Structural Studies of Lanthanidhe(III) Succinates

G. ECHEVERRÍA; G. POZZI; G. PUNTE; M. C. BERNINI; E. V. BRUSAU; G. E. NARDA; J.A. ELLENA

Puerto Madryn – Chu

Congreso; II Reunión de la Asociación Argentina de Cristalografía; 2006

The last years have witnessed a great effort dedicated to investigate, design and predict open hybrid organic/inorganic frameworks, the structural agents being metal cations and  multifunctional anions.  Such molecular-based porous materials exhibit potential utility in catalysis, optics, gas separation/storage and host-guest chemistry. Within this field, the observed wide use of dicarboxylates to build open framework systems owes to the versatility of these anions interactions. Their many ways of metal coordination, which can coexist, and their ability to act as hydrogen bonds acceptors and donors make them capable to generate supramolecular assemblies and account for their multifunctionality. The synthesis of specific rare-earth dicarboxylates allows to expand the application field of this kind of compounds by the construction of smart materials with attractive optical or magnetic properties, depending on the nature of the metal ion. A large variety of coordination polymers has been synthesized by crystallization under mild conditions; hydrothermal techniques allow the variety of structural types to be considerably extended. Though synthesis conditions are known to play an important role in the framework dimensionality and hydration grade, the way in which all the variables influence on framework development is not clear yet. According to the literature hydrothermal conditions are more suitable for preparing 3D solids with extended M–O–M networks, which favors better thermal stability, since they enhance metal–ligand interactions rather than metal–water coordination. Here we will present a review of the structural knowledge of lanthanide succinates. According to our investigations and results existent in the literature, four structural varieties can be recognized. The analysis indicates that compounds of type 1, crystallize in the triclinic system ―space group P1¯   and do not host hydration waters, in spite of that they present a framework with important structural filiations with the type 2 compounds, which crystallize in the monoclinic C2/c space group. Both structural types include infinite 1D inorganic networks (running along a crystallographic axis), which are built up from edge-sharing LnO8(H2O) polyhedra interconnected by succinate dianions, and embody channels parallel to the chains. On the other hand, lutetium succinate structure, type 3, consists of isolated LuO6(H2O)2 irregular polyhedra linked by the organic moieties to form a three-dimensional framework. We have recently reported the fourth type, a new triniclinic phase, which is a 2D hybrid polymer and its crystal structure is unprecedented for rare-earth succinates. This phase, which has been found in several Ln(III) compounds, consists of centrosymmetric dimers of edge-sharing LnO7(H2O)2 polyhedra linked by succinate ions in the [100] and [101] directions, hydrogen-bonded hydration water molecules connect layers ensuring the three-dimensional stability. In this paper we analyze all the observed conformations and coordination modes of the succinate anions and the geometry of the Ln(III) cations, O-Ln(III)-O inorganic networks, and of hydrogen bond networks observed. The influence of synthesis conditions on the obtained frameworks and the compatibility of trends in network densities and ionic radii with cell volumen is also examined.