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 β‐diketones (HL) in alkaline methanol solution represents a good synthetic route to hexairon(III) clusters [MFe6‐(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 [Fe6(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 23Na and 7Li 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 1.5 K. 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. Highly symmetric iron(III) clusters [MFe6(OCH3)12(L)6]+ (MNa, Li; HLβ‐diketone), which encapsulate alkali metal ions in a crown‐ ether‐like fashion, can be synthesized by controlled methanolysis reactions, and their magnetism can be studied in considerable detail. A clear picture of the spin manifold and of the sources of magnetic an‐ isotropy can be obtained from high‐ field magnetization and singlecrystal susceptibility measurements; the figure shows a typical plot of differential magnetization against field strength.