Crystal structure and magnetic properties of a pseudo-cubic close-packed array of oxalate linked {Fe-6(II)(mu(3)-OH)(6)}(6+) clusters

The ionic hexanuclear cluster aggregate [Fe-6(II)(mu(3)-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 Ba-4[Fe-6(II)(mu(3)-OH)(6)(C2O4)(6)]Br-2 center dot 6H(2)O shows that the [Fe-6(II)(mu(3)-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-6(II) 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range antiferromagnetic ordering mediated by the oxalate bridges and a molecular-based description of the magnetism is no longer valid.