American Chemical Society
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Effects of Ligand Substitutions on the Rotation Rate of Indenyl Ligands in Bis(2-arylindenyl)zirconocenes by NMR Line-Shape Analysis and Relaxation in the Rotating Frame

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journal contribution
posted on 2004-05-10, 00:00 authored by Gregg M. Wilmes, Marcia B. France, Stephen R. Lynch, Robert M. Waymouth
Unbridged bis(2-arylindenyl)metallocene complexes, such as bis(2-phenylindenyl)zirconium dichloride (1a), when treated with appropriate activators are active catalyst precursors for the production of elastomeric polypropylene. The proposed mechanism for polymerization involves isomerization of the catalyst between geometries of differing stereoselectivity on a time scale slower than polymer chain growth. As the rotation of the 2-arylindenyl ligand is proposed to result in the different catalyst geometries, the rate at which this rotation occurs is of interest for the interpretation of the behavior of these complexes. A series of zirconocenes {bis(2-phenylindenyl)zirconium dimethyl (1b), bis(2-phenylindenyl)zirconium dibenzyl (1c), bis(2-(3,5-di-tert-butylphenyl)indenyl)zirconium dimethyl (2b), and bis(2-(3,5-di-tert-butylphenyl)indenyl)zirconium dibenzyl (2c)}, varying both in the indenyl ligand framework and the σ-ligand, has been synthesized. The rotation rates of the methyl-substituted metallocenes are too fast to be determined by standard NMR techniques such as EXSY, line-shape analysis, or magnetization transfer experiments. On-resonance spin−lattice relaxation in the rotating frame (T1ρ) has been used to determine the rate of rotation of the aryl ligands in 1b, 1c, and 2b and allowed comparison with the rates obtained for the benzylated species by line-shape analysis. The nature of both the π and the σ ligand systems affects the rate, with sterically bulky substituents resulting in slower rotation. The effect of the σ ligand is substantially stronger than that of the π ligand.