Jiang Meng-Xu, Liu Chun-Guang
College of Chemical Engineering, Northeast Dianli University, Jilin City, 132012, PR China.
College of Chemical Engineering, Northeast Dianli University, Jilin City, 132012, PR China.
J Mol Graph Model. 2017 May;73:8-17. doi: 10.1016/j.jmgm.2016.12.012. Epub 2016 Dec 21.
Although epoxidation of alkenes by NO catalyzed by Mn-substituted polyoxometalates (POMs) has been studied both experimental and theoretical methods, a complete catalytic cycle has not been established currently. In the present paper, density functional theory (DFT) calculations were employed to explore possible reaction mechanism about this catalytic cycle. Our DFT studies reveal that the reaction pathway starts from a low-valent Keggin-type POM aquametal derivative [PWOMnHO]. In the presence of NO pressure, the formation of the active catalytic species [PWOMnO] involves a ligand-substituted reaction about replacement of the aqua ligand with NO to generation of POM/NO adduct [PWOMnON] and dissociation of N from this adduct. The calculated free energy indicates that the ligand-substituted reaction is endergonic both in gas phase or various solvents. The partial optimization method reveals that the dissociation of N from [PWOMnON] involves crossing of the quintet state with a low-lying triplet state. Due to the high reactivity, the high-valent Mn-oxo species, [PWOMnO], may react with the excess NO and alkenes. Thus, two alternative reaction pathways corresponding to activation of NO and epoxidation of alkenes have been considered in this work. The calculated free energy profile indicates that epoxidation of alkenes pathway is the favorable routes. Finally, a complete catalytic cycle for this reaction has been proposed. The rate-determining step in this catalytic cycle is the dissociation of N from the low-valent POM/NO adduct according to our DFT-M06L calculations.
尽管通过实验和理论方法对锰取代的多金属氧酸盐（POMs）催化一氧化氮（NO）使烯烃环氧化进行了研究，但目前尚未建立完整的催化循环。在本文中，采用密度泛函理论（DFT）计算来探索该催化循环可能的反应机理。我们的DFT研究表明，反应途径始于低价Keggin型POM水合金属衍生物[PWOMnHO]。在有NO压力的情况下，活性催化物种[PWOMnO]的形成涉及一个配体取代反应，即用水合配体被NO取代，生成POM/NO加合物[PWOMnON]，以及该加合物中N的解离。计算得到的自由能表明，配体取代反应在气相或各种溶剂中都是吸热的。部分优化方法表明，[PWOMnON]中N的解离涉及五重态与低能三重态的交叉。由于高反应活性，高价锰氧物种[PWOMnO]可能与过量的NO和烯烃反应。因此，在这项工作中考虑了两条分别对应于NO活化和烯烃环氧化的替代反应途径。计算得到的自由能剖面图表明，烯烃环氧化途径是有利的途径。最后，提出了该反应的完整催化循环。根据我们的DFT-M06L计算，该催化循环中的速率决定步骤是低价POM/NO加合物中N的解离。
