Multisubstituted C₂-symmetric ansa-metallocenes bearing nitrogen heterocycles: influence of substituents on catalytic properties in propylene polymerization at higher temperatures

In this work we systematically studied the effects of modifications of substituents on the performance of the isospecific zirconocene-based catalyst family, Me2Si(2-Alk-4-(N-carbazolyl)Ind)ZrX2 (X = Cl, Me), wherein the progenitor was shown to be particularly suitable in high-temperature propylene polymerization processes. In order to obtain the required zirconocenes, we developed a novel synthetic pathway to 4-(N-carbazolyl)indenes through Pd-catalyzed cyclizations of 2,2′-dibromobiaryls with 4-aminoindenes, which were synthesized via Buchwald–Hartwig reaction or electrophilic amination of 4-indenyl Grignard reagents with trimethylsilylmethyl azide. By a number of examples, the anion-promoted rac-to-meso isomerization method was shown to work reliably well for preparation of rac-ZrMe2-complexes. Certain zirconocenes among the 21 tested in propylene polymerization at 70 and 100 °C under MAO or borate activation outperformed the parent catalyst in molecular weight capability, regio- or stereoselectivity.