Heterometallic cooperativity in divalent metal ProPhenol catalysts: combining zinc with magnesium or calcium for cyclic ester ring-opening polymerisation
Heterometallic cooperativity in divalent metal ProPhenol catalysts: combining zinc with magnesium or calcium for cyclic ester ring-opening polymerisation
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DOI：
10.1039/d1cy01914g
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发表时间：
2021-11-16
影响因子：
5
通讯作者：
Garden, Jennifer A.
中科院分区：
文献类型：
--
作者：
Gruszka, Weronika;Sha, Haopeng;Garden, Jennifer A.
同质双金属锌配合物在丙交酯 (LA) 开环聚合 (ROP) 中表现出极高的活性。相比之下，将锌与其他二价异金属（例如镁和钙）结合的异双金属 ROP 催化剂是未知的，尽管异金属协同作用导致与其他异质组合的催化剂性能得到改善。在这里，我们报告了第一个仅具有二价金属的异双金属 LA ROP 催化剂，通过用 Mg 或 Ca 系统地取代双 Zn ProPhenol 络合物中的一个 Zn 中心。 Mg/Zn 和 Ca/Zn 配合物具有高活性，并且优于 rac-LA 和 ε-己内酯 (ε-CL) ROP 中的同质双金属配合物。 Ca/Zn复合物表现出最高的活性，转化为48当量。 rac-LA 在 5 秒和 380 eq 内完成。 epsilon-CL 在室温下 30 秒内完成。活性的增强归功于路易斯酸性镁和钙增强单体配位以及通过增加 Zn-Et 引发基团的极性来改变催化剂电子。异双金属催化剂还在 rac-LA ROP 中表现出不寻常的两步动力学图，这归因于初始催化剂偏好 rac-LA 立体异构体之一的配位和插入。
Homobimetallic zinc complexes have shown extremely high activity in lactide (LA) ring-opening polymerisation (ROP). In contrast, heterobimetallic ROP catalysts combining zinc with other divalent heterometals such as Mg and Ca are unknown, despite heterometallic cooperativity leading to improved catalyst performance with other heterocombinations. Here, we report the first heterobimetallic LA ROP catalysts featuring only divalent metals, by systematically replacing one Zn centre in a bis-Zn ProPhenol complex with Mg or Ca. The Mg/Zn and Ca/Zn complexes are highly active and outperform their homobimetallic counterparts in rac-LA and epsilon-caprolactone (epsilon-CL) ROP. The Ca/Zn complex exhibits the highest activity, converting 48 eq. rac-LA in 5 s and 380 eq. epsilon-CL in 30 s at room temperature. The activity enhancements are credited to Lewis acidic Mg and Ca enhancing monomer coordination as well as altering the catalyst electronics by increasing the polarity of the Zn-Et initiating group. The heterobimetallic catalysts also display unusual two-step kinetic plots in rac-LA ROP, which is attributed to the initial catalyst preference for the coordination and insertion of one of the rac-LA stereoisomers.