Imidazolin-2-iminato titanium complexes: Synthesis, structure and use in ethylene polymerization catalysis

The Staudinger reaction of the imidazolin-2-ylidenes, 1,3-di-tert- butylimidazolin-2-ylidene (1a), 1,3-diisopropylimidazolin-2-ylidene (1b), 1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene (1c), 1,3-bis(2,4,6- trimethylphenyl)imidazolin-2-ylidene (1d) and 1,3-bis(2,6- diisopropylphenylimidazolin-2-ylidene (1e), with trimethylsilyl azide furnishes the corresponding N-silylated 2-iminoimidazolines 2a-e, which react with [(η-C₅H₅)TiCl₃] to afford half-sandwich cyclopentadienyl titanium complexes of the type [CpTi(L)Cl₂] (3) (L = imidazolin-2-iminato ligand). Similarly, the reactions of 1,3-di-tert-butyl-2- (trimethylsilylimino)imidazoline (2a) with [(η-tBuC₅H ₄)TiCl₃] results in the formation of [(η-tBuC ₅H₄)Ti(L)Cl₂] (4) (L = 1,3-di-tert- butylimidazolin-2-imide). Bis(1,3-di-tert-butylimidazolin-2-iminato)titanium dichloride (5) is obtained from the reaction of two eq. of 2a with TiCl ₄. Treatment of 5 with methyllithium results in the formation of the corresponding dimethyl complex [L₂Ti(CH₃)₂] (6), whereas [CpTi(L)(CH₃)₂] (7) is similarly obtained from 3a. The molecular structures of 3a, 3b, 3c, 3e·C₇H ₈, 4 and 7 are reported revealing linearly coordinated imidazolin-2-iminato ligands together with very short Ti-N bond distances. All dichloro complexes (3a-e, 4 and 5) can be activated with methylaluminoxane (MAO) to give active catalysts for ethylene homopolymerization. In most cases, moderate to high activities are observed together with the formation of high (HMWPE) or even ultra high molecular weight polyethylene (UHMWPE).