The Role of Low Valent Transition Metal Complexes in Homogeneous Catalysis: An EPR Investigation
The Role of Low Valent Transition Metal Complexes in Homogeneous Catalysis: An EPR Investigation
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DOI：
10.1007/s11244-015-0417-6
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发表时间：
2015-09-01
影响因子：
3.6
通讯作者：
Murphy, Damien M.
中科院分区：
文献类型：
--
作者：
Carter, Emma;Murphy, Damien M.
电子顺磁共振（EPR）光谱是一种非常通用的技术，用于表征涉及顺磁中心的均相催化剂。顺磁性母体预催化剂、活化的催化剂本身或任何所得的反应性中间体都可以原位监测，以描述形式金属氧化态的作用和配体结构对所得催化活性的影响。在我们最近的研究结果的这篇综述中，我们描述了如何高反应性，低价过渡金属配合物的正式氧化态Ni（I），Fe（I）和Cr（I）协调的N-杂环卡宾（NHC）或膦配体，这是积极的一系列交叉偶联反应或乙烯低聚，已被调查的EPR。EPR谱的分析揭示了（i）NHC环尺寸的影响如何影响Ni（I）中心的电子性质，（ii）低自旋Fe（I）中间体如何在交叉偶联反应中是催化活性的循环上物种，以及（iii）在向反应介质中添加助催化剂后，涉及Cr（I）中心的分子内结构重排如何发生。这些结果表明，EPR探测顺磁性均相催化剂的结构-反应性关系的实用程序，提供的信息不容易通过其他技术。
Electron paramagnetic resonance (EPR) spectroscopy is an extremely versatile technique for the characterisation of homogeneous catalysts involving paramagnetic centres. The paramagnetic parent pre-catalyst, the activated catalyst itself or any resulting reactive intermediates, can all be monitored in situ in order to delineate the role of the formal metal oxidation state and the influence of ligand structure on the resulting catalytic activity. In this review of our recent results, we describe how highly reactive, low valent transition metal complexes of formal oxidation states Ni(I), Fe(I) and Cr(I) coordinated by N-heterocyclic carbene (NHC) or phosphine ligands and which are active for a range of cross-coupling reactions or ethylene oligomerisation, have been investigated by EPR. Analysis of the EPR spectra revealed (i) how the influence of the NHC ring size affects the electronic properties of the Ni(I) centre, (ii) how low spin Fe(I) intermediates are catalytically competent on-cycle species in cross-coupling reactions and (iii) how intramolecular structural rearrangements involving Cr(I) centres occur following the addition of a co-catalyst to the reaction medium. These results demonstrate the utility of EPR to probe the structure-reactivity relationships in paramagnetic homogeneous catalysts, providing information not readily accessible by other techniques.