title: Cobalt-containing silicotungstate sandwich dimer [{Co3(B-beta-SiW9O33(OH))(B-beta-SiW8O29(OH)2)}2]22-.
abstract: The 6-cobalt-substituted [{Co3(B-beta-SiW9O33(OH))(B-beta-SiW8O29(OH)2)}2]22- 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-SiW8O29(OH)2)}2].49H2O (KNa-1), which crystallizes in the monoclinic system, space group P2(1)/n, with a=19.9466(8) A, b=24.6607(10) A, c=34.0978(13) A, beta=102.175(1) degrees, 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-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 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.
authors.affiliation: School of Engineering and Science, International University Bremen, P. O. Box 750 561, 28725 Bremen, Germany.
authors.forename: Bassem S
authors.initials: BS
authors.lastname: Bassil
authors.forename: Ulrich
authors.initials: U
authors.lastname: Kortz
authors.forename: Anca S
authors.initials: AS
authors.lastname: Tigan
authors.forename: Juan M
authors.initials: JM
authors.lastname: Clemente-Juan
authors.forename: Bineta
authors.initials: B
authors.lastname: Keita
authors.forename: Pedro
authors.initials: P
authors.lastname: de Oliveira
authors.forename: Louis
authors.initials: L
authors.lastname: Nadjo