A series of [2 × 2] square grid Ln III4 clusters: a large magnetocaloric effect and single-molecule-magnet behavior

Abstract

A series of [2 × 2] square grid Ln^III₄ clusters based on a multidentate chelating ligand N′-(2-hydroxybenzylidene)-6-(hydroxymethyl)picolinohydrazide (H₃L), namely [Ln₄(HL)₄(μ₂-CH₃O)₄]·4CH₃OH (Ln^III = Gd (1), Tb (2), Dy (3), and Ho (4)), were successfully synthesized and characterized by single-crystal X-ray diffraction and SQUID magnetometry. The X-ray structural analysis reveals that clusters 1–4 are mainly composed of four Ln^III ions which are interconnected through four μ-O bridging picolinoyl hydrazine ligands and four μ-O bridging methoxide ions. Magnetic studies show that cluster 1 exhibits antiferromagnetic exchange interactions between nearby Gd^III centers and displays a large magnetic entropy (−ΔS_m = 34.46 J kg⁻¹ K⁻¹ for ΔH = 7.0 T at 2.0 K). Alternating current (ac) magnetic susceptibility measurements demonstrate that slow relaxation behavior occurs in 3 under a zero direct-current (dc) field, with an effective energy barrier (ΔE/k_B) of 42.76 K and a pre-exponential factor of τ₀ = 1.02 × 10⁻⁶ s, while in clusters 2 and 4, no out-of-phase (χ′′) signals are observed.