Record High Single-Ion Magnetic Moments Through 4fⁿ5d¹ Electron Configurations in the Divalent Lanthanide Complexes [(C₅H₄SiMe₃)₃Ln]⁻

The recently reported series of divalent lanthanide complex salts, namely [K­(2.2.2-cryptand)]­[Cp′₃Ln] (Ln = Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm; Cp′ = C₅H₄SiMe₃) and the analogous trivalent complexes, Cp′₃Ln, have been characterized via dc and ac magnetic susceptibility measurements. The salts of the complexes [Cp′₃Dy]⁻ and [Cp′₃Ho]⁻ exhibit magnetic moments of 11.3 and 11.4 μ_B, respectively, which are the highest moments reported to date for any monometallic molecular species. The magnetic moments measured at room temperature support the assignments of a 4fⁿ⁺¹ configuration for Ln = Sm, Eu, Tm and a 4fⁿ5d¹ configuration for Ln = Y, La, Gd, Tb, Dy, Ho, Er. In the cases of Ln = Ce, Pr, Nd, simple models do not accurately predict the experimental room temperature magnetic moments. Although an LS coupling scheme is a useful starting point, it is not sufficient to describe the complex magnetic behavior and electronic structure of these intriguing molecules. While no slow magnetic relaxation was observed for any member of the series under zero applied dc field, the large moments accessible with such mixed configurations present important case studies in the pursuit of magnetic materials with inherently larger magnetic moments. This is essential for the design of new bulk magnetic materials and for diminishing processes such as quantum tunneling of the magnetization in single-molecule magnets.