Heteropolymolybdates

Heteropolymolybdates are a subset of polyoxometalates consisting of molybdenum oxide units linked to form discrete, highly symmetric anionic clusters that incorporate a central heteroatom. In these compounds, the addenda atoms are molybdenum in octahedral MO6 units, and the heteroatom X (such as phosphorus, silicon, germanium, or boron) is integrated into the framework to give a closed, well-defined structure. The term typically refers to both the Keggin-type and Dawson-type architectures that contain Mo as the metal framework.

Two common structural motifs are observed. The Keggin-type [XM12O40]n− features a tetrahedral central heteroatom X surrounded by a shell of twelve MoO6 octahedra connected through shared oxygen atoms. The Dawson-type [X2Mo18O62]n− consists of two heteroatoms linked by MoO6 units arranged to form a larger, yet related, polyoxometalate framework. Lacunary variants arise by removing one or more MoO6 units, creating vacancies that can bind metal cations or form heterometal-substituted derivatives, expanding catalytic and redox chemistry opportunities.

Properties and reactivity are characterized by high negative charge, strong Brønsted acidity in acidic solutions, and redox-active Mo centers (MoVI/MoV couples). They are typically prepared and studied in aqueous media under acidic conditions, exhibiting good thermal and hydrolytic stability within their operating pH range.

Applications are broad and include homogeneous catalysis (oxidations and oxidative dehydrogenations, often with hydrogen peroxide), electrochemical and photoelectrochemical systems, materials science (catalytic supports and sensors), and the development of heterometal-substituted POMs with tailored activity. Their well-defined structures and tunable compositions make heteropolymolybdates a foundational class in polyoxometalate chemistry.