Metal–oxo-cluster-based crystals as solid catalysts

This review discusses the catalysis of metal–oxo-cluster-based crystals. The catalytically active metal–oxo cluster anions (polyoxometalates [POMs]) can be solidified with inorganic cations, organic cations, cationic metal complexes, or cationic coordination polymers (CPs). The spatial periodicity of POMs, which is closely associated with their catalysis, can be engineered via electrostatic interaction, hydrogen bonds, or coordination bonds by appropriately choosing the cations above. We also refer to less common cationic metal–oxo clusters. The catalytic properties of metal–oxo clusters, such as acid/base and redox, can be controlled by the charge, electron configuration, ionic radius, or electronegativity of their constituent elements. Similarly, the properties of their counterparts (inorganic or organic ions, metal complexes, and CPs) can be designed by their nuclearity, denticity, or ligands. Therefore, the infinite combination of anions and cations would enable construction of a large variety of crystals, contributing to the development of catalytic science.