Frontiers and applications of polyoxometalates-based porous ionic crystals

Polyoxometalates (denoted as POMs) are discrete metal-oxide anions of V, Mo, W, etc., with variable structures and sub-nanometer sizes. Plenty of POMs and their derivatives have been reported due to their oxygen-enriched surface and abundant substitutional chemistry. However, few studies have focused on the modulation of the counter cations so far. Different from common cations, such as Na⁺, NH₄⁺ and other organic ammonium ions (tetramethylammonium, tetrabutylammonium, etc.), cation clusters with larger size can also be used as counter ions of POMs. They are arranged alternately through ionic bonds or hydrogen bonds to form the solid ionic crystal materials with special properties, which are named POM-based porous ionic crystals (PPICs). The use of cation clusters with different compositions greatly enriches the structure and type of PPICs, which further boosts the development of polyoxometalates chemistry. The POMs anions and large metal complex cations in PPICs are regularly arranged into a porous honeycomb or layered structure. Some PPICs also contain monovalent cations such as H⁺ and alkali metal ions to balance their extra negative charges. The use of ion clusters facilitates the formation of pore structures in the PPICs lattice because they can reduce the electrostatic interaction between cations and anions. Thus, the pore structure of PPICs is much larger than that of POMs. Note that the pore size in PPICs can be easily adjusted by changing the shape, size and charges of cation and anions. Apart from the electrostatic interaction, anisotropic π-π stacking and hydrogen bonding network among the components of PPICs also contribute to their assembly. All these interaction modes will affect the arrangement of anions and cations in the PPICs lattice, resulting in the formation of different hole sizes and various channels characteristics in the crystal lattice, such as hydrophilic, hydrophobic and amphiphilic pores. In addition, the long-range Coulomb interaction works isotropically, leading to the easy transformation of the flexible PPICs structure. Hence, the adjustment of the channel provides a useful strategy for constructing PPICs with unique structures. Most importantly, PPICs show better performance than individual components because they inherit the advantages from both anions and cations. Briefly, PPICs not only retain good redox reversibility, rich multi-electron transfer characteristics and strong Brønsted acidity of POMs, but also reserve the magnetic properties of large cation clusters. Therefore, the physical and chemical properties of PPICs can be modulated by the rational design of each component. The future research on PPICs should not be limited to expanding the categories of cations, and the innovative structural type of POMs is also an important aspect. In addition to the Keggin POMs, other POMs structures, such as Dawson, Anderson, and Preyssler, can also be used in PPICs, resulting in some unique properties. Besides, various POM-based materials (modified POMs or POM-based composites, etc.) can also be adopted to fabricate PPICs, which may bring unexpected performance. Thus, changing anions and cations makes PPICs have great potential in many interdisciplinary fields such as chemistry, materials science, and biomedicine. This review systematically summarizes the structural characteristics and composition of PPICs, which is essential for understanding the characteristics of PPICs, such as adjustable pore structure, unique redox behavior, strong acidity and magnetic properties. In general, PPICs with different properties can be constructed by using diverse POMs and distinct cation clusters, which will be widely applied in many fields such as guest adsorption, ion exchange, photoelectric catalysis, bioimaging and medical materials.