Utility of Lithium in Rare-Earth Metal Reduction Reactions to Form Nontraditional Ln2+ Complexes and Unusual [Li(2.2.2-cryptand)]1+ Cations.

The utility of lithium compared to other alkali metals in generating Ln2+ rare-earth metal complexes via reduction of Ln3+ precursors in reactions abbreviated as LnA3/M (Ln = rare-earth metal; A = anionic ligand; M = alkali metal) is described. Lithium reduction of Cp'3Ln (Cp' = C5H4SiMe3; Ln = Y, Tb, Dy, Ho) under Ar in the presence of 2.2.2-cryptand (crypt) forms new examples of crystallographically characterizable Ln2+ complexes of these metals, [Li(crypt)][Cp'3Ln]. In each complex, lithium is found in an N2O4 donor atom coordination geometry that is unusual for the cryptand ligand. Magnetic susceptibility data on these new examples of nontraditional divalent lanthanide complexes are consistent with 4fn5d1 electronic configurations. The Dy and Ho complexes have exceptionally high single-ion magnetic moments, 11.35 and 11.67 μB, respectively. Lithium reduction of Cp'3Y under N2 at -35 °C forms the Y2+ complex (Cp'3Y)1-, which reduces dinitrogen upon warming to room temperature to generate the (N2)2- complex [Cp'2Y(THF)]2(μ-η2:η2-N2). These results provide insight on the factors that lead to reduced dinitrogen complexes and/or stable divalent lanthanide complexes as a function of the specific reducing agent and conditions.