Bis(tetramethylaluminate) Lanthanide Complexes Supported by Bi- and Tridentate Amidinate Ligands: Performance in Isoprene Polymerization

The isolable bis(tetramethylaluminate) lanthanide complexes [2-R-2,6-XC6H4NC(tBu)(2,6-(R1C6H3)-C-2)]Ln(AlMe4) (2) (R-1 = R-2 = X = Me, Ln = La (1); R-1 = R-2 = X = Me, Ln = Nd (2); R-1 = iPr, R-2 = H, X = OMe, Ln = La (3); R-1 = iPr, R-2 = H, X = OMe, Ln = Nd (4)) coordinated by bi- and tridentate amidinate ligands were synthesized by the reactions of the amidines 2,6-Me2C6H3N-C(tBu)NH-2,6-Me2C6H3 and 2,6-iPr(2)C(6)H(3)N=C(tBu)NH-2MeOC(6)H(4) with Ln(AlMe4)(3) (Ln = La, Nd) and were isolated in 81, 75, 86, and 74% yields, respectively. X-ray analyses revealed that complexes 3 and 4 feature intramolecular coordination of an OMe group to the metal ions. Both AlMe4 fragments in complexes 3 and 4 coordinate to the metal centers in a kappa(2) mode, while in Nd complex 2 one AlMe4 group adopts a kappa(2) and the second a kappa(3)-coordination mode. Complexes 1-4 were evaluated as precatalysts for isoprene polymerization. The ternary catalytic systems 1-4/[Ph3C][B(C6F5)(4)],[PhNHMe2][B(C6F5)(4)]/nAlR(3) (AlR3 = AlMe3, n = 10; AliBu(2)H, n = 1; AliBu(3), n = 10) enable 82-100% conversions of 1000 equiv of monomer at 25 degrees C within 5-60 min. cis-1,4-Selectivity of the ternary catalytic systems proved to be affected by the type of ligand, the borate, the AlR3 cocatalyst, and the nature of the metallo center and varied in the range of 34.4-91.8%. The binary systems 1-4/[Ph3C][B(C6F5)(4)], [PhNHMe2][B(C6F5)(4)] also catalyze isoprene polymerization and afford polyisoprenes containing 96.2-99.0% of cis-1,4 units in polymer chains. The stereoselectivity of binary systems containing complex 1 dramatically depends on the type of borate: 66.8% of trans-1,4 for [Ph3C][B(C6F5)(4)] and 87.8% of cis-1,4 for [PhNHMe2][B(C6F5)(4)].