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 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 organic ligand. 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 [SiW12O40]4– counterpart. Hence, the thermal stability of the used organic ligand must be considered, since when we used pyridine-2,6-bis(monothiocarboxylate) for synthesizing 4, instead of chelidamic acid, under the same synthesis conditions decomposition of the ligand to pyridine was observed and led to formation of colorless crystals of pseudo hybrid inorganic–organic assembly. The luminescent properties of compounds 1 and 3 have been investigated and their solid state architectures have been analyzed. We have found that strong anion– interactions between the used 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.

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