Magnetic Blocking at 10 K and a Dipolar-Mediated Avalanche in Salts of the Bis(η8‑cyclooctatetraenide) Complex [Er(COT)2]−

The structures and magnetic properties of [K­(18-crown-6)]+ (1) and [K­(18-crown-6)­(THF)2]+ (2) salts of the η8-cyclooctatetraenide sandwich complex [Er­(COT)2]− (COT2– = cyclooctatetraene dianion) are reported. Despite slight differences in symmetry, both compounds exhibit slow magnetic relaxation under zero applied dc field with relaxation barriers of ∼150 cm–1 and waist-restricted magnetic hysteresis. Dc relaxation and dilution studies suggest that the drop in the magnetic hysteresis near zero field is influenced by a bulk magnetic avalanche effect coupled with tunneling of the magnetization. Through dilution with [K­(18-crown-6)­(THF)2]­[Y­(COT)2] (3), these phenomena are substantially quenched, resulting in an open hysteresis loop to 10 K. Importantly, this represents the highest blocking temperature yet observed for a mononuclear complex and the second highest for any single-molecule magnet. A comprehensive comparative analysis of the magnetism of [K­(18-crown-6)]­[Ln­(COT)2] (Ln = Sm, Tb, Dy, Ho, Yb) reveals slow relaxation only for [K­(18-crown-6)]­[Dy­(COT)2] (4) with weak temperature dependence. Collectively, these results highlight the utility of an equatorial ligand field for facilitating slow magnetic relaxation in the prolate ErIII ion.