A polymeric form of basic iron(III) acetate with an acetic acid ligand

A new crystalline compound, catena-poly[hexa-μ-acetato-(acetic acid)-μ₃-oxido-triangulo-triiron(III)]-μ-acetato], [Fe₃(C₂H₃O₂)₇O(C₂H₄O₂)]_n, incorporating the basic ferric acetate unit, has been obtained from an acetic anhydride solution of hydrated iron(III) nitrate. The crystals have the composition Fe₃O(OAc)₇(HOAc) (HOAc is acetic acid) and include the well-known [Fe₃O(OAc)₆]⁺ unit, in which the Fe^III centres are linked to a central coplanar μ₃-oxido ligand. Acetate ions provide bridges between pairs of Fe^III centres. These individual [Fe₃O(OAc)₆]⁺ units are linked by additional bridging acetate anions to form zigzag chains. The bridging acetate ions coordinate to a position trans to the oxido group on two of the Fe^III centres. Remarkably, the trans site on the third Fe^III centre is occupied by the carbonyl group of an acetic acid molecule. This is the first reported case of an acetic acid molecule coordinating to an Fe^III centre. Not surprisingly, the acetic acid molecule is only weakly coordinating, resulting in a short Fe-O(oxido) bond trans to the carbonyl group. The trans influence apparent in this structure provides an interesting contrast with the structurally similar Mn^III analogue, in which the corresponding pair of trans bonds are both elongated because of the Jahn-Teller effect.