Cation‐Directed Host–Guest Assembly and Reversible Wheel to Cluster Interconversion in a 20‐Molybdate System

We report on the synthesis and structural characterization of the polyoxo‐20‐molybdate wheel, [Mo VI 20 O 60 {AsO 2 (CH 3 ) 2 } 12 ] 12− ( Mo 20 ), which can be isolated with an empty cavity (diameter ca. 2 nm) or containing two dimethylarsinate guests, depending on the type of cations used during synthesis. The polyanion Mo 20 was characterized by single‐crystal XRD, FT‐IR, and TGA, and in solution by multinuclear NMR ( 1 H, 13 C, 95 Mo) and 1 H DOSY, as well as x‐ray scattering (SWAXS) and mass spectrometry (ESI‐MS). We demonstrated a fast, quantitative, and fully reversible transformation of Mo 20 to [Mo VI 4 O 12 (OH){AsO 2 (CH 3 ) 2 }] 2− ( Mo 4 ) in aqueous solution, whereas Mo 20 is cleanly and quantitatively recrystallized from such solution, indicating a unique phenomenon, which is fully supported by all experimental techniques used as well as DFT and MD calculations.