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 Tb2+, Pr2+, Gd2+, and Lu2+ have been isolated, which demonstrate that Ln2+ ions are accessible in soluble molecules for all of the lanthanides except radioactive promethium. The first molecular Tb2+ 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)K][Cp′3Tb], {[(18-crown-6)K][Cp′3Tb]}n, and {[K(18-crown-6)]2(μ-Cp′)}{Cp′3Tb}. The first complex is analogous to previously isolated Y2+, Ho2+, and Er2+ complexes, the second complex shows an isomeric structural form of these Ln2+ complexes, and the third complex shows that [(18-crown-6)K]1+ alone is not the only cation that will stabilize these reactive Ln2+ 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 Tb2+ was produced as well as more stable Y2+, Ho2+, and Er2+ analogs: [K(2.2.2-cryptand)][Cp′3Ln]. Exploration of this 2.2.2-cryptand-based reaction with the remaining lanthanides for which Ln2+ had not been observed in molecular species provided crystalline Pr2+, Gd2+, and Lu2+ complexes. These Ln2+ 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 Ln3+ precursors, Cp′3Ln. These data, as well as density functional theory calculations and EPR spectra, suggest that a 4fn5d1 description of the electron configuration in these Ln2+ ions is more appropriate than 4fn+1.