Crystal structure and magnetic properties of a pseudo-cubic close-packed array of oxalate linked {FeII6(mu3-OH)6}6+ clusters.

The ionic hexanuclear cluster aggregate [FeII6(mu3-OH)6]6+ has been synthesised using hydrothermal conditions starting from ferrous oxalate in the presence of barium and bromide or iodide ions. A single crystal X-ray structure on the compound Ba4[FeII6(mu3-OH)6(C2O4)6]Br2.6H2O shows that the [FeII6(mu3-OH)6]6+ units are held together by bridging oxalates in a pseudo-cubic close-packed array. The barium ions in conjunction with oxalate groups provide a barrel-shaped cage between the FeII6 aggregates containing the bromide counterions and lattice waters and corresponding to an 'octahedral hole' in the pseudo-cubic close-packed structure. A magnetic susceptibility study shows that the FeII centres are antiferromagnetically coupled. Below 10 K the system displays a long range antiferromagnetic ordering mediated by the oxalate bridges and a molecular-based description of the magnetism is no longer valid.