Alumina as a Simultaneous Support and Co Catalyst: Cationic Hafnium Complex Evidenced by Experimental and DFT Analyses

Abstract

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 γ-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−CH2tBu. Three coexisting surface complexes have been fully characterized and quantified: a monoaluminoxy [(≡AlIVO)Hf(CH2tBu)3], a neutral bis-aluminoxy [(≡AlIVO)(AlsO)Hf(CH2tBu)2], and a zwitterionic bis-aluminoxy complex [(≡AlIVO)(AlsO)Hf(CH2tBu)2]+[(CH2tBu)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.