The objective of the present work is to explore the ratio of polyoxometalate charge to size together with the effect of the cation in coordination assemblies. In this regard, 12-borotungstic acid as a Keggin-type polyoxometalate which has a high negative charge density and is very rare in hybrid categories is used to synthesize six new hybrid networks, [M3(H2O)13(pydc-OH)2][BW12O40]·xH2O (H2pydc-OH = chelidamic acid; 4-hydroxy-2,6-pyridinedicarboxylic acid, M = Sm (1), Nd (2), and x = 17 (1), 19 (2)), [Ce3(H2O)11(pydc-OH)2(μ2-OH)][HBW12O40]·20H2O (3), and {K[M(H2O)4(pydc)]4}[BW12O40]·2H2O (M = Sm (4), Nd (5), Ce (6), H2pydc = 2,6-pyridinedicarboxylic acid) under hydrothermal conditions and characterized by elemental analysis, FT-IR spectroscopy, thermogravimetric analysis, and single-crystal X-ray diffraction. Single crystal X-ray diffraction analyses show that complexes 1 and 2 feature a 2D hybrid coordination polymer which further expands to a 3D supramolecular network through a dense layer of water chains, while compound 3 exhibits a 3D hybrid inorganic–organic network through μ2-OH bridges. Compounds 4–6 are isostructural and each consist of tetramer units to construct cationic sheets where the POMs interconnect these sheets to make 3D covalent hybrid frameworks. Furthermore, a CSD survey was carried out on the cation and ligand roles in the structure formation.

Keywords