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 [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 2 Si(η 5 -C 5 H 4 )(η 5 -C 5 H 3 -3-R)]M(CH 2 CHR‘)H (M = Nb, Ta; R = CHMe 2, CMe 3; R‘ = H, C 6 H 5; M = Ta; R = CHMe 2, CMe 3; R‘ = Me), rac- and meso- [Me 2 Si(η 5 -C 5 H 3 -3-R)(η 5 -C 5 H 3 -3-R)]Nb(CH 2 CH 2 )H (R = CMe 3 ), and [Me 2 Si(η 5 -C 5 H 4 )(η 5 -C 5 H 2 -2,4-(CHMe 2 ) 2 )]Ta(CH 2 CHR‘)H (R‘ = H, C 6 H 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 5 H-3,5-(CHMe 2 ) 2 )(η 5 -C 5 H 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 5 H-3,5-(CHMe 2 ) 2 )(η 5 -C 5 H 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 5 H-3,5-(CHMe 2 ) 2 )(η 5 -C 5 H 2 -4-CMe 3 )]Ta(CH 2 CHR‘)H (R‘ = CH 3, C 6 H 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.