Facile high yield, excellent catalytic performance of polyoxometalate-based lanthanide phosphine oxide complexes: Syntheses, structures, photocatalysis and THz spectra

Abstract Water pollution, which continuously threatens human health and the sustainable development of society, has become a major concern. Photocatalytic degradation is an effective strategy to remove organic dyes from wastewater. For this strategy, it is crucial to select the appropriate catalyst. Using triphenylphosphine oxide (OPPh3) as the ligand, phosphomolybdic acid as the anion template, three new lanthanide complexes [Ln(OPPh3)4(H2O)3](PMo12O40)∙4C2H5OH (1–3) (Ln = Sm, Gd, Tb) were synthesized. The raw materials for the reaction are cheap and readily available. The convenient synthesis method is environmentally friendly, with high yield (70%–80%). Complexes 1–3 are all seven-coordinated mononuclear structures centered on lanthanide ions, [PMo12O40]3- anions and solvent molecules are not coordinated with metal ions. These mononuclear structures eventually form complicated 3D supramolecular structures through hydrogen bonds, Mo–O … π or C–H … π weak interactions. Complexes 1–3 photocatalytic degradation of MB have high removal rates, as catalysts have enough stability to be reused, and can be used as excellent catalysts for the degradation of dye molecules in sewage. Among them, the removal rate of MB by photodegradation of complex 2 was highest (99.50%). In addition, the effects of different initial concentrations of MB solution and different types of organic dyes on the photocatalysis experiment were investigated. The photocatalytic reaction mechanism of complexes 1–3 was also studied. Due to the similar structures of complexes 1–3, they have almost the same THz absorption spectra with different absorption intensity, which may be attributed to the difference of the number of weak interactions. Therefore, terahertz spectroscopy can be used as a sensitive method to distinguish and determine small differences between lanthanide-organic complexes. This is the first time that this spectrum has been used to characterize lanthanide phosphine oxide complexes modified by [PMo12O40]3-.