C1-Symmetric {cyclopentadienyl/indenyl}-metallocene catalysts: synthesis, structure, isospecific polymerization of propylene and stereocontrol mechanism

Abstract

A series of new Me₂Si-bridged cyclopentadiene/indene proligands {Me₂Si(R^2′,5′₂-R^3′,4′₂-Cp)(R²,R⁴,R⁵,R⁶-Ind)H₂} (1a–j) with various substitutions both on the indene and cyclopentadiene moieties was prepared. The corresponding C₁-symmetric group 4 ansa-metallocene complexes (M = Zr, Hf), namely, {Me₂Si(Me₄Cp)(Ind)}ZrCl₂ (2a-Zr), {Me₂Si(Me₄Cp)(2-Me,4-Ph-Ind)}MCl₂ (2b-M), {Me₂Si(Me₄Cp)(2-Me,4-Ph,6-tBu-Ind)}ZrCl₂ (2c-Zr), {Me₂Si(Me₄Cp)(2-Me,4-Ph,5-OMe,6-tBu-Ind)}MCl₂ (2d-M), {Me₂Si(Me₄Cp)(2-R′,4-(3′,5′-tBu₂,4′-OMe-C₆H₂),5-OMe,6-tBu-Ind)}ZrCl₂, R′ = Me (2e-Zr), R′ = Et (2f-Zr), {Me₂Si(2,5-Ph₂-3,4-Me₂-Cp)(2-Me,4-(3′,5′-tBu₂,4′-OMe-C₆H₂),5-OMe,6-tBu-Ind)}ZrCl₂ (2g-Zr), {Me₂Si(Me₄Cp)(2-Me,4-(3′,6′-tBu₂-carbazol-4′-yl)-Ind)}ZrCl₂ (2h-Zr), {Me₂Si(2,5-Me₂,3,4-iPr₂-Cp)(2-Me,4-Ph-Ind)}ZrCl₂ (2i-Zr), {Me₂Si(2,5-Me₂,3,4-iPr₂-Cp)(2-Me,4-Ph,6-tBu-Ind)}ZrCl₂ (2j-Zr) and {Me₂Si(Me₄Cp)(2-Me-4,5-[a]anthracene-Ind)}MCl₂ (2k-Zr) were synthesized and characterized by NMR spectroscopy and mass spectrometry. The solid-state molecular structures of 2b-Zr, 2d-Zr, 2e-Zr, 2f-Zr, 2j-Zr and 2k-Zr were determined by X-ray crystallography. The zirconocene complexes, once activated with MAO in toluene solution, exhibited propylene polymerization activities at 60 °C up to 161 000 kg(PP) mol(Zr)⁻¹ h⁻¹, affording highly isotactic polypropylenes (iPP) with [m]⁴ up to 96.5% and T_m up to 157 °C. Also, metallocene complexes 2b–e-Zr were supported on SiO₂-MAO and evaluated in slurry bulk propylene polymerization at 70 °C, producing iPPs with [m]⁴ = 91.7–96.6 mol% and low regiodefects (0.2–0.3 mol%) content, with productivities up to 636 000 kg(PP) mol(Zr)⁻¹ h⁻¹. DFT calculations allowed rationalizing a polymerization reaction mechanism occurring through “chain-stationary” enchainment with preference for 1,2-insertions.