| Summary: | A calix-shaped polyoxometalate, [V<sub>12</sub>O<sub>32</sub>]<sup>4−</sup> (<b>V12</b>), stabilizes an anion moiety in its central cavity. This molecule-sized container has the potential to control the reactivity of an anion. The highly-reactive cyanate is smoothly trapped by <b>V12</b> to form [V<sub>12</sub>O<sub>32</sub>(CN)]<sup>5−</sup>. In the CH<sub>3</sub>NO<sub>2</sub> solution, cyanate abstracts protons from CH<sub>3</sub>NO<sub>2</sub>, and the resultant CH<sub>2</sub>NO<sub>2</sub><sup>−</sup> is stabilized in <b>V12</b> to form [V<sub>12</sub>O<sub>32</sub>(CH<sub>2</sub>NO<sub>2</sub>)]<sup>5−</sup> (<b>V12(CH<sub>2</sub>NO<sub>2</sub>)</b>). A crystallographic analysis revealed the double-bond characteristic short bond distance of 1.248 Å between the carbon and nitrogen atoms in the nitromethane anion in <b>V12</b>. <sup>1</sup>H and <sup>13</sup>C NMR studies showed that the nitromethane anion in <b>V12</b> must not be exchanged with the nitromethane solvent. Thus, the <b>V12</b> container restrains the reactivity of anionic species.
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