Metallacrowns as Templates for Diabolo-like {LnCu₈ } Complexes with Nearly Perfect Square Antiprismatic Geometry

A series of diabolo-like nonanuclear {Ln^III Cu^II₈ } (Ln=Tb, Dy, Ho, Er, Tm, Yb, and Y) clusters were prepared in which the Ln^III ion is capped by two 8-MC-4 metallacrown ligands to form a nearly ideal square antiprismatic (SAP) coordination geometry with D_4d symmetry. Despite the lack of crystallographic symmetry, these molecules engender the lanthanide ions with highly axial m_J states. The axial/equatorial nature of the crystal field in environments close to ideal SAP geometry is very subtle and influenced by the nature of the ligand lone pairs. Slow magnetic relaxation behaviour was observed for the Dy^III , Er^III , Tm^III , and Yb^III analogues, and the obtained effective energy barriers are not consistent with excitations on the Ln^III ion, suggesting a more nuanced situation.