Following the Reaction of Heteroanions inside a {W18O56} Polyoxometalate Nanocage by NMR Spectroscopy and Mass Spectrometry**

By incorporating phosphorus(III)-based anions into a polyoxometalate cage, a new type of tungsten-based unconventional Dawson-like cluster, [W18O56(HP(III)O3)2(H2O)2](8-), was isolated, in which the reaction of the two phosphite anions [HPO3](2-) within the {W18O56} cage could be followed spectroscopically. As well as full X-ray crystallographic analysis, we studied the reactivity of the cluster using both solution-state NMR spectroscopy and mass spectrometry. These techniques show that the cluster undergoes a structural rearrangement in solution whereby the {HPO3} moieties dimerize to form a weakly interacting (O3PH⋅⋅⋅HPO3) moiety. In the crystalline state the cluster exhibits a thermally triggered oxidation of the two P(III) template moieties to form P(V) centers (phosphite to phosphate), commensurate with the transformation of the cage into a Wells-Dawson {W18O54} cluster.