Experimental and Computational Investigation of the Mechanism of Carbon Dioxide/Cyclohexene Oxide Copolymerization Using a Dizinc Catalyst
Experimental and Computational Investigation of the Mechanism of Carbon Dioxide/Cyclohexene Oxide Copolymerization Using a Dizinc Catalyst
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DOI：
10.1021/ma300803b
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发表时间：
2012-09-11
期刊：
影响因子：
5.5
通讯作者：
Williams, Charlotte K.
中科院分区：
文献类型：
--
作者：
Buchard, Antoine;Jutz, Fabian;Williams, Charlotte K.
详细研究了二锌催化剂共聚环己烯氧化物和二氧化碳的机理。催化剂，先前由 Williams 等人发表。 (Angew. Chem. Int. Ed. 2009, 48, 931)，仅在 1 bar 的 CO2 压力下就显示出高活性。这项工作应用原位衰减全反射红外光谱（ATR-FTIR）来研究催化剂与环己烯氧化物反应以及随后与二氧化碳反应时结构的变化。使用具有溶剂化校正的 DFT 进行计算研究，用于计算由该双核配合物催化的交替共聚循环中各种过渡态和中间体的相对自由能。还研究了两种潜在的竞争副反应，连续环氧化物链和连续二氧化碳链。这两个副反应在热力学上被证明是不利的，合理化了使用 1 的实验研究中表现出的高选择性。此外，DFT 计算表明，速率决定步骤是锌键合碳酸酯基团对配位环氧化物分子的亲核攻击，与之前的实验结果一致（Delta Delta G(353) = 23.5 kcal/mol；Delta G(353)双匕首 = 25.7千卡/摩尔）。原位光谱学和 DFT 计算均表明每个二锌催化剂分子仅引发一条聚合物链。该催化剂采用“碗”形构象，配位在凹面上的乙酸基为旁观配体，配位在凸面上的乙酸基为引发基团。旁观者羧酸根基团在催化循环中发挥着重要作用，平衡反面的链增长。 DFT 用于预测两种新型催化剂的活性，观察到实验周转数与 DFT 预测之间具有良好的一致性。
A detailed study of the mechanism by which a dizinc catalyst copolymerizes cyclohexene oxide and carbon dioxide is presented. The catalyst, previously published by Williams et al. (Angew. Chem. Int. Ed. 2009, 48, 931), shows high activity under just 1 bar pressure of CO2. This work applies in situ attenuated total reflectance infrared spectroscopy (ATR-FTIR) to study changes to the catalyst structure on reaction with cyclohexene oxide and, subsequently, with carbon dioxide. A computational investigation, using DFT with solvation corrections, is used to calculate the relative free energies for various transition states and intermediates in the cycle for alternating copolymerization catalyzed by this dinuclear complex. Two potentially competing side reactions, sequential epoxide enchainment and sequential carbon dioxide enchainment are also investigated. The two side-reactions are shown to be thermodynamically disfavored, rationalizing the high selectivity exhibited in experimental studies using 1. Furthermore, the DFT calculations show that the rate-determining step is the nucleophilic attack of the coordinated epoxide molecule by the zinc-bound carbonate group in line with previous experimental findings (Delta Delta G(353) = 23.5 kcal/mol; Delta G(353)double dagger = 25.7 kcal/mol). Both in situ spectroscopy and DFT calculations indicate that just one polymer chain is initiated per dizinc catalyst molecule. The catalyst adopts a "bowl" shape conformation, whereby the acetate group coordinated on the concave face is a spectator ligand while that coordinated on the convex face is the initiating group. The spectator carboxylate group plays an important role in the catalytic cycle, counter-balancing chain growth on the opposite face. The DFT was used to predict the activities of two new catalysts, good agreement between experimental turn-over-numbers and DFT predictions were observed.