Co-Ln mixed-metal phosphonate grids and cages as molecular magnetic refrigerants

The synthesis, structures, and magnetic properties of six families of cobalt-lanthanide mixed-metal phosphonate complexes are reported in this Article. These six families can be divided into two structural types: grids, where the metal centers lie in a single plane, and cages. The grids include [4 × 3] {Co 8Ln 4}, [3 × 3] {Co 4Ln 6}, and [2 × 2] {Co 4Ln 2} families and a [4 × 4] {Co 8Ln 8} family where the central 2 × 2 square is rotated with respect to the external square. The cages include {Co 6Ln 8} and {Co 8Ln 2} families. Magnetic studies have been performed for these compounds, and for each family, the maximum magnetocaloric effect (MCE) has been observed for the Ln = Gd derivative, with a smaller MCE for the compounds containing magnetically anisotropic 4f-ions. The resulting entropy changes of the gadolinium derivatives are (for 3 K and 7 T) 11.8 J kg -1 K -1 for {Co 8Gd 2}; 20.0 J kg -1 K -1 for {Co 4Gd 2}; 21.1 J kg -1 K -1 for {Co 8Gd 4}; 21.4 J kg -1 K -1 for {Co 8Gd 8}; 23.6 J kg -1 K -1 for {Co 4Gd 6}; and 28.6 J kg -1 K -1 for {Co 6Gd 8}, from which we can see these values are proportional to the percentage of the gadolinium in the core. © 2011 American Chemical Society.