Hexalacunary [α-H2P2W12O48]12- Wells-Dawson Anion: X-ray Crystal Structural Evidence and Oligomerization to WO(OH2)4+-Bridged Dimer and Trimers

We report the first single-crystal X-ray structural evidence of the potassium salt of the hexalacunary [& alpha;-H2P2W12O48](12-) (abbreviated as {P2W12}) anion after its discovery by Contant and Ciabrini in 1977. In addition, we observed oligomerization of {P2W12} into a {WO(OH2)}(4+)-bridged Pacman-shaped [{WO(OH2)}(& alpha;-HP2W12O48)(2)](22-) ({P4W25}) dimer and a cyclic [{WO(OH2)}(3)(P2W12O48)(3)](30-) ({P6W39}) trimer. The three phosphotungstate anions were synthesized through recrystallization of (NH4)(12)[& alpha;-H2P2W12O48] from slightly alkaline (HOCH2)(3)CNH2/KCl, CH3NH3Cl/KCl, and CH3NH3Cl/NH4Cl solutions. The structure of {P2W12} is derived from [& alpha;-P2W18O62](6-) that has six tungsten atoms one from each polar group and four from the belt-removed, and the center of the lacunary site is capped by a potassium cation. Structures of {P4W25} and {P6W39} are constructed by connecting two and three {P2W12} units with {WO(OH2)}(4+), respectively. The isolation of a pure {P6W39} phosphotungstate framework without coordination with transition metal cations is unprecedented. Powder X-ray diffraction confirmed the bulk purity of these compounds, indicating that selective crystallization was achieved through the selection of countercations and pH.

Automated summary

This study provides the first single-crystal X-ray structural evidence of the existence of α-H2P2W12O48 potassium salt and discovers two dimeric and one trimeric structures that it forms in solution. These phosphotungstate anions were synthesized through recrystallization of (NH4)12[α-H2P2W12O48] in alkaline solutions. The study also confirms that selective crystallization of these compounds can be achieved through the selection of appropriate counterions and pH values.