Recent advances in transition-metal-containing Keggin-type polyoxometalate-based coordination polymers

As a new field in POM-based functional materials, polyoxometalate-based coordination polymers (POMCPs), especially transition-metal-containing POMCPs (TM-POMCPs), have undergone substantial advancements over the past few decades for their impressive structural features and desirable properties in optics, electrochemistry, and organic catalysis. Notably, TM-POMCPs based on Keggin-type POM building blocks have attracted widespread research interest and account for more than half of the compounds reported in this class. Keggin-type POMs, strong Lewis acids with adjustable redox properties, can interact with transition metals via self-assembly in the presence of organic ligands, combining the advantages of the three constituents and resulting in many improved properties. This review focuses on Keggin-type TM-POMCPs, which are extended structures with covalently bound metal-oxide clusters with 1D chains, 2D layers, and 3D frameworks. Such coordination polymers not only enrich the structural diversity of Keggin-type POM derivatives but also provide a suitable pathway for designing functional materials with outstanding properties directed by structure–property relationship. In this review, we highlight and discuss the structural features of Keggin-type TM-POMCPs based on various dimensionalities. Furthermore, synthetic strategies and relevant applications, especially in the field of catalysis, are overviewed.