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 = Bu-t, Ph) gives [Fe6O2(O2CR)(6)(hmp)(6)](NO3)(2) containing a Fe-6(III) octahedron with two opposite faces bridged by mu(3)-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 [Fe-8(mu(3)-O)(4)] core that can be described as four triangular [Fe2O2] 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 Fe-6 and Fe-8 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. (C) 2000 Elsevier Science S.A. All rights reserved.