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This work reports the interesting and unique cation‐exchange behaviors of the first indium‐bridged purely inorganic 3D framework based on high‐nuclearity polyoxoniobates as building units. Each nanoscale polyoxoniobate features a fascinating near‐icosahedral core–shell structure with six pairs of unique inorganic "molecular tweezers" that have changeable openings for binding different metal cations via ion‐exchanges and exhibit unusual selective metal‐uptake behaviors. Further, the material has high chemical stability so that can undergo single‐crystal‐to‐single‐crystal metal‐exchange processes to produce a dozen new crystals with high crystallinity. Based on these crystals and time‐dependent metal‐exchange experiments, we can visually reveal the detailed metal‐exchange interactions and mechanisms of the material at the atomic precision level. This work demonstrates a rare systematic and atomic‐level study on the ion‐exchange properties of nanoclusters, which is of significance for the exploration of cluster‐based ion‐exchange materials that are still to be developed. [ABSTRACT FROM AUTHOR]

This work demonstrates that the use of high‐negative and high‐symmetry lacunary polyoxometalates (POMs) for the clustering of alkali metal ions is a feasible strategy not only for the formation of rare high‐nuclearity alkali‐metal clusters but also for the construction of new‐type all‐inorganic ionic porous materials. By the strategy, an unprecedented high‐nuclearity K‐H2O cluster {K42(H2O)60} with core–shell K6@K36 configuration is stabilized by 8 C3v‐symmetry trivacant POMs [GeW9O34]10−, forming a novel giant ionic alkali‐metal‐POM composite cluster {K42Ge8W72O272(H2O)60} with more than 100 metal centers. The incorporated 42‐nuclearity K‐H2O cluster {K42(H2O)60} exhibits the highest‐nuclearity alkali‐metal‐water cluster known to date in POM chemistry. Further, the giant {K42Ge8W72O272(H2O)60} clusters can be linked by another kind of alkali metal ions Na+ to generate a fascinating three‐dimensional all‐inorganic ionic porous framework with high chemical stability, proton conductivity, and water vapor adsorption. Giant ionic cluster: The utilization of highly negative polyoxometalates for the clustering of alkali metal cations has allowed the synthesis of a rare all‐inorganic ionic porous material built from novel giant ionic clusters {K42Ge8W72O272(H2O)60} (K42W72), which exhibits high proton conductivity. The K42W72 not only is among the largest ionic clusters but also contains unprecedented core–shell K6@K36‐H2O cage. [ABSTRACT FROM AUTHOR]