Binuclear zirconocene cations with μ-CH₃-bridges in homogeneous Ziegler-Natta catalyst systems

Abstract

Binuclear zirconocene cations are observed by 1H-NMR in C6D6 solutions containing B(C6F5)3 and an excess of a zirconocene dimethyl complex. The CH3-bridged cation [((C5H5)2ZrCH3)2(μ-CH3)]+, solvent-separated from the anion H3CB(C6F5)−3, is present in equilibrium with (C5H5)2Zr(CH3)2 and the mononuclear ion pair [(C5H5)2ZrCH+3***H3CB(C6F5)−3]; in more concentrated solutions, a binuclear ion pair [((C5H5)2ZrCH3)2(μ-CH3)+***H3CB(C6F5)−3] is the dominant species. Similar equilibria are observed in C6D6 solutions containing B(C6F5)3 and (CH3)4C2(C5H4)2Zr(CH3)2, (CH3)2Si(C5H4)2Zr(CH3)2 or rac-(CH3)2Si(indenyl)2Zr(CH3)2. Complexes with sterically more demanding ligands, such as (C5(CH3)5)2Zr(CH3)2 or rac-(CH3)2Si(2-methyl-benz[e]indenyl)2Zr(CH3)2 do not form any binuclear species under these conditions. In the catalyst system rac-(CH3)2Si(indenyl)2Zr(CH3)2Bu3NH+B(C6F5)−4, activities for the polymerization of propene increase with excess of the dimethyl zirconocene complex. This effect is due in part to a sacrifice of some dimethyl zirconocene for the removal of impurities from the catalyst system and in part to a stabilization of the catalyst in the form of the binuclear cation [((CH3)2Si(indenyl)2ZrCH3)2(μ-CH3)]+. The latter appears to act, in the presence of propene, as a source of the mononuclear cation [(CH3)2Si(indenyl)2ZrCH3(C3H6)]+, rather than as a polymerization catalyst by itself.