Crystal structure and magnetic properties of a pseudo-cubic close-packed array of oxalate linked {FeII6(μ3-OH)6}6+ clusters

Abstract

The ionic hexanuclear cluster aggregate [Fe^II₆(μ₃-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 compond Ba₄[Fe^II₆(μ₃-OH)₆(C₂O₄)₆]Br₂·6H₂O shows that the [Fe^II₆(μ₃-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 Fe^II₆ 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 Fe^II 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range antiferromagnetic ordering mediated by the oxalate bridges and a molecular-based description of the magnetism is no longer valid.