Electronic effects in homogeneous Ziegler–Natta catalysts: Zr–Rh early–late heterobimetallic complexes

Abstract

The correlation between electron density at zirconium and catalytic activity in olefin polymerization was investigated with exclusion of any steric factors. Two novel Zr–Rh heterobimetallic complexes, meso-(η⁵-C₉H₇)Rh{(η²-CH₂CH)₂Si(η⁵-C₅H₂Me₂)₂}ZrCl₂1m and (η⁵-C₉H₇)Rh{(η²-CH₂CH)₂Si(η⁵-C₅Me₄)₂}ZrCl₂2 (C₉H₇ = indenyl), were synthesized and 2 was structurally characterized. These complexes and the racemic isomer of 1 (1r) were investigated in terms of effects of the rhodium units on the electron density of the zirconium. The redox behavior examined by cyclic voltammetry and NMR study of the dimethylated complexes indicated the electron-rich character of zirconium in the bimetallic complexes 1r, 1m and 2. The catalytic activity of 1r and 1m for hexene polymerization was found to be significantly higher than for the corresponding monometallic complexes, and they formed polyhexene of higher molecular weight, while 2 did not show significant differences in its catalytic performance compared to those of the monometallic ones. The results implied that the electron-donating rhodium units give rise to higher catalytic activity in olefin polymerization unless they have sterically hindered structures.