Hydrothermal routes to new sodium hydrogen polytungstates

Abstract

The new acidic sodium polytungstates, Na₅[H₇W₁₂O₄₂]·20H₂O (1) and Na₂[H₁₀W₁₂O₄₂]·20H₂O (2), have been synthesised under hydrothermal conditions where variations of pH and temperature allow control of the protonation of the [W₁₂O₄₂]¹²⁻cluster in the products. The polytungstate cluster, [H_nW₁₂O₄₂]^(12−n)−, in each compound consists of the well known polyhedral unit based on 12 edge- and corner-sharing WO₆ octahedra, though the decoration of this unit with varying levels of protons results in significant changes in the W–O distances. The polytungstate clusters exist in arrays separated by sodium ions and water molecules forming well separated, (polytungstate centre to centre distance of 10.87 Å), rhombohedrally stacked units in 1 and strongly hydrogen bonded and more tightly packed units, (centre to centre distance of 9.34 Å), due to the high H : Na ratio in 2.