The preference of open chain of growing macromolecule vs. possible cyclic form was examined for the bifunctional cobalt(III)-salen catalyst for the copolymerization of CO2 with epoxides. A variety of possible isomers was considered (resulting from trans/cis-β salen arrangement, different mutual orientation of quaternary ammonium-chains, and possible binding modes). To explore the conformational space, a combined approach was applied, utilizing semiempirical (PM7) MD and the DFT calculations. The preference of the open and cyclic macromolecules attached to the metal center was compared with the corresponding results for isolated model macromolecules, and the systems built of the macromolecule interacting with the tetra-butyl ammonium cation. Result shows that the cyclic structures are strongly preferred for isolated ions, with relatively low cyclization barriers. In the field of positive point charge, the open structures are strongly preferred. For the ions interacting with tetrabutyl ammonium cation, the cyclic structures are preferred, due to delocalization of the positive charge in the cation. For the complexes involving model and "real" Co(III)-salen catalysts, the open structures are strongly preferred. The possible cyclization by dissociation of alkoxide and its transfer to the neighborhood of quaternary ammonium cation is characterized by high activation barriers. Further, the transfer of alkoxide from the metal center to the cation is less likely than the transfer of carbonate, since the metal-alkoxide bond-energy energy is much stronger than energy of metal-carbonate bonding, as shown by ETS-NOCV results. The conclusions are in qualitative agreement with experimental data showing high selectivity towards copolymer formation in the copolymerization processes catalyzed by bifunctional Co(III) salen-complexes. Graphical abstract.

中文翻译：

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钴（III）-salen双功能催化剂在CO2 /环氧共聚中开放聚合物相对于环状产物的偏爱的理论研究。

研究了用于CO2与环氧化物共聚的双官能钴（III）-salen催化剂对正在生长的大分子相对于可能的环状形式的开链的偏爱。考虑了多种可能的异构体（由于反式/顺式-βSalen排列，季铵链的互不相同的取向以及可能的结合方式）。为了探索构象空间，使用了组合方法，利用了半经验（PM7）MD和DFT计算。将与金属中心相连的开环和环状大分子的偏好与分离的模型大分子的相应结果进行了比较，并比较了由大分子与四丁基铵阳离子相互作用的系统。结果表明，环状结构非常适合分离离子，具有相对较低的环化障碍。在正电荷的领域中，强烈优选开放结构。对于与四丁基铵阳离子相互作用的离子，由于正电荷在阳离子中的离域，因此优选环状结构。对于涉及模型和“真实” Co（III）-salen催化剂的配合物，强烈优选开放结构。醇盐解离并转移到季铵阳离子附近可能导致的环化反应具有很高的活化势垒。此外，如ETS-NOCV结果所示，因为金属-醇盐键的能量比金属-碳酸盐键的能量强得多，所以醇盐从金属中心到阳离子的转移比碳酸盐的转移更不可能。该结论与实验数据在质量上吻合，该实验数据显示了在双官能Co（III）salen-络合物催化的共聚过程中对共聚物形成的高选择性。图形概要。