Bis-pyrazolone-based dysprosium(iii) complexes: zero-field single-molecule magnet behavior in the [2 × 2] grid Dy III4 cluster

Abstract

We assembled two new Dy(III) compounds, [Dy₄(L1)₄(OH)₄]·2CH₃OH (1) and [Dy₂(L2)₂(HL2)₂(CH₃CN)₂]N(C₂H₅)₃ (2) (where H₂L1 and H₂L2 are methyl- and p-tolyl-substituted ligands, respectively) using bis-pyrazolone carbohydrazide-based multi-dentate hydrazone ligands. Structurally, the ligands coordinated with the Dy(III) spin centers in eight coordination environments in two ways. While four deprotonated L1 ligands with carbonyl oxygen (μ₂-O_carbonyl) and four hydroxyl (μ₂-OH⁻) as bridging motifs afforded [2 × 2] grid 1 with un-symmetric adjacent centers, the two partially deprotonated HL2 and two deprotonated L2 ligands, as well as two coordinated solvents, afforded defected helicate-like compound 2 with symmetric adjacent centers. Interestingly, the p-tolyl-substituted ligand generates di-nuclear compound 2 with C_2v symmetry, while the methyl-substituted ligand produces tetra-nuclear compound 1 with D_2d symmetry. Magnetic analyses revealed that compound 1 exhibited a zero-field single-molecule magnet (SMM) behavior with a temperature-dependent multi-step relaxation of the magnetization consistent with the presence of independent centers with slightly different coordination geometries owing to the asymmetric μ₂-O_carbonyl and μ₂-OH⁻ bridging motifs. However, the absence of the above bridging motifs, as well as the involvement of an extra third anionic pyrazolone oxygen (O_pyrazolone) and solvents in the coordination environment creating centrosymmetric centers, led to magnetically unfavorable conditions that exhibit slow magnetic relaxation under zero applied field in compound 2.