MIIIDyIII3 (M = FeIII, CoIII) Complexes: Three-Blade Propellers Exhibiting Slow Relaxation of Magnetization

[Dy-III(HBpz(3))(2)](2+) moieties (HBpz(3)(-) = hydrotris(pyrazolyl)borate) and a 3d transition-metal ion (Fe-III or Co-III) have been rationally assembled using an dithiooxalato dianion ligand into 3d-4f [MDy3(HBpz(3))(6)(dto)(3)]center dot 4CH(3)CN center dot 2CH(2)Cl(2) (M = Fe (1), Co (2) complexes. Single-crystal X-ray studies reveal that three eight-coordinated Dy-III centers in a square antiprismatic coordination environment are connecting to a central octahedral trivalent Fe or Co ion forming a propeller-type complex. The dynamics of the magnetization in the two isostructural compounds, modulated by the nature of the central M-III metal ion, are remarkably different despite their analogous direct current (dc) magnetic properties. The slow relaxation of the magnetization observed for 2 mainly originates from isolated Dy ions, since a diamagnetic Co-III metal ion links the magnetic Dy-III ions. In the case of 1, the magnetic interaction between S = 1/2 Fe-III ion and the three Dy-III magnetic centers, although weak, generates a complex energy spectrum of magnetic states with low-lying excited states that induce a smaller energy gap than for 2 and thus a faster relaxation of the magnetization.