Singlet versus Triplet Reactivity in an Mn(V)-Oxo Species: Testing Theoretical Predictions Against Experimental Evidence.
Singlet versus Triplet Reactivity in an Mn(V)-Oxo Species: Testing Theoretical Predictions Against Experimental Evidence.
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DOI：
10.1021/jacs.6b05027
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发表时间：
2016-09-28
影响因子：
15
通讯作者：
de Visser SP
中科院分区：
文献类型：
--
作者：
Yang T;Quesne MG;Neu HM;Cantú Reinhard FG;Goldberg DP;de Visser SP
识别控制高价金属氧物种的反应活性的因素对于理解金属酶的反应活性和相关的过渡金属催化剂都是至关重要的。计算研究表明，一些金属氧物种中激发的高自旋态可以为氧化反应提供较低的能垒，从而得出结论，这种未被观察到的高自旋态络合物应该被认为是活性氧化剂。然而，通过实验来验证这些计算预测是困难的，而且很少被实现。在这里，我们对高价锰(V)-氧配合物与对-Z-取代的硫代苯甲醚的反应活性中自旋态的作用进行了详细的计算研究，并利用实验证据来区分理论结果。计算表明，与对-Z取代基的给电子性质相关的主要单态自旋态发生了反常的机理变化，而三重态自旋态则没有观察到这种变化。最小能量交叉点计算预测了较小的自旋-轨道耦合常数，使得自旋态不太可能从低自旋转变为高自旋。对位取代的硫代苯甲醚衍生物的反应性趋势为研究锰-氧-腐拉津配合物的自旋态反应性提供了一种实验方法。因此，计算表明，V形Hammett图是由单重态表面重现的，而不是由三重态趋势重现的。用价键模型解释了取代基效应，证实了通过取代基的改变从亲电机理转变为亲核机理。
Discerning the factors that control the reactivity of high-valent metal–oxo species is critical to both an understanding of metalloenzyme reactivity and related transition metal catalysts. Computational studies have suggested that an excited higher spin state in a number of metal–oxo species can provide a lower energy barrier for oxidation reactions, leading to the conclusion that this unobserved higher spin state complex should be considered as the active oxidant. However, testing these computational predictions by experiment is difficult and has rarely been accomplished. Herein, we describe a detailed computational study on the role of spin state in the reactivity of a high-valent manganese(V)–oxo complex with para-Z-substituted thioanisoles and utilize experimental evidence to distinguish between the theoretical results. The calculations show an unusual change in mechanism occurs for the dominant singlet spin state that correlates with the electron-donating property of the para-Z substituent, while this change is not observed on the triplet spin state. Minimum energy crossing point calculations predict small spin–orbit coupling constants making the spin state change from low spin to high spin unlikely. The trends in reactivity for the para-Z-substituted thioanisole derivatives provide an experimental measure for the spin state reactivity in manganese–oxo corrolazine complexes. Hence, the calculations show that the V-shaped Hammett plot is reproduced by the singlet surface but not by the triplet state trend. The substituent effect is explained with valence bond models, which confirm a change from an electrophilic to a nucleophilic mechanism through a change of substituent.