The recent exponential interest in the synthesis and characterization of new hybrid materials with 'organic-inorganic' open-framework has revealed the diversity of such materials in terms of structure, topology, composition. One of the most innovating approaches in microporous materials arises from metal-organic or metal-coordination chemistry where organic moieties act as spacer or linker between metal blocks. The use of oxalates, oxocarbon entities with aromaticity or aliphatic dicarboxylates has increased providing spectacular examples of how the organic ligand symmetry as well as the metal geometry can be exploited for conceiving novel architectures with a view to exploiting interesting properties as magnetism, porosity, ion-exchange ability. The introduction of lanthanides centers within the skeleton creates unusual prototype open-frameworks and may give unique properties stemming from their f-f electronic transitions for optical properties (oxalic acid has been reported to form, with neodymium, a chiral mixed carboxylate exhibiting NLO properties). For the preparation of new 3D frameworks with potential properties, the use of two anions of different geometry and chemical functions offers immense scope, but has not been investigated systematically with the squarate dianion. The group presents, with the rare-earth elements, a high diversity of unusual coordination modes, including chelation. Our aim is to create novel mixed materials based on lanthanides combined with two anions but wherein an organic rigid moiety is one of the components. By using hydrothermal methods, the first hydrated lanthanum sulfato-squarate has been obtained [1]. Otherwise, as the oxalate acid has been often used to form novel porous metal dicarboxylates, our synthetic efforts have also focused on the possibility to prepare oxalato-squarates. Among our successful productions, pure isostructural Ln dicarboxylates [Ln₂(C₂O₄)₂(C₄O₄)(H₂O)₂, Ln = La, Ce, Pr, Nd] have been obtained hydrothermally, their crystal structure solved by X-ray diffraction. For Ln = La and Ce, the thermal decomposition from the hydrated precursor to the finely divided oxide [La₂O₃ (550°C)/CeO₂ (250°C)] has been investigated by X-ray thermodiffractometry. The TDXD results show a common first step of dehydration at 200°C leading to the formation of unknown well crystallized phase, Ln₂(C₂O₄)₂(C₄O₄). After, the two decomposition mechanisms proceed through different steps. For a better understanding of the processes, complementary TG/DTA combined with simultaneous evolved gas analysis using mass spectrometry were carried out. The structure determination from X-ray powder diffraction of the stable anhydrous monoclinic phase La₂(C₂O₄)₂(C₄O₄) is in progress. Finally, an isostructural (La/Eu) phase doped with europium(III) was recently prepared allowing for an extension of our interest to the field of luminescent hydrid phases based on rare earth metals.

[1] H. Akkari et al., Solid State Sci. 8 (2006) 704.

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