Structures, magnetic refrigeration and single molecule-magnet behavior of five rhombus-shaped tetranuclear Ln(III)-based clusters

A serie of coplanar tetranuclear Ln(III)-based clusters, namely [Ln4(acac)4(L)6(μ3-OH)2]·[solvent] (Ln = Eu (1), Gd (2), Tb (3), Dy (4) and Er (5)), have been synthesized by using an 8-hydroxyquinoline Schiff base ligand (HL = 5-(2-thenylidene)-8-hydroxylquinoline) and acetylacetone. The structures and magnetic properties of them have been completely characterized. The structures determinations show that clusters 1-5 are isostructural with a rhombus-shaped Ln4 core. The magnetic studies reveal that Gd4 cluster (2) displays cryogenic magnetocaloric effects with maximum −ΔSm value of 22.09 J kg−1 K−1 at 2.5 K and ΔH = 7.0 T; while ac susceptibility measurements show that single molecule-magnet behavior occurs for Dy4 cluster (4) with the anisotropic barrier (ΔE/kB) of 75.01 K. Interestingly, more than one maxima out-of-phase ac signals (χ″) were observed in 4, suggesting the operation of multiple magnetic relaxation processes in 4. In addition, the solid state luminescence properties reveal that clusters 1, 3, and 4 display the characteristic lanthanum luminescence at room temperature.