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Characterization of a Rhodium(I) σ-Methane Complex in Solution

Science  23 Oct 2009:
Vol. 326, Issue 5952, pp. 553-556
DOI: 10.1126/science.1177485

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Abstract

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

  • * Present address: Department of Chemistry, Brown University, Providence, RI 02912, USA.

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