Structure, Spectral, Thermal, Magnetic Properties, and Hydrolytic Stability of Three Novel Fluorinated MOFs Based on Ho(III), Dy(III), and Yb(III) Ions

Abstract

Three novel fluorinated Ln(III) metal-organic frameworks (Ln-F-MOFs) containing holmium for UPJS-21(Ho), dysprosium for UPJS-21(Dy), and ytterbium for UPJS-22(Yb) were prepared and studied. The formulas of compounds were: {[Ho₂(L)₃(DMF)_0.65(DMA)_1.35]∙0.75DMF∙0.75DMA}_n, {[Dy₂(L)₃(DMF)_0.65(DMA)_1.35]∙0.75DMF∙0.75DMA}_n, {[Yb₂(L)₂(CH₃COO)₂]∙1DMF∙1DMA}_n. The structures of UPJS-21(Ho) and UPJS-22(Yb) were solved from single-crystal measurements. Both structural forms consist of two-dimensional polymeric layers interacting through weaker intramolecular interactions, like hydrogen bonds involving fluoro, nitrogen and hydrogen atoms. The single crystals of UPJS-21(Dy) with sufficient quality were not obtained, but based on the powder XRD measurements, the third compound, UPJS-21(Dy) is isostructural with UPJS-21(Ho). The surface of UPJS-21(Ho) shows hydrophobic behaviour referred to as a “rose petal effect.” All complexes are thermally stable up to 300 °C. The photoluminescent properties of the complexes were also investigated. While for UPJS-21(Ho), UPJS-21(Dy), a distinct 4f-4f electron transitions unique to the respective lanthanide ion were observed, for UPJS-22(Yb), only weak π−π* transitions of the aromatic ring associated with the (L)^2- ligand were recorded. The magnetic behaviour of UPJS-21(Ho) and UPJS-21(Dy) is characterised by a substantial reduction of the magnetic moment at low temperatures due to the low mJ of the ground state Stark level. On the other hand, the deformed pentagonal bipyramidal coordination of Yb(III) ions in UPJS-22(Yb) induced a strong axial anisotropy, yielding an energy gap between the ground and the first excited doublet of 191.4 cm^-1. Moreover, adsorption of N₂, CO₂, and H₂ was also studied for UPJS-21(Ho) and UPJS-21(Dy) complexes.