Synthesis, crystal structure, magnetism, and magnetic anisotropy of cyclic clusters comprising six iron(III) ions and entrapping alkaline ions

The reaction of ferric chloride and beta-diketones (HL) in alkaline methanol solution represents a good synthetic route to hexairon(III) clusters [MFe(6)(OCH3)(12)(L)(6)](+) (M = Na, Li), which exhibit an unusual sixfold molecular symmetry Single-crystal X-ray diffraction showed that the six octahedrally coordinated iron(III) ions define a ring and are linked by twelve bridging methoxide ligands. The resulting [Fe-6(OCH3)(12)] skeleton has the remarkable property of acting as a host for an alkali-metal ion both in the solid state and in organic solution, as demonstrated by Na-23 and Li-7 NMR experiments. The magnetic behavior of these systems is consistent with the presence of a nonmagnetic S = 0 ground state and of antiferromagnetic exchange interactions between the high-spin ferric ions. The energy of the excited states was studied in detail by high-field DC and pulsed-field differential magnetization experiments at 0.7 and 1.5K. Single-crystal susceptibility measurements at variable temperature revealed a sizeable magnetic anisotropy, which has been successfully analyzed in terms of single-ion and dipolar contributions. The results are relevant to research into the origin of superparamagnetic-type behavior in transition-metal clusters.