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

A new crystalline compound, catena -poly[hexa-μ-acetato-(acetic acid)-μ 3 -oxido- triangulo -triiron(III)]-μ-acetato], [Fe 3 (C 2 H 3 O 2 ) 7 O(C 2 H 4 O 2 )] 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 Fe 3 O(OAc) 7 (HOAc) (HOAc is acetic acid) and include the well-known [Fe 3 O(OAc) 6 ] + unit, in which the Fe III centres are linked to a central coplanar μ 3 -oxido ligand. Acetate ions provide bridges between pairs of Fe III centres. These individual [Fe 3 O(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.