Reversible Heterolytic Methane Activation of Metal‐Free Closed‐Shell Molecules: A Computational Proof‐of‐Principle Study
Reversible Heterolytic Methane Activation of Metal‐Free Closed‐Shell Molecules: A Computational Proof‐of‐Principle Study
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DOI：
10.1002/ejic.201000242
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发表时间：
2010-05
影响因子：
2.3
通讯作者：
G. Lu;Lili Zhao;Haixia Li;Fangcheng Huang;Zhi‐Xiang Wang
中科院分区：
文献类型：
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作者：
G. Lu;Lili Zhao;Haixia Li;Fangcheng Huang;Zhi‐Xiang Wang
同时利用酸碱效应是过渡金属配合物活化H-H/C-H σ键的基本原理之一。原则上，无金属FLP（受抑刘易斯对）对H2的高反应性也可归因于这些效应。在我们提出的集成FLP的基础上，我们将效应推到更高（如果不是极限的话）的水平，在该水平下，非常不活泼的甲烷C-H σ键可以被我们设计的无金属闭壳分子激活。报道的三个分子（M3 c，M4 b和M4 c）的活化自由能（分别为22.4，20.0和20.2 kcal mol-1）与具有Ti=N双键的TM模型络合物的22.3 kcal mol-1相当（或小于）。TM模型的衍生物已被实验证明能够活化甲烷。此外，有些活化反应是（或接近）热中性的。例如，M3 c、M4 b和M4 c的甲烷活化分别是放热的，自由能为-1.9、-4.5和-5.2 kcal mol-1。动力学和热力学表明，分子可以进一步发展，实现催化甲烷活化。电子结构分析表明，虽然我们的无金属分子和TM模型配合物在激活C-H键方面具有相同的原理，但它们如何保持有效的活性位点存在差异。所报道的分子可能成为实验实现的目标。该策略可应用于类似分子的设计，实现更一般的烷烃C-H键活化。
Utilization of the acid/base effects simultaneously is one of the basic principles used by transition-metal (TM) complexes to activate H–H/C–H σ bonds. In principle, the high reactivity of metal-free FLPs (frustrated Lewis pairs) towards H2 can also be attributed to these effects. On the basis of our proposed integrated FLPs, we pushed the effects to a higher (if not a limit) level at which the extremely unreactive methane C–H σ bond can be activated by our designed metal-free closed-shell molecules. Three molecules (M3c, M4b, and M4c) among the reported have activation free energies (22.4, 20.0, and 20.2 kcal mol–1, respectively) comparable with (or less than) the 22.3 kcal mol–1 of a TM model complex that features a Ti=N double bond. The derivative of the TM model has been experimentally shown to be capable of activating methane. Moreover, some of the activation reactions are (or nearly) thermoneutral. For example, the methane activations of M3c, M4b, and M4c are exothermic by –1.9, –4.5, and –5.2 kcal mol–1 of free energies, respectively. The kinetics and thermodynamics imply that the molecules could be further developed to realize catalytic methane activation. The electronic structure analyses reveal that, although our metal-free molecules and the TM model complex share the same principle in activating the C–H bond, there are differences as to how they go about maintaining the effective active sites. The reported molecules could be the targets for experimental realizations. The strategy could be applied to the design of similar molecules to realize more general C–H bond activation of alkanes.