Synthesis and Structures of Tris(cyclononatetraenyl) Rare-Earth Complexes [Ln(C₉H₉)₃] (Ln = Y, Gd, Tb, Dy, Ho, Er, Tm)

The article reports the synthesis and structural characterization of a series of Ln(C9H9)3 complexes with the cyclononatetraenyl (Cnt, C9H9) ligand (Ln = Y, Gd, Tb, Dy, Ho, Er, Tm). The Yb and Sm complexes were not obtained, and the reaction of the potassium salt of the Cnt ligand with trivalent halide salts of the corresponding metals led to the known bis-Cnt sandwich compounds Ln(C9H9)2. The X-ray diffraction studies on the trivalent complexes show that the Cnt ligand is significantly bent in order to accommodate the large size of the ligand while it maintains its aromaticity. When the size of the lanthanide ion decreases, the ligand does not switch away but swings over the metal ion in order to maximize the electrostatic interactions. 1H NMR and UV-visible spectra, as well as the solid-state magnetism, were recorded. UV-visible spectroscopy highlights a remarkable charge-transfer band in the Tm complex, while ligand-based transitions are principally observed with all other metal ions. The magnetic behavior of the series agrees with trivalent lanthanide ions, and the computations at the CASSCF level confirm the trivalent electronic structure.