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

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(CH(2)tBu)(4), 1, on gamma-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-CH(2)tBu and Al-CH(2)tBu. Three coexisting surface complexes have been fully characterized and quantified: a monoaluminoxy [( AlIVO)Hf(CH(2)tBu)(3)] a neutral bis-aluminoxy [( AlIVO)(AlsO)Hf(CH(2)tBu)(2)], and a zwitterionic bis-aluminoxy complex [( AlIVO)(AlsO)Hf(CH(2)tBu)(2)](+)-[(CH(2)tBu)Al-s](-) in 40%, 26%, and 34% yield, respectively. In C-13 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 (> 10(3) 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.