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_2M-R where X = Cl or Cp = (η^5-C_5H_5); where M = Sc and Ti^+ (and also Ti); and where R = H, CH_3, or SiH_3. The general trend is that the M = Sc compounds strongly prefer a planar configuration, whereas the M = Ti^+ 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)^2(3d)^1, whereas Ti^+ is (4s)^1(3d)^2. The nonplanar geometry for [Cp_2Ti-R]^+ suggests an explanation for the origin of stereospecificity in the syndiotactic polymerization by unsymmetric metallocene catalysts. These results suggest that {(η^5-C_5H_4)CMe_2(η^5-fluorenyl)}Sc-R would not catalyze syndiotactic polymerization under these conditions.