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