Heteropolyanions

Heteropolyanions (HPAs) are a class of polyoxometalates that consist of metal-oxide frameworks in which a heteroatom or heteroatom-containing unit is incorporated into the lattice. They are discrete, highly negative inorganic anions built mainly from early transition metals in high oxidation states, such as tungsten, molybdenum and vanadium, connected by oxide bridges to a central heteroatom, typically phosphorus, silicon or germanium. The heteroatom sits at the core of the structure and influences geometry, acidity and redox properties. HPAs are typically obtained in aqueous solution under acidic conditions and can be isolated as salts with various countercations.

Two well-studied archetypes are the Keggin and Dawson families. The Keggin ion has the form [XM12O40]n−, featuring a central XO4 tetrahedron surrounded by twelve MO6 octahedra in near-spherical symmetry. The Dawson ion, [X2M18O62]n−, contains two heteroatoms and a larger framework. In addition, lacunary derivatives—frameworks with one or more metal-oxide units removed—can accommodate transition metals or organometallic fragments, expanding reactivity.

Properties and behavior: HPAs exhibit pronounced, reversible multi-electron redox chemistry and high chemical and thermal robustness. Their electronic structure gives rise to strong oxidizing powers in catalytic cycles and the ability to mediate electron transfer in electrochemical systems. They also display notable Brønsted and Lewis acidity and can form stable sols and crystalline salts.

Applications: HPAs are used as homogeneous and heterogeneous catalysts for oxidation, epoxidation and other multi-step oxidations; they serve in electrochemical and photoelectrochemical cells, including water-splitting catalysts; and are employed in sensors, ion-exchange materials and as building blocks for inorganic-organic hybrid materials through lacunary derivatives.