Factors influencing the thermal decomposition of transition metal complexes with 2-OH-aryloximes under nitrogen

Article abstract of DOI:10.1007/s10973-005-0965-2

The thermal behavior of copper(II), nickel(II) and palladium(II) complexes with two anionic varieties of 2-OH-aryloximes (ox), [M(ox)₂] (2-hydroxypropiophenonoxime and 2-hydroxy-4-methoxy-benzophenonoxime) was studied by using simultaneous TG/DTG-DTA technique under nitrogen in the temperature range 40-700°C. The behavior was compared with that in static air, which had been previously studied. It was found that the metal, the substituents on the ligand and the heating rate influenced their thermal decomposition. The thermal stability of the complexes with the same ligand depended on the metallic cation, following the order Pd(II)>Ni(II)>Cu(II). It also depended on the type of ligand, increasing with bulky substituents on the oximic carbon and the benzene ring. The sample mass almost did not affect their decomposition mode. The residues at 700°C of all complexes consisted of a carbonaceous oxide, determined by energy dispersive spectrometry (EDS) and IR spectroscopy.

Full text of DOI:10.1007/s10973-005-0965-2

Journal of Thermal Analysis and Calorimetry, Vol. 82 (2005) 791–796
FACTORS INFLUENCING THE THERMAL DECOMPOSITION OF
TRANSITION METAL COMPLEXES WITH 2-OH-ARYLOXIMES UNDER
NITROGEN
M. Lalia-Kantouri*
Aristotle University of Thessaloniki, Department of Chemistry, Laboratory of Inorganic Chemistry, P.O. Box 135,
Thessaloniki 54124, Greece
The thermal behavior of copper(II), nickel(II) and palladium(II) complexes with two anionic varieties of 2-OH-aryloximes (ox),
[M(ox)2] (2-hydroxypropiophenonoxime and 2-hydroxy-4-methoxy-benzophenonoxime) was studied by using simultaneous
TG/DTG-DTA technique under nitrogen in the temperature range 40–700°C. The behavior was compared with that in static air,
which had been previously studied. It was found that the metal, the substituents on the ligand and the heating rate influenced their
thermal decomposition. The thermal stability of the complexes with the same ligand depended on the metallic cation, following the
order Pd(II)>Ni(II)>Cu(II). It also depended on the type of ligand, increasing with bulky substituents on the oximic carbon and the
benzene ring. The sample mass almost did not affect their decomposition mode. The residues at 700°C of all complexes consisted of
a carbonaceous oxide, determined by energy dispersive spectrometry (EDS) and IR spectroscopy
Keywords: copper complexes, nickel complexes, 2-OH-aryloximes, palladium complexes, thermal properties TG/DTG-DTA, tran-
sition metal complexes
Introduction
studies for some divalent metal (Cu, Ni, Pd)
2-OH-aryloximates, [M(ox)2], reveal a slightly dis-
torted trans square-planar configuration with the
M–N bonds being longer than the M–O ones. Strong
intramolecular hydrogen bonds between the phenolic
oxygen and the hydrogen of the oximic hydroxyl were
observed in all the cases [10–11, 13–15].
The thermal behavior of some transition metal
complexes of 2-hydroxyaryloximates has received
considerable attention [12, 17 and references therein].
Their thermal stability and decomposition mode have
been studied in air atmosphere, using the TG-DTA
technique, while mass spectrometry (MS) was very
helpful for determining the various fragmentation pat-
terns. Recently, there has been an increasing research
interest on the thermal properties of transition metal
complexes, with ligands having N,O donor abilities
like the 2-hydroxyoximes, studied by TG-DTA or
TG-MS/DTA techniques [18–19].
The strong coordinating properties of 2-hydroxy-
aryloximes (ox) with transition metals [1–2] have stim-
ulated researches in finding significant applications in
solvent extraction systems and in hydrometallurgy
[3–5]. Knowledge of the structural features and the pa-
rameters influencing bond formation and chelate sta-
bility might help in the design of ligands with predeter-
mined properties. Substituents, aryl or alkyl, attached
to the oximic carbon of the hydroxyoximes engender
stereoelectronic effects that may influence distinctive
physicochemical and biological properties. For in-
stance, Moreley suggested, from theoretical studies,
that steric factors were responsible for the different
extraction behaviours of various 2-hydroxyaryl-
oximes [6]. Moreover, correlation of the structure of
bis(hydroxyaryloximates) with biological activities
have been reported [7–8].
In view of these significant applications we have
initiated several years ago, a systematic research pro-
ject on some 3d-transition metal complexes with
2-hydroxyaryloximes in order to establish the chemi-
cal, electronic and redox properties of the various co-
ordination geometries obtainable within the oxime
system [8–12, 16]. The metals Cu, Ni and Pd were se-
lected on the basis of their different electronic config-
urations and their biological activities. The X-ray
Continuing our investigations, the present study
involving the thermal behavior of transition metal
2-OH-aryloximates was undertaken in order to ex-
plore the factors influencing their thermal decomposi-
tion process, such as: i) the substituents of the oxime
ligands and the structure of the complexes, ii) the
heating rate, iii) the sample mass, and iv) the atmo-
sphere around the sample. For this purpose, the
*
lalia@chem.auth.gr
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2005 Akadémiai Kiadó, Budapest

LALIA-KANTOURI
Methods
The simultaneous TG/DTG-DTA curves for the com-
plexes [M(ox)2] were obtained on a SETARAM in-
strument, model SETSYS-1200. The samples were
heated in platinum crucibles, in nitrogen atmosphere,
within the temperature range 40–700°C. The heating
rates were 5, 10, 15 and 20°C min–1 and the sample
size ranged in mass from 10–15 mg. The stoichio-
metry of the residues was checked by Energy Disper-
sive Spectrometry (EDS) JSM-840A type analytical
system Oxford ISIS-300.
Results and discussion
Six 2-hydroxyaryloxime complexes of the type
[M(ox)2] (M(II)=Cu, Ni, Pd) were subjected to simul-
taneous TG/DTG-DTA analysis from 40 to 700°C in
nitrogen atmosphere and at a heating rate 10°C min–1.
The temperature ranges, determined percentage mass
losses and thermal effects accompanying the decom-
position reactions for all the complexes are given in
Table 1. Representative examples of thermoanalytical
curves at a heating rate of 10°C min–1 for complexes
with the same ligand and different metals, such as
[Cu(opox)2], [Ni(opox)2] and [Pd(opox)2] are pre-
sented in Fig. 2.
The main decomposition stage of [M(opox)2]
complexes was shown by a sudden mass loss in the
TG curves accompanied by a strong exothermic effect
in the DTA curves. In the case of palladium this hap-
pens after its melting point (shown by an endothermic
DTA peak at 304°C), while in the case of copper and
nickel the compounds decompose without melting.
The profile of the thermoanalytical curves is different
for each metal, while their thermal stability, based on
the temperature at which the first mass loss stage be-
gins, follows the order Pd>Ni>Cu.
The first decomposition stage was followed by
one or two weak endothermic decomposition stages
accompanied by further gradual mass loss. The de-
composition proceeds with rupture of the coordina-
tion bonds by the elimination of one oxime molecule,
or with the rupture of bonds inside the ligands, proba-
bly by elimination of nitrile, nitriloxide or alcohol
molecules as calculated from mass loss on the
TG/DTG curves (details for each studied compound
are given in Table 1). The kind of evolved molecules
was inferred from the Mass Spectral data previously
reported for these complexes [17].
As far as the thermal decomposition of the com-
plexes with the same metal and different ligands is
concerned, there is a correlation between their ther-
mal stability and the substituents on the ligands.
When the substituents were bulky and occur in both,
Fig. 1 Structural formulas of the two 2-OH-aryloximes used in
the present study
TG/DTG-DTA measurements of some complexes of
the type [M(ox)2], where M was Cu, Ni and Pd, were
recorded in nitrogen atmosphere and in different heat-
ing rates. The results are reported and discussed.
Experimental
Materials
Synthesis of the ligands
2-hydroxyaryloximes (ox) were synthesized employ-
ing literature methods [20–21]. Figure 1 illustrates the
two oximes used in the present work, which are deriv-
atives of 2-hydroxybenzaldoxime (named also
salicylaldoxime,
2-hydroxyphenyl-1-propanonoxime, known also as
2-hydroxypropiophenonoxime (Hppox) and
Hsaox).
They
are
2-hydroxyphenyl-4-methoxy-1-benzophenonoxime,
known also as 2-hydroxy-4-methoxy-benzophenon-
oxime (Hopox).
Synthesis of [M(ox)2,], where M=Cu(II), Ni(II), Pd(II)
The metal salts (MCl2·xH2O, M=Cu, Ni, Pd) were
purchased from Fluka in extra pure grade. Samples of
bis(2-hydroxyaryloximato) complexes were prepared
by the reaction between the metal(II) chloride and
oxime in 1:2 metal to ligand ratio as previously re-
ported [9–10].
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FACTORS INFLUENCING THE THERMAL DECOMPOSITION
J. Therm. Anal. Cal., 82, 2005
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LALIA-KANTOURI
the oximic carbon and the benzene ring, as in the case
of opox, with a phenyl group attached to the oximic
carbon and a methoxy group attached to the benzene
ring, the thermal stability of the complex is greater
compared with the analogous complex of the ppox
ligand, where there is a small ethyl group attached to
the oximic carbon and no methoxy group attached to
the benzene ring.
The intermediates formed after the first decom-
position stage, which was the main degradation step,
were not stable and upon further heating the residues
of all the complexes at 700°C they were composed of
a carbonaceous metal oxide, as it was inferred from
energy dispersive spectrometry (EDS) and IR data.
In order to determine the effect of heating rate on
the decomposition process of the studied compounds
[M(ox)2], the following complexes [Cu(opox)2],
Ni(ppox)2] and Pd(ppox)2] were chosen and subjected
for thermal measurements at different heating rates (5,
10, 15 and 20°C min–1) with the same mass sample.
From the thermoanalytical data of [Cu(opox)2] complex
at different heating rates, it was deduced that the residue
CuO at 700°C had smaller amount of carbon at low
heating rates (5 or 10°C min–1), than at higher (15 or
20°C min–1). Evidence for this observation came from
the estimated total mass loss on the TG curves of the
complexes and verified from the Energy Dispersive
Spectrometry (EDS) data of their residues. That means
that, under nitrogen atmosphere, the decomposition of
oximates was incomplete and that there was a relation
between the heating rate and the extent of decomposi-
tion. The position of the exothermic peak on DTA
Fig. 2 Thermoanalytical curves of a – [Cu(opox)2],
b – [Ni(opox)2] and c – [Pd(opox)2] recorded under
nitrogen, at a heating rate of 10°C min–1
Fig. 3 Thermoanalytical curves of [Pd(ppox)2] at different heating rates recorded under nitrogen
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FACTORS INFLUENCING THE THERMAL DECOMPOSITION
corded at 5°C min–1 under nitrogen atmosphere, was
observed at 193°C. However, when the heating rate
for the same complex was 10 or 20°C min–1, two en-
dothermic peaks instead of one were observed at 190
and 200°C. It was also observed that the temperature
of the exothermic peak for the main decomposition
stage at 290°C at a heating rate of 5°C min–1, shifted
to 314°C at a heating rate of 20°C min–1. This phe-
nomenon may be explained if we take into consider-
ation the high degree of crystallinity of this com-
pound and the shape of the endothermic peaks of the
DTA curves. The compound crystallizes in the
monoclinic crystal system, containing in the unit cell
two Ni(ppox)2 units with strong hydrogen bonds [10].
At small heating rates, the compound has the suffi-
cient time to absorb heat and to transfer from the solid
to the liquid phase by loosening some bonds in the
crystal lattice. At higher heating rates, this process
cannot be completed at the same time but the recorded
temperature rises, resulting in the appearance of two
endotherms instead of one, the second being at a
higher temperature. Another possible explanation
would be that with high heating rate the original sam-
ple is transformed first to a meta-stable solid phase,
which melts only at elevated temperatures.
The influence of the atmosphere on the thermal
decomposition of the studied complexes is very signif-
icant leading to different final products as it was ex-
pected, due to the oxidation process that takes place in
air atmosphere. So, in air atmosphere, pure metal oxides
are formed at ~500°C [17], in accordance with other
similar results on salicylate and cefadroxil transition
metal complexes [22–23], while in nitrogen the residues
at 700°C consist of carbonaceous metal oxides.
From X-ray studies, it is well known that in the
complexes under investigation there are strong
intramolecular bonds between the phenolic oxygen
and the hydrogen of the oximic hydroxyl. The hydro-
gen bond distances (O–O’) of Ni(ppox)2 [10],
Cu(ppox)2 [13] and Pd(ppox)2 [11], which are 248,
258 and 259 pm respectively, are in agreement with
the negative shift of the oximic hydroxyl group (510,
385 and 380 cm–1 respectively) that was observed in
the IR spectra of these complexes [9–10].
The activation energies, calculated from the TG
data for the first decomposition stage in air atmo-
sphere of the nickel, palladium and copper
propiophenoxime (ppox) complexes, were found to
be 274, 215 and 196 kJ mol–1, respectively [17]. Con-
sidering the X-ray, spectroscopic (IR) and thermal
(TG/DTG-DTA) results of the investigated ppox
complexes, it was concluded that their thermal stabil-
ity in air, which depends on the activation energies, is
correlated with the strong hydrogen bonds of the
complexes. From the present results it seems that
Fig. 4 Thermoanalytical curves of [Ni(ppox)2] at different
heating rates recorded under nitrogen
curves was observed at higher temperatures with in-
creasing heating rate, as it was expected.
The same results were obtained for the complex
[Pd(ppox)2] when the TG/DTG-DTA curves were re-
corded at four different heating rates, as is illustrated
in Fig. 3. It is worthwhile to mention, however, that
the temperature of the melting point did not alter dra-
matically with the heating rate, whereas the decompo-
sition was observed at higher temperatures.
In order to determine the influence of mass size
on the thermal decomposition mode of the palladium
oximates, the thermoanalytical curves of the complex
[Pd(ppox)2] were recorded with three different mass
10, 15 and 20 mg. The results showed that there was
no influence of the sample size on the final residue, as
well as on the melting point temperature, but there
was a small effect on the decomposition temperature,
which became higher with increasing sample mass.
This effect is best observed in the case of [Ni(ppox)2]
complex (Fig. 4).
The melting point of [Ni(ppox)2] complex, deter-
mined on a Reichert heated plate apparatus was found
to be 189–190°C [10], and on the DTA curve, re-
J. Therm. Anal. Cal., 82, 2005
795

LALIA-KANTOURI
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The DTA and TG/DTG curves of the
2-OH-aryloxime complexes with Cu, Ni and Pd re-
corded under nitrogen, show that these substances
possess a sharp intense exothermic decomposition
followed by one or two very weak endothermic de-
compositions accompanied by mass loss. The decom-
position mode depends on the metal and the substitu-
ents of the ligand involved.
The thermal stability under nitrogen, based on the
initial decomposition temperature for the complexes
with the same ligand follows the series Pd>Ni>Cu,
while for the complexes with the same metal it be-
comes greater with bulky substituents in both the
oximic carbon and the benzene ring of the ligand.
In air atmosphere, pure metal oxides are formed
at ~500°C, while under nitrogen the residues at
~700°C for all the complexes consist from a carbona-
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The thermal stability, based on the estimated ac-
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air, is correlated with the existed strong
intramolecular hydrogen bonds of the complexes.
Different heating rates resulted in different de-
composition temperatures and in different amounts of
carbon contents in the metal oxide residue.
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DOI: 10.1007/s10973-005-6884-4
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