Single-Molecule Magnetism in Three Related {Co^III₂Dy^III₂}‑Acetylacetonate Complexes with Multiple Relaxation Mechanisms

Three new heterometallic complexes with formulas ­of ­[Dy^III₂­Co^III₂­(OMe)₂­(teaH)₂­(acac)₄­(NO₃)₂]­ (<b>1</b>),­ [Dy^III₂Co^III₂­(OH)₂­(teaH)₂­(acac)₄­(NO₃)₂]­·4H₂O (<b>2</b>), and [Dy^III₂­Co^III₂­(OMe)₂­(mdea)₂­(acac)₄­(NO₃)₂] (<b>3</b>) were characterized by single-crystal X-ray diffraction and by dc and ac magnetic susceptibility measurements. All three complexes have an identical “butterfly”-type metallic core that consists of two Dy^III ions occupying the “body” position and two diamagnetic low-spin Co^III ions occupying the outer “wing-tips”. Each complex displays single-molecule magnet (SMM) behavior in zero applied magnetic field, with thermally activated anisotropy barriers of 27, 28, and 38 K above 7.5 K for <b>1</b>–<b>3</b>, respectively, as well as observing a temperature-independent mechanism of relaxation below 5 K for <b>1</b> and <b>2</b> and at 3 K for <b>3</b>, indicating fast quantum tunneling of magnetization (QTM). A second, faster thermally activated relaxation mechanism may also be active under a zero applied dc field as derived from the Cole–Cole data. Interestingly, these complexes demonstrate further relaxation modes that are strongly dependent upon the application of a static dc magnetic field. Dilution experiments that were performed on <b>1</b>, in the {Y^III₂Co^III₂} diamagnetic analog, show that the slow magnetic relaxation is of a single-ion origin, but it was found that the neighboring ion also plays an important role in the overall relaxation dynamics