Polyoxometalate chemistry of {M[P4Mo6]2}: From structure assembly to functional application

Reduced hourglass-shaped polyoxometalate [Mⁿ(P₄Mo^V₆O₃₁)₂]^(24−n)− (abbr. {M[P₄Mo^V₆]₂}) clusters, represent an emerging subfield of polyoxometalates (POMs) that has garnered widespread interest across various multidisciplinary domains, including structural chemistry, coordination chemistry, catalytic chemistry, inorganic chemistry, and material chemistry. This interest stems from their totally reduction state, unique electron storage/release characteristics, photochemical behavior and structural tailorability. Despite over four decades of research on {M[P₄Mo^V₆]₂} clusters and the identification of more than 200 functional compounds based on this structure, this area remains relatively unexplored compared to other POM family members and there is a noticeable absence of focused reviews summarizing this field. This review offers a comprehensive survey on the synthetic strategies, structural characteristics and assembly mechanism of {M[P₄Mo^V₆]₂} POMs, as well as their applications in various functional areas such as catalysis, magnetism, fluorescence, proton conduction, energy conversion. Additionally, it also discusses the structure–activity relationships of these materials, highlights the current challenges, and provides an outlook for future research directions, aiming to inspire new investigations and advancements in this fascinating realm of POM chemistry.