Ab Initio Theoretical Study on Ethylene Polymerization with Homogeneous Silylene-Bridged Group 4 Metallocene Catalysts. Ethylene Insertion and .beta.-Elimination
Ab Initio Theoretical Study on Ethylene Polymerization with Homogeneous Silylene-Bridged Group 4 Metallocene Catalysts. Ethylene Insertion and .beta.-Elimination
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均质甲硅烷基桥联 4 族茂金属催化剂乙烯聚合的从头理论研究。
DOI：
10.1021/om00002a024
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发表时间：
1995
期刊：
影响因子：
2.8
通讯作者：
K. Morokuma
中科院分区：
文献类型：
--
作者：
Tohru Yoshida;N. Koga;K. Morokuma
Ti，Zr，Hf）的分子轨道从头算方法，重点研究了两个重要的反应：乙烯重复插入金属-烷基键沿着Cossee的直接插入机理和β-消除，被认为是一个终止或链转移步骤.在RHF水平上对所有的驻点结构进行了优化（对Zr的一些驻点结构也在RMP 2水平上进行了优化），并在RQCISD水平上进行了能量计算。乙烯插入到M-Me键中是通过一个四中心过渡态进行的，其活化能为7-10 kcal/mol，导致动力学产物具有约30 kcal/mol的双极性丙基配体。在Ti和其它金属之间的反应能量有明显的差异，Ti是放热最少的，与插入活性的实验相对顺序一致：Zr> Hf> Ti。Ti的这种特殊性被认为是尺寸效应的结果，Ti的小半径使其过渡态和中间体比其它金属的过渡态和中间体更紧凑、更拥挤。β-正丙基产物异构化为更稳定的β-正丙基络合物需要高活化能，特别是对于Hf，这是与产生具有更高分子量的聚合物的能力相关的因素。3-从β-Agostic丙基络合物中消除是吸热的，约为50 kcal/mol，并且难以进行。此外，还与最近的理论计算作了比较，这些计算没有给出势垒。
Ti, Zr, Hf) has been studied by the ab initio MO method, with focus on two important reactions: the repetitive insertion of ethylene into the metal—alkyl bond along Cossee’s direct insertion mechanism and the/3-elimination, considered to be a termination or chain-transfer step. All stationary point structures are optimized at theRHF level (some for Zr at the RMP2 level as well), and the energetics is calculated at the RQCISD level. The insertion of ethylene into the M-Me bond proceeds through a four-centered transition state with a small activation energy of 7-10 kcal/mol, leading to the kinetic product having they-agostic propyl ligand with an exothermicity of about 30 kcal/mol. There is a clear difference in energy of reaction between Ti and the other metals, Ti being the least exothermic, consistent with the experimental relative order of insertion activity: Zr> Hf> Ti. The peculiarity of Ti hasbeen attributed to the size effect; thesmall radius of Ti makes its Ti transition states andintermediates more compact and crowded than those of the other metals. Isomerization of they-agostic product to the more stable/3-agostic propyl complex requires a high activation energy, especially for Hf, a factor related to the ability to produce polymer with higher molecular weight./3-Elimination from the/3-agostic propyl complexis endothermic by about 50 kcal/mol and takes place withdifficulty. In addition, comparison has been made with recent theoretical calculations which gave no barrier.