Syntheses, structures, properties and DFT study of hybrid inorganic–organic architectures constructed from trinuclear lanthanide frameworks and Keggin-type polyoxometalates

In this paper we report the synthesis and X-ray characterization of four novel hybrid inorganic-organic assemblies generated from H4SiW12O40 as Keggin-type polyoxometalates (POM) and, in three of them, a trinuclear lanthanide cluster of type {Na(H2O)3[Ln(HCAM)(H2O)3]3}(4+) is formed, where Ln metal is La in compound 1, Ce in compound 2, and Eu in compound 3 (H3CAM = chelidamic acid or 2,6-dicarboxy-4-hydroxypyridine). These compounds represent the first POM-based inorganic-organic assemblies using chelidamic acid as an organic ligand. The thermal stability of the organic ligand is crucial, since pyridine-2,6-bis(monothiocarboxylate) instead of chelidamic acid is used (compound 4) under the same synthesis conditions, the decomposition of the ligand to pyridine was observed leading to the formation of colorless crystals of a pseudo hybrid inorganic-organic assembly. In compound 4 the hybrid inorganic-organic assembly is not formed and the organic part simply consists of four molecules of protonated pyridine acting as counterions of the [SiW12O40](4-) counterpart. The luminescent properties of compounds and have been investigated and their solid state architectures have been analyzed. Whereas compound only shows ligand emission, the Eu(3+) emission in compound 3 is discussed in detail. We have found that unprecedented anion-π interactions between the POM, which is a tetra-anion, and the aromatic rings play a crucial role in the crystal packing formation. To the best of our knowledge, this is the first report that describes and analyzes this interaction in Keggin-type POM based inorganic-organic frameworks. The energetic features of these interactions in the solid state have been analyzed using DFT calculations in some model systems predicted by us.