The cyclic 48-tungsto-8-phosphate [H7P8W48O184]33− Contant–Tézé polyanion and its derivatives [H6P4W24O94]18− and [H2P2W12O48]12−: structural aspects and reactivity

Abstract

Polyoxometalates (POMs) are discrete, anionic metal-oxo clusters of early transition metals in high oxidation states (e.g., W^VI, Mo^VI, V^V) usually comprised of edge- and corner-shared MO₆ octahedra. Lacunary POMs are defect heteropolyanions mainly of the Keggin or Dawson type, and they can be formed by the loss of one or more MO₆ octahedra by controlled base hydrolysis. The largest subclass of POMs are tungstophosphates, and several lacunary derivatives are known, such as the Keggin-based [PW₁₁O₃₉]⁷⁻ and [PW₉O₃₄]⁹⁻, and the Dawson-based [P₂W₁₇O₆₁]¹⁰⁻ and [P₂W₁₅O₅₆]¹²⁻. This review is based on the cyclic 48-tungsto-8-phosphate [H₇P₈W₄₈O₁₈₄]³³⁻ (P₈W₄₈) as well as its smaller derivatives [H₆P₄W₂₄O₉₄]¹⁸⁻ (P₄W₂₄), and [H₂P₂W₁₂O₄₈]¹²⁻ (P₂W₁₂), with a focus on structural aspects, solution stability and reactivity. All three polyanions can be considered as inorganic multidentate O-donor ligands that coordinate with d, f or p-block metal ions. Here we provide a comprehensive overview of guest metal-containing derivatives of the P₈W₄₈ wheel, the P₄W₂₄ half-wheel and the P₂W₁₂ quarter wheel. The structures containing P₂W₁₂ as a building unit are presented in a sequence of increasing number of POM units in the resulting assembly. Transition metal-containing POMs have been of interest for decades due to their remarkable capability of forming novel and unexpected structures associated with interesting and relevant physicochemical properties (e.g., catalysis, magnetism, biomedicine, electrochemistry), and this also applies for derivatives containing P₈W₄₈, P₄W₂₄ and P₂W₁₂.