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 η(8)-cyclooctatetraenide sandwich complex Er(COT)2(-) (COT(2-) = 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)2Y(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 Er(III) ion.