Synthesis, structure and magnetism of a new dimeric silicotungstate: K9Na2Cu0.5[γ-Cu2(H2O)SiW8O31]2·38H2O

Abstract

A new structurally distinct dimeric silicotungstate K₉Na₂Cu_0.5[γ-Cu₂(H₂O)SiW₈O₃₁]₂·38H₂O (1) has been synthesized and characterized by infrared spectroscopy, elemental analysis, and variable-temperature magnetic measurements. Blue needle-like crystals of 1 were obtained by reaction of K₈[γ-SiW₁₀O₃₆] with 2 equivalents of Cu(II) in a 0.5 M sodium acetate solution (pH 4.2) and subsequent addition of an equal volume of ethylene glycol. The structure of 1 was determined by single-crystal X-ray diffraction: final R₁ = 3.41% based on 9709 independent reflections. The structure consists of two [γ-Cu₂(H₂O)SiW₈O₃₁]⁶⁻ Keggin-like units with the [Cu₂O(H₂O)(μ₃-O)(μ₂-O)] moiety of one unit bonded to the top of a [W₂O₆] moiety of the other unit. Magnetic susceptibility measurements indicate competing ferro- and antiferromagnetic intramolecular coupling between the four S = 1/2 Cu(II) centers in the cluster anion.