Halobenzene Adducts of a Dysprosocenium Single-Molecule Magnet

Dysprosium complexes with strong axial crystal fields are promising candidates for single-molecule magnets (SMMs), which could be used for high-density data storage. Isolated dysprosocenium cations, [Dy(Cp ^R) ₂] ⁺ (Cp ^R = substituted cyclopentadienyl), have recently shown magnetic hysteresis (a memory effect) above the temperature of liquid nitrogen. Synthetic efforts have focused on reducing strong transverse ligand fields in these systems as they are known to enhance magnetic relaxation by spin-phonon mechanisms. Here we show that equatorial coordination of the halobenzenes PhX (X = F, Cl, Br) and o-C ₆H ₄F ₂ to the cation of a recently reported dysprosocenium complex [Dy(Cp ^ttt)(Cp*)][Al{OC(CF ₃) ₃} ₄] (Cp ^ttt = C ₅H ₂ ^tBu ₃-1,2,4; Cp* = C ₅Me ₅) reduces magnetic hysteresis temperatures compared to that of the parent cation. We find that this is due to increased effectiveness of both one- (Orbach) and two-phonon (Raman) relaxation mechanisms, which correlate with the electronegativity and number of interactions with the halide despite κ ¹-coordination of a single halobenzene having a minimal effect on the metrical parameters of [Dy(Cp ^ttt)(Cp*)(PhX-κ ¹-X)] ⁺ cations vs the isolated [Dy(Cp ^ttt)(Cp*)] ⁺ cation. We observe unusual divergent behavior of relaxation rates at low temperatures in [Dy(Cp ^ttt)(Cp*)(PhX)][Al{OC(CF ₃) ₃} ₄], which we attribute to a phonon bottleneck effect. We find that, despite the transverse fields introduced by the monohalobenzenes in these cations, the interactions are sufficiently weak that the effective barriers to magnetization reversal remain above 1000 cm ^-1, being only ca. 100 cm ^-1 lower than for the parent complex, [Dy(Cp ^ttt)(Cp*)][Al{OC(CF ₃) ₃} ₄].