Three-dimensional supramolecular networks constructed from Keggin anions and metal–organic complex with unpredicted oxidation of an N-heterocycle ligand

Two novel Keggin polyoxometalate compounds, [Cu2(PCA)4(H2O)2][SiW12O40]·8H2O (1) and [Cu(PCA)2(H2O)4][Cu(PCA)3(H2O)][SiW12O40]·H2O (2) (PCA = pyridine-4-carboxylic acid), were synthesized by in situ generation of pyridine-4-carboxylic acid ligand from 1,3-bis(4-pyridyl)propane precursor under hydrothermal conditions. Compounds 1 and 2 were structurally characterized by elemental analysis, IR and UV spectrography, TGA and single-crystal X-ray diffraction analysis. The variable-temperature magnetic susceptibility was measured at 2–300 K for 1. The compound 1 consists of polyoxoanions [α-SiW12O40]4−, dimer cations [Cu2(PCA)4(H2O)2]4+ and lattice water molecules. The water molecules link [α-SiW12O40]4− and [Cu2(PCA)4(H2O)2]4+ into three-dimensional (3D) architecture via hydrogen bonds. Polyoxoanions of 2 are linked into 1D chains through hydrogen bonds. Moreover, 1D chains are further extended into 3D supramolecular networks by hydrogen bonds and π···π stacking interactions of pyridine-4-carboxylic acid molecules. The variable-temperature magnetic measurement of 1 shows an antiferromagnetic interaction in the 2–300 K temperature range.