Polyoxometalates (POMs) are a large family of anionic metal–oxygen clusters of early transition metals and stimulated many current research activities in broad fields of science including oxidation catalysis. In addition, POMs are thermally and oxidatively stable in comparison with common organometallic complexes and enzymes.
We have successfullysynthesized novel tungsten-based POMs such as diperoxotungsten dimer, Se-bridged diperoxotungsten dimer, lacunary silicodecatungstate, and S-shaped silicodecatungstate dimer for H2O2-based oxidation of alkenes, silanes, and sulfides. By using lacunary POMs as “structural motifs”,we havesuccessfully synthesizedseveral POM-based molecular catalysts (metal-substituted POMs)with controlled active sites by introducing metal cations (V, Fe, Cu, Mn, Ti, Zn, Al, Zr, and Hf) into the vacant sites. Generally,metal-substituted POMs have been synthesized by the reaction of alkali-metal salts of lacunary POMs and the corresponding transition-metal salts in aqueous media, while the isomerization and decomposition of lacunary POMs sometimes proceeds in aqueous (acidic) media. We have developed new synthetic procedures for metal-substituted POMs in organic media using appropriatelacunaryprecursors, which can completely avoid the isomerization and decomposition of the original lacunary frameworks. This finding made the design of well-defined active sites with POMs much easier, and various molecular catalysts, e.g., 3d metal and rare-earth metal cations, which are previously very difficult to synthesize in aqueous media, have been synthesized.By using thesemolecular catalysts, various unique oxidations, e.g.,regioselective oxidation of alkanes and alkenes, oxidation of alcohols, oxidative homocoupling, and hydrolytic oxidation of silanes have been realized.
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