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 alk. MeOH soln. represents a good synthetic route to hexairon(III) clusters [MFe6(OCH3)12(L)6]+ (M = Na, Li), which exhibit an unusual 6-fold mol. symmetry. Single-crystal x-ray diffraction of [NaFe6(OMe)12(pmdbm)6]ClO4·xCHCl3·yMeOH (pmdbmH = 1,3-bis(p-methoxyphenyl)-1,3-propanedione) showed that the six octahedrally coordinated Fe(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 org. soln., as demonstrated by 23Na and 7Li NMR expts. 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 d.c. and pulsed-field differential magnetization expts. at 0.7 and 1.5 K. Single-crystal susceptibility measurements at variable temp. revealed a sizeable magnetic anisotropy, which was 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.