Square versus tetrahedral iron clusters with polyoxometalate ligands.

Two new insoluble transition metal substituted phosphotungstates, (C2N2H10)11[{(B-alpha-PW9O34)Fe3(OH)3}4(PO4)4Fe].38H2O(1) and K4(C2N2H10)12[(alpha-PW10Fe2O39)4].30H2O(2), have been isolated by the hydrothermal reaction of [A-alpha-PW9O34]9-, Fe(III) ions and ethylenediamine. Compound 1 has a tetrahedral symmetry and contains a Fe13 core built from the assembly of four Fe(III) trisubstituted [B-alpha-PW9O34]9- anions around a central disordered iron ion via four phosphato ligands. The anion in 2 can be described as a square of disubstituted [PW10O37]9- anions linked by Fe(III)-O-Fe(III) bridges. Magnetic measurements performed on 1 and 2 have shown the occurrence of antiferromagnetic interactions between the iron ions and have allowed the coupling constants between the magnetic centers to be determined.