Nickel(II) and Palladium(II) Polymerization Catalysts Bearing a Fluorinated Cyclophane Ligand: Stabilization of the Reactive Intermediate

Abstract

The synthesis and characterization of Ni(II) and Pd(II) α-diimine olefin polymerization catalysts bearing a fluorinated cyclophane-based ligand were performed. Fluorine was placed in such a manner as to interact with the metal center from the axial direction. The catalysts were active in the polymerization of ethylene, showing substantial differences in both catalytic behavior and polymer size and structure as compared to their nonfluorinated analogues. Both catalysts afforded polymer of comparatively low branching density and high molecular weight. The Ni(II) catalysts, from precursor [Ni(acetylacetonato)(F-Cyc)]+ salts (F-Cyc = fluorinated cyclophane), exhibited enhanced thermal stability by remaining active after 70 min with little loss in polymerization activity at 105 °C. The Pd(II) catalysts from salts of [Pd(F-Cyc)Me(NCR)]+ (NCR = nitrile) afforded polymer of molecular weights far higher than the nonfluorinated analogue. Additionally, polymerization activity was directly related to ethylene feed pressure for the Pd(II) system, and NMR analysis could not detect the presence of bound olefin, indicating that the polymerization proceeded via different kinetics involving an olefin-free 14 e− complex as the catalyst resting state. Furthermore, NMR 1H−19F coupling data provide clear evidence that the fluorine atoms were indeed interacting with the metal axial site. The unusual properties of these new complexes are thus attributed to stabilization of the highly reactive 14 e− intermediate by donation of the fluorine lone pair to the metal center.

Recent advances in Ni-mediated ethylene chain growth: Nimine-donor ligand effects on catalytic activity, thermal stability and oligo-/polymer structure

“…Indeed, 7b remained active after 70 minutes at 105 ºC with scarcely any loss in polymerization activity. In addition, the polymer formed displayed a bimodal molecular weight distribution, consisting of a high molecular weight major fraction (Mn ≈ 10 5 g mol -1 ) and a minor proportion based on ultrahigh molecular weight polymer (Mn > 10 6 gmol -1 ) [57]. The same group also synthesized the alkyl-bridged macrocyclic-containing 8 ( Fig.…”

“…4) [56,57]. Complex 7a exhibits excellent activity for ethylene polymerization with a productivity of 4.2 × 10 7 g(PE) mol -1 (Ni) h -1 [56].…”

(α-Diimine)palladium catalyzed ethylene polymerization and (co)polymerization with polar comonomers

“…The unique structure of the cyclophane ligand, especially its ability to shield the axial binding sites, significantly suppresses the associative monomer/comonomer exchange, thus enhances the incorporation of the polar comonomer [53,54]. The Pd(II) complex 4 bearing a fluorinated cyclophane ligand was also investigated [55]. It afforded the polymer of a comparatively low branching density and a high molecular weight.…”

The performance of methallyl nickel complexes and boron adducts in the catalytic activation of ethylene: a conceptual DFT perspective

“…Depending on the secondary ligand framework in a complex structure, it is possible to control molecular weight, polydispersity and branching of produced polyethylene [11][12][13]. However, many nickel catalyst systems still need to support a cocatalyst to enhance their activity [11][12][13][14][15][16].…”