Synthesis and thermal decomposition kinetics of Gd(III) complex with unsymmetrical Schiff base Ligand

Article abstract of DOI:10.14233/ajchem.2013.13624

A new unsymmetrical solid Schiff base (LLi) was synthesized using L-lysine, o-vanillin and 2-hydroxy-l-naphthaldehyde. Solid gadolinium(III) complex of this ligand [GdL(NO₃)]NO₃2H₂O has been prepared and characterized by elemental analyses, IR spectra, 1H NMR spectra, UV spectra and molar conductance. The thermal decomposition kinetics of the complex for the second stage was studied under non-isothermal condition by TG and DTG methods. The kinetic equation may be expressed as: d?/dt = Ae-E/RT(1-?α) ₃. The kinetic parameters (E, A), activation entropy δS≠and activation free-energy δG≠were also gained.

Full text of DOI:10.14233/ajchem.2013.13624

Asian Journal of Chemistry; Vol. 25, No. 6 (2013), 3267-3270
http://dx.doi.org/10.14233/ajchem.2013.13624
Synthesis and Thermal Decomposition Kinetics of
Gd(III) Complex with Unsymmetrical Schiff Base Ligand
1
2
2
2
2
2,*
SHUANGYU BI , JIAN ZUO , NAN ZHANG , PENGFEI ZHANG , ZHEN ZHANG and YUHUA FAN
1Center for Theoretical Biology, Academy for Advanced Interdisciplinary Studies, Peking University, Haidian District, Beijing 100871, P.R. China
2Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, College of Chemistry and Chemical Engineering,
Ocean University of China, Qingdao 266100, P.R. China
*Corresponding author: E-mail: fanyuhua301@163.com
(Received: 18 February 2012;
Accepted: 17 December 2012)
AJC-12563
A new unsymmetrical solid Schiff base (LLi) was synthesized using L-lysine, o-vanillin and 2-hydroxy-l-naphthaldehyde. Solid
gadolinium(III) complex of this ligand [GdL(NO3)]NO3·2H2O has been prepared and characterized by elemental analyses, IR spectra, 1H
NMR spectra, UV spectra and molar conductance. The thermal decomposition kinetics of the complex for the second stage was studied
under non-isothermal condition by TG and DTG methods. The kinetic equation may be expressed as: dα/dt = Ae-E/RT(1-α)2. The kinetic
parameters (E, A), activation entropy ∆S≠ and activation free-energy ∆G≠ were also gained.
Key Words: Unsymmetrical Schiff base, Gd(III) complex, Thermal decomposition.
Unsymmetrical Schiff base (LLi): Mono-Schiff base
(HR) (1.402 g, 5.0 mmol) and lithium hydroxide (0.120 g, 5.0
mmol) were dissolved in 60 mL anhydrous methanol and
isopropanol in the proportion of 1:5 (v/v) and stirred for 1 h at
50-55 ºC. 2-Hydroxy-l-naphthaldehyde (0.861 g, 5.0 mmol)
dissolved in 10 mL isopropanol was added drop-wise to the
above solution and stirred for 4 h at 50-55 ºC to give a yellow
precipitate. The precipitate was collected by filtration, washed
with ethanol and dried in vacuum. The yield of the Schiff base
(LLi) was 1.563 g (71 %) and the purity was higher than 99 %.
Calcd. (%) for C25H25N2O5Li (440.4): C, 68.18; H, 5.72; N,
6.36, found (%): C, 67.24; H, 5.77; N, 6.34.
INTRODUCTION
Some Schiff base complexes derived from amino acids
are particularly active in biology. Recently, studies of such
metal complexes of mono-Schiff bases have been reported1-4.
In this study, a new unsymmetrical Schiff base ligand has been
synthesized starting from L-lysine, o-vanillin and 2-hydroxy-
l-naphthaldehyde by a new method. Since this ligand dose not
exist in literature, this paper deals with the preparation and
characterization of the complex formed from this Schiff base
ligand with Gd(III). As thermal aspects are essential in the
complex, the thermal decomposition process of [GdL(NO3)]NO3·
2H2O by TG-DTG is described in this paper and the corres-
ponding non-isothermal kinetics are discussed.
Preparation of the complex: The unsymmetrical Schiff
base (1.321 g, 3.0 mmol) dissolved in 65 mL anhydrous metha-
nol and isopropanol in the proportion of 1:5 (v/v) was mixed
with Gd(NO3)3·6H2O (1.355 g, 3.0 mmol) dissolved in 15 mL
anhydrous ethanol and stirred for 3 h at 50-55 ºC to give yellow
precipitate. The precipitate was filtered, recrystallized with
anhydrous methanol and isopropanol in the proportion of 1:5
(v/v) and dried in vacuum. The yield of the complex was 1.532
g (68 %) and the purity was higher than 99 %. Calcd. (%) for
C25H29N4O13Gd (750.8): C, 39.99; H, 3.89; N, 7.46; Gd, 20.95,
found (%): C, 40.09; H, 3.76; N, 7.52; Gd, 21.96.
EXPERIMENTAL
All the reagents used in this work were of analytical grade.
Hydrated gadolinium(III) nitrate was prepared by the reaction
of gadolinium(III) oxide with nitric acid.
Mono-Schiff base (HR): L-lysine (2.193 g, 15 mmol)
was dissolved in 230 mL of anhydrous ethanol and methanol
in the proportion of 1:1 (v/v) and heated for 1.5 h at 55-50 ºC,
filtered. o-Vanillin (2.282 g, 15 mmol) dissolved in 40 mL of
hot ethanol was added drop-wise to the above filtered solution
and stirred for 2 h at 50-55 ºC to give a light yellow precipitate.
The precipitate was collected by filtration, washed with ethanol
and dried in vacuum. Yield 2.943 g (70 %); m.p. 232-234 ºC.
Elemental analyses were carried out with a model 2400
Perkin-Elmer analyzer. The metal content was determined
gravimetrically. Infrared spectra of the ligand and complex
were recorded in KBr pellets using a Bio-Rad FTS 165

3268 Bi et al.
Asian J. Chem.
spctrophotometer. The ultraviolet spectra were recorded on a
Shimadzu UV-3000 spctrophotometer in DMSO. The molar
conductance was measured with a Shanghai DDSJ-308A
conductivity meter. 1H NMR spectra were recorded in DMSO-
d6 as the solvent at 600 MHz with a JNM ECP-600 spectro-
meter using tetramethylsilane (TMS) as an internal reference.
Thermogravimetric measurements were made using a Perkin-
Elmer TGA7 instrument. The heating rate was programmed
to be 10 ºC/min with a protecting stream of N2 flowing at a
rate of 40 mL/min. The mass spectrogram of the ligand was
recorded on a Finnegan MAT-212 mass spectrometer.
Fig. 2. Mass spectrum of LLi
RESULTS AND DISCUSSION
the complex, five additional bands, which are not present in
the spectrum of the ligand, were observed. Of these, the bond
of 1035 cm-1 is assigned to the ν2 mode of the nitrate group.
The bands of 1489 and 1270 cm-1 in the complex are the two
split bands of ν4 and ν1, respectively, of the coordinated nitrate
group. The magnitude of ν4-ν1 is more than 180 cm-1 for the
complex, which indicates that the nitrate group in coordinated
to the metal ion in a bidentate fashion. The bands at 1387 and
819 cm-1 are assigned to the non-coordinated nitrate group6.
The shift of νas(COO–) and νs(COO–) from 1633 and 1398 cm-1
in the ligand to 1644 and 1398 cm-1 in the complex, respec-
tively, suggests the coordination of the oxygen in the carboxy-
late group to the metal ion. The magnitude of νas(COO–)-
νs(COO–) is more than 200 cm-1 in the complex, which indicates
that the -COO– group is coordinated to the metal ion in a
monodentate fashion7. The broad bands at 3144 cm-1 in the
complex is attributed to ν(O-H) of phenol and water molecules.
Electronic spectra: The electronic spectrum of the
complex in DMSO exhibits two spectral bands at 262 and 382
nm, having the molar extinction coefficients ε = 2.77 × 104,
7.21 × 103 L mol-1 cm-1, respectively. These bands occur at
270, 375 nm (ε = 3.65 × 104, 9.33 × 103 L mol-1 cm-1) in the
spectrum of the ligand. The change of the molar extinction
coefficients suggests that the ligand is coordinated to the metal.
1H NMR spectra: The 1H NMR spectra of the ligand and
complex [GdL(NO3)]NO3·2H2O were recorded in DMSO-d6.
In the spectrum of the ligand the phenolic OH proton appears
at 14.21 ppm. This signal shifts to 12.68 ppm in the complex
spectrum, which indicates that the coordination of phenolic
oxygen to metal ions. The peak at 9.37 ppm in the ligand can
be assigned to CH=N proton. It shifts to 8.74 ppm in the
spectrum of the complex, which confirms the coordination of
azomethine group to metal ion. In the spectrum of the complex
the multisignals within the 7.36-7.58 ppm range were assigned
to aromatic H protons. In the spectrum of the ligand the -OCH3
proton appears at 3.36 ppm and it appears at 3.34 ppm in the
complex spectrum, which indicates the -OCH3 is not bound to
metal ion.
The reaction activity and steric hindrance of the two
-NH2 in L-lysine is different and the -NH2 in a seat have higher
activity than the -NH2 in e seat because of the induced effect
of -COO– in L-lysine. When the molar ratio of L-lysine and
o-vanillin was 1:1, the o-vanillin reacted with the -NH2 in a
seat first forming the mono-Schiff base. Then the mono-Schiff
base reacted with 2-hydroxy-l-naphthaldehyde forming the
unsymmetrical di-Schiff base. The synthesis reactions of the
ligand are shown in Fig. 1. The synthesis of the complex may
be represented as:
-
H3N+CH2CH2CH2CH2CHCOO
CHO
N
H2NCH2CH2CH2CH2CHCOOH
HO
+
CH
NH2
H3CO
HO
H3CO
-
-
H3N+CH2CH2CH2CH2CHCOO
N
+
CH2 CH2 CH2 CH2 CH COO Li
CHO
N
N
LiOH
OH
CH
+
HC
CH
HO
HO
OH
H3CO
H3CO
Fig. 1. Preparation of the ligand
Gd(NO3)3·xH2O + LLi = [GdL(NO3)]NO3·2H2O
+ LiNO3 + (x-2)H2O
The molar conductance value of the complex determined
in DMSO is 50.8 S cm2 mol-1, which is expected for 1:1 elec-
trolyte5. This suggests that one nitrate ion is within the coordi-
nation sphere and the second is ionic and not coordinated.
The complex is stable in air and soluble in DMSO and DMF;
however insoluble in benzene, acetone or diethyl ether.
Mass spectrum: The mass spectrum of LLi is shown in
Fig. 2. The molecular weight of LLi is 440, which indicates
that the reaction product of L-lysine with o-vanillin and 2-
hydroxy-l-naphthaldehyde is an unsymmetrical di-Schiff base.
IR Spectra: The shift of ν(C=N) from 1633 cm-1 in the
ligand to 1644 cm-1 in the complex, suggests the formation of
a C=N-La bond system. The vibration ν(Ar-O) of LLi occurs
at 1228 cm-1 and the shift to lower frequency ca. 13 cm-1 in the
complex indicates the coordination of hydroxyl oxygen to
metal ion. The shift of ν(C-O-C) from 1093 cm-1 in the ligand
to 1085.8 cm-1 in the complex, which indicates the coordination
of the oxygen in the methoxyl to metal ion. In the spectrum of
Thermal decomposition studies: The TG and DTG
curves of the complex are shown in Fig. 3, which indicates
that the complex decomposes in three steps. The first weight
loss stage has a decomposition temperature range of 61-141
ºC, with a weight loss of 5.12 %, which corresponds to the
loss of two molecules of water (calcd. 4.80 %). The fact that
the water molecule was lost at a low temperature suggests that

Vol. 25, No. 6 (2013)
100
Synthesis and Thermal Decomposition Kinetics of Gd(III) Complex 3269
The original kinetic data for the second step obtained form
the TG and DTG curves are listed in Table-1, in which Ti is
the temperature at any point i on the TG and DTG curves, αi is
the corresponding decomposition rate. (dα/dt)i = [β/(W0 –W1)]
× (dW/dT)i in which (dW/dT)i is the height of the peak in the
DTG curve, β is the heating rate and W0 and W1 are the initial
and final weight at that stage, respectively. The calculated
kinetic parameters (E, A) and correlation coefficients (r) of
steps (2) are listed in Table-2.
50
TABLE-1
DATA FOR STEP (2) OF THE
THERMODECOMPOSITION OF [GdL(NO3)]NO3·2H2O
OBTAINED FROM TG AND DTG OF CURVES
0
60.
220
380
Temperature (ºC)
540
700
Fig. 3. TG-DTG curves of the complex
Ti (K)
α
(dα/dt)i
0.0918
0.1210
0.1354
0.1422
0.1497
0.1585
0.1667
0.1592
0.1510
0.1449
0.1347
0.1204
0.0912
i
543
547
549
550
551
552
553
554
555
556
557
559
563
0.2342
0.3092
0.3223
0.3861
0.4064
0.4882
0.4950
0.5258
0.5479
0.5660
0.5787
0.6293
0.7288
the water is crystal water. The second weight loss stage showed
a continuous weight loss between 141 and 429 ºC, with a
weight loss of 58.26 %, which corresponds to the loss of
unsymmetrical Schiff base ligand (calcd. 57.74 %). The third
stage showed a continuous weight loss between 429 and 770 ºC
and 23.88 % of the original sample remained.With its calculated
weight percentage of 24.15 %, Gd2O3 is the final product.
On the basis of 30 kinetic functions in both differential
and integral forms commonly used in recent reviews8, the non-
isothermal kinetics of the steps were investigated using theAchar
differential method9 and the Coats-Redfern integral method10.
TABLE-2
RESULTS OF ANALYSIS OF THE DATA FOR STEP (2) IN TABLE-1 BY
ACHAR DIFFERENTIAL METHOD AND COATS-REDFERN INTEGRAL METHOD
No.
1
E (kJ/mol)
153.5
211.9
235.4
280.4
105.5
415.5
135.9
65.54
30.36
-4.83
ln (A/s-1)
31.08
43.63
47.34
54.36
18.26
87.43
28.24
12.68
4.82
r
E (kJ/mol)
294.3
328.8
342.4
369.8
261.18
459.9
201.9
131.6
96.37
61.18
43.59
170.2
180.3
142.5
66.67
41.38
28.74
275.6
58.35
91.96
413.0
624.2
835.3
261
ln (A/s-1)
49.81
56.82
58.35
64.47
40.15
84.52
30.83
15.68
8.10
r
0.7747
0.8695
0.8937
0.9257
0.66
0.9717
0.9763
0.9779
0.9806
0.9689
0.9862
0.9831
0.9823
0.9814
0.9796
0.9774
0.9776
0.9797
0.9699
0.9657
0.9607
0.9545
0.9886
0.9737
0.9679
0.9838
0.984
2
3
4
5
6
0.9679
0.8647
0.6851
0.4207
0.0782
0.3511
0.6234
0.7372
0.0087
0.6819
0.7957
0.8352
0.9879
0.9396
0.5058
0.9527
0.9677
0.9734
0.9837
0.7792
0.7627
0.8923
0.9297
0.0555
0.3626
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
-3.17
0.520
-3.27
23.07
24.92
17.59
1.48
-22.42
68.37
90.88
0.820
-75.04
-100.3
-113.0
270.0
203.5
-49.75
347.0
558.2
769.3
406.1
102.1
-134.3
-269.4
-404.5
-3.87
-7.24
12.51
17.12
-1.83
-18.64
-24.41
-27.39
58.30
42.58
-12.98
74.39
120.26
166.01
89.08
19.33
-31.21
-60.88
-90.66
-3.35
-3.89
-6.58
49.69
0.410
6.85
76.29
121.75
167.21
45.51
25.8
0.9842
0.9787
0.9806
0.9475
0.9158
0.8745
0.9774
0.9750
185.6
98.34
66.67
44.42
85.57
80.51
8.39
1.67
-3.09
5.15
30.44
3.76
4.22

3270 Bi et al.
Asian J. Chem.
The results obtained from the two different methods are
ACKNOWLEDGEMENTS
approximately the same when based on function No. 18. The
kinetic equation is expressed as: dα/dt = A e-E/RT (1-α)2, E =
272.8 kJ/mol, ln A = 54.00, r = 0.9883.
This work was supported by the National Science Foun-
dation of China (No. 20971115 and No. 21071134).
The activation entropy ∆S≠ and activation free-energy ∆G≠
are calculated by the following equations11: A = kTs exp (∆S≠/
R)/h, Ae-E/RT = kTs exp (∆S≠/R) exp (-∆H≠/RT)/h, ∆G≠ = ∆H≠-
T∆S≠, in which Ts is the temperature at the top of peak (2), k is
Boltzmann constant, R is gas constant, h is Plank constant.
The activation entropy ∆S≠ and activation free-energy ∆G≠ for
second thermal decomposition stage were gained, ∆S≠ = 198.9
J/mol K, ∆G≠ = 162.8 kJ/mol.
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Conclusion
The results presented here indicate that L-lysine can
react with o-vanillin and 2-hydroxy-l-naphthaldehyde forming
unsymmetrical Schiff base LLi and gadolinium nitrate can form
stable solid complex with this ligand. The composition of the
complex is confirmed to be [GdL(NO3)]NO3·2H2O. The kinetic
equation for second decomposition step may be expressed as:
dα/dt = A e-E/RT(1-α)2, E = 272.8 kJ/mol, ln A = 54.00, r =
0.9883, ∆S≠ = 198.9 J/mol K, ∆G≠ = 162.8 kJ/mol.
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(1987).
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