Square versus tetrahedral iron clusters with polyoxometalate ligands

Two new insoluble transition metal substituted phosphotungstates, (C2N2H10)(11)[{(B-alpha-PW9O34)- Fe-3(OH)(3)}(4)(PO4) Fe-4]center dot 38H(2)O (1) and K-4(C2N2H10)(12)[(alpha-PW10Fe2O39)(4)]center dot 30H(2)O (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 Fe-13 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.