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

Abstract

Interaction of the dilacunary polyanion precursor [gamma-GeW(10)O(36)](8-) with Fe(3+) 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 [gamma-GeW(10)O(36)](8-) ratio of 1:1, the asymmetric anion [K(H(2)O)(beta-Fe(2)GeW(10)O(37)(OH))(gamma-GeW(10)O(36))](12-) (1) is formed, whereas [{beta-Fe(2)GeW(10)O(37)(OH)2}2]12- (2) is formed when using a ratio of 2:1. Single-crystal X-ray analyses were carried out on Cs(3)K(9)[K(H(2)O)(beta-Fe(2)GeW(10)O(37)(OH))(gamma-GeW(10)O(36))].19H(2)O (CsK-1), which crystallizes in the triclinic system, space group P1, a = 11.4547(2), b = 19.9181(5), c = 21.0781(6) A, alpha = 66.7977(12), beta = 89.1061(12), gamma = 84.4457(11) degrees, and Z = 2 and on Cs(7)K(4)Na[{beta-Fe(2)GeW(10)O(37)(OH)(2)}(2)].39H(2)O (CsKNa-2), which crystallizes in the monoclinic system, space group C2/m, a = 32.7569(13), b = 12.2631(5), c = 14.2895(5) A, beta = 104.135(2) degrees , and Z = 2. Polyanion 1 consists of (beta-Fe(2)GeW(10)O(37)) and (gamma-GeW(10)O(36)) units linked via two Fe-O-W bridges and a central potassium ion. Two equivalent FeO(6) octahedra complete the belt of the beta-Keggin unit and link to the (gamma-GeW(10)O(36)) fragment. On the other hand, 2 consists of two {beta-Fe(2)GeW(10)O(37)(OH)(2)} units with four bridging hydroxo groups linking the four Fe(3+) 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 Fe(3+) 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 Fe(3+) 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.