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.