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-SiW(10)O(36))(2)](12-) (3) and a trimeric C(2)-symmetric polyanion, [{Fe(6)(OH)(9)(H(2)O)(2)(OAc)(2)}(gamma-SiW(10)O(36))(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-SiW(10)Fe(2)}, adopts the "out-of-pocket" structural motif (with two corner-sharing FeO(6) coordination polyhedra no longer connected to the internal SiO(4) tetrahedron of the Keggin unit) also observed in the {gamma-SiW(10)Fe(2)}(-)type structures isolated from nonbuffered aqueous solutions. Following hydrolysis, 3 is converted to 4 as confirmed by (29)Si NMR. Magnetic measurements establish that in both 3 and 4 all exchange interactions are antiferromagnetic.