Isomerically Pure α1-Monosubstituted Tungstodiphosphates: Synthesis, Characterization and Stability in Aqueous Solutions

Abstract

Isomerically pure samples of α1-P2W17O61M were prepared, in which M represents CaII and mainly the first-row transition metal cations: MnII, FeIII, CoII, NiII, CuII, ZnII. The formula is α1-P2W17O62M for VV and MoVI. The compounds were characterised by elemental analysis, IR, UV/Vis and 31P NMR spectroscopy. The stability of these complexes was monitored by cyclic voltammetry and/or UV/Vis spectroscopy. All the compounds proved to be stable for at least 24 h in a pH = 3 medium, even in the presence of 0.2 M Na+, except for the precursor lacunary species itself and the CaII-substituted derivative. In this medium, the transformation of the lacunary heteropolyanion yielded a mixture of α-[P2W18O62]6− and [H2P2W12]12−. The same species were obtained, but at a much slower rate, through the decomposition of α1-P2W17O61Ca. This difference in stability between the lacunary and the CaII-substituted derivatives might be ascribed to size and charge effects, along with the rigidity of the vacant site. At pH = 5, the final product of the transformation of α1-[P2W17O61]10− is α2-[P2W17O61]10−. The presence of Li+ appears to be more favourable than Na+ for the stability of these complexes. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)