Photocatalysis, terahertz time domain spectroscopy and weak interactions of six polyoxometalate-based lanthanide phosphine oxide complexes

Abstract

Using triphenylphosphine oxide (OPPh₃) or tetraethyl ethylenebisphosphonate (L) as a ligand and phosphomolybdic acid hydrate as an anion template, six new lanthanide complexes [Nd(OPPh₃)₄(H₂O)₃](PMo₁₂O₄₀)·4CH₃CN (1a), [Ln(OPPh₃)₄(H₂O)₃](PMo₁₂O₄₀)·4C₂H₅OH (2a–4a) (Ln = Dy, Ho, Er), [NdL₄](PMo₁₂O₄₀)·CH₃CN (1b), and [ErL₃(H₂O)](PMo₁₂O₄₀)·CH₃CN (4b) were synthesized. They were characterized by single crystal X-ray diffraction, elemental analysis, IR spectroscopy, powder X-ray diffraction, thermogravimetric analysis and THz spectroscopy. Also, [LnL₃(H₂O)](PMo₁₂O₄₀)·CH₃CN (2b, 3b) (Ln = Dy, Ho) have been published previously and they are cited in this paper for comparison. Complexes 1a–4a and 2b–4b are all seven-coordination mononuclear structures centered on lanthanide ions, while complex 1b is of eight-coordination. These mononuclear structures eventually form complicated supramolecular structures through hydrogen bonding, Mo–O⋯π or C–H⋯π weak interactions. The photocatalytic experiments show that the photocatalytic degradation removal rates of methylene blue are 94.93–99.55% for 1a–4a, 1b and 4b, and they have good stability and can be recycled, so they can be used as catalysts to degrade dye molecules in sewage. Due to the similar structures of complexes 1a–4a, 1b and 4b, they have almost the same THz absorption spectra with different absorption intensities, which may be attributed to the difference in the number of weak interactions. So THz spectroscopy can be used as a sensitive way to distinguish and determine small differences between lanthanide–organic complexes. In summary, the structures, photocatalytic properties, adsorption and terahertz spectra of a series of lanthanide complexes based on phosphine oxide ligands and Keggin type polyoxometalates were studied in this paper.