Unprecedented Halide-Ion Binding and Catalytic Activity of Nanoscale Anionic Metal Oxide Clusters.

One halide ion (X- ) can bind on the surface of nanoscale Anderson-type polyoxometalate (POMs) clusters [(n-C4 H9 )4 N]3 {AlMo6 O18 (OH)3 [(OCH2 )3 CCH3 ]}, and form stable complexes in solution with binding constant K=1.53×103 . Single-crystal structural analysis showed that this binding behavior occurs through multiple hydrogen bonding between X- and three hydroxy groups on the uncapped side of the cluster. This supramolecular interaction in the cluster systems means that their catalytic activities, evaluated from the oxidation of alcohols to aldehydes, can be switched upon the introduction of halide ions and water molecules. The halide ions work as inhibitors by blocking the active sites of the clusters while they can be re-activated by the addition of water.