Light Lanthanide Metallocenium Cations Exhibiting Weak Equatorial Anion Interactions

As the dysprosocenium complex [Dy(Cp^ttt )₂ ][B(C₆ F₅ )₄ ] (Cp^ttt =C₅ H₂ tBu₃ -1,2,4, 1-Dy) exhibits magnetic hysteresis at 60 K, similar lanthanide (Ln) complexes have been targeted to provide insights into this remarkable property. We recently reported homologous [Ln(Cp^ttt )₂ ][B(C₆ F₅ )₄ ] (1-Ln) for all the heavier Ln from Gd-Lu; herein, we extend this motif to the early Ln. We find, for the largest Ln^III cations, that contact ion pairs [Ln(Cp^ttt )₂ {(C₆ F₅ -κ¹ -F)B(C₆ F₅ )₃ }] (1-Ln; La-Nd) are isolated from reactions of parent [Ln(Cp^ttt )₂ (Cl)] (2-Ln) with [H(SiEt₃ )₂ ][B(C₆ F₅ )₄ ], where the anion binds weakly to the equatorial sites of [Ln(Cp^ttt )₂ ]⁺ through a single fluorine atom in the solid state. For smaller Sm^III , [Sm(Cp^ttt )₂ ][B(C₆ F₅ )₄ ] (1-Sm) is isolated, which like heavier 1-Ln does not exhibit equatorial anion interactions, but the Eu^III analogue 1-Eu could not be synthesised due to the facile reduction of Eu^III precursors to Eu^II products. Thus with the exception of Eu and radioactive Pm this work constitutes a structurally similar family of Ln metallocenium complexes, over 50 years after the [M(Cp)₂ ]⁺ series was isolated for the 3d metals.