Synthesis and Magnetic Characterization of Fe(III)-Based 9-Metallacrown-3 Complexes Which Exhibit Magnetorefrigerant Properties.

The structural characterization and magnetic properties of three related 9-metallacrown-3 (9-MC-3) structures are reported. Each of these iron complexes is shown to exhibit significant magnetic refrigerant properties. FeIII(acetate)3[9-MCFeIIIN(shi)-3](MeOH)3·MeOH·7H2O (1-OAc) and FeIII(benzoate)3[9-MCFeIIIN(shi)-3](MeOH)3·MeOH·4H2O (1-OBz) are structurally analogous tetranuclear iron(III) clusters which exhibit drastically different magnetic properties, due to differences in intermolecular and intramolecular π interactions which affect superexchange. 1-OAc displays a magnetocaloric effect with a maximum entropy change of -ΔSm = 15.4 J kg-1 K-1 at T = 3 K and an applied field change of μoΔH = 7 T, whereas 1-OBz exhibits a maximum -ΔSm = 7.4 J kg-1 K-1 at T = 7 K and μoΔH = 7 T and displays an inverse magnetocaloric effect at lower temperatures and field changes. 1-OAc has -ΔSm values comparable to those of other Fe-based MCE materials and displays a significant MCE at lower applied fields, with -ΔSm = 11.2 J kg-1 K-1 at 3 K and μoΔH = 3 T. The tetranuclear core of 1 may be linked with isophthalate to form an octanuclear FeIII2(isophthalate)3[9-MCFeIIIN(shi)-3]2 dimer (2) that crystallizes in a honeycomb packing arrangement and exhibits solvation-dependent magnetic properties. The MCE for this molecule ranges from -ΔSm = 9.9 J kg-1 K-1 at T = 5 K and μoΔH = 7 T, when the pores of the material are highly occupied with solvent, to -ΔSm = 5.4 J kg-1 K-1, when the system is fully desolvated.