[Mn^III₄Ln^III₄] Calix[4]arene Clusters as Enhanced Magnetic Coolers and Molecular Magnets

The use of methylene-bridged calix[4]arenes in 3d/4f chemistry produces a family of clusters of general formula [MnIII4LnIII4(OH)4(C4)4(NO3)2(DMF)6(H2O)6](OH)2 (where C4 = calix[4]arene; Ln = Gd (1), Tb (2), Dy (3)). The molecular structure describes a square of LnIII ions housed within a square of MnIII ions. Magnetic studies reveal that 1 has a large number of molecular spin states that are populated even at the lowest investigated temperatures, while the ferromagnetic limit S = 22 is being approached only at the highest applied fields. This, combined with the high magnetic isotropy, makes the complex an excellent magnetic refrigerant for low-temperature applications. Replacement of the isotropic GdIII ions with the anisotropic TbIII and DyIII ions “switches” the magnetic properties of the cluster so that 2 and 3 behave as low-temperature molecular magnets, displaying slow relaxation of the magnetization.