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Two Iron-Containing Tungstogermanates:  [K(H2O)(β-Fe2GeW10O37(OH))(γ-GeW10O36)]12- and [{β-Fe2GeW10O37(OH)2}2]12-

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journal contribution
posted on 15.10.2007, 00:00 by Nadeen H. Nsouli, Sib Sankar Mal, Michael H. Dickman, Ulrich Kortz, Bineta Keita, Louis Nadjo, Juan M. Clemente-Juan
Interaction of the dilacunary polyanion precursor [γ-GeW10O36]8- with Fe3+ ions in aqueous buffer medium (pH 4.8) results in the formation of two dimeric tungstogermanates depending on the reactant ratios. When using an Fe3+ to [γ-GeW10O36]8- ratio of 1:1, the asymmetric anion [K(H2O)(β-Fe2GeW10O37(OH))(γ-GeW10O36)]12- (1) is formed, whereas [{β-Fe2GeW10O37(OH)2}2]12- (2) is formed when using a ratio of 2:1. Single-crystal X-ray analyses were carried out on Cs3K9[K(H2O)(β-Fe2GeW10O37(OH))(γ-GeW10O36)]·19H2O (CsK-1), which crystallizes in the triclinic system, space group P1̄, a = 11.4547(2), b = 19.9181(5), c = 21.0781(6) Å, α = 66.7977(12), β = 89.1061(12), γ = 84.4457(11)°, and Z = 2 and on Cs7K4Na[{β-Fe2GeW10O37(OH)2}2]·39H2O (CsKNa-2), which crystallizes in the monoclinic system, space group C2/m, a = 32.7569(13), b = 12.2631(5), c = 14.2895(5) Å, β = 104.135(2)°, and Z = 2. Polyanion 1 consists of (β-Fe2GeW10O37) and (γ-GeW10O36) units linked via two Fe−O−W bridges and a central potassium ion. Two equivalent FeO6 octahedra complete the belt of the β-Keggin unit and link to the (γ-GeW10O36) fragment. On the other hand, 2 consists of two {β-Fe2GeW10O37(OH)2} units with four bridging hydroxo groups linking the four Fe3+ ions, forming an eight-membered ring. The magnetic properties of CsK-1 and CsKNa-2 have been studied by magnetic susceptibility and magnetization measurements and fitted according to an isotropic exchange model. Both polyanions 1 and 2 exhibit diamagnetic ground states with a small amount of paramagnetic impurity. Electrochemistry studies on 1 and 2 were carried out in a pH 5 acetate medium. The two polyanions have in common the simultaneous reduction of all of their Fe3+ centers. This observation suggests the existence of identical or almost-identical influences on these centers and their equivalence, especially in the reduced state. Controlled potential coulometry results indicate the presence of two Fe3+ centers in 1 and four in 2. The splitting of the tungsten wave of 1 compared to the single tungsten wave of 2 is attributed to a difference in acid−base properties of the polyanions. Voltammetric peak-potential shifts as a function of pH were studied in the case of 2.