UoC-14: MOFs with trivalent lanthanides and fluorinated BTB-linkers

A new series of MOFs (metal-organic frameworks) based on the lanthanides Ln = Sm-Lu and a fluorinated variant of benzene-1,3,5-tribenzoate (BTB^3-) is presented. These compounds, named UoC-14(Ln) (UoC = University of Cologne), crystallize as pseudo-merohedral twins in the space group P2₁/c and possess permanent porosity as confirmed by gas sorption measurements (S_BET = 723.8 m² g^-1 for Ln = Eu). Thermogravimetric analyses reveal the release of solvent molecules below 200 °C under inert conditions. The wide range of metal cations which can serve as nodes for this type of MOF enables the synthesis of luminescent framework materials whose properties are examined for Ln = Eu, Tb in detail. The 3F-BTB^3- linker was shown to serve as an antenna, increasing the spectral absorption width of the material significantly, whereas the electron withdrawing fluorine substituents lead to the expected bathochromic shift. The crystal structure of UoC-14(Ln) is similar to a recently published structure for [Ln(BTB)DMF] (Ln = Y, Eu, Tb) with the unfluorinated linker. However, subtle differences occur. In order to gain a deeper understanding of the effect of the fluorination on the preferred geometry of the linker, DFT calculations and a detailed meta-analysis of the BTB linker system are carried out.