Tetrairon and Hexairon Hydroxo/Acetato Clusters Stabilized by Multiple Polyoxometalate Scaffolds. Structures, Magnetic Properties, and Chemistry of a Dimer and a Trimer

Investigation of the catalytically relevant γ-diiron(III) Keggin complexes in aqueous acetate buffer leads to a dimeric
C2v-symmetric polyanion, [{Fe(OH)(OAc)}4(γ-SiW10O36)2]12- (3) and a trimeric C2-symmetric polyanion, [{Fe6(OH)9(H2O)2(OAc)2}(γ-SiW10O36)3]17- (4). Polyanion 3 incorporates a hydroxo/acetato-bridged tetrairon(III) core, while 4
incorporates a trigonal prismatic hydroxo/acetato-bridged hexairon(III) core. The monomeric building unit of 3 and
4, {γ-SiW10Fe2}, adopts the “out-of-pocket” structural motif (with two corner-sharing FeO6 coordination polyhedra
no longer connected to the internal SiO4 tetrahedron of the Keggin unit) also observed in the {γ-SiW10Fe2}-type
structures isolated from nonbuffered aqueous solutions. Following hydrolysis, 3 is converted to 4 as confirmed by
29Si NMR. Magnetic measurements establish that in both 3 and 4 all exchange interactions are antiferromagnetic.