Mn-PNP催化碳酸亚乙酯氢化制甲醇的机理研究  

Study on the mechanism for the hydrogenation of ethylene carbonate to methanol catalyzed by a Mn-PNP pincer complex

作　　者：李婧婧 阳园 王金昭 LI Jing-jing;YANG Yuan;WANG Jin-zhao(Basic Courses Department,Shanxi Agricultural University,Jinzhong 030801,China;Institute of Theoretical Chemistry,Shandong University,Jinan 250100,China)

机构地区：[1]山西农业大学基础部,山西晋中030801 [2]山东大学理论与计算化学研究所,山东济南250100

出　　处：《分子科学学报》2023年第1期49-59,共11页Journal of Molecular Science

基　　金：山西农业大学博士科研启动项目(2017YJ37)。

摘　　要：催化氢化CO_(2)及其衍生物转化为高值化学品甲醇符合“绿色化学”的理念,是CO_(2)资源利用中最具应用前景的方法之一.本文基于密度泛函理论系统阐明了PNP配位的金属Mn配合物催化CO_(2)衍生物——碳酸亚乙酯氢化的反应机制.计算结果表明,Mn-PNP首先与氢气反应生成活性催化剂,接着碳酸亚乙酯在Mn—H和N—H物种的协助下经历3次氢化过程得到产物甲醇和副产物乙二醇,其中可能涉及outer-sphere和inner-sphere 2种反应机理,后者的总反应势垒为107.9 kJ·mol^(-1),比前者低了27.0 kJ·mol^(-1).催化氢化反应沿着inner-sphere路径进行更有利,这可能归因于inner-sphere路径有稳定中间体CH3O—Mn配合物的形成.进一步探究了配体的取代基效应对金属Mn配合物催化活性的影响,证实强供电子能力的咪唑基团能够增加金属Mn中心的电子密度,一方面不利于Mn—H成键,另一方面可以提高Mn—H物种的亲核性.研究结果为筛选有效催化CO_(2)衍生物氢化的有机金属催化剂提供一定的理论指导.Catalytic hydrogenation of carbon dioxide(CO_(2)) and its derivatives into high value chemicals is one of the most promising ways to use CO_(2)resources, which conforms to the concept of “green chemistry”. Choosing the [Mn(CO)2N(C2H4PiPr2)2](Mn-PNP pincer complex) as a representative of a non-noble metal catalyst, the catalytic mechanism for the hydrogenation of ethylene carbonate, which is one of CO_(2)derivatives were systematically studied via density functional theory(DFT) calculations at the M06/6-31+G(d, p)/LanL2DZ level. Further single-point energies were calculated using the 6-311+G(2d, 2p)/lanL2TZ(f).The outer-sphere pathway proposed in the previous experimental work has been examined, and the calculated free energy barrier of the bottleneck step is as high as 134.9 kJ·mol^(-1), which seems not to completely rationalize the experimental findings. Alternatively, an optimized inner-sphere mechanism was proposed by performing an exhaustive search on the potential energy surface. The inner-sphere pathway can be divided into four main steps:(1) the pre-catalyst reacts with hydrogen to generate an active catalyst;(2) hydride transfer from the active catalyst to the carbonyl group of ethylene carbonate results in the formation of the intermediate with a Mn—O unit followed by subsequent proton transfer from N—H unit to the adjacent epoxide group produces the chain intermediate 2-hydroxyethyl formate;(3) nucleophilic attack the carbonyl group of the 2-hydroxyethyl formate by Mn-bound hydride leads to another intermediate with a Mn—O unit which upon proton transfer liberates ethylene glycol and formaldehyde;(4) formaldehyde undergoes a similar hydrogenation step to afford the methoxide complex which then releases methanol and regenerates the pre-catalyst through a similar protonation step. The rate-determining step is the reduction of ethylene carbonate with an activation barrier of 107.9 kJ·mol^(-1), which is substantially lower than the overall barrier involved in outer-sphere pathway, indicating tha

关 键 词：金属锰配合物 碳酸亚乙酯氢化 反应机理 密度泛函理论 

分 类 号：O643[理学—物理化学]