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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
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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 that the inner-sphere pathway is more dominant. The low reaction barrier of the inner-sphere pathway may be attributed to the formation of the stable methoxide complex, which has been detected in the experiments. In addition, the catalytic performance of Mn-PNP pincer complex was compared with the imidazole-based PNN-ligated manganese pincer complex(Mn-PNN). NBO analysis shows that the charge on HMn—Hatom in Mn-PNN hydride complex is more negative than that in Mn-PNP hydride complex. On the other hand, hydride dissociation energy(HDE) of Mn—H species in the former is smaller than that in the latter. This is attributed to the stronger electron-donating imidazole group, which is beneficial to increase the electron density at the Mn center and hence enhances the nucleophilicity of Mn—H species in Mn-PNN hydride complex. Moreover, the free energy barriers of the heterolytic cleavage of H2and reduction of ethylene carbonate catalyzed by Mn-PNN hydride complex are determined to be 97.2 and 99.5 kJ·mol^(-1), which are 4.8 kJ·mol^(-1)higher and 8.4 kJ·mol^(-1)lower than those in the PNP-ligand system, respectively. This result further indicates that the electron-rich PNN-pincer ligand on the Mn complex is not beneficial to H2activation, but could improve the reactivity of Mn—H species. The present work rationalizes the experimental findings and provides theoretical guide to screen out the efficient organic metal catalysts for the hydrogenation of CO_(2)derivatives.
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)
出处 《分子科学学报》 CAS 北大核心 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)。
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 manganese complex hydrogenation of ethylene carbonate reaction mechanism density functional theory
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