Department of Chemistry, The Johns Hopkins University , Baltimore, Maryland 21218, United States.
Manchester Institute of Biotechnology and School of Chemical Engineering and Analytical Science, The University of Manchester , 131 Princess Street, Manchester M1 7DN, United Kingdom.
J Am Chem Soc. 2016 Sep 28;138(38):12375-86. doi: 10.1021/jacs.6b05027. Epub 2016 Sep 14.
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.
辨别控制高价金属氧物种反应性的因素对于理解金属酶反应性和相关过渡金属催化剂至关重要。计算研究表明，在许多金属氧物种中，激发的高自旋态可以为氧化反应提供更低的能垒，从而得出结论，应该将这种未观察到的高自旋态配合物视为活性氧化剂。然而，通过实验来检验这些计算预测是困难的，并且很少实现。在此，我们描述了一项关于自旋态在高价锰(V)-氧配合物与对-Z 取代的硫代苯甲醚反应性中的作用的详细计算研究，并利用实验证据来区分理论结果。计算表明，对于主要的单重态自旋态，发生了一种不寻常的机制变化，这与对-Z 取代基的供电子性质相关，而三重态自旋态则没有观察到这种变化。最低能量交叉点计算预测自旋轨道耦合常数较小，使得低自旋到高自旋的自旋态变化不太可能发生。对-Z 取代的硫代苯甲醚衍生物的反应性趋势为锰-氧咯嗪配合物中的自旋态反应性提供了实验测量。因此，计算表明，V 形哈米特图是由单重态表面重现的，但三重态趋势则不然。取代基效应用价键模型解释，该模型通过取代基的变化证实了从亲电机制到亲核机制的转变。
