Insight into the Mechanism of Thermal Stability of α‑Diimine Nickel Complex in Catalyzing Ethylene Polymerization

The union of experimental and computational methods can accelerate the development of polymerization catalysts for industrial applications. Herein, we report complementary experimental and computational studies of the thermal stability of α-diimine nickel complexes by using thermally stable Cat. 1 and a typical Brookhart catalyst (B-Cat) as models. Experimentally, we found that many more nickel atoms could be activated for Cat. 1 at elevated temperature during the ethylene polymerization process compared to those for B-Cat. Computationally, first-principle calculations showed that the decomposition energies of Cat. 1 were found to be higher than those of B-Cat, contributing to the activation observed for Cat. 1. We found that the presence of ethydene evidently affected the conformation of C1–N1–Ni–N2–C2 five-membered ring (where the nickel center is located) of Cat. 1, turning the envelope conformation (B-Cat) into a half-chair conformation (Cat. 1). According to calculations, the decomposition energy of the latter was 17.4 kJ/mol higher than that of the former. These results provide information to elucidate the mechanism of thermal stability of α-diimine nickel catalyst and significantly advance the development of thermally stable α-diimine nickel catalysts used in industry.

实验与计算方法联用可加速工业聚合催化剂的开发进程。本文以热稳定型Cat.1与典型布鲁克哈特催化剂（Brookhart catalyst，简称B-Cat）为模型，开展了α-二亚胺镍配合物（α-diimine nickel complexes）热稳定性的互补性实验与计算研究。实验结果显示，相较于B-Cat，在高温乙烯聚合过程中，Cat.1可使更多镍原子得到活化。计算层面，第一性原理计算表明，Cat.1的分解能高于B-Cat，这与实验中观测到的Cat.1更高的镍原子活化率相契合。研究发现，乙叉（ethydene）可显著影响Cat.1的C1–N1–Ni–N2–C2五元环（含镍中心）的构象，将B-Cat的信封式构象转变为Cat.1的半椅式构象。计算结果证实，后者的分解能较前者高出17.4 kJ/mol。本研究阐明了α-二亚胺镍催化剂的热稳定机制，有力推动了工业用热稳定型α-二亚胺镍催化剂的研发进程。