Dendritic Zirconium-Peroxotungstosilicate Hybrids: Synthesis, Characterization, and Use as Recoverable and Reusable Sulfide Oxidation Catalysts

A series of dendrimer-containing polyoxometalates (DENDRIPOMs) was synthesized by coupling zirconium-peroxotungstosilicate [Zr-2(O-2)(2)(SiW11O39)(2)](12-) with ammonium dendrons by electrostatic bonding. These DENDRIPOMs were successfully characterized by standard physicochemical techniques (e.g. IR and NMR spectroscopy and MS), and they represent the first examples of dendritic POMs based on zirconium-substituted polytungstates. The data obtained are consistent with structures in which the anionic POM is surrounded by cationic ammonium dendrons. In contrast to the potassium salt of [Zr-2(O-2)(2)(SiW11O39)(2)](12-), the dendritic counterparts are soluble in common organic solvents, an important feature for the use of DENDRIPOMs in homogeneous catalysis. Our DENDRIPOMs are stable, efficient, recoverable, and reusable catalysts for the oxidation of sulfides in aqueous/CDCl3 biphasic media, with hydrogen peroxide as the oxidant, in contrast to their nondendritic n-butyl ammonium counterpart. Two cycles of catalytic reactions were performed without any appreciable loss of activity We also discovered that the reaction kinetics and selectivity of the DENDRIPOMs are influenced by the structure of the countercation used.