REGULAR PAPERS
Vibrational And Thermal Study Of Hexaaquatris (Malonato) Dieuropium (III) Dihydrate
Brusau, E.V.1; Pedregosa, J.C.1; Narda, G.E.1; Ayala, E. P.2; Oliveira, E.A.2
1Área de Química
General e Inorgánica “Dr. G. F. Puelles”, Facultad de Química, Bioquímica y
Farmacia, Universidad Nacional de San Luis, Chacabuco y Pedernera, 5700 San
Luis, Argentina.
FAX: +54(2652)430224;
*E-mail: gnarda@unsl.edu.ar.
2Departamento de
Física, Universidade Federal do Ceará, Fortaleza (CE), Brazil.
Received February, 25, 2004. In Final Form March 12, 2004
Dedicated
to Professor Dr. Pedro J. Aymonino on the occasion of his 75th
birthday.
Abstract
The vibrational spectra
(FTIR-Raman) of the hexaaquatris(malonato)dieuropium(III) dihydrate are
reported and discussed. An assignment of H2O, CH2 and OCO
modes in the complex is proposed on the basis of the structural data previously
reported. Valuable information about the water librational modes has been
obtained. The different types of interaction between the carboxylate groups of
the malonate ions and Eu(III), four- and six-membered chelate rings and
monodentate bridging interactions, have been also evidenced. The mechanism of the thermal dehydration is
analysed and the results are in agreement with the crystal structure.
Resumen
En este trabajo se presentan y
discuten los espectros vibracionales (FTIR-Raman) de
hexacuotris(malonato)dieuropio(III) dihidrato. Se propone una asignación de
los modos asociados a H2O, CH2 y OCO de acuerdo con los
datos estructurales reportados en literatura. Fue posible obtener información
valiosa sobre los modos libracionales de agua. Se evidenciaron también los
diferentes tipos de interacción entre grupos carboxilato del malonato con el
ion Eu(III), con presencia de anillos de cuatro y seis miembros e interacciones
monodentadas. El mecanismo de la deshidratación térmica resultó consistente con
las interacciones descriptas en la estructura cristalina.
Introduction
The binding of metal ions to the dicarboxylate residues has been a
subject of interest because of the importance of such interactions in a wide
variety of metalloproteins. Metal complexes containing malonate or
α-substituted malonates have been taken as models for blood and bone
calcium proteins that present the modified amino acid residues γ-carboxyglutamic
acid and β-carboxyaspartic acid. In addition, some lanthanide (Ln(III))
dicarboxylates are applied as metal probe techniques [1-3].
Specific lanthanide dicarboxylates are also applied to other areas
such as materials with attractive optical and magnetic properties [4-7] and
recently, the study on the Ln-dicarboxylate complexes has been expanded to the design of new solid-state architectures which are characterized as organic/inorganic hybrids with polymeric layer structures [8-11].
The discovery of these interesting applications of
Ln(III)-dicarboxylates has stimulated the enthusiasm of inorganic chemists in
obtaining these complexes and studying the binding of metal ions and
carboxylate groups. Within a general research program focused on understanding the
factors that influence the way of binding and the formation and stability of
polymeric structures with different dimensionality, e.g. Ca(II) and Cd(II)
dicarboxylates, we have previously reported the structural, vibrational and
thermal analysis of some metal-OOC(CH2)nCOO complexes
with n = 1-4 [12-15].
Structural data of Eu(III) and Nd(III) malonates hydrates has been
reported [16-18]. On the basis of these investigations, the lanthanide
malonates, M2mal3∙nH2O, can be
divided into three different isostructural groups, one with n = 8 and M
= Ce – Gd (referred as octahydrate type I), another with n = 6 and M =
Ce – Eu, and a third type with n = 8 and M = Eu – Lu (referred as
octahydrate type II). However, a novel malonato
- gadolinium(III) complex with n = 6 was reported recently [19]. In spite of its structure being known,
neither the complete vibrational assignment of the title compound nor its thermal analysis in air were found in the literature.
Then, in order to obtain a wider
insight into metal dicarboxylate complexes properties, FTIR and Raman analysis
and TGA-DTA measurements were carried out and the results are presented in
connection with a brief description of the single-crystal X-ray study.
Experimental
[Eu2(mal)3(H2O)6] 2H2O
crystals were obtained by precipitation from solution at room temperature
according to the procedure described in reference [16] for the octahydrates
type I. Reagents of analytical grade were used in the synthesis.
Europium(III) oxide (0.5 mmol, Aldrich) was dissolved in 50-mL of
water with the aid of hydrochloric acid (Merck) and then it was slowly added to
a 50-mL aqueous solution containing 1.5 mmol of malonic acid (Sigma) under
stirring. The pH of each solution was adjusted to about 5 by the addition of
sodium hydroxide (Merck) before mixing. The resulting solution was left to
evaporate slowly at room temperature under inert atmosphere. A large number of
prismatic colorless single crystals was obtained after several days. They were
filtered, washed with distilled water and dried at room temperature.
FTIR spectra were recorded on a Nicolet PROTÉGÉ 460 spectrometer in
the 4000-225 cm-1 range with 32 scans, using the KBr pellet
technique; spectral resolution was 4 cm-1. Raman spectrum was
scanned on a Raman Jobin-Yvon T64000 provided with a N2 - cooled CCD
and operating with an Ar laser. Two excitation wavelengths (488 and 514.5 nm)
were used to avoid the strong europium luminiscence.
TGA-DTA curves were obtained with a Shimadzu TGA-50H/DTA-50 thermal
analyzer apparatus using flowing air at 50 mL min-1 at a heating
rate of 10°C min-1. Appropriate combinations of X-ray powder
diffraction and FTIR spectroscopy were used for further characterization of the
pyrolysis residue.
X-ray powder
diagrams were obtained with a Rigaku D-MAX-IIIC diffractometer using CuKa radiation
(Ni-filter) and NaCl and quartz as external calibration standards.
Results
And Discussion
Structure Description
The powder X-ray pattern of [Eu2(mal)3(H2O)6] 2H2O shows an excellent agreement with that simulated by the Fullprof refinement program [20] on the basis of the single-crystal X-ray diffraction data reported previously in literature [16]. This fact allows us to conclude that the title compound crystallizes as the octahydrate type I. The crystals are orthorhombic, space group Pbcn. The structure is a three-dimensional europium malonate network. Figure 1 shows the Eu(III) coordination polyhedra. Each europium ion is coordinated by six carboxylate and three water oxygen atoms which form a monocapped square antiprism. There are two independent malonate ions in the structure, both non planar. One of them has one strict twofold symmetry and is bonded to four metal ions by six Eu – O bonds, i.e. two of its oxygens form two Eu – O bonds each. The other malonate ion forms a six-membered chelate ring, namely “malonate type” binding, with europium and one bridge Eu – OCO – Eu. One of the coordinated water molecules forms two hydrogen bonds to carboxylic oxygens and accepts one hydrogen bond from the uncoordinated (hydration) water molecule oxygen. The remaining coordinated water molecules each form one intralayer hydrogen bond to a carboxylic oxygen and one to the uncoordinated water molecule. The hydration water molecule is situated between the europium malonate layers. It is bonded to the coordinated water oxygens of one layer and to a carboxylic oxygen of an adjacent layer.
Vibrational Spectra
The vibrational study has been carried out taking into account the structural characteristics outlined above. The internal vibrations can be described by means of the following building units: carboxylate groups, methylene groups and water molecules. The interpretation of the FTIR and Raman spectra is supported by their comparison with the spectroscopic data of malonic acid and related compounds found in literature [21, 22].
Figure 1: Coordination
polyhedra of Eu(III) in [Eu2(mal)3(H2O)6] 2H2O
Figure 2: FTIR
spectrum of [Eu2(mal)3(H2O)6] 2H2O
Figure 2 and Figure 3 show the FTIR and Raman spectra respectively. The proposed assignment is detailed in Table 1. The features of the principal modes assigned to H2O and OCO groups are discussed below.
Figure 3:
Raman spectrum of [Eu2(mal)3(H2O)6] 2H2O
Table 1. Assignment of the FTIR-Raman spectra of [Eu2(mal)3(H2O)6] ∙ 2H2O (values in cm-1)
FTIR |
Raman |
Assignment |
3440 (m, br) |
|
n (OH) W |
3375 (m, br) |
|
n (OH) W |
3260 (m, br) |
|
n (OH) W |
3180 (m, br) |
3120 (w) |
n (OH) W |
3070 (m, br) |
3075 (m) |
n (OH) W |
3010 (m, ov) |
3015 (s) |
nas (CH) |
2995 (m, ov) |
2995 (m) |
nas (CH) |
|
2955 (m) |
nas (CH) |
2915 (m, ov) |
2915 (vs) |
ns (CH) |
1690 (sh, ov) |
|
d(H2O) |
1625 (s) |
1635 (w) |
nas (OCO) |
1585 (s) |
1580 (vw) |
nas (OCO) |
1550 (s) |
1560 (vw) |
nas (OCO) |
1535 (vs) |
|
nas (OCO) |
1475 (m) |
1480 (m) |
d (CH2) |
1450 (s) |
1455 (m) |
d (CH2) |
|
|
d (CH2) |
1415 (m) |
1410 (m) |
d (CH2) |
1365 (s) |
1365 (m) |
ns (OCO) |
|
1300 (vw) |
rw (CH2) |
1275 (m) |
1275 (w) |
rw (CH2) |
|
1270 (vw) |
rw (CH2) |
1185 (m) |
1190 (w) |
nas (CC) |
1170 (m) |
1170 (vw) |
nas (CC) |
975 (m) |
980 (vw) |
ns (CC) |
950 (m) |
955 (m) |
ns (CC) |
|
920 (vw) 900 (vw) |
ns (CC) |
845 (m) |
845 (m) |
d (CC) |
820 (m) |
825 (vw) |
d (OCO) |
775 (m) |
775 (m) |
rr (H2O) |
710 (m) |
715 (w) |
rw (OCO) |
665 (m) |
655 (vw) |
rr (H2O) |
645 (m) |
|
p (OCO) |
625 (m) |
|
rr (H2O) |
595 (m) |
595 (vw) |
rr (H2O) |
560 (m) |
565 (w) |
rw (H2O) + d (CCO) |
465 (w) |
460 (vw) |
Lattice modes no associated with water molecules |
|
430 (vw) |
|
420 (w) |
420 (vw) |
|
340 (m) |
405 (vw) 385 (vw) |
|
255 (m) 250 (m) 245 (m) |
280 (vw) 250 (vw) 240 (vw) |
n (M-O) |
Key to Table 1: vs: very strong s: strong m: medium w: weak vw: very weak ov: overlap br: broad nas: asymmetric stretching; ns: symmetric stretching; d: deformation; rw: wagging mode; rr: rocking mode; p : out of plane deformation
Water Modes.
The assignment of these modes, particularly the librational ones,
was performed considering the spectra of partial and completely dehydrated
samples. These samples were obtained by thermal treatment of the original
compound at selected temperatures (the thermal diagram was taken into account
for the selection). Figure 4 shows the FTIR
spectra of the dehydration course in the IR range of n(O-H) modes (3a), d(H2O)
mode (3b) and the zone of water librational modes (3c). The spectrum of the
sample heating at 200ºC was removed in Figure 3c for a better viewing since it
is similar to that obtained at 250 ºC.
The n(O-H) can be identified in the 3500-3000 cm-1 IR zone
with one markedly asymmetric, broad band which involves several components.
This fact suggests the presence of different kinds of water molecules in the
structure. The frequency values are lower than those expected for free water
stretching vibrations owing not only to coordination but also to the presence
of hydrogen bonds in the lattice. Some n(O-H) signals were identified
in the Raman spectrum. The strong europium luminiscence does not allow us to
record the complete region of the n(O-H) modes. Deformation modes are not
observed in the FTIR spectrum but the progressive thinness of the nas(OCO) bands on thermal dehydration might reveals that d(H2O)
bands are overlapped by the former ones. Thus, the shoulder that appears at
1690 cm-1 in the spectrum of the sample treated at 200ºC in Figure
3b can be assigned to d(H2O).
Librational modes which usually present low intensity and are
overlapped with other modes, are spread out in the 800-500 cm-1 range of vibrational frequencies. Several bands associated to these modes are
observed in both Raman and IR spectra supporting the presence of water
molecules differently involved in the structure. They were assigned taking into
account the nature of the modes and water molecules interactions in the
structure. The IR bands associated to these modes are marked in Figure 3c where
it can be seen that they disappear with increasing temperature. The frequency
values of these modes are lower than those observed for i.e.
diaquamalonatocalcium(II) [12] in response to the different masses of the
respective cations, since these values depend on the inverse of the inertial
moment.
OCO Group Modes. nas(OCO) stretching modes are present in the FTIR and Raman spectra as broad, asymmetric bands with various components included, whereas the ns(OCO) mode exhibits no splitting. The splitting of the former mode can be explained in terms of the different types of interaction between the carboxylate groups of the malonate ions and Eu(III), four- and six-membered chelate rings and monodentate bridging interactions.
Figure 4 : FTIR
spectra of the dehydration course of [Eu2(mal)3(H2O)6] ∙ 2H2O Room temperature
200 ºC
250 ºC
350 ºC *Librational modes of water
Figure 5: TGA-DTA
curves of [Eu2(mal)3(H2O)6] 2H2O
The nas(OCO) mode appears in a 1625-1550 cm-1 range being the
band at the highest frequency expected in compounds where the “malonate”
binding mode is present [23]. Taking into account the spectroscopic criterion
to select the vibrational modes associated to the different metal-carboxylate
interactions [24], the resulting Dn from the FTIR spectrum are 265 cm-1 and 190 cm-1 and correspond to the presence of six- and four-membered chelate rings
respectively. Robinson and Uttley [25] have characterized
unidentate and bidentate modes of binding by OCO group frequencies. They found
that nas(OCO) and Dn values are closer to those in the free ion (i.e. sodium malonate: nas(OCO) 1600, 1562 cm-1 and ns(OCO) 1390, 1370 cm-1 [26]) for chelate four-membered
ring coordination, while Dn values for six-membered ring coordination are much larger, and nas(OCO) bands appear at higher frequencies. This systematic was
observed for several M(III) malonate complexes [27-31].
The presence of malonic acid or bimalonate ion is discarded because
no evidence of IR carbonyl (n(C=O) 1735, 1705 cm-1 [26]) absorption is present.
No significant differences are observed in frequency
values of equivalent IR and Raman bands in our spectra, in disagreement with
the prediction for a centrosymmetric space group. This behavior could probably
be ascribed to unimportant coupling effects between different oscillators in
the unit cell.
Thermal Behavior
The
TGA-DTA curves of hexaaquatris(malonato)dieuropium(III) dihydrate are shown in
Figure 5. Dehydration proceeds via three stages associated with DTA endothermic
peaks at 99, 162 and 195°C. The first signal corresponds to the loss of the two
hydration water molecules. The experimental weight loss in the TGA curve is
4.75% (theoretical, 4.77%). The second stage is assigned to the departure of
four water molecules with an experimental weight loss of 9.59% (theoretical,
9.55%). Two water molecules are eliminated in the further step with an
experimental weight loss of 4.53% (theoretical, 4.77%). The second loss can be
associated with coordinated water molecules which exhibit two H-bonds according
to the structural description while the third one corresponds to those with
stronger H-bond interactions.
On further heating, decomposition of the anhydrous compound occurs
via three steps related with DTA exothermic peaks at 370, 381, 447 and 660°C.
The complete weight loss for this process is 34.42% (theoretical, 34.23%). The
final product, stable up to 1000°C, has been confirmed by XRD as europium(III)
oxide (cubic) (JCPDS card 43-1008) [32]. Nagase et al.[33] have observed that in the decomposition course of the Eu2mal3
6 H2O, none of the weight loss values could be assigned to a
distinct product possibly owing to side reactions; besides, their
identification is difficult because they result amorphous to X-rays.
Conclusions
This work reports the vibrational and thermal behavior of [Eu2(mal)3(H2O)6]
2H2O obtained from solution at room temperature. A complete
assignment of the FTIR and Raman signals was performed for malonate and water
modes, particularly the librational contribution. The results were consistent
with the reported structural data. The splitting of the nas(OCO) mode
can be explained in terms of the different types of interaction between the
carboxylate groups of the malonate ions and Eu(III), four- and six-membered
chelate rings and monodentate bridging interactions. Higher frequencies for the
nas(OCO) mode seem to be a common feature for metal malonates where the
"malonate" binding mode is present.
The shift to lower frequencies of water stretching modes indicates
the presence of strong hydrogen bonds in the lattice.
Dehydration process confirms the different nature of the
interactions of water molecules in the lattice: hydration water, coordinated
water molecules which exhibit two H-bonds and those ones with stronger H-bonds.
Thus, vibrational and thermal information is in complete agreement with the
structural description.
Acknowledgments
The authors thank the Consejo Nacional de Investigaciones Científicas y Técnicas (PID-CONICET 02482), the Agencia Nacional de Promoción Científica y Tecnológica (PICT 03041), Universidad Nacional de San Luis and the Brazilian agencies CNPq (Prosul) and CAPES, for financial support, J. C. P. and G. E. N. are members of the CONICET.
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