Polyoxometalates, the other discovery by D’Elhuyar brothers

Not many people know that polyoxometalates (POMs) discovery dates back to 1783 and that they were the D´Elhuyar brothers (Juan José D´Elhuyar(Logroño, 1754) and Fausto D´Elhuyar (Logroño, 1755) who discovered a yellow spicy/bitter tasting salt from the reaction of ammonium molybdate with phosphoric acid which is now known as ammonium 12-phosphomolybdate, (NH₄)₃[PMo₁₂O₄₀] ¹ (Figure 1).

Although in 1826 it was Berzelius who first reported the system ². And In 1933, the structure H₃[PW₁₂O₄₀]was solved by Keggin by powder X-ray diffraction, reporting the first polyoxoanion which is now known as Keggin polyanion ³.

What are they?

They are discrete structurally and chemically diverse nanosized metal-oxygen molecular anions of early transition metals (groups V and VI) in their highest oxidation states ⁴.They have a wide range of molecular structural diversity, existing in a wide variety of shapes, sizes and compositions. They are composed by the condensation of MO_n units, where ‘n’ indicates the coordination number of M (n = 4, 5, 6 or 7).

How can we describe them?

In general, POMs, can be described by the formula [X_xM_mO_y]^q– where X is the heteroatom and M is a transition metal of V and VI groups, principally Mo, W and V and less frequently Nb, Ta, Cr. There are no limitations for the heteroatom as about 70 elements of the periodic table are known, with the exception of noble gases.

How can we classify them?

Isopolyoxometalates, [M_mO_y]ⁿ⁻, are metal oxide cluster species that are composed of a single metal type, with no additional heterogroups or supporting ligands.

Heteropolyoxometalates, are represented with the general formula [X_xM_mO_y]^q–where the heteroatom X, can be one of the many elements of the periodic table. It is the most explored group inside POM chemistry, with more than 5000 articles in the past four years.

Molybdenum blue and brown reduced POMs, inside this group, ring or sphere shape giant clusters are found, composed by a combination of Mo(VI) and reduced Mo(V) centers and connected by oxygens. Mo-Blues are the most remarkable species of polyoxoanion generated by self assembly procedures by the condensation of small buildings blocks to giant discrete molecules up to 368 molybdenum centers described by Prof A.Muller ⁵ (Figure 2).

Which is the most studied structure?

The Keggin structure is the most widely studied structure for tungsten POMs. It has the general formula [XM₁₂O₄₀]^n– (heteropolyanion) where most commonly metals (M) are Mo^VI or W^VI and the heteroatom (X) are B^III, Al^III, Si^IV, P^V, Fe^III, Ge^IV and As^V. It is based on a central heteroatom XO₄ tetrahedron surrounded by four trimers, M₃O₁₃ groups, of three edge-shared MO₆ octahedra. These trimers are linked via corner-sharing to each other and to the central XO₄ tetrahedron via μ₂– and μ₄– oxo bridges respectively, resulting in a high symmetry (T_d) as shown in Figure 3.

What else?

Keggin-type heteropolyanions can have five different rotational isomers known as Baker-Figgis ⁶. The α isomer has the overall T_dsymmetry. By a 60° rotation of one, two, three or all four trimers, the isomers β, γ, δ and ε can be formed,generating structures of C_3v, C_2v, C_3v and T_d symmetry respectively (Figure 4). In general, γ, δ and ε are less stable than α and β isomers because some of the trimers are edge-sharing, decreasing linear M-O-M bonds and so d_π-p_π interactions ⁷.

Lacunary species or vacant species of the Keggin structures are obtained by removal of one, two, or three MO₆ octahedra under basic conditions forming what so-calledmono-, di-, and tri-lacunary species. There are only known lacunary species of the Baker-Figgis isomers, α, β and γ.

The plenary α–[XM₁₂O₄₀]^n– Keggin is the most stable species in acid media but it can lose adjacent MO₆ octahedra to form mono- and tri-lacunary species by increasing the pH of the solution. The monovacant {α-XM₁₁O₃₉} is generated when removing one of the twelve equivalent octahedra. The trilacunary species can be either {A-α-XM₉O₃₄} in which three corner-shared octahedral (triad, M₃O₁₅) have been lost or {B-α-XM₉O₃₄} in which three edge-shared octahedral (trimer, M₃O₁₃) are removed. In contrast, due to C_3v symmetry of the β isomer, the octahedral are non-equivalent so this implies the formation of different types of lacunary species depending on the position and the type of octahedra removed. Thus, three monovacant species can be generated: {β₁-XM₁₁O₃₉}, if the vacancy is located in the opposite triad to the rotated one (unconnected with the rotated triad), {β₂-XM₁₁O₃₉} when the vacant is located in the central belt (connected with the rotated triad), and {β₃-XM₁₁O₃₉} by the removal of one of the three octahedra that is part of the rotated trimer. In the same way, two trilacunary species can be obtained from the plenary β isomer. {A-β-XM₉O₃₄} by removal the opposite triad to the rotated trimer and {B-β-XM₉O₃₄} any trimer can be removed with the exception of the rotated one.The removal of two adjacent octahedra from α– and β–[XW₁₂O₄₀]ⁿ⁻ isomers has not been observed. But, in the case of γ isomer the dilacunary species, {γ-XM₁₀O₃₆}, is only generated when removing two vertex-sharing octahedral belonging to two rotated trimers (Figure 5).

The lacunary species can be formed either by the acidification of solutions containing X and M salts or by the basification of the heteropolianion XW₁₂O₄₀, breaking M-O bonds. The plenary polyanions are predominant in a very acid pH (0-2), the monolacunary species in a lightly acid pH (4-6) and the trilacunary ones at a basic pH (8-10).

Where can they find applications?

Given the vast array of structures we find applications in many different fields like catalysis, biomedicine or material science, to name a few. I am more familiar with this last case, where POMs act as inorganic ligands and with the inclusion of metalorganic units, the generation of new hybrid materials is guaranteed. The development of such compounds, appear as a creative alternative for obtaining new materials with unusual features and properties⁸.