DENDRI-POM hybrids based on the Keggin, Dawson, Preyssler and Venturello polyanions and their catalytic evaluation in oxidation reactions

Abstract

A series of four dendritic polyoxometalate hybrids (DENDRI-POMs) have been synthesized by electrostatic coupling of diallyl carbinol benzyl ammonium dendron with the Keggin [PW12O40]3−, Dawson [P2W18O62]6−, Preyssler [NaP5W30O110]14− and Venturello [PO4(WO(O2)2)4]3− polyanions, respectively. The structures of these DENDRI-POMs were examined in solution by standard physicochemical techniques such as IR and NMR spectroscopy as well as elemental analysis. Particular focus was devoted to 31P and 183W NMR spectroscopy of these hybrids, as these techniques offer evidence for the presence of the intact polyanion in the corresponding hybrids. The 183W NMR spectrum of the Keggin-based DENDRI-POM showed the expected signal, whereas no signal was observed for the other hybrids. On the other hand, the expected 31P NMR signal was observed for all four DENDRI-POMs. These POM-based dendrimers oxidized methyl phenyl sulfide to the corresponding sulfoxide and sulfone, with the Venturello-based DENDRI-POM being the most efficient catalyst. In addition, the latter catalyzed efficiently the oxidation of cyclooctene to the corresponding epoxide, whereas hybrids based on the other three polyanions were inert. Catalyst recovery experiments showed that, after two cycles of oxidation reactions, the activity of the Keggin- and Venturello-based DENDRI-POMs were not changed, whereas that of the Dawson-based hybrid decreased dramatically after the first cycle.