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 Y...