Chiral ansa zirconocene compounds with [Me2Si] and [Me4Si2] bridges and with tert-butyl ring substituents: Synthesis and structural characterization of the racemo complexes rac-[Me2Si(C5H2-2,4-But2)2]ZrCl2 and rac-[Me4Si2(C5H2-2,4-But2)2]ZrCl2

A series of chiral ansa zirconocene compounds that feature (i) [Me 2 Si] and [Me 4 Si 2 ] bridges and (ii) two tert-butyl substituents on each cyclopentadienyl ring, namely, rac-[Me 2 Si(Cp B u t 2) 2 ]-ZrCl 2 , rac-[Me 2 Si(Cp B u t 2) 2 ]Zr(Ph)Cl, rac-[Me 2 Si(Cp B u t 2) 2 ]ZrMe 2 , rac-[Me 2 Si(Cp B u t 2) 2 ]Zr(CO) 2 , rac-[Me 4 Si 2 (Cp B u t 2) 2 ]ZrCl 2 , and rac-[Me 4 Si 2 (Cp B u t 2) 2 ]Zr(CO) 2 , has been synthesized. In each case, the complex exists as the racemo isomer due to inter-ring steric interactions between the But substituents proximal to the ansa bridge that destabilize the meso isomer. rac-[Me 4 Si 2 -(Cp B u t 2) 2 ]ZrCl 2 is the first structurally characterized ansa zirconocene complex with a [Me 4 -Si 2 ] bridge that adopts a racemo geometry. IR spectroscopic studies on the dicarbonyl complexes (Cp 1 , 3 - B u t 2) 2 Zr(CO) 2 , rac-[Me 2 Si(Cp B u t 2) 2 ]Zr(CO) 2 , and rac-[Me 4 Si 2 (Cp B u t 2) 2 ]Zr(CO) 2 demonstrate that the [Me 2 Si] and [Me 4 Si 2 ] ansa bridges have opposite electronic influences in this system, with the [Me 2 Si] bridge exerting an electron-withdrawing effect and the [Me 4 -Si 2 ] bridge exerting an electron-donating effect. The [Me 2 Si] and [Me 4 Si 2 ] bridges also exert a different influence with respect to olefin polymerization, with {rac-[Me 2 Si(Cp B u t 2) 2 ]ZrCl 2 / MAO} being a more active catalyst system for polymerization of ethylene than is either {(Cp 1 , 3 - B u t 2) 2 ZrCl 2 /MAO} or {rac-[Me 4 Si 2 (Cp B u t 2) 2 ]ZrCl 2 /MAO}.