Osmium‐Catalyzed Selective Oxidations of Methane and Ethane with Hydrogen Peroxide in Aqueous Medium

Various transition metal chlorides including FeCl₃, CoCl₂, RuCl₃, RhCl₃, PdCl₂, OsCl₃, IrCl₃, H₂PtCl₆, CuCl₂ and HAuCl₄ were studied for the selective oxidations of methane and ethane with hydrogen peroxide in aqueous medium. Among the metal chlorides investigated, osmium(III) chloride (OsCl₃) exhibited the highest turnover frequency (TOF) for the formation of organic oxygenates (mainly alcohols and aldehydes) from both methane and ethane. For the OsCl₃‐catalyzed oxidation of methane with hydrgen peroxide, methyl hydroperoxide was also formed together with methanol and formaldehyde. The effects of various kinetic factors on the catalytic behavior of the OsCl₃‐H₂O₂ system were investigated, and TOF values of 12 and 41 h⁻¹ could be obtained for oxygenate formation during the oxidations of methane and ethane, respectively. In the presence of OsCl₃, NaClO, NaClO₄ or NaIO₄ as oxidant was incapable of oxidizing methane and ethane to the corresponding oxygenates, and the use of tert‐butyl hydroperoxide (TBHP) instead of H₂O₂ provided remarkably lower rates of formation of oxygenates. UV‐Vis spectroscopic measurements suggested that OsCl₃ was probably oxidized into an Os(IV) species by H₂O₂ in aqueous medium, and the Os(IV) species might be involved in the oxygenation of methane or ethane. The result that the conversions of both methane and ethane to oxygenates were suppressed by the addition of a radical scavenger suggested that the reactions proceeded via a radical pathway.