Organolanthanide Complexes Supported by Thiazole-Containing Amidopyridinate Ligands: Synthesis, Characterization and Catalytic Activity in the Isoprene Polymerization

Neutral bis(alkyl)-organolanthanide complexes supported by tridentate (N-,N,N) monoanionic 5-methylthiazole- or benzothiazole-amidopyridinate ligands have been prepared and completely characterized: (LThiaMe2)Ln(CH2SiMe3)2 [Ln = Lu3+ (3), Er3+ (7), Yb3+ (8)] and (LBnThMe2)Lu(CH2SiMe3)2 (5). Similarly to related Y3+ systems, the nature of the thiazole unit controls the ultimate catalyst stability in solution. In the diamagnetic Lu3+ complex 5, a progressive and complete rearrangement of its metal coordination sphere takes place through a metal-to-ligand alkyl migration with subsequent benzothiazole ring-opening and generation of the Lu3+ mono(alkyl)-arylthiolate species stabilized by a tetradentate (N-,N,N,S)- dianionic ligand. On the other hand, the 5-methylthiazole containing complexes 3, 7 and 8 showed no evidence of any ligand rearrangement. Complexes 3-8 have been tested as homogeneous catalysts in the isoprene (IP) polymerization, after activation with selected organoborates. Binary systems 3/TB and 7/TB [TB = tritylium tetrakis(pentafluorophenyl)borate] show the highest activity and living character toward the IP polymerization affording polymers with relatively high trans-1,4-selectivity (up to 76.4 %), moderate molecular weights (Mn up to 146 000 g/mol) and narrow polydispersities (Mw/Mn). Depending on the rare-earth ion of choice, a prevalent trans-1,4 (Lu3+, Er3+, Yb3+; up to 76.4 %) or a dominant 3,4 (Y3+; 92.7 %) polymer structure is observed. The influence of the ligand type, metal ion and activator(s) on the ultimate catalyst activity and selectivity are discussed.