Lanthanide complexes of 3-methoxy-salicylaldehyde: Thermal and kinetic investigation by simultaneous TG/DTG-DTA coupled with MS

Article abstract of DOI:10.1007/s10973-009-0640-0

The reaction of a lanthanide(III) nitrate (Ln = Pr, Nd, Gd, Dy, Er) with 3-methoxy-salicylaldehyde(3-OCH₃-saloH), afforded neutral complexes of the general formula [Ln(3-OCH₃-salo)₃], which were characterized by means of elemental analysis, FT-IR spectra, TG-DTA curves, and magnetic measurements. The released products, due to the thermal decomposition were analyzed by on-line coupling MS spectrometer to the thermobalance in argon, allowed to prove the proposed decomposition stages. In order to confirm the stability scale provided on the basis of the onset decomposition temperature, a kinetic analysis of the three decomposition stages was made using the Kissinger equation, while the complex nature of the decomposition kinetics was revealed by the isoconvertional Ozawa-Flynn-Wall method.

Full text of DOI:10.1007/s10973-009-0640-0

J Therm Anal Calorim (2010) 99:931–938
DOI 10.1007/s10973-009-0640-0
Lanthanide complexes of 3-methoxy-salicylaldehyde
Thermal and kinetic investigation by simultaneous TG/DTG–DTA coupled with MS
•
Christos Papadopoulos Nikolaos Kantiranis
•
•
Stefano Vecchio Maria Lalia-Kantouri
MEDICTA2009 Special Issue
´
´
Ó Akademiai Kiado, Budapest, Hungary 2010
Abstract The reaction of a lanthanide(III) nitrate (Ln = Pr,
Nd, Gd, Dy, Er) with 3-methoxy-salicylaldehyde(3-OCH3-
saloH), afforded neutral complexes of the general formula
[Ln(3-OCH3-salo)3], which were characterized by means of
elemental analysis, FT-IR spectra, TG-DTA curves, and
magnetic measurements. The released products, due to the
thermaldecompositionwereanalyzedbyon-linecouplingMS
spectrometer to the thermobalance in argon, allowed to prove
the proposed decomposition stages. In order to confirm the
stability scale provided on the basis of the onset decomposi-
tion temperature, a kinetic analysis of the three decomposition
stages was made using the Kissinger equation, while the
complex nature of the decomposition kinetics was revealed by
the isoconvertional Ozawa–Flynn–Wall method.
Introduction
Lanthanide complexes are of continuing interest mainly due
to their structural and catalytic properties [1] and their
applications in diagnostic pharmaceuticals [2] and laser
technology [3, 4]. Besides, the strong coordinating proper-
ties of 2-hydroxy-phenones or 2-hydroxy-benzaldehydes
(salicylaldehyde, saloH) and their derivatives with 3d tran-
sition metals have stimulated research in these compounds,
which find applications in both pure [5] and applied chem-
istry fields, especially in extractive metallurgy as analytical
reagents [6]. These ligands are known to coordinate in a
bidentate manner with divalent transition metals in the
mono-anionic form, adopting square-planar geometry,
[Cu(5-Br-salo)2] [7], [Fe(salo)2] [8], or with vanadium
octahedral arrangement [VO(3-OCH3-salo)2(H2O)] [9].
There is no reference of any salicylaldehyde or 2-hydroxy-
phenone complex with trivalent 3d metal or trivalent
lanthanides structurally characterized. It is recently found,
however, by us that under proper conditions, 3-OCH3-sali-
cylaldehyde with Fe(III) ion can coordinate with two dif-
ferent modes and as bridging ligand, forming polynuclear
complexes [Fe2(3-OCH3-salo)8Na5]8H2O [10], while
2-OH-benzophenones give the simple complexes [Fe(2-OH-
benzophenone)3] [11]. Recently, there is an increasing
research interest on the thermal properties of the lanthanide
complexes with ligands having O and/or N donor ability,
studied by TG-DTA techniques [12–15].
Keywords Lanthanide(III) complexes ꢀ
3-Methoxy-salicylaldehyde ꢀ Coupled TG-MS ꢀ
Decomposition kinetics ꢀ TG/DTG–DTA ꢀ PXRD
C. Papadopoulos ꢀ M. Lalia-Kantouri (&)
Department of Chemistry, Laboratory of Inorganic Chemistry,
Faculty of Sciences, Aristotle University of Thessaloniki,
P.O. Box 135, Thessaloniki 54124, Greece
e-mail: lalia@chem.auth.gr
N. Kantiranis
In the course of our studies on the lanthanide chemistry
[16, 17], we report here the synthesis, spectral (IR), and
thermal studies (TG/DTG–DTA) of five new lanthanide
complexes with the ligand 3-methoxy-salicylaldehyde
(3-OCH3-saloH). The evolved gaseous products from the
thermal decomposition were analyzed by MS coupled on
line with the TG to verify the decomposition mode.
Department of Mineralogy-Petrology-Economic Geology,
Faculty of Sciences, Aristotle University of Thessaloniki,
Thessaloniki 54124, Greece
S. Vecchio
Department of Chemical Engineering (I.C.M.A.), University
of Rome, ‘‘La Sapienza’’, Via del Castro Laurenziano 7,
00161 Rome, Italy
123

932
C. Papadopoulos et al.
The thermal stability cannot be assessed only on the basis
of the decomposition temperature, since its value can be
influenced by several operative conditions (i.e., sample
mass, heating rate, atmosphere used, and its flux). On the
other hand, some authors used kinetic analysis to provide
stability scales of a series of lanthanide complexes, but
they consider the activation energy only as a stability
parameter [18, 19]. Actually, this procedure can be
considered questionable since usually higher activation
energies E (which means high stability from an enthalpic
point of view) are associated to higher pre-exponential
factors A (which correspond to lower stability from an
entropic point of view). This problem can be overcome by
considering the rate constant, obtained from the knowledge
of both E and A values using the Arrhenius equation. In this
study, a kinetic analysis was applied to the first three stages
of decomposition to provide a stability scale comparing
with the one constructed on the basis of the onset decom-
position temperature.
Magnetic susceptibility measurements on powdered
samples were performed at 25 °C employing the Faraday
method in a home build balance calibrated against
Hg[Co(SCN)4]. Molar conductivities were measured in
DMF solutions, employing a WTW conductivity bridge and
a calibrated dip-type cell. Infrared spectra were recorded on
a Perkin-Elmer FT-IR 1650 spectrometer, in the region
4000–200 cm-1 using KBr pellets. The simultaneous TG/
DTG–DTA curves were obtained on a SETARAM thermal
analyzer, model SETSYS-1200. The samples of approxi-
mately 10 mg were heated in platinum crucibles, in nitro-
gen atmosphere at a flow rate of 80 ml min-1, within
the temperature range 30–1000 °C. For the kinetic study of
decomposition processes, the TG/DTG–DTA experiments
were carried out at several heating rates (5, 10, 15, and
20 °C min-1) using the isoconversional Ozawa–Flynn–
Wall (OFW) [21, 22], which is based on Eq. 1:
lnðbÞ ¼ a þ bð1=TaÞ
ð1Þ
where a = ln[(AaꢀEa)/R] – ln g(a) - 5.3305 and b =
-1.052ꢀ(Ea/R) assuming that the Doyle’s approximation
[23] is valid over the entire range of a. Ea and Ta are the
activation energies and temperatures, respectively,
calculated at each extent of reaction, a, and R is the gas
constant. Once the conversion dependence on activation
energy is established for each decomposition stage the
Kissinger method [24] was applied, using Eq. 2:
Experimental
The hydrated nitrate salts of lanthanide (Ln(NO3)3ꢀxH2O,
where Ln = Pr, Nd, Gd, Dy, Er) and the organic compound
2-hydroxy, 3-methoxy-benzaldehyde (3-methoxy-salicy-
laldehyde, commonly known as o-vanilline) were pur-
chased as reagent grade from Aldrich and were used
without further purification. All solvents were distilled
prior to use.
À ꢀ
Á
ln b Tm2 ¼ a0 þ b0ð1=TmÞ
ð2Þ
where a0 = ln[(A R)/E] and b0 = -Eꢀ103/R, E, and A being
the activation energy and pre-exponential factor associated
to each decomposition stage, respectively. The kinetic
parameters thus obtained were subsequently used to cal-
culate the specific rate constant at the mean of the corre-
sponding temperature range using the Arrhenius equation.
The thermal behavior was investigated using TG data at
10 °C min-1. Mass spectra (MS) of the gaseous evolved
moiety during the experiments were recorded since the TG
apparatus was coupled with a Quadrupole Mass Spec-
trometer (QMS) model Thermostar. The TG was linked to
a heated gas cell of the MS by means of a heated transfer
line, at temperature 180 °C. Data were processed using
online connected computer system with commercial soft-
ware Quadstar. Powder XRD was performed using a
Philips PW 1710 diffractometer with Ni-filtered CuKa
radiation on randomly oriented samples. The samples were
scanned within the range 2h = 5–80°.
Synthesis of the complexes [Ln(3-methoxy-
salicylaldehyde)3]
[Ln(3-Methoxy-salicylaldehyde)3] complexes were pre-
pared by the following general procedure: to a water–
ethanol solution (15 mL) of lanthanide nitrate (1 mmol)
was added drop-wise a methanolic solution (25 mL) of the
ligand 3-methoxy-salicylaldehyde (4 mmol), deprotonated
by CH3ONa. After 4 h stirring, microcrystalline yellow
solids were obtained, filtered off, washed with small por-
tions of ethanol, and dried in vacuum over anhydrous
calcium chloride. The compounds are soluble in MeOH,
CH2Cl2, DMF but virtually insoluble in H2O, EtOH, and
Et2O.
Instrumental methods
Stoichiometric analyses (C, H) were performed on a
Perkin-Elmer 240B elemental analyzer. Metal content was
determined by EDTA titration after decomposition with
nitric acid using as indicator xylenol orange and buffer
hexamethylenetetramine [20].
Results and discussion
The reaction of the lanthanide nitrate with the ligand
3-methoxy-salicylaldehyde (3-OCH3-saloH) afforded in
123

Lanthanide complexes of 3-methoxy-salicylaldehyde
933
Table 1 Analytical and physicochemical data for the complexes [Ln(3-OCH3-salo)3]
Complex a/a
Ln
Yield/%
C/%
H/%
Ln/%
leff/MB
IR bands/cm-1
m(C=O)
m(Ln–O)
1
2
3
4
5
a
Pr
74
75
62
69
77
48.42 (48.48)a
48.21 (48.24)
47.12 (47.19)
46.78 (46.80)
46.25 (46.45)
3.52 (3.53)
3.52 (3.52)
3.43 (3.44)
3.41 (3.42)
3.39 (3.40)
23.70 (23.72)
24.05 (24.15)
25.72 (25.77)
26.37 (26.40)
26.95 (26.97)
4.20
3.75
1625
1627
1623
1624
1620
383
384
388
389
392
Nd
Gd
Dy
Er
9.04
10.62
9.39
Calculated values in parenthesis
good yield very stable in air solid compounds. The char-
acterization of their molecular structure was made by ele-
mental analyses, conductivity and magnetic measurements,
as well as by infrared spectroscopy. All the studied com-
plexes are neutral and possess 1:3 metal-to-ligand com-
position, as it is indicated from elemental analyses and the
absence of electrical conductivities (*3 S cm2 mol-1) in
DMF solutions. The room temperature magnetic moments
(leff) are characteristics of trivalent lanthanide ions,
showing very close agreement with the theoretical values
calculated from Van Vleck formula. Similar values have
been observed with substituted benzoate complexes of
lanthanides [25]. The complexes may be formulated as
[Ln(3-OCH3-salo)3] evidence also arisen from the inter-
pretation of the IR data of the ligand and the complexes.
Upon coordination, the bands stemming from the stretching
and bending vibrational modes of the phenolic OH (3410
and 1390 cm-1) in the free ligand, disappear indicating the
removal of the hydrogen atom, while the band at
1641 cm-1 attributable to stretching vibration of the C=O
bond is shifted to lower frequencies (*1623 cm-1),
indicative of their coordination mode [26]. The coordina-
tion bond is also inferred from the bands due to the Ln–O at
*388 cm-1. Their physicochemical characteristics are
listed in Table 1.
first derivative of the thermogravimetric curve allows to
distinguish the initial release of either one salicylaldehyde
molecule(HL), as in the case of the Gd, Dy, and Er com-
plexes, or of the fragment PhO (derived from the salicyl-
aldehyde ligand) as in the case of the Nd complex. The
DTA curve shape indicates that no melting takes place
before decomposition for these complexes.
For the Pr compound, however, the DTA curve (Fig. 1)
shows a sharp endothermic peak at 279 °C, which is
seemingly followed by a sharp exothermic at 290 °C. The
first peak is attributed to simultaneous melting and
decomposition of the compound, as it was evidenced with
automated melting point capillary tube system in static air
(m.p. at 274–276 °C). The decomposition commences with
a sudden mass loss of 15% with DTGmax at 290 °C,
attributable to the PhO moiety. In order to explain the sharp
exothermic peak of the first stage, the same experiment was
recorded at a heating rate of 5 °C min-1 (Fig. 3). As it is
seen from the thermoanalytical curves, the endothermic
peak at 275 °C is followed from a rather broad and with
smaller heat flow value exothermic peak at 283 °C, sug-
gesting that it could be regarded as a continuation of the
endothermic one and that the first stage is accompanied by
an endothermic decomposition along with the melting of
the Pr compound.
Upon increasing the temperature, the unstable interme-
diates undergo further decomposition (at *390 or 400 °C)
with mass loss attributable to the elimination of carbon
monoxide plus the methoxy moiety for all the complexes.
This second degradation stage is accompanied by an exo-
thermic thermal effect as it is evidenced from the DTA
curve, which is maybe due to the rapid elimination of the
gaseous decomposition products (as it happens to explo-
sions) or to partially oxidation of the released carbon
monoxide to carbon dioxide. Efforts to isolate the inter-
mediates were not successful due to their continuous
decomposition, as it was evidenced from the TG curves.
The third degradation stage follows beyond 470 °C until
1000 °C, with a continuous mass loss, possibly attributable
to successive elimination of the second and third ligands.
The smaller quantities found may be considered as
Thermal and kinetic behavior
The temperature ranges, the determined percentage mass
losses, and the thermal effects accompanying the decom-
position are given in Table 2. Representative thermal
curves are depicted for the Pr and Er complexes in Figs. 1
and 2, respectively.
The simultaneous TG and DTA results of the Pr com-
plex are similar as those of the set of complexes Nd, Dy,
Gd, and Er. It is obvious that there is no crystallized water
or alcohol solvent molecule in the compounds. Both Pr and
the set of complexes undergo three-stage decomposition to
give the respective oxide as the final products. In the first
stage, the complexes 2–5 undergo an endothermic
decomposition with DTGmax at about 300 °C (Fig. 2). The
123

934
C. Papadopoulos et al.
Table 2 Thermoanalytical results (TG/DTG–DTA) for the complexes [Ln(3-OCH3-salo)3] in nitrogen
a/a
Stage
Trange/°C
Evolved moiety formula
DTGmax/°C
DTA/°C
Mass loss exp/%
(mass loss calc/%)
1
I
250–330
330–470
470–1000
[950
PhO
290
390
–
279(-)
15.0 (15.65)
II
{CO ? OCH3}
2{L–O}
390(?)
10.2 (4.71 ? 5.22)
24.0 ? x (45.44)
49.7 (27.78, 28.67 ? x)
16.3 (15.58)
III
640(?), 780(?), 820(?)
Residue
Mixture of (Pr2O3, Pr6O11) ? C
PhO
2
3
I
250–340
340-470
470–1000
[950
295
380
–
295(-)
II
{CO ? OCH3}
2{L–O}
380(?)
10.0 (4.69 ? 5.19)
19.0 ? x (45.22)
55.0 (28.14 ? x)
25.0 (24.92)
III
600(?), 750(?), 850(?)
Residue
Nd2O3 ? C
HL
I
280–340
340–470
470–800
800–1000
[980
300
385
680
–
300(-)
385(?)
720(?)
850(?)
II
{CO ? OCH3}
PhO
10.0 (4.59 ? 5.08)
15.0 (15.24)
IIIa
b
Unknown
8.0
Residue
Gd2O3 ? C
HL
42.0 (29.67 ? x)
24.7 (24.69)
4
5
I
285–350
350–500
500–1000
[980
305
400
680
305(-)
400(?)
680(?)
II
{CO ? OCH3}
Unknown (or PhO)
Dy2O3 ? C
HL
9.9 (4.55 ? 5.04)
13.4 (15.11)
III
Residue
52.0 (30.30 ? x)
24.8 (24.52)
I
285-340
340–490
490–1000
[980
310
420
680
310(-)
II
{CO ? OCH3}
Unknown (or PhO)
Er2O3 ? C
420(?)
10.0 (4.52 ? 5.00)
15.0 (15.0)
III
720(?), 840(?)
Residue
50.2 (30.80 ? x)
x non-combusted carbon, (-) endothermic thermal effect, (?) exothermic
PhO = C6H5O, HL = 3-OCH3-salicylaldehyde ligand, L–O = ligand–oxygen
100
95
90
85
80
75
70
65
60
55
50
45
40
10
30
20
10
0
100
95
DTA
5
Exo
Exo
0
DTA
90
85
80
75
70
65
60
55
50
–5
–10
–15
TG
TG
–10
0
–20
0
–2
–4
DTG
DTG
–2
–4
–6
–8
–10
100 200 300 400 500 600 700 800 900 1000
Temperature/°C
100 200 300 400 500 600 700 800 900 1000
Temperature/°C
Fig. 1 Thermoanalytical curves (TG/DTG–DTA) of Pr (1) complex
in nitrogen. Heating rate 10 °C min-1
Fig. 2 Thermoanalytical curves (TG/DTG–DTA) of Er (5) complex
in nitrogen. Heating rate 10 °C min-1
123

Lanthanide complexes of 3-methoxy-salicylaldehyde
935
0
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
3
100
Exo
DTG
Er2O3
2
Er2O3: 65-3175 ICDD card
1
–1
–2
–3
–4
Er2O3
0
95
–1
–2
–3
–4
–5
–6
DTA
90
Dy2O3: 86-1327 ICDD card
Dy2O3
Dy2O3
Dy2O3
TG
Gd2O3
Gd2O3: 86-2477 ICDD card
85
230 240 250 260 270 280 290 300 310 320
Temperature/°C
Gd2O3
Gd2O3
Gd2O3
Gd2O3
Gd2O3
Fig. 3 Thermoanalytical curves (TG/DTG–DTA) for the first decom-
position stage of Pr (1) complex in nitrogen. Heating rate 5 °C min-1
Nd2O3: 74-2139 ICDD card
Nd2O3
Nd2O3
Nd2O3
Nd2O3
Nd2O3
non-combustible residues of the organic fragments of the
compounds. This speculation can be enhanced from the
composition of the final residues. The residues at 1000 °C
for all the studied complexes are carbonaceous solids
consisting of the respective lanthanide oxide Ln2O3 plus
carbon in a ratio of *20% of the complex, evidence arisen
Pr2O3
Pr2O3
Pr2O3: 47-1111 ICDD card
Pr2O3
Pr2O3
Pr2O3
Pr2O
3Pr2O3
from the TG/DTG curves and IR peak at *385 cm-1
,
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80
2θ/°
attributable to Ln–O bond, in agreement with other anal-
ogous lanthanide compounds [27]. The formation of the
oxides Ln2O3 was observed at *750 °C as it was expected
[28]. Powder XRD diagrams for the final residue of all the
complexes are given in Fig. 4, confirming the existence of
the lanthanide oxide as Ln2O3. When the purging flow is
changed to static air the obtained residues at 1000 °C were
pure oxides without carbon content. An example is given for
Fig. 4 Powder XRD for the thermal decomposition residues in
nitrogen of the lanthanide complexes at 1000 °C
100
Exo
10
5
90
80
70
60
50
40
30
DTA
-1
the praseodymium complex at heating rate 20 °C min
(Fig. 5). The experimental total mass loss of the thermal
decomposition 70% collaborates with the expected residue
Pr6O11 (calculated 28.7%), evidence arisen from its XRD
diagram (Fig. 6).
0
TG
–5
0
By the analysis of the evolved gases, performed by on-
line coupling a MS spectrometer to the thermobalance, the
proposed decomposition stage can be confirmed. Exami-
nation of the ionic current profiles during decomposition in
inert, under nitrogen or argon atmosphere of the investi-
gated compounds has shown the presence of small mole-
cules, released at the 280–350 and 350–500 °C temperature
ranges, which correspond to the decomposition stages I and
II, respectively.
–1
–2
–3
–4
–5
–6
–7
–8
–9
–10
DTG
As seen in the MS spectra of the thermal decomposition
released gases of the erbium complex, given as an example
in Fig. 7, the detected masses for the first stage with m/z:
30, 32, and 44 are assigned to formaldehyde (H2CO),
methanol (CH3OH), and carbon dioxide (CO2), respec-
tively. For the second stage, the masses m/z: 18, 28, and 44
are assigned to water vapor (H2O), carbon monoxide (CO),
and carbon dioxide (CO2), respectively. The existence of
the carbon dioxide explains the exothermic nature of the
100 200 300 400 500 600 700 800 900
Temperature/°C
Fig. 5 Thermoanalytical curves (TG/DTG–DTA) of Pr (1) complex
in static air. Heating rate 20 °C min-1
second decomposition, possibly due to the oxidation of the
released carbon monoxide. All the detected ions derived
from the fragmentation of the 3-methoxy-salicylaldehyde
123

936
C. Papadopoulos et al.
Pr6O11
Pr6O11: 42-1121 ICDD card
Pr2O3: 22-0880 ICDD card
a
b
c
1500
1200
900
Pr6O11
600
300
Pr6O11
Pr6O11
Pr6O11
Pr6O11
0
1500
5 10 15 20 25 30 35 40 45 50 55 60 65 70 75
2θ/°
1200
Fig. 6 Powder XRD for the thermal decomposition residue in static
air of the Pr complex at 1000 °C
900
600
Ion current/E-11A
0.95000
18.13
300
28.16
0.90000
30.13
0.85000
0.80000
32.09
44.09
0
0.75000
0.70000
0.65000
0.60000
0.55000
0.50000
0.45000
0.40000
0.35000
0.30000
0.25000
0.20000
0.15000
0.10000
1500
1200
900
600
300
0
0
10 20 30 40 50 60 70 80 90 t/min
0.0
0.2
0.4
0.6
0.8
1.0
100 200 300 400 500 600 700 800 900 1000
Temperature/°C
α
Fig. 8 Conversion dependence of activation energy for thermal
decomposition processes: a first stage, b second stage, and c third
stage. Filled square—PrL3, filled diamond—NdL3, filled triangle—
GdL3, filled circle—DyL3, and cross—ErL3
Fig. 7 Ionic current profiles recorded during TG–MS analysis of the
Er complex (5) in argon atmosphere
ligand (molecular ion 152), so confirmed the proposed
fragmentation pattern by the mass loss on the TG/DTG
curves. At the third stage, beyond 490 °C the detected
masses (m/z: 18, 32, and mainly 28) are again fragments of
the ligand.
As far as the first stage is concerned (plot a), noticeably
(and only partially realistic) higher and variable E values
can be observed for Er(III), Dy(III), and Gd(III) complexes
substantially confirming the stability order previously
assessed. Furthermore, the remarkable high E values and
their significant variation (about 600–700 kJ mol-1) rela-
ted to the first stage of these complexes can be partially
ascribable to an artifact, but is unquestionably be explained
by the negligible shift of the TG curves toward higher
temperatures with increasing the heating rate. Lower, more
reasonable and comparable E values are calculated for the
first stage of Nd(III) and Pr(III), in agreement with the
stability scale based on decomposition temperature. The
highest activation energies related to the second and third
stages (plots b and c) are those of the Er(III) complex,
whose variation demonstrates the complex nature of
decomposition processes and the need to use more than one
method for a more complete kinetic characterization.
The analysis of thermal behavior enable to conclude that
the lanthanide (III) complexes (1–5) are stable, showing a
TG plateau, up to at least 250 °C, when the break-down
starts. The stability of the complexes examined can be
assessed according to the thermal profile recorded in
nitrogen atmosphere. The onset decomposition tempera-
tures (Table 2) indicate that the thermal stability of the
studied lanthanide complexes follows the series: ErL3 &
DyL3 [ GdL3 [ Nd & Pr.
The kinetic analysis of the first three decomposition stages
by isoconversional OFW method provides a conversion
dependence of activation energy for each decomposition
stage. The results are summarized in Fig. 8a–c for the first,
second, and third decomposition stages, respectively.
123

Lanthanide complexes of 3-methoxy-salicylaldehyde
937
Table 3 Regression and kinetic parameters for the decomposition processes obtained by the Kissinger method
Complexes
Stage
ln (b//T2m) = a0 ? b0ꢀ(T-m1
)
R2
E/kJ mol-1
ln(A/min-1
)
-ln(k/min-1 a
)
a0
b0/K
[Pr(L)3]
I
31
3
4
4
4
3
4
7
9
3
7
2
1
–23
–35
–39
–22
–34
–39
–180
–54
–25
–123
–26
–23
2
3
3
3
3
3
4
6
3
4
1
1
0.9899
0.9860
0.9858
0.9905
0.9855
0.9876
0.9987
0.9770
0.9782
0.9988
0.9961
0.9989
0.9814
0.9584
0.9859
194 14
293 25
321 27
185 25
284 25
328 25
1500 35
453 50
41
53
3
4
4
4
3
4
7
9
3
7
2
1
–0.3 (562 K)
–0.2 (664 K)
–0.5 (831 K)
–0.3 (563 K)
–0.4 (664 K)
–0.4 (832 K)
–1.5 (576 K)
–0.3 (669 K)
1.2 (935 K)
–1.2 (582 K)
0.5 (666 K)
1.3 (935 K)
–1.7 (585 K)
0.6 (668 K)
1.0 (940 K)
II
III
I
42
35
46
[Nd(L)3]
[Gd(L)3]
[Dy(L)3]
[Er(L)3]
30
40
II
III
I
41
52
36
47
302
71
310
82
II
III
I
15
211
3
25
201
29
1030 40
218 10
213
39
II
III
I
14
194
5
24
320 30
–193 19
1610 150
190 30
250 20
330 30
II
III
24
21
5
3
–23
–30
3
2
34
31
5
3
The associated uncertainty is standard deviations
L the anion of the 3-OCH3-salicylaldehyde ligand
a
Calculated at the mean of experimental temperature range indicated in parentheses
Conclusions
Activation energies, pre-exponential factors, and rate
constant k (calculated at the mean of the respective tem-
perature ranges) obtained by the Kissinger method are
given in Table 3.
From the obtained results it appears that the lanthanide(III)
complexes with the ligand 3-methoxy-salicylaldehyde were
synthesized as neutral complexes. The ligand behaves as
bidentate chelating agent in its mono-anionic form. The
complexes upon heating up to 1000 °C in inert atmosphere
decompose in three stages leaving as final residue carbo-
naceous oxides of Ln2O3, proved by powder XRD mea-
surements. On heating the Pr complex in static air up to
1000 °C, the Pr6O11 oxide was obtained as final residue,
proved by powder XRD.
In the last column on the right the decreasing values of
–ln k (mainly the values related to the first stage) can
provide a more reliable decreasing stability order. Taking
into account this assumption, the lower values (which
identify the faster process) can be found for the first stage
regardless which complex is considered (only slight dif-
ferences are observed). For the Er(III), Dy(III), and Gd(III)
complexes the third stage seems to be the rate-determining
stage, while for Pr(III) and Nd(III) complexes no signifi-
cant differences are found by comparing the rate constant
values of the three stages. By considering the –ln k values
related to the third stage, the comparable higher stability
can be attributed to Er(III), Dy(III), and Gd(III) complexes
and the comparable lower stability to Pr(III) and Nd(III)
complexes.
The released decomposition gases, analyzed by coupled
MS spectrometer to the thermobalance, were found to be
small fragments of the ligand at specific temperatures,
proved the proposed decomposition stages. Kinetic analy-
sis of decomposition processes performed in these lantha-
nide(III) complexes confirms that thermal decomposition
has usually a complex nature. Also, it is possible to assess
a stability scale in a more reliable way by comparing the
rate constant values, which compensate enthalpic and
entropic stability contributions of activation energy and
pre-exponential factor, respectively.
These conclusions are in substantial agreement with the
results derived by considering the decomposition temper-
ature and partially agree with the results obtained by con-
sidering the thermal decomposition of another series of
lanthanide(III) complexes taken from literature [19], whose
–ln k values were calculated from the Arrhenius parame-
ters for the slower decomposition stage:
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