New hexanuclear and octanuclear iron(III) oxide clusters: octahedral [Fe6O2]14+ species and core isomerism in [Fe8O4]16+ complexes

The synthesis, X-ray crystal structures and magnetic properties of two new classes of hexanuclear and octanuclear iron(III) clusters are reported. The reaction of hmpH (2-(hydroxymethyl)pyridine) with [Fe3O(O2CR)6(H2O)3](NO3) (R=But, Ph) gives [Fe6O2(O2CR)6(hmp)6](NO3)2 containing a FeIII6 octahedron with two opposite faces bridged by μ3-O2− ions. Similar reactions employing FeCl3/NaO2CR (R=Me, Ph) give [Fe8O4(O2CPh)11(hmp)5] and [Fe8O4(O2CMe)12(hmp)4]. Both contain a similar [Fe8(μ3-O)4] core that can be described as four triangular [Fe3O] units joined together by edge or vertex sharing or, alternatively, as two [Fe3O] units bridged by a [Fe2O2] rhombus; however, the dispositions of the [Fe3O] units about the [Fe2O2] central unit differ in the two complexes and can be described as syn and anti core isomers. The Fe6 and Fe8 complexes have been found to have S=0 ground states when studied by variable-temperature, solid-state magnetic susceptibility measurements, consistent with the expected antiferromagnetic exchange interactions between the Fe(III) ions.