Pyridylamido Hafnium and Zirconium Complexes: Synthesis, Dynamic Behavior, and Ethylene/1-Octene and Propylene Polymerization Reactions

Hafnium and zirconium pyridylamido complexes were prepared by the reaction of deprotonated ligand with MCl4 followed by alkylation with MeMgBr. 1H NMR analysis of the isolated products revealed the presence of two complexes in each sample in about a 93:7 ratio. Both complexes were shown by 1D NOESY experiments to be in dynamic equilibrium with each other at ambient temperature. An analysis of the chemical shift differences between major and minor isomers together with low-temperature NOE experiments revealed that the difference between major and minor isomers is due to the rotation of the 2-iPr-phenyl group attached to the chiral center. Magnetization transfer experiments conducted at temperatures between 10 and 40 °C yielded ΔH‡ and ΔS‡ of 14.3(6) kcal/mol and −11(2) cal/mol·K, respectively. Density functional theory calculations resulted in very similar activation parameters. Additionally, this fluxional process was calculated to occur in the cationic species with a rate comparable to that of propylene propagation kinetics. Both complexes have been studied as procatalysts for ethylene/1-octene copolymerization and propylene polymerization reactions. 13C NMR analysis of polypropylene obtained under high-temperature polymerization conditions allowed for the unequivocal determination of the identity of a second 2,1-regioerror.本研究通过去质子化配体与四氯化金属（MCl₄）的反应，再经甲基溴化镁（MeMgBr）烷基化，制备得到吡啶酰胺基铪与锆配合物。对分离得到的产物进行氢核磁共振（¹H NMR）分析发现，每份样品中均存在两种配合物，其摩尔比约为93:7。通过一维核Overhauser效应谱（1D NOESY）实验证实，两种配合物在室温下处于动态平衡中。对比主要与次要异构体的化学位移差异，并结合低温NOE实验分析，结果表明二者的结构差异源于连于手性中心的2-异丙基苯基（2-iPr-phenyl）基团的旋转。在10至40℃温度区间内开展的磁化转移实验测得其活化焓（ΔH‡）为14.3(6) kcal/mol，活化熵（ΔS‡）为−11(2) cal/(mol·K)。密度泛函理论（DFT）计算得到的活化参数与实验结果极为接近。此外，计算表明该流变过程在阳离子物种中亦可发生，且其反应速率与丙烯聚合动力学速率相当。本研究对两种配合物作为乙烯/1-辛烯共聚与丙烯聚合的前催化剂进行了考察。通过对高温聚合条件下得到的聚丙烯进行碳核磁共振（¹³C NMR）分析，明确鉴定出了第二种2,1-区域错误结构。