电子自旋交叉对铬系催化乙烯聚合/齐聚反应影响的实验与理论模拟研究

Spin crossover and its effects on the catalytic reactivity of Cr-catalyzed ethylene coordinative oligo-/polymerization still remain as one of the key problems.Industrial Phillips CrOx/SiO2 ethylene polymerization catalyst and Cr(2-ethylhexanoate)3/2,5-dimethylpyrrole/Et3Al/Et2AlCl trimerization catalyst will be considered in this work. The objective of this project is to elucidate the plausible mechanism of spin crossover reaction in the Cr-catalyzed ethylene oligo-/polymerization using the multiple techniques by combining experiments and computational molecular modeling. By introducing a magnetic field during ethylene oligo-/polymerization reactions, we could possibly control or change the spin state of the chromium center and thus the effects of the magnetic field on the catalytic reactions could be evaluated. Futhermore, we are trying to unravel the effects of spin block and/or spin acceleration on the catalyst reactions. Meanwhile, the dynamic information of the minium-energy crossing point, which connects the two adjacent poententical energy surfaces, could be obtained at molecular level through a combination of the characterizations using paramagnetic solid state NMR, X-ray absorption fine structure (EXAFS) and Raman spectroscopies as well as molecular modeling using DFT calculations and ab initio molecular dynamics simulation. Thus, the profound mechanistic understanding could be achieved on the spin-crossover mechanisms for the Cr-catalyzed ethylene oligo-/polymerization reactions. The successful implementation of this project will be possible to achieve breakthough in the field of Cr-catalyzed coordination oligo-/polymerization theory and thus to guide the further development of new Cr-based catalysts with better performance for ethylene oligo-/polymerizations.

电子自旋交叉及其对催化反应的影响是铬系乙烯配位聚合与齐聚催化反应中仍未解明的重要难题。本课题将采用工业Phillips无机铬系CrOx/SiO2聚乙烯催化剂和有机铬系乙烯三聚催化剂体系，通过采用实验与理论模拟计算相结合的方法阐明电子自旋交叉及其对催化反应影响的分子机理。实验研究拟考察外加磁场对铬系催化乙烯聚合、齐聚反应中的电子自旋交叉现象及其对催化反应的影响规律，同时结合多种现代光谱表征手段，对催化反应过程中的活性物种进行表征和原位跟踪；理论模拟计算拟采用密度泛函理论和从头算分子动力学方法，力争在分子原子水平捕获电子自旋交叉的动态信息，实现对铬系催化乙烯聚合、齐聚反应体系中自旋交叉机理的深刻认识。最后，基于实验与计算结果的组合分析获得机理认识的新突破。本项目的实施将实现烯烃聚合和齐聚反应中有关电子自旋交叉及其对催化反应影响机理的新认识，为新型高效乙烯聚合及齐聚催化剂的开发提供理论指导。

本项目研究了i) 乙烯三聚中的自旋交叉以及半稳定配体的作用；ii) 乙烯四聚中的自旋交叉以及三聚、四聚转换机理；iii）乙烯共三聚中的自旋交叉，及C10烯烃的形成机理；iv) 基于超快时间分辨红外光谱，捕获Fe，V，Mn过渡金属体系中三重态物种的转化信号，结合理论计算探讨自旋交叉反应的动力学规律。主要研究结果及关键数据如下：.1）铬/吡咯体系的活性中心为中性模型。乙烯三聚反应遵循Cr(I/III)催化循环（∆∆G‡ = 19.0 kcal/mol），并发生一次势能面交叉。配体中的垂坠氯原子与铬中心之间存在缔合-解离的相互作用。采用AlMe3基团代替AlMe2Cl基团，可以得到活化能更低的反应路径，预示该催化体系可以向不含卤素的绿色催化体系发展。.2）铬系(2-C5H4N)2NR-Cr的活性中心为二价铬单核中心B和D。反应在单乙烯-铬处发生势能面交叉，降低了反应能垒。模型B上发生乙烯四聚反应，其速率控制步骤为铬金属七元环向九元环的环增长步骤（∆∆G‡ = 15.4 kcal/mol），模型D上发生乙烯三聚反应，其速率控制步骤为铬金属五元环向七元环的环增长步骤（∆∆G‡ = 18.6 kcal/mol）。当配体中双吡啶环中取代基从氮原子对位移到邻位，催化体系从乙烯四聚向乙烯三聚发生了转化，与实验结果一致。.3）铬系Cr-PNPOMe催化乙烯与1-己烯共齐聚反应，遵循金属环状反应机理。癸烯的生成遵循铬金属七元环上agostic-assisted β-H还原消去一步法。通过计算各反应路径的TOF值，得到产物分布比例为：0.2:74.7:25.1，与实验结果基本一致。乙烯三聚及共齐聚体系自旋态的转变，降低了速率决定步骤的活化能，加快了反应的进行。.3）Fe(CO)4, CpMn(CO)2 和CpV(CO)3三种络合物在三重态基态下的保留时间分别为140 ns、17 ns和350 ns。体系发生自旋交叉的过程主要取决于Fe(CO)4, CpMn(CO)2 和CpV(CO)3三种络合物从三重态向一重态的转变，在B3LYP**水平下计算得到的反应速率常数分别为9.13 × 106 s-1、2.68 × 108 s-1、1.17 × 108 s-1，这与实验测得值7.14 × 107 s-1、5.88 × 108 s-1、2.86 × 107 s-1之间的偏差均在一个数量级以内。