Family of Isomeric Cu-Ln (Ln = Gd, Tb, and Dy) Complexes Presenting Field-Induced Slow Relaxation of Magnetization Only for the Members Containing Gd.

The strategic design and synthesis of two isomeric Cu complexes, [CuL] and [CuL], of asymmetrically dicondensed NO-donor Schiff-base ligands (where HL and HL are -salicylidene-'-3-methoxysalicylidenepropane-1,2-diamine and -3-methoxysalicylidene-'-salicylidenepropane-1,2-diamine, respectively) have been accomplished via a convenient Cu template method. These two complexes have been used as metalloligands for the synthesis of three pairs of Cu-Ln isomeric complexes [CuL(μ-NO)Ln(NO)(HO)]·CHCN (for complexes -, L = L, and for complexes -, L = L and Ln = Gd, Tb, and Dy, respectively), all of which have been characterized structurally. In all six isomorphous and isostructural complexes, the decacoordinated Ln centers and pentacoordinated Cu centers possess sphenocorona and square-pyramidal geometries, respectively. The isomeric pair of Cu-Gd compounds shows field-induced slow relaxation of magnetization, although they present the typical isotropic behavior of Gd complexes, indicating that slow relaxation is not due to the usual energy barrier originating from the magnetic anisotropy. The isostructural derivatives with the ion-anisotropic lanthanides Tb and Dy do not show slow magnetic relaxation with or without a direct-current bias field, demonstrating that the magnetic response of the isotropic system Cu-Gd occurs through different mechanisms than the rest of the Ln cations.