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.