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

The correlation between electron density at zirconium and catalytic activit y in olefin polymerization was investigated with exclusion of any steric fa ctors. Two novel Zr-Rh heterobimetallic complexes, meso-(eta (5)-C9H7)Rh{(e ta (2)-CH2=CH)(2)Si(eta (5)-C5H2Me2)(2)}ZrCl2 1m and (eta (5)-C9H7)Rh{(eta (2)-CH2=CH)(2)Si(eta (5)-C5Me4)(2)}ZrCl2 2 (C9H7= indenyl), were synthesize d and 2 was structurally characterized. These complexes and the racemic iso mer 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 cycl ic voltammetry and NMR study of the dimethylated complexes indicated the el ectron-rich character of zirconium in the bimetallic complexes 1r, 1m and 2 . The catalytic activity of 1r and 1m for hexene polymerization was found t o be significantly higher than for the corresponding monometallic complexes , and they formed polyhexene of higher molecular weight, while 2 did not sh ow significant differences in its catalytic performance compared to those o f the monometallic ones. The results implied that the electron-donating rho dium units give rise to higher catalytic activity in olefin polymerization unless they have sterically hindered structures.