α -Diimine Iron(II) Complexes Featuring Axial Anagostic Interaction: A High Cis -1,4-Selective System for 1,3-Butadiene Polymerization

Abstract Iron complexes are renowned for promoting vinyl-selective polymerization of diene monomers, whereas systems capable of delivering high cis-1,4-selectivity remain relatively scarce. Herein, a series of α-diimine FeCl2 complexes featuring axial Fe...H–C anagostic interactions were synthesized and evaluated for 1,3-butadiene polymerization. Upon activation with AlR3/[Ph3C][B(C6F5)4], these complexes produce polybutadienes with much-enhanced cis-1,4-selectivity of up to 86%. Notably, the cis-1,4-selectivity remains largely insensitive to steric variations at distal ligand positions across the examined series (H vs tBu, cis-1,4-content of 81–84%), showing clear contrast to conventional α-diimine iron systems (cis-1,4-content <50%).