Key Metal-Methane Complex Observed

A transition-metal-methane complex believed to mimic a key intermediate in the conversion of natural gas into liquid fuels has finally been observed (Science 2009, 326, 553). In the species, the electrons of a C–H bond serve as a donor ligand to the metal, an arrangement called a σ complex. Such σ complexes are well-known for H₂ and likely occur as intermediates in hydrogenation and hydrogenolysis reactions but have not been documented for methane in solution. A research team led by Maurice Brookhart of the University of North Carolina, Chapel Hill, protonated a Rh–CH₃ complex to yield a stable Rh–CH₄ ⁺ species. They characterized the complex by nuclear magnetic resonance spectroscopy and found a sharp resonance for the methane protons, indicating that they are in very rapid exchange. The species is isoelectronic with a Pt(II) σ-methane complex that is believed to be an intermediate in Pt(II)-mediated methane oxidation reactions. The σ-methane complex “furnishes details about the weak interaction of methane with a transition-metal center before C–H bond scission,” the authors write.