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 gamma-diiron(III) Keggin complexes in aqueous acetate buffer leads to a dimeric C-2v-symmetric polyanion, [{Fe(OH)(OAc)}(4)(gamma-SiW10O36)(2)](12-) (3) and a trimeric C-2-symmetric polyanion, [{Fe-6(OH)(9)(H2O)(2)(OAc)(2)}(gamma-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, {gamma-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 {gamma-SiW10Fe2}-type structures isolated from nonbuffered aqueous solutions. Following hydrolysis, 3 is converted to 4 as confirmed by Si-29 NMR. Magnetic measurements establish that in both 3 and 4 all exchange interactions are antiferromagnetic.