The thermal behaviour of nickel, copper and zinc complexes with the Schiff bases cis- and trans-N,N′-bis(salicylidene)-1,2-ciclohexadiamine (Salcn)

Article abstract of DOI:10.1016/S0040-6031(00)00777-2

The Schiff base complexes were prepared and characterised by UV, IR and NMR (¹H and ¹³C) spectroscopy, elemental analysis and X-ray powder diffractometry. Free ligands and some new complexes were submitted to thermal analysis (TG and

Full text of DOI:10.1016/S0040-6031(00)00777-2

Thermochimica Acta 370 (2001) 129±133
The thermal behaviour of nickel, copper and zinc complexes with
the Schiff bases cis- and trans-N,N0-bis(salicylidene)-1,2-
ciclohexadiamine (Salcn)
*
Eder T.G. Cavalheiro , Francisco C.D. Lemos,
Â
Â
Julio Zukerman Schpector, Edward R. Dockal
 Â
Departamento de Quõmica, Universidade Federal de SaÄo Carlos, Via Washington Luõs,
km 235, Caixa Postal 676, CEP 13565-905, SaÄo Carlos, SP, Brazil
Received 30 October 2000; accepted 12 December 2000
Abstract
The Schiff base complexes were prepared and characterised by UV, IR and NMR (1H and 13C) spectroscopy, elemental
analysis and X-ray powder diffractometry. Free ligands and some new complexes were submitted to thermal analysis (TG and
DSC) under dynamic air atmosphere. The differences in the decomposition pro®les were related to the structure of isomers
and decomposition intermediates were characterised according to their X-ray diffraction pattern. # 2001 Elsevier Science
B.V. All rights reserved.
Keywords: Nickel; Copper; Zinc; Schiff bases; Thermogravimetry; Differential scanning calorimetry
1. Introduction
In this paper, we describe the differences observed
in the thermal behaviour of nickel, copper and zinc
complexes with cis-N,N0-bis(salicylidene)-1,2-cyclo-
hexadiamine, (c-Salcn) and trans-N,N0-bis(salicyli-
dene)-1,2-cyclohexadiamine, (t-Salcn) in relation to
the structure of the isomers, using thermogravimetry
(TG) and differential scanning calorimetry (DSC).
Structural representations of the ligands and their
abbreviations are given in Fig. 1.
Schiff bases are widely used as analytical reactants
since they allow simple and inexpensive determination
of several organic and inorganic substances [1]. They
also form stable complexes with metals that perform
important role in biological systems [2].
Due to their importance in analytical and bioinor-
ganic chemistry, complexes of tetra-coordinated
Schiff bases and transition metals are extensively
studied. Such complexes present many applications
in catalysis and oxygen storage devices [3]. Many
present antitumoral, antiviral and antibacterial activity
[4] and also used as mimetic systems for enzyme
models [5].
2. Experimental
2.1. Ligand preparation
The ligands were prepared according to Felicio [6].
To a hot ethanol solution of 6.368 g (30 mmol) of cis-
1,2-diaminocyclohexane sulphate or of 5.613 g
* Corresponding author. Fax: ‡55-16-2608-350.
Â
E-mail address: cavalheiro@dq.ufscar.br (E.T.G. Cavalheiro).
0040-6031/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 0 4 0 - 6 0 3 1 ( 0 0 ) 0 0 7 7 7 - 2

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E.T.G. Cavalheiro et al. / Thermochimica Acta 370 (2001) 129±133
130
Fig. 1. Structural representations of the ligands: A Ð (c-Salcn) and B Ð (t-Salcn).
(30 mmol) of trans-1,2-diaminocyclohexane dihy-
drochloride and 8.165 g (60 mmol) of sodium acetate
trihydrate was added 6.4 cm3 (60 mmol) of the sali-
cylaldehyde. After stirring and heating at 808C for 2 h,
the resulting solution, a yellow crystalline solid, was
obtained. The ligands were used without further pur-
i®cation. (c-Salcn): yield, 8.605 g, 89%; m.p., 132±
1348C; found: C, 74.7; H, 6.9; N, 8.7. Ca. for
C20H22O2N2: C, 74.5; H, 6.9; N, 8.7. (t-Salcn): yield,
8.221 g, 85%; m.p., 115±1178C; found: C, 74.2; H,
6.9; N, 8.8. Ca. for C20H22O2N2: C, 74.5; H, 6.9; N,
8.7.
& Braun spectrophotometer, equipped with an Arid
Zone detector.
The residues and intermediates of thermal decom-
position were characterised by their X-ray diffraction
patterns, powder method, using Cu Ka radiation
between 5 and 758 with a D5000 Siemens diffract-
ometer.
2.4. Thermal measurements
Thermogravimetric curves of recently dried sam-
ples were recorded using a TGA-2050 modulus
coupled to a 2000 thermal analyser, TA Instruments,
using ca. 7 mg sample mass and a platinum sample
2.2. Complex preparation
1
holder. Heating rates of 5, 10 and 158C min under
The complexes were prepared and puri®ed using an
adaptation of a method described previously [7]. A
solution of the appropriate metal acetate, (5.0 mmol)
in 10 cm3 of distilled water was added to a hot
methanol solution, 50 cm3 containing 5.0 mmol of
the Schiff base ligand. Although, a precipitate formed
almost immediately, the mixture was re¯uxed with
stirring for 4 h. After cooling slowly to room tem-
perature, the reaction mixture was held at 08C for 12 h.
The resulting precipitate was collected by ®ltration,
washed twice with 20 cm3 of distilled water and twice
with of methanol. The complexes were puri®ed by
Soxhlet extraction using the appropriate solvent. The
puri®ed complexes were dried in a desiccator over
silica gel.
dynamic air atmosphere (90 cm3 min 1) were inves-
1
tigated. In all experiments a heating rate of 158C min
was chosen since it reduces the experiment time
without signi®cant change in the curve shape.
DSC curves were recorded on a DSC 910 modulus
coupled to a 2000 thermal analyser, TA Instruments, in
a covered aluminium sample holder with a central pin
1
hole and a sample mass of ca. 5 mg under a 108C min
1
heating rate and 100 cm3 min dynamic air ¯ow.
3. Results and discussion
3.1. Compound characterisation
The analytical data for ligands and complexes are
resumed in Table 1. The results agree with the formula
C20H22N2O2 for the ligands and with a general for-
mula C20H20N2O2MÁxH2O (where M ˆ Zn, Cu, Ni
and x ˆ 0 or 2) for the complexes.
2.3. Characterisation of the complexes
Elemental analyses (C, N, H) were performed in an
EA-1108 CHNS-O Fisons apparatus. The infrared
spectra were recorded in the range, 4000±200 cm
using CsI pellets with a MB-102 Bomem Hart Mann
1
,
The IR spectra of the free ligands and the complexes
1
exhibit various bands in the 200±4000 cm region.

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E.T.G. Cavalheiro et al. / Thermochimica Acta 370 (2001) 129±133
131
Table 1
while the differential scanning calorimetric (DSC)
curves are in Fig. 3.
Analytical data for the complexes
Mass loss (TG and calculated), temperatures ranges
and a description of thermal events observed in the
studies are summarised in Table 3.
Compound
Found (calculated) (%)
C
N
H
[Zn(c-Salcn)]Á2H2O
[Zn(t-Salcn)]Á2H2O
[Cu(c-Salcn)]
57.0 (56.9)
57.3 (56.9)
61.1 (62.6)
62.4 (62.6)
63.4 (63.4)
63.3 (63.4)
6.2 (6.1)
6.2 (6.1)
7.3 (7.3)
7.4 (7.3)
7.1 (7.4)
7.3 (7.4)
5.4 (5.7)
5.6 (5.7)
5.2 (5.3)
5.2 (5.3)
5.3 (5.3)
5.3 (5.3)
From the thermal investigation (TG/DTG) it is
possible to observe that the decomposition occurs in
different ways for the cis- and trans-isomers of ligand.
The cis-isomer decomposed in two steps producing a
carbonaceous residue previous to the ®nal burning.
The trans-isomer decomposed in a single step without
residue in the sample holder. However, both start
decomposition at around 150±1608C, suggesting simi-
lar thermal stability.
The zinc complexes were both hydrated and lost
one molecule of water in the ®rst mass loss and
afterwards decomposed in two steps. The ®nal decom-
position product was ZnO according to the X-ray
diffraction pattern. The nickel complexes also decom-
posed in two steps resulting in NiO and Ni as the ®nal
residue. The presence of metallic Ni residue suggests
the presence of carbonaceous material as decomposi-
tion intermediate.
[Cu(t-Salcn)]
[Ni(c-Salcn)]
[Ni(t-Salcn)]
The most relevant infrared peaks for ligands and
complexes are in Table 2. The O±H stretching fre-
quency of the free ligand is expected in the 3300±
1
3800 cm region, however this frequency is gener-
1
1
ally displaced to the 2555 cm cis, and 2589 cm
trans, region due to the internal hydrogen bond OH±
N=C [6,7]. The nC=N band characteristic for Schiff
bases [8±13], is observed at 1631 cm for c-Salcn
1
and 1637 cm for t-Salcn.
1
The nC±O are generally observed in the region
1270±1330 cm
1
for free ligands [6±8,12,14] and
Copper complexes presented similar TG pro®les
resulting in CuO as main residue after two decom-
position steps.
1
1305±1330 cm for the complexes [6±8]. The shift
of C=N stretching frequencies of the free ligand to
lower value as well as of C±O stretching frequencies to
higher values in the corresponding complexes was
taken as evidence for the coordination of both imino
and hydroxyl groups [6,7,12,14±16].
DSC curves presented a melting process for both
ligands, at 1218C for trans and 1358C for the cis-
isomer, followed by decomposition represented by
exothermic processes at 3508C. In agreement with
TG curves, the cis-isomer completed the decomposi-
tion above 6008C. The trans-isomer presented decom-
position peaks at 349 and 5688C in the closed DSC pan
while in the open TG crucible it decomposed com-
pletely between 147±2828C.
3.2. Results from thermoanalytical techniques
The thermogravimetic (TG/DTG) curves for the
ligands and their complexes are presented in Fig. 2
Table 2
1 a
)
Infrared data for ligands and complexes (cm
Compound
nC=N
nC±N
nC±O
M±N
M±O
c-Salcn
1631 (s)
1637 (s)
1634 (s)
1635 (s)
1625 (s)
1632 (s)
1602 (s)
1622 (s)
1391 (m)
1382 (m)
1387 (m)
1397 (m)
1393 (m)
1389 (m)
1387 (m)
1388 (m)
1280 (m)
1283 (m)
1314 (m)
1305 (m)
1326 (m)
1318 (m)
1330 (m)
1321 (m)
±
±
t-Salcn
±
±
[Zn(c-Salcn)]Á2H2O
[Zn(t-Salcn)]Á2H2O
[Cu(c-Salcn)]
[Cu(t-Salcn)]
[Ni(c-Salcn)]
[Ni(t-Salcn)]
357 (w)
357 (m)
375 (w)
380 (w)
384 (s)
384 (w)
475 (m)
472 (m)
459 (m)
458 (m)
471 (m)
478 (w)
a s: strong; m: medium; w: weak.

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E.T.G. Cavalheiro et al. / Thermochimica Acta 370 (2001) 129±133
132
Fig. 2. Thermogravimetric curves for ligands and Ni, Cu and Zn
complexes: (a) cis-isomers and (b) trans-isomers. Heating rate:
1
.
158C min 1; Pt-sample holder; synthetic air: 90 cm3 min
The zinc complexes presented similar DSC curves,
with characteristic endothermic water loss and decom-
position in agreement with TG data. An intense
exothermic decomposition peak was observed for
trans copper (3158C) and nickel complexes
(3568C). The cis-isomers presented a less intense peak
at same temperature. The ®nal decomposition is
retarded in cis copper complex.
Fig. 3. DSC curves for ligands and Ni, Cu and Zn complexes: (a)
cis-isomers and (b) trans-isomers. Heating rate: 108C min 1; Al-
1
sample holder; synthetic air: 100 cm3 min
.
According to the data in Table 3 after dehydration
zinc complexes presented the higher decomposi-
tion temperatures for the series of compounds
here investigated. The order for thermal stability

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E.T.G. Cavalheiro et al. / Thermochimica Acta 370 (2001) 129±133
133
Table 3
TG mass losses, temperature range and DSC peaks related to the thermal decomposition processes of the ligands and metal complexes under air
Process
TG data
DSC peaks (8C)a
Temperature
range (8C)
Mass loss a or residue b (%)
TG
Calculated
Cis-isomers
c-Salcn (s) ! c-Salcn (l)
±
±
±
135 (endo)
c-Salcn (s) ! CRb
156±330
330±607
274±523
248±357
121±205
292±569
90.8 a
9.1 a
±
350, >600 (exo)
568 (exo)
CR burning
9.2
±
[Ni(c-Salcn)] ! NiO
[Cu(c-Salcn)] ! CuO
[Zn(c-Salcn)]ÁH2O ! [Zn(c-Salcn)] ‡ H2O
[Zn(c-Salcn)] ! ZnO
16.3 b
19.0 b
4.55 a
20.3 b
332, 339, 533 (exo)
286, 322, 338, >500 (exo)
203 (endo)
20.7
4.46
20.1
350, 533 (exo)
Trans-isomers
t-Salcn (s) ! t-Salcn (l)
t-Salcn (l) ! decomposition
[Ni(t-Salcn)] ! NiO ‡ Ni
[Cu(t-Salcn)] ! CuO
±
±
±
121 (endo)
147±282
295±533
247±533
147±201
327±650
1.10 b
17.43 b
21.3 b
4.24 a
21.6 b
0
350, 568 (exo)
±
20.7
4.46
20.5
315, 327, 559 (exo)
214 (endo)
[Zn(t-Salcn)]ÁH2O ! [Zn(t-Salcn)] ‡ H2O
[Zn(t-Salcn)] ! ZnO
503 (exo)
a Exo: exothermic process; endo: endothermic process.
b CR: carbonaceous residue.
[3] N. Chantarasiri, T. Tuntulani, P. Tongraung, R. Seangprasert-
kit-Magee, W. Wannatong, Eur. Poly. J. 36 (2000) 695.
[4] R. Pignatello, A. Panicol, P. Mazzone, M. Pinizzotto, A.
Garozzo, P. Furneri, Eur. J. Med. Chem. 29 (1994) 781.
[5] L. Guofa, S. Tongshun, Z. Yonghian, J. Mol. Struct. 412
(1997) 75.
found is:
Znꢀt-Salcn† > Znꢀc-Salcn† > Niꢀt-Salcn†
> Niꢀc-Salcn† > Cuꢀt-Salcn† ꢁ Cuꢀc-Salcn†
This fact should be related with the structure of the
ligand and suggests the cis as a more unstable isomer.
Although, trans-isomers started their decomposition
process at higher temperatures, the intermediate ther-
mal events associated to these compounds are more
intense and occurred at lower temperatures than those
observed for cis-isomers (see DSC data in Fig. 3).
[6] R.C. Felicio, E.T.G. Cavalheiro, E.R. Dockal, Polyhedron, in
press.
[7] R.C. Felicio, G.A. da Silva, L.F. Ceridorio, E.R. Dockal,
Synth. React. Inorg. Met.-Org. Chem. 29 (1999) 171.
Â
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M.F.C. Guedes da Silva, M.A.N.D.A. Lemos, A.J.L. Pom-
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[9] L.J. Bellamy, The Infrared Spectra of Complex Molecules,
3rd Edition, Chapman & Hall, London, 1975, p. 52.
Â
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Acknowledgements
[11] A. Vogt, S. Wolowiec, R.L. Prasad, A. Gupta, J. Skarzewski,
Polyhedron 17 (1998) 1231.
Author are indebted to Brazilian agencies FAPESP
and CNPq, for ®nancial support.
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È
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