Characterization of a rhodium(I) sigma-methane complex in solution.

Abstract

Numerous transition metal-mediated reactions, including hydrogenations, hydrosilations, and alkane functionalizations, result in the cleavage of strong sigma bonds. Key intermediates in these reactions often involve coordination of the sigma bond of dihydrogen, silanes (Si-H), or alkanes (C-H) to the metal center without full scission of the bond. These sigma complexes have been characterized to varying degrees in solid state and solution. However, a sigma complex of the simplest hydrocarbon, methane, has eluded full solution characterization. Here, we report nuclear magnetic resonance spectra of a rhodium(I) sigma-methane complex obtained by protonation of a rhodium-methyl precursor in CDCl2F solvent at -110 degrees C. The sigma-methane complex is shown to be more stable than the corresponding rhodium(III) methyl hydride complex. Even at -110 degrees C, methane rapidly tumbles in the coordination sphere of rhodium, exchanging free and bound hydrogens. Kinetic studies reveal a half-life of about 83 minutes at -87 degrees C for dissociation of methane (free energy of activation is 14.5 kilocalories per mole).