Observations and Mechanistic Insights on Unusual Stability of Neutral Nickel Complexes with a Sterically Crowded Metal Center

A series of novel nickel(II) methyl pyridine complexes based on β-ketiminato and phenoxyiminato ligands, [(2,6-iPr2C6H3)NCH(8-R1C10H7)O]Ni(Me)(Py) (5a, R1 = phenyl; 5b, R1 = norbornyl), [(2,6-iPr2C6H3)NCH(8-R1C10H5)O]Ni(Me)(Py) (6a, R1 = phenyl; 6b, R1 = norbornyl), and [(2,6-iPr2C6H3)NCHCHC(2′-R2C6H4)O]Ni(Me)(Py) (7a, R2 = H; 7b, R2 = phenyl), have been synthesized and characterized. Molecular structures of 5a and 6b were further confirmed by X-ray crystallographic analysis. These complexes showed very different catalytic properties for ethylene polymerization. Design and construction of a special catalyst structure like 5a with a phenyl group directed toward the nickel(II) center proved to be an effective strategy for improving catalyst stability. Remarkably, according to 1H NMR spectroscopy, bis-ligated deactivation of complex 5a was entirely avoided, while a rapid deactivation rate was observed with regard to the parent complex C without a phenyl group in the R1 position. As a result, higher activities of complex 5a were obtained relative to complex C under the same polymerization conditions. In addition, the ligand backbone was found to have a great influence on polymerization behaviors. Specifically, the polyethylenes with greatly decreased molecular weights were produced by neutral nickel phenoxyiminato catalysts 6a,b in comparison with values for the corresponding β-ketiminato catalysts 5a,b. This is best explained by DFT results that the ethylene insertion and chain termination barriers for complex 6a are very different from those of complex 5a.

本研究合成并表征了一系列基于β-酮亚胺基（β-ketiminato）与苯氧亚胺基（phenoxyiminato）配体的新型二价镍甲基（Me）吡啶（Py）配合物，具体包括：[(2,6-iPr₂C₆H₃)N=CH(8-R¹C₁₀H₇)O]Ni(Me)(Py)（5a，R¹=苯基；5b，R¹=降冰片基）、[(2,6-iPr₂C₆H₃)N=CH(8-R¹C₁₀H₅)O]Ni(Me)(Py)（6a，R¹=苯基；6b，R¹=降冰片基）以及[(2,6-iPr₂C₆H₃)N=CHCHC(2′-R²C₆H₄)O]Ni(Me)(Py)（7a，R²=H；7b，R²=苯基）。通过X射线晶体衍射分析（X-ray crystallographic analysis）进一步确证了配合物5a与6b的分子结构。该系列配合物在乙烯聚合反应中表现出显著差异化的催化性能。研究证实，构建带有指向二价镍中心的苯基取代基的特殊催化剂结构（如5a）是提升催化剂稳定性的有效策略。尤为关键的是，核磁共振氢谱（¹H NMR spectroscopy）检测结果显示，配合物5a完全避免了双配体失活现象；而R¹位未连苯基的母体配合物C则呈现出快速失活的特征。在相同聚合条件下，配合物5a的催化活性显著高于配合物C。此外，配体骨架对聚合行为具有显著影响：与对应β-酮亚胺基催化剂5a、5b相比，中性镍苯氧亚胺基催化剂6a、6b所制备的聚乙烯分子量大幅降低。这一现象可通过密度泛函理论（DFT, Density Functional Theory）计算结果得到合理解释：配合物6a的乙烯插入能垒与链终止能垒与配合物5a存在显著差异。