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 ₂₀O₆₀{AsO₂(CH₃)₂}₁₂]^12- (Mo₂₀), 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 Mo₂₀ was characterized by single-crystal XRD, FT-IR, and TGA, and in solution by multinuclear NMR (¹H, ¹³C, ⁹⁵Mo) and ¹H DOSY, as well as x-ray scattering (SWAXS) and mass spectrometry (ESI-MS). We demonstrated a fast, quantitative, and fully reversible transformation of Mo₂₀ to [Mo^VI ₄O₁₂(OH){AsO₂(CH₃)₂}]^2- (Mo₄) in aqueous solution, whereas Mo₂₀ 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.