C–H bond activation reactions with transition metals typically proceed via the formation of alkane σ-complexes, where an alkane C–H σ-bond binds to the metal. Due to the weak nature of metal–alkane bonds, σ-complexes are challenging to characterize experimentally. Here, we establish the complete pathways of photochemical formation of the model σ-complex Cr(CO)₅-alkane from Cr(CO)₆ in octane solution and characterize the nature of its metal–ligand bonding interactions. Using femtosecond optical absorption spectroscopy, we find photoinduced CO dissociation from Cr(CO)₆ to occur within the 100 fs time resolution of the experiment. Rapid geminate recombination by a fraction of molecules is found to occur with a time constant of 150 fs. The formation of bare Cr(CO)₅ in its singlet ground state is followed by complexation of an octane molecule from solution with a time constant of 8.2 ps. Picosecond X-ray absorption spectroscopy at the Cr L-edge and O K-edge provides unique information on the electronic structure of the Cr(CO)₅-alkane σ-complex from both the metal and ligand perspectives. Based on clear experimental observables, we find substantial destabilization of the lowest unoccupied molecular orbital upon coordination of the C–H bond to the undercoordinated Cr center in the Cr(CO)₅-alkane σ-complex, and we define this as a general, orbital-based descriptor of the metal–alkane bond. Our study demonstrates the value of combining optical and X-ray spectroscopic methods as complementary tools to study the stability and reactivity of alkane σ-complexes in their role as the decisive intermediates in C–H bond activation reactions.

中文翻译：

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时间分辨光学和 X 射线吸收光谱中烷烃 σ 配合物的光化学形成和电子结构


与过渡金属的 C-H 键活化反应通常通过形成烷烃 σ-配合物进行，其中烷烃 C-H σ-键与金属结合。由于金属-烷烃键的弱性质，σ-配合物的实验表征具有挑战性。在这里，我们建立了在辛烷溶液中从 Cr(CO) ₆ 光化学形成模型 σ-配合物 Cr(CO) ₅ -烷烃的完整路径，并表征了其性质金属-配体键合相互作用。使用飞秒光学吸收光谱，我们发现光诱导 CO 从 Cr(CO) ₆ 中解离，发生在实验的 100 fs 时间分辨率内。发现一小部分分子的快速双生重组发生的时间常数为 150 fs。在单线态基态下形成裸露的 Cr(CO) ₅ ，随后与溶液中的辛烷分子络合，时间常数为 8.2 ps。 Cr L 边和 O K 边的皮秒 X 射线吸收光谱从金属和配体的角度提供了有关 Cr(CO) ₅ -烷烃 σ 配合物电子结构的独特信息。基于清晰的实验观察结果，我们发现在 C-H 键与 Cr(CO) ₅ -烷烃 σ 配合物中的欠配位 Cr 中心配位时，最低未占据分子轨道显着不稳定，并且我们将其定义为金属-烷烃键的通用的、基于轨道的描述符。我们的研究证明了结合光学和 X 射线光谱方法作为补充工具来研究烷烃 σ 配合物作为 C-H 键活化反应中决定性中间体的稳定性和反应性的价值。