Mechanistic Studies of Nickel(II) Alkyl Agostic Cations and Alkyl Ethylene Complexes:  Investigations of Chain Propagation and Isomerization in (α-diimine)Ni(II)-Catalyzed Ethylene Polymerization

Abstract

The synthesis of a series of (alpha-diimine)NiR(2) (R = Et, (n)Pr) complexes via Grignard alkylation of the corresponding (alpha-diimine)NiBr(2) precursors is presented. Protonation of these species by the oxonium acid [H(OEt(2))(2)](+)[BAr'(4)](-) at low temperatures yields cationic Ni(II) beta-agostic alkyl complexes which model relevant intermediates present in nickel-catalyzed olefin polymerization reactions. The highly dynamic nature of these agostic alkyl cations is quantitatively addressed using NMR line broadening techniques. Trapping of these complexes with ethylene provides cationic Ni alkyl ethylene species, which are used to determine rates of ethylene insertion into primary and secondary carbon centers. The Ni agostic alkyl cations are also trapped by CH(3)CN and Me(2)S to yield Ni(R)(L)(+) (L = CH(3)CN, Me(2)S) complexes, and the dynamic behavior of these species in the presence of varied [L] is discussed. The kinetic data obtained from these experiments are used to present an overall picture of the ethylene polymerization mechanism for (alpha-diimine)Ni catalysts, including effects of reaction temperature and ethylene pressure on catalyst activity, polyethylene branching, and polymer architecture. Detailed comparisons of these systems to the previously presented analogous palladium catalysts are made.

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.…”

Integrating Catalyst and Co‐Catalyst Design in Olefin Polymerization Catalysis: Transferable Dianionic Ligands for the Activation of Late Transition Metal Polymerization Catalysts

“…The quest for new, MAO-free homogeneous polymerization catalysts has led to many important advances, facilitating the progress of mechanistic investigation and revealing the important role of the counter-anion in the polymerization process. [4][5][6] However, most of these systems rely in the use of sensitive transition metal alkyl complexes and sophisticated boron-based Lewis or protic acids, that are unattractive for practical applications. In this contribution we propose a strategy for the activation of simple and easily handled precursors, based on the complementary design of both the pre-catalyst and the co-catalyst components, devised to favor an efficient generation of active polymerization catalysts.…”