Bernskoetter Wesley H, Schauer Cynthia K, Goldberg Karen I, Brookhart Maurice
Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599-3290, USA.
Science. 2009 Oct 23;326(5952):553-6. doi: 10.1126/science.1177485.
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).
许多过渡金属介导的反应，包括氢化反应、硅氢化反应和烷烃官能团化反应，都会导致强σ键的断裂。这些反应中的关键中间体通常涉及氢气、硅烷（Si-H）或烷烃（C-H）的σ键与金属中心的配位，而键并未完全断裂。这些σ配合物在固态和溶液中已得到不同程度的表征。然而，最简单的烃——甲烷的σ配合物尚未得到完整的溶液表征。在此，我们报告了通过在-110℃的CDCl2F溶剂中对铑-甲基前体进行质子化得到的铑（I）σ-甲烷配合物的核磁共振谱。结果表明，σ-甲烷配合物比相应的铑（III）甲基氢化物配合物更稳定。即使在-110℃时，甲烷在铑的配位球中也会快速翻滚，使游离氢和配位氢发生交换。动力学研究表明，在-87℃时，甲烷解离的半衰期约为83分钟（活化自由能为每摩尔14.5千卡）。
