The use of methylene-bridged calixarenes in 3d/4f chemistry produces a family of clusters of general formula [MnIII4Ln III4(OH)4(C4)4(NO3) 2(DMF)6(H2O)6](OH)2 (where C4 = calixarene; Ln = Gd (1), Tb (2), Dy (3)). The molecular structure describes a square of LnIII ions housed within a square of Mn III 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. © 2010 American Chemical Society.