| Abstract
| - Ligand rotation rates of a series of bis(2-arylindenyl)zirconcenes, catalyst precursors for the production of elastomeric polypropylene, have been measured by NMR line-shape analysis and longitudinal magnetization relaxation in the rotating frame (T1ρ). Steric bulk on either the indenyl ligand framework or directly attached to the metal significantly slows the rate of rotation.
- Unbridged bis(2-arylindenyl)metallocene complexes, such as bis(2-phenylindenyl)zirconiumdichloride (1a), when treated with appropriate activators are active catalyst precursors forthe production of elastomeric polypropylene. The proposed mechanism for polymerizationinvolves isomerization of the catalyst between geometries of differing stereoselectivity on atime scale slower than polymer chain growth. As the rotation of the 2-arylindenyl ligand isproposed to result in the different catalyst geometries, the rate at which this rotation occursis 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 frameworkand the σ-ligand, has been synthesized. The rotation rates of the methyl-substitutedmetallocenes are too fast to be determined by standard NMR techniques such as EXSY,line-shape analysis, or magnetization transfer experiments. On-resonance spin−latticerelaxation in the rotating frame (T1ρ) has been used to determine the rate of rotation of thearyl ligands in 1b, 1c, and 2b and allowed comparison with the rates obtained for thebenzylated species by line-shape analysis. The nature of both the π and the σ ligand systemsaffects the rate, with sterically bulky substituents resulting in slower rotation. The effect ofthe σ ligand is substantially stronger than that of the π ligand.
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