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

• Published in: Organometallics 2011-03, Vol.30 (5), p.925-934
• Main Authors: Song, Dong-Po, Wang, Yong-Xia, Mu, Hong-Liang, Li, Bai-Xiang, Li, Yue-Sheng
• Format: Article
• Language: English
• ISSN: 0276-7333; 1520-6041
• DOI: 10.1021/om100725h

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.