Slow Relaxation of the Magnetization in Dysprosium-Aluminum Metallacrowns

A series of Dy^III-Al^III metallacrowns (MC) with the ligand salicylhydroxamic acid (H₃shi) were investigated for magneto-structural interactions. The four MCs demonstrate that slight variations in reaction conditions and molecular components can lead to different classes of MCs. The Dy^III-Al^III MCs include an archetype Dy ^III Al ^III ₄ [12-MC-4], the dimer Dy ^III ₂ Al ^III ₈ [12-MC-4] _2, a [3.3.1] metallacryptand (MCr) Dy ^III Al ^III ₆ [3.3.1], and Dy ^III ₂ Al ^III ₆ [18-MC-6], representing a new type of MC. Dy ^III ₂ Al ^III ₆ [18-MC-6] consists of a MC ring with six ring Al^III ions that have an octahedral propeller configuration with a stereoisomer pattern ΔΛΛΛΔΔ about the MC ring. The 18-MC-6 captures a [Dy^III ₂(μ₃-OH)₂]⁴⁺ core, where each Dy^III ion is nine-coordinate with muffin geometry (C _s). Both the static and dynamic magnetic properties of the complexes were investigated. Models of the static magnetic data reveal that Dy ^III Al ^III ₄ [12-MC-4] and Dy ^III ₂ Al ^III ₈ [12-MC-4] ₂ do not have a significant axial ligand field, as the Dy^III ions have a square antiprism geometry (D _4d). However, a similar analysis demonstrates that Dy ^III Al ^III ₆ [3.3.1] and Dy ^III ₂ Al ^III ₆ [18-MC-6] do have a significant axial component to the ligand field, as the Dy^III ions in these complexes have spherical capped square antiprism (C _4v) or muffin geometry, respectively. The Dy^III geometry differences lead to differing dynamic magnetic susceptibility behavior. Dy ^III Al ^III ₄ [12-MC-4] and Dy ^III ₂ Al ^III ₈ [12-MC-4] ₂ do not display a frequency-dependent out-of-phase magnetic susceptibility signal in the absence of a static magnetic field. A frequency-dependent signal is observed only with the application of an 800 Oe magnetic field. However, Dy ^III Al ^III ₆ [3.3.1] and Dy ^III ₂ Al ^III ₆ [18-MC-6] do exhibit SMM behavior in the absence and presence of a static magnetic field. For Dy ^III Al ^III ₆ [3.3.1] and Dy ^III ₂ Al ^III ₆ [18-MC-6], the effective energy barrier to magnetization relaxation (U _eff) with an 800 Oe magnetic field is 59 ± 2 and 35 ± 2 cm^-1 with τ₀ = 2.5 ± 0.7 × 10^-8 and 3.5 ± 1.1 × 10^-8 s, respectively.