N‐Functionalized Pentamethyldihydrobenz[f]Indenyl Rare‐Earth‐Metal Complexes for Highly cis ‐1,4‐Selective Polymerization of Conjugated Dienes

ABSTRACT Three N‐functionalized pentamethyldihydrobenz[f]indenyl ligands bearing dimethylamino, pyrrolidinyl, or piperidinyl substituents and 16 corresponding rare‐earth‐metal complexes were synthesized and characterized. The molecular structures of 1a and 2a were confirmed by single‐crystal X‐ray diffraction. Upon activation with an alkylaluminum reagent and an organoborate, these complexes exhibited high activity and selectivity in butadiene and isoprene polymerization, with their catalytic performance strongly dependent on the ligand substituent, central metal, activation system, and temperature. Notably, the Gd‐based system 2d afforded polybutadiene in > 99% yield within 1 min with > 99.0% cis ‐1,4 content. The same complex afforded polyisoprene in 96% yield within 2 min with 98.1% cis ‐1,4 selectivity, which increased to 99.0% at −20°C.