Binding methane to a metal centre

Abstract

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, [η⁵-CpOs(CO)₂(CH₄)]⁺, 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 ¹H NMR resonance (δ -2.16), and the highly shielded carbon resonance (δ -56.3) shows coupling to the four attached protons (¹J_C-H = 127 Hz). The methane complex has an effective half-life of about 13 hours at -90 °C.