Binding methane to a metal centre

The σ-alkane complexes of transition metals, which contain an essentially intact alkane molecule weakly bound to the metal, have been well established as crucial intermediates in the activation of the strong C–H σ-bonds found in alkanes. Methane, the simplest alkane, binds even more weakly than larger alkanes. Here we report an example of a long-lived methane complex formed by directly binding methane as an incoming ligand to a reactive organometallic complex. Photo-ejection of carbon monoxide from a cationic osmium–carbonyl complex dissolved in an inert hydrofluorocarbon solvent saturated with methane at −90 °C affords an osmium(II) complex, [η 5 -CpOs(CO) 2 (CH 4 )] + , containing methane bound to the metal centre. Nuclear magnetic resonance (NMR) spectroscopy confirms the identity of the σ-methane complex and shows that the four protons of the metal-bound methane are in rapid exchange with each other. The methane ligand has a characteristically shielded 1 H NMR resonance ( δ –2.16), and the highly shielded carbon resonance ( δ –56.3) shows coupling to the four attached protons ( 1 J C–H = 127 Hz). The methane complex has an effective half-life of about 13 hours at –90 °C.