Tetramethylcyclopentadienyl Ligands Allow Isolation of Ln(II) Ions across the Lanthanide Series in [K(2.2.2-cryptand)][(C5Me4H)3Ln] Complexes

Although previous studies of the stabilization of Ln­(II) ions across the lanthanide series have relied on Me3Si-substituted cyclopentadienyl ligands, we now find surprisingly that these ions can also exist surrounded by three tetramethylcyclopentadienyl ligands. Reduction of the 4fn Ln­(III) complexes, Cptet3Ln (Cptet = C5Me4H) using potassium graphite in the presence of 2.2.2-cryptand (crypt) produces the Ln­(II) complexes, [K­(crypt)]­[Cptet3Ln] for Ln = La, Ce, Pr, Nd, Sm, Gd, Tb, and Dy, all of which were characterized by X-ray crystallography. These complexes display intense absorptions in the UV–visible–near IR region that are red-shifted compared to those of previously characterized (Cp′3Ln)1– complexes (Cp′ = C5H4SiMe3). The thermal stability of these new Ln­(II) complexes decreases with the size of the metal.