Electron-poor transition metal complexes are of high interest in polymerization or oligomerization, but they require the use of a Lewis acid cocatalyst in order to reach the cationic active structure. The structure of the surface complexes obtained by grafting Hf(CH2tBu)4, 1, on y-alumina has been resolved by a combined experimental (mass balance analysis, labeling, in situ IR, NMR) and theoretical (DFT calculations) study. Thermolysis, oxidation, and hydrogenolysis reactions have unambiguously proved the presence of two kinds of neopentyl-metal bonds: Hf-CH2tBu and Al-CH 2tBu. Three coexisting surface complexes have been fully characterized and quantified: a monoaluminoxy [(≡AlIVO) Hf(CH2tBu)3], a neutral bis-aluminoxy [(≡Al IVO)(AlsO)Hf(CH2tBu)2], and a zwitterionic bis-aluminoxy complex [(≡AlIVO)(Al sO)Hf(CH2tBu)2]+-[(CH 2tBu)Als]- in 40%, 26%, and 34% yield, respectively. In 13C NMR calculations the important effect of spin-orbit coupling has been underlined on the chemical shifts of the carbon atoms directly linked to hafnium. Hence, a large fraction of the grafted complex is in a cationic structure, explaining why this system is active in polymerization (>103 kg of PE/mol of Hf·h·atm) without the need of a cocatalyst, since alumina plays the dual role of solid support and Lewis acid.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films