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 FeIII centres are linked to a central coplanar μ3-oxido ligand. Acetate ions provide bridges between pairs of FeIII 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 FeIII centres. Remarkably, the trans site on the third FeIII 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 FeIII 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 MnIII analogue, in which the corresponding pair of trans bonds are both elongated because of the Jahn-Teller effect.