Polymerization of Isoprene Promoted by Aminophosphine(ory)-Fused Bipyridine Cobalt Complexes: Precise Control of Molecular Weight and cis-1,4-alt-3,4 Sequence

Ligands N-(dialkyl or arylphosphino)-(2,2′-bipyridin)-6-amine (L1, aryl = Ph; L2, alkyl = tBu; L3, alkyl = adamantyl (Ad)) as well as the corresponding oxidized N-(2,2′-bipyridin-6-yl)-P,P-dialkyl or aryl phosphinic amide (L4, aryl = Ph; L5, alkyl = tBu; L6, alkyl = Ad) congeners were designed and coordinated to cobalt dichloride. The structures of formed complexes were characterized by IR and elemental analyses, as well as characterizations of the X-ray diffractions for complexes Co4 and Co6, which revealed the cobalt center is expectedly pentacoordinated in a distorted trigonal bipyramidal configuration with a prolonged Co—O(═P) bond. In combination with MMAO, complex Co2 was highly active in cis-1,4-alt-3,4 enchained polymerization. The hemilabile nature of O═P is possible for the alternating η4-cis-1,4 and η2-3,4 coordination, and insertion at the metal–carbon bond ensued. In combination with AlEt2Cl, each of complexes Co4, Co5, and Co6 was capable of converting isoprene to polyisoprene in a control m...