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

Binuclear zirconocene cations are observed by H-1-NMR in C6D6 solutions containing B(C6F5)(3) and an excess of a zirconocene dimethyl complex. The CH3-bridged cation [((C5H5)(2)ZrCH3)(2)(mu-CH3)](+), solvent-separated from the anion H3C-B(C6F5)(3)(-), is present in equilibrium with (C5H5)(2)Zr(CH3)(2) and the mononuclear ion pair [(C5H5)(2)ZrCH3+ ... H3C-B(C6F5)(3)(-)]; in more concentrated solutions, a binuclear ion pair [((C5H5)(2)ZrCH3)(2)(mu-CH3)(+) ... H3CB(C6F5)(3)] is the dominant species. Similar equilibria are observed in C6D6 solutions containing B(C6F5)(3) and (CH3)(4)C-2(C5H4)(2)Zr(CH3)(2), (CH3)(2)Si(C5H4)(2)Zr(CH3)(2) or rac-(CH3)(2)Si(indenyl)(2)Zr(CH3)(2). Complexes with sterically more demanding ligands, such as (C-5(CH3)(5))(2)Zr(CH3)(2) or rac-(CH3)(2)Si(2-methyl-benz[e]indenyl)(2)Zr do not form any binuclear species under these conditions. In the catalyst system rac-(CH3)(2)Si(indenyl)(2)Zr(CH3)(2)/Bu(3)NH(+)(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)(2)Si(indenyl)(2)ZrCH3)(2)(mu-CH3)](+). The latter appears to act, in the presence of propene, as a sourer of the mononuclear cation [(CH3)(2)Si(indenyl)(2)ZrCH3(C3H6)](+), rather than as a polymerization catalyst by itself.