Cobalt-containing silicotungstate sandwich dimer [{Co3(B-β-SiW9O33(OH))(B-β-SiW8O29(OH)2)}2]22-

The 6-cobalt-substituted [{CO3(B-beta-SiW9O33(OH))(B-beta-SiW8O29(OH)(2))}(2)](22-) (1) has been characterized by IR and UV-vis spectroscopy, elemental analysis, magnetic studies, electrochemistry, and gel filtration chromatography. A single-crystal X-ray analysis was carried out on K10Na12[{CO3(B-beta-SiW9O33(OH))(B-beta-Siw(8)O(29)(OH)(2))}2](.)49H(2)O (KNa-1), which crystallizes in the monoclinic system, space group P2(1)/n, with a = 19.9466(8) angstrom, b = 24.6607(10) angstrom, c = 34.0978(13) angstrom, = 102.175(1)degrees, and Z = 2. Polyanion 1 represents a novel class of asymmetric sandwichtype polyanions. It contains three cobalt ions, which are encapsulated between an unprecedented (B-beta-SiW9O34) fragment and a (B-beta-SiW8O31) unit. Polyanion 1 is composed of two sandwich species via two Co-O-W bridges in the solid state and almost certainly in solution as well based on gel filtration chromatography. UV-visible spectroscopy and cyclic voltammetry also confirmed its stability. Two well-separated groups of waves appeared in the voltammetric pattern: the wave observed in the negative potential range versus a saturated calomel electrode features the redox processes of W-VI centers; the two reversible redox couples observed in the positive potential domain are attributed to the redox processes of Co2+ centers and indicated that the two types of Co2+ centers in the structure are oxidized in separate waves. Such reversibility of Co2+ centers within multi-Co-substituted polyoxometalates is uncommon. The magnetic properties of KNa-1 are also discussed. The ferromagnetic ground state has been studied by magnetic susceptibility and magnetization measurements and fitted according to an anisotropic exchange model.