| Summary: | In ongoing attempts to efficiently utilize abundant natural gas, there has been steady scientific and industrial interest in using an environmentally benign and inexpensive oxidant (dioxygen O<sub>2</sub>) for the direct catalytic oxidation of methane to oxygenate products under mild conditions. Here, we report the homogeneous bis(tetramethylammonium) tetrachlorocobaltate ([Me<sub>4</sub>N]<sub>2</sub>CoCl<sub>4</sub>)-catalyzed methane oxidation to methyl trifluoroacetate (MeTFA) with dioxygen O<sub>2</sub> in trifluoroacetic acid (HTFA) media. [Me<sub>4</sub>N]<sub>2</sub>CoCl<sub>4</sub> had the highest catalytic activity among previously reported homogeneous cobalt-based catalyst systems; the turnover of methane to MeTFA reached 8.26 mol<sub>ester</sub> mol<sub>metal</sub><sup>−1</sup>h<sup>−1</sup> at 180 °C. Results suggest that the ionic form of the catalyst makes the Co species more soluble in the HTFA media; consequently, an active catalyst form, [CoTFA<sub>x</sub>Cl<sub>y</sub>]<sup>2−</sup>, can form very rapidly. Furthermore, chloride anions dissociated from CoCl<sub>4</sub><sup>2−</sup> appear to suppress oxidation of the solvent HTFA, thereby driving the reaction toward methane oxidation. The effects of reaction time, catalyst concentration, O<sub>2</sub> and methane pressure, and reaction temperature on MeTFA production were also investigated.
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