Four-Shell Polyoxometalates Featuring High-Nuclearity Ln₂₆ Clusters: Structural Transformations of Nanoclusters into Frameworks Triggered by Transition-Metal Ions

A series of polyoxometalates (POMs) that incorporate the highest-nuclearity Ln clusters that have been observed in such structures to date (Ln₂₆ , Ln=La and Ce) are described, which exhibit giant multishell configurations (Ln⊂W₆⊂Ln₂₆⊂W₁₀₀). Their structures are remarkably different from known giant POMs that feature multiple Ln ions. In particular, the incorporated Ln–O clusters with a nuclearity of 26 are significantly larger than known high-nuclearity (≤10) Ln–O clusters in POM chemistry. Furthermore, they also contain the largest number of La and Ce centers for any POM reported to date and represent a new kind of rare giant POMs with more than 100 W atoms. Interestingly, the La₂₆-containing POM can undergo a single-crystal to single-crystal structural transformation in the presence of various transition-metal ions, such as Cu²⁺, Co²⁺, and Ni²⁺, from an inorganic molecular nanocluster into an inorganic–organic hybrid extended framework that is built from POM building blocks with even higher-nuclearity La₂₈ clusters bridged by transition-metal complexes.