This is evident by the fact that distances
between the V and O atoms of the cis (2.354 Å) and trans
(4.196 Å) conformers are larger than a V−O single bond
(1.791 Å).44 Therefore, it is adequate to continue the investigation on the stable trans conformer without water coordinated as it has been already confirmed experimentally.45
The equilibrium geometry of the complex has been determined by the energy minimization with the same basis set,
i.e., 6-311G. The relative energies, dipole moments, HOMO,
LUMO, and the energy gaps for the two conformers are
given in Table 6. The relative energies show that the trans
conformer, shown in Fig. 4, is the stable structure of the
target complex. The energy difference between conformers is 7.88 kJ/mol, which is larger by about three times of
the thermal energy kT (at room temperature where k is Boltzmann constant). As a result, there is no possibility of coexistence of these two conformers at room temperature.
From the other side of view, it can be found that (Table 6)
the cis conformer having higher dipole moment and less
stable HOMO electronic state is not favorable as the trans
conformer.
The calculated electron density on the rings containing
Br indicates that the electron orbital of HOMO is localized over the rings having Br, while of the LUMO is distributed over the rings. As a result, the HOMO-LUMO
energy gap of the trans complex increases. The stability of
the trans conformer can be in part accounted by considJournal of the Korean Chemical Society
Synthesis, Ab initio Calculations and Properties
Table 6. Calculated properties for cis and trans conformers of the
[VO(L1)2] complex at HF/6-311G level of theory
EHF (a. u.)
µ (Debye)
point group
HOMO (a.u.)
LUMO (a.u.)
HOMO-LUMO gap (eV)
Trans
−7415.950
1.471
C1
−0.110
0.049
4.318
Cis
−7415.947
3.0217
C1
−0.108
0.049
4.272
69
Vibrational Assignment
For the coformer [VO(L1)2] shown in Fig. 4, the number of normal mode is 150 [(3N-6), where N is the number of atoms N=52]. The conformation obtained from the
geometry optimization exhibits no special molecular symmetries, and hence the molecule belongs to the C1 point
group. Consequently, all the 150 fundamental vibrations
of the gas phase molecule belong to the A irreducible representation and are both IR and Raman active.
Vibrational Frequencies
Comparison of the calculated frequencies at HF/6-311G
level with experimental values (Table 2) reveals an overestimation of the wavenumber of the vibrational modes,
which can be attributed to the neglect of the anharmonicity present in a real system. The calculated infrared
absorption spectra are shown in Fig. 6.
The spectrum predicted by HF method shows the fin-
Fig. 4. The structure of the trans conformer of VO(L1)2.
ering that its electric dipole moment is less than that of the
cis. Fig. 5 shows the distribution of the HOMO and LUMO
orbitals over the stable conformer.
Fig. 6. Calculated infrared absorption spectra of the [VO(L1)2]
complex versus frequency (cm−1) in vacuo.
Fig. 5. HOMO (left) and LUMO (right) orbitals of the trans conformer of VO(L1)2 in vacuo.
2013, Vol. 57, No. 1
70
Mozaffar Asadi, Mohammad Hadi Ghatee, Susan Torabi, Khosro Mohammadi, and Fatemeh Moosavi
gerprint of the complex in the range of 400−1820 cm−1
and the C−H stretching can be seen around 1820 cm−1.
C−X vibrations, X=O, C, H, N, Br
The predicted stretching modes at 1165 and 1451.71 cm−1
correspond to bands of the IR spectra. The C−O bond in
this complex is located near the center of the molecule
(vanadium). C=C stretching bands can be seen at 1253.93,
1783, 1779.59, and 1181.37 cm−1. The C=C bond near the
Br group vibrates at a lower frequency compared to the
vibration of C=C bond on the ring in the farrest position to
Br. The C=C stretching mode in the benzene ring is 1783
cm−1, which occurs at a higher frequency relative to the
C=C bond in the ring involving Br (1779.59 cm−1).
The bending vibration of C−H groups connected to the
rings occurs at 1631 cm−1. The C−H modes also depend
upon the location of the bond. Near the electron acceptor
bromide, it can be observed at 959.9, 1056.57, and
1260.52 cm−1, while far from this electron acceptor, it
can be found in a wide range from 970 cm−1 to 1300 cm−1.
In addition, the C−H bond near the nitrogen atom of the
complex vibrates at 1570 cm−1. The stretching mode of
C=N bond is calculated to be at 1811.97 and 1820.38 cm−1.
The smaller wavenumbers at 96.95 and 165.95 cm−1 represent the torsional mode of C=N. The stretching vibration of CN group on the ring has the strongest band
in the IR spectrum. The normal vibrational mode of
C−Br in the bromobenzene ring was predicted to be at
1179.85 cm−1.
V−Y vibrations, Y=O, N
The four modes of V=O vibrations can be identified by
the bands at 593.04, 727.35, 732.69, and 1164 cm−1. The
first one corresponds to the bending motion and the second and the third ones to the symmetric and asymetric
stretchings, respectively. The forth one could hardly be
identified. The vibrational modes of the V−N bond are
seen at 65.85, 378.52, and 5477.96 cm−1 of the IR spectra.
Lattice Vibrations
The lattice vibration can not be associated with any
vibrational mode of the single molecule. Interestingly, the
lattice vibrations are usually observed below 200 cm−1.
The lattice modes associated with the translations and
librations of the whole molecule can be only observed by
far infrared, dispersive Raman, or tetrahertz spectroscopy.46
Since HF method overestimates these modes, the lattice
vibrational mode in trans conformer of [VO(L1)2] are
observed at frequencies below 350 cm−1.
Interaction Energy Between Complex and Water
To provide more insights into the nature of the interaction between [VO(L1)2] complex and water molecule, a
systematic approach was taken into account. The interaction energy of complex is defined as the difference between
the energy of the complex with water (E[VO(L1)2·H2O]) and the
sum of the energies of the pure H2O(EH2O) and [VO(L1)2]
(E[VO(L1)2]) species:
–1
E ( kJmol ) =
2625.50[ E[VO(L1)
2 ⋅ H2 O ]
( a.u. ) – EH2O ( a.u. ) – E[VO(L1) ]( a.u. ) ]
2
(2)
For this purpose each isolated [VO(L1)2.H2O] and its
corresponding species ([VO(L1)2] and H2O) were optimized at HF/6-311g level of theory as mentioned above.
The calculated interaction energies are −25.420 and
−23.883 kJ/mol for the trans and the cis conformers,
respectively. Since the interaction energy for the trans
(−25.420 kJ/mol) and for the cis (−23.883 kJ/mol) conformers is small, it can be confirmed again that this complex does C contain water molecule coordinated to the
vanadium.
Charge Distribution on the Complex
The atomic charges calculated by ab initio method at
HF/6-311g level of theory by Natural Bond Orbital (NBO)
for two conformers indicate that the most positive charge,
+2.0216 C, is on the center of the trans complex (vanadium atom). This charge value may be one of the effective
factors in determining the stability of trans conformer
over the cis conformer.
CONCLUSIONS
The structural, geometrical and the thermodynamic
properties of the oxovanadium(IV) complexes have been
investigated. Geometry optimization on the cis and trans
conformers shows that the cis conformer is less stable and
the energy difference between these two conformers is
7.88 kJ/mol. Thus, calculations using HF mathod with 6311g basis set show that the energy difference between the
conformers is much larger than kT, such that there is no
possibility of coexistence of conformers.
In conformer trans, there does not exist the possibility
of intramolecular bond formation between V−O (the oxygen of the water) as indicated by the large distance between
V and O atoms, e.g., 4.196 Å. This means that no water is
coordinated to vanadium as confirmed by the calculated
Journal of the Korean Chemical Society
Synthesis, Ab initio Calculations and Properties
interaction energy. Vibrational spectroscopy and computational chemistry have been applied for investigating the
most stable conformer of bis(5-bromo-salicylideneanilinato) oxovanadium (IV). Infrared spectrum was recorded,
and vibrational bands were assigned based on the HF calculations. In general, the calculated modes observed were
overestimated. Using the elaborated method, which includes
the correlations like DFT, shall improve the results.
A striking feature of the present work is the study of the
interaction energy, which reveals small interaction between
H2O and [VO(L1)2] in [VO(L1)2.H2O], e.g., −25.420 kJ/
mol. This energy lies below a V−O bond energy and it can
be concluded that bis(5-bromo-salicylideneanilinato)oxovanadium (IV) does not have a coordinated water. According to the thermodynamic studies, the formation constant
of the complexes depends upon the steric and the electronic characteristic of the ligands. Moreover, the molecular
electronic structure of each complex plays an important
role on its thermodynamic properties. It is evident that
there is a close relationship between these various properties.
Acknowledgments. We are grateful to Shiraz University
Research Council for their financial support.
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