Title: Spectroscopic study of uranyl soaps
Authors: De Houwer, Sandy
Jongen, Liesbet
Ferrando Mangrinyan, Mayte
Görller-Walrand, Christiane
Binnemans, Koen #
Issue Date: 2003
Conference: 8th International Symposium on Metallomesogens (ISM2003) location:Namur (Belgium) date:28-31 May 2003
Article number: P3
Abstract: We report on the thermal and spectroscopic behaviour of uranyl soaps. Uranyl compounds of the following composition were synthesized: UO2(CxH2x+1CO2)2.nH2O, where x = 6 – 18 and n = 0 – 2. x represents the number of carbon-atoms in the alkyl chain and n the number of water molecules coordinated. Thermal behaviour of the complexes was studied with DSC and thermo-optical microscopy. The complete series of synthesized uranyl alkanoates exhibits mesomorphism, although the mesophase was identified as being plastic crystal. Whereas the melting point increases with increasing chain length, the temperature of isotropization decreases. The length of the alkyl chain causes the increase of the melting temperature: the longer the chain, the more thermal energy is needed to melt. The decrease in clearing point can be seen less favourable interactions between the molten alkyl chains. This corresponds with the behaviour of the larger Ln(III) soaps. La(III) to Nd(III) alkanoates exhibit a clear smectic A phase, where the same chain length dependence is observed [1].
From the IR-spectra the analyses of the C=O stretching frequency was obtained. The strong carbonyl peak around 1700 cm-1 disappeared and two new bands were seen, resulting from the carboxylate group. The splitting between this symmetrical and asymmetrical vibration of the carboxylate holds a lot of structural information about the complex. Also, the wagging vibrations of the CH2-groups of the alkyl chain indicate that all chains are present is a fully stretched conformation (all-trans), except in the case of C18 and C19.
UV-VIS spectra between 300 and 600 nm in DMF at room temperature were measured, where the typical vibrational progressions of the central uranyl ion were clearly observed.Luminescence and excitation spectra at several excitation and detection wavelengths were recorded at 77 K in KBr-pellets, ranging from 450 nm until 650 nm. The spectroscopic properties differ with the chain length. Together with the IR-data, we divided the compounds into four groups, each with there own specific properties. From each group, we picked one model complex to explain the observed characteristics. Figure 1 shows the luminescence spectra of these model complexes. Group I (C7, C8) exhibits a progression of high (A) and low (B) intensity peaks. At higher excitation wavelengths, a third, very low intense peak appears in the spectrum. In group II (C11, C14) a very strong peak (C) is present, although the intensity of C varies with the chosen excitation wavelength. For groups III (C12, C13, C16, C17) and IV (C18, C19), no dependence to the excitation wavelength is observed. The spectra of group IV show broad A peaks and weak shoulders where peaks B and C arose in the previous groups.
Monodentate coordination of the complexes can be excluded, whilst no clear distinction can be made between a chelating bidentate or bridge-forming bidentate coordination. It is also possible that the differences seen in the spectra arise from a mixed coordination form, which is often the case for the complex formation of uranyl with shorter carboxylates, from formiate to butyrate [2]. For the C11, C13, C15 and C17 complexes a chelating and bridge-forming mixed coordination was already proposed by Solanski et al. [3]. The spectral differences reflect the changes in the first coordination sphere resulting from the different coordination forms.

[1] K. Binnemans, L. Jongen, C. Bromant, D. Hinz, G. Meyer, Inorg. Chem. 39 (2000) 5938.
[2] J. Leciejewicz, N. W. Alcock, T. J. Kemp, Struct. Bond. 82 (1995) 43.
[3] A. K. Solanski, A. M. Bhandari, Tenside Detergents 18 (1981) 1.
Description: poster presented by Sandy De Houwer
Publication status: published
KU Leuven publication type: IMa
Appears in Collections:Molecular Design and Synthesis
# (joint) last author

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