Lanthanide Coordination Polymers with Tetrafluoroterephthalate as a Bridging Ligand: Thermal and Optical Properties

By slow diffusion of triethylamine into a solution of
2,3,5,6-tetrafluoroterephthalic
acid (H2tfBDC) and the respective lanthanide salt in EtOH/DMF
single crystals of seven nonporous coordination polymers, ∞2[Ln­(tfBDC)­(NO3)­(DMF)2]·DMF (Ln3+ = Ce, Pr, Nd,
Sm, Dy, Er, Yb; C2/c, Z = 8) have been obtained. In the crystal structures, two-dimensional
square grids are found, which are composed of binuclear lanthanide
nodes connected by tfBDC2– as a linking ligand.
The coordination sphere of each lanthanide cation is completed by
a nitrate anion and two DMF molecules (CN = 9). This crystal structure
is unprecedented in the crystal chemistry of coordination polymers
based on nonfluorinated terephthalate (BDC2–) as
a bridging ligand; as for tfBDC2–, a nonplanar conformation
of the ligand is energetically more favorable, whereas for BDC2–, a planar conformation is preferred. Differential
thermal analysis/thermogravimetric analysis (DTA/TGA) investigations
reveal that the noncoordinating DMF molecule is released first at
temperatures of 100–200 °C. Subsequent endothermal weight
losses correspond to the release of the coordinating DMF molecules.
Between 350 and 400 °C, a strong exothermal weight loss is found,
which is probably due to a decomposition of the tfBDC2– ligand. The residues could not be identified. The emission
spectra of the ∞2[Ln­(tfBDC)­(NO3)­(DMF)2]·DMF compounds
reveal intense emission in the visible region of light for Pr, Sm,
and Dy with colors from orange, orange-red, to warm white.