What Triggered the Switching from Ethylene-Selective Trimerization into Tetramerization over the Cr/(2,2′-Dipicolylamine) Catalysts?,ACS Catalysis

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What Triggered the Switching from Ethylene-Selective Trimerization into Tetramerization over the Cr/(2,2′-Dipicolylamine) Catalysts?
ACS Catalysis ( IF 11.3 ) Pub Date : 2019-10-22 , DOI: 10.1021/acscatal.9b03340
Lin Liu ₁ , Zhen Liu ₁ , Siyang Tang ₂ , Ruihua Cheng ₁ , Xuelian He ₁ , Boping Liu ₃

Affiliation

The mechanism of switching from ethylene-selective trimerization into tetramerization catalyzed by Cr/(2,2′-dipicolylamine) catalysts has been investigated using a combined density functional theory and a high-level ab initio DLPNO methods. The oxidation states of metal, the steric position of the ligand, and the position of the methyl substituents in the pyridine rings were fully explored to unravel the key factors that trigger the switchable selectivity. The calculations support the metallacycle mechanism for ethylene tri-/tetramerization with the Cr(I)/Cr(III) catalytic cycle. Conformational analysis shows that the conformers of the metallacycloheptane and metallacyclononane intermediates are of crucial importance in understanding the selectivity of ethylene tri-/tetramerization. The out-of-plane models are responsible for ethylene trimerization and tetramerization. NBO charge analysis indicates that the lower charge at the Cr center has a positive effect on the metallacycle expansion. The steric position of the ligand and the chromacycle, and the methyl substituent in the proximal ortho or the distal para position on the aromatic rings significantly changes the energy of two key transition states for metallacycloheptane decomposition and metallacycloheptane expansion, thereby triggering the switchable selectivity between ethylene trimerization and tetramerization.

中文翻译：

是什么引发了在Cr /（2,2'-Dipicolylamine）催化剂上从乙烯选择性三聚向四聚的转变？

使用组合密度泛函理论和高级从头算DLPNO方法研究了由Cr /（2,2'-二甲基吡啶胺）催化剂催化的乙烯选择性三聚转化为四聚反应的机理。充分探索了金属的氧化态，配体的空间位置以及吡啶环中甲基取代基的位置，以揭示触发可切换选择性的关键因素。该计算支持具有Cr（I）/ Cr（III）催化循环的乙烯三/四聚的金属环化机理。构象分析表明，金属环庚烷和金属环壬烷中间体的构象异构体对于理解乙烯三-/四聚的选择性至关重要。平面外模型负责乙烯三聚和四聚。NBO电荷分析表明，Cr中心的较低电荷对金属环的膨胀有积极影响。配体和色环的空间位置以及芳环上邻位或远端对位的甲基取代基显着改变了金属环庚烷分解和金属环庚烷膨胀的两个关键过渡态的能量，从而触发了乙烯之间的可切换选择性三聚和四聚。

更新日期：2019-10-23

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