Some novel tetradentate Schiff base complexes VO(IV) and Cu(II) involving fluorinated heterocyclic β-diketones and polymethylene diamines of varying chain length : Synthesis, spectral, coordination and thermal aspects

Article abstract of DOI:10.1007/s10973-007-8645-z

The present article describes the synthesis, spectral, coordination and thermal aspects of N,N'-polymethylene bis(1-phenyl-3-methyl-4- trifluoroacetylimino-2-pyrazoline-5-ol)oxovanadium(IV) or copper(II) Schiff base complexes with alkyl backbones ranging

Full text of DOI:10.1007/s10973-007-8645-z

Journal of Thermal Analysis and Calorimetry, Vol. 94 (2008) 2, 567–577
SOME NOVEL TETRADENTATE SCHIFF BASE COMPLEXES VO(IV) AND
Cu(II) INVOLVING FLUORINATED HETEROCYCLIC b-DIKETONES AND
POLYMETHYLENE DIAMINES OF VARYING CHAIN LENGTH
Synthesis, spectral, coordination and thermal aspects
1*
C. K. Modi and B. T. Thaker
2
1Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar, 388 120, Gujarat, India
2Department of Chemistry, Veer Narmad South Gujarat University, Surat, 395 007, Gujarat, India
The present article describes the synthesis, spectral, coordination and thermal aspects of N,N’-polymethylene
bis(1-phenyl-3-methyl-4-trifluoroacetylimino-2-pyrazoline-5-ol)oxovanadium(IV) or copper(II) Schiff base complexes with alkyl
backbones ranging from two to four carbons have been characterized on the basis of elemental analysis, magnetic moments, molar
conductivity measurements, spectra (FTIR, ESR, UV-Visible, MS), VPO and thermal studies. The vapour pressure osmo-
metry (VPO) and mass spectral studies indicate that the complexes are monomeric. An ESR study of all these complexes of VO(IV)
and Cu(II) are consistent with the square pyramidal and square planar geometries of these metal ions, respectively. In addition, the
kinetics and thermodynamic parameters for the different thermal decomposition steps of the complexes have been studied employ-
ing Horowitz–Metzger and Freemen–Carroll methods.
Keywords: mononuclear VO(IV) and Cu(II) complexes, novel tetradentate Schiff bases (H2TSB-n), spectroscopic, TG/DTG and
DSC studies, VPO
Introduction
Transition metal complexes of tetradentate Schiff base
ligands find applications as models of certain metal en-
zymes and in catalysis and materials chemistry [1]. A
series of tetradentate Schiff base complexes of the type
M(Sal-n) [where M=oxovanadium(IV) and copper(II)]
with n=2–10 have been studied extensively [2–5].
Thermogravimetry is a process in which a sub-
stance is decomposed in the presence of heat, which
causes bonds the molecules to be broken [6, 7].
The thermal analysis techniques were widely applied
in studying of thermal behaviour of metal complexes
[8–11]. The thermal behaviors of mixed-ligand com-
plexes involving pyrazolone derivatives were studied
by Xu et al. [12]. In continuation of our earlier work
[13–16] herein we describe synthetic, thermal, spec-
troscopic and coordination aspects of some novel
tetradentate Schiff base complexes of VO(IV) and
Cu(II) involving fluorinated heterocyclic b-diketones
and polymethylene diamines. The suggested structure
of the Schiff bases (H2TSB-n) is shown in Scheme 1.
Scheme 1 Suggested structure of the Schiff bases (H2TSB-n)
*
Author for correspondence: chetank.modi1@gmail.com
1388–6150/$20.00
© 2008 Akadémiai Kiadó, Budapest
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands

MODI, THAKER
Experimental
1620s (C=Nazomethine), 1590w (C=Ncyclic), 1352s
(C–O). 1H NMR (300 MHz, DMSO-d6):
d (ppm)=2.19 (s, 3H, –CH3); 1.21–2.16 (m, 6H,
–CH2– bridge); 7.17–7.82 (m, 5H, Ph); 9.26 (s, 1H,
OH…N).
Materials
Reagent and solvents
All the chemicals used were of analytical grade and
used without further purification. The organic sol-
vents were purified by standard methods [17].
Synthesis of N,N’–butylene bis(1-phenyl-3-
methyl-4-trifluoroacetylimino-2-pyrazoline-5-ol)
(H2TSB-3)
Synthesis of tetradentate Schiff base ligands
The Schiff base H2TSB-3 was prepared the same way
as H2TSB-1 was prepared. Yield 70%, m.p. 99°C.
Anal. calc. for C28H26N6O2F6: C, 56.75; H, 4.39;
N, 14.18. Found: C, 56.71; H, 4.32; N, 14.12%. FTIR
(cm–1), nmax (KBr): 3340b (OH–N), 1622s
(C=Nazomethine), 1585w (C=Ncyclic), 1355s (C–O).
1H NMR (300 MHz, DMSO-d6): d (ppm)=2.18 (s, 3H,
–CH3); 1.44–2.00 (m, 8H, –CH2– bridge); 7.07–7.85
(m, 5H, Ph); 9.42 (s, 1H, OH…N).
(H2TSB-n)
The Schiff base ligands (H2TSB-n) used in the present
investigation is the condensation products of
HPMTFP and alkyl diamines [ethylene diamine (en),
propylene diamine (pn) or butylene diamine (bn)] in
2:1 molar ratio. The reaction leading to the formation
of the Schiff bases as discussed below.
Synthesis of 1-phenyl-3-methyl-4-trifluoro-
acetyl-2-pyrazoline-5-one (HPMTFP)
Synthesis of metal complexes
The ligand HPMTFP was synthesized by using a pro-
cedure that reported by Okafor earlier [18]. Yield:
74%, m.p. 144°C. Anal. calc. for C12H9N2O2F3:
C, 53.34; H, 3.35; N, 10.49. Found: C, 53.27; H, 3.36;
N, 10.49%. 1H NMR (300 MHz, CDCl3):
d (ppm)=2.47 (s, 3H, –CH3); 7.28–7.86 (m, 5H, Ph);
12.20 (s, 1H, –OH).
The solution of VOSO4·5H2O (1.45 g, 5 mmol) or
CuSO4·5H2O (1.25 g, 5 mmol) in 25 mL of hot metha-
nol was added to a 25 mL of methanolic solution of
the various Schiff bases (H2TSB-n) [H2TSB-1
(2.82 g, 5 mmol), H2TSB-2 (2.89 g, 5 mmol) and
H2TSB-3 (2.96 g, 5 mmol)] in 1:1 molar ratio. The pH
of the solution was adjusted 5–6 by drop wise addi-
tion of methanolic solution of sodium acetate in it.
The resulting mixture was heated under reflux on a
water bath for 2–3 h. The obtained coloured precipi-
tate was filtered off, washed with hot water, hot meth-
anol and diethyl ether and finally dried in a vacuum
desiccator over anhydrous CaCl2.
Synthesis of N,N’–ethylene bis(1-phenyl-3-
methyl-4-trifluoroacetylimino-2-pyrazoline-5-ol)
(H2TSB-1)
An ethanolic solution (50 mL) of 1-phenyl-
3-methyl-4-trifluoroacetyl-2-pyrazoline-5-one (2.70 g,
10 mmol) and an ethanolic solution (25 mL) of ethylene
diamine (5 mmol, 0.33 mL) in 2:1 molar ratio were
mixed with constant stirring. Refluxing was carried out
for 2 h. The solution was cooled overnight at room tem-
perature. The formed pinkish brown crystals were col-
lected and dried in air, yield 80%, m.p. 90°C. Anal. calc.
for C26H22N6O2F6: C, 55.30; H, 3.90; N, 14.89. Found:
C, 55.30; H, 3.88; N, 14.82%. FTIR (cm–1), nmax (KBr):
3354b (OH–N), 1623s (C=Nazomethine), 1588w (C=Ncy -
Physical measurements
Elemental analyses (C, H, N) were performed at
CDRI, Lucknow with a Carlo Ebra 1108 analyzer.
The copper content of the complexes were analyzed
by the EDTA titration technique [19]. Infrared spectra
were recorded with a Perkin-Elmer IR spectro-
photometer (4000–400 cm–1) using KBr pellets.
1H NMR and ESR spectra were recorded at RSIC, IIT,
Powai, Mumbai. The number average molecular mass
(Mn) of the complexes were measured by a VPO
(Knaur, Germany) using DMF as a solvent and poly-
styrene (PS) as a calibrant. The electronic spectra
were recorded on Shimadzu 160A UV-visible
spectrophotometer using DMSO as the solvent blank.
The magnetic moments were obtained by the Gouy’s
method using mercury tetrathiocyanato cobaltate(II)
as a calibrant (cg=16.44·10–6 c.g.s. units at 20°C).
The FAB mass spectra were recorded on a
JEOL SX 102/DA-6000 Mass Spectrometer. A simul-
1
clic), 1350s (C–O). H NMR (300 MHz, DMSO-d6):
d (ppm)=2.21 (s, 3H, –CH3); 3.40–3.47 (m, 4H, –CH2–
bridge); 7.17–7.96 (m, 5H, Ph); 9.25 (s, 1H, OH…N).
Synthesis of N,N’–propylene bis(1-phenyl-3-
methyl-4-trifluoroacetylimino-2-pyrazoline-5-ol)
(H2TSB-2)
The Schiff base (H2TSB-2) was prepared the same
way as H2TSB-1 was prepared. Yield 72%, m.p.
92°C. Anal. calc. for C27H24N6O2F6: C, 56.15;
H, 4.13; N, 14.53. Found: C, 56.10; H, 4.10;
N, 14.49%. FTIR (cm–1), nmax (KBr): 3384b (OH–N),
568
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SOME NOVEL TETRADENTATE SCHIFF BASE COMPLEXES
taneous TG/DTG have been obtained by a model
VPO constant K, the Mn value of the complexes were
Pyris-1 TGA, Perkin Elmer, USA. The DSC was re-
corded using DSC 2920, TA Instrument, USA.
calculated (Table 1).
Mass spectral studies
Results and discussion
The recorded mass spectra and the molecular ion peak
for the complex [VO(TSB-1)]·0.5H2O have been used
to confirm the molecular formulae. Calculated and
found molecular masses are given in Table 1. The
mass spectrum of [VO(TSB-1)]·0.5H2O shows multi-
peaks corresponding to the successive degradation of
the molecule as shown in Fig. 1 and its fragmentation
pattern is given in Scheme 2. The first peak at m/e 629
represents the molecular ion peak of the complex with
9.72% abundance. The sharp peak (base peak) ob-
The analytical and physical data of the complexes are
presented in Table 1. The complexes are colored and
stable in air. They are insoluble in water and in most
organic solvents but soluble in DMF. The molar con-
ductances of 10–3 M DMSO solutions of the com-
pounds lie in the range 0.02–0.05 W–1 cm2 mol–1show-
ing that the complexes are non-electrolytes. The
formations of the complexes are assumed according
to the balanced chemical Eq. (1).
VOSO4·5H2O+H2TSB-n®
[VO(TSB-n)]·0.5H2O+H2SO4+4.5H2O
CuSO4·5H2O+H2TSB-n®
[Cu(TSB-n)]·1.5H2O+H2SO4+3.5H2O
Infrared spectra
Fig. 1 The mass spectrum of [VO(TSB-1)]·0.5H2O
In order to study the binding mode of Schiff base to the
metal ion in the complexes, the IR spectra of Schiff bases
(H2TSB-n) were compared with the spectra of the corre-
sponding complexes. In the investigated Schiff bases, a
broad band observed between 3340–3384 cm–1 is attrib-
uted to n(OH). However free n(OH) is generally observed at
or above 3450 cm–1; the observed lower value is due to
intramolecular H-bonding and therefore structure [I] in
Scheme 1 is favoured. The n(C–O) band is raised by
10–15 cm–1 in the spectra of complexes to the position
observed in the Schiff bases (H2TSB-n) suggesting the
coordination via this oxygen [20]. The strong band at
~1620 cm–1 attributable to the C=N stretching vibration
of the Schiff base ligands is shifted to a region of
1600–1605 cm–1 in the complexes, indicating the coordi-
nation of the azomethine nitrogen [21]. The
oxovanadium(IV) complexes exhibit an additional strong
absorption band near ~958 cm–1, which has been as-
signed to n(V=O) [22]. In the far–IR region, two new bands
at 515–550 and 412–482 cm–1 in the complexes are as-
signed to n(M–N) [23] and n(M–O)pyrazolone groups, respec-
tively. All of these data confirm the fact that H2TSB-n be-
haves as a dinegative tetradentate ligand forming a
conjugated chelate ring, with the ligand exists in the
complexes in the enolized form.
Vapour pressure osmometry (VPO)
The number average molecular mass (Mn) of some
complexes was estimated by vapour pressure
osmometry [24]. The plot of millivolts vs. concentra-
tion was drawn. With the help of the slope and the
Scheme 2 Suggested fragmentation pattern of
[VO(TSB-1)]·0.5H2O
J. Therm. Anal. Cal., 94, 2008
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MODI, THAKER
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SOME NOVEL TETRADENTATE SCHIFF BASE COMPLEXES
served at m/e 157 represents the stable species
[C9H5N2O]+ ion with 99.41% abundance.
anisotropic spectra of the complexes exhibit axial rather
than rhombic symmetry. The Hamiltonian parameters
g||, g^, |g|, A|| and A^ were calculated and are included in
Table 3. The values are in accordance with that of a
molecule which exists in square pyramidal geometry
[26–28]. The molecular orbital coefficients a2 (covalent
in-plane s-bonding) and b2 (covalent in-plane p-bond-
ing) were calculated by using the reported equations
[29] (Table 2). This suggests the equal amount of
covalency between M–L bonding in the present com-
plexes. The g, the bonding coefficient for dxy and dyz
orbitals, measures the covalency of the V=O bonds.
For Cu(II) complex, the g values indicate that
g||>g^>2.0023 in all the cases. Further, |g| values
2.08–2.13 are indicative of square planar geometry
for Cu(II) complexes. G values measure the exchange
interaction between copper centers in polycrystalline
state. In the present complexes G>4 suggests that the
exchange interaction is negligible and the ligand
strength is moderate [30]. The ESR spectra of Cu(II)
complexes in solution state at RT are isotropic proba-
bly due to tumbling motions of the molecules. The
isotropic value |g| is calculated at the center of the
four lines spectrum. The isotropic nuclear hyperfine
constant |A| is calculated from the separation between
two adjacent lines of the spectrum. However, these
compounds, in solution at 77 K (frozen state) show
three well resolved, moderate intensity peaks, in the
parallel part and eight lines in the perpendicular part,
is due to super hyperfine splitting of nearby nitrogen
of Schiff base ligand. Hamiltonian parameters g||, g^,
|g|, A|| and A^ were calculated and are summarized in
Table 2. For Cu(II) complex, g|| is a parameter sensi-
tive enough to indicative covalence. Generally g||
value for covalent complexes is g||<2.3, and for ionic
g||³2.3. The g|| values (Table 2) for all the complexes
are almost same, i.e. 2.27, which indicates that the co-
valent nature of M–L bond in these complexes. The
ratio of g||/A|| is used to find the structure of a complex.
In the present Cu(II) complexes, the ratio obtained is
in the range 126.57–130.92 cm, which falls in the
range 90–140 cm for square-planar copper(II) com-
plexes [31].
Magnetic properties and electronic spectra
The [VO(TSB)]·0.5H2O complexes exhibit meff value
(Table 1) between 1.68–1.79 B.M., which are very
1
close to spin only value for d case [25]. This indi-
cates that the compounds are monomeric and there
may not be intermolecular association in solid state.
The electronic spectra of the VO(IV) complexes show
one prominent maxima between 22421–22988 cm–1
and a red shoulder at 17544–18518 cm–1. The third
band
is
very
weak
and
observed
at
10471–11737 cm–1. In principle, oxovanadium com-
pound could either be mononuclear with V=O or
could be polynuclear with –V–O–V–O bridge bond.
In the present complexes, evidences FAB-mass and
VPO studies suggest that the compounds are
mononuclear. Thus keeping in view, the tetradentate
nature of the Schiff bases, 5-coordinated square-py-
ramidal structures with C4V symmetry, in which the
Schiff base lies in the basal plane and O at the apical
position may be suggested for all the complexes.
The magnetic moment of [Cu(TSB)]·1.5H2O
complexes are observed in the range of
1.72–1.78 B.M., which corresponds to a single un-
paired electron with a very slight orbital contribution.
The electronic spectra of [Cu(TSB)]·1.5H2O com-
plexes show well resolved band observed at 12270,
13263 and 12063 cm–1 in [Cu(TSB-1)]·1.5H2O,
[Cu(TSB–1)]·1.5H2O and [Cu(TSB-1)]·1.5H2O com-
plexes, respectively. On the other hand, the broad
shoulder observed at 16393, 16129 and 16000 cm–1.
These d–d transitions are shifted to lower energy as
the methylene (CH2) numbers increases. Absence of
any band below 10000 cm–1, ruled out the possibility
of tetrahedral structure for the complexes. Thus,
square-planar geometry is proposed for all the com-
plexes in the present study.
ESR spectral studies
[VO(TSB)]·0.5H2O and [Cu(TSB)]·1.5H2O
Molecular orbital co-efficient, a2 (covalent
in-plane s-bonding) and b2 (covalent in-plane
p-bonding) (Table 2) were calculated [32]. For pres-
ent [Cu(TSB)]·1.5H2O complexes, the values of a2
are in the range of 0.79–0.81, indicating moderate
covalency for s-bonding. The in-plane p-bonding pa-
rameter b2 has more potential interest that a2 as it
should be sensitive to the back donation from dxy or-
bital of the Cu to the p* anti-bonding orbital of the
ligand. The calculated values of b2 are found between
0.59–0.66. This suggests that the back donation in the
present complexes is significant.
ESR
spectra
for
[VO(TSB)]·0.5H2O
and
[Cu(TSB)]·1.5H2O complexes have been recorded for
polycrystalline and solution (50:50, DMSO:CHCl3) at
RT and LNT. Spectra of polycrystalline samples give
one signal in oxovanadium(IV) compounds. The g val-
ues ~1.99 are invariably lower than the free electron
value (ge=2.0023). This lowering is related to the
spin-orbit interaction of the ground state, dxy level. The
spectra of DMSO:CHCl3 (50:50) mixed solvents
yielded isotropic asymmetrical eight lines at RT except
[VO(TSB-3)]·0.5H2O. In the frozen solutions, the
J. Therm. Anal. Cal., 94, 2008
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SOME NOVEL TETRADENTATE SCHIFF BASE COMPLEXES
Thermal studies
Each step of decomposition in most of the thermal re-
actions of the solids follows the trend
heat
solid-1¾¾®solid-2+gas
(1)
This process comprises of several stages such as
the chemical act of breaking of bonds, breakdown of the
solid-1 crystal lattice, formation of the crystal lattice of
the solid-2, absorption–desorption of the gaseous prod-
ucts, diffusion of gas and heat transfer etc. The rate of
the thermal decomposition is determined by the rate of
one or more of these stages and sometimes, the rate de-
termining step changes during the process.
The first order reaction may be described by the
equation,
Fig. 2 Horowitz–Metzger plot for the thermal degradation of
da/dt=k(1–a)n
(2)
[VO(TSB-1)]·0.5H2O
where a is the volume fraction reacted at time t and n
is the order of reaction. The constant k is related to the
absolute temperature by an Arrhenius type equation,
[(–Ea/2.303R)D(1/T)]/Dlogwr=
–n+Dlog(dw/dt)/Dlogwr
(6)
k=Aexp(–Ea/RT)
(3)
where T is the temperature in K, R is gas constant,
wr=wc–w; wc is the mass loss at the completion of the re-
action and w is the total mass loss up to time t. Ea and n
are the energy of activation and order of reaction, re-
spectively. A typical curve of [Dlog(dw/dt)/Dlogwr] vs.
[D(1/T)/Dlogwr] for the [VO(TSB-1)]·0.5H2O complex
is shown in Fig. 3. The slope of the plot gave the value
of Ea/2.303R and the order of reaction (n) was deter-
mined from the intercept.
where A is constant and Ea is the energy of activation.
For the determination of various kinetic parame-
ters, numerous methods have been described in the
literature. However, in the present communication,
the methods reported by Freeman–Carroll [33] and
Horowitz and Metzger [34] have been adopted. The
later method is based on two assumptions: (a) thermal
and diffusion barriers are negligible and (b) the
Arrhenius equation is valid (Eq. (3) above). Horowitz
and Metzger have derived the relation:
The thermal behavior of the prepared complexes
log[1–(1–a)1–n/(1–n)]=Eaq/2.303RTs for n¹1 (4)
2
The thermal behaviour of the synthesized complexes
has been characterized on the basis of TG/DTG and
DSC methods. TG curve of [VO(TSB-1)]·0.5H2O and
[Cu(TSB-1)]·1.5H2O are taken as representative ex-
amples for the decomposition of these complexes.
Thermoanalytical data of the complexes are given in
Table 3.
For the first order relation, i.e. n=1, the left hand
side of Eq. (4) would be log[–ln/1–a]. Therefore, for
the first order kinetic process the Horowitz–Metzger
equation may be written in the form:
2
log[logw¥/wr]=Eaq/2.303RTs –log2.303
(5)
where wr=w¥ – w; w¥ is the mass loss at the comple-
tion of the reaction; w is the mass loss up to time t;
q=T–Ts, T is the absolute temperature at time t and Ts
is the peak temperature which is directly taken from
the DTG curve, R is a gas constant and Ea is the en-
ergy of activation. The plots of log[logw¥/wr] vs. q
were drawn and the energy of activation Ea was calcu-
lated from the slopes of these plots for a particular
stage. The typical Horowitz–Metzger plot for the
thermal degradation of [VO(TSB-1)]·0.5H2O is
shown in Fig. 2.
In the differential method of Freeman–Carroll,
the equation was used in the following form:
Fig. 3 Freeman–Carroll plot for the thermal degradation of
[VO(TSB-1)]·0.5H2O
J. Therm. Anal. Cal., 94, 2008
573

MODI, THAKER
[VO(TSB-1)]·0.5H2O
vanadium [35] atom, has the observed mass 8.27% as
vs. the calculated value of 7.98%.
TG/DTG and DSC curves of the complex
[VO(TSB-1)]·0.5H2O represented in Figs 4 and 5, re-
spectively. The decomposition of the complex under-
goes in three stages. The first mass loss (obs. 1.44%;
calcd. 1.41%) in the temperature range 40–150°C,
corresponds to the loss of half mole of lattice water
molecule. The loss of half mole of lattice water mole-
cule is of zero order reaction and the value of the en-
ergy of activation for the dehydration process is found
to be 3.21 kJ mol–1. With heating; the decomposition
of the ligand moiety was followed. Two well-sepa-
rated peaks are displayed on the DTG curve; this indi-
cates that the decomposition of the ligand divides into
two steps. The second stage, which occurs in the tem-
perature range 150–300°C, with a DTG peak at
280°C, corresponds to the decomposition of one part
of the ligand. The observed mass loss for this stage is
50.37%. It is further supported by an exothermic peak
at 261°C in DSC curve. The third stage occurs be-
tween 300 and 810°C, corresponding to the decompo-
sition of the remaining part of the ligand, with the
mass loss of 39.91%. The maximum rate of mass loss
is indicated by the DTG peak at 336°C. The total mass
loss (91.72%) coincides with the theoretical value of
92.02% (Table 3). The final residue, estimated as free
[Cu(TSB-1)]·1.5H2O
The thermal decomposition of the complex
[Cu(TSB-1)]·1.5H2O undergoes in three stages. The
first endothermic peak [36] in DSC at 115°C, in the
temperature range 40–150°C, with the mass loss
(obs. 4.06%; calcd. 4.14%) corresponds to the loss of
one and half moles of lattice water molecules. The
loss of one and half moles of lattice water molecules
is of zero order reaction and the value of the energy of
activation for the dehydration process is found to be
4.86 kJ mol–1. The second stage, which occurs in the
temperature range 150–530°C with a DTG peak at
256°C, is corresponding to the decomposition of one
part of the ligand. The observed mass loss for this
stage is 47.45%. This process is further supported by
an exothermic peak at 239°C in DSC curve. The third
stage is related to the decomposition of the remaining
part of the ligand in the temperature range of
530–760°C with a DTG peak at 689°C, accompanied
by a mass loss of 36.26%. The overall mass losses are
observed to be 87.77%, which is in well agreement
with the calculated value of 87.86%. The final resi-
due, estimated as copper oxide, has the observed mass
12.12% as against the calculated value of 12.01%.
Non-isothermal calculations were used exten-
sively to evaluate kinetic parameters such as activa-
#
tion entropy (DS ), activation enthalpy (DH ) and the
#
#
free energy of activation (DG ) for the different ther-
mal decomposition steps in the complexes employing
the Horowitz–Metzger equations [34] are given in
Table 4. The kinetic parameters calculated by the
Horowitz–Metzger method revealed no significant
difference with that evaluated by the Freeman–
#
Carroll method. The negative DS values for all de-
composition steps in the complexes indicate that the
complexes are more ordered [37]. The complexes de-
composed in mainly three-step processes and it was
observed that the first dehydration step in all com-
plexes are of zero order reactions whereas, the re-
maining decomposition steps are of first order reac-
tions. The activation energy values of third step, the
high value of Ea for all the complexes, indicate that
the removed part is strongly bonded to the metal ion.
The kinetic parameters, especially activation energy
(Ea) is helpful in assigning the strength of the bonding
of ligand moieties with the metal ion. The relative
high Ea values of Cu(II) complexes than VO(IV) com-
plexes (Table 4) indicates that the ligand is strongly
bonded to the metal ion [38]. Based on the activation
energy values, VO(IV) complexes show a lower ther-
mal stability than Cu(II) complexes. This can be dis-
cussed in terms of repulsion among electron pairs in
Fig. 4 TG/DTG curves of [VO(TSB-1)]·0.5H2O
Fig. 5 DSC curve of [VO(TSB-1)]·0.5H2O
574
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SOME NOVEL TETRADENTATE SCHIFF BASE COMPLEXES
J. Therm. Anal. Cal., 94, 2008
575

MODI, THAKER
5 L. C. Nathan, J. E. Koehne, J. M. Gilmore,
K. A. Hannibal, W. E. Dewhirst and T. D. Mai,
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Fig. 6 The tentative structure of the complexes
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the valence shell of the central ion. Both vanadium
and copper ions in the investigated complexes have
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shells. But due to small ionic size of vanadium which
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square-pyramidal, giving lower stability [39]. Based
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are investigated. The complexes decomposed in
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#
tions (except dehydration steps). The negative DS
values for all decomposition steps in the complexes
indicate that the complexes are more ordered.
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SOME NOVEL TETRADENTATE SCHIFF BASE COMPLEXES
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Received: November 25, 2007
Accepted: June 19, 2008
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OnlineFirst: Septemer 20, 2008
DOI: 10.1007/s10973-007-8645-z
J. Therm. Anal. Cal., 94, 2008
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