The role of bulky substituents in Brookhart-type Ni(II) diimine catalyzed olefin polymerization: A combined density functional theory and molecular mechanics study
The role of bulky substituents in Brookhart-type Ni(II) diimine catalyzed olefin polymerization: A combined density functional theory and molecular mechanics study
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DOI：
10.1021/ja970226a
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发表时间：
1997-07-02
影响因子：
15
通讯作者：
Ziegler, T
中科院分区：
文献类型：
--
作者：
Deng, LQ;Woo, TK;Ziegler, T
用密度泛函理论、量子力学和分子力学(QM/MM)相结合的模型研究了大分子配体在Ni(II)-二亚胺催化乙烯聚合中的作用。具体地，我们考察了(Arn=C(R)-C(R)=NAR)Ni-II-R‘(+)型的催化中心，其中R=Me，Ar=2，6-C6H3(i-Pr)(2)。Ar基团和R基团用分子力力场处理，其余部分用密度泛函理论处理。用杂化方法研究了链的扩展、链的支化和链的终止过程，得到了双H剑势垒分别为11.8、15.3和18.4千卡/摩尔，这在绝对和相对条件下都与实验符合得很好。这与纯QM模型形成了鲜明的对比，在纯QM模型中，忽略了大体积Ar和R基团的影响，甚至没有再现势垒的既定顺序。大体积取代基所起的作用具有双重性。首先，Ar和R基团在空间上阻碍了Ni中心的轴向配位。这对占据两个轴向位置的静止态和终止过渡态具有最显著的不稳定效应。除了空间因素外，我们还发现芳基环与二亚胺环以共面方式定向的电子择优导致插入过渡态相对于休止态稳定。与“裸露”的纯QM模型系统相比，这两个因素都起到了降低传播势垒和增加终止势垒的作用。
The role of the bulky ligands in Ni(II) diimine catalyzed ethylene polymerization has been examined with a combined density functional theory quantum mechanics and molecular mechanics (QM/MM) model. Specifically, we have examined the catalytic center of the type (ArN=C(R)-C(R)=NAr)Ni-II-R'(+), where R = Me and Ar = 2,6-C6H3(i-Pr)(2). The Ar and R groups were treated by a molecular mechanics force field while density functional theory was applied to the remainder of the system. The chain propagation, chain branching, and chain termination processes have been investigated with the hybrid method and found to have barriers of Delta H-double dagger = 11.8, 15.3, and 18.4 kcal/mol, respectively, which is in excellent agreement with experiment in both absolute and relative terms. This is in stark contrast to the pure QM model in which the influence of the bulky Ar and R groups was neglected and the established order of the barriers is not even reproduced. The role played by the bulky substituents is dual in nature. First, the Ar and R groups act to sterically hinder the axial coordination sites of the Ni center. This has the most dramatic destabilizing effect on the resting state and termination transition states, in which both axial positions are occupied. In addition to the steric factor, we find that the electronic preference for the aryl rings to orient themselves in a coplanar fashion with the diimine ring results in a stabilization of the insertion transition state relative to the resting state. These two factors act to both lower the propagation barrier and increase the termination barrier compared to the ''naked'' pure QM model system.