Selective methane oxidation by molecular iron catalysts in aqueous medium

Using natural gas as chemical feedstock requires efficient oxidation of the constituent alkanes-and primarily methane(1,2). The current industrial process uses steam reforming at high temperatures and pressures(3,4) to generate a gas mixture that is then further converted into products such as methanol. Molecular Pt catalysts(5-7) have also been used to convert methane to methanol(8), but their selectivity is generally low owing to overoxidation-the initial oxidation products tend to be easier to oxidize than methane itself. Here we show that N-heterocyclic carbene-ligated Fe-II complexes with a hydrophobic cavity capture hydrophobic methane substrate from an aqueous solution and, after oxidation by the Fe centre, release a hydrophilic methanol product back into the solution. We find that increasing the size of the hydrophobic cavities enhances this effect, giving a turnover number of 5.0 x 10(2) and a methanol selectivity of 83% during a 3-h methane oxidation reaction. If the transport limitations arising from the processing of methane in an aqueous medium can be overcome, this catch-and-release strategy provides an efficient and selective approach to using naturally abundant alkane resources.