Dichotomous hydrogen atom transfer vs proton-coupled electron transfer during activation of X-H bonds (X = C, N, O) by nonheme iron-oxo complexes of variable basicity.
Dichotomous hydrogen atom transfer vs proton-coupled electron transfer during activation of X-H bonds (X = C, N, O) by nonheme iron-oxo complexes of variable basicity.
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DOI：
10.1021/ja408073m
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发表时间：
2013-11-13
影响因子：
15
通讯作者：
Shaik S
中科院分区：
文献类型：
--
作者：
Usharani D;Lacy DC;Borovik AS;Shaik S
本文描述了FeIV-oxo和feii -oxo配合物（1 - 4）的氢原子转移(HAT)/质子耦合电子转移（PCET）反应性，这些配合物激活了9,10二氢蒽（S1）、二甲基甲酰胺（S2）、1,2二苯基肼（S3）、对甲氧基酚（S4）和1,4-环己二烯（S5）中的C-H、N-H和O-H键。在1-3中，铁与三[N'-叔丁基脲醛)-N-乙烯]氨基（[H3buea]3−）或其衍生物五配位。这些复合物是碱性的，顺序是3b> > 1> 2。氧化剂4 [FeIVN4Py(O)]2+ （N4Py: N，N-双（2-吡啶基甲基）-双（2-吡啶基）甲胺）是碱性最低的氧化剂。DFT结果符合实验趋势，并显示了从协同的HAT和PCET反应到协同的异步质子转移(PT) /电子转移（ET）机制，一直到PT的机制谱。TS中沿O—H—X （X= C， N， O）部分的单占轨道清楚地表明，在PCET情况下，电子与质子分开转移。Bell-Evans-Polanyi原理不能解释观察到的反应性模式，正如计算的势垒与反应驱动力图中的散点所证明的那样。然而，TS与各自势垒的变形能量图提供了HAT/PCET二分法的清晰标志。因此，在所有的C-H键激活中，势垒来自于产生TS所需的变形能，而在N-H/O-H键激活中，变形能远远大于相应的势垒，表明TS片段之间存在稳定相互作用。用一个价键模型将观察到的结果与反应物的碱度/酸度联系起来。
We describe herein the hydrogen-atom transfer (HAT)/ proton-coupled electron-transfer (PCET) reactivity for FeIV-oxo and FeIII-oxo complexes (1–4) that activate C-H, N-H, and O-H bonds in 9,10 dihydroanthracene (S1), dimethylformamide (S2), 1,2 diphenylhydrazine (S3), p-methoxyphenol (S4), and 1,4-cyclohexadiene (S5). In 1–3, the iron is pentacoordinated by tris[N'-tert-butylureaylato)-N-ethylene]aminato ([H3buea]3−) or its derivatives. These complexes are basic, in the order 3 >> 1 > 2. Oxidant 4, [FeIVN4Py(O)]2+ (N4Py: N,N-bis(2-pyridylmethyl)-bis(2-pyridyl) methylamine), is the least basic oxidant. The DFT results match experimental trends and exhibit a mechanistic spectrum ranging from concerted HAT and PCET reactions to concerted-asynchronous proton transfer (PT) / electron transfer (ET) mechanisms, all the way to PT. The singly occupied orbital along the O---H---X (X= C, N, O) moiety in the TS shows clearly that in the PCET cases, the electron is transferred separately from the proton. The Bell-Evans-Polanyi principle does not account for the observed reactivity pattern, as evidenced by the scatter in the plot of calculated barrier vs. reactions driving forces. However, a plot of the deformation energy in the TS vs. the respective barrier provides a clear signature of the HAT/PCET dichotomy. Thus, in all C-H bond activations, the barrier derives from the deformation energy required to create the TS, whereas in N-H/O-H bond activations, the deformation energy is much larger than the corresponding barrier, indicating the presence of stabilizing interaction between the TS fragments. A valence bond model is used to link the observed results with the basicity/acidity of the reactants.
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