The role of agostic interaction in the mechanism of ethylene polymerisation using Cr(iii) half-sandwich complexes: What dictates the reactivity?

Abstract

Chromium-based catalysts play a significant role in the production of ultra-high molecular weight polyethylene, and half-sandwich functionalised-metallocene complexes were proven to be one of the most suitable candidates as catalysts for generating such large polymeric-length olefins. Earlier experimental studies on olefin polymerisation using a series of catalysts such as [L^1-2CrCl₂] (where L¹ = 1-((pyridin-2-yl)methyl)indenyl (1) and L² = 2-methyl-1-{[4-(yridinene-1-yl)yridine-2-yl]methyl}-1H-indenyl (2)) reveal significant variation where peripheral substitution on the ligand was found to influence the reactivity significantly. However, the specific ligand position that affects the reactivity has not been established. As these reactions are fast and robust, it is challenging to establish reactive intermediates via experiments, and therefore, mechanistic clues for such reactions are elusive. Here we have undertaken a detailed computational study by employing an array of DFT (uB3LYP-D3/def2-TZVP, CASSCF/NEVPT2, and DLPNO-CCSD(T) methods to explore the substituted and non-substituted pyridine-cyclo-pentadienyl chromium complexes and their influence on the catalytic activity in ethylene polymerisation. Our study not only unravels the catalytic pathway for olefin polymerisation for such Cr(III)-half-sandwich complexes but also reveals that the energetics of the formation of pseudo-three-coordinate alkyl intermediates is key to the variation in the reactivity observed. A detailed examination using MO and NBO analysis unveils the presence of a C–H⋯Cr agostic interaction that is found to significantly stabilise this intermediate when the pyridine ligand has strong electron-donating groups at its para position. The other substitutions, such as on the cyclopentadienyl ligand, neither yield the desired stability nor the desired interaction. Further studies on models support this proposal. In order to improve the efficiency and selectivity of catalytic systems in olefin polymerisation, we strongly advocate for the integration of agostic interactions as a crucial criterion in the design of future catalysts. Considering the prevalence of electron-deficient metal centres in successful olefin polymerisation catalysts, this research prompts a broader mechanistic inquiry to propose a unified approach for this industrially crucial reaction.