Preparation of ansa-niobocene and ansa-tantalocene olefin hydride complexes as transition state analogues in metallocene-catalyzed olefin polymerization

To examine the effects of cyclopentadienyl and olefin substitution on preferred stereochemistry, a series of singly [SiMe₂]-bridged ansa-niobocene and -tantalocene olefin hydride complexes has been prepared via reduction and alkylation of the corresponding dichloride complexes. In this manner, [Me₂Si(η⁵-C₅H₄) (η⁵-C₅H₃-3-R)] M(CH₂= CHR′)H (M = Nb, Ta; R = CHMe₂, CMe₃; R′ = H, C₆H₅; M = Ta; R = CHMe₂, CMe₃; R′ = Me), rac- and meso-[Me₂-Si(η⁵-C₅ H₃-3-R)(η⁵-C₅H₃-3-R)]Nb (CH₂= CH₂)H (R = CMe₃), and [Me₂Si(η⁵-C₅H₂-2,4- (CHMe₂)₂)] Ta(CH₂=CHR′)H (R′ = H, C₆H₅) have been prepared and characterized by NMR spectroscopy and, in some cases, X-ray diffraction. The doubly [SiMe₂]-bridged ansa-tantalocene ethylene hydride complex [(1,2-SiMe₂)₂(η⁵-C₅H-3,5- (CHMe₂)₂)(η⁵- C₅H₂-4-CMe₃)]-Ta(CH₂=CH₂) H has been prepared from thermolysis of the methylidene methyl complex [(1,2-SiMe₂)₂(η⁵-C₅H-3,5- (CHMe₂)₂)(η⁵-C₅H₂- 4-CMe₃)]Ta(CH₂)CH₃. Addition of an excess of propylene or styrene to the tantalocene ethylene hydride results in olefin exchange and formation of the olefin hydride complexes [(1,2-SiMe₂)₂(η⁵-C₅H-3,5- (CHMe₂)₂) (η⁵-C₅H₂-4-CMe₃)]Ta (CH₂=CHR′)H (R′ = CH₃, C₆H₅). These compounds serve as stable transition state analogues for the much more kinetically labile group 4 metallocenium cationic intermediates in metallocene-catalyzed olefin polymerization. Characterization of the thermodynamically preferred isomers of metallocene olefin hydride complexes reveals that alkyl substitution on the cyclopentadienyl ligand array may have a significant effect on the stereochemistry of olefin coordination.