Structure and Magnetism of Ln^III₂ (Ln = Gd, Tb, Dy, and Ho) Assemblies Constructed from a Bis(Hydrazone) Compartmental Ligand: Slow Magnetic Relaxation in the Dy^III₂ Analogue

The aim of the paper was to synthesize a series of Ln^III₂ (Ln = Gd, Tb, Dy, and Ho) complexes that offer a great perspective to construct functional magnetic materials. Here, we have ventured a Schiff base ligand by combining 2,6-diformyl-4-methylphenol with 2-hydroxy-benzoic acid hydrazide, and the ligand is reacted with Ln(NO₃)₃·6H₂O (1:1), which affords [Ln₂(H₂L)₃](NO₃)₃ (Ln = Gd, Dy, and Ho) and [Ln₂(H₂L)₃](NO₃)₃(CH₃OH) (Ln = Tb) complexes. Structural analyses illustrate that the four complexes have a stoichiometry of 2:3 (M:L), wherein three phenolic oxygen atoms are simultaneously coordinated with two metal atoms to form Ln^III–O_phenoxo–Ln^III bridges. Variable-temperature magnetic susceptibility measurements have been performed in the 2–300 K region for all of the compounds. Magnetic studies indicate that the Dy^III₂ analogue exhibits the slow relaxation of magnetization behavior under 700 Oe external dc field with an energy barrier of 2.97–7.73 K. The alternating current (ac) magnetic susceptibility measurements were also performed on the 90:10 (Y:Dy) diluted sample analogue and they depicted that the slow magnetic relaxation properties originated mostly from the single ion but not the coupling between Dy^III ions. The best-fitting results provided U_eff values of 1.61–2.90 K with τ₀ values of 1.31 × 10^–5–6.97 × 10^–4 s for the diluted sample of Dy^III under 500 Oe dc field.