Dinuclear Tb and Dy complexes supported by hybrid Schiff-base/calixarene ligands: synthesis, structures and magnetic properties

Abstract

The synthesis of the new lanthanide complexes [HNEt₃][Dy₂(HL¹)(L¹)] (5), and [Ln₂(L²)₂] (Ln = Tb^III (7), Dy^III (8)) supported by the hybrid Schiff-base/calix[4]arene ligands H₄L¹ (25-[2-((2-methylphenol)imino)ethoxy]-26,27,28-trihydroxy-calix[4]arene) and H₃L² (25-[2-((2-methylpyridine)imino)ethoxy]-26,27,28-trihydroxy-calix[4]arene) are reported. Spectroscopic data (for 5) and X-ray crystallographic analysis (for 7·4MeCN, 8·4MeCN) reveal the presence of dimeric structures, featuring doubly-bridged NO₄Ln(μ-O)₂LnO₄N (5) or N₂O₃Ln(μ-O)₂LnO₃N₂ cores (7, 8) with seven-coordinated Ln³⁺ ions. The magnetic properties of polycrystalline samples of 5, 7 and 8 were studied by variable temperature dc and ac magnetic susceptibility measurements. The χ′′(T) vs. T plots show no maxima in zero field, but the maxima can be detected under a 3 kOe dc field. The relaxation times τ obey the Arrhenius law above 5 K. Anisotropy barriers of ∼18 cm⁻¹ (26 K) for 5 and ∼23 cm⁻¹ (33 K) for 8 were determined.