Rare-earth metal bis(alkyl)s that bear a 2-pyridinemethanamineligand: Dual catalysis of the polymerizations of both isoprene and ethylene

Abstract

New pyridinemethanamido-ligated rare-earth metal bis(alkyl) complexes [C₅H₄N-CH(Me)-NC₆H₃(ⁱPr)₂]Ln(CH₂SiMe₃)₂(THF) (Ln = Sc (1), Y (2), Lu (3)) have been prepared at 0 °C via a protonolysis reaction between rare-earth metal tris(alkyl)s and the corresponding 2-pyridinemethanamine ligand and fully characterized by NMR and X-ray diffraction analysis. Bis(alkyl) complexes 1–3 are analogous monomers of THF solvate, where the ligand bonds to the metal center in a κN:κN-bidentate mode. Complexes 1–3, in combination with [Ph₃C][B(C₆F₅)₄], showed a good activity towards isoprene polymerization to give polyisoprene with a main 3,4-selectivity (60%–66%); in particular the yttrium catalyst system, 2/[Ph₃C][B(C₆F₅)₄], displayed a living mode. By contrast, only the precatalyst 2 exhibited activity for isoprene polymerization in the presence of [PhNMe₂H][B(C₆F₅)₄]. The influence of alkylaluminium (AlR₃, R = Me, Et, ⁱBu) and the metal center on the polymerization of isoprene was also studied, and it was found that addition of AlMe₃ to the catalyst systems could lead to a dramatic change in the microstructure of the polymer from 3,4-specific to 1,4-selective (89%–95%), but the ionic radius of the central metal had little influence on the selectivity. In addition, by using the 1(Sc)/[Ph₃C][B(C₆F₅)₄]/10 AlⁱBu₃, the polymerization of ethylene was also achieved with moderate activity (up to 3.2× 10⁵ g (PE) mol_Sc⁻¹ h⁻¹bar⁻¹) and narrow polydispersity (M_w/M_n = 1.19–1.28); while the effect of temperature on the activity was discussed. Such dual catalysis for the polymerizations of both isoprene and ethylene is rare.