Cobalt-Containing Silicotungstate Sandwich Dimer [{Co₃(<i>B-</i>β-SiW₉O₃₃(OH))(<i>B-</i>β-SiW₈O₂₉(OH)₂)}₂]^22-

The 6-cobalt-substituted [{Co3(B-β-SiW9O33(OH))(B-β-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-β-SiW9O33(OH))(B-β-SiW8O29(OH)2)}2]·49H2O (KNa-1), which crystallizes in the monoclinic system, space group P21/n, with a = 19.9466(8) Å, b = 24.6607(10) Å, c = 34.0978(13) Å, β = 102.175(1)°, and Z = 2. Polyanion 1 represents a novel class of asymmetric sandwich-type polyanions. It contains three cobalt ions, which are encapsulated between an unprecedented (B-β-SiW9O34) fragment and a (B-β-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 WVI 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.