Testing the Limits of Imbalanced CPET Reactivity: Mechanistic Crossover in H-Atom Abstraction by Co(III)–Oxo Complexes
Testing the Limits of Imbalanced CPET Reactivity: Mechanistic Crossover in H-Atom Abstraction by Co(III)–Oxo Complexes
复制标题
测试不平衡 CPET 反应性的极限：Co(III)−Oxo 配合物提取 H 原子的机械交叉
DOI：
10.1021/jacs.2c10553
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发表时间：
2023
影响因子：
15
通讯作者：
Anderson, John S.
中科院分区：
文献类型：
--
作者：
Zhao, Norman;Goetz, McKenna K.;Schneider, Joseph E.;Anderson, John S.
过渡金属-氧代配合物是各种氧化转化的关键中间体，特别是C-H键活化。由过渡金属-氧代络合物介导的C-H键活化的相对速率通常在具有协同质子-电子转移（CPET）的情况下基于底物键解离自由能来预测。然而，最近的工作表明，替代逐步热力学贡献，如酸度/碱度或氧化还原电位的基板/金属氧代可能占主导地位，在某些情况下。在这种情况下，我们已经发现了碱性控制的协同激活C-H键与终端钴III-氧代配合物PhB（tBuIm）3CoIIIO。我们一直有兴趣在测试这种碱性依赖的反应性的限制，并合成了一个类似的，更基本的配合物，PhB（AdIm）3CoIIIO，并研究了其与H-原子供体的反应性。该复合物显示出比PhB（tBuIm）3CoIIIO与C-H底物更高程度的不平衡CPET反应性，并且苯酚底物的O-H活化显示出逐步质子转移-电子转移（PTET）反应性的机械交叉。质子转移（PT）和电子转移（ET）的热力学分析揭示了一个独特的热力学交叉点协调和逐步反应。此外，逐步和协调的反应性的相对速率表明，最大限度地不平衡的系统提供最快的CPET速率的机械交叉点，这导致较慢的产品形成。
Transition metal–oxo complexes are key intermediates in a variety of oxidative transformations, notably C–H bond activation. The relative rate of C–H bond activation mediated by transition metal–oxo complexes is typically predicated on substrate bond dissociation free energy in cases with a concerted proton–electron transfer (CPET). However, recent work has demonstrated that alternative stepwise thermodynamic contributions such as acidity/basicity or redox potentials of the substrate/metal–oxo may dominate in some cases. In this context, we have found basicity-governed concerted activation of C–H bonds with the terminal CoIII–oxo complex PhB(tBuIm)3CoIIIO. We have been interested in testing the limits of such basicity-dependent reactivity and have synthesized an analogous, more basic complex, PhB(AdIm)3CoIIIO, and studied its reactivity with H-atom donors. This complex displays a higher degree of imbalanced CPET reactivity than PhB(tBuIm)3CoIIIO with C–H substrates, and O–H activation of phenol substrates displays mechanistic crossover to stepwise proton transfer–electron transfer (PTET) reactivity. Analysis of the thermodynamics of proton transfer (PT) and electron transfer (ET) reveals a distinct thermodynamic crossing point between concerted and stepwise reactivity. Furthermore, the relative rates of stepwise and concerted reactivity suggest that maximally imbalanced systems provide the fastest CPET rates up to the point of mechanistic crossover, which results in slower product formation.