Following the Reaction of Heteroanions inside a {W(18)O(56)} 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, [W(18)O(56)(HP(III)O(3))(2)(H(2)O)(2)](8−), was isolated, in which the reaction of the two phosphite anions [HPO(3)](2−) within the {W(18)O(56)} 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 {HPO(3)} moieties dimerize to form a weakly interacting (O(3)PH⋅⋅⋅HPO(3)) 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 {W(18)O(54)} cluster.