Trimetallo derivatives of lacunary 9-tungstosilicate heteropolyanions. Part 1. Synthesis and charaterization

Abstract

Salts of trimetallo derivatives of 9-tungstosilicate heteropolyanions have been prepared from the lacunary tungstates α- and β-[SiW₉O₃₄]^10– and characterized by elemental analysis, spectroscopy and electrochemistry. Tungsten-183 NMR spectra of the (four) diamagnetic derivatives of Al and Ga consist of two lines (intensity ratio 1 : 2) as expected for the C_3v structures of trisubstituted α- and β-Keggin anions. Molecular weight determinations by negative-ion fast atom bombardment mass spectrometry indicate the presence of small quantities of dimeric species (Si₂W₁₈M₆) in some of the products. Electronic spectra of the derivatives of Co, Cr, Ni and Cu are consistent with octahedral co-ordination and therefore the probable attachment of a water molecule to each cation. Magnetic measurements at 5–292 K demonstrate moderate to strong antiferromagnetic interactions in the derivatives of Cr^III, Mn^II and Cu^II. The complexes are stable in aqueous solution between pH ca. 5 and 10. Below pH 5 they are slowly transformed into [SiW₁₁O₃₉M(H₂O)]^n–. In addition to the expected reducibility of all of the new complexes to heteropoly blue species at negative potentials, the complexes of Fe, Cu and Mn exhibit voltammetric redox features corresponding to reduction of Fe^III to Fe^II, Cu^II to Cu⁰, and oxidation of Mn^II to Mn^III and Mn^IV.