Thermal analysis combined with X-ray diffraction/Rietveld method, FT-IR and UV-vis spectroscopy: Structural characterization of the lanthanum and cerium (III) polycrystalline complexes

Article abstract of DOI:10.1016/j.tca.2020.178662

In this work, structural, spectroscopical and thermal studies of lanthanum(III) and cerium(III) complexes with the 3,5-dimethoxybenzoate (DMBz) monocarboxylate ligand were performed. The metal-ligand minimum stoichiometry and polymeric arrangement of the complexes were respectively defined by TGA-DSC and X-ray powder diffraction as [M(DMBz)₃.n]x, wherein “M” represents the lanthanides, “n” the water molecules in the lanthanum compound and “x” the basic unit of repetition in monoclinic polycrystalline system with space group P2₁/c. The products of the thermal decomposition of the material were also monitored by TGA-DSC/FT-IR both in air and N₂ atmospheres in order to suggest the thermal decomposition mechanism. Theoretical calculations based in experimental results were performed to assess the coordination mode and spectroscopic properties of lanthanide complexes involving a monocarboxylate ligand. From theoretical calculations it was possible to generate FT-IR theoretical vibrational spectra and relate the level of correspondence to the experimental data inherent to the metal-ligand coordination mode. From the HOMO/LUMO orbitals obtained by TD-DFT calculations, the main electronic transitions responsible for the absorption and emission bands of the complexes were determined.

Full text of DOI:10.1016/j.tca.2020.178662

ThermochimicaActa690(2020)178662
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Thermal analysis combined with X-ray diffraction/Rietveld method, FT-IR
and UV-vis spectroscopy: Structural characterization of the lanthanum and
cerium (III) polycrystalline complexes
K.V. Tenorioa, A.B. Fortunatoa, J.M. Moreiraa, D. Romana, K.A. D’Oliveirab, A. Cuinb, D.M. Brasilc,
L.M.C. Pintoc, T.A.D. Colmana, C.T. Carvalhoa,*
a Federal University of Grande Dourados, UFGD, CP 364, 79.804-970, Dourados, MS, Brazil
b Federal University of Juiz de Fora, UFJF, 36036–330, Juiz de Fora, MG, Brazil
c Federal University of Mato Grosso do Sul, 79074-460, Campo Grande, MS, Brazil
A R T I C L E I N F O
A B S T R A C T
Keywords:
In this work, structural, spectroscopical and thermal studies of lanthanum(III) and cerium(III) complexes with
the 3,5-dimethoxybenzoate (DMBz) monocarboxylate ligand were performed. The metal-ligand minimum stoi-
chiometry and polymeric arrangement of the complexes were respectively defined by TGA-DSC and X-ray
powder diffraction as [M(DMBz)3.n]x, wherein “M” represents the lanthanides, “n” the water molecules in the
lanthanum compound and “x” the basic unit of repetition in monoclinic polycrystalline system with space group
P21/c. The products of the thermal decomposition of the material were also monitored by TGA-DSC/FT-IR both
in air and N2 atmospheres in order to suggest the thermal decomposition mechanism. Theoretical calculations
based in experimental results were performed to assess the coordination mode and spectroscopic properties of
lanthanide complexes involving a monocarboxylate ligand. From theoretical calculations it was possible to
generate FT-IR theoretical vibrational spectra and relate the level of correspondence to the experimental data
inherent to the metal-ligand coordination mode. From the HOMO/LUMO orbitals obtained by TD-DFT calcu-
lations, the main electronic transitions responsible for the absorption and emission bands of the complexes were
determined.
Synthesis
Thermal analysis
X-ray diffraction
Polymeric structure
Theoretical calculation
dicarboxylate ligands to the lanthanides. Among the research groups
with an expressive number of scientific papers published on lanthanide
carboxylates, it is worth mentioning those of the researchers Ionashiro
and Ferenc and their collaborators, represented here by the references
[1–3], which can support some of the characterizations used in this
study.
TGA/DSC/FT–IR analysis and refinement by Rietveld method
•
Polymeric structure of monoclinic system with space group P21/c
•
Elucidation of the type of carboxylate-metal coordination in the
•
complexes
Computational calculations
•
HOMO – LUMO molecular orbital contributions
•
In recent years, metal carboxylates, often referred to as coordinating
polymers [3], have attracted interest from the scientific community
around the world since they can be used to produce a fascinating class
of materials, the MOFs (Metal-Organic Frameworks), promising struc-
tures for application in catalysis, gas separation, gas storage, ion ex-
change, luminescence, among others [4,5]. For application of these
materials, it is equally interesting to know their structural arrangement,
however, in most cases they are difficult to be obtained due to the
absence of single crystals [1–4]. One way researchers have found to
overcome this is by adding a nitrogenous co-ligand in the synthesis
1. Introduction
Lanthanides complexes obtained with carboxylate ligands in their
structure have been extensively studied in the solid state.
Characterization of most of these compounds has been done using
thermal analysis techniques, mainly thermogravimetric, to establish the
dehydration grade, thermal stability, thermal behavior, stoichiometric
and purity, as well as infrared spectroscopy (FT-IR) technique to pro-
vide information concerning the coordination mode of mono or
⁎ Corresponding author.
E-mail address: claudiocarvalho@ufgd.edu.br (C.T. Carvalho).
https://doi.org/10.1016/j.tca.2020.178662
Received 9 March 2020; Received in revised form 12 May 2020; Accepted 23 May 2020
Availableonline26May2020
0040-6031/©2020ElsevierB.V.Allrightsreserved.

K.V. Tenorio, et al.
ThermochimicaActa690(2020)178662
process [6,7]. In such syntheses, the researchers Zheng and collabora-
tors and Ling-Yan and collaborators added the 2,2′-bipyridine and 1,10-
phenanthroline co-ligands respectively, so that through this metho-
dology they were able to obtain crystalline complexes. These materials
were then characterized using x-ray diffraction for single crystal,
thermal and spectroscopic techniques, as well as kinetic and lumines-
cence studies.
the precipitate was dried in an oven at 60 °C for 10 h and then kept
stored in a desiccator until the moment of the analyses.
2.2. Instrumental analysis
FT-IR/ATR spectra of the [NaDMBz] salt and complexes were per-
formed using a Nicolet iS10 FT-IR spectrophotometer using ATR ac-
cessory with Ge window.
On the other hand, the synthesis of complexes using only carbox-
ylate ligands to obtain single crystals can also be found in the literature.
Those syntheses carried out under hydrothermal conditions are made
under high temperature and pressure, however it is not always easy to
obtain crystals through this method [8,9].
The TGA-DSC curves were obtained using a thermal analytical
system, model STA 449 F3 Jupiter® and the experimental data were
obtained by Proteus® Software. For the analysis of the samples, masses
near to 10 mg and alumina crucibles under purge gas flow (air or N2) of
50 ml min−1 with heating rate optimized to 10 °C min-1 were used.
The percentages of carbon, hydrogen, oxygen and metal were de-
termined by the mass losses data in TGA curves, since the decomposi-
tion of the complexes occurs with total thermal decomposition of the
organic content and further production of the respective metal oxides of
known stoichiometry, La2O3 and CeO2.
Finally, an elegant way of elucidating the structure of metallic
complexes obtained in powder form is using the Rietveld method for
analysis of crystalline parameters.To take full advantage of this method
in providing reliable information, though, it is important to make
previous use of an appropriate thermogravimetric characterization to
determine the stoichiometry of the material, especially carried out
under air atmosphere, so that the XRD theoretical and experimental
data obtained are convergent. In this sense, Katia and collaborators
[10] using the 3,5-dimethoxybenzoate ligand as complexing agent for
the praseodymium metal obtained this compound in the form of crys-
talline powder and resolved its structure combining theoretical calcu-
lations, TGA-DSC characterization techniques and powder X-ray dif-
fraction (XRD) analysis and refinement by the Rietveld method.
Another study can be cited to exemplify this application, which is
that of Roel and collaborators [5]. They synthesized lanthanide com-
plexes using two different types of dicarboxylate ligand so that one
compound was obtained in the form of monocrystal using hydrothermal
synthesis, while another four compounds were obtained in the form of
crystalline powder. The elucidation of the structures of the compounds,
obtained in powder form, was solved using the Rietveld method ac-
companied by thermal and spectroscopic characterizations, as well as
porosity analysis.
The gaseous fragments of the thermal decomposition of the com-
plexes were monitored using a Mettler® TGA-DSC system coupled to a
Nicolet® FT-IR spectrophotometer equipped with gas cell and DTGS KBr
detector. The gas cell was kept at 250 °C and the 120 cm transfer line at
225 °C. The mass of the samples for the TGA-DSC curves recording were
about 10 mg with heating rate of 10 °C min−1 in alumina crucibles. The
online FT-IR spectra were recorded at 4 cm−1resolution.
The sample image (DSC video) was obtained by the Mettler-Toledo
DSC 1 Stare System equipment with a SC30 digital camera which in-
corporates a 3.3 Megapixel CMOS sensor, Navitar 1-6232D mechanical
optical subassembly with 6.5X zoom. The sample mass used was ap-
proximately 2 mg heated at the ratio of 10 °C min−1
.
2.3. Crystal structure
The [La(DMBz)3·2H2O] complex, before being analyzed by powder
X-ray diffraction, was dehydrated at 200 °C in a muffle furnace, tem-
perature selected according to TGA-DSC previous data. Then, the [La
(DMBz)3] and [Ce(DMBz)3] samples were carefully grounded in an
agate mortar and measured by powder diffraction methods, depositing
them on a glass sample-holder plate. The diffraction data were collected
by overnight scans in the 2θ range of 5−105° with step of 0.02°, using a
Bruker AXS D8 Da Vinci Advanced diffraction equipped with Ni-filtered
CuKα radiation (λ = 1.5418 Å) and Lynxeye linear position-sensitive
detector (2.94°).The optical parameters were primary-beam Soller slits
(2.94°), fixed divergence slit (0.3°) and receiving slit 8.0 mm. The unit
cell parameters were checked using about 20 low-angle peaks, followed
by indexing through the single-value decomposition approach by
Coelho [11,12]. The space group P21/c was chosen for both complexes
and after checking the systematic absences and the cell parameters,
they were then refined using diffraction data up to the range of 55° (2θ)
using Pawley method [13]. In both cases, no higher symmetry trans-
formations were suggested by Spek [14]. The structure solution process
of each complex was performed by the simulated annealing technique
[15], also implemented in TOPAS. The 3,5-dimethoxybenzate ion rigid
body model based on single-crystal data [16] was defined by the Z-
matrix formalism, Fig. 1, as well as in previous works [17,18].
The optical and specimen parameters and angles of torsion, trans-
lation and rotation of the lanthanide ions and ligands were adjusted by
the Chebyshev polynomial function and refined by the Rietveld method
[19]. The crystal structure models found were treated by SA routine in
conjunction with new assays. The final Rietveld refinement plots of
both complexes are described as supplementary material, Fig. S1a and
S1b.
Based on the results presented in the literature and given the ab-
sence of studies on the elucidation of the crystalline structure of the 3,5
lanthanum(III) and cerium(III) dimethoxybenzoates in the literature,
we were motivated to synthesize and characterize them using thermal
and spectroscopic techniques as a basis for elucidation of the structure
of these materials using X-ray powder diffraction and refinement by the
Rietveld method. From the result obtained, other important theoretical-
experimental correlations were evaluated using experimental and the-
oretical FT-IR data and Time-Dependent Density Functional Theory
(TD-DFT) calculations in order to obtain insights related to the optical
properties of the materials, not found in the literature, for future ap-
plications.
2. Experimental
2.1. Synthesis of the complexes
The DMBz, 3,5-dimethoxybenzoic acid (CH3O)2C6H3CO2H 97 %
purity, was purchased from Sigma-Aldrich Brazil. For the synthesis, a
solution of 0.100 mol L−1 of sodium salt [NaDMBz] was prepared at the
pH of 8.0 from the DMBz acid using a 0.100 mol L−1NaOH solution,
whereas the metal salts (chloride or nitrate) solutions were prepared
from lanthanum oxide with concentrated hydrochloric acid and from
cerium nitrate by direct dissolution, both at the pH adjusted between 5
and 6. The pH measurements were assisted by a pH-meter with glass
electrode [10]. Approximately 500 milligrams of each complex [M
(DMBz)3] in solid state, wherein M = lanthanum or cerium, were ob-
tained by mixing both solutions at 27 °C so that the slight excess of the
ligand aqueous solution was used until complete precipitation of the
rare earth ion. The precipitate obtained was washed with distilled water
for elimination of chloride or nitrate and sodium ions and filtered
through Whatman® quantitative filter paper (Grade 42). After washing,
Absorption spectra in the ultraviolet/visible region were recorded
using a Digital UV/Visible scanning spectrophotometer, Model IL-592S-
BI – KASUAKI, in wavenumber range of 190–1000 nm and spectral
band width of 2 nm.
2

K.V. Tenorio, et al.
ThermochimicaActa690(2020)178662
stated that the complexes in their anhydrous form are thermally stable
up to around 326 °C (La) and 315 °C (Ce). Above those temperatures,
the thermal decomposition of the ligand occurs basically in two stages
of mass loss. In these processes, a fact to be highlighted is that the
thermal decomposition of the cerium complex occurs in a temperature
range lower (315–381 °C) than that for the lanthanum complex
(326–401 °C). Based on these experimental data, it can be suggested
that the result shown is intrinsically linked to the change of the oxi-
dation state of cerium metal from 3+ to 4+ during the thermal de-
composition. This exothermic process probably occurs simultaneously
with the decomposition of the organic matter (intense peak at 371 °C),
so that it helps raising the temperature inside the sample and con-
comitantly contributes to the thermal decomposition of the organic
matter occurring at a lower temperature and through a fast process
when compared to other lanthanide ions complexed with the same li-
gand. These data can be observed in other studies [2,31,32].
Fig. 1. Sketch of the 3,5-dimethoxybenzate ion. The 5 torsion angles required
in the simulated annealing and refinement routines were defined as τ1 to τ5.
The last two steps of mass loss (TGA/DTG) for lanthanum complex,
which comprise a slow mass loss between 401 and 639 °C, is related to
thermal decomposition of the carbonized material without peak asso-
ciated with the DSC and DTG curves, while the mass loss between 639
and 700 °C, associated with an endothermic peak at 673 °C, is related to
For the fluorescence studies,
a Varian Cary Eclipse spectro-
fluorimeter was used applying an excitation scan in the range of
210 nm–600 nm at intervals of 10 nm. A xenon lamp with two mono-
chromators was used as excitation source, one of which was intended to
select the excitation wavelength and the other to select the wavelength
emitted by the sample, with fluorescence detection by a photo-
multiplier tube. Solutions containing 5 mg of each compound in N,N-
dimethylformamide (DMF) 99 % purity (J.T. Baker) were analyzed at
the temperature of 25 °C.
carbonate decomposition from the dioxycarbonates (La2O2CO3
→
La2O3 + CO2) mixture. In this reaction, as CO2 is stable under oxidizing
atmosphere, it is concluded, based on thermodynamic laws, that the
absorbed energy (bond breaks) is greater than the energy released
(product formation), that is, with delta H > 0 for this system. This
thermal event at 673 °C can be better observed by the 5x magnified
image on the y-axis for the DSC curve inserted in Fig. 2. In addition,
dioxycarbonates formation, especially for the lanthanum metal co-
ordinated to carboxylate ligands, is common to occur, so that a quali-
tative test of the residue, obtained at this temperature with diluted HCl
solution was used to indicate the presence of carbonate [2,31]. For
cerium complex, the last step between 381 and 560 °C (TGA) occurs
associated with an exotherm (similar to an extended exothermic peak)
in DSC curve between 465 and 550 °C. Furthermore, similarly to the
slow stage of mass loss for lanthanum compound, the DTG curve also
does not peak in this region, because gradual changes are produced
without an inflection point. Lastly, considering the end of the steps for
each of the complexes, the residues formed were the La2O3 and CeO2
oxides, which at high temperatures and in an oxidizing atmosphere
(air) are obtained free of carbonaceous material. Therefore, from the
experimental results obtained using an oxidizing atmosphere in relation
to the percentages of water loss, ligand and oxide formation per step
(Table 1), when compared to the theoretical results (Table S1), it was
possible to establish the empirical formula of these compounds as fol-
lows: [La(DMBz)3·2H2O] and [Ce(DMBz)3].
For the FT-IR theoretical calculation, the quantum chemical ap-
proach employed to determine the molecular structure was Becke's
three parameter hybrid exchange functional combined with Lee-Yang-
Par correlation functional (B3LYP) [20,21]. The molecular calculations
in this study were made using the Gaussian 09 routine [22] and the
theoretical infrared spectrum was calculated using a harmonic field
[23,24] based on C1 symmetry (electronic state 1A). The geometry
optimization was computed using the Berny's optimization algorithm
and the calculations of vibrational frequencies were also implemented
to determine whether the optimized geometry constitutes minimum or
saddle points. The principal assignments and descriptions of the in-
frared active fundamental mode were provided by Gauss View 5.0.2 W
graphic routines [25].
Absorption theoretical calculations were performed within TD-DFT
using the def2-SVP [26] basis set for the light atoms (C, O, and H), and
the def2-TZVP [26] for the lanthanides with the Stutttgart-Dresden ECP
(MBW28) for lanthanum [27] and (MBW47) for cerium [27,28] im-
plemented in the ORCA 4.0.1 package (Neese F 2012). Molecular or-
bital diagrams were reproduced using Avogadro (version 1.2.0)
[29,30].
TGA-DSC/DTG analysis (Fig. 2) in N2 inert atmosphere for the
complexes shows a different profile of the curves when compared to
that presented in air atmosphere. This difference is due to the inter-
action of oxygen with the released volatiles, resulting in degradation
and/or combustion of the organic matter, while in nitrogen atmosphere
there is only energy transference (heat) to the substance with higher
production of volatile compounds at high temperatures. On the other
hand, the thermal events related to the hydration water will always
have similar profiles under both atmospheres. This observation ratifies
the explanation of the interaction between the atmosphere and the
volatile products, since the loss of hydration water only requires energy
absorption. In relation to the thermal events of the second step, the
differences in the decomposition profile of the organic matter (ligand)
are significant under air and N2. Under air atmosphere, it is possible to
observe intense exothermic peaks, while in N2 atmosphere; en-
dothermic peaks of low intensity are noticed.
3. Results and discussion
3.1. Thermal study
The TGA-DSC/DTG thermal analyses data, Fig. 2, under air and
inert (N2) atmospheres are shown in the summary form in Table 1 and
Table S1 in supplementary material, so that it is possible to make a
more accurate analysis of the material studied as follows. For the lan-
thanum complex under air atmosphere, a mass loss starting at 64 °C
(TGA/DTG) is observed, which is associated with a small endothermic
peak in the DSC at 91 °C. This event can be attributed to dehydration
that occurs in a single step and through a slow process. After this mass
loss step, the complex in the anhydrous form is thermally stable over a
wide temperature range.
Another interesting result observed only in N2 inert atmosphere is
the sharp endothermic peak at 356 °C (La) in the DSC curve, Fig. 2, with
no corresponding mass losses in the TGA/DTG curves. This type of
event, identified as a physical one, is due to the phase transition of the
Therefore, from this point onwards the description of the thermal
events for both complexes can be summarized in anhydrous form. Thus,
by examining the TGA-DSC/DTG curves in the air atmosphere it can be
3

K.V. Tenorio, et al.
ThermochimicaActa690(2020)178662
Fig. 2. TGA-DSC/DTG curves in air (for stoichiometry) and N2 (pyrolysis) atmospheres using a heating ratio of 10 °C min−1 and initial mass near 10.0 mg for
lanthanum and cerium complexes study. Peaks up (exo) and peaks down (endo).
lanthanum complex in its anhydrous form, as elucidated by DSC ana-
lysis coupled to a microscope shown in the video (supplementary ma-
terial). In addition, the endothermic peaks at 396 and 436 °C (DSC) are
probably related to the thermal decomposition/pyrolysis of the organic
matter, which occurs within a single rapid step according to DTG peak,
between 350 and 439 °C. Thus, examining the relationship between the
thermal events observed in the DTG and DSC curves, it can be suggested
that the rapid decomposition in this region seems to be favored by the
physical event. The last mass loss step, above 439 °C without DTG peak,
is related to the thermal decomposition of the pyrolyzed material
(carbonaceous material), which is signaled by simultaneous en-
dothermic events in the DSC curve between 670 and 943 °C.
Considering the first thermal decomposition step of the cerium
complex, between 262–416 °C (TGA), unlike lanthanum complex, it is
possible to observe three endothermic peaks in this region (DSC), being
the first two at 306 and 326 °C overlapped, which coincide with a small
mass loss in the TGA curve, as well as with the beginning of the opening
of two large overlapping DTG peaks, Fig. 2. Thus, it is possible that the
beginning of the thermal decomposition is associated with decarbox-
ylation in parallel to the mass losses of the methoxy groups of the li-
gand, while the adjacent step with DSC peak at 356 °C and overlapping
DTG peak (364 °C) can be possibly attributed to the thermal decom-
position/pyrolysis of the organic matter, remaining aromatic residue.
The last step, between 416 and 1000 °C (TGA) and without a defined
Table 1
TGA-DSC/DTG thermoanalytical data of the lanthanum and cerium complexes performed in air and N2 atmospheres, where the mass losses and metal oxides formed
in air were used for stoichiometric calculus.
Steps of
Events
[La(DMBz)3·2H2O]
air
[La(DMBz)3·2H2O]
N2
[Ce(DMBz)3]
air
[Ce(DMBz)3]
N2
mass loss
1st
θ ºC
64−100
0.45 mg
(4.50 %)
91↓
72−102
299−381
262−416
*mg (%)
0.41 mg
6.61 mg (66.10 %)
3.8 mg (38.00 %)
(4.10 %)
Tp (ºC)
θ ºC
91↓
371↑
306↓, 326↓,356↓
416−1000
2nd
326−401
6.12 mg
(61.30 %)
392↑
350−439
4.159 mg (41.59 %)
381−560
0.82 mg (8.20 %)
*mg (%)
3.61 mg (36.10 %)
Tp (ºC)
θ ºC
356↓↓, 396↓, 436↓
439−943
465−550 exotherm
909↓
3rd
401−639
0.855 mg
(8.55 %)
–
–
–
–
–
*mg (%)
3.101 mg (31.01 %)
Tp (ºC)
θ ºC
–
–
–
–
–
–
–
–
–
4th
639−700
0.315 mg
(3.15 %)
673↓
*mg (%)
Tp (ºC)
–
–
–
Residue
*mg (%)
La2O3
La2O3
CeO2
CeO2+R
2.25 mg (22.50 %)
2.33 mg (23.30 %)
2.57 mg (25.70 %)
2.59 mg (25.90 %)
Temperature range (θ), *mass loss observed in TGA curves in milligrams (mg) and percentage (%), peak temperature (Tp) observed in DSC curves, exothermic event↑,
endothermic event↓, phase transition↓↓. DMBz = 3,5-dimethoxybenzoate and R = carbonaceous residue.
4

K.V. Tenorio, et al.
ThermochimicaActa690(2020)178662
peak in the DTG, but with a wide and intense peak at 909 °C is due to
partial decomposition of the pyrolyzed material.
the ligand (1*, 2*, 3* and 4*). Among the products monitored under air
atmosphere, observed in a relatively larger quantity, the fragments of
two groups released simultaneously in the first mass loss steps (TGA) up
to 360 °C stand out, which are methanol (CH3OH), with peaks at 1008,
1033 e 1062 cm−1 originating from methoxy groups, and carbon di-
oxide (CO2), Fig. 3(a), with characteristic peaks in doublet form at
2360, 2400 cm-1 and 666 cm-1, assigned to the decarboxylation of the
carboxylate group. In addition to these two main fragments, there is
also the presence of peaks of very small intensity, inherent to fragments
of dimethoxybenzene (C8H10O2) with bands at 1046, 1150, 1202, 1286,
1486 and 1594 cm-1, and methoxybenzene (C7H8O) with bands at 1054,
1254, 1498, 1602 cm−1 and 1062 cm-1, as shown in Fig. 3 (b, c). From
360 °C, an increase in the proportion of CO2 in relation to the other
products is observed, so that from the slow mass losses steps at 401 °C
(La) and 381 °C (Ce), Fig. 3(d), only methane (peaks at 2995 and 3018
cm-1) and carbon dioxide (CO2) are observed in the FT-IR spectra.
Possibly, most of the gaseous products CH4 and CO2 originate from the
fragmentation of the carbon skeleton.
In summary, from the TGA-DSC/DTG data in both atmospheres it
was possible to observe physical and chemical events such as: thermal
behavior, stoichiometric ratio of the complexes, phase transition,
thermal stability, decomposition steps and stable residue formation
temperature. For the identification of the decomposition products re-
leased in each step, a TGA-DSC/FT-IR system was used. Specifically for
this analysis type, the nitrogen atmosphere is more efficient than the air
atmosphere, since in air the fragmentation of the organic matter leads
to an excessive production of carbon monoxide (CO) and carbon di-
oxide (CO2), so that these excess can overlap the bands of organic
fragments.
3.1.1. EGA - evolved gas analysis
In parallel with the thermal behavior study and stoichiometry de-
termination by TGA-DSC, we also monitored the gaseous products from
thermal decomposition in air and nitrogen atmospheres through a TGA-
DSC/FT-IR system, so that in both atmospheres the same products were
detected. However, as previously mentioned, the monitoring of gaseous
products is more advantageous in an inert atmosphere because the
amount of CO and CO2 produced from the organic matter decomposi-
tion is lower, i.e., inherent only to the ligand decarboxylation. The
combination of these results now offers us the possibility of obtaining a
set of complementary information about the complexes that can be used
to evaluate the likely thermal decomposition mechanism those com-
plexes follow.
The monitoring in nitrogen atmosphere, slightly different from that
observed in air atmosphere, initially shows methanol as the only pro-
duct released and, as mentioned earlier, it comes from the methoxy
group of the ligand. This process occurs almost simultaneously with
decarboxylation, Fig. 3(a), a coherent attribution because the atmo-
sphere used was inert (N2). Subsequently, with the increase in tem-
perature, intense peaks attributed to fragments of dimethoxybenzene
and methoxybenzene are observed, Fig. 3(b, c), which continue to be
released in decreasing proportion until the end of this step together
with fragments of methanol and carbon dioxide. The slow mass loss
steps, for both compounds, starting at 439 °C (La) and 416 °C (Ce), show
peaks for the fragments only in the 3000 cm−1 region, corresponding to
CeH group stretching that may indicate the presence of different sizes
of carbon chain. Furthermore, it is more likely that these fragments
originated from the aromatic skeleton, since it was not possible to ob-
serve bands related to benzene in the FT-IR spectra, considering the
monitoring between 30 and 1000 °C performed for both compounds.
The gaseous products of the thermal decomposition of the com-
plexes [La(DMBz)3] and [Ce(DMBz)3] in anhydrous form, Fig. 3 (a–d),
were continuously monitored between 30 and 1000 °C (TGA). The ex-
perimental FT-IR spectra obtained were compared with literature data
[33,34] as well as evaluated and compared through OMNIC® software
with a broad spectral library of gaseous products. The bands identified
in FT-IR spectra (numbered 1–6), Fig. 3(a–d), are also associated with
illustrations of the possible position of fragmentation in the molecule of
Fig. 3. a–d). FT-IR spectra of the main gaseous products monitored in air and N2 atmospheres from the thermal decomposition of the complexes [La(DMBz)3·2H2O]
and [Ce(DMBz)3] at the temperatures (TGA): (a) > 310 °C (b and c) > 360 to 400 °C and (d) above 381 °C. DMBz: 3,5-dimethoxybenzoate.
5

K.V. Tenorio, et al.
ThermochimicaActa690(2020)178662
Fig. 4. Illustration based on the most products released, for the main path thermal decomposition follows in lanthanum and cerium 3,5-methoxybenzoate complexes,
monitored by TGA-DSC / FT-IR in air and nitrogen atmospheres.
This statement can be made due to the absence of characteristic bands
of benzene, located in 1802 and 1950 cm−1. From these data, it was
possible to qualitatively suggest, based on the gaseous products re-
leased in greater quantity, the main way in which fragmentation occurs,
as shown in simplified form in Fig. 4.
Table 2
Experimental and theoretical FT-IR data assigned to the 3,5-dimethoxybenzoate
salt and lanthanum 3,5-dimethoxybenzoate complex as representative of all
complexes.
Assignment
NaDMBz
[La(DMBz)3·2H2O]
Exp.
Theor.
Exp.
Theor.
3.2. Structural study
ν C = C (ring)
1653w
,
1645,
1363
1211,
1170
1542
1399
157
1610m
1331m
1202m
1148s
1526s
1381s
145
,
,
1646, 1371
1211, 1174
3.2.1. Infrared spectroscopy: theoretical and experimental
1340m
δ CH3 + δ C– H (ring) + 1205s,
The experimental and theoretical FT-IR spectra for the ligand salt
and lanthanum complex, as representatives of both complexes, are
shown in Fig. 5 due to similarity of spectra. The optimized structures
used to generate the theoretical vibrational spectra are shown in the
supplementary material, Fig. S2. The identification of the main bands in
the FT-IR spectra, Table 2, was performed from the spectroscopic
ν C–OCH3
1148s
1561s
1385s
176
νasym COO–
1527
1412
115
νsym COO–
Δν (νasym – νsym
)
ν: stretching vibration; δ: bending vibration; asym: asymmetrical; sym: sym-
metrical; w: weak; m: medium; S: Strong; Theor.: Theoretical; Exp.:
Experimental; DMBz: 3,5–dimethoxybenzoate.
correlations by computational animations and by checking the level of
correspondence with the FT-IR experimental data. As it can be observed
in Fig. 5, the positions of the main bands between the theoretical (a*,
b*) and experimental (a, b) spectra are highly concordant, suggesting
that the proposed structures are very close to the real structure. How-
ever, due to the approximate nature of the computational technique,
small differences observed were expected, since the theoretical calcu-
lations do not include harmonic effects in the infrared vibrations, be-
sides the complexes have been considered in the gaseous phase for the
calculations [35]. In this work, as the experimental infrared spectra of
the complexes are very similar, that allows us to state that the co-
ordination occurs equally for both compounds.
In order to study the coordination mode of the carboxylate groups to
the metal centers, as indicated by the experimental and theoretical
calculation results, the methodology described by Deacon and Phillips
[36] was applied. The differences between the symmetric and asym-
metric stretching values for carboxylate groups of complexes and the
ionic compound (sodium salt) were compared. From the combined re-
sults, presented in Table 2, it can be suggested that the carboxylate
group is coordinated to the lanthanide ions by bidentate chelating mode
[33–35]. In addition to the complexation study by FT-IR data, further X-
ray powder diffraction is intended to provide accurate structural in-
formation on the crystalline structures.
3.3. Crystalline structure
In the absence of suitable single crystals, the complexes were pre-
viously characterized by thermogravimetry and simultaneous differ-
ential scanning calorimetry (TGA-DSC) to determine stoichiometric
ratio and thermal stability. From this data, the structural arrangement
was determined using X-ray powder diffraction after dehydration of
lanthanum compound at 200 °C.
Fig. 5. Experimental FT-IR spectra of the 3,5-dimethoxybenzoate sodium salt
(a) and lanthanum complex (b), correlated to the FT-IR theoretical spectra of
3,5–dimethoxybenzoate sodium salt (a*) and lanthanum complex (b*). The
lanthanum complex was used as representative of cerium complex due to their
similarity.
6

K.V. Tenorio, et al.
ThermochimicaActa690(2020)178662
Table 3
information could be afforded and the crystallographic parameters of
both complexes were summarized in Table 3, while the eight oxigen-
lanthanides bond distances present in the coordination sphere of cerium
and lanthanum are given in Table 4. The molecular structure of both
complexes, drawn using OLEX2 [37], are shown in Fig. 6(a, b), as well
as the overall packing and hydrogen bond are presented in Figures S3
(a–d) and S4 (a–e) as supplementary material. The DMBz is a carbox-
ylate ligand and there are three coordination modes for carboxylate
ligands [16,17]. The bidentate chelating (mode a) is easily described as
both oxygen atoms bonded to the same lanthanide ion. Another co-
Principal data of crystalline parameters obtained for [Ce(DMBz)3] and [La
(DMBz)3] complexes.
[Ce(DMBz)3]
[La(DMBz)3]
Empirical formula
C
27H27O12Ce
C27H27O12La
682.40
Formula weight (gmol−1
)
683.61
298
T(K)
298
λ(CuKα) (Å)
Crystal system
Space group
a(Å)
1.5418
Monoclinic
P21/c
1.5418
Monoclinic
P21/c
14.829(8)
24.351(2)
7.7758(1)
90
14.790(1)
24.294(3)
7.836(2)
90
ordination mode is when
a lanthanide ion carboxylate forms a
b,(Å)
monoatomic bridge or μ-oxo bridge (mode b). The third mode is when
each oxygen in a carboxylate group binds to a different lanthanide ion,
syn, syn-η1:η1:μ2 bidentate bridging fashion.
c(Å)
α=γ (o)
β (o)
97.85(6)
2775.2(4)
4
98.16(1)
2787.0(9)
4
V (Å3)
As described in Fig. 6(a, b), both structures show two ligands
bonded to lanthanide ions through the carboxylate group in mode a and
b only the other DMBz ligand is coordinated to lanthanide ion in mode
b. Besides the lanthanide-oxigen bond lengths, both complexes also
exhibit important and strong interaction, that is, 3.879 Å and 3.944 Å,
between Ce-Ce and La-La metal ions respectively.
Z
d
calc(g cm−3
)
1.636(2)
13.2(8)
78
1.626(5)
12.4(4)
73
μ (mm−1
)
Number of parameters
R
Bragg, Rwp
0.069 / 0.0106
0.0359 / 0.0827
In the supplementary material, Figures S3 and S4 show the main
hydrogen bonds for the [Ce(DMBz)3] and [La(DMBz)3] complexes, re-
spectively. For [La(DMBz)3], there are intramolecular H-bonds between
H11 and H21 with O21, H31 and H38b with O31 and O32 with H21
and two other intermolecular H-bonds between H33…O64 and H13…
O63. Similarly, H-bonds fashion are observed in [Ce(DMBz)3], where
the two intermolecular H-bonds are assembled by O31…H31 and
O54…H18a and the intramolecular ones are located between O12…
H21 and O31…H31.
Table 4
Main Ln-O bond lengths of oxygen atoms around lanthanide ions (Ln).
[Ce(DMBz)3]
Lengths/Å
O11
O12
O21
O22
2.420(2)
2.618(3)
2.466(2)
2.629(3)
O31
O32
O32i
2.597(2)
2.459(3)
2.718(1)
[La(DMBz)3]
3.4. UV–vis absorption study
O11
O12
O21
O22
2.501(5)
2.585(3)
2.232(3)
2.648(3)
O31
O32
O11i
2.541(2)
2.394(3)
2.733(2)
For both complexes studied, the electronic experimental spectra
recorded in DMF solution show a single absorption band that reaches its
maximum at 298 nm. For the DFT calculations, unfolding in two bands
of different intensities in the UV–vis spectra is observed; an intense
band at 255 nm and a lower intensity one at 300 nm, as shown in Fig. 7,
so that such differences may probably be due to experimental condi-
tions like complex-solvent interaction or even by limitation of the
monochromator system (bandpass limitation) in the equipment used. In
addition, for the theoretical calculations a single molecule, free of in-
teractions, is taken into account.
The crystal structure models of the [La(DMBz)3] and [Ce(DMBz)3]
complexes in anhydrous form were solved using state-of-the-art powder
diffraction data measured in conventional laboratory equipment. In
order to solve the present crystal structures, a rigid body for organic
moiety and a high number of parameters were extensively used. After a
long procedure as described before, very important crystallochemical
From HOMO → LUMO orbital transitions, Fig. S5, it is noticed that
Fig. 6. (a, b). Asymmetric units and distorted pentagonal bipyramidal geometry around (a) La3+ and (b) Ce3+ respectively. Ln-O bonds are represented in both
structures, whereas Ln represents La3+ and Ce3+ lanthanide ions.
7

K.V. Tenorio, et al.
ThermochimicaActa690(2020)178662
complexes were obtained by Time-Dependent Density Functional (TD-
DFT) Theory. However, lanthanum (III) ion, in contrast to other lan-
thanide ions, is theoretically non-fluorescent because of its 3d104f°
electronic configuration, so only the fluorescence for the cerium com-
plex is discussed in this section. The emission (fluorescence) of the Ce
(III) results from 4f-5d optical transitions, which gives rise to much
more intense bands than intraconfiguration transitions of 4f-4f type and
the emission is generally observed in the ultraviolet region, although it
may also occur in the visible region [39].
In Fig. 8(a, b), the major HOMO →LUMO orbitals contributions and
experimental excitation and emission spectra are shown, which are
correlated with each other to identify the electronic transitions re-
sponsible for the system's fluorescence. The H-1(93 %) and H-2(98 %)
→ L + 3 transitions (Fig. 8(a)) show that there is one ligand-metal
charge transfer (LMCT), as well as contributions of intraligand charge
transfer (ILCT and LMCT) originated from a mixture of transitions H-
1(69 %); H-2 (88 %); H-4 (93 %) and H-5 (86 %) → LUMO. The con-
tributions ILCT and LMCT, especially the LMCT, can be justified since
the orbital of the organic part of the molecule has also charge dis-
tribution on the metallic center. In addition, from the experimental and
theoretical data (Fig. 8(a,b)) and summary in Table S2), we have not
excluded the possibility of ligand-ligand charge transfer (LLCT) as a
function of this overlapping of orbitals on the metallic center.
Fig. 7. Absorption spectra in UV–vis (experimental: black; theoretical: red) for
the [La(DMBz)3·2H2O] and [Ce(DMBz)3] complexes. DMBz: 3,5-dimethox-
ybenzoate (For interpretation of the references to colour in this figure legend,
the reader is referred to the web version of this article).
The excitation and emission spectra, Fig. 8(b), were obtained re-
spectively by monitoring of λmax at 310 nm and λmax emission at
355 nm. Hence, the emission band in the spectrum observed at 355 nm
is an overlapping (summation) of n-ᴨ* and ᴨ-ᴨ* transitions of the li-
the charge density on the organic part of the complex is due to ᴨ-ᴨ* and
n-ᴨ* transitions, the latter being usually of low intensity. According to
literature, ᴨ-ᴨ* transitions for organic ligands are recorded in the region
between 196 and 313 nm and correspond to the C]C chromophore
group of aromatic ring [38]. The overlapped band in the UV–vis ex-
perimental data, apparent at 300 nm in the theoretical calculation, is
probably due to n-ᴨ* transitions. Regarding the charge transfer on the
metal complexes, the overlapping of oxygens p-orbital of the carbox-
ylate group (Fig. S5) should have favored the interligand charge
transfer like in HOMO + 1 → LUMO (74 %), HOMO + 2 → LUMO (89
%) for lanthanum and HOMO + 1 → LUMO (69 %), HOMO + 2 →
LUMO (89 %) for cerium. The main contributions of UV–vis radiation
absorption occur through the aromatic system and the carboxylate
group interactions, although singlet states of charge transfer of ligand-
metal can also contribute. Anyway, those contributions are discussed in
the fluorescence section.
gand, as well as LMCT contributions from the antenna effect, transitions
2
attributed to energy levels of the Ce(III) metal center F7/2 →2F5/2
.
4. Conclusions
The lanthanum(III) and cerium(III) complexes using
a mono-
carboxylate ligand, 3,5-methoxybenzoate, were synthesized in aqueous
medium and characterized in the solid state or solution by thermo-
analytical methods, X-ray diffraction, FT-IR and UV–vis.
Based on the thermoanalytical data from the TGA-DSC curves, it
was possible to define the stoichiometric ratio of the synthesized
complexes as M(DMBz)3, where M represents the lanthanides and DMBz
the 3,5-dimethoxybenzoate ligand. Among the complexes synthesized
and characterized, only that of lanthanum was obtained in hydrated
form. A probable explanation can be attributed to the lower lanthanide
contraction effect for lanthanum, which would make the coordination
sphere more available. Another relevant information from TGA-DSC/
DTG to the study of these materials was the thermal stability, decom-
position temperatures, as well as the temperature of formation of the
3.5. Fluorescence study
The molecular orbitals (HOMO/LUMO) with the highest probability
of charge transfer on the 3,5-dimethoxybenzoate lanthanum and cerium
Fig. 8. (a) Molecular orbitals for [Ce(DMBz)3] according to the TD-DFT calculations, (b) excitation and emission spectra in. UV–vis.
8

K.V. Tenorio, et al.
ThermochimicaActa690(2020)178662
final residue used in the calculation of stoichiometry and the mon-
itoring, by TGA/FT-IR, of the main volatile products related to thermal
decomposition and consequently the probable path thermal decom-
position follows.
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methoxybenzoate monocarboxylate ligand coordinates with lanthanide
ions in chelating form, while by X-ray diffractograms with Rietveld
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2
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Declaration of interests
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The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Acknowledgements
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The authors thank the Brazilian Foundations CNPQ (grant nº
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04.13.0448.00/2013) for financial support, and are grateful to
Professor Massao Ionashiro for allowing us to carry out measurements
in the Ivo Giolito Thermal Analysis Laboratory (LATIG) – UNESP.
Daniel M. Brasil and Leandro M. C. Pinto acknowledge CENAPAD–SP
(Centro Nacional de Processamento de Alto Desempenho em São Paulo)
for granting computing time and Professional Noemi Marques de
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