Theoretical studies on the role of bridging group of Cp ligands for the Ziegler-Natta catalysis

The homogeneous Ziegler-Natta reaction mechanisms of olefin with metallocenes by incorporation of nine bridging atoms or groups at the cyclopentadienyl (Cp) ring as ligands were studied by ab initio MO and density functional methods. The formation energy of the complex between ethylene and the metallocene by incorporation of bridging of oxygen atom is the most stable, and the difference from that of non-bridged system is about 5 kcal/mol. The energy barriers at the transition states from the reactants for all systems treated here indicate negative values, except for the MP3/6-31G(d) calculation values for the AlH and GaH bridged systems. The largest difference of the energy barriers at the transition states from the complexes is only 1-2 kcal/mol at the B3LYP calculation level. It was established that the distance between Ti atom and the bridging atom and/or the electronic density of the bridging group are closely related to the potential energy.