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-Bu-2(t))2]ZrCl2 and rac-[Me4Si2(C5H2-2,4-Bu-2(t))(2)]ZrCl2

Abstract

A series of chiral ansa zirconocene compounds that feature (i) [Me2Si] and [Me4Si2] bridges and (ii) two tert-butyl substituents on each cyclopentadienyl ring, namely, rac-[Me2Si(CPBut2)(2)]ZrCl2, rac-[Me2Si(CPBut2)(2)]Zr(Ph)Cl, rac-[Me2Si(CPBut2)(2)] ZrMe2, rac-[Me2Si(CPBut2)(2)]Zr(CO)(2), rac-[Me4Si2(Cp-But2)(2)] ZrCl2, and rac-[Me4Si2(CPBut2)(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-[Me4Si2(Cp-But2)(2)]ZrCl2 is the first structurally characterized ansa zirconocene complex with a [Me4Si2] bridge that adopts a racemo geometry. IR spectroscopic studies on the dicarbonyl complexes (Cp-1,Cp-3-But2)(2)Zr(CO)(2), rac-[Me2Si(Cp-But2)(2)]Zr(CO)(2), and rac-[Me4Si2(Cp-But2)(2)]Zr(CO)(2) demonstrate that the [Me2Si] and [Me4Si2] ansa bridges have opposite electronic influences in this system, with the [Me2Si] bridge exerting an electron-withdrawing effect and the [Me4Si2] bridge exerting an electron-donating effect. The [Me2Si] and [Me4Si2] bridges also exert a different influence with respect to olefin polymerization, with {rac-[Me2Si(Cp-But2)(2)]ZrCl2/MAO} being a more active catalyst system for polymerization of ethylene than is either {(Cp-1,Cp-3-But2)(2)ZrCl2/MAO} or {rac-[Me4Si2(Cp-But2)(2)]ZrCl2/MAO}.