Completing the Series of +2 Ions for the Lanthanide Elements: Synthesis of Molecular Complexes of Pr2+, Gd2+, Tb2+, and Lu2+

The first examples of crystallographically characterizable complexes of Tb (2+), Pr (2+), Gd (2+), and Lu (2+) have been isolated, which demonstrate that Ln (2+) ions are accessible in soluble molecules for all of the lanthanides except radioactive promethium. The first molecular Tb (2+) complexes have been obtained from the reaction of Cp'3Ln (Cp' = C5H4SiMe3, Ln = rare earth) with potassium in the presence of 18-crown-6 in Et2O at -35 °C under argon: (18-crown-6) KCp'3Tb, (18-crown-6) KCp'3Tbn, and K (18-crown-6) 2 (μ-Cp') Cp'3Tb. The first complex is analogous to previously isolated Y (2+), Ho (2+), and Er (2+) complexes, the second complex shows an isomeric structural form of these Ln (2+) complexes, and the third complex shows that (18-crown-6) K (1+) alone is not the only cation that will stabilize these reactive Ln (2+) species, a result that led to further exploration of cation variants. With 2. 2. 2-cryptand in place of 18-crown-6 in the Cp'3Ln/K reaction, a more stable complex of Tb (2+) was produced as well as more stable Y (2+), Ho (2+), and Er (2+) analogs: K (2. 2. 2-cryptand) Cp'3Ln. Exploration of this 2. 2. 2-cryptand-based reaction with the remaining lanthanides for which Ln (2+) had not been observed in molecular species provided crystalline Pr (2+), Gd (2+), and Lu (2+) complexes. These Ln (2+) complexes, K (2. 2. 2-cryptand) Cp'3Ln (Ln = Y, Pr, Gd, Tb, Ho, Er, Lu), all have similar UV-vis spectra and exhibit Ln-C (Cp') bond distances that are ~0. 03 Å longer than those in the Ln (3+) precursors, Cp'3Ln. These data, as well as density functional theory calculations and EPR spectra, suggest that a 4f (n) 5d (1) description of the electron configuration in these Ln (2+) ions is more appropriate than 4f (n+1).