Cis-1,4-Selective Polymerization of Isoprene Catalyzed by Guanidinate Rare-Earth Metal Complexes

Regioselective polymerization of isoprene enables the synthesis of polymers with well-defined microstructures, in which the ancillary ligand at the metal center plays a critical role in modulating polymerization performance. To investigate the effect of the ligand framework on polymerization reactivity, in this work, a series of guanidinate rare-earth metal bis(silylamide) complexes {(2,6-ⁱPr₂C₆H₃NH)[PhNC(2,6-ⁱPr₂C₆H₃N)]}Ln[N(SiHMe₂)₂]₂(THF) (Ln = Sc (1), Y (2), Lu (3), Sm (4), Nd (5)), {(2,6-ⁱPr₂C₆H₃NH)[(2-OMe-C₆H₄)NC(2,6-ⁱPr₂C₆H₃N)]Nd[N(SiHMe₂)₂]₂(THF) (6), and {(2,6-ⁱPr₂C₆H₃NH)[(2-F-C₆H₄)NC(2,6-ⁱPr₂C₆H₃N)]Nd[N(SiHMe₂)₂]₂(THF) (7) were synthesized via amine elimination reactions involving sterically demanding guanidines and one equivalent of Ln[N(SiHMe₂)₂]₃(THF)_n (Ln = Sc, n = 1; Ln = Y, Lu, Sm, Nd, n = 2). Upon activation with 1 equiv of [Ph₃C][B(C₆F₅)₄] and in the presence of excess AlMe₃, these complexes exhibited catalytic activity for isoprene polymerization. The cis-1,4 selectivity was found to be strongly dependent on both the rare-earth metal center and the electronic properties of the ligand framework. Notably, the catalytic system 7/[Ph₃C][B(C₆F₅)₄]/AlMe₃ not only afforded polyisoprene with high cis-1,4 selectivity (up to 99%) but also demonstrated well-controlled polymerization behavior at room temperature. Furthermore, a heterometallic Nd/Al alkyl complex, like {(2,6-ⁱPr₂C₆H₃NH)[(2-OMe-C₆H₄)NC(2,6-ⁱPr₂C₆H₃N)]}Nd[(μ-Me)₂AlMe₂]₂ was experimentally identified as the precatalyst for the cationic polymerization of isoprene.