SYNTHESIS, CRYSTAL-STRUCTURE, MAGNETISM, AND MAGNETIC-ANISOTROPY OF CYCLIC CLUSTERS COMPRISING 6 IRON(III) IONS AND ENTRAPPING ALKALINE IONS

The reaction of ferric chloride and beta-diketones (HL) in alkaline me thanol solution represents a good synthetic route to hexairon(III) clu sters [MFe(6)(OCH3)(12)(L)(6)](+) (M = Na, Li), which exhibit an unusu al 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 fo r 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 behav ior of these systems is consistent with the presence of a nonmagnetic S = 0 ground state and of antiferromagnetic exchange interactions betw een the high-spin ferric ions. The energy of the excited states was st udied in detail by high-field DC and pulsed-field differential magneti zation experiments at 0.7 and 1.5K. Single-crystal susceptibility meas urements at variable temperature revealed a sizeable magnetic anisotro py, which has been successfully analyzed in terms of single-ion and di polar contributions. The results are relevant to research into the ori gin of superparamagnetic-type behavior in transition-metal clusters.