High Valent Metal Oxo Complexes as Oxidation Catalysts

A series of new dioxomolybdenum(VI) and -tungsten(VI) complexes of tripodal tetradentate and tridentate amino bis- and monophenolate ligands have been synthesized and characterized. The majority of the complexes are mononuclear, but dimeric and trimeric oxo-bridged M(VI) (M = Mo, W) were also obtained. The reactivities of all complexes have been investigated in oxygen atom transfer (OAT) processes, e.g. epoxidation and sulfoxidation, with different organic substrates. Good results were obtained for epoxidation of cyclooctene, but low to moderate yields were observed for more challenging substrates (1-octene, styrene, limonene and α-terpineol). The tungsten complexes exhibited very high activity in cyclooctene epoxidation when H2O2 was used as an oxidant and dimethyl carbonate was used as a solvent. Several molybdenum complexes were found to be good sulfoxidation catalysts, but no activity could be detected for the tungsten complexes that were tested.
For comparative purposes, several new vanadium(V) oxo complexes of amino bis- and monophenolate ligands have been synthesised and investigated as catalysts for the epoxidation and sulfoxidation reactions. The overall catalytic efficiencies were low for epoxidation but excellent for sulfoxidation.
Finally, new oxotungsten(VI) complexes have been prepared from aminobisphenol and aminomonophenol ligands and examined as catalysts for haloperoxidation of various organic substrates, using hydrogen peroxide as oxidant. They showed good catalytic performances for the bromoperoxidation reaction. Some of the more active catalysts were also investigated as chloroperoxidation catalysts; these reactions were more sluggish than the analogous bromination reactions, but the catalytic performances was better than for other reported tungsten catalysts.