Orbital-specific mapping of the ligand exchange dynamics of Fe(CO)5 in solution
Orbital-specific mapping of the ligand exchange dynamics of Fe(CO)5 in solution
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DOI：
10.1038/nature14296
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发表时间：
2015-04-02
期刊：
影响因子：
64.8
通讯作者：
Foehlisch, A.
中科院分区：
文献类型：
--
作者：
Wernet, Ph.;Kunnus, K.;Foehlisch, A.
过渡金属配合物长期以来一直引起人们对基本化学反应性研究的兴趣，并可能在太阳能转换中使用（1,2）。电子激发，金属中心的配体损失，或两者的结合，会在金属位点（3-11）产生电荷和自旋密度的变化，需要对这些变化进行控制，以优化配合物的光催化产氢(8)和选择性碳氢键激活（9-11）。在分子水平上理解过渡金属配合物是如何催化反应的，特别是所涉及的短寿命和反应性中间态的作用，将对这种优化至关重要。然而，缺乏合适的方法来详细表征电子激发态。在这里，我们使用基于x射线激光的飞秒分辨率光谱和先进的量子化学理论来探测基准过渡金属配合物Fe(CO)(5)在溶液中的反应动力学，光诱导去除CO产生16个电子的Fe(CO)(4)，一种均相催化剂（12,13），在Fe中心（14,15）缺乏电子。在一种迄今未被报道过的激发单重态中，它要么转化为三重态基态，要么与CO或溶剂分子结合，在亚皮秒的时间尺度上再生出五配位的铁。这一发现解决了关于不同自旋通道在Fe(CO)(5)光化学中的相对重要性的争论（参考文献4,16 -20），这一发现是通过飞秒x射线光谱探测具有原子特异性的边界轨道相互作用的能力而成为可能的。我们期望该方法能广泛应用于化学科学，并补充在超快过程中探测结构动力学的方法。
Transition-metal complexes have long attracted interest for fundamental chemical reactivity studies and possible use in solar energy conversion(1,2). Electronic excitation, ligand loss from the metal centre, or a combination of both, creates changes in charge and spin density at the metal site(3-11) that need to be controlled to optimize complexes for photocatalytic hydrogen production(8) and selective carbon-hydrogen bond activation(9-11). An understanding at the molecular level of how transition-metal complexes catalyse reactions, and in particular of the role of the short-lived and reactive intermediate states involved, will be critical for such optimization. However, suitable methods for detailed characterization of electronic excited states have been lacking. Here we show, with the use of X-ray laser-based femtosecond-resolution spectroscopy and advanced quantum chemical theory to probe the reaction dynamics of the benchmark transition-metal complex Fe(CO)(5) in solution, that the photo-induced removal of CO generates the 16-electron Fe(CO)(4) species, a homogeneous catalyst(12,13) with an electron deficiency at the Fe centre(14,15), in a hitherto unreported excited singlet state that either converts to the triplet ground state or combines with a CO or solvent molecule to regenerate a penta-coordinated Fe species on a sub-picosecond timescale. This finding, which resolves the debate about the relative importance of different spin channels in the photochemistry of Fe(CO)(5) (refs 4, 16-20), was made possible by the ability of femtosecond X-ray spectroscopy to probe frontier-orbital interactions with atom specificity. We expect the method to be broadly applicable in the chemical sciences, and to complement approaches that probe structural dynamics in ultrafast processes.