Syntheses, structures and properties of two Keggin polyoxometalates [H5PCo(4,4′-bipy)Mo11O39][H3PMo12O40]·3.75(4,4′-bipy)·1.5H2O and [H3PMo12O40]·2(4,4′-bipy)·1.5H2O

Two novel Keggin polyoxometalates [H5PCo(4,4′-bipy)Mo11O39][H3PMo12O40]·3.75(4,4′-bipy)·1.5H2O (1) and [H3PMo12O40]·2(4,4′-bipy)·1.5H2O (2) (bipy=bipyridine) were prepared by the hydrothermal method for the first time and characterized by elemental analyses, IR spectra and X-ray single crystal diffraction, showing that compound 1 is a novel organic–inorganic hybrid compound which consists of a cobalt-monosubstituted Keggin polyoxoanion [PCo(4,4′-bpy)Mo11O39]5− with one pendant ligand 4,4′-bipy, a well-known Keggin polyoxoanion [PMo12O40]3−, 4,4′-bipy and lattice water molecules. Additionally, the two different heteropolyoxoanions in 1 with alternating alignment, combined with the discrete organic substrates 4,4′-bipy and water molecules by hydrogen bond interactions to afford a pseudo 3D network structure. While compound 2 is an intermolecular compound between polyoxoanion unit [PMo12O40]3− and organic substrate 4,4′-bipy. Furthermore, both the compounds exhibit strong photoluminescence properties in the solid state at room temperature. The catalytic activities of the two compounds were also determined by the oxidation of benzaldehyde to benzoic acid using H2O2 as oxidant in a liquid–solid multiphase system. Meanwhile, the catalytic property of the compound 2 was evaluated by the esterification of MeCO2H (acetic acid) with n-BuOH (n-butyl alcohol).