Rare-earth dichloro and bis(alkyl) complexes supported by bulky amido–iminoligand. Synthesis, structure, reactivity and catalytic activity in isoprene polymerization

Abstract

A monoanionic amido–imino ligand system [(2,6-iPr₂C₆H₃)NC(Me)C(CH₂)N(C₆H₃-2,6-iPr₂)]⁻ was successfully employed for the synthesis of monomeric dichloro [(2,6-iPr₂C₆H₃)NC(Me)C(CH₂)N(C₆H₃-2,6-iPr₂)]LnCl₂(THF)₂ (Ln = Y, 2Y; Lu, 2Lu) and bis(alkyl) [(2,6-iPr₂C₆H₃)NC(Me)C(CH₂)N(C₆H₃-2,6-iPr₂)]Ln(CH₂SiMe₃)₂(THF) (Ln = Y, 4Y; Lu, 4Lu) species of yttrium and lutetium. Dichloro complexes 2Y and 2Lu turned out to be unstable in aromatic solvents. The ligand symmetrization reaction in the case of 2Y affords the yttrium complex coordinated by dianionic [(2,6-iPr₂C₆H₃)NC(CH₂)C(CH₂)N(C₆H₃-2,6-iPr₂)]²⁻ ligand, (2,6-iPr₂C₆H₃)NC(Me)C(Me)N(C₆H₃-2,6-iPr₂) and YCl₃. On the contrary, bis(alkyl) species 4Y and 4Lu are rather stable and do not undergo such a transformation or thermal decomposition. The treatment of complex 4Y with DME resulted in C–O bond cleavage and the formation of a dimeric methoxy-alkyl species {[(2,6-iPr₂C₆H₃)NC(Me)C(CH₂)N(C₆H₃-2,6-iPr₂)]Y(CH₂SiMe₃)(μ-OMe)}₂ (5). The ternary systems 4Ln/AliBu₃/borate (borate = [HNMe₂Ph][B(C₆F₅)₄] and [CPh₃][B(C₆F₅)₄]; molar ratio 1 : 10 : 1) performed high catalytic activity in isoprene polymerization and ability to convert into polymer 1000–5000 equivalents of isoprene in 20–120 min with quantitative conversion. The obtained polyisoprenes possessed high molecular weights (2.9 × 10⁴–4.1 × 10⁴) and moderate polydispersities (2.14–3.52). Predominant 3,4-regioselectivity (up to 78%) was observed.