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 Ln(2+) 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'(3)Ln (Cp' = C5H4SiMe3, Ln = rare earth) with potassium in the presence of 18-crown-6 in Et2O at -35 degrees C under argon: [(18-crown-6)K][Cp'Tb-3], {[(18-crown-6)K][Cp'Tb-3]}(n), and {[K(18-crown-6)]2(mu-Cp')}{Cp'Tb-3}. The first complex is analogous to previously isolated Y2+, Ho2+, and Er2+ 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 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'(3)Ln/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'(3)Ln]. 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 Pr2+, Gd2+, and Lu2+ complexes. These Ln(2+) complexes, [K(2.2.2-cryptand)][Cp'(3)Ln] (Ln = Y, Pr, Gd, Tb, Ho, Er, Lu), all have similar UVvis spectra and exhibit LnC(Cp') bond distances that are 0.03 angstrom longer than those in the Ln(3+) precursors, Cp'(3)Ln. These data, as well as density functional theory calculations and EPR spectra, suggest that a 4f(n)5d1 description of the electron configuration in these Ln(2+) ions is more appropriate than 4f(n)+1.