Frontiers and progress in cation-uptake and exchange chemistry of polyoxometalate-based compounds

Abstract

Cation-uptake and exchange has been an important topic in both basic and applied chemistry relevant to life and materials science. For example, living cells contain appreciable amounts of Na⁺ and K⁺, and their concentrations are regulated by the sodium-potassium pump. Solid-state cation-exchangers such as clays and zeolites both natural and synthetic have been used widely in water softening and purification, separation of metal ions and biomolecules, etc. Polyoxometalates (POMs) are robust, discrete, and structurally well-defined metal-oxide cluster anions, and have stimulated research in broad fields of sciences. In this perspective, cation-uptake and exchange in POM and POM-based compounds are categorized and reviewed in three groups: (i) POMs as inorganic crown ethers and cryptands, (ii) POM-based ionic solids as cation-exchangers, and (iii) reduction-induced cation-uptake in POM-based ionic solids, which is based on a feature of POMs that they are redox-active and multi-electron transfer occurs reversibly in multiple steps. This method can be utilized to synthesize mixed-valence metal clusters in metal ion-exchanged POM-based ionic solids.

Fig. 1. Cation-uptake and exchange in POM and POM-based compounds categorized in three groups: (i) POMs as inorganic crown ethers, (ii) POM-based ionic solids as cation-exchangers, and (iii) reduction-induced cation-uptake in POM-based ionic solids.

Fig. 2. Preyssler–Pope–Jeannin-type POM [Xⁿ⁺(H₂O)P₅W₃₀O₁₁₀]^(15–n)– with flexible W₅O₅ cavity for cation encapsulation. Light green and purple polyhedra show the [WO₆] and [PO₄] units, respectively.

Fig. 3. Coordination environments of protonated urea (left, black: C and yellow: N/O) and Ce³⁺ (right, red) in the nanoporous POM capsule., Blue and green polyhedra show the [MO₆] and [SO₄] units, respectively.

Fig. 4. Microporous solid composed of {ε-ZnMo₁₂O₄₀Zn₂} units and exchangeable counter cations.

Fig. 5. (a) Crystal structure of Cs_xH_4–x[α-SiW₁₂O₄₀]. Light green and purple polyhedra show the [WO₆] and [SiO₄] units, respectively. Purple spheres are exchangeable cations (Cs⁺). (b) Anion vacancies (in light yellow) along the [100] and [111] directions. Red and purple spheres show the O^2– of POM and Cs⁺, respectively. (c) Cation-exchange: time courses of uptakes of alkali metal ions. (d) Elemental mapping images.

Fig. 6. (Left) Crystal structure of K₂[Cr₃O(OOCH)₆(4-methylpyridine)₃]₂[α-SiW₁₂O₄₀]·nH₂O. Light green and orange polyhedra show the [WO₆] and [CrO₅N] units, respectively. Purple and blue spheres show the alkali metal ions and oxygen atoms of the water of crystallization, respectively. (Right) Arrhenius plots of the temperature dependent proton conductivities at RH 95% (303–323 K). Proton conductivities of the compound with Li⁺ and Cs⁺ as counter cations were 1.9 × 10^–3 and 1.2 × 10^–7 S cm^–1, respectively (323 K).

Fig. 7. (Upper) Molecular structure of [P₈W₄₈O₁₈₄]^40– comprising a nanometer-size cavity. (Lower) The molecular unit is linked by Mn²⁺ resulting in a 3D-POM framework.

Fig. 8. (a) Crystal structure of (etpyH)₂[Cr₃O(OOCH)₆(etpy)₃]₂[α-SiMo₁₂O₄₀]·nH₂O (etpy = 4-ethylpyridine). Each void (in yellow-brown) has a size of ca. 6.5 Å × 12.5 Å. (b) Amounts of cations incorporated by the reduction-induced method. Note that there is a color change due to the reduction of POM upon Cs⁺ uptake.

Fig. 9. Schematic illustration of the reduction-induced ion-exchange of Cs⁺ with Ag⁺ in the one-dimensional channel (in yellow-brown) to form small mixed-valence luminescent silver clusters. PL spectrum and time course of PL images of a single crystal of Cs₂-red in AgNO₃(aq.) (excitation at 405 nm).

Sayaka Uchida