Transition metal complexes with uninegative bidentate Schiff base : SSSynthetic, thermal, spectroscopic and coordination aspects

Article abstract of DOI:10.1007/s10973-006-7682-3

The present article describes the synthesis, structural features and thermal studies of the complexes of the type [M(SB)₂(H ₂O)₂]?nH₂O [where HSB=pyridine-m- carboxaldene-o-aminobenzoic acid and M=Mn(II), Co(II), Ni(II), Cu(II), Zn(II) and Cd(II)]. The complexes have been characterized on the basis of elemental analyses, magnetic susceptibility measurements, (FTIR and electronic) spectra and thermal studies. The nature of the bonding has been discussed on the basis of infrared spectral data. Magnetic susceptibility measurements and electronic spectral data suggest a six-coordinated structure of these complexes. The complexes of Mn(II), Co(II), Ni(II), Cu(II) are paramagnetic, while Zn(II) and Cd(II) are diamagnetic in nature. The thermal decomposition of the complexes have been studied and indicates that not only the crystallization and coordinated water are lost but also that the decomposition of the ligand from the complexes is necessary to interpret the successive mass losses. The kinetic parameters such as order of reaction (n) and the energy of activation (E _a) have been reported using Freeman-Carroll method. The entropy (S*), the pre-exponential factor (A), the enthalpy (H*) and the Gibbs free energy (G*) have been calculated. Springer-Verlag 2007.

Full text of DOI:10.1007/s10973-006-7682-3

Journal of Thermal Analysis and Calorimetry, Vol. 87 (2007) 2, 441–448
TRANSITION METAL COMPLEXES WITH UNINEGATIVE BIDENTATE
SCHIFF BASE
Synthetic, thermal, spectroscopic and coordination aspects
C. K. Modi, S. H. Patel and M. N. Patel*
Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar, Gujarat 388 120, India
The present article describes the synthesis, structural features and thermal studies of the complexes of the type
[M(SB)2(H2O)2]×nH2O [where HSB=pyridine-m-carboxaldene-o-aminobenzoic acid and M=Mn(II), Co(II), Ni(II), Cu(II), Zn(II)
and Cd(II)]. The complexes have been characterized on the basis of elemental analyses, magnetic susceptibility measurements,
(FTIR and electronic) spectra and thermal studies. The nature of the bonding has been discussed on the basis of infrared spectral
data. Magnetic susceptibility measurements and electronic spectral data suggest a six-coordinated structure of these complexes. The
complexes of Mn(II), Co(II), Ni(II), Cu(II) are paramagnetic, while Zn(II) and Cd(II) are diamagnetic in nature.
The thermal decomposition of the complexes have been studied and indicates that not only the crystallization and coordinated
water are lost but also that the decomposition of the ligand from the complexes is necessary to interpret the successive mass losses.
The kinetic parameters such as order of reaction (n) and the energy of activation (Ea) have been reported using Freeman–Carroll
method. The entropy (S*), the pre-exponential factor (A), the enthalpy (H*) and the Gibbs free energy (G*) have been calculated.
Keywords: divalent transition metal complexes, DSC studies, DTA studies, spectroscopic analyses, TG/DTG
uninegative bidentate Schiff base
Introduction
Experimental
Materials
Schiff bases and their metal complexes played a key
role in our understanding of the coordination chemistry
of transition metal ions. The field of Schiff base com-
plexes was fast developing on account of the wide vari-
ety of possible structures for the ligands depending
upon the aldehydes and amines [1]. The combination
of distinctly different metal ion binding sites within
one ligand could lead to materials with interesting new
properties [2, 3] e.g. specific sensors, molecular wires,
magnetic and optical devices. The thermal analysis
techniques were extensively applied in studying of the
thermal behaviour of metal complexes [4–10]. Differ-
ential scanning calorimetry (DSC) and differential
thermal analysis (DTA) procedures permit the determi-
nation of non-isothermal transformation kinetics [11].
In continuation of our earlier work [12, 13] herein we
describe synthetic, spectral, thermal and coordination
aspects of transition metal complexes with uninegative
bidentate Schiff base (HSB). The structure of the
Schiff base is shown in Fig. 1.
Reagent and solvents
All the chemicals used were of analytical grade. Metal
nitrates and o-amino benzoic acid were purchased
from E. Merck (India) Ltd., Mumbai. Pyridine-
3-carboxyaldehyde was purchased from Fluka. The or-
ganic solvents were purified by standard methods [14].
Synthesis of pyridine-m-carboxaldene-o-aminobenzoic
acid (HSB)
An ethanolic solution (100 mL) of pyridine-3-
carboxyaldehyde (10 mmol, 1.07 g) and an ethanolic
solution (100 mL) of o-aminobenzoic acid (10 mmol,
1.37 g) in 1:1 molar ratio were mixed with constant
stirring. Refluxing was carried out for 5 h. The solu-
tion was cooled overnight at room temperature. The
formed light brown crystals were collected and dried
in air. Yield: 1.85 g (76%); m.p.: 232°C.
Synthesis of the metal complexes
The preparation of complexes was carried out by mix-
ing methanolic solution (100 mL) of metal nitrate
(10 mmol) and a hot DMF solution of the Schiff base
(20 mmol, 4.52 g) in 1:2 molar ratio. The resulting
Fig. 1 Structure of the Schiff base (HSB)
*
Author for correspondence: jeenenpatel@yahoo.co.in
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2007 Akadémiai Kiadó, Budapest

MODI et al.
mixture was stirred under reflux for 4 h at 70°C. The
plexes are coloured and stable in air. They are insoluble
in water, alcohol and DMF but least soluble in DMSO.
The formations of the complexes are assumed according
to the following balanced chemical equations:
obtained coloured crystals were collected by filtra-
tion, washed with water, methanol and finally with di-
ethyl ether and dried in air.
M(NO3)2×nH2O+2HSB®
[M(SB)2(H2O)2]×nH2O+2HNO3
Methods
The metal content of the complexes were determined by
the EDTA titration technique [15] after decomposing
the organic matter with a mixture of HClO4, H2SO4 and
HNO3 (1:1.5:2.5). Carbon, hydrogen and nitrogen were
analyzed with the Perkin Elmer, USA 2400-II CHN an-
alyzer. 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). Diamagnetic corrections were made using
Pascal’s constant.
where M=Mn(II), Co(II), Ni(II), Cu(II), Zn(II) and
Cd(II).
Infrared spectra
IR spectra of the Schiff base (HSB) and its metal com-
plexes were recorded as KBr disks and are summa-
rized with some tentative assignments of important
characteristic bands. The infrared spectral data of the
metal complexes are shown in Table 2.
A simultaneous TG/DTG and DTA had been ob-
tained by a model 5000/2960 SDT, TA Instruments,
USA. The experiments were performed in N2 atmo-
sphere at a heating rate of 10°C min–1 in the temperature
range 50–800°C, using Al2O3 crucible. The sample sizes
are ranged in mass from 4.5–10 mg. DSC was recorded
using DSC 2920, TA Instrument, USA. DSC curves
were obtained at a heating rate of 10°C min–1 in N2 at-
mosphere over the temperature range of 50–400°C, us-
ing aluminium crucible. The IR spectra were recorded
on a FTIR Nicolet 400D spectrophotometer using KBr
pellets. The reflectance spectra of the complexes were
recorded in the range of 1700–350 nm (as MgO discs)
on a Beckman DK-2A spectrophotometer.
In the investigated complexes, the bands observed
in the region 3200–3400, 1270–1310, 830–860 and
700–720 cm–1 are attributed to OH stretching, bending,
rocking and wagging vibrations, respectively due to
the presence of water molecules [16]. In the spectra of
the Schiff base, a strong band at 1669 cm–1 is attribut-
able to the –C=N group. On coordination, due to possi-
ble drift of the lone pair density towards the metal ion,
the azomethine –C=N bond is expected to absorb at
~1622–1629 cm–1 indicates the coordination of the
azomethine nitrogen to the metal ion [17]. In all the
metal chelates, a new band is seen in the 500–510 cm–1
region, which is probably due to the formation of M–N
bonds [18].
The concept of the thermal stability order of the
metal complexes can be explained on the basis of the
separation (D) between carboxylate anti-symmetric
and symmetric stretching frequencies for the C–O
bonds of the metal complexes. As the covalent char-
acter of the M–O bond increases, the anti-symmetric
Results and discussion
The analytical and physical data of the ligand and its
metal complexes are presented in Table 1. The com-
Table 1 Analytical and physical data of the Schiff base (HSB) and its metal complexesa
Found (calc.)/%
meff/B.M.
Compound
Colour
Yield/%
C
H
N
M
–
69.10
(69.02)
4.45
12.39
Ligand
light green
dark brown
dark brown
yellowish green
dark brown
light yellow
light yellow
–
76
66
66
74
72
68
70
(4.42)
(12.38)
53.26
(53.24)
4.56
9.58
9.39
(9.37)
[Mn(SB)2(H2O)2]×2.5H2O
[Co(SB)2(H2O)2]×2.5H2O
[Ni(SB)2(H2O)2]×3H2O
[Cu(SB)2(H2O)2]×2.5H2O
[Zn(SB)2(H2O)2]×2.5H2O
[Cd(SB)2(H2O)2]×3H2O
5.97
(4.60)
(9.55)
52.96
(52.88)
4.59
9.50
10.03
(9.98)
4.02
(4.50)
(9.49)
52.14
(52.11)
4.71
9.37
9.86
(9.80)
2.86
(4.67)
(9.35)
52.52
(52.47)
4.57
9.44
10.72
(10.68)
1.78
(4.54)
(9.41)
52.32
(52.31)
4.58
9.35
10.98
(10.96)
diamag.
diamag.
(4.52)
(9.38)
47.83
(47.82)
4.33
8.64
17.26
(17.23)
(4.29)
(8.58)
aHSB=pyridine-m-carboxaldene-o-aminobenzoic acid (the same in Tables 2–5)
442
J. Therm. Anal. Cal., 87, 2007

TRANSITION METAL COMPLEXES WITH UNINEGATIVE BIDENTATE SCHIFF BASE
Table 2 The characteristic IR bands of the ligand and the complexes
nCOO
nOH
nC=N
nC=C aromatic
nM–N
nM–O
Compound
antisymmetric symmetric
Dn
Ligand
3440
3420
3447
3444
3435
3442
3429
1669
1626
1622
1628
1625
1629
1628
1550
1578
1570
1555
1560
1574
1568
1624.00
1582.18
1596.25
1595.50
1592.10
1590.20
1585.54
1384.00
1379.30
1362.00
1374.00
1379.00
1382.50
1384.30
240.00
202.88
234.25
221.50
213.10
207.70
201.24
–
–
[Mn(SB)2(H2O)2]×2.5H2O
[Co(SB)2(H2O)2]×2.5H2O
[Ni(SB)2(H2O)2]×3H2O
[Cu(SB)2(H2O)2]×2.5H2O
[Zn(SB)2(H2O)2]×2.5H2O
[Cd(SB)2(H2O)2]×3H2O
500
502
509
502
510
505
485
474
490
478
485
485
stretching frequency should also increase and the
symmetric stretching frequency should decrease.
More importantly the separation between the two fre-
quencies increases (Table 2), the thermal stability of
the metal complexes are also increases [19, 20].
Based on the separation (D) between stretching fre-
quencies for the carboxylate in HSB, the thermal sta-
bility of the metal complexes in decreasing order is:
Co>Ni>Cu>Zn>Mn>Cd.
at ~15200 cm–1 ( 2Eg®2T2g transition) in the electronic
spectra of Cu(II) complex [25] suggest an octahedral
geometry. The Zn(II) and Cd(II) complexes are dia-
magnetic as expected for d10 systems. The values of the
electronic parameters, such as the ligand field splitting
energy (10 Dq), Racah interelectronic repulsion pa-
rameter (B), nephelauxetic ratio (b) and ratio n2/n1 are
summarized in Table 3. The suggested structure of the
complexes is shown in Fig. 2.
Magnetic moments and electronic spectra
The electronic spectra of the Mn(II) complex shows
absorption bands at ~15000, ~20000 and ~25000 cm–1
assignable to A1g®4T1g , A1g®4T2g and A1g®4A1g,
4Eg transitions, respectively, in an octahedral environ-
ment around the Mn(II) ion [21]. The magnetic mo-
ment value of the Mn(II) complex is 5.99 B.M. due to a
high-spin d5-system with an octahedral geometry [22].
The observed magnetic moments 4.02 and 2.86 B.M.,
for the Co(II) [23] and Ni(II) [24] complexes, respec-
tively, are within the range for an octahedral geometry.
The electronic spectra of the Co(II) complex exhibits
three bands at ~9400, ~18500 and ~19000 cm–1, which
6
6
6
Fig. 2 Suggested structure of the metal complexes where
M=Mn(II), Co(II), Cu(II) and Zn(II), n=2.5H2O;
M=Ni(II) and Cd(II), n=3H2O
Thermal studies
4
may reasonably be assigned to T1g(F)®4T2g(F),
In general, the stages of thermal decomposition of the
complexes can be written as shown:
4
4
4T1g(F)® A2g(F) and T1g(F)®4T1g(P) transitions, re-
spectively. The electronic spectra of the Ni(II) com-
plex also shows three bands at ~10500, ~17500 and
~24600 cm–1 assignable to 3A2g(F)®3T2g(F),
50–140°C, dehydration
[M(SB)2 (H2O)2 ]×nH2O¾¾¾¾¾¾®
[M(SB)2 (H2O)2 ]+nH2O
3
3A2g(F)®3T1g(F) and A2g(F)®3T1g(P) transitions, re-
where M=Mn(II), Co(II), Cu(II) and Zn(II) (n=2.5),
M=Ni(II) and Cd(II) (n=3).
spectively. The observed magnetic moment value of
the Cu(II) complex is 1.78 B.M. and the band observed
Table 3 Electronic parameters of the Co(II) and Ni(II) complexesa
Observed bands/cm–1
Complex
n2/n1
b0
B
b
10 Dq
n1
n2
n3
[Co(SB)2(H2O)2]×2.5H2O
[Ni(SB)2(H2O)2]×3H2O
9400
18500
17500
19000
24600
1.97
1.66
715
707
0.73
0.69
26.4
31.4
10521
10500
10500
aThe ligand field splitting energy (10 Dq), interelectronic repulsion parameter (B) and covalency factor (nephelauxetic ratio) (b) for
the Co(II) and Ni(II) complexes were calculated using the secular equations given by König [26]:
For Co(II) complex
For Ni(II) complex
10 Dq=n1
10 Dq = 1/2[(2n1–n3)+(n2+n1n3–n2)1/2
]
3
1
15B=n3–2n1+10 Dq
b=B/B0 [B0 (free ion)=971]
b0=(1–b)×100
15B=(n2+n3)–3n1
b=B/B0 [B0 (free ion)=1030]
b0=(1–b)×100
J. Therm. Anal. Cal., 87, 2007
443

MODI et al.
For the anhydrous complexes:
The thermodynamic activation parameters of the
decomposition process of dehydrated complexes such
as entropy (S*), preexponential factor (A), enthalpy
(H*) and free energy of the decomposition (G*), were
calculated using the following relations [28, 29]:
140–280°C, removal of coordinate water molecules
[M(SB)2 (H2O)2 ]¾¾¾¾¾¾¾¾¾¾¾®
[M(SB)2 ]+2H2O
280–500°C
[M(SB)2 ]¾¾¾®intermediate (unstable)
Ea /RTs2 = A/Fexp(–Ea /RTs )
(2)
500–780°C
intermediate¾¾¾®metal oxide
S*=2.303(logAh/KT)R
H*=Ea–RT
(3)
(4)
(5)
The anhydrous Schiff base complexes show great
thermal stability up to 250°C; and in the third and sub-
sequent stages, the decomposition and combustion of
ligand (HSB) in the complexes occur. The nature of the
intermediates formed in the third and later stages are
not known, but they undergo violet decomposition to
give the respective metal oxides as the final product.
G*=H*–TsS*
where F is the heating rate, K is the Boltzman con-
stant, h is the Plank constant and Ts is the peak temper-
ature from DTG curve. According to the kinetic data
obtained from DTG curves, all the complexes have
negative entropy, which indicates that the studied com-
plexes have more ordered systems than reactants [30].
Calculation of activation thermodynamic parameters
of the decomposed complexes
The thermodynamic activation parameters of the de-
composition process of the complexes such as energy
of activation (Ea) and order of reaction (n) were evalu-
ated graphically by employing the Freeman–Carroll
[27] method using the following relation:
The thermal behavior of the prepared complexes
In the following paragraphs the thermal behaviour of
the synthesized complexes, characterized on the basis
of TG/DTG, DTA and DSC methods are described.
Thermal data and kinetic parameters of the complexes
are given in Tables 4 and 5, respectively.
[(–Ea/2.303R)D(1/T)]/Dlogwr=
= –n+Dlog(dw/dt)/Dlogwr
(1)
where T is the temperature in K, R is gas constant,
wr=wc–w; wc is the mass loss at the completion of the
reaction and w is the total mass loss up to time t. Ea
and n are the energy of activation and order of reac-
[Zn(SB)2(H2O)2]×2.5H2O
TG/DTG, DTA and DSC curves of the complex
[Zn(SB)2(H2O)2]×2.5H2O are represented in Figs 4
and 5, respectively. The [Zn(SB)2(H2O)2]×2.5H2O com-
plex undergoes decomposition in four stages. The first
DTG peak at 69°C, in the temperature range 50–140°C,
with the mass loss (obs. 7.57%; calc. 7.54%) corre-
sponds to the loss of two and half moles of lattice wa-
ter molecules. DTA curve shows an endothermic peak
at 76°C. This process is further supported by an endo-
thermic peak [31] at 128°C in DSC curve. The loss of
two and half moles of lattice water molecules is a first
order and the value of the energy of activation for the
dehydration process is found to be 3.851 kJ mol–1.
The second stage between 140 and 280°C corresponds
to the decomposition of two coordinated water mole-
cules [32]. The observed mass loss is 5.96% which is
consistent with the theoretical value of 6.03%. This
process is accompanied by an endothermic peak
at 226.47°C in DSC curve [33]. With heating; the de-
composition of the ligand moiety was followed. Two
well-separated peaks are displayed on the DTG curve;
this indicates that the decomposition of the
ligand (HSB) divides into two steps. The third stage,
which occurs in the temperature range 280–400°C,
with a DTG peak at 379°C, corresponds to the decom-
position of one part of the ligand. The observed mass
loss for this stage is 17.38%. It is further supported by
tion,
respectively.
A
typical
curve
of
[Dlog(dw/dt)/Dlogwr] vs. [D(1/T)/Dlogwr] for the
Zn(II) complex is shown in Fig. 3. The slope of the
plot gave the value of Ea/2.303R and the order of reac-
tion (n) was determined from the intercept.
Fig. 3 Freeman–Carroll plot for thermal dehydration of
[Zn(SB)2(H2O)2]×2.5H2O
444
J. Therm. Anal. Cal., 87, 2007

TRANSITION METAL COMPLEXES WITH UNINEGATIVE BIDENTATE SCHIFF BASE
Table 4 Thermoanalytical results (TG, DTG and DTA) of the divalent metal complexes of HSB
TG range/ DTGmax
/
DTAmax
°C
/
Mass loss/%
obs. (calc.)
Complex
Assignment
°C
°C
50–130
130–250
250–470
470–650
68
189
420
573
75(+)
–
393(–)
570(–)
7.58 (7.67)
6.26 (6.14)
38.52
35.40
87.76* (87.87)
loss of 2.5 lattice water molecules
loss of 2 coordinated water molecules
removal of one part of the ligand
removal of remaining part of the ligand
leaving MnO residue
[Mn(SB)2(H2O)2]×2.5H2O
50–190
190–490
490–780
79
313
526
82(+)
–
521(–)
13.89 (13.73)
17.67
40.28
71.75* (71.87)
loss of 4.5 water molecules
removal of one part of the ligand
removal of remaining part of the ligand
leaving Co2O3 residue
[Co(SB)2(H2O)2]×2.5H2O
[Ni(SB)2(H2O)2]×3H2O
50–130
130–240
240–390
390–490
490–650
74
198
282
438
535
81(+)
–
294(–)
–
9.28 (9.01)
5.97 (6.01)
18.1
19.48
34.92
loss of 3 lattice water molecules
loss of 2 coordinated water molecules
removal of one part of the ligand
removal of second part of the ligand
removal of remaining part of the ligand
leaving NiO residue
527(–)
87.65* (87.44)
50–130
130–230
230–370
370–780
70
186
268
462
78(+)
7.61 (7.56)
5.98 (6.05)
17.31
55.66
86.56* (86.60)
loss of 2.5 lattice water molecules
loss of 2 coordinated water molecules
removal of one part of the ligand
removal of remaining part of the ligand
leaving CuO residue
–
281(–)
–
[Cu(SB)2(H2O)2]×2.5H2O
[Zn(SB)2(H2O)2]×2.5H2O
[Cd(SB)2(H2O)2]×3H2O
50–140
140–280
280–400
400–780
69
216
379
668
76(+)
–
390(–)
679(–)
7.57 (7.54)
5.96 (6.03)
17.38
55.61
86.31* (86.34)
loss of 2.5 lattice water molecules
loss of 2 coordinated water molecules
removal of one part of the ligand
removal of remaining part of the ligand
leaving ZnO residue
50–130
130–250
250–520
520–650
67
194
361
568
72(+)
–
366(–)
555(–)
8.22 (8.27)
5.34 (5.51)
34.15
32.37
80.18* (80.30)
loss of 3 lattice water molecules
loss of 2 coordinated water molecules
removal of one fragment of the ligand
removal of remaining part of the ligand
leaving CdO residue
(+) – endothermic; (–) – exothermic; *total mass loss
Fig. 5 DSC curve of [Zn(SB)2(H2O)2]×2.5H2O
[Mn(SB)2(H2O)2]×2.5H2O
Fig. 4 TG, DTG and DTA curves of [Zn(SB)2(H2O)2]×2.5H2O
The thermal decomposition of the complex
[Mn(SB)2(H2O)2]×2.5H2O undergoes in four stages.
The thermal dehydration begins at 50°C and ends
at 130°C with a DTG peak at 68°C. The observed mass
loss (7.58%) is attributed to the decomposition of two
and half moles of lattice water molecules (theoretical
mass loss: 7.67%). The DTA curve shows an endother-
mic peak at 75°C. The thermal dehydration is of first
order and the energy of activation is found to
an endothermic peak at 373°C in DSC curve. The
fourth stage occurs between 400 and 780°C, corre-
sponding to the decomposition of the remaining part of
the ligand, with the mass loss of 55.61%. The maxi-
mum rate of mass loss is indicated by the DTG peak
at 550°C. The total mass loss (86.31%) coincides with
the theoretical value of 86.34% (Table 4). The final
residue, estimated as zinc oxide.
J. Therm. Anal. Cal., 87, 2007
445

MODI et al.
Table 5 Thermodynamic data of the thermal decomposition of metal complexes
Complex
TG range/°C Ea/kJ mol–1
n
A/s–1
S*/J K–1 mol–1
H*/kJ mol–1
G*/kJ mol–1
50–130
130–250
250–470
470–630
3.81
4.79
8.85
0.96
1.48
1.00
0.010
0.093
0.103
–284
–268
–271
–177
0.974
0.949
3.092
72.06
97.9
125
191
[Mn(SB)2(H2O)2]×2.5H2O
79.09
1.00 1.01×104
222
50–190
190–490
490–780
4.89
9.34
100.8
0.98
1.20
0.008
0.223
–286
–263
–141
1.969
4.477
94.13
102
159
207
[Co(SB)2(H2O)2]×2.5H2O
[Ni(SB)2(H2O)2]×3H2O
1.00 0.73×106
50–120
120–240
240–380
380–480
480–650
4.59
4.83
10.81
24.38
131.2
0.96
1.49
1.10
0.009
0.090
0.438
–285
–269
–257
–242
–103
1.70
0.922
6.196
18.46
124.5
101
128
149
190
207
1.01 0.35×101
1.00 0.73×108
50–130
130–230
230–370
370–780
3.90
5.12
10.55
55.81
0.97
1.39
1.00
0.010
0.112
0.452
–284
–267
–257
–195
1.052
1.308
6.052
49.07
98.5
124
145
[Cu(SB)2(H2O)2]×2.5H2O
[Zn(SB)2(H2O)2]×2.5H2O
[Cd(SB)2(H2O)2]×3H2O
1.00 1.02×103
192
50–130
130–280
280–400
400–780
3.85
4.92
13.06
59.74
0.98
1.51
1.00
0.010
0.083
0.412
–284
–270
–259
–212
1.006
0.857
7.646
51.91
98.2
133
176
1.00 1.66×102
252
50–130
130–210
210–510
510–650
3.77
4.81
8.10
0.97
1.49
1.10
0.010
0.091
0.112
–284
–269
–269
–172
0.948
0.929
2.829
80.98
97.6
126
174
88.41
1.00 1.95×104
235
be 3.810 kJ mol–1. The second stage is related to the
liberation of two coordinated water molecules from the
complex in the temperature range of 130–250°C, ac-
companied by a mass loss of 6.26% (calc. 6.14%). The
maximum rate of mass loss is indicated by the DTG
peak at 189°C. The third stage, which occurs in the
temperature range 250–470°C with a DTG peak
at 420°C, corresponding to the decomposition of one
part of the ligand. The observed mass loss for this stage
is 38.52%. This process is further supported by an exo-
thermic peak at 393°C in DTA curve. The fourth stage
is related to the decomposition of the remaining part of
the ligand in the temperature range of 470–650°C with
a DTG peak at 573°C, accompanied by a mass loss
of 35.40%. The overall mass losses are observed to
be 87.76%, which is in well agreement with the calcu-
lated value of 87.87%. The final residue, estimated as
manganese oxide, has the observed mass 12.23% as
against the calculated value of 12.10%.
nied by endothermic effects at 82 and 159°C in DTA
and DSC curves, respectively. The total loss of two
and half moles of lattice water molecules and two
moles of coordinated water molecules is a first order
reaction and the value of the energy of activation for
the dehydration process is 4.896 kJ mol–1. With heat-
ing, the decomposition of the ligand was followed.
Two well-separated peaks are displayed on the DTG
curve; this indicates that the decomposition of the
ligand divides into two steps. The temperature range
190–490°C with a DTG peak at 313°C, correspond-
ing to the decomposition of one part of the ligand.
The observed mass loss for this stage is 17.67%. The
endothermic peak at 318°C corresponds to this stage
is given by DSC curve. The third stage, which occurs
in the temperature range 490–780°C, corresponds to
the decomposition of the remaining part of the ligand.
The observed mass loss for this stage is 40.28%. The
overall mass loss 71.75% is as against the theoretical
value (71.87%). The end product estimated as Co2O3,
has the observed mass of 28.25% compared with the
calculated value of 28.11%.
[Co(SB)2(H2O)2]×2.5H2O
The thermal decomposition of the complex
[Co(SB)2(H2O)2]×2.5H2O undergoes in three stages.
The thermal dehydration of this complex takes place
in a single step between 50 and 190°C, with a mass
loss of 13.8% (calc. 13.73%). The maximum rate of
mass loss is indicated by the DTG peak at 79°C. Two
and half moles of lattice water molecules and two
moles of coordinated water molecules are removed in
this stage of dehydration. This process is accompa-
[Ni(SB)2(H2O)2]×3H2O
The thermal decomposition of the complex
[Ni(SB)2(H2O)2]×3H2O undergoes in five stages. The
first DTG peak at 74°C, in the temperature range
50–130°C, with the mass loss (obs. 9.28%; calc. 9.01%)
corresponds to the loss of three lattice water mole-
cules [34, 35]. This process is further supported by en-
446
J. Therm. Anal. Cal., 87, 2007

TRANSITION METAL COMPLEXES WITH UNINEGATIVE BIDENTATE SCHIFF BASE
[Cd(SB)2(H2O)2]×3H2O
dothermic effects at 81 and 116°C in DTA and DSC
curve, respectively. The loss of three lattice water mole-
cules is a first order and the value of the energy of acti-
vation for the dehydration process is 4.594 kJ mol–1. The
second stage occurs in the temperature range
130–240°C, with the mass loss (5.97%) is due to the re-
moval of two coordinated water molecules from the
complex (calc. 6.01%). The DTG peak corresponding to
this stage is found at 198°C. This process is further sup-
ported by an endothermic peak at 190°C in DSC curve.
With heating, the decomposition of ligand was fol-
lowed. The third (from 240–390°C), fourth (from
390–490°C) and fifth (from 490–650°C) stages may be
due to the decomposition of the ligand molecule. The
observed mass losses for these temperature ranges are
18.1, 19.48 and 34.82%, respectively. The overall mass
losses are observed to be 87.65%, which is in well
agreement with the calculated value of 87.44%. The fi-
nal residue, estimated as nickel oxide, has the observed
mass 12.32% as against the calculated value of 12.47%.
The complex [Cd(SB)2(H2O)2]×3H2O undergoes ther-
mal decomposition in four stages. The thermal dehy-
dration of this complex takes place between 50
and 130°C, with a mass loss of 8.22% (calc. 8.27%).
The maximum rate of mass loss is indicated by the
DTG peak at 67°C. Three moles of lattice water mole-
cules are removed in this stage of dehydration. This
process is accompanied by endothermic effects at 72
and 122°C in DTA and DSC curve, respectively. The
loss of three lattice water molecules is a first order re-
action and the value of the energy of activation for the
dehydration process is 3.777 kJ mol–1. This is fol-
lowed immediately by the second mass loss
(obs. 5.3%; calc. 5.5%) within the temperature
range 130–250°C with a DTG peak at 194°C, corre-
sponds to the liberation of two coordinated water
molecules. This process is accompanied by an endo-
thermic peak at 228°C in DSC curve. The third
(from 250–520°C) and the fourth (from 520–650°C)
stages may be due to the pyrolysis of ligand moiety.
The observed mass losses for these temperature
ranges are 34.15 and 32.37%, respectively. The over-
all mass losses are observed to be 80.18%, which is in
well agreement with the calculated value of 80.30%,
and the final residue estimated as CdO.
In order to discuss the thermal stability in the
solid-state, a plot of the reciprocal ionic radii of the
divalent metal ions against the atomic number exhib-
its some definite trends as shown in Fig. 6. The metal
ions follow the sequence: Ni(II)>Cu(II)>Zn(II)>
Co(II)>Mn(II)>Cd(II).
The initial temperatures of decomposition (dehy-
dration) of the complexes were taken for comparative
purposes. A plot of the initial decomposition tempera-
tures of the complexes against the atomic number ex-
hibits some definite trends as shown in Fig. 7. The
thermal stabilities of the complexes follow the se-
quence: Co(II)>Ni(II)>Cu(II)>Zn(II)>Mn(II)>Cd(II).
These orders are in good agreement with the order
predicted from the IR spectra, on the basis of separa-
[Cu(SB)2(H2O)2]×2.5H2O
The thermal decomposition of the complex
[Cu(SB)2(H2O)2]×2.5H2O takes place in four distinct
stages. The first stage starts at 50 and ends at 130°C,
with a DTG peak at 70°C. The observed mass
loss (7.61%) is attributed to the decomposition of two
and half moles of crystallized water molecules
(calc. 7.56%). This process is further supported by an
endothermic peak at 78°C in DTA curve. The activa-
tion energy for this dehydration step is equal
to 3.902 kJ mol–1 and the order of reaction for the de-
hydration process is found to be one. The second stage
is related to the decomposition of two coordinated wa-
ter molecules from the complex in the temperature
range 130–230°C with a DTG peak at 186°C. The ob-
served mass loss is 5.98%, which is consistent with the
theoretical value of 6.05%. The broad endothermic
peak at 123°C corresponding to the removal of water
molecules from the complex is given by DSC curve.
The third stage, which occurs in the temperature
range 230–350°C with a DTG peak at 268°C, corre-
sponding to the decomposition of one part of the
ligand. The observed mass loss is 17.31%. The DSC
peak corresponding to this stage is 279°C. The fourth
stage is related to the decomposition of remaining part
of the ligand, in the temperature range of 350–780°C
with the DTG peak at 462°C, accompanied by a mass
loss of 55.66%. The total mass loss (86.56%) is coin-
cides with the theoretical value of 86.60%. The end
product estimated as copper oxide.
Fig. 6 Reciprocals of ionic radii of divalent metal ions vs.
atomic number
J. Therm. Anal. Cal., 87, 2007
447

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Acknowledgements
We wish to express our gratitude to the Head, Department of
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