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] {Co₈Ln₄}, [3 × 3] {Co₄Ln₆}, and [2 × 2] {Co₄Ln₂} families and a [4 × 4] {Co₈Ln₈} family where the central 2 × 2 square is rotated with respect to the external square. The cages include {Co₆Ln₈} and {Co₈Ln₂} 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 {Co₈Gd₂}; 20.0 J kg^–1 K^–1 for {Co₄Gd₂}; 21.1 J kg^–1 K^–1 for {Co₈Gd₄}; 21.4 J kg^–1 K^–1 for {Co₈Gd₈}; 23.6 J kg^–1 K^–1 for {Co₄Gd₆}; and 28.6 J kg^–1 K^–1 for {Co₆Gd₈}, from which we can see these values are proportional to the percentage of the gadolinium in the core.