Two isostructural densely packed squarato-bridged lanthanide based 3D metal-organic frameworks (MOFs) [Ln(5)(mu(3)-OH)(5)(mu(3)-O)(CO3)(2)(HCO2)(2)(C4O4)(H2O)(2)] [Ln = Gd (1) and Dy (2)] show giant cryogenic magnetic refrigeration (for 1) and slow magnetic relaxation (for 2). The structural analyses reveal the presence of a self-assembled crown-shaped building unit with a cubane-based rectangular moiety that leads to a special array of metal centers in 3D space in the complexes. Magnetic investigations confirm that complex 1 exhibits one of the largest cryogenic magnetocaloric effects among the molecular magnetic refrigerant materials reported so far (-Delta S-m = 64.0 J kg(-1) K-1 for Delta H = 9 T at 3 K). The cryogenic cooling effect (of 1) is also quite comparable with that of the commercially used magnetic refrigerant gadolinium-gallium garnet, whereas for complex 2, slow relaxation of magnetization was observed below 10 K.