Syntheses, structures and magnetic properties of tetra- and heptanuclear iron(III) clusters incorporating phosphonate ligands

Two iron(III) complexes, [Fe4OCl(O2CMe)3(O3PC6H9)3(py)5] (1) and [Fe7O2(O2CPh)9(O3PC6H9)4(py)6] (2), have been prepared through solution reactions of [Fe3O(O2CR)6(H2O)3]Cl (R = Me, Ph) with cyclohexenephosphonic acid. Both compounds contain triangular oxo-centered [Fe3(μ3-O)]7+ units. In complex 1, the fourth iron atom is capped on this triangular unit through O–P–O bridges, forming a tetranuclear cluster with a tetrahedral arrangement of iron atoms. In complex 2, two equivalent [Fe3(μ3-O)]7+ units are connected by the fourth iron atom through four phosphonate ligands, forming a heptanuclear cluster. Variable temperature susceptibility measurements were performed for 1 and 2. Both exhibit dominant antiferromagnetic interactions between the Fe(III) centers.