Polyoxometalate

In chemistry, a polyoxometalate (abbreviated POM) is a polyatomic ion, usually an anion, that consists of three or more transition metal oxyanions linked together by shared oxygen atoms to form closed 3-dimensional frameworks. The metal atoms are usually group 6 (Mo, W) or less commonly group 5 (V, Nb, Ta) transition metals in their high oxidation states. They are usually colorless or orange, diamagnetic anions. Two broad families are recognized, isopolymetalates, composed of only one kind of metal and oxide, and heteropolymetalates, composed of one metal, oxide, and a main group oxyanion (phosphate, silicate, etc.). Many exceptions to these general statements exist. In chemistry, a polyoxometalate (abbreviated POM) is a polyatomic ion, usually an anion, that consists of three or more transition metal oxyanions linked together by shared oxygen atoms to form closed 3-dimensional frameworks. The metal atoms are usually group 6 (Mo, W) or less commonly group 5 (V, Nb, Ta) transition metals in their high oxidation states. They are usually colorless or orange, diamagnetic anions. Two broad families are recognized, isopolymetalates, composed of only one kind of metal and oxide, and heteropolymetalates, composed of one metal, oxide, and a main group oxyanion (phosphate, silicate, etc.). Many exceptions to these general statements exist. The oxides of d0 metals such as V2O5, MoO3, WO3 dissolve at high pH to give orthometalates, VO3−4, MoO2−4, WO2−4. For Nb2O5 and Ta2O5, the nature of the dissolved species is less clear. As the pH is lowered, these orthometalates protonate to give oxide–hydroxide compounds such as W(OH)O−3 and V(OH)O2−3. These species condense via the process called olation. Condensation proceeds via loss of water and the formation of M–O–M linkages. An abbreviated condensation sequence illustrated with vanadates is: When such acidifications are conducted in the presence of phosphate or silicate, then one obtains a heteropolymetalate. For example, the phosphotungstate anion PW12O3−40 consists of a framework of twelve octahedral tungsten oxyanions surrounding a central phosphate group. The assembly of polyoxometalates upon acidification of solutions is an example for covalent self-assembly. This process produces homogeneous solutions of highly organized structures. Under a specific set of conditions only one polyoxometalate structure, or a small subset thereof will form. Evidence shows that this occurs via a dense-phase type mechanism whereupon small oxometalate ions first assemble non-covalently to form supramolecular structures that may then condense to form a covalently bound polyoxometalate. The first example of a polyoxometalate compound was ammonium phosphomolybdate, containing the PMo12O3−40 anion, discovered in 1826. This anion has the same structure as the phosphotungstate anion, whose structure was reported in 1934. This structure is called the Keggin structure after its discoverer. Following this discovery, other fundamental structures such as the Wells–Dawson ion were found, and their chemistry and applications as catalysts were determined. Ramazzoite, the first example of a mineral with a polyoxometalate cation, was discovered in 2016 in Mt. Ramazzo Mine, Liguria, Italy. This polyoxometalate has not been reported in a synthetic compound. Certain structural motifs recur. The Keggin ion for example is common to both molybdates and tungstates with different central heteroatoms. Examples of some fundamental polyoxometalate structures are shown below. The Lindqvist ion and the rest of the first raw structures in the following figure are iso-polyoxometalates (isopolyanions in this case), since only one type of transition metal atoms is involved in their composition. The other structures are of the hetero-polyoxometalate type (heteropolyanions) since they involve more than one type of metal atom. The Keggin and Dawson structures have tetrahedrally-coordinated heteroatoms, such as P or Si, and the Anderson structure has an octahedral central atom, such as aluminium. Polyoxomolybdates include the wheel-shaped molybdenum blue anions and spherical keplerates. Numerous hybrid organic–inorganic materials that contain POM cores, new potential applications based on unusual magnetic and optical properties of some POMs, and potential medical applications such as antitumor and antiviral uses. The typical framework building blocks are polyhedral units, with 4-, 5-, 6- or 7-coordinate metal centres. These units share edges and/or vertices, or, less commonly, faces (such as in the ion CeMo12O8−42, which has face-shared octahedra with Mo atoms at the vertices of an icosahedron).