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

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. Fe^III(acetate)₃[9-MC_Fe-3](MeOH)₃·MeOH·7H₂O (1-OAc) and Fe^III(benzoate)₃[9-MC_Fe-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 -ΔS_m = 15.4 J kg^-1 K^-1 at T = 3 K and an applied field change of μ_oΔH = 7 T, whereas 1-OBz exhibits a maximum -ΔS_m = 7.4 J kg^-1 K^-1 at T = 7 K and μ_oΔH = 7 T and displays an inverse magnetocaloric effect at lower temperatures and field changes. 1-OAc has -ΔS_m values comparable to those of other Fe-based MCE materials and displays a significant MCE at lower applied fields, with -ΔS_m = 11.2 J kg^-1 K^-1 at 3 K and μ_oΔH = 3 T. The tetranuclear core of 1 may be linked with isophthalate to form an octanuclear Fe^III₂(isophthalate)₃[9-MC_Fe-3]₂ dimer (2) that crystallizes in a honeycomb packing arrangement and exhibits solvation-dependent magnetic properties. The MCE for this molecule ranges from -ΔS_m = 9.9 J kg^-1 K^-1 at T = 5 K and μ_oΔH = 7 T, when the pores of the material are highly occupied with solvent, to -ΔS_m = 5.4 J kg^-1 K^-1, when the system is fully desolvated.