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

Abstract

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 Thia Me 2 )Ln(CH 2 SiMe 3 ) 2 [Ln = Lu 3+ ( 3 ), Er 3+ ( 7 ), Yb 3+ ( 8 )] and (L BnTh Me 2 )Lu(CH 2 SiMe 3 ) 2 ( 5 ). Similarly to related Y 3+ systems, the nature of the thiazole unit controls the ultimate catalyst stability in solution. In the diamagnetic Lu 3+ 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 Lu 3+ 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 (Lu 3+, Er 3+, Yb 3+; up to 76.4%) or a dominant 3,4 (Y 3+; 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.