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.