Bis(“ferrocene-saliminato”)group 4 metal complexes: synthesis, structural features and use in homogenous Ziegler–Natta polymerization catalysis

Abstract

Reaction of the (p-S) enantiomers of the N-aryl-substituted planar chiral α-hydroxy-ferrocene carbaldimines [(p-S)-3 (aryl = 2,6-diisopropylphenyl (a), –C₆F₅ (c)] with LDA gave the corresponding lithiated “ferrocene-saliminato” systems (p-S)-4a and (p-S)-4c, respectively. Both were characterized by X-ray diffraction. In the crystal, compound (p-S)-4a features an associated cyclotrimeric structure with a six-membered alternating Li₃O₃ core while (p-S)-4c shows a cyclotetrameric structure around a central cubic Li₄O₄ core unit. The reagent (p-S)-4a was treated with the group 4 metal tetrahalides to yield the bis(ferrocene-saliminato)MCl₂ complexes 5a (Ti), 6a (Zr), and 7a (Hf). In the crystal the latter feature single diastereomeric octahedral (p-S,p-S,Λ)-6a (Zr) and (p-S,p-S,Λ)-7a (Hf) structures, whereas the titanium analogue shows both the (p-S,p-S,Λ)-5a and the (p-S,p-S,Δ)-5a diastereoisomers in the crystal. Upon dissolving, we observe a rapid Δ-5a to Λ-5a isomerization to occur. The latter is the only isomer observed in solution under thermodynamic control. In contrast, the pure Λ-6a and Λ-7a isomers equilibrate in solution with a second isomer to a ca. 1 : 1 mixture. Treatment of (p-S)-4c (aryl = C₆F₅) with TiCl₄ or ZrCl₄ gave the analogous octahedral complexes 5c and 6c, respectively, as single diastereoisomers. Treatment of the neutral ferrocene-salimines (p-S)-3a–c with ZrCl₄ or HfCl₄ gave the respective N-aryl-substituted zwitterionic (ferrocene-salimine)₂MCl₄ complexes (8a: Zr, 2,6-diisopropylphenyl; 9a: Hf, 2,6-diisopropylphenyl; 8b: Zr, mesityl; 8c: Zr, C₆F₅), that contain the zwitterionic iminium-tautomers O-bonded to the metal. Activation of a variety of the complexes 5–7 and 8 with methylalumoxane gave homogeneous ethylene polymerization catalysts that exhibit low to moderate catalyst activities in the 20 °C to 125 °C temperature range.