(“Ferrocene-salaldiminato”)zirconium Complexes for Ethylene Polymerization Catalysis: The Role of the Bulky Substituents

We prepared the 2-hydroxy-5-trimethylsilylferrocenecarbaldimine
systems 13 enantioselectively by a short synthetic route
starting from the chiral ferrocene carbaldehyde acetal 5. The imines 13a–c were used to synthesize the
zwitterionic (“ferrocene-salaldiminato”)2ZrCl4 complexes 14a–c. Activation with MAO gave homogeneous Ziegler–Natta catalysts,
which showed only a moderate ethylene polymerization activity, similar
to the unsubstituted parent “ferrocene-salaldiminato”
zirconium systems. Consequently, a synthetic route was devised and
carried out to make the corresponding 2-hydroxy-3,5-bis­(trimethylsilyl)­ferrocenecarbaldimines
(21) available. The enantiomerically highly enriched
ligands (pS)-21a and
-d derived from the aldehyde and aniline or cyclohexylamine,
respectively, were used to synthesize the corresponding (“ferrocene-salaldiminato”)2ZrCl4 complexes 22a,d. Both gave highly active homogeneous Ziegler–Natta catalysts
upon activation with MAO for the formation of linear low molecular
weight polyethylene. Complex 22d and many ligand precusors
from both sequences were characterized by X-ray diffraction.