Insights into the Mechanism of Carbon Dioxide and Propylene Oxide Ring-Opening Copolymerization Using a Co(III)/K(I) Heterodinuclear Catalyst.
Insights into the Mechanism of Carbon Dioxide and Propylene Oxide Ring-Opening Copolymerization Using a Co(III)/K(I) Heterodinuclear Catalyst.
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DOI：
10.1021/jacs.2c06921
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发表时间：
2022-10-05
影响因子：
15
通讯作者：
Williams, Charlotte K.
中科院分区：
文献类型：
--
作者：
Deacy, Arron C.;Phanopoulos, Andreas;Lindeboom, Wouter;Buchard, Antoine;Williams, Charlotte K.
用Co(III)K(I)异双核配合物()对二氧化碳和环氧丙烷开环共聚的催化循环进行了理论和实验相结合的研究。，(()，)，−)。该络合物是一种罕见的双核催化剂，对二氧化碳和大规模商业产品环氧丙烷的共聚反应具有活性。了解产物和副产物的形成机理对于合理改进催化剂是必不可少的，但很少有其他使用这些单体的机理研究。研究表明，钴既起到活化环氧丙烷的作用，又起到稳定催化中间体的作用，而钾则提供了一种瞬时的碳酸盐亲核试剂，使活化的环氧丙烷开环。密度泛函理论(DFT)计算表明，金属的反转作用具有难以接近的高能垒，在实验条件下不太可能发生。速率决定步骤计算为环氧丙烷的开环(ΔGcalc†=+22.2kcal mol-1)，与实验测量结果(ΔGexp†=+22.1kcal moL-1，50°C)一致。计算出的选择性限制步骤的势垒，即从醇盐中间体反咬生成碳酸亚丙酯(ΔGcalc†=+21.4kcalm ol-1)，与环氧化物开环的势垒(ΔGcalc†=+22.2kcal m ol-1)竞争，暗示了醇盐和碳酸盐中间体之间的平衡。这一想法经过了实验验证，并由二氧化碳压力或温度控制，以适中选择性。在理论和实验研究的支持下，催化机理将有助于指导未来的催化剂设计和优化。
A combined computational and experimental investigation into the catalytic cycle of carbon dioxide and propylene oxide ring-opening copolymerization is presented using a Co(III)K(I) heterodinuclear complex (. , ( (), ), −). The complex is a rare example of a dinuclear catalyst, which is active for the copolymerization of CO2 and propylene oxide, a large-scale commercial product. Understanding the mechanisms for both product and byproduct formation is essential for rational catalyst improvements, but there are very few other mechanistic studies using these monomers. The investigation suggests that cobalt serves both to activate propylene oxide and to stabilize the catalytic intermediates, while potassium provides a transient carbonate nucleophile that ring-opens the activated propylene oxide. Density functional theory (DFT) calculations indicate that reverse roles for the metals have inaccessibly high energy barriers and are unlikely to occur under experimental conditions. The rate-determining step is calculated as the ring opening of the propylene oxide (ΔGcalc† = +22.2 kcal mol–1); consistent with experimental measurements (ΔGexp† = +22.1 kcal mol–1, 50 °C). The calculated barrier to the selectivity limiting step, i.e., backbiting from the alkoxide intermediate to form propylene carbonate (ΔGcalc† = +21.4 kcal mol–1), is competitive with the barrier to epoxide ring opening (ΔGcalc† = +22.2 kcal mol–1) implicating an equilibrium between alkoxide and carbonate intermediates. This idea is tested experimentally and is controlled by carbon dioxide pressure or temperature to moderate selectivity. The catalytic mechanism, supported by theoretical and experimental investigations, should help to guide future catalyst design and optimization.
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