A Spontaneous Structural Transition of {U 24 Pp 12 } Clusters Triggered by Alkali Counterion Replacement in Dilute Solution

A transition between two isomeric clusters involving the change of the main skeleton structure of a well‐defined, rigid molecular cluster [(UO 2 ) 24 (O 2 ) 24 (P 2 O 7 ) 12 ] 48− , {U 24 Pp 12 }, is achieved by simply introducing proper alkali cations into its dilute aqueous solution. While the unique structural transition can be triggered by introducing any of the Na + /K + /Rb + /Cs + alkali ions, the two isomers, Li/Na‐{U 24 Pp 12 } and Na/K‐{U 24 Pp 12 }, as typical macroions, can accurately choose among different alkali counter‐cations based on their hydrated sizes, and the ion selectivity process clearly showed endothermic features. The preferred K + and Rb + ions have suitable sizes to be incorporated into the proper windows on {U 24 Pp 12 } nanocapsules, as supported by the transition points in both ITC studies and IR measurements.