Preparation of ansa-Niobocene and ansa-Tantalocene Olefin Hydride Complexes as Transition State Analogues in Metallocene-Catalyzed Olefin Polymerization

o examine the effects of cyclopentadienyl and olefin substitution on preferred stereochemistry, a series of singly [SiMe_2]-bridged ansa-niobocene and -tantalocene olefin hydride complexes has been prepared via reduction and alkylation of the corresponding dichloride complexes. In this manner, [Me_2Si(η^5-C_5H_4)(η^5-C_5H_3-3-R)]M(CH_2⚌CHR‘)H (M = Nb, Ta; R = CHMe_2, CMe_3; R‘ = H, C6H5; M = Ta; R = CHMe2, CMe3; R‘ = Me), rac- and meso-[Me_2Si(η^5-C_5H_3-3-R)(η^5-C_5H^3-3-R)]Nb(CH_2⚌CH_2)H (R = CMe_3), and [Me_2Si(η^5-C_5H_4)(η^5-C_5H^2-2,4-(CHMe_2)_2)]Ta(CH_2⚌CHR‘)H (R‘ = H, C_6H_5) have been prepared and characterized by NMR spectroscopy and, in some cases, X-ray diffraction. The doubly [SiMe_2]-bridged ansa-tantalocene ethylene hydride complex [(1,2-SiMe_2)_2(η^5-C_5H-3,5-(CHMe_2)_2)(η^5-C_5H_2-4-CMe_3)]Ta(CH_2⚌CH_2)H has been prepared from thermolysis of the methylidene methyl complex [(1,2-SiMe_2)_2(η^5-C_5H-3,5-(CHMe_2)_2)(η^5-C_5H_2-4-CMe_3)]Ta(CH_2)CH_3. 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-SiMe_2)_2(η^5-C_5H-3,5-(CHMe_2)_2)(η^5-C_5H_2-4-CMe_3)]Ta(CH_2⚌CHR‘)H (R‘ = CH_3, C_6H_5). 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.