Theoretical Studies of Ziegler-Natta Catalysis: Structural Variations and Tacticity Control

Models for the likely active catalysts in homogeneous Ziegler-Natta systems have been studied using ab initio quantum chemical methods. We investigated the geometries of the isoelectronic model complexes, X[sub 2]M-R where X = Cl or Cp = ([eta][sup 5]-C[sub 5]H[sub 5]); where M = Sc and Ti[sup +] (and also Ti); and where R = H, CH[sub 3], or SiH[sub 3]. The general trend is that the M = Sc compounds strongly prefer a planar configuration, whereas the M = Ti[sup +] cases generally prefer pyramidal geometries. This difference in geometry can be related to the differing ground-state electronic configurations for the metals: Sc is (4s)[sup 2](3d)[sup 1], whereas Ti[sup +] is (4s)[sup 1](3d)[sup 2]. The nonplanar geometry for [Cp[sub 2]Ti-R][sup +] suggests an explanation for the origin of stereospecificity in the syndiotactic polymerization by unsymmetric metallocene catalysts. These results suggest that ([eta][sup 5]-C[sub 5]H[sub 4])CMe[sub 2]([eta][sup 5]-fluorenyl)Sc-R would not catalyze syndiotactic polymerization under these conditions. 31 refs., 5 figs., 7 tabs.