Low-Temperature Spectroscopic Observation of Chain Growth and Migratory Insertion Barriers in (α-Diimine)Ni(II) Olefin Polymerization Catalysts

Ethylene Oligomerizations by Sterically Modulated Salicylaldimine Cobalt(II) Complexes Combined with Various Alkyl Aluminum Cocatalysts

“…5) for the salicylaldimine Co(II) complexes, as there are a lot of similarities between cobalt and nickel metal centers in the way they behave in catalysis. The alkyl ethylene complex in this mechanism constitutes the catalyst resting state and which has been identified by monitoring the ethylene chain growth at late-transition metals by low-temperature NMR spectroscopy [11,33]. This mechanism clearly proposes the barrier to chain transfer is very low as the open and less crowded structure does not hinder the olefinic units which can axially orient to form the transition state for b-hydrogen transfer process [17].…”

Agostic interactions in transition metal compounds

“…The catalyst resting states are generally the alkyl ethylene species, but after migratory insertion, ␤-agostic alkyl complexes have been shown to be intermediates through independent synthesis and spectroscopic characterization at low temperatures (Scheme 7) (80,(85)(86)(87)(88)(89). These ␤-agostic species undergo rapid ''chain walking'' by means of a series of formally ␤-elimination/ readdition reactions as shown in Scheme 7; however, density functional theory (DFT) studies suggest that a true olefin hydride intermediate actually never forms in these isomerizations (90).…”

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

“…An additional effect of the alkyl-substituted backbone of the ligand is that the molecular weight distributions of the polyethylenes formed are less than those produced by the planar acenaphthene systems [46]. The mechanism of polymerization catalyzed by nickel complexes bearing α-diimine ligands is generally considered to be based on an active species comprising a cationic nickel-alkyl/hydride [47,48]; chain-walking (β-H elimination/reinsertion) during the chain propagation step are the principal reasons for the formation of branched 6 polyethylene [49].…”

“…Furthermore, the axial pyridine unit in 9 reversibly interacts with the metal center, providing electrons for the active 14 valence electron metal center in the axial direction to stabilize it, suppressing β-H elimination [47][48][49], thereby preventing the occurrence of a chain transfer reaction and achieving temporary protection to the catalytic intermediates.…”