Organolanthanide Complexes Supported by Thiazole-Containing Amidopyridinate Ligands: Synthesis, Characterization, and Catalytic Activity in 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: (L^ThiaMe₂)­Ln­(CH₂SiMe₃)₂ [Ln = Lu³⁺ (3), Er³⁺ (7), Yb³⁺ (8)] and (L^BnThMe₂)­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 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 IP polymerization, affording polymers with relatively high trans-1,4-selectivity (up to 76.4%), moderate molecular weights (M_n up to 146 000 g/mol), and narrow polydispersities (M_w/M_n). 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 is discussed.