Chiral-at-ansa-Bridged Group 4 Metallocene Complexes {(R1R2C)-(3,6-tBu2Flu)(3-R3-5-Me-C5H2)}MCl2: Synthesis, Structure, Stereochemistry, and Use in Highly Isoselective Propylene Polymerization

International audienceNew chiral racemic, sterically crowded fluorenyl-cyclopentadienyl compounds having monoaryl-substituted methylene bridges (R1R2C)-((3,6-tBu2Flu)H)(3-R3-5-Me-C5H3) (R1 = H, R2 = Ph, R3 = tBu: 2a; R1 = H, R2 = 2,4,6-Me3C6H2 (Mes); R3 = CMe2Ph (cumyl): 2b; R1 = H, R2 = 3,5-(CF3)2C6H3, R3 = tBu: 2c) or a disubstituted (R1 = CF3, R2 = Ph, R3 = tBu: 2d) as well as an unsubstituted (nonstereogenic) methylene bridge (R1 = R2 = H, R3 = tBu: 2e) were synthesized via nucleophilic additions to substituted fulvenes. The zirconium and hafnium dichloro complexes derived from proligands 2a, 2b, and 2e, namely, {Ph(H)C-(3,6-tBu2Flu)(3-tBu-5-Me-C5H2)}ZrCl2 (3a), {Ph(H)C-(3,6-tBu2Flu)(3-tBu-5-Me-C5H2)}HfCl2 (4a), {Mes(H)C-(3,6-tBu2Flu)(3-cumyl-5-Me-C5H2)}ZrCl2 (3b), and {H2C-(3,6-tBu2-Flu)(3-tBu-5-Me-C5H2)}ZrCl2 (3e), were prepared (63−88% yields). Attempted metalations at the fluorinated proligands 2c and 2d were not successful. Due to the presence of the stereogenic center of the ansa-bridge, 3a, 4a, and 3b exist as mixtures of two diastereomers in which the most bulky substituent in the methylene bridge is either anti (major product in the case of 3a and 4a) or syn (only one observed for 3b) to the 5-methyl substituent in the Cp ring. Diastereomerically pure anti-3a, anti-4a and syn-4a, and syn-3b were isolated and characterized by elemental analysis, NMR spectroscopy, and X-ray crystallography. When activated with MAO, metallocene complex anti-3a showed very high activity in the polymerization of propylene (14 000 kg(PP)*mol(Zr)−1*h−1, toluene, 60 °C) to yield highly isotactic polypropylene (iPP) with [m]4 = 95.8% and Tm = 153 °C. Both diastereomers of hafnium analogues exhibited lower activities (260−270 kg(PP) mol(Hf)−1*h−1) but distinct catalytic behavior: anti-4a yielded low molecular weight and moderately isotactic PP ([m]4 = 88.8%), while highly isotactic PP ([m]4 = 94.0%) with a bimodal distribution was produced with syn-4a. Precursor syn-3b was inactive under standard polymerization conditions, presumably due to deactivation of the cationic metal center by the cumyl Cp substituent. Methylene-bridged 3e produced highly isotactic PP ([m]4 = 96.9%; Tm = 154 °C) with moderate activity (2 320 kg(PP)*mol(Zr)−1*h−1)