Dawson Type Heteropolyanions. 3.<sup>†</sup> Syntheses and <sup>31</sup>P, <sup>51</sup>V, and <sup>183</sup>W NMR Structural Investigation of Octadeca(molybdo−tungsto−vanado)diphosphates Related to the [H<sub>2</sub>P<sub>2</sub>W<sub>12</sub>O<sub>48</sub>]<sup>12-</sup> Anion

The synthesis and multinuclear NMR characterization of mixed molybdenum−vanadium−tungsten polyoxometalates [P<sub>2</sub>Mo<i><sub>x</sub></i>V<i><sub>y</sub></i>W<sub>18</sub><sub>-</sub><sub>(</sub><i><sub>x</sub></i><sub>+</sub><i><sub>y</sub></i><sub>)</sub>O<sub>62</sub>]<i><sup>n</sup></i><sup>-</sup> (<i>x</i> + <i>y</i> ≤ 8) related to the Dawson structure are reported. The mixed species are obtained from the hexavacant anion [H<sub>2</sub>P<sub>2</sub>W<sub>12</sub>O<sub>56</sub>]<sup>12-</sup> by successive condensation and hydrolysis reactions. The strategy of synthesis is mainly based on the steric control of hydrolysis reactions by the nature and the strength of the base, the relative kinetic lability of molybdenum and tungsten in hydrolysis reactions, and the conservation of the framework when vacant sites are refilled by new metal atoms. Rather good values of <sup>31</sup>P chemical shift variations can be predicted by an additive model taking into account the contribution of substituting groups, depending on their position in the structure. The influence of Mo/W and V/W substitutions on <sup>183</sup>W chemical shifts of the remaining W atoms has been discussed and seems to be preferentially passed on through corner junctions.