Linear-shaped Ln III4 and Ln III6 clusters constructed by a polydentate Schiff base ligand and a β-diketone co-ligand: structures, fluorescence properties, magnetic refrigeration and single-molecule magnet behavior

Abstract

Herein, ten new linear-shaped Ln^III₄ and Ln^III₆ clusters, with the formula [Ln₄(acac)₆L₂(CH₃O)₂(CH₃OH)₄]·xCH₃OH (Ln = Nd (1), Sm (2), Eu (3), Gd (4), Tb (5), Dy (6), and Tm (8), Hacac = acetylacetone), [Ln₆(acac)₄L₄(CH₃O)₆]·xCH₃OH (Er (7) and Yb (9)), and [Lu₄(acac)₆L₂(OH)₂]·2CH₂Cl₂ (10), based on a polydentate Schiff base ligand, H₂L, and a β-diketone co-ligand were successfully synthesized and structurally characterized. Single crystal X-ray diffraction measurements reveal that the structures of the clusters 1–6, 8 and 10 are very similar and their central Ln(III) ions are linearly arranged Ln₄; however, the clusters 7 and 9 possess a rare linearly arranged Ln₆. The investigations on the solid-state fluorescence properties show that the clusters 2, 3, 5 and 6 display the characteristic lanthanum luminescence at room temperature. Magnetic studies reveal that weak antiferromagnetic interactions exist between adjacent Gd(III) ions in cluster 4. More importantly, the cluster 4 exhibits significant MCE with the maximum −ΔS_m value of 27.96 J kg⁻¹ K⁻¹ at 2.0 K and 7.0 T, whereas the cluster 6 displays a slow magnetic relaxation behavior under a zero dc field with the effective energy barrier ΔE/k_B = 8.64 K and τ₀ = 6.98 × 10⁻⁶ s.