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

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 P2(1)/c. The products of the thermal decomposition of the material were also monitored by TGA-DSC/FT-IR both in air and N-2 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.