Synthesis, spectral and thermal properties of some transition metal(II) complexes with a novel ligand derived from thiobarbituric acid

Article abstract of DOI:10.1007/s10973-009-0290-2

Four novel metal(II) complexes, Ni(L)₂, Co(L)₂, Cu(L)₂, and Zn(L)₂ (L = 5-(2-(1,5-dimethyl-3-oxo-2-phenyl- 2,3-dihydro-1H-pyrazol-4-yl)hydrazono)-1,3-diethyl-2-thioxo-dihydropyrimidine-4, 6(1H,5H)-dione), were s

Full text of DOI:10.1007/s10973-009-0290-2

J Therm Anal Calorim (2009) 98:387–394
DOI 10.1007/s10973-009-0290-2
Synthesis, spectral and thermal properties of some transition
metal(II) complexes with a novel ligand derived
from thiobarbituric acid
Xiaoyi Li Æ Yiqun Wu Æ Donghong Gu Æ
Fuxi Gan
Received: 16 September 2008 / Accepted: 13 January 2009 / Published online: 28 July 2009
´
´
Ó Akademiai Kiado, Budapest, Hungary 2009
Abstract Four novel metal(II) complexes, Ni(L)2, Co(L)2,
Cu(L)2, and Zn(L)2 (L = 5-(2-(1,5-dimethyl-3-oxo-2-phe-
nyl-2,3-dihydro-1H-pyrazol-4-yl)hydrazono)-1,3-diethyl-
2-thioxo-dihydropyrimidine-4,6(1H,5H)-dione), were syn
thesized using the procedure of diazotization, coupling and
metallization. Their structures were identified by elemental
Introduction
With the emerging of high-definition television broad-
casting, the storage of over two-hour program has been
required and a new high density optical storage disc for-
mat, which is called Blu-ray disc (BD), has been devel-
oped. BD system uses a 405 nm blue-violet laser and an
objective lens with numerical aperture (NA) of 0.85 that
can write small bits and thus increases the data storage
capacity [1, 2]. Organic dyes, especially metallized organic
dyes which are with modifiable molecular structures, low
cost, and easy fabrication, have attracted much attention
[3–5]. Recently, metal(II) complexes based on barbituric
acid were found with interesting electronic and geometrical
features in connection with their application in nonlinear
optical elements, printing system, and this kind of com-
pounds may be a potential medium for information storage
[6–8]. As is known, the optical and thermal recording
mechanism is governed by the changes of the optical
properties of the recording material induced by the irre-
versible local thermal decomposition or thermal deforma-
tion, which are mainly depended on the absorption and
thermal properties, etc. [4, 5], so understanding of the basic
properties are important for a material used as a optical
recording media. But little study has been done on these
properties of transition metal complexes derived from
thiobarbituric acid. In this work, four novel metal(II)
complexes with blue-violet light absorption were synthe-
sized by a new ligand from 1,3-diethyl-2-thiobarbituric
acid and 4-aminoantipyrine reacting with transition metals
(nicket(II), cobalt(II), copper(II) and zinc(II)) ions. The
chemical structures of both the free ligand and its metal(II)
complexes were studied. The effect of different central
metal(II) ions on absorption bands of the metal(II) com-
plexes was researched. Furthermore, the thermal properties
1
analyses, H NMR, ESI-MS and FT-IR spectra. The effect
of different central metal(II) ions on absorption bands of the
metal(II) complexes was researched. The thermal properties
of the metal(II) complexes were investigated by thermo-
gravimetry (TG) and differential scanning calorimetry
(DSC). Furthermore, the thermodynamic parameters, such
as activation energy (E*), enthalpy (DH*), entropy (DS*)
and free energy of the decomposition (DG*) are calculated
from the TG curves applying Coats–Redfern method. The
results show that the metal(II) complexes have suitable
electronic absorption spectra with blue-violet light absorp-
tion at about 350–450 nm, high thermal stability with sharp
thermal decomposition thresholds.
Keywords Thiobarbituric acid ꢀ Metal(II) complexes ꢀ
Absorption spectra ꢀ Thermal property ꢀ Thermodynamic
parameters
X. Li ꢀ Y. Wu (&) ꢀ D. Gu ꢀ F. Gan
Key Laboratory of High Power Laser Materials, Shanghai
Institute of Optics and Fine Mechanics, Chinese Academy
of Sciences, No. 390, Qinghe Road, Jiading District,
Shanghai 201800, People’s Republic of China
e-mail: yqwu@siom.ac.cn
Y. Wu
Key Lab of Functional Inorganic Material Chemistry
(Heilongjiang University), Ministry of Education,
Haerbin 150080, People’s Republic of China
123

388
X. Li et al.
Coupling
of the metal(II) complexes were investigated and the
activation thermodynamic parameters, such as activation
energy, enthalpy, entropy and free energy of the decom-
position were calculated. All these properties imply that
this kind of metal(II) complexes has a promising use in blu-
ray disc-recordable systems for next generation optical
storage.
The coupling component (1,3-diethyl-2-thiobarbituric acid,
4.4055 g, 0.022 mol) was dissolved in 200 mL sodium
hydroxide solution (2%, pH = 14) and cooled to -5 to 0 °C
in an ice–salt bath. The above diazonium solution was added
to the stirred coupling component solution at -5 to 0 °C over
30 min, maintaining the pH at 8–10. The mixture was
allowed to rise to room temperature over 4 h and the pH was
lowered to about 5. The precipitated solid was filtered
with suction, washed with water and then vacuum dried.
The rough product was finally purified by column chro
matography to give 5-(2-(1,5-dimethyl-3-oxo-2-phenyl-2,3-
dihydro-1H-pyrazol-4-yl)hydrazono)-1,3 -diethyl-2-thioxo-
dihydropyrimidine-4,6(1H,5H)-dione ligand (HL).
Experimental
Materials
All the reagents and solvents in this work were of reagent-
grade quality. 4-Aminoantipyrine and 1,3-diethyl-2-thio-
barbituric acid were purchased from Sinopyarm Chemical
Reagent Co., Ltd and Acros Chemical Co. respectively and
used without further purification. A facile route was
adopted in the synthesis of the ligand and its metal(II)
complexes and the synthetic schemes together with sug-
gested structures are shown in Fig. 1.
General procedure for the synthesis of the metal(II)
complexes
The resulting ligand (0.69 mmol) was dissolved in 40 mL of
ethanol together with 0.691 mmol of sodium acetate. After
heating up to reflux, the metal(II) acetate (0.35 mmol) dis-
solved in 2 mL water was added dropwise under vigorous
stirring, whereupon a suspension of the metal(II) complex
dye results. The precipitated solid was collected by filtration,
washed with water and then vacuum dried to obtain the
metal(II) complex.
Synthesis of 5-(2-(1,5-dimethyl-3-oxo-2-phenyl-2,3-
dihydro-1H-pyrazol-4-yl) hydrazono)-1,3-diethyl-2-
thioxo-dihydropyrimidine-4,6(1H,5H)-dione ligand
(HL)
Some physico-chemical and elemental analysis data of
the synthesized ligand and its metal(II) complexes are
given in Table 1.
Diazotization
4-Aminoantipyrine (4.0650 g, 0.020 mol) was dissolved in
40 mL concentrated phosphoric acid (85%) at room tem-
perature. The solution was then cooled to -5 to 0 °C in an
ice–salt bath and maintained at this temperature, solution
of sodium nitrite (1.4861 g, 0.021 mol) in water (10 mL)
was added dropwise within 1 h under continuous stirring
and the mixture was stirred at 0–5 °C 1 h. The resulting
diazonium solution was used directly in the coupling step.
Instrument and methods
The melting points of the compounds were determined
using an XT-4 microscopic melting point apparatus (made
in China) and were uncorrected. Elemental analyses of
C, H and N were carried out on a Vario EL elemental
Fig. 1 Synthetic schemes of
ligand and its metal(II)
complexes
O
O
CH2CH3
S
CH3
CH2CH3
O
O
CH3
N
N
CH3
N
N
a) H3PO4, NaNO2, -5-0oC
b) NaOH, pH=14, -5-0oC
N
N
CH3
N
H
N
N
NH2
+
S
N
CH2CH3
CH2CH3
O
O
keto-hydrazone
O
CH2CH3
O
CH2CH3
CH3
N
N
CH3
N
CH3
N
S
CH3
M(Ac) 2
S
N
NaAC
N
N
Na+
N
N
N
N
N
ethanol
-O
O
CH2CH3
CH2CH3
O
O
M1/2
azo-enol
M = Co(II), Ni(II), Cu(II), Zn(II)
123

Synthesis, spectral and thermal properties of some transition metal(II) complexes
389
Table 1 Physico-chemical and elemental analysis data of the ligand and its metal(II) complexes
Compound (formula)
Yield/% mp/°C
Calc. (found)/%
m/z Calc. (found)
k
max/nm in chloroform
C
H
N
HL (C19H22N6O3S)
83
220–225 55.06 (54.98) 5.35 (5.29) 20.28 (20.04) 414.1 (415.1)[M ? H]? 461
Co(L)2 (C38H42N12O6S2Co) 64
Ni(L)2 (C38H42N12O6S2Ni) 71
Cu(L)2 (C38H42N12O6S2Cu) 45
Zn(L)2 (C38H42N12O6S2Zn) 59
[320
[327
[230
[300
51.52 (51.46) 4.78 (4.70) 18.97 (18.91) 885.2 (886.2)[M ? H]? 432
51.53 (51.49) 4.78 (4.69) 18.98 (18.92) 884.2 (885.2)[M ? H]? 435
51.25 (51.13) 4.75 (4.67) 18.87 (18.91) 889.2 890.1[M ? H]? 443
51.15 (51.09) 4.74 (4.69) 18.84 (18.68) 890.2 (891.2)[M ? H]? 441
analyzer. FT-IR spectra were obtained in KBr pellets on
a Perkin-Elmer FT-IR 1650 spectrometer in the 4000–
400 cm-1 region. 1H NMR spectra (DMSO solutions) were
recorded on a Brucker 400 MHz Spectrometer in DMSO-
d6 using TMS as an internal reference. ESI mass spectra
were performed using a Surveyor LCQ spectrometer. UV–
vis spectra were measured using a Perkin-Elmer Lambda
9UV/VIS/NIR spectrophotometer. Thermal properties were
analyzed with a TA instrument, the SDT Q600 Simulta-
neous DSC/TG Analyzer, at a heating rate of 10 °C min-1
from 50 to 700 °C under nitrogen atmosphere, using
2–6 mg of powdered samples in a nitrogen atmosphere.
the carbonyl stretching vibrations, have been of consider-
able importance in establishing that the compounds exist
exclusively in the keto-hydrazone form. The solid state IR
spectra of the synthesized ligand shows three intense car-
bonyl bands (1689 (mC=OꢀꢀꢀH), 1660 (mC=O), 1629
(mC=O) cm-1) consistent with a keto-hydrazone form. It
can be suggested that the ligand exists as keto-hydrazone
form with extensive six-membered intramolecular hydro-
gen bonding, and this has been confirmed by a number of
previous published reports of keto-hydrazone analogues
[10, 12, 13]. Another typical feature of the free ligand is the
existence of an NH vibration around 3100 cm-1 which
affords proof of the H-bonded hydrazone structure in the
solid state also [9, 14]; the remaining vibrational frequen-
cies obtained and their tentative assignments are shown in
Results and discussion
1
Table 2. In the H NMR spectra of the ligand measured in
Synthesis and characterization
DMSO-d6, the hydrazone proton appears as a singlet at
14.5 ppm, which corresponds to imine NH proton reso-
1
nance of the hydrazone form. Furthermore, the H NMR
The ligand was synthesized by diazo coupling reaction. It
may exist in keto-hydrazone and azo-enol tautomeric forms
as shown in Fig. 1. It has been shown, conclusively from a
series of investigations using various techniques, such as
IR, NMR and X-ray crystallography, that hydrazones
containing 4,6-dicarbonyl groups exist exclusively in the
keto-hydrazone form both in solution and in the solid state
[9–12]. An intramolecular hydrogen bridge linking one of
the carbonyl groups to the NH-moiety of the hydrazone
unit was found to be a characteristic feature of this com-
pound class. The six-membered intramolecular hydrogen
bonding ring is possible in the keto-hydrazone tautomer as
showed in Fig. 1. The infrared spectra of hydrazone based
on a substituted 4,6-dione and, in particular the position of
spectra of some dyes reported in DMSO-d6 exist in the
hydrazone form and show NH peaks within the range of
1
13.50–15.10 ppm [15–17]. The H NMR other spectra of
the ligand show a singlet at 3.27 ppm (N–CH3 of antipy-
rine ring), a singlet at 2.48 ppm (C–CH3 of antipyrine
ring), a triplet at 1.17–1.21 ppm (CH3 of thiobarbituric
acid ring), a quartet at 4.06–4.42 ppm (CH2 of thiobarbi-
turic acid ring), multi signals at 7.39–7.54 ppm (CH of
pheny). The above results suggest that the ligand prepared
exists in the keto-hydrazone form in the solid state, and the
1
obtained IR spectra, ESI mass spectra, H NMR and ele-
mental analytical data agree well with the formulae of the
ligand.
Table 2 Significant FT-IR bands and tentative assignments of the ligand and its metal(II) complexes
Compound
mH–N (hydrazone)
mAro–H
mAl–H
mC=O
mN=N
mC–O(enol hydroxyl)
mM–N
mM–O
HL
3100
3020
3053
3056
3059
3048
2868–2977
2872–2976
2872–2977
2871–2976
2868–2977
1689, 1660, 1629
1662, 1603
–
1414
1415
1417
1415
–
–
–
Co(L)2
Ni(L)2
Cu(L)2
Zn(L)2
–
–
–
–
1264
1263
1265
1262
454
461
474
448
509
509
508
505
1664, 1606
1653, 1602
1652, 1603
123

390
X. Li et al.
The metal(II) complexes were synthesized by reaction
about 1629 cm-1 corresponding to the mC=O of the anti-
pyrine ring, which was shifted to a lower frequency by
about 20–30 cm-1 after complexation in all metal(II)
complexes, suggesting that the oxygen atom of the anti-
pyrine ring also contributes to complexation [20, 22]. The
mode of bonding of the metal(II) ion is further supported
by the broad absorption bands in the region 505–509 and
448–474 cm-1, which can be assigned to mM–O and mM–N
stretching vibrations, respectively [22–25]. Therefore, from
these IR spectra, it is concluded that the ligand may exist in
azo-enol form during complexation and behave as a
O,N,O0-monobasic tridentate ligand coordinated to meta-
l(II) ions via the enolic hydroxyl O, azoic group N and O of
the antipyrine ring.
of the relevant metal(II) acetate with the ligand. Due to the
possible keto-hydrazone and azo-enol tautomerism of the
prepared ligand [11], the action of the sodium acetate on
the ligand in solution is to convert the keto-hydrazone form
into the azoanion form [18, 19]. Consequently, metal(II)
complexes were easily synthesized by the chelation of the
metal(II) ion and ligand in the azo-enol form (see Fig. 1).
The elemental analytical data, ESI mass spectra and FT-IR
spectra of the metal(II) complexes agree well with their
formulae. The metal-to-ligand ratios of the four metal(II)
complexes were found to be 1:2. Figure 1 shows the sug-
gested structure of the metal(II) complexes.
FT-IR spectra of the ligand and its metal(II) complexes
UV–vis electronic absorption spectra of the ligand
and its metal(II) complexes
The important IR characteristic absorption bands, along
with their proposed assignments, are summarized in
Table 2. As seen from Table 2, the IR spectra of the
metal(II) complexes were similar to each other, except for
some slight shifts and intensity changes of a few vibration
bands caused by different metal(II) ions, which indicates
that the complexes were of similar structure. However,
there were some significant differences between the free
ligand and the metal(II) complexes. The coordination mode
and sites of the ligand to the metal(II) ions were investi-
gated by comparing the infrared spectra of the free ligand
with its metal(II) complexes. Upon coordination, it is
noteworthy that two strong absorption bands in the region
1602–1664 cm-1 attributed to mC=O vibration were shifted
by 20–30 cm-1 on complexation, while the third carbonyl
absorption band at 1689 cm-1, appearing in the spectra of
the free ligand with the extensive six-membered intramo-
lecular hydrogen bonded structure, was not observed.
These results indicate that the ligand are in the azo-enol
form in the complex which naturally forms an enolic
hydroxyl oxygen and an enolic carbony with consequent
replacement of the enolic hydrogen by the metal(II) ion [3].
A new band due to C–O vibration appearing at 1262–
1265 cm-1 in the spectra of all the metal(II) complexes can
be further evidence for the bonding of the enol hydroxyl
oxygen to the metal(II) ion [9, 11, 20]. Furthermore, the
medium-intensity band at 3100 cm-1 of the free ligand,
due to mNH stretching, disappears in the complexes, sug-
gesting that the NH proton is lost via enolization and the
resulting enolic oxygen and azoic nitrogen take part in
coordination [5, 9]. Moreover, another new band due to
–N=N– vibration appears around 1414–1417 cm-1 in the
spectra of all the complexes, which supports the azo-enol
form of the ligand in the metal(II) complexes; the
appearance of this peak at relatively lower field may
indicate coordination via the N=N group [3, 21]. In addi-
tion, the IR spectrum of the ligand reveals a sharp band at
Figure 2 gives the electronic absorption spectra of the
ligand and its metal(II) complexes in CHCl3 solutions. It
can be seen that the ligand has three absorption bands. The
first one appearing below 250 nm may be attributed to
p ? p* transition of the heterocyclic moiety and/or phenyl
ring. The second band observed at 330 nm is attributed to
p ? p* transition of the C=O groups [20]. The third band
appearing around 461 nm can be assigned to n ? p*
transition involving the whole electron system of the ligand
[22]. In the spectra of the metal(II) complexes, their
absorption bands are hypsochromically shifted comparative
to the corresponding bands of the free ligand. The results in
our experiments can be explained as follows: the visible
absorption band of the ligand as the azoanion form are
observed a hypsochromically shift comparative to that of
the hydrazone ligand [19, 26]. The above effects in visible
spectra of the metal(II) complexes may support the
1.6
d
1.2
e
a
b
0.8
0.4
0.0
c
300
400
500
Wavelength/nm
600
700
Fig. 2 Absorption spectra of the ligand and its metal(II) complexes
in chloroform: (a) ligand (HL); (b) Ni(L)2; (c) Co(L)2; (d) Cu(L)2; (e)
Zn(L)2
123

Synthesis, spectral and thermal properties of some transition metal(II) complexes
391
participation as anion of the ligand. Otherwise this
20
0
100
(a)
TG
DTG
340
assumption is very reasonable [18, 27, 28].
90
From Fig. 2, it also can be observed that the maximal
80
absorption peak of the free ligand is at 461 nm, but those of
DSC
its metal(II) complexes are at 432, 435, 443 and 441 nm for
Co(L)2, Ni(L)2, Cu(L)2 and Zn(L)2, and blue shift 29, 26,
18 and 20 nm comparative to the free ligand, respectively.
This also indicates that center metals have influence on the
absorption bands of the complexes. According to modern
molecular orbital theory [26], any factor, such as the
electronegativity, the radius and electronic configuration of
the metal(II) ion, that can influence the electronic density
of a conjugated system must result in the difference of the
absorption band. For metal(II) ions, in generally, the ability
to attract the electron enhances with the electronegativity
increasing and the radii reducing. In the complexes Co(L)2,
Ni(L)2, Cu(L)2, the order of electronegativity metal(II) ions
is 1.88 Co2?(d7) \ 1.91 Ni2?(d8) \ 2.0 Cu2?(d9), the
70
60
50
40
30
-20
-40
-60
337
100
200
300
400
500
600
700
Temperature/°C
100
90
80
70
60
50
40
30
(b)
20
0
TG
324
DTG
DSC
-20
-40
-60
2?
2?
order of radii is Co2? (0.74 A) [ Ni (0.72 A) [ Cu
˚
˚
˚
(0.69 A), so the ability to attract the electrons from the
oxygen and nitrogen should be Co(II) \ Ni(II) \ Cu(II),
and the n electronic density in the conjugated system is
increased from Co(L)2 to Cu(L)2, and the energy gaps of
n ? p* is reduced from Co(L)2 to Cu(L)2, Therefore, the
blue shift is Co(L)2 [ Ni(L)2 [ Cu(L)2, which is well in
agreement with our experiment results. Except for Zn(L)2,
because of its fully filled d-orbit (d10). Furthermore, from
the Fig. 2 it can be found that these complexes have suitable
absorption at 405 nm, and could well matched with blue
laser. These characteristics imply that these complexes may
have potential application for blu-ray optical storage.
321
100
200
300
400
500
600
700
Temperature/°C
100
20
0
(c)
TG
245
90
80
70
60
50
40
30
20
DTG
DSC
-20
-40
-60
Thermal properties of the metal(II) complexes
242
The thermal properties of the metal(II) complexes were
investigated by thermogravimetric (TG) analysis, differen-
tial thermogravimetric (DTG) and differential scanning
calorimetry (DSC). Figure 3 presents the recorded TG/DTG
and DSC curves of four metal(II) complexes in nitrogen
atmosphere. It can be seen that the TG curves of the com-
plexes show no any mass loss up to 200 °C, indicating the
absence of water molecule and any other adsorptive solvent
molecules in the coordination sphere. As the temperature is
increased, the TG/DTG curves of the four metal(II) com-
plexes exhibit a sharp mass loss at temperature of about
240–340 °C, which are accompanied with a sharp exo-
thermic peak in the DSC curves. The correlations between
the different decomposition steps of the metal(II) com-
plexes with the corresponding mass losses are discussed in
terms of the proposed formulae of the metal(II) complexes.
The Ni(L)2 complex shows only one-step decomposition.
This step shows drastic mass loss of 47.89% within a wide
100
200
300
400
500
600
700
Temperature/°C
20
0
100
90
80
70
60
50
40
30
(d)
310
TG
DTG
DSC
-20
-40
-60
309
300
100
200
400
Temperature/°C
500
600
700
Fig. 3 TG/DTG and DSC curves of the metal(II) complexes: a
Ni(L)2; b Co(L)2; c Cu(L)2; d Zn(L)2
123

392
X. Li et al.
Kinetic studies of non-isothermal decomposition
temperature range 319–375 °C with the DTG peak at
337 °C, also giving rise to a sharp exothermic peak at 340 °C
in DSC curve. This process can be readily interpreted as
loss of one ligand molecule (Ni(L)2 ? Ni(L) ? L) (calcd.
46.69%). The following decomposition of the Ni(L)2
complex exhibits a gradual mass loss process in the rest of
temperature range, and shows no obvious peaks both in the
DTG and DSC curves.
The kinetic parameters of the first decomposition process
of the metal(II) complexes were calculated from the non-
isothermal TG curves, with heating rate of 10 °C min-1
using Coats–Redfern method [29]. The equations are used
,
in the forms:
"
ꢂ
#
ꢂ
ꢀ
ꢁꢃ
1ꢁn
1 ꢁ ð1 ꢁ aÞ
AR
bEꢂ
2RT
Eꢂ
Eꢂ
The Co(L)2 complex is thermally decomposed in two
successive decomposition steps within the temperature
range 50–700 °C. The first decomposition step of estimated
mass loss 26.70% within the temperature range 305–334 °C
may be attributed to the loss of the C11H11N4O molecule
(diazo part of the ligand) (calcd. 24.40%). Simultaneously,
the DTG peak corresponding to this stage is observed at
321 °C. The recorded DSC curve reveals sharp exothermic
peak at 324 °C. The second step occurs with the tempera-
ture range 336–700 °C, which may be accounted for the
decomposition of the remaining ligand molecules, and
shows no obvious peaks in DSC curve.
ln
ln
¼ ln
1 ꢁ
ꢁ
ðn ¼ 1Þ
ð1Þ
T2ð1 ꢁ nÞ
RT
ꢃ
ꢂ
ꢃ
ꢁ lnð1 ꢁ aÞ
ꢁEꢂ
AR
bEꢂ
¼
þ ln
ðn ¼ 1Þ:
ð2Þ
T2
RT
Where a is the fraction of sample decomposed at
temperature T, R is the gas constant, A is the pre-
exponential factor, n is reaction order, E* is the
activation energy in kJ mol-1, b is the heating rate and
(1 - (2RT/E*) ffi 1.
Plots of ln{[1 - (1 - a)1-n]/[T2(1 - n)]} against 1/T for
different values of n have been tested and ln[-ln(1 - a)/T2]
against 1/T has also been tested. The correct value of n for
the thermal reaction is the one which gives the best straight
line (shown in Fig. 4). From the slope of the straight line
the activation energy E* could be calculated, where the
slope = -E*/R. A was determined from the intercept. The
entropy of activation (DS*), enthalpy of activation (DH*)
and the free energy change of activation (DG*) were calcu-
lated using the following equations:
The Cu(L)2 complex is thermally decomposed in two
successive decomposition steps. The first decomposition
step of estimated mass loss 25.78% within the temperature
range 219–272 °C may be attributed to the loss of the
C11H11N4O molecule (calcd. 24.27%). The DTG peak
corresponding to this stage is observed at 242 °C. The
recorded DSC curve reveals sharp exothermic peak at
245 °C. The second step occurs with the temperature range
272–700 °C with a DTG peak at 335 °C, which may be due
to the decomposition of the remaining ligand molecules,
and shows no obvious peaks in DSC curve.
ꢀ
ꢁ
Ah
DSꢂ ¼ 2:303R log
DHꢂ ¼ Eꢂ ꢁ RTS
ð3Þ
kTs
The Zn(L)2 complex shows decomposition pattern of two
stages. The first step displays a sharp mass loss of 24.67%
within the temperature range 290–326 °C with a DTG peak
at 309 °C, which may be attributed to the loss of the
C11H11N4O molecule (calcd. 24.12%). The recorded DSC
curve reveals sharp exothermic peak at 310 °C. After the
rapid mass loss of the Zn(L)2 complexes, the second step
exhibits a gradual loss in mass within the remaining tem-
perature range with a DTG peak at 369 °C, which may be
attributed to the decomposition of the remaining ligand
molecules. This step shows no obvious peaks in DSC curve.
The above TG and DTG data reveal that the decompo-
sition pattern is different. The Ni(L)2 complex exhibits
ð4Þ
ð5Þ
DGꢂ ¼ DHꢂ ꢁ TSDSꢂ
where k, h and Ts are the Boltzman constant, Planck con-
stant and the DTG peak temperature, respectively.
-14
-15
d
c
b
-16
-17
-18
only one-step decomposition, whereas
a two-stage
a
decomposition is observed in the case of Co(L)2, Cu(L)2
and Zn(L)2 complexes. According to the beginning tem-
perature of the decomposition of the metal(II) complexes
the following order of thermal stability may be proposed:
Ni(L)2 [ Co(L)2 [ Zn(L)2 [ Cu(L)2. The difference in
thermal stability of the metal(II) complexes reveals that the
metal(II) ion may have some marked influence on the
thermal stability.
1.6
1.7
1.8
1000/T/K-1
1.9
2.0
Fig. 4 Coats and Redfern plots for the metal(II) complexes: (a)
Ni(L)2; (b) Co(L)2; (c) Cu(L)2; (d) Zn(L)2
123

Synthesis, spectral and thermal properties of some transition metal(II) complexes
393
Table 3 The thermal behavior and kinetic parameters of the metal (II) complexes
Complex
Temperature
range/°C
Mass loss found
(calc.)/%
E*/kJ mol-1
A/s-1
DS*/J K-1 mol-1
DH*/kJ mol-1
DG*/kJ mol-1
Ni(L)2
Co(L)2
Cu(L)2
Zn(L)2
319–375
305–334
221–272
290–326
47.89 (46.69)
26.70 (24.30)
25.48(24.18)
24.67 (24.12)
565.35
404.39
165.45
563.68
3.89 9 1044
3.02 9 1031
1.35 9 1012
2.34 9 1046
602.85
352.07
-17.23
636.91
560.28
399.45
161.16
558.61
192.45
190.27
170.04
170.00
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assigning the strength of the bonding of ligand moieties with
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