Synthesis and Magnetic Properties of Bis-Halobenzene Decamethyldysprosocenium Cations

The decamethyldysprosocenium cation, [Dy(Cp*)₂]⁺ (Cp* = {C₅Me₅}), was a target single-molecule magnet (SMM) prior to the isolation of larger dysprosocenium cations, which have recently shown magnetic memory effects up to 80 K. However, the relatively short Dy···Cp*_centroid distances of [Dy(Cp*)₂]⁺, together with the reduced resonance of its vibrational modes with electronic states compared to larger dysprosocenium cations, could lead to more favorable SMM behavior. Here, we report the synthesis and magnetic properties of a series of solvated adducts containing bis-halobenzene decamethyldysprosocenium cations, namely [Dy(Cp*)₂(PhX-κ-X)₂][Al{OC(CF₃)₃}₄] (X = F or Cl) and [Dy(Cp*)₂(C₆H₄F₂-κ²-F,F)(C₆H₄F₂-κ-F)][Al{OC(CF₃)₃}₄]. These complexes were prepared by the sequential reaction of [Dy(Cp*)₂(μ-BH₄)]_∞ with allylmagnesium chloride and [NEt₃H][Al{OC(CF₃)₃}₄], followed by recrystallization from parent halobenzenes. The complexes were characterized by powder and single crystal X-ray diffraction, NMR and ATR-IR spectroscopy, elemental analysis, and SQUID magnetometry; experimental data were rationalized by a combination of density functional theory and ab initio calculations. We find that bis-halobenzene adducts of the [Dy(Cp*)₂]⁺ cation exhibit highly bent Cp*···Dy···Cp* angles; these cations are also susceptible to decomposition by C-X (X = F, Cl, Br) activation and displacement of halobenzenes by O-donor ligands. The effective energy barrier to reversal of magnetization measured for [Dy(Cp*)₂(PhF-κ-F)₂][Al{OC(CF₃)₃}₄] (930(6) cm^-1) sets a new record for SMMs containing {Dy(Cp*)₂} fragments, though all SMM parameters are lower than would be predicted for an isolated [Dy(Cp*)₂]⁺ cation, as expected due to transverse ligand fields introduced by halobenzenes and the large deviation of the Cp*···Dy···Cp* angle from linearity promoting magnetic relaxation.